Onset of aridity in southern Western Australia—a preliminary palaeomagnetic appraisal

Abstract

The timing of the onset of full arid conditions in southern Western Australia during the late Cenozoic remains uncertain. The playas and associated sedimentary sequences preserved as part of the Tertiary palaeodrainage networks, which are widely developed in Western Australia, provide the stratigraphic evidence necessary to resolve this issue. Lake Lefroy forms part of a chain of playas that occur in the eastern Yilgarn Craton. These lake chains are the remnants of a once external palaeodrainage system, developed in pre-Eocene times. Eocene non-marine to marginal marine sequences were deposited in the palaeodrainage as channel infills. The low relief area of the palaeodrainage featured a permanent to semi-permanent lacustrine environment during post-Eocene times, and fine-grained red–brown clastic clay up to 10 m in thickness was deposited over an extensive area. A significant hydrological transition, as inferred by the litho-sedimentary change from freshwater clay to evaporitic gypsum-dominated sedimentation, took place in the late Cenozoic. The extensive freshwater system changed to the saline/deflation playas that characterises this landscape today. A detailed palaeomagnetic study was carried out on the lacustrine clay unit and the overlying evaporitic gypsum unit in Lake Lefroy. Results from drill core and pit wall exposures have provided the first time constraints for these sequences. Age estimates, based on extrapolation from the Brunhes/Matuyama geomagnetic boundary, suggest that the gypsum-dominated sedimentation and by inference, full arid conditions in Lake Lefroy, commenced within the Brunhes Normal Polarity Chron, probably within the last 500 Ka. This age is considerably younger than previously thought, but appears to bear some correspondence to similar claims to the age of the onset of aridity in southeast and central Australia. Evidence emerging from the inland dune field to the surrounding oceans suggests a trend of increasing aridity during the Quaternary in Australia. The onset of full aridity may well indicate that the impact of global glacial–interglacial cycles on Australian climate, especially the large scale glacial `dryness' resulted from the 100 Ka astronomic variations reached beyond its threshold.