Southern Australian breakup history is divisible into three phases. The first phase began with Callovian (c.159–165 Ma) rifting in the western Bight Basin. During the Tithonian (c.142–146 Ma), rifting extended eastwards into the Duntroon, Otway and Gippsland Basins. By the Valanginian (c.130–135 Ma), ocean crust formed between India and western Australia. Structural style in the western Bight changed to thermal subsidence. However, fluvio-lacustrine rift sedimentation continued in Duntroon, Otway and Gippsland until the Barremian (c.115–123 Ma) when these basins also changed to thermal subsidence. The diachronous progression of basin fill types produces a progressive shift in ages of potential source, seal and reservoir intervals along the margin.The second phase began during the Cenomanian (c.92–97.5 Ma) with uplift in eastern Australia, stress reorganisation and divergence of basin development. The Otway, Sorell and Great South Basins formed in a transtensional regime. These tectonics resulted in trap generation through faulting, inversion and wrenching. During the Santonian, oceanic spreading began in the southern Tasman Sea (c.85 Ma). Slow extension caused thinning of continental crust in the Bight and Otway Basins and subsidence into deeper water. Ocean crust formed south of the Bight Basin in the Early Campanian (c.83 Ma) and also started extending up the eastern Australian coast.The third stage in development was caused by Eocene changes to fast spreading in the Southern Ocean (c.44 Ma), final separation of Australia and Antarctica, and cessation of Tasman Sea spreading. These events caused collapse of continental margins and widespread marine transgression. The resultant loading, maturation and marine seal deposition are critical to petroleum prospectivity in the Gippsland Basin.

Reconnaissance field traverses in Seram have led to major revisions in the stratigraphy, structure, tectonic history and geological maps. The island is composed of 4 principal stratigraphical-structural elements:(1) metamorphic continental -I of uncertain structural status and palaeogeographical affinity, (2) an entirely marine early TiiassicMiocene imbricate succession regarded as para-autochthonous, (3) an allochthon composed of several different thrust sheets, including metamorphic rocks, Triassic limestone and a late Miocene olistostrome, (4) a Plio-Pleistocene post-orogenic autochthon. The apparently overthrust slices of metamorphic basement complex can be interpreted as derived from either the Asian or Australian craton. The Australian shelf, slope and rise sediments, possibly including some oceanic sediment, are regarded as para-autochthonous. Remarkably close correlation is demonstrated between the stratigraphical breaks reported from the Mesozoic-Cenozoic succession of the NW Australian shelf, from Misool, and the para-autochthonous rocks of Seram and Timor. This emphasizes the presence of the Australian craton underlying these 3 islands. Close correlation is also found between the allochthonous rocks of Seram and Timor. Some of these thrust sheets are interpreted as having been derived from the Asian continental margin. The ultrabasic rocks of SW Seram and Ambon seem to form the highest thrust sheets. The main period of orogenesis, involving over-thrusting, olistostrome emplacement and imbrication of the underlying Australian cover-rock sequences occurred in the late Miocene-early Pliocene (N.18). The structural position of the volcanic rocks of Ambon is uncertain. A tentative interpretation is that they are in sim, having been extruded from deep-seated fractures that penetrate the 'Asian' thrust sheets and the underlying Australian continental basement.In our continuing study of the Banda Arcs, Seram ( Fig. 1) was considered the most important island to investigate after Timor, since it lies on the opposite side of the 180" curved Banda Arc, is the next largest island (being 400 X 75 km), and has geological sketch map coverage (with sample descriptions), which allow reinterpretation on the basis of airphotos, ERTS photos and field studies. The geology of Seram is a mirror image of the geology of Timor in many important respects, such as the supposed directions of overthrusting and the contrasting Mesozcic faunas and facies showing affinities with either Australian (basement) or 'Asian' (overthrust) elements. Published reports on the geology of Seram (Valk 1945; Germeraad 1946; van der Sluis 1950; Zillman & Paten 1975) suggested that lithologies and faunas were similar to those in Timor. Compared with Timor (AudleyCharles 1968; Carter et al. 1976; Barber er al. 1977), accounts of the pre-Neogene stratigraphy and structure of Seram seem confused, with apparently unrelated stratigraphical divisions and dissimilar structural elements lumped together unconvincingly. Furthermore, no unambiguous evidence of ...

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Increased exploration activity in Area A of the Timor Gap Zone of Cooperation between Australia and Indonesia (ZOCA) has created the need for revision of the existing stratigraphic framework of the region. A chronostratigraphic approach to the analysis of the Mesozoic and Cainozoic succession of Western ZOCA provides a framework for improved stratigraphic prediction. The framework is based on the identification of depositional sequences by the integration of seismic and well data. Genetically related depositional sequences have been grouped into seven 'megasequences' which reflect distinct stages in the tectonic development of the basin.The Mesozoic and Cainozoic succession in the Northern Bonaparte Basin was deposited in a marginal sag basin that was affected by Triassic to Lower Cretaceous extension related to continental separation along the northwest margin of Australia. Four stages are seen in the evolution of the basin since the end of the Permian. Relative tectonic quiescence during the Triassic preceded two cycles of extension related to continental separation during the Jurassic to Earliest Cretaceous. Continental separation was followed by the development of a Cretaceous/Tertiary passive margin and a subsequent phase of tectonism related to the Miocene/Pliocene collision of the Indo-Australian and Eurasian plates. A tentative correlation has been made between the megasequence framework of Western ZOCA and the geological succession exposed on Timor Island.The framework forms the basis for a system of common stratigraphic nomenclature for the Timor Gap. The model also assists in understanding the tectono-strati-graphic evolution of the basin and is a foundation for the development of new play concepts that will support continuing exploration activity in the area.

The northwest-trending Otway Basin in southeast Australia formed during the separation of Australia and Antarctica between the latest Jurassic and the Early Cainozoic. A new, deep-seismic data set shows that the basin comprises two temporally and spatially overlapping rift components:the mainly Late Jurassic to mid-Cretaceous, east-west trending, inner Otway Basin—comprising the onshore basin and most of the continental shelf basin; andthe northwest–southeast to north–south trending depocentres beneath the outer shelf and continental slope, extending from eastern South Australia to the west coast of Tasmania, and a relatively minor and ill-defined sub-basin underlying the continental rise in water depths greater than about 4,500 m. This rift system was most active from the mid-Cretaceous to Palaeogene, and was strongly affected by sinistral strike-slip motion as Australia and Antarctica separated.The continental slope elements contain the bulk of the sediment volume in the basin. From northwest to southeast, these elements comprise the Beachport and Morum Sub-basins, the north-south trending Discovery Bay High, and the Nelson Sub-basin which appears to be structurally and stratigraphically continuous with the Sorell Basin off west Tasmania.The reflection character of the crust and upper mantle varies widely across the basin, and there is a strong correlation between that character and the basin configuration. It appears that accommodation space beneath the slope basin was created largely by extension and removal of most of the laminated deep continental crust.There is encouragement for hydrocarbon exploration in the deep-water basin. Firstly, there are indications of diagenesis related to fluid flow in and above the strongly faulted Cretaceous section in the Morum Sub-basin. As an Early Cretaceous petroleum system is already proven beneath the continental shelf, this suggests that the same system is also active in deep-water. Secondly, existing sample data suggest that a second, Late Cretaceous petroleum system could be active where any source rocks are sufficiently deeply buried; this condition would probably be met in the Nelson Sub-basin.