GEOLOGICAL FRAMEWORK OF THE CONTINENTAL SHELF OFF NORTHWEST AUSTRALIA

Abstract

A simplified synthesis of exploration results in permits covering a large area of the broad continental shelf off northwest Australia (between Bathurst Island, N.T. and the Monte Bello Islands. W.A.) is presented and a preliminary analysis of the geological framework of the shelf is offered. Companies with working interests in these permits are B.O.C. of Australia Limited. Shell Development (Australia) Pty. Ltd., Woodside (Lakes Entrance) Oil Co. N.L., Mid-Eastern Oil N.L., B.P. Petroleum Development Australia Pty. Ltd. and. in the permits In Western Australia, California Asiatic Oil Co.Reconnaissance aeromagnetic and seismic surveys have provided evidence of substantial thicknesses of sedimentary rocks, and revealed the presence of several major structural/ depositional features. The geological significance of these features has been partly rationalized by the integration of stratigraphic and seismic data, and by interpolation of established onshore geology. Ashmore Reef No. 1 revealed a substantially complete Tertiary-Upper Cretaceous carbonate- clay sequence, a thin early Cretaceous-late Jurassic section, containing detritus from underlying Upper Jurassic basic lavas, and part of a thick Triassic sedimentary sequence. Legendre No. 1 penetrated Tertiary, Cretaceous, and Jurassic sedimentary sequences, with gross stratigraphic similarities to sequences in parts of the Carnarvon basin.The shelf has a block-faulted basement substructure of Precambrian rocks, related in part to the extension of major structure in Precambrian rocks of the northwest Australian mainland. The geological superstructure of the shelf consists of Phanerozoic rocks whose essential structural framework is related to rejuvenated Precambrian structure. Warped and faulted Phanerozoic sequences have two major strike components, one sub-parallel and the other sub-normal to the present coastline. The effects of tectonics on sedimentation are outlined.