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

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 δ¹³C_carb-δ¹⁸O_carb crossplots, δ¹⁸O_carb, and Mn/Sr ratios, confirmed that the δ¹³C_carb 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.