LAKE GREENLY, EYRE PENINSULA, SOUTH AUSTRALIA - SEDIMENTOLOGY, PALEOCLIMATIC AND PALAEOHYDROLOGIC CYCLES

Lake Greenly is a continental playa located in a mid-latitude region ( 34 degrees 30'S), 33 m above mean sea-level on Lyre Peninsula, South A ustralia. The uppermost 3 m of central Lake Greenly sediments contain two complete cycles and one half cycle of carbonate-evaporite depositi on. The sequence is represented by alternating units of protodolomite and fine-grained gypsarenite capped by 30 cm thick surficial low magne sian calcite mud which grades laterally into skeletal peloidal sand an d beachrock near the lake margin. The favoured depositional model sugg ests that calcite was deposited at the sediment-water interface during ''freshening'' episodes marked by higher lake levels and lower salini ties. At the same time individual flooding events deposited skeletal p eloidal sand as thin interbeds which thicken and coarsen laterally tow ards the lake margin. The dolomite was probably deposited during perio ds of low levels and high brine concentrations caused by increased ari dity and evaporation and represents a ''dolomitization front''. During this stage water was drawn up by evaporative pumping with subsequent precipitation of gypsum and halite at the sediment-air interface. Alth ough gypsum precipitation did not control dolomitization, it probably assisted the process by raising the Mg/Ca ratio and reducing the numbe r of competing ions until formation of protodolomite by early diagenet ic replacement of calcite took place.