The Early and Middle Jurassic Marrat Formation in Kuwait was deposited along the southwestern margin of the Neo-Tethys Ocean. The formation has been extensively cored and sampled in deep wells by Kuwait Oil Company for sedimentological and biostratigraphic analyses. Based on mainly long-ranging benthic species, the Middle Marrat Member is assigned in published articles to the Toarcian Stage. The formation has also been sampled for stable carbon and oxygen isotopic analyses in 19 wells across a depositional profile extending from sabkha to intrashelf basin. Stable carbon isotope analyses from these wells (∼2000 samples) reveal the signature of the negative Pliensbachian-Toarcian carbon-isotope excursion (Pl-T-CIE) in the uppermost part of the Lower Marrat Member. The younger early Toarcian carbon-isotope excursion (T-CIE) is recognized in the lower part of the Middle Marrat Member. A composite δ13Ccarb curve spanning the Middle Marrat Member is constructed by combining overlapping δ13C segments from six wells guided by correlations of regional sequence boundaries and gamma-ray logs. Cyclostratigraphic analyses of gamma-ray logs in these six wells reveal a periodic signal formed by multiples of the 0.405-Ma long-eccentricity cycle. This result suggests the Middle Marrat Member in Kuwait may represent the complete Toarcian Stage consisting of depositional sequences controlled by sea-level cycles with durations ranging between 0.8 and 1.6 Ma. Correlations of the Toarcian succession between Kuwait, Saudi Arabia and Oman are hampered by imprecise biostratigraphic control. This limitation is bypassed by correlating the T-CIE in Kuwait to published δ13C records in the Marrat Formation in central Saudi Arabia and the Mafraq Formation in Wadi Sahtan in Oman. The correlations indicate that whereas the Toarcian Stage is apparently complete in Kuwait, most of the middle and upper Toarcian and the Aalenian are absent in Oman and Saudi Arabia.

中文翻译：

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科威特 Toarcian 的碳同位素地层


科威特的早侏罗世和中侏罗世 Marrat 组沉积在新特提斯洋的西南缘。科威特石油公司已在深井中对该地层进行了广泛的取芯和取样，用于沉积学和生物地层学分析。主要基于长分布的底栖物种，Middle Marrat 成员在已发表的文章中被归入 Toarcian 阶段。该地层还被采样，在从 sabkha 到大陆架盆地的沉积剖面中的 19 口井中进行稳定的碳和氧同位素分析。来自这些井（∼2000 个样品）的稳定碳同位素分析揭示了下 Marrat 段最上部负 Pliensbachian-Toarcian 碳同位素偏移 （Pl-T-CIE） 的特征。较年轻的早期 Toarcian 碳同位素偏移 （T-CIE） 在中 Marrat 段的下部被发现。在区域层序边界和 γ 射线测井的相关性指导下，通过组合来自 6 个孔的重叠 δ13C 段，构建了跨越中 Marrat 段的复合 δ13Ccarb 曲线。对这六个井中伽马射线测井的旋回地层分析揭示了由 0.405-马 长偏心率周期的倍数形成的周期信号。这一结果表明，科威特的Middle Marrat成员可能代表了完整的Toarcian阶段，由海平面周期控制的沉积序列组成，持续时间在0.8至1.6 马之间。科威特、沙特阿拉伯和阿曼之间的 Toarcian 演替的相关性受到不精确的生物地层控制的影响。通过将科威特的 T-CIE 与沙特阿拉伯中部的 Marrat 组和阿曼 Wadi Sahtan 的 Mafraq 组已发表的 δ13C 记录相关联，可以绕过这一限制。 相关性表明，虽然科威特的 Toarcian 阶段显然已经完成，但阿曼和沙特阿拉伯没有大部分中上 Toarcian 和 Aalenian 阶段。