The Namib Plains: Gypsum Crusts and Stone Pavements

Abstract

In the central Namib gypsum crusts (gypcrete) are a widespread surface material, particularly in drier areas. Their origin is complex. One model to explain the presence of sulphate in the coastal tracts of the central Namib is that marine biogenic hydrogen sulphide (H2S), developed on the highly productive Namibian shelf, erupts from time to time and is carried inland by south westerly winds. Another suggested source of sulphate is anhydrite and bedrock sulphide. Some of the Namib gypsum may be distributed across the desert surface by dust storms that have deflated evaporite material from the many small pans and sabkhas identified on satellite images. Another important surface type in the Namib Plains is the stone or desert pavement. These are armoured surfaces composed of a mosaic of fragments, usually only one or two stones thick, set on or in matrices of finer material. They are formed by a range of processes that cause coarse particle concentration at the surface: (a) deflation of fine material by wind; (b) removal of fines by surface runoff and/or creep; and (c) processes causing upward migration of coarse particles to the surface.