干旱区生态环境知识资源中心

Geological and sedimentological studies form the basis of discussion of development of endorheic scarpfoot depressions of the geological Bilma-basin of Southern Central Sahara. Since Miocene tectonic movements fixed the position and direction of the scarps as well as of the depressions, however not as defined by structural depression but in a direct spatial relation to the depression to formation boundaries and/or lateral and vertical gradation of facies. Therefore a causative correlation is given between the deepening/rate of deepening resp. the existence/missing of depressions and the stratigraphic/facies change from Upper Cretaceous/Lower Tertiary and from the southern to the northern/north-eastern part of the Bilma basin. The further development of the scarpfoot depressions was characterized by fluvial/limnic and eolian denudation. Local faults, joint tectonics and karstification marked the geomorphological way of the exogene factors. Ground water influenced by rock weathering the forming of the depressions: in warm humid climatic periods of Lower Tertiary deep weathering combined with high ground water levels and tectonical inactivity resulted in a saprolitisation, in Miocene decreasing ground water levels due to tectonical activity and semi-arid climatic conditions led to a deep oxidation.