EVOLUTION OF LANDSCAPES IN NORTHERN SOUTH AUSTRALIA IN RELATION TO THE DISTRIBUTION AND FORMATION OF SILCRETES

Geomorphological features on the southwestern margins of the Lake Eyre Basin provide a basis for interpreting the evolution of old landscape s containing pedogenic and groundwater silcretes. and thick bleached a nd weathered profiles. Recurrent sequences of cut-and-fill and duricru st formation have been identified and related to major sea level chang es and tectonic movements in the Lake Eyre Basin. An extensive high pe diment had formed around a basement inlier by the late Eocene. An armo ur of pedogenic silcrete developed on this pediment under alternating dry and wet climates during the late Eocene and Oligocene. The charact eristic kaolinite + opal + alunite + gypsum assemblage of the bleached profiles formed in acid saline groundwaters during the Miocene at a t ime of regional low water table and arid climate. Groundwater silcrete s formed in the bleached profile in response to dissection of the high pediment. They are related to a period of high groundwater tables, hu mid climate, and gradual sinking of the Lake Eyre basin. In the meanti me, widespread low pediments (glacis) formed in Lake Eyre catchment. p ossibly in the early Pliocene. There is a good correspondence between a geomorphic approach, taking in to account the relationship between w eathering features and palaeolandscapes, and the information provided from stratigraphic studies in the region.