塔里木盆地柯坪地区玉尔吐斯组同位素地层学研究——基于碳、锶同位素的化学地层对比与时代限定

Isotope stratigraphy of Yurtus Formation in Keping area, Tarim Basin: chemical stratigraphic correlation and age constraints based on Carbon and Strontium isotopes

  • 摘要: 新元古代晚期—寒武纪早期是地质历史上的关键时期。新疆塔里木盆地柯坪地区埃迪卡拉系—寒武系地层出露较好,但化学地层学研究还相对薄弱。选取了新疆塔里木盆地柯坪地区的肖尔布拉克西沟和昆盖阔坦剖面下寒武统玉尔吐斯组碳酸盐岩为研究对象,通过碳、锶同位素的地球化学研究进行化学地层学的区域地层对比以及全球地层对比,限定了玉尔吐斯组地层的形成时代。样品的δ13Ccarbδ18Ocarb关系性图、δ18Ocarb、Mn/Sr等地球化学特征表明,绝大多数样品的δ13Ccarb87Sr/86Sr未受到后期成岩作用的改造,保留了沉积时的海水特征。样品的87Sr/86Sr与Sr含量、Rb含量的关系图显示溶样过程中没有大量硅酸盐组分的加入。无机碳同位素结果显示,两个剖面玉尔吐斯组碳酸盐岩中记录了两次碳同位素负漂移事件,分别位于灰岩—页岩互层段底部和上部泥质灰岩段,并能够与柯坪地区其他剖面进行对比。玉尔吐斯组碳酸盐岩87Sr/86Sr表现出“先下降,后上升”的变化特征,可以与摩洛哥地区和西伯利亚东南部地区的87Sr/86Sr在托莫特阶的变化对应。在玉尔吐斯组内未识别出全球碳同位素正异常ZHUCE事件,结合87Sr/86Sr变化特征和古生物资料,认为玉尔吐斯组内存在沉积间断并导致第二阶中下部地层的缺失。通过碳、锶同位素地层对比并结合前人研究资料,认为玉尔吐斯组中部白云岩段位于幸运阶与第二阶界限附近,灰岩—页岩互层段直到玉尔吐斯组顶部属于第二阶晚期到第三阶早期。

     

    Abstract: The Late Neoproterozoic to Early Cambrian represents a critical period in geological history. In the Keping area of the Tarim Basin, Xinjiang, the Ediacaran to Cambrian strata are well-exposed, but research on chemostratigraphy is relatively limited. This study selected the Lower Cambrian Yurtus Formation in the Xiaoerbulake Xigou and Kungaikuotan sections in the Keping area as the research subject. Carbon and Strontium isotope geochemistry were used to establish regional and global stratigraphic correlations and constrain the depositional age of the Yurtus Formation. The geochemical characteristics of the samples, including δ13Ccarb-δ18Ocarb crossplots, δ18Ocarb, and Mn/Sr ratios, confirmed that the δ13Ccarb and 87Sr/86Sr of most samples were not altered by later diagenesis and preserved the seawater characteristics at the time of deposition. Crossplots of 87Sr/86Sr versus Sr content and Rb content of the samples indicated that there was no significant addition of silicate components during the dissolution process. The inorganic carbon isotope results showed that two negative carbon isotope excursions were recorded in the carbonates from the Yurtus Formation in both sections, located at the base of limestone and shale interbeds and in the upper argillaceous limestone member, respectively, which could be correlated with other sections in the Keping area. The 87Sr/86Sr ratios of Yurtus Formation carbonates exhibited a "decline-rise" pattern, which could be correlated with the 87Sr/86Sr variations in the Tommotian Stage of Morocco and southeastern Siberia. The global positive carbon isotope excursion ZHUCE event was not identified within the Yurtus Formation. Combined with 87Sr/86Sr variation characteristics and paleontological data, it is inferred that there was a sedimentary hiatus within the formation, resulting in the absence of the middle and lower Stage 2 strata. Based on carbon and strontium isotope stratigraphic correlations combined with previous studies, it is concluded that the middle dolomite member of the Yurtus Formation lies near the Fortunian and Stage 2 boundary, and the upper limestone and shale interbeds to the top of the Yurtus Formation belong to the late Stage 2 and early Stage 3 of the Cambrian.

     

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