Carbon And Oxygen Isotopic Characteristics And Paleoenvironmental Significance Of Majiagou Formation Carbonate Rocks,Ordos Basin

In this paper,the Majiagou Formation in the Ordos Basin is studied based on the data of stable carbon and oxygen isotopes,strontium isotopes,major elements,trace elements,and conodonts,and discusses the palaeoenvironmental characteristics of the Ordovician Majiagou Formation in the study area.The paper mainly achieved the following understanding results:(1)The?¹³C(‰PDB)value of the Ordovician Majiagou Formation in the study area is between-3.21‰?1.2‰,the?¹⁸O(‰PDB)value is between-5.76‰?-9.20‰,and the ⁸⁷Sr/⁸⁶Sr value is 0.708873?0.727977,which is basically consistent with the global isotope stratigraphy?¹³C(‰PDB),?¹⁸O(‰PDB),⁸⁷Sr/⁸⁶Sr value distribution characteristics.The carbon isotope curve of the Majiagou Formation showed an overall downward trend and then an upward trend.The strontium isotope evolution curve showed an overall upward trend and then a downward trend,indicating that the water environment of the Majiagou Formation experienced relatively high salinity concentration during the deposition period.During the period and relative desalination period,the stratigraphic development showed a cyclical trend,responding to the multi-phase transgression and regression process.(2)Conodonts are dominated by claw conodonts,beautiful conodonts and other species,with composite conodonts as the main features,reflecting the shallow water environment of the land and sea at that time.Using the content of trace element Sr,the paleotemperature of the Majiagou Formation is calculated to be approximately 25.73?,and it is inferred that the Majiagou Formation in the study area has a relatively warm climate.The rare earth element data has the characteristics of relatively depleted heavy rare earth elements,relatively enriched light rare earth elements,and obvious negative anomalies of Eu.It is inferred that the Ordovician Majiagou Formation is a weak oxidation-weak reduction environment.(3)The average Sr/Ba value of the Majiagou Formation is 7.42,which has strong sea-source properties.The ⁸⁷Sr/⁸⁶Sr value of the study area is slightly higher than that of the seawater⁸⁷Sr/⁸⁶Sr in the same period.Combined with the high Si and Al content of the study area,the comprehensive analysis is due to the inclusion of terrigenous debris.