ABSTRACT
Monogenetic calcretes, up to 30 m thick, are widespread in the Selima Sand Sheet and also occur in the Great Sand Sea, whereas in the hydrologically favoured area of Bir Tarfawi, Bir Sahara und Bir Dibis there have been several phases of calcrete formation, resulting in thick, complex, calcareous crust profiles. In the Bir Tarfawi area a more or less dolomitized horizon (dolocrete) sometimes occurs in thick crust profiles. Calcrete formation was due to a high groundwater table (deriving from carbonate accumulation in small lakes or ponds or carbonate-cementation of sandy material). Radiocarbon and U/Th dating of the calcretes yielded a late Pleistocene age.
