Geochemistry of Ahoko claystone-shale sequence, south eastern Bida Basin, Nigeria: implications for tectonic setting, provenance and source area weathering

Abstract

Results of geochemical investigation of shale and claystone samples from two different sections of the exposed Ahoko claystone-shale sequence along the Koton-Karifi/Abaji highway are presented and discussed. Chemical variation of SiO2/Al2O3 and Na2O/K2O is ascribed to varying degrees of weathering of plagioclase rich rock either at the source and/or during transportation to the depositional basin. The high Chemical Index of Alteration (CIA) and Chemical Index of Weathering (CIW) values indicate intense alteration/weathering and removal of mobile cations. The CIW/CIA ratios indicate material that has undergone short distance transportation before final deposition. Besides, the overall Index of Compositional Variability (ICV) values and lower contents of Rb and Cs than upper continental crust (UCC) and post Archean Australian shale (PAAS) of both profiles (i.e. AH-1 and AH-2) indicate intense weathering. The average Th/U ratio is less than the average values for UCC and PAAS, which indicates that the source area is rich in uranium. Furthermore, the average Zr/Sc values are considerably higher than the average value for both UCC and PAAS, suggesting a high recycled sedimentary source. The geochemistry of our samples is consistent with felsic rocks, mixed with some old sediment components and should be derived from UCC. On the log (Fe2O3/K2O) versus log (SiO2/Al2O3) scattergram, the Ahoko claystone-shale samples mainly plot in the shale and Fe-shale fields. The variability in the geochemical classification of our samples is attributed to variation in the relative proportion of matrix, feldspar and lithic components. The major, trace and rare-earth elements assemblage of the studied Ahoko claystone-shale samples plot on various discrimination diagrams, revealing that the tectonic setting belongs to a passive continental margin and a continental island arc.