A very unconventional hydrocarbon play: The Mesoproterozoic Velkerri Formation of Northern Australia

Collins, Alan S.1, Cox, Grant M.1, Jarrett, Amber J.M.2, Blades, Morgan L.1, Shannon, April, V.1, Yang, Bo.1, Farkas, Juraj1, Hall, P. Tony1, O’Hara, Brendan3, Close, David3, Baruch, Elizabeth, T.4, Altmann, Carl4, Evans, David5, Bruce, Alex5

1Tectonics and Earth Systems Group, The University Of Adelaide, Adelaide, Australia, 2MinEx CRC, , , 3Geoscience Australia, Canberra, Australia, 4Santos Limited, Adelaide, Australia, 5Origin Energy Ltd, Brisbane, Australia, 6Empire Energy, Sydney, Australia

The ca. 1.5–1.3 Ga Roper Group of the greater McArthur Basin is a component of one of the most extensive Precambrian hydrocarbon‐bearing basins preserved in the geological record, recently assessed as containing 429 million barrels of oil and eight trillion cubic feet of gas (in place). It was deposited in an intra‐cratonic sea, referred to here as the McArthur‐Yanliao Gulf.

The Velkerri Formation forms the major deep‐water facies of the Roper Group. Trace metal redox proxies from this formation indicate that it was deposited in stratified waters, in which a shallow oxic layer overlay suboxic to anoxic waters. These deep waters became episodically euxinic during periods of high organic carbon export. The Velkerri Formation has organic carbon contents that reach ~10 wt%. Variations in organic carbon isotopes are consistent with organic carbon enrichment being associated with increases in primary productivity and export, rather than flooding surfaces or variations in mineralogy.

Although deposition of the Velkerri Formation in an intracontinental setting has been well established, recent global reconstructions show a broader mid to low latitude gulf, with deposition of the Velkerri Formation being coeval with the widespread deposition of organic rich rocks across northern Australia and North China. The deposition of these organic‐rich rocks may have been accompanied by significant oxygenation associated with such widespread organic carbon burial during the Mesoproterozoic.


Biography

Alan Collins is a tectonic geologist with wide ranging interests in basin analysis and plate tectonic controls on the development of the earth system

Tasmanian landslide fatalities and some implications for landslide risk

Roberts, Dr  Nicholas1

1Mineral Resources Tasmania, Rosny Park, Australia

Tasmania experiences frequent, diverse landslides ranging from extremely slow failures that gradually damage structures to extremely fast ones capable of claiming lives. Their impacts in recent decades might give the false impression that Tasmanian landslides threaten only infrastructure and not lives. However, life-loss risk is poorly constrained – and generally underappreciated – because the state’s landslide fatalities have not been fully inventoried. Details from a growing catalogue of Tasmanian landslide fatalities (excluding underground failures) show that deaths, although sporadic, do occur and were surprisingly common during the nineteenth and earliest twentieth centuries. Landslides have injured at least 23 people in the last 100 years. Fourteen of the injuries and four of the five confirmed landslide fatalities from that period occurred in 2001 when a road shoulder collapsed under the weight of a bus. Although sometimes conflated with mass movement, the rainfall-triggered burst of a rock-filled concrete dam above Derby in 1929 that claimed 14 lives was unrelated to landsliding. Tasmania’s at least eight landslide fatalities prior to 1920 provide additional insight into life-loss potential. Deadly landslides occurred during construction of Port Arthur’s Convict Church (ca. 1836), quarrying in Hobart (1848) and Queenstown (1905), railway expansion through Kelly Basin (1889), and open-pit mining at Mount Bischoff (1900). All but the 1848 failure, which killed four, were single-fatality events, providing a stronger historical basis for calibrating quantitative estimates of individual risk compared to group risk. Commonalities between these early events highlight settings and processes of particular concern as well as a population of elevated exposure. Each failure affected very recently excavated slopes, involved extremely rapid sliding and flow of soil-strength materials, and exclusively claimed lives of workers (convicts before 1850, employees thereafter). Coronial Inquests into each of the events from 1848 to 1905 provide more detail about failure metrics and behaviour than is commonly available for landslides of that era. All eight deaths were ruled accidental despite knowledge that many sites were failure-prone. Several close calls prior to 1920 highlight risks of members of the public being engulfed by debris flows or impacted by landsides at home. However, fatalities from hazardous phenomena that commonly influence (bushfires) or accompany landslides (flash flooding) remain more common. Although Tasmanian landslide fatalities superficially appear to have decreased, determining and explaining trends in these records is complicated by data sparsity. Advances over the past century in engineering, workplace safety, and regulation undoubtedly affect risk levels, but evaluating other possible influences such as long-term drought-rain cycles requires further work. Decreases in per-capita landslide fatalities elsewhere are attributed to progressive improvements (e.g. British Columbia) or drastic reforms (e.g. Hong Kong) in local risk management; circumstances in Tasmania are closer to the former, although the low number of deaths complicates comparisons. Notwithstanding these challenges, it is noteworthy that fatal landslide in Tasmania show several commonalities, are generally underestimated, and are possible in the future, particularly as population and development increase.


Biography

Nick is a natural hazards geologist and Quaternary geoscientist in the Geological Survey Branch of Mineral Resources Tasmania. A large part of his work centres on characterizing landslides and their impacts across Tasmania through a combination of field investigations, diverse remote sensing techniques, analytical tools, and reviewing historical documents.

On the foundering of carbonate platforms and coral reefs

Wilson, Moyra E.J.1, Arosi, Hamed, A. 1, Loche, Marco1, Webster, Jody2,

1School of Earth Sciences, University of Western Australia, Perth WA, 2University of Sydney, Sydney, Australia. Correspondence e-mail: Moyra.wilson@uwa.edu.au. 

Carbonate systems and coral reefs build amongst the largest edifices on the planet, are able to keep-up with most tectonic or glacio-eustatic induced sea-level rises and consequently the foundering of many platforms is often enigmatic.  The cause of demise of platforms and the deposition of potential overlying seal units are critical for understanding thresholds for carbonate platform survival as well as petroleum systems evaluations in better understanding relationships between reservoirs and caprocks.

The paradox of foundering of carbonate platforms has been variously linked to ‘drowning’ via (1) fast glacio-eustatic sea-level rise, (2) tectonic induced sea-level rise, (3) nutrient and/or clastic poisoning and (4) subaerial exposure, shut-down of the carbonate factory and a subsequent inability to ‘catch-up’ on subsequent reflooding. Despite better understanding of the foundering of carbonate platforms being critical for their survival, evaluations of the sedimentary, geochemical and petrophysical signatures of each of the potential causes for demise remain understudied.

This study will evaluate the sedimentary, geochemical and diagenetic signatures across key outcrop analogue sections and subsurface reservoirs to understand the impacts of different causes of foundering on reservoir and caprock development. The research investigates: (1) both short- and longer-term (~> 1 Ma) transgressive drowning successions of carbonate platforms, (2) nutrient/or and clastic influenced land-attached, nearshore carbonate foundering, (3) carbonate platforms affected by karstification prior to drowning, and (4) volcanogenic smothered systems.


Biography

Moyra is a Senior Lecturer at UWA, Perth, with 30 years academic/industry experience focusing on the carbonate and reefal systems of Australasia, marine palaeoenvironmental change and reservoir potential.  Moyra’s awards include the Lyell Fund (Geological Society of London), Wiley Best ‘Sedimentology’ Paper, Curtin University Fellowship, and Australian Bicentennial Award. 

Shallow Conduit and Vent Processes during the 1886 Basaltic Plinian Eruption at Tarawera, New Zealand

Moore, Hannah1, Carey, Dr Rebecca1, Houghton Dr Bruce2, Jutzeler, Dr Martin1, White, Dr James3

1School of Natural Sciences and CODES, University of Tasmania, TAS, Australia 2Department of Earth Sciences, University of Hawaii at Mānoa 3Department of Geology, University of Otago

The 1886 eruption of Tarawera, New Zealand, is one of four known examples of basaltic Plinian eruptions. During the climactic phase, a high Plinian eruption column was produced, fed by vents in four segments along an 8-km-long fissure across Mt Tarawera. This eruptive activity was simultaneous with other adjacent vents across the mountain that were in a low-intensity style of eruption. Here we present a detailed re-examination of microtextures from pyroclasts to constrain the ascent and degassing histories of the magma which influenced the Plinian versus low intensity styles of eruption along the Mt. Tarawera portion of the fissure. With this study, we aim to understand how this basaltic magma erupted at such high mass eruption rates. Scoria clasts and ash particles selected from stratigraphy proximal to the vent represent (1) a widespread endmember, sedimented from the Plinian column margin and (2) a localised endmember, sedimented from low intensity explosions, representing the lowest mass eruption rates. We also study clasts from a medial section, from which clasts were absolutely entrained into the Plinian plume and represent the highest mass eruption rates. The main differences in scoria clasts from different sites along the fissure segment are in microlite crystallinities: these are low within the proximal localised material (35–55 %), high within the proximal widespread material (95–99 %), and intermediate within the medial material (69–84 %). We suggest that, for vents erupting at Plinian intensity, there was a strong parabolic velocity profile across the conduit, which ensured that magma near conduit margins ascended slower, cooled faster and became more viscous than magma along the axis, leading to longer residence times and therefore more advanced degrees of outgassing and crystallisation. The highly viscous magma at the margins may have reduced permeability and therefore outgassing from magma along the axis, causing a build-up of pressure within the conduit, driving higher eruption rates, and leading to a Plinian eruption. Eruption products sedimented into the widespread proximal environment represent the collar of cooler, more crystallised magma, whereas products entrained into the high plume and sedimented into the medial location represents magma from the central axis. For vents erupting at low intensities, there were transient discrete explosions, where the most intense explosive eruptions cleared out the shallow conduit. These
episodic explosions allowed efficient outgassing from magma in the shallow conduit, but viscosity remained relatively low compared to Plinian magma.


Biography

Hannah Moore conducted her MSc degree in Volcanology at the University of Bristol. Hannah’s PhD research at the University of Tasmania is focussed on the 1886 basaltic Plinian eruption of Tarawera. She uses techniques such as field geology, textural analysis and physical volcanology to understand this unusual eruption.

The tectonostratigraphic evolution of the South Nicholson region, Northern Territory and Queensland: key discoveries from the Exploring for the Future and implications for resource exploration

Carson, Chris1, Henson, Paul1, Lidena, Carr1, Southby, Chris1 and Anderson, Jade1.

1Geoscience Australia, Canberra, Australia

Proterozoic rocks of the South Nicholson region, which straddle the NT and QLD border, are juxtaposed between the Proterozoic Mount Isa Province to the east and the southern McArthur Basin to the northwest. The McArthur Basin and Mount Isa Province are comparatively well-studied and prospective for energy and mineral resources. In contrast, rocks of the South Nicholson region are mostly undercover and, as such, there is incomplete understanding of their geological evolution, relationship with adjacent geological provinces and resource potential. To address this gap, two deep crustal seismic reflection surveys, the South Nicholson and Barkly surveys (completed in 2017 and 2019, respectively), were conducted across the South Nicholson region by Geoscience Australia, under the federally funded Exploring for the Future (EFTF) initiative, in collaboration with the Northern Territory Geological Survey, the Geological Survey of Queensland and AuScope (e.g. Carr et al., 2019, 2020). While the Barkly seismic data are still being interpreted, these seismic datasets, together with other complementary regional studies, provides an improved understanding of the geological evolution and resource potential across this poorly understood region.

Both seismic surveys targeted both suspected undercover sedimentary basins and known crustal structures to resolve regional subsurface fault geometry. A key finding from the South Nicholson seismic survey is the discovery of a large concealed sedimentary sag basin that is up to 8 km deep, around 120 km wide and 190 km from north to south, called the Carrara Sub-basin (e.g. Carr et al., 2019). The sub-basin is interpreted to contain Mesoproterozoic to late Paleoproterozoic rocks equivalent to those outcropping in the Lawn Hill Platform and Mount Isa Province. The eastern end of the one of the lines (17GA-SN1), connects with a legacy seismic line that intersects the world class Pb-Zn Century deposit on the Lawn Hill Platform, the late Paleoproterozoic host rocks of which can be traced into the Carrara Sub-basin.

The South Nicholson profiles also reveal a series of ENE–trending, north-dipping half grabens which evolved during two episodes of crustal extension, at ca. 1725 Ma and ca. 1640 Ma, broadly coinciding with structural and basin forming events identified from the Lawn Hill Platform and the Mount Isa Province. Inversion of the half-graben bounding faults, resulting in south–verging thrusts, probably commenced during N-S crustal contraction characteristic of the early Isan Orogeny at ca. 1600-1580 Ma to at least the Paleozoic Alice Springs Orogeny (ca. 400-300 Ma).

Furthermore, our comprehensive regional geochronology program proposes extensive revision of regional stratigraphic relationships. Some successions, previously mapped as Mesoproterozoic South Nicholson Group may instead represent late Paleoproterozoic successions, that form part of the highly prospective Isa Superbasin (and the broadly stratigraphic equivalent McArthur Group in the McArthur Basin), which hosts numerous viable base metal deposits and is prospective for energy commodities (e.g. Jarrett et al., 2020; MacFarlane et al., 2020). Our findings significantly expand the extent of highly prospective late Paleoproterozoic stratigraphy across the South Nicholson region, which, possibly, extends an as yet unknown distance west beneath the Georgina and Carpentaria basins.

Carr, L.K., et al., 2019. Exploring for the Future: South Nicholson Basin Geological summary and seismic data interpretation. Record 2019/21. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/Record.2019.021

Carr, L.K., et al., 2020. South Nicholson seismic interpretation. In: Czarnota, K., et al., (eds.) Exploring for the Future Extended Abstracts, Geoscience Australia, http://dx.doi.org/10.11636/132029

Jarrett A.J.M., et al., 2020. A multidisciplinary approach to improving energy prospectivity in the South Nicholson region. In: Czarnota, K., et al., (eds.) Exploring for the Future Extended Abstracts, Geoscience Australia, http://dx.doi.org/10.11636/134164

MacFarlane, S. et al., 2020. A regional perspective of the Paleo- and Mesoproterozoic petroleum systems of northern Australia In: Czarnota, K., et al., (eds.) Exploring for the Future Extended Abstracts, Geoscience Australia, http://dx.doi.org/10.11636/133716


Biography

Chris has worked in Antarctica, Canadian Arctic, Alaska, New Caledonia and northern and central Australia, specialising in metamorphic petrology, geochronology and structural geology. Joining Geoscience Australia in 2006 he dabbled in SHRIMP geochronology and, in 2017, joined the Onshore Energy program, working in the South Nicholson region of the NT.

Trace element distributions in carbonate rocks: a sedimentologist’s perspective on sample targeting versus technique

Webb, Gregory E.1

1The University of Queensland, Brisbane, Australia

Trace element geochemistry is useful increasingly in ancient and not so ancient carbonate rocks where it provides the basis for radiometric dating and many palaeoenvironmental proxies. High precision U-Th and U-Pb analyses in carbonate samples are converging to close the ‘undatable’ window past 500 ka. A variety of palaeothermometers are used commonly (e.g., Sr/Ca, Mg/Ca, U/Ca, etc.) and rare earth elements (REEs) inform the source of ancient water masses, water quality and redox states. Other elements (e.g., Ba, Mn, V, etc.) provide proxies for biological processes and palaeoproductivity as well as terrestrial processes, including firing. Redox sensitive elements (e.g., Mo, V, U, etc.) inform complex oxygenation scenarios in Precambrian seas. Trace elements are providing an ever increasing tool kit for sedimentologists, stratigraphers and palaeoenvironmentalists.

However, a variety of pitfalls accompany the expanding use of trace element geochemistry in carbonate rocks. As carbonate minerals are metastable at the Earth’s surface, sedimentologists are highly attuned to the problem of diagenetic alteration. Effective sample vetting is crucial, but new core scanning technologies and SEM approaches are easing sample selection. Regardless, many samples are complex mixtures of sources with differing elemental concentrations and distributions that require detailed understanding of the sample, which elements are being targeted for analysis and the reservoir for which they are meant to serve as proxies. For example, trace elements in marine precipitates are sourced from ambient seawater, but depending on the sample, elements also may reflect siliciclastic detritus (contamination) and organic components, which may or may not cause fractionations or enrichments. Additionally, analysed element distributions may record local microenvironments rather than the ambient water masses in which they occur. Where ambient water chemistry is targeted, elemental contributions of all other sources must be identified and removed. For bulk samples (dissolution ICP-MS), contaminants can be removed post-analysis using mixing lines to quantify contamination in each sample. Alternately, sequential etching may attempt to analyse separate sources independently during dissolution. Increasingly, laser ablation (LA) ICP-MS combined with LA mapping can be used to identify and sample increasingly small, specific targets while avoiding contaminants. Other elemental mapping approaches (e.g., synchrotron based x-ray fluorescence, SEM electron dispersive spectroscopy) also aid sample vetting and targeting of appropriate sources. The source of the proxy elements is critical.

Although ICP-MS is increasingly common, significant technical issues remain past adequately low blanks and high count rates. As sample size decreases (e.g., LA analysis), low sample volume exacerbates low element concentrations leading to poor data quality. However, even apparently ‘low quality’ data for some elements, like the REEs, provide useful information owing to their self-normalising behaviour. Cohesive REE data suggest adequate precision to carry information, regardless of calculated detection limits and groups of less cohesive data can be analysed statistically to provide some information.

Overall, successful interpretation of trace element proxies requires more than a good geochemical laboratory; it requires knowledge of the finite relationship between the targeted proxy elements and the sample being analysed (i.e., context) along with application of the most appropriate technique for the job.


Biography

Gregory Webb is a palaeontologist and carbonate sedimentologist who specialises in ancient and modern corals, coral reefs and microbialites. He uses petrographic and geochemical means to understand how organisms make rocks and how those rocks record environmental information in their geochemistry and morphology.

Paleozoic and Triassic crustal evolution of the proto-Andes from detrital heavy minerals

Bahlburg,Heinrich1, Panca,Fernando 1

1Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität, Münster, Germany

The central Andean margin of South America evolved as an external accretionary orogen since the Late Neoproterozoic. During the Paleozoic and Triassic evolution, the evolution of the margin and its orogenic basins was linked to alternating extensional and compressional states. The most prominent arc system is the Late Cambrian-Ordovician Famatinian arc which extended along the entire margin. The Devonian is here a time of tectonic, magmatic and metamorphic quiescence allowing for the steady accumulation of shallow marine facies on a stable platform west of the Ordovician orogen. This setting appears like a passive margin but in the geologic context has to be considered enigmatic.

Connected to the Gondwanide Orogeny, a Late Carboniferous to Triassic arc system with episodic activity was present in northern Chile and absent from Bolivia. In southern Peru it is preserved as Late Carboniferous to Early Permian backarc magmatic rocks.

In the late Permian and Triassic, extensional alluvial basins hosting within-plate magmatic rocks developed here. The latter evolution is represented by the Mitu Group. Its depocenters are considered rift and not backarc basins, mainly because a contemporaneous magmatic arc is nowhere exposed and unknown.

Accretionary orogens are considered to be the sites of the production of juvenile continental crust. The Famatinian and Late Carboniferous to Early Permian magmatic arcs, however, are characterized by a marked scarcity of mafic intrusive and extrusive rocks. The detrital heavy mineral spectra reflect this with Nb/Cr ratios in detrital rutiles, for example, indicating predominant input from felsic rocks. Detrital zircon U-Pb age distributions denote the arcs as prolific zircon sources. The epsHf(t) isotope values of the zircons derived from the arcs, and the Famatinian arc in particular, are strongly negative. In both cases a pronounced recycling of older continental crust is indicated. Detrital zircons of Devonian age are very scarce.

The late Permian and Triassic Mitu Group basin system extends for 1500 km in NW-SE orientation along the length of Peru. It is characterized by highly variable alluvial facies with angular unconformities at base and top. The group appears to be arranged in several subbasins. Mafic alkaline and calc-alkaline lavas, and felsic ignimbrites are intercalated episodically. U-Pb zircon ages of the latter allow for a stratigraphic subdivision of the subbasin fills.

Detrital zircon age distributions of the Paleozoic central Andes show a typical South American, Amazonian provenance, with ages ranging from the Archean to the maximum ages of deposition. In the Mitu Group, maximum depositional ages indicate a variable onset of deposition between 260 and 216 Ma, locally continuing into the Jurassic. The epsHf(t) isotope values of late Permian and Triassic zircons form vertical arrays between ca. -10 and +6 and indicate a significantly more pronounced juvenile component in Mitu rocks than in the older arc rocks.


Biography

Heinrich Bahlburg studies clastic sedimentary basins and the crustal evolution of accretionary orogens. He combines field work and facies analysis with the geochemical and geochronological analysis of heavy minerals. This interest took him to orogens in Alaska, Europe, and for over 30 years to the central Andes.

Lithospheric-scale magnetotellurics over the Eastern Goldfields Superterrane, Yilgarn Craton

Selway, Kate1, Dentith, Michael2, Gessner, Klaus3

1Department of Earth and Environmental Sciences, Macquarie University, Australia; 2Centre for Exploration Targeting, School of Earth Sciences, The University of Western Australia, Crawley, WA 6009, Australia; 3Geological Survey of Western Australia, East Perth, WA 6004, Australia

The Eastern Goldfields Superterrane, in the Yilgarn Craton, Western Australia, is one of the most highly mineralised regions on Earth, hosting world-class orogenic gold and nickel-sulfide deposits. Mineral systems models for both of these deposit types suggest that lithospheric-scale processes are involved in their formation. Therefore, lithospheric-scale geophysical imaging is a promising tool to improve understanding of the formation of the deposits and to aid future exploration.

Long-period magnetotelluric (MT) data were collected over an approximately 250 km x 200 km area covering the western part of the Eastern Goldfields Superterrane and the eastern Youanmi Terrane. The survey region covers the Kalgoorlie and St Ives gold camps and the Kambalda nickel camp, as well as the Ida Fault, a prominent isotopic boundary between the older Nd model ages of the Youanmi Terrane and the younger Nd model ages of the Eastern Goldfields Superterrane. A 3D conductivity model was produced from the data, with good resolution to depths of 150 to 200 km.

Results show that the lithospheric mantle from depths of approximately 100 to 150 km is more conductive (~10 to 100 ohm m) beneath the Youanmi Terrane than the Eastern Goldfields Superterrane (>100 ohm m). Crustal conductivity is more heterogeneous but most of the strongly conductive regions (<100 ohm m) are located in the Eastern Goldfields Superterrane. The resolution of the model in the near-surface is insufficient to make a detailed comparison with the locations of known deposits, but most upper crustal conductors are spatially correlated with regional-scale faults, which are inferred to be important in the formation of orogenic gold deposits.

Anomalously conductive zones in tectonically stable regions often indicate past metasomatism, either through the hydration of nominally anhydrous minerals or the growth of conductive mineral phases such as amphibole or phlogopite.

Quantitative interpretation of the MT model shows that the mantle conductors in the Youanmi Terrane are too conductive to be explained purely by hydrated peridotite and imply the presence of hydrous metasomatic minerals. The observed patterns of lithospheric conductivity suggest a more complex relationship between mantle metasomatism and gold and nickel mineral systems than expected from previous studies.


Biography

Kate, Mike and Klaus are all interested in the multi-disciplinary application of geophysical, geological and geochemical data to understanding tectonic evolution and mineral systems.

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