Abstract Cenomanian – Turonian (CT) Afowo shales selected from offshore X, and coastline Orimedu-1 and Ise-2 exploratory wells in the Dahomey Basin, southwestern Nigeria, were analyzed for foraminifera, major oxide and trace elements to evaluate their provenance and paleoenvironments. The hydrocarbon rich calcareous CT shale is about 100m thick in the coastline area and 300m thick in offshore area with abundant of marine planktonic and benthonic foraminifera. Benthonic species were significantly present in the coastline wells suggesting a shallow marine condition probably a neritic to inner shelf environments while abundant deep water Rotaliporid sp. and shallow water fauna heterohelix sp. in X well suggest inner shelf to bathyal (ca. >350 m) depositional environments. SiO2 and Al2O3 are the most abundant oxides of CT shale with average value of 46.4% and 12.7% in X, 57.1% and 15.65% in Orimedu-1 and 55.3% and 13.1% in Ise-2 respectively suggesting high influx of terrigenous and argillaceous sediments at the coastal area. Average Al2O3/TiO2 of 19.2 in X, 17.8 in Orimedu-1 and 19.3 in Ise-2 indicates that the sediments were sourced from intermediate igneous rock. Favorable oxic conditions for chemical weathering is more pronounce in the manganese concentration of 658–937 ppm in Orimedu-1 and 829–838 ppm in Ise-2 than 364–604 ppm in the shales from X well. Their degree of weathering estimated from chemical index of alteration (CIA) averaging 97 in the coastline wells is higher than 61 in X well, thus indicating high degree of oxidation in the depositional environments. Vanadium to nickel ratio ranging from 2.88 to 5.0 in X well suggest mixed marine and terrigenous source under dysoxic to oxic conditions for the shales while 0.48–1.2 values in Orimedu-1 further indicates a prevailing oxidizing condition at the time of deposition. Dysoxic-oxic and moderate deep-water conditions were more favorable in the offshore area and probably a significant paleo-factor for organic matter preservation of CT shales in the basin.

中文翻译：

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尼日利亚达荷美盆地 Cenomanian-Turonian Afowo 组页岩的地球化学和古生态学：对物源和古环境的影响

摘要 对从尼日利亚西南部达荷美盆地的海上 X、海岸线 Orimedu-1 和 Ise-2 探井中选取的 Cenomanian - Turonian (CT) Afowo 页岩中的有孔虫、主要氧化物和微量元素进行了分析，以评估它们的来源和古环境。海岸线区富烃钙质 CT 页岩厚约 100m，近海区厚约 300m，海洋浮游和底栖有孔虫丰富。底栖物种在海岸线井中显着存在，表明浅海条件可能是内陆架环境的浅海环境，而深水 Rotaliporid sp。和浅水动物heterohelix sp。在 X 井中建议内陆架到深海 (ca. >350 m) 沉积环境。SiO2 和 Al2O3 是 CT 页岩中含量最丰富的氧化物，X 中的平均值分别为 46.4% 和 12.7%，Orimedu-1 中的平均值为 57.1% 和 15.65%，Ise-2 中的平均值分别为 55.3% 和 13.1%，表明陆源和泥质的大量涌入沿海地区的沉积物。X 中的平均 Al2O3/TiO2 为 19.2，Orimedu-1 中为 17.8，Ise-2 中为 19.3，表明沉积物来自中级火成岩。Orimedu-1 中 658-937 ppm 和 Ise-2 中 829-838 ppm 的锰浓度比 X 井页岩中的 364-604 ppm 对化学风化有利。根据化学蚀变指数（CIA）估计，海岸线井平均为 97 的风化程度高于 X 井的 61，因此表明沉积环境中的氧化程度很高。钒镍比为 2。X 井中的 88 至 5.0 表明页岩在缺氧至含氧条件下海相和陆源混合，而 Orimedu-1 中的 0.48 至 1.2 值进一步表明沉积时普遍存在氧化条件。近海区缺氧-好氧和中等深水条件更有利，可能是盆地CT页岩有机质保存的重要古因素。