The pleistocene palaeoenvironmental record of Lake Buchanan: An atypical Australian playa

Abstract

Lake Buchanan is an “intermontane” playa located in subdued topography astride the Great Dividing Range in central Queensland. Undated multiple beach ridges on the western shore and cliffed lacustrine sediments on the eastern shore, both up to 5 m above the present lake-floor, attest to previous permanent-lake phases. Due to its position astride the continental divide for much of the Tertiary, Lake Buchanan probably escaped the acid weathering regime and attendant gypsum-formation of the continental interior. Present surface- and groundwaters (140–210% salinity) are atypical of other Australian salt lakes which are commonly Mg 2+-rich and similar to evaporated ocean water. For Lake Buchanan, ionic Ca 2+/Mg 2+ ≈, and the Sr 2+ (≈95 ppm) and F − (≈ 12 ppm) contents are very high, and Br − exceedingly low (<0.02 ppm). The unusual water compositions are considered to reflect the chemical composition of the attachment rocks which are Mesozoic and Tertiary clay-rich sandstones. This relationship argues against the universal applicability of marine aerosol transport and marine-salt recycling as sources of salts for Australian playas. Sediments cored from the lake floor are uniform non-laminated sandy clays composed of quartz (35 weight%), kaolinite (30%) and illite (25%). There is no primary chemically-precipitated carbonate or gypsum. Low-magnesium calcite occurs as a biogenic detritus of shelly fauna, charophyte gyrogonites and pedogenic nodules. Apart from the upper 20–50 cm, the sediment is devoid of organic carbon. A sequence of palaeosols indicates exposure of the lake floor during at least 13 episodes in the Pleistocene. For a 15-m core (BU-1), palaeomagnetic analysis has identified the Brunhes-Matuyama boundary (0.73 Ma) at 5.0 m; and probably the top of Jaramillo subchron (0.92 Ma) at 6.3 m. An extremely slow sedimentation rate of ≈7 m/Ma is indicated for the last 1 Ma. From 6.5 to 10.5 m in this core, intense iron-staining (pedogenesis) has confounded designation of palaeomagnetic polarity. Extrapolation might suggest an age of between 1.5 and 2 Ma for the base of this core. Fossil biota from BU-1 and BU-9 include abundant ostracods and charophyte remains, and less commonly gastropods, fish fragments and foraminifers. Water depth is interpreted from ostracod and charophyte assemblages. For the last 0.73 Ma we recognize four major wet phases. The early history of BU-1 (below 12 m, ≈16 Ma?) alternated between dry playa and shallow lake.