Williams, M.A.J.¹ Prescott, J.R. ² Adamson, D.A.³ Lawson, E ⁴ and Cock, B¹

The presence of fine-grained lacustrine sediments in and upstream of Brachina Gorge in the central Flinders Ranges dated by AMS radiocarbon and OSL to 35-13.5 y BP raises some interesting questions. This interval spans the Last Glacial Maximum and was a time of glacial aridity when most of the lakes in southern and central Australia were becoming dry. The Flinders Ranges are presently arid to semi-arid, with very high rates of evaporation in summer, and there are no lakes within the ranges today. The streams draining the ranges are ephemeral or highly seasonal coarse bed-load streams. During sporadic but intense summer storms, these streams may flow for several hours to depths of several metres, and are competent to move rocks up to a metre or more in diameter. The sedimentary structures and granulometry of the piedmont alluvial fans along the western and eastern margins of the ranges are characteristic of ephemeral desert stream channels. It is therefore surprising to find that the late Quaternary valley fills within the ranges consist primarily of very fine sandy clays to depths of 10-15m, with minor cut-and-fill structures of fine to medium gravel close to the main ridges. Almost all of these valley-fill clays are now deeply dissected by ephemeral coarse bed-load channels. The answer to how and why did Pleistocene Lake Brachina exist during a time of glacial aridity is complex. Initial damming by a plug of fine sediment from a major tributary valley immediately downstream of a very narrow section of the gorge may have led to ponded drainage and reed growth. The progressive demise of River Red Gums along the valley floor in response to increasing cold, windiness and lower atmospheric carbon dioxide levels may have initiated a steady rise in local ground-water levels, resulting in lateral seepage from valley-side springs. Calcareous tufas accumulated on either side of the bottleneck as well as within the fine-grained fluvio-lacustrine sediments. The shallow dam consisting of tufa, reeds and fine sediment led to ponding of drainage and the gradual creation of a small lake within and upstream of Brachina Gorge. The diatoms, shells and ostracods associated with the tufa and the lake clays are consistent with relatively shallow, sluggish, ponded drainage. The ratio of catchment area to lake area is very high, so that once the dam had formed the lake would have functioned as an amplifier lake. The ranges lie athwart one of the major late Quaternary dust paths and were effective traps for aeolian dust blown from the west. Some of the fine sediment within the lake appears to be reworked from hillslope dust mantles. Remnants of these mantles occupy the interstices on the rocky summits of the surrounding Cambrian quarzite ridges.