Geochemistry And Depositional Characteristics Of Maestrichtian Shales In Parts Of South Western Nigeria

Abstract

A geochemical study of the Maestrichtian shales from Auchi, Akinbo/Ewekoro and Ifon areas belonging to same sedimentary basins within southwestern Nigeria was carried out to determine their geochemical characteristics and environmental conditions of deposition.
The results indicate that the shales have their own distinct geochemical characteristics. For example the shales from Auchi areas are characterized by relatively higher TiO2, Rb, Ni, Co, Cu, Cr, V, Mo, Zr as well as Rb/K20, Cu/Zr and V/Cr ratios than the other shales. On the other hand, Akinbo shales are enriched in MgO, CaO, P2O5 and Sr generally attributable to the associated carbonates and phosphates in the area. The Ifon shales are intermediate geochemically between the Akinbo and the Auchi shales.
Variations in the element ratios have been used to obtain sedimentological/ecological information from the geochemical data. The relatively high Cu/Zn and (Cu+Mo)/Zn ratios in the shales from Auchi area indicate confined circulation in the depositional environment. The lower Cu/Zn ratios in the shales from Ifon and Akinbo areas indicate a more oxidizing condition of deposition, although the presence of glauconitic sands or green concretions in the Akinbo shales provide evidence for subsequent reducing conditions. Basal phosphatic bands associated with the Akinbo shale are indicative of lithological changes during deposition. It is concluded that changes in source rock characteristics, depositional environment and diagenesis are responsible for the variations in the geochemistry of these shales.


Journal of Mining and Geology Vol. 43 (2) 2007: pp. 167-174