GEOLOGICAL FRAMEWORK OF THE GREAT AUSTRALIAN BIGHT

Abstract

Geophysical exploration carried out in the Great Australian Bight since 1966, combined with geological fieldwork in the adjacent land areas, has made it possible to outline the broad geological framework of the area.

The "basement" consists of two major units, an offshore extension of the locally metamorphic Cambrian Kanmantoo Group in the south-east and the extension of the West Australian Archaean shield in the north-west. The boundary is thought to follow a trend extending westerly from the Cygnet-Snelling fault zone on Kangaroo Island.

In two areas the basement has been downfaulted, thus creating depositional areas for thick sequences of sediments, namely the Elliston trough to the west of Eyre Peninsula and the Duntroon basin, south of Eyre Peninsula and west of Kangaroo Island.

The geological setting of the Duntroon basin appears to be comparable with the Otway basin and a Jurassic- Cretaceous age is assumed for the folded sequence of sediments overlying the basement and underlying the Tertiary with angular unconformity. The basin was possibly partially and temporarily closed to the south and open to marine influences from the west.

In the Elliston trough the lower part of the section which has low to medium velocity seismic character, is probably Mesozoic, as is evidenced by the Upper Jurassic encountered in its onshore extension. Proterozoic-Cambrian sediments may overlie the basement in the eastern part of the trough. Deformation of the Mesozoic is limited to the mouth of the trough where there is indication of a base- Tertiary unconformity. This trough was probably also open to marine influences to the west.

Along the continental margin between the basins and also south of the Eucla basin a thin Mesozoic section, conformably underlying the Tertiary, is probably present, gradually thickening towards the continental slope.

In the onshore area Tertiary sedimentation started with local deposition of clastics during the Middle Eocene, which also may have been the case off the Eucla basin, in the Elliston trough and in the Duntroon basin. Carbonate sedimentation took place from the Middle-Upper Eocene onwards, to reach its widest areal extent during the Lower Miocene. A hiatus during the Oligocene may have occurred in the western part of the Bight as is the case in the Eucla basin.

Only weak deformation of the Tertiary in the offshore area has been observed. This generally occurs over Mesozoic structures in the Duntroon basin and as draping over topographic basement highs at the mouth of the Elliston trough.

No significant hydrocarbon indications are known from the surrounding land areas, but the well-documented bitumen strandings along the coast point to offshore seepages indicating generation of hydrocarbons in the general area.

At this stage prospects must be regarded as speculative.

although a folded probable Mesozoic sequence forms an objective in the Duntroon basin while prospective Mesozoic-Tertiary section appears to be present in the Elliston trough, where structural evaluation is still at a relatively early stage.