EVOLUTION OF THE LEFROY AND COWAN PALAEODRAINAGE CHANNELS, WESTERN-AUSTRALIA

A dendritic system of palaeodrainage channels is a prominent feature of the Yilgarn Craton of Western Australia. The history of the Cowan and Lefroy palaeodrainage channels described herein is typical of the mid-lower reaches of these features. Incision of the palaeodrainage system occurred during Jurassic times. Reversal of the Cowan palaeodrainage channel occurred prior to Jurassic deposition in the Bremer Basin, resulting in the Cowan palaeodrainage channel flowing south to the Bremer Basin and the Lefroy palaeodrainage channel flowing east to the Eucla Basin. Earliest preserved strata consist of the Early to Middle Eocene non-marine Hampton Sandstone and Pidinga Formation in the Lefroy palaeodrainage channel, and the Werillup Formation in the Cowan palaeodrainage channel. Subsequently, two transgressive sequences (Tortachilla and Aldinga transgressions) were deposited. During the Tortachilla transgression, estuarine-deltaic facies of the Pidinga Formation and Hampton Sandstone were deposited in the Lefroy palaeodrainage channel, with estuarine-deltaic facies of the Werillup Formation and the shallow marine Norseman Limestone deposited in the Cowan palaeodrainage channel. The shallow marine Princess Royal Spongolite was deposited in both palaeodrainage channels during the Aldinga transgression. In post-Eocene times, the palaeodrainage channels were fragmented into a chain of lakes, accompanied by deposition of the fluvio-lacustrine Redmine Group. Deposition of the clastic-dominated Revenge Formation took place during the Miocene. Limited carbonate deposition also occurred in both lakes, forming the Cowan Dolomite in Lake Cowan, and the Gamma Island Formation in Lake Lefroy. From the Pliocene onwards, increasing aridity led to deposition of the evaporitic Roysalt Formation in Lake Lefroy and Polar Bear Formation in Lake Cowan.