Geology and Geochemistry of Sediments from Lewumeji and Idogun Wells, Eastern Dahomey Basin Southwestern Nigeria

Abstract

Selected composited samples from Lewumeji (0-111m) and Idogun (0- 54m) Abeokuta Group, Eastern Dahomey Basin, were subjected to detailed lithologic and geochemical studies. The studies aimed at determining the lithological sequence, paleoenvironments of deposition, tectonic settings, provenance, weathering history and the classification of sediments. The core samples were subjected to lithological description and geochemical analysis were done following standard procedure through the use of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) through lithium metaborate fusion method.The lithologies from both wells consist of reddish to brown colour, rounded to sub rounded sandstone, brownish clay, and dark grey shale denoting possibly fluvial, brackish, lagoonal and marine environments. The sandstone revealed dominance of quartz which were classified as quartz arenites to sublitharenite and subarkoses, this suggests a recycled orogen source for the sandstones. Shale in the study area shows patches of CaCO₃ indicating the presence of biogenic materials. And the reddish brown colouration of clay suggests ferruginization.  The geochemistry revealed low values for K₂O/Al₂O₃ ratio coupled with high average values of Plagioclase Index of Alteration (81.64% and 73.80%), Chemical Index of Alteration (79.22% and 71.52%) and Chemical Index of Weathering (82.41 % and 75.03%) for Idogun and Lewumeji wells respectively. This values indicate intense weathering condition. The plots of Log (Fe₂O₃/K₂O) against Log (SiO₂/Al₂O₃) revealed sediments dominated by Fe - sand and Fe - shale. Also, Al₂O₃-(K₂O+CaO+MgO)-(Fe₂O₃+MgO), (AKF) ternary plots reveals that the sediments are derived from continental environment. The Log (K₂O/Na₂O) against SiO₂ and the discriminant function plots indicate sediments deposited within the passive margin. Also, the Trace elements ternary plots of Th-Sc-Zr/10 and Th-Co-Zr/10 reveal deposition within Continental Island Arc and Passive Margin. The discriminant function plots characterized the sediments as Mafic Igneous rock and quartoze sedimentary provenances with minor input from Intermediate and felsic igneous provenances. The chondrite normalized REE plots show enrichment of Light REE over Heavy REE in the study area with negative Europium and Cerium anomalies greater than 1. This indicated an oxidising and a shallow marine environments. The REE pattern is similar to those for the Upper Continental Crust sediments (UCC).  Conclusively, the study shows that the sediments in the study area have multiple provenances subjected to high weathering conditions and were deposited within an oxidizing and continental to shallow marine settings.

Keywords: Dahomey Basin, lithostratigraphy, paleoenvironment, Provenance, Tectonic setting.

DOI: 10.7176/JEES/10-5-03

Publication date:May 31^st 2020