Petrography and geochemistry of Paleocene-Eocene (Ewekoro) limestone, eastern Benin basin, Nigeria: implications on...

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‰ V_PDB while the oxygen isotope δ¹⁸O is from -3.5‰ to -0.9‰ V_PDB 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.