Palaeoenvironment Of The Carboniferous-Permian Carbonate Rocks In Mandula-Xilinhot,Inner Mongolia

Large numbers of researches about sedimentary environment of the Late Paleozoic in this area have beencompleted.However,there is no unified understanding of the Late Paleozoic carbonate stratigraphic distribution and paleoenvironment in Inner Mongolia area.A large number of overlying strata a are covered on the Paleozoic strata with complex types.The distribution of the Paleozoic carbonate strata and the formed paleoenvironment are the problems to be solved in this study.Usually,the Inner Mongolia region is divided into several large units for research,or for a formation in a certain region.In this case,it is difficult to estublish an integral layer system model in the region,and also impossible to provide more support for the sedimentary environment research.The changes of paleo-ocean environment such as temperature,salinity and paleoproductivity of ancient seawater can be recorded according to the carbon,oxygen isotope and elemental geochemical characteristics of carbonate rocks.However,the current research on the comprehensive geochemical characteristics of carbonate rocks in Inner Mongolia area is not systematic,and the geochemical characteristics and sedimentary environment of the Carboniferous-Permian in this area cannot be summarized.The sedimentary microfacies,isotopes and trace element geochemical characteristics of the Carboniferous-Permian carbonate strata in Mandula-Xilinhot area are systematically analyzed in this paper based on the sampling analysis,thin section analysis,isotope geochemistry and trace element geochemical experiments,combined with geology,geochemistry,and carbonate microscopy.The samples of the Benbatu formation carbonate rocks are mostly micrite in the Mandula and Erenhot areas.No ooides are found in the carbonate rocks of Benbatu formation.The content of palaeontological species is high with the kinds of echinodermatas,bivalves,foraminiferas and crinoids.The characteristics of the Amushan formation carbonate samples are similar to those of the Benbatu formation.The lithologies of the Amushan formation are mostly sparite with sandstone fragment and biodetritus.No ooides are found in the carbonate rocks of Amushan formation.There are few species of carbonate rocks in the Amushan Formation,and mostly are crinoids.The samples of the Zhesi formation carbonate rocks are mostly sparite with a large number of ooides and palaeontological species.The species are echinodermatas,bivalves,brachiopods,gastropods,foraminiferas,anthozoans and crinoids.The contents of crinoids,anthozoans and foraminiferas are high and the algeas are mostly dasycladales.Overall,the water level becomes deeper from the sedemtary period of Benbatu formation to Zhesi formation and the ocean range becomes larger,increasing from the platform to the restricted sea-slope.At the same time,the species are increasing in this period.Based on the experiments of carbon,oxygen isotope and trace elements,the geochemical characteristics of Benbatu,Amushan and Zhesi formations are analyzed in this paper.The average value of?¹³C of the carbonate rock in Benbatu formation is 1.37‰,the average value of?¹³C of the Amushan formation is 3.26‰,and the average value of?¹³C of the Zhesi formation is 2.62‰.The average value of?¹³C gradually increases from the sidementary period of Benbatu formation to Amushan formation and then decreases to the Zhesi formation.The results of trace element analysis indicate that Zr and Th are positively correlated in Benbatu,Amushan and Zhesi formations.The Er/Nd values are less than 0.27 and some are less than 0.1.Moreover,the distribution characteristic of?Ce and?Eu is discrete,and is also the same of?Ce and?REE.There is no positive correlation which indicates that rare earth elements are less affected by metharmosis.The paleotemperatures of the Benbatu formation range from 13.86°C to 22.46°C with average of 19.58°C based on the calculation of oxygen isotope value.The paleotemperatures of Amushan formation is from 11.58°C to 24.16°C with average of16.75°C,and that of Zhesi formation is from 14.44°C to 35.66°C with average of26.29°C.The Z values of the Benbatu formation carbonate are between 121.76‰and134.69‰,the Amushan Formation values are between 124.24‰and 136.27‰,and the Z values of Zhesi Formation are between 117.11‰to 135.55‰,all values are greater than120‰.The values of Sr/Ba are almost greater than 1,and the values of B/Ga are almost greater than 4.5 which indicate a salt water environment.The results of trace element ratio method are the same as that of isotope method which both indicate a salt water environment.U/Th,Ni/Co,V/Cr,V/?V+Ni?values are used to determine the redox conditions of the Carboniferous-Permian carbonate rocks in the study area.The carbonate rocks of Benbatu formation have the characteristics of U/Th<0.75,Ni/Co<7,V/Cr<2;V/?V+Ni?<0.5,?Ce<1,and the value of Ceanom is<-0.1.All these characteristics indicate an oxidizing environment.The U/Th,Ni/Co,V/Cr,V/?V+Ni?values of the Amushan formation indicate an anoxic and reducing environment in Darhan Muminggan Joint Banner,Sonid Left Banner and West Ujimqin banner.The values of U/Th,Ni/Co,V/Cr,V/?V+Ni?in the Zhesi formation indicate a hypoxic reduction environment in Mandula and Sonid Left Banner area,while the values of the samples in Xilinhot and West Ujimqin banner area indicate an oxidizing environment.The paleoenvironmental evolution model of Carboniferous-Permian is established in this paper based on carbon and oxygen isotope characteristics and paleoenvironmental characteristics.The Benbatu formation may be deposited at the end of the ice age.When the temperature rises,the glaciers begin to melt.As a result,the water level rises with an oxidizing environment.The Amushan formation may have experienced another glacial period,the increasing values in?¹³C and?¹⁸O indicate a decrease in temperature and an increase in salinity.The Zhesi formation may have experienced many environmental changes that have caused an increase in paleotemperature and paleo-salt.