LATE CENOZOIC STRATIGRAPHY AND HYDROLOGIC HISTORY OF LAKE AMADEUS, A CENTRAL AUSTRALIAN PLAYA

This study of stratigraphy, chronology and sedimentology at Lake Amade us, a major playa lake in central Australia, provides for the first ti me a Late Tertiary and Quaternary sedimentary sequence from the contin ent's centre. The Cenozoic sediments of the lake basin consist of two major units: the Uluru Clay; and the overlying Winmatti Beds. At least 60 m of Uluru Clay overlies Proterozoic dolomitic limestone and consi sts of uniform clay horizons with minor intercalated gypsum. The clay was deposited in a shallow lacustrine and fluvial environment. Conditi ons were periodically saline and frequently dry. The basal Uluru Clay is estimated to be over 5 Ma old. The Winmatti Beds, comprising the to p several metres of basin sediments, are characterized by aeolian sand , gypsum-clay laminae and thick gypsum sands. The dominance of saline groundwater marks the first development of a groundwater discharge pla ya system. The association with aeolian deposits signals, for the firs t time, the dominance of major aridity. The basal Winmatti Beds seem t o coincide with the Jaramillo subchrone (0.91 Ma). On the landward mar gin of Lake Amadeus, two rings (older and younger) of gypseous dunes a nd at least one quartz dune unit are present. The older gypseous dune possibly formed soon after the Uluru Clay and the younger one correlat es with a gypseous clayey sand layer within the Winmatti Beds. They we re deposited by deflation of near-shore gypsum accumulating in the gro undwater seepage zone during a period of high regional watertable asso ciated with a wetter climate. The younger gypseous dune formed around 45-60 Ka BP (thermoluminescence dates). A period of regional dune acti vation followed the younger gypseous dune formation resulting in an ae olian sand deposit in the playa and the thick quartz sand mantle on th e gypseous dunes. This represents a drier period which may correlate w ith the low lake level period of 25-16 Ka in southern Australia. The s ubsequent deposition of a shallow water gypsum layer, which now forms marginal terraces and low terrace islands, represents a relatively hig h water level period. This may correlate with the relatively high earl y Holocene lake levels of southern Australia and western Victoria.