Structure and Sedimentary History of Exmouth Plateau

N. F. Exon, P. E. Williamson, U. Von Rad

The large, deep-water Exmouth Plateau off northwestern Australia has been actively explored for petroleum, and a giant gas accumulation has been found. Data from industry and research institutions have established its geological framework. The plateau has a basement of continental crust that was thinned and extended in the Permian. This is overlain by 10 km of Phanerozoic strata, with an average of more than 3 km of Triassic, about 1 km of Jurassic/Cretaceous, and 0.5 km of Cenozoic strata.

The plateau separated from other parts of the northern margin of Gondwanaland in the Mesozoic. Latest Triassic and Jurassic rifting formed large north-northeast-trending fault blocks; in the Oxfordian a microcontinent drifted away to the northwest, forming the plateau's northern margin. The other margins developed in the Neocomian as "Greater India" separated from Australia--the western margin by rifting and the southern by shearing. Terrigenous input declined greatly at that time.

This old continental margin, with its relatively thin Mesozoic and Cenozoic sediments, was selected by the Ocean Drilling Program (ODP) for comprehensive and fully integrated sedimentologic, bio-stratigraphic, paleobathymetric, and subsidence studies. Following successful presite surveys (seismic and dredging) by F. S. Sonne and R. V. Rig Seismic, ODP Leg 122 drilled six continuously cored holes on the plateau, two near industry wells (1,974 m of coring) and four on the tectonically complex northern Wombat subplateau (1,600 m of coring). These holes intersected Upper Triassic siliciclastic sediments and shallow-water carbonates including reefs, Neocomian deltaic claystones, Aptian marine claystones, and younger Cretaceous and Cenozoic marls, chalks, and oozes. The drilling has yielded exciting new results bearing on the sedimentary and tectonic histories of the plateau.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.