Sedimentary geochemical study of Ledia claystone: implication on provenance, paleoenvironment of deposition and claystone classification, Gwandu Formation, Sokoto Basin, Northwestern, Nigeria

Abstract

This study combines sedimentary and geochemical studies of Ledia Claystone outcrops in Argungu area, Sokoto Basin, Nigeria. The study was carried out to improve current understanding of the depositional conditions, provenance as well as the classification of the sediments. The methods employed include both field description and geochemical study using the Atomic Absorption Spectrometry and flame photometry. The lithostratigraphic section consists of a fining-upward sequence of beds, where the basal bed is made up of fine laminated herringbone structure sandstone, followed by claystone facies; intercalation of silty claystone and a massive variegated bioturbated claystone, capped by oolitic ironstone. Geochemical analytical results of the claystone revealed a quantitative concentration of SiO₂ and Al₂O₃ varying from 57.19 to 83.4 % and 3.15 to 29.5 % respectively. Relatively high concentrations of Fe₂O₃ (4.6 %) indicate ferruginization, while the CaO and P₂O values of 1.62 mg/l and 0.37 mg/l respectively are indicative of deposition in marine environment due to dissolved diagenetic calcite cement. Relatively moderate values of MgO (8.63 mg/l) and CaO (1.62 mg/l) in the samples indicated the presence of calcite. All other oxides decreased with an increase in SiO₂ (67.75 %) due to mineral dissolution with distance of deposition. In particular, the Al₂O₃/TiO₂ ratio from Samples I and 2 of Ledia Claystone had 23.64 and 22.69 values respectively: suggestive of felsic igneous rock source. However, Sample 3 with Al₂O₃/TiO₂ ratio of 3.16 is indicative of mafic igneous rock source. The low Ni content (1.034) was suggestive of a felsic provenance source, while low values in the Cu/Zn and Ni/Co ratios 4.310 and 0.301 respectively were indicative of deposition under oxidizing conditions. The Log (Fe₂O₃/K₂O) against Log (SiO₂/Al₂O₃) shows that Samples 1 and 2 are claystone while Sample 3 is sandstone, classified between sub-arkose and litharenites facies.