Mineralogy and Geochemistry of Shales of Mamu Formation in Nigeria: Effects of Deposition, Source Rock, and Tectonic Background

Abstract

Study of lithofacies identification, geochemical characterization of shales is vital to the provenance, paleoweathering, and tectonic setting reconstruction. The combination of morphological analysis, bulk chemical analysis and in-situ multi-element analysis was used to investigate the provenance, source area weathering, and depositional setting of outcropped Maastrichtian shale sequence of the Mamu Formation, Anambra Basin in Nigeria. Ten representative shale samples were examined by scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS). Geochemical analysis was performed by X-ray fluorescence (XRF) Spectroscopy and Laser Ablation-Induced Coupled Plasma Mass Spectrometry (LA-ICPMS) techniques. The structural and morphological development of kaolinite in the outcropped shale samples of Mamu Formation is due to mechanical disintegration during transportation and re-deposition. Major oxides such as SiO2, Al2O3 and Fe2O3 constitute greater than 86% of the bulk composition. The weathering indices suggest highly weathered source materials. The plot of Cr versus Ni indicated the studied samples are Late Archean shale. Binary plots of trace elements suggest derivation from acidic or felsic sources rather than intermediate or basic source rocks. Ternary plot of Na2O + K2O, SiO2/10 and CaO + MgO indicated multiple sources such as felsic igneous rocks or recycled residues of quartz-rich. Tectonic discrimination diagram depict a typical Passive Margin field.