Abstracts No.69: 16th Victorian Universities Earth Sciences Conference, 2002 by GSAustralia

Geological Society of Australia

ABSTRACTS Number 69

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16th Victorian Universities Earth Sciences Conference The University of Melbourne School of Earth Sciences September, 2002

The University of Melbourne School of Earth Sciences September, 2002

THE UNIVERSITY OF

MELBOURNE

A meeting organised by the students of the University of Melbourne, School of Earth Sciences

Geological Society of Australia Abstract Volume Number 69

ISSN 0729 001X © Geological Society of Australia Incorporated, 2002 Citation for this volume: Schneider, C.L. (editor), 2002, 16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69, 59 p.

Example of citations for papers in this volume: Bettio, L.M., Power, S., and Walsh, K., 2002, The Dynamics of El Nino-Southern Oscillation in a Sophisticated Computer Model of the Earth's Climate, in: Schneider, C.L. (editor), 16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69, p. 5.

Copies of this publication may be obtained from: The Business Manager, Geological Society of Australia Incorporated, Suite 706 Thakral House, 301 George Street, Sydney NSW 2000, Australia

16th Victorian Universities Earth Sciences Conference Program Schedule Session/Speakers

Time

Andrew Gleadow - School of Earth Sciences. The Univerisity of Melbourne 9:00 - 9:20 9:30-10:15

Addresses

David Taylor - Victorian Earth Sciences Alliance Sue White - Geological Society of Australia - Victoria Division Session 1 Climatology

9:30 - 9:45

Josephine Brown and Ian Simmonds, Modelling Oxygen Isotopes and Tropical Climate Variability

9:45-10:00

J.M. Mav, Processes Controllina the Antarctic Ice Sheet

10:00-10:15

Cecelia MacFarlina David Etheridqe, Tas van Ommen, The Natural Variations of Atmospheric C02, CH4, and N20 During the Holocene from Ice Core Analysis Break

10:15-10:30 10:30 -12:00 10:30-10:45 10:45-11:00 11:00-11:15 11:15-11:30 11:30-11:45 11:45-12:00

Session 2 Crustal Evolution/Geochronology/Metamorphism G.M. Swain, Isotopic, Geochemical, Tectonic, and Metamorphic Character of the Late Archean Mulgathing Complex, Gawler Craton A.J. Woodhouse, M. Hand, K.M. Barovich, and M. Schwarz, Archean Crustal Development of the Southern Gawler Craton M. Lorencak, B.P. Kohn, K.G. Osadetz, and A.J.W. Gleadow, Unravelling the Phanerozoic Thermal History of Cratons: A Case Study of the Southern Canadian Shield A. Fowler, The Denudation History of Southeastern Tibet: A Presentation of New Apatite Fission Track Anayses B.J. Tiddv, R.W. White, and R. Powell, Upper Amphibolite Fades Partial Melting of the Metasedimentary Package at Southern Cross, Broken Hill D. Kelsey, From Mineral Reaction Textures To Pressure-Temperature Evolution Paths

12:00-1:00

Lunch

1:00-2:30

Session 3 Geochemistry/Ore Deposit Geology/Geophysics

1:00-1:15 1:15-1:30 1:30-1:45 1:45-2:00 2:00-2:15 2:15-2:30 2:30 - 3:45

D.J. Evans, Nature and Paragenesis of the Lithogeochemical Halo Surrounding the Broken Hill Pb-Zn Orebody M.L. Carey and D.C. McPhail, Convective Groundwater Flow Around Playa Lakes - Influence On Anomalous Groundwater Distributions Around Mineralization in the Eastern Goldfields, W.A. M.D. Edwards, J.A. Webb, and D. Heislers, Hydrogeochemical Characterization of the Salinised Kamarooka Catchment, Central Victoria S. Santomartino, Experimental Testing of the Effectiveness of Anoxic Limestone Drains for the Treatment of Acid Mine Drainage J.A. Hollambv and R.J. Musqrave, Investigations into the Bacterially Generated Rock-Magnetic Properties of Gas Hydrate-Bearing Deep Marine Sediments Break Session 4 Sedimentology/Volcanology

2:30 - 2:45

N. Raven, Controls on Sonic Velocity in the Ashmore Reef Area, Brose Basin, North West Shelf

2:45 - 3:00

A. Powell, Carbonate Sonic Velocity for the Wells; Pitcairn 1, Mutineer 1B, and Bounty 1, Northwest Shelf

3:00-3:15

K.A. Potter, C.M. Gray, and P.G. Jackson, Geology of the Mt. Hotham Region, Eastern Highlands, Victoria

3:15-3:30

A. Pittari, R.A.F. Cas, and J. Marti, The Origin of Prominent Massive, Pumice-Rich Ignimbrite Lobes within the 2 ka Abrigo Ignimbrite, Tenerife, Canary Islands

3:30 - 3:45

O. Reubi and I. Nicholls, Eruptive Dynamics of the Caldera-Forming Eruptions at Batur Volcano, Bali, Indonesia Break

3:45 - 4:00 4:00 - 5:00 4:00-4:15 4:15-4:30 4:30 - 4:45 4:45 - 5:00 5:00 - 5:30

Session 5 Structure/Tectonics/Remote Sensing S. Howman and M. Hall, The Structural and Thermal History of the Skenes Creek - Wild Dog Valley Region, Eastern Otway Basin M.R.P. Tinaav, R.R. Hillis, C.K. Morlev, R.E. Swarbrick, and E.C. Okpere, The Present-Day Stress Field In Brunei Darussalam: A Snapshot of Prograding Tectonics in a Delta J. Celerier, L. Elliott, K. Harper, and A. Petts, Neotectonics in the Northern Flinders Ranges, South Australia; The Structural, Sedimentary, and Geomorphic Evidence K.E. Fitzsimmons and K.L., Harper, Imaging the Dunes: Desert Dune Dynamics and ASTER Satellite Imagery

Poster Session (Posters will be on display all day)

5:30 - 5:45

Judge's Deliberation

5:45-6:15

Awards

The Organising Committee of the 16th Victorian Universities Earth Sciences Conference would like to thank the following sponsors for their kind support:

THE UNIVERSITY OF MELBOURNE The University of Melbourne - School of Earth Science

Geological Society of Australia

£sso Esso Australia Pty. Ltd.

Department of

Natural Resources Victoria and Environment

State Government

Victoria Department of Natural Resources - Petroleum Development Branch

MONASH U

Monash University - School of Earth Sciences

Geological Survey of Victoria

MELBOURNE The University of Melbourne - Faculty of Science

Petroleum Exploration Society of Australia Ltd Victoria/!asmanm Branch

Petroleum Exploration Society of Australia Ltd. (Vic/Tas Branch)

PIACER DOME ASIA PACIFIC Placer Dome Asia Pacific

Victorian Institute of Earth and Planetary Sciences

Victorian Division of the Geological Society of Australia

Geotrack International Pty Ltd

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V L

A M I T Df tO v RD Pf "i • U N I V E R S I T Y

La Trobe University - Department of Earth Sciences in

Geological Society of Australia - Specialist Group on Solid-Earth Geophysics

Newcrest Mining Ltd.

THE UNIVERSITY OF

MELBOURNE

Thermochronology Group - The University of Melbourne

Australian Society of Exploration Geophysicists

pmd*CRC Predictive Mineral Discovery Cooperative Research Centre

SN9WDEN MINING INDUSTRY CONSULTANTS Snowden

The Australian Meteorological and Oceanographic Society

CONTENTS Program Schedule

SESSION 1: CLIMATOLOGY

Modelling Oxygen Isotopes and Tropical Climate Variability Josephine Brown and Ian Simmonds

Processes Controlling the Antarctic Ice Sheet J. M. May

The Natural Variations of Atmospheric C02, CH4, and NzO During the Holocene from Ice Core Analysis Cecelia MacFarling, David Etheridge, Tas van Ommen 4 The Dynamics of El Nino-Southern Oscillation in a Sophisticated Computer Model of the Earth's Climate Bettio, L.M., Power, S., Walsh, K

Decadal Climate Variability in the Pacific Region Sae-Koe, S

SESSION 2: CRUSTAL EVOLUTION / GEOCHRONOLOGY / METAMORPHISM / IGNEOUS PROCESSES

Isotopic, Geochemical, Tectonic and Metamorphic Character of the Late Archaean Mulgathing Complex, Gawler Craton Swain, G.M

Archean Crustal Development of the Southern Gawler Craton Woodhouse, A.J., Hand, M., Barovich, K.M. & Schwarz, M

Unravelling the Phanerozoic Thermal History of Cratons: A Case Study of the Southern Canadian Shield M. Lorencak, B.P. Kohn, K.G. Osadetz and A.J.W. Gleadow

The Denudation History of Southeastern Tibet: a Presentation of New Apatite Fission Track Analyses A. Fowler 11 Upper Amphibolite Facies Partial Melting of the Metasedimentary Package at Southern Cross, Broken Hill Bryden J. Tiddy, Richard W. White & Roger Powell

From Mineral Reaction Textures to Pressure-Temperature Evolution Paths Dave Kelsey

SHRIMP U-Pb Geochronology of Pegmatites from the Harts Range, Central Australia Storkey, A., Williams, I. & Buick, 1

A Three Dimensional Investigation of the Hiltaba Granitoid Suite Geometry: Implications for Pluton Emplacement Mechanisms McLean M.A & Betts, P.G 15 Textures, Metamorphism, and Partial Melting of the Metapelitic Granulites from the Larsemann Hills, East Antarctica L. Tong vii

SESSION 3: GEOCHEMISTRY/ ORE DEPOSITS GEOLOGY/ GEOPHYSICS

Nature and Paragenesis of the Lithogeochemical Halo Surrounding the Broken Hill Pb-Zn Orebody Evans, D.J Convective Groundwater Flow around Playa lakes - Influence on Anomalous Groundwater Distributions Around Mineralisation in the Eastern Goldfields, W.A. M.L. Carey and D.C. McPhail 19 Hydrogeochemical Characterization of the Salinised Kamarooka Catchment, Central Victoria Edwards, M.D., Webb, J.A. & Heislers, D 20 Experimental Testing of the Effectiveness of Anoxic Limestone Drains for the Treatment of Acid Mine Drainage Santomartino, S., Webb, J.A 21 Investigations into the Bacterially Generated Rock-Magnetic Properties of Gas HydrateBearing Deep Marine Sediments Hollamby, J.A., Musgrave, R.J

Silcrete and its Relationship to Basalt in the Morrisons Region, South-central Victoria Damiani, L.C. and Webb, J.A

Attenuation of Cadmium by UK Triassic Sandstone Aquifer Materials Al-Hosni, T., Tellam, J

The Belltopper Hill Prospect, Malmsbury: Case for Intrusive-Related Gold Systems in Victoria Robert Whittam, Frank Bierlein, Stafford McKnight, Andrew Radojkovic

The Broken Hill Pb-Zn-Ag Orebody Richard B. Tully

Structural Controls on the Timing of Gold Emplacement in the Bendigo Gold Field D.G. Wood, F.P. Bierlein, G.S. Lister, L. Ailleres

SESSION 4: SEDIMENTOLOGY / STRATIGRAPHY / VOLCANOLOGY

Controls on Sonic Velocity in the Ashmore Reef Area, Browse Basin, Northwest Shelf Nicole Raven

Carbonate Sonic Velocity for the Wells Pitcairn 1, Mutineer 1B, and Bounty 1, Northwest Shelf Anne Powell

Geology of the Mt. Hotham Region, Eastern Highlands, Victoria Potter, K.A., Gray, C.M., Jackson, P.G

The Origin of Prominent Massive, Pumice-Rich Ignimbrite Lobes within the 2 Ka Abrigo Ignimbrite, Tenerife, Canary Islands A. Pittari, R. A. F. Cas and J. Marti

Eruptive Dynamics of the Caldera-Forming Eruptions at Batur Volcano, Bali, Indonesia Olivier Reubi, Ian Nicholls

Miocene Foraminifera of the South-eastern Australian Basins: Proxies for Palaeoclimatic Change and Ecostratigraphic Correlation Tiffany Gourley 34 Three-Dimensional Computer Modelling of Sediment Deposition on the Enderby Terrace, Carnarvon Basin, Western Australia Paraschivoiu E., Griffiths C.M., Dyt C., and Lang S 35 viii

The Geodynamic Evolution of the Owen Group, West Coast Range, Western Tasmania Noll, C.A. and Hall, M

Contact relationships of the Mt Keith Komatiite Horizon, Mt Keith region, Western Australia N. Rosengren, B.A. Grguric, G S. Beresford, R. Cas 37 The Highly Explosive Ramadas Volcanic Centre: Plinian Activity in the Central Andes; Puna Plateau, Salta Province, NW Argentina M.A. Tait, R.A.F Cas and J.G. Viramonte 38 Intrusive Komatiites: Field Evidence from the Kalgoorlie Terrane, Yilgarn Craton, Western Australia J. Trofimovs, R. Cas and B.K. Davis 39 The Evolution of the Mt. Eccles Volcanic Complex, Newer Volcanics Province, Victoria, Australia Rebecca Trowbridge

SESSIONS: STRUCTURE / TECTONICS / REMOTE SENSING / GEOMORPHOLOGY

The Structural and Thermal History of the Skenes Creek-Wild Dog Valley Region, Eastern Otway Basin Howman, S.A. & Hall, M 42 The Present-Day Stress Field in Brunei Darussalam: A Snapshot of 'Prograding Tectonics' in a Delta Tingay, M.R.P., Hillis, R.R., Morley, C.K., Swarbrick, R.E., Okpere E.C 43 Neotectonics in the Northern Flinders Ranges, South Australia, The Structural, Sedimentary, and Geomorphic Evidence Julien Celerier, Lauren Elliott, Kate Harper and Anna Petts 44 Imaging the Dunes: Desert Dune Dynamics and ASTER Satellite Imagery Kathryn. E. Fitzsimmons and Kate. L. Harper

The Proterozoic Structural Evolution of the Eldee West Region Neilson, I.E., Betts, P.G, Lister, G.S

Synchronous Development of Type-2 and Type-3 Fold Interference Patterns: Evidence For Recumbent Sheath Folds in Broken Hill, NSW Caroline Forbes, Pete Betts and Gordon Lister 47 Back-arc Deformation in the Kuril basin and Sea of Okhotsk, Northwest Pacific Schellart, W.P., Jessell, M.W. & Lister, G.S

Tertiary N-S Direction Crustal Shortening of Central Tibet, China Liu, S., Wang,C.S., Ding,X.L

Fluid Dynamical Modelling of Subduction in the Upper Mantle Schellart, W.P. & Lister, G.S

New Insights on the Motion of Tasmania Relative to Australia Vos, I.M.A., Rosenbaum, G., Lister, G.S. and Bierlein F.P

The Origin of the Arc of the Western Alps Rosenbaum, G. and Lister, G.S

Upper Crustal Cenozoic-Recent Deformation Across the Yidun Arc, Eastern Tibet: Assessing the Indian Impact Reid, A.J. and Wilson, C.J.L 53 Routine Processing Techniques for Seismological and Earthquake Engineering Applications Allen, T 54 ix

Neotectonics and Intraplate Deformation in the Northern Flinders Ranges, South Australia 55 Julien Celerier The Morphotectonic Study of the Paralana Escarpment and Yudnamutana Gorge, Northern Flinders Ranges, S.A. 56 Anna E. Petts Dissection of Alluvial Fans: Evidence for Recent Uplift in the Northern Flinders Ranges Lauren Elliott

Application of ASTER Satellite Imagery Northern Flinders Ranges, South Australia Kate Harper

Desert Dune Dynamics in the Lake Frome Region, South Australia Kathryn E. Fitzsimmons

An Apparent Stratigraphic Reversal in Poorly-Bleached Quartz Fluvial Sediments Resolved by Single-Grain OSL Measurements GresleyA. Wakelin-King 60

Conference Organising Committee Conference Convenor

Georgia Boyd

Abstract Volume Editor

Craig Schneider

Website Coordinator

Lynette Bettio

Sponsorship Committee

Fiona Elmer Robert Bolhar

Audiovisual

Anthony Reid

Publicity

Andrew Fowler

Catering

Elisabetta Carrara

Poster Session

Tiffany Gourley

General Support

Margaret Lauricella Elizabeth Condilis Silvana Santomartino Helen Russell Richard Young Susanne Baxter-Smith Malcolm Wallace Brett Holman

SESSION 1: CLIMATOLOGY

MODELLING OXYGEN ISOTOPES AND TROPICAL CLIMATE VARIABILITY Josephine Brown and Ian Simmonds School of Earth Sciences, University of Melbourne Oxygen isotope ratios in tropical marine records provide information about past tropical climates. Coral and ocean core reconstructions provide estimates of palaeo-sea surface temperatures, local hydrological balance and global changes in sea level. However such reconstructions require an understanding of the complex interaction between temperature, atmospheric circulation, hydrological changes and the isotopic record. The use of an isotopic tracer scheme in an atmospheric General Circulation Model allows the isotopic response to different climate conditions to be tested. The isotopic signal produced in the upper ocean can also be investigated using a slab ocean isotopic scheme coupled to the atmospheric model. In order to test the ability of the model to capture present day isotopic climate signals, a simulation of recent (1950-1999) climate was carried out using the GISST2.3b sea surface temperature and sea ice monthly data as boundary conditions. The model reproduces many of the features of present climate including interannual variability associated with El Nino-Southern Oscillation. Comparison with isotopic records from observational stations over the same period shows that the accuracy of the modelled isotopic distribution is limited by the representation of tropical precipitation. However averages of modelled precipitation and isotopic ratios over large areas of the tropical Pacific are significantly correlated with indices of El Nino-Southern Oscillation. This suggests that confidence can be placed in the model for investigating tropical climate sensitivity and carrying out palaeoclimate simulations. Keywords: water isotopes, climate modelling, palaeoclimate, ENSO

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

PROCESSES CONTROLLING THE ANTARCTIC ICE SHEET J. M. May School of Earth Sciences, University of Melbourne Antarctica is by far the largest present day ice sheet. The processes which have controlled it throughout the Quaternary Period will be presented, and the reasons for its continued existence whilst the ice sheets of the Northern Hemisphere have grown and decayed will be discussed. A particular focus of the study is the relationship between the continental ice volume and eustatic sea level. Based on Johanns Oerlemans' simple model, a one-dimensional mass balance model has been formulated which assesses the relationship between the long-term change in net evaporation from the world's oceans, and hence sea level, and the state of the Antarctic Ice Sheet. Preliminary results suggest that the ice sheet is marginally unstable, with some dependence on the fluctuation of ice sheets in the Northern Hemisphere. The results of further development will be presented.

SESSION 1: CLIMATOLOGY

THE NATURAL VARIATIONS OF ATMOSPHERIC C02, CH4, AND N20 DURING THE HOLOCENE FROM ICE CORE ANALYSIS Cecelia MacFarlinq123, David Etheridge2, Tas van Ommen3 1

School of Earth Sciences, University of Melbourne, Victoria, 3010 2 CSIRO Atmospheric Research, Aspendale, Victoria, 3195 Antarctic CRC & Australian Antarctic Division, GPO Box 252-80, Hobart, Tasmania, 7001

Bubbles of air trapped in ice sheets provide an important record of past atmospheric gas composition. Understanding of climate change and atmospheric composition over the last 400,000 years has been greatly improved using ice core analysis. Ice cores from Law Dome in Antarctica have several attractive properties due to the high snow accumulation rate, low temperatures, low impurity levels and stable glaciology. The cores are generally well dated, have a narrow air age spread, and contain air spanning from recent decades to thousands of years before present. The gas records already produced from Law Dome cores have tracked variations in C0 2 , 513C02, and CH4 over the past 1000 years with high precision and age resolution. Holocene climate is considered to be stable compared to the rapid climate changes of the previous ice ages. Published records of methane and carbon dioxide generally show small and slow changes during this period. The age resolution of the Law Dome ice core air, and the high measurement precision for several trace gases, allow subtle changes to be revealed. An abrupt cooling event 8,200 years BP has recently been found in several paleoclimate records, including Greenland ice cores. One of the project aims is to locate the 8,200-year event in the Law Dome "DSS" ice core. Uncertain ice dating through this period means the event must be identified using methane. The event is clear and well dated in the methane record in Greenland ice, and since methane is a globally well-mixed gas, the Antarctic and Greenland methane records may be synchronised. As a result, records such as climate in the DSS ice core may be accurately dated. Furthermore, the global atmospheric C0 2 response to the cooling may be found by using the well resolved and preserved C0 2 record in Law Dome ice. Keywords: ice cores, trace gases, Holocene, paleoclimate, Law Dome

16th Victorian Universities Earth Sciences Conference, September 2002,

Geological Society of Australia Abstracts 69

THE DYNAMICS OF EL NINO-SOUTHERN OSCILLATION IN A SOPHISTICATED COMPUTER MODEL OF THE EARTH'S CLIMATE 1

Bettio, L.M., 2Power, S., 1Walsh, K.

School of Earth Sciences, University of Melbourne, Melbourne, Australia 2 Bureau of Meteorology, Melbourne, Australia

The term El Nino was originally used to describe a localised current that flowed along the west coast of South America during Christmas. This name is now associated with large scale warming in the tropical Pacific Ocean. The anomalous changes in sea surface temperature affect the large-scale atmospheric circulation over a large part of the world. The atmospheric component is known as the Southern Oscillation, which is indexed by the difference in sea level pressure between the western and eastern tropical Pacific. El Nino-Southern Oscillation (ENSO) is the largest interannual climate signal and has been shown to be associated with various climate phenomena, for example, incidences of drought in northeastern Australia. An understanding of the dynamics of the ENSO phenomenon and improved representation of ENSO in climate models could lead to enhanced predictions, important for planning purposes. This project uses data from a so-called Coupled General Circulation Model (CGCM) from the Bureau of Meteorology Research Centre. Such models are complex, mathematical, physically based models of the earth's climate. Equations for a wide range of physical processes are included, e.g. for changes in rainfall, ocean currents, temperature and salinity as well as land surface temperatures and sea-ice distribution. One of the main uses of such models is to better understand the complex role that ocean dynamics play in the initiation, maintenance and predictability of ENSO events. Model studies, combined with observational analyses, have led to the development of various physical paradigms, which attempt to explain the cyclic nature of ENSO events. One of the current leading paradigms for ENSO is the delayed action oscillator. This theorizes that interactions between the ocean and the atmosphere are at the heart of ENSO variability. The delayed oscillator paradigm involves a positive feedback between ocean temperatures and winds as well as the reflection of oceanic waves at the western boundary of the tropical Pacific Ocean. ENSO in this climate model is analysed to see if it is consistent with this theory. The analysis will be used to illustrate how increased understanding could lead to improved climate predictions. Keywords: ENSO, CGCM, delayed action oscillator

SESSION 1: CLIMATOLOGY

DECADAL CLIMATE VARIABILITY IN THE PACIFIC REGION Sae-Koe, S. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia. Evidence of the decadal variability in the Pacific region is the large-scale change in the winter circulation pattern over the North Pacific during the mid-1970s and the El Nino-Southern Oscillation (ENSO) like mode pattern in the global sea surface temperatures (SST) during the 1990s. Several authors have attempted to identify the mechanisms involved. One of the main features involved in the decadal climate variability is the low frequency SST variability. Recent studies have suggested two plausible SST patterns. One is called 'the decadal ENSO-like pattern'. This pattern is very similar to the interannual ENSO pattern but with variability longer than 10 years. The other SST pattern is described as the North Pacific decadal variability and is not related to the above decadal ENSO-like pattern. This project aims to study the decadal variability in the Pacific region by using the CSIRO coupled ocean-atmosphere general circulation model. Empirical Orthogonal Function (EOF) studies of the SST pattern in the model suggest that it is able to simulate both the decadal ENSO-like SST pattern and the North Pacific decadal SST variability. The physical mechanisms involved in generating these decadal SST variability patterns in the model and in the observations will be further assessed in the future. Keywords: decadal variability, Pacific, decadal ENSO-like SST

SESSION 2: CRUSTAL EVOLUTION / GEOCHRONOLOGY / METAMORPHISM / IGNEOUS PROCESSES

ISOTOPIC, GEOCHEMICAL, TECTONIC AND METAMORPHIC CHARACTER OF THE LATE ARCHAEAN MULGATHING COMPLEX, GAWLER CRATON Swain, G.M. Department of Geology and Geophysics, University of Adelaide SHRIMP U-Pb zircon, Sm-Nd isotope, geochemical and metamorphic data from Archaean metavolcanic, plutonic and metasedimentary rocks of the Mulgathing Complex, Central Gawler Craton, are used to trace the processes of crustal evolution, tectonic development and subsequent granulite facies metamorphic events. Juxtaposed to the Lake Harris Greenstone Belt mafic-ultramafic lavas, komatiites and interlayered metasedimentary volcaniclastics (2527±7.5 Ma) is the metasedimentary quartzo-feldspathic Kenella Paragneiss and more distant outcropping Christie Paragneiss. The Kenella Paragneiss is dominated by ca. 2560 and ca. 2700 Ma inherited zircons and a broad ca. 2440 Ma peak correlative with metamorphic overgrowths. Lack of ca. 2520 Ma zircon population in the Kenella Paragneiss places it as a sedimentary precursor of the Lake Harris Volcanics. Two near concordant zircon analyses from the Kenella Paragneiss yielded ages of 1747±24 and 1729±10, fingerprinting the regional extent of the Kimban Orogeny. The dominance of a late Archaean sedimentary source is confirmed by consistent £ N C I values from -1.9 to -1.0 with a model age range of ca. 2650-2720 Ma. Uniform HREE depletion in the Kenella Paragneiss (La/Ybn = 17-75) relative to the Christie Paragneiss (La/Ybn = 3-29) is attributed to a dominantly felsic volcanic and/or granitic source province as opposed to a mixture of felsic and mafic-ultramafic source rock input respectively. Peak regional granulite facies metamorphism of the Sleafordian Orogeny is recorded in zircon overgrowths from the Kenella Paragneiss only, suggesting lower level crustal exposure from Proterozoic thrusting (Daly et al, 1998). Komatiite volcanology and subsequent high temperature low pressure metamorphism point toward a high thermal gradient consistent with back-arc basin extensional tectonics. Keywords: SHRIMP U-Pb zircon; Sm-Nd isotope; Geochemistry; Metasedimentary; Komatiite; Archaean; Gawler Craton

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

ARCHEAN CRUSTAL DEVELOPMENT OF THE SOUTHERN GAWLER CRATON Woodhouse, A.J.1. Hand, M.1, Barovich, K.M.1 & Schwarz, M.2 1

Department

of Geology and Geophysics, The University of Adelaide, 2 Minerals Resource Group, PIRSA

Adelaide,

Australia

The Sleaford Complex in the southern Gawler Craton is comprised of poly-metamorphosed Archaean units characterised by limited outcrop. Two metasedimentary units, the Wangary Gneiss and the Carnot Gneisses, have previously been correlated based on similar lithologies. Aside from their synchronous thermal histories there is currently little known about this stage of the Gawler Craton's crustal development. This study uses geochemical and isotopic tools to establish the similar provenance of these sequences and places constraints on possible sources. Two distinct provenance sources have been identified through the combined use of SHRIMP U/Pb detrital zircon ages, REE geochemistry and sNd isotopic analysis. Detrital zircon patterns yield a dominant input of ca 2520 Ma zircons with a significant 2720 Ma peak. This identifies the Hall Bay Volcanics as a likely source, which have a SHRIMP U/Pb magmatic age of ca 2520 Ma with significant zircon inheritance at 2720 Ma. However, steep HREE depleted patterns of the metasediments cannot be explained by an exclusive Hall Bay Volcanics provenance. A highly fractionated felsic source displaying REE patterns with high (La/Yb)N ratios typical of Archaean mantle-derived felsic rocks is required. It is reasonable to allocate an approximate age of ca 2700 Ma to this source due to significant zircon growth during this period in combination with Sm-Nd depleted mantle model ages of the metasediments between 2650-2800 Ma. There are currently no known 2700 Ma felsic rocks on the Gawler Craton. Late Archaean felsic rocks with high (La/Yb)N ratios occur in the eastern Yilgarn Craton. It is speculated that this craton may have contributed to the Gawler Archaean metasediments. Keywords: Archaean, Provenance, Gawler Craton

SESSION 2: CRUSTAL EVOLUTION / GEOCHRONOLOGY / METAMORPHISM / IGNEOUS PROCESSES

UNRAVELLING THE PHANEROZOIC THERMAL HISTORY OF CRATONS: A CASE STUDY OF THE SOUTHERN CANADIAN SHIELD M. Lorencak1. B.P. Kohn1, K.G. Osadetz2 and A.J.W. Gleadow1 1) School of Earth Sciences, The University of Melbourne, VIC 3010 2) Geological Survey of Canada, 3303 33rd St. N.W., Calgary, Alberta T2L 2A7, Canada Phanerozoic stratigraphic or tectonic information is commonly scarce or missing on most cratons. However, an investigation using low-temperature sensitive methods (e.g. apatite fission track (AFT) analysis and (U-Th)/He thermochronometry) could potentially be a very powerful tool for unravelling information about their Phanerozoic evolution. Such an approach, on aliquots of the same mineral separate, could allow the derivation of thermal histories independent of the AFT system. Additionally, the complementary range of temperature sensitivities (~120-60°C and 80-40°C, respectively) of the two systems allows further constraints on modelling of time-temperature cooling paths in such environments. To illustrate this potential, we report the results of a combined apatite (U-Th)/He and FT study on the southern Canadian Shield. Vertical profiles allow additional constraints on the data, since different levels successively enter or leave critical palaeotemperatures throughout the thermal history of the section constituting the profile. Further, the profile as a whole would be expected to display an internally consistent set of thermal history trends. To use this advantage, a 3440 m deep exploration borehole, drilled by INCO Limited, in the relatively undisturbed north range of the Palaeoproterozoic Sudbury structure in southern Ontario, was sampled at -200 m intervals. 19 apparent AFT ages and track length measurements were obtained, 8 apatite samples were also analysed for their chlorine content. Apatite aliquots from 10 of the samples were chosen for (U-Th)/He thermochronometry. The apparent He ages define a trend younger than the AFT ages and can mostly be replicated within error limits of 1DD Interpretation of both datasets and t-T modelling of representative samples results in a well constrained and comprehensive thermal history. The results suggest burial of the shield under Alleghenian foreland sediments during Permian time and a late Cretaceous - early Tertiary heating event. The subtle thermal imprint of the latter event can only be recognised using apatite (U-Th)/He thermochronometry. Keywords: apatite fission track analysis, (U-Th)/He thermochronometry, craton, Canadian Shield, Sudbury

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

THE DENUDATION HISTORY OF SOUTHEASTERN TIBET: A PRESENTATION OF NEW APATITE FISSION TRACK ANALYSES A. Fowler School of Earth Sciences, The University of Melbourne, Vic 3010, Australia. Apatite fission track analysis (AFTA) is a useful geological tool that can unlock the denudation history of an area. In orogenic belts, the denudation history is intimately linked with the tectonic history. Thus, AFTA indirectly provides quantitative information about the tectonic history of a region. The eastern margin of the Tibetan Plateau is characterised, along an east-west transect, by a sharp rise in topography up to the plateau in the north, while in the south the topography rises more gradually along an east-west transect. This southeastern topographic feature is interpreted to be the result of a free boundary where lateral extrusion takes place due to the indentation of India into Asia. In northeastern Tibet, however, lateral extrusion is inhibited by the existence of the Proterozoic Yantze Craton. This craton is the basement to the low-lying areas adjacent to the plateau and it outcrops on thrust faults along the plateau margins. The plateau itself is predominantly composed of Paleozoic sediments and Mesozoic igneous intrusions. All of these lithologies were sampled for AFTA The new, preliminary AFT analyses from southeastern Tibet demonstrate three important points: • Approximately 2 km of rock cover has been eroded from the plateau region since the Cretaceous. • There was a major cooling event in the Early Cretaceous, which correlates well with estimates on the timing of the Lhasa Block collision with the Asian mainland. • The current, rapid episode of denudation began in most parts of southeastern Tibet at 5.9 ± 3.3 Ma (2a) and was caused by the Cenozoic-Recent, Himalayan tectonic event. Keywords: apatite fission track, denudation, southeast Tibet.

SESSION 2: CRUSTAL EVOLUTION / GEOCHRONOLOGY / METAMORPHISM / IGNEOUS PROCESSES

UPPER AMPHIBOLITE FACIES PARTIAL MELTING OF THE METASEDIMENTARY PACKAGE AT SOUTHERN CROSS, BROKEN HILL Brvden J. Tiddv, Richard W. White & Roger Powell School of Earth Sciences, The University of Melbourne, Parkville, Victoria 3052, Australia Chemical differences within the metasedimentary package in the Southern Cross area of the Willyama supergroup, Broken Hill, has resulted in seemingly different melting histories between rocks that have clearly witnessed the same deformational events, and presumably the same PT path. The peak metamorphic conditions experienced by the rocks is about 750°C and 400-500 MPa. This places the suite in the Upper Amphibolite facies where wet-solidus melting occurs. ViaTHERMOCALC, bulk compositions for the two rock types (metapelitic schist and felsic gneiss) give phase diagrams which account for the differences in the observed melting relationships. The breakdown of muscovite provides the key: if muscovite breaks down supra-solidus, the water released promotes melting; if subsolidus, the water is lost from the rock so that it is not available for this. The Na content of the Felsic Gneiss is high relative to the Pelitic rocks and has depressed the solidus by approximately 30°C. When overlain by the anti-clockwise PT path witnessed by the area, the muscovite breakdown reaction occurs supra-solidus for the felsic gneiss and sub-solidus for the metapelite, and hence a greater degree of partial melting is seen in the felsic gneiss than the metapelite. Keywords: Broken Hill, partial melting, leucosome, NCKFMASH

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

FROM MINERAL REACTION TEXTURES TO PRESSURE-TEMPERATURE EVOLUTION PATHS Dave Kelsey School of Earth Sciences,

University

of Melbourne,

Victoria,

3010

Interpretation of mineral reaction textures typically provides the basis for elucidating the pressuretemperature (P-7) evolution of metamorphic rocks. From the P-T information large-scale tectonic models are commonly proposed. However, preserved reaction textures rarely define a unique P-T path. The determination of a P-T path is further complicated since mineral reaction textures commonly involve fewer minerals than are required by discontinuous (univariant) reactions depicted on "standard" total P-7 grids. Importantly, total P-7 grids are unable to successfully predict mineral assemblage development for a single rock composition. Therefore a method is needed to reduce the non-uniqueness of P-7 paths consistent with the development of a mineral reaction texture. Quantitative P-T diagrams, in the form of calculated pseudosections, can provide a more rigorous approach to the determination of a metamorphic P-7 path. Calculated pseudosections quantify: 1) the composition of the equilibration volume, thereby accounting for non-univariant mineral assemblages, 2) equilibrium mineral proportions and 3) mineral chemistry. The integration of the above information preserved within a thin section with the calculated pseudosections is critical for narrowing down a possibly large range of P-T paths. The power of using calculated pseudosections for a terrain increases with the integration of information from several different or similar mineral reaction textures. Only with a restricted range of P-7 paths that satisfy all the necessary constraints of petrographic and chemical information can broader-scale tectonic models be more safely hypothesised. Of course, field structural information should always provide independent support for the conclusions of a metamorphic study. The presentation will demonstrate how the information from a petrographic thin section can be integrated with the calculable information of pseudosections to explore the P-7 path possibilities of lower-crustal granulite-facies rocks from the Rauer Group in Prydz Bay, east Antarctica. Keywords: metamorphism, mineral reaction texture, pressure-temperature path, pseudosection

SESSION 2: CRUSTAL EVOLUTION /GEOCHRONOLOGY/ METAMORPHISM / IGNEOUS PROCESSES

SHRIMP U-Pb GEOCHRONOLOGY OF PEGMATITES FROM THE HARTS RANGE, CENTRAL AUSTRALIA 1

Storkev. A., 2Williams, I. & 1Buick, I.

department of Earth Sciences, La Trobe University, Bundoora 3086, VIC, Australia Research School of Earth Sciences, Australian National University, Canberra 0200, ACT, Australia

The Harts Range forms part of the southeastern Arunta Inlier and has a complex Palaeoproterozoic and Palaeozoic metamorphic and deformation history. The timing of high-grade metamorphism is now well established in the Harts Range as being Ordovician (-470 Ma). However the age of emplacement of extensive pegmatite swarms (Harts Range pegmatites) is largely unstudied. The pegmatites form sills and dykes up to 2km long and locally 50m wide and intrude all three major lithological associations in the Harts Range. The pegmatites contain weakly to strongly developed foliations and display discordant to concordant relationships with the surrounding rocks, implying that the timing of pegmatite emplacement is broadly late syn-kinematic. However, it is unclear whether the pegmatites were emplaced during single or multiple events. Only two pegmatites have been dated previously, giving ages of -300 Ma and -387 Ma. This study presents preliminary SHRIMP U-Pb monazite and zircon data to further constrain the age of emplacement of the Harts Range pegmatites. Initial SHRIMP U-Pb analyses of five pegmatites from the Harts Range Metamorphic Complex (HRMC) gave a range of Palaeozoic ages. Monazite from pegmatites at three different locations gave ages of -360 Ma, -415 Ma and -440 Ma. Zircon from a pegmatite gave an age of -466 Ma, and zircon from a pegmatitic vein within a metabasic rock gave an age of -474 Ma. These ages are similar to those from previously dated Harts Range pegmatites and may indicate that pegmatite emplacement did not occur during a single event, but during several events over a period of about 150 million years. The pegmatites are particularly abundant in the upper metasedimentary unit of the HRMC (Brady Gneiss) compared to the mafic unit of the HRMC, suggesting that they may represent partial melts formed during peak metamorphism and therefore may be locally derived. However, if the pegmatites are locally derived, this implies that the HRMC underwent repeated episodes of anatexis. It is possible that several influxes of an externally-derived fluid entered the terrain in order to facilitate more than one melting episode. Keywords: SHRIMP U-Pb geochronology; Harts Range; pegmatites; monazite; zircon

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

A THREE DIMENSIONAL INVESTIGATION OF THE HILTABA GRANITOID SUITE GEOMETRY: IMPLICATIONS FOR PLUTON EMPLACEMENT MECHANISMS McLean M.A & Betts, P.G. Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton Vic, 3800, Australia At ~1595-1585 Ma, a large thermal event within the Gawler Craton resulted in the extrusion of the Gawler Range Volcanics, and widespread emplacement of the Hiltaba Suite Granitoids. In the central - western Gawler Craton, major Archaean-aged east-west trending shear zones are spatially and structurally associated with the Mesoproterozoic Hiltaba Granitoid Suite. This association and recent geochronological data suggests that these structures were important conduits during pluton emplacement. The geometry of a pluton can provide inferences about the emplacement mechanism. However, determining the three-dimensional geometry of a pluton is complex because the subsurface distribution of plutons is difficult to constrain. A thick veneer of overburden compounds the pluton geometry problem. Modelling of aeromagnetic and gravity data enables assessment of the subsurface geometries of these plutons allowing inferences on the emplacement mechanism. Using the forward modelling package GM-SYS, two and three quarter dimensional modelling of gravity and aeromagnetic data reveals that shallow level Hiltaba Suite Granitoids typically display a flat silllike geometry. These granites are up to 25 km in diameter and are less than 6 km deep. We propose that the Hiltaba Suite Granitoids were emplaced along the major east - west trending shear zones and then propagated out laterally to be shallowly emplaced in the form of a thick sill. This emplacement mechanism is typical of a laccolith intrusion. Keywords: Hiltaba, Gawler Craton, aeromagnetic, Pluton Emplacement, Shear Zones

SESSION 2: CRUSTAL EVOLUTION / GEOCHRONOLOGY / METAMORPHISM / IGNEOUS PROCESSES

TEXTURES, METAMORPHISM, AND PARTIAL MELTING OF THE METAPELITIC GRANULITES FROM THE LARSEMANN HILLS, EAST ANTARCTICA L. Tong School of Earth Sciences, The University of Melbourne, Vic 3010, Australia. A peak assemblage of garnet - cordierite - orthopyroxene - K-feldspar was identified with cordierite or cordierite + orthopyroxene corona on garnet in a structurally early Al-rich metapelite lens from the Larsemann Hills, east Antarctica, suggesting that M, peak assemblage was involved a melt phase, and this has been modelled in the KFMASH system with quartz in excess. An initial prograde heating path is suggested by the crossing of the univariant reaction garnet + biotite = cordierite + orthopyroxene + K-feldspar + liquid in the divariant field, and the first partial melting and melt loss occurred during or near M1 peak metamorphism and resulted in the preservation of M^ peak assemblage. The average P-T calculations provide IVh peak metamorphic P-T conditions of 7-9 kbar and >900 °C, followed by M2 cooling and decompression to 6-7 kbar and 800-860 °C. Extensive development of syn-D2 garnet-K-feldspar-dominated leucosomes in the Fe-rich metapelites suggest that a second prograde high-grade event caused further partial melting during the D2 high-grade transpression event, which resulted in the formation of syn-D2 Progress Granite. Subsequent decompression - cooling led to the formation of narrow cordierite or quartz coronae on spinel and/or garnet grains, garnet or biotite + sillimanite coronae on spinel grains during the D3/M3 event. D4/M4 may be associated with late development of myrmekitic textures formed at the waning stage of D3 shear deformation. Geochronological data and field relationships suggest that the early D ^ high-grade event is related to heating by extensive syn-Di intrusion of mafic-felsic orthogneisses at -1100 Ma. D2/M2 to D3/M3 evolution is associated with regionally extensive high-grade transpression, subsequent decompression and cooling, and is accompanied by the widespread intrusion of the Progress Granite and pegmatites during the early Palaeozoic -500 Ma orogeny. Decompression and cooling occurred during D3 extension shear event as a result of the exhumation of the terrain. Keywords: Metamorphism, Partial Melting, Metapelites, Larsemann Hills, East Antarctica

SESSION 3: GEOCHEMISTRY/ ORE DEPOSITS GEOLOGY / GEOPHYSICS

SESSION 3: GEOCHEMISTRY / ORE DEPOSITS GEOLOGY / GEOPHYSICS

NATURE AND PARAGENESIS OF THE LITHOGEOCHEMICAL HALO SURROUNDING THE BROKEN HILL Pb-Zn OREBODY Evans, D.J. Department of Earth Sciences, La Trobe University, Bundoora VIC 3068, Australia The Broken Hill ore deposit is the world's largest single concentration of lead and zinc yet discovered. Various genetic models for ore body emplacement have been proposed, yet consensus has not been reached. Difficulty in resolving precursor assemblages and environments of deposition have been caused by regional amphibolite-granulite facies metamorphism, coupled with multiple deformation events and retrograde metamorphism. Adjacent to the ore body itself, metamorphic processes have produced a lithogeochemical halo in the form of an unusual garnet-rich lithology. This project investigates the geochemistry and mineralogy of a cross-section through the Broken Hill ore deposit to determine the nature of the lithogeochemical halo, and whether a spatial and genetic link can be found between the ore body and the surrounding host rocks. Two kilometres of drill core were re-logged within Section 110 of the Broken Hill South Mine. Detailed core logs and petrological samples were taken to conduct optical microscopy, electron microprobe analysis, and XRF whole rock analysis. The host rocks were reclassified in terms of metamorphic mineral assemblages, resulting in eleven lithological units which were used to re-interpret existing mine logs and produce an interpretive cross-section. Initial examination of the petrology has revealed metasomatic features in the vicinity of the ore body, with an overall systematic variation in the number of mineral phases within the host rock as the ore body is approached. Composition-paragenesis diagrams will be constructed incorporating petrography and mineral chemical data. Petrographic features of the lithogeochemical halo will be presented with relevant paragenesis diagrams. Keywords: Broken Hill, lithogeochemical halo, alteration, genetic models, hydrothermal, metasomatism, paragenesis

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

CONVECTIVE GROUNDWATER FLOW AROUND PLAYA LAKES - INFLUENCE ON ANOMALOUS GROUNDWATER DISTRIBUTIONS AROUND MINERALISATION IN THE EASTERN GOLDFIELDS, W A M.L. Carey1,2 and D.C. McPhail3 1

School of Geosciences, Monash University, Melbourne Australia 2 WMC Resources, Peth Western Australia 3 Geology Department, ANU, Canberra, Australia

Much of Australia's mineral wealth is situated in its semi-arid interior where playa lakes are common, continued exploration success is reliant on identifying strategies effective in these terrains. Although hydrogeochemistry is an effective exploration technique, density-driven convective groundwater flow associated with playa lakes influences the distribution of elements in groundwater around mineralised positions, making interpretation of hydrogeochemical survey data difficult. In this study both numerical models and a physical sandbox model were designed to study this phenomena in more detail. Hydrological and geochemical data were used to develop a numerical groundwater flow model for the Lake Lefroy system. Groundwater flows into the playa lake where it evaporates and then due to its increased density sinks until it reaches a permeability barrier at which point it flows back beneath the regional groundwaters forming a convective mixing zone up to 4km wide. The Argo deposit is located within this mixing zone, shallow groundwaters overlying mineralisation have anomalous gold values, but deeper samples that would have been interpreted to be down flow of Argo in a simple flow model did not show anomalous gold values. In fact these waters were hypersaline brines flowing back from the playa and had not interacted with Argo mineralisation. The sandbox model is a scaled 2-D representation of the playa lake groundwater system and is designed to illustrate how the location of mineralisation relative to the convective mixing zone influences the distribution of anomalous groundwaters. Dye representing a mineralised source is injected into the sandbox model in a series of locations; movies made up of a series of time-lapse images illustrate how the location of the mineralisation within the flow regime can result in very different groundwater anomaly patterns. Results of the numerical and physical analogue models can be used to relate groundwater anomalies back to their source, and sampling strategies targeting groundwaters most likely to have interacted with mineralisation can be designed. Keywords: density-driven convection, exploration, hydrogeochemistry, playa lakes

SESSION 3: GEOCHEMISTRY / ORE DEPOSITS GEOLOGY / GEOPHYSICS

HYDROGEOCHEMICAL CHARACTERIZATION OF THE SALINISED KAMAROOKA CATCHMENT, CENTRAL VICTORIA 1

Edwards, M.D., 1Webb, J.A. & 2Heislers, D.

Department of Earth Sciences, La Trobe University, Bundoora VIC 3086, Australia 2 Centre for Land Protection Research, NRE Victoria, Epsom VIC 3551, Australia

The Kamarooka area has been enduring increasing soil salinisation due to rising water tables since the 1940's. On current estimates, at least 800ha are severely affected with some 10,000ha having saline groundwater (up to 26,000mg/L) within 2 meters of the ground surface. A detailed geochemical study, incorporating extensive major element analyses, soil-water chloride profiles, stable and radiogenic isotope analyses (1sO, 2H, 3H, 14C and 36CI) and high resolution airborne electromagnetics (TEMPEST), has been undertaken to investigate the distribution, movement and origin of salt within the catchment. Field measurements reveal a complex hydrogeochemical system, characterised by intense spatial variability. Anomalous pH values may indicate the presence of very localised pyritic sediments while a high level of Eh variability may suggest that localised organic rich sediments or confining layers are present, all alluding to a more complex geology or geomorphological history than previously envisaged. The specific causes of saline discharge at Kamarooka, eg. hydraulic conductivity changes, bedrock highs, terrain structure and the role of the Whitelaw fault, interpreted to occur in the vicinity of the discharge zone, will be investigated. The volumes of salt import and export will be quantified with an attempt to delineate the relative contributions of rainfall from past and present dry deposition, principally using 36CI. Keywords: Kamarooka, 1sO, 2H, 3H, 14C, 36CI, dryland salinity, groundwater, TEMPEST, airborne electromagnetics

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

EXPERIMENTAL TESTING OF THE EFFECTIVENESS OF ANOXIC LIMESTONE DRAINS FOR THE TREATMENT OF ACID MINE DRAINAGE Santomartino, S., Webb, J.A Department of Earth Sciences, La Trobe University, Bundoora VIC 3086, Australia Anoxic limestone drains (ALD's) have been widely used in the passive treatment of acid mine waters, but their effectiveness has often been less than anticipated. To determine the factors that make for an effective ALD, a small-scale laboratory system was built to enable a detailed examination of the geochemical processes that occur within an ALD and the contribution that exsolution of C0 2 from effluent waters makes to acid neutralisation. Synthetic anaerobic acid drainage containing Fe2+ (130 ppm), Al3+ (80 ppm), SO24~and H+ was pumped at a rate of 30 mL/min up a 90 cm tall, 15 cm diameter vertical column filled with limestone gravel of 98 % purity. Three sampling ports were fitted at equal intervals along the column; water was periodically extracted for analysis at the sampling ports over both 12-hour and 5 day periods. At the outlet end of the column, effluent water was directed down a series of steps to promote C0 2 exsolution. Experiments were run with different influent pH levels. Results show that 1.2 hours of water/limestone contact was required for pH levels to rise from 2 - 4 to above 6. A longer contact time generated a greater concentration of alkalinity and hence a higher rate of C0 2 exsolution from effluent waters. With a starting pH of 2, sufficient C0 2 was generated in the column from limestone dissolution to neutralise the excess acidity produced upon Fe + oxidation. Almost 100 % of the influent Al was retained in the bottom of the column as Al hydroxide whilst only 20 % of the Fe precipitated in the column. A discussion of a pre-treatment method for the removal of Fe is provided. Keywords: passive treatment, acid rock drainage, metal oxidation, laboratory experiment.

SESSION 3: GEOCHEMISTRY / ORE DEPOSITS GEOLOGY / GEOPHYSICS

INVESTIGATIONS INTO THE BACTERIALLY GENERATED ROCK-MAGNETIC PROPERTIES OF GAS HYDRATE-BEARING DEEP MARINE SEDIMENTS Hollambv, J.A., Musgrave, R.J. Department of Earth Sciences, LaTrobe University, Melbourne, Australia Gas hydrates are ice-like solids composed of water and low molecular weight gases (most commonly methane, a highly efficient greenhouse gas). Due to the high pressures and low temperatures required for their formation and stability, oceanic gas hydrates occur dominantly in the sediments underlying the deep-water basins of continental margins. The base of hydrate stability is often signified by the occurrence of a bottom-simulating reflector (BSR), a seismically detectable acoustic interface between solid hydrate and underlying free gas. Increases in bottom-water temperature cause vertical migration of the BSR and dissociation of hydrates. This can result in the rapid release of large volumes of methane into the atmosphere, potentially affecting global climate cycles. The ability to identify zones of hydrate stability, and track their temperature dependent migration history has direct implications for palaeoceanographic and palaeoclimate studies. Recent Ocean Drilling Program (ODP) studies of oceanic gas hydrates have revealed the occurrence of deep-level sedimentary bacterial activity, apparently stimulated by the presence of hydrates and associated methane gas. Subsequent rock-magnetic analyses of these sediments have also highlighted a relationship between variations in rock-magnetic properties across hydrate-bearing sequences and related in situ bacterial activity. Bacteria are believed to be utilising iron oxide and sulfide minerals in hydrate-stimulated metabolism. The consequent changes in grain-size, concentration and magnetic state of the iron species resulting from such processes are readily identified via magnetic hysteresis studies and other rock-magnetic analytical techniques. These techniques are potentially useful in characterising hydrate zones (past and present), and in tracking any temperature-related variation in the depth of hydrate stability. The viability of using rock-magnetic properties to identify past and present hydrate zones is the focus of current research. Rock-magnetic analyses are being conducted on sediment samples from known ODP hydrate-bearing sites, and will also be undertaken on samples from forthcoming geomicrobiological and/or hydrate focussed ODP Legs. Results from these studies will be compared to previous studies and to other hydrate-related data, to assess the applicability of such techniques in characterising deep-marine sediment hydrate zones. Keywords: Gas Hydrates, Rock-Magnetics, ODP, Bacteria

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

SILCRETE AND ITS RELATIONSHIP TO BASALT IN THE MORRISONS REGION, SOUTH-CENTRAL VICTORIA Damiani, L.C. and Webb, J.A. School of Earth Sciences, LaTrobe University, Melbourne, Australia Silcrete occurs commonly in the Morrisons area and always outcrops adjacent to basalt. Three different rock types have been silicified, the Ordovician bedrock, the overlying regolith and Tertiary sediments, which show rounded detrital quartz grains. The silicified regolith displays angular pieces of broken up quartz veins but still in continuous lines, in a fine grained microquartz matrix. The Ordovician bedrock silcretes are distinguished by intact quartz veins and evidence of bedding, and display a more uniform texture of interlocking quartz crystals. Some silcretes display strong anatase banding, and anatase is often concentrated on the upper side of individual quartz clasts. Geochemical analyses will be used to test whether the anatase represents titanium remaining after the dissolution and replacement of clay in the matrix. The age and formation of the silcretes are controversial; some authors believe the silcretes have a strong genetic link to the near by basalt flows, others propose that the basalt was extruded over a preexisting silcrete surface. This project will use geochemical analyses to assist in answering some of these questions. Keywords: silcrete, regolith, anatase banding, basalt flows

SESSION 3: GEOCHEMISTRY / ORE DEPOSITS GEOLOGY / GEOPHYSICS

ATTENUATION OF CADMIUM BY UK TRIASSIC SANDSTONE AQUIFER MATERIALS 1

AI-Hosni.T., 2 Tellam, J.

Department of Earth Sciences, Sultan Qaboos University, Oman. 2 School of Earth Sciences, University of Birmingham, UK.

Sorption behaviour of cadmium (Cd) in Triassic sandstone aquifer is an important process, which exerts a major influence on its mobility and toxicity. Fourteen rock horizons from various depths of the aquifer were subjected to Cd treatment at various concentrations (0.1 to 10 mg/l) and equilibrated at ambient temperature and unadjusted pH to investigate their sorption potential. The Cd sorbed by each sample was calculated as the difference between the initial dissolved amount and the remaining at the end of the experiment. All the fourteen samples used in this study approximately fitted all equilibrium sorption isotherms with highly positive correlations (R2), particularly Freundlich model. In the range 5 < pH < 8, the distribution coefficients varied from 0.1580 to 0.9739 l/g suggesting effective retardation. In Birmingham aquifer, boreholes with highly Cd contaminated groundwater can be correlated with land use, with highest total Cd concentrations being associated with metalwork/engineering and chemical works sites. The study suggests that sorption is capable to retard Cd migration. However, it provides quantitative description for specific range of conditions and so further experiments at a variety of circumstances is necessary to confirm the findings. Keywords: cadmium, sorption, batch experiments, isotherms, sorption coefficients

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

THE BELLTOPPER HILL PROSPECT, MALMSBURY: CASE FOR INTRUSIVERELATED GOLD SYSTEMS IN VICTORIA Robert Whittam1, Frank Bierlein1, Stafford McKnight2, Andrew Radojkovic3 1

VIEPS School of Geosciences, Monash University, PO BOX 28E, Melbourne Victoria 3800 2 School of Sciences, University of Ballarat, PO Box 663, Ballarat Victoria 3353 3 Geological Survey of Victoria, 240 Victoria Parade, East Melbourne Victoria 3002

Although Ordovician-aged orogenic lode gold mineralisation is evident in numerous mineralised fault structures within the Bendigo Zone of the Western Lachlan Fold Belt, this style of gold deposition is unlikely to be responsible for the 'atypical' mineralisation observed at Belltopper Hill (Leven Star deposit). The timing of this gold-bearing fluid event coincides with the Mid-Late Devonian magmatic event where large felsic intrusive bodies were emplaced throughout central Victoria. Current evidence suggests that the Mid-Late Devonian magmatic event and the Leven Star mineralisation share a genetic relationship, where the mineralising fluid was driven by a magmatic heat source, with Leven Star being similar in style to an 'intrusive-related gold deposit'. The Leven Star mineralisation at Belltopper Hill is a shear zone-hosted, finely disseminated gold deposit, where gold is precipitated within the altered host rock, closely associated with zones of stockwork veinlets. The mineralisation style of Leven Star is characterised by a strong Au-As-Sb (Zn) metal association, with complex minor enrichment in Bi-Te-Ni-Co-Cu. Evidence for an intrusiverelated genesis includes a distinctive biotite-cordierite thermal alteration that has been overprinted by a generally low salinity, H 2 0-C0 2 hydrothermal fluid, with a temperature range of ~170-340°C. Although a relatively new class of deposit, investigation and exploration of these intrusion-related gold systems has intensified due to their global distribution and the large number of gold deposits that contain >30 t Au. While Leven Star is not a world class deposit, the genetical implications of this occurrence present new possibilities for gold exploration in the Western Lachlan Fold Belt. Keywords: Malmsbury, ore genesis, geochemistry, fluid inclusions, intrusive-related gold mineralisation, Lachlan Fold Belt, Victorian gold province.

SESSION 3: GEOCHEMISTRY / ORE DEPOSITS GEOLOGY / GEOPHYSICS

THE BROKEN HILL Pb-Zn-Ag OREBODY Richard B. Tully School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia, 3010 The Palaeoproterozoic Broken Hill Pb-Zn-Ag deposit has produced more than 200 Mt of ore at average grades of 10% Pb, 8.5% Zn and 148 g/t Ag, making it the world's largest Zn-Pb deposit. It formed in a rift setting broadly confined to 1690±5 Ma. Accumulation of such an enormous quantity of ore is inferred to have been promoted by a local long-lived, anomalously high geothermal gradient, combined with focussed, possibly fault-bound fluid flow, efficient and rapid episodic submarine hydrothermal deposition, and burial by impermeable pelitic cap rocks. Massive sulphide precipitation, possibly in wet, unconsolidated sediments occurred coeval with bimodal volcanism and possibly with movement on growth faults. It produced a stacked system of lodes, comprising 5 orebodies at The Pinnacles; Tin Street Mineralisation, Potosi Mine and numerous smaller deposits in the Parnell Formation; Western Mineralisation; C Lode and the main Broken Hill lodes, B Lode, A Lode, 1 Lens, 2 Lens and 3 Lens. Isotope studies show that the sulphide rocks contain magmatic S, mantle/lower crustal Pb and Sr, magmatic C and O in carbonates and biogenic C in graphite. By comparison with modern submarine hydrothermal systems, major element and REE data suggests that the Western Mineralisation, B Lode, A Lode, 2 Lens and 3 Lens are very similar to modern hydrothermal sediments, whereas C Lode and 1 Lens have both a hydrothermal and clastic sediment signature. The data suggests that, apart from C Lode and 1 Lens, the rate of hydrothermal precipitation exceeded that of the intercalated turbidite clastic sedimentation. Lode horizon rocks (quartz-gahnite rocks, garnetites, quartz-garnet rocks, iron formations) occur interlayered with and as strike equivalents to massive sulphide rocks. Their major element and REE chemistry suggests that they are oxidised distal submarine hydrothermal precipitates and this chemistry can be used as a vector to mineralised sulphide rocks. Keywords: Broken Hill, Palaeoproterozoic, hydrothermal precipitates, massive sulphide, exhalite.

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

STRUCTURAL CONTROLS ON THE TIMING OF GOLD EMPLACEMENT IN THE BENDIGO GOLD FIELD D.G. Wood, F.P. Bierlein, G.S. Lister, L. Ailleres Australian Crustal Research Centre, School of Geosciences, PO Box 28E, Monash University, Vic 3800

The Bendigo Gold Field is situated in the central Victorian gold province of southeastern Australia, and is the site of concentrated orogenic lode-gold mineralisation within the Palaeozoic Lachlan Fold Belt. Historically Bendigo is Victoria's largest gold producer with ~600t gold produced from hard rock mining. Compilation of historic mining data, along with a comprehensive drilling and exploration program undertaken by Bendigo Mining N.L. (BMNL) has led to the recent Swan Decline construction. This has allowed a unique opportunity to study excellent underground exposure of an auriferous system in conjunction with detailed cross sections and data sets produced from extensive drill core. Knowledge of overprinting relationships and relative timing of structural deformation and gold precipitation are essential to the understanding of any mineralised system. Extensive analysis carried out by BMNL geologists on drill core and exposed auriferous structures highlight the complexities associated with gold mineralisation at Bendigo. These complexities include the 'nugget effect', with gold occurring as free milling coarse grains up to 2mm in size in quartz veins. Gold is precipitated from hydrothermal fluids introduced along fault-controlled fluid pathways, however the exact source(s) of both the fluids and the gold are not well understood. Available geochronological data constrain the major mineralising event to 440Ma; subsequent later fluid events have only minor associated gold precipitation. Pervasive alteration is associated with the mineralising fluid events, and includes silicification, carbonatisation, sericitisation and a strong arsenopyrite correlation within metres of the quartz-gold reefs. Mineralised quartz vein arrays are closely associated with steeply dipping reverse fault zones that transect the hinges of NNW trending chevron-folded anticlines. The reverse fault zones have extremely variable gold grades between anticlines. Grades in historically mined ore shoots range from <1 g/t to >300g/t Au. The geometry controlling the higher gold content of these structures is not well understood. Gold is also concentrated in vein shoots that can persist along strike for tens of metres. Clearly, these shoots carry particular importance in exploration, however their relatively narrow diameter and commonly discontinous formation mean that consistent targeting is often difficult. Re-interpretation of existing drill core data and examination of new core show that arrays of eastdipping reverse faults transect the larger west dipping fault zones. The areas of offset along the westdipping zones between east-dipping faults form vertical zones of dilation. These dilational sites host concentrated gold mineralisation. Although these observations are indicated by the drilling of only a small portion of the New Bendigo gold project, this correlation between structural development and ore formation is also seen in a three-dimensional model of the Sheepshead anticline built in gOcad™, where a vertical zone of elevated gold grades occurs metres below the intersection of two such faults. A scale-integrated approach that clarifies how micro- and macro-structural data relate to ore genesis will aid in understanding the formation and post-ore history of the Bendigo gold deposit. Our approach utilises a combination of microstructural analysis, detailed two-dimensional interpretation and threedimensional modelling of ore-bearing structures. A better understanding of the structural controls on mineralisation at Bendigo has implications for analogous deposits elsewhere in Victoria, as well as orogenic lode-gold deposits in fold belt terrains worldwide. Acknowledgements We are grateful for support and input provided by Bendigo Mining N.L. (especially P.Quigley and D. Turnbull)

SESSION 4: SEDIMENTOLOGY / STRATIGRAPHY / VOLCANOLOGY

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

CONTROLS ON SONIC VELOCITY IN THE ASHMORE REEF AREA, BROWSE BASIN, NORTHWEST SHELF Nicole Raven School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia Sonic velocity is an important factor in the exploration for hydrocarbons. Velocity variations in carbonate rocks can cause severe distortions of structural features in seismic interpretation. On the North West shelf carbonates overlie reservoir sequences and velocity distortion is a problem. The Ashmore Reef-1 well, in the northern part of the Browse Basin, has sonic velocity values of between 4000 and 9000m/s at very shallow depths (350 to 850m), before dropping back to the more normal values of 3000-4000m/s at greater depths. This project aims to determine the cause of the high velocity interval in the Ashmore Reef-1 well. In Copernicus-1, Londonderry-1 and Brecknock-1 carbonate content is found to be the major control on sonic velocity. High carbonate intervals have higher velocities than high quartz intervals. However, the high sonic velocity intervals in Ashmore Reef-1 do not appear to be strictly related to carbonate content. It had been suggested that the high velocity interval in Ashmore Reef-1 was due to a significant amount of uplift (4-5km). However the structures required to accommodate this uplift do not exist. Preliminary analysis suggests that the high velocity intervals may be related to dolomitization and calcite cementation. Keywords: Ashmore Reef, Browse Basin, Carbonates, North West Shelf, Sonic Velocity

SESSION 4: SEDIMENTOLOGY / STRATIGARPHY / VOLCANOLOGY

CARBONATE SONIC VELOCITY FOR THE WELLS PITCAIRN 1, MUTINEER 1B, AND BOUNTY 1, NORTHWEST SHELF Anne Powell School of Earth Sciences, The University of Melbourne, Parkville, Victoria 3052, Australia The Northwest Shelf extends 2400 km along the Australia's northwestern continental margin and is composed of a series of basins, the most southerly offshore basin being the Northern Carnarvon Basin (Purcell and Purcell, 1988). The wells Bounty 1, Pitcairn 1 and Mutineer 1B are located in the Northern Carnarvon Basin's, Dampier Sub-basin. Oil has been discovered in Late Jurassic Angel Sands within the Mutineer 1B and Pitcairn 1 wells. However, velocity variations within the Tertiary carbonates distort underlying trap structures in seismic sections. This study aims to provide a detailed analysis of velocity controlling factors in the carbonates within the wells Bounty 1, Pitcairn 1 and Mutineer 1B, to increase the accuracy of seismic interpretation. Geophysical data such as well logs and seismic sections have been combined with petrological observations to determine carbonate velocity controls. Previous studies have found that porosity and factors that determine porosity significantly control carbonate sonic velocity. These factors may include lithology, composition, grainsize, cementation, burial-depth and sediment age. Keywords: Carbonates, Sonic Velocity, Northwest Shelf

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

GEOLOGY OF THE MT. HOTHAM REGION, EASTERN HIGHLANDS, VICTORIA Potter, K.A., Gray, C.M., Jackson, P.G. School of Earth Sciences, La Trobe University, Bundoora, Australia 2

An area of 45km was mapped to determine the petrology and stratigraphy of the Ordovician turbidites and overlying alkali basalts at Mt. Hotham. Ordovician turbidites form the basement of much of the Eastern Highlands and locally, at least three deformations have affected the turbidites creating a complex folding regime. The turbidites are separated into two distinct domains on the basis of grain size and feldspar abundance. The lower flanks of Mt. Hotham are dominated by silt and fine to medium sand turbidites whilst the upper slopes have significantly more coarse-grained components. The Tertiary Basalts of the Eastern Highlands, and in particular the Mount Hotham region, are Early to Middle Oligocene in age, and can be found overlying and intercalated with Tertiary fluviatile and lacustrine sediments. The basalts occur as restricted lava flows and are thought to represent valleyfilling flows. Nine separate basaltic flows have been identified that are laterally discontinuous, separating basalt in outcrop into two domains. Significant time breaks in eruption have been identified and are expressed as deeply weathered basaltic regolith profiles and fluviatile and lacustrine sediments. Keywords: Ordovician turbidites, alkali basalt, petrology, stratigraphy

SESSION 4: SEDIMENTOLOGY / STRATIGARPHY / VOLCANOLOGY

THE ORIGIN OF PROMINENT MASSIVE, PUMICE-RICH IGNIMBRITE LOBES WITHIN THE 2 Ka ABRIGO IGNIMBRITE, TENERIFE, CANARY ISLANDS A. Pittari1, R. A. F. Cas1 and J. Marti2

1 School of Geosciences, PO Box28E, Monash University, Victoria, 3800, Australia Instituto de las Ciencias de la Tierra 'Juame Almera', Consejo Superior de Investigaciones Cientificas, Llius Sole Sabaris s/n, 08028, Barcelona, Spain.

The lithic-rich Abrigo Ignimbrite was deposited during the last major explosive and caldera-forming eruption on Tenerife around 2ka. High relief lobes of massive, pumice-rich ignimbrite, which form ridges parallel to the pyroclastic flow direction, are a prominent feature within the ignimbrite. In crosssection, they are convex upwards bedforms with a flat base and can be symmetrical, asymmetrical or highly irregular. The major components are rounded pebble- to cobble-sized phonolitic pumice clasts within an ignimbritic matrix of ash, fine lithics and minor crystals which is either lithic-rich or lithic-poor. A vertical increase in pumice concentration from matrix-supported clasts at the base to clastsupported at the top accompanied by an increase in pumice clast size is common. The lobes often thin and grade laterally perpendicular to current flow into planar pumice concentration zones. They occur at one or more stratigraphic levels as either solitary lobes associated with flat topography or as multiple onlapping lobes or within a complex pumice concentration stratified facies near palaeotopographic highs. The lobes are original depositional features, not erosional in origin and were derived from a larger pyroclastic flow system. Pumice was likely to have been segregated to the upper and outer regions of the parent flow causing a significant rheological contrast with the lower lithic-rich zone. The more mobile pumice-rich part detached from the parent flow as it decelerated onto gentler slopes or interacted with topographic highs and raced ahead as derivative pyroclastic flow lobes. The flowparallel ridge shape of the lobes may be a result of fingering within these flows or concentration of pumice within the intermediary clefts. Deposition occurred "en masse" at the termination of the flow front. The resultant lobate deposits were then overridden and mantled by normal ignimbrite facies. Keywords: pumice-rich lobe, pyroclastic flow, Abrigo ignimbrite, Tenerife

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

ERUPTIVE DYNAMICS OF THE CALDERA-FORMING ERUPTIONS AT BATUR VOLCANO, BALI, INDONESIA Olivier Reubi, Ian Nicholls School of Geosciences, Monash University, Vic 3800, Australia Batur is an active stratovolcano on the island of Bali, Indonesia, with two well-developed concentric calderas. Its volcanic activity may be divided into five main periods: (1) Building stage dominated by basaltic to andesitic lava flows. (2) First caldera collapse associated with the eruption of dacitic ignimbrites. (3) Formation of an andesitic to dacitic lava-dome complex and an associated tuff ring within the caldera. (4) Second caldera collapse episode with eruption of a dacitic ignimbrite. (5) Building of a basaltic andesite stratovolcano within the second caldera. The eruption associated with the collapse of the first caldera, resulted in the deposition of a 100 m thick sequence of partially to densely welded ignimbrites and minor pumiceous fallout deposits within the caldera. Three cooling units separated by non-welded fallout deposits and pyroclastic flows are recognized. Each cooling unit comprises 2 to 8 flow units. These deposits are lithic-rich and comprise a wide range of lithic types. The deposits associated with the second caldera collapse consist, within the caldera, of a 40 m thick sequence of intercalated phreatomagmatic and magmatic surges, pyroclastic flows and fallouts overlain by a 20-30 m thick, massive ignimbrite, which locally comprises a basal coarse lithic lag breccia. The lithics within these deposits consist essentially of black glassy dacite. These observations suggest that the two calderas resulted from contrasting eruption dynamics. The first eruption was characterized by moderate to high discharge rate, possibly from multiple vents, over a prolonged period of time, which formed a collapsing fountain feeding pyroclastic flows. The second eruption occurred from a single vent and started with low discharge rate that resulted in the development of an unstable wet to dry plinian column. The discharge rate drastically increased during caldera collapse leading to the collapse of the column and deposition of a massive ignimbrite. Keywords: Batur volcano, Bali, caldera, ignimbrite

SESSION 4: SEDIMENTOLOGY / STRATIGARPHY / VOLCANOLOGY

MIOCENE FORAMINIFERA OF THE SOUTH-EASTERN AUSTRALIAN BASINS: PROXIES FOR PALAEOCLIMATIC CHANGE AND ECOSTRATIGRAPHIC CORRELATION Tiffany Gourley Department of Earth Sciences, The University of Melbourne, Victoria e-mail: t.gourley@pgrad.unimelb.edu.au A biofacies investigation involving quantitative foraminiferal analysis of subsurface boreholes intersecting the Oligo-Miocene marine carbonates of the Murray Basin in Victoria, has uncovered evidence for palaeoclimatic change during the Miocene that has global significance. Within bryozoal calcarenites and calcisiltites of the Duddo Limestone, the appearance of the larger foraminiferal species Amphistegina lessoni and Operculina victoriensis during the Early Miocene (planktonic foraminiferal Zones N6-N7), signals the first phase of warming of the 'Miocene Climatic Optimum' when the climate was sub-tropical and the ocean warm. Around the Early-Middle Miocene boundary (Zones N7-N8a), a foraminiferal assemblage characterised by an abundance of infaunal species, in particular Ehrenbergina marwicki, Epistominella sp. and the Cassidulinids, indicates a shift in climatic conditions to a cooler and more eutrophic oceanic regime and corresponds to the onset of the Monterey carbon excursion. The reappearance of the large foraminiferids and an influx of the planktonic species Cassigerinella chipolensis during the early Middle Miocene (Zone N8b), signals the second phase of warmth of the 'Miocene Climatic Optimum' that occurred during the peak of carbon build-up (CM3) of the Monterey carbon excursion. These biofacies signals are of ecostratigraphic importance since foraminiferal studies of other Oligo-Miocene carbonates, in particular those carried out in the Port Phillip and Gippsland Basins, have revealed evidence for similar biofacies events. Biofacies analysis has also enabled the isolation of lithologically and faunally distinct units within the previously undivided Duddo Limestone. Correlation of facies equivalents to the more detailed stratigraphic framework outlined for South Australian Oligo-Miocene carbonates has subsequently been achieved. Keywords: Foraminifera, Miocene, Murray Basin

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

THREE-DIMENSIONAL COMPUTER MODELLING OF SEDIMENT DEPOSITION ON THE ENDERBY TERRACE, CARNARVON BASIN, WESTERN AUSTRALIA Paraschivoiu E.1. Griffiths C.M.2, Dyt C.2, and Lang S.1 1

National Centre for Petroleum Geology and Geophysics, The University of Adelaide, SA 5005, Australia 2 CSIRO, Division of Petroleum Resources, PO Box 1130, Bentley WA 6102, Australia

The 3D depositional modelling program 'Sedsim' that has been used in this study simulates the transport, deposition and erosion of uncompacted sediment on a bathymetric surface. The algorithm employed is based on numerical approximations to fluid flow equations, while the hydraulic regime governing the fluid flow is controlled by a range of geomorphologic, climatic, tectonic and oceanographic factors. The simulation output, at each grid node, consists of a succession of equal time layers, each characterized by the thickness of each of the sediment grain size classes used as input. An area of 50x70 km on the central-southern Enderby Terrace, Carnarvon Basin, Western Australia, provided the setting for a modelling study that re-played sedimentation and erosion over a geological time interval spanning 2.5 million years in the Early Cretaceous. The modelled sedimentary volume represents a marine siliciclastic succession, formed in depositional environments ranging from proximal nearshore to basin-floor settings, over a period of overall marine transgression during the post-rift phase in the evolution of the continental passive margin. The Sedsim input was based on geological interpretation of seismic profiles, well data from around 30 wells, high-resolution sea-level curves, tectonic activation history, modern analogues, and palaeoclimatic reconstructions. The outputs from the simulation include 3D interactive display of sediment grain size, facies and distance from shore, 3D chronostratigraphy and 2D maps of sediment thickness, sand percentage and palaeo-topography. The results were matched against seismic stratal geometries and architecture, and compared to lithological profiles available in the control wells. By quantifying the conceptual depositional model we were able to generate a predictive model of facies distribution for the inter-well volume, with implications for the petroleum exploration in the Enderby area. These results are being further used as the basis of a reservoir-scale depositional model over the Stag oil field - to predict and quantify stratigraphically controlled facies heterogeneity below seismic resolution. Keywords: stratigraphy, quantitative, computer modelling, depositional model, siliciclastic

SESSION 4: SEDIMENTOLOGY / STRATIGARPHY / VOLCANOLOGY

THE GEODYNAMIC EVOLUTION OF THE OWEN GROUP, WEST COAST RANGE, WESTERN TASMANIA Noll. C.A. and Hall, M. School of Geosciences, Monash University, Melbourne, Victoria 3800. The Late Cambrian - Early Ordovician Owen Basin of the West Coast Range, western Tasmania, contains an overall upward fining, transgressive stratigraphy comprised of coarse grained siliclastics. Early basin fill is dominated by alluvial slope deposits that exhibit distinctly unidirectional palaeoflow from eastern highlands. This fluvial system was replaced by a sandy braided river system in the south and a laterally equivalent marine environment consisting of pro-delta and thickly bedded proximal turbidite deposits. The basin was subsequently inundated by a prograding fluvial and braid delta succession that was transgressed by regionally extensive, thinly bedded clastics (Pioneer Sandstone) and overlying thick limestone (Gordon Group Limestone), that record the onset of Ordovician marine sedimentation in western Tasmania. Basin fill thickens substantially towards the western basin margin fault system, and progressively onlaps basement to the east. Structural control of the basin is evident with a characteristic wedge shaped basin geometry, although the precise nature of the present basin margin fault system, known as the Great Lyell Fault, remains elusive and enigmatic. However, the key to understanding Late Cambrian basin architecture lies in clearly identifying post-depositional deformation episodes. Two distinct Devonian trends are identified within the Owen Basin, including north-south oriented reverse faults and associated folds (D^, and a pronounced west-northwest oriented fault and fold trend (D2). The west-northwest trend overprints all other structures and is the only episode with a well developed penetrative cleavage (S2), although locally developed axial planar cleavage (S^ associated with Di folding has been mapped in some areas. D2 fold patterns are largely dependent on earlier fold geometry, with variable plunges associated with pre-existing fold limbs, an interaction which lead to the spectacular local development of Type 1 interference patterns within the Owen Basin. The most revealing structures, however, are associated with the reactivation of east-dipping faults that appear to have also controlled deposition. Folding of beds against faults across which there are significant stratigraphic thickness changes is interpreted as evidence for basin inversion and provides crucial insights into the tectonostratigraphic evolution of the Owen Basin. Keywords: stratigraphy, structural architecture, inversion, Owen Group, western Tasmania

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

CONTACT RELATIONSHIPS OF THE MT KEITH KOMATIITE HORIZON, MT KEITH REGION, WESTERN AUSTRALIA N. Rosenqren1, B.A. Grguric2, G S. Beresford1, R. Cas1 1

School of Geoscience, Monash University, Clayton, Australia 2 WMC Mt Keith Operation, W.A, Australia

The Mt Keith region is located in the northern section of the Archaean Agnew Wiluna Greenstone Belt, which comprises the northern-most subdivision of the Norseman-Wiluna Greenstone Belt, Eastern Goldfields Province, Yilgarn Craton, Western Australia. The Agnew Wiluna Greenstone Belt is exceptionally well endowed with economic nickel sulphide mineralization, hosting both type I and type II nickel deposits (Barnes 1995). Mt Keith is a world-class example of a Type II disseminated or 'Giant komatiitic dunite hosted' deposit with an estimated resource of approximately 300 million tons at 0.6% Ni. The Mt Keith region contains three komatiite horizons; the Western, Central and Eastern Ultramafic units. The Western and Central ultramafic units are comprised of multiple stacked spinifex and cumulate textured komatiite lava flows, similar to the Munro-type komatiites originally described by Pyke, 1973. The Eastern ultramafic unit exhibits different characteristics to the Western and Central ultramafic horizons. This unit forms the focus of this study. The Eastern Ultramafic unit is characterized by an orthocumulate to adcumulate olivine/serpentine textured, layered dunite body. The internal complexity of the unit has not previously been described in detail. The three Ultramafic horizons are separated by horizons comprised predominantly of basaltic to rhyolitic coherent lavas and associated fragmental products (Hepinstall 1991., Palich 1994., Bongers 1994). Primary hyaloclastite facies together with fine-grained, carbonaceous sedimentary units and the lack of pyroclastic fragmentation are indicative of a deep marine depositional environment for the lavas and associated sediments. This presentation summarises the varied and complex facies architecture and lithological relationships and interactions between the Eastern Ultramafic and the enclosing mafic and felsic strata

SESSION 4: SEDIMENTOLOGY / STRATIGARPHY / VOLCANOLOGY

THE HIGHLY EXPLOSIVE RAMADAS VOLCANIC CENTRE: PLINIAN ACTIVITY IN THE CENTRAL ANDES; PUNA PLATEAU, SALTA PROVINCE, NW ARGENTINA M.A. Tait1, R.A.F Cas1 and J.G. Viramonte2 1

School of Geosciences, Monash University, Clayton, PO Box 28E Melbourne, 3800 2 Institutio Geonorte, Universidad Nacional de Salta, Salta, Argentina

The Ramadas Volcanic Centre (RVC) in the Central Andean Volcanic Zone of NW Argentina erupted in the Late Miocene to produce one of the most widespread and unique pyroclastic sequences found in the Andes, namely the Corte Blanco Tuff (CBT). Explosive silicic activity in the region has been widespread since the Late Tertiary, but has typically produced widespread pyroclastic flow deposits, or ignimbrites, emanating from some of the largest collapse calderas currently recognised. A very different style of explosive activity is proposed for the eruption of the RVC. Until now, the nature of many of the internal facies of the CBT has been poorly understood, a problem complicated by a lack of continuous outcrop. This work considers the internal facies of the CBT and presents new interpretations for their emplacement, based largely on detailed field geology. Outcrops of the CBT are exceptional in thickness, reaching over 20m, 20km from source. The dominant facies in the sequence is a stratified, well sorted, clast supported pumiceous lapilli-stone, with a highly variable lithic content. This unit has previously been described as a fines depleted ignimbrite, but the extreme angularity of clasts and stratified nature of the units is more consistent with aerial transport and deposition from a sustained, plinian style eruption column and a strongly deflected downwind plume. Correctable, >1m thick, ash layers are recognised over 300km to the east of the RVC, which is consistent with this interpretation. Other facies are recognised within the CBT eruptive sequence, including basal surge deposits and subordinate massive, ash-rich units, interpreted as intra-plinian ignimbrites. A new model is presented for the eruptive history of the RVC, consistent with a highly dynamic eruption, somewhat unique in terms of Andean volcanology. Keywords: Andes, Explosive, Plinian, Pyroclastic, Ignimbrite

16th Victorian Universities Earth Sciences Conference, September 2002,

Geological Society of Australia Abstracts 69

INTRUSIVE KOMATIITES: FIELD EVIDENCE FROM THE KALGOORLIE TERRANE, YILGARN CRATON, WESTERN AUSTRALIA J. Trofimovs1, R. Cas1 and B.K. Davis2 1

School of Geosciences, Monash University, P.O. Box 28E, Clayton, Victoria 3800, Australia 2 Auriongold, P.O. Box 862, Kalgoorlie, Western Australia, 6433

The Kalgoorlie Terrane of the Norseman-Wiluna Greenstone Belt, Eastern Goldfields Province, Western Australia, is characterised by large volumes of komatiite. Within the Eastern Goldfields Province, including the Kalgoorlie Terrane, komatiites have typically been described as extrusive lava flows primarily associated with basaltic stratigraphy, e.g. Kambalda of the Kalgoorlie Terrane. The Boorara Domain, the northeastern-most subsection of the Kalgoorlie Terrane, provides the bestpreserved komatiite textures and contact relationships within the Eastern Goldfields Province and is a key area for the study of primary komatiite-substrate relationships. A re-evaluation of the stratigraphy of the region primarily attributes the complexity observed to the associations between the komatiite horizons and the predominantly felsic substrate. Two komatiite emplacement styles are recognised in the Boorara Domain. Large volume, dominantly cumulate-textured komatiite bodies that show symmetrically zoned chilled margins together with apophyses and xenoliths on top and basal margins, are interpreted as high-level intrusions. Extrusive komatiite lavas are also documented showing asymmetrical zonation within each lava flow, where the top margin shows a fine-grained, chilled margin that grades into a well developed spinifex zone and then to a cumulate zone above a thin basal chilled contact. Komatiite magmatism/volcanism is interpreted to have been contemporaneous with dacitic magmatism and volcanism. Textures such as irregular, fluidal morphology contacts between the two lithologies, komatiitic apophyses in the dacite and magma mingling, are documented at key localities within the Boorara Domain. These textural features clearly demonstrate an autochthonous relationship between the two lithofacies, which allows for preliminary interpretation into the tectonic regime of the Boorara Domain. Keywords: Archaean, komatiite, dacite, Boorara Domain, Kalgoorlie Terrane

SESSION 4: SEDIMENTOLOGY / STRATIGARPHY / VOLCANOLOGY

THE EVOLUTION OF THE MT. ECCLES VOLCANIC COMPLEX, NEWER VOLCANICS PROVINCE, VICTORIA, AUSTRALIA Rebecca Trowbridge School of Geosciences, Monash University The Quaternary Mt Eccles Volcanic Complex is located in the south-west Newer Volcanics Province (NVP) of Victoria, Australia. The main eruptive styles that characterised the complex were Hawaiian fire-fountaining, Strombolian explosive eruption, and phreatomagmatism, as part of a fissure system. Activity at the complex was initially Hawaiian fire-fountaining, however variations in conduit widths along the fissure produced local variations in eruptive style. Strombolian-style eruption occurred at points where the conduit was narrow, inhibiting magma ascent rate, promoting cooling and ponding, and eruption became pistonlike, or transitional fire-fountaining. Fissure widening northward, through a narrow conduit, allowed rising magma to mix efficiently with the water-laden sediments of an underlying aquifer, resulting in the phreatomagmatic deposits of the Mt Eccles cone. Magma ascent fluctuated in the Mt Eccles cone, resulting in pulsatory phreatomagmatism, and when the aquifer dried up or was sealed, Strombolianstyle volcanism predominated. The phreatomagmatic deposits of the complex are 'dry', indicating no excess water in the eruption, or that the near vent deposits that are the focus of this study were emplaced at high temperatures. The main modes of transport were base surge and air-fall. Continued fissure widening developed the Lake Surprise vent, and final activity at the complex was rapid lava effusion, producing the lava channels that breached the vent, and the extensive lava field. Based on geochemical analysis and stratigraphic correlation, the cones of Little Mt, Mt Eccles and the Lake Surprise crater, are considered to have formed coevally. The Mt Eccles Volcanic Complex comprises a range of basaltic rocks, with a strong Ocean Island Basalt character. The geochemical signatures of the complex are consistent with the geochemical signatures of the NVP. The complex plots as part of the young alkalic/transitional basalts of the NVP, consistent with the previous young age estimates of the complex. Keywords: eruption style, fissure, Newer Volcanics Province, Strombolian, phreatomagmatic

SESSION 5: STRUCTURE / TECTONICS / REMOTE SENSING / GEOMORPHOLOGY

THE STRUCTURAL AND THERMAL HISTORY OF THE SKENES CREEK-WILD DOG VALLEY REGION, EASTERN OTWAY BASIN Howman, S.A. & Hall, M.

School of Earth Sciences, Monash University, Melbourne, Australia. The Skenes Creek- Wild Dog Valley region is situated on the southeastern coastal margin of the Otway Ranges, immediately east of Apollo Bay and approximately 160 km southwest of Melbourne. The stratigraphy is dominated by fluvial sandstone and overbank and lacustrine mudstone of the Aptian-Albian Eumeralla Formation and includes the Skenes Creek Coal Measures. These comprise a thick (>1000m) sequence of interbedded fine sandstone and mudstone with abundant thin coal seams (maximum thickness c.50cm). The region is structurally complex, containing a number of NE trending thrust faults, associated monoclines and asymmetric folds on the southern limb of a broad, regional scale anticline which extends along the northern part of the region. Bedding on the coastal platform generally dips 20°-40° SSE, with local complications related to a series of major top-to-the-SE thrust faults trending close to the coast. To the northwest the structure develops into the Skenes Creek Monocline which is interrupted by faulting and asymmetric folding in the east of the area. Approximately 4km inland, the NW trending Wild Dog Valley turns abruptly NNE and lies parallel to the adjacent Barham Valley. A number of brecciated outcrops, along with fault planes containing strike slip slickensides, lead to the interpretation that these deeply incised valleys follow large shear zones. The structural framework of the region, as determined by field mapping, is constrained by VR analyses and aeromagnetic data. VR samples analysed range from 0.65 on the coastal platform to 1.72 1.5km inland and have been useful for pinpointing faults where outcrop is locally poor. They will also enable the geology to be "reconstructed" and the throw on the faults to be determined. Aeromagnetic data highlight the major faults, their trend through the region and their relationship to other major structures. Keywords: Otway Basin, structure, thermal history, inverted basin, VR analysis

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

THE PRESENT-DAY STRESS FIELD IN BRUNEI DARUSSALAM: A SNAPSHOT OF 'PROGRADING TECTONICS' IN A DELTA 1

Tinqav. M.R.P., 1Hillis, R.R., 2Morley, C.K., 3Swarbrick, R.E., 4Okpere E.C.

National Centre for Petroleum Geology & Geophysics, University of Adelaide, Adelaide, Australia 2 Department of Petroleum Geoscience, University of Brunei Darussalam, Brunei Darussalam 3 Department of Geological Science, Durham University, Durham, United Kingdom 4 Brunei Shell Petroleum, Seria, Brunei Darussalam

The Baram Basin of Brunei Darussalam (northwest Borneo) is unusual when compared to other Tertiary deltas, as it has been built up upon an active margin. Hence, the structures observed in Brunei are the result of a complex interaction between gravity-driven deltaic tectonics and transpressive tectonics associated with the active margin. Image and dipmeter logs have been examined for breakouts and drilling-induced tensile fractures (DITFs) in 46 wells throughout the basin. Breakouts and DITFs observed in 19 wells suggest that the maximum horizontal stress is oriented margin-normal (NW-SE) in the proximal parts of the basin and margin-parallel in the distal region. The margin-normal maximum horizontal stress direction is perpendicular to the strike of Miocene-Pliocene normal growth faults in the delta. Hence, there has been an approximately 90° rotation of the maximum horizontal stress over time, in addition to the observed present-day spatial horizontal stress rotation across the delta. The Baram Basin also shows a variation in vertical stress gradient from 18.3 MPa/km at the delta front to 24.3 MPa/km in the delta hinterland, equivalent to a change in bulk density from 2.07 to 2.48 g/cm3 across the delta. The present-day and temporal stress rotation and variation in vertical stress yield a 'snapshot' of a delta that is inverting and self-cannibalising. The proximity of the active margin has resulted in uplift of the hinterland with successive inversion of normal-faults in a basin-ward direction. The region of active growth faulting is 'prograding' as demonstrated by the margin-parallel maximum horizontal stress direction in the distal region of the delta. Keywords: Brunei, Stress, Tectonics

SESSION 5: STRUCTURE / TECTONICS / REMOTE SENSING / GEOMORPHOLOGY

NEOTECTONICS IN THE NORTHERN FLINDERS RANGES, SOUTH AUSTRALIA, THE STRUCTURAL, SEDIMENTARY, AND GEOMORPHIC EVIDENCE Julien Celerier, Lauren Elliott, Kate Harper and Anna Petts School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia The classic flat, desert, landscapes of central Australia, together with the lack of other overt signs of active tectonism, have all contributed to a generally held perception of Australia as a technically inactive realm. In the Flinders and Mount Lofty Ranges relatively high earthquake activity, combined with the youthful nature of the topography are evidence of contemporary tectonism. Structural, sedimentary and geomorphic evidence from the north-eastern Flinders Ranges, indicates a comparatively rich neotectonic record in this part of Australia. Detailed mappping along the Paralana escarpment has located numerous thrusts showing late Neogene displacement of at least 30m. Fault exposures juxtapose Proterozoic basement against modern hill scree, suggesting substantial movement along these range bounding faults has occurred during the Pleistocene. As well as these faults, the foreland Tertiary to Quaternary detrital strata have been mildly deformed in front of the propagating Paralana fault system. In addition to being deformed, the alluvial fan systems have undergone dissection, exposing Mesozoic, Tertiary and early Quaternary sediments. Palaeocurrent directions from the early Tertiary Eyre Formation, exposed at the base of the Paralana escarpment, suggest at least some fluvial systems were flowing to the position of the ranges today. This implies that the ranges could not be at their present elevation during the early Tertiary. Analysis of the development and subsequent erosion of the surfaces along the piedmont zone , as well as the drainage patterns of the Yudnamutana and East Painter Gorges, reveals a rich morphotectonic record of uplift and incision in the region. The use of ASTER multispectral satellite data provides regional scale coverage of the study area at a minimum spatial resolution of 15m. This imaging along with digital elevation models generated from ASTER allows for detailed interpretation of such features that could not be achieved by conventional mapping. Keywords: alluvial fans, ASTER, dissection, neotectonics, palaeocurrents, piedmont, thrust faults

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

IMAGING THE DUNES: DESERT DUNE DYNAMICS AND ASTER SATELLITE IMAGERY Kathrvn. E. Fitzsimmons and Kate. L. Harper School of Earth Sciences, University of Melbourne, Parkville, Victoria, 3010 The largely arid continent that is Australia is popularly perceived as barren, alien and hostile. Geomorphologists however, recognising the diversity present in the desert landscape, are learning to understand the morphology and landform processes involved. In the Lake Frome region east of the northern Flinders Ranges, the close association between transverse lake shorelines and longitudinal dunes displays the dynamic interrelationship between different aeolian landforms. Elongate sand ridge systems extend across the continent at the scale of planetary climate systems, and run parallel to the resultant strong wind directions at the time of their formation. The linear dunefields in the Lake Frome area form a region of variable orientation, extending from the more east-west oriented dunes in the south, and running more northeasterly further north near Lake Callabonna. In addition, the association between the longitudinal dunes and numerous transverse palaeoshorelines, evidence of higher lake levels in the past, clearly illustrates the process of dune dynamics and evolution. Field observations combined with the analysis of multispectral ASTER satellite imagery give valuable insights to dune morphology and process dynamics in the linear dunefields east of Lake Frome. ASTER satellite imagery has allowed for the viewing and interpretation of over 30,000 km2 of desert landforms, at a minimum spatial resolution of 15 m. ASTER also provides us with another 'dimension' for viewing the landscape, allowing for observations made in 14 spectral bands of the visible and infrared sections of the electromagnetic spectrum. The scale and resolution of the ASTER satellite imagery vividly depicts these spatial relationships, thus allowing for better understanding of desert geomorphology. This approach permits a reconstruction of the geomorphological history of the area during late Quaternary time. Keywords: ASTER, desert landforms, dune, geomorphology, Lake Frome, Quaternary

SESSION 5: STRUCTURE / TECTONICS / REMOTE SENSING / GEOMORPHOLOGY

THE PROTEROZOIC STRUCTURAL EVOLUTION OF THE ELDEE WEST REGION Neilson, I.E., Betts, P.G, Lister, G.S. Australian Crustal Research Centre, Monash University, P.O. Box28E, Victoria, Australia 3800 The Proterozoic structural evolution of the north western region of Broken Hill, the Eldee West structure, is investigated using structural mapping in conjunction with potential field interpretation. The Eldee West region underwent poly deformation resulting in the complex geometry formed by the overprinting of recumbent and upright folds. Four periods of deformation have been preserved in the metasedimentary and igneous rocks of Eldee West Region. D1 and D2 deformations occurred under amphibolite-granulite facies conditions giving rise to a bedding parallel foliation (S1) and a subhorizontal shallow dipping fabric (S2). The first fabric (S1) is observed layer parallel to compositional layering (SO), defined by quartz, biotite, K- feldspar and sillimanite. A mineral lineation (L1) defined by sillimanite ± mica is associated with the S1 foliation. Amphibolite bodies are preserved within the S1 foliation and have developed a strong LS fabric, defined by amphiboles and feldspars. The second generation fabric (S2) is dominantly horizontal to shallowly inclined in the hinges of later fold generations, and is defined by sillimanite, K - feldspar and white micas. S2 is axial planar to recumbent to inclined, isoclinal F2 folds. A mineral lineation, defined by sillimanite, occurs within S2. The third generation folds (F3) are associated with an axial planar spaced cleavage (S3), defined by biotite. F3 folds are close, upright to reclined, "similar" style shallow plunging folds with axial surfaces trending NE - SW. An S4 fabric is defined by a spaced styolitic cleavage overprinting S3. S4 is axial planar to upright to reclined, close "similar" folds with an E - W trend. Type 2 interference structures are observed associated with D2 and D3 structures, while Type 3 features are attributed to D2 and D4 superposition. The geometry of the Eldee West Region is interpreted to have developed during D2 large scale, noncylindrical nappe tectonics overprinted by subsequent D3 and D4 deformations further producing complicated fold interference patterns. Keywords: Eldee West Region, Broken Hill, non-cylindrical, nappe tectonics

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

SYNCHRONOUS DEVELOPMENT OF TYPE-2 AND TYPE-3 FOLD INTERFERENCE PATTERNS: EVIDENCE FOR RECUMBENT SHEATH FOLDS IN BROKEN HILL, NSW Caroline Forbes, Pete Betts and Gordon Lister Australian Crustal Research Centre, School of Geosciences, Monash University, Clayton VIC 3800, Australia Fold interference patterns are common in polydeformed terranes and are useful in establishing the geological history and kinematic controls on the deformation. Three 'classic' endmember examples of fold interference patterns have been defined: Type 1 (dome and basin); Type 2 (mushroom crescent); and Type 3 (convergent - divergent). Unusual fold interference patterns between F2 and F3 fold generations have been identified in the Allendale Area, northern Broken Hill Block, New South Wales. The Allendale Area has undergone early extension (Gibson, 2000) associated with hightemperature/low pressure metamorphism (up to amphibolite facies), followed by multiple folding events during later crustal shortening. The structural geometry of the area is the result of fold interference of early recumbent F2 folds superimposed by later ~N-S trending, F3 upright folds. Overprinting relationships on the eastern limb of the F3 Paps Synform shows evidence of a preexisting macroscale F2 recumbent fold hinge that has now been "squashed" and "opened" during horizontal crustal shortening. The F2 hinge is now incorporated into the eastern limb of the Paps Synform and the structural geometry reflects a modified Type-3 fold interference pattern. Meso- and macro-scale Type-2 fold interference patterns between F2 and F3 folds are preserved in the Allendale Mine Area. The synchronous development of Type-2 and Type-3 fold interference patterns between F2 and F3 folds in the Allendale Mine Area raises the question of how such a structural geometry could have developed. The consistent ~N-S trending orientation of F3 folds across the Allendale Area suggests variation in F2 axial trace by as much as 90°, resulting in co-axial and non-coaxial folding during the same sequence of deformation events. This implies a regional-scale recumbent sheath fold geometry developed during the D2 event in the Allendale Area. Keywords: fold interference patterns, Broken Hill Block, sheath folding

SESSION 5: STRUCTURE / TECTONICS / REMOTE SENSING / GEOMORPHOLOGY

BACK-ARC DEFORMATION IN THE KURIL BASIN AND SEA OF OKHOTSK, NORTHWEST PACIFIC Schellart, W.P.1, Jessell, M.W.2 & Lister, G.S.1 1

Epsilon Laboratory, Australian Crustal Research Centre, School of Geosciences, P.O. Box 28E, Monash University, Melbourne, VIC 3800, Australia Laboratoire des Mecanismes de Transfert en Geologie, UMR 5563 - CNRS -Universite Paul Sabatier-OMP, 38, rue des 36-Ponts, 31400 TOULOUSE CEDEX, France

The Kuril arc is located in the Northwest Pacific, where the Pacific plate is subducting underneath the East Asian continental margin. On the back-arc side, the Kuril arc is flanked by the Kuril Basin and the Sea of Okhotsk, which have formed due to asymmetric spreading and extension in the Eocene to Middle Miocene. We propose that these structures have formed due to asymmetric anticlockwise rollback of the Pacific slab and collapse of the overriding plate towards the retreating hinge-line of the Pacific slab. Deformation in the Kuril region has been simulated with scaled analogue experiments to test the applicability of the tectonic model proposed above. The experiments simulate the progressive asymmetric extension of the overriding plate due to asymmetric retreat of the subducting Pacific plate. The analogue results show the formation of a N-S to NE-SW oriented dextral shear zone near the far edge of the retreating boundary analogous to the Sakhalin-Hokkaido dextral shear zone. Contemporaneously, normal faults and grabens form, striking parallel to the retreating boundary near the far edge but striking more oblique near the hinge-point and away from the retreating boundary. This is similar to extensional structures observed in the Kuril Basin and the Sea of Okhotsk. Furthermore, the model shows that the amount of extension progressively decreases away from the retreating boundary. This appears to have also happened in the Kuril - Okhotsk region, where the bathymetry progressively decreases from the Kuril Basin towards the north, most likely reflecting the decrease in thickness of the thinned continental crust. Also, the model results show that with progressive deformation, extension is increasingly accommodated by the region close to the retreating boundary. This can account for the Eocene - Early Miocene extension in the Sea of Okhotsk followed by Middle Miocene spreading in the Kuril Basin. Keywords: Kuril arc, Kuril Basin, Sea of Okhotsk, asymmetric rollback, asymmetric extension, analogue modelling

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

TERTIARY N-S DIRECTION CRUSTAL SHORTENING OF CENTRAL TIBET, CHINA 1,2

Liu, S.. 2Wang,C.S., 2Ding,X.L.

School of Earth Sciences, University of Melbourne, Melbourne, Australia 2 Chengdu University of Technology,Chengdu,China

Some hypotheses have been proposed to explain the uplift and crustal thickening of the Tibetan plateau during the Cenozoic. We test the hypotheses using geological, geophysical and geochemical data, in particular, crustal shortening data. Acquired crustal shortening data are mostly from southern and northern Tibetan plateau. There are rare reported data from central Tibet. Our acquired data show there is -17.5% crustal shortening in the interior Qiangtang block and >41% in the Hoh Xil orogenic belt. The crustal shortening mainly takes place at the end of the Oligocene, with some occurring at the end of the Miocene. These data are obtained based on setting up balanced geological profiles in Fenghuoshan Group, Hoh Xil orogenic belt, and in Kangtuo Formation, Qiangtang block, both of which belong to the lower Tertiary strata. Paleomagnetic studies show that the Lhasa block has moved about 2000 km northward since 45 Ma. Such a displacement requires crustal shortening of approximately 40-50% averaged over the large area between the northern margin of Lhasa block and the southern margin of Siberia block. Our data show, on average, only about 29% crustal shortening in central Tibet, suggesting that there is 11%-21% of shortening of the above areas unaccounted for by penetrative crustal shortening. Therefore, other mechanisms are required to accommodate the northward movement of the Lhasa block. A possible mechanism is intracrustal subduction along some large fault zones, such as the northern edge of Kunlun fault, the southern edge of Tianshan fault, among others. Extrusion seems not to take place in the above areas. Keywords: Central Tibet,Qiangtang ,Hoh Xil, Crustal Shortening, Tertiary

SESSION 5: STRUCTURE / TECTONICS / REMOTE SENSING / GEOMORPHOLOGY

FLUID DYNAMICAL MODELLING OF SUBDUCTION IN THE UPPER MANTLE Schellart, W.P. & Lister, G.S. Australian Crustal Research Centre, Department of Earth Sciences, P.O. Box 28E, Monash University, Melbourne, VIC 3800, Australia We present results of three-dimensional fluid dynamical experiments to model the kinematic and dynamic behaviour of a subducting lithosphere in the upper mantle. With these experiments, several important features of slab behaviour have been investigated: first, the temporal variability of hinge-line migration; second, the slab induced three-dimensional sub-lithospheric upper mantle flow; third, the magnitude of tensional stress transmission (slab pull) from the sinking slab across the subduction hinge towards the horizontal part of the subducting lithosphere; and fourth, the kinematic behaviour of the slab. Results show that without an externally applied "ridge push force" hinge-line migration is in all cases regressive. The migration rate increases exponentially during the free sinking of the slab until it approaches the 670 km discontinuity, resulting in a decrease in migration rate followed by a renewed increase and finally approaching a steady state. Slab retreat results in forced convection of sublithospheric mantle from underneath the slab towards the mantle wedge. The experimental results show that flow underneath the slab tip is negligible and almost all flow occurs laterally around the edges of the slab. This lateral flow forces the hinge-line of the subducting lithosphere to attain a convex shape towards the direction of retreat. During the free sinking of the slab, it transmits tensional stresses to the horizontal part of the slab, which increase with increasing slab length and slab dip angle but constitute only a small percentage of the total negative buoyancy force of the slab. These slab pull stresses decrease when the slab hits the 670 km discontinuity. Finally, the slab preferentially retreats when its dip has an angle close to 90°. In this position, the slab does not sink vertically downwards. Rather, it sinks at an inclined angle in a regressive manner in order to minimise internal slab deformation and asthenosphere flow. Keywords: lithospheric slab, subduction, upper mantle, hinge-line migration, convection, modelling

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

NEW INSIGHTS ON THE MOTION OF TASMANIA RELATIVE TO AUSTRALIA Vos, I.M.A., Rosenbaum, G., Lister, G.S. and Bierlein F.P. School of Geosciences, Australian Crustal Research Centre, Monash University, Victoria, Australia An alternative reconstruction of the motion of Tasmania relative to mainland Australia since the Paleozoic is presented using the newly released PlatyPlus V.2 software package. This reconstruction visualizes crustal extension in Bass Strait, which separates Tasmania from the Australian mainland. The Bass Strait, underlain by the Otway, Bass and Gippsland intracratonic basins, is considered to contain the key to understanding the mechanism of Late Jurassic to Middle Cretaceous rifting between Tasmania and the Australian mainland. We consider the <150km southeastward motion of Tasmania relative to mainland Australia to be governed by sinistral strike-slip faulting ahead of sea-floor spreading that led to the development of a failed rift along the Australian southeastern margin. Time-slice images are presented, displaying strike-slip faulting and extension in Bass Strait from Late Jurassic to Mid-Cretaceous times. By the end of the Jurassic, the EW-directed Southeast Indian Ridge (SIR) was positioned northwest of Tasmania giving rise to sinistral strike-slip in the Bass Strait region ahead of sea-floor spreading between Australia and Antarctica. In the Early Cretaceous the strike-slip regime in Bass Strait transferred to a NE-SW extensional regime related to episodic rifting. Extension in this region ceased in the Middle Cretaceous and rifting was repositioned to the west and south of Tasmania. The described mechanism of rifting shows distinct similarities with later south to north opening of the Tasman Sea on the eastern margin of Australia. Our reconstruction model presents a challenging hypothesis on the opening of Bass Strait and its controlling mechanisms. Insight is gained on the position of Tasmania during the break-up of Gondwana and the evolution of sea-floor spreading between Antarctica and Australia. Moreover, a southeastward motion for Tasmania relative to the Australian mainland in the Mesozoic further constrains correlations between the Paleozoic of Tasmania and SE Australia. Keywords: reconstruction; Tasmania; Bass Strait; rifting; crustal extension; sea-floor spreading

SESSION 5: STRUCTURE / TECTONICS / REMOTE SENSING / GEOMORPHOLOGY

THE ORIGIN OF THE ARC OF THE WESTERN ALPS Rosenbaum, G. and Lister, G.S. School of Geosciences Australian Crustal Research Centre, Monash University, Victoria, Australia The arc of the western Alps is a collisional orogen suturing the Adriatic African promontory (Italy) with Europe. It consists of several terranes of continental origin (e.g. Briangonnais Zone, Internal Crystalline Massifs, Austroalpine) and remnants of ocean basins (e.g. Piedmont, Valais). These terranes underwent different tectono-metamorphic evolution during Alpine orogeny with several distinct episodes of high-pressure and ultra-high-pressure metamorphism affecting each terrane at different time. In the western Alps, the present tectonic configuration shows ribbons that run parallel to the arcuate strike of the orogen. The association of allochthonous continental ribbons in an arcuate orogenic belt is strikingly similar to other orogens that formed in retreating subduction zone setting. We therefore suggest that the arc of the western Alps formed during post-Oligocene westward subduction rollback accompanied by anticlockwise rotations of continental ribbons, which eventually collided and accreted onto the orogenic edifice. The anticlockwise rotations are confirmed by palaeomagnetic results that imply increasingly amount of block rotations from north to south. The kinematic model also explains the present orientations of kinematic indicators, which are consistently perpendicular to the strike of the orogen, forming an arcuate array of lineations. The northern boundary of the rollback/back-arc system accommodated 100-150 km of westward motion along the dextral strike-slip Insubric Line, which was active between 25-16 Ma. At the same time, the relative motion of Africa with respect to Europe was particularly slow and the whole Mediterranean region was subjected to widespread back-arc extension as a result of rapid subduction rollback. In conclusion, we suggest that the arc of the western Alps formed contemporaneously with Mediterranean back-arc basins during a period of relative quiescence in plate convergence. During this period, rapid rollback of subduction zones could not be supported by plate convergence, leading to the development of Alpine-Mediterranean arc systems. Keywords: Alpine orogen; western Alps; Mediterranean Sea; subduction rollback

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

UPPER CRUSTAL CENOZOIC-RECENT DEFORMATION ACROSS THE YIDUN ARC, EASTERN TIBET: ASSESSING THE INDIAN IMPACT Reid, A.J, and Wilson, C.J.L. School of Earth Sciences, University of Melbourne, Parkville 3052, Australia The uplift of the Tibetan Plateau occurred in response to the collision of India with the southern margin of Asian c. 45Ma. Upper crustal responses to the ongoing Indo-Asian collision are heterogeneous across the Tibetan Plateau and range from the generation of Tertiary-aged gneisses to brittle strikeslip faulting. This observed range in deformation style across the Tibetan Plateau is a function of both (i), proximity to the Indian indentor, the principal driving force of Cenozoic tectonism in the region and (ii), the interaction between structures developed during the complex pre-Tertiary orogenic history of the region and the evolving Cenozoic-Recent stress field. The Tibetan Plateau itself is composed of a number of tectonic blocks accreted to the southern margin of Eurasia, including in eastern Tibet, the Yidun Volcanic Arc. The Yidun Arc is a Permo-Triassic volcanic arc incorporated into the central Asian landmass during the early Jurassic. Presently the Yidun Arc lies some 300-400km east of the Eastern Himalayan Syntaxis. While the majority of ductile structures within the Yidun Arc are attributed to the early Jurassic basin closure event, post-Jurassic, and in particular Cenozoic-Recent deformation has also played an important role in shaping the structural character of the arc. Cenozoic-Recent structures observed across the Yidun Arc include large-scale strike slip Quaternary faults, the transpressive reactivation of Jurassic thrusts and, in the southern regions, the development of low-grade ductile strike-slip shear zones. Importantly, no pervasive crustal thickening is observed. Palaeostress reconstructions from fault slip data generally show east-west compression within the Yidun Arc and are consistent with the observed regional-scale clockwise rotation of the principle compressive stress about the Eastern Himalayan Syntaxis. Thus Cenozoic-Recent deformation within the Yidun Arc is attributed to block rotation between major strike-slip fault zones developed in response to the progressive northwards motion of the Indian indentor. Keywords: Tibet; Cenozoic-Recent deformation; palaeostress

SESSION 5: STRUCTURE / TECTONICS / REMOTE SENSING / GEOMORPHOLOGY

ROUTINE PROCESSING TECHNIQUES FOR SEISMOLOGICAL AND EARTHQUAKE ENGINEERING APPLICATIONS 1

Allen, T.

School of Geosciences, Monash University, Melbourne, Australia

Advances in earthquake data acquisition and processing techniques have allowed for improved quantification of source parameters and seismic attenuation associated with local Australian earthquakes. Until recently, only hypocentral locations and local (Richter) magnitudes (ML) had been determined routinely, with little attention given to other source parameter determination (e.g. seismic moment, moment magnitude, stress drop, source dimensions) or attenuation studies (e.g. seismic quality factor and kappa). Development of improved earthquake analysis software, which uses time and frequency domain analysis, forms the basis for this study. The software, eqSource, allows simple and rapid calculation of earthquake source parameters, seismic attenuation and conversion of digital time series recordings to true ground motion in terms of displacement, velocity and acceleration. These features are complimented with the addition of Duhamel's Integral, which is commonly used in structural engineering studies and predicts the dynamic response of a structure when exposed to an external ground motion such as that due to an earthquake or blast. Studies employing eqSource indicate that stress drop for southeastern Australian intraplate earthquakes increases with seismic moment to unusually high levels, indicative of high compressive crustal stress, and strong rocks and faults. In addition, new frequency dependent attenuation functions have been developed from the decay of seismic source spectra, enabling more precise magnitude calculations. These new source data calculated using eqSource will help to improve earthquake hazard estimates and further extend our understanding of the in situ stress conditions of the Australian continental landmass.

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

NEOTECTONICS AND INTRAPLATE DEFORMATION IN THE NORTHERN FLINDERS RANGES, SOUTH AUSTRALIA Julien Celerier School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia The classic flat, desert, landscapes of central Australia, together with the lack of other overt signs of active tectonism, have all contributed to a generally held perception of Australia as a technically inactive realm. In the Flinders and Mount Lofty Ranges relatively high earthquake activity, combined with the youthful nature of the topography are evidence of contemporary tectonism. Detailed mapping along the Paralana escarpment in the northern Flinders Ranges, indicates a comparatively rich neotectonic record in this part of Australia. The eastern range-bounding Paralana fault forms part of a major topographic escarpment with relief commonly in excess of 500m. The province is flanked by a realm of low depressions occupied by salt lakes. Numerous thrusts were located showing significant late Neogene displacement of at least 30m. Fault exposures juxtapose Proterozoic basement against modern hill scree, suggesting substantial movement along the range bounding faults has occurred during the Pleistocene. As well as these faults, the foreland Tertiary detrital strata have been mildly deformed in front of the propagating Paralana fault system. The ongoing deformation in the northern Flinders Ranges represents a unique opportunity to investigate the poorly understood occurrence and mechanisms of intraplate deformation. Using regional gravity data, the possibility that the ranges are but one expression of large scale lithospheric processes affecting much of the topography of South Australia is investigated. Gravity highs over the ranges, Broken Hill block, and Gawler Craton, seperated by relative gravity lows over the intervening salt lakes, indicate that such geometries may result from lithospheric folding producing crust-mantle upwarps. Keywords: Flinders Ranges, intraplate deformation, lithosphere folding, neotectonics, thrust faults

SESSION 5: STRUCTURE / TECTONICS / REMOTE SENSING / GEOMORPHOLOGY

THE MORPHOTECTONIC STUDY OF THE PARALANA ESCARPMENT AND YUDNAMUTANA GORGE, NORTHERN FLINDERS RANGES, S.A. Anna E. Petts School of Earth Sciences, The University of Melbourne, Parkville, Victoria 3052, Australia The tectonic and geomorphic history of the Northern Flinders Ranges, reveals ongoing uplift and dissection of the ranges themselves as well as the alluvial fan system that bound them; this study highlights the evidence for neotectonic activity in a geomorphic context, i.e. how the landscape has evolved to what is observed today and the morphotectonic processes involved. The landforms of the Northern Flinders Ranges have been greatly influenced by tectonic activity in the present as well as the past. Methods used in this study include field mapping and manipulation of ASTER imaging, for an accurate geomorphic map and digital elevation model. The Paralana High Plains border the eastern range front of the Flinders Ranges and form quite prominent features, while Yudnamutana Gorge dissects both the range-front and the Freeling Heights Surface, which is an exhumed surface that has been dissected by Yudnamutana Creek. Focusing on the landscape features of the range-front from Yudnamutana Gorge to the edge of the Paralana High Plains, an analysis of the geomorphology of the study area reveals whether these features are aggradational or degradational. The development and subsequent erosion of the surfaces along the Paralana zone reveals the neotectonic record that has shaped the range front. The digital elevation model can be used to study landforms, such as the drainage patterns of Yudnamutana Gorge and East Painter Gorge. The late Neogene to present tectonism contributes substantially to the relief and topography of the Northern Flinders Ranges, and by investigating the examples of Quaternary faulting and the history of denudation in the area, the timescales associated with the erosional processes can be deduced. Keywords: geomorphic, neotectonics, Flinders Ranges, ASTER

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

DISSECTION OF ALLUVIAL FANS: EVIDENCE FOR RECENT UPLIFT IN THE NORTHERN FLINDERS RANGES Lauren Elliott School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia Sedimentary evidence in the north eastern Flinders Ranges provides support for the notion that Australia has undergone recent, neotectonic activity. Deposition and subsequent incision of alluvial fans in the late Tertiary, early Quaternary, has exposed Mesozoic, Tertiary and early Quaternary sediments. Paralana and Four Mile Creek are two of the main fluvial systems of where good exposure occurred. Detailed stratigraphic sections were measured along these creeks and in the Mount Babbage region. Mount Babbage consisted of late Cretaceous, fluvial to marine sediments; Paralana and Four Mile Creek of the Tertiary sequence, Eyre, Namba and Willawortina Formations. Palaeocurrent directions from the early Tertiary Eyre Formation suggest fluvial systems were flowing towards the current position of the ranges. For this to occur, the Flinders ranges could not be at their present elevation, implying a much more recent uplift. Mesozoic sediments exposed along the Paralana fault scarp also suggest fluvial direction to the present ranges. Aggradational conditions allowed the deposition of the Willawortina Formation alluvial fans. These conditions are a complex interaction of technically and climatically controlled factors, including a steep hydraulic gradient and large sediment and water supply. As the fans have since been dissected this is thought to be due to a change in either climatic or tectonic conditions. Numerical modelling of alluvial fans explores how these factors impact on alluvial fan formation. Keywords: alluvial fans, incision, palaeocurrent, Tertiary, uplift

SESSION 5: STRUCTURE / TECTONICS / REMOTE SENSING / GEOMORPHOLOGY

APPLICATION OF ASTER SATELLITE IMAGERY NORTHERN FLINDERS RANGES, SOUTH AUSTRALIA Kate Harper School of Earth Sciences,

University

of Melbourne,

Parkville,

Victoria, 3010

Multispectral ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) satellite data has been evaluated for lithologic, geomorphic and structural interpretations of the Northern Flinders Ranges, South Australia. ASTER is a new remote sensing instrument on board the Terra satellite, launched in December, 1999. The system collects electromagnetic data from 14 spectral bands covering the visible, near infrared, short wave infrared, and thermal infrared sections of the electromagnetic spectrum, with a minimum spatial resolution of 15m. ASTER has the ability to produce digital elevation models (DEM), important for understanding the dynamics of the landscape, by draping false colour images over topography. The Northern Flinders Ranges is one of the most diverse geological settings on the Australian continent, making it an ideal 'test site' to demonstrate the capabilities of the ASTER instrument. The area contains a range of geology from Precambrian basement to Quaternary desert landforms. Analysis of spectral data has proved useful in differentiating between the main lithologies within the study area. The potential for mapping certain individual mineral groups is assessed. Structural and geomorphic features have been interpreted at a range of scales, some likely to have gone unnoticed using conventional mapping methods. The results indicate the potential significance of ASTER data to minerals exploration and also environmental monitoring. Keywords: ASTER; DEM; Flinders Ranges; geomorphology; geology; multispectral; remote sensing

16th Victorian Universities Earth Sciences Conference, September 2002, Geological Society of Australia Abstracts 69

DESERT DUNE DYNAMICS IN THE LAKE FROME REGION, SOUTH AUSTRALIA Kathryn E. Fitzsimmons School

of Earth

Sciences,

University

of Melbourne,

Parkville,

Victoria,

3010

The largely arid continent that is Australia is popularly perceived as barren, alien and hostile. Geomorphologists however, recognising the diversity present in the desert landscape, are learning to understand the morphology and landform processes involved. In the Lake Frome region east of the northern Flinders Ranges, the close association between transverse lake palaeoshorelines and longitudinal dunes displays the dynamic interrelationship between different aeolian landforms. Linear sand ridge systems run parallel to the resultant strong wind directions at the time of their formation. The linear dunefields in the Lake Frome area form a region of variable orientation, trending more eastwest in the south, and running more northeasterly further north near Lake Callabonna. Field observations conducted in the linear dunefields northeast of Lake Frome have enabled a reconstruction of dune stratigraphy and structure. Dune asymmetry is evident both in slope steepness and in the exposure of palaeodune sediments on the steeper southern slopes, contrasting with modern, mobile accumulations on the gentler northern slopes. Mobile dune crests have diverged from the orientation of the main palaeodune trend, and run in a more northerly direction. With the exception of the mobile crestal quartz sands, the linear dune sediments have a significant clay component. The clay was probably sourced from the clay-rich interdune swales and required cyclical saline groundwater activity to bind clay into pellets, thus enabling its transport. At least two palaeosol horizons have been observed within the linear dunes; these have been exposed due to scalding on the steeper southern flanks. Optically stimulated luminescence dating techniques have been used to estimate the ages of two dune palaeosols, and have yielded ages of 15ka and 65ka1. These observations have implications for the climatic and hydrologic conditions in operation during the dune building episodes, and enable a reconstruction of the geomorphological evolution of the area. Keywords: Desert landforms, longitudinal dunes, geomorphology, Lake Frome, optically stimulated luminescence dating (OSL), palaeosols, Quaternary.

More complete results, including margins of error, will be available by September.

SESSION 5: STRUCTURE / TECTONICS / REMOTE SENSING / GEOMORPHOLOGY

AN APPARENT STRATIGRAPHIC REVERSAL IN POORLY-BLEACHED QUARTZ FLUVIAL SEDIMENTS RESOLVED BY SINGLE-GRAIN OSL MEASUREMENTS GresleyA. Wakelin-King Department of Earth Sciences, La Trobe University, Bundoora, Melbourne, Victoria, Australia 3088 Across the drainage basin of Fowlers Creek, western NSW, steep river banks reveal a consistent fining-upwards stratigraphic pattern of fluvial sedimentation. The top layer is a distinctive, red-brown fine sandy silt, often finely laminated. Optically-Stimulated Luminescence dating is being used to address the question: is this silty layer a consequence of normal arid-zone fluvial process, or is it the result of landscape disturbance following European settlement? At one site, two sequential samples collected from this unit appeared stratigraphically reversed, with the top sample showing a considerably greater optical age than the bottom. Single-grain analysis demonstrates that both samples were partially-bleached, but the top sample contained significantly more partially-bleached grains and anomalously high-PD grains. The true age of the unit in this location (but not everywhere) is <150 ybp (i.e. post-European). Keywords: optical dating, landscape history, fluvial process, arid zone

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