Abstract:
During Ocean Drilling Program (ODP) legs 122 and 123, six sites were continuously cored on the Exmouth and Wombat plateaus, and two sites on or adjacent to the abyssal plains nearby. Altogether, 3372 m of section were cored at the Exmouth-Wombat sites in the Upper Triassic, Cretaceous and Cainozoic sequences. At an Argo Abyssal Plain site, 935 m of uppermost Jurassic, Cretaceous and Cainozoic sediments, and 280 m of oceanic crust, were cored through. At a site between the Exmouth Plateau and the Gascoyne Abyssal Plain, 458 m of Cretaceous and Cainozoic sediments and about 70 m of diabase sills were cored through. The Upper Triassic sequence cored on the Wombat Plateau consists of about 600 m of marine Carnian and Norian siliciclastic, northward-prograding, fluvio-deltaic sediments (Mungaroo Formation), and 300 m of Rhaetian reefal and lagoonal carbonates. The porous reefal rocks were the first of this age discovered in Australia, but similar rocks have since been dredged from the outer margin of the Canning Basin. A period of associated rift volcanism has been proven on the margins of the Exmouth Plateau. No Jurassic sediments were cored by ODP, apart from about 10 metres of Tithonian-Kimmeridgian claystone at the base of the sedimentary section at Site 765 on the Argo Abyssal Plain. However, Lower Jurassic continental and shallow marine siliciclastic rocks and shelf carbonates are widely known from wells and dredge hauls on and near the Exmouth Plateau; similar Middle Jurassic siliciclastic rocks and coal measures overlie the Lower Jurassic. A major tectonic episode, with thermal uplift, faulting and erosion (and volcanism in the outer Canning Basin) preceded the Callovian-Oxfordian breakup that led to formation of the Argo Abyssal Plain. This early phase of Gondwanan breakup is precisely dated as 155 Ma by K/Ar dating of oceanic crust at Site 765. Much of the area remained high and was subject to little or no sedimentation in the Late Jurassic. However, the thick Tithonian-Berriasian Barrow Group delta prograded rapidly northward from a southern area that was thermally uplifted just before the Valanginian breakup. That breakup led to the formation of the Gascoyne and Cuvier Abyssal plains, and defined the Exmouth Plateau as a recognisable entity. About 850 m of Cretaceous sediments were cored at the two central Exmouth Plateau sites. Berriasian deltaic mudstone (Barrow Group) underlies the breakup unconformity. Hauterivian to lower Aptian marine mudstone (Muderong Shale) was deposited during a marine transgression. A late Aptian hiatus was followed by deposition of Albian mudstone and marl, upper Albian to Coniacian marl, and younger Cretaceous and Cainozoic chalks.