Geochemical Investigation of the Quaternary Coastal Plain Sediment Deposit, Ikoyi-Victoria Island Lagos, Eastern Dahomey Basin, Southwestern Nigeria. – I. M. Akaegbobi, A. O. Ola-Buraimo, and P.E.Ajaino
Geochemical study was undertaken on coastal sediments retrieved from 3 wells- AP1 (60m), AP2 (48m), and AP3 (65m) deep; of which 42 representative samples were selected for lithological description and paleoenvironment of deposition. Ten samples were used for geochemical analyses with the view towards determining their provenance, extent of weathering, paleoenvironment of deposition, tectonic setting and paleoclimatic condition of the quaternary sediments in Ikoyi-Victoria Island Lagos, Nigeria. Laboratory analyses include the adoption of Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) and Inductively Coupled Mass-Electron Spectroscopy (ICPES).Lithologic sections of the three wells are composed mainly of clay, sandstones and heteroliths of the facies varying from coarse to fine size particles. The argillaceous sediments were deposited in lagoonal setting of relative low energy compared with the sandstones deposited in barrier islands characterized by higher marine water currents. The geochemical results reveal maturity index of 8.9 % and high values (>75 %) of Plagioclase as indicated by Index of Alteration (PIA), Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW); Na2O/Al2O3, Na2O/K2O, Na2O/TiO2 and Sr/Ba indicate intense weathering and humid climate at source. Discrimination function plots support that the sediments were derived from quartzose sedimentary provenance of recycled source and matured in nature. The plots of Th-Co-Zr/10 and log(K2O/Na2O) versus SiO2 showed and defined the sediments to belong to passive margin setting and partly active marginal region. Condrite normalized REE plots depict sediment enrichment in REE over heavy REE with strong negative EU anomaly which suggests felsic source for the sediments. However, variation plots show a similar and regular accumulation pattern for Al, K, Ti, Na, Mg and Fe; implying contemporaneous sediment influx rate. The mature quartzose sediments of the studied wells were derived from the adjoining basement rocks, felsic in nature and sourced under intense weathering of a tropical humid climatic condition.