Structure and tectonic history of the northern Exmouth Plateau and Rowley Terrace: outer North West Shelf

A series of regional seismic reflection lines has been recorded, which link three key exploration wells on the northern Exmouth Plateau and Rowley Terrace to the lower continental slope; this slope has also been extensively dredged. This study primarily examines these new data, combined with all existing seismic, well and dredge data for some 200 000 km 2 of the northwest Australian margin, to come up with an improved structural and stratigraphic interpretation, and hence a better understanding of geological history. The study area lies between the shallow North West Shelf and the Argo Abyssal Plain and is underlain by continental crust. Water depths increase slowly seaward from 500 to 3000 m and then rapidly to 5500 m at the edge of the abyssal plain.  The northern Exmouth Plateau and Rowley Terrace margin is underlain by thinned continental crust, and contains 5000-10 000 m of Permian and younger sediments. The sequences we analyse here are the top 1000 m of Upper Triassic sedimentary rocks, 1000-2000 m of Jurassic sedimentary and volcanic rocks, 500-1 000 m of Cretaceous sedimentary rocks and 500-2000 m of Cainozoic sedimentary rocks. Triassic sediments were laid down as a fluvio-deltaic blanket across this part of Gondwana, which had continental crust extending over the area presently occupied by the Argo Abyssal Plain. Coral reefs existed near the open ocean, but we have no unequivocal seismic evidence of them except on the Wombat Plateau. Toward the end of the Triassic a period of faulting on the Exmouth Plateau led to the formation of the F unconformity, but the Rowley Terrace remained undisturbed. In the Early Jurassic, a thick sequence of marine clastics with some carbonates was deposited, which, after a regression, was overlain unconformably near the sea by a thick Middle Jurassic coal measure sequence.  At the end of the Middle Jurassic, there was a period of thermal uplift, normal and strike-slip faulting, volcanism and erosion over a zone within 100- 150 km of the future abyssal plain, making the widespread E unconformity particularly angular in this area. This tectonism culminated in breakup in the Callovian-Oxfordian, and the "Argo Landmass " drifted north-westward leaving oceanic crust behind. Continental volcanics are interpreted to cover about 25 000 km2 of the northwestern Rowley Terrace below the E unconformity. An Upper Jurassic to Berriasian deltaic sequence was commonly confined to structural lows, suggesting that rapid subsidence of the margin commenced later, accompanied by a marine transgression that led to the formation of the Valanginian D unconformity. Rapid subsidence of oceanic crust led to the collapse of the continental margin along faults in a zone 30-40 km wide, with the continent-ocean boundary positioned somewhat shallower than the abyssal plain, and about 10-20 km seaward of the magnetic anomaly that previous authors used to locate it.  Maximum vertical displacement of the Mesozoic sequences across this collapsed zone is now about 2000 m.  No major tectonic events occurred on the continental margin after the Valanginian, with steady subsidence leading to a general deepening of environments.  Continental movement northward led to an increasing proportion of biogenic carbonate in the sediments. Much of the area was in bathyal depths by the Paleocene, but thereafter increased carbonate production and oceanward progradation led to a shallowing of water with time in many areas.

Tags: Earth Sciences, GA Publication, Journal, Published_External, WA, marine
Organization: Geoscience Australia Data