Extensional Systems

Presented by Kevin C. Hill, Associate Professor, University of Melbourne Gravity modelling of Australia's southern margin reveals that the initial rift with Antarctica was beneath the current Ceduna Delta. A regional, high-quality seismic traverse from the coast to oceanic crust across the Bight Basin has been assembled and interpreted in detail, then balanced, restored, decompacted, and replaced at paleo-water depths. The Late Cretaceous Ceduna Delta developed above a Late Jurassic-Early Cretaceous rift basin in three stages punctuated by significant pulses of uplift and erosion across areas >100 km wide and with up to 1 km of erosion. The Cenomanian White Pointer delta prograded into deepening water and hence underwent gravitational collapse. This was terminated in the Santonian when the Antarctic margin was pulled out from below, thus supplying heat to a remnant thicker outer margin crust, causing doming and erosion. Importantly, this established the saucer-shaped geometry of the Ceduna Delta that persisted throughout its development, so that any hydrocarbons generated in the southern half of the basin would have migrated towards this outer margin high. The Tiger Formation was deposited in shallow water in a full rift basin prior to breakup, which was followed by regional thermal subsidence. The Hammerhead delta developed on the newly formed passive margin but was terminated by another pulse of uplift and erosion, perhaps associated with a change in plate motion at the end of the Cretaceous. The finite element modelling of this proposed tectonic evolution will test its validity and predict hydrocarbon generation and migration through time.