Geochemistry of the paleocene limestones of ewekoro formation, eastern Dahomey Basin, southwestern Nigeria: implication on provenance and depositional conditions

Abstract

Thirty three limestone samples of the Paleocene Ewekoro Formation from exploratory drill cores within Ibeshe cement quarry, southwestern Nigeria were investigated for major, trace and rare earth elements abundances using inductively coupled plasma spectrometry (ICP-MS) analytical method. The aims were to investigate the depositional settings and source of rare earth element (REE). The major oxides concentration showed significant variations among the limestone facies. Higher SiO₂, Al₂O and Fe₂O₃ contents were observed in the marly and sandy facies than the fossiliferous limestone, whereas the fossiliferous facies were enriched in CaO content. The trace elements content of the limestones normalized with the Post-Archean Australian Shale (PAAS) values revealed significant enrichment in Nb, Sr and U, whereas Ba, Th, Rb and other trace elements contents were depleted. The PAAS normalized REE + Y of the limestones displayed relatively uniform patterns of (i) slightly enriched LREE; (ii) positive Ce anomaly; (iii) positive Gd anomaly; (iv) positive Eu anomaly (v) high Y/Ho ratio. The characteristics non-seawater-like REE patterns, elevated ΣREE, high La_N /Yb_N ratios and high Y/Ho ratios, suggested that variations in REE contents were mainly controlled by the amount of terrigenous contaminations in the limestones. Ratios of La/Sc, La/Co, Th/Co, Th/Cr, Cr/Th and Th/Sc suggested that the terrigenous inclusions in these limestones were derived mainly from intermediate to felsic source rocks. The negative correlation of Sr with Mn and positive correlations of Eu with elements such as Zr, Y, Th and Hf suggested that the observed positive Eu anomalies in the limestones might be due to diagenetic processes. The geochemical parameters such as Ce anomaly, authigenic U, Mn*, V/(V+Ni), V/Ni ,Th/U ratios indicated that the limestones of Ewekoro Formation were deposited in restricted to open shallow marine environments under fluctuating oxidizing to anoxic conditions.

Keywords: Ewekoro Formation, Rare Earth Elements, Depositional Condition, Provenance, Dahomey Basin