The Gassa sandstone near Tillaberi in western Niger, shown on the 1960 geologic map of western Africa (l:2,000,000) as provisionally assigned to the Paleozoic, is an Infracambrian formation deposited on the Precambrian basement and overlain by the Infracambrian Ydouban group. The Gassa sandstone thus is the stratigraphic equivalent of the Firgoun sandstone in the Gourma basin.

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