Geochemical Investigation of Enugu Shale, Anambra Basin Nigeria: Implication for Provenance and Paleoenvironment

Abstract

The shale units of the Campanian-Maastrichtian Enugu Formation were sampled from road cuts at Enugu, Anambra Basin, southeastern Nigeria. The study is focused on examining the source rock weathering history, tectonic setting and paleo-environmental conditions of deposition using a high quality bulk elemental composition data of the shale samples. Obtained geochemical data reveal that the protoliths of these shales were most probably felsic in nature based on the average values of SiO (55.67%), Al O (14.00%), K O (1.22%), CaO (0.38%), MgO (0.95%), K O 2 2 3 2 2 (1.22%) and TiO (1.39%) and other geochemical proxies. The values are relatively lower except Fe O (10.63%) 2 2 3 when compared to (NASC) and post-Archean Australian Average Shales (PAAS) values. The normalized REE distribution patterns which displayed relatively depleted LREE but enriched HREE coupled with small concentration of Eu/Eu* also reveal felsic protolith for the shales. The ratio of values of certain trace elements in the studied shales; La/Sc (7.27), Th/Sc (3.07), Th/Co (1.01), Th/Cr (0.65) and Cr/Th (1.67) are comparable to the published average values of the sediments derived from felsic and basic rocks (fine fraction), Upper Continental Crust (UCC) and PAAS and therefore suggest felsic provenance. The plot of Th/Co versus La/Sc and TiO versus Ni is also indicative of felsic protolith for the sediments Weathering indices (mean Chemical Index of 2 Alteration CIA 87.65 and Chemical Index of Weathering CIW 95.21),low K O content (1.22% average), Th/U (7.58 2 average), Th (11.1ppm average), and U (1.57ppm average), and A-CN-K plot suggest these shales were derived from a source subjected to intense weathering. The geochemical parameters such as U, U/Th, V/Cr, Ni/Co and Cu/Zn values obtained suggest that the shales were deposited under oxic environment. The immobile trace element ratios and geochemical discrimination diagrams (K O/Na O vs. Si O; La-Th-Sc) reveals that the protoliths of the Enugu 2 2 2 Formation were derived from a semi-humid to humid region under intense weathering.