Carbon and Oxygen Isotopic Characteristics of Longmendong Formation in Muxu River Section of Qishan, Shanxi Province and Its Significance

Abstract

The lower and upper Longmen cave formation of Late Ordovician in the Muxu river section of Qishan in the southwest margin of Ordos are Sandaogou formation of Middle Ordovician and beiguoshan formation of Upper Ordovician respectively, with continuous and complete strata. The average values of carbon and oxygen isotopes of carbonate rocks are 0.57‰ and -10.73‰ respectively, with poor correlation, which basically preserves the characteristics of original isotopic composition. The paleosalinity Z is between 120 and 130, which reflects that the late Ordovician was a marine sedimentary environment, which is consistent with the geological research results. According to the base value characteristics and drift changes of carbon isotope curve of Longmenshan formation, it can be divided into XH1-XH4 cycle segments. Generally speaking, the early carbon isotope values are mainly positive drift, with large scale and long time; The late stage is dominated by negative drift, with small scale and short time. This reflects that the sea level fluctuated greatly in the early Late Ordovician, with strong activity, and the sea level fluctuated slightly in the late Ordovician, showing a downward trend as a whole. Carbon and oxygen isotope variation curve of longmendong formation and global Ordovician δ13C variation curves shows that longmendong formation is deposited in the late Ordovician, which is equivalent to Sandbian and Katian periods. There is a positive drift event of GICE carbon isotope, which is regionally comparable.