Stable isotope records of the Ediacaran Doushantuo Formation in the eastern Yangtze Gorges and its significance for choronostratigraphy

Abstract: 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 the δ13C profile between different geographic positions of the Eidiacara Doushantuo Formation in the eastern Yangtze Gorges indicate that there are five negative δ13C 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 negative δ13C excursions of the SN1 and SN4 are most obvious, their minimum δ13C 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.