LAKE LEFROY, A PALAEODRAINAGE PLAYA IN WESTERN-AUSTRALIA

Lake Lefroy is a medium-sized playa lake within the Lefroy Palaeodrainage on the Yilgarn Craton of Western Australia. The climate is semi-arid with summer and winter rainfall maxima. Evaporation exceeds precipitation by a factor of 10. Prevailing strong winds are westerly in winter, and northerly in summer. Lake shorelines are erosional along northern and western shores and depositional along eastern and southern shores. Islands consist of gypsum dunes, siliciclastic dunes and bedrock. The evaporitic succession is up to 9 m thick. Deposition has occurred episodically since the earliest Pliocene. Wind and wave-driven currents rework sediments that are then deflated by the wind to be redeposited in marginal dunes. The gypsum content of the sediment increases away from the shore and much of the lake is underlain by crystalline gypsum within a siliciclastic to slightly calcareous matrix. Surface water in Lake Lefroy is supplied largely by surface runoff, yet there is also a limited contribution from near-surface aquifers. The surface brine pool is partly isolated from the main palaeodrainage aquifers and regional basement groundwater by essentially impermeable sediments of probable Miocene age. Lake brines are of sodium chloride type and salinities are as high as 462 000 ppm total dissolved solids. The pH ranges from neutral to weakly acid. The probable origin of the solutes is marine salt derived from rainfall.