Weathering and tectonic settings of sediments, source area, maturity, metal enrichment and distribution in parts of Oban massif, southeastern Nigeria

This study explores weathering and tectonic settings of sediments, source area, maturity, metal enrichment and distribution using geochemical data from elemental analysis of major oxides and trace metals in sediments within parts of the Oban Massif, southeastern Nigeria. Stream sediment samples were collected and taken to the Department of Pure and Applied Chemistry laboratory, University of Calabar for analysis using Atomic Absorption Spectrophotometer (AAS). The major oxides are SiO2, Al2O3, Na2O, FeO, MgO, K2O, CaO, and TiO2, while trace metals analyzed include Fe, Cr, Co, Cu, Pb, Ni, and Zn. Result showed variation in the concentration of major oxides is in the order; SiO2 > Al2O3 > Na2O > Fe2O3 > K2O > CaO > FeO > MgO > TiO2., while that of trace metals is in the order; Fe > Co > Pb > Cr > Ni > Cu > Zr. Bivariate plots revealed relatively high amount of silica (SiO2) content which is typical of a matured sediment. Indexical approach revealed that values obtained for pollution load index (PLI) and geo-accumulation index (Igeo) indicates there are no pollution, suggesting that the sediments are geochemically pristine. There is also low contamination of all the trace metals analyzed, the only exception is Co which revealed considerable contamination owing to the computed values obtained. However, localized enrichment of trace metals obtained may possibly be due to natural mineralization or lithological variations other than anthropogenic inputs, since the study area consists predominantly of uninhabited farmlands and forest where human activities are minimal. The A-CN-K (Al2O3-CaO+Na2O-K2O) ternary plot indicates that the dominant crust from whence these sediments are derived consists of felsic to intermediate igneous rocks. The diagram of chemical index of weathering (CIW) versus chemical index of alteration (CIA) showed that the samples plotted between the low silicate weathering to no silicate weathering fields, and this indicates that there was low to moderate chemical alteration of primary silicates. The plot of K2O/Na2O versus SiO2 indicates a mixed tectono-provenance, where majority of the samples (75 %) come from the passive margin (PM), while the remaining (25 %) are source from active continental margin (ACM).