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 ar eas of the Australian Precambrian Shield-a vast planar surface masked by the regolith. Much of the geological history of the shield subseque nt to its early development is recorded in ancient cover rocks and you nger basin sediments which occur in important stratigraphic sequences, particularly on the margins of the shield. Within these sequences, st ratigraphically 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 form ation since the Proterozoic. Laterite and silcrete profiles are seen t o be assemblages of paleosols, stratigraphically associated deposits a nd companion materials which were formed in response to changes in gro undwater conditions at particular times in the past. The paleosols rec ord the evolution of the regolith: Older weathering zones and bleached rocks were features of successive landscapes after the early Palaeozo ic; ferruginous mottling, ferricrete and silcrete pans were formed aft er 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 ov erlain in places by calcretes formed after the early Pleistocene and b y younger soils. The assemblages are distinctive and are characteristi c of particular morpholithogical provinces.