Petrography and geochemistry of Paleocene-Eocene (Ewekoro) limestone, eastern Benin basin, Nigeria: implications on depositional environment and post-depositional overprint

Petrographic and geochemical studies were undertaken on the Paleocene-Eocene limestones of the Ewekoro Formation in order to infer the depositional and post-depositional imprints on the limestone. Thin section petrographic studies revealed three distinct microfacies; grainstone, packstone and wackestone, and there were variations in the depositional environments from meteoric vadose to shallow marine environments with evidences of post-depositional processes of compaction, micritization and dolomitization. The total rare earth element concentration (ƩREE) of the limestone samples ranged from 39.93 to 103.11ppm with negative Ce/Ce∗ values of 0.67–0.87 and depleted europium anomalies of 0.48–0.69. The carbon isotopic signals δ13C ranged from -1.4‰ to +1.9‰ VPDB while the oxygen isotope δ18O is from -3.5‰ to -0.9‰ VPDB except for a positive value of 0.5‰ observed in one limestone sample. The δ13C vs δ18O plot and Mn/Sr ratio suggested that the δ13C measured values are primary in nature and remain unaltered during diagenesis. The δ18O values showed slight alteration with negative values and indicating meteoric diagenesis and the estimated paleotemperature ranged from 19 °C to 39 °C. Ewekoro; Limestone; Stable isotope; Diagenesis; REE.