Mesozoic sedimentary and volcaniclastic rocks dredged from the northern Exmouth Plateau and Rowley Terrace, offshore northwest Australia

An extensive program of dredging was undertaken by AGSO in 1990 on the northern Exmouth Plateau and Rowley Terrace margin, mainly to provide geological control for seismic interpretations. Forty-two major Mesozoic lithofacies types were recovered. The rocks indicate that: 1. During the Late Triassic (Norian-Rhaetian), the Rowley Terrace margin was, like the adjacent northern Exmouth Plateau, host to shelf carbonates, including reef and peri-reefal deposits. Volcanics were emplaced along the margin (particularly in the north) during the Late Triassic-Mid Jurassic, probably as a result of the commencement of rifting between Australia and Greater India. The volcanics were laid down partly or reworked into shallow water , producing hyaloclastites and tuffaceous mudstones with marine microfossils. By the Early Jurassic, carbonate deposition was restricted to relatively small areas with most of the outer Rowley Terrace being covered by thick, fluvial - paralic siliciclastics with marine interbeds. These deposits continued to be deposited until the early Late Jurassic. They were subjected in the ?Late Jurassic to subaerial exposure under arid conditions resulting in cementation by Fe-oxides. With the early Late Jurassic (Oxfordian- Callovian) breakup and formation of the Argo Abyssal Plain, the margin started to subside and thin, shallow-marine, Late Jurassic-Early Cretaceous sediments were deposited. These were followed, with continuing margin subsidence, by progressively deeper-water deposits, including Cretaceous and younger hemipclagic-eupelagic claystones, chalks, and marls. 2. Reefs grew in the area of the outer part of the Rowley Terrace during the Late Triassic. The potential therefore exists for Upper Triassic, reefal petroleum reservoirs to be present within the main part of the Rowley Sub-basin to the east, if reefs in that area were not prevented from growing by sand influx from rivers. Similarly, Lower Jurassic shallow-water platform carbonates, which are present on Triassic horst blocks beneath the central northern part of the Exmouth Plateau, could act as petroleum reservoirs if porosity is preserved. 3. During the Late Triassic and Early Jurassic there is a clear similarity between the facies and associated foraminiferal and ostracod microfaunas of the northern Exmouth Plateau/ Rowley Terrace area and thos e of other southern Tethyan margins, including the Northern Calcareous Alps. This indicates that broadly similar depositional conditions existed along much of Neo-Tethys at that time.