Adelabu I O, Opeloye S A, Oluwajana O A
Department of Applied Geology, Federal University of Technology, Akure, Nigeria.
Department of Earth Sciences, Adekunle Ajasin University, Akungba-Akoko, Nigeria.
Heliyon. 2021 Dec 9;7(12):e08579. doi: 10.1016/j.heliyon.2021.e08579. eCollection 2021 Dec.
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 δC ranged from -1.4‰ to +1.9‰ V while the oxygen isotope δO is from -3.5‰ to -0.9‰ V except for a positive value of 0.5‰ observed in one limestone sample. The δC vs δO plot and Mn/Sr ratio suggested that the δC measured values are primary in nature and remain unaltered during diagenesis. The δO values showed slight alteration with negative values and indicating meteoric diagenesis and the estimated paleotemperature ranged from 19 °C to 39 °C.
对埃韦科罗组古新世-始新世石灰岩进行了岩相学和地球化学研究，以推断石灰岩上的沉积和沉积后印记。薄片岩相学研究揭示了三种不同的微相：颗粒灰岩、泥粒灰岩和泥晶灰岩，沉积环境从大气渗流环境到浅海环境存在变化，并有压实、微晶化和白云石化等沉积后过程的证据。石灰岩样品的总稀土元素浓度（ƩREE）范围为39.93至103.11ppm，Ce/Ce∗负值为0.67 - 0.87，铕亏损异常为0.48 - 0.69。碳同位素信号δC范围为-1.4‰至+1.9‰V，而氧同位素δO范围为-3.5‰至-0.9‰V，但在一个石灰岩样品中观察到一个正值0.5‰。δC与δO的关系图和Mn/Sr比值表明，测得的δC值本质上是原生的，在成岩过程中未发生改变。δO值显示出轻微变化，呈负值，表明是大气淡水成岩作用，估计古温度范围为19℃至39℃。
