Stratigraphic Geochemistry Of The Late Carboniferous To Early Permian In Kake,Xinjiang

The composition and distribution of elements in carbonate rocks are controlled by their own chemical properties and paleo-seawater environment.The use of this property of carbonate to invert the sedimentary environment and the compositional evolution of paleo-seawater is a hot topic in sedimentary stratigraphy.The Carboniferous-Permian section of the Kake area in the northwestern margin of the Tarim Basin in Xinjiang has continuous carbonate deposition and good outcrop,and is rarely modified by late tectonic movement.It is a scientific issue for studying carbonate sedimentation and diagenetic environment and restoring ancient seawater composition.Based on field investigations,this study carried out sedimentary facies and sequence stratigraphy on typical sections of the study area,and systematically collected samples for fossils of paleontology and major traces and rare earth elements.The results are as follows:According to the fossils zone identified by the paleontological fossils taken in this section,the geological age of the study interval was determined.The Beedeina-Fusulinella was identified in the third section of the Biegentawu Formation,and its era was determined to be the late Moscovian Stage;the Triticites distributed in the Kangkelin Formation,the era was determined to be the Gzhelian Stage;the Pseudoschwagerina distributed in the second section of the Kangkelin Formation And the frist section of Kunkelaqie Formation,which identified its era as the Gescher Stage to the Asselian Stage;the Pseudoschwagerina and the Eoparafusulina-Parafusulina identified in the second section of the Kunkelaqie Formation identified its era as the Asselian Stage to the Sakmar Stage.Through the analysis of rare earth elements,the difference between the third section of the Biegentawu Formation and the second section of the Kunkelaqie Formation showed LREE loss(mean?Nd/Yb?_SN=0.31 to 0.69)and positive Gd?1.03 to 1.08?.It is obviously higher than the typical Y/Ho ratio in the chondrite?47.92⁵1.02?and other typical seawater rare earth partitioning patterns,indicating that the rare earth source mainly inherits from seawater;the rare earth partitioning pattern of the Kangkelin Formation and the Kunkelaqie Formation,Although it also shows certain anomalies,the anomaly tends to be mild,and the rare earth curve is almost flat,indicating that the rare earth is derived from seawater but is affected by terrestrial materials.The?Ce and?Eu anomalies combined with sedimentary facies analysis suggest that the carbonate rocks in this area are mainly deposited in shallow sea shelf,carbonate platform and localized platform and other sedimentary environments where the water depth is shallow and seawater can be oxidized.Comparing the third segment of the Biegentawu Formation with the less interference from terrestrial debris and the second segment of the Kunkelaqie Formation:the negative?Ce anomaly in the rare earth and the concentration of each rare earth element are significantly larger in the deeper part of the water body.The Carboniferous-Permian is in the global glacial period.Since the Late Carboniferous,there have been many small glacial-interglacial cycles in various regions,and sea levels have risen and fallen many times during this period.This in turn affects the increase and decrease of terrigenous debris.During this period,there is a good coupling between the Y/Ho,Er/Nd,?Nd/Yb?_SN,?La and?¹³C isotopes on the profile.From the third section of the Biegentawu Formation to the second section of the Kunkelaqie Formation,there is a regular change.Y/Ho and Er/Nd decrease with the increase of the degree of mixing,and?Nd/Yb?_SN and?La are mixed.The increase in the degree of dyeing increases,indicating that they are effective indicators?ratio?of whether seawater is contaminated with terrestrial debris.Through trace element analysis,carbonates contaminated with terrestrial materials are enriched in Li,Be,Cr,Cu,Zn,Ga,Rb,Sr,Cs,Ba,Sc,and Th compared to purer carbonates.Sc U,Ti elements,analysis of the correlation of enriched elements can be clearly seen:?REE,Sc,Th,Ti,Be,Ga,Cs,Li and Rb and other elements have a good correlation,the correlation coefficient can reach 0.80 the above.The correlation between some elements is more prominent,and the correlation coefficient can reach 0.90 or more.It indicates that the high content of water insoluble elements?REE,Sc,Th,Ti,Be,Ga,Cs,Li and Rb is better coupled with carbonate rocks with higher terrigenous content.These elements can be used as indicator carbonic acid.Whether the salt rock is contaminated by terrestrial materials.