--> Abstract: Modern Cool-Water Siliciclastic/Carbonate Sediments, Lacepede Shelf, South Australia, by Y. Bone, N. P. James, C. C. Von Der Borch, and V. Gostin; #91004 (1991)

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Modern Cool-Water Siliciclastic/Carbonate Sediments, Lacepede Shelf, South Australia

BONE, YVONNE, University of Adelaide, Adelaide, South Australia, NOEL P. JAMES, Queen's University, Kingston, Ontario, Canada, C. C. VON DER BORCH, Flinders University, Bedford Park, South Australia, and VICTOR GOSTIN, University of Adelaide, Adelaide, South Australia

The Lacepede Shelf is a 130 km X 100 km open embayment along the southern passive continental margin of Australia. The shelf includes the mouth of the River Murray--Australia's largest drainage system--the extensive arcuate Coorong strand, the 50-70 m deep and flat plateau of the shelf proper, the nonrimmed shelf break, and the upper slope to depths of 200 m. The shelf bathymetry is locally interrupted by seafloor highs, reflecting the underlying rugged terrain of deformed Precambrian and early Paleozoic bedrock in the west and Tertiary limestones and Quaternary dunes in the east.

The late Pleistocene/Holocene sediment blanket is formed by discrete sedimentary facies. Quartz sands cover a significant cross-shelf zone opposite the river mouth, with current generated offsets. High-resolution seismic profiles reveal buried lowstand channels. The mid-shelf is an area of conspicuously coarse-grained, mud-free loose sediments composed of variable amounts of bryozoans and bivalves. The shelf break and upper slope bryozoan sands are similar, apart from species differences, from 40 to 100 m, with an increase in mud below 100 m. The seafloor highs are sites of prolific bryozoan, calcareous algae, sponge, and bivalve growth, the skeletons of which are shed onto the shelf. Bryozoan distribution is moderated by water depth and substrate type. Most forms are low-Mg calcite t high-Mg calcite, but two major groups are aragonitic. Distribution of these different mineralogical types is important for later diagenesis.

Both terrigenous clastic and carbonate sediments are a mixture of relict and modern components, depending upon location, and reflect Holocene glacio-eustatic sea-level changes.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)