Geochemical characterisation and organic matter enrichment of Upper Cretaceous Gongila shales from Chad (Bornu) Basin, northeastern Nigeria: Bioproductivity versus anoxia conditions

摘要

The Upper Cretaceous Gongila Formation in the Chad (Bornu) Basin, northeastern Nigeria has always been considered as a transitional sequence between the underlying continental Bima Formation and the overlying marine Fika Formation. So far, little information has been available on this formation, which is regarded as one of the prospective source rocks in this Basin. Elemental and organic geochemical analyses were performed on the shales recovered from this formation in order to define the origin and type of organic matter within the sediments and also to examine the processes controlling organic matter enrichment. Based on their bulk geochemical characteristics and biomarker compositions, the selected Gongila shales were interpreted to be deposited in marine environment. Their biomarkers and chemical compositions also indicate enhanced bioproductivity within the photic zone of the water columns and a contribution by mixed aquatic algae/microorganisms and terrigenous organic matter input. The increased bioproductivity in the selected Gongila shales may be related to enhanced semi-arid/humid to humid-warm climate conditions. Low salinity stratification and suboxic to relatively anoxic bottom water conditions are also supported by Sr/Br, V/Ni, and Pr/Ph ratios. Therefore, a stratified water column with low salinity and suboxic to relatively anoxic bottom water conditions may have contributed to the preservation of organic matter in the sediments. (C) 2015 Elsevier B.V. All rights reserved.