THE EYRE SUB-BASIN: RECENT EXPLORATION RESULTS

Abstract

The Eyre Sub-basin of the Great Australian Bight Basin comprises a series of half-grabens with a maximum sediment thickness in the order of 6 000 m. It is bounded to the north by high-standing basement with a sedimentary cover about 550 m thick. To the west, sedimentary cover gradually thins and onlaps rising basement. To the south, a high- standing basement ridge separates the sediments within the Eyre Sub-basin from those of the Great Australian Bight Basin proper. The sedimentary pile apparently thickens south of the basement ridge where water depth increases to more than 1 400 m.

The high basement trend bounding the sub-basin to the south plunges gradually to the east where it is eventually broken up by faulting. Seismic data from the eastern end of the sub-basin show progressive down-faulting of basement and increasing sediment thickness to the south.

Jerboa 1 was drilled on a tilted basement fault block. It penetrated 1 739 m of sedimentary section, which is believed to be a condensed sequence representative of most of the total sedimentary fill of the sub-basin. Middle to Late Jurassic (Callovian-Kimmeridgian) sediments were encountered above basement, and the sequence continued almost unbroken into the Late Cretaceous (Cenomanian). Minor unconformities occur between the non-marine Aptian sequence and the overlying marine Albian, and between the Albian and Cenomanian. A major unconformity separates the Cenomanian from the overlying Tertiary section, interpreted to have been deposited after the separation of Australia from Antarctica.