Depositional conditions of the Upper Cretaceous shales from the Anambra Basin, SE Nigeria: Constraints from mineralogy and geochemistry

Abstract

Thirty-four core shale samples from the Upper Cretaceous Mamu Formation, southeastern Nigeria, were studied using XRD and geochemical analyses to constrain their provenance, tectonic setting, palaeoweathering and palaeoredox conditions during deposition. X-ray diffraction analysis revealed the presence of clay minerals (kaolinite, illite and illite-smectite mixed layer) and non-clay minerals (quartz and feldspar). The dominance of kaolinite and the high kaolinite/illite (K/I) contents indicate warm and humid climatic conditions during sediment accumulation. The total organic carbon (TOC) content of the shales ranges between 0.31 and 55.34%. The high Chemical Index of Weathering (CIW; mean∼98.8%), Chemical Index of Alteration (CIA; mean∼92%) and Plagioclase Index of Alteration (PIA; mean∼98.7%) for the shales, including a high content of kaolinite, indicate intense chemical weathering in the source area. The positive correlations between V, Cr, Ni, Co, Ga, Cd, Be, Sc, TiO2, MgO with Al2O3 and Ba with K2O suggest that these elements in the shales are mainly held in clay minerals. The elevated contents of Zr and the high Zr/Cr and Zr/Sc ratios imply that the shales were derived from predominantly felsic source rocks. However, TiO2/Zr and Y/Ni ratios suggest a derivation from an admixture of complex felsic and mafic basement rocks. The low average enrichment factor values of Mo (MoEF), V (VEF) and U (UEF), along with low concentrations of redox-sensitive trace element ratios (Mo/U, Ni/Co, V/Cr and Cu/Zn) and high TOC contents infer oxic conditions during deposition. Furthermore, Fe2O3–S-TOC relationships, along with DOPT, V/(V + Ni) and Fe2O3/Al2O3 ratios, suggest dominantly oxic to slightly dysoxic/anoxic conditions during the accumulation of the Mamu Formation shales. Binary plots of K2O/Na2O ratio versus SiO2 indicate a passive margin tectonic setting for the shales.