The Lioness, Bonaparte's Tongue and Cool-water Carbonates on the South Australian Continental Margin

Noel P. James, Yvonne Bone, Steven J. Hageman, Victor A. Gostin, and David. A Feary

The 120-170km-wide, 50-150m-deep Lincoln Shelf, in the center of the continental margin, is wave-dominated, high-energy and cool-water. A m-scale veneer of late Quaternary carbonate sediment is biogenic and composed of bryozoans, molluscs, foraminifers and coralline algae. Sediment patterns illustrate the importance of large-scale oceanic current circulation and movement of locally derived water masses in controlling carbonate sediment facies on otherwise open continental margins.

Cool (<17°C), open marine shelf waters are seasonally modified by 1) west-to-east, warm-water flow of the Leeuwin (Lioness) Current, making the region marginally subtropical, and 2) north-to-south flow of cold, saline bottom water (Bonaparte's Tongue) out of Spencer Gulf across the shelf. Leeuwin Current water leads to increased oligotrophy, occasional large foraminifers and abundant corallines in an otherwise bryozoan-mollusc assemblage. The saline Bonaparte's Tongue promotes downwelling, increased mesotrophy, and sediments dominated by benthic foraminfers and molluscs. A shelf-edge, robust bryozoan biostrome thrived during the last eustatic lowstand and in the initial stages of the following transgression when both the Leeuwin Current and Bonaparte Tongue were absent and mes trophic conditions prevailed. It was terminated and left stranded as the Leeuwin Current developed and Spencer Gulf was flooded about 10ka..

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California