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We combine two- and three-dimensional seismic stratigraphic interpretation with paleobathymetric analysis from benthic foraminifera to understand the genetic significance of prominent seismic discontinuity surfaces typically mapped as "sequence boundaries" and "flooding surfaces" in the late Paleogene-early Neogene Northern Carnarvon Basin. The progradational succession, dominated by heterozoan carbonate sediments, is divided into five northwest-prograding clinoformal sequences and 19 sub-sequences. Clinoform fronts progress from smooth to highly dissected, with intense gullying apparent only after the mid Miocene optimum. Once initiated, gullies become the focus for sediment distribution across the front. Bottomsets remain relatively sediment-starved without the development of aprons on the lower slope and basin. Small-scale variability suggests heterogeneous sediment dispersal through the slope conduits. Along-strike sediment transport superimposed on progradation changes from south-directed in the late Oligocene to north-directed in the late mid-Miocene suggesting a major reorganization of circulation in the southeastern Indian Ocean. Prominent seismic discontinuity surfaces represent both intervals of shallow paleo-water depth and flooding of the shelf. Partial exposure of the shelf indicated by karst morphology is coeval with middle to outer neritic paleo-water depths on the outer shelf. Rather than build to sea-level, progradation occurs with shelf paleo-water depths at the clinoform rollover >100 m. Therefore, in the Northern Carnarvon Basin onlap onto the clinoform front is not coastal and the sensitivity of the clinoforms to sea-level changes is muted.

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Conference volume and CD are available through the Petroleum Exploration Society of Australia

Two- and three-dimensional (2D and 3D) seismic stratigraphic interpretation, palaeobathymetric analysis from benthic foraminifera, and 2D forward tectonic modelling are combined to understand the genetic significance of prominent seismic discontinuity surfaces typically mapped as ?sequence boundaries? and ?flooding surfaces?, and their intervening sequences. Integration of these data has allowed interpretation of the Tertiary, heterozoan (i.e., non-photozoan) carbonate-dominated succession detailing the evolution of five prograding clinoformal sequences (2-5 m.y. duration), and 19 sub-sequences (<0.5-1 m.y. duration), along the Rankin Trend. Variations in accommodation space as modelled across the Dampier Sub-basin using 2D kinematic and flexural modelling are the combined result of synrift and postrift thermal subsidence, inversion and eustatic variations. The major observations and implications of this study are: ? Onlap onto the clinoform front of primary mappable surfaces is submarine with minimum estimated palaeo-water depths > 100 m at the shelf edge. Exposure surfaces identified in the middle Miocene are seismically less prominent, with potential karstification identified 6-8 km inboard of shelf edges. ? Systems tracts could not be consistently identified in the progradation-dominated succession. Lowstand basin-floor fans/aprons and transgressive systems tracts are largely absent on the seismic scale, resulting in downlap directly onto sequence boundaries. ? Linear, 30-80 km along strike, two-dimensional mapped sequences, are the integration of local sedimentary lobes up to 10 km in diameter. ? Canyon development may be controlled by inclination on gully failure walls rather than variations in sea level. Gully initiation is coincident with the mid-Miocene climatic Optimum. However, once established, erosion paths are maintained and enlarged by downslope sediment flows, derived from headward failure, regardless of proposed sea-level variations. ? The magnitude of inversion-related uplift is small, reaching a maximum of ~50-70 m at anticlinal crests focussed along the Rankin, Madeleine and Rosemary trends. Although this is of a similar scale to postulated eustatic variations that increase or decrease accommodation space across the entire margin, unconformities and onlap discontinuity surfaces related to these inversion structures are areally restricted.

Seismic sequence analysis across the Northern Carnarvon basin of the northwest Australian margin has been combined with a kinematic and flexural model for the deformation of the lithosphere and palaeobathymetric analysis of benthic foraminifera to define the history, distribution and magnitude of inversion within the Dampier Sub-basin during the Cretaceous and Tertiary. The large palaeo-water depths (>1000 m) developed across the outer margin in the late Oligocene-Miocene questions the results from earlier well-based backstripping studies. This accommodation was created by a combination of thermal subsidence engendered primarily by Tithonian-Valanginian extension and continental break-up and sediment loading associated with the progradation of Neogene clinoforms. Discrete inversion events characterize the Santonian, late early Miocene, middle Miocene, late Miocene, latest Miocene, and Plio-Pleistocene of the Northern Carnarvon basin. Compression-induced inversion creates and destroys accommodation space at different spatial wavelengths compared with thermal subsidence, sediment loading and eustatic variations and thus can be spatially separated. While brittle deformation in the upper crust results in relatively short-wavelength uplift, the flexural response to this tectonic loading produces a longer-wavelength regional subsidence adjacent to the inversion anticline. In general, the flexural component is negligible. Inversion tends to be focused along pre-existing rift fault systems. However, the spatial distribution of inversion varied through time, with Cretaceous inversion concentrated along the northeast-southwest oriented Rankin, Madeleine, and Rosemary trends while the locus of Miocene inversion was located ~20 km northwest of the Rosemary Trend. Clearly, different fault zones were involved in the inversion process at different times. We surmise that intraplate stresses generated from the readjustment of the Indo-Australian plate were a possible mechanism for Santonian inversion. Tertiary inversion, interpreted to have commenced in the middle Miocene (~17 Ma), continued to occur through to the Plio-Pleistocene. The onset and continued compression is interpreted to be related to the Australian/Indonesian continent-continent collision. Total shortening of the lithosphere during the Santonian and Tertiary was modelled to be 2.6 and 0.16 km, respectively.

A new sequence stratigraphic framework has been developed for the Otway Basin based on the interpretation and integration of offshore wells, key onshore wells, new biostratigraphic results and a regional grid of 2D seismic data. In the new tectonostratigraphic framework, seven major basin phases and their eight component supersequences are recognised as follows: 1) Tithonian?-Barremian rifting of the Crayfish Supersequence 2) Aptian-Albian post-rift deposition of the Eumeralla Supersequence 3) mid-Cretaceous compression and inversion 4) Late Cretaceous rifting of the Shipwreck and Sherbrook Supersequences 5) latest Maastrichtian to Middle Eocene basin reorganisation and early thermal subsidence of the Wangerrip Supersequence 6) local inversion and thermal subsidence of the Nirranda Supersequence (Middle Eocene to Early Oligocene) followed by thermal subsidence and progressive compression of the Heytesbury Supersequence (Late Oligocene to Late Miocene) leading to Late Miocene uplift and erosion and 7) Plio-Pleistocene deposition of the Whalers Bluff Supersequence. Basin phases are distinguished by their different tectonic driving mechanisms as the primary control on the creation of accommodation space. The supersequences are bounded by regional unconformities and define major episodes of sedimentation within each basin phase. Supersequences are related to second-order transgressive-regressive cycles within the basin and are regionally mappable. The new sequence stratigraphic framework is then used as the basis for correlation to deep-water regions where well-control is limited or absent. The framework is also used to help place existing, complex, facies-dependent lithostratigraphic schemes into depositional and petroleum systems context.

The Bass Basin is a moderately explored Cretaceous to Cainozoic intracratonic rift basin on Australia?s southeastern margin. A basin-wide integration of seismic data, well logs, biostratigraphy and sequence stratigraphy has resulted in the identification of six basin phases and related megasequences/ supersequences. These sequences correlate to three periods of extension, a rift-transition phase, and two subsidence phases. The complex nature of facies relationships across the basin is attributed to the (mostly) terrestrial setting of the basin until the Middle Eocene, multiple phases of extension, strong compartmentalisation of the basin due to underlying basement fabric, and differential subsidence during extension and early subsidence phases. Evidence of the initial rift phase (Otway Megasequence) is only clearly observed in the Durroon Sub-basin and in the southwestern Cape Wickham Sub-basin. The second rift phase (Durroon Megasequence) is pervasive throughout the Bass Basin, although a full succession of this megasequence was only penetrated in the Durroon Sub-basin. The third-rift phase (Bass Megasequence) is also pervasive throughout the basin, but appears to have affected only particular depocentres such as the Pelican, Cormorant and Yolla troughs. Here, expanded syn-tectonic growth sections have been intersected. There is wide variation in facies type, environment and thickness of the Bass Megasequence due to differential rates of subsidence. Three component sequences have been recognised within the Bass Megasequence (Furneaux, Tilana and Narimba sequences), with each component sequence correlated to discrete periods of increased accommodation. The shift from rift-to-post-rift conditions (Aroo Megasequence) was signaled by waning subsidence rates and an increasing brackish influence. A wide variation in facies types, environments and thicknesses is also observed. The frequency and thickness of coals began to increase during the deposition of this megasequence, lasting from Early Eocene until the mid-Middle Eocene. A slowdown in subsidence rates allowed the aggradation of coaly facies (many geochemically characterised as ?hydrogen-rich?), indicating there was a balance between accommodation, sediment supply and peat production. The most important sequences for petroleum generation and trapping are the Bass and Aroo megasequences. Most of the coaly source rocks now typed to liquid hydrocarbon generation were deposited during the period of late Early Eocene to Middle Eocene rift-transition phase. The critical factor in sourcing accumulations from the coaly succession appears to be effective primary and secondary expulsion from the source rock and the volume of charge. Biostratigraphic studies have identified lacustrine cycles during the Late Cretaceous to Middle Eocene, with geological evidence indicating these lakes developed during times of increased accommodation. Lacustrine shales are likely to be more important as seal facies, rather than as potential source rocks. The Middle Eocene (Demons Bluff Sequence) and younger marine successions (Torquay Sequence) show low source potential and do not lie within the oil window. Optimal conditions for seal deposition occurred during lacustrine cycles in the Late Cretaceous to Early Eocene, and the mid-Eocene. Untested plays include reservoir/seal pairs associated with seven maximum flooding events in the western Bass Basin. The petroleum systems elements of the Durroon Sub-basin differ significantly from the Cape Wickham Sub-basin owing to the cessation of tectonically-driven subsidence in the eastern Bass Basin (Durroon Sub-basin) from the mid-Campanian onward.

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The conjugate margins of Wilkes Land, Antarctica, and the Great Australian Bight (GAB) are amongst the least understood continental margins. Break up along the GAB-Wilkes Land part of the Australian-Antarctic margin commenced at approximately 83 Ma. Using recent stratigraphic interpretations developed for the GAB, we have established a sequence stratigraphy for the Wilkes Land margin that will, for the first time, allow for a unified study of the conjugate margins. By reconstructing the two margins to their positions prior to break up we were able to identify comparable packages on the Wilkes Land margin to those recognised on the GAB margin. Excluding the glacial sediments on the Antarctic margin, the sedimentary sequence along the Wilkes Land margin is very thin compared to the GAB margin, which has substantially more syn- and post-rift sediments. Despite the differences in thickness, the syn-rift sedimentary package on the Wilkes Land margin exhibits a similar style of extensional faulting and seismic character to its GAB margin counterpart. In comparison, post-rift sequences on the Wilkes Land margin are markedly different in geometry and seismic character from those found on the GAB margin. Isopach mapping shows substantial differences in the thickness of the post-breakup sediments, suggesting different sediment sources for the two margins. The Late Cretaceous Hammerhead Supersequence provides much of the post-rift thickness for the GAB margin as a result of large sediment influx into the basin. This supersequence is characterised by a thick progradational succession and was deposited in fluvio-deltaic and marine environments. The equivalent succession on the Wilkes Land margin has a different seismic character. It is thinner and aggradational, suggesting a distal marine environment of deposition.