Mesozoic sedimentary and volcanic rocks dredged from the northern Exmouth Plateau: petrography and microfacies

The deeply incised northern margin of the Exmouth Plateau has been dredged extensively along seismic reflection profiles, in water 2000- 5600 m deep, by R. V. Sonne (Cruise SO-8) and R. V. Rig Seismic (BMR Cruise 56). Geological samples obtained have greatly increased our understanding of the Late Triassic- Recent history of the margin. Detailed petrography and microfacies analysis have enabled us to define seven major lithofacies associations. Three Late Triassic to Middle Jurassic associations were laid down roughly coevally on this southeastern margin of Tethys: (1) a Late Triassic- early Liassic volcanic and volcaniclastic association of early rift volcanics, (2) a Late Triassic- Middle Jurassic shallow water carbonate association, and (3) a ?Late Triassic- Middle Jurassic coal measure association. The coal measures were uplifted and weathered to form a ?Jurassic ferruginous sediment and ironstone association. We distinguish 14 Late Triassic- Callovian microfacies types of shallow water carbonates, which can be correlated with the facies of coeval platform carbonates in the Alps and Mediterranean area of the Tethys ocean. During Late Triassic times intertidal to shallow-subtidal carbonates were deposited in the Swan Canyon area close to the palaeo-coastline in the east, and deeper subtidal and shelf lithologies in the Wombat Plateau area in the west. During the latest Triassic and earliest Jurassic, the carbonate platform subsided and was structured into shoals with red biomicrites, and basinal areas with hemipelagic autochthonous micrites and redeposited calcarenitic turbidites. Locally, uplifted blocks, such as the Wombat Plateau horst, were subaerially eroded during Jurassic or earliest Cretaceous times. Carbonate platform deposition continued in places until Middle Jurassic time. Following breakup to form the Argo Abyssal Plain in the earliest Cretaceous, the margin started to subside and a Lower Cretaceous marginal-marine claystone association was deposited, followed by a hemipelagic late Lower Cretaceous radiolarian claystone. As subsidence continued, from Turonian times onwards, there was increasingly pelagic deposition of a Late Cretaceous to Cainozoic association of hemipelagic to eupelagic variably silicified marls and chalks. Complex diagenetic transformations involve silica, silicates, carbonates, and phosphates.