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Sedimentary Basin Framework of Exmouth Plateau, Northwest Australia

Ron Boyd, Neville Exon, Paul Williamson

The Exmouth Plateau is a marginal plateau lying off northwest Australia. Water depths range between 800 m and 4,000 m, and the area shallower than 2,000 m covers approximately 150,000 km2. The plateau consists of rifted and deeply subsided continental crust, with a Phanerozoic sedimentary sequence around 10 km thick, deposited in the Canning and Carnarvon basins. The plateau is separated from the Northwest Shelf by the Kangaroo syncline and is bounded to the north, west, and south by oceanic crust of Cretaceous and Jurassic age.

The present structural configuration of Exmouth Plateau was initiated by rifting in the Triassic to Middle Jurassic, followed by northwest-oriented sea floor spreading. The western margin has a normal rifted structure, while the southern margin structure was dominated by transform motion. The complex rifted and sheared northern margin contains at least one crustal block of post-breakup igneous origin.

Below a rift onset unconformity of Neocomian age lies a thick Triassic paralic sequence to the south, while farther north the unconformity is of Callovian age and overlies a Jurassic sequence of Tethyan carbonates, coal measures, and volcanics. The post-breakup sequence consists of Late Jurassic-Cenomanian deltaic and shelf clastics, overlain by thin Late Cretaceous-Tertiary shallow marine to pelagic carbonates. Exmouth Plateau therefore represents classic rift to mature ocean stage development of a sediment-starved passive margin. The large fault blocks in the rifted Triassic-Jurassic sequences and large areal closures in the Cretaceous deltas encouraged petroleum exploration over the last two decades. The rifted section was shown to be gas prone, while the overlying section proved t be largely immature. The opportunity to continuously core a complete rift to mature ocean stage passive margin sequence and the ability to test the Vail sea level curve in the Indian Ocean have made Exmouth Plateau an important site on Leg 121 of the Ocean Drilling Program, scheduled for 1988.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.