Lake Greenly, Eyre Peninsula, South Australia: sedimentology, palaeoclimatic and palaeohydrologic cycles

Lake Greenly is a continental playa located in a mid-latitude region (34°30′S), 33 m above mean sea-level on Eyre Peninsula, South Australia. The uppermost 3 m of central Lake Greenly sediments contain two complete cycles and one half cycle of carbonate-evaporite deposition. The sequence is represented by alternating units of protodolomite and fine-grained gypsarenite capped by 30 cm thick surficial low magnesian calcite mud which grades laterally into skeletal peloidal sand and beachrock near the lake margin. voured depositional model suggests that calcite was deposited at the sediment-water interface during “freshening” episodes marked by higher lake levels and lower salinities. At the same time individual flooding events deposited skeletal peloidal sand as thin interbeds which thicken and coarsen laterally towards the lake margin. The dolomite was probably deposited during periods of low levels and high brine concentrations caused by increased aridity 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. Although gypsum precipitation did not control dolomitization, it probably assisted the process by raising the MgCa ratio and reducing the number of competing ions until formation of protodolomite by early diagenetic replacement of calcite took place.