In situ rounding of quartz grains within an African surface weathering profile in North West Ngamiland, Botswana

摘要

A basal sandstone was regarded as the lowest component of the thick, unconsolidated sands and silts assigned to the Kalahari Sediments in the north west Ngamiland area of Botswana. The roundness of the quartz grains in the sandstone was taken to indicate physical transport. This seriously conflicts with recent evidence which indicates that the sandstone is a component of an in situ weathering profile. Here the results of ESEM examination of the sandstone, of unconsolidated material above it and of the underlying weathered Damaran metasediments are presented. Quartz grain rounding and shrinkage are achieved in situ in the basal sandstone by repeated alteration of a thin surface layer to an amorphous form which becomes detached, exposing a shallowly etched surface. Within the underlying Damaran metasediments, dominated by surviving interlocking crystalline textures, the more weathered partings show similar alteration of the originally angular quartz crystals. Discrete areas of interlocking crystalline textures survive within the basal sandstone, providing further evidence of a continuity of alteration processes across the boundary between the Damaran metasediments and the basal sandstone. The process of quartz grain rounding continues in the overlying unconsolidated silts and sands. No evidence was found to support the assumption that the grain rounding here indicates mechanical transport. The rounding is attributed to an in situ process of repeated alteration and exfoliation of the altered rind.