High (ground) water levels and dune development in central Australia: TL dates from gypsum and quartz dunes around Lake Lewis (Napperby), Northern Territory

An episode of high lake levels prior to the last maximum glaciation has been identified at many localities in wastern Australia. Similar events have been recognized at playa lakes in central Australia, where gypsum dunes along playa margins formed during one or more episodes of high groundwater discharge, with a large influx of calcium sulphate.At Lake Lewis, exposures at two islands show similar sediment sequences: three pedogenic gypcrete layers interbedded with aeolian quartz and gypsum sand horizons form three units within gypsum dunes up to 7 m high. The lowest unit has cliffed edges buried by the upper units, indicating a significant time break. Four TL dates (coarse-grained quartz) show that this lowest unit was deposited at or before 70–80 ka. The middle unit of mixed gypsum and quartz sand capped by gypcrete represents the major phase of gypsum dune formation, and 6 TL dates range from 33 to 46 ka with overlapping error bars. These are slightly younger but statistically similar to TL dates (from 39 to 59 ka) of the shoreline gypsum dune at Lake Amadeus in the same region. The top unit of the two islands, up to 1 m thick, has not yet been well dated. One date is inconsistent with the well dated middle layer below, possibly because of incomplete bleaching, and has been rejected. The other date (17 ± 5 ka) is much younger which possibly indicates a minor and local reactivation of old gypsum sediments. At the lake margin, there are quartz dunes overlying the gypsum dunes, and a buried aeolian quartz sand layer occurs in a lake-margin terrace. These represent reactivation of the regional quartz dune field after the major gypsum dune formation. Two consistent TL dates (21 ± 4 ka and 23 ± 6 ka) indicate that regional dunes were active at about the time of the Last Glacial Maximum.