PALEOSOLS IN LATERITE AND SILCRETE PROFILES EVIDENCE FROM THE SOUTH EAST MARGIN OF THE AUSTRALIAN PRECAMBRIAN SHIELD

Laterite and silcrete profiles are common in the arid and semi-arid areas of the Australian Precambrian Shield-a vast planar surface masked by the regolith. Much of the geological history of the shield subsequent to its early development is recorded in ancient cover rocks and younger basin sediments which occur in important stratigraphic sequences, particularly on the margins of the shield. Within these sequences, stratigraphically associated or as companion materials, weathering zones and paleosols were developed which individually and as assemblages of layers and horizons record the history of weathering and of soil formation since the Proterozoic. Laterite and silcrete profiles are seen to be assemblages of paleosols, stratigraphically associated deposits and companion materials which were formed in response to changes in groundwater conditions at particular times in the past. The paleosols record the evolution of the regolith: Older weathering zones and bleached rocks were features of successive landscapes after the early Palaeozoic; ferruginous mottling, ferricrete and silcrete pans were formed after the early Cainozoic; ferricretes and mottled clay paleosols - some of which have been described as ''laterite'' - were formed during and after the Pliocene. Materials in laterite and silcrete profiles are overlain in places by calcretes formed after the early Pleistocene and by younger soils. The assemblages are distinctive and are characteristic of particular morpholithogical provinces.