Abstract
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 SiO2 having a percentage (%) range value of 45.35%-96.98% and Al2O3 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 Al2O3 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, P2O5, V and Mo, the rocks analyzed have been subdivided into two (2) geochemical packages and three (3) geochemical units. Ratios of SiO2 and Al2O3 indicate three (3) sedimentary facies namely sandstone, siltstone and claystone while the variation in the ratios of Ga/Rb and Al2O3/(Na2O+CaO+K2O+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/Al2O3, TiO2/Nb, Nb/Al2O3 as well as the discriminant function analysis.
Highlights
Sedimentary rocks are faithful records of the sensitive and subtle changes in provenance, environment of deposition and post-depositional changes
The results presented above are interpreted to show both the lateral and vertical changes in the inorganic geochemistry of the lithostratigraphic units present in the study area
From the changes in the inorganic geochemistry of the study area, it can be concluded that the study area is composed of three main sedimentary facies which are; Sandstones, Siltstones and Claystones
Summary
Sedimentary rocks are faithful records of the sensitive and subtle changes in provenance, environment of deposition and post-depositional changes. A stratigraphic record is the outcome of an exogenic system (one derived externally) consisting of geologic setting, changes in sea level, changes in geochemical reactions between the sea water and rock, climate and processes of sediment formation (Srinivasan, 1989), the ensuing sedimentary record, shows differences in bulk chemistry. As these changes produce different combinations of minerals, there are distinctive major and trace elemental compositions of the sediments as well which are observable and can be used for the correlation and characterization of these rocks; this in turn forms the basis of chemostratigraphic application in sub-dividing strata (Das, 1997). The concept of chemostratigraphy holds good, with an ability to judge these differences, www.ccsenet.org/jgg
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