Geological history of the outer North West Shelf of Australia: a synthesis

This paper uses information contained in other papers in this issue of the AGSO Journal together with the scientific literature to produce a synthesis of the geological development of the outer continental margin of northwest Australia, and the adjacent plateaus and abyssal plains, with special emphasis on the northern Exmouth Plateau and Rowley Terrace. In the Palaeozoic, the outer North West Shelf formed part of the continental crust of East Gondwana. The region was stretched in the Late Palaeozoic, and subsequently subsided to form the Westralian Superbasin on the southern margin of Tethys. The superbasin filled with thick Triassic sediments, and variable thicknesses of Jurassic sediments, before progressive breakup in Callovian-Valanginian time.  The Late Triassic was dominated by fluvio-deltaic sedimentation but, on what ultimately became the outer shelf, carbonates  were deposited. These included reefal buildups on what is now the northern Exmouth Plateau and Rowley Terrace. Rift volcanics were erupted in areas of future breakup, in the latest Triassic and earliest Jurassic. The Early Jurassic saw a similar sedimentary situation to the Late Triassic, with mixed siliciclastic and carbonate deposition. Restricted circulation allowed organic-rich, potential petroleum source-rocks to form in local depressions throughout the Jurassic. In the Middle Jurassic, shallow-marine conditions predominated and siliciclastic sedimentation was ubiquitous ; coaly sequences were laid down in swamps on parts of what is now the northern Exmouth Plateau and outer Rowley Terrace.  The late Middle Jurassic saw thermal uplift and erosion prior to breakup of Gondwana in the north, and a major period of faulting and rift volcanism. Callovian breakup led to the genesis of the Argo Abyssal Plain by seafloor spreading, but there was very little sedimentation on the plain until the Early Cretaceous. Late Jurassic and earliest Cretaceous sedimentation was shallow marine and siliciclastic.  The remainder of East Gondwana broke up in the Valanginian, and the Gascoyne, Cuvier and Perth Abyssal Plains were initiated as seafloor spreading began. The present-day physiography started to take shape. Volcanic buildups of the Wallaby Plateau and Joey Rise formed in the Early Cretaceous on oceanic crust. In the Neocomian, rift-style volcanics were erupted on the Scott Plateau, 30 m.y. after breakup in that area, and volcanic ash was a major source of sediment on and near the northern Exmouth Plateau.  The Early Cretaceous was a period of transgression with siliciclastic shelf sedimentation on the subsiding continental crust gradually giving way to marls. In the Santonian, deposition of shelf carbonates became established, and thenceforth shelf and bathyal carbonate deposition characterised the margin. On the Argo Abyssal Plain, there was a steady rain of pelagic clay, and major influxes of carbonate turbidites and debris flows in the Early Cretaceous and the Mio-Pliocene.  A tilt to the south developed in the Early Miocene, presumably as the fore-bulge south of the Java Trench affected the plain as it moved northward with the Australian Plate.

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