Geochemistry of rocks (Late Cretaceous) in the Anambra Basin, SE Nigeria: insights into provenance, tectonic setting, and other palaeo-conditions

The geochemistry of Late Cretaceous sandstones and shales in the Anambra basin has been investigated by relatively few subsurface data-based studies, thus, there is a need for subsurface geochemical characterisation of the basin's fill provenance, tectonics, and palaeo-conditions (source rocks palaeo-weathering, palaeo-climate, and palaeo-redox). This paper presents the geochemistry of sandstones and shales from cored sections (n = 10) of the Amansiodo_1 well (located in the Anambra basin) with the intent of unravelling the provenance, tectonic setting, and palaeo-conditions. Major elements (n = 10) and the trace element concentrations of Sc, Be, V, Sr, Y, Zr, and Ba were obtained using fusion inductively coupled plasma. Trace (n = 24) and rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) concentrations were obtained using fusion mass spectrometry analytical method. Based on geochemical discrimination diagrams and element ratios such as Al2O3/TiO2, TiO2/Zr, Th/Sc, and La/Th; it is suggested that the studied samples from the Anambra Basin were derived from mixed sources (mafic, felsic, and intermediate) from the adjoining basement rocks, and recycled sediments from the Abakaliki uplift or high. The high-silica and low-silica tectonic multidimensional discrimination diagrams showed that the examined rocks might have originated from rift (69 % probability), collision (24 % probability), and continental arc (7 % probability) settings. The plagioclase index of alteration (PIA), chemical index of weathering (CIW), and chemical index of alteration (CIA) values (average > 80) of studied rock specimens indicate an intensive degree of chemical weathering of the source rocks. The plot of Al2O3 + K2O + Na2O versus SiO2, weathering indices (PIA, CIW, and CIA), and Sr/Cu and Rb/Sr ratios; as well as U, Mo, Th/U, V/Cr, V/Sc, U/Th, and Ce/Ce∗ all suggested warm to hot, humid and/or arid palaeo-climate in an oxic palaeo-redox condition during the accumulation of the examined samples.