STRUCTURE AND DEVELOPMENT OF THE WEST TASMANIAN OFFSHORE SEDIMENTARY BASINS - RESULTS OF RECENT MARINE AND AEROMAGNETIC SURVEYS

The Sorell and southernmost Otway Basins, off west Tasmania, cover an area of 100 000 km(2). At least 6 km of Cretaceous-Tertiary section is present in places and more than 50% of the region has a sediment thic kness greater than 2 km. Free oil traces were found in Cape Sorell 1, drilled in the Strahan Sub-basin and geochemical surveys of sea-floor sediments indicate thermogenic hydrocarbons and mature source rocks at depth. New data on the basin have come from swath-mapping, deep seism ic and aeromagnetic surveys recently conducted by AGSO. The Sorell Bas in initiated in the latest Jurassic-earliest Cretaceous, largely by wr ench tectonics within the Southern Rift System between the Australian and Antarctic cratons. Post-Cenomanian shallow marine to fluvial depos ition was disrupted by major Maastrichtian-Early Paleocene tectonism a ssociated with Australia-Antarctica breakup off west Tasmania. This wa s the last major structuring event in the Sorell Basin, producing a ba sement high adjacent to the transtensional plate boundary and extensiv e faulting throughout the Sorell Basin, including wrench reactivation of the depocentres now beneath the shelf. The outer high is characteri sed by intense block-faulting and igneous intrusion. Margin collapse r esulted first in deposition of thick, prograding Paleocene sequences i n the north, and then of similar thick Eocene sequences farther south as the spreading axis migrated southward (relative to Australia). In t he mid-late Eocene the rate of Southern Ocean sea-floor spreading incr eased dramatically and changed its direction to north-south; associate d transform movement along the western margin of the South Tasman Rise produced Eocene wrench deformation that affected mainly the southern Sorell Basin. The west Tasmanian margin sagged rapidly in post-Eocene time and was starved of sediment, with terrigenous sediments giving wa y to temperate carbonates. New offshore aeromagnetic data generally su pport the seismic structural interpretations and modelling confirms th at basin thickness is up to 6 lan in places and indicates that Late Ce nozoic volcanics underlie parts of the continental shelf.