Australian Journal of Earth Sciences (2000) 47, 337–340

INTRODUCTION

Palaeoproterozoic sedimentary rocks of the McArthur –Mt Isa – Cloncurry mineral belt of northern Australia hostthe world’s most important zinc repository, several world-class Cu and Cu–Au deposits and Australia’s principal uranium reserves. Advances in exploration techniques in the 1980s led to the discovery of several major deposits,including Cannington, Century, Ernest Henry and Osborne,and several prospects, including Walford Creek and Grevillea.

Over the past 25 years the area has been the subject of significant geoscientific investigation. Commonwealth,State and Territory geoscientists have mapped the Palaeo-proterozoic outcrop belt at a scale of 1:100 000, andresearchers from several universities, including JamesCook University, Monash University and the University ofTasmania, have undertaken a variety of integrated studies aimed at better understanding the tectonics andmineralisation of the region. Despite this extensive historyof recent scientific endeavour, exploration results duringthe 1990s were disappointing.

During 1994 the Australian Geological Survey Organis-ation (AGSO) and its National Geoscience Mapping Accordpartners, the Queensland Department of Mines and Energyand the Northern Territory Geological Survey, met withrepresentatives of the mineral-exploration industry toevaluate the existing exploration strategies in the northAustralian region. The outcome was the North AustralianBasins Resource Evaluation (NABRE) project. The projectcombined mineral- and petroleum-exploration techniquesand focused on a number of key tasks that could have thepotential to generate new exploration concepts.

The main objectives of the NABRE project were to:(i) establish the evolution of ‘basement’ and its influenceon the architecture and stratal geometry in the many basins of the region; (ii) establish a regional time–space or tectonostratigraphic framework for the basins usingstructural, stratigraphic and age-dating techniques; and(iii) understand basin architecture and potential fluidmigration pathways at the time of each major pulse offluid movement. The petroleum concepts of plays and play elements were central to the NABRE project approach.A ‘source’ for the fluids, timing of ‘generation’ of a fertile brine, timing and pathway of fluid ‘migration’, a‘trap’ for the deposit, a ‘seal’ that may have controlled the migration path and formed a component of the trap and the ‘reservoir’ as a component of some types of traps.

This work has led to: (i) the development of a new geo-dynamic model for Palaeoproterozoic crustal evolution andbasin development in northern Australia, (ii) a chrono-stratigraphic basin framework for the interval 1730–1575 Ma,and (iii) the identification of a number of play concepts thatwill assist in future exploration in the area. The results ofthe NABRE project are reported here and in a comple-mentary set of AGSO Records, in CD ROM format, docu-menting the stratigraphic (Bradshaw & Scott 1999; Krassayet al. 1999; Southgate et al. 1999; Jackson et al. 2000) and geo-physical databases (Tarlowski & Scott 1999).

Overview of the thematic issue

This thematic issue comprises 12 papers organised under the three main objectives or themes of the NABREproject. The first two manuscripts consider the evolutionof metamorphic and stratigraphic basement to the prospec-tive intervals of the Mt Isa, McNamara, Fickling andMcArthur Groups. The next two papers document SHRIMPzircon and palaeomagnetic datasets used to constrain thesequence stratigraphic results. The remaining set of eightpapers report the results of the integrated chronostrati-graphic analyses. The sequence-stratigraphic papers arearranged in approximate chronological order and progressfrom southeast to northwest across the outcrop belt (Figures 1, 2). The framework studies documented hereinwill be of use to geoscientists worldwide. The processes ofbasin evolution will assist geoscientists to unravel the earlyphases of basin evolution in Palaeoproterozoic successionsnow preserved as structurally dismembered and meta-morphosed provinces in Australia and overseas.

THEME 1: BASEMENT STUDIES

Scott et al. (Figures 1, 2) integrate geochemical, geochrono-logical and geophysical datasets for north-central Australiato develop an understanding of basement evolution across the region. The new geodynamic model for Palaeo-proterozoic crustal evolution and basin development(1800–1575 Ma) relates the development of thick accumula-tions of sediments (superbasins) in northern Australia toconvergent, subduction and magmatic arc processes in central Australia. The model compares and contrasts base-ment grain and fabric across northern Australia andrelates basin development to variations in crustal responseduring times of compression. While acknowledging theexistence of local transtensional to extensional structures

Carpentaria – Mt Isa Zinc Belt: basement framework, chronostratigraphy and geodynamic evolution of Proterozoic successions

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the new model proposes that these structures are bestexplained by a combination of flexural, thermal anddynamic processes related to an active southern marginwhere subduction and orogeny took place. The convergentmodel is significantly different to previous extensional con-cepts involving repeated episodes of rift and sag.

Jackson et al. (Figures 1, 2) present new stratigraphic,geochemical and palaeomagnetic data from the MurphyInlier and extrapolate the improved understanding torevise correlations for the successions of northern Aus-tralia deposited between 1800 and 1700 Ma. The plethora oflithostratigraphic units, currently used to describe theserocks, are rationalised into two major superbasins: rocksof the 1800–1750 Ma interval comprise the Leichhardt Superbasin and those from the 1730–1690 Ma interval,the Calvert Superbasin. An age gap of approximately 20 million years separates the two superbasins. Eightpseudo-chronostratigraphic basin phases are recognised inthe two superbasins.

THEME 2: CONSTRAINING THE AGE OF PROTEROZOIC SEDIMENTS

Two methods were used to constrain and test the sequencestratigraphic correlations: palaeomagnetism and SHRIMPzircon geochronology. Palaeomagnetic research by Idnurm(Figures 1, 2) focused on improving the resolution of theapparent polar wander path, primarily to better understandthe chronology of intraplate tectonic events across north-

ern Australia. Poles from the Gunpowder Creek Formationhelp define the intraplate tectonic event responsible for closure of the Calvert and opening of the Isa Superbasins.Seven new primary poles upgrade the apparent polar wander path for northern Australia. Primary and sec-ondary poles, previously restricted to sampling sites in theMcArthur region, have now been extended to the Lawn HillPlatform in the east, permitting better correlations betweenthe two regions.

Page et al. (Figures 1, 2) document 55 new SHRIMP zircon ages obtained from basement rocks and basin sedi-ments of northern Australia. These ages provide a time calibration for events principally in the Calvert and IsaSuperbasin phases. Zircon ages are determined for nine ofthe formations previously used to document primary poleson the apparent polar wander path and provide a detailedtime calibration for the apparent polar wander path.

THEME 3: SEQUENCE-STRATIGRAPHIC STUDIES

The first paper by Southgate et al. (Figures 1, 2) provides aregional chronostratigraphic overview for sediments of the Calvert and Isa Superbasins. This paper integratessequence-stratigraphic interpretations of outcrop, drill-core, wireline-log and seismic datasets with SHRIMP zircon and palaeomagnetic determinations to provide adetailed chronostratigraphic basin framework for thesouthern McArthur, Lawn Hill and Mt Isa regions ofnorthern Australia. Nine 2nd-order unconformity-bounded

338 P. N. Southgate

Figure 1 Map showing theregion of interest for eachof the papers in this issue.

supersequences are identified, each with a duration of10–20 million years. Nested 3rd- and 4th-order sequences aredefined within the supersequences. Sequence-stratigraphicsubdivision at the 4th-order scale provides correlations toan accuracy of approximately 1 million years. The analy-sis relates accommodation history and times of fluid flowto major intraplate tectonic events evident on the apparentpolar wander path for northern Australia.

Domagala et al. (Figures 1, 2) document the sequence-stratigraphic evolution of the Surprise Creek Formationand Mt Isa Group (upper Calvert and lower Isa Super-basins) of northwest Queensland. Southgate et al. integratethe results of this work and provide a regional depositionalmodel for the ca 1670–1650 Ma Gun Supersequence. Rocksof the lower McNamara Group (Gunpowder Creek toEsperanza Formations) and Mt Isa Group (upper WarrinaPark Quartzite to Kennedy Siltstone) are interpreted toform in a marine environment, on a broad southeast-facing ramp. The depocentre occurred in the Mt Isa Valleywhere the deepest water facies are recorded. Host sedi-ments for the Mt Isa deposit are interpreted as accumulat-ing in a basin dominated by sinistral strike-slip tectonism.

Papers by Krassay et al. (Figures 1, 2) synthesise thesequence-stratigraphic evolution of the upper McNamaraGroup (upper Lady Loretta to Lawn Hill Formations). Fivesupersequences are recognised in this stratigraphic inter-val. South of the Murphy Inlier the supersequences definetwo south-thickening depositional wedges, separated by amajor intrabasin structure, the Elizabeth Creek FaultZone. Both wedges are internally partitioned into a seriesof smaller strike-slip sub-basins whose sedimentary fill iscommonly affected by synsedimentary faulting. Recon-struction of the stratigraphic architecture in the twowedges shows basin inversion, depocentre switching andregional changes in provenance and depositional systemsthat characterise the Lawn Hill Platform from RiversleighSiltstone- to Lawn Hill Formation-time. Organic-matterrich shales in the depocentres of fault-bounded sub-basinshost the Pb–Zn–Ag base-metal mineralisation at the Century mine and the Grevillea and Walford Creekprospects.

Papers by Bradshaw et al. and Jackson and Southgate(Figures 1, 2) discuss the evolution of depositional systemsin attenuated basin-margin settings. Bradshaw et al. inte-grate seismic, drillcore and outcrop datasets to documentstratigraphic architecture on the southern flank of theMurphy Inlier. The combined effects of onlap and trunca-tion of sequences over the Murphy Inlier basement highare responsible for the thinning of strata in the Calvert andIsa Superbasins from 11 km in the depocentre to 1 km at thebasin margin. In the Fickling Group unconformity surfaceswith up to 25 million years of missing rock record are man-ifest at supersequence boundaries as conglomerate bedscomposed of silicified detritus from older strata. Jacksonand Southgate provide a revised interpretation of deposi-tional systems in the Nathan Group. The integration ofgamma-ray datasets, facies and SHRIMP zircon ages per-mits the identification of three attenuated supersequencesin a lithostratigraphic unit previously interpreted as form-ing a conformable, gradually evolving succession, of shal-low-water lacustrine and lagoonal carbonates. Eachsupersequence is a few hundred metres thick and records

Introduction 339

Figure 2 Bar graph showing the temporal range for each of thepapers in this issue.

deposition over a few million years. Stratigraphic breaksapproaching 10 million years in duration separate eachsupersequence.

Abbott and Sweet (Figures 1, 2) provide a sequence-stratigraphic model for siliciclastics in the middle parts ofthe marine Roper Superbasin. They develop a sequencemotif for the large-scale coarsening-upward cycles that isdifferent from the models used in most other papers. How-ever, like supersequences and sequences in the underlyingCalvert and Isa Superbasins, those in the Roper Superbasinare also attributed to tectonically driven cycles. In the RoperSuperbasin these cycles are tentatively related to flexuralloading, possibly associated with the Isan Orogeny ofwestern Queensland or the Anmatjira Uplift of central Australia.

ACKNOWLEDGEMENTS

Many people have contributed to the NABRE project andto bringing the results of five years of basin research tofruition. As editor of this thematic issue and project leaderfor the project I would like to express my gratitude to the loyal, energetic and creative team of researchers andtechnicians (particularly Andrew Retter, Inge Zeilinger andKurt Barnett) whose work is represented in these papers.The project was originally conceived by Tom Loutit, whogenerated initial support, both within AGSO and the Australian mineral-exploration industry. The support,advice and mentoring provided to me by Tom Loutit andChris Pigram are gratefully acknowledged. I also wish tothank the many reviewers for their time and constructivecomments, which improved the published works: acknow-ledgements for individual reviewers are found in eachpaper. The fruitful discussions with mineral industry geo-scientists—Mark Hinman, Graeme Broadbent, SteveAndrews, John Wright, Andrew Allan, Jonathan Clarke,Graeme Drew, Tony Hespe, Simon Tear, John Dunster, IanGarsed, Martin Neudert, Alan Goode and Geoff Derrick—are gratefully acknowledged. Access to confidential drill-

core provided by Rio Tinto, Pasminco, MIMEX, WMC Cor-poration, Western Metals, BHP, North Exploration andCoolgardie Gold assisted greatly in the stratigraphic synthesis. The project also benefited from discussions held with our many colleagues, familiar with the geologyof northern Australia, in AGSO, Geological Survey ofQueensland, Northern Territory Geological Survey,University of Tasmania, James Cook University and University of Queensland. Finally, I wish to express my sincere thanks to Tony Cockbain, editor of the journal,for his time and patience in advising me on matters editorial and finding those inevitable mistakes that I over-looked.

REFERENCES

BRADSHAW B. E. & SCOTT D. L. 1999. Integrated basin analysis of theIsa Superbasin using seismic, well-log and geopotential data: anevaluation of the economic potential of the northern Lawn HillPlatform. Australian Geological Survey Organisation Record1999/19.

JACKSON M. J., SOUTHGATE P. N., WINEFIELD P. R., BARNETT K. & ZEILINGER

I. 2000. Revised sub-division and regional correlations of theMcArthur Basin succession based on NABRE’s 1995–8 sequencestratigraphic studies. Australian Geological Survey OrganisationRecord 2000/03.

KRASSAY A. A., BRADSHAW B. E. DOMAGALA J. ET AL. 1999. Measured sections and sequence stratigraphic interpretations: upper McNamara and Fickling Groups. Australian Geological SurveyOrganisation Record 1999/15.

SOUTHGATE P. N., SAMI T. T., JACKSON M. J. ET AL. 1999. Measured sections and sequence stratigraphic interpretations: lowerMcNamara, Mt Isa and Fickling Groups. Australian GeologicalSurvey Organisation Record 1999/10.

TARLOWSKI C. Z. & SCOTT D. L. 1999. Geophysical Investigations in north central Australia: images, algorithms and crustal studies.Australian Geological Survey Organisation Record 1999/29.

P. N. SOUTHGATEAustralian Geological Survey Organisation,

GPO Box 378, Canberra, ACT 2601, Australia

340 P. N. Southgate