Geochemical studies of shales from the Asu River Group, Lower Benue Trough: Implications for provenance and paleo-environment reconstruction

Abstract

Combined use of trace elements, rare earth elements (REEs) with hydrogen (δH) and oxygen (δ¹⁸O) stable isotope composition to elucidate origin and paleo-environment-conditions and reconstruction is a contemporary trend in the field of geochemistry. Geochemical investigations were carried out on the shale deposit from five wells within the Asu River Group Formation, exposed at Ikwo, Lower Benue Trough (LBT). Results of trace elements for the shale deposits examined showed averages of Co (19.10 ppm), Th (16.50 ppm), Zn (103.73 ppm), Sr (203.71 ppm) and Zr (292.80 ppm) compared to PAAS and UCC. Observed negative Eu anomalies, enriched LREEs and depleted HREEs patterns have shown that the shales are from rocks of continental origin. This is provided by Al/(Al + Fe + Mn) with a value > 0.2. Plots of Zr/Sc vs Th/Sc, La/Th vs. Hf and Cr/V vs. Y/Ni indicated felsic igneous rock precursors for the shale samples. C-parameter, ratios of Rb/Sr and Sr/Cu signify paleo-climatic conditions of semi-humid to arid, Ba/Al showed low paleo-productivity of the basin during shale deposition, largely within the freshwater setting according to the ratio of Sr/Ba. V/(V + Ni), U/Th, Ni/Co ratios and Ce/Ce∗ anomalies have revealed an oxic depositional environment with Fe/Ti > 20 suggesting hydrothermal activity. The temperature of formation (120 °C–∼195 °C) coupled with δ¹⁸O (+17 to + 23‰) and δH (−46.90 to −38.80‰) is consistent with materials of sedimentary origin from chemically weathered felsic precursors under humid climatic conditions with an influence of hydrothermal activity.