GSA Special Publication No.16: The Adelaide Geosyncline, 1990 by GSAustralia

THE EVOLUTION OF A LATE PRECAMBRIAN EARLY PALAEOZOIC RIFT COMPLEX: THE ADELAIDE GEOSYNCLINE

EDITORS: J.B. JAGO & P.S. MOORE

SPECIAL PUBLICATION NO. 16 GEOLOGICAL SOCIETY OF AUSTRALIA INCORPORATED JANUARY 1990

National Library of Australia ISBN 0 909869 71 5 ISSN 0072 1085

GEOLOGICAL SOCIETY OF AUSTRALIA INCORPORATED

President: I.R. Johnson Vice-Presidents: D.M. Boyd, D.H. Green Hon. Secretary: G.F. Taylor Hon. Treasurer: P.S. Moore Hon. Administrative Officer: D.H. Probert

Copies of Special Publications and the Society's Journal are available from:

The Business Manager, Geological Society of Australia Incorporated Room 1001, Challis House, 10 Martin Place Sydney 2000, N.S.W. Australia

Printed by Watson Ferguson & Company, Brisbane, QUEENSLAND Typesetting by METAGraphics Pty Ltd, Brisbane, QUEENSLAND

DEDICATION

This volume is dedicated to Brian Daily who died on March 6, 1986. After doing his Honours Degree at the University of Adelaide in 1952, Brian went on to do his Ph.D on the Cambrian stratigraphy and palaeontology of South Australia. This work, published as a paper in the proceedings of the 20th International Geological Congress (Mexico, 1956), has been the basis of most subsequent studies on the Cambrian of South Australia. The faunal assemblages established by Brian on this work have been used as a reference succession for Australian Lower Cambrian sequences. Brian's employment with the South Australian Museum (1956-60) and the University of Adelaide (1961-1986) saw him continue his research on the Precambrian and Cambrian palaeontology, stratigraphy, and sedimentology of South Australia. His very substantial contribution to these fields is shown by the large number of references to his work contained in the papers of this volume. Brian's main forte was as a dedicated and observant field geologist. Although he published over 40 papers, a considerable amount of his work was unpublished at the time of his death. However, his influence on the understanding of the Precambrian and Early Palaeozoic geology of the Adelaide Geosyncline is shown by the quality of the papers in this volume, almost all of which have been written by his former colleagues and students.

PREFACE This volume contains 27 papers on the Adelaidean-Ordovician geology of the Adelaide Geosyncline. The content and variety of the papers reflect the considerable amount of new and exciting work currently being undertaken, particularly in the Mt Lofty and Hinders Ranges, by geologists from the South Australian Department of Mines and Energy, various mining companies, Adelaide and Hinders Universities and the South Australian Institute of Technology. Many of the papers are either on sedimentology and stratigraphy or deal with Cambrian sequences, which is appropriate in a volume dedicated to the late Dr. Brian Daily. However, there are also significant contributions on igneous, metamorphic and tectonic history. The current rapid rate of progress in the understanding of the geology of the Adelaide Geosyncline is reflected in the fact that much of the work in the present volume has been done since the publication of Preiss' (1987) monumental work on the Adelaide Geosyncline (Geological Survey of South Australia Bulletin 53). It also should be noted that Bulletin 53 did not deal with the Cambrian rocks. Over half of the papers in the present volume deal with Cambrian sediments and Delamerian granites and tectonics. The Late Proterozoic-Cambrian sedimentary sequences of the Hinders Ranges are superbly exposed, are essentially unmetamorphosed and represent one of the best, and most easily accessible, areas in the world to study sedimentary sequences of these ages. In the Hinders Ranges the Adelaidean sequence is up to 20 km thick while the Cambrian sediments of the Heurieu Peninsula/Kangaroo Island area may be up to 20 km thick. The excellent exposures of the Adelaidean rocks of the Hinders Ranges, plus the widespread occurrence of the Ediacara metazoan fossil assemblage, have led to several suggestions that the upper part of the Adelaidean sequence should be considered as a reference area for a proposed Late Proterozoic Ediacar(i)an Period and System. This matter was discussed in detail during 1981 by Jenkins (Transactions of the Royal Society of South Australia, 105,179-194) and Cloud and Glaessner (Science, 217,783-792). The Precambrian-Cambrian boundary as exposed in the Adelaide Geosyncline has attracted considerable attention as explained by Glaessner in this volume. Among other significant features of the Late Precambrian sequences are the two phases of glacial activity represented in the Adelaide Geosyncline. A recent, and fascinating, development has been the recognition of widespread impact ejecta horizon within the Bunyeroo Formation of the Hinders Ranges, plus a probable impact structure on the Gawler Craton. Possible Late Proterozoic submarine canyons have also been described from the Hinders Ranges. The Cambrian successions of the Adelaide Geosyncline are significant in that they include the best exposed and most complete Early Cambrian sequence in Gondwanaland. They are also of historical interest in that one of the earliest major works on the Archaeocy atha was done by Taylor (1910, Memoirs of the Royal Society of South Australia, 2, 55-188) from specimens collected in the Mt Lofty and Hinders Ranges. As with the Proterozoic rocks the Cambrian sequences show complex palaeogeographic patterns, e.g. there is a transition from the non-marine/shallow marine sequences of Yorke Peninsula to the submarine fan complexes within the Kanmantoo Group on Heurieu Peninsula and Kangaroo Island.

During the Late Proterozoic and Cambrian the sediments of the Adelaide Geosyncline were deposited on the eastern edge of the Australian/Antarctic craton. The type of crust to the east of the Adelaide Geosyncline during this period of time is still a matter of debate. The Late Cambrian-Early Ordovician Delamerian Orogeny affected not only the rocks of the Adelaide Geosyncline but is part of a much more extensive event which is reflected in the rocks of the Transantarctic Mountains. Recent developments in conceptual thinking on the geology of the Adelaide Geosyncline include attempts to apply plate tectonic models and sequence stratigraphy to both the Adelaidean and Cambrian sequences. In summary the Adelaide Geosyncline is a very large basin with a long depositional history, which spans a very significant time in the development of the history of life.

ACKNOWLEDGEMENTS The editors wish to thank all authors for their contributions and for their patience during the various stages of the production of this volume. Helpful and constructive reviews were given by many referees. Considerable assistance was received from the South Australian Department of Mines and Energy and the Department of Applied Geology, South Australian Institute of Technology. J.B. Jago P.S. Moore *

Adelaide, May, 1989

Table of Contents 1.

W. V. PREISS A stratigraphic and tectonic overview of the Adelaide Geosyncline, South Australia

D. HILYARD Willouran Basic Province: Stratigraphy of Late Proterozoic flood basalts, Adelaide Geosyncline, South Australia

A. J. CRAWFORD and D. HILYARD Geochemistry of Late Proterozoic tholeiitic flood basalts, Adelaide Geosyncline, South Australia

B. G. FORBES Geology of the Willouran Ranges

A. P. BELPERIO Palaeoenvironmental interpretation of the Late Proterozoic Skillogalee Dolomite in the Willouran Ranges, South Australia

R. K. UPPILL Sedimentology of a dolomite - magnesite - sandstone sequence in the late Precambrian Mundallio Subgroup, South Australia

105

A. J. PARKER, W. M. COWLEY and B. P. THOMSON The Torrens Hinge Zone and Spencer Shelf with particular reference to early Adelaidean volcanism

129

N. M. LEMON and V. A. GOSTIN Glacigenic sediments of the late Proterozoic Elatina Formation and equivalents, Adelaide Geosyncline, South Australia

149

P. S. PLUMMER Late Precambrian wave-to tide-dominated delta evolution in the west-central Adelaide Geosyncline, South Australia

164

10. P.W.HAINES A late Proterozoic storm-dominated carbonate shelf sequence: The Wonoka Formation in the central and southern Flinders Ranges, South Australia

. . 177

11. M. F. GLAESSNER Problems of the base of the Cambrian: A review of Daily's contributions and of further tasks

199

12. S. TOTEFF The Adelaide Supergroup - Kanmantoo Group contact, eastern Mount Lofty Ranges, South Australia

207

13. B. DAILY Cambrian stratigraphy of Yorke Peninsula

215

14. J. D.A.CLARKE Slope facies deposition and diagenesis of the Early Cambrian Parara Limestone, Wilkawillina Gorge, South Australia

230

15. J. D.A.CLARKE Platform carbonate deposition and diagenesis, Woodendinna Dolomite and lower Wilkawillina Limestone (Early Cambrian), Wilkawillina Gorge, South Australia

. . 247

16. E.M. ALEXANDER and D.I. GRAVESTOCK Sedimentary facies in the Sellick Hill Formation, Fleurieu Peninsula, South Australia

269

17. F. DEBRENNE and D. I. GRAVESTOCK Archaeocyatha from the Sellick Hill Formation and Fork Tree Limestone on Fleurieu Peninsula, South Australia

290

18. P.D. KRUSE Are archaeocyaths sponges, or are sponges archaeocyaths?

310

19. R.C. SPRIGG Implications of the discovery of Archaeocyatha in the Macclesfield Marble, Mount Lofty Ranges

324

20.

P.S.MOORE Origin of redbeds and variegated sediments, Cambrian, Adelaide Geosyncline, South Australia

334

21. C. G. GATEHOUSE, J. B. JAGO and B. J. COOPER Sedimentology and stratigraphy of the Carrickalinga Head Formation (low stand fan to high stand systems tract), Kanmantoo Group, South Australia

351

22. N. S. MANCKTELOW The structure of the southern Adelaide Fold Belt, South Australia

369

23. R. J. F. JENKINS The Adelaide Fold Belt: Tectonic reappraisal

396

24. A. R. MILNES The Encounter Bay Granites, Fleurieu Peninsula and Kangaroo Island

421

25. R. S. MORRISON and J. D. FODEN A zoned Middle Cambrian pluton in the Peake and Denison Ranges, South Australia

. . . 450

26. J. D. FODEN, S. P. TURNER and R. S. MORRISON Tectonic implications of Delamerian magmatism in South Australia and western Victoria

. 465

27. M. SANDIFORD, R. L. OLIVER, K. J. MILLS and R. V. ALLEN A cordierite-staurolite-muscovite association, east of Springton, Mt Lofty Ranges; Implications for the metamorphic evolution of the Kanmantoo Group

482

A stratigraphic and tectonic overview of the Adelaide Geosyncline, South Australia by W. V. Preiss

South Australian Department of Mines and Energy P.O. Box 151, Eastwood, S. A. 5063, Australia The Adelaide Geosyncline contains an extremely thick, folded late Proterozoic* to middle Cambrian* sequence deposited initially in rifted troughs and later in broad zones of regional subsidence. These are partly analogous to the Mesozoic-Cainozoic system of basins associated with the southern continental margin of Australia, but relationship to an ocean is speculative since no oceanic-crust remnants have been discovered. Age constraints on deposition are based on dating of rare igneous rocks near the base of the Adelaidean and on a few Rb-Sr whole-rock shale ages. Contradictions between some of the data have yet to be resolved. The Callanna Group commences with platformal deposition of clastics and carbonates followed by regional extensional mafic volcanism (Arkaroola Subgroup); thereafter evaporitic clastics and carbonates of the Curdimurka Subgroup were deposited in rift-valleys with limited marine access. These commonly occur in a disrupted state in the Hinders Ranges diapirs. The Burra Group commences with largely fluvial sandstone and conglomerate, followed by eastward-prograding deltaic cycles alternating with marginal-marine and lagoonal carbonates, including sedimentary magnesite, and with fine-grained basinal sediments. A regional unconformity separates the older rocks from glaciogenic sediments at the base of the Umberatana Group. Shelf deposits of glaciomarine tillite pass laterally into basinal clastics. Post-glacial marine transgression inundated the geosyncline as well as much of the Stuart Shelf, with uniform deposition of basinal carbonaceous silt. Regression led to marginal carbonate deposition, then renewed clastic influx from the west. During Marinoan glaciation, the main source of ice was to the north, while conditions on the Stuart Shelf were periglacial. The Wilpena Group records two major marine transgressive- regressive cycles, during the second of which submarine canyons were cut and, later, the metazoa of the Ediacara assemblage became prolific. After withdrawal of the sea, renewed marine transgression in the early Cambrian led to widespread platform carbonate deposition (Hawker and Normanville Groups). Redbed sedimentation followed in the north, while in the south, the Kanmantoo Trough developed and was rapidly filled with a very thick clastic sequence. The Delamerian Orogeny commenced towards the end of the Cambrian with folding and the intrusion of granites in the south. Syntectonic and post-tectonic intrusives follow the Nackara and Fleurieu Arcs and a north-northwest lineament that was probably an active basement shear during folding. Deformation was most intense in the south, with high-grade metamorphism also being controlled by this shear.

Key words: Adelaide Geosyncline, stratigraphy, sedimentation, tectonics, Callanna Group, Burra Group, Umberatana Group, Wilpena Group, Normanville Group, Hawker Group, Kanmantoo Group, Lake Frome Group, Delamerian Orogeny.

INTRODUCTION Folded rocks of the Adelaide Geosyncline are exposed in the Flinders and Mount Lofty Ranges,

forming a continuous highland chain that is mainly meridional but with arcuate segments and branches (Fig. 1). The State's pioneer geologists, such as R. Tate, H. Y. L. Brown, R. L. Jack, W.

*The subdivisions early, middle and late for the Proterozoic and Cambrian are used in this paper in an informal sense.

W. V. Preiss

Howchin and D. Mawson, examined and mapped various parts of the highland chain and recognised the extent of its dominantly sedimentary rocks. The age of the sediments, however, remained contentious for many years, and even today the onset of sedimentation has not been unequivocally dated. The early recognition of tillite (Howchin 1901) and Cambrian fossils (Howchin 1897) in the sequence profoundly influenced stratigraphic interpretation. Howchin (1904) successfully erected a stratigraphic column based on sections near Adelaide; although this area includes some of the most strongly deformed rocks of the whole fold belt, Howchin's scheme has survived essentially unchanged today. However, upward and downward extensions became necessary when Howchin's stratigraphy was applied to the Flinders Ranges. Howchin realised that the tillite he had discovered forms a stratigraphic marker throughout the basin of deposition, allowing ready subdivision on a regional scale. Although Cambrian fossils occur only in the upper part of the sedimentary succession, Howchin continued to regard all or most of the sequence as Cambrian. Subsequent workers interpreted rocks below the fossiliferous Cambrian, including the glacials, as late Proterozoic, but the position of the CambrianPrecambrian boundary was not refined till much later. When the now famous Ediacara metazoan assemblage was discovered by Sprigg (1947), the sandstone containing the fossils was still regarded as Cambrian; a late Precambrian age was proposed by Glaessner & Daily (1959) and adopted in subsequent mapping by the South Australian Geological Survey. The pioneer geologists recognised that the sedimentary rocks of the Flinders and Mount Lofty Ranges were deposited in a single, large basin but the term Adelaide Geosyncline was first formalised by Sprigg (1952) in what was the earliest attempt at palaeogeographic reconstruction for the region. Today, the genetic connotations of geosynclinal theory have been supplanted by mobilistic concepts of plate tec-

tonics, and many authors have argued against continued use of "Adelaide Geosyncline". The term is, however, one of historical long standing and convenience; it is used to describe the basin of deposition (as distinct from the Delamerian fold belt whose eroded remains are exposed today), but without genetic implication. Sufficient data are not yet available for unambiguous plate-tectonic interpretation of either the basin or the fold belt; multiple hypotheses should therefore be considered until eliminated by new factual evidence. Little is gained by replacing a well established, non-genetic name with new terminology requiring commitment to a particular tectonic model. STRATIGRAPHIC FRAMEWORK Systematic mapping of the Adelaide Geosyncline by the Geological Survey commenced soon after World War II, firstly at a scale of 1:63 360 and later at 1:250 000. Stratigraphic research in the universities has continued at the same time, while in more recent years, drilling and geophysical surveys by government and private enterprise have provided vast amounts of subsurface data. With initial 1:250 000 scale mapping in the Adelaide Geosyncline nearing completion, a compilation of the Proterozoic geology of the region became possible (Preiss 1987); work on the Cambrian geology is still in progress and a similar synthesis has yet to be attempted. The chronostratigraphic term Adelaide System for the Proterozoic rocks of the Adelaide Geosyncline, and the subdivisions Torrensian (oldest), Sturtian and Marinoan (youngest) Series, were the first formal stratigraphic names of higher rank to be applied (Mawson & Sprigg 1950). However, Daily (1963) and Thomson et al (1964) recognised the need for true lithostratigraphic grouping of formations. Daily (1963) suggested changing the old series names to groups. This scheme worked well in the Adelaide region, but Thomson etal (1964), aware of difficulties in applying these terms to mapping

Adelaide Geosyncline Overview

• Marree Chintapanna Dam* RANGES Arkaroola Village /

Olympic Dam •Copley

Frome Harris.

Woomera •

• Blinman

Torrens

FLINDERS

,Mount Gunson Everard

Wilpena 'Pound

i Lake \Gairdner

Hawker

Lake^ Acraman

RANGES

'Worumba' Depot Creek PORT AUGUSTA

RANGES

• Quorn •Yunta

'Streaky Bay

• Orroroo WHYALLA"

Cowell 1

Peterborough .PORT MID-NORTH PIRIE • Jamestown t 'Gladstone , Crystal REGION # ^Brook, Spalding > \ Burra* ( \ *Clare •Wallaroo Port | Wakefield

Maitland®

Kapunda

Murray

Renmarki

MOUNT

UPort Lincoln^

•Gawler LOFTY ADELAIDE®/

RANGES L Murray Bridge •Strathalbyn

KILOMETRES Kingscotee KANGAROO ISLAND

—Victor^* Harbor x Encounter

Bay

Fig. 1. Locality map showing areas of outcropping Adelaidean and Cambrian rocks in the Flinders, Mount Lofty, Willouran, Peake and Denison, and Barrier Ranges.

W. V. Preiss

Table I. Summary of stratigraphic units of Adelaidean and Cambrian age in the Adelaide Geosyncline, and their presently accepted correlations.

Adelaide Geosyncline Overview

W. V. Preiss

in the Flinders Ranges, proposed a new group classification. The latter scheme incorporated downward and upward extensions of Mawson & Sprigg's (1950) Adelaide System to include Sprigg's (1952) Willouran Series at the base and the fossiliferous Pound Quartzite (now Subgroup) at the top. This classification is, with minor refinement, still in use on regional geological maps of the Adelaide Geosyncline. The stratigraphy of the late Proterozoic groups (Callanna, Burra, Umberatana and Wilpena) as well as of Cambrian sediments, is summarised for various regions of the Adelaide Geosyncline and environs in Table 1. The older "Series" terms have been retained by many authors in a generalised time sense, but it is recognised that precise time correlations between different regions are not possible at present. The time term Adelaidean for the period of late Proterozoic sedimentation in the Adelaide Geosyncline was introduced by Dunn et al (1966) but their evidence for the dating of the beginning of the Adelaidean is no longer accepted. As mapping proceeded away from Mawson and Sprigg's type area near Adelaide, new formation names came to be applied long before any regional synthesis was possible. The profusion and complexity of nomenclature existing today is regrettable, but the names occur in an extensive literature and cannot be deleted or changed without causing confusion. Were it possible to start afresh, no doubt a simpler and more logical scheme of formation and subgroup nomenclature could be devised. The groups are, however, quite consistent and can be readily applied. Mapping and correlation of the Proterozoic units have always relied heavily on lithostratigraphic comparison. The glacial horizons have served as reliable markers, but even these are subject to facies changes and major uncertainties in correlation remain in some areas. Intertonguing relationships, lateral facies changes and the recognition of widespread breaks in the sequence have made possible an attempt at correlation of individual units across the basin. The correlations accepted here (Table 1) and by

Preiss (1987) are at variance with some of those published by Thomson et al (1964) in a number of important respects. The Adelaide Geosyncline and environs may be subdivided into a number of regions characterised by facies variations in the sediments and by differences of tectonic style (Fig. 2). Some of these regions have been grouped in Table 1 for convenience to show stratigraphic relationships. It is important to note that, with minor exceptions, these technically controlled subdivisions do not have sharp, uniquely defined boundaries, nor do they normally have faulted contacts. They are therefore not "terranes" in the currently popular sense and spelling of the term to denote technically accreted fragments of crust that may have originally been widely separated. The integrity of the Adelaide Geosyncline as a single basin is strongly indicated by the overall stratigraphic consistency of its sediments and by the shifting positions of the boundaries between facies belts indicated by intertonguing relationships.

TECTONIC FRAMEWORK Tectonic structures mapped in the Adelaide Geosyncline were largely imposed during the Cambro-Ordovician Delamerian Orogeny (Thomson 1969), rocks as young as at least middle Cambrian being affected. Moreover, rejuvenation of some Delamerian structures during the Mesozoic and Cainozoic has resulted in the uplift of the Flinders and Mount Lofty Ranges and the depression of surrounding basins, which are spatially coincident with some older tectonic elements. For example, the Cainozoic St Vincent and Pirie-Torrens Basins overlie the Torrens Hinge Zone that was active at various times in the Adelaidean and became a synclinal depression during the Delamerian Orogeny, while the Frome Embayment (Mesozoic and Cainozoic) overlies Adelaidean and Cambrian sediments on the moderately stable Cumamona Cratonic Nucleus.

Adelaide Geosyncline Overview

Fig. 2. Tectonic elements of the Adelaide Geosyncline and environs.

W. V. Preiss

The present highlands coincide with regions of maximum subsidence during evolution of the Adelaide Geosyncline. It is suggested that the faults controlling syndepositional troughs (possibly listric normal faults) suffered reversal of movement during Delamerian compressive deformation to form thrusts. Renewed activity in the Cainozoic may be associated with the breakup of Gondwana, but the detailed tectonic mechanisms are yet to be clarified. The study of tectonic features in the Delamerian fold belt and in the little-deformed adjacent cratonic regions (Stuart Shelf and Curnamona Cratonic Nucleus) provides insights into the tectonic controls operating during sedimentation. However, it is necessary to view these tectonic controls within a time framework provided

by the stratigraphic record. From plots of facies

distribution and thickness variation of formations, troughs and highs within the Adelaide Geosyncline can be identified. These zones shifted in time, so that a region characterised by a veiy thick sequence of one formation may show evidence of much reduced sedimentation at another time. This makes the estimation of total thickness at any one point very difficult, and a general statement on total thickness for the whole basin almost meaningless. Moreover, only a few sections have been measured in detail; elsewhere, thicknesses have been estimated only from maps. There are some regions, for example parts of the inner Nackara Arc, where several Adelaidean units are apparently greatly thickened in a single locality. In these cases, if future detailed section measurement confirms that these are primary thicknesses, a cumulative total of Adelaidean in excess of 20 km is possible. One of the most obvious regions of pronounced sediment accumulation is the Cambrian Kanmantoo Trough, whose metasediments are best preserved in the eastern Mount Lofty Ranges. While this is a very important feature, its apparent tectonic uniqueness within the Adelaide Geosyncline may have been overemphasised in the past. Equally significant faultcontrolled troughs, some involving only

shallow-water sediments and others deep basinal deposition by turbidity currents, were active in other parts of the geosyncline at different times; these troughs have now been named (Preiss 1987 and Fig. 2). Studies of geophysical and surface lineaments and corridors have led to the recognition of linear zones of weakness in the Earth's crust, which have had a long history of recurrent movement (O'Driscoll 1973). While some of these features can also be recognised in regional mapping, others are difficult to relate to mapped tectonic structures. Some can be interpreted to have been loci of active faulting during Adelaidean and Cambrian deposition, as seen from their close coincidence with changes in thickness or facies, or with distribution limits for some units. Although no comprehensive geophysical synthesis of the Adelaide Geosyncline has been published, attempts have been made at tectonic interpretation of aspects of the gravity and magnetic anomaly patterns. Thomson (1970) first published a contour map showing depth to magnetic basement for the whole Geosyncline, and interpreted a prominent gravity high to the north of the Flinders Ranges as a ridge of shallow basement (Muloorina Ridge). This gravity ridge joins two other linear gravity highs at a point in the northeastern Flinders Ranges, forming a three-armed structure interpreted by von der Borch (1980) as the triple junction of a continental rift system, marked by deep-seated basic magmas. However, the Muloorina Ridge extends northwestwards into outcropping basement (east of the Peake and Denison Ranges) and southeastwards into pre-Adelaidean metamorphics of the Mount Babbage Inlier at the northeastern extremity of the Flinders Ranges. Limited drilling suggests that the eastern arm of the threearmed gravity high is also shallow basement. Facies distributions in some Adelaidean units suggest that the Muloorina Ridge may have been a sediment source during at least the early Adelaidean. The reason for the southern arm of the gravity high is much less certain, since there are no associated known basement rocks or shal-

['QUEENSLAND ["NEW SOUTH 1 WALES

>pu

a et> o

n B' ft) O

1. a> 3 Fault Fault

Post-Delamerian basins

Depth to basement (metres)

Generalised isopachs of lower Wilpena Group to illustrate an example of differential subsidence (metres)

Bathymetric contours (metres) V

V V

+ Pre-Adelaidean basement _ _ Drn E.A.

-I-

Cainozoic basalt Precambrian and Palaeozoic rocks_ .

V V

87-797 SADME

Fig. 3. Comparison of scale, morphology and tectonic setting of the Adelaide Geosyncline with Mesozoic to Cainozoic structures of southeastern Australia. vo

W. V. Preiss

low magnetic anomalies; Adelaidean and Cambrian sequences in the central and northern Flinders Ranges are thick and basement cannot be regionally shallow. Another regional geophysical interpretation by Gunn (1984) infers an original rift zone running meridionally along the axis of the Flinders and Mount Lofty Ranges, with an east-northeasterly spur towards Olary. Although based on comparison with the geophysical characteristics of other rift zones, this model suffers from lack of stratigraphic and sedimentological control. The evolutionary history of the Adelaide Geosyncline described by Preiss (1987) resulted from a synthesis of stratigraphic data with mappable tectonic features, some Delamerian, and some inferred to have had an earlier history. Geophysical features such as depth to magnetic basement anomalies and gravity highs and lows were used as a general guide, but a detailed and comprehensive reconciliation of all geophysical features with a tectonic model is not yet possible. As pointed out by von der Borch (1980), the Adelaide Geosyncline shows many similarities with passive continental margins (in its southern portion) and with intracontinental rifts or aulacogens (in the northern region). A useful comparison of scale and morphology can be drawn with the Mesozoic and Cainozoic structural features of the southern continental margin of Australia, including the Otway and Bass Basins (Fig. 3). In this analogy, the Australian mainland corresponds to the Gawler Craton, and Tasmania to the Curnamona Cratonic Nucleus. The arcuate southern continental margin with thick sediment wedges such as in the Otway Basin is analogous to the Nackara Arc, while the Bass Basin occupies a similar position to the Central Flinders Zone. While such comparisons are attractive, it must be stressed that there is no factual proof that either the Nackara Arc or the Kanmantoo Trough represent a continental margin, since no oceanic crust of late Precambrian or Cambrian age has been identified anywhere in South Australia. However, some recent models for the evolution of the Palaeozoic Lachlan Fold

Belt in eastern Australia support the existence of ocean-floor volcanics of Cambrian age in Victoria (e.g. Crawford etal 1984), although the ages of volcanics and unfossiliferous sediments, as well as of orogenic events, in the western Lachlan Fold Belt are poorly constrained. Since many lines of evidence suggest the present-day existence of Precambrian basement at depth beneath the Lachlan Fold Belt, Rutland (1976) argued that uninterrupted sialic crust extends eastward from the Adelaide Geosyncline, which Rutland et al (1981) interpreted as a multiple-rifted arch. The more recent concepts of accretion of Precambrian crustal blocks and overthrust Cambrian oceanic remnants implied by the models of Crawford et al (1984) and Scheibner (1985) obviate the need for continuous Precambrian crust east of the southern part of the Adelaide Geosyncline during the late Precambrian and Cambrian, and make a continental margin setting for this region plausible, though not proven. As discussed below, the polarity of sedimentation trends in the southern region of the Geosyncline does not provide evidence of an eastern source area. Nor has any cratonic basement been found to the east, where limited drilling beneath the Murray Basin has intersected various Adelaidean and Cambrian sediments, metasediments, volcanics and intrusives.

AGE CONTROL FOR THE ADELAIDE GEOSYNCLINE The timing of the onset of sedimentation in the Adelaide Geosyncline, and hence the age of the beginning of the Adelaidean, are still in doubt after more than two decades of attempts to solve the problem. The scarcity of igneous rocks in the succession and the invariable secondary alteration of those that do exist, have made radiometric age determination difficult. Attempts to use stromatolite biostratigraphy to correlate Adelaidean units with the standard Riphean and Vendian sequences of the USSR, themselves subject to considerable dating uncertainty, have met with only limited success, largely because of endemism of the Australian stromatolite taxa.

Adelaide Geosyncline Overview The appearance of the Ediacara metazoan assemblage near the top of the Adelaidean sequence has been used for correlation with similar assemblages from other parts of the world, but there is as yet no guarantee of precision for such correlations. Glacial events are important timemarkers within Australia, but while tillites of late Precambrian age are widespread on most continents, there is still debate about the relative synchroneity or diachroneity of the glaciations. Only in the Cambrian are more precise biostratigraphic zonation and correlation possible, but much of even the Cambrian sequence is only very sparsely fossiliferous, especially the Lake Frome and Kanmantoo Groups.

Compston et al (1966) attempted to date the Willouran Wooltana Volcanics and obtained an imprecise age of around 830 Ma (recalculated using A, Rb = 1.42 x lO^V ), but were uncertain whether this represented an age of extrusion or subsequent alteration. Thomson (1966) preferred correlation of the Wooltana Volcanics with the Roopena Volcanics of the Stuart Shelf, dated by Compston et al (1966) at about 1300 Ma, and this correlation formed the basis on which Dunn et al (1966) placed the beginning of the Adelaidean at 1400 Ma. Mason et al (1978) instead proposed correlation with the Beda Volcanics which had recently been discovered in mineral exploration drilling on the Stuart Shelf. Webb & Coats (1980) dated the Beda Volcanics at 1076 ± 34 Ma, revising an earlier estimate by Webb & Horr (1978), and the base of the Adelaidean has consequently been placed at about 1100 Ma by many authors. 87

In the Willouran Ranges, a thin, lenticular porphyritic dacite in the Rook Tuff (lower part of Curdimurka Subgroup: Table 1) is interpreted from limited stratigraphic relationships to be slightly younger than the Wooltana Volcanics. If this relationship is correct, the concordant U-Pb age determination of 802 ±10 Ma on zircon from the dacite (Fanning et al 1986) lends renewed support to the original age interpretation of the Wooltana Volcanics by Compston et al (1966) but renders correlation of the latter with the apparent-

ly much older Beda Volcanics very uncertain. This contradiction has still to be resolved. Other age determinations are restricted to RbSr shale dates on late Adelaidean rocks from the Stuart Shelf; as summarised by Webb etal (1983), these fall in the range 750 to 600 Ma. Although they have large errors and are based only on whole-rock analyses and not on separated diagenetic clay minerals, all are plausible within the accepted late Precambrian time framework and are consistent with the stratigraphic order. No geochronological data are available for beds containing the Ediacara assemblage. STRATIGRAPHY AND EVOLUTION OF THE ADELAIDE GEOSYNCLINE Callanna Group Arkaroola Subgroup The oldest deposits assigned to the Adelaidean are relatively coarse, well sorted siliciclastics such as the Paralana Quartzite preserved around the Mount Painter Inlier and the Lady Don Quartzite and Christine Judith Conglomerate of the Barrier Ranges. The Paralana Quartzite is greatly thickened east of the Paralana Fault, and contains local conglomerate pockets apparently deposited in fault-angle depressions, suggesting syndepositional rifting in that area (Coats & Blissett 1971). Equivalents are the much thinner Younghusband Conglomerate of the Peake and Denison Ranges, and possibly the quartzitic Cutana Beds occurring as outliers on the Willyama Inlier (Pitt 1979). Carbonate-dominated units follow the basal clastics in each of the three main areas. The Wywyana Formation overlying the Paralana Quartzite consists of calc-silicates and laminated limestone marble. The Boco Formation of the Barrier Ranges and Coominaree Dolomite of the Peake and Denison Ranges are partly stromatolitic, fine-grained dolomites, while the highly deformed Black Knob Marble of the Wil-

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louran Ranges is a probable equivalent. Carbonates of this age may also occur as xenoclasts in diapirs but would be difficult to distinguish in this context from younger Willouran carbonates of the Curdimurka Subgroup. The Wooltana Volcanics and equivalents are the only widespread and substantial volcanic unit recognised in the Adelaidean. Hilyard (this volume) regards these altered basic lavas as typical flood basalts. Equivalents occur in the Barrier Ranges, Peake and Denison Ranges, and Willouran Ranges (Table 1), as well as in numerous xenoclasts in Flinders Ranges diapirs. Whether the Beda Volcanics of the eastern Stuart Shelf are to be correlated will depend on future geochronological studies, but the geochemistry of the lavas is similar (Giles & Teale 1979; Crawford & Hilyard, this volume). The volcanics are almost exclusively basaltic but invariably strongly altered; amygdaloidal flow tops are common and varying amounts of clastic sedimentary material may be interbedded. The subaerial environment suggested by Hilyard (this volume) is also supported by the relationships of the Cadlareena Volcanics (Table 1), which in places fill palaeo-valleys cut into Younghusband Conglomerate and basement (Ambrose et al 1981). Curdimurka Subgroup The Wooltana Volcanics are unconformably covered by Torrensian and Sturtian rocks but equivalent volcanics are inferred to have been overlain by thick evaporitic mixed carbonate and clastic sequences of the Curdimurka Subgroup, which were deposited in a series of northwest to north-northwest trending troughs through the Flinders, Willouran and Peake and Denison Ranges. Sedimentary contacts at the base of the Curdimurka Subgroup are almost never preserved; this stratigraphic level may mark a major decollement surface above which the evaporitic sediments have been mobilised and brecciated to form the Flinders Ranges diapirs. Only near Chintapanna Dam in the Willouran Ranges is a sedimentary contact between Noranda Volcanics

and overlying probable Dome Sandstone preserved, and it is apparently conformable. The type area for the Curdimurka Subgroup (Forbes et al 1981) contains the basal Dome Sandstone (possibly largely fluvial), Rook Tuff (siltstone with lenticular porphyritic dacite), Dunns Mine Limestone with diagenetic evaporite pseudomorphs, Recovery Formation micaceous siltstone and sandstone with abundant halite casts, Hogan Dolomite (in part stromatolitic), Cooranna Formation sandstone, siltstone and dolomite with evaporite pseudomorphs, and Boorloo Siltstone (carbonaceous siltstone with platy thin dolomite interbeds near the top). Similar though not completely correlatable sequences have been assembled for the Peake and Denison Ranges (Ambrose etal 1981), Worumba Anticline (Preiss 1985), and as the River Broughton Beds of the Spalding Inlier (Preiss 1974). Diapirs in the Flinders Ranges contain numerous xenoclasts, of which many can be related to units defined at Worumba. The Curdimurka Subgroup exists in the Adelaide Geosyncline in vaiying stages of disruption, possibly because of the original presence of highly evaporitic horizons which have acted as decollement surfaces. The timing of this deformation and disruption is still debated (Preiss 1985). The lithological association and abundant evaporites of the Curdimurka Subgroup are consistent with deposition in alkaline lakes in a continental riftvalley setting (Rowlands et al 1980), but the paucity of very coarse clastics suggests limited relief on surrounding highlands. Limited vulcanicity persisted during deposition of the Curdimurka Subgroup (e.g. Rook Tuff, basalt in River Broughton Beds at Spalding). The altered basalt at Depot Creek could also belong to the Curdimurka Subgroup, though this is far from certain and most authors have correlated the basalt with the Wooltana Volcanics.

Adelaide Geosyncline Overview

Burra Group Sedimentary contacts between the Curdimurka Subgroup and the Burra Group are exceedingly rare, the contact horizon possibly acting as another regional decollement surface during deformation. However, where Curdimurka Subgroup is absent, probably because of non-deposition outside the Willouran rift valleys, the Burra Group unconformably overlies either basement (e.g. near Adelaide and near Olary) or Willouran volcanics (e.g. near Arkaroola Village). Elsewhere, the base of the Burra Group is a tectonic contact, with the possible exception of the Spalding Inlier, where a poorly exposed contact below the Rhynie Sandstone may be conformable. Deposition of the Burra Group was confined to a region between the Torrens Hinge Zone, along which syndepositional faults were active in the Torrensian, and the MacDonald Corridor near Olary.

Emeroo Subgroup and River Wakefield Subgroup The Burra Group commences with generally coarse, feldspathic sandstones, locally pebbly, with abundant trough cross-bedding and finingupward sequences. In the south and northeast, titaniferous hematite is an abundant heavymineral component in the basal clastics, which are interpreted as largely fluvial, although local herringbone cross-bedding and halite casts may indicate marginal-marine influence. Together with possibly deltaic sandstones (Bungaree Quartzite and equivalents listed in Table 1), these basal clastics are grouped as the Emeroo Subgroup, based on Mawson's (1947) stratigraphic scheme for the western Flinders Ranges. The Emeroo Subgroup intertongues eastwards with the River Wakefield Subgroup, a more silty sequence containing dolomite near the base and top and the Ingomar Quartzite in the middle. The coarsening-upward cycles of the lower and upper River Wakefield Subgroup (the latter into the Bungaree Quartzite) are interpreted

as deltaic progradation from west to east. The partly magnesite-bearing Wirreanda Dolomite Beds at "Worumba" are considered to have been deposited at the same time as the Blyth Dolomite at the base of the River Wakefield Subgroup. In the north, River Wakefield Subgroup equivalents include the mainly silty Willawalpa Formation of the Willouran Ranges, Fountain Spring Beds of the Peake and Denison Ranges and dolomitic siltstone of the Opaminda Formation near Arkaroola Village. The Blue Mine Conglomerate (Table 1) resembles parts of the Rhynie Sandstone and may indicate persistence of fluvial deposition adjacent to elevated basement, probably to the east.

Mundallio Subgroup The Emeroo Subgroup is overlain by the carbonate-dominated Mundallio Subgroup (Uppill 1979). In the Flinders Ranges, this interval is represented entirely by the Skillogalee Dolomite, often divisible informally into upper and lower members, although Uppill (1979) elevated these to formation status. The lower member commonly contains pale-coloured micritic dolomite with local stromatolites, green siltstone, and feldspathic sandstone, the clastic components being dominant in the northern Flinders, Willouran and Peake and Denison Ranges. The upper member is dominated by thinly bedded organicrich, blue-grey dolomicrite with stromatolites, cryptalgal lamination, mudcracks, tepee structures, early diagenetic black cherts (commonly microfossiliferous) and sedimentary magnesite. Magnesite occurs occasionally as beds of micrite but is most commonly reworked as sheets of fine to coarse intraclast packstone (magnesite conglomerate). There is a major sand component along the western margin of the Hinders Ranges. Towards the south, in the Mid-North region, the Skillogalee Dolomite is overlain by the silty and fine-sandy Woolshed Rat Shale, which laterally replaces the Skillogalee Dolomite in the southern Mount Lofty Ranges. However, facies typical of the Skillogalee are still present locally near Adelaide, as the lenticular pale-coloured

W. V. Preiss

dolomite of the Castambul Formation and the blue-grey, magnesite-bearing, cherty Montacute Dolomite, which intertongue with the carbonaceous Balhannah Shale Member of the Woolshed Hat Shale. Carbonates of the Mundallio Subgroup are dominated by dolomite, commonly preserving fine detail of primary fabric (Forbes 1960, 1961; Preiss 1972, 1973; Uppill 1979). This argues against secondary replacement of limestone. The presence of magnesite to the general exclusion of evaporite minerals suggests a lacustrine environment without access to sea-water (Uppill, this volume). Von der Borch & Lock (1979) drew attention to an analogy with deposition of dolomite and hydromagnesite in modern ephemeral lagoons associated with the Coorong, and von der Borch (1980) suggested that most of the Burra Group may have been deposited in lacustrine environments. However, the Coorong example is instructive in showing that groundwater-fed magnesite-precipitating lagoons can exist in very close proximity to an open ocean. In the Skillogalee Dolomite, relatively small changes in sea-level could have caused alternating marine and non-marine environments because of the very low depositional relief and rapid lateral shift of facies belts (Belperio, this volume). The existence of Burra Group in the central Hinders Ranges between Copley and "Worumba" is uncertain, since the level of erosion of anticlinal cores is not deep enough in this region to expose Burra Group. The local unconformable relationship of the Umberatana Group on disrupted or diapiric Callanna Group suggests at least local absence of Burra Group except, perhaps, for a veiy thin erosional remnant near "Oraparinna" (Preiss 1987). The wedges of unstratified and unsorted sedimentary megabreccia in the northern part of the area mapped by Preiss (1985) suggests the presence of syndepositional fault scarps north of "Worumba" (Preiss 1979). These may have marked the southern limit, during the mid to late Torrensian, of an area of uplift in the Central Flinders Zone, from which

much of the previously deposited Burra Group may have been stripped.

Upper Burra Group In the western part of the southern Adelaide Geosyncline, the Mundallio Subgroup is succeeded by, and probably intertongues with, coarse feldspathic sandstone of the Undalya and Stonyfell Quartzites. These eastward-lensing sand wedges have lobate outlines determined from their outcrop distributions, suggestive of prograding deltas. Several upward-coarsening cycles from siltstone to coarse, clean sandstone are commonly represented within these formations. Renewed transgression of the sea followed with widespread deposition of basinal shale and siltstone of the Saddleworth Formation, in places carbonaceous, over the deltaic sands. Lenticular silty and carbonaceous dolomite with occasional small chert blebs was deposited as dolomite mud under basinal conditions; these carbonates (Auburn and Beaumont Dolomites) lack the obvious shallow-water features of the Skillogalee Dolomite. The eastward-lensing Watervale Sandstone Member of the Mid-North and the Cradock Quartzite of the central Hinders Ranges are interpreted as progradational sand wedges similar to the Undalya Quartzite, within the Saddleworth Formation, while the Leasingham Quartzite Member and Minburra Quartzite respectively of the same regions, occur near the top. In the northern Hinders Ranges, the Skillogalee Dolomite is overlain by green siltstone, with minor quartzite and dolomite interbeds, of the Myrtle Springs Formation. It is uncertain whether this correlates only with the Saddleworth Formation, or Undalya Quartzite and Woolshed Hat Shale as well. The equivalent Kalachalpa Formation of the Peake and Denison Ranges is abundantly stromatolitic and may represent a slightly shallower environment. Abundant and well preserved microfossils have been recovered

Adelaide Geosyncline Overview

from cherts of the Kalachalpa Formation and underlying transition to Skillogalee Dolomite by Fairchild (1975). In the south, the Saddleworth Formation coarsens upward into the well laminated silty Mintaro Shale of the Mid-North and Glen Osmond Slate near Adelaide. As discussed by Preiss (1987), these units have been reported to contain rare, isolated "lonestones" of sandstone, limestone and quartzite, of unknown origin. Ice-rafting has not been proven, and no tillites of this age are known, but occasional local river or shore ice may be responsible.

Belair Subgroup The base of the Sturtian Series was defined by Mawson & Sprigg (1950) at the relatively abrupt base of the Mitcham Quartzite, basal unit of the Belair Subgroup in the Adelaide region. They believed that the fresh, coarse feldspar and varvelike lamination of parts of the Subgroup suggested glacial affinity, but this view is not now widely held, since Belair Subgroup sediments are quite typical for Burra Group clastics (Coats 1967). The Torrensian-Sturtian chronostratigraphic boundary is, however, left as originally defined, the uppermost part of the Burra Group being regarded as Sturtian. In the Mid-North, the Gilbert Range Quartzite and overlying Kadlunga Slate are equivalent, but are progressively eroded northwards by the early Sturtian unconformity. Umberatana Group Sedimentation ceased throughout the Adelaide Geosyncline after deposition of the Belair Subgroup.The erosional surface at the base of the Sturtian glacial sequence varies from a disconformity to a low-angle unconformity over large areas; locally, especially in southern to central Flinders Ranges, there are even highangle unconformities associated with diapirs and syndepositional faulting. The basal contact of the

Umberatana Group is commonly sharp and irregular in detail; locally, regoliths are preserved suggesting that ice did not everywhere erode the Sturtian landscape (e.g. Hopton 1983; Preiss 1985). However, at Merinjina Well in the northern Flinders Ranges, a small glaciated pavement is preserved (mainly as a cast on the sole of the overlying bed) on an elevated block from which the Burra Group has been stripped (Mirams 1964; Preiss 1987). Sturtian glacial deposits are relatively thin over most of the western Adelaide Geosyncline (commonly less than 500 m), but greatly thickened in the Yudnamutana Trough of the northeastern Hinders Ranges and Baratta Trough adjacent to the southwestern edge of the Curnamona Cratonic Nucleus. The interglacial sequence also varies considerably in thickness, while the Marinoan glacials attain their thickest development in the Nackara Arc.

Older Sturtian glacials and Yudnamutana Subgroup The oldest known Sturtian glacials are the thick diamictite deposits with interbedded orthoquartzite, locally heavy-mineral-laminated, of the Pualco Tillite (Forbes & Cooper 1976), deposited in the Baratta Trough (Fig. 2). These are overlain by the laminated Benda Siltstone, containing occasional ice-rafted erratics (Forbes 1970), and both units pass laterally into a lenticular ferruginous facies, termed informally the Braemar ironstone facies. Magnetite-enrichment, which is sufficient in the "Braemar" area for the rock to have been considered as potential iron ore (Whitten 1970), may be due to low-grade metamorphism. The equivalent unit in the central Flinders Ranges is the relatively unmetamorphosed hematitic Holowilena Ironstone. In the northeastern Flinders Ranges, the oldest glacial sequence overlying eroded Burra and Callanna Groups as well as basement is the very thick Yudnamutana Subgroup (Thomson et al 1964; Coats & Blissett 1971). Deposited in the

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Yudnamutana Trough (Fig. 2), this comprises the Fitton Formation at the base (consisting of basal granite conglomerate, and well bedded siltstone and sandstone with dropstones), massive Bolla Bollana Tillite intertonguing with bedded siltstone (Belperio 1973) and dark grey siltstone, with dropstones, of the Lyndhurst Formation at the top. Although these authors correlated the Yudnamutana Subgroup with other Sturtian tillites further south, Coats (1973) suggested instead that it equates with the older Sturtian glacials of the Olary region, a correlation based on two concepts. Firstly, the older glacials and the Yudnamutana Subgroup were found to lack distinctive clast-types (red, porphyritic acid volcanics and liesegang-ring-weathered reddish quartzites) that are abundant in the younger glacials. Secondly, the unconformity between the Yudnamutana Subgroup and overlying Serle Conglomerate (reinterpreted as a glaciogenic deposit) was suggested to be equivalent to the unconformity between the Benda Siltstone and overlying Wilyerpa Formation. However, the Yudnamutana Subgroup lacks ironstones and could equally represent a trough-filling glacial deposit of the younger Sturtian phase, as originally envisaged, though perhaps with a different provenance. Younger Sturtian glacials In the Baratta Trough, the Benda Siltstone, and in places Pualco Tillite, are overlain with very low-angle unconformity by turbiditic sandstone and siltstone of the Wilyerpa Formation, containing dropstones. Outside the trough, diamictites of the Sturt and Appila Tillites are widespread, extending west to the Torrens Hinge Zone and locally onto the Stuart Shelf, where they pinch out. A local occurrence of agglomerate near Depot Creek (Hopton 1983) indicates minor volcanism. The Merinjina Tillite and the reworked glacial debris of the Serle Conglomerate are equivalents in the northern Flinders Ranges, as is the Calthorinna Tillite of the Peake and Denison Ranges. The Appila Tillite intertongues eastwards with

the Wilyerpa Formation which attains a thickness of over 3 km in the Baratta Trough. The diamictites of the Sturtian have been considered as glaciomarine deposits since the work of Mawson (1949a). Erratics suggest a multiple provenance; some of the most distinctive clasttypes, including acid volcanics and quartzites resembling the Pandurra Formation, may have been derived from both the Gawler Craton and Cumamona Cratonic Nucleus. The abundance of gneissic and granitic clasts in the Sturt Tillite near Adelaide and in the diamictites near Olary suggest derivation from the Gawler Craton and Willyama Inlier respectively; very large (800 m) megaclasts of granite in the Pualco Tillite adjacent to the MacDonald Fault (Wiltshire 1983) suggest sliding from an actively rising fault scarp. Deposition of the diamictites is likely to have been from wet-based glaciers debouching into the marine basins of the Adelaide Geosyncline. During the second glacial phase, the ice-sheet advanced from the Gawler Craton and broke up into icebergs over the Baratta Trough. Interglacial sequence: Farina Subgroup and Willochra Subgroup At the end of the Sturtian glaciation, melting of major ice-sheets resulted in marine transgression of the Adelaide Geosyncline and, for the first time, the Stuart Shelf. The Tapley Hill Formation, consisting dominantly of very thinly laminated carbonaceous siltstone, deposited below wave base, occurs throughout the Adelaide Geosyncline and Stuart Shelf, except where it lapped out against topographic highs, e.g. the eastern part of the Muloorina Ridge near Mount Babbage, the Curnamona Cratonic Nucleus, and parts of the northern Stuart Shelf near Olympic Dam. The basal Tindelpina Shale Member, characterised by extremely thinly laminated pyritic and carbonaceous silty shale with interbedded dark grey laminated dolomite, is very widespread. Locally, conglomerates were shed from submarine diapiric and fault escarpments. The upper part of the Tapley Hill Formation is

Adelaide Geosyncline Overview

more carbonate-rich, with cross-bedding and local stromatolites indicating a higher-energy, shallower-water environment. The oolitic and stromatolitic Brighton Limestone along the western margin of the Adelaide Geosyncline overlies and intertongues with the Tapley Hill Formation. The upper member of the Brighton Limestone is a micritic and intraclastic dolomite with widespread tepee structures (Preiss & Kinsman 1978). These units are thus part of an upward-shallowing sequence deposited towards the end of the Sturtian, the Sturtian-Marinoan boundary being defined in the Adelaide region at

the gradational top of the Brighton Limestone. At the beginning of Marinoan time, the western and northeastern margins of the Adelaide Geosyncline and much of the adjacent platform areas on the Stuart Shelf and Curnamona Cratonic Nucleus were occupied by extensive red tidal mudflats with abundant evidence of desiccation and subaerial oxidation (Angepena Formation) ; the Curnamona area was first transgressed at this time, with local deposition of basal conglomerate. In the more basinal regions along the axis of the Adelaide Geosyncline, deeper-water grey and green silts continued to accumulate, some below wave base (e.g. Amberoona Formation of the northern Hinders Ranges), and some (flaser-bedded Tarcowie Siltstone of the MidNorth and Olary regions) probably deposited under intermediate water depths. In the central Flinders Ranges, an extensive carbonate platform developed during the latest Sturtian to earliest Marinoan, with deposition of the Etina Formation, consisting of sandy, oolitic and stromatolitic limestones interbedded with siltstone (Preiss 1973; Lemon 1986). Deposition was modified locally around the rising Enorama Diapir, with clasts of Willouran sedimentary and igneous rocks being shed as the diapir became exposed (Dalgamo & Johnson 1968; Lemon 1985). The Etina Formation lenses out southwards near Orroroo into Tarcowie Siltstone, and northwards (where its equivalent is represented by the Balcanoona Formation, Weetootla Dolomite Member and Wundowie Limestone Member) into Amberoona Formation. The Balcanoona consists

of oolitic and stromatolitic limestone with a tepee-bearing dolomite upper member, representing a similar sequence to that of the Brighton Limestone. South of Arkaroola Village, the Balcanoona Formation terminates abruptly at an interpreted submarine escarpment (Coats & Blissett 1971). The carbonate platform was inundated by a widespread transgression represented by the Enorama Shale in the central Flinders Ranges; renewed upward shallowing is recorded by the stromatolitic and intraclastic Trezona Formation ("hieroglyphic limestone" of Mawson 1938), the petrography and geochemistry of which have been described by Singh (1987). Along the western margin of the Adelaide Geosyncline, the Angepena Formation passes up into a sandier shallow-water sequence (Wilmington Formation); equivalents of the latter on the southern Stuart Shelf grade up into coarse, well rounded Whyalla Sandstone, interpreted as aeolian (Gersteling & Heape 1975; W. Garlick, pers. comm. 1982; G. E. Williams, pers. comm. 1984). Near Mount Gunson, however, a topographic high of silicified Pandurra Formation (the Pernatty Upwarp) remained exposed until deposition of the Whyalla Sandstone; prior to being covered by these wind-blown sands, this area was subjected to periglacial conditions, producing sand-wedge polygons as described by Williams (1986a). Further east, in the Geosyncline, the Wilmington Formation passes apparently conformably up into the Elatina Formation, mostly a well sorted sandstone but containing lenses of gritty red siltstone (Reynella Siltstone Member) and diamictite. First recognised by Mawson (1949b), these represent the Marinoan glacials in the central and western Flinders Ranges, a region of dominantly shallowwater shelf deposition. Deltaic, shallow-marine and lacustrine environments are represented; Williams (1985) described cyclic varves in siltstone and fine sandstone, interpreted by him to reflect sunspot cycles. The Elatina Formation has been included within the Willochra Subgroup, largely because of its gradational contact

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with the Wilmington Formation. However, in the central and northern Flinders Ranges there is evidence of disconformity at the base of the Elatina.

Lower Wilpena Group

Wilpena Group

The Marinoan post-glacial transgression was even more extensive onto adjacent platforms than that of the Sturtian. The base of the Wilpena Group is marked by the lenticular but persistent Nuccaleena Formation, a thin, distinctive, cream to reddish, laminated, micritic dolomite, with interbedded shale in the upper part. The unit is thickest (about 10 m) and most continuous in the north-east. Near Adelaide, along parts of the Torrens Hinge Zone and near Olary, the Seacliff Sandstone intertongues with the Nuccaleena Formation. Both units grade up into marine siltstone and fine sandstone of the Brachina Formation, commonly red-brown near the western edge of the Adelaide Geosyncline and red-brown to olive-green in the central Flinders Ranges, where three members have been recognised (Leeson 1970; Plummer 1978). Deposition of the lowest of these, the Moolooloo Siltstone Member, continued onto the Stuart Shelf as the Tregolana (=Woomera) Shale Member of the Tent Hill Formation. No shoreline facies are known that would indicate the limit of this transgression, either on the Stuart Shelf or Curnamona Cratonic Nucleus. The upper members, the Moorillah Siltstone Member and Bayley Range Siltstone Member, are generally more sandy, the former containing the enigmatic fossil Bunyerichnus dalgarnoi (see Cloud & Glaessner, 1982, for discussion). On the Stuart Shelf, the cross-bedded, red Corraberra Sandstone Member of the Tent Hill Formation is probably a nearer-shore time-equivalent of these members, which form an upward-shallowing sequence (Plummer 1978; Dyson 1986). Note, however, that Dyson (1986) proposed a somewhat greater depth of water as recorded by the turbidite-like facies of the Brachina Formation. The green Ulupa Siltstone is a probably deeperwater equivalent of the Brachina Formation occurring in the eastern and northern parts of the basin.

The Wilpena Group records two major postglacial transgressive-regressive cycles, the younger of which includes the fossiliferous beds of the Pound Subgroup.

The progradational phase culminated with deposition of veiy shallow-water, clean sandstones derived from the Gawler Craton. The lower contact is generally gradational, with

Yerelina Subgroup: thick Marinoan glacial sequences. The more basinal regions of the Adelaide Geosyncline in the Nackara Arc and North Flinders Zone continued to receive thick sedimentation during the Marinoan glaciation. Arenites such as the Gumbowie Arkose and Grampus Quartzite in the Nackara Arc and the Balparana Sandstone of the northern Flinders Ranges may represent tongues of sandstone facies of the Elatina Formation. In the northern Flinders Ranges, the Yerelina Subgroup commences disconformably with laminated siltstone containing scattered small dropstones - the Fortress Hill Formation - but correlation with the southern region where sedimentation appeared to be continuous with the Farina and Willochra Subgroups is uncertain. Thick diamictites of the Pepuarta Tillite in the south and Mount Curtis Tillite in the north represent the maximum of the Marinoan glaciation, but clasts are generally more sparsely distributed than in Sturtian tillites and tend to decrease in frequency from north to south. The main source of clasts is likely to have been from the north and east, where cold conditions apparently set in earlier than in the south, as evidenced by the Fortress Hill Formation. The top of the Yerelina Subgroup in the north is largely erosional, but in the Nackara Arc sedimentation continued with a relatively thick unnamed siltstone containing occasional clasts.

Adelaide Geosyncline Overview

regional interbedding and intertonguing suggesting diachronism. Plummer (1978) suggested that the very thick ABC Range Quartzite of the southwestern Flinders Ranges was equivalent to the Moorillah and Bayley Range Siltstone Members plus ABC Range Quartzite further north and east. In a later paper, Plummer (1983) rejected the widely held correlation of the Simmens (Arcoona) Quartzite Member (Tent Hill Formation, Stuart Shelf) with the ABC Range Quartzite (Coats 1965) in favour of equivalence with the Pound Subgroup, but this is not accepted here. Indeed, if the clean sandstones of this progradational cycle are diachronous from west to east, the Simmens is likely to be slightly older than the ABC Range Quartzite of its type area in the central Flinders Ranges. The abundance of desiccation cracks, mud-clasts and medium-scale cross-bedding, including herringbone cross-beds, suggests an intertidal sand-flat environment as proposed by Plummer (1978). The ABC Range Quartzite lenses out along an approximately meridional boundary situated some 100 km east of the Torrens Hinge Zone. To the east and northeast of this boundary, deposition of the deeper water green Ulupa Siltstone continued.

Upper Wilpena Group The second major cycle of the Wilpena Group commenced with rapid, renewed, marine transgression. The maroon silty shales of the Bunyeroo Formation overlie the ABC Range Quartzite with an abrupt transition in which gritty sandstone at the top of the Quartzite is interbedded with shale. The Bunyeroo Formation is of uniform facies across the basin, and has equivalents in the Yarloo Shale preserved on the northeastern Stuart Shelf (Johns 1968) and a thin maroon shale near Sellick Hill (Jenkins & Gostin 1983). Gostin et al (1986) have described a thin but continuous layer, a few centimetres thick, of coarse, feldspathic detritus in the lower part of the Bunyeroo, extending from near Quom to west of Leigh Creek. The presence of clasts of porphyritic acid volcanics of similar petrology and age to the Gawler Range Volcanics, and of quartz

grains showing evidence of shock metamorphism, were used to demonstrate an origin by meteorite impact and settling of the ejecta in relatively deep, quiet waters during Bunyeroo Formation deposition. Shattered volcanics within the circular depression of Lake Acraman in the Gawler Ranges (Williams 1986b), were interpreted to indicate the point of impact. No other coarse-grained sediments are known from the Bunyeroo Formation and, as with the Moolooloo Siltstone Member/Tregolana Shale, no shoreline facies are known. The Bunyeroo is not preserved on the Curnamona Cratonic Nucleus. In the northern Flinders Ranges, Coats (1973) recorded the thin but persistent cupriferous Wearing Dolomite Member and local occurrences of black shale, while phosphatic chert and chert breccia occur locally near the base. Over most of the central Flinders Ranges, there is a continuous passage from Bunyeroo Formation to the overlying Wonoka Formation. Dalgamo & Johnson (1966) placed the boundary at the gradational colour change from red to dominantly grey sediments and an abrupt increase in carbonate content. Gostin & Jenkins (1983) suggested a revised boundary, lower than that of Dalgarno & Johnson (1966), which places the Wearing Dolomite Member at the base of the Wonoka. This revision also places the sandy beds (possible turbidites) in the upper part of the original Bunyeroo into the Wonoka Formation. While placement of a boundary in a transitional sequence is debatable, the present author is cautious to accept the revision until boundary relationships are clarified in parts of the northern and southern Flinders Ranges where deeply incised channels (over 1 km deep) were cut into underlying sediments (Coats 1964,1973; vonder Borch et al 1982). The stratigraphic position of the erosional contact may correspond more closely to the original Bunyeroo-Wonoka boundary than to the revised one. Work is in progress at both Adelaide and Hinders Universities on the Wonoka Formation and its erosional features. The kilometre-deep channels are filled with sediments containing

W. V. Preiss

some shallow-water structures such as wave ripples and tidally influenced cross-bedding (pointed out by participants of the 1986 Twelfth International Sedimentological Congress excursion), yet no break in sedimentation can be discerned away from these canyon-like incisions. This apparent contradiction has yet to be explained. Haines (1986) has divided the Wonoka Formation into eleven informal units, of which the lower three had been included in the original definition of the Bunyeroo Formation. These units reflect an overall upward-shallowing marine sequence, passing from basinal fine clastics through calcareous and sandy turbidites to storm-deposited shelf carbonates, and culminating with very shallow-water ooid grainstones surrounding small stromatolite bioherms. The Billy Springs Formation of the extreme northeastern Hinders Ranges may be a distal, silty facies equivalent in part to both Wonoka Formation and Bonney Sandstone, and contains some sandstones of possible turbiditic origin as well as slump deposits. The Pound Subgroup (redefined by Jenkins 1975) gradationally overlies the Wonoka; these units are confined to the Flinders Ranges. A thin sandstone interbed within the uppermost shallow-water carbonates of the Wonoka probably represents intertonguing. The lower, soft, red Bonney Sandstone was deposited on coastal sand-flats under oxidising conditions. Gehling (1982) reported a disconformity in the central Flinders Ranges between the Bonney Sandstone and the overlying Rawnsley Quartzite, a cleaner white sandstone and quartzite. The Ediacara Member, containing siltstones and flaggy sandstones with impressions of metazoan fossils, is widespread in the Flinders Ranges (Jenkins 1975) but has been variously interpreted to represent either intertidal deposits (Jenkins et al 1983) or a submarine channel-filling turbidite sequence (Gehling 1982).

Early Cambrian sedimentation: Hawker and Normanville Groups The end of the Precambrian was marked by a complete withdrawal of the sea from the Adelaide Geosyncline, for a period of unknown length. Renewed widespread transgression very early in the Cambrian is recorded by the Parachilna Formation of the Flinders Ranges, Mount Terrible Formation of the Mount Lofty Ranges and Winulta Formation of Yorke Peninsula. An even earlier but more restricted sedimentation event is recorded in parts of the northern Flinders Ranges by the partly channel-filling Uratanna Formation, an upward-coarsening silty and sandy unit containing early Cambrian trace fossils (Daily 1973). The Uratanna Formation and Pound Subgroup are disconformably overlain by the Parachilna Formation; the disconformity has cut down to varying levels in the Rawnsley Quartzite and locally into the Bonney Sandstone. The Parachilna Formation contains the first abundant sediment-penetrating worm burrows such as Diplocraterion, and was deposited as a transgressive tidal sand-sheet grading up, with increasing carbonate content, into the dolomitic, oolitic and stromatolitic Woodendinna Dolomite. The overlying Wilkawillina Limestone is a diachronous pure limestone facies largely constructed of highenergy skeletal grainstone, and archaeocyathid and algal buildups, in an open marine shelf environment (D. I. Gravestock, pers. comm. 1987). A reddened disconformity surface is widespread in the Flinders Ranges at the top of the lower Wilkawillina Limestone, while platform carbonate of the upper Wilkawillina intertongues laterally with the more basinal dark, thinly bedded Parara Limestone, Oraparinna Shale, Bunkers Sandstone, Edeowie Limestone Member and Moorowie Formation. The Moorowie includes a megabreccia facies deposited north of an east-northeast-trending faulted hingeline established during the early Cambrian in the central to northern Flinders Ranges. These units together comprise the Hawker Group, while equivalents in the Mount Lofty Ranges are the Normanville Group (Table 1), discussed in more detail by

Adelaide Geosyncline Overview Alexander & Gravestock (this volume). Carbonaceous basinal siltstone with phosphate nodules (Heatherdale Shale) occurs at the top of the Normanville Group, and is overlain with abrupt but conformable contact by the Kanmantoo Group. In the northeastern Mount Lofty Ranges the Normanville Group contains the Truro Volcanics (Forbes et al 1972), possibly intertonguing westwards with the carbonate and shale facies, but their precise stratigraphic position is not certain. Gravestock and Hibburt (pers. comm.) suggest that a thin tuff band in the Parara Limestone on Yorke Peninsula may be related to the Truro Volcanics. Drilling in the western Murray Basin has intersected basic volcanics that may also be associated (Gidley 1983), although no direct evidence for their age is available.

Group; a turbidite origin has been proposed for this flysch-like sequence by Sprigg & Campana (1953), Thomson (1969), Flint (1978) and Boord (1985) among others, but Daily & Milnes (1971, 1973) preferred a shallow-water interpretation. Strong contrasts between some of the units suggest that both shallow and deep-water facies may be represented, e.g. the cleaner feldspathic sandstones of the Backstairs Passage Formation and Middleton Sandstone contain large-scale cross-bedding, perhaps suggestive of deltaic deposition, while the relatively clay-rich sandstones with interbedded shales represented by the Tappanappa and Balquhidder Formations are more flysch-like. Although Bouma sequences are locally recognisable, graded bedding is rare. The shallow-marine clastics of the northern coast of Kangaroo Island described by Daily et al (1980) are broadly equivalent to the Kanmantoo Clastic sediments of the Billy Creek Group and may represent a nearer-shore facies; Formation, Lake Frome Group and the occurrence of boulders of fossiliferous limeKanmantoo Group stone and basement gneiss in the White Point The carbonate-dominated early Cambrian Conglomerate indicate stripping of an uplifted Hawker and Normanville Groups are overlain by carbonate shelf overlying basement just to the clastic-dominated sequences of contrasting facies north of Kangaroo Island (Daily et al 1979) ; in the north and south. In the Flinders Ranges, the similar conglomerate persists as lenses within the tidal-flat and deltaic redbeds of the Billy Creek Tappanappa Formation on eastern Kangaroo IsFormation overlie the Hawker Group with local land and in the Minlaton Formation on Yorke disconformity (Moore 1979,1982) and pass with Peninsula. renewed marine transgression into the shallowwater shelf carbonates of the Wirrealpa Limestone (Youngs 1977). This is followed by a thick DELAMERIAN OROGENY terrigenous redbed sequence, the Lake Frome Group, very sparsely fossiliferous and repre- Folding and metamorphism senting shallow marine to terrestrial environments (Daily 1956; Dalgarno & Johnson 1962). The Delamerian Orogeny (named by Thomson 1969) brought to an end Cambrian deposition A redbed sequence with minor limestone and in the Adelaide Geosyncline, though sedimentaevaporite interbeds on Yorke Peninsula is broadly tion may have continued without major interrupcomparable to the Billy Creek Formation, Wir- tion in other basins (e.g. Amadeus, Officer, realpa Limestone and Lake Frome Group of the Georgina and Warburton Basins). The younger Flinders Ranges (Daily 1956,1972, this volume). age limit of the Delamerian Orogeny is not yet In the Mount Lofty Ranges, however, the clastic well defined and some of the tectonic events may sediments of the Kanmantoo Group, overlying have overlapped in time with Palaeozoic the carbonate-rich Normanville Group, are orogenies in eastern Australia (Milnes et al 1977). dominantly of deeper water facies. There has Deformation of the thick sedimentary pile of the been considerable debate on the environments Adelaide Geosyncline was most intense in the and modes of deposition of the Kanmantoo southern Mount Lofty Ranges (Figs 4 and 5). In

W. V. Preiss

Cainozoic sediments

Kapund<r

Permian glacial sediments \Truro» Delamerian intrusives K A N M A N T O O G R O U P : sandstone, limestone, siltstone, pyritic shale _

Nuriootpal.H

NORMANVILLE GROUP: limestone, shale, volcanics W I L P E N A G R O U P : siltstone, sandstone U M B E R A T A N A G R O U P : siltstone, limestone, diamictite B U R R A GROUP:siltstone, sandstone, dolomite

Williamstown •

Stonyfell and Undalya Quartzites Emeroo Subgroup Basement: gneiss, phyllonite

Houghton

Fault (including thrust)

ADELAIDE

F, anticline F, overturned anticline F 2 and F 3 syncline F 2 and F 3 anticline

Murray Bridge KILOMETRES

Strathalbyn Sellicks YWWksZX

Normanville

LAKE ALEXA NDRINA

Myponga;

^Victor Harbor

LAKE ALBERT 8 8 - 9 4 SADME

F i g . 4 . Simplified geological map of the Mount Lofty Ranges showing the distribution of major lithostratigraphic units and structural features.

Adelaide Geosyncline Overview

A n t i c l i n a l axis of first d e f o r m a t i o n

A n t i c l i n a l axis o f s e c o n d a n d third d e f o r m a t i o n s D i r e c t i o n of a s y m m e t r y

PEAKE AND A DENISON RANGES "

Thrust fault P o s t - D e l a m e r i a n basin '

MOUNT WOODS INLIER

Pre-Adelaidean basement

EROMANGA

BASIN

CURNAMONA CRATONIC STUART

NUCLEUS

SHELF

0 I

100 1 KILOMETRES

200

Fig. 5. Anticlinal axes of the Delamerian fold belt, illustrating the effect of fold interfence in the Nackara Arc and Central Flinders Zone. The interpreted shear zone along the Crystal Brook Lineament marks the boundary between west-directed and east-directed asymmetry of structures.

W. V. Preiss

the western zone of this region, overthrusts and asymmetrical folds, overturned towards the west and northwest, dominate within the Burra Group, while younger beds to the southwest become progressively less strongly deformed. Crystalline basement was involved in this deformation, appearing as a series of anticlinal cores, commonly with faulted western margins (Figs 2 and 4). Both basement and cover in this zone were metamorphosed to greenschist facies; gently east-dipping slaty cleavage is ubiquitous in suitable finegrained rock types, and the slates become phyllitic in narrow zones of high strain. Folds of the first generation dominate in this zone, but there is local evidence of crenulation and refolding by a second fold phase without large-scale redistribution of rock units. A few kilometres east of the basement inliers, metamorphic grade increases abruptly to amphibolite facies across the Kitchener-Meadows Fault (Figs 2 and 4). This fault has surprisingly little stratigraphic throw, mainly within the Burra Group, suggesting that the metamorphic isograds here are steeply dipping and not related to stratigraphic level. The highest grade of metamorphism (upper amphibolite facies) and development of migmatites are confined to a north-northwest-trending belt in the eastern Mount Lofty Ranges (Offler & Fleming 1968), possibly related to a major continental shear zone (Coward 1976; see Crystal Brook Lineament, Fig. 5). The regional distribution of rock units in the high-grade belt (dominantly in the Kanmantoo Group) is determined mainly by second- and third-generation folds (Offler & Fleming 1968; Fig. 4). Within the amphibolite facies belt, the Kanmantoo Group is brought in contact with Adelaidean rocks by the Naime Fault (Fig. 4), but further work is required to ascertain whether this is a late, steeply dipping fault (Toteff, this volume) or an early (? first deformation) thrust refolded by the regional later fold phases (Marlow 1975). The latter possibility is suggested by the apparent persistence of a faulted contact between Adelaidean and Kanmantoo Group rocks

around the Macclesfield Syncline and Strathalbyn Anticline, determined by Offler & Fleming (1968) to be post-Dl folds, and by the absence of a change in metamorphic grade across this boundary. The contact has also been interpreted as an unconformity (Thomson 1969), but no outcrops are known that prove such a relationship. South of Normanville on Fleurieu Peninsula, the Kanmantoo Group is in fault contact with crystalline basement (B. Daily, pers. comm. 1963), and this may be an extension of the Naime Fault. Such a thrust would be unusual in that younger strata have been moved westwards over older rocks, effectively cutting downwards in the stratigraphic pile. A possible explanation for this might be that the floor of the Kanmantoo Trough had previously been lowered to deeper levels of the crust by normal faulting during sedimentation; west-directed thrusting before the main metamorphism may have brought these rocks up to override the Adelaidean at a higher structural level. To the northeast near Sedan, the metamorphic grade again drops abruptly, the isograds trending northwesterly and transecting obliquely the continuous stratigraphy of the Kanmantoo Group (Offler & Fleming, 1968). North and east of this region, the direction of overturning of folds changes to easterly, and most of the long, arcuate folds of the Mid-North have steeply west-dipping axial planes with slaty cleavage. The change in asymmetry has been interpreted by Preiss (1987) as a response in the cover to a north-northwest-trending basement wrench fault such as Coward's (1976) proposed shear zone. The change in direction of fold trends to east-west on Kangaroo Island appears to be related to the orientation of rifts that produced the Kanmantoo Trough. Mancktelow (1981) attributed the very intense deformation of parts of Fleurieu Peninsula to compression against a curved southeasterly promontory of the Gawler Craton. The tectonic styles of the Flinders Ranges differ markedly from that of the southern Mount Lofty Ranges, while the Mid-North region is of intermediate character. The Mid-North folds

Adelaide Geosyncline Overview

form the southern arm of the Nackara Arc, and are characteristically long, gently arcuate from north-northwest to north-northeast-trending, broadly doubly plunging, and slightly asymmetrical towards the east; fine-grained lithologies display mostly steeply west-dipping slaty cleavage. A largely concealed fault of considerable stratigraphic throw trends north-northwest along the western margin of the fold belt between Gawler and Port Augusta, and separates gently folded Wilpena Group and Cambrian sediments in the Torrens Hinge Zone from strongly deformed early Adelaidean rocks. Folds near this fault are partly overturned towards the west, and the fault may itself be a west-directed overthrust.

Fault, which may be a steep reverse fault, in places bringing Burra Group in contact with Cambrian sediments, and the Paralana Fault, actually a system of north to northeast-trending arcuate splinters whose activity appears to date from earliest Adelaidean time. Like the Nackara Arc, the North Flinders Zone has tighter, arcuate folds with slaty cleavage at least locally developed. The Mount Painter and Mount Babbage Inliers of pre-Adelaidean rocks occur in anticlinal culminations in the east; although there has been some shearing along the basementcover contact, the basal unconformity is locally well preserved. Although Richert (1976) claimed that there was major thrusting onto the Mount

A probable northerly extension of the fault has

Painter

been intersected in drilling at Depot Creek, where Willouran volcanics and sediments are thrust westwards over Wonoka Formation (Preiss & Faulkner 1984), although regional folds in that area are slightly overturned eastwards, and more than one phase of deformation may be involved.

Adelaidean rocks adjacent to the inlier are characterised by upright folds and steeply dipping schistosity. The grade of Delamerian metamorphism locally reached lower amphibolite facies around the Mount Painter Inlier (Coats & Blissett 1971).

The eastern arm of the Nackara Arc is characterised by east-northeasterly regional fold trends, shown by Berry et al (1978) to be due to a second Delamerian fold phase overprinting more meridionally oriented first phase folds. Preiss (1987) suggested that this scheme could be extended to other parts of the fold belt to account for formation of the Nackara Arc and fold interference patterns through much of the Flinders Ranges (Fig. 5).

Diapirs

The Central Flinders Zone between Wilpena and Copley constitutes the least-deformed province of the Delamerian fold belt, with broad, open, dome and basin folds (Fig. 5) resulting from the interfence of east-northeast and northnorthwest fold trends (Richert 1976; Preiss 1987). The sediments of this region are almost totally uncleaved and display no obvious sign of metamorphism. Arcuate fold patterns again dominate in the North Flinders Zone, trending from northwesterly in the west to northeasterly in the east. Major faults include the northwest-trending Norwest

Inlier,

strongly

deformed

early

Large bodies of carbonate-cemented breccia containing irregularly orientated blocks of Willouran sedimentary and igneous rocks in the Flinders Ranges were first interpreted as diapirs by Webb (1960), and this concept was applied in subsequent mapping by the South Australian Geological Survey. In a recent review of diapirism in the Adelaide Geosyncline, Preiss (1985) concluded that the Curdimurka Subgroup was the main source of incompetent carbonate, clastic and evaporitic sediments that contributed to the formation of the diapiric breccias. Mount (1975) gave a detailed account of the mechanisms by which fluid-saturated and mobile breccias were intruded into overlying sequences during tensional phases late in the folding. However, considerable evidence also exists for syndepositional diapirism during the period from Sturtian to Cambrian (Dalgarno & Johnson 1968; Coats 1973; Haslett 1976; Lemon 1985), especially where clasts of Callanna Group lithologies were shed into contemporaneous sediments. However, uncertainty remains about timing of the deforma-

W. V. Preiss

tion of some diapiric xenoclasts and largely intact Willouran sequences, which locally display isoclinal folding and thrusting (Parker 1983; Preiss 1985). Igneous activity Plutonic rocks provide the only direct dating of the Delamerian Orogeny. Granitoid intrusives are confined to specific zones within the fold belt (Fig. 2): • a discontinuous belt of mainly syn-tectonic intrusives follows the sigmoidal pattern of the eastern edge of the Nackara and Fleurieu arcs, from the southern coast of Kangaroo Island, through Encounter Bay and the eastern Mount Lofty Ranges, to the Bendigo and Anabama Granites in the northeast • a north-northwest trending belt of syn- and post-orogenic intrusives in the Padthaway Ridge, possibly continuing the trend of granitoids in the highest-grade zone of the eastern Mount Lofty Ranges. • granitoid intrusives in the Mount Painter and Mount Babbage Inliers and adjacent Adelaidean metasediments

the inception of folding. Within the high-grade zone, some granitoids are highly foliated and appear to have been emplaced before or during the most intense deformation. In contrast, the plutons along the western margin of the Murray Basin are mostly massive and appear to post-date the major deformation (e.g. Murray Bridge Granite, Anabama Granite, Black Hill Norite). However, intrusive relationships for this group are rarely seen; only the Anabama Granite is observed to intrude Sturtian rocks, and others may intrude either Adelaidean or Kanmantoo Group. Granitoids of the northern part of the Delamerian fold belt intrude pre-Adelaidean and basal Adelaidean rocks (e.g. in the Mount Painter Inlier) or Burra Group (Bungadillina Monzonite of the Peake and Denison Ranges). The latter also occurs as xenoclasts in diapiric breccia, and has yielded a range of K-Ar ages from 470 to 500 Ma (Ambrose et al 1981). Metamorphosed mafic dykes are common in the eastern Mount Lofty Ranges, often with northwesterly orientation (Parker et al 1987). A few unmetamorphosed Delamerian dykes cut early Adelaidean sediments in the Peake and Denison Ranges, but no mafic igneous activity is associated with the orogeny over most of the Flinders Ranges.

• intrusives in lower Adelaidean rocks in the Willouran Ranges and Peake and Denison Ranges.

CONCLUSIONS

Not all of these plutonic rocks have exposed contact relationships and their timing with respect to phases of folding is still controversial. Published radiometric dating is confined to K-Ar and Rb-Sr techniques; in many cases, the dates are based only on separated minerals and are therefore unable to distinguish unequivocally between age of intrusion and ages of subsequent resetting (Milnes et al 1977). These authors argued that the Encounter Bay Granites intruded into essentially undeformed sediments of the Kanmantoo Group, now moderately metamorphosed, and used the Rb-Sr whole rock isochron method to date intrusion (504 ± 8 Ma) and hence

The Adelaide Geosyncline is the repository of a comprehensive sedimentary record for the late Proterozoic and early to middle Cambrian. The dominance of sediments over igneous rocks and the paucity of fossils through most of the succession have rendered correlation and dating difficult. However, mapping of the well exposed Flinders Ranges, in particular, has allowed the erection of a consistent stratigraphic scheme whereby facies can be related to an approximate time framework, and hence palaeogeographic evolution can be reconstructed. The Adelaide Geosyncline is host to many unusual aspects of sedimentation, such as sedimentary magnesite, glaciogenic sediments, periglacial features, syn-

Adelaide Geosyncline Overview

depositional diapirism, persistence of some laterally uniform facies while others are confined to specific belts, probable meteorite-impact debris in one thin layer, submarine canyons with unexpected shallow-water structures in their fill, and the famous Ediacara metazoan assemblage. The early Adelaidean history is dominated by successive rifting events forming troughs with different locations and orientations. The earliest sediments record an initially stable phase of platform sedimentation, the Willouran mafic lavas indicating the first major regional extension of the crust. Subsequent Willouran troughs were orientated mainly northwest or north-northwest, while Torrensian rifts bordered the Torrens Hinge Zone and allowed subsidence of the Adelaide region. Both Callanna Group and Burra Group are mixed carbonate and clastic sequences, the former characterised by evaporite mineral casts and pseudomorphs. The base of the Burra Group marks a major clastic influx from the Gawler Craton, while the remainder of Torrensian deposition is cyclic, with eastward-prograding sand wedges and lagoonal to marginal-marine carbonates dominated by syndepositional dolomite. The Umberatana Group everywhere unconformably overlies older rocks and reflects a change in sedimentary and tectonic style. Thin shelf sequences of Sturtian glaciomarine diamictite and reworked glacials covered the sites of the older troughs, while the new Baratta and Yudnamutana Troughs were generated much further east. The interglacial sequences record widespread transgression and then upward shallowing, with complexly intertonguing relationships of redbeds, carbonates and deeper-water fine clastics. The Marinoan glaciation appears to have had ice centred to the north of the Geosyncline, while the Stuart Shelf was subject only to periglacial conditions. The lower Wilpena Group resulted from an even more widespread post-glacial transgression, followed by easterly progradation of sands from the Gawler Craton. Renewed transgression again

inundated the basin, allowing deposition of the fine-grained sediments of the Bunyeroo Formation into which the meteorite-impact debris fell. The youngest units of the Wilpena Group are more or less confined to the Flinders Ranges and record upward shallowing to near-shore environments represented by the fossiliferous Pound Subgroup. After a basin-wide hiatus, deposition resumed with renewed transgression in the early Cambrian, depositing shelf and slope carbonates of the Hawker and Normanville Groups; in the later early Cambrian to middle Cambrian, the cratonic areas and Flinders Ranges region received redbed deposition, while the deeperwater Kanmantoo Trough in the south records renewed tensional activity. The Delamerian Orogeny brought sedimentation to a close in the late Cambrian and subjected the Adelaide Geosyncline sediments to several fold phases, of which the first was probably the most widespread and intense. West-directed overfolds and small to medium-scale thrusts characterise much of the Mount Lofty Ranges, but deformation in the Hinders Ranges was much milder. Many of the dome-and-basin folds and intersecting fold trends appear to result from interference of two fold phases, the first dominantly with north-northwesterly axes and the second with east-northeasterly. Associated igneous activity is confined to specific zones within the fold belt, and includes syntectonic granitoids in the zone of highest metamoiphic grade.

ACKNOWLEDGEMENTS I am indebted to the late Dr Brian Daily for his first introduction to me of the geology of the Adelaide Geosyncline, and to the numerous other earth scientists who have over many years shared their ideas. Most of the concepts in this paper are summarised from a recent larger publication (Preiss, 1987), based on manuscripts by B. G. Forbes, R. P. Coats and W. V. Preiss, which includes a more comprehensive acknowledgement.

W. V. Preiss

The present paper was greatly improved by helpful comments from A. J. Parker, C. G. Gatehouse, D. Gravestock, R. B. Hint and A. P. Belperio, as well as from referees D. B. Hilyard and V. A. Gostin, and is published with the permission of the Director General, South Australian Department of Mines and Energy.

REFERENCES ALEXANDER E. M. & GRAVESTOCK D. I. (this volume). Sedimentary facies in the Sellick Hill Formation, Fleurieu Peninsula, South Australia. AMBROSE G. J., FLINT R. B. & WEBB A. W. 1981. Precambrian and Palaeozoic geology of the Peake and Denison Ranges. Geological Survey of South Australia, Bulletin 50. BELPERIO, A. P. 1973. The stratigraphy and facies of the late Precambrian lower glacial sequence, Mount Painter, South Australia. B. Sc. (Hons) thesis, University of Adelaide (unpubl.). BELPERIO A. P. (this volume). Palaeoenvironmental interpretation of the late Proterozoic Skillogalee Dolomite in the Willouran Ranges, South Australia. BERRY R. F., FLINT R. B. & GRADY A. E. 1978. Deformation history of the Outalpa area and its application to the Olary Province, South Australia. Transactions of the Royal Society of South Australia 102,43-53. BOORD R. A. 1985. Sedimentology of the Cambrian upper Kanmantoo Group, southern Fleurieu Peninsula. B. Sc. (Hons) thesis, University of Adelaide (unpubl.). CLOUD P. E. & GLAESSNER M. F. 1982. The Ediacarian Period and System: Metazoa inherit the Earth. Science, 217,783-792. COATS R. P 1964. Large scale Precambrian slump structures, Flinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 11, 1-2. COATS R. P. 1965. Tent Hill Formation correlations, Port Augusta and Lake Torrens areas. Quarterly Geological Notes, Geological Survey of South Australia 16,9-11.

COATS R. P. 1967. The "Lower Glacial Sequence" Sturtian type area. Quarterly Geological Notes, Geological Survey of South Australia 23,1-3. COATS R. P. 1973. COPLEY, South Australia. Explanatory notes, 1:250 000 geological series. Sheet SH/54-9. Geological Survey of South Australia. COATS R. P. & BLISSETT A. H. 1971. Regional and economic geology of the Mount Painter Province. Geological Survey of South Australia, Bulletin 43. COMPSTON W., CRAWFORD A. R. and BOFINGER V. M. 1966. Aradiometric estimate of the duration of sedimentation in the Adelaide Geosyncline, South Australia. Journal of the Geological Society of Australia 13, 229-276. COWARD M. P. 1976. Large scale Palaeozoic shear zone in Australia and present extension to the Antarctic Ridge. Nature 259, 648-649. CRAWFORD A. J., CAMERON W. E. & KEAYS R. R. 1984. The association of low-Ti andesite-tholeite in the Heathcote Greenstone Belt, Victoria; ensimatic setting for the early Lachlan Fold Belt. Australian Journal of Earth Sciences 31, 161-175. CRAWFORD, A. J. & HILYARD, D. B. (this volume). Geochemistry of late Proterozoic tholeiitic flood basalts, Adelaide Geosyncline, South Australia. DAILY B. 1956. The Cambrian in South Australia. In El Sistema Cambrico, su paleogeografia y el problema de su base. Vol. 2, pp. 91-147, Report of the 20th International Geological Congress, Mexico. DAILY B. 1963. The fossiliferous Cambrian succession on Fleurieu Peninsula, South Australia. Records of the South Australian Museum 14, 579-601. DAILY B., 1972. The base of the Cambrian and the first Cambrian faunas. Centre for Precambrian Research, University of Adelaide, Special Paper 1,13-41. DAILY B. 1973. Discovery and significance of basal Cambrian Uratanna Formation, Mount Scott Range, Flinders Ranges, South Australia. Search 4,202-205. DAILY B. (this volume). Cambrian stratigraphy of Yorke Peninsula.

Adelaide Geosyncline Overview DAILY B. & MILNES A. R. 1971. Stratigraphic notes on Lower Cambrian fossiliferous metasediments between Campbell Creek and Tunkalilla Beach in the type section of the Kanmantoo Group, Fleurieu Peninsula, South Australia. Transactions of the Royal Society of South Australia 95, 199-214. DAILY B. & MILNES A. R. 1973. Stratigraphy, structure and metamorphism of the Kanmantoo Group (Cambrian) in its type section east of Tunkalilla Beach, South Australia. Transactions of the Royal Society of South Australia 97, 213-242. DAILY B„ MILNES A. R., TWIDALE C. R. & BOURNE J. A., 1979. Geology and geomorphology. In Tyler M. J., Twidale C. R. and Ling J. K. eds. Natural History ofKangaroo Island, pp. 1-38, Royal Society of South Australia, Adelaide. DAILY B. MOORE P.S. & RUST B. R. 1980. Terrestrial-marine transition in the Cambrian rocks of Kangaroo Island, South Australia. Sedimentology 27, 379-399. DALGARNO C. R. & JOHNSON J. E. 1962. Cambrian sequence of the western Flinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 4, 2-3. DALGARNO C. R. & JOHNSON J. E. 1966. PARACHILNA map sheet, Geological Atlas of South Australia, 1:250 000 series. Sheet SH/54-13. Geological Survey of South Australia. DALGARNO C. R. & JOHNSON J. E. 1968. Diapiric structures and late Precambrian-early Cambrian sedimentation in the Flinders Ranges, South Australia. American Association of Petroleum Geologists, Memoir 8,301-314. DUNN P. R., PLUMB K. A. & ROBERTS H. G. 1966. A proposal for time-stratigraphic subdivision of the Australian Precambrian. Journal of the Geological Society of Australia 13, 593-608. DYSON I. A., 1986. Geology of the late Precambrian Brachina Subgroup at Hallett Cove, South Australia. Geological Society of Australia, Abstracts 15, 233.

FAIRCHELD T. R. 1975. The geologic setting and paleobiology of a late Precambrian stromatolitic microflora from South Australia. Ph.D. thesis, University of California (unpubl.). FANNING C. M., LUDWIG K. R., FORBES B. G. and PREISS W. V., 1986. Single and multiple grain U-Pb zircon analyses for the early Adelaidean Rook Tuff, Willouran Ranges, South Australia. Geological Society of Australia, Abstracts 15, 71-72. FLINT D. J. 1978. Deep sea fan sedimentation of the Kanmantoo Group, Kangaroo Island. Transactions of the Royal Society of South Australia 102, 203-222. FORBES B. G. 1960. Magnesite of the Adelaide System: petrography and descriptive stratigraphy. Transactions of the Royal Society of South Australia 83,1-9. FORBES B. G. 1961. Magnesite of the Adelaide System: a discussion of its origin. Transactions of the Royal Society of South Australia 85, 217-222. FORBES B. G. 1970. Benda Siltstones. Quarterly Geological Notes, Geological Survey of South Australia 33, 1-2. FORBES B. G., COATS R. P. & DAILY B. 1972. Truro Volcanics. Quarterly Geological Notes, Geological Survey of South Australia 44, 1-5. FORBES B. G. & COOPER R. S. 1976. The Pualco Tillite of the Olary region, South Australia. Quarterly Geological Notes, Geological Survey of South Australia 60, 2-5. FORBES B. G., MURRELL B. & PREISS W. V., 1981. Subdivision of lower Adelaidean, Willouran Ranges. Quarterly Geological Notes, Geological Survey of South Australia 79, 7-16. GEHLING J. G., 1982. The sedimentology and stratigraphy of the late Precambrian Pound Subgroup, central Hinders Ranges, South Australia. M.Sc. thesis, University of Adelaide (unpubl.). GERSTELING R. & HEAPE J. M. 1975. The Cattlegrid Orebody, Mt Gunson, South Australia. In South Australian Conference, 1975. Part A: Adelaide and Port Pirie. Australasian Institute of Mining and Metallurgy, Conference Series 4, 103-112.

W. V. Preiss

GIDLEY, P. R. 1983. A review with interpretation and geological assessment of the Barker-Pinnaroo areas of South Australia, EMR 5/83. South Australian Department of Mines and Energy open file envelope 4719 (unpubl.).

GILES C. W. & TEALE G. S. 1979. A comparison of the geochemistry of the Roopena Volcanics and the Beda Volcanics. Quarterly Geological Notes, Geological Survey of South Australia 71,13-18. GLAESSNER M. F. & DAILY, B. 1959. The geology and late Precambrian fauna of the Ediacara Fossil Reserve. Records of the South Australian Museum 13, 369-401. GOSTIN V. A., HAINES P. W., JENKINS R. J. F., COMPSTON W. & WILLIAMS I. S., 1986.1mpact ejecta horizon within late Precambrian shales, Adelaide Geosyncline, South Australia. Science 233,198-200. GOSTIN V. A. & JENKINS R. J. F. 1983. Sedimentation of the early Ediacaran, Flinders Ranges, South Australia. Geological Society of Australia, Abstracts 9, 196-197. GUNN P. J. 1984. Recognition of ancient rift systems: examples from the Proterozoic of South Australia. Exploration Geophysics 15, 85-97. HAINES P. W. 1986. Late Proterozoic carbonate shelf to shale basin transition, Wonoka Formation, Hinders Ranges, S. A. Geological Society of Australia, Abstracts 15,92-93.

HASLETT P. G. 1976. Lower Cambrian carbonate stratigraphy and sedimentology, Old Wirrealpa Springs, Flinders Ranges, South Australia. Ph.D. thesis, University of Adelaide (unpubl.). HILYARD, D. B. (this volume). Willouran basic province: stratigraphy of Late Proterozoic flood basalts, Adelaide Geosyncline, South Australia. HOPTON D. L. 1983. Environmental analysis of the late Precambrian Appila Tillite equivalent at Depot Flat, southern Flinders Ranges, South Australia. B. Sc. (Hons) thesis, University of Adelaide (unpubl.).

HOWCHIN W. 1897. On the occurrence of Lower Cambrian fossils in the Mount Lofty Ranges. Transactions of the Royal Society of South Australia 21,74-86. HOWCHIN W. 1901. Preliminary note on the existence of glacial beds of Cambrian age in South Australia. Transactions of the Royal Society of South Australia 25,10-13. HOWCHIN W. 1904. The geology of the Mount Lofty Ranges. Part I - the coastal district. Transactions of the Royal Society of South Australia 28,253-280. JENKINS R. J. F. 1975. An environmental study of the rocks containing the Ediacara assemblage in the Flinders Ranges. In Abstracts, 1st Australian Geological Convention, Adelaide, 1975 - Proterozoic Geology, pp. 21-22, Geological Society of Australia. JENKINS R. J. F. & GOSTIN V. A. 1983. Marinoan and Ediacaran type sections in the context of tectonic cycles in the Adelaide Geosyncline. Geological Society of Australia, Abstracts 10, 39-44. JENKINS R. J. F., FORD C. H. & GEHLING J. G., 1983. The Ediacara Member of the Rawnsley Quartzite: the context of the Ediacara assemblage (late Precambrian, Flinders Ranges). Journal of the Geological Society of Australia 30, 101-119. JOHNS R. K. 1968. Geology and mineral resources of the Andamooka-Torrens area. Geological Survey of South Australia, Bulletin 41. LEESON B. 1970. Geology of the Beltana 1:63 360 map area. Geological Survey of South Australia, Report of Investigations 35. LEMON, N. M. 1985. Physical modelling of sedimentation adjacent to diapirs and comparison with late Precambrian Oratunga breccia body in central Flinders Ranges, South Australia. American Association of Petroleum Geologists, Bulletin 69, 1327-1338. LEMON N. M. 1986. Blinman to Enorama - a one day excursion. In: Proterozoic clastic sedimentation in relation to glaciations and tectonism, Flinders Ranges, South Australia. Excursion 27B, 12th International Sedimentological Congress (unpubl.).

Adelaide Geosyncline Overview

MANCKTELOW N. S. 1981. Variation in fold axis geometry and slaty cleavage microfabric associated with a major fold arc, Fleurieu Peninsula, South Australia. Journal of the Geological Society of Australia 28, 1-12.

MOORE P. S. 1979. Stratigraphy and depositional environments of the Billy Creek Formation (Cambrian), central and northern Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 103,197-211.

MARLOW P. C. 1975. Structural investigations near Macclesfield, South Australia. M.Sc. thesis, University of Adelaide (unpubl.).

MOORE P. S. 1982. Ripple mark analysis of a fine-grained epeiric-sea deposit (Cambrian, South Australia). Journal of the Geological Society of Australia 29,71-81.

MASON M. G., THOMSON B. P. & TONKIN D. G. 1978. Regional stratigraphy of the Beda Volcanics, Backy Point Beds and Pandurra Formation on the southern Stuart Shelf, South Australia. Quarterly Geological Notes, Geological Survey of South Australia 66, 2-9.

MAWSON D. 1938. Cambrian and sub-Cambrian formations at Parachilna Gorge. Transactions of the Royal Society of South Australia 62, 255-262.

MAWSON D. 1947. The Adelaide Series as developed along the western margin of the Flinders Ranges. Transactions of the Royal Society of South Australia 71, 259-280.

MAWSON D. 1949a. The late Precambrian ice-age and glacial record of the Bibliando Dome. Journal and Proceedings of the Royal Society of New South Wales 82, 150,-174.

MAWSON D. 1949b. The Elatina glaciation. Transactions of the Royal Society of South Australia 73, 117-121.

MAWSON D. & SPRIGG R. C. 1950. Subdivision of the Adelaide System. Australian Jourrial of Science 13, 69-72.

MDLNES A. R., COMPSTON W. & DAILY B., 1977. Pre- to syn-tectonic emplacement of Palaeozoic granites in southeastern South Australia. Journal of the Geological Society of Australia 24, 87-106.

MIRAMS R. C. 1964. A Sturtian glacial pavement at Merinjina Well, near Wooltana. Quarterly Geological Notes, Geological Survey of South Australia 11,4-6.

MOUNT T. J. 1975. Diapirs and diapirism in the Adelaide "Geosyncline", South Australia. Ph.D. thesis, University of Adelaide (unpubl.). O'DRISCOLLE. S.T. 1973. A structural syndrome for ore emplacement at Broken Hill, Australia. Zeitschrift der Deutschen Geologischen Gesellschaft 124, 257-266. OFFLER R. & FLEMING P. D., 1968. A synthesis of folding and metamorphism in the Mt Lofty Ranges, South Australia. Journal of the Geological Society of Australia 15, 245-266. PARKER A. J. 1983. Tectonic development of the Adelaide fold belt. Geological Society of Australia, Abstracts 10, 23-28. PARKER A. J., RICKWOOD P. C., BAELLIE P. W., McCLENAGHAN M. P., BOYD D. M., FREEMAN M. J., PIETSCH B. A., MURRAY C. G. & MYERS J. S. 1987. Mafic dyke swarms of Australia. Geological Association of Canada, Special Paper 34, 401-417. PITT G. M. 1979. The Cutana Beds. Quarterly Geological Notes, Geological Survey of South Australia 71, 19-23. PLUMMER P. S. 1978. Stratigraphy of the lower Wilpena Group (late Precambrian), Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 102, 25-38. PLUMMER P. S. 1983. Correlation of the uppermost late Precambrian succession across the Torrens Hinge Zone in the Port Augusta region of South Australia. Transactions of the Royal Society of South Australia 107,171-175.

W. V. Preiss

PREISS W. V. 1972. The systematics of South Australian Precambrian and Cambrian stromatolites. Part I. Transactions of the Royal Society of South Australia 96, 67-100. PREISS W. V. 1973. Palaeoecological interpretations of South Australian Precambrian stromatolites. Journal of the Geological Society of Australia 19, 501-532. PREISS W. V. 1974. The River Broughton Beds - a Willouran sequence in the Spalding Inlier. Quarterly Geological Notes, Geological Survey of South Australia 49, 2-8. PREISS W. V. 1979. Megabreccia in Burra Group, Worumba Anticline. Quarterly Geological Notes, Geological Survey of South Australia 72, 6-12. PREISS W. V. 1985. Stratigraphy and tectonics of the Worumba Anticline and associated intrusive breccias. Geological Survey of South Australia, Bulletin 52. PREISS W. V. (compiler) 1987. The Adelaide Geosyncline - late Proterozoic stratigraphy, sedimentation, palaeontology and tectonics. Geological Survey of South Australia, Bulletin 53. PREISS W. V. & FAULKNER P. 1984. Geology, geophysics and stratigraphic drilling at Depot Creek, southern Flinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 89,10-19. PREISS W. V. & KINSMAN J. D. 1978. Stratigraphy andpalaeoenvironmental interpretation of the Brighton Limestone south of Adelaide and its equivalents in the Orroroo region. Geological Survey of South Australia, Report of Investigations 49. RICHERT J. P. 1976. Thrust faulting in the northern Flinders Ranges, South Australia. Journal of the Geological Society of Australia 23, 361-366. ROWLANDS N. J., BLIGHT P. G., JARVIS D. M. & VON DER BORCH C. C. 1980. Sabkha and playa environments in late Proterozoic grabens, Willouran Ranges, South Australia. Journal of the Geological Society of Australia 27, 55-68. RUTLAND R. W. R. 1976. Orogenic evolution of Australia. Earth Science Reviews 12, 161-196.

RUTLAND R. W. R., PARKER A. J., PITT G.M., PREISS W. V. & MURRELL B. 1981. The Precambrian of South Australia. In Hunter D. R. ed. Precambrian of the Southern Hemisphere. Developments in Precambrian Geology, vol. 2, pp. 309-360, Elsevier, Amsterdam. SCHEIBNER E. 1985. Suspect terranes in the Tasman fold belt system, eastern Australia. In Howell D. G. ed., Tectonostratigraphic terranes of the Circum-Pacific region, pp. 493-514. Circum-Pacific Council for Energy and Mineral Resources, Houston. SINGH U. 1987. Ooids and cements from the late Precambrian of the Flinders Ranges, South Australia. Journal of Sedimentary Petrology 57, 117-127. SPRIGG R. C. 1947. Early Cambrian (?) jellyfishes from the Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 71,212-224. SPRIGG R. C. 1952. Sedimentation in the Adelaide Geosyncline and the formation of the continental terrace. In Glaessner, M. F. and Rudd, E. A. eds. Sir Douglas Mawson Anniversary Volume, pp. 153-159, University of Adelaide, Adelaide. SPRIGG R. C. & CAMPANA B. 1953. The age and facies of the Kanmantoo Group. Australian Journal of Science 16, 12-14. THOMSON B. P. 1966. The lower boundary of the Adelaide System and older basement relationships in South Australia. Journal of the Geological Society of Australia 13, 203-228. THOMSON B. P. 1969. The Kanmantoo Group and early Palaeozoic tectonics. In Parkin L. W. ed. Handbook of South Australian Geology, pp. 97-108, Geological Survey of South Australia, Adelaide. THOMSON B. P. 1970. A review of the Precambrian and lower Palaeozoic tectonics of South Australia. Transactions of the Royal Society of South Australia 94,193-221. THOMSON B. P., COATS R. P., MIRAMS R. C., FORBES B. G., DALGARNO C. R. & JOHNSON J. E. 1964. Precambrian rock groups in the Adelaide Geosyncline: anew subdivision. Quarterly Geological Notes, Geological Survey of South Australia 9, 1-19.

Adelaide Geosyncline Overview

UPPILL R. K. 1979. Sedimentology of the late Precambrian Mundallio Subgroup (new name) in the late Precambrian Burra Group of the Mount Lofty and Binders Ranges. Transactions of the Royal Society of South Australia 103, 25-43. UPPILL R. K. (this volume). Sedimentology of a dolomite-magnesite-sandstone sequence in the late Precambrian Mundallio Subgroup, South Australia. VON DER BORCH C. C. 1980. Evolution of late Proterozoic to early Palaeozoic Adelaide Foldbelt, Australia: comparisons with post-Permian rifts and passive margins. Tectonophysics 70, 115-134. VON DER BORCH C. C. & LOCK D. 1979. Geological significance of Coorong dolomites. Sedimentology 26, 813-824. VON DER BORCH C. C., SMIT R. & GRADY A. E. 1982. Late Proterozoic submarine canyons of Adelaide Geosyncline, South Australia. American Association of Petroleum Geologists, Bulletin 66, 332-347. WEBB A. W. & COATS R. P. 1980. A reassessment of the age of the Beda Volcanics on the Stuart Shelf, South Australia. South Australian Department of Mines and Energy report 80/6 (unpubl.). WEBB A. W., COATS R. P., FANNING C. M. & FLINT R. B., 1983. Geochronological framework of the Adelaide Geosyncline. Geological Society of Australia, Abstracts 10, 7-9.

WEBB A. W. & HORR G. 1978. The Rb-Sr age and petrology of a flow from the Beda Volcanics. Quarterly Geological Notes, Geological Survey of South Australia 66,10-13. WEBB B. P. 1960. Diapiric structures in the Flinders Ranges, South Australia. Australian Journal of Science 22,9. WHITTEN G. F. 1970. The investigation and exploitation of the Razorback Ridge iron deposit. Geological Survey of South Australia, Report of Investigations 33. WILLIAMS G. E. 1985. Solar affinity of sedimentary cycles in the late Precambrian Elatina Formation. Australian Journal of Physics 38, 1027-1043. WILLIAMS G. E. 1986a. Precambrian permafrost horizons as indicators of palaeoclimate. Precambrian Research 32, 233-242. WILLIAMS G. E. 1986b. The Acraman impact structure: source of ejecta in late Precambrian shales, South Australia. Science 233, 200-203. WILTSHIRE R. G. 1983. Basement blocks in Umberatana Group - Olary. Geological Society of Australia, Abstracts 10,19. YOUNGS B. C. 1977. The sedimentology of the Cambrian Wirrealpa and Aroona Creek Limestones. Geological Survey of South Australia, Bulletin 47.

Willouran Basic Province: Stratigraphy of Late Proterozoic flood basalts, Adelaide Geosyncline, South Australia David Hilyard*

Department of Applied Geology, South Australian Institute of Technology, The Levels, SA. 5095, Australia The Wooltana Metabasalt, a subaerial continental tholeiite basalt lava formation, lies near the base of the Late Proterozoic Adelaide Geosyncline. Stratigraphic equivalents from elsewhere in the geosyncline, and the adjacent Stuart Shelf, consist of very similar sequences of lava. The stratigraphy of these units is dominated by sheet-like subaerial flows, with minor interbedded fluvial and lacustrine sediments. Available major, trace, and REE geochemistry for these rocks, and dolerite dykes of the Gairdner Dyke Swarm, are very sunilar, and indicate a continental tholeiitic composition. The basalts were subjected to hydrothermal alteration by hypersaline brines derived from evaporite-bearing sediments within the early Adelaidean sequence. These metabasic rocks define a basic igneous province of Willouran age that covered an area of c. 210 000 sq.km, comparable to the size of the Columbia Plateau basalts. Eruption occurred as flood basalts across the Willouran basic province. The stratigraphic sequence within the Wooltana Metabasalt shows many similarities to the classic flood basalt sequences of the Columbia River Basalt Group and the Keweenawan lavas. A constructional volcanic geomorphology formed, so that although the basalts were erupted in a grossly extensional tectonic environment, there were no morphological rift valleys. Widespread, massive graben subsidence and formation of rifts occurred after volcanism had ceased. The rifts were filled by fluvial and lacustrine sediments. The Willouran basic province documents extensive continental tholeiite flood volcanism associated with the early extension and rifting to form the Adelaide Geosyncline.

Key words: Adelaidean, Adelaide Geosyncline, Late Proterozoic, Willouran, Wooltana Metabasalt, flood basalts, South Australia. INTRODUCTION The basal (Willouran) sequence of the Adelaide Geosyncline and adjacent Stuart Shelf contains a widespread component of basic igneous rocks (Figs 1,2; Rutland et al 1981). These units occur as lavas within normal stratigraphic sequences (Wooltana Metabasalt, Beda Volcanics, Cadlareena Volcanics, Wilangee Basalt, and possibly the Boucaut Volcanics); as parts of sequences disrupted by carbonate breccia intrusions ("diapirs") - the Noranda Volcanics, basalts of the River Broughton beds, and unnamed xenoclasts; and as dykes intruding preAdelaidean sediments on the Stuart Shelf

(Gairdner Dyke Swarm). The lavas in normal stratigraphic sequences all lie above basal Adelaidean (Late Proterozoic) sequences unconformably overlying Lower to Middle Proterozoic crystalline basement. In each case the Adelaidean sediments below the lavas consist of basal coarse clastics, usually overlain by calcareous and dolomitic siltstones, limestone, and dolomite. In the Mount Painter region, the lower Adelaidean rocks have been metamorphosed to lower amphibolite facies, and the carbonate sediments are now calcsilicate rocks. The Wooltana Metabasalt in the northeastern part of the Adelaide Geosyncline is the most

* Present address: Geological Survey of Papua New Guinea, P.O. Box 778, Port Moresby, Papua New Guinea

Willouran Basic Province

extensively outcropping metabasic unit. It consists of variably metamorphosed metabasaltic lavas with minor dolerite-gabbro intrusions, and minor interbedded sediments (Hilyard 1986). The extensive sheetlike form of the lavas, general lack of pillows or pyroclastic rocks, and occurrence of

interbedded fluvial sandstones, indicate that the lavas were erupted subaerially by quiescent effusion of low viscosity magma. Geochemistry of Willouran metabasalts is described by Crawford & Hilyard (this volume).

Fig. 1 Location map of the Willouran Basic Province in the Adelaide Geosyncline and Stuart Shelf, South Australia, showing distribution of the Wooltana Metabasalt, Beda Volcanics, Gairdner Dyke Swarm and other Willouran basic units including the Cadlareena Volcanics (Ca), Noranda Volcanics (No), Willangee Basalt (Wi), and Boucaut Volcanics (Bo). Also shown is location of Shell Reedy Lagoon drillhole SRL1, and metabasic xenoclast- bearing intrusive breccias.

MOUNT PAINTER REGION

WILLOURAN RANGES

PEAKE AND DENISON RANGES

BARRIER RANGES

OLARY REGION

STUART SHELF

WORUMBA ANTICLINE

MID-NORTH REGION

U> o\

I 1

a C <O ol

§ I

£ i Fig. 2 Correlation of Willouran sequences from the Adelaide geosyncline and Stuart Shelf. Adapted from Forbes et al (1981) with additional data from Mason et al (1978), and Preiss (1983a, 1985).

Willouran Basic Province

The purpose of this paper is to compare the stratigraphy of the Wooltana Metabasalt with other Willouran basic rocks, and to assess their mode of eruption and significance in the early tectonic development of the Adelaide Geosyncline.

COMPARISON OF THE WOOLTANA METABASALT WITH OTHER WILLOURAN METABASIC ROCKS Stratigraphy The distribution of Willouran metabasic rocks is shown in Fig. 1 and the stratigraphy of these sequences in Fig. 2. Most of the igneous units occur at the top of the Arkaroola Subgroup of the Callanna Group. Where the stratigraphy has not been largely dismembered, and field relationships are well understood, the metabasalts lie at a similar stratigraphic horizon - above a sequence of basal Adelaidean coarse clastics and carbonate or metacarbonate sediments, commonly showing evidence of evaporites. They are unconformably overlain by Torrensian or Sturtian sediments except where the Curdimurka Subgroup is present and the original nature of the contacts is poorly understood. Table 1 compares the physical features of these units with the Wooltana Metabasalt. Most of these units consist of metabasalt, though the Gairdner Dyke Swarm is doleritic, and some more silicic rocks are present in places, though usually associated with basic rocks. These metabasalts are generally very similar to the Wooltana Metabasalt, described in detail by Hilyard (1986). It consists of a pile of superimposed, laterally extensive, sheetlike lava flows with amygdaloidal flow tops; tuffaceous rocks are absent. The lavas consist of albite-chlorite-actinolite-magnetite rocks, with evidence of potassic alteration. Interbedded sediments are minor, consisting mainly of thin lenses of sandstone, though locally more abundant metapelite and calcsilicate rocks are present. The sediments are compositionally and texturally quite mature; they are derived from Early to Middle Proterozoic

crystalline basement, with virtually no contribution of pyroclastic material from the volcanism. From an analysis of thickness and facies changes in the Wooltana Metabasalt, and the unconformably overlying Burra Group clastics, Hilyard (1986) concluded that graben subsidence was active during and after eruption of the Wooltana Metabasalt. However, thick rift valley facies sediments, such as alluvial fan-playa lake sequences are absent at the level of the basalts throughout the geosyncline, though they appear for the first time above the basalts. This is taken to indicate that rift valleys, as morphological features, were absent during eruption of the basalts. Volcanism kept pace with subsidence, forming a constructional geomorphology of lava plateaux. Eruption was followed by major graben subsidence, producing rift valleys in which the Curdimurka Subgroup and Burra Group were deposited. Following Swanson et al (1975), subsidence was probably due to evacuation of voluminous magma chambers, isostatic adjustment due to the thick lava pile, and regional extension. The intrusive breccias ('diapirs') of the Flinders Ranges (Table 1, Fig. 1) are megabreccias with a carbonate matrix, containing abundant xenoclasts of sedimentary rocks, metadolerite, less abundant metabasalt, and rare crystalline basement (Dalgarno & Johnson 1968). A variety of origins has been proposed for these bodies. The consensus opinion is that they are remobilized Callanna Group sediments that intruded higher in the sequence as passive, rheid-like viscous flows, by virtue of their high ductility, imparted partly by their evaporite content (Mount 1975, 1980; Preiss 1985; Lemon 1985). They carry a distinctive xenoclast suite of sandstones with halite casts, metabasics, and stromatolitic and evaporitic carbonates, that closely resembles Callanna Group rock types in normal stratigraphic context or in partly dismembered sequences. Most breccia bodies carry some metabasic xenoclasts; during the course of this project some of these were sampled. Metadolerites are more abundant than metabasalts. All the rocks seen are

David Hilyard

Wooltana Metabasalt

Beda Volcanics

Cadlareena Volcanics

Noranda Volcanics

Stratigraphic relationships

Conformably (gradationally) overlies calc-silicate metasediments. Disconformably overlain by basal Burra Group Sandstone

Conformably overlies and interfingers with arkosic sandstone and polymict conglom. Unconformably overlain by Sturtian Tapley Hill Formation. At Depot Creek, flows conformably overlie dolomitic sediments; disconformably overlain by basal Burra Gp sandstone

Conformably overlies dolomite and sandstone; locally disconformably on basal Adelaidean polymict conglom., or preAdelaidean metamorphics. Top everywhere is intrusive breccia contact. Minor blocks, out of strat. context, in intrusive breccia

Sequence thoroughly disrupted by intrusive breccia; strat. relationships inferred. Probably overlie black carbonaceous calcitic marble, which is intruded by dolerite. Occurs within intrusive breccia, and has faulted and sedimentary contacts with basal set of Curdimurka Subgp (i.e.upper part of Callanna Gp)

Thickness

c. 220-2000 m; highly variable

Less than 200 m

730 m maximum preserved

80 m (?) preserved

Known areal extent

230 sq. km

9200 sq. km

c. 20 sq. km

c. 1 sq. km

Rock types

Amygdaloidal metabasalt, lesser dolerite-gabbro, chlorite-albite rock; LGMA, KA; lower amphibolite facies metamorphism in west. Continental tholeiite geochemistry

Amygdaloidal metabasalt; LGMA, KA; geochemically similar to Wooltana Metabasalt

Amygdaloidal metabasalt, dolerite, minor andesite, rare thin "rhyolite" and dacite: LGMA, KA; geochemically similar to Wooltana Metabasalt

Amygdaloidal metabasalt, LGMA, KA

1-55 m

LGMA - low-grade metabasic assemblage (i.e.albite, chlorite, epidote, carbonate, actinolite) KA potassic alteration Flow thickness

10 m average

20-30 m

Flow structures

Sheet-like, unpillowed Amygdaloidal tops and flows; amygdaloidal bases; brecciated flow tops; peperite at tops contacts with sediments

Massive and amygdaloidal flows

Pillow basalt reported at one locality

Associated sediments

Thin quartz sandstone lenses; more abundant calc-silicate and pelitic metasediments in west

Thin sandy lenses between many flows

Interbedded minor mudstone, lapilli tuff and tuffaceous sediments, quartzite. Basaltic clasts in mudstones

"Tuffs with intercalated sediments" reported

Comments

Most extensively outcropping Willouran igneous unit; base not exposed in low-grade areas

Correlation between Stuart Shelf drillhole sections and Depot Creek outcrop is uncertain

Note presence of more silicic rocks, not developed in Wooltana Metabasalt. Possible equivalents intersected in Boorthana No.l, 30 km to SW of Peake and Denison Ranges

Unit is poorly known and restricted to a few outcrops, disrupted by intrusive breccia

References

Hilyard (1986), Crawford (1963), Coats (1971), Crawford & Hilyard (this volume)

Mason et al (1978), Webb & Horr (1978), Preiss & Faulkner (1984), Horr (1977)

Ambrose et al (1981)

Forbes etal (1981)

Table 1: Summary of Willouran Igneous Rock Units

Willouran Basic Province

Willangee Basalt

Xenoclasts in Breccia Intrusions ("Diapirs")

Gairdner Dyke Swarm

Rock Tuff

Boucaut Volcanics

Conformable within lower part of Curdimurka Subgp. U-Pb zircon age of 802 ± 10 Ma

Underlies and faulted Disconformably(?) against basal Burra Group overlies stromatolitic limestone and dolomite, sandstone. Other unconformably relationships obscured by Quaternary overlies crystalline basement in places. cover Unconformably overlain by Umberatana Group equivalent

15-60 m

Indeterminate; total outcrop width at type section 762m, but structure uncertain

160-200 m

Limited

Few sq. km

c. 5 sq. km

26 650 sq. km

Anomaly swarm covers 44 000 sq. km

Flagstones, tuffaceous greyshale, minor porphyritic tuff (ignimbrite?)

"Rhyolite" dominant, lesser amygdaloidal metabasalt-andesite. Multiply deformed; biotite grade metamorphism

Fine amygdaloidal metabasalt; LGMA, probable KA

Metadolerite dominant; metabasalt recorded in places (e.g.Worumba Anticline); LGMA, KA; petrographically similar to Wooltana Metabasalt

Ophitic dolerite in Reedy Lagoon 1 is much less altered than usual Willouran metabasics; trace and REE geochemistry of one sample analyzed is indistinguishable from Wooltana Metabasics

Association of sediments within breccias indicates derivation from Callanna Gp - mostly Curdimurka Subgp. Some of the metabasics, especially the dolerites, may be younger than the Wooltana Metabasalt (Preiss 1983a). Breccias show low-grade hydrothermal alteration cf.Wooltana Metabasalt (Mount 1980)

Stratigraphic relationships indicate early Adelaidean age. Presumably feeders for Beda Volcanics

Dalgarno & Johnson (1968), Preiss (1974, 1983a), Mount (1980), Coats (1964), S.A. Geol. Survey 1:63360 sheets of Flinders Ranges

Preiss (1983b), Brash <et al (1983)

Range from basalts within NW trending dolerite dykes, outlined by linear partly dismembered magnetic anomalies stratigraphic sequences on Stuart Shelf; dolerites (Spalding inlier) to intersected in drillholes xenoclasts in intrusive (e.g.Shell Reedy carbonate breccias Lagoon DDH1). Dykes (e.g.Blinman, Arkaba, and Burr intrusion, intrude pre-Adelaidean units; unconformably Worumba Anticline) overlain by Late Adelaidean sediments

15-30 m Flow structure (?) in qtz- Amygdaloidal musc. rock metavolcanic (?); amygdaloidal and layered rhyolite See above

"Phyllitic rocks"

Within Curdimurka Subgroup, so younger than other volcanic units of Callanna Gp. Correlatives outside Willouran Ranges uncertain

Poorly known unit; considerably more silicic than usual Willouran volcanics. May be early Adelaidean or preAdelaidean

Forbes et al (1981), Fanning et al (1986)

Forbes(1978)

Thin, crossbedded, ripplemarked quartzite interbeds; minor limestone at base

Cooper et al (1978)

David Hilyard

Flow thickness (m)

Plateau flood basalt complexes

Basaltic lava plains

Shield volcanoes

5-10

less than 10

less than 4

Willouran basic province 10 m average

Volume x 106 km3

0.2-1.0

0.1-0.4 estimated

Area covered x 106 km 2

0.5-1.5

0.2

Flow morphology

Sheet-like; colonnadeentablature jointing (lava pools)

Lava tube flows, pit craters

Pahoehoe near vent; aa downslope; hummocky tube-fed pahoehoe

Vent morphology

Fissures (often buried by their own eruptions)

Small shields, fissures

Initially fissure eruptions; subsequent restriction to point source builds cone. Calderas common

Surface morphology

Mainly featureless plateaus

Coalescing shields with intervening hummocky flows (collapsed lava tubes) & flat fissure flows

Broad gently sloping cones, cinder cones, collapse pits, rifts and fissures

Transitional between plateau flood basalt complexes and shield volcanoes

Recognized largely by external form, so may be hard to recognize when eroded or buried

Snake River Plain, Western District of Victoria

Mauna Loa

Comments

Examples

Columbia Plateau, Ethiopia, Keeweenawan

Compiled from Basaltic Volcanism Study Project (1981)

Table 2: Comparison of Basaltic Lava Volcanoes

Sheet-like amygdaloidal flows

Subdued topography

Willouran Basic Province

petrographically very similar to the Wooltana Metabasalt, and show typical alteration assemblages of albite-chlorite-carbonate-epidotehaematite actinolite. This petrographic similarity, and their association with typical Callanna Group sediments, suggests that they are probably correlatives of the metabasic units in normal stratigraphic sequence. The Gairdner Dyke Swarm (Table 1) occurs on the Stuart Shelf, west of the Adelaide Geosyncline (Preiss 1983b). It is represented by a northwest-trending swarm of linear magnetic anomalies; mineral exploration drilling of some of these magnetic anomalies has revealed that their source is dolerite dykes (Preiss 1983b; Brash et al 1983). The dykes cut basement (including Pandurra Formation) to the Adelaidean sequence, but nowhere are they known to intrude Burra Group or younger sediments. Thus they appear to be Willouran in age or older. Geochemistry of the dyke intersected in Shell Reedy Lagoon DDH1 is discussed by Crawford & Hilyard (this volume);

_____—-——

' - ^ m -+-

— +

— -+-

the trace and REE geochemistry of the dyke is indistinguishable from that of the Wooltana Metabasalt. Their stratigraphic distribution and geochemical similarity indicate that the Reedy Lagoon dolerite, and probably the remainder of the Gairdner Dyke Swarm, are correlatives of the Wooltana Metabasalt, and the Beda Volcanics of the Stuart Shelf. They probably represent feeders to the Beda Volcanics, which presumably were more widespread prior to erosion. Geochemistry Crawford & Hilyard (this volume) studied the geochemistry of Willouran basic rocks, and concluded that the Wooltana Metabasalt has strong compositional similarities with the Beda and Cadlareena Volcanics, and one sample of the Gairdner Dyke Swarm. The suite consists of continental tholeiites, and is very similar in composition to basalts erupted immediately prior to, and during, initial opening of the South Atlantic Ocean. Alteration of the Willouran basic volcanics was caused by saline fluids enriched in K, Na, Rb, Ba, halides, and boron, that were derived from evaporite-bearing sediments of the Callanna and Burra Groups. Within the broad geochemical unity of the Willouran basalts, Crawford & Hilyard (this volume) point out subtle geochemical variations between the Wooltana and Beda lavas, that reflect variations seen in the South American suites. These variations suggest that the Beda Volcanics are closer to true continental flood basalts; the Wooltana Metabasalt is transitional to N-MORB. By analogy with the South Atlantic lavas, these differences imply that the Wooltana Metabasalt was erupted in rift basins 'oceanward' of the Beda Volcanics, and are possibly slightly younger. Isotopic dating

Fig. 3 Schematic tectonic evolution of the Adelaide Geosyncline during the Willouran and Torrensian.

Numerous attempts have been made to isotopically date components of the Willouran basic suite to provide some control on the age of formation of the Adelaide Geosyncline (Compston et al 1966; Thomson 1966; Mason et al

David Hilyard

1978; Webb & Horr 1978; Webb et al 1983). Results were inconclusive due to alteration effects or resetting during the Delamerian orogeny. More recently Fanning et al (1986) conducted a U-Pb zircon study on the Rook Tuff, a partly volcanogenic silty unit within the Curdimurka Subgroup in the Willouran Ranges. The Rook Tuff conformably overlies the Dome Sandstone, which in turn overlies the Noranda Volcanics (Table 1, Fig .2), possibly conformably. A U-Pb age of 802±10Ma for dacite from the Rook Tuff (Fanning et al 1986) is an absolute minimum age for the Noranda Volcanics, and thus possibly for the remainder of the early Willouran basic lavas. How much older the basalts are is unclear at present. The 802 Ma U/Pb age casts some doubt on the Wooltana-Beda correlation, if the 1076 Ma age of the Beda Volcanics is accepted. Crawford & Hilyard (this volume) show close geochemical similarity between the units, and on this basic argue for correlation. The Rook Tuff may lend support to the c. 830 Ma minimum age for the Wooltana Metabasalt determined by Compston et al (1966), but stratigraphic relationships between the Rock Tuff and underlying Noranda Volcanics remain ambiguous. The questions of Wooltana Metabasalt age, and its correlation with the Beda Volcanics, remain unresolved at present.

Discussion The overall similarity of stratigraphic position, rock types, alteration, and geochemistry, indicates that most of the units in Table 1 can be correlated. The obvious exception is the Rook Tuff, which lies at a higher level in the Callanna Group. With this exception, the remainder represent a Willouran basic province in the Adelaide Geosyncline and Stuart Shelf. The original area of the province was c. 2.1xl0 5 sq. km. This was estimated by unfolding the structures depicted on 1:250 000 scale geological maps within the currently known limits of Willouran basalt distribution, without any allowance for flattening or cleavage- related strain. If the lavas were continuous across the province with an average thickness of 2000 m, the volume would be 4.2x105 cu. km; for an average thickness of 500 m, the volume is c. lxlO 5 cu. km. Obviously these estimates are very poorly controlled, given the degree of segmentation and disruption present in the system; however they do serve as 'order-ofmagnitude' indications of the volume of basaltic lava in the province. The Wooltana Metabasalt and the Reedy Lagoon dolerite dyke lie some 300 km apart across the Adelaide Geosyncline (Fig. 1). The extraordinary similarity between the compositions of these rocks suggests a very widespread igneous event involving rapid ascent

Province

Location

Age (106 km 2 )

Area (105 km 3 )

Volume

Columbia River

USA

Miocene

0.2

2.0

1.4

4.2

Karoo

South Africa

Jurassic

Trap Series

Ethiopia

Early Tertiary

Keweenawan

USA

Late Proterozoic

0.2

1.0

Deccan

India Palaeocene

Late Cretaceous-

0.52

7.0

Antrim Plateau

Australia

Cambrian

0.15

From: Basaltic Volcanism Study Group (1981)

Table 3: Dimensions of some Rood Basalt Provinces

1.5

Willouran Basic Province

and eruption of magma, with little differentiation or crustal contamination. Snavely et al (1973) described lavas from the Columbia River flood basalt province, erupted 500 km apart, that also displayed very similar geochemistry. The Columbia River flood volcanism involved widespread, rapid eruption of lavas on a large scale and is discussed further below.

ERUPTION STYLE AND TECTONIC SETTING Nature of the Willouran volcanism The Wooltana Metabasalt is a subaerial basalt suite (Hilyard 1986). The other members of the Willouran basic province have similar internal stratigraphy and rock types, and thus are also likely to be subaerial lavas. Four styles of subaerial basaltic volcanism have been recognized (Basaltic Volcanism Study Project 1981): (i)

Plateau flood basalt complexes;

and thus may be difficult to find. Eruptive centres have not been recognized in the Wooltana Metabasalt. The relatively primitive composition (Crawford & Hilyard, this volume) suggests rapid ascent and eruption of the lava, which is typical of plateau flood basalts (Swanson et al 1975). Hilyard (1986) presented stratigraphic evidence to suggest that the Paralana Fault (Fig. 1) lies along a major crustal structure that may have localized ascent and eruption of the basaltic magmas in that part of the geosyncline. Dickinson & Sprigg (1953) suggested that the Paralana Lineament, an extensive structural feature that includes the Paralana Fault, was a long-lived major crustal feature that controlled Adelaidean deposition and subsequent deformation. Thus the indications are that the Willouran basic province erupted as a flood basalt. To critically assess this, the following sections will describe the characteristics of flood basalt sequences and compare them with the Wooltana Metabasalt.

(ii) Basaltic lava plains;

Flood basalts

(iii) Shield volcanoes, and

Rood basalts or plateau basalts are the most voluminous subaerial basalts on earth; they range in age from Proterozoic to Cainozoic, though most are less than 200 Ma old. Characteristic dimensions of some flood basalt provinces are listed in Table 3. These sequences are marked by piles of thick, laterally extensive flows, with subdued topography. Fluid, rapidly erupted lavas spread over great distances, burying pre-basalt topography. Individual flows in the Deccan province can be traced for 160 km (Choubey 1973); in the Columbia River Basalt Group flows 550 km long are known (Hooper 1982).

(iv) Basaltic cones These classes are based on morphology, and are gradational with one another. They have been defined mainly from Cainozoic examples, and it may be difficult to apply them strictly to old, partly exposed sequences. The basaltic cones are dominantly built of scoria and ash, and are thus not relevant to the pyroclastic-free Wooltana Metabasalt. Some features of the other three styles are summarized in Table 2. The Wooltana Metabasalt most resembles plateau flood basalt complexes, with its uniform, featureless sheet-like lavas, and lack of associated features such as scoria cones and lava tubes. Vents in flood basalt fields are narrow elongate fissure systems that are commonly buried by the erupted lava (Swanson et al 1975)

Most descriptions of flood basalt provinces concentrate on the geochemistry of the rocks, with little description of the internal stratigraphy and facies variations of the basaltic pile. Cas & Wright (1982) presented a general facies model for continental basaltic sequences. The main elements of their model are:

(i)

David Hilyard

Cinder cones, maars, tuff rings, shield volcanoes;

(ii) Flood and valley-fill lavas; (iii) Alluvial and lacustrine sediments, usually of limited lateral extent; (iv) Palaeosol horizons, and (v) The volcanic sequence may commonly be underlain and overlain by alluvial sequences. Flood basalt provinces should be dominated by lava flows; the features in point 1 above are more typical of basaltic lava plains (Table 2; Greeley 1982; Basaltic Volcanism Study Group 1981). Lavas are fed by fissure systems, that are commonly buried by subsequent flows; consequently they are usually difficult to locate. Flood basalts are typically continental tholeiites, though more alkaline basalts may also occur. Most Phanerozoic flood basalts occur near continental margins and are associated with the separation of continental plates. The Columbia River Basalt Group of the Columbia Plateau has been intensively studied, and its internal stratigraphy is now well known (Waters 1961; Swanson 1967; Schminke 1967; Swanson et al 1975, 1979; Hooper 1982). The tectonic setting of the Columbia Plateau is more complex than the setting of other flood basalt provinces (Hooper 1982; Goles 1986). However, in terms of physical processes operating during eruption, it is a typical flood basalt province; due to the abundance of information available, it may be regarded as the "type" flood basalt sequence for developing a volcanic facies model. The Keweenawan lavas are an extensive suite of Late Proterozoic lavas which crop out around Lake Superior in Canada and north-central U.S.A. Data on the suite are summarized by Green (1977) and Basaltic Volcanicm Study Group (1981). They lie along part of the midcontinent gravity high of the central U.S.A. which Dickas (1986), interpreted as an aborted continental rift. Both of these se-

quences are typical flood basalt provinces. The physical features of the Columbia River Basalt Group and the Keweenawan lavas (Table 4) show many similarities to the Willouran basic province. The Willouran province covers c. 211 000 sq. km, which is similar to the Columbia River Basalt Group, and considerably larger than the exposed portion of the Keweenawan lavas, though substantially more of these probably exist in the buried part of the midcontinent gravity high. The volume of the Willouran basic province is uncertain, due to incomplete preservation and exposure, but a range of c. 14xl0 5 cu.km may be estimated, depending on the average thickness of basalt through the geosyncline. This range is the same order of magnitude as the volumes of the Columbia River Basalt Group and the Keweenawan lavas. Individual flows in the Wooltana Metabasalt have been traced along strike for c. 1 km, whereas those in the other provinces can be traced for up to several hundred kilometres. This may reflect real differences in flow extent and eruption rate. However, the shortness of Wooltana flows may only be apparent - the sequence is dismembered by faulting, and distinctive, traceable flows are absent. All three provinces are dominated by subaerial basaltic lava flows. Vertical profiles through all three sequences show lava flows stacked one above the other, with thin sedimentary lenses interbedded in places. All sequences contain discontinuous interbeds of fluvial and lacustrine sediments. In summary, all three sequences share many features of their gross stratigraphy and rock types; these similarities indicate that the Willouran basic igneous province is a flood basalt sequence erupted during early development of the Adelaide Geosyncline.

WILLOURAN-TORRENSIAN TECTONIC DEVELOPMENT OF THE ADELAIDE GEOSYNCLINE Von der Borch (1980) and Rutland et al (1982) described the Adelaide Geosyncline in terms of rift-type environments. The continental

Willouran Basic Province

Columbia River Basalt Group

Keweenawan Lavas

Age

Miocene

Late Proterozoic c. 1100-1120 Ma

Late Proterozoic c. 1076 Ma

Area

220 000 sq. km

125 000 sq. km originally in Lake Superior region, continues along MCGH

c. 211 000 sq. km

Estimated volume

200 000 cu. km

Over 300 000 cu. km

Estimated range 100 000-400 000 cu. km

Thickness

1500 m average; over 3000 m maximum

Average 5000 m; maximum 800012 0 0 0 m

Less than 100-2000 m preserved

Basement

Mesozoic-Cenozoic orogen

Archaean-Lower Proterozoic craton

Lower-Middle Proterozoic crystalline rocks

Rock types

Olivine-quartz tholeiite

Olivine-quartz tholeiite, alkaline basalt, andesite, minor rhyolite

Tholeiitic metabasalt, metadolerite. Extensive alteration

Associated intrusives

Abundant dyke swarms

Dyke swarms and mafic plutons

Dyke swarms

Stratigraphic sequence

Superimposed lava flows with minor interbedded sediments; some flows fill valleys cut in underlying units

Superimposed lava flows with minor interbedded sediments

Superimposed lava flows with generally minor interbedded sediments

Flow thicknesses

25 m average; 15-35 m typical

3-20 m

10 m average; range 1-20 m

Willouran Basin Province

Flow extent

5000-20 000 sq. km typical; Roza Member 40 000 sq. km

Up to 90 km

c. 1 km mapped

Flow volume

10-20 cu. km typical; Roza Member 1500 cu. km (usually 2 flows)

Up to 400 cu. km +

Vents

Linear vent systems less than 15 km wide, 100 km long; individual vents marked by minor spatter, pumice

None described

None recognized

Flow structures

Columnar jointing widespread, vesicular tops; pillow-palagonite complexes at base; palaeosoils

Poorly developed columnar jointing, amygdaloidal tops. Minor pillows

Sheet-like, unpillowed flows: amygdaloidal tops; peperite at contacts with sediments

Interbedded sediments

20-60m thick lenses interbedded with lavas. Basalt pyroclastics; epiclastics derived largely from metamorphic-plutonic terrain. Basaltic detritus also common. Dominantly fluvial-lacustrine, minor shallow marine

Thin fluvial sandstones

Thin fluvial sandstones; some lacustrine calc-silicate. Epiclastic rocks derived from crystalline basement. Basaltic detritus very rare

Table 4:Comparison of the Columbia River and Keweenawan Flood Basalt Provinces with the Willouran Basic Province

David Hilyard

tholeiitic composition of the Wooltana Metabasalt and other units in the Willouran basic province adds further support to a rift hypothesis for the Adelaide Geosyncline. The Wooltana Metabasalt formed during early extension in a continental setting, erupting as plateau flood basalts. After volcanism ceased, large-scale downfaulting occurred, perhaps as a response to evacuation of voluminous magma chambers, isostatic adjustment due to the thick lava pile, and regional extension. Deposition of fluvial and lacustrine sediments of the Curdimurka Subgroup and Burra Group continued in the actively forming rift basins. The East African Rift region may be a Cainozoic analogue for the Adelaide Geosyncline in the early Adelaidean. The Willouran and Torrensian history of the

Adelaide Geosyncline can be described as fol-

lows (Fig. 3) :

1. Regional doming in the early Willouran, possibly as a response to rise of upper mantle diapirs. Deposition of sheet sands (Paralana Quartzite and correlatives) and carbonates (Wywyana Formation, etc.) occurred in broad "sag" basins along the crest of the dome, with minor half-graben development in places. 2. Eruption of tholeiitic plateau basalt lavas occurred along deep-seated faults (Wooltana Metabasalt and equivalents) together with intrusion of dolerite dykes (Gairdner Dyke Swarm). Formation of constructional volcanic topography kept pace with graben subsidence, and the geomorphology consisted of broad volcanic plateaux, without significant rift valleys. Hypersaline brines derived by dissolution of evaporites caused hydrothermal alteration of the basic rocks.

The Callanna and Burra Groups document the early rift phase prior to establishment of continental margins, and regional subsidence. Within this early phase, the Willouran basic province records the volcanism associated with the initiation of rifting. ACKNOWLEDGEMENTS I am grateful to Dr J.B. Jago for supervision of this study, and to Prof D.H. Stapledon for generous financial assistance. The Royal Society of South Australia also provided a research grant toward fieldwork expenses in 1982. Wolfgang Preiss and Jim Jago critically reviewed an early draft of the manuscript. Miriam Lakou and Ancilla Vali typed the maniscript, Boida Omoru drafted the figures.

REFERENCES AMBROSE G.J., FLINT R.B. & WEBB A.W. 1981. Precambrian and Palaeozoic Geology of the Peake and Denison Ranges. Geological Survey of South Australia, Bulletin 50. BASALTIC VOLCANISM STUDY PROJECT 1981. Basaltic Volcanism on the Terrestrial Planets. Pergamon Press, New York. BRASH A.H., BAILEY D.P. & HUNGERFORD H.N. 1983. Progress and final reports from 12-4-1981 to 12-1-1983, on EL774, Reedy Lagoon. Shell Company of Australia, Geological Survey of South Australia. Open File Envelope 4113 (unpubl.). CAS R.A.F. & WRIGHT J.V. 1982. Modern and ancient volcanic successions. Department of Earth Sciences, Monash University, Clayton, Victoria.

3. Cessation of volcanism was succeeded by foundering on a massive scale. The grabens formed were filled by alluvial fan-playa sequences (Curdimurka Subgroup and Burra Group).

CHOUBEY V.D. 1973. Long-distance correlation of Deccan flow central India. Geological Society of America, Bulletin 84, 2785-2790.

Subsequent development of the geosyncline is described by Preiss (1983c) in terms of intracratonic and ?Atlantic-style margin settings.

COATS R.P. 1964. The geology and mineralization of the Blinman Dome diapir. Geological Survey of South Australia, Report of Investigations 26.

Willouran Basic Province

COATS R.P. 1971. Regional geology of the Mount Painter Province. In Coats R.P. & Blisset A.H. Regional and Economic Geology of the Mount Painter Province. Geological Survey of South Australia, Bulletin 43, 15-40. COOPER P.F., TUCKWELL K.D., GILLIGAN L.B. & MEARES R.M.D. 1978. Geology of the Torrowangee and Fowlers Gap 1:100000 sheets 7135, 7235. N.S.W. Geological Survey, Sydney.

FORBES B.G., MURRELL B. & PREISS W.V. 1981. Subdivision of lower Adelaidean, Willouran Ranges. Quarterly Geological Notes, Geological Survey of South Australia 79, 7-16. GOLES G.G. 1986. Miocene basalts of the Blue Mountains Province in Oregon: Compositional types and their geological settings. Journal of Petrology 27, 495-520.

COMPSTON W., CRAWFORD A.R. & BOFINGER V.M. 1966. A radiometric estimate of the duration of sedimentation in the Adelaide Geosyncline, South Australia. Journal of the Geological Society of Australia 13, 229-276.

GREEN J.C. 1977. Keweenawan plateau volcanism in the Lake Superior Region. In Barrager W.R.A., Coleman L.C. & Hall J.M. Volcanic Regimes in Canada. Geological Association of Canada, Special Paper 16, 407-422.

CRAWFORD A.J. & HILYARD D. (this volume). Geochemistry of Late Proterozoic tholeiitic flood basalts, Adelaide Geosyncline, South Australia.

GREELEY R. 1982. The Snake River Plain, Idaho: Representative of a new category of volcanism. Journal of Geophysical Research 87(B4), 2705-2712.

CRAWFORD R. 1963. The Wooltana volcanic belt, South Australia. Transactions of the Royal Society of South Australia 87, 123-154.

HILYARD D.B. 1986. The stratigraphy and tectonic setting of the Wooltana Metabasalt, South Australia. M.App.Sci. thesis, South Australian Institute of Technology, Adelaide (unpubl.).

DALGARNO C.R. & JOHNSON J.E. 1968. Diapiric structures and Late Precambrian-Early Cambrian Sedimentation in Flinders Ranges, South Australia. American Association of Petroleum Geologists, Memoir 8,301-314. DICKAS A.B. 1986. Comparative Precambrian stratigraphy and structure along the mid-Continent rift. American Association of Petroleum Geologists, Bulletin 16,225-238. DICKINSON S.B. & SPRIGG R.C. 1953. Geological structure of South Australia in relation to mineralisation. In Edwards A.B. ed. Geology of Australian Ore Deposits, Vol.1, pp.426-448. Australasian Institute of Mining and Metallurgy, Melbourne.

HOOPER P.R. 1982. The Columbia River Basalts. Science 219,1463-1468. HORR G.M. 1977. Precambrian spilites and the Pandurra Formation of the Stuart Shelf, South Australia. B.Sc.(Hons) thesis, University of Adelaide (unpubl.). LEMON N.M. 1985. Physical modelling of sedimentation adjacent to diapirs and comparison with Late Precambrian Oratunga Breccia Body in Central Flinders Ranges, South Australia. American Association of Petroleum Geologists Bulletin 69, 1327-1338.

FANNING C.M., LUDWIG K.R., FORBES B.G. & PREISS W.V. 1986. Single and multiple grain U-Pb zircon analyses for the early Adelaidean Rook Tuff, Willouran Ranges, South Australia. Geological Society of Australia, Abstracts 15, 71-72.

MASON M.G., THOMAS B.P. & TONKIN D.G. 1978. Regional stratigraphy of the Beda Volcanics, Backy Point Beds and Pandurra Formation on the Stuart Shelf, South Australia. Quarterly Geological Notes, Geological Survey of South Australia 66, 2-9.

FORBES B.G. 1978. The Boucaut Volcanics. Quarterly Geological Notes, Geological Survey of South Australia 65, 6-10.

MOUNT T.J. 1975. Diapirs and diapirism in the Adelaide 'Geosyncline', South Australia. Ph.D thesis, University of Adelaide (unpubl.).

David Hilyard

MOUNT T.J. 1980. The Arkaba breccia intrusion and the Arkaba Hill Beds, Flinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 74,4-11.

PREISS W.V. (Compiler) 1983a. Adelaide Geosyncline and Stuart Shelf 1:600 000 Geological Sheet. Geological Survey of South Australia, Adelaide.

SNAVELY P.D., MACLEOD N.S. & WAGNER H.C. 1973. Miocene tholeiitic basalts of coastal Oregon and Washington and their relations to coeval basalts of the Columbia Plateau. Geological Society of America, Bulletin 84, 387-424. SWANSON D.A. 1967. Yakima Basalt of the Tieton River area, south- central Washington. Geological Society of America, Bulletin 78,1077-1110.

PREISS W.V. 1983b. Depositional and tectonic contrasts between Burra Group and Umberatana Group sedimentation. Geological Society of Australia, Abstracts 10,13-16.

SWANSON D.A., HOOPER P.R. & BENTLEY R.D. 1979. Revisions in stratigraphic nomenclature of the Columbia River Basalt Group. United States Geological Survey, Bulletin 1457G.

PREISS W.V. 1985. Stratigraphy and tectonics of the Worumba Anticline and associated intrusive breccias. Geological Survey of South Australia Bulletin 52.

SWANSON D.A., WRIGHT T.L. & HELZ R.T. 1975. Linear vent systems and estimated rates of magma production and eruption for the Yakima Basalt on the Columbia Plateau. American Journal of Science 215, 875-905.

PREISS W.V. & FAULKNER P. 1984. Geology and stratigraphic drilling at Depot Creek, Southern Flinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 89,10-19.

RUTLAND R.W.R., PARKER A.J., PITT G.M., PREISS W.V. & MURRELL B. 1981. The Precambrian of South Australia, In Hunter D.R. ed. Precambrian of the Southern Hemisphere. Developments in Precambrian Geology, vol.2, pp.309-360. Elsevier, Amsterdam.

SCHMINKE H.-U. 1967. Stratigraphy and petrography of four upper Yakima Basalt flows in South-Central Washington. Geological Society of America, Bulletin 78,1385-1422.

THOMSON B.P. 1966. The lower boundary of the Adelaide System and older basement relationships in South Australia. Journal of the Geological Society of Australia 13, 203-228. VON DER BORCH C.C. 1980. Evolution of Late Proterozoic to Early Palaeozoic Adelaide Foldbelt, Australia: Comparisons with post- Permian rifts and passive margins. Tectonophysics 70,115-134. WATERS A.C. 1961. Stratigraphic and lithologic variations in the Columbia River Basalt. American Journal of Science 259, 583-611. WEBB A.W., COATS R.P., FANNING C.M. & FLINT R.B. 1983. Geochronological framework of the Adelaide Geosyncline. Geological Society of Australia, Abstracts 10, 7-9. WEBB A.W. & HORR G. 1978. The Rb-Sr age and petrology of a flow from the Beda Volcanics. Quarterly Geological Notes, Geological Survey of South Australia 66, 10-13.

Geochemistry of Late Proterozoic tholeiitic flood basalts, Adelaide Geosyncline, South Australia Anthony J. Crawford1 and David Hilyard2* 1

Geology Department, University of Tasmania, P.O. Box 252C, Hobart, Tas 7001, Australia Department of Applied Geology, South Australian Institute of Technology, The Levels, SA. 5095, Australia

The Willouran basic province is constituted by continental flood basalts with tholeiitic affinities, and covered an area similar to the Columbia River basalts, but only half the size of the Parana or Deccan flood basalt provinces. A geochemical study of basalts from the Wooltana Metabasalt shows they have strong compositional similarities with other Willouran basalts in the Adelaide Geosyncline, including the Beda and Cadlareena Volcanics, and the Gairdner Dyke Swarm. Immobile trace element ratios suggest that the Beda Volcanics are geochemically akin to true continental flood tholeiites, such as the Parana basalts of Brazil, whereas the Wooltana Metabasalts are compositionally closer to tholeiitic basalts erupted immediately prior to opening of the South Atlantic Ocean, and preserved in Brazilian continental margin and the High Atlas in Morocco. Alteration of the Willouran basic volcanics involved saline fluids enriched in K, Na, Rb, Ba, halides and boron, which produced widespread K-feldspar, and scapolite and tourmaline in higher grade assemblages in the Wooltana Metabasalt. It is thought that these fluids derived their unusual composition from either dewatering of, or reaction with, evaporites within the Callanna and Burra Groups of the Adelaide Geosyncline. An analyzed amphibolite dyke from just north of East Painter Gorge is considered to be representative of the swarm of similar dykes which intrude the Mount Painter Block. It is notably more LREE- enriched than any analyzed Willouran basic lavas or dykes, and has immobile element ratios very similar to the Roopena Volcanics, which Giles & Teale (1979) correlate with the pre-Adelaide Geosyncline Gawler Range Volcanics. However, the age and stratigraphic relationships of this dyke swarm remain unresolved.

Key words: Adelaide Geosyncline, flood basalts, geochemistry, Wooltana Metabasalt, alteration. INTRODUCTION

GEOLOGICAL SETTING

Late Proterozoic (Adelaidean) basic volcanics occur near the base of the Adelaide Geosyncline and constitute the most extensive suite of igneous rocks of this foldbelt. They were erupted as flood basalts immediately prior to, and during, the initial rifting stage of development of the geosyncline. In this paper, we document the major and trace element geochemistry of these lavas, with emphasis on the Wooltana Metabasalt, and compare these with other Late Proterozoic and younger continental flood basalt provinces.

Late Proterozoic-Early Palaeozoic sediments in South Australia were deposited in a major crustal downwarp, the Adelaide Geosyncline, between c. 1100 Ma and Middle Cambrian (Rutland et al 1981). Up to 20 km of clastic and carbonate sediments were deposited in this geosyncline, which has flanking "shelf' areas underlain at shallow levels by Early and Middle Proterozoic crystalline basement to the west (Stuart Shelf) across the Torrens Hinge Zone, and northeast (Cumamona Cratonic Nucleus). Relative to similar aged sequences in the geosyncline, sedimentary sequences on the flanking shelves

* Present address: Geological Survey of Papua New Guinea, P.O. Box 778, Port Moresby, Papua New Guinea

A. J. Crawford & D. Hilyard

Peake and Denison Block

Willouran sequences: in normal stratigraphic context in breccia intrusions Pre-Adelaidean basement

Basalt Delamerian fold trend

Borehole *

Wooltana Metabasalt

Shell Reedy Lagoon

V, DDH1 - SRL1 STUART

Mount Painter Block Paralana , Fault Curnamona Cratonic Nucleus

ADELAIDE GEOSYNCLINE

Beda A Volcanics

Broken Hill / ^ Block [ +

GAWLER

South Australia

ADELAIDE

Fig. 1 Location map of the Willouran Basic Province in the Adelaide Geosyncline and Stuart Shelf in South Australia, showing distribution of Wooltana Metabasalt, Beda Volcanics, Gairdner Dyke Swarm and other Willouran basic units including the Cadlareena Volcanics (Ca), Noranda Volcanics (No), Willangee Basalt (Wi), and Boucaut Volcanics (Bo). Also shown is location of Shell Reedy Lagoon drillhole SRL1, and metabasic xenoclast-bearing intrusive breccias.

Geochemistry of Adelaide Geosyncline Flood Basalts

are thinner and essentially flat-lying. Regional facies studies of, less complete, the AdelaideanCambrian sediments of the Adelaide Geosyncline have concluded that they are largely rift sequences deposited during the early stages of formation of an aborted Atlantic-style rifted margin (Von der Borch 1980; Rutland et al 1981; Preiss 1983a). The basal geosynclinal sequence, the Callanna Group, of Willouran age, includes a widespread suite of basic igneous rocks which extends over large areas of the Adelaide Geosyncline and Stuart Shelf (Fig.l; Preiss 1983b). Beside the focus of the present study, the Wooltana Metabasalt, these include the Gairdner Dyke Swarm and Beda Volcanics on the Stuart Shelf (Webb & Horr 1978), the Cadlareena Volcanics in the Peake and Denison Ranges (Ambrose et al 1981), the Noranda Volcanics in the Willouran Ranges (Forbes et al 1981), the Willangee Basalt near Broken Hill (Cooper et al 1978) and possibly the Boucaut Volcanics in western South Australia (Forbes 1978). In addition, metadolerite and metabasalt occur as xenoclasts in intrusive breccias derived from Callanna Group sediments throughout the northern and central parts of the Adelaide Geosyncline (Dalgarno & Johnson 1968; Preiss 1985). Where the Willouran rocks are preserved in normal stratigraphic sequence, the basalts are separated from crystalline basement by thin basal coarse clastics and conformably overlying stromatolitic carbonates or calc-silicate metasediments (Fig.2). The Willouran basic province covered at least 210 000 km 2 , only one third of the area covered by the Ethiopian flood basalt province and one fifth of the area of the Deccan and Parana flood basalt provinces (Mohr 1983), yet larger than the Tertiary flood basalt province of East Greenland associated with proto-Atlantic opening (Brooks et al 1976), and similar to the area covered by the Columbia River basalt province (Basaltic Volcanism Study Project, 1981). Due to the variable exposed thickness (<100 m to >2 000 m) of basalt in the Willouran basic province, estimates of the volume of

magma erupted during this phase of continental flood basalt magmatism are poorly constrained. Taking an average thickness estimate of 500 m, the volume of basalts erupted is c. 105 000 km3, only 30% of the volume of the North America Keeweenawan flood basalts of Late Proterozoic age, and 15% of the volume of the Parana flood basalt province in South American (Basaltic Volcanism Study Project, 1981).

m < O

< LU 9 < LU

D <

z< CO D o

I d

. • •• • •• •% •*•

•/. • % • sO • ••

EARLY-MIDDLE + + + + + + + + + + + + H PROTEROZOIC

Fluvial, lacustrine and marine clastics and carbonates Wooltana Mteabasalt and correlates Carbonates and calc-silicate metasediments Sandstone, conglomerate Metamorphic basement Fig. 2 Generalized stratigraphy of the Adelaide Geosyncline showing detail for the Early Adelaidean Willouran succession in the Wooltana area.

A. J. Crawford & D. Hilyard

THE WOOLTANA METAB ASALT: OCCURRENCE AND PETROGRAPHY The Wooltana Metabasalt in the NE part of the geosyncline (Fig.l) is the most extensively outcropping unit of the Willouran basin province, and consists of a pile of basaltic lava flows with minor interbedded fluvial and lacustrine sediments (Hilyard 1986). The Paralana Fault separates essentially unmetamorphosed rocks in the east from lower amphibolite facies rocks in the west (Coats & Blissett 1971). It is a Delamerian structure which lies along a zone of pronounced facies and thickness changes in Willouran-Torrensian rocks, and probably represents re-activation of an older structure which may have localized fissure eruptions. Basalts in the unmetamorphosed sequence show the effects of pervasive low-grade hydrothermal alteration, and are dominantly amygdaloidal flows. Basalts in the eastern low-grade area are generally albite-chlorite-actinolite-leucoxene rocks. While original calcic plagioclase is mainly pseudomorphed by albite, it is also commonly partly altered to epidote and carbonate. Sparse small olivine phenocrysts have been altered to chlorite, carbonate and Fe-oxides, while augite phenocrysts and groundmass plates are generally altered to actinolite. Amygdales are commonly rimmed by Fe-oxide dust and filled by quartz, K-feldspar, chlorite, epidote, stilbite and minor tourmaline. Most low-grade lavas have been extensively K-metasomatized, as shown by staining for K-feldspar. In higher grade sequences, metabasic rocks are medium-grained, subidioblastic- to xenoblastic-textured plagioclaseactinolite-scapolite-epidote-carbonate-opaque oxide rocks; associated pelitic rocks have cordierite-biotite-andalusite(fibrolite) assemblages, suggesting a lower amphibolite facies of metamorphism for these rocks. Basic dykes intrude crystalline basement of the geosyncline at many localities; near East Painter Gorge, a dyke 2 km long and 20 m thick is the only dyke known to intrude basal Adelaidean sediments underlying the Wooltana

Metabasalt (Coats & Blissett 1971). This dyke is amphibolite, composed of medium-grained hornblende and calcic plagioclase; relict subophitic textures are preserved in places. The petrographical similarity and proximity of this dyke to the Wooltana Metabasalt flows higher in the sequence led Coats & Blissett (1971) to suggest that it may be an intrusive feeder for the Wooltana basalts. Geochemical data reported herein suggest that this is unlikely to be true. A dyke intersected by the Shell Reedy Lagoon No.l drillhole on the Stuart Shelf (Fig. 1) is part of the Gairdner Dyke Swarm (Preiss 1983b). This is a NW-trending swarm of dykes, marked by linear magnetic anomalies, which cut the crystalline basement to the Adelaidean sequence but are nowhere known to intrude Adelaidean strata above the basal Callanna Group, suggesting a Willouran age for dyke intrusion. These dykes occur within and adjacent to the area of subcrop of the Beda Volcanics (Fig.l, the most extensive unit in the Willouran basic province. The Reedy Lagoon Drillhole dyke is remarkably fresh, and consists of subophitic augite and calcic plagioclase with interstitial Fe-Ti oxides and quartz-albite granophyric intergrowths. Plagioclase shows only minor cloudy alteration, and some clinopyroxene crystals have narrow actinolitic rims. Geochemical data (see later) indicate that this dyke, and by implication, the Gairdner Dyke Swarm, is geochemically closely similar to the Wooltana and Beda basalts.

GEOCHEMISTRY Material and techniques For the Wooltana Metabasalt, we have analyzed 18 samples, comprising 13 metabasalts from the low-grade sequence and 5 high-grade rocks, including one gabbro, two coarse ophitic lavas and two formerly glassy lavas (Table 1.). Also, two dykes have been analyzed, including the Reedy Lagoon dyke from the Gairdner Dyke Swarm, and the amphibolite dyke from north of East Painter Gorge. In selecting samples from

5479 W1

5337 W2

5481 W3

5478 W4

5477 W5

5314 W6

5468 W7

5470 W8

5472 W9

5480 W10

5257 Wll

5469 W12

5282 W13

5345 W14

5474 W15

5473 W16

5482 W17

5471 5483 5398 W18 RL Dyke EPG Dyke

P2O5

51.90 1.70 14.23 11.96 0.19 6.92 7.65 4.71 0.62 0.14

46.90 1.11 12.98 14.22 0.40 14.71 6.59 2.19 0.80 0.11

50.27 1.78 15.18 11.57 0.26 9.40 5.96 4.02 1.40 0.15

51.83 1.62 15.96 10.97 0.22 9.19 5.38 3.09 1.62 0.12

51.54 1.60 15.68 11.23 0.24 9.00 4.12 3.98 2.47 0.14

53.79 1.42 15.56 13.73 0.21 7.26 1.57 3.28 3.06 0.13

49.81 1.49 14.89 11.55 0.22 9.48 5.82 2.79 2.56 0.11

48.90 1.44 15.64 14.08 0.76 11.93 1.87 2.80 2.13 0.11

49.95 1.37 14.54 11.89 0.21 8.62 9.65 1.92 1.50 0.11

50.82 1.44 15.02 12.80 0.28 7.71 7.32 2.18 1.44 0.13

49.98 1.64 15.05 13.46 0.64 9.08 4.23 2.66 3.72 0.14

47.19 1.08 11.97 13.82 0.64 12.48 8.75 2.11 1.56 0.10

49.67 1.59 14.03 13.45 0.33 9.59 4.24 3.47 3.21 0.14

50.70 1.81 13.68 13.73 0.30 6.43 8.71 3.88 0.30 0.17

50.45 1.45 14.66 12.78 0.30 8.37 5.59 5.14 0.87 0.11

50.59 1.61 13.96 16.87 0.33 9.54 3.74 2.74 0.15 0.14

49.62 1.83 14.11 15.01 0.28 8.10 6.16 3.37 1.43 0.16

50.47 1.38 14.00 11.66 0.66 7.31 11.06 2.73 0.31 0.11

50.48 1.67 14.02 12.80 0.28 7.07 10.85 2.10 0.32 0.14

LOI

8.05

5.34

3.41

7.37

3.54

4.03

5.07

7.74

2.50

2.03

2.81

4.45

3.06

1.75

3.54

3.39

2.44

1.13

0.46

1.05

2.14

1.53

1.77

1.85

1.60

1.81

2.39

Si0 Ti0 AI O FeO* MnO MgO CaO Na 0 KO 2

51.22 2.32 14.41 13.29 0.19 5.56 7.21 2.42 1.53 0.41

FeO*/MgO

1.73

0.97

1.23

1.19

1.25

1.89

1.22

1.18

1.38

1.66

1.48

1.11

1.40

Cr V Sc Zr Nb Y Sr Rb Ba

234 314 45 83 5 21 134 24 94

285 205 29 56 3 17 69 36 100

278 305 46 92 5 28 151 45 144

265 310 45 83 4 23 112 50 191

270 362 47 85 4 26 73 70 338

260 335 41 76 4 18 97 103 815

233 307 39 75 4 25 125 87 221

231 331 34 71 3 16 99 41 448

255 325 38 75 5 22 115 58 112

205 340 43 79 5 23 195 86 322

235 355 46 88 5 25 94 122 1034

193 232 28 57 4 16 84 75 316

346 334 46 87 5 27 125 107 668

141 401 40 113 8 29 357 9 57

233 334 43 78 4 23 119 41 163

226 321 47 76 4 27 56 5 125

209 374 44 106 6 30 179 42 328

207 305 42 76 4 23 426 5 237

236 379 43 95 5 27 164 13 45

67 368 47 256 16 67 169 90 774

Ti/Zr Zr/Nb Zr/Y Zr/Sc Y/Nb

123 16.60 3.95 1.84 4.20

119 18.67 3.29 1.93 5.67

116 18.40 3.29 2.00 5.60

117 20.75 3.61 1.84 5.75

113 21.25 3.27 1.81 6.50

112 19.00 4.22 1.85 4.50

119 18.75 3.00 1.92 6.25

122 23.67 4.44 2.09 5.33

110 15.00 3.41 1.97 4.40

109 15.80 3.43 1.84 4.60

112 17.60 3.52 1.91 5.00

114 14.25 3.56 2.04 4.00

110 17.40 3.22 1.89 5.40

96 14.13 3.90 2.83 3.63

111 19.50 3.39 1.81 5.75

127 19.00 2.81 1.62 6.75

103 17.67 3.53 2.41 5.00

109 19.00 3.30 1.81 5.75

105 19.00 3.52 2.21 5.40

54 16.00 3.82 5.45 4.19

Table 1: Whole-rock analyses recalculated volatile-free of Wooltana Volcanics (Wl-18), a Gairdner Dyke dolerite (RL Dyke) and an amphibolite dyke from north of E Painter Gorge

CD o o cr <T>

>o pro, gf Cu n>

OCD o 05

5' CD

o Q-

A. J. Crawford & D. Hilyard

5 5 t %Si02

%CaO

10 8' u *•©

50-

<yo

- #a

642-

45-1

2.5i

oTi02

%Na20

~ bi

0.5-1 207 %AI203

%K2O

15"

10-

MgO%

10 MgO%

Cadlareena Volcanics

•

Reedy Lagoon dyke (Gairdner Dyke Swarm)

•

Wooltana Metabasalt (low-gr.)

•

Mount Painter Gorge dyke

Wooltana Metabasalt (high-gr.)

Limit of field for Continental Flood Basalts

Beda Volcanics

Fig. 3 Element versus MgO diagrams for Willouran basic rocks. Beda Volcanics data from Horr (1977); Cadlareena Volcanics data from Ambrose et <2/ (1981). Limits of compositional fields (marked by ticks along field limit line) for continental flood basalts (CFB) from literature in references.

Geochemistry of Adelaide Geosyncline Flood Basalts

17.00 • 16.00 •

15.00 •

FeO'

14.00 •

• •

"X "

13.00 • 12.00 -

11.00 +

10.00

0.50

1.00

1.50

-- * _ XX-

2.50

3.00

3.50

2.00

2.50

3.00

3.50

2.00

2.50

2.00

2.40 2.20 2.00

Ti02 %

1.80

1.60

1.40 +

x . 1.20 1.00

0.50

1.00

1.50

450 400 350

X •

V ppm 300 ••

••

250 200

150 0.50

1.00

1.50

FeOVMgO

3.00

3.50

Fig. 4 Plots of FeO*, Ti02 and V versus FeO*/MgO for Willouran basic rocks, showing strong enrichment of these elements with increasing differentiation, typical of tholeiitic suites. Symbols as forFig.3.

A. J. Crawford & D. Hilyard

amygdaloidal flows, interior portions of flows were generally sampled to avoid obvious alteration associated with vesicle filling, brecciated flowtops, and quartz/calcite veining. Sample localities are listed in Appendix 1.

(ii) That although some minor but significant compositional differences are present, the Wooltana and Beda Volcanics are almost certainly lateral correlates, as proposed by Giles & Teale (1979); and

In addition to our analyzed Wooltana basalts, we have also used 55 published analyses, including 6 Wooltana basalts (Coats & Blissett 1971), 10 Cadlareena Volcanics from the Peake-Denison Ranges (Ambrose et al 1981) and 39 analyses of Beda Volcanics (Horr 1977).

(iii) That low-grade alteration of the Wooltana Metabasalt was caused by reaction with fluids derived from evaporites within the same sequence, leading to anomalous enrichments in, notably, K, Ba and Rb for CFB.

Analyses were carried out at the Geology Department, University of Adelaide, using standard XRF techniques. All volatiles were measured as loss on ignition (LOI), and Na20 was determined by atomic absorption spectrometry. To minimize compositional variation attributable to varying extents of hydration, all analyses are quoted recalculated to 100% volatile-free. Rare earth element analyses were performed in the Geology Department, University of Tasmania, using a combined ion-extraction XRF technique (Robinson et al 1986), for which precision and accuracy are estimated to be better than 5%. Major and trace element geochemical features The geological setting and areal extent of the Wooltana Metabasalt and correlated Willouran basic rocks demand classification as a continental flood basalt (CFB) province. In the following discussion, we address three arguments concerning the geochemical features and affinities of the Wooltana Metabasalt and related metabasic rocks in the Adelaide Geosyncline. These are: (i)

That the primary compositional range of these Late Proterozoic lavas is very similar to the low-Ti suites in more recent CFB provinces, such as the Parana and SE Brazilian continental margin basalts which were erupted immediately preceding, and during, initial rifting associated with opening of the south Atlantic Ocean (Fodor & Vetter 1984);

Geochemical affinities of Willouran basic lavas and dykes: major elements Element-MgO plots for the Wooltana Metabasalt and other Willouran basic volcanics (Beda and Cadlareena Volcanics) are shown in Fig.3. The majority of samples contain 6-10% MgO, and within this range Si02 varies irregularly from 49-54%, probably reflecting in large part the low-grade alteration of these lava suites. Relative to other continental flood basalt suites, the Willouran basic volcanics are somewhat more MgO-rich overall. The Parana and Ferrar (Antarctica), CFB suites have average MgO contents from 4-6% (Bellieni et al 1984) and the Keeweenawan flood basalts of northern USA have an average of 5.8% MgO (Basaltic Volcanism Study Project 1981) ; only the Karoo basalts have a higher average MgO (8%) content than the Willouran basalts, probably reflecting a skew to higher values due to unrepresentative sampling of volumetrically limited picrites and limburgites in the Karoo Province (Basaltic Volcanism Study Project 1981). The abundance of relatively primitive basalts within the Wooltana Metabasalts may be due to their proximity to the Paralana Fault, a probable fissure eruption source zone for these basalts. This zone may have provided easy access to eruption for mantlederived primitive magmas, so that crustal pooling and extensive fractionation, and fractionation during surface flow, were relatively limited. Although mobility of Si02, CaO and alkalies, plus alteration of primary Fe 3+ /Fe 2+ ratios render norm determinations meaningless, the Willouran

Geochemistry of Adelaide Geosyncline Flood Basalts

Zr ppm

450

S r

P P m

400 • 350 • 300 • 250 • 200 '

•o •

150 •

100 50 4.00

4.00

6.00

10.00

8.00

12.00

6.00

8.00

10.00

12.00

14.00

16.00

10.00

12.00

14.00

16.00

14.00

Ba ppm

1400

Nb ppm

1200 12 •

1000

10 '

800

600 400

5.00

6.00

7.00

8.00

• • (O

9.00

200

10.00

11.00

12.00

13.00

14.00

15 0C

4.00

6.00

8.00

Y ppm 125

Rb ppm

105

5.00

6.00

7.00

8.00

9.00

10.00

11.00

12.00

13.00

14.00

15.00

MgO %

4.00

6.00

8.00

10.00

MgO%

12.00

14.00

16.00

F i g . 5 Trace element versus MgO diagrams for Willouran basic rocks. Symbols as for Fig.3. Limits of compositional fields (marked by ticks along field limit line) for continental flood basalts (CFB) from literature in references.

basic lavas have major and trace element characteristics which clearly indicate an original tholeiitic parentage for these rocks. The strong FeO*, Ti and V enrichment (Fig.4) with increasing differentiation (measured by FeO*/MgO) is typical of strongly-differentiated tholeiitic suites in which fractionation was dominated by the lowpressure assemblage olivine, clinopyroxene and plagioclase. The low-grade lavas form a diffuse field, due probably to mobility (addition?) of FeO* during alteration (see later). The absence of highly differentiated ferrobasalts with

FeO*>16% and FeO*/MgO>3 suggests the parental magmas of the Willouran basic volcanics did not stagnate in sub-volcanic magma chambers; rather, regular fresh inputs of more primitive magma suppressed the trend toward high FeO* and Ti02 and resulted in eruption of slightly less differentiated, more magnesian lavas than CFB in general (see above). There is abundant petrographic and compositional evidence to indicate that Si02, CaO, FeO*, MnO, Na20 and K 2 O were mobilized to varying

A. J. Crawford & D. Hilyard

Zr/Nb

Fig. 6 Relationships between ratios of small, highly-charged immobile elements Zr, Nb and Y. Fields for N-, Tand P-MORB are shown as N, T, and P, and are from LeRoex (1987). Data for Parana and Brazilian continental margin basalts from Fodor et al (1985) and Fodor & Vetter 1984), Karoo data from Duncan et al (1984), Morocco data from Bertrand et al (1982).

degrees during metamorphic degradation of the Willouran basic volcanics. This will be discussed in more detail in the section dealing with alteration. Trace elements Plots of abundances of various trace elements versus MgO are given in Fig.5. There is some evidence that under conditions of intense CO2dominated, hypersaline metasomatism, the normally immobile elements Ti, Zr, Nb, Y and Sc

(Pearce 1974) may be mobilized to variable degrees (Murphy & Hynes 1986). However, for the carefully-selected Wooltana basalts studied here, these elements display generally coherent trends explicable by magmatic processes such as low-pressure fractionation and perhaps limited magma mixing. We note that the spread in Nb contents at 5 ppm Nb reflects analytical uncertainty at low abundances of Nb. In contrast, Sr, Rb and Ba abundances are mainly highly variable, and have been modified to some degree by alteration processes. Only the immobile elements

Geochemistry of Adelaide Geosyncline Flood Basalts

WOOLTANA BASALTS FIELD AND GAIRDNER DYKE S W A R M DOLERITE DYKE

S A M P L E

WOOLTANA BASALTS AND EAST PAINTER GORGE DYKE

Dyke

C 0 N D R T E

A La

Keeweenawan

FIELDS for WOOLTANA and PARANA BASALTS

FIELDS for WOOLTANA and KEEWEENAWAN BASALTS

Fig. 7 a. Chondrite-normalized REE patterns for three Wooltana basalts and the East Painter Gorge dyke. b. REE field for Wooltana basalts showing pattern for Reedy Lagoon dyke from the Gairdner Dyke Swarm, c. Upper and lower field limits for REE patterns of Wooltana and Parana CFB. Data from this study and Fodor et al (1985). d. Upper and lower field limits for REE patterns of Wooltana and Keeweenawan CFB. Data from this study and

Sample La Ce Pr Nd Sm Eu Gd Dy Er Yb

5479 5.17 12.0 1.64 8.1 2.32 0.80 2.83 3.04 1.87 1.50

5282

5345

Reedy Lagoon 5483

7.00 19.0 2.58 13.0 3.62 1.35 4.54 5.02 3.22 2.78

10.60 26.8 3.50 15.8 4.23 1.61 5.06 5.34 3.40 2.62

7.65 19.3 2.75 13.2 3.58 1.42 4.53 4.95 3.15 2.47

Table 2: REE Contents (ppm) of Wooltana Metabasalts and Two Dykes 5483 and 5471

E. Painter Gorge 5471 42.70 86.4 11.80 46.8 10.10 2.34 10.80 11.20 7.73 6.16

A. J. Crawford & D. Hilyard

are considered in the following discussion of the affinities and petrogenesis of the Willouran basic volcanics. W O O L T A N A B A S A L T S AND AV. CONTINENTAL THOLEIITE

W O O L T A N A AND P A R A N A - E A R L Y ATLANTIC OPENING BASALTS

"•„••••

Numerous recent studies of the geochemistry of CFB tholeiites have shown that these lava series have trace element characteristics intermediate between depleted (N-type) MORB and enriched plume (P-type) tholeiites (e.g. Fodor et al 1985; Dupuy & Dostal 1984; Dostal etal 1986; Duncan et al 1984), and show pronounced compositional similarities to transitional (T-type) oceanic basalts. This is clearly shown with respect to the highly-charged immobile elements Zr, Nb and Y in Fig.6. The great majority of CFB tholeiites fall between the fields for N-MORB and T-type tholeiites, except for some of the Kand Ti-enriched picritic and associated lavas from the Karoo, which require a source with higher Zr/Nb that found in typical P-type tholeiites.

Fig. 8 a. Primitive mantle-normalized element diagram for average altered Wooltana basalt (filled squares) and relatively fresh Reedy Lagoon dolerite dyke from the Gairdner Dyke Swarm (open squares) compared with pattern for average continental flood basalt (Holm 1985). The lower normalized abundances of the Willouran basic rocks relative to the average CFB is due to the more primitive (i.e.less fractionated) nature of the former relative to the latter, b. Primitive mantle-normalized element diagram for average altered Wooltana basalt and relatively fresh Reedy Lagoon dyke (dashed lines) compared with patterns for an average Parana (open circles) CFB (Fodor et al

Fig. 9 Chondrite-normalized element/Y diagram showing trends for average Roopena Volcanics (Giles & Teale 1979), average Wooltana Metabasalt, and East Painter Gorge dyke.

Three important points are evident from Fig.6. Firstly, the field defined by the Willouran basic lavas in Fig.6 is almost identical to that defined by the Parana tholeiites, and tholeiites drilled from the eastern continental margin of Brazil which define the transition from CFB (Parana) volcanism to initial rifting associated with Atlantic opening (Foder & Vetter 1984). Secondly, while the Beda Volcanics broadly cover the same compositional field as the Parana tholeiites, the Wooltana basalts and the initial Atlantic rifting tholeiites fall closer to the NMORB field than do the Beda Volcanics or Parana basalts. This reflects a gradual trend toward steady-state N-MORB volcanism as continental crust is attenuated, then ruptured, and eventually ocean crust is generated at an established spreading centre. Sedimentary facies indicate that open ocean conditions were not developed during this evolutionary stage of the Adelaide Geosyncline. Consideration of the above factors implies that the Wooltana Metabasalt lavas were erupted closer to the site of eventual rupturing of continental crust than the Beda Volcanics, in a tectonic setting analogous to the rift basins of offshore Brazil associated with initial Atlantic opening. The Beda Volcanics, in

Geochemistry of Adelaide Geosyncline Flood Basalts

contrast, are true CFB, more comparable to the Parana tholeiites. Thirdly, the two analyzed dykes from the Adelaide Geosyncline fall within the field for the Willouran basalts, implying on this evidence at least, broad magmatic similarities; this point is taken up further following discussion of the REE data. We have determined rare earth element (REE) abundances (Table 2) in three Wooltana basalts and the Reedy Lagoon and East Painter Gorge dykes. Chondrite-normalized REE patterns are shown in Fig.7. The Reedy Lagoon dyke, from the Gairdner Dyke Swarm, has a REE pattern identical within analytical error to low-grade Wooltana basalt 5282. Abundances of V, Sc, Zr, Nb, Y and Ti in these two samples are also very similar, strongly implying a broadly comagmatic genesis for the Wooltana Metabasalts (and by implication, although REE data are presently unavailable, the Beda Volcanics) and the Gairdner Dyke Swarm. The primitive mantle-normalized element variation patterns (Fig.8) for the Wooltana basalts and the Reedy Lagoon dyke further emphasize the strong compositional similarites for these rocks. The East Painter Gorge dyke (5471) has a REE pattern (Fig.7a) significantly different from those of the other four analyzed samples, being considerably enriched in total REE contents and having notably higher chondrite-normalized La/Sm (2.6 compared with 1.3-1.5) and La/Yb (4.6 compared with 2.2-2.5) ratios. This sample also has chondrite-normalized element/Y ratios unlike the Willouran basic volcanics (Fig.9), and is thus unlikely to be magmatically related to them. However, these element/Y ratios are very similar to the average trend shown by the Roopena Volcanics of the southern Stuart Shelf (Giles & Teale 1979). The latter authors argued convincingly that the Roopena Volcanics are correlates of basalts within the extensive Gawler Range Volcanics (c. 1 500 Ma). Numerous amphibolite dykes intrude the Mount Painter Block north of the main outcrop area of the Wooltana Metabasalt (Coats & Blissett 1971). At least some of these are geochemically similar to the Roopena

Volcanics (G.Teale, pers. comm. to DH, 1984). The analyzed East Painter Gorge dyke (5471) may be part of this dyke swarm. The geochemical similarities between the East Painter Gorge dyke and the Roopena and Gawler Range Volcanics raises a problem with regard to the timing of eruption of the Gawler Range Volcanics and deposition of the basal Adelaidean sediments. Two alternative explanations are offered. First, the geochemical similarities may be fortuitous, and the Mount Painter dyke (swarm) is younger, perhaps Cambro-Ordovician. Alternatively, the dyke may correlate with the Roopena Volcanics, and the quartzites which it intrudes predate Adelaidean deposition. In this regard, it is interesting to note that G. Teale (pers. comm. to DH, 1984) has recognized a major unconformity within the basement sequence of the Mount Painter Block, where Early Proterozoic metamorphics are overlain by Middle Proterozoic volcanics and clastics equivalent to the Gawler Range Volcanics. This problem has not been resolved. In Figs 7c and d, REE patterns of the Wooltana basalts are compared with the fields for Parana low-Ti basalts, Keeweenawan basalts, and basalts associated with the initial stages of opening of the Atlantic (from the Santos Basin on the Brazilian continental margin and Morocco). Fig. 8 compares primitive mantle-normalized element patterns of these basalts with those of an average CFB (Holm 1985). As has already been shown for the highly-charged elements Ti, Zr, Nb and Y, there is a pronounced similarity between the Wooltana basalt REE patterns and those from the Parana and Keeweenawan CFB provinces. The former, however, tend to flatten slightly between La and Pr relative to other CFB REE patterns shown. As this flattening is a feature of both the altered Wooltana basalts and the relatively unaltered Reedy Lagoon dyke, it is likely to be primary, and to be a characteristic of the source. Despite this very minor difference in LREE patterns, the overall geochemical similarity (Figs 6, 7 and 8) of the Wooltana basalts (and other Willouran basic volcanics) and CFB (especially

A. J. Crawford & D. Hilyard

those associated with initial Atlantic opening) argue strongly for similar petrogenetic scenarios for these Proterozoic and Mesozoic tholeiitic flood basalts. Similar mantle sources, primary magmas, fractionation assemblages and tectonomagmatic scenarios are implied. Element mobility, alteration and crustal contamination In the preceding discussion we deliberately avoided interpretation of abundances of the elements Ca, Na, K, Sr, Rb and Ba, as these elements are generally considered to be highly mobile during low-grade alteration processes affecting basalts. In addition, there is some evidence (see later) that Fe and Mn were also mobile during alteration of the Wooltana basalts. Verification of the mobility of these elements during alteration of the Willouran basic volcanics comes from their diffuse and generally 'un-magmatic' compositional fields on element MgO diagrams (Figs 3 and 5). In the following section, we examine the alteration styles shown by the Wooltana, Cadlareena and Beda Volcanics. Calcium and strontium. The CaO and Sr versus MgO diagrams show highly variable CaO and Sr contents in the Willouran basic volcanics, with the great majority of samples falling below the fields defined by relatively unaltered Proterozoic and Mesozoic flood basalts. The well-preserved Reedy Lagoon dyke falls within the unaltered' field on both diagrams, and suggests that the primary CaO and Sr contents of the Wooltana basalts at around 7% MgO may have been approximately 11% and 160 ppm respectively. A depletion in CaO and Sr contents is a characteristic feature of low-grade altered ('spilitized') basalts, resulting from liberation of these elements during albitization of calcic plagioclase. The widespread occurrence of veins and vesicle-fillings dominated by calcite, epidote or prehnite is testimony to the effective removal of CaO (and Sr) during alteration of basalts. 4

Sodium. An interesting feature of the Na20-Mg0 diagram (Fig.3) is the coherent field shown by Beda Volcanics at high Na20 contents relative to other Willouran basalts. High Na20 contents (46%) are typical of greenschist facies metabasalts, and are considered to be due to Ca-Na exchange following introduction of Na from seawaterderived metamorphic fluids. Once again, the Reedy Lagoon dyke falls within the field for fresh CFB, and indicates that the Wooltana basalts had around 2% Na20 at 7% MgO. The difference in the Na20 contents of the Beda, versus other Willouran, volcanics is discussed below in conjunction with data for K2O. Potassium, rubidium and barium. Generally, CFB contain less than 1.5% K2O (Fig.3), and values around 0.5% seem to be typical (although the presence of high-K suites, particularly amongst the Karoo basalt suites, should be noted). The well-preserved Reedy Lagoon dyke, with 0.32% K2O, may be close to the pristine value for Wooltana basalts with around 7% MgO. If this is so, it suggests that most Willouran basalts have been strongly enriched in K2O, with the Wooltana basalts suffering generally greater K20-enrichment than the Beda or Cadlareena Volcanics. Rubidium abundances in CFB are generally less than 40 ppm (Fig.5), although some strongly LREE-enriched Karoo suites extend to much higher values. Enrichment of Rb in Willouran basalts seems to mimic that of K, with the Wooltana basalts mainly showing significantly greater enrichment than the Beda Volcanics. Abundances of Ba are highly variable in CFB suites, but are generally between 100 and 400 ppm. Again, Wooltana basalts (Fig.5) are seen to be enriched in Ba relative to both CFB and the Beda Volcanics; Ba contents of the latter are low, mainly 200 ppm, and may not reflect any diagenetic enrichment.

Geochemistry of Adelaide Geosyncline Flood Basalts

The enrichment in K2O (and Rb and Ba), to values between 1.5 and 3.7% in basalts with 8-10% MgO and low chondrite-normalized La/Sm ratios (in contrast to high La/Sm Karoo picrites which have high-K characteristics), is exceptional and requires explanation. It is significant that metamorphic assemblages of the Wooltana (and probably other Willouran) metabasalts are unlike those imposed on basaltic assemblages during alteration (spilitization) involving seawater-derived brines; such assemblages rarely contain K-feldspar or scapolite and tourmaline (e.g. Seyfried 1987). The occurrence of Wooltana Metabasalt flows showing ubiquitous low-grade alteration (including common K-feldspar) within a sequence of unaltered sediments is notable, and may indicate that the sediments were in equilibrium with fluids which reacted with an altered the basalts. Alteration is characterized by significant addition of K2O, while the scapolite and tourmaline indicate metamorphic fluids were chlorine- and boronrich. Early Adelaidean sediments enclosing the metabasalts were originally partly evaporitic (Rutland et al 1981; Uppill 1980; Rowlands etal 1980), and fluids in equilibrium with these sediments would be rich in Na, K, carbonate and halides. Infiltration of such fluids through joints and along basalt flow boundaries induced the observed large-scale re-equilibration of basalt mineral assemblages. It is notable that the least altered member of the suite studied, the Reedy Lagoon dyke, occurs in an area where evaporitebearing early Adelaidean sediments are absent. The reason for the higher K2O and lower Na20 contents of the Wooltana Metabasalts relative to other Willouran basic volcanics is probably due to operation of one or the other of the following two processes. If the Willouran basalts were universally "spilitized" during reaction with hot, seawater-derived fluids at or near the sea-floor, post-burial diagenetic exchange of evaporitederived K for Na may have occurred to a greater degree in the Wooltana basalts than in other Willouran lavas. Alternatively, the Willouran basalts may never have undergone a spilitic-style

degradation, so that variable K/Na ratios now shown amongst the different Willouran suites simply reflect the K/Na ratios of the evaporitederived fluids responsibile for alteration of these lavas. Mount (1980) described an alteration assemblage of an intrusive breccia body derived from Callanna Group sediments which included carbonate, chlorine, adularia, hematite, quartz, stilpnomelane, talc, and minor analcite, brucite, epidote, phlogopite, ilmenite and tourmaline. Fluids responsible for this assemblage were considered by Mount (1980) to be low-temperature (150°C-250°C) hypersaline, CC>2-rich aqueous solutions derived in part by dissolution of typical Callanna sediments. Similar fluids were probably responsible for the alteration of the Wooltana Metabasalt. Lindley & Chapin (1986) described potassic metasomatism of volcanic rocks from the Rio Grande Rift, which involved strong enrichment in K, Rb and Ba, and depletion in Ca, Mg, Sr and Mn. The elements Ti, Zr, Y, Th and REE were considered to have remained immobile during alteration, which was attributed to alkaline, saline brines derived from hydrographic basins in downwarps in arid areas of crustal thinning associated with regional extension. The regional setting and alteration style of the Wooltana Metabasalt are very similar to the Rio Grande Rift sequence described by Lindley & Chapin (1986), and probably originated via the same processes.

Iron and manganese. Relative to typical CFB and the unaltered Reedy Lagoon dyke, a number of Wooltana Metabasalts show notable enrichment in Fe (Fig.4), and some samples contain up to 0.76% MnO, attesting to the mobility of these elements under the hypersaline alteration conditions. There is no correlation between enrichments in Fe, Mn or K.

A. J. Crawford & D. Hilyard

We are unable to constrain the timing of the extensive low-temperature alteration which has affected the Wooltana Metabasalts, although by analogy with the Rio Grande Rift example, we suggest that alteration of the basalts probably occurred within 10-20 m.y. of their eruption. Hydrothermal alteration and deposits as young as Tertiary in age are common along the Delamerian Paralana Fault, which still localizes hot spring activity. One final point worthy of consideration is whether or not the Willouran basic magmas were contaminated by crustal rocks during their preemptive history. Limited crustal contamination is implicated in the genesis of some CFB (Deccan Traps: Cox & Hawkesworth 1985; Skye-Mull: Thompson et al 1982; Ferrar province: Siders & Elliot 1985), and is ruled out for others (some Parana suites: Hawkesworth et al 1986; most of the Karoo: Duncan et al 1984). Unfortunately, those elements most informative in testing for crustal contamination (K, Ba, Rb and Sr) have been strongly mobilized during post-magmatic alteration of the Willouran basic volcanics, so that on the basis of available data we are unable to determine the extent, if any, of crustal contamination in the petrogenesis of these lavas. Given, however, the identical REE patterns and P2O5 contents of the relatively unaltered Reedy Lagoon dyke and Wooltana Metabasalt 5282, it is unlikely to be a crustal contamination effect that the basalt has an order of magnitude more K2O than the dyke. We believe that the high and variable K-group element abundances of the Wooltana Metabasalts are very largely alteration-related, and are not due to extensive crustal contamination.

for the Beda Volcanics of the Stuart Shelf confirms the conclusion of Giles & Teale (1979) that the Beda Volcanics are Wooltana Metabasalt correlates; however, subtle compositional differences between these units indicate that the latter may have been generated during initial stages of continental rupturing and rifting preceding establishment of a spreading centre, while the former represent true continental flood basalts. The absence of open marine sediments in the overlying Callanna Group sequences, however, indicates that rifting was aborted (or jumped eastward) prior to continental separation and marine transgression. The entire Willouran basic volcanic suite exhibits pronounced compositional similarities to the low-Ti Parana CFB and basalts erupted oceanward of the Parana, during rifting associated with earliest Atlantic Ocean opening.

ACKNOWLEDGEMENTS John Foden provided analytical facilities in the Geology Department, University of Adelaide and J.Stanley and PMcDuie carried out the XRD analyses; Philip Robinson and Carol Hall-Jones assisted with analytical work in Hobart. DH acknowledges generous financial support for this project from Prof.D.H. Stapleton, School of Applied Geology, SAIT, and the Royal Society of South Australia, and thanks Wolfgang Preiss, Jim Jago, Rick Rogerson and Eric Finlayson for valuable discussions. Samples from Shell Reedy Lagoon drillhole No.l were supplied by the Core Library, South Australian Dept of Mines. AJC acknowledges financial support from a Queen Elizabeth II Fellowship.

REFERENCES CONCLUSIONS Geochemical studies of the Wooltana Metabasalt and a fresh dolerite from the Gairdner Dyke Swarm indicate that the parental magmas of these units were essentially identical in terms of immobile element ratios and REE patterns. A comparison of this new data with available data

AMBROSE G.J., FLINT R.B. & WEBB A.W. 1981. Precambrian and Palaeozoic geology of the Peake and Denison Ranges. Bulletin of the South Australian Geological Survey 50. BASALTIC VOLCANISM STUDY PROJECT 1981. Basaltic Volcanism on the Terrestrial Planets. Pergamon Press, New York, 1286pp.

Geochemistry of Adelaide Geosyncline Flood Basalts

BELLIENI G., BROTZU P., COMIN-CHIARAMONTI P., ERNESTO M., MELFI A., PACCA I.G. & PICCIRDLLO E.M. 1984. Flood basalt to rhyolite suites in the Southern Parana Plateau Brazil: palaeomagnetism, pedogenesis and geodynamic implications. Journal of Petrology 25, 579-618. BERTRAND H., DOSTAL J. & DUPUY C. 1982. Geochemistry of early Mesozoic tholeiites from Morocco. Earth and Planetary Science Letters 58, 225-239. BROOKS C.K., NIELSEN T.F.D. & PETERSEN T.S. 1976. The Blossville Coast basalts of East Greenland: their occurrence, composition and temporal variations. Contributions to Mineralogy and Petrology 58, 279-292. COATS R.P. & BLISSETT A.H. 1971. Regional and economic geology of the Mount Painter Province. Geological Survey of South Australia Bulletin 43. COOPER P.F., TUCKWELL K.D., GILLIGAN L.B. & MEARES R.M.D. 1978. Geology of the Torrowangee and Fowlers Gap 1:100,000 sheets 7135 and 7235. New South Wales Geological Survey, 164pp. COX K.G. & HAWKES WORTH C.J. 1985. Geochemical stratigraphy of the Deccan Traps at Mahabaleshwar, Western Ghats, India, with implications for open system magmatic processes. Journal of Petrology 26, 355-377. DALGARNO C.R. & JOHNSON J.E. 1968. Diapiric structures and Late Precambrian-Early Cambrian sedimentation in Flinders Ranges, South Australia. American Association of Petroleum Geologists MemoirS, 301-314. DOSTAL J., BARAGAR W.R.A. & DUPUY C. 1986. Pedogenesis of the Natkusiak continental basalts, Victoria Island, NW Territories, Canada. Canadian Journal of Earth Sciences 23, 622-632. DUNCAN A.R., ERLANK A.J. & MARSH J.S. 1984. Regional geochemistry of the Karoo Igneous Province. Special Paper of the Geological Society of South Africa 13, 355-388. DUPUY C. & DOSTAL J. 1984.. Trace element geochemistry of some continental tholeiites. Earth and Planetary Science Letters 67, 61-69.

FODOR R.V., CORWIN C. & ROISENBERG A. 1985a. Petrology of Serra Geral (Parana) continental flood basalts, southern Brazil: crustal contamination, source material and South Atlantic magmatism. Contributions to Mineralogy and Petrology 91, 54-65. FODOR R.V., CORWIN C. & SIAL A.N. 1985b. Crustal signature in the Serra Geral flood basalt province, southern Brazil: O- and Sr-isotope evidence. Geology 13, 763-765. FODOR R.V. & VETTER S.K. 1984. Rift-zone magmatism: petrology of basaltic rocks transitional from CFB to MORB, southeastern Brazil margin. Contributions to Mineralogy and Petrology 88, 307-321. FORBES B.G. 1978. The Boucaut Volcanics. Quarterly Geological Notes, Geological Survey of South Australia 65, 6-10. FORBES B.G., MURRELL B. & PREISS W.V. 1981. Subdivision of the Lower Adelaidean, Willouran Ranges. Quarterly Geological Notes, Geological Survey of South Australia 79, 7-16. GILES C.W. & TEALE G.S. 1979. A comparison of the geochemistry of the Roopena Volcanics and the Beda Volcanics, Quarterly Geological Notes, Geological Survey of South Australia . HAWKESWORTH C.J., MANTOVANI M.S.M., TAYLOR P.N. & PALACZ Z. 1986. Evidence from the Parana of South Brazil of a continental contribution to Dupal Basalts. Nature 322, 356-359. HILYARD D.B. 1986. The stratigraphy and tectonic setting of the Wooltana Metabasalt, South Australia. M.App.Sci. thesis (unpubl.). South Australian Institute of Technology. HOLM P.E. 1985. The geochemical fingerprints of different tectonomagmatic environments using hygromagmatophile element abundances of tholeiitic basalts and basaltic andesites. Chemical Geology 51, 303-323. HORR G.M. 1977. Precambrian spilites and the Pandurra Formation ot the Stuart Shelf, near Port Augusta, South Australia. B.Sc.(Hons.) thesis (unpubl.), Univ. of Adelaide.

A. J. Crawford & D. Hilyard

LE ROEX A.P. 1987. Source regions of mid-ocean ridge basalts: evidence for enrichment processes. In Menzies M. and Hawkesworth C.J. eds. Mantle Metasomatism, 398-422. LINDLEY J.I. & CHAPIN C.E. 1986. Potassium metasomatism of volcanic rocks in areas of regional extension. (Abstr.), International Volcanological Congress, Symposium 5: Volcanism, hydrothermal systems and related mineralization, 100. MOHR P. 1983. Ethiopian flood basalt province. Nature 303, 577-584. MOUNT T.J. 1980. The Arkaba breccia intrusion and the Arkaba Hill Beds, Flinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 74, 4-11. MURPHY J.B. & HYNES A.J. 1986. Contrasting secondaiy mobility of Ti, P, Zr, Wb and Y in two metabasaltic suites in the Appalachians. Canadian Journal of Earth Sciences 23, 1138-1144. PEARCE J.A. 1974. Basalt geochemistry used to investigate past tectonic environments of Cyprus. Tectonophysics 25, 41-67. PREISS W.V. 1983a. Depositional and tectonic contrasts between Burra Group and Umberatana Group sedimentation. Geological Society of Australia Abstracts 10, 13-16. PREISS W.V. (compiler) 1983b. Adelaide Geosyncline and Stuart Shelf 1:600 000 Geological Sheet. South Australian Geological Survey.

ROWLANDS N.J., BLIGHT P.G., JARVIS D.M. & VON DER BORCH C.C. 1980. Sabkha and playa environments in Late Proterozoic grabens, Willouran Ranges, South Australia. Journal of the Geological Society of Australia Journal 27, 55-68. RUTLAND R.W.R., PARKER A.J., PITT G.A., PREISS W.V. & MURRELLB. 1981. The Precambrian of South Australia. In Hunter D.R. ed. Precambrian of the Southern Hemisphere. Developments in Precambrian Geology Vol. 2, 309-360. SEYFRIED W.E. 1987. Experimental and theoretical constraints on hydrothermal alteration processes at mid-ocean ridges. Annual Review of Earth and Planetary Sciences 15, 317-335. SIDERS M.A. & ELLIOT D.H. 1985. Major and trace element geochemistry of the Kirkpatrick Basalt, Mesa Range, Antarctica. Earth and Planetary Science Letters 72, 54-64. THOMPSON R.N., DICKIN A.P., GIBSON I.L. & MORRISON M.A. 1982. Elemental fingerprints of isotopic contamination of Hebridean Palaeocene mantle-derived magmas by Archaean sial. Contributions to Mineralogy and Petrology 79, 159-168. UPPILL R.K. 1980. A Late Precambrian sedimentary dolomite and magnesite sequence in the Mundallio Subgroup, South Australia. Geological Society of Australia Programmes with Abstracts, Fourth Australian Geological Convention, Hobart, 75. VON DER BORCH C.C. 1980. Evolution of the Late Proterozoic to Early Palaeozoic Adelaide Foldbelt, Australia: comparisons with post-Permian rifts and passive margins. Tectonophysics 70, 115-134.

PREISS W.V. 1985. Stratigraphy and tectonics of the Worumba Anticline and associated intrusive breccias. Geological Survey of South Australia, Bulletin 52.

WEBB A.W. & HORR G. 1978. The Rb-Sr age and petrology of a flow from the Beda Volcanics. Quarterly Geological Notes, South Australian Geological Survey 66, 10-13.

ROBINSON P., HIGGINS N.C. & FRYER B.J. 1986. A combined ion extraction-XRF procedure for the determination of rare earth elements in rocks and minerals. Chemical Geology 44, 221-227.

WOOD D.A. 1979. A variably-veined suboceanic upper mantle - genetic significance for mid-ocean ridge basalts from geochemical evidence. Geology 7, 499-503.

Geochemistry of Adelaide Geosyncline Flood Basalts

APPENDIX Is WOOLTANA METABASALT SAMPLE LOCALITIES Sample No.

AMG Grid Ref.

Locality

5479 5337 5481 5478 5477 5314 5468 5470 5472 5480 5257 5469 5282 5345 5474 5473 5482 5398

470324 48983642 48213667 34914065 51254460 51014421 51534594 512451 51044586 50414680 49714710 49684694 50054556 46824896 468503 47995167 485490 427488

Merinjina Well area Mount Jacob Mount Jacob Woodlamulka Mine Arkaroola Creek Arkaroola Creek Groan Creek area Arkaroola Creek Groan Creek area Groan Creek area Groan Creek area Groan Creek Groan Creek S of Lady Buxton mine S of Lady Buxton mine East Painter Gorge S of Lady Buxton mine S of Echo Camp

5471

480529

5483

407246

YUDNAMUTANA 1:50 000 East Painter Gorge (dyke) KINGOONYA 1:250 000 Reedy Lagoon DDH1, 647.5 m

Sample numbers refer to the rock collection of the Department of Applied Geology, SAIT. Grid references refer to WOOLTANA 1:50 000 sheets unless otherwise stated.

Geology of the Willouran Ranges Bryan G. Forbes* South Australian Department of Mines and Energy, P.O. Box 151, Eastwood, S.A. 5063, Australia Adelaidean (Late Proterozoic) rocks are exposed in the Willouran Ranges southwest of Mairee and in inliers within Mesozoic and younger rocks as far west as the gorges of Stuart Creek. These approximate the junction of the Stuart Shelf (to the west) with the Torrens Hinge Zone, which extends east to the Norwest Fault, the western edge of the fractured and folded Adelaide Geosyncline. Oldest rocks of the geosyncline are the Callanna Group, over 6000 m of partly evaporitic rocks with a basaltic flow, and a thin dacite with U-Pb zircon age of 802+10 ma. Overlying sequences are the Burra Group, over 9000 m of partly intertidal clastics and carbonates; Umberatana Group, over 3000 m of glacial and marine sediments, and Wilpena Group of over 1500 m of predominantly intertidal clastics. The ranges are cut by northwesterly-trending fracture zones, paralleling Delamerian (lower Palaeozoic) folds, accompanied by intrusive, partly diapiric, breccias. Minor copper deposits are mainly related to quartz- carbonate veins just outside breccia zones.

Key words: Late Proterozoic stratigraphy, northwest Flinders Ranges, South Australia, structural geology, diapir, Adelaide Geosyncline, paralic environment, glacial sediments, copper deposits.

INTRODUCTION The term 'Willouran Ranges' is an informal extension of 'Willouran Range', the accepted name for the easternmost of the low ranges southwest of Marree (Fig. 1), which includes Willouran Hill. The purpose of this contribution is to review the pre-Mesozoic geology of the Willouran Ranges as far west of Marree as the vicinity of Cadnia Hill and as far southwest and south as the neighbourhoods of Mount Norwest and Twenty Mile Hill, a region of folded and greatly fractured Adelaidean rocks, and a northwesterly extension of the Mount LoftyFlinders Ranges. The western extension of this region forms part of the Stuart Shelf and will be given some mention here. Rocks west of the Norwest Fault represent the Torrens Hinge Zone and east of there is the Adelaide Geosyncline/Delamerian fold zone. Adelaidean rocks extend into the adjacent published 1:250 000 map areas of Marree, Copley and Andamooka. Features of interest are volcanics in the Callanna Group, the faulted relationship between the lower Adelaidean Cal-

lanna and Burra Groups, several units of glacial origin in the Umberatana Group and intrusive tectonic breccias associated with regional fracture zones.

PREVIOUS INVESTIGATIONS The earliest recorded geological observations are those of Scoular (1887) who travelled between Marree and Anna Creek noting geological features including Proterozoic rocks at Davenport Springs and Humphreys Springs. The Record of Mines (Brown 1908) indicates visits by the Government Geologist to copper mines in the 1890's. Brown (1892) reported on country south of Lake Eyre. Howchin (1926) described a brief visit to the Willouran Range near Marree in 1906 and noted glacial erratics, "Sturtian Tillite", probably within the upper and lower diamictites of the Umberatana Group. Likewise Mawson (1927) visited the Willouran Range in 1920, describing his "Willouran Series" north of Willouran Hill: this would have included lower Burra Group

^Present address: 11 Bruce Avenue, Rostrevor, S.A. 5073, Australia

Geology of the Willouran Ranges

KILOMETRES

30o00' I Tectonic breccia, partly diapiric containing blocks or intrusives of gabbroic rock (v) and Arkaroola Subgroup = Noranda Volcanics (nv) and Black Knob Marble (bk).

Fault

•

Wilpena Group.

Anticlinal fold showing plunge Synclinal fold showing plunge Trend of bedding showing facing direction

Eu*

• Eb v

Umberatana Group.

]

Strike and dip of bedding, vertical bedding 80

Burra Group. X

Strike and dip of cleavage , vertical cleavage Copper mine or prospect

Callanna Group (Upper) = Curdimurka Subgroup, partly brecciated.

Fig. 1 Plan showing distribution of the main pre-Mesozoic rock groups in the northern Willouran Ranges. Section A-A 1 is shown in Fig. 2. The strippled area containing the 40 degree dip sign 12 km WNW of Twenty Mile Hill contains Umberatana Group rocks rather than Callanna Group rocks as shown.

Bryan G. Forbes

rocks. He also noted breccias, faulting, dolerite and copper deposits as at Dunn's Mine. The first relatively detailed map of the Willouran Ranges was produced by Sprigg (1950) who made a structural and photogeological study. Miles (1952) made a mineral reconnaissance but did not find the region promising for large mineral deposits. Geological mapping by the Geological Survey in 1960 resulted in publication of the Callanna 1:63 360 geological map (Webb et al 1963). This covers the northern fringe of the Willouran Ranges and shows a basic lithological outline of the Precambrian sequence.

regarded complex breccia zones to be diapiric in nature, with features similar to diapirs elsewhere in the Adelaide Geosyncline. Dampier's work included mapping by Carthew and Murrell, and analytical and petrological reports. Carthew (1975) regarded gabbroic rocks as basement to the Adelaidean and did not consider megabreccias to be of diapiric origin. Later exploration sought Mid-Proterozoic, Olympic-Dam type copper-uranium-gold mineralisation in the eastern Stuart Shelf. Late Proterozoic overburden was found to be unfavourably thick, but the wells Newmont SRI7/2 and WMCFHD1 provide useful stratigraphic data.

Australian Selection (Sampey & Drissen 1966a, b) searched for copper in the Willouran Range and near Tarlton Knob and appear to be the first to note a dacite layer in the Callanna Group near the old Rook workings. Anaconda (Dalgarno 1966; Ruker 1966) in their exploration for base metals produced excellent photogeological maps of a wide region. Some other companies making valuable geological contributions were Noranda (Thomas & Dunlop 1968), Mount Isa Mines (Fairburn 1969), Finance Facilities (Dewar 1974; Gillespie 1974a, b), Dampier Mining (Bischoff 1975; Carthew 1975) and Utah Development Company (Rowlands et al 1978, 1980, 1983). Fairbum concluded that although some anomalous copper appeared to be stratigraphically controlled, copper deposits were related to fracturing and were essentially epigenetic. He

Further geological traversing and stratigraphic studies were carried out by Survey geologist R.P.Coats as a contribution to a preliminary Curdimurka 1:250 000 geological map prepared by Daly (1970). There was also minor work by Coats and Preiss in 1971 and 1972 and a report by Preiss (1971). Murrell (1977) made a regional study of the area as a Ph.D. project and established the stratigraphy of the Callanna Group: this was later published in summary (and slightly modified) form (Forbes etal 1981). Mapping and correlation by Murrell and Utah geologists were valuable contributions, although their interpretations of crystalline basement inliers, basal Burra Group unconformities and a largely olistostrome origin of Callanna Group metabreccias have not been adopted here.

Section A-A' T O R R E N S HINGE ZONE

ADELAIDE

GEOSYNCLINE

I KILOMETRES

10 I

Fig. 2 Geological Section A-A showing inferred structure at natural scale. Location is shown in Fig. 1 and symbols are explained in Table 1. Areas in solid black are inferred intrusives or blocks of gabbroic rock. 1

Geology of the Willouran Ranges

Rowlands et al (1980) interpreted the Callanna Group as a sabkha association and recognised a suite of evaporitic minerals and structures. Rayner & Rowlands (1980) presented a picture of stratiform copper in a delta (Umberatana Group): this received comment from Dalgarno et al (1981). Parker (1983) described overprinting of folds and thrusts within the Rischbieth structural complex and suggested significant tectonism prior to or during early sedimentation of the Burra Group. Coats & Dalgarno (1986) featured large-scale slumping in the Umberatana Group; Belperio (1986) described the stratigraphy and sedimentology of the Skillogalee Dolomite.

STRATIGRAPHY Stratigraphy is summarised in Table 1 and the description below forms a supplement to this. The specimen numbers referred to in the text are those of the Geological Survey of South Australia. Callanna Group The Callanna Group (Warrina Supergroup) is the oldest rock group represented in the region (Figs 1, 2, 3). Although crystalline basement is not known in outcrop in the CURDIMURKA region, it does underlie Callanna Group (Arkaroola Subgroup) in the Peake and Denison Ranges and the Mount Painter area. The upper limits of the Callanna Group (Curdimurka Subgroup) are technically disturbed and it is not known whether the original boundary with the overlying Burra Group was an unconformity or conformable passage. Nomenclature used here is that of Forbes et al (1981) after Murrell (1977). Details of type sections appear in Forbes (1980). Aggregate thickness of the Callanna Group is more than 7000 m in places and it is thought to represent a continental evaporitic to shallow marine, partly sabkha, environment. Unlike equivalent sequences of the Peake and Denison Ranges and Mount Painter area, the

Arkaroola Subgroup is not known as an undisturbed succession in the Willouran Ranges, but is represented only by faulted or distorted and isolated blocks of Noranda Volcanics and Black Knob Marble. The Black Knob Marble is tentatively correlated with the Wy wyana Formation of the Mount Painter region (Forbes et al 1981). Its type section is at Black Knob, southwest of Rischbieth Well, where it is 25 m thick. Here there are several blocks, some over 150 m in length, showing tight irregular folding, within strongly brecciated and folded quartzite beds correlated with the Dome Sandstone. Near the margins of blocks the dark grey marble may be bleached to a white or pink colour, possibly as the result of contact with fluids within the surrounding megabreccia. Blocks of dark marble correlated with the Black Knob Marble occur within megabreccias at and near the Clara St Dora Copper Mine, near The Dome and near the Boorloo Copper Mine. The Noranda Volcanics (Forbes et al 1981) have been correlated with the Wooltana Volcanics and Cadlareena Volcanics and have their type section at the former site of Noranda Aust. Pty Ltd exploration camp near the Boorloo Copper Mine, 10 km southeast of "Callanna". Near Chintapanna there are pillow structures within an approximate thickness of 80 m of basaltic rock. These were noted by Utah Development Co. in 1979. Pillow core has been described as altered andesite (6438RS169; Radke, 1980). The pillow is rimmed by vesicular basalt with plagioclase phenocrysts. Carthew (1975) mentions possible pillow structures in his "Horseshoe Volcanics", probably equivalent to the Noranda Volcanics, south of The Dome. Farrand & Parker (1986) have described probable Noranda Volcanics in the Rischbieth structural complex as amygdaloidal basalt and basic pyroclastics, originating as rapidly-chilled thin flows. Chemically the basalts are sodic and fall within the Hawaiite/Mugearite field on a total akali-silica diagram. Trace elements are consistent with a within-plate environment.

Bryan G. Forbes

The Curdimurka Subgroup is least disturbed and best exposed in its type area west of Boorloo Copper Mine where its thickness is 4700 m. It is composed of clastics and carbonates deposited under partly evaporitic conditions, probably in fault-controlled basins (Rowlands etal 1980) and has been correlated with the Rockwater Beds to Duff Creek Beds of the Peake and Denison Ranges and the Wirrawilka Beds to Worumba Dolomite Beds of the Worumba Anticline (Forbes et al 1981). Details of type sections are reported in Forbes (1980). Upper and lower limits of the Curdimurka Subgroup are generally fault planes. An exception is at Chintapanna Dam, where Dome Sandstone appears to have a sedimentary contact with Noranda Volcanics (Preiss, pers. comm.). Lowermost beds of the Burra Group contain no sedimentary evidence of unconformity and are broadly similar to the upper Curdimurka Subgroup. The oldest known unit in the Curdimurka Subgroup is the Dome Sandstone, which is 1480 m thick in its type section, 3 km southwest of "Callanna". Most of the prominent quartzite blocks within megabreccias are correlated with the Dome Sandstone. The Rook Tuff is a somewhat variable darkweathering, silty to sandy, partly tuffaceous sequence conformably above the Dome Sandstone and 15 to 60 m in thickness. Near the Rook workings it contains a porphyritic dacite layer, a possible welded tuff, three metres thick, with gradational contacts. Zircon crystals from this have been dated by the U-Pb method at 802±10 Ma (Fanning et al 1986). The Boorloo Siltstone (519m) is the youngest known formation of the Curdimurka Subgroup and is characterised by dark siltstone. The type section comprises ridges west of the Boorloo copper workings and immediately west of a megabreccia zone. At the top is a distinctively well-bedded alteration of 153m of limestone, dolomite and siltstone with possible tepee structures. The upper limit of the Boorloo is deter-

mined by faulting against an intrusive breccia or against the lower Burra Group. Burra Group Sedimentary relationships of the Burra Group (upper Warrina Supergroup) with the underlying Callanna Group in the Willouran Range are obscured by tectonic disruption. However, similar siltstone-carbonate-sandstone lithologies in the upper Callanna and lower Burra do not suggest that there was a great change or major sedimentary unconformity at their original juncture. Total exposed thickness of the Burra Group is partly controlled by post- depositional erosion or tectonic disruption and varies from 2540 m north of Willouran Hill, thickening to 5370 m south of Willouran Hill and forming a belt of greatest thickness (8450 m in the north, 8470 m in the south) east of the Norwest Fault. West of the Norwest Fault it thins again to 2990 m near Mount Norwest. The Burra Group appears to reflect shallow water depositional environments but with much less frequent evidence of evaporative conditions than in the Callanna Group. The Emeroo Subgroup comprises basal, predominantly quartzitic, beds of the Burra Group and reaches its greatest exposed thickness of 3920 m south of Willouran Hill. It is thinnest (1310 m) north of Willouran Hill (possibly near the basin margin) and reaches intermediate thicknesses elsewhere. Subdivisions originally set up by Murrell (1977; Forbes etal 1981) are generally recognisable except near Mount Norwest. Depositional environments may have included fluvial, lacustrine (partly evaporative) and deltaic. North of Mount Norwest limited cross-bedding evidence suggests sediment transport to the north and northwest. Some low-angle cross beds suggest a beach environment here. Carbonate content decreases upward from 30 to 12 percent in the lower and upper Top Mount, 7 percent in the Willawalpa and probably less in the Witchelina. Mundallio Subgroup is a term defined by Uppill (1978) to refer to the Woolshed Hat Shale

Geology of the Willouran Ranges

(probably not present) and Skillogalee Dolomite. Uppill also proposed terms Yadlamalka, Nathaltee and Nankabunyana Formations which refer to various facies of the Skillogalee Dolomite, and has recognised the Yadlamalka (upper unit) and Nankabunyana (lower unit) in the area. Other terms not used here were proposed by Murrell (1977), e.g."Camel Flat Shale" and "Tilterana Sandstone" for local basal black shale and overlying sandstone, and "Mirra Formation" for an eastern sandy facies of the Skillogalee. The Skillogalee Dolomite conformably overlies the Witchelina Quartzite. It has been studied by Forbes (1960, 1961), Preiss (1973), Murrell (1977), Uppill (1979, 1980), Rowlands et al (1983) and Belperio (1986, 1987a, b, c) and is consequently the best-known formation in the region. Information here is derived largely from Belperio. Four regions of differing subsidence and accumulation have been recognised and give evidence of early activity on the Norwest, Bungarider and Kingston-Callanna fracture zones. Belperio (1987a, b, c) infers a major northwesttrending fault- bounded sedimentary basin containing a number of sub-parallel, differentially-subsiding, fault-bounded subbasins. Interfingering of carbonates, sands and silts occurred as a result of continuing subsidence, rapid oscillations of a shallow marine shoreline and seasonal sheet flooding. Presumably conditions were more persistently continental to the west, where a greater proportion of magnesite was produced. Ripple marks suggest mainly north-south wave directions and this was also apparent in the Willouran Range in the Marree 1:250 000 area (Forbes 1960) where some detritus and slumping were directed in a southeasterly direction. Belperio (this volume) reports oxygen and strontium isotope and carbon/pyritic sulphur determinations which are compatible with a marine origin for the carbonates. The Myrtle Springs Formation is the uppermost unit of the Burra Group in the Curdimurka region and, as on Copley, is characterised by greenish siltstone. Places where it can be more conveniently seen are north of Mirra Bore and

east and west of Chintapanna Dam. It conformably overlies the Skillogalee Dolomite, which contains much more carbonate, and it has a similar thickness distribution to the Skillogalee. It is not present in the Willouran Range (near Willouran Hill) presumably because of early glacial or preglacial erosion, but is 1000 m thick to the southeast (Copley 1:250 000) area. It is over 4000 m thick each side of the Bungarider Fault but thins again to the west, where near Mount Norwest it is less than 400 m. A detailed section appears in Forbes (1984). Limited observations of cross-bedding indicate transport in a southeasterly direction. The sedimentary environment appears to resemble that of the Skillogalee, that is, intertidal, but with greater flooding by fine clastics. Rowlands et al (1978) also suggest this environment and recognise shallow tidal channel deposits.

Umberatana Group Because of the anticlinal nature of the central ranges, the Umberatana Group (lower Heysen Supergroup) is exposed mainly in the northeast Willouran Range (3100 m thickness) and near Mount Norwest (3290 m) and in a few synclinal areas within the ranges. The group is composed of lower glacial diamictites, an interglacial shaly sequence and upper glacial diamictites or sandstone. The lower boundary is commonly an unconformity or locally a fault. Conditions were probably largely subtidal marine, influenced early and late in that time by marginal ice sheets. The Bolla Bollana Tillite is tentatively identified as massive diamictite locally present below the Wilyerpa Formation and lying unconformably on the Burra Group. The Wilyerpa Formation is more widespread than the Bolla Bollana and may unconformably overlie it, but there is no convincing evidence for unconformity. The Wilyerpa is characterised by interbedded greenish siltstone, quartzite and diamictite, and reaches its greatest thickness (584 m) in the Mount Norwest range.

Bryan G. Forbes

74 Stratigraphic unit, symbol

Lithology

ADELAIDEAN? ORDOVICIAN Tectonic breccia

Carbonate breccia; Callanna Group and gabbroic blocks

ADELAIDEAN Wilpena Group Ew Wonoka Formation Ewo

Yellow, reddish flaggy limestone, green calcareous shale, siltstone, limestone breccia, sandstone

Thickness (m)

Remarks Partly intrusive, diapiric

19 000+ 1 600+ 300+

West of Mt Norwest; shallow marine

Bunyeroo Formation Ewb

Reddish, greenish siltstone

300

West of Mt Norwest; may contain Wearing Dolomite Member

ABC Range Quartzite Ewa

Grey, reddish flaggy quartzite, shale clasts

1 000

West of Mt Norwest; ? deltaic, cross-bedding

Brachina Formation Ewr

Grey, greenish, reddish siltstone, quartzite

1 000

West of Mt Norwest; flute casts, mud cracks, current lineation; tidal

Nuccaleena Formation Ewn

Pale yellowish, reddish laminated dolomite

Umberatana Group Ph Elatina Formation Ehl

Reddish, white sandstone, siltstone, breccia

Mt Norwest, "Callanna" areas 3 000+ 130

Mt Norwest area and west Northeast of "Callanna"; Yerelina Subgroup; glacial

Mount Curtis Tillite Eec

Bouldery diamictite

200

Fortress Hill Formation Eef

Siltstone with dropstones

760

Enorama Shale Efe

Grey-green silty, dolomitic shale

100

Etina Formation Ehe

Sandy limestone, dolomite

Amberoona Formation Efa

Greenish, grey siltstone; shale; slump breccia

1 700

Erosional base

Brighton Limestone Efh

Limestone, calcitic sandstone

Stuart Shelf

Tapley Hill Formation Eft

Grey laminated 800 siltstone; dark, finely laminated basal dolomite, siltstone (Tindelpina Shale Member); local basal coarse sandstone

Local erosional base; marine

Wilyerpa Formation Euw

Pebbly sandstone, diamictite, green siltstone

Possible Sturt Tillite equivalent; widespread lower glacial beds

? Bolla Bollana Tillite Eyb

Dark pebbly siltstone, sandstone

Table 1: Pre-Mesozoic Stratigraphy, Willouran Ranges

580 200

Farina Subgroup West of Mt Norwest

Limited distribution

Geology of the Willouran Ranges

Stratigraphic unit, symbol

Lithology

Burra Group Pb Myrtle Springs Formation Ebm

Green siltstone, sandstone, grey dolomite

Skillogalee Dolomite Pmk

Dark dolomite, magnesite, sandstone, siltstone

Thickness (m)

Remarks

9 000 1 500

Widespread in Willouran Ranges

2 0003 700

Widespread in Willouran Ranges; lagoonal to shallow marine

UNCONFORMITY

Emeroo Subgroup Witchelina Quartzite Eow

Flaggy to medium800 bedded feldspathic quartzite, siltstone; local carbonate

Prominent ridges in Willouran Range; ? deltaic

Willawalpa Formation Poi

Flaggy to platy quartzite, siltstone, minor dolomite

2 200

Ripple marks, mud cracks, lenticular bedding; possible equivalent of River Wakefield Subgroup; ? tidal

Top Mount Sandstone Beds Pot

Medium-bedded to platy quartzite, siltstone, dolomite, shale

750

Ripple marks, mud cracks, halite and ?gypsum casts, wavy bedding

6 000+

Callanna Group Pk Curdimurka Subgroup Boorloo Siltstone Pkb.

Grey siltstone, dolomite, limestone, minor sandstone Grey, greenish flaggy siltstone, minor sandstone

4 700 520

? Marine, evaporitic

780

Halite, gypsum casts, mud cracks; ? lacustrine

Hogan Dolomite Pkh

Light brownish, grey flaggy to medium-bedded dolomite, partly sandy; chert, sandstone, siltstone

640

Stromatolites, cauliflower chert, ripples; ? sabkha

Recovery Formation Pkr

Grey, greenish flaggy siltstone, sandstone, minor dolomite

2 220

Halite, gypsum casts; mud cracks, ripple marks; lacustrine or tidal; evaporitic

Dunns Mine Limestone Pku

Medium-bedded to flaggy limestone, partly sandy; sandstone, chert

330

Wavy bedding, lenticular bedding; ? marginal marine, sabkha

Rook Tuff Pkk

Dark tuffaceous siltstone, sandstone, minor carbonate

Dacite flow, Willouran Range

Dome Sandstone Pkd

Thin to medium-bedded 1 480 quartzite, greenish siltstone, limestone; partly hematitic, pebbly

Cooranna Formation Pkc

Ripple marks, crossbedding, mud cracks; ? fluviatile to deltaic

Arkaroo la Subgroup Noranda Volcanics Pan

Altered basic lava

? Equivalent of Wooltana Volcanics

Black Knob Marble Pab

Laminated grey to white calcitic marble

Distorted blocks in diapirs

Bryan G. Forbes

Geology of the Willouran Ranges

The Farina Subgroup is typified by grey shale and siltstone of the Tapley Hill Formation and Amberoona Formation and includes the Enorama Shale (and Brighton Limestone on the Stuart Shelf). Near the Boorloo Copper Mine the Wilyerpa Formation is overlain with angular unconformity by coarse sandstone which passes transitionally up into dark siltstone of the Tindelpina Shale Member of the Tapley Hill Formation. The pebbly sandstone is unit B3 of Rayner & Rowlands (1980), and is thought to be deltaic. South of Glen Hill the basal Tindelpina Shale is a sedimentary breccia two metres in thickness lying with angular unconformity on Myrtle Springs Formation. The Amberoona Formation widely shows pronounced angular unconformity with the Tapley Hill Formation or older rocks and this has been related to large-scale slumping (Coats & Dalgarno 1986). The erosional base of the Amberoona is most evident near the Bungarider Fault: from 3.5 to 1 km south of Chintapanna Dam the base descends through Tapley Hill Formation and Wilyerpa Formation on to Myrtle Springs Formation. Two kilometres northwest of North Bungarider Dam the Amberoona siltstones contain an isolated block of dolomite 6 by 12m, possibly attributable to slumping.

Wilpena Group The Wilpena Group (upper Heysen Supergroup) has limited exposure (285m thickness) in a syncline northeast of "Callanna" but is more widely evident in scattered outcrops west of Mount Norwest (1640 m) and near Stuart Creek. The Brachina Formation appears in scenic gorges

and waterholes of Stuart Creek west of "Stuart Creek" (west of the area shown in Fig. 1). Flat-lying Brachina Formation in the Stuart Creek gorges (Belperio, pers. comm.) shows features characteristic of an intertidal environment. The rocks include flaggy reddish and greenish laminated micaceous siltstone, sandy siltstone, silty sandstone, fine-grained sandstone with carbonate patches, and quartzite. Sedimentary structures are rippled trending 030degrees, ripple-drift cross-lamination, starved ripples, interference ripples, tabular cross-beds, herring-bone crossstratification, shallow sand channels, streaming lineation trending 120degrees, 090degrees, flutes indicating an easterly current flow, mud cracks and mud balls. Fine sandstones northeast of "Stuart Creek" have similar features and also display problematic circular structures on bedding planes, and tubular structures. In western exposures the Brachina Formation contains much cross-bedded quartzite with clay pellets, and cracks and streaming lineation. Quartzite members of the Brachina may represent intertonguing ABC Range Quartzite.

INTRUSIVE ROCKS This refers to tectonic breccias and igneous rocks within structural and diapiric complexes mainly associated with northwesterly-trending fracture systems. Tectonic breccias are of variable grain size and composition and may form the main part of what appear to be intrusive, fault-bounded, bodies such as that west of the Boorloo Copper Mine (Breaden Hill Diapir) or may form a minor

Fig. 3 Reproduction of an aerial photograph (S. A. Department of Lands, Survey 209) showing part of the Willouran Range east of "Callanna". Numbered features are as follows: l.Top of the Recovery Formation (Callanna Group) and base of the Hogan Dolomite; 2.Base of the Cooranna Formation; 3.Base of the Boorloo Siltstone; 4.Contorted blocks of the Black Knob Marble within tectonic breccia (Breaden Hill Diapir), just east of well bedded carbonates of the uppermost known Boorloo Siltstone; 5.Coarse basal arkose of the Tapley Hill Formation, with copper carbonate (Boorloo Copper Mine), lying unconformably on diamictite of the Wilyerpa Formation; and 6.Fault-bounded intrusion of tectonic breccia cutting across Tapley Hill Formation and probably an off-shoot of the Breaden Hill Diapir.

Bryan G. Forbes

matrix between large blocks of folded rock such as in the Rischbieth structural complex. A common breccia matrix is pale coloured and rich in carbonate, described by Dewar (1974) as an impure calcareous rock with sand-sized and larger fragments and sometimes containing areas of coarsely-recrystallised carbonate. Thomas & Dunlop (1968) record a micaceous, calcitic and dolomitic matrix of breccia in the Breaden Hill Diapir. This rock is generally poorly exposed. Similar intrusive breccias are described in the Peake and Denison region (Ambrose et al 1981) and the Worumba Anticline (Preiss 1985). Time of intrusion may have been as early as Torrensian, but also post-dated deposition of the Amberoona Formation. Torrensian movement of breccia or related faults can possibly be inferred from thickness variations in the Skillogalee Dolomite and the thickening of the Witchelina Quartzite in the southwestern Marree map area. This may be analogous to basins adjacent to rising diapirs modelled by Lemon (1985). Gabbroic and associated igneous rocks, like other megabreccia elements, occur as scattered small bodies within the tectonic breccias. Dewar (1974) noted in some southeastern diapiric zones that dolerite bodies (one with a quartzite xenolith) are cut by faults filled with intrusive breccia, suggesting some movement after emplacement of the dolerite. Brecciation has also been observed in gabbroic rocks in the Mirra Diapir southwest of "Callanna", in the West Bungarider Diapir and in the Rischbieth area (Parker 1983). The dolerites commonly appear as rounded, plug-like bodies in plan, but may also occur as elongate bodies. Sills have been observed within a block of ?Cooranna Formation within megabreccia northwest of Kingston Dam and were possibly emplaced prior to dislodgement of the block. Northwest of Glen Hill a northwest-trending dolerite dyke 30 m wide cuts a gabbro mass 300 by 100 m in plan. The gabbro lies within brecciated siltstone, carbonate and sandstone of the Callanna Group. A complex of gabbroic rocks ("Euromina Window" of Murrell 1977) is over 1 km in length, near the edge of the Mirra Diapir, but contains a variety of rock types including

gabbro, albitite, porphyry (trachyte or trachyandesite 6438RS128) and scapolitic rocks. Preiss (pers. comm.) described a probable intrusive contact here, and some brecciation of igneous rocks, and suggested that the porphyry could be hematised and partly albitised Rook Tuff. A number of previous workers have suggested the sodic nature (e.g.albite, scapolite component) of some gabbroic rocks to have resulted from reaction with the evaporitic content of the Callanna Group (Pontifex & Rowlands in Rowlands et al 1978, 1983; R.Morrison pers. comm.). Other intrusive bodies are albitic dykes and sills, few of which have been observed by the writer. One example is a narrow carbonateveined dyke cutting quartzite of the Wilyerpa Formation adjacent to intrusive breccia near the Boorloo workings. Southwest of The Dome a pale 1m thick cupriferous dyke cuts dolerite. Another better-documented example is a sill (5m thick, Marree 1:250 000 map area, southeast of Kingston Dam; Forbes 1966, p. 10) of micaceous albitite within Willawalpa Formation. Thick veins of crystalline carbonate are a common feature of megabreccia zones and adjacent areas. Southwest of Mount Norwest, northeast-trending fractures are commonly filled with carbonate up to a width of 30cm. Minor cupriferous quartz-carbonate veins cut some gabbroic rocks.

STRUCTURAL GEOLOGY The larger elements in the pre-Mesozoic structural framework of the region are the Stuart Shelf in the west, the Torrens Hinge Zone west of the Norwest Fault, a region of thick, folded and fractured Adelaidean (Delamerian fold belt, Adelaide Geosyncline) forming the ranges, and the Muloorina Ridge in the east. Most structural elements trend northwesterly. As indicated by Milton & Morony (1975), the Muloorina Ridge is reflected in a gravity high and is paralleled to its southwest by a gravity low corresponding to the Adelaide Geosyncline-Delamerian fold belt. Variations within the gravity low are interpreted to be related to diapiric structures. The Norwest

Geology of the Willouran Ranges

Fault separates areas of differing gravity and magnetic properties and an estimate of depth to magnetic basement suggests a downward displacement of the order of 4000 m on the southwest side. The Stuart Shelf is inferred to extend westwards from the vicinity of the Stuart Creek gorge, west of "Stuart Creek", where there is gently-dipping Brachina Formation cut by northerly-trending faults. In the Torrens Hinge Zone to the east of there Adelaidean rocks are folded and thicken. The Norwest Fault occurs in a zone of markedly increased thickness in the lower Adelaidean (Warrina Supergroup) and where there has been upward movement of disturbed Callanna Group in a complex, fractured anticlinorium. This is the westernmost zone of diapiric activity. Northwest and southeast of Black Knob there are large segments of faultbounded steeply-dipping east-facing quartzite of the Dome Sandstone. As suggested by Dalgarno (1966) a nappe-like structure here suggests possible southeasterly movement of an eastern block, which may also have influenced folding northeast of Top Mount Well. The segments of Dome Sandstone are surrounded, as at Black Knob, by strongly disturbed sandstone and carbonate of the Callanna Group. The terms Stony Range, Rocky Point, High Hill and Clara St Dora Diapir have been applied by Dewar (1974) to the more disturbed zones along the Norwest Fault. The Clara St Dora Diapir comprises an inner core of megabreccia which includes blocks of Black Knob Marble, one of which is host for the Clara St Dora Copper Mine, surrounded by strongly disturbed quartzite and siltstone correlated with the Recovery Formation. Sprigg (1950) described the Norwest Fault as a sinistral tear fault. The West Bungarider Diapir, between the Norwest and Bungarider Faults, sharply intrudes generally east-facing Skillogalee Dolomite. It is probably the simplest diapiric structure in the

region and is associated with a minor parallel synclinal fold on its southwestern margin. The Bungarider Fault, with associated Rischbieth structural complex and other breccia zones, lies at the western edge of a platform-like area of moderately folded Burra Group rocks (cross section AA'; Fig. 2). The Rischbieth complex resembles the complex around Black Knob in that it contains large folded segments of Dome Sandstone interspersed with tectonic breccia. The margins are faults, with no evidence of sedimentary unconformities. If the stratigraphic elements within the structure have been correctly identified, there must have been considerable disruption, since the stratigraphically well-separated Dome Sandstone and Boorloo Siltstone have been brought close together. Parker (1983) reports overprinting of earlier tight to isoclinal folds within the Rischbieth complex by later folding of probable Delamerian age. The Bungarider, West Bungarider and Norwest fractures approach each other to the northwest near Clara St Dora Diapir. The eastern edge of the 'platform' is bounded by a sinuous fracture zone extending from the "Callanna" area southerly toward Twenty Mile Hill. The major fault forming the western limit of the Emeroo Subgroup in the Willouran Range does not seem to have been named by earlier workers and is termed here the West Willouran Fault (close to the West Willouran copper prospect). Thomas & Dunlop (1968) suggest some strike-slip movement on this fault, evidenced by drag effects in sediments to the west, perhaps indicating east block moving southeasterly. The South Hill "dextral fault" of Sprigg (1950) appears to involve northward movement of an eastern block (Andamooka 1:250 000 area). Thick sedimentation near this may be associated with sedimentary sinks near a diapir (Preiss, pers. comm.). Megabreccias asociated with the fracture zone cut across or intrude rocks as young as Amberoona Formation (Sturtian). Relationships of the West Willouran Fault suggest an easterly dip as shown in Fig. 2. The easterly dip of this fault is consistent with the

Bryan G. Forbes

intrusion of megabreccia on its western side. By analogy the concentration of megabreccia zones on the eastern side of the Bungarider and Norwest Faults would suggest westerly dip of these faults at depth. Another explanation might be that they acted as barriers to westerly-flowing pulses of breccia. Coats (1973) shows the Norwest Fault dipping easterly in the Copley area, with diapiric intrusion immediately on the western side. Regional folding is ascribed to the Delamerian Orogeny which produced generally northwesterly-trending axial planes with a vertical or steeply northeasterly-dipping attitude. There is a host of minor fractures cutting the pre-Mesozoic rocks. These fractures are mostly oriented at a high angle to the main faults and may be clustered into fracture zones as near Mirra Bore and Mirra Dam. There is a minor strike-slip component of movement on these. Some similarly-oriented fractures are carbonate-filled and may thus represent a late tensional phase in the Delamerian Orogeny. ECONOMIC GEOLOGY Although there has been much exploration for minerals in the Willouran Ranges and other preMesozoic prospects, only small deposits of copper and gold have so far been discovered and there are no prospects being currently worked. Exploration has been directed mainly toward copper, including Olympic Dam-type deposits, but there have also been unsuccessful searches for diamonds. Anomalous copper, zinc and lead have been reported from some formations (e.g.up to 0.47 per cent Cu in the Rook Tuff-Dunns Mine Limestone, 0.11 per cent Cu in the Boorloo Siltstone: Rowlands et al 1983), but economic concentrations of metals are largely related to quartz-carbonate veins which developed during folding and faulting, principally Delamerian. A number of mines in post-Willouran rocks lie just east of megabreccia zones. Numerous workings

southeast of "Callanna", including the Rook, Dunns, Dome, Euchre Pack and Callanna Mine, follow fractured Dunns Mine Limestone adjacent to intrusive breccia. The Clara St Dora Copper Mine was worked in the 1890's as a series of open cuts and shallow shafts (Brown 1908, p.42) and at one stage had produced over 30t ore containing 39 percent Cu. Chalcocite, cuprite, copper carbonates and chalcopyrite fill fractures and vugs in blocks of Black Knob Marble (impure dolomite) and also occur in softer adjacent calcitic rocks, near the edge of the central breccia core of the Clara St Dora Diapir. This setting is analogous to the Blinman Copper Mine, within a carbonate block near the edge of the Blinman Dome Diapir (Coats 1964). Wells (1976) reports that the mine was worked for 21 years, up to 1915, producing over 2000t ore, worth over $18 000, and of generally high grade. The Warra Warra Copper Mine (Miles 1952; Dalgarno 1966) is the most extensive in the region and consists of shafts up to 68 m deep and open cuts extending along the southern limb of an easterly-pitching anticline in Skillogalee Dolomite. Malachite, native copper, chalcopyrite and cuprite are associated with quartz-limonite veins parallel to bedding in silty calcareous shale and sandstone. Secondary enrichment extends down to 18 m depth. Reported production is less than 180t ore containing up to 20-24 percent Cu. The West Willouran copper workings (with some associated alluvial gold) occur with quartzhematite veins in tightly folded and fractured Top Mount Sandstone Beds just east of the West Willouran Fault. The Rook copper workings lie east of Mirra Bore. Secondary copper minerals are associated with quartz, hematite and carbonate veins in sheared and partly brecciated carbonate, siltstone and sandstone of the Curdimurka Subgroup (partly Dunns Mine Limestone) just west of the West Willouran Fault.

Geology of the Willouran Ranges Dunns Copper Mine (or Willouran Mine) is associated with the Dunns Mine Limestone, similarly to the East Rook workings. Recorded production is 1411 of 2.4-8.5 percent Cu (18801883). Copper carbonates occur on joint faces and there are ferruginous quartz-carbonate veins sub-parallel to strike (Sampey & Driessen 1966a). Pits follow the lode for more than 3 km. Some open cuts are in fault breccia (Dewar 1974). The Callanna Mine (just southeast of "Callanna ) comprises shallow pits in folded Dunns Mine Limestone with ironstone, quartz and minor copper mineralisation. Utah (Rowlands et al 1983) regarded this area as the most prospective for stratiform copper. In a drillhole (WP1220) north of the mine they note 1.2 percent Cu over small intervals containing pyrite, pyrrhotite and chalcopyrite. M

DISCUSSION Earliest known sedimentation in the Willouran Ranges is evidenced by the Black Knob Marble, possibly representing a shallow, partly evaporitic marine shelf about 850 Ma ago. This is inferred to be followed by crustal fracturing and attendant basaltic volcanism (Noranda Volcanics) by analogy with the Peake and Denison region and the Arkaroola area (Preiss 1985). Within the resultant grabens there was deposited a thick, partly lacustrine and evaporitic sedimentary sequence represented by the Curdimurka Subgroup. Later periodic marine incursions gave rise to the carbonate and magnesitic beds of the Skillogalee Dolomite within a fault controlled series of basins trending northwesterly and bordered to west and east by the Stuart Shelf and Muloorina Ridge.

Thickness variation of the Skillogalee The Boorloo Copper Mine was worked in the suggests differential syndepositional early 1900's and extensively investigated by Dolomite subsidence of fault-bounded sub-basins. Activity Australian Selection, Noranda and Utah (Sampey of basement faults and sediment loading may & Driessen 1966a; Dalgarno 1966; Rayner & initiated lateral and upward movement of Rowlands 1980). Ageochemical anomaly 900 by have evaporite-bearing sediments of the 120 m is centred on sandy conglomerate and less-dense Callanna Group. Intrusion of basic rocks may dolomitic shale of the basal Tapley Hill Forma- also have commenced at about this time and tion (Fig. 3) containing chalcopyrite, malachite, contributed to upward movement. Contemporary azurite, chalcocite and native copper. Mineralisa- movement is also expressed in erosional surfaces tion is mostly associated with quartz-carbonate below the Umberatana Group and within it. This veins and better intersections encountered over a was accompanied by glacial conditions (Sturtian) few metres during testing include 0.4 percent, up when floating and grounded ice gave rise to to 1 percent, and 1.4 percent. There are some diamictites and marine muds containing stratiform crystalline malachite layers. At depth dropstones. Up to this time, in the Willouran and chalcopyrite is exclusively present, mainly along Torrensian, deposition was probably confined to joints and fractures. Rayner & Rowlands (1980) a trough with western limits in the Torrens Hinge report 2.5 percent Cu over a i m interval in a Zone. Later deposition extended over the Torrens chalcopyrite-pyrite-quartz vein in Utah Hinge Zone and Stuart Shelf. After a marine DDHWD004. They also note fine dissemination incursion under a milder climate there was further of chalcopyrite and pyrite which they regard to glaciation in upper Umberatana time (Marinoan). be of sedimentary derivation within a delta fan. This was sharply followed by shallow marine The Breaden Hill Copper Mine is in a similar conditions with carbonate deposition (Nucstratigraphic and structural position in lower caleena Formation) and sedimentation of interTapley Hill Formation faulted against the tidal sands, silts and minor carbonate (upper Breaden Hill Diapir. Wilpena Group).

Bryan G. Forbes

There is no record of Cambrian rocks in the area of Fig. 1 but there may have been some sedimentation at this time, terminated in the Cambro-Ordovician by the Delamerian Orogeny. This gave rise to folds with generally steep, northwest-trending axial planes and may have re-activated major faults and megabreccias and given rise to minor fractures and quartz-carbonate veins, accompanied in places by copper and gold mineralisation. The megabreccias of the Willouran Ranges possess many of the features of structures labelled "diapirs" elsewhere in the Flinders Ranges, and have seemingly been mobile and intrusive. This, however does not tell the whole story, and there is need for more work such as that carried out in the Rischbieth area by Parker (1983).

ACKNOWLEDGEMENTS Information used here was partly gained during mapping of the Marree and Curdimurka 1:250 000 geological maps with colleagues in the Geological Survey of South Australia, but also rests heavily upon the large bulk of information gathered by geologists with exploration companies and the universities. This paper is published with the approval of the Director-General of Mines and Energy, South Australia. Preparation of figures by the Drafting Branch of the Department of Mines and Energy is gratefully acknowledged.

REFERENCES

BELPERIO A.P. 1987a. Mineralogical, chemical, isotopic and source-rock analyses of Burra Group (Adelaidean) sedimentary rocks from the Willouran Ranges. South Australian Department of Mines and Energy, Report Book 87/40 (unpubl.). BELPERIO A.P. 1987b. Stratigraphic sections measured in Adelaidean (Burra Group) rocks in the Willouran Ranges, CURDIMURKA area. South Australian Department of Mines and Energy, Report Book 87/75 (unpubl.). BELPERIO A.P (this volume). Palaeoenvironmental interpretation of the Late Proterozoic Skillogalee Dolomite in the Willouran Ranges, South Australia. BISCHOFF, B.G.R. 1975. Exploration for base metals in the Willouran Ranges EL 143 South Australia, final report. Exploration Dept, B.H.P. Co. Ltd (for Dampier Mining Co. Ltd). South Australian Department of Mines & Energy open file Envelope 2436 (unpubl.). BROWN H.Y.L. 1892. Report by the Government Geologist on country in the neighbourhood of Lake Eyre. Parliamentary Paper South Australia No. 141. BROWN H.Y.L. 1908. Record of the mines of South Australia. Government Printer, Adelaide. CARTHEW S.J. 1975. Sedimentary environs of stratiform copper mineralization in the Callanna Beds, Willouran Ranges. B.Sc. (Hons) thesis, University of Adelaide (unpubl.). COATS R.P. 1964. The geology and mineralisation of the Blinman Dome Diapir. Geological Survey of South Australia, Report of Investigations 26. COATS R.P. 1973. Copley map sheet, Geological Atlas of South Australia, 1:250 000 series. Sheet SH/54-9. Geological Survey of South Australia.

AMBROSE G.J., FLINT R.B. & WEBB A.W., 1981. Precambrian and Palaeozoic geology of the Peake and Denison Ranges. Geological Survey of South Australia, Bulletin 50.

COATS R.P & DALGARNO R. 1986. Large scale slumping in the Umberatana Group, Willouran Ranges. Geological Society of Australia, Abstracts 15, 223.

BELPERIO A .P. 1986. Stratigraphy and sedimentology of the Precambrian Skillogalee Dolomite, Northern Flinders Ranges. Geological Society of Australia Abstracts 15, 29.

DALGARNO C.R. 1966. Report on Special Mining Leases 111 and 114. Willouran Ranges area South Australia (for Anaconda Aust. Inc.). South Australian Department of Mines and Energy open file Envelope 637 (unpubl.).

Geology of the Willouran Ranges

DALGARNO C.R., COATS R.R & PREISS W.V. 1981. A discussion of the paper by R.A.Rayner and N.S.Rowlands "Stratiform copper in the late Proterozoic Boorloo delta, South Australia". Mineralium Deposita 16, 185-186. DALY S.J. (Compiler) 1970. Curdimurka map sheet, Geological Atlas of South Australia, 1:250 000 preliminary series. Geological Survey of South Australia. DEWAR G.J. 1974. Report on reconnaissance exploration Tarlton Knob (EL52) and Cadnia Hill (EL53) Willouran Ranges, South Australia (for H.R.Gillespie). South Australian Department of Mines and Energy open file Envelope 2289 (unpubl.). FAIRBURN W.A. 1969. Mt Isa Mines Ltd, Willouran Ranges, SML169. South Australian Department of Mines and Energy open file Envelope 1145 (unpubl.). FANNING C.M., LUDWIG K.R., FORBES B.G. & PREISS W.V. 1986. Single and multiple grains U-Pb zircon analyses for the early Adelaidean Rock Tuff, Willouran Ranges, South Australia. Geological Society of Australia, Abstracts 15, 71-72. FARRAND M.G. & PARKER A.J. 1986. Callanna Group basic volcanics and sediments in DDHWD-038, Rischbieth structural complex, Willouran Ranges. South Australian Department of Mines and Energy, Report Book 86/82 (unpubl.). FORBES B.G. 1960. Magnesite of the Adelaide System: petrography and descriptive stratigraphy. Transactions of the Royal Society of South Australia 83, 1-9. FORBES B.G. 1961. A photogeological study of the Coober Pedy-Lake Eyre region. South Australian Department of Mines and Energy, Report Book 53/142 (unpubl.).

FORBES B.G. 1984. Progress in mapping the Precambrian of the Curdimurka area. South Australian Department of Mines, Report Book 84/33 (unpubl.). FORBES B.G., MURRELL B. & PREISS W.V. 1981. Subdivision of lower Adelaidean, Willouran Ranges. Quarterly Geological Notes, Geological Survey of South Australia 79,7-16. GILLESPIE H.R. 1974a. Aurora Oil N.L., EL52 and EL53. Tarlton Knob progress reports. South Australian Department of Mines and Energy open file Envelope 2289 (unpubl.). GILLESPIE H.R. 1974b. Aurora Oil N.L., EL53, Cadnia Hill. South Australian Department of Mines & Energy open file Envelope 2290 (unpubl.). HOWCHIN W. 1926. The Sturtian Tillite in the Willouran Ranges, near Marree (Hergott), and in the northeastern portions of the Flinders Ranges. Report of the Australian and New Zealand Association for the Advancement of Science 17, 67-76. LEMON N.M. 1985. Physical modeling of sedimentation adjacent to diapirs and comparison with late Precambrian Oratunga breccia body in central Flinders Ranges, South Australia. American Association of Petroleum Geologists, Bulletin 69, 1327-1338. MAWSON D. 1927. Geological notes on an area along the northeastern margin of the northeastern portion of the Willouran Range. Transactions of the Royal Society of South Australia 51, 386-390. MILES K.R. 1952. Reconnaissance mineral survey of the Willouran Ranges. South Australian Department of Mines and Energy Report Book 34/81 (unpubl.).

FORBES B.G. 1966. The geology of the Marree 1:250 000 map area. Geological Survey of South Australia, Report of Investigations 28.

MILTON B.E. & MORONEY G.K. 1975. A regional interpretation of 1:1 000 000 gravity and aeromagnetic maps of the Great Artesian Basin in South Australia. Geological Survey of South Australia, Report of Investigations 46.

FORBES B.G. 1980. Three sections measured in lower Adelaidean (Proterozoic) rocks, southeastern Curdimurka area. South Australian Department of Mines and Energy, Report Book 79/159 (unpubl.).

MURRELL B. 1977. Stratigraphy and tectonics across the Torrens Hinge Zone between Andamooka and Marree, South Australia. Ph.D. thesis, University of Adelaide (unpubl.).

Bryan G. Forbes

PARKER A J . 1983. Tectonic development of the Adelaide fold belt. Geological Society of Australia Abstracts 10, 23-28. PREISS W.V. 1971. Geological reconnaissance and stromatolites of the Northern Flinders and Willouran Ranges. South Australian Department of Mines and Energy Report Book 71/173 (unpubl.). PREISS W.V. 1973. Palaeoecological interpretations of South Australian Precambrian stromatolites. Journal of the Geological Society of Australia 19, 501-532. PREISS W.V 1985. Stratigraphy and tectonics of the Worumba Anticline and associated intrusive breccias. Geological Survey of South Australia, Bulletin 52. RADKE F. 1980. Petrography of two basic rocks. Amdel report GS920/81 (unpubl.). RAYNER R.A. & ROWLANDS N.J. 1980. Stratiform copper in the Late Proterozoic Boorloo delta South Australia. Mineralium Deposita 15, 139-149. ROWLANDS N.J., BLIGHT P.G., JARVIS D.M. & VON DER BORCH C.C. 1980. Sabkha and playa palaeoenvironments in Late Proterozoic grabens, Willouran Ranges, South Australia. Journal of the Geological Society of Australia 27, 55-68. ROWLANDS N.J., JARVIS D.M„ TEDDER I.J., RAYNER R.A. & BLIGHT P.G. 1978. Utah Development Company, EL277 Willouran Ranges. South Australian Department of Mines and Energy open file Envelope 2915 (unpubl.). ROWLANDS N.J., JARVIS D.M., RAYNER R.A., BLIGHT P.G., MANN S.T. & CIRCOSTA G. 1983. Utah Development Co. EL461, 850, Willouran Ranges. South Australian Department of Mines and Energy open file Envelope 3507 (unpubl.). RUKER R. 1966. Memorandum report photogeological and field evaluation Willouran SML111 and 114 South Australia (for Anaconda Aust. Inc.). South Australian Department of Mines and Energy open file Envelope 637 (unpubl.).

SAMPEY D. & DRIESSEN A.J.B. 1966a. Australian Selection Pty Ltd SM165, Willouran Ranges. South Australian Department of Mines and Energy open file Envelope 389 (unpubl.).

SAMPEY D. & DRIESSEN A.J.B. 1966b. Australian Selection Pty Ltd SM170. Tarltons Knob. South Australian Department of Mines and Energy open file Envelope 599 (unpubl.). SCOULAR G. 1887. Sketch of the geology of the southern and western parts of the Lake Eyre Basin. Transactions of the Royal Society of South Australia 9, 39-54. SPRIGG R.C. 1950. Thrust structures of the Witchelina area, South Australia. Transactions of the Royal Society of South Australia 73,40-47. THOMAS A. & DUNLOP A. 1968. Report on Special Mining Lease No. 165 Breaden Hill area Willouran Ranges, South Australia (for Noranda Aust. Pty Ltd). South Australian Department of Mines and Energy open file Envelope 884 (unpubl.). UPPILL R.K. 1979. Stratigraphy and depositional environments of the Mundallio Subgroup (new name) in the late Precambrian Burra Group of the MtLofty and Flinders Ranges. Transactions of the Royal Society of South Australia 103, 25-44. UPPILL R.K. 1980. Sedimentology of the Late Precambrian Mundallio Subgroup: A clastic-carbonate (dolomite, magnesite) sequence in the Mt Lofty and Flinders Ranges, South Australia. Ph.D. thesis, University of Adelaide (unpubl.). WEBB B.P., HORWITZ R.C.H. & COATS R.P. 1963. Callanna map sheet, Geological Atlas of South Australia, 1:63360 series. Geological Survey of South Australia. WELLS R. 1976. Mines in the Willouran Ranges. South Australian Department of Mines and Energy open file Envelope 3593 (unpubl.).

Palaeoenvironmental interpretation of the Late Proterozoic Skillogalee Dolomite in the Willouran Ranges, South Australia A.P. Belperio South Australian Department of Mines and Energy, P.O. Box 151, Eastwood, SA. 5063, Australia The Skillogalee Dolomite is a thick (up to 4 000 m), predominantly peritidal carbonate sequence (dolomite, magnesite) of the Late Proterozoic Adelaide Geosyncline, representing a major marine inundation of an evolving, intracratonic rift basin. In the Willouran Ranges, rapid deposition occurred in a complex of sub-parallel, differentially subsiding, extensional sub-basins. Sedimentation kept pace with subsidence to maintain a subdued topography within the basin. Repetitive shoaling and exposure resulted in a change from subtidal stromatolite biostromes and cryptalgal laminates through intertidal oncolites, intraclastic dolomites and dolomitic grainstones, to extratidal intraformational magnesite and continental arenaceous deposits. Syn- and post-depositional piercement diapirism of underlying strata occurred preferentially along bounding faults. Strontium isotope values of cryptalgal dolomites and magnesites (0.7090-0.7119) are similar to those for other Precambrian carbonates of accepted marine origin. Cryptalgal dolomites have oxygen isotopic values of between -5 and -10 per mil versus PDB, also compatible with a marine origin. Some samples indicate evaporative enrichment, consistent with the sedimentological evidence that indicates intermittent exposure and desiccation. Intraformational magnesite-mud-pellet conglomerates are enriched in 1 8 0 and 13C relative to coeval dolomites. The lateral and vertical uniformity of carbonate mineralogy, fabric detail and preserved sedimentary structures indicate a primary sedimentary or penecontemporaneous origin of dolomite and magnesite. Key words: Stromatolite, dolomite, magnesite, Late Proterozoic, rift, diapir, isotopes, Burra Group, Adelaide Geosyncline. INTRODUCTION Intracratonic and marginal cratonic, rift-related sequences of Late Proterozoic age occur through much of central Australia (Preiss & Forbes 1981). In the Adelaide Geosyncline (Fig. 1), up to 15 km of mainly shallow water, Late Proterozoic sediments were deposited on rifted Early and Middle Proterozoic basement. The preserved strata record an evolutionary sequence spanning some 600 m.y., from pre-rift mafic dyke intrusion and basaltic volcanism (Parker 1983), to discontinuous syn-rift intracratonic graben sedimentation (Rowlands et al 1980), and subsequent epeirogenic shelf and continental margin sedimentation (Von der Borch 1980; Rutland et al 1981; Preiss 1983). The extension and subsidence mechanism that initiated the rifting is still unclear, but is believed by Von der Borch (1980) to be fundamentally similar to the pre-oceanic

rifting process that formed many contemporary passive continental margins. Dichotomy of opinion exists for the finer details of the palaeogeography and tectonosedimentaiy evolution of the Adelaide Geosyncline, and in particular for the Burra Group and Skillogalee Dolomite (Fig. 2). Very thick deposits, of between 7 000 and 10 000 m, are known from the Willouran Ranges and the Peake and Denison Ranges (Ambrose et al 1981; Belperio 1986). The Group is constrained by radiometric ages of 802±10 and 750±53 Ma for older volcanics and younger shales respectively (Fanning et al 1986). The Skillogalee Dolomite is a prominent, repetitive siliciclasticstromatolitic dolomite-intraclastic magnesite succession within the Burra Group. Forbes (1960, 1961), Preiss (1973) and Preiss & Forbes (1981) considered the stromatolites and dolomite to be

A.P. Belperio

essentially of shallow marine origin, with magnesite forming in marginal alkaline lagoons, and ascribed the cyclicity to repeated marine transgressions and regressions. Conversely, Murrell (1977), Von der Borch (1980) and Uppill (1980, 1983) considered the sequence to be dominantly or entirely non-marine, essentially cyclic playa lake and perennial lacustrine deposits. Von der Borch (1980) equated the deposits with an early rift setting not unlike the present East African Rift Valley. He considered continental break-up and the development of fully marine conditions did not occur until about the Precambrian-Cambrian boundary time.

BURRA GROUP SEDIMENTS WARBURTON^, BASIN^^

Dominantly siliciclastics Mixed quartz-carbonate grainstones Cyclic p e r i t i d a l carbonates

Rutland et al (1981) envisaged the Adelaide Geosyncline as representing only the western part of a multiple-rifted arch system that extended several hundred kilometres to the east. Facies gradients within the Burra Group were thus considered to reflect an evolving continental shelf with a continental margin well to the east. The "break-up" unconformity separating rift and postrift sediments was placed by Preiss (1983) at the top of the Burra Group sequence, some 200 m.y. prior to that selected by Von der Borch (1980). The purpose of this study is to examine in detail the stratigraphy and sedimentology of the Skillogalee Dolomite and its lateral variation within the Willouran Ranges. Specific data on facies development and palaeogeographic setting for this region, particularly a marine or nonmarine interpretation for the Skillogalee Dolomite, are pertinent to the overall tectonostratigraphic interpretation of the Adelaide

Faults and lineaments

sediments

750 ± 5 3 M a (T.R. Rb-Sr) unconformity

M Y R T L E S P R I N G S F O R M A T I O N (Bbm)

/, A 7 7

/ / /, 7 7 7

Z I 7 /

7 Z T

7 7 / , / , / , / A 7 / A 7 / / Z Z

777"

Qor. ZD

/'/;/'/ 7 v 7 /wywl

S K I L L O G A L E E D O L O M I T E (Bmk)

Tilterana Sandstone (Bmkt) Camel Flat Shale (Bmkc) E M E R O O S U B G R O U P (Bo) structural break 802 ± l O M a (U-Pb Zircon)

Stromatolite b i o s t r o m e __

Fig. 1 Location of the Willouran Ranges and distribution of thicker Burra Group sediments in central South Australia. Structural elements of the Adelaide Geosyncline modified after Flint & Parker (1982).

Z Z Z

Dolomite

Intraclastic magnesite _

Sandstone

Volcanics_

Siltstone

Drn E.A

7777/ ~ZZL

87-619 SADME

Fig. 2 Schematic Burra Group lithostratigraphy and age constraints for the Willouran Ranges.

Palaeoenvironments of the Skillogalee Dolomite

Geosyncline. In addition to geological field mapping, twelve detailed stratigraphic sections of Burra Group strata were measured at select localities across the ranges. Features recorded on these transects included lithology and mineralogy of sediment and intraclasts, biogenic and sedimentary structures, palaeocurrent directions, desiccation criteria, and nature of bed contacts. Samples from representative lithofacies were collected for petrographic examination, X-ray diffractometry, carbonate and organic carbon analysis, geochemistry, isotopic measurement and hydrocarbon source-rock analysis. Details of methodology and results are given in Belperio (1987a, b, c, d).

REGIONAL SETTING The Willouran and Peake and Denison Ranges are inliers of folded and fractured Proterozoic (predominantly Adelaidean) sedimentary rocks, that delineate a northwesterly extension of the former Adelaide Geosyncline (Fig. 1). An associated gravity low, and sub-parallel gravity highs flanking both sides of the Willouran Ranges, define the former narrow depositional trough that was continuous through to the Peake and Denison Ranges. Within this zone, deposition of Burra Group sediments occurred in a discontinuous series of half-grabens (Murrell 1977). The Burra Group represents a major tectonosedimentary cycle of siliciclastic and carbonate deposition bounded above, and possibly below, by major hiatuses (Fig. 2). A three-fold subdivision of the Burra Group (Emeroo Subgroup, Skillogalee Dolomite, Myrtle Springs Formation) has been applied in the Willouran Ranges (Forbes 1984). The basal Emeroo Subgroup is an arenaceous sequence of thick sandstones, quartzites, siltstones and shales. The base is taken as the first major sand body above the evaporative carbonates and clastics of the more deformed Callanna Group sediments (Forbes & Preiss 1987). A phyllitic mudstone marker bed (the Camel Flat Shale of Murrell 1977) is taken as the base of the carbonate-

dominated Skillogalee Dolomite. Together with the Tilterana Sandstone (Fig. 2), these comprise a "basal siliciclastic fades" to the Skillogalee Dolomite. The main phase of sedimentation for the Skillogalee Dolomite is characterised by a repetitive sequence of dolomites, magnesites and arenaceous beds. The overlying Myrtle Springs Formation is recognised by the return to dominance of carbonate-poor sandstones and siltstones.

LITHOLOGIES The Skillogalee Dolomite is characterised by repetitive (but not cyclic) lithologies of i n f o r mational carbonates (dolomite and magnesite), cryptalgal laminated carbonates and argillaceous siltstones and sandstones. Diagenetic chert replacement is common but geographically and stratigraphically variable, being most abundant in the cores of stromatolites. Also present, but more rare, are oncolites, algal bioherms, halite and shortite moulds, siliciclastic conglomerates and sedimentary chert beds. Desiccation features such as mudcracks, tepees and disrupted carbonate crusts are abundant, as are wavy and flaser bedding and trough cross-stratification. Field classification of sediments is corroborated by mineralogical analysis using X-ray diffractometry (Fig. 3) and chemical analysis (Belperio 1987a). Dolomitic grainstones, cryptalgal dolomites and stromatolites have a variable composition depending upon the degree of incorporation of silicate grains (predominantly quartz and feldspar). Pelletal magnesites also have a variable matrix of finer grained clastic dolomite and silicates. Finely laminated micritic dolomites (dolosiltite and dololutite) with crenulated to stratiform organic laminae (ciyptalgal dolomites) are fossil algal laminites analagous to modern peritidal algal sediments (Scholle etal 1983). Stromatolite biostromes, up to 2m thick, represent greater development of algal growth structures. Algal layers are now represented by very fine dolomite

A.P. Belperio

crystals, partly obscured by organic staining and carbonised organic matter (micrinite). Sub-parallel bands of detrital quartz, dolomite, feldspar and mica separate the organic layers. The columnar stromatolite Baicalia burra is virtually ubiquitous (Preiss 1973). "Reef-like" stacking of biostromes occurs in marginal areas that presumably underwent slower subsidence, although discrete bioherm structures are rare. Thin interbedded quartzose siltstones and sandstones consist of detrital quartz, feldspar and mica with a variable content of detrital dolomite grains and pellets. Sorting varies from good to poor and small-scale ripples and trough crossbedding are common. Mudcracks and euhedral cubic cavities after evaporite minerals are abundant and most beds are in sharp contact with overlying and underlying carbonates.

quences up to 3.5 m thick, and grade up to 98% pure magnesite (Belperio 1987a). They consist of a framework of aphanatic magnesite mudstone clasts (typically 2-20 mm) in a poorly sorted sandy matrix of variable dolomite-magnesitequartz composition (Fig. 4). Intermixing of dolomite and magnesite clasts is common. Talc and authigenic feldspar are also common constituents. Magnesite clasts show variable shape and rounding, and little internal structure. Rarely, a disrupted, micritic, laminated magnesite crust is preserved at the base of some intraformational

Magnesite-mud-pellet conglomerates are abundant, particularly on the western side of the study area. Individual magnesite beds are up to 1 m thick, with composite torrent-bedded seMAGNESITE AND TALC

•

// •

**\ \

• •

•

" O

——QQ

\ ° o

\ QQ

SILICATES

DOLOMITE FIELD C L A S S I F I C A T I O N

Chert Siliciclastics Pelletal magnesite. Drn E.A.

_ •

dolomites and dolomitic grainstones

87-620 SADME

Fig. 3 Mineralogy of 35 Skillogalee Dolomite samples from Belperio (1987a). Ternary plot of normalised XRD peak heights.

Fig. 4 Outcropping, 30 cm thick magnesite conglomerate bed consisting of coarse grained, micritic magnesite pellets in a matrix of finer magnesite, dolomite and silicate grains (SADME negative 36147). Sharply overlies finely laminated cryptalgal dolomite. Overlain by a sedimentary chert bed consisting of peloids of chalcedonic silica, detrital quartz and feldspar and minor micritic magnesite and dolomite clasts in a light grey, siliceous, sandy matrix. Chert is sample 6438 RS381 in Table 1. For stratigraphic position, see Fig. 8.

Palaeoenvironments of the Skillogalee Dolomite

beds. Torrent bedding, graded bedding and inverse-graded bedding are all common, as are basal mudcracks, tepees, shrinkage cracks, plastic injection features and rip-up structures. Chert occurs as primary nodules, nodular conglomerates and lenses within the sedimentary sequence, but more commonly as a replacement silicification in the cores of stromatolites. Chert nodules are complexly intertwined with a dolomite and magnesite matrix. In addition to the predominant lithofacies described above, shallow channelised and scour and fill structures are also common. They contain variable quantities of stromatolitic debris and oncolites in a quartzose to oolitic sandy matrix.

STRATIGRAPHY AND REPETITIVE SEQUENCE DEVELOPMENT Twelve sections measured through the Burra Group at selected localities across the ranges (Fig. 5) are described in detail by Belperio (1987b). Summaries of the overall thickness of strata are shown in Fig. 6. The Burra Group has a maximum thickness (of exposed strata) of between 7 000 and 8 000 m in the central Willouran Ranges (Fig. 6a). Correlation along and across the ranges reveal non-uniformity of accumulation and marked facies changes. The arenaceous Emeroo Subgroup has a maximum thickness of 4 000 m in the east, whereas the carbonatedominated Skillogalee Dolomite is thickest (3921 m) in the central region (Figs 5b, c). The younger Myrtle Springs Formation is thickest (4436 m) to the northwest of the study area (Fig. 6d). Dramatic thickness changes with apparent onlap of Burra Group strata onto older, technically disrupted regions are clearly visible (Fig. 5). Examples of repetitive strata development over vertical scales of 15 and 50 m are shown in Figs 7 and 8. Desiccation features such as tepees, carbonate crusts (dolomite and/or magnesite) and mudcracks occur most commonly at the base of

magnesite conglomerate beds, and are also associated with siliciclastic grainstones. Stromatolite biostromes and cryptalgal laminated micritic dolomite commonly grade up into intraformational cryptalgal dolomites, oncolites and dolomitic grainstones. Sandstone beds and magnesite conglomerates generally display sharp basal and upper contacts. Their thickness and repetition appears to be random. Apart from channel deposits, most beds are laterally extensive, and even decimetre thick beds may be traced along strike for many kilometres. Magnesitemud-pellet conglomerate beds, however, display mesoscale interfingering and lensing (Fig. 7). Chronological constraints imply a net sedimentation rate for the Burra Group as a whole of at least 0.05 to 0.15 mm per year, a figure that is in the upper range of recorded sedimentation rates of platform carbonates (Sadler 1981) but is well within the range for extensional rift-valley basins. The measured strati graphic sections allow for intra-basinal comparison of lithofacies. These have been combined to recreate a west to east palinspastic section using the top of the Skillogalee Dolomite as an approximately isochronous surface (Fig. 9). Some of the general observations that can be deduced from this have been noted by previous workers in this area (Murrell 1977; Uppill 1980; Utah Development Company 1983). Sedimentation occurred in a series of discrete, axial basins (numbered sub-basins 1-4 on Fig. 9). Individual lithologies and sedimentary structures are essentially similar in each subbasin, but there are major differences in the predominant lithologies. Thus in the east, siliciclastic deposition dominated, whilst the two central basins are dominated by much greater accumulation of carbonates (stromatolites, cryptalgal dolomites, dolomitic grainstones). Magnesite is best developed in the two western sub-basins (Fig. 6f, Fig. 9). A "lower siliciclastic sequence" was recognised throughout the Willouran Ranges but is thickest towards the east (Fig. 6e, Fig. 9). An upper zone of siliciclastic

A.P. Belperio

dominance is evident in the two eastern subbasins. The axial sub-basins are clearly reflected in the present outcrop distribution of the Burra Group, separated by northwest-trending zones of tectonically-disturbed megabreccia and diapiric strata (Fig. 5). Murrell (1977) and Utah Development Company (1983) considered these tectonic complexes to have been basement palaeo-highs onto which the Burra Group sea transgressed. However, detailed mapping has not revealed interdigitating marginal coarse clastics which would be expected to have shed off any elevated basement blocks. In addition, some 4 or 5 km of onlap of peritidal deposits is required using a simple transgressive model. A more plausible

explanation is that these structurally complex corridors represent the former bounding faults to rapidly subsiding and sedimenting extensional sub-basins. The faults became preferential sites for syn- and post-depositional diapiric piercement, modified by later (Cambro-Ordovician) orogenesis. PALAEOENVIRONMENT EVIDENCE Evidence of palaeoenvironment at the outcrop scale is provided by observation of lithology, mineralogy and sedimentary structures, and by additional chemical and isotopic analyses. Of particular interest is the need to distinguish marine depositional environments from evapora-

CALLANNA

WILPENA AND UMBERATANA GROUPS BURRA GROUP Myrtle Springs Formation Skillogalee Dolomite Emeroo S u b g r o u p CALLANNA GROUP (partly t e c h n i c a l l y disrupted) _ \\v.\v.\Y

Structural corridors Fault Stratigraphic section

10 KILOMETRES

87-621 SADME

Fig. 5 Simplified geological plan and east-west cross-section of the Willouran Ranges showing locations of detailed stratigraphic transects.

Palaeoenvironments of the Skillogalee Dolomite

(a) Burra Group

(b) Emeroo Subgroup

(d) Myrtle Springs Formation

KILOMETRES

j°

(e) Skillogalee Dolomite Lower siliciclastic sequence Thickness in metres

(f) Skillogalee Dolomite Magnesite facies •100

For geology see Fig.5

Fig. 6 Geographic variation within the Willouran Ranges, in the estimated thickness of the Burra Group(A) and measured thickness of the Emeroo Subgroup(B), Skillogalee Dolomite(C) and Myrtle Springs Formation(D). Also shown are the measured thicknesses of the lower siliciclastic facies(E) and magnesite dominated facies(F) of the Skillogalee Dolomite. Measurements are related to sections and geology as shown on Fig. 5.

A.P. Belperio

I6-1 £

14-

• • •

Sample 6 4 3 8 R S 3 8 3

• • • 200

300

STRIKE LENGTH (metres)

Stromatolite Carbonate crust Mud cracks Tepee structure Trough cross-bedding _

Cryptalgal dolomite . —

-vv

Dolomitic grainstone_

Magnesite conglomerate

ttt ' • • •

Sample 6 4 3 8 R S 3 8 2

Siltstone 87-623 SADME

^S-v

Fig. 7 Outcrop plan of vertically dipping Skillogalee Dolomite strata over a strike length of 600 m. Location approximately midway along Section 9, central Willouran Ranges. tive lacustrine and playa environments. The relevant analytical results are summarised in Table 1, and details of methodology and results are given in Belperio (1987a). Isotopic analyses were undertaken on selected whole-rock specimens, and measurements are given in per mil deviations from PBD standards.

Q n

• • •

Sample 6 4 3 8 R S 3 2 9

IXz

Dolomite _ • • • • • • •

• • • • • • •

Sandy dolomite Stromatolites Algal lamination

_ TTT _ • • •

Pelletal magnesite _

_ vv

Chert Carbonate crust

Interpretation from outcrop observations

Intraformational crust -

Lithologies, mineralogies and sedimentary structures indicate peritidal deposition with repeated, intermittent exposure for the entire Skillogalee Dolomite succession. Finely laminated micritic dolomites with crenulate to stratiform organic laminae (cryptalgal dolomites) are analogous to modern peritidal algal laminites. Because of their extensive development, and lack of desiccation features, they are here interpreted as shallow subtidal in origin. Equivalent intertidal deposits are also present and are recognised by the varying degree of disruption and resedimentation as intraformational dolomite breccias, the presence of oncolites, and associated tepees,

<<v

Mud cracks Tepee structure

•vnvn

Sample 6 4 3 8 R S 3 8 1 See Fig.4

TTT" TTT • • •

Sample 6 4 3 8 R S 3 8 0

87-624 SADME

Fig. 8 Example of repetitive sequence development. Detailed stratigraphic section for part of the Skillogalee Dolomite, Screechowl Creek (Section 8). Samples refer to analyses in Table 1.

Sample No. (6438-)

RS276 RS280A RS282 RS284B RS289B RS298A RS298A RS299 RS301A RS328 RS329E RS332 RS333 RS334 RS334 RS380 RS381 RS382 RS383 RS423 RS424 RS425 RS426 RS427

Lithology

Mudstone Stromatolite Mudstone Stromatolite Stromatolite Cryptalgal dolomite Cryptalgal dolomite Pelletal magnesite Mudstone Chert Pelletal magnesite Cryptalgal dolomite Siltstone Mudstone Mudstone Pelletal magnesite Pelletal chert Pelletal magnesite Pelletal magnesite Cryptalgal dolomite Cryptalgal dolomite Cryptalgal dolomite Cryptalgal dolomite Cryptalgal dolomite

Organic carbon

S(%)

Sulphide sulphur S($)

Organic carbon delta 13C (per mil PDB)

<0.005 0.005 <0.005 0.010 0.005 0.025

<0.005 0.005 <0.005 0.010 0.005 <0.005

Acid evolved C02(%)

Total sulphur

0.10 0.16 0.13 0.83 0.13 0.27

0.05 23.7 0.28 38.1 14.2 34.3

0.08 0.04 0.02 0.03 0.69 0.15 0.02

32.0 0.57 0.48 42.2 38.0 0.44 1.65

0.005 0.015 0.055 0.005 0.020 0.46 0.015

0.03 0.19 <0.02 0.03 0.10 0.82 1.38 0.59 0.28

40.8 7.35 41.9 46.4 7.45 28.0 2.10 9.65 8.45

0.010 0.020 0.005 0.010 0.39 0.23 1.49 0.72 8.35

0.005 <0.005 0.030 <0.005 <0.005 0.020 0.005 -

0.010 0.020 <0.005 0.005 0.38 0.17 1.48 0.66 8.30

-19.85 -8.46 -10.07 -16.63

Carbonate delta ,3 C (per mil PDB)

Carbonate delta , 8 0 (per mil PDB)

Carbonate 87 SrrSr

0.74057 ± 0.00014 -

0.71194 ±0.00014

+2.36 +2.52 +2.26

-7.8 -6.6 -5.4

£EL

0.70932 ± 0.0009 0.70927 ± 0.00007

+7.27

-5.5

+1.38 +5.55 +2.61

-2.8 -2.9 +2.2

-9.03 -15.15 -

-23.81 -

-15.98 -

-20.22 -20.08

+0.02

+1.3

+3.40 +3.08 +5.51 +5.89 +3.38 +2.68 +0.39 +2.16

-4.2 -7.5 -2.9 -1.0 -9.5 -6.8 -9.4 +2.7

0.71009 ± 0.00012

CD D <

3O 3CD 3

0.71377 ± 0.00009 0.71392 ± 0.00009 -

0.70903 ± 0.00008 -

00 2f. o CTQ eL Ct> (V a

o o

Table 1: Skillogalee Dolomite; Results of analyses for carbon, carbonate and sulphur, and isotopes of organic carbon and carbonates. For details of location and methodology, see Belperio (1987a)

A.P. Belperio

NORWEST STRUCTURAL CORRIDOR

RISCHBIETH STRUCTURAL CORRIDOR Sub-basin Myrtle

3 Springs

CALLANNA STRUCTURAL CORRIDOR Sub-basin

Sub-basin

Formation

DOMINANT LITH0L0GY Mudstone Sandstone _ Dolomitic grainstone Cryptalgal dolomite Magnesite c o n g l o m e r a t e

o-i

KILOMETRES

m • • • •

Intra-basin faults Laterally continous bed _ 87-625

SADME

Fig. 9 Palinspastic reconstruction of east-west section across the Willouran Ranges showing variation in thickness and dominant facies of the Skillogalee Dolomite. Constructed from detailed stratigraphic sections shown on Fig. 5. Separating depositional sub-basins are complex structural corridors of disrupted older strata, tectonic breccia and diapiric megabreccia that mark former intra-basinal faults. Vertical exaggeration 5x.

mudcracks and euhedral evaporite moulds. Stromatolite biostromes represent greater development of algal growth structures and, because of their extensive stratiform development, are also interpreted as largely of shallow subtidal origin. Intertidal stromatolites are also clearly recognisable from the degree of reworking, more common chert replacement, and associated dolomite grainstones and intertidal channel deposits. Sedimentary magnesite beds are also intimately associated with desiccation features

such as basal disrupted carbonate crusts, tepees and mudcracks. Together with torrent, graded and inverse-graded bedding features, these imply repetitive subaerial exposure and erosion of semiconsolidated magnesium carbonate mud. Preferential intermixing with dolomite rather than siliciclastics further implies a marginal marine setting. By direct analogy with the modern Coorong (Forbes 1961; Von der Borch & Lock 1979), precipitation probably occurred in marginal ephemeral lakes that underwent seasonal evaporation. Magnesium may have been provided by sporadic marine flooding, by marine

Palaeoenvironments of the Skillogalee Dolomite

aerosols or by magnesium-rich groundwaters. Erosion of unconsolidated magnesite mud and dolomicritic algal mats may have resulted from repetitive shoreline migration across very low gradient, depositional flats. Inorganic precipitation of gelatinous silica also occurs within the carbonate muds of ephemeral Coorong lagoons where it is intimately associated with both dolomite and magnesite (Peterson & Von der Borch 1965; Yon der Borch & Jones 1976). Chert is also abundant in lacustrine deposits of rift valley environments as an alteration product of volcaniclastic material. However, widespread penecontemporaneous volcanism is not recognised for Skillogalee Dolomite time. Sandstone beds show characteristics of subaerial sheet-flood deposition. These include thin, flaggy, poorly sorted beds, sharp contacts, and abundant mudcracks and remnant evaporites. If a marine setting is accepted for the carbonates, then sandstone beds represent intermittent sheet-flood deposition across playa-like, marginal marine flats. Textural and petrographic evidence, in particular the intermixing of dolomite and magnesite intraclasts in a variety of grain sizes, and the preservation of fine algal structures, indicate that both dolomite and magnesite were formed penecontemporaneously with deposition rather than by late-stage diagenesis of precursor carbonate minerals. Modern environments of penecontemporaneous dolomite formation are poor models for explaining the great thickness and lateral uniformity of ancient dolostones such as the Skillogalee Dolomite. Tucker (1982) also considers modem environments of dolomite formation to be poor analogues for interpreting Precambrian dolomites and has suggested primary precipitation may have been the norm at that time due to a seawater of different composition to that of the present. The combination of lithologies, mineralogies and sedimentary structures evident at the outcrop

scale are thus consistent with a shallow marine to peritidal setting for the Skillogalee Dolomite, with primary or penecontemporaneous formation of dolomite on an extensive carbonate platform. Repeated, small-scale transgressions and regressions concomittant with shoaling and subsidence resulted in complex repetitive vertical sequences of peritidal marine and marginal playa-like deposits, with vertical facies changes on the scale of decimetres to metres. Shoaling generated a sequence change from subtidal, laminated, cryptalgal dolomites and stromatolites through trough-crossbedded dolomitic grainstones, to intraformational, intertidal dolomites, supratidal to ephemeral magnesites, and arenaceous sheetflood deposits. This ideal cycle is, however, frequently disrupted by other variables. Uppill (1980) recognised a similar preferred cycle of shoaling, exposure and disruption within magnesite-dominated facies. The thick terrigenous sequence below the Skillogalee Dolomite (the Emeroo Subgroup) was examined only briefly in this study. Interbedded flaggy sandstones, feldspathic quartzites, laminated siltstones and shales with abundant ripple marks, clay intraclasts, mudcracks, lenticular bedding and moulds after shortite and halite are consistent with deposition in alluvial and perennial and playa lacustrine environments. Palaeosalinity Desiccation features such as mudcracks, tepees and disrupted carbonate crusts are abundant throughout the Skillogalee Dolomite, but without supporting evidence, provide no direct evidence of palaeosalinity. Halite and shortite moulds were recorded but are relatively rare in the dolomitic facies. Von der Borch & Lock (1979) attributed the lack of evaporites in the Burra Group dolomites to a more humid climate or a better developed through-flushing hydrological system compared with underlying sabkhastyle dolomites of the Callanna Group. Pyrite is common within the Skillogalee Dolomite, particularly in arenaceous beds. Authigenic pyrite forms under anoxic or euxinic conditions from

A.P. Belperio

bacterial sulphate reduction within bottom sediments. Organic carbon and sulphur relationships have been used to distinguish oxic marine, euxinic marine and non-marine freshwater depositional environments (Berner & Raiswell 1984; Leventhal 1987). Of 22 samples analysed (Table 1), five have a discriminatory content of organic carbon and pyritic sulphur (Berner & Raiswell 1984) and plot in the range established for modern marine oxic and euxinic sediments (Fig. 10). No samples plot definitively within the range for modern freshwater lacustrine sediments. However, the C/S palaeosalinometric method does not differentiate between saline lacustrine and marine environments. Organic composition Thirty-five samples from various sediment facies of the Skillogalee Dolomite were analysed for total organic carbon (Table 1; Belperio 1987a). Cryptalgal dolomite and stromatolite samples record the highest residual organic content, of up to 1.38% T.O.C. Intraclastic magnesite samples consistently record organic contents of <0.1% by weight. The Skillogalee Dolomite, and the coeval or slightly older Bitter Springs Formation of the Amadeus Basin, contain a diverse preserved microbiota of cyanobacteria, algae and fungi

(Barghoorn & Schopf 1965; Schopf 1968). Because of metamorphism (lower greenschist facies), the Typel kerogens have undergone extensive dehydrogenation and are heavily altered (McKirdy & Watson 1986). The kerogen extracted from stromatolites and algal dolomites 13

shows enrichment of the heavier C isotope (delta 13 C -8 to -20 per mil PDB) relative to the established range for biogenic carbon preserved in Phanerozoic and Proterozoic sedimentary rocks (Fig. 11). Enrichment of 13 C in Precambrian carbonaceous matter has been attributed to fundamentally different environmental conditions such as lower oceanic pH or temperatures (Degens 1969) and to post-depositional metamorphism (McKirdy et al 1975; McKirdy & Powell 1974). The probable carbon isotopic shift due to metamorphism of the Burra Group is estimated by Hayes et al (1983) at about 4 per mil. Even allowing for such a correction, most samples remain anomalously heavy and correspond with the established range (-8 to -21 per mil PDB) for extant cyanobacterial mat communities (Schidlowski et al 1983). In recent sediments, organic matter has a similar isotopic composition to the organisms living in the environment of deposition. Sediments with organic material from freshwater

j Cyanobacteria Phanerozoic

Cryptalgal dolomite _ o Chert

Euxinic

and

1-1.2

-10 DC1

kerogens

-0 8 £

Cryptalgal

marine

Proterozoic

•

dolomite

Siliciclastic

Chert

0.6 -0.4

J CD

Norma/

-0.2

marine -40

-30

-20 <J13C(PDB, o/oo)

0 87-627

SADME

13 samples

Freshwater

>•0 0

lacustrine

oo 1 2 WEIGHT % ORGANIC CARBON

87-626

SADME

Fig. 10 Plot of organic carbon versus pyritic sulphur for 22 samples from the Skillogalee Dolomite. Modern environment fields from Berner & Raiswell (1984).

Fig. 11 Plot of organic carbon isotope values versus organic carbon content for 10 Skillogalee Dolomite samples. General range of isotopic values for biogenic carbon preserved in Phanerozoic and Proterozoic strata after Degens (1969) and Oehler et al (1972). Range for extant marine and freshwater cyanobacterial mat communities after Schidlowski et al (1983).

Palaeoenvironments of the Skillogalee Dolomite 13 lacustrine environments usually have a lower C content, with isotopic ratios generally in the range -20 to -30 per mil PDB. Marine organic carbon is isotopically heavier, with a mean of -20 per mil (Degens 1969). Therefore, with due allowance for isotopic fractionation during diagenesis, the simplest conclusion to be drawn from the organic isotopic compositions is that the preserved kerogens from the Skillogalee Dolomite reflect the dominance of a photosynthetic cyanobacterial community in a marine or marginal marine environment. Carbonate isotopes

largely forms by replacement of a precursor carbonate (Land 1984). Dolomite is rare in Holocene unrestricted marine sediments and, in coastal 18 evaporative deposits (sabkhas), is enriched in O as a result of evaporation of seawater (Fig. 12). Similarly, Holocene dolomite formation from evaporative concentration of continental to meteoric-seawater mixing zone environments such as the ephemeral Coorong lakes and lagoons 18 also result in enrichment of O relative to normal marine carbonates. Many ancient platform dolomites, whilst displaying a general secular constancy of carbon isotopic composition are, however, significantly depleted in 0 with respect to Holocene dolomites (Land 1980) and Holocene limestones (Fig. 12). This anomaly is further accentuated in late Precambrian 1 8

Numerous studies of Phanerozoic dolomites have resulted in clear evidence that dolomite

& C(PDB, 0/00) Chert

Siliciclastic

. A

Brighton Limestone_

Magnesite

Cryptalgal dolomite _ .

. o

Skillogalee Dolomite

_• .0 87-628

i •j

SADME

Fig. 12 Plot of delta C versus delta O (versus PDB) for 16 Skillogalee Dolomite samples. Included are data from Schidlowski et al (1975) and Veizer & Hoefs (1976) for the Skillogalee and for the younger, marine Brighton Limestone. Data ranges are indicated for: 1. Precambrian platform carbonates after Schidlowski et al (1975) and Veizer & Hoefs (1976) ; 2. Extant shallow marine bioclasts after Milliman (1974) ; 3. Lacustrine stromatolites from the Magadi-Natron basin after Hillaire-Marcel & Casanova (1987) ; 4. The regressive Coorong lagoon showing trend from aragonitic to dolomitic sediments after Botz & Von der Borch (1984) ; and (5) Coorong ephemeral lake magnesites after Botz & Von der Borch (1984). I o

A.P. Belperio

dolomites (Schidlowski et al 1975; Veizer & Hoefs 1976). The Skillogalee Dolomite is no exception (Table 1, Fig. 12). Analyses reported here confirm the general range of previous analyses obtained on samples from the Skillogalee Dolomite by Schidlowski et al (1975) and Veizer & Hoefs (1976), and provide additional data. One sample of terrigenous mudstone (with 3% dolomite) is enriched in 1 8 0. All but two of the cryptalgal dolomite and magnesite samples are depleted in O by some 10 to 12 per mil relative to Holocene examples. Two samples of cryptalgal dolomite, however, are clearly enriched in O relative to other samples and plot within the fields for lacustrine or evaporitic mixing-zone dolomites. Magnesite samples are all distinctly enriched in C relative to coeval dolomites, but only marginally enriched in 1 8 0. This may indicate simple evaporative enrichment rather than meteoric mixing. The isotopically light cryptalgal dolomites and stromatolites are significantly different to Quaternary lacustrine stromatolites as exemplified by those from the Magadi-Natron basin (Hillaire-Marcel & Casanova 1987). They are, however, notably similar in isotopic signature to marine biosparites of the younger Brighton Limestone (Schidlowski et al 1975).

Veizer & Compston (1974, 1976), Burke et al (1982) and Veizer et al (1983) have established a secular trend for the Phanerozoic and Late Proterozoic (Fig. 13). These studies included dolomite samples from the Skillogalee Dolomite and the younger Brighton Limestone and Wonoka Formation of the Adelaide Geosyncline, and the Bitter Springs Formation of the Amadeus Basin in central Australia. Eight additional analyses were undertaken as part of this study (Table 1). 87 Sr/ 86 Sr ratios (normalised to 88 Sr/ 86 Sr= 8.3752) were measured on 0.1N HCl-soluble fractions of these samples in order to eliminate the influence of radiogenically enriched strontium in any non-carbonate detritus that may be present.

•t

Sr/ 86 Sr

0.715AA i,

0.710-

Secular trend of seawater c o m p o s i t i o n

^ _

Strontium isotopes The isotopic ratio 87 Sr/ 86 Sr shows no detectable variation in present-day ocean and coastal water, but shows a slow, systematic variation over geological time. In the absence of secondary alteration, Sr contained in biogenic carbonates records the isotopic composition of the coeval waters in which they formed. Previous studies by

}1 11 / — ? // v/

0 705

Unaltered, penecontemporaneous carbonates should have isotopic signatures that reflect the composition of the host waters from which they were formed (Veizer & Hoefs 1976). The average delta 1 8 0 value of -7.6 for the six isotopicallylight cryptalgal dolomites and stromatolites may approximate the value of basin or oceanic water at that time.

( \ f S \ f S ^ ^ " V \

200

400

600

1 800

1 1000

Time (Ma) THIS STUDY

OTHER DATA

(SKILLOGALEE DOLOMITE) Magnesite Cryptalgal dolomite Siliciclastic Drn E.A.

, r Wonoka Fm

•

Brighton Limestone

•

Skillogalee Dolomite

1AI

_ A

Bitter Springs Fm

• 177771 8 7 - 6 2 9 SADME

Fig. 13 Strontium isotope data for eight Skillogalee Dolomite samples (age ca.775 ±25Ma) compared with the secular trend of seawater composition established after Burke et al (1982), Veizer et al (1983) and Veizer & Compston (1974). Also shown are data ranges and accepted values of previous analyses by Veizer et al (1983) and Veizer & Compston (1976) for the Skillogalee Dolomite, the marine Brighton Limestone (ca.700±50 Ma) and marine Wonoka Formation (ca.650±50 Ma) of the Adelaide Geosyncline, and the Bitter Springs Formation (ca.800±50 Ma) of the Amadeus Basin.

Palaeoenvironments of the Skillogalee Dolomite

The "preferred" value of seawater 87 Sr/ 86 Sr composition of Skillogalee time is 0.7091 (Veizer et al 1983). It is also considered unlikely that the continental river flux of Sr in the Late Proterozoic was either less or very much more radiogenic than present-day counterparts (0.711). Three cryptalgal dolomite and two magnesite samples plot within the range for the Skillogalee Dolomite previously established by Veizer & Compston (1976). The range was considered to result from a varying degree of secondary alteration. Three samples were considerably more radiogenic (Fig. 13) ; all were terrigenous mudstones from the basal Camel Flat Shale (Figs 2,9). The higher values may be indicative of non-marine deposition, or may have resulted from post-depositional alteration and isotopic equilibration of Sr between carbonate and non-carbonate components.

PALAEOGEOGRAPH Y AND S YNDEPOSITIONAL TECTONICS The Burra Group generally, and Skillogalee Dolomite in particular, have been interpreted by several investigators as dominantly or entirely of non-marine origin. Murrell (1977) and Uppill (1980) inferred deposition in a large, shallow inland sea with a chemistry somewhat removed from ocean water. Von der Borch (1980) suggested the cycles within the Skillogalee Dolomite represented a repetitive playa-lacustrine sequence similar to that of the Laney Shale Member of the Green River Formation. Others, such as Forbes (1960, 1961), Preiss (1983) and Preiss et al (1981), considered the ciyptalgal dolomites as essentially penecontemporaneous shallow marine carbonates, with magnesite deposition in marginal alkaline lagoons. Lacustrine basins, particularly those in a rift valley setting, are characterised by high deposition and subsidence rates, variable carbonate and clastic sedimentation, and by preservation of organic matter, features already noted in the Skillogalee Dolomite. In such settings, however, lacustrine dolomites and stromatolites are also intimately associated and interdigitate with al-

luvial fanglomerates and aeolian and fluvial quartzose sandstones (e.g. White & Youngs 1980; Elmore 1983; Smoot 1983). In more arid settings, they are associated with bedded evaporites and saline sediments. Lacustrine environments also produce characteristic "bath-tub ring" patterns of stromatolite growth as a result of fluctuating water levels (e.g. Hillaire-Marcel & Casanova 1987). The extensive and regular formation of stromatolite biostromes, the lack of interdigitating coarse continental deposits, and the absence of evaporitic strata argue against a playa or lacustrine origin for the carbonates. Sedimentary structures are consistent with a shallow marine to peritidal setting for the Skillogalee Dolomite, with repetitive shoaling, exposure and desiccation. With due regard to possible post-depositional changes, the isotopic compositions of carbonates and residual organics provide supportive evidence of sedimentation in a marine environment. In particular, the coincidence of 87 Sr/ 86 Sr ratios of cryptalgal dolomites and magnesites with those for Precambrian limestones generally accepted as marine in origin, such as the Brighton Limestone (Preiss & Kinsman 1978) and Wonoka Formation (Haines 1986), indicates a similar marine association. Mudstone samples which are more radiogenic indicate continental sedimentation, or may indicate post-depositional exchange of Sr between silicate and carbonate phases (Veizer & Compston 1974, 1976). Oxygen isotopic composition of dolomites, magnesites and cherts similarly display a general equivalence with values previously obtained from "accepted" marine limestones. Two anomalous samples indicate evaporative enrichment and concur with the sedimentological evidence that indicates periodic exposure and desiccation, though unaccompanied by sulphate or halide precipitation. C/S ratios provide further, though limited, support for a marine origin for the cryptalgal carbonates. The Burra Group and Skillogalee Dolomite in the Peake and Denison Ranges to the northwest (Fig. 1) display a similar megascale stratigraphy and cyclicity to that in the Willouran Ranges. However, algal laminations and stromatolites are

100

A.P. Belperio

much less developed and intraclastic magnesites are thinner and finer grained. Also, the Skillogalee Dolomite is dominated by stacked, decimetre-thick, planar bedded grainstones of mixed quartz-dolomite composition. This is consistent with the existence of a through-going shallow seaway extending northwestwards from the Willouran Ranges, with cryptalgal dolomite and magnesite production centred about a southeastern entrance about the present Willouran Ranges. The terrigenous lower Burra Group sequence in the Peake and Denison Ranges is similarly consistent with an alluvial and perennial to ephemeral lacustrine origin.

tional, fault-controlled subsidence of the narrow, axial, extensional sub-basins.

Despite high rates of subsidence, sedimentation essentially kept pace throughout Burra Group time. As a consequence, little positive or negative relief was generated between sub-basins and individual beds show little lithological change when traced marginwards. Differential subsidence occurred along intra-basinal faults that are preserved today as structural corridors of tectonically disturbed megabreccia and shaleand-dolomite-hosted diapiric breccia of older strata (Fig. 5). Syn- and post- depositional diapirism of partially evaporitic Callanna Group often been invoked to explain the The marked variations in the total thickness carbonates ofhas such piercement megabreccias (2 000-8 000 m) of the Burra Group and its facies presence through much of Adelaide Geosyncline (e.g. across the Willouran Ranges (Fig. 9) are inter- Webb 1961; Coatsthe1965; & Johnson preted as resulting from differential syn-deposi- 1968; Lemon 1985; Preiss Dalgarno 1985). Sedimentation tional subsidence between at least four in modem extensional basins is often intimately northwest-trending sub-basins. Sedimentation of associated with diapirism. In the northern Red predominantly continental clastics of the Emeroo Sea for example, many elongate diapirs ascend Subgroup was greatest (4 000 m+) in the eastern along bounding faults between numerous narrow, ranges. For the Skillogalee Dolomite, the locus of axially-trending, depositional sub-basins, under sedimentation shifted to the central ranges overburden as little as 1 km thick (Mart & Ross (3900 m) with predominantly shallow marine 1987). With accumulation of 8 000 to 10 000 m carbonates deposited. Maximum sedimentation of sediment in Burra Group time in sub-basins of shifted again, this time to the northwest (4400 m) the order of 10 km width, gravitational instability i) • the return to dominantly terrigenous deposition and concomittant diapirism are to be expected of the Myrtle Springs Formation. Shallow water given even minimal density inversion or converconditions and intermittent exposure prevailed sion. throughout these major depositional cycles. For the Skillogalee Dolomite, variation in facies development across the basin implies a dominant CONCLUSIONS source of terrigenous sediment from the northeast, a major central zone of in-situ algal carearliest Adelaidean (Callanna Group) in bonate production and shallow, paralic flats the The Willouran Ranges records syn-rift, inconducive to magnesite precipitation to the tracratonic graben sedimentation that includes southwest. volcanics, carbonates, clastics and evaporitic sequences deposited in sabkha and playa environWithin the sub-basins, the Burra Group strata ments (Rowlands et al 1980; Preiss & Forbes thin dramatically when traced marginwards, in 1981). The Burra Group represents a continuation some cases from 4 000 or 5 000 m to zero over of this intracontinental extensional tectonic lateral distances of less than one kilometre. Al- regime, but with a major change to marine conthough some of this is due to later (Cambro-Or- ditions particularly for the Skillogalee Dolomite. dovician) deformation and thrusting (Sprigg The depositional trough extending from the Wil1950), much of the thinning is due to syn-deposi- louran Ranges to the Peake and Denison Ranges

Palaeoenvironments of the Skillogalee Dolomite and beyond, was a persistently subsiding basin with subdued topography masking a complex of differentially subsiding sub-basins. Syn- and post-Burra Group diapirism occurred along bounding faults between sub-basins. Interaction between sea level fluctuations, shoaling and subsidence produced a complex repetitive sequence of algal dolomites, intraclastic dolomites, dolomitic grainstones, intraclastic magnesites and arenaceous beds. Cryptalgal dolomites and stromatolite biostromes were formed in shallow marine and intertidal waters, whilst magnesite muds were deposited in marginal flats and lagoons. Desiccation, intraformational brecciation and sheet flooding occurred as a result of shoaling, shoreline migration and exposure. Dolomite was probably precipitated directly from seawater or formed penecontemporaneously with sedimentation, possibly as a result of cyanobacterial photosynthesis. Early diagenetic silicification of stromatolites and cryptalgal dolomites, and precipitation of magnesite muds, occurred preferentially on very shallow, intermittently exposed, marginal mud flats and lagoons. This spatial association is somewhat analogous to the modern association of dolomite deposition marginal to aragonite/magnesian calcite marine sedimentation as exemplified by the Coorong lagoon and ephemeral lakes, though the tectonic setting is more akin to that of the northern Red Sea rift.

101

REFERENCES AMBROSE G.J., FLINT R.B. & WEBB A.W. 1981. Precambrian and Palaeozoic geology of the Peake and Denison Ranges. Geological Survey of South Australia, Bulletin 50. BARGHOORN E.S. & SCHOPF J.W. 1965. Microorganisms from the Late Precambrian of central Australia. Science 150, 337-339. BELPERIO A.P. 1986. Stratigraphy and sedimentology of the Precambrian Skillogalee Dolomite, northern Flinders Ranges. Geological Society ofAustralia, Abstracts 15, p.29. BELPERIO A.P. 1987a. Mineralogical, chemical, isotopic and source-rock analyses of Burra Group (Adelaidean) sedimentary rocks from the Willouran Ranges. Department of Mines and Energy, South Australia, Report Book SUM, 1-17. BELPERIO A.P. 1987b. Stratigraphic sections measured in Adelaidean (Burra Group) rocks in the Willouran Ranges, Curdimurka area. Department of Mines and Energy, South Australia, Report Book 87/56,1-58. BELPERIO A.P. 1987c. Hydrocarbon potential of Late Proterozoic graben sediments, South Australia. Department of Mines and Energy, South Australia, Report Book 87/75, 1-7. BELPERIO A.P. 1987d. Hydrocarbon potential of Late Proterozoic graben sediments. Australian Journal of Earth Sciences 34,403-404.

ACKNOWLEDGEMENTS

BERNER R.A. & RAISWELL R. 1984. C/S method for distinguishing freshwater from marine sedimentary rocks. Geology 12, 365-368.

This paper is published with permission of the Director-General, South Australian Department of Mines and Energy.

BOTZ R.W. & VON DER BORCH C.C. 1984. Stable isotope study of carbonate sediments from the Coorong area, South Australia. Sedimentology 31, 837-849. BURKE W.H., DENISON R.E., HETHERINGTON E.A., KOEPNICK R.B., NELSON H.F. & OTTO J.B. 1982. Variation of seawater 87Sr/86Sr throughout Phanerozoic time. Geology 10, 516-519. COATS R.P. 1965. Diapirism in the Adelaide Geosyncline. APEA Journal (1965), 98-102.

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DALGARNO C.R. & JOHNSON J.E. 1968. Diapiric structures and late Precambrian-Early Cambrian sedimentation in Flinders Ranges, South Australia. American Association of Petroleum Geologists, Memoir 8, 301-314. DEGENS E.T. 1969. Biogeochemistry of stable carbon isotopes. In Eglinton G. & Murphy M.T.J. Organic Geochemistry, pp.304-329. Springer-Verlag, Berlin. ELMORE R.D. 1983. Precambrian non-marine stromatolites in alluvial fan deposits, the Copper Harbor Conglomerate, upper Michigan. Sedimentology 30, 829-842. FANNING C.M., LUDWIG K.R., FORBES B.G. & PREISS W.V. 1986. Single and multiple grain U-Pb Zircon analyses for the early Adelaidean Rook Tuff, Willouran Ranges, South Australia. Geological Society of Australia, Abstracts 15, 71-72. FLINT R.B. & PARKER A.J. 1982. Tectonic Map South Australia, 1:2 000 000 Scale. South Australian Department of Mines and Energy, Adelaide. FORBES B.G. 1960. Magnesite of the Adelaide System: Petrography and descriptive stratigraphy. Transactions of the Royal Society of South Australia 83, 1-9. FORBES B.G. 1961. Magnesite of the Adelaide System: A discussion of its origin. Transactions of the Royal Society of South Australia 85, 217-222. FORBES B.G. 1984. Progress in mapping the Precambrian of the Curdimurka area. Department of Mines and Energy, South Australia, Report Book 84/33,1-8. FORBES B.G. & PREISS W.V. 1987. Stratigraphy of the Burra Group. In Preiss W.V. (Compiler). The Adelaide Geosyncline: Late Proterozoic Stratigraphy, Sedimentation, Palaeontology and Tectonics. Geological Survey of South Australia, Bulletin 53, 73-123. FORBES B.G., MURRELL B. & PREISS W.V. 1981. Subdivision of lower Adelaidean, Willouran Ranges. Quarterly Geological Notes, Geological Survey of South Australia 79, 7-16.

HAINES P.W. 1986. Late Proterozoic carbonate shelf to shale basin transition, Wonoka Formation, Flinders Ranges, S.A. Geological Society of Australia, Abstracts 15, 92-93.

HAYES J.M., KAPLAN I.R. & WEDEKING K.W. 1983. Precambrian organic geochemistry, preservation of the record. In Schopf J.W. ed. Earth's Earliest Biosphere, pp.93-134. Princeton University Press, Princeton.

HILLAIRE-MARCEL C. & CASANOVA J. 1987. Isotopic hydrology and paleohydrology of the Magadi (Kenya) - Natron (Tanzania) Basin during the Late Quaternary. Palaeogeography, Palaeoclimatology, Palaeoecology 58, 155-181. LAND L.S. 1980. The isotopic and trace element geochemistry of dolomite: the state of the art. In Zenger D.H., Dunham J.B. & Ethington R.L. eds. Concepts and Models of Dolomitization, pp.87-110. Society of Economic Paleontologists and Mineralogists, Special Publication No.28. LAND L.S. 1984. Dolomitization. Associations of Petroleum Geologists, Course Note Series No. 24.

American Education

LEMON N.M. 1985. Physical modeling of sedimentation adjacent to diapirs and comparison with Late Precambrian Oratunga breccia body in central Flinders Ranges, South Australia. American Association of Petroleum Geologists, Bulletin 69, 1327-1338. LEVENTHAL J.S. 1987. Carbon and sulphur relationships in Devonian shales from the Appalachian Basin as an indicator of environment of deposition. American Journal of Science 287, 33-49. MART Y. & ROSS D.A. 1987. Post-Miocene rifting and diapirism in the northern Red Sea. Marine Geology 74,173-190. McKIRDY D.M. & POWELL T.G. 1974. Metamorphic alteration of carbon isotopic composition in ancient sedimentary organic matter: new evidence from Australia and South Africa. Geology 2, 591-595.

Palaeoenvironments of the Skillogalee Dolomite

McKIRDY D.M. & WATSON B.G. 1986. Source rock analysis of Skillogalee Dolomite, Burra Group, Adelaide Geosyncline. Australian Mineral Development Laboratories, Report 6341/86 (unpubl). McKIRDY D.M., SUMARTOJO J., TUCKER D.H. & GOSTIN V.A. 1975. Organic, mineralogic and magnetic indications of metamorphism in the Tapley Hill Formation, Adelaide Geosyncline. Precambrian Research 2, 345-373. MELLIMAN J.D. 1974. Springer-Verlag, Berlin.

Marine

Carbonates.

MURRELL B. 1977. Stratigraphy and tectonics across the Torrens Hinge Zone between Andamooka and Marree. Ph.D thesis, University of Adelaide (unpubl.). OEHLER D.Z., SCHOPF J.W. & KVENVOLDEN K.A. 1972. Carbon isotopic studies of organic matter in Precambrian rocks. Science 175, 1246-1248. PARKER A.J. 1983. Tectonic development of the Adelaide fold belt. Geological Society of Australia, Abstracts 10, 23-28. PETERSON M.N.A. & VON DER BORCH C.C. 1965. Chert: Modern inorganic deposition in a carbonate-precipitating locality. Science 149, 1501-1503. PREISS W.V. 1973. Palaeoecological interpretations of South Australian Precambrian stromatolites. Journal of the Geological Society of Australia, 19, 501-532. PREISS W.V. 1983. Depositional and tectonic contrasts between Burra Group and Umberatana Group sedimentation. Geological Society of Australia, Abstracts 10, 13-16. PREISS W.V. 1985. Stratigraphy and tectonics of the Worumba anticline and associated intrusive breccias. Geological Survey of South Australia, Bulletin 52, 1-85. PREISS W.V. & FORBES B.G. 1981. Stratigraphy, correlation and sedimentary history of Adelaidean (Late Proterozoic) basins in Australia. Precambrian Research 15, 225-304.

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PREISS W.V. & KINSMAN J.E. 1978. Stratigraphy and palaeoenviron-mental interpretation of the Brighton Limestone south of Adelaide and its equivalents in the Orroroo region. Geological Survey of South Australia, Report of Investigations 49,1-34. ROWLANDS N.J., BLIGHT P.G., JARVIS D.M. & VON DER BORCH C.C. 1980. Sabkha and playa environments in Late Proterozoic grabens, Willouran Ranges, South Australia. Journal of the Geological Society of Australia, 27, 55-68. RUTLAND R.W.R., PARKER A.J., PITT G.M., PREISS W.V. & MURRELLB. 1981. The Precambrian of South Australia. In Hunter D.R. ed. Precambrian of the Southern Hemisphere. Developments in Precambrian Geology, Vol.2, pp.309-360. Elsevier, Amsterdam. SCHIDLOWSKIM., EICHMANN R. & JUNGE C.E. 1975. Precambrian sedimentary carbonates: carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget. Precambrian Research 2, 1-69. SCHIDLOWSKI M., HAYES J.M. & KAPLAN I.R. 1983. Isotopic inferences of ancient biochemistries: carbon, sulfur, hydrogen and nitrogen. In Schopf J.W. ed. Earth's Earliest Biosphere, pp. 149-186. Princeton University Press, Princeton. SCHOLLE P.A., BEBOUT D.G. & MOORE C.H. 1983. Carbonate Depositional Environments. American Association of Petroleum Geologists, Memoir 33. SCHOPF J.W. 1968. Microflora of the Bitter Springs Formation, Late Precambrian, central Australia. Journal of Paleontology 42, 651-688. SMOOT J.P. 1983. Depositional subenvironments in an arid closed basin; the Wilkins Peak Member of the Green River Formation (Eocene), Wyoming, U.S.A. Sedimentology 30, 801-827. SPRIGG R.C. 1950. Thrust structures of the Witchelina area, South Australia. Transactions of the Royal Society of South Australia 73, 40-47. TUCKER M.E. 1982. Precambrian dolomites: Petrographic and isotopic evidence that they differ from Phanerozoic dolomites. Geology 10, 7-12.

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UPPILL R.K. 1980. Sedimentology of the late Precambrian Mundallio Subgroup: a clastic-carbonate (dolomite, magnesite) sequence in the MtLofty and Flinders Ranges, South Australia. Ph.D thesis, University of Adelaide (unpubl.). UPPILL R.K. 1983. Depositional environments of the dolomite-magnesite facies association of the Mundallio Subgroup. Geological Society of Australia, Abstracts 10, 17-18. UTAH DEVELOPMENT COMPANY 1983. Progress, annual and surrender reports, Exploration Licenses 461, 850, Willouran Ranges. South Australian Department of Mines and Energy, Open File Envelope 3507 (unpubl.).

VEIZER J., COMPSTON W., CLAUER N. & SCHIDLOWSKI M. 1983. ^Sr/^Sr in Late Proterozoic carbonates: evidence for a "mantle" event at c. 900 Ma ago. Geochimica et Cosmochimica Acta 47, 295-302. VON DER BORCH C.C. 1980. Evolution of Late Proterozoic to early Palaeozoic Adelaide Foldbelt, Australia: comparisons with post-Permian rifts and passive margins. Tectonophysics 70, 115-134. VON DER BORCH C.C. & JONES J.B. 1976. Spherular modern dolomite from the Coorong area, South Australia. Sedimentology 23, 587-591.

VEIZER J. & COMPSTON W. 1974. Sr/ Sr composition of seawater during the Phanerozoic. Geochimica et Cosmochimica Acta 38, 1461-1484.

VON DER BORCH C.C. & LOCK D. 1979. Geological significance of Coorong dolomites. Sedimentology 26, 813-824.

VEIZER J. & COMPSTON W. 1976. ^Sr/^Sr in Precambrian carbonates as an index of crustal evolution. Geochimica et Cosmochimica Acta 40, 905-914.

WEBB B.P. 1961. Diapiric structures in the Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 85,1-6.

VEIZER J. & HOEFS J. 1976. The nature of 0 / 0 13 12 and C /C secular trends in sedimentary carbonate rocks. Geochimicia et Cosmochimica Acta 40, 1387-1395.

WHITE A.H. & YOUNGS B.C. 1980. Cambrian alkali playa-lacustrine sequence in the northeastern Officer basin, South Australia. Journal of Sedimentary Petrology 50,1279-1286.

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1 6

Sedimentology of a dolomite - magnesite - sandstone sequence in the late Precambrian Mundallio Subgroup, South Australia. Robin K. Uppill 30 Kosciusko

St. Middle Park, Queensland

4074,

Australia

Dark-grey dolomite, magnesite and sandstone comprise the dominant facies association of the widespread Mundallio Subgroup, a late Precambrian unit of the Adelaide Geosyncline in South Australia. Magnesite facies comprise up to 21% of the facies association, with intraclastic magnesite being dominant. Magnesite mudstone, inter-laminated magnesite - dolomite mudstone, and nodular magnesite sourced the intraclastic beds. The nodular magnesite developed by replacement of magnesite mudstone during periods of subaerial exposure and non-deposition. Dolomite mudstone is the dominant facies, minor dolomite facies present include intraclastic, ooid, oncoid and peloidal grainstones, and stromatolitic dolomite. Locally in the central southern Flinders Ranges, massive diagenetic textured dolomites and associated megabreccias record periods of local tectonic uplift and exposure. Terrigenous facies are dominated by dolomitic sandstones, muddy facies being relatively minor. Where the major facies are all present in significant amounts, a 1-4 m thick cycle of intraclastic magnesite - dolomitic sandstone - dolomite mudstone - magnesite mudstone nodular magnesite is indicated. It records a depositional history of high energy events such as sheet flooding or wave undercutting of magnesite mudflats leading to deposition of intraclastic magnesite and the introduction of sand to the basin by land derived sheet flooding or the migration of sand dunes into the basin. As protected areas developed behind offshore sand bars or the basin shallowed due to progradation, widespread dolomite mud depositon occurred, largely on submerged mudflats. With further shallowing and progradation, the developing isolated ephemeral lagoons became sites of magnesite deposition. Textural evidence from the dolomite and magnesite facies, e.g. inter-laminated dolomite and magnesite mudstones, dolomite as laminae within and matrix to intraclastic magnesite, and magnesite intraclasts as nuclei to dolomite ooids and oncoids, suggests that dolomite and magnesite were present as discrete minerals virtually contemporaneously with deposition.

Key words : Magnesite, dolomite, Mundallio Subgroup, Yadlamalka Formation, facies descriptions, depositional environments. INTRODUCTION The Mg-carbonate magnesite is an uncommon component of both modern and ancient sediments. Generally the ancient occurrences are strongly recrystallised with little preserved information regarding primary sedimentary textures. Discrete occurrences of magnesite which are probably sedimentary, include the bedded deposits of Precambrain age in Manchuria (Nishihara 1956), Triassic deposits of Washinton State, U.S.A. (Fox & Reinhart 1968), lenses in the Gangolihat Dolomites of the Kumaun Himalayas, India (Valdiya 1968), a number of recrystallised deposits in the Carpathain Mountains and the Urals (Lesko 1972) and statabound recrystallised deposits of the West Pyrenees,

Spain (Petrascheck et al 1977; Kralik & Hoeffs 1978). However none of the above occurrences contain magnesite as texturally well preserved or as widespread as the late Precambrian Mundallio Subgroup of the Adelaide Geosyncline in South Australia. The most widespread facies association in this subgroup is a dark-grey dolomite magnesite conglomerate - sandstone association in which dolomite is the dominant carbonate, magnesite comprises up to 21% of the sequence, limestones are absent, and evaporites or their traces are very minor. The magnesite is preserved largely as interbeds of intraclastic conglomerate, but also as thinner and less extensive beds of magnesite mudstone. Dolomite occurs predominantly as cabonate mudstone, but also as stromatolitic dolomite and dolomite grainstones.

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Robin K. Uppill

Terrigenous facies are largely sandy with minor terrigenous mudstones. Features of the dolomitic facies support the assertion by Tucker (1982) that many Precambrian dolomites were precipitated and behaved in a manner identical to calcium carbonate in Phanerozoic limestone sequences. GEOLOGICAL SETTING The facies association discussed in this paper was deposited in the Adelaide Geosyncline, an extensive late Precambrian - Cambrian sedimentary basin, the geology of which is summarised by Forbes & Preiss (1987). The sediments of this basin are now preserved in a somewhat arcuate outcrop belt extending from Kangaroo Island in the south to the Mount Painter area and the Willouran Ranges in the north (Fig. 1).

Cambrian

and

q l d

younger A d e l a i d e a n . sediments B u r r a and Callanna

Groups

Sedimentary history began with deposition of a sequence of shallow water and possibly nonmarine clastics, carbonates and evaporites, the Callanna Group, in isolated basins or halfgrabens within an intracontinental rift (von der Borch 1980). The overlying Burra Group, of which the Mundallio Subgroup forms a part, accumulated in more laterally continuous .basins. This group comprises prograding deltaic sandsheets, shallow water carbonates almost exclusively dolomitic, and low energy shale sequences. Subsequent Precambrian and Cambrian sedimentation was in a relatively continuous basin, and was accompanied by periodic transgressions onto the adjacent more tectonically stable Stuart Shelf (Fig. 1). A major tectonic episode, the Delamerian Orogeny, terminated the sedimentary history of the Adelaide Geosyncline. Metamorphism preceding and associated with the orogeny resulted in the development of lower greenschist facies thoughout much of the geosyncline. Locally (e.g. at Mount Painter) higher metamorphic grades were reached, and the Mundallio SubMT

ERN FLINDERS RN L FROME

LOFTY

RANGES

STHN

FLINDERS

Central Upper ro a a 3 0

SOUTHERN FLINDERS

(3 CO

O) -Q

cn "5 c 3 5

Woolshed

B u r na

Flat

Montacute / Dolomite (

Sh.

) \

\Skillogalee Doi.

Castambul^ Formation\_

Western

Group

Yadlamalka Formation <TNathaltee

00 Emeroo

Subgroup

COPLEY

GAWLER CRATON WILLOURAN

Myrtle

Springs

ARKAROOLA

Formation

Yadlamalka

Formatii

kms I

VIC

Fig. 1 Location map showing present day area of outcrop of Adelaide Geosyncline. Locations shown and referred to in text are CP - Copley, MS - Myrtle Springs, A - Arkaroola, W - Worumba, YDA Yednalue Anticline, YD - Yednalue, YE - Yacka.

^ C.F.Shr)

<CFS^ Emeroo

Nankabunyana

Formation

Subgroup

Fig. 2 Stratigraphy of the Mundallio Subgroup and immediately underlying and overlying intervals of Burra Group. Tilt. Ss. and TS = Tilterana Sandstone; C.F.Sh. and CFS = Camel Flat Shale.

Dolomite - magnesite sequence, Mundallio Subgroup

group was metamorphosed to upper greenschist and amphibolite facies. The Burra Group, within which the Mundallio Subgroup occurs, is a sequence characterised by cyclical carbonate - clastic deposition. Sedimentation of this group commenced with quartzo-feldspathic sandstones deposited on coalescing alluvial fans and prograding deltas. Subsequent cyclical deposition involved four major facies associations: (i) black shales, (ii) fine sandy silts, (iii) medium to coarse-grained cross-bedded feldspathic sandstones, and (iv) micritic dolomites with associated stromatolites, magnesite and desiccation features; (Preiss 1983, 1985; Forbes & Preiss 1987). The stratigraphic relationships of the Mundallio Subgroup are presented in Figure 2 and are discussed in detail by Uppill (1979, 1980) and Forbes & Preiss (1987). In the revised statigraphic nomenclature proposed by Uppill (1979), three new formations, the Nathaltee Formation, Nankabunyana Formation and Yadlamalka Formation, were proposed for the interval previously referred to as the Skillogalee Dolomite in the southern and northern Flinders Ranges where this formation had commonly been subdivided into two unnamed members. The term Skillogalee Dolomite was retained for outcrops in the northern Mount Lofty Ranges. The dolomite-magnesite-sandstone facies association is encompassed within the Yadlamalka Formation which outcrops extensively in the southern and northern Hinders Ranges, and in the more lenticular Montactue Dolomite which is preserved only near Adelaide. A thin grey dolomitic unit with rare magnesite at the top of the Skillogalee Dolomite in the Mount Lofty Ranges north of Adelaide, and the dolomiteshale-sandstone sequence of the Mirra Formation in the eastern Willouran Ranges, are closely related and in part laterally equivalent sequences to the Yadlamalka Formation and Montacute Dolomite. Aspects of the Mundallio Subgroup have been described previously by Forbes (1960, 1961) and Preiss (1972,1973), they proposed that deposition occurred on a shallow marine platform with magnesite forming in marginal alkaline

107

lakes. Coats & Blisset (1971), Fairchild (1975), Murrell (1977), Ambrose et al (1980) and Belperio (1986) also refer to various features of the Mundallio Subgroup.

SEDIMENTARY FACIES: DESCRIPTIONS AND INTERPRETATION The Yadlamalka Formation and Montacute Dolomite are characterised by a very similar suite of facies throughout their areas of occurrence. Dolomite mudstone is the dominant facies, other less abundant dolomite facies are stromatolitic dolomite, and intraclastic, peloidal, oncoid and ooid grainstones. Magnesite is dominantly intraclastic with magnesite mudstones much less common. Terrigenous clastics are dominated by dolomite-cemented sandstones. These facies are arranged in somewhat monotonous repetitious sequences, 200-800 m thick in the southern Flinders Ranges, and 500-1 100 m and rarely 3 000 m thick in the northern Hinders Ranges. Magnesite Facies Magnesite facies form a significant component of the sequence in only a few areas; Depot Creek (11%), Arkaroola (6%), Myrtle Springs to Copley ( 1 8 - 2 1 % ) , and the southwestern Willouran Ranges (14%). Elsewhere magnesite comprises less then 4%. They are readily distinguished from the grey to dark-grey dolomitic facies by cream to yellow weathering outcrop surfaces, although fresh rock is cream to dark grey. The presence of magnesite was also confirmed by X-ray analysis and the staining of rock slabs using the method of Freidman (1959). The facies present are described below in relation to the evolution of sedimentary environments, rather than in order of abundance.

Interlaminated Magnesite-Dolomite Mudstones This rare facies comprises alternating dolomite and magnesite laminae 1-10 mm thick. The former are microsparitic and generally con-

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tain quartz silt and small magnesite intraclasts; the latter are commonly desiccated or upturned into tepees. The micritic magnesite laminae represent carbonate crusts formed on an exposed mudflat. Evaporative concentration of porewaters during exposure resulted in precipitation of magnesite at the sediment suface. The tepees formed on these crusts contain fractured, disrupted and occasionally crumpled magnesite laminae, with dolomitic sediment infilling the fractures and centres of the tepees. The apparent upward movement of dolomitic sediment into tepee centres, and its incorporation of pieces of magnesite crust, suggest that upwelling groundwaters may have contributed to tepee formation (von der Borch & Lock 1979). The associated dolomite laminae appear to be of detrital origin, and may represent sheetwash deposits resulting from flooding across a dry mudflat, or from sheetwash floodwaters entering ponded water on the muflat (Hardie et al 1978)

Magnesite Mudstone This minor facies of laminated to thinly bedded mudstone consisting largely of micritic magnesite but commonly containing a few per cent dolomite and very minor terrigenous sediment, is most abundant in the Copley-Myrtle Springs area (1-3%). Beds of magnesite mudstone generally overly dolomite mudstone and are most commonly erosionally overlain by intraclastic magnesite, although less commonly they are gradational into nodular magnesite. The mudstones are flat to wavy laminated with many small tepees (4-1 cm, rarely 40 cm in height) with buckled and fractured laminae, and minor lenses of intraclastic magnesite. Deposition of magnesite mudstones occurred in very low energy environments virtually free from detrital influx, possibly peripheral mudflats well shoreward of the zone of wave agitation, or lagoons physically separated from a larger basin. There is little evidence for detrital reworking of the unlithified magnesite mud, hence it may have been precipitated from lagoon waters and deposited from suspension, resulting in the formation of

homogeneous mud laminae similar to the protodolomite yoghurt muds forming in ephemeral lakes associated with the Coorong (von der Borch & Lock 1979). Further expansive precipitation within the sediment during exposure of these muds produced tepees. The micritic character of the sediment and the preservation of structures and textures suggest that the primary precipitate was probably a magnesium carbonate. This may have been one of the hydrated varieties rather than magnesite; the former, although metastable, are more easily synthesized as dehydration of the Mg-ion is not required during their precipitation.

Nodular Magnesite Minor interbeds of laminated magnesite mudstone are partly or completely replaced by a mosaic of cream magnesite nodules (Fig. 3b). The nodules consist of micritic magnesite which, in thin section, has a lustre-mottled appearance and is much paler than the generally slightly dolomitic host micritic magnesite (Fig. 3c). Some nodules occur as isolated botryoidal structures within the host sediment; others have a 'folded' appearance resembling the enterolithic structure of nodular anhydrite. They both disrupt and cross-cut lamination suggesting displacive and replacive growth. With continued nodular growth, the magnesite mudstone was replaced by a mass of coalescing nodules, separated by stringers of the host sediment. Completely replaced beds resemble intraclastic beds as some of the latter consist of close-packed nodulesourced intraclasts and have little matrix. However, the two facies are intergradational where erosion and transportation of intraclasts derived from nodular magnesite was minimal. Microscopic examination suggests nodule growth occurred by the formation of small discrete areas (0.15-0.3 mm) of pale magnesitic micrite within the darker host micrite. With continued growth, these small areas coalesced to form individual nodules which internally have a vague granular texture. This texture and the

Dolomite - magnesite sequence, Mundallio Subgroup

109

Fig. 3 a. Magnesite mudstone (cream coloured outcrop surface) with wavy lamination and possibly small tepees, overlying well laminated grey dolomite mudstone with a sharp boundary, Copley. Pen 15 cm in length; b. Nodular magnesite with botryoidal cream coloured nodules, Copley. Top of bed is to the left. Pen 15 cm in length; c. Magnesite mudstone (micritic) at left replaced by nodular magnesite at right, with isolated patches in left half. Nodular magnesite is lighter in colour, is micritic, and has a mottled lustre. White grains are authigenic albite. Sample from Copley, field of view is 8.5 mm in width, plane polarised light; d. Inverse graded intraclastic magnesite with very close-packed intraclasts at the base, and a greater amount of sandy matrix associated with coarser intraclasts at the top of the bed, Depot Creek. Pen 14 cm in length; e. Ripple cross-laminated intraclastic magnesite with granule sized intraclasts, southwestern Willouran Ranges. Pen 14 cm in length; f. Bedded intraclastic magnesite with variable clast size and content of sandy matrix between adjacent beds, some of which have an open framework. Inraclasts elongate to equidimensional. Location is Depot Creek, up is to left, hammer is 33 cm in length.

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Robin K. Uppill

lustre-mottled appearance are characteristic of all nodular magnesite, and enable easy recognition of intraclasts derived from it. Although this facies is now minor, intraclasts of this type are often dominant in beds of intraclastic magnesite.

tepees in magnesite mudstones represents shorter periods of exposure.

The nodular fabrics formed diagenetically within the sediment, probably at and immediately below the sediment surface as indicated by the frequent erosion of this facies. Nodules grew during periods of non-deposition following subaerial exposure of magnesite mudstones. Evaporative pumping (Hsu & Siegenthaler 1969) may have induced the migration of interstitial solutions into the vadose zone where micrite was precipitated as nodules. Lithification resulting from nodule growth protected the sediment surface from aeolian deflation.

This facies consists almost entirely of clastsupported beds of generally rounded magnesite intraclasts; matrix supported beds are uncommon. Individual horizons range from less than 0.1 to 5 m in thickness; the thicker horizons are internally bedded (Fig. 3f). Planar bounding surfaces, or an irregular erosional lower boundary on magnesite mudstones, are characteristic on outcrop scale for all except the thinnest interbeds (generally less than 10 cm, rarely to 50 cm), which are more lenticular. Outcrops are generally only continuous for a few tens or hundreds of metres, but some thicker units (1-2 m) can be followed for several kilometres (e.g. at Copley and Arkaroola). Hence, beds of intraclastic magnesite were deposited in extensive sheets (Forbes 1960).

The micritic character of magnesite in nodules, the similarity in texture of nodules at all stages of growth, and the preservation of identical texture in intraclasts reworked from nodular magnesite, suggest its precipitation as a primary magnesium carbonate. Although the nodules are similar in appearance to nodular anhydrite, the above evidence and the lack of relic sulphates or pseudomorphs after sulphates, argue against an origin involving the replacement of anhydrite nodules. The morphology is also similar to that of the nodular zones of some calcrete profiles. Nodular calcrete profiles 1-2 m in thickness, which occur in continental environments where they are formed by percolating rainwater, may take up to 100 000's of years to form (Gardner 1972; Chapman 1974). However, other nodular horizons, formed where there was a continous supply of groundwater, may develop much more rapidly. For example, nodular anhydrite has developed in 0.6-1.0 m of supratidal carbonate sediments in a 4 km wide zone of the Trucial Coast in the last 3 000 years (Shearman 1978). Hence interbeds of this facies may record a period of exposure and non-deposition of the order of several thousands of years during which nodular magnesium carbonate was precipitated in the sediment by displacive growth and replacement of magnesite mudstones. The development of

Intraclastic Magnesite

Elongate rounded intraclasts up to a maximum size of 5 cm and, in some cases 20 cm, are enclosed in a matrix of dolomite, dolomite and magnesite, or dolomite and siliciclastic sand. Beds of granule to small pebble size are generally well sorted and may contain current wave-formed structures such as tabular cross-bedding, ripple cross-lamination (Fig. 3e) and symmetrical ripple marks. Coarser beds are less well sorted and some have a bimodal distribution. Others are inversely graded (Fig. 3d) or, very rarely, normally graded. Inverse grading may be due to a high concentration of clasts and matrix in the transporting medium during high energy depositional events. As a result the larger intraclasts move to regions of least shear, away from the sediment surface, whilst the smaller clasts remain near the sediment surface (Davies & Walker 1974). The occurrence of intraclastic magnesite as extensive sheet deposits, and the lack of channelling, indicate low depositional gradients and erosion of magnesite from large, flat exposed

Dolomite - magnesite sequence, Mundallio Subgroup areas. In nearshore areas adjacent to the basin margin, landward derived unconfined sheet floods (Hardie et al 1978) may have caused extensive erosion of magnesite mudstone and nodular magnesite, and introduced the minor sand present as matrix. However, intraclastic magnesite is not solely confined to those areas probably close to the former basin margin. Within the basin, exposed magnesite mudflats, inundated during transgressive events which increased the wave fetch and energy, may have been eroded by wave undercutting during severe storms analogous to the process described by Hardie & Ginsburg (1977). Poorly sorted and inversely graded beds may represent deposition from single high energy events in which the clasts were transported largely in suspension and rapidly deposited, probably close to the source. Finer beds with intraclasts arranged approximately parallel to bedding indicate lower flow strengths with transport largely as bedload. Locally waves and currents reworked the magnesite intraclasts improving sorting and producing the sedimentary structures observed. Dolomite Facies Dolomite Mudstones Dolomite mud-supported sediments comprise 50-80% of outcrop in all areas with the exception of the Copley-Myrtle Springs and Yacka areas. Although most dolomite mudstones contain less than 25% terrigenous sediment and some are almost pure dolomite, impure shaly dolomites are also present in most areas, and are commonly more weathered than the purer dolomite mudstones. Flat to wavy lamination and thin bedding are characteristic, with planar to lenticular laminae of terrigenous sediment. Silt and sand also occur at the bases of some graded laminae. The dolomite is generally microsparitic, showing differences in grain size between adjacent laminae or through graded laminae (Fig. 4a). Clotted or grumous laminae, some grading into more distinctly peloidal laminae, and wispy

111

carbonaceous laminae which may be the relics of sediment poor algal mats, are also present. The flat, even to slightly wavy laminated mudstones appear to have formed by detrital deposition of dolomite mud and minor terrigenous detritus. Weak current activity transported silt and sand as bedload with the finer material in suspension. Mudstones with a more irregular wavy lamination may reflect the greater role of algae in the trapping of sediment and production of the laminated structure (Zamarreno 1975). Desiccation cracks, although generally present, are only locally abundant, and tepees are relatively uncommon (Fig. 4b). The latter are generally small, being in the embryonic class of Assereto & Kendall (1977). Disrupted sediment in the centre of some tepees, their small size, and association with desiccated beds, suggest formation under the influence of groundwater upwelling through indurated mudstone crusts (von der Borch & Lock 1979). Other features of dolomite mudstones include the presence of slump structures ranging from softsediment folds to semi-brittle fractures, diagenetic cross-cutting chert nodules, and rare indications of former evaporite minerals. The latter include small centimetre sized white nodules of quartz and dolomite spar, possibly after anhydrite, and rarely rosettes and lathshaped pseudomorphs after gypsum. Quartz within the nodules consists of coarse-grained mosaics of commonly cloudy crystals with undulose and flamboyant extinction and highly sutured boundaries, which often grade into radiating elongate crystals. Mosaic chert and length-slow chalcedony are minor components. Nodules containing quartz of similar texture and morphology and associated with relic sulphates, have been described in the literature as sulphate replacements (Siedlecka 1972, 1976). In summary, although this facies contains evidence of exposure in the form of desiccation cracks and tepees, these structures are not generally abundant and when combined with the lack of fenestral features, may indicate deposition

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Robin K. Uppill

Fig. 4 a. Dolomite mudstone with graded laminae containing scattered quartz silt grains and coarser dolomicrospar at base of laminae. Up is to left, sample from Copley. Plane polarised light, field of view 3.2 mm in width; b. Small eroded tepee in desiccated dolomite mudstone (weathering light-grey), underlain and overlain by grey dolomite-cemented very fine-grained sandstone, Copley. Lens cap 55 mm in diameter; c. Extensively silicified oncoid grainstone (bed identified with "o" and containing black chert), overlying dolomite mudstone ("M"). the boundary between silicified oncoid grainstone and unsilicified dolomite mudstone is sharp. Location Depot Creek, pen 14 cm in length; d. Extensively silicified elongate oncoids, lenticular lamination is thickened on the ends of grains and contains finely disseminated carbonaceous material. Very fine relic dolomite grains in partly silicified dolomite intraclasts forming oncoid nuclei. Sample from Copley, plane polarised light, field of view 2 mm; e. Horizon of gently domal stromatolites with minor small cross-cutting lenses of black chert, and overlain by a more fissile dolomite mudstone, Depot Creek. Pen 14 cm in length; f. Stromatolite microstructrure with grumous laminae at base containing dark carbonaecous clots of micrite enclosed in clear fine spar; overlain by microspar laminae with a more homogeneous texture (indistinctly grumous), variable carbonaceous staining and containing scattered quartz grains. Sample from a columnar stromatolite, central Willouran Ranges, plane polarised light, field of view 2 mm in width.

Dolomite - magnesite sequence, Mundallio Subgroup

in predominantly submerged environments in which dolomite mud, generally with minor terrigenous detritus, was deposited as thin detrital laminae. Algal mat-covered surfaces also trapped mud from suspension. On adjacent mudflats, storm-induced flooding deposited laminae of sand, silt, dolomite intraclasts and mud following erosion of desiccated crusts. As currents waned or in areas of ponded water on the mud-flats, laminae of dolomite mud formed, and were subsequently desiccated on exposure.

Dolomite Grainstones Oncoid grainstones, which are commonly partly replaced by black chert (Fig. 4c) form up to 1 % of the outcrop, and occur as plane bounded, thin to thick, rarely cross-bedded units interbedded with dolomite mudstone or as lenses in stromatolitic dolomite and intraclastic grainstone. Some thicker beds may be quite extensive, one 0.5 m horizon at Copley extends for at least 12 km. Textures of oncoid grainstones are well preserved in some silicified outcrops, and the majority of the grains fall in the micro-oncoid category of Dahanyake & Krumbein (1986). Most abundant are elongate oncoids, 0.2-2 mm in size, with lenticular laminae stained with carbonaceous matter (Fig. 4d) on nuclei of dolomite and magnesite intraclasts or quartz sand grains. These oncoids may form the nuclei for slightly larger oncoids with a relic radial structure and laminae of more even thickness. Oncoids with a more structureless appearance are approximately equidimensional grains with concavo-convex margins, and may enclose the concentrically laminated varieties. These textures are generally indistinct in unsilicified oncoid grainstones; compaction and recrystallisation of dolomite has obscured textural detail. The presence of carbonaceous material, lenticular lamination and the destruction of oncoid shape during diagenesis, suggest the oncoid grains were not rigid structures formed by inorganic precipitation, but consisted of an aggregate of carbonate mud and algal material. Formation

113

occurred in variably agitated submerged environments, the more concentrically laminated oncoids reflect the most agitated conditions. In lower energy environments with only intermittent wave or current activity, the more elongate oncoids formed, whilst the massive oncoids represent lowest energy conditions (Kaufftnan 1977; Dahanyake 1978). Intraclastic dolomite grainstone is a relatively minor facies, occurring as thin lenses associated with dolomite mudstones, ranging to beds 0.5 m in thickness, commonly with a sandy matrix, and which may grade to dolomitic sandstone with scattered intraclasts. Also of minor significance are peloidal grainstones which grade to packstones. They are similar in outcrop appearance to dolomite mudstones, particularly where the peloids are of silt size. The dark carbonaceous staining of some peloids may suggest an organic influence in their formation, but many were probably derived by erosion of dolomite mudstone. Ooid grainstones are the least abundant grain supported facies, the ooids are generally in part recrystallised with concentric and in some instances radial structure. Nuclei are most commonly grains of recrystallised dolomite; however, intraclasts of micritic dolomite and magnesite, and sand grains, also occur. The textural preservation and presence of magnesitic nuclei suggest ooid formation as a Ca-Mg carbonate, which was subsequently replaced by stoichiometric dolomite

Stromatolitic Dolomites Stromatolites are generally minor except at Depot Creek (9% of the sequence), however their common occurrence as massively outcropping, partly silicified biostromes results in conspicuous outcrops. Columnar, and small and large domal forms are present (Preiss, 1972). Columnar stromatolites occur in both bioherms and biostromes. Some of the latter are quite extensive, for example one 0.5-2 m thick biostrome at Copley is continuous for at least 12 km. Dolomite mudstones generally overly the bioherms,

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Robin K. Uppill

whereas grainstones (e.g. intraclastic and oncolitic) generally enclose biostromes. Small domal stromatolites, both as isolated and laterally linked domes, are also associated with dolomite mudstones (Fig. 4e). Large domal structures, 0.64 m in width and 0.4-4 m in height, were observed only in the Willouran Ranges, although similar structures were described by Fairchild (1975) from the Peake and Denison Ranges (Fig. 1). Columnar and domal stromatolites are finely laminated, the laminae are horizontal to steeply inclined and overturned, indicating the influence of algal trapping of sediment and/or in situ precipitation. Lamination is due to an alternation of lighter dolomicrospar laminae, darker finergrained laminae, and clotted or grumous laminae of irregular overlappping clots of carbonaceousstained micrite in clear microspar and fine spar (Fig. 4f). Detrital grains, peloids, oncoids and terrigenous silt are present in some of the coarser laminae. Clotted fabrics are common in modern stromatolites and algal laminated sediments

SUBMERGED

The extensive nature of stromatolite biostromes indicates uniform substrates and depositional environments over extensive areas. Association with dolomite mudstones suggests a low energy environment. Desiccation cracks and micro-unconformities are uncommon, hence exposure and erosional events were infrequent. In environments of greater wave and current action, stromatolite growth became restricted to bioherms, although the finer nature of the detritus within the stromatolites compared with the interspace and interbiohermal sediment, suggests that once columns developed, their growth may have been maintained by the ability of algal mats to trap sediment from suspension.

EXPOSED Above

Deposition of dolomite mudstones

(Monty 1967, 1976; Hardie & Ginsburg 1977), and are also described in ancient stromatolites (Fairchild 1980). Proposed origins involve algal trapping of peloids, precipitation of carbonate around algal filaments, and compaction of the algal mat. In this sequence the framework of micritic clots and the very dark carbonaceous staining in some laminae, suggest precipitation of carbonate around algal filaments.

water table (vadose)

Below water table (phreatic) Undersaturated

Lithif i c a t i o n of surface sediments? possibly some fenestra! development

Dissolution give

breccia

Precipitation of coated grains, ? peloids Laminated carbonate precipitated on exposed surfaces

(Erosion)-

Fig. 5 Summary of the formation of diagenetic textures in massive dolomites.

Saturated Acicular

rim

cements

Dolomite - magnesite sequence, Mundallio Subgroup

Massive Diagenetic-Textured Dolomites The dolomite facies described above have generally experienced a simple diagenetic history; lithification and compaction apparently occurred without significant destruction of primary texture, although pervasive recrystallisation may have destroyed finer detail. Outcrops in the central part of the southern Flinders Ranges however, contain interbeds of massively outcropping, poorly bedded dolomites with complex diagenetic textures. Pseudobreccias are most common, with angular and irregular fragments of micrite, microspar and peloids, enclosed in a thin rim cement and more equant blocky spar. Textures indicate in situ brecciation and dissolution of lithified mudstones, with subsequent cementation of the brecciated sediment. Other dolomite beds consist of peloidal grains in association with larger more irregular but rounded fragments of micrite-microspar, and minor coated grains. Rim and blocky cements again fill the highly irregular pore space. These 'peloidal' textures also appear to be diagenetic, and formed following dissolution of mudstones and redeposition of micrite as peloids and coated grains analagous to the formation of inorganic peloids and coated grains in calcrete profiles (Harrison 1977; Harrison & Steinen 1978). Coated grain formation in situ is apparently unique to vadose zone diagenesis under exposed conditions (Harrison 1977). In contrast, the observed cement textures appear to be phreatic in origin (e.g. lack of incomplete rims, meniscus cements, or gravitational pendant cements; Longman 1980). Vadose diagenesis involving both dissolution and precipitation to give pseudobreccia and peloidal fabrics is indicated (Fig. 5), possibly due to prolonged exposure of low relief islands within the basin. These apparently lacked the fringing exposed mudflats which elsewhere developed as sites of magnesite and nodular magnesite deposition. In the Worumba Anticline (Fig. 1), these islands appear to have been features of greater relief, and were the source of major tongues of sedimentary megabreccia which were deposited in large channels possibly in a subaerial environ-

115

ment (Preiss 1985). The enormous size-range of clasts in the megabreccias suggests the emerging islands were the sites of abrupt active fault scarps (Preiss pers. comm. 1986). Terrigenous Facies Dolomitic Sandstones In the southern half of the study area, sandstones are largely fine to medium grained, and dolomite or rarely quartz-cemented, whereas to the north they are predominantly dolomitic, very fine-grained and grade to dolomitic siltstones (siliciclastic). The coarser grained sandstones are dominantly flat to slightly wavy laminated and thin bedded, with minor tabular cross beds occurring as isolated sets to 30 cm, rarely 2 m, and with laminae generally tangential to the lower bounding surfaces. Ripple marks are largely symmetrical, and their internal lamination appears to be wave modified current ripple lamination. Occasional flaser lamination is defined by dolomite mudstones lenses, whereas thin planar interbeds and laminae of the latter are occasionally desiccated. This range of sedimentary structures is similar to that present in waveformed sand bars (de Raaf et al 1977; Roep et al 1979). Flat lamination reflects deposition of sand from suspension during storms, while cross-beds and ripples formed in shallower more turbulent water. In the nearshore zone, dolomite mud laminae were deposited from waning currents on shallow to emergent sandflats. Finer grained sandstones commonly contain dolomite peloids, and there is a gradation from dolomitic sandstone to sandy dolopelsparite. The sandstones contain intervals of flat lamination alternating with sets of ripple cross-lamination. The latter form complexly interwoven, commonly bidirectional sets with laminae tangential to the erosive bases, resembling the lamination formed in wave ripples or combined wave-current ripples (Harms et al 1975; de Raaf et al 1977). This interpretation is supported by the dominance of symmetrical ripple marks, and the rounded profiles and straight crests of asymmetrical rip-

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pies. Some flaser lamination is defined by dolomite mud laminae, the latter also infill synaeresis cracks and the uncommon desiccation cracks. The finer grain size of sandstone interbeds in the northern Flinders Ranges is probably a function of sediment source, however the sedimentary structures indicate a similar environment of deposition to that of the coarser sandstones to the south.

Non-Dolomitic Shales and Siltstones The fine-grained component of this facies association is dominated by dolomite mudstones, albeit at times somewhat shaly. Terrigenous muddy sediments are much less abundant, largely occurring as rare 5-15 m thick units which are non-dolomitic, and at times form the lower part

a™ u -i I< Z V ) C"Do 2o Z uu ® (0 C O5 ' E a ™ .2 .2 Eg J s CO CE <D £ g> V) c a ? - ® ® ® >Q) <S O £ UJ Z l5/>£D — u. >> 5 o^ oC 5 Q </) OQ u j (/)K XZ

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I II I II

Fig. 6 Section through part of the Yadlamalka Formation at Copley, representing the interval from 425-495 m which is the most magnesite rich portion of the sequence. The drill hole intersected the down dip equivalent of the costean exposure. Both were completed in an attempt to ascertain the economic potential of the magnesite. Some discrepances between the costean and drill hole are due to the very weathered nature of some magnesite outcrops and the lack of 100% core recovery.

of coarsening upward shale-sandstone cycles. Locally however, shales and siltstones, some dolomitic, are more significant (e.g. Yacka, Yatina, central-east Willouran Ranges). The lack of sedimentary structures indicates a low energy submerged environment of deposition. FACIES ARRANGEMENT The facies described above are generally arranged in somewhat monotonous sequences characterised by rapid vertical alternations between the various carbonate and terrigenous facies, on the scale of tens of centimetres to a few metres at most. Due to the dominance of only one or two facies in many areas (e.g. dolomite mudstones with either dolomitic sandstones or intraclastic magnesite), there is commonly no clearly defined cyclicity. However, magnesite mudstones where present are almost invariably overlain by intraclastic magnesite, and interbeds of the latter are commonly associated with sandstones where both are present. In addition, where the major facies are all present in significant amounts, Markov Chain Analysis using the method of Mi all (1973) indicates a preferred cycle of intraclastic magnesite - dolomitic sandstone - dolomite mudstone - magnesite mudstone (- nodular magnesite) (Figs. 6,7). This cycle represents a progressive change from clastic to chemical deposition, and a shallowing environment, at least in the upper part. Magnesite mudstones were deposited in shallow protected lagoons; their deposition led to vertical accretion and exposure, and the subsequent replacement of bedded mudstones by nodular magnesite. Exposure may also have been accompanied by a lowering of basin water level (e.g. due to external factors such as a more arid climate). Conversely, increasingly humid conditions may have resulted in inundation and extensive erosion of magnesite mudflats, and the subsequent deposition of intraclastic magnesite. Siliciclastic sand was introduced during, and subsequent to, deposition of intraclastic magnesite. The lack of associated terrigenous mud and the rounded to well-rounded nature of medium- to

117

Dolomite - magriesite sequence, Mundallio Subgroup

Costean

Drillhole

CREEK

Pw 2*km S -

Costean

LITHOLOGY

location

SEDIMENTARY STRUCTURES _ ^ [

[

^ Chert nodules

Flat l a m i n a t i o n t h i n bedding W a v y lamination t h i n bedding M e d i u m to t h i c k bedding Ripple cross lamination

Disrupted

'At

Tepees

bedding

AA Stromatolites

COLOUR DG

Dark

Grey

Light

Cream

White

-v- D e s i c c a t i o n

cracks

Synaeresis

cracks

M a x i m u m clast s i z e in c m

grey

LITHOLOGICAL BOUNDARIES .

Sharp Gradational Irregular

Fig. 7 Summary of the intraclastic magnesite-dolomitic sandstone-dolomite mudstone-magnesite mudstone-nodular magnesite cycle. Depositional environments A - high energy erosional and depositional events, sheet flooding and wave undercutting; B - submerged sand sheets and sand shoals, wave processes; C - submerged dolomite mudflats with fringing exposed areas; D - ephemeral magnesite lakes; E - exposed magnesite mudflat.

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Robin K. Uppill

veiy coarse-grained sand, suggest that it may have been derived from aeolian deposits adjacent to the basin of deposition. An expanding basin may have encroached upon adjacent dune fields, or the dunes themselves may have prograded into the basin (Fig. 8) as occurs along parts of the Persian Gulf today (Fryberger et al 1983). The sand was subsequently reworked, largely by waves on shallow sandflats close to the source, and into wave-formed bars further along shore. Only minor sand reached the more central areas of the basin. The transition from deposition of sandstones to dolomite mudstones may have resulted from changes in the source area, or within - basin topographic changes as sand shoal development produced sheltered lagoons and mudflats.

Progradation of mudflats by simple vertical accretion led to the development of more isolated ephemeral lagoons in which dolomite deposition was subsequently replaced by magnesite deposition.

FACIES DISTRIBUTION Northern Flinders and Willouran Ranges At Arkaroola, submerged to emergent dolomite mudflats were the dominant environment during deposition of the lower part of the Yadlamalka Formation (Fig. 9). Intraclastic magnesite is generally fine-grained, indicating that the source magnesite lagoons lay largely outside the present outcrop area, although occasional in-

prevailing w i n d ^ sand dunes interdune / depressions

fanning sheet

out

flow

1 0 - 3 0 km

Sandstone

Dolomite

peloids

Dolomite mudstone

Dolomite

intraclasts

Intraclastic

magnesite

Magnesite mudstone

Nodular

magnesite

Stromatolites

O A

Desiccation cracks Tepees Wave

oscillation

Fig. 8 Schematic block diagrams illustrating the formation of the major facies. A - Erosion of magnesite mudstone and nodular magnesite by sheet flooding and wave action as the basin expands. Some introduction of sand. B Magnesite mudstone is now largely eroded, and sand deposition is more widespread as aeolian sand is transported into the basin where it is reworked by wave processes. C - Sand supply largely exhausted, and energy levels in the basin reduced due to shallowing and progradation, resulting in the widespread deposition of dolomite muds. Minor magnesite deposition occurs in isolated lakes formed as the water level falls. D - With continued fall in the water level, magnesite deposition becomes more widespread, and there are extensive exposed mudflats.

Dolomite - magnesite sequence, Mundallio Subgroup terbeds of magnesite mudstone do record their presence. Interbeds of dolomitic sandstonesiltstone are more abundant higher in the sequence reflecting source area changes and/or more frequent sheet flooding. Lateral facies changes within the 40 km strike extent in this area are minor. Along a similar stike extent in the CopleyMyrtle Springs area, facies changes are also minor apart from an increase in the abundance of sandstone (22-33%, Figure 9). However the thickness of the formation increases by 50%, which, along with the lack of facies changes and the continuity of some thin units, indicates that the rate of deposition generally matched the rate of subsidence, and shallow water environments and low palaeoslopes were maintained over the

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whole area. As at Arkaroola, dolomite mudflats were the dominant depositional environment in the lower part of the Yadlamalka Formation, whilst sand influx and magnesite deposition increased upwards possibly reflecting increased proximity to the basin margin. In the Willouran Ranges, outcrops occur in four approximately strike-parallel zones within which the proportions of facies remain grossly similar. However, they vary considerably across strike. Grain size and the abundance of intraclastic magnesite, and the abundance of magnesite mudstone, decrease from southwest to northeast. Magnesite is rare in the eastern half of the ranges, where dolomitic sandstones and siltstones dominate, reflecting a major source area on that side of the basin. Major thickness changes across Depositional Environments

/o-«/o * V ? 7°

mm h ° °IQ °i

Outcrop limits Adelaidean & Cambrian

Mundallio outcrops Basement

Subgroup inliers

Fig. 9 Distribution of the major facies associations and depositional environments, Mundallio Subgroup, northern Flinders Ranges. Depositional environments: A - alternating deposition of dolomite mudstones on submerged to occasionally emergent mudflats, sandstones on wave agitated sand shoals, magnesite in ephemeral lagoons, and intraclastic magnesite during high energy events (Yadlamalka Formation); B - deposition of dolomite mudstones, sandstone and siltstones in largely submerged environments (Mirra Formation, central Willouran Ranges); C deposition of sandstones and dolomite mudstones in largely submerged environments (Mirra Formation, eastern Willouran Ranges); D - areas of limited deposition on Callana Group highs.

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strike are apparently due to extensional block faulting during deposition and although the basin was probably a continous feature in this area (Belperio 1986), locally rapid thickness and associated facies changes suggest at least the intermittent presence of syndepositional highs (Murrell 1977). In the northern Flinders Ranges a similar spectrum of depositional environments is indicated at Arkaroola, Copley and in the southwestern half of the Willouran Ranges (Fig. 10). The only interruption to the deposition of dolomite mudstone, dolomitic sandstone and intraclastic magnesite, was the minor but locally widespread deposition of shale or coarsening upward shale to sandstone sequences (Fig. 10) possibly reflecting transgressions over a basin with featureless topography so that large areas were deepened to below wave base.

Southern Flinders Ranges Within this region, similar facies are again present in all areas, although the thickness of the Yadlamalka Formation and the proportion of facies within it varies significantly (Fig. 11). Very sandy sequences are present near Yacka, with sandstone abundance decreasing to both the north and east. West of Yacka, the Yadlamalka Formation appears to pass laterally into a sandy sequence included within the underlying Bungaree Quartzite. Signs of desiccation are also relatively common in this area, hence it may have been relatively close to the basin margin. Northward at Depot Creek in the Emeroo Range, intraclastic magnesite is more abundant than in other outcrops of this region, and commonly contains clasts in the 5-15 cm range reflecting deposition close to source. However, magnesite mudstones were rarely preserved.

CP2

CP1

Fig. 10 Summary stratigraphic sections of the Yadlamalka Formation in the northern Flinders and Willouran Ranges showing the percentages of the major facies. Dolomite - all dolomite facies, Magnesite - all magnesite facies; Sandstone - sandstone and coarse-grained siltstones; Shale - all other siltstones and shales, generally non-dolomitic. For locations, see Fig. 10.

Dolomite - magnesite sequence, Mundallio Subgroup In the central southern Flinders Ranges, dolomite mudstones predominate (Fig. 11), but often contain abundant evidence of exposure. Sandstones are thinner and less abundant than to the west, but where present are commonly associated with intraclastic magnesite or desiccated

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dolomite mudstones. In the Yednalue, Yednalue Anticline and Worumba Anticline areas, periods of non-deposition and diagenesis under subaerial conditions, possibly on small islands resulting from local uplift, are indicated by interbeds of massive diagnetic textured dolomites.

Depositional Environments

Outcrop

limits

Adelaidean & Cambrian

Mundallio Subgroup outcrops

Basement inliers

100 km

Fig. 11 Distribution of major facies associations and depositional environments, upper Mundallio Subgroup, southern Flinders and Mt Lofty Ranges. Depositional environments: A - sandstone deposition with minor shales and dolomites (Bungaree Quartzite); B - grey dolomite mudstones deposited on submerged to emergent mud-flats, with associated deposition of magnesite and sandstone (Yadlamalka Formation and Montacute Dolomite) ; C deposition as above but with periods of exposure following minor uplift (Yadlamalka Formation) ; D - grey dolomite mudstones deposited on submerged to occasionally emergent mudflats (upper part of the Skillogalee Dolomite); E - laminated grey shales deposited in submerged environments (Woolshed Flat Shale).

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Mount Lofty Ranges In the Adelaide region, the Montacute Dolomite contains the typical dolomite mudstone-sandstone-magnesite facies. The vertical facies distribution indicates the predominance of submerged dolomite mudflats during deposition, although marginal exposed areas were locally sites of magnesite deposition. Transgressive-based cycles of sandstone with magnesite intraclasts followed by dolomite mudstone, and interlaminated dolomite-magnesite mudstone with desiccation cracks and tepees, are preserved in this area.

widespread distribution of very shallow water facies and continuity of some individual beds imply low palaeoslopes. Hence, the higher energy deposits formed as extensive sheets and bars, rather than as channellised deposits, and small changes in water level would have caused alternate flooding and exposure of large areas both within and marginal to the basin. The common abrupt vertical facies changes imply a constantly shifting mosaic of sedimentary environments. Origin of Carbonates

General Environmental Comments

Textural evidence from the various dolomite and magnesite facies and the association of dolomite and magnesite indicate that both dolomite and magnesite were either present as discrete minerals at the sediment surface and hence were essentially primary minerals, or replaced closely related minerals virtually comtemporaneously with deposition. The preservation of fine details of detrital texture in dolomite facies suggests the primary mineral was a Ca-Mg carbonate (e.g. calcian dolomite or high Mg-calcite) since early dolomitization of these is fabric retentive, whereas dolomite replacement of aragonite or low Mg-calcite precursors tends to be more fabric destructive (Tucker 1983). A similar conclusion may be drawn from the preservation of stromatolitic microstructures and ooid textures.

The dolomite-magnesite-siliciclastic facies association discussed above was deposited in a very shallow, low relief basin extending across the southern and northern Flinders Ranges; southward deeper water environments, which were sites of shale deposition, dominated (Figs. 2,11). Within the carbonate basin, facies with features indicative of exposure occur in all areas and are not confined to outcrops around the margin of the geosyncline as defined by present day outcrop limits. Hence shallow water conditions prevailed thoughout, and the depositional basin is envisaged to have comprised a series of shallow perennial and smaller ephemeral lagoons and lakes containing or partly enclosed by exposed mudflats or low relief islands. The

Dolomite mudstones contain thin beds and laminae of homogeneous dolomicrospar with little terrigenous component; these may have originated by a process analagous to that producing yoghurt muds in the Coorong Lake system. Here, in small physically isolated lagoons, precipitated carbonate mud is stirred into a water saturated slurry by wind driven circulation. As the water level in the lagoons falls, this slurry is deposited as layers of homogeneous structureless micrite, or yoghurt muds (von der Borch & Lock 1979). However, aspects of the texture of dolomite mudstone (e.g. scattered dolomite peloids in dolomicrospar, the presence of terrigenous silt and fine sand often in lenticular laminae or at the base of graded laminae, and

In the northern part of the Mount Lofty Ranges, a thin lenticular unit dominated by grey wavy laminated dolomite mudstones in the upper Skillogalee Dolomite may be equivalent to the Montacute Dolomite and Yadlamalka Formation. Minor stromatolitic interbeds are present, and the mudstones themselves may be largely algal laminates. Magnesite is very rare and desiccation features are minor, indicating deposition on low energy subaqueous mudflats with little terrigenous input. DISCUSSION

Dolomite - magnesite sequence, Mundallio Subgroup

small scours), indicate that detrital deposition predominated over direct deposition following precipitation. Detrital fabrics within carbonate mudstones have been described by other workers (Zamarreno 1975; Hardie & Ginsburg 1977; Smoot 1978), and generally result from periodic depositional events which transfer sediment from sites of carbonate precipitation to adjacent areas. Within the Mundallio Subgroup erosion of lithified dolomite crusts and tepees, and stromatolitic dolomites, may have contributed minor amounts of dolomite mud and peloids to the shallow submerged mud flats which were the dominant environment of deposition of dolomite mudstones. Ultimately, however, most of the dolomite must have been derived by chemical precipitation within the basin. Inorganic carbonate precipitation of carbonate muds occurs in several modern lacustrine envionments (Kelts & Hsu 1978; von der Borch & Lock 1979). Precipitation may be induced by a number of factors, for example by evaporative concentration (with the resultant CO2 degassing and pH increase; Eugster & Jones 1979), by a temperature increase, by mechanical release of CO2, or by biogenic factors such as removal of CO2 by algal photosynthetic activity. Supersaturation may also result from the mixing of alkaline brines with dilute inflow waters (Eugster & Maglione 1979). Climatic conditions may result in variation in one or more of the above factors on a seasonal or longer term basis. Carbonate (dolomite or a precursor) precipitation may have been greatest in the shallower environments, subjected to greater salinity variations. The sediment formed was then subjected to several periods of transportation, deposition and re-erosion as energy and water levels varied. In contrast to dolomite mudstones, magnesite mudstones experienced little reworking by currents prior to lithification. Some magnesite formed on mudflats as thin micritic crusts overlying detrital laminae of silty dolomite mud. During exposure, evaporative concentration causing increased pH and CO32" concentration led to dolomite and then magnesite precipitation. Most magnesite, however, appears to have

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formed in shallower ephemeral lagoons with little detrital influx; such lagoons may have been partly fed by groundwater seepage from adjacent larger water bodies. The water level thus rose and fell due to seasonal changes in evaporation rates, and at times fell below the sediment surface. Storm flooding across mudflats was probably only a minor contributor as indicated by the very limited detrital influx. Although magnesite mudstones generally contain minor dolomite, boundaries between magnesite mudstones and underlying dolomite mudstones are generally sharp. Hence much of the magnesite was precipated as a primary magnesium carbonate rather than forming as a dolomite replacement. The initial precipitate may have been a metastable hydrated Mg-carbonate, however during periods when the lagoons were dry, further evaporative concentration of interstitial solutions (and hence reduced water activity) would have resulted in the inversion of the hydrated Mg-carbonate to magnesite. Efflorescent crusts of saline minerals may have formed during exposure of the mudflats, but were removed by aeolian deflation or during subsequent flooding of the lakes. During more prolonged periods of exposure in which the water table remained consistently below the sediment surface (thus inhibiting the formation of surface evaporites), magnesite mudstones were partly replaced by nodular magnesite. A sequence of widespread and extensive dolomite of 'primary' origin with associated magnesite (locally up to half as abundant as dolomite), in which calcite is absent, and in which sulphates appear to have been a minor constituent, is unlikely to have formed in a basin with waters of a marine chemistry. Although widespread dolomite deposits have formed in a marine environments, because of the excess of Ca 2+ and Mg 2+ over CO32" plus HCO3" and the abundance of SO42" in seawater, magnesite is only a minor component of such sequences, and its precipitation is preceded by or occurs in association with Ca-sulphates or even more soluble salts. The mineral sequence forming on evapora-

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tion in the system Ca-Mg-C03-SC>4-H20 at 25°C (Eugster & Hardie 1978) can only contain extensive dolomite and magnesite without coprecipitation of sulphates, if CO3 2 " is present in solution in significant amounts compared with SO42". However magnesite in the Mundallio Subgroup does not contain associated sulphate minerals and there is little evidence that they were ever present in abundance. Hence this sequence was probably deposited in an extensive shallow lake or inland sea with basin waters containg Mg 2+ , Ca 2 + and HCO3" as the major ions. The Mg 2 + + Ca 2 + content was probably approximately balanced by HCO3" (Eugster & Maglione 1978). Because of the absence of calcite, the Mg/Ca ratio exceeded one. The sulphate content relative to carbonate was probably lower than that of seawater, since low sulphate activity enhances the precipitation of dolomite (Baker & Kastner 1981).

SUMMARY During deposition of the upper part of the Mundallio Subgroup, a shallow elongate intracratonic basin existed in the area of the Mt Lofty and Hinders Ranges in South Australia, and was the site of both clastic and carbonate deposition (Fig. 12). The uniform carbonate mineralogy of dolomite and magnesite, and similarity in carbonate facies over wide areas of the basin, indicates uniform sedimentary processes and water chemistry in a basin at least 600 km in length and 100-200 km in width. The basin probably had a non-marine chemistry, and circulation with the open ocean was limited. Within the basin there was little variation in water depth over wide areas, and environments which experience subaerial exposure developed at times in most areas.

YE2

Dolomite Magnesite Sandstone Shale

YD A

Nathaltee Formation

Fig. 12 Summary stratigraphic sections of the Yadlamalka Formation, southern Flinders Ranges. Dolomite = all dolomite facies, magnesite = all magnesite facies, sandstone = sandstones and minor coarse-grained siltstones, shales = all other siltstones and shales, generally non-dolomitic.

Dolomite - magnesite sequence, Mundallio Subgroup Dolomite was derived largely from the precipitation of mud and peloids within the basin, with the initial precipitates probably containing protodolomite and high Mg-calcite which subsequently inverted to dolomite during early diagenesis. The dolomite mud commonly experienced reworking before final deposition. Magnesite mud, which may have been initially precipitated as hydrated Mg-carbonates, experienced only minor stirring by wave action, and was deposited at essentially the same site as the initial precipitation. Prolonged periods of exposure led to lithification of magnesite mudstones following which they were extensively eroded into intraclastic beds. ACKNOWLEDGEMENTS This paper is based on work undertaken during the tenure of a Commonwealth Postgraduate Research Award at the University of Adelaide. The author wishes to thank her supervisors, Brian Daily and John Jones. Thanks must also go to Wolfgang Preiss, Victor Gostin and Grant Young for reviewing a draft of this paper. REFERENCES AMBROSE G.J., FLINT R.B. & WEBB A.W. 1981. Precambrian and Proterozoic Geology of the Peake and Denison Ranges. Geological Survey of South Australia, Bulletin 50. ASSERETO R.L.M. & KENDALL C.G. St. C. 1977. Nature, origin and classification of peritidal tepee structures and related breccias. Sedimentology 24, 153-210. BAKER PA. & KASTNER M. 1981. Constraints on the formation of sedimentary dolomite. Science 213, 214-217. BELPERIO A.P. 1986. Stratigraphy and sedimentology of the Precambrian Skillogalee Dolomite, northern Flinders Ranges. Geological Society of Australia, Abstracts 15,29.

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CHAPMAN R.W. 1974. Calcareous duricrusts in Al-Hasa, Saudi Arabia. Geological Society ofAmercia,

Bulletin 85,119-130.

COATS R.P. & BLISSETT A.H. 1971. Regional and economic geology of the Mount Painter Province. Geological Survey of South Australia, BulletinB 43. DAHANYAKE K. 1978. Sequential position and environmental significance of different types of oncoids. Sedimentary Geology 20, 301-316. DAHANYAKE K. & KRUMBEIN W.E. 1986. Micriobial structures in oolitic iron formations. Mineralium Deposita 21, 85-94. DAVIES I.C. & WALKER R.G. 1974. Transport and deposition of resedimented conglomerates. The Cap Enrage Formation, Cambro-Ordovician, Gaspe, Quebec. Journal of Sedimentary Petrology 44, 1200-1216. DE RAAF J.F.M., BOERSMA J.R. & VAN GELDER A. 1977. Wave-generated structures and sequences from a shallow marine succession, Lower Carboniferous, County Cork, Ireland. Sedimentology 24,451-483. EUGSTER H.P. & HARDIE L.A. 1975. Sedimentation in an Ancient Playa-Lake Complex: the Wilkins Peak Member of the Green River Formation of Wyoming. Geological Society ofAmerica, Bulletin 86, 319-334 EUGSTER H.P. & HARDIE L.A. 1978. Saline Lakes. In Lerman A. ed. Lakes - Chemistry, Geology, Physics. pp. 237-293, Springer-Verlag, New York. EUGSTER H.P. & JONES B.F. Behaviour of major solutes during closed-basin brine evolution. American Journal of Science 279, 609-631. EUGSTER H.P. & MAGLIONE G. 1979. Brines and evaporites of the Lake Chad Basin. Geochimica Cosmochimica Acta 43, 973-981. FAIRCHILD I.J. 1980. Sedimentation and origin of a late Precambrian 'Dolomite' from Scotland. Journal of Sedimentary Petrology 50, 423-446.

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FAIRCHILD T.R. 1975. The Geologic Setting and Paleobiology of a Late Precambrian Microflora from South Australia. Ph.D. thesis, University of California, Los Angeles (unpubl.). FORBES B.G. 1960. Magnesite of the Adelaide System: petrography and descriptive stratigraphy. Transactions of the Royal Society of South Australia 83,1-9. FORBES B.G. 1961. Magnesite of the Adelaide System: discussion of its origin. Transactions of the Royal Society of South Australia 86, 217-222. FORBES B.G. & PREISS W.V. 1987. Stratigraphy of the Burra Group. In Preiss W.V. compiler. The Adelaide Geosyncline: Late Proterozoic Stratigraphy, Sedimentation, Palaeontology and Tectonics. Geological Survey of South Australia, Bulletin 53, 73-123 FOX K.F. & REINHART C.C. 1968. Geology of the magnesite deposits in northern Okanogan County, Washington - a preliminary report. United States Geological Survey, Bulletin 1272B. FRIEDMAN G.M. 1959. Identification of carbonate minerals by staining methods. Journal of Sedimentary Petrology 29, 87-97. FRYBERGER S.G., AL-SARI A.M. & CLISHAM T.J. 1983. Eolian dune, interdune, sand sheet and siliciclastic sabkha sediments of an offshore prograding sand sea, Dhahran area, Saudi Arabia. American Association of Petroleum Geologists, Bulletin 67, 280-312. GARDNER L.R. 1972. Origin of Mormon Mesa Caliche, Clark County, Nevada. Geological Society of America, Bulletin 83, 143-156.

HARMS J.C., SOUTHARD J.B., SPEARING D.R. & WALKER R.G. 1975. Depositional environments as interpreted from primary sedimentary structures and stratification sequences. Society of Economic Paleontologists and Mineralogists, Short Course 2. HARRISON R.S. 1977. Caliche profiles: indicators of near-surface sub-aerial diagenesis, Barbados, West Indies. Bulletin of Canadian Petroleum Geology 25, 123-173. HARRISON R.S. & STEINEN R.P. 1978. Subaerial crusts, caliche profiles, and breccia horizons: comparisons of some Holocene and Mississippian exposure surfaces, Barbados and Kentucky. Geological Society of America, Bulletin 89, 384-396. HSU K.J. & SIEGENTALER C. 1969. Preliminary experiments on hydrodynamic movement induced by evaporation and their bearing on the dolomite problem. Sedimentology 12,11-25. KAUFFMAN M.E. 1977. Oncoliths and ooliths comparable indicators of environment. Geological Society of America, Abstracts with Programs 9 (5), 613-614. KELTS K. & HSU K.J. 1978. Fresh water Carbonate sedimentation. In Lerman A. ed Lakes - Chemistry, Geology, Physics, pp. 295-323, Springer-Verlag, New York. KRALIK M. & HOEFS J. 1978. Die isotopen-zusammensetzung der karbonate in der magnesitlagerstotte Eugui (Westpyrenaen) (The isotopic composition of carbonates in magnesite deposit Eugui (Western Pyrenees). Tschermarks Mineralogische und Petrographische Mitteilungen 25, 185-193. LESKO I. 1972 Uber die bildung von magnesit lagerstatten. Mineralium Deposita 7, 61-72.

HARDIE L.A. & GINSBURG R.N. 1977. Layering: the origin and environmental significance of lamination and thin bedding. In Hardie L.A. ed Sedimentation of the Modern Carbonate Tidal Flats of Northwest Andros Island, Bahamas. John Hopkins University Studies in Geology 22, 50-123.

LONGMAN M.W. 1980. Carbonate diagenetic textures from nearsurface diagenetic environments. American Association of Petroleum Geologists, Bulletin 64,461-487.

HARDIE L.A., SMOOT J.P. & EUGSTER H.P. 1978. Saline lakes and their deposits: a sedimentological approach. International Association of Sedimentologists, Special Publication 2,7-41.

MACGREGOR B.I. & BLISS N.W. 1968. The Barton Farm Magnesite Depost Gatooma, Rhodesia. Geological Society of South Africa, Transactions 71, (Annexure), 159-174.

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MIALL A.D. 1973. Markov Chain analysis applied to an ancient alluvial plain succession. Sedimentology 20, 347-364.

SHEARMAN D.J. 1978. Evaporites of coastal sabkhas. Society of Economic Paleontologists and Mineralogists, Short Course 4, 6-42.

MONTY C.L.V. 1967. Distribution and structure of Recent stromatolitic algal mats, eastern Andros Island, Bahamas. Societie Geologique de Belgique, Annates 90, 55-100.

SIEDLECKA A. 1972. Length slow chalcedony and relics of sulphates - evidence of evaporitic environments in the Upper Carboniferous and Permian beds of Bear Island, Svalbard. Journal of Sedimentary Petrology 42, 812-816.

MONTY C.L.V. 1976. The origin and development of cryptalgal fabrics. In Walter M.R. ed Stromatolites. Developments in Sedimentology 20, pp 193-249, Elsevier, Amsterdam MURRELL B. 1977. Stratigraphy and tectonics across the Torrens Hinge Zone between Andamooka and Marree. Ph.D. thesis, University of Adelaide (unpubl.). NISHIHARA H. 1956. Origin of the bedded magnesite deposits of Manchuria. Economic Geology 51, 698-711. PETRASCHECK W.E., KRALIK M. & RANZENBACHER A. 1977. The stratabound magnesite deposit of Eugui-Asturreta in the Spanish Pyrenees. In Klemm D.D. and Schneider M.J. eds. Time and Stratabound Ore Deposits, pp. 254-259, Springer-Verlag, Berlin. PREISS W.V. 1972. The systematics of South Australian Precambrian and Cambrian stromatolites. Part 1. Transactions of the Royal Society of South Australia 96, 67-100. PREISS W.V. 1973. Palaeoecological interpretations of South Australian Precambrian stromatolites Journal of the Geological Society of Australia. 19, 501-532. PREISS W.V. 1983. Depositional and tectonic contrasts between Burra Group and Umberatana Group sedimentation. Geological Society of Australia, Abstracts 10, 13-16.

SIEDLECKA A. 1976. Silicified Precambrian evaporite nodules from northern Norway: a preliminary report. Sedimentary Geology 16,161-175. SMOOT J.P. 1978. Origin of the carbonate sediments in the Wilkins Peak Member of the lacustrine Green River Formation (Eocene), Wyoming, U.S.A. International Association of Sedimentologists, Special Publication2, 109-127. TUCKER M.E. 1982. Precambrian dolomite: petrographic and isotopic evidence that they differ from Phanerozoic dolomites. Geology 10, 7-12. TUCKER M.E. 1983. Diagenesis, geochemistry and origin of a Precambrian dolomite: the Beck Spring Dolomite of eastern California. Journal of Sedimentary Petrology 53, 1097-1119. UPPILL R.K. 1979. Stratigraphy and depositional environments of the Mundallio Subroup (new name) in the late Precambrian Burra Group of the Mt Lofty and Flinders Ranges. Transactions of the Royal Society of South Australia 103, 25-43. UPPILL R.K. 1980. Sedimentology of the late Precambrian Mundallio Subgroup: a clastic-carbonate (dolomite, magnesite) sequence in the Mt. Lofty and Flinders Ranges, South Australia. Ph.D. thesis, University of Adelaide (unpubl.)

PREISS W.V. 1985. Stratigraphy and tectonics of the Worumba Anticline and associated intrusive breccias. Geological Survey of South Australia, Bulletin 52.

VALDIYAK.S. 1968. Origin of the magnesite deposits of the southern Pithoragarch, Kumaun Himalaya, India. Economic Geology 63, 924-934.

ROEP TH. B., BEETS, D.J. DRONKET H. & PANGDER H. 1979. Aprograding coastal sequence of wave built structures of Messinian age, Almerica, Spain. Sedimentary Geology 22, 135-164.

VON DER BORCH C.C. 1980. Evolution of late Precambrian to early Paleozoic Adelaide foldbelt: comparison with post-Permian rifts and passive margins. Tectonophysics 70,115-134.

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VON DER BORCH C.C. & LOCK D. 1979. Geological significance of Coorong dolomites. Sedimentology 26, 813, 824.

ZAMARRENO I. 1977. Early Cambrian algal carbonates in southern Spain. In Flugel E ed Fossil Algae: Recent Results and Developments, pp 360-365, Springer-Verlag, Berlin.

The Torrens Hinge Zone and Spencer Shelf with particular reference to early Adelaidean volcanism A.J. Parker, W.M. Cowley and B.P. Thomson* South Australian Department of Mines and Energy, P.O. Box 151, Eastwood SA. 5063, Australia Detailed reappraisal of drillcore from the Port Pirie region of northern Yorke Peninsula indicates that there are several basalt flows interbedded with the Emeroo Subgroup of the Burra Group. This is the first substantiated evidence of basic volcanism within the Burra Group in addition to that occurring within the Callanna Group, and raises some doubts about previous correlations of volcanic units across the Torrens Hinge Zone from the Stuart Shelf into the Adelaide Geosyncline. Nevertheless, combined with stratigraphic data from Depot Creek and the northern part of the geosyncline, there is ample evidence for protracted basic volcanism associated with the rift-related phase of Adelaidean tectonism. The Torrens Hinge Zone was a locus of faulting during deposition of the Callanna and Burra Groups in the early Adelaidean but rapid thickening of the overlying Umberatana Group suggests that the zone acted as a complex monoclinal flexure during the late Adelaidean. Key words: South Australia, Yorke Peninsula, Torrens Hinge Zone, Adelaide Geosyncline, Stuart Shelf, Spencer Shelf, volcanism, stratigraphy, tectonics, basalts, Burra Group, Callanna Group, Umberatana Group, Wilpena Group, Beda Volcanics. INTRODUCTION Between Bute and Port Germein (Fig. 1), the northern Spencer Shelf and parts of the Torrens Hinge Zone have been investigated on several occasions (1968-1983) because of the perceived potential for base metal mineralisation. Due to the extensive Cainozoic cover, these investigations have been restricted to geophysical techniques and drilling. The Department of Mines and Energy Torrens Hinge Zone Project was initiated by B.P. Thomson in the late 1960's; its purposes were to demonstrate the existence of prospective metalliferous rocks beneath the extensive Cainozoic cover, to stimulate company interest in exploration in this region, and to investigate the transition, in the vicinity of the Torrens Hinge Zone, between the Adelaidean sequences of the wellexposed Adelaide Geosyncline and those of the flat-lying Stuart Shelf and Spencer Shelf.

For this project, twenty-one diamond-drill holes and sixteen rotary-drill holes (with bottom hole coring) were completed near Bute and south of Port Broughton between 1968 and 1977. Drilling was supported by ground magnetic, gravity and resistivity surveys (Department of Mines, S. Aust. 1968-1974; Thomson 1973; Parker & Thomson 1978; Gerdes, 1978; Cowley & Parker 1988b. Exploration licences were subsequently taken up over this area by North Broken Hill Ltd and Jododex Australia Pty Ltd (now St Joe Australia Pty Ltd). North Broken Hill Ltd drilled thirtythree diamond-drill holes and three percussiondrill precollars (not diamond-drilled) around Bute from 1976 to 1982. St Joe Aust. Pty Ltd took up the remaining area and drilled several diamond-drill holes south of Port Broughton, three of which intersected Adelaidean rocks. When North Broken Hill Ltd ceased exploration in 1982, St Joe Aust. Pty Ltd took over their Bute licence, and completed one further diamond-drill

*Present Address: 4 Myrtle Ave, Myrtle Bank, SA 5064

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Fig. 1. Location of drillholes and major tectonic structures referred to in text.

Torrensian Volcanism in Torrens Hinge Zone

hole. Uranerz Australia Pty Ltd is the current operator of the Port Broughton E.L. From 1977 to 1982, North Broken Hill Ltd also held an exploration licence in the Port Pirie region, where they completed nineteen diamond drill-holes. Summary geological logs have been prepared by the authors by combining their own notes and observations on the drillcore with the most detailed logs available in company open-file envelopes and Departmental reports. These logs, together with an overview of the stratigraphy and mineralisation encountered, are presented in Cowley & Parker (1988b). The original geological logs for drillholes in the Port Pirie region (North Broken Hill Ltd. 1976-1982) identified a sequence of basic volcanics and conglomerates which were correlated with the Beda Volcanics and Backy Point Beds of the Stuart Shelf. This correlation infers that the volcanics of the Port Pirie region are approximately 1100 Ma old (Webb & Coats 1980; N.B. all Rb-Sr age dates have been recalculated using X = 1.42 x 10' n a _ 1 ). However, there are also similar basic volcanics in the Adelaide Geosyncline. U-Pb dating of a tuffaceous unit above such basic volcanics in the Willouran Ranges indicates an age for this basalt of about 800 Ma (Fanning et al 1986). There is therefore a dilemma regarding the age of basic volcanics near Port Pirie which has a very significant implication for the base of the Adelaidean in the Adelaide Geosyncline (Preiss, this volume). The stratigraphic name "Beda Volcanics" at present may have to be restricted on the Stuart Shelf to the volcanics intersected in Beda Bore and nearby drillholes (eg BDH-2, Fig. 1). Since the Port Pirie region straddles the boundary between the Stuart Shelf and Adelaide Geosyncline, it presents an opportunity to examine not only the transition of shelf sequences into the geosyncline, but also the base of the Adelaidean in relation to the Beda Volcanics and to volcanics associated with the Backy Point Beds and Emeroo Subgroup. This paper describes the stratigraphy of the transitional units in some

131

detail, and presents a reconstruction of the geological and tectonic events within this portion of the Torrens Hinge Zone during the Adelaidean. It will be concluded that the volcanics near Port Pirie are not "Beda Volcanics" but are largely of Torrensian age which makes them very significant as the first demonstrable record of Torrensian basic volcanism in the Adelaide Geosyncline.

REGIONAL TECTONIC SETTING Northern Yorke Peninsula and the Port Pirie region lie astride the Torrens Hinge Zone, which defines the eastern margin of the Gawler Craton, a stable block of Archaean to Middle Proterozoic basement (Fig. 1). Although the craton has been essentially stable since approximately 1500 Ma (Webb et al 1986), it has nevertheless been subject to local epeirogenic movement and episodic platformal sedimentation and volcanism. The last major phase of sedimentation on the Gawler Craton prior to the commencement of Adelaidean sedimentation is represented by the fluvial Pandurra Formation, which was deposited in northwest-trending grabens on the northeastern part of the craton at about 1450-1400 Ma (Fanning et al 1983). Structures which formed these grabens extend to the southeast onto northern Yorke Peninsula. Adelaidean sedimentation also extended onto the eastern margin of the Gawler Craton, commencing with clastic sedimentation and basic volcanism of the interpreted early Adelaidean "Beda Volcanics" and Backy Point Beds (Thomson et al 1976; Mason et al 1978). These and subsequent Adelaidean units are thin and flatlying across the northeastern Gawler Craton and were deposited on a continental marginal platform or shelf designated as the Stuart Shelf north of Why alia and as the Spencer Shelf on northern Yorke Peninsula (Sprigg 1952). The Torrens Hinge Zone (Thomson 1970; Thomson et al 1976) which defines the boundary between the Gawler Craton and Adelaide

132

A J . Parker, W.M. Cowley & B.P. Thomson

Geosyncline is a zone, approximately 25 km wide at Port Pirie, which was technically active during Adelaidean time. It has been variously represented as part of a monoclinal crustal flexure (Thomson 1980), a complex half-graben fault system (Thomson et al 1976), the margin of a major crustal shear zone and, at its eastern margin, a frontal thrust (Jenkins, 1986). Irrespective of the mechanism by which it formed, the Torrens Hinge Zone is a major crustal feature which at various times during the Adelaidean controlled sedimentation. Adelaidean sediments of the Torrens Hinge Zone are intermediate in character between the thin, flat-lying, incomplete record of the Stuart Shelf, and the thicker, more comprehensive, folded sequence of the Adelaide Geosyncline. In general, the former have escaped the major effects of the Late Cambrian-Early Ordovician Delamerian Orogeny which folded and metamorphosed, to varying degrees, the entire geosyncline. Nevertheless, attempts to isotopically date Brachina Formation shales within the zone just east of Port Broughton yielded a 503±56 Ma Delamerian age suggesting that the hinge zone was subjected to minor deformation/metamorphism sufficient to equilibrate the Rb-Sr system (Webb 1978). Because the Torrens Hinge Zone has been the locus of Cainozoic graben formation (the PirieTorrens Basin and Spencer Gulf), the Adelaidean record and precise nature of the zone are obscured by superficial deposits. Therefore, interpretation of these transitional sequences has only come about through extensive stratigraphic and exploration drilling supported by detailed geophysical surveys.

STRATIGRAPHY Pre-Adelaidean Basement Basement to the Adelaidean sequence in the Port Pirie region is predominantly grey, purple and red-grey, fine-grained, finely-bedded hematitic metasiltstone of the Early Proterozoic Wandearah Metasiltstone (Parker, 1980; Parker,

in press.). Near Bute, however, this unit is much more variable, with interbedded chloritic (greengrey), dolomitic (grey), carbonaceous (black) and siliceous (grey-white) metasiltstone, sometimes phyllitic, being present in addition to grey metasiltstone. Buff, grey, pink and red, siliceous dolomites are also common around Bute. A distinctive feature of the metasiltstones is the presence of pale cream, pink or greenish bleached spots and bands. Brecciation is also common. Interbedded with the metasiltstones near Bute are fine to medium-grained, green to green-grey massive and amygdaloidal basalt and tuffaceous sediments. These were previously described as the Willamulka Yolcanics (Thomson 1973) and were considered to be possibly equivalent to the Roopena Volcanics and basal Callanna Group volcanics. The correlation with Roopena Volcanics was reiterated by Mason etal. (1978) who rightly interpreted the Roopena and Willamulka Volcanics as pre-Adelaidean, and, in fact, as predating Pandurra Formation. Details of these relationships are discussed more fully elsewhere (Parker, in press.), but it is emphasized that throughout much of the Port Pirie-Bute region, there is a sharp, angular unconformity between the steeply-dipping and locally-folded Wandearah Metasiltstone/Willamulka Volcanics sequence and the flat-lying Adelaidean sequence. Other basement lithologies include stronglyaltered dolerites, with abundant chlorite, carbonate, and minor feldspar-hematite replacement, and altered gabbros, amphibolites and metadolerites (Bute Amphibolite; Thomson 1973) which, together with the intrusive dolerites, have been considered to be the feeders for the Willamulka Volcanics. Massive porphyritic rhyolite (grading to dacite) was also intersected in several drillholes near Bute. It is variably chloritic or hematitic and may be equivalent to either the 1737+5 Ma Moonta Porphyry (Fanning et al in press) or the 1592±2 Ma Gawler Range Volcanics. In either case the rhyolites are definitely pre-Adelaidean. The Tickera Granite, which crops out along the coast between Wallaroo and Port Broughton (Fig. 2), is a

Torrensian Volcanism in Torrens Hinge Zone

CAMBRIAN

133

(€)

K u l p a r a L i m e s t o n e and W i n u l t a F o r m a t i o n ADELAIDEAN-WILPENA

GROUP

(Pw)

A B C Range Quartzite (Pwa) Brachina Formation S e a c l i f f S a n d s t o n e and N u c c a l e e n a F o r m a t i o n ADELAIDEAN-UMBERATANA

GROUP

W i l l o c h r a Subgroup B r i g h t o n L i m e s t o n e (Pfh) Tapley Hill Formation. Woocalla Dolomite M e m b e r S t u r t i a n t i l l i t e (Pus) ADELAIDEAN-BURRA

GROUP

undifferentiated Skillogalee Dolomite(Pbk) Emeroo S u b g r o u p ( i n c l u d i n g v o l c a n i c s ) ADELAIDEAN-CALLANNA undifferentiated

GROUP

(including v o l c a n i c s )

PRE A D E L A I D E A N B A S E M E N T

(GAWLER

CRATON)

Tickera Granite Wandearah

Metasiltstone,

W i l l a m u l k a V o l c a n i c s and B u t e A m p h i b o l i t e Doora S c h i s t and M o o n t a P o r p h y r y

Outcrop

Interpreted s u b - o u t c r o p b o u n d a r i e s

Fault

T h r u s t fault (teeth on upper plate)

Drillhole

Fig. 2. Pre-Tertiary geological subcrop map derived from drillhole information and geophysical interpretation (in part after Parker, 1983).

134

A.J. Parker, W.M. Cowley & B.P. Thomson

massive to weakly foliated medium- to coarsegrained adamellite, considered to be equivalent to the Middle Proterozoic Charleston Granite (Parker 1980). Moonta Porphyry and Doora Schist (Parker, 1980) do not occur in any drillholes in the Port Pirie-Bute region but do occur to the southwest (Fig. 2).

which is only observed elsewhere in the Adelaide Geosyncline near Spalding (Preiss 1974) where River Broughton Beds (Callanna Group) are structurally concordant with overlying Rhynie Sandstone (equivalent to the lower part of the Emeroo Subgroup). Burra Group

Callanna Group Emeroo Subgroup and (?) Backy Point Beds Drillholes PP12 (Fig. 3) and PP13 north of Port Broughton intersected a sequence of dominantly massive, red, irregularly flaserbedded, calcareous or dolomitic siltstone grading to fine-grained sandstone, locally gritty, with thin, grey dolomite or fawn calcareous interbeds. Pale green reduction spotting is sometimes present. Similar lithologies occur in an unnamed Callanna Group siltstone at Depot Creek (Preiss & Faulkner 1984). These sequences are believed to be equivalent and to represent the lowest known Adelaidean sequence in this region. As is the case at Depot Creek, a massive, green-grey basalt flow (perhaps equivalent to the 'Beda Volcanics'; Mason et al 1978) occurs within the sequence at Port Pirie (drillhole PP12, Fig. 3). Laminated shale and cherty limestone with chert pods above and below the flow may reflect tuffaceous input to the sediments immediately preceding and following eruption or, quite possibly, chemical sedimentation directly related to volcanic activity much the same as is present in the large lakes of the East African rift system. Below the red dolomitic siltstones in PP13 (Cowley & Parker, 1988b), there is a sequence of maroon to pink, micaceous, variably-hematitic siltstone and sandstone with local heavy-mineral banding and minor feldspathic grit and gravel conglomerate. In places this sequence is similar to lithologies found in the lower part of the overlying Emeroo Subgroup. This fact, together with transitional contacts observed between the Callanna Group and Emeroo Subgroup in both of these holes, suggests that there is no break between these units. Should these stratigraphic interpretations be correct, then these two drillholes preserve a conformable and gradational contact

Most drillholes in the Port Pirie to Port Broughton region intersected a distinctive sequence of interbedded feldspathic quartzite and granule- to pebble-conglomerate. The sequence can be divided into an upper unit containing pink and buff, prominently-feldspathic quartzite and granule to pebble conglomerate with common, but not ubiquitous, heavy-mineral banding, and a lower unit (only present near Port Pirie) comprising variably hematitic, red to pink, feldspathic, fine- to medium-grained sandstone, and redbrown, hematitic and micaceous siltstone grading to fine-grained sandstone, with thin granule- and pebble-conglomerate interbeds. The proportion of conglomerate in the upper unit generally decreases southwards from Port Pirie and rare, red or greenish siltstone interbeds are locally present near its base. Clasts within the conglomerates are dominated by quartz, orange to pink K-feldspar, and pink, red and brown quartzite or sandstone, with lesser hematitic metasiltstone, acid volcanics, granite, jasper, chert and siltstone, and rare basic and intermediate igneous rocks, pegmatite, and carbonaceous siltstone. Clasts of hematitic basalt are locally common near interbedded basalt flows (see below). In some holes south of Port Pirie (e.g. PP6, Fig. 3), either at the boundaiy between the upper and lower units of the quartzite/ conglomerate sequence or within the upper unit, there are interbedded flows of green-grey to reddish, variablyhematitic, amygdaloidal basalts. The basalt intervals range in thickness from 4 to 81 m and generally comprise several flows, with amyg-

PP4

PP6

PP12

BURRA GROUP

Interbedded black and green-grey SHALE, grey DOLOMITIC SANDSTONE, green-grey DOLOMITE. GRIT and thin, grey GRANULE CONGLOMERATE

SKILLOGALEE DOLOMITE Grey-white DOLOMITE; minor c breccia and grit.

BRACHINA FORMATION

Red brown SHALE; green grey shale and fine sandstone interbeds

Transitional Unit Grey-white CONGLOMERATE; minor green grey calcareous shale " CARBONACEOUS SHALE overlying red maroon CALCAREOUS SHALE i with PEBBLE CONGLOMERATE

SEACLIFF SANDSTONE Interbedded off-white, partly dolomitic SANDSTONE and red-brown SHALE

NUCCALEENA FORMATION

Interbedded DOLOMITIC SANDSTONE and DOLOMITIC SANDY SHALE.

mm,

Pink, feldspathic CONGLOMERATE, n feldspathic quartzite and rare green

REYNELLA SILTSTONE

Brown and green DOLOMITIC SHALE, interbedded with thin FELDSPATHIC GRIT.

Interbedded red-brown, haematitic BASALT, amygdaloidal in part, and red, haematitic CONGLOMERATE, carbonate-matrix in part

C> V

A A ^

V V V V /V V V

Massive, grey DOLOMITIC SILTSTONE and intraformational CONGLOMERATE; minor black shale.

tew*-;

Massive, pebbly DIAMICTITE; minor conglomerate, dolomitic siltstone and black shale

BURRA GROUP

Massive, grey, GRITTY DOLOMITE, DOLOMITIC GRIT; minor black shale

EMEROO SUBGROUP E.OH.524.3m

Red-pink, locally gritty FELDSPATHIC SANDSTONE. Local heavy mineral banding. Weakly haematitic.

•

< cT p

• •

D 1

v'y'v'vV

Massive, red black. HAEMATITIC AMYGDALOIDAL BASALT. Red-pink PEBBLE CONGLOMERATE; local carbonate-rich matrix.

Green-grey, fine to medium-grained BASALT, amygdaloidal in part.

Interbedded, red, flaser bedded SILTSTONE and fine-grained SANDSTONE; local grit beds; pink conglomerate interbeds in upper part.

Coarse CONGLOMERATE or BRECCIA, carbonate-rich matrix. Includes basalt clasts (bombs?) Pink, fine to medium-grained, feldspathic QUARTZITE. Haematitic, or heavy-mineral banded in part. Rare haematitic gritty mudstone.

Red, flaser bedded DOLOMITIC SILTSTONE; massive, grey DOLOMITE near base. i-i

Red, haematitic, gritty, silty SANDSTONE, interbedded with haematitic, gritty SILTSTONE and MUDSTONE Basal conglomerate.

Interbanded, white, grey and maroon LIMESTONE and CALCAREOUS SILTSTONE; rare chert

r^f.

Interbanded, laminated SHALE and LIMESTONE. Local chert pods (magadiite?) Massive, red, flaser bedded CALCAREOUS SILTSTONE, with abundant, thin pale brown LIMESTONE interbeds and rare laminated limestone interbeds. Locally sandy in lower part.

- x - r -

E. O H . 6 2 7 . 4 m 88-44

Fig. 3. Summary geological logs of drillholes PP4, PP6 and PP12 from the Wandearah area.

CfQ n> £

Massive, grey-green, medium-grained BASALT

n -

Very fine-grained HAEMATITIC METASILTSTONE and SILTSTONE

1 Maroon SHALE and DOLOMITE.

V V VV VV V V V VV V Vv V V

WANDEARAH METASILTSTONE

o D p '

Red CONGLOMERATE or BRECCIA, sandy carbonate-haematitic matrix. Includes basalt clasts.

STURTIAN TILLITE

> <7 >

Green-grey, fine-grained BASALT, amygdaloidal at top

BRIGHTON LIMESTONE

Laminated, black CARBONACEOUS SHALE; minor dolomitic siltstone.

Pink, sandy, FELDSPATHIC. PEBBLE to COBBLE CONGLOMERATE. Local pink white GRITTY QUARTZITE interbeds. Local heavy-mineral banding, haematitic towards base. Rare reddish siltstone.

. •• • ••

WHYALLA SANDSTONE

TAPLEY HILL FORMATION

Interbedded black and green DOLOMITIC SHALE and pink CONGLOMERATE.

•. •

Massive, maroon. GRITTY DIAMICTITE Brown SILTY MUDSTONE and SILTSTONE Locally gritty.

Grey white, medium grained. DOLOMITIC. FELDSPATHIC SANDSTONE; local grit and conglomerate . Interbedded, black SHALE and white FELDSPATHIC SANDSTONE and GRIT.

Transitional Unit

SADME

136

A.J. Parker, W.M. Cowley & B.P. Thomson

daloidal zones, grainsize variations and thin red interflow sediments delineating the individual flows. Amygdales are largely dolomite-chloritehematite-filled, and may be interconnected by extensive, irregular carbonate veins containing local hematite or feldspar. In some holes, the conglomerates enclosing the basalts have a partly calcareous matrix, and occasionally contain clasts of hematitic basalt, some of which may have been volcanic bombs. Elsewhere the basalts are interlayered with conglomerate and sandstone typical of the sequence above and below. Previous interpretations of this interbedded conglomerate - basalt interval have correlated it with the Backy Point Beds and 'Beda Volcanics' of the Callanna Group (Parker 1980) or as 'Beda Volcanics' with or without Backy Point Beds, overlain by Emeroo Subgroup (Preiss 1987). These interpretations were supported by the presence of carbonate in the matrix of some of the conglomerates, apparently common in the Backy Point Beds on Eyre Peninsula, and by the similarity of the basalts to those of the Beda Volcanics south, west, and northwest of Port Augusta (Mason et al 1978). This interpretation is not supported here, since, in the Port Pirie holes, the basalts are interlayered with feldspathic conglomerates lithologically indistinguishable from those of the overlying sequence which is typical of the Emeroo Subgroup. Furthermore, heavy-mineralbanded quartzites, also typical of the Emeroo Subgroup, are present beneath the basalts in several holes. The carbonate and hematite content of the enclosing sediments, though unusual for the Emeroo Subgroup, is not unexpected in view of the high carbonate and hematite content of the basalts, which have been veined extensively by calcite and which are the obvious source of iron. Therefore, given that this feldspathic quartzite and conglomerate sequence, apart from the hematite and carbonate, is remarkably similar to typical Emeroo Subgroup from, for example, the Emeroo Range, there is good justification for correlating it with that sequence.

This correlation is further enforced by interbedding at the top of the sequence with Skillogalee Dolomite units and by the absence of significant breaks in the sequence. Consequently this then constitutes the first recognition of substantial basic volcanism taking place during deposition of the Burra Group. The only previous identifications of possible Burra Group volcanism are by Drexel & McCallum (1986), who report acid to intermediate tuff, lava and agglomerate within Skillogalee Dolomite at the Burra Mine, and by Preiss (1987), who reports altered amygdaloidal basalts subcropping within an area of tightly folded Rhynie Sandstone 10 km west-southwest of Clare. However, at Clare the sandstone shows opposing facings at different outcrops, so it is possible that the volcanics are not in sequence, but have been faulted into place. Because the evidence at Port Pirie favours the interpretation that the basalts are interbedded within the Emeroo Subgroup of the Burra Group, much like the interbedded volcanics and conglomerates at Backy Point, it is proposed to tentatively equate the Backy Point Beds with the Emeroo Subgroup (see Discussion). The lower unit of the Emeroo Subgroup, as described in some drillholes from the Port Pirie region, was previously assigned to the Middle Proterozoic Pandurra Formation (North Broken Hill Ltd 1976-1982; Preiss, 1987; Parker 1980). This interpretation is not supported here since there is no obvious major break in sedimentation at the supposed boundary between the Pandurra Formation and Emeroo Subgroup, especially where there is no intervening basalt. Assuming an age ca 1400 Ma for Pandurra Formation (Fanning et al 1983) and ca 800-750 Ma for Burra Group, a very major erosional break would be expected at this level, but only a gradual down-hole increase in hematite content is noted, with a change in colour from pink to red. The common distinctive lithology of the Pandurra Formation on the Stuart Shelf is a red to purple and whitish- mottled, strongly hematitic and sericitic, poorlysorted gritty sandstone. This lithology was not seen in any of the drill core although the finer,

Torrensian Volcanism in Torrens Hinge Zone

hematitic sediments in the lower unit of the Emeroo Subgroup do resemble the subordinate finer-grained intervals of the Pandurra Formation. Where this lower unit of the Emeroo Subgroup is absent, the upper unit rests either unconformably on Wandearah Metasiltstone and equivalent basement units, or with gradational contact on Callanna Group. Abasal conglomerate with clasts of the immediately-underlying basement is commonly present where Callanna Group is absent (eg. Wokurna No. 4). An extremely altered, orange (K-feldspar alteration?), massive dolerite present between the upper and lower units of the Emeroo Subgroup in PP10 is either an intrusive phase of the volcanics, or a thick single flow which cooled sufficiently slowly to allow a coarse grainsize to develop. The latter interpretation is preferred because of the contrast between the apparently unaltered sediments above the dolerite, and the hematite + feldsparveined, altered, whitish appearance of those immediately below the dolerite.

137

terbedded with typical Burra Group dolomitic siltstones. This interbedding strongly supports correlation of the underlying sequence with the Emeroo Subgroup and hence supports a Torrensian age for the enclosed volcanics. Anomalous gold has been discovered within the transitional interval in six of the diamond drill-holes south of Port Pirie (Cowley & Parker 1988). This occurrence of low but persistent gold values in coarse clastics of fluvial origin and carbonaceous shales invites comparison with the Witwatersrand gold mining province in South Africa, despite the much greater age of the latter. Outcrops of Emeroo Subgroup pink and grey, feldspathic quartzite and conglomerate with heavy-mineral banding are known southwest of Bute, on the coast near Moonta and Port Broughton and in the Hummocks Range near Kulpara. Except for a small outcrop of Sturtian tillite in a quarry eight kilometres south of Bute, the next youngest Adelaidean unit known to crop out in the region is the Willochra Subgroup, in the Hummocks Range (Preiss 1987).

Overlying the Emeroo Subgroup in most holes is a transitional unit consisting of intercalated, pink, grey or white gritty sandstone to pebble conglomerate (rarely black and carbonaceous), green, red or grey shale or siltstone which may be micaceous, calcareous or dolomitic, grey to buff dolomite, and black, weakly-uraniferous, carbonaceous shale (Fig. 3). Trace to minor syngenetic pyrite is present in the carbonaceous shale, and disseminated pyrite and chalcopyrite can occur within the more porous lithologies or in quartz-carbonate veining. The transitional unit grades upwards into the Skillogalee Dolomite, or where that is absent, into undifferentiated Burra Group. In a few cases, the contact between the Emeroo Subgroup and the overlying Burra Group is sharp, and lacks a recognisable transitional unit, but in general the transitional unit is present.

Undifferentiated Burra Group

The real significance of the transitional unit is that it contains granule and pebble conglomerate bands, identical to the underlying sequence, in-

Gradationally overlying the Skillogalee Dolomite, or the unit transitional to Emeroo Subgroup, is a sequence of interbedded, grey, mas-

Skillogalee Dolomite The Skillogalee Dolomite is a massive, grey to whitish dolomite with common, thin, black, carbonaceous shale interbeds. Stylolites are a common feature, and silicified oolitic limestone is rare. It grades in places to a dolomitic siltstone or gritty dolomite, and has local feldspathic grit, sandstone or intraformational breccia interbeds. It is not present south of Wokurna, where it, together with the overlying undifferentiated Burra Group, has either been eroded prior to deposition of the Sturtian tillite, or was never deposited.

ADELAIDE GEOSYNCLINE-

-STUART SHELF-

-TORRENS HINGE ZONESPENCER SHELF-

(this paper, Mason et a I., 1978) (Drill Holes SAS1 and BDH2) composite section

CULTANA INLIER

YORKE PENINSULA

DEPOT CREEK

(this paper, Mason et a!.. 1978)

(this paper, Parker and Thomson. 1978)

(Preiss and Sweet. 1966. Thomson et a!.. 1976/

Z^Z^IZ-fdi U/

v v vyy

Backy Point Beds

Tapley

Formation

VC0/V/r(

—rryrrw-jvr^ > 700Ma (Webb. 1978) (SAS1)

composite section

composite section interglacial

UMBERATANA GROUP

tillite

?Auburn Dolomite ?Undalya Quartzite Skillogalee Dolomite V V V V v v y y . v v v V V

*CP

BURRA GROUP

Emeroo Subgroup (Aldgate Sandstone equivalent)

cT ^ & Cd

dolomitic siltstones

? CALLANNA GROUP

carbonate breccia

1076Ma (Webb et a I.. 1983) (BDH2)

Beda Volcanics

Fig. 4. Correlation of stratigraphic sequences from the Stuart Shelf across the Torrens Hinge Zone into the Adelaide Geosyncline.

o tJ

Torrensian Volcanism in Torrens Hinge Zone

sive dolomite, dolomitic siltstone and dolomitic sandstone, grey and white sandstone, green to grey siltstone and black carbonaceous and pyritic shale, with thin grit and feldspathic granule conglomerate interbeds. This sequence is assigned to the Burra Group but its precise stratigraphic position is uncertain. Towards its southern limit, near Wokurna, sandstones become more prominent; they are feldspathic, or lithic (with chert and shale grains), in places calcareous or dolomitic, and are interbedded with green-grey dolomitic or pyritic siltstone, or with rare carbonaceous siltstone. Rutland etal (1981) note that dolomites indistinguishable from the Skillogalee Dolomite occur in the River Wakefield Subgroup, which intertongues with the central part of the Emeroo Subgroup in the southern Flinders Ranges. Therefore it is possible that the units assigned here to the Skillogalee Dolomite and undifferentiated Burra Group are instead River Wakefield Subgroup, and that clastics here ascribed to the Emeroo Subgroup would then be equivalent to the Rhynie Sandstone, at the base of the Emeroo Subgroup.

Umberatana Group

Sturtian Tillite This widespread basal unit of the Umberatana Group was deposited unconformably on older units and comprises glacial diamictite with a finegrained, grey, often dolomitic, rock-flour matrix, which is occasionally sandy or carbonaceous (Fig. 3). Variable thicknesses of intraglacial sediments comprising buff or pink dolomite, grey dolomitic siltstone, quartzite, carbonaceous shale or conglomerate intertongue with the diamictite in places. Clasts within the diamictite are of various basement lithologies, and include wellrounded to angular granite, gneiss, schist, quartzite, dolomite, marble, metasediments, dolerite, quartz and feldspar and local dolomitic sandstone and dolomitic mudstone probably reworked from older Adelaidean units. It is postulated that a local magnetic horizon within Stur-

139

tian tillite, approximately 70 m above the base of the tillite in Wokurna No. 4, may be laterally equivalent to the Braemar ironstone facies. The term "McLeay Beds" was first used by Gersteling & Heape (1975) for the Mount Gunson area, where a discontinuous unit of shale, sandstone, quartzite and conglomerate rests unconformably on Pandurra Formation, and is overlain by the Woocalla Dolomite Member of the Tapley Hill Formation. On the Spencer Shelf, there is occasionally a sandy or conglomeratic unit between diamictites of the Sturtian tillite and the Woocalla Dolomite Member, and this has been referred to as the "McLeay Beds" (Parker & Thomson 1978). It comprises grey, lithic, dolomitic or gritty sandstone and grey dolomitic siltstone, often with a basal conglomerate, or may be dominantly grey conglomerate, often with a pyritic matrix. Superficially, these sediments appear similar to the intraglacial sediments of the Sturtian tillite, and since they were probably deposited in the same fluvioglacial environment, they may be better correlated with the tillite and its associated sediments. Therefore, in view of the uncertain status of this unit, it is suggested the name not be used.

Tapley Hill Formation and Woocalla Dolomite Member The Tapley Hill Formation is a monotonous, distinctive, sequence of grey to black, laminated, carbonaceous, dolomitic siltstone or shale, often pyritic, with thin interbeds of grey dolomite or sandy to silty dolomite which become more common down sequence. Often these rocks grade downwards into the Woocalla Dolomite Member, composed of grey, laminated or massive dolomite, often quite sandy or silty, with local carbonaceous siltstone interbeds in the upper part.

140

A J . Parker, W.M. Cowley & B.P. Thomson

Brighton Limestone Overlying and interbedded with the upper part of the Tapley Hill Formation is the Brighton Limestone, a sequence of grey limestone, dolomite and dolomitic or calcareous siltstone. Basal interbeds comprise mainly grey to pink intraformational breccias but merge upwards into pale grey algal-laminated and relatively-massive stromatolitic limestone. The uppermost section of the unit is a sandy, dolomitic limestone often grading up into the Willochra Subgroup.

Willochra Subgroup (undifferentiated) The Willochra Subgroup has been encountered in drilling east of Wokurna and north to near Port Pirie and crops out in the Hummocks Range near Kulpara (Preiss 1987). It is likely that it is continuous between these occurrences, buried below Cainozoic deposits along the western edge of the Barunga and Hummocks Ranges. In drillcore, it consists predominantly of brown-maroon and green-grey siltstone and shale, interbedded with pink feldspathic grit or granule conglomerate, often dolomitic or calcareous. In some holes, pale pink, grey or brown calcareous or dolomitic, feldspathic sandstone dominates part of the section, and may be interbedded with polymict grit or conglomerate or rarely with buff, cross-bedded, oolitic limestone or black shale.

interbeds of fine-grained calcareous sandstone, maroon siltstone and shale (with possible varves), and laminated, buff-pink, very-fine-grained quartzite have been intersected. Wilpena Group Nuccaleena Formation and Seacliff Sandstone These two formations are lateral equivalents, and intertongue with one another; one or the other is present in each drillhole intersection of the base of the Wilpena Group and the unit appears to be as persistent here as it is in the Geosyncline. The Seacliff Sandstone in drillholes near Wokurna and Wandearah (Fig. 3) is an off-white, pale greypink or red-brown, fine-grained sandstone or quartzite, occasionally dolomitic or feldspathic, with local interbeds of red and green-grey shale marking the transitional contact to the Brachina Formation. Interbeds of pink and cream dolomite and dolomitic sandstone or shale represent the Nuccaleena Formation. In Wokurna No. 6, pink dolomite clasts about 75 m above the base of the sandstone represent intraformational brecciation of coeval dolomite sedimentation. The base of the Seacliff Sandstone in Wokuma No. 6 is abrupt, locally bleached and is the locus of minor pyrite/chalcopyrite precipitation. Cream, feldspathic, cross-bedded sandstone outcropping near Kulpara is assigned to the Seacliff Sandstone (Preiss 1987).

Reynella Siltstone Member (of Elatina Formation)

Brachina Formation

The upper part of the Willochra Subgroup is represented by the Reynella Siltstone Member, consisting mainly of glacial diamictite, with dispersed grit and granules of a variety of lithologies in a massive, red-maroon sandy siltstone or redbrown to grey-green sandstone matrix. With an increase in the clast content the diamictite can grade to grit or granule conglomerate, and local

The Brachina Formation was only intersected in drillholes east of Wokurna, southwest of Port Pirie and northwest of Port Germein. It comprises red-brown to maroon laminated shale, with thin interbeds of grey-green shale to siltstone, and buff-grey to red or greenish, fine-grained, crossbedded sandstone, occasionally calcareous. It is gradational into (and interbedded with) underly-

Torrensian Volcanism in Torrens Hinge Zone

ing Seacliff Sandstone, and into the overlying ABC Range Quartzite.

ABC Range Quartzite Resistant ABC Range Quartzite forms the bulk of the Hummocks and Barunga Ranges, where in outcrop it is a white quartzite and red to purple silty sandstone with local heavy mineral bands (Thomson 1969; Mirams, 1964). It overlies purple and red, laminated shale and siltstone of the Brachina Formation near Kulpara (Preiss 1987). This formation was intersected in one drillhole southwest of Port Pirie, and in one drillhole northwest of Port Germein, where it is a white, medium-grained quartzite, with local cross-bedding, and heavy-mineral laminations. Towards the base, it is in places reddish or greenish in colour, and has thin interbeds of green-grey and maroon, micaceous siltstone and shale typical of the Brachina Formation, suggesting intertonguing of these units. Cambrian Stratigraphy Early Cambrian strata are only found in a small area surrounding Bute, where they unconformably overlie thin Adeaidean cover and preAdelaidean rocks. Their maximum thickness is 108 m, in B18. Winulta Formation This thin, basal unit is equivalent to the Mt Terrible Formation (Daily, 1963) and consists of white, grey or buff, medium-grained sandstone or quartzite grading down to a gritty granule sandstone or pebbly sandstone, in places micaceous or feldspathic. There are local interbeds of buff dolomite or greenish siltstone, and occasionally a basal conglomerate is developed, with quartzite, metasiltstone and quartz pebbles.

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Kulpara Limestone Gradationally overlying the Winulta Formation is the Kulpara Limestone, consisting of grey to brown, massive fine- to medium-grained dolomite and dolomitic siltstone with local siltstone and sandstone interbeds near the base. It is cavernous or brecciated in places. Scattered outcrops are known near Bute. Cainozoic Sediments Tertiary and Quaternary sediments blanket most of the Spencer Shelf region. Depths to Cambrian or older rocks vary from 100-170 m around Wandearah and southwest of Port Broughton to generally less than 80 m (mostly 10-50 m) inland between Port Broughton and Bute. In a few holes around Bute and on the beach at Port Broughton, Cainozoic cover is very thin or absent.

MINERALISATION Departmental and company drilling in the Spencer Shelf region has recovered widespread, but weak, copper, lead, zinc, molybdenum and gold mineralisation at various strati graphic levels (Cowley & Parker 1988 a, b). Drilling near Bute revealed anomalous copper, lead and zinc concentrations in Wandearah Metasiltstone (in B23 and B46), copper in Willamulka Volcanics (Bute No. 5) and in Sturtian tillite (Bute No. 7). Whereas the chalcopyrite in this area mainly occurs in veinlets with calcite and varying amounts of pyrite, hematite, chlorite or quartz, that within the Willamulka Volcanics may be primary, and related to basic volcanism. Weakly anomalous copper was recovered within basal Tapley Hill Formation (Wokurna No. 2), the Emeroo Subgroup (Wokurna No. 4) and in basal Seacliff Sandstone (Wokurna No. 6) southeast of Port Broughton.

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South of Port Pirie, several drillholes (PP2, PP3, PP5, PP6, PP8 and PP10) intersected Emeroo Subgroup or undifferentiated Burra Group, parts of which host disseminated chalcopyrite + chalcocite mineralisation. Significant anomalous gold accompanies the copper in PP3, PP5, PP9 and PP10; these gold occurrences are all within the same transitional interval between the Emeroo Subgroup and either the Skillogalee Dolomite or undifferentiated Burra Group (Cowley & Parker 1988a). In PP5, traces of molybdenum are present with copper in an Emeroo Subgroup conglomerate intercalated between two basalts, while in PP10, an intensely altered dolerite within the Emeroo Subgroup carries disseminated chalcopyrite. Tapley Hill Formation in PP4 and PP15 (Woocalla Dolomite Member) hosts anomalous lead accumulations.

DISCUSSION Early Adelaidean Volcanism Until now, Precambrian basic volcanics of the Stuart Shelf and Adelaide Geosyncline have been considered to be almost exclusively of Willouran age. The only exceptions to this have all been in areas of highly disrupted sequences (eg diapirs; disrupted sequence at Clare) where it is not clear of what age the basics are; some may even be pre-Adelaidean. However, drilling in the Port Pirie region of the Torrens Hinge Zone has clearly established evidence supporting Torrensian volcanism synchronous with deposition of the Emeroo Subgroup of the Burra Group. This is an important and very significant finding because although many authors have argued that the Burra Group was deposited in a rifting environment, maybe analogous to modern continental rifting of eastern Africa (von der Borch 1980; Preiss 1987; Belperio 1988), there has not, until now, been any substantial evidence for volcanism associated with Torrensian rifting. Age constraints from the northern Adelaide Geosyncline suggest that, unless the Burra Group - Callanna Group boundary

is diachronous (i.e. older in the south), this volcanism must be younger than 802±10 Ma, the U-Pb age for the Rook Tuff in the Callanna Group (Fanning et al 1986). A minimum age for the volcanics at Port Pirie is given by Rb-Sr ages from the Stuart Shelf, for Tapley Hill Formation (750±53 Ma; Webb et al 1983) and for Willochra Subgroup siltstones (724±40 Ma; Webb et al 1983). Therefore, the Torrensian volcanism of the Port Pirie region must have occurred approximately at 800-750 Ma. While it has been argued in the past (Mason et al 1978) that basic volcanic units from the Stuart Shelf ('Beda Volcanics'), Depot Creek, Port Pirie, Arkaroola (Wooltana Volcanics), Willouran Ranges (Noranda Volcanics) and Peake and Denison Inlier (Cadlareena Volcanics) are petrologically and geochemically similar and that they are all therefore of early Adelaidean age and probably equivalent, there is substantial evidence to now question those correlations. For example, there have been uncertainties about the absolute ages of these basic volcanics as determined by isotopic dating; the 1076+33 Ma age for the 4 Beda Volcanics' (Webb et al 1983) clearly conflicts with the 830±50 Ma age for the Wooltana Volcanics (Compston et al 1966) and the 802±10 Ma age for the Rook Tuff (Fanning et al 1986). Stratigraphically, the latter are by definition within the Callanna Group, however, there are few stratigraphic constraints on the 'Beda Volcanics', since they are everywhere unconformably overlain by Tapley Hill Formation and do not contain any interbedded lithologies or sequences distinctively of early-Adelaidean age. Preiss (1987) discusses this dilemma and concludes that the 'Beda Volcanics' and the 1076 Ma age may represent pre-Adelaidean to early-Willouran volcanism, while basic volcanism in the Adelaide Geosyncline may have occurred at the same time or much later (ca 830 Ma, as suggested by dating on the Wooltana Volcanics). Preiss then suggests that the 802 Ma age may represent lateWillouran volcanism. Given the time difference (approximately 300 Ma), and accepting that the 802 Ma age is the most reliable of all the age determinations so far, it is highly unlikely that

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Fig. 5. Geological cross-sections across and along the Torrens Hinge Zone. Refer to Figure 1 for location.

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A J . Parker, W.M. Cowley & B.P. Thomson

Callanna Group sedimentation commenced as early as 1076 Ma. Therefore, it is also highly unlikely that either the 1076 Ma age is correct, or that the 'Beda Volcanics' as defined in Beda Bore and in BDH-2 are early Willouran. The assumption, that the basalts of the Stuart Shelf are all Willouran, is now queried. Originally, the 4Beda Volcanics' were considered to be 697±70 Ma old based on Rb-Sr isotopic analyses of basalts from Australian Selection Pty Ltd DDH SAS-1 at Sugarloaf Hill west of Port Augusta (Webb & Horr 1978). However, this date was considered too young and unreliable due to intense alteration of the basalts. Consequently, lessaltered basalts from another drillhole further to the north, Delhi-Aquitaine BDH 2, were analysed and found to yield the much older 1076 Ma age which was then assumed to be representative of all 'Beda Volcanic' basalts of the Stuart Shelf (Webb et al 1983). It should be noted here that the 1076 Ma is a minimum age and that if samples with substantial potassic enrichment are disregarded, then an older isochron, yielding approximately 1200 Ma, is defined (Page et al 1984). In view of the stratigraphic relationships at Port Pirie, and the lack of any unequivocal stratigraphic relationships on the Stuart Shelf, it is suggested that the basalts of the Stuart Shelf, previously all assigned to the 'Beda Volcanics', may actually represent two separate volcanic sequences. In this context, basalts of the southern Stuart Shelf (viz. those at Backy Point and in DDH SAS-1) may be, in fact, equivalent to those of the Port Pirie region (and hence Burra Group volcanics) whereas those to the north could indeed be pre-Adelaidean. The latter would be the 'Beda Volcanics' as defined in Beda Bore. If this is correct then the correlation of basic volcanics across the Torrens Hinge Zone into the Adelaide Geosyncline must be revised. In so doing, the Backy Point Beds could be considered equivalent to the Emeroo Subgroup of Emeroo Range and Port Pirie/Port Broughton, while the volcanics at Depot Creek and those in the lower part of PP12 (Fig. 3) could be considered either equivalent to

the Wooltana Volcanics or an independent early Torrensian or late Willouran volcanic phase; and the 'Beda Volcanics' in Beda Bore and BDH-2 (and Gairdner Dyke Swarm) could be considered as part of the pre-Adelaidean basement to the Stuart Shelf (Fig. 5). The latter is based purely on the 1076 Ma age from BDH-2 drillhole whereas in a nearby drillhole, SLT-106, the same volcanics are associated with a red siltstone much the same as at Depot Creek. Therefore, while we are confident of the Port Pirie/Depot Creek relationships, the stratigraphic position of the 'Beda Volcanics' (as in BDH-2 and SLT-106) remains very uncertain. Influence of the Torrens Hinge Zone Following consolidation of the Gawler Craton ca 1500 Ma there was a protracted period of continental sedimentation and volcanism that resulted in the formation of northwest-trending grabens on the northeastern portion of the craton (Pandurra Formation ca 1420 Ma) and intrusion/extrusion of basic magmas (Gairdner Dyke Swarm and Beda Volcanics, ca 1100 Ma). None of these events appear to have been influenced in any way by the Torrens Hinge Zone since they are oblique to it in the Port Pirie/Whyalla/Port Augusta area. At the onset of Adelaidean sedimentation in the Port Pirie/Bute region, the Torrens Hinge Zone had become an important and very major crustal tectonic structure (Preiss 1987). Early Torrensian sediments and volcanics (accepting that the Backy Point Beds are Burra Group) extend as a relatively thin veneer west of the Zone between Backy Point and Port Augusta (eg SAS-1) but thicken rapidly (as the Emeroo Subgroup) east of the Zone. Younger Burra Group sediments (eg Skillogalee Dolomite) are strictly confined to the Adelaide Geosyncline within and east of the Torrens Hinge Zone (Fig. 2). The Torrens Hinge Zone has long been recognised as a prominent bounding feature between the Adelaide Geosyncline and the Spencer and Stuart Shelves. Thomson et al (1976) described it

Torrensian Volcanism in Torrens Hinge Zone

as the zone between the Torrens Lineament on the west (Sprigg 1952) and the Cainozoic fault system forming the western edge of the present-day Mt Lofty and Flinders Ranges. As noted earlier, it is a complex structure for which several different models have been proposed: a monoclinal flexure, a series of normal faults progressively stepping down the pre-Adelaidean basement, and, more recently, as the front of a major, westdirected thrust system. All are likely to be true but at different stages of development of the Geosyncline. While thrust faulting has been identified at the eastern margin of the Torrens Hinge Zone at Depot Creek (Preiss & Faulkner 1984) and in Bute drillhole B26, it does not relate directly to early syndepositional tectonics of the Hinge Zone north of Bute to Port Pirie/Port Augusta where normal faults account for much of the observed displacement of Burra Group sediments (Fig. 5) (Parker & Thomson 1978; Parker 1983). Near Wandearah, thickness variations of Emeroo Subgroup over relatively short distances suggest syndepositional movement on northwest-trending faults. To the south near Port Broughton and southwest of Bute, thick accumulations of Emeroo Subgroup are respectively bounded by northwest- and east-west-trending faults. Though this latter faulting may post-date Emeroo deposition, it is likely to predate Umberatana Group (see below). Observed thicknesses of Skillogalee Dolomite and undifferentiated Burra Group siltstones are not well constrained by drilling information. However, they do not appear to vary as much as the Emeroo Subgroup, suggesting that syndepositional faulting was less important, consistent with the substantial decrease in clastic input from the Gawler Craton. A period of uplift and erosion probably preceded deposition of the Umberatana Group which unconformably overlies either older Adelaidean units or basement. Pre-Tertiary outcrop/subcrop of both the Umberatana and Wilpena Groups (Fig. 2) defines a continuous belt aligned north-south suggesting that they were deposited under a regime of steady subsidence of

145

the area east of the central part of the Torrens Hinge Zone, without appreciable faulting. In terms of extensional tectonic models (Lister et al in press; Lister 1987) this stage of development represents the sag phase with the basal Umberatana Group unconformity representing the post-rift unconformity. In such a model the Torrens Hinge Zone represents a monoclinal flexure to the east of which there was substantial crustal thinning that ultimately enabled a great thickness of sag phase sediments (the Umberatana and Wilpena Groups) to be deposited in the Adelaide Geosyncline. Shallow-marine, fluvial and deltaic facies are characteristic of several Umberatana and Wilpena Group units along the Torrens Hinge Zone north of Bute (clastic units in Sturtian tillite, stromatolitic Brighton Limestone, thick Seacliff Sandstone) and represent the near-shore equivalents of thicker, deeper-marine facies to the east. Direct evidence of faulting associated with the Torrens Hinge Zone is rare within drillholes of the Spencer Shelf; sheared, steeply-dipping Burra Group (including Skillogalee Dolomite) in PP14 indicates close proximity to a major fault but, elsewhere, faults can only be inferred from stratigraphic and facies correlations aided by aeromagnetic interpretation. Many of the faults, those trending northeast, are of Tertiary age since they control the distribution and thickness of Tertiary sediments (eg southwest of Port Broughton; Parker, in press). Drillhole B26, southeast of Bute, intersected a reverse fault, with Kulpara Limestone and Tapley Hill Formation repeated across a zone of brecciated limestone. Reverse faulting (thrusting ?) is also present at Depot Creek and was formed as a result of compressional tectonics during the Late Cambrian-Early Ordovician Delamerian Orogeny.

ACKNOWLEDGEMENTS W.V. Preiss and D.B. Hilyard kindly reviewed the paper, which benefitted greatly from incorporation of their critical comments and suggestions. The typing and drafting staff of the

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Department of Mines and Energy are thanked for their contributions, and for their patience with numerous changes. A J . Parker and W.M. Cowley publish with permission of the Director-General of Mines and Energy.

REFERENCES BELPERIO A.P. (this volume) Palaeoenvironmental interpretation of the Late Proterozoic Skillogalee Dolomite in the Willouran Ranges, South Australia. COMPSTON W., CRAWFORD, A.R. and BOFINGER V.M. 1966. A radiometric estimate of the duration of sedimentation in the Adelaide Geosyncline, South Australia. Journal of the Geological Society of Australia 13, 229-276. COWLEY W.M. & PARKER A.J. 1988a. Gold in Burra Group sediments, Port Pirie Region. Quarterly Geological Notes, Geological Survey of South Australia 104, 5-13. COWLEY W.M. & PARKER A.J. 1988b. Drilling in the Torrens Hinge Zone - Spencer Shelf region between Bute and Port Germein, 1968-1983. South Australian Department of Mines and Energy Report Book (unpublished). DAILY B. 1963. The Fossiliferous Cambrian succession on Fleurieu Peninsula, South Australia. Records of the South Australian Museum 14,579-601. DEPARTMENT OF MINES, South Australia 1968-1974. Reports on exploration, EL 75, 207, 375. South Australian Department of Mines and Energy open file Envelopes 2429,2772, 3353 (unpubl.).

FANNING C.M., LUDWIG K.R., FORBES, B.G. & PREISS W.V. 1986. Single and multiple grain U-Pb analyses for the early Adelaidean Rook Tuff, Willouran Ranges, South Australia. Geological Society of Australia, Abstracts 15, 71-72. FANNING C.M., FLINT R.B., PARKER A.J., LUDWIG K.R. & BLISSETT, A.H., in press. Refined Proterozoic evolution of the Gawler Craton, South Australia, through U-Pb zircon geochronology. Precambrian Research. GERDES R.A. 1978. A geophysical study of the Broughton area in Wallaroo and Blyth 1:100 000 sheet areas. South Australian Department of Mines and Energy Report Book No. 78/109. (unpubl.). GERSTELING R.W. & HEAPE J.M. 1975. The Cattlegrid Orebody, Mt Gunson, South Australia. In: South Australian Conference, 1975. Part A: Adelaide and Port Pirie. Australasian Institute of Mining and Metallurgy, Conference Series 4, 103-112. JENKINS R.J.F. 1986. Ralph Tate's enigma - and the regional significance of thrust faulting in the Mt Lofty Ranges. Geological Society ofAustralia, Abstracts 15, 101.

LISTER G.S. 1987. The nature and origin of structures formed during extensional orogeny. In Applied Extension Tectonics. 16th BMR Research Symposium, Canberra, 1987. Bureau of Mineral Resources Record 1987/51. LISTER G.S., ETHERIDGE M.A. & SYMONDS P.A. in press. Detachment model for the formation of passive continental margins. Tectonics.

DREXEL J.F. & McCALLUM W.S. 1986. Origin and age of the Burra copper orebody. Quarterly Geological Notes, Geological Survey of South Australia 98.

MASON M.G., THOMSON B.P. & TONKIN D.G. 1978. Regional stratigraphy of the Beda Volcanics, Backy Point Beds and Pandurra Formation on the southern Stuart Shelf, South Australia. Quarterly Geological Notes, Geological Survey of South Australia 66,2-9.

FANNING C.M., FLINT R.B. & PREISS, W.V. 1983. Geochronology of the Pandurra Formation. Quarterly Geological Notes, Geological Survey of South Australia 104, 5-13.

MIRAMS R.C. 1964. BURRA map sheet. Geological Atlas of South Australia, 1:250 000 series. Sheet SI 54-5. Geological Survey of South Australia.

Torrensian Volcanism in Torrens Hinge Zone NORTH BROKEN HILL Ltd. 1976-1982. Reports on exploration, Bute EL 248, 420, 577, 972. South Australian Department of Mines and Energy open file Envelopes 2749, 3311, 3767 (unpubl.). PAGE R.W., McCULLOCH M.T. & BLACK L.P. 1984. Isotopic record of major Precambrian events in Australia. Proceedings, 27th International Geological Congress, Moscow, 1984, 5, 25-72. PARKER A.J. 1980. The six 1:100 000 compilation sheets of WHYALLA - A progress report. South Australian Department of Mines and Energy Report Book No. 80/93 (unpubl.). PARKER A J. 1983. Tectonic development of the Adelaide Fold Belt. Geological Society of Australia, Abstracts 10,23-28. PARKER A.J. in press. WHYALLA, South Australia. Explanatory Notes, 1:250 000 geological series. Sheet SI 53-8. Geological Survey of South Australia. PARKER A.J. & THOMSON B.P. 1978. Preliminary report on stratigraphic drilling in northern Yorke Peninsula. Mineral Resources Review, South Australia 147,61-67. PREISS W.V. 1974. The River Broughton Beds - A Willouran sequence in the Spalding Inlier. Geological Survey of South Australia, Quarterly Geological Notes 49, 2-8. PREISS W.V. (compiler) 1987. The Adelaide Geosyncline - Late Proterozoic stratigraphy, sedimentation, palaeontology and tectonics. Geological Survey of South Australia, Bulletin 53.

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RUTLAND R.W.R., PARKER A.J., PITT G.M., PREISS W.V. & MURRELL B. 1981. The Precambrian of South Australia. In Hunter, D.R. ed. Precambrian of the Southern Hemisphere, pp. 309-360 Elsevier, Amsterdam, pp. 309-360. SPRIGG R.C. 1952. Sedimentation in the Adelaide Geosyncline and the formation of the continental terrace. In Glaessner, M.F. and Rudd, E.A. eds. Sir Douglas Mawson Anniversary Volume, pp. 153-159, University of Adelaide, Adelaide. THOMSON B.P. 1969. ADELAIDE map sheet, Geological Atlas of South Australia, 1:250 000 series. Sheet SI 54-9. Geological Survey of South Australia. THOMSON B.P. 1970. A review of the Precambrian and lower Palaeozoic tectonics of South Australia. Transactions of the Royal Society of South Australia 94, 193-221. THOMSON B.P. 1973. Torrens Hinge Zone project: Bute Region. Report No. 1. South Australian Department of Mines and Energy Report Book No. 73/8 (unpubl.). THOMSON B.P. (compiler) 1980. Geological Map of South Australia, 1:1 000 000 scale. South Australian Department of Mines and Energy, Adelaide. THOMSON B.P., DAILY B., COATS R.P. & FORBES B.G. 1976. Late Precambrian and Cambrian geology of the Adelaide "Geosyncline" and Stuart Shelf, South Australia. 25th International Geological Congress, Sydney, 1976, Excursion Guide, 33A.

PREISS W.V. (this volume) A stratigraphic and tectonic overview of the Adelaide Geosyncline, South Australia.

VON DER BORCH C.C. 1980. Evolution of late Proterozoic to early Palaeozoic Adelaide Fold Belt, Australia : comparisons with post-Permian rifts and passive margins. Tectonophysics 70,115-134.

PREISS W.V. & FAULKNER P. 1984. Geology, geophysics and stratigraphic drilling at Depot Creek, southern Hinders Ranges. Quarterly Geological Notes, Geological Survey ofSouth Australia 89,10-19.

WEBB A.W. 1978.Geochronologyofstratigraphically significant rocks from South Australia: Progress Report No. 21. South Australian Department of Mines and Energy open file Envelope 1689 (unpubl.).

PREISS W.V. & SWEET LP. 1966. Geology of the Depot Creek area. University of Adelaide B.Sc. (Hons.) thesis (unpubl.).

WEBB A.W. & COATS, R.P. 1980. A reassessment of the age of the Beda Volcanics on the Stuart Shelf, South Australia. South Australian Department of Mines and Energy Report Book No. 80/6 (unpubl.).

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WEBB A.W., COATS R.P., FANNING, C.M. & FLINT R.B. 1983. Geochronological framework of the Adelaide Geosyncline. Geological Society of Australia, Abstracts 10, 7-9.

WEBB A.W. & HORR G. 1978. Rb-Sr age and petrology of a flow from the Beda Volcanics. Quarterly

Geological Notes, Geological Survey of South Australia 66,10-13.

WEBB A.W., THOMSON B.P., BLISSETT A.H., DALY S.J., FLINT R.B. & PARKER A J. 1986. Geochronology of the Gawler Craton, South Australia. A ustralian Journal of Earth Sciences 33, 119-143.

Glacigenic sediments of the late Proterozoic Elatina Formation and equivalents, Adelaide Geosyncline, South Australia. 1 2 by N.M. Lemon and V.A. Gostin 1

National Centre for Petroleum Geology and Geophysics, P.O. Box 498, Adelaide, S.A. 5001. Dept. of Geology and Geophysics, University of Adelaide, P.O. Box 498, Adelaide, SA. 5001.

The Elatina Formation, of late Proterozoic age, consists of sandstones with minor mudstones and diamictites, deposited under the influence of the Marinoan glaciation, the second major glacial period recorded in the sediments of the Adelaide Geosyncline. A series of detailed sedimentary sections were measured in the vicinity of the type section in the Central Flinders Ranges of South Australia to fully account for the considerable local lateral facies variation and to include additional section preserved at the base of the formation nearby. This work showed that the entire formation was deposited under glacial conditions. A locally developed conglomerate or tillite at the base of the formation rests on a regional unconformity eroded during the waxing stages of the Marinoan glaciation. The Elatina Formation was deposited from the peak of the glaciation through the waning stages. Sea level changes during this time affected facies distribution basin wide. A three fold subdivision of the formation in the type section of a lower slumped sandstone, a middle diamictite unit and an upper current reworked diamictite is recognized. This allows correlation with the Marinoan type section south of Adelaide and with the upper parts of the much thicker, more complete sequences of diamictites and associated sediments deposited in the NE and SE of the geosyncline. Final deglaciation is marked by the deposition of a widespread "cap" dolomite associated with a major transgression.

Key words: Elatina Formation; Marinoan glaciation; diamictite; Central Flinders Ranges. INTRODUCTION

The late Proterozoic succession in the Central Hinders Ranges was first described in detail by Mawson (1939). However, the glacial nature of the sediments around Elatina Hut (see Fig. 1) was not recognized until 1944 when facetted and striated pebbles were collected nearby (Mawson 1949). That paper recognized a second, separate glacial event well above the major Sturt Tillite, correctly correlating it with the Marinoan section at Hallett Cove, just south of Adelaide. The stratigraphy of the Central Flinders Ranges (Fig. 2) was formalized by Thomson et al (1964) and the section in Elatina Creek made the type section for the Elatina Formation (Dalgarno & Johnson 1964 a, b). Coats in Thomson et al (1964) correlated the Elatina Formation with the Yerelina Formation (later Yerelina Subgroup, Coats & Blissett 1971) in the northeast of the Adelaide Geosyncline. Thomson (in Parkin 1969) made a basin wide correlation between the

Yerelina Subgroup, the Elatina Formation, the Wilmington Formation of the Willochra Subgroup along the western margin of the Geosyncline, the Pepuarta Tillite of the Olary region and the Reynella Siltstone in the Marinoan type section at Hallett Cove. An age of 680 - 690 Ma. has been suggested for this sequence (Coats 1981). Rapid facies changes along strike and the preservation of additional units nearby mean that the type section at Elatina Hut is not fully representative of the formation. This paper describes a series of closely spaced measured sections up to 20 km north of Elatina with interpretations of the environments of deposition based on the explosion of recent articles in the literature on glacially associated rocks, (Gravenor et al 1984; Eyles & Miall 1984). Lithofacies codes devised by Miall (1978) and expanded by Eyles et al (1983), (Table 1), have been used to relate these descriptions to those of the recent literature. The

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N.M. Lemon & V.A. Gostin

new descriptions have also allowed a reinterpretation of some of the details of correlation with the other units deposited under the influence of the Marinoan glaciation. BACKGROUND Coordinates published in Thomson et al (1964), place the type section of the Elatina For-

mation in Etina Creek. This is the position shown on the Oraparinna 1:63 360 scale map and the Parachilna 1:250 000 scale map. The Mawson section, however, was in Elatina Creek near Elatina Hut, about 5 km to the south of that location. However, the thicknesses mentioned in both articles do not match that measured in Etina Creek and there is also some fault repetition at the base of this type section. The type section in Etina Creek was consequently remeasured (Section 19 in this paper) to clarify this problem. Local variation is such that it became necessary to measure a series of sections to adequately describe the formation. DETAILS IN THE LOCAL AREA. The Elatina Formation in the area around the type section rests on a marked regional unconformity. Activity on the nearby Enorama Diapir (Fig. 3) during the Marinoan glaciation locally changed the shape of the basin allowing deposition and preservation of additional section in the peripheral sink that developed around the margin of the active diapir (Lemon 1985). The Elatina Formation in the local area is described below in terms of the nature of the unconformity, the section preserved within the sink, the overlying more widespread units, and the nature of the contact with the succeeding formation. Nature of the unconformity

Fig. 1. Location map.

Recent measurements of detailed sections (NML) within the Trezona Formation, which underlies the Elatina Formation, show that 120 m of section has been removed by erosion on the unconformity between Sections 17 and 23. Sections 19 and 36, to the south of Section 23 are similar to Section 17, showing the removal of about 120 m of section. Mapping in the Nuccaleena area, 45 km to the north, (Brenchley-Gaal 1985), shows that the entire Trezona Formation, over 450 m thick, has been removed, and the Elatina Formation is in contact with the Enorama Shale, the unit that underlies the Trezona Formation. Progressive truncations of beds within the

Elatina Formation glacigenic sediments Trezona Formation indicate that the unconformity is more pronounced to the north, west and south of Section 23. At the local scale, there is no visible erosional relief on the unconformity surface, although there is some possible solution karsting evident around Section 19. The limestone surface often develops a small scale breccia up to 1 m thick on the contact which may be related to frost shattering. Pebbles of eroded Trezona limestones are usually included in the basal conglomerate of the Elatina Formation. Section 17 shows a weathered zone extending some 10 m down from the unconformity into the limestones of the Trezona Formation. Distinct reddening, limonitization, manganese enrich-

< cr

LATE MIDDLE

<-I o:e< Q.

EARLY

EDIACARAN

MARINOAN

O O OcrN o cc CL

STURTIAN

<9 _i LU O<

TORRENSIAN

WILLOURAN

MORALANA SUPERGROUP

<LU

HEYSEN SUPERGROUP

o INI o

ment and some silicification are associated with this weathering. Elsewhere, minor dolomitization is evident. Immediately above this weathered zone, a massive mudstone with Trezona carbonate clasts probably represents a regolith. Section preserved within the peripheral sink

Large exotic boulders up to 1 m in diameter are seen in places within the uppermost beds of the Trezona Formation (Fig. 4a). This clast suite contains granite gneiss, dolerite and well rounded gritty quartzites. The boulders are surrounded by a matrix of contorted red sandy mudstone and white sandstones with red mudstone clasts (the dominant lithology of the upper beds of the Trezona Formation). Elsewhere, the uppermost 1-2 m of the Trezona Formation is contorted

LITHOSTRATIGRAPHY

WARRINA SUPERGROUP

CHRONOSTRATIGRAPHY

151

LAKE FROME GROUP

ABC RANGE QTZ

HAWKER GROUP

NUCCALEENA FM

BRACHINA FM

WILPENA GROUP

ELATINA FM

TREZONA FM ENORAMA SHALE

UMBERATANA GROUP

ETINA FM

BURRA GROUP

CALLANNA GROUP

TAPLEY

WILYERPA FM

STRATIGRAPHY - CENTRAL FLINDERS RANGES

Fig. 2. Broad outline of late Proterzoic and Cambrian stratigraphy, Flinders Ranges.

HILL

FORMATION

152

N.M. Lemon & V.A. Gostin

without any boulders. It appears that this interval has been in contact with glaciers that have both transported the exotic boulders and bulldozed blocks into the only partially lithified underlying sediments. A massive boulder diamictite is seen in contact with the underlying Trezona Formation on Section 23 at Trezona Bore (Fig. 4b). This diamictite contains exotic clasts as well as some material from the underlying beds. This could be a true ice push tillite, (pers comm., G. Young), the only example of this facies in the area. This suggestion is supported by the recognition of

Dm Dc Dm Ds Dg Dr Dc Ds

Diamictite. D (_) matrix supported clast supported massive stratified graded resedimented current reworked sheared

Gm Gs

Gravel. G J J massive stratified Sands. S J J

Sr St Sh Sm Sg SI Sd

rippled trough cross bedded horizontal lamination massive graded low angle cross stratified soft sediment deformed

F1 Fm Fd

Muds, Fine grained. F JJ laminated massive with dropstones

Table 1. Lithofacies Codes, after Miall (1978) and Eyles etal (1983).

esker sandstones (Fig. 4b) of the form described by Visser etal (1987). Along strike from Section 23 to the north, a polymict conglomerate is seen resting on the Trezona Formation. This material is distributed in discrete channels, apparently oriented east-west. The conglomerate has a bimodal source with 60% of the clasts being reworked from the underlying Trezona Formation and containing distinctive black ooid limestones and red algal and intraclastic limestones. The other 40% of the clasts are exotic, consisting of granite gneiss, red porphyritic dacite, schist, metamorphic quartzite, vein quartz, basic intrusive, iron formation and sediments. There is a lag of large boulders at the base of the conglomerate. It is apparent that the conglomerate consists of reworked Trezona Formation mixed with clasts derived from the basal tillite. It is interesting to note that the exotic clast suite examined at Bulls Gap and Trezona Bore could all have been derived from an area within a 30 km radius of Iron Knob, a very likely glacial source area 200 km to the southwest. Middleback iron formation, the distinctive Gawler Range Volcanics, granite gneisses, schists and quartzite can all be found in outcrop there. One large boulder at Bulls Gap is a distinctive well rounded quartz granule conglomerate with bright green shale flakes and well defined quartz overgrowth cement. This bears a striking similarity to a bed within the Corunna Conglomerate, 5-10 km north of Iron Knob (Lemon 1972). The basal polymict conglomerate is almost laterally equivalent to the tillite and in Section 23, the latter is overlain and cut into by a relatively clean, white channellized gritty sandstone (Fig. 4c). The sandstone and conglomerate appear to represent fluvial channels with a lag of granule to small pebble conglomerate at the base. Current directions are from the N and NW and the sinuous nature of the creek at Trezona Bore allows individual channels to be picked up in successive outcrops along the creek. These sediments grade into a muddy flaser bedded silty

NORTH

SOUTH

SECTION 16 WERTA

SECTION 23 TREZONA BORE

SECTION 36 SECTION 19 ETINA CREEK

SECTION 17 SIX SPRINGS

ffl If 5' p •n o

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crq CD D o> QCD 3 TREZONA FORMATION

| silt/mudstone |

\o o | ooids intraclasts/clay galls

| sandstone conglomerate/granules

|§SgSS| stromatolites

||' I ' ! | limestone

| cross bedding

| ^

| ripples

1 dolomite

| ball and pillows

\ A a | tillite/dropstones

I slumping

\$> v 9 \ breccia

dolomite nodules/tepees

— ~

unconformity

25 20 15 -10

Vertical Scale Metres

5 L0

YVV~ k a r s t surface

Fig. 3. Correlation of sections measured north from the type section.

Horizontal Scale 1 2

(See table 1 for lithofacies codes) Section 19 is the type section

MEASURED SECTIONS E LATIN A FORMATION CENTRAL FLINDERS RANGES

Kilometres U\ LO

154

N.M. Lemon & V.A. Gostin

Fig. 4. a. Hammer lies on a boulder of granite gneiss bulldozed into contorted redbeds of the upper Trezona Formation overlain by the basal conglomerate of the Elatina Formation, near Section 16. b. Basal contact of the Elatina Formation (Section 23) at 20cm mark on staff. Red sandstone overlain by diamictite with a boulder lm left of staff. Dark slumped channels in the centre are esker sandstones, c. Detail of the fluvial channel sands seen at the top of Fig. 4b. d. Large ball and pillow structures, (Section 23), developed as sands prograded into a muddy lake. e. Slumped cross bedded sands; usually the slumping destroys obvious bedding. Photo near Section 23. f. Bimodal, "granule train" sandstone, typical of widespread bottom third of the Elatina Formation, near Section 16.

Elatina Formation glacigenic sediments

155

Fig. 5. a. Massive diamictite, Section 19. b. Facetted and striated pebbles are common in the upper part of the Elatina Formation, 1 km N of the type section, c. Massive diamictite is overlain by a thin gravel lag, then a well bedded, ripple cross laminated sandstone grading up into a massive diamictite, between Sections 10 and 23. d. Cobble of quartzite in ripple cross laminated fine sandstone, top of the Elatina Formation, Section 15, Moolooloo. e. The contact between red, fine sandstone of the Elatina Formation and overlying buff dolomite of the Nuccaleena Formation is gradational over 10 cm., 1 km N of type section, f. Large "tepoid" structure in dolomite of the Nuccaleena Formation (hammer scale), 1 km N of type section.

156

N.M. Lemon & V.A. Gostin

sandstone indicating a change from sub-aerial to sub-aqueous deposition. The rate of sedimentation then became very rapid with the influx of sand bodies up to 1 m thick into the muddy basin. This led to the development of dramatic ball and pillow structures as the sands collapsed into the underlying muds (Fig. 4d). Sand influx into the basin began to dominate at this point. A series of coarse, cross bedded sandstones outcrop at Section 10, some 6 km north of Section 23 and are equivalent to the massive diamictite, fluvial channel sandstones, flaser bedded mudstones and ball and pillow intervals as described above. These are interpreted as a thicker development and amalgamation of the fluvial channels. Section preserved away from the peripheral sink As sandstones became more prominent in the formation, deposition and/or preservation of the sediments moved away from the deeper parts of the basin to become far more widespread. All sections measured in the Central Flinders Ranges show the units described below. Above the ball and pillow section lies a thick interval of poorly sorted sandstones that display a number of unusual characteristics, making the unit readily recognizable in all sections measured by the authors and allowing a ready correlation with sections measured by other workers. In most places, these sandstones lie directly above the basal unconformity (see Sections 36, 19, 16 and 17, Fig. 3). The dominant characteristics of these sandstones are the absence of distinct bedding, the presence of contorted "trains" of granules (Fig. 4e) and a generally slumped appearance. In the less deformed intervals, slumped trough cross beds up to 1 m thick occur with obvious grading of the sandstone into fining upwards packages. Despite slumping, the cross bedding can be seen

by truncations at the tops of beds (Fig. 4f). Rare outsize boulders are also present in these sandstones but the slumped nature of the sediment has destroyed any hint of how these boulders came to be incorporated. Small wispy sandstone patches also occur within the sediment and probably represent water escape zones which have been washed clean of silt by the water movement. Small irregular patches of coarsely crystalline dolomite, or dolomite "knots", up to 1 cm across may be another representation of zones cleaned of mud by water escape, and later cemented. The sandstone described above is distinctly bimodal, with a silt to fine sand fraction together with a veiy coarse to granule size fraction. The medium and coarse sands are missing here, but the equivalent unit on the Stuart Shelf, the Whyalla Sandstone, is of medium to coarse sand size, described as possibly being an aeolianite (Williams & Tonkin 1985). This fractionation has been noted with other glacially associated sands and it is suggested that the sediment has been involved in aeolian reworking and removal of the medium to coarse sand fraction prior to final deposition. A similar fractionation has been recorded by Gostin (1966). This whole interval can be interpreted as having been due to rapid deposition of a poorly sorted sand in a subaqueous environment with almost continuous slumping and dewatering. The sandstone grades upward into a massive and well laminated red siltstone with a few dropstones or, in other places, into a red sparse diamictite (Fig. 5a). There is usually some interfingering of the sandstone with the finer sediments. The diamictites in this interval have clasts randomly distributed in both massive and laminated sandy siltstones. These appear to have been rafted into the basin by floating ice as there is no obvious deformation of the beds. The depth of water must have been sufficient to allow icebergs to float freely without grounding. The boulders which are found as isolated dropstones are dominated by dolerites and

Elatina Formation glacigenic sediments

vesicular basalts which are identical to rocks presently outcropping two to five km east in the Enorama Diapir. The remainder of the coarse suite consists of dolomites and heavy mineral banded sandstones which are also found within the diapir. Many clasts are marked by facets and striations, attesting to their glacial character (Fig. 5b). Current reworked beds dominate the top third of the Elatina Formation. The diamictites and dropstone muds have undergone obvious reworking with prominent beds of distictive gravel lags (Fig. 5c) capped by ripple cross-laminated sandstone within otherwise massive units. The formation is generally a recessive weathering unit, but near its top, well sorted, ripple crosslaminated fine sandstones (Fig. 5d) often form a boldly outcropping ridge. Sections measured close to the diapir show two further modifications to the sequence not common elsewhere. There is a further diamictite at the top of the formation which again shows local derivation from an exposed nearby diapir. Between Sections 19 and 23, several channels of polymict, small pebble to cobble conglomerate occur within the current reworked interval. These contain clasts derived directly from the diapir together with diapiric clasts reworked from the diamictites plus clasts of ooid and algal limestones from the Trezona Formation. This shows that the diapir must have been actively rising to both supply clasts from itself and to drag to the surface Trezona sediments previously deposited around its margin. The upper contact of the Elatina Formation varies from sharp to gradational over 20 cm, and is overlain by massive dolomites of the Nuccaleena Formation. Such cap dolomites to the glacial sequences of this age are known worldwide (Williams 1979, Hambrey & Harland 1981). The Nuccaleena Formation usually commences with a thin interval of pink silty dolomite ( Fig. 5e), that grades into a generally massive, micritic dolomite with occasional evidence of current activity suggesting that the dolomite was

157

deposited as detrital grains. Unusual "tepoid" structures can be seen around Sections 19 and 36. These are large tepee-like culminations in the dolomite bedding and appear to have formed during sedimentation. The angular peaks in the bedding are up to 1.5 m high and have sediments draped over them so that they disappear up section (Fig 5f). There is no evidence of exposure and no associated intraclastic material so that the structures must have formed while completely submerged. The massive dolomites grade upward through silty dolomite to dolomite interbedded with red shale, to nodular dolomite in red shale and eventually into red shale (Plummer 1978b). The dolomite in this position marks the end of the glacial influence, but much ot the Elatina Formation contains dolomite, both as cement to some of the sandstones and as concretionary "knots" in the cleaner parts of the slumped sandstones. In contrast, there is no carbonate in the several thousand metres of the Brachina Formation above the nodular dolomite. In summary, except for pockets of locally preserved additional section, the Elatina Formation can be subdivided into three parts over much of the western side of the geosyncline: lower part of slumped sandstone, a middle interval of dropstone diamictites and an upper interval of current reworked diamictites. These divisions can be explained in terms of water level in the basin. The slumped sandstones were deposited as the sea level deepened after deposition of the basal ice contact tillite and fluvial channel sandstones. As water depth increased, finer sediments were deposited and icebergs were able to raft gravel into the basin. Sea level then fell or the basin began to fill without subsidence and the diamictites were reworked by current activity. Water levels began to deepen again at the close of the glacial period and the Nuccaleena Formation dolomites were deposited, grading up into red shales with continued transgression. Thus, in general, it appears that the Elatina Formation was deposited following the peak of the Marinoan glaciation. Onset of the actual

158

N.M. Lemon & V.A. Gostin

glaciation resulted in a drop of base level. That led to the creation of a major disconformity at the base of the Elatina Formation in the shallower parts of the basin. In deeper parts, a minor local ice-contact tillite was preserved, with the remainder of the formation deposited under subaqueous conditions. Water levels deepened through deposition of the first two units, shallowed during the deposition of the upper unit then deepened again at the close of the glacial period with deposition of the Nuccaleena Formation and the rest of the Wilpena Group. A tillite on the basal contact and a dropstone diamictite in contact with the overlying Nuccaleena Formation attest to the fact that the whole of the Elatina Formation was deposited under glacially influenced conditions.

entire western margin of the Adelaide Geosyncline from northwest of Blinman to Hallett Cove in the south (Fig. 7). The threefold subdivision of the Elatina Formation, related as this is to regional sea level fluctuations, forms a basis for correlation. As suggested by Mawson (1949), outcrops along the coast at Hallett Cove are a distant "echo" of the Elatina glaciation. Regional mapping of the Adelaide Geosyncline has shown the dolomite of the Nuccaleena Formation to be a persistent regional marker horizon and this allows many measured sections of the S E C T I O N 10

SECTION 30

BULLS G A P

ELATINA FORMATION EAST OF THE TYPE AREA Section 30 was measured 20 km east of the line of sections described above (Fig. 1). This is near the centre of the basin of deposition and on the other side of the line of diapiric islands which acted as a partial barrier during deposition of the Elatina Formation. Despite the geographic separation of these sections, there are a number of similarities between the areas. Section 30 (Fig. 6) has a thin basal conglomerate followed by a massive diamictite containing a selection of exotic boulders similar to the suite in Bulls Gap. This diamictite is followed by the distinctive pink slumped sandstone. The rest of the section is much thicker than Section 10 and is dominated by ripple cross-laminated very fine to fine sandstone with rare dropstones. The marker dolomite horizon, Nuccaleena Formation, caps the Elatina Formation as it does elsewhere.

Sm/Sp Sm/Smd

_ Gmd/Smd Dmm Dmsc

CORRELATION THROUGHOUT THE ADELAIDE GEOSYNCLINE A remarkably similar section to that seen in the Central Flinders Range outcrops along the

Dmm

Fig. 6. Sections 10 and 30.

Elatina Formation glacigenic sediments

159 Yerelina

Fig. 7. Correlation of the Elatina Formation and other Marinoan glacigenic sequences throughout the Adelaide Geosyncline. Yerelina section from Coats (1973), Eudunda from Morris (1972), Pepuarta Bluff from Forbes (1975), Buckaringa Gorge from Miller (1975), Spring Creek from Plummer & Gostin (1976) and Hallett Cove from Alexander (1984).

160

N.M. Lemon & V.A. Gostin

Elatina Formation to be hung from a reliable datum. Details of the Marinoan type section as measured by Alexander (1984), show a similar three fold subdivision of the sequence below the Nuccaleena Formation. The basal interval consists of dark green dolomitic sandstone with distinctive slumped bedding, capped by a pink trough cross-bedded sandstone. This in turn is overlain by red and purple siltstones, diamictites and channelized sandstones with dolomitic clasts of the Reynella Siltstone. The overlying Seacliff Sandstone contains ripple cross-laminated and trough cross-bedded sandstones with water escape features and interbedded dolomite lenses with dolomite chips reworked into the overlying sandstones. The authors agree with the interpretation of Plummer (1978a), that, where more than one dolomite layer is present, as at the type Marinoan section, the uppermost and best developed dolomite is taken as the Nuccaleena Formation. This tops a sequence of generally dolomitic sandstones and is below the purely clastic shales and sandstones of the Brachina Formation. Thomson (1969) and others, however, suggest that the Seacliff Sandstone in the Marinoan type section, is equivalent to the Nuccaleena Formation and therefore in the Wilpena Group overlying the Elatina Formation in the Umberatana Group. Detailed sections measured by Alexander (1984), show that the dolomites in the type Marinoan section within the Seacliff Sandstone have reworked tops suggesting deposition in shallowing upward conditions. Only the uppermost and thickest dolomite was deposited in deepening upward conditions, as is the Nuccaleena Formation in the Flinders Ranges. The authors thus consider that the Seacliff Sandstone is equivalent to the current-reworked section at the top of the Elatina Formation. Miller (1975) and Jablonski (1975), recorded a threefold subdivision of the Elatina Formation

in the Buckaringa Gorge and Warren Gorge areas of the southern Flinders Ranges and correlated these to the Marinoan type section (Fig. 7). Alexander (1984), correlated the Seacliff Sandstone, Reynella Siltstone and underlying massive sandstone of the Marinoan type section with the three subdivisions of Miller and Jablonski. The section measured by Plummer (Plummer & Gostin 1976), at Spring Creek near Wilmington, shows a massive slumped sand beneath a siltier unit which together were considered to belong to the Elatina Formation. In that area there is a very poor development of a sand beneath the Nuccaleena Formation. The threefold subdivision is also evident in sections measured by one of us (NML) at Pichi Richi (Fig. 7). Although the overall section is quite thin near the Torrens Hinge Line which forms the western margin to the Adelaide Geosyncline, the distinctive slumped sandstones with granule "trains" overlie an interval of well bedded sandstones with granule beds which are considered to be part of the Wilmington Formation. The slumped sandstones are overlain by an interval of mudstones and diamictites. Current reworked sandstones with shale intervals and occasional dropstones make up the rest of the sequence. It is this interval that contains the spectacular cyclicly banded deposits described by Williams (1981,b 1988). The varves suggest a freshwater, lacustrine environment and emergence around the margin of the basin. The threefold subdivision is again evident at Parachilna Gorge and further north at Moolooloo but sections measured a little west of there by Brenchley-Gaal (1985), show the bottom half of the Elatina Formation to be composed of slumped sandstones and the upper half of ripple cross laminated sandstones with a minor diamictite at the very top of the formation. The relatively thin sections of the Elatina Formation exposed along the western edge of the geosyncline are partially equivalent to the very

Elatina Formation glacigenic sediments thick sections exposed along the eastern side of the basin. Over 1300 m of sediments comprise the Yerelina Subgroup in its type area in the northeast of the geosyncline. This has been correlated with about 850 m of sediments of the Pepuarta Tillite in the Olary region and at least 1 500 m of Pepuarta Tillite near Eudunda (Morris 1972). At least part of the thickening of the glacially associated rocks in these areas is due to local syndepositional downfaulting. This is shown by rapid thickness changes within each area. During the time of erosion along the western margin of the geosyncline, the deeper parts of the basin accumulated thick sections of fine grained sediments. The proposed correlation is shown in Fig. 7. The tillite seen in contact with the unconformity on Sections 23 and 30 may be equivalent to the Mount Curtis Tillite (Coats & Blissett 1971), with the majority of the Elatina Formation equivalent to the Balparana Sandstone. There is some problem with the definition of the underlying Fortress Hill Formation. Both Coats (in Coats & Blissett 1971) and Morris (1972) defined the base of the Yerelina Subgroup as the first appearance of sandstone above the Amberoona Formation and Tarcowie Siltstone respectively. This definiton includes a considerable thickness of sandy siltstone before the presence of the first definite glacial characteristic, such as rare dropstones. With the approach of a major glaciation, sea levels would drop and sediments within the basin would coarsen upward. If the Yerelina Subgroup and equivalent Elatina Formation are defined as having been deposited under glacially influenced conditions, then the base of the formation should be defined as the first appearance of those conditions. The authors agree with Coats (1981) that the Fortress Hill Formation was deposited as the glaciation waxed, the Mount Curtis Tillite represents the peak of the glaciation and the Balparana Sandstone, the waning stages. The Elatina Formation was certainly deposited during the waning stages and for this reason it is correlated with the Balparana Sandstone.

161

SUMMARY Measurement of detailed sections of the Elatina Formation near the type section in the Central Flinders Ranges, have shown that the entire formation was deposited under glacially influenced conditions. Additional section preserved in locally deeper parts of the basin assist in the correlation with the Yerelina Subgroup in the northeastern Flinders Ranges. Away from the local deeps, the formation can be subdivided into three units. These are a distinctive, usually pink, slumped sandstone with common granule "trains", a red siltstone with dropstones and interbeds of dropstone diamictite, then a red ripple cross-laminated sandstone derived from the reworking of diamictites under shallower water, higher energy conditions. This threefold subdivision can be traced southward along the western margin of the Adelaide Geosyncline to the type section of the Marinoan at Hallett Cove. The onset of the Marinoan glaciation was attended by a lowered sea level expressed as a shoaling upward top to the Fortress Hill Formation in the northern Flinders Ranges (Adelaide Geosyncline), and to the Trezona Formation in the central area, as well as a regional unconformity/disconformity along the shallower western margins of the geosyncline at the base of the Elatina Formation. The peak of the glaciation is represented by the Mount Curtis Tillite and Pepuarta Tillite. Remnants of the peak glaciation are represented in local deeps in the central area by minor diamictites and basal gravels. The lithologies of the megaclasts indicates that the area around Iron Knob, on the Gawler Craton, was a source area for glacial erosion and entrainment. The source area for the northeastern Mount Curtis Tillite was probably the Curnamona Cratonic Nucleus, but detailed study is warranted. Sea levels rose with deglaciation and widespread deposition occurred of slumped sands at the base of the Elatina Formation and of the Balparana Sandstone. A renewed cooling created some local ice caps such as those sited on diapiric islands in the

162

N.M. Lemon & V.A. Gostin

central part of the geosyncline, and deposited diamictites with a mixed assemblage of glacial transported clasts derived both from the old cratonic areas and from the diapirs. These sediments were reworked by shallow marine currents, resulting in several intervals of lag gravels. A waning of the ice age created large volumes of glacial outwash, shallow marine reworking of sediments, grain flow proximal turbidites, (as at Hallett Cove), and deposition and erosion of dolomite lenses. Final deglaciation was marked by a major transgression and deposition of an undisturbed and very widespread "cap" dolomite of the Nuccaleena Formation.

ACKNOWLEGEMENTS The authors wish to thank Esso Australia Ltd, for some financial assistance, Mr. and Mrs. G. Mcintosh for accomodation in the field, Dr. Grant Young, for suggestions and comments on a joint field trip, Sherry Proferes for draughting and Rick Barrett for photography. The authors also benefitted from numerous comments from participants of a 12th International Sedimentological Congress field trip. The work was done as part of a Ph.D thesis, (NML), under the supervision of VAG.

REFERENCES

COATS R.P. 1981. Late Proterozoic (Adelaidean) tillites of the Adelaide Geosyncline. In Hambrey M.J. & Harland W.B. eds, Earth's Pre-Pleistocene glacial record, pp.537-548, Cambridge University Press. COATS R.P. & BLISSETT A.H. 1971. Regional and economic geology of the Mount Painter province. Geological Survey of South Australia, Bulletin 43. DALGARNO C.R. & JOHNSON J.E. 1964a. The Wilpena Group. In Thomson et al, Precambrian rock groups in the Adelaide Geosyncline: a new subdivision. Quarterly Geological Notes, Geological Survey of South Australia 9, 1-19. DALGARNO C.R. & JOHNSON J.E. 1964b. Glacials of the Marinoan Series, Central Flinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 11. FORBES B.G. 1975. Stratigraphic sections through the uppermost Umberatana Group near Yunta and Manna Hill, South Australia. South Australian Department of Mines and Energy, Report Book 75/13 (unpubl.). EYLES N., EYLES C.H. & MIALL A.D. 1983. Lithofacies types and vertical profile analysis; an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences. Sedimentology 30, 393-410. EYLES N. & MIALL A.D. 1984. Glacial Facies. In Walker R.G. ed. Facies Models (second edition), Geoscience Canada, Reprint Series 1, 15-38.

ALEXANDER E.M. 1984. Sedimentology of the Marinoan type section, Marino Rocks to Hallett Cove area, South Australia. B.Sc (Hons) thesis,University of Adelaide (unpubl.).

GOSTIN V.A. 1966. Tertiary stratigraphy of the Mornington District, Victoria. Proceedings of the Royal Society of Victoria 79, 459-512.

BRENCHLEY-GAAL A.J. 1985. The influence of faulting on late Proterozoic sedimentation and Delamerian tectonic development within the Nuccaleena area, Central Flinders Ranges, S.A. B.Sc (Hons) thesis, University of Adelaide, (unpubl.).

GRAVENOR C.P., VON BRUNN V., & DREIMANIS A. 1984. Nature and classification of waterlain glaciogene sediments, exemplified by Pleistocene, Late Paleozoic and late Precambrian deposits. Earth-Sciences Reviews 20, 105-166.

COATS R.P. 1973. COPLEY, South Australia. Explanatory Notes. 1:250,000 geological series. Sheet SH/54-9, Geological Survey of South Australia.

HAMBREY M.J. & HARLAND W.B., eds 1981. Earth's Pre-Pleistocene glacial record. Cambridge University Press.

Elatina Formation glacigenic sediments

JABLONSKI H. 1975. Late Precambrian geology of the Warren - Buckaringa Gorge area, Flinders Ranges, South Australia. B.Sc, (Hons) thesis, University of Adelaide (unpubl.). LEMON N.M. 1972. A sedimentological approach to the geology of the Corunna Conglomerate. B.Sc(Hons) thesis, University of Adelaide, (unpubl). LEMON N.M. 1985. Physical modelling of sedimentation adjacent to diapirs and comparison with late Precambrian Oratunga breccia body in Central Flinders Ranges, South Australia. Bulletin of the American Association of Petroleum Geologists 69, 1327-1338. MAWSON D. 1939. The late Proterozoic sediments of South Australia. Report of Australian and New Zealand Association for the Advancement of Science 24, 79-88. MAWSON D. 1949. The Elatina Glaciation. A third recurrence of glaciation evidenced in the Adelaide System. Transactions of the Royal Society of South Australia 73,117-121. MIALL A.D. 1978. Lithofacies types and vertical profile models in braided rivers: a summary. In Miall A.D. ed. Fluvial Sedimentology. Canadian Society of Petroleum Geologists, Memoir 5, 597-604.

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PLUMMER P.S. 1978b Note on the paleoenvironmental significance of the Nuccaleena Formation (upper Precambrian), Central Flinders Ranges, South Australia. Journal of the Geological Society of Australia 25 395-402. PLUMMER P.S. & GOSTIN V.A. 1976. Faulting contemporaneous with Umberatana Group sedimentation (late Precambrian), Southern Hinders Ranges, South Australia. Transactions of the Royal Society of South Australia 100 29-37. THOMSON B.P., COATS R.P., MIRAMS R.C., FORBES B.G., DALGARNO C.R., & JOHNSON J.E. 1964. Precambrian rock groups in the Adelaide Geosyncline: new subdivision. Quarterly Geological Notes, Geological Survey of South Australia 9,1-19. VISSER J.N.J., LOOCK J.C. & COLLISTON W.P. 1987. Subaqueous outwash fan and esker sandstones in the Permo-Carboniferous Dwyka Formation of South Africa. Journal of Sedimentary Petrology 57, 467-478. WILLIAMS G.E. 1979. Sedimentology, stable isotope geochemistry and paleoenvironment of dolostones capping late Precambrian glacial sequences in Australia. Journal of the Geological Society of Australia 26, 377-386.

MILLER R.K. 1975. The late Precambrian geology of the Wyacca Bluff - Buckaringa Gorge area, Flinders Ranges, South Australia. B.Sc(Hons) thesis, University of Adelaide, (unpubl.).

WILLIAMS G.E. 1981a. Reply: Sedimentology, stable isotope geochemistry and paleoenvironment of dolostones capping late Precambrian glacial sequences in Australia. Journal of the Geological Society of Australia 28,102-105.

MORRIS L.J. 1972. The geology of the Eudunda Hansborough area of South Australia with emphasis on the Pepuarta Tillite Formation of Upper Precambrian age. B.Sc(Hons) thesis, University of Adelaide, (unpubl.).

WILLIAMS G.E. 1981b. Sunspot periods in the late Precambrian glacial climate and solar-planetary relations. Nature 291, 624-628.

PARKIN L.W. ed 1969. Handbook of South Australian Geology. Geological Survey of South Australia, Adelaide. PLUMMER P.S. 1978a Stratigraphy of the lower Wilpena Group (late Precambrian), Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 102, 25-38.

WILLIAMS G.E. 1988. Cyclicity in the late Precambrian Elatina Formation, South Australia: Solar or Tidal signature? Climatic Change 13, 117-128. WILLIAMS G.E. & TONKIN D.G. 1985. Periglacial structures and palaeoclimatic significance of a late Precambrian block field in the Cattle Grid copper mine, Mount Gunson, South Australia. Australian Journal of Earth Sciences 32, 287-300.

Late Precambrian wave- to tide-dominated delta evolution in the west-central Adelaide Geosyncline, South Australia P. S. Plummer Shell Company of Australia, 155 William St., Melbourne, Vic, 3000, Australia. The Late Precambrian Brachina Subgroup was deposited in the Adelaide Geosyncline in response to a massive influx of coarse detritus generated on the Gawler Craton to the west by a phase of tectonism and volcanic activity. The palaeogeographic configuration of the geosyncline at the time of initial detrital influx was that of a vast aerobic tidal shelf in the western and central regions skirting a shallow anaerobic basin in the eastern region. The sediment debouchment accumulated as an intertidal deltaic pile which prograded across the western region, halting at the gentle palaeoslope leading into the deeper eastern region. Wave activity acted upon the leading edge of the delta creating a fringing barrier-bar system. Delta characteristics indicate this delta to have been a fluvial and tide modified, wave-dominated system fed by stable distributary channels. Basin shallowing accompanied the continued sediment influx and the eastern and central regions shallowed to form an extensive intertidal shelf. Tidal activity thus replaced wave action as the dominant influence on delta morphology, and the delta evolved to an unbarred fluvial modified, tide-dominated system fed by braided distributary channels. Delta development ceased when tectonic instability directly affected the western margin of the basin, causing partial erosion of the delta with reworking and dispersal into a vast, thin intertidal sandflat deposit throughout the central region of the basin. Key words: Late Precambrian, Adelaide Geosyncline, Brachina Subgroup, palaeogeography, delta, wave-dominated, tide-dominated. INTRODUCTION The Adelaide Geosyncline, extending some 750 km north from Kangaroo Island to the Mount Painter Province (Fig. 1), is host to a maximum of 15 000 m (Preiss & Forbes 1981) of essentially shallow water sedimentary rocks. These accumulated over the closing phases of the Precambrian and into the Middle Cambrian, approximately 800 to 500 million years ago. The sequence has been relatively untouched by metamorphism during the compressive deformations that created and elevated the present ranges which, due to a semi-arid climate, provide excellent exposure of the complete Adelaidean and Ediacaran succession. Worldwide attention was thrust upon this sequence with the discovery of the Ediacaran fossil assemblage of soft-bodied macroscopic life forms (Sprigg 1947; Glaessner & Wade 1966).

This fauna is preserved within the uppermost coarsening-upward megacycle of the Wilpena Group (Fig. 2), which defines the succession between the last Precambrian glaciation and the base of the Cambrian. The lower megacycle within the Wilpena Group, the Brachina Subgroup (Plummer 1978a; Gatehouse 1980), was subjected to detailed scrutiny and analysis in the hope of extending the macropalaeontological record further back in time. Although features resembling body and trace fossils of soft-bodied organisms were located, inorganic origins were concluded to have been their cause (Plummer 1980; Jenkins etal 1981). The detailed geological analysis revealed, however, that this subgroup was not simply three lithotypes overlying one another (i.e. a basal dolomite, an intervening shale and a capping quartzite), but a complex arrangement of ten distinct sedimentary facies resultant from the inter-

P.S. Plummer

play of environments during two distinct depositional phases. The first phase involved the development of a previously unrecognised delta system in the western region of the basin (Fig. 1) which covered upwards of 10 000 sq. km with at least 2 000 m of sediment. The later phase involved the partial erosion of this delta and northward removal of the reworked sediment to form a vast, thin intertidal sandflat throughout the central region of the basin. This paper deals with the palaeogeographic evolution of the deltaic phase of Brachina Subgroup deposition.

METHODOLOGY Fieldwork was carried out on the Brachina Subgroup over some 28 000 sq. km of the Adelaide Geosyncline (Fig. 1) with 77 localities visited, including 24 measured and recorded sections. In addition to noting the lithologies and

Fig. 1 Study area location map and subdivision of the Adelaide Geosyncline into structural regions.

165

sedimentary structures present, the sedimentary facies arrangement, both vertically and horizontally, was analysed using a Markov Chain technique (see Harms et al 1975) to ascertain the relevant facies transitions within the succession as an assistance in determining the overall palaeogeographic evolution over such a vast area. The analysis of palaeocurrent data is a crucial aid in such studies and readings were taken wherever directional structures revealed unambiguous bearings. Analysis of these data was undertaken using the technique of Plummer & Leppard (1979), wherein complete analysis could be made of uni-, bi-, or tri-modal data. Field samples of all sandstone facies were collected and microscopic petrologic analyses were made of their bulk composition and heavy mineral components. The sandstone classification scheme used is presented in Fig. 3.

Fig. 2 Stratigraphy of the upper Adelaide Geosyncline section highlighting the Brachina Subgroup and the two coarsening-upward megacycles of the Wilpena Group (I and II).

166

Late Precambrian delta evolution

GEOLOGIC SETTING Following the uppermost Precambrian Elatina glaciation the Adelaide Geosyncline became a vast peritidal basin. Deposition of the Brachina Subgroup commenced with the accumulation of a predominantly intertidal dolomite, the Nuccaleena Formation. Locally, around low relief islands in the central region of the basin, supratidal conditions prevailed (Fig. 4a). This dolomite heralds the onset of a new cycle of basin subsidence and sediment accumulation which culminated with the deposition of the ABC Range Quartzite (Fig. 2). Gradationally overlying this dolomite is a thick sequence of shales and siltstones which constitute the Moolooloo Formation. This formation is present in the central and western regions of the basin as an aerobic tidal shelf deposit and in the eastern region as an anaerobic basinal facies (Fig. 4b). Deep open water is postulated to have been present to the southeast. Sedimentation of the Moolooloo Formation occurred from episodic turbid bottom currents, as evidenced by numerous small scour structures, flute and tool marks, load structures, graded bedding, ripple cross-lamination and rare flaser bedding (Fig. 5). It was into this essentially quiet depositional environment that a massive debouchment of coarse detritus occurred from the west, originating from tectonic instability on the Gawler Craton (see Fig. 1), and thereby initiating the development of a large delta.

Dissecting these deposits are three west-east trending zones within which the coarsest fraction of the detritus accumulated as purple, trough crossbedded, heavy mineral bearing, medium feldspathic quartzose arenites. Within the ubiquitous trough crossbedding recumbent foreset faces are common, indicative of drag by strong sediment-laden currents upon water-saturated quasistable megaripples (Hendry & Stauffer 1977). Palaeocurrent analysis reveals a predominantly easterly flow direction (Fig. 6a), although minor southerly or northerly deviations are attributed to the geographic location of each zone on the growing sediment pile. These three west-east zones represent high energy distributary channels that dissect the purple silty deposits of subaqueous interdistributaiy levees and earlier broad distal distributary shoals. On reaching the gentle palaeoslope leading into the deeper anaerobic basin to the east, the distal distributary channel deposits fanned out into broad, possibly coalescing distributary mouth shoals, providing occasional oxygenated (purple) sandy interbeds that flowed and slumped into the otherwise reduced (green) shales and siltstones of the subtidal basin, which now constituted a deltaic bottomset (Fig. 7a). Within the central region of the basin, adjacent to, and northeast of the emerging delta and overlying the Moolooloo Formation, the initial QUARTZ Quartzarenite-^3%, Lithjc Feldspathic Quartzose Arenite Subarkose

DELTA INCEPTION

Quartzose

25% Lithic

Deposition of a dark purple, massive to laminated siltstone to fine subarkose heralded the detrital influx into the western region of the basin. Although generally structureless, occasional scours, filled with shale and siltstone intraclasts set in a coarse siltstone matrix, load and flame structures, and heavy mineral depicted ripple cross-lamination, are infrequently encountered in this unit.

Arenite

Sublitharenite

FELDSPAR

50%

Subarkose

10% ROCK FRAGMENTS

Fig. 3 Sandstone classification scheme.

P.S. Plummer

detrital influx is recorded by the presence of dark purple massive siltstones, often soft-sediment deformed, and/or white and red laminated, fine to medium feldspathic quartzose arenites (Figs 8a, b). These deposits accumulated under environmental conditions similar to those of the underlying Moolooloo Formation, namely those of an aerobic tidal shelf (Fig. 7a). Interbedded with these sediments are dark purple, massive, crossbedded or intraformational conglomeratic tuffaceous siltstones (Figs 8c, d). These volcanogenic deposits originated as ashfall

167

from volcanic activity to the west of the basin associated with the tectonic instability responsible for the generation of the coarse detritus (Plummer 1978b).

DELTA DEVELOPMENT Coarse sediment continued to be produced on the Gawler Craton and input into the basin. Consequently, a delta topset developed and prograded into the western region above the previously submerged distal deltaic system. The distributary

Fig. 4 Palaeogeographic reconstructions at: (a) Nuccaleena Formation time (after Plummer 1978c) ; and (b) Moolooloo Formation time.

168

Late Precambrian delta evolution

channels continued their existence, but now medium sand, silt and clay spilled out across this topset plain to accumulate in a succession of thinly bedded purple shales, siltstones and fine to medium subarkoses cyclically interposed with medium bedded purple, planar crossbedded, medium feldspathic quartzose arenites. The arenites generally comprise from 50 to 80 percent of the cycles, which range in thickness from 3 to 12 m (Fig. 9). Shaly partings throughout this sequence provide a natural plane of section revealing a wealth of environmentally significant sedimentary structures. Periodic atmospheric exposure is evident by the common occurrence of polygonal desiccation cracks, sometimes with possible hailstone

impressions (Fig. 10a), surfaces displaying raindrop impressions and rare bubble tracks. Load casts are not uncommon and can be associated with spindle-shaped synaeresis cracks. Such cracks also developed between the crests of starved ripples or partially connected lenticular sandstone beds (Fig. 10b), or on the basal surface of essentially flat sandstone beds. Continuous sinuous synaeresis cracks, on the other hand, occur along the troughs of symmetric ripple marks. Asymmetric and interference current ripples are also well represented. The most ubiquitous sedimentary structure, however, is planar crossbedding, frequently displaying a herringbone arrangement. Trough crossbedding is occasionally encountered,

Fig. 5 Sedimentary structures of the Moolooloo Formation: (a) graded bedding; (b) flute marks; and (c) load casts.

P.S. Plummer probably originating as scour, or lunate megaripples in contrast to the straight crested megaripples which generated the planar crossbedding. The foreset faces often display slumping, or ripple marks (Fig. 10c), are disrupted by reactivation surfaces, or merge into ripple-drift cross-lamination. The basal foreset of each arenite unit frequently intertongues with the underlying shale, a relationship that indicates the migration of discrete areas of high energy sand accumulation over discrete areas of lower energy clay/silt accumulation (and vice versa at the top of each arenite unit). Towards the top of the arenite units the individual sandy beds become thinner and ripple 14

7-"-

marks are well preserved. Klein (1970) purports this to be indicative of slackening currents evidential of intertidal conditions of deposition. In such an environment the arenite units of any cycle represent high energy megaripple shoals (perhaps within shallow tidal channels in the broad expanses between the distributary channels) separated by slightly elevated, and hence lower energy areas of mud accumulation. This cyclical intertidal deltaic plain, with its discrete distributary channels, developed across the entire western region of the basin, but halted at the gentle palaeoslope leading into the deeper submerged eastern region. At the eastern limit of the deltaic plain, facing the shallow submerged basin, north-south trending elongate sand bodies

169

// \\

, i \ i > i / i / / /

47NUMBER OF READINGS c CROSSBEDDING r ASYMMETRIC RIPPLES p PARTING LINEATION

(a)

(b)

Fig. 6 Palaeocurrent data at the time of: (a) delta inception; (b) wave-dominated delta; and (c) time-dominated delta. Compare with Fig. 7.

170

Late Precambrian delta evolution

Fig. 7 Palaeogeographic reconstructions at the time of: (a) subaqueous delta inception; (b) wave-dominated delta; and (c) tide- dominated delta.

P.S. Plummer

developed composed predominantly of pink planar crossbedded, medium to coarse quartz arenite. Included within these sand bodies are rare polygonal desiccation cracks, surfaces displaying raindrop impressions, synaeresis cracks, both symmetric and asymmetric ripple marks, fluterill marks and wedges of white, medium to coarse quartz arenite which occasionally display planar crossbeds up to 3m thick. These north-south trending sand bodies are the reworked and winnowed coarser fraction of the deltaic plain deposits acted upon by wave activity, generated over the submerged eastern region, that formed a barrier-bar system at the seaward limit of the deltaic topset. To the northeast, within the central region, a sequence of predominantly laminated to thinly bedded intercalations of purple shales, siltstones and pink fine feldspathic quartzose arenites continued to accumulate. Interrupting this succession

171

are medium to thick beds of purple siltstone that frequently display an abundance of soft-sediment deformation, including convolute bedding, load and flame structures, slumps and ball-and-pillow structures (Fig. lOd). Ripple marks are also abundant, including symmetric wave ripples, polygonal interference ripples, microripples (wavelength less than 5 mm), symmetric current ripples and flat-topped ripples (Fig. 11a). Lenticular bedding and solitary crossbedded sandstones, sometimes with scoured bases, are also encountered. Numerous subcylindrical to conical pot-and-gutter casts (see Jenkins et al 1981) also occur (Fig. lib), being indicative of rapidly changing currents typical of intertidal environments. Palaeocurrent data for this period of delta growth and development (Fig. 6b) are suggestive of a strong easterly directed current in the western region being influenced by a secondary,

Fig. 8 Sedimentary structures of the Moorillah Formation: (a) soft- sediment deformed siltstone; (b) red and white scour crossbedded sandstone; (c) soft-sediment deformed tuffaceous siltstone; and (d) tuffaceous intraformational conglomerate.

172

Late Preeambrian delta evolution

dominantly northerly directed current. Meanwhile, within the central region a bimodal, essentially southeast-northwest current system is indicated, along with a secondary northerly directed flow. These current systems are interpreted to represent the strong easterly fluvial outwash current across the deltaic plain and a northerly longshore current which transported much of the final detritus into the central region where a southeast-northwest tidal system dominated on an intertidal shelf (Fig. 7b).

DELTA EVOLUTION With continued coarse detritus influx from the west and high sedimentation rates exceeding basin subsidence, basinwide shallowing occurred. Consequently, energy conditions of deposition altered, with a corresponding change in the depositional environments. Within the central region the purple intertidal shelf deposits were replaced by a succession of green shales, siltstones and white feldspathic quartzose arenites. Within this sequence, sand surfaces displaying raindrop impressions, flattopped ripple marks, sometimes with a secondary parallel set of ripples developed within the troughs, attest to the continuation of intertidal conditions of deposition. The green (reduced) colouration, therefore, is interpreted as resulting from the reduction activity of abundant microscopic organisms, as is common on present-day anaerobic intertidal mudflats which lack macroscopic burrowing organisms and thereby ensure the preservation of abundant sedimentary structures.

somewhat scant, shows a predominantly southeasterly directed current with a minor northwesterly counter-current flowing across the intertidal shelf. This intertidal environment eventually developed throughout both the central and eastem regions as basin infilling continued, thereby skirting the entire delta of the western region (Fig. 7c). Continued progradation of the delta, in company with the shallowing of the eastern region, caused a change in energy conditions at the edge of the topset plain. Tidal activity became dominant over wave activity, resulting in the dispersal of the fringing barrier-bar system and abandonment of the discrete distributary channels in favour of a braided distributary system. A

-DESICCATION CRACKS

RIPPLED FORESET FACES

-HERRINGBONE CROSSBEDDING

Other sedimentary structures include interference ripples (both wave-wave and wave-current interference) ; shallow erosional channels filled with ripple cross-laminated (Fig. 11c), or crossbedded sandstones, often displaying a herringbone arrangement; and soft-sediment deformation structures, especially load casts and ball-and-pillow structures (Fig. lid). Palaeocurrent data from this succession (Fig. 6c), although

c o

-SOFT-SEDIMENT ~ DEFORMATION c/5

__WAVY TO PARALLEL LAMINATED

25"

-REACTIVATION SURFACES -SHALE PEBBLES

Fig. 9 Typical sandstone/shale and siltstone cycle of the ABC Range Quartzite deltaic topset.

173

P.S. Plummer

strong bimodality in palaeocurrent data from this deltaic plain, ranging from essentially northwestsoutheast to southwest- northeast (Fig. 6c), is indicative of a fluvially influenced, tidally dominated topset environment. This configuration continued until an uplifting of the western margin of the basin occurred (possibly a migration of the continuing tectonic activity responsible for the coarse detrital influx), causing partial erosion of the deltaic succession and a total rearrangement of both the shoreline and basin configuration.

Brachina Subgroup delta system (Fig. 12 & Tablel). The initial barred delta, with discrete and relatively stable distributary channels, was dominated by wave activity at the distal edge of the deltaic plain, producing a fringing barrier-bar system and a northerly littoral drift which caused delta growth parallel to the depositional strike. Moderate fluvial and tidal activity dominated the deltaic plain itself. On such deltas the distributary channels are relatively few (three in this case), generally originating from a nearly common point at the head of the delta, but with only one channel dominant at any one time (Coleman & Wright 1975). Such systems are termed "fluvial and

TREND OF DELTA EVOLUTION Using the criteria of Coleman & Wright (1975) and Galloway (1975), the variations in the delta-forming processes and resultant evolution in delta morphology can be traced for this

tide

modified,

wave-dominated

deltas"

(Fig. 12). The Burdekin and Nile deltas are modem day equivalents. As the basin shallowed with continued sediment input, so the conditions of delta formation changed (Tablel). With the emergence of the

Fig. 10 Sedimentary structures of the ABC Range Quartzite (a to c) and Moorillah Formation (d) : (a) large polygonal desiccation cracks with moulds of possible hail imprints; (b) casts of synaeresis cracks between starved ripple crests; (c) ripple marks on crossbed foreset face; and (d) ball-and-pillow structure.

Late Precambrian delta evolution

174

Characteristic Offshore slope Fluvial activity Tide activity Wave activity Littoral drift Delta growth Framework facies Distributary channel type 9. Distributary channel shifting pattern 10. Type of delta

1. 2. 3. 4. 5. 6. 7. 8.

Modern day equivalents

Delta I Moderate Moderate Moderate High High Parallel to depositional strike Barrier-bars Single active

Delta II Low Moderate High Low Low Parallel to depositional slope Tidal ridges Braided

Extension

Switching

Fluvial and tide modified, wave-dominated Burdekin and Nile Deltas

Fluvial modified, tide-dominated Mahakam Delta

Table 1: Delta characteristics (after Coleman & Wright 1975; Galloway 1975) applied to the Brachina Subgroup deltas

Fig. 11 Sedimentary structures of the Moorillah Formation (a and b) and Bayley Range Formation (c and d ) : (a) flat-topped ripple marks; (b) pot-and-gutter casts; (c) ripple-drift cross-laminated sandstone with scoured base; (d) soft-sediment deformation.

P.S. Plummer

adjacent submerged mudflat into the intertidal zone, the previously dominant wave activity and littoral drift was replaced by tidal activity. This change caused the dispersal of the protective barrier-bar system, thus enabling tidal currents to sweep across the deltaic topset resulting in a major alteration in the distributaiy system, with braided channels replacing the previously discrete channels. Channel switching within such braided systems is common, generally originating far upstream in the intertidal deltaic plain and creating a new river course each time (Coleman & Wright 1975). The lack of any littoral drift

175

resulted in delta growth parallel to the depositional slope. Thus the delta evolved from a fluvial and tide modified, wave-dominated delta to a "fluvial modified, tide-dominated delta" (Fig. 12), not unlike the present-day Makakam delta of Kalimantan.

ACKNOWLEDGEMENTS Dr Victor A. Gostin of the Department of Geology and Geophysics, Adelaide University, is thanked for his supervision of the Ph.D. on which

Mississippi (Plaquemines)

b: Burdekin c: Copper cl: Colorado d: Danube e: Ebro gb: GangesBrahmaputra m: Mekong

mh: Mahakam mp: MississippiPlaquemines n: Nile ng: Niger o: Orinoco od: Ord r: Rhone sf: Sao Francisco

* f#^ESTUAR[NE

KEY TO DELTA TYPES ( F T ) W : F l u v i a l and T i d e M o d i f i e d , W a v e Dominated. (F)T • ' Fluvial M o d i f i e d , Tide - Dominated. A S

E v o l u t i o n a r y T r e n d of the S u b g r o u p Delta.

Brachina

Fig. 12 Ternary delta classification (after Galloway 1975) showing the evolutionary trend of the Brachina Subgroup delta.

176

Late Precambrian delta evolution

this paper is based and for his critical review of the manuscript. Thanks are also due to Ms Sharon Proferes and Ms Andrea Griffin, also of the Department of Geology and Geophysics, Adelaide University, for their respective drafting and photographic reproduction of the figures.

REFERENCES COLEMAN J.M. & WRIGHT L.D. 1975. Modern river deltas: variability of processes and sand bodies. In Broussard M.L. ed. Deltas: models for exploration, pp.99-149. Houston Geological Society.

GALLOWAY W.E. 1975. Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems. In Broussard M.L. ed. Deltas: models for exploration, pp.87-98. Houston Geological Society.

GATEHOUSE C.G. 1980. New stratigraphic names in South Australia. Quarterly Geological Notes, Geological Survey of South Australia 74, 2-4.

GLAESSNER M.F. & WADE M. 1966. The Late Precambrian fossils from Ediacara, South Australia. Palaeontology 9, 599-622.

HENDRY H.E. & STAUFFER M.R. 1977. Folded crossbedding. In Bolton T.E. ed. Current Research in the Geological Sciences in Canada, May 1978-April 1977, p.98. Canadian Geoscience Council, Paper 77-5.

JENKINS R.J.F., PLUMMER P.S. & MORIARTY K.C. 1981. Late Precambrian pseudofossils from the Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 105,29-37. KLEIN G. de V. 1970. Tidal origin of a Precambrian quartzite - the Lower Fine-grained Quartzite (Middle Dalradian) of Isaly, Scotland. Journal of Sedimentary Petrology 40, 1095-1127. PLUMMER P.S. 1978a. Stratigraphy of the lower Wilpena Group (late Precambrian), Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 102, 25-38. PLUMMER P.S. 1978b. Evidence of volcanism contemporaneous with late Adelaidean sedimentation. Quarterly Geological Notes, Geological Survey of South Australia 88, 15-19. PLUMMER P.S. 1978c. Notes on the palaeoenvironmental significance of the Nuccaleena Formation (Upper Precambrian), central Flinders Ranges, South Australia. Journal of the Geological Society of Australia 25, 395-402. PLUMMER P.S. 1980. Circular structures in a Late Precambrian sandstone: fossil medusoids or evidence of fluidization? Transactions of the Royal Society of South Australia 104, 13-16. PLUMMER P.S. & LEPPARD P.I. 1979. An analytical technique for uni-, bi-, and tri-modal palaeocurrent data. Computers and Geosciences 5,157-172. PREISS W.V. & FORBES B.G. 1981. Stratigraphy, correlation and sedimentary history of Adelaidean (late Precambrian) basins in Australia. Precambrian Research 15, 255-304. SPRIGG R.C. 1947. Early Cambrian (?) jellyfishes from the Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 71,212-224.

A late Proterozoic storm-dominated carbonate shelf sequence: The Wonoka Formation in the central and southern Flinders Ranges, South Australia Peter W. Haines* Department of Geology and Geophysics, The University ofAdelaide, G.P.O. Box 498, Adelaide, SA. 5001, Australia

The late Proterozoic Wonoka Formation is a mixed carbonate/siliclastic regressive sequence of marine origin. In the central and southern Flinders Ranges the Wonoka Formation averages about 700 m in thickness and has been subdivided into eleven mappable units that can be traced over wide areas despite some facies changes. The vertical sequence, overlying deep water shales of the Bunyeroo Formation, provides evidence for the gradual development and progradation of a storm-dominated carbonate ramp, deposited under open shelf conditions. The ramp deepened to the north and east, and relatively deep water mud deposition continued in the northern Flinders Ranges at this time. The ramp sequence is capped by two lagoonal carbonate units separated by tidal to supratidal siliciclastics genetically related to the overlying Bonney Sandstone. The lagoons were separated from somewhat deeper open marine conditions to the north by a series of islands related to diapiric intrusion. This appears to be the first well documented example of a late Proterozoic storm-dominated carbonate shelf sequence.

Key words: Adelaide Geosyncline, late Proterozoic, Wonoka Formation, storm sedimentation, hummocky cross-stratification, carbonates, shelf.

INTRODUCTION It has been in relatively recent years that sedimentologists have realised that major storms are a very important sedimentary process, particularly on continental shelves, and have learnt to recognise their signature in the ancient record. One of the most important steps was the recognition of hummocky cross-stratification (HCS) (Harms et al 1975) as a wave formed structure. There has followed a flood of papers describing ancient storm-dominated sequences from many different areas, and from all geological ages. The great majority of these are largely or entirely dominated by siliciclastic sediments. Most are regressive sequences where storm-dominated sedimentation is restricted to a relatively thin

interval (tens of metres) between sediments deposited below storm wave base, and those deposited in the littoral zone. The purpose of this paper is to describe an unusually thick mixed carbonate/siliciclastic storm-dominated shelf sequence within the context of a regressive cycle within the late Proterozoic Wonoka Formation of the central and southern Flinders Ranges of South Australia. The concept of storm sedimentation and the significance of HCS, although still incompletely understood, are now firmly entrenched in geological thought and it is unnecessary to give a detailed review here. The reader is referred to reviews in Dott & Bourgeois (1982), Walker et al (1983), Walker (1984), Aigner (1985), Brenchley

^Present address: Northern Territory Geological Survey, P.O. Box 2901, Darwin, N.T. 0801, Australia

178

Peter W. Haines

(1985) and Duke (1985). Haines (1988) discusses HCS with particular reference to the Wonoka Formation. The term 'tempestite', coined by Ager (1974), is synonymous with the term 'storm bed' of other workers, and is used in this paper as a general term to distinguish all beds considered to be the product of storm processes. The terms proximal and distal tempestite are used in the sense of Einsele & Seilacher (1982), and Aigner (1982, 1985). Storm effects decrease with increasing depth, thus producing a lateral variation in frequency of tempestite deposition, bed thickness and internal sedimentary structures. Proximal tempestites, deposited in relatively shallow water, comprise thick bedded amalgamated sequences generally displaying HCS. Distal tempestites are thinner bedded and finer grained, often display planar and wave ripple laminations rather than HCS, and are generally interbedded with mudstone. The depths at which proximal and distal tempestites will be deposited are not fixed, but will vary considerably depending on the local wave climate. SW

REGIONAL GEOLOGICAL SETTING The Wonoka Formation forms part of the upper of two major coarsening-upward regressive marine cycles that compose the late Proterozoic Wilpena Group (Fig. 1) of the Adelaide Geosyncline (Fig. 2) in South Australia. The lower cycle, with a maximum thickness of about 2200 m, comprises a thin basal dolostone (Nuccaleena Formation) overlain by a thick shallowing-upward storm-dominated siliciclastic sequence (Brachina Formation and Ulupa Siltstone) and is capped by shallow marine sands of the ABC Range Quartzite. Sediments entered the basin largely from the west as part of an extensive delta complex (Plummer 1978a).

Fig. 1 Stratigraphy of the Wilpena Group in the Flinders Ranges.

Fig. 2 Locality map showing the position of the study area (enlarged in Figs 5,7, 8) in the northern Adelaide Geosyncline. The structural zones of Preiss et al (in Rutland etal 1981) referred to in the text are also shown (SFZ= South Flinders Zone, CFZ= Central Flinders Zone, NFZ= North Flinders Zone).

Proterozoic storm-dominated shelf carbonates The ABC Range Quartzite is abruptly but apparently conformably overlain by monotonous maroon (and lesser green) shales of the Bunyeroo Formation, which averages about 400 m in thickness in the central Flinders Ranges. Details of the depositional setting are poorly known, but apparently involved relatively deep water in an environment generally starved of coarse sediment, probably at the peak of transgression. The succeeding Wonoka Formation (as redefined by Gostin & Jenkins 1983) indicates a prolonged regressive event, and in the central and southern Flinders Ranges can be summarised as the development of a storm-dominated carbonate

ramp that shallowed with time, culminating in the deposition of lagoonal carbonates, that intertongue with tidal to supratidal siliciclastics of the overlying Bonney Sandstone. The ramp sequence grades laterally via a tectonically-controlled shelf edge, into a somewhat deeper shale-dominated marginal basin sequence in the northern Flinders Ranges (Haines 1986). In this area a number of major channel fill sequences have been described (Coats 1964; von der Borch et al 1982, 1984, 1985). These were cut and filled during early Wonoka times, and in some cases have been incised more than 1 km into underlying sediments. Although somewhat controversial (Eickhoff et al 1986), the majority of observations made by the author support the submarine canyon hypothesis of earlier workers. The Bonney Sandstone is composed of dominantly red coloured siliciclastics of a prograding tidal mud flat and delta sand ridge complex, which pass up into alluvial plain sediments (Gehling 1982). It is disconformably overlain by more mature marine sands and minor siltstones of the Rawnsley Quartzite, together comprising the Pound Subgroup of Jenkins (1975) which completes Precambrian sedimentation in the Adelaide Geosyncline. In the far northeastern Flinders Ranges the Wonoka Formation is overlain by a thick, poorly studied sequence of mudstones, sandstones and carbonates known as the Billy Springs beds. The

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depositional environment and stratigraphic relationship to the southern sequence is uncertain, but it probably represents deeper water lateral equivalents of the upper Wonoka Formation and Pound Subgroup. Preiss et al (in Rutland et al 1981) have divided the Adelaide Geosyncline into a number of convenient structural zones. The South, Central and North Flinders zones (Fig. 2) clearly had somewhat different histories during at least part of Wonoka time, and thus form an ideal spatial subdivision when discussing the Wonoka Formation. Hence these zones are referred to throughout this paper.

LITHOSTRATIGRAPHY AND FACIES IN THE CENTRAL AND SOUTH FLINDERS ZONES The best exposure of the Wonoka Formation that is both fault-free and easily accessible is the 690 m thick Bunyeroo Gorge section in the Heysen Range (Fig. 3). In most aspects it is typical of sections in the Central Flinders Zone with respect to both thickness and facies development, and thus forms an ideal reference section for the subdivision of units, and as a standard by which to compare lateral facies and thickness changes. The type section at Brachina Gorge, 9.5 km to the north, is very similar, but somewhat less well exposed and complicated by minor faulting. Bunyeroo Gorge is also the type section of the Ediacaran of Jenkins (1981), and the Ediacarian of Cloud & Glaessner (1982) (see Fig. 1). It has been convenient to subdivide the Wonoka Formation into eleven mappable units at Bunyeroo Gorge. Unit boundaries were defined at points of major lithological change. However, these positions do not necessarily correspond to obvious changes in the hydrodynamic regime of the depositional environment. Once defined at the reference section, it was found that the majority of units could be easily traced throughout much of the Central and South Flinders Zones. However, only the lower three

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units are recognisable in the North Flinders Zone. When examined regionally, individual units may show a lateral variation from proximal to distal depositional environments, while retaining the overall lithological characteristics of the unit. Subdivisions based purely on hydrodynamic

processes (e.g. depth-related proximality trends) are generally observed to cut diagonally across lithological unit boundaries. These changes, reflecting physical aspects of the water body, are often highly gradational and thus more difficult to map. Inferred

depth Tidal

£ CO

Bonney Sandstone

£ u.

Depositional environment

r^D 1

« ^

Lagoonal Limestone

Tidal/supratidal Lagoonal

tVTVI

sandy-

WAW w t t

silty- -

Inner s h e l f

Dolostone Mudstone Intraclasts

Middle

Ooids

shelf

Planar beds T u r b i d itbs HCS S m a l l - s c a l e H C S only • Wave ripples Trough cross

bedding-

Wavy

bedding

Soft

sediment

defm -

Desiccation

cracks • •

Stylonodular

texture-

Stromatolites Thickening Thinning

O u t e r shelf

Bunyeroo Formation

mud sand grav.

Fig. 3 Measured reference section through the Wonoka Formation at Bunyeroo Gorge with inferred relative depth curve and environmental interpretations (SWB= storm wave base, FWB= fairweather wave base, MSL= mean sea level). Modified after Haines (1988).

Proterozoic storm-dominated shelf carbonates

Basal contact with Bunyeroo Formation The stratigraphic position of the base of the Wonoka Formation has been somewhat controversial and clarification of this point is essential before discussion of the Wonoka Formation can proceed. Dalgarno & Johnson (1964, p. 19) formally defined the Wonoka Formation and nominated Brachina Gorge, where it is 4 of the order of 1 500 feet (457 m) in thickness', as the type section. The formal definition equates the Wonoka Formation with units 61-65 of the measured section of Mawson (1939) (approximately units 7-11 of this paper). However, this interval comprises only 780 feet (238 m) by Mawson's measurements. It appears from Dalgarno and Johnson's description, thickness and the co-ordinates given for the base of the type section, that Mawson's units 59-65 (1 563 feet by Mawson's measurements) were intended (approximately units 4-11 of this paper). At the type section this basal position approximately corresponds to the beginning of a very gradational colour change from dominantly reddish to dominantly greenish sediments, with interbedding of both colours over a considerable vertical interval. The need for a distinct and regional significant basal datum for the Ediacaran (Jenkins 1981) led Gostin & Jenkins (1983) to redefine the base of the Wonoka Formation, lowering it to the base of a thin but essentially basin-wide dolostone that is probably the equivalent of the 'Wearing Dolomite member' formerly of the upper Bunyeroo Formation. This change has not been universally supported (e.g. Forbes & Preiss 1987), however it is retained here for the following reasons: (a) the position is unique and essentially basin-wide; (b) it is usually well exposed and easily picked in the field and on aerial photographs; (c) it has considerable sedimentological and possible tectonic significance as it demarcates a sudden regional change in the depositional system, separating uniform shales below from sandy turbidites and tempestites immediately above, and generally carbonate poor sediments below from a trend of increasing car-

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bonate content above. The original boundary position is generally poorly exposed, often hard to locate precisely, and has not been picked consistently on published maps. It may also be of somewhat lesser sedimentological significance. Unit 1 Unit 1 (2.5 m thick at Bunyeroo Gorge) is a thin though remarkably persistent dolomitic horizon. It is always present within the study area, and also widespread to the north and south, except where locally removed by canyon erosion or slumping. It varies from a maximum thickness of about 10 m in the southwest of the study area to as thin as 5 cm at many northern localities. At sections in the northern South Flinders Zone unit 1 consists of up to 10 m of finely laminated, rhythmically bedded dolomicrite (individual beds 1-2 cm thick) interbedded with green (locally maroon) dolomitic mudstone (Fig. 9a). Most beds display a gradational base, but have a sharp planar or slightly eroded top. At most localities in the southern half of the Central Flinders Zone, unit 1 averages about 2 m in thickness, and consists of rhythmical dolomicrites interbedded with mudstones in a thickening upward package. The dolomicrites are generally reworked into one or more lenticular intraclast horizons at the top in this area. At the Brachina Gorge type section and other nearby localities to the north, the entire unit consists of a single, or locally several, sharp based beds of intraformational conglomerate varying from 10 to 40 cm in thickness. In the northern Central Flinders Zone, unit 1 is generally restricted to a single thin (5-10 cm) sharp based bed of allochthonous (sometimes intraclastic) dolomicrite. In most areas the intraclastic beds have a matrix containing angular polymict sand grains and fine clastic dolomite, cemented by spary calcite distinguishable into early ferroan seafloor cement, and later void-filling iron-poor calcite. Unit 1 is notably cupriferous as indicated by the frequent occurrence of malachite staining in outcrop. Pyrite and chalcopyrite are sometimes observed in fresh samples.

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The sandstones generally occur in thickening upward packages, superimposed on a general thickening upward sequence accompanied by an upward increase in the sandstone/mudstone ratio. There is also a gradation from thin bedded TBCE, TB-E (divisions of Bouma 1962) (Fig. 4b) beds with relatively small sole marks and some smallscale HCS in the lower half, to dominantly thicker bedded massive and planar sandstones, often with dewatering structures and large sole marks, near the top. In upper parts of the unit single beds may reach 1 m in thickness with flute moulds up to 60 cm long. The top few metres of unit 2 display a rapid thinning of beds and transition into the overlying mudstones of unit 3.

Fig. 4 a. Unit 1 near Mayo Gorge, South Flinders Zone. Rhythmically interbedded planar laminated dolomicrite (light bands) and dolomitic shale. Note the gradational base and sharp tops to some dolomicrite bands, b. A 12 cm graded TB-E turbidite typical of lower unit 2. Smallflutemoulds are present on the base. Brachina Gorge section. Unit 2 Unit 2 (74 m) thick at Bunyeroo Gorge) is characterised by the alternation of brown, sharp based, fine grained calcareous and dolomitic sandstone and coarse siltstone, with maroon mudstone. The sandstones frequently display erosional sole marks (most commonly flute moulds), may be slightly graded, and internally often display Bouma sequences (Bouma 1962) suggestive of turbidites. However small-scale HCS is also present at many localities. In the far western South Flinders Zone abundant HCS of typical scale associated with wave ripples occur in the upper part of unit 2.

Petrographically the fine calcareous and dolomitic sandstones of unit 2 are arkosic (quartz and feldspar often occur in equal proportions), with mica content (including biotite or its alteration products) varying up to 20%. This siliciclastic assemblage is characteristic of all sandstones between units 2 and 6, and sandstones within the submarine canyons. Unit 2 varies in thickness from a maximum of about 160 m in the northwestern South Hinders Zone, to less than 10 m in the far east and north of the study area. Palaeocurrents from directional sole marks and current ripples are strictly unidirectional at most localities (directed from the west and southwest) and are generally approximately orthogonal to the isopach trend (Fig. 5). Regionally they display a fanning pattern best developed across the Central Flinders Zone. At individual localities the variation in readings is not random, but can be resolved as a small but consistently northerly shift in current direction upward through the unit. A large shift is noted at some localities near the Central/North Flinders Zone boundary. Unit 3 Unit 3 (92 m at Bunyeroo Gorge) is one of the most widely distributed, yet conservative of all units, displaying very similar facies over a wide area. It is characterised by finely laminated red-

Proterozoic storm-dominated shelf carbonates

dish calcareous mudstone, containing varying proportions of thin bedded pink micritic limestones (Fig. 6a). The pink limestones (generally 2 cm or less in thickness) usually occur in pairs, or packets of three or four. Individual beds com-

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monly display gradational bases and sharp tops (sometimes erosional), much like the thin dolomicrites of unit 1. Internally they are massive, or display fine planar occasional cross laminations which are also seen in the associated

Fig. 5 Palaeocurrent and isopach map of unit 2 for the study area outlined in Fig. 2. All data are from current aligned sole marks such as flute moulds. This is also a locality map for specific sites mentioned in the text.

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mudstones. Bedding plane exposures of upper surfaces often display a variety of features including elongate erosional scours and current crescents (Fig. 6b), but most abundant are elongate ridges whose bulbous terminations show clear evidence of current scour and all evidence suggests that these features are current aligned. Sharp based calcareous siltstones having the characteristics of distal turbidites appear near the top of the unit in many sections. Along the Central/North Hinders Zone boundary there is local evidence of a considerable palaeoslope which deepened to the north, particularly in the lower part of the unit. The evidence includes thick debris flows containing pink limestone intraclasts and local slump

removal of the underlying units 1 and 2, and parts of the Bunyeroo Formation. Palaeocurrent readings from the upper surfaces of the pink limestones are unimodal and usually highly consistent at individual localities (Fig. 7). At most western localities the average palaeocurrent vector is approximately orthogonal to that obtained from sole maiks in unit 2 below and unit 4 above (i.e. orthogonal to the probable local palaeoslope), and directed towards the south. In the east the limited data indicates currents directed towards the north and northeast. The palaeocurrent vector generally approximately parallels the local isopach contours. The thickness increases steadily to the east in complement to the steady decrease in thickness of unit 2. Unit 4

Fig. 6 a. Typical unit 3 displaying packets of thin bedded pink limestones interbedded with maroon calcareous shale. Old Wirrealpa area. b. Current crescents on the upper surface of a bed, unit 3, Old Wirrealpa.

The base of unit 4 (150 m at Bunyeroo Gorge) is taken at the first major occurrence of greenish limestone in the section. This may be a sudden change with a prominent basal marker at some northern and eastern localities, or quite subtle and gradational at many western and southern localities. The basal beds of the unit are notably cupriferous at most sections, as indicated by the common occurrence of malachite nodules and staining. Pyrite and minor chalcopyrite, chalcocite and galena may be present in fresh samples from the interval. The unit is characterised by the cyclic alternation of packets of thin bedded green silty to fine sandy limestones interbedded with green mudstone, with packets of brownish calcareous siltstones, pink limestones and maroon or brown mudstone resembling upper unit 3. The green limestones and brown calcareous siltstones are sharp based and often display unidirectional erosional sole marks and internal Bouma sequences suggesting distal turbidites ( T B C ( D ) E and T C ( D ) E ) , although some are massive or planar laminated throughout. There is a general increase in the proportion of greenish sediments towards the top of the unit, where some beds display wave ripples and multidirectional tool marks on soles. Thicker beds may display small-scale HCS. At a few localities large HCS occurs near the base.

Proterozoic storm-dominated shelf carbonates

Bedding generally thins to the east and north, and very thin bedded limestone/shale rhythmites interspersed with intervals of massive or laminated green mudstones occur at this level in the far northern Central Flinders Zone. A mudstone

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dominated sequence occurs in the North Binders Zone. Evidence of locally increased palaeoslope to the north along the Central/North Flinders Zone boundary continues into unit 4. The evidence includes debris flows and major in-

I N

Fig. 7 Palaeocurrent and isopach map of unit 3 for the same area as Fig. 5. All readings are from current aligned ridges, crescents etc. on top surfaces. A legend can be found in Fig. 5.

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Fig. 8 Palaeocurrent map of unit 4 for the same area as Figs. 5 and 7. All readings are from current aligned sole marks, generally flute moulds. A legend can be found in Fig. 5.

Proterozoic storm-dominated shelf carbonates

traformational truncation surfaces interpreted as being of slope induced slump origin. Palaeocurrent readings from sole marks (mainly flute moulds) give similar directions to those obtained from unit 2, but differ in generally showing a slight shift to the north in the Central Flinders Zone and a slight shift to the south in the South Flinders Zone (Fig. 8). The variation in readings at individual localities is greater than that of unit 2, however like unit 2 the variation is not random, but represents a systematic shift to the north upwards through the unit.

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with brown and grey mudstone. Minor green and red limestone are also present. At the Bunyeroo Gorge section and further south the sandstones are medium to thickly bedded and commonly display HCS (Fig. 9a), while upper surfaces often show interference wave ripples. To the east and north unit 6 becomes notably more distal in character, with a thinning of the sandstone beds and increase in the proportion of interbedded mudstone. Occasional thicker beds display HCS, but most have the characteristics of distal tempestites. Unit 7

UnitS Unit 5 is laterally highly variable. At Bunyeroo Gorge (105 m) it is dominated by medium to thickly bedded green silty limestones, with very little interbedded mudstone. Planar bedding, HCS and vertically climbing symmetrical ripple laminations are abundant, and large scale ball and pillow structures and other forms of soft sediment deformation are common. Intraformational conglomerates are also present. Minor stylonodular bedding and red limestones (see unit 7) are also present near the top. To the north and east of Bunyeroo Gorge lateral equivalents of unit 5 displays a successively more distal character, grading from thin bedded greygreen limestones displaying wave ripples and small scale HCS, to limestone/mudstone rhythmites indistinguishable from the northern equivalents of unit 4. In the northwestern South Flinders Zone typical unit 5 is rapidly replaced by a highly cyclic sequence characterised by the alternation of fine grained calcareous HCS sandstones with green limestones of similar facies to unit 5 at Bunyeroo Gorge. This sequence is described and discussed in detail by Haines (1988). Unit 6 Unit 6 (53 m at Bunyeroo Gorge) is characterised by brown fine grained calcareous sandstones (and coarse siltstones) interbedded

Unit 7 is very consistent in thickness (averaging 100 m) and facies across most of the Central and South Flinders Zones. It is characterised by grey-green argillaceous and silty micritic limestone, with lesser red glauconitic limestone. There is very little interbedded mudstone and the limestones generally contain less intermixed siliciclastic material than those of lower units. Planar laminations, small-scale and typical HCS, climbing wave ripple laminations (polygonal interference ripples in plan view (Fig. 9b), and various forms of soft sediment deformation are the most common sedimentary structures. Stylonodular bedding is abundant (effecting up to 80% of the green limestones in some sections) and often destroys, or partially destroys the primary sedimentary fabric. Stylonodular bedding (Flugel 1982) (Fig. 9b) is a secondary structure in which the rock is divided into small ovoid nodules of millimetre to centimetre size by a series of horizontally anastomosing stylolitic sheets. See Haines (1988) for a discussion of the origin of this feature. Infraformational conglomerates, either as continuous beds or lenses, or filling pot and gutter structures (Fig. 9c), are common and increase in abundance towards the top of the unit. The first clear evidence of very shallow deposition (within the photic zone) is found in the upper few metres of unit 7 in the form of a widespread stromatolite bearing horizon. Red limestones also occur in units 5 and 6, but are generally most prominent in unit 7, where

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their occurrence is notably cyclic. These cycles are described in detail and illustrated in Haines (1988). Individual red limestones are commonly

less than one metre thick (maximum 3 m) and generally display prominent HCS, and occasionally swaly cross-stratification. Petrographically they consist of fine sand-sized calcite, glauconite, francolite (microcrystalline carbonate fluorapatite) and detrital silicates cemented by haematite. The glauconite and francolite are intimately associated and occur as irregular to highly wispy grains.

Unit 8 Unit 8 (54 m at Bunyeroo Gorge) consists of a thinning upward sequence of medium to thin bedded grey limestones, interbedded and capped by green calcareous mudstones. The limestones are sharp based and commonly display planar laminations and interference wave ripples. Intraformational conglomerates are abundant and occur as pavements of intraclasts, as basal lags to limestone beds, or fill wave-formed pot and gutter structures. HCS occurs in some of the thicker beds near the base. Green calcareous mudstone becomes dominant in the upper third of the unit, which in the Central Flinders Zone contains lenses of grey and olive green silty limestones with algal laminations near the top. Unit 8 is notably glauconitic throughout, but glauconite is most abundant within graded layers in a widespread marker horizon (averaging 4 m in thickness) characterised by the presence of distinctive purple mudstone. Like unit 7, the glauconite is intimately associated with francolite. Petrographic examination indicates the presence of possible volcanic quartz and volcanic rock fragments in some samples from this level. At a number of localities, including Bunyeroo and Brachina Gorges, a thin zone (averaging 1 m) Fig. 9 a. HCS in fine grained calcareous sandstone, unit near the top of unit 8 contains a fossil preserved 6, Bunyeroo Gorge. This bed is interbedded with as dark films on bedding surfaces within silty mudstone and displays the complete HCS sequence limestones (Haines 1987; Jenkins etal 1988). Its including wave ripples on the upper surface, b. distinct pattern is characterised by series of paralVertically climbing wave ripple laminae overlain by lel curved bars (Fig. 10a, b). Successive bars may stylonodular bedding in argillaceous limestone, unit 7, in length leading to a widening swath Brachina Gorge, c. Top surface of a bed displaying increase bifurcate gutters and pot holes infilled with small that is capable of branching. The fossil bears at least superficial resemblance to the Russian form intraclasts, unit 7, Brachina Gorge.

Proterozoic storm-dominated shelf carbonates

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Palaeopascichnus (Fedonkin 1977), which is generally interpreted as a trace fossil. The pattern also bears close resemblance to certain modern Phaeophyta (brown algae) such as Padina (e.g. Lobban & Wynne 1981, fig.2.5). The relationship to Palaeopascichnus remains equivocal, but a trace fossil explanation is here rejected in favour of an algal origin. Unit 9 Unit 9 is the first of two shallow water carbonate units, separated by siliciclastics, that mark the top of the Wonoka Formation. It is only 1 m thick at Bunyeroo Gorge, near the southern limit of its distribution, but thickens to at least 40 m in the far north. Unit 9 is dominantly composed of thin bedded, algal laminated, grey limestones, interbedded with thicker, massive to cross bedded, black calcarenites. The dominant particles are identified as small intraclasts and a variety of peloids of uncertain origin. Scattered coarse crystals and clusters of saddle dolomite (Radke & Mathis 1980) are common (apparently replacing some previous carbonate particle), and in areas of faulting or other structural complexity the entire unit is usually dolomitised. Late stage dedolomitisation of secondary dolomite has also taken place. The carbonates are occasionally sandy or pebbly, with a major increase in the proportion of siliciclastic sand towards the boundary with the North Flinders Zone. Beyond this boundary unit 9 is not recognised due to rapid deepening-related facies changes. Unit 10

Fig. 10 a, b. Probable algal fossil collected near top of unit 8, Parachilna Gorge area. Bar scale 10 mm. c. Plaster cast of 'fan-shaped fossil' from the top of unit 10 east of Beltana.

Unit 10 comprises an entirely siliciclastic sequence lying between the carbonates of units 9 and 11. It bears a close similarity to certain facies of the overlying Bonney Sandstone, and in the far west and south of the study area cannot be differentiated from the Bonney Sandstone in the absence of unit 11. It is 30 m thick at Bunyeroo Gorge, and generally thins to the north and east, while thickening to the south (up to 73 m near the

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southern edge of the study area). In the west and south the unit is dominated by red micaceous silty sandstones with abundant mud pellets and desiccation cracks. Thinly bedded greenish sandstones and mudstones may occur near the base, and the top is generally marked by several metres of laminated olive green calcareous mudstone. To the east and northeast the central red bed unit is reduced in thickness until the unit is composed entirely of pale and greenish sandstones interbedded with green mudstones. The sandstones may be trough crossbedded, or display wave and current ripples. Jenkins (1984) reports the discovery of a possible frond-like fossil and poorly preserved medusoids from the upper surface of a sandstone bed near the top of unit 10 at Bunyeroo Gorge. In addition the author has located a large fan-shaped fossil (Haines 1987) (Fig. 10c) of unknown affinities at the top of unit 10 about 30 km east of Beltana in the northern part of the study area. Unit 11 The top of unit 11, which is 16 m thick at Bunyeroo Gorge, marks the top of the Wonoka Formation throughout most of the Central and South Hinders Zones. The unit generally thickens to the north and east and reaches 80 m along the northern edge of the Central Flinders Zone. Like unit 9 it is dominated by grey and black limestones, and is generally dolomitised in any faulted or structurally complex areas. However the cyclic nature of these carbonates is much more evident in unit 11. A typical coarsening upward cycle has a lower interval consisting of thinly bedded, lenticular and wavy grey to black limestones interbedded with thin planar bedded silty micritic grey limestones. The black limestones are commonly fine grained peloidal grainstones. Some of the peloids have the characteristics expected of faecal pellets. There is usually a rapid transition into the coarser calcarenite forming the upper part of each cycle. Here the carbonate particles consist of ooids, large peloids, intraclasts and unidentified particles that can be assigned to catagraphs. Scattered coarse grains of saddle dolomite, ap-

parently replacing earlier carbonate particles, are usually present in abundance. These calcarenites are frequently crossbedded and thicker examples may contain internal hardgrounds associated with several centimetres of partial dissolution and dolomitisation. They may be overlain by intraclasts and material resembling calcrete. Phosphorites are associated with some surfaces. The top of each cycle may also be marked by an erosional hardground surface. Stromatolites, where present, are usually found growing on internal hardground surfaces or at the top of cycles, most commonly in the lower part of the unit. A variety of columnar stromatolites are present, the most common form being identified as Tungusia cf. T.julia by Walter etal (1979). Oncolites are also present locally. At several localities tepee structures, generally associated with probable desiccation cracks and intraformational conglomerates, occur in fine grained calcarenites at the top of some cycles. Ooids are common to abundant in unit 11 but have not been noted lower in the Wonoka Formation in the study area. The ooids may be composed of calcite or replaced by diagenetic saddle dolomite, or rarely francolite. More rarely ooids may be composed of fine grained dolomite which preserves the primary fabric to varying degrees. The ooids are almost exclusively concentric. Calcite ooids are composed of neomorphic calcite with crystal fabrics varying from a micritic mosaic, significantly finer than the surrounding cement, to well developed brick textures. Some ooids may be replaced by single crystals of pseudospar showing relic laminae. Ooids with combined radial and concentric fabrics are very rare and have only been found associated with hardground surfaces. Contact with the Bonney Sandstone The contact between unit 11 and the overlying Bonney Sandstone is generally sharp but always conformable. Wherever unit 11 is present the basal beds of the Bonney Sandstone usually com-

Proterozoic storm-dominated shelf carbonates

prise a unit of greenish siltstone and fine sandstone (unit A of Gehling 1982). This interval contains rare medusoids and other problematical fossils (Haines 1987). Where units 9 and 11 are not present in the western and southern South Flinders Zones, red clastics of the Bonney Sandstone conformably overlie mudstones of unit 8. As indicated earlier, unit 10 cannot be distinguished in these areas.

DISCUSSION The significance of the widespread thin dolostones of unit 1 is uncertain. It clearly demarcates a sudden regional change in the style of sedimentation, separating relatively deep water uniform shales of the Bunyeroo Formation from turbidites and tempestites of the lower Wonoka Formation. It also marks the change from carbonate poor sedimentation below, to a trend of increasing carbonate content above. Thus it is possible that the base of the Wonoka Formation (as amended by Gostin & Jenkins 1983) is laterally approximately synchronous, reflecting some regional tectonic or isostatic event. Von der Borch et al (1988) consider the possibility that the base represents a downlap surface or combined deep water sequence boundary and downlap surface. The rhythmical nature of the dolomicrites and their fine laminations imply deposition generally below storm wave base. Seafloor cementation of dolomicrite intraclasts with calcite suggests that the dolomite is primary, or at least a very early diagenetic replacement (prior to disruption and intraclast formation). The gradational base, but sharp (and sometimes erosional) top to the dolomicrite beds may be explained if dolomitisation of the seafloor occurred during periods of non-deposition or minor erosion. Recent evidence indicates that primary dolomite is forming on modern continental shelves in areas with relatively low rates of sedimentation and high (0.5wt.%) organic carbon contents (Baker & Burns 1985). The organic carbon content of unit 1 is unknown, however the associated green shales and sulphides indicate the presence of

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temporary reducing conditions that would be conducive to the preservation of organic carbon. If this mechanism is operating, it implies that unit 1 may be a condensed interval, as also suggested by von der Borch et al (1988). Extreme storm activity may be responsible for local bed erosion and intraclast formation. Unit 2 displays evidence of both turbidite and storm activity. Evidence that storm sedimentation increases in southwestern outcrops is consistent with the inferred palaeoslope from palaeocurrent data. It is likely that all sand beds were initiated by storm activity, but that some were carried below the level of wave influence by storm generated turbidity currents before final deposition. Others displaying strictly unidirectional erosional sole marks, but containing small-scale HCS may have been deposited under conditions of combined turbidity and storm induced flow, while beds displaying typical HCS probably represent largely in situ reworking by storm waves. As such, most of unit 2 was probably deposited in an outer shelf setting similar to that interpreted for the Mallacoota Beds of eastern Australia, which also display fan-like as well as shallow water characteristics (Fenton & Wilson 1985). The terms outer, middle- and inner-shelf are used here purely in the sense of sedimentary processes and do not imply any specific distance from shore. Unfortunately several problems exist with the above interpretation. The strongly unidirectional nature of palaeocurrents from most localities, and the large size of many of the sole marks, would tend to suggest the existence of a substantial palaeoslope. However, if this were the case, evidence for storm reworking should be confined to western outcrops, whereas eastern areas should have been beneath storm wave base. In fact, structures interpreted as small-scale HCS are present even in far eastern outcrops. Either the strong unidirectional currents flowed down a very low palaeoslope, small-scale HCS has been incorrectly identified, or it can be generated below storm wave base. Prave (1985) has recently described HCS from turbidites considered on

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palaeontological and sedimentological grounds to have been deposited in water greater than 500 m deep, and discusses its possible origin in terms of internal waves breaking below effective storm wave base. If such a mechanism is possible, only the large scale HCS and associated wave ripples in southwestern areas may have been deposited above storm wave base. The fanning palaeocurrent pattern and curvature of isopachs in the Central Flinders Zone indicates a fan-shaped morphology for unit 2 in this area, and suggests a point source of sediments to the west. This is best explained if the exposed portion of unit 2 represents the distal edge of one or more major deltas. The rapid thinning of beds at the top of unit 2 and transition into the overlying shales of unit 3 suggests either (a) sudden removal of sediment source, (b) rapid rise in relative sealevel causing a westward shift in the point of sediment input, or (c) sediment diversion (for example channelling of sediments such that they bypass the shelf). Study of the Wonoka Formation along the Central/North Flinders Zone boundary suggests that major downfaulting of the northern area was initiated at this time. This also coincides with the time of incision of possible submarine canyons in this area. Basal canyon sands are similar to the sands of unit 2. These observations suggest that a bypass event was the most likely cause of sand cutoff. Unit 3 represents a return to dominantly pelagic mud deposition similar to that of the Bunyeroo Formation, but with a considerable increase in carbonate content. Currents were periodically active, but the limestones themselves show little evidence of being deposited by currents, and are possibly the product of carbonate cementation of the seafloor during periods of non-deposition or following minor erosional events (like the dolomicrites of unit 1). The variety of current features were probably formed in mud prior to carbonate generation. Deposition apparently occurred largely below storm wave base. Palaeocurrent measurements perpendicular to the inferred palaeoslope imply that currents were following isobaths and thus may be related

to storm-generated geostrophic flows. Geostrophic flows evolve from the influence of coriolis force on the seaward oriented relaxation flow that follows coastal set-up (storm surge) produced during major storms (Walker 1984). Units 4 to 7 consist of a lateral and vertical gradation of facies associated with a single cycle of development and progradation of a gently inclined carbonate ramp. The major facies changes consisting of a northward reduction in bed thickness, changing sedimentary structures, and decreasing carbonate/mudstone ratio, are consistent with a gradual deepening to the north and the sequence becoming more distal with respect to the carbonate source area. In most areas the shelf apparently took the form of a gently inclined ramp that was distally steepened due to tectonic activity along the Central/North Hinders Zone boundary region during early stages, but became more homoclinal with time (using the carbonate platform terminology of Read 1982, 1985). In most areas the majority of unit 4 was apparently deposited below storm wave base in an outer shelf setting, with increasing wave influence indicated towards the top in the south. Local highs were present however, indicated by the local abundance of HCS near the base. The limestone turbidites were probably initiated by storm activity within shallower areas of carbonate generation to the west of present outcrops. The greenish colouration of packets of limestone turbidites suggests reducing conditions in the source area higher on the shelf. The reddish colouration of the thicker interbedded mudstone dominated intervals indicates predominantly oxidising conditions on the outer shelf. This is a reversal of the more common situation of oxidising conditions in shallower water, with reducing conditions at depth. It may be explained by circulation of deep shelf currents beneath a stagnant middle- to inner-shelf zone. An identical situation has been described from the Cambro-Ordovician Cow Head Group of Newfoundland (James & Stevens 1986).

Proterozoic storm-dominated shelf carbonates

Unit 5, as developed at Bunyeroo Gorge, comprises a proximal calcareous tempestite facies generated by frequent storm wave reworking in an environment supplied with abundant carbonate mud. To the north and east there is a transition into more distal tempestites indicating less frequent and less intense storm influence probably related to increasing water depth. The gradual transition from proximal to distal without evidence of steep slopes implies that a ramp morphology continues at this time. The laterally restricted cyclical carbonate/siliciclastic facies in the southwest also represents a proximal tempestite environment. Haines (1988) argues that climatic cyclicity (Milankovitch cycles) involving either alternate wet and dry periods, or minor sealevel fluctuations along a low gradient shoreline, are the most likely causes of the cyclicity. Unit 6 represents a similar environment of deposition to that of unit 5 (and overlying unit 7) but the nature of the available sediment has changed from carbonate mud-dominated, to fine siliciclastic sand-dominated. As interbedded mudstone is also present, this is probably due to a regional reduction in the rate of carbonate precipitation. Like unit 5 there is a gradual transition from proximal to distal tempestites to the north and east of the Bunyeroo Gorge region. Unit 7 was also deposited in a proximal tempestite environment, but at this level the lateral continuity of facies and thickness over a wide area indicates a very low palaeoslope throughout the South and Central Flinders Zones, and uniform basin subsidence almost balancing the sedimentation rate. The increased proportion of carbonate suggests a decrease in siliciclastic input and/or increased rate of carbonate precipitation. The upward increase in intraclastic material, wave erosion features such as pot and gutter structures, and the presence of stromatolites in the uppermost beds is consistent with a gradual shallowing upward at all localities. Haines (1988) suggests that the cyclicity observed in unit 7 may once again be climatically controlled. Haines (1988) also suggests the possibility that some of

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the highly irregular and wispy grains of glauconite/francolite in the red limestones may be a replacement of original volcanic shards. Alteration of basic volcanic contributions may also explain the abundance of iron oxides in these beds. Similar hematite-rich horizons containing probable altered shards have been reported from the Brachina Subgroup (Plummer 1978b). The base of unit 8 seems to record a slight deepening after the very shallow conditions suggested for the top of unit 7. The lower two thirds of the unit is dominated by tempestites, but shows an upward decrease in the strength and frequency of wave influence. It seems unlikely that this is related to a continuing increase in water depth as very shallow water algal laminites are present near the top of the unit and in unit 9. Diapiric structures are common in the Adelaide Geosyncline and many were periodically active, with the core often exposed on the seafloor or as islands during deposition (Dalgamo & Johnson 1966; Lemon 1985). Equivalents of unit 8 along the Central/North Hinders Zone boundary show the first clear evidence of syndepositional diapiric intrusion into the Wonoka Formation (Haines 1986). This evidence takes the form of interbeds of sandy and pebbly detritus of identical composition to rocks exposed in the cores of nearby diapiric breccias. Sometimes notable facies and thickness changes and progressive unconformities are present in the adjacent sediments. Haines (1986) suggests that rising diapiric islands within the Central/North Flinders Zone boundary region produced a wave barrier that progressively increased in efficiency during the deposition of unit 8, culminating in lagoonal conditions in which mud deposition became dominant near the top of the unit. This lagoon terminated to the south against marginal marine and terrestrial siliciclastics deposited during the first advance of the Bonney Sandstone. Diapiric islands continued to act as major barriers during the deposition of units 9 to 11 which are restricted to areas south of the Central/North Flinders Zone boundary. The

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black, organic rich nature of many of the limestones suggests restricted circulation and low oxygen conditions behind the barrier. Open marine conditions prevailed to the north. The cyclic sequence developed within unit 11 is similar to the classic shallowing and coarsening upward cycles found in many ancient shallow water carbonates (e.g. James 1984; Tucker 1985b). The well developed oolite capped cycles resemble the grainy sequence of James (1984), indicating relatively low energy conditions. The lower micritic and peloidal portion of each cycle was probably deposited in a shallow subtidal lagoon. The lagoon shallowed to produce ooid shoal and beach environments, with repeated periods of exposure and cementation. In unit 9 cycles occurred on a smaller scale and in most cases did not reach the level of exposure.

Unlike units 9 and 11, the abundant carbonate present in lower units contain no shallow water particles, and no other indicators of very shallow conditions. If this carbonate was reworked from shallow banks, as is common in Phanerozoic sequences, allochthonous shallow particles should be present. Their absence has led Haines (1988) to argue that the middle-shelf zone represents the main carbonate generation area, and that carbonate was directly precipitated (probably inorganically) as fine carbonate mud. It is suggested that inorganic precipitation as 'whitings' from warm CaC03 saturated surface water would have been more prevalent during the Precambrian because of the absence of carbonate removal by carbonate secreting organisms.

The great majority of published descriptions of ancient storm-dominated sequences are comprised largely or entirely of siliciclastics. The best Brick textures in ooids were first described by documented carbonate example is the Upper Assereto & Folk (1976), and later by Tucker Muschelkalk (Middle Triassic) of the South Ger(1985a) and Singh (1987). These workers provide man Basin, described in detail by Aigner (1982, pettographic and geochemical evidence that this 1985). In this example, deposited in an epicontexture results from the replacement of original tinental seaway, the carbonate was generated on aragonite by calcite. Neomorphic pseudospar shallow banks and is composed largely of shallow replacing ooids has been described by Rich water particles of gravel to mud size. Individual (1982), Tucker (1985a) and Singh (1987), and is tempestites are frequently well graded from a also considered to indicate replacement of coquina base to muddy top. Wave ripples and pot aragonite. Original aragonite in the Wonoka For- and gutter structures are common, as in the mation is consistent with the recent suggestion by Wonoka Formation, but HCS is generally not several authors (e.g. Sandberg 1983) that late well developed. In the Wonoka Formation the Precambrian seawater had similar chemistry to coarsefractionis represented by intraclasts rather than bioclastic material. Here the similarity ends. that of today. In the Muschelkalk individual coarsening upward to proximal cycles, interpreted as relating Unit 10 is considered to represent a variety of distal repeated small-scale transgressive/regressive shallow water facies ranging from subtidal, toshifts of the ramp system, are very thin (generally through intertidal to supratidal in origin. The red less than 7 m), and the entire sequence of many bed facies may be non-marine in part. During the vertically stacked cycles is usually less than deposition of unit 10 this facies prograded from 100 m. Hanford (1986) has described a stormthe west across large areas of the Central and dominated carbonate shelf system within the South Flinders Zones, but did not reach far east- Mississippian Fayetteville Shale and Pitkin ern areas where shallow subtidal and tidal reduc- Limestone couplet of Arkansas. This succession, ing conditions continued throughout. The deposited on a prograding ramp, is interpreted as widespread shale unit at the top of unit 10 representing 'deep' muddy shelf grading to provides evidence of a minor transgressive event storm-dominated muddy shelf, and capped by ooid skeletal shoal and shoreface facies. The enand return to widespread lagoonal conditions.

Proterozoic storm-dominated shelf carbonates tire sequence is of the order of 40 m in thickness, and contains a relatively thin storm-dominated interval composed of reworked shallow water particles. Although dominated by carbonates, the Wonoka shelf sequence appears to have more in common with the storm-dominated siliciclastic shelf model (e.g. Hamblin & Walker 1979; Walker 1984; Brenchley 1985). This may be because the Wonoka shelf carbonates were inorganic precipitates not directly related to organic productivity which increases with decreasing depth, creating relatively rapid regressions and thin regressive sequences. The general lack of interbedded mudstone in the middle-shelf zone (except unit 6) is probably related to the continuous nature of carbonate generation, as distinct from episodic supply from shallow regions that operated in Phanerozoic examples. CONCLUSION In summary, the late Proterozoic Wonoka Formation has been subdivided into 11 mappable units in the Central and South Flinders Zones of the Adelaide Geosyncline. The typical vertical sequence begins with outer shelf turbidites and mudstones, possibly with minor storm influence, and grades upwards with increasing carbonate content into a thick storm-dominated mixed carbonate/siliciclastic sequence of middle- to innershelf origin. The sequence is capped by lagoonal and tidal carbonates and siliciclastics. The carbonate shelf took the form of a shallowly inclined ramp deepening to the north and east. The Wonoka shelf sequence appears to represent the first well documented example of a late Proterozoic storm-dominated carbonate shelf. It differs markedly from Phanerozoic examples in its thickness and in the nature of the constituent carbonates. These differences may be explained if the carbonate was not reworked from shallow banks, but represents direct slow inorganic precipitation into the middle-shelf environment.

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ACKNOWLEDGEMENTS Research for this paper was completed while the author was in receipt of a University of Adelaide Scholarship for Postgraduate Research. I thank Drs Vic Gostin and Richard Jenkins for supervising the project, and gratefully acknowledge financial support for fieldwork from an Esso Australia Ltd research grant to my supervisors. I would also like to acknowledge the National Parks and Wildlife Service for granting permission to work within the Flinders Ranges National Park, and the cheerful cooperation of the many landowners on whose properties I have undertaken fieldwork. I thank Alfredo Camacho for generous provision of word processing facilities. REFERENCES AGER D.V. 1974. Storm deposits in the Jurassic of the Moroccan. High Atlas. Paleogeography, Paleoclimatology, Paleoecology 5, 83-89. AIGNER T. 1982. Calcareous tempestites: storm dominated stratification in the Upper Muschelkalk Limestones (Middle Trias, SW Germany). In Einsele G. & Seilacher A. eds. Cyclic and Event Stratification, pp. 180-198. Springer-Verlag, Berlin. AIGNER T. 1985. Storm Depositional Deposits, Springer-Verlag, Berlin. ASSERETO R. & FOLK R.L. 1976. Brick-like texture and radial rays in Triassic pisolites of Lombardy, Italy. Sedimentary Geology 16, 205-222. BAKER P.A. & BURNS S.J. 1985. Occurrence and formation of dolomite in organic-rich continental margin sediments. American Association of Petroleum Geologists, Bulletin 69, 1917-1930. BRENCHLEY RJ. 1985. Storm influenced sandstone beds. Modern Geology 9, 369-396. BOUMA A.H. 1962. Sedimentology of Some Flysch Deposits. Elsevier, Amsterdam.

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COATS R.P. 1964. Large-scale Precambrian slump structures, Flinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 11, 1-2.

FLUGEL E. 1982. Microfacies Analysis of Limestones. Springer-Verlag, Berlin.

CLOUD P. & GLAESSNER M.F. 1982. The Ediacarian Period and System: metazoa inherit the earth. Science 217, 783-792.

FORBES B.G. & PREISS W.V. 1987. Stratigraphy of the Wilpena Group. In Preiss W.V. (compiler). The Adelaide Geosyncline - Late Proterozoic Stratigraphy, Sedimentation, Palaeontology and Tectonics. Geological Survey of South Australia, Bulletin 53, 211-254.

DALGARNO C.R. & JOHNSON J.E. 1964. Wilpena Group. In Thomson B.P. et al. Precambrian rock groups in the Adelaide Geosyncline: a new subdivision. Quarterly Geological Notes, Geological Survey of South Australia 20, 12-15. DALGARNO C.R. & JOHNSON J.E. 1966. Diapiric structures and late Precambrian - Early Cambrian sedimentation in Flinders Ranges, South Australia. American Association of Petroleum Geologists, Memoir 8, 301-314. DOTT R.H. Jr. & BOURGEOIS J. 1982. Hummocky stratification: significance of its variable bedding sequences. Geological Society of America, Bulletin 93, 663-680. DUKE W.L. 1985. Hummocky cross-stratification, tropical hurricanes, and intense winter storms. Sedimentology 32, 168-194. EICKHOFF K.H., VON DER BORCH C.C. & GRADY A.E. 1986. Incised meandering canyon in the late Proterozoic Wilpena Group, problems of timing and thalweg migration. Twelfth International Sedimentological Congress, Canberra, Abstracts, p.92. EINSELE G. & SEILACHER A. 1982. Palaeogeographic significance of tempestites and periodites. In Einsele G. & Seilacher A.eds. Cyclic and Event Stratification, pp.531-536. Springer-Verlag, Berlin. FEDONKIN M.A. 1977. Precambrian-Cambrian ichocoenoses of the east European platform. In Crimes T.P. & Harper J.C. eds. Trace Fossils 2. Geological Journal Special Issue 9, 183-193. Seel House Press, Liverpool. FENTON M.W. & WILSON C.J.L. 1985. Shallow water turbidites: an example from the Mallacoota Beds, Australia. Sedimentary Geology 45, 231-260.

GEHLING J.G. 1982. The sedimentology and stratigraphy of the late Precambrian Pound Subgroup, central Flinders Ranges, South Australia. M.Sc.thesis, University of Adelaide (unpubl.).

GOSTIN V.A. & JENKINS R.J.F. 1983. Sedimentation of the early Ediacaran, Flinders Ranges, South Australia. Geological Society of Australia, Abstracts 9, 196-197.

HAINES P.W. 1986. Late Precambrian carbonate shelf to shale basin transition, Wonoka Formation, Flinders Ranges, S.A. Geological Society of Australia, Abstracts 15, 92-93.

HAINES P.W. 1987. Carbonate shelf and basin sedimentation, late Proterozoic Wonoka Formation, South Australia. Ph.D.thesis, University of Adelaide (unpubl.).

HAINES, P.W. (1988). Storm-dominated mixed carbonate/siliciclastic shelf sequence displaying cycles of hummocky cross-stratification, late Proterozoic Wonoka Formation, South Australia. Sedimentary Geology 58, 237-254.

HAMBLIN A.P. & WALKER R.G. 1979. Storm-dominated shallow marine deposits: the Fernie-Kootenay (Jurassic) transition, southern Rocky Mountains. Canadian Journal of Earth Sciences 16, 1673-1690.

HANFORD C.R. 1986. Facies and bedding sequences in shelf-storm-deposited carbonates - Fayetteville Shale and Pitkin Limestone (Mississippian), Arkansas. Journal of Sedimentary Petrology 56, 123-137.

Proterozoic storm-dominated shelf carbonates

HARMS J.C., SOUTHARD J.B., SPEARING D.R. & WALKER R.G. 1975. Depositional environments as interpreted from primary sedimentary structures and stratification sequences. Society of Economic Paleontologists and Mineralogists, Short Course, No. 2, 161pp. JAMES N.P. 1984. Shallowing-upward sequences in carbonates. In Walker R. G. ed. Fades Models. Geoscience Canada, Reprint Series, No.l, second edition, pp.213-228. JAMES N.P. & STEVENS R.K. 1986. Stratigraphy and correlation of the Cambro-Ordovician Cow Head Group, western Newfoundland. Geological Survey of Canada, Bulletin 366, 143pp. JENKINS R.J.F. 1975. An environmental study of the rocks containing the Ediacara assemblage in the Flinders Ranges. Geological Society of Australia, First Australian Geological Convention Abstracts, 21-22. JENKINS R.J.F. 1981. The concept of an "Ediacaran Period" and its stratigraphic significance in Australia. Transactions of the Royal Society of South Australia 105, 179-194. JENKINS R.J.F. 1984. Interpreting the oldest fossil cnidarians. Palaeontographica Americana 54,95-104. JENKINS R.J.F., HAINES P.W. & GOSTIN V.A. 1988. The Ediacaran revisited. Geological Society of Australia, Abstracts 21, 203-204. LEMON N.M. 1985. Physical modeling of sedimentation adjacent to diapirs and comparison with late Precambrian Oratunga Breccia Body in the Central Hinders Ranges, South Australia. American Association of Petroleum Geologists, Bulletin 69, 1327-1338. LOBBAN C.S. & WYNNE M.J. 1981. The Biology of Seaweeds. University of California Press, Berkley. MAWSON D. 1939. The late Proterozoic sediments of South Australia. Report of the Australian and New Zealand Association for the Advancement of Science 24, 78-88.

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PLUMMER P.S. 1978a. Stratigraphy of the lower Wilpena Group (Late Precambrian), Hinders Ranges, South Australia. Transactions of the Royal Society of South Australia 102, 25-38. PLUMMER P.S. 1978b. Evidence of volcanism contemporaneous with Adelaidean sedimentation. Quarterly Geological Notes, Geological Survey of South Australia 68,15-19. PRAVE A.R. 1985. Can hummocky cross-stratification be formed below effective wave base? Geological Society of America, Abstracts with programs 17, 693. RADKE B.M. & MATHS R.L. 1980. On the formation and occurrence of saddle dolomite. Journal of Sedimentary Petrology 50, 1149-1168. READ J.F. 1982. Carbonate platforms of passive (extentional) continental margins: types, characteristics and evolution. Tectonophysics 81, 195-121. READ J.F. 1985. Carbonate platform facies models. American Association of Petroleum Geologists, Bulletin 69, 1-21. RICH M. 1982. Ooid cortices composed of neomorphic pseudospar: possible evidence for ancient originally aragonitic ooids. Journal of Sedimentary Petrology 52, 843-847. RUTLAND R.W.R., PARKER A.J., PITT G.M., PREISS W.V.&MURRELLB. 1981. The Precambrian in South Australia. In Hunter D.R. ed. Precambrian of the Southern Hemisphere, pp.309-360. Elsevier, Amsterdam. SANDBERG PA. 1983. An oscillating trend in Phanerozoic non-skeletal carbonate mineralogy. Nature 305, 19-22. SINGH U. 1987. Ooids and cements from the late Precambrian of the Flinders Ranges, South Australia. Journal of Sedimentary Petrology 57, 117-127. TUCKER M.E. 1985a: Calcitized aragonite ooids and cements from the late Precambrian Biri Formation of southern Norway. Sedimentary Geology 43, 67-84.

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TUCKER M.E. 1985b: Shallow-marine carbonate facies and facies models. In Brenchley P.J. & Williams B.P.J. eds. Sedimentology - Recent developments and applied aspects, pp. 147-169. Blackwell Scientific Publications, Oxford. VON DER BORCH C.C., SMIT R. & GRADY A.E. 1982. Late Proterozoic submarine canyons of the Adelaide Geosyncline, South Australia. American Association of Petroleum Geologists, Bulletin 66, 332-347.

Proterozoic Adelaide Geosyncline, South Australia. Sedimentology 32,507-518. VON DER BORCH C.C., CHRISTIE-BLICK N. & GRADY A.E. 1988. Depositional sequence analysis applied to upper Proterozoic Wilpena Group, Adelaide Geosyncline, South Australia. Australian Journal of Earth Sciences 35, 59-72. WALKER R.G. 1984. Shelf and shallow marine sands. In Walker R.G. ed. Facies Models. Geoscience Canada, Reprint Seriesl, second edition, pp. 141-170.

VON DER BORCH C.C. & GRADY A.E. 1984. Mechanisms of sandstone deposition in a late Proterozoic submarine canyon, Adelaide Geosyncline, South Australia. American Association of Petroleum Geologists, Bulletin 68, 648-689.

WALKER R.G., DUKE W.L. & LECKIE D.A. 1983. Hummocky stratification: significance of its variable bedding sequence: discussion. Geological Society of America, Bulletin 94, 1245-1249.

VON DER BORCH C.C., GRADY A.E., ALDAM R., MILLER B., NEUMANN R., ROVIRA A. & EICKHOFF K. 1985. A large-scale meandering submarine canyon: outcrop example from the late

WALTER M.R., KIYLOV I.N. & PREISS W.V. 1979. Stromatolites from Adelaidean (late Proterozoic) sequences in central and South Australia. Alcheringa 3, 287-305.

Problems of the base of the Cambrian: A review of Daily's contributions and of further tasks Martin F. Glaessner Department of Geology and Geophysics, University of Adelaide, G.P.O. Box 498, Adelaide, SA. 5001, Australia The main observations and conclusions recorded in Daily's work, from his Ph.D.thesis to his last publications, concerning the transition from Precambrian to Cambrian are considered. World-wide organised study of this period is proceeding now and planned for several more years. Old and new methods and principles discussed and applied are likely to influence future Australian time scales and correlations.

Key words: Precambrian-Cambrian boundary, biostratigraphic principles.

INTRODUCTION In their obituary of Brian Daily, two of his former students (Jago & Pledge 1987) remarked that Daily 'made substantial contributions to the study and understanding of the PrecambrianCambrian boundary, both in Australia and overseas'. It seems appropriate to give now a brief summary of these contributions and to place them in the context of the present world-wide activities that were proceeding when Daily's life came to a regrettably premature end. The foundations of a modern stratigraphy of the Precambrian-Cambrian transition in South Australia were presented in Daily's doctoral thesis. This work, which he started in 1953, received the unusual recognition of being rapidly published as a contribution to a symposium on the Cambrian at the 20th International Geological Congress in Mexico (Daily 1956, 1957). These publications contain an 'Historical Introduction' listing various views on the subject developed during the preceding 50 years. Interested readers are referred to it. In a section on 'The base of the Cambrian in South Australia' Daily (1957, p. 134) 'tentatively followed Mawson by including the Pound Sandstone and the Stokes Bay Sandstone of Kangaroo Island in the Cambrian'. He noted the then prevailing view that, in all areas except those of marginal overlap, fossiliferous Cambrian

rocks pass down conformably into very thick rock sequences which are either unfossiliferous or do not contain conventional Cambrian fossils. Aware of Sprigg's (1947, 1949) finds of jellyfishes of (originally queried) 'Early Cambrian' age in the Pound Quartzites and Sandstones of Ediacara, which were then still under investigation (see Glaessner & Daily 1959), Daily (1957, p. 134) noted correctly and significantly that 'The Archaeocyatha are the first authentic Lower Cambrian fossils to appear in the sequence but their relationships to Lower Cambrian faunas in other provinces have not yet been satisfactorily fixed'. The earliest Archaeocyatha of the eastern Siberian Lower Cambrian standard sequence are still unrecorded elsewhere. In his correlation chart, Daily (1957; Fig. 2) gave two queried positions, above or below the Pound Sandstone, for the Precambrian-Cambrian boundary. Only with the regional mapping (Dalgarno 1964; Dalgarno & Johnson 1964) in the Flinders Ranges could it become clear that the clastics considered by previous observers as transitional from the Pound to the limestones, actually followed the Pound Quartzite dis- or unconformable Daily accepted in his subsequent publications the newly established Parachilna Formation. In his comments on the base of the Cambrian system in South Australia (Daily 1963, p.597), he stated this important principle:

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'It has been the practice in this State to make time boundaries coincide with mapped rock unit boundaries. This is contrary to established stratigraphic principles and any attempt to define the base of the Cambrian as a rock unit boundary should be resisted. For this paper the CambrianPrecambrian boundary is placed within the Mount Terrible Formation below the first appearance of the Hyolithes and associated fauna. Future study may decide that this position is incorrect but the boundary proposed is more realistic and compatible with known fact than one which is forced to fit the lithology.' In fact, difficulties with application to the local as well as the general situation remain to this day not completely resolved. The 1963 publication marked the beginning of a significant extension of Daily's work. Together with A.R. Milnes, correlation of the fossiliferous Cambrian strata with the metamorphosed Kanmantoo Group of the southeastern Mt Lofty Ranges, Fleurieu Peninsula and Kangaroo Island led to important stratigraphic and tectonic conclusions. Details are outside the scope of this paper. There can be little biostratigraphic input to the Precambrian-Cambrian boundary problem from correlation with metamorphics. The next step in the collection of data relevant to this problem came with Daily's discovery of the Uratanna Formation (Daily 1972, 1973, 1976a, b). In the Hinders Ranges it rests unconformably on the Pound Quartzite and is followed by the regionally unconformable Parachilna Formation. The Uratanna Formation consists of greenish silts and shales interbedded with thin quartzite and sandstone lenses. In them the trace fossils Rusophycus, to which Daily (1973, p.204) refers as trilobite burrows, occur with Curvolithus? davidi Webby, Didymaulichnus, Phycodes pedum, and others (Daily 1972). Correlations based on trace fossils will be discussed below. Three articles deal with far-reaching conclusions on world-wide correlations of the Early Cambrian sediments and fossils (Daily 1972, 1976a, b). These contributions mark Daily's ac-

tive participation in the work of the PrecambrianCambrian Working Group, IUGS-IGCP Project 29 and his own excursions in North America, Scandinavia, Morocco and Siberia, to which he later added observations in Argentina and China. Unfortunately, most results of his wide-ranging studies and collecting tours remain unpublished. What he had observed in South Australia led him to draw conclusions about the PrecambrianCambrian boundaiy in other places (Daily 1972; figs. 1, 5, 6). While much detailed evidence has been obtained since by critical modem studies, two conclusions drawn by Daily in his papers should be pointed out here. In most sections exposing Proterozoic sediments below Lower Cambrian strata he depicted the boundary as an unconformity. He did not go so far as to deny the possibility of a transition which would qualify as a boundary stratotype, but he did not propose one to the Boundary Working Group. His other important observation, with its now much discussed potential for stratigraphic correlation, is the sudden increase in abundance of trace fossils, including some restricted Cambrian ichnospecies, above the assumed base of the Cambrian. THE PRECAMBRIAN-CAMBRIAN BOUNDARY WORKING GROUP The work of the Boundary Working Group since 1972 has been abundantly documented in Circulars, IGCP and IUGS Reports and in the literature (Cowie & Glaessner 1975; Cowie 1985, 1986). It has brought to our attention many stratigraphic successions through the transition from Late Proterozoic to Early Palaeozoic sediments. It has shown the soundness of Daily's judgment that for the definition of the base of the Cambrian which is essential for any discussion of the history of this transition, the classical and time-honoured biostratigraphical approach should be retained' (Daily 1972, p.13). 4

What are now the tasks of the Boundary Working Group and what principles are being discussed, scrutinized, and hopefully applied by it? The first task is the selection and definition of

Problems of the base of the Cambrian a stratotype for the Precambrian-Cambrian boundary. World-wide discussion of geological observations relevant to it and the point in time it represents must be based on such a definition. In its absence, participants in discussions may use the same terms with different meanings, according to personal preference or experience. The problem of drawing practical conclusions from events in a distant geological transition period is now seen as an interdisciplinary one of exceptional difficulty. These interlinked events reflect transitions in the evolution of the biosphere, the lithosphere, hydrosphere and atmosphere. In concrete terms, their explanation requires relevant data from biostratigraphy, geochemistry, palaeogeography, palaeomagnetism, and of course isotopic geochronometry. These very disparate fields require much specialist knowledge being concentrated on a specific interval of geological time. The specificity of this interval might appear now to lie more in a temporary speeding up of known processes rather than in the assumption of a singular, exceptional planetary or cosmic event. This dilemma is still unsolved. Continued comparative and objective studies of other Era boundaries first recognised in the last centuiy could help. At present the Precambrian-Cambrian boundary Working Group includes some 63 specialists from different fields. It can also call for advice from other commissions and affiliated organisations of the IUGS, international programmes such as IGCP, and has to request confirmation of its decisions from the International Geological Congress. Regrettably, but not surprisingly, this fearsome apparatus is seen by some as more bureaucratic than scientific. Its composition is based on practical experience rather than personal ambition. It aims at consensus, necessarily expressed as majority decisions (of about half the members of the Working Group designated as Voting Members). It seems that consensus is ultimately necessary; scientific discussion can continue endlessly, but we must express geological observations in the form of maps that show boundaries such as the one between Precambrian and Cambrian rocks. They are further generalised to

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show configurations and placings of land and sea, climatic conditions, biogeography, etc. To make sense, the 'point' or interval of time to which the map refers, must be defined in the same manner for all parts of the Earth's surface. In science, majorities may be and are often wrong. Dissidents must remain free to express their divergent opinions, provided they give their own reasons, based either on new observations or on better founded theories than those on which majority opinion was based. If tests prove them right, then maps will have to be changed: bureaucratic decisions cannot for long prevail over sound scientific conclusions. It has been rightly said that facts are stubborn. PROBLEMS OF STRATIGRAPHIC CORRELATION One of the established qualifications of stratotypes is their biostratigraphic correlatability, particularly that of acceptable boundary stratotypes. Problems are seen in Daily's correlation of the Australian basal Cambrian with other parts of the world. Constraints on speculation have been suggested in the subsequent construction of similar correlation charts. Only selected problems can be included in this review, particularly those concerning the biostratigraphic means of placing the Precambrian-Cambrian boundary in the regional geological context of South and central Australia, its position in the international stratigraphic scale, and its geochronological calibration, with remarks on current work in other regions. The first problem is the lack of detail in Daily's original Faunal Assemblages which he positioned in his chart of 1972. He had planned to describe them in his later years. Several groups of researchers are now undertaking to fill that particular gap. The generality of the basal unconformity of the Cambrian in South and central Australia (Daily 1972; fig. 5) is now being questioned (pers. comms. from various investigators). The South Australian correlations were refined in the latest papers (Jago & Daily 1982; Jago etal 1986).

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Problems of the earliest Cambrian fossils Among the many Early Cambrian fossils collected by Daily are hyolithids and sabelliditids. In some countries, particularly the USSR and Poland, some of these fossils are considered important for correlation. None of those from South Australia has been identified specifically, described or figured. Modern generic nomenclature has not yet been applied to South Australian hyolithids. An early report by Daily on the abundant hyolithids from Myponga Beach remains unpublished. In Daily (1976a, p.50) there is an interesting discussion on the correlative significance of some of the oldest fossils found in the Mt Terrible Formation, the Sabelliditidae. It does not appear to have been published in English or commented upon since. None of these fossils has been described. The recent transfer of the Rovno Horizon with Sabelliditidae, originally designated by Sokolov as the base of his Baltic Series, from the lowest Cambrian to the top of the Vendian (Sokolov & Fedonkin 1984) has confused the argument. Clearly, careful recollecting and description of these fossils from the Mt Terrible Formation is urgently required. It may enable us to decide whether there is a Sabellidites zone at the base of the Cambrian in Australia, as it exists on the margins of the Baltic Shield and on the Baltic Platform. Most of the Uratanna Formation is shown as Precambrian in Cowie's correlation table (1985, after Xing & Luo 1984). No fossil evidence is available for this correlation of the lower Uratanna. There is no evidence for and much against the speculation by Crimes (1987, p.l 13) 'that much of the Arumbera Formation and some of the Pound Quartzite may yet prove to be of Tommotian to Lower Atdabanian age'. The general problems of small 'shelly' fossils S.Bengtson has promoted and undertaken much work on establishing the stratigraphic ranges of taxa of small 'shelly' fossils, i.e.those with calcareous, phosphatic or phosphatisied shells (as distinct from those with agglutinated or organic tubes). Their first occurrence in some abundance

near the base of the Tommotian Stage in Siberia was taken (or mistaken) as a general indication of Tommotian age. It was subsequently shown that some Tommotian species range up into the trilobite- or Archaeocyatha-bearing strata of Atdabanian or younger age. Only those of 'uniquely earliest Cambrian age' (i.e. of restricted stratigraphic range) are vital for biostratigraphic guidance concerning the position and correlation of the boundaiy (Cowie, IGCP Working Group Report 1983-7, March 1987). Detailed work on these fossils is now in progress in many institutions. The distribution of fossil species is to a varying extent controlled by the facies of the sediments containing them. This problem which can affect the use of their local stratigraphic ranges can be solved when facies dependence can be studied with actualistic models derived from living species with similar ecological requirements. Few of the first 'shelly' fossils can be convincingly interpreted in this manner. Trace fossils The same principles should also apply to trace fossils but in this field the problems are even greater. They cannot be fully discussed here (see Glaessner 1984). In many sequences of Early Cambrian clastic sediments there is a sudden increase in abundance of trace fossils. It has often been considered as the result of an evolutionaiy burst of diversity in early Metazoa and used for correlation of this horizon. Some authors have proposed it as suitable for defining the base of the Cambrian. It is difficult to apply the stringent biostratigraphic principles required in the use of body fossils indiscriminately for trace fossils, for the following reasons. Firstly, they represent the interaction with the sediments of animals, most of which were still alive. Most trace fossils show more of the activities rather than the morphology of the producers. Especially in the study of Precambrian or Early Cambrian ichnofossils, taxonomic comparisons can be made, if any, with modern phyla or, rarely, with classes but hardly ever with lower-rank systematic categories of

Problems of the base of the Cambrian

living organisms. Genera and species of trace fossils are in no way comparable with those of body fossils, yet they are occasionally included with names of animal taxa in lists drawn up for comparison of diversity of faunas of different ages or environments. The meaning of 'abundant trace fossils' is often left unexplained: individual traces or different kinds, per square metre of bedding plane or for a cubic measure of rock? Classifications are often typological, by agreement with a named specimen, or topological, by general configuration, rather than biological. The absence of trace fossils in a stratigraphic unit, even on a regional scale, can be quite meaningless for correlation, just as unfossiliferous strata cannot be correlated as such. The abundance and variety of ichnofossils depends on lithofacies far more than that of body fossils. Conclusions about depth and dynamic conditions have been drawn from trace fossil assemblages. These are not consistently reliable; apparent deep water indicators occur in rocks formed under conditions of rapid sedimentation in shallow water. Crimes (1987) has made very substantial contributions to the possibilities of zoning and correlation by trace fossil assemblages in Precambrian-Cambrian transition strata. Most of them can be seen as helpful and as useful suggestions for further work, but they should not be accepted uncritically. Marks resembling Monomorphichnus cannot be considered as distinctive fossils on the phylum level. Not only arthropod claws but also stiff bristles on other invertebrates make scratches in sediments. Burrows of Diplocraterion occur rarely in the Pound, at places where they could have been made during the transgression of the Parachilna Formation before its diagenetic silicification. (The drawing of a worm burrow in the reconstruction of the Ediacara fauna, Glaessner 1961, p.73, is based on such a specimen.) Daily accepted the view that the trace fossil Rusophycus was made by a trilobite or an animal that had attained the trilobite grade of evolution (Daily 1972, p.29). While there is still no evidence of trilobite body fossils in the lowest Cambrian and no precise evidence of trilobite body fossils in the lowest

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Cambrian and no precise evidence of morphology or behaviour of this trace fossil's originator (such as exists in younger Rusophycus and Cruziana) it is likely to have been made by some otherwise unknown arthropod. Crimes quotes Rusophycus from his trace fossil zone IE (Upper Tommotian-Lower Atdabanian). There is no evidence for his extended conclusion "that the Uratanna Formation and the top of the Pound Quartzite are very probably no older than upper Tommotian and might even be Atdabanian" (Crimes 1987, p. 112). In the last 30 years the Ediacara fauna has not been shown to include any distinctive Cambrian fossils. It remains stratigraphically below any reasonable base of the Cambrian, a view accepted by the Boundary Working Group. Nor are there any indications of annihilation of this fauna by a mass extinction. Comparative morphological analyses have shown that the body plans of certain younger and living cnidarians, polychaet worms and arthropods had existed in the Late Proterozoic Ediacara faunas and that distinctive, detailed structural homologies could be demonstrated. The time represented by the post-Ediacaran unconformities in Australia and by the Kotlin Stage of the Vendian in the USSR may allow time for the evolution in a suitable environment of the ancestors of the first 'shelly' fossils. Evidence of their first representatives known in the USSR and China suggests that they were probably few and small. The variety of body plans of animals undoubtedly increased in early Cambrian time. The first appearance of macrophagous predators accounts for the disappearance from the fossil record of the Ediacaran large soft-bodied sessile and slow-moving invertebrates. Correlation by plant microfossils Long before trace and body fossils of animal origin attracted interest in biostratigraphy of sediments of the Precambrian-Cambrian transition which had been considered as unfossiliferous, micropalaeontologists found remains of ancient spore-like microfossils and attempted correlations. This has developed wide-ranging applications not only to biostratigraphic zoning but also

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to studies of environmental changes during the transition period. Only limitations of this method will be briefly listed here. It should not be expected to solve all problems, because: (1) the biological nature of the acritarchs is not clear, (2) they are poorly preserved in weakly metamorphosed sediments and destroyed by oxydative weathering processes; Late Precambrian and Cambrian redbeds which are common in Australia cannot be correlated by this method; (3) many plant microfossils of this period are relatively poor in distinctive morphological characters; taxonomy and systematics and particularly nomenclature are often controversial; and (4) zone boundaries based on assemblages of plant microfossils, even when regionally recognisable, may not coincide with other biostratigraphic zones.

A SUMMARY OF BIOSTRATIGRAPHIC PRINCIPLES APPLICABLE TO THE PRECAMBRIAN-CAMBRIAN TRANSITION (1) The origination of biomineralisation was neither geologically instantaneous nor dominant in the evolution of the biosphere. It has been said that two-thirds of living marine invertebrates are soft-bodied or poorly skeletonized. It was important for the emergence of new biocoenoses (e.g. reefs) and for the preservation of abundant fossils. (2) Diversification was not directional (channelled towards existing phyla), nor simultaneous in different lineages but based on experimental (stochastic?) modification of genomes, resulting in bush-like divergent lineages, many of which were short-ranging. (3) Patterns of evolution are not directly comparable with adaptive radiations which in later times were firmly based on established body plans,. The view is often expressed as 'origination of most existing phyla in Cambrian-Ordovician times'. (This author found it confirmed in the form stated above in early studies on the evolu-

tion of Foraminifera, Crustacea, and Precambrian Metazoa.) (4) After the establishment of the eukaryote level in cell organisation in mid-Proterozoic time, the coelomate grade must have been reached before the latest Proterozoic. A long, unknown, evolution of animals in that time span of perhaps 500 million years duration is therefore more likely than a sudden appearance of all Ediacarian animals. (5) Many biologists have concluded that the diversification of the biota in the more distant past was the result not only of adaptation to changing environments by natural selection but also of the emergence of increasingly coordinated organ systems in animals. Selection in this wider sense will result in greater efficiently in individual development and in adult functioning, in either specialised or variable environments. Rapid changes in the biota are not generally due to environmental changes that are recognisable by geology and sedimentology but also to coordinated organismic (biochemical, biomechanical, etc.) factors. Only some 600 million years ago did animals become sufficiently abundant, diversified and preservable to enable us to record past events by biostratigraphic correlation and classification.

THE STANDARD STRATIGRAPHIC SCALE AND ITS CALIBRATION To be useful for geologists working on Precambrian-Cambrian rock sequences, the Standard Stratigraphic Scale must not have a break at the Precambrian-Cambrian boundary. The experienced workers of the Subcommission on Precambrian Stratigraphy have proposed 'a purely chronometric subdivision of Precambrian time with boundaries defined in years'. Their latest published proposal (Plumb & Gee 1987) leaves a blank space, apparently for chronostratigraphic subdivision of their table in the future. The proposal of special treatment for

Problems of the base of the Cambrian

the transition time from about 700 million years to the beginning of the Cambrian (Glaessner 1984) included classification of the Vendian as an Era. So far it has not received any consideration. The rank of a category in the scale such as Eon, Era, Period, etc. should depend not on its length but on its information content. The proposal referred to above included a renaming of the Palaeozoic-to-Recent time span as Holozoic because the Vendian fits the definition of Phanerozoic but not its geological or palaeontological characteristics. (The term Phanerozoic belongs to a different scale, with Cryptozoic and Azoic, without definition in years or boundary stratotypes). Sokolov (1984) considered the establishment of an Ediacarian (Ediacaran) Period and System without definition of its top boundary as 'nonsense' (using the English word in his Russian text). At the same time he confused the issue by shifting the top of the Vendian to include the former basal Horizon of his Baltic Series (as stated above) and leaving two versions of the base of the Vendian still undecided. Holland (1986) insisted that only lower boundaries need boundary stratotypes, thus assuring the absence of gaps or overlaps in the scale.

CURRENT WORK ON COMPLETION AND CALIBRATION OF THE SCALE A Working Group on a Terminal Precambrian System is being established by the International Commission on Stratigraphy (pers. comm. from Dr J.W. Cowie). This System may be delimited by both geochronometry and biostratigraphy. With the current work on biostratigraphic zoning of the Lower Cambrian in progress, there is hope of a resulting International Stratigraphic Scale without gaps or overlaps. This scale must be calibrated by chronometric dating of as many boundaries of standard stratigraphic units as possible. Here again there are more difficulties than solutions. A recent review by Krasnobaev & Semikhatov (1986) quotes a Rb/Sr date of 627±15 Ma for the fossiliferous Redkino Horizon of Middle Vendian

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age and 621+12 Ma for a minimum age of the Lower Vendian. The authors conclude from various datings that the lower boundary of the Vendian sensu lato may be close to 650 Ma The isotopic age of the Precambrian-Cambrian boundary in the USSR cannot be determined without thorough supplementary studies. Definitely stated datings of PrecambrianCambrian sections in China are conflicting with other datings and are also subject to revision. Jenkins (1986) reports that Compston (unpublished) supports an age for the Ediacaran {sensu stricto, i.e.base of the Wonoka Formation) of about 590 Ma Things are getting better but they are not yet good enough to determine the rates of processes that interest biostratigraphers.

REFERENCES COWIE J.W. 1985. Continuing work on the Precambrian-Cambrian boundary. Episodes 8, 93-98. COWIE J.W. 1986. Guidelines stratotypes. Episodes 9, 78-82.

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COWIE J.W. & GLAESSNER M.F. 1975. The Precambrian-Cambrian boundary: A symposium. Earth-Science Reviews 11, 209-251. CRIMES T.P. 1987. Trace fossils and correlations of late Precambrian and early Cambrian strata. Geological Magazine 124, 97-119. DAILY B. 1956. The Cambrian in South Australia. In El sistema Cambrico, su paleogeografia y el problema de su base, vol.2, pp.91-147. Report of the 20th International Geological Congress, Mexico, 1956. DAILY B. 1957. The Cambrian in South Australia. Bureau of Mineral Resources, Geology and Geophysics, Australia, Bulletin 49, 91-147. DAILY B. 1963. The fossiliferous Cambrian succession of Fleurieu Peninsula, South Australia. Records of the South Australian Museum 14,579-601. DAILY B. 1972. The base of the Cambrian and the first Cambrian faunas. Centre for Precambrian Research, University of Adelaide, Special Paper 1, 13-37.

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DAILY B. 1973. Discovery and significance of basal Uratanna Formation, Mt Scott Range, Flinders Ranges, South Australia. Search 4, 202-205. DAILY B. 1976a. New data on the base of the Cambrian in South Australia. Izvestiya Akademiyi Nauk SSR, Seriya Geologiya 3,45-52 (In Russian). DAILY B. 1976b. The Cambrian in the Flinders Ranges. 25th International Geological Congress, Excursion Guide 33A, 15-19. DALGARNO C.R. 1964. Report on the Lower Cambrian stratigraphy of the Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 88, 129-144. DALGARNO C.R. & JOHNSON J.E. 1962. Cambrian sequence of the western Flinders Ranges, South Australia. Quarterly Geological Notes, Geological Survey of South Australia 4, 2-3. GLAESSNER M.F. 1961. Precambrian animals. Scientific American 204, 72-78.

JAGO J.B. & PLEDGE N.S. 1987. Obituary. Brian Daily, 1 April 1931-6 March 198 ^.Records of the South Australian Museum 21, 65-68. JENKINS R.J.F. 1986. Project 29. Title: International correlation of Precambrian-Cambrian boundary. The Australian Geologist 61, 16-17. KRASNOBAEV A.A. & SEMIKHATOV M.A. 1986. Geochronological scale of the Upper Proterozoic (Riphean and Vendian) of the USSR: The present state. In Metody isotopnoy geologii i geokhronologicheskaya shkala, pp. 159-183. Academy of Sciences of the USSR, Vernadsky Institute of Geochemistry and Analytical Chemistry. Nauka, Moscow. (In Russian). PLUMB K.A. & GEER.D. 1987. Nomenclature for the Proterozoic- Recent actions by the Subcommission on Precambrian Stratigraphy. Precambrian Research 36, 185-187.

GLAESSNER M.F. 1984. The dawn of animal life, Cambridge University Press.

SOKOLOV B.S. 1984. The Vendian System: Position in the stratigraphic scale. 27th International Geological Congress, Stratigraphy, Section C.OI, 1, Proceedings, 111-127 (in Russian).

GLAESSNER M.F. & DAILY B. 1959. The Geology and Late Precambrian fauna of the Ediacara Fossil Reserve. Records of the South Australian Museum 13, 369-401.

SOKOLOV B.S. & FEDONKIN M.A. 1984. The Vendian as the terminal System of the Precambrian. Episodes 7,12-19.

HOLLAND H.D. 1986. Does the golden spike still glitter? Journal of the Geological Society 143, 3-21. JAGO J.B. & DAILY B. 1982. South Australia. In Cooper R.A. & Grindley G.W. eds. Late Proterozoic to Devonian sequences of southeastern Australia, Antarctica and New Zealand and their correlation. Geological Society ofAustralia, Special Publication 9, 6-12. JAGO J.B., GEHLING J.G. & DAILY B. 1986. Cambrian sediments of the Sellick Hill-Carrickalinga Head area. Fleurieu Peninsula, South Australia. In Parker A.J. compiler, pp.67-81. One day geological excursions of the Adelaide region. Eighth Australian Geological Convention.

SPRIGG R.C. 1947. Early Cambrian(?) jellyfishes from the Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 71,212-229. SPRIGG R.C. 1949. Early Cambrian jellyfishes of Ediacara, South Australia, and Mount John, Kimberley District, Western Australia. Transactions of the Royal Society of South Australia 73,72-99. XING YUSHENG & LUO HUILIN 1984. Precambrian-Cambrian boundary candidate, Meishnean, Jinning, Yunnan, China. Geological Magazine 121, 143-154.

The Adelaide Supergroup - Kanmantoo Group contact, eastern Mount Lofty Ranges, South Australia S. Toteff

Aberfoyle Resources Limited, 91 Beulah Road, Norwood, SA. 5067, Australia Examination of a critical segment of the contact between the late Precambrian Adelaide Supergroup and the Cambrian Kanmantoo Group in the eastern Mount Lofty Ranges, South Australia, has demonstrated a faulted relationship. This work has restated the existence of the Nairne Fault and disputes some earlier hypotheses advocating an unconformable boundary.

Key words: Stratigraphy, Adelaide Supergroup, Kanmantoo Group, Nairne Fault, Mount Lofty Ranges, South Australia.

INTRODUCTION In South Australia there is an extremely thick accumulation of late Precambrian sediments, the Adelaide Supergroup, which formed in an extensive elongate basin termed the Adelaide Geosyncline. Sedimentation continued into the early Palaeozoic, the end of the Precambrian being accompanied by a series of movements (Duttonian Folding of Thomson, 1969a), resulting in widespread disconformity between the earliest Cambrian deposits and the underlying Precambrian rocks. In the southern part of the Adelaide Geosyncline (eastern Mount Lofty Ranges, Fleurieu Peninsula and Kangaroo Island) the Cambrian sequence comprises two main cycles of sedimentation. A carbonate-dominated depositional cycle (Normanville Group, Daily & Milnes 1973) is followed with apparent conformity by a thick sequence of clastic sediments named the Kanmantoo Group (Sprigg & Campana 1953). Mainly poorly sorted terrigenous detritus rapidly accumulated in the Kanmantoo Trough which was initiated by the Kangarooian Movements commencing in Early Cambrian time (Daily & Forbes 1969; Daily & Milnes 1971). The unconformable nature of the contact between the Precambrian Adelaide Supergroup and

Cambrian Normanville Group is well documented. In the eastern Mount Lofty Ranges, however, where the Adelaide Supergroup and Kanmantoo Group are in juxtaposition, relationships have been problematical. Since the first reference to the "Kanmantoo Series" on the legend of the Adelaide 1:63 360 sheet by Sprigg et al (1951), there has been controversy regarding the relationships of this Cambrian metasedimentaiy sequence with older strata in the eastern Mount Lofty Ranges, where the bulk of the Kanmantoo Group is exposed. Initially, Sprigg et al (1951) proposed a fault contact, with late Precambrian (Adelaidean) rocks lying to the west of a major structure which they termed the "Nairne Fault", shown to pass through the township of this name. This interpretation was then extended into the adjoining Echunga and Gawler 1:63 360 sheets by Sprigg & Wilson (1954) and Campana (1953) respectively. Later, however, much emphasis was placed on an alternative interpretation of an unconformable relationship with the Adelaidean rocks, for example by Campana & Horwitz (1956), Horwitz et al (1959) and more recently by Thomson (1969a). On western Fleurieu Peninsula, the Kanmantoo Group conformably overlies the fossiliferous Lower Cambrian Normanville Group which is

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transgressive onto the Adelaidean sequence (Daily 1963, 1969). The presence of extensive "basal Cambrian" (Normanville Group) rocks in the eastern Mount Lofty Ranges was advocated by Horwitz et al (1959). Their correlations suggested unconformity, with local conformity, between Normanville Group rocks and the Adelaidean sequence whereas conformity between the Normanville Group and the overlying Kanmantoo Group was inferred. Thomson (1969b), however, interpreted a double unconformity bounding these "basal Cambrian" rocks. A related issue in this controversy has been the stratigraphic position of a prominent orthoquartzite unit, the Mount Barker Quartzite, so named due to its best development in the vicinity of Mount Barker summit. Suggestions have been made of a "basal" Cambrian age (Normanville Group) or a higher level within the younger Kanmantoo Group (e.g. on the Barker 1:250 000 sheet: Thomson & Horwitz 1962). Daily & Milnes (1971) strongly disputed many of the previous correlations involving not only the Kanmantoo Group but also the inferred "basal Cambrian". Errors in Adelaidean correlations were also noted. Furthermore, they re-stated the existence of the Nairne Fault on the Adelaide 1:250 000 sheet area, predicting that in the eastern Mount Lofty Ranges, the boundary between the Kanmantoo Group and mainly late Precambrian rocks lying to the west would be a series of en echelon faults. This paper outlines the results of investigations through an area in the eastern Mount Lofty Ranges covering a large part of the problem contact zone (Toteff 1977). Stratigraphic interpretation was greatly aided by the detailed observations reported by Daily & Milnes (1971, 1972) which provided a new and complete stratigraphic scheme for the Kanmantoo Group in its type area along the south coast of Fleurieu Peninsula. With this information came the necessary insight to overcome the correlation difficulties in the eastern Mount Lofty Ranges, due in part to poor outcrop and in part to metamorphic

grade differences from the type sections of Adelaidean and Cambrian formations.

STUDY AREA Investigations of relationships between Adelaidean and Kanmantoo Group rocks was undertaken between the township of Birdwood and Mount Barker Creek (Fig. 1), representing a distance of some 32km along the line of contact. The disputed fault contact (Nairne Fault) in this area has attracted several interpretations. An angular unconformity has been suggested in the north, with Early (or "basal") Cambrian rocks being shown as transgressive onto deformed Adelaidean strata (Horwitz et al 1959; Thomson 1969b). In the south, however, conformable (Horwitz et al 1959; Thomson & Horwitz 1962), or unconformable (Thomson 1969b) relationships have been proposed. Medium to high grade metamorphism has affected rocks in the area, sillimanite being identified at a number of localities (Toteff 1977). As mentioned above, this has caused some correlation difficulties as the metamorphic grades in the various stratotypes, particularly of the Adelaidean units, are considerably lower. This has not, however, presented an insurmountable problem.

STRATIGRAPHY Results of geological mapping are summarized in Fig. 1. An approximately north-south line, locally sigmoidal, marks the contact between Adelaidean rocks (Burra Group to Wilpena Group) to the west, and younger Kanmantoo Group metasediments to the east. No "basal Cambrian" (Normanville Group) rocks have been recognised. Some facies variation is evident in the Kanmantoo Group when compared with the type area, but it does not pose an obstacle to regional correlation. The most marked difference is the increased carbonate component in these northern counterparts reflected as minor calc-sili-

Adelaide Supergroup-Kanmantoo Group Contact

cate bands within the dominantly clastic metasediments in both the Carrickalinga Head Formation and the lower levels of the Backstairs Passage Formation. On a large scale, the relationship between the Cambrian rocks and the Adelaidean sequence indicates a faulted contact, as the oldest formation of the Kanmantoo Group, namely the Carrickalinga Head Formation, disappears and reappears along this contact. For example, some 6 km south of Mount Torrens township, the middle and upper members of the Carrickalinga Head Formation, the Blowhole Creek Siltstone Member and Campana Creek Member respectively, disappear against the structure. The lowest member is absent altogether. Laminated metasiltstones and fine-grained metasandstones of the distinctive Campana Creek Member (Daily & Milnes 1971) reappear south of Nairne, being exposed by excavations for the South Eastern Freeway. The well developed small-scale sedimentary structures characteristic of the type section can also be observed, particularly in the Mount Torrens area. Also near Mount Torrens, the underlying Blowhole Creek Siltstone Member contains numerous calc-silicate bands and a lens of white marble also outcrops amidst these generally poorly exposed metasiltstones. This marble may be equated with more extensively developed carbonates in the north (Milendella Limestone Member) which occur within a monotonous siltstone section of the Carrickalinga Head Formation (Gatehouse et al, this volume). On a smaller scale, evidence of faulting between the Adelaidean and Kanmantoo Group sequences is recorded by brecciation on the line of contact. Due to a virtual absence of natural outcrop along this contact zone, these observations are limited to sites where excavations have, by chance, been undertaken in the critical locations. One exposure of brecciated rocks occurs in an old railway cutting 1.5 km NNW of Mount Torrens township (Fig. 1) whereas a well defined fault zone was revealed by excavations in 1976 for the South Eastern Freeway. Here, there is a steep easterly-dipping normal fault zone (as indicated

209

by sense of drag) consisting of a three to four metre wide breccia (Fig. 2). This dislocation occurs at the predicted position of the Nairne Fault, between the Mount Barker Quartzite and leached metasiltstones and fine metasandstones of the Carrickalinga Head Formation. The base of the Mount Barker Quartzite is clearly gradational into the underlying scapolite-rich metasiltstones and calc-silicates which, in turn, as observed in Mount Barker Creek, are conformable with the Adelaidean sequence below.

DISCUSSION The geometric relationships between Adelaidean and Kanmantoo Group rocks shown in Fig. 1 offer a compelling argument for a structural break between these sequences. That younger rocks strike into the contact zone, with rock units appearing and disappearing along the contact yet maintaining their lithological integrity demonstrates a faulted relationship. These observations are incompatible with either a conformable or unconformable contact. Near Mount Barker summit, the exposed fault between the Mount Barker Quartzite and welllaminated metasediments of the Cambrian Carrickalinga Head Formation, and the clearly gradational contact between the Mount Barker Quartzite and the underlying Adelaidean metasediments disproves some earlier interpretations that the Mount Barker Quartzite is of Cambrian age (e.g. Horwitz et al 1959). The occurrence of scapolite-rich metasiltstones and calc-silicates in association with the Mount Barker Quartzite is compatible with its correlation with the Adelaidean ABC Range Quartzite. Occurring towards the top of the Brachina Subgroup of the Wilpena Group, the ABC Range Quartzite is best developed far to the north in the Flinders Ranges and is well represented in the western Mount Lofty Ranges but thins to the east (Rutland et al 1981). Sedimentation of the upper Brachina Subgroup is considered to have been under very shallow waters, above fairweather wavebase, with accompanying deposition of sul-

210

S. Toteff

Fig. 1 Geology along the Adelaide Supergroup-Kanmantoo Group contact between Birdwood and Mount Barker Creek. (Pages 210-211)

Adelaide Supergroup-Kanmantoo Group Contact T A P A N A P P A

FORMATION: IMPURE POORLY LAMINATED FINE TO MEDIUM GRAINED METASILTSTONES COMMON TOWARDS BASE.

METASANDSTONES.

TALISKER CALC-SILTSTONE: METASILTSTONES AND QUARTZ-MICA SCHISTS, WHICH BECOME PYRITIC TOWARDS THE TOP. THE UPPERMOST INTERVAL CONSISTS OF WEAKLY LAMINATED PYRITIC METASILTSTONES WHICH FORM TYPICAL GOSSANOUS IRONSTAINED OUTCROPS. THIS INTERVAL IS TERMED THE "NAIRNE PYRITE MEMBER" IN THE VICINITY OF BRUKUNGA.

CL z>

cr CD <

211

cr o

BACKSTAIRS MEDIUM-GRAINED

LAMINATED

PASSAGE FELDSPATHIC

METASILTSTONE

FORMATION:

METASANDSTONES GRADING

MASSIVE LAMINATED AND META-ARKOSES WITH THIN

TO QUARTZ-MICA

SCHIST.

COMMON

FINE TO INTERBEDS LOW

ANGLE

CROSS-BEDDING.

IN SOME AREAS, DEVELOPMENT OF BASAL AND UPPER MORE PELITIC, LESS

SORTED

WITH

INTERVAL

WEAKER

LAMINATION

IN MICACEOUS

FINE

TO MEDIUM

GRAINED

FELDSPATHIC METASANDSTONES, GRADING INTO METASILTSTONES. LAYERED CALC-SILICATES PROMINENT IN NORTHERN PART OF AREA. QUARTZITES (Q) NEAR BASE IN VICINITY OF MOUNT CHARLES

O O

CARRICKALINGA 4B

o: <

LU CL ?SS Q. =D O cr o

?HA

FORMATION

CAMPANA CREEK MEMBER: FINELY LAMINATED METASILTSTONES AND MINOR QUARTZ-MICA SCHISTS. SCAPOLITE-RICH METASILTSTONES AT SOME LEVELS, WITH ASSOCIATED CALC-SILICATES IN THE NORTH. BLOWHOLE CREEK SILTSTONE: LOCALLY LAMINATED METASILTSTONES AND QUARTZ-MICA SCHISTS. MINOR ANDALUSITE SCHISTS AND PHYLLITE HORIZONS. COMMON WEAKLY LAMINATED MASSIVE METASILTSTONE TO FINE METASANDSTONE INTERBEDS IN UPPER PORTION. MARBLE AND C A L C - SILICATE LENSES NEAR MT TORRENS (MILENDELLA LIMESTONE M E M B E R ) .

LOWER PORTION OF ANDALUSITE SCHISTS AND QUARTZ-MICA SCHISTS. METASILTSTONES WITH MINOR ANDALUSITE SCHISTS AND QUARTZ-MICA SCHISTS IN THE UPPER PORTION. LENTICULAR GLASSY MT BARKER QUARTZITE AT THE TOP, WELL BEDDED AND INTERLAYERED WITH SCAPOLITE METASILTSTONES AND CALC SILICATES TOWARDS BASE IN VICINITY OF MT B A R K E R . UNDERLYING SCAPOLITE METASILTSTONES AND CALC-SILICATES IN NAIRNE MT BARKER CREEK AREA. VARIABLY LAMINATED METASILTSTONES, ANDALUSITE SCHISTS AND FINE FELDSPATHIC METASANDSTONES. INTERBEDDED UNITS OF FINE TO COARSE META-ARKOSES AND LAYERED CALC-SILICATE. (SEACLIFF SANDSTONE - NUCCALEENA FORMATION (SS) AND MARINO ARKOSE (HA) EQUIVALENTS?).

(3)

BRIGHTON

LIMESTONE LAYERED CALC-SILICATE IN NORTH. MASSIVE POORLY LAYERED PALE GREY MARBLE WITH MINOR CALC-SCHIST AT MT BARKER CREEK.

cr < o cr cr LU LU S?

HEAD

THIN LENTICULAR

EAST.

CO>SCr

TAPLEY

HILL FORMATION : LAMINATED METASILTSTONES WITH SCAPOLITE-RICH BEDS. INTERBEDDED LAYERED CALC-SILICATES. WEAKLY LAMINATED GREY METASILTSTONE WITH FINE TO COARSE GRAINED PEBBLY META-ARKOSE AND FELDSPATHIC METASANDSTONE AND QUARTZITE NEAR TOP (EUDUNDA ARKOSE MEMBER). BELAIR SUBGROUP FELDSPATHIC FINE TO MEDIUM QUARTZITES WITH INTERBEDDED METASILTSTONES AND FINE

cr CD

METASANDSTONES.

CL =>

O (J cr LU cr LU CL Q

SADDLEWORTH IB

FORMATION : FELDSPATHIC METASILTSTONES WITH QUARTZMICA SCHISTS. WELL LAMINATED IN MOST HORIZONS. INTERBEDDED MEDIUM TO COARSE GRAINED QUARTZITES. LOWEST QUARTZITES AT MT TORRENS INTERBEDDED WITH LAYERED CALC-SILICATES

< _J LU o < <cr cr z> OQ i— <

BELOW QUARTZITES AT MT TORRENS INTER-BEDDED THIN QUARTZ-MICA CALC-SILICATES.

STONYFELL MEDIUM-GRAINED

: WELL SCHISTS

QUARTZITE QUARTZITES

UNITS. • CALC-SILICATES

AND

LAMINATED FINE METASILTSTONES AND LENTICULAR METASANDSTONES

INTERBEDDED

METASILTSTONES.

COMMON

UNITS

LAYERED

WITH AND

FELDSPATHIC CALC-SILICATE

212

S. Toteff

phates (Dyson & Von der Borch 1983). A delta platform depositional environmment is suggested for the overlying ABC Range Quartzite in the western Mount Lofty Ranges (Dyson & Von der Borch 1986). The Adelaidean age of the Mount Barker Quartzite raises severe doubts concerning the correlations made by Horwitz et al (1959) and Thomson (1969b) over the interval referred to as "basal Cambrian". In particular, thin quartzites near Mount Charles East (Fig. 1) have been incorrectly correlated with the Mount Barker Quartzite. Rather, these siliceous metasandstones occur amidst impure silty metasandstones of the basal Backstairs Passage Formation and are clearly strati graphically above the characteristically finely laminated metasiltstones of the Carrickalinga Head-Formation which appear immediately east of the Nairne Fault just north of Mount Charles East. Further evidence of a faulted contact between the Adelaidean and Kanmantoo Group sequences in the eastern Mount Lofty Ranges has been recognised to the south, in the Macclesfield area, where Mariow (1975) interpreted a strongly sinuous fault line. In this area splay faulting appears to be associated with the Nairne Fault although time relationships are uncertain. A curved fault trace is also evident in the Bird wood-Mount Charles East segment of the structure. Hence the Nairne Fault must predate at least the last phase of large scale folding in the region (i.e. late in the Delamerian Orogeny - D3 of Offler & Fleming 1968). Yet the structure observed at the Mount Barker freeway cutting is a normal fault rather than a thrust which might be expected during the relatively ductile conditions of the orogeny. It is suggested that the Nairne Fault may represent a reactivated growth fault developed towards the flank of the Kanmantoo Trough. Possibly movement along this structural feature was periodic. Major movements for example may have occurred at some time during deposition of the Middle Cambrian Tapanappa Formation of Daily & Milnes (1971). These movements may be recorded by the presence of numerous con-

glomerate lenses in this formation, ascribed by Daily & Milnes (1971) to movement of adjacent fault blocks. Conceivably, thrusting may have been initiated along this zone of weakness early in the Delamerian Orogeny (Di of Offler & Fleming 1968). The extent and scale of thrusting is unknown but it may have been more pronounced in the Macclesfield area than in the north. In any case, later deformation (D3) would result in a regionally sigmoidal fault zone trace, with still younger reactivation causing renewed movement

- X*

Fig. 2 The Nairne Fault exposed by the South Eastern Freeway near Mount Barker summit. Fault zone (arrowed) marks the contact between kaolinised laminated metasiltstones of the Carrickalinga Head Formation (left) and the Mount Barker Quartzite (right). The Quartzite is sharply truncated at the western margin of the fault zone which dips steeply east, considerably steeper than that of the strata on either side. Drag is obvious in the adjacent quartzite beds and angular blocks of this unit have been incorporated into the breccia. Figure is the late Brian Daily.

Adelaide Supergroup-Kanmantoo Group Contact

and brecciation as observed near Mount Torrens and Mount Barker.

CONCLUSIONS Considering all evidence now available, it is apparent that a regional fault marks the boundary between the Adelaidean sequence and the Cambrian Kanmantoo Group through at least most of the eastern Mount Lofty Ranges. While there may be minor readjustments in the position of the Nairne Fault initially shown by Sprigg et al (1951) on the Adelaide 1:63 360 sheet, and later in adjoining areas, the basic concept is believed to be correct.

ACKNOWLEDGEMENTS I am indebted to the late Dr Brian Daily for his assistance during the undertaking of this study. Were it not for the efforts of both Dr Daily and Dr Robin Oliver of the Geology Department, University of Adelaide, the opportunity to carry out this work would not have eventuated.

REFERENCES CAMPANA B. 1953. Gawler 1:63, 360 geological map. Geological Survey of South Australia.

CAMPANA B. & HORWITZ R.C. 1956. The Kanmantoo Group of South Australia considered as a transgressive sequence. Australian Journal of Science 18, 128-129.

DAILY B. 1963. The fossiliferous Cambrian succession on Fleurieu Peninsula, South Australia. Records of the South Australian Museum 14, 579-601.

DAILY B. 1969. Fossiliferous Cambrian sediments and low grade metamorphics, Fleurieu Peninsula, South Australia. In Daily B.ed. Geological Excursions Handbook, pp.49-54. ANZAAS, Section 3, 41st Congress, Adelaide.

213

DAILY B. & FORBES B.G. 1969. Notes on the Proterozoic and Cambrian, southern and central Flinders Ranges, South Australia. In Daily B. ed. Geological Excursions Handbook, pp.22-30. ANZAAS, Section 3,41st Congress, Adelaide. DAILY B. & MILNES A.R. 1971. Stratigraphic notes on Lower Cambrian fossiliferous metasediments between Campbell Creek and Tunkalilla Beach in the type section of the Kanmantoo Group, Fleurieu Peninsula, South Australia. Transactions of the Royal Society of South Australia 95, 199-214. DAILY B. & MILNES A.R. 1972. Revision of the stratigraphic nomenclature of the Cambrian Kanmantoo Group, South Australia. Journal of the Geological Society of Australia 19, 197-202. DAILY B. & MILNES A.R. 1973. Stratigraphy, structure and metamorphism of the Kanmantoo Group (Cambrian) in its type section east of Tunkalilla Beach, South Australia. Transactions of the Royal Society of South Australia 97, 213-251. DYSON I.A. & VON DER BORCH C.C. 1983. Evidence of evaporite minerals in the late Precambrian Brachina Subgroup, Hallett Cove, South Australia. Geological Society of Australia, Abstracts 10, 68-69. DYSON I.A. & VON DER BORCH C.C. 1986. A field guide to the geology of the late Precambrian Wilpena Group, Hallett Cove, South Australia. In Parker A.J. (Compiler). One day geological excursions of the Adelaide region, pp. 17-40. 8th Australian Geological Convention, Geological Society of Australia, South Australian Division, Adelaide 1986. GATEHOUSE C.G., JAGO J.B. & COOPER B.J. (this volume). Sedimentology and stratigraphy of the Carrickalinga Head Formation, Kanmantoo Group, South Australia. HORWITZ R.C., THOMSON B.P. & WEBB B.P. 1959. The Cambrian-Precambrian boundary in the eastern Mount Lofty Ranges region, South Australia. Transactions of the Royal Society of South Australia 82,205-218. MARLOW P.C. 1975. Structural investigations near Macclesfield, South Australia. M.Sc. thesis, University of Adelaide (unpubl.).

214

S. Toteff

OFFLER R. & FLEMING RD. 1968. A synthesis of folding and metamorphism in the Mount Lofty Ranges, South Australia. Journal of the Geological Society of Australia 15, 245-266.

RUTLAND R.W.R., PARKER A.J., PITT G.M. & MURRELL B. 1981. The Precambrian of South Australia. In Hunter D.R. ed. The Precambrian of the Southern Hemisphere, pp.309-360. Elsevier, New York.

SPRIGG R.C., & WILSON B. 1954. Echunga 1:63 360 geological map. Geological Survey of South Australia. THOMSON B.P. 1969a. The Kanmantoo Group and Early Palaeozoic Tectonics. In Parkin L.W. ed. Handbook of South Australian Geology, pp.97-108. Government Printer, Adelaide. THOMSON B.P. 1969b. Adelaide 1:250 000 geological map. Geological Survey of South Australia.

SPRIGG R.C. & CAMPANA B. 1953. The age and facies of the Kanmantoo Group. Australian Journal of Science 16, 12-14.

THOMSON B.P. & HORWITZ R.C. 1962. Barker 1:250 000 geological map. Geological Survey of South Australia.

SPRIGG R.C., WHITTLE A.W.G. & CAMPANA B. 1951. Adelaide 1:63 360 geological map. Geological Survey of South Australia.

TOTEFF S. 1977. Geology of the Adelaidean -Kanmantoo Group sequences in the eastern Mount Lofty Ranges. Ph.D. thesis, University of Adelaide (unpubl.).

Cambrian stratigraphy of Yorke Peninsula B. Daily* Department

of Geology and Geophysics, University of Adelaide, G.P.O. Box 498, SA. 5001, Australia

Adelaide,

A poorly outcropping platform cover of up to 1 400 m of flat to gently folded Cambrian strata rests unconformably on Precambrian crystalline rocks of the Gawler Block over most of Yorke Peninsula. However, in northern Yorke Peninsula in the vicinity of Kulpara a similar Cambrian sequence disconformably overlies strongly folded Late Precambrian (Adelaidean) strata. The Cambrian sequence commences with the transgressive clastic sequence of the Winulta Formation which is overlain by limestones and dolomites (Kulpara Formation) followed by dark silty limestones of the Parara Limestone which contain a clean archaeocyathid-rich tongue, the Koolywurtie Limestone Member. The Winulta, Kulpara and Parara Formations contain Early Cambrian fossils. After deposition of the Parara Limestone, the first phase of the Kangarooian Movements caused uplift of Yorke Peninsula and resulted in a widespread erosion surface. Deposition recommenced with siltstones, sandstones, evaporites and conglomerates of the Minlaton Formation which show progressive onlap from north to south over the eroded surface. The Minlaton Formation is overlain by the Ramsay Limestone, the sandstones, siltstones and shales of the Corrodgery Formation, the Stansbury Limestone, the shales and sandstones of the Moonan Formation, the Coobowie Limestone and the fine to coarse clastics of the Yuruga Formation. The Ramsay, Stansbury and Coobowie Limestones contain early Middle Cambrian fossils. In Edithburgh No. 1, the Minlaton, Ramsay, Corrodgery and Stansbury Limestones are absent and the Moonan Formation rests directly on the Kulpara Limestone. This indicates that the area of greatest uplift during the Kangarooian Movements was in the present area of Investigator Strait and southern Yorke Peninsula and that the transgression which commenced with the deposition of the Minlaton Formation did not reach southern Yorke Peninsula until the time of deposition of the Moonan Formation. The Koolywurtie Limestone Member, Minlaton Formation, Corrodgery Formation, Stansbury Limestone, Moonan Formation, Coobowie Limestone and Yuruga Formation are described in detail for the first time.

Key words: Cambrian, stratigraphy, Yorke Peninsula, Winulta Formation, Kulpara Formation, Parara Limestone, Koolywurtie Limestone Member, Minlaton Formation, Ramsay Limestone, Corrodgery Formation, Stansbury Limestone, Moonan Formation, Coobowie Limestone, Yuruga Formation. INTRODUCTION

Fossiliferous rocks which were subsequently shown to be Cambrian in age were first discovered in Australia in 1878 by J.G.O.Tepper at Ardrossan (Horse Gully) on Yorke Peninsula (Tepper 1879, 1881). Cambrian rocks are widespread but outcrop poorly (Fig. 1). Of the surface exposures, the most extensive are found in the Hummocks, between Kulpara and the head of Gulf St Vincent. Other important outcrops are

found near Winulta, in Horse Gully near Ardrossan, and around Curramulka township and southwards from these. Summary accounts of the geology of these areas can be found in Howchin (1925), Daily (1956), Horwitz & Daily (1958), Horwitz (1961) and Crawford (1965). Many new observations on the Cambrian are reported here. The most important advances in our knowledge of the Cambrian succession on Yorke Peninsula came with the drilling of the Minlaton

*This paper was found amongst Brian Daily's papers after his death on March 6, 1986. It was written in about 1974 and is published unaltered except for minor editorial amendments.

216

B. Daily

Fig. 1 Locality map showing Cambrian outcrops and positions of wells on Yorke Peninsula.

Cambrian Stratigraphy of Yorke Peninsula

G R E A T , ARTESIAN BASIN

MT.

PAINTER BLOCK

217

important exception is in the Kulpara region where markedly folded Late Precambrian Adelaide Supergroup sediments intervene between the basement and the sympathetically folded Cambrian. However, even there a disconformity, marked by conglomerates, is observed between the Adelaide Supergroup sandstones and the Cambrian (Thomson 1966). Horwitz (1959, 1961) mapped a thrust fault separating the folded Adelaidean and its cover from the less steeply dipping Cambrian platform cover lying to the west. Thus the Kulpara rocks have been interpreted as "geosynclinal" Proterozoic (Crawford 1965, p.23) thrust westwards over a platform cover.

STRATIGRAPHY AND SEDIMENTATION

Fig. 2 Cambrian tectonic elements of South Australia.

No. 1 stratigraphic bore in the Stansbury Basin by the South Australian Department of Mines, and the drilling of several wells by Beach Petroleum No Liability. The stratigraphic results of these exploration activities are summarised in Fig. 3. and petroleum well core data are presented in Table 1. A fairly comprehensive account of the Palaeozoic history of the Gulf St Vincent region based on drilling, a study of the surface outcrops, and especially geophysical evidence gathered in surveys by the South Australian Department of Mines and Beach Petroleum No Liability, is given in Stuart & von Sanden (1972). On Yorke Peninsula all known Cambrian rocks are unmetamorphosed and are either flat lying or gently dipping, except in the vicinity of faults where they may even be overturned. For the most part they rest unconformably on the Precambrian crystalline rocks which form the south-eastern portion of the Gawler Cratonic Nucleus of Sprigg (1952) or the Gawler Block (Fig. 2) in the sense of Daily et al (1973). An

The following account is based largely on unpublished data obtained by the present author as a result of his studies of the subsurface geology and palaeontology of the Stansbury Basin. The study, particularly the palaeontology, is still incomplete as there remains a considerable amount of cored rock yet to be exploited. Some further data gleaned from new field observations and recently acquired fossil collections complement the latter work. Several new formation names and one new member name are erected to facilitate description of the stratigraphy of the region. Winulta Formation (new name)* The basal reddish and paler coloured crossbedded sandstones, arkoses and conglomerates and minor intercalated shales and siltstones which rest unconformably on the older Precambrian crystalline basement and conformably below the Kulpara Formation in the Winulta district and elsewhere (Howchin 1918, 1925; Crawford 1965) are herein named the Winulta Formation. Around Winulta, where the unconformity with the basement can be demonstrated, the formation attains a thickness of nearly 100 m. Daily (1956) and Daily in Horwitz

*Since the preparation of the original manuscript, this formation was described by Daily (1976a).

Well

Edithburgh 1

Stansbury Town 1

Stansbury West 1

Operator

Beach Petroleum N.L.

Core No.

(Recovery)

Stratigraphic Unit

(m)

(this paper)

520.0- 523.0

694.3- 700.4

(2.4)

Coobowie Limestone

976.9- 980.2

(2.3)

Kulpara Formation

1049.1-1050.0

(0.8)

Proterozoic gneiss

283.8- 286.8

(0.3)

Yuruga Formation

368.8- 371.9

Yuruga Formation

467.6- 470.6

(1.8) (2.2)

550.8- 553.8

(0.03)

Yuruga Formation

632.2- 635.2

(0.05)

Yuruga Formation

635.2- 638.3

(0.2)

Yuruga Formation

716.3- 719.3

(2.7)

Yuruga Formation

798.0-

810.0

(2.8)

Yuruga Formation

888.8-

890.0

(0.9)

Stansbury Limestone

981.5- 984.5

(3.0)

Ramsay Limestone

1074.1-1077.2

(0.5)

1173.2-1176.2

(2.4)

Kulpara Formation

1266.2-1268.6

(1.2)

Proterozoic gneiss

178.3-

(1.5)

Permian pebbly clay Yuruga Formation

Minlaton Formation

346.6- 348.7

(2.1)

529.7- 531.6

(1.3)

Yuruga Formation

616.6- 619.7

760.8- 763.8

(2.1) (3.0)

Ramsay Limestone

956.8- 959.8

(2.0)

Parara Limestone

Laws & Heisler (1967)

Stansbury Limestone

1010.4-1013.5

0.3)

Koolywurtie Limestone Mbr

1165.0-1167.1

1291.8-1295.7

(1.7) (2.0)

Kulpara Formation

1440.8-1443.9

1466.7-1472.8

(3.0)

Kulpara Formation

1642.0-1643.5

Winulta Formation

1743.5-1744.7

(1.5) (0.2)

(0.2)

Watts (1967)

Yuruga Formation

182.9

Reference

Yuruga Formation

Table 1: Petroleum well core data

Depth

Kulpara Formation Kulpara Formation

Proterozoic gneiss

Watts & Gausden (1966)

Cambrian Stratigraphy of Yorke Peninsula

& Daily (1958) erroneously correlated these transgressive deposits with the Pound Quartzite of the Flinders Ranges. Crawford (1960, 1965) mapped all known occurrences of the formation on Yorke Peninsula and referred to them under the heading of "Shield Proterozoic". However, a relogging of the Minlaton No. 1 stratigraphic bore suggested to B.P.Thomson that the clastics below the Kulpara Formation in which the drill terminated were not Pound Quartzite but instead the initial Lower Cambrian deposits (Ludbrook 1965). Palaeontological proof of Cambrian age was established by the present writer with the discovery of fragments of hyolithids and spicules of the sponge Chancelloria Walcott in well cuttings taken through a 3 m interval of glauconitic and pyritic sandstones with grey siltstone interbeds, commencing 30 m above the base of the Winulta Formation in the Stansbury West No.l well. Later, black phosphatic steinkerns of hyolithids and other fossil fragments were found in cuttings of similar lithologies taken from 33 m to 39 m above the base of the formation in the Stansbury Town No. 1 and Edithburgh No. 1 wells. In the sub-surface the Winulta Formation consists of grey, pink and red coloured feldspathic sandstones, arkoses and conglomerates often with a dolomitic matrix. Thin intercalations of dolomite and siltstone may also be present. In the Stansbury Town No.l well the Winulta Formation occurs in a more dolomitic facies than is seen in the other wells cited. The fauna recovered from the sub-surface in the Stansbury Basin has not been located in surface exposures. However, palaeontological proof of the Cambrian age of these exposures is given with the recent finds (by Mr B.Bowman and the author) of numerous trace fossils in exposures in the Winulta and Maitland districts and from stone heaps in the region south-west of Ardrossan. Most of the trace fossils are found on sandstone slabs associated with slightly siltier intervals. Among the many characteristic trace fossils are the widespread ichnospecies Plagiogmus arcuatus Roedel and Phycodes pedum Seilacher. Rare examples of Diplocraterion Torell have also been found, for example in Dinham Quarry, 3 km

219

north-west of Ardrossan. In the Winulta area the trace fossils occur about 20 m above the base of the formation. The assemblage allows a firm correlation with the Early Cambrian Parachilna Formation of the Flinders Ranges, South Australia, the Allua Formation within the Amadeus Basin, central Australia and the Grant Bluff Formation in the western Georgina Basin in the Northern Territory (Daily 1974). In the Kulpara area, below the Kulpara Formation, there is a sequence of cross-bedded and dark, heavy mineral banded sandstones which are followed by lighter coloured sandstones and conglomerates. Daily (1956) correlated all these sandstones with the Pound Quartzite which at that time was considered by most Australian geologists to be Early Cambrian in age and in which occurred the Ediacara Fauna discovered by Sprigg (1947, 1949). However, stratigraphic studies by Coats (1965) and Thomson (1966) suggested that the Kulpara sandstones consisted of the Late Precambrian ABC Range Quartzite disconformably overlain by basal Cambrian conglomerates and sandstone. Palaeontological proof of this is now given with the discovery (by B.Bowman and the present writer) of the Early Cambrian Scandinavian ichnospecies Plagiogmus arcuatus Roedel in association with markedly bioturbated and ferruginised sandstones above the transgressive conglomerates, and about 20 m above the base of the Winulta Formation in a creek that flows east through sections 497 and 498 in the Hundred of Kulpara (see map of Barnes & Kleeman 1934). The importance of the disconformity at the base of the Winulta Formation can be readily appreciated, for in the Brachina Creek area, Flinders Ranges, about 2000 m of sediments occur between the top of the ABC Range Quartzite and the top of the stratigraphically younger Pound Quartzite immediately below the earliest Cambrian, which is represented there by the Parachilna Formation and in which, in nearby Wilpena Pound, Plagiogmus has again been found (Daily et al 1969).

220

B. Daily

AGE

MINLATON No. 1 STRATIGRAPHIC BORE

STANSBURY WEST No. 1 W E L L TOP YURUGA

MOONAN

TOP FORMATION

COOBOWIE

LIMESTONE

MOONAN

FORMATION + 55m

MINLATON

R A M S A Y L I M E S T O N E "T" LIMESTONE » * «, » t 3 2 m K "k < x % *.* * * * * * * t 8 5 m FORMATION

MINLATON

FORMATION

MOONAN FORMATION

STANSBURY

LIMESTONE+ 8 1m

CORRODGERY

FORMATION 56m

RAMSAY LIMESTONE

ERODED FORMATION

COOBOWIE LIMESTONE

LIMESTONE

FORMATION

ERODED

RAMSAY

YURUGA

STANSBURY LIMESTONE

TOP

EDITHBURGH No. 1 W E L L

ERODED

YURUGA

FORMATION

COOBOWIE

MIDDLE CAMBRIAN

S T A N S B U R Y TOWN No. 1 W E L L TOP

ERODED

21fi 6 n

* * * * * * * * * 7 5m MINLATON FORMATION 24m

1 20m

1 39m

LOWER CAMBRIAN

PARARA

LIMESTONE

PARARA

?±

K U L P A R A j ; imesto^e member_ 109 m) (Dolomite member

FORMATION

IIIIl IiI l OLDER P R E C AMBRIAN

BASEMENT

187r 146m)

(Do,om|te member

354m)

KULPARA

, n . m

FORMATION 5Q0m WINULTA FORMATION 98m

WINULTA FORMATION B A S E M E N T NOT

LIMESTONE (Limestone member

195m)

43m

REACHED GNEISS

• • Hyolithid fauna

KULPARA FORMATION (Dolomite member only J

WINULTA FORMATION

49m

3 0 2rr

WINULTA FORMATION

43n

I I I I I I 11 I I IITTTT I I I l I I I III I ITTTT I I I I I I l I I I l ITTTT 3 BASEMENT

GNEISS

Shelly faunas

BASEMENT *

GNEISS

BASEMENT

, F a u n a l A s s e m b l a g e N o . 2 of

DAILY(1956)

F a u n a l A s s e m b l a g e N o . 1 0 of

DAILY(1956)

GNEISS

Fig. 3 Correlation of sub-surface Cambrian sequences, Yorke Peninsula, South Australia (modified from Daily 1972a). Note that no scale is implied as thicknesses are indicated for the rock units intersected.

Kulpara Formation There is a gradual passage from the Winulta Formation into the overlying carbonates of the Kulpara Formation which is known to attain a thickness of 500 m in the subsurface. Kulpara Formation is used herein to replace Kulpara Limestone of Daily (1956) because it can be demonstrated that the formation over large areas consists of a lower dolomite member and an upper limestone member. It is possible that many of the lower member dolomites are dolomitized secondarily as indicated by the numerous vugs and high porosity. Nevertheless, some bands of fine grained dolomite are better regarded as "primary" dolomites. The carbonates are generally clean and are dominantly grey to brown-grey in colour, but there are also pink, yellow and white varieties. Some more siliceous intervals are indicated by concretions and irregular layers of chalcedonic silica. Stromatolites are the only fos-

sils found so far in the lower member. The absence of any evidence of a shelly fauna within this member is no doubt related to the very restrictive conditions which must have obtained at the time of deposition. The upper member, whose occurrence is restricted compared with that of the lower member, comprises light and dark grey, pink and white limestones, dark blue-grey siliceous limestones, some of which are oolitic, and prominent intervals of intraformationally brecciated limestones. Associated flat algal laminates and birdseye limestones are interpreted as representing products of intertidal and supratidal deposition respectively. In contrast to the lower member, parts of the upper member, especially in its uppermost levels, are richly fossiliferous. They contain the widespread and easily recognised Faunal Assemblage No.2 of Daily (1956). It was from out-

Cambrian Stratigraphy of Yorke Peninsula

crops in the Ardrossan and Curramulka districts that Tate (1892) described several fossils from this member. The following have been reported or are known to occur in this interval on Yorke Peninsula: Archaeocyatha, the enigmatic fossil redescribed by Walcott (1912) as Micromitra (Paterina) etheridgei (Tate), the tommotiid "Ambonychia" macroptera Tate, brachiopods including "Nisusia" compta (Tate), hyolithids among which is Hyolithes conularoides Tate, and undescribed species of trilobites (Redlichiacea), gastropods and many problematica. Parara Limestone Poorly outcropping fossiliferous grey to darkblue grey, mottled and rubbly argillaceous limestones and calcareous shales of the Parara Limestone overlie Kulpara Formation in the Curramulka, Ardrossan, Dowlingville, Clinton and Kulpara districts. A third lithology, namely a very clean and massive pink to light-grey coloured limestone with Archaeocyatha, herein termed the Koolywurtie Limestone Member (not indicated on Fig. 3, but see Fig. 4), is intercalated between the darker limestones in the subsurface in the Stansbury Basin. Similar limestones with abundant Archaeocyatha occur in Horse Gully interbedded in darker coloured limestones at a stratigraphically lower level. The contact between the Kulpara Formation and Parara Limestone can be examined in Horse Gully, about 4 km south of Ardrossan, where it is found to be somewhat irregular, reddened, and in places phosphatised. These features suggest a hiatus or unconformity between the two formations. Besides the sharp change from pale coloured to dark coloured limestones across the formation boundary, there is also a marked change in fauna at the contact, which may however simply reflect a change in the depositional environment rather than give supporting evidence for a hiatus in deposition. Only detailed faunal studies could help solve the problem. The Koolywurtie Limestone Member was intersected in the Minlaton No.l strati graphic bore (thickness 66 m) and Stansbury West No.l well

221

(thickness 127 m). This clean limestone is typical of a stable shelf environment and was deposited at shallower depths than the darker coloured argillaceous limestones which were laid down contemporaneously in the more negative parts of the shelf. It is important to note that the member thickens southwards at the expense of the darker limestones and indeed there is strong evidence suggesting that it was perhaps only the Koolywurtie Limestone Member that was developed in what is now Investigator Strait and the south-western portion of Gulf St Vincent. Such evidence is given by the lack of dark Parara Limestone pebbles in the younger Minlaton Formation conglomerates on Yorke Peninsula and in the White Point Conglomerate and Boxing Bay Formation on Kangaroo Island. Instead, pale coloured carbonate pebbles of Kulpara Formation and Koolywurtie Limestone Member there dominate the pebble assemblages in these late Early Cambrian conglomerates. Five faunal assemblages, all containing trilobites, were reported by Daily (1956) from outcrops of Parara Limestone on Yorke Peninsula. Prominent in Faunal Assemblage No.3 is the trilobite Yorkella australis (Woodward) which occurs abundantly in the basal metre of the formation. Also common are the gastropods Helcionella tatei Resser, Pelagiella subangulata (Tate), hyolithids including Hyolithes planoconvexa (Tate) and Hyolithes communis Billings, brachiopods and the bivalve Pojetaia runnegari (see Jell 1980). Archaeocyatha are rare but are common in younger beds. Higher in the sequence Faunal Assemblage No.4 contains several species of Pararaia Kobayashi in association with species of Redlichiacea, these including fragments of what is possibly Redlichia Cossmann itself. If so, this is the earliest occurrence of the genus in Australia. The associated non-trilobite faunas are of similar aspect to those below. In the Kulpara area about 135 m of Parara Limestone, developed in a more argillaceous facies than to the south, outcrop in the single section. The basal part was investigated by Barnes & Kleeman (1934) who reported trilobites in

222

B. Daily

weathered shales dug from a trench. Daily (1956) distinguished three faunal assemblages (Nos. 5,6 and 7) based on trilobites from the uppermost 85 m of the exposed sequence. These are stratigraphically above the beds from which Barnes and Kleeman collected. Unfortunately, none of the species occurring in these assemblages were located in the Minlaton No.l stratigraphic bore. Possibly the three assemblages occupy the time span covered by the Archaeocyatha-rich Koolywurtie Limestone Member intersected in that bore. The trilobites recovered from the top levels of the Parara Limestone in the Minlaton No.l stratigraphic bore do not at present provide for correlation with sequences elsewhere in South Australia. Thus the age of the top of the formation for the Yorke Peninsula region cannot be established. Minlaton Formation The Minlaton Formation as defined herein includes all the sediments between the Parara Limestone and the overlying Middle Cambrian Ramsay Limestone intersected by the Minlaton No.l stratigraphic bore, at depths between 220 m (721 feet 6 inches) and 258.75 m (1177 feet). Previously the geographical term Minlaton (Minlaton Conglomerate) has been used by Daily (see Wopfner 1969; fig. 33), Daily (1972) and Stuart & von Sanden (1972), in reference only to the conglomeratic portion of the formation, first found in this bore and then recognised later in other wells. The formation, which is known with certainty only in the subsurface, is highly variable lithologically and consists of inter-fingering tongues of red, chocolate, grey and green shales, silts, arkoses, breccias and conglomerates. Thin beds of limestone and dolomite and bands of gypsum associated with red-beds occur in the upper part of the formation in the Minlaton No. 1 stratigraphic bore. The log of this bore (Ludbrook 1965) when compared with that of the Minlaton No.2 stratigraphic bore (Blissett 1970) readily demonstrates the rapid lateral variability in lithol-

ogy and thickness of the many beds comprising the formation. Evidence from the Stansbury Town No. 1 well (Fig. 3) shows that there is a major unconformity below the 24 m thick conglomerate-sedimentary breccia which in that well comprises the Minlaton Formation. Core taken from near the base of the unit revealed only slightly rounded clasts of Kulpara Formation dolomites, up to 8 cm in size, and smaller clasts of basement gneiss. All are set in a matrix of red-coloured clastics (mainly silt but with angular sand and granule-sized fragments) which is calcareous, quartzose and feldspathic, micaceous and rich in dark heavy minerals. The crude horizontal bedding of the rocks seen in the core is emphasised by thin pebble-deficient layers. It would seem that the conglomerate-breccias had a dual source: (a) the carbonate fragments were stripped from adjacent ridges (probably fault scarps) of Kulpara Limestone, and (b) the matrix and basement gneiss pebbles were derived from the crystalline Older Precambrian, with much of the matrix presumably being derived from a more distant source. Notable is the absence of pebbles of Parara Limestone lithologies in the conglomerates of this well. The unconformity is readily traceable in the subsurface. In the Stansbury West No.l well, chocolate coloured siltstone overlain by mainly limestone conglomerates marks the base of the formation. In the Minlaton No.l stratigraphic bore, 18 m of green-grey and chocolate coloured shales lie above the unconformity surface and are overlain by 9 m of coarse conglomerates with clasts of mainly carbonates up to 10 cm across, and with smaller clasts of quartzite and sandstone. The carbonate clasts are both Kulpara Formation dolomites and Koolywurtie Limestone Member pebbles. Many of these latter pebbles contain Archaeocyatha and trilobites. In the Minlaton No.2 stratigraphic bore Blissett (1970) logged 34.5 m of breccia-conglomerate resting unconformably on the Archaeocyatha-rich Koolywurtie Limestone Member. Clasts reported were limestones, sandstones and angular and sub-

Cambrian Stratigraphy of Yorke Peninsula

angular fragments of reddish-brown silty shale. The maximum clast size was at least 15 cm across. Thin red finer-grained clastic interbeds and a reddish-brown calcareous and "gritty" matrix in the conglomerates were also reported. The unconformity at the base of the Minlaton Formation and the deposition of the succeeding conglomerates are a record of some of the earliest phases of the Kangarooian Movements (Daily & Forbes 1969; Daily & Milnes 1971) which commenced in the latter half of the Early Cambrian and which played a vital role in building up the vast sedimentary pile known as the Kanmantoo Group, which is now found on Kangaroo Island, in the eastern Mount Lofty Ranges and on Fleurieu Peninsula (see Fig. 2). No fossils have yet been found in the Minlaton Formation and it is therefore difficult to state anything definite about the environment of deposition. It is conceivable that all parts of the sequence such as the red-bed clastics and the conglomerate-breccias are non-marine, particularly as the clasts in the latter show little rounding. On the other hand it seems likely that the paler coloured clastics and the carbonates and gypsum horizons reflect deposition in a restricted marine environment. Possibly a paralic environment, such as is seen in the upper Lower Cambrian deposits of Kangaroo Island, existed on the Yorke Peninsula north of the Edithburgh region at that time. Pritchard (1892) reported possible Cambrian rocks 6 km south of Curramulka, but their Cambrian age was not verified until Archaeocyatha were discovered in them by Ward (1944). The rock suite consists of dark, blue-grey, mottled, argillaceous limestones which are lithologically indistinguishable from the Lower Cambrian Parara Limestone. However, they contain Redlichia aff. nobilis Walcott, Helcionella aff. rugosa chinensis Walcott, "Obolella" wirrealpensis Etheridge and Girvanella Nicholson and Etheridge which allows correlation (Fig. 4) with the early Middle Cambrian Aroona Creek Limestone and Wirrealpa Limestone of the

223

Hinders Ranges (Horwitz & Daily 1958). Crawford (1960, 1965) named the formation Ramsay Limestone. No other outcrops of the formation are known on Yorke Peninsula but it can be traced into the subsurface in the wells within the Stansbury Basin. A maximum thickness of 85 m was intersected in Stansbury West No.l well where oolitic and sandy limestones and dolomitic limestones containing Redlichia, Helcionella and Chancelloria (identified by the present author) were found about 15 m below the top of the formation. Samples of core from the top of the Ramsay Limestone in Stansbury Town No.l well showed a mottled rock consisting of grey limestone interbedded with black siltstone which is pyritic, calcareous and micaceous. Splashes and veins of galena occur in the limestone and fossil fragments are replaced by this sulphide. Honey coloured ciy stals of sphalerite are also prominent. Fossils identified by the present writer included Redlichia, species of Hyolithes, Lingulella, "Obolella", Chancelloria and abundant eocystid ossicles. The Ramsay Limestone represents the southernmost record of the very extensive early Middle Cambrian transgression within the Adelaide 'Geosyncline'. The transgression virtually drowned the source areas which had provided the clastics for the Minlaton Formation and the Kanmantoo Group found in the southern portion of the Adelaide 'Geosyncline' and for the widely developed Billy Creek Formation of the Flinders Ranges and surrounding areas. However, it would seem that some highs persisted for there are small amounts of sand sized quartz and feldspar within the Ramsay Limestone. Corrodgery Formation (new name) This new stratigraphic name (named after Corrodgery Triangulation Station) is applied to the 85 m thick clastic sequence found in the Stansbury West No.l well between the Ramsay Limestone and the overlying Stansbury Limestone. Therein, the formation, as deduced from cuttings, consists of grey, pink and red coloured micaceous and feldspathic sandstones and

to K>

OLDER PRECAMBRIAN CRYSTALLINE

BASEMENT

Fig. 4 Correlation of the Cambrian of Yorke Peninsula with the other areas of South Australia. This diagram was also published in Daily (1976b).

Cambrian Stratigraphy of Yorke Peninsula

arkoses, all with calcareous cements, and interbedded grey shales and siltstones. The only other certain occurrence of the formation is in the Stansbury Town No.l well. No fossils have been found in the Corrodgery Formation in either well. It appears likely that the red sandstone float reported by Crawford (1965) from within the Ramsay Limestone syncline represents this formation. The Corrodgery Formation is interpreted as representing the erosional debris resulting from renewed uplifts, possibly in the Edithburgh area where rocks of this age are unknown, and in Gulf St Vincent. Stansbury Limestone The Stansbury Limestone (Daily 1969,1972) refers to the 55 m thick sequence of pale to darkgrey oolitic limestones with thin shale interbeds which occurs above the Corrodgery Formation and which was first drilled in the Stansbury West No.l well. The formation thickens to 81 m in its only other known occurrence, in Stansbury Town No.l well, where there are oolitic limestones containing very small amounts of quartz sand. Dark coloured, almost black, pyritic and weakly calcareous siltstones occur near the base and green shales are interbedded with the limestones near the top of the formation. Core from the middle part of the formation showed an oolitic limestone, pyritiferous, and containing large blebs of sphalerite, some galena and traces of chalcopyrite and fluorite, a suite of minerals which probably indicates an epigenetic rather than a syngenetic origin. Fragments of Redlichia were the only fossils recovered by the writer from this core, but core from the top of the formation in Stansbury West No. 1 well produced Redlichia, Chancelloria, Hyolithes and Girvanella. The fauna suggests correlation with the Wirrealpa Limestone of the Flinders Ranges. The Stansbury Limestone is a shallow water deposit, laid down after the highs which shed the Corrodgery Formation debris had either been eroded to near base level or largely resubmerged

225

- there is hardly any sand-sized clastic material in the formation. Moonan Formation (new name) Minor uplifts followed the deposition of the Stansbury Formation, giving rise to a thin clastic sequence herein called the Moonan Formation (named after Moonan Triangulation Station). It is 24 m thick in Stansbury West No. 1 well where 18 m of dark grey shale are succeeded by 6 m of green-grey to red sandstones. Lithologies are quite similar in Stansbury Town No. 1 well, but in Edithburgh No. 1 well only green-grey sandstone and red arkose are present and these rest unconformably on white unfossiliferous dolomites of the Kulpara Formation. Thus in the Edithburgh area the unconformity at the base of the Moonan Formation is of considerable significance. It would appear that the Edithburgh area was subjected to erosion, possibly from the time of Minlaton Formation deposition, and finally became a negative area to receive the clastics of the Moonan Formation. The Edithburgh region, where erosion did not reach to basement, probably occupied a position marginal to the region of greatest uplift, namely the Investigator Strait. In other areas evidence shows that erosion had already exposed large regions of basement rocks in the late Early and early Middle Cambrian. This is seen in the arkoses above the Ramsay Limestone and to a lesser extent in the gneiss pebbles and arkoses in the Minlaton Formation. Coobowie Limestone The Coobowie Limestone (Daily 1972) was named for the 12 m thick pale grey oolitic limestone which occurs above the Moonan Formation in the Stansbury West No.l well. The formation thickens to 21 m in the Edithburgh No.l well where white dolomitic bands are interbedded. Fragments of indeterminate brachiopods and algae were recovered from cuttings in this well and are the only fossils known from the formation. They do not permit an age determination. Like the Stansbury Limestone, the Coobowie Limestone is a clean oolitic carbonate which

226

B. Daily

reflects either a period of general stability of the surrounding source areas or else shallow depositional sites (as indicated by the ooids and algae) removed from the influence of transported clastics because of their relative elevation within the depositional basin. Yuruga Formation (new name) The 550 m thick dominantly sandstone sequence which lies strati graphically above the Coobowie Formation in the Stansbury Town No. 1 well is herein named the Yuruga Formation, the name being taken from the Yuruga Triangulation Station. The formation is overlain unconformably by Permian glacigenes. Eight cores cut in the formation have assisted in the interpretation of the sequence. Its basal part is marked by well laminated dark-grey shale and siltstone which isbioturbated and contains trilobite tracks. Above are chocolate micaceous siltstones with thin interbeds of pink, reddish and grey micaceous and mafic-rich sandstones. These are succeeded by reddish and grey very micaceous and feldspathic sandstones interbedded with grey shales and siltstones, some of which are also strongly micaceous. Trilobite tracks were located in mud-cracked purple siltstone in this interval. Desiccation breccias occur in the same beds. The upper 425 m of the formation consist of chocolate coloured micaceous and feldspathic sandstones (sub-greywackes) and red, pink and brown cross-bedded feldspathic sandstones and arkoses. Most of these sandstones are finegrained, contain variable amounts of mafic minerals and their matrices have a variable carbonate cement. Rock fragments are common constituents. Rare green-grey coloured sandstones and thin interbeds of chocolate coloured micaceous silts and shales within the coarser clastics have been noted. Trilobite tracks and other trace fossils in association with mud-cracked chocolate micaceous siltstones were located about half-way through this interval. Similar finegrained, angular and poorly sorted sandstones

and arkoses occur in a comparable stratigraphical position in the Stansbury West No. 1 well. In the Edithburgh No.l well thin conglomerates containing pebbles of fossiliferous limestone and metamorphic rocks were found about 30 m above the base of the formation. Another conglomeratic horizon occurs, interbedded with red-brown feldspathic and micaceous sandstones and reddish calcareous arkoses, about 20 m below the contact with the Permian tillite. It would seem that the 140 m red sandstonearkose sequence intersected at the bottom of the South Australian Department of Mines Stansbury No.l stratigraphic bore (between 227.4 m (910 feet) and 417.6 m (1370 feet) should be correlated lithologically with the Yuruga Formation rather than with the Minlaton Formation as tentatively suggested by Johnson (1960) and Crawford (1965), or with Upper Proterozoic rocks of the Marinoan Series as provisionally suggested by Thomson & Ludbrook in Ludbrook (1965, p. 87). Further the sedimentary breccias and arkoses of the Pine Point and the limestone conglomerate of Rocky Point are more likely to be Yuruga Formation than Minlaton Formation as tentatively suggested by Crawford (1965). If so, these are the only surface exposures of the formation known. The lack of shelly fossils within the formation precludes the dating of the upper part of the formation. Nevertheless, in view of the early Middle Cambrian age of both the Ramsay Limestone and Coobowie Limestone it would seem that the Yuruga Formation is still well down in the Middle Cambrian. A correlation with the lower part of the Lake Frome Group seems warranted (Fig. 4). The interbedded red and grey clastics of the lower part of the Yuruga Formation reflect continuous changes in the depositional environment. Shallow water conditions are indicated because periodic emergence is shown both by the mudcracked horizons and the abundant clay galls and clay balls as seen in cored intervals. Some marine influence is indicated by bioturbated beds and trilobite tracks.

Cambrian Stratigraphy of Yorke Peninsula

The upper thick red clastics contain quartz and feldspars as the major constituents and arkosic rocks are prominent, but sub-greywackes with significant mica content, including authigenic chlorites and rock fragments are also conspicuous. All of the clastics contain detritus interpreted as having been derived largely from nearby crystalline basement highs. Grains are typically angular, much of the quartz shows undulose extinction and metamorphic rock fragments are common. Occasional conglomerates with pebbles of fossiliferous Cambrian prove that areas with Cambrian cover were still being elevated and eroded. The red pigment in the clastics is hematite and this coats all detritals and lightly stains the cleavage planes within sericitized alkali feldspars which are the prevalent feldspars in the rocks. Some unaltered feldspar is, however, present. Detrital mafic minerals are not common and were possibly removed by chemical weathering at the source, although post depositional weathering may have caused their removal and alteration to iron oxides to give a red pigment. However, some chemical weathering in the source is suggested by the sericitized nature of most feldspars. Their subsequent coating and partial invasion with hematite would have developed after deposition. This part of the formation was also laid down under very shallow water conditions and emergence at times is given by mud-cracked horizons. Again, some marine influence is indicated by the presence of trilobite tracks and other trace fossils. The accumulation of the Yuruga Formation clastics is not merely of local significance but is part of the pattern of widespread and very thick clastic sedimentation (mainly reddish coloured sediments) which took place within the Adelaide 'Geosyncline' commencing in the early Middle Cambrian and continuing possibly to the end of the Cambrian or even into the Early Ordovician, as for example in the Wirrealpa and Brachina areas, Flinders Ranges. All these clastic sequences are a reflection of the very widespread and continuing Kangarooian Movements that elevated large areas of crystalline rocks, for the majority of the clastics can be shown to contain

227

detritals from such sources. In the case of Yorke Peninsula, the clastics were probably derived mainly from an arcuate region now covered by the waters of Gulf St Vincent and Investigator Strait.

CONCLUSIONS Cambrian sedimentation on Yorke Peninsula was initiated by widespread deposition of the fossiliferous marine Winulta Formation, everywhere unconformably overlying either older Precambrian crystalline basement or unmetamorphosed sediments of the Late Precambrian Adelaide Supergroup. A period of shelf carbonate deposition followed, at first under very restrictive shallow water conditions (Kulpara Formation, dolomite member), but less restrictive fossiliferous clean shelf or bank carbonates and slightly deeper water argillaceous carbonates succeeded this (upper limestone member of the Kulpara Formation, and the Parara Limestone). Along the Torrens Lineament of Sprigg (1952) and other faults, late Early Cambrian faulted uplifts, regarded as the first phase of the Kangarooian Movements, initiated erosion which continued into the Middle Cambrian in the Edithburgh region. Elsewhere however, deposition recommenced in the later part of the Early Cambrian with the progressive onlapping of the Minlaton Formation over eroded Lower Cambrian strata. In the early Middle Cambrian, widespread shallow marine limestone deposition was punctuated by new influxes of clastics stripped off highs elevated by continuing phases of the Kangarooian Movements. The most intense phases of these movements recorded on Yorke Peninsula are indicated by the ill-sorted clastics of the Yuruga Formation which were most likely deposited under deltaic conditions, at times subaerial. The sediments preserved are probably only a remnant of a formerly much thicker clastic sequence.

228

B. Daily

The Late Cambrian-Early Ordovician Delamerian Orogeny (Thomson 1969), produced only broad open folds on Yorke Peninsula. Areas of more complex folding are known along the eastern side of Yorke Peninsula in association with faults, for example in the Pine Point area. In these situations, vertical and overturned limbs and confused fold plunges have resulted. Such folds are of local importance only. No metamorphic or igneous rocks of Cambrian age are known from Yorke Peninsula. REFERENCES BARNES TA. & KLEEMAN A.W. 1934. Notes on fossiliferous Cambrian near Kulpara, South Australia. Transactions of the Royal Society of South Australia 58, 7-9. BLISSETT A.H. 1970. Exploration for argillaceous material on Yorke Peninsula. Department of Mines, South Australia, Mineral Resources Review 129, 142-152. COATS R.P. 1965. Tent Hill Formation correlations Port Augusta and Lake Torrens. Quarterly Geological Notes, Geological Survey of South Australia 16, 9-11. CRAWFORD A.R. 1960. Maitland map sheet Geological Atlas of South Australia, 1:253 440 series, Sheet I 53-12. Geological Survey of South Australia. CRAWFORD A.R. 1965. The geology of Yorke Peninsula. Geological Survey of South Australia, Bulletin 39. DAILY B. 1956. The Cambrian in South Australia In El sistema Cambrico, su paleogeografia y el problema de su base, Report of the 20th International Geological Congress, Mexico, 1956 2,91-147. DAILY B. 1969. Fossiliferous Cambrian sediments and low-grade metamorphics, Fleurieu Peninsula, South Australia In Daily B.ed. Geological Excursions Handbook. 41st ANZAAS Congress, Section 3, Adelaide, 49-54.

DAILY B. 1972. The base of the Cambrian and the first Cambrian faunas. Centre for Precambrian Research, University of Adelaide, Special Paper 1, 13-41. DAILY B. 1974. The Precambrian-Cambrian boundary in Australia. Geological Society ofAustralia Specialist Group in Biostratigraphy and Palaeontology, 'Precision in Correlation', Hobart, Abstracts, 4. DAILY B. 1976a. Novye dannye ob osnovanii kembriya v yuchnoy australii (New data on the base of the Cambrian in South Australia). Izvestiya Akademiyi Nauk SSSR, Seriya Geologiya 3,45-52 (In Russian). DAILY B. 1976b. The Cambrian of the Flinders Ranges. 25th International Geological Congress, Excursion Guide 33A, 15-19. DAILY B. & FORBES B.G. 1969. Notes on the Proterozoic and Cambrian, southern and central FlindersRanges, South Australia In Daily B. ed. Geological Excursions Handbook. 41st ANZAAS Congress, Section 3, Adelaide, 23-30. DAILY B., JAGO J.B. & MILNES A.R. 1973. Large-scale horizontal displacement within AustraloAntarctica in the Ordovician. Nature Physical Science 244, 61-64. DAILY B. & MILNES A.R. 1971. Stratigraphic notes on Lower Cambrian fossiliferous metasediments between Campbell Creek and Tunkalilla Beach in the type section of the Kanmantoo Group, Fleurieu Peninsula, South Australia. Transactions of the Royal Society of South Australia 95, 199-214. DAILY B., TWIDALE C.R. & ALLEY N.F. 1969. Occurrences of Lower Cambrian sediments in Wilpena Pound, central Flinders Range, South Australia. Australian Journal of Science 31, 301-302. HORWITZ R.C. 1959. Wakefield map sheet, Geological Atlas of South Australia, 1:63 360 series. Geological Survey of South Australia. HORWITZ R.C. 1961. The geology of the Wakefield military sheet (explanation of the geological map). Geological Survey of South Australia, Report of Investigations 18.

Cambrian Stratigraphy of Yorke Peninsula

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HORWITZ R.C. & DAILY B. 1958. Yorke Peninsula In Glaessner M.F. & Parkin L.W. eds. The geology of South Australia. Journal of the Geological Society of Australia 5, 46-70.

STUART W.J. Jnr & VON SANDEN A.T. 1972. Palaeozoic history of the St Vincent Gulf region. Australian Petroleum Exploration Association Journal 12,9-16.

HOWCHIN W. 1918. Notes on the geology of Ardrossan and neighbourhood. Transactions of the Royal Society of South Australia 42,185-225.

TATE R. 1892. The Cambrian fossils of South Australia. Transactions of the Royal Society of South Australia 15,183-189.

HOWCHIN W. 1925. The geographical distribution of fossiliferous rocks of Cambrian age in South Australia with geological notes and references. Transactions of the Royal Society of South Australia 49, 1-26. JELL, P.A. 1980. Earliest known pelcypod on Earth a new Early Cambrian genus from South Australia. Alcheringa, 4, 233-239. JOHNSON W. 1960. Future oil exploration, Gulf St Vincent region. Department of Mines, South Australia, Mining Review 110, 126-133. LAWS R.A. & HEISLER H.H. 1967. Stansbury Town No. 1 Well, Well completion report. South Australian Department of Mines, Envelope 784 (unpubl.). LUDBROOK N.H. 1965. Minalton and Stansbury Stratigraphic Bores. Sub-surface stratigraphy and micropaleontology. Appendix to Crawford A.R., The geology of Yorke Peninsula. Geological Survey of South Australia, Bulletin 39, 83-96. PRITCHARD G.B. 1892. On the Cambrian rocks at Curramulka. Transactions of the Royal Society of South Australia 15, 179-182. SPRIGG R.C. 1947. Early Cambrian(?) jellyfishes from the Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 71,212-224. SPRIGG R.C. 1949. Early Cambrian "Jellyfishes" of Ediacara, South Australia and Mount John, Kimberley District, Western Australia. Transactions of the Royal Society of South Australia 73, 72-99. SPRIGG R.C. 1952. Sedimentation in the Adelaide Geosyncline and the formation of the continental terrace In Glaessner M.F. and Rudd E.A. eds. Sir Douglas Maws on Anniversary Volume, University of Adelaide, 153-159.

TEPPER J.G.O. 1879. Introduction to the cliffs and rocks at Ardrossan, Yorke's Peninsula. Transactions and Proceedings of the Royal Society of South Australia 2, 71-79. TEPPER J.G.O. 1881. Sketch of a geological and physical history of Hundred Cunningham and neighbouring regions. Transactions and Proceedings of the Royal Society of South Australia 4, 61-70. THOMSON B.P. 1966. Stratigraphic relationships between sediments of Marinoan age - Adelaide region. Quarterly Geological Notes, Geological Survey of South Australia 20, 7-9. THOMSON B.P 1969. The Kanmantoo Group and Early Palaeozoic tectonics In Parkin L.W. ed. Handbook of South Australian Geology, pp 97-108, Geological Survey of South Australia, Adelaide. WALCOTT C.D. 1912. Cambrian Brachiopoda. United States Geological Survey Monograph 51, 2 vols. WARD L.K. 1944. The search for oil in South Australia. Geological Survey of South Australia, Bulletin 22. WATTS T.R. 1967. Edithburgh No.l Well completion report. South Australian Department of Mines, Envelope 702 (unpubl.). WATTS T.R. & GAUSDEN J. 1966. Stansbury West No.l Well completion report. South Australian Department of Mines, Envelope 656 (unpubl.). WOPFNER H. 1969. The Cambrian Period In Parkin L.W. ed. Handbook of South Australian Geology, pp 84-87, Geological Survey of South Australia, Adelaide.

Slope facies deposition and diagenesis of the Early Cambrian Parara Limestone, Wilkawillina Gorge, South Australia Jonathan D. A. Clarke* School of Earth Sciences, Flinders University, Bedford Park, SA, 5042, Australia. The Early Cambrian Parara Limestone forms an important part of the carbonate dominated Hawker Group in the Flinders Ranges. At Wilkawillina Gorge the formation represents a carbonate slope facies. Most of the sediment is composed of platform and shelf derived carbonate grains transported downslope by turbidity currents and debris flows. Slope facies can be traced laterally into shallow water facies across a platform margin. The bulk of deposition occurred from low energy currents depositing fine grained turbidites. Coarse grained deposits from higher density and energy density currents are less common, as are debris flows. Coarse grained facies are more common close to the source platform. The Parara Limestone represents a major depositional contrast to the underlying Wilkawillina Limestone, which formed in a shallow water setting. Diagenesis of the formation has been most prominent in the burial realm under the influence of saline and mainly reducing pore waters. Most diagenesis occurred during deep burial with some shallow burial effects. Key words: Parara Limestone, Cambrian, South Australia, carbonate sedimentology, slope facies, diagenesis. INTRODUCTION This paper proposes a slope depositional model for the Parara Limestone at Wilkawillina Gorge (Fig. 1). The occurrence of the formation in the area was recognised by Daily (1956) although it is lithologically distinct from that occurring in the type locality at Horse Gully near Ardrossan on Yorke Peninsula. The Parara Limestone has been previously described as deep water, low energy sediments (Haslett 1969; Daily 1976) with input of allochthonous material. Mount (1970) recognised carbonate breccias in the Parara Limestone which indicated active slope processes.

REGIONAL GEOLOGY The Parara Limestone outcrops in many parts of the Flinders Ranges and is highly variable in thickness. Maximum thicknesses occur in the northern Flinders Ranges. It is thin to absent in

the central Flinders Ranges. Two main lithological associations occur. The first is composed of stylonodular limestone in a silty matrix while the second is composed of interbedded limestone and siltstone. The first lithology is characteristic of outcrops near Hawker and Quorn, in the Chace and Druid Ranges and of some of the thin sequences north of Wilkawillina Gorge. The second association forms the very thick sequences in the Bunkers Graben (so called by Dalgarno 1983), at Wirrealpa and in the Arrowie syncline. This paper deals with the second lithologic association as it occurs at Wilkawillina Gorge (Fig. 2). The Parara Limestone at Wilkawillina Gorge is thickest in the synsedimentary Bunkers Graben. Originally 8 km across, this structure was formed by movement of the underlying Oraparinna Diapir (Dalgarno 1983; Dalgarno & Johnson 1968). The tectonic history of the graben has had a profound impact on the depositional history of the sediments within it. It was sporadically active during the Precambrian and Cambrian (Clarke

*Present address: Western Mining Corporation, Kambalda, WA 6442, Australia.

Slope Facies, Parara Limestone

231

1986a). The higher rate of subsidence within the graben during active periods resulted in the deposition of a thicker sequence of sediments compared to that deposited outside. Comparison of the sequences inside and outside the graben indicates that during the Cambrian subsidence was most rapid during deposition of the Parara Limestone.

PARARA LIMESTONE STRATIGRAPHY AT WILKAWILLINA GORGE

Fig. 1. Locality map of South Australia showing location of study area.

TYPE

The age of the Parara Limestone is poorly constrained but it contains the Faunal Assemblages 3,4 and 8 of Daily (1956). It is underlain by Wilkawillina Limestone containing the Faunal Assemblage 2(FA2) fauna of Atdabanian age (Gravestock 1984). The Parara has been correlated with the Botomian stage on the Siberian Platform by Rozanov & Sokolov (1984). GORGE Edeowie

Limestone

| Oraparinna [;•;•;•;! B u n k e r s e

The Parara Limestone at Wilkawillina Gorge forms part of the Hawker Group (Dalgarno 1964), as shown in Table 1. In this area it conformably overlies the Second Plain Creek Member of the Wilkawillina Limestone (Clarke 1986b, c). The formation also interfingers with the upper part of the Wilkawillina Limestone along the southeastern margin of the Bunkers Graben (Dalgarno 1964; Clarke 1986d). At Wilkawillina Gorge the Parara Limestone is disconformably overlain by the Bunkers Sandstone in all areas except the north-west, where it is conformably overlain by the Oraparinna Shale.

Shale

Wilkawillina Limestone & Woodendinna Dolomite

KILOMETRES PARARA L I M E S T O N E T h i r d P l a i n Ck Mem. L i n n s S p r i n g s Mem. S i x Mile B o r e M e m . Fault

Fig. 2. Outcrop of the Parara Limestone in the Bunkers Graben at Wilkawillina Gorge (modified from Clarke 1986c).

The lithostratigraphy of the Parara Limestone in the Bunkers Graben has been discussed by Clarke (1986c) who formally defined three members. The members are summarised below and shown in Fig. 3. The type section for these members lies in the reference section of the Hawker Group defined by Dalgarno (1964). The members all pinch out along strike and pass into the Wilkawillina Limestone (Fig. 4).

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Jonathon D.A. Clarke

PLATFORM SEQUENCE

GRABEN FILL

Edeowie Limestone i5m Oraparinna Shale

500m

Bunkers Sandstone 319m Parara Limestone

Wilkawillina Limestone (Upper) 820.9m

ing from both less overall dolomite and less ferroan dolomite) and a generally more homogeneous lithology. In most sections this member makes up the bulk of the formation. Laterally the member becomes coarser to the south-east. Third Plain Creek Member

802m

Wilkawillina Limestone (Lower) 340.9m Woodendinna Dolomite 58.5m Table 1. Hawker Group stratigraphy in the Bunkers Graben (maximum thicknesses only)

Six Mile Bore Member The Six Mile Bore Member forms the base of the formation. The member is composed of interbedded black to dark grey nodular dolomitic limestones and shales which weather to a reddish brown grey colour. The shale and limestone beds form couplets 5 to 20 cm thick. The member is 207 m thick in the type section. Ferroan dolomitisation is common, resulting in the characteristic weathering colour. Individual beds can change rapidly along strike, with some beds traceable for only a few tens of metres. A few beds extend for several kilometres. Overall lithology does not change significantly until near the southeastern margin of the graben. The member becomes coarser grained and more dolomitised. Sedimentary structures are less well preserved in this member than in the overlying members. Linns Springs Member The Linns Springs Member conformably overlies the Six Mile Bore Member. It is composed of interbedded flaggy (rarely nodular) dark grey to black limestones and shales which weather to a sandy grey colour. The member is 361 m thick in the type section. It can be distinguished from the underlying member by its more flaggy bedding, paler weathering colour (result-

The Third Plain Creek Member conformably overlies the the Linns Springs Member. The member is 233 m thick in the type section. It is very similar to the underlying member in lithology but can be distinguished by more irregular bedding and a greater abundance of peloidal, oolitic, sandy and silty lithologies. The member laterally grades southeast into a tongue of oolitic and sandy Wilkawillina Limestone that extends towards the graben centre from the marginal buildup of Wilkawillina Limestone. Parara/Wilkawillina transition The interfingering relationship between the Parara and Wilkawillina Limestones along the southeastern margin of the Bunkers Graben was first recognised by Dalgarno (1964) and then mapped in more detail by Walter (1967). Although not a formal member, the lateral transition between the Wilkawillina and Parara Limestones is characterised by a suite of distinctive lithologies. The Wilkawillina Limestone is composed of skeletal, oolitic and peloidal carbonates with boundstones and lime mudstones. The transition between these lithologies and the black flaggy and nodular carbonates interbedded with shales of the Parara Limestone is characterised by considerable interfingering and many transitional lithologies (Fig. 4).

LITHOFACIES The Parara Limestone is composed of three main carbonate lithofacies interbedded with shales. These are lime mudstones and wackestones, packstones and grainstones, and floatstones and rudstones.

Slope Facies, Parara Limestone

233

S P R I N G S MEMBER

THIRD PLAIN C R E E K MEMBER 801 -

SIX MILE BORE MEMBER 207200

Ijljl I I II

100

-e-

a a

-L

-J-

—

a X

S 3

jzzj

nu.

I i I I I II

1/ V 3 ?

e7SZ3 207

569

Rudstone/floatstone with reworked b l o c k s Intraclast rudstones and f l o a t s t o n e s P a c k s t o n e s or grainstones Wackestones and interbedded siltstone Lime mudstones and interbedded s i l t s t o n e FOSSILS Archaeocyaths Trilobites Hyoliths

-9-

Trace fossils

S k e l e t a l fragments (undifferentiated)

1=2^7 ^7

SEDIMENTARY STRUCTURES s^ft

Irregular bedding

arc

Nodular bedding Channel fill Intraclasts C r o s s bedding Oolites/pisolites Slump structures L o a d structures Graded beds Intraformational truncation surfaces

i=i O 1/ ^

Fig. 3. Stratigraphy of the Parara Limestone in the type section of its members (modified from Clarke 1986c).

Jonathon D.A. Clarke

234

^ ^

Edeowie Limestone METRES

500 I

Oraparinna Shale

Upper Wilkawillina Limestone (platform facies) 1=3

Bunkers S a n d s t o n e

Lower Wilkawillina Limestone Woodendinna Dolomite

Karstic Horizons o o o o o OOP

Parara/Wilk. Trans.

TTTT

Tuff

Fault

Third Plain Creek Member

Track

Linns Springs Member

Park Fence

Six Mile Bore Member

Fig. 4. Outcrop map showing the transition between the platform facies of the Wilkawillina Limestone and the slope facies of the Parara Limestone along the southeastern margin of the Bunkers Graben.

Slope Facies, Parara Limestone

235

The proportions of these three lithologies vary from member to member (Fig. 3). Lime mudstones make up the bulk of all three (over 60%) but are most common in the Linns Springs Member (80%). Coarse grained lithologies (packstones, grainstones, floatstones and rudstones) are most common in the Six Mile Bore Member and least common in the Linns Springs Member. The Third Plain Creek Member contains an intermediate proportion of these lithologies.

loading, nodular cementation and stylolitisation. Where preserved, sedimentary structures include sharp based normally graded beds with coarser grained basal lags, rare water escape structures, ripple cross laminae and (most common of all) parallel laminae. Laminae are often disrupted by synsedimentary microfaulting of soft sediment deformation. Trace fossils are rare, but include simple horizontail trails and sub-vertical branching burrows, tentatively assigned to the Nereites ichnofacies.

All three lithologies have been effected by synsedimentary deformation. Slump folds (Fig. 5a) and intraformational truncation surfaces (Fig. 5b) are common. They are most common in the Six Mile Bore Member and least common in the Linns Springs Member.

The lime mudstones and wackestones tend to coarsen towards the platform sequence and are replaced in the transitional zone by wackestones and packstones. Shallow water derived intraclasts, aggregates and coated grains also become more common towards the platform.

Boulder sized clasts are scattered throughout the Six Mile Bore Member but form only a minor proportion of the total sequence.

Packstones and Grainstones

Lime Mudstones and Wackestones This is the predominant lithology of the Parara Limestone (Fig. 3). It generally comprises 80% of the Parara Limestone. Grains present in this lithology are most commonly quartz silt (particularly in the Third Plain Creek Member), and spicules from calcisponges, demosponges, hexactinellids and Chancelloria. Fragmentary fossils include molluscs, brachiopods, echinoderms and trilobites. Small peloids, derived from disaggregated calcified algae such as Renalcis, are also present. The lime mudstones and wackestones tend to be the darkest coloured of all lithologies present. Fine grained organic matter and pyrite are visible in thin section, often concentrated along laminae. Total organic carbon analyses quoted by Youngs & Moorcroft (1982) showed up to 0.34% TOC present. Bedding varies from flaggy to nodular. Nodules are superimposed on the original depositional fabric through a combination of boudinage,

These lithologies make up about 10% of the thickness of the Parara Limestone in much of the study area except near the platform where they predominate. Packstones are more common than grainstones. The beds occur both as lenticular channel fills and as laterally continuous beds which are up to 2 m thick and extend along strike for several hundred metres (Fig. 5c). The largest can be traced for several km. Some pass laterally into shallow marine platform facies of the Wilkawillina Limestone. Normally graded beds with sharp erosional bases are the most common sedimentary structures. Flute casts are rare. The beds are generally massive but ripple cross laminae are occasionally present in the upper part of some beds. When present, clasts tend to have a sub-parallel orientation. A wide range of grains is present, including fossil fragments, coated grains, quartz sand and silt, intraclasts and peloids. Fossils are most commonly trilobites, echinoderms and spicules from demosponges, calcisponges, hexactinellids and Chancelloria. Less common fossils are archaeocyaths, clumps and clasts of calcified algae, molluscs, hyoliths and brachiopods. Most fossils are severely abraded. Coated grains include both

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Jonathon D.A. Clarke

radial and concentric ooids, cortoidal grains and aggregates. Quartz grains are generally well rounded or sub-rounded and of metamorphic and possibly volcanic provenance. Quartz sand and

silt are most common in the Third Plain Creek Member and least common in the Linns Springs Member. Intraclasts are less common than other grain types. They are composed of a wide range

Fig. 5 (a). Slump fold in the Six Mile Bore Member of the Parara Limestone, near type section (photograph by G. Jenkins), (b) Intraformational truncation surface developed within interbedded lime muds tones and shales, type section, Third Plain Creek Member, (c) Massive grainflow beds of shallow water derived grainstones and packstones (some amalgamated), Linns Springs Member, 2 km east of platform, (d) Outcrop of debris flow with randomly orientated tabular clasts of laminated Girvanella bindstone. Matrix of dolomitic and carbonaceous lime mudstone, (e) Thin section of debris flow showing skeletal grainstone clast in matrix of spicular silty carbonaceous lime mudstone, crossed polars, 15 X 10 mm, (f) Boulder of skeletal packstones (approximately 1.5 m thick) lying parallel to bedding in the Linns Springs Member.

Slope Facies, Parara Limestone

of sediment types. They include algal boundstones and oolitic limestones derived from the Wilkawillina Limestone on the platform, clasts similar to various lithologies within the Parara Limestone and a few clasts reworked from the underlying Wilkawillina Limestone. Peloids are predominantly derived from calcified algae. The grainstones have been cemented by blocky equant calcite, often ferroan. Echinoderm plates generally show syntaxial overgrowths. In some samples the grains have been neomorphosed to ferroan calcite. Minor silicification has also occurred, and is most evident near the platform sequence. Carbonate grain boundaries are often represented by non-sutured seam microstylolites. Rudstones and Floatstones These lithologies generally compose about 10% of the sequence studied. Most are massive beds 10 cm to 2 m thick composed of either randomly orientated or sub-parallel clasts in a lime mudstone or wackestone matrix (Figs 5d, 5e) and in a few instances show crude normal grading. Rudstones and floatstones occur as pods, lenses and laterally continuous beds. The pods depress the underlying beds and are draped by the overlying beds. In some cases evenly bedded Parara lithologies can be traced laterally into convolute bedded intervals and then into rudstones and floatstones.

237

predominating. Hyoliths, echinoderms, archaeocyaths, brachiopods and molluscs may also be present. Quartz sand and silt, ooids and aggregates compose the rest of the grains. Terrigenous grains are most common in the Third Plain Creek Member. The matrix usually fills the spaces between clasts but shelter cavities do occur. Where present they contain coarse calcite, ferroan calcite or ferroan dolomite spar and very rarely they may contain fluorite. Boulders Rare boulders are scattered throughout the Parara Limestone. Some occur as large (up to 1.5 m) clasts within coarse grained beds. Most however are isolated boulders. Boulders may rest parallel to bedding with little or no disruption of the surrounding beds (Fig. 5f) or may be associated with depressed and draped beds, debris tails and grabenward contorted beds. One example (Fig. 6) has penetrated vertically into the underlying bed. The boulders are composed of coarse grained lithologies such as packstones and grainstones or rudstones and floatstones and resemble similar lithologies found elsewhere in the Parara Limestone. Rarely, blocks from the underlying Wilkawillina Limestone (containing FA 2), are found.

PALAEOECOLOGY The clasts are of two types. The first type consists of skeletal, oolitic and peloidal grainstones and algal boundstones lithologically similar to those in the laterally equivalent Wilkawillina Limestone to the south-east. The second comprises a range of dark grey to black grainstones to lime mudstones reworked from within the Parara Limestone. The rudstones and floatstones can be composed entirely of either Wilkawillina or Parara derived clasts or of a mixture of the two lithologies. The grains in the carbonaceous lime mudstone and wackestone matrix are also heterogeneous. The most common are fossil fragments, with sponge spicules

In comparison with the underlying and interfingering Wilkawillina Limestone the Parara Limestone is less fossiliferous. The body fossils that do occur have all been reworked to some extent. Some fossils have been severely fragmented and abraded. Unlike the Wilkawillina Limestone there are no in situ buildups and bioherms. Well preserved archaeocyaths and calcified algae are very rare or absent. Echinoderm plates, Chancelloria and calcisponge spicules, trilobites and brachiopods are more abundant than in laterally equivalent Wilkawillina lithologies. Siliceous sponge spicules (both

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Jonathon D.A. Clarke

hexactinellids and demosponges) appear confined to the Parara Limestone. Archaeocyaths, calcified algae and algal derived peloids are most common in packstones and grainstones and in intraclasts and have most likely been derived from the Wilkawillina Limestone. The remaining fossil types are most common in the coarse grained lithologies but are also present in lesser numbers in the lime mudstones and wackestones. Fossils are therefore most common in the parts of the Parara with the greatest proportion of coarse grained lithologies, these being the Parara/Wilkawillina transition and Six Mile Bore Member. They are least common in the predominantly fine grained Linns Springs Member. As mentioned above, rare trace fossils are present in the Parara Limestone, particularly the Linns Springs and Third Plain Creek Members. The trace fossils take the form of simple horizontal trails and sub-vertical to sub-horizontal simple branching burrows and belong to the Nereites ichnofacies (Frey & Pemberton 1984). The absence of common sedimentary structures in the Six Mile Bore Member and the Parara/Wilkawillina Transition is interpreted to result from exten-

sive bioturbation. In contrast, the Linns Springs and Third Plain Creek Members are relatively undisturbed by bioturbation and consequently structures are common and well preserved. The severity of bioturbation is related with the availablity of dissolved oxygen in the water. The Parara Limestone ecosystem was characterised by low biological diversity compared to the Wilkawillina Limestone. It lacked calcified algae and bioherms and had a sponge dominated fauna. Associated organisms were other filter feeders such as brachiopods and deposit feeding trilobites and molluscs. The echinoderms may have been both filter and deposit feeders. There was extensive input of sediment and biological material from the laterally equivalent Wilkawillina Limestone which would support such a deposit and filter feeding community. The Six Mile Bore Member and Parara/Wilkawillina transition were deposited in water with sufficient dissolved oxygen to support both an abundant soft bodied infauna and a modest skeletal epifauna. The predominantly fine grained Linns Springs Member supported a veiy sparse fauna dominated by siliceous sponges and a limited infauna, which may indicate a very limited amount of oxygen. The Third Plain Creek Member has a slightly more diverse and abundant fauna than the Linns Springs Member, suggesting more dissolved oxygen. In all three cases the environment became reducing below the sediment-water interface allowing the preservation of organic matter, formation of pyrite and ferroan calcite precipitation and neomorphism. Comparison with the model of Byers (1977) suggests deposition in aerobic, anaerobic and dysaerobic environments respectively.

DEPOSITION A L ENVIRONMENT

Fig. 6. Boulder in Third Plain Creek Member orientated vertically to bedding, penetrating underlying sequence and draped by overlying beds.

By integrating lithostratigraphic, outcrop and palaeoecologic data the Parara Limestone at Wilkawillina Gorge is interpreted as a carbonate slope sequence. The Parara Limestone interfingers with, and grades into, a build-up of

Slope Facies, Parara Limestone

Wilkawillina Limestone along the south-eastern margin of the Bunkers Graben. The Wilkawillina Limestone was clearly deposited in a shelf and platform setting (Clarke 1986d), characterised by shelf edge carbonate sand shoals, and small bioherms with lagoonal muddy carbonates and nearshore peritidal sediments. The sequence has been been extensively dolomitised. In contrast, the Parara Limestone lacks any features which indicate a shallow marine setting. The Parara Limestone is composed of interbedded black limestones and shales, with the majority of the limestones composed of graded sequences with sharp basal contacts and gradational upper contacts with the shales. The overwhelming majority of grains within the Parara Limestone have been transported. Most of these can be indentified as being platform derived. The fine carbonate muds are also likely to be of platform derivation due to the lack of Early Cambrian calcareous pelagic organisms. Even off modern platforms, such as the Bahamas, much of the fine grained sediment is derived from shallow water despite input from calcareous plankton (Boardman & Neumann 1984). Such fine grained, deep water, platform derived carbonates have been termed "peri-platform oozes" (Schlanger & James 1978). Graded limestone beds are interpreted as carbonate turbidites and the interbedded black shales as being deposited by hemipelagic fallout forming the Bouma E division (Bouma 1962). The wackestones and lime mudstones are interpreted as representing the D division (Walker, 1984) and representing classes Dl.l, D1.2, D2.1, D2.2 and D2.3 deep water sediments of Pickering et al (1986) and were deposited by low energy and density turbidity currents. Classes Dl.l and D2.1 occur only in the Linns Springs Member, D1.2 in Six Mile and Linns Springs Members and classes D2.2 and D2.3 in the Third Plain Creek Members. Beds with well preserved sedimentary structures resemble the "organised turbidite facies" described by Stow & Piper (1984) from many ancient and modern slope sequences. The fine laminations and scour and fill structures within

239

each bed are interpreted as being produced by internal hydrodynamic variations within single flow events as suggested by Stow & Bowen (1978) from their studies of fine grained, deep water sediments. The coarse graded sequences pass up from grainstones or packstones at the base into wackestones and lime mudstones. They represent the A and B Bouma divisions of coarse grained high density turbidites grading up into C and D divisions deposited by major events transporting material off the platform and down slope. These sediments fall into class Bl, B2 and B2.1 of Pickering etal (1986). The rudstones and floatstones, with their heterogeneous clasts and matrix grain types, are interpreted as being debris flows which have sampled a range of lithologies including platform, upper slope and slope sediments. Many contain a mixture of lithologies. The debris flows are presumed to have originated in the slumping of sediment on an unstable slope. Outcrop transitions between unslumped and slumped sediments are however rare in the Parara Limestone in the study area. Some debris flows fill small channels, these either represent the final stages of a flow that was originally more erosive or flows that travelled down pre-existing channels. Isolated boulders are interpreted as having undergone different types of down slope movement. The boulders orientated parallel with bedding are interpreted as being transported by simple basal sliding. Those having up slope tails and contorted bedding towards the graben centre represent more energetic slides and are generally associated with debris flows. The third (and rarest) type consist of those which have penetrated the underlying sediment. These boulders appear to have moved very rapidly downslope, apparently rolling or bouncing before coming to rest. The boulders contain a wide range of lithologies but they are generally composed of heterolithic rudstones, floatstones and packstones. As such they generally represent cemented and partially cemented debris flows and coarse turbidites that have been reworked down slope.

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Jonathon D.A. Clarke

In addition to the four sediment transport processes mentioned above (low and high density turbidity currents, debris flows and block falls and slides) there is considerable evidence for small scale movement of large sediment masses. On the smallest scale, synsedimentary normal microfaults and sedimentary boudinage of beds has resulted from extensional stresses within a poorly lithified to unlithified sediment mass. These stresses were most likely to have resulted from rapid sedimentation on an unstable slope. On a larger scale, slump folds and intraformational truncation surfaces indicate downslope movement of plastic to semi-rigid sediment masses. The largest intraformational truncation surfaces involve sediment thicknesses of several tens of metres and extend for several hundred metres along strike. Palaeo water depths for the Parara Limestone at this locality are difficult to calculate. The sedimentary association and evidence of post depositional down slope movement suggest the presence of a reasonable slope while palaeoecological data indicate deposition ranging from the aerobic to anaerobic zones. Assuming a slope of 4 degrees, common on modern continental slopes, the centre of the Bunkers Graben could have reached a depth of approximately 300 m. Analysis of outcrop relationships between the overlying Bunkers Sandstones and the Parara and Wilkawillina Limestones suggests a depth of about 280 m. A schematic reconstruction of the major sedimentological and ecological features of the Parara palaeoenvironment is shown in Fig. 7. In more detail, the history of the Parara Limestone at this locality falls into three phases, each represented by the three stratigraphic members. The three stages were controlled by the interaction of graben tectonics, sedimentation rate and sea level. Prior to the first phase, a rise in sea level and increased rate of graben subsidence drowned the shallow shelf and platform sediments of the underlying Wilkawillina Limestone. Only along the southeastern margin of the Bunkers Graben was the relative sea level rise sufficiently slow for

sedimentation to keep pace (Clarke 1986a). Initially a drowned shelf sequence was deposited, forming the Second Plain Creek Member of the Wilkawillina Limestone (Clarke 1986b). The first phase of Parara Limestone deposition occurred when platform derived slope facies prograded across the drowned shelf, resulting in deposition of the Six Mile Bore Member. This was accompanied by relatively common high energy currents transporting sediment down slope and reasonably high slope instabilities resulting in debris flows and block falls. Deposition was within the aerobic zone in this phase. The second stage of deposition, corresponding with the Linns Springs Member, was marked by much quieter conditions and was characterised by low energy and low density turbidites with few high energy events. Slope stabilty was higher, resulting in fewer slump folds and debris flows. With deeper and lower energy conditions the water mass became stratified, with anaerobic conditions predominating. The final phase occurred when a fall in sea level resulted in grabenward progradation of platform facies. At the same time there was an increase in supply of terrigenous sand and silt. The water mass remained stratified and deposition took place in the anaerobic and dysaerobic zones. This resulted in the deposition of the Third Plain Creek Member. Deposition of the Parara Limestone ceased when falling sea level exposed the platform, shutting off the supply of carbonate sediment. Terrigenous sedimentation continued, resulting in deposition of the largely shallow marine, tide-dominated Bunkers Sandstone.

DIAGENESIS In comparison with correlative shelf carbonates like the Wilkawillina Limestone the Parara Limestone has undergone limited diagenesis. The different stages of diagenesis will be discussed in turn using Choquette & Pray's (1970) classification of limestone diagenetic en-

241

Slope Fades, Parara Limestone

vironments. The diagenetic history of representative lithologies from the Parara Limestone is shown in cartoon form in Fig. 8.

Mesogenesis

Eogenesis refers to the sea floor diagenetic environment (James & Choquette 1983). It is difficult to infer what processes have occurred in this environment as the general occlusion of pore spaces by a micritic matrix means that cement generations (if any) cannot be observed. In lithologies without such a matrix (mainly grainstones), the earliest cement seen is a much later stage burial cement. The nodular fabric which cuts across sedimentary features (especially in the Six Mile Bore Member) suggests early nodular cementation, similar to that described by Mullins (1980) from Bahaman slopes. Such cementation would be microcrystalline. Exotic grains (intraclasts, aggregates and fossils) derived from the platform sequence have undergone more extensive sea floor diagenesis. Micritic envelopes, filled moulds and fibrous sea floor cements have been observed in these grains. The most important aspect of sea floor and shallow burial diagenesis has been mechanical compaction of both terrigenous shales and lime muds.

Most of the diagenetic changes observed in the Parara Limestone appear to have occurred during burial. No evidence of shallow burial (meteoric) diagenesis (James & Choquette 1984) has been observed except in platform derived grains. Observable intragranular cements are generally confined to grainstones. Without exception they appear to have been formed during burial in the saline phreatic environment and are characteristically blocky equant calcite. Syntaxial overgrowths of echinoderm plates, and occasionally calcisponge spicules, are common. Much of the cementation of the micritic lithologies is also presumed to have taken place in this environment. Some cements are composed of ferroan calcite, indicating reducing groundwater conditions (Choquette & James 1987). In some cases grains of non-ferroan calcite (such as fossils) have been neomorphosed to ferroan calcite. Larger cavities (such as shelter cavities in rudstones) can be filled with dolomite and ferroan dolomite spar. Veiy rarely fluorite is present. Dolomite is most common as fine grained replacive dolomite, possibly formed through sulphate reduction in a reducing environment (Kelts & MacKenzie 1984).

PARARA LIMESTONE

WILKAWILLINA LIMESTONE

Eogenesis

LOWER SLOPE

BASIN

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Graded Lime Mudstones wackestones

Fig. 7. Schematic diagram of facies pattern and palaeoecological zonation of the Parara Limestone.

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Jonathon D.A. Clarke

Slope Facies, Parara Limestone

243

The Parara Limestone has been patchily silicified. Most affected are fossils and intraclastmatrix contacts. The silica usually replaces part of the grain, disrupting its fabric. The silica is in the form of microcrystalline quartz. Silicification is most common in beds containing abundant siliceous sponge spicules, now devitrified to quartz. Silicification probably occurred as silica was mobilised during devitrification of the spicules, during deep burial. Chemical compaction has been limited to non-sutured seam microstylolitic boundaries round many grains.

widely known Palaeozoic carbonate slope sequences, such as the Cambro-Ordovican Cow Head Group of Newfoundland (Hubert et al 1977; Hiscott & James 1985; Coniglio 1986) as well as other, more recent slope sediments (reviewed by Enos & Moore 1983; Cook & Mullins 1983).

Telogenesis

(i)

The arid climate of the Flinders Ranges has resulted in limited telogenesis. This has been aided by the rubbly nature of the outcrop of the Parara Limestone which has resulted in mechanical erosion out-stripping chemical erosion through solution. Telogenetic effects are limited to the formation of carbonate crusts on the undersides of loose blocks and the development of a calcrete soil profile.

CONCLUSIONS In conclusion, the preserved facies of the Parara Limestone resemble those forming depositional carbonate margins (classification of MacBreath & James 1984). The Parara Limestone has many features in common with more

It is clear that the Parara Limestone at Wilkawillina Gorge represents a typical deep water slope carbonate deposit. The key evidence in this interpretation are the following. The sequence is composed of limestone/shale couplets, showing features indicative of turbidites.

(ii) Frequent debris flows, intraformational truncation surfaces and slump folds, suggest deposition on an unstable slope. (iii) A gradational and interfmgering relationship with the Wilkawillina Limestone to the south-east which represents a high energy carbonate platform. (iv) The presence of many exotic clasts of Wilkawillina Limestone lithologies, often mixed with Parara Limestone clasts, in debris flows. Some exceed several metres in diameter. (v) The absence of in situ evidence of shallow water deposition of the type present in the Wilkawillina Limestone, such as ooid shoals, bioherms, algal limestones, shallow

Fig. 8. Schematic cartoon illustrating diagenetic history of representative lithologies of the Parara Limestone. A, grainstones; 1. original fabric (i, archaeocyath; ii, echinoderm plate; iii, Chancelloria spicule; iv, ooid; v, algally derived peloid; vi, trilobite fragment; vii, quartz sand grain; viii, calcisponge spicule). 2. early burial (i, mechanical compaction of grain fabric; ii, neomorphism of unstable elements such as Chancelloria and hyoliths; iii, siliceous sponge spicule brought into view through compaction. 3. Deep burial - processes overlap with 2. (i, cementation of grains by equant blocky calcite - often ferroan calcite; ii, syntaxial calcite overgrowths around echinoderm plates and calcisponge spicules; iii, devitrification of silica glass in siliceous sponge spicules forming microquartz). B, floatstones; 1. original sedimentary fabric (i, archaeocyath; ii, slope derived lime mud intraclast; iii, quartz sand grain; iv, platform derived lithoclast containing calcified algae and fossil fragments cemented by seafloor and meteoric cements; v, siliceous sponge spicules; vi, mollusc; vii, trilobite; viii, carbonaceous lime mud matrix). 2. shallow burial (i, neomorphism of fossils with unstable -? aragonitic- mineralogies, ii mechanical compaction of lime mud matrix). 3. deep burial- processes overlap with 2 (i, partial silicification of some fossils; ii, formation of styloboundaries to many grains through chemical compaction; iii, partial silicification of some clasts; iv, devitrification of siliceous spicules, a possible source of secondary silica).

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Jonathon D.A. Clarke

marine bioclastic limestones and subaerial disconformities. On the basis of field observations by the author it appears that the slope model can be applied to some (though not all) outcrops of the Parara Limestone in the Flinders Ranges. The slope model is most applicable where thick sequences of Parara Limestone have been deposited.

ACKNOWLEDGEMENTS It would be impossible to thank all the people who assisted in the preparation of this paper. Particular thanks however must go to Chris von der Borch, Pete Di Bona and John Dunster for reviewing and commenting on drafts of the manuscript. Gail Jackson and the drafting department at Kambalda Nickel Operations prepared the figures. I should like to thank Nick and Sande Hay ward, Dom Barrington and Chris Banasik for the use of their various computers and word processing packages. Finally I should like to thank the unknown reviewer who made many essential comments and corrections.

REFERENCES BOARDMAN M. R & NUEMANN A. C. 1984. Source of periplatform carbonates, Northwest Province channel, Bahamas. Journal of Sedimentary Petrology 54,1110-1112.

CHOQUETTE P.W & PRAY L.C. 1970. Geological nomenclature and classifaction of porosity in sedimentary carbonates. American Association of Petroleum Geologists, Bulletin 54, 207-250. CLARKE J.D.A. 1986a. Depositional tectonics of a synsedimentary graben, Wilkawillina Gorge, South Australia. Geological Society of Australia, Abstracts 15, 222. CLARKE J.D.A. 1986b. Stratigraphy of the Second Plain Creek Member of the Wilkawillina Limestone at Wilkawillina Gorge. Quarterley Geological Notes, Geological Survey of South Australia 100, 2-7. CLARKE J.D.A. 1986c. Subdivision of the Early Cambrian Parara Limestone at Wilkawillina Gorge, Flinders Ranges. Quarterley Geological Notes, Geological Survey of South Australia 99, 2- 7. CLARKE J. D. A. 1986d. Early Cambrian platform-slope transition at Wilkawillina Gorge, South Australia. 12 th International Sedimento logical Congress, Abstracts, 61. CONIGLIO M. 1986. Synsedimentary submarine slope failure and tectonic deformation in deep water carbonates, Cow Head Group, western Newfoundland. Canadian Journal of Earth Sciences 23, 476-490. COOK H.E & MULLINS H.T. 1983. Basin Margin. In Scholle P.A., Bebout D.G. & Moore C.H. 1983 eds. Carbonate Depositional Environments. American Association of Petroleum Geologists, Memoir 33, 539-618.

BOUMA A. H. 1962. Sedimentology of Some Flysch Deposits. Elsevier, Amsterdam.

DALGARNO C.R. 1964. Lower Cambrian stratigraphy of the Flinders Ranges. Transactions of the Royal Society of South Australia 88,129-144.

BYERS C. W. 1977. Biofacies patterns in euxinic basins: a general model. In Cook E. H & Enos P. eds. Deep Water Carbonate Environments. Society of Economic Paleontologists and Mineralogists, Special Publication 25, 5-17.

DALGARNO C.R. 1983. Sturtian and Cambrian growth faults, Oraparinna Diapir. Geological Society of Australia, Abstracts 10, 71-72.

CHOQUETTE P. W & JAMES N. P. 1987. Diagenesis of Limestones-3. The deep burial environment. Geoscience Canada 14,3-35

DALGARNO C.R. & JOHNSON I.E. 1968. Diapiric structures and Late Precambrian-Early Cambrian sediments in Flinders Ranges, South Australia. American Association of Petroleum Geologists, Memoir®, 301-314.

Slope Facies, Parara Limestone DAILY B. 1956. The Cambrian in South Australia. In El Sistema Cambrica su plaeogeografia y el problema de su base,Vol. 2, pp. 97-147 . Report of the 20th Inernational Geological Congress, Mexico, 1956. DAILY B. 1976. The Cambrian of the Flinders Ranges. 25th International Geological Congress Excursion Guide, 33A, 15-18. ENOS P. & MOORE C.H. 1983. Fore Reef facies. In Scholle P.A., Bebout D.G. & Moore C.H. 1983 eds. Carbonate Depositional Environments. American Association of Petroleum Geologists, Memoir 33, 507-538. FREY R.W. & PEMBERTON S.G. 1984. Trace fossil facies models. In Walker R.G. 1984 ed. Facies Models. Geoscience Canada, Reprint Series 1, 2nd Edition, pp. 189-207. GRAVESTOCK D.I. 1984. Archaeocyatha from lower parts of the Lower Cambrian carbonate sequence in South Australia. Australasian Association of Palaeontologists Memoir 2. HASLETT P.G. 1969. The Cambrian geology north of Wirrealpa Diapir, Flinders Ranges. University of Adelaide, B.Sc.(Hons) thesis (unpubl.). HISCOTT R.N & JAMES N.P. 1985. Carbonate debris flows, Cow Head Group Western Newfoundland. Journal of Sedimentary Petrology 55,735-745. HUBERT J.K., SUCHECKI R.K. & CALLAHAN R.K.K. 1977. The Cow Head Breccia, Sedimentology of the Cambro-Ordovican continental margin. In Cook H.E. & Enos P. 1977 eds. Deep Water Carbonate Environments. Society of Economic Paleontologists and Mineralogists, Special Publication 25, 125-154.

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KELTS K. & MACKENZIE J.A. 1984. Diagenetic dolomite formation in Quaternary anoxic diatomaceous muds of Deep Sea Drilling Project Leg 64, Gulf of California. International Reports of the Deep Sea Drilling Project 64,553-569. McILREATH I.A. & JAMES N.P. 1984. Carbonate Slopes. In Walker R.G. 1984 ed. Facies Models, Geoscience Canada, Reprint Series 1, 2nd Edition pp. 245-258. MOUNT T.J. 1970. Geology of the Mount Chambers Gorge, Flinders Ranges. University of Adelaide, B.Sc.(Hons) thesis (unpubl.). MULLINS M.T., NEUMANN A.C., WILBER R.J. & BOARDMAN M.R. 1980. Nodular carbonate sediment on Bahama slopes, possible precursors to nodular limestones. Journal of Sedimentary Petrology 54, 17-131. PICKERING K.T., STOW D.A.V., WATSON M.P. & HISCOTT R.N. 1986. Deep water facies, processes and models, a review and classification scheme for modern and ancient sediments. Earth-Science Reviews 23,75-174. ROZANOV A.Y. & SOKOLOV B.S. 1984 eds. Lower Cambrian Stage subdivision. USSR Academy of Sciences, Moscow. SCHLANGER W. & JAMES N.P. 1978. Low magnesium calcite limestones forming at the seafloor, Tongue of the Ocean, Bahamas. Sedimentology 25, 675-702. STOW D.A.V. & BOWEN A.J. 1978. Origin of lamination in deep sea fine grained sediments. Nature 274, 324-328.

JAMES N.P. & CHOQUETTE P.W. 1983. Diagenesis 6. Limestones-the seafloor diagenetic environment. Geoscience Canada 10, 162-179.

STOW D.A. V. & PIPER D.J.W. 1984. Deep-water fine grained sediments; facies models. In Stow D.A.V & Piper D.J.W. 1984. Fine Grained Sediments: Deep-Water Processes and Facies, pp. 611-646. Blackwell Scientific, Oxford.

JAMES N.P. & CHOQUETTE P.W. 1984. Diagenesis 9. Limestones-the meteoric diagenetic environment. Geoscience Canada 11, 161-194.

YOUNGS B.C. & MOORCROFT E. 1982. The petroleum potenial of the eastern Arrowie Basin. APEA Journal 22, 82-101.

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WALKER R.G. 1984. Turbidites and associated coarse grained clastic deposits. In Walker R.G. 1984 ed. Fades Models, pp. 171-188, Geoscience Canada reprint series 1,2nd edition.

WALTER M.R. 1967. Archaeocyatha and biostratigraphy of the Lower Cambrian Hawker Group, South Australia. Journal of the Geological Society of Australia 14,139-152.

Platform carbonate deposition and diagenesis, Woodendinna Dolomite and lower Wilkawillina Limestone (Early Cambrian), Wilkawillina Gorge, South Australia Jonathan D. A. Clarke

School of Earth Sciences, Flinders University, Bedford Park, SA 5042, Australia The Early Cambrian Woodendinna Dolomite and Wilkawillina Limestone at Wilkawillina Gorge were deposited as a transgressive sequence within the synsedimentary Bunkers Graben of the Flinders Ranges. The sediments were deposited in the full range of shallow marine environments, from peritidal to shelf edge bioherm and skeletal sand shoal settings. The sequence was probably deposited in one transgressive cycle. The final phase of sedimentation occurred in deeper water on a shelf drowned by increased tectonic subsidence following subaerial exposure.

The carbonates have undergone a complex diagenetic history. Seafloor cementation was generally extensive, but greatest in biohermal sediments and grainstones. Meteoric dissolution and reprecipitation was generally more limited except below intraformational subaerial erosion surfaces. Deep burial effects include neomorphism, dolomitisation and final occlusion of primary porosity by saddle dolomite and blocky calcite. The present arid climate has led to telogenetic effects being generally confined to surface karst and calcrete formation. Key words: Wilkawillina Limestone, Woodendinna Dolomite, South Australia, Early Cambrian, Platform facies, Reefs, Palaeoecology, Carbonate Diagenesis. INTRODUCTION The Cambrian of the Flinders Ranges comprises the youngest sediments in the Late Precambrian- Cambrian Adelaide Geosy ncline in South Australia. The Adelaide Geosycline is a passive margin sequence with a wide range of sediments from the earliest rift to the post rift stages (von der Borch 1980). The basal Cambrian sediments are dominated by the carbonate-rich Hawker Group (Dalgarno 1964). The Wilkawillina Limestone (Daily 1956) and Woodendinna Dolomite (Haslett 1975) are an important part of the Hawker Group in the Flinders Ranges. However, little detailed work has been published on their sedimentology and diagenesis. This paper describes these formations from the type section of the Wilkawillina Limestone at Wilkawillina Gorge (Fig. 1), where the sequence is very well exposed.

The basal Cambrian carbonates in this area disconformably overlie Adelaidean sediments and comprise the Woodendinna Dolomite at the base, overlain by the Wilkawillina Limestone. The Wilkawillina Limestone at Wilkawillina Gorge is overlain by the Parara Limestone. The sediments at Wilkawillina Gorge were deposited in the diapirically controlled Bunkers Graben. The tectonic control of Early Cambrian sedimentation at Wilkawillina Gorge was discussed by Clarke (1986a). This paper concentrates on the Wilkawillina Limestone and Woodendinna Dolomite as they occur at Wilkawillina Gorge. Regional aspects of these sediments are discussed by Mawson (1939), Daily (1972), Haslett (1986a, b), and Gravestock & James (1986).

^Present address: Western Mining Corporation, Kambalda WA 6442, Australia

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STRATIGRAPHY Lithostratigraphy The lithostratigraphy of the Early Cambrian Hawker Group within the synsedimentary Bunkers Graben is shown in Fig. 1. Originally defined by Daily (1956), the Wilkawillina Limestone was subdivided by Haslett (1975). The basal non-fossiliferous carbonates were renamed Woodendinna Dolomite and Wirrapowie Limestone at the base (only the Woodendinna Dolomite is found at Wilkawillina Gorge), while the Wilkawillina Limestone was restricted to the fossiliferous upper part. The Wilkawillina Limestone was further subdivided by Clarke (1986b,c), who recognised four members at Wilkawillina Gorge (Fig. 2).

In ascending order the members are: Mount Mantell Member:Predominantly oolitic limestones, with some wackestones and sandy limestones. Hideaway Well Member:Composed almost entirely of mottled carbonates and is usually poorly fossiliferous. Winnitinny Creek Member:Composed of interbedded skeletal carbonate sands and bioherms. The top of the member is truncated by a largely subaerial disconformity. Second Plain Creek Member:Comprises an upward fining sequence of grainstones to wackestones.

249

Gorge.The Woodendinna Dolomite is non-fossiliferous. Gravestock (1984) correlated FA 1 and 2 with the Atdabanian Stage on the Siberian Platform.

LITHOLOGY Woodendinna Dolomite The Woodendinna Dolomite at Wilkawillina Gorge is often poorly exposed and extensively replaced by coarse dolomite which has largely destroyed the original fabric. Where original textures are locally preserved, the Woodendinna Dolomite is composed of a range of lithologies including dolomitic, sandy, ooid and peloidal wackestones and packstones; dolomitic, intraclastic floatstones and rudstones and fenestral dolomitic lime mudstones form well bedded and channel-filling carbonates. The dolomite is all veiy fine grained except where altered and replaced by coarse dolomite spar during diagenesis. Sand grains are generally well rounded and composed of quartz with an undulose extinction indicating a metamorphic provenance. The ooids generally show a radial fabric. Intraclasts composed of laminated, dolomitic lime mudstone vary from angular to well rounded and equant to tabular. Sandy, oolitic and intraclastic lithologies are most common in channels. Sediments of similar composition to those forming intraclasts are found in the surrounding well bedded carbonates. Some intraclastic horizons have been recemented by cryptalgal crusts.

Biostratigraphy Daily (1956) recognised twelve faunal assemblages in the Cambrian of South Australia, which have been the basis of subsequent biostratigraphic correlation. Of these, Faunal Assemblages (FA) 1 and 2 occur within the Wilkawillina Limestone at Wilkawillina

Angular lithoclasts of Late Proterozoic Rawnsley Quartzite up to 20 cm in diameter occur in the Woodendinna Dolomite along the southeastern margin of the Bunkers Graben. Cements in the Woodendinna Dolomite are composed of microcrystalline dolomite.

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Jonathon D.A. Clarke

Wilkawillina Limestone Mount Mantell Member The Mount Mantell Member is composed of two main lithologies. The first and lowermost is composed of skeletal wackestones containing intact fossils along with clumps of calcified algae. Small algal clumps and archaeocyaths in growth position occur in this lithology along with some stromatolites. The wackestones are overlain by grainstones and packstones dominated by oolitic components. Sedimentary structures include trough cross bedding, scour and fill structures, and normally graded bedding. The amount of quartz sand is variable with the greatest percentage occurring in packstones associated with abundant skeletal fragments. Aggregate grains and rare intraclasts are also present. Both radial and concentric ooids occur in the Mount Mantell Member. Concentric ooids are most common in the ooid grainstones while radial ooids occur in the lithologies containing aggregates, quartz sand and fossil fragments. Some parts of the oolitic sequence show a bindstone texture suggesting binding by cyanobacteria. These are most common in the upper part of the sequence and are closely associated with oncolites. Minor lithologies occuring near the top of the Mount Mantell Member include oncolitic rudstones and floatstones and red peloidal and pisovadoidal grainstones and packstones. The oncolites are generally small and have formed round nuclei of fossil fragments, including possible corals (Gravestock pers. comm. 1984). The red limestones are laterally equivalent to the oncolitic lithologies. They are interpreted as being of subaerial origin for three reasons, (i) their composition (which is dominated by vadose pisoliths and peloids, (ii) the presence of primary micritic meniscus and drusy calcite cements (indicative of meteoric vadose and phreatic zone cementation) and (iii) the associated dissolution features in the underlying oolitic limestones filled by similar sediments.

Three generations of cement are present within the Mount Mantell Member. The earliest phase of cementation is represented by isopachous fibrous calcite orientated normally to grain surfaces. The second generation is composed of drusy calcite and the third phase by blocky equant calcite. Lithologies with a lime mudstone matrix have been cemented by microcrystalline calcite. Dolomitisation of the member has been limited and confined largely to grain selective replacement and scattered sucrosic dolomitisation. Sporadic stylolite bounded stromatactoid cavities are filled by iron stained quartz and saddle dolomite. Limited calcite veining post dates cementation. The final phase of fracturing has been accompanied by fracture infill by clay-rich iron and manganese oxides.

Hideaway Well Member The Hideaway Well Member is composed of peloidal and skeletal lime mudstones and wackestones with minor packstones. Superimposed on these is a very strong stylonodular fabric. The nodules are composed of limestone within a silty and dolomitic matrix, and have boundaries. The nodular fabric is locally truncated by erosion surfaces indicating that initiation of the fabric occurred syndepositionally. The peloids resemble dissaggregated Renalcis and Epiphyton thalli and

thus resemble the algally derived peloids described by Coniglio & James (1985). Fossils are generally rare. Small thrombolitic bioherms with bored upper surfaces (Fig. 3a) occur locally. The thrombolites are surrounded by intraclastic rudstones and floatstones derived from them. The most common cement in the Hideaway Well Member is microcrystalline calcite. Archaeocyaths adjacent to thrombolites are covered by thin rinds of isopachous cement and some of the bioherm-flanking rudstones have cements of blocky equant calcite. Originally aragonitic fossils (such as corals) have been leached and the moulds filled in by drusy calcite. Minor calcite

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veining again post dates the formation of the nodular fabric.

Winnitinny Creek Member This member is composed predominantly of skeletal grainstones, with interbedded bioherms and lesser proportions of skeletal packstones. Minor lithologies include oncolitic floatstones and fenestral wackestones. The sediments of the Winnitinny Creek Member contain localised fissures infilled by multiple generations of sediment and cement, forming neptunian dykes. These are most common along the southeastern margin of the Bunkers Graben.

The grainstones are planar to trough cross bedded with individual sets approximately 0.5 m thick. Concentrations of white tabular algal intraclasts occur locally, sometimes showing a fanwise imbrication. The bioherms range in size from less than 1 m to 20 m across and 12 m thick. Their dark grey colour contrasts strongly with the surrounding white or pale grey grainstones. Cavities are numerous and are filled by internal sediments and cements. Packstones occur in the uppermost few metres of the member and are locally capped by oncolitic and fenestral sediments. These occur just beneath the calcretised top of the member and were presumably originally more extensive than at present.

Fig. 3. (a) Thin section of thrombolite margin showing boring (A), crossed polars, 15X10 mm. (b) Thin section of grainstone composed of archaeocyath fragments and elongated Girvanella intraclasts cemented by isopachous marine and blocky meteoric cements. Winnitinny Creek Member, crossed polars, 1 5 X 1 0 mm. (c) Clumps of Epiphyton in bioherm with internal sediments (A) and internal cavity (B) filled by drusy (meteoric) calcite and saddle dolomite (dark). Winnitinny Creek Member, crossed polars, 15X10 mm. (d) Invaginated hyolithids from Second Plain Creek Member, plane light, 15 X 10 m.

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Fig. 4. Polished slab from bioherm in Winnitinny Creek Member. Archaeocyaths overgrown by Renalcis. Extensive internal cavities (over half original volume) have been filled in by internal sediments, marine cement and finally by dolomite. Grainstones and packstones are largely composed of fragmentary and intact fossils, and occasional algal intraclasts (Fig. 3b). Some of these have been reworked from the bioherms. The lime mudstone matrix in the packstones is red due to the presence of disseminated microcrystalline iron oxides and has formed geopetal fills within many fossils and sheltered cavities. Bioherms are predominantly composed of archaeocyaths overgrown by a framework of encrusting calcified algae (Fig. 4). The contact between the bioherms and the grainstones is usually obscured by stylolitisation but the bioherms are generally surrounded by sediments rich in fossils and intraclasts which have been derived from the bioherms. Internal sediments are composed of lime mudstones with scattered fossils. Some cavities are floored by crusts of cal-

cified algae. The cavities may exceed 10 cm in width and are filled by internal sediments and fibrous and isopachous calcite cements. The extensive porosity in the grainstones (and to a lesser extent the packstones) and in the bioherms has been filled by four generations of cement. The earliest cement occurs as multiple isopachous rinds of fibrous calcite. This cement has a sweeping extinction. Contemporaneous with the fibrous cements were some botryoidal cements, also of fibrous calcite. Rare syntaxial overgrowths of echinoderm plates are also present. This initial phase was followed by cavity- lining by drusy calcite (rare, except in the oncolitic lithologies) and ultimately by cavity filling first by equant blocky calcite and then by saddle dolomite. Lime muds were cemented by microcrystalline carbonate at an unknown stage.

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Dolomitisation was variable and patchy, with skeletal fragments commonly replaced. Scattered sucrosic dolomitisation of lime mudstones is also common. Complete replacement of the calcite fabric by dolomite is rare. Cementation was followed by calcite veining, late stage fracturing and infill by iron and manganese oxide-rich clays. The neptunian dykes in the Winnitinny Creek Member are most common along the southeastern margin of the Bunkers Graben and decrease in occurrence to the northwest. These fissure fills are distinct from those associated with the calcrete horizon at the top of the member in that they are truncated by the calcrete horizon rather than being developed down from it. The neptunian infills contain skeletal grainstones, packstones and wackestones of fossil fragments containing the FA 2 index fossil Micrina etheridgei (Laurie 1986). This bivalved phosphatic fossil was formerly commonly referred to as "Micromitra". These fissure-fills are very different from the hematitic lime mud, hematitic peloid and lithoclastic breccia fissure-fills associated with the calcrete horizon. The dykes penetrate down through the Winnitinny Creek Member to depths exceeding 20 m and are of varying width. The fissures are often anastomosing and bedding within the blocks dragged up along the margins, indicating fissure formation before sediment lithification was complete. Fissure fills are of skeletal grainstones (often rich in Micrina etheridgei) and cement. Skeletal packstones and wackestones are sometimes present. The muddy sediments are often dolomitised. The earliest cements are composed of multiple rinds of normally orientated fibrous calcite with sweeping extinction. Later cements are of very coarse equant calcite and saddle dolomite, the latter often partially dedolomitised. Relationships between cements and sediments are often complex, indicating multiple phases of fissure opening, infill and reopening. A few of the fissures have a final infill of red peloidal lime mudstone similar to that found in fissures associated with the calcrete horizon. This suggests that some fissures received their final fills under

meteoric conditions when the calcrete and karstic horizon was formed. The calcretised and karstic surface that caps and truncates the top of the Winnitinny Creek Member is characterised by a single, or multiple horizons of red limestone with laminated, brecciated or microstromatolitic fabrics. Vertical fissures filled with similar material are developed down from the horizon. Microscopically the horizon displays a range of features indicating meteoric and subaerial diagenesis, including vadose cements, vadose pisolites, microstromatolitic and laminar crusts, and microkarstic disconformities.

Second Plain Creek Member The sediments in this member are distinctively different from those found in the underlying members of the Wilkawillina Limestone. The lower part of the member is largely composed of skeletal and peloidal packstones and grainstones, with locally developed ripple cross laminae and very strong sparry stylonodular bedding. These grade up-section into interbedded skeletal and peloidal grainstones, packstones, lime mudstones and wackestones, with the latter dominant. Apart from stylonodular bedding, few structures are normally visible. The upper part of the sequence contains numerous intraclastic rudstone beds and the intervening beds contain abundant pull-apart structures and synsedimentary boudinage. The skeletal grains in the Second Plain Creek Member are commonly disaggregated and well fragmented. Most peloids resemble disaggregated fragments of calcified algae. Some ironrich, dolomitic, glauconitic and phosphatic peloids occur in the lower part of the member. These are associated with phosphatic fossil fills, small hardgrounds and neomorphism by ferroan dolomite and calcite. Cements in the Second Plain Creek Member are of two types. The first is an extensively neomorphosed cement, possibly originally

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isopachous and fibrous but now normally microspar. This cement occurs in patches throughout the member. The second cement type is equant blocky spar, which occurs as either a first generation cement, or as a second generation filling in the remaining porosity. This cement forms syntaxial overgrowths on echinoderm plates. Some blocky equant cements are composed of iron-poor calcite while others are composed of ferroan calcite, or zoned with alternating laminae of iron-rich and iron-poor calcite.

laminae. The assemblage is therefore considerably more diverse than that in the Woodendinna Dolomite. The fossils are most common in the lower part of the Mount Mantell Member and are rare in the upper oolitic part where only large robust archaeocyaths are present along with cryptalgally bound ooids indicating the presence of cyanobacteria. The bound ooids occur near the top of the member in close association with oncolites.

Neomorphism is relatively common in the member, with the replacement of some cements by microspar and of some grains by ferroan calcite and dolomite. Hyolithid shells have been replaced by coarse sparry calcite, or more rarely by glauconite and phosphate.

Hideaway Well Member

PALAEOECOLOGY Because of the abundant faunal remains throughout much of the basal carbonates of the Hawker Group, palaeoecology has been an important tool in reconstructing the sedimentary environment. Woodendinna Dolomite The Woodendinna Dolomite does not contain any fossil remains apart from relatively uncommon cryptalgal laminations, stromatolites and possible thrombolites. Wilkawillina Limestone Mount Mantell Member The member contains a range of shelly fossils including growth-position archaeocyaths, hyolithids, brachiopods, calcareous sponge spicules, and possible solitary corals similar to those described from the Middle Cambrian by Jell & Jell (1976). These remains are commonly intact. Also present are clumps of the calcified algae Epiphyton and Renalcis and occasional hemispherical stromatolites with crinkly or wavy

Fossils are less common throughout most of this member. Algally derived peloids are the most common. Scattered calcareous sponge spicules, archaeocyaths and brachiopods also occur. Fossils are common only in and surrounding thrombolitic bioherms. The thrombolites are composed of clotted to laminar cryptalgal fabrics, with locally bored upper surfaces. The borings are very shallow and there is no evidence of the identity of the endolithic organisms. If the identification of the borings is correct, then these are probably the earliest known. Associated with the bioherms are archaeocyaths, brachiopods, possible corals, calcareous sponge and Chancelloria spicules, Renalcis and Epiphyton. The biohermal thrombolites are formed by the intergrown mosaic of these organisms, with the main frame being constructed by the thrombolitic algae and minor contribution from calcified algae. Attached to the bioherm surface were brachiopods, sponges, Chancelloria, and archaeocyaths. Erosion of the thrombolites supplied intraclastic and skeletal debris to the surrounding environment as an apron of talus.

Winnitinny Creek Member There are two main palaeoecological associations in the Winnitinny Creek Member. The first occurs in the skeletal grainstones and packstones and the second occurs in the bioherms. The skeletal grainstones are dominated by both fragmentary and intact archaeocyaths with lesser

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proportions of the enigmatic Micrina etheridgei (Laurie 1986), trilobite fragments and rare echinoderm plates. They also contain local concentrations of Girvanella-rich intraclasts that were probably eroded from nearby bioherms. Packstones contain a similar fauna with the addition of possible molluscs and calcareous sponge spicules. Micrina etheridgei has been found only in the grainstones and packstones and therefore was confined to the interbiohermal environment. The other fossils are presumed to have been derived from either the bioherms or from both the biohermal and interbiohermal environments. The bioherms are composed of a rigid supporting framework that has grown over an archaeocyath substrate. The dominant alga is Renalcis but lesser amounts of Epiphyton and Girvanella occur as intergrowths while Epiphyton also forms small bioherms on its own. Associated with the bioherms are echinoderm plates and calcareous sponge spicules, both presumably from organisms attached to the bioherm surface. Also present are hyolithids and trilobites, both probably motile organisms on the surface and within the cavities of the bioherm. The fauna within the fissures closely resembles that from the grainstones with localised concentrations of phosphatic organisms such as Micrina etheridgei.

Member largely reflects environmental rather than stratigraphic factors. Laterally equivalent units to the south east contain a similar fauna to the Winnitinny Creek Member and were deposited in a similar environment.

DEPOSITIONAL ENVIRONMENTS Woodendinna Dolomite The Woodendinna Dolomite contains many features indicative of peritidal environments, as reviewed by Shinn (1983). The channel fills of intraclastic sandy and oolitic dolomitic wackestones represent low energy tidal channel facies filled by mixed terrigenous, tidal flat and offshore sediments. The cryptalgal and fenestral lithologies that make up most of the Woodendinna Dolomite represent tidal flat environments. The absence of known shelly fossils from the Woodendinna Dolomite, when such organisms are known to have existed elsewhere is a problem. Mount & Signor (1985) pointed out that many very shallow marine sequences of Early Cambrian age lack a shelly fauna, and suggested that shelly organisms had not yet evolved to exploit these environments. Wilkawillina Limestone Mount Mantell Member

Second Plain Creek Member Fossil remains in the Second Plain Creek Member are largely fragmentary and well worn. The most common are algally derived peloids. Trilobites, echinoderm plates, molluscs, calcareous sponge spicules and hyolithids are more common than they are in the underlying Winnitinny Creek Member. Minor components include Chancelloria spicules and various phosphatic problematica such as lapworthellids. Intact and in situ algal structures are absent. The increased abundance of trilobites, hyolithids and sponges in the Second Plain Creek

The Mount Mantell Member contains a relatively abundant and diverse fauna including calcified algae and archaeocyaths, indicating that it was deposited in a subtidal environment. The presence of growth-position archaeocyaths within the skeletal wackestones of the lower part of the member indicates deposition in a very low energy environment. Growth-position archaeocyaths are not known from other parts of the Wilkawillina Limestone, except from within bioherms, due to post-mortem toppling. The presence of the overlying oolitic and underlying peritidal facies suggests that the lower part of the Mount Mantell Member was deposited in a low energy, shallow subtidal, near-shore lagoon.

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Fig. 5. Schematic environmental model, Mount Mantell Member.

Fig. 6. Schematic environmental model, Hideaway Well Member.

Fig. 7. Schematic reconstruction of bioherm from the Winnitinny Creek Member showing principle ecological constituents and relationship to surrounding lithologies.

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The overlying cross-stratified, oolitic lithologies were clearly deposited in predominantly high energy, ooid shoal environments. Well sorted, clean grainstones composed of concentric ooids were deposited in the higher energy regions of the environment. Lithologies containing quartz sand, skeletal debris, radial fabric ooids and aggregate grains forming grainstones and packstones are interpreted as being characteristic of lower energy, shoal environments. Near the top of the member, algally bound, oncolitic and reddened pisovadoid lithologies indicate very shallow to subaerial environments. These are interpreted as being deposited on, and adjacent to, a stable sand cay with the development of a karstic and protocalcrete profile on the exposed areas, or they may relate to a relative fall in sea level. A schematic environmental interpretation of the member is shown in Fig. 5.

Hideaway Well Member Environmental interpretation of the Hideaway Well Member is difficult as the imposition of the secondary stylonodular fabric has destroyed most depositional textures. The predominantly lime mudstone to wackestone lithology indicates a low energy environment, while the absence of abundant fossils may indicate an unfavourable environment for shelly organisms. The syndepositional origin of the mottled fabric, although greatly accentuated by later diagenesis as suggested by (Haslett 1976a), is shown by internal erosion surfaces. Such fabrics are common in carbonates, and have been attributed to burrowing (Enos 1983), early nodular cementation, as by Mullins (1980) or sedimentary boudinage and loading of interbedded sediments with slightly differing rheologies. Locally developed bioherms are the exception to the general paucity of fossils and contain a diverse and abundant flora and fauna. The environment was probably relatively deep, low energy and possibly slightly reducing, resulting in lime mudstones to wackestones with few shelly fossils. Thrombolitic bioherms grew in

more favourable areas. The sediment is interpreted as representing a midshelf facies. A schematic diagram of the environment of the Hideaway Well Member is shown in Fig. 6.

Winnitinny Creek Member The bulk of the member is characterised by interbedded skeletal grainstones and bioherms and is interpreted as being deposited in a high energy, shallow marine environment. The grainstones correspond with the death assemblage of Brasier (1976) while the biohermal lithology corresponds to the life assemblage. The existence of the algal-archaeocyath bioherms in such a shallow and high energy environment indicates that they were rigid and wave resistant structures. Equivalent biohermal facies have been reported from the Cambrian of Newfoundland (Debrenne & James 1981), the Yukon (Read 1980), California and Nevada (Rowland 1984) and elsewhere. The facies present are very similar to those which make up high energy shelf and bank margins as summarised by James & Mountjoy (1983) and Halley et al (1983). The mosaic of organisms forming the bioherms and the relationship between bioherms and surrounding sediments is shown in Fig. 7. The uppermost part of the member is composed of packstones overlain by fenestral and oncolitic sediments. It is interpreted that these were deposited in a lower energy shallower marine to peritidal environment. Upward shallowing of the environment culminated in subaerial exposure and formation of karsted and calcreted surfaces. Neptunian dykes within the Winnitinny Creek Member pose a separate problem. Similar dykes have been reported from the Devonian of Poland (Szulczewski 1973) and elsewhere. These neptunian dykes were interpreted as opening under synsedimentary stress. A similar origin is likely for those in the Winnitinny Creek Member as they are most common along the south-eastern margin of the Bunkers Graben where the most extensive

Platform carbonates, Wilkawillina Gorge

faulting occurred. Similar dykes occur in the Wilkawillina Limestone at Wirrealpa Spring (Haslett 1976a).

Second Plain Creek Member The Second Plain Creek Member consists of a broadly defined upward fining sequence with the base bearing some resemblance to lithologies in the Winnitinny Creek Member. The upper part resembles the Parara Limestone. The lack of in situ algal remains indicates deeper water deposition than was the case for the underlying members. Fragmentation of skeletal remains and invagination of hyolithids (Fig. 3d) suggests extensive reworking. Glauconitic peloids, phosphatic fossil infills, peloids and thin localised hardgrounds in the lower part of the sequence, imply low deposition rates. Together, these factors indicate deposition in a deeper water, lower energy, shelf environment with extensive winnowing and reworking of relict carbonate grains and some lateral input from shallower facies. Low rates of deposition resulted in the formation of authigenic glauconite and phosphate. The percentage of lime mud increases up section, while beds begin to form graded sequences with common pull-apart structures and sedimentary boudinage. These lithologies are interbedded with intraclastic rudstones and floatstones which are interpreted as reperesenting debris flows. The upper part of the Second Plain Creek Member bears some resemblance to the slope facies of the Parara Limestone (Clarke 1986d), and is probably an incipient slope deposit. The overall sequence of the member represents a rapidly drowned carbonate shelf or platform, with a karsted and calcreted disconformity overlain by relict shelf sands, passing upward into incipient slope deposits before burial by the debris flows and fine and coarse grained turbidites of the Parara Limestone.

259

DIAGENESIS Choquette and Pray (1970) divided carbonate diagenesis into three phases; eogenesis (syndepositional and early vadose processes), mesogenesis (burial-related, phreatic diagenesis) and telogenesis (uplift and renewed subaerial and meteoric processes). The three phases of James & Kobluk (1978) were: Phase 1, synsedimentary diagenesis; Phase 2, near surface (meteoric) diagenesis and Phase 3, deep burial diagenesis. These subdivisions are used to classify the diagenetic phases observed in the lower part of the Wilkawillina Limestone. Eogenesis Eogenesis has been extensive in most of the lithologies studied. It has been greatest in sediments that possessed high primary porosities such as grainstones, bioherms and neptunian dykes. Eogenetic features include micritic envelopes round originally aragonitic grains, possible partial micritisation of algal-derived peloids, incipient cementation in peritidal environments to form crusts (subsequently reworked as intraclasts), possible sabkha dolomitisation of peritidal lithologies and precipitation of multiple rinds of fibrous and botryoidal calcite. It is the multiple cement rinds which are best developed in grainstones, bioherms and neptunian dykes. Such rinds were identified by Brasier (1976) as pseudostromatolitic coniatolites formed in the marine vadose zone. These high energy sediments lacked matrix and had high primary porosities. Consequently there would have been extensive pumping of seawater through the sediment due to combined wave, tide and current action (James et al 1979). The presence of erosional surfaces truncating both grains and cements in the Mount Mantell Member, the neptunian dykes and the intercalation of calcified algae, cement rinds and internal sediments in bioherms and dyke-fills of the Winnitinny Creek Member indicate that cementation was contemporaneous with deposition. This renders the coniatolite model described of Brasier (1976) as a possible origin of these

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fabrics unlikely. Marine cements have occluded most of the primary porosity in the Winnitinny Creek Member. In some grainstones from the Second Plain Creek Member seafloor cements are absent and the earliest cements are mesogenetic. This may be the result of low current activity on the drowned shelf and subsequently stagnant pore water conditions, leading to limited or no cementation. These cements comprise phase 1 (synsedimentary) of diagenesis. Eogenetic subaerial exposure occurred at the top of the Mount Mantell and Winnitinny Creek Members. This resulted in the reddening and formation of solution cavities and fissures in the underlying sediments and the formation of laminar and brecciated calcrete fabrics similar to the types described by Read (1976) and James (1972). Aragonitic skeletal components have been leached out and filled by meteoric cements or neomorphosed to calcite spar. Together with the early mesogenetic cements discussed below this completes phase 2 (near surface and meteoric processes) of diagenesis. Mesogenesis Final cementation occurred during mesogenesis, resulting in occlusion of the remaining porosity. Mesogenetic cements formed during three generations: void lining drusy calcite; void filling equant blocky calcite; and a final fill of either equant dolomite spar or saddle dolomite. Drusy calcite cements indicate phreatic cementation by meteoric waters (James & Choquette 1984). The blocky equant calcite cements are interpreted to have formed in either the meteoric phreatic environment or during deep burial with saline groundwater. Equant blocky calcite cements in the Second Plain Creek Member are often composed of ferroan calcite, or of alternately zoned iron-rich and iron-poor calcite. This would indicate precipitation under mildy reducing or alternating reducing and oxidising groundwater chemistry (Choquette & James 1987). Saddle dolomite has been interpreted to

have formed by precipitation by saline reducing phreatic water at temperatures of 50-150°C (Radke & Mathis 1980). Saddle dolomite is best developed in large spaces which remained open until relatively deep burial. These include bioherm cavities and neptunian dykes. The carbonates of the Wilkawillina Limestone and Woodendinna Dolomite have undergone extensive neomorphism and dolomitisation. The bulk of this probably occurred during mesogenesis. Neomorphism involved the recrystallisation of aragonitic components to sparry calcite and the formation of microspar from lime mud and microcrystalline skeletal components (such as archaeocyaths). The Second Plain Creek Member shows apparent neomorphism of micritic peloids to ferroan dolomite and ferroan calcite. Dolomitisation occurred as fabric selective (such as ooids and fossils) and non-fabric selective sucrosic dolomitisation. Dolomitisation of the Wilkawillina Limestone tends to be concentrated into discrete but irregular patches. Neomorphism and dolomitisation of the Wilkawillina Limestone and Woodendinna Dolomite was most intense adjacent to the main bounding faults of the Bunkers Graben. In these localities both formations are composed of sparry dolomite and calcite, with almost total destruction of the orginal texture. There was minor silica mobilisation with growth of scattered euhedral quartz, minor grain replacement by chalcedony and quartz infill of stromatactoid stylolitic cavities. The age of silicification is unknown except for the quartz replacement which predated infill by saddle dolomite which forms the final phase of vug fill. Pressure solution fabrics (mainly stylolites) occur throughout the area but are best developed in the Hideaway Well and Second Plain Creek Members. In these members a stylonodular fabric has obliterated original sedimentary structures, although microfabrics have been preserved. In the Hideaway Well Member the nodular fabric is truncated along erosion surfaces, indicating the presence of a precursor nodular fabric. The nodules are at present bounded by stylolites or

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261

Fig. 9. Evolution of the upper part of the Wilkawillina Limestone (Second Plain Creek Member). 1. Deposition of shelf edge facies (Winnitinny Creek Member). 2. Regression and subaerial exposure. 3. Transgression and fault controlled subsidence drowns shelf. Deposition of Second Plain Creek Member. 4. Relict platforms shed sediments into deeper water, resulting in progradation of Parara Limestone over the Second Plain Creek Member.

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microstylolites which have enhanced the original texture through chemical compaction. Compaction also resulted in partial rotation of nodules with respect to their neighbours. In the Second Plain Creek Member, extensional solution features occur in fold hinges, forming localised sparry fabrics of stylolite-bounded nodules cemented by coarse, isopachous bladed calcite spar. This completes the third phase of diagenesis (deep burial).

biohermal facies. The cycle culminated with a relative fall in sea level and formation of a karsted and calcreted surface. Slope equivalents (Parara Limestone) of these shallow marine platform and shelf lithologies occur at Eregunda Creek (Gravestock pers. comm. 1985), north of Old Wirrealpa Mine (Daily 1976) and probably at Nantawarinna Bore. The lateral facies succession corresponds to the ramp model based on the Persian Gulf described by Wilson & Jordan (1983).

Telogenesis Telogenetic effects are moderate in the study area. Many of the rocks have been fractured, probably through tensional stress related to unloading, with the fractures being infilled by iron and manganese oxides and clays. Calcareous soils are ubiquitous with loose boulders partially coated and cemented into the soil by calcrete rinds. Surface solution sculpture of the carbonates is common, resulting in solution flutes and pits. Small caverns and travertine flows occur locally but there are no large caves. The largest karst feature in the area is Wilkawillina Gorge itself where it cuts through the Wilkawillina Limestone. The absence of large scale karst may be due to the semi-arid climate, intermittent rainfall and the lack of preserved aragonitic components in the sediments.

DISCUSSION Depositional History The sedimentary history of the Woodendinna Dolomite and lower Wilkawillina Limestone is interpreted as a single upward deepening sequence (Fig. 8). The sequence begins with the peri tidal lithologies of the Woodendinna Dolomite and passes vertically into the shallow, low energy, lagoonal sediments of the lower Mount Mantell Member. These are capped by ooid shoal facies and then by the deeper water middle shelf nodular limestones of the Hideaway Well Member. The Winnitinny Creek Member represents the shelf-edge skeletal shoal and

The fact that the succession making up the Woodendinna Dolomite and Wilkawillina Limestone is upward deepening (transgressive) rather than the more common upward shallowing (regressive) cycles (James 1979a), indicates that subsidence was very rapid. The actual shelf margin during FA 1 and 2 times is not exposed at Wilkawillina Gorge. The overall facies pattern indicates that the margin was in the offlap mode (James & Mountjoy 1983), where subsidence exceeded sedimentation rate but not to such an extent that a rimmed or drowned shelf formed (Kendall & Schlanger 1981). The facies pattern resembles that of a shoal-rimmed platform (James 1979b). Following the drowning of the subaerial disconformity at the top of the Winnitinny Creek Member there was a marked change in basin evolution. The Second Plain Creek Member shows a rapid transition from shallow marine sediments with localised condensed sequences at the base, to deeper marine sediments with incipient slope processes at the top. This pattern is characteristic of drowned shelf sediments (Schlanger 1981). The drowning was a combination of a major transgression and an increase in both the rate of regional subsidence through platform break-up and local subsidence of the Bunkers Graben (Clarke 1986a). The drowning event took the shelf below the zone of main carbonate production. Only along the south-eastern margin of the Bunkers Graben was subsidence slow enough to maintain shallow water conditions. In this area there was the build-up of a small platform from which shallow marine car-

Platform carbonates, Wilkawillina Gorge

SKELETAL WACKESTONE brachiopod

263

NODULAR LIME MUDSTONE silty lime mudstone matrix

Epiphyton

mmmmm hyolith

lime mud

lime mudstone (microspar)

V *

r o t a t e d nodules

Fig. 10. Pedogenesis, matrix-supported lithologies. Skeletal wackestone (left); (A), fabric during deposition; (B), fabric following meteoric and burial diagenesis with recrystalisation of aragonite components (hyolithids) and neomorphism of lime mud. Nodular lime mudstone (right) ; (C) fabric shortly after deposition showing protonodules (burrows or sedimentary boudinage). Some protonodules truncated by erosion surface; (D) Meteoric diagenesis results in recrystalisation of aragonite in mollusc; (E) Chemical compaction results in formation of microstylolites and formation of stylonodular fabric.

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OOID GRAINSTONE A.

radial o o i d

concentric ooid

quartz grain

fibrous calcite

SKELETAL GRAINSTONE

PACKSTONE archaeocyath

Uw^lm-W t i M

mollusc

echinoderm

cementation of lime mud

syntaxial overgrowth

drusy calcite

dolomitization

saddle dolomite

c a l c i t e vein

b l o c k y calcite

neomorphic microspar

Fig. 11. Pedogenesis, grain-supported lithologies. Ooid grainstone:(A) original fabric, (B) seafloor fibrous isopachous calcite cement, (C) meteoric and deep burial diagenesis results in occlusion of remaining porosity by blocky calcite and formation of veins (some features shown here also apply to other grainstones). Packstones:(D) original fabric, (E) seafloor cementation of lime mud, (F) deep burial results in neomorphism of lime mudstone and veining. Skeletal grainstone:(G) original fabric, (H) seafloor cementation and micritisation, (I) meteoric cementation by drusy calcite, (J) deep burial diagenesis with occlusion of porosity by blocky calcite and saddle dolomite, some veining (some features may also apply to ooid grainstones).

265

Platform carbonates, Wilkawillina Gorge

ALGAL - ARCHAEOCYATH BIOHERM trilobite

Girvanella

fibrous calcite

archaeocyath Renalcis Girvanella

Epiphyton

internal sediments

•

hyolith lime mud m a t r i x

drusy c a l c i t e

blocky calcite

partial dolomitization

- saddle d o l o m i t e

n e o m o r p h o s e d microspar

authigenic quartz

Fig. 12. Petrogenesis, archaeocyath-algal bioherm. (A) synsedimentary cementation and infill by internal sediments. (B) meteoric exposure results in porosity lined by drusy calcite and replacement of aragonite by calcite. Some components micritised. (C) deep burial results in neomorphism of lime mudstone to microspar and final occlusion of porosity by blocky calcite and saddle dolomite. Partial replacement dolomitisation and formation of authigenic quartz has also occurred.

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bonate sediments were transported into deeper water (Fig. 9). The subaerial disconformity at the top of the Winnitinny Creek Member represents a major sequence boundary between the shelf/platform sediments beneath and the deeper water slope sediments which occur almost everywhere above. The sequence boundary may have originated through relative sea level fall and subsequent erosion being followed by break-up and drowning of the carbonate platform.

Petrogenesis Given a range of initial lithologies and the various diagenetic phases identified, it is possible to reconstruct the petrogenetic history of the different characteristic lithologies of the Wilkawillina Limestone. These are summarised in Figs 10-12. Only the more general eogenetic and mesogenetic processes (phases 1-3) are shown. Extensive dolomitisation or recrystallisation are of local significance only and telogenetic effects visible in thin section are limited to fractures filled by iron and manganese oxides and clays. Three basic lithologies are illustrated: matrixsupported (lime mudstones and wackestones; characteristic of the lower Mount Mantell, Hideaway Well and upper Second Plain Creek Members) in Fig. 10; grain-supported (grainstones and packstones from the upper Mount Mantell, Winnitinny Creek and lower Second Plain Creek Members) in Fig. 11 and bioherms (Hideaway Well and Winnitinny Creek Members) in Fig. 12.

subaerial exposure, leading to the formation of a karsted and calcreted surface. A rapid rise in relative sea level through widespread transgression and an increase in subsidence resulted in the formation of a drowned platform sequence prior to the progradation of slope facies derived from remnant platform areas.

ACKNOWLEDGEMENTS Many people were of help in the preparation of this paper. Chris von der Borch, Greg Jenkins and Pete Di Bona commented on early versions of it while Gail Jackson of Hinders University and Kathy Grow of the draughting department of Kambalda Nickel Operations prepared the figures. I would like to thank Nick and Sande Hay ward, Chris Banasik and Peter Van Hyum for allowing the author free rein on the PC in the preparation of the manuscript. The research was funded from the Flinders University Research Budget. I would like to acknowledge the invaluable comments of Peter Moore (Esso Australia Ltd.) and an unknown reviewer.

REFERENCES BRASIER M. D. 1976. Early Cambrian intergrowths of Archaeocyaths, Renalcis and pseudostromatolites from South Australia. Palaeontology 19, 223-245.

CHOQUETTE P. W. & PRAY L. C. 1970. Geological Nomenclature and Classification of Porosity in Sedimentary Carbonates. American Association of Petroleum Geologists, Bulletin 54,207-250.

CONCLUSIONS The carbonate sediments of the Woodendinna Dolomite and Wilkawillina Limestone at Wilkawillina Gorge are interpreted as forming an upward deepening sequence deposited on a shelf to ramp setting, with a high energy shoal (and bioherm) rimmed margin to ramp succession. The top of the succession has been modified by

CHOQUETTE P. W. & JAMES N. P. 1987. Diagenesis 12. Diagenesis in limestones-3. The deep burial environment. Geoscience Canada 14, 3-35.

CLARKE J. D. A. 1986a. Depositional tectonics of a synsedimentary graben Wilkawillina Gorge, South Australia. Geological Society of Australia, Abstracts 15,222.

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CLARKE J. D. A. 1986b. Subdivision of the lower part of the Wilkawillina Limestone, Eastern Flinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 97, 12-17.

GRAVESTOCK D. 1.1984. Archaeocyaths from lower parts of the Lower Cambrian carbonate sequence in South Australia. Australasian Association of Palaeontologists, Memoir 2,139p.

CLARKE J. D. A. 1986c. Stratigraphy of the Second Plain Creek Member of the Wilkawillina Limestone at Wilkawillina Gorge, Hinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 100, 2-7.

GRAVESTOCK D. I. & JAMES N. P. 1986. Archaeocyathan distribution on the Lower Cambrian carbonate shelf, Hinders Ranges, South Australia. Geological Society of Australia, Abstracts 15, 239.

CLARKE J. D. A. 1986d. Subdivision of the Early Cambrian Parara Limestone at Wilkawillina Gorge, Hinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 99, 2-7.

HALLEY R. B., HARRIS P. M. & HINE A. C. 1983. Bank Margin. In Scholle P. A., Bebout D.A. and Moore C. H. eds. Carbonate Depositional Environments. American Association of Petroleum Geologists, Memoir 33, 463-506.

CONIGLIO M. & JAMES N. P. 1985. Calcified algae as sediment contributors to Early Palaeozoic limestones: evidence from deep water sediments of the Cow Head Group, Western Newfoundland. Journal of Sedimentary Petrology 55, 746-754.

HASLETT P. G. 1975. The Woodendinna Dolomite and Wirrapowie Limestone, -two new Lower Cambrian Formations, Hinders Ranges, South Australia. Transactions of the Royal Society of South Australia 99,211-219.

DAILY B. 1956. The Cambrian in South Australia. In El Sistema Cambrico, su paleogeografia y el problema de su base, Vol. 2, pp. 97-147, Report of the 20th International Geological Congress, Mexico, 1956.

HASLETT P. G. 1976a. Lower Cambrian Carbonate Stratigraphy Old Wirrealpa Spring, Hinders Ranges, South Australia. Ph.D. thesis, University of Adelaide (unpubl.).

DAILY B. 1972. Aspects of Carbonate Sedimentation in the Cambrian of South Australia. Joint Specialist Groups Meeting Canberra, February, 1972, Abstracts. Geological Society of Australia, CIO-14.

HASLETT P. G. 1976b. Lower Cambrian stromatolites from open and sheltered intertidal environments, Wirrealpa, South Australia. In Walter M. R. ed. Stromatolites, Elsevier, Amsterdam, pp.567-584.

DAILY. B. 1976. The Cambrian of the Hinders Ranges. 25th International Geological Congress Excursion Guide 33A, 15-19.

JAMES N. P. 1972. Holocene and Pleistocene calcareous crust (caliche) profiles: criteria for subaerial exposure. Journal of Sedimentary Petrology 42, 817-836.

DALGARNO C. R. 1964. Lower Cambrian Stratigraphy of the Hinders Ranges. Transactions of the Royal Society of South Australia 88, 129-144. DEBRENNE F. & JAMES N. P. 1981. Reef-associated archaeocyathans from the Lower Cambrian of Labrador and Newfoundland. Palaeontology 24, 343-378. ENOS P. 1983. Shelf. In Scholle P. A., Bebout D. G and Moore C. H. eds. Carbonate Depositional Environ ments. American Association of Petroleum Geologists, Memoir 33, 267-286.

JAMES N. P. 1979a. Shallowing upward sequences in carbonates. In Walker R. G. ed. Fades Models. Geoscience Canada, Toronto, Reprint Series 1, pp. 105-108. JAMES N. P. 1979b. Reefs. In Walker R. G. ed. Fades Models. Geoscience Canada, Toronto, Reprint Series l,pp. 121-132. JAMES N. P. & CHOQUETTE P. W. 1984. Diagenesis 9. Limestones-the meteoric diagenetic environment. Geoscience Canada 11, 161-194.

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JAMES N. P., GINSBURG R. N., MARSZALEK D. S. & CHOQUETTE P. W. 1979. Belize fades and fabric specifity of early subsea cements in shallow (British Honduras) reefs. Journal of Sedimentary Petrology 46, 523-614. JAMES N. P. & KOBLUK D. R. 1978. Lower Cambrian patch reefs and associated sediments, southern Labrador, Canada. Sedimentology 25,1-32. JAMES N. P. & MOUNTJOY E. W. 1983. Shelf-slope break sedimentation in fossil carbonate platforms. Society of Economic Paleontologists and Mineralogists, Special Publication 33,189-206. JELL P. A. & JELL J. S. 1976. Early Middle Cambrian corals from western New South Wales. Alcheringa 1, 181-196. KENDALL C. G. St. & SCHLANGER W. 1981. Carbonates and relative rises in sea level. Marine Geology 44, 181-242. LAURIE J. R. 1986. Phosphatic fauna of the Early Cambrian Todd River Dolomite, central Australia. Alcheringa 10,431-454. MAWSON D. 1939. The Cambrian sequence in the Wirrealpa Basin. Transactions of the Royal Society of South Australia 63, 331-347. MOUNT J. F. & SIGNOR P. W. 1985. Early Cambrian innovation in shallow subtidal environments: Palaeoenvironments of Early Cambrian shelly fossils. Geology 1,730-733. MULLINS H. T. 1980. Nodular carbonate sediment on Bahaman slopes: possible precursors to nodular limestones. Journal of Sedimentary Petrology 50, 117-131.

RADKE B. M. & MATHIS R. L. 1980. On the formation and occurrence of saddle dolomite. Journal of Sedimentary Petrology 50, 1149-1168. READ B. G. 1980. Lower Cambrian archaeocyathid buildups, Pelly Mountains, Yukon. Geological Survey of Canada, Paper 78-18. READ R. F. 1976. Calcretes and their distinction from stromatolites. In Walter M. R. ed. Stromatolites, pp.55-71, Elsevier, Amsterdam. ROWLAND S. M. 1984. Were there framework reefs in the Cambrian? Geology 12, 181-183. SCHLANGER W. 1981. The paradox of drowned reefs and carbonate platforms. Geological Society of American, Bulletin 92, 197-211. SHINN E. A. 1983. Tidal Flat. In Scholle P. A., Bebout D. A., and Moore C. H. eds. Carbonate Depositional Environments. American Association of Petroleum Geologists, Memoir 33, 171-210. SZULCZEWSKI M. 1973. Fammenian-Tournasian neptunian dykes and their conodont fauna from Dalania in the Holy Cross Mountains. Acta Geologica Polonica 23,15-59. VON DER BORCH C. C. 1980. Evolution of Late Proterozoic to Early Palaeozoic Adelaide Fold Belt, Australia. Comparison to post Permian rifts and passive margins. Tectonophysics 70,115-134. WILSON J. C. & JORDAN C. 1983. Middle Shelf Environments. In Scholle P. A. Bebout D. G., and Moore C., H. eds. Carbonate Depositional Environments. American Association of Petroleum Geologists, Memoir 33,297-348.

Sedimentary facies in the Sellick Hill Formation, Fleurieu Peninsula, South Australia E.M. Alexander and D.I. Gravestock South Australian Department of Mines & Energy, P.O. Box 151, Eastwood, SA. 5063, Australia Five facies associations are described for the Early Cambrian Sellick Hill Formation. The basal associations, i.e. tabular cross-bedded sands and bioturbated heterolithic shales, silts and sands are in part laterally equivalent. They are interpreted as shallow marine migrating sand ridge and inter-ridge sediments composed of craton-derived siliciclastics reworked under tidal influence. Succeeding facies associations are carbonate-dominated. Ribbon limestone, bioclastic packstone, and isolated bioherms were deposited in deeper water on a mildly unstable ramp attached to a shallow shelf. Periods of non-deposition allowed development of hardgrounds veneered by phosphate and capped by lag conglomerates with concentrations of phosphate-coated skeletal debris. Lithified sediments were reworked by storms as sandy conglomerates, and entrained with soft sediment in debris flows. However, a number of conglomerates with ramp-derived lithoclasts are of uncertain origin.

Key words: Sellick Hill Formation, Normanville Group, Early Cambrian, carbonate ramp, Archaeocyatha, bioherms.

INTRODUCTION

Ninety years ago Howchin (1897) traced massive ' Archaeocyathinae marble' by means of an adjacent flaggy unit with a veiy distinctive serrated weathering pattern. This was subsequently mapped by Abele & McGowran (1959) as Sellick Hill Formation, and the overlying archaeocyathan unit as Fork Tree Limestone. No detailed facies study has been made of the Sellick Hill Formation and firm biostratigraphic correlation was only recently achieved with archaeocyaths from its upper levels (Debrenne & Gravestock, this volume). Tantalising glimpses of facies, fauna and sedimentary fabrics in the Sellick Hill Formation have nevertheless been revealed in the literature. These include bedding continuity and presence of lamination (Howchin 1897), abundance at Myponga Beach of horizontal trails, concentrations of hyoliths and development of arenaceous facies (Madigan 1925, 1926), intraformational breccias (Abele & McGowran 1959), and discovery of domed archaeocyathan mounds at the same locality by Daily (1969).

Detailed mapping between Sellick Hill and Carrickalinga Head by Abele & McGowran (1959) led to the separation of four Lower Cambrian carbonate and mixed carbonate/clastic formations, and these, together with the Mount Terrible Formation (Daily 1963), establish the

K A N M A N T O O GROUP

Q_ D O 0c CD -J —J

Heatherdale Shale Fork Tree Limestone Sellick Hill Formation

o z

Wangkonda Formation Mount Terrible Formation M A R I N O GROUP

Dm P A A

87-772 SADME

Fig. 1 Formations of the Normanville Group on Fleurieu Peninsula.

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Lower Cambrian succession shown in Fig. 1. The formations constitute the Normanville Group (Daily & Milnes 1973) which conformably underlies Kanmantoo Group siliciclastics (Sprigg & Campana 1953). The purpose of this paper is to document sedimentary facies of the Sellick Hill Formation, briefly describe characteristics of the abundant intraformational conglomerates, and show that the vertical facies sequence represents an upward-deepening marine setting consistent with the development of a carbonate ramp in the sense of Ahr (1973). GEOLOGICAL SETTING Madigan (1927) recognised that the Cambrian sequence at Sellick Hill was overturned, but Abele & McGowran (1959) showed

that coastal outcrops flanking Myponga Beach are on the southern limb of a gently plunging anticline with an east-west offshore axis, and that the succession is normally facing. The superb, continuous coastal exposures are the chief sources of our data. Mapped formations are shown on Fig. 2 together with the locations of four stratigraphic sections. A fifth section (not shown) was measured in a road cutting on Sellick Hill. Two new outcrops of Wangkonda Formation have been discovered. One is exposed 2 km east of Myponga Beach (previously mapped as Fork Tree Limestone), the other occurs at the base of section 5. At this locality, and southeast of Carrickalinga Head, the contact with Sellick Hill Formation is sharp but conformable, in contrast to the disconformity found at Sellick Hill (Daily 1963). Thus the whole of the Sellick Hill Formation can be studied on the coast, enabling comparison of facies there with more limited exposures at Sellick Hill itself.

T 17 w

Port

I KILOMETRES

Carrickalinga Head Formation

Measured section

Heatherdale Shale

S t e e p reverse, thrust f a u l t

Fork Tree Limestone S e l l i c k Hill F o r m a t i o n Wangkonda Formation

Normal, ? w r e n c h fault

—"" —

A n t i c l i n e , p l u n g e of axis

I *8

S y n c l i n e , p l u n g e of axis

* f

Monocline

S t r i k e a n d d i p of b e d d i n g

^25

S t r i k e a n d d i p of overturned bedding

*8o 87-768

Fig. 2 Geological map showing locations of measured sections 2 to 5.

SADME

Sellick Hill Formation

FACIES DESCRIPTION The Sellick Hill Formation can be readily divided into five facies associations designated A to E in ascending order as shown in Fig. 3. Each association comprises a set of two or more facies identified by their lithology, sedimentary and biogenic structures, and in some cases by surfaces of non-deposition, and syn-sedimentary disruption. The basal facies association A sharply overlies the Wangkonda Formation in section 5, but the contact is submerged in sections 2 to 4. However in section 1 at Sellick Hill, association B disconformably overlies the Wangkonda Formation (Daily 1963) suggesting that facies association A is lenticular and intertongues with the lower part of association B. In contrast, the remaining facies associations show no evidence of intertonguing.

271

(terminology of Harms et al 1982) which are typically 3 m in wavelength and 0.5 m in amplitude. Horizontal lamination. Fine-medium grained sandstones at the base of facies association A in section 5 show horizontal to wavy lamination in beds 2-5 cm thick. Elsewhere basal units have not been seen but in section 5 this facies sharply overlies ooid grainstone of the Wangkonda Formation. Ripple cross lamination. Fine-medium diagenetically mottled and differentially weathered calcareous sandstones are generally ripple cross-laminated. In less weathered outcrops herringbone ripples and lenticular crosssets can be distinguished (Fig. 4c).

Facies Association A (Sand ridges)

Interpretation

A complete section has been measured only in section 5 (Fig. 3) where a thickness of 41 m was recorded. It is dominated by coarse arkosic sandstone with minor, thin micaceous red-brown muddy siltstone. Clasts of fine sand to granule size quartz and feldspar (orthoclase, microcline) are subangular to well-rounded and cemented by buff dolomite. A significant calcarenite component (up to 30%) is indicated in some beds by ooids, peloids and rare shelly fragments, while dolomitic mud clasts are scattered throughout. Bioturbation is limited to sinuous trails on some surfaces, and small vertical burrows in scarce thin .silty interbeds. The following sedimentary structures have been observed.

Interpretation of sand body geometry is difficult on the rugged coastline. Sand bodies occur on sections 2 and 5 with a third between sections 2 and 3. Palaeocurrent measurements from facies associations A and B (Fig. 5), show a broadly east-west bimodal distribution, which with herringbone cross-stratification, ooids and fragmentary fossils suggest a tidal influence. Subangular quartz and very fresh feldspar indicate a nearby fluvial source with an additional input of well rounded grains from a beach environment.

Tabular cross beds. Outcrops are characterised by planar tabular cross-beds, 0.2 to 3.5 m thick (Fig. 4a, b) containing the coarsest sands and even small pebbles. Small scale tabular crosssets are bounded by truncation surfaces, show rare bipolar cross-bedding and are often overlain by fine-medium grained cross-laminated sandstones. Cosets of 3-4 tabular cross-sets have lenticular bedforms with well-exposed undulating form sets of large two-dimensional ripples

The thickest bedforms deposited as a single set imply water depths of 3 m or more, suggesting tidal and rip currents may have transported and rapidly deposited the sand into deeper water. Truncation surfaces bounding tabular cross-sets are the only preserved erosional features. The lack of obvious scours or channels and abundance of scattered rather than concentrated clasts tends to eliminate subtidal channel migration unless channel width is much larger than observed outcrop (approx. 50 m). The paucity of bioturbation compared with facies association B suggests a shifting sand body composed of migrating large two-dimensional ripples. These formed a sand

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Fig. 3 Detail of measured sections hung on base of Fork Tree Limestone. Grain size scales refer only to siliciclastics of facies associations A and B.

Sellick Hill Formation

ridge system moving over lithified Wangkonda Formation under the influence of tidal currents. Hein (1987) developed such a model to account

273

for facies in the Lower Cambrian Gog Group of Canada that are broadly similar to facies associations A and B.

Fig. 4 Facies association A (Sand ridges), (a) Planar-tabular cross beds with opposing palaeocurrent directions. Location 20 m southwest of section 5, pen scale 10 cm. (b) Weathered carbonate-rich planar-tabular cross beds. Location section 5, divisions on staff 10 cm. (c) Herringbone ripple cross-lamination with scattered granules and small pebbles. Location 80 m southwest of section 5, pen scale 10 cm. Facies association B (Inter-ridge sediments), (d) Isolated sand cross-set underlain and overlain by thinly interbedded bioturbated sand and silt. Location 50 m southwest of section 5, hammer 27 cm long, (e) Interlaminated sand, silt and mud with sand filled horizontal burrows. Location on section 2,0.2 m above base. Plane polarized light, field of view 21 mm. (f) Wavy, lenticular, bioturbated thin interbeds of silt and fine-coarse sand. Location on section 1, pen scale 7 cm.

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E.M. Alexander & D.I. Gravestock Cross-bedded sandstone. Tabular crossbedded coarse grained sandstone forms a minor component of association B. Beds are lenticular to continuous over tens of metres, have sharp bases and tops (Fig. 4d) and are typically less than 0.4 m thick. In some instances cross-beds interfinger with and overlie conglomerate bands.

TN n=30

Prn P A A

87-771

SADME

Fig. 5 Palaeocurrent rose, 30 measurements from facies associations A and B.

Facies Association B (Inter-ridge clastics) This association is present in all measured sections reaching a thickness of 46 m in section 1 (Fig. 3) where it overlies fenestral and oolitic Wangkonda Formation with an irregular contact. On the coast, association B sharply overlies A, and in contrast is quite heterolithic with thinly interbedded calcareous sand, silt and shale. Mineralogically both A and B sandstones are similar, the latter facies association being distinguished on the basis of its generally finer grain size, sedimentary and biogenic fabrics, phosphate surfaces and intraformational conglomerates.

Interbedded shale, silt and sand. Micaceous shale, siltstone and fine to medium grained sandstone, interlaminated to finely interbedded (Fig. 4e) form the dominant rock type. Abundant phosphate coated fossil fragments (Hyolithes, Tannuella, Chancelloriapeloids and ooids occur in sandy interbeds. Carroll (1982) determined ooid composition to be calcite and calcium phosphate. Sand stringers only a few grains thick are very common while thicker sands display herringbone ripple cross-lamination and tabular cross-sets on 1-5 cm scale. Bedding-plane exposures of interference ripples are best seen at the coast (Fig. 6f). As carbonate becomes more abundant in the laminated argillaceous and silty interbeds, diagenetic nodules and ribbon limestone are evident (Fig. 6c). Intraformational slumping is observed in coastal outcrops.

Intraformational conglomerate. This is a rare component of association B, the best example outcropping 20 m southwest of section 5. This example has a sharp flat base with no obvious basal scour. Clasts are stacked subhorizontally up to 0.1-0.2 m thick extending laterally several tens of metres and interfingering with cross-bedded arkosic sandstone. The same sand, fine to coarse grained, granule rich and carbonate cemented, forms the matrix in the clast-supported fabric. Clasts are ovoid to tabular (Fig. 6a, b) and consist of intraformational ooid-peloid calcarenite, laminated calcarenite/siltite and lithified lime mud. Phosphate surfaces and stains. An irregular phosphate surface caps the Wangkonda Formation at section 1 where several such surfaces recur in association B (Carroll 1982) (Fig. 6e). On the coast, irregular multiple surfaces coat crossbedded sandstone (Fig. 6d). Amalgamated surfaces are common, forming irregular anastomosing seams in cross section. Sometimes phosphate-rich dark stains rather than distinct surfaces are seen. Almost invariably, surfaces are overlain by astronomical numbers of phosphatecoated hyoliths. Biogenic structures. Prolific horizontal trails and burrows, and less common vertical burrows have disturbed the interbedded shales, silts and sands, but not to the extent that bedding has been destroyed. As noted by Madigan (1926) burrows and trails are filled with fine to coarse sand (Fig. 4e) and are best seen on bedding surfaces at the coast. Jago et al (1986) have listed the ichnogenera Diplichnites, Treptichnus, Planolites and Laevicyclus, but as these have not been formally described from the Sellick Hill Formation, we prefer to figure the most spectacular examples

Sellick Hill Formation

275

Fig. 6 Facies association B (Inter-ridge sediments), (a) Conglomerate with horizontal ovoid-tabular clasts and coarse sand-granule matrix with minor spar filled voids. Location 250 m southwest of section 5, pen scale 10 cm. (b) Thin section of above conglomerate showing variety of calcareous clast lithologies (calcisiltiteiquartz sand± lime mud) and poorly sorted quartz-rich matrix. Plane polarized light, field of view 32 mm across, (c) Thin limestone ribbons and nodules in laminated calcareous silt with sand stringers. Location on section 1, pen scale 7 cm. (d) Cross-bedded coarse sandstone irregularly scoured, veneered with phosphate and overlain by phosphate-coated hyoliths. Hyolithid debris inserted into wedge-shaped openings between some cross-sets. Location on section 5, pen scale 7 cm. (e) Phosphatised surface just above pen cap over bioturbated coarse-granule sandstone. Surface abruptly overlain by laminated silt with thinly interbedded sand. Location on section 1, pen scale 7 cm. (f) Polygonal interference ripples 100 m southwest of section 3, hammer 27 cm long.

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Fig. 7 Facies association B (Trace fossils), (a) Solitary U-shaped burrow inclined to bedding. Location fallen block approx. 600 m west of section 2, hammer scale 27 cm. (b) Straight to slightly curved horizontal burrows. Location on section 5 approx. 50 m above base, (c) Lobate horizontal burrow. Location fallen block approx. 50 m northwest of section 4, pen scale 7 cm. (d) Inclined burrows preferentially oriented on stoss sides of ripples. Location fallen block 2 km northeast of section 2, pen scale 10 cm. Facies association C (Carbonate slope), (e) Pseudobedding giving serrated outcrop pattern. Locality adjacent to old Sellick Hill Quarry, 2.5 km northeast of sectionl, coin 25 mm diameter, (f) Deformed soft and semi-lithified argillaceous and ribbon limestone in discordant zone striking obliquely into slump roll (Fig. 10b). Location 17 m above base of section 3, pen scale 15 cm. (g) Discordant zone approx. 1 m thick between undisturbed ribbon limestone. Location 200 m southwest of section 2.

Sellick Hill Formation

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(Fig. 7a-d) without formal taxonomic assignment.

carbonates (Mcllreath & James 1984). Compaction is particularly striking in nodules 'strung out' like beads in more argillaceous units.

Interpretation

Truncation surfaces, intraformational slumps and synsedimentary bedding distortion are frequent in association C. Truncation surfaces, some of which can be traced several hundred metres, are revealed by dip discordances bounded above and below by undisturbed sediments (Fig. 7g) while partly lithified carbonate beds and argillaceous limestone are buckled within discordant zones (Fig. 7f). On the coast approximately 0.5 km east of Myponga Beach, a lithified slab has been squeezed up, puncturing and dooming the overlying ribbon limestone (Fig. 8a). Intraformational slumping has resulted in severe deformation of entrained material which displays a range of brittle, 'firm' and soft-sediment deformation. In section 3 one such slump 3 m thick (Fig. 8b) has cut down into underlying ribbon limestone resulting in a pillow-like undersurface with up to 0.5 m of relief. Depending on location, underlying sediments were either lithified (laminae sharply truncated) or unlithified (sediments squeezed and contorted) at the slump interface.

Though it appears certain that facies association A and B are in part laterally equivalent, crucial outcrops are rock-covered or submerged. Thin stringers and cross-beds of sandstone persist as a minor component of B, indicating some input from the shallow, tidally influenced sand ridges of association A. The far greater proportion of bioturbated silt and mud in association B implies an environment in the inter-ridge zone influenced by current activity and crossed only rarely by small migrating ripples. Phosphate surfaces, stains and coated fossil debris suggest periods of reduced sedimentation. Rare storms are likely to have been the cause of the sandy intraformational conglomerates. Facies Association C (Carbonate slope) Association C is present over the exposed strike length of Sellick Hill Formation, reaching 90 m thickness in section 1. Association B, having become more calcareous in its upper part, grades into C through a short passage of 2-3 m. Sand stringers and cross-beds are absent and the lithology is dominated by calcareous silt, with generally only minor proportions of quartzose silt to very fine sand. Principal features are given below. Ribbon and nodular limestone. The most striking aspect of this facies is thin 2-5 cm, less often 5-10 cm pseudobedding, resulting from diagenetic alteration and differential weathering (Abele & McGowran 1959) (Fig. 7e). Calcareous silt, minor quartz silt, very fine sand, and lime mud are interlaminated. Planar, wavy and ripple cross-lamination are present as well as occasional sinuous and 'feather stitch' f mis. Though abundant in some beds, trace fos. Is tend to be rarer than in association B. Early lithification is evident, while ribbon, 'pinch and swell' and nodular limestones result from submarine cementation, downslope creep and compaction typical of slope

Intraformational conglomerates. The most puzzling sedimentary structures in association C are the very frequent conglomerate beds, typically 0.1 to 0.2 m thick, which can either be traced continuously for several hundred metres, or which disappear abruptly over a few tens of metres. Clasts consist of quartzose and peloidal calcarenite and laminated calcisiltite with varying proportions of fine quartz sand. Matrix compositions vary between conglomerate beds, and include calcarenite, calcisiltite, scattered quartz sand, lime mud and bioclastic debris. The fabric is clast-supported, clasts are tabular to platy, subequant up to 15 cm across, arranged from subhorizontal to vertical often with fanwise arrangement (Fig. 8c, d). Vertically oriented clasts or 'fans' exhibit a crude periodicity along strike, repeating every metre or so among otherwise more or less horizontal clasts. On bedding surfaces, clasts protruding vertically from each 'fan' either form short sinuous trains or crudely

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Fig. 8 Facies association C (Carbonate slope), (a) Ribbon limestone punctured and domed by upthrust more competent sheet of underlying material with same lithology. Location 700 m northeast of Myponga Beach, hammer 27 cm long, (b) Slump roll 3 m thick, scoured into variably lithified ribbon limestone. Location 17.3 m above base of section 3, hammer 27 cm long, (c) Conglomerate with amalgamated 'fans' of stacked platy clasts. Location 1 km northeast of Myponga Beach, hammer 27 cm long, (d) 'Fanwise' stacking of tabular clasts in conglomerate bed of even thickness. Location 300 m southwest of section 2, hammer 27 cm long, (e) Bedding surface showing top edges of vertically stacked clasts. Location 400 m southwest of section 2, hammer 27 cm long, (f) Small pebbles stacked vertically around periphery of large horizontal clast. Location approx. 300 m southwest of section 2, hammer 27 cm long.

Sellick Hill Formation

279

Fig. 9 Facies association C (Carbonate slope), (a) Deformed sheet-clasts at terminal nose of conglomerate. Location 23 m above base of section 3, pen scale 16 cm. (b) Clasts fanning to horizontal at conglomerate termination. Location 100 m southwest of section 4, hammer 27 cm long, (c) Chevron-stacked sheet clasts in fallen block, 800 m southwest of section 5, pen scale 7 cm. (d) Rubbly lag conglomerate on submarine hardground surface. Location 44.6 m above base of section 2, pen scale 10 cm. (e) Isolated crest of stacked tabular clast conglomerate. Location 1 km northeast of Myponga Beach, hammer scale 27 cm. (f) Smoothly sculpted hardground surface overlain by muddy hyolithic packstone and well-rounded limestone pebbles. Location as for D, field of view 32 mm across.

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radiate from localised centres (Fig. 8e) comparable with those shown by Sepkoski (1982). Pebbles also stack vertically around larger horizontal clasts (Fig. 8f) in the same manner as described by Sanderson & Donovan (1974) for scallop shells and slates on recent beaches. Rapid thickness changes are expressed as isolated (Fig. 9e), periodic, or even laterally amalgamated 'crests' up to 0.4 m thick. These contain multiply stacked clast sheets with fanwise arrangement. In extreme cases sub vertical 'chevron packing' occurs (Fig. 9c). Conglomerate beds appear to have originally had flat bases beneath crests, but compaction has commonly reduced crest amplitude and increased downward relief into underlying ribbon limestones (Fig. 8c). Between adjacent crests (where not laterally amalgamated) clasts are packed subhorizontally in beds as thin as 5 cm, but vertical or inclined packing is sometimes seen. Conglomerate beds often terminate abruptly, some resembling the 'snouts' of Hiscott & James (1985) at the leading edges of debris flows (Fig. 9a). More often the clasts simply fan to horizontal and are abutted by ribbon limestone identical to the enclosing facies (Fig. 9b). Phosphate hardgrounds. These occur veneering smoothly eroded or smooth-sided 'pit and pinnacle' surfaces sculpted into the underlying lithified sediment with a few cms relief (Fig. 9f). Lags of veneered pebbles and concentrations of phosphate coated hyoliths (Fig. 9d) overlie the phosphate skins, one such layer being prominent at the boundary between associations B and C at Myponga Beach.

Interpretation Facies association C is characterised by dominance of fine-grained carbonates over siliciclastics, deposited as calcareous silt and mud in planar, wavy or ripple laminae. Intraformational slumps and discordant surfaces are clear evidence of instability typical of carbonate slopes

(e.g. Wilson 1969; James & Mountjoy 1983). However, the slumps are not spectacular in size as in settings adjacent to platform margins (e.g. Cow Head Group, Coniglio 1986), nor does the entrained debris include clasts of shallow water origin as found for instance in carbonate slope facies adjacent to the Early Cambrian platform in the Flinders Ranges (Mount 1970; Clarke 1986a, b). Association C conglomerates are locally derived but their peculiar stacking geometry is not known in slumps and debris flows which commonly are crudely stratified or are chaotic (e.g. Krause & Oldershaw 1979). Some characteristics are more typical of shallow marine tempestites (Markello & Read 1981; Sepkoski 1982; Park et al 1986; Meng et al 1986). They may be a unique type of debris flow and they warrant further study. Episodes of non-deposition are shown by irregular surfaces veneered with phosphate, lag conglomerates and coated fossil debris. The surfaces are clearly hardgrounds although there is no evidence of boring activity or of substrate colonisation by benthic organisms. We propose that association C represents a sedimentary sequence deposited at a moderate depth below fair weather wave base on a gentle slope. However, this proposition is best explained in the context of the entire facies sequence, together with the overlying and laterally equivalent units described in a later section. Facies Association D (Bioherm constructive/destructive complex) Though no more than 2.8 m thick, association D is remarkably persistent, cropping out from the old Sellick Hill quarry northeast of section 1 to the last coastal exposures southwest of Myponga Beach, a distance of almost 12 km. The thin sequence records a complex history of constructive and destructive processes, often on centimetre scale. Distilled into its most important components, association D comprises in ascending order: (i)

establishment of bioherms over laminated calcisiltites of association C;

Sellick Hill Formation

281

(ii) extensive destruction of bioherms and erosion of the lithified substrate (Fig. 10a);

glauconite locally imparting a greenish hue to the pale grey outcrops.

(iii) development of phosphatic surfaces during episodes of reduced sedimentation (Fig. 10b);

Interpretation

(iv) deposition of planar to cross-bedded bioclastic packstone (Fig. lOd). Bioherms. Only remnants several decimetres thick remain of bioherms constructed by regular archaeocyaths, thickets of Epiphyton and crusts of Girvanella (Fig. 10c). Hexactinellid sponges dwelt in niches between archaeocyaths. Centimetre-size cavitities floored with geopetal lime mud and micritic microbial debris, are roofed by pendant Epiphyton and encrusting Girvanella surrounded by turbid calcite cement, while final cavity infill is blocky calcite. Preserved interbiohermal debris includes small blocks of lithified framestone (Fig. lOe) and fossil-rich muddy wackestone overgrown by thickets of Epiphyton with loose Girvanella rafts. Phosphate surfaces. The first of these veneers the erosion surface cut irregularly into association C. Unlike earlier examples, this is not covered with concentrated hyolithid debris, although undercut niches do contain hyolith and mollusc fragments. Instead there are either reworked biohermal blocks or quartz sandy bioclastic wackestones studded with Epiphyton shrubs (Fig. 10a). Subsequent phosphate surfaces are multiply developed, often anastomosing in fossiliferous, increasingly phosphatic lime mud. Some burrowing of these surfaces is evident (Fig. 10b). Bioclastic packstone. The bulk of association D consists of planar to cross-bedded bioclastic packstone composed principally of small and fragmentary regular archaeocyaths, hyoliths, chancelloriides, spicules, gastropods and tommotiids (Fig. 10c). Rare trilobite fragments, reworked Epiphyton, phosphatic ooids, glauconite and detrital quartz also occur, the

Unlike the shallow water, high energy archaeocyathan-calcified microbial buildups of the Flinders Ranges (James & Gravestock 1986), the preserved bioherm remnants contain no large cavities rimmed with multiple submarine cements, yet the deeper water, low energy archaeocyath-sponge mounds of the Flinders Ranges also differ in some crucial aspects. Those in association D, as well as complete bioherms in association E, and coalescing bioherm complexes in the overlying Fork Tree Limestone, all lack stromatactis, are composed of framestone rather than floatstone and were dominated by few species of regular archaeocyaths (Debrenne & Gravestock this volume) which bound the framework with exothecal tissue. Such binding was not in response to high energy conditions since both biohermal and interbiohermal sediments are mud-dominated. Preservation of sponge spicule frameworks also points to low energy conditions. Bioherms were lithified as witnessed by blocks which have sloughed gently into the surrounding muddy Epiphyton thickets. However, the abundance of mud and absence of thick cements appear to have rendered the buildups vulnerable to destruction by rare, high energy events. Such an event, or series of events took place, almost obliterating the lowest bioherms and redepositing the most resistant biohermal constituents (archaeocyaths) as well as interbiohermal material, as bioclastic packstones. How frequent the alternation from low-energy bioherm construction to high-energy destruction had been, is unknown, but the processes were most likely sporadic, with intervals of reduced deposition indicated by multiple phosphate surfaces. We see no need to invoke a return to shallow subtidal conditions to account for association D as explained below.

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Facies Association E (Isolated bioherms) Association E which is up to 84 m thick, consists of isolated bioherms surrounded and draped by argillaceous and nodular ribbon limestone. The base of the conformably overlying

Fork Tree Limestone is marked by disappearance of the ribbon limestone facies and apparent coalescence of bioherms into complex buildups. The isolated bioherms and complex buildups have the same components as the bioherms in association D. Lack of observed bioherms in sec-

Fig. 10 Facies association D (Bioherm constructive/destructive complex). Sedimentary facing to top of page, (a) Eroded lithified laminated calcareous silt capped with phosphatised layer. Eroded hollows filled with skeletal packstone; erosion surface overlain by Epiphyton wackestone with detrital quartz. Location 37.7 m above base of section 4, field of view 30 mm high, (b) Anastomosing phosphatised surfaces in muddy skeletal wackestone with basal packstone layers on each surface. Surfaces overlain by quartzose skeletal wackestone with small fallen lithified bioherm blocks; same thin section and scale as A. (c) Archaeocyaths - Epiphyton, encrusting Girvanella, and floored with muddy calcified microbial debris. Framework filled with lime mud rich in sponge spicules. Location 59 m above of section 3,fieldof view 31 mm high, (d) Bioclastic packstone of bioherm and interbioherm debris. Location 150 m above base of section 1,fieldof view 20 mm across, (e) Small lithified block from bioherm (Girvanella and Epiphyton enclosing cavity with geopetal and submarine cement fill) fallen into quartzose muddy skeletal wackestone; same thin section and scale as A.

Sellick Hill Formation

tions 1 and 2 (Fig. 3) is a function of poor or restricted exposure since they occur along strike from both measured section lines. Ribbon limestone. The complete absence of conglomerates (other than debris aprons flanking bioherms) and scarcity of intraformational slumps distinguishes ribbon limestones in association E from those stratigraphically below. In other respects - planar and ripple lamination, presence of sinuous trails, diagenetic nodules the facies are much alike. Phosphatised surfaces are absent, however. Bioherms. Principal constituents of the bioherms are described above, but their geometry is best seen in E due to lack of erosion. Buildups are plano-convex, ranging from 0.5 to 2 m in thickness and 0.5 to 4 m in extent. Small debris aprons flanking bioherms usually wedge out within a few metres. The bioherms are interbedded with argillaceous ribbon limestones, with drape of overlying beds enhanced by compaction.

Interpretation Entombment of bioherms in lime mud and silt displaying only ripple lamination indicates low current energy. A stable substrate with minimal evidence of downslope movement coupled with the lack of high energy events and continuity of sedimentation are key factors ensuring colonisation and subsequent domination (in the lower Fork Tree Limestone) of frame-building organisms.

DEPOSITIONAL MODEL Facies sequence The vertical facies sequence in the Sellick Hill Formation is best viewed in context with other formations of the Normanville Group. Although detailed studies of these units have yet to be published, the following points are known.

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Wangkonda Formation. The 100 m thick Wangkonda Formation contains siliciclastics and carbonates deposited in a supratidal to shallow subtidal complex. Facies include oolites, oncolites, fenestral limestones and thrombolites. Sandy units are extensively bioturbated with locally abundant Skolithos and possible Diplocraterion (Carroll 1982; Daily 1972). At least 5 irregular phosphatised surfaces have been recognised in this unit by Carroll (1982) who points out correctly that the phosphatised 'cut and fill' surface between the Wangkonda and Sellick Hill Formations is not an isolated feature. Fork Tree Limestone. The basal 30 m of the 240-300 m thick Fork Tree Limestone are composed of bioherm complexes of the same type as found in the upper Sellick Hill Formation. The upper 10-30 m of the Fork Tree consist of sparsely fossiliferous mottled limestone (Abele & McGowran 1959). The intervening 200 m are often extensively recrystallised but good preservation on the coast southwest of Myponga Beach reveals thinly bedded, unfossiliferous (?)graded calcarenites sometimes with dewatering structures. Bioherms occur high in the sequence on section 4. We suggest that these are deep water deposits with allochthonous material distributed as sandy turbidites. Heatherdale Shale. A lower calcareous member transitionally overlies mottled Fork Tree Limestone. Initially carbonate rich, Heatherdale Shale passes upward into black pyritic shale and siltstone with nodules and stringers of phosphate, although argillaceous rubbly limestone persists to the south (Daily 1969; Daily et al 1976). The formation is sparsely fossiliferous but a recently discovered conocoryphid trilobite (Jago et al 1984) supports an offshore, deeper water environment. We suggest this unit was deposited beyond the carbonate slope in a deep (100+m) basinal setting. Lateral correlation Archaeocyaths in the upper Sellick Hill Formation and lower to mid Fork Tree Limestone

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(Debrenne & Gravestock this volume) are assigned to Faunal Assemblage 2 of Daily (1956). Thus direct correlation is possible with the upper Kulpara Formation and lower Parara Limestone on Yorke Peninsula 60 km to the west across Gulf St Vincent. The Kulpara Formation is of immediate interest because of its shallow to emergent facies consisting of cryptalgal laminates, stromatolites, cross-bedded oolite shoals and grainstones composed of short stubby segments of stick-like calcified microbial microfossils. These calcified segments, resembling Proaulopora (see e.g. Drozdova 1980), are exceedingly abundant in the upper ten metres of Kulpara Formation, and have been recognised mixed with ooids in a bottom hole core from Port Gawler Observation Bore F, 60 km north of Adelaide. The implication is that high energy upper Kulpara shelf carbonates once extended considerably further east, and may occur at depth beneath the waters of Gulf St Vincent. The maximum known thickness of Kulpara Formation on Yorke Peninsula is 373 m; immediately to the west is Early Proterozoic Gawler Craton. Thus by Faunal Assemblage 2 time the craton was fringed by a thick sequence of shallow shelf carbonates which, although not known to be closer than 60 km north and west of Fleurieu Peninsula, may be reasonably inferred to have extended much further east and south. Since the north-south extent of Kulpara Formation on Yorke Peninsula is at least 116 km (Kulpara-Edithburgh) this inference is not unreasonable. Depositional model Two stages in development of the depositional model are proposed, each in a transgressive marine setting. Stage one established a shallow subtidal to supratidal carbonate shelf several tens of kilometres wide and at least 100 km long (Wangkonda, 80% of Kulpara). Most likely this was continuous with a broader platform, remnants of which remain in the Flinders Ranges (total length 600 km). Intermittent supply and reworking of arkosic detritus from the Gawler

Craton resulted in cross-bedded sandstone, conglomerate and bioturbated sandstone and siltstone units in the Winulta, Mount Terrible, Wangkonda and lower Sellick Hill Formations. Facies associations A and B are part of a shallow shelf sequence dominated by clastic input from presumably fluvial sources and reworked under tidal influence as actively migrating sand ridges and inter-ridge sediments. The latter, though containing some coarse crossbedded sands, are generally fine grained, muddy and intensely bioturbated mainly by horizontal burrowers. Carbonate deposition was effectively swamped by siliciclastic input nearshore, but scattered peloids, ooids, and calcarenite-calcisiltite clasts in storm-derived conglomerates indicate prevailing carbonate sedimentation offshore. Fig. 11 illustrates the processes and products of stage one of the model. Stage two (Fig. 11) is represented by facies associations C to E in which carbonate is the dominant sediment. Calcisiltites, lime mudstones and minor fine-grained siliciclastics were deposited in a low energy mildly unstable slope setting in association C. Pioneer bioherms grew, were partially demolished then became re-established in associations D and E, finally dominating in the lower Fork Tree Limestone. During this time (Faunal Assemblage 2) shallow, high energy ooid and skeletal sand shoals rich in calcified microbial debris were forming somewhere not far north and west of the Sellick Hill-Myponga Beach region. Shallow conditions prevailed as sedimentation rate kept pace with the continuing marine transgression. There are no known high energy mound complexes, nor any evidence of an abrupt shelf/slope break from which debris could be derived as peri-platform talus. Instead, isolated low energy archaeocyath-microbial-sponge mounds encased in silt and mud formed downslope on a substrate which eventually became stable enough to ensure colonisation. The preserved characteristics of the lateral and vertical facies sequence: shallow, high energy carbonate shelf; presumably attached gentle

Sellick Hill Formation

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Fig. 11 Stage 1 (facies associations A, B) and Stage 2 (facies associations C-E), showing sedimentary processes and products. See text for detail of depositional model.

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carbonate slope with neither shelf-edge bioherm complexes nor abrupt shelf/slope break; downslope low energy calcisiltites and lime mudstones with isolated bioherms, conform to those of Ahr's (1973) ramp model for facies associations C to E of the Sellick Hill Formation. A lateral shelf-ramp transition is nowhere seen in outcrop, being somewhere beneath Gulf St Vincent, and the vertical transition is complicated by siliciclastic input from the Gawler Craton (facies associations A, B). A preliminary sketch showing the extent of the Early Cambrian facies belts is given in Fig. 12, the basinal sequence being represented by Heatherdale Shale. This sketch is certain to change substantially to the south when the Cambrian sequence on northern Kangaroo Island can be better correlated, and to the northeast when the Truro Volcanics (almost 500 m thick, at least 20 km in strike length), which are interbedded with Fork Tree Limestone and Heatherdale Shale (Forbes et al 1972; Cobb & Farrand 1984), receive the study they so richly deserve.

CONCLUSIONS Facies associations of the Sellick Hill Formation, in sequence and in relation to the Kulpara Formation on Yorke Peninsula, indicate progressive development of a shelf-ramp complex. A shallow carbonate shelf extended several tens of kilometres east of the Gawler Craton landmass which episodically supplied siliciclastic sediments to the Myponga Beach region. Continued marine transgression allowed progressive onlap of the craton by shelf facies while a carbonate ramp developed at the outer shelf margin. Fine grained carbonate was supplied to the ramp in suspension from the shelf by storm and tide activity. Unconsolidated sediments were redistributed on the ramp by weak but persistent currents, while intermittent storms reworked lithified ribbon limestones and caused partial destruction of pioneer bioherms. Soft to lithified sediments were redistributed en masse by slope processes. Periods of non-deposition caused the development of hardgrounds, lag conglomerates, and precipitation of calcium phosphate. Isolated bioherms finally were established on the ramp, eventually coalescing as complex buildups in water perhaps several tens of metres deep. Numerous thin, persistent conglomerate beds occur in the mid-part of the Sellick Hill Formation. Clasts are derived from lithified ramp sediment, but their mode of emplacement, bizarre stacking geometry and abrupt terminations require further study to determine whether or not they are indeed a special type of debris flow.

ACKNOWLEDGEMENTS

Fig. 12 Relative positions of facies belts on eastern margin of Gawler Craton superimposed on present-day coastline.

Among those who have (all too briefly) visited the outcrops and shared their ideas, we would like to thank J.B. Jago, N.P. James, R.J.F. Jenkins, J. Mount and P.O. Roehl. This paper is published with permission of the DirectorGeneral, South Australian Department of Mines & Energy.

Sellick Hill Formation

REFERENCES ABELE C. & McGOWRAN B. 1959. The geology of the Cambrian south of Adelaide (Sellick Hill to Yankalilla). Transactions of the Royal Society of South Australia 82, 301-320. AHR W.M. 1973. The carbonate ramp: an alternative to the shelf model. Gulf Coast Association of Geological Societies, Transactions 23, 221-225. CARROLL P.G. 1982. Phosphates in Early Cambrian limestones. Fleurieu and Yorke Peninsulas, South Australia. B.Sc. (Hons) thesis, University of New England, Armidale (unpubl.). CLARKE J.D.A. 1986a. Subdivision of the Early Cambrian Parara Limestone at Wilkawillina Gorge, Hinders Ranges. Quarterly Geological Notes, Geological Survey of South Australia 99, 2-7. CLARKE J.D.A. 1986b. Early Cambrian carbonate platform-slope transitions at Wilkawillina Gorge, South Australia. 12th International Sedimentological Congress, Canberra, Abstracts, 61. COBB M.A. & FARRAND M.G. 1984. A new occurrence of the Truro Volcanics. Quarterly Geological Notes, Geological Survey of South Australia 89, 8-10. CONIGLIO M. 1985. Synsedimentary submarine slope failure and tectonic deformation in deep-water carbonates, Cow Head Group, western Newfoundland. Canadian Journal of Earth Sciences 23,476-490. DAILY B. 1956. The Cambrian in South Australia. In El Sistema Cambrico, su Paleogeografia y el Problema de su base, Vol.2, pp.97-147. Report of the 20th International Geological Congress, Mexico, 1956. DAILY B. 1963. The fossiliferous Cambrian succession on Fleurieu Peninsula, South Australia. Records of the South Australian Museum 14, 579-601. DAILY B. 1969. Fossiliferous Cambrian sediments and low-grade metamorphics, Fleurieu Peninsula, South Australia. In Daily B. ed. Geological Excursions Handbook, pp.49-54. Australian and New Zealand Association for the Advancement of Science, 41st Congress, Adelaide.

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DAILY B. 1972. Aspects of carbonate sedimentation in the Cambrian of South Australia. Geological Society of Australia, Joint Specialists Groups Meetings, Abstracts, Canberra, February, C10-C14.

DAILY B. 1976. Novye dannye ob osnovannii Kembriya v Yuzhnoy Avstralii. (New data on the base of the Cambrian in South Australia.) Izvestiya Akademiya Nauk SSSR, Seriya Geologiya 1976(3) , 45-52 (In Russian).

DAILY B., FIRMAN J.B., FORBES B.G. & LINDSAY J.M. 1976. Geology. In Twidale C.R., Tyler M.J. & Webb B.P. eds. Natural History of the Adelaide Region, pp.5-42. Royal Society of South Australia, Adelaide. DAILY B. & MELNES A.R. 1973. Stratigraphy, structure and metamorphism of the Kanmantoo Group (Cambrian) in its type section east of Tunkalilla Beach, South Australia. Transactions of the Royal Society of South Australia 97,213-242. DEBRENNE F. & GRAVESTOCK D.I. (this volume). Archaeocyatha from the upper Sellick Hill Formation and lower Fork Tree Limestone on Fleurieu Peninsula, South Australia.

DROZDOVA N.A. 1980. Vodorosli v organnogennikh postroykakh nizhnego Kembriya zapadnoy Mongolii. (Algae in Lower Cambrian organic mounds of western Mongolia.) Sovmestnaya Sovetsko-Mongolskaya paleontologicheskaya ekspeditsiya. Trudy vuip 10, Nauka Moskva, 137pp. (In Russian).

FORBES B.G., COATS R.P. & DAILY B. 1972. Truro Volcanics. Quarterly Geological Notes, Geological Survey of South Australia 44,1-5. HARMS J.C., SOUTHARD J.B. & WALKER R.G. 1982. Structures and sequences in clastic rocks. Society of Economic Paleontologists and Mineralogists, Short Course 9 (Lecture Notes). HEIN F.J. 1987. Tidal/littoral offshore shelf deposits Lower Cambrian Gog Group, southern Rocky Mountains, Canada. Sedimentary Geology 52, 155-182.

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HISCOTT R.N. & JAMES N.P. 1985. Carbonate debris flows, Cow Head Group, Western Newfoundland. Journal of Sedimentary Petrology 55, 735-745.

MADIGAN C.T. 1926. Organic remains from below the Archaeocyathinae limestone at Myponga Jetty, South Australia. Transactions of the Royal Society of South Australia 50,31-35.

HOWCHIN W. 1897. On the occurrence of Lower Cambrian fossils in the Mount Lofty Ranges. Transactions of the Royal Society of South Australia 21, 74-86.

MADIGAN C.T. 1927. The geology of the Willunga Scarp. Transactions of the Royal Society of South Australia 51, 398-409.

JAGO J.B., DAILY B., VON DER BORCH C.C., CERNOVSKIS A. & SAUNDERS N. 1984. First reported trilobites from the Lower Cambrian Normanville Group, Fleurieu Peninsula, South Australia. Transactions of the Royal Society of South Australia 108, 207-211. JAGO J.B., GEHLING J.G. & DAILY B. 1986. Cambrian sediments of the Sellick Hill-Carrickalinga Head area, Fleurieu Peninsula, South Australia. In Parker A.J. compiler. One Day Geological Excursions of the Adelaide Region, pp.67-81. Geological Society of Australia, South Australian Division. JAMES N.P. & GRAVESTOCK D.I. 1986. Lower Cambrian carbonate shelf and shelf margin buildups, South Australia. 12th International Sedimentological Congress, Canberra, Abstracts, 154. JAMES N.P. & MOUNTJOY E.W. 1983. Shelf-slope break in fossil carbonate platforms: an overview. Society of Economic Paleontologists and Mineralogists, Special Publication 33, 189-206. KRAUSE F.F. & OLDERSHAW A.E. 1979. Submarine carbonate breccia beds - a depositional model for two-layer, sediment gravity flows from the Sekwi Formation (Lower Cambrian), Mackenzie Mountains, Northwest Territories, Canada. Canadian Journal of Earth Sciences 16, 189-199.

MARKELLO J.R. & READ J.F. 1981. Carbonate ramp-to-deeper shale shelf transitions of an Upper Cambrian intrashelf basin, Nolichucky Formation, Southwest Virginia Appalachians. Sedimentology 28, 573-597. MENG X., QIAO X., GE M. & YAN K. 1986. Study of ancient shallow sea carbonate storm deposits (tempestites) in North China and Dingjiatan model of facies sequences. 12th International Sedimentological Congress, Canberra, Abstracts, 210. MOUNT T.J. 1970. Geology of the Mt Chambers Gorge Region, Flinders Ranges, S.A. B.Sc.(Hons) thesis, University of Adelaide (unpubl.). PARK B. & HAN S. 1986. Sedimentology of carbonate flat pebble conglomerate of the Upper Cambrian Hwajol Formation, Choson Supergroup, Korea. 12th International Sedimentological Congress, Canberra, Abstracts, 234. ROEHL P.O. 1967. Stony Mountain (Ordovician) and Interlake (Silurian) facies analogs of Recent low-energy marine and subaerial carbonates, Bahamas. American Association of Petroleum Geologists Bulletin 51,1979-2032. SANDERSON D.J. & DONOVAN R.N. 1974. The vertical packing of shells and stones on some recent beaches. Journal of Sedimentary Petrology 44, 680-688.

McILREATH I.A. & JAMES N.P. 1984. Carbonate Slopes. In Walker R.G. ed. Fades Models. Second Edition, pp.245-257. Geoscience Canada, Reprint Series 1.

SEPKOSKI J.J. 1982. Flat-pebble conglomerates, storm deposits, and the Cambrian bottom fauna. In Eiensele G. & Seilacher A. eds. Cyclic and Event Stratification, pp.371-385. Springer-Verlag, Berlin.

MADIGAN C.T. 1925. The geology of the Fleurieu Peninsula. Parti - The coast from Sellick's Hill to Victor Harbour. Transactions of the Royal Society of South Australia 49,198-212.

SPRIGG R.C. & CAMPANA B. 1953. The age and facies of the Kanmantoo Group, eastern Mount Lofty Ranges and Kangaroo Island. Australian Journal of Science 16,12-14.

Sellick Hill Formation WILSON J.L. 1969. Microfacies and sedimentary structures in "Deeper Water" lime mudstones. In Friedman G.M. ed. Depositional Environments in Carbonate Rocks. Society ofEconomic Paleontologists and Mineralogists, Special Publication 14, 4-17.

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Fleurieu Peninsula archaeocyaths

Archaeocyatha from the Sellick Hill Formation and Fork Tree Limestone on Fleurieu Peninsula, South Australia 1

Francoise Debrenne and David I. Gravestock

l ER 154 CNRS, Institut de Paleontologie, Paris, France South Australian Department of Mines and Energy, Eastwood, S.A. 5063, Australia

An oligotypic archaeocyathan fauna, consisting entirely of Regulares, occurs in the upper Sellick Hill Formation and lower to middle Fork Tree Limestone. In the upper Sellick Hill Formation thin, laterally persistent archaeocyath-rich bioclastic packstone has eroded pioneer bioherms, but succeeding biohermal buildups are interbedded with and draped by argillaceous ribbon to nodular limestone. Lower and middle levels of the conformably overlying Fork Tree Limestone consist of probably coalescing bioherms of the same construction. Regular archaeocyaths with extensive outgrowths held together an open framework also occupied by sponges, patches of Epiphyton and encrusting Girvanella, with external cavities floored by geopetal lime mud. Anaptyctocyathus sellicksi (Taylor) is revised and thirteen additional taxa described, five questionably to genus and species rank. Robertocyathus Rozanov and Kymbecyathus Debrenne & Kruse are recorded in Australia for the first time. Three species allow correlation with the lower Wilkawillina Limestone (Flinders Ranges). The fauna is no older than mid-Atdabanian and arguably Botomian in age. New species are Aldanocyathus bulbosus, A.inaequabilis and Mennericyathus latus.

Key words: Archaeocyatha, Regulares, Sellick Hill Formation, Fork Tree Limestone, Early Cambrian, bioherms, Anaptyctocyathus sellicksi, Aldanocyathus bulbosus, Aldanocyathus inaequabilis, Mennericyathus latus.

INTRODUCTION Discovery of an archaeocyathan fauna near Carrickalinga Head by Edgeworth David and at Sellick Hill in the same year by Howchin (1897) established a Cambrian age for part of the sedimentary sequence exposed on Fleurieu Peninsula south of Adelaide. The Sellick Hill archaeocyaths were found in vertical to slightly overturned massive limestone, and consequently have been somewhat recrystallized and deformed. Their poorer preservation compared with exquisitely silicified specimens from the Ajax Mine (Flinders Ranges) appears to be one reason for their lack of systematic study. An equally important reason may be due to the recognition of only a single species by Taylor (1910) in the 250 specimens collected by him from Sellick Hill. Taylor (1910) noted this extreme contrast in diversity compared with Ajax Mine specimens and made a detailed study of the abundant exothecal tissue characteristically surround-

ing cups he named Archaeocyathus sellicksi. In the following pages we redescribe Taylor's species as Anaptyctocyathus sellicksi and provide further observations on the distribution, facies setting and systematics of thirteen additional species.

GEOLOGICAL SETTING Howchin and Taylor made their Sellick Hill collections in the vicinity of an old quarry cut into the upper 20 m of flaggy argillaceous limestone and basal 20 m of massive limestone which were subsequently mapped as Sellick Hill Formation and Fork Tree Limestone by Abele & McGowran (1959). Two additional formations mapped by them, and a basal Cambrian arkose recognized by Daily (1963), constitute the Lower Cambrian Normanville Group (Daily & Milnes 1973). In summary they are in ascending order: Mount Terrible Formation consisting of sandstone and

F. Debrenne & D.I. Gravestock

siltstone becoming calcareous at the top (Daily 1963) ; Wangkonda Formation, a complex suite of peritidal limestones and lenticular bioturbated sandstones (Abele & McGowran 1959; Daily 1972a, b; Carroll 1982); Sellick Hill Formation, initially shallow subtidal but deepening upward with isolated bioherms near the top (Abele & McGowran 1959; Alexander & Gravestock this volume) ; Fork Tree Limestone, open marine carbonates with bioherm complexes in the lower part and sparsely fossiliferous mottled limestone in the upper part (Abele & McGowran 1959; Daily 1963) ; Heatherdale Shale, variably calcareous with characteristic phosphate nodules, black, pyritic, with a sparse fauna (Abele & McGowran 1959, Jago etal 1984, 1986). The Normanville Group is overlain conformably but sharply by siliciclastics of the very thick Lower to Middle Cambrian Kanmantoo Group, the basal

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unit of which, the Carrickalinga Head Formation, is described by Gatehouse et al (this volume).

SAMPLE LOCATIONS Formations of the Normanville Group are exposed between Sellick Hill and the coast southwest of Myponga Beach (Fig.l). Five fossil collection sites are shown, site 1 being at the type locality of the Normanville Group. Collections were made there in 1976 on the back wall and south flank of the old quarry in the top 20 m of Sellick Hill Formation and basal 20 m of Fork Tree Limestone. Site 2 is 84.2 m below the top of the Sellick Hill Formation which is 236 m thick in a cutting on the Main South Road. Specimens at site 3 were collected 18.9 to 26.6 m below the top of the Sellick Hill Formation on the flank of a low hill at the eastern end of Myponga Beach.

Heatherdale Shale Fork Tree Limestone Sellick Hill Formation Wangkonda Formation Mount Terrible Formation 87-773

SADME

Fig. 1 Geological map of the Normanville Group in the Sellick Hill-Myponga area, and location of 5 fossil collection sites. Map modified from Abele & McGowran (1959).

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Fleurieu Peninsula archaeocyaths

Three collections were made at site 4, 27.5 to 66 m below the top of the Sellick Hill Formation, while two widely spaced collections were made at site 5, 71.9 m below the top of the Sellick Hill Formation (thickness 221.6 m) and 169.5 m above the base of the Fork Tree Limestone (thickness 301.2 m). Sample sites 2, 4 and 5 are on stratigraphic sections 1, 3 and 4 of Alexander & Gravestock (this volume). Syntypes from Taylor's Sellick Hill collection (T1560, T1582) housed at the South Australian Museum were restudied from newly prepared thin sections. All figured specimens from our collections are lodged at the same institution and are labelled SAMP29084-SAMP29107. Remaining specimens are housed in the Museum National d'Histoire Naturelle Paris, with the labels L82014-L82036.

gregarius (Gravestock), Erismacoscinus uratannensis (Gravestock), lErugatocyathus cf. mawsoni Gravestock and Mennericyathus dissitus Kruse are known from the Ajax Limestone in the Mount Scott Range at a level correlated with the lower Wilkawillina Limestone (Gravestock 1984). M. dissitus Kruse was first described from the upper Mount Wright Volcanics and lower Cymbric Vale Formation in western New South Wales, in Fauna 1 of Kruse (1982).

CENTRAL FLINDERS RANGES

FLEURIEU PENINSULA

WIRREALPA LIMESTONE

BILLY CREEK FORMATION

CARRICKAUNGA HEAD FORMATION

BIOSTRATIGRAPHIC CORRELATION Until now only the oldest and youngest formations of the Normanville Group have yielded fossils useful for correlation. Shelly fossils from the Mount Terrible Formation were considered by Daily (1976) to be earliest Cambrian (Tommotian) in age and significantly older than his Faunal Assemblage 1 (Daily 1956). Recent discovery of a conocoryphid trilobite from the Heatherdale Shale has enabled correlation with the upper Parara Limestone in the Flinders Ranges (Jago etal 1984). The presence of helcionellid and tannuellid gastropods in the basal Sellick Hill Formation led Horwitz & Daily (1958) and Daily (1972b) to suggest correlation with Faunal Assemblage 2 or 3, the latter being favoured in their correlation charts. Greater precision is now possible based on the presence of two archaeocyathan species and two comparable genera found elsewhere only in Faunal Assemblage 2. It is thus probable that the entire Sellick Hill Formation and at least half the Fork Tree Limestone correlate with the lower Wilkawillina Limestone in the Flinders Ranges as shown in Fig.2. The taxa Kaltatocyathus cf.

ORAPARINNA SHALE TRURO VOLCANICS>

PARARA LIMESTONE

HEATHERDALE SHALE

FORK T R E E LIMESTONE

SELLICK HILL FORMATION

WANGKONDA FORMATION

MT TERRIBLE FORMATION

F.A.2

WILKAWILLINA LIMESTONE

WOODENDINNA DOLOMITE

PARACHILNA FORMATION

A WILPENA GROUP

MARINO GROUP 88-243

SADME

Fig. 2 Suggested correlation of Normanville Group (Fleurieu Peninsula) with Hawker Group (central Flinders Ranges), showing currently known range of Faunal Assemblage 2 of Daily (1956). Modified from Jago et al (1986).

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Fig. 3 SAMP29107. Bioclastic packstone from site 2, 84.2 m below top of Sellick Hill Formation, X4. From a global study of the distribution of regular archaeocyathan genera, Debrenne & Rozanov (1983) tabulated generic stratigraphic ranges with respect to the Siberian Platform Stage scheme. They subdivided each stage into numbered intervals (Tommotian 1-4, Atdabanian 1-4, Botomian 1-3, Toyonian (ex-Elankian) 1-3) on the basis of the best stratigraphic and palaeogeographic data available at that time. Stratigraphic ranges provided in the systematic palaeontology conform with this numerical scheme. Firm correlation with established Siberian Platform stages is a matter of debate. Gravestock (1984) maintains that archaeocyaths from Faunal Assemblages 1 and 2 are no older than mid-At-

dabanian but points out that typically Botomian species do not appear until higher in the sequence, regrettably not studied in detail. Zhuravlev (1986) considers Assemblage 2 archaeocyaths are Botomian, a view shared by the senior author. It is important to note that a specimen is tentatively assigned below to Inacyathella Debrenne, a Moroccan genus from a stratigraphic level correlated with the Botomian (Debrenne 1977). What is now abundantly clear is that many South Australian archaeocyaths and other shelly fossils have strongly facies controlled stratigraphic ranges. For example, not one specimen of Micrina etheridgei (Tate) recently redescribed by Laurie (1986) has been found to date in the Sellick Hill Formation or Fork Tree

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Fleurieu Peninsula archaeocyaths

Fig. 4 SAMP29106. Archaeocyath framestone from site 4,66.0 m below top of Sellick Hill Formation, X4.

Fig. 5 Encrusting Girvanella forming roof of cavity in small block from bioherm. Block interpreted to have fallen gently into lime mud on bioherm flank. Site 5, 71.9 m below top of Sellick Hill Formation.

F. Debrenne & D.I. Gravestock

Limestone . This species, long assumed to be characteristic of Faunal Assemblage 2, is indeed abundant in high energy platform and platform margin settings, but Alexander & Gravestock (this volume) propose a low energy moderately deep water carbonate ramp setting for the upper Sellick Hill Formation and basal Fork Tree Limestone. This contrasting setting presumably accounts for the absence of M.etheridgei.

FACIES ANALYSIS Two facies have yielded the archaeocyaths described in the systematic palaeontology. The first is a bioclastic packstone representing a rare high energy deposit in the upper Sellick Hill Formation. The packstone, 1.5-2 m thick, is exposed at the type locality 62 m below the top of the formation, and either the same unit, or one allied to it, has been mapped discontinuously to the southernmost outcrop of the formation southwest of Myponga Beach. Apparent lateral continuity of the packstone suggests wholesale destruction and reworking of somewhat older bioherms by a single high energy event. Eroded remnants of these pioneer bioherms occur in outcrop.

295

usually lensing out into surrounding sediments within a few metres. Bioherm dimensions vary from subequant 0.5x0.5 m up to 3-4 m wide by 1.5-2 m high. Synoptic relief is unlikely to have been more than a few decimetres but this has been enhanced by drape and compaction of the overlying argillaceous limestone. The basal 30 m of Fork Tree Limestone at the type locality may consist of a complex of coalescing mounds but we have been unable to determine individual bioherm boundaries. Unfortunately exposures of Fork Tree Limestone at Myponga Beach are too strongly reciystallised for useful study and better exposures will have to be sought elsewhere. The description given below must be regarded as preliminary.

The packstone (Fig.3) consists of lime mud mostly recrystallized containing numerous archaeocyathan cups, none larger than 4 mm diameter and often with thickened outer walls and one or two layers of abraded metaldetimorphatype outgrowths. Equally common are fragments of larger cups, sponge spicules, chancelloriides and hyoliths. Rare bioclasts include fragments of trilobites and small Epiphyton bushes.

As shown in Figs 4 and 5 the bioherms are true framestones constructed by archaeocyaths, sponges and shrubby patches of Epiphyton. Internal cavities of archaeocyaths and sponges are floored with geopetal lime mud, while external cavities of irregular shape are similarly mud floored and roofed by archaeocyaths, Epiphyton and occasionally by encrusting Girvanella. Stromatolite-like encrustations (Fig.8b) may also be due to Girvanella but recrystallization has obscured details. Downward-growing Epiphyton., occasional archaeocyaths and inverted Ushaped problematica are pendant into cavity interiors which are now filled withblocky calcite. Archaeocyaths dominate the overall framework, but hexactinellid sponge spicules (sometimes as intact frameworks) locally occur in roughly equal proportions. Epiphyton comprises perhaps 30 per cent of the framework but is patchy, while cryptalgal mottling of interstitial lime mud is common.

The second facies is biohermal. These occur in the upper Sellick Hill Formation as isolated piano -convex mounds enclosed in ribbonlaminated and argillaceous nodular limestone. Debris aprons of fallen material are not extensive,

The main characteristic of the archaeocy athan fauna is the complete lack of Irregulares which because of their polymorphism and common exostructures are usually important frame builders. For instance, the ramifying exostructures of

*Note added in proof - One specimen of Micrina has been found recently. It occurs infloatwith archaeocyaths a few metres below the mottled upper member of the Fork Tree Limestone, southwest of Sellicks Beach.

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Metaldetes profundus (Billings) act as 'coenchyme', imparting a massive habit and binding the framework in the manner of reef corals (Debrenne & James 1981). A similar role has been assumed by Regulares in the Sellick Hill and Fork Tree bioherms, with several genera developing extensive metaldetimorpha-type outgrowths as noted by Taylor (1910). The fauna is oligotypic, with Anaptyctocyathus sellicksi and Erismacoscinus uratannensis most abundant in Fork Tree bioherms at Sellick Hill, and Mennericyathus dissitus as the principal frame builder at Myponga Beach (collection site 4). The archaeocyaths most commonly expand to 'saucerlike' shaped from cylindroconical initial stages. In a global study of archaeocyathan palaeobiogeography Zhuravlev (1986) noted a sharp increase in generic diversification in the late Atdabanian followed at the base of the Botomian by the appearance of framework reefs. This general trend is not followed in the bioherms described here which appear to have increasingly colonised a stable substrate in a moderately deep, calm marine setting. By analogy with modern coral reefs (Connell 1978) the relatively low diversity coupled with local dominance by different archaeocyathan species suggests the attainment of competitive equilibrium in the absence of frequent or catastrophic disturbances. SYSTEMATIC PALAEONTOLOGY Class REGULARES Vologdin 1937 Family MONOCYATH3DAE Bedford & Bedford 1934 ARCHAEOLYNTHUS Taylor 1910

Type-species. Monocyathus porosus Bedford & Bedford 1934. Diagnosis. One-walled cup. The wall, simply porous, is often affected by secondary thickening generally due to the presence of exothecal structures. Stratigraphic range. Tommotian 1 to Botomian 3. Distribution. America.

Cosmopolitan

except

North

ARCHAEOLYNTHUS cf. POROSUS (Fig. 6a, b) 1934 Monocyathus porosus Bedford & Bedford, 1982 Archaeolynthus porosus Kruse p.2, pl.l, fig. 1., p. 142-143, text-fig.7A-E cum. syn. Material. Fifteen specimens: L82024-82026 from site 2, SAMP29084-29085, L82027 from site 4 (66 m below top SHF), L82032 from site 5 (71.9 m below top SHF), upper Sellick Hill Formation. Description. Cup narrowly conical. The largest known here is 2 mm in diameter for a height of about 7 mm. Wall perforated by round pores separated by broad lintels. Wall thickness depends on the development of secondary layers. Dimensions. D (cup diameter) 2 mm, H (cup height) 7 mm, (|) (diameter of wall pores) 0.08 to 0.11 mm, 1 (lintel width) 0.18-0.20 mm, t (thickness) 0.10-0.15 mm.

Fig. 6 a .Archaeolynthus cf.porosus (Bedford & Bedford), SAMP29084,3different cups surrounded by Epiphyton bushes, b. Archaeolynthus cf. porosus (Bedford & Bedford), SAMP29085, small cup with secondarily thickened wall and Epiphyton. c. Kaltatocyathus cf. gregarius (Gravestock), SAMP29086, colonial cup, oblique tranverse section, d. Dokidocyathus sp., SAMP29087, longitudinal section, with tangential view of the outer wall, e. Kymbecyathus sp., SAMP29088, tranverse section, f. Robertocyathus sp., SAMP29089, longitudinal section; growth of upper part is disturbed by expanded portion of anaptyctocyathid. Young cups and exostructures attached to Robertocyathus. g. Aldanocyathus grandipora (Taylor), SAMP29090, tranverse oblique section, h. Aldanocyathus bulbosus Debrenne & Gravestock sp.nov., SAMP29091, oblique section with bulges; hyolith and trilobite fragments.

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Discussion. Small diameter cups measured by Kruse (1982, p. 142) have similar dimensions. Family DOKIDOCYATfflDAE Bedford & Bedford 1936

(outer wall) ty 0.15 mm, 1 0.11 mm, t 0.11 mm, IW (inner wall) <j) 0.11 mm, 1 0.11 mm, t 0.08 mm. Discussion. The particular disposition of outer wall pores is different from other species.

DOKIDOCYATHUS Taylor 1910 Type-species. Dokidocyathus Taylor 1910.

simplicissimus

Diagnosis. Outer and inner walls with simple pores. Intervallum crossed by radial bars, circular or flattened in cross-section; successive bars arranged in vertical radial planes equivalent to septa. Recent revision (Debrenne & Rozanov 1985) shows that Dokidolynthus Debrenne 1974 must be reincorporated within Dokidocyathus Taylor 1910. Stratigraphic range. Tommotian 1 to Botomian 3. Distribution. America.

Cosmopolitan

except

North

Family KALTATOCYATHIDAE Rozanov in Zhuravleva, Konyushkov & Rozanov 1964 KALTATOCYATHUS Rozanov in Zhuravleva, Konyushkov & Rozanov 1964 Type-species. Kaltatocyathus kashinae Rozanov in Zhuravleva, Konyushkov & Rozanov 1964. Diagnosis. Cup with two walls connected by radial bars. Outer wall pores covered by simple tumuli, inner wall with simple pores. Applying the same reasoning as for DokidocyathusDokidolynthus (Debrenne & Rozanov 1985) we now have to consider Aroonacyathus Gravestock 1984 as junior synonym of Kaltatocyathus Rozanov. Stratigraphic range. Atdabanian 1 to Botomian 2.

DOKIDOCYATHUS sp. (Fig. 6d)

Distribution. Siberian Platform, Altai Sayan, Mongolia, North Africa, Australia.

Material. Twenty specimens: L82015, 82023 from site 1, SAMP29107, L82024-82026 from site 2, SAMP29087, L82027 from site 4 (66 m below top of SHF), L82032 from site 5 (71.9 m below top of SHF), upper Sellick Hill Formation and basal Fork Tree Limestone.

KALTATOCYATHUS aff. (Gravestock 1984) (Fig. 6c)

GREGARIUS

1984Aroonacyathus gregarius Gravestock, p.46, fig. 31E-L.

Description. Small conical cups up to 18 mm diameter. Outer surface shows vertical ridges between adjacent rows of vertically elongated pores which are themselves arranged in horizontal rows. Inner wall simple with quincuncial arrangement of pores; scarce bars of unknown section. Exothecal structures often present at the base.

Material. Ten specimens: L82035, 82036 from site 3, SAMP29086, L82027 from site 4 (66 mm below top SHF), L82032 from site 5 (71.9 m below top of SHF), upper Sellick Hill Formation.

Dimensions. D 18 mm, I (intervallum width) 1.5 mm, IK (intervallum coefficient) 0.08, OW

Description. Small cylindrical cups, sometimes ceratoid in shape or colonial. Outer wall tumuli

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rather scarce and low, and difficult to observe. Inner wall simply porous. Radial bars are rare.

Family AJACICYATHIDAE Bedford & Bedford 1939

Dimensions. D 4.6 mm, 11.2 mm, IK 0.26, OW<|) 0.15 mm, 1 0.11 mm, t 0.20 mm, IW<|) 0.15 mm, 1 0.15 mm, 10.08 mm.

ALDANOCYATHUS Voronin in Debrenne & Voronin 1971

Discussion. For the same diameter, the intervallum is larger in the specimens observed here than in the type material of K.gregarius.

Type-species. Ajacicyathus Zhuravleva 1960.

sunnaginicus

Diagnosis. Outer and inner walls simple, connected by radial porous septa.

Family KYMBECYATHIDAE Debrenne, Rozanov & Zhuravlev (1989)

Stratigraphic range. Tommotian 1 to Botomian 3.

KYMBECYATHUS Debrenne & Kruse 1986

Distribution. Cosmopolitan.

Type-species. Kymbecyathus avius Debrenne & Kruse 1986. Diagnosis. Outer wall with pore canals, inner wall with simple pores. Intervallum traversed by radial bars. Stratigraphic range. Equivalent of Botomian stage in Antarctica. Distribution. Antarctica, Australia. KYMBECYATHUS sp. (Fig. 6c) Material. Ten specimens: SAMP29088, L82020 from site 1, SAMP29107, L82024-82026 from site 2. Upper Sellick Hill Formation and basal Fork Tree Limestone. Description. Small cup about 4 mm diameter with thick outer wall perforated with straight pore canals, simply porous inner wall and rare thin radial bars. Dimensions. D 3.8 mm, 11.0 mm, IK 0.26, OW(|) 0.18 mm, 1 0.08 mm, 10.22 mm, IW(|) 0.11 mm, 1 0.15 mm, 10.1 mm. Discussion. Differs from the holotype by its comparatively thin inner wall. Material is too rare to erect a new species.

ALDANOCYATHUS GRANDIPORA (Taylor 1910) (Fig. 6g) 1910 Archaeocyathus grandipora Taylor, p.120, pl.7, figs 41-42. Material. Nineteen specimens: SAMP29090, L82014, 82020, 82023 from site 1, L82024 from site 2, L82034-82035 from site 3, L82029-82030 from site 4 (27.5 m below top of SHF), L82032 from site 5 (71.9 m below top SHF), upper Sellick Hill Formation and basal Fork Tree Limestone. Description. Conical cup with rather wide intervallum and relatively distant septa. Outer wall with numerous rows of small pores (5-6 per intersept). Inner wall with 2 rows of pores between adjacent septa. Septa perforated by large pores. Dimensions. D 15.7 mm, I 2 mm, Is (distance between 2 septa) 0.9 mm, N (number of septa) about 50, IK 0.12, RK (radial coefficient) about 3, OW(j> 0.11 mm, 1 0.08 mm, t 0.11 mm, IW(J) 0.18 mm, 1 0.11 mm, t 0.11 mm, S (septa) <>| 0.22 mm, 1 0.11 mm, 10.11 mm. Discussion. Type A.grandipora (Taylor) from the Ajax Mine has similar IK and RK (respectively

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0.12 and 2.6) but larger septal pores than in the present form. ALDANOCYATHUS BULBOSUS Debrenne & Gravestock sp.nov. (Fig. 6h)

301

Material. Thirty specimens: holotype SAMP29092, paratypes SAMP29093, L82032 from site 5, 71.9 m below top of Sellick Hill Formation, L82026 from site 2, L82027, 82030 from site 4 (66 m, 27.5 m below top of SHF respectively), upper Sellick Hill Formation.

Derivatio nominis. Latin bulbosus = with bulbs. Material. Two specimens: holotype SAMP29091 and paratype L82031 from site 4, 71.9 m below top of Sellick Hill Formation. Description. Conical cup with external bulges affecting the outer wall and intervallum. Short inner wall spines present. Dimensions. D about 15 mm, 13.8 to 6.2 mm, Is 1.0 mm, IK variable, RK unknown, OW 6-8 rows, cj> 0.11 mm, 1 0.04 mm, t 0.11 mm, IW 4 rows, (J) 0.22 mm, 1 0.08 mm, t 0.19 mm, S(|> 0.4 mm, 1 0.2 mm, 10.15 mm. Discussion. Differs from other species of Aldanocyathus by the presence of outer wall bulges. As the inner wall is not affected it could not be included within Orbicyathus Vologdin. Gravestock's reported Pycnoidocyathus (in Jago et al 1986) is A.bulbosus.

ALDANOCYATHUS INAEQUABILIS Debrenne & Gravestock sp.nov. (Fig.7a, b) Derivatio nominis, Latin inaequabilis regular.

= ir-

Description. Outer wall irregularly thickened, perhaps in connection with the presence of exothecal structures at the base. Inner wall thin, simply porous. Septa rare, sometimes bifurcating, perforated by large pores. Dimensions. Cups have been observed from 3 to 13.6 mm diameter. The IK varies from 0.2 in small cups to 0.29 in larger ones. The RK varies from 1.5 to 2 independently of cup diameter as the distribution of septa is very irregular (Is varying from 1.5 to 2 mm at the same level in transverse section). OW 5-6 rows, ([) 0.18 mm, 1 0.15 mm, t 0.20 to 0.37 mm, IW 2-3 rows, (J) 0.26 mm, 1 0.20 mm, t 0.15 mm, S 5-6 rows, (j) 0.30 mm, 1 0.30 mm, 10.10 mm. Discussion. This form is frequent in the Sellick Hill Formation. Thickening of the outer wall is unique compared with other species of Aldanocyathus, and no known species without thickening have similar coefficients. Consideration was given to housing the species within the genus Fallocyathus Rozanov in Zhuravleva, Korshunov & Rozanov 1969 the outer wall of which consists of straight poretubes. However, it was impossible in the present case to fine true pore-tubes in longitudinal sections which reveal only thick pores.

Fig. 7 a. Aldanocyathus inaequabilis Debrenne & Gravestock sp.nov., holotype, SAMP29092, transverse section with branching septa, b. Aldanocyathus inaequabilis Debrenne & Gravestock sp.nov., paratypes S AMP29093, two cups of the same species, c. Mennericyathus dissitus Kruse, SAMP29094, detail of intervallum and double outer wall (at top), d. Mennericyathus dissitus Kruse, SAMP29095, ribbon-like fragment of intervallum of adult cup. e. Erismacoscinus uratannensis (Gravestock), SAMP29096, oblique section. Outer wall slightly bulges within each interseptum. f. Erismacoscinus uratannensis (Gravestock), SAMP29097, upper part of expanding cup. g. Erismacoscinus uratannensis (Gravestock), SAMP29098, detail of tabular porosity, h. ? Inacyathella sp., SAMP29099, longitudinal section; S-tubes of the inner wall are observed in the part not affected by secondary thickening; young cups growing at top of ?Inacyathella.

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Family ROBERTOCYATfflDAE Rozanov 1969

Family GUMBYCYATHIDAE Debrenne & Kruse 1986

ROBERTOCYATHUS Rozanov 1969 INACYATHELLA Debrenne 1977 Type-species. Robertocyathus polaris Rozanov 1969. Diagnosis. Outer wall with carcass and independent microporous sheath. Inner wall with simple pores. Porous septa in the intervallum. Stratigraphic range. Atdabanian 2 to Botomian 3.

Type-species. Inacyathella pulchra Debrenne 1977. Diagnosis. Outer wall with non-independent microporous sheath attached to the carcass. Inner wall with non communicating S-shaped poretubes, one row per intersept. Septa sparsely porous. Stratigraphic range. Botomian 2.

Distribution. Siberian Platform, Altai Sayan, Mongolia, Far East. This is the first Australian occurrence.

Distribution. Morocco, ? Australia.

ROBERTOCYATHUS sp. (Fig. 6f)

? INACYATHELLA sp. (Fig. 7h)

Material. Five specimens: SAMP29107 from site 2, SAMP29089, L82027 from site 4 (66 m below top SHF), upper Sellick Hill Formation.

Material. SAMP29099, only 1 specimen from site 4,66 m, below top of Sellick Hill Formation.

Description. Conical cup slightly ceratoid and with some very slight vertical undulations. One specimen (SAMP29089) shows signs of injury at the top of the cup (Fig.6f) at the contact with an anaptyctocyathid cup. Outer wall has a strong carcass with independent microporous sheath. Inner wall simply porous. Septal pores vertically elongated, separated by stout lintels. Dimensions. D 6 mm, I 1.2 mm, Is 0.35 mm, N unknown, IK 0.19, OW carcass 2 rows, (|) 0.15 mm, 1 0.08 mm, t 0.15 mm, micropores 4 for one carcass-pore <>| 0.04 mm, 1 0.04 mm, t 0.03 mm, IW 2 rows (j> 0.15 mm, 1 0.04 mm, t 0.05 mm, S<|) 0.08 mm, 1 0.11 mm, 10.05 mm. Discussion. Only longitudinal sections enable Robertocyathus without tabulae to be distinguished from Mennericyathus with flat tabulae. Probably some tranverse sections observed in the material studied could also be attributed to Robertocyathus, but without corresponding longitudinal sections we cannot determine them.

Description. Tapering cup surrounded by one layer of exothecal tissue which sometimes produces buds. External parts of septa thickened at contacts with the embedded outer wall. The outer wall itself is double, with sheath micropores probably limited to carcass pores. Inner wall with thin oblique straight to slightly curved canals sometimes hidden by thickening. Septa porous. Dimensions. D 3.6 mm, 10.51 mm, Is 0.3 mm, N unknown, IK 0.18, OW<|) 0.11 mm, 1 0.11 mm, t 0.11 mm, microspores 4,ty0.08 mm, 1 0.04 mm, 10.04 mm, IW<t> 0.08 mm, 1 0.02 mm, 10.15 mm, S<|) 0.08 mm, 1 0.11 mm, 10.05 mm. Discussion. The lack of tranverse sections deprives us of information on some important characteristics such as the number of pores per intersept at the inner wall (generic) and the radial coefficient (specific). A more precise determination is thus not possible.

F. Debrenne & D.I. Gravestock

Family ASTEROCYATfflDAE Vologdin 1956 ERISMACOSCINUS Debrenne 1958 Type-species. Erismacoscinus Debrenne 1958.

marocanus

Diagnosis. Outer wall and inner wall simple. Septa porous; tabulate independent from the walls, generally flat or slightly curved, appearing late in development. Forms with arched tabulae participating in the construction of one or two walls are included in the genus Coscinocyathus. Stratigraphic range. Tommotian 2 to Botomian 3. Distribution. America.

Cosmopolitan

except

North

303

0.04 mm. For larger cups (12 mm diameter) the intervallum coefficient is smaller (IK:0.07) as is the radial coefficient. Septa are always 0.500.60 mm apart. The interseptal camera (IC) is constant (1/1.8 to 1/1.5). Discussion. Preservation in the studied material is not very good. Spines, which are described in type material from the Mount Scott Range, are not observed here. Ribbon-like forms seem to be characteristic from Sellick Hill whereas only conical forms are known at Mount Scott. Other characteristics however (especially numerical data for the same diameter) are very close and are sufficient to identify the species.

Family POLYCOSCINIDAE Debrenne 1964 MENNERICYATHUS Debrenne & Rozanov 1974

ERISMACOSCINUS URATANNENSIS (Gravestock 1984)(Fig 7e-g) 1984 Coscinocyaths uratannensis Gravestock, p.72, fig. 41A-E. Material. One hundred and forty-nine specimens: SAMP29096- 29098, L82017-82023 from site 1, L82034, 82036 from site 3, upper Sellick Hill Formation and basal Fork Tree Limestone. Description. Conical cup becoming plate-like with increasing growth. Intervallum narrow with ribbon fragments of upper parts of cups very frequent at site 1 where this species dominates. Walls may slightly bulge between septa. Septa porous, tabulae flat with net-like porosity, irregularly spaced. Root processes and exothecal structures are often developed at the lower part of cups. Dimensions. Conical part D 5-6 m, I 1.0 mm, Is 0.51-0.61 mm, IK 0.17, RK 5.4-5.8, OW 6 rows, <|) 0.08-0.10 mm, 1 0.05-0.08 mm, 10.05 mm, IW 4 rows, <>0.11 mm, 1 0.08 mm, t 0.05 mm, S 5 rows, <|) 0.08 mm, 1 0.08 mm, t 0.05 mm, T (tabulae) 6 rows, <J> 0.1 mm, I 0.04 mm, t

Type-species. Tomocyathus kundatus Rozanov in Rozanov & Missarzhevskiy 1966. Diagnosis. Outer wall with independent microporous sheath. Inner wall with simple pores. Septa porous. Tabulate flat or slightly domed, irregularly spaced. Stratigraphic range. Atdabanian 2 to Botomian 3. Distribution. Siberian Platform, Altai Sayan, Mongolia, Far East, North Africa, Western Europe, Australia.

MENNERICYATHUS DISSITUS Kruse 1982 (Fig. 7c, d) 1982 Mennericyathus dissitus Kruse, p. 191-193, pi. 12, figs 7-11, pl.13, figs 1-7. Material. Fifteen specimens: SAMP29094, L82034, 82036 from site 3, L82029-82030 from site 4 (27.5 m below top SHF), SAMP29095, L82031, 82033 from site 5 (71.9 m below top

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F. Debrenne & D.I. Gravestock

SHF and 169.5 m above base FTL), upper Sellick Hill Formation and mid Fork Tree Limestone. Description. Cup conical with plate-like expansion in adult stage, mostly found as wavy ribbonlike fragments in the rock. Outer wall double, inner wall simply porous. Tabulae irregularly spaced. Dimensions. D larger than 17 mm, I 1.0 mm, Is 0.41-0.51 mm, IC 1/1.6-1/1.2, OW n 4, f 0.11 mm, 1 0.04 mm, t 0.08 mm, IW 3 rows, (|) 0.08 mm, 1 0.07 mm, t 0.05 mm, S and T<j) 0.08 mm, 1 0.04 mm, 10.04 mm. Discussion. Bracts of the inner wall of the type material have not been observed here, but other characteristics are very close.

Fig. 9 Anaptyctocyathus sellicksi (Taylor), syntype T1560, part of longitudinal section showing tabula (arrowed), x3.5.

MENNERICYATHUS LATUS Debrenne & Gravestock sp. nov. (Fig. 8a)

Discussion. Differs from other species except M.echinus (Debrenne) by the width of the intervallum. M.echinus has a lower radial coefficient and more frequent tabulae.

Derivatio nominis. Latin latus = large, referring to width of intervallum.

Family ANAPTYCTOCYATHIDAE Debrenne 1970

Material. One specimen: holotype SAMP21900 from site 3, upper Sellick Hill Formation.

ANAPTYCTOCYATHUS Debrenne 1969

Description. Outer wall double, with thin independent sheath. Large intervallum crossed by numerous porous septa and rare tabulae. Inner wall spines present. Dimensions. D 11.3 mm, 12.6 mm, Is 0.41, N 70, IK 0.23, RK 6, OW 4 rows, <> 0.11 mm, 1 0.04 mm, t 0.11 mm, microspores 4, $ 0.06 mm, 1 0.02 mm, t 0.05 mm, IW 2 rows, <j) 0.11 mm, 1 0.08 mm, t 0.11 mm, S and T<|) 0.11 mm, 1 0.15 mm, 10.04 mm.

Type-species. Coscinocyathus ford & Bedford 1934.

cribripora Bed-

Diagnosis. Outer wall with non-independent microporous sheath attached to the carcass. Inner wall simply porous, smooth or sometimes spinose. Septa and tabulae porous. Stratigraphic Botomian 3.

range.

?Atdabanian

Distribution. Australia, China, Antarctica.

Fig. 8 a. Mennericyathus latus Debrenne & Gravestock sp.nov., SAMP29100, tranverse oblique section, b. Stromatolitic structure encrusting archaeocyathan cup, SAMP29101. c. lErugatocyathus cf. mawsoni Gravestock, SAMP29102, two cups in tranverse section, d. Anaptyctocyathus sellicksi (Taylor), SAMP29103, lower part and expanding cup. e. Anaptyctocyathus sellicksi (Taylor), SAMP29104, lower part with tabula; on left, part of llnacyathella sp.

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Fleurieu Peninsula archaeocyaths

ANAPTYCTOCYATHUS SELLICKSI (Taylor 1910) (Figs 8d, e; 9) 1910 Archaeocyathus sellicksi Taylor p. 126, pl.2 figs 2, 3, pl.3 figs 9, 12, pl.9 figs 48-52, text figs 5,9,10,16-20. 1935 Archaeocyathus sellicksi Okulitch, p.92. 1937 Archaeocyathus sellicksi Bedford R. & Bedford J., p.32. 1939 Archaeocyathus (Archaeocyathellus) sellicksi Simon, p.53. 1983 ? Ajacicyathus sellicksi Gravestock, p.68. Material. Syntypes T1560, T1582. Thirty additional specimens: SAMP29103, L82015, 82017, 82019, 82020, 82023 from site 1, SAMP29107, L82026 from site 2, L82034 from site 3, SAMP29104,29105, L82027,82029 from site 4, L82032, 82033 from site 5 (71.7 m below top SHF and 169.5 m above base FTL). Upper Sellick Hill Formation to mid Fork Tree Limestone. Description. Conical cup with narrow intervallum and large central cavity at the top. Ribbon fragments present in the rocks correspond to cups larger than 20 mm diameter for intervallum width of 2 mm. Cups anchored by exotheca developing into several successive zones of buttresses radiating outwards from septa. Lower part of central cavity filled with endothecal tissue. Outer wall main pores covered by sieves of micropores. Inner wall simple or with spiny processes. Septa pierced by round to oval pores, thickened by stereoplasma. Tabulae sporadic, irregularly porous.

the inner and outer walls. However, syntype T1560 clearly showed the presence of simply porous, fiat, remote tabulae (Fig.9). This, plus the presence of an attached outer wall microporous sheath and spinose inner wall on new specimens has led to our assignment of the species to Anaptyctocyathus.

ERUGATOCYATHUS Debrenne 1969 Type-species. Coscinocyathus papillatus Bedford & Bedford 1934. Diagnosis. Outer wall with several rows of pores per intersept covered by attached microporous sheath. Inner wall pores shielded by bracts. Septa and tabulae fully porous. Tabulae flat to slightly arched, irregularly spaced. Stratigraphic range. Atdabanian 3 to Botomian 3. Distribution. Australia, China, Antarctica, South Africa.

? ERUGATOCYATHUS cf. MAWSONI Gravestock 1984 (Fig. 8c) 1984 Erugatocyathus mawsoni Gravestock, p. 83, figs 13E-G, 45E-I. Material. Nine specimens: SAMP29102, L82035 from site 3, L82027, 82029 from site 4, L82032, L82033 from site 5 (71.9 m below top SHF and 169.5 m above base FTL). Upper Sellick Hill Formation to mid Fork Tree Limestone.

Dimensions. D 4.12 mm, 10.9 mm, Is 0.51 mm, N 12, IK 0.22, RK 3, OW carcass 4 rows, <|) 0.08 mm, 1 0.11 mm, 10.11 mm, microspores 4, (j) 0.05 mm, 1 0.03 mm, 10.03 mm, IW 3 rows, <>| 0.15 mm, 1 0.11 mm, t 0.15 mm, S 7 rows, <j) Description. Cylindroconical cup. Outer wall with microporous sheath of unknown type. Inner 0.10 mm, 1 0.10 mm, t 0.11 mm, T unknown, D wall spinose. Septa porous. Tabulae with reticular 20 mm, 12.0 mm, Is 1.0 mm, IK 0.10, IC 1/2. porosity. Discussion. Only two syntypes were available for Dimensions. D 5 mm, 11 mm, Is 0.51 mm, N 24, examination. Fine detail was either too recrystallised or too obscured by endo- and exothecal IK 0.20, RK 4.8, OW carcass cj> 0.11 mm, 1 tissue to be absolutely certain about the nature of 0.07 mm, 10.07 mm, IW<|) 0.11 mm, 1 0.08 mm,

F. Debrenne & D.I. Gravestock

10.11 m m , septal porosity not observed, T 6 rows, <> | 0.08 mm, 1 0.04 mm, 10.04 m m . Discussion. The spinose inner wall recalls that of E. mawsoni Gravestock, but other characteristics are not well enough preserved to be absolutely sure of the generic attribution.

307

BEDFORD R. & BEDFORD W.R. 1934. New species of Archaeocyathinae and other organisms from the Lower Cambrian of Beltana, South Australia. Memoirs of the Kyancutta Museum 1, 1-7. CARROLL P.G. 1982. Phosphates in Early Cambrian limestones, Fleurieu and Yorke Peninsulas, South Australia. B.Sc.(Hons) thesis, University of New England (unpubl.).

ACKNOWLEDGEMENTS We are very grateful to Dr Jim Jago for accompanying Max and Francoise Debrenne to the Sellick Hill area in 1976 and our late friend Dr Brian Daily for his help and hospitality during that period. Recent collections were made possible by the willing collaboration of Elinor Alexander. Special thanks are due to Neville Pledge (South Australian Museum) who readily made syntypes of A. sellicksi available for examination. Thanks to Michel Lemoine for the prompt fabrication of thin sections and Max Debrenne for photography.

REFERENCES ABELE C. & McGOWRAN B. 1959. The geology of the Cambrian south of Adelaide (Sellick Hill to Yankalilla). Transactions of the Royal Society of South Australia 82,301-320. ALEXANDER E.M. & GRAVESTOCK D.I. (this volume). Sedimentary facies in the Sellick Hill Formation, Fleurieu Peninsula, South Australia. BEDFORD R. & BEDFORD J. 1936. Further notes on Cyathospongia (Archaeocyathi) and other organisms from the Lower Cambrian of Beltana, South Australia. Memoirs of the Kyancutta Museum 3, 21-26. BEDFORD R. & BEDFORD J. 1937. Further notes on Archaeos (Pleospongia) from the Lower Cambrian of South Australia. Memoirs of the Kyancutta Museum 4, 27-38. BEDFORD R. & BEDFORD J. 1939. Development and classification of Archaeos (Pleospongia). Memoirs of the Kyancutta Museum 6, 67-82.

CONNELL J.H. 1978. Diversity in tropical rain forests and coral reefs. Science 199, 1302-1310. DAILY B. 1956. The Cambrian in South Australia. In El Sistema Cambrico, su paleogeografia y el problema de su base, Vol.2, pp.97-147. Report of the 20th International Geological Congress, Mexico, 1956. DAILY B. 1963. The fossiliferous Cambrian succession on Fleurieu Peninsula, South Australia. Records of the South Australian Museum 14, 579-601. DAILY B. 1972a. Aspects of carbonate sedimentation in the Cambrian of South Australia. Geological Society of Australia, Joint Specialists Groups, Abstracts, 10-14. DAILY B. 1972b. The base of the Cambrian and the first Cambrian faunas. University of Adelaide Centre for Precambrian Research Special Paper 1, 13-41. DAILY B. 1976. New data on the base of the Cambrian in South Australia. Izvestiya Akademiya Nauk SSSR seriya geologiya 3,45-52. DAILY B. & MILNES A.R. 1973. Stratigraphy, structure and metamorphism of the Kanmantoo Group (Cambrian) in its type section east of Tunkalilla Beach, South Australia. Transactions of the Royal Society of South Australia 97, 213-251. DEBRENNE F. 1958. Sur quelques Archaeocyatha du Jbel Tai'ssa (Anti-Atlas occidental). Notes et Memoirs Services Mines Carte Geologique Maroc 16, 59-67. DEBRENNE F. 1964. Archaeocyatha. Contribution a 1'etude des faunes cambriennes du Maroc, de Sardaigne et de France. Notes et Memoirs Services Mines Carte Geologique Maroc 179.

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Fleurieu Peninsula archaeocyaths

DEBRENNE E 1969. Lower Cambrian Archeocyatha from the Ajax Mine, Beltana, South Australia. Bulletin of the British Museum of Natural History (Geology) 17(7), 295-376.

GRAVESTOCK D.I. 1983. Structure and function of the exothecal tissue of Somphocyathus coralloides Taylor and allied regular Archaeocyatha. Memoir of the Association of Australasian Palaeontologists 1,67-74.

DEBRENNE F. 1970. A revision of Australian genera of Archaeocyatha. Transactions of the Royal Society of South Australia 94,21-49.

GRAVESTOCK D.I. 1984. Archaeocyatha from lower parts of the Lower Cambrian carbonate sequence in South Australia. Memoir of the Association of Australasian Palaeontologists 2, 1-139.

DEBRENNE F. 1977. Archaeocyathes du Jbel Irhoud (Jebilets, Maroc). Bulletin societe geologique et mineralogique de Bretagne 7, 93-136. DEBRENNE F. & JAMES N.P. 1981. Reef-associated archaeocyathans from the Lower Cambrian of Labrador and Newfoundland. Palaeontology 24, 343-378. DEBRENNE F. & KRUSE RD. 1986. Shackleton Limestone archaeocyaths. Alcheringa 10, 235-278. DEBRENNE F. & ROZANOV A.Yu. 1974. Mennericyathus, a new Tomocyathus-like archaeocyathid. Journal of Paleontology 48, 607-608. DEBRENNE F. & ROZANOV A.Yu. 1983. Paleogeographic and stratigraphic distribution of regular Archaeocyatha (Lower Cambrian fossils). Geobios 16(6), 727-736. DEBRENNE F. & ROZANOV A.Yu. 1985. On Dokidocyathus Taylor. Akademiya Nauk SSSR, Seriya geologiya 3, Moskva, 111-112. DEBRENNE F. & VORONIN Yu.J. 1971. Significance of septal porosity for the classification of ajacicyathids. Paleontologicheskiy Zhurnal 1971(3), 26-31. DEBRENNE F., ZHURAVLEV A.Yu & ROZANOV A.Yu 1989. Regular Archeocyatha. Trudy Paleontologichshiy Institut Akademiya Nauk SSSR Moskva 233. GATEHOUSE C.G., JAGO J.B. & COOPER B.J. (this volume). Sedimentology and stratigraphy of the Carrickalinga Head Formation (low stand fan to high stand systems tract), Kanmantoo Group, in South Australia.

HORWITZ R. & DAILY B. 1958. Yorke Peninsula. In Glaessner M.F. & Parkin L.W.eds. The geology of South Australia. Journal of the Geological Society of Australia 5, 46-60.

HOWCHIN W. 1897. On the occurrence of Lower Cambrian fossils in the Mount Lofty Ranges. Transactions of the Royal Society of South Australia 21, 74-86.

JAGO J.B., GEHLING J.G. & DAILY B. 1986. Cambrian sediments of the Sellick Hill-Carrickalinga Head area, Fleurieu Peninsula, South Australia. In Parker A J.compiler. One day geological excursions of the Adelaide region. Eighth Australian Geological Convention, 67-81.

KRUSE P.D. 1982. Archaeocyathan biostratigraphy of the Gnalta Group at Mt Wright, New South Wales. Sonder-Abdruck aus Paleontographica beitrage zur naturgeschichte der vorzeit, Abteilung, A. Bild 177, 129-212.

LAURIE J.R. 1986. Phosphatic fauna of the Early Cambrian Todd River Dolomite, Amadeus Basin, central Australia. Alcheringa 10,431-454.

OKULITCH V.J. 1935. Cyathospongia - a new class of Porifera to include the Archaeocyathinae. Transactions of the Royal Society of Canada series 3, section 4 40, 73-86.

F. Debrenne & D.I. Gravestock

ROZANOV A. Yu. 1969. Some problems on the systematics of Archaeocyatha. In Zhuravleva I.T. ed. Biostratigraphy and palaeontology of the Lower Cambrian of Siberia and the Far East. Nauka Moskva, 106-113. ROZANOV A. Yu. & MISSARZHEVSKIY V.V. 1966. Biostratigraphy and fauna of the lower horizons of the Cambrian. Trudy Geologicheskiy Ins tit ut 148. SIMON W. 1939. Archaeocyathacea. I.Kritische Sichtung der Superfamilie. II.Die fauna im Kambrium der Sierra Morena (Spanien). Abhandlungen der Senckenbergischen naturforschenden Gesellschaft, Abhandlung 448, 1-87. TAYLOR T.G. 1910. The archaeocyathinae from the Cambrian of South Australia, with an account of the morphology and affinities of the whole class. Memoirs of the Royal Society of South Australia 2, 55-188. VOLOGDIN A.G. 1937. Archaeocyatha, and the results of their study in the USSR. Problemy Paleontologicheskiy 2-3,453-500, Moskva.

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VOLOGDIN A.G. 1956. On the classification of the phylum Archaeocyatha. Doklady Akademiya Nauk SSSR 3(4), 877-880. ZHURAVLEV A.Yu. 1986. Evolution of archaeocyaths and palaeobiogeography of the Early Cambrian. Geological Magazine 123, 377-385. ZHURAVLEVA I.T. 1960. Archaeocyatha of the Siberian Platform. Akademiya Nauk SSSR Moskva, 344p. ZHURAVLEVA I.T., KONYUSHKOV K.N. & ROZANOV A.Yu. 1964. Archaeocyatha of Siberia: Two-walled Archaeocyatha. Akademiya Nauk SSSR Institut Geologii i Geofiziki Sibirskovo Otdeleniya Paleontologicheskiy Institut, Moskva, 132p. ZHURAVLEVA I.T., KORSHUNOV V.I. & ROZANOV A.Yu. 1969. The Atdabanian Stage and its significance based on Archaeocyatha from the stratotypical section. In Zhuravleva I.T. ed. Biostratigraphy and Palaeontology of the Lower Cambrian of Siberia and the Far East. Nauka, Moskva, 5-59.

Are archaeocyaths sponges, or are sponges archaeocyaths? Peter D. Kruse Northern

Territory Geological

Survey, P.O. Box 2901, Darwin, N.T. 0801,

Australia

Discoveries of living sclerosponges having a calcareous skeleton (sclerosome), with or without associated siliceous spicules, have led to a reevaluation of the affinities of many fossil groups of uncertain systematic position. Should the Archaeocyatha be included among the sponges? Closest similarities are with the sphinctozoan sponges. Exopore modifications of some sphinctozoans have analogues in the wall pores of certain archaeocyaths. Some gross morphological comparisons between selected taxa of sphinctozoans and archaeocyaths show striking similarities. Archaeocyathan microstructure is similar if not identical to the irregular type of sphinctozoan microstructure. Archaeocyathan soft parts are best reconstructed on a filter feeding sponge model. However, the laminated stereoplasm of archaeocyaths is not known with certainty among sphinctozoans, and there are differences in initial ontogenetic stages. Recent discoveries in the Cambrian of Australia suggest a possible pathway for the derivation of sphinctozoans from monocyathine archaeocyaths via modification of the pelta and skeletal microstructure.

Key words: Archaeocyatha, Porifera, Sphincotozoa, skeletal morphology, skeletal microstructure, Archaeata, classification. INTRODUCTION

Recent discoveries of living sclerosponges having a calcareous skeleton or sclerosome, with or without associated siliceous spicules, have led to a reevaluation of the affinities of many fossil groups of uncertain systematic position. The spicules of sclerosponges, where present, may be either incorporated into the sclerosome and subsequently resorbed, or remain free within the living matter. In either case, the spicules are unlikely to be fossilised. Preserved spicules are thus no longer a prerequisite for assignment of a fossil group to the sponges. Consequently, some groups of

sessile

organisms,

for

example

the

stromatoporoids (Stearn 1972) and chaetetides (Hartman & Goreau

1972) have now

been

referred to the Porifera. The tabulates have gained a reprieve with the discovery of preserved coral polyps in Favosites

colonies (Copper 1985).

What of the archaeocyaths? Some leading archaeocyath exponents (Debrenne & Vacelet 1984) are now proposing their inclusion within the phylum Porifera.

Fig. 1 Archaeocyath Mawsonicoscinus sigmoides Debrenne & Kruse, holotype MG513, longitudinal section (outer wall to right) x5. Early Cambrian (Botomian stage equivalent), Shackleton Limestone, Nimrod Glacier, Antarctica.

Are archaeocyaths sponges? Comparisons of archaeocyaths with sphinctozoan sponges are particularly apt. The Archaeocyatha (Early-Late Cambrian) have been related variously to the sponges, corals, radiocyaths, aphrosalpingoids, stromatoporoids, foraminifers, receptaculitaleans or dasycladacean algae. The sphinctozoans (Cambrian Recent) are unquestionably sponges, as the living matter and spermatogenesis of the extent Vaceletia crypta have aspects in common with demosponges (Vacelet 1979), and as well, some Cretaceous genera have preserved calcareous spicules (Reid 1968), while a Triassic example has calcite pseudomorphs after siliceous spicules (Reitner 1987). This article examines the relationships between archaeocyaths and sphinctozoans, both benthic organisms with a calcareous skeleton, in the areas of skeletal morphology, microstructure, soft parts and stereoplasm, with comments on classification. ARCHAEOCYATHS Gerbicanicyathus

•

\A K •

• !

Tabulacyathus

o SPHINCTOZOANS

V .H V—N

V~t

v—<

Barroisia

Vaceletia

Fig. 2 Gross morphological comparison of longitudinal sections: archaeocyaths Gerbicanicyathus and Tabulacyathus (both Early Cambrian), sphinctozoans Barroisia (Cretaceous) and Vaceletia (Recent). Not to scale. [After Belyaeva & Nikitina (1984), Hill (1972) and Vacelet (1979) ].

311

SKELETAL MORPHOLOGY Sphinctozoans are basically a series of stacked chambers, with or without an axial siphon. Such metamerism is exhibited also in archaeocyaths, as evidenced by episodic (periodic?) annulations of the cup, regularly spaced tabulae (or arched septal pore rows in atabulate forms) and concentrically zoned exothecal structures (Zhuravleva 1974; Debrenne 1980). In some tabulate archaeocyaths (Gerbicanicyathidae, Mawsonicoscinacea and Clathricoscinacea in Regulares, some Archaeosyconina in Irregulares), the outer wall is non-independent, consisting of the downtumed outer edges of tabulae (Fig.l). In gross morphology, these forms approach the typical sphinctozoan architecture, with the tabulae analogous to the sphinctozoan interwall and exowall. Gerbicanicyathus and Tabulacyathus (Botomian stage, USSR) are striking examples of this (Zhuravleva & Myagkova 1974; Belyaeva & Nikitina 1984; Fig.2). Even radial partitions superficially similar to septa are now known among sphinctozoans, in the Triassic Phragmocoelia Ott 1974 and Devonian Radiothalamos Pickett & Rigby 1983. In detail, the "pores" in the partitions of the latter genus are unlike the regular, ordered pores of archaeocyathan septa and might equally be regarded as interstices between radially flattened pillars. In wall pore architecture, parallels can be drawn also between the centripetally directed marginal spines of living Vaceletia and Cambrian Jawonya Kruse 1987, and the inner wall spinose screen of Veronicacyathus Debrenne 1973 (Fig.3); between the exaules and labripores (Finks 1983) of sphinctozoans and the simple tumuli of archaeocyaths (Fig.4); and between the external sieves of Wagima Kruse 1987 and the multiperforate tumuli of archaeocyaths (Fig.5). The perennial dilemma is whether these gross similarities should be taken to imply a close phylogenetic relationship, or are merely convergences in otherwise distant groups of organisms sharing similar lifestyles. Both views have had

312

P.D. Kruse

their adherents: Ott (1967) favoured the latter view, while Taylor (1910), Zhuravleva (1959; Zhuravleva & Rezvoy 1956) and Finks (1970) suggested that archaeocyaths were possibly ancestral to some sphinctozoans. Some Jurassic and Cretaceous sphinctozoans in addition to the sclerosome possess calcareous spicules (Reid 1968; Wendt 1979) ; the living Vaceletia crypta has none. No spicules have been found in archaeocyath-bearing deposits which cannot be attributed to the hexactinellide or heteractinide sponges (Finks 1967).

MICROSTRUCTURE The primary archaeocyathan skeleton invariably consists of interlocked microgranular calcite polyhedra with randomly oriented c-axes (Hill 1964), the dark tone of which Vologdin (1962) and Zhuravleva (1963) uncritically attributed to included dispersed organic matter. In

Archaeocyathus atlanticus the polyhedra of the primary skeleton are in the size range 4-S\im, while polyhedra of the stereoplasm (secondary skeleton - see section below) are smaller, in the range 2-3|im; the boundary between primary and secondary skeleton is sharp (Lafuste & Debrenne 1977; Debrenne & James 1981). As the primary skeleton microstructure is finer than that expected from recrystallised aragonite, an original calcite mineralogy is presumed (see also James & Klappa (1983).

Primary mineralogy of the sphinctozoan skeleton has varied through the Phanerozoic (Wendt 1979) : almost exclusively aragonite in Carboniferous-Triassic, calcite in JurassicCretaceous, aragonite in Recent time. Wendt (1979) recognised four distinct primary microstructures: spherulitic, clinogonal, orthogonal, irregular. Only the last type approaches that observed in archaeocyaths: it is an aggregate of loosely packed rodlike crystals, generally randomly oriented, of length 4-8|im. This

microstructure is present in Vaceletia crypta and some Triassic sphinctozoans (Wendt 1984).

Fig. 3 Comparison of centripetal spines in wall pores of sphinctozoans and archaeocyaths. A, Recent sphinctozoan Vaceletia crypta Vacelet, specimen from lies Glorieuses, Indian Ocean, exowall xl80. B, Middle Cambrian sphinctozoan Jawonya gurumal Kruse, holotype P8553 from Tindall Limestone, Northern Territory, Australia, exowall x8. C, Early Cambrian archaeocyath Veronicacyathus concavus Kruse, holotype SUP75283 from Cymbric Vale Formation, New South Wales, Australia, inner wall xl8. [A after Vacelet (1979) ].

Are archaeocyaths sponges? Micritisation of this and other microstructures can produce a granular pseudostructure of interlocked crystals l-5|im in diameter, not unlike the archaeocyathan microstructure. Secondary skeleton (filling tissue) in sphinctozoans may or may not have a microstructure different to that of the primary skeleton. SOFT PARTS In the extant sphinctozoan Vaceletia crypta the living matter is largely internal to the skeleton, with only a fine film 2-20jam thick sheathing the external surface. Growth is by episodic addition of chambers at the distal end; the skeleton is secreted as an organic matrix, with rapid progressive calcification. Living matter in the newly formed chamber is continuous with that in the subjacent chamber (Vacelet 1979).

SPHINCTOZOAN

313

Interpretation of the living matter of the entirely fossil archaeocyaths begins with the assumption that the wall pores serve a water filtering function. The alternatives, that the pores act as apertures for pseudopodia, filaments or gamete dispersion, make no sense in view of the presence of porous elements in the intervallum, which would be without functional significance under those conditions (Debrenne 1983). Most exponents of the group share the view that archaeocyaths were filter feeders. The filtrative function of the wall pores accepted, the observation that these pores are present on both walls of two-walled forms functionally constrains the living matter largely to the confines of the intervallum, with at least a thin film of living matter coating the skeleton (including a thin film on the external surface (Fig.6). Zhuravleva(1959,1963,1974; Zhuravleva& Elkina 1974; Zhuravleva & Myagkova 1979) has

ARCHAEOCYATH

Angullongia Tumuliolynthus Fig. 4 Comparison of exaules of sphinctozoan Angullongia vesica Webby & Rigby (Late Ordovician, New South Wales, Australia) xl .5 and simple tumuli of archaeocyath Tumuliolynthus tubexternus Vologdin (Early Cambrian, Siberian Platform and Altay Sayan Fold Belt) xl5. [A.vesica figure reproduced from Alcheringa with permission; Tumuliolynthus figure after Zhuravleva (1963) ].

314

P.D. Kruse

speculated extensively on the nature of the archaeocyathan living matter. In her model, the living matter differs from the protoplasm of protozoans and the mesoglea of sponges, being confined to the intervallar loculi, and consisting of generalised cells grouped in specialised associations (though not at the tissue grade) in individual loculi, with intracellular nutrition (digestion) and no excretory organs, nervous system or muscular chambers. There were perhaps sex cells in some loculi.

ing to him that the skeleton developed from initially non-calcified soft parts in the growth region. This inferred mode of skeletogenesis is compatible with that operating in Vaceletia crypta. Much debate has been joined regarding the direction of water flow in archaeocyaths. Zhuravleva (Zhuravleva & Elkina 1974) for example, on the basis of a single Siberian specimen showing what is patently a subsequent diagenetic encrustation, proposed that these structures were in fact calcified living matter indicating a flow direction from the central cavity and inner wall through the intervallum and outer wall to the exterior.

The inference of Zhuravleva & Myagkova (1979) that archaeocyaths and some other problematic benthic groups (their Archaeata - see section below) had an "elastic" skeleton, later calcified, is supported for the former by Brasier's (1976) observations on archaeocyaths in the Wilkawillina Limestone of South Australia. In these, Brasier found cups were often distorted where juveniles had attached to the wall, suggest-

Debrenne & Vacelet (1984), in (hopefully) providing the final word on this issue, pointed out that the inner wall pores are typically larger than those of the outer wall and hence filtration was possible only with a water flow in the direction

Wagima

Lenocyathus

Fig. 5 Comparison of sieves of sphinctozoan Wagima galbanyin Kruse, holotype P8598 (Middle Cambrian, Tindall Limestone, Northern Territory, Australia) xlO and multiperforate tumuli of archaeocyath Lenocyathus lenaicus Zhuravleva (Early Cambrian, Siberian Platform) xl20. [Lenocyathus figure after Zhuravleva (1960) ].

Are archaeocyaths sponges?

outer wall-intervallum-inner wall, exiting from the central cavity. This is the flow direction in sponges. It is supported by Brasier's (1976) ob-

315

servation that in the Wilkawillina Limestone, juveniles preferentially settled on the outer wall rather than the inner. With the specific addition of a canal system and choanocyte chambers in accord with the earlier views of Vologdin (1937, p.490-492), it is but a short step from Zhuravleva's views on archaeocyathan soft parts to a functional model fully compatible with that of sponges.

STEREOPLASM Stereoplasm is a secondary thickening in parallel laminations on the primary skeleton of archaeocyaths (Debrenne & Rozanov 1978). As such it includes the exo- and endothecal structures of Debrenne (1964; cf. Taylor 1910), the pellis, and at least some if not all dissepiments. Its microstructure is granular, but finer than that of the primary skeleton (Lafuste & Debrenne 1977). It is well illustrated by Debrenne & James (1981) for Archaeocyathus atlanticus. Laminations are distinguished by zones of differing tone under the light microscope, or zones of granules of differing size under SEM. There is continuity of individual laminae between exo-, intervallar and endostructures, with uniform microstructure throughout.

Fig. 6 Interaction of archaeocyaths Bractocyathus labiosus Kruse (below) and Aporosocyathus gnaltaensis Kruse (above). The B. labiosus cup has secreted a thin aporose sheet of skeletal material (arrowed) in response to the proximity of the A. gnaltaensis cup. The gap between the sheet and the outer wall of A. gnaltaensis is an indication of the maximum thickness of any film of living matter coating the external surface of the latter cup. SUP75243, Early Cambrian, Cymbric Vale Formation, Mt Wright, New South Wales, Australia xl5.

Several possible functions of the stereoplasm have been proposed: fixation (Taylor 1910 and others); response (perhaps pathological - Vologdin 1962) to a stimulus (Brasier 1976), calcified living matter (Vologdin 1948) ; dimorphism (Zhuravleva 1974); epibiont organisms (Vologdin 1962) ; coenosteum (Debrenne & James 1981). The latter three are inherently speculative and are difficult to assess with available evidence. The laminated nature of stereoplasm indicates episodic accretion, and renders untenable notions of accidental post-mortem calcification of living matter and "internal organs" in the central cavity (Vologdin, 1948,1957,1962). More recent observations (Brasier 1976; Gravestock 1983) point to

316

P.D. Kruse

an intimate link with the substrate, be it sediment or archaeocyathan cup. Brasier (1976) found an association of exothecal stereoplasm with attachment sites of juvenile regular archaeocyaths or with the close growth of two or more adult cups in Wilkawillina Limestone biocoenoses. Debrenne & Rozanov (1978) noted that in cases where stereoplasm enveloped two or more cups, it was produced by one of them, and hence was epibiont on the others. It seems on present evidence that stereoplasm played a fixation-support role, perhaps as a "proliferative response of ectodermal cells in contact with foreign-cell surfaces" (Brasier 1976, p.238) or an inorganic substrate. Dissepiments, although generated by the same fundamental mechanism as other forms of stereoplasm, differ in being confined to the intervallum and central cavity of archaeocyathan cups. They are more common in the lower parts of cups, and seal off areas of the intervallum or central cavity, rendering them functionally inoperative. This implies a progressive atrophy and withdrawal of the living matter from the initial parts of the cup with growth (Vologdin 1962; Ziegler & Rietschel 1970), as is observed in the sphinctozoan Vaceletia crypta.

According to Debrenne & Vacelet (1984), the laminated microstructure seen in archaeocyathan stereoplasm is not known in living or fossil sphinctozoans. One possible exception is Triassic Cryptocoelia zitteli described by SenowbariDaiyan & Reid (1987, p.894) with "trabecular tissue and chamber walls...laminated in places". Apart from vesicles (=dissepiments in archaeocyathan terminology), Ott (1967) recognised three types of sphinctozoan filling tissue: reticular, trabecular, tubular. Not all of these can be compared to archaeocyathan stereoplasm some appear to be part of the primary skeleton as understood herein. Vesicles are certainly secondary, and are known from Late Ordovician time (Angullongia Webby & Rigby 1985). In Vaceletia crypta, a secondary thickening covers the dead basal parts, in some cases completely covering the exopores. Vacelet (1979) suggests the internal thickening is secreted by the sphinctozoan, while the external thickening may be due to physicochemical action associated with the environment or attached organisms. Personal examination of a specimen from the Great Barrier Reef establishes that the external and internal thickenings are continuous and optically indistinguishable, and hence attributable to a single origin: secretion by the sphinctozoan itself. The thickening consists of successive vesicle-shaped

Fig. 7 Vaceletia crypta from the Great Barrier Reef. A, longitudinal section x6. B, detail of longitudinal section showing secondary thickening toward base of skeleton x27.

Are archaeocyaths sponges?

additions of optically microgranular aragonite, unlike the laminated stereoplasm of archaeocyaths (Fig.7).

ARCHAEATA In a series of publications, Zhuravleva & Myagkova (1972, 1974, 1979, 1981, 1983; Myagkova 1985) have proposed a new kingdom Archaeata, to include the following fossil groups: phylum Archaeocyatha with subphyla Euarchaeocyatha (Cambrian) and Aphrosalpingata (Siluro-Devonian) ; phylum Receptaculita with classes Soanitida (Ordovician), Receptaculitida (Ordovician-?Carboniferous) and Radiocyatha (Cambrian) (Zhuravleva & Myagkova 1981, modified by Myagkova 1985). Zhuravleva & Myagkova (1972) listed the following definitive characteristics of the kingdom: sessile marine filter-feeding multicellular organisms with porous calcareous skeleton, cuplike form, two walls with interwall space and central cavity, and stereoplasm in some. These authors have further postulated that archaeates were organisms without differentiation into tissues and organs, wherein physiological functions (digestion, excretion, respiration) were intracellular, lacking a true nervous system and the double layering characteristic of the 'Eumetazoa', and capable of dior polymorphism involving cups and stereoplasm; cells were totipotent as in the sponges (Zhuravleva & Myagkova 1974,1981).

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"... a thorough morphological comparison of the skeleton of some stromatoporoids and archaeocyaths (Archaeata) does not exclude a probable interpretation of Palaeozoic stromatoporoids, or some of them (Amphipora, etc.), as a group close to Archaeata or included with them" (1974, p.68); "The Porifera could be partially included [in Archaeata], particularly certain sponges with a non-spicular calcareous skeleton..." (1979, p.525). Such utterances are now paradoxical in the light of their proposal for an independent kingdom. In their most recent contribution, Zhuravleva & Myagkova (1987) grouped the Archaeata together with the Porifera as subkingdoms within a new kingdom Inferibionta. This was felt necessary in order to accommodate recent views (Debrenne & Vacelet 1984; Zhuravlev 1985; Zhuravlev & Nitecki 1985; Nitecki 1986) favouring a closer relationship of archaeocyaths with sponges, and conversely little or no connection between archaeocyaths and receptaculitaleans. The kingdom Inferibionta was justified as including organisms with loose associations of cells, structurally intermediate between single celled organisms (Procaryota, Protista) and multicelled organisms with tissues and organs (Fungi, Plantae, Animalia).

"[Archaeata] differ from sponges (Porifera) in the absence of spicules..." (1972, p.9);

Are the Archaeata any more than a gathering of otherwise unrelated groups united by a sessile lifestyle? The receptaculitaleans and soanitides are treated by some (Nitecki 1970, 1972; Rietschel 1977; Nitecki & Toomey 1979) as calcareous algae, while of course the archaeocyaths have recently been referred to the Porifera by Wendt (1980) and Debrenne & Vacelet (1984).

"Differences in the structure of the skeleton between Archaeata and Palaeozoic Sphinctozoa are practically absent; yet the discovery in recent years... of spicules in Mesozoic representatives of this group shows that we cannot consider Sphinctozoa as part of the Archaeata" (1972, p. 10);

There are difficulties in Zhuravleva & Myagkova's definition of the Archaeata - not all the included groups consistently display all the features attributed to the (sub) kingdom (above), and some groups not included in the (sub) kingdom display some of the features. Zhuravleva & Myagkova appealed to Whittaker's (1969)

On the question of relationships of Archaeata with f1 , Porifera, Zhuravleva & Myagkova have offeied the following comments:

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five-kingdom system in establishing the Archaeata. In that system, apart from the procaryotic Monera and eucaryotic unicellar Protista, the three eucaryotic multicellular kingdoms (Plantae, Fungi, Animalia) are distinguished by their mode of nutrition: photosynthesis, absorption, ingestion respectively. Zhuravleva & Myagkova (1972,1983) place the Archaeata close to Animalia and Fungi in invoking both ingestive and absorptive aspects to archaeate nutrition. In the absence of a workable list of distinguishing criteria, such speculations seem insufficient to justify a separate kingdom or subkingdom. In fact there is nothing among Zhuravleva & Myagkova's less controversial speculations regarding archaeocyaths in particular which is inconsistent with the sponge model. DISCUSSION Hill (1972, p.E50) defined the phylum Archaeocyatha as follows: "Skeleton in form of calcareous cup composed of porous wall, or more commonly porous outer and inner walls, with central cavity; porous septa, tabulae, and nonporous rods, bars, and dissepiments may form in intervallum. Plates of skeleton not spiculate, of microgranular calcite". With the possible exception of the last, all the characters listed by Hill are present in porate sphinctozoan sponges. All of the sponge-archaeocyath distinctions advanced by Okulitch & de Laubenfels (1953) in first proposing the phylum Archaeocyatha have been rendered irrelevant by the arguments of Ziegler & Rietschel (1970) - largely with reference to sphinctozoans - or by subsequent research.

Late Palaeozoic and younger sphinctozoans, one of which (corresponding in part to the Sebargasiidae and Verticillitidae of Ott 1967) he derived questionably from the Archaeocyatha. Zhuravlev (1985) elaborated Finks' view in drawing attention to similarities between archaeosyconine archaeocyaths (sensu Hill 1972), and verticillitid sphinctozoans (sensu Boyko 1981), incorporating elements of the Verticillitidae and Cryptoceliidae of Ott (1967) and including Vaceletia crypta). Such similarities had been noted previously (Belyaeva & Nikitina 1984). The verticillitids are retrosiphonate sphinctozoans with flattened chambers supported by pillars ("trabecular" filling tissue of Ott (1967)), the interwalls and pillars analogous to the tabulae and rods of archaeosyconines. Their microstructure is irregular or, in Zhuravlev's view, a granular diagenetic derivative of that (Wendt 1979), Zhuravlev further noted similarities between the ontogenies of 'archaeosyconines' (Korovinellids) such as Altaicyathus Korovinellids, and Praeverticillites.

On the basis of these perceived similarities Zhuravleva postulated a probable phylogenetic link between the two groups. However, there is some difference in ontogenetic development; among regular archaeocyaths the outer wall forms first, then the intervallum structures and inner wall, while in Vaceletia crypta all parts of a new chamber, including pillars, are added simultaneously. Zhuravlev attempted to fill the stratigraphic gap between the latest archaeocyaths (Cambrian) and earliest verticillitids (Permian) by appeal to the stromatoporoids or more fancifully, the anthaspidellids. (In this context, the Middle Devonian Alaskan sphinctozoan Hormospongia Rigby & Blodgett 1983 has been referred to the Verticillitidae (sensu Ott 1967) by those authors. Its filling tissue is reticulate, and quite unlike the trabecular pillars of archaeosyconine ar-

The sphinctozoans are polyphyletic: while calcareous triactinal spicules are present in some Mesozoic genera (Reid 1968), the extant chaeocyaths, Verticillites or Vaceletia.) ArVaceletia crypta has demosponge features in its chaeocyathan-sphinctozoan connections, if real, histology and larval structure (Vacelet 1979). should be sought in the Early Cambrian, during Finks (1970) recognised three major lineages in the period of acme and rapid demise of the ar-

Are archaeocyaths sponges? chaeocyaths, and concurrent early history of the sphinctozoans. A unique Atdabanian- or Botomian-age specimen from the archaeocyathbearing Ajax Limestone of South Australia (Kruse 1987; Fig.8) suggests a possible developmental pathway from one-walled archaeocyaths to sebargasiid sphinctozoans, via modification of the inwardly projecting archaeocyathan pelta as an ambisiphonate rim (Fig.9).

319

chaeocyatha and Porifera as independent phyla (Whittaker 1969; Nitecki & Debrenne 1979), perhaps within a higher-level taxon such as Parazoa (or Inferibionta?). The latter option could only be countenanced if the even more distinctive hexactinellides were accorded phylum status. Such a course has been advocated by Bergquist (1985)

CONCLUSION

Granted the similarities of archaeocyaths and sphinctozoans in gross morphology and inferred functional model of the living matter, the following distinctions remain: 1. Archaeocyathan wall pore elaborations such as microporous sheaths, pore canals and annuli are not known in sphinctozoans. 2. The archaeocyathan microgranular structure is similar but not identical to the irregular microstructure of some sphinctozoans - although the archaeocyathan microstructure may be a diagenetic derivative of an irregular microstructure. 3. Laminated stereoplasm of archaeocyaths is not known with certainty in sphinctozoans or any calcified sponge.

Fig. 8 Possible sphinctozoan ?Jawonya tiro Kruse, holotype P85103, Early Cambrian (Atdabanian or Botomian), Ajax Limestone, Mount Scott Range, South Australia xl.

4. In archaeocyaths the outer wall ontogenetically precedes the intervallum and inner wall; in sphinctozoans, all parts of a chamber are added simultaneously.

/ \

The first and third distinctions could be eliminated by further discoveries, while the second and fourth may be modified by additional research. So how are we to classify the archaeocyaths? There are two basic options: the first, to place the Archaeocyatha as a subphylum or class within the phylum Porifera (Wendt 1980,1984; Debrenne & Vacelet 1984) ; the second, to retain the Ar-

pelta

one-walled archaeocyath

ambisiphonate rim

? Jawonya tiro

Fig. 9 Fanciful phylogenetic development of a simple sphinctozoan such as ?Jawonya tiro from one-walled archaeocyaths via modification of an inwardly projecting toric archaeocyathan pelta as an ambisiphonate rim.

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in elevating Reiswig & Mackie's (1983) subphylum Symplasma (including the hexactinellides) to the rank of phylum, thus recalling the classification of Bidder (1929). Whether as phylum, subphylum or class, the Archaeocyatha seem closely allied to the sphinctozoans, and hence to the Calcarea and/or Demospongia. We cannot say at present exactly to which sphinctozoans they are related. That must await the elucidation of Early-Middle Palaeozoic sphinctozoan lineages, following further discoveries.

ACKNOWLEDGEMENTS J.Pickett (Geological Survey of New South Wales, Sydney), B.D.Webby (University of Sydney) and F. Debrenne (Museum National d'Histoire Naturelle, Paris) commented on preliminary drafts of the manuscript. Aileen Potter printed the photographic figures and Anja Lohi produced the line drawings. Mary Vassallo typed the manuscript. Clive Wilkinson (Australian Institute of Marine Science, Townsville) supplied the Great Barrier Reef specimen of Vaceletia crypta. John Hooper (Northern Territory Museum of Arts and Sciences, Darwin) assisted with literature. This paper is published with the approval of the Secretary, Northern Territory Department of Mines and Energy.

REFERENCES BELYAEVA G.V. & NIKITINA N.P. 1984. Sfinktozoa Dal'nego Vostoka. [Sphinctozoa of the Far East]. Doklady Akademii Nauk SSSR 276, 711-713. BERGQUIST P.R. 1985. Poriferan relationships. In Conway Morris S., George J.D., Gibson R. and Piatt H.M. eds. The origins and relationships of lower invertebrates, 14-27. Clarendon, Oxford. BIDDER G.P. 1929. Sponges. In Encyclopaedia Britannica (14th ed.) Vol.21, 254-261. Encyclopaedia Britannica Company, London.

BOYKO E.V. 1981. O semeystve Verticillitidae Steinmann, 1882, ego sostave i sistematicheskom polozhenii. [On the family Verticillitidae Steinmann, 1882, its composition and systematic position]. Trudy Instituta Geologii i Geofiziki, Sibirskoe Otdelenie 481, 74-82. BRASIER M.D. 1976. Early Cambrian intergrowths of archaeocyathids, Renalcis and pseudostromatolites from South Australia. Palaeontology 19, 223-245. COPPER P. 1985. Fossilized polyps in 430-Myr-old Favosites corals. Nature, 316,142-144. DEBRENNE F. 1964. Archaeocyatha. Contribution a l'etude des faunes cambriennes du Maroc, de Sardaigne et de France. Service des Mines et de Carte Gologique du Maroc, Notes et Memoires 179 (2 volumes). DEBRENNE F. 1973. Modifications de la porosite primaire de la muraille externe chez les archeocyathes reguliers. Annales de Paleontologie 59(1), 3-24. DEBRENNE F. 1980. PhenomSnes de croissance periodique chez les Archaeocyatha (fossiles du cambrien inferieur). Bulletin de la Socite Zoologique de France 105, 285-292. DEBRENNE F. 1983. Archaeocyathids: morphology and affinity. In Rigby J.K. and Stearn C.W. eds Sponges and spongiomorphs. Notes for a short course. University of Tennessee, Department of Geological Sciences, Studies in Geology 7, Knoxville. DEBRENNE F. & JAMES N.P. 1981. Reef-associated archaeocyathans from the Lower Cambrian of Labrador and Newfoundland. Palaeontology 24, 343-378. DEBRENNE F. & ROZANOV A.Yu. 1978. Associations at interactions organiques chez les Archeocyathes (Cambrien inferieur). Sociite Geologique de France, Comptes Rendus des Seances 5, 235-237. DEBRENNE F. & VACELET J. 1984. Archaeocyatha: is the sponge model consistent with their structural organisation? Palaeontographica Americana 54, 358-369.

Are archaeocyaths sponges?

FINKS R.M. 1967. Phylum Porifera Grant 1836. In Harland W.B. et al eds. Thefossil record. A symposium with documentation, pp.333-341. Geological Society of London, London. FINKS R.M. 1970. The evolution and ecologic history of sponges during Palaeozoic times. Symposium of the Zoological Society of London 25,3-22. FINKS R.M. 1983. Pharetronida: Inozoa and Sphinctozoa. In Rigby J.K. and Stearn C.W. eds. Sponges and spongiomorphs. Notes for a short course. University of Tennessee, Department of Geological Sciences, Studies in Geology 7, Knoxville. GRAVESTOCK D.I. 1983. Structure and function of the exothecal tissue of Somphocyathus coralloides Taylor and allied regular Archaeocyatha. Memoirs of the Association of Australasian Palaeontologists 1, 67-74. HARTMAN W.D. & GOREAU T.F. 1972. Ceratoporella (Porifera: Sclerospongiae) and the chaetetid "corals". Transactions of the Connecticut Academy of Arts and Science 44, 132-148. HILL D. 1964. The phylum Archaeocyatha. Biological Reviews 39, 232-258. HILL D. 1972. Archaeocyatha. In Teichert C. ed. Treatise on invertebrate paleontology, Part E, Vol.1, 158p. Geological Society of America, Inc. & The University of Kansas, Boulder and Lawrence. JAMES N.P. & KLAPPA C.F. 1983. Pedogenesis of Early Cambrian reef limestones, Labrador, Canada. Journal of Sedimentary Petrology 53, 1051-1096. KRUSE P.D. 1987. Further Australian Cambrian sphinctozoans. Geological Magazine 124, 543-553. LAFUSTE J. & DEBRENNE F. 1977. Presence de deux types de microstructure chez Archaeocyathus atlanticus Billings (Cambrien inferieur, Labrador, Canada). Geobios 10,103-106. MYAGKOVA E.I. 1985. Tip Receptaculita. [Phylum Receptaculita]. Trudy Instituta Geologii i Geofiziki, Sibirskoe Otdelenie 632,44-51. NITECKI M.H. 1970. North American cyclocrinitid algae. Fieldiana, Geology 21, 182p.

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NITECKI M.H. 1972. North American Silurian receptaculitid algae. Fieldiana, Geology 28,108p.

NITECKI M.H. 1986. Receptaculitids and their relationship to other problematic fossils. In Hoffman A. and Nitecki M.H. eds. Problematic fossil tarn, pp.27-34. Oxford University Press, New York.

NITECKI M.H. & DEBRENNE F. 1979. The nature of radiocyathids and their relationship to receptaculitids and archaeocyathids. Giobios 12, 5-27.

NITECKI M.H. & TOOMEY D.F. 1979. Nature and classification of receptaculitids. Bulletin du Centre de Recherche Exploration-Production Elf-Aquitaine 3(2), 725-732.

OKULITCH V.J. & DE LAUBENFELS M.W. 1953. The systematic position of Archaeocyatha (Pleosponges). Journal of Paleontology 27,481-485.

OTT E. 1967. Segmentierte Kalkschwamme (Sphinctozoa) aus der alpinen Mitteltrias und ihre Bedeutung als Riffbildner im Wettersteinkalk. Bayerische Akademie der Wissenschaften, Mathematisch-Naturwissenschaftliche Klasse, Abhandlungen 131, 1-96.

OTTE. 1974. Phragmocoelia n.g. (Sphinctozoa), ein segmentierter Kalkschwamme mit neuem Fiillgewebetyp aus der Alpinen Trias. Neues Jahrbuch fur Geologie und Palaontologie, Monatschaften 12, 712-723.

PICKETT J. & RIGBY J.K. 1983. Sponges from the Early Devonian Garra Formation, New South Wales. Journal of Paleontology 57,720-741.

REID R.E.H. 1968. Tremacystia, Barroisia, and the status of Sphinctozoida (Thalamida) as Porifera. University of Kansas Paleontological Contributions 34,1-10.

REISWIG H.M. & MACKIE G.O. 1983. Studies on hexactinellid sponges. III. The taxonomic status of Hexactinellida within the Porifera. Philosophical Transactions, Royal Society of London B301,419-428.

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REITNDER J. 1987. Anew calcitic sphinctozoan sponge belonging to the Demospongiae from the Cassian Formation (Lower Carnian; Dolomites, northern Italy and its phylogenetic relationship. Giobios 20, 571-589. RIETSCHEL S. 1977. Receptaculitids are calcareous algae but not dasyclads. In Flugel E. ed. Fossil algae, 212-214. Springer- Verlag.

WEBBY B.D. & RIGBY J.K. 1985. Ordovician sphinctozoan sponges from central New South Wales. Alcheringa 9,209-220. WENDT J. 1979. Development of skeletal formation, microstructure, and mineralogy of rigid calcareous sponges from the Late Palaeozoic to Recent. In Levi C. and Boury-Esnault N. eds. Biologie des spongiaires. Colloques internationaux du CNRS No291,449-457.

RIGBY J.K. & BLODGETTR.B. 1983. Early Middle Devonian sponges from the McGrath quadrangle of west-central Alaska. Journal of Paleontology 57, 773-786.

WENDT J. 1980. The non-spicular skeleton. In Ginsburg R.N. ed. Living andfossil sponges. Notes for a short course, 196-214. University of Miami, Miami.

SENOWBARI-DARYAN B. & REID R.P. 1987. Upper Triassic sponges (Sphinctozoa) from southern Yukon, Stikinia terrane. Canadian Journal of Earth Sciences 24, 882-902.

WENDT J. 1984. Skeletal and spicular mineralogy, microstructure and diagenesis of coralline calcareous sponges. Palaeontographica Americana 54, 326-336.

STEARN C.W. 1972. The relationship of the stromatoporoids to the sclerosponges. Lethaia 5, 369-388. TAYLOR T.G. 1910. The Archaeocyathinae from the Cambrian of South Australia, with an account of the morphology and affinities of the whole class. Memoirs of the Royal Society of South Australia 2(2), 55-188. VACELET J. 1979. Description et affinity d'une eponge sphinctozoaire actuelle. In Levi C. and Boury-Esnault N. eds. Biologie des spongiaires. Colloques internationaux du CNRS No.291,483-493. VOLOGDIN A.G. 1937. Arkheotsiaty i rezul'taty ikh izucheniya v SSSR. [Archaeocyatha and the results of their study in the USSR]. Problemy Paleontologii 2-3, 453-500. VOLOGDIN A.G. 1948. K stroeniyu tela pravil'nykh arkheotsiat. [On the structure of the soft parts of the regular Archaeocyatha]. Izvestiya Akademii Nauk SSSR, Seriya Biologicheskaya 1,93-99.

WHITTAKER R.H. 1969. New concepts of kingdoms of organisms. Science 163, 150-160. ZHURAVLEV, A.Yu. 1985. Sovremennye arkheotsiaty? [Recent archaeocyaths? ]. Trudy Instituta Geologii i Geofiziki, Sibirskoe Otdelenie 632, 24-33. ZHURAVLEV, A.Yu. & NITECKI M.H. 1985. O sravnitel'noy morfologii arkheotsiat i retseptakulitov. [On the comparative morphology of archaeocyaths and receptaculitids]. Paleontologicheskiy Zhurnal 4, 121-123. ZHURAVLEVA I.T. 1959. O polozhenii arkheotsiat v filogeneticheskoy sisteme. [On the position of archaeocyaths in the phylogenetic system]. Paleontologicheskiy Zhurnal 4, 30-40. ZHURAVLEVA I.T. 1960. Arkheotsiaty Sibirskoy Platformy. [Archaeocyatha of the Siberian Platform]. Akademiya Nauk SSSR, Moscow, 1-344.

VOLOGDIN A.G. 1957. K stroeniyu vnutrennogo organa arkheotsiat. [On the structure of the internal organ of the Archaeocatha]. Doklady Akademii Nauk SSSR 114,1105-1108.

ZHURAVLEVA I.T. 1963. Arkheotsiaty Sibiri. Odnostennye arkheotsiaty (otrady Monocyathida i Rhizacyathida) . [Archaeocyatha of Siberia. One-walled Archaeocyatha (orders Monocyathida and Rhizacyathida)]. Akademiya Nauk SSSR, Moscow, 1-138.

VOLOGDIN A.G. 1962. K anatomii arkheotsiat. [On the anatomy of Archaeocyatha]. Paleontologicheskiy Zhurnal 2,9-20.

ZHURAVLEVA I.T. 1974. Biologiya arkheotsiat. [Biology of archaeocyaths]. Trudy Instituta Geologii i Geofiziki, Sibirskoe Otdelenie 276, 107-124.

Are archaeocyaths sponges?

ZHURAVLEVA I.T. & ELKINA V.N. 1974. Arkheotsiaty Sibiri. Etmofilloidnye arkheotsiaty. [Archaeocyatha of Siberia. Ethmophylloid archaeocyaths]. Trudy Instituta Geologii i Geofiziki, Sibirskoe Otdelenie 230,166p.

ZHURAVLEVA I.T. & MYAGKOVA E.I. 1972. Archaeata - novaya gruppa organismov Paleozoya. [Archaeata - a new group of Palaeozoic organisms]. Mezhdunarodnyy Geologicheskiy Kongress, XXIV Sessiya, Doklady Sovetskikh Geologoy, 7,7-14. ZHURAVLEVA I.T. & MYAGKOVA E.I. 1974. Sravnitel'naya kharakteristika Archaeata i Stromatoporoidea. [Comparative characteristics of Archaeata and Stromatoporoidea]. In Sokolov B.S. ed. Drevnie Cnidaria, Toml. [Ancient Cnidaria, Vol.1], 63-70. Nauka, Novosibirsk. ZHURAVLEVA I.T. & MYAGKOVA E.I. 1979. Comparaison entre les Archaeata et les Porifera. In Levi C. and Boury-Esnault N. eds. Biologie des spongiaires. Colloques internationaux du CNRS N0.291, 521-526.

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ZHURAVLEVA I.T. & MYAGKOVA E.I. 1981. Materialy k izucheniyu Archaeata. [Materials on the study of Archaeata]. Trudy Instituta Geologii i Geofiziki, Sibirskoe Otdelenie 481,41-74. ZHURAVLEVA I.T. & MYAGKOVA E.I. 1983. Polozhenie Archaeata v sisteme razvitiya organicheskogo mira. [The position of Archaeata in the system of development of the organic world]. Trudy Instituta Geologii i Geofiziki, Sibirskoe Otdelenie 538, 60-65. ZHURAVLEVA I.T. & MYAGKOVA E.I. 1987. Nizshie mnogokletochnye fanerozoya. [Primitive Phanerozoic multicellular organisms]. Trudy Instituta Geologii i Geofiziki, Sibirskoe Otdelenie 695,224p. ZHURAVLEVA I.T. & REZVOY P.D. 1956. K sistematike iskopaemykh gubok i arkheotsiat. [On the systematics of fossil sponges and archaeocyaths]. Doklady Akademii Nauk SSSR 111, 449-451. ZIEGLER B. & RIETSCHEL S. 1970. Phylogenetic relationships of fossil calcisponges. Symposium of the Zoological Society of London 25, 23-40.

Implications of the discovery of Archaeocyatha in the Macclesfield Marble, Mount Lofty Ranges Reg C. Sprigg Arkaroola Village, via Port Augusta, SA. 5700, Australia The discovery of Cambrian Archaeocyatha widely throughout the Flinders-Mt Lofty Ranges in South Australia went far to establishing the broad anticlinorial structure of the Adelaide Geosyncline. The author's recent discovery of Archaeocyatha in the Macclesfield Marble, southeast of Adelaide, has removed a remaining uncertainty and confirmed clearly that commencement of deposition of the Kanmantoo Group followed this development almost immediately. Key words: Cambrian, Archaeocyatha, Macclesfield Marble, Kanmantoo Group, Adelaide Geosyncline. INTRODUCTION Archaeocyatha were first discovered in South Australia near Ardrossan on Yorke Peninsula by Tepper (1879). During the next fifty years they were located widely throughout the Flinders and Mt Lofty Ranges over an extent of more than 600 km longitudinally by 80 km width, but relatively narrow in vertical range and almost wholly Early Cambrian in age. Following the original discovery, David and Howchin (Howchin 1897) next located comparable Archaeocyatha-bearing limestones at Normanville, 50 km south of Adelaide on what was later proven to be the overthrusted and locally overturned western anticlinal flank of the southern Mt Lofty Ranges. Subsequently Howchin (1922) demonstrated them also to occupy both anticlinal flanks of the central Flinders Ranges. By 1925 Howchin was able to produce an archaeocyathan distribution map disclosing eleven separate, widely scattered localities throughout the ranges (Fig.l). A significant, late contribution to regional stratigraphic understanding was Sprigg's discovery in 1946 (Sprigg 1947) of Archaeocyatha in the limestones at Ediacara directly overlying his original Adelaidean ("Ediacarian") metazoan fossil discovery.

Archaeocyatha in South Australia.

Archaeocyatha in the Macclesfield Marble

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Near Mt Wright in far western New South Wales, east of the Carpentarian Willyama Inlier, Archaeocyatha were first recorded by Fletcher (1964), while Kruse (1978) was first to provide detailed descriptions of these. They occur in close association with volcanic accumulations suggesting proximity to the initiating Tasman or eastAustralian eugeosynclinal complex.

limestone boulders, the latter resembling some of the Cambrian 'marbles'. Madigan (1928) subsequently conducted two student excursions to Emu Bay and identified probable trilobite tracks in the Emu Bay Shale, but found no body fossils. Students R.G.Thomas and W.Ham did, however, discover remanie Archaeocyatha in the White Point Conglomerate (White Point Limestone of Sprigg 1955).

CAMBRIAN OVERLAP ONTO SHELVES BORDERING THE ADELAIDE GEOSYNCLINE

In 1952, Sprigg mapped the whole of the island in rapid broad scale reconnaissance (Sprigg et al 1954). While mapping he located a prolific trilobite-bearing horizon within the Emu Bay Shale (Glaessner 1952; Sprigg 1955). These indicated a latest Early Cambrian age. Impending diastrophism was assumed from the presence of penecontemporaneously flattened Archaeocyatha-bearing limestone boulders forming much of the overlying White Point Conglomerate. The copious red granite boulders in the limestone (Daily 1956) suggested the nearby presence of contemporaneous faulting of the Cambrian to expose also granitic bedrock.

Forty years after Tepper's discovery of archaeocyathan limestones at Ardrossan on eastern Yorke Peninsula, Howchin (1918) established a gentle monoclinal relationship for these Cambrian beds in westerly overlap directly onto "Archaean" (=Lower Proterozoic) granitic basement. The recognition of similar flat-lying Cambrian limestones in a comparable shelf-edge and (Torrens) hinge-line situation, 400 km to the north near the head of Lake Torrens, followed much later (Johns etal 1966). Here R.LJack (1926) had previously reported Archaeocyatha in his log of the Yarra Wurta Creek Bore. Jack (1927) referred to similar flat-lying limestones near Andamooka, actually on the Stuart Shelf, but which in the absence of fossil evidence he attributed to "probable Ordovician", a notion that persisted for the next decade or so. It was Segnit (1935, 1939) who, still in the absence of fossils, described them as Cambrian. Johns etal (1966) subsequently revealed them to be Archaeocy athabearing. On Kangaroo Island at the exposed southwestern extremity of the Adelaide Geosyncline, the discovery of Cambrian fossils, also in a flatlying shelf-type situation, came still later. Howchin (1899) and H. Y.L.Brown both reconnoitered the island in 1899 noting little altered subhorizontal shales at Emu Bay. Spectacular breccia-conglomerates formed the cliffs east to White Point. Both units were pronounced unfossiliferous, but Howchin recorded the conglomerates to contain prolific granite and

Credit for the original insight into local diastrophism however, must go to Madigan (1928, p.212) who, in a remarkably prophetic passage wrote "The author considers the quartzite with its included lenticular boulder beds to be of shallow water or even terrestrial origin, derived by torrential streams from neighbouring highlands. These highlands were composed of Cambrian and Pre-Cambrian rocks, which furnished the boulders of the Archaeocyathinae limestone and the slate and schist fragments and pebbles of gneissic and granitic rock. A torrential stream a few miles long flowing across the Willunga scarp could furnish the materials of exactly such a conglomerate." Despite the fact that Madigan considered the formation to be postCambrian and most likely Ordovician in age (p.215), he recognised the depositional process, the lenticularity of the conglomerate bands and continuity of the 'Point Marsden series' which he compared with the Old Red Sandstone. Implicit in this was Madigan's recognition that the con-

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Fig. 2 The Adelaide Geosyncline showing the distribution of Kanmantoo Group and other Cambrian sediments.

Archaeocyatha in the Macclesfield Marble

glomerate was emplaced during episodes of intermittent tectonism. Sprigg (1955) recognised the likelihood of syn- or early post- depositional tectonism. However, he could not fully reconcile this with the conglomerate fabric, particularly the occurrence of rounded pebbles of granite and schist together with flattened soft limestone boulders. Sprigg suggested the boulders with Archaeocyathae were derived from a fore-reef facies. Daily (1956, p. 125) disputed this viewpoint and suggested that the White Point Conglomerate represented normal conglomerates derived from the active erosion of the adjacent land mass. This is quite in conformity with Madigan's original view. Such prediction of contemporary tectonism north beneath Gulf St Vincent was subsequently clearly confirmed by Geosurveys of Australia Pty Ltd, via marine seismic surveys carried out between Kangaroo Island and Yorke Peninsula on behalf of Beach Petroleum N.L. (Yakunin & Sprigg 1968; Stuart & von Sanden 1972). It represented a late Early Cambrian phase of the onsetting Delamerian Orogeny. Daily (1956) originally termed this presumably local, regionalised tectonism, the Kangaroo Island Orogeny, but later modified it to his Kangarooian Movements (Daily 1969). At the north-eastern extension of the geosyncline beneath the Frome Embayment, seismic surveys by Delhi-Santos and the drilling of Yalkalpo 2 in 1977 by the S.A. Mines Department (Youngs & Moorcroft 1982) have demonstrated that Cambrian limestones underlie much of the Frome Embayment. Originally these shelved eastward as far as Mt Arrowsmith in western New South Wales (Youngs & Moorcroft 1982, fig. 4A), but in part were interrupted nondepositionally and/or erosionally by the Benagerie Ridge. At Wilkatana near Port Augusta within the narrow reverse-faulted graben confronting the

327

western Flinders Ranges, drilling by Santos Ltd in 1955, disclosed archaeocyathan limestones near the Torrens Hingeline. In summary then, it is clear that Archaeocyatha-bearing limestones originally blanketed much, if not all, of the Adelaide Geosyncline, and spread far onto platforms both to the east and west of the central trough. Only along the northern Mt Lofty Range-Olary Arc are the limestones now completely absent. However, the occurrence of Pound Subgroup forming the keel of the Waroona Hill syncline lying 15 km northwest of Yunta, and stratigraphically only a few hundred metres below the predicted limestone base, leaves little doubt of their original spread completely across the geosyncline (Fig.2).

THE KANMANTOO GROUP PROBLEM AND DEFINITION OF THE MT LOFTY ANTICLINORIUM The main mass of sediments constituting the eastern Mt Lofty Ranges and southern Kangaroo Island are devoid of recognisable fossils. These accumulations were primarily flysch-type, muddy, unstable slope deposits (Sprigg & Campana 1953) apparently hostile to contemporary benthic fossil life. It was an environment of considerable interest to Dr Brian Daily (1963; Daily & Milnes 1971, 1972,1973) who invested much time in searching for fossils within them and in defining the successions. So dominating are the southeasterly dip attitudes of these 20 000 m or so of Kanmantoo Group metasediments from Gulf St Vincent to Lake Alexandrina, that their structural interrelationships with the local Cambrian archaeocyathan beds remained enigmatic for more than half a century. For the foregoing reason and because also of relatively intense metamorphism, Professor Ralph Tate late in the last century, not unreasonably assumed the bulk of these rocks to be Archaean in age. He failed to recognise a major anticlinal axis occupying the longitudinal igneometamorphic core of the range. Accordingly he

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(Tate 1893, p.47) reported..."The grandest exemplification of the Archaeans is in the Mount Lofty Ranges of South Australia. These rocks there occupy a vast monocline, with a dip to the south-east, of not less than ten miles (c. 16 000 m) in thickness." Howchin (1926) subsequently exposed the fallacy of this and noted that Woolnough (1908) fell into the same error. Earlier Howchin (1907, p.415) had correctly described the structure of the ranges as constituting an "Anticlinorium of great intensity (that) was developed with numerous contortions, overfolding and thrust planes." This indeed was a remarkable insight for Howchin's day and age. He correctly traced many of the Upper Proterozoic formations around the northern anticlinorial perimeter of the older Precambrian inliers into the vicinity of Barossa Valley. Despite this he did not recognise the probable Cambrian age of the Angaston Marble high in the stratigraphic succession. This marble is overlain directly in turn by typical Kanmantoo Group schists. Not surprisingly, considerable uncertainty continued to exist amongst contemporary workers as to Adelaide System and CambrianKanmantoo Group inter-relationships. Woods

(1862, p.20) earlier had inadvertently confused the issue by reporting that, as quoted by Howchin (1897, p. 81), "No fossils have been found except at one portion of the range about 30 miles south of Adelaide. I was informed that the fossil was a Pentamerus oblongus. This would be a characteristic of the lowest division of the Upper Silurian rocks. The person who found it is since deceased..." Howchin shrewdly concluded that such a fossil, if it ever existed, would have been from the Cambrian of the Sellicks Hill Range where Professor David and he (Howchin 1897) had previously located Archaeocyatha in the ranges for the first time. Howchin independently traced the Archaeocyatha-bearing limestones north to Sellick Hill and measured a cross-section through the western range escarpment (Fig.3). The succession there dips steeply eastwards but the Cambrian Normanville Group faces west, actually being locally overturned under the Carpentarian core of the ranges (Fig.4). This structural complication was not recognised by Howchin. In consequence his westernmost "Argillites" that physically underlie down to the shores of Gulf St Vincent were interpreted to be the oldest, rather than really to stratigraphically supercede the east-adjoining Archaeocyathabearing beds.

Fig. 3 Howchin's (1897) section through the Sellicks Hill area.

Archaeocyatha in the Macclesfield Marble

Madigan(1925,1927,1928) went far towards resolving the foregoing enigma. Unfortunately by measuring and linking up geological cross sections undertaken only at wide intervals along the regional strike, he erroneously depicted the whole westerly rock succession, lying south of about Willunga, to be overturned westward to as low as 45 degrees east-dip. This was from Sellick Hill to Carrickalinga Head. Such indication of laterally continuous overturning was further propagated on the Yankalilla sheet by Campana etal (1954). It was Abele & McGowran (1959) who first determined the overall facing of the regional succession as well as extensive dip attitudes. Near Myponga the Normanville Group actually dips east and faces east along one third of the local strike length, forming part of a landward syncline caught up in major overthrusting (see Alexander & Gravestock, this volume). Alexander and Gravestock have further been able to map the Wangkonda Formation at two points on the coast north and south of Myponga Beach. This reveals that the southeastern limb of Abele and McGowran's offshore anticline (sequence dipping and facing southeast) is in northerly fault contact with a syncline whose southeastern limb faces northwest, but locally dips southeast (as at Sellick Hill). That is, it is overturned to the northwest just where both Howchin and Madigan

329

ran sections. Madigan (1925) clearly saw a dilemma when he stated (p.203) "This series (at Carrickalinga Head) corresponds exactly with the series of the western side of Myponga Jetty, but in the reverse order of superposition, so that there is no doubt that the change in dips is merely a local reversal, unless, indeed, the whole of the rest of the area shows reversed dip, and this is normal." The solution was not found until the work of Abele & McGowran. Madigan (1925) also suggested that the coarsely recrystallised, cream-coloured marbles of Rapid Bay were simply the metamorphosed extensions of the Sellick Hill-Normanville "Archaeocyathinae beds". This was something that colleague Sir Douglas Mawson strongly rejected. Madigan (1928, p.216) predicted that "The Rapid Bay marble should underlie the north coast of the island (Kangaroo Island) and here we find a conglomerate (White Point) ... a strong point in favour of the argument put forward for the southerly extension of the Archaeocythinae limestone" Madigan had by then also been able to demonstrate that his "coprolitic" black (Heatherdale) shales, which stratigraphically overlie the Archaeocyatha-bearing Fork Tree Limestone (Abele & McGowran 1959) in the region, actually continues south into the environs of the main

Fig. 4 Cross-section through the Fleurieu Peninsula from Carrickalinga Head to Currency Creek (after Campana et al 1954; Abele & McGowran 1959 and Alexander & Gravestock, this volume).

330

Reg C. Sprigg

Kanmantoo sequence constituting the backbone of Fleurieu Peninsula. However he did not demonstrate that the Heatherdale Shale also extended onto Kangaroo Island. Finally it was Campana etal (1954) who conclusively mapped structural continuance of the Cambrian south around the nose of the south pitching regional anticline near Cape Jervis. There is little doubt that Madigan clearly believed his "coprolitic" (Heatherdale) shales to pass directly up into the subgrey wacke sequence of Fleurieu Peninsula. Also, on student excursions he insisted that the greywackes immediately overlying the Archaeocyatha limestones at Carrickalinga Head were indeed indicative of mounting tectonism and contemporary, rapid, nearby erosion in late Early Cambrian time. It was not until the early 1950's that Sprigg etal (1951) introduced the term "Kanmantoo Series" for the subgreywacke succession. They deduced it to be of early Palaeozoic age in their production of the Adelaide 1:63 360 Map Sheet. Sprigg & Wilson (1954) in subsequently completing mapping of the adjoining Echunga Map Sheet, refined the term to Kanmantoo Group status and assigned it a Cambrian age. They commenced the sequence as immediately succeeding deposition of the Macclesfield Marble. Their mapping had also clearly confirmed regional anticlinorial turn-over of the Adelaidean-Cambrian succession across the range. This was a contention originally advanced by Howchin and finally convincingly confirmed by Campana et al (1954) in their mapping of the Jervis Map Sheet. On the Echunga Map Sheet the commencement of the Cambrian was drawn by the authors at the base of a prominent sandstone formation that arches around Macclesfield and also forms the monadnock, Mt Barker. An overlying schistose slate is overlain in turn by the 100 to 200 m thick Macclesfield Marble which continues to and beyond Paris Creek. Diligent checks for fossils along the marble in outcrop proved unsuccessful at the time. This led Sprigg & Wilson to hesitate in correlating the formation directly with the Archaeocyatha limestones of Normanville and Sel-

lick Hill. In substantiation of discordant relationships obvious along the Macclesfield-Charleston trend, and also the development there of extensive well aligned quartz reefing and associated doleritic dyke intrusions, they mapped in a prominent Nairne Fault. Much of the structural/stratigraphic discordance is now known to relate to both contemporary fault tectonism and consequential erosion unconformity developed during Early Cambrian time. It is a history still to be resolved in final detail. The work of Daily (1956) on the base of the Cambrian was monumental and outstanding for its day. Still he was disadvantaged in the Kanmantoo region, as also were Horwitz et al (1959), by the lack of fossil evidence across the ranges basic to any final resolution of the Kanmantoo Group/Adelaidean relationships. Horwitz argued adroitly for the Macclesfield Marble to belong above the equivalent phosphatic shales of the Yankalilla region and thus well up into the Kanmantoo succession and accordingly much higher than the main Archaeocyatha beds (Fork Tree Limestone) - a contention no longer sustainable. The complicating Nairne Fault in this region has itself experienced a chequered history of successive additions and eradications from a sequence of relevant map sheets. Kleeman & Skinner (1959) in particular, almost eradicated any suggestion of a significant fault in this zone. As so often is the case, the reality is somewhere between. The Nairne Fault and associated contemporary uplift was a significant factor in late Early Cambrian diastrophism. It contributed to a complex pattern of erosion and deposition that can only be fully resolved by still more detailed field mapping. On November 15th, 1986, while searching the Macclesfield Marble for fossils, the writer was at last successful. About the Paris Creek marble quarry, surprisingly clearly recognisable, silicified, examples of Archaeocyatha were located in outcrop. Prominent amongst these was what appeared to be (^)Anatyctocyathus sellicksi as revised after Archaeocyathus sellicksi Taylor by Debrenne & Gravestock (this volume), com-

Archaeocyatha in the Macclesfield Marble plete with exo- and endo-thecal tissue. It is a common form preserved across the range at Sellick Hill in the Fork Tree Limestone. Other genera tentatively since recognised by David Gravestock (pers. comm.) include Dokidocyathus and (l)Erugatocyathus. The fossils located range to ten centimetres or more in length. Some appear complete and to be preserved in original situation. However, many are extensively broken, currentwashed, sorted and comminuted. In view of their close palaeogeographic and stratigraphic association with the fly sch sequence of the developing nearby Kanmantoo Trough or eu-geosyncline (Sprigg 1952, 1984), it is presumed that the Archaeocyatha fauna here populated an outer, unstable shelf edge. CONCLUSIONS Early Cambrian Archaeocyatha meadows, isolated mounds and reef-like complexes obviously once expanded widely across the length and breadth of the Adelaide (mio)Geosyncline and much of its marginal broad shelves. To the southeast they spread at least to the lip of the developing Kanmantoo Trough, or incipient Tasman (eu)Geosyncline, and emphasised also the relatively rapid onset of Kanmantoo flysch-type sedimentation in this direction. Accordingly, other than the Adelaidean Sturt Tillite, the Archaeocyatha-bearing limestones provide by far the most widespread and significant marker horizon in the whole of the tens of kilometres thick Adelaidean-Cambrian geosyncline. By comparison the trilobite-bearing beds of Kangaroo Island, Yorke Peninsula, and of the Arrowie Basin in the North Flinders Ranges, are quite limited in preserved extent. Such ubiquity of a relatively thin zone of archaeocyathan limestones, and now marbles, spread across the geosyncline provide also a major control in structural understanding.

331

ACKNOWLEDGEMENTS Appreciation is expressed for the generous assistance afforded to the writer by the late Dr Brian Daily in many mutually rewarding discussions, particularly in seeking to define the base of the Cambrian and Kanmantoo successions generally. Also appreciation is extended to Margaret Welsh for her part in the final successful search for Archaeocyatha in the Macclesfield Marble at Paris Creek, and to David Gravestock for his invaluable suggestions and contributions to the finalisation of this presentation. REFERENCES ABELE C. & McGOWRAN B. 1959. The geology of the Cambrian south of Adelaide (Sellick Hill to Yankalilla). Transactions of the Royal Society of South Australia 82, 301-320. ALEXANDER E.M. & GRAVESTOCK D.I. (this volume). Sedimentary facies in the Sellick Hill Formation, Fleurieu Peninsula, South Australia. CAMPANA B., WILSON R.B. & WHITTLE A.W.G. 1954. The geology of the Jervis and Yankalilla Military Sheets. Geological Survey of South Australia, Report of Investigations 3. DAILY B. 1956. The Cambrian in South Australia. In El Sistema Cambrico su paleogeografia y el problema de su base, Vol.2, pp.97-147. Report of the 20th International Geological Congress, Mexico, 1956. DAILY B. 1963. The fossiliferous Cambrian succession on Fleurieu Peninsula, South Australia. Records of the South Australian Museum 14,579-601. DAILY B. 1969. Fossiliferous Cambrian sediments and low grade metamorphics, Fleurieu Peninsula, South Australia. In Daily B. ed. Geological Excursions Handbook, pp.49-54. ANZAAS, Section 3, 41st Congress, Adelaide.

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DAILY B. & MILNES A.R. 1971. Stratigraphic notes on Lower Cambrian fossiliferous metasediments between Campbell Creek and Tunkalilla Beach in the type section of the Kanmantoo Group, Fleurieu Peninsula, South Australia. Transactions of the Royal Society of South Australia 95,199-214. DAILY B. & MILNES A.R. 1972. Revision of the stratigraphic nomenclature of the Kanmantoo Group, South Australia. Journal of the Geological Society of Australia 19, 197-202. DAILY B. & MILNES A.R. 1973. Stratigraphy, structure and metamorphism of the Kanmantoo Group (Cambrian) in its type section east of Tunkalilla Beach, South Australia. Transactions of the Royal Society of South Australia 97, 213-251. DEBRENNE F. & GRAVESTOCK D.I. (this volume). Archaeocyatha from the Sellick Hill Formation and Fork Tree Limestone on Fleurieu Peninsula. FLETCHER H.O. 1964. New Linguloid shells from the Lower Ordovician and Middle Palaeozoic rocks of New South Wales. Records of the Australian Museum 26, 283-294. GLAESSNER M.F. 1952. A Cambrian fauna from Kangaroo Island, South Australia. South Australian Department of Mines, Report (unpubl.). HORWITZ R.C., THOMSON B.P. & WEBB B.P. 1959. The Cambrian-Precambrian boundary in the eastern Mt Lofty Ranges region, South Australia. Transactions of the Royal Society of South Australia 82, 205-218. HOWCHIN W. 1897. On the occurrence of Lower Cambrian fossils in the Mount Lofty Ranges. Transactions of the Royal Society of South Australia 21,74-86. HOWCHIN W. 1899. Notes on the geology of Kangaroo Island, with special reference to evidences of extinct glacial action. Transactions of the Royal Society of South Australia 23, 198-207. HOWCHIN W. 1907. A general description of the Cambrian Series of South Australia. Report of the Australasian Association for the Advancement of Science 11,414-422.

HOWCHIN W. 1918. The Geology of South Australia. South Australian Education Department, Adelaide. HOWCHIN W. 1922. A geological traverse of the Flinders Range from the Parachilna Gorge to the Lake Frome Plains. Transactions of the Royal Society of South Australia 46,46-82. HOWCHIN W. 1925. The geographical distribution of fossiliferous rocks of Cambrian age in South Australia with geological notes and references. Transactions of the Royal Society of South Australia 49,1-26. HOWCHIN W. 1926. The geology of the Barossa Ranges and neighbourhood in relation to the geological axis of the country. Transactions of the Royal Society of South Australia 50,1-16. JACK R.L. 1926. Note on two occurrences of Archaeocyathinae near Lake Torrens. Transactions of the Royal Society of South Australia 50, 317. JACK R.L. 1927. The prospects of obtaining additional underground water supply on Andamooka Station (west of Lake Torrens). South Australian Department of Mines, Report Book 10, 247 (unpubl.). JOHNS R.K., HEIRN M.N. & NIXON L.G. 1966. Andamooka Map Sheet, 1:250, 000 Series. Sheet M53-12. Geological Survey of South Australia. KLEEMAN A.W. & SKINNER B.J. 1959. The Kanmantoo Group in the Strathalbyn-Harrogate Region, South Australia. Transactions of the Royal Society of South Australia 82, 61-71. KRUSE P.D. 1978. New Archaeocyatha from the Early Cambrian of the Mt Wright area, New South Wales. Alcheringa 2, 27-47. MADIGAN C.T. 1925. The geology of the Fleurieu Peninsula. Parti - The coast from Sellick's Hill to Victor Harbour. Transactions of the Royal Society of South Australia 49, 198-212. MADIGAN C.T. 1927. The Geology of the Willunga Scarp. Transactions of the Royal Society of South Australia 51, 298-409.

Archaeocyatha in the Macclesfield Marble

MADIGAN C.T. 1928. Preliminary notes of new evidence as to the age of formations on the north coast of Kangaroo Island. Transactions of the Royal Society of South Australia 52, 210-216. SEGNIT R.W. 1935. Andamooka Opal Field, South Australia. South Australian Department of Mines, Mining Review 62, 51-56. SEGNIT R.W. 1939. The Pre-Cambrian-Cambrian succession. Geological Survey of South Australia, Bulletin 18. SPRIGG R.C. 1952. On the nature of the Adelaide Geosyncline; studies in mio-geosynclinal sedimentation, palaeogeography and orogeny. Text for Geological Survey of South Australia, Bulletin30 (not published). Copies with Adelaide University Library and Basser Library, Canberra. SPRIGG R.C. 1947. Early Cambrian (?) Jellyfishes from the Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 71, 212-224. SPRIGG R.C. 1955. The Point Marsden Cambrian Beds, Kangaroo Island, South Australia. Transactions of the Royal Society of South Australia 78, 165-168. SPRIGG R.C. 1984. Arkaroola-Mt Painter in the Flinders Ranges; the last billion years. Lutheran Press, Adelaide. SPRIGG R.C. & CAMPANA B. 1953. The age and facies of the Kanmantoo Group. Australian Journal of Science 16, 12-14. SPRIGG R.C., CAMPANA B. & KING D. 1954. Kingscote map sheet, Geological Atlas of Australia, 4 mile series. Sheet I 53-16. Geological Survey of South Australia.

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SPRIGG R.C., WHITTLE A.W.G. & CAMPANA B. 1951. Adelaide map sheet, Geological Atlas of South Australia, 1 mile series. Geological Survey of South Australia.

SPRIGG R.C. & WILSON R.B. 1954. Echunga map sheet, Geological Atlas of South Australia, 1 mile series. Geological Survey of South Australia. STUART W.J. & VON SANDEN A.T. 1972. Palaeozoic history of the St Vincent Gulf region, South Australia. APEA Journal 12,9-16. TATE R. 1893. Inaugural address. Australasian Association for the Advancement of Science 5, 1-69. TEPPER J.G.O. 1879. Introduction to the cliffs and rocks at Ardrossan, Yorke Peninsula. Transactions and Proceedings of the Philosophical Society of South Australia 2, 71-79. WOODS J.E.TENISON 1862. Geological observations in South Australia: principally in the district south-east of Adelaide. Longman Green, London. WOOLNOUGH W.G. 1908. Notes on the geology of the Mount Lofty Ranges chiefly the portion east of the Onkaparinga River. Transactions and Proceedings of the Royal Society of South Australia 32, 121-137. YAKUNIN A. & SPRIGG R.C. 1968. St Vincent Gulf marine seismic survey, O.E.L.24 South Australia. Unpublished Report for Beach Petroleum N.L. YOUNGS B.C. & MOORCROFT E. 1982. The petroleum potential of the eastern Arrowie Basin and Frome Embayment. APEA Journal 22, 81-101.

Origin of redbeds and variegated sediments, Cambrian, Adelaide Geosyncline, South Australia P.S. Moore 15 Coventry Rd, Strathfield West, N.S.W. 2140, Australia. Red hematite pigment is common in Cambrian sediments of the Adelaide Geosyncline. Two processes of formation are postulated for this pigment: (1) syndepositional conversion of limonitic clay to hematite and (2) early post-depositional weathering of iron-bearing grains. The relative influence of each of these processes is a function of sediment grainsize. In the first case, deep weathering in the source area formed iron-rich limonitic clay which was incorporated into fine-grained deposits on intertidal and supratidal mudflats. Dehydration of the limonite resulted in syndepositional formation of hematite cement. In the second case, sandy sequences containing abundant iron-rich grains such as biotite have undergone alteration, leading to the formation of early post-depositional hematite. Variegated sequences, where red clastics are interbedded with green shales and siltstones and grey to buff limestones and dolomites, best illustrate the relationships between sediment colour and depositional environment. Typically, the red deposits show evidence of subaerial exposure, whereas the non-red clastics and carbonates reflect deposition in subtidal environments. Original sediment colour has subsequently been overprinted in many cases by the effects of several processes, including surface weathering, diagenetic alteration of tuffs, fluid transmission in porous units and changed chemical conditions associated with the mobilization and alteration of anhydrite. Key words: Billy Creek Formation, Frome Group, Adelaide Geosyncline, Redbeds, hematite, Flinders Ranges, Kangaroo Island.

INTRODUCTION

Red-coloured sediments dominate the upper part of the Cambrian sequence of the Adelaide Geosyncline (Fig. 1). They crop out in the Flinders Ranges (Fig. 2) as the Billy Creek Formation and Lake Frome Group, and extend westwards beyond Lake Torrens (Yarra-Wurta Shale). Four hundred kilometres to the south on Yorke Peninsula, where similar aged sediments are preserved in the subsurface, redbeds occur in the Minlaton Formation. Red coloured sediments are less common on Kangaroo Island, but are well represented by parts of the Stokes Bay Sandstone, White Point Conglomerate and Boxing Bay Formation. Thus, this Cambrian redbed complex is both thick (up to 3000 m) and arealy extensive (originally covering as much as 200 000 sq. km).

This paper examines the nature and origin of Cambrian redbeds in the Adelaide Geosyncline. It is based on a detailed study of the Billy Creek Formation, supplemented by outcrop or core studies of the other units mentioned above. The Billy Creek Formation has been chosen as the focus for several reasons: (a) its sedimentology has been studied in detail (Moore 1979a, 1979b, 1979c, 1980a, 1980b, 1982), (b) subsurface data supplement excellent outcrop exposures, and (c) the formation consists of red, non-red and variegated deposits of varying grain sizes, thus providing adequate controls on the mechanisms of redbed formation. The study is based on macroscopic and microscopic examination of red and non-red sediments (including 900 thin sections), but is not concerned with the molecular structure or form of the hematite cement (as indicated, for example, by scanning electron microscopy).

335

Origin of Cambrian redbeds

YORKE

ORDOVIC

PENINSULA

KANGAROO

IS.

MT L O F T Y RANGES

STUART SHELF

FLINDERS

RANGES

REAPHOOK

HILL

IAN Grindstone

Range Sst

Pantapinna

Sst

LATE CAMBRIAN

MIDDLE CAMBRIAN

Coobowie Moonak

Moodlatana

Frr

Lst Fm

Stansbury

Lst

Corrodgery

Ramsay

Balcoracana

Lst Eregunda

Minlaton

Boxing LATE

Emu

EARLY

White

CAMBRIAN

Sst

Mbr

Nildottie

Frr

nth

Bay

Frr

Kanmantoo

Shale

Point

Cong.

Bay

Shale

S t o k e s Bay Sst rickalinga Head

Si'tstone

Mbr

Group Yarra Wurta Shale

P a ra r a

Normanville

Lst

Group

Wa r r a g e

E rud ina Si I t s t o n e

Coads

Member

Mbr

Member

"^[mr Andamooka Lst

Hawker Group

Fig. 1. Cambrian stratigraphy, Adelaide Geosyncline (after Daily 1976, and Moore 1979a, 1980a)

GEOLOGICAL SETTING

The Billy Creek Formation The Billy Creek Formation consists of an Early to Middle Cambrian, predominantly redbed sequence of shale, siltstone and sandstone, with minor limestone, dolomite and tuff. It crops out sporadically throughout the central and northern Flinders Ranges of South Australia (Fig. 3), and has been identified in the subsurface to the east of the ranges (Moore 1979a, 1980a).

Cambrian - Adelaide

Geosyncline

f'•*y*|

Cambrian - Kanmantoo

I ^ W j

Adelaidean

^ ^ ^

Early

Proterozoic

|v v v |

Acid

volcanics

& Stuart

Shelf

Trough

Fig. 2. Location of Cambrian sediments, Adelaide Geosyncline.

Deposition of the Billy Creek Formation occurred in response to the Kangarooian Movements; a complex and persistent tectonism which was most pronounced to the south of the Flinders Ranges. Initial tectonic activity terminated a major phase of Early Cambrian carbonate deposition (the Hawker Group; Fig. 1), and promoted the development of the Billy Creek Formation. In most areas, the Billy Creek Formation consists of a thick, coarsening-up wards sequence of redbeds, which is subdivided into three units (Warragee, Nildottie Siltstone and Eregunda Sandstone Members; Fig. 1). At Reaphook Hill, however,

336

P.S. Moore

environment than the Warragee Member. Minor volcanic activity, probably in the Mt Wright region of New South Wales, is recorded as thin tuffaceous interbeds in the lower half of the formation.

Fig. 3. Outcrop and subsurface distribution of the Billy Creek Formation, Flinders Ranges. Outcrop locations are Chambers Gorge (CG), Hey sen Range (HR), Mernmerna (M), Mount Frome (MF), Mt Scott Range (MSR), Reaphook Hill (RH), Wertaloona (W) and Wirrealpa Basin (WB).

Further uplifts in the source area released silt and fine sand into the basin of deposition, forming the laterally equivalent Nildottie and Erudina Siltstone Members. The redbed facies of both members were deposited mainly in the intertidal to supratidal zones under the influences of weak wave and current activity, while cyclically interbedded dolomites in the more easterly outcropping Erudina Siltstone Member were probably deposited in sheltered coastal lagoons in the shallow subtidal zone (Fig. 4b). Further uplifts increased topographic relief in the source area and a complex of fluvial-dominated deltaic sands (the Eregunda Sandstone Member) prograded across the basin from the southeast (Fig. 4c). Palaeocurrent and petrographic data indicate that the main source of the sediment was the Broken Hill-Olary basement high. Lateral equivalents of the Billy Creek Formation

the stratigraphy is more complex and a mixed suite of facies (the Coads Hill Member) is overlain by an interbedded sequence of redbeds and thin dolomites (Erudina Siltstone Member). In the subsurface, the upper part of the Billy Creek Formation has been penetrated in Lake Frome 1 and 2 (Moore 1980a) while the lower part has been penetrated in Yalkalpo 2 (Youngs 1978).

West of the Hinders Ranges, near Lake Torrens, the poorly exposed Yarra-Wurta Shale is lithologically and sedimentologically similar to the Warragee Member, being fine-grained and showing evidence of intermittent subaerial exposure. However, lateral equivalents of the Billy Creek Formation occur mainly to the south of the Flinders Ranges.

Most of the formation was deposited under shallow marine conditions, commonly in the intertidal zone. During the early stages of deposition a broad muddy tidal flat developed in the west (the Warragee Member), while to the east a complex stratigraphy (the Coads Hill Member) was evolving (Fig. 4a). The great variety of facies in the Coads Hill Member emphasises the instability of this eastern region during the late Early Cambrian, although in general the sequence was deposited in a more open marine

Tectonic movements were pronounced in the southern portion of the Adelaide Geosyncline during the latter part of the Cambrian. Uplift occurred, with erosion of the area to the north of Kangaroo Island. Limited deposition of conglomeratic redbeds (Minlaton Formation; Fig. 1) occurred over the erosion surface, while faultcontrolled subsidence of the areas to the east and south led to the development of the Kanmantoo Trough. On the northeast coast of Kangaroo Island, six units (Carrackalinga Head to Boxing

Origin of Cambrian redbeds

Bay Formations) were deposited on a shallow shelf adjacent to the southern shoreline of the uplifted block. The area was subject to strong tidal, minor wave, and at times alluvial influence. The palaeoenvironment has thus been interpreted as a shoreline along which alluvial deposits, including alluvial fan complexes, interfingered with tidal deposits (Daily et al 1980). Shoreline migration was in response to the interplay of fault uplift of the northern source area and basinal subsidence, which determined the nature and amount of coarse alluvial detritus shed southwards into the basin.

337

Lake Frome Group The Billy Creek Formation is conformably overlain by the Wirrealpa and Aroona Creek Limestones, which were deposited during a brief, but widespread marine transgression. The overlying Lake Frome Group consists of a thick sequence of mainly red sediments, divided into four formations (Fig. 1). The lowermost Moodlatana Formation is similar to the Nildottie Siltstone Member of the Billy Creek Formation, consisting of interbedded siltstones and sandstones, with minor shales and a few thin dolomites. The Balcoracana Formation is characterised by cyclically

Fig. 4. Interpreted palaeogeography, Billy Creek Formation, during deposition of the (A) Warragee Member and laterally equivalent Coads Hill Member; (B) Erudina Siltstone Member and laterally equivalent Nildottie Siltstone Member; (C) Eregunda Sandstone Member. Arrows refer to predominent palaeocurrent directions. Data points are Chambers Gorge (CG), Heysen Range (HR), Lake Frome borecores (LF), Mt Frome (MF), Mt Scott Range (MS), Reaphook Hill (RH), Wirrealpa Basin (WB) and Yarra-Wurta (YW).

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interbedded redbeds and green shales which grade into buff-coloured dolomites. Thus, it is very similar to the Erudina Siltstone Member of the Billy Creek Formation. The Pantappina and Grindstone Range Sandstones are probably fandeltaic deposits, displaying evidence of both marine conditions (with minor arthropod markings) and braided-alluvial deposition. The overall coarsening-upwards nature of the Lake Frome Group probably reflects increasing tectonic activity in the source area, as the final cycle of deposition in the Adelaide Geosyncline reached its culmination.

Folk (1976) considered that there are three basic prerequisites for redbed formation:

THE REDBED PROBLEM

The origin of hematite pigment in ancient redbeds has long been a focus of interest for geologists, and a centre of controversy. For a review of research carried out prior to 1962 see Van Houten (1961). Until about 1966, the view was held that redbeds were valuable palaeoclimatic indicators, and that most redbeds were formed in tropical-humid and warm savannah climates. Proponents of this hypothesis argue that the hematite cement forms in lateritic soils in the source area and is subsequently transported to desert basins where it is preserved. However, the redbed problem was re-opened by Walker (1963, 1967a, 1967b) when he showed that redbeds could develop diagenetically in hot desert climates, mainly by in situ, intrastratal weathering of heavy minerals during deep burial, aided by ageing. A third alternative for redbed formation, which has since received considerable attention (see Van Houten 1973) related redbed cyclicity in the Catskill Mountains to variations in the oxidation-reduction potential due to watertable fluctuations (Friend 1966). Walker (1974) expanded this idea, and suggested that it may be possible to form redbeds in moist climates by processes of intrastratal alteration. In summary, research over the last two decades has repeatedly emphasised the danger in

assigning hematite-pigmented sediments to a particular climatic or sedimentological regime. Rather, redbeds form where the physical and chemical conditions are suitable, regardless of climate.

(a) a source of abundant iron (either heavy minerals or ferruginous clays); (b) deposition above or not very far below the water table, for readier access of oxygen;

In the Cambrian redbeds of the Adelaide Geosyncline, all of these prerequisites are met. This is particularly the case for the Billy Creek Formation where the sediments are both micaceous and arkosic, containing up to 10 percent biotite (average 2-3 percent; Table 1). These sediments are commonly associated with desiccation cracks or halite pseudomorph casts, indicating periodic subaerial exposure. Because deposition occurred prior to the evolution of land plants, the rate of organic accumulation may have been low, considering the restricted, tidal flat environment of deposition of the sequence. Furthermore, the abundant evidence of evaporite precipitation in the redbeds suggests that the climate was warm and dry and, as pointed out by Folk (1976, p.605) "the lack of vegetation and low water table in deserts provides ideal circumstances for oxidation".

SOURCE OF THE RED PIGMENT Numerous studies on ancient redbeds have shown that the source of the red pigment is amorphous and microcrystalline hematite. Possible sources of the hematite are (a) detrital, (b) in situ, intrastratal weathering of iron-rich minerals, and (c) in situ dehydration of limonite. These three alternatives are discussed below.

Origin of Cambrian redbeds

339

Detrital hematite

Hematite as an in situ weathering product

Although detrital hematite is a source of pigment for some redbeds (e.g. Schluger 1976), Van Houten (1973, p.50) notes that "actively eroded uplands in almost any climate, and regardless of the colour of their soils, generally deliver brown to greyish-brown sediment". This observation implies that the red colour of most redbeds develops after deposition.

Walker et al (1967) showed that much of the clay in Pliocene redbeds in Baja, California, comprises iron-rich montmorillonite formed by in situ intrastratal solution of hornblende. Walker & Honea (1969) later extended these results to suggest that other iron-bearing silicates (such as amphiboles, pyroxenes and biotite) also produce iron-bearing clay. In their study of Recent alluvium eroded from upland areas in the Sonoran Desert, Walker & Honea (1969) showed that the clay fraction was rich in iron, due to initial concentration of iron-bearing clay minerals and biotite in fine-grained sediment.

Study of the Billy Creek Formation confirms an in situ origin for the red colour in this unit. For example, in poorly sorted sandstones and siltstones of the unit, pigment is commonly present between grains, suggesting introduction of free ferric oxide during sedimentation. Ferric oxide cements are also common surrounding quartz overgrowths, indicating that formation of pigmenting oxide continued even after very early diagenetic silica cementation.

In the Billy Creek Formation, there are several lines of evidence to suggest that in situ intrastratal weathering was important as a mechanism for producing free iron in coarsegrained sediments:

Modal composition

BC-K1 Red sandstone (pt. count %)

Quartz Orthoclase Microcline Plagioclase Muscovite Biotite Opaque oxides Sedimentary rock fragments Metamorphic rock fragments Indeterminate rock fragments Overgrowths Iron cement Indeterminate matrix Q:F:R Rock name

39.8 18.2 0.2 1.0 1.8 1.0 0.8 0.8 0.6 1.0 9.2 12.4 13.2 65:31:4 arkose

BC-K11 Green sandstone (pt. count %) 41.6 20.6 1.8 3.0 4.0 1.8 1.0 0.2 0.6 16.6 0.8 8.0 63:33:3 arkose

Table 1. Modal analyses, red and green sandstones, Eregunda Sandstone Member, Wirrealpa Basin. Comparison of many such analyses (Moore 1979c) suggests that hematite cement is formed post-depositionally in sandstones, by alteration of biotite, opaque oxides and rock fragments.

340

P.S. Moore

(a) grains of biotite are opacitized, and have poorly defined, fuzzy or very ragged margins. The alteration must be post-depositional, since the grains could not have withstood transportation in this form; (b) opaque oxides commonly have poorly defined margins, with weak hematite rims developed in rare cases (cf. Walker 1967a); (c) hematite coatings are rarely present at grainto-grain contacts, indicating that development of the pigment was post-depositional; (d) ferric pigments may be developed both beneath, and as coatings on silica overgrowths, indicating that formation and migration of iron-rich pigment occurred over a prolonged period of time, and at least into the early stages of diagenesis of the sequence; and (e) the ZTR (heavy mineral) index for arkosic redbeds in the Billy Creek Formation is anomalously high (average 95 percent), considering that these sediments were derived from Precambrian crystalline basement rich in amphibole, pyroxene and biotite. The high ZTR index probably indicates removal of unstable iron silicates (e.g. pyroxene, amphibole) by post-depositional weathering and intrastratal solution (Table 1). A similar example of heavy mineral enrichment is reported by Schulger (1976). In situ oxidation of iron-rich clays The origin of the red pigment in shaly units in the Billy Creek Formation is difficult to determine. However, silt and clay fractions in the Billy Creek Formation redbeds generally are more strongly stained by hematite than associated coarser sediments (Fig. 5). This is a general characteristic of many redbeds (including most of the Cambrian redbeds in the Adelaide Geosyncline) and has led authors (e.g. Krynine 1949; Stock 1974; Van Houten 1964, 1968) to conclude that the clay fraction in these fine-grained deposits is derived from iron-oxide-rich soils, formed in regions characterised by intense weathering. Weathering in the source area would have

produced yellow-brown limonite (which comprises mainly goethite; HFeC>2), since goethite is stable relative to hematite-plus-water below about 40 degrees centigrade (Berner 1969). Dehydration of limonite to form the red hematite pigment then occurs after burial. This is a common and well documented process for forming red pigment in ancient sequences, as discussed by Berner (1969,1971), Walker (1967b), and Hubert & Reed (1978). In summary, it is believed that two processes were probably responsible for the red pigment in the Billy Creek Formation. Source area weathering and subsequent oxidation of iron-rich clays is considered to be the origin of most of the red pigment in the shales and fine-grained siltstones of the sequence. Moderately sorted, coarse siltstones and sandstones, however, were largely

stained red by intrastratal solution of iron-bearing grains. There is no evidence supporting a detrital origin for any of the red pigment. Limited petrographic data (Stock 1974; Moore 1979c), combined with field observations, suggest that this conclusion can be extended to other Cambrian redbeds of the Adelaide Geosyncline. ORIGIN OF VARIEGATED SEQUENCES According to Walker & Honea (1969, p542), "the vital factor for the formation of redbeds is the occurrence within the depositional basin of special interstitial chemical conditions (for example, favourable Eh and pH) that favour the formation and preservation of hematite". Valuable clues as to the nature of these special conditions are obtained from variegated sequences, where red and non-red intervals are closely associated. Variegated units are common in the Warragee and Erudina Siltstone Members of the Billy Creek Formation in outcrop, and also occur in the Yalkalpo 2 and Lake Frome borecores. Spectacular cyclic sequences of red and non-red sediments are present in the Lake Frome Group, particularly in the Balcoracana Formation. Cyclically interbedded red and grey-green sediments are also common in parts of the Lower Cambrian section on the northeast coast of Kangaroo Island

Origin of Cambrian redbeds

(Smith Bay Shale, Emu Bay Shale and Boxing Bay Formations; Moore 1979c; Daily etal 1980). Examination of these sequences has revealed several origins for the variegation, which can be grouped into two major categories: those essentially syndepositional, and those clearly postdepositional.

341

Syndepositional formation of variegated sequences Fining-upward cycles (or parasequences), from greyish green sandstone to red mudstone, are a common feature of many of the Cambrian formations in this study. The parasequences typi-

SHALE

WAVY

BEDDING

RIPPLE-LAMINATED

SST

PLANAR-LAMINATED

SST

CONTORTED

SST

T R O U G H CROSS S T R A T I F I E D

ARTHROPOD

TRACKS

DESICCATION MUDSTONE

~15~

SST

BURROWS

LOAD

CRACKS

INTRACLASTS

CASTS

R I P P L E OR CROSS BED D I R E C T I O N LINEATION

DIRECTION

Fig. 5. Stratigraphic column, middle part of Boxing Bay Formation, White Point, Kangaroo Island. Note the alternatioi of red and non-red colours and their association with cycles of sedimentation. Red intervals are typicall; desiccation-cracked.

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P.S. Moore

cally grade from cross-stratified or rippledlaminated units, showing evidence of deposition under subtidal conditions, to evenly laminated or massive red mudstone, bearing signs of subaerial exposure. Such parasequences are best developed on the northeast coast of Kangaroo Island, especially in the Smith Bay Shale and Boxing Bay Formation (Moore 1979c; Daily et al 1980). However, they also occur in Cambrian redbeds of the Flinders Ranges. While there is clear evidence in many of these examples that the green to red fining-upward transition represents a regressive, progradational event culminating in subaerial exposure, there remains the doubt that the green intervals may originally have been red, gaining their outcrop colour as a result of Recent subaerial weathering. In order to further study this problem, borecore data from the Billy Creek Formation was inspected. Fining-upward units, from greyish-green sandstones to red mudstones, are well developed in the Billy Creek Formation in the Yalkalpo 2 borecore. The parasequences are typically 2-5 m in thickness and represent regressive events associated with tidal flat progradation. Mediumgrained, calcareous, green sandstones rest sharply on finer grained sediment. The contact is commonly erosional and the calcareous sandstone may contain red or green mudstone intraclasts in the basal portion, which are interpreted as channel lag deposits. The overlying drab sandy unit is evenly laminated or rippled and typically bioturbated. Grey-green siltstones grade upwards into evenly laminated shaly redbeds containing halite imprints, desiccation cracks, minor arthropod tracks and rare anhydrite. The fining-upward parasequences are thus divisible into a coarse, drab unit and a fine, red unit and in this respect are similar to variegated fluvial parasequences described by Allen (1965, 1970), Friend (1966), Van Houten (1973) and Braunagel & Stanley (1977), despite their different sedimentary origin. Red intraclasts in the green sandstones indicate that the red pigment developed and stabilized as hematite prior to erosion of the mudstone. Thus, the hematite formation is essentially syndepositional. Because

the process of intrastratal solution of iron-rich grains to form hematite cement (discussed above) requires thousands and possibly millions of years (Walker 1967a, 1974), it is extremely unlikely that in situ intrastratal solution was important in the formation of these shaly redbeds. Rather, the process of oxidation of iron-rich clay minerals and limonite in an intertidal mudflat environment was responsible for the red pigmentation. This observation is consistent with, and lends support to the thesis presented earlier, that intrastratal solution is a significant process mainly in coarse silty and sandy units. Other types of variegated units occur in the Billy Creek Formation. Parts of the Yalkalpo 2 borecore are characterised by cyclic repetition of red and green shale. Similar, although rather poorly developed cycles, occur in the Warragee Member in the Flinders Ranges. Cycles vary from 0.5-15 m in thickness, and average 1-2 m. Basal scour surfaces are absent, and red and green intervals pass gradationally into each other. In the Yalkalpo 2 borecore, green sandstones are evenly laminated, with laminae 1-5 mm in thickness, commonly graded from medium or fine siltstone to shale. Arthropod tracks are present and indicate a marine influence. Coarser grained intervals contain starved ripples or lenticular bedding, with common bioturbation. Red mudstones are only slightly finer grained, and comprise evenly laminated shale with common halite imprints, desiccation. cracks and minor anhydrite. The cycles are interpreted as resulting from minor transgressions and regressions along a very low energy, muddy shoreline. It is interpreted that in this type of finegrained sequence sufficient clay minerals were present to stain the entire cycle red, given the right chemical conditions. Thus, the green colour of the low energy subtidal deposits is related entirely to the Eh-pH environment at the site of deposition. Arguments that the green intervals contain insufficient clay minerals to supply the neccessary iron for pigmentation, or that the coarse grain-size of the green intervals allowed migration of groundwater and subsequent reduc-

Origin of Cambrian redbeds tion, are not relevant to these fairly uniformly fine-grained variegated sequences. However, what is relevant is that reduction of limonite to ferrous oxide and stabilization of iron-bearing clay minerals as illite-montmorillonite and chlorite is favoured particularly by low Eh (Berner 1971). Such conditions commonly pertain below the water table, particularly in organicrich sediment. Finally, oxidation of the redbeds and reduction of the green intervals must have occurred essentially syndepositionally, otherwise the variegation would have been masked by the changing physicochemical conditions during early burial, and also by subsequent fluctuations in the water table.

343

the Adelaide Geosyncline. In outcrops of the Billy Creek, Moodlatana and Balcoracana Formations in the Binders Ranges, clastic-carbonate parasequences contain additional evidence of syndepositional hematite. The parasequences typically grade from red paralic shales, siltstones or sandstones, through green shale, into buffcoloured subtidal stromatolitic carbonates (Figs 6a, 6b). Within these cycles in the Billy Creek Formation, fining-upward couplets of green shaly siltstone and red, desiccation cracked shale are present, repeated on the scale of 1-5 cm (Fig. 6c). The green shales are evenly laminated to blocky and, at two locations in the Wirrealpa Basin, contain abundant trilobites (Dalgarno 1964; Dalgarno etal 1964; Moore 1979a, 1979c). The trilobites are mostly intact and include a large

The essentially syndepositional stabilization of iron (either as hematite or ferrous oxide) is a feature of the fine-grained Cambrian redbeds of B

Shale

V~~

Desiccation

7^1 S i l t s t o n e

•

Halite

Mudstone

H I

Laminated &/or stromatolitic carbonate Calcite |

(?after evaporites)

C a l c i s i l t i t e , with minor

siltstone

Ripple F

cracks

imprints intraclasts

marks

Body f o s s i l s

(trilobites)

Fig. 6. Variegated cycles, Billy Creek Formation. (A) symmetrical cycle, upper Warragee Member, Wirrealpa Basin (Ten Mile Creek) type section; (B) highly asymmetrical cycle, upper Warragee Member, Lake Frome. (These cycles also occur in the Erudina Siltstone Member and Balcoracana Formation); (C) small scale cycle, upper Warragee Member type section.

344

P.S. Moore

proportion of juvenile forms (Pocock 1970). The fossils occur at the boundary between green and red shale (Fig. 6c). The overlying red shales con-

tain abundant desiccation cracks, suggesting that mass mortality was due to desiccation as the trilobites were stranded by a falling tide.

UPPER

INTERTIDA L

S U P R A T I DAL

(TRANSITIONAL)

MID TO LOWER

LOWER

INTERTIDAL

I NTERTIDAL SUBTIDAL EAST

WEST

I RED S H A L E

AND

SILTSTONE

GREY-GREEN

GREY-GREEN |

BUFF

SILTY

D O L O M IT IC |

DOLOMITE

SHALE

jLIGHT

GREY

| D O L O M I T IC | LIMESTONE

Desiccation Halite

cracks

imprints

Gypsum - anhydrite

Wave

ripples

Current

ripples

Mudflake

Bed

intraclasts

thickness

Flaser

bedding

Wavy bedding

Silt-streaked

mudstone

Sorting

Maturity

D e g r e e of

Algal

oxidation

laminations

C arbonate

Dolomite - calcite

ratio

Fig. 7. Facies model, Warragee Member, Billy Creek Formation. This model has wide applicability to variegated shale-carbonate sequences within the Cambrian of the Adelaide Geosyncline.

Origin of Cambrian redbeds

345

Fig. 8. Large-scale variegated sequences, Balcoracana Formation, Heysen Range. Cycles grade from red shale, siltstone and minor sandstone, into green shale and eventually into algal-bound buff dolomite. The transition back into redbeds tends to be more rapid (cf Fig. 6a).

Fig. 9. Basal view of variegated and desiccation-cracked shale-siltstone couplets, Warragee Member, Wirrealpa Basin. Desiccation cracks in reddish shales have been infilled with green siltstone. In a few examples, families of trilobites have undergone mass mortaliity and are preserved adjacent to the green shale - red shale boundary (cf Fig. 6c).

346

P.S. Moore

Fig. 10. Small variegated cycle within the Balcoracana Formation, Brachina Creek, Heysen Range. Red shale passes rapidly into green shale, with the colour transition being conformable with the bedding plane over the extent of the outcrop. Green shale passes upwards into green calcareous shale then shaly dolomite. Typically in such cycles, the upper contact is sharp, with carbonate deposition being terminated by a sudden influx of sandy clastics (cf Fig. 6b).

Fig. 11. Post-depositional formation of redbeds in fan-deltaic deposits, Eregunda Sandstone Member, Wirrealpa Basin. Channel sandstones and thinner crevasse splay sandstones are red due to post-depositional alteration of iron-rich minerals. Interbedded marginal marine shales were originally green, but have been locally oxidised by groundwater percolating down from the main channel sand. Note how the red-green boundary in these shales follows the base of the channel sand, and cuts across bedding planes within the shales.

Origin of Cambrian redbeds

The desiccation cracks in the red shale are infillled with green shaly siltstone of the next cycle, with essentially no transfer of pigment from one unit to the next (Fig. 9). The couplets are interpreted as single depositional events associated with flooding of upper intertidal mudflats and subsequent desiccation. The fact that the green shaly siltstones remain green indicates that ferrous oxide had formed and stabilized prior to the desiccation of the tidal flat. The fact that the red desiccation-cracked shales remained red during the next marine incursion indicates that the conversion to hematite had also occurred essentially syndepositionally. The depositional model for these variegated clastic-carbonate sequences is shown in Fig. 7. The large scale cycles (Fig. 6a, 6b, 8,10) are also interpreted as representing transgressive and regressive events. Redbeds in these cycles typically show signs of subaerial exposure, with desiccation cracks and halite imprints common. Such features are absent from the green siltstones and shales, which tend to be evenly laminated or ripple laminated, with common arthropod scratch marks. Buff-coloured dolomites are evenly laminated to algal bound, with low domal stromatolites developed at the top of many units. An intertidal to shallow subtidal origin is suggested for the carbonates. This interpretation is supported by several other lines of evidence which indicate that the carbonates represent the deeper water end of the environmental spectrum: (a) an easterly progression occurs in the Billy Creek Formation, from paralic redbeds of the Nildottie Siltstone Member to interbedded redbeds, green shales and dolomites of the Erudina Siltstone Member (Figs 1, 4b). Based on facies changes that occur in the main redbed sequence and abundant palaeocurrent indicators, the deeper water facies are interpreted to occur in the east in association with the non-red sediments (Moore 1979a, 1979b, 1979c, 1982); (b) in the Wirrealpa Basin (Fig. 3), variegated deposits with interbedded dolomites occur at the top of the Warragee Member. The sequence thick-

347

ens into a prominent graben (Ten Mile Creek Graben) that was active throughout most of the Early Cambrian. As the Warragee Member thickens into this graben, redbeds pass into variegated deposits, and laminated and stromatolitic dolomites become thicker and more numerous. The thickening is markedly in excess of thickening of the sequence overall, and is attributed to a basinward change in facies. Post-depositional formation of variegated sequences Post-depositional formation of redbeds by alteration of green-coloured sediment is known to occur. As discussed previously, the process of alteration of iron-rich grains to form hematite occurs over a prolonged period of time. This process is best documented in sandy sequences, where it is probably aided by groundwater movement. However, it also occurs to a limited extent in shaly units. For example, Fig. 11 shows a fan-deltaic deposit in the Eregunda Sandstone Member, where the variegation is clearly postdepositional. Green interdistributary shales with arthropod tracks are truncated by an overlying, red sandy channel deposit. All of the sandstones are red, including thin crevasse splay sandstones within the green shales, presumably due to early post-depositional alteration of iron-rich minerals. Note, however, that the sandstones have a zone of red shale beneath them, suggesting that groundwater percolated into these shales, causing intrastratal solution and post-depositional redbed formation. Post-depositional bleaching of red sediment also occurs and is responsible for the production of some variegated sequences. Generally, however, post-depositional bleaching is easy to recognise, since it transgresses primary depositional structures. Most post-depositionally bleached intervals are related to Recent weathering. Green aureoles around fractured blocks of red sandstone or siltstone are common, and in some cases the alteration has progressed to the stage where the

348

P.S. Moore

original colour of the sediment is difficult to determine.

sediment, but these late-stage effects are generally easy to recognise.

A somewhat unusual example of bleaching of redbeds is present in the lower portion of the Billy Creek Formation where salmon pink and bright green tuff beds occur (Moore 1979a). The green tuffs consist of fine ash with only minor crystal shards. The green colour is due to diagenetic alteration of the volcanic ash to chlorite and clay minerals (Williams et al 1954). In the Nildottie Siltstone Member, rare green tuffs are the only intervals which are not stained red, and thus it is apparent that the tuffaceous detritus was deposited in an oxidising environment. It is concluded that the green colour developed postdepositionally by diagenetic alteration of the chemically unstable, fine-grained ash.

Variegated sequences form where the environment of deposition is alternatively oxidising and reducing. Red-green alternations vary from centimetres to several tens of metres in thickness. In vertical profile, the large scale cycles may be symmetrical or markedly asymmetrical. Symmetrical cycles probably originate in response to autocyclic mechanisms and eustatic variations. Strongly asymmetrical cycles typically pass from red clastics through green shale to buff-coloured stromatolitic dolomite which, in turn, are sharply overlain by sandy or silty redbeds of the next cycle. This asymmetry, with gradual deepening followed by rapid shallowing, is attributed to a tectonic overprinting on the eustatic and autocyclic mechanisms that were operating in the basin.

Green haloes around anhydrite in the Billy Creek Formation in the Lake Frome wells are also interpreted as post-depositional reduction of red sediment. Some of the anhydrite has undergone post-depositional migration, since it occurs as irregular veins and patches, commonly disrupting bedding. Reduction haloes around these veins are therefore also interpreted as post-depositional in origin. Bemer (1971) has shown that reduction of hematite can occur by lowering either the Eh or pH of the sediment. Release of SO4 " ions from anhydrite (CaSC>4) during diagenesis could thus be responsible for creation of local reducing conditions surrounding anhydrite patches and veins. 2

DISCUSSION AND CONCLUSIONS The data presented in this paper support a syndepositional and early post-depositional origin for the Cambrian redbeds of the Adelaide Geosyncline. For shaly sequences, red colouration is closely tied to the environment of deposition, with most of the deposits showing evidence of intermittent subaerial exposure. In coarser grained sediments, this pattern is complicated by post-depositional formation of hematite due to alteration of iron-rich minerals. Further complications result from subsequent alteration of the

ACKNOWLEDGMENTS The data in this paper were largely collected during the course of postgraduate studies by the author at the University of Adelaide. The work was funded by a Commonwealth Postgraduate Scholarship, and supervised by the late Dr Brian Daily. I gratefully acknowledge Brian Daily's contribution in providing encouragement, enthusiasm, and abundant helpful discussions. I also acknowledge discussion and helpful criticism by Bob Dalgarno, David Gravestock, Phil Plummer and Brian Rust. REFERENCES ALLEN J.R.L. 1965. Fining upward cycles in alluvial successions. Liverpool Manchester geological Journal 4,229-246. ALLEN J.R.L. 1970. Studies in fluviatile sedimentation: a comparison of fining-up wards cyclothems, with special reference to coarse-member composition and interpretation. Journal of Sedimentary Petrology 40, 298-332.

Origin of Cambrian redbeds BERNER R.A. 1969. Goethite stability and the origin of red beds. Geochimica et Cosmochimica Acta 33, 267-273. BERNER R.A. 1971. Principles of chemical sedimentology. McGraw-Hill, New York. BRAUNAGEL L.H. & STANLEY K.O. 1977. Origin of variegated redbeds in the Cathedral Bluff tongue of the Wosatch Formation (Eocene), Wyoming. Journal of Sedimentary Petrology 47, 1201-1219. DAILY B. 1976. The Cambrian of the Flinders Ranges. 25th International Geological Congress, Excursion Guide 33A, 15-19.

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MOORE P.S. 1979a. Stratigraphy and depositional environments of the Billy Creek Formation (Cambrian), central and northern Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 103,197-211. MOORE PS. 1979b. Deltaic sedimentation - Cambrian of South Australia. Journal of Sedimentary Petrology 49,1229-1244. MOORE P.S. 1979c. Stratigraphy and sedimentology of the Billy Creek Formation (Cambrian, Flinders Ranges) and its equivalents on the northeast coast of Kangaroo Island, South Australia. PhD thesis, University of Adelaide (unpubl.).

DAILY B, MOORE P.S. & RUST B.R. 1980. Continental-marine transition in the Cambrian rocks of Kangaroo Island, South Australia. Sedimentology 27, 379-399.

MOORE P.S. 1980a. Stratigraphy and depositional environments of the Billy Creek Formation (Cambrian), east of the Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 104,117-132.

DALGARNO C.R. 1964. Lower Cambrian stratigraphy of the Flinders Ranges. Transactions ofthe Royal Society of South Australia 88, 129-144.

MOORE P.S. 1980b. Fine grained clastic sedimentation in an epeiric sea - a model from the Cambrian of South Australia. Geological Society of Australia, Abstracts 2, 39-40.

DALGARNO C.R., JOHNSON J.E. & COATS R.P. 1964. BLINMAN map sheet, Geological Atlas of South Australia, 1 63 360 series. Geological Survey of South Australia.

MOORE PS. 1982. Ripple-mark analysis of a fine-grained epeiric sea deposit (Cambrian, South Australia). Journal of the Geological Society of Australia 29, 71-81.

FOLK R.L. 1976. Reddening of desert sands: Simpson Desert, N.T., Australia. Journal of Sedimentary Petrology 46, 604-615.

POCOCK K.J. 1970. The Emuellidae, a new family of trilobites from the Lower Cambrian of South Australia. Palaeontology 13, 522-562.

FRIEND RF. 1966. Clay fractions and colours of some Devonian red beds in the Catskill Mountains, U.S.A. Quarterly Journal, Geological Society ofLondon 122, 273-292.

SCHLUGER P.R. 1976. Petrology and origin of the red beds of the Perry Formation New Brunswick, Canada, and Maine, U.S.A. Journal of Sedimentary Petrology 46, 22-37.

HUBERT J.F. & REED A.A. 1978. Red-bed diagenesis in the East Berline Formation, Newark Group, Connecticut Valley. Journal of Sedimentary Petrology 48, 175-184. KRYNINE P.D. 1949. The origin of red beds. Transactions of the New York Academy of Science 11, 60-68.

STOCK E.C. 1974. The clay mineralogy, petrology and environments of deposition of the Cambrian Lake Frome Group, Flinders Ranges, South Australia. M Sc thesis, University of Adelaide (unpubl.). VAN HOUTEN F.B. 1961. Climatic significance of red beds. In Nairne A.E.M. ed, Descriptive palaeoclimatology. Interscience Publications Incorporated, New York.

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VAN HOUTEN F.B. 1964. Origin of red beds - some unsolved problems. In Nairne A.E.M. ed, Problems in palaeoclimatology, pp. 647-661. Proceedings, NATO Palaeoclimates conference 1963. Interscience Publications Incorporated, New York. VAN HOUTEN F.B. 1968. Iron oxides in red beds. Geological Society ofAmerica, Bulletin 79, 399-416. VAN HOUTEN F.B. 1973. Origin of red beds - a review 1961-1972. Earth and Planetary Sciences, Annual Review 1, 39-61. WALKER T.R. 1963. In situ formation of red beds in an arid to semi-arid climate. Geological Society of America, Special Paper 76, 174-175.

WALKER T.R. 1974. Formation of red beds in moist tropical climates: a hypothesis. Geological Society of America, Bulletin 85, 633-638. WALKER T.R. & HONEAR.M. 1969. Iron content of modern deposits in the Sonoran Desert: a contribution to the origin of red beds. Geological Society of America, Bulletin 80, 535-544. WALKER T.R., RIBBS P.H. & HONEA R.M. 1967. Geochemistry of horneblende alteration of Pliocene rocks of the Baja California. Geological Society of America, Bulletin 78, 1055-1060.

WALKER T.R. 1967a. Formation of red beds in modern and ancient deserts. Geological Society of America, Bulletin 78, 353-368.

WILLIAMS H., TURNER F.J. & GILBERT C.M. 1954. Petrography. An introduction to the study of rocks in thin sections. Freeman & Company, San Francisco.

WALKER T.R. 1967b. Colour of red sediments in tropical Mexico: a contribution to the origin of red beds. Geological Society of America, Bulletin 78, 917-920.

YOUNGS B.C. 1978. The petrology and depositional environments of the Middle Cambrian Wirrealpa and Aroona Creek Limestones (South Australia). Journal of Sedimentary Petrology 48, 63-74.

Sedimentology and stratigraphy of the Carrickalinga Head Formation (low stand fan to high stand systems tract), Kanmantoo Group, South Australia 1 2 1 Colin G. Gatehouse , James B. Jago and Barry J. Cooper 1

South Australian Department Department

of Mines and Energy, P.O. Box 151, Eastwood, S.A. 5063,

Australia

of Applied Geology, South Australian Institute of Technology, The Levels, SA. Australia

5095,

The Early Cambrian Carrickalinga Head Formation, the basal unit of the Kanmantoo Group in the Fleurieu Peninsula-Mt Lofty Ranges area, rests conformably on the Heatherdale Shale (the top unit of the Normanville Group) or is faulted against Late Proterozoic (Adelaidean) sediments. The base of the Carrickalinga Head Formation represents a sequence boundary. On Fleurieu Peninsula, the Carrickalinga Head Formation comprises three members: Madigan Inlet Member (base), Blowhole Creek Siltstone Member, and the Campana Creek Member (top). The Madigan Inlet Member comprises sharp-based sandstone-mudstone couplets, the sandstone portions of which were deposited as a series of sediment gravity flows with the mudstone portions representing hemipelagic sedimentation. It is postulated that the Madigan Inlet Member was deposited as a submarine fan (low-stand fan) at a time of low sea level. The Blowhole Creek Siltstone Member comprises mainly siltstone and represents basin plain (lowstand wedge, trasngressive and part of the highstand systems tracts) sedimentation at a time of rising sea level. The sandstones of the Campana Creek Member represent a shallowing upwards succession which is a precursor to the shallow water, high-energy deposits of the conformably overlying Backstairs Passage Formation which prograded across the present area of outcrop of the Kanmantoo Group in the Mt Lofty Ranges, Fleurieu Peninsula and Kangaroo Island. Together the Campana Creek Member and the Backstairs Passage Formation represent part of the highstand systems tract At Sedan Hill in the northeastern Mt Lofty Ranges the three members of the Carrickalinga Head Formation are recognised but the whole sequence represents a shallower water environment than on Fleurieu Peninsula. In the northeastern Mt Lofty Ranges the Blowhole Creek Siltstone Member includes a carbonate-rich unit - the Milendella Limestone Member. The Carrickalinga Head Formation is the start of a new cycle of sedimentation which was triggered off by the Kangarooian Movements. One result of this tectonism was the uplift of parts of the Gawler Craton which supplied the clastics of the Carrickalinga Head Formation.

Key words: Cambrian, Mount Lofty Ranges, Fleurieu Peninsula, Kanmantoo Group, Kangarooian Movements, submarine fan, basin plain, prograding sand body, sequence boundary.

INTRODUCTION

of the basal Kanmantoo Group and a sedimentological study of a Cambrian submarine delta-

The purpose of this paper is to describe the

fan system. Ancient submarine fan systems

nature and depositional setting of the basal unit

constitute

of the Early Cambrian Kanmantoo Group, the

various parts of the world (Shanmugam & Moiola

major

hydrocarbon

reservoirs

Carrickalinga Head Formation. This work repre-

1988). Mineralisation is apparent throughout

sents the first detailed study of the unit on

much of the Kanmantoo Group; copper, gold,

Fleurieu Peninsula and in the Mt Lofty Ranges.

silver, lead, and other metals, have been produced

It provides an insight into the nature and origins

in the past (Parker 1986).

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The first useful description of the Carrickalinga Head Formation is that of Madigan (1925) who referred to "yellow arenaceous beds with minute mica flakes" or "greywacke" at Carrickalinga Head (Fig. 1) and a "dark hornstone" at Blowhole Creek on the South Coast of Fleurieu Peninsula (referred to herein as the South Coast). Campana etal (1955, pl.6) described a greywacke from Carrickalinga Head, while Abele & McGowran (1959) also used the term "greywacke", published a thin-section deposition and gave a general outline of the structure at Carrickalinga Head and surrounding areas.

of an unnamed unit below the Inman Hill Formation. The Strangway Hill Formation of Thomson (1969a, b) corresponds in part to the Carrickalinga Head Formation, but as noted by Daily & Milnes (1971), most of Thomson's Strangway Hill Formation belongs to rocks of the Late Proterozoic Adelaide Supergroup. The stratigraphic nomenclature used here for the Kanmantoo Group (Table 1) is that of Daily & Milnes (1971, 1972a, 1973), who incorporated some of the terms used by Thomson & Horwitz (1962) and Thomson (1969a, b) in their subgroup nomenclature.

Thomson & Horwitz (1962) on the Barker 1:250 000 sheet included the South Coast sections of the Carrickalinga Head Formation as part

The term Carrickalinga Head Formation was introduced by Daily (1963) and formally defined by Wopfner (1969) and Daily (1969) from the

Carrickalinga Head Formation Sedimentology

TABLE 1 Cambrian stratigraphic nomenclature of southern Fleurieu Peninsula

11 11 £§

MIDDLETON SANDSTONE PETREL COVE FORMATION BALQUHIDDER FORMATION TUNKALILLA FORMATION TAPANAPPA FORMATION TALISKER CALC-SILTSTONE

i? BACKSTAIRS PASSAGE FORMATION

CAMPANA CREEK MEMBER CARRICKALINGA HEAD FORMATION

BLOWHOLE CREEK SILTSTONE MEMBER MADIGAN INLET MEMBER

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on the north coast of Kangaroo Island, NW of Mount McDonnell where it was named the Mount McDonnell Formation by Daily (1969). Daily & Milnes (1971, p.204) suggested that the name, Mount McDonnell Formation, be discarded because of its identity with the Carrickalinga Head Formation. Moore (1983) gave a detailed description and sedimentological analysis of 520 m of Carrickalinga Head Formation on the north coast of Kangaroo Island, near Hummocky Point.

HEATHERDALE SHALE FORK TREE LIMESTONE SELLICK HILL FORMATION WANGKONDA FORMATION MOUNT TERRIBLE FORMATION

Table 1 Cambrian stratigraphic nomenclature of southern Fleurieu Peninsula.

outcrop at Carrickalinga Head, which thus became the type area. Only the basal 80 m of the unit is exposed at Carrickalinga Head, where the formation is underlain conformably by the carbonaceous shales, siltstones and limestones of the Heatherdale Shale, the top unit of the Normanville Group. Daily & Milnes (1971) established and produced brief descriptions of three members of the Carrickalinga Head Formation near Blowhole Creek on the South Coast of Fleurieu Peninsula, an area they proposed as a subsidiary type section for the entire formation. The three members named by Daily & Milnes (from oldest to youngest) were the Madigan Inlet Member, the Blowhole Creek Siltstone Member and the Campana Creek Member (Table 1). Under this scheme, the type section at Carrickalinga Head includes only the basal part of the Madigan Inlet Member, since the upper part of the formation does not outcrop at this locality. Along the South Coast of Fleurieu Peninsula, and elsewhere, the Carrickalinga Head Formation is overlain conformably by the Backstairs Passage Formation. Daily (1969) and Daily & Milnes (1971) note that the Carrickalinga Head Formation also crops out

There is no palaeontological control for the various sections of the Carrickalinga Head Formation. However, they are well-controlled lithostratigraphically in that at Carrickalinga Head, on the South Coast, and at Sedan Hill, the Carrickalinga Head Formation is underlain by the Heatherdale Shale. On the South Coast, at Sedan Hill and elsewhere in the Mt Lofty Ranges the Carrickalinga Head Formation is overlain by the very distinctive Backstairs Passage Formation. On the north coast of Kangaroo Island the Carrickalinga Head Formation is overlain by the Stokes Bay Sandstone, an equivalent of the Backstairs Passage Formation (Daily & Milnes 1971, p.205); the base of the Carrickalinga Head Formation is not exposed on the north coast of Kangaroo Island.

GEOLOGICAL SETTING The Carrickalinga Head Formation rests conformably on the blue-black, phosphatic, carbonaceous Heatherdale Shale, the top unit of the Normanville Group. The Normanville Group comprises mainly very shallow marine sandstones, shales and limestone, although the Heatherdale Shale is of deeper water origin. Limited palaeontological evidence suggests that the Heatherdale Shale is of middle Early Cambrian age, thus providing an older limit to the age of the Kanmantoo Group (Jago et al 1984, 1986). Very few body fossils, none of which are biostratigraphically significant, have been found in the Kanmantoo Group.

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The Kanmantoo Group (Sprigg & Campana 1953) comprises a succession of up to 18 000 m of medium to fine-grained clastic sediments with minor conglomerates and carbonates. Outcrops occur in the Mt Lofty Ranges, Fleurieu Peninsula, and Kangaroo Island regions of South Australia, and extend over an area about 365 km by 35 km. The type section of the Kanmantoo Group is well exposed along the wave-cut platform and cliffs of the South Coast of Fleurieu Peninsula from near Coalinga Creek (also known as Campbell Creek) to Middleton (Fig. 1). However, some of the outcrop is of difficult access. Daily & Milnes (1971,1973) defined and described eight formations from the type section (Table 1). In the eastern part of the Mt Lofty Ranges the Kanmantoo Group appears to be, at least in part, faulted against Adelaidean rocks (Toteff 1977, this volume; Marlow 1975). Due to a cover of Cainozoic Murray Basin sediments no eastern limit of the Kanmantoo Group has yet been mapped. However, metamorphosed sediments intruded by granites of the Delamerian Orogeny along the Padthaway Ridge in southeastern South Australia may be equivalent to the Kanmantoo Group, as may be the Glenelg River Beds of western Victoria (Cooper & Grindley 1982). The generally accepted origin of the Kanmantoo Group sediments is given by Thomson (1975). In this model, the present area of Yorke Peninsula and Investigator Strait was uplifted (Cassinian Uplift of Thomson 1969a) during middle Early Cambrian times providing a source of sediments for the Kanmantoo Group, which were deposited in the Kanmantoo Trough formed by the Waitpingan Subsidence of Thomson (1969a). Daily & Forbes (1969) used the term Kangarooian Movements to describe these tectonic events. Daily & Milnes (1971, 1972a, b, 1973) considered that the Kanmantoo Group sensu lato was rapidly deposited in a shallow marine environment by strong traction currents. However, other workers (e.g.Thomson 1969a, 1975; Flint 1978; Mancktelow 1979; Von der Borch 1980; Jago & Daily 1982) have suggested that at least

some units were deposited as proximal turbidites possibly in deep-sea fans. Von der Borch (1980) has interpreted the Kanmantoo Group sediments as the initial phase of fully developed continental margin sedimentation, which typified the eastern flank of Australia during most of the Palaeozoic. The basal part of the Carrickalinga Head Formation at Carrickalinga Head lies north of Normanville where Von der Borch (1980, p. 126) proposed that the Houghton Anticlinal Zone acted as a separating feature between shallow-water sediments to the west and continental slope and rise environments to the east. Scheibner (1986, fig. 4) implies that the Kanmantoo Group sediments were at least in part deposited on oceanic crust and that the "Kanmantoo Trough" was part of a marginal sea, extending eastwards to the Stavely volcanic belt of western Victoria. Parker (1986) suggested that the Kanmantoo Group sediments were probably deposited in an extensional tectonic regime on a marginal shelf reflecting tectonic activity to the west and a continental margin to the east. However, the general lack of outcrop east of the present Mt Lofty Ranges makes all such interpretations speculative, a point noted by Parker and others. In summary the Kanmantoo Group represents the start of a new cycle of sedimentation which was initiated by the Kangarooian Movements and was probably stopped by the onset of the Late Cambrian-Early Ordovician Delamerian Orogeny (Thomson 1969a). Prior to the culmination of the first phase of this deformation the Encounter Bay Granites (504-495Ma) were intruded and crystallised (Milnes et al 1977). Delamerian folding and associated regional metamorphism up to sillimanite grade continued for a further 50Ma with a further weak metamorphic event in the late Silurian-Early Devonian (Milnes et al 1977; Webb 1976). Three phases of folding and low-pressure intermediate metamorphism affected the Kanmantoo Group over a large area (Offler & Fleming 1968).

Carrickalinga Head Formation Sedimentology

PRESENT STUDY This study involved measuring sections at Carrickalinga Head (80 m), Myponga Beach (76 m), Coalinga Creek to Blowhole Creek (1200 m), and Sedan Hill (1700 m) (Fig. 1). This stratigraphic and sedimentological work has been supported by a limited amount of petrology. At the type section at Carrickalinga Head only about the basal 80 m of the lowest member, the Madigan Inlet Member, is well exposed. This section was first measured by two of us (CGG, JBJ) in late 1985, with part of the section being published in Jago et al (1986). A short section near Myponga Beach, 5 km north of Carrickalinga Head includes the basal part of the Madigan Inlet Member though the contact with the underlying Heatherdale Shale, is not exposed. The section is located in the first creek south of the road to Myponga Beach 300 m from its mouth.

355

A poorly-exposed section of the Carrickalinga Head Formation measured by Toteff (1977), near Nairne is included in Figure 2.

The subsidiary reference section of the Carrickalinga Head Formation in the Coalinga Creek-Blowhole Creek area is well-exposed along the wave-cut platform, but faulting and folding have complicated the outcrop so that it is not possible to measure a complete section, particularly of the Madigan Inlet Member. Mancktelow (1979), following comparison with the section at Fishery Beach between Coalinga Creek and Cape Jervis (Fig. 1), suggested that part of this section may be duplicated, but this is yet to be proven. A further subsidiary reference section (measured by CGG and BJC) for the Carrickalinga Head Formation is herein established at Sedan Hill (Fig. 1) where exposure is excellent and tectonic disturbance is minimal. The recent widening of the Keyneton to Sedan road at Sedan Hill has revealed excellent vertically dipping exposures of the Backstairs Passage Formation as well as almost all the Carrickalinga Head Formation. Only the basal few metres of the latter appear to be cut out by faulting.

Fig. 2 Summary sections of the Carrickalinga Head Formation from Blowhole Creek, Carrickalinga Head, Myponga Beach, Nairne and Sedan Hill.

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C.G. Gatehouse, J.B. Jago & B J . Cooper

Carrickalinga Head Formation - Type Section

COALINGA CREEK CARRICKALINGA HEAD MYPONGA BEACH

Fig. 3 Comparison of the lower parts of the Madigan Inlet Member from sections at Coalinga Creek, Carrickalinga Head and Myponga Beach.

The type section of the Carrickalinga Head Formation occurs on the foreshore at Carrickalinga Head (Fig. 1). Exposure is best at low tide during calm weather. Excellent exposures exhibiting only very low-grade metamorphism allow a detailed section to be measured (Fig. 3) of the basal 80 m of the Madigan Inlet Member. The sharp, but conformable, contact between the base of the Carrickalinga Head Formation and the dark phosphatic and calcareous shales and siltstones of the underlying Heatherdale Shale outcrops at the southern end of a small cove immediately north of Carrickalinga Head. The basal metre of the Carrickalinga Head Formation at Carrickalinga Head comprises grey-green siltstone which includes a 20 cm thick bed of micaceous fine sandstone. Apart from this basal metre, the Madigan Inlet Member at Carrickalinga Head comprises a sequence of units from one to nine metres thick (Fig. 3). Each unit rests sharply on the underlying unit (Figs. 4a,b) and comprises a lower, thicker part composed of fine to medium grained, greygreen, poorly sorted sandstone which grades rapidly up over a few centimetres to a thinner component of olive-green mudstone. Thin section studies reveal that the grain size in the sandstone remains constant until just below the mudstone part of the unit. Within each sandstone/mudstone couplet the sandstone is interpreted as representing deposition from a sediment gravity flow, or series of gravity flows, with the mudstone being of hemipelagic origin. The mudstones are massive and show no indications of current or wave activity.

Fig. 4 a. Sharp-based sandstone overlying mudstone of underlying couplet, 15.2 m above base of formation, Carrickalinga Head. Staff is 1.5 m long; b. Amalgamated sandstone units, 13.5 m above base of formation, Carrickalinga Head. Hammer is 30 cm long; c. Laminated sandstone, 5.5 m above base of formation, Carrickalinga Head. Hammer is 30 cm long; d. Clasts within sandstone, 15.6 m above base of formation, Carrickalinga Head; e. Water escape structures within sandstone, 50.5 m above base of formation, Carrickalinga Head; f. Top of large water escape structure within sandstone, 37 m above base of formation, Carrickalinga Head; g. Slightly cleaner Fe-rich sandstone horizons within larger sandstone unit, 44.8 m above base of formation, Carrickalinga Head; h. Large flame structure, main disrupted horizon, 62.5 m above base of formation, Carrickalinga Head.

Carrickalinga Head Formation Sedimentology

357

358

C.G. Gatehouse, J.B. Jago & B.J. Cooper

In some of the sandstone/mudstone couplets the sandstone portion seems to have been deposited by a single sediment gravity flow. However, in other sandstones, two or more gravity flows seem to have been involved, as is suggested by either the presence of distinct erosional surfaces within the sandstones, or by abrupt vertical lithological changes. These changes include abrupt transitions from massive or laminated sandstones to horizons with disrupted bedding. Many of the sandstone beds show a crude to well-developed lamination (Fig. 4c) while others are massive. In some sandstone horizons there is evidence of post-depositional movement. The amount of movement varied from slight with little contortion of the laminae to substantial with some beds probably having been redeposited as a slurry-like flow with the original laminae being almost completely disrupted. In the latter case individual laminated clasts are identifiable within the redeposited slurry (Fig. 4d). Many of the sandstone beds also include scattered angular dark siltstone clasts up to 1 m across, but generally 10 to 20 mm across. These clasts result from the scouring of previously deposited sediments by the sediment gravity flow. Dewatering structures are prominent within some of the sandstone units. These vary from thin vertical, almost worm-like structures of cleaner sand through the sandstone (Fig. 4e) to more massive columnar structures (Fig. 4f). Possible dish structures are found at some horizons. Discontinuous horizons of cleaner, more iron- and quartz-rich, generally laminated sandstones occur at some levels (Fig. 4g). These cleaner sands show load structures in places. At the base of each sandstone/mudstone couplet the sandstone of the upper unit rests sharply on the mudstone of the lower unit (Figs. 4a,b). No sole marks such as flute casts and other scour structures have been seen. However, in a few cases the sandstone units from successive couplets are amalgamated (Fig. 4b) thus suggesting that the hemipelagic mud from the underlying

unit was either scoured out by the later sediment flow, or that successive sediment flows occurred before much hemipelagic material could settle out. The latter possibility is favoured because at one such amalgamated contact there are quite large load and flame structures that are attributed to the unconsolidated character of the underlying sand. There is massive disruption at the contact between two sandstone/mudstone couplets about 63.5 m above the base of the Carrickalinga Head Formation (Fig. 4h). At this level the sandstone of the upper couplet has, in places, sunk right through the underlying mud horizon which in turn was squeezed upwards into the overlying sandstone via a series of large flame structures (Fig. 4h). The result is a discontinuous, disrupted mudstone horizon, originally from the lower couplet, which occurs as isolated blocks about 1 m thick and several metres long within the sandstone. This feature is exposed for about 200 m along strike. The description given above suggests that most of the sandstones described above belong in Facies B of Mutti & Ricci Lucchi (1972). However, some of the amalgamated horizons, particularly those containing coarse clasts, would fall into Facies A of Mutti & Ricci Lucchi. The sandstones at Carrickalinga Head show some of the characteristics of fluidised sediments and debris flows as outlined by Middleton & Hampton (1973,1976). These include poor grainsize grading, poor sorting, water-escape structures, large clasts "floating" in a finer matrix and the sharp-based nature of the sandstones. Stauffer (1967) reported the presence of very large clasts suspended in a sandy or muddy matrix in beds of grain flow origin. He also noted the absence of erosional sole markings, convolute laminations, cross-bedding, and ripple marks, features which are generally absent at Carrickalinga Head. However, Lowe (1982) has reinterpreted Stauffer's "type" grain-flow deposits as a high-density turbidity current deposit. As noted by Shanmugam & Moiola (1988) there has been considerable

Carrickalinga Head Formation Sedimentology

discussion and reinterpretation of sediment gravity flow deposits in recent years. At present we prefer to use the term sediment gravity flow, rather than a more specific term, in discussing the mode of deposition of the sandstones of the Madigan Inlet Member at Carrickalinga Head, Myponga Beach, and on the South Coast of Fleurieu Peninsula. Myponga Beach The section measured near Myponga Beach, although faulted at top and bottom, is similar to that exposed at Carrickalinga Head. However, sandstone beds are thicker while the mudstone beds are thinner and less frequent (Fig. 3). Clasts are less frequent in the sections of which only about 76 m are exposed. The Myponga Beach section corresponds with the basal part of the section at Carrickalinga Head because a correlation of individual mudstone units can be made between the two localities (Fig. 3). Thin sections (Farrand 1988) at the top and base of several sandstone beds show that they comprise very poorly sorted, very angular, fine- to medium-grained quartz sandstone with feldspar and mica. The maximum measured grainsize was 0.3 mm but the average is of the order of 0.05 to 0.1 mm. As at the type section there is no evidence of graded bedding or sole markings. Coalinga Creek The section from about 1.2 km west of Blowhole Creek to 300 m east of that creek (Fig. 1) was designated the type section for the three members of the Carrickalinga Head Formation by Daily & Milnes (1971). This section was measured as completely as possible and is summarised in Fig. 2. Problems of accessibility along the coastline combined with structural complications meant that a complete section of the basal Madigan Inlet Member could not be obtained. The thickness of about 550 m shown in Fig. 2 should be considered as a reasonable approximation, although Mancktelow (1979) has suggested

359

that this section is partly duplicated. Deformation and metamorphic grade are appreciably higher than at Carrickalinga Head, but sedimentary structures and other depositional features are well preserved. Daily & Milnes (1971) noted common actinolite-garnet nodules and segregations within sandstone units of the Madigan Inlet Member and considered that the grade of metamorphism is at the low stage of Winkler (1970) between the almandine and staurolite isograds. The formation and member boundaries are as described by Daily & Milnes (1971). As at Carrickalinga Head, the Madigan Inlet Member comprises a sequence of sandstone/mudstone couplets, although along the South Coast these are metasandstone/phyllite couplets. In the following discussion the terms sandstone and mudstone are used in order to facilitate comparison with the other sections. Each couplet comprises fine to medium-grained sandstone overlain by mudstone, with the base of the sandstone differentiated sharply from the mudstone of the underlying couplet. In Figure 3 the lowermost 80 m of the South Coast section is compared with the section at Carrickalinga Head. The most striking difference between the two sections is that the couplets are much thinner on the South Coast than at Carrickalinga Head. Many couplets in the South Coast section are less than a metre thick; the thickest couplet in the basal 80 m is only 2.8 m thick. The mudstone intervals are also thicker and the clasts are less abundant. As at Carrickalinga Head the clast assemblage comprises almost entirely dark siltstone. Thin sections from several sandstone units show no discemable difference in grain size from top to bottom. There is a rapid gradation from the top of each sandstone upwards into the mudstone of the couplet. The grains are very angular and very poorly sorted; rounded to subrounded grains rarely are present. Quartz, feldspar and mica are present in approximate proportions of 85%, 10% and 5%. Minor chert, tourmaline, and sericite are present. The sandstone beds have sharp bases,

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C.G. Gatehouse, J.B. Jago & B.J. Cooper

with rare examples of load structures. Although some of the basal sand contacts show minor undulation, no scour structures such as flute casts have been found. Most of the sandstone beds are massive, but there are many which are either

diffusely or clearly laminated. Near the top of the Madigan Inlet Member many of the finer sands show either small scale ripples or small slumps. Small-scale cross lamination is found at some

Fig. 5 a. Base of Blowhole Creek Siltstone Member. The figure is standing on the top sandstone of the Madigan Inlet Member. South Coast section 100 m west of Blowhole Creek; b. Folding in Campana Creek Member, about 37 m above base of Member, South Coast Section; c. Bedding characteristics, Campana Creek Member, about 33 m above base of Member, South Coast Section; d. Lenticular bedding , Campana Creek Member, 33 m above base of Member, South Coast Section; e. Graded bedding, Campana Creek Member, about 1 m above base of Member, South Coast Section; f. ?hyolithid, Campana Creek Member, about 107 m above base of Member, South Coast Section. Coin diameter, 25 mm.

Carrickalinga Head Formation Sedimentology

levels, for example about 200 m above the base of the member. The sandstone units within the sandstone/mudstone couplets vary from less than 0.1 m to about 2.4 m thick. A plot of sandstone thicknesses from the basal 300 m of the Madigan Inlet Member shows that there are several thick intervals (up to 40 m) in which the sandstone units rarely exceed a thickness of 1 m. There appears to be a regularity of bed thickness variation which consists mainly of thinning upwards sequences but with some thickening upwards sequences.

361

The Campana Creek Member, which sharply overlies the Blowhole Creek Siltstone Member, is about 135 m thick although structural complications are present (Fig. 5b). The basal 20 m comprises laminated fine to medium grained metasandstone, with some low angle cross-bedding. Petrologically it is a fine-grained biotiterich quartz arenite; the recrystallisation of clay to biotite during metamorphism masks the original mineralogy (Farrand 1988).

BLOWHOLE CREEK

SEDAN HILL BACKSTAIRS PASSAGE FORMATION

The thickest sandstone units within the Madigan Inlet Member occur 55-140 m below the top of the unit. Here, some of the sand units are up to 10 m thick, and at one horizon about 100 m below the top of the member one sand has channelled into an underlying cross-bedded sandstone. This latter case is the only clear example of channelling observed in the Madigan Inlet Member. Unfortunately the upper part of the member is more structurally complex and less accessible than the basal 300 m. The Blowhole Creek Siltstone Member is about 400-450 m thick in its type section. As noted by Daily & Milnes (1971) it consists almost entirely of grey laminated phyllite and is sharply differentiated from the Madigan Inlet Member (Fig. 5a). In places there are very thin (1-2 mm) discontinuous paler fine sandstone laminae while about 100 m above the base of the member there are a few thin fine sandstone horizons up to 20 cm thick. About 130-150 m above the base of the member, immediately west of the mouth of Blowhole Creek, the laminated phyllite is interbedded with fine to medium grained sandstone. The uppermost 100 m of the Blowhole Creek Siltstone contains some thin scattered lenses of fine sandstone, with what appear to be starved ripples in a few places. Daily & Milnes (1971) report worm casts in the upper part of the Member.

_LL

87 845 SADME

Fig. 6 Comparison of the upper parts of the Campana Creek Member from sections at Blowhole Creek and Sedan Hill.

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C.G. Gatehouse, J.B. Jago & B.J. Cooper

Above this is about 20 m of thin-bedded to laminated metasiltstone with numerous sandstone horizons up to 20 mm thick (Figs. 5c,d). The overlying 70 m comprises essentially interbedded fine-medium grained sandstone with thin siltstone horizons or laminated dark siltstone with thin (1 mm) discontinuous laminae of pale grey sandstone. Some of the sandstones show graded bedding (Fig. 5e). At some levels there are medium grained 5-10 cm thick sandstone horizons within the siltstone. Possible burrows are found in a sandy siltstone towards the top of this 70 m section. The top 25 m of the Campana Creek Member comprises mainly laminated fine sandstone with minor siltstone (Fig. 6). There are also some medium-grained, thinly-bedded and crossbedded sandstones. Some horizons show soft sediment slumping and flame structures. About 10 m below the top of the member possible hyolithids up to 25 mm long occur on some bedding planes (Fig. 5f). The sandstones of the Campana Creek Member become more quartz-rich up sequence. About 20 m below the top of the member Farrand (1988) has described from thin section a fine-grained quartz arenite which contains biotite, chlorite, feldspar, and muscovite. As noted by Daily & Milnes (1971) the boundary between the Campana Creek Member and the overlying Backstairs Passage Formation is apparently conformable but is not clearly exposed. The basal 20 m of the Backstairs Passage Formation comprises well-bedded, fine to medium-grained sandstone with abundant ripple marks and cross-bedding. Some channelling is also present. The sandstones of the Backstairs Passage Formation are more quartz-rich than those of the Campana Creek Member. A thin section prepared from a sample near the base of the Backstairs Passage Formation comprises a recrystallised mosaic of quartz grains up to 0.2 mm across with traces of plagioclase, abundant biotite, minor muscovite with heavy mineral grains (zircon, tourmaline) concentrated in bands (Farrand 1988). The biotite and muscovite are probably of metamorphic origin.

Sedan Hill A 1575 m thick sequence of sandstone, siltstone and subordinate limestone at Sedan Hill is equated with the Carrickalinga Head Formation. Most of the sequence is well-exposed in the cuttings of the newly widened Sedan-Keyneton road. This sequence is overlain conformably by strongly cross-bedded clean sandstones of the Backstairs Passage Formation. Although the Heatherdale Shale occurs at Sedan Hill, the contact with the Carrickalinga Head Formation is not exposed. However, 8 km further north at Red Creek the Carrickalinga Head Formation rests conformably on the Heatherdale Shale. At Sedan Hill there is a three-fold subdivision of the Carrickalinga Head Formation corresponding to that exposed in the South Coast of the Fleurieu Peninsula. At Sedan Hill the Madigan Inlet Member is approximately 850 m thick and comprises predominantly siltstone with subordinate sandstone beds and minor carbonate horizons. Sedimentary features throughout the unit include flaser and linsen bedding, slumping, crossstratification, and ripples. In detail the siltstone contains many very thin sandstone laminae displaying load-casting and rarely lenticular bedding. This suggests that the unit was deposited under relatively shallow-water conditions. The Blowhole Creek Siltstone Member here is quite thin (190 m) and differs from the type-section because it has within it 65 m of impure limestone known as the Milendella Limestone Member (Fig. 2). The latter is a locally named unit of lenticular form with strike length of 60 km. Thomson (1969b) shows the distribution of the unit while White (1959) discussed aspects of its metamorphism. The Campana Creek Member at Sedan Hill is 535 m thick. It comprises light to medium grey micaceous sandstone units up to 30 m thick which are interbedded with thinner siltstone (Fig. 6). The sandstones are fine- to medium-grained. Some of the sandstones contain an appreciable amount of silt while the siltstone units are in

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places sandy. The sandstones contain abundant sedimentary structures including parallel laminations, asymmetric ripples, slumping, and crossbedding. The regular laminations of sandstone and siltstone are similar to those described by Daily & Milnes (1971) in the type section of the Campana Creek Member.

INTERPRETATION The Carrickalinga Head Formation is the basal unit of the Kanmantoo Group. As such, it represents the start of a new cycle of sedimentation which was triggered by the Kangarooian Movements in about the middle Early Cambrian (Daily & Milnes 1971). The sediments of the Carrickalinga Head Formation were derived from an uplifted land mass in the southeastern part of the Gawler Craton to the west and including the present areas of Invetigator Strait and Gulf St Vincent and part of Yorke Peninsula (Jago & Daily 1982; Figs. 1,7). The exact relationship between the Cambrian sequences on Yorke Peninsula (see paper by Daily, this volume), and those on Fleurieu Peninsula is uncertain. Seismic data indicate that an erosional surface at the top of the Parara Limestone (equivalent to part of the Normanville Group on Fleurieu Peninsula, Table 1) extends east from Yorke Peninsula into Gulf St Vincent (Stuart & Von Sanden 1972). The seismic data do not allow us to determine how far the Carrickalinga Head Formation at Carrickalinga Head extends out into Gulf St Vincent, as the seismic sections do not extend to shore. This paper does not deal with the Carrickalinga Head Formation from the north coast of Kangaroo Island where Moore (1983) described equivalents of the Blowhole Creek Siltstone and Campana Creek Members. Further field work is required before analysis of the overall depositional conditions of the Carrickalinga Head Formation and its equivalents (as exposed on Kangaroo Island, Fleurieu Peninsula, and the eastern side of the Mount Lofty Ranges) can be produced. Nevertheless some speculative remarks are warranted at this time.

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The Madigan Inlet Member on Fleurieu Peninsula (Myponga Beach, Carrickalinga Head, South Coast) is herein interpreted as part of a submarine fan sequence. This interpretation is supported by the repetitive nature of the sandstone/mudstone couplets, the inclusion of scattered clasts (up to 1 m across at Carrickalinga Head) within the couplets, and the deposition of the sandstone portions of the couplets as sediment gravity flows. The exposed thickness of the Madigan Inlet Member along the South Coast is considerably greater than at Carrickalinga Head or Myponga Beach where only the basal 80 m occurs. However, as noted in the descriptions, the South Coast outcrops are interpreted as being further from the source than the Carrickalinga Head/Myponga outcrops. No flute casts or other directional sole markings have been found. Most of the sandstones at Carrickalinga Head belong in Facies B of Mutti & Ricci Lucchi (1972) with some belonging in Facies A. The sandstones along the South Coast also generally belong in Facies B. The fact that most sandstones

Fig. 7 Suggested broad depositional environments at the time of deposition of the Madigan Inlet Member (based on Gravestock, unpubl.).

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seem to belong in Facies B combined with a distinct dominance of thinning upwards sandstone sequences as compared to thickening upwards sequences in the bottom 300 m of the Madigan Inlet Member on the South Coast suggests that we are dealing with either the Inner or Middle Fan Sub-Association of Mutti & Ricci Lucchi (1972). The general lack of obvious channelling in the South Coast sections suggests that this area belongs to the Middle Fan Sub-Association. Thin section studies of the sandstones from the Madigan Inlet Member at both Carrickalinga Head and Myponga Beach show that it comprises quartz of both plutonic and metamorphic origin, chert, microcline, plagioclase, biotite, muscovite, and clay (Farrand 1988). The grains are poorly sorted and generally very angular, although a few grains are subrounded. This is consistent with the inference of Thomson (1969a, 1975) that the Early Proterozoic metamorphics and igneous intrusives of the Gawler Craton, as exposed on Yorke Peninsula, were a source of sediment for the Kanmantoo Group. The presence of chert and sericitic lithic fragments suggests that the Adelaidean and/or pre-Kanmantoo Group Cambrian sediments of Yorke Peninsula and perhaps elsewhere were a source of some of the material. At this time there is a substantial break in the Cambrian sequence on at least part of Yorke Peninsula. This is the break recorded by Daily (1972, this volume), between the Parara Limestone (equivalent to part of the Normanville Group) and the overlying Minalton Conglomerate. It is the same break, recorded from seismic data by Stuart & von Sanden (1972) at the top of the Parara Limestone, which extends into Gulf St Vincent. In other parts of Yorke Peninsula non-marine or shallow marine sediments may have been deposited contemporaneously with the Madigan Inlet Member (see correlation chart in Cooper & Grindley 1982). Whatever the detailed situation the implication is that there was a fairly abrupt lateral transi-

tion from shallow shelf to fan sedimentation. Such an abrupt lateral transition has been described from the Eocene of Oregon by Chan & Dott (1983). These workers used a model with sand being supplied from a series of deltas across a narrow (80 km or less) continental shelf and then successively into the inner, middle, and outer parts of the fan before reaching the basin plain. Working on the same sequence in Oregon Heller & Dickinson (1985) suggested a similar model, but proposed a more abrupt transition by having deltas feeding directly down the slope into the fan system. At our present state of knowledge, both with respect to the Madigan Inlet Member on Fleurieu Peninsula and to the Cambrian palaeogeography of the Yorke Peninsula area, either the Chan-Dott or Heller-Dickinson models could fit the situation for the Madigan Inlet Member on Fleurieu Peninsula. In terms of the models presented by Haq etal (1987) and other works referred to in that paper, the Madigan Inlet Member would represent a low stand fan in the present Fleurieu Peninsula area. At the same time there was non-deposition over at least part of the shelf areas of Yorke Peninsula and Gulf StVincent as represented by the unconformity at the top of the Parara Limestone noted above. Deposition of the Madigan Inlet Member required a constant supply of fine- to mediumgrained sand, presumably derived from a stream system (or systems) draining the Gawler Craton. Heller & Dickinson (1985) suggested that the onset of sand-rich fan sedimentation was due to rapid uplift along technically active basin margins. In Oregon the tectonic activity took place at a convergent plate boundary. The cause of the Kangarooian Movements which triggered the deposition of the Carrickalinga Head Formation and subsequent units of the Kanmantoo Group is unclear at this stage. At Sedan Hill the Madigan Inlet Member consists of an overall finer-grained rock containing siltstone and sandstone, with carbonate-rich siltstone beds in places. The presence here of linsen and flaser bedding, and the thinness of the units show that water depth is very much less and

Carrickalinga Head Formation Sedimentology

may even be close to sub-tidal. Outcrops of this unit between here and the South Coast section are limited and have yet to be studied. A very broad outline of the suggested depositional environments at the time of deposition of the Madigan Inlet Member is shown in Figure 7 where it is assumed that the Madigan Inlet Member near Mt McDonnell on Kangaroo Island was deposited under very shallow water conditions. Although there is no outcrop of Madigan Inlet Member equivalent at this locality, the equivalents of both the Blowhole Creek Siltstone and Campana Creek Members are of very shallow water origin (Moore 1983). In the South Coast section, some of the thicker sandstone units occur in the upper parts of the Madigan Inlet Member where distinct channelling has been recorded. The sudden input of muds of the Blowhole Creek Siltstone may be due to fan switching with the Blowhole Creek Siltstone Member representing basin plain sedimentation. It is suggested that the deposition of the Blowhole Creek Siltstone on the South Coast essentially reflects the start of a rise in sea level with the increasing number of scattered thin sandstone lenses in the upper part of the sequence reflecting the commencement of the prograding sandstone bodies of the Campana Creek Member and the Backstairs Passage Formation higher on the shelf. In the model of Haq et al (1987) it probably forms the lowstand wedge systems tract, the transgressive systems tract and part of the highstand systems tract. As shown by Christie-Blicke etal (1988, fig. 1) the transgressive systems tract would be a condensed interval in this part of the basin. The Blowhole Creek Siltstone Member at Sedan Hill differs from the section on the South Coast in that it can be subdivided into upper and lower parts separated by the Milendella Limestone Member. This is interpreted as showing that at Sedan Hill, the Blowhole Creek Siltstone Member was deposited in shallower water than further to the south and that clastic sediment supply was limited. However, on the north coast of Kangaroo Island Moore (1983) reported more

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sandstone in an equivalent of the Blowhole Creek Siltstone than found elsewhere. This may result from the Kangaroo Island sections being closer to the source area. Lateral variations of the Blowhole Creek Siltstone Member require further investigation. The Campana Creek Member, both in the South Coast section and at Sedan Hill represents a shallowing upwards succession as compared with the sub-wave base deposits of the Blowhole Creek Siltstone Member. The shallowing upwards nature of the Campana Creek Member is a precursor to the shallow, high energy deposits of the Backstairs Passage Formation. Together the Campana Creek Member and the Backstairs Passage Formation represent a prograding sand body at a time of rising and high sea level. In the scheme of Haq et al (1987) the Campana Creek Member and the Backstairs Passage Formation together may represent the upper part of the highstand systems tract. The shallow water, highenergy deposits of the Backstairs Passage Formation form a distinctive unit across the area of outcrop of the Kanmantoo Group from the north coast of Kangaroo Island, where it is represented by the Stokes Bay Sandstone, to Australia Plains (Fig. 1).

ACKNOWLEDGEMENTS The pioneering work done by Brian Daily with A.R.Milnes on the Kanmantoo Group was first impressed on C.G. Gatehouse and B.J. Cooper when walking part of the South Coast section with Daily. It is to him and his inspirational work that this current paper is dedicated. We wish to thank W. V.Preiss, D.I. Gravestock and an unnamed referee whose interest in the work provided thought-provoking comments and caution where and when needed. The South Australian Department of Mines and Energy provided logistical support for J.B. Jago. This paper is published with the permission of the Director General of the S.A. Department of Mines and Energy.

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REFERENCES ABELE C. & McGOWRAN B. 1959. The geology of the Cambrian south of Adelaide (Sellick Hill to Yankalilla). Transactions of the Royal Society of South Australia 82, 301-320. ALEXANDER E. & GRAVESTOCK D.I. (this volume). Sedimentary facies in the Sellick Hill Formation on Fleurieu Peninsula, South Australia. CAMPANA B., WILSON B. & WHITTLE A.W.G. 1955. The geology of the Jervis and Yankalilla military sheets. Explanation of the geological maps. Geological Survey of South Australia, Report of Investigations 3. CHAN M.A. & DOTT R.H. Jr. 1983. Shelf and Deep-Sea sedimentation in Eocene forearc basin, Western Oregon - fan or non-fan? American Association of Petroleum Geologists, Bulletin 67, 2100-2116. CHRISTIE-BLICK N., GROTZINGER J.P. & VON DER BORCH C.C. 1988. Sequence stratigraphy in Proterozoic successions. Geology 16,100-104. COOPER R.A. & GRINDLEY G.W. eds. 1982. Late Proterozoic to Devonian sequences in southeastern Australia, Antarctica and New Zealand and their correlation. Geological Society of Australia, Special Publication 9, 103pp. DAILY B. 1963. The fossiliferous Cambrian succession on Fleurieu Peninsula, South Australia. Records of the South Australian Museum 14(3) , 579-601. DAILY B. 1969. Fossiliferous Cambrian sediments and low grade metamorphics, Fleurieu Peninsula, South Australia. In Daily B. ed. Geological Excursions Handbook, 49-54. Australian and New Zealand Association for the Advancement of Science, 41st Congress, Adelaide. DAILY B. 1972. The base of the Cambrian and the first Cambrian faunas. Centre for Precambrian Research, University ofAdelaide, Special Paper 1,13-41. DAILY B. (this volume). Cambrian stratigraphy of Yorke Peninsula.

DAILY B. & FORBES B.G. 1969. Notes on the Proterozoic and Cambrian, southern and central Flinders Ranges, South Australia. In Daily B. ed. Geological Excursions Handbook, 22-30. Australian and New Zealand Association for the Advancement of Science, 41st Congress, Adelaide. DAILY B. & MILNES A.R. 1971. Stratigraphic notes on Lower Cambrian fossiliferous metasediments between Campbell Creek and Tunkalilla Beach in the type section of the Kanmantoo Group, Fleurieu Peninsula, South Australia. Transactions of the Royal Society of South Australia 95,199-214. DAILY B. & MILNES A.R. 1972a. Revision of the stratigraphic nomenclature of the Cambrian Kanmantoo Group, South Australia. Journal of the Geological Society of Australia, 19, 197-202. DAILY B. & MILNES A.R. 1972b. Significance of basal Cambrian metasediments of andalusite grade, Dudley Peninsula, Kangaroo Island, South Australia. Search 3, 89-90. DAILY B. & MILNES A.R. 1973. Stratigraphy, structure and metamorphism of the Kanmantoo Group (Cambrian) in its type section east of Tunkalilla Beach, South Australia. Transactions of the Royal Society of South Australia 97, 213-251. FARRAND M. 1988. A brief exploratory survey of lithological variation in part of the Kanmantoo Group in the south of Fleurieu Peninsula, South Australia. South Australian Department of Mines and Energy, Report Book (unpubl.). FLINT D.J. 1978. Deep sea fan sedimentation of the Kanmantoo Group, Kangaroo Island. Transactions of the Royal Society of South Australia 102, 203-222. HAQ B.U., HARDENBOL J. & VAIL P.R. 1987. Geomorphology of fluctuating sea levels since the Triassic. Science 235, 1156-1167. HELLER P.L. & DICKINSON W.R. 1985. Submarine ramp facies model for delta-fed, sand-rich turbidite systems. American Association of Petroleum Geologists, Bulletin 69,960-976.

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JAGO J.B. & DAILY B. 1982. South Australia. In Cooper R.A. & Grindley G.W. eds. Late Proterozoic to Devonian sequences of South-eastern Australia, Antarctica and New Zealand and their correlation. Geological Society ofAustralia, Special Publication 9, 6-12. JAGO J.B., DAILY B., VON DER BORCH C.C, CERNOVSKIS A. & SAUNDERS N. 1984. First reported trilobites from the Lower Cambrian Normanville Group, Fleurieu Peninsula, South Australia. Transactions of the Royal Society of South Australia 108,207-211. JAGO J.B., GEHLING J.G. & DAILY B. 1986. Cambrian sediments of the Sellick Hill-Carrickalinga Head area, Fleurieu Peninsula, South Australia. In Parker A.J. compiler. One Day Geological Excursions of the Adelaide Region. Geological Society of Australia, South Australian Division, 67-81. LOWE D.R. 1982. Sediment gravity flows: II.Depositional models with special reference to the deposits of high-density turbidity currents. Journal of Sedimentary Petrology 52,279-297. MADIGAN C.T. 1925. The geology of the Fleurieu Peninsula. Part I- The coast from Sellick's Hill to Victor Harbour. Transactions of the Royal Society of South Australia 44, 198-212.

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MELNES A.R., COMPSTON W. & DAILY B. 1977. Pre- to syn-tectonic emplacement of early Palaeozoic granites in southeastern South Australia. Journal of the Geological Society of Australia 24, 87-106. MOORE P.S. 1983. Geological field guide to the northeast coast of Kangaroo Island. Australasian Sedimentologists Group, Adelaide. MUTTI E. & RICCI LUCCHI F. 1972. Le torbiditi deH'Appennine settentrionale introduzione all'analise di facies. Memorie Societa Geologica Italiana 11, 161-199. Translated into English by T.H.Nilsen, 1978, International Geology Review 20, 125-166. OFFLER R. & FLEMING P.P. 1968. A synthesis of folding and metamorphism in the Mt Lofty Ranges, South Australia. Journal of the Geological Society of Australia 15,245-266. PARKER A.J. 1986. Tectonic development and metallogeny of the Kanmantoo Trough in South Australia. Ore Geology Reviews 1, 203-212. SCHEIBNER E. 1986. Suspect terranes in the Tasman Fold Belt System, Eastern Australia. In Howell D.G. ed. Tectonostratigraphic terranes of the Circum-Pacific Region. Circum Pacific Council for Energy and Mineral Resources, Earth Science Series 1,493-514.

MANCKTELOW N.S. 1979. The structure and metamorphism of the southernmost Adelaide Fold Belt. Ph.D. thesis, University of Adelaide, (unpubl.).

SHANMUGAM G. & MOIOLA R.J. 1988. Submarine Fans: Characteristics, Models, Classification, and Reservoir Potential. Earth-Science Reviews 24, 383-428.

MARLOW P.C. 1975. Structural investigations near Macclesfield, South Australia. M.Sc. thesis, University of Adelaide (unpubl.).

SPRIGG R.C. & CAMPANA B. 1953. The age and facies of the Kanmantoo Group. Australian Journal of Science 16,12-14.

MIDDLETON G.V. & HAMPTON M.A. 1973. Mechanics of flow and deposition. In Middleton G.V. & Bourne A.H. eds. Turbidites and Deep-Water Sedimentation. Society of Economic Paleontologists and Mineralogists, Pacific Section, Short Course, Anaheim, 1-38.

STAUFFER P.H. 1967. Grain-flow deposits and their implications, Santa Ynes Mountains, California. Journal of Sedimentary Petrology 37,487-508.

MIDDLETON G.V. & HAMPTON M.A. 1976. Subaqueous sediment transport and deposition by sediment gravity flows. In Stanley D.G. & Swift D.J. eds. Marine Sediment Transport and Environmental Management. John Wiley, New York, 197-217.

STUART W.J. & VON SANDEN A.T. 1972. Palaeozoic history of the St Vincent Gulf Region, South Australia. APEA Journal 12, 9-16. THOMSON B.P. 1969a. The Kanmantoo Group and Early Palaeozoic tectonics. In Parkin L.W. ed. Handbook of South Australian Geology, pp. 97-108. Geological Survey of South Australia, Adelaide.

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THOMSON B.P. 1969b. Adelaide map sheet, Geological Atlas of South Australia, 1:250 000 Series. Sheet SI 54-9. Geological Survey of South Australia, Adelaide.

VAILP.R., MITCHUM R.M., TODD R.G., WIDMffiR J.M, THOMPSON S., SANGREE J.B., BUBB J.N. & HATLELID W.G. 1977. Seismic stratigraphy and global changes of sea level. American Association of Petroleum Geologists, Memoir 26,49-212.

THOMSON B.P. 1975. Kanmantoo Trough - Regional Geology and comments in mineralization. In Knight C.L. ed. Economic Geology of Australia and Papua New Guinea - I Metals. Australasian Institute of Mining and Metallurgy, Monograph Series 5,555-565.

VON DER BORCH C.C. 1980. Evolution of Late Proterozoic to Early Palaeozoic Adelaide Foldbelt, Australia: Comparison with post-Permian rifts and passive margins. Tectonophysics 70,115-134.

THOMSON B.P. & HORWITZ R.C. 1962. Barker map sheet. Geological Atlas of South Australia, 1:250 000 Series. Sheet I 54-13. Geological Survey of South Australia, Adelaide.

WEBB A.W. 1976. The use of the potassium-argon method to date a suite of granitic rocks from south-eastern Australia. Amdel Bulletin 21,25-35. WHITE A.J.R. 1959. Scapolite-bearing marbles and calc-silicate rocks from Tungkillo and Milendella, South Australia. Geological Magazine 96, 285-306.

TOTEFF S. 1977. Geology of the Adelaid ean-Kanmantoo Group sequences in the eastern Mt Lofty Ranges. Ph.D. thesis, University of Adelaide (unpubl.).

WINKLER H.G.F. 1970. Abolition of metamorphic facies, introduction of the four divisions of metamorphic stage, and of a classification based on isograds in common rocks. Neues Jahrbuch fur Mineralogie, Monatshefte 5, 189-248.

TOTEFF S. (this volume). The AdelaideanKanmantoo Group contact, eastern Mt Lofty Ranges.

WOPFNER H. 1969. The Cambrian Period. In Parkin L.W. ed. Handbook of South Australian Geology, 84-97. Geological Survey of South Australia, Adelaide.

The structure of the southern Adelaide Fold Belt, South Australia Neil S. Mancktelow Geologisches Institut, ETH-Zentrum, CH-8092 Zurich, Switzerland Units of the southern Adelaide Fold Belt, comprised of platformal, generally clastic Proterozoic sediments (the Adelaide Supergroup), a lowermost Cambrian carbonate platform sequence (the Normanville Group), and an overlying thick sequence of immature flysch-like clastic sediments (the Kanmantoo Group) were deposited with marked unconformity on an older crystalline continental basement, and have undergone one major orogeny (the Delamerian Orogeny) of Cambro-Ordovician age. The first deformation of this tectonic event was the most important and pervasive; it was responsible for the development of a regional slaty cleavage, axial plane to major, generally fairly open folds with variably oriented, but shallowly plunging, fold axes. The folds vary from inclined to overturned towards the stable foreland (the Gawler Craton), to fairly upright and symmetric in the eastern and southern internal parts of the fold belt. Evidence of extensive thrust and nappe tectonics associated with or preceding this deformation is lacking. The regional strike of the slaty cleavage defines a broad arc (the Fleurieu Arc), gradually changing from near E-W on Kangaroo Island in the south to N-S east of Adelaide. The slaty cleavage has been overprinted in limited areas, usually within the medium to high metamorphic grade region, by a second crenulation cleavage which may develop into a penetrative new schistosity. The peak of regional metamorphism coincided with the initiation of this second deformation. A second crenulation can be observed in some areas, but this third deformation, together with occasional subsequent deformation (crenulations, kinking, faulting) is not of great regional significance. The dominant first deformation occurred around the Cambro-Ordovician boundary (ca.505 Ma) and the higher metamorphic grade regions had already cooled through the ductile-brittle transition by the Middle to Late Ordovician to close the Delamerian Orogeny.

Key words: Regional structure, metamorphism, Adelaide Fold Belt, deformation, Delamerian Orogeny. INTRODUCTION

migmatites (e.g. Fleming & White 1984) is limited to this southern region.

The Adelaide Fold Belt forms a long sigmoidal zone of Late Precambrian to Early Palaeozoic rocks stretching from Olary in the northeast to Kangaroo Island in the southwest, and encompasses an area of more than 40 000 km (Figs 1,4). This large area has had a similar tectonic history, culminating in one major period of extensive deformation and metamorphism - the Cambro-Ordovician Delamerian Orogeny (Thomson 1969; Glen et al 1977). Flysch-facies Cambrian sediments of the Kanmantoo Group are only exposed south of latitude 34°S (Fig. 1), and as a descriptive term this southerly segment of the belt is here referred to as the southern Adelaide Fold Belt. High grade metamorphism, including the development of

This study presents a regional synthesis of the structure of the southern Adelaide Fold Belt. It embraces a large body of earlier work, mainly from relatively small areas, though Offler & Fleming (1968) made a significant contribution to a more regional approach. There is considerable disagreement about the relative importance of the various deformation events in the southern Adelaide Fold Belt. Studies in low grade metamorphic terrains (e.g. Mills 1964; Talbot 1964) conclude that the dominant deformation is the first, slaty cleavage-forming event (Di), with only limited overprinting by later structures. Workers in the medium and high-grade metamorphic areas generally consider that the second (D2,

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e.g. Marlow 1975; Abbas 1975) and/or third (D3, e.g. Offler 1966; Fleming 1971) deformations represent important macroscopic folding events. In their regional synthesis, Offler & Fleming (1968) conclude that Di folds are dominant in the western and northern regions of the Mount Lofty Ranges, but that D3 folds dominate the structural pattern to the east. Outcrop suitable for structural work is limited mainly to the coast, road cuttings, and creek sections, the quality decreasing in that order. As outcrop within the Kanmantoo Group is generally better than elsewhere, most field work for this

study has been done in this unit. There is little outcrop suitable for structural work in the Adelaide Supergroup east of the basement inliers (Figs 1, 5).

STRATIGRAPHY The stratigraphic nomenclature of the Adelaide Fold Belt is in a continuing state of flux (Preiss 1982; Jago & Hilyard 1983), and a detailed description is beyond the scope of this paper. A comprehensive summary is given in Preiss etal (1981). Overall, the sedimentation in

Fig. 1. Locality map showing the approximate limits of outcrop and the broad distribution of stratigraphic units within the southern Adelaide Fold Belt.

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Fig. 2. Metamorphic zone boundaries in the southern Adelaide Fold Belt, modified after White (1956), Mills (1964) and Offler & Fleming (1968).

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the southern Adelaide Fold Belt can be considered in terms of four broad sequences (Preiss et al 1981; Von der Borch 1980): initial fluviatile to shallow water marine platformal sedimentation (the Burra Group) of shales, magnesitic dolomites and quartzites with a basal, heavymineral-laminated sandstone, which was deposited with profound unconformity upon polydeformed continental basement (Talbot 1964); a disconformable glacial and postglacial sequence (the Umberatana and Wilpena Groups respectively) ; a shallow water carbonate-rich Cambrian platform sequence above a marked erosional disconformity (the Normanville Group); and a very thick, rapidly deposited flysch-like sequence of immature sandstones and siltstones (including turbidites, e.g. Mancktelow 1979a; Flint 1978; Sprigg & Campana 1953), of the Kanmantoo Group. This whole sequence has been variably metamorphosed, but even at the highest metamorphic grades, sedimentary features are often preserved (cross-beds, grading, load casts, slump and dewatering structures, and climbing ripples). These features can be used to establish younging directions and thereby provide additional constraint on structural interpretation.

METAMORPHISM One major, regional metamorphic event can be documented within the southern Adelaide Fold Belt ranging from very low-grade metamorphism (Winkler 1979) on Yorke Peninsula to upper amphibolite facies in the Palmer region east of Adelaide. The metamorphic facies series is of a low pressure/high temperature type, with the aluminosilicate polymorphs andalusite and sillimanite predominant; kyanite is only found at a few scattered locations (Fig. 2). Peak temperatures, associated with migmatites and the incipient instability of muscovite + quartz, are in the order of 640-650°C with pressures of 3.5-

4kbar (Mancktelow 1979a; Offler & Fleming 1968). This implies a rather high geothermal gradient of around 45°C/km within the migmatite zone. Granitoid intrusions are common within the highest grade regions (Fig. 1) and can be divided into associations on the basis of mineralogy, major and trace element geochemistry, and age and isotopic composition (Mancktelow 1979a; White et al 1967; Milnes et al 1977; Webb 1976). Many of these associations (e.g. Palmer, Murray Bridge, Reedy Creek, Tintinara) have I-type granite affinities and could have been instrumental in transferring heat from lower to upper crustal levels, to establish the observed high geothermal gradient. Others are more S-type (e.g. Encounter Bay Granites) and may have developed by higher level partial melting of continental crust. The regional variation of metamorphic grade can be outlined in terms of metamorphic mineral zones, the boundaries of which mark the first appearance of a particular zone mineral in suitable rock compositions. The zone boundaries of Fig. 2 differ significantly from those of Offler & Fleming (1968), particularly on southern Fleurieu Peninsula, and are more similar to those from the earlier syntheses of White (1956) and Mills (1964). Outside the immediate vicinity of the Encounter Bay Granites (Fig. 2), andalusite and staurolite occurrences are unknown from southern Fleurieu Peninsula, with the single exception of an andalusite-staurolite-garnet bearing schist found during this study SSE of Carrickalinga Head (Loc. 1 on Fig. 2, Yankalilla 622685*). Lithostratigraphic units on the southern Fleurieu Peninsula can be correlated with andalusitestaurolite bearing units further north on the mainland and from Dudley Peninsula, Kangaroo Island (Daily & Milnes 1972), and whole-rock chemical analyses (Mancktelow 1979a) demonstrate the common occurrence of pelitic compositions suitable for the development of aluminous porphyroblasts. The lower grade, compositionally equivalent assemblage biotitemuscovite-chlorite±garnet is developed instead.

^Coordinates refer to the Australian Map Grid, as used on the 1:50 000 series topographic maps published by the South Australian Department of Lands.

Structure of the Adelaide Fold Belt

Aluminous quartz-rich segregations, which are common throughout the southern Adelaide Fold Belt, contain chlorite (±garnet) rather than andalusite, such as is found above the andalusitestaurolite zone boundary in the Encounter Bay area to the east and further north (Mills 1964). In the absence of andalusite and staurolite, Offler & Fleming (1968) based their zone boundary on the occurrence of plagioclase with An>17 in association with epidote. The composition of the plagioclase was determined optically. However, microprobe analyses of samples from this region show a very wide range of plagioclase compositions in individual probe mounts (in the range albite-andesine), reflecting only partially reset detrital compositions. The homogenization of plagioclase compositions approximately corresponds to the first appearance of andalusite and staurolite in pelitic compositions. Milnes (1973) reported quartz-andesine-hornblende assemblages from calc-silicate nodules on the south coast of the Fleurieu Peninsula west of Blowhole Creek (Loc. 2 of Fig. 2, Cape Jervis 423505), and suggested that this association indicated that the area was within the andalusite-staurolite zone as proposed by Offler & Fleming (1968). Winkler (1979) and Wenk & Keller (1969) have shown, however, that plagioclase of An>17 can coexist with hornblende at temperatures probably 30°C lower than the first appearance of staurolite at the expense of muscovite + chlorite. Winkler (1979) concludes that field observations suggest that actinolite changes to hornblende at about the same P-T conditions as the first appearance of almandine garnet in metapelitic rocks for medium and high pressures. It would appear, therefore, that at the present exposure level on the southern Fleurieu Peninsula there is a very flat temperature gradient from Encounter Bay in the east towards the west, with a drop of perhaps only ca.30°C over a distance of 30 km. Minor perturbations to this gradient could lead to small areas of isolated andalusite-staurolite development (e.g. Loc. 1, Fig. 2). The zone boundaries on Kangaroo Island are not well defined and must be considered as only preliminary. The uppermost preserved Kanman-

373

too Group unit, the Middleton Sandstone, covers much of the eastern and southern part of the island and contains few pelitic units suitable for the development of aluminous zone minerals. As on the mainland, however, the pattern appears to be more complex than the broad zones proposed by Offler & Fleming (1968). This is clear, for example, on the northeast coast of Dudley Peninsula, where porphyroblastic chlorite-bearing phyllites (Fig. 8b, Loc. 3 of Fig. 2) occur east of the andalusite (+minor staurolite) schists exposed near Alex Lookout (Daily & Milnes 1972). Work is also required on the lower temperature metamorphic zonation. The chlorite zone, as delineated on the inner, western and northern side of the Fleurieu Arc in Fig. 6 of Offler & Fleming (1968), is absent from Fig. 2; rocks from these areas contain metamorphic biotite. On Yorke Peninsula, however, glauconite is still preserved in Cambrian sandstones (Winulta Formation) and K-Ar biotite ages from the basement have not been reset since ca.1400 Ma (Webb etal 1986). There is, then, clearly a fall in metamorphic grade towards the foreland region of Yorke Peninsula; the preserved biotite mineral ages suggest that this region did not exceed ca.300°C during the Delamerian Orogeny (Wagner etal 1977). A broad study of illite crystallinity in the pelitic units could help delineate this transition to the lowest grades of metamorphism (e.g. Kubler 1967).

THE Di DEFORMATION The present study indicates that the first deformation, Di, is the major regional deformation event within the southern Adelaide Fold Belt (Figs 4, 5). Reconnaissance mapping in the Olary, Flinders Ranges and Mount Painter regions, together with previous work (e.g. Bell 1978), suggests that this first deformation dominates the Adelaide Fold Belt as a whole, with overprinting by later events occurring only in limited areas (typically of higher metamorphic grade; Fig. 2). The Di event is most readily recognised and studied in low grade regions where the

Fig. 3. Bedding trends and stratigraphic boundaries in the southern Adelaide Fold Belt. The orientation of bedding So and fold axes in bedding(L? and L2 ) is given by the lower hemisphere, equal area stereographic projections for representative domains (as defined in the inset). Contour levels are 1,3,5,7.... times uniform.

Fig. 4. Map of the major structures (faults and axial traces of regional folds). Tectonic setting is presented in inset. Stereograms for the domains of Fig. 3 give the orientation of the regional slaty cleavage (Si), the elongation lineation (Li), and the second (S2) and third (S3) crenulation cleavages. Stereograms are plotted similarly to Fig. 3. Symbols on the axial traces indicate the direction of dip of the axial plane; opposing symbols imply steep to vertical dips.

o j L/i

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Neil S. Mancktelow

microstructure is not obscured by later porphyroblast growth and mica recrystallization (Mancktelow 1979b). The southern coast of the Fleurieu Peninsula provides an excellent section of almost total outcrop across the fold belt which is dominated by the effects of Di (Daily & Milnes 1973).

Major structures On a broad scale, the regional structure of the southern Adelaide Fold Belt is rather simple: on the mainland it consists of a major anticlinesyncline pair (Figs 3,4,5). The western anticlinal zone is comprised in detail of a series of noncylindrical, domal anticlines which form an enechelon pattern successively offset to the

southeast (Thomson 1969, anticlines 1, 2, 3 & 4 in Fig. 4). The oldest rocks of the southern Adelaide Fold Belt, the basement inliers, outcrop in these domal culminations. The regional anticlinal culmination can be projected across Backstairs Passage onto Kangaroo Island, where it continues as the American River Anticline (structure 5 in Fig. 4). On Kangaroo Island, outcrop extends north of this anticline towards the stable foreland of the Gawler Craton and additional structures can be recognized; a major syncline (West Bay Syncline) followed by a broad culmination (Cassini Anticline) due to the general shallowing of bedding dip in the northernmost outcrops on Kangaroo Island, west of White Point (Figs 1, 3, 4).

B' Normanville Group

/ / / X Umberatana Group

Cambro-Ordovician granitoids

.. . . ,

Burra Group

Adelaide Supergroup

|:; 1; |

Middleton Sandstone

^ ^

Petrel Cove Formation

[

Pre-Adelaidean Crystalline Basement

Balquhidder Formation

)—H

Tunkalilla Formation

[ v v':']

Tapanappa Formation

Kanmantoo Group

Talisker Formation F, Axial Plane

•' -'j

Backstairs Passage Formation

p 2 Axial Plane

Izggj

Carrickalinga Head Formation

Fig. 5. Interpreted geological profiles, without vertical exaggeration, from Adelaide to Palmer (A-A ) and along the southern coast of Fleurieu Peninsula (B-B ). Location of profile lines is given in inset of Fig. 3. Stratigraphic scheme is based on Thomson (1964) for the Proterozoic Adelaide Supergroup, on Daily (1963) and Abele & McGowran (1959) for the Cambrian Normanville Group, and on Daily & Milnes (1971, 1973) for the overlying Kanmantoo Group. 1

Structure of the Adelaide Fold Belt

As is clear from Fig. 4, the axial traces of the major Di folds define a broad arc (the Fleurieu Arc), varying gradually in strike from north-south east of Adelaide to east-west on Kangaroo Island. For simplicity, the sides of this arc adjacent to the Gawler Craton (Fig. 4, inset), that is the western (Adelaide), northwestern (Fleurieu Peninsula) and northern (Kangaroo Island) sides respectively, are referred to here as the external region, and the opposite sides as the more internal region of the fold belt. A synoptic cross-section from the external region, transitional to the stable Gawler Craton, into the more internal part of the fold belt can be established by combining the observations from Kangaroo Island with those from the mainland. Penetrative ductile deformation of the shallowwater proximal facies Cambrian sediments of the northernmost, external region of Kangaroo Island (Daily et al 1980) is weak (Fig. 6a). Folding is very open, with limb dips of less than 30 degrees and little or no cleavage development. Deformation is discontinuous and is largely accommodated by faulting (e.g. Dinnick 1985). In the adjacent, more internal regions, the intensity of penetrative deformation (Fig. 6b) and the

377

amplitude of folding increases, producing a series of markedly asymmetric folds inclined towards the cratonic foreland (Fig. 7). Deformation is heterogeneous. The steep to overturned limbs of the major folds are generally more highly strained, with boudinage and attenuation of sedimentary units and occasional thrusting (e.g. Campana 1955; Offler & Fleming 1968). This is particularly marked in the north near Adelaide, where folding west of the regional anticlinal culmination consists of long, shallow to near horizontal, east-younging limbs and shorter, steep to overturned west-younging limbs, with axial planes commonly dipping less than 50 degrees towards the east. Individual fold hinges are discontinuous; adjoining anticlines and synclines commonly coalesce and annihilate each other (Fig. 4). More internally still, across the regional anticlinal culmination (anticlines 1-5; Fig. 4), the structures become more symmetric and upright, and deformation more homogeneous. Mesoscopic structures The recognizable mesoscopic structures associated with Di are a dominant slaty cleavage or

Fig. 6. Conglomerates from White Point (Fig. 6a) and from west of Alex Lookout on Dudley Peninsula, Kangaroo Island (Fig. 6b, Penneshaw 657434), showing the increase in penetrative ductile strain towards the more internal region of the fold belt. Fig. 6b is perpendicular to the foliation, parallel to the lineation, and is the section showing the maximum two-dimensional strain.

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Neil S. Mancktelow

schistosity, Si; a bedding-cleavage intersection lineation, L? (notation follows the convention of Bell & Duncan 1978); folds in bedding, Fi, with fold axes F?; and a mineral and/or elongation lineation Li (Fig. 8a). In low grade metamorphic regions (e.g. along the western side of Fleurieu Peninsula), Si is readily recognized as a penetrative slaty cleavage strongly developed in the pelites (Figs 8a, b), and weakly to moderately developed in the psammites and carbonate-rich units. In regions of higher metamorphic grades, the Si schistosity may be difficult to distinguish in hand specimen from later, crenulation cleavages, although the difference is usually clear in thin section. Refraction of Si between pelitic and psammitic layers can be greater than 40 degrees, particularly in regions of shallowly-dipping Si. In some zones, largely confined to the lowergrade western side of the fold belt, Si is very strongly developed and displays a waviness on a scale of several centimetres. This is particularly clear in high strain, overturned limb regions in the area north and south of Rapid Bay. The waviness is probably due to the development of a mechanical instability during continued deformation of a highly foliated, strongly anisotropic rock (Cobbold etal 1971). The intersection lineation, L?, appears as a compositional variation on Si (Fig. 8a), or as a mineral lineation and parting on So. The lineation on So is commonly accentuated by a preferred orientation of sedimentary structures, such as load casts, flame structures and climbing ripples (fig. 19, Daily & Milnes 1973), due to shortening of these structures perpendicular to Si. Mullion structures elongate parallel to L? are also observed at the interface of units of marked competence difference (cf. Ramsay 1967). The mineral lineation, Li, is not always recognizable and with the exception of the Rapid Bay area (Mancktelow 1981b), is generally weak (Fig. 8a). The fold belt is characterized by planar Di fabrics, with only a limited linear component. Where measurable, the Li lineation consistently

Fig. 7. Asymmetric Di folds, inclined towards the cratonic foreland, on the north coast of Kangaroo Island west of Stokes Bay (Western River Cove, Snug Cove 784502). The axial plane strikes 080 degrees and dips 48 degrees S. These folds occur in the hinge of a major Fi syncline, which forms part of a syncline-anticline pair parasitic on the macroscopic West Bay Syncline (see Fig. 4).

pitches steeply in Si (Figs 4,8a), most commonly to the south, and is usually defined on slaty cleavage surfaces by an alignment of mica. In coarser-grained sandstones, gritty siltstones and conglomerates it can be demonstrated that the mineral lineation is parallel to the elongation direction of the clasts and represents the elongation direction of the local strain ellipsoid. Mesoscopic Fi folds are rare in the Kanmantoo Group, reflecting the predominance of thickbedded sandstones. The best examples occur in the more thinly-bedded pelitic units, particularly the Talisker Formation on the south coast of Fleurieu Peninsula and on Dudley Peninsula, Kangaroo Island, and the Campana Creek and Blowhole Creek Siltstone Members of the Carrickalinga Head Formation around Blowhole Creek (Loc. 2; Fig. 2). Mesoscopic folds are more common in the generally thinner-bedded Adelaide Supergroup. Most mesoscopic folds maintain an approximately constant bed thickness and thus approach a parallel fold style (Type IB of Ramsay 1967), but in areas of higher strain (as indicated by stronger deformation fabrics, common boudinage, and tighter folding,

Structure of the Adelaide Fold Belt

379

e.g. the Rapid Bay region), the fold limbs are Burra Group. However, the observation that the markedly thinned and the folds approach a basement inliers occur as Di anticlinal culminasimilar style (Type 2 of Ramsay 1967). In extreme tions and that both the basement and cover are cases, the limbs of some small scale folds may be similarly affected by this deformation argues thinned to such an extent that continuity is lost against major detachment between basement and and So is effectively transposed into the Si orien- cover during Di within the internal part of the fold tation (Allen 1977). belt. Planar and linear elements of identical field aspect and orientation to Si and Li of the cover lithologies are developed as a retrograde deformation fabric within the basement inliers (Spry 1951; Talbot 1963). The planar fabric in the basement is axial plane to open, shallowly plunging folds in a compositional layering defined by variation in the quartz and feldspar content and by ferromagnesian mineral-rich layers. The basement and cover have apparently folded as one unit, with no evidence of major detachment along the unconformity surface. The unconformity surface itself is quite irregular in detail, with harder bands from the basement protruding into the basal cover beds (e.g. Mills 1973). If detachment has occurred, it is more likely to be found at a higher stratigraphic level within the shales of the

Orientation The variation in orientation of Di structures along the fold belt is summarized in the stereograms of Figs 3 and 4. Neglecting the effects of the overprinting crenulation deforma-

tions, the strike of Si defines a broad arc (the

Fleurieu Arc, see above). The first folds F? and associated intersection lineation, Li, vary in plunge within the Si axial plane, though over most of the southern Adelaide Fold Belt, the variation is less than 30 degrees to either side of the horizontal (e.g. Fig. 8a). Southerly plunges dominate in the Kanmantoo Group, and consequently the highest units of the Kanmantoo Group are exposed in the south, on Fleurieu Peninsula and Kangaroo Island.

Fig. 8. Regional Si slaty cleavage, as developed in the phyllitic Petrel Cove Formation in Antechamber Bay, Dudley Peninsula (Loc. 3, Fig. 2). From the field exposure (Fig. 8a), note variability in orientation of the bedding/cleavage intersection L? and the weak Li mineral lineation pitching steeply within the planar cleavage. In thin section (Fig. 8b), note lack of mineralogical differentiation when compared to the later crenulation cleavages occurring locally in the fold belt (Fig. 10), and the growth of post-Si chlorite porphyroblasts.

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Neil S. Mancktelow

There are several localities where the F? fold axes pitch steeply or vary markedly in orientation within Si. In the regional Fi synclinal closure which marks the northern limit of Kanmantoo Group outcrop near Australia Plains (Figs 1,3,4), the intersection lineation and mesoscopic fold axes consistently plunge around 50 degrees south. Mills (1964, figs 75, 77) reported almost 180 degrees variability in fold plunge in carbonate-rich areas east of Springton, though this may be due to overprinting by later deformation. The Rapid Bay area on southern Fleurieu Peninsula, within the 'hinge' of the Fleurieu Arc, is also anomalous. The orientation of the mesoscopic fold axes and intersection lineations changes from an external region typical of the southern Adelaide Fold Belt, where axes show only a limited variation from the horizontal, through a region of great variability within a more constantly oriented Si, and into a region around Rapid Bay of reclined folds with a strong alignment of axes near parallel to the stretching lineation Li (Mancktelow 1981b). This transition is accompanied by a strengthening of the linear component of the Si fabric, and by a decrease in the dip of Si. Variable fold axis orientation, with common reclined folds, is also observed on Dudley Peninsula, Kangaroo Island (Fig. 3). Microstructure The first deformation fabric, Si, is generally defined by aligned phyllosilicates (Fig. 8b), the relative proportions of biotite, muscovite and chlorite varying with metamorphic grade. Biotite is usually dominant, with chlorite and muscovite more common at lower grades before their total or partial consumption in the formation of aluminous porphyroblasts. Preservation of elongate single quartz grains is only rarely observed (Mancktelow 1981a), as quartz readily recrystallizes into aggregates of unstrained new grains. The Si fabric consists of two mica populations for which the orienting mechanisms have been quite different: older grains, whose shape and orientation have been modified by intracrystalline slip and intracrystalline diffusion ('pres-

sure solution' e.g. Rutter 1976), and new, elongate, apparently undeformed grains with a very strong crystallographic preferred orientation and shape fabric (Mancktelow 1979b). Electron microprobe analyses of the old biotite grains show only a limited variation in chemical composition, suggesting that they are of metamorphic and not of detrital origin (Mancktelow 1979b). There is, therefore, clear evidence that much of the south coast of Fleurieu Peninsula, was already within at least the biotite zone of metamorphism prior to and during Di. Along the south coast of Fleurieu Peninsula, the grainsize and proportion of new biotite grains to old grains increases steadily towards the east, where in the vicinity of Newland Head, assemblages containing cordierite, andalusite, garnet, and rare staurolite and fibrolitic sillimanite first appear. As discussed in Mancktelow (1979b), this suggests that the temperature gradient towards the east already existed during the development of the Si slaty cleavage, although the peak of metamorphism was attained later. Age of the Di deformation Diagnostic Lower Cambrian fossils are found only below the base of the Kanmantoo Group (Daily 1963; Daily & Milnes 1971). The age of the youngest exposed Kanmantoo Group formation (the Middleton Sandstone) has not been directly determined, although an upper limit, coinciding approximately with the Cambro-Ordovician boundary, is provided by the intrusive Encounter Bay Granites (see below). Even in these upper formations, the metamorphic assemblages indicate considerable overburden, probably about 10 km (Daily & Milnes 1973; Mancktelow 1979a). The sediments were lithified prior to Di and had crystallized muscovite, chlorite, biotite and perhaps staurolite, andalusite (see Fleming & Offler 1968) and cordierite (Daily & Milnes 1973) before this first deformation. On the south coast of Fleurieu Peninsula, the Kanmantoo Group was locally invaded by numerous quartz veins which were

Structure of the Adelaide Fold Belt

folded and boudinaged during Di. Several of the amphibolite dykes, which are not uncommon throughout the fold belt, were clearly boudinaged during Di (Daily & Milnes 1971). Events younger than Di provide a minimum age for this event. The 'stripy layering' at Petrel Cove (Talbot & Hobbs 1968) has a strong spatial and probably temporal relationship to the Encounter Bay Granite, which intruded and cooled around 495 Ma (Milnes et al 1977). This layering consistently cuts across Si at a low angle and transects sedimentary structures (e.g. load casts) strained during Di, but is never folded with Si axial plane. The stripy layering, therefore, clearly post-dates Di (and pre-dates D2, by which it is folded). Higher metamorphic grade porphyroblasts, particularly andalusite and cordierite (+minor staurolite), appear to form a well developed aureole around the Encounter Bay Granite. Indeed, cordierite and andalusite porphyroblasts are lacking in the continuation of the Petrel Cove Formation ca.2-3 km north of the granitic outcrops at Port Elliot (Daily & Milnes 1973; fig. 4 & p.221). In the Encounter Bay region, andalusite generally appears to post-date, and form at the expense of, cordierite (e.g. Daily & Milnes 1973). The andalusite is often concentrated along the selvedges to stripy layers (Talbot & Hobbs 1968, plate 3); these particular porphyroblasts must, therefore, have grown synor post- the stripy layering formation, and therefore clearly post-Di. Indeed, the microstructure suggests that andalusite grew during the early stages of D2 crenulation cleavage development in the Encounter Bay area. Minor fibrolitic sillimanite and staurolite are also present, but their microstructural relationship to Di and D2 is unclear. Common chlorite porphyroblasts clearly grew post-D2. From these observations, it is clear that amphibolite facies metamorphic conditions, well within the field of plastic deformation of quartz, were established in the Encounter Bay area prior to Di and continued through D2. The Encounter

381

Bay Granites themselves, however, show little evidence of penetrative internal deformation which could be compared to the intensity of Di in the surrounding metasediments. The Si foliation strikes into the Encounter Bay Granites and is not bowed around, as would be expected if the Encounter Bay Granites acted as a rigid inclusion in a more ductile matrix during Di. With the exception of rare narrow zones of high strain immediately adjacent to the contact (Wright Island; fig. 65, Daily & Milnes 1973), which may be related to the heterogeneously developed D2, strain within the granites is limited to undulose extinction in quartz (with partial recovery by subgrain formation). This limited intracrystalline strain is unlikely to be due to Di, which occurred during a period of sustained high temperatures conducive to recrystallization and annealing, as is clearly seen in the microstructure of the metasediments. The conclusion is that the Di deformation preceded emplacement of the Encounter Bay Granites into their presently exposed level. The pre-Di mineral growth, which appears to increase in metamorphic grade towards the Encounter Bay Granites, may reflect a higher heat flow in this region already prior to Di, but with the culmination of metamorphism and the intrusion of the Encounter Bay Granites occurring essentially post-Di (and probably pre- to early syn-D2). It follows that the Di deformation of the Delamerian Orogeny in the southern Adelaide Fold Belt occurred at about 500-505 Ma, near the Cambro-Ordovician boundary.

THE D 2 CRENULATION DEFORMATION The D2 deformation structures exhibit a range of morphologies in field exposure, from weak crenulations of the pre-existing Si slaty cleavage, through to a very strong S2 schistosity difficult to distinguish from Si. Effects of D2 are not ubiquitous throughout the fold belt, but occur within limited areas. Overprinting by D2 is most com-

*A11 Rb-Sr ages discussed in this paper have been recalculated for a Rb half life of 1.42 x 10"11 a"1.

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Neil S. Mancktelow

mon in the higher-grade metamorphic zones. It is important to emphasize that, due to the disjointed areal extent of D2, the overprinting relationships of D2 on Di structures establish only a local time sequence, and that it has not been unequivocally established that these D2 structures represent a regionally isochronous event. Four structural elements associated with D2 are discernible in the field; the crenulation microfold axes F i , the crenulation cleavage S2, folds in bedding with S2 axial plane F§, and occasionally a steep-plunging mineral lineation within S2 (L2). L2 is defined by aligned biotite, and in higher-grade aluminous lithologies, by aligned sillimanite in the matrix and as fibres on andalusite porphyroblasts. Orientation and associated folding On the mainland, the orientation of the D2 crenulation cleavage maintains a broad relationship to the orientation of the pre-existing slaty cleavage, following Si as it curves around the Fleurieu Arc. The strike of S2 changes progressively from approximately N-S in the northern areas east of Adelaide to more NE-SW on the south coast of Fleurieu Peninsula (e.g. around Petrel Cove), and is usually within 30 degrees of the strike of Si. S2 generally dips more steeply than Si (Fig. 4). This pattern is modified slightly to the northeast in the region of the major Fi Kanmantoo Syncline (Fig. 4). S2 on the limbs of this fold diverge slightly to either side of the Si axial plane. Consequently, in the Nairne to Mount Torrens area, on the western limb of the major Fi syncline, S2 strikes west of the Si orientation, whereas in the Palmer to Monarto area on the other limb, S2 strikes east of the Fi syncline's axial plane. S2 may continue to follow Si around the Fleurieu Arc onto Kangaroo Island, for the dominant and generally only crenulation cleavage on Kangaroo Island is oriented approximately east-west, close to the orientation of Si (Fig. 4). Unfortunately, this cleavage is indistinguishable in orientation from S3 as measured

on the mainland (see below). It is, therefore, uncertain whether the major crenulation cleavage on Kangaroo Island correlates with S2 or S3 (if either) on the Fleurieu Peninsula, particularly as S3 is well developed in Middleton Quarry just north of the peninsula coast. In style, the crenulation structures on Kangaroo Island are similar to those of D2 on the mainland. The crenulation fold axes and folds in bedding are shallowly plunging (Fig. 4), commonly towards the south or southwest, both on the Fleurieu Peninsula and on Kangaroo Island. Major folds associated with D2 are not common and are largely restricted to the higher-grade metamorphic zones. Minor folds of bedding are observed in scattered areas throughout the belt, but the effect on the overall envelope is small. The most extensive region of D2 folding, representing a major syncline-anticline pair, forms a broad north-south zone west of Palmer (Fig. 4). In this study, these major D2 folds have only been mapped within the Kanmantoo Group, but from the regional geometry (e.g. Preiss 1983) it is possible that they extend still further north into the Adelaide Supergroup (the possible D2 structures west of Australia Plains in Fig. 4). As the orientation of Si and S2 are often little different, the assignment of major folds in bedding to either Di or D2 can be difficult, particularly in areas where S2 has formed a new schistosity. Minor F2 folds and an associated S2 crenulation cleavage superimposed on a larger amplitude, pre-existing Fi fold may result in bedding-S2 relationships which could be interpreted as indicating that the whole structure developed during D2 (Fig. 9a). The crucial observation is whether the So/Si sense changes across the fold. The determination of the correct So/Si vergence may, however, be difficult in strongly crenulated zones in which a new, differentiated S2 has developed (Figs 9a, c). The schematic geometry of Fig. 9 is directly relevant to the field relationships observed around the Strathalbyn-Macclesfield anticline-syncline pair, which were interpreted by Marlow (1975) as F2 folds. In the present study, they are considered to be Fi struc-

Structure of the Adelaide Fold Belt tures modified during D2. The axial trace of these major structures can be followed to the south into areas of lower metamorphic grade (Fig. 4), where the crenulation effects are largely absent. Here, the axial plane cleavage in the hinge of this major structure is the regional slaty cleavage Si. The style of the D2 deformation is characteristic. D2 deformation does not refold Fi folds on a macroscopic scale and refolding on a mesoscopic scale is rare. Instead D2 tends to modify the shape of pre-existing folds. This results in a shallowing of the dip of the overall bedding envelope on the limbs of Fi folds (e.g. Fig. 9a). Excellent examples can be seen on a mesoscopic scale along Torrens Gorge and in Petrel Cove near Victor Harbor, and on a macroscopic scale in the Mount Torrens-Birdwood, Strathalbyn, and Palmer-Monarto areas. This geometry is inconsistent with a regional shortening perpendicular to the developing S2 crenulation cleavage, which would have tightened the Fi folds and thereby steepened the average envelope of the bedding on the fold limbs. Because of the small angle between the regional orientation of the uncrenulated slaty cleavage Si and the crenulation cleavage S2, shortening perpendicular to S2 could not have produced the F2 crenulation microfolds by buckling of S1; the S1 cleavage would have been in the field of extension, and there is no evidence that S is an extensional crenulation cleavage (cf. Piatt & Vissers 1980). Instead, the strongly crenulated regions appear to represent broad zones of steep shear associated with approximately vertical relative displacements (Fig. 9a).

383

metamorphic conditions. The two forms of crenulation cleavage are not mutually exclusive and often occur in association.

Age of the D2 deformation The peak of regional metamorphism within the southern Adelaide Fold Belt occurred during the early stages of S2 crenulation cleavage development. This can be established from the preservation, as an internal fabric within porphyroblasts (usually andalusite and staurolite), of open F2 crenulations which are continuous with larger amplitude, more strongly developed F2 crenulations in the external matrix. It is also indicated by the aligned growth of fibrous sillimanite as "pressure shadows" on andalusite porphyroblasts, defining the L2 lineation within S2. Post-S2 mineral growth appears to be exclusively

Microstructure The microstructure of the S2 crenulation cleavage is gradational between two end-members: discrete and zonal crenulation cleavage (Gray 1977) and commonly involves marked mineralogical differentiation (Fig. 10; Mariow & Etheridge 1977). Discrete cleavage tends to dominate at lower grades of metamorphism in the southern Adelaide Fold Belt, although both cleavage types do occur over the full range of

(a)

(c)

Fig. 9. Sketch outlining the geometry of the D2 deformation. Folding and cleavage development associated with D2 generally occur within limited zones and lead to an overall shallowing in dip of the bedding envelope (Fig. 9a). In field exposure, the orientation of Si within the more quartzo-feldspathic microlithons between the S2 cleavage domains is the most immediately obvious (Fig. 9b), and can lead to a false interpretation of the So/Si vergence (Figs 9b, c).

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retrograde (e.g. post-S2 chlorite porphyroblasts at Petrel Cove). An absolute age for D2 can be approximately determined. D2 crenulations fold albitic veins (the 'stripy layering' of Talbot & Hobbs 1968), which appear to be late stage effects related to intrusion of the Encounter Bay Granites (cf. Milnes 1973; Milnes et al 1977). The Palmer Granite has also been deformed during D2, probably producing its slightly elongate shape and resulting in the development of a linear deformation fabric in the granite parallel to the crenulation line at ion in the surrounding meta-arkose (Kleeman & White 1956). Northwest of Pallamana, dykes of Monarto Granite protruding from the rather irregular contact are folded with the S2 schistosity as axial plane. All these granitic intrusions therefore predated the D2 deformation. The Encounter Bay and Palmer Granites have both been isotopically dated at around 495 Ma (Milnes et al 1977; White et al 1967). In road cuttings near Summerfield, aplitic sills related to the Murray Bridge Granites (Mancktelow 1979a) truncate F2 folds (Fig. 11). The Murray Bridge Granites have been dated at around 470-475 Ma (Webb 1976). Therefore, D2 must have occurred within the time period 495 to 470 Ma, within the Early to Middle Ordovician.

associated with tight F3 folding and the development of crenulation cleavage. This has not been observed elsewhere. D3 crenulations of S2 (F^) commonly plunge between 30 degrees and 70 degrees east to southeast. Significant D3 folds in bedding have been observed only in Middleton Quarry, where they plunge ca. 40 degrees towards 112 degrees. Overall the effects of D3 along the fold belt are minor. Microscopic effects of D3 range in different areas from open kink-like crenulations to welldeveloped, differentiated, discrete and zonal crenulation cleavages. The better developed cleavages are identical in microstructure to those of D2. The weaker kink-like crenulations, however, show little differentiation, and commonly involve bending of mica around crenulation hinges. This suggests that D3 at such localities may have occurred at relatively low temperatures.

In the Kanmantoo-Harrogate region, the S3 crenulation cleavage is commonly overprinted by open, rather kink-like crenulations with steeplydipping axial planes striking between north and northwest. In thin section, the crenulations are often sharply angular, with micas either still bent around the hinges or recrystallized into an interlocking mosaic. Associated mineralogical difTHE D CRENULATION DEFORMATION ferentiation has not been observed and a new S4 crenulation cleavage has not developed. The conMinor folds and crenulations of the D3 defor- sistently oriented D4 crenulations have only been recognized in the Kanmantoo-Harrogate area, mation are not common, and on the mainland have been observed only in the Strathalbyn to where all the crenulation deformation fabrics are Harrogate region, the hinterland of Wil- more strongly developed. Detailed work elseliamstown, and in Middleton Quarry. On the where may find further evidence of D4 crenulamainland, the S3 crenulation cleavage and the tions, but the effect of this event on the fold belt axial plane to D3 crenulations have a consistent as a whole appears to have been insignificant. orientation trending between east and northeast and generally dipping moderately to steeply The post-D2 crenulation events have not been south (Fig. 4). This orientation appears to remain systematically deciphered on Kangaroo Island, constant despite the swing in Si orientation along but there are at least four deformational events the Fleurieu Peninsula. In the Warren Reservoir recorded in schists at the mouth of Breakneck area, Mills (1973) described a change in S3 strike River on the west coast: an early, differentiated with increasing intensity of D3, from east to crenulation cleavage (S2?), which is folded by northeast to north, the northerly orientation being both N-S and E-W crenulations (Flint 1974). 3

Structure of the Adelaide Fold Belt

Cooling of the southern Adelaide Fold Belt through the blocking temperature for Rb-Sr and K-Ar isotopic systems in biotite (300±50°C, Wagner etal 1977) occurred during the Middle to Late Ordovician (ca.460 Ma, Milnes etal 1977). This temperature coincides approximately with the gradational brittle-ductile transition for quartz-rich rocks (Atkinson 1982) and the facility for significant ductile deformation, and consequently the Delamerian Orogeny, must be considered to have ended by this time.

FAULTING Faulting is quite extensive in the southern Adelaide Fold Belt, but only a few faults exhibit significant total displacements (see Figs 3,4, 5). Some of these faults may have been active, or reactivated, over a considerable timespan ranging from the Proterozoic to recent times. Sedimentary studies in the Flinders Ranges (Plummer & Gostin 1976) and stratigraphic drilling and surface mapping of the change from platformal sediments on the Gawler Craton to thicker basinal

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deposits in the fold belt (e.g. Thomson et al 1976) have shown that syn-depositional faults were important in controlling the sedimentation. Thickness changes in stratigraphic units across these early faults can be dramatic (Plummer & Gostin 1976) and may have strongly influenced the development of later tectonic structures. The Talisker Fault on Fleurieu Peninsula (Fig. 4) may represent an example of a reactivated syn-sedimentary fault. There is a very marked change in stratigraphic thickness across this structure, which appears to be too great to attribute entirely to tectonic thinning. On the south coast of Fleurieu Peninsula, arkosic sandstones of the Backstairs Passage Formation are ca.700 m thick west of the Talisker Fault, only a few metres thick immediately to the east of the fault but dramatically thicken again to the east to attain around 500 m east of Blowhole Creek (Daily & Milnes 1971; Fig. 2). A related geometry may be observed along strike on Dudley Peninsula, Kangaroo Island (Fig. 4). There, a major reverse fault (a continuation of the Talisker Fault?) has brought up an upper Adelaide Super-

Fig. 10. Morphology of S2 crenulation cleavage in thin-section, perpendicular to h \ . In Fig. 10a, the discrete S2 crenulation cleavage truncates the microfolded Si fabric in a low metamorphic grade carbonaceous siltstone of the Burra Group, from Torrens Gorge near Adelaide (Adelaide 926400). In Fig. 10b, from south of Macclesfield (Echunga 038027), the S2 crenulation cleavage shows very marked mineralogical differentiation into microlithons rich in quartz and phyllosilicates. This is a common morphology above the andalusite-staurolite zone boundary.

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group-Normanville Group sequence overlain, above a discontinuity, by the lowermost unit of the Kanmantoo Group (the Carrickalinga Head Formation, Daily & Milnes 1972, fig. 2). The discontinuity appears to cut down sequence from the Normanville Group to the Adelaide Supergroup going north towards the coast. Daily & Milnes (1972) interpret this discontinuity as a fault, but the geometry does not exclude an erosional disconformity. The region immediately east of the Talisker Fault may have been one of more positive relief during deposition, with thinned sequences preserved on Fleurieu Peninsula and an erosional disconformity on Dudley Peninsula.

Knapman's Creek Fault on the west coast of Kangaroo Island (Fig. 4) is clearly a major structure, but constraints on the timing, direction and amount of displacement have yet to be established. This low-angle fault separates sillimaniteK feldspar grade grey metasiltstones and sandstones, which dip near parallel to the fault plane, from steep to vertically dipping lightcoloured meta-arkoses in the hangingwall above the fault. Its regional structural position, on the northern limb of the American River Anticline, is similar to that of the Talisker Fault on Fleurieu Peninsula and its probable continuation on Dudley Peninsula (Fig. 4).

Faulting associated with Di is common, particularly on the steep to overturned limbs of asymmetric Fi folds towards the cratonic foreland. On these high strain limbs there is often considerable ductile thinning of units and sometimes failure, usually of particular competent layers to produce boudinage structures, but occasionally along discrete planes to form major reverse faults (e.g. the steep western limb of the Mt Compass Anticline, Anderson 1975). The

The Nairne and Meadows-Williamstown Faults are major structures in the north, with a total displacement of 3-5 km proposed for the latter (Figs. 4,5; Mills 1973). Both cross-cut and offset D2 folds; significant displacement must have occurred post-D2, although they could have been initiated still earlier. The Meadows-Williamstown Fault also truncates the metamorphic zone boundaries (Fig. 2; Mills 1973), and in many areas of poor outcrop the fault has been mapped on the basis of this juxtaposition of lowand medium-grade rocks. Mills (1964) mapped a major fault in the region north of Palmer which also offsets the zone boundaries. Some of these faults may have been initiated or at least rejuvenated during the Tertiary. The Nairne Fault at Mount Barker has associated Tertiary conglomerates, suggesting it formed a fault scarp at that time (Daily, Marlow & Toteff, pers. comm., 1978).

Fig 11. Aplitic vein with geochemical affinities with the Murray Bridge Granite suite (Mancktelow 1979a) truncating F2 folds. Road cutting near Summerfield (Tepko 335346).

The present Mount Lofty Ranges are a rejuvenated Cenozoic topographic feature that was uplifted relative to adjacent basinal areas along steeply dipping northeast-trending normal and reverse faults (Thomson 1969). These faults are important in limiting the outcrop of Adelaide Supergroup and Kanmantoo Group rocks to both the east and west, but in profile the vertical offsets are generally small (Fig. 5), and do not have a major effect on the overall structure.

Structure of the Adelaide Fold Belt DISCUSSION The possibility of pre-Di deformation

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(e.g. Fig. 10; Marlow & Etheridge 1977, fig. 10b). The Si fabric throughout the Adelaide Fold Belt shows little evidence of comparable differentiation (e.g. Fig. 8b).

The features attributed to Di, which have been described above, are those associated with Jenkins (1986) and Clarke (1987), in conthe dominant, regional, penetrative foliation sidering the Adelaide Fold Belt on a more within the Adelaide Fold Belt. It has recently been regional scale, were concerned by the apparent suggested that there may be an additional defor- thickness of the Kanmantoo Group succession mation phase preceding this event. Steinhardt (see below) and by the occurrence of peak (1987) studied the porphyroblast relationships in metamorphic zones within the uppermost the andalusite-cordierite-bearing schists around stratigraphic units. They have suggested an addiPetrel Cove (Fig. 1). The cordierite por- tional phase of early thrusting to explain these phyroblasts within this area often have practically observations. As discussed above, limited thrustplanar internal inclusion trails, which are con- ing, usually associated with highly strained overtinuous with, but rotated relative to, the external turned limbs of asymmetric Fi folds towards the fabric (Daily & Milnes 1973, figs 69 & 70). Away foreland on Fleurieu Peninsula and Kangaroo from zones of later crenulation, this external Island has indeed been recognized for some time foliation (which is correlated with the ubiquitous (e.g. Talbot 1964; Offler & Fleming 1968; Daily Si throughout the fold belt) is not clearly & Milnes 1972). However, an unrecognized large mineralogically differentiated (e.g. Daily & Mil- scale thrusting event, with the development of a nes 1973, figs 69-70). Daily & Milnes (1973) nappe pile of allocthonous sheets, is in conflict considered these microstructural relationships to with the relatively good lithostratigraphic corbe indicative of pre- to syn-Di growth of correlation possible within the Kanmantoo Group dierite and Offler & Fleming (1968) indicative of and Adelaide Supergroup. It also does not help syn-Di growth. Steinhardt (1987) interprets the explain the metamorphic isograd pattern, which internal porphyroblast fabric as preserving the is clearly oblique to the overall structural trend original, unrotated orientation of an earlier pre-Si (Fig. 2; Offler & Fleming 1968), traversing fold tectonic foliation, the external foliation repre- hinges and lithological boundaries. senting an overprinting crenulation cleavage which has developed into a non-differentiated penetrative schistosity, Si (Stage 6 of Bell & Deformational control of stratigraphic thickness Rubenach 1983). There has been considerable discussion and The basis of this interpretation is open to controversy over the relative importance of synquestion. It relies heavily on the inability of por- depositional basinal tectonics, reflected in facies phyroblasts to rotate, even during a rotational and sedimentary thickness variations, and postbulk deformation (Bell 1985, but see e.g. Ghosh depositional ductile and brittle (fault) tectonics in & Ramberg 1976), and on the interpreted ability controlling the observed stratigraphic sequence of crenulation cleavages to develop into a in the southern Adelaide Fold Belt (cf. Daily & penetrative stage in which their domainal original Milnes 1971). Discussion has concentrated on the form is no longer discernible (Bell & Rubenach variation in preserved thickness of the Adelaide 1983). Evidence from regions of strong post-Si Supergroup, which ranges from ca.4000 m east crenulation cleavage development elsewhere in of Adelaide to nothing at Little Gorge Creek, the Adelaide Fold Belt suggest that within pelitic southern Fleurieu Peninsula (Fig. 1 ; Campana units (such as the Petrel Cove Formation, as con- 1955, fig. 1; 1958, fig. 2; Campana & Horwitz sidered by Steinhardt 1987), the mineralogical 1956; Daily & Milnes 1971), the Cambrian Nordifferentiation is never completely eliminated manville Group, which is ca.900 m thick in the

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type section south of Sellick Hill (Abele & McGowran 1959, fig. 1; Daily 1963), but often absent between the Adelaide Supergroup and Kanmantoo Group at many other localities along the length of the Adelaide Fold Belt (e.g. see Preiss 1983), and the surprising thickness of the Kanmantoo Group (perhaps>20 km?). With the exception of considerations of the thickness of the Kanmantoo Group, the problem is generally one of the reduction of stratigraphic thickness and the loss of stratigraphic units from the sequence. Such thinning cannot be achieved by thrust faulting of a normal stratigraphic sequence; this would lead to imbricate stacking, a repetition of stratigraphy and an overall thickening. If faulting alone is invoked, it must involve extensional normal faults. Such extensional tectonics may be associated with crustal thinning during sedimentation (e.g. Von der Borch 1980) or with the post-Delamerian uplift history, including the Cenozoic fault rejuvenation. Faulting certainly is involved in the case of Little Gorge (e.g. Daily & Milnes 1971), where a major structure (the Talisker Fault; Fig. 4) can be traced as far as the south coast of Fleurieu Peninsula, and probably extends to Dudley Peninsula, Kangaroo Island. However, this fault appears to be affecting an already much reduced Adelaide Supergroup stratigraphic thickness (Preiss 1983). Due to the shallow plunge of regional fold axes in the southern Adelaide Fold Belt, stratigraphic sequences are generally observed on the dipping limbs of the regional folds, and it could be argued that both the Adelaide Supergroup and overlying Normanville Group have undergone ductile thinning and boudinage on these fold limbs. However, in the hinge of the Di Mount Compass Anticline near Rapid Bay (Fig. 4), where the fold style tends to similar folds with thickened hinges (cf. Ramsay 1967), the Adelaide Supergroup is still much reduced. A similar observation can be made with regard to the absence of Normanville Group units in the hinge of the Karinya Syncline near Australia Plains (Preiss 1983). These units must have been thin or absent prior to the regional Di folding.

The minimum thickness of the Kanmantoo Group sediments, as summed from the components of the type section along the south coast of Fleurieu Peninsula, is of the order of 12 000 m. The upper limit to this sequence is not exposed and the uppermost formations are intruded by granitoids whose contact aureole mineral assemblage indicates emplacement depths of ca.10 km (Daily & Milnes 1973; Mancktelow 1979a). Direct evidence for significant tectonic thickening by major thrust imbrication has not been established (see above), although it may be difficult to recognize within the rather monotonous turbidite and sandstone sequences. The extent of tectonic thickening of the stratigraphic column due to more homogeneous layer parallel shortening and small scale folding has not yet been evaluated in detail, though it is probably significant (e.g. Daily & Milnes 1971, p.208; Mancktelow 1981a). Considering the extensive and ubiquitous development of axial plane cleavage, the tectonic thickening could well exceed 50% (cf. Wood 1974; fig. 4). It is also significant that the type section for the Kanmantoo Group is a composite one, which extends over a horizontal distance of more than 50 km. The section does not, therefore, necessarily represent the vertical stratigraphic sequence at any one specific locality within the depositional basin. The summed thickness of the component members of the Kanmantoo Group in its type section may be a gross overestimate. In summary, the variability in preserved stratigraphic thickness in the Adelaide Supergroup and Normanville Group and the great thickness of the Kanmantoo Group appear to be original pre-Delamerian depositional features, which have been amplified by faulting and ductile strain. Regional structure and tectonics The arcuate form of the southern Adelaide Fold Belt is its most striking regional feature. Palaeomagnetic results from Yorke Peninsula indicate that there has been little rotation of the basement adjacent to the arc (Giddings &

Structure of the Adelaide Fold Belt

Embleton 1974), and the arcuate form must be largely original rather than due to oroclinal bending of the fold belt. The geometry of Di folds, which dominate the overall structure, may be explained by compression in a NW-SE direction against a curved cratonic margin (Fig. 12). This would produce a component of sinistral shear to the deformation in the north-south oriented segment around Adelaide, consistent with the observed en echelon fold pattern developed in this region (Fig. 4). Adextral shear component would be imposed on the east-west oriented segments, with a tendency to constrictional deformation on the inner arcs (Fig. 12) due to the opposed shear components. As shown in Fig. 12, the NW-SE orientation of the principal compression direction across the arcuate fold belt is also consistent with the development of similarly oriented extension cracks in the Gawler Craton, preserved as a swarm of basic dykes (inset; Fig. 4). The subsurface geology of the southern Adelaide Fold Belt is poorly known and the basement behaviour during deformation can only be interpreted from the available outcrop and the limited seismic reflection coverage of Gulf St Vincent (Stuart & von Sanden 1972). This behaviour clearly changes between the cratonic basement of the foreland on Yorke and Eyre Peninsulas, which is apparently unaffected by the Delamerian Orogeny, and the strongly overprinted, retrogressed basement inliers of the anticlinal culmination east and southeast of Adelaide. Between these two regions of basement exposure, the style of the cover deformation also clearly changes, from a little deformed, thin platformal sequence, to a foreland fold and thrust belt involving strongly heterogeneous deformation and asymmetric folds vergent towards the foreland and then, gradually, to a more upright and homogeneous fold style on the internal side of the basement inliers. These observations on deformation intensity and style in basement and cover are obviously related, and may well reflect a change from an external region of thin-skinned detachment tectonics, with the cover deforming independently of a relatively rigid crystalline basement, to an internal region of high geother-

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mal gradient in which the thermally weakened, thinned (due to syn-depositional extension) crystalline basement has been fully involved in the deformation. The high geothermal gradient was probably established during the extensional phase, and certainly prior to the first deformation, as shown by extensive pre-Di mineral growth. The present overall shape of the fold belt, with the stratigraphically higher units exposed in the south and southeast, probably reflects the original sedimentary basin shape, which has been amplified during Di. The compression and vertical extension of an already deep basin would have led to marked overthickening of the crust. This was followed by high grade metamorphism, peaking early syn-D2, which would have greatly reduced the yield strength of the overthickened crust. As stated by Ramberg (1981), the gravitational potential in a stratigraphic sequence with a normal density profile (i.e.density steadily increasing downwards) would tend to reduce the amplitude of major buckle folds, which represent perturbations in the equipotential surface. The isostatic recovery may have occurred by displacement across broad, steep, ductile shear zones, leading to the development of the characteristic form of the D2 folds, which is seen as a tendency to flatten out the overall dip of beds steepened during Di. The post-Di deformation may be a gravity-driven response, during a period of high temperature and enhanced ductility, to the crustal shortening and thickening caused by Di. The preserved segment of the southern Adelaide Fold Belt represents the transition form of stable continental foreland to the ProterozoicLower Palaeozoic passive margin of the Australian segment of the Gondwanaland supercontinent (e.g. Veevers 1976; Von der Borch 1980). The width of this continental margin is purely conjectural, as deeper structural levels are not exposed east of the basement inliers along the anticlinal culmination near Adelaide (Fig. 1, inset Fig. 4). Von der Borch (1980) argues that it approximately corresponds to the western limit of Kanmantoo Group exposure. According to Rutland (1976), however, the entire sequence of the

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Adelaide Fold Belt was deposited on older Precambrian basement. Certainly, there is currently no direct evidence to imply the development of new oceanic basement. The nearest greenstones and calc-alkaline volcanics of Cambrian age outcrop as three linear belts in Victoria (the Stavely, Heathcote & Mt Wellington Belts, e.g. fig. 9, Cooper & Grindley 1982; Crawford & Keays 1978), over 500 km southeast of the currently exposed Adelaide Fold Belt. The extension of crystalline basement of Precambrian age well to the east of the current study area is also supported by the distribution and geochemistry of I- and S-type granites in eastern Australia (White et al 1976; McCulloch & Chappell 1982), though this crystalline basement may not always have represented a single continuous block. Deposition on this passive margin during the Proterozoic was dominated by shallow water platformal sedimentation which continued (after an erosional hiatus) into the Cambrian with the

Fig. 12. Tectonic interpretation of the southern Adelaide Fold Belt as developed by compression in a NW-SE direction against a curved cratonic margin. This model is consistent with the orientation of basic dykes on the Gawler Craton, the en-echelon pattern of major fold structures in the N-S segment, and regions of constrictional deformation within the inner arcs.

development of an extensive carbonate shelf. Within the Lower Cambrian, however, there was a fairly sudden collapse of the basin, associated with rapid accumulation rates and common turbiditic sedimentation to form the Kanmantoo Group. Near the stable foreland, conglomerates with clasts up to a metre in size and clast lithologies ranging from the Cambrian to the crystalline basement developed locally (e.g. the White Point Conglomerate on northern Kangaroo Island, (Fig. 6a). This rapid subsidence (the Kangarooian Movements of Daily & Forbes 1969) may have been associated with the transition from extensional to compressional tectonics, leading to the development of fairly narrow deep basins (e.g. Matter & Home wood 1980). Deposition, at least at the levels exposed, was closed by the pervasive Di slaty cleavage-forming event of the Delamerian Orogeny, which presumably resulted in the development of a mountain chain of considerable relief. These mountains may have acted as the source for the sudden influx of immature turbiditic sediments into western and central Victoria in the Lower Ordovician. The gradual fining from west to east of these immature clastic sediments in Victoria is indicative of a source to the west, in the region of the southern Adelaide Fold Belt. This correlation of renewed sedimentation of reworked Kanmantoo Grouptype greywackes with the initiation of tectonic activity in the Adelaide Fold Belt accords well with the interpreted age of the Di deformation at around the Cambro-Ordovician boundary. The timing of the intrusion of the various granitoids (and in particular the isotopically dated Encounter Bay and Palmer Granites) compared to the main phases of deformation is still equivocal, with arguments for both pre- (Daily & Milnes 1973; Milnes et al 1977) andpost-Di possible. In practice, however, the predicted ages based on these diverging arguments for the all important Di deformation of the Delamerian Orogeny do not differ markedly, and must approximately coincide with the Cambro-Ordovician boundary around 500-505 Ma.

Structure of the Adelaide Fold Belt

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Regional extent

ACKNOWLEDGEMENTS

Traditionally, the Adelaide Fold Belt has been linked with the Ross Orogen of the Transantarctic Mountains in Antarctica on the basis of preCenozoic Gondwanaland reconstructions and tectonic similarity (e.g. Craddock 1982; Veevers 1976). This similarity is indeed striking, and features of the Ross Orogen which beg direct comparison with the southern Adelaide Fold Belt include:

The basic field and laboratory work for this paper was carried out at the Department of Geology, University of Adelaide, between 1975 and 1979, while in receipt of a Commonwealth Postgraduate Scholarship. Thanks are due to my supervisors, Pat James and Roy Rutland, and to Robin Oliver, Graeme Teale, Vic Gostin, Brian Daily and Tim Bell for many stimulating discussions. Pat James and Peter Fleming are also thanked for helpful criticism of an earlier draft.

the development of an extensive turbiditic sandstone/siltstone sequence of probable Cambrian age (Robertson Bay Group, Field & Findlay 1983); ii) the complete lack of known ophiolitic complexes; iii) a main deformation phase at ca.500 Ma (e. g. Table 1, Grindley & Oliver 1983); iv) generally upright folding with variable but usually shallow-plunging fold axes, and with the development in the more extensive, lower metamorphic grade areas of a single dominant axial plane cleavage; v) areas of low pressure-high temperature metamorphism involving more extensive polydeformation and the development of gamet-andalusite-sillimanite-cordierite bearing mineral assemblages; aluminous quartz-rich segregations, containing coarse andalusite porphyroblasts, have also been reported (Kleinschmidt 1981); vi) 'post-tectonic' intrusion of a granite-tonalite series (the Granite Harbour Intrusives) of age ca.475 Ma (Kreuzer etal 1981) ; this may be compared to the Rb-Sr isochron age of472±5 Ma for the Murray Bridge Granites (Webb 1976; Mancktelow 1979a), which are 'post-tectonic' to the Delamerian Orogeny (post-F2, relationship to F3 unknown). The Adelaide Fold Belt is, therefore, probably only a small segment, preserved on the Australian continent, of a very extensive, elongate CambroOrdovician orogenic belt, of dimensions similar to the present Alpine-Himalayan chain but with a distinctly different structural style.

REFERENCES ABBAS S.A.F. 1975. Granitic and migmatitic rocks of the Cooke Hill area, South Australia, and their structural setting. Ph.D. thesis, University of Adelaide (unpubl.). ABELE C. & McGOWRAN B. 1959. The geology of the Cambrian south of Adelaide (Sellick Hill to Yankalilla). Transactions of the Royal Society of South Australia 82, 301-320. ALLEN R. 1977. Metamorphic phase relationships in the Kanmantoo. B.Sc. (Hons) thesis, University of Adelaide (unpubl.). ANDERSON J.A. 1975. Structural and strain analysis of the nose of the Myponga-Little Gorge Inlier, Fleurieu Peninsula, S.A. B.Sc. (Hons) thesis, University of Adelaide (unpubl.). ATKINSON B.K. 1982. Subcritical crack propagation in rocks: theory, experimental results and applications. Journal of Structural Geology 4, 41-56. BELL T.H. 1978. The development of slaty cleavage across the Nackara Arc of the Adelaide Geosyncline. Tectonophysics 51,171-201. BELL T.H. 1985. Deformation partitioning and porphyroblast rotation in metamorphic rocks: a radical reinterpretation. Journal of Metamorphic Geology 3, 109-118. BELL T.H. & DUNCAN A.C. 1978. Arationalized and unified shorthand terminology for lineations and fold axes in tectonites. Tectonophysics 47, T1-T5.

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BELL T.H. & RUBENACH M.J. 1983. Sequential porphyroblast growth and crenulation cleavage development during progressive deformation. Tectonophysics 92,171-194. CAMPANA B. 1955. The structure of the eastern South Australian ranges: the Mt Lofty-Olary arc. Journal of the Geological Society of Australia 2,47-62. CAMPANA B. 1958. The Mt Lofty-Olary region and Kangaroo Island. Journal of the Geological Society of Australia 5, 3-27. CAMPANA B. & HORWITZ R. 1956. The Kanmantoo Group of South Australia considered as a transgressive sequence. Australian Journal of Science 18, 128-129. CLARKE G.L. 1987. Basement control of cover folding in the Adelaide Geosyncline, South Australia. Geological Society ofAustralia, Abstracts 19,125. COBBOLD P.R., COSGROVE J.W. & SUMMERS J.M. 1971. Development of internal structures in deformed anisotropic rocks. Tectonophysics 12,23-53. COOPER R.A. & GRINDLEY G.W. (Editors) 1982. Late Proterozoic to Devonian sequences of southeastern Australia, Antarctica and New Zealand and their correlation. Geological Society of Australia, Special Publication 9, 1 -103.

CRADDOCK C. 1982. Antarctica and Gondwanaland. In Craddock C. ed. Antarctic Geoscience, pp.3-13. The University of Wisconsin Press, Madison, Wisconsin.

FIELD B.D. & FINDLAY R.H. 1983. The sedimentology of the Robertson Bay Group, Northern Victoria Land. In Oliver R.L., James P.R. & Jago J.B. eds. Antarctic Earth Science, pp. 102-106. Cambridge University Press, Cambridge.

CRAWFORD A.J. & KEAYS R.R. 1978. Cambrian greenstone belts in Victoria: marginal sea-crust slices in the Lachlan fold belt of southeastern Australia. Earth and Planetary Science Letters 41, 197-208.

FLEMING P.D. 1971. Metamorphism and folding in the Mt Lofty Ranges, South Australia, with particular reference to the Dawesley-Kanmantoo area. Ph.D. thesis, University of Adelaide (unpubl.).

DAILY B. 1963. The fossiliferous Cambrian succession on Fleurieu Peninsula, South Australia. Records of the South Australian Museum 14,579-601. DAILY B. & FORBES B.G. 1969. Notes on the Proterozoic and Cambrian, Southern and Central Flinders Ranges, South Australia. In Daily B. ed. Geological Excursions Handbook, 41st ANZAAS Conference, Adelaide, 1969, pp.23-30.

FLEMING P.D. & OFFLER R. 1968. Pre-tectonic metamorphic crystallization in the Mt Lofty Ranges, South Australia. Geological Magazine 105, 356-359. FLEMING P.D. & WHITE A.J.R. 1984. Relationships between deformation and partial melting in the Palmer migmatites, South Australia. Australian Journal of Earth Sciences 31, 351-360.

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FLINT D.J. 1974. Geological investigations of Kanmantoo Group metasediments between West Bay and Breakneck River, Kangaroo Island, South Australia. B.Sc. (Hons) thesis, Flinders University, Adelaide (unpubl.).

KLEINSCHMIDT G. 1981. Regional metamorphism in the Robertson Bay Group area and in the southern Daniels Range, north Victoria Land, Antarctica - a preliminary comparison. Geologisches Jahrbuch B41, 201-228.

FLINT D.J. 1978. Deep sea fan sedimentation of the Kanmantoo Group, Kangaroo Island. Transactions of the Royal Society of South Australia 102, 203-222.

KREUZER H., HOHNDORF A., LENZ H., VETTER U., TESSENSOHN F., MULLER P., JORDAN H., HARRE W. & BESANG C. 1981. K/Ar and Rb/Sr dating of igneous rocks from north Victoria Land. Geologisches Jahrbuch B41, 2167-2273.

GHOSH S.K. & RAMBERG H. 1976. Reorientation of inclusions by combination of pure shear and simple shear. Tectonophysics 34, 1-70. GIDDINGS J.W. & EMBLETON B.J.J. 1974. Large-scale horizontal displacements in southern Australia - contrary evidence from palaeomagnetism. Journal of the Geological Society of Australia 21, 431-436. GLEN R.A., LAING W.P., PARKER A.J. & RUTLAND R.W.R. 1977. Tectonic relationships between the Proterozoic Gawler and Willyama orogenic domains, Australia. Journal ofthe Geological Society of Australia 24, 125-150.

KUBLER B. 1967. La crystallinite de Finite et les zones tout h. fait superieures du metamorphisme. In Shaer J.P. ed. Etages Tectoniques, Colloque a Neuchatel, pp. 105-122. Editions de la Baconniere, Neuchatel. McCULLOCH M.T. & CHAPPELL B.W. 1982. Nd isotope characteristics of S- and I-type granites. Earth and Planetary Science Letters 58, 52-64. MANCKTELOW N.S. 1979a. The structure and metamorphism of the southern Adelaide Fold Belt. Ph.D. thesis, University of Adelaide (unpubl.).

GRAY D.R. 1977. Morphologic classification of crenulation cleavage. Journal of Geology 85,229-235.

MANCKTELOW N.S. 1979b. The development of slaty cleavage, Fleurieu Peninsula, South Australia. Tectonophysics 58,1-20.

GRINDLEY G.W. & OLIVER P.J. 1983. Post-Ross Orogeny cratonization of northern Victoria Land. In Oliver R.L., James P.R. & Jago J.B. eds. Antarctic Earth Science, pp. 133-139. Cambridge University Press, Cambridge.

MANCKTELOW N.S. 1981a. Strain variation between quartz grains of different crystallographic orientation in a naturally deformed metasiltstone. Tectonophysics 78,73-84.

JAGO J.B. & HDLYARD D.B. 1983. Comment: Late Precambrian-Cambrian stratigraphic nomenclature in the Adelaide Geosyncline. Transactions of the Royal Society of South Australia 107, 131-133.

MANCKTELOW N.S. 1981b. Variation in fold axis geometry and slaty cleavage microfabric associated with a major fold arc, Fleurieu Peninsula, South Australia. Journal of the Geological Society of Australia 28,1-12.

JENKINS R.J.F. 1986. Ralph Tate's enigma - and the regional significance of thrust faulting in the Mt Lofty Ranges. Geological Society ofAustralia, Abstracts 15,

MARLOW P.C. 1975. Structural investigations near Macclesfield, South Australia. M.Sc. thesis, University of Adelaide (unpubl.).

KLEEMAN A.W. & WHITE A.J.R. 1956. The structural geology of portion of the eastern Mt Lofty Ranges. Journal of the Geological Society of Australia 3, 17-31.

MARLOW P.C. & ETHERIDGE M.A. 1977. Development of a layered crenulation cleavage in mica schists of the Kanmantoo Group near Macclesfield, South Australia. Bulletin of the Geological Society of America 88, 873-882.

101.

394

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MATTER A. & HOMEWOOD P. 1980. Flysch and molasse of Western and Central Switzerland: Introduction. In Trumpy R. An Outline of the Geology of Switzerland. Guide Book, 26th International Geological Congress, Paris, pp.261-269. Wepf and Co., Basel. MILLS K.J. 1964. The structural petrology of an area east of Springton, South Australia. Ph.D. thesis, University of Adelaide (unpubl.).

PREISS W.V., RUTLAND R.W.R. & MURRELL B. 1981. The Stuart Shelf and Adelaide Geosyncline. In Hunter D.R. ed. Precambrian of the Southern Hemisphere, pp.327-354. Elsevier, Amsterdam. RAMBERG H. 1981. Gravity, Deformation and the Earth's Crust. Academic Press, London. RAMS AY J.G. 1967. Folding and Fracturing of Rocks. McGraw-Hill, New York.

MILLS K.J. 1973. The structural geology of the Warren National Park and the western portion of the Mount Crawford State Forest, South Australia. Transactions of the Royal Society of South Australia 97, 281-315.

RUTLAND R.W.R. 1976. Orogenic evolution of Australia. Earth Science Reviews 12,161-196.

MILNES A.R. 1973. The Encounter Bay Granites. Ph.D. thesis, University of Adelaide (unpubl.).

RUTTER E.H. 1976. The kinetics of rock deformation by pressure solution. Philosophical Transactions of the Royal Society of London, Series A 283, 203-219.

MILNES A.R., COMPSTON W. & DAILY B. 1977. Pre- to syn-tectonic emplacement of Early Palaeozoic granites in southeastern South Australia. Journal of the Geological Society of Australia 24, 87-106. OFFLER R. 1966. The structure and metamorphism of the Pewsey Vale area northeast of Williamstown, South Australia. Ph.D. thesis, University of Adelaide (unpubl.). OFFLER R. & FLEMING P.D. 1968. A synthesis of folding and metamorphism in the Mt Lofty Ranges, South Australia. Journal of the Geological Society of Australia 15, 245-266. PLATT J.P. & VISSERS R.L.M. 1980. Extensional structures in anisotropic rocks. Journal of Structural Geology 2,397-410. PLUMMER P.S. & GOSTIN V.A. 1976. Faulting contemporaneous with Umberatana Group sedimentation (Late Precambrian), southern Flinders Ranges, South Australia. Transactions of the Royal Society of South Australia 100, 29-37.

SPRIGG R.C. 1952. Sedimentation in the Adelaide Geosyncline and the formation of the continental terrace. In Glaessner M.F. & Rudd E.A. eds. Sir Douglas Mawson Anniversary Volume, pp. 153-159. University of Adelaide. SPRIGG R.C. & CAMPANA B. 1953. The age and facies of the Kanmantoo Group. Australian Journal of Science 16, 12-14. SPRY A.H. 1951. The Archean complex at Houghton, South Australia. Transactions of the Royal Society of South Australia 74,115-134. STEINHARDT C. 1987. Lack of porphyroblast rotation in non-coaxially deformed schists from Petrel Cove, South Australia, and its implications. Geological Society of Australia, Abstracts 19,102. STUART W.J. & VON SANDEN A.T. 1972. Palaeozoic history of the St Vincent Gulf region, South Australia. APEA Journal 12,9-16.

PREISS W.V. 1982. Supergroup classification in the Adelaide Geosyncline. Transactions of the Royal Society of South Australia 106, 81-83.

TALBOT J.L. 1963. Retrograde metamorphism of the Houghton Complex, South Australia. Transactions of the Royal Society of South Australia 87,185-196.

PREISS W.V. (Compiler) 1983. Adelaide Geosyncline and Stuart Shelf: Precambrian and Palaeozoic Geology (with special reference to the Adelaidean). 1:600 000 scale. Department of Mines and Energy, Adelaide.

TALBOT J.L. 1964. The structural geometry of rocks of the Torrens Group near Adelaide, South Australia. Journal of the Geological Society of Australia 11, 33-48.

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TALBOT J.L. & HOBBS B.E. 1968. The relationship of metamorphic differentiation to other structural features at three localities. Journal of Geology 76, 581-587.

WEBB A.W. 1976. Geochronology of the granitic rocks of southeastern South Australia. Report Nr.l 138, Australian Mineral Development Laboratories, Adelaide (unpubl.).

THOMSON B.P. 1964. Precambrian rock groups in the Adelaide Geosyncline: a new subdivision - general outline. Quarterly Geological Notes, Geological Survey of South Australia 9, 1-3.

WEBB A.W., THOMSON B.P., BLISSETT A.H., DALY S.J., FLINT R.B. & PARKER A.J. 1986. Geochronology of the Gawler Craton, South Australia. Australian Journal of Earth Sciences 33, 119-143.

THOMSON B.P. 1969. The Kanmantoo Group and Early Palaeozoic tectonics. In Parkin L.W. ed. Handbook of South Australian Geology, pp.97-108. Government Printer, Adelaide.

WENK E. & KELLER F. 1969. Isograde in Amphibolitserien der Zentralalpen. Schweizerische Mineralogische und Petrographische Mitteilungen 49, 157-198.

THOMSON B.P., DAILY B., COATS R.P. & FORBES B.G. 1976. Late Precambrian and Cambrian geology of the Adelaide "Geosyncline" and Stuart Shelf, South Australia. Excursion Guide 33A, 25th International Geological Congress, Sydney, Australia.

WHITE A.J.R. 1956. The granites and associated metamorphic rocks of Palmer, South Australia. Ph.D. thesis, University of London (unpubl.).

VEEVERS J.J. 1976. Early Phanerozoic events on and alongside the Australasian-Antarctic platform. Journal of the Geological Society ofAustralia 23,183-206. VON DER BORCH C.C. 1980. Evolution of Late Proterozoic to Early Palaeozoic Adelaide Foldbelt, Australia: comparisons with post-Permian rifts and passive margins. Tectonophysics 70, 115-134. WAGNER G.A., REIMER G.M. & JAGER E. 1977. Cooling ages derived by apatite fission-track, mica Rb-Sr and K-Ar dating: the uplift and cooling history of the Central Alps. Memorie degli Istituti di Geologia e Mineralogia dell'Universita di Padova 30.

WHITE A.J.R., COMPSTON, W. &KLEEMAN, A.W. 1967. The Palmer Granite - a study of a granite within a regional metamorphic environment. Journal of Petrology 8, 29-50. WHITE A.J.R., WILLIAMS I.S. & CHAPPELL B.W. 1976. The Jindabyne Thrust and its tectonic, physiographic and petrogenetic significance. Journal of the Geological Society ofAustralia 23,105-112. WINKLER H.G.F. 1979. Petrogenesis of Metamorphic Rocks, Fifth Edition. Springer-Verlag, New York. WOOD D.S. 1974. Current views of the development of slaty cleavage. Annual Reviews of Earth and Planetary Science 2, 369-401.

The Adelaide Fold Belt: Tectonic reappraisal R J.F. Jenkins Department of Geology and Geophysics, University of Adelaide, G.P.O. Box 498, Adelaide, 5001, Australia

SA.

The late Precambrian to Cambrian sedimentary prism of the ca.750 km long Adelaide Fold Belt of South Australia is readily explicable in terms of a model of cyclical lithospheric extension that was subsequently followed by a compressive phase. Volcanics in the Callanna Beds suggest significant stretching possibly with a p value >2. Apparent absence of volcanics in thick piles of Burra Group rocks suggests a value of fk2 for the associated extensional event. The thick' mixtites' of the older glaciation (Sturtian) clearly reflect basin instability and both of the two principal glacigenic episodes may be related to extensional cycles; formation of rift shoulders or thermal doming probably influenced refrigeration through the altitude effect. The link between glaciation and tectonic processes casts doubt on the utility of these cycles for inter-regional correlation. Renewed lithospheric attenuation or stretching characterised the start of the Ediacaran, and the Parara Limestone and Heatherdale Shale/ Kanmantoo Group record a closing extension in the Early Cambrian. (3 values apparently were buffered -2. Cambrian compressive deformation in the Mount Lofty Ranges generated upright to recumbent folds and an imbricate pattern of mainly southeasterly dipping thrust faults. Sub-horizontal, raft-like masses of the Stonyfell Quartzite enveloped by phyllites define a major thrust-faulted anticlinorium. The oblique intersection between the axis of the anticlinorium and the pattern of thrusts suggests that the inliers of basement gneiss and entire sedimentary prism represent an allochthon. Thermal metamorphism may have been related to early deformation in the possible thrust stack of the Kanmantoo Complex. The foreland basin lying within the approximate frame of the present Gulf St Vincent includes southerly derived molassic conglomerates on the north coast of Kangaroo Island. Variation of clast materials up-section may document unroofing of more internal parts of the tectogene. Trilobite biostratigraphy indicates that the molassic deposits date from the Bottomian, or late Early Cambrian, and suggests that the Delamerian Orogeny commenced earlier than previously suspected. Key words: Adelaide Fold Belt, tectonic reappraisal. INTRODUCTION Why do some areas of the Earth's crust that originally were shallowly sited, as shown by fluvial deposits or evaporites, come to receive great piles of sediment while analogous adjacent regions remain stable? The brilliant inductive reasoning of Hutton and Lyell pioneered modern ideas that sedimentation is episodic and cyclical, punctuated by great orogenies with attendant intrusive magmatism and metamorphism, the whole apparently driven by the internal heat engine of the Earth. However, explanations for the formation of sedimentary basins have remained imprecise and largely anecdotal until recently. Hall's concept of the geosyncline was the great 19th century breakthrough in describing the ar-

chitecture of major basins. A good part of the effort of modern marine exploration and a significant aspect of the plate-tectonic theory has been to focus on finding and understanding modern analogues of geosynclines. Attempts to mathematically model processes of basin formation, based on geophysical information and high temperature and pressure petrological experiments relating the interactive components of the mantle, lithosphere and crust, date only from the mid 1970's (e.g. Beaumont 1978; McKenzie etal 1980; Houseman & England 1986). The present study is a brief reappraisal of the Adelaide Fold Belt in terms of modern ideas of the development of complex major basins, and encompasses investigations of

The Adelaide Fold Belt: Tectonic Reappraisal sedimentation and of the major tectonic processes leading to the existing structural configuration. Necessarily this treatment will be incomplete in many aspects, not least because of the dimensions of this ca.750 km long basin and its tiered complexity. Limitations of space preclude historical treatment of ideas; a major overview and references are given by Preiss (1987).

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BASIN DEVELOPMENT There are three major categories of continental sedimentary basins, subcircular 'bull's eye' basins of enigmatic origin, foreland basins with downwarping induced by the load of an advancing thrust flake or thick wedges of sediment, and basins formed by lithospheric stretching (Dewey 1982; Dewey & Pitman 1986). This latter category by far predominates. Diffuse zones of lithospheric attenuation necessary for formation of stretch basins are intrinsically confined to continental lithosphere and the sialic crust. The olivine-rich petrology of oceanic rocks promotes brittle fracture rather than pervasive stretching.

of hot mantle material nearer to the surface. This in turn induces upward displacement of isotherms, heating the sedimentary pile accumulating in the surficial depression. Lateral migration of heat causes thermal expansion of the adjacent lithosphere and crust, leading to uplift of shoulders or escarpments at the margins of the zone of thinning. Cooling of the emplaced mantle material under lower environmental pressures creates new, strengthened lithosphere leading to "thermal sag" of the basin, and the thermal shoulders subside. Strengthening of the emplaced mantle material on cooling tends to buffer processes of lithospheric extension at (3 values near 2 (e.g. England 1983). At ordinary original crustal and lithospheric thicknesses (c ca.31.2 km, l ca.125 km) episodes of homogeneous stretching of (3 <ca.4 at geologically rapid strain rates (Jarvis & McKenzie 1980) engender patterns of sediment accumulation (thickness) following an asymptotic function. At values of p >4-6, physical disruption of lithosphere and crust commonly occurs with dyke injection and subsequent breakup of the continent (Dewey 1982).

Attempts to mathematically model basins of general 'rift' origin that satisfy the characteristics of sedimentation actually observed, include a theory of lithospheric attenuation developed by McKenzie (1978) and study of tectonic and thermal effects induced by rising mantle plumes (Houseman & England 1986; Houseman & Hegarty 1987). Massive dyke injection approaches conditions of total rifting (Royden et al 1980). In the McKenzie (1978) model, lithospheric stretching is related to geophysical parameters including the original thickness of the lithosphere, l , and original thickness of the crust, c , the inferred temperature at the mantle-lithosphere transition (1333°C) and the stretching factor induced in the zone of lithospheric thinning, P (McKenzie 1978; Dewey 1982; fig.2). With homogeneous stretching the thickness of the crust and lithosphere are each reduced by a factor of 1/p. Where c j \ <0.1375 uplift results (some old cratonic areas); in the more normal instance of Cz/lz >0.1375 subsidence results (Dewey 1982, fig. 13). Lithospheric thinning introduces a body

As the curve of subsidence induced by any particular uniform lithospheric stretching is predictable, it follows that by "backstripping" intervals of deposition of a basin and compensating for the isostatic depression induced by the weight increments of sediment, and compaction resulting from loading, a curve representing the depth to basement without a sediment load (driving subsidence curve) may be calculated and p estimated, assuming rapid strain rates (e.g. Dewey 1982; fig. 30). This approach is applicable to Phanerozoic basins where palaeontological studies in conjunction with chronostratigraphy give control of rates of sedimentation, but clearly is of little use for the Precambrian, where fossils are lacking and isotopic ages are commonly of limited reliability, because of imperfect geological information or assumptions concerning the provenance and/or permeability of dated sediments. However, studies of Phanerozoic basins might usefully be used as paradigms for patterns of sedimentation in the Precambrian (cf. McKenzie et al 1980).

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Of particular application in this respect is the finding that at stretching values of p>2 with strain rates exceeding thermal re-equilibration, volcanism is increasingly likely to accompany sedimentation because the lowering of the basin causes the confining weight of the lithosphere to fall below the pressure limiting generation of magma in the mantle (Dewey 1982, pp.391-392; fig. 21). Continental breakup is commonly associated with outpourings of flood basalts. Lithospheric extension leading to the formation of major basins is commonly polyphase and such regions may later localise zones of strikeslip and/or shortening deformation. In extreme instances, such as the Alps, thrust restacking is located along previously thinned continental crust (Dewey 1982). It is noteworthy that where deformation is concentrated along such zones of thinned crust a considerable amount of strain, including slaty cleavage development, may take place prior to the emergence of the deformed pile (Dewey 1982, p.400).

in the central and northern parts of the Fold Belt (Forbes etal 1981; Preiss 1987), and the halite and gypsum casts commonly present suggest deposition near base level. The Burra Group, also extremely thick in a large trough extending along the western central part of the Fold Belt, and in the northwestern Flinders Ranges and the Peake and Denison Ranges, consists of repeated sandstones and supratidal dolomites, and thick siltstones of shelf aspect. Such patterns of deposition are consistent with the rapid accumulation of paralic or shallow marine sediments during formation of intracratonic stretch-basins, the 'rift' systems of other workers (Rowlands etal 1980; Von der Borch 1980; Rutland etal 1981; Preiss 1987).

The cumulative thickness of the sediments within the Adelaide Fold Belt (Fig. 1) is difficult to estimate overall because the oldest parts of the succession are commonly sited in regions of complexity bounded by tectonic margins and may contain zones of attenuation with associated listric faulting (Dalgarno 1986) as well as thrusts that repeat parts of the sequence. The sedimentary pile included within the Kanmantoo Group possibly comprises a stack of thrust sheets. The simplified compilation of estimated thickness for the Proterozoic and Cambrian presented in Fig. 2 is based on the summary given in Rutland et al (1981).

The prevalence of volcanics in lower parts of the Callanna Beds and occurrences of monzonitesyenite-diorite and doleritic intrusives is suggestive of a relatively high degree of lithospheric attenuation. However the volcanics vaiy rapidly in thickness and are discontinuous, certainly not indicative of flood basalts. Thus the maximum degree of stretching was probably no greater than |3=ca.3. At a p value of ca.3, and assuming pure shear over a short interval, the known thickness of approximately 6 km or more of sediment comprising the Callanna Beds could have been deposited in a time as brief as ca.15 Ma (see Dewey 1982, fig. 14). At |3<2, deposition may have occurred over ca.40 Ma. These estimates are both less than the interval between a Rb-Sr age of 849±31 Ma (Cooper & Compston 1971) obtained from pre-Burra Group basement in the Mount Lofty Ranges and a U-Pb zircon date of 802+10 Ma (Fanning et al 1986) for a tuff in the Callanna Beds, Willouran Ranges. Complex disruption of the Callanna Beds may reflect early development of listric tensional faults (cf.Parker 1983) and their subsequent reactivation as thrusts during the Delamerian Orogeny.

Thick sections of Callanna Beds (so maintained by Brian Daily because the bottom and top of the sequence are not preserved in the type area of the Willouran Ranges) or approximate equivalents are developed in several local basins

The deep depressions that received Burra Group sediments probably formed as a result of a renewed episode of lithospheric attenuation. The apparent absence of volcanics suggests values of p<2, assuming pure shear, and unless

SEDIMENTATION OF THE ADELAIDE FOLD BELT

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Fig. 1 Locality Maps. a. "G2 gravity corridor" (e.g. O'Driscoll 1986). b. Interlinking of late Proterozoic-older Cambrian depositional centres via postulated Dundas-Flinders Shear; elements of Palaeozoic suture of Baillie (1985) lying to the east indicated for comparison, c. Tectonic zones of Adelaide Fold Belt after Rutland etal( 1981).

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Fig. 2 Stratotectonic cycles in Adelaide Fold Belt (modified after Jenkins & Gostin 1983). Cumulative thickness of stratigraphic units shown are largely from data given in Rutland etal (1981). Informal nomenclature of extensional cycles is based on names of Aboriginal tribes who inhabited areas of sub-basins or native words for geographical entities. Estimated values of (3 assume homogeneous strain at rates exceeding thermal re-equilibration.

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Fig. 3 Tectonic framework of the southern part of the Adelaide Fold Belt, Gulf St Vincent and Yorke Peninsula. Tectonic elements beneath Gulf St Vincent after Stuart & Von Sanden (1972). Insert shows occurrence of molassic rocks on the north coast of Kangaroo Island (after Dinnick 1985), and the area of the central Mount Lofty Ranges portrayed in Fig. 6 is outlined. the ca.7 km thickness of sediments in the major basin in the central part of the Fold Belt is compounded by repetition due to significant thrusts, the time represented by the deposition of the Group cannot be less than about 50 Ma. Early during the formation of the Burra Group prominent thermal shoulders marginal to basins were probably actively stripped to provide the widely distributed heavy-mineral-laminated, gritty sands and arkoses, and more restricted

pebbly arkoses so prevalent at this time. Such a shoulder evidently extended in a S.E.-N.E. direction through the region of the present Mount Lofty Ranges, where sandstones and conglomerates (Fleurieu Peninsula) or sandstones rich in heavy minerals onlap or overlie crystalline basement. A north-south ridge formed the western margin of the Southern Flinders Zone, where Rutland etal (1981) and Preiss (1987) indicate thick clastic wedges prograding

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outward migration of flexural hinge and disconformities

onlap as basin edge migrates evaporite/carbonate cap

extensional phase

Fig. 4 "Steer's Head" morphological characteristic of many basins resulting from lithospheric attenuation. After Dewey (1982).

eastward. The basement highs in the Mount Lofty Ranges terminate to the northeast near or at the margin of the "Gambier-Anson Bay Gravity Lineament" of O'Driscoll (1974) or "G2 gravity corridor" (e.g. O'Driscoll 1986; figs. 1 and 3) and the postulated high on the western side of the Southern Hinders Zone also flanked "G2". It seems possible that an early formed shear within the gravity corridor localised this particular depositional centre of the Burra Group (cf. O'Driscoll 1986). Distinctive intervals of the stratigraphy of the mid-part of the Burra Group, such as the Skillogalee Dolomite or its near equivalents, can be traced widely and thick shales probably representing marine deposition characterise higher aspects of the Group. Episodes of lithospheric extension promoting formation of deep faulted troughs are commonly followed by a longer phase or thermal subsidence or 'flexural phase' (Dewey 1982) resulting in progressive basin deepening and lateral onlap; cross-sections of the fill tend to show a 'steer's head' configuration (Fig. 4). In view of their wide distribution and indications of progressive deepening of the depositional regimen, mid and later parts of the Burra Group probably record such a 'thermal sag'. Based on the preceding discussion, postBurra Group sedimentation is likely to have begun some 65 Ma or more later than the

ca.800 Ma datum established in the Callanna Beds. Deposition of early 'tillitic' sediments and other deep basinal deposits (Yudnamutana Subgroup) of the Umberatana Group mark a further major episode of lithospheric stretching, with principal depositional centres in the northeastern Flinders Ranges and southern Nackara Arc. Syndepositional faulting occurred at both centres. Through the altitude effect, rising thermal shoulders or regional doming related to a mantle plume may have helped nucleate a ring of ice fields surrounding the basins, and glaciers cut through these rising lands, generating an abundant supply of tills that were widely rafted by bergs broken from floating ice fronts. Slumping of tills from the steepened sides of the subsiding basins generated flows which deposited as 'mixtite' - turbidites (cf. Preiss 1985, 1987, pp.132, 135, 138). These deposits and the comparable fill of late Proterozoic basins in southern, central Africa and elsewhere clearly reflect the tectonic influence that has so long puzzled Schermerhorn (1974, 1983) and others. Interglacials resulted in sediment starving with deposition of iron-rich black shales (e.g. Benda Siltstone of Nackara Arc). Minor volcanism is represented in an equivalent of the Sturt Formation (Hopton 1983). The wider distribution of subaqueous tills towards the end of the glacial epoch (Sturt Formation) indicates the transition from the active stretching phase into the time of thermal subsidence. Gradual lessening of glacial activity upsequence may be a reflection of subsidence of the

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Fig. 5 Hypothetical model for the formation of a submarine rift and its subsequent filling during the time of deposition of the Wonoka Formation. A deep seated low-angle slide induced within incompetent sediments by crustal movements localised at a depositional ramp may have led to rapid extension and thinning of overlying sediments and listric collapse of the uppermost beds. The resultant 'canyon' shows an initial fill with deepening upward facies and major slumps of wall materials; its subsequent filling is of progradational storm deposits and shelf carbonates.

surrounding thermal dome. Beds of dolomite became common as the climate ameliorated. The Tapley Hill Formation, the most widely transgressive unit in the tectogene, principally reflects the phase of thermal subsidence; lateral flexural bulges starved the basin of sediment; euxinic conditions developed widely at the commencement of deposition of thick shales that settled commonly at depths below storm wave base. The Brighton Limestone and equivalents, with bird's eye dolomites, ooids and stromatolites, mark the filling of the flexural trough. A complex pattern of subsidence and progradational filling characterises much of the sedimentation of the remainder of the Umberatana Group. Rb-Sr studies of shales date the 'thermal sag' phase of the cycle just described at 713±38 Ma (Compston etal 1987), and this seems in fair agreement with the limits that might be expected based on projection of inferred rates of subsidence from the ca.800 Ma datum in the Callanna Beds. Sediments of the Yerelina Subgroup, up to 1300 m thick and deposited in developing troughs in the northern Flinders Ranges and Nackara Arc, may indicate further lithospheric

extension. This interval comprises a major expression of the Elatina Glacigenic Cycle, and again it may be possible that rising thermal highs played a part in the onset of refrigerated conditions. Glacigenic sediments spread in a rather wide apron, with periglacial conditions present on adjacent lands (Williams & Tonkin 1985) and extending to sea level. Succeeding the glacigenic deposits, basinal shales and siltstones and packets of storm beds in the Ulupa Siltstone and Brachina Formation signify widening and deepening of the basin. The persistent presence of tuffaceous deposits in the Brachina Formation up to the base of the ABC Range Quartzite (e.g. Plummer 1978; N.M.Lemon pers. comm.) suggests values of |i>ca.2 or a little greater. The ABC Range Quartzite poured into a local trough in the southwestern Flinders Ranges and formed a wide apron, its feldspar-rich sands probably derived from the stripping of an adjacent thermal high. The widely distributed Bunyeroo Formation, consisting of silts deposited below wave base and rare slumped sands, indicates the thermal subsidence phase, with flexural bulges starving the basin of sediment. 4

A thin, persistent cupriferous dolomite ,the Wearing Dolomite' at the type section of Preiss

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(1987), indicates the base of the Wonoka Formation (Gostin & Jenkins 1983; Haines 1986) ; passage through storm deposits to turbidites and then basinal shales including localised euxinic intervals signals rapid basin subsidence marking the last major phase of lithospheric stretching during the Precambrian. This subsidence was confined to the Flinders Ranges. The region of the southern Mount Lofty Ranges remained stable, with minor erosion occurring. In the southern Flinders Ranges localised compression formed a coarse fracture cleavage in the Bunyeroo Formation prior to sedimentation of onlapping Wonoka Formation (Jenkins 1981). This episode of stretching was accompanied by deposition of sparse tuffs in the Wonoka Formation (Haines 1987). Basin filling followed the progradational pattern common in extensional basins. Early stages of deposition of the Wonoka Formation were coeval with the formation of widespread 'submarine' or 'erosional' canyons (Von der Borch et al 1985; Haines 1987; Eickhoff et al 1988). Attention might be given to the possibility of these structures being localised or formed through attenuation of underlying strata related to bedding plane slippages within incompetent beds in certain narrow geographical belts subject to unusual strain (Fig. 5). The observation of upward deepening sediment facies in certain 'canyons' (Gostin pers. comm. 1986; Eirckhoff et al 1988) is consistent with sagging of underlying strata and the uneven geometry so developed may go some way to explaining the reversing current directions recorded (e.g. Von der Borch et al 1985) in the most complete known 'canyon' complexes. Transgressive sediments of the Early Cambrian may be considered as indicating the thermal subsidence following the terminal Proterozoic extension. In the south, transition to a euxinic shale facies (Heatherdale Shale) and a thick turbidite succession (Carrickalinga Head Formation) are suggested to mark renewed lithospheric attenuation. This is substantiated by the intimate association of the Truro Volcanics with Early Cambrian sediments (? Fork Tree Lime-

stone, Heatherdale Shale and ? lower Kanmantoo Group; Forbes et al 1972) in the northwestern part of the Kanmantoo 'Trough' (Parker 1986). The conocoryphid trilobite recently discovered high in the Heatherdale Shale (Jago et al 1984) may be referable to the genus Ivshiniellus Korobov, 1966, described from the mid Early Cambrian of Siberia (Jenkins & Hasenohr 1989). The Kanmantoo Group includes major turbidite fans that prograded towards the east or southeast (Flint 1978; Boord 1985), sulphide rich euxinic facies, and subtidal sandstones (Backstairs Passage Formation, Middleton Sandstone). The limited occurrence of volcanics in the Early Cambrian suggests values of (3>2, assuming pure strain. Lack of fossils in the Kanmantoo Group and evident differences in sedimentary settings outside the region of its type area preclude any certain correlation with sequences in the rest of the Fold Belt. As the cycles of sedimentation herein linked to phases of lithospheric attenuation may reflect fundamental processes influencing the local history of the crust, an informal nomenclature is presented for the convenience of future discussion (Fig. 2).

COMPRESSIVE CYCLE Tectonic movements that followed the phase of sedimentation just described culminated in the regional folding and metamorphism of the Cambro-Ordovician Delamerian Orogeny (e.g. Thomson 1970; Preiss 1987). The writer suggests that the major shear broadly located within the 'G2 gravity corridor' and partly responsible for the great'S'-shaped form of structural trends in the southern half of the Fold Belt (O'Driscoll 1968; Sprigg 1984; Preiss 1987), extended further south and north. This Dundas-Flinders Shear (Fig. 1) probably gave rise to the line of weakness that localised the rifting of the Tasmanian region from the Antarctic Plate during the Mesozoic and likely incorporates the 'Torrens Hinge Zone' of Thomson (1970). The ca.1075 Ma old Beda Volcanics, centred on southern Lake Torrens and the head of Spencer

The Adelaide Fold Belt: Tectonic Reappraisal

Gulf in a north-south alignment (Fanning et al 1983), may mark the instigation of the DundasFlinders Shear, and its episodic movements possibly localised areas of extensional tectonics during depositional phases of the Fold Belt. A tensional model has recently been proposed for late Precambrian to Cambrian deposition and related volcanism in western Tasmania (Varne & Foden 1987). The movement of the Shear during the Delamerian Orogeny was evidently in a sinistral sense. The complex deformational history of the Flinders Ranges requires considerable additional study. The interfering pattern of open folds in the central Flinders Ranges has been linked to an inferred relatively shallow depth to basement (Rutland et al 1981) and differing directions of compressive movements (Preiss 1987). While an arcuate, generally southward directed pattern of thrust faulting may be present in the northern Flinders Ranges, actual location of shallow dipping thrusts has proved elusive, unless the major elongate 'diapirs' of Callanna Beds are considered to represent thrust-faulted anticlines (e.g. W. Nyman, pers. comm. 1986). If this is so, the associated thrusts presumably tail into bedding plane faults in the adjacent sediments. The study of Scotford (1984) emphasized sinistral displacements of early formed folds along an important set of NE-SW trending faults in the central northern region and he considered that late stress was directed from the northeast, the same conclusion Nyman (pers. comm. 1986) reached from detailed study of the Wirrealpa Diapir. Intense compressive deformation with thrusting and localised high grades of metamorphism characterises the central Mount Lofty Ranges, and Fleurieu Arc/Kangaroo Island. The intrinsic enigma posed by the structural architecture of this region was recognised more than a century ago by Tate (1879) who noted that in respect of the huge cumulative thickness of sediments present in the Mount Lofty Ranges "it is remarkable that the apparently less metamorphosed strata occupy the lowest position". Seismic studies in the region (Stewart 1972; Shackleford & Sutton 1981;

405

Singh 1985) do not substantiate a deep crustal root (nor is there any large gravity anomaly) and some of the older parts of the sedimentary pile, the upper interval of the central belt of Burra Group rocks arcing from the Yankakilla area to the vicinity of Wakefield-Hoyleton, show the greatest degree of deformation. Howchin (1918, p.362-363) recognised that rocks of the central Mount Lofty Ranges comprise a great antiform and this is apparent on modern maps. Although the axis of this antiform does not plunge uniformly, on a regional basis it shows an overall shallow plunge towards the south. The great antiform is intersected by numerous thrust faults (Howchin 1904,1906, 1918; Sprigg 1946, 1984; Mills 1973) dipping to the east or southeastward (Figs. 3 and 6). Presumably a major fault separates the deformed rocks of the Fold Belt from nearly flat-lying Cambrian sediments inferred from seismic studies to underly Gulf St Vincent (Stuart & von Sanden 1972). Further important southeasterly dipping thrusts are encountered extending through the Coromandel Valley, the Clarendon area, Kangarilla and Meadows. The dislocations between Coromandel Valley and Kangarilla displace a major marker for the great antiform, the Stonyfell Quartzite; in the hill near Adelaide, huge masses of the Stonyfell Quartzite are repeatedly cut by shear zones and displaced, showing dextral strike separation (Fig. 6). Actual outcrops of the shears are rare in the vicinity of the Stonyfell Quartzite, but where seen, comprise foliated rock with tight and disrupted small folds showing attenuated limbs parallel to the cleavage. In one such zone at Scott Bottom, the foliation dips 22°SE. Sprigg (1946) described the quartzite masses as being displaced by 'repeated dip faulting' and occurring as 'isolated "slabs" (...or "islands"...) surrounded by slates and limestones'. Away from the quartzite the shears are commonly manifest by foliated and disrupted phyllites with anastomosing and sigmoid shaped zones of cleavage enveloping remnants of deformed sandstone and quartzite with attenuated tails in the direction of major shear. Foliated rocks with rotated bedding-

RJ.F. Jenkins

406

Shear zone Plunging antiform

E o

o N o

cc O

oc Q_

BELAIR SUBGROUP

I c | CAINOZOIC

BURRA GROUP (Undifferentiated)

| P [PERMIAN

Stonyfell Quartzite

Aldgate Sandstone Basement

^ < O CL LU I<-

Plunging synform Major axis of anticlinorium

KANMANTOO GROUP Nairne Pyrite NORMANVILLE GROUP

s\s] MARINOAN-

Thrust fault ^ ""

Fault inferred

^ S T U R T I A N 7 7 ^ Sturt Formation

Fig. 6 Pattern of faulting in the southern Mount Lofty Ranges based on Department of Mines and Energy 1:50, 000 geological map sheets of Adelaide, Onkaparinga, Noarlunga and Echunga and the author's observations of parts of the fault system.

The Adelaide Fold Belt: Tectonic Reappraisal cleavage intersections form lineations in SE-NW directions. Quartz segregations are common. One such shear exposed in the excavation for a house near the elbow of Valley Road, Teringie, dips approximately 8°SE. Near Clarendon, shears commonly dip 30-34°SE, though foliation in chloritic phyllites in the main street of the town dips about 20°SE. The shear passing near Kan-

g BELAIR O SUBGP UMBERATANA GP

KANMANTOO COMPLEX

407

garilla dips at 16-20°SE, while "Palaeozoic" shear planes associated with the WilliamstownMeadows Fault, where it is exposed several kilometres north of Echunga, dip 48-55°E. Study of the thrust-faulted Stony fell Quartzite discloses that the shear bounded western edges of blocks tend to describe antiforms, the main body

\ \ \ \x\ \ \

^ g <

Fig. 7 Interpretive geological section of the Mount Lofty Ranges between Marino and Bletchley, based on Department of Mines and Energy 1:50,000 map sheets of Noarlunga and Echunga and measured attitude of major thrust faults. Construction assumes a SS W declination of the upper and lower boundaries of the Stonyfell Quartzite at 11.5-12°. Individual thrusts may actually comprise zones of shearing up to ca.l km wide. Note disharmonic styles of deformation between lower Burra Group and Belair Subgroup/Umberatana Group. Relationship between older Palaeozoic thrusts and late Phanerozoic faults (hachured) is enigmatic.

408

R.J.F. Jenkins

of the mass is subhorizontal and eastern margins tend to be synformal, with trailing 'tails' in vertical section. Such a configuration characterises thrust-faulted anticlinoria (e.g. Dahlstrom 1969; fig. 5); the sense of movement is towards the northwest. Where shears intersect the crystalline basement cores of the Ranges, they may steepen appreciably to form east dipping reverse faults, as near Carey Gully, or still dip relatively shallowly as in the vicinity of the mouth of the Congeratinga River and joining Anacotilla River, where at least three thrusts inferred to dip at 35-4CTSE repeat the unconformity between crystalline basement and pebbly sandstones of the overlying Burra Group (cf. P.R.James; fig. 20 in Hardy 1986). The shears and thrusts just described are not to be confused with the post-Mesozoic faults that have generated the present escarpments of the Ranges. These younger faults are commonly seen to be subvertical, and may be marked by quartz segregations, or more usually, shattered and fractured rock. Using Department of Mines and Energy 1:50, 000 geological map sheets of Adelaide, Noarlunga and Echunga, mapped faults which the writer recognises as thrusts were projected on to an arbitrary transect from Marino to Meadows. Intersections between foliation and bedding show that the regional folds plunge ca.lO-12°SSW in this area. By projecting lower and upper surfaces of the Stonyfell Quartzite down the line of plunge to the reference transect, and utilizing other available geological data, the form of the major thrustfaulted anticlinorium can be constructed as shown in Fig. 7. Phyllitic rocks of the Burra Group in the western Mount Lofty Ranges between Gawler and Willunga show a prominent foliation dipping ca. 10-40° SE and commonly a parallel compositional layering that is sometimes indicated as bedding on maps. Fainter relict layering representing the true bedding reveals recumbent folds with the foliation axial plane; lower limbs are

commonly sheared. Because of inversion of fold limbs and shearing, these rocks do not form a continuous stratigraphic succession. Moreover, there do not appear to be any exposures where so called 'Torrensian' rocks (Mawson & Sprigg 1950; Thomas 1964) can be traced continuously into the Belair Subgroup (cf. Barnes & Kleeman 1934). The style of deformation in the Belair Subgroup between Belair and Eden Hills comprises inclined similar folds and higher parts of the succession show tight to open inclined folds; a continuous stratigraphy is evident. Thus a disharmonic relationship apparently exists between 'Torrensian' rocks and the Belair Subgroup/Sturtian and a possible explanation is that this constitutes a major early thrust surface that has been refolded to envelop the core of the major anticlinorium. This postulated 'roof thrust' has been cut subsequently by further movement of thrusts propagating from below; it possibly corresponds to the Williamstown-Meadows Fault in the east (Fig. 7) and the southerly branch of the latter extending near Mt Magnificent (Fig. 6). On a regional scale, thrusts and major faults form an imbricate pattern within the central region of the Mount Lofty Ranges (Fig. 6). The major anticlinal axis of the Ranges (indicated as a double trace in Figs. 2 and 6) is intersected by the imbricate thrust pattern at acute angles. Individual fault slices or 'horses' (Butler 1982) in major thrust belts commonly show additive individual movement in 'transfer zones' (e.g. Dahlstrom 1969; fig. 12); because of the maintenance of relative volume in individual horses all parts of thrust complexes move by a similar amount. The observation that the trace of the major antiform of the Ranges approximates a simple curve subject to only minor offset of individual thrusts indicates such a transfer zone, implying that the basement cores and entire sedimentary prism are allochthonous, and have presumably moved northwestward above an underlying master-thrust or decollement. The necessary constraints of stratigraphic thickness and fault attitude involved in the construction of a cross-section of the Ranges show that the decollement is possibly located between depths of 8

CP > CP1 p

% | c c C1 CHLORITE ZONE |

| BIOTITE ZONE

1 SILLIMANITE ZONE

s ttf

ANDALUSITE-STAUROLITE ZONE 1^7;^ MIGMATITE

///

Dundas-Flinders

Shear

MDALUSITE

CHLORITOID

STAUROLITE 8 KYANITE (DEEP SEATED NAPPES S MASSIFS) iDIOPSIDE 8 CALCITE , 1 SILLIMANITE 8 FIBROLITE

^ ^ ADUjjA. N A P P E ^

/AAR MASSIF

Fig. 8 a. Block diagram of Mount Lofty Ranges, Fleurieu Arc and vicinity showing interpretive geological sections and regional metamorphism (view southward, frontal zone schematic). Metamorphic zonation after Offler & Fleming ( 1 9 6 8 ) ; seismic section below Gulf St Vincent based on data of Stuart & Von Sanden (1972). b. Composite section of Swiss Alps and attendant metamorphism compiled from Schweizerishe Geologische Kommission (1980).

o B. o"

CP £

410

R .J.F. Jenkins

and 10 km (Figs. 7 and 8). The simplest possible geometry of the central western Mount Lofty Ranges constitutes a "leading imbricate fan" in the thrust terminology of Boyer & Elliott (1982), or if the 'Torrensian'/Belair Subgroup contact zone and Meadows Fault are considered as parts of a major roof thrust, a "hindward dipping duplex" is represented. A "weakly emergent thrust front" seems to have been present in the classification of Morley (1986; fig. 9A).

KANMANTOO COMPLEX The writer follows Daily & Milnes (1971) in considering that the Kanmantoo Complex of the eastern Ranges is bounded on its western side by a major fault. Early Cambrian metasandstones and marbles cropping out in the vicinity of this fault show a discontinuous distribution and occasional masses of vein quartz are present (Paris Creek). South of Yankalilla Hill, rocks of the Kanmantoo Group are thrust over crystalline basement; here the fault dips ca.30° SE and sigmoidal shapes outlined by the foliation define minor duplexes. The fault has been folded after its formation (Macclesfield; Mariow 1975, pp.45-52), indicating it to be one of the earliest major structural elements of the Ranges and reentrants, such as near Ashbourne, possibly indicate amalgamation of thrust sheets. Though the easterly occurrence of Kanmantoo Group rocks evidently comprises a great synform, it is suggested that the apparent thickness of the Group may be greatly augmented by unmapped thrusts repeating parts of the sequence. Exposure is commonly rather poor. The thrusts may be indicated by zones of phyllites with bedding parallel foliation. This being so, attempts to apply stratigraphic nomenclature in different parts of the Kanmantoo synform may be hazardous. In the vicinity of Mt Pleasant and immediately east, an improbable succcession forms a large synformal structure:

(c) Rathjen Gneiss - granitic gneiss with penetrative fabric. (b) Metasandstone with pegmatite sills and veins. (a) Layered muscovite-biotite gneiss with ptygmatic pegmatites. The foliation of the muscovite-biotite gneiss is folded parallel to the presumed 'bounding surface' of the synform. The penetrative fabric of the granitoid comprising the Rathjen Gneiss implies intense strain, and the foliation defines a closed synform (steeper dipping on the eastern side), indicating that the present thickness of the gneiss mass is perhaps about 0.8 km. However, the sandwiched metasediments show open folds suggesting relatively low strain. This is the classical allochthonous situation of intensely deformed crystalline rocks occurring above sediments. The Rathjen Gneiss may represent acidic metavolcanics (White 1966; Fleming & White 1984), but certainly is an unusual lithology in respect of the Kanmantoo Group (but not perhaps some of the other granitoids). I advance the notion that it is part of a thrust sheet and may represent a flake derived from the crystalline basement. As there is no obvious origin for such a flake within more than 5 km, the presumed thrust stack may be considered as a nappe complex.

PROPAGATION OF THRUSTS Traced to the north, the major thrust faults of the Mount Lofty Ranges reach or overlap the inferred Dundas-Flinders Shear at acute angles, and most likely initiated as Riedels as the cover deformed in response to the presumed sinistral movements of the underlying basement shear. The faults probably propagated southwards and may have been rotated into flatter orientations as increasingly intense compressive movements from the southeast generated flake tectonics. A pattern of divergent faults in the Kanmantoo Complex to the north of Palmer probably constitutes a late formed 'flower structure' resem-

The Adelaide Fold Belt: Tectonic Reappraisal

411

bling patterns of dislocation such as Emmons (1969) demonstrated in sandbox experiments.

to a classical ensialic thrust-mountain belt such as the Alps (Fig. 8).

METAMORPHISM

FORELAND BASIN AND MOLASSIC DEPOSITS

Traversing west to east across the central Mount Lofty Ranges, an increase in metamorphic grade occurs at each major structural boundary; little-deformed Proterozoic/Cambrian sediments evidently occur below the floor of Gulf St Vincent (Stuart & von Sanden 1972); at Marino, chlorite grade is encountered, east of Coromandel Valley the biotite grade is present (with injection of minor pegmatites), east of the Meadows Fault the grade is andalusite-staurolite, and in the Kanmantoo Complex the sillimanite grade is developed (Offler & Fleming 1968). Thus thrusting evidently followed a major thermal event towards the southeast and the individual thrust sheets were emplaced 'hot'. Support for this is lent by structural investigations in the Palmer area indicating partial melting and emplacements of migmatite veins before and after the first folding event (Fleming & White 1984). Indeed, perhaps the earliest emplaced thrust sheet represented by the single outlier of the Rathjen Gneiss, may have been a hot slab of basement that was subsequently carried northwestward piggy-back above the northeastern aspect of the developing thrust complex. This may have acted as a 'thermal lid' engendering localised high metamorphic grades (kyanite-sillimanite). In contrast, the deepest, and probably younger, parts of the thrust stack, were well removed and well insulated from the overlying sheets of heated rock, and show a relatively low degree of metamorphism (incipient biotite grade), but high strain manifest by sheared recumbent folds. Limited exposures of the internal parts of the Fold Belt revealed by inliers in the Tertiary cover of the Murray Basin disclose a north-south trending zone of mylonite near the mouth of the Mame River (Wegmann 1980) and indicate the presence of mafic rocks (Black Hill). It is interesting to note the placement of these elements in relation

While there are repeated claims that the spectacular conglomerates in upper parts of the Kangaroo Island Group on the north coast of Kangaroo Island (Figs. 3 and 9) were derived from a northern source (Daily etal 1979; Daily etal 1980), I consider that there is stronger evidence that these sediments mainly came from the south, eroded off near parts of the Island itself. One line of evidence advanced as supporting transport of the conglomerates from the north is the imbrication of cobbles in some beds of the White Point Conglomerate (Daily etal 1980). However, cobbles are sometimes aligned along faint, large scale crossbedding, presumably indicating an opposite flow direction. New mapping at Cape D'Estaing (Dinnick 1985) indicates that all the major conglomerates there are within the White Point Conglomerate. Evidence for a general southeasterly or southerly origin for the probable mud-flow deposits in the White Point Conglomerate are: (i)

At Cape D'Estaing a number of massive conglomerate beds thin towards the north and tail off towards marine shale and sandstone southwestward; conglomerates can be demonstrated to intertongue into probable marine sandstones towards the west, towards the north and towards the east.

(ii) Near Cape D 'Estaing and west of the mouth of Big Gully massive beds of conglomerate repeatedly show abrupt facies changes (over distances of only 2 or 3 m) into crossbedded coarse grits, in a northerly direction. West of Big Gully the boundary demarcating such a facies change has been measured tending 87° true, and the margin of a channel sand abruptly truncating the northern part of a conglomerate bed trends at 112° true. Crossbedding of the grits, presumably reflecting reworking by tidal currents (Daily

412

R J.F. Jenkins

et al 1980) indicates flow towards the west, between ca.240° and ca.310° true, (iii) West of Big Gully storm beds associated with the top of the White Point Conglomerate show internal crossbedding indicating northward flow, and presumably deposition on a slope in that direction. Sets of crossbedding in sandstones of adjacent paralic facies also commonly indicate northward flow. Conglomerates in the Boxing Bay Formation east of White Point are associated with festoon crossbedding showing a mean trend of ca.315° true. However at Point Marsden, crossbedding

o ^ O n

O0 ° 0

At Bald Rock, conglomeratic sandstones emplaced above the White Point Conglomerate over either a reactivation unconformity or a thrust fault include abundant well rounded cobbles of metapsammites resembling lithologies of the Kanmantoo Group. Crossbedding in channel deposits shows northward flow and climbing ripples in a rather flat thrust sheet further to the west also indicate northward flow. Leaving aside precarious lithological correlations based on occurrences of certain conglomerates (Daily et al 1979, pp. 19-20), there FLINDERS RANGES

FLEURIEU PENINSULA

NORTH COAST KANGAROO ISLAND

within conglomerates indicates transport from the north.

Delamerian Orogeny

(5S,

GRINDSTONE

RANGE

BALD ROCK BEDS

PANTAPINNA SANDSTONE

BOXING BAY FM (UPPER)

EMU BAY S H A L E '

• REDLICHIA

CONGLOMERATE

00 DC

MOODLATANNA

ANADOXIDES

WIRREALPA LIMESTONE

ESTAINGIA

BALCORACANIA

" U SMITH BAY S H A L E "

BALCORACANA

BILLY

CREEK

BALCORACANIA

Kangarooian Movement REDLICHIA

LOWER KANGAROO IS. GROUP

—

REDLICHIA

—

-Co I 0

cf. ATOPS KOOTENIA CALODISCUS PAGE TIDES

HEATHERDALE

NORMANVILLE GROUP

cf.

IVSHINIELLUS WILKAWILLINA LIMESTONE PARACHILNA

URATANNA

Fig. 9 Approximate stratigraphic equivalence of Cambrian rocks of north coast of Kangaroo Island, Fleurieu Peninsula and Flinders Ranges suggested in present paper, and the possible relationship of these rocks to the beginning of compressive tectonics, the Kangarooian Movement and subsequent Delamerian Orogeny. Occurrences of trilobites potentially useful for biostratigraphic purposes are indicated.

The Adelaide Fold Belt: Tectonic Reappraisal

seems no firm evidence to link the Kangaroo Island Group with the turbidite dominated facies of the Kanmantoo Group. West of Cape D'Estaing a steep, south-dipping reverse fault occurs between rocks of the 'Lower' and 'Upper' Kangaroo Island Group. The Snelling Fault (steeply dipping 50-62°S) bounds the large area of Kanmantoo Group rocks to the south. It is suggested that the 'Upper' Kangaroo Island Group, together with equivalents on Yorke Peninsula and presumed correlatives below Gulf St Vincent comprise a molassic apron within a foreland basin that experienced basement faulting and thence subsided through flexure loading due to advance of a thrust front from the south and southeast during the Delamerian Orogeny. Percentages of different lithologies in clasts occurring within the 'Upper' Kangaroo Island Group change up section (Dinnick 1985); dolomites, limestones and archaeocyathal limestones with rare gneiss, porphyry and granitoids are present in the White Point Conglomerate, dolomites, foliated granites and schist occur in the Boxing Bay Formation, and quartzite, metapsammitic cobbles and granites are common in beds at Bald Rock. This may be interpreted as indicating progressive unroofing of the tectogene involving erosion of Early Cambrian limestone and dolomite from a former shelf that probably extended through present northern Kangaroo Island (cf. Daily et al 1979, p. 12), exposure of the basement or an axial basement core with a geology comparable to the older rocks of Yorke Peninsula, and the subsequent beginning of downcutting of an advancing thrust front compromising metamorphised Kanmantoo Complex. Low-angle thrusts intersect the molassic deposits at the eastern end of Emu Bay.

TIMING OF LATE SEDIMENTATION AND THE START OF OROGENESIS In terms of general ideas of tectonics, the flyschoid Kanmantoo Group likely predates the molassic rocks on the north coast of Kangaroo Island (Fig. 9). It is noteworthy that in the Kan-

413

mantoo Group and presumed older parts of the Kangaroo Island Group the dominant direction of sediment transportation appears to have been towards the east (Flint 1978; Moore 1983). In higher parts of the Kangaroo Island Group (Smith Bay Shale and above) the dominant direction of current transportation is diametrically opposite, towards the west, suggesting a complete alteration of sediment provenance and basin geometry. If the White Point Conglomerate and Emu Bay Shale are late Early Cambrian as has been considered (Pocock 1970; Glaessner 1979), then there is seemingly only a rather short time available for deposition of the Kanmantoo Group and initiation of thrusting. In a zone of thinned lithosphere appreciable compressive shortening, deformation of the covering sedimentary prism and metamorphism may occur prior to emergence of the tectogene above sea level (Dewey 1982). Perhaps in part the Kanmantoo Group is synorogenic. Though there remains considerable uncertainty as to the timing of the closing phases of sedimentation in the Fold Belt in relation to the beginning of orogenesis, trilobite biostratigraphy sheds some light in helping to elucidate these questions. A new conocoryphid trilobite in the upper part of the Parara Limestone in the western Flinders Ranges (Jago et al 1986; Fig. 9) seems to be a near ally of the Early Cambrian genus Atops Emmons 1844. It occurs in association with Kootenia Walcott, 1888, Calodiscus (Serrodiscus) daedalus (Opik 1975), Meniscuchus Opik, 1975, and Page tides Rasetti 1945, an assemblage suggesting near correlation with beds at Opik's (1975b) site 'A' in the late Early Cambrian Cymbric Vale Formation of the Mootwingee Ranges, western New South Wales. Pagetides cf. minitus Rasetti 1945, Neocobboldia Rasetti, 1952, another pagetiid affJukonides Fritz 1972, and a new species of emuellid occur in nodules in the succeeding Oraparinna Shale. Kootenia, Pagetides and Yukonides are elements of the Nevadella Zone in the mid Early Cambrian of western North America (Fritz 1972). The Oraparinna Shale probably equates with older

414

R.J.F. Jenkins

parts of the Bottomian on the Siberian platform, perhaps overlapping the Bergeroniellus micmacciformis - Erbiella and the Bergeroniellus gurarii zones. The White Point Conglomerate and the Billy Creek Beds, which are disconformable or paraconformable above the Oraparinna Shale (Daily 1976), both include Balcoracannia Pocock, 1970, but the species represented differ. This suggests but does not fully substantiate a possible correlation. Redlichia Cossmann, 1902, occurs commonly in both the 'Lower' and 'Upper' Kangaroo Island Group (Daily et al 1979; Moore 1983) ; the trilobite-bearing Emu Bay Shale is now recognised as being disconformable above the White Point Conglomerate (Dinnick 1985). Estaingia bilobata Pocock, 1964, from the Emu Bay Shale is not the same taxon as Opik (1975b) described from the Cymbric Vale Formation under the same name, the two differing in that the former has a relatively narrower glabella and appreciably wider preglabellar field. Opik (1975a, b) allied Estaingia with the Middle Cambrian Xystridura Whitehouse, 1936. In the Flinders Ranges Anadoxides Matthew 1839, occurs in the archaeocyatha bearing Wirrealpa Limestone (Jell 1983) and above in the Moodlatana Formation, closely paralleling its distribution with and above archaeocyatha in Sardinia (Rasetti 1972; Brasier 1976), where its occurrence probably equates with the Sanashtygol horizon (Bottomian) of the Altay-Sayan region, Siberia. Hundreds of metres (ca.750 m) of unfossiliferous carbonates separate the Anadoxides levels in Sardinia from subsequent Middle Cambrian trilobite assemblages, the oldest of which may approximate the Paradoxides hicksi zone of the Atlantic region. In terms of the biostratigraphic comparisons just outlined, all of the archaeocyathid limestones and surface occurrences of trilobites in the Flinders Ranges probably predate the St David's Epoch or classical Middle Cambrian, and the balance of evidence suggests that the White Point Conglomerate and Emu Bay Shale are Bottomian.

The problem of the apparently short time interval available for the deposition of the Kanmantoo Group is largely obviated if the phase of rapid deepening that led to the accumulation of this clastic sequence is equated with the basinal subsidence responsible for the substantial thickness of sediments of the Hawker Group in the northeastern central Flinders Ranges, particularly the offshore (dark-nodular) facies of the Parara Limestone, and the Midwerta Shale, Nepabunna Siltstone, Bunkers Sandstone, Oraparinna Shale and Narina Grey wacke, the cumulative thickness of these units being as much as ca.4 km. The lateral facies changes so characteristic of this interval (Daily 1976) may be only part of the expression of more profound changes in the depositional regime of the Fold Belt. Recent U-Pb zircon and sphene ages of granites and metamorphics in the Armorican Massif of Brittany, France, suggest it is unlikely that the transgressive Early Cambrian of Western Europe is older than about 540 Ma (Peucatt 1986). ARb-Sr whole rock age of 526±25 Ma for the Rathjen Gneiss (Milnes et al 1977) is consistent with beginning of unroofing of the southern part of the Adelaide Fold Belt during the late Early Cambrian, but it is difficult to evaluate the statistical significance of this number in relation to the Rb-Sr ages of 511±3 - 496±6 Ma (Milnes etal 1977) for other synorogenic intrusives emplaced during the Delamerian tectogenesis.

DISCUSSION The depositional centres formed as a result of lithospheric stretching during the history of the Adelaide Fold Belt varied in configuration and placement. Newly emplaced mantle material below dilational zones presumably cooled and hardened, strengthening the lithosphere, so that with resumption of tension the reactive zones of weakness tended to shift laterally. Each instance of lithospheric stretching was attended by clastic sedimentation, probably as a result of erosion of rising rift shoulders. During the oldest cycles this clastic input was terrestrial (Callanna Beds) or

The Adelaide Fold Belt: Tectonic Reappraisal paralic (Burra Group). The extraordinarily deep basins formed during the Sturtian were filled by complex mixtites derived partly from glacigenic sources and partly from intrabasinal slumping and reworking. This tectonic or flyschoid aspect also characterises the Ulupa Siltstone, intervals of the Wonoka Formation and parts of the Kanmantoo Group. In contrast, the 'thermal sag' or flexural phases of basin development are marked by more widely transgressive sediments formed seemingly in quiet marine regimes. Widening flexural shoulders deflected drainage; grainsize of clastic input became much finer, the basins tended to become starved or euxinic, and carbonate regimes commonly began to predominate. This 'thermal sag' aspect is represented by the carbonates and upper shales of the Burra Group, the Tapley Hill Formation and succeeding carbonates (Brighton Limestone), and the widespread Bunyeroo Formation. In contrast, the thermal subsidence phase of the Ediacaran was more progradational, with paralic and shallow marine sands or silt. The Early Cambrian cycle was interrupted by the instigation of congressional movements during the Bottomian. It is noteworthy that the suggested phases of lithospheric stretching and thermal subsidence correspond closely with the major lithostratigraphic divisions recognised by Mawson & Sprigg (1950) and Sprigg (1952) or timerock intervals of Thomson (1964) and subsequent workers (Fig. 2). This suggests that the currently perceived major stratal divisions of the Fold Belt are close to natural breaks in terms of the controlling mechanisms of sedimentation. Exceptions to this are the recognition of the start of a tensional cycle at the base of the Yerelina Subgroup and the major stretching phase within the Early Cambrian. The base of the Cambrian seems to lie in a 'thermal sag' phase, with perhaps a eustatic overprint. As tectonic processes evidently have a continuous aspect over different parts of the Earth's crust, the finding that the glacigenic cycles of the Adelaidean are linked closely to processes of basin formation, a relationship now equally evi-

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dent in North America (Grant Young pers. comm. 1987), suggests that utility of such glacial events for interregional correlation is open to question. Since the nominate base of the Ediacaran is similarly defined by a tectonic event, a comparable criticism may be levelled. However, it seems reasonably apparent that this division is broadly underpinned biostratigraphically by the early radiation of animal life. The observation of the complexity of the closing tectogenic phases of the Fold Belt cements its focus for geological interest within the wider framework of the development of the Australian continent, and provides a new example of a deeply eroded sector of a convergent alpine regime. ACKNOWLEDGEMENTS Brian Daily fostered the writer's early schoolboy interests in geology and later as a teacher, provided a model for observation of geological phenomena in the field. As a colleague and companion he will be long remembered for adventures in the Flinders Ranges and central Australia. Drs V.A.Gostin and RRJames are acknowledged for joint excursions, discussion and help with literature. I am grateful for the constructive comments of an anonymous reviewer and especially thank Dr M.Sandiford for his considerable help in improving the manuscript. Professor L.A.Frakes arranged funding necessary for attendance at the lecture series of Professor J.F.Dewey, Durham, England, and Dr W.C.Pitman, LamontDoherty, New York, held in April, 1986 at the Australian Mineral Foundation, Adelaide, and their Workshop is complemented for its depth and stimulation. Dr Hartmut and Sabine Krietemeyer are warmly thanked for their tour of the Austrian Alps. Meiyl Parker helped research the figures and Mrs. Sherry Proferes assisted with drafting. Funding from ESSO Australia Ltd. aided in supporting this study.

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REFERENCES BAILLIE P.W. 1985. A Palaeozoic suture in eastern Gondwanaland. Tectonics 4, 653-660. BARNES T.A. & KLEEMAN A.W. 1934. The Blue Metal Limestone and its associated beds. Transactions of the Royal Society of South Australia 58, 80-85. BEAUMONT C. 1978. The evolution of sedimentary basins on a viscoelastic lithosphere: theory and examples. Geophysical Journal of the Royal Astronomical Society 55,471-497. BOORD R.A. 1985. Sedimentation of the Cambrian, upper Kanmantoo Group, southern Fleurieu Peninsula, South Australia. B.Sc.(Hons) thesis, University of Adelaide (unpubl.). BOYER S.E. & ELLIOTT D. 1982. Thrust systems. American Association of Petroleum Geologists Bulletin 66, 1196-1230. BRASIER M.D. 1976. An archaeocyathid-trilobite association in Sardinia and its stratigraphic significance. Rivista Italiana Di Paleontologia E Stratigrafia 82, 267-278. BUTLER R.H.W. 1982. The terminology of structures in thrust belts. Journal of Structural Geology 4, 239-245. COMPSTON W., WILLIAMS, I.S., JENKINS R.J.F., GOSTIN V.A. & HAINES P.W. 1987. Zircon age evidence for the Late Precambrian Acraman ejecta blanket. Australian Journal of Earth Sciences 34, 435-445. COOPER J.A. & COMPSTON W. 1971. Rb-Sr dating within the Houghton Inlier, South Australia. Australian Journal of Earth Sciences 17, 213-219. DAHLSTROM C.D.A. 1969. Balanced cross sections. Canadian Journal of Earth Sciences 6,743-757. DAILY B. 1976. The Cambrian of the Hinders Ranges. 25th International Geological Congress Excursion Guide 33A, 15-19.

DAILY B., FIRMAN J.B., FORBES B.G. & LINDSAY J.M. 1976. Geology. In Twidale C.R., Tyler M.J. & Webb B.P. eds. Natural History of the Adelaide Region, pp.5-42. Royal Society of South Australia, Adelaide. DAILY B. & MILNES A.R. 1971. Stratigraphic notes on Lower Cambrian fossiliferous metasediments between Campbell Creek and Tunkalilla Beach in the type section of the Kanmantoo Group, Fleurieu Peninsula, South Australia. Transactions of the Royal Society of South Australia 95,199-214. DAILY B., MILNES A.R., TWIDALE C.R. & BOURNE J.A. 1979. Geology and geomorphology. In Tyler M.J., Twidale C.R. & Ling J.K. eds. Natural History of Kangaroo Island, pp. 1-38. Royal Society of South Australia, Adelaide. DAILY B., MOORE P.S. & RUST B.R. 1980. Terrestrial-marine transition in the Cambrian rocks of Kangaroo Island, South Australia. Sedimentology 27, 379-399. DALGARNO C.R. 1986. Syndepositional faults, Adelaide Geosyncline. Geological Society of Australia, Abstracts 15, 52. DEWEY J.F. 1982. Plate tectonics and the evidence of the British Isles. Journal of the Geological Society of London 139, 371-412. DEWEY J.F. & PITMAN W.C. 1986. The Origin and Evolution of Sedimentary Basins. Workshop course, Adelaide, 3rd-5th April, 1986. Australian Mineral Foundation, Adelaide. DINNICK P.M. 1985. Stratigraphy, sedimentology and palaeontology of a Cambrian molassic sequence, Cape D'Estaing to Point Marsden, North East Coast of Kangaroo Island, South Australia. B.Sc.(Hons) thesis, University of Adelaide (unpubl.). EICKHOFF K.H., VON DER BORCH C.C. & GRADY A.E. 1988. Proterozoic canyons of the Flinders Ranges (South Australia): submarine canyons or drowned river valleys? Sedimentary Geology 58, 217-235. EMMONS R.C. 1969. Strike-slip rupture patterns in sand models. Tectonophysics 7, 71-87.

The Adelaide Fold Belt: Tectonic Reappraisal ENGLAND P.C. 1983. Constraints on extension of continental lithosphere. Journal of Geophysical Research B, 88, 1145-1152. FANNING C.M., FLINT R.B. & PREISS W.V. 1983. Geochronology of the Pandurra Formation. Quarterly Geological Notes, Geological Survey of South Australia 88, 11-16. FANNING C.M., LUDWIG K.R., FORBES B.G. & PREISS W.V. 1986. Single and multiple grain U-Pb zircon analyses for the early Adelaidean Rook Tuff, Willouran Ranges, South Australia. Geological Society ofAustralia, Abstracts 15, 71-72.

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HAINES P.W. 1987. Carbonate shelf and basin sedimentation, Late Proterozoic Wonoka Formation, South Australia. Ph.D.Thesis, University of Adelaide (unpubl.). HARDY B. 1986. Normanville to Anacotilla Beach. In Hasenohr P. & Corbett D. eds. A Field Guide to the Coastal Geology of Fleurieu Peninsula, Chapter 6, pp.63-72. The Field Geology Club of South Australia Inc., Adelaide. HOPTON D.L. 1983. Environmental analyses of the late Precambrian Appila Tillite equivalent at Depot Flat, southern Flinders Ranges, South Australia. B.Sc.(Hons) thesis, University of Adelaide (unpubl.).

FLEMING P.D. & WHITE A.J.R. 1984. Relationships between deformation and partial melting in the Palmer migmatites, South Australia. Australian Journal of Earth Sciences 31,351-360.

HOUSEMAN G.A. & ENGLAND P.C. 1986. A dynamical model of lithosphere extension and sedimentary basin formation. Journal of Geophysical Research B, 91, 719-729.

FLINT D.J. 1978. Deep sea fan sedimentation of the Kanmantoo Group, Kangaroo Island. Transactions of the Royal Society of South Australia 102, 203-222.

HOUSEMAN G.A. & HEGARTY K.A. 1987. Did rifting on Australia's southern margin result from tectonic uplift. Tectonics 6, 515-527.

FORBES B.G., MURRELLB. & PREISS W.V. 1981. Subdivision of lower Adelaidean, Willouran Ranges. Quarterly Geological Notes, Geological Survey of South Australia 79, 7-16.

HOWCHIN W. 1904. The geology of the Mount Lofty Ranges, Part I - The coastal district. Transactions and Proceedings ofthe Royal Society ofSouth Australia 28, 253-280.

FRITZ W.H. 1972. Lower Cambrian trilobites from the Sekwi Formation type section, Mackenzie Mountains, northwestern Canada. Geological Survey of Canada, Bulletin 212,90pp.

HOWCHIN W. 1906. The geology of the Mount Lofty Ranges, Part II. Transactions and Proceedings of the Royal Society of South Australia 30,227-262.

GLAESSNER M.F. 1979. Lower Cambrian crustacea and annelid worms from Kangaroo Island, South Australia. Alcheringa 3, 21-31.

HOWCHIN W. 1918. The Geology of South Australia. Education Department, Adelaide, 543pp.

GOSTIN V.A. & JENKINS R.J.F. 1983. Sedimentation of the Early Ediacaran, Flinders Ranges, South Australia. Geological Society ofAustralia, Abstracts 9, 196-197.

HAINES P.W. 1986. The upper Wilpena Group at Bunyeroo Gorge. In Dalgarno C.R. ed. Proterozoic to Cambrian sedimentary environments and resources potential, Flinders Ranges. Eighth Australian Geological Convention, February, 1986, pp.20-26. Geological Society of Australia, Adelaide.

JAGO J.B., GEHLING J.G. & DAILY B. 1986. Cambrian sediments of the Sellick Hill-Carrickalinga Head area, Fleurieu Peninsula, South Australia. In Parker A.J.ed. One Day Geological Excursions of the Adelaide Region. Eighth Australian Geological Convention February 1986, pp.67-81. Geological Society of Australia, Adelaide.

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JARVIS G.T. & McKENZIE D.R 1980. The development of sedimentary basins with finite extension rates. Earth and Planetary Science Letters 48,42-52. JELL RA. 1983. The Early to Middle Cambrian boundary in Australia. Geological Society of Australia, Abstracts 9, 236. JENKINS R.J.R 1981. The concept of an "Ediacaran Period" and its stratigraphic significance in Australia. Transactions of the Royal Society of South Australia 105,179-194. JENKINS R.J.F. 1986. Ralph Tate's enigma - and the regional significance of thrust faulting in the Mt Lofty Ranges. Geological Society ofAustralia, Abstracts 15,

MOORE P.S. 1983. Geological Fieldguide to the Northeast Coast of Kangaroo Island. Australasian Sedimentologists Group, Adelaide. MORELY C.K. 1986. A classification of thrust fronts. American Association of Petroleum Geologists Bulletin 70,12-25. O'DRISCOLL E.S.T. 1968. Notes on the structure of the Broken Hill lode, and its tectonic setting. Australasian Institute of Mining and Metallurgy, Monographs 3, 97-102. O'DRISCOLL E.S.T. 1974. Basement tectonics and fold patterns in the Australian continental structure. Australasian Institute of Mining and Metallurgy, Bulletin 376-377,35-36.

101.

JENKINS R.J.F. & GOSTIN V.A. 1983. Marinoan and Ediacaran type sections in the context of tectonic cycles in the Adelaide Geosyncline. Geological Society of Australia, Abstracts 10, 39-44. JENKINS R.J.F. & HASENOHR P. 1989. Trilobites and their trails in a black shale, Early Cambrian of the Fleurieu Peninsula. Transactions of the Royal Society of South Australia 113 (in press). MAWSON D. & SPRIGG R.C. 1950. Subdivision of the Adelaide System. Australian Journal of Science 13, 69-72.

O'DRISCOLL E.S.T. 1986. Observations of the lineament-ore relation. Philosophical Transactions of the Royal Society of London A, 317, 195-218. OFFLER R. & FLEMING P.D. 1968. A synthesis of folding and metamorphism in the Mt Lofty Ranges, South Australia. Journal of the Geological Society of Australia 15, 245-266. OPIK A.A. 1975a. Templetonian and Ordian Xystridurid trilobites of Australia. Bureau of Mineral Resources, Geology and Geophysics, Bulletin 121, 84pp.

McKENZIE D. 1978. Some remarks on the development of sedimentary basins. Earth and Planetary Science Letters 40,25-32.

OPIK A.A. 1975b. Cymbric Vale Fauna of New South Wales and Early Cambrian biostratigraphy. Bureau of Mineral Resources, Geology and Geophysics, Bulletin 159,78pp.

McKENZIE D., NISBET E. & SCLATER J.G. 1980. Sedimentary basin development in the Archaean. Earth and Planetary Science Letters 48, 35-41.

PARKER A.J. 1983. Tectonic development of the Adelaide Fold Belt. Geological Society of Australia, Abstracts 10,23-28.

PARKER A.J. 1986. Tectonic development and metallogeny of the Kanmantoo Trough in South Australia. Ore Geology Reviews 1,203-212.

MILNES A.R., COMPSTON W. & DAILY B. 1977. Pre- to syntectonic emplacement of early Palaeozoic granites in south-eastern South Australia. Journal of the Geological Society of Australia 24, 87-106.

PEUCAT J.J. 1986. Behaviour of Rb-Sr whole rock and U-Pb zircon systems during partial melting as shown in migmatitic gneisses from the St Malo Massif, RE. Brittany, France. Journal of the Geological Society of London 143, 281-315.

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419

PLUMMER RS. 1978. Evidence of volcanism contemporaneous with Adelaidean sedimentation. Quarterly Geological Notes, Geological Survey of South Australia 68, 15-19.

SCHERMERHORN L.J.G. 1983. Late Proterozoic glaciation in the light of CO2 depletion in the atmosphere. Geological Society of America, Memoir 161, 309-315.

POCOCK K.J. 1970. The Emuellidae, a new family of trilobites from the Lower Cambrian of South Australia. Palaeontology 13, 522-562.

SCHWEIZERISHE GEOLOGISCHE {COMMISSION 1980. Geology of Switzerland. Parts A &B. Wepf & Co., Basel, 334pp.

PREISS W.V. 1983. Depositional and tectonic contrasts between Burra Group and Umberatana Group sedimentation. Geological Society of Australia, Abstracts 9, 13-16.

SCOTFORD G.L. 1984. Sedimentation of late Proterozoic sediments with syn-depositional diapirisim, and Delamerian thrust faulting, Warraweena, Northern Flinders Ranges, South Australia. B.Sc.(Hons) thesis, University of Adelaide (unpubl.).

PREISS W.V. 1985. Stratigraphy and tectonics of the Worumba Anticline and associated intrusive breccias. Geological Survey of South Australia, Bulletin 52, 85pp. PREISS W.V. (Compiler) 1987. The Adelaide Geosyncline - late Proterozoic stratigraphy sedimentation, palaeontology and tectonics. Geological Survey of South Australia, Bulletin 53, 438pp. RASETTI F. 1972. Cambrian trilobite faunas of Sardinia. Atti Delia Accademia Nazxionale Dei Lincei Memorie 11, 100pp. ROWLANDS N.J., BLIGHT P.G., JARVIS D.M. & VON DER BORCH C.C. 1980. Sabkha and playa environments in late Proterozoic grabens, Willouran Ranges, South Australia. Journal of the Geological Society ofAustralia 27,55-68. ROYDEN L., SCLATER J.G. & VON HERZEN R.P. 1980. Continental margin subsidence and heat flow important parameters in formation of petroleum hydrocarbons. American Association of Petroleum Geologists Bulletin 64, 173-187. RUTLAND R.W.R., PARKER A.J., PITT G.M., PREISS W.V. & MURRELLB. 1981. The Precambrian of South Australia. In Hunter D.R. ed. Precambrian of the Southern Hemisphere, pp.309-360. Elsevier, Amsterdam. SCHERMERHORN L.J.G. 1974. Late Precambrian mixtites: glacial and/or nonglacial? American Journal of Science 274, 673-824.

SHACKLEFORD P.R.J. & SUTTON D J. 1981. A first interpretation of crustal structure in the Adelaide Geosyncline in South Australia using quarry blasts. Journal of the Geological Society of Australia 28, 491-500. SINGH R. 1985. Seismicity and crustal structure of South Australia. M.Sc.thesis, Flinders University (unpubl.). SPRIGG R.C. 1946. Reconnaissance geological survey of portion of the western escarpment of the Mount Lofry Ranges. Transactions of the Royal Society of South Australia 70, 313-347. SPRIGG R.C. 1952. Sedimentation in the Adelaide Geosyncline and the formation of the continental terrane. In Glaessner M.F. & Sprigg R.C. eds. Sir Douglas Mawson Anniversary Volume, pp. 153-159. University of Adelaide. SPRIGG R.C. 1984. Arkaroola-Mount Painter in the Northern Flinders Ranges, SA: The Last Billion Years. Arkaroola Pty. Ltd., Adelaide. STEWART I.C.F. 1972. Seismic interpretation of crustal structure in the Flinders-Mt Lofty Ranges and Gulf regions, South Australia. Journal of the Geological Society of Australia 19, 351-362. STUART W.J. & VON SANDEN A.T. 1972. Palaeozoic history of the St Vincent Gulf Region, South Australia. APE A Journal 12, 9-16.

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TALBOT J.L. 1964. The structural geometry of rocks of the Torrens Group near Adelaide, South Australia. Journal of the Geological Society of Australia 11, 33-48. TATE R. 1879. The anniversary address of the President. Transactions and Proceedings of the Philosophical Society, Adelaide, South Australia 1878-9. THOMSON B.P. 1964. General outline. Quarterly Geological Notes, Geological Survey of South Australia 9, 1-3. THOMSON B.P. 1970. A review of the Precambrian and lower Palaeozoic tectonics of South Australia. Transactions of the Royal Society of South Australia 94,193-221. VARNE R. & FODEN J.D. 1987. Tectonic setting of Cambrian rifting, volcanism and ophiolite formation in western Tasmania. Tectonophysics 140,275-295. YON DER BORCH C.C. 1980. Evolution of late Proterozoic Adelaide Fold Belt, Australia; comparisons with post-Permian rifts and passive margins. Tectonophysics 70,115-134.

VON DER BORCH C.C., GRADY A.E., ALDAN R., MILLER D., NEUMANN R., ROVIRA A. & EICKHOFF A. 1985. A large-scale meandering submarine canyon: outcrop example from the late Proterozoic Adelaide Geosyncline, South Australia. Sedimentology 32, 507-518. WEGMANN D. 1980. Pre-Tertiary geology of the Black Hill Region in the Western Murray Basin of South Australia; with special emphasis on the petrology and geochemistry of the gabbroic rocks. B.Sc.(Hons) thesis, University of Adelaide (unpubl.). WHITE A.J.R. 1966. Petrology and structure of the Rathjen Granite Gneiss of the Palmer Region, South Australia. Journal of the Geological Society of Australia 13,471-489. WILLIAMS G.E. & TONKIN D.G. 1985. Periglacial structures and palaeoclimatic significance of a late Precambrian block field in the Cattle Grid copper mine, Mount Gunson, South Australia. Australian Journal of Earth Sciences 32, 287-300.

The Encounter Bay Granites, Fleurieu Peninsula and Kangaroo Island A.R. Milnes CSIRO Division of Soils, Private Bag No. 2, Glen Osmond, SA 5064, Australia. The field relationships, petrology, mineral chemistry, petrochemistry and Sr isotope composition of the Encounter Bay Granites are described from detailed studies at Encounter Bay (Fleurieu Peninsula) and Cape Willoughby (Kangaroo Island). The granites were emplaced into the Kanmantoo Group in a mobile, crystal-rich condition between 506-515 Ma ago, resulting in the formation of megacrystic granites. A border facies littered with fragments of Kanmantoo Group country rock contains a fine-grained granophyric groundmass produced by rapid cooling or a sudden loss of volatiles. An inner facies has a coarse-grained groundmass free of granophyric intergrowths: it appears to have cooled more slowly in environments away from the influence of the country rock, and contains pegmatitic masses indicating the retention of volatiles. Features including a wide variety of inclusions, accumulations of potash feldspar megacrysts, and mineral layering, provide clues to conditions in the crystallising magma and aspects of its pre-emplacement history. Subordinate granite varieties include fine and medium granites, red leucogranites, miarolitic granophyres and hybrid granites. Late-stage alteration produced widespread albitites and less common greisens. Major element analyses of whole-rock samples of the granite varieties delineate a trend which largely reflects contamination of an original magma by metasedimentary rock. The contamination is also reflected in the composition of biotites from the various granite types: increasing contamination is registered by an increase in FeO + MgO in total rocks and by a decrease in the Fe/(Fe+Mg) ratio of constituent biotites. The Encounter Bay Granites probably originated from a water-deficient magma generated by partial melting of crustal rocks. They were emplaced at relatively shallow depths in the crust, based on considerations of the structural and metamorphic environment. Key words: Encounter Bay Granites, Encounter Bay, Cape Willoughby, field relationships, petrology, mineral chemistry, petrochemistry, Sr isotope composition.

INTRODUCTION The Encounter Bay Granites, which crop out along the coast of Encounter Bay on southern Fleurieu Peninsula, and at Cape Willoughby on the eastern end of Kangaroo Island, intruded the youngest formations of the Early Cambrian Kanmantoo Group during the Early Palaeozoic Delamerian Orogeny (Daily & Milnes 1973; Milnes etal 1977; Daily et al 1979). Their contact with Kanmantoo Group metasedimentary rocks at Rosetta Head in Encounter Bay (Daily & Milnes 1973; Milnes etal 1977) and Cape Willoughby (Daily etal 1979), may mark part of the northwestern wall of an extensive granite mass.

Early work on the Encounter Bay Granites included a brief petrological description (Moulden 1895) and chemical analyses of potash feldspar megacrysts (Gartrell 1903). Tilley (1919a, b) described the field relationships and petrological features of the granites at Cape Willoughby while Browne (1920) conducted similar studies in Encounter Bay. Mawson (1926) summarised earlier investigations of the Encounter Bay Granites in the context of a brief report on the igneous rocks of South Australia. The socalled "dioritic" varieties of the granites on Granite Island in Encounter Bay were described by Kleeman (1937), who considered them various stages in the assimilation of metasedimentary rock xenoliths. Bowes (1954) investigated granites and associated metasedi-

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ments at Rosetta Head and later (Bowes 1959) described the granites at Port Elliot. In addition, there are various unpublished studies of jointing (Asthana 1958), accessory minerals (Fander 1960) and potash feldspar megacrysts (Slade 1962).

syntectonic emplacement of granites followed by overprinting by regional folding and metamorphism. Fleming & White (1984) have since reported evidence in migmatite terrains in the metamorphic belt of thermal activity before penetrative deformation.

More recent studies have been concerned with the field relationships, petrology and geochemistry of the Encounter Bay Granites (Milnes 1973), their stratigraphic and metamorphic environment (Daily & Milnes 1971a, b; 1972a, b; 1973; Daily et al 1979), and radiogenic isotope data for the granites and adjacent Kanmantoo Group metasediments as a means for determining their age (Dasch et al 1971; Milnes etal 1977). This work has indicated that the granites concordantly intruded the youngest formations of the Kanmantoo Group prior to the culmination of the first phase of folding and associated schistosity development during the Delamerian Orogeny. In fact, a reappraisal of radiometric data for other granitic rocks in southeastern South Australia led Milnes (1973) and Milnes etal (1977) to argue that the whole orogen may have been characterised by pre- to

This paper deals with the characteristics and field relationships of the Encounter Bay Granites as seen in the coastal cliff sections at Encounter Bay and Cape Willoughby, and with penological, mineralogical and geochemical data for the various granite facies. The work was completed in the early 1970's when Brian Daily and the author were actively engaged in studies of the Kanmantoo Group, and so some of the concepts and conclusions may be at variance with contemporary ideas about granites. Nevertheless, it was on the basis of some of these observations and data that arguments relating to the timing of granite emplacement and the nature of the metamorphic environment were made in earlier papers (Daily & Milnes 1973; Milnes et al 1977; Daily etal 1979). The focus of this paper is the description and interpretation of the character and associations of the various granite facies.

contiguous Kanmantoo Group metasediments.

The Encounter Bay Granites

THE GRANITES AT ENCOUNTER BAY Megacrystic granite _

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also occur within the megacrystic granite and, with the exception of the hybrid granites, are generally less than 1 m in diameter.

The major granite variety is a coarsegrained , non-schistose granite containing large, ovoid potash feldspar megaciysts which invariably enclose fine-grained, randomly or zonally arranged inclusions of plagioclase, quartz and biotite. Many potash feldspar megacrysts are mantled by plagioclase as in "wiborgite" rapakivi granites (Wahl 1925). Opalescent blue quartz is distinctive as subhedral megacrysts. Plagioclase occurs as medium to coarse subhedral megacrysts, and biotite is present as mediumgrained laths and clusters of finer crystals. The megacrystic granite contains a large variety of xenoliths. The most conspicuous and abundant of these are unaltered fragments of metasedimentary rocks, many of which can be reliably identified as fragments of Kanmantoo Group metasediments. Such xenoliths vary in size from small chips several millimetres in diameter to very large blocks tens of metres in length, and are invariably disoriented with respect to the structural fabric of the contiguous country rock. They are fine-grained and biotiterich, and commonly have well-defined bedding laminations and small-scale sedimentary structures. The preferred orientation of micas is significantly less well developed in the xenoliths than in countiy rock Kanmantoo Group metasediments, and in some is not discernible at all. This observation forms one important aspect of the argument that the granites were emplaced and incorporated blocks of Kanmantoo Group metasediments prior to penetrative structural deformation in the regional environment. Xenoliths of hornfelses, hybrid granites, finegrained granites and granophyric leucogranites

Types of megacrystic granite The megaciystic granite can be subdivided into two types on the basis of texture and the distribution of xenoliths. The dominant type occurs on West Island, Rosetta Head, Wright Island, Granite Island and in the vicinity of Green Bay at Port Elliot, and is considered to be a border facies. It is littered with metasedimentary rock xenoliths (Fig. 2), and has a bimodal grain size distribution such that coarsegrained feldspar, quartz, and less commonly biotite crystals, are set in a fine-grained granophyric groundmass. Pyrite and pyrrhotite are relatively abundant in parts of the granite.

Fig. 2. Dark-coloured xenoliths of metasedimentary rock within border facies megacrystic granite in small quarry adjacent to road on northern side of Granite Island. Spectacles 13 cm long.

^Classification and nomenclature generally follows the scheme proposed by Streckeisen (1967). **Coarse is the term used for granites and their minerals with an average grain size greater than 10 mm. Medium and fine are the terms used to describe granites and their minerals with average grain sizes between 2 mm - 10 mm, and less than 2 mm, respectively. ***The term granophyric is used according to the definition of Barker (1970).

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Potash feldspar megacrysts consist of an euhedral core sharply bounded by a zone of granophyric quartz-feldspar intergrowth grading outwards into a rim which appears to be continuous with the interstitial groundmass potash feldspar. Such structures are like those found in potash feldspar phenocrysts in porphyries and were recognised by both Browne (1920) and Bowes (1954). The granophyric zones probably mark changes consequent upon rapid cooling, or a sudden loss of water vapour, or a combination of these conditions, during emplacement of the granite into its present environment. Subsequent crystallisation would have resulted in the formation of the outer zones of the megacrysts and the groundmass minerals, and culminated in the crystallisation of interstitial potash feldspar. The euhedral cores of many megacrysts have concentric zones of fine-grained plagioclase and biotite inclusions, as well as randomly-disposed inclusions of plagioclase, biotite and quartz that are significantly finer than the same minerals in the groundmass of the granite. The zonal arrangement of inclusions is usually considered to be a feature of magmatic growth of the megacrysts as phenocrysts (Schermerhorn 1956; Hibbard 1965; Kerrick 1968). X-ray diffraction data show that the megacrysts contain both triclinic and monoclinic or near monoclinic phases. Of those megacrysts examined in detail (Milnes 1973), several have orthoclase cores grading to intermediate and maximum microcline rims. Others appear to have an erratic and non-systematic distribution of structural states. Potash feldspar in the groundmass and around the margins of the megacrysts is generally maximum microcline. The presence of orthoclase in the euhedral megacryst cores is consistent with a magmatic origin, and its preservation reflects relatively rapid cooling and crystallisation, in the absence of volatiles, from temperatures at least as high as 375°C (Steiger & Hart 1967; Wright 1967). The megacrysts are typically perthitic, and exhibit one or more of the film-, vein- or patch-perthite textures. Plagioclase rims on potash feldspar megacrysts may consist of several plagioclase subhedra or a single plagioclase crystal, and are of variable width.

Plagioclase megacrysts are commonly subhedral, and have pronounced oscillatory zoning. The cores of the megacrysts have a composition of about An45 and may be extensively saussuritized. Many oscillatorily zoned plagioclase crystals are untwinned, or exhibit only very fine lamellar twinning, and the intermediate zones of such crystals contain bleb-like and vermicular quartz in myrmekitic intergrowths. The opalescent blue colour of quartz is particularly distinctive, both in subhedral to euhedral megacrysts and in fine to medium-grained groundmass crystals. Quartz megacrysts display concentric zonation of the blue colour, a feature initially recorded by Browne (1920). The zoning

Fig. 3. Megacryst of quartz in border facies megacrystic granite 4-55 showing zonal arrangement of blue colour parallel to euhedral margins of crystal. Crystal 1 cm long.

The Encounter Bay Granites

is occasionally seen in crystals in outcrops and is especially clear in cut and polished surfaces of hand specimens (Fig. 3), but is not normally visible in thin sections. It is regarded as a growth phenomenon, but the cause of the colour zoning is not known. Border facies granite textures are well displayed in exposures near Green Bay at Port Elliot (Fig. 4), where there are also abundant diffuse schlieren of light coloured granite of variable grain size. The schlieren commonly encompass metasediment xenoliths, and small tourmalinerich pegmatite clots and pods, and may represent the products of inhomogeneous crystallisation due to variations in water content. The character of the border facies megacrystic granite suggests that it was emplaced in a mobile, yet crystal-rich condition while being simultaneously contaminated by stoped fragments of country rock Kanmantoo Group metasediments. Subsequent crystallisation

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resulted in the formation of the fine-grained groundmass typical of this facies. The second type of megacrystic granite is exposed to the east of Green Bay at Port Elliot, and is regarded as an inner facies. It is characterised by abundant circular, clot- or pod-like pegmatite bodies consisting predominantly of tourmaline and quartz, but with minor constituents including muscovite, feldspar, apatite, and rarely pyrite. There are also inclusions of fine-grained granites and granophyric leucogranites. It can be distinguished from the border facies by the absence of granophyric border zones around potash feldspar megacrysts and by the lack of a fine-grained groundmass. Moreover, potash feldspar megacrysts do not contain orthoclase. Macro-modal analyses (Fig. 5; data in Milnes 1973) of the two granite varieties indicate differences in the proportions of the major minerals. Abundant pyrrhotite and pyrite with some marcasite occur in abundance in parts of the inner facies granite. Some schist-like inclusions contain powellite (CaMoCU). The con-

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Fig. 5. Triangular diagram showing modal compositions of granite facies in terms of quartz, plagioclase and potash feldspar.

tact between the two varieties of megacrystic granite (about 40 m east of Green Bay; Fig. 4) appears to be a southwesterly-trending fault along which there has been considerable alteration. The inner facies megacrystic granite is thought to have cooled slowly and crystallised without the influence of stoped fragments of Kanmantoo Group country rock, and was possibly shielded from the country rock by the border facies granite. The pegmatites indicate a concentration of late-stage components (particularly volatiles). Other inclusions (horafelses, hybrid granites and various leucogranites) also occur in the border facies granite and represent processes or events in a homogeneous parent granite magma prior to its emplacement.

Other features

Fig. 6. Concentration of coarse potash feldspar megacrysts around hybrid granite xenoliths in border facies megacrystic granite near breakwater on Granite Island. Note dark masses of biotite in places between megacrysts. Pen 14 cm long.

Zones in which there are concentrations of potash feldspar megacrysts occur in many parts of the border facies granite. The megacrysts are generally clustered against groups of hornfels and hybrid granite xenoliths, and vary in shape from subhedral and euhedral to rounded (Fig. 6). They are considered to represent zones of entrapment and accumulation of early-formed potash feldspar crystals during movement of the xenoliths through an essentially liquid magma. The general paucity of quartz and plagioclase in such zones, coupled with the large size of the potash feldspar megacrysts, suggests that potash feldspar dominated the solid phases for a significant time during the early crystallisation history of the magma. Plagioclase, quartz and biotite inclusions in potash feldspar megacrysts were clearly crystallising at the same time, but the fine size of the inclusions compared with the same minerals in the groundmass of the granite suggests that the major deposition of these minerals post-dated the formation of potash feldspar megacrysts. Several examples of biotite layering have been identified in both varieties of the megacrys-

The Encounter Bay Granites

tic granite. These zones are usually less than 3 m in length and generally contain planar biotite-rich layers alternating with quartzo-feldspathic layers in which potash feldspar megacrysts may be concentrated (Fig. 7). In one locality on Granite Island a series of concentric biotite layers alternate with quartzo-feldspathic layers packed with potash feldspar megacrysts (Fig. 8). Mineral layering of this type in granites has been attributed to partial assimilation of country rock xenoliths (Pitcher & Read 1958), variations in water vapour pressure and subsequent crystal accumulation governed by density contrast (Harry & Emeleus 1960; Emeleus 1963; Bateman etal 1963; Claxton 1968; Coats & Wilson 1971), and intense differential movement (Coats & Wilson 1971) in a crystallising magma. Some layered zones delineated in the megacrystic granites have features that are consistent with gravity settling

427

of early-formed biotite crystals. However, because of their relatively low specific gravity, it is unlikely that the coarse-grained potash feldspar megacrysts concentrated in bands in some of the layered zones are magmatic cumulates. Perhaps there was an effect of magmatic currents to concentrate and segregate the feldspar megacrysts and the biotite, and to produce biotite-rich schlieren in parts of the crystallising granite. A poorly developed preferred orientation of biotite crystals, and in rare cases, feldspar crystals, occurs in places in both varieties of the megacrystic granite. The layering appears to be variable in orientation but is difficult to detect in many outcrops, and no measurements have been attempted. Fine and medium, even-grained granites Pink to buff and grey coloured, medium, even-grained granites occur adjacent to the bor-

Fig. 7. Zone of biotite layering in border facies megacrystic granite near breakwater on Granite Island. Arrow shows concentration of coarse potash feldspar megacrysts in light coloured bands between biotite layers. Note patches of high biotite concentration each end of layered zone, and contiguous metasediment xenolith.

Fig. 8. Spiral-shaped zone of biotite layering in border facies megacrystic granite on coast west of breakwater, Granite Island.

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der facies megacrystic granite at Knights Beach and the inner facies megacrystic granite on the coastline about 320 m north of Commodore Point at Port Elliot (Fig. 4). Their contact with the border facies megacrystic granite is gradational over about 1 m whereas that with the inner facies granite is about 30 m wide. The medium grained granites contain mainly microcline and blue quartz, with plagioclase (the average composition of the unaltered parts of several crystals is about An27) and minor biotite. In some samples, post-ciystallisation deformation is indicated by subgrains along microcline boundaries and by a marked subgrain development in quartz crystals. Abundant pyrrhotite occurs in some patches of the medium-grained granite northeast of Commodore Point. These granites appear to be younger than the megacrystic granites, but grada-

very fine-grained hematite granules which appear to be responsible for the red colour in hand specimen. Fine-grained, miarolitic granophyre Afine-grained,granophyric-textured rock exposed in a small quarry inland from the red leucogranite and also on the beach in Fishermans Bay at Port Elliot (Fig. 4) was referred to by Browne (1920) as a "greisenised soda granophyre", although it contains abundant potash feldspar. The striking feature of the rock is the abundant miarolitic cavities in which coarse grained muscovite and quartz occur together with tourmaline and feldspars. In some miaroles muscovite has extensively replaced microcline to form granophyric quartz-muscovite inter-

tional contacts between the two indicate a degree

growths.

of intermixing before the megacrystic granite was completely solidified.

plagioclase, many with myrmekitic borders, have a composition of about An29- Except for the miarolitic cavities, the granophyre is similar in texture to the red leucogranite and may in fact be gradational with it.

Two sheet-like bodies of frne to medium, even-grained granite within the border facies megacrystic granite just east of Green Bay are truncated by the fault that separates the inner and border facies megacrystic granites. They have a series of dyke-like offshoots with sharp contacts with the border facies granite, and are clearly younger. In terms of macro-modal composition (Fig. 5), the fine and medium even-grained granites partly overlap the field of inner facies megacrystic granite, but generally contain more potash feldspar. Red leucogranite A fine, even-grained, brick-red granite with a granophyric texture crops out along the coast immediately south of Fishermans Bay at Port Elliot (Fig. 4). Its contact with the medium, evengrained granite is gradational over 2 - 3 m. The red leucogranite ("red aplite" of Browne 1920) is composed mainly of microcline and blue quartz, with plagioclase (oligoclase composition) and minor micas. Microcline crystals are clouded by

Lamellar-twinned

crystals

As around the borders of potash feldspar megacrysts in the border facies megacrystic granite, spectacular granophyric intergrowths in the red leucogranites practically obliterate microcline crystals and indicate rapid crystallisation. Similar features are present in the miarolitic granophyre. Rapid crystallisation may have been achieved by a sudden loss of volatiles, evidence for which is provided by the abundant clots and miaroles filled with tourmaline, quartz and muscovite. Browne (1920) suggested that the miarolitic granophyre resulted from the late-stage greisenisation of an original granophyric leucogranite. Rock textures including the replacement of feldspars by muscovite and the muscovite- and quartz-filled miaroles support this hypothesis. Hybrid granites Hybrid granites are the rocks referred to by Browne (1920) and Kleeman (1937) as "diorite". They are fine-grained, dark-coloured rocks that

The Encounter Bay Granites contain megacrysts of opalescent blue quartz, potash feldspar and plagioclase (of similar appearance to those present in the megacrystic granite) in a groundmass rich in biotite and/or hornblende. The hybrid granites occur as a dykelike body and associated large, irregular masses up to 20 m in diameter within the border facies megacrystic granite on Granite Island (Fig. 9). Smaller subrounded inclusions are common within both varieties of the megacrystic granite throughout the Encounter Bay area. Abundant hornfels and laminated metasedimentary rock inclusions within the hybrid granites produces an enigmatic xenolith-within-xenolith relationship. Rocks similar in texture and mineralogy to the hybrid granites have been widely recorded in granites (Grantham 1928; Wells & Wooldridge 1931; Thomas & Smith 1932; Bateman etal 1963; Phillips 1968). In many cases they were interpreted to be modified xenoliths of mafic country rock. However, Bateman etal (1963)

suggested they might represent either "clots of early-formed crystals" or "refractory material that was not melted when the magma was formed". Type-A hybrid granite Typical type-A hybrid granite crops out on the northern side of Granite Island (Fig. 9) along the roadway and also on the coast on the southern side in a locality known as "Natures Eye". It is a dark grey coloured, fine-grained rock with megacrysts of plagioclase, blue quartz and potash feldspar, and biotite and hornblende clots. The potash feldspar megacrysts are commonly mantled by plagioclase. In places there is layering produced by differences in megacryst abundance and in the grain size of groundmass minerals. Inconspicuous dark coloured and fine grained hornfels inclusions without megacrysts are also present.

L E G E N D | IXN^L

MEGACRYSTIC A L B I T E - C H L O R I T E ROCK ALBITISED

MEGACRYSTIC

GRANITE

|T T TL

BORDER FACIES MEGACRYSTIC GRANITE

fHHf

HYBRID

[LV/L I

METASEDIMENTARY

•3-26 NOTE

Quarry edge Sample, locality Xenoliths less than Inn in diameter have not been maj

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GRANITES ROCK. XENOLITHS

Fig. 9. Map showing field relationships of granites on Granite Island.

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An intertonguing contact between hybrid granite-A and the megacrystic granite at "Natures Eye" (Fig. 10) is characterised by biotite schlieren in the marginal zones of the megacrystic granite. Large potash feldspar megacrysts (as well as quartz and plagioclase megacrysts) that appear to be identical to megacrysts of the same minerals within the megaciystic granite commonly straddle the boundary. However, some potash feldspar megacrysts in the hybrid granite contain abundant groundmass mineral inclusions and are more like poikiloblasts. Both the "hornblende diorite" and the "quartzmica diorite" of Kleeman (1937) are classed as

hybrid granite-A because they cannot be distinguished in the field or in hand specimen. The proportion of megacrysts to groundmass is approximately 1:10, with plagioclase megacrysts (in which the cores have a composition about A1142) the most abundant. Some of these contain small, rectangular potash feldspar inclusions arranged in concentric zones: others contain randomly arranged hornblende inclusions. There are also large poikilitic megacrysts mantled by plagioclase. The cores are usually a patch-work of equidimensional plagioclase and untwinned potash feldspar crystallites rimmed by anhedral quartz, or in some cases, sphene. The quartz and sphene in these intergrowths exhibit optical continuity over relatively large areas, in a form like graphic texture. Inclusions of hornblende, biotite, sphene, opaque minerals, epidote, calcite and apatite are abundant. Quartz megacrysts are typically zoned with respect to their blue colour in the same fashion as those in the megacrystic granite. Fine-grained groundmass minerals include biotite, hornblende (in some samples) and sphene. Hornblende is also present as skeletal and poikilitic megacrysts.

Type-B hybrid granite

Fig. 10. Interfingering contact between hybrid granite-A and border facies megacrystic granite near "Natures Eye" on Granite Island. Note granitic schlieren (incorporating feldspar and quartz megacrysts) in hybrid granite, veining of hybrid granite by megacrystic granite, and "rafts" of hybrid granite with associated biotite-rich schlieren in megacrystic granite adjacent to contact. Pen 14 cm long.

Type-B hybrid granite is a fine-grained, light grey coloured rock which contains abundant coarse potash feldspar, blue quartz and plagioclase megacrysts (megacryst to groundmass ratio about 1:2), and is littered with fragments of altered metasedimentary rock (including some of white quartz) and hornfels inclusions (Fig. 11). It commonly occurs as an intermediate phase between hybrid granite-A and the megacrystic granite (and is thus regarded as a modification of hybrid granite-A brought about by recrystallisation and metasomatism during crystallisation of the megacrystic granite), but is also widely distributed as discrete xenoliths within the megacrystic granite. The texture of hybrid granite-B is variable, but it approaches that of the megacrystic granite. Under these circumstances the contacts between the two rock types are difficult to locate precisely in some

The Encounter Bay Granites

outcrops. As in the case of hybrid granite-A, large potash feldspar megacrysts (and those of other minerals) commonly straddle the contact between hybrid granite-B and the megacrystic granite (Fig. 12). Potash feldspar megacrysts are the most abundant in hybrid granite-B; the absence of hornblende and the paucity of sphene are also distinctive features. Potash feldspar megaciysts are usually orthoclase- and microcline-perthite; some have granophyric intergrowths of quartz and microcline forming partial borders around euhedral cores. Cores of normal and oscillatorily zoned plagioclase megacrysts have a composition around An45.

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Megacrysts within the hybrid granites are thought to have crystallised as porphyroblasts at the same time as the megacrysts formed in the contiguous megacrystic granite as phenocrysts. A process such as this required conditions of chemical equilibration, and so the parent material for the hybrid granites is considered to have been incorporated into the megacrystic granite at an early stage in its history, and certainly prior to its emplacement into the Kanmantoo Group. The hornfels inclusions and the layering in hybrid granite-A, together with the similar but more extensively modified xenoliths in hybrid graniteB, are taken to indicate that hybrid granite parent rock may have been of metasedimentary origin, or was some pre-existing intrusive igneous rock in the region. Minor granite varieties

Aplite dykes up to 50 cm wide occur at Port Elliot. They are generally pink to buff coloured, granular-textured, fine-grained rocks, but in places they contain feldspar and blue quartz

Fig. 11. Typical hybrid granite-B exposed on coast west of "Natures Eye", Granite Island. Note abundant xenoliths of layered metasedimentary rock and hornfels, and scattered potash feldspar megacrysts. Pen 14 cm long.

Fig. 12. Contact between hybrid granite-A and border facies megacrystic granite near "Natures Eye" on Granite Island. Note potash feldspar megacrysts straddling contact (fine biotite inclusions randomly scattered or in zonal arrangement in megacrysts). Plagioclase and quartz megacrysts also straddle contact. Small layered hornfels xenolith in hybrid granite-A above potash feldspar megacrysts. Pen 14 cm long.

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megacrysts. In the vicinity of Green Bay, tourmaline-rich pods (Fig. 13) and pegmatitic masses of quartz are commonly associated with the dykes. Northeast of Commodore Point bulbous granophyric masses protrude into the surrounding inner facies megacrystic granite from the margins of one aplite dyke. Quartz veins occur along joint-fracture planes in many of the granites.

The albitised zones are thus dyke-like in form (Fig. 14). Some zones in the set are composed of closely-spaced fractures which display polished surfaces and slickensides indicative of post-albitisation movement (Fig. 15). On the southern side of Wright Island, intense cataclastic deformation is evident in some zones.

Albitisation of the granites

Albitised megacrystic granite

Late-stage albitisation has affected all granite varieties along a prominent set of near-vertical joints and fractures striking 300°, approximately normal to the contact between the granites and the country rock Kanmantoo Group metasediments.

In all cases, the albitisation has proceeded without substantially altering the texture and fabric of the original granite (Fig. 16). For example, the albitised megacrystic granite is texturally identical to the unaltered granite but is composed of albite, quartz and biotite: all pre-existing potash feldspar and plagioclase has been replaced by albite. It can be distinguished by the white or orange colour of the albite megacrysts compared with the pink to grey colour of feldspars in the unaltered granite. In many samples, peripheral granulation and brittle fracturing of quartz and albite, kinking of lamellartwinned albite, and pronounced undulose extinction or deformation lamellae in quartz provide evidence of post-albitisation deformation. Some specimens contain thin cataclastic zones of granulated quartz and albite. Pre-existing potash feldspar megacrysts, groundmass crystals and interstitial material are albite characterised by finescale, chess-board twinning. Original twin boundaries, zonal arrangements of inclusions, granophyric border zones (in the border facies megacrystic granite), inclusion shapes, and perthite textures are perfectly preserved. Biotite occurs as aggregates of fine-grained, ragged laths with small rutile inclusions, and as mediumgrained laths and plates with abundant rutile granules. In more extensively altered samples, pre-existing blue quartz is pale blue in colour or colourless. Biotite may be progressively altered and replaced by chlorite which contains abundant rutile granules.

Fig. 13. Black mass of tourmaline crystals in lens-shaped pod within leucogranite mass in inner facies megacrystic granite, east of Green Bay, Port Elliot. Lens cap 5.5 cm diameter.

The Encounter Bay Granites

Megacrystic albite-chlorite rock In some localities, for example the western end of Granite Island and in many places at Rosetta Head, the border facies megaciystic granite has been altered to a significantly greater degree than is evident in the albitised joint zones. The resulting rock type is composed essentially of albite and chlorite and yet retains the texture and fabric, at least in broad aspect, of the pre-existing megacrystic granite. Browne (1920) referred to such rocks as the "albite-mica syenite" and Bowes (1954) used the term "coarse albite chlorite rocks of igneous aspect". At Rosetta Head the albite-chlorite rock crops out in a number of places along the contact between the granite and the country rock, and commonly con-

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tains albitised metasedimentary rock xenoliths. At the southwestern tip of Rosetta Head the albite-chlorite rock interfingers with megacrystic granite along joints that have the same orientation as the albitised joint-fracture zones. In general, samples consist of albite (both chessboard- and lamellar-twinned megaciysts and groundmass crystals) with interstitial aggregates of fine chlorite. Many albite crystals are fractured and partly granulated, and some contain kinked twin lamellae. Chlorite laths also have well defined kink bands. Fine-grained zircon, apatite and rutile crystals are common inclusions in chlorite. Some samples contain abundant muscovite closely associated with interstitial chlorite aggregates. Relict biotite and phlogopite laths may be present, and are invariably littered with rutile granules. Pre-existing quartz megacrysts composed of albite can be recognised by their subhedral, equidimensional shape. Colourless anhedral quartz with pronounced undulose extinction occurs in most samples, and many such grains are partly replaced by albite. The contact between the border facies granite and the albite-chlorite rock is gradational over about 5 cm, and is marked by a significant decrease in quartz content and a change in colour of the quartz from blue to colourless as the albitechlorite rock is approached. Albitisation of potash feldspar and plagioclase occurs to varying degrees within the megacrystic granite up to 1 m from the contact. In addition, the alteration of biotite to chlorite in the megaciystic granite is generally apparent several centimetres from the contact. The same alteration has affected metasedimentary rocks along the shore platform around Rosetta Head and in the Middleton area to produce distinctive zones of albite-chlorite schists.

Fig. 14. Dyke-like zone of albitised inner facies megacrystic granite developed adjacent to specific fracture set east of Green Bay, Port Elliot. Dark bands marking edges of albitised zone composed of very fine

Albitisation is considered to be a late-stage hydrothermal phenomenon. However, a source for sodium-rich fluids is difficult to envisage and further critical data are required.

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Other examples of albitisation Northeast of Commodore Point at Port Elliot, alteration of the red leucogranite along the characteristic northwesterly-trending joint-fracture set has produced dyke-like bands of white albite leucogranite identical in texture to the red leucogranite but composed mainly of chessboard- and lamellar-twinned albite and pale blue quartz. The borders of both quartz and albite crystals are often granulated, and quartz crystals commonly exhibit deformation lamellae. In some specimens there are thin cataclastic zones formed during post-albitisation deformation.

Fig. 15. Closely-spaced fractures in albitised joint-fracture set in medium grained granite northeast of Commodore Point, Port Elliot. Fracture surfaces slickensided, indicating post-albitisation brittle deformation. Dark bands mark edges of albitised zone at right. Pen 14 cm long.

The contacts between the albitite and the unaltered granite are gradational over about 2 cm, but numbers of conspicuous dark coloured bands or layers (each about 2 cm wide) usually occur within the albitite parallel to the contact (Fig. 17). The bands contain very fine-grained clay-size material, and are generally less distinct with distance from the contact. They may represent times of hiatus in the alteration process, and thus record a series of separate stages of albitisation. Metadolerite dykes Metadolerite dykes intruding the granites were recognised by Chewings (1894), Moulden

Fig. 16. Border facies megacrystic granite near breakwater on Granite Island showing preferential albitisation of specific potash feldspar megacrysts (arrows) crossed by thin fracture zones. Pen 14 cm long.

The Encounter Bay Granites

(1895), Browne (1920) and Bowes (1954,1959). A medium grained metadolerite mass, presumed to be part of a dyke, crops out amongst the boulders on the eastern side of the rocky cove east of Green Bay at Port Elliot. Several metadolerite dykes are exposed at Rosetta Head. Of these, two have intruded the border facies megaciystic granite (Fig. 18). The northernmost dyke can be traced almost continuously from the contact between the granite and the Kanmantoo Group metasediments southeastwards through the granite to the coast where it is well exposed in a high cliff face at the head of a narrow gulch. It was not identified within the metasediments. The southernmost dyke can only be followed for a short distance through the granite from the main

435

contact, but it can be traced into the metasediments where it abruptly changes direction. The contact between the dyke and the granite are partly exposed, and indicate significant alteration and reciystallisation of the granite in a zone up to 30 cm wide. The margins of the dyke are significantly finer grained than the main mass, and contain scattered quartz and feldspar xenocrysts. A folded and boudinaged metadolerite exposed on the shore platform in Petrel Cove represents a possible extension of the dyke.

THE GRANITES AT CAPE WILLOUGHBY Megacrystic granite At Cape Willoughby (Fig. 19) the megacrystic granite is similar to but finer grained than the border facies megacrystic granite in the Encounter Bay area. Its affinity to granites at Encounter Bay is immediately suggested by the blue colour of the quartz, a feature not found in other Palaeozoic granites in southeastern South Australia. The granite also contains coarsegrained potash feldspar megacrysts that are rimmed by plagioclase and contain abundant fine-grained, randomly- or zonally-arranged inclusions of quartz, plagioclase and biotite. It is quite variable in texture, despite the statement by Tilley (1919b) to the contrary. Fine-grained varieties consist of scattered medium to coarse megacrysts of potash feldspar, quartz and plagioclase in a fine groundmass of these minerals plus biotite. Conspicuous granophyric intergrowths surround euhedral potash feldspar cores in both megacrysts and groundmass crystals. Coarse-grained varieties of the granite are similar to the border facies megaciystic granite at Encounter Bay.

Fig. 17. Relict lens of red leucogranite in zone of albitised red leucogranite near Fishermans Beach, Port Elliot. Dark bands represent former positions of margins of albitised zone. Maximum width of lens about 1.2 m.

The megacrystic granite encompasses a variety of inclusions, although they are not as abundant as in the border facies megacrystic granite at Encounter Bay. Xenoliths of metasedimentary rock (rarely greater than 1 m in length, and generally with sharp contacts with the

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A.R. Milnes

enclosing granite) are usually dark-coloured, well-laminated, biotite sandstones that can be identified as fragments of the contiguous Middleton Sandstone (Daily & Milnes 1973; Daily et al 1979). In some examples the margins have been considerably modified to produce a rock type like hybrid granite. Homfelses, hybrid granites, fine-grained granophyric leucogranites, and rare quartz-feldspar pegmatites also occur as inclusions within the megacrystic granite.

Zones in which biotite-rich bands alternate with quartzo-feldspathic layers are present, although of limited extent. The biotite crystals commonly have a preferred orientation parallel to the plane of the layers, and small tabular hornfels xenoliths sometimes present in such zones are oriented in the same fashion.

Fig. 18. Map showing field relationships of granites at Rosetta Head.

The Encounter Bay Granites

Other granite varieties A northeasterly-dipping sheet-like body of red leucogranite crops out in a complex zone in the inlet immediately west of Cannon Hill. To the west the red leucogranite is a fine, even-grained rock but further eastwards it contains abundant medium-grained megacrysts of feldspar and blue quartz. It is characterised by abundant and spectacular granophyric intergrowths which practi-

437

cally obliterate groundmass microcline crystals. Microcline, quartz and plagioclase are the dominant minerals; micas (biotite andmuscovite) are minor components. Tilley (1919b) described the red leucogranite as "an aplite with development in part of a distinct granite porphyry fades". He also characterised the distinctive quartz-tourmaline segregations within the leucogranite (Tilley 1919a).

Fig. 19. Map showing field relationships of granites at Cape Willoughby, Kangaroo Island.

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The upper contact of the sheet with the megacrystic granite is gradational over about 10 cm. At the lower contact, the red leucogranite appears to be interlayered with the megacrystic granite and the contacts are gradational over only 1-2 cm. Just seawards of the sheet there are several 3-4 m diameter masses of leucogranite, with irregular dyke offshoots that have intruded the megacrystic granite. Hybrid granites of similar character to those in the Encounter Bay area occur as a large body westwards along the coast from Barn Bluff. Similar rocks are common as xenoliths within the megacrystic granite, for example just south of Pink Bay. Thin, red-coloured aplite veins with small tourmaline-rich segregations intrude both the red leucogranite and the megacrystic granite along the coast west of Cannon Hill. Red aplite dykes have intruded the megacrystic granite in other parts of the area. Late-stage alteration of the granites Albitisation Tilley (1919b) described several varieties of "white pegmatite" ("albitite", "quartz-albitite" and "muscovite-albitite") at Cape Willoughby. He mapped some of them as veins and others as irregular masses within the megacrystic granite, and regarded them as the "final differentiate or end product of the residual magma", having at first rejected the possibilities that they might be "an immiscible phase of the liquid residual magma" or the result of "albitisation of original potassic rocks". However, they are clearly the result of albitisation of pre-existing potassic rocks. As in the Encounter Bay area, albitisation of all granites (including the red coloured aplite dykes) has been controlled by a near-vertical joint and fracture set striking at 300°, approximately normal to the contact between the granites and the countiy rock. Considerable post-alteration defor-

mation including thin cataclastic zones occur within the joint-fracture set. The albitites are distinguished from the unaltered granites by their white colour due to the predominance of white albite. The alteration is most intense immediately adjacent to fracture planes and is progressively less pronounced away from these structures. In fact, the width of the albitised zones varies from a few centimetres to several metres depending on the intensity of fracturing. In the case of the megacrystic granite, the contact with the albitite is marked by the replacement of quartz by albite over 1-2 cm as the albitised zone is approached. However, alteration of potash feldspar to albite can occur several centimetres from the contact. Such partly albitised zones containing albite, blue quartz and biotite correspond to Tilley's "quartz albitite". A prominent jagged mass of albite-muscovite rock ("muscovite albitite" of Tilley 1919b) occurs on the eastern side of Bam Bluff (Fig. 19) in a complexly fractured and jointed zone. Its contact with the megacrystic granite is gradational over about 30 cm. The albite-muscovite rock occurs at the intersection of two joint-fracture sets: one striking 300° and nearly vertical corresponds to the albitised zones; the other, which strikes 115° and dips at a very shallow angle southwards, has controlled late-stage greisenisation.

Greisenisation Tilley (1919b) described the effects of greisenisation only in relation to the red leucogranite. However, this alteration has affected all granite types at Cape Willoughby along the shallow-dipping joint-fracture set striking 115°. Thin quartz-muscovite veins are common in these zones in the megacrystic granite, and alteration of the granite immediately adjacent to the veins has produced a dark-green quartz-muscovite-albite-epidote rock without major disruption to the texture and fabric of the original rock. Away from the main joint-fracture zone extensive sericitisation has produced an altered granite composed mainly of quartz, muscovite, potash

The Encounter Bay Granites

Fe a s FeO

feldspar and plagioclase: biotite and many feldspar crystals are substantially replaced by muscovite. Epidote and rutile appear to be important by-products of the alteration of plagioclase and biotite respectively. The alteration zones vary in width depending on the nature of the joint-fracture zone, but are commonly 5-10 cm wide. They are separated from the unaltered granite by a thin, brick-red zone that appears to obtain its colour from the intense clouding of the potash feldspar by very fine-grained hematite. In many localities there has been deformation

of the megacrystic granite along the joint-fracture set controlling greisenisation, resulting in the formation of a moderate schistosity defined by the alternation of quartz-rich and mica-rich zones (in which micas have a well-developed preferred orientation). In contrast to the albitised joint-fracture set, brittle deformation and cataclastic features have not been observed.

MINERALOGY Micas, chlorite and hornblende The compositions of biotite from a variety of granite facies and from metasedimentary rock and homfels xenoliths have been plotted in Fig. 20 (data tabulated in Milnes 1973). Biotites in samples of the inner facies megacrystic granite and the fine and medium, even-grained granites are uniform in composition. On the other hand, biotites in samples of the border facies megacrystic granite have a significant range in composition and are more MgO-rich (Fe/[Fe+Mg] ratios between 0.58-0.67). Within individual samples of the border facies granite, however, electron probe microanalyses demonstrate that the biotite has the same composition irrespective of whether it occurs in the groundmass, as inclusions in feldspar megacrysts, or as crystals in homfels xenoliths.

439

• ° ° * A °

border facies megacrystic granite inner facies megacrystic granite fine and medium even-grained granites metasedimentary rock xenoliths V3 Kanmantoo Group metasedimentary rock hybrid granite - A

hybrid granite - B

•

miarolitic granophyre

•

biotite-rich inclusion

schist-like inclusion

K 2 0 + Na20

Fig. 20. Triangular diagram showing compositions of coexisting micas, hornblende and chlorite in Encounter Bay Granites in terms of total FeO, alkalis and MgO.

Biotites in large metasedimentary rock xenoliths differ in composition to those in the border facies megacrystic granite by being more MgO-rich, but they plot on an extension of the linear composition field of the granite biotites, close to biotite * from one specimen (V3 ) of contiguous Kanmantoo Group metasediments. The compositional trend is best explained as the result of variable degrees of contamination of magma (with the approximate composition of the inner facies granite or the fine to medium, even-grained granites) by metasedimentary rocks, like the Kanmantoo Group metasediments, that contain comparatively MgO-rich biotites. As the composition of biotite within a given sample of the border facies is uniform, the presence of xenocrysts of biotite derived directly from metasedimentary rocks during assimilation can be discounted. Therefore it seems likely that the crystallisation of comparatively MgO-rich biotites in the border facies granite was caused by a compositional change consequent upon contamination. Muscovite in the miarolitic granophyre falls within the composition range of muscovite from samples of the border facies megacrystic granite.

^Numbers refer to samples housed in the collections of the Department of Geology & Geophysics, University of Adelaide. For sample locations refer to geological maps.

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Biotites in hybrid granite-A are similar in composition to the most MgO-rich biotite in the border facies granite and to biotite from metasedimentary rock xenoliths and the country rock Kanmantoo Group (Fig. 20). This is consistent with the suggestion that the hybrid granites represent either an early intrusive igneous facies contaminated with metasedimentary rock, or material of sedimentary origin incorporated into the megacrystic granite at an early stage of its crystallisation history and subsequently modified by metasomatism and recrystallisation to varying degrees. Biotites in hybrid granite-B are more FeO-rich, like those in the border facies megacrystic granite, as expected of more extensively assimilated material. Hornblende is present in two samples of hybrid granite-A examined and has a similar Fe/(Fe+Mg) ratio to that of coexisting biotite. One of the expressions of albitisation of the megacrystic granites is the appearance of numbers of fine rutile granules within biotite crystals. With increasing degree of albitisation, biotite is progressively altered to chlorite and the altered crystals are littered with rutile inclusions. The progressive change in the compositions of biotite and co-existing chlorite and muscovite are shown in Fig. 21 (data given in Milnes 1973), together Fe as FeO

Fig. 21. Triangular diagram showing compositions of total rocks and coexisting micas and chlorite in albitised granites in terms of total FeO, alkalis and MgO.

with changes in total-rock composition. Biotites in samples 4-79, 4-26 and 7-55 fall within the composition field of the border facies megacrystic granite: those in 7-53, 8-13 and 7-54 are significantly depleted in FeO. Feldspars Electron probe microanalyses of potash feldspar and plagioclase were made at intervals in traverses across megacrysts in samples of both facies of the megacrystic granite (data are given in Milnes 1973). The compositions of exsolution bodies and inclusions encountered in the traverses were also determined. In summary, the results indicate that: (a) Ba tends to be enriched in the inner zones of potash feldspar megacrysts (up to 1.5 mol% Cs) relative to their margins (around 0.2 mol% Cs); (b) groundmass potash feldspar has Ba concentrations comparable with the margins of potash feldspar megacrysts; (c) exsolution lamellae in potash feldspar megacrysts are albitic; (d) plagioclase inclusions within potash feldspar megacrysts have inner zones as calcic as about An39 but are surrounded by albitic borders; (e) plagioclase megacrysts are oscillatorily zoned with cores of about An45 and albitic margins; and, (f) plagioclase crystals within small hornfels xenoliths are zoned with cores of An29 and margins about An24. The enrichment of Ba in the cores of potash feldspar megacrysts relative to their margins and to the groundmass potash feldspar supports an early-formed origin (Taylor 1965; Kerrick 1968). The similar composition of the inner parts of plagioclase megacrysts and plagioclase inclusions in potash feldspar megacrysts is consistent with the crystallisation of plagioclase during the formation of potash feldspar megacrysts.

The Encounter Bay Granites

Co-existing feldspar megacrysts in two samples of hybrid granite-A (3-27, 3-29) were also analysed (Milnes 1973). One large megacryst, typical of potash feldspar megacrysts in hybrid granite-A, is a patch-work intergrowth of potash feldspar and andesine, and also contains quartz, sphene, hornblende and biotite. It may have formed by simultaneous crystallisation of potash feldspar and plagioclase in an essentially solid medium, or alternatively by the replacement of one feldspar by another. The Ba content of the potash feldspar (up to 1.1 mol% Cs) compares well with that of the inner zones of potash feldspar megacrysts in the megacrystic granites. However, the plagioclase in these intergrowths is not zoned, and is significantly more sodic than adjacent plagioclase megacrysts. Such megacrysts are oscillatorily zoned from about An42 in the core to An3i margins, similar to plagioclase megacrysts in the megacrystic granite with the exception that the latter have distinctly albitic rims. The compositions of co-existing feldspars in samples of hybrid granite-B are similar to those in hybrid granite-A (Milnes 1973). The average compositions of potash feldspar megacrysts and coexisting plagioclase calculated from electron probe microanalyses provide no unequivocal information on temperature or pressure of formation ( Milnes 1973) because of the opposite effect of pressure to temperature on the orientation of the tie-lines (Seek 1971; Yoder et al 1957). However, potash feldspar megacrysts in the hybrid granites have similar chemical and X-ray diffraction properties to those in the border facies megacrystic granite (Milnes 1973), and are thus considered to record a similar crystallisation history.

Petrochemistry of the granites Chemical analyses of several samples of most varieties of the granites are plotted in Fig. 22 (data tabulated in Milnes 1973) and define a curvilinear trend extending from the total alkalis apex towards the FeO - MgO side of the triangular

441

composition diagram. The inner facies megacrystic granite occupies a small field part way along the along the trend from the total alkalis apex, and the border facies megacrystic granite from Cape Willoughby is approximately coincident with this. However, the border facies megacrystic granite from Encounter Bay delineates an elongate field that overlaps the inner facies granite, but extends significantly towards the FeO - MgO side of the diagram into the composition field of metasedimentary rock xenoliths and Kanmantoo Group metasediments. Many authors have ascribed such trends in granite suites to differentiation (Oba 1962; Butler & Ragland 1969). However, in the Encounter Bay Granites, the extensive field occupied by the border facies megacrystic granite is thought to reflect variable assimilation of metasedimentary rock fragments like those presently existing as xenoliths. Samples of hybrid granite-A, the least modified facies, have compositions within the field of Encounter Bay border facies megacrystic granites close to the metasedimentary rock composition field. More extensively assimilated hybrid granite-B, including the hybrid granite at Cape Willoughby, plot near the field of the inner facies megacrystic granite. Within the border facies granite metasedimentary rock xenoliths recognised as fragments of contiguous Kanmantoo Group metasediments appear to have suffered little or no chemical modification. The medium even-grained granites, the fine even-grained granites, the granophyric leucogranites and the red leucogranites fall progressively towards the total alkalis apex of the diagram, and outline a limited compositional trend due to enrichment of alkalis (to form rocks containing progressively more potash feldspar). These rocks are likely to be late-stage products of the differentiated granite magma, and the numerous aplites and pegmatites are also in this category. Systematic changes in composition of granite varieties along the curvilinear trend delineated in Fig. 22, for example away from the total alkalis

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A.R. Milnes

Fe as FeO

• o •

• •

border fades megacrystic granite inner fades megacrystic granite metasedimentary rock xenoliths Kanmantoo Group metasedimentary rocks Cape Willoughby megacrystic granite hybrid granite - A hybrid granite - B

O • v V °

medium, even-grained granites red leucogranite granophyric leucogranite inclusions fine, even-grained granites aplites

A D

Field of border fades megacrystic granite Field of inner fades megacrystic granite Field of medium evengrained granites Field of Kanmantoo Group metasedimentary rocks and metasedimentary rock xenoliths

K 2 0 + Na 2 0

Fig. 22. Triangular diagram showing compositions of Encounter Bay Granites and associated rocks in terms of total FeO, alkalis and MgO. apex of the triangular diagram, are accompanied by a consistent increase in the MgO content of the constituent biotites (Fig. 23). The presence of hornfels and hybrid granite xenoliths in the inner facies megacrystic granite indicates that it has been contaminated to some extent by sedimentary rock material. Based on the compositional trend, which is thus regarded as a reflection of degree of contamination of the granites, the composition of the medium even-grained granites may approximate the composition of the uncontaminated parent magma of the Encounter Bay Granites. K/Rb data for the variety of granites and metasedimentary rocks (Fig. 24) also show a consistent change from the alkali-rich red leucogranites and medium even-grained granites (K/Rb around 125), through the inner facies megacrystic granites, to the border facies

granites, type A hybrid granites and Kanmantoo Group metasedimentary rocks (K/Rb apFe as FeO

Fig. 23. Triangular diagram showing compositions of total rocks and constituent biotites in Encounter Bay Granites in terms of total FeO, alkalis and MgO.

The Encounter Bay Granites

proximately 180). The average K/Rb ratio for crustal rocks is reported by Taylor (1965) and Kolbe & Taylor (1966) to be about 220. Coincidence of the K/Rb ratio for contaminated varieties of the Encounter Bay Granites with Kanmantoo Group metasedimentary rocks indicates that country rock of similar composition to the Kanmantoo Group was the dominant contaminant. The K/Rb data exhibit an abrupt change in trend at K concentrations of about 4%. For example, small increases in K concentration above 4% correspond to comparatively large increases in Rb concentration, in contrast to the relationship below 4% K. This discontinuity is probably related to the maximum potash content of granitic liquids dictated by the position of the ternary minimum in the Q-An-Ab-Or-EfeO system, whereas there are no such constraints on the concentration of Rb in granitic liquids. The medium even-grained granites and the red leucogranite have K/Rb ratios near 125,

•

which is significantly lower than the average value for crustal rocks. According to Kolbe & Taylor (1966), this suggests that these facies were derived through processes of marked differentiation. A similar enrichment of Rb relative to K in the Snowy Mountains leucogranites in eastern Australia was considered by Kolbe & Taylor to be characteristic of granitic rocks that have a composition approaching the ternary eutectic.

DISCUSSION AND CONCLUSIONS Field observations of the Encounter Bay Granites need to be assimilated with data obtained from petrological studies, the compositions of selected mineral phases, and petrochemical considerations (including Sr isotope data) if a reasonable interpretation of the origin and environment of emplacement is to be made. Daily & Milnes (1973) and Milnes etal (1977) described the contacts of the Encounter Bay Granites and used stratigraphic and structural observations, together with Rb-Sr isotope

border facies megacrystic granite

o inner facies megacrystic granite

O medium, even-grained granites _ • red leucogranite •

hybrid granite - A hybrid granite - B

Kanmanioo Group metasedimentary rocks Cape Willoughby megacrystic granite A fine, even-grained granites aplites

A — •

100

443

200

300

Rb (ppm) Fig 24. Plot of Rb versus K concentrations in Encounter Bay Granites

A.R. Milnes

444

data, to place these and other Delamaride granites in the region into a time-framework and structural context, suggesting that they were essentially preto syn-tectonic. Age data and initial 87 Sr/ 86 Sr ratios As reported by Milnes (1973) and Milnes et al (1977), Rb-Sr isotope dilution analyses of several varieties of the Encounter Bay Granites and their constituent minerals indicate that the border facies megacrystic granite and various other granite facies crystallised at about the same time between 506 - 515 Ma ago. A similar age was established for pegmatites and their constituent muscovites from Kangaroo Island. It is argued on the basis of field and petrographic evidence that the border facies granite was emplaced as a crystal-rich liquid which then crystallised rapidly. The crystallisation age will thus approximate the age of emplacement. As there is clear evidence for contamination of the border facies granite by metasedimentary rock, there is a possibility that the isochron for this granite represents a mixing line. However, the concordance in age between the uncontaminated granites and the border facies granite indicates that either significant mixing of metasedimentary rock with granite magma did not occur, that the granite magma and the metasedimentary rock contaminant had a similar Sr isotope composition at the time of mixing, or that isotopic redistribution was completely effective. The first possibility, that significant mixing did not occur, can be ruled out on the basis of field observations and total-rock chemical data. Similarly, complete isotopic redistribution can be discounted because of the disequilibrium beon

o(L

tween initial Sr/ Sr ratios for the border facies granite (0.7169) and ^Sr/^Sr ratios for the metasedimentary rock xenoliths (0.7234) and hybrid granite samples (0.7150) at the time of granite emplacement (Milnes 1973). Unfortunately, the analytical errors are sufficiently large to preclude a distinction between the contaminated and uncontaminated granites, and further data are required. However, the disposition of these data on an isochron diagram suggests that the uncontaminated granites have a lower initial

87 Sr/86Sr ratio which is expected to reflect the composition of the parent magma of the Encounter Bay Granites (Milnes 1973).

The interpolated 87Sr/86Sr ratio for metasediment xenoliths and Kanmantoo Group metasedimentary rocks (0.7231±0.0003) at the time of emplacement of the border facies megacrystic granite is higher than the initial ratio for the granite (0.7169±0.0006), according to Milnes et al (1977). Samples of hybrid granite-A have an initial ratio of 0.7150±0.0007, about the same as the granite, indicating that they are not related to Kanmantoo Group metasedimentary rocks of the type measured (Milnes 1973). One interpretation is that hybrid granite-A represents the final composition of metasedimentary rocks of unknown source assimilated by granitic magma with a lower initial 87 S r / 86 Sr ratio than the border facies megacrystic granite. Alternative hypotheses are that the hybrid granites represent an early igneous intrusion contaminated by metasedimentary rock fragments, or the refractory residue of crustal melting or igneous source material, variously assimilated by the parent magma of the Encounter Bay Granites, and carried upwards to the site of emplacement. There are not enough Sr isotope data at this stage to distinguish between the various possibilities. However, it is clear that the megacrystic granites have experienced a complex contamination history involving Kanmantoo Group metasedimentary rocks at the present level of exposure and other rock types represented by the hybrid granites from deeper levels in the crust, perhaps nearer the site of magma generation. Origin of the Encounter Bay Granites On the basis of Rb-Sr data, there are three likely mechanisms for the origin of granites. a.

Liquids of granitic composition may be derived by fractional crystallisation of magmas of basic or intermediate composition generated by partial melting of mantle material.

The Encounter Bay Granites

Granitic liquids may be derived as in (a), but become contaminated with radiogenic Sr by assimilation of crustal rocks.

Granitic liquids may be derived by partial melting of crustal rocks.

The granites resulting from each of these simple models may be distinguished on the basis of their initial 87Sr/86Sr ratios. For example, model (a) granites have a low initial ratio within the range exhibited by oceanic basalts (Hurley et al 1962; Faure & Hurley 1963; Faure & Powell 1972), whereas models (b) and (c) granites would have significantly higher initial ratios. However, models (b) and (c) cannot be distinguished on the basis of initial ratios alone. The initial 87Sr/86Sr ratio for the uncontaminated varieties of the Encounter Bay Granites which, on the basis of field observations and chemical data, are considered to represent the composition of the parent granite magma, is 0.7115. This ratio is significantly higher than values for oceanic basalts of mantle origin (Faure & Powell 1972) and is consistent with contamination of the primary granitic liquids according to one or other of models (b) and (c). A model (c) origin requires that the parent magma was derived by partial melting of crustal rocks with an initial 87Sr/86Sr ratio of about 0.711, which excludes the Kanmantoo Group metasedimentary rocks but is consistent with hybrid granite-A. The granites plot close to the cotectic line in the Q-An-Ab-0r-H20 system (Milnes 1973), and thus approach the composition of either the first liquid produced by partial melting of crustal rocks, or the final product of fractionation of a magma of basic or intermediate composition. However, the granite varieties with a composition close to that assumed for the parent magma plot in the potash feldspar field of the Q-An-Ab-OrH 2 0 system and could not have originated by fractionation of a magma of basic or intermediate composition (Barth 1966; James & Hamilton 1969) unless assimilation of potash-rich material was involved.

445

If the parent magma was derived by partial melting of crustal rocks, as in model (c), the two limiting cases to be considered relate to partial melting of crustal rocks containing excess water and the generation of water-deficient magmas. In the first case, granitic liquids may be produced at temperatures as low as 630°C at depths around 35 km (10 kbar pressure) and will be watersaturated (Luth et al 1964; Kleeman 1965; Boetcher & Wyllie 1968). Such magmas are expected to crystallise rapidly on rising from the generation site due to the negative P-T of the solidus curve, but can move upwards in the crust to a lower pressure regime if sufficient superheat is available (Tuttle & Bowen 1958; Luth 1969). Water-deficient magmas are more likely to be generated by partial melting of crustal rocks in an anatectic environment because the only water available is from the breakdown of hydrous minerals (Brown 1973). The generation of such magmas is likely to occur at depths between 25-30 km at temperatures of 700-750°C, assuming normal geothermal gradients. In rising to lower pressure regimes, the melting point of water-deficient magmas decreases until they reach a critical pressure at the water-saturation boundary and thereafter the melting point of the magma increases with decreasing pressure. If the magma could migrate rapidly upwards from the site of generation without significant loss of heat to the surrounding environment, the critical pressure may be reached prior to crystallisation and the magma could thus crystallise entirely in a water-saturated condition. Alternatively, with slow upward migration, heat loss to the environment could be sufficient to cause the onset of crystallisation before the magma reaches the critical pressure: only the final stage of crystallisation may thus occur under conditions of water-saturation. Whatever the case, water-deficient magmas are potentially able to closely approach the Earth's surface, and are capable of assimilating vaiying amounts of the rocks with which they come into contact. The Encounter Bay Granites contained vapour phases (and were therefore probably water-saturated) at least during the later stage of

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A.R. Milnes

their crystallisation history. This is clearly demonstrated by the occurrence of miarolitic cavities and abundant pod-like tourmaline-rich pegmatites in many granite varieties. Granophyric textures and the presence of a finegrained groundmass in the border facies megacrystic granite are considered to reflect rapid crystallisation of a crystal-rich magma consequent upon loss of volatiles from the margins of the intrusion. The absence of pegmatite pods in the border facies granite but their presence in the inner facies megacrystic granite supports this suggestion. Unfortunately, none of these features indicate what proportion of the crystallisation history took place in the presence of a vapour phase. Metamorphic assemblages containing andalusite and cordierite occur in the Kanmantoo Group metasediments adjacent to the Encounter Bay Granites in the Encounter Bay area, and are considered to have crystallised during and after the emplacement of the granites (Milnes 1973; Daily & Milnes 1973; Milnes etal 1977). Based on the experimental alumino-silicate equilibria and a likely paragenesis for cordierite, the depth of emplacement of the granites was probably not greater than about 14 km (4 kbar pressure), and may have been less than 10 km (3 kbar pressure). The granites are out of equilibrium with their environment in terms of mineralogy and Sr isotope composition, and were clearly not formed in situ. Therefore an estimate of their depth of origin must be based on one of the following 3 models. (1). The parent magma may have been watersaturated, and in this case could not have been generated by partial melting of crustal rocks more than a few kilometres away from the site of emplacement for the reasons discussed. Thus, a very high geothermal gradient would be required to obtain the temperatures necessary for melting at these comparatively shallow depths. A geothermal high is indicated by the regional metamorphic assemblages in Kanmantoo Group and older metasediments adjacent to the granites (Daily &

Milnes 1971a, 1971b, 1973; Daily etal 1979; Milnes et al 1977). However, the metamorphism appears to have largely post-dated the emplacement of the granites and therefore the geothermal high was probably not responsible for their origin. (2). The parent magma may have been derived by partial melting of water-deficient rocks at depths between 25-35 km, assuming a normal geothermal gradient of 20-20°C km"1. Alternatively, partial melting could occur at shallower depths with higher geothermal gradients. In either case the magma would have been waterdeficient, and may have migrated some distance to reach the present site of emplacement. (3). The parent magma may have been derived by crustal contamination of the product of fractional crystallisation of magma of basic or intermediate composition. The granite magma in this case would also be water deficient, and could have migrated some distance to the present site of emplacement. Either of models (2) or (3) provides the most likely origin for the parent magma of the Encounter Bay Granites, but the possibilities cannot be distinguished on the basis of the available data.

ACKNOWLEDGEMENTS My grateful thanks are due to the late A.W. Kleeman for his advice and discussion on all aspects of the study; to J.B. Jones for assistance with the feldspar work; and especially to the late Brian Daily, with whom I spent many weeks in the field, and who contributed a great deal to my knowledge of geology. Many others, including K. Norrish, W. Compston, R.L. Oliver, R.W. Nesbitt and J.A. Cooper, kindly contributed their time and expertise. The diagrams were prepared by the Publications Group, CSIRO Division of Soils.

The Encounter Bay Granites REFERENCES ASTHANA D.B. 1958. Structural study on granites on Encounter Military sheet. South Australian Department of Mines Report Book 47/90 (unpubl.). BARKER D.S. 1970. Compositions of granophyre, myrmekite and graphic granite. Geological Society of America, Bulletin 81, 3339-3350. BARTH T.W.F. 1966. Aspects of the crystallisation of quartzo-feldspathic plutonic rocks. Tschermaks mineralogische und petrographische Mitteilungen 11, 209-222. BATEMAN P.C., CLARK L.D., HUBER N.K., MOORE J.G. & RINEHART C.D. 1963. The Sierra Nevada Batholith: a synthesis of recent work across the central part. United States Geological Survey, Professional Paper 414D, 1-45. BOETTCHER A.L. & WYLLIE P.J. 1968. Phase relationships in the system NaAlSi04-Si02-H20 to 35 kilobars pressure. American Journal of Science 267, 875-909. BOWES D.R. 1954. The metamorphic and igneous history of Rosetta Head, South Australia. Transactions of the Royal Society of South Australia 77,182-214. BOWES D.R. 1959. Distribution and field relationships of the granitic rocks of Port Elliot, South Australia. Transactions of the Royal Society of South Australia 82, 7-9. BROWN G.C. 1973. Evolution of granite magmas as destructive plate margins. Nature Physical Science 241, 26-28.

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CHEWINGS C. 1894. Beitrage zur Kenntnis der Geologie Sud- und Zentral-Australiens nebst einer Uebersicht des Lake Eyre Beckens und seiner Randgebirge. Heidelberg Universitats Buchdruckerei, von J. Horning, 41pp. CLAXTON C.W. 1968. Mineral layering in the Galway Granite, Connemara, Eire. Geological Magazine 105, 149-159. COATS J.S. & WILSON J.R. 1971. The eastern end of the Galway Granite. Mineralogical Magazine 38, 138-151. DAILY B. & MILNES A.R. 1971a. Stratigraphic notes on Lower Cambrian fossiliferous metasediments between Campbell Creek and Tunkalilla Beach in the type section of the Kanmantoo Group, Fleurieu Peninsula, South Australia. Transactions of the Royal Society of South Australia 95,199-214. DAILY B. & MILNES A.R. 1971b. Discovery of Late Precambrian tillites (Sturt Group) and younger metasediments (Marino Group) on Dudley Peninsula, Kangaroo Island, South Australia. Search 2,431- 433. DAILY B. & MILNES A.R. 1972a. Significance of basal Cambrian sediments of andalusite grade, Dudley Peninsula, Kangaroo Island. Search 3, 89-90. DAILY B. & MILNES A.R. 1972b. Revision of the stratigraphic nomenclature of the Cambrian Kanmantoo Group, South Australia. Journal of the Geological Society ofAustralia 19, 197-202. DAILY B. & MILNES A.R. 1973. Stratigraphy, structure and metamorphism of the Kanmantoo Group (Cambrian) in its type section east of Tunkalilla Beach, South Australia. Transactions of the Royal Society of South Australia 97, 213-242.

BROWNE W.R. 1920. The igneous rocks of Encounter Bay South Australia. Transactions ofthe Royal Society of South Australia 44, 1-57.

DAILY B., MILNES A.R., TWIDALE C.R. & BOURNE J.A. 1979. Geology. In Tyler M.J., Twidale C.R. & Ling J.K. eds Natural History of Kangaroo Island, pp. 1-38. Royal Society of South Australia, Adelaide.

BUTLER J.R. & RAGLAND P.C. 1969. A petrochemical study of plutonic intrusions in the Piedmont, south-eastern Appalachians, U.S.A. Contributions to Mineralogy and Petrology 24, 164-190.

DASCH E.J., MILNES A.R. & NESBITT R.W. 1971. Rubidium-strontium geochronology of the Encounter Bay granite and adjacent metasedimentary rocks, South Australia. Journal of the Geological Society of Australia 18, 259-266.

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EMELEUS C.H. 1963. Structural and petrographic observations on layered granites from southern Greenland. Mineralogical Society of America, Special Paper 1,22-29.

KLEEMAN A.W. 1937. The nature and origin of the so-called diorite inclusions in the granite of Granite Island. Transactions of the Royal Society of South Australia 61,207-220.

FANDER H.W. 1960. Accessory minerals of South Australian granites. M.Sc. thesis, University of Adelaide (unpubl.).

KLEEMAN A.W. 1965. The origin of granite magmas. Journal of the Geological Society of Australia 12, 35-52.

FAURE G. & HURLEY P.M. 1963. The isotopic composition of strontium in oceanic and continental basalts: application to the origin of igneous rocks. Journal of Petrology 4,31-50.

KOLBE P. & TAYLOR S.R. 1966. Geochemical investigation of the granitic rocks of the Snowy Mountains area, New South Wales. Journal of the Geological Society of Australia 13, 1-25.

FAURE G. & POWELL J.L. 1972. Strontium Isotope Geology. Springer-Verlag, New York. GARTRELL H.W. 1903. The Port Victor granite. Transactions of the Royal Society of South Australia 27, 256-260. GRANTHAM, D.R. 1928. The petrology of the Shap Granite. Proceedings of the Geologists Association 34, 299-331. HARRY W.T. & EMELEUS C.H. 1960. Mineral layering in some granite intrusions of S.W. Greenland. International Geological Congress, XXI Session (Norden I960), part. XIV, 172-181. HIBBARD M.J. 1965. Origin of some alkali feldspar phenocrysts and their bearing on pedogenesis. American Journal of Science 263,245-261. HURLEY P.M., HUGHES H., FAURE G., FAIRBAIRN H.W. & PINSON W.H. 1962. Radiogenic strontium-87 model of continent formation. Journal of Geophysical Research 67, 5315-5334. JAMES R.S. & HAMILTON D.L. 1969. Phase relations in the system NaAlSi308-KAlSi30sCaAl2Si208-Si02 at 1 kilobar water vapour pressure. Contributions to Mineralogy and Petrology 21, 111-141. KERRICK D.M. 1968. Experiments on the upper stability of pyrophyllite at 1.8 kilobars and 3.9 kilobars water pressure. American Journal of Science 266, 204-214.

LUTH W.C. 1969. The systems NaAlSi 3 0 8 -Si02 and KAlSi308-Si02 to 20kb and the relationship between H2O content, PH20 and Ptotai in granitic magmas. American Journal of Science 267, 325-341. LUTH W.C., JAHNS R.H. & TUTTLE O.F. 1964. The granite system at pressures of 4 to 10 kilobars. Journal of Geophysical Research 69, 759-773. MAWSON D. 1926. A brief resume of the present knowledge relating to the igneous rocks of South Australia. Report of the Australasian Association for the Advancement of Science 18, 230-274. MILNES A.R. 1973. The Encounter Bay Granites, South Australia, and their environment. Ph.D. thesis, University of Adelaide (unpubl.). MILNES A.R., COMPSTON W. & DAILY B. 1977. Pre- to syn-tectonic emplacement of Early Palaeozoic granites in southeastern South Australia. Journal of the Geological Society of Australia 24, 87-106. MOULDEN J.C. 1895. Petrographic observations on some South Australian rocks. Transactions of the Royal Society of South Australia 19,70-78. OB A N. 1962. Petrochemical studies of the Kyushu Outer Zone granites, Japan. Journal of the Geological Society of Japan 68, 162-171. PHILLIPS E.R. 1968. Mafic hybrid rocks from the New England Batholith, New South Wales. Geological Magazine 105,160-165. PITCHER W.S. & READ H.H. 1958. The main Donegal Granite. Quarterly Journal of the Geological Society of London 102, 389-446.

The Encounter Bay Granites

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SCHERMERHORN L.J.G. 1956. The granites of Trancoso (Portugal) : a study in microclinisation. American Journal of Science 254,329-348.

TDLLEY C.E. 1919a. The occurrence and origin of certain quartz-tourmaline nodules in the granite of Cape Willoughby. Transactions of the Royal Society of South Australia 43,156-165.

SECK H.A. 1971. Koexistierende Alkalifeldspate und Plagioklase im System NaAlSi30sKAlSi308-CaAl2Si208-H20 bei Temperaturen von 650° bis 900°C. Neues Jahrbuch fur Mineralogie Abhandlungen 115, 315-345.

TILLEY C.E. 1919b. The petrology of the granitic mass of Cape Willoughby, Kangaroo Island. - Part I. Transactions of the Royal Society of South Australia 43,316-341.

SLADE P.G. 1962. A study of the alkali feldspars from a granite - xenolith contact at Victor Harbour. B.Sc.(Hons) thesis, University of Adelaide (unpubl.). STEIGER R.H. & HART S.R. 1967. The microcline orthoclase transition within a contact aureole. American Mineralogist 52, 87-116.

STRECKEISEN A.L. 1967. Classification and nomenclature of igneous rocks. Neues Jahrbuch fur Mineralogie Abhandlungen 107, 144-240.

THOMAS H.H. & SMITH W.C. 1932. Xenoliths of igneous origin in the Tregastel - Ploumanac'h Granite, Cotes du Nord, France. Quarterly Journal of the Geological Society of London 88, 274-296.

TUTTLE O.F. & BOWEN N.L. 1958. Origin of granite in the light of experimental studies in the system NaAlSi30g-KAlSi308-Si02-H20. Geological Society of America, Memoir 74,1 -153. WAHL W. 1925. Die gesteine des Wiborger rapakivigebietes. Fennia 45, No. 20. WELLS A.K. & WOOLRIDGE S.W. 1931. The rock groups of Jersey with special reference to intrusive phenomena. Proceedings ofthe Geologists Association 42,178-215. WRIGHT T.L. 1967. The microcline-orthoclase transition in the contact aureole of the Eldora Stock, Colorado. American Mineralogist 52, 87-116. YODER H.S., STEWART D.B. & SMITH J.R. 1957. Ternary feldspars. Carnegie Institution of Washington, Year Book 56, 206-214.

A zoned Middle Cambrian pluton in the Peake and Denison Ranges, South Australia Robert S. Morrison and John D. Foden Department of Geology and Geophysics, The University of Adelaide, G.P.O. Box 498, Adelaide, SA. 5001, Australia A suite of monzonitic rocks, including quartz monzonite, syenite, alkali syenite, monzogabbro, syenogabbro, as well as their altered equivalents, intrude Burra Group sediments of the Adelaide Geosyncline in the northern section of the Margaret Inlier in the Peake and Denison Ranges, S. A. The style of emplacement of these plutons and the nature of their alteration, in particular marginal albitization, suggest they were intruded into a thick sedimentary pile in the presence of a circulating meteoric water system. The petrography and chemistry of the pluton indicate that initial crystallization and progressive fractionation was controlled by clinopyroxene and hornblende, and subsequently by calcic plagioclase. Trace element trends support the involvement of two different stages of fractionation. Rb/Sr and U/Pb geochronology of the zoned pluton imply that this magmatism may coincide with late Early to Middle Cambrian volcanism in the Adelaide Geosyncline and thus pre-date the Delamerian Orogeny.

Key words: Hypabyssal anorogenic granites, geochemistry, Adelaide Geosyncline, Delamerian Orogeny, albitization. INTRODUCTION

The Peake and Denison Ranges consist of a series of Adelaidean and middle Proterozoic inliers 200 km northwest of the Willouran Ranges. The northern section of the largest inlier, the Margaret Inlier, is host to a suite of intrusive bodies up to 3 km in diameter collectively referred to as the Bungadillina Monzonite (Ambrose et al 1981) (Fig.l). K-Ar geochronology conducted on this suite by Ambrose et al (1981) recorded variable ages ranging from 679469 Ma, but they suggest that the date of emplacement was broadly synchronous with the Delamerian Orogeny. The Bungadillina suite is unique in the Adelaide Geosyncline as it is composed of lithologies ranging from quartz monzonite and quartz syenite to monzogabbro and syenogabbro with minor alkali syenite and biotite lamprophyre. They intrude slightly folded Burra Group sedimentary rocks. The intrusives have undergone varying degrees of alteration, most notably partial to complete albitization and the associated formation of leucocratic albitite bodies.

The most westerly exposed pluton of the Bungadillina suite is largely composed of monzonite, but is unusual in that it has distinctive syenogabbro cores, some monzogabbro margins and small relatively silica-rich areas of quartz monzonite. A smaller homogeneous quartz monzonite pluton is exposed to the immediate northeast. This paper presents the petrology and geochemistry of these plutons from the Bungadillina suite and presents new isotopic data for their age and genesis.

FIELD RELATIONS

The most westerly exposed pluton of the Bungadillina suite (Fig.2) has an irregular shape roughly 4 km by 2 km and passively intrudes Burra Group (Willouran-Torrensian) siltstones and quartzites of the Fountain Spring Beds and Mount Margaret Quartzites (Ambrose et al 1981). Although Ambrose et al (1981) cited the presence of feldspar porphyroblasts in dolomite as evidence of low greenschist facies metamorphism in the northern Denison Inlier, there is no such indication of regional metamorphism in the

Zoned Middle Cambrian Pluton, Peake and Denison Ranges

sediments of the northern section of the Margaret Inlier. Within the margin of the pluton, many larger host Burra Group xenoliths retain orientations similar to that of the immediate strata. The beds are generally undeformed, and like most other intrusions of the suite, lack any evidence of contact metamorphism. To the southeast, the pluton intrudes distorted chaotically oriented beds com-

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monly referred to as diapiric breccia. A small oval-shaped pluton roughly 600 m in diameter intrudes Mount Margaret Quartzite beds to the immediate northeast. In contrast to the passive nature of intrusion of the larger body, this massive, coarse grained and homogeneous pluton intruded forcibly as evident from the bowing apart of the surrounding beds (Reyner 1955). A series of sub-parallel alkali syenite and syenite dykes up to 4 m wide intrude both the northern section of the zoned pluton and the host sediments. These dykes represent a separate latestage alkaline magmatic event, and are equivalent to aegirine-augite-bearing ultrapotassic intrusives of the Bungadillina suite found elsewhere in the inlier. In the zoned pluton, the most northern of these dykes show extensive albitization. A small zone of metasiltstone breccia hosted by albite-carbonate outcropping near the centre of the pluton is the only indication of late-stage magmatic volatile activity. There are no associated pegmatites, but a narrow elongate megacrystic monzonite zone within the pluton is composed of feldspar phenocrysts grading up to 8 cm in length.

PETROLOGY AND MINERALOGY The larger zoned pluton varies from a coarse grained poikiolitic monzogabbro and syenogabbro to a medium grained equigranular monzonite and quartz monzonite according to Sorensen's (1974) classification scheme. In this scheme the definitive components are the modal concentration of: (1) the mafic constituents (hornblende, clinopyroxene, biotite, sphene and magnetite) ; (2) alkali feldspar; (3) plagioclase; and (4) quartz (Fig.3). The contacts between the gabbroic and more felsic lithologies in the zoned pluton are gradational over a narrow distance of less than one metre.

Fig. 1 Location of the Peake and Denison Ranges showing general outline of the Adelaide Geosyncline.

Alkali feldspar occurs as both slightly zoned euhedral orthoclase and microcline grains, or up to 15 mm subhedral unzoned water-clear orthoclase crystals enclosing all other mineral com-

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ponents, giving the rock a ghost-poikiolitic texture. Microprobe analyses of alkali feldspar compositions range from Or62 to OT99 although compositions that co-exist with plagioclase are typically more potassic than Orso (Fig.4). In the smaller quartz monzonite pluton, feldspars often display perthitic texture and minor micrographic intergrowths with quartz.

600m

LEGEND Mesozoic and Cainozoic sediments MID CAMBRIAN TO EARLY ORDOVICIAN Alkali syenite dykes Quartz monzonite Megacrystic monzonite

Plagioclase crystals are zoned with labradorite-andesine cores (no more calcic than An67) and oligoclase-albite rims (Fig.4). Calcic plagioclase cores are often extensively sericitized and carbonatitized. More altered lithologies have suffered albitization with both calcic and potassic feldspars replaced by almost pure albite, but retaining relict zoning. Co-existing plagioclase with alkali feldspar have undergone some degree of subsolidus re-equilibration as they record temperatures in the range 200-500°C (Powell & Powell 1977; Whitney & Stormer 1976). Magnetite is common and the sole oxide forming small euhedral crystals. It occurs as inclusions in all other major mineral phases and lacks any Ti exsolution textures. Rarely, magnetite contains inclusions of chromite. Minor alteration to hematite exists along fractures and margins. Ti02 and Cr2C>3 are less than 0.20wt%, MnO and M g O less than 0.28%, AI2O3 less than 0.35wt% and V2O3 up to 0.51wt%.

Fountain Spring Beds

Clinopyroxene forms early 0.2 to 1 mm euhedral pale green prisms, often with slight concentric normal zonation from diopside to ferrosalite or irregular patchy subsolidus zonation from Fe- to Mg-rich compositions (Fig.5). No exsolution lamellae are present. Of the minor constituents, TiC>2 is usually in low abundance (less than 0.5 wt%), but AI2O3 and Na20 can be as high as 4.5wt% and 1.5wt% respectively with acmite and Ca-Tchermakite components no greater than 2.5%, and jadeite less than 1.3%. In more altered lithologies, actinolite or chlorite form around margins and along cleavage.

Fig. 2 Geology of the western intrusives of the Peake and Denison Ranges.

Amphiboles are the main mafic mineral, composed largely of euhedral green and brown-green

— I- + + + + "O ocs CD c =J C O

Monzonite Monzogabbro Syenogabbro

ADELAIDEAN D

Diapiric breccia

13 O O 03

Mount Margaret Quartzite

Zoned Middle Cambrian Pluton, Peake and Denison Ranges hornblende up to 5 mm in diameter. Clinopyroxene and magnetite often occur as inclusions. The amphiboles often display concentric oscillatory zoning or patchy zoning Quartz

Fig. 3 Modal mineral proportions of quartz, alkali feldspar(F), plagioclase(P) and total(*) mafics for selected rocks of the Bungadillina suite in the Peake and Denison Ranges. Symbols: a-quartz syenite, b-quartz monzonite, c-quartz monzodiorite, d-alkali syenite, e-syenite, f-monzonite, g-monzodiorite, h-syenogabbro, i-monzogabbro. Or

represent rims. Alkali feldspar compositions presented co-exist with rim plagioclase.

453

between Mg- and Fe-rich phases (Fig.5). These primary amphiboles are invariably altered to actinolite, which is incomplete, forms rims or replacements along cleavage planes. Actinolitization of pyroxene and hornblende has been described in altered gabbroic rocks as a hydration reaction resulting from the influx of surrounding meteoric water (Mongkoltip & Ashworth 1986). The aluminium content of unaltered hornblende margins in one suitable sample (9598) suggests maximum pressure of crystallization in the order of 7 kbars (Hammarstrom & Zen 1986; Hollister etal 1987) though the final depth of emplacement is less than this. The application of hornblende geothermometry on the basis of Ti content would be erroneous as the early crystallization of magnetite indicates melts of high oxygen fugacity which strongly lowers the Ti content of hornblende (Helz 1973; Otten 1984). Biotite is locally a major phase occasionally forming in preference to amphibole as the sole ferro-silicate phase in more felsic lithologies. In some cases it replaces hornblende, probably in response to decreasing temperature and increasing potassium activity (Wones & Gilbert 1982). It is commonly partially altered to either actinolite or more often to chlorite and has a restricted composition ranging between phlogopite 65-annite 35 and phlogopite 53-annite 47 (Fig.5). Sphene occurs as the main Ti-bearing mineral phase, crystallizing early as euhedral prisms showing slight yttrium enrichment within some cores from electron microprobe analyses of zoned crystals. Sphene is also occasionally found associated with anatase and calcite which are considered to be alteration products, or as a minor secondary mineral replacing clinopyroxene. Like sphene, apatite is a locally abundant early accessory phase and it also occurs in secondary calciteepidote patches. This may imply some phosphate mobility during alteration. Substantial mobility of high-yield strength elements (Ti, Y, P, Zr and Nb) has been noted as the result of greenschist facies metamorphism in the presence of a CO2bearing phase (Hynes 1980; Murphy & Hynes

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1986). This may have been a significant factor during the Delamerian Orogeny. Zircon is a minor phase occurring as tiny stubby prisms and quartz, when present, occurs as small anhedral grains. Ca

Fig. 5 Compositional variations for pyroxene, amphibole and biotite. Numbers in brackets refer to number of individual microprobe analyses.

Alteration of the pluton also includes localized and patchy recrystallization of calcite, epidote, chlorite, sericite, albite and actinolite. Albite-epidote-calcite form subspherical patches up to 0.5 m in diameter. Such assemblages are those of the lower greenschist facies of metamorphism (e.g. Miyashiro 1979). Alteration involving albitization is more prevalent along pluton margins with small leucocratic albitized dykes noted along the western margin. Albitite occurs along the northern margin and syenogabbro altered to monzogabbro along the eastern, northern and western margins. In contrast, the quartz monzonite pluton has undergone little albitization. GEOCHEMISTRY Whole-rock major and trace element geochemistry was conducted on 29 samples from the pluton (Table 1). Sample numbers refer to a rock identification code at the University of Adelaide. All elements were determined by XRF at the University of Adelaide with the exception of sodium (atomic absorption), fluorine (specific ion electrode) and ferric iron (titration).

The plutons have a strong metaluminous calcalkaline character (Figs. 6,7,8,9,10) with silica contents ranging from 48-67wt%. The Harker diagrams of the alkali oxides demonstrate that some samples have undergone extensive sodic metasomatism (albitization) and in these Na20/Na20+K20>0.8 (Fig.8). This process involves the progressive replacement of K by Na as well as the conspicuous depletion of the large ion lithophile elements Rb and Ba, and to a lesser extent Sr. The average Ba concentration in albitized samples is commonly 600 ppm less than the unaltered equivalent (910 ppm vs 300 ppm), and Rb has experienced depletions of a similar magnitude (94 ppm down to 19 ppm; Fig.9). Other incompatible trace elements (e.g. Zr or Nb) and the halogens appear to be unaffected by albitization (Fig. 10). Such alteration is not noted in the immediate surrounding sediments and is in keeping with the interaction of low-temperature saline groundwater and the cooling pluton (e.g. Baker 1955), and does not involve magmatic volatile fluids as in the case of fenitization (e.g. Krosten & Morogan 1986). Iron may also have been mobilized during albitization reflected in hematitization and general Fe depiction. The abundance of secondary calcite in albitized lithologies indicates that Ca was liberated as part of the alteration reaction but immediately recrystallized by interaction with CO2. One albitized sample (9630) has relatively high incompatible trace element concentrations. This is a marginal felsic differentiate located directly adjacent to the equally albitized more mafic main lithology (9585), and is the only evidence for late-stage REE enrichment. 2+

The Harker diagrams for the alkali oxides of relatively unaltered samples show a general linear trend of increasing Na20 and K2O with increasing silica as would be anticipated in a fractionating system. Calcium is only slightly effected by alteration and shows a negative linear correlation with silica. These trends, in conjunction with the positive correlation of Fe, Mg and Ca, reflect the progressive fractionation of hornblende and pyroxene. AI2O3 is generally very high (>16wt%) and shows an initial negative

Zoned Middle Cambrian Pluton, Peake and Denison Ranges

455

Major elements in weight percentage Sample 7540 Si0 2 48.55 TiCh 1.07 AI2O3 15.29 Fe203 6.92 FeO 4.58 MnO 0.07 MgO 5.48 CaO 8.51 Na 2 0 4.55 K20 1.67 P2C*+ 0.79 H2O 1.38 Total 98.86

7541 49.23 1.06 11.61 6.74 4.80 0.08 7.82 10.38 4.00 1.51 0.73 1.14 99.10

7525 51.07 1.26 14.07 6.89 4.47 0.17 4.08 8.88 4.44 2.57 0.69 0.65 99.24

7537 51.91 1.02 15.78 5.57 3.74 0.11 3.77 7.23 4.31 3.96 0.56 1.24 99.20

F CI Sc V Cr Ni Ga Rb Sr Y Zr Nb Ba Ce Nd

800 290 29.7 317 35 27 22 61 1254 28.5 84 5.8 564 72 35

660 385 42.0 323 201 74 17 52 852 27.7 71 5.4 313 61 42

700 310 27.7 340 11 14 21 79 1218 44.0 106 10.8 998 98 55

700 265 21.0 236 30 16 22 88 1189 29.4 87 8.5 669 72 37

480 270 28.6 253 100 36 16 104 1014 24.7 112 6.3 933 82 39

Sample 7533 Si0 2 59.06 0.78 Ti02 AI2O3 15.58 Fe2C>3 4.97 FeO 1.60 MnO 0.04 MgO 3.24 CaO 3.20 Na 2 0 7.20 K20 1.33 P2O5+ 0.45 H2O 2.01 Total 99.46

9599 59.73 0.76 17.47 2.03 1.07 0.05 2.26 6.26 7.71 0.60 0.36 0.83 99.13

7550 60.50 0.47 17.28 3.12 1.26 0.07 1.59 4.22 5.59 3.78 0.25 0.53 98.86

7542 60.68 0.59 16.87 3.35 1.81 0.09 1.80 4.48 4.38 4.40 0.28 0.59 99.32

9630 61.09 0.65 18.19 0.63 0.73 0.03 2.07 5.71 8.15 0.81 0.30 1.02 99.39

F CI Sc V Cr Ni Ga Rb Sr Y Zr Nb Ba Ce Nd

350 130 17.5 138 10 9 19 13.9 901 31.3 133 10.3 218 73 35

200 125 10.3 101 9 6 19 58 1320 22.1 107 8.0 1409 74 17

700 225 13.5 120 12 8 21 124 834 22.8 134 8.7 893 49 20

9598 51.98 0.86 13.18 5.55 4.36 0.12 5.56 8.44 4.37 2.81 0.76 1.02 99.03

7523 53.92 1.02 14.77 6.02 3.24 0.17 3.27 7.01 3.62 5.34 0.55 0.65 99.58

7544 54.23 0.92 16.52 5.18 2.98 0.11 3.09 6.53 4.46 3.61 0.49 1.08 99.20

9595 55.16 0.80 16.95 4.28 2.31 0.07 2.67 6.29 5.98 2.14 0.39 1.93 98.97

9582 55.69 0.83 16.92 3.96 2.25 0.07 3.09 7.02 6.72 1.33 0.40 1.39 99.67

9583 55.73 0.86 16.66 4.39 3.06 0.11 2.78 6.29 4.67 3.87 0.42 0.74 99.57

7543 55.96 0.80 18.39 4.08 2.09 0.08 1.74 5.99 5.23 3.46 0.30 0.77 98.89

7534 57.63 0.67 18.00 3.74 2.03 0.08 1.86 5.29 4.88 3.61 0.29 1.08 99.16

9588 58.28 0.68 18.65 3.49 1.77 0.13 1.28 5.40 5.33 3.62 0.22 0.54 99.39

7521 58.71 0.59 17.22 5.41 1.37 0.11 1.75 3.61 4.90 4.15 0.29 1.75 99.86

9585 58.92 0.64 17.51 3.61 2.00 0.05 1.97 4.45 8.06 1.01 0.25 0.70 99.17

500 315 12.5 134 <5 7 22 63 1141 34.0 150 12.0 1046 60 25

410 370 13.5 133 <5 5 21 96 1103 27.7 130 8.4 834 55 23

440 215 11.1 105 <5 5 21 92 1067 32.5 152 11.8 790 66 28

220 220 13.7 135 6 5 20 83 1098 24.9 134 8.6 1270 60 18

810 210 11.5 147 19 9 21 22.4 804 27.1 138 9.0 276 50 26

9311 63.49 0.35 17.35 2.51 0.77 0.05 0.74 2.62 5.30 5.51 0.11 0.48 99.27

9592 64.94 0.29 17.18 1.70 1.25 0.07 0.44 2.38 5.51 4.54 0.08 0.84 99.22

9593 64.98 0.29 17.11 1.90 0.70 0.05 0.55 2.53 5.47 4.66 0.09 1.13 99.46

9590 67.10 0.23 16.54 1.29 0.73 0.04 0.37 2.37 5.43 4.33 0.07 1.06 99.56

200 165 7.3 73 10 6 20 123 1211 18.9 140 8.1 1048 51 19

400 145 6.3 61 5 6 20 114 1023 13.8 133 6.7 856 40 13

60 115 6.2 63 <5 1 19 108 974 14.1 128 7.0 820 37 13

240 150 5.0 50 <5 2 19 118 791 13.5 133 7.2 736 36 12

Trace elements in parts per million 510 450 22.7 266 10 9 18 153 1178 33.0 124 9.8 1467 59 33

650 370 23.5 208 18 14 23 70 1062 29.7 104 8.7 680 70 24

700 155 21.0 181 11 11 20 43 968 30.9 146 9.3 1297 71 24

580 165 18.1 193 23 17 21 25.2 1214 31.2 121 10.3 404 77 36

640 310 21.3 178 20 17 21 101 1250 30.5 120 9.5 678 74 31

Major elements in weight percentage 7531 61.10 0.53 17.37 3.05 1.43 0.09 1.42 4.25 4.87 4.35 0.19 0.76 99.41

7522 61.43 0.48 17.13 2.83 1.50 0.08 1.40 3.85 4.82 4.40 0.18 1.48 99.58

7539 62.34 0.55 17.09 0.72 0.46 0.01 2.97 6.11 8.07 0.59 0.30 0.67 99.88

9313 62.69 0.74 18.45 1.00 0.77 0.03 1.34 2.94 9.31 0.69 0.21 1.22 99.40

9591 63.23 0.33 17.26 1.88 0.83 0.05 0.45 2.66 5.38 5.81 0.09 1.50 99.46

Trace elements in parts per million 310 350 23.0 186 10 11 19 32 312 25.3 135 7.2 420 69 29

180 265 13.1 104 17 5 20 9.2 809 56 140 8.2 300 69 51

330 410 11.1 101 <5 6 20 106 894 21.8 146 8.6 190 50 16

220 180 11.0 90 5 3 20 114 859 22.1 138 8.3 905 55 17

240 585 11.5 107 46 12 19 10.3 646 25.0 162 10.3 196 46 31

100 90 10.5 106 6 4 21 19.8 510 22.7 156 13.9 286 48 20

n.a. 135 6.6 86 <5 5 19 148 1149 15.3 117 7.3 1053 49 14

Table 1: Major and Trace Element Geochemistry - Western Intrusives of the Peake and Denison Ranges

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correlation with CaO in more mafic samples (CaO>5wt%) indicating fractionation of primarily hornblende and to a lesser degree pyroxene. For rocks with <5 wt% CaO, the inverse holds true suggesting incoming and fractionation of calcic plagioclase. Incompatible trace elements have low concentrations in comparison to silica-saturated alkaline anorogenic intrusives and A-type granites, and are more in common with I-type granites (e.g. Bedard et al 1987). Variation diagrams of the high field strength incompatible elements (Zr, Y, Nb) against CaO show enrichment with initial CaO-depletion followed by Al

Fig. 6 Whole geochemistry utilizing Shand's (1950) classification scheme: a-peraluminium, b-metaluminous, c-peralkaline. Squares represent analyses from larger zoned pluton, circles from smaller quartz monzonite pluton, diamonds from albitized (Na20/K20+Na20>0.8) samples in zoned pluton.

depletion once CaO has dropped to <5.5%. The small quartz monzonite body (Fig.2) has a composition which always places it at the most fractionated end of the compositional spectrum shown by the main monzonite-monzogabbro body (e.g. Figs. 8, 10 & 11). This suggests it is simply a separate intrusion of a batch of the same magma. The incompatible elements Ba and Sr are generally in greater abundance than in typical Iand A-type granites. Furthermore, by comparison with typical I- or S-type suites from the Lachlan Fold Belt (e.g. Chappell & White 1974), this suite shows much more extensive continuity towards relatively mafic end-members. In this sense, there is some similarity with the Boggy Plain Suite recently described by Wyborn et al (1987). Sc and V variations are consistent with fractional crystallization (Fig. 11), showing initial rapid depiction with falling CaO, but with an inflection in the trend at about 5% CaO, Ni and Cr concentrations show exponential depletion from quite high values in the most mafic rocks (e.g. 7541, Tablel) to very low values in the quartz monzonites (e.g. 9593, Tablel). This variation again is consistent with fractional crystallization. Nd and Ce generally decrease in concentration with increasing silica content with about 12 ppm in the quartz monzonites and over 50 ppm and 40 ppm respectively in the syenogabbros. Ga occurs in concentrations comparable to A-type granites (Collins etal 1982). This reflects the generally higher aluminium content, possibly reflecting enrichment during early Fe-Mg silicate fractionation.

ISOTOPE GEOCHEMISTRY

b-calc-alkaline trend, c-alkaline trend. Symbols as in Fig.6.

Whole-rock Sr isotope geochemistry was conducted on a Thompson mass-spectrometer and detailed whole-rock Rb concentrations were determined by XRF at the University of Adelaide, analyses of which are summarized in Table2. The resultant isochron records a Model 1 age of 521±35 Ma with an initial ratio of 0.70648±0.00014. The Model 3 isochron displays a severe error range of ±114 Ma with little variation in the error range of the initial ratio.

Zoned Middle Cambrian Pluton, Peake and Denison Ranges

1.0

Na20/K20+Na20

MgO

o <><*> o o

0.8

457

•D

••

• •

•

D •

•

•ID

( A d d

•

0.4

FeO*

Na20

6 -

••

•D

• a n

I—I

•-

<& §

°a

• ff

•

o -J

K20

AI203

ft cm]

•

••

%> o

<g o ••

o 12

CaO d

•

••

• 6

-L-D

Ti02

1.2

• ••

o •

0.7

•

• 2I

S i 0 2 wt.% 0/

• • •

• o

•nu

•o oo

Fig. 8 Selected Harker diagrams. Symbols as in Fig.6.

0.2

CaO wt.%

458

R.S. Morrison & J.D. Foden

Extensively albitized samples were not included as the preferential loss of Rb over Sr would erroneously shift resultant points towards the 87

Sr/ Sr axis. In conjunction with the South Australia Department of Mines and Energy, U/Pb zircon geochronology was conducted for one sample (9583). Samples from the same site were responsible for a K/Ar date of 679 Ma (Ambrose et al 1981). The broad error range of the resultant isochron of 525±35 Ma (Fanning pers. comm. 1987) was incapable of defining the age of the intrusion as pre- or syn-Delamerian, but it possibly indicates that the K/Ar date is erroneous while supporting the Rb/Sr results. These dates suggest that the intrusion may predate the Delamerian Orogenic ages assumed for the suite (c.495 Ma) by Ambrose et al (1981). Instead are they more equivalent to an age obtained from U/Pb dating of zircons in a 1 m thick sequence of late Early to Middle Cambrian tuffaceous volcanics in the Ar-

rowie Basin in the Flinders Ranges (532±12 Ma, Fanning 1987). This volcanic event corresponds with the formation of the Kanmantoo Trough along the southern and eastern margins of the MtLofty Ranges. The Truro Volcanics in the Mount Lofty Ranges, the Mount Wright Volcanics of the Barrier Ranges and the Mooracoochie Volcanics in the Warburton Basin in northern South Australia represent a slightly 160

130

100

70 1300 •

Sr ppm

•

Nb ppm

• dd •o

o 800

Rb ppm 300

0.711

100

200

Y ppm

87Sr/86Sr

0.709

A—O • CT

30 0.707

initial ratio = 0.70648 87

0.705 0.2

D / 0

•

/DCtP

Rb/86Sr 0.4

Fig. 9 Rb and Sr systematics. Symbols as in Fig.6. Filled circles represent analyses from relatively unaltered samples (Table2). Initial 87Sr/86Sr ratio for linear regression is 0.70648.

10 6

CaO wt.%

Fig. 10 Selected trace element variation diagrams. Dashed line represents 5wt% CaO. Symbols as in Fig.6.

Zoned Middle Cambrian Pluton, Peake and Denison Ranges

459

earlier period Early Cambrian volcanism (Preiss 1987; Gatehouse 1986). The initial ratio obtained (0.7065) is similar to that of the Anabama Granite (0.705) of the Nackara Arc, and the Palmer Granite (0.70680.7071) of the southern Mount Lofty Ranges, but much less than that of the Encounter Bay suite (Milnes et al 1977). The Black Hill norite body has virtually the same ratio (0.7066) as the Peake and Denison intrusives (Milnes et al 1977).

Fig. 11 Selected trace element variation diagrams. Symbols as in Fig.6.

Sample

Rb (ppm)

Sr (ppm)

7525 7531 7539 7540 7540 (R1) 7540 (R2) 7541 7542 7550 9582 9583 9588 9590 9592 9593

79.68 108.23 9.90 60.13 60.13 60.13 51.65 124.68 58.17 24.97 100.92 91.36 119.40 114.83 107.77

1229.68 896.33 639.83 1277.00 1277.00 1277.00 861.75 843.21 1346.17 1216.73 1247.71 1070.50 801.81 1037.93 984.15

Figure 12 illustrates age-initial 87Sr/86Sr relationships for granitic rocks in the Gawler Craton and southern Lachlan Fold Belt in N.S.W., Victoria and Tasmania. The South Australian Delamerian and Peake and Denison data are also included on this diagram. The Bungadillina monzonite has a lower initial 87Sr/86Sr value than all S- and most I-type granites from the Lachlan Fold Belt in N.S.W., Victoria and Tasmania. It is still however, markedly more radiogenic than the mantle growth curve at this time. In South Australia, the last major pre-Adelaidean phase of crustal growth and evolution took place during and immediately after the middle Proterozoic Kimban Orogeny (ca. 1700-1400 Ma). 87

Rb/ 86 Sr

0.32181 0.34311 0.04613 0.14075 0.14074 0.14074 0.17658 0.43026 0.12713 0.06006 0.23379 0.24949 0.43173 0.32247 0.32086

0.70872 0.70910 0.70739 0.70781 0.70726 0.70744 0.70717 0.70939 0.70729 0.70697 0.70802 0.70842 0.71019 0.70907 0.70884

Table 2: Rb-Sr isotope whole-rock data for Western Intrusives of the Peake and Denison Ranges

Sr/86Sr

460

R.S. Morrison & J.D. Foden

+ Bungadillina o Anabama

_ 0.715

•

O Palmer

0 Encounter Bay X Black Hill • Gawler Craton

Lachlan Fold Belt S--Type i A •

• • • •

1--Type |

•

00 0.709

•

0.703 -

Carnot Gneiss

l .

•

2000

Rb/Sr =^0.0285

•

2500

Gawler Range Volcanics

•

00 —

7 1

1500

1000

500

AGE (Ma)

Fig. 12 Hlustration of age-initial relationships for granitic rocks in the Gawler Craton and southern Lachlan Fold Belt in comparison with the South Australian Delamerian granites and the Bungadillina intrusive suite. Data from Blissett (1986), McCulloch & Chappell (1982), Milnes et al (1977), Richards & Singleton (1981) and Webb et al (1986).

Age-initial ratio data from Webb et al (1986) are shown on Fig. 12. These data extend from very radiogenic samples to some which are very close to the mantle growth curve. The Gawler Range Volcanic province (e.g. Blissett 1986) is shown to be bimodal with both crustal and mantle inputs. The Yardea Dacite for instance, has recently been demonstrated to be pigeonite-bearing, geothermometric results indicating extrusion temperatures of the order of 1100°C (R. Creaser, pers. comm.). This suggests major emplacement of mantle-derived melts in the lower crust at this time. Though the Sr evolution diagram (Fig. 12) only places limited constraints on the evolution of the Bungadillina suite, it does permit the following source models:

(1). An origin by re-melting of mafic rocks with moderate Rb/Sr ratios (0.05) emplaced in the lower crust during the middle Proterozoic (Kimban Orogeny). (2). An origin by partial melting of anomalously enriched contemporaiy lithospheric mantle. (3). An origin by partial melting of contemporary mantle, the composition of which is close to the mantle evolutionary trend, and subsequently contaminated by more radiogenic crustal material. EMPLACEMENT Unlike other plutons intruding Adelaidean strata such as the Anabama and Bendigo granites (Blissett & Reid 1973; Langsford 1972), the

Zoned Middle Cambrian Pluton, Peake and Denison Ranges

western-most exposed pluton of the Peake and Denison Ranges has neither a contact metamorphic aureole nor a dyke network intruding into the surrounding host beds. In contrast to the granites of the Kanmantoo Trough, the Bungadillina intrusive suite has no associated migmatites and does not occur within a zone of high-grade metamorphism (Fleming & White 1984). The contrast between strongly zoned euhedral minerals such as pyroxene, hornblende and plagioclase in low-temperature oikiociystic feldspars suggests initial crystallization at a greater depth than that of final emplacement. The presence of large rafts of sediments along the larger pluton margins indicate passive stoping as the main mode of intrusion. The local site of these intrusions may be partly determined by zones of weakness as dictated by the presence of diapiric breccia. Small circular quartz monzonite enclaves in the monzonite pluton may represent stoping of earlier formed marginal facies of the pluton. Although not in the area of immediate concern, monzonite and monzogabbro sills occur within light, steeply dipping Delamerian folds immediately northeast of the largest single pluton of the Bungadillina suite, indicating pre-orogenic emplacement. Similarly, a sill swarm intruding less competent shales interbedded in massive Mount Margaret Quartzite to the immediate northeast of the area of study suggests that emplacement was not dictated by regional Delamerian structural weaknesses, but is stratigraphically controlled. The shallow intrusion of a hot magma may have been expected to cause contact metamorphic aureoles, yet none are present around most intrusives of the Bungadillina suite, the exception being the western side of the largest single body. In order to prevent the formation of a contact metamorphic aureole, there must be an effective dispersal of the latent heat of the magma body. This may be made possible by circulating meteoric water. The interaction of water, especially if it is bearing Na+ ions, could also provide the mechanism for localized albitization (e.g. Baker 1985). If the intrusion occurred

461

during the Delamerian Orogeny, much of the fluid content of the country rocks may be expected to have already been squeezed out due to increased heat and pressure. Thus emplacement probably occurred prior to orogenic activity, intruding a wet sedimentary pile. Up to 15 km of sediments were deposited into intracratonic basins and later in a continental shelf environment comprising the Adelaide Geosyncline (Preiss & Forbes 1981). The subsequent Delamerian Orogeny, although not intense enough to significantly deform the intrusive bodies, did cause patchy low grade metamorphism as evident from abundant epidote nodules (Smith 1977) as opposed to syn-intrusive epidote veins (Marzouki et al 1979). Mafic anorogenic margins originating in lower crust can be readily emplaced to high levels via fractures due to crustal isostatic readjustments (Castro 1987). In the Peake and Denison Ranges, the area of plutonism in the northern section of the Margaret Inlier has been postulated to coincide with a major east-west shear zone, the Karari fault lineament, sporadically active since the Early Proterozoic (Ambrose et al 1981; Flint & Parker 1982; Rankin et al 1987). Yet there is no evidence to indicate the presence of any such structure in the Late Proterozoic strata. If a major crustal shear zone does exist, it must occur between the Margaret and Denison Inliers and is thus not exposed. The zoned nature of the pluton is best explained by autointrusion of a fractionating diapir (e.g. Nabelek et al 1986). Crystallization and fractionation would have occurred at depth prior to emplacement. The expanding diapir model involving reverse zonation would develop in response to a denser more mafic core collapsing into a felsic intruding magma from a progressively fractionating source. This style of emplacement by cauldron subsidence is typical in anorogenic environments (Castro 1987). Relict mafic margins are preserved on both sides of the pluton as more altered monzogabbro, the margins being more accessible to the metasomatizing effects of circulating meteoric water.

462

R.S. Morrison & J.D. Foden

CONCLUSIONS U/Pb and Rb/Sr isotope systematics in conjunction with the nature of emplacement, alteration effects and regional metamorphism, indicate that the intrusion predates the Delamerian Orogeny. It is probably more closely associated with late Early to Middle Cambrian volcanic activity of limited extent within the Adelaide Geosyncline, occurring in response to a renewal of extensional tectonism. This magmatism must have either involved a contemporary input from the mantle or possibly partial fusion of mafic rocks from the lower-most crust. Cambrian rifting probably provided the basin into which the Kanmantoo sediments were deposited and was also associated with the intrusion of pre-Fi mafic dykes in the Mount Lofty Ranges (Fleming & White 1984). This extensional phase was terminated by compression at the onset of the Delamerian Orogeny.

ACKNOWLEDGEMENTS XRD analysis was presided over by J. Stanley; wet chemistry directed by R McDuie; aid in mass-spectrometry, D. Bruce; aid in isotope preparation, P. Macdonald & J. Cooper. The original manuscript was immensely improved by M. Sandiford. Fieldwork completed with financial and logistical assistance from Union Oil Development Corporation and the Regional Geology Division of the South Australia Department of Mines and Energy under the authority of Dr A.J. Parker.

REFERENCES AMBROSE G.J., FLINT R.B. & WEBB AW. 1981. Geology of the Peake and Denison Ranges. Geological Survey of South Australia, Bulletin 50, 1-71. BAKER J.H. 1985. Rare earth and other trace element mobility accompanying albitization in a Proterozoic granite, W. Bergslagen, Sweden. Mineralogical Magazine 49, 107-115.

BEDARD J.H.J., LUDDEN J.N. & FRANCIS D.M. 1987. The Megantic intrusive complex, Quebec: a study of the derivation of silica- oversaturated anorogenic magmas of alkaline affinity. Journal of Petrology 28, 355-388. BLISSETT A.H. 1986. Subdivision of the Gawler Range Volcanics in the Gawler Craton. Quarterly Geological Notes of the Geological Survey of South Australia 97,2-11. BLISSETT A.H. &REID J.A. 1973. Geological survey investigation of the Anabama copper and molybdenum prospect, Olary, Wadnaminga. South Australia Department of Mines and Energy Report Book 73/4 (unpubl.). CASTRO A. 1987. On granitoid emplacement and related structures. A review. Geologische Rundschau 76,101-124. CHAPPELL B.W. & WHITE A.J.R. 1974. Two contrasting granite types. Pacific Geology 8,173-174. COLLINS W.J., BEAMS S.D., WHITE A.J.R. & CHAPPELL B.W. 1982. Nature and origin of A-type granites with particular reference to southeastern Australia. Contributions to Mineralogy and Petrology 80,189-200. FANNING C.M. 1987. Amdel report G6988#07 & G6677/86. South Australian Department of Mines and Energy Envelope 5530 (unpubl.). FLEMING P.D. & WHITE A.J.R. 1984. Relationships between deformation and partial melting in the Palmer migmatites, South Australia. Australian Journal of Earth Sciences 31, 351-360. FLINT R.B. & PARKER A.J. 1982. Tectonic map of South Australia 1:2000000. Geological Survey of South Australia, Adelaide, S A. GATEHOUSE C. 1986. The geology of the Warburton Basin in South Australia. Australian Journal of Earth Sciences 33, 161-180. HAMMARSTROM J.M. & ZEN E. 1986. Aluminium in hornblende: An empirical igneous barometer. American Mineralogist 71,1297-1313.

Zoned Middle Cambrian Pluton, Peake and Denison Ranges

HELZ R.T. 1973. Phase relations of basalts in their melting range at PH20=5kb as a function of oxygen fugacity. Parti. Mafic phases. Journal of Petrology 14, 249-302. HOLLISTER L.S., GRISSOM G.C., PETERS E.K., STOWELL H.H. & SISSON V.B. 1987. Configuration of the empirical correlation of A1 in hornblende with pressure of solidification of calc-silicate plutons. American Mineralogist 72, 231-239. HYNES A.J. 1980. Carbonatization and mobility of Ti, YandZr in Ascot Formation metabasalts, S.E. Quebec. Contributions to Mineralogy and Petrology 75,79-87.

463

NABELEK P.I., PAPIKE J.J. & LAUL J.C. 1986. The Notch Peake granite stock, Utah: origin of reverse zoning and pedogenesis. Journal of Petrology 27, 1035-1069. OTTEN M.T. 1984. The origin of brown hornblende in the Artfjallet gabbro and dolerites. Contributions to Mineralogy and Petrology 86,189-199. POWELL R. & POWELL M. 1977. Plagioclase-alkali feldspar geothermometry revisited. Mineralogical Magazine 41, 253-256.

KRESTEN P. & MOROGAN V. 1986. Fenitization at the Fen complex, southern Norway. Lithos 19, 27-42.

PREISS W.V. (compiler) 1987. The Adelaide Geosyncline. Geological Survey of South Australia, Bulletin 53,1-438.

LANGSFORD N.R. 1972. Kia-Ora-Southdam project. BURRA 1:250000 sheet reconnaissance drilling. South Australia Department of Mines and Energy Report Book 72/131 (unpubl.).

PREISS W.V. & FORBES B.G. 1981. Stratigraphy correlation and sedimentary history of the Adelaidean (Late Proterozoic) basins in Australia. Precambrian Research 15,255-304.

MARZOUKI F., KERRICH R. & FYFE W.S. 1979. Epidotization of diorites at A1 Hadah, Saudi Arabia: fluid influx into cooling plutons. Contributions to Mineralogy and Petrology 68, 281-284.

RANKIN L.R., MARTIN A.R. & PARKER A.J. 1987. Identification of a major crustal shear zone, northwest Gawler Craton, S.A. South Australia Department of Mines and Energy Report Book 87/30 (unpubl.).

McCULLOCH M.T. & CHAPPELL B.W. 1982. Nd isotope characteristics of S- and I-type granites. Earth and Planetary Science Letters 58, 51-64. MILNES A.R., COMPSTON W. & DAILY B. 1977. Pre- to syn-tectonic emplacement of Early Palaeozoic granites in southeastern South Australia. Journal of the Geological Society of Australia 24, 87-106. MIYASHIRO A. 1979. Metamorphism and metamorphic belts (4th edition). George Allen & Unwin Ltd, London. MONGKOLTIP P. & ASHWORTH J.R.L. 1986. Amphibolitization of metagabbros in the Scottish Highlands. Journal of Metamorphic Geology 4, 261-283. MURPHY J.B. & HYNES A.J. 1986. Contrasting secondary mobility of Ti, P, Zr, Nb and Y in two metabasaltic suites in the Appalachians. Canadian Journal of Earth Sciences 23, 1138-1144.

REYNER M.L. 1955. The geology of the Peake and Denison region. Geological Survey of South Australia, Report of Investigations 6,1-16. RICHARDS J.R. & SINGLETON O.P. 1981. Palaeozoic Victoria, Australia: igneous rocks ages and their interpretation: Journal of the Geological Society of Australia 28, 395-422. SHAND S.J. 1950. The Eruptive Rocks, their genesis, composition, classification, and their relation to ore-deposits with a chapter on meteorites (4th edition). Thomas Murby & Co., London. SMITH R.E. 1977. Petrography and geochemistry of epidote alteration patches in gabbro dykes at Matagami, Quebec: discussion. Canadian Journal of Earth Sciences 14, 505-507. SORENSEN H. (ed.) 1974. The Alkaline Rocks. John Wiley & Sons, London.

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WEBB A.W., THOMSON B.P., BLISSETT A.H., DALY S.J., FLINT R.B. & PARKER A.J. 1986. Geochronology of the Gawler Craton. Australian Journal of Earth Sciences 33,119-144. WHALEN J.B., CURRIE K.L. & CHAPPELL B.W. 1987. A-type granites: geochemical characteristics, discrimination and pedogenesis. Contributions to Mineralogy and Petrology 95,407-419. WHITNEY J.A. & STORMER J.C. 1976. Geothermometry and geobarometry in epizonal granitic intrusions: a comparison of iron-titanium

oxides and co-existing feldspars. American Mineralogist 61, 751-761. WONES D.R. & GILBERT M.C. 1982. Amphiboles in the igneous environment. In Verblen D.R. & Ribbe P.H. eds. Amphiboles: Petrology and experimental phase relations. Mineralogical Society of America, Reviews in Mineralogy 9B, 355-389. WYBORN D., TURNER B.S. & CHAPPELL B.W. 1987. The Boggy Plain Supersuite: a distinctive belt of I-type igneous rocks of potential significance in the Lachlan Fold Belt. Australian Journal of Earth Sciences 34, 21-43.

Tectonic implications of Delamerian magmatism in South Australia and western Victoria J.D. Foden, S.P. Turner and R.S. Morrison Department of Geology and Geophysics, University of Adelaide, G.P.O. Box 498, Adelaide, 5001, Australia

SA.

A comparison of the early Palaeozoic igneous history of the Glenelg Metamorphic Complex in western Victoria with that of the Delamerian rocks in southern South Australia supports the suggestion that they are part of a single province. In both areas, Early Ordovician, syn-tectonic, I-type dioritegranodiorite-granite magmas intrude Cambrian metasediments. With these syn-Delamerian intrusives, a younger Ordovician series of high-level, silicic, A-type granites and acid volcanics occur across SE South Australia and into western Victoria. This post-tectonic phase of Ordovician felsic magmatism was associated with mafic dyke emplacement and the intrusion of large mafic plutons in a probable extensional environment. The highly fractionated, felsic, post-Delamerian magmas are enriched in F, Ga, Nb, LREE and Y. These were generated in a second-stage melting event as the products of re-melting of the same mafic lower crustal source that produced the syn-tectonic granites. This resulted from the rise of post-tectonic, mantle-derived, mafic plutons. Cambrian sedimentation before the Delamerian orogeny was associated with contemporary basaltic to trachytic volcanism and the intrusion of mafic sills. Cambro-Ordovician magmatism in this "Delamerian province" therefore records a tectonic history of extension, collision and then further extension. In a reconstructed Gondwanaland, the Delamerian granites of South Australia and western Victoria form part of an extensive belt which extends through western Tasmania and the eastern part of the Wilson terrane in Victoria Land.

Key words: Delamerian Orogeny, geochemistry, A-type granite, I-type granite, syn-tectonic, post-tectonic.

INTRODUCTION

Recent studies, including those of Von der Borch (1980), Jenkins (1986, this volume) and Clarke & Powell (1989) have promoted a critical appraisal of the tectonic character of the Adelaide Fold Belt. This fold belt has a sedimentation history extending from the Adelaidean through to the Late Cambrian (Daily & Milnes 1972) and experienced the effects of the Delamerian Orogeny in the Late Cambrian to Early Ordovician. This orogenic event resulted in complex, polyphase deformation (e.g. Mancktelow 1980) and a Buchan-style metamorphism extending at the highest grade zones to upper amphibolite facies (Offler & Fleming 1968; Sandiford et al this volume).

Von der Borch (1980) proposed that Adelaidean sedimentation proceeded in a developing rift. This tensional environment continued into the Cambrian where the Kanmantoo Group, at least in part, developed an open marine character with some sand-silt units probably representing proximal turbidites. By this stage, in the southern part of the fold belt, accretion of some oceanic crust may have occurred to the east. The extensional phase was arrested and reversed by the Delamerian Orogeny in the Early Ordovician and we propose that following this event, minor lithospheric relaxation and crustal extension followed during the Middle Ordovician.

466

J.D. Foden, S. Turner & R.S. Morrison

In western Victoria, the Glenelg River Complex (1956; Vandenberg 1978) is considered an easterly extension of the South Australian "Delamerian province". Unfortunately the intervening areas in the southeast of South Australia, to the east of the Murray River, are covered by Cainozoic and Mesozoic marine and terrestrial sediments. The only basement outcrops are small, resistant exposures of mainly granitic rocks and more rarely, acid volcanics (Fig. 1). Syn- and post-Delamerian igneous activity is recorded by felsic and mafic intrusions which outcrop to the east of the sinusoidal axis of the Adelaide Fold Belt. Cambrian pre-tectonic magmatism also occurred, resulting in syn-sedimentary volcanism and probable early sill injection. These igneous rocks have been the subject of relatively few studies. It is the aim of this paper to present new reconnaissance data and to provide a synopsis of that already existing. It has been recognised for some time (e.g. Milnes et al 1977; Mancktelow 1979) that the South Australian Palaeozoic granite suites included both syn- and post-tectonic associations. In this paper we intend to discuss the timing and compositional relations of Early Palaeozoic magmatism in the southern Adelaide Fold Belt as a means of assessing the tectonic model described above.

GEOLOGICAL SETTING OF THE IGNEOUS ROCKS Mt Lofty Ranges In the southern Mt Lofty Ranges, most intrusive igneous rocks are restricted to the Kanmantoo Group. Granitic intrusives occur at Palmer, Reedy Creek, Monarto, in the Victor Harbor area (Encounter Bay Granites) and at Cape Willoughby and Remarkable Rocks on Kangaroo Island (Fig. 1). There are also smaller intrusions in the Tanunda Creek and Cookes Hill areas north of Palmer and at Long Ridge, Man-

num and Murray Bridge in the Murray Valley, just to the east of the Mt Lofty Ranges. The Rathjen Gneiss (Fig. 1) (White 1966) is a highly foliated, sheet-like body of strongly deformed granite in the Palmer Area. It has a prominent north-trending lineation and is very like the granitic gneiss at Tanunda Creek (Chinner 1955). The Black Hill Norite (Wegmann 1980) is a major, but poorly exposed mafic intrusion to the east of the Mt Lofty Ranges north of Mannum (Fig. 1). No contacts are exposed, but it is thought to be emplaced in Kanmantoo Group metasedimentary rocks. Diamond drilling here has intersected several granitic or tonalitic bodies (Wegmann 1980), some of which cross-cut the norite and others with strong deformational fabrics, which may be roof septa or xenoliths. The Black Hill Norite is not metamorphosed and observed deformation is restricted to discrete shear zones. We therefore regard this body as effectively post-tectonic. Though almost all the Cambro-Ordovician igneous rocks in the southern part of the Adelaidean Fold Belt are restricted to the Kanmantoo Group, the pegmatitic Mt Crawford granite is an exception. This intrudes the Adelaidean rocks close to the contact with a Mid-Proterozoic crystalline basement inlier, the Barossa Complex (Mills 1973). Mafic intrusive rocks occur in the southern Mt Lofty Ranges mainly as sub-alkaline, dolerite dykes many of which are post-tectonic. These form swarms mostly oriented NW-SE, and are well represented in the Mt Barker-Woodside area and near Mt Kitchener south of the Barossa Valley. Likewise, the strongly foliated Reedy Creek granodiorite, the much less deformed and crosscutting Reedy Creek diorite and the post-tectonic Mannum granite are all cut by late-stage, undeformed mafic dykes. These mafic dyke rocks are apparently confined to the Cambrian Kanmantoo and Norman-

Delamerian Magmatism

467

LOCATION Truro /vj A/olcanics0]J

$ Sedan Q^Tanunda Ck.

GULF VINCENT

• fethi6n • Black Hill Norite Crawford ^ j ^ G n e i s s ^ S m C o o k e Hill

£ Palmer^ ADELAIDE^ 0 0 / d s j d e ( / ( Mt. / / Barker *

$ Mannum Reedy Ck. Monarto Murray Bridge

Victor Harbor ^ C o o n a l p y n granites (^Tintinara granites KEITH Mt. Monster

KANGAROO ISLAND

Didicoolum

Christmas Rocks Kongal Rocks ^•Willalooka granites

Cape Willoughby

Taratap

^Marcollat

<5> Bin Bin <S> Papineau Rocks

Remarkable Rocks KINGSTON

Harrow

LEGEND Syn-tectonic Granite Post-tectonic Granite Volcanic or Subvolcanic

Dergholm Hummocks Serpentinite CASTERTON

;: Wando ::. tonalite gneiss)

Wando Granodiorite

Kanmantoo Trough Sediments (incl. Glenelg Metamorphic Complex)

Fig. 1 Location of Delamerian granitic rocks in southern Mt Lofty Ranges, southeastern South Australia and western Victoria.

468

J.D. Foden, S. Turner & R.S. Morrison

ville Groups. Even where the Adelaidean is adjacent to dyke-rich Cambrian sequences (as in the Woodside area) these dykes do not intrude the Proterozoic rocks. Many of the amphibolite bodies however appear to be deformed and have Fi or F2 fabrics (Fleming & White 1984). Some of these syn- or pre-tectonic dykes occur in the Cookes Hill area south of Springton (Abbas 1975), and they are also observed in the Monarto area where they are cut by pegmatitic granite (Hoesni 1985). Likewise, in the Tungkillo area to the west of Palmer, numerous amphibolite bodies up to 3 m thick, are conformable with and have suffered the same deformation as Kanmantoo Group metasediments. These must therefore be either early sills, or basaltic lava flows. Clear evidence of pre-tectonic magmatic activity is provided by the basaltic to trachytic Truro Volcanics (Forbes et al 1972). These are conformably interbedded with shales and sandstones which form part of a sequence which is equated with the Normanville Group (Jago pers. comm.). The volcanic rocks indicate shallow water deposition. They are highly vesiculated, have brecciated flow tops and display ballistic bombsag features. This magmatic activity is broadly contemporaneous with extensive volcanism in western Tasmania (Vame & Foden 1987) and with high level monzonite to gabbro intrusions in the Peake and Denison Ranges in the northern Adelaide Fold Belt (Morrison 1988; Morrison & Foden this volume).

Victoria to the southeast. This basement high is known as the Padthaway Ridge (Rochow 1971). The granite at Taratap, near Kingston, lies to the south of this belt and differs petrographically from those granites. Most of the "southeast" granites are siliceous, post-tectonic bodies, though that at Taratap (Fig. 1) is a biotite granodiorite with a mica-defined fabric. Post tectonic volcanic rocks of rhyolitic or dacitic composition (Henstridge 1970) outcrop with the Padthaway Ridge granites, suggesting that comagmatic granites were high level. Some sub-surface drilling at various localities on the Padthaway Ridge suggests that between the Adelaide Fold Belt and the Glenelg River district in western Victoria, there is an extensive concealed tract of Early Palaeozoic mafic to felsic magmatic rocks and associated meta-sediments (e.g. Parker 1986). The Glenelg Metamorphic Complex The Glenelg Metamorphic Complex represents the most westerly exposure of Palaeozoic rocks in Victoria (Fig. 1). The geology of the region has been studied by Wells (1956) and Turner (1986). Here a metasedimentary sequence of pelitic to psammo-pelitic composition has suffered polyphase deformation under low pressure, amphibolite facies conditions. Pre- to syn-tectonic igneous intrusive suites include the Wando Granodiorite and a series of gabbroic dykes or sills. The I-type Wando Granodiorite is compositionally like the Reedy Creek granodiorite in the Mt Lofty Ranges.

Southeastern South Australia From east of the Murray River to the Victorian border, the only occurrences of Palaeozoic rocks are scattered outcrops of granite and very rare acid volcanic rocks, protruding from Quaternary sediments. Particular localities include Mt Boothby, Willalooka, Gip Gip Rocks, Marcollat, Mt Monster, Christmas Rocks and Kongal. This series of isolated outcrops forms a linear SEtrending ridge including the Sedan and Mannum granites to the north and the Dergholm Granite in

In the same area, the Harrow migmatitic granite is also syn-tectonic. This is an S-type intrusion which contains numerous metapelitic xenoliths (Fig. 1). The Glenelg igneous-metasedimentary sequence is in faulted contact with ultramafic rocks (the Hummocks Serpentinite). These have harzburgite compositions and relict textures suggestive of olivine-orthopyroxene primaiy mineralogy. Preserved Cr-spinels are the same as

Delamerian Magmatism

those from the tectonite-harzburgite zone of ophiolites. Intruding this sequence is a post-tectonic pluton, the Dergholm Granite, which is texturally as well as compositionally similar to the post-tectonic Delamerian granites to the west. The metasedimentary rocks in the Glenelg Metamorphic Complex preserve early isoclinal folds with an axial-planar, Si schistosity which was refolded by at least one other event (Turner 1986). These structures are much more like the Delamerian structures exhibited by the Kanmantoo Group in the Mt Lofty Ranges, than those of the Benambran Orogeny seen in Ordovician rocks further east in Victoria (in the Bendigo area for instance; Vandenberg 1978; King 1985).

GEOCHRONOLOGY AND ISOTOPE GEOLOGY Webb (1976) provides the most comprehensive review of isotopic and geochronological studies carried out on Delamerian granitic and volcanic rock from South Australia. Other studies of specific plutons include those of Milnes et al (1977) (Encounter Bay granites) and White et al (1967) (Palmer Granite). The only data available are those for the K/Ar and Rb/Sr systems. As discussed by Milnes et al (1977) Rb/Sr dating of the syn-tectonic granites (Encounter Bay and Palmer) is complicated by resetting. The general conclusion is that the syn-tectonic granites (Palmer, Encounter Bay and Taratap) record slightly greater ages (515-475 Ma) than the post-tectonic granites and volcanics (479-460 Ma). Significantly, the initial 87Sr/86Sr ratios of the posttectonic group are very low (0.7047-0.7096) implying that derivation took place by melting of young contemporary source rocks which had low Rb/Sr ratios (i.e. mafic rocks). The syn-tectonic granites have higher and more variable initial 7 Sr/86Sr ratios and may reflect the influence of contamination by country rock material such as the Kanmantoo Group.

469

PETROLOGY Syn-tectonic granites The syn-tectonic granite suites such as Wando and Reedy Creek show wide compositional ranges from dioritic end members through tonalite and granodiorite to granite (Table 1). The more mafic end members are plagioclase- (An53-Anio) quartz-hornblende-biotite quartz diorites. Sphene, zircon and apatite are important accessory phases, together with epidote. Felsic endmembers of the syn-tectonic series are composed of meso-perthitic microcline, quartz, plagioclase (oligoclase), biotite, sphene, magnetite and apatite and range from granodiorite (Reedy Creek) to quite siliceous true granite (the Encounter Bay granites). The syn-tectonic granites show development of biotite-defined foliation. At Reedy Creek this fabric is strong in the granodiorite and quite weak in the younger diorite which cuts it. Milnes et al (1977) describe strong, gneissic fabrics in the margins of granites at Victor Harbor. The syn-tectonic granites contain xenoliths of diorite and amphibolite as well as metapelitic ones derived locally from the Kanmantoo Group. Geological relationships between the dioritic and more felsic end members of these suites are in places complex. At Reedy Creek, the diorite intrudes the granodiorite, though they exhibit good compositional continuity (e.g. Moeller 1980 and Fig. 2), suggesting they are related magmas. At Encounter Bay, there are extensive rafts of quartz diorite included in the felsic granite. At Wando Vale, tonalite and granodiorite are deformed and support the early, regional fabric and are intruded by the more felsic Wando Granodiorite. The most mafic Wando granitoids include diorite which is hornblende- (Ca 30:Mg 40:Fe 30, Mg/Mg+Fe 0.45-0.59, Ti0 2 0.7-1.2%) and plagioclase (An5o)-bearing and of cumulate origin. In both the Reedy Creek and Wando syntectonic suites, compositional evolution from

Sample

Si0 Ti0

53.58 1.46 18.61 8.35 0.14 4.13 7.52 4.15 1.75 0.50

62.66 0.46 18.08 4.56 0.13 2.25 5.99 3.67 2.04 0.15

64.45 0.73 16.76 4.46 0.09 2.31 4.76 4.19 2.00 0.25

65.47 0.76 16.61 4.16 0.07 1.78 3.87 4.43 2.60 0.24

68.65 0.74 14.74 4.27 0.07 1.51 2.42 2.18 5.21 0.19

71.59 0.28 15.48 2.02 0.02 0.61 1.93 4.74 3.24 0.08

73.21 0.21 15.02 1.30 0.01 0.47 1.52 4.39 3.82 0.06

73.39 0.26 14.30 2.26 0.06 0.94 1.44 2.88 4.30 0.17

73.48 0.41 13.22 2.47 0.02 1.01 1.32 3.43 4.52 0.06

73.47 0.32 14.10 1.59 0.02 0.61 1.19 3.98 4.56 0.05

74.60 0.27 13.34 1.83 0.06 0.37 1.49 4.01 4.01 0.06

75.58 0.26 12.47 1.66 0.04 0.74 0.82 2.96 5.40 0.06

47.00

48.00

43.00

39.00

35.00

39.00

43.00

n.d.

2 2

203

FeO MnO MgO CaO Na.0 KO PA 2

Mg#

19 12 82

28 16 11 56

73 880 719 n.a.

80 1389 522 n.a.

210 827 192 21

16 175 17

21 304 23

14 237 34

31 26 215

19 8 15 99

21 13 104

Rb Ba Sr Ga

56 791 737 n.a.

74 474 386 20

Nb Zr Y

22 229 31

8 134 16

Cr Ni Sc V

n.d.

42.00

41.00

27.00

44.00

n.d.

n.a. 3 n.a.

27 12 10 36

95 1396 592 n.a.

115 1605 487 n.a.

178 654 174 18

189 735 113 n.a.

86 579 95 n.a.

139 253 66 n.a.

314 340 59 n.a.

16 211 7

25 163 7

11 116 26

13 113 27

22 249 21

14 155 28

15 112 58

n.a. 3 n.a.

n.a. 6 n.a.

16 4 16

n.a. 5 n.a.

La Ce Nd

n.a. 56 30

23 46 15

n.a. 50 21

n.a. 87 29

50 82 36

n.a. 90 n.a.

n.a. 88 17

22 35 13

n.a. 20 n.a.

n.a. 115 43

n.a. 49 n.a.

n.a. 57 30

n.a.

142

n.a.

944

n.a.

354

n.a.

Rb/Sr Ba/Rb K 0/Na 0 Ce/Y Nb/Y

0.08 14.02 0.42 1.81 0.72

0.19 6.36 0.56 2.89 0.53

0.10 11.99 0.48 2.83 0.91

0.15 17.30 0.59 3.74 0.91

0.16 14.62 0.68 12.71 2.29

0.24 13.90 0.87 12.57 3.57

1.02 3.67 1.49 1.35 0.42

1.68 3.88 1.32 0.74 0.48

0.90 6.69 1.15 5.38 1.05

2.11 1.81 1.00 1.75 0.50

5.27 1.08 1.82 0.98 0.26

Table l:Chemistry of selected Syn-Tectonic granites

0.10 3.93 2.39 2.38 0.41

1. Reedy Creek diorite; 2. Reedy Creek Granodiorite; 3. Average Wando Granodiorite; 4. Syn-tectonic Black Hill tonalite; 5. Taratap Granite;

6. South Monarto Granite; 7. North Monarto Granite; 8. Average Harrow Granodiorite; 9. Syn-tectonic Black Hill Granodiorite; 10. Cape Willoughby Granite;

11. Palmer Granite; 12. Average Encounter Bay Granite; n.d. = not detected; n.a. = not analysed.

I Tj

CD P 00 H c 3

3. O P

Delamerian Magmatism tonalite or diorite to granodiorite, is controlled by plagioclase-hornblende crystallisation. Mineralogically and geochemically these granites are I-type according to the criteria of Chappell & White (1974,1982). Mafic end members are hornblende-bearing while even very felsic granites such as the Encounter Bay series are magnetite-sphene bearing. Aluminous phases such as cordierite do not occur and most rocks are metaluminous. 19 AI203 %

Si02% 55

6 CaO%

MgO%

Fig. 2 MgO- variation diagrams (versus CaO, AI2O3

and S1O2), illustrating the compositional range of I-type, syn-tectonic suites from Wando in the Glenelg Metamorphic Complex and Reedy Creek in the Mt Lofty Ranges (Moeller 1980). Open circles= felsic Wando granodiorite; filled circles= Wando tonalite; open squares= Reedy Creek granodiorite; filled squares= Reedy Creek diorite.

471

Mafic end members of the syn-tectonic granite spectrum have high FeO, MgO, CaO, Ti02and AI2O3 (Fig. 2). Their Mg/Mg+Fe values are close to 0.50 and Sc, Ni and V levels are high. These rocks have moderate K2O contents, low K20/Na20 ratios, high Sr and moderately high Ba contents. Their Rb/Sr ratios are basalt-like (<0.25) (Fig. 3). Granodiorite in this province (e.g. Reedy Creek or Taratap, Table 1) is very similar in composition to the average Lachlan Fold Belt I-type granite (e.g. Chappell & White 1982; Whalen etal 1987). These are quite calcic, sodic granodiorites, with silica contents between 62 and 69% and Mg/Mg+Fe ratios about 0.37-0.47 (Fig. 2). Concentrations of Ni, V, Sc and Ti are much lower than those of associated diorites, but are still fairly high. Likewise, Sr levels are lower, but still quite high, and Rb/Sr ratios are quite low (<1.0). Ba concentrations are high, and Ba/Rb ratios are very high (Fig. 3). The incompatible element patterns of these granodiorites are very similar to that of the average Lachlan Fold Belt I-type (Fig. 4). The granites in the Encounter Bay area (Victor Harbor, Cape Willoughby), the Palmer granite and those in the Monarto area to the west of Murray Bridge, are very felsic members of this syn-tectonic suite (Table 1). These are true granites with high Si02 (73-75%), K20/Na20 ratios >1, low CaO, MgO and FeO and with low Mg/Mg+Fe ratios in the range 0.3-0.4. They have lower Sr and higher Ba contents and higher Rb/Sr and Ba/Sr values than the granodiorites (Fig. 3). Their incompatible trace elements patterns are very similar to that of the average felsic Lachlan Fold Belt I-type granite (Fig. 5) given by Whalen etal (1987). By comparison with the granodioritic members of the syn-tectonic group, these felsic granites have lower Ti contents and similar or depleted Nb, Zr and Y. By comparison with the post-tectonic granites, which are also very siliceous, these felsic syn-tectonic granites are less depleted in Sr, are markedly more Ba-rich, have much lower

472

J.D. Foden, S. Turner & R.S. Morrison

Rb/Sr ratios and have much lower Nb, Zr and Y contents. These contrasts are illustrated in Fig. 6b. They have lower Fe/Mg ratios and have lower F and Ga contents. Evidence for local syn-tectonic melting

Migmatite and pegmatite is abundant in the highest metamorphic grade zones of the Mt Lofty Ranges. These are derived by partial melting of pelitic and feldspathic units of the Kanmantoo or Normanville Groups (White 1966). The generation of such melts spans the deformational history of the belt from the deformation of earliest migmatite leucosomes by Fi, through to the formation of tourmaline-rich, post-F3 pegmatite

(Fleming & White 1984). At Reedy Creek, the foliated granodiorite cross-cuts the migmatite. There is evidence that this migmatite underwent some remelting during intrusion of both the granodiorite and the diorite. The granodiorite and diorite at Reedy Creek are also cross-cut by later pegmatite, aplite and leucogranite veins. In the Mt Lofty Ranges, the suggestion made by Clarke & Powell (1989) that the Kanmantoo Group was emplaced as a hot allochthon has interesting implications for the generation of migmatite. Was this local melting due entirely to thermal effects, or was it also promoted by fluid flux from the adjacent cold and hydrated autochthon?

100

Nb p p m

Log FeO*/MgO

Fig. 3 (a). Ba/Rb versus ppm Nb variation for average or typical examples of South Australian and western Victorian plutons for which data are available. The analyses plotted in each of Figs. 2, 3,4, 6 and 7 are given in Tables 1 and 2. Symbols: open squares = syn-tectonic granites; filled diamonds= post-tectonic granites, (b). Ba/Rb versus ppm Y variation for the same typical representative Delamerian granites as in Fig. 3a. Asterix = average Lachlan Fold Belt A-type granite; half-filled square = average Lachlan Fold Belt I-type granite; open circle= average felsic I-type granite (Whalen etal 1987). (c). Log Rb/Sr versus ppm Nb variation for the same representative Delamerian granite compositions from South Australia and western Victoria as in Fig. 3a. (d). Log FeO/MgO versus Log Rb/Sr variation.

Delamerian Magmatism The syn-tectonic granites also include the Harrow Granodiorite, amigmatitic granite which occurs near the Wando area (Fig. 1). This is an S-type granite, apparently derived by melting of the local metapelites. The granite contains abundant biotite-rich schleiren which are the residue of this melting. In contrast to most other granites considered in this paper, the Harrow Granodiorite has Al203/Ca0+Na20+K20>l.l. The biotite compositions are also different from those of the other granites in that they are much more aluminous. They contain about 3.5 atoms of Al/formula (/22 oxygens) while the other (I-type) granites contain less than 3 (Table 3).

473

Amongst the very felsic Encounter Bay granites there are some that have Al203/Ca0+Na20+K20 ratios slightly greater than 1.1. As White et al (1986) point out this is not a necessary indication of S-type character. Metaluminous melts may evolve to become peraluminous close to their solidi in equilibrium with hornblende (e.g. Ellis & Thompson 1986). Furthermore, there is a strong possibility that the Encounter Bay granites have become contaminated by digestion of Kanmantoo Group metasedimentary country rock. This suggestion is both supported by field observation and Srisotope data (Milnes et al 1977). The post-tectonic granites

~ Wando Granodiorite

0.1

—

Rb K Nb La Sr Zr Ba Ce Nd -

Y b

Black Hill tonalite

Members of the post-tectonic group are all very siliceous, true granites. They are often very homogeneous with interlocking, xenomorphicgranular textures. Some are one feldspar granites (e.g. Marcollat, Table 2) and all are alkali feldspar-, quartz-rich and relatively plagioclasepoor. Plagioclase is often included in alkali feldspar. These granites are characterised by smokey quartz. The alkali-feldspar is very coarse perthite, in which the abundant albite exsolution is often contiguous with albite rims.

W) \

- Wando Granodiorite -

ill

i iiiI i

Ce Nd

Reedy Creek Granodiorite 1

Fig. 4 Average continental crust-normalised incompatible element variation diagrams showing: (a)Wando granodiorite and the average Lachlan Fold Belt I-type granite (Whalen et al 1987); and (b) typical tonaltic or granodioritic members of the South Australian and western Victorian Delamerian province.

Fig. 5 Average continental crust-normalised incompatible element variation diagram showing the average Lachlan Fold Belt felsic I-type granite (Whalen etal 1987) and a typical example of a very felsic granite from the Delamerian province (Cape Willoughby).

SiO,

Sample

75.40

77.37

77.35

77.87

74.39

72.57

75.56

73.63

73.26

74.14

Ti02

0.19

0.08

0.06

0.09

0.22

0.38

0.19

0.37

0.20

AI 2 O 3

12.92

12.20

12.37

11.68

13.70

13.83

12.22

13.29

13.58

12.59

FeO

1.21

1.44

1.11

1.59

1.34

1.93

2.13

2.61

2.14

3.32

MnO

0.02

0.01

0.05

0.07

0.05

MgO

0.05

0.37

0.01

0.10

0.49

0.55

0.08

0.23

0.28

0.11

CaO

0.53

0.15

0.45

0.56

1.33

0.87

Na 2 0

3.45

4.03

3.82

3.32

3.94

4.03

3.77

3.74

3.79

K2O

3.87

5.92

4.69

4.81

4.73

4.49

5.69

5.36

5.45

5.18

4.76

0.01

0.02

0.01

0.04

0.08

0.02

0.03

0.07

0.03

6.00

14.00

19.00

6.00

n.d.

Mg#

35.00

1.00

10.00

39.00

34.00

n.d.

0.59

0.78

1.27

0.94

n.a.

188

378

298

347

210

197

109

186

279

188

119

110

495

643

384

431

303

100

136

n.a.

a a> p

H c

3 a>

197

202

152

109

159

335

392

246

280

249

160

102

n.a.

n.a. 166

n.a.

106

152

251

166

118

119

n.a.

523

1754

1364

384

714

553

777

n.a.

1122

Rb/Sr

5.17

25.23

28.74

17.56

2.10

1.45

10.38

Ba/Rb

3.60

0.63

0.29

0.26

0.20

2.35

3.25

0.70

2.06

1.55

K 2 0/Na 2 0

1.71

1.16

1.26

1.42

1.14

1.41

1.42

1.46

1.37

1.23

Ce/Y

1.19

0.49

0.73

1.14

2.31

2.45

6.58

3.27

1.89

Nb/Y

2.27

0.34

0.68

0.32

0.41

0.76

0.79

0.51

0.39

0.56

0.35

Table 2:Chemistry of selected post-tectonic granites

-J

1. Post-tectonic Black Hill Granite; 2. Christmas Rocks Microgranite; 3. Dergholm Granite; 4. Kongal Rocks Granite;

5. Sedan Granite; 6. Mannum Granite; 7. Marcollat Granite; 8. Mount Monster Porphyry;

2.97

3.28 1.61

9. Murray Bridge Granite; 10. Willalooka Granite; n.d. = not detected; n.a. = not analysed.

O 00

3. C/3

O 3

Amphibole analyses

Biotite analyses 94

Sample

36.90

40.50

41.89

44.00

44.80

Si0 2

33.44

38.52

36.48

33.08

35.07

35.11

36.30

2.25

2.36

2.41

2.31

3.24

2.93

1.10

1.13

0.95

3.53

1.56

Ti0 2

11.68

18.31

15.01

15.83

15.10

9.37

11.30

10.89

14.19

6.61

9.26

AI2O3

15.20

19.30

18.20

18.70

18.77

21.57

18.25

35.37

FeO

23.77

20.21

0.24

0.98

0.62

0.28

0.22

0.86

1.19

11.07

9.46

12.60

0.46

11.00

8.47

7.49

11.80

7.64

15.15

8.91

MgO

0.21 12.10

0.50

MnO

0.00

16.20 0.16

18.00

15.99

0.00

10.14

11.80

11.70

0.00

11.44

CaO

0.00

K2O

9.41 0.24

10.11

9.06

9.39

9.64

9.95

9.36

0.00

0.27

0.00

0.17

0.00

1.92

1.32

1.41

0.29

1.18

Na20

93.17

92.40

91.94

92.80

94.97

96.95

91.40

97.11

90.27

99.21

95.58

Total

95.89

39.8 0.237

52.0

46.6

55.0

2.3

0.081

n.d.

55.0 n.a.

52.00

38.8 1.29

56.60

38.7 0.45

62.8 1.94

45.8

Mg#

n.a.

n.d.

n.a.

9.85

1.04

1.20

5.5251

5.9701

1.02

0.64

(Based on 23 Oxygen)

5.9104

5.5636

5.5575

5.5320

5.5930

6.6626

6.5122

6.7240

6.6470

0.2745

0.3710

0.3440

0.1935

0.1115

0.1240

0.1030

2.9536 2.5544

2.7140

1.2824

0.9961

1.6690

p CfQ

2.3240

2.3680

4.8660

2.4408

2.0310

2.2770

0.0270 2.7270

0.1654

0.0655

0.0210

0.0450

0.1126

2.0647

2.6450

2.4710

0.4388

0.1318

0.2737

0.2856

A1 Fe

2.7641

2.0640

2.2300

2.0720

3.2201

3.4738 2.7212

2.8072

2.9801

Mn Mg

0.0340 1.8811

0.1283 3.5004

0.0857 2.0444

0.0376 1.7964

0.0291 2.6189

0.1152

0.0000

2.2315

2.8530

Ca K

0.0000

0.0000 1.9989

0.0000 1.8732

0.0000

1.9832

1.9293

1.9518

0.0000 2.0084

0.0000 1.8190

0.0784

0.0000

0.0902

0.0835

0.0000

0.0510

Table 3: Selected electron microprobe analyses

5.3265

0.2872

0.35

CD, Structural Formulae (Based on 22 Oxygen)

0.91

2.4216

2.7080

1.6370

0.0000

1.7877

1.9063

1.8980

1.8960

1.9030

0.2512

0.2059

0.1220

0.1970

0.0000

0.6139

0.3564

0.3850

0.4140

1. Taratap Granite 94. Mannum Granite; 90. Dergholm Granite; 13. Harrow Granodiorite;

86. Wando Tonalite; 42. (felsic) Wando Granodiorite; 5. Reedy Creek Diorite; 51. Reedy Creek Granodiorite;

08. Marcollat Granite; n.d. = not detected; n.a. = not analysed.

-J Ux

476

J.D. Foden, S. Turner & R.S. Morrison

The late-stage granites are magnetite-bearing generally with Fe-rich biotite, although some (including the Marcollat Granite) are amphibole (ferro-hastingsite) and even hedenbergitefayalite-bearing. Their ferro-magnesian silicate phases have very high Fe/Mg ratios (Table 3). Amphibples and biotites have low A1 (total) and high Al(lv) /A1(V1) ratios. Biotite, amphibole and apatite all contain significant fluorine contents while those in the syn-tectonic suite are F-free (Table 3). Accessory phases comprise apatite, sphene, zircon and fluorite. Zircons are large, euhedral and apparently unzoned. Zoned allanite is also a common accessory mineral. Some even have

Ce Nd

Fig. 6 Average continental crust-normalised incompatible element variation diagram showing: (a) average Lachlan Fold Belt A-type granite (Whalen et al 1987) and post-tectonic Delamerian granites from Mannum and Dergholm; and (b) a comparison of felsic syn-tectonic granites (Monarto) and post-tectonic granites (Mannum).

other minerals as cores (plagioclase). High fluorine concentrations in these magmas have resulted in high zircon solubility, probably eliminating inherited populations. The post-tectonic granites are siliceous (7377% SiC>2), relatively Al2C>3-poor, alkali-rich rocks. They have markedly higher K20/Na20(ratios (1 to 2.5) than the syn-tectonic group and have low CaO and very low MgO contents. Mg/Mg+ Fe values are extremely low (<0.35) and are much lower than those of the syn-tectonic group. Cr, Ni, Sc and V concentrations are all extremely low. Sr concentrations are very low and Rb/Sr ratios are very high (Figs 3, 6). By comparison with the syn-tectonic granites

Fig. 7 Trace element discrimination diagrams for the tectonic interpretation of granites: (a) log Nb versus log Y; and (b) log Rb versus log Y + Nb. Symbols represent the same rocks as in Fig. 3. ORG= field of ocean ridge granites, VAG= volcanic arc granites, syn-COLG= syn-collisional granites, WPG= within plate granites.

Delamerian Magmatism

they are depleted in Ba (Figs 3,6) and have lower Ba/Rb ratios, whilst Ga, Nb, Zr and Y concentrations are considerably enriched. The trend of evolution towards one-feldspar magmas indicates fractionation at low pressures where the feldspar solvus does not intersect the solidus (they are hypersolvus granites). This implies in addition that they have already fractionated plagioclase, a fact demonstrated by their very low Sr contents and Ba depletion. These must have been dry magmas by comparison with the syn- tectonic group. For example the Marcollat Granite (Fig. 1, Table 3) shows good evidence of an extended history of plagioclase fractionation which would have enriched the water content of the residual melt. Olivine and clinopyroxene relicts are still present, but amphibole is present rather than biotite. The parent magmas of this series must have been very dry. By contrast, the syn-tectonic suites show evidence of higher water activities with the replacement of hornblende by biotite in quite mafic, alkali-poor magmas. These post-tectonic suites are classical A-type granites on the basis of the criteria of Collins et al (1982) and White et al (1982).

477

syn-tectonic group falls into the volcanicarc/syn-collisional field on Fig. 7a whereas on Fig. 7b, where this field is divided into syn-collisional and volcanic-arc fields, these granites fall into the volcanic-arc field. Mineralogical and geochemical factors lead to the following conclusions about the origin of the post-tectonic granites or their magmas: (i) they are highly fractionated; (ii) they have evolved from quite dry parent magmas; (iii) they have been emplaced at high crustal levels, and (iv) the magmas from which they crystallised were relatively hot.

SOURCE ROCK AND TECTONIC IMPLICATIONS Data presented here suggest a close similarity between both the nature and sequence of granitic magmatism in the Glenelg Metamorphic Complex and in the southern Lofty Ranges. In both localities the intrusion of syn-tectonic dioritegranodiorite-granite suites (I-type) are followed by post-tectonic A-type granites.

The syn-tectonic suites show differentiation trends of Y, Zr and Nb depletion. This is not the case in the post-tectonic group (Figs 3, 5, 6, 7). White etal (1982) have discussed the influence of high F/OH ratios on the crystallisation history of granite magmas. They recognised that Nb, Zr and Ga probably formed magmatic alkali-fluoro complexes and that these delayed saturation of, for instance, zircon. In the syn-tectonic magmas with higher aH20, the early saturation of zircon, sphene and hornblende results in depletion of these high field strength elements.

The Early Palaeozoic igneous history of this belt implies a sequence of (1) Cambrian extension (pre-Delamerian) ; (2) Early Ordovician compression (Delamerian) ; and then (3) later Ordovician extension (post-Delamerian). Mafic magmatism is associated with both of the extensional phases, but the post- Delamerian period was bi-modal, with A-type granite intrusion and rhyolitic volcanism accompanying (spatially and temporally) mafic intrusion.

The tectonic discriminant diagrams (Fig. 7) of Pearce et al (1984) provide support for the synand post-tectonic subdivisions of the Delamerian granites. The post-tectonic group without exception fall into the within-plate granite field. The

During the compressional phase associated with the Delamerian Orogeny, mafic magmas were excluded from the upper crust. The suggestion that the Kanmantoo Group is allochthonous (Clarke & Powell 1989) prompts the question:

Syn-tectonic magmatism

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Where was the contemporary locus of synDelamerian crustal fusion? Clarke & Powell (1989) considered the thrusting may have taken place early and from the east. In this case, some of the syn-tectonic plutons may have been emplaced before folding and faulting produced crustal thickening. On the Rb versus Nb + Y discriminant diagram (Fig. 7) the syn-tectonic granites fall into the "volcanic arc granite" field which is the field of granitic rocks formed at sites where a significant mafic source component is present, usually during continent-ocean collision. In the Casterton area, the Hummocks Serpentinite provides some evidence for the development of oceanic crust, a consequence of the preDelamerian extensional phase. It is also a strong possibility that an extensive lower crustal or upper-mantle mafic layer (underplate) had already developed beneath the Adelaide Fold Belt before the Cambrian extensional phase. This mafic layer may have begun development during phases of mafic magmatism at the start of the Adelaidean (e.g. the Depot Creek Volcanics, the Beda Volcanics, the Gairdner Dyke Swarm and the Wooltana Volcanics) (Crawford & Hilyard this volume). The existence of this underplate is supported by low initial 87Sr/86Sr ratios determined for the preDelamerian monzonites of the Peake and Denison Ranges (Morrison & Foden this volume). We propose that the syn-tectonic granites developed by melting of this underplate material as a result of heat input during the preDelamerian extensional phase. These magmas moved up into the upper crust during the compressional stages of the orogeny and suffered some contamination there. As there is a considerable variation in the degree of tectonic fabric development in the syn-tectonic granites in the Adelaide Fold Belt (Rathjen Gneiss very strong, Reedy Creek Diorite very weak) this deliveiy of melts from the source to their emplacement site probably continued over a protracted period of the orogeny. It is not expected that these types of

granite would result from melting of evolved Proterozoic continental crust. Post-tectonic magmatism The Sr-isotopic characteristics of the post-tectonic granites are consistent with a source that included even less evolved crustal material than the syn-tectonic series. Their relationship to source-rock chemistry is difficult to assess because of their extensively fractionated character. The very high levels of some trace elements may reflect enrichment during extreme fractionation rather than particularly high levels of these elements in the source region. However, the posttectonic source must have lower water contents than the syn-tectonic. This is consistent with the sequential derivation of syn- and post-tectonic granites from the same source. Restites or cumulates left by first stage I-type magmatism could include biotite, hornblende, magnetite, apatite, zircon, sphene and allanite. These have a collective capacity to host many of the trace elements enriched in the post-tectonic magmas. An initial melting event may have fractionated the OH/F ratios of the residue after which second stage, relatively F-enriched, hightemperature melts were produced. These were able to rise and fractionate much more before freezing. Several features support the hypothesis that the post-tectonic magmas were relatively hot and were therefore able to be emplaced at high crustal levels. Their association with volcanic rocks in fact suggests that some may have been subvolcanic. Having low aH20 levels initially would also allow them to fractionate extensively at the granitic minimum without reaching water saturation. The terrains that host these post-tectonic granites therefore represent shallower Ordovician crustal levels than those in which the syn-tectonic granites are exposed. Sandiford et al (this volume) have suggested that in the Mt Lofty Ranges, the D2 deformation with upright folds and a near vertical stretching

Delamerian Magmatism

lineation was part of a crustal thickening event. The post-tectonic granites were emplaced after this at a stage when lithospheric extension followed on from crustal overthickening. This extensional phase also produced mafic intrusion and there is evidence that mafic and felsic magmas were in close spatial and temporal proximity. The quarry at Mannum, for instance, exposes the A-type Mannum Granite which hosts swarms of mafic xenoliths. These xenoliths show extensive signs of digestion and hybridisation with the host granite. Some show disaggregating, tear-drop shapes. There is also evidence of blockstoping of a chilled marginal granite phase. These features suggest magma mixing is taking place, perhaps during turbulent flow as the magma ascended a conduit. The post-tectonic granites generally have very low Mg and Fe contents and high Fe/Mg ratios. This is reflected in the very low Mg/Mg+ Fe ratios of ferromagnesian silicates in these rocks. By contrast, the Mannum Granite, a member of this group, has very magnesian biotites (Mg#>60, Table 3), as a reflection of the magma mixing observed. In his study of the post-tectonic mafic intrusion at Black Hill, Wegman (1980) has shown that a component of magmatic differentiation involves large-scale potassium enrichment towards monzo-norite. Many of the norites contain biotite and orthoclase. It is probable that this potassium enrichment is a result of the combined effect of assimilation of locally formed crustal melts and fractional crystallisation. A speculative conclusion may be that the A-type granite plutons are high level reflections of deeper seated, hot, mafic bodies, like the Black Hill intrusion. Some mixing of these systems occurs, as at Mannum, a situation which is promoted by lithospheric extension. Although the post-tectonic suite can be explained by re-melting of the syn-tectonic source due to heating by contemporaneous mafic magmas, it is not possible to entirely discount an origin by direct fractionation from these mafic magmas.

479

THE DELAMERIAN GRANITES AND THE EAST GONDWANALAND RECONSTRUCTION The reconstruction of Gondwanaland places eastern Antarctica, southeastern Australia and Tasmania in close, pre-Mesozoic proximity. However, there is some controversy over the detailed nature of this fit. In eastern Victoria Land, Cambro-Ordovician I- and S-type granites intrude the Wilson Terrane (Vetter & Tessensohn 1987; Stump et al 1986). The eastern Wilson Terrane is composed of metamorphosed, probable Cambrian sediments (e.g. Findlay 1987). Those granites which intrude closest to the boundary between the Wilson and Bowers Terranes to the east are mainly I-type. In Tasmania the I-type Darwin and Murchison granites are emplaced in the Cambrian Mt Read Volcanics and the Dove Granite intrudes Adelaidean basement. Each of the eastern Antarctic, Tasmanian and South Australian-Victorian Delamerian granite terranes is flanked to the east by areas of deeper-water Ordovician sedimentation. We equate the three Delamerian granite terranes and would therefore favour a reconstruction which allows them to form a single belt in the then eastern margin of Gondwanaland. In this respect, Baillie's (1985) reconstruction, which moves Tasmania to the west with respect to Australia and which places Victoria Land south of Tasmania, is favoured (see Baillie 1985; figs. 3,4). The South Australian-western Victorian "Delamerian province" on this basis is equivalent to the eastern Wilson terrane. In order to achieve correlation between Devonian granites of NE Tasmania and SE Victoria this reconstruction implies eastwards translation of Tasmania by an E-W sinistral fault before the Late Devonian, an interpretation quite different from that of Stump et al (1986) who ignore the constraints implied by the Tasmanian Delamerian granites.

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CONCLUSIONS

REFERENCES

The conclusions of this study are as follows:

ABBAS S.A.F. 1975. Granitic and migmatitic rocks of the Cookes Hill area, South Australia and their structural setting. Ph.D. thesis, University of Adelaide (unpubl.).

1. Similarities in the structural and magmatic histories of early Palaeozoic terranes in western Victoria (The Glenelg Metamorphic Complex) and in the southern Adelaide Fold Belt suggest that these are part of a single province. 2. Three distinct phases of Cambro-Ordovician magmatic activity are recognised. These comprise: (a) an at least partly syn-sedimentary, pre-deformational phase; (b) a syn-deformational phase; and (c) a post-tectonic extensional phase. 3. The pre-deformational magmatic activity is dominantly mafic, the syn-tectonic period is mainly one of I-type granite generation and the post-tectonic phase is one of combined maficand A-type granitic magmatism. The syn-tectonic phase is also associated with crustal melting producing migmatite, pegmatite and S-type granite. The tectonic cycle implied is one of: a, rifting; b, collision and uplift; and c, post-collisional extension. This continental marginal tectonic activity formed a "Delamerian province" which is also preserved in western Tasmania and eastern Antarctica.

BAILLIE P.W. 1985. A Paleozoic suture in eastern Gondwanaland. Tectonics 4, 653-660. CHAPPELL B.W. & WHITE A.J.R. 1974. Two contrasting granite types. Pacific Geology 8,173-174. CHAPPELL B.W. & WHITE A.J.R. 1982. I- and S-type granites in the Lachlan Fold Belt, southeastern Australia. In Geology of granites and their metallogenetic relations, pp.87-101. Proceedings of the International Symposium, Nanjing University, China. CHINNER G.A. 1955. The granitic gneisses of the Barossa Ranges. B.Sc. (Hons) thesis, University of Adelaide (unpubl.). CLARKE G.L. & POWELL R. 1989. Basement-cover interaction in the Adelaide Foldbelt, South Australia: the development of an arcuate foldbelt. Tectonophysics, 158,209-226. COLLINS W.J., BEAMS S.D., WHITE A.J.R. & CHAPPELL B.W. 1982. Nature and origin of A-type granites with particular reference to southeastern Australia. Contributions to Mineralogy and Petrology 80,189-200. CRAWFORD A.J. & HILYARD D. (this volume). Geochemistry of Late Proterozoic tholeiitic flood basalts, Adelaide Geosyncline, South Australia.

ACKNOWLEDGEMENTS The authors thank Mr J. Stanley and Mr P. McDuie at the Department of Geology and Geophysics, University of Adelaide, for their vital role in rock analysis. We also acknowledge helpful comments provided by Drs R. Both, R. Oliver, M. Sandiford, J. Parker, J. Jago, C. Gatehouse and two anonymous reviewers.

DAILY B. & MJLNES A.R. 1972. Revision of the stratigraphic nomenclature of the Cambrian Kanmantoo Group, South Austrlaia. Journal of the Geological Society of Australia 19, 197-202. ELLIS D.J. & THOMPSON A.B. 1986. Subsolidus and partial melting reactions in the quartz-excess Ca0+Mg0+Al203+Si02+H20 system under water-excess and water-deficient conditions to lOkb: Some implications for the origin of peraluminous melts from mafic rocks. Journal of Petrology 27,91-121.

Delarnerian Magmatism FINDLAY R.H. 1987. A review of the problems important for interpretation of the Cambro-Ordovician palaeogeography of Northern Victoria Land (Antarctica), Tasmania, and New Zealand. American Geophysical Union 40,49-66. FLEMING P.D. & WHITE A.J.R. 1984. Relationships between deformation and partial melting in the Palmer migmatites, South Australia. Australian Journal of Earth Sciences 31, 351-360. FORBES B.G., COATS R.P. & DAILY B. 1972. Truro Volcanics. Quarterly Geological Notes, Geological Survey of South Australia 44, 1-5. HENSTRIDGE D.A. 1970. The petrology and chemistry of the upper southeast granites, South Australia. B.Sc. (Hons) thesis, University of Adelaide (unpubl.). HOESNIM. J. 1985. The granitoids and migmatites of the Monarto area, South Australia. B.Sc. (Hons) thesis, University of Adelaide (unpubl.). JENKINS R.J.F. 1986. Ralph Tate's enigma - and the regional significance of thrust faulting in the Mt Lofty Ranges. Geological Society ofAustralia, Abstracts 15, 101.

JENKINS R.J.F. (this volume). The Adelaide Foldbelt: tectonic reappraisal. KING R.L. 1985. Ballarat 1:250 000 geological map explanatory notes. Geological Survey of Victoria, Report 75.

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MILNES A.R., COMPSTON W. & DAILY B. 1977. Pre- to syn-tectonic emplacement of early Palaeozoic granites in southeastern South Australia. Journal of the Geological Society ofAustralia 24, 87-106. MOELLER T. 1980. The petrology and geochemistry of the Reedy Creek granitoids and migmatites. B.Sc. (Hons) thesis, University of Adelaide (unpubl.). MORRISON R.S. 1988. Igneous intrusive rocks of the Adelaide Geosyncline. Ph.D. thesis, University of Adelaide (unpubl.). MORRISON R.S., & FODEN J.D. (this volume). A zoned Middle Cambrian pluton in the Peake and Denison Ranges, South Australia. OFFLER R. & FLEMING P.D. 1968. A synthesis of folding and metamorphism in the Mt Lofty Ranges, South Australia. Journal of the Geological Society of Australia 15,245-266. PARKER A.J. 1986. Tectonic development and metallogeny of the Kanmantoo Trough in South Australia. Ore Geology Reviews 1,203-212. PEARCE J.A., HARRIS N.B.W. & TINDLE A.G. 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology 25,956-983. ROCHOW K.A. 1971. Naracoorte, South Australia. Explanatory Notes, 1:250000 geological series. Geological Survey of South Australia.

MANCKTELOW N.S. 1980. The development of slaty cleavage, Fleurieu Peninsula, South Australia. Tectonophysics 58, 1-20.

SANDIFORD M., OLIVER R.L., MILLS K.J. & ALLEN R. (this volume). A cordieritestaurolite-muscovite association, east of Springton, Mt Lofty Ranges; Implications for the metamorphic evolution of the Kanmantoo Group.

MANCKTELOW N.S. 1979. The structure and metamorphism of the southern Adelaide Fold Belt. Ph.D. thesis, University of Adelaide (unpubl.).

STUMP E., WHITE A.J.R. & BORG S.G. 1986. Reconstruction of Australia and Antarctica: evidence from granites and recent mapping. Earth and Planetary Science Letters 79, 348-360.

TURNER S.P. 1986. Early Palaeozoic plutonism in western Victoria and eastern South Australia: implications. B.Sc. (Hons) thesis, University of Adelaide (unpubl.).

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VANDENBERG A.H.M. 1978. The Tasman Fold Belt system in Victoria. Tectonophysics 48, 267-297. VARNE R. & FODEN J.D. 1987. Tectonic setting of Cambrian rifting, volcanism and ophiolite formation in western Tasmania. Tectonophysics 140, 275-295. VETTER U. & TESSENSOHN F. 1987. S- and I-type granitoids of North Victoria Land, Antarctica, and their inferred geotectonic setting. Geologische Rundschau 76,233-243. VON DER BORCH C.C. 1980. Evolution of late Proterozoic to Early Palaeozoic Adelaide foldbelt, Australia: comparisons with post- Permian margins. Tectonophysics 70,115-134. WEBB A.W. 1976. Geochronology of the granitic rocks of southeastern South Australia. Amdel Report 1138. WEGMANN D. 1980. Pre-Tertiary geology of the Black Hill region in the western Murray Basin of S.A., with special emphasis on the petrology and geochemistry of the gabbroic rocks. B.Sc. (Hons) thesis, University of Adelaide (unpubl.). WELLS B.E. 1956. Geology of the Casterton district. Proceedings of the Royal Society of Victoria 68, 85-110.

WHALEN J.B., CURRIE K.L. & CHAPPELL B.W. 1987. A-type granites: geochemical characteristics, discrimination and pedogenesis. Contributions to Mineralogy and Petrology 95, 40-419. WHITE A.J.R. 1966a. Petrology and structure of the Rathjen granitic gneiss of the Palmer region, South Australia. Journal of the Geological Society of Australia 13,471-489. WHITE A.J.R. 1966b. Genesis of migmatites from the Palmer region of South Australia. Chemical Geology 1,165-200. WHITE A.J.R., COMPSTON W. & KLEEMAN A.W. 1967. The Palmer granite - a study of a granite within a regional metamorphic environment. Journal of Petrology 8,29-50. WHITE A.J.R., COLLINS W.J. & CHAPPELL B.W. 1982. Influence of melt structure on the trace element composition of granites. In Geology of granites and their metallogenetic relations, pp.737-751. Proceedings of the International Symposium, Nanjing University, China. WHITE A.J.R., CLEMENS J.D., HOLLOWAY J.R., SILVER L.T., CHAPPELL B.W. & WALL V.J. 1986. S-type granites and their probable absence in southwestern North America. Geology 14, 115-118.

A cordierite-staurolite-muscovite association, east of Springton, Mt Lofty Ranges; Implications for the metamorphic evolution of the Kanmantoo Group Michael Sandiford 1 , Robin L. Oliver 1 , Kingsley J, Mills 2 and Rosemary V. Allen 1 1

Department of Geology and Geophysics, University of Adelaide, GPO Box 498, Adelaide, 5001, Australia 2

SA.

Department of Geology, University of Sydney, N.S.W. 2006, Australia

Acordierite, staurolite, muscovite, biotite, sillimanite, andalusite, quartz schist from the Kanmantoo Group, east of Springton in the Mt. Lofty Ranges preserves textural evidence for the breakdown of staurolite by the divariant reactions staurolite + biotite + quartz = cordierite + muscovite staurolite + quartz = cordierite + sillimanite Inclusion trails in cordierite porphyroblasts suggest these prograde reactions occurred late in the deformation sequence (syn-S2). Preservation of early formed andalusite in quartz-muscovite-staurolite absent zones suggests that reaction took place with increasing temperature between 530°C and 680°C, at pressures of 3-5 kbars. A minimum temperature of 600°C is suggested for nearby K2O deficient schists containing orthoamphiboles with compositions in the range of the anthophyllite-gedrite solvus. The P-T conditions reflect a severely perturbed thermal regime well in excess of that expected for the conductive heating of tectonically thickened crust. Additional heat for the metamorphism of the Kanmantoo Group appears likely to have been introduced by syntectonic magmas.

Key words: Springton, Delamerian Orogeny, metapeiites, cordierite, petrogenetic grid, Kanmantoo Group INTRODUCTION

Reaction textures in metamorphic rocks record temporal variations in the physical and/or chemical environment of metamorphism. The interpretation of reaction textures therefore assumes considerable significance in the elucidation of the thermal and baric history of metamorphic terrains and of the mass transfer processes that affect them. In this paper we describe reaction textures in a cordieritestaurolite-muscovite schist, and in associated cordierite-orthoamphibole rocks, which form part of the Kanmantoo Group sequence east of Springton in the Mt Lofty Ranges (Fig. 1). These reaction textures provide an insight into the thermal and baric evolution of this low pressure-high temperature "Buchan style" terrain near the peak

of metamorphism, and therefore provide some constraints on the cause of metamorphism.

REGIONAL SETTING AND PREVIOUS WORK The rocks described in this paper form part of a metamorphosed sedimentary sequence situated approximately 60 km north-east of the city of Adelaide, South Australia, on the eastern side of the Mount Lofty Ranges (Fig. 1). Stratigraphically, the rocks belong to the Kanmantoo Group; a Cambrian sequence of rapidly deposited "flyschlike" sediments (Milnes 1982), consisting predominantly of metasandstones but with some intercalated metashales and siltstones and minor carbonates. Metasediments of the Kanmantoo

484

Cordierite, staurolite, muscovite association

iKappalunta^

Marne g \Reserve

LEGEND Metasiltstone & Metasandstone Micaceous

schist

Impure limestone Quartz albite rock "Gravale

I111JJ Calc silicate bed Migmatite vium S ofA l l uMurray Basin

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MAP—, AREA J

\Springton|-j (I > kDELAIDd'£«o Palmer

° ° o 1st a n d a l u s i t e * * * 1st s t a u r o l i t e — 1st s c a p o l i t e x . x 1st d i o p s i d e in calc s c h i s t s • • • 1st d i o p s i d e in l i m e s t o n e 1st c o r d i e r i t e +- a n t h o p h y l l i t e 1st s i l l i m a n i t e - — k y a n i t e zone sillimanite + p o t a s h feldspar F \

Kangaroo Is.

Fault Road

50km

Fig. 1. Geological map of the Springton area. Modified from Mills, plate 1,1964.

Railway

M. Sandiford, R.L. Oliver, K.J. Mills & R.V. Allen

Group are commonly regarded as the youngest rocks of the "Adelaide Geosyncline" (Sprigg 1952), post dating Early Cambrian and late Proterozoic Adelaidean sediments. There is controversy concerning the nature of the boundary between the Adelaide Supergroup and the Kanmantoo Group; we are inclined to the opinion of Daily and Milnes (1972) that, in the eastern and southern Mt. Lofty Ranges, the main bulk of the Kanmantoo is in fault contact with older, Adelaidean, rocks. Commencing probably in the Early Cambrian, and continuing into the Early Ordovician (Daily et al, 1976), the Kanmantoo Group and the Adelaidean sequence in the Mount Lofty Ranges were intensely deformed and metamorphosed during the Delamerian Orogeny (Parkin 1969; Daily et al, 1976). The nature of the Delamerian folding and metamorphism in the Mt. Lofty Ranges was outlined by Offler & Fleming (1968). According to them, metamorphism is of the low pressure-intermediate type, with four progressive zones of metamorphism, namely chlorite, biotite, andalusite-staurolite, and sillimanite zones, culminating in a migmatite zone near Palmer (e.g. Fleming & White, 1984). Offler and Fleming considered the zone boundaries to be parallel to the structural trends of the fold belt. Three fold generations were recognised, the latter two being moderately tight, upright structures, best developed on the eastern side of the Mt. Lofty Ranges, in association with the highest metamorphic grades. Fleming & Offler (1968) argued that the metamorphic climax occurred early in the tectonic history, in some regions apparently predating foliation development. More recently, Mancktelow (1979), in his review of the structure and metamorphism of the southern Adelaide Fold Belt, found that the metamorphic zone boundaries transgress the trend of the fold belt. In contrast with Fleming & Offler (1968), Mancktelow argued that development of the peak metamorphic assemblages postdated the first and most intense period of deformation (Di) and was broadly coeval with the second deformation event (D2). Mancktelow suggested that the Kanmantoo sequence was

485

greatly thickened during Di which he considered to have occurred at 515-500 Ma, with D2, which he argued produced no significant bulk strain on the regional scale, occurring in the interval 495470 Ma. The Springton area has previously been mapped and studied by one of us (Mills 1964). Mapping resulted in the delineation of a succession of mineral isograds, summarised in Fig. 1, and the mineral paragenesis and associated deformation history were discussed. More recently, metamorphic phase relationships in parts of the area displayed in Fig. 1 were investigated by Allen (1977).

PETROGRAPHY Cordierite is relatively abundant in potassium-deficient aluminous schists but is uncommon in true pelites in the Kanmantoo sequence to the east of Springton (Mills 1964; Joplin 1973) In this region metamorphic grade increases from north-east to south-west, with cordierite first appearing near the top of the andalusite-staurolite zone. It is best developed between the 1 st sillimanite and sillimanite + K-feldspar isograds (Fig. 1). Cordierite typically occurs as ovoid xenoblasts up to 1 cm diameter sieved with inclusions of quartz, biotite, plagioclase, tourmaline and, less commonly, staurolite, andalusite, sillimanite and spinel. In K20-deficient schists, cordierite-orthoamphibole assemblages are relatively common. The orthoamphibole, which has a compositional range within that of the anthophyllite-gedrite solvus (sample 498-299 has XAiiv = 0.7 -1.3, Table 1), typically occurs as radiating aggregates intergrown with cordierite, and formed at the expense of an earlier foliation defined by quartz-biotitestaurolite and quartz-biotite-andalusite-sillimanite associations. In these textures staurolite and andalusite are now completely armoured by cordierite and nowhere are they in direct contact with biotite.

486

Cordierite, staurolite, muscovite association

Fig. 2. (a) Photomicrograph showing xenoblastic cordierite with gently curved inclusion trails of biotite, ilmenite, quartz and muscovite. Crossed nicols. Width of field is ~ 3 mm. (b) backscattered scanning electron photomicrograph showing inclusions of muscovite, biotite, plagioclase, quartz and ilmenite in cordierite porphyroblast. Width of field is - 0.2 mm. (c) Photomicrograph showing late muscovite replacing biotite at margin of cordierite porphyroblast. Width of field is ~ 0.6 mm.

Fig. 3. (a) Photomicrograph of xenoblastic cordierite, with relict staurolite core, sillimanite fibres, tourmaline and ilmenite inclusions, in schistose matrix of biotite and quartz. Crossed nicols. Width of field ~ 3 mm. (b) Backscattered scanning electron photomicrograph showing staurolite, ilmenite and biotite inclusions in c o r d i e r i t e . W i d t h of view is ~1 m m . (c) Photomicrograph showing large late muscovite intergrown with cordierite porphyroblast replacing staurolite. Crossed nicols. Width of view is ~3 mm.

M. Sandiford, R.L. Oliver, K.J. Mills & R.V. Allen

Si0 2 Ti0 2 AI2O3 FeO MgO MnO CaO Na20

49.25 0.01 7.31 23.86 15.39 0.54 0.12 0.77

44.83 0.16 13.09 24.09 13.41 0.60 0.13 1.33

Total

97.25

97.64

0.73

1.36

Table 1. Electron microprobe analyses of orthoamphiboles in sample 498-299. A11V calculated on the basis of 8 tetrahedral sites. In potassium-rich, metapelitic schists cordierite is uncommon, and the equilibrium intergrowth of cordierite and muscovite has been observed in only one sample (185/389), discussed in some detail here. In this, cordierite poikiloblasts are of two mineralogically and texturally distinct types. The largest (up to 1 cm) poikiloblasts are flattened in the plane of the foliation and contain abundant inclusions of biotite, ilmenite, plagioclase (Ab92) and quartz and minor muscovite which define gently curved foliation trails (Figs 2a,b). Somewhat smaller and more equidimensional porphyroblasts, generally with fewer inclusions, contain embayed staurolite relicts, as well as rounded tourmaline inclusions and, frequently, splays of fibrous sillimanite (Figs 3a,b). In addition to its occurrence as part of the foliation-defining fabric both within the larger cordierite porphyroblasts (Fig. 2b) and in the matrix, muscovite occurs as somewhat irregularly oriented blasts along the margins of, and intergrown with, the cordierite porphyroblasts of both types (Figs 2c,3c). Rare andalusite poikiloblasts in 185/389 occur only in contact with matrix biotite and plagioclase in quartz-muscovite-deficient clots. The andalusite shows no replacement by sillimanite.

487

The textures in 185/389 indicate the progressive replacement of an early formed staurolitebiotite-ilmenite-quartz association by cordierite and muscovite and, perhaps, finally by cordierite and sillimanite. While the textures suggest that andalusite formed at an early stage in the metamorphic cycle it does not appear to have ever formed part of a stable assemblage with muscovite, staurolite and quartz in 185/389. However the assemblage andalusite-staurolite-muscovitequartz is relatively common in pelites elsewhere in the Springton region (Mills 1964). The gently curved inclusion trails in cordierite can be traced into the external S2 foliation suggesting cordierite growth occurred during the development of this foliation.

PETROGENETIC INTERPRETATION OF CORDIERITE REACTION TEXTURES Muscovite-bearing textures in sample 185-389 Ignoring tourmaline and plagioclase, which do not show characteristic reaction relationships with other phases, the eight remaining phases present in sample 185/389, i.e. ilmenite, quartz, muscovite, biotite, andalusite, sillimanite, cordierite and staurolite, can be represented by the seven component system K20-Fe0-Mg0als and bt chl cd gt ilm mu oa pi qtz siU st

= = =

= = = = = =

Al2Si05 polymorph andalusite biotite chlorite cordierite garnet ilmenite muscovite orthoamphibole plagioclase quartz sillimanite staurolite

Table 2. List of abbreviations used in the text and in figures

488

Cordierite, staurolite, muscovite association

Al 03-Si02-H 0-Ti02 (KFMASHT). In the following discussion we ignore TiC>2, which is present in significant quantities only in ilmenite and biotite. Ignoring Ti02 has little effect on the phase relations described below since any Ti produced or consumed by biotite reaction can be balanced by ilmenite. It will however, have some (probably minor) effect on the P-T location of the equilibria. Table 2 shows the abbreviations used in this paper. 2

We begin our analysis by considering the phase relationships in the Fe-end member system KFASH. In this system, with quartz and an aqueous fluid phase in excess, and simplifying by treating the AbSiOs polymorphs as the one phase (als), the following univariant reactions can be defined: (mu, bt) st+q=als+crd+H20 (dT/dP - 75°/kbar) (crd) st+mu+q=als+bt+H20 (dT/dP ~ 50°/kbar) (als) st+bt+q=mu+crd+H 0 (dT/dP - 90°/kbar) (st) q+als+bt=crd+mu+H20 (dT/dP ~ 150°/kbar) 2

1. Si0

Ti02

AI2O3

FeO MnO MgO CaO K2O

Na20 Total X Fe

47.55 -

33.52 7.42 -

9.06 -

0.36 97.91 0.31

2. 35.36 2.11 19.75 16.66 -

10.61 -

8.67 0.25 93.41 0.47

In accordance with convention we have labelled the reactions by the absent phase in round brackets. The high temperature associations are indicated on the right hand side. The approximate slopes of these reactions have been calculated using thermodynamic data for Fe-cordierite, Festaurolite, annite, quartz and sillimanite from an updated version of the Holland & Powell (1985) data set. The KFASH invariant assemblage, represented by the intersection of the four reactions (see Fig. 4), is univariant in KFMASH. The order of Fe/Mg partitioning amongst the phases (see Table 3), viz. XFe,st>XFe,bt>XFe,crd (where XFe=Fe/Mg+Fe), constrains this KFMASH univariant reaction to extend from the KFASH invariant point into the staurolite stability field (Fig. 4). In accordance with Harte & Hudson (1979), we consider this Fe/Mg univariant reaction to have a steep positive slope, which probably terminates at a KFMASH invariant point involving the additional phase chlorite (the alternative possibility that it terminates in an invariant point involving garnet is not likely in view of

3.* 24.61 0.59 55.61 13.31

4. 44.61 0.22 37.76 1.36

1.86

0.64

95.98 0.80

7.50 1.11 93.20 -

1. cordierite 2. biotite 3. staurolite 4. muscovite 5. plagioclase 6. ilmenite * Zn detected but not analysed. Table 3. Electron microbrobe analyses of minerals in sample 185-389.

5. 65.42 -

21.77 0.14 -

1.66 0.10 10.72 99.80 -

6. _

52.23 0.14 44.36 0.77 0.34 -

97.84 0.98

M. Sandiford, R.L. Oliver, K.J. Mills & R.V. Allen apparent absence in nature of the assemblage garnet-cordierite-muscovite-quartz, e.g. Hudson & Harte 1979). A schematic KFMASH grid involving chlorite is shown in Fig. 4. The absence of chlorite as a prograde phase in the metapelites descibed here (chlorite does occur with muscovite in metapelites to the north of the Springton region, (e.g. Mills 1964) suggests that the prograde assemblages formed in the lower pressure, higher temperature region illustrated in Fig. 4, specifically in the area bounded by the KFMASH reactions (als) and (st), which constitute the lower temperature and upper pressure limits of cordierite stability in the presence of muscovite, biotite and quartz.

489

st als cd chl

Fig. 5. In the Springton biotite-muscovite-quartz bearing metapelites it is possible to show the compositional variation as a binary. Such a projection scheme is illustrated in this figure.

chl st

KFMASH KFASH all assemblages with mu, q, H 2 0

Fig. 4. Schematic P-T grid showing stability relationships amongst the phases biotite, staurolite, AhSiOs, cordierite and chlorite for rocks containing excess muscovite and quartz in the system KFMASH (light lines) and KFASH (heavy lines). Arrows on KFMASH univariant curves indicate the direction in which the magnesia content of the femic phases increases. Compatibility diagrams show the KFMASH assemblages coexisting with biotite, muscovite and quartz (see Fig. 5). Large shaded arrows indicate the sense of displacement of invariant points with decreasing aH20 (see text for discussion).

Because the Kanmantoo metapelites contain abundant biotite (and ilmenite) it is possible to show the effective compositional variation, that controls the stability relations, as a binary projected from biotite, in addition to muscovite, quartz and H2O (Fig. 5). The P-T-XFe-Mg space for this binary system consists of large trivariant one phase volumes, smaller divariant two-phase volumes, univariant three phase planes, and invariant four-phase lines. Sections at constant bulk composition through P-T-XFe-Mg space therefore show the distribution of phases coexisting with biotite, muscovite and quartz on the P-T plane. Fig. 6 shows three different schematic P-T sections for a range of bulk compositions appropriate to the Springton metapelites. Fig. 6 clearly shows that reaction of staurolite to cordierite in the presence of muscovite, biotite and quartz occurs over a divariant field for specific bulk compositions and is likely to occur with significant decompression and/or heating. A hypothetical P-T path is indicated in Fig. 6b, which shows how such reaction textures may arise in metapelites of intermediate Fe-Mg composition. Note that in rocks of significantly higher XFe (Fig. 6c) or lower XFe (Fig. 6a) reaction

Cordierite, staurolite, muscovite association

490

relationships are unlikely to develop along the same P-T segment that produced reaction textures in 185/389. The development of fibrous sillimanite in many of the staurolite-cordierite textures is probably related to the divariant reaction (mu, bt) which, during either heating or decompression, would be expected to follow (als) once staurolite had been effectively isolated from matrix biotite by the developing cordierite armour. This explanation for the late sillimanite suggests that the reaction involving replacement of staurolite by cordierite accompanied a transition from andalusite to sillimanite stability fields. The preservation of early formed andalusite in 185-389 which shows no sign of replacement by sillimanite may be attributed to the notorious sluggishness of the AteSiOs polymorphic transformations. The formation of late sillimanite by this mechanism favours an essentially isobaric heating path or heating with only minor decompression for the reaction interval observed in 185-389.

t st

Reaction textures involving orthoamphibole and cordierite Reaction textures involving the growth of orthoamphibole and cordierite at the expense of earlier formed biotite bearing assemblages are abundant in the Springton region (Mills 1964). Unlike the muscovite producing reactions in 185389 described above, the reactions producing orthoamphibole cannot easily be interpreted in terms of the changing physical environment alone as they do not appear to have conserved K2O (Mills 1964). Our purpose here is to illustrate that these reactions may have formed along the P-T path determined for sample 185389 by treating K2O as a perfectly mobile component. A schematic T - J I K O grid, topologically equivalent to the |IH O "M-K O grid, for the KFMASH system involving muscovite, biotite, staurolite, aluminosilicate polymorph, orthoamphibole, cordierite, quartz and H2O is shown in Fig. 7 (the order of Fe/Mg partitioning amongst the phases depicted in Fig. 7 is XFe,st>XFe,oa>XFe,bt>XFe, cd). This grid shows 2

Fig. 6. Schematic P-T sections for constant bulk composition (XFe) showing stability relations amongst the minerals staurolite, AbSiOs, cordierite and chlorite in biotite, muscovite, quartz rocks. Bulk rock XF decreases from A through C. The arrow indicates a P-T path which will produce the observed reaction sequence in the Springton metapelites of intermediate Fe/Mg composition. c

M. Sandiford, R.L. Oliver, K.J. Mills & R.V. Allen that reaction sequences involving replacement of staurolite-biotite and andalusite-biotite by cordierite and orthoamphibole are consisent with an essentially isobaric heating path (or decreasing M-H O at constant P and T) when K2O is treated as a mobile component. 2

491

corresponding to a depth of 10-17 km, and 530680°C. The occurrence, in K20-deficient schists, of orthoamphiboles with compositions in the range of the anthophyllite-gedrite solvus suggests temperatures in excess of 600°C, at least by the time of orthoamphibole formation (Spear 1980).

PRESSURE-TEMPERATURE CONDITIONS OF REACTION The P-T conditions for the reaction textures observed in 185-389 are constrained between the low pressure, high temperature limiting KFASH univariant (mu, bt) and the high pressure, low temperature limiting A^SiOs triple point, with temperatures likely to be in the vicinity of the andalusite-sillimanite boundary (Fig. 8). Using the Holland & Powell (1985) dataset modified to include Fe-staurolite and Fe-cordierite, these constraints provide a P-T range of 3-5 kbars, Temperature (°C)

Fig. 8. P-T diagram showing the preferred pressure-temperature path for the sample 185-389. The location of the reaction boundaries and errors (shaded regions) have been calculated using the program THERMOCALC (Powell & Holland, 1988). The possibility of a P-T path initiating in the kyanite stability field is indicated by relict kyanite assemblages elsewhere in the Springton region (see Fig. 9, and text for discussion).

all assemblages with bt, q, H 2 0

Fig. 7. Schematic T-jik 0 grid, topologically equivalent to ^h 0-J^k 0, for the KFMASH system containing the phases cordierite, orthoamphibole, staurolite, aluminosilicate, biotite, muscovite, quartz and H20-fluid. Path A illustrates the reaction sequence expected with increasing temperature (or decreasing JiH o for a biotite-aluminosilicate-quartz schist. Path B shows the reaction sequence along an identical heating path for a rock with the lower jik o assemblage biotite-staurolite-quartz. In both cases the initial assemblages are expected to react to cordierite and 2

Fig. 9. Photomicrograph showing early kyanite (ky) replaced by andalusite (and) and finally fibrous sillimanite (sill) in quartz-rich segregation from the Marne Reserve (see Fig. 1).

492

Cordierite, staurolite, muscovite association

DISCUSSION The formation of a cordierite-muscovite association at the expense of an early formed staurolite-biotite association in the sample described here testifies to significant heating, possibly accompanied by decompression, late in the deformation history (syn-S2) of the Kanmantoo sequence to the east of Springton. It is not yet known whether the P-T history of this sample reflects the P-T history of the Kanmantoo Group as a whole or whether it has been substantially modified during the development of the macroscopic structure on the local scale, as, for instance, has been established in other amphibolite facies terrains (e.g. Chamberlain 1986). Some indication that the P-T path deduced here is representative of the Springton region in general is provided by evidence, in quartz-segregations from outcrops near the Mame Reserve (Fig. 1), for the progressive replacement of early formed kyanite by andalusite and then fibrous sillimanite (Fig. 9). These segregations are concordant with the earliest formed foliation (Si) in the host schists and thus reaction textures contained within them record the thermal changes following Si. Whatever the precise nature of the P-T history, it is clear from the assemblages documented here that the metamorphism of the Kanmantoo Group in the region east of Springton occurred during an extremely perturbed thermal regime with temperatures well in excess of those expected for steady state thermal conditions in the continental crust. Substantial thermal perturbations are to be expected during orogensis as highlighted by thermal models of continental collision zones (e.g. England & Thompson, 1984). Modelling shows that significant departures from the steady state thermal regime can occur where rocks are buried within thickened crust for periods of time approaching the thermal time constant of the lithosphere (i.e. of the order of 50-100 Ma), especially where such rocks are subsequently subject to excavation at rates fast compared to the thermal time constant. The heat-

ing is largely a consequence of the increased radiogenic heat production of the thickened crust. While such models may adequately account for the metamorphism observed in many medium-high temperature, medium-high pressure ("Barrovian-style") metamorphic terrains, they do not readily account for the metamorphism

Temperature

(°C)

Fig. 10. P-T-t paths calculated for the Kanmantoo metamorphism using timing constraints and structural constraints outlined by Mancktelow (1979). For discussion of model parameters see text, and Appendix. The box labelled K indicates the P-T field determined for the Kanmantoo metamorphics described in this paper.

in lower pressure "Buchan-style" terrains such as decribed here. To demonstrate this we have modelled the thermal consequences of deformation and erosion in the Kanmantoo (using timing and structural arguments presented by Mancktelow, 1979) as shown in Fig. 10 (for an outline the model rationale see the Appendix). Model parameters are: an initial surface heat flux of 75 mWm"2, appropriate to a young continental margin sequence; crustal thickening deformation

M. Sandiford, R.L. Oliver, KJ. Mills & R.V. Allen

(Di) producing a total crustal thickness of 60 km from 525 Ma to 505 Ma (i.e. from the end of sedimentation to the intrusion of the Encounter Bay granites); erosion at 0.25 km/Ma following D2 at 475 Ma. A limiting crustal thickness during Di of 60 km has been calculated by summing the present-day erosional level (10-15 km) and present-day crustal thickness (35-45 km). The initial surface heatflow, the crustal thickness and the deformation timing have been chosen so as to maximise the amount of, and time allowed for, heating within the thickened crust, prior to the onset of erosion The Di crustal thickening has been modelled as a homogeneous deformation. The model is, however, insensitive to the deformation geometry on the timescales involved here, as long as the deformation geometry does not radically change the distribution of crustal heat sources. The thermal model clearly shows that the conditions estimated for the Kanmantoo metamorphism in the region to the east of Springton lie well outside the field of P-T co-ordinates accessible during the deformationerosion cycle. In order to explain the thermal perturbation necessary for the metamorphism in the Kanmantoo we must appeal to additional heat transfer mechanisms such as the intrusion of magma or infiltration of heat transporting fluids. There is abundant field evidence to support the existence of syn-tectonic magmatic intrusions within the Kanmantoo (Mancktelow 1979). The striking correlation between the location of these magmatic bodies and the highest metamorphic grades, supports the notion that magmatism and metamorphism are closely linked within the Kanmantoo. Despite the fact that many of the granites are syntectonic, foliated and surrounded by migmatitic haloes - e.g. Palmer granite, Reedy Creek granodiorite and Rathjen gneiss - the granites' chemistry does not permit derivation from a dominantly sedimentary source such as the Kanmantoo (e.g. Foden etal this volume). The emplacement of these granites from a deeper level source region must have contributed significantly to the thermal budget of the currently exposed crustal levels. Fig. 10 illustrates that, at

493

very deep crustal levels during Delamerian time, temperatures as high as 850°C may have been attained between the Di and D2 episodes, simply as a consequence of the thermal equilibration of the thickened crust. Such temperatures are well in excess of those required to initiate melting in the crust, and may be sufficient for the generation

st+cd

I cd

1 st+ als+ als/ cd

alst

Fig. 11. (a) Schematic projection onto the plane H20-Fe0-Mg0 from muscovite, biotite, quartz showing some of the assemblages in the metapelites from the region east of Springton. Three phase assemblages buffer the aH2o. At the temperature and pressure of reaction in sample 185-389, the assemblage andalusite-staurolite (plus, muscovite, biotite and quartz) cannot coexist with a pure H2O fluid, (b) Schematic an2o-XFe diagram treating H2O as an intensive variable rather than the apex of a compatibility diagram (e.g. Fig. 11a) showing that the assemblage staurolite-cordierite- aluminosilicate (plus quartz, biotite, muscovite) buffers aH20-

494

Cordierite, staurolite, muscovite association

of significant volumes of crustal melts. Alternatively, elevated crustal temperatures resulting in crustal melting may have been caused by the emplacement of mantle derived magmas, such as are now observed at Black Hill, 20 km east of the area illustrated in Fig. 1. The thermal significance, if any, of fluid migration during the Kanmantoo metamorphism has yet to be clearly evaluated. The possibility of significant fluid infiltration during prograde metamorphism is suggested by our interpretation of cordierite-orthoamphibole intergrowths in K20-deficient schists, which requires a mechanism for the local removal of K2O. One consequence of large scale fluid infiltration of the type needed to significantly perturb the local thermal regime is that fluid compositions would be expected to be similar in all rocks throughout the sequence. That this is not the case in the region east of Springton is suggested by the occurrence of 185-389 at higher grades than the first occurrence of the assemblage staurolite-andalusitemuscovite-biotite-quartz (Mills 1964). The staurolite-andalusite metapelite assemblage can only exist at equivalent or lower grades than the 185-389 assemblage (staurolite-cordierite-muscovite-biotite-quartz) if it were in equilibrium with a fluid, or a fluid-absent assemblage, defining a lower aH20 (Fig. 11). The effect of reducing aH20 of the position of reaction boundaries which define the stability limits of these assemblages is shown schematically in Fig. 4 by the shaded arrows. Rather than supporting pervasive infiltration of fluid during metamorphism, these occurrences suggest that in much of the terrain fluid composition was internally bufferred and fluid infiltration must have been strongly focussed into narrow pathways.

ACKNOWLEDGEMENTS M.S. acknowledges the support of a C.S.I.R.O. post-doctoral fellowship. K.J.M. acknowledges receipt of a University of Adelaide research grant.

REFERENCES ALLEN R.VA. 1977. Metamorphic phase relations in the Kanmantoo. B.Sc.(Hons) thesis, University of Adelaide. CHAMBERLAIN C.P. 1986. Evidence for repeated folding of isotherms during regional metamorphism. Journal of Petrology 27, 63- 89. DAILY B., FIRMAN J.B., FORBES B.G. & LINDSAY J.M. 1976. Geology. In Twidale C.R., Tyler M.J. & Webb B.R eds. Natural History of the Adelaide Region, pp. 17-18, Royal Society of South Australia. DAILY B. & MILNES A.R. 1971. Stratigraphic notes on Lower Cambrian fossiliferous metasediments between Campbell Creek and Tunkalilla Beach in the type section of the Kanmantoo Group, Fleurieu Peninsula, South Australia. Transactions of the Royal Society of South Australia 95,199-214. ENGLAND PC. & THOMPSON A.B. 1984. Pressure-temperature-time paths of regional metamorphism, I. Heat transfer during the evolution of regions of thickened crust. Journal of Petrology 25, 894-928. FODEN J.D., TURNER S.P. & MORRISON, R.S., (this volume) Tectonic implications of Delamerian magmatism in South Australia and western Victoria. FLEMING P.D. & OFFLER R. 1968. Pretectonic crystallisation in the Mt. Lofty Ranges, South Australia. Geological Magazine 105, 356-359 FLEMING P.D. & WHITE A.J.R. 1984. Relationships between deformation and partial melting in the Palmer migmatites, South Australia. Australian Journal of Earth Sciences 31, 351-360. HARTE B. & HUDSON N.F.C. 1979. Pelite facies series and the temperatures and pressures of Dalradian metamorphism in E. Scotland. In Harris A.L., Holland C.H. and Leake B.E. eds. The Caledonides of the British Isles - reviewed. pp. 323-337, Geological Society of London. HOLLAND TJ.B. & POWELL R. 1985. An internally consistent thermodynamic data set with uncertainties and correlations: 2 Data and results. Journal of Metamorphic Geology 3, 343-370.

M. Sandiford, R.L. Oliver, K J . Mills & R.V. Allen

HUDSON N.F.C. & HARTE B. 1979. K 2 0 poor, aluminous assemblages from the Buchan Dalradian, and the variety of orthoamphibole assemblages in aluminous bulk compositions in the amphibolite facies. American Journal of Science 285, 224-266. JOPLIN G.A. 1973. A petrography of Australian metamorphic rocks. Angus and Robertson Pty. Ltd., Australia. MANCKTELOW N.S. 1979. The structure and metamorphism of the southern Adelaide fold belt. PhD thesis, University of Adelaide, South Australia (unpubl).

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SPEAR, F.S. 1980. The gedrite-anthophyllite solvus and the compositional limits of orthoamphibole from the Post Pond volcanics, Vermont. American Mineralogist 65, 1103-1118.

SPRIGG R.C. 1952. Sedimentation in the Adelaide Geosyncline and the formation of the Continental Terrace. In Glaessner M.F. & Rudd E.A. eds. Sir Douglas Mawson Memorial Volume, pp. 153-159, University of Adelaide.

Appendix MILLS K.J. 1964. The structural petrology of an area east of Springton, south Australia. PhD thesis, University of Adelaide (unpubl). MILNES A.R. 1982. The Encounter Bay Granites and their relationship to the Kanmantoo Group. In Oliver R.L. and Gatehouse C.G. eds. Guide to Excursions Bl, B2,B3, B4, Geology of the Adelaide Reg ion,pp. 16-29; Fourth International Symposium on Antarctic Earth Sciences. OFFLER R. & FLEMING P.D. 1968. A synthesis of folding and metamorphism in the Mt. Lofty Ranges, South Australia. Journal of the Geological Society of Australia 15, 245-266. PARKIN L. W. 1969. ed. Handbook ofSouth Australian Geology. Geological Survey of South Australia. POWELL R. & HOLLAND T.J.B. 1988. An internally consistent data set with uncertainties and correlations: 3. Applications to geobarometry, worked samples, and a computer program. Journal ofMetamorphic Geology 6, 173-204.

The thermal model illustrated in Fig. 10 uses a Crank-Nicolson implicit finite difference approximation for the equation of heat transfer in 1-dimension in a heat conducting, deforming medium : dT_ d2 T_ d2T H V dt V dX+pCp

where T is temperature, t is time, X is depth, K is diffusivity, H is internal heat production, C p is heat capacity and p is density. Temperature has been calculated at 1 km depth intervals with time stepping intervals of 1 Ma. A constant mantle heat flux of 20 mWm" 2 was maintained during the deformation. A uniformly distributed crustal heat source of 1.8 jiWm" 3 was used so as to give an initial surface heat flow of 75 mWm" 2 . The values used for diffusivity and conductivity were 1 mms" 1 and 3Wm" 1 K~ 1 , respectively.