GEOMORPHOLOGY OF THE KAMBALDA REGION, WESTERN-AUSTRALIA

Landforms of the Kambalda area are typical of the greenstone belts of the southern Yilgarn Craton. The landscape consists of palimpsest landforms that originated in different climates. The gently undulating land surface forms a partial etchplain, topographic relief is controlled largely by two factors, the elevation of the palaeoweathering front and erosional topography of the incised palaeodrainage. Low hills are composed of mafic rocks where the palaeoweathering front was shallow whereas valleys occur on felsic volcanics and sedimentary rocks where the front was deeper. The palaeodrainage system has been incised across the structural-stratigraphic fabric of the area. Hills are composed of fresh rock, while valley slopes, pediments and pediplains are composed of fresh to saprolitic rock. Residual ferricretes form low mesas. Palaeodrainage fill locally reaches 100 m; the palaeodrainages are currently occupied by playa lakes separated by sand dunes. This study confirms the great age of the landscape of the Yilgam Craton proposed by earlier authors. Correlation of depositional events in sedimentary basins with weathering and erosion on adjacent cratonic areas allows reconstruction of the history. Formation of the weathered regolith largely predates the incision of the palaeodrainage which, in its deepest parts, has cut through to fresh rock. The relict saprolite dates back to the Late Permian-Middle Jurassic. Stripping of this regolith occurred in the Middle Jurassic-Early Eocene during the formation of the palaeodrainage system. About 400 m of material was denuded from the Yilgam Craton, to infill the Perth, Eucla and Great Australian Bight Basins. Further deep weathering occurred concurrently with erosion. Increasing aridity led to disorganization of the drainage to form a chain of lakes. Pliocene times saw increased aridity in the Kambalda area, with the establishment of the current semi-arid geomorphic regime. Arid conditions have persisted for only 2% of the history of the Kambalda landscape. Thus arid geomorphic models are inappropriate for understanding all but the most recent evolution of the regolith. The present is not the key to the past history of the Kambalda landscape, rather the past is the key to the present.