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Geological Society of Australia

ABSTRACTS Number 62

Victorian Universities Earth Sciences Conference Monash University September, 2000

14'" VICTORIAN UNIVERSITIES EARTH SCIENCES CONFERENCE

Monash University September, 2000

Geological Society of Australia Abstract Volume Number 62

ISSN 0729-11X © Geological Society of Australia Incorporated 1999 Copies of this publication may be obtained from the Geological Society of Australia Incorporated, 706 Thakral House, 301 George St, Sydney, NSW, Australia, 2000. Example citation for papers in this Volume: Arcaro, H., 2000. Structural relationships between the Proterozoic assemblages of the Moonta Subdomain, Gawler Craton. Geological Society of Australia, Abstracts No. 62, pp 19.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 14*'' Victorian Universities Earth Sciences Conference, Monash University, September 2000

The Organising Committee for the Victorian Universities Earth Sciences Conference, IVIonash University, September 2000, would like to thank the following sponsors for their support

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 14"" Victorian Universities Earth Sciences Conference, IVIonasli University, September 2000

La Trobe University Department of Earthi Sciences

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Table of Contents Sponsors of the 14th VUESC

Index of Authors

Symposium: Atmosphere and the Environment

Influences on Precipitation in Australia at the Last Glacial Maximum - Hope, P., Simmonds, I.H. & Bowler, J

Transport and degradation of sub-surface hydrocarbons at Digman Reserve, Newport, Melbourne - Moore, B.G., Lavis, A.J., Cartwright, L, Weaver, T.R. & Reid, R

Reassessing the stable water isotope record in understanding past climate - Noone, D. & Simmonds, 1

Variability in the Southeast Indian Ocean: results from an upper ocean heat budget (19851997) -Ansell, T., Reason, C.J.C. & Meyers, G

The nature of Australian rainfall variability - a statistical analysis - Bailey, M

The response of oxygen isotope ratios in precipitation to changes in global atmospheric circulation - Brown, J., Simmonds, I. & Noone, D

The occurance of dryland salinity in the Creswick-Ballarat area of the Loddon Catchment, Victoria - McAliece, S

Comparison of methods for the determination of reduced inorganic sulphur in acid sulphate soils - Santomartino, S.L

The effects of pH and adsorption on metal speciation and mobility and its relation to heavy metal hyperaccumulation by native Australian flora. - Menon, D., McPhail, D.C & Hallam, N

Evaluating categorical rainfall forecasts of Melbourne using various skill scores. - Phan, T.

Processes controlling groundwater chemistry and identification of water resource vulnerability: Yarra Catchment, Victoria - Tweed, S., Weaver, T. R. & Cartwright, 1

Symposium: Crustai/Mantie Dynamics

Role of fluids in the metamorphism of the Alpine Fault Zone, New Zealand - Vry, J.K., Storkey, A C . & Harris, C

Continental margin escarpment retreat: can we measure this or not? -Dimas, V.-A

TABLE OF CONTENTS

Trace element characteristics of mafic and ultramafic meta-igneous rocks from the 3.5 ga Warrawoona Group: evidence for plume-lithosphere interaction beneath Archaean continental crust? - Bolhar, R., Hergt, J. & Woodhead, J

Missing pieces of the Proterozoic Jigsaw; The Peake and Denison Inlier, South Australia, and its part in the Proterozoic Evolution of Australia. - Richardson, J. A

The post-break up history of the Damara Sector of Namibia: a low temperature thermochronology perspective - Raab, MJ., Brown, R.W. & Cockburn, H.A.P

Structural relationships between the Proterozoic assemblages of the Moonta Subdomain, Gawler Craton, Western Australia. -Arcaro, H

Mountains and molehills, how different are they? Mechanical implications of residence time in the brittle crust -Belton, D.X.

Late Miocene uplift across the Southeast Australian Margin - Dickinson, J. A., Wallace, M. W., Holdgate, G. R., Gallagher, S. J. & Thomas, L

Modelling the fracture of rock during caldera collapse - Gray, J. P. & Monaghan, J. J

Dyke emplacement in Southeast Australia during the Mesozoic -Halas, D

Timing of Tertiary extension in the Southern Canadian Cordillera: Constraints from fission-track analysis in the Shuswap Metamorphic Complex - Lorencak M., Seward D. & Burg J.-P

The potential of apatite fission track dating of vertical profiles in Namibia and the implications for passive margin evolution - Raab, M.J. & Brown, R.W.

Large scale fluid infiltration in Palaeozoic shear zones of the Arunta Inlier, Central Australia. - Read, C. M. & Cartwright, I

Physical modelling of back-arc extension - Schellart, W.P., Lister, G.S. & Jessell, M.W.

West bounding faults associated with the Heathcote Greenstone Belt, Victoria: is this only true for the central segment? - Shawcross, M

Island-arc and accretionary prism collision, western/central Lachlan Orogen, South Eastern Australia - Spaggiari, C. V., Gray, D. R. & Foster, D. A

Apatite fission track dating of the Northern Western Shield, Western Australia - Weber, U.D., Kohn, B.P., Nelson, D.R. & Gleadow, A.J.W.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62

vii

14*^ Victorian Universities Earth Sciences Conference, Monash University, September 2000

Symposium: Economic Geology

Late Palaeozoic high resolution biostratigraphy, sequence stratigraphy and palaeogeography of the Tarim Basin, Northwest China: an integrated approach to hydrocarbon exploration - Chen Z. Q. and Shi G. R

Gold occurrence on contrasting structures in the Tasman Flat Gold Field, Central Victoria - Rosengren, N

Deformation Processes at the Magdala Gold Mine, Stawell.

- Witham, B., Wilson, CJ.L

& Miller, J. McL

Mineralisation & structural history of Hill 800, Mount Useful Slate Belt, Eastern Victoria - Morey, A.A., Bierlein F.P., Turner G. & Cherry D.P

Thermal and burial history of the Abbeytown Pb-Zn deposit, Ireland - Bayrak, AA

GIS analysis of the potenetial for gold exploration St Arnaud, Western Victoria. - Emmett, S

Relative timing of deformation, metamorphism and mineralisation within the Willyama Complex, New South Wales -Hills, Q.G

Origin of Irish Zn-Pb mineralisation: syngenetic vs epigenetic - Reed, C. & Leonard, \N.

Effects Of Heavy Metal Speciation On Partial Leach Geochemistry - White, D., McPhail, D.C.& Lawie, D

Symposium: Geophysics

structural development and petroleum potential of the South Wanganui Basin, New Zealand - Daber, R. & Hall, M

Focal Mechanisms of the Thomson Reservoir-Thggered Earthquakes - Allen, T, Gibson, G. & Hill, C

Rock magnetism and magnetostratigraphy of cave sediments in the Wellington Caves, New South Wales. - Christensen, T.

Geological evolution of the South Wanganui Basin, New Zealand - Daber, R. & Hall, M

TEM's "Full Monty" - Grant, A.J. & Cull, J

viii

TABLE OF CONTENTS

Modelling petrophysics and potential field data - Huynh T.H. & Ailleres, L

Symposium: Surface Processes

Variability of transport mechanisms for Volcanic Debris Avalanches: Insight from the Maronne valley (Cantal Volcano, France). - Reubi, O. & Hernandez, J

The microvertebrate fauna of Bullock Creek, Northern Territory. - Schwartz, L

Discovery of a Middle Permian (Guadalupian) brachiopod fauna from Peninsular, Malaysia - Campi, M. J., Shi, G. R., & Mohd S. L

Cainozoic submarine canyons of the Otway Basin, Victoria - Leach, A.S

Glacial-marine sedimentation and dolomite formation in Permian sediments of the Bacchus Marsh Formation, Bald Hill. - Spence, E.

Luminescence dating of rock art using mud-wasp nests -Ashton, M.R., Roberts, R.G. & Yoshida, H

Timing of deposition and provenance of stromatolitic carbonates from the 2.7 Ga Belingwe Greenstone Belt/ Zimbabwe Craton: constraints from Pb-Pb and Sm-Nd systematics - Bolhar, R., Hofmann, A., Hergt, J., Woodhead, J., & Dirks, P

First record of permochonetes afanasjeva (brachiopoda) from Sichuan, China; a new species - ecology and palaeobiogeography - Campi, M. J. &Shi, G. R

Seismic architecture of the Seaspray Group: evolution of the Gippsland Shelf and Slope through the Tertiary - Daniels, J., Gallagher, S.J., Holdgate, G.R., Keene, J.B., Smith, A.J. & Wallace, M.W.

Sedimentology and ichnology of Mount Bepcha, Western Grampians, Victoria - Gouramanis, C

Variations within the large volume Cerro Galan Ignimbrite, Northwest Argentina: Implications for eruption and emplacement processes. - Hildyard, S.C., Cas, R.A.F. & Viramonte, J.G

Establishing the stratigraphy of the Werribee Plains, Newer Volcanic Province, Victoria: an interdisciplinary approach - Moore, A.G. & Cas, R.A.F

The Structural, Stratigraphic and Thermal History of the Skenes Creek - Wongarra Region, Otway Ranges, Eastern Otway Basin. -Noll, C

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62

Victorian Universities Earth Sciences Conference, Monash University, September 2000 Palynology of Bacchus Marsh glacigene sediments - Pierson, R

Sedimentological constraints on Cu-Zn-(Pb) sulphide deposits from the Archaean Whim Creek Belt, Pilbara Craton, Western Australia - Pike, G. & Cas, R.A.F

Global distribution of Permian Conulariids: a preliminary report - Weldon, E. A

INDEX OF AUTHORS

Index of Authors A Ailleres, L Allen, T Ansell, T Arcaro, H Arden, P. J Ashton, M. R

49 45 5 19 41 57

B Bailey, M Bayrak, A. A Belton, D. X Bierlein, F. P Bolhar, R Bowler, J Brown, J Brown, R. W Burg, J.-P

54, 59 3, 12, 26 62, 63, 66 32 35 46 18 48

D Daber, R Daniels, J Dickinson, J. A Dimas, V.-A Dirks, P

44, 47 60 21 15 58

E Emmett, S

F Foster, D. A

21, 60 45 30 61 48 29 22

H Halas, D Hall, M Hallam, N Harris, C Hergt, J Hernandez, J

23 44, 47 10 14 16,58 52

K Keene, J. B

Kohn, B. P

L Lavis, A.J Lawie, D Leach, A. S Leonard, W Lister, G. S Lorencak, M M Maher, S McAliece, S McPhail, D. C Menon, D Meyers, G Miller, J. McL Mohd S. L Monaghan, J. J Moore, A. G Moore, B. G Morey, A.A

3 40 55 39 27 24 41 8 10, 40 10 5 34 54 22 63 3 35

N Nelson, D. R Noll, C Noone, D

30 64 4, 7

O OIley, J

G Gallagher, S. J Gibson, G Gleadow, A. J. W Gouramanis, C Grant, A.J Gray, D. R Gray, J.P

62 45 38 58 21, 60 2 49

J Jessell, M. W

6 36 20 35, 41 16, 58 2 7 18, 25 24

C Campi, M. J Cartwright, 1 Cas, R. A. F Chen, Z. Q Cherry, D. P Christensen, T Cockburn, H. A. P Cull, J

Hildyard, S. C Hill, C Hills, Q. G Hofnnann, A Holdgate, G. R Hope, P HuynhT.H

P Phan, T Pierson, R Pike, G

11 65 66

R Raab, M. J Read, C. M Reason, C.J.C Reed, C Reid, R Reubi, 0 Richardson, J. A Roberts, R Roberts, R. G

18, 25 26 5 39 3 52 17 67 57

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62

14*" Victorian Universities Earthi Sciences Conference, Monasli University, September 2000 Rosengren, N

Santomartino, S.L Schellart, W. P Schwartz, L Seward, D Shawcross, M Shi, G. R Simmonds, 1 Simmonds, I.H Smith, A. J Smith, B Spaggiari, C. V Spence, E Storkey, A.C

9 27 53 24 28 32,54,59 4, 7 2 60 67 29 56 14

T Thom, J. L Thomas, L Turner, G

Tweed, S

41 21 35

Viramonte, J. G

Vry, J. K

W Wallace, M. W Weaver, T. R Weaver, T.R Webb, J Weber, U.D Weldon, E. A White, D Wilson, C. J. L Witham, B Woodhead, J Y Yoshida, H

21, 60 12 3 67 30 68 40 34 34 16. 58 57

XII

CONFERENCE ORGANISING COMMITTEE Conference Convenor

Geoff Pike

Co-Convenors

Stuart Hildyard Sam Watkins

Editor

Matthew Purss

Sub-Editors

Alison Moore Miciielle Carey Priscilla Gaff Desmond Menon Daniel Halas

Web Page Design

Nick Rawlinson Caroline Forbes

Catering

Miranda Mayle Lara Spina Jessica Trofimovs Mark Tait

Audiovisual

Adrian Pittari

The organising committee would also like to thank students of the Monash University Geological Society (MUGS) for their assistance on the day.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 14"^ Victorian Universities Earth Sciences Conference, IVIonash University, September 2000

SYMPOSIUM: ATMOSPHERE AND THE ENVIRONMENT

ATMOSPHERE AND THE ENVIRONMENT

Influences on Precipitation in Australia at the Last Glacial Maximum Hope, P., Simmonds, I.H., & Bowler, J. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia.

Palaeo proxy data indicate drier and cooler conditions in nnost locations across Australia during the last glacial maximum (LGM). This raises the question of what mechanisms are modified to produce this different hydrological regime. The motion and processes in the atmosphere are driven by a range of forcings, from the Sun, the surface, and composition of the air. In a general circulation model of the atmosphere (AGCM), these forcings can be prescribed to simulate conditions at the LGM. The Melbourne University AGCM is used to simulate the LGM, using boundary conditions from the palaeo modelling intercomparison project (PMIP). The changes in the climate will be shown. Sensitivity studies on each of the prescribed boundary conditions have been run, and it is seen that modifications to the available sea surface and the temperature of that surface have the most significant influence on the resulting change in the precipitation. An analysis of the shifts in storm tracks and the source of the precipitation will also be discussed. Keywords: Australia, precipitation source, last glacial maximum, general circulation model.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Transport and degradation of sub-surface hydrocarbons at Digman Reserve, Newport, Melbourne ^'^Moore, B.G., ^ \ a v i s , A J . , ^ ^ C a r t w r i g h t , I., ^'^Weaver, T . R . & ^'^Reid, R. ^Hydrogeology and Environment Research Group ^Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia. ^School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia.

Digman Reserve is located next to the Shell Australia Newport Terminal in southwest Melbourne. During the 80 years of operation at the Terminal, the groundwater in the underlying fractured basaltic aquifer system has been impacted by Light Non-Aqueous Phase Liquids (LNAPLs) and associated dissolved phases. The transport and degradation of these hydrocarbons is studied to assist the identification of potential receptors of the environmental impact and to assist the review of remediation strategy. Observation well gauging and sampling data were used to delineate the extent of the Phase Separated Hydrocarbon (PSH) plume beneath the reserve, and the associated dissolved phase hydrocarbon plume extending towards the Yarra River and nearby residential areas. The hydrocarbon migration behaviour is characterised by the development of continuous plumes, rather than spot concentrations connected by discrete flow paths such as fractures. This implies that the study area may be treated as a representative porous medium at this scale. The important transport mechanism for the PSH plume is immiscible phase flow. Dissolved phase transport is governed by advection, molecular diffusion and mechanical dispersion on the study site scale. At a smaller scale, lava flow fractures, joints and flow boundaries of the aquifer probably direct both the PSH and the dissolved phase hydrocarbons. The PSH plume undergoes immiscible phase partitioning into volatile organic compounds in the unsaturated zone, and into a dissolved phase within the saturated zone. Annual variation in the groundwater surface smears the PSH plume vertically and elevates the rate of its dissolution into the groundwater. Attenuation of the hydrocarbons primarily results from biodegradation and sorption. When compared to the local groundwater, observation wells impacted by PSH generally showed lower pH, reducing instead of oxidising conditions, higher alkalinity and dissolved C02, and lower dissolved oxygen. Such conditions reflect elevated microbial activity degrading the hydrocarbons. Gas chromatography and hydrocarbon fingerprinting analyses constrained the source location and identified the source composition to be a mixture of diesel and gasoline. It also verified the loss of lighter fractions due to volatilisation and hydrocarbon degradation. The remediation strategies will focus on pumping and treating the LNAPL and groundwater, and on the expected proficiency of intrinsic bioremediation to attenuate potential environmental impact. Keywords: groundwater, phase separated hydrocarbons, dissolved phase hydrocarbons, hydrocarbon transport, hydrocarbon degradation, intrinsic bioremediation, Newport.

ATMOSPHERE AND THE ENVIRONMENT

Reassessing the stable water isotope record in understanding past climate Noone, D. & Simmonds, I. School of earth Sciences, The University of Melbourne, Parkville VIC, 3010, Australia.

The impact of atmospheric circulation on the stable water isotope record has been examined using an atmospheric general circulation model to reassess the validity of using isotopes to reconstruct Earth's climate history. Global temperature changes are classically estimated from the variations in (polar) isotopic values assuming a simple linear relationship. Such a relationship can be justified from first order theoretical considerations given that the isotopic fractionation at the deposition (ice core) site is temperature dependent. However, it is found that the history of a given air mass is more important that local processes because of the net effect of condensation events active along the transport pathway from the source region. Modulations in the hemispheric flow are seen to be crucial to Antarctic precipitation and the isotopic signal. Similarly, both transient and stationary disturbances influence the pathways of the air masses associated with Antarctic precipitation. During different climate regimes, such as that of the Last Glacial Maximum, the properties of these types of disturbances may not be assumed to be the same. As such, we may not assume that the condensation histories are the same as under different climate conditions. Therefore, the veracity of the linear climate reconstructions becomes questionable. Notwithstanding this result, the types of changes to the circulation regime that are expected generally correspond to changes in the global temperature. This fortunate result does not disallow the use of regressional reconstruction, however, the uncertainties associated with these circulation changes are of the same magnitude as the differences suggested by conventional linear regression in climate reconstruction. This indicates that interpretation of ice core data must be accompanied by detailed examination of the atmospheric processes and quantification of the impacts of their changes. Keywords: Pelaeoclimate, stable water isotope, general circulation model

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Variability in the Southeast Indian Ocean: results from an upper ocean heat budget (1985-1997) ^AnselL T., ^Reason, C J . C . , & ^Meyers, G. ^School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia. ^CSIRO, Division of Marine Research

A study of variability in the southeast (SE) Indian Ocean is presented. Expendable bathythermograph observations (XBT) along line 1X1 (Shark Bay to Sundra Strait) and heat flux data are used to examine an upper ocean heat budget of the region, with the aim of furthering understanding of sea surface temperature (SST) variability here. Net heat flux is estimated from NCEP reanalysis data and is compared with Southampton Oceanographic Centre surface flux data. Heat storage is calculated using two methods; firstly, by integrating down from the surface to fixed depths, and secondly integrating down from the surface to an isotherm that was one degree less than the coldest observed monthly SST. Heat content to isotherm was found to more closely correspond to SST variations in the region. Correlations of monthly heat flux and heat storage rates (1985-1997) are highly significant, accounting for over 60% of the variance. At the northern end of XBT line 1X1 (off the southern coast of Java) this correlation weakens, suggesting some other processes are effecting SST variability here. Upwelling and vertical advection are also processes that affect SST. Upwelling has been observed along the Java coast in winter, associated with the southeasterly longshore wind stress. Some preliminary findings of the contribution of upwelling and horizontal advection at these northern grid points and the errors associated with these calculations will be presented. Keywords: Southeast Indian Ocean, SST variability, heat budget, surface fluxes, upwelling.

A TMOSPHERE AND THE ENVIRONMENT

The nature of Australian rainfall variability - a statistical analysis Bailey, M. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia.

Many studies have attempted to interpret the inherent spatial and temporal variability of the Australian climate. This study mathematically analyses how much the variation changes across the nation, and what the dominant influences on such perturbations in the record are. A gridded monthly rainfall data set, established through interpolation of the station data from January 1925 to April 2000 is used to draw frequency distributions at each grid point for each monthly total, each seasonal total and the annual total rainfall. Typically, the frequency distribution of monthly and seasonal rainfall takes the form of a gamma distribution, characterised by a considerable asymmetry and positive skewness. This is especially true for arid regions of low and highly variable precipitation, where there is a high frequency of low rainfall events and a few larger outliers. In regions or seasons of high rainfall and low variability, the distribution is more likely to be Gaussian ('normal'). To describe the extent of asymmetry, and the likelihood of extremes, the distribution is analysed in three stages. The distribution is first transformed by taking the square root (SR) and cube root (CR) of the raw data. A normality test then establishes which of the untransformed, SR transformed or CR transformed data yields a normal distribution. Thus, the level of transform required to generate a normal distribution from the raw data explains how skewed the untransformed distribution is. In the second stage of this study, the gamma function is fitted to the distribution by calculating the two descriptive gamma parameters, alpha and beta. These explain the location of the maximum and the skewness, and the extent to which the tail of the distribution is stretched to the right, respectively. The last stage of the analysis is the calculation of the coefficient of skewness and coefficient of variation. A regression analysis is also applied to reveal any persistent trends in the 75-year data set, and a spectral analysis is done to view any underlying periodicities. The weather experienced by different regions in then investigated to gain an understanding of what systems dominate the described distribution. Keywords; Australia, gamma distribution, rainfall variability, transforms, trends.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

The response of oxygen isotope ratios in precipitation to changes in global atmospheric circulation Brown, J., Simmonds, I. & Noone, D. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010 Australia.

Stable isotopes of oxygen and hydrogen present in water are known to undergo fractionation at phase transitions, with heavy isotopes tending to evaporate less readily and to undergo condensation more readily than lighter isotopes. The combination of fractionation processes for a given air parcel therefore produces the known relationships between precipitation amount, surface temperature and the isotopic ratio in precipitation. An additional factor in determining the isotopic ratio is the effect of changes in vapour source region and vapour path due to changes in atmospheric circulation. In order to explore the effect of changes in circulation related to El Nino/Southern Oscillation (ENSO) events, an atmospheric general circulation model fitted with a diagnostic module to predict the stable isotopic ratios of meteoric water (both HDO and H2180) is used to investigate the effect of warm sea surface temperature (SST) anomalies on moisture transport and hence on isotopic ratios in precipitation. Observed isotopic ratios obtained from the Global Network of Isotopes in Precipitation (GNIP) are also correlated with Troup SOI values and compared with the model results. Keywords: oxygen isotopes; general circulation models; ENSO; climate modeling

A TMOSPHERE AND THE ENVIRONMENT

The occurrence of dryland salinity in the Creswick-Ballarat area of the Loddon Catchment, Victoria McAliece, S. Department of Earth Sciences, La Trobe University, Bundoora VIC 3083, Australia.

Dryland salinity has become a major problem in Victoria, particularly in the southeast and central north of the state. Although dryland salinity in Victoria has been investigated for a long time and solutions and treatment have been the topic of much research, the hydrochemistry has not been studied in detail.. The area around Creswick in the Loddon catchment affected by dryland salinity was selected for detailed study. This area is characterized by volcanic cones and basalt lava flows overlying Ordovician sedimentary bedrock. Recharge occurs on the slopes of the volcanic cones, through fractures in the basalt. Discharge occurs at the base of slopes. This project aims to develop a conceptual model for the process of salinisation around Creswick. Over 20 bores were selected from a large area from a variety of rock types and topographic settings. In addition to collecting water samples, the soil conductivity and salinity were measured at a number of sites. Preliminary results show that, in general, groundwater salinity increases downslope, probably as a result of the progressive input of highly saline water infiltrating slowly through the very low permeability, highly saline soils on the Paleozoic bedrock. Recharge occurs largely though the thin, higher permeability soils on the volcanic cones. Because fracture permeability characterizes both basalts and Paleozoic sediments, flow pathways for groundwater are not always predictable. Thus, fresher groundwater is sometimes found in the higher elevation Paleozoic outcrops, underlying more saline groundwater in the basalt lavas. In general, soil permeability appears to have a controlling influence on groundwater salinity. Soils with a low hydraulic conductivity have slower water movement though the soil, allowing evapotranspiration to concentrate the infiltrating rainwater. Any aeolian accessions of salt will be readily flushed away. Thus, the underlying groundwater will have a higher salinity.

Keywords: dryland salinity, bores, geochemistry, hydrogeology

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Comparison of methods for the determination of reduced inorganic sulphur in acid sulphate soils Santomartino, S.L. Department of Earth Sciences, La Trobe University, Bundoora VIC 3083, Australia.

The management of acid sulphate soils requires analytical methods that provide accurate data on the quantity of reduced inorganic sulphur within a soil, as it is this fraction that produces acid upon oxidation. This study uses sulphidic Coode Island Silt samples to compare common analytical methods including POCAS (Peroxide Oxidation-Combined Acidity and Sulphate) which consists of TSA (Total Sulphidic Acidity), Spos (Peroxide Oxidisable Sulphur), TOS (Total Oxidisable Sulphur) and chromium-reducible sulphur. The determination of total sulphur by Leco sulphur is strongly correlated with, but slightly less than, that analysed by XRF. Comparison of soil sulphide content by chromium-reducible sulphur, TSA and TOS methods indicates that TOS values are substantially higher than both other methods. The problem with the TOS method lies in the sulphate extraction procedure. Hot distilled water and HCI are commonly used as extractants, however hot distilled water fails to remove organic sulphur, thereby overestimating the sulphide content of the soil. Leco carbon analyses verify that a substantial proportion of organic matter exists within the samples. The HCI extraction process, which uses Ion Chromatography to analyse for sulphate, produces highly inaccurate results due to the interference of the sulphate peak by the chloride peak during analysis. An alternative method involving HCI extraction and XRF analysis of the soil residue is currently being undertaken. The use of KCI to extract sulphate generally produces values similar to the hot distilled water method. The sulphidic content measured by TSA is strongly correlated with, but slightly higher than that determined by the chromium-reducible sulphur method. This is attributed to the use of hydrogen peroxide in the TSA method, which oxidises organic matter to organic acids in addition to oxidising sulphides. These preliminary findings indicate that the chromium-reducible sulphur method is the most suitable technique for the analysis of acid sulphate soils. Keywords: analytical techniques, acid sulphate soils, sulphide, sulphate, organic sulphur

A TMOSPHERE

AND THE

ENVIRONMENT

The effects of pH and adsorption on metal speciation and mobility and its relation to heavy metal hyperaccumulation by native Australian flora Menon, P., McPhail, D. C. & Hallam, N. Department of Earth Sciences. Monash University. Clayton VIC 3800. Australia.

The purpose of this research is to establish a clearer understanding of the relations that exist between heavy metal activity, speciation, availability and their uptake into plants. A combination of experimental modeling and field studies using toxic heavy metals (cadmium, lead, mercury, copper, chromium and zinc) will be carried out. Some of the external factors that will be investigated comprise pH, redox, metal concentration and adsorption. Mathematical and geochemical (PHREEQC) models will be used to vary these factors and emulate the conditions under which experimentation will be taking place. The variations will help determine the possible heavy metal speciation that would occur in both soil and hydroponic based experiments under those conditions. The rhizosphere, soil solution and the plant material will also be analysed by Inductively Coupled Plasma Mass Spectroscopy (ICPMS). In the experiments, native Australian flora {Eucalyptus, Acacia, Casuarina, Atriplex) adapted to adverse environmental conditions will be used. Scanning electron microscope (SEM) techniques will help ascertain the distribution, concentration and species of heavy metal taken up into the plant tissue. Information derived from these data will be used to determine and understand how these varying environmental conditions and heavy metal speciation affect their bioavailability in the medium, and how this might affect their acceptance, accumulation and storage within such plants. Preliminary experimentation showed that, for lead and cadmium, the variation in soil solution pH affected the quantity of heavy metal accumulated into selected native Australian flora. ICP-MS analysis of plants subjected to a known soil solution spiked with 200 inmol of lead and cadmium show that the quantities of lead and cadmium accumulated into plant sections (roots, stems and leaves) varied in the selected woody species. A. tieteroclita, A. saligna, and A. burbanyana also showed no or little signs of phytotoxicity upon accumulating the toxic heavy metals into their tissues under both acidic and alkaline pH conditions. It is possible that these species have the potential to phytoextract toxic heavy metals in high quantites. Keywords: Hyperaccumulation, phytoremediation, phytoextraction, phytomining,

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Evaluating categorical rainfall forecasts of Melbourne using various skill scores. Phan, T. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia.

Many skill scores are used in the evaluation of categorical forecasts of meteorological parameters such as rainfall. Currently, common skill scores, e.g. Hanssen-Kuipers discriminant (HKSS) and Heidke (HSS) skill scores, are used to evaluate forecasts, along with other less common skill scores such as the Critical Skill Index (CSI) and Probability of Detection (POD). Recently, the Odds Ratio (ORSS) skill score, widely used in the medical studies, has been shown to be suitable for evaluation of categorical forecasts in the meteorological field. No skill scores completely quantify forecast skill due to the many aspects of forecast 'accuracy' or 'quality'. Consequently, each skill score has its advantages and disadvantages when applied to forecast evaluation. Using rainfall data from the Bureau of Meteorology (BoM) for Melbourne, Australia, the advantages and disadvantages of the most common skill scores are illustrated, along with the newly suggested ORSS. Trends in forecast skill are analysed. In recent years, skill at forecasting "rain/no rain" situations has increased significantly, but the same could not be said about the forecasting of "extreme" rainfall events (i.e. > 10 mm). Keywords: skill score, categorical forecasts, Hanssen-Kuipers discriminant, Heidke, Odds Ratio.

A TMOSPHERE AND THE ENVIRONMENT

Processes controlling groundwater chemistry and identification of water resource vulnerability: Yarra Catchment, Victoria ^'^Tweed, S., ^ ^ W e a v e r , T . R. & ^ ^ C a r t w r i g h t , I. ^Hydrogeology and Environment Research Group (HERG) ^School of Earth Sciences, The University of l\/lelbourne, Parkville VIC 3010, Australia. ^Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

Assessment of processes controlling groundwater chemistry is essential for water resource management and identification of water resource vulnerability. The Yarra Catchment (4045 km2) extends 120 km east of Melbourne and is considered a significant area for Melbourne's water resources. This catchment is characterised by fractured rock aquifers, a high baseflow component, high, localised recharge, and diverse land use. Therefore, identification of processes controlling groundwater chemistry is important for determining areas where groundwater, and inevitably surface water, are most vulnerable to contamination. The regional groundwater flow system is affected by high, localised recharge in areas of high surface elevations. Hence, a local groundwater flow system is investigated in detail to aid regional interpretation. Processes causing spatial heterogeneity of groundwater chemistry include the extent of recharge, water-rock interaction and land use processes. At high surface elevations Na:CI = 1, and direct recharge of precipitation into sedimentary or granitic aquifers results in groundwater with lower TDS values 50 to 100 mg/L), compared to groundwater where precipitation directly recharges younger basaltic aquifers (TDS -- 100 to 150 mg/L). At lower surface elevations Na:CI ratios decrease, and major ion concentrations, pH and TDS are significantly higher (TDS -- 400 to 500 mg/L) reflecting cumulative effects of water-rock interaction and agricultural land use lower in the catchment. Groundwater samples were also analysed for stable isotopes, the d6180 and d62H values (-6.2 to 5.50, and -38.1 to 32.20 VSMOW respectively) lie on the Melbourne Meteoric Water Line, indicating that groundwater has not been subjected to evaporation or high-temperature water rock interaction. Slightly lower d5180 and d62H values for groundwater compared to nearby surface water indicates that recharge of colder rains occurs preferentially, both on local and more regional scales. Keywords: water resources, recharge, water-rock interaction, land use, stable isotopes.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 14*'' Victorian Universities Earth Sciences Conference, Monash University, September 2000

SYMPOSIUM: CRUSTAL/MANTLE DYNAMICS

CRUSTAUMANTLE DYNAMICS

Role of fluids in the metamorphism of the Alpine Fault Zone, New Zealand Vry, J.K., ^Storkev, A.C. & ^Harris, C. ^ School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand ' Department of Earth Sciences, La Trobe University, Bundoora VIC 3083, Australia. ' Department of Geological Sciences, University of Cape Town, South Africa.

Models of fluid/rock interaction in and adjacent to the Alpine Fault in the Hokitika area, South Island, New Zealand, were investigated using hydrogen and other stable isotope studies, together with field and petrographic observations. All analysed samples from the study area have similar whole-rock 6D values = - 5 6 to - 3 0 % o , average = - 4 5 % o , n = 2 0 ) , irrespective of rock type, degree of chloritisation, location along the fault, or across-strike distance from the fault in the garnet zone. The green, chlorite-rich fault rocks, which probably formed from Australian Plate precursors, record nearly-isothermal fluid/rock interaction with a greyschist-derived metamorphic fluid at high temperatures, near 4 5 0 - 5 0 0 ° C . 5D of water in equilibrium with the green fault rocks (50^20. green)) 's 18%o; 8D of water in equilibrium with the greyschists and greyschist-derived mylonites (5DH2o,grey) "s 19%o at 5 0 0 ° C . 5 0 ^ 2 0 , g r e e n -^7%o^, 5 0 ^ 2 0 . g r e y "S -14%o at 4 5 0 ° C . There is no indication of an influx of a meteoric or mantle-derived fluid in the Alpine Fault Zone in the study area. The Alpine Fault Zone at the surface shows little evidence of late-stage retrogression or veining, which might be attributed to down-temperature fluid flow. It is probable that prograde metamorphism in the root zone of the Southern Alps releases metamorphic fluids that, in some regions, rise vertically rather than following the trace of the Alpine Fault up to the surface, due to the combined effects of the fault, the disturbed isotherms under the Southern Alps, and the brittle-ductile transition. Such fluids could mix with meteoric fluids to deposit quartz-rich, possibly gold-bearing veins in the region ~ 5-10 km back from the fault trace. These results and interpretations are consistent with interpretations of magnetotelluric data obtained in the South Island GeopHysical Transects (SIGHT) program. Keywords: Alpine Fault Zone; stable isotopes; fluid flow; New Zealand; South Island GeopHysical Transects (SIGHT) program.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62

Victorian Universities Earth Sciences Conference, Monash University, September 2000

Continental margin escarpment retreat: can we measure this or not? Dimas, V.-A. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia.

The aim of this project is to quantify rates of escarpment retreat using in-situ-produced cosmogenic isotopes and low temperature thermochronologic data. Major escarpments are primary morphotectonic features formed by the process of continental rifting. They are characteristic topographic features of many continental margins around the world. The origin, behaviour and preservation of continental margin escarpments have been highly controversial over the last century. The SE Australian escarpment is a prime example and has played an important role in the debate. The combination of cosmogenic isotope analysis of quartz with (U-Th)/He and fission track analysis of apatites offers a powerful opportunity to finally test the range of conflicting models of escarpment evolution. Cosmogenic nuclides (e.g. ''°Be & A!) are produced in-situ in bedrock and surficial materials by interaction of highly energetic cosmic-ray-derived particles with nuclei in rock forming minerals. The concentration of these cosmogenic nuclides tells us about 'dwell time' in the attenuation zone (top few metres of surface) from which we can determine the rate of denudation. This technique has been successfully used in determining the rate of erosion along the southern African continental margin. Helium diffusion in U- and Th-rich minerals (e.g. apatite) is able to provide constraints on the thermal history of the sample. (U-Th)/He is a new tool for measuring the low temperature thermal history of the upper crust. The low closure temperature of helium dating of apatites provides us with a valuable thermochronometer. The method of apatite He thermochronometry gives insight to the cooling of apatites from --75° C through to -40° C. Currently cosmogenic isotope and (U-Th)/He analysis are being carried out on samples collected from southern NSW between the coast and Brown Mountain to constrain controversial debate on the evolution of the SE Australian continental margin. Together with already existing fission track data this will help in further constraining the model. By using these techniques, a more powerful means of interpreting the evolution of the landscape will become known. Keywords:

escarpment retreat, erosion rate, cosmogenic thermochronology, (U-Th)/He, apatite fission track

isotopes,

low

temperature

CRUSTAUMANTLE DYNAMICS

Trace element characteristics of mafic and ultramafic metaigneous rocks from the 3.5 Ga. Warrawoona Group: evidence for plume-lithosphere interaction beneath Archaean continental crust? Bolhar, R., Hergt, J. & Woodhead, J. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia.

Magnesian- to Fe-rich tholeiitic basalts represent the dominant lithology in the Marble Bar Greenstone Belt, E-Pilbara Craton, and are locally associated with komatiitic basalts and rare komatiitic cumulates. Based on trace element characteristics, the extrusive and intrusive rocks from all three major stratigraphic units can be subdivided into LREE enriched and unfractionated to weakly LREE depleted groups. The former group is characterized by La/Smpm = 1.7-4.6, Gd/Ybpm = 1-23.2 and Nb/Thpm = 0.1-0.5, while the latter rocks possess ratios of La/Smpm = 0.5-1.7, Gd/Ybpm = 0.8-1.9 and Nb/Thpm = 0.4-1.3. Nb/La -Nb/Th relationships in the LREE enriched samples indicate 7-28% contamination by crustal material similar in composition to Pilbara granitoids. LREE enrichment and strong negative HFSE anomalies, along with MgO = 2.2-22.0 wt% and Si02 = 39.263.5 wt%, have been observed in numerous Archaean greenstone belts, and can be successfully modeled in this study by AFC processes. In contrast, strong HFSE depletion combined with unfractionated to slightly depleted LREE in rocks of the latter group require different processes. Melting of mantle material previously depleted by melt extraction, enrichment of LILE and LREE relative to the HFSE in an arc-like environment and HFSE fractionation as a result of garnet retention in the melting source cannot account for negative Nb, Ta, Ti, P and strong positive Pb anomalies. Introduction of small amounts of crustal material into a depleted or primitive mantle, as possibly indicated by Nb/Ta ratios between 12 and 18, also fails to reproduce the trace element abundances of the second group of rocks. Recycling of oceanic crust previously processed through a subduction zone (low Th/Nb, La/Nb) and sub-arc lithospheric mantle (high Th/Nb, La/Nb), and subsequent mixing into the Archaean mantle has been recently invoked by several workers (e.g. Kerrich et al., EPSL, 168, 101-115; 1999) to explain the origin of compositionally similar volcanic greenstones in the Superior Province (Canada). However, this concept is problematic for two reasons: (1) Modern oceanic crust is typically associated with overlying terrigenous/ pelagic sediments, both of which are introduced into the mantle via subduction. Mixing with mantle and subsequent partial melting invariably produces compositions with HFSE depletion and LREE enrichment at low to moderate degrees of melting. (2) Mixing of subduction-modified lithosphere into the mantle followed by melting should be detectable in volcanic rocks with strong depletions in elements such as Nb and Ti, but increased abundances in the LILE and LREE (La/Smpm » 1). Compositionally, the Warrawoona meta-igneous rocks resemble compositions found in modern oceanic plateaus (e.g. Broken Ridge) which incorporated variable amounts of continental lithospheric mantle (CLM). Variability in trace element ratios (e.g. Nb/Ta, Ce/Pb, and Nb/U) may reflect source heterogeneity or the coexistence of tectonically accreted oceanic fragments with differing petrogenetic histories. However, well-defined co-variations in major and trace elements of samples from all three major stratigraphic units point to a common magmatic origin. In an attempt to link Archaean rocks to present day analogues, we conclude that the spatial association of ultramafic and mafic volcanics and crustally contaminated high-Mg, Fe rocks most resembles melting of a plume head with incorporation of CLM-components and volcanic outpouring within a (rifted?) continental environment. Support for the existence of preexisting continetal crust comes from published studies which report on xenocrystic zircons in basalts, underlying granitoids and sediments of pre-Warrawoona age and mafic inclusions within granitoid bodies. Temporal decreases in La/Smpm and Nb/Thpm ratios, along with unfractionated HREE may be interpreted as adiabatic upwelling of plume material and a decreasing influence of the lithospheric component ('CLM erosion'), with melt segregation outside the garnet stability field (< ca. 75km). Keywords: Pilbara Craton, crustal contamination, Th-U-Nb systematics, subcontinental lithosphere, mantle-plume

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Missing pieces of the Proterozoic Jigsaw; The Peake and Denison Inlier, South Australia, and its part in the Proterozoic Evolution of Australia. Richardson, J.A. Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

The Peake and Denison Inlier is located 000km north of Adelaide and comprises Early to MidProterozoic metamorphic basement rocks and late Proterozoic sediments of the Adelaide Geosyncline (Hopper 1998). Reconstructions of Proterozoic Australia by earlier authors have not considered any major form of rotation during the rafting and amalgamation of the Australian continent during the Proterozoic. Recent work (Giles and Betts, in press) have rotated the South Australian Craton 55® counter clockwise and translated it eastward with respect to its present position. A consequence of this is that the northern part of the Gawler Craton, including the Peake and Denison Inlier, may be a relict part of the North Australian Craton. This hypothesis is tested in this project. There is complex structure of the basement rocks with four (4) deformational events recognized. D1 is a layer parallel S1 foliation, with no recognized folding. During D2east-west oriented isoclinal F2 folds formed. An associated axial planar S2 layer parallel crenulation cleavage is formed. East-west oriented F3 isoclinal folds re-fold F2 folds into a Type 3 interference pattern. S3 is a layer parallel axial planar cleavage. D4 folding is oriented perpendicular to the previous deformation, resulting in north south axial planes, producing gentle warping of the basement, and the overlying Adelaidean sediments. The Peake and Denison Inlier may have been a fragment of the North Australian Craton during the Palaeo to Meso-Proterozoic. The Peake, Denison, and Mt Isa Inliers share a similar structural style of deformation. The two areas also show similarities in geochemistry (Primordial mantle normalised of igneous units) and geochronology (U-Pb zircon) deposition of volcanic units. This may indicate a close proximity of the two areas, and is consistent with the Proterozoic configuration of Giles and Betts (in press).

CRUSTAUMANTLE DYNAMICS

The post-break up history of the Damara Sector of Namibia: a low temperature thermochronology perspective 1,2

Raab, M.J., ^ ^Brown, R.W. & ^Cockburn, H A P .

Unstitut f" ur Geologie and Dynamik der Lithosph " are,Goldschmidtstrasse 3, 37077 G " ottingen, Germany ^School of Earth Sciences, The University of Melbourne, Parl<ville VIC 3010, Australia.

The great escarpment is one of the most prominent geomorphological features in Namibia. It runs parallel to, and 200 km inboard of the present coastline. The escarpment is interrupted by the Damara Zone, some 160 km wide, characterised by NE-SW trending regional lineaments. These lineaments originated in the Neoroterozoic and some of them have experienced multiple episodes of reactivation in the Phanerozoic. This area has been targeted with over sixty apatite fission track sampling sites across all major shear zones. Our new apatite fission track data now provide improved constraints on the chronology of tectonism. These data identify and date reactivation on a subcontinental scale commencing in the Late Cretaceous, and furthermore provide estimates of relative vertical displacements of the continental crust. We now propose a two-stage denudation history of this area since the break up of Gondwanaland with two phases of accelerated denudation: the first phase, influenced by rift-flank uplift due to South Atlantic opening, occurred at 130 Ma and ceased shortly afterwards. A second phase of accelerated denudation commenced 50 Ma later in the Late Cretaceous, caused by a reactivation of at least one of the shear zones, the Omaruru Lineament-Waterberg Thrust. The latter phase is coeval with a very well recorded change in spreading geometry within the Central Atlantic between anomalies C34 and C24. It records the geomorphic response of a global change in plate kinematics with kilometer scale denudation. Subsequent to this later phase of reactivation, average denudation rates diminished to less than 20 m/my., and remained low throughout the Tertiary. In situ-produced cosmogenic isotopes of Be and Al along a transect across the escarpment confirm that low rates of denudation have characterised the western margin of Namibia over the last 1-2 Ma. Deep erosion associated with the period of reactivation has obliterated the escarpment within the Damara zone and the regional topography now rises gently from sea level towards the interior forming a regional ramp of less than 1°. Keywords: analysis

Namibia, tectonics, fission track dating, thermochronology, and cosmogenic isotope Passive margin evolution, landscape evolution

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62

Victorian Universities Earth Sciences Conference, Monash University, September 2000

Structural relationships between the Proterozoic assemblages of the Moonta Subdomain, Gawler Craton, Western Australia Arcaro, H. Australian

Crustal Research Centre, Department VIC 3168,

of Earth Sciences, iVIonash University, Australia.

Clayton

The Proterozoic assemblage in the Moonta area South Australia has undergone at least four stages of deformation. The oldest unit is the Doora Metasediment (1741 ± 9 Ma) (Zang, pers. comm.), which outcrops on the coast at Point Riley. Metamorphosed ash flow t u f f , ignimbrite, and rhyolite of the Moonta Porphyry (1737 ± 5 Ma) (Zang, pers. comm.) are not exposed in the region. Analysis of drill core shows that these units preserve part of the deformation history of the Doora Schist. The Doora Schist and the Moonta Porphyry are intruded by the foliated and unfoliated suites of the Tickera Granite, which yield emplacement ages of 1600±16 Ma and 1575 ± 7 Ma (Zang, pers. comm.) respectively. Southwards, North Beach has outcrops of polydeformed gneiss and migmatite. The highest-grade (amphibolite facies) Proterozoic rocks are preserved at North Beach. The earliest phase of deformation is preserved within the metasedimentary unit and is characterised by alignment of quartz and feldspar aggregates. The second phase of deformation (D2) involved tight to isoclinal folding and the development of an intense gneissocity and schistocity. D2 structures are subsequently refolded during Ds and D4. These events involved the development of a minor fracture cleavage, which is generally manifest in the metasediments as a spaced fracture cleavage. Da fold structures are open. D4 folds are characteristically very open warps (interlimb angle of - 1 7 0 ° ) with approximate n o r t h - s o u t h trending axial trace. At Point Riley, the Doora Metasediments preserve slaty S I cleavage which is overprinted by tight to isoclinal F2 folds with an axial planar schistose cleavage. D3 and D4 are open structures and are characterised by fracture and spaced cleavages. Rock types at Point Riley are separated from those at North Beach by a strike-slip fault ( - 5 m wide). It is possible that the North Beach rocks are part of the Lincoln Complex and were juxtaposed with the rocks at Pont Riley during later movement along the strike-slip fault. Further work is needed. Keywords:

Moonta Subdomain, Hiltaba Suite, Lincoln Complex, Kimban Orogeny, Gawler Craton, structure, tectonics.

CRUSTAUMANTLE DYNAMICS

Mountains and molehills, how different are they. Mechanical implications of residence time in the brittle crust Belton, D. X. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia.

Crustal response to stress and strain has long been addressed by means of a two-phase crust. Within the ductile lower crust, temperature and strain rate are key parameters in a power-law rheological model, while in the upper, brittle crust, the response is defined in terms of a linear, friction law. For the brittle crust, this produces a threshold response independent of time - once the friction envelope is intersected, failure occurs. Here I focus on the upper 10km and present a model that treats the brittle crust as a viscoelastic solid, such that it responds elastically to sudden application of stress but relaxes in response to long-term stress. The viscous analogy is used to describe the evolution of stress in the rock as it undergoes static fatigue or sub-critical crack-growth. Static fatigue is the process of creep strain by microcrack growth. It acts to reduce the stress while propagating cracks and occurs at stresses significantly below ultimate failure. An increase in crack density also changes the elastic properties. In this viscoelastic analogue, stress reduces over time as microcracks propagate. Thus, rock properties such as rigidity and porosity will vary as a function of burial duration while the rock sits at depth in the crust. Brittle creep in this model is most pronounced at depths close to the brittle-ductile transition but decreases in a nonlinear fashion near the surface. The rate of exhumation of the rock column controls the exposure time to a given depth/stress regime, such that long crustal residence should produce a softer, more porous rock, highly susceptible to the actions of weathering and erosion. In contrast, a low residence time caused by rapid exhumation should produce tougher rocks, resistant to the attack of surface processes and with the ability to produce substantial edifices. The emphasis here is on the evolution of physical properties rather than the end point of ultimate failure. Thus, a mechanical stratigraphy can be defined for a crustal section where we have thermochronological control such as apatite and zircon fission track data.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, IVIonash University, September 2000

Late Miocene uplift across the Southeast Australian Margin Dickinson, J. A., Wallace, M. W., Holdgate,G. R., Gallagher, S. J. & Thomas, L. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia.

An unconformity occurs at around the Mio-Pliocene boundary in the onshore and near-shore portions of the Otway, Port Phillip/Torquay and Gippsland basins of SE Australia. The unconformity is angular (generally < 1-5° angularity) in nature, with the underlying Miocene units having been deformed (gentle folding and reverse faulting) and eroded prior to deposition of the Pliocene succession. The unconformity also marks the change from Oligo-Miocene cool water carbonate- and brown coal-dominated successions to more siliciclastic-rich Pliocene sediments. The Mio-Pliocene boundary therefore represents a period of significant regional uplift in the SE Australian basins. The amount of section removed is greatest around the Otway and Strzelecki Ranges in Victoria, where up to a kilometer of section may have been removed. In most onshore sections of the Victorian basins, hundreds of meters of section have been eroded. In distal offshore locations, the boundary becomes conformable. The timing of uplift and erosion is best constrained in the Otway and Port Phillip basins, where Late Miocene (N16, - 1 0 Ma) sediments underlie the unconformity and earliest Pliocene ( - 5 Ma) sediments overlie it. This timing coincides with a change in the dynamics of the Australian plate, beginning at around 12 Ma. SE Australia is presently under a NW-SE compressional regime and this has probably persisted since the Late Miocene. In the basins (as opposed to the basement-dominated highland areas), the Late Miocene uplift event is more significant than later Pliocene-Recent uplift. We propose the use of the term 'Moorabool Uplift' for this period of Late Miocene tectonism, to distinguish it from the younger Kosciusko Uplift of the Eastern Highlands.

CRUSTAUMANTLE

DYNAMICS

Modelling the fracture of rock during caldera collapse Gray, J.P. & Monaghan, J J . Department of Mathematics and Statistics, Monash University, Clayton VIC 3166, Australia.

We investigate how rock around a magma chamber fractures during caldera subsidence. Using the method of Smooth Particle Hydrodynamics we look at stress fields around a simplified chamber subject to various external conditions. The aim being to realistically model the events leading to the collapse of the chamber. Our chamber consists of a hole inside a two-dimensional plate of rock. Our numerical method incorporates a statistical damage model that we use to investigate the growth of cracks and fracture. We outline the effects that chamber pressure, geometry and surrounding stresses have on crack formation and subsequent collapse events. Keywords: Numerical Simulation, rock fracture, caldera subsidence

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, IVIonash University, September 2000

Dyke emplacement in Southeast Australia during the Mesozoic Halas, D. Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

During continental breakup, high degrees of mantle melting produce large scale basic volcanic activity near the margins of major zones of crustal extension forming the well known Continental Flood Basalt (CFB) Provinces. In regions of Antarctica, South America and South Africa there exist vast areas of basic Mesozoic igneous rocks which together form the most notable CFB province known as the Ferrar Large Igneous Province. Major rifting and mantle melting episodes during the Middle Jurassic eventually led to the breakup of Gondwana which included the separation of Africa from South America, and Australia, India and Africa from Antarctica. In South Eastern Australia however (with the exception of Tasmania), magmatism was not as extensive and produced only numerous smaller scale regions of Mesozoic igneous activity in the form of basic dykes, plugs, pipes, minor lava flows. It is these dykes and pipes that are the most interesting and of major importance as a result of their style of emplacement and the inclusions they contain. The two main occurrences of this type of intrusion in Victoria are the lamprophyre dykes of Bendigo (including the surrounding goldfields) and the Freestone Creek dykes of East Gippsland, both emplaced during the Middle Jurassic. Collectively these dykes are all highly alkaline and have Mg # in the range of 60-50 suggesting low degrees of fractionation from the source magma. The Bendigo dykes contain mantle xenoliths (fragments of the subcontinental lithospheric mantle incorporated during emplacement) which contain important information regarding the geochemical nature of the mantle below the crust at the time of rifting. Common geochemical signatures in primitive mantle-normalised and chondritenormalised REE data suggests that both the Bendigo and Freestone Creek dykes share a similar melting history and mantle source. Keywords: Mesozoic, basic dykes, Gondwana, rifting, mantle xenoliths, source magma.

CRUSTAUMANTLE DYNAMICS

Timing of Tertiary extension in the Southern Canadian Cordillera: Constraints from fission-track analysis in the Shuswap Metamorphic Complex 1,2

Lorencak, M., ^Seward, D. & ^Burg, J.-P.

^Institute of Geology, ETH Zentrum, Zurich, CH 8092 ^now at: School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia.

Nine zircon and eighteen apatite fission-track analyses have been made in order to determine the low temperature cooling history of the Shuswap metamorphic core complex, western Canada. The zircons vary in apparent age from 53.9 ± 5.6 to 37.5 ± 5.0 Ma and the apatites from 48.5 ± 3.2 to 27.7 ± 3.4 Ma. In the footwall of the detachment faults defining the core complex, the cooling histories show a similarity until temperatures of --250D C were reached at about 45 Ma. From then on, activity on two normal faults, the Columbia River Fault and the Victor Creek Fault, controlled the regional cooling pattern. The ages and the combination of ages fall into four groups and on the basis of the fission-track data, we suggest that the region can now be divided into four thermotectonic units which are the result of differing tectonic controls during regional extension. Additionally, a complete cooling history of the Shuswap core complex can now be reconstructed, using constraints from U-Pb, Rb-Sr and K-Ar age data from several authors as well as the fission track results presented here. Keywords: Fission-track analysis, Canada, Shuswap metamorphic core complex

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

The potential of apatite fission tracit dating of vertical profiles in Namibia and the implications for passive margin evolution ^ ' ^ R a a b J V U L & ^'^Brown, R . W . ^Institut f" ur Geologie und Dynamik der Lithosph " are, Goldschmidtstrasse 3, 37077 G " ottingen, Germany ^School of Earth Sciences, The University of Melbourne, Parl<ville VIC 3010, Australia.

The on- and offshore geology of the Namibian passive continental margin has experienced kilometer scale erosion since South Atlantic opening in Lower Cretaceous times. A vertical apatite fission track profile of four samples in the Namibian highland has been analysed to constrain the low temperature thermal history of that area since the Pan-African Damara Orogeny at about 550 Ma. As a temperature sensitive thermochronological technique apatite fission track analysis is a powerful tool in constraining the low temperature history of rocks over a range of 60-110°C. These temperatures, depending on the geothermal gradient, equal a burial depth of 3-5 km so the method can reconstruct the cooling history of rocks as they approached the surface in response to erosion and tectonic processes. The four apparent apatite cooling ages are taken over a vertical distance of 300 m from the Windhoek Graben 40 km north of Windhoek. Forward modelling of the age and track length distribution has shown that these samples experienced high palaeotemperatures from ca. 90 to 95°C in the Late Cretaceous. This information was used to calculate the palaeogeothermal gradient at that time (20°C/km) which gives an estimate of the sedimentary cover of about 4.5 km which has been removed over a few million years in the Late Cretaceous. It was previously thought that the Namibian highland has been exposed at the surface more or less since the Permo-Carboniferous. In fact the samples provide evidence for a post Carboniferous reburial history of several kilometers followed by a short period of accelerated denudation in the Late Cretaceous at about 70 Ma. This might imply a far larger extent of the Etendeka flood basalts (132 Ma) and/or an underestimated sedimentary Karoo (Permian to Jurassic) thickness. The wider importance of these four data is that they detect the geomorphic impact of a global change of plate motion along the passive margin of Namibia which is known from a more comprehensive dataset of another 180 fission track samples over a wide area of the country. Keywords: Namibia, tectonics, fission track dating, thermochronology, passive margin evolution, landscape evolution, Windhoekgraben

CRUSTAUMANTLE DYNAMICS

Large scale fluid infiltration in palaeozoic shear zones of the Arunta Inlier, Central Australia. Read, C. M. & Cartwright, I. Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

This project involves studying the internal plumbing of intracontinental orogenic belts Uom the perspective of fluid-rock interaction. Importantly, fluid-rock interaction processes involving fluid flow through metamorphic belts affects the tectonic evolution of the belts. So, therefore, the retrogressive evolution (post-peak metamorphism) of metamorphic provinces can be assessed by studying the areas of focussed deformation and fluid infiltration. The study area for this project is the Arunta Inlier in central Australia, which is a multiply deformed and metamorphosed Proterozoic Inlier dissected by many faults and shear zones. Faults and shear zones play an important role as fluid conduits during reworking of metamorphic provinces owing to increased transient permeabilities and rehydration reactions accrued during active faulting and shearing. Therefore, much of this project is focussed on looking in detail at fluid involvement during shearing. This presentation summarises the results from several shear zones in the Arunta Inlier. The shear zones form part of a crustal scale pop-up structure which exhumed the northern Arunta Inlier during the Palaeozoic Alice Springs Orogeny (300-400Ma). Results from oxygen isotope analyses suggest some of the fluids involved in shearing during this event came from the surface. This involves a major downward migration of possibly 10km into the ductile crust. Therefore, the surface derived fluids have infiltrated the middle crust without loosing their isotopic signature through water-rock interaction. This implies large volumes of fluids are involved, which is also reflected in the major geochemical changes seen across the shear zones. So there is evidence in central Australia for large volume and large scale fluid flow through shear zones active during Alice Springs age reworking of the Arunta Inlier. Keywords: Fluid-rock interaction, metamorphism, shearing, oxygen isotopes.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Physical modelling of back-arc extension S c h e l l a r t , W . P . , ^Listen G.S. & ^ ^ Jessell, M.W. ^Epsilon Laboratory ^Australian Crusta! Research Centre, Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

Back-arc extension is related to rollback of the subducting plate, where rollback results in the creation of space at the trench. This space is filled by the overriding plate under the release of potential energy, leading to extension in the back-arc region. We have performed 3-D physical experiments to model the process of back-arc extension, where we have investigated three basic models. All three models consist of a two-layered brittle-ductile system, situated in a rectangular box. In one of the four sides of the box a removable part is situated which, after removal, allows the layered system to flow out. In the first model back-arc extension is unconstrained and the outward flow is allowed in all directions. In the second model, the outward flow is allowed in one direction, perpendicular to the gate. In the third model, the outward flow is asymmetric and rotates around a hinge, where the opening of the gate is analogous to an opening door. The resulting strain pattern of these three basic models is significantly different. The general patterns of deformation of these different models can be compared with natural examples of back-arc extension, where they can give new insights or provide alternative interpretations to structural patterns observed in back-arc regions. The first model can be compared the Carpathian Orocline, providing an alternative interpretation for the occurrence of strike-slip faults in the western region of the Pannonian Basin, compared to the possible explanation of these structures by the lateral escape in the Eastern Alps. The second model can be compared with the Mariana Arc, where extension is predominantly in one direction, resulting in a convex shaped Mariana Ridge, Mariana Trough and West Mariana Ridge. The third model can be compared with the New Hebrides Arc, where the back-arc region clearly shows asymmetric extensional structures.

Keywords: Back-arc extension, subduction, rollback, physical modelling.

CRUSTAUMANTLE DYNAMICS

West bounding faults associated with the Heathcote Greenstone Belt, Victoria: is this only true for the central segment?

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Shawcross, M. Department of Earth Sciences, La Trobe University, Bundoora VIC 3083, Australia.

The Heathcote Greenstone Belt (HGB) in central Victoria lies in the Cambrian-Ordovician BendigoBallarat Zone is approximately 1000m thick, has predominantly meridonal north-south strike of between 110km and 150km. It is divided into three segments (northern, central and southern segments). The Mt. Ida-Mclvor-Mt. William fault defining the eastern margin of the Heathcote Greenstone Belt (HGB) juxtaposes Silurian-Devonian volcanics and sediments of the Melbourne Zone with the Bendigo-Ballarat Zone and its location is characterised by the occurrence of patches of salt resistant Spiny Bull Rush along the fault. Seismic reflections indicate that faulting has steeply dipping reverse movement and listric form with potentially around 14 kilometres of displacement. The western fault, identified in the central segment has conjecture surrounding its continuation into the northern and southern segments. The accepted interpretation is that the northern and southern segments are conformable from greenstones through shale and volcaniclastic sediments and chert into the lower Ordovician greywacke-shale succession of the eastern Bendigo-Ballarat Zone. Recent mapping in the northern segment north of the Tolleen mine has identified ferruginisation and patchy manganese staining with associated brecciated quartz between outcrops of upper Cambrian Goldie Shales and lower Ordovician, micaceous sandstone . These features suggest that the west bounding faulting of the HGB may extend into the northern segment. Within the ferruginised breccia zone, foliation appears to parallel the stratigraphy within the HGB and while Spiny Bull Rush is present around the identified areas, landscape relief is low and extension of saline flats continues into the Cambrian as opposed to trending westward away from the Cambrian ridge.

Keywords: Heathcote Greenstone Belt, Spiny Bull Rush, ferruginisation

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Island-arc and accretionary prism collision, western/central Lachlan Orogen, South Eastern Australia ^Spaggiari, C. V., ^Gray, D. R. & ^Foster, D. A. ^Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia, ^Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, U.S.A.

Regional profiles across the western and central Lachlan Orogen (LO) show that these two subprovinces have roughly opposite senses of tectonic vergence, and have undergone collision. The boundary between the two subprovinces can be defined as a suture zone, and records the collision of an Ordovician/Silurian accretionary prism (Tabberabbera Zone, central LO) with a Cambrian, oceanic island-arc (as part of the Melbourne Zone, western LO). Structural relationships, metamorphic conditions, lithological associations, and geochronology all indicate that: 1) the early Palaeozoic history (Cambrian - Silurian) of this region developed in an oceanic setting, and the basal Cambrian "greenstones" are oceanic crustal rocks (i.e. ophiolitic fragments), 2) the Tabberabbera Zone can be interpreted as an accretionary prism, 3) the frontal portion of the Tabberabbera Zone (Governor Fault Zone) consists of an underplated tectonic melange which includes blocks of blueschist metavolcanics, and an accreted ophiolitic sliver, 4) the eastern margin of the western LO (Mount Wellington Fault Zone) contains fault slices/duplexes of Middle to Late Cambrian island-arc volcanics and associated sedimentary rocks, which collided with, and are at least partially overthrusted onto, the Tabberabbera Zone. Several factors make it clear that the Cambrian "greenstones" represent oceanic crust. These include conformable sequences of pillow basalt, chert/shale and turbidite, a predominance of tholeiitic basalts, dolerites, boninites and associated ultramafics, and their geochemical signatures (MORB and backarc basin) (e.g. Howqua, Dolodrook). In contrast, andesites with arc geochemical signatures are associated with andesitic breccias and volcaniclastics, some of which include limestone olistoliths (e.g. Jamieson, Licola). U-Pb zircon ages, in conjunction with fossil data, demonstrate that the oceanic crust formed between --510 and 495 Ma (Middle to Late Cambrian). Features that are characteristic of accretionary prism complexes can be found throughout the Tabberabbera Zone. Regional scale, southwest vergent folds are cut by reverse faults with tectonic melange zones that show evidence of deformation of partially lithified sediments, stratal disruption, and sedimentary blocks in a scaly mud-matrix. One major fault zone consists of a large, fault-bounded serpentinite body associated with pillow basalt, ophiolitic sedimentary rocks, cherts/shales, and melange/broken formation (Dolodrook inlier). The structural architecture and metamorphic PT (low grade/intermediate to high pressure) of the frontal portion of the Tabberabbera zone (Governor Fault Zone) is consistent with underplated sequences and ophiolitic slivers documented in accretionary complexes in other parts of the world, (e.g. Kodiak accretionary prism, Alaska). Collision between the two fault zones has produced the dominant structural trend (NW/SE), including late folding of the melange zone and overthrusting of the oceanic arc and related sedimentary rocks. The timing is estimated to have been at around 395 to 380 Ma, and correlates with what has been termed the Tabberabberan Orogeny. This "event" is interpreted as an island arc and accretionary prism collision following closure of the backarc or marginal basin that formed in the Middle to Late Cambrian.

Keywords: Cambrian ophiolites, melange, collisional margin

CRUSTAUMANTLE DYNAMICS

Apatite fission track dating of the Northern Western Shield, Western Australia ^Weber, U.D., ^Kohn, BP., ^Nelson, D.R. & ^Gleadow, AJ.W. ^Australian Geodynamics Cooperative Research Centre, School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia 'Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia

The investigation of the thermotectonic evolution of the northern part of the Precambrian Western Shield of Western Australia using apatite fission track (AFT) thermochronology is the main focus of this study. The study area encompasses Precambrian rocks of the Pilbara Craton and the northern part of the Yilgarn Craton including the Narryer Gneiss Complex. AFT data, mostly from the Archaean cratons reveal cooling ages ranging between 260±8 Ma and 400±20 Ma. Mean confined horizontal track lengths fall between -12 and 13 |jm with standard deviations ranging from 1.1-2.2 |jm. Forward modelling of time-temperature history paths for representative samples reveals a period of regional cooling of at least ~50«C in the late Palaeozoic. Most paths also show a second period of cooling of • 2 5 0 from temperatures <~80-85°C. This later cooling episode occurred in the Mesozoic but its timing is less well constrained. Assuming that the average present day geothermal gradient of ~18±2°C.km-"' was prevalent since the late Palaeozoic, then the minimum of ~75°C of cooling predicted by the fission track modelling suggests overall denudation of at least -3.7-4.6 km of section since that time. Phanerozoic basins (Perth, Carnarvon and Canning) adjacent to the north and west of the northern Western Shield mostly continue offshore and form complex structures containing up to -15 km of predominantly clastic sediments of early Ordovician to late Cretaceous age. The basins are likely to have been depocentres for much of the detritus derived from the denudation inferred from the cooling recorded by the AFT. Possible causative events which could be linked to the observed late Palaeozoic cooling are tectonism related to the collision of Gondwanaland with Laurussia in Carboniferous time forming the supercontinent Pangea or a possible far-field effect related to the Alice Springs Orogeny. Further low temperature thermochronological studies using the ""^Ar/^'Ar and (U-Th)/He systems will provide additional information on the post Mesoproterozoic thermal history of the northern Western Shield. Keywords: Low temperature thermochronology, apatite fission track dating, denudation, Western Australia, tectonics

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 14"^ Victorian Universities Earth Sciences Conference, Monash University, September 2000

SYMPOSIUM: ECONOMIC GEOLOGY

ECONOMIC GEOLOGY

The Tarim Basin is a frontier petroliferous basin, covering an area of 560,000 km^. It lies in the southern part of Xinjiang Province, NW China and is bounded by three mountain ranges, the Kunlun, Tienshan and Altyntagh Mountains to the south, north and southeast respectively. Tectonically, the basin is between the Tienshan and Kunlun fold belts and also bounded by the Altyntagh deep fault in the southeast. Recent geological surveys and petroleum exploration demonstrate that the potential reserves of oil and gas for the basin is up to 19.1 billion tonnes, which accounts for one-fifth of the total reserves in China. To date, more than ten high rate productive oil fields have been discovered at Tarim since 1988. Most of these trap structures are hosted at the latest Devonian to Permian rocks. Improvement of biostratigraphy is critical for increasing and refining resolution of stratigraphical correlation and correlation of geological events, which in turn is essential for resources exploration. As such, this study establishes the composite latest Devonian to Permian biostratigraphic and chronostratigraphic frameworks based on the detailed marine and terrestrial fossil data from outcrops as well as boreholes. Four Supersequences reflecting the 2nd cycles were recognized by microfacies analysis across the basin. Of these, the first is the Bachu Supersequence of the latest Famennian to earliest Visean, within which three sequences have been recognizable. The overlying Heshilafu Supersequence is early Visean to Serpukhovian, from which two sequences are distinguished. The Bashkirian-Moscovian Azigan Supersequence is composed of five sequences. The Keziliqiman Supersequence corresponding to the Kasimovian to the Artinskian comprises three sequences. The remaining marine sediments have been assigned to the Qipan Supersequence, including three sequences. Thirty (30) palaeogeographical maps from the latest Devonian through to Early Permian have been reconstructed for the basin. By means of sequence stratigraphical and palaeogeographical analysis, we conclude that five major transgressions and four major regressions took place in Tarim through the Late Palaeozoic. The transgressive events occurred at the late Famennian, early Tournaisian, late Visean-Serpukhovian, Moscovian, and earliest Artinskian respectively, while the regressive events correspond to the earliest Visean, latest Serpukhovian, earliest Kasimovian, late Artinskian and late Kungurian respectively. After the Kunguhan, deposition within the basin was characterized by terrestrial sediments. Several most important reservoirs are composed of the quartz sandstones of the Lowstand System Tract (LST); the black mudstones and shales of Transgressive System Tract (TST) containing maximum flooding surfaces (mfs) are also ideal source rocks; while the gypsum beds of the Highstand System Tract (HST) are actually the cap rocks for many oil fields.

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Gold occurrence on contrasting structures in the Tasman Flat Gold Field, Central Victoria Rosengren, N. Department of Earth Sciences, La Trobe University, Bundoora Vic 3083, Australia. Detailed outcrop mapping of the Tasman Flat gold field has reveled structural characteristics and stratigraphy similar to that of the main Bendigo field. A major feature of the field is the presence of prominent east west cross-course faults inferred by bedding offset. There are many historic mines in the area with the largest generally clustered around the intersection of north south shear zones and east west cross-course faults. Gold grains recovered from samples from several localities indicate two contrasting gold grain morphologies, nugget shapes (both free and on quartz) and very fine wirey grains. The very fine grains may indicate supergene gold; electron microprobe work is underway to determine if grains of different morphological character also have different compositions. Decrepitometer studies of quartz samples gathered from veins and shears of different orientation show a marked difference in character between some samples and striking similarities in others. This information will be used to determine which "generation" of quartz carried Au. This then becomes a tool for assessing other quartz bearing structures for Au potential and has been applied to the cross-course faults to examine their relationship to other quartz and Au bearing structures. Detailed soil geochemistry carried out by Bendigo Mining N.L. will be used in an attempt to identify and pinpoint anomalies and their sources. A series of RAB holes have also been drilled by BMNL showing lithology, Au grade and quartz %. These will be used to interpret the geology and contour Au anomalies to provide a 3 dimensional view of this section of the Tasman Flat gold field in an attempt to gain an insight into the structural controls in other parts of the field.

Keywords: Gold, structure, decrepitometer, electron microprobe, supergene.

ECONOMIC GEOLOGY

Deformation processes at the Magdala Gold Mine, Stawell. Witham. B.. Wilson, C.J.L. & Miller, J. McL.

School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia. I

The Magdala deposit lies in the most western tectonic corridor, the Stawell Zone, of the Victorian Lachlan Fold Belt. The mine sequence is underlain by tholeiitic basalt domes. Lodes consist of an extensive network of shears and veins that are concentrated in volcanogenic sediments. These are overlain by a thick package of turbidites. Gold is localised where there are contrasts between the rheology and/or geochemistry of the host rocks. The deformation history is primarily controlled by lithological competency contrasts. Large amounts of vein material formed early in the deformation history and had quite different expressions in the different lithological units. This accentuated the already high competency contrasts between adjacent lithological units. These contrasts eventually caused deformation to be limited to narrow zones and they had a strong influence on the final architecture of the ore system. In the past, the deformation history of this deposit has been described as a series of discrete events. Current work indicates that this is difficult to do. In particular, it has become apparent that the different alteration and mineralisation assemblages cannot unambiguously be tied to separate deformation events. Alteration and gold mineralisation phases show repetitive overprinting relationships, implying that the mineralisation processes occurred in cycles over more than one of the broadly defined deformation episodes. In general, the evolution of the system shows that the style of deformation changed from regional folding to the development of extensive networks of shears against the basalt domes. Changes to the style of veining, from being cleavage parallel to being large, discrete and shear-hosted, are broadly synchronous with the deformation changes. The relationship between styles of veining and the different assemblages of alteration and mineralisation also appears to be intimate. The culmination of the gold mineralisation system appears as late fracture fill veins associated with an alteration assemblage of silica -carbonate-stilpnomelanearsenopy rite-gold.

Keywords: rheology, competency contrasts, vein processes, shearing, gold mineralisation

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GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Mineralisation & structural history of Hill 800, Mount Useful Slate Belt, Eastern Victoria ^Morev, A . A . , ^Bierlein, F.P., ^Turner, G. & ^Cherry, D P . ^Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia. ^Mount Wellington Gold N/L, 5 Nolan St, Bendigo VIC 3550, Australia. Hill 800 is a gold and base metal prospect located within the Cambrian Jamieson window, a greenstone thrust-sequence comprising andesitic, dacitic and rhyolitic volcanics and siliclastic sediments. Principal features of the Hill 800 prospect include intense zones of hydrothermal alteration, structural deformation, alongside anomalous Au ± (Cu, Pb, Zn) mineralisation, up to 30 g/t Au being recorded within gossanous material. Given the well-developed mineralisation models of Victorian sediment-hosted lode-gold deposits, this prospect represents a previously unidentified economic source of gold associated with Cambrian greenstone. The hydrothermal mineralisation style(s) accountable for Hill 800 are yet to be constrained, with possibilities ranging from Cambrian volcanic-hosted massive sulphide mineralisation to Devonian porphyry/epithermal ore system(s). This study attempts to constrain the timing and nature of the hydrothermal fluid events via optical microscopy, fluid inclusion and oxygen isotope analysis of critical vein and silicified rock samples from the Jamieson window. Drill core analysis of the mineralised zone has yielded that the sulphides (dominantly pyrite) occur as disseminated grains hosted by andesitic volcanics, which are associated with sericiticpropylictic alteration assemblages. Multi-staged deformation fabrics and dynamic sequences of quartz/carbonate veining overprint these sulphides. No mineralisation is yet to be found within the vein occurrences sampled. Fluid inclusion geothermometry and microscopy indicate that the late-stage quartz veins are derived predominantly from low-temperature, low-salinity fluids. Oxygen isotope studies of the late-stage veins, Cambrian volcanics and various silicified rocks will strengthen evidence which proposes that silicification and vein formation comprise of intense, multi-staged fluid influxes that post-date the mineralisation. This suggests that the mineralisation is synvolcanic in origin and the late-stage fluid infiltration processes do not significantly contribute to the mineralisation at Hill 800. Work to be undertaken will determine whether latestage hydrothermal fluids are capable of transporting yet to be recognised Au and base metal phases within the Mount Useful Slate Belt. Ore genesis models developed on the basis of these findings can then be used in exploration to assess the economic potential of the Cambrian greenstones within the Mount Wellington Fault Zone.

Keywords: Gold, Cambrian, greenstone. Mount Useful Slate Belt, Victoria.

ECONOMIC GEOLOGY

Thermal and burial history of the Abbeytown Pb-Zn deposit, Ireland Bayrak, A. A. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia. The aim of this study is to examine the relationship between Pb-Zn mineralisation and organic maturation and its application to the Abbeytown area (Ireland). Thermal basin effects are main factors influencing both the mineralisation of Pb-Zn deposits and organic maturation. In this study, a detailed thermal history for a selected district has been examined and its effects through time compared for Pb-Zn mineralisation and organic maturation. The thermal history of the basins is established and examined by means of various techniques. In classical models, thermal history estimations and calculations are based on present day geologic situations, vitrinite reflectance and bottom hole temperatures. In many studies, there have not been critical temperature control points through geological time. In any thermal history there may be many geologic scenarios that may fit the present day data. For this reason in order to analyse the real thermal history, temperature data through geologic time should be assessed as well. Thus, the fluid inclusion technique has been applied to the study area in order to constrain thermal evolution. In the Irish Midland area, lead-zinc ore deposits have been producing for the last 2 decades. Irish lead-zinc deposits are one of the most economically important deposits of this type in the world. Much research has been done on these deposits and their geology. However, up until now the thermal history of the basin was not properly understood due to lack of geological constraints. The structure of the study area is complicated due to the effects of the Caledonian Orogeny in the Ordovician-Devonian and the Hercynian Orogeny in the Upper Carboniferous-Permian times. These orogenic movements influenced the sedimentation history and the deposition profile of the area as well as the thermal history. The problem with the thermal history of the Irish Midlands is that Mesozoic-Tertiary sequences are of limited distribution except for the Ulster Basin and some offshore areas. The Carboniferous in the offshore area contains proven oil and gas source intervals. Since the structural and thermal history of the basin is complex, hydrocarbon generation and organic maturation within the basin is poorly understood. It has long been thought that organic materials in host rocks or generated hydrocarbons from source rocks may affect Pb-Zn ore mineralisation. The role of organics are very important to understand the development of mineralisation and diagenesis on account of their inter-relations in basin evolution. Considering the importance of the subject very little work has been completed to date. In the Abbeytown area, there has been no attempt to constrain this relationship to date. In this study, Pb-Zn mineralisation and the role of organics during diagenesis have been examined in the Abbeytown area by means of fluid inclusion and basin modelling techniques.

Keywords: organic maturation, basin modelling, fluid inclusion, thermal history, diagenesis.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 14*'' Victorian Universities Earth Sciences Conference, Monash University, September 2000

GIS analysis of the potenetial for gold exploration St Arnaud, Western Victoria. Emmett, S. School of Ecology and Environment, Deakin University, Rusden Campus, 662 Blackburn Road Clayton VIC 3168, Australia.

St Arnaud is located approximately 200 km northwest of Melbourne, and was a major gold producer from 1852 to 1915. Since its discovery some 16,445 kg of gold have been produced (Krokowski de Vickerod, 1997) from five major gold fields within the area. Of these five 89% has come from the St Arnaud gold field located near the township of St Arnaud. The goldfields of Stuart Mill and RedbankMoonambel account for another 8% (1250kg/Au). This project is constrained by information given by the Geological Survey of Victoria in the St Arnaud Geological Data Set. Within this data set are fifteen thematic layers as well as Vicmine data, geophysical and geochemical data. The geophysical data is of magnetic, radiometric and gravity images, while geochemical data consists of soil and stream sediment surveys from past exploration. Primary gold mineralisation within region is controlled by a number factors. Firstly, primary gold mineralisation occurs almost exclusively within a 5km central zone. This zone encompasses the goldfields of St Arnaud, Stuart Mill and Redbank-Moonambel. Primary gold mineralisation is concentrated in quartz reefs associated within brittle fracture zones and the most abundant auriferous quartz veins are associated with major NW reverse faults and WNW cross faults. Secondly, gold mineralisation is associated with elevated base metal values and orebodies become pyritic below the watertable. By cross correlating data from previous exploration two areas of a possible four show anomalous levels for primary gold mineralisation. These areas are located, east of Navarre and North of the township of Stuart Mill, along or in near proximity to major reverse faults and cross faults trending which appear to be a precursor to gold mineralisation.

Keywords: GIS, gold, St Arnaud.

ECONOMIC GEOLOGY

Relative timing of deformation, metamorphism and mineralisation within the Willyama Complex, New South Wales Hills, Q.G Australian Crusta! Research Centre, Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

Drillcore samples from two prospects situated north of the Willyama Inlier, which intersect highly prospective Olary and Broken Hill Domain metasediments of the Willyama Supergroup were investigated to establish the relative timing of the deformation, metamorphism and mineralisation of these metasediments. The rocks in these prospects are less deformed and metamorphosed (midamphibolite facies) than most rocks in the exposed Broken Hill & Olary Blocks and are therefore more suitable for studying the early tectonic history of the Willyama Complex. Microstructural and petrographical analyses show a first static metamorphism (M1) defined by a quartz foam texture and randomly oriented biotite and muscovite. This assemblage appears to have been overgrown by prograde growth of garnet and andalusite (M2). Many M2 andalusite grains that preserve inclusions of M1 assemblage also preserve inclusions of a quartz-biotite-muscovite foliation (S1) in their outer rim, indicating that the M2 peak was synchronous with deformation (D1). A second quartz-biotitemuscovite foliation (S2) crenulates S1. The S2/S1 overprinting relationship is commonly included within AlSisOs porphyroblasts (M3- defined by andalusite at Polygonum and sillimanite at Thunderdome) indicating that the D1 event was overprinted by another high-grade metamorphism (M3) and synchronous deformation (D2). The S2 foliation is locally overprinted by a strong, finegrained muscovite fabric (S3) most commonly preserved in discrete shear zones, suggesting that D3 occurred as metamorphic conditions were retrograding from the peak-metamorphic M3 (associated with D2) conditions. The early M1 assemblage may predate the Olarian Orogeny and associated with crustal extension. Zinc, lead and silver mineralisation, characteristic of Broken Hill Type deposits, that is preserved within these prospects was present prior to D2-M3, suggesting a pre- to syntectonic relative timing. Copper and gold mineralisation overprints all deformation microstructures and metamorphic assemblages indicating that it either formed during the retrogression from M3 P-T conditions or resulted from a later hydrothermal.

Keywords: Broken Hill, Olary Block, mineralisation, deformation, metamorphism, overprinting

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, IVIonash University, September 2000

Origin of Irish Zn-Pb mineralisation: syngenetic vs. epigenetic Reed, C. & Leonard, W. ^ School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia. The Lower Carboniferous marine carbonate sequence of the Midland and Limerick provinces in Ireland is host to one of the world's major zinc orefields. The transgressive carbonate sequence overlies a wedge of Upper Devonian continental red beds. Following the discovery of several major base metal deposits, researchers during the 1960's debated the origin of the ore, which is confined largely to particular geological horizons. The last three decades has seen a synsedimentary "Irishtype" model dominate the literature, combining sedimentary exhalative (SEDEX) mineralisation with a component of replacive mineralisation precipitated during shallow, pre-lithification burial. Results from this study on the Ballinalack and Courtbrown deposits show the precipitation of these two Irish Zn-Pb deposits occurred after significant burial of the host rocks. Paragenetic evidence places the mineralising event after precipitation of marine and burial cements. Mineralisation also post-dates extensive pressure solution indicating that the deposits formed at depths in excess of 800 metres burial. Ballinalack and Courtbrown can be classified as Mississippi Valley-type (MVT) deposits. Burial history reconstruction for the Courtbrown area indicates a maximum early-Chadian (350 Ma) age for the Courtbrown deposit. A minimum age of 307 Ma for Courtbrown has been assigned based on Variscan deformation structures. The epigenetic nature of the Ballinalack and Courtbrown mineralisation and their similar stratigraphic level to many classic Irish-type deposits places doubt on the validity of the problematic Irish-type model. Currently the Irish-type model is used as an exploration tool, which limits the search for deposits to stratigraphy of pre-Arundian (345 Ma) age. Furthermore, it has been used by some researchers to imply a genetic link between SEDEX and MVT ore genesis. A reclassification of the Zn-Pb deposits in Ireland as epigenetic would therefore have ramifications for both practical exploration procedures as well as ore genesis theory.

Keywords: zinc, lead, Ireland, Ballinalack, Courtbrown, Mississippi Valley-type, Irish-type.

ECONOMIC GEOLOGY

Effects of Heavy Metal Speciation on Partial Leach Geochemistry ^White, P., ^McPhail, D.C. & \ a w i e , D. ^Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia. ^Pasminco Exploration, Level 1, Royal Domain Centre, 380 St Kilda Rd, Melbourne VIC 3001, Australia.

The ability of partial soil leaches to selectively target elements mobilised from orebodies has led to their increasing popularity as a method of analysis in exploration geochemistry. This study aimed to determine quantitatively the phases that mobile elements such as Zn and Cd reside in, and if there are any differences between anomalous and background soils taken from a partial leach anomaly over buried mineralisation. The arid Northern Pods prospect near Cobar NSW and the high rainfall Beatrice prospect near Queenstown in Tasmania were chosen due to the presence of a strong and defined partial leach anomaly, that is associated with mineralisation at depth. Anomalous and background soils from each of the regions were sampled and sieved into size fractions of <63um, 63-250um and >250um. These size fractions, along with a bulk sample, were subjected to a sequential and selective leach scheme. Optical microscopy and XRD and were used to define the mineral phases present in the soils. The highest concentrations of Zn and Cd were present in the <63um size fractions. The amount of element removed by each leach divided by the total concentration of element in each sample was similar for the each of the size fractions analysed. The "adsorbed" phase as determined by a 0.1 M CaCI^ leach was found to be a reservoir for up to 90 wt. % of Cd, while 40-90 wt. % of Zn present in the samples was not removed by the less aggressive extraction schemes before the acid digestion. The selectivity of the leach schemes was also investigated. A 0.1 M Na-pyrophosphate leach targeted at organic matter was also found to remove up to 25% of the iron and manganese present in the sample. The results indicate that leaches on arid soils should be targeted at the adsorbed phase, while in wet temperate soils the leach should target the amorphous Fe or Mn oxides.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Characteristics of sediment-hosted disseminated gold mineralisation at Daylesford, Victoria, Australia: Potential for a new type of low-grade, high-tonnage gold deposit in slate belts? ^Thom, J.L., ^Bierlein, F. P., 'Maher, S. & 'Arden, P J. ^Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia ^Geological Survey of Victoria, 250 Victoria Parade, Fitzroy VIC 3065, Australia ^Range River Gold, NL, 15 Queen Street, Melbourne VIC 3000, Australia

Until recently, significant gold deposits in slate belt provinces have been perceived as being located exclusively within structurally controlled quartz veins. Recent findings, however, have demonstrated that disseminated accumulations of gold within sediments adjacent to quartz veins may constitute a significant proportion of the total gold reserve in slate belt gold provinces. Disseminated ore is generally low-grade but may embody extensive sedimentary packages with evenly dispersed gold grades. In contrast, quartz vein ore bodies often have erratic gold grade distribution and yield relatively low tonnages of ore. A better understanding of the genetic relationship between quartz vein and sediment-hosted, disseminated gold mineralisation is of vital importance in order to optimise existing mining operations and assess the potential for new discoveries in slate belts. At Daylesford, disseminated gold has been recognised in drill core in lower greenschist metamorphosed sandstones, siltstones and carbonaceous shales proximal to the auriferous quartz reefs of the Ajax Anticline. Preliminary studies on some samples (ranging in grades between 0.3 and 1.6g/t) indicate a multi-stage sulphide paragenesis and clearly demonstrate that the gold is associated with a late-stage, relatively arsenic-poor phase of pyrite. The gold-bearing pyrites typically show marked compositional zonation and a gold-sphalerite-galena assemblage in the central, arsenic-poor portions. A carbonaceous shale sample from within a reverse fault contains visible gold, associated with small, late stage pyrite veinlets. Another sample contains pyrite/sphalerite-hosted free gold within the matrix of coarse-grained sandstone. The existence of disseminated gold within the wallrock alteration haloes surrounding quartz veins suggests that disseminated gold is genetically related to auriferous quartz vein mineralisation. Results from oxygen isotope analysis of the auriferous sedimentary rocks and quartz veins should confirm this and further constrain a genetic model for sediment-hosted, disseminated gold formation in slate belts. A better understanding of such a model is vital for future exploration and may be of significance to sediment-hosted, disseminated gold occurrences in other slate belts such as the Meguma Terrane, Canada and the Buller Terrane, New Zealand.

Keywords: Sediment-hosted, disseminated gold, slate belts, Daylesford.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 14**^ Victorian Universities Eartii Sciences Conference, Monash University, September 2000

SYMPOSIUM: GEOPHYSICS

GEOPHYSICS

Structural development and petroleunfi potential of the South Wanganul Basin, New Zealand Daber, R. & Hall, M. Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

The Wanganui Basin is located along the southern west coast of the North Island of New Zealand. Its proximity to the oil and gas producing Taranaki Basin to the west makes the Wanganui Basin a logical target for hydrocarbon exploration. Previous exploration in the southern onshore portion of the basin resulted in the drilling of two wells, in 1943 and 1964, and the acquisition of 500 kilometers of 6 fold seismic data in the early 1970's. These data were recently supplemented by 10 kilometers of 48 fold seismic data and a 100 kilometer, 400m spaced gravity survey, designed to fill in gaps in the seismic coverage, and to calibrate the gravity data with the seismic results and extend the geophysical coverage to the south and east. The Wanganui Basin is a very young, Pliocene-Pleistocene basin, which has experienced rapid subsidence, resulting in the accumulation of up to 4000 meters of largely shallow water sediments. It lies on the Pacific Plate side of the west-directed Hikurangi subduction zone and is currently undergoing compressional deformation, as illustrated by the conspicuous geomorphic features caused by a number of actively growing folds. The present depocentre of the basin lies about 25 km offshore, but has moved gradually southwards to its present position as a result of regional uplift in the central North Island. Detailed interpretation of the seismic data shows that the anticlines are asymmetric and lie on the upthrown side of dominantly NNE striking reverse faults, which dip both to the east and west. These structures were clearly active during deposition and comparison of sediment thicknesses on anticlinal crests and in synclinal troughs show that their growth has been episodic. Although the basin has been referred to as a foreland basin, it is different from that model in a number of respects. The faults underlying the fold belt are high angle reverse faults as opposed to thrusts, and may have been inherited from old basement faults. Also, there is no evidence that the depocentre has migrated westwards in front of the fold belt. Instead the basin is underlain by a basement with a rugged topography which has been progressively onlapped by sediments extending from the west, so that only the youngest sediments were deposited in the eastern part of the basin where the major uplift has occurred. There is no transition from deep marine (flysch) through marginal sediments to continental deposits (molasse), but instead cyclic deposition of shallow marine and terrestrial sediments controlled by glacially-influenced sea-level changes and major influxes of felsic pyroclastic material make up the basin fill. Numerous possible structural traps have resulted from continuous deformation and combined with sandstone reservoirs and mudstone seals make a good combination for potential petroleum prospects. Although potential source rocks with 1% TOC have been transected in the Parakino-1 well, 45 km to the north of the Santoft 1A well, these source rocks do not extend far enough onshore as deposition occurred during the early part of basin formation.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Focal Mechanisms of the Thomson Reservoir-Triggered Earthquakes ^Allen, T., ^Gibson, G. & ^Hill, C. ^Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia. ^Seismology Research Centre, 2 Park Drive, Bundoora VIC 3083, Australia ^Melbourne Water Corporation, PO Box 4342, Melbourne VIC 3001, Australia

There are few better documented examples of reservoir triggered earthquakes than those recorded about the Thomson Reservoir, east of Melbourne. In 1977, the Melbourne and Metropolitan Board of Works, predecessor to the current Melbourne Water Corporation, commissioned the Seismology Research Centre to install a network of seismographs to monitor seismicity around the Thomson Dam area during the initial filling cycle at the reservoir. About three years following the commencement of filling in 1983, shallow earthquakes began occurring immediately under the reservoir, with magnitudes up to ML 3.0, at rates of up to 5 events per week. These events were almost certainly reservoir-induced. During the period from 1990 to September 1996, the seismic activity near the Thomson Reservoir migrated to greater depths and away from the reservoir, with decreasing activity levels. On the 25 September 1996, a magnitude 5.0 earthquake occurred at a depth of 12 km in close proximity to the dam wall. The mechanism of this event shows southeast to northwest compression, consistent with it having occurred on the nearby Yallourn Fault. This was followed by an intensive period of seismicity with approximately 200 aftershocks recorded over more than a year. Recent records indicate that the area now seems to be approaching the end of the period when triggered earthquakes are likely with activity levels now similar to those existing prior to commencement of filling.

GEOPHYSICS

Rock magnetism and magnetostratigraphy of cave sediments in the Wellington Caves, New South Wales. Christensen, T. Department of Earth Sciences, LaTrobe University, Bundoora VIC 3083, Australia.

The Wellington Caves are renowned as a site of vertebrate palaeontology. Since 1830, fossils from sediments within the caves have attracted international interest from numerous scientists, including Cuvier, Lyie and Owen. Despite the long history of investigation of the caves and the bones found within them, the stratigraphy and ages of the deposits are poorly defined. The fossils have been assigned an age range from Late Pliocene to Late Pleistocene based on palaeontological correlation. A palaeomagnetic survey to provide absolute ages and internal correlation was undertaken in the Phosphate Mine of the Wellington Caves system. Detailed mapping in the Phosphate Mine indicate that the stratigraphy of the Mitchell Cave Beds is more complicated than previously supposed. The Mitchell Cave Beds comprise 3 distinct units containing red aeolian silt. The Pig Sty Unit is characterised by layers rich in phosphate derived from bat guano. The younger West Loop Unit is represented by a chaotic facies incorporating blocks of the underlying Pig Sty Unit to the east and by fine grained entrance facies deposits to the west. The Bone Cave Breccia, overlying the West Loop Unit to the east, may represent deposition from more than one talus cone. A simple reversed magnetic polarity has been established for the West Loop Unit, indicating an age of Early Pleistocene. The palaeomagnetism of the West Loop Unit is simple, contrasting with the very pervasive overprinting observed in cave sediments of similar age and appearance from Buchan. The disparity in magnetic behaviour reflects a difference in the reduction of iron phases related to contrasting sediment provenance or post-depositional history. Both factors are likely to have palaeoenvironmental significance.

Keywords: palaeomagnetism, aeolian. Pleistocene, cave sediments

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Geological evolution of the South Wanganui Basin, New Zealand Daben R. & Hall, M. Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

The South Wanganui Basin is a Plio - Pleistocene sedimentary basin lying along the southern west coast of the North Island of New Zealand. To the east it is bounded by the axial Ruahine and Tararua ranges and to the west the Patea-Tongaporutu High, while no definite northern or southern boundaries have been defined. The basin sits above the westerly subducting Pacific Plate, and compression due to the subduction zone is causing active reverse faulting and asymmetric folding which controls the topography of the eastern, onshore part of the basin. Rapid sedimentation began in the central part of the Wanganui Basin at about 5Ma. The timing of basin fill and its relationship with the continuous structural deformation has been illustrated by mapping the distribution of units which onlap onto the eastern part of the basin. This progression is illustrated by isopach maps for the total thickness between basement and the mid Opoitian (4.3Ma), and basement and the base of the Waipipian (3.7Ma), Nukumaruan (2.6Ma), and Castlecliffian (1.6Ma) stages. The isopach maps show an onlap onto a rugged basement topography from the east in the early stages of deposition. Continued filling of the basin extended the area of deposition westward until the early Nukumaruan when the last onlap onto basement is seen at the highest point of the Himatangi Anticline. Continued sedimentation after the Nukumaruan filled the basin to its current level. Together with expansion diagrams detailing fault movement and fold growth, both the structural and filling history of the Wanganui Basin has been compiled to give an overall view of the geological evolution of this unusual basin.

GEOPHYSICS

TEM's "Full Monty" Grant A.J. & Cull, J. Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

Exploration using electromagnetic methods can give an insight into the conductivity, depth, orientation and location of a target conductor. However, if the target conductor is buried in a conductive host environment, then the determination of these parameters can be very complicated. There are several ways of removing a host-response effect and these are generally referred to as stripping techniques. Most stripping methods normally involve filtering (especially low and high pass filters), stripping of theoretical host and current channelling power laws, and the stripping of a layered forward model. Techniques, such as data modelling (forward and inverse) and current filament tracking, are also used to model and interpret the host response. These methods have been adapted for TEM data obtained from a highly conductive prospect in South Africa. This method is applied to the Duddar, Pakistan dataset, with the best models producing a three layered system of low-high-low resistivities, however the resistivities calculated using the inversion program showed systematic trends with lateral variations from west to east. These trends are evident in the inverted 2D parasections and the 3D paraslices; these appear to be dominated by the Khauri Fault which runs roughly north-south, suggesting a major structural boundary for mineralisation. After developing a general approximation to the host response, better models can be obtained by stripping from the down hole and surface TEM data. These methods provide an enhanced response for the target conductor, with only a small residual signature from the layered host. Hence, a freespace model can be used to closely model the final stripped data. These stripped models suggest an increase in the conductor's strike length (or radius) along with greater depth extent.

Keywords: Electromagnetics, TEM, resistivity, conductivity, stripping, host-response, modelling, inversion.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Modelling petrophysics and potential field data Huvnh T.H. & Ailleres, L. Australian Crusta! Research Centre, Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

The specific comparison of modeled geological structures and their related geophysical responses are commonly limited by the inability to integrate complex petrophysical rock property distributions. Solutions for potential field data are inherently non-unique and the spatial effects of superimposed, contrasting deformation geometries are at best, difficult to characterise. In addition, the sparse sampling yielded by most petrophysical field measurements required to constrain quantitative geophysical interpretations frequently display an inhibited or random and often clustered distribution. This presents an important statistical problem for estimating the variable of interest across the study region and limits our understanding of potential variabilities rarely perceived in localised structures. We present an investigation of geostatistical modelling of petrophysical rock properties and potential field data. A three-dimensional geometric model of the Cannington Ag-Pb-Zn deposit was developed through interpreted geological and geophysical datasets. The principal rock properties examined in this investigation are rock density and magnetic susceptibility. These properties were determined by direct measurement of samples from drillcore and, where possible, from in-situ underground exposures. The external rock property constraints were integrated into the model and implemented using geostatistics. Spatial correlation between values of the property vahable was characterised through variogram analysis. The local trends identified from the variograms were spatially interpolated in the model using the method of kriging. The model objectively provides recognition of anisotropic patterns in geometry and spatial orientation that can be assessed for influences of structurally controlled anomaly asymmetries. Preliminary results have yielded encouraging models for characterising the rock property distributions and propagation trends in the local mine structure. This has lead to refined images of the calculated potential field anomalies, which show a greater level of correlation with the target data than with initial models.

Keywords: potential field data, petrophysical rock properties, geostatistics, variogram, kriging.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 14"^ Victorian Universities Earth Sciences Conference, Monash University, September 2000

SYMPOSIUM: SURFACE PROCESSES

SURFACE PROCESSES

Variability of transport mechanisms for Volcanic Debris Avalanches: Insight from the Maronne valley (Cantal Volcano, France). ^Reubi, O. & ^Hernandez, J. ^Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia. ^Institutde Minora log ie et Geochimie, Universite de Lausanne, BFSH2, 1015 Lausanne, Suisse

A detailed study of the deposits of two volcanic debris avalanches (VDA I and II) that occur in the upper Maronne valley, northwest sector of Cantal Volcano, France, shows that these two VDA clearly differ in regard to their structures, textures and extensions. VDA I is voluminous and widespread. The deposits display a two-layer structure. The intermediate and upper part of the flow (block facies) consists of more or less brecciated megablocks (<150 m) set in essentially monomictic breccias resulting from block dissaggreption. Differential movements are almost absent in this part of the VDA. The basal layer (mixed facies) is made of polymictic breccias rich in fine-grained particles that originate from block dissaggregation, and material picked up from the substrate during movement. This part of the flow is interpreted to have been turbulent. These data strongly indicate that VDA I is purely gravitational and argue for a model in which the initial sliding mass transforms into a flow due to differential in situ fragmentation caused by shear stress. VDA II is restricted to low topography areas. The deposits consist of brecciated, rounded blocks and megablocks (<20 m) set in a fine grained polymictic matrix composed essentially of volcanic glass. This unit is stratified, with the basal and intermediate part comprising a massive layer that contains all the megablocks. The upper part is a normally graded layer that contains blocks <1m. These observations suggest that VDA II may be of Bezymianny-type and that it underwent a flow transformation from a turbulent to a stratified flow consisting of a basal hyperconcentrated laminar body overlain by a dilute layer. Superimposition of these two VDAs clearly demonstrates that this type of deposit results from a spectrum of formation and transport mechanisms, and that a single volcano can be affected by successive VDAs with contrasting mechanical behaviour.

Keywords: volcanic debris avalanche, transport mechanism, Cantal Volcano.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

The microvertebrate fauna of Bullock Creek, Northern Territory. Schwartz, L. Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

Bullock Creek is a fossil site in the Northern Territory of approximately the same age as the Riversleigh World Heritage site. This study examines the microvertebrate fauna of this site, emphasizing the mammalian microfauna. A more complete faunal list of this site is considered important due to its position as the most northerly Tertiary fauna known on the Australian continent. The Bullock Creek Local Fauna is thought to have been deposited during a period of major changes in Australian climates, ecosystems and biodiversity, about which the need for more information is critical. The Camfield Beds, in which the Local Fauna is found, can be divided into two distinct lithologies, conglomeritic limestone and calcilutite, which have been interpreted to represent streamchannel and lacustrine fades respectively. A preliminary faunal list for the Bullock Creek mammalian microvertebrates is presented here, and the presence in the fauna of forms not previously known from this site, such as Ganawamaya ornata and a species of ektopodontid is noted. The geological and palaeontological context of these remains is investigated and the relevance of this site and its fauna is examined in terms of the stage-of-evolution time scale that has been developed for Australia's Tertiary terrestrial mammalian faunas. This time scale is reviewed in the light of recent studies such as that by Woodburne et al. (1993) on the Etadunna Formation of South Australia, and the conceptual groundwork for such a chronology is critiqued. Further, it is suggested, partly on the basis of the new elements of the fauna listed in this project, that the currently accepted date for this site is incorrect, and that it would be more accurate to situate this site earlier in the Miocene, between Riversleigh Systems B and C.

Keywords: Microvertebrates, Bullock Creek, Camfield Beds, Tertiary mammals, biochronology

SURFACE PROCESSES

Discovery of a Middle Permian (guadalupian) brachiopod fauna from Peninsular, Malaysia ^ C a m p i , M. J., ^Shi, G. R. & ^ M o h d S. L ^School of Ecology and Environment, Deakin University, Rusden Campus, 662 Blackburn Road, Clayton VIC 3168, Australia. ^Jabatan Geologi, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia.

A Middle Permian (Guadalupian) brachiopod fauna is described from the 'Leptodus shales' of the Gua Musang Formation in the Central Belt of Peninsular Malaysia. The 'Leptodus shale' brachiopod fauna is dominated by lyttonids and productids, with characteristic genera including Vediproductus, Uncinunellina, Leptodus, Transennatia, Neochonetes, Meekella and Derbyia. Overall, the fauna shows strong Palaeoequatorial (Cathaysian) affinities and can be correlated with other Middle Permian brachiopod faunas from Indochina such as the Cambodian Sisophon fauna and from the South China block (e.g. Lengwu fauna from Zhejiang Province). This fauna is important for two reasons. Firstly, brachiopod faunas from the 'Leptodus shales' are currently thought to be Changhsingian in age, but the discovery of genera such as Vediproductus suggests an older Guadalupian age is more likely for this fauna. Secondly, the presence of a Guadalupian brachiopod fauna with strong Palaeoequatorial affinities in this area of Peninsular Malaysia is important for the study of the tectonic history of the South-east Asia region, as it adds evidence to the theory that east and west Peninsular Malaysia were separated during the Early and Middle Permian. It indicates that the eastern half of Peninsular Malaysia was within the Palaeoequatorial Realm in the Middle Permian, while faunas from the western part are characteristic of the Sibumasu Province and were allied with the cool water Gondwanan faunas. The discovery of this Guadalupian brachiopod fauna from the Central Belt of Peninsular Malaysia is important in understanding the somewhat controversial tectonic and palaeobiogeographic history of the South-east Asian region at the close of the Palaeozoic. Keywords: Brachiopoda, Guadalupian, Permian, Palaeobiogeography, Palaeo-equatorial Realm, Gondwana Realm

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 14*" Victorian Universities Earth Sciences Conference, Monash University, September 2000

Cainozoic submarine canyons of the Otway Basin, Victoria Leach, A.S. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia.

Submarine canyoning has played a significant role in cool-water carbonate shelf growth along the Victorian coast from the Oligocene to the Recent. Using seismic profiles, two distinct regions of canyons in the Otway Basin have been mapped. They are confined within the Pliocene to Recent, Whalers Bluff Formation in the west, and the Oligocene to Miocene, Heytesbury Group to the east. During the Oligocene to Miocene period large, U-shaped submarine canyons were continually eroding and infilling on a gently dipping upper slope. The Oligo-Miocene canyons all migrate laterally towards the west. Stacked canyons have long been a problem for petroleum companies as canyon fill causes lateral variation in sonic velocity. These lateral variations manifest themselves as 'velocity pull-ups' that can be misinterpreted as important seismic structures. The Pliocene to Recent canyons are smaller, V-shaped and irregular and represent the transition between the canyon heads on the outer shelf and the main canyon bodies, eroding the upper slope. The canyons young towards the southern boundary of the basin. Submarine canyons are still forming and infilling on the present day continental slope. Erosion of the older canyons initiated during a low stand in sea level at the end of the Oligocene, during deposition of the Clifton Formation, the basal unit of the Heytesbury Group. A nodular band at the base of this unit may represent a hiatus due to the regressive event. Presumably, canyoning then ceased for a short time. However, uplift within the Otway Basin caused a rise in base level initiating further erosion and canyon cutting within the Pliocene. Some of the larger Pliocene to Recent canyons correspond to the location of ancient rivers systems, (i.e. Gleneig River has had three exit points).

Keywords: submarine canyons, Otway Basin, seismology, Heytesbury Group

SURFACE PROCESSES

Glacial-marine sedimentation and dolomite formation in Permian sediments of the Bacchus Marsh Formation, Bald Hill. Spence, E. Department of Earth Sciences, La Trobe University, Bundoora VIC 3083, Australia.

Bald Hill, 1.5km north-east of Bacchus Marsh, Victoria, has excellent outcrop of Permian sediments. Sandstones and pebbly sandstones (diamictites) represent deposition in glacial outwash fans or subglacial channels. They are purple or yellow muddy sands with quartz and feldspar grains and clasts of granite, quartz and meta-sandstones up to 20cm in size. Structures in the sandstones include current ripples, planar bedding and large scale deformation. The sandstones also include fossil leaf and stems of a Gangamopteris sp. The large scale irregular folds were caused by grounding of the glacier over the partially consolidated sediments. Overlying the sandstones and diamictites is the Mortons Conglomerate Member, which contains the same clasts as the diamictites. Marine fossils have been found in the conglomerate. A full marine incursion into the area occurred during deposition of the conglomerate, which was deposited by reworking of a diamictite by wave action in a shallow marine environment. Overlying the Morton's conglomerate member are thinly bedded sandstones and siltstones for up to 125m. They are muddy, quartz and feldspar rich sandstones, siltstones and mudstones which exhibit cross bedding, soft sediment deformation, large scale runzle marks. Bioturbation of the medium sandstones occurs throughout the sediments and increases towards the top of the sequence. Marine fossils have been found in some bed. Deposition occurred in a shallow enclosed bay into which the streams of the glacier drain. Gangamopteris sp. leaf and stem fossils can be found at the top of the sequence. The conglomerate and overlying beds contain dolomite as a partial cement and as ferroan dolomitic concretions. The origin of the dolomite in this cool water environment is from direct precipitation from pore water or form seawater circulation through the sediments.

Keywords: glacial marine, fluvioglacial, diamictite, dolomite and conglomerate

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Luminescence dating of rock art using nfiud-wasp nests ^Ashton, M.R., ^Roberts, R.G. & ^Yoshida, H. ^Department of Earth Sciences, The University of Melbourne, Parl<ville VIC 3010, Australia. ^Department of Earth Sciences, La Trobe University, Bundoora VIC 3083, Australia.

Mud-wasp nests are common to rock shelters and may be associated with prehistoric rock paintings. Mud-wasp nests overlie and underlie the "Palorchestes" painting in Deaf Adder Gorge, Western Arnhem Land. The mud forming the nests contains quartz grains allowing the age of the nests to be established using optically stimulated luminescence (OSL). OSL, the successor of thermoluminescence (TL) provides a measure of time since minerals such as quartz were last exposed to sunlight. This exposure, known as the bleaching event resets the pre-existing OSL signal to near zero. Subsequently, when light is blocked by surrounding sediment deposition, the OSL signal builds up again through exposure to the weak flux of the ionising radiation in the sediment. An implicit assumption when dating sediments is that all mineral grains were exposed to sunlight just prior to deposition, and once deposited were protected from sunlight to prevent partial bleaching of the OSL signal. This is an inherent problem in dating mud wasp nests as a percentage of all nests have been exposed to sunlight. This problem is overcome by analysing the distribution of data for partial bleaching evidence and correcting for this, and using mainly single grain analysis to establish the most accurate age for the mud-wasp nests.

Keywords: Optically Stimulated Luminescence, Prehistoric Rock Paintings,

SURFACE PROCESSES

Timing of deposition and provenance of stromatolitic carbonates from the 2.7 Ga. Belingwe Greenstone Beit/ Zimbabwe Craton: constraints from Pb-Pb and Sm-Nd systematics

^ B o l h a r R., ^ H o f m a n n A., ^Hergt, J., ^ W o o d h e a d , J. & ^Dirks, P. 'School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia. ^Department of Geology, University of Zimbabwe, P.O. Box MP 167, Harare, Zimbabwe.

Pb-Pb isochron ages have been obtained for stromatolitic limestones from the late Archaean Belingwe Greenstone Belt, providing direct age constraints on the deposition of these shallow water marine sediments. Samples from the Manjeri Formation and stratigraphically upper Cheshire Formation yield ages of 2700±49 Ma (MSWD = 13) and 2543±53 Ma (MSWD = 5.6), respectively. These data are in agreement with published U-Pb zircon and Pb-Pb whole rock ages of stratigraphically enclosing volcanics, and we interpret our Pb-Pb ages as the timing of diagenesis and minimum age of carbonate precipitation. A 2400±53 Ma age (MSWD = 0.66) for one sample from the Cheshire Formation is considered to be unrelated to a depositional/ diagenetic event. We infer that this age may reflect a craton wide thermal/ chemical disturbance event or tectonic activity along the crustal-scale Mtshingwe fault. Carbonate samples collected from a sheared contact have also been subjected to Pb isotope analysis in an attempt to obtain absolute age constraints on deformation. Geological information precludes this age from being meaningful and this, along with a high MSWD value, indicates open-system behaviour and/or mixing relationships between isotopic end-members. A step-dissolution procedure using dilute acetic and hydrochloric acids has been applied to sub-samples from individual hand-specimens. This technique enables to recover radiogenic and common Pb, and thus is of particular utility where single hand-specimens are to be dated. However, the spread in Pb ratios is insufficient to suppress the uncertainty of isochron ages below ±50 Ma in our study. Calculated model |li1-values for Manjeri and Cheshire limestones are 8.4±0.02 and 9.02±0.01, similar to values for ca. 3.5 and 2.9 Ga. old Archaean basement units adjacent to the Belingwe Belt. Negative SNd (Tdeposition) and /sm/Nd suggest derivation of the REE from old, LREE enriched continental crust. 2-stage Nd model ages for the carbonates indicate that precursor rocks were extracted from the mantle 3.5 - 3.8 Ga. (Cheshire Formation) and 3.3 Ga. (Manjeri Formation) ago. Although in general agreement with mantle-extraction ages for basement units (model Tdm: 3.5 - 3.3 Ga.), the 3.8 Ga. 2-stage Tdm of one Cheshire sample requires an even older source, for which there is evidence from published Tdm and zircon xenocryst ages of ca. 3.8 Ga. for basement units.

Keywords: Nd systematics, Nd-model ages, Pb-Pb isochrons, Belingwe Greenstone Belt

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

First record of permochonetes afanasjeva (brachiopoda) from Sichuan, China; a new species - ecology and palaeobiogeography Campi, M. J. & Shi, G. R. School of Ecology and Environment, Deakin University, Rusden Campus, 662 Blackburn Road Clayton VIC 3168, Australia.

First record of the genus Permochonetes and a new species, P. elfinis, is described from the Early Permian (Artinskian) Liangshan Formation of the South China block. The new species is placed within Permochonetes Afanasjeva on the basis of close similarity of dorsal internal morphology and presence of external fine capillation; and is separated from the type species, Permochonetes pamiricus Afanasjeva, by the presence of a distinct posteromedian sinus on the ventral umbo, delayed capillation and a much smaller size. The discovery of the new species has extended the known palaeogeographic range of this genus to include South China, in addition to earlier records from the SE Pamirs and Karakorum Ranges. The presence of this genus in both the Palaeoequatorial Realm and the peri-Gondwanan region indicates the presence of a faunal link between the former and the incipient transitional Cimmerian Region. The local environment during the deposition of the Liangshan Formation appears to have been a restricted tidal flat or lagoon, which experienced frequent sea-level fluctuations associated with the onset of the Yanghsingian transgression. The new species, P. elfinis, appears to have several morphological adaptations enabling successful exploitation of this environment. It was typically a very small and thin-valved species with a high surface area to volume ratio, an advantage in an oxygen restricted environment. The small size and numerous body spinules would have aided the individuals to remain suspended at the top of the fine, soft substrate. It also dominated the brachiopod assemblage in the Liangshan Formation, comprising up to 94% of specimens within a bed. These factors indicate that the new species appears to have some of the characteristics of an opportunistic species.

Keywords: Brachiopoda, Permian, South China, Palaeoecology

SURFACE PROCESSES

Seismic architecture of the Seaspray Group: evolution of the Gippsland Shelf and Slope through the Tertiary ^Daniels, J., ^Gallagher, S.J., ^Holdgate, G.R., ^Keene, J.B., ^Smith, A.J., & ^Wallace, M.W. ^School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia. ^School of Geosciences, University of Sydney, NSW 2006, Australia.

The Seaspray Group in the Offshore Gippsland Basin is composed of deep-water marls and limestones that record the growth and evolution of the continental shelf over the last 30 million years. The Gippsland basin has had a complex subsidence history during Seaspray Group deposition. Overall, the basin has undergone passive subsidence with episodes of compression and reactivation of previous structures. The timing of these compressive events may be linked to periods of more intense submarine canyoning. Tertiary inversion and uplift of the oil- and gas-bearing structural traps of the underlying Latrobe Group have influenced the evolution of the overlying continental slope. In particular, uplift around the Marlin-Turrum field possibly during the Mid-Miocene has influenced the growth and decay of submarine canyons. Five seismic architectural sequences mapped in this study show submarine canyons were largely absent from the continental slope during the Latest Oligocene and Earliest Miocene. During the Mid-Miocene a marked change in sedimentation style across the basin is associated with the onset of submarine canyons in the offshore Gippsland Basin. These canyons were much larger, with meandering morphology and associated deep-water sediment lobes in the Central Deep. Latest Miocene and Pliocene canyons show much shorter morphology associated with a steeper continental slope and a marked seaward shift of the continental shelf. Overall, the morphology of the continental shelf and slope with the associated canyons in Gippsland has changed markedly since the Earliest Miocene. The position of the base of the slope has shifted seaward only gradually compared to the position of the shelf break during the last 30 million years, which has shifted seaward markedly. The continental slope has become much steeper since the earliest Miocene with progressive progradation of the continental shelf seaward.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Sedimentology and ichnology of Mount Bepcha, Western Grampians, Victoria Gouramanis, C. Department of Earth Sciences, La Trobe University, Bundoora VIC 3083, Australia.

The Silurian to Devonian Grampians Group sandstones of Mount Bepcha, west of the main Grampians Range in Victoria, are referable to the Major Mitchell Sandstone and unconformably overlie Cambrian Greenstones. The sandstones show a marine to fluvial transition with at least four Nthofacies of which each has a distinct trace fossil assemblage (ichnocoenose). The lowermost shallow marine lithofacies contains tabular cross beds with some pebbly quartz lag deposits and mudstone rip-up clasts. Often laminae of fine sand and mud occur with occasional scours, water escape structures, normal and reverse graded bedding and localised soft sediment deformation. The lower parts of this lithofacies contain burrow mottlings (Thalassinoides sp. or Ophiomorpha sp.(it)), large intrastratal trails, one occurrence of Rhizocorallium with its characteristic horizontal spreite, and a small Cruziana-Wke trace. This assemblage is characteristic of the shallow-midshelf Cruziana ichnofacies. The overlying beds lack mud and contain densely spaced vertical Skolithos linearis, forming Skolithos piperock. The Palaeozoic Skolithos ichnofacies generally occurs in relatively high energy shoreface environments with continuously shifting sands. This marine unit is conformably overlain by sediments deposited in a braided river system with large scale, laterally discontinuous, trough cross bedding, vein quartz lag horizons, planar laminated sandstones and minor current rippling. These fluvial sandstones contain large intrastratal trails and Daeda/as-like burrows. Overlying these fluvial sandstones are planar laminated coarse sandstones with a strong unimodal current lineation. These sandstones contain one fish swimming trace, Undichnia, and at least three types of large straight trackways, of which two are probably arthropodan, and the third is a suspect vertebrate trackway. The arthropodan tracks include Diplichnites and all travel perpendicular to the current direction. They were probably produced by large eurypterids, xiphosurids and myriapods. The (invertebrate trackway was probably formed by an ancestor of Acanthostega.

Keywords:

Mount Bepcha, braided stream, marine shelf, Skolithos ichnofacies, Cruziana ichnofacies, arthropod trackways, vertebrate trackway.

SURFACE PROCESSES

Variations within the large volume Cerro Galan Ignimbrite, Northwest Argentina: Implications for eruption and emplacement processes. ^Hildvard, S.C., ^Cas, R A F . & ^Viramonte, J.G. ^Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia. ^Institute Geonorte, Facultad de Ciencias Naturales, Universidad Nacional de Salta, Buenos Aires 171, Salta, Argentina.

The large volume (c.1000 km^) Cerro Galan Ignimbrite was erupted from the Cerro Galan caldera, NW Argentina (26°00'S 67°00'W) approximately 2 Ma. The ignimbrite consists of two main depositional units that form a single compound cooling unit. A plinian airfall deposit is absent from the ignimbrite. The lower unit is lithic- (--30%) and ash-hch but pumice-poor, whereas the upper unit is pumice- and crystal-rich. Pumice content (--40%) is greatest in the middle of the upper unit, and towards the top of the upper unit pumice size and quantity decrease and crystal content increases (-50%). A fines depleted deposit usually separates the two main depositional units. However, within proximal exposures a sharp flow break separates the lower and upper flow units and the fines depleted deposit appears to have been eroded. In some exposures, the nature of the contact is gradational but the change from lithic- to pumice-rich facies takes place over a few metres, and the internal stratigraphy remains unchanged. In some localities the ignimbrite displays several stratigraphic textural variations and locally has multiple flow units. Each unit is texturally distinct, and these units occur in the basal part of the ignimbrite. These units range from fines depleted basal deposits, sometimes gradational with the lower unit, to pumice- and ash-rich units with a range of thickness variations. Their evolution appears intricately associated with irregular topography and possibly results from a decoupling process during pyroclastic density current transportation or the passage of multiple separate flow lobes/pulses over a single point. The Ignimbrite has both a compositional and textural range of juvenile clasts. The four main types are: dominant, white, highly vesicular, dacitic pumice clasts (66-69% Si02), subordinate banded dacitic pumice clasts, dense, volumetrically minor, mingled pumice clasts and rare, grey andesitic pumice clasts (61% Si02). The dense, volumetrically minor mingled pumice clasts are dominantly mafic in hand specimen and occur in the basal units. Their presence, together with the rare andesitic pumice clasts which occur near the top of the upper unit, suggest an input of a separate mafic magma batch or batches may have influenced (or even triggered) the eruption process. The crystal-rich nature of the magma, absence of a plinian airfall deposit, and occurrence of mingled juvenile clasts, all have important implications on the eruption mechanism and subsequent emplacement of the pyroclastic density currents.

Keywords: large volume, Cerro Galan Ignimbrite, internal stratigraphy, magma batches.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Establishing the stratigraphy of the Werribee Plains, Newer Volcanic Province, Victoria: an interdisciplinary approach Moore, A.G. & Cas, R A F . Department of Earth Sciences, Monash University, P.O. Box 28E, Clayton, VIC 3800, Australia.

The Newer Volcanic Province (NVP) of southeastern Australia is an intraplate continental plainsbasalt province. The province is east-west trending, extending 330 km west of Melbourne into South Australia, and covers an area of over 15,000 km^. Products range in age from 4.6 Ma - -4.5 ka. The NVP is one of only two volcanic provinces in Australia which is presently classified as active, yet there are aspects of the geology that have not been explored, such as the frequency of past eruptions and the likelihood of future volcanic activity. Conventional mapping techniques are not useful for establishing a stratigraphy in the NVP, because lavas are flat-lying, the topography is flat, weathering has been extensive, and erosional incision is limited. This study involves establishing a stratigraphy for the Werribee Plains in the east of the province by employing a number of different techniques. A network of diamond drill holes (containing up to --lOOm basalt thickness) are being logged, and geophysical well logs interpreted to constrain the palaeotopography. Flow unit boundaries have been identified in drill core, and palaeomagnetic studies are being performed on each flow unit. Eruption packages can be correlated between drill holes, based on the polarity of the earth's magnetic field at the time of eruption, which is recorded within the basalts. Within eruption packages separate volcanic facies will be identified by analysing the internal structure of the basalt flow units, including vesicularity trends and petrographic textural zonations. Quarry exposures will act as a guide to the scale of lateral facies variations, and identified facies will be interpreted in terms of physical emplacement processes. Whole rock and trace element geochemistry will further aid in correlating eruption packages, particularly in distinguishing between alkali and tholeiitic basalts. Once a stratigraphy has been established, geochronological studies will provide absolute ages of the products, and the frequency of eruptions.

Keywords: Newer Volcanic Province, basalt, stratigraphy

SURFACE PROCESSES

The Structural, Stratigraphic and Thermal History of the Skenes Creek - Wongarra Region, Otway Ranges, Eastern Otway Basin. Noll, C. Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia.

The Skenes Creek - Wongarra region lies on the coastal margin of the Otway Ranges in the eastern Otway Basin, about 160 km southwest of Melbourne. The region hosts spectacular coastal outcrops of Aptian-Albian Eumeralla Formation, deposited during Early Cretaceous extension between Australia and Antarctica. The region was mapped at 1:10,000 and 1:5,000 scale, in order to define the structural and stratigraphic framework. Vitrinite reflectance studies were used to further constrain fault displacement. A number of fault and fold trends were identified. Northeast-southwest and east-west trending faults are inferred to be associated with Early Cretaceous extension, while north-northwest trending faults are likely to be related to Late Cretaceous extension. Northeast trending faults, including the Skenes Creek, Beacon Point and Browns Creek Faults, were reactivated during mid-Cretaceous and Miocene-Recent inversion. This inversion lead to the development of northeast trending hanging wall anticlines and monoclines that are responsible for the present day topography of the Otway Ranges. The stratigraphic framework of the study area comprises mudstone dominated overbank and lacustrine successions and high flow regime fluvial sandstone of volcaniclastic composition. Overbank Fines (OF) deposits are characterised by the 1020 m thick Skenes Creek Coal Measures, a structurally controlled lacustrine deposit comprising of mudstone, thin coal seams, siltstone, minor fine grained sandstone and laminated sheetlike (LS) elements. Fluvial architecture is dominated by channel elements (CH) that comprise channel sandbodies (SB), downstream (DA) and lateral accretion (LA) elements, and are bound by fourth- and fifth-order bounding surfaces. The braided fluvial system comprises intraformational and exotic channel lag conglomerate, massive and planar laminated, trough and tabular cross-bedded and ripple laminated sandstone. Predominantly unidirectional palaeocurrents trend to the southwest and reflect the control of active, northeast trending intrabasinal faults on fluvial drainage and channel distribution. Organic matter in the region is dominated by vitrinite-rich coal and shaly coal, with a predominantly trimaceral microlithotype assemblage. An elevated palaeothermal gradient of -56°C/km existed in the Early Cretaceous, while Vitrinite Reflectance (VR) values indicate rocks in the hanging wall of the Skenes Creek Fault have been buried to at least 3500 m. Northeast trending faults on Skenes Creek Road have accumulated up to 880 m of vertical displacement, while displacements of up to 400 m are estimated across east-west and north-northwest trending faults on the coast.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No. 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Palynology of Bacchus Marsh glacigene sediments Pierson, R. Department of Earth Sciences, Deakin University, Clayton VIC 3168, Australia.

The Permian glacially derived sediments of the Bacchus Marsh area have attracted the interest of palaeontologists since the 1850s. Initially interest was restricted to leaves of the Gangamopteris Glossopteris flora. During the 1960s, Notoconularia inorta, Paraconularia sp, and Trigonotreta victoriae marine fossils were discovered. Sporomorphs have been extracted and studied by a number of researchers from 1939 to the 1980s. None of these palynological investigations was a comprehensive study of the sporomorphs of the area. The present work examines palynomorphs from samples taken at Bald Hill in material from old quarry sites, the Lerderderg River banks near Morven Farm, and from the shore of Merrimu Reservoir near Coimadai. Representative Early Permian spores, pollen of monosaccate, disaccate and monocolpate forms, as well as algal spores are figured in this poster. Reworked Devonian spores, probably of Antarctic origin, are also figured. The presence of the index forms Granulatisporites tentula, G. confluens (it.), and Pseudoreticulatisporites pseudoreticulata indicate a palynostratigraphic age for the Bacchus Marsh glacigene sediments of Asselian to Early Sakmarian. Using current palynostratigraphic nomenclature this is considered comparable to Australian Pollen and spores Permian APP12 to APP21.

Keywords:

palynology, Bacchus Marsh

SURFACE PROCESSES

Sedimentological constraints on Cu-Zn-(Pb) sulphide deposits from the Archaean Whim Creek Belt, Pilbara Craton, Western Australia Pike, G. & Cas, R A F . Department of Earth Sciences, Monash University, Clayton VIC 3800. Australia.

Polymetallic Cu-Zn-(Pb) sulphide mineralisation of the c.3,010-2,940Ma Whim Creek Belt, Pilbara Craton is dominated by three deposits of >1 Mt ore at Whim Creek, Mons Cupri and Salt Creek. The stratigraphic relationship, age and mineralogy of the three deposits differ and there is no reason to assume a common genetic model. However, all three have traditionally been interpreted as Volcanic Hosted Massive Sulphide (VHMS) deposits. No deposit preserves definitive evidence for primary mineralisation and a VHMS origin is not proven for any mineralised horizon within the Whim Creek Belt. Two main stratigraphic subdivisions are recognised in the Whim Creek Belt. The lower Sherlock Group contains dominantly volcanic and volcaniclastic rocks. These are further subdivided into a resedimented, arc related volcaniclastic association and a rift-related, bimodal association. Disconformably overlying the Sherlock Group is the Whim Creek Group, a package of siliciclastic sedimentary rocks and basaltic lava formed within a continental rift basin. Sedimentary rocks of the Whim Creek Group are host to the Whim Creek, Mons Cupri and Salt Creek deposits. However, the inferred source rocks are large-volume dacitic intrusions of the Sherlock Group. Sedimentary and volcanic textures are described that indicate a high-level intrusive origin for the dacitic intrusions of the Sherlock Group. This interpretation is inconsistent with the development of a significant hydrothermal system, as overlying sediment was probably only metres in thickness. In addition, new geochronology suggests that mineralisation is c. 60-70 Ma younger than the dacitic rocks and may be related to a significant heat event recorded in the Pilbara at c. 2,945 Ma.

GEOLOGICAL SOCIETY OF AUSTRALIA, ABSTRACTS No, 62 Victorian Universities Earth Sciences Conference, Monash University, September 2000

Optical luminescence ages from gypsum dunes in the Raak Boinka, Southeastern Australia: geomorphology, gypsum diagenesis and periodic water-table fluctuations over the past 100ka. ^Smith, B., ^ W e b b , J., ^Roberts, R. & ^Olley, J. ^Department of Earth Sciences, La Trobe University, Bundoora VIC 3083, Australia. ^CSIRO Land & Water, Canberra, ACT, Australia.

A series of quasi-prograding aeolian dunes bordering a groundwater discharge site in northern Victoria, Australia, have been optically dated using single and multiple grains of quartz. The dunes are composed of gypsum and quartz and are thought to have formed during drying phases following periods of high water level. A series of discrete Pleistocene ages have been obtained for this dune sequence, which are consistent with a framework of water-table reconstructions from nearby localities. The optical ages also correlate well with what are thought to be time-dependent variations in copi-gypsum crystal morphologies. Copi is believed to form as a result of internal weathering of the gypsum profile whereby sand sized gypsum is progressively converted to acicular prismatic copi crystals, then to stubby prismatic crystals, and finally to a microcrystalline discoidal variety. Porosity reduction, and possibly the addition of organics, are thought to control this sequence.

Keywords: OSL dating, gypsum, copi dunes, palaeoclimates, groundwater, salinity. Quaternary.

SURFACE PROCESSES

Global distribution of Permian conulariids: a preliminary report Weldon, E. A. School of Ecology and Environment, Deakin University, Rusden Campus, 662 Blackburn Road, Clayton VIC 3168, Australia.

This study examines a collection of four specimens of conulariids from the Permian. Two of the specimens are from the Sydney Basin. Identification of the Early Permian conulariid is indeterminable due to poor preservation. The second conulariid is from the Middle Permian and assigned to the genus Notoconularia (Thomas, 1969). Also of Middle Permian age is a cConularia sp. from the Phosphoria Basin, Idaho, USA and a Paraconularia sp. from the Russian Platform. The global distribution of the collection prompted investigation into the palaeobiogeography of Permian conulariids. In 1979, Waterhouse concluded that, in the Permian, the genera Paraconularia and Notoconularia could be regarded as cold- to cool- water fauna. A broader investigation into the palaeogeographic distribution of Permian genera may support Waterhouse's assertion of a cold- to cool- water fauna, and indicate Bipolar and/or Boreal Realm affinities for the fauna. In addition, temporal distribution data may demonstrate diachronous dispersal, which correlates to migration patterns and ocean currents.

Keywords: Permian, Conulariids, Palaeobiogeography