Geochemical elements as provenance and paleoenvironmental indicators in siliciclastic sediments of Neogene Gwandu formation exposed in Birnin Kebbi Area, Northwestern Nigeria

Abstract

Sediments from the exposed Gwandu Formation in Birnin Kebbi area (Northwest Nigeria) were analysed for their geochemical composition in order to determine their provenance, and paleoenvironments of deposition. Inductively Coupled plasma -mass spectrometer (ICP-MS) and -emission spectrometer (ICP-ES) were used for trace and major elements quantification. Also, samples were subjected to X-ray diffractometric analysis for the determination of their mineralogical constituents, and total sulphur (TOS) and total organic carbon (TOC) were determined using LECO analysis. High intensity of chemical weathering was inferred, as calculated values for chemical index of alteration (CIA), plagioclase index of alteration (PIA) and chemical index of weathering (CIW) were all above 90. Quartz, kaolinite, hematite and rutile in relative order of decreasing abundance, are the mineral constituents of the sediments. Bivariate plot of Fe₂O_3T vs MgO and the ternary diagram of MgO- Fe₂O_{3T –}SiO₂ /Al₂O₃ revealed that sediments were sourced from non-marine and deltaic environments. High LREE/HREE ratios indicate relative depletion in the heavy rare earth elements (HREE), and with all values of europium anomaly below unity, it is logical to infer that sediments are predominantly constituted by felsic materials. Bivariate plot of TiO₂ vs Zr showed that approximately 93 % of the samples are dominantly felsic 2 constituents. Also, ratios of Cr/V, Y/Ni, Th/U and the scatter plot of Th-Sc have suggested that sediments are constituted substantially by Upper continental crustal materials, and predominance of lower continental crustal components in few cases. The ratios of TOC/S suggested non-marine to marginal marine environments of deposition. Whereas, U/Th ratios implied oxic paleoenvironment for all strata, the V/Cr ratios inferred 71.43 % of the strata from Gwandu Formation were emplaced under oxidizing condition. Conversely, V/Sc ratios revealed that 78.57 % of the samples are from strata emplaced in oxygen depleted environment. Inferring from the various provenance indices, sediments of Gwandu Formation composed dominantly of materials from the upper continental crust, and low but considerable proportion from lower continental crust. Also, the application of ratios of redox sensitive elements has unveiled that sediments were emplaced principally under oxic conditions.