Cainozoic evolution and hydrogeology of Lake Lewis Basin, Central Australia

Abstract

Research in Lake Lewis basin in central Australia has investigated the geology, geomorphology and hydro geology of this intermontane Cainozoic basin that overlies the Proterozoic Arunta Craton. The evolution of the area from before Tertiary basin inception to the present-day has been examined. Observations of the structural architecture of the basin have been integrated with models for intracratonic basin evolution, involving reactivation of ancient basement faults. Prior to basin inception, the Palaeozoic and Mesozoic landscape was a highly irregular topography of steep bedrock ranges, valleys and inselbergs. Analysis of geophysical and sedimentological data has enabled reconstruction of the patterns of Tertiary infill within the 200 m deep basin. Lacustrine sedimentation dominated the Palaeogene, infilling a deep structural trough north of the MacDonnell Ranges. Subsequent sedimentation during the Neogene involved accumulation of thick alluvial fan deposits over the basal clay, with the lacustrine depocentre moved northward to the present site of Lake Lewis. A drying phase occurred towards the end of the Tertiary, when fluvial and lacustrine sediments were calcretised, then silicified. At least 80 m of lacustrine clay, the Anmatyerre Clay, accumulated in Lake Lewis in the Neogene-Pleistocene. This uniform clay was deposited in perennial lakewaters, infilling depressions in the heterogeneous basement topography. The Anmatyerre palaeolake was probably an overflow lake during most of its history, with westward outflow at 570 m AHD (Australian Height Datum). The catchment was evidently highly efficient in delivery of runoff from the mountains to the lake and no depositional breaks are apparent. This lake, at its maximum capacity, was up to 19 m deep and covered an area in excess of 3,000 km² , over 12 times the size of the present playa. The BrunhesMatuyama palaeomagnetic polarity reversal (780 ka) is identified in the Anmatyerre Clay at 8 m depth beneath the playa surface. A major switch in sedimentation style followed the Anmatyerre lacustral phase, with hydrologic closure of the basin during more arid conditions. The Early Tilmouth beds were deposited in a <9 m deep lake, the Tilmouth palaeolake, that extended to the 560 m AHD level and covered an area of >l375 km² , 5.5 times the size of the present playa. Hydro logic closure of the basin and high evaporation rates resulted in accumulation of solutes in the system. Saturation with respect to CaC03 and CaS04.2H20 was reached and large volumes of calcium carbonate (calcrete) and gypsum consequently precipitated in the depocentre. The Early Tilmouth beds progressively interacted with saline groundwaters during variable climatic and hydrologic conditions. The timing of the pivotal switch from the perennial Anmatyerre lacustral phase to the more variable Tilmouth lacustral phase is poorly constrained. Extrapolation from records from analogous monsoon dominated lake basins suggests that deposition of the Anmatyerre Clay may have ceased in the Middle Pleistocene, possibly during the severe Marine Isotope Stage (MIS) 10 glacial period. Much of the overlying Early Tilmouth beds may have been deposited during the MIS 7 interglacial period. Attenuation of the main Tilmouth lacustral phase was followed by a deflationary episode. Resumed lacustrine sedimentation involved more intense interaction with saline groundwaters in a greatly contracted, shallow lake setting, and deposition of the highly gypseous Late Tilmouth beds. This lacustral phase may have occurred during early MIS 5, the last interglacial period. An optically stimulated luminescence (OSL) date from a regional linear dunefield in the basin indicates an episode of dune mobility before 95 ka. Arid periods and the continued dominance of highly saline groundwaters followed the Tilmouth lacustral period at Lake Lewis, heralding playa conditions during the Late Pleistocene. Large volumes of gypsum in the form of thick aeolian sand deposits in playa islands and playa-fringing dunes are a legacy of high groundwater tables, moderate or high recharge, evaporative pumping at the playa and possible episodes of enhanced windiness. Interspersed stable periods and the influence of meteoric waters on playa processes are indicated by gypcretisation of respective gypsum sand units in the playa islands. Widespread quartz dunes across the basin landscape represent a period of maximum aridity, at around 23-21 ka. Subsequent ameliorated climatic conditions are represented by strandlines surrounding Lake Lewis that attest to inundation by floodwaters, and substantial fluvial sedimentation along the creeks. OSL dates indicate that the latter sediments span the period 19 - 0 ka. This latest period in the evolution of the basin is additionally represented by reduction of aeolian activity, major disruption of dunefields by floodwaters, and karstic degradation of calcrete ground. Groundwater evolution since hydrologic closure of the basin has followed a CaC0₃ - CaS0₄.2H₂0 path. Precipitation of large volumes of calcium carbonate and gypsum during various phases in the Quaternary has led to near-depletion of dissolved Ca stores in the system. The present-day brine is enriched in Na-Cl-S0₄ and has a salinity of 210,000 mgL⁻¹ (Total Dissolved Solids). High concentrations of dissolved silica, combined with the availability of a favourable host in the form of karstified calcrete, and continued high evaporation rates have promoted precipitation of large amounts of opaline and chalcedonic silica around the playa. Groundwaters flowing lakeward from the silicified calcrete aureole have consequently become silica-depleted. This has greatly impinged upon the brine composition and on diagenetic processes beneath the playa. The zeolite mineral, analcime, Na(A1Si2)06.H20, is precipitating authigenically in the Anmatyerre Clay in the face of silica-deficient, sodium-enriched brine. These findings have resulted in the first detailed investigation of the formation of zeolites in an Australian salt lake setting. Analysis of diagenetic minerals at Lake Lewis has underscored the importance of the antithetic relationship between active silica precipitation, shoreward, in the playa margins, and active analcime crystallisation lakeward, beneath the playa. Despite being the furthest from the coast of Australia's salt lakes, lying at the southern edge of influence of the Australian monsoon, the research reveals that Lake Lewis has, through long periods of its evolution, been a well-watered and efficient lake basin. Available rainfall continues to be efficiently delivered to the lake in today's semi-arid climatic regime. This is attributed to the orographic influence of the adjacent ranges, the high catchment area to lake area ratio and the centripetal drainage system that feeds the lake directly from the encompassing mountains. Notwithstanding the effective delivery of available rainfall, the lake system evolved to a groundwater dominated regime during the Late Pleistocene and has continued to be substantially governed by groundwater processes. The distinctive diagenetic processes currently operating at the depocentre are driven by both the chemical composition of groundwaters and the intensity of evaporation in the current climatic regime. Show more