Geology of the Exmouth and Wallaby Plateaus off northwest Australia: sampling of seismic sequences

In this paper we present the first detailed geological information about the margins of the Exmouth and Wallaby Plateaus. New single-channel seismic profiles helped to select sampling targets, which were related to the established seismic stratigraphy. We obtained pre-Quaternary rocks, mainly by dredging, from 29 stations on the Exmouth Plateau, and from 12 stations on the Wallaby Plateau and the Cuvier Abyssal Plain. The results have led to major changes in our understanding of the northern margin of the Exmouth Plateau, which behaved more as part of the offshore Canning Basin than of the Carnarvon Basin. Middle and Late Triassic paralic, detrital sediments are generally overlain by an average of 2500 m of Early Jurassic shallow-water carbonates and mid-Jurassic coal measures and ferruginous sediments. The coal-measure sequence (lithofacies association A) includes carbonaceous silty claystones with seams of an immature sub-bituminous vitreous coal and coaly mudstone, carbonaceous quartz  siltstones and very fine sandstones, medium to coarse-grained quartz arenites, and pyrite concretions. The barren ferruginous sediments (B) consist of brown clayey ironstones, sandy ironstones, ferruginous concretions, and ironstones breccias. The Liassic transgressive shallow-water carbonates (C) are a heterogenous group of micritic limestones, biocalcarenites, very coarse crinoidal biosparites, calcareous quartz arenites, recrystallised sparry limestones and dolosparites. On the northern margin of the, Wombat Plateau (a sub-plateau of the Exmouth), at least 300 m of Late Triassic to earliest Jurassic flows of early-rift alkali rhyolite and undersaturated trachyte s.l. (213-192 m.y.) underlie the early Jurassic carbonate sequence.  Above the main unconformity, which corresponds to the Callovian breakup of this margin, Early Cretaceous shallow-marine claystones (facies D) indicate the formation of a juvenile ocean. The mature ocean stage is indicated by a condensed sequence of pelagic foraminiferal nanno chalks of Aptian, Albian and Tertiary age (facies E). On the northwestern Exmouth Plateau we sampled Aptian and Miocene chalks. On the southern margin Mesozoic sandstone and shale, latest Jurassic or earliest Cretaceous marine shale, and Cainozoic chalks were recovered. On the eastern and southern Wallaby Plateau, and on the Sonne Ridge which extends  northward into the Cuvier Abyssal Plain, the layered sequence beneath the main Neocomian unconformity consists of interbedded weathered tholeiitic and differentiated alkali basalts, tuffs, basalt breccias and thick volcaniclastic sandstones and conglomerates. A minimum mid-Cretaceous age (K/ Ar age: 89 m.y.) was determined for a somewhat altered basalt from the southern Wallaby Plateau. This suggests that intense volcanism and associated deposition of volcaniclastic debris flows formed the plateau, during or after the Neocomian breakup of this region. Therefore, the plateau appears to have no petroleum potential. Quaternary cores in the central Exmouth Plateau contain methane in very small amounts; the u13C isotope results (-14 to -40%) and the absence of higher hydrocarbons tend to downgrade the petroleum potential of this part of the plateau. Manganese nodules from the southern and eastern margins of the Wallaby Plateau have combined Cu + Ni + Co values of about 0.76%, and are of no commercial interest.

Tags: Earth Sciences, GA Publication, Journal, Published_External, WA, marine
Organization: Geoscience Australia Data