The central Australian groundwater discharge zone: Evolution of associated Calcrete and Gypcrete deposits

The groundwater discharge zone of central Australia is marked by a chain of playas extending 500 km from Lake Hopkins in Western Australia through Lake Neale, Lake Amadeus and numerous smaller playas to the Finke River in the Northern Territory. This great valley is devoid of surface drainage, and the playas and their associated landforms and chemical sediments are a result of groundwater transmission, and discharge from a large regional flow system. The valley contains extensive groundwater calcrete deposits, which are commonly silicified. The playas contain gypsum and glauberite deposits resulting from the evaporative concentration of discharging groundwater to 250 g/1 total dissolved solids. Thin gypcrete crusts have developed on the playa margins and islands as a result of induration of bordering gypsum dunes. Diagenetic changes in the calcrete and gypcrete are caused by dissolution and reprecipitation through groundwater movement. Calcrete and gypcrete samples from playas near Curtin Springs, NT, dated by electron spin resonance (ESR) — calibrated by¹⁴C dates on calcrete and the uranium content of gypcrete - are mostly in the range 8000-16 000 years bp, and are evidence of groundwater discharge conditions similar to those of the present day. Calcrete in the vadose zone, above the present water-table, gives ESR dates in the range 22 000-27 000 years bp suggesting episodes of high intensity rainfall at that time. Phreatic calcrete, below the water-table, with ages in the range 34 000-75 000 years bp, provides evidence of older episodes of calcretization. The ESR and¹⁴C dates for the chemical sediments suggest that this groundwater flow system has been an important feature in the region for much of the Late Quaternary. Although there are problems with the interpretation of ESR dating for chemical sediments with diagenetic alteration, the technique shows promise as a dating tool.