CARBONATE HARDGROUNDS IN MAARS OF WESTERN VICTORIA, AUSTRALIA: A GLIMPSE AT MODERN LACUSTRINE DOLOMITE FORMATION

The Western Victorian Plains physiographic province of southern Australia contains the greatest concentration and diversity of salt lakes in the entire Southern Hemisphere. The paleolimnology of many of these basins has received considerable attention because of their excellent high-resolution stratigraphic records, which provide important clues about the past climate and hydrology of Australia. In contrast, the modern sediments in these lakes are less well-studied despite the occurrence of a great spectrum of unusual carbonate and evaporite-precipitating environments.

Lakes Gnotuk, Bullenmerri and Keilambete occupy small, deep craters in the central part of the Volcanic Plains. The lakes are topographically closed, and because of their small drainage basins and limited groundwater component, can be viewed as large ‘rain-gauge’ basins. Modern water levels and brine salinities fluctuate dramatically and even greater fluctuations have been deduced from their offshore stratigraphies.

The modern shoreline and nearshore sediments in these basins are dominated by well-indurated carbonate hardgrounds. These dolostones and limestones show a great variety of textures, fabrics and compositions. Morphologically, they range from flat, featureless and laminated wackestone pavements with variable polygonal fragmentation, to algal boundstones and microbialites having relief of more than a metre. The rocks, ranging from modern to ~4000 yr BP, are composed of variable amounts of siliciclastic and carbonate-clastic debris cemented by dolomite, aragonite, magnesite, and, more rarely, calcite, monohydrocalcite, hydromagnesite and kutnahorite.

The complex climate-driven hydrological changes, coupled with multiple exposure and associated diagenetic effects have created an exceedingly complex petrologic record in these hardgrounds. Although their genesis and diagenesis are complex, there is abundant evidence to support mineral formation by both primary inorganic precipitation and by biologically-induced cementation. Similarly, both transformation (chemical and structural alteration of pre-existing minerals) and neoformation (precipitation directly from pore or lake fluids) processes have been important in creating and diagenetically altering these shoreline carbonates.