GEOLOGICAL AND PETROGRAPHIC STUDIES OF LENSOIDAL PAPALANTO GYPSUM DEPOSIT IN DAHOMEY BASIN, SOUTH WESTERN NIGERIA

Papalanto gypsum occurs as lenses within the Ewekoro formation of the Dahomey basin, southwestern Nigeria. The lithologic units encountered include clay, shale and marl/limestone. The clay unit is mottled in color and often lateritized at top. The mixed clay-shale layer is grey with black staining of dark carbonaceous materials. Iron concretions are found associated with the clay-shale member. The shale unit is grey, generally fissile and the fissility increases with depth. The marl/limestone occurs as intercalation within the shale. The gypsum units are localized within the shale beds and with concordant veinlets on lenses. Petrographic studies on papalanto gypsum show large conspicuous crystals of gypsum in a micro crystalline matrix of anhydrite. The evaporite minerals notably observed are laminated and nodular to lensoidal gypsum and anhydrite. The modular to lensoidal gypsum are found in association with micro crystalline anhydrite, forming cryptocrystalline texture plates. The unfossiliferous (micro) nature of the sediment suggests hyper saline conditions for the formation of the sulphates, gypsum and anhydrites. The presence of minerals such as anhydrite and polyhalite as fine-grained matrix suggest that the gypsum is later-formed and laminated.