A NEW LOOK AT THE OTWAY BASIN

Abstract

The Otway Basin in western Victoria and eastern South Australia is a primarily offshore, Upper Cretaceous to Recent basin overlying a Lower Cretaceous basin. The major stratigraphic break developed after the deposition of the Upper Cretaceous Sherbrook Group. The offshore part of the basin in Victorian and Tasmanian waters has been explored since 1960 with a series of seismic surveys which improved in quality after 1971. Further offshore seismic data has been provided by the Bureau of Mineral Resources (BMR) Continental Margins Survey and by Shell Petrel lines. Seven wells have been drilled in Victorian/Tasmanian waters and six in South Australian waters.

Stricter control of recording noise levels and continuous velocity analyses in processing, plus the use of refraction in seismic surveys since 1971 has enabled an improved interpretation of the Victorian/Tasmanian part of the basin.

The Upper Cretaceous clastic sediments were deposited in an intracratonic basin with a maximum thickness of approximately 20 000 feet. The northeast flank is formed by a series of down-to-basin faults and increased movement on these faults and overall tilting gave a marginal basin in Tertiary time so that the section comprises a series of outbuilding sedimentary wedges.

A major basin started to form across southern Australia in Lower Cretaceous times. In the Upper Cretaceous the three discrete Bass Strait basins developed from the major basin with Australia and Antarctica still in close relationship. Plate separation was associated with the change from an intracratonic basin to a marginal basin. Further separation produced no major change in the nature of deposition in the marginal basin.