Mineralogy and geochemistry of the sandstone facies of Campanian Lokoja formation in the Southern Bida basin, Nigeria: implications for provenance and weathering history

The upper Cretaceous Lokoja Formation is the earliest deposited stratigraphic unit in the intracratonic Bida Basin, Nigeria. It consists predominantly of sandstone and offers a potential hydrocarbon reservoir in the basin. In this study, we investigated the bulk elemental (major, trace and rare earth elements) and mineralogical compositions of the sandstones for the determination of their compositional maturity, tectonic setting, source and weathering history. The sandstone geochemical data indicates that the samples contain moderate amount of SiO2 and Al2O3 with average values of 78.3 % and 9.75 % respectively. Relatively high average values of K2O (1.59 %) and Na2O (1.52 %) and low Fe2O3 (2.56 %), MnO (0.05 %), TiO2 (0.31 %) and P2O5 (0.018 %) were obtained. Plots of relevant geochemical elements reveal that the sandstones are predominantly arkose and litharenite types with minor sub-litharenites. Application of some trace element geochemical proxies; La/Co (11.78), Th/Co (1.32), La/Th (7.04) and Eu/Eu* (0.82) to constrain the source history suggest protolith dominated by felsic rock. Relatively low average values of Sigma REEs (12.81) and Sigma HREEs (1.52) support the felsic provenance of the sandstones and the tectonic setting discriminant function plots indicate passive margin depositional basin. Paleo-weathering plot reveals a moderate humid climatic condition and the mean values of CIA (79.21), CIW (90.51), PIA (66.55) and A-CN-K diagram indicates low to moderate chemical weathering in the source area and compositional immaturity for the sediments. This study concludes that the sandstones of the Lokoja Formation are immature and were derived from the faulted blocks of granitic basement rocks at the margin of the basin and deposited proximally.