The Arafura Basin contains a sequence of Palaeozoic rocks lying north-east of Darwin, and which extends from onshore Australia, to perhaps as far as the Irian Jaya mainland. There are over 9 km of Palaeozoic rocks preserved along the southern bounding fault of a major graben (the Arafura Graben) located in the southern part of the basin. In the uplifted centre of the graben, there is less than 3 km of the Palaeozoic section preserved. The basin is underlain by a Middle to Late Proterozoic sequence which thickens to the east, and is probably equivalent to the onshore McArthur Basin. Overlying the Arafura Basin is the Mesozoic Money Shoal Basin, which is approximately 1 km thick over the central parts of the graben, thickening rapidly to the west and thinning to the east and north. The structural cross-section that has been drawn is located entirely offshore. It has been compiled using modern seismic and well control. It runs from south-east of Tasman 1 in a general north and north-east direction tying with Torres 1 and Arafura 1. It passes through the central and north-eastern parts of the graben, and the north-eastern part of the basin. Two-way time to depth conversions were based on the velocity surveys from the wells within the graben, but were modified locally outside the graben. The structural analysis presented is largely based on the evidence found along the line of the section. A more complete structural analysis would require a regional examination of the entire seismic network. Information from the recently published Petroleum Basin Study on the Arafura Basin (Northern Territory Geological Survey) has been incorporated into this report, although there are major differences between some of their findings and the interpretations presented here. To date the major risk in hydrocarbon exploration has been finding adequate reservoir conditions and seal. Contradictory interpretations are present between the maturation and structural modelling of the graben. Untested plays include possible Permian and Triassic sediments (up to 5 km thick) which exist along the flanks of the graben and which will probably contain good source potential and improved reservoir conditions. To the north outside the graben, there are poorly explored areas where it is speculated that there are thick Palaeozoic and Proterozoic sequences.

Multibeam sonar mapping, drill cores and underwater video data have confirmed the existence of a previously unknown reef province in the Gulf of Carpentaria, Australia. Seven reefs, comprised of coral limestone that support living corals have been mapped so far and as many as 50 other reefs may exist in the region. U/Th ages show that reef growth commenced shortly after limestone pedestals were submerged by rising sea level around 10.5 kyr BP, making them the oldest reefs known in Australia. Reef growth persisted for ~2.0 kyr but it had ceased at most locations by ~8.0 kyr BP. Measurements of reef growth rates (0.95 to 4 m kyr-1), indicate that the reefs were unable to keep pace with contemporaneous rapid sea level rise (>10 m kyr-1), which is consistent with a 'give up' reef growth history. Core samples from reef platforms demonstrate that Pleistocene limestone is exposed in depths of 27 and 30 m below present mean sea level. These depths represent regionally significant phases of reef growth during a prolonged sea level still stand. We conclude that the reefs are therefore mostly relict features, whose major phase of growth and development relates to an earlier, pre-Holocene sea level stillstand.

The area northwest of Elura mine, northwest of Cobar, has diverse surficial materials reflecting a complex landscape history. This paper examines some of this history and its possible effects on surface geochemical sampling programs.

Situated just inboard of the late Neoproterozoic Australian rift margin (Tasman Line), the Mount Isa region occupies a critical position in reconstructions of Rodinia, combining an older crystalline basement (Kalkadoon-Leichhardt Block) affected by the 1850 Ma Palaeoproterozoic Barramundi orogeny with three variably deformed and vertically stacked supracrustal sequences ranging in age from 1790-1595 Ma (Leichhardt, Calvert and Isa superbasins). Orogenesis in these three superbasins peaked around 1585Ma (Isa orogeny) and obscures an earlier history of syn-extensional magmatism, deformation and low-pressure/high-temperature metamorphism linked to basin formation and normal faulting at higher crustal levels. Basin geometry and normal fault patterns indicate a change in the direction of extension from east-west to ENE-WSW at Calvert time and a corresponding shift in the depositional environment from narrow intracontinental rift to passive continental margin by 1685 Ma. Coincidently, fluviatile to near-shore sedimentary facies (Haslingden Group and Surprise Creek Formation in the west) gave way to deep water marine sediments, including turbidites (Soldiers Cap Group in the east), and basaltic magmas evolved from continental to oceanic tholeiites in composition as the crust became increasingly attenuated. This pattern of basin formation and near continuous extension is also evident in the 1720-1640 Ma Willyama Supergroup at Broken Hill but is difficult to reconcile with existing reconstructions of Rodinia in which the Broken Hill and Mount Isa terranes are juxtaposed against rocks of similar age in southwest Laurentia that preserve a record of contractional deformation related to plate convergence, terrane accretion and collision. Equally difficult to reconcile are palaeogeographic reconstructions of Australia which place the Broken Hill and Mount Isa terranes adjacent to each other in a back-arc position while maintaining an along-strike continuity with the rocks of southwest Laurentia (Mojave, Yavapai and Mazatzal Provinces). An alternative reconstruction of Proterozoic eastern Australia is proposed in which back-arc extension in the Broken Hill and Mount Isa terranes was linked to retreat of a west-dipping (present-day coordinates) subduction zone and associated magmatic arc that now resides partially or wholly within North America. Eastern Australia and Laurentia ceased to be part of a single continental landmass soon after 1800 Ma and thereafter followed increasingly divergent tectonic paths until re-amalgamated during collision at ca. 1600 Ma. Extension in the Broken Hill and Mount Isa terranes produced a North American Basin and Range-style crustal architecture that has no obvious counterpart in southwest Laurentia.

As part of the Australian Government's National CO2 Infrastructure Plan (NCIP), Geoscience Australia undertook a CO2 storage assessment of the Vlaming Sub-basin. The Vlaming Sub-basin a Mesozoic depocentre within the offshore southern Perth Basin located about 30 km west of Perth, Western Australia. The main depocentres formed during the Middle Jurassic to Early Cretaceous extension. The post-rift succession comprises up to 1500 m of a complex fluvio-deltaic, shelfal and submarine fan system. Close proximity of the Vlaming Sub-basin to industrial sources of CO2 emissions in the Perth area drives the search for storage solutions. The Early Cretaceous Gage Sandstone was previously identified as a suitable reservoir for the long term geological storage of CO2 with the South Perth Shale acting as a regional seal. The Gage reservoir has porosities of 23-30% and permeabilities of 200-1800 mD. The study provides a more detailed characterisation of the post Valanginian Break-up reservoir - seal pair by conducting a sequence stratigraphic and palaeogeographic assessment of the SP Supersequence. It is based on an integrated sequence stratigraphic analysis of 19 wells and 10, 000 line kilometres of 2D reflection seismic data, and the assessment of new and revised biostratigraphic data, digital well logs and lithological interpretations of cuttings and core samples. Palaeogeographies were reconstructed by mapping higher-order prograding packages and establishing changes in sea level and sediment supply to portray the development of the delta system. The SP Supersequence incorporates two major deltaic systems operating from the north and south of the sub-basin which were deposited in a restricted marine environment. Prograding clinoforms are clearly imaged on regional 2D seismic lines. The deltaic succession incorporates submarine fan, pro-delta, delta-front to shelfal, deltaic shallow marine and fluvio-deltaic sediments. These were identified using seismic stratigraphic techniques and confirmed with well ties where available. The break of toe slope was particularly important in delineating the transition between silty slope sediments and fine-grained pro-delta shales which provide the seal for the Gage submarine fan complex. As the primary reservoir target, the Gage lowstand fan was investigated further by conducting seismic faces mapping to characterise seismic reflection continuity and amplitude variations. The suitability of this method was confirmed by obtaining comparable results based on the analysis of relative acoustic impedance of the seismic data. The Gage reservoir forms part of a sand-rich submarine fan system and was sub-divided into three units. It ranges from canyon confined inner fan deposits to middle fan deposits on a basin plain and slump deposits adjacent to the palaeotopographic highs. Directions of sediment supply are complex. Initially, the major sediment contributions are from a northern and southern canyon adjacent to the Badaminna Fault Zone. These coalesce in the inner middle fan and move westward onto the plain producing the outer middle fan. As time progresses sediment supply from the east becomes more significant. Although much of the submarine fan complex is not penetrated by wells, the inner fan is interpreted to contain stacked channelized high energy turbidity currents and debris flows that would provide the most suitable reservoir target due to good vertical and lateral sand connectivity. The middle outer fan deposits are predicted to contain finer-grained material hence would have poorer lateral and vertical communication.

Legacy product - no abstract available

Legacy product - no abstract available

A small collection of marine Lower Cretaceous Mollusca from Mount Samuel area, central Western Australia, consists of six genera and seven species which are known from the Roma Formation beds of the Great Artesian Basin. It is suggested that a communicating seaway existed between the Great Artesian Basin and Mount Samuel area during Aptian times. Migration of marine organisms probably took place along a southern route through Officer and Eucla Basins, but may have possibly proceeded directly to the west from Rumbalara. It is also suggested that at the same time another seaway existed between Mount Samuel area and the Dampier Peninsula.The sediments of the Great Artesian Basin occur in two areas in the Northern Territory, viz, in the vicinity and to the east of Rumbalara, at the southern extreme of the Northern Territory where they are of Aptian age and are correlated with beds of Roma Formation in Queensland; and up to about one hundred miles south of the Gulf of Carpentaria, close to the Queensland border, where they are of Aptian and of Upper Albian age and can be correlated with the Roma and Tambo Formation beds of Queensland.

The Early Cretaceous South Perth Shale has been previously identified as the regional seal in the offshore Vlaming Sub-basin. The South Perth Shale is a deltaic succession, which unfilled a large palaeotopographic low in the Early Cretaceous through a series of transgressive and regressive events. The new study undertaken at Geoscience Australia has shown that the seal quality varies greatly throughout the basin and at places has very poor sealing properties. A re-evaluation of the regional seal based on seismic mapping determined the extent of the pro-delta shale facies within the South Perth Shale succession, which are shown to provide effective sealing capacity. New sequence stratigraphic interpretation, seismic facies mapping, new and revised biostratigraphic data and well log analysis were used to produce palaeogeographic reconstructions which document the distribution of depositional facies within the South Perth Shale Formation and reveal evolution of the Early Cretaceous deltas. Our study documents spatial variations in the seal quality and re-defines the extent and thickness of the regional seal in the central Vlaming Sub-basin. It provides an explanation for the lack of exploration success at some structural closures and constraints for possible location of the valid plays.

Legacy product - no abstract available