The Geochemical Characterization of Shale Using Multivariate Statistical Technique, Northern Anambra Basin, Nigeria, Implications for Provenance, Paleoweathering and Tectonic Settings

The geochemical study of Campanian-Maastritchtian shales from Mamu and Nsukka Formations, Northern Anambra Basin, Nigeria was conducted aimed at extracting information on their provenance, sedimentary history, source rock area, tectonic settings as well as establishing the influence of palaeoweathering and redox conditions. TiO₂/Al₂O₃ ratio indicates a basaltic-granitic source. TiO₂/Zr ratio also suggests a felsic igneous source. Major oxide dominance showed association with clay. The low MgO and CaO concentration reflects no associated carbonates and dolomitization. The Th/Co, Th/Sc and La/Sc ratios compared with those of basic and felsic rocks from upper continental crust (UCC) and PAAS indicates a felsic source for the shale. Enrichment of some trace elements relative to UCC reflects the abundant occurrence of mafic components in the source area. The high LREEs/HREEs ratio also confirms felsic source rocks. Average concentration of Zr and Ti indicates possible fractionation between source and the point of deposition. The Cu/Zn ratio of <2 and Cu/Mo+Zn of less than 3, suggests oxidization and shallow marine accumulation conditions for these shales. Both A-CN-K and A-CNK-FM plots suggests intense weathering for the shale. The average CIA and CIW values of 93.79 and 99.31 respectively reflect intense source rock weathering and a provenance of clay as well as an absence of detrital feldspars. High alteration indices also imply geochemically and texturally matured sediments formed under semi-humid conditions. K₂O/Na₂O Vs SiO₂ falls within active and passive margin fields. The sediments appear to have been derived mostly from unstable continental areas.