The geological development of the passive margins of the Exmouth Plateau off northwest Australia

Abstract

The Exmouth Plateau is a very large sunken continental block off Australia's Northwest Shelf, whose culmination lies 800 m below sea level. It is elongated in a northeasterly direction, and bounded by abyssal plains on three sides. Its broad tectonic framework is controlled by Mesozoic rifting and transform faulting, which accompanied the breakup of Australia and “Greater India”, after which the plateau sank to bathyal depths. The pre-breakup sequence consists of more than 5000 m of Palaeozoic and early Mesozoic shallow marine to terrestrial detrital sediments. The post-breakup sequence consists of about 2000 m of late Mesozoic and Cainozoic marine sediments, mainly carbonates. The northern margin came into existence in the Callovian (155 m.y. ago), when a continental fragment separated from it and moved off to the northwest. An early phase of rifting gave rise to Triassic—Jurassic intermediate and acid volcanics (213-192 m.y. ago), which overlie a thick Triassic paralic sequence. Steady subsidence north of an east—west hinge line allowed several thousand metres of early and middle Jurassic carbonates and coal measures to accumulate before breakup. Breakup occurred along a series of rifted and sheared segments, and the margin is further complicated by northeast-trending Callovian horsts and grabens. The horsts were planed off in late Jurassic and early Cretaceous times, and the whole margin was covered by a few hundred metres of late Cretaceous and Cainozoic pelagic carbonate as it sank steadily to its present average depth of 2000–2500 m. The northeast-trending western margin formed by rifting in the Neocomian (120–125 m.y. ago), as “Greater India” moved off to the northwest. Callovian normal faults parallel the margin. A thick Triassic paralic sequence is unconformably overlain by thin late Jurassic and younger marine sequences, indicating that the area was high in the early and middle Jurassic. Thin late Cretaceous and Cainozoic pelagic carbonates cover the margin, which now lies more than 2000 m below sea level. The northwest-trending southern margin formed by shearing in the Neocomian, at the same time as the western margin. It is cut by northeast-trending normal faults, which formed in the late Triassic and Callovian, and is paralleled by Neocomian and later normal faults. A thick Triassic paralic sequence is unconformably overlain by a thick late Jurassic and Neocomian delta, suggesting that the area was high in the early and middle Jurassic, but a depocentre before and afterwards. There was thermal uplift of more than 1000 m during the Neocomian period of shearing, and igneous intrusions buttressed the margin. Later normal faulting lowered the outermost margin, and turned the uplift into a marginal anticline trending northwest. The anticline had sunk beneath the sea by late in the Cretaceous, and thereafter this margin was covered by a thin sequence of pelagic carbonate, which now lies beneath more than 1500 m of water.