峡东埃迪卡拉系陡山沱组稳定碳同位素记录及其年代地层意义

摘要

提基于峡东秭归青林口、泗溪和宜昌晓峰河埃迪卡拉系下部陡山沱组剖面岩石、层序和精细的碳同位素地层研究，在埃迪卡拉系下部陡山沱组底部、下部、中部、上部和顶部分别识别出5次以灰岩出现或黑色页岩增厚为标志的海侵事件，划分出5个可以进行区域对比的三级层序，自下而上分别命名为BDSS，LDSS，MDSS，UDSS和TDSS。以层序界面为对比标志，峡东埃迪卡拉纪不同古地理部位碳同位素组成变化的对比显示峡东埃迪卡拉系陡山沱组中内部发育5次可以进行区域乃至全球对比的碳同位素负异常，分别是分布在BDSS下部的SN1，LDSS上部的SN2，MDSS上部和顶部的SN3和SN4，以及分布在UDSS上部的SN5。其中SN1和SN4碳同位素负偏离最为明显，最小δ13C分别达到−4‰~−5‰和−9‰~−10‰，分别与Marinoan冰期上覆盖帽白云岩以及Gaskier冰碛沉积上、下地层中的碳同位素组成特点接近。SN4是埃迪卡拉系最晚冰期的产物。与SN1是埃迪卡拉系底界划分对比的标志相对比，SN4的出现应该是埃迪卡拉系内部再划分中另一个不可忽视的界线标志。%The litho-, sequence and high-resolution stable isotopic stratigraphy of the Lower Eidacaran Doushantuo Formation in the Qinglingkou segment as well as Zigui, Xiaofeng and Sixi segments of Yichang were studied in this paper. Five transgressive events characterized by the occurrence of limestone or thickened black shale and five third-order sequences in ascending order named BDSS, LDSS, MDSS, UDSS and TDSS were recognized in the base, lower, middle, upper and top parts of the Lower Eidacaran Doushantou Formation. According to the features of the sequence boundary, the correlations of theδ13C profile between different geographic positions of the Eidiacara Doushantuo Formation in the eastern Yangtze Gorges indicate that there are five n negativeδ13C excursions, which are comparable in the region and even in the global scale. They are SN1 in the lower part of the BDSS, SN2 in the upper part of the LDSS, SN3 and SN4 in the upper and top parts of the MDSS, and SN5 in the upper part of the UDSS. Among them, the negativeδ13C excursions of the SN1 and SN4 are most obvious, their minimumδ13C are separately close to−4‰ to −5‰ and −9‰ to −10‰, and separately very similar to data of the carbon isotope in the cap dolostone overlaying the Marinoan tillite and the strata related to the Gaskier glacial period. The formation of the SN4 was attributed to the results of Gaskier glacial period. A comparison with the significance of the SN1 as the mark of the basal Ediacaran System shows that SN4 might have been another important boundary mark in the subdivision of the Ediacaran System.