Newly identified mafic and felsic tuffs of the Shoalhaven and Talaterang Groups, southern Sydney Basin: their volcanic significance and palaeoecological impacts

Bann, Dr Glen1, Graham, Ian2, Jones, Brian1

1University of Wollongong, Wollongong, Australia, 2University of NSW, Kensington, Australia

A suite of newly identified mafic and felsic tuffs are described from the Shoalhaven and Talaterang Groups of the southern Sydney Basin. This includes the Clyde Coal Measures, Wasp Head, Pebbly Beach and Snapper Point Formations, Wandrawandian Siltstone, Nowra Sandstone, Berry Siltstone and the lower Broughton Formation. The tuffs are readily observed from outcrops in the field however, so far, have proved very difficult to discern in drill cores.

The mafic tuffs commonly comprise abundant biotite and muscovite grains, which are often deformed, K feldspar, plagioclase, volcanic quartz, with embayments, quartz shards and rare euhedral zircons. The felsic tuffs contain abundant volcanic and metamorphic quartz, plagioclase, more common pumaceous material, less micas and very rare or absent, shards. All tuffs contain carbonaceous material of various amounts and both are commonly reworked, although a lack of abrasion on the phenocrysts in the mafic tuffs suggests that the material has not travelled far from its source. Numerous dropstones of the same tuff material are common throughout the sequence, with volcanic types dominating in the east and metamorphic cratonic types in the west.

The Koo Lee Tuff Member, the largest of the mafic eruptions with a maximum thickness of almost 3m, is stratigraphically located within the lower Broughton Formation and due to its explosiveness and volume, has been deposited across a large area of the basin, hence outcrops in a number of locations. This provides the opportunity to identify eruption and emplacement mechanisms plus lateral changes in the deposit as well as providing a chronological time line through the southern Sydney Basin.

Volcanic detritus from island volcanoes to the south-east inundated sediment derived from the craton to the west during this period. Evidence from the presence of predominantly Cruziana ichnogenera and glendonites throughout the succussion, in addition to wavy contact surfaces beneath coarser sands and sporadic volcanic derived clasts suggest deposition was dominated by episodic storm activity in cold climate conditions with periodic coastal ice sheets depositing the clasts, or dropstones. Very fine-grained carbonaceous horizons indicate that deposition was also periodically dominated by extended low energy conditions. These deposits represent small or distant components of much larger volcaniclastic aprons surrounding a series of vents to the south-east. Evidence suggests a proximal source from island volcanoes ranging from mild Strombolian to the violently explosive Vulcanian or Plinean phreatomagmatic type eruptions. The association with the Late Permian Gerringong Volcanics and these earlier eruptions is presently unclear. The felsic eruptions are more distal, possibly associated with a large felsic provenance in the Zealandia craton to the south east.

The tuffs are often associated with trace fossil escape burrows, both successful and unsuccessful, and marine body death assemblages, commonly within the tuffs themselves but also found both below and above the tuffs. The effects of the eruptions and the tuffs on the local biota at the time will include changes in pH and Eh, elevated water and substrate temperatures, chemical toxicities such as Hg and As, during and post eruption, and an increase in turbidity. Effects will impact different species, with the more significant eruptions impacting everything. The more proximal eruptions, such as the Koo Lee Tuff, will also destroy habitat, displacing the animals.

It is therefore apparent that volcanism was controlling and dominating the deposition and conditions during early stages of the formation of the southern Sydney Basin.


Biography

This work has been ongoing since completing an Honours thesis on the early volcanism of the southern Sydney Basin in 1999, was drastically highjacked for many years by a PhD which had nothing to do with this, glad to be back and making interesting and challenging discoveries

Gravity inversion constrained by probabilistic magnetotellurics models: methodology and application

Giraud, Dr Jeremie Eugene Cyril1,2, Seillé, Dr Hoël3, Ciolczyk,Damien4, Grose, Dr Lachlan5, Visser, Dr Gerhard3, Lindsay, Dr Mark1,2, Jessell, Dr Mark1,2, Ogarko, Dr Vitaliy6

1Centre For Exploration Targeting, School Of Earth Sciences, University of Western Australia, Crawley, Australia, 2Mineral Exploration Cooperative Research Centre, School Of Earth Sciences, University of Western Australia, Crawley, Australia, 3CSIRO Deep Earth Imaging Future Science Platform, Kensington, Australia, 4Université de Strasbourg, School and Observatory of Earth Sciences (EOST), Strasbourg, France, 5School of Earth, Atmosphere and Environment, Monash University, Clayton, Australia, 6The International Centre for Radio Astronomy Research, The University of Western Australia, Crawley, Australia

In this contribution we introduce an inversion workflow where we integrate magnetotelluric (MT) data, which are primarily sensitive to horizontal resistivity interfaces, together with gravity  measurements, which are well suited for the recovery of lateral density variations. We connect these data using petrophysical prior information and geological principles. The approach we
present relies on a flexible, cooperative workflow where the different datasets are modelled using standalone algorithms. The workflow consists of the following three steps.

First, we perform the inversion of MT data in a 1D probabilistic fashion. For each MT site, the results include an ensemble of plausible 1D models, from which the probability of having an
interface between rock units of varying electrical conductivity is calculated. Second, these probabilities are interpolated to the whole study area using the implicit geological simulator LoopStructural. During this process, geological rules and principles (stratigraphy, superposition and cross-cutting relationships) are used to ensure that the models resulting from such probabilities are geologically plausible. Finally, domains corresponding to different rock units are derived using interfaces’ probabilities and are used in combination with petrophysical information to define the range of density contrast values allowed at every location in the model. These ranges are used to constrain deterministic gravity inversion using the alternating direction method of multipliers as implemented in the Tomofast-x engine.

We first summarise the methodology and present the proof-of-concept using a realistic synthetic model built using geological data from the Mansfield area (Victoria, Australia). Results demonstrate that the proposed workflow can effectively leverage the complementarity between geophysical methods relying on different physical phenomena such as MT and gravity and improve imaging. We then show preliminary results modelling existing real-world data that crosses the Eucla Basin (Western Australia), which is a region that is receiving increasing attention for its undercover mineral potential and where prior geological knowledge suggests complementarity between MT and gravity.

We acknowledge the support of the MinEx CRC and the Loop: Enabling Stochastic 3D Geological Modelling (LP170100985) consortia, and of the CSIRO Deep Earth Imaging Future Science Platform. The work has been supported, in part, by the Mineral Exploration Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Programme. This is MinEx CRC Document 2020/xx


Biography

Jeremie Giraud is a research fellow at the Centre for Exploration Targeting. His research efforts focus on the integration of geophysics and geology and on multi-physics integration.

How to computationally include regional interpretations into the seismic imaging process

Rashidifard, Mahtab1,3, Giraud, Dr Jérémie1,3, Ogarko, Dr Vitaliy1,2, Lindsay, A/Prof Mark1,3, Jessell, Prof Mark1,3

1Centre for Exploration Targeting, University Of Western Australia, 35 Stirling Highway, WA Crawley 6009, Perth,, Australia, 2International Centre for Radio Astronomy Research (ICRAR), University Of Western Australia, 35 Stirling Highway, WA Crawley 6009, Perth,, Australia, 3Mineral Exploration Cooperative Research Centre, School of Earth Sciences, University of Western Australia, 35 Stirling Highway, WA Crawley 6009, Perth,, Australia

Understanding the regional evolution of the Earth and subsurface processes is a key for mineral exploration. Crucial to this understanding is accounting for geoscience knowledge obtained from
integrated interpretation of geological and geophysical data. Reflection seismic data, although sparsely distributed due to the high cost of acquisition, is the only data that gives a high resolution image of the crust to reveal deep subsurface structures and the architectural complexity that may vector attention to minerally prospective regions. Without an iterative depth migration and a depth conversion step, seismic images remain in time domain and are totally data-driven. However, for reconstructing the architecture and history of the area it is necessary to have these images in depth. To obtain the depth of seismic events, depth correlations need to be applied to them from other sources of information. The limitation here is that existing depth conversion methods rely on borehole information which is rarely available for deep crust studies.

We introduce a new methodology which allows inclusion of deep regional interpretations from potential field and geological sources of information in depth migration of seismic data. This fast algorithm aims at reducing the ambiguity in time-to-depth conversion and reduce the time spent on depth migration of seismic data by numerically including geologists’ interpretations.
A modelling-ray approach, not dependent on borehole information and accounting for lateral variations in velocity models, is used for switching from a time-migrated to depth-converted domain. An imageray approach is used for the reverse process to move from a depth-migrated to a time-converted domain. On the other hand, primary geological models are directly used in potential field inversion algorithms without re-parametrization of the model using an existing generalized level-set algorithm. Integrating solutions of the Eikonal equation in the form of Hamilton-Jacobi for both potential-field levelset inversion and seismic migration leads to a simultaneous modelling through which seismic, potential field, and geological data mitigate each other’s limitations. An investigation of the proposed methodology and a proof-of-concept using a fairly advanced realistic synthetic dataset are presented.

The proposed workflow is a novel approach for questioning the geological meaning of the sparsely distributed seismic sections and to integrate them in a 3D volume following the regional geological data and potential field results. The primary outcome of this study is a step toward adding equal weight to geological data not only as primary models but also as additional quantitative information within geophysical modelling and seismic imaging algorithms.

Our results lead to a significant improvement in the final model consistency with available sparsely distributed data sets. As a result, seismic migration is influenced by complementary information from potential field inversion results while simultaneously respecting sparse seismic sections in the 3D model obtained from regional interpretation and potential field inversion.

We acknowledge the support of the MinEx CRC and the Loop: Enabling Stochastic 3D Geological Modelling (LP170100985) consortia. The work has been supported by the Mineral Exploration
Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Programme. This is MinEx CRC Document 2020/44.


Biography

I completed my undergraduate and masters in petroleum exploration engineering. I started my Ph.D. at UWA (CET) as a minexCRC student focusing on data fusion methodologies for integrated inversion of geological and geophysical data.  This presentation is part of my PhD project for integrating seismic and gravity with different coverage.  

Mapping the near surface architecture of the Amadeus Basin using magnetic data: Petrophysical properties and geophysics pitfalls

Austin, James1, Schmid, Susanne 2 and Foss, Clive1

1Potential Fields Geophysics, CSIRO Mineral Resources, Lindfield, NSW 2070, 2Multidimensional Geoscience, CSIRO Mineral Resources, Kensington, WA 6151

The Amadeus Basin in central Australia is prospective for stratiform base metal deposits and hydrocarbons. The Basin displays subtle magnetic anomalies that trace strata for considerable distance, highlighting complex folding patterns. Magnetic modelling techniques can be utilised on these stratiform anomalies to extrapolate the near-surface structure of the basin. Generally magnetic anomalies are assumed to have predominantly induced magnetisation, and with this assumption dip can be reasonably estimated using magnetic data alone. However, where the magnetisation is not purely induced (i.e., includes remanent magnetisation) the mathematical trade-off between the dip and magnetisation of bodies means that the dip of a body cannot be known unless the magnetisation is also known. Normally it would be optimal to measure the magnetisation, but this is not always possible or feasible, e.g., due to land access issues. In this study, we investigate the relationships between dip and magnetisation using an approach that would generally be considered a little backward. Rather than constrain structure using petrophysics, we use structural geology to constrain petrophysics. Three study areas were chosen to investigate numerous stratigraphic horizons in three major study areas, the Waterhouse Range, Glen Helen Station and Ross River areas. Modelling results suggest that a paucity of layers retain predominantly induced magnetisation, remanence is dominant in some, but both induced and remanent magnetisation are typically present. Remanence is mainly associated with relatively oxidised units that contain only hematite (e.g. Arumbera Sandstone), and comparisons with known apparent polar wander paths suggest that these magnetisations pre-date major folding in the basin. In some cases, magnetic anomalism reflects redox zonation within units, e.g. the Pertatataka Formation near Glen Helen, where discrete magnetic layers coincide with thin grey (reduced, magnetite-rich) horizons interbedded with more prevalent red (oxidised, hematite-rich) horizons, which are only very weakly magnetised. We also found that where magnetised units are relatively thin and occur near the surface, their magnetic response is sharp. However, in coincident aeromagnetic data, adjacent anomalies commonly overlap to form a single anomaly, thus misrepresenting the magnetic field, and mis-mapping the dip of the magnetic horizons. This study highlights some major pitfalls in attempting to map structure using magnetics. Near surface sedimentary units tend to be variably oxidised, and their petrophysical properties are inconsistent along strike. Their total magnetisation is commonly comprised of a significant component of remanent magnetisation, and therefore due to the mathematical trade-off between dip and magnetisation direction, industry standard inversions will commonly mis-map surface structure. Remanent magnetisation pre-dates major folding in many cases therefore, opposite limbs of the same fold can have completely different magnetic signatures. Our ability to target mineral systems in sedimentary systems is contingent on our ability to map the structure of such systems. This study demonstrates that petrophysical knowledge is a pre-requisite constraint for successfully informing structural and tectonic studies using geophysics.


Biography

James Austin is specialised in structural geology and potential fields geophysics, but he dabbles in many aspects of geology. His research is  focused on understanding  relationships between crustal processes and geophysical fields. His recent work is focused on the development of integrated technologies for mapping mineral systems.

Reconstructing the Soldiers Cap Group – Kuridala Group basin: Implications for BHT and IOCG mineralisation

Connors, Karen1

1Sustainable Minerals Institute, The University of Queensland, Brisbane, Australia

The vast basin hosting the 1700-1650 Ma Soldiers Cap and Kuridala groups (SCG-KG), eastern Mount Isa Province, NW Queensland, extends >300 km east to include the ca 1700-1610 Ma Etheridge Group, Georgetown Inlier. The present-day extent and thickness represents only part of the original depocentre following inversion, uplift and erosion (1610-1500 Ma). Whilst the importance of extension has long been recognised, pervasive compression, voluminous 1535-1490 Ma granites, and limited seismic integration, has prevented elucidation of the extensional architecture and its influence on inversion and mineral systems. Integrated interpretation of seismic and potential field data, and review of geochronology has provided a new understanding of the extensional architecture, potential age range and thickness of the basin, and the tectonic evolution.

The preserved thickness, extent, age range, erosion estimates, and the crustal architecture provide first-order constraints on basin reconstruction. The SCG-KG basin overlies several crustal-scale boundaries, including the Gidyea Suture where the thinned eastern margin of the poorly reflective, Mount Isa crust is thrust over the thinner Numil crust. The Numil comprises a series of moderate to low-angle fault blocks, many only 5-15 km thick, and typically has pervasive, dipping reflections.

While outcrop mapping suggests the SCG group is 2-5 km thick, seismic data indicates 3-5 sec TWT, implying 12-15 km preserved thickness. Mapping indicates localised isoclinal folding, nappes and structural repetition within some highly-deformed zones. Although regional structural repetition can’t be ruled out, the seismic data suggests many areas are dominated by limited repetition and thickening on inverted, normal faults.

The minimum age for the SCG-KG is generally accepted as 1650 Ma. However, the 1650-1610 Ma units of the Tommy Creek Domain and Etheridge Group are likely to have been widespread across the basin. In addition, zircon populations from drainages along the eastern outcrop margin and SCG show peaks in juvenile mafic magmatism at 1630-1625 Ma, as well as 1667 Ma. Although the upper SCG-KG unit (Toole Creek Volcanics (TCV)) is attributed to thermal relaxation from ca 1670 Ma, coeval felsic and mafic magmatism at 1655 and 1625 Ma, and the large volume (20-30%) of high-Fe mafic sills within the TCV suggest extension continued or was episodic.

Prior to inversion and erosion, the 12-15km SCG-KG basin was thicker as well as wider than the present ~350 km. While the stretching factor and total extension are unknown, the thin low-angle fault blocks of the Numil, are consistent with highly-thinned to hyperextended crust (i.e. 10 km thickness or less), and exhumation of lower crust or mantle may have occurred. The resulting high geothermal gradient has implications for BHT mineralisation, and raises questions regarding controls on metal deposition.

The extensional fault system and preliminary reconstruction provide insights into the extensional evolution and controls on later inversion. The structural framework and its links to the underlying basement blocks and crustal-scale structures that form the first-order conduits of the plumbing system provide insights for both syn-sedimentary BHT mineral systems and later IOCG deposits.


Biography

Karen has had a varied career in mineral and petroleum exploration. She specialises in integrated interpretation of seismic with potential field data to understand 3D crustal architecture, structural inheritance and influence of basement on basin evolution, 3D modelling, and controls on mineral systems.

Review of Australian Mesoproterozoic basins: Geology and resource potential

Anderson, Jade1, Carr, Lidena1, Henson, Paul1, Carson, Chris1

1Geoscience Australia, Canberra, Australia

Australian cratons underwent substantial tectonism and cratonic reorganisation during the Mesoproterozoic, coinciding globally with the transition from Nuna to Rodinia (e.g. Li et al., 2008; Pisarevsky et al., 2014). The full extent and nature of this tectonism remains contentious (e.g. Bagas, 2004; Betts and Giles, 2006; Cawood and Korsch, 2008; Maidment, 2017).

During the Mesoproterozoic several sedimentary basin systems were deposited, and are now variably preserved, in the Northern Territory, Queensland, Western Australia, South Australia and Tasmania; providing an invaluable indirect record of the evolving Australian lithosphere and tectonic processes. Most of these basins were deposited on or at the margins of Archean to Paleoproterozoic cratons (North Australian Craton, West Australian Craton and South Australian Craton; e.g. see Myers et al. 1996; Cawood and Korsch, 2008 for spatial geography and constituents of these cratons). The remnants of these basins vary from weakly-deformed, relatively continuous units, such as the Roper Group of the McArthur Basin in the Northern Territory, to basins that were subsequently deformed and metamorphosed under high grade conditions, such as the Arid Basin of the Albany Fraser Orogen in Western Australia.

Individual basins are typically studied in isolation or in subsets, for which available geological datasets are commonly disparate with markedly different levels of knowledge. Mineral and energy resources have been identified in some of these basins; including oil and gas resources hosted in the Roper Group in the Beetaloo Sub-basin; manganese deposits in the Collier Basin and Manganese Group (Western Australia); and polymetallic, stratabound, hydrothermal mineralisation in the late Paleoproterozoic to early Mesoproterozoic Edmund Basin (Western Australia). Typically, these more overtly prospective basins, or groups, have been studied in greater detail than other Mesoproterozoic basins or groups.

This study provides a holistic overview of Australian Mesoproterozoic sedimentary basin systems, integrating geological, geochronological, and publically available resource data. As part of this collated approach, we also discuss potential inter-basin correlations for Mesoproterozoic-aged successions in Australia. This study aims to assist future work targeted at improving the geological understanding of these Mesoproterozoic sedimentary provinces and their resource prospectivity.

References

Bagas, L., 2004. Proterozoic evolution and tectonic setting of the northwest Paterson Orogen, Western Australia. Precambrian Research 128(3-4), 475-496.

Betts, P. G. and Giles, D., 2006. The 1800-1100 Ma tectonic evolution of Australia. Precambrian Research 144(1), 92-125.

Cawood, P. A. and Korsch, R. J., 2008. Assembling Australia: Proterozoic building of a continent. Precambrian Research 166(1-4), 1-38.

Li, Z. X., Bogdanova, S. V., Collins, A. S., Davidson, A., De Waele, B., Ernst, R. E., Fitzsimons, I. C. W., Fuck, R. A., Gladkochub, D. P., Jacobs, J., Karlstrom, K. E., Lu, S., Natapov, L. M., Pease, V., Pisarevsky, S. A., Thrane, K. and Vernikovsky, V., 2008. Assembly, configuration, and break-up history of Rodinia: A synthesis. Precambrian Research 160(1-2), 179-210.

Maidment, D. W., 2017. Geochronology from the Rudall Province, Western Australia: implications for the amalgamation of the West and North Australian Cratons. Geological Survey of Western Australia, Perth, 95 pp.

Myers, J. S., Shaw, R. D. and Tyler, I. M., 1996. Tectonic evolution of Proterozoic Australia. Tectonics 15(6), 1431-1446.

Pisarevsky, S. A., Elming, S.-Å., Pesonen, L. J. and Li, Z.-X., 2014. Mesoproterozoic paleogeography: Supercontinent and beyond. Precambrian Research 244, 207-225.


Biography

Jade Anderson completed a PhD at the University of Adelaide in the areas of metamorphic geology, geochronology and Proterozoic Australia tectonics. She currently works as a Geoscientist in Basin Systems at Geoscience Australia.

Quantifying the Dolomite problem and its impacts on Mg/Ca change through time

Opdyke, Dr Bradley1

1The Australian National University, Australia

Global dolomite deposition has declined globally throughout the Cenozoic.  While the volumes of other sedimentary rocks increase from the Paleocene to the Holocene.  This anomaly has been called the ‘dolomite problem’ and recognized since the time of Darwin.  Recently my team discovered that crustose coralline algae  (CCA) does precipitate calcium-magnesium carbonate with a dolomite chemistry.  CCA is not as abundant on immature reefs as mature reefs, in fact CCA ‘crusts’ only become thick and widespread once a significant portion of the reef flat has been located within the tidal zone for many thousands of years.  In a world where sea level is moving up and down like a Milankovitch driven yo-yo it is rare for these algal facies to become established, hence dolomite precipitation is rare. Recent sea level studies and compilations of stable isotope records from the Eocene to the Holocene allow us to model the probable trajectory of dolomite deposition over this time interval and calculate the probable Magnesium-Calcium ratio change in sea water.  At the present time we only have a few ‘tie points’ for the Mg/Ca over this time.  Using the stability of sea level as a driver of dolomite production we can predict more precisely how the Mg/Ca ratio has increased from 2.5 at the Eocene/Oligocene boundary to 5.1 in modern sea water.


Biography

AB Geochemistry Columbia University 1984

MS Geology The University of Michigan 1987

PhD Geology The University of Michigan 1990

Characteristics and diagenesis of the Upper Permian Beekeeper Formation from the Perth Basin, Western Australia

Adhari, Muhammad Ridha.1 Wilson, Moyra E. J.1

1School of Earth Sciences, The University of Western Australia, Perth, Australia

The Upper Permian Beekeeper Formation is a proven reservoir in the Woodada field, Perth Basin, Western Australia, yet the nature and sedimentary features of this formation are not well understood. The Beekeeper Formation is about 20 km wide and 70 km long and the thickness of this sedimentary unit is up to 134 m. This formation is interpreted to be deposited in a cool-water ramp setting and has been identified not only in the Woodada field, but also in the Beharra Spring field, Perth Basin. This study aims to better understand the characteristics of the Beekeeper platform and the diagenetic processes that occurred during its evolution. Sixty metres of conventional cores are available from five wells from the Perth Basin and 127 thin sections were made from those cores. Sedimentary logging, acetate peels, thin section petrography, and acid digestion have been conducted on the available dataset. Results from this study show that the Beekeeper Formation is a mixed carbonate-siliciclastic system, with both coeval and reciprocal mixing during sequence development. Bryozoas, brachiopods, and crinoids are the main bioclasts in packstones, grainstones, and rudstones. Primary matrix porosity is minimum in the Beekeeper Formation, but the secondary fracture porosity is of the highest quality. The fracture system is interpreted to be generated through tectonic activity on the basis of the relative timing of the paragenetic events, offset along the fractures, common sub-vertical fracture orientations, multi-size and multi-episode fracture development, and multi-phase fracture cements. The main diagenetic processes affecting the Beekeeper Formation include micritisation, boring, mechanical compaction, syntaxial overgrowth, granular-blocky calcite cementation, chemical compaction, dolomitisation, recrystalisation, and replacement, whereas bioclast and calcite vein dissolution are as minor features. These findings are expected to advance our understanding of this Upper Permian mixed carbonate-siliciclastic system and reservoir.

Keywords: Upper Permian, Mixed carbonate-siliciclastic, Beekeeper Formation, Fracture porosity


Biography

Muhammad Ridha Adhari is a student at the School of Earth Sciences, UWA sponsored by AAS. Adhari holds a bachelor degree from Institut Teknologi Bandung, Indonesia and a master degree from Curtin University, Australia. Currently, he is conducting a research on fractured carbonate-siliciclastic reservoir from the Perth Basin, Australia.

Deep water cuspate stromatolites in the Cryogenian Trezona Formation, South Australia

O’Connell, Brennan1, Wallace, Dr Malcolm W.1, Hood, Dr. Ashleigh v.s.1, Rebecchi,Luke1

1University of Melbourne, , Australia

Deep water stromatolite horizons are well developed in the Cryogenian Trezona Formation and were deposited in an open marine, mid- to outer-ramp setting. Stromatolite horizons predominantly occur in association with shales that contain intraclastic horizons interpreted as mass flow deposits, and are associated with iron oxides and irregular surfaces of erosion. These stromatolite horizons—developed as elongate structures with cuspate mm-scale laminae—are interpreted as condensed sections in sediment starved settings. Documentation of these deep water stromatolites adds to a small collection of deep water stromatolites, which appear to be largely represented by cuspate forms. Deep water stromatolites could be linked to specific redox environments such as anoxic/suboxic ferruginous waters, and/or may be related to carbonate saturation, sediment starvation, or other factors.


Biography

Brennan is a PhD candidate at the University of Melbourne.

Descending into the “snowball”: Improving interpretations of Tonian palaeoenvironments with multi-proxy elemental and isotopic geochemistry

Virgo, Georgina1,2, Collins, Alan2, Farkas, Juraj3, Blades, Morgan2, Amos, Kathryn1, Lloyd, Jarred2

1Australian School of Petroleum and Energy Resources, the University of Adelaide, SA 5005, Australia, 2Tectonics and Earth Systems (TES) and Mineral Exploration CRC, Department of Earth Sciences, the University of Adelaide, SA 5005, Australia, 3Metal Isotope Group (MIG), Department of Earth Sciences, the University of Adelaide, SA 5005, Australia

The Tonian–Cryogenian transition represents a period of significant physiochemical change in Earth history. It involved variations in oceanic and atmospheric oxygenation, significant changes in the biosphere, tectonic reorganisation, and the onset of the global ‘Sturtian’ glaciation. Tonian and Cryogenian sedimentary rocks in the Adelaide Superbasin, South Australia (SA), represent some of the most well-exposed, continuous and thick sections of this interval globally, recording major environmental shifts through distinct variations in lithology and isotope chemistry. Although this transition is geologically significant, it remains enigmatic due to a distinct lack of comprehensive, contemporary Tonian–Cryogenian research in South Australia.

We present robust palaeoenvironmental interpretations for a complete pre- to post- Sturtian glacial succession near Copley in the northern Flinders Ranges, SA. During fieldwork, a ~3km sedimentary log was measured for facies and sequence stratigraphic analyses, and 350 samples were collected for elemental and isotopic geochemical analyses. Our study reveals multiple regressive-transgressive cycles, recorded by deltaic rippled and cross-stratified sandstones, through lagoonal intraclastic magnesite and stromatolitic carbonates, to subtidal laminated siltstone and platform carbonates. These pre-glacial formations are unconformably overlain by subglacial to ice contact pebbly diamictites with quarzitic and dolomitic interbeds, which grade into proglacial laminated mudstone and sandstone with dropstones. We suggest that these facies reflect glaciomarine conditions. The post-glacial formation consists of subtidal laminated shales and carbonates, reflecting widespread transgression after the glaciation.

Elemental chemistry, along with C- and Sr-isotope signatures were analysed to determine primary basin water chemistries or palaeo-seawater compositions, and to further constrain the depositional setting. Results demonstrate a nearshore/restricted, dysoxic setting with indications for moderate hydrothermal input, which supports the sedimentological data. Furthermore, there is an inverse relationship between 13C and 87Sr/86Sr data, ranging from 7.37‰ to -6.68‰ and 0.7088 to 0.7182, respectively. In addition, the studied carbonates also exhibited relatively light 88Sr values (≤0.211‰). These isotopic observations could reflect a drop in relative sea level, increased weathering of carbonates and sufficient input of continental material, which is consistent with sequence stratigraphic interpretations. Such settings might be analogous to the modern Coorong depositional environment, SA, where interaction of seawater with brackish continental waters facilitate precipitation of primary dolomite and magnesite. Insights into the water chemistry and isotope signatures of these primary Mg-rich carbonates will assist with interpretations of isotope data collected from the studied Tonian dolomites and magnesites. This multi-proxy study presents new palaeoenvironmental insights into a key Tonian–Cryogenian succession, which sheds light in our understanding of how the world descended into one of the most severe glaciations ever recorded.


Biography

Third year Phd student, research focused on sedimentology and geochemistry of Neoproterozoic sediments in South Australia

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About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.