Abstract

OKUNLOLA, Olugbenga A  and  IDOWU, Olubunmi. The geochemistry of claystone-shale deposits from the Maastritchian Patti formation, Southern Bida basin, Nigeria. Earth Sci. Res. J. [online]. 2012, vol.16, n.2, pp.139-150. ISSN 1794-6190.

An inorganic geochemical study of a claystone and shale sequence from the Patti Formation around Ahoko in the Southern Bida basin, Nigeria, was carried out to determine the basin’s depositional conditions, provenance and tectonics. Representative samples underwent mineralogical and geochemical analysis involving major, trace and rare earth element analysis. Mineralogical studies using X-ray diffraction analysis revealed prominent kaolinite, dickite and illite peaks; accessory minerals included quartz and microcline. Major element abundance showed that the shale samples had SiO₂ (61.26%), Al₂O₃ (16.88%) and Fe₂O₃ (3.75%) constituting more than 79% of bulk chemical composition whilst claystone samples contained SiO₂ (67.79%), Al₂O₃ (17.81%) and Fe₂O₃ (1.67%). Higher SiO₂, Ba, Sr, V, and Nb concentrations were observed in claystone samples rather than shale whereas the shale samples were observed to be more Zr-, Ni- and Zn-enriched than the claystone ones. The shale and claystone samples showed slightly light rare earth enrichment and slightly flat heavy rare earth depleted patterns having a negative Eu and Tm anomaly. High TiO₂ and Rb/K₂O values also indicated that the shale and claystone samples were matured. Geochemical parameters such as U, U/Th, Ni/Co and Cu/Zn ratios indicated that these shales were deposited in oxic conditions; the Al₂O₃/TiO₂ ratio suggested that intermediate igneous rocks were probable source rocks for the shales, while mafic rocks were suggested as being source rocks for the claystone. However, the La/Sc, Th/Sc, Th/ Co ratios and shales and claystone plots revealed that they came within the range given for felsic rocks as provenance, thereby suggesting mixed provenace for the sediments. A passive-margin tectonic setting was adduced for the sedimentary sequences.

Keywords : geochemical; inorganic; shale; claystone; provenance; tectonic.

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