THE STRUCTURE AND CARNARVON TERRACE STRATIGRAPHY OF THE AND WALLABY PLATEAU

Abstract

The Carnarvon Terrace, Wallaby Plateau and adjacent slopes cover about 350 000 km2 beyond the continental shelf of central Western Australia. Water depths within the region range from about 600 m on the Carnarvon Terrace to 5000 m over the Cuvier Abyssal Plain.

The dominant structural grain of the Carnarvon Terrace varies from north-northeast in the north to north-northwest in the south. A Palaeozoic structural high (Gascoyne Sub-basin) extends across the central Carnarvon Terrace. This feature is controlled by a Precambrian basement high. which is onlapped by sedimentary rocks of Ordovician to Carboniferous age. The high is bounded in the north by a major northeast trending fault, which may be an extension of the Rough Range Fault. Troughs trending parallel to the fault are part of the southern Exmouth Sub-basin; the largest trough contains up to 2000 m of possible Triassic and Jurassic sediments. A north-northwest trending half-graben 600 km long and up to 100 km wide, bounds the Palaeozoic high to the south. Normal faults, downthrown to the west, are common within this half-graben, which appears to connect with the Abrolhos Sub-basin and thus probably contains Permian, Triassic and Jurassic section.

Northwest and possibly north-northeast trends occur on the Wallaby Plateau. Much of the Plateau consists of folded (?)Palaeozoic rocks and possibly shallow Proterozoic basement, with igneous bodies abundant on its western and southern margins.

Rifting of the margin probably commenced in the Permian and culminated in a final phase from the Late Jurassic to Neocomian. This was followed by a period of intense erosion and peneplanation during the Neocomian. Formation of oceanic crust which now underlies the Cuvier and Perth Abyssal Plains, began in the Neocomian and was accompanied by a marine transgression which resulted in deposition of Aptian to Cenomanian shallow-marine clastic sediments over both the Carnarvon Terrace and Cuvier Abyssal Plain. After a hiatus, deposition of a post-Senonian shelf carbonate sequence commenced and has continued until the present day with only minor interruptions.

The existence of a large trough of Mesozoic sediments that is probably a northwestern extension of the Abrolhos Sub-basin considerably upgrades the hydrocarbon potential of the southern Carnarvon Terrace. Traps along the possible offshore extension of the Rough Range Fault and Devonian reef complexes within the Palaeozoic high may also be prospective for hydrocarbons.