Sedimentology, mineralogy and isotopic analysis of Pellet Lake, Coorong region, South Australia

Gravity cores of Holocene sediments from a shallow ephemeral lake in the Coorong region (Pellet Lake, southeastern coastal Australia) show a mineral assemblage and sequence particular to its hydrology. The mineralogical sequence above an initial dolomitic siliciclastic sand reflects conditions of increasing salinity in the lower portions of the core (i.e. organic-rich aragonite to magnesite + hydromagnesite + aragonite) followed by a relative decrease in salinity (i.e. magnesite + aragonite + hydrornagnesite to aragonite + hydromagnesite) in the upper portions of the core. This sequence is capped by -0.4 m of micritic dolomite and minor amounts of hydromagnesite, with the relative abundance of dolomite increasing upwards. Three stratigraphically and spatially distinct dolomite units (upper, lower and margin) are recognized using stable carbon and oxygen isotope data, unit cell calculations and MgC03 mole per cent data of the dolomite. Detailed X-ray diffraction (XRD) analyses of samples with more than 80% dolomite shows that the dolomite is ordered. Average unit cell parameters, calculated from the XRD patterns, indicate that the upper dolomite unit has crystal lattices expanded in the c, direction (c, = 16.09 A) relative to ideal dolomite (c, = 16.02 A) and contracted in the a, direction (a, = 4.796 A) relative to ideal dolomite (a, = 4.812 A). The mol fraction of MgC03 in the upper dolomite shows up to 4.0 f 2.0 mole per cent excess Mg in the dolomite crystal lattice (calculated from XRD). This unusual dolomite crystal chemistry is probably generated by rapid precipitation from solutions which have greatly elevated Mg/Ca ratios. Transmission electron microscopy reveals that the upper dolomite has a heterogeneous microstructure which also suggests rapid precipitation from solution. The modulated microstructure found in calcium-rich dolomite is completely lacking. Dolomite ordering reflections are present in electron diffraction patterns, but are weak. Stable oxygen and carbon isotope values of the upper dolomite are tightly grouped (ave. 6l80 x + 7.55%,, 6I3C = + 4.10%,), yet show three upward-lightening oxygen cycles. The oxygen cycles correlate with three upward decreases in the calculated Mg content of the dolomite zone. These cycles may indicate the increased importance of rain-water dilution of the brine at times when the water in the lake was at its shallowest levels. Analyses of the lower dolomite and the margin dolomite suggest that these units precipitated more slowly from less evaporitic brines than the upper dolomite unit. The lower dolomite is close to stoichiometric, has less evaporitic stable isotope values than the upper dolomite, and has only a slightly expanded c,-axis. The margin dolomite is Ca-rich, has a more homogeneous microstructure, and has expanded a, and c, axes. The abundance of relatively soluble Mg-bearing phases, such as hydromagnesite and magnesite, may supply additional magnesium for the dolomitization of aragonite and calcite during subsequent diagenesis and burial of the sediment. This process may leave a finely laminated dolomicrite deposit which retains little, if any, evidence of evaporite minerals.