Abstract
Two basins (Southern Bida and Northern Anambra Basins) were investigated to deduce weathering, paleooxygenation, provenance, depositional environment and tectonic setting, as well as to establish a relationship between the two basins. The obtained high values of calculated weathering indices such as Chemical index of alteration (CIA > 90), Chemical Index of Weathering (CIW > 90), Plagioclase Index of Alteration (PIA > 90) and the Al2O3-(CaO + Na2O)-K2O ternary relationship for the clay – shale sediments from both basins indicate intense weathering in the source area. Important geochemical ratios such as V/Cr, Cu/Zn, Ni/Co, (Cu+Mo)/Zn, revealed predominantly oxic conditions for the clay – shale sediments from both basins, although, a more reducing or an anoxic condition cannot be ruled out for the clay – shale sediments from the Southern Bida basin due to high ratios of U/Th (1.93-5.67) and Cu/Zn (0.19-5.00). In addition, the Sr/Ba ratios (0.16–3.50) for the clay-shales from the Southern Bida basin indicated an alternated marine and continental paleo-depositional settings and only continental setting (Sr/Ba ratios = 0.22 – 0.50) for the Northern Anambra basin. The Th/Sc, La/Sc, Th/Co and the LREE/HREE ratios showed a derivation of the shale and clay deposits from similar felsic-rich source rock while the log of (K2O/Na2O) vs SiO2, revealed a Passive Margin tectonic setting for the two Basins. There is insignificant differences between the geochemical classifications, weathering, source rock/provenance and tectonic settings of clay-shale sediments of both sedimentary basins, however, there exist slight disparity in their salinity conditions and redox settings.
 Keywords: Geochemistry, Clay-shale, Provenance, Tectonic Setting, Northern Anambra and Southern Bida Basins
Highlights
Geochemical signatures are crucial for determination of prehistoric and depositional events in rocks
The beads are analyzed by wavelength dispersion X-ray fluorescence spectrometry (WD-XRF) for the following oxides and elements determination; SiO2, Al2O3, Fe2O3, CaO, MgO, TiO, P2O5, K2O, MnO and loss on ignition (LOI), Trace elements such as As, Ba, Co, Cr, Cu, Ga, Hf, Nb, Ni, Pb, Sr, Th, U, Zr and rare earth elements such as Heavy rare earth elements (HREE) e.g. Y and Sc and Light rare earth elements (LREE) e.g. Ce, La, Nd and Sm
The major oxide concentrations, values of plagioclase index of alteration (PIA), chemical index of alteration (CIA) and chemical index of weathering (CIW) as well as calculated geochemical ratios for the studied Cretaceous sediments from Southern Bida and Northern Anambra Basins are presented in Tables 1 and 2
Summary
Geochemical signatures are crucial for determination of prehistoric and depositional events in rocks. Previous researchers have addressed questions such as depositional environment, provenance, weathering conditions and tectonic settings of the clay-shale deposits in Southern Bida and Northern Anambra Basins of Nigeria (Okunlola and Idowu, 2012; Odoma et al, 2015; Bolarinwa et al, 2019). The relationship between the two basins on the basis of environment of deposition and variation in chemical constituents remain unresolved. TiO2 with La, Y, Sc, Cr, Th, Zr, Hf and Nb trace elements in combination are powerful tool for provenance and tectonic setting determination due to their relatively low mobility and they are not significantly redistributed in the course of sedimentation, lithogenesis, and metamorphism. Atabo Nathaniel Odoma, Department of Geology, Federal University, Lokoja, kogi State, Nigeria. Sunday Ojochogwu Idakwo, Department of Geology & Mining, Ibrahim Badamasi Babangida University, Lapai, Niger State, Nigeria
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