Abstract

Seismic images of the continental margin in the western Great Australian Bight reveal the internal anatomy of a long-lived Cenozoic cool-water carbonate shelf. The Cenozoic succession is divisible into seven seismic sequences that reflect four depositional phases. Phase A (Paleocene-middle Eocene) is a progradational siliciclastic wedge deposited in a depositional sag, and represents lowstand and transgressive sedimentation. Phase B (middle Eocene-earliest middle Miocene) contains cool-water ramp carbonates with biogenic mounds (Eocene-Oligocene), overlain by a warm-water, flat-topped platform rimmed by the early middle Miocene(?) Little Barrier Reef. Coeval deep-water carbonate deposition formed a multi-lobed sediment apron. This phase was terminated by gentle uplift and tilting throughout southern Australia in the late middle Miocene. Phase C (late Miocene-early Pliocene) represents cool-water lowstand wedge and ramp deposition, and contains numerous biogenic mounds in the youngest sequence. This phase is terminated by an unconformity attributed to marine erosion. Phase D (Pliocene-Quaternary) is a thick succession of cool-water carbonates with spectacular clinoform ramp geometry that forms most of the modern outer shelf, and contains large deep-water biogenic mounds. This platform, the first large cool-water carbonate shelf imaged by high-quality seismic reflection data, demonstrates the interaction between regional tectonic and local and global paleo-oceanographic processes. As Australia drifted northward during the Cenozoic, the Great Australian Bight moved from high to middle latitudes, and the regional oceanographic regime remained cool water largely because of coeval development of the Antarctic Circumpolar Current in the evolving Southern Ocean. Short episodes of warm-water deposition probably reflect incursions of a proto-Leeuwin current from the Indian Ocean, whereas growth of the Miocene coral-algal "Little Barrier Reef" resulted from a short-term global increase in sea-surface temperatures. This platform, dominated by stacked carbonate ramps, is most similar to the West Florida Shelf, but contains many more biogenic mounds. The western Great Australian Bight carbonate platform is an excellent modern analog for the mesoscale structure of cool-water platforms in the older geologic record.

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