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

Lying in a semi-arid region, the Flinders Ranges have rocky, weathering-lim ited hillslopes and streams within the ranges are active only during rare d ownpours, when they transport boulders. gravel and coarse sand. However, si lt- and clay-rich valley-fill deposits, incised by present streams, occur a s terraces and terrace remnants. Such deposits are not accumulating today. In and upstream of Brachina Gorge, in the central ranges. these remnant val ley-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 s ilt that can be traced for several hundred metres, and contain gastropods, diatoms and phytoliths consistent with sluggish, shallow water flow under f resh to brackish conditions. AMS C-14 and OSL dating shows that these valle y-fill deposits accumulated between similar to 33 and 17 ka, an interval th at embraces the Last Glacial Maximum (LGM). Formerly thought to be take bed s (Aust. J. Earth Sci. 46 (1999) 61), the valley fills are shown here to ha ve 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 sub stantial reduction or both rainfall variability and evaporation. which woul d be favoured by the low temperatures and reduced incursions of summer rain fall. This is consistent with climatic reconstructions by Miller et al. (Na ture 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 h eralded by a major influx of coarse alluvium followed by channelling and di ssection. Erosion was interrupted by an episode of aggradation and floodpla in widening, represented by remnants of a wide terrace inset below the prim ary wetland surface. This episode, which is interpreted as a return to lowe r climatic variability. ceased with establishment of the present climatic r egime. which has resulted in stripping of the Late Pleistocene deposits fro nt much of the Brachina valley. (C) 2001 Elsevier Science Ltd and INQUA. Al l rights reserved.