GENESIS AND PALEOENVIRONMENTAL CONSIDERATIONS OF SHORELINE MICROBIALITES, HARDGROUNDS AND OFFSHORE CARBONATE SEDIMENTS IN TWO ADJACENT MARINE-MARGINAL SALINE LAKES IN SOUTH AUSTRALIA

The Eyre Peninsula of South Australia is a large 80,000 km2, sub-humid to semi-arid region of gently rolling terrain in the south-central part of Australia. The poorly integrated drainage of the area, high E/P ratios, and presence of numerous marine-marginal lagoons and cutoff embayments have given rise to many saline and hypersaline lakes. Pillie Lake and Sleaford Mere occupy small adjacent lacustrine basins at the southernmost tip of the Peninsula. Both lakes are topographically closed. However, because of its larger catchment and influx of meteoric-derived groundwater, Sleaford is a permanent lake, whereas Pillie exhibits playa characteristics.

Both lakes contain abundant shoreline and nearshore carbonate hardgrounds and well-indurated carbonate sediments. These nearshore carbonates in Sleaford, dominantly in the form of lithified algal boundstones and microbialites having relief of more than a metre, comprise mainly aragonite. Pillie Lake carbonates are morphologically more complex, with flat, featureless wackestone pavements having variable polygonal fragmentation, broad biohermal structures, and unlithified mud. Mineralogically, the Pillie carbonates also show a greater diversity than Sleaford, but are mainly a mixture of Ca-rich protodolomite and aragonite.

Preliminary 14C dating and stable isotopic examination of the rocks and carbonate muds in these two lakes indicate that the thrombolites in Sleaford are modern, whereas deposition in the Pillie basin ceased about 2000 yr BP. Carbonate sedimentation in both basins occurred under strongly evaporitic conditions. The composition of the modern and sub-modern carbonates in the two basins is reflective of hydrological differences: throughout the Holocene until about 2k BP, the basins were dominated by marine groundwater influx, which provided an adequate source of Mg+2 to drive the dolomite-precipitating mechanisms. However, during the last several millennia, the lakes have been cutoff from nearly all marine influence, resulting in lower Mg/Ca ratios in Sleaford (i.e., aragonite precipitation) and non-deposition in the smaller Pillie basin.