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Abstract
This study of stratigraphy, chronology and sedimentology at Lake Amadeus, a major playa lake in central Australia, provides for the first time a Late Tertiary and Quaternary sedimentary sequence from the continent'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 consists of uniform clay horizons with minor intercalated gypsum. The clay was deposited in a shallow lacustrine and fluvial environment. Conditions were periodically saline and frequently dry. The basal Uluru Clay is estimated to be over 5 Ma old.
The Winmatti Beds, comprising the top 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 playa system. The association with aeolian deposits signals, for the first time, the dominance of major aridity. The basal Winmatti Beds seem to coincide with the Jaramillo subchrone (0.91 Ma).
On the landward margin of Lake Amadeus, two rings (older and younger) of gypseous dunes and at least one quartz dune unit are present. The older gypseous dune possibly formed soon after the Uluru Clay and the younger one correlates with a gypseous clayey sand layer within the Winmatti Beds. They were deposited by deflation of near‐shore gypsum accumulating in the groundwater seepage zone during a period of high regional watertable associated with a wetter climate. The younger gypseous dune formed around 45–60 Ka bp (thermoluminescence dates).
A period of regional dune activation followed the younger gypseous dune formation resulting in an aeolian sand deposit in the playa and the thick quartz sand mantle on the gypseous dunes. This represents a drier period which may correlate with the low lake level period of 25–16 Ka in southern Australia. The subsequent deposition of a shallow water gypsum layer, which now forms marginal terraces and low terrace islands, represents a relatively high water level period. This may correlate with the relatively high early Holocene lake levels of southern Australia and western Victoria.