Geochemical characterisation of Fika Formation in the Chad (Bornu) Basin, northeastern Nigeria: Implications for depositional environment and tectonic setting

• Fika shales were deposited in suboxic marine conditions.
• Aquatic algae and microorganisms represent major contribution in organic matter with minor terrigenous input.
• Moderate salinity stratification water conditions are also evident in the Fika shales.
• An active continental margin tectonic setting for the provenance of the Fika shales.

Late Cretaceous shales of the Fika Formation in the Chad (Bornu) Basin, northeastern Nigeria, were analysed to define paleoenvironment and source of the organic matter, and their relation to tectonic setting. The organic carbon and sulphur contents of Fika shale samples are in the range of 0.51–2.13 and 0.31–1.65 wt.%, respectively, pointing that these shales were deposited in suboxic-anoxic marine conditions. The biomarker and chemical compositions provide evidence for a major contribution of aquatic algae and microorganisms with minor terrigenous organic matter input. Moderate salinity stratification and relatively anoxic-suboxic bottom water conditions are also likely in the Fika shales. Therefore, stratified water column with moderate salinity and relatively anoxic-suboxic bottom water conditions have contributed to organic matter (OM) preservation in the Fika shale layer. Fika shale samples are rich in SiO2 (54.80 wt.%), followed by Al2O3 (23.75 wt.%) and Fe2O3 (10.19 wt.%). Compared with average shale, the analysed shale samples are obviously enriched in Al2O3 (23.75 wt.%), TiO2 (1.34 wt.%), and P2O5 (0.30 wt.%), indicating that these sediments are rich in clay minerals and represent a good possibility for enhanced organic matter production and enrichment.Plots of Fika shale on bivariate discriminant function diagram suggest an active continental margin setting for the provenance. The inferred tectonic setting for the late Cretaceous shales of the Fika Formation of the Chad (Bornu) Basin is in agreement with the tectonic evolutionary history of the west and central Africa during the Cretaceous period.