Georgina Basin Research Articles

Exploring unconventional plays in the Georgina Basin, central Australia: will the real Arthur Creek Formation hot shale please stand up!

The search for hydrocarbons in the southern Georgina Basin in central Australia is now focused on assessing the unconventional potential of the Middle Cambrian lower Arthur Creek Formation. Recent regional well correlation studies, supported by a review of nominated type sections and of regional seismic data, and integrating core-based sedimentological, palaeontological, chemostratigraphic, and geochemical information, suggest that the organic rich hot shale at the base of this formation, encountered in wells drilled in the Dulcie Syncline, is not a correlative of the organic rich hot shale encountered in wells drilled in the Toko Syncline. Although similar in lithofacies and mineralogy, the former is significantly older and is more restricted in geographic distribution to an area west of a prominent northwest to southeast trending shear zone in the basement. This interpretation is contrary to correlations presented in recent publications on the geology and resource potential of the southern Georgina Basin by government agencies, various petroleum consultancy firms that have provided estimates of the volumetric potential of the unconventional hydrocarbon resources in the lower Arthur Creek Formation, and several petroleum companies actively exploring in the basin. Any historical inference that simply assumed the lower Arthur Creek Formation hot shale in the Toko domain should be an attractive unconventional target because of the lithological properties of the lower Arthur Creek Formation hot shale observed in the Dulcie domain is not supported by these revised correlations. Pursuit of the basal lower Arthur Creek Formation unconventional play in the Dulcie Syncline and in the Toko Syncline should be done on their respective merits.

Shale gas potential of the major marine shale formations in the Upper Yangtze Platform, South China, Part II: Methane sorption capacity

The marine black shale formations on the Upper Yangtze Platform (UYP), South China are exploration targets for shale gas. Here, we report on the methane sorption capacity of thermally overmature samples from the Lower Silurian and Lower Cambrian black shale intervals in the UYP (UYP-samples). Two immature shale samples from the Middle Cambrian formation in the Georgina Basin, North Australia (AU samples) were also tested to investigate the effect of thermal maturity on sorption isotherms. Excess sorption isotherms were performed over a pressure range of 0–25MPa at 46°C. The effects of TOC content, thermal maturity, clay minerals, moisture content, pore properties, particle size, temperature, and pressure on methane sorption capacity were analysed. In addition, thermovaporisation gas chromatography (Tvap-GC) was used to measure the residual gas that is stored in the samples under atmospheric pressure and temperature conditions.The results indicate that the maximum methane excess sorption of the Lower Silurian samples is between 0.045 and 0.064mmol/g rock and that of Lower Cambrian samples is between 0.036 and 0.210mmol/g rock. The Langmuir sorption capacity of the Lower Silurian samples ranges from 0.096 to 0.115mmol/g rock, whereas that of the Lower Cambrian shale ranges from 0.077 to 0.310mmol/g rock. These results are close to the sorption capacities of the Barnett (U.S.), Devonian–Mississippian (Western Canada), and Alum (Southern Scandinavia) shale samples. The shape of the sorption isotherms and methane sorption capacity vary from sample to sample. Under the measured pressure range, the isotherms of the selected immature AU Cambrian samples increase monotonously with pressure, whereas the overmature UYP samples exhibit maxima. The methane sorption capacity of the measured samples positively correlates with TOC content and exhibits a distinct linear relation. The TOC-normalised sorption capacity shows a positive correlation with thermal maturity; however, the corresponding pressure of maximum excess sorption and Langmuir pressure decrease substantially with increasing thermal maturity. The clay minerals show a positive effect on the TOC-normalised sorption capacity. The sorption capacity of clay minerals, however, should have been reduced by the moisture content. The two Lower Cambrian samples that have similar maturities were measured for porosity and pore-size distribution. The sample with a high TOC content shows a high total cumulative pore volume, surface area, total porosity and thus a higher sorption capacity than the sample with less TOC. In addition, larger-sized particles show slightly less sorption capacity than smaller-sized particles. The Tvap-GC results show that the residual gas content of core samples is evidently higher than that of the outcrop samples, which implies a remarkably negative effect of the weathering process.

Timing and origin of megabreccia and folds along the Early Middle Cambrian margin of the Georgina Basin, Australia

Megabreccia and related folds are two of the most interesting features of the Lawn Hill Outlier, a small carbonate platform situated in the northeastern part of the Georgina Basin in Queensland, Australia. Field studies and stable isotopic data were used to assess the timing and origin of folds and megabreccias in this carbonate plateau, and understand its possible relationship to an asteroid impact. Together with field and isotope data, the reconstruction of the sequence of events that led to the cratonization of the Centralian Superbasin supports a synsedimentary timing of formation for the folds and breccias. Some of the brittle faulting and veining accompanying strain localisation within the Thorntonia Limestones may represent, however, post-sedimentary, syntectonic deformation, possibly linked to the Late Devonian Alice Springs Orogeny. An origin for the folding and megabreccias linked to an asteroid impact cannot be completely discounted. Nevertheless, observed field relationships concerning the spatial distribution and typology of breccias occurring in basement and cover agree with stable isotopic signatures, suggesting that multiple intrabasinal processes contributed to platform destabilisation. Processes such as karstification, solution collapse, and fault reactivation, were the most likely mechanisms responsible for the formation of intrastratal breccias and slump folds in the Lawn Hill Outlier.

The times they are a-changin': Australian biozones, petroleum basins, and the international geologic time scale (GTS) 2012

The international Geologic Time Scale (GTS) continually evolves due to refinements in age dating and the addition of more defined stages. The GTS 2012 has replaced GTS 2004 as the global standard timescale, resulting in changes to the age and duration of most chronological stages. These revisions have implications for interpreted ages and durations of sedimentary rocks in Australian basins, with ramifications for petroleum systems modelling. Accurate stratigraphic ages are required to reliably model the burial history of a basin, hence kerogen maturation and hydrocarbon expulsion and migration. When the resolution of the time scale is increased, models that utilise updated ages will better reflect the true basin history. The international GTS is largely built around northern hemisphere datasets. At APPEA 2009, Laurie et al. announced a program to tie Australian biozones to GTS 2004. Now, with the implementation of GTS 2012, these ties are being updated and refined, requiring a comprehensive review of the correlations between Australian and International biozonation schemes. The use of Geoscience Australia’s Timescales Database and a customised ‘Australian Datapack’ for the visualisation software package TimeScale Creator has greatly facilitated the transition from GTS 2004 to GTS 2012, as anticipated in the design of the program in 2009. Geoscience Australia’s basin biozonation and stratigraphy charts (e.g. Northern Carnarvon and Browse basins) are being reproduced to reflect the GTS 2012 and modified stratigraphic ages. Additionally, new charts are being added to the series, including a set of onshore basin charts, such as the Georgina and Canning basins.

Physical properties of petroleum formed during maturation of Lower Cambrian shale in the upper Yangtze Platform, South China, as inferred from PhaseKinetics modelling

Lower Cambrian shale in the Upper Yangtze Platform (UYP), South China, is an important source rock of many conventional petroleum fields and was recently recognized as a promising unconventional shale reservoir. In this paper, hydrocarbon generation kinetics and petroleum physical properties were investigated using the PhaseKinetics approach (di Primio and Horsfield, 2006) and a Cambrian shale sample from the Georgina Basin, North Territory Australia (NTA), as similar paleogeological and sedimentary environments in Cambrian are found for the UYP and NTA.The source rock comprises type II kerogen and belongs to an organofacies generating Paraffinic–Naphthenic–Aromatic low wax oil. Bulk petroleum generation can be described by a single frequency factor A = 8.43E + 14 (1/s) and a dominant activation energy at 56 kcal/mol, which is characteristic for sulphur-poor organic matter deposited in an anoxic marine environment. Onset (transformation ratio TR = 10%) and end (TR = 90%) of bulk hydrocarbon generation was calculated to take place at 120 °C and 165 °C respectively for an assumed average geological heating rate of 1.5°C/Ma. Based on the thermal history of a local “model”-well, onset temperature was not reached until the Middle Triassic (241 ma) when sediments were buried more than 2000 m and basalt eruptions caused enhanced heat flows. The main generation stage of primary petroleum took place during the Middle–Late Triassic and ended in the Early Jurassic (187 ma) for burial depths exceeding 4000 m (TR 90%; 165 °C). Temperatures increased to more than 200 °C in the Middle–Late Jurassic leading to secondary cracking of primary products.Hydrocarbons formed at the onset (TR = 10%) of petroleum generation can be characterized by a gas-oil-ratio (GOR) of 63 Sm3/Sm3, a saturation pressure (Psat) of 101 bar, and a formation volume factor (Bo) of 1.2 m3/Sm3. Those parameters stay low during primary petroleum generation before 203 ma, at temperatures < 145 °C, and at burial depths <3400 m (GOR = 176 Sm3/Sm3, Psat = 189 bar, Bo = 1.6 m3/Sm3 at 90% TR). However, predicted parameters increase rapidly (GORs >> 10,000 Sm3/Sm3, Psat > 250 bar and Bo > 2.0 m3/Sm3) during secondary cracking starting roughly at 200 ma, 152 °C and 3500 m burial. Assuming zero expulsion, the shale reservoir position within the sedimentary basin indicates that bubble point pressure was always below reservoir pressure, and fluids in the shale reservoir occurred only as a single, undersaturated phase throughout maturation history. Black oil and volatile oil phases dominated during the primary cracking period, whereas wet gas and dry gas phases dominated during the secondary cracking period.

High grade metamorphism of sedimentary rocks during Palaeozoic rift basin formation in central Australia

Exhumation of middle and lower crustal rocks during the 450–320Ma intraplate Alice Springs Orogeny in central Australia provides an opportunity to examine the deep burial of sedimentary successions leading to regional high-grade metamorphism. SIMS zircon U–Pb geochronology shows that high-grade metasedimentary units recording lower crustal pressures share a depositional history with unmetamorphosed sedimentary successions in surrounding sedimentary basins. These surrounding basins constitute parts of a large and formerly contiguous intraplate basin that covered much of Neoproterozoic to early Palaeozoic Australia. Within the highly metamorphosed Harts Range Group, metamorphic zircon growth at 480–460Ma records mid-to-lower crustal (~0.9–1.0GPa) metamorphism. Similarities in detrital zircon age spectra between the Harts Range Group and Late Neoproterozoic–Cambrian sequences in the surrounding Amadeus and Georgina basins imply that the Harts Range Group is a highly metamorphosed equivalent of the same successions. Maximum depositional ages for parts of the Harts Range Group are as low as ~520–500Ma indicating that burial to depths approaching 30km occurred ~20–40Ma after deposition. Palaeogeographic reconstructions based on well-preserved sedimentary records indicate that throughout the Cambro–Ordovician central Australia was covered by a shallow, gently subsiding epicratonic marine basin, and provide a context for the deep burial of the Harts Range Group. Sedimentation and burial coincided with voluminous mafic magmatism that is absent from the surrounding unmetamorphosed basinal successions, suggesting that the Harts Range Group accumulated in a localised sub-basin associated with sufficient lithospheric extension to generate mantle partial melting. The presently preserved axial extent of this sub-basin is >200km. Its width has been modified by subsequent shortening associated with the Alice Springs Orogeny, but must have been >80km. Seismic reflection data suggest that the Harts Range Group is preserved within an inverted crustal-scale half graben structure, lending further support to the notion that it accumulated in a discrete sub-basin. Based on palaeogeographic constraints we suggest that burial of the Harts Range Group to lower crustal depths occurred primarily via sediment loading in an exceptionally deep Late Cambrian to Early Ordovician intraplate rift basin. High-temperature Ordovician deformation within the Harts Range Group formed a regional low angle foliation associated with ongoing mafic magmatism that was coeval with deepening of the overlying marine basin, suggesting that metamorphism of the Harts Range Group was associated with ongoing extension. The resulting lower crustal metamorphic terrain is therefore interpreted to represent high-temperature deformation in the lower levels of a deep sedimentary basin during continued basin development. If this model is correct, it indicates that regional-scale moderate- to high-pressure metamorphism of supracrustal rocks need not necessarily reflect compressional thickening of the crust, an assumption commonly made in studies of many metamorphic terrains that lack a palaeogeographic context.

An updated stratigraphic framework for the Georgina Basin, NT and Queensland

The Georgina Basin is a Neoproterozoic to Lower Devonian sedimentary basin covering 325,000 km2 of western Queensland and the NT. It is a northwest-southeast-trending extensional basin, with prospective conventional and unconventional hydrocarbon targets in Cambrian and Ordovician carbonate and siliciclastic rock units. The unconventional gas and oil potential of the basin has led to recent exploration interest, although the basin has been relatively less explored in the past. At the southern end of the basin, depocentres contain up to 2.2 km of Cambrian to Devonian sedimentary rocks, overlying Neoproterozoic sedimentary rocks more than 1.5 km thick. The basin succession thins toward the north, where Cambrian sediments overlie the McArthur Basin sediments in the Beetaloo Sub-basin. Biostratigraphic interpretations of the prospective southern, central and eastern regions of the basin have been revised to reflect the 2012 Geological Time Scale (Gradstein, Ogg, Schmitz, and Ogg, 2012), resulting in an updated chronostratigraphic framework for the basin. The revised biostratigraphic interpretations have implications for important hydrocarbon source rocks. For example, the limestone unit in the southern parts of the basin, generally regarded as the Thorntonia Limestone, is of a different age to the type section for this unit, located in the Undilla Sub-basin. Additionally, the basal hot shale of the Arthur Creek Formation is diachronous across the Dulcie and Toko synclines, which may have ramifications for hydrocarbon exploration. This revised chronostratigraphic framework (by Geoscience Australia) for the Georgina Basin provides a baseline for the first basin-wide assessment of the unconventional hydrocarbon potential of the basin.

Unconventional petroleum resource evaluation in Queensland

The search for unconventional petroleum resources requires a shift in the way the petroleum potential of sedimentary basins is assessed. Gas in source rocks and tight reservoirs has largely been ignored in preference for traditional conventional gas plays. Recent developments in technology now allow for the extraction of gas trapped in low-permeability reservoirs. Assessments of the unconventional petroleum potential of basins, including estimates of the potential resource are required to guide future exploration. The Geological Survey of Queensland is collaborating with Geoscience Australia (GA) and other state agencies to undertake regional assessments of several basins with potential for unconventional petroleum resources in Queensland. The United States Geological Survey methodology for assessment of continuous petroleum resources is being adopted to estimate total undiscovered oil and gas resources. Assessments are being undertaken to evaluate the potential of key formations as shale oil and gas and tight-gas plays. The assessments focus on mapping key attributes including depth, thickness, maturity, total organic carbon (TOC), porosity, gas content, reservoir pressure, mineralogy and regional facies patterns using data from stratigraphic bores and petroleum wells to determine play fairways or areas of greatest potential. More detailed formation evaluation is being undertaken for a regional framework of wells using conventional log suites and mudlogs to calculate porosity, TOC, maturity, oil and gas saturations, and gas composition. HyLoggerTM data is being used to determine its validity to estimate bulk mineralogy (clay-carbonate-quartz) compared with traditional x-ray diffraction methods. These methods are being applied to key formations with unconventional potential in the Georgina and Eromanga basins in Queensland.

Key features of new deep seismic reflection lines across frontier sedimentary basins in central Australia: the Arckaringa, Officer, Amadeus and Georgina Basins

The Onshore Energy Security Program, funded by the Australian Government and conducted by Geoscience Australia, in conjunction with State and Territory geological surveys, has acquired deep seismic reflection data across several frontier sedimentary basins to stimulate petroleum exploration in onshore Australia. Here, we present the key aspects of the stratigraphy and structural architecture identified using seismic data from the Arckaringa, Officer, Amadeus and Georgina basins on seismic lines 09GA-GA1 and 08GAOM1.

Mineral Systems in the Mount Isa Inlier

Northwest Queensland contains several world class mineral deposits, being one of the worlds leading producers of Zn, Pb, Cu and Ag. Rather than focus on mineral deposit models, as has been done in the past, we are using the mineral system approach (Barnicoat, 2008), where the whole system is studied at a variety of scales and a variety of processes, which culminate in the deposition of mineralisation. Seven Mineral Systems are identified, namely: 1. Shale/siltstone/dolomite hosted Zn-Pb-Ag systems - Western Fold Belt. 2. Ag-Pb-Zn in high-grade metamorphic terrains - Eastern Fold Belt Province. 3. Structurally-controlled epigenetic iron oxide-Cu-Au - Eastern Fold Belt and Kalkadoon-Ewen Provinces. 4. Structurally-controlled epigenetic Cu±Au mineralising system - Western Fold Belt Province. 5. Phosphate Mineralisation in the basal Georgina Basin sequence. 6. U and Rare Earth element (REE) mineralisation. 7. Fe Ore - South Nicholson Group.

Energy potential of the Millungera Basin: a newly discovered basin in north Queensland

Deep seismic reflection surveys in north Queensland that were collected in 2006 and 2007 discovered a previously unknown sedimentary basin, now named the Millungera Basin, which is completely covered by a thin succession of sediments of the Jurassic–Cretaceous, Eromanga-Carpentaria Basin. Interpretation of regional aeromagnetic data suggests that the basin could have areal dimensions of up to 280 km by 95 km. Apart from regional geophysical data, virtually no confirmed geological information exists on the basin. To complement the seismic data, new magnetotelluric data have been acquired on several lines across the basin. An angular unconformity between the Eromanga and Millungera basins indicates that the upper part of the Millungera Basin was eroded prior to deposition of the Eromanga-Carpentaria Basin. Both the western and eastern margins of the Millungera Basin are truncated by thrust faults, with well-developed hangingwall anticlines occurring above the thrusts at the eastern margin. The basin thickens slightly to the east, to a maximum preserved subsurface depth of ˜3,370 m. Using sequence stratigraphic principles, three discrete sequences have been mapped. The geometry of the stratigraphic sequences, the post-depositional thrust margins, and the erosional unconformity at the top of the succession all indicate that the original succession across much of the basin was thicker–by up to at least 1,500 m–than preserved today. The age of the Millungera Basin is unknown, but petroleum systems modelling has been carried out using two scenarios, that is, that the sediment fill is equivalent in age to (1) the Neoproterozoic-Devonian Georgina Basin, or (2) the Permian–Triassic Lovelle Depression of the Galilee Basin. Using the Georgina Basin analogue, potential Cambrian source rocks are likely to be mature over most of the Millungera Basin, with significant generation and expulsion of hydrocarbons occurring in two phases, in response to Ordovician and Cretaceous sediment loading. For the Galilee Basin analogue, potential Permian source rocks are likely to be oil mature in the central Millungera Basin, but immature on the basin margins. Significant oil generation and expulsion probably occurred during the Triassic, in response to late Permian to Early Triassic sediment loading. Based on the seismic and potential field data, several granites are interpreted to occur immediately below the Millungera Basin, raising the possibility of hot rock geothermal plays. Depending on its composition, the Millungera Basin could provide a thermal blanket to trap any heat which is generated. 3D inversion of potential field data suggests that the inferred granites range from being magnetic to nonmagnetic, and felsic (less dense) to more mafic. They may be part of the Williams Supersuite, which is enriched in uranium, thorium and potassium, and exposed just to the west, in the Mount Isa Province. 3D gravity modelling suggests that the inferred granites have a possible maximum thickness of up to 5.5 km. Therefore, if granites with the composition of the Williams Supersuite occur beneath the Millungera Basin, in the volumes indicated by gravity inversions, then, based on the forward temperature modelling, there is a good probability that the basin is prospective for geothermal energy.

Chapter 67 Neoproterozoic glacial deposits of central Australia

Abstract There are two distinct stratigraphic levels of Neoproterozoic glacigenic deposits in central Australia, both Cryogenian in age, spread over an area greater than 2.5×10 6 km 2 . They were deposited in a once continuous intracratonic sag basin and are now preserved in four major structural basins: the Officer, Amadeus, Ngalia and Georgina Basins. In all four basins there are units that correlate with the (older) Sturt Tillite and equivalent glacial deposits in the Adelaide Rift Complex – the Sturt glaciation ( Preiss et al. 2011 ) – and with the (younger) Elatina Formation (Fm.) and equivalents – the Elatina glaciation ( Williams et al. 2008 , 2011 ). The clearest evidence for glacial activity is the occurrence of diamictites that contain clasts of lithologically diverse origin which are often striated, faceted and polished, and the occurrence of dropstones. Most glacial deposits were deposited in shallow marine to fluvio-lacustrine palaeoenvironments. In all basins, the Elatina glacial deposits are overlain (at least locally) by dolostone units that mark the onset of post-glacial transgression and contain unique sedimentary and geochemical features. The cap dolomite units are distinct from dolomite beds within glaciogenic sediments, and those that occur near the top of Sturt glacial units in the Amadeus (Areyonga Fm.) and eastern Officer Basins (Chambers Bluff Tillite). None of the central Australian glacial units have direct geochronological constraints. There are, however, radiometric dates for a Sturt glacial unit in the Adelaide Rift Complex (Wilyerpa Fm.) and post-glacial shales in the Amadeus Basin (Aralka Fm.), Stuart Shelf (Tapley Hill Fm.) and Adelaide Rift Complex (Tapley Hill Fm.) that indicate a c. 660 Ma age for the Sturt glaciation in Australia ( Kendall et al. 2006 , 2007 ; Fanning &amp; Link 2008 ). The age of the Elatina glaciation in Australia is constrained only by the age of the Sturt glaciation and the presence of the Ediacara fauna in overlying strata of all the basins except the Ngalia Basin. Consequently, correlations have been mainly established by means of lithostratigraphy, chemostratigraphy, palynology, and to a lesser extent, stromatolite biostratigraphy, mainly on the successions above and below the glacial units. Results from each of the above techniques show a remarkable consistency, and indicate that the two major Cryogenian glacial episodes are of similar age across Australia.

Chapter 65 Neoproterozoic glacial deposits of the Kimberly Region and northwestern Northern Territory, Australia

Abstract Neoproterozoic glaciogenic formations are preserved in the Kimberley region and northwestern Northern Territory of northern Australia. They are distributed in the west Kimberley adjacent to the northern margins of the King Leopold Orogen, the Mt Ramsay area at the junction of the King Leopold and Halls Creek orogens, and east Kimberley, adjacent to the eastern margin of the Halls Creek Orogen. Small outlier glaciogenic deposits are preserved in the Litchfield Province, Northern Territory (Uniya Fm.) and Georgina Basin, western Queensland (Little Burke Fm.). Glaciogenic strata comprise diamictite, conglomerate, sandstone and pebbly mudstone and characterize the Walsh, Landrigan and Fargoo/Moonlight Valley formations. Thin units of laminated dolomite sit conformably at the top of the Walsh, Landrigan and Moonlight Valley formations. Glaciogenic units are also interbedded with the carbonate platform deposits of the Egan Fm. and Boonall Dolomite. δ 13 C data are available for all carbonate units. There is no direct chronological constraint on these successions, and dispute over the regional correlation of the Neoproterozoic succession has been largely resolved through biostratigraphic, chemostratigraphic and lithostratigraphic analysis. However, palaeomagnetic results from the Walsh Fm. are inconsistent with sedimentologically based correlations. Two stratigraphically defined glaciations are preserved in northwestern Australia: the ‘Landrigan Glaciation’, characterized by SW-directed continental ice-sheet movement and correlated with late Cryogenian glaciation elsewhere in Australia and the world; and, the ‘Egan Glaciation’, a more localized glaciation of the Ediacaran Period. Future research focus should include chronology, palaeomagnetic constraint and tectonostratigraphic controls on deposition.

Structural and stratigraphic architecture of Australia's frontier onshore sedimentary basins: the Arckaringa, Officer, Amadeus, and Georgina basins

The Onshore Energy Security Program—funded by the Australian Government and conducted by Geoscience Australia—has acquired deep seismic reflection data in conjunction with state and territory geological surveys, across several frontier sedimentary basins to stimulate petroleum exploration in onshore Australia. Here, we present data from two seismic lines collected in SA and NT. Seismic line 08GA-OM1 crossed the Arckaringa and Officer basins in SA and the southern-most Amadeus Basin in NT. Seismic line 09GA-GA1 crossed the northeastern part of the Amadeus Basin and the complete width of the southern Georgina Basin in NT. Structural and sequence stratigraphic interpretations of the seismic lines will be presented here, followed by an assessment of the petroleum potential of the basins. Seismic line 08GA-OM1 also crosses the Neoproterozoic to Devonian eastern Officer Basin. The basin is structurally complex in this area, being dominated by south-directed thrust faults and fault-related folds—providing potential for underthrust petroleum plays. The northern margin of the basin is overthrust to the south by the Mesoproterozoic Musgrave Province. To the north, the Moorilyanna Trough of the Officer Basin is a major depocentre of up to 7,000 m deep. Both seismic lines cross parts of the eastern Amadeus Basin. Seismic line 08GA-OM1 shows that the southern margin of the basin is overthrust to the north by the Musgrave Province with the main movement during the Petermann Orogeny. In the northeast, seismic line 09GA-GA1 crosses two parts of the basin separated by the Paleoproteroozic to Mesoproterozoic Casey Inlier (part of the Arunta Region). The northern margin of the basin is imaged seismically as a southward-verging, thinned-skinned thrust belt, showing considerable structural thickening of the stratigraphic succession. Seismic line 09GA-GA1 was positioned to cross that part of the southern Georgina Basin that was considered previously to be in the oil window. Here, the basin has a complex southern margin, with Neoproterozoic stratigraphy being thrust interleaved with basement rocks of the Arunta Region. The main part of the basin, containing a Neoproterozoic to Devonian succession, is asymmetric, thinning to the north where it overlies the Paleoproterozoic Davenport Province. The well, Phillip–2, drilled adjacent to the seismic line, intersected basement at a depth of 1,489 m, and has been used to map the stratigraphic sequences across the basin.

Neoproterozoic rifting in the southern Georgina Basin, central Australia: Implications for reconstructing Australia in Rodinia

[1] A system of northwest striking Neoproterozoic rift basins underlies Paleozoic strata in the southern Georgina Basin of central Australia. Normal faults bounding these rift basins were selectively reactivated during the mid-Paleozoic Alice Springs Orogeny and are now expressed as high-angle reverse faults that invert the preexisting rift basins. Exhumed and eroded rift basin remnants are present in the hanging wall of the Oomoolmilla, Lucy Creek, Tarlton, and Toomba reverse faults, and rift basins may be preserved in the subsurface beneath the Toko Syncline and Burke River Structural Belt. Rift basin fill indicates two periods of extension: a major rift-forming episode between approximately 700 and 650 Ma (coeval with Sturtian glacial deposits) and a second episode of extension at approximately 600 Ma (coeval with Marinoan glacial deposits). This northwest striking rift system in central Australia supports results from other regions, indicating that the Neoproterozoic continental margin of Australia consisted of northwest striking rift segments offset by northeast striking transform faults. Such a configuration is geometrically incompatible with a Laurentian continental margin consisting of northeast striking rift segments and conflicts with reconstructions such as SWEAT and AUSWUS that match Australia with western Laurentia in the Rodinia supercontinent.

Substrate control of benthos in a Middle Cambrian near-shore, epeiric palaeoenvironmental setting

Late Middle Cambrian Mail Change Limestone, a thin (< 30 m) lime mudstone unit of the epeiric Middle and Late Cambrian Georgina Basin, Australia is unusually rich in epifaunal linguliform brachiopods, especially acrotretids. It is the only unit of this abundantly fossiliferous Cambrian basinal succession where the representation of this group is more prolific than that of trilobites. The diminutive size of acrotretids, generally less than 3 mm in shell diameter in the case of common taxa from the Mail Change Limestone, raises questions of how an epizoic lifestyle was achieved on muddy substrates. Facies analysis indicates that lime mud substrates associated with deposition of the Mail Change Limestone included microbial firm-ground which developed in upper shoreface settings, and ephemeral evaporate horizons of supratidal or high intertidal association, during the shallow-water phases of repeating parasequence cycles. Lime muds of shoreface association are brachiopod-rich, suggesting that firm-ground substrates were favoured attachment sites for populations of anchoring epizoic acrotretids. The conical shell form typical of this group evolved in part to minimize current drag, reducing stress on pedicle attachment, and in part to induce turbulence at the shell gape to facilitate feeding. The high incidence of undamaged acrotretid valves, representing a spectrum of growth stages, obtained from the Mail Change Limestone suggests that failure of pedicle attachment was a major contributor to mortality in this group. Firm-ground was also characterised by ubiquitous fine-scale, Planolites-like, open burrows which housed substantial populations of wormlike suspension feeders (?polychaetes) which also took advantage of firm-ground substrates. Near-shore substrates were also host to suspension feeding Lingulella and Protospongia and a deposit feeder responsible for Thalassinoides ichnofossils. Substrates associated with deposition of the Mail Change Limestone were unfavorable to colonization by vagile, seafloor feeding ptychopariid trilobites which are poorly represented in the unit. In contrast, agnostid trilobites of nektonic habit were less limited by trophic opportunities and are more commonly represented. Diverse agnostids ranged into very shallow marine, shoreface, environments represented by the Mail Change Limestone. The association of abundant epifaunal linguliforms and infaunal suspension feeders with firm-ground suggests that substrate stability strongly influenced the nature and biomass of Middle Cambrian sessile benthos. Evolutionary strategies among sessile benthos to take full advantage of shallow marine level bottom environments were not realised until the Ordovician biodiversity event.

New data on molluscs and their shell microstructures from the Middle Cambrian Gowers Formation, Australia

Numerous new cases of preserved shell micro- structure were discovered in molluscs from the Middle Cam- brian Gowers Formation (Ptychagnostus atavus ⁄Peronopsis opimus Zone, Floran Stage) in the Georgina Basin, Australia. The new data provide further evidence that, by the Middle Cambrian, molluscan shell microstructures were diverse, and many molluscs had a complex shell with multiple types of shell microstructure. In addition, many new occurrences of laminar microstructures are described herein. For many, the nature of these laminar microstructures is not known, but in three species the microstructure is foliated calcite, and in at least two the microstructure is more likely to have been cal- citic semi-nacre, a type of microstructure known in brachio- pods and bryozoans but unknown in modern molluscs. This commonality among these three closely related lophotroch- ozoans underscores a similar mechanism of biomineraliza- tion. Moreover, these observations suggest a prevalence of calcite-shelled lineages among molluscs from the Middle Cambrian, a time of calcite seas. In addition, the broad occurrence of laminar, nacre-like microstructures in many of these fossils reveals how widespread these strong (fracture- resistant) microstructures were in Middle Cambrian mol- luscs. Additionally, a few specimens of Yochelcionella preserve imprints of a bilaterally symmetrical pair of muscle scars. New taxa described here include Corystos thorntoniensis gen. et sp. nov., Yochelcionella snorkorum sp. nov., Yochelcionella saginata sp. nov., and Anhuiconus? agrenon sp. nov.

The Australian desert dunefields: formation and evolution in an old, flat, dry continent

Abstract A new map, the first based on interpretation of satellite imagery, reveals both the complexity of Australia's dunefields and their relationships with topography, climate and substrate. Of the five main sand seas, the Mallee, Strzelecki and Simpson in eastern Australia cover Quaternary sedimentary basins whereas the Great Victoria and Great Sandy dunefields in the west are formed by reworking of valley and piedmont sediments in a non-basinal landscape of low-relief ridge and valley topography. These dunefields cover large areas of the arid zone and semi-arid zone and small areas of dunes in sub-humid areas around the margins of the continent reflecting past expansion of arid climates during glacial stages of the last several glacial cycles. Several areas of low relief stand out as being largely dune-free: the limestone Nullarbor Plain, clay plains of the Georgina Basin and floodplains of rivers in the Carpentaria, Lake Eyre and Murray–Darling drainage basins where sand is rare or not transported by diminished Late Quaternary rivers. The Yilgarn Block of southwestern Australia is also surprisingly free of dunes, possibly as a result of long, deep weathering. Everywhere the history of climate change is evident in dune morphology and distribution, including large areas where the sand dune orientations are markedly divergent from modern sand moving wind directions.

The role of deep seismic reflection data in understanding the architecture and petroleum potential of Australia's onshore sedimentary basins

The onshore energy security program, funded by the Australian Government and conducted by Geoscience Australia, has acquired deep seismic reflection data across several frontier sedimentary basins to stimulate petroleum exploration in onshore Australia. Detailed interpretation of deep seismic reflection profiles from four onshore basins, focussing on overall basin geometry and internal sequence stratigraphy, will be presented here, with the aim of assessing the petroleum potential of the basins. At the southern end of the exposed part of the Mt Isa Province, northwest Queensland, a deep seismic line (06GA–M6) crosses the Burke River structural zone of the Georgina Basin. The basin here is &gt;50 km wide, with a half graben geometry, and bounded in the west by a rift border fault. Given the overall architecture, this basin will be of interest for petroleum exploration. The Millungera Basin in northwest Queensland is completely covered by the thin Eromanga Basin and was unknown prior to being detected on two seismic lines (06GA–M4 and 06GA–M5) acquired in 2006. Following this, seismic line 07GA–IG1 imaged a 65 km wide section of the basin. The geometry of internal stratigraphic sequences and a post-depositional thrust margin indicate that the original succession was much thicker than preserved today and may have potential for a petroleum system. The Yathong Trough, in the southeast part of the Darling Basin in NSW, has been imaged in seismic line 08GA–RS2 and interpreted in detail using sequence stratigraphic principles, with several sequences being mapped. Previous studies indicate that the upper part of this basin consists of Devonian sedimentary rocks, with potential source rocks at depth. In eastern South Australia, seismic line 08GA–A1 crossed the Cambrian Arrowie Basin, which is underlain by a Neoproterozoic succession of the Adelaide Rift System. Stratigraphic sequences have been mapped and can be tied to recent drilling for mineral and geothermal exploration. Shallow drill holes from past petroleum exploration have aided the assessment of the petroleum potential of the Cambrian Hawker Group, which contains bitumen in the core, indicating the presence of source rocks in the basin system.

What can deep seismic reflection data tell us about hydrocarbon prospectivity in onshore

Now in its third year, Geoscience Australia’s Onshore Energy Security Program has acquired several suites of regional geological and geophysical data. The data includes several deep seismic reflection surveys that have been designed to image: basement provinces with high geothermal gradients that may contain uranium enrichments and are potential candidates for geothermal energy; geological terrane boundaries; and, sedimentary basins that are known to host petroleum system elements but are under-explored. Seismic signals are recorded down to 20 seconds two-way-time (TWT) which corresponds to 25–35 km depth depending on dominant lithologies. Basinal sections normally extend down to 6–8 sec TWT and the data is of such high quality that any section of the seismic profile can be enlarged without significant loss of resolution. Deep reflection surveys are able to image the relationship between crystalline basement and overlying basin sequences very clearly and also allow interpretations of structural styles as well as impacts of deformational processes on the basin-fill. A new basinal section was discovered beneath the Eromanga Basin suite of sediments. Named the Mullangera Basin,its structural style and basement relationship seem to indicate some affinity with the Georgina Basin, located further west. The succession is clearly composed of several sequences that contain both fine and coarse-grained sediments. If a geological relationship with the Georgina Basin can be ascertained, a new hydrocarbon prospective area could be delineated. Another new section was discovered beneath the Devonian section of the Darling Basin. Judging by the fast acoustic velocities, the entire basin-fill sequence appears to be very dense and therefore largely non-porous and of low permeability. While low poro-perm characteristics would normally be translated to low prospectivity for oil and gas, the potential for unconventional hydrocarbons such a tight-gas and shale-gas warrants further attention.