ONSET OF ARIDITY IN SOUTHERN WESTERN-AUSTRALIA - A PRELIMINARY PALEOMAGNETIC APPRAISAL

The timing of the onset of full arid conditions in southern Western Au stralia during the late Cenozoic remains uncertain. The playas and ass ociated sedimentary sequences preserved as Dart of the Tertiary palaeo drainage networks, which are widely developed in Western Australia, pr ovide the stratigraphic evidence necessary to resolve this issue. Lake Lefroy forms part of a chain of playas that occur in the eastern Yilg arn Craton. These lake chains are the remnants of a once external pala eodrainage system, developed in pre-Eocene times. Eocene non-marine to marginal marine sequences were deposited in the palaeodrainage as cha nnel infills. The low relief area of the palaeodrainage featured a per manent to semi-permanent lacustrine environment during post-Eocene tim es, and fine-grained red-brown elastic clay up to 10 m in thickness wa s deposited over an extensive area. A significant hydrological transit ion, as inferred by the lithe-sedimentary change from freshwater clay to evaporitic gypsum-dominated sedimentation, took place in the late C enozoic. The extensive freshwater system changed to the saline/deflati on playas that characterises this landscape today. A detailed palaeoma gnetic study was carried out on the lacustrine clay unit and the overl ying evaporitic gypsum unit in Lake Lefroy. Results from drill core an d pit wall exposures have provided the first time constraints for thes e sequences. Age estimates, based on extrapolation from the Brunhes/Ma tuyama geomagnetic boundary, suggest that the gypsum-dominated sedimen tation and by inference, full arid conditions in Lake Lefroy, commence d within the Brunhes Normal Polarity Chron, probably within the last 5 00 Ka. This age is considerably younger than previously thought, but a ppears to bear some correspondence to similar claims to the age of the onset of aridity in southeast and central Australia. Evidence emergin g from the inland dune field to the surrounding oceans suggests a tren d of increasing aridity during the Quaternary in Australia. The onset of full aridity may well indicate that the impact of global glacial-in terglacial cycles on Australian climate, especially the large scale gl acial 'dryness' resulted from the 100 Ka astronomic variations reached beyond its threshold. (C) 1998 Elsevier Science B.V. All rights reser ved.