HOLOCENE CARBONATE SEDIMENTATION ON THE WEST EUCLA SHELF, GREAT-AUSTRALIAN-BIGHT - A SHAVED SHELF

The southern continental margin of Australia is a cool-water carbonate sedimentary province located in a high-energy, swell-dominated oceano graphic setting. A vibrocore transect of ''C-dated sediments across th e centre of the Eucla Shelf is the first record of Holocene shelf depo sition in the Great Australian Bight. Much of the seafloor shallower t han 70 m water depth, the base of wave abrasion, is bare Cenozoic lime stone, in some places encrusted by (?) Late Pleistocene, coral-rich, l imestone that is cemented by high-magnesium calcite (12 mole% MgCO3). The areally extensive, 100 km-wide, hard, bored substrate supports an epibiota of coralline algae, minor bryozoans and soft algae or is cove red by patches of Holocene sediment up to 1.5 m thick; generally a bas al bivalve lag (< 3 ka) overlain by quartzose-bioclastic palimpsest sa nd. This pattern of active carbonate production but little accretion o n the wave-swept mid- to inner-shelf is similar to that on other parts of the southern Australian continental margin. The term shaved shelf is proposed for this style of carbonate platform, formed by alternatin g periods of sediment accretion, cementation and erosion. The palimpse st sand is typically rich in bivalves, coralline algae and locally, de trital dolomite. Outer shelf Holocene sediment, below the base of wave abrasion but inboard of the shelf edge, is a metre-thick unit of fine , microbioclastic muddy sand with minor delicate bryozoans overlying a 9-13 ka rhodolith gravel. Some of this outer shelf sediment appears t o have been resedimented. The shelf edge is a sandy and rocky seafloor with active bryozoan growth and sediment production. The Holocene sed iments are enriched in coralline algal particles and conspicuous large foraminifers (cf. Marginopora) and depleted in bryozoans, as compared to coeval deposits on the Lacepede and Otway shelves off southeastern Australia. These differences are interpreted to reflect warmer waters of the Leeuwin Current and prevalent downwelling in this area as oppo sed to the general upwelling and colder waters in the east.