Abstract
Three examples of extensive diamictites within glacial sequences well exposed in the sedimentary cover of the West African cratonic platform are described. They belong to the Late Precambrian and Upper Ordovician glacial periods and the paleogeography is summarized. The Late Precambrian diamictites of the Jbeliat Group in Adrar of Mauritania displays characteristics of tillite deposited as lodgement or melt-out till, Detail studies on structure, clast shape nature and fabric, texture in thin section, clay mineralogy and carbonate content are given and discussed in order to delimitate useful criteria for tillite identification. Compared to this first example, the two other diamictites from the Late Precambrian Bakoye Group in western Mali and from the Upper Ordovician Tichit Group in southeastern Mauritania have obviously been deposited in an aquatic environment (waterlain glacial diamictites) by the action of ice-rafting, bottom current, and flow processes. These examples show that for such ancient deposits sedimentological interpretations are partly solved by broad scale stratigraphical relationships. However, detailed sedimentological analyses remain essential for precise facies identification. Especially structural and petrographic studies must be encouraged in the lights of modern glaciodynamic processes.
Published Version
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