Abstracts No.47: ASG Advances in Sedimentology, 1997, Melbourne

Geological Society of Australia

ABSTRACTS Number

47

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Advances in Sedimentology

ASG-Conference '97 MELBOURNE December 3-4, 1997

Geological Society of Australia Abstracts No. 47

Advances in Sedimentology

ASG-Conference '97 December 3 - 4, 1997 School of Earth Sciences, University of Melbourne

Meeting convened by Dr Tom Bernecker (Petroleum Development, DNRE)

Geological Society of Australia, Abstracts No. 47

Australasian Sedimentologists Group Conference Advances in Sedimentology Melbourne, December 1997

ISSN 0729-01IX Copyright - Geological Society of Australia Incorporated 1997

Copies of this publicatiori may be obtained from the Geological Society of Australia Incorporated, 1203 Wynyard House, 301 George St, Sydney, NSW, Australia 2000

Example citation for papers in this volume: Hinman, M., 1997, The Essential Interplays between Deformation, Sedimentation, and Fluid Flow for the Formation of the HYC Ag-Pb-Zn Deposit at McArthur River, Northern Territory. Geological Society of Australia, Abstracts No. 47, p. 16-18.

This abstract volume was compiled and edited by Tom Bernecker

Sponsorship The ASG-committee is very grateful for the financial support provided by the following organisations:

CULTUS Petroleum NL DNRE, Petroleum Development Unit GEO-ENG Pty Ltd GEOTRACK International Pty Ltd PASMINCO Exploration PROSPECTORS Earth Sciences Pty Ltd SANTOS Limited WMC Resources Ltd

ASG Conference Committee: Tom Bernecker Convenor Malcolm Wallace Chair, ASG Guy Holdgate Secretary John Webb Treasurer Therese Van der Linden

(DNRE, Petroleum Development) (University of Melbourne) (University of Melbourne) (La Trobe University) (La Trobe University)

December 3-4,1997 Fritz Loewe Theatre School of Earth Sciences, University of Melbourne Programme Day 1, Wednesday 3rd December 8.30 - 9.00 am: 9.00 - 9.10 am

REGISTRATION Opening, Welcome Tom Bemecker

9.10 -10.00

Keynote Address: Origin of the Great Barrier Reef - Prospective Play and Global Catalyst Prof. Peter Davies, University of Sydney

10.00 -10.20

Syngenetic Karst in Pleistocene Dunes in Southwestern Victoria Susan White

10.20 -10.40

MORNING TEA

10.40 -11.00 Lachlan

Transported Carbonate Debris in the Late Silurian Bamby Hills Shale, NE Fold Belt Lis Morgan

11.00 -11.20

Neoproterozoic Salt Glaciers in the Adelaide Geosyncline Ian Dyson Oiairparson - NeU Tupper, SANTOS Ltd

SILICICLASnC SEDIMENTS: 11.30 -11.50

Quaternary Sediments of the Bonaparte Gulf Jonathan Clarke

11.50 -12.10

Rapid Sedimentation by Sustained High-Density Turbidity Currents: The Cambrian Kaiunantoo Group, South Australia Peter Haines. J.C. Gum and J.B. Jago

12.10 -12.30

Sedimentology of the Early Cambrian Stokes Bay Sandstone, Kangaroo Island, S.A. Elinor Alexander, Dave Gravestock and G.Weste

12.30 - 2.00pm

LUNCH

PROVENANCE STUDIES:

Chaiiperson - iMDuddy, GEOTRACK Ihtfemational Pty Ltd

2.00 - 2.20

Comparison of Early Cretaceous Volcano-Sedimentary Successions along the Eastern Australian Continental Margin: Implications for the Break-Up of Eastern Gondwana Scott Bryan, Andrew Constantine, Chris Stephens, Tony Ewart and J. Parianos

2.20 - 2.40

Is the Gold Coast from Antarctica? Provenance Implications from Detrital Mineral Geochronology of Eastern Australian Sediments Keith Sircombe

2.40 - 3.00 3.00 - 5.30 7.00 pm

AFTERNOON TEA I

POSTER SESSION and CORE DISPLAYS CONFERENCE DINNER II Gambero Italian Restaurant, Lygon Street

(see next page for details)

Poster Presentations: The Eyam Limestone of Derbyshire, England Late Visean Mudmounds, Carbonate Banks, Lacustrine and Lagoonal Carbonates Maxwell Brown The Evolution of Salt Tectonics in the Adelaide Geosyncline Ian Dyson Sedimentology of an Eocene Estuary: the North and South Maslin Sands at Maslin Beach, South Australia Ian Dyson Sedimentology and Stratigraphy of the Early Triassic Rewan Group, Bowen Basin Paul Grech Burial Diagenesis of Deep Shelf Calcareous Mudstone of the Early Devonian Buchan Group, Southeast Australia Rao R. Munuganti and T, A. Webb A Study of Sequence Stratigraphy Based on the Oligo-Miocene Torquay Group, Southern Australia; a Mid-Shelf Mixed Siliciclastic Temperate Carbonate System Therese Van der Linden. John A. Webb and Peter J. Davies Examples of Syn-Sedimentary Deformation Related to Slope Development in Proterozoic Carbonates Peter R, Winefield, David Selley and Stuart W. Bull

Core Displays: Tertiary Carbonate Sequence at Torquay Port Phillip Embayment: Continuously Cored Section of Tertiary Sediments from Altona

Wine and Cheese will be served during this afternoon session.

Programme Day 2, Thursday 4th December 8.30 - 9.00 am: 9.00 - 9.10 am

REGISTRATION Opening Tom Bemecker

9.10 -10.00

Keynote Address: The Essential Interplays between Deformation, Sedimentation and Fluid Flow for the Formation of the HYC-Deposit at McArthur River, Northern Territory Dr Mark Hinman, M.I.M. Exploration, Brisbane

10.00 - 10.20

Carbonate-Hosted Zn-Pb Mineralisation within the U-lens at Navan, Ireland Wendy Peace, J.H. Ashton, M. Holdstock and J. Geraghty

10.20 - 10.50

MORNING TEA

10.50 - 11.10

Diagenesis of Carbonate-Hosted Ore Deposits Malcolm Wallace

11.10 -11.30

Three Generations Gold-Bearing Stephen Carey and of Martin Hughes Fluvial Systems at Ararat, Victoria

11.30 -11.50

"White Hill Gravel" of Bamganie-Dereel: Calivil Formation Gold Leads Overlain by Moorabool Viaduct Sand Michael Smith, Martin Hughes and Stephen Carey

12.00-1.30pm

LUNCH

vossmj^ts

E-VIC

31

1.30 -1.50

Tertiary Lignite Deposits in Australia, New Zealand and Germany: Timings, Correlation and Depositional Factors Guy Holdgate

1.50 - 2.10

Late Tertiary Reefs of New Guinea and the Philippines Alan Lloyd

2.10 - 2.30

Carbonate Sequence Stratigraphy: Northern Carnarvon Basin Hamish Young and Johnny Hull

2.30-2.50 Example

Sequence Stratigraphic Interpretation of Carbonate Wireline Log Motifs: an from the Northwest Shelf of Australia Tohnny Hull and Hamish Young

2.50 - 3.20

AFTERNOON TEA

3.20 - 3.40

Foraminiferal Sequence Stratigraphy of the Oligo-Miocene Janjukian Section at Torquay, southeastern Australia Gianyu Li, Peter Davies, Brian McGowran and Therese van der Linden

3.40 - 4.00

Accommodation History and Gamma Logs: A1610 Ma Disordered Evaporitic, Stromatolitic Fades Mosaic or a Gradually Deepening Ramp? Tim Tackson and Peter Southgate

eA^gtmd^

Chairpersan-JohnWebb,LaTtebeUniversity

4.10 - 4.30

Impact Structures in the Eromanga Basin: Relevance to Fetrolexun Exploration Vic Gostin

4.30 -4.50

The Genesis of the Anomalous Mid to Late Eocene Section at Fohn-1, Zone of Cooperation, Northwestern Australia John Gorter

4.50 - 5.10

The Hulopoe Gravel, Lanai, Hawaii: New Data and their Bearing on the "Giant Wave" Emplacement Hypothesis Alison Kay, Barbara Keating and Keith Crook

5.10 - 5.20

Concluding Remarks Malcolm Wallace

ASG-Conference '97

Advances in Sedimentology

Sedimentology of the Early Cambrian Stokes Bay Sandstone, Kangaroo Island, South Australia

E.M. Alexander^ D.L Gravestock^ and G. Weste^ 1. Petroleum Division, Department of Primary Industries and Resources South Australia, GPO Box 2355, Adelaide, SA, 5001 2. Geoweste Pty Ltd, 8 Cumberland Ave, Aldgate, SA, 5155. The Stansbury Basin consists of Early-Middle Cambrian clastics and carbonates which occur at depth beneath Gulf St Vincent and outcrop on Yorke and Fleurieu Peninsulas and Kangaroo Island, SA. The Early Cambrian Stokes Bay Sandstone is exposed on the north coast of Kangaroo Island and may extend beneath Gulf St Vincent as a potential hydrocarbon reservoir. The region is currently being explored for hydrocarbons reservoired in both clastics and carbonates by Canyon (Australia) P/L. The contacts between the Stokes Bay Sandstone, the underlying Mount McDonnell Formation and overlying Smith Bay Shale are transitional. The Stokes Bay Sandstone is a thick uniform unit with little apparent change in grain size, provenance, or facies. It consists of fine-medium grained arkose with minor thin shale and siltstone interbeds. Quartz is the dominant component, followed by potassium feldspar and plagioclase, with traces of mica, heavy minerals and zircon. Lithic grains are rare and are typically reworked shale and siltstone intraclasts, but there are also rare clasts of reworked metamorphic basement. Planar cross bedding, often forming classical herringbone cross bedding is the most common sedimentary structure. Both upper (current lineations) and lower flow regime planar beds occur. Deformed beds are also common, with slumps, convolute beds and load casts, sometimes on metre-scale. Horizontal trails and small sand-filled vertical burrows occur in rare finer grained interbeds. Ripple cross beds at the tops of planar bed sets and planar cross bed sets were deposited during periods of waning or opposing flow. A shallow subtidal environment of deposition is indicated by bipolar palaeocurrent data, bioturbation, current flow strength variations (waning flow ripples), and the lack of evidence of both subaerial exposure and deeper water deposition. The dominant palaeocurrent direction indicates that sediment was supplied from the Gawler Craton in the W I ^ and a north-south oriented coastline is interpreted. Sand was dumped rapidly onto the shelf from river deltas, which have yet to be identified or located, and prograded to the ESE. Although WNW directed tidal currents reworked some sands, delta-processes dominated. Small scale ESE directed progrades, common amalgamation of beds, poor sorting, lack of roundness of grains and convolute bedding indicate abundant, rapid deposition coupled to adequate accommodation space on the shelf. Trilobites collected in April 1996 suggest the top of the Stokes Bay Sandstone is approximately coeval with archaeocyathan-calcimicrobe reefs known from drillholes and outcrop on Yorke Peninsula. This implies that clastic input was quite separate from these lagoon and reef tracts. Correlations between Kangaroo Island and Fleurieu and Yorke Peninsulas remain controversial, due to limited biostratigraphic control and structural complications on Kangaroo Island. The apparent juxtaposition of carbonate-free clastics and clastic-free carbonates remains unexplained and may be resolved when Canyon's exploration drilling programme conmiences in Gulf St Vincent in 1998.

ASG-Conference '97

Advances in Sedimentolopy

The Eyam Limestone of Derbyshire, England - Late Visean Mudmounds, Carbonate Banks, Lacustrine and Lagoonal Carbonates

M.C.Brown, 51 Debenham St., Mawson, A.C.T. 2607 The late Visean Eyam Limestone of Derbyshire comprises carbonate mud mounds, carbonate banks, and lagoonal and lacustrine carbonates. It rests conformably on older carbonates and is overlain conformably by late Visean and Namurian marine shales. The immediate substrate, the Monsal Dale Limestone, resembled some of the larger reefs of the Great Barrier Reef. A lagoon near Ashford was surrounded by a broad shallow subtidal platform, analogous to a modern reef flat, with a steep outer margin. Eyam Limestone sedimentation began with a rapid sea level rise of 40 m or more. Numerous mud mounds with a bryozoan-brachiopod fauna, 25-30m high and surrounded by crinoid forests, grew up rapidly from the platform substrate. A fall of sea level then left the tops of the mounds exposed; and a thin limestone with a coral-brachiopod fauna was deposited on their flanks - passing laterally into a thin yellow lime mudstone away from the mounds. A further fall of sea level left the former platform exposed; and the Ashford lagoon became a hypersaline lake in which laminated carbonate muds accumulated. Later marine carbonates accumulated mainly as shallow banks prograding out from mud mounds toward lagoons located between groups of mud mounds. The carbonate banks accumulated during periods of relatively high sea level, punctuated by low stands in which they were planed off at low tide level. Earlier banks were mainly carbonate muds with a gastropod-nautiloid fauna. Later banks are a higher energy facies with a fauna of giant brachiopods and corals, which eventually covered the early mud mounds. Some younger mud mounds occur within the bank sequence. In the lagoons much smaller thicknesses of dark marly lime mudstones and shales accumulated slowly in deeper water, uninterrupted by the erosion surfaces of the bank sequence. The carbonate sequence was finally drowned by the Namurian marine transgression. The sea level fluctuations were comparable with those of the Pleistocene, and may mark the beginning of the late Palaeozoic Gondwana glaciation.

ASG-Conference '97

Advances in Sedimentology

A Comparison of Early Cretaceous Volcano-Sedimentary Successions along the Eastern Australian Continental Margin: Implications for the Break-Up of Eastern Gondwana. S.E. A.E. Constantine^ CJ. Stephens^'^^ Ewart^, J. Parianos^'^ iDepaitment of Earth Sciences, Monash University, Clayton, Victoria 3168 ^Centre for Microscopy and Microanalysis, Uni. of Queensland, St Lucia, Queensland 4072 ^Department of Earth Sciences, University of Queensland, St Lucia, Queensland 4072 ^Central Norseman Gold Corporation Ltd, PO Box 56, Norseman, Western Australia 6443 5QNI Limited, PO Box 7879 Waterfront Place, Brisbane, Queensland 4001 *email: ST-bryan@artemis.earth.monash.edu.au We report on three large volume Early Cretaceous volcanic and sedimentary provinces: the Whitsunday Volcanic Province and Great Artesian Basin system, both of northeastem Australia, and the Otway/Gippsland basin system along the southeastern margin of Australia. The Whitsunday Volcanic Province is part of a mafic to silicic, high-K calc-alkaline pyroclastic volcanic belt that extends for more than 900 km along the central and southern Queensland coast. Estimated extrusive volumes are >10^ km^. Volcanic and intrusive activity shows a broad range of ages from 132 to 95 Ma, but ages are dominated by an event between --120 and 105 Ma. Contemporaneous with volcanism along eastem Queensland, sedimentary basins in interior and eastem Queensland were receiving large volumes (>10^ km^) of volcanogenic sediment. The Otway and Gippsland basins 1500 km to the south, were initiated by the breakup of Antarctica and Australia. These basins contain >4 x 10^ km^ of Aptian-Albian extrabasinal volcanogenic sediment supplied from the east. This volcanogenic sedimentation postdates rift-related volcanism within the basin system. These three provinces are each significant for: (1) the accumulation of large volumes of volcanic and/or coeval volcanic-derived material (>1.5 x 10^ k m h (2) the compositional similarity between phenocryst and detrital plagioclase, augite and hornblende; and (3) age data recording a major volcanic episode between 125-105 Ma. We propose these provinces record volcanism related to the break-up of eastem continental Gondwana and the formation of the modem eastem Australian passive margin. Volcanism occurred along the length of the present eastem Australian margin, a distance of >2,500 km, with the Whitsunday Volcanic Province interpreted to represent the northem extension of this volcanic belt. Volcanogenic material was shed westwards into the basin systems of eastem Australia. Silicic pyroclastic volcanism characterised this event, and the pyroclastic mode of fragmentation and dispersal was an important factor in the availability of large volumes of fine grained sediment to be rapidly delivered into these basin systems. This volcanic event, now preserved mainly in the sedimentary basins of eastem Australia, is similar to many large volcanic provinces in its large volume of products, scale, and association with a regional extensional regime. This study supports the notion that the Early Cretaceous was a period of widespread rifting, global magmatism and continental break-up.

ASG-Conference '97

Advances in Sedimentology

Three Generations of Gold-Bearing Fluvial Systems at Ararat, Victoria Stephen P. Carey^ and Martin J. Hughes^ ^Geology Department, University of Ballarat, P.O. Box 663, Ballarat Vic 3353 ^Martin Hughes and Associates, 1034 Geelong Road, Mount Clear Vic 3350 Three generations of gold-bearing fluvial systems are clearly distinguishable in the Ararat district of western Victoria. Large yields of gold, including gold recycled from older fluvial systems into younger ones, have been produced from each generation. Parameters used to separate the three systems include: • relative elevation • presence/absence of duricrusts and pallid/bleached zones • erosional or other superpositional relationships among the different fluvial generations and between the latter and basalt flows. Each of the three generations, described as follows from oldest to youngest, may be further subdivided: 1. The oldest lead system (e.g. Canton/Port Curtis Hill/Hard Hill/Red Hill leads) resides at the highest level of the three generations in the landscape and typically has a deep weathering profile imposed upon it as well as adjacent basement rocks. It can be subdivided into an older component of lithic ± quartz conglomerate and minor sandstone, and a younger, more widespread, component of quartz pebble gravel/conglomerate with minor mudstone and sandstone. Coarse (cobbles and boulders) angular lithic conglomerates in the headwaters give way downstream to deposits dominated by more rounded quartz pebbles. 2. The intermediate-level lead system (e.g. Wet, Blackman's, Spring, Cathcart, Langi Logan leads) is largely quartz pebble gravel with subordinate sand, silt and palaeosols. Localised lithic pebble conglomerate and sandstone contains ironstone clasts. 3. Floodplains and terrace systems are situated at low elevations close to those of the modem incised drainages (e.g. Denicull Creek). The deposits are dominated by mud (especially clay) with channelised quartz pebble gravel; sand is virtually absent. The deposits lack deepweathering profiles but locally contain ironstone clasts (pisoliths and chips). It is difficult to subdivide the deposits of the older and modem floodplains and terraces lithologically, but a geomorphic distinction can be made on the basis of their different relative elevations (1-2 m). The first generation corresponds to what has been termed the White Hills Gravel (Cretaceousearly Tertiary?), the second to the Calivil Formation (mid-late Tertiary) and the third to the Shepparton/ Coonambidgal Formation (Pliocene-Recent), although the usefulness of the first two terms on a regional scale is questioned. At the close of sedimentation of the first-generation deposits, the palaeodivide was 600 m lower than the Mesozoic palaeosurface, suggesting a likely Tertiary age. Burial of second-generation deposits beneath previously dated basalt south of Ararat indicates a minimum age of 6.07 ± 0.11 Ma (Miocene). Although the White Hills Gravel of the Central Highlands has been described as sheet-like in form, fluvial sedimentation of the three generations of deposits in the Ararat area was, and is, confined by prominent ridges with a dominant north-south strike. The ridges are controlled by the major Mount Ararat, Cathcart and Copes Hill faults. Palaeozoic thrusts which may have been reactivated during rift and drift of Australia from Antarctica. The position of the Great Divide in the Ararat area has been stationary since prior to the deposition of the first fluvial system. Acknowledgements: We thank Metex Resources NL, Forrestania Gold NL, Centaur Mining & Exploration Ltd and, in particular, Dr Neil Phillips for their support of and comments on this work.

ASG-Conference '97

Advances in Sedimentology

Quaternary Sediments of the Bonaparte Gulf

Jonathan D. A. Qarke WMC Resources Lid, PO Box 850K, Melbourne 3001 Bonaparte Gulf is a large, macro-tidal shallow marine embayment (van Andel^f al 1967) off the Kimbeiley region of Westem Australia. Sea diamond exploration in the Gulf has elucidated Quatemary evolution of the inner part of the embayment. The study focused on the westem shores of the Gulf. Drowned rivers, cliffs, and limited beach development characterise the regioa The Holocene sediments are predominantly clastic, supplied by the Qrd (Coleman and Wright 1978) and Berkeley rivers. Tidal mud banks up to 10m high occur along the coast. Away from the coast the sea bed is extremely fiat. Coral reefs and seagrass banks are absent because of the highly turbid water. Mangroves are rare along the steep coast. The area off Cambridge Gulf is has very strong tidal currents and result in the development of the tidal sand bars of the King Shoals and MedusaBank (Lees 1992). Numerous seismic-stratigraphic units (Fugro 1995a, b) appear in the subsurface, bounded by erosional breaks featuring incised valley systems. Tlie largest of these valley systems are offshore extensions of the Berkeley and Ord rivers. The incis^ valleys are up to 15km wide and 30m deep. The both the Berkeley and Qrd incised valleys show anabranching. The meandering of the Qrd palaeochannel, presumably within the incised valley, is clearfy evident in magnetic data (Gunn et al 1995a, b). The most recent incised valleys were cut into fine-grained sandy calcaienites weathered during the Late Pleistocene and filled by fluvial sands and flood plain black soils to estuarine gravelly sands. The estuarine sediments are dominated by coarse quartz, reworked bioclasts, and bored and encrusted calcaienite lithoclasts. Elsewhere the Late Pleistocene land surface is buried by a transgressive lag of calcaieous gravelly sand containing skeletal fragmaits and lithoclasts and localfy exceeding 50% carbonate. The transgressive lag is typically Im thick or less. The bioclastic component is dominated by molluscs and bryozoans. Subordinate components include forams, echinoids, solitary corals and ostracods. Colonial corals are rare and large benthic forams are absent Near shore these calcaieous sands thickm to form drowned strands. Thicknesses of the strandlines are typically 2-3m. Up to four strandlines occur discontinuously parallel to the westem shoreline of Bonaparte Gulf. Transgressive sedimaits are overlain by highstand sediments of olive silts with typically 5% carbonate as bioclasts. These are similar in composition to those found in the transgressive sands. Plant debris is disseminated throu^out the sediments and concentrated into beds and laminae. Authigpnic pyrite is common in the high stand silts both filling and staining fossils. The highstand sedimaits may exceed 15m in thickness in the inshore mudbanks but are more typically l-5m in thickness. Furtha- offshore from the study area the highstand sediments become more calcaieous and thinno* (Jones and Burgis 1974). In the centre of the Bonaparte depression muddy calcaienites overlie Late Pleistocene lagooni sediments deposited in a large enclosed water body isolated from the ocean by the Sahul Shelf (van Andel and Veeveis 1965, 1967). The Melita v^ley cuts across the Sahul shelf and provided a narrow connection from the lagoon to the sea. Coral and calcaieous algae-rich sediments formed banks and reefs on the Sahul Shelf early during the Holocene. These were subsequently drowned. Many Tertiary marine sedimaits in the Murray and Qtway Basins of southo-n Australia consist of marls and calcaieous siltstonesto clay stones. No modern analogues exist in southan Australia for these fine-grained and clastic rich sedimaits. They were deposited in partly restricted shelfdepth embayments. Their facies and biota resemble that of the modern Bonaparte Gulf, except that they pass laterally into cool-water rather than warm-water carbonates. The Bonaparte Gulf may provide a useful analogue for these sediments.

ASG-Conference '97

REFERENCES

Advances in Sedimentology

COLEMAN, J. M. and WRIGHT, L. D. 1978. Sedimentation in an arid macrotidal alluvial river system: Ord River, Western Australia. Journal of Geology 86: 621-642. FUGRO SURVEY LTD. 1995a. Reconnaissance marine geophysical surveys in Joseph Bonaparte Gulf (WA/2(yMEL, WA/13MEL, and WA/9/MEL), unpublished confidential report FUGRO SURVEY LTD. 1995b. Reconnaissance marine geophysical surveys in Joseph Bonaparte Gulf (E80/1581, E80/1751, E80/1752), unpublished confidential report GUNN, P. J., BRODIE, R. C. MACKAY, T., and O'BRIEN, G. W. 1995a. Evolution and structuring of the Joseph Bonaparte Gulf as deUneated by aerom^netic data. Exploration Geophysics 26: 255-261. GUNN, P. J., BRODEE, R. C., and MACKAY, T. 1995b. Interpretation of aeromagnetic data over the southan Bonaparte Gulf in the context of petroleumprospectivity. AGSO Record 1995/40. JONES, H. A. and BURGIS, W. 1974. Timor Sea continental shelf sedunents map. BMR Bulletin 83A. LEES, B. G. 1992. Recent terrigenous sedimaitation in Joseph Bonaparte Gulf, northwestern Australia. Marine GeoloQ/ 103: 199-213. VAN ANDEL, T. H. and VEEVERS, J. J. 1965. Submarine morphology of the Sahul Shelf, northwestern Australia. Geolo^cal Society of America Bulletin 76: 695-700. VAN ANDEL, T. H., HEATH, G. R., MOORE, T.C., ANDMcGEARY, D. F. R. 1967. Late Quatemary history, climate, and oceanography of the Timor Sea, northwestern Australia. American Journal of Science 265: 737-758. VAN ANDEL, T. H. and VEEVERS, J. J. 1967. Morphology and sediments of the Timor Sea. BMR Bulletin 83.

ASG-Conference '97

Advances in Sedimentology

Origin of the Great Barrier Reef Peter J.Davies Department of Geology and Geophysics, University of Sydney, NSW 2006 This paper addresses the fundamental questions -"When and why was the Great Barrier Reef initiated" and" What were the principle factors effecting its growth?" It uses the results of the ODP drilling and the 1995 drilling on Ribbon 5 and Boulder Reefs as the basis for relating cause and effect. The regional and global implications are discussed. In 1991, the Ocean Drilling Program drilled a series of crucial cores in the slope of the Great Barrier Reef, in Grafton Passage east of Cairns. These allowed the definition of an event history critical for understanding the initiation and evolution of the Great Barrier Reef. Oxygen isotope data obtained from core 820 in Grafton Passage indicates that two major changes in environment occurred approximately 400,000 years ago ie a change in the orbital factors governing global climate from 19000 obliquity to 100,000 precessional cycles and a fundamental increase in sea surface temperature. At the same time, shelf composition and morphology changed from laterally prograding to vertically aggrading and implying a massive change in accumulation rates. In 1995, two holes were drilled on the continental shelf east of Cooktown, one on the inner shelf at Boulder Reef, and the second on the outer shelf at Ribbon 5. The drill hole at Boulder Reef bottomed at 86m with reef changing to subreef siliciclastics at a depth of 32m. The main reef section above a drill hole depth of 34m is comprised of 4 individual superimposed reefs. The drill hole at Ribbon 5 bottomed at 210m while the change from reef to subreef sequence occurs at a depth of 96m with a further change occurring at 156m. The reef sequence is comprised of five superimposed reef tracts composed of two distinctive coral assemblages which grew in periods of sealevel highstand. Each reef was wiped out by a succeeding glacidly induced sea level fall. The subreef sequence between 96m and 156m is comprised of subtropical foram^ryozoan grainstones with thin coral horizons and/or deep water rhodoliths while between 156m and 210m algal/foram/bryozoan grainstones dominate and these are interpreted to represent an outer shelf environment. Changes in the coralline algae throughout the core allow the recognition of three distinct assemblages, ie.tropical shallow water, cool shallow water and deep water shelf assemblages. Their distribution clearly accord with sea level changes prompted by global climate change. Correlation of the Ribbon 5 and ODP hole 820 indicates that the Great Barrier Reef was initiated some 400,000 years ago, catalysed by a substantial seawater temperature change associated with a fundamental switch in the earth's orbital parameters. The Great Barrier Reef may therefore represent a regional signal of a global statement heralding a massive increase in calcification in the ocean, seen in benthic and planktonic components. Assuming the above to be correct, then oxygen isotope stage 11 (around 400,000 years BP) defines the start of a positive natural influx of C02 into the atmosphere, with feedback to ensuing ocean temperatures and global climate. Reefs, far from being potential C02 sinks, are one part of an ocean cause of natural greenhouse. The record of that story lies in the rocks. Further work in progress is defining in detail the coral reef response to rising and falling sea level and in particular the attendant temperature variations. During the last rise in sea level from 18000 to the present, sea water temperatures have changed substantially and the record of that change is seen in both the isotope signals and in the interaction of coralline algae and corals as reef constructors. Acknowledgements: Plenary addresses are given by one person. This address is no exception but the work reported is very much the work of many. I wish to pay particular thanks to the many colleagues I have worked with since 1990, particularly my students Frank Perderman, Jody Webster and Alexa Troedson, and the international colleagues who worked with me to fund and complete the GBR drilling, especially Juan Carlos Braga, Dick Kroon, Lin Kay, Lucian Montaggioni and Judy McKenzie. Finally, to Phil Manning, without whose skill, patience and good humour we would not have recovered 89% of the core from Ribbon Reef 5.

ASG-Conference '97

Advances in Sedimentology

Neoproterozoic Salt Glaciers in the Adelaide Geosyncline

LA. Dyson National Centre for Petroleum Geology & Geophysics, University of Adelaide, S.A., 5005 Salt glaciers are a subaerial or subaqueous sheet-like extrusion from an exposed diapir. Salt glaciers of Neoproterozoic age have to date been identified at three different stratigraphic levels in the Adelaide Geosyncline. One is the 'Etina Dolomite', a tongue of yellow dolomite that c ^ be traced for some 10 km along strike on the western flank of the Enorama Diapir where in places it rests unconformably on diapiric breccia and ranges in thickness from a few decimetres up to some 50 m. Two glaciers have also been found at different stratigraphic levels on the south-eastern flank of the Pinda Diapir where they occur as laterals of the Christmas tree diapir near Pinda Springs. Perhaps the most exciting of these is represented by a tongue of brecciated Callanna Group sediments of evaporite affinity that is some 7 km long and up to 500 m thick within the upper Bunyeroo Formation. It consists of calcareous shale, quartzite, sandstone, limestone and dolomite within a matrix of grey green shale and yellow dolomitic limestone. The base of the 'salt' tongue rises step-like towards its distal end, suggesting progradation during a period of slow sediment aggradation. Its upper surface is overlain by reddish brown shale of the Bunyeroo Formation. The tongue trends downdip from the position of the former feeder stock. Edgewise conglomerates are present in lenticular sand sheets that are found adjacent to the feeder stock as shoreface deposits within incised valley fills, or within the distal tongue of the glacier where they are interpreted to be gravity flows. A large syn-sedimentary fold occurs about two-thirds down the length of the tongue where overlying Bunyeroo shales are deformed together with the slumped breccia. The slump is thought to have been triggered by renewed diapiric uplift. As a result, the upper boundaty of the Bunyeroo Formation was tilted and represents a disconformity prior to deposition of the Wearing Dolomite. This angular discordance is significant because it is correlated with the timing of earliest canyon incision in the Wonoka Formation. As such, it represents a deep water sequence boundary. Sedimentation of the Wonoka Formation was followed by another phase of active diapirism, resulting in a major unconformity (third-order sequence boundary) at the base of the Bonney Sandstone where an incised valley fill some 30 m thick comprises shoreface sandstones. Another glacier, though somewhat smaller, occurs about 200 m above the base of the Bonney Sandstone. Further sedimentation resulted in another phase of passive diapirism and formation of a large turtle anticline within the overlying Rawnsley Quartzite. Another glacier is also found within the lower Bunyeroo Formation at nearby Mucatoona Diapir. The 'salt' tongue within the Bunyeroo Formation at Pinda Springs is interpreted as a subaqueous 'salt' glacier. The glacier emerged as a 'salt' fountain during a phase of passive diapirism that was accompanied by slow sediment accumulation, i.e. as part of the transgressive systems tract. Like the 'Etina Dolomite', it displays characteristics similar to debris flows such as inverse and graded bedding that in turn suggest relatively low viscosity. However, at Beltana Diapir about 3 km south of Trebilcock Gap and some 50 km to the west of Pinda Springs, an olistostrome-like body of diapiric breccia (500 m x 300 m in size) slumped into a former incised valley that was cut and partially filled prior to deposition of the Bunyeroo shales, suggesting that in this case the breccia exhibited greater viscosity. These examples further suggest that formation of 'salt' glaciers within Neoproterozoic depositional sequences of the Adelaide Geosyncline appears to have been dependent on the relative effects of extensional tectonics versus sedimentary loading and sediment viscosity.

ASG-Conference '97

Advances in Sedimentology

The Evolution of Salt Tectonics in the Adelaide Geosyncline

LA. Dyson National Centre for Petroleum Geology & Geophysics, University of Adelaide, S.A., 5005 Diapirs were first recognised in the Adelaide Geosyncline of South Australia over 50 years ago when they were referred to as "plug upthrusts" and "complex crush zones" in the Flinders Ranges. Since then, these intrusive "salt" bodies have been studied by the Geological Survey, universities and exploration companies. More than 100 diapirs are spectacularly exposed in the Adelaide Geosyncline. Their large size (as much as 20 km across), abundance, spectacular degree of exposure and correlatable stratigraphy constitute a remarkable array features that rival the Great Kavir of Iran, the largest salt desert in the world. The evolution of salt tectonics reveals a familiar pattern of science. Once dominant models have been superseded by new ones. Looking back over the past 50 years to the first perceptions of salt tectonics in the Adelaide Geosyncline, we see a scientific landscape strewn with outmoded ideas in which specific aspects have been recycled to seed the next conceptual breakthrough. From the perspective of the 1990's, three eras of disparate length reflect progress in the understanding of Adelaidean diapirs. The pioneering era (1960-1973) was responsible for a general hypothesis of diapirism that was to have a profound effect on other studies that followed. The recognition of the tectonic breccia at Blinman Dome as a diapir by Bruce Webb in the late-1950's was to prove the springboard for a concentrated study of diapirs by the Geological Survey of South Australia for the next decade and a half. In the fluid era (1974-1992), differential loading and density and viscosity contrasts were emphasised, whereas the strength and nature of faulting were largely ignored. A source bed stratigraphy was erected, and the diapirs were variably proposed as slumps or tectonic breccias. Along with the timing of intrusion, many of these ideas were fiercely debated at the Adelaide Geosyncline Sedimentary Environments and Tectonics Settings symposium in 1983. From 1985-1992, palinspastic reconstructions and sandbox modelling attempted to show the development of peripheral sinks, turtle anticlines, mushroom diapirs and salt upwelling below thin overburden. The basic driving force was fault tectonics, and marked an overlap with the brittle era. The 1990's have revealed the effect of rifting and extensional tectonics on diapirism. Whereas diapirism was previously thought to be a consequence of sedimentation, current theory (Dyson, 1996, 1997) suggests that diapirism dominated and, in many cases, initiated sedimentation in the Adelaide Geosyncline. It advocates a strong relationship between unconformities and diapirism, a concept first identified by both Ron Coats and Bob Dalgamo in the mid-1960's. Extensional tectonics is considered to be the driving force behind the formation of major sequence boundaries, and not glacio-eustasy as expounded by the classic Exxon models for sequence stratigraphy. However, glacio-eustasy is thought to have been a significant control on sequence boundaries that cap Sturtian and Marinoan glacial units, as well as higher frequency disconformities and transgressive-regressive cycles within Neoproterozoic depositional sequences. Perhaps the most exciting development in recent times has been the identification of allochthonous salt sheets in the Flinders Ranges, interpreted to represent subaqueous salt glaciers. Their formation within the Neoproterozoic succession of the Adelaide Geosyncline appears to be dependent on the relative effects of extensional tectonics versus sedimentary loading. Here again, the idea of differential loading halokinesis has resurfaced to explain the extrusion of salt glaciers during the passive stage of diapirism.

ASG-Conference '97

Advances in Sedimentology

Sedimentology of an Eocene Estuary: the North and South Maslin Sands at Maslin Beach, South Australia LA. Dyson National Centre for Petroleum Geology & Geophysics, University of Adelaide, S.A., 5005 The North Maslin Sand and overlying South Maslin Sand, both of mid-Eocene age, represent basal Tertiary deposition in the Willunga Embayment of the St. Vincent Basin. Here, they are exposed in the coastal cliffs at Maslin Beach and nearby sand quarries, and have been intersected in many stratigraphic boreholes. In the past, the North Maslin Sand has been interpreted to be of fluvial origin based on the occurrence of quartz sands and gravels that are often stained and occasionally cemented with iron oxides. However, a deltaic environment of deposition or marginal marine influence towards the top of the unit has been suggested previously, and was possibly based on the identification of arenaceous foraminifera and sponge spicules at the top of the unit in the Noarlunga Embayment. A disconformity was also recognised between the North Maslin Sand and the glauconitic South Maslin Sand. The greensand was thought to indicate the first sign of marine influence in the Willunga Embayment. The present study has focused on the relationship between the North and South Maslin sands within a sequence stratigraphic framework, based on a detailed sedimentological synthesis of the cross-bedded sands and gravels. At Maslin Beach, stratigraphic boreholes suggest that the North Maslin Sand is contained within a narrow incised valley of fluvial origin. The quartz sands and gavels are typically crossbedded and well-rounded to sub-angular. Grain size is highly variable and commonly ranges from fine sand to pebbly gravel. The North Maslin Sand is overall upward-fining, and pebbly gravels and coarse sands at the base display abundant large-scale trough crossbedding, small to large-scale sigmoidal and tangential planar-tabular crossbedding, and minor horizontal planar stratification. Reactivation surfaces, tidal bundles and clay clasts interbedded with foresets are common. Wave-rippled surfaces within medium-grained, planar-tabular crossbedded sand display a metre-scale wavelength and are defined by a pebbly gravel lag. Palaeocurrents collected from the planar-tabular cross-bedded sands and gravels are dominantly towards the east. This facies (--lOm thick) is interpreted to have been deposited as a flood-tidal delta and mouth bar under wave influence in a tide-dominated estuary. It is overlain by a 14m-thick unit of small to medium sets of planar-tabular crossbedding characterised by tidal bundles and abundant reactivation surfaces. Palaeocurrents are dominantly oriented westward in the interpreted offshore direction, suggesting deposition as a prograding estuarine sand wave complex. It is in turn overlain by a similar facies some 7m thick in which palaeocurrents are dominantly oriented eastward, reflecting local ebb-flood asymmetry in the distal reaches of an estuary. Greensand of the South Maslin Sand is contained within a channel about 200m wide and showing some 20m of relief. Its base is marked by a 20cm-thick lag of well-rounded pebbly gravel and is deeply incised into the prograding estuarine sand wave complex of the North Maslin Sand. The medium-grained glauconitic sediments display large-scale trough crossbedding, and planar-tabular to sigmoidal crossbedding that is dominantly oriented towards the east. The greensand facies is interpreted as tidal channel and tidal delta sands that were deposited within a constricted estuary-mouth during transgression. The base of the South Maslin Sand at Maslin Beach is therefore interpreted as a tidal ravinement surface within a mixed wave and tide-dominated estuary.

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ASG-Conference '97

Advances in Sedimentology

The Hulopoe Gravel, Lanai, Hawaii: New Data and their Bearing on the "Giant Wave" Emplacement Hypothesis

E. Anne Felton^, E. Alison Kay^, Barbara Keating^ and Keith A. W. Crook^. ISOEST Hawaii Institute of Geophysics and Planetology, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822, U.S.A. ^Department of Zoology, University of Hawaii, 2538 The Mall, Honolulu, Hawaii, 96822, U.S.A. ^Hawaii Undersea Research Laboratory, MSB#303, 1000 Pope Road, Honolulu, HI 96822 U.S.A. There is currently considerable controversy about the origin of Pleistocene emerged marine gravel deposits on south Lanai (Hulopoe Gravel). Deposition above the present shoreline to 365 m by one or more giant waves (tsunami) has been proposed. It has been suggested that the same wave impacted the Australian east coast. Other hypotheses include abandoned shoreline deposition, due to sea-level falls from former high stands, or due to island uplift resulting from lithospheric flexure. Resolution of this controversy is important for correctly interpreting the geological history of oceanic and volcanic arc islands, for recognition of ancient tsunami deposits, and for interpretation of "normal" coarse-grained deposits on high energy coasts of continents and islands. The present study is focused on gravel and conglomerate deposits on the lower slopes of south Lanai, where up to 8 m of section is exposed in gullies incised by modem ephemeral streams, down to unweathered basalt bedrock in places. The deposits are well-bedded and consist of several sedimentary facies distinguished by variations in clast size, shape, sorting, clast fabric, presence or absence of interstitial matrix, and clast composition. Individual beds are slightly to strongly carbonate-cemented. Abrupt vertical changes in facies and a range of bounding discontinuities between depositional units are present. Beds containing basalt and limestone, mainly coral, clasts are interspersed with beds containing no limestone clasts. Basalt clast sizes in beds with >70% basalt range from large granules to large boulders (>1.5 m), with poor to moderate sorting, and are mainly subangular to subrounded. Clast fabrics in basalt-dominated beds are usually random. Clasts in rudstones (>90% limestone clasts) are well-sorted and subrounded. Some rudstones are plane bedded, and have aligned clast fabrics. Unlithified rudstones at one locality may contain an in situ molluscan fauna including adult and juvtmltStrombus spp., a benthic infaunal genus. Here the lowest bed, a thin grainstone, is bioturbated, and overlies a paleosol with erosional unconformity. The presence of a paleosol indicates that terrestrial as well as marine environments and processes are represented in the Hulopoe Gravel, and that at least one long time break occurred during its deposition. While the deposits may include tsunami-laid intervals, our data indicate that they are not exclusively tsunamigenic. We suggest that the Hulopoe Gravel includes products of the normal range of processes occurring on high-energy, storm and tsunamidominated rocky coasts of oceanic islands.

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Advances in Sedimentology

The Genesis of the Anomalous Mid to Late Eocene Section at Fohn-1, Zone of Cooperation, Northwestern Australia John D. Gorter Hardy Petroleum Limited, P.O. Box 1265, West Perth, Western Australia, 6872. The top Eocene Hibemia Formation is a relatively homogenous carbonate sequence with a generally low radioactive blocky gamma ray profile throughout the Timor Sea off northem Australia. However, at Fohn 1, the sediments are very much more radioactive, much more silty and heterogeneous, and have a markedly different sonic log character to neighbouring wells. Petrographic work indicated a large part of the anomalous sediments were altered glassy volcanics of probably basaltic composition. Samples are characterised by an amygdaloidal texture with spherical to occasionally elongate amygdules usually <1000um in diameter, which were probably vesicles or beads of glass but are now altered to illitic or smectitic clay, or have been replaced by calcite of ?zeolite.' Some contain larger, irregular-shaped vesicles sometimes rimmed by zeolite or chlorite and often infilled by calcite, or cut by irregular veinlets containing secondary minerals including ?zeolite, chlorite, calcite, illite and/or smectite. Reworked Campanian and Maastrichtian foraminifera and nannoplankton are seen in different proportions in samples from the anomalous zone. The abundance of recycled Campanian forms is unique from this part of the Timor Sea. The section overlying the anomalous interval is dated as early Miocene. The anomalous section is dated as late middle Eocene. A hiatus spanning the late Eocene and the entire Oligocene is present at Fohn 1 and a minimum age of late Eocene for the anomalous section. Seismic mapping of the Eocene-Paleocene section shows a regionally anomalous, oval shaped thick to the northwest of Fohn 1. Seismic profiles through this thick show an anomalous Paleocene section in the centre of the thick characterised by high amplitudes and a distinctly 'channelled' appearance. The intra-Turonian to base Paleocene isochron also shows an anomalous slightly offset to the northwest of the 'thick' in the Eocene-Paleocene. The base Paleocene horizon is difficult to map in the vicinity of this thick, but the base Turonian reflector and a series of reflectors originating from carbonates overlying the base Turonian horizon continue through with little disturbance. The interpretation of the origin of the anomalous section must account for all the unusual features noted in lithology, age dating, regional log correlations and seismic mapping. The questions of how abundant Campanian nannoplankton was transported into the Eocene section and the origin of the 'volcanics' must be answered. An origin from the impact of an extraterrestrial body satisfies these criteria, with the Cretaceous strata uplifted during rebound after the impact, which formed the glassy 'volcanics'. The dating of the anomalous section in Fohn 1 suggests that a potential impact occurred on the Sahul Platform during the late Eocene to Burdigalian interval (ca 37.5 to 24 Ma, AGSO timescale). Circumstantial evidence suggests that the impact occurred about 34 or 37.5 Ma, possibly coincidental with the Popigai crater in Russia and the coeval Chesapeake Bay and Toms Canyon impact craters in Virginia, the North American microtektite swarms, shocked quartz with PDFs, and an iridium anomaly seen in widely scattered localities.

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ASG-Conference '97

Advances in Sedimentoloqy

Impact Structures in the Eromanga Basin: Relevance for Petroleum Exploration.

V.A. Gostin, Dept Geology & Geophysics, University of Adelaide A.M. Therriault, Geological Survey of Canada, Ottawa, Ontario, Canada Two large circular areas centred on Tookoonooka and Talundilly in southwest Qsl., affect all Jurassic and Cretaceous sediments deposited before the upper Cadna-owie Formation (Hauterevian,-'128Ma), and predate the extensive marine inundations of the Aptian/Albian. The better studied '-66km wide Tookoonooka structure was first recognised from anomalous seismic profiles as a set of concentric anticlines and synclines, surrounding a complex central dome ~22km wide. A gravity low and a central gravity high characterize this structure, and thin sections obtained from a centrally located exploration well reveal an impact melt breccia of quartz-mica schist. Planar deformation features in quartz grains show 64 lamellae sets in 25 grains, corresponding to those found from impact structures in porous sedimentary rock targets, and confirm an impact origin for Tookoonooka. Hydrocarbon prospectivity is limited to the basinward (NW) half of the impact structure, underlying the disruption boundary; but some traps may occur within the intemal listric fault blocks and in the uppermost wavereworked sandstones under the transgressive late Early Cretaceous mudstones.

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ASG-Conference '97

Advances in Sedimentology

Sedimentology and Stratigraphy of the Early Triassic Rewan Group, Bowen Basin. Paul V. Grech National Centre for Petroleum Geology and Geophysics, Thebarton Campus, University of Adelaide, SA 5005. e-mail: pgrech @ ncpgg.adelaide.edu.au Marginally economic gas-charged reservoirs within the Early Triassic Rewan Group have been consistently discovered along the western flank of the Bowen Basin. An inadequate understanding of the stratigraphy of the Rewan Group has contributed to the lack of significant discoveries. The Early Triassic in the Bowen Basin has managed to evade close scrutiny due to the poor response of the rocks in the various data collecting techniques. This overview is part of a study that integrates all available data and applies stratigraphic principles to outcrop, wireline log, palynology and core data. It attempts to re-assemble all the data into a coherent and useable stratigraphic framework that will assist in defining facies and consequently potential reservoirs below seismic resolution within the Rewan Group. Eight stratigraphic field sections (total ~ 1.2km) were correlated with conventional core data (Taroom #10 - 240m). In turn, this data was related to wireline log character and biostratigraphic data. More than 90km of seismic section was also interpreted. Since this study, a further 4km of conventional core has been logged in detail and will form the basis for further research. The Rewan Group consists of the Sagittarius Sandstone and the Arcadia Formation. Depositional systems of the two formations were affected by major tectonic and climatic changes. Each formation is marked at its base by a third-order sequence boundary. The Sagittarius Sandstone is overall regressive and was deposited in a dominantly lacustrine environment with a possible marine incursion during maximum flood. The base of the overlying Arcadia Formation is marked by the Brumby Sandstone Member. It was deposited in a fluvially dominated shoreface setting. Red beds of fluvial origin in the Arcadia Formation are characterised by overbank fines and lenticular channel sandstone. Highfrequency incised channel fills occur in the uppermost Arcadia Formation. It is in turn erosively overlain by pebbly sandstone Clematis Group. Sandstone-filled incised valleys at the base of the Sagittarius Sandstone, the Brumby Sandstone Member and the channels in the upper Ai-cadia Formation offer the best reservoir potential in the Rewan Group. REFERENCES: GRECH, P.v., 1997. Sequence Boundaries of the Early Triassic Rewan Group, Bowen Basin. PESA (Qld) Symposium, 1997, p47 - 53. GRECH, P.V. and DYSON, I.A., 1997. An Integrated approach to the study of the Early Triassic Rewan Group, Bowen Basin. The APPEA Journal, Vol37 (1), pl92 - 204.

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Advances in Sedimentology

Rapid Sedimentation by Sustained High-Density Turbidity Currents: The Cambrian Kanmantoo Group, South Australia

Peter W. Haines, J. C. Gum and J. B. Jago Department of Applied Geology, University of South Australia, The Levels, SA 5095, Australia The Kanmantoo Group is a thick (--7-8 km) predominantly clastic marine succession which was deposited within a localised extensional basin (Kanmantoo Trough) close to the palaeo-Pacific margin of Gondwana during the Early Cambrian. Radiometric constraints indicate very rapid sedimentation, with the group deposited in possibly less than 10 My, followed immediately by convergent deformation of the Cambro-Ordovician Delamerian Orogeny. The sediments were variably metamorphosed from lower greenschist to amphibolite facies, and deformed by westdirected imbricate thrusting. Due to these complications, few studies of the sedimentological aspects of this economically significant (Cu, Zn, Pb) succession have been undertaken, and the regional stratigraphy of the group is still incompletely known. Despite metamorphism, a number of sedimentary facies can be recognised within the Kanmantoo Group. Here we focus on the dominant facies, which comprises monotonous repetitions of sharp-based, grey, mineralogically immature sandstone (greywacke) capped by mudstone. The origin of this facies has been somewhat controversial, for although it has been widely recognised as "flysch-like", there is a general rarity of "classical turbidites", or of welldeveloped grading. Most beds, which may reach several metres in thickness, are massive, or only weakly stratified, and of essentially even grainsize (typically fine to medium sand), except at the top. We suggest that the sandstone beds were deposited by sustained high-density turbidity currents (see Kneller & Branney 1995). Currents were probably initiated at the mouths of high load capacity rivers, probably draining an active orogenic terrain, and capable of periodically generating hyperpycnal sediment plumes in seawater (see Mulder & Syvitski 1995). Palaeocurrent measurements from erosional sole marks indicate north-directed troughparallel currents. In contrast, rippled silts at bed tops commonly indicate currents directed to the east or southeast, probably relating to reflection of waning currents off basin controlling fault scarps and other trough-parallel structures. A southerly provenance is contrary to traditional palaeogeographic models, which assume a local basement source and trough-normal sediment dispersal. It is, however, consistent with the lack of a suitable orogenic source terrain in adjacent regions of South Australia, and with recent detrital zircon dating (Ireland et al. 1995) which rules out the local basement or underlying sediments as significant contributors to Kanmantoo Group sands. A prominent young (latest Neoproterozoic to Early Cambrian) component to the detrital zircon spectra suggests considerable penecontemporaneous magmatic activity. The probable source was a belt of convergent deformation and magmatism occurring along the active palaeo-Pacific margin of formerly contiguous Antarctica.

REFERENCES

Ireland T. R., Fanning C. M., Flottmann T., Weaver S. D., Bradshaw J. D. & Adams C. 1995. Continental structure on the Pacific margin of Gondwana: detrital zircon ages from Lachlan Fold Belt, Kanmantoo Group and Adelaide Fold Belt of Australia, and Victoria Land correlatives. 7th Internat. Symp. Antarctic Earth Sci., Siena, Italy, Abstracts, p.202. Kneller B. C. & Branney M. J. 1995. Sustained high-density turbidity currents and the deposition of thick massive sands. Sedimentology 42, 607-616. Mulder, T. & Syvitski, P. M. 1995. Turbidity currents generated at river mouths during exceptional discharges into the worlds oceans. The Journal of Geology 103, 285-299.

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Advances in Sedimentology

The Essential Interplays between Deformation, Sedimentation, and Fluid Flow for the Formation of the HYC Ag-Pb-Zn Deposit at McArthur River, Northern Territory Mark Hinman M M Exploration Pty. Ltd., PO Box 1042, Brisbane, QLD. 4001 The HYC deposit at McArthur River, Northern Territory is a world class stratiform Ag-Pb-Zn deposit containing a mineral resource of 103.7Mtonnes @ 14.1%Zn, 6.4%Pb, 64^tAg of which mining reserves (proven and probable) of 22.9 Mtonnes grading 13.6%Zn, 6.3%Pb and 65g/t Ag (pers comm: Damien Nihill, MRM) are currently being exploited in an underground room and pillar operation. The deposit is hosted by dolomitic-carbonaceous-pyritic silts and shales of the Palaeoproterozoic Bamey Creek Formation - the lowest unit of significant organic accumulation and preservation within the 4-6km thick, dolomitic-evaporitic McArthur Group. Surface, underground and drillcore structural work at McArthur River over the last four years has constrained the timing of mineralisation relative to sedimentation, diagenesis and deformation and suggested, firstly, a complex interplay between these elements and mineralising fluid flow (Hinman, 1994; 1995; 1996) and secondly, tight constraints on the hydrodynamic and geochemical processes of stratiform base metal mineralisation at HYC. In northern Australia, the McArthur Group makes up most of the lower portion of "Cover Sequence 3". This cover sequence accumulated under broadly NNW-SSE extension above an older and inverted, mafic-dominated "Cover Sequence 2" which formed as a rift succession controlled by broadly NE-SW extension (Etheridge & Wall, 1994). The McArthur Group and "Cover Sequence 3" comprise an early clastic-dominated rift succession that rapidly evolves into a carbonate-dominated, low accommodation, ramp succession that contains a number of significant re-activations . The most significant of these re-activations is the "OP2 Event" (APWP; Irdnum et al., 1993) that results in localised foundering and the accumulation of the Bamey Creek Formation in the McArthur River region - the ultimate host of the HYC deposit. In the McArthur region, throughout the accumulation of most of the lower McArthur Group, the Emu Fault Zone behaved as an accommodation zone in a NNW-SSE extensional regime. Sometime prior to Bamey Creek-time a radical shift in the drift of the Australian Proterozoic proto-continent significantly altered the intra-plate stresses of the McArthur Basin (Idnurm et al., 1993; Loutit et al., 1994). This resulted in a shift in the extension direction which, coupled with the sinuosity of a relatively immature Emu Fault system, put different sections of the Emu Fault Zone into transpression and transtension. Up-thrown and down-warped portions of the carbonate ramp along the Emu Fault system appear to have been closely juxtaposed with the downwarps receiving sediment through Barney Creek-times. At HYC, the Barney Creek Formation accumulated within a downwarp immediately west of the Emu Fault Zone but also within a listric fault-bounded graben (Neudert & McGeough, 1996) along the Bald Hills trend at a high angle to the Emu system. At HYC, accommodation exceeded supply for most of Bamey Creek-time and "basinal facies" Bamey Creek Formation (Bull, 1995) comprising turbiditic, dolomitic, silt-mud rhythmites and sedimentary breccia mass flow deposits deposited until transpressive inversion and failure late in Bamey Creek-time (Hinman, 1995) released accumulated stresses, producing a shoal and ending the Bamey Creek cycle. Two subsequent drowning-shoaling cycles (the Reward and Caranbarini cycles; Neudert & McGeough, 1996) have also been the focus of base metal exploration. At HYC, in samples of "mineralised shale", base metal mineralisation formed after carbonate "crusts", "nodular carbonates", diagenetic pyrite and dolomitic concretions, but before the formation of abundant and kinematically distinctive stmctures that occur within the ore sequence on all scales from drillcore and microscopic (Hinman, 1995) to mine-scale (Coutts, 1996). These stmctures are consistent with layer-parallel failures driven by more rapid subsidence (and reflected in thicker accumulation) in the northeast at HYC during Bamey Creek-time. Previously reported stmctural relationships at HYC have demonstrated that the vast bulk of mineralisation formed below the sediment-water interface within semi-consolidated Bamey Creek sediments prior to this phase of gravity-driven deformation (Hinman et al., 1994; Hinman, 1995). Hydrothermal fluids, having formed the stratiform HYC Pb-Zn orebodies, 16

ASG-Conference '97

Advances in Sedimentology

subsequently refocussed, during the subsequent, transpressive inversion event that closed the Barney Creek cycle of sedimentation, to form overprinting styles of base metal mineralisation. At HYC, this inversion culminated in the formation of the Cu-Pb-Zn mineralised "Cooley Breccias" on the western edge of the Westem Fault Block. The Western Fault Block (rather than a hanging wall block marking the eastern edge of the Barney Creek basin as previously interpreted (Williams, 1978)) is now understood to be composed of a largely coherent fold limb of pre-Bamey Creek dolomites that has been transpressively, reverse-faulted over stratigraphically higher Barney Creek Formation. The upper time constraint on these events is afforded by the onlapping of upper Barney Creek Formation sediments over the steeplydipping, brecciated and mineralised dolomitic units of the Westem Fault Block just prior to the Reward drowning. The syn-diagentic model of ore formation proposed by Hinman (1996) is a refinement of the models of Williams (1978) and Eldridge et al. (1993). Base-metal minerahsation is considered to have formed relatively shallowly (~10-20meters) below the sediment-water interface from a dense, sulphate-bearing brine that flowed within the available permeability of the rhythmically interbeded, organic-rich, silts and shales of the Barney Creek Formation. Redox reactions between very immature organic matter and brine sulphate produced reduced sulphide resulting in the precipitation of base metal sulphides. Ultra thin section work of Hinman (1996) suggest base metal mineralisation is texturally associated with intense stylolamination. This stylolamination appears to only effect the originally silty portions of the sediments and not the muddy tops of the typical Barney Creek Formation silt-shale rhythmites which remain texturally unmodified and unmineralised. Mass balance work suggests that carbonate is dissolved to form the stylolaminated "mineralised shale" and that this dissolved carbonate, in the instantaneous view, is redeposited down-flux and above the stylolaminated mineralised zone as secondary "nodular carbonates" (previously considered to be carbonate-replaced evaporites (Logan, 1979)) and cements along with additional components from the brine and products of the organic-sulphate redox reactions. This model implies significant sediment-brine interaction and is incompatible with exhalitive models (Large et al., 1996).

References

Bull, S.W., 1995. Towards a regional depositional model for the Palaeoproterozoic Bamey Creek Formation, southern McArthur Basin, Northern Territory. AMIRA Project Report, Coutts, B.P., 1996. Geological controls on mining at McArthur River. MIC96 Abstracts Volume, Townsville Etheridge, M. & Wall, V., 1994. Tectonic and structural development of the Australian Proterozoic. Geological Society of Australia, Abstracts No 37 Eldridge, S.C., Williams, N. & Walshe, J.L., 1993. Sulphur isotope variability in sedimenthosted massive sulphide deposits using the ion microprobe SHRIMP: 11 A study of the HYC deposit at McArthur River, NT., Australia. Economic Geology 88, 1-26 Hinman, M.C., Wall, V.J. and Heinrich, C., 1994. The interplay between sedimentation, deformation and mineralisation at the McArthur Pb-Zn(-Cu) deposit. Geological Society of Australia, Absts No 37 Hinman, M.C., 1995. Base metal mineralisation at McArthur River: Structure and kinematics of the HYC-Cooley zone at McArthur River. AGSO Record 1995/5 Hinman, M.C., 1996. Constraints, timing and processes of stratiform base metal mineralisation at the HYC Ag-Pb-Zn deposit, McArthur River. MIC96 Abstracts Volume, Townsville Idnurm, M., Giddings, J.W. & Plumb, K.A., 1993. Palaeomagnetism of the southeastern McArthur Basin: poles, overprints and reversals. Exploration Geophysics, 24, 227230 Large, R., Bull, S.W., Cooke, D. & McGoldrick, P., 1996. Review of genetic models from HYC: constraints from sedimentology, alteration halo and fluid chemical modelling. MIC96 Abstracts Volume, Townsville Logan, R.G., 1979. The geology and mineralogical zoning of the HYC Ag-Pb-Zn deposit, McArthur River, NT. UnpubL Msc Thesis, Australian National University Neudert, M. & McGeough, M., 1996. A new tectonostratigraphic framework for the deposition of the upper McArthur Group, Northern Terrirory. MIC*96 Abstracts Volume, Townsville 17

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Advances in Sedimentology

Loutit, T.S., Wybom, L.A., Hinman, M.C. & Idnurm, M., 1994. Palaeomagnetic, tectonic, magmatic and mineralisation events in the Proterozoic of northern Australia. AusIMM Abstracts, Darwin, 123-128 Williams, N., 1978. Studies of the base metal sulphide deposits at McArthur River, Northern Territory, Australia: I. The Cooley and Ridge deposits. Economic Geology 73, 1005-1036 Acknowledgments: Many MIM geologists at McArthur River produced the extensive data sets on which the new work presented here has built. Stimulating discussions with geologists at both MIM and AGSO have helped hone the ideas presented here. In particular, Chris Heinrich, Vic Wall, Ross Logan, Martin Neudert and Graham Logan have contributed with their insights and prejudices. This work is presented with the permission of MIM Exploration Pty Ltd.

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ASG-Conference '97

Advances in Sedimentology

Tertiary Lignite Deposits in Australia, New Zealand and Germany: Timings, Correlations and Depositional Factors

G. R. Holdgate School of Earth Sciences, University of Melbourne, Parkville, Victoria, 3052. Fourteen Tertiary aged brown coal/lignite deposits in Australia, New Zealand and Germany are compared from the viewpoint of sequence stratigraphy, timing, palaeogeography, palaeoclimatic setting, and coal quality. Major points of similarity between the lignite deposits are: -the concentration of major coal forming phases into relatively short time periods within the full Tertiary time range. -the close relationship of these major coal forming phases to the Haq et al. (1988) peak periods of coastal onlap where sea-levels were relatively higher than for other periods in the Tertiary. -common periods of thick seam development are confined to three sequence third order cycles or groupings. These are: i) the late Early to early Middle Miocene sequence cycles TB2.1-2.3 (15.5-21.0 m.yrs). ii) the late Upper Eocene sequence cycle TA4.2 (37.0-38.0 m.yrs). iii) the late Middle Eocene sequence cycle TA3.5 (40.5-42.5 m.yrs). Additional factors for thick seam development include structural setting, the palaeoenvironment, and stacking of coal lithotype cycles. Data available on some of the deposits indicates the following points of significant correlation: -coal deposits often form in graben-like embayments adjacent to the major basinal developments. -the embayments are often barred from the main marine basinal sedimentation by a barrier sand build-up across the entrance, effectively isolating the peat swamps for long periods from marine transgression. -a correlation between thick brown coals and near-tropical environmental conditions prevailing at the time. -evidence for raised mire ombrotrophic conditions developed within the coals similar to the modem tropical peats in Indonesia. Haq, B.U., Hardenol, J. & Vail, P.R., 1988: Mesozoic and Cenozoic chronostratigraphy and cycles of sea-level change, in Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., and Van Wagoner, J.C., (eds.). Sea-level changes: an integrated approach: The Soc. Eco. Paleo. & Mineral, Spec. Publ. No. 52, pp. 71-109.

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Advances in Sedimentology

Sequence Stratigraphic Interpretation of Carbonate Wireline Log Motifs: an Example from the North West Shelf of Australia. Johnny HulL Stuart Smith (University of Adelaide) and Hamish Young (University South Australia), National Centre for Petroleum Geology and Geophysics, Thebarton Campus, University of Adelaide, SA 5005. An integrated biostratigraphic, wireline and seismic sequence stratigraphic study has been conducted to constrain the timing and evolution of Late Cretaceous to Tertiary depocentres along the North West Shelf of Australia. During this study a model for the sequence stratigraphic interpretation of wireline logs in this carbonate-dominated regime has been developed. A series of readily identifiable, lowstand clastic deposits interspersed within the predominantly carbonate passive margin section of the North West Shelf provide well-defined correlatable events with which to divide the section. Biostratigraphic data have indicated the presence of missing section at the base of these clastic deposits and their shelfal equivalents. These events have been correlated to define sequence boundaries that are represented on wireline log data by a sharp increase in the gamma signature. Lowstand systems tracts exhibit an irregular sonic and upwardly increasing gamma signature. Transgressive systems tracts show characteristically upward-decreasing gamma and sonic profiles. Maximum flooding surfaces have been identified as the point of cleanest carbonate sedimentation represented by gamma minima on wireline logs. Log motifs exhibiting little character have been interpreted as highstand systems tracts. On seismic these sequence stratigraphic events are represented by stratal geometries that would be expected for these systems tracts. The model has enabled the definition of a higher resolution chronostratigraphic framework for the Mid Cretaceous to Recent section of the North West Shelf than has previously been possible. Forty basin-wide events have been identified from the biostratigraphic and wireline log analysis, thirty of which can be tied throughout the Barrow, Dampier and Roebuck basins.

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Advances in Sedimentolopy

Accommodation History and Gamma Logs: A 1610 Ma Disordered Evaporitic, Stromatolitic Facies Mosaic or a Gradually Deepening Ramp?

M J Jackson & P N Southgate Australian Geological Survey, GPO Box 378, Canberra, ACT, 2601 Based on detailed lithostratigraphic studies in the late 1970's the uppermost --ISOOm of the McAthur Basin succession was defined as the Nathan Group, and sub-divided into two formations and several informal members (Muir in BMR Bulletin 220, p 136-145). The lower 1300m consists of a basal 50m thick fluvial sandy unit succeeded by carbonates - a 250m thick evaporitic unit, a 280m thick stromatolitic unit and a 350m thick recrsytallised unit. Due to the common presence of halite and sulphate evaporites, intraclast beds, desiccation cracks, and stromatolites, the carbonates were interpreted as being deposited in a complex of lagoons, continental playas, sabkhas and possibly alkaline lake environments. We re-measured the succession in detail in 1995 using a Jacob's staff and Abney Level and also recorded gamma ray measurements at 50 cms intervals using a hand held spectrometer. Continuous gamma ray curves, based on objective geophysical parameters of the rocks, represent a powerful compliment to more subjective lithostratigraphic facies logs - they emphasise systematic variations and often show the stacking pattems of sedimentary cycles. Integration of the lithofacies and gamma logs within the context of accommodation is a powerful tool for revising sedimentation histories. These techniques have been extensively tested and are routinely used for the study of Phanerozoic rocks in the petroleum industry. We believe this represents their first application to surface sections in the Proterozoic and reconmiend wide-spread adoption of the techniques.. In sequence stratigraphic terms the Nathan Group can now be considered as one large supersequence made up of seven individual sequences (probably of order rank) ranging from 80 to 340m thick. The gamma log motif for individual sequences is generally an asymmetric curve with a thin peak in values near the base followed by a gradual decline; younger sequences develop a prominent gamma peak near their middles. Sequence boundaries (sharp shifts in ganrnia ray value) and maximum flooding surfaces(mfs) or condensed intervals (broad peaks or troughs in gamma values) are clearly evident in the gamma logs for most sequences. The lithofacies relating to these features vary systematically through the supersequence. The high gamma values near the base of the older cycles are produced by "dirty", evaporitic, coastal-plain lowstand deposits; whereas the high gamma values near the middles of the younger cycles are produced by shaley, storm-deposited fine-grained clastics near the mfs. Stromatolite facies also change systematically through the supersequence. Sequences 1 and 2 are dominated by lowrelief forms that are intraclastic and contain evaporite pseudomorphs. Stromatolites become progressively larger upwards and form distinctive, laterally-persistent biostomes - the Balbirini prima and Kussiella kussiensis marker beds. These were deposited in lower energy, deeper water environments at the mfs of sequences 3 and 4. Overall the supersequence is dominantly transgressive. The facies on the large (ie supersequence) scale progressively deepen upwards as accommodation rates gradually increased so that by the uppermost two sequences shales replace most of the shallower water carbonates as the ramp was drowned. The maximum flooding for the supersequence is marked by concretions and glauconite in storm-affected fine dolomitic siltstones in sequence 6. A regional unconformity at the base of the overlying -1500Ma Roper Group truncates these deposits and erosion at this level has probably removed most of the highstand of the Supersequence.

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Advances in Sedimentology

Foraminiferal Sequence Stratigraphy of the Oligo-Miocene Janjukian Section at Torquay, Southeastern Australia O. Li 1. PJ. Davies 2, B. McGowran 1 and T. Van der Linden 2,3 1 Department of Geology & Geophysics, The University of Adelaide, Adelaide, SA 5005 ^Department of Geology & Geophysics, The University of Sydney, Sydney, NSW 2006 ^School of Earth Sciences, La Trobe University, Bundoora, VIC 3083 The coastal section of marls and limestones at Torquay in southeastern Australia were the basis for erecting the regional Janjukian Stage; more recently, they have had a role in proselytizing sequence stratigraphy. Based on a recent stratigraphic drilling and coring, we present foraminiferal biofacies studies at (i) third order resolution and (ii) at higher frequencies. (i) Foraminiferal biofacies closely follow third-order sequences under eustatic control. The biofacies recovered from Holes 9 and 17 are dominated by inner to mid shelf benthic species and groups, especially the cibicidids, discorbids and miliolids. Planktonic species are rare and rarely age-diagnostic. A cluster analysis of taxa and relative abundances in both sections simultaneously yielded four assemblages (A to D upsection). Strongly successional, the assemblages also closely correspond to lithological changes characterizing hthostratigraphic units, as follows: Angahook Formation (assemblage A), lower and upper Jan Juc Formation (B, C) and Puebla Clay (D). Biofacies trends based on the relative abundances of inner and outer neritic taxa invited the recognition of third-order sequences and boundaries equivalent to TBl.l to TB1.4, confirming a previous identification based on physical stratigraphy (Fig. 1). We conclude that these foraminiferal assemblages were directly influenced by third order fluctuations in sea level. A climatic warming at upper Jan Juc time brought such subtropical species as Amphistegina lessonii and Pararotalia mackayiAnto the studied area and tmncated the prevailing cool-water depositional environments. Faunal differences between the two sites became stronger, suggesting a higher availability of microhabitats in warmer conditions. This event is at least regional, for similar faunas have been found also in coeval sequences from other basins along the southern Australian margin. The long-uncertain regional Oligocene/Miocene boundary is placed at the Jan Juc/Puebla contact, across which there was a major faunal change. Several benthic forms disappeared, at least temporarily: Cibicidoides perforatus, Amphistegina lessonii and Pararotalia mackayi. The planktonic Subbotina euapertura also became extinct, but typical Miocene taxa Globoquadrina dehiscens and Globoturborotalita brazieri made their first appearance. These events were associated with an increase in inner neritic benthos signalling a low sea level, consistent with the global pattern of glaciation MiL (ii) Biofacies at higher frequencies reflect both depth changes and bioturbation. Successional changes in foraminiferal assemblages occur in a sediment package of m from the lower part of the Jan Juc Formation in Hole 9, corresponding to a lithological change at Milankovitch scale (Fig. 1). Variations in the abundance of individual species and species groups characterise these assemblages. Shallow-water species dominated the fauna in (partially) lithified, shelly layers, whereas deeper-water species are more abundant in soft, marly samples. The contrast between shallow- and deeper-water taxa allowed an estimate of changes in relative palaeodepths. Maximum water depth ocurred in the middle part of the soft layer, while a shallowest depth was recorded from near the top of the shelly layer, close to the lithocycle's upper boundary. Several pairs of samples representing background sediment and burrow infills show that burrows transported material in three directions: laterally, downcore and upcore. A higher faunal similarity in some pair samples was probably related to a relatively stable bottom condition before and during most burrows were filled, although more aerobic conditions were likely associating with the burrow-fill process. A wider range of faunal similarity between other pair samples suggests a complex bioturbation mechanism, which may not be resolved by studying faunas alone. 22

standard stratigraphy Epoch zones

"eustatic" curve

Torquay Embayment Hole 9 high resolution (estimated paleodepth)

Fig. 1. Using foraminiferal biofacies to reconstruct sea level and climatic history. Standard stratigraphy with zones of (1) planktonic foranfiinifera and (2) nannoplankton (Berggren et al., 1995), eustatic curve (Haq et al., 1987), deepsea isotopic curve and Oi and Mi glacial events (Miller et al., 1991; Wright and Miller, 1993), and curves constructed by foraminifera in Holes 9 and 17. Note the parallel swings between them at third order scale (TBI .1 to TBI .4). The high resolution reflects sealevel fluctuations at Milankovitch scale, -80,000 years in this example.

ASG-Conference '97

Advances in Sedimentology

Late Tertiary Reefs of New Guinea and the Philippines

Alan R. Lloyd Alan R. Lloyd & Associates, 29 Foston drive, Duncraig, WA 6023 A reef is defined in the dictionary as "A narrow ridge of rocks or sand, often of coral debris, at or near the surface of water. " When people think of reefs they usually think of the Great Barrier Reef of Queensland or the atolls of the south Pacific. These are what are usually called biohermal reefs. The living reef on the surface is made up of coral and algae which require warm, shallow, clear water to survive. They therefore form on submarine volcanoes or structural highs. About 14 years ago I attended a lecture on limestones and reefs by Gerald Freedman of New York in Jakarta. He showed a series of slides taken of cores cut in a modern day reef. There was nothing in the cores below the top living reef that would indicate that the hole penetrated a reef. Most people seem to think a reef consists of solid masses of coral and algae. A paper was published on reefs of the Salawati Basin, western Irian Jay a which are the main oil reservoirs. It was based on a report by an Indonesian geologist who used only electric logs to draw his conclusions. He did not examine any actual samples. There was no palaeontology or petrology carried out on any of the wells. This meant that there had been no petrological, age or environmental data available on which to base their conclusions. Based on my work in Papua New Guinea, my own results from Irian Jaya and other published data I came to the conclusion that these so called reefs were not reefs in the sense of biohermal reefs and were in fact deep water deposits made up mainly of reworked shoal limestone. The "reefs" are believed to be mounds of this reworked limestone or pinnacles formed by erosion of the limestone and represent a paleo karst topography. Geophysicists see a big bump on a seismic section and therefore call it a reef. This could be a valid geophysical term but it unfortunately conjures up visions of bioherms. I think this is unfortunate because they may not know exactly what they are looking for, the time of oil migration and accumulation. I have found a similar situation in the Gulf of Papua and offshore Palawan Island in the Philippines. I think it is time for a major review of the late Tertiary limestones of this region because I believe it would lead to the discovery of a lot more oil and gas. Being pinnacles of a karst limestone rather than isolated reefs I believe that the large grids presently used in seismic surveys must miss a lot of prospects. Costs, however, could ht the limiting factor to shooting small grids.

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Transported Carbonate Debris in the Late Silurian Barnby Hills Shale, NE Lachlan Fold Belt Elisabeth J. Morgan Geological Survey of New South Wales, PO Box 53 Orange, NSW 2800 A number of discrete shallow water carbonate deposits are present south of Wellington New South Wales, on the eastern margin of the Molong High. The carbonate debris is embedded within deep water, non-calcareous shales of the Late Silurian Bamby Hills Shale and occurs sporadically over a strike length of 30 km. Carbonate facies, fossil species and faunal assemblage preserved in the debris are identical to the shallow water Mid to Late Silurian Narragal Limestone that conformably underlies the Bamby Hills Shale. The calcareous debris occurs at least 200 m above the top of the Narragal Limestone and is of similar Late Silurian (early - late Ludlow) age to enclosing graptolitic shales. The debris has a similar depositional history to the well documented Nubrigyn Formation (Conaghan et al 1976) that was deposited in the same area during the Early Devonian. The carbonate debris deposits consist of allochthonous limestone mega blocks, calc-lithic sandstones and limestone breccias. The limestone blocks vary in shape from long, thin lenses to broad, elongate bodies and range up to 180 m in length. The blocks generally lie parallel or subparallel to the strike of enclosing strata, although one block lies perpendicular to bedding. All blocks lie at the same stratigraphic level. Enclosing strata is devoid of calcareous debris suggesting the limestone blocks slid or tumbled down slope as free moving objects, rather than as a coherent mass flow. The calc-lithic sandstones exhibit well preserved sedimentary structures and are characteristic of beds deposited by turbidity currents. The turbidites range from crudely bedded coarse grained sandstones to well bedded, medium to fine grained sandstones. The limestone breccias are characteristic of debris flow deposits, with disorganised fabric, poor sorting and sharp upper and lower contacts. Lithics range from mm size to 9 m in length and the breccias reach 30 m in thickness. During Mid to Late Silurian time, the Molong High began to rapidly subside. Associated sea level rise together with the influx of fine grained sediment effectively terminated development of the Narragal Limestone. The presence of calcareous horizons stratigraphically above the Narragal Limestone suggests however that during deposition of the Bamby Hills Shale parts of the limestone were exposed and subject to erosion and gravitational collapse. The localised nature of the calcareous debris suggests the dislodged material was transported into deeper water along restricted pathways, in the context of submarine fans.

REFERENCES

CONAGHAN, P. J., MOUNTJOY, E. W., EDGECOMBE, D. R., TALENT J. A. & OWEN, D. E., 1976. Nubrigyn algal reefs, eastern Australia: allochthonous blocks & megabreccias. Geol Soc. Am. Bull, 87, 515-530. Acknowledgements: Published with the permission of the Director General, New South Wales Department of Mineral Resources. Ian Percival, John Pickett and Lawrence Sherwin (GSNSW) are thanked for their identification of macrofossils in the study area.

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Advances in Sedimentolopy

Burial Diagenesis of Deep Shelf Calcareous Mudstone of the Early Devonian Buchan Group, Southeast Australia

Rao R. Munuganti and J. A. Webb School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083 The Early Devonian (Emsian) Buchan Group outcrops extensively in the Buchan Basin, eastern Victoria, and comprises both shallow and deeper shelf carbonates. The deeper water strata, known as the Taravale Formation, consist of thin-bedded black to dark grey lime-mudstone and marl, with occasional thicker wackestone/packstone beds. Fossils tend to be concentrated in thin beds, and are dominated by solitary rugose corals and large gastropods, with scattered nautiloids and gastropods, and occasional colonial tabulate corals. The chambers within the fossils frequently contain well-preserved carbonate cement generations which served as longlived host areas for the diagenetic fluid migration; these cements were documented using petrographic, geochemical and fluid inclusion studies. The initial cement phase in most fossils is a cloudy, isopachous layer of dully luminescent calcite with a radial-fibrous to radiaxial-fibrous fabric. The oxygen isotope composition (mean 5.52 %o) and 87sr/86sr ratio (0.708164) are similar to those of Emsian sea-water and the host limestone. This cement resembles marine calcite, which has been widely reported from reefs, and it probably precipitated on the sea floor, but has clearly formed in a much quieter water environment than most previously described marine calcite. Syntaxially overgrowing the cloudy, fibrous cement is a generation of clear, equant calcite cement, which shows altemating dull and thin brightly luminescent zones under the cathodoluminoscope. The 87sr/86sr ratio of this cement (0.708155) matches that of the underlying fibrous cement, but the oxygen isotope composition is more depleted (mean -7.63 %o), indicating precipitation from marine fluids at elevated temperatures, perhaps due to burial. In some corals the fibrous cloudy calcite cement is overgrown by a thin horizon of subhedral to anhedral, brightly luminescent dolomite with a similar oxygen isotopic composition to the underlying calcite. However, a much more common ferroan saddle dolomite cement infills porosity in all fossil types. This dolomite is coarse-grained, with well-developed sweeping extinction, and has a slightly heavier oxygen isotopic composition (-5.74 %o) than the equant calcite cement, and a slightly more radiogenic ^^Sr/^^Sr ratio (0.708270), implying precipitation at elevated temperatures from slightly evolved Early Devonian sea water trapped in the pore spaces. Intimately associated with the saddle dolomite are fluorite and strontianite; both have very similar 87sr/86sr ratios (0.708264 and 0.708281 respectively) to the saddle dolomite, and were probably also deposited during a deep burial diagenetic event. Fluid inclusion data from the fluorite in the fossil cavities indicate that the precipitating fluids had a minimum temperature of 195-300 °C, with a salinity of 6 to 12 wt% NaCl equivalent. Many fossil chambers have central cavities, but some are filled by a final phase of very coarse, clear, blocky, dully luminescent calcite, with a heavier oxygen isotopic composition (-3.01 %o), and a slightly less radiogenic ^^Sr/^^Sv ratio (0.708246). This cement generation may record lower temperature precipitation during uplift of the Buchan Basin in the mid-Devonian. There is little evidence of extemal fluid input; the fossil cavities within the lime-mudstone appear to have acted almost as a closed system, at least on a hand specimen scale. The Buchan Caves Limestone underlying the Taravale Formation contains base-metal mineralization localized along faults; the lack of mineralization within the lime-mudstones indicates that the metals were not derived from the clays of the Taravale Formation, but must have had an extemal source.

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Advances in Sedimentology

Carbonate-Hosted Zn-Pb Mineralisation within the U-lens at Navan, Ireland

W. Peacel M.W. Wallace^, J.H. Ashton^, M. Holdstock^, & J. Geraghty^ ^School of Earth Sciences, University of Melbourne, Parkville, Vic, 3052 ^Outokumpu - Tara Mines Ltd., Navan, Co. Meath, Ireland The Navan Zn-Pb orebody is located approximately 50 km northwest of Dublin, near the northern margin of the Dublin Basin. Mineralisation is hosted within a shallow water carbonate sequence of Lower Carboniferous (Courceyan) age informally known as the Pale Beds (Philcox, 1984) [formally termed the Meath Formation (Strogen et aL, 1990)], on the northern flank of a strongly faulted, northeast trending anticlinal structure at the southwestern margin of the Lower Palaeozoic Longford-Down inlier (Ashton et aL, 1986). Mineralisation occurs as a series of six stratabound lenses which dip up to 15° to the southwest. The stratigraphically highest (and most recently discovered) of these is the U-lens. U ('upper')-lens mineralisation is defined as mineralisation which occurs in the stratigraphic interval between the base of the Upper Sandstone Marker (USM) marker horizon, and the overlying Shaley Pales [Moathill Formation (Strogen et aL, 1990)]. The thickness of this unit varies, from a maximum of approximately 40 m, to completely absent due to faulting or down cutting by the pre-Arundian (Ashton et aL, 1992) submarine erosion surface and associated Boulder Conglomerate debris flows. Prior to mineralisation, the U-lens has underpne dolomitisation. This has resulted in sections of the U-lens being completely replaced by fine grained, rhombic dolomite. This dolomitisation has probably partially influenced the style and occurrence of the sulphide mineralisation. The U-lens is stratabound in nature, with a pod or lens-like morphology. Lateral continuity of the mineralisation has been affected by offsets due to post-mineralisation movement of E-NE trending extensional faults. Sphalerite is the dominant sulphide in the U-lens, as in the rest of the orebody, with lesser galena, pyrite and marcasite also present. Due to the nature of the mineralisation, gangue minerals are relatively rare, but where present, is typically a combination of barite, saddle dolomite, and calcite, with barite normally dominant. Sulphide mineralisation is mainly by replacement of the carbonate host and disseminated sulphides in inter particle voids. The remaining sulphides occur as open-space cements, both cavity and vein/fracture fill, and to a lesser extent, as internal sediments. Ashton, J. H., Downing, D.T., & Finlay, S., 1986. The geology of the Navan Zn-Pb orebody. In_ Geology and genesis of mineral deposits in Ireland. Andrew, C. J., Crowe, R.W.A., Finlay, S., Pennell, W.M., & Pyne, J.F., eds., Irish Association for Economic Geology, Dublin, p. 243-280. Ashton, J. H., Black, A., Geraghty, J., Holdstock, M., & Hyland, E., 1992. The geological setting and metal distribution pattems of Zn-Pb-Fe mineralization in the Navan Boulder Conglomerate. IiL The Irish minerals industry 1980 - 1990. Bowden, A. A., Earls, G., O'Connor, P.G., & Pyne, J.F., eds., Irish Association for Economic Geology, Dublin, p. 171-210. Philcox, M. E., 1984. Lower Carboniferous lithostratigraphy of the Irish Midlands. Irish Association for Economic Geology, Dublin, 89 p. Strogen, P., Jones, G.L., & Somerville, I.D., 1990. Stratigraphy and sedimentology of Lower Carboniferous (Dinantian) boreholes from West Co. Meath, Ireland. Geological Journal, 25, p. 103-137.

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Advances in Sedimentology

Is the Gold Coast from Antarctica? Provenance Implications from Detrital Mineral Geochronology of Eastern Australian Sediments

K.N. Sircombe Research School of Earth Sciences, AustraUan National University, Canberra, ACT 0200 A distal Precambrian provenance for detrital heavy mineral grains in eastern Australian beach sands has long been a subject of speculation based on traditional heavy mineral analyses. In order to resolve this speculation, the Sensitive High Resolution Ion MicroProbe (SHRIMP) has been used to determine the Pb/U and Pb/Pb ages of single detrital zircon, monazite and rutile grains from beach sand and sediments in eastern Australia. These ages are in effect a direct measurement of the protosource of the grains and thus are powerful provenance indicators. With the SHRIMP'S ability to rapidly perform single-grain analyses, a statistically valid (n > 60) set of measurements can be relatively easily acquired for study. The Late Proterozoic-Cambrian ages attributed to the Pacific-Gondwana Protosource are commonly seen in nine eastem Australian beach samples from Mallacoota (Victoria) to Rockhampton (Queensland), yet no suitable protosource exists in the region. However, Late Proterozoic-Cambrian grain ages are seen in the Hawkesbury Sandstone of the Sydney Basin and this is regarded as a significant intermediate sedimentary repository of the PacificGondwana Protosource in eastern Australia. The actual location of the Pacific-Gondwana Protosource remains enigmatic, although geochronological and palaeocurrent evidence suggest a position now inland of the Transantarctic Mountains, Antarctica. The importance of recycling heavy minerals from intermediate sedimentary repositories is also seen in Westem Australia, where the geographically dominant Archean craton is only a minor contributor to beach sand. Instead, Neoproterozoic ages probably recycled from Mesozoic Perth Basin sediments dominate detrital ages. In the Murray Basin, some samples display the significant presence of Jurassic-Cretaceous ages attributed to a volcanic arc system in Queensland and possibly along the rest of the eastem margin. Similar ages are not seen in the modem eastem beach sands, indicating that although the source was once physically located on the eastem margin, unfavourable sedimentary pathways have prevented this being seen in the detrital grains. The suspected Precambrian distal source for heavy minerals on the eastern coast has been confirmed by SHRIMP analysis with a sedimentary pathway traced through the Hawkesbury Sandstone. The importance of favourable sedimentary pathways in determining what provenance is or is not 'seen' in detrital grains is also illustrated in the Murray Basin and Westem Australia, and has implications for wider provenance research and heavy mineral placer exploration.

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ASG-Conference '97

Advances in Sedimentology

^^White Hills Gravel" of Bamganie-Dereel: Calivil Formation Gold Leads Overlain by Moorabool Viaduct Sand Michael Smith\ Martin J Hughes^ and Stephen P Carey^ ^Geology Department, University of Ballarat, PO Box 663, Ballarat VIC 3353 ^Martin Hughes and Associates, 1034 Geelong Road, Mount Clear VIC 3350

The prevailing perception of the White Hills Gravel is that it represents scree slope, outwash plain and braided river deposits, which are remnants of an extensive sheet which filled broad, shallow valleys within an early Tertiary palaeosurface. Source material was thought to be, in significant part, accumulated gravel lags on a Mesozoic palaeosurface. Recent studies in the Ararat area indicate instead that these sediments were deposited in broad but moderately deep valleys confined by prominent fault-controlled ridges, with their provenance in headwaters at least 500 m lower than the Mesozoic palaeosurface. This prompted a re-evaluation of rocks in the southern part of the BALLARAT 1:100,000 map sheet, where past mapping indicated the White Hills Gravel to have a clearly sheet-like form. These rocks fringe the southem margin of the Palaeozoic highland in the vicinity of the ESE-striking Enfield Fault, which coincides with the northern limit of deposition of the marine Moorabool Viaduct Sand of Pliocene age. This unit was originally in continuous contact to the north with the rocks mapped as White Hills Gravel, over a strike-length in excess of 50 kilometres. Cross-sections of the "White Hills Gravel" and contiguous Moorabool Viaduct Sand show that there is a close correspondence in the elevations of the bases of both formations over this distance, despite their supposed different origins and ages. Field studies in the Dereel area have now shown that areas mapped as White Hills Gravel are instead Moorabool Viaduct Sand of probable marine origin, and that alluvial gold workings in this area have mined underlying fluvial channels, rather than the outcropping cover of Moorabool Viaduct Sand. Even Aese older fluvial rocks might be younger Calivil Formation, rather than White Hills Gravel. Similar relationships exist between these marine and fluvial units immediately to the south at Grassy Diggings, possibly extending southwards to Corindhap-Rokewood (gold was mined from the fluvial rocks at both localities), and also 19 km to the east at the Cargerie Creek quarry. "White Hills Gravel" exposed in a quarry between Dereel and Cargerie Creek (west of the Leigh River Bridge) is also reinterpreted as Moorabool Viaduct Sand. Much of the extensive area previously mapped as White Hills Gravel in the vicinity of the Enfield Fault is therefore reinterpreted as Moorabool Viaduct Sand; there is some doubt that the former is present at all where mapped south of the fault, although it is present only a few kilometres north of the fault at Enfield. Sub-basaltic gravels and sands which immediately underlie and interfinger with the Moorabool Viaduct Sand to the east and south of Dereel may instead be Calivil Formation and younger Tertiary units (e.g. broadly equivalent to the Ballark Conglomerate and Hanson Plains Sand of late Miocene to Pliocene age).

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ASG-Conference ^97

Advances in Sedimentolopy

A Study of Sequence Stratigraphy Based on the Oligo-Miocene Torquay Group, Southern Australia; a Mid-Shelf Mixed Siliciclastic Temperate Carbonate System

Therese Van der Linden. University of Sydney, Sydney Australia; John A. Webb., LaTrobe University, Melbourne, Australia; Peter J. Davies., University of Sydney, Sydney, Australia The Torquay Group, exposed along the beach cliffs near Torquay, southern Australia, formed part of the basis of the Haq et al (1988) sealevel curve and has been portrayed as a classical example of sequence stratigraphic development as a consequence of global eustatic oscillations. We have tested this assertion using drill core data from inaccessible cliff sections together with conventional analysis of accessible cliff-site information. This study has found that basic building blocks, and defining criteria of the depositional sequences differ from the classic definitions of the EXXON model in that: 1) not all cycles shoal upwards; deepening upwards cycles cannot be defined as parasequences; 2) packages of cycles bound by flooding surfaces do not always contain a consistent stacking pattern, and therefore cannot be defined as parasequence sets; 3) sequence boundaries are not always defined by subaerial exposure and onlap; instead they are consistently identified by a change from progradational to retrogradational stacking pattems. These differences largely reflect the very detailed scale of observation employed and th^e fact that the Torquay Group is mostly mid-shelf in character, and suggest that some aspects of the sequence stratigraphic model need to be adapted in this setting. Four depositional sequences have been defined from the base of the Angahook Formation to the middle of the Puebla Formation within the Torquay Group. Transgressive and highstand systems tracts can be recognised; however, since the transgressive surface immediately overlies the sequence boundary in all four sequences, lowstand systems tracts cannot be defined. Biostratigraphic dating of the Torquay succession has been provided primarily by nannofossils (S. Shafik, work in progress), and is more precise than previously available. Compared to the Haq et a/.(1988) sealevel curve and the previous sequence stratigraphic interpretations of the Torquay Group, the present study identifies two less sequences for the same time interval. The apparent sealevel change for the Torquay section is broadly comparable with the global eustatic curve recorded by oxygen isotopes. However, Late Oligocene onlap is recorded within the Torquay section at the same time as the oxygen isotopes suggest eustatic sealevel was falling. Tectonic activity within the basin at this time is probably responsible; the field area lies on the fault-bounded Torquay Horst, and off-shore seismic lines record fault-propagated folds of Oligocene age. Suggesting that sequence development at Torquay occurred as a consequence of the interplay of tectonic activity and eustatic sealevel change.

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ASG-Conference '97

Advances in Sedimentology

Burial Diagenesis and Carbonate-Hosted Ore deposits

Malcolm Wallace*. Dennis Amet and Tracy Fouet* ^University of Melbourne, Parkville, Vic. 3052 tBallarat University College, PO Box 663, Ballarat, Vic. 3353 In their landmark paper, Jackson and Beales (1967) suggested that Mississippi Valley-type (MVT) sulphide precipitation was a normal consequence of basinal brine evolution. However, in many recent papers, compressional tectonic events have been considered as the prime factor in producing MVT sulphide deposits. We have assessed these two contrasting genetic models for MVT deposits by examining the timing of MVT sulphides relative to the regional diagenetic, burial and structural history of the MVT districts. Remarkably similar diagenetic phases and sequences of phases are present in many MVT districts. MVT sulphides are commonly found to be relatively early in the burial diagenetic histoty of the carbonate sequence. Furthermore, coarsely crystalline burial dolomites almost invariably precede mineralisation. The diagenetic and burial histories of two contrasting Australian MVT provinces (Lennard Shelf, WA and Buchan, Victoria) are examined in detail in this paper. Despite the large differences in the structural (extensional vs compressional) and burial (low temperature shallow burial vs +200 °C) history, the two sequences share a very similar diagenetic and mineralisation sequence. This suggests that a compressional tectonic regime is not a prime control on MVT sulphide precipitation. We suggest that burial dolomitization and MVT sulphide precipitation are two components of a burial diagenetic sequence which is very common in carbonate rocks. The prime control on the development of MVT sulphides appears to be sedimentary burial, rather than a particular tectonic regime.

REFERENCES

Jackson, S.A., and Beales, F.W., 1967, An aspect of sedimentary basin evolution; the concentration of Mississippi Valley-type ores during late stages of diagenesis: Bulletin of Canadian Petroleum Geology, v. 15, p. 383-433.

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ASG-Conference '97

Advances in Sedimentology

Syngenetic Karst in Pleistocene Dunes in Southwestern Victoria

Susan White School of Earth Sciences, La Trobe University, Bundoora, Victoria, 3083 Most studies of karst landscapes and their processes have been concerned with consolidated, often well jointed limestones. These studies assume the diagenesis of the limestone occurs prior to the speleogenesis and that caves develop mainly in massive limestones of sufficient age for the diagenesis to occur prior to speleogenesis. Previous studies in aeolian calcarenite indicated these problems and Jennings (1968) developed the scheme of syngenetic speleogenesis in aeolian calcarenite. The poorly consolidated Pleistocene carbonate dunes in southern Australia have developed extensive karst systems. The karst features and processes at Bats Ridge and Codrington are integral parts of the landscape of a mid Pleistocene calcarenite dune system. The resolution of problems of the rapid subaerial speleogenesis in the area is achieved by the synthesis of the known karst features of the ridge and an appreciation of diagenesis in such calcareous dunes. Karst development on the extensive Pleistocene aeolianite ridges in south eastern Australia is dependent on several interrelated conditions: lithological conditions such as the purity of the limestone, its porosity and its ability to support a cavity as well as the availability of aggressive water capable of solution. The dune ridge must have a sufficiently high proportion of soluble material, pure enough for the solution processes not to be impeded by insoluble residues left after solution has occurred. The development of underground karst forms, is dependent on the ability of the limestone to support a cavity. Insufficient tensional and compressive strength in the limestone will result in solutional cavities collapsing before they are very large. It is necessary also that there be a suitable climate with sufficient water available for solutional processes. The percolating water is made more aggressive by C02 enrichment from soil air and this is enhanced by the vegetation covering the dunes. Adequate moisture must be available for karst processes to be operative. The Pleistocene calcareous ridge systems of the Otway Basin were deposited as the sea retreated from the coast during the mid Pleistocene. Karst landforms are found in ridges at Bats Ridge, Mumbannar, Strathdownie, Puralka, Ardno and Codrington but some show more intensive karst development than others. The karst features at Bats jfidge have been described in detail by White (1989), and those of Codrington by Berryman and \^ite (1995). As the dunes were deposited during the mid Pleistocene (between about 235 ka and 300 ka), it is difficult to envisage the evolution of this karst landscape from the standpoint of the usual speleogenetic schemes which depend on the presence of compact, well jointed limestones with sufficient time for diagenesis and subsequent karst development. The study of dune karst at Bats Ridge and Codrington develops further the thesis that diagenesis and karstification can and does occur simultaneously in less consolidated limestones. The diagenesis of the calcarenite is occurring now and must have been occurring by the mid Pleistocene. This simultaneous lithification of the carbonate dunes into aeolian calcarenite rock and the development of solutional karst features in the dunes is the characteristic feature of the speleogenesis in this area. Berryman, S. and White, S. 1995. Karst in recent dunes Codrington, Victoria. In: Baddeley G. (Ed) 1995, Vulcon Precedings Vic. Spel. Assoc., Inc., p. 21-31. Jennings, J.N., 1968. Syngenetic Karst in Australia. In: Jennings, J.N. and P.W. Williams, Contributions to the study of Karst. Res.Sch.Pac.St. A.N.U. 5, p. 41-110. White, S., 1989. Karst Features in Pleistocene Dunes, Bats Ridges, Western Victoria. Helictite 27(2), p. 53-7.

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ASG-Conference '97

Advances in Sedimentology

Examples of Syn-Sedimentary Deformation Related to Slope Development in Proterozoic Carbonates Peter R. Winefield^ David Selley^ and Stuart W. Bull^ ^Centre for Ore Deposit Research, University of Tasmania ^North Exploration Limited, P.O. Box 231, Cloverdale, WA 6105 Email: P.Winefield@utas.edu.au; David.Selley@north.com.au; sbull@utas.edu.au The McArthur Basin is an extensive Proterozoic depositional system located along the southern and western margins of the Gulf of Carpenteria in northern Australia. The southern McArthur Basin contains the best exposure of Palaeoproterozoic McArthur Group carbonates that are host to the large Pb-Zn-Ag deposit at McArthur River. The detailed study of McArthur Group carbonates along the Kilgour River, approximately 60 km southwest of the McArthur River Mine, has shown a complex history of syn-sedimentary deformation. Dolarenites and dolomitic siltstones of the Bamey Creek Fm. within the study area are interpreted as a slope facies association recording slope development. Several syn-sedimentary deformation features associated with the development of the slope are described here. Numerous sediment- and fibrous cement-infilled fractures (or neptunian dykes) have been recognised in the Emmerugga and Teena Dolomites within the study area. They are commonly infilled by successive generations of radial-axial fibrous dolomite cement and intemi dolomicrite sediment. The fibrous dolomite cement displays identical characteristics to Phanerozoic carbonate cements known to be of marine origin. Laminated intemal sediment occasionally overlies fibrous dolomite cement supporting a syn-sedimentary origin. Lamination within the dykes is conunonly at an angle to bedding implying rotation of the host strata prior to infilling. Occasionally, large (up to 4 cm in diameter) clasts of dolostone (mostly Teena Dolomite) rimmed with fibrous cement are recognised within some of the larger fractures or fissures. Individual neptunian dykes display fitted and regular features with no evidence of dissolution associated with subaerial exposure. Localised extension or dilation related to the onset of basin subsidence is inferred as the mechanism responsible for fracturing and fissuring, prior to infilling by marine cements and intemal sediments. Within thinly bedded dolomitic siltstones of the Bamey Creek Fm. are a number of lensoidal stromatolitic dolostone blocks, the largest of which can to be traced along for several lOO's of metres. Internally, the dolostone blocks exhibit pervasive chaotic deformation in marked contrast to the relatively coherent dolomitic siltstone host. Commonly, stromatolites contained within these blocks are rotated and ductilely sheared, indicating the deformation occurred prior to lithification. The contrast in strain between these two lithotypes and pervasive pre-lithification deformation within the dolostone blocks, suggests emplacement or redeposition of marginal/shoal facies debris as exotic slide blocks. Matrix-rich brecciated Teena Dolomite is directly overlain by the Bamey Creek Fm. at a number of localities. The breccia matrix is commonly chaotically laminated while the clasts are angular and contain Coxco needles. It is thought that they formed as a result of liquefaction-like processes associated with syn-sedimentary faulting. The lack of brittle deformation features (ie. neptunian dykes) in this case supports brecciation prior to complete cementation. In addition, close to isoclinal upright folds within dolarenite-dolomitic siltstones of the Bamey Creek Fm. have anticlinal closures that are conmionly cored by massive sandstone. Fold development must therefore have occurred whilst the sediment was incompletely lithified to have allowed sand to migrate into the hinge positions. This style of deformation is confidently identified as being syn-sedimentary and related to slide generation.

32

ASG-Conference '97

Advances in Sedimentology

Carbonate Sequence Stratigraphy: Northern Carnarvon Basin

Hamish Young (University of South Australia) and Johnny Hull (University of Adelaide), National Centre for Petroleum Geology and Geophysics, Thebarton Campus, University of Adelaide, SA 5005. The North West Shelf of Australia contains a thick succession of Late Cretaceous to Recent passive margin carbonate dominated sediments. Current research interests at the NCPGG are concentrating on this phase of the evolution of the Northern Camarvon Basin. The study aims to gain a quantitative understanding of the rate of accommodation space change during the basins evolution. To date a high-resolution chronostratigraphic framework has been defined for the late Cretaceous to Tertiary section of the basin. Combining the results of the chronostratigraphic analysis with sequence stratigraphic models and forward stratigraphic modelling a detailed facies analysis will be undertaken. This work will provide valuable insights into the newly recognized hydrocarbon play of the Tertiary succession. Data utilised in the project include a good quality regional seismic grid of 6000 km, wireline logs for 100 wells and biostratigraphic reports of varying quality. Integration of high resolution biostratigraphic data, wireline logs and theoretical considerations (Schlager 1992; Sarg 1993; Tucker 1991; Loucks 1993 and R. Handford 1993) has enabled the definition of a new model for the sequence stratigraphic interpretation of carbonate wireline log motifs. Carbonate sequence stratigraphy has many elements that contradict siliciclastic sequence stratigraphy. This stems largely from the fact that carbonate systems have a dominant biological component and different chemical stability. The resulting production and redistribution of carbonate sediment occurs at different rates and with varying geometries to their siliciclastic counterparts. During lowstand in situ carbonate accumulation is dramatically reduced, whilst clastic input is often increased. During transgressions carbonates are seen to backstep similar to siliciclastics, but often aggrade vertically to match sea-level. During highstand massive volumes of sediment are produced that are exported off the platform top and redeposited in slope and basinal areas as prograding wedges. The North West Shelf provides an excellent opportunity to investigate the passive margin transition from siliciclastic to carbonate sedimentation, ramp to rimmed shelf evolution and the change from temperate to tropical dominated styles of carbonate accumulation.

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