Patti formation shales at Agbaja Plateau: A geochemical window into the provenance and tectonic setting history of the Southern Bida Basin, Nigeria

Abstract

Agbaja Plateau is one of the mesas that dotted the topography of north centrally located Bida Basin. Seventeen shale samples from the exposed Maastrichtian Patti Formation at Agbaja Plateau were analyzed using x-ray fluorescence (XRF) and inductive coupled plasma mass spectrophotometry (ICPMS) with the aim of unroofing the provenance, weathering and paleotectonic history of the basin. The provenance-sensitive elemental ratios; Al2O3/TiO2 , Cr/Ni, Th/Cr, La/Sc, Th/Sc and Th/Co and the binary diagrams of Al2O3 vs TiO2, TiO2 vs Zr, reveal that the shales originated from felsic source rock. The Light rare earth elements (LREE) enrichment, Flat heavy rare earth elements (HREE) model, a negative Eu anomaly and positive Ce anomalies, and La/Yb-REE bivariate diagram also reveal sediments derivation predominantly from felsic protolith. The plots of SiO2 versus K2O/Na2Oand Ti/Zr versus La/Sc, and Ti/Zr–La/Sc, Th-Sc-Zr/10, Th–Co-Zr/10, and La-Th-Sc discrimination diagrams show that the shales plotted in the passive margin. The Zr/Sc and Th/Sc ratios and low ICV values show first cycle recycling with heavy mineral enrichment. LREE, with flat HREE distributions, as well as the normalized Gd/Yb and La/Yb enrichment indicate similarity with the post-Archean Australian Average Shales (PAAS). The high values of chemical index of weathering, chemical index of alteration and Plagioclase Index of Alteration indicate that the protolith experienced strong chemical weathering. In conclusion, the study suggest that the investigated shales were derived mainly from felsic source rocks and deposited in passive margin setting similar to the adjacent sedimentary basins in Nigeria and west Africa.