ABSTRACT

Extensive supratidal flats composed dominantly of non-calcareous muds occur on the landward side of thick mangrove swamps around the margins of Broad Sound, central Queensland coast. During inundation by spring or storm tides, abundant algal growth takes place on the mudflats but for the rest of the time they are sub-aerially exposed and subject to desiccation. As a result, a thin veneer of evaporites covers the surface of the flats and large gypsum crystals form just below the surface. Interstitial pore waters are hypersaline, with salinities in excess of 120 ppt being recorded. The Mg/Ca and K/Ca ratios become particularly high in these brines, attaining values of up to 9.9 and 2.2 respectively, primarily as a result of the removal of Ca from solution, in the form of gypsum. The sup atidal muds are generally low in calcium carbonate but dolomitic concretions do occur in the Charon Point area. These concretions are composed of low magnesium calcites, and calcian dolomites with a compositional range of Ca_0.59 Mg_0.41 CO₃ to Ca_0.65 Mg_0.35 CO₃. Electron microprobe, X-ray diffraction and whole rock analyses all suggest that the dolomites have formed by dolomitization of calcite. The dolomitization is associated with ferruginization, and possibly also phosphatization. Radiocarbon age determinations on associated wood and shell material and on the concretions, indicate that the dolomitization has occurred within the past 3,000 years.