Geochemical Investigation of the Nanka Formation in Southeastern Nigeria: Proxy for Sediment Provenance and Tectonic Setting

Abstract

This study investigates the geochemical characteristics of the Nanka Formation in the Niger Delta Basin, southeastern Nigeria. The research was carried out in order to reconstruct it's sediment provenance, weathering history, depositional environment, and tectonic setting. Representative sandstone samples collected from the outcrop sections of the formation were analyzed for major and trace elements using X-ray Fluorescence (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) techniques. The results revealed that the major elements are dominated by silica with concentration value ranges of 72.95 and 82.14wt%. Comparison of the major and trace elements concentrations in the investigated sediment with the Upper Continental Crust (UCC) and Post-Archean Australian Shale (PAAS) indicated silica enrichment and depletion of Al2O3, CaO, MgO, K2O and Na2O. Trace elements such as Ni, Co, Zn, Th and Zr were depleted with respect to PAAS. There is also minor enrichment of U and Ba. The sandstones were classified as litharenites and sublitharenites. Geochemical indices such as the Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Alteration (CIW) have value ranges of 46.94 to 57.94%, 55.28 to 74.02% and 50.77 to 66.61% respectively. The value ranges of the indices are consistent with A- CN- K tenery plot of the sandstone, suggesting moderate weathering. The Index of Compositional Variability (ICV) values of between 1.37 and 1.60% indicate immature source rock. The paleo- climatic condition is semi- humid. Nanka Formation is sourced from felsic and intermediate igneous rocks from passive margin and was deposited in a marginal marine setting.