Engineers Australia

The Guildford Formation re-evaluated

Bob Gozzard

Abstract

The Guildford Formation extends over a significant area of the Swan Coastal Plain from Cervantes north of Perth to Busselton in the southwest of Western Australia and forms an easily recognizable geomorphological unit — the Pinjarra Plain. The Guildford Formation was deposited as a series of coalescing alluvial fans at the foot of the Darling Scarp. These fans interfinger in the west with sandy fluvial and shallow marine sediments of the Gnangara Sand and Bassendean Sand. Along the middle reaches of the Swan River fluvial and estuarine sediments of a later alluvial complex infill a deep inset valley cut into and through the Guildford Formation. These deposits are defined as the Perth Formation. Along the middle and lower reaches of the Swan and Canning Rivers alluvial and estuarine sediments of a younger alluvial complex are found infilling a series of younger inset valleys cut into and through the Perth Formation and into the underlying bedrock. These sediments were previously considered to be lateral equivalents of the Guildford Formation but this correlation can no longer be accepted and they are defined as the Swan River Formation. Between Cervantes in the north and Bunbury in the south extensive drilling confirms a relatively simple model for the Guildford Formation. In contrast, the alluvial histories of the Perth Formation and of the Swan River Formation are complex and reveal alluvial regimes that have varied significantly over time. The complicated interrelationships between the sedimentary bodies of the Perth Formation and of the Swan River Formation are a result of both local-scale and regional-scale changes of river and channel morphology that were influenced by changes in sea level caused by climatic fluctuations during the Late Pleistocene.