THE REGIONAL GEOLOGY OF THE EXMOUTH PLATEAU

Abstract

The Exmouth Plateau and adjacent continental slopes cover 300 000 km 2 beyond the Northwest Shelf, in water depths ranging from 800 to 5000 m. The regional geology has been interpreted from 18 000 km of seismic reflection profiles, and ties to exploration wells.

The plateau is formed over a major arch and syncline. These structures parallel the Rankin Platform and lie 250 and 100 km respectively to the northwest of it. The dominant structural grain is northeasterly, but easterly and southeasterly cross-trends occur along the northern and southwestern margins respectively. Extensive normal faulting affects the pre-Cretaceous sequence.

Basement is overlain by up to 10 000 m of Phanerozoic strata, about half of which probably comprises shallow marine and terrestrial sediments of Silurian to Permian age. As much as 4 000 m of Triassic shallow marine to fluvial sediments overlie the Palaeozoic. The Triassic surface is uneven, extensively block-faulted, and unconformably overlain by up to 2 000 m of Middle Jurassic to Neocomian deltaic sediments. An average of 200 m of mid-Cretaceous shallow marine clastic sediments overlies the Neocomian, and are in turn unconformably overlain by a carbonate sequence of Santonian to Recent age containing two major hiatuses and averaging 700 m thick.

The period of block-faulting preceded formation of a northeasterly-trending spreading-centre in the Late Jurassic, which separated the area from the adjacent part of Gondwanaland to the west. An associated easterly-trending transcurrent fault probably formed the northern margin of the Exmouth Plateau. In the Late Cretaceous the area southwest of the plateau collapsed rapidly along a northwesterly-trending fault. Gradual subsidence of the plateau took place throughout the Cainozoic and, at the northern margin, collapse along old easterly trending fault-lines gave rise to grabens south of marginal sub-plateaux. Late Cainozoic warping formed the Exmouth Plateau Arch.

Petroleum source rocks, especially Palaeozoic to Neocomian shales and siltstones, and reservoir rocks, especially Triassic and Neocomian sandstones, appear to exist in the Exmouth Plateau area. The depth of burial has probably been adequate to form hydrocarbons from pre-Cretaceous source rocks. Numerous fault traps in Triassic sediments, analogous to those of the Rankin Platform, appear to exist. Other likely petroleum targets are stratigraphic traps in the Jurassic—Neocomian deltaic sequence.