Chemostratigraphic Characterization of Siliciclastic Rocks in Parts of the Eastern Dahomey Basin, Southwestern Nigeria

摘要

Chemostratigraphy, involving the application of inorganic geochemistry for the characterization of sedimentary sequences has been applied in the study of siliciclastic rocks in parts of the eastern Dahomey basin, Southwest Nigeria, with the aim of characterizing the rocks based on variations in inorganic geochemistry. The present study is based on 15 surface samples carefully selected from exposed sedimentary sequence near Igbile and 5 subsurface samples retrieved from cores of a drilled hole in Ijagun and these were subjected to geochemical analyses using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) to determine the major and trace elements concentration. Results of the geochemical analyses revealed eleven (11) Major elements of which SiO 2 having a percentage (%) range value of 45.35%-96.98% and Al 2 O 3 with a percentage (%) range value of 0.8%-37.49% being the highest major element concentrations. Twenty-four (24) Trace elements were represented with Zr ranging value of between 150.7 ppm-1651.6 ppm being the most significant trace element. Sixteen (16) Rare Earth elements of which Ce ranging in value between 10.8ppm-251.3ppm show the highest concentration. The high percentage concentration of Al 2 O 3 is an indication of abundant clay minerals whereas the high concentration of Zr in parts per million suggests sedimentary reworking of the sediments. High Ce concentration is indicative of oxic conditions. Based on variations in elements and element ratios such as Th/U, Cr, Zr, P 2 O 5 , V and Mo, the rocks analyzed have been subdivided into two (2) geochemical packages and three (3) geochemical units. Ratios of SiO 2 and Al 2 O 3 indicate three (3) sedimentary facies namely sandstone, siltstone and claystone while the variation in the ratios of Ga/Rb and Al 2 O 3 /(Na 2 O+CaO+K 2 O+MgO) depicts fluctuating paleoclimate during the deposition of the sedimentary sequence. Sediment provenance became increasingly felsic from an initial mafic source as inferred from the variation in ratios of Cr/Al 2 O 3 , TiO 2 /Nb, Nb/Al 2 O 3 as well as the discriminant function analysis.