Paleohydrological indicators in playas and salt lakes, with examples from Canada, Australia, and Africa
Abstract The sedimentary records of playas and salt lakes provide some of the best evidence for hydrological change in a region. These records are comprised of siliciclastic sediments and chemically-precipitated minerals that typically vary temporally and spatially. In some cases, the interpretation is straightforward, with mineralogical sequences related to changing brine concentrations and hydrology, as well as to climate. In other cases, it is difficult to identify the controlling factors, which include climate, groundwater, geological setting, and basin morphology. In addition, diagenesis commonly affects the sedimentary record. Models of lake level fluctuations are given for basins in the Canadian Prairies and southeastern Australia. The Lake Manitoba model is based mainly on secondary changes associated with pedogenesis that alters the deeper-water sediment during low water stages. Smaller Canadian lakes in more arid regions display a larger variety of diagnostic paleohydrological indicators, such as specific carbonate and salt mineralogy, nature of bedding, grain size, and secondary alterations. Decreasing water levels in shallow lakes tend to show a progression from clastics to carbonates, to salts, and then back to clastics as groundwater levels fall below the lake floor. Similar parameters are used in Australian lakes. The hydrological model for the Lake Tyrrell playa in Victoria, whose present ionic composition is similar to seawater, shows clastics being deposited when average lake levels are high, followed by deposition of primary carbonates, gypsum, and halite; once seasonal drying on the lake floor begins, secondary gypsum grows within already-deposited sediment, and clay pelletization in the capillary fringe of surface sediment leads to basin deflation and the accumulation of adjacent eolian lunettes. Continued water level decline leads to pedogenesis. Subsequent lake level rises in all of these models may initiate sedimentation in any part of the hydrological cycle, and a complete cycle in the sedimentary record appears to be uncommon.