Pauline.English@anu.edu.au

Lake Lewis is a 250 km² salt lake, or playa, in central Australia (Figure 1). The lake is fed by groundwater discharge and a centripetal array of ephemerally active creeks that rise in major mountain ranges in the north and south (Figure 2). The basin is hydrologically closed. The area today receives almost 300 mm average annual rainfall and has an average annual potential evaporation rate of 3065 mm. With an evaporation-precipitation ratio of >10, the area is classified as arid. Rainfall is derived from the Australian summer monsoon system and tropical cyclones that originate in coastal regions to the north.

Figure 2

The landscape of Lake Lewis basin is composed of steep mountains of granite, gneiss and quartzite; widespread alluvial fans; a lacustrine plain; dunefields and playas. Up to 80 m of Cainozoic lacustrine sediment, Anmatyerre Clay, underlies Lake Lewis playa, infilling a pre-existing mountainous topography of crystalline basement rocks. The detrital clastics of Anmatyerre Clay reflect the composition of the granites and amphibolites that dominate the catchment lithologies. Imprints of expanded perennial lacustrine conditions of the Pleistocene are expressed in the geomorphology and stratigraphy of the basin. In particular, the landscape bears palimpsest evidence of a high lake stand at the 560 m contour; this palaeolake represents a water body several metres deep and 1375 km² in area, i.e. 5.5 times the size of the present playa. The continuity of Anmatyerre Clay suggests perennial lacustrine conditions through the Middle Pleistocene, culminating with the 560 m megalake phase and contractional stages thereof. The "Anmatyerre lakebed" subsequently dried up and was scoured before a new, distinctive regime overtook basinal processes after the last interglacial. The gypseous Tilmouth Beds, along with gypsum-rich aeolian deposits and relatively recent episodic flood alluvium (Figure 3), represent approximately the last 100 000 years of sedimentation at Lake Lewis.

The evolution of Lake Lewis from a large perennial lake to a shrunken dry playa has been accompanied by major changes in the groundwater system of the basin. With reduced rainfall and associated diminished runoff and attenuated recharge since the last interglacial, groundwater flow in Lake Lewis basin became sluggish and residence times in aquifers became more prolonged. Acquisition of solutes by stagnant or slow-moving groundwaters, coupled with increased evaporation rates, resulted in the waters becoming progressively more solute-rich in the closed hydrologic system. A >5 km wide halo of phreatic calcrete (CaCO₃) precipitated around the contracting lake, nucleating on surface and near-surface sediments in the lakebed of the former 560 m megalake. With continued and intensified aridity, the waters evolved to gypsum (CaSO₄) saturation, in response to evaporative concentration. The Tilmouth Beds of intercalated grey-olive clay and sedentary gypsum are a legacy of perhaps the earliest manifestation of high levels of salinity in the basin.

The evolution of contemporary groundwaters in shallow aquifers of Lake Lewis basin follows the calcium carbonate to calcium sulphate path that has prevailed since the last interglacial. The groundwater flow rate is of the order of a few thousand years over the 60 km distance from the ranges to the playa. In response to intense evaporative concentration, fresh waters sourced from the mountains and floodplains evolve to CaCO₃ saturation levels as they approach the playa. With depletion of carbonate-bicarbonate, the remaining Ca is coupled with sulphate to precipitate gypsum at the discharge zone. Displacive (intrasedimentary) gypsum precipitates from interstitial brine in both the Anmatyerre Clay and the overlying flood alluvium. Because the playa surface is within the capillary fringe of the high water table, efflorescent crusts of gypsum and minor halite (NaCl) precipitate as the brine evaporates. Calcium availability seems to be the limiting factor to the volume of gypsum being precipitated. The salinity gradient over a few kilometres, from CaCO₃ saturation to residual brine that is enriched in Na-Cl-SO₄, is steep, because of the high intensity of evaporation at the playa.

Additional to accumulation of carbonates, sulphates and chlorides in the closed hydrologic system, silica concentrations also increase along the groundwater flow path. Aqueous silica (H₄SiO₄), derived from weathering of feldspar, mica and kaolinite in bedrock and alluvium, are at supersaturation levels with respect to quartz in groundwaters approaching the centre of the basin. Opaline and chalcedonic silica and cryptocrystalline quartz precipitate in calcrete in the playa margins, largely in response to intense evaporation. Consequent depletion of SiO₂ lakeward from the playa margin, in the brine pool beneath the playa, results in aqueous compositions that are undersaturated with respect to quartz. This depletion of available SiO₂ in the brine pool, coupled with decreased H⁺ activity due to consumption of H⁺ during weathering of aluminosilicate clays, and increased aqueous Na⁺ due to evaporative concentration of this very soluble ion, result in the SiO₂-deficient zeolite mineral, analcime, attaining equilibrium in the system. Analcime, Na(AlSi₂)O₆·H₂O, precipitates below the water table in Anmatyerre Clay as an authigenic mineral, crystallising from amorphous aluminosilicate clays or gels whilst accessing Na⁺ ions and water molecules from interstitial brine. This is a distinctive Late Pleistocene diagenetic development that is an outcome of high levels of salinity at the playa, coupled with chemical compositions of lacustrine clays and brine that are optimum for zeolite formation. The abundance of authigenic analcime at Lake Lewis is likely to increase with the duration of hydrologic closure of the basin under prevailing arid climatic conditions.