Palynological studies of Triassic-Jurassic well sections in the Offshore North Perth Basin have helped to reveal a more complicated geological history than previously recognised. This work is part of a major Geoscience Australia project studying the geological history and petroleum prospectivity of the basin. Seismic and well log interpretations have been combined with the sedimentological data to develop a high resolution sequence stratigraphic framework. This work is heavily reliant on the palynological data to provide the necessary age control, palaeoenvironmental interpretations and well correlations. Abstract continues (no space in field).

The Perth Basin formed as part of an obliquely-oriented extensional rift system on Australia's southwestern margin during the Paleozoic to Mesozoic breakup of eastern Gondwana. The Houtman Sub-basin is situated in the offshore portion of the northern Perth Basin, located about 200 km northwest of Perth. It is an elongate, northwest-southeast trending depocentre containing up to 14 km of Early Triassic to Late Jurassic sedimentary strata. A detailed sequence stratigraphic study has been undertaken on the three wells in the Houtman Sub-basin: Gun Island 1, Houtman 1 and Charon 1. The purpose of this study was to investigate facies variations between the wells to gain a better understanding of potential source, reservoir and seal distribution and to assist regional palaeogeographic reconstructions of the Perth Basin. The study focussed on the Early-Late Jurassic succession comprising the Cattamarra Coal Measures, Cadda Formation and Yarragadee Formation. Wireline log character, cuttings, sidewall core and conventional core lithologies and palynological data were used to identify facies and paleoenvironments. Palynology for all wells has been reviewed, including new data collected by Geoscience Australia for Gun Island 1 and Charon 1. Facies stacking patterns were used to define systems tracts and subsequently ten third-order depositional sequences. Collectively these sequences define a larger scale, second-order (supersequence) transgressive-regressive cycle. The Cattamarra Sequence Set forms a regional transgression which culminates in an extensive marine maximum flooding event within the Cadda Sequence Set. These sequence sets are followed by the regressive highstand Yarragadee Sequence Set. The third-order sequences characterised in this study overprint this supersequence and control the local distribution of facies. The combined influence of these third- and second-order sequences on facies distribution has significant implications for the distribution of potential reservoirs and seals, particularly in the northern Houtman Sub-basin where well and seismic data are relatively sparse.

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The following abstract was written in order to facilitate the compilation of the Queensland four-mile geological sheets and the explanatory notes accompanying them. The area described covers the Springsure, Emerald, Jericho and partly the Tambo and Baralaba four-mile sheets.

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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.

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.

The rifted margins of eastern and southern Australia formed during multiple periods of extension associated with the fragmentation and dispersal of Gondwana in the Late Jurassic to Early Eocene (Veevers & Ettreim 1988; Veevers et al. 1991). The sedimentary basins of the Southern Rift System (Stagg et al. 1990) extend from Broken Ridge in the west, to the South Tasman Rise (STR) in the east. Collectively, these depocentres cover an area in excess of 1 million square kilometres (excluding the STR), with the thickest sediments (up to 15 km) occurring in the Ceduna Sub-basin of the Bight Basin. Early phases of the extension during the late Middle Jurassic to Early Cretaceous resulted in the formation of a series of west-northwesterly trending continental rift basins along the southern margin of Australia and a series of north-northwest trending transtensional basins along the western margin of Tasmania. The amount of upper crustal extension varied between basins of the rift system. This phase of upper crustal extension preceded eventual breakup between the Australian and Antarctic plates off the Bight Basin in the latest Santonian to earliest Campanian (Sayers et al. 2001). The nature of source rocks within the rift basins reflects the eastward propagation of the rift system through time, with largely terrestrial systems dominating in the early rift stages, followed by marine inundation from the Aptian onwards (west of the Otway Basin). In the Otway Basin, the first marine influence is recorded during the early Turonian, while in the Sorell and Bass basins marine conditions prevailed from ?Maastrichtian and middle Eocene time, respectively. Terrestrial progradational systems in the Late Cretaceous are important in the maturation of potential source rocks in the Bight and Otway basins, while Neogene carbonate-dominated systems are important in the Sorell, Bass and Gippsland basins. Outside of the Gippsland Basin where exploration has reached a mature status, the southern margin basins remain frontier to moderately exploration areas, with an overall drilling density (excluding the Gippsland Basin) of approximately 1 well per 6,000 square kilometres. Key Words: Australian Southern Margin, Southern Rift System, petroleum systems References SAYERS, J., SYMONDS, P.A., DIREEN, N.G. and BERNARDEL, G., 2001. Nature of the continent-ocean transition on the non-volcanic rifted margin of the central Great Australian Bight. In, Wilson, R.C.L., Whitmarsh, R.B., Taylor, B., and Froitzheim, N., (Eds), Non-Volcanic Rifting of Continental Margins; A Comparison of Evidence from Land and Sea. Geological Society, London, Special Publications, 187, 51?77. STAGG, H.M.J., COCKSHELL, C.D., WILLCOX, J.B., HILL, A., NEEDHAM, D.J.L., THOMAS, B., O?BRIEN, G.W. and HOUGH, P., 1990. Basins of the Great Australian Bight region, geology and petroleum potential. Bureau of Mineral Resources, Australia, Continental Margins Program Folio 5. VEEVERS, J.J. and ETTREIM, S.L., 1988. Reconstruction of Australia and Antarctica at breakup (95 ? 5 Ma) from magnetic and seismic data at the continental margin. Australian Journal of Earth Sciences, 35, 355?362. VEEVERS, J.J., POWELL, C.MCA. and ROOTS, S.R., 1991. Review of seafloor spreading around Australia, I. Synthesis of the patterns of spreading. Australian Journal of Earth Sciences, 38, 373?389. WILLCOX, J.B. and STAGG, H.M.J., 1990. Australia?s southern margin, a product of oblique extension. Tectonophysics, 173, 269?281.

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