SURFICIAL COOL-WATER CARBONATE SEDIMENTS ON THE OTWAY CONTINENTAL-MARGIN, SOUTHEASTERN AUSTRALIA

The Otway continental margin of southern Australia is a swell-dominated, open, cool-water carbonate platform. It is partitioned into a series of depth-related zones of biological sediment production and accumulation by three hydrodynamic interfaces: (1) abrasion depth (-70 m), (2) swell wavebase (-130 m), and (3) storm wavebase (> -250 m). The Otway shelf profile drops steeply from the strandline. The shallow shelf lies largely between -30 and -70 m, and is a high energy zone of carbonate sediment production, intensive destructive diagenesis and erosion. The seafloor is characterized by widespread exhumed limestone substrates which host dense encrusting assemblages of molluscs, sponges, bryozoans and red algae. Loose sediment accumulation is patchy, and consists of a thin, quartz-rich, palimpsest gravel veneer. The middle shelf (-70 to -130 m) is a zone of swell wave shoaling, and production and accumulation of coarse mega-rippled (2-D dunes) bryozoan sands. Deep shelf areas (-130 to -180 m) accumulate finer bioclastic sands (foraminiferal/echinoid/bryozoan), that are intensely bioturbated and episodically reworked by storms. Grains are variably fragmented and marginally bored. At the shelf edge and top of slope (-180 to -350 m) nutrient-rich upwelling currents support laterally extensive aphotic biostromal/hermal bryozoan/sponge/coral communities which generate and accumulate thick successions of muddy skeletal sand. Sediments of the upper slope (-350 to -500 m) are a bioturbated mixture of periplatform bioclastic debris and pelleted foraminiferal/nanno-fossil mud. Turbidites and resedimentation features are common. Bioturbation and shelf-derived skeletal content decrease progressively downslope and pelagic muds dominate below -500 m. Lower slope areas are cross-cut by gullies and have low accumulation rates. The base of slope is characterized by sediment apron deposits consisting of interbedded shelf-derived, coarse-grained turbidites, and pelagic ooze.