The enigma of a late Pleistocene wetland in the Flinders Ranges, South Australia

Abstract

Lying in a semi-arid region, the Flinders Ranges have rocky, weathering-limited hillslopes and streams within the ranges are active only during rare downpours, when they transport boulders, gravel and coarse sand. However, silt- and clay-rich valley-fill deposits, incised by present streams, occur as terraces and terrace remnants. Such deposits are not accumulating today. In and upstream of Brachina Gorge, in the central ranges, these remnant valley-fills are exposed in bank sections up to 18 m high. At some localities, exposures show horizontal, centimetre-scale beds of fine sand and clayey silt that can be traced for several hundred metres, and contain gastropods, diatoms and phytoliths consistent with sluggish, shallow water flow under fresh to brackish conditions. AMS 14C and OSL dating shows that these valley-fill deposits accumulated between ~33 and 17 ka, an interval that embraces the Last Glacial Maximum (LGM). Formerly thought to be lake beds (Aust. J. Earth Sci. 46 (1999) 61), the valley fills are shown here to have accumulated in a fluvial wetland that extended westward from the middle reaches of Brachina Creek, through Brachina Gorge and joined with aggraded fan deposits beyond the ranges. Aggradation of this wetland requires a substantial reduction of both rainfall variability and evaporation, which would be favoured by the low temperatures and reduced incursions of summer rainfall. This is consistent with climatic reconstructions by Miller et al. (Nature 385 (1997) 241) and Johnson et al. (Science 284 (1999) 1150) for Lake Eyre, north of the Flinders Ranges, at the LGM. Demise of the wetland was heralded by a major influx of coarse alluvium followed by channelling and dissection. Erosion was interrupted by an episode of aggradation and floodplain widening, represented by remnants of a wide terrace inset below the primary wetland surface. This episode, which is interpreted as a return to lower climatic variability, ceased with establishment of the present climatic regime, which has resulted in stripping of the Late Pleistocene deposits from much of the Brachina valley.