Onset of aridity and dune-building in central Australia: sedimentological and magnetostratigraphic evidence from Lake Amadeus
Abstract Sediments from Lake Amadeus, a groundwater discharge playa in central Australia, comprise a surface playa sequence (the Winmatti Beds), varying between 0.6 and 3 m in thickness, overlying a long sequence of fairly uniform fluvial-lacustrine clays (the Uluru Clay). The latter extends down to at least 15 m (the maximum depth cored during this study), and possibly down to 65 m below the present playa surface. In the cores studied the Brunhes-Matuyama boundary (0.73 Ma) was identifiable in all the upper sediments, typically at a depth of between 1 and 2 m, but below this there are two plausible chronological interpretations of the palaeomagnetic data. Deposition rates are very low, typically no more than 1.5 cm/ka in the Uluru Clay Beds, and down to 0.2 cm/ka in the Winmatti Beds. These rates suggest that the fluvial-lacustrine phase lasted for at least 5 Ma. The boundary between the Uluru Clay and the Winmatti Beds bears a close correspondence to the boundary between the Blanchetown Clay (a lacustrine sequence) and the Tyrrell Beds (a playa sequence) in palaeo-lake Bungunnia, in what is now the Murray Basin in southeastern Australia. However the transition occurs at about 0.9 Ma (or possibly 1.6 Ma) at Lake Amadeus, whereas it has been dated palaeomagnetically at less than 0.73 Ma (probably 0.5 Ma) in Lake Bungunnia. Records from other locations in southeastern Australia likewise support a major change from wetter to drier conditions at, or shortly after, the Matuyama-Brunhes polarity transition. Thus the current evidence from Lake Amadeus indicates that in central Australia the onset of aeolian and saline gypseous deposits, characteristic of arid-zone facies, may have pre-dated the corresponding change in the southeastern part of the continent by about 0.4 Ma, or possibly by about 1.1 Ma. In common with most playas in central Australia, Lake Amadeus is surrounded by a ring of gypseous dunes derived from material deflated from the surface of the playa. Magnetostratigraphic evidence from Auger Island, in the middle of Lake Amadeus, suggests that the oldest gypseous dune formation on the island must have pre-dated the start of the Jaramillo subchron at 0.98 Ma (or possibly the start of the Brunhes epoch, 0.73 Ma), thus making these the oldest dated dunes in Australia. The age of the Auger Island dunes is probably characteristic of other gypseous dunes around the margins of playas in central Australia.