Demystifying provenance signatures and paleo-depositional environment of mudrocks in parts of south-eastern Nigeria: Constraints from geochemistry

Abstract

Geochemical signatures of sedimentary rocks provide reliable proxies that allow a better understanding of sedimentary processes. The geochemical compositions of Awgu Shale (southern Benue Trough) and Enugu Shale (Anambra Basin), south-eastern Nigeria, were investigated to appraise the degree of source area weathering and reconstruct the provenance and paleo-depositional environment. High values of several weathering indices such as Chemical index of alteration (CIA; 91.26 ± 3.26), Chemical Index of Weathering (CIW; 98.78 ± 1.42), Plagioclase Index of Alteration (PIA; 98.64 ± 1.62) and the Al2O3-(CaO + Na2O)–K2O ternary relationship indicate intense weathering in the source area. The remarkable enrichment in light rare-earth elements (LREE), the negative Eu anomalies (0.77 ± 0.06) and elemental ratios (La/Sc, La/Co, Th/Co, Th/Cr, Cr/Th, and Th/Sc) diagnostic of provenance suggest derivation from predominantly felsic sources. The results also provide clues of the tectonic settings and suggest deposition on a passive margin. The signals of redox proxies (Ni/Co, V/Cr, Cu/Zn, U/Th, Ce/Ce*) reflect predominantly oxic conditions while the Sr/Ba ratios (0.25–2.04) suggests alternated marine and continental paleo-depositional settings. The inorganic geochemical proxies of weathering, provenance, salinity, and redox conditions reflect an insignificant disparity between the pre-Santonian Awgu Shale and the post-Santonian Enugu Shale.