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 δ(13)C ranged from -1.4‰ to +1.9‰ V(PDB) while the oxygen isotope δ(18)O is from -3.5‰ to -0.9‰ V(PDB) except for a positive value of 0.5‰ observed in one limestone sample. The δ(13)C vs δ(18)O plot and Mn/Sr ratio suggested that the δ(13)C measured values are primary in nature and remain unaltered during diagenesis. The δ(18)O values showed slight alteration with negative values and indicating meteoric diagenesis and the estimated paleotemperature ranged from 19 °C to 39 °C.