Structure and development of the west Tasmanian offshore sedimentary basins: Results of recent marine and aeromagnetic surveys

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Abstract
The Sorell and southernmost Otway Basins, off west Tasmania, cover an area of 100 000 km2. At least 6 km of Cretaceous‐Tertiary section is present in places and more than 50% of the region has a sediment thickness 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 seismic and aeromagnetic surveys recently conducted by AGSO. The Sorell Basin initiated in the latest Jurassic ‐ earliest Cretaceous, largely by wrench tectonics within the Southern Rift System between the Australian and Antarctic cratons. Post‐Cenomanian shallow marine to fluvial deposition was disrupted by major Maastrichtian ‐ Early Paleocene tectonism associated with Australia‐Antarctica breakup off west Tasmania. This was the last major structuring event in the Sorell Basin, producing a basement high adjacent to the transtensional plate boundary and extensive faulting throughout the Sorell Basin, including wrench reactivation of the depocentres now beneath the shelf. The outer high is characterised by intense block‐faulting and igneous intrusion. Margin collapse resulted first in deposition of thick, prograding Paleocene sequences in the north, and then of similar thick Eocene sequences farther south as the spreading axis migrated southward (relative to Australia). In the mid‐Late Eocene the rate of Southern Ocean sea‐floor spreading increased dramatically and changed its direction to north‐south; associated 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 way to temperate carbonates. New offshore aeromagnetic data generally support the seismic structural interpretations and modelling confirms that basin thickness is up to 6 km in places and indicates that Late Cenozoic volcanics underlie parts of the continental shelf.