Mesozoic outcrops on the lower continental slope off Exmouth, Western Australia

Seismic reflection profiles over the southwestern margin of the Exmouth Plateau, which have been tentatively tied to the Northwest Shelf stratigraphic sequence, indicate that rocks of probable Permian to Early Tertiary age form bedrock on the lower continental slope and probably crop out in places. Water depths range from 1400 to 4000 m. It is surmised that the rocks present include pre-Late Jurassic fluvial, deltaic and shallow marine sandstone, siltstone and shale; Late Jurassic to Neocomian deltaic sandstone, siltstone and shale; mid Cretaceous shallow marine shale; Late Cretaceous shelf limestone and Cainozoic foraminiferal ooze. The Exmouth Plateau is cut by normal faults which are predominantly downthrown oceanwards and trend north-northeasterly across most of the area, except near the southwestern margin where they trend northwesterly. Faulting occurred primarily in the Middle to Late Jurassic and led to formation of the north-western and northern margins by seafloor spreading. A major anticline, which parallels the southwestern margin, has one limb truncated by the lower continental slope, probably resulting in older rocks cropping out in places. Deep Sea Drilling Project results on the Cuvier Abyssal Plain indicate that the southwestern margin formed in the Late Cretaceous, either as a transcurrent fault associated with seafloor spreading or by collapse along normal faults. Sampling of outcrops along the southwestern margin would enable the sequence to be dated more reliably and this would permit the theories of geological evolution and favourable petroleum potential of the Exmouth Plateau to be tested.