Trace Fossil Assemblage And Bioturbation Dolomite Of The 5₅ Submember Of Ordovician Majiagou Formation In Xingxian County, Shanxi Province

A large number of invertebrate trace fossils are preserved in the stratum of the 5₅Submember of Ordovician Majiagou Formation in the Guanjiaya section,Xingxian county,identified types of domichnia,fodinichnia and grazing trace,eight ichnogenera and fifteen ichnospecies.There are three ichnogenera and six ichnospecies in domichnia,including Arenicolites isp.?Cylindricum isp.?Thalassinoides isp.1?Thalassinoides isp.2?Thalassinoides isp.3?Thalassinoides isp.4;two ichnogenera and four ichnospecies in grazing trace,consisting of Helminthopsis abeli?Helminthopsis isp.1?Helminthopsis isp.2?Scolicia isp.;four ichnogenera and nine ichnospecies in fodinichnia,including Lorenzinia isp.?Planolites isp.?Planolites beverleyensis?Planolites montanus?Teichichnus rectus?Thalassinoides isp.1?Thalassinoides isp.2?Thalassinoides isp.3?Thalassinoides isp.4.Three trace fossil assemblages under different sedimentary environments have been recognized as follows:1)Helminthopsis–Planolites trace fossil assemblages mainly consists of fodinichnia and domichnia from intertidal and subtidal zone;2)Thalassinoides–Teichichnus trace fossil assemblages,are relatively simple mainly consists of fodinichnia from intertidal and supratidal zone;3)Thalassinoides–Helminthopsis trace fossil assemblages occur in intertidal zone.Based on the distribution characteristics of the trace fossil assemblages,the sedimentary environment model of the 5₅ Submember of the Ordovician Majiagou Formation in the eastern margin of Ordos Basin was proposed.A large number of bioturbated dolomites/bioturbated limestone have been developed in the Cambrian and Ordovician of North China.Through field geological investigation and laboratory related experimental means and methods,the macro-structure and micro-structure of bioturbation and surrounding rocks were analyzed by polarizing microscope,scanning electron microscope.It was found that dolomite was dominant in the burrows,and calcite was the main element in the matrix.The burrows in the outcrops represented different color patches.There are abundant Thalassinoides in the two studied sections,the three-dimensional burrowing system of Thalassinoides provide a good transport channel for dolomitization fluids,providing favorable conditions for reflux osmosis and promoting dolomitization of filling,thus forming a large number of bioturbated dolomites plaques.