The 1999 release of offshore petroleum exploration acreage in the Great Australian Bight and the acquisition of high quality seismic datasets covering the Bight and Duntroon Basins, have provided a timely opportunity to reassess the stratigraphic and tectonic evolution of the area. A sequence stratigraphic framework for the Great Australian Bight region has been developed based on the interpretation of exploration wells in the Bight and Duntroon basins and a grid of new and reprocessed seismic data in the Bight Basin. Previous formation-based nomenclature has emphasised lithostratigraphic correlations rather than the chronostratigraphic relationships. The new sequence framework underpins an analysis of play elements and petroleum systems and is helping to identify new exploration opportunities.Deposition in the Bight and Duntroon Basins commenced in the Late Jurassic during a period of lithospheric extension. Extensive half graben systems were filled with fluvial and lacustrine clastic sediments (Sea Lion and Minke supersequences). Potential source rocks within these supersequences are immature at Jerboa-1 in the Eyre Sub-basin, however higher maturities are expected within adjacent half graben and in the Ceduna and Recherche Sub-basins. The syn-rift successions are overlain by widespread Berriasian to Albian fluvio-lacustrine to marine sediments of the Southern Right and Bronze Whaler supersequences. The onlapping sag-fill geometry of these Early Cretaceous packages in the Eyre, Ceduna and inner Recherche Sub-basins suggests that they were deposited during a period of thermal subsidence.Accelerated subsidence commencing in the late Albian led to the deposition of the marine shales of the Blue Whale supersequence, followed by a period of gravity-controlled faulting and deformation in the Cenomanian. The White Pointer supersequence is characterised by growth strata associated with a series of listric faults that sole out in underlying ductile shales of the Blue Whale supersequence. Open marine conditions during the Turonian-Santonian (Tiger supersequence) were followed by the development of massive shelf margin delta complexes in the late Santonian-Maastrichtian (Hammerhead supersequence). The progradational to aggradational stratal geometries within the Hammerhead supersequence suggest initial high rates of sediment input that subsequently waned during this period. An overall transgressive phase of sedimentation in the Early Tertiary (Wobbegong supersequence) was followed by the establishment of open marine carbonate shelf conditions from the Early Eocene onward (Dugong supersequence). Organic geochemical studies show that the Bronze Whaler to White Pointer supersequences have good source rock potential in the relatively proximal facies intersected by existing petroleum exploration wells. Our sequence stratigraphic model predicts the likelihood of widespread late Aptian, Albian, Cenomanian-Santonian, and Campanian marine shales, which underpin four potential marine petroleum systems.

The interpretation of two regional seismic reflection profiles and the construction of a balanced cross section through the southern Australian margin (Bight Basin) are designed to analyze the influence of the Australia‐Antarctica continental breakup process on the kinematic evolution of the Cretaceous Ceduna delta system. The data show that the structural architecture of this delta system consists of two stacked delta systems. The lower White Pointer delta system (Late Albian‐Santonian) is an unstable tectonic wedge, regionally detached seaward above Late Albian ductile shales. Sequential restoration suggests that the overall gravitational sliding behavior of the White Pointer delta wedge (∼45 km of seaward extension, i.e., ∼27%) is partially balanced by the tectonic denudation of the subcontinental mantle. We are able to estimate the horizontal stretching rate of the mantle exhumation between ∼2 and 5 km Ma−1. The associated uplift of the distal part of the margin and associated flexural subsidence in the proximal part of the basin are partially responsible for the decrease of the gravitational sliding of the White Pointer delta system. Lithospheric failure occurs at ∼84 Ma through the rapid exhumation of the mantle. The upper Hammerhead delta system (Late Santonian‐Maastrichtian) forms a stable tectonic wedge developed during initial, slow seafloor spreading and sag basin evolution of the Australian side margin. Lateral variation of basin slope (related to the geometry of the underlying White Pointer delta wedge) is associated with distal raft tectonic structures sustained by high sedimentation rates. Finally, we propose a conceptual low‐angle detachment fault model for the evolution of the Australian‐Antarctic conjugate margins, in which the Antarctic margin corresponds to the upper plate and the Australian margin to the lower plate.

Acreage release by the Australian Government in 2010 offers exploration opportunities in the frontier Mentelle Basin for the first time. The Mentelle Basin is a large deep-water basin on the southwest Australian margin. It consists of a large, very deep water (2,000—4,000 m) depocentre in the west and several depocentres in the east, in water depths of 500–2,000 m. The major depocentres are estimated to contain 7–11 km of sediments. Initial rifting in the Mentelle Basin occurred in the Early Permian, followed by thermal subsidence during the Triassic to Early Jurassic. In the Middle Jurassic renewed extension led to the accumulation of very thick sedimentary successions in half-graben depocentres. Early Cretaceous continental breakup was accompanied by extensive volcanism resulting in a thick syn-breakup volcanic succession in the western Mentelle Basin. Assessment of the petroleum prospectivity of the Mentelle Basin is based on correlations with the adjacent Vlaming Sub-basin. These correlations suggest that the Mentelle Basin depocentres are likely to contain multiple source rock intervals associated with coals and carbonaceous shales, as well as regionally extensive reservoirs and seals within fluvial, lacustrine and marine strata. Petroleum systems modelling suggests that potential source rocks are thermally mature and commenced generation in the Early Cretaceous. The Mentelle Basin offers a wide range of play types, including faulted anticlines and fault blocks, sub-basalt anticlines and fault blocks, drape and forced fold plays, and a large range of stratigraphic and unconformity plays.

Terra Nova, 24, 167–180, 2012 Abstract The rifting history of the magma‐poor conjugate margins of Australia and Antarctica is still a controversial issue. In this article, we present a model for lithosphere‐scale rifting and deformation history from initial Jurassic rifting to Late Cretaceous breakup for the conjugate Bight Basin–Terre Adélie section of the margin, based on the interpretation of two regional conjugate seismic profiles of the margins, and the construction of a lithosphere‐scale, balanced cross‐section, sequentially restored through time. The model scenario highlights the symmetric pattern of initial stretching resulting from pure shear at lithospheric‐scale accompanied by the development of four conjugate detachments and crustal half‐graben systems. This system progressively evolves to completely asymmetric shearing along a single south‐dipping detachment at the scale of the lithosphere. Antarctica plays the role of the upper plate and Australia, the lower plate. The detachment accounts for the exhumation of the mantle part of the Australian lithosphere, and the isolation of a crustal klippe separated from the margin by a serpentinized peridotite ridge. The total elongation amount of the Australian–Antarctic conjugate system reaches ∼473 km (178%). Elongation was partitioned through time: ∼189 and ∼284 km during symmetric and asymmetric stages respectively. During the symmetric stage, both margins underwent approximately the same degree of crustal stretching [∼105 km (75%) and ∼84 km (67%) for Australia and Antarctica respectively]. Again, both margins accommodated relatively the same elongation during the asymmetric stage: the Antarctic upper plate records an elongation amount of ∼284 km (88%) as crustal/mantle stretching, above the inferred low‐angle south‐dipping detachment zone, whereas the Australian lower plate underwent ∼270 km (206%) of elongation through mantle exhumation. Although the restoration process does not allow reconstruction of the precise geometry before deformation, we propose that the Jurassic early geometric evolution of the margins may have been controlled by the inherited structure or rheological heterogeneities of the continental crust; its later evolution is thought to relate to the mechanical evolution of the crustal and mantle material during exhumation, with a strong increase in localization of shear in the lower crust and mantle part of the Australian margin. The geometry of the rifted margins is comparable to other magma‐poor rifted margin such as the Newfoundland–Iberia margins or the exhumed Alpine Tethys margin exposed in the Central Alps.

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