RIFT AND DRIFT IN THE DAMPIER SUB-BASIN, A SEISMIC AND STRUCTURAL INTERPRETATION

Abstract

Quality improvements in marine reflection seismic data over recent years have lead to a better understanding of the relationships between seismo-stratigraphical sequences present in the Dampier Sub-basin and those in adjacent areas. The "drift-onset" unconformity, which separates the syntectonic rift sequence from the post-tectonic drift sequence, can now be seismically recognised as a single unfaulted surface. Previously this unconformity was interpreted to be faulted. In places this surface had some 2400 m of palaeotopography in the form of an escarpment. This escarpment was formed by tectonic movements and subsequent erosion some time in the Callovian, probably as a consequence of the opening of the Indian Ocean.

The presence of Callovian and Upper Jurassic marine sands on the Rankin Platform shows that the Rankin Platform was in places submerged during Callovian and Upper Jurassic times. Furthermore, the Dampier Sub- basin must have been more than 2000 m deep immediately following the causal tectonic event. The escarpment was rapidly buried, with 1200 m of sediments locally deposited by the end of the Callovian and was finally buried by Neocomian times. Thereafter the Rankin Platform and Dampier Sub-basin have subsided.