PALEOZOIC BASINS OF SOUTHERN SOUTH-AUSTRALIA - NEW INSIGHTS INTO THEIR STRUCTURAL HISTORY FROM REGIONAL SEISMIC DATA

Interpretation of three recently recorded offshore seismic lines provi des a regional picture of the geology from the Gawler Craton across th e Stansbury and Troubridge Basins to the Otway Basin in South Australi a. The 300 km transect crosses most of the Cambrian Stansbury Basin, w hich consists of a marginal platform in the west and the Kanmantoo Tro ugh in the east. The Kanmantoo Trough is filled by an eastward deepeni ng sedimentary prism formed by the Kanmantoo Group. The little-deforme d platformal Cambrian sedimentary rocks onlap the Gawler Craton and un derlie the Gulf St Vincent. The eastern margin of the platform is sepa rated from the western Kanmantoo Trough by a northeast-trending zone o f intense deformation. This transition zone comprises numerous southea st-dipping faults which are correlated to faults and shear zones with reverse displacement that are mapped in outcrop on Kangaroo Island and Fleurieu Peninsula. These faults constitute contractionally reactivat ed former extensional faults, which controlled the western margin of t he Kanmantoo Trough. A southeast limit to the Kanmantoo Trough could b e indicated immediately north of the Otway Basin by an opposing, north west-dipping structural grain within the presumed Cambrian succession. Faults, which were active only during the Cambro-Ordovician Delameria n Orogeny, and the top of the Cambrian sedimentary rocks are truncated by a Permian glacial event during which the Permian Troubridge Basin sequence was deposited. This sequence is generally 200-400 m thick but this increases to 2000 m in a major trough east of Backstairs Passage . This feature may be of interest for petroleum exploration. A Tertiar y sequence covers the region with a thickness of up to 600 m. Tertiary -Holocene compression reactivates the western boundary faults of the K anmantoo Trough and is attested by marked uplift that causes the prese nt day topographic relief of Kangaroo Island, Fleurieu Peninsula and t he southern Flinders Ranges. In this framework the depression of the G ulf St Vincent could be a shallow, contemporaneous foreland basin.