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Overall, the cruise met its objectives of studying rift and drift sedimentation, and obtaining cores for palaeo-oceanography. The east Tasmanian seismic program was completely successful. The planned sampling program was somewhat curtailed by bad weather, equipment failures and other factors. It was least successful off east Tasmania. A total of about 1300 km of 8-fold multichannel seismic data were acquired along 8 transects across the east Tasmanian margin. The quality of the seismic profiles was excellent, with good resolution and penetration, given the bad weather and the limitations of the acquisition system. The seismic source comprised 2 GI airguns (each 45/105 cu. in. capacity) giving a penetration of 2-2.5 s twt (2.5-3 km) in places. The seismic profiles indicate a structurally complex margin with rugged basement relief that includes large-scale horst/graben structures and volcanic intrusions. The sedimentary section on the continental slope is at least 1.5 s twt thick in some graben and includes Campanian-Paleocene early sag-phase deposits, which are 0.5-1.0 s twt thick. Regional compressive tectonism in the Late Paleocene-Early Eocene has produced widespread inversion (folding/faulting) in this succession. A wedge of Neogene shallow-water carbonates underlies the continental shelf. It shows seaward progradation and attains a maximum thickness of ~700 m beneath the shelf edge. Oceanic basement (?Campanian) adjacent to the margin lies at a depth of 7.0-7.5 s twt. The continental rise and Tasman Abyssal Plain in this zone are underlain by 1.5-2.0 s twt of post-breakup sedimentary section. The East Tasman Saddle is underlain by `transitional? basement and contains a sedimentary section of similar thickness. During the sampling program 58 of 86 stations were successful: 38 gravity cores (21 successful), 4 piston cores (3), 16 dredges (7) and 28 grabs (28). Total core recovery was 81.4 metres from the 16 successful cores taken in soft sediments, an average recovery of about 5 metres. The fairly low success rate with the gravity corer can be ascribed to problems with foram sand east of Tasmania, and shelly sand in Storm Bay. The low success rate with the dredge was related to the lightness of the gear. The deployment of the heavy piston corer for the first time on Franklin was successful. However, we did not attempt to piston core in deep water. East of Tasmania we recovered 8 gravity cores, and 7 dredge hauls. Deepwater dredging and coring were surprisingly unsuccessful. The upper slope stations, designed to sample older rocks, were reasonably successful. From these results and some existing information, general conclusions can be drawn about changes along the margin with increasing water depth. The shelf and upper slope wedge of Neogene, seaward-prograding sediments was sampled out to 1640 m. The sediments recovered include muddy sand, clayey sandstone with siliceous nodules, siliceous sandstone and calcarenite. The calcarenite is presumably part of the Middle Miocene shelf limestone sequence that is widespread off St Helens. Somewhat deeper on the upper slope, basement outcrops occur in steep slopes: granite, arkose, metasediments, conglomerate, quartz sandstone and gritty mudstone. The granites are probably from Devonian batholiths like those onshore up the east coast. Volcanic rocks and conglomerate form a basement block in deeper water on a ridge off northeast Tasmania at ~3750m. Deepwater outcrop ridges support manganese nodules and crusts. Nannofossil oozes cling to the slope, particularly in local basins, and are ubiquitous in deep water. The East Australian Current apparently winnows many of the oozes to form a blanket of foram sand.

Since the publication in 1967 of the monograph on the marine geology of the Timor Sea,1 the Bureau of Mineral Resources has initiated a program of systematic reconnaissance geological surveys of the continental shelf. The results of this work are being published in the BMR Bulletin series accompanied by 1:1 000 000 lithofacies maps of the shelf sediments. Three sheets (Rowley Shoals, W.A.2; Scott Reef, W.A.2 ; and Arafura Sea, N.T.8 ) have been printed by early 1974, and work on two further sheets covering part of the east Australian continental shelf is well advanced. Users of the map should refer to Bulletin 83 (GeoCat # 163) to assist in interpretation. For instance, wide areas of the shelf are non-depositional, or even subject to erosion, and therefore the variations in lithology portrayed are not exclusively the result of variations in the modern depositional regime. Also the map does not distinguish sediments which are relics of earlier regimes from modern ones; however, some information of the distribution of these older sediments can be obtained from Bulletin 83 (GeoCat # 163) and inferred from a study of the gravel content in relation to the bathymetry.

The Australian Southern Margin SEEBASE® Compilation represents many years of work by SRK in southern Australia in the petroleum, mineral and coal sectors. During this time SRK has undertaken numerous projects in southern Australia with both the private and government sectors. These projects have resulted in the development of a model of the geological evolution of southern Australia from Archean to Recent that is summarised in this GIS and report. The model is consistent with a wide range of datasets including airborne and satellite remote sensing, seismic, well and outcrop observations. The basins of Australia's Southern margin formed by the repeated reactivation of long-lived basement structures. By understanding the genesis and geometry of the old basement structures, we have produced a model for the evolution of the Southern Margin basins that explains their structural framework and architecture. This SEEBASE model and structural interpretation can now be used as the basis for a new understanding of the sequence stratigraphy and petroleum systems of the margin.

In early 2008 Geoscience Australia and Mineral Resources Tasmania acquired 141,234 km of high resolution (800m line spacing) aeromagnetic data over Bass Strait and the offshore marginal basins of western Tasmania. The data fill a gap in the existing aeromagnetic coverage between Tasmania and mainland Australia and provide fresh insights into basement structure and its control on basin architecture and sedimentation patterns during Gondwanan continental break-up and the separation of Australia from Antarctica. Prominent in the new data are several northwest-trending basement faults that extend from the mainland into westernmost Tasmania and the South Tasman Rise; they appear to represent an offshore extension of previously mapped structures in western Victoria (Hummocks and Yarramyljup Faults). These structures postdate, truncate and offset in a sinistral sense many older north- and northeast-trending basement structures, including the late Neoproterozoic Arthur lineament in Tasmania, the Bambra fault in central Victoria and the boundary between the Lachlan and Delamerian Orogens (Moyston Thrust) in western Victoria. The Hummocks Fault coincides with a narrow belt of ultramafic rocks and possibly continues offshore as a series of prominent magnetic anomalies whereas the Yarramyljup Fault may form the western limit of Proterozoic (Tyennan) basement in Tasmania. The distribution and geometry of Mesozoic-Tertiary offshore sedimentary basins in western Tasmania and the South Tasman Rise is consistent with reactivation of the older basement structures in a north-south-directed transtensional tectonic regime. Magmatic rocks intruded into the Bass, Otway and Sorell Basins and Torquay Sub-Basin are clearly delineated in the new aeromagnetic data.

In 2010 and 2011, the Australian Government released exploration acreage in the Perth, Mentelle and Southern Carnarvon basins off the southwest margin of Australia. This release was underpinned by two new marine geophysical surveys (GA-310 and GA-2476) that were conducted by Geoscience Australia in late 2008 and early 2009 as part of the Australian Government's Offshore Energy Security Program. These surveys acquired a range of pre-competitive geological and geophysical data that included seismic reflection, gravity, magnetic and swath bathymetry measurements, as well as seafloor dredge samples. The new surveys provided a total of about 26 000 line km of new gravity and magnetic data that add to existing data from around 150 previous marine surveys conducted off the southwest margin since 1960. This Record describes the integration and levelling of the new gravity and magnetic data with existing data, both offshore and onshore, to produce a unified gravity and magnetic dataset for use in constraining regional tectonics, basin structure and petroleum prospectivity. Levelling is a key step in processing ship-borne gravity and magnetic data. This process minimises the mistie errors at ship-track cross-overs that arise from factors such as positioning errors, instrument drift and lack of diurnal corrections to magnetic data. Without accounting for these cross-over errors, gridded data can be rendered un-interpretable by artefacts and distortions at line cross-overs.

Presentation delivered on 8 March 2012 at the Tasman Frontier Petroleum Industry Workshop, 8-9 March 2012, Geoscience Australia, Canberra.

The Jurassic-Cretaceous Bight Basin is situated along the western and central parts of the southern Australian continental margin. The largely offshore basin extends from the southern tip of Western Australia in the west, to just south of Kangaroo Island in the east, where it adjoins the Otway Basin. The thickest depocentre in the basin is the Ceduna Sub-basin, which contains a sedimentary section in excess of 15 km thick. The deepwater Recherche Sub-basin adjoins the Ceduna Sub-basin and extends west along the southern margin as far as the Leeuwin Fracture Zone. Perched half-graben systems of the Denmark, Bremer and Eyre sub-basins lie to the north of the Recherche Sub-basin. The Duntroon Sub-basin adjoins the Ceduna Sub-basin to the east, and consists of a series of oblique extensional depocentres. The Bight Basin evolved through repeated episodes of extension and thermal subsidence leading up to, and following, the commencement of sea-floor spreading between Australia and Antarctica. The basin was initiated during a period of Middle-Late Jurassic to Early Cretaceous upper crustal extension. A northwest-southeast to north-south extension direction, superimposed on east-west and northwest-southeast-oriented basement structures, resulted in oblique to strongly oblique extension and the formation of en echelon half graben in the Denmark, Bremer, Eyre, inner Recherche, eastern Ceduna and Duntroon sub-basins. The areal extent of the early extensional structures beneath the thick Ceduna Sub-basin cannot be determined at present. The anomalously thick nature of the Ceduna Sub-basin may indicate, however, that Jurassic-Early Cretaceous rifts are present at depth. Post-rift thermal subsidence was followed by a phase of accelerated subsidence, which commenced in the Late Albian and continued until continental break-up in the Late Santonian-Early Campanian. During this phase of enhanced subsidence, the dominant structural feature was a system of gravity-driven, detached extensional and contractional structures, which developed in the Ceduna Sub-basin during the Cenomanian as a result of deltaic progradation. Evidence for upper crustal extension during this basin phase is limited to Turonian-Santonian extensional faulting, and the reactivation and propagation of Cenomanian growth faults. The commencement of sea-floor spreading at ~83 Ma was followed by a further period of thermal subsidence and establishment of a passive margin

Australian passive margins display a large variety of styles, including narrow, wide, volcanic and non-volcanic margins. Their tectonic history has been complicated by tectonic reactivation and anomalous subsidence/uplift, widespread at various times during the post-rift phase. Defining the exact location of the boundary between continental and oceanic crust (COB) is of key importance to understanding the structure and evolution of continental passive margins. Here, we review the history of Australian passive margins and the location of the COB, and we assess rift-related magmatism and anomalous post-rift subsidence based on recently acquired seismic reflection data, available industry data, and current tectonic models for margin evolution.

Integration of conventional interpretation of deep-seismic data with potential field modelling is a powerful tool for elucidating the geology of continental margins and particularly the continent-ocean transition zone (COT). Recent work carried out on the Wilkes Land margin of East Antarctica using new seismic and potential field data shows the power of combining these techniques. In this study, the initial deep-seismic interpretation was combined with sonobuoy- and stacking-derived velocity information to provide a starting model for the potential field modelling; the potential field model and aspects of the seismic interpretation were iterated until a consistent interpretation was reached. The most important observations from the COT zone on the Wilkes Land margin include: ? The outer edge of the COT, that is the point at which the crustal type becomes 100% oceanic, is much further offshore than previously interpreted from seismic data and seafloor spreading anomalies, and lies beneath the deep ocean basin. ? The COT is dominated by a basement ridge complex which may represent serpentinised, unroofed mantle peridotites and associated intrusions and extrusions related to decompression melting, similar to features inferred on the conjugate southern Australian margin. ? The lower crust is inhomogeneous, probably due to massive localised intrusion landward of the COT; however, pre-existing inhomogeneities cannot be ruled out. ? The base of the crust has considerable relief landward of the COT, increasing from ~10 km to 16 km depth over a distance of about 40 km.