Study Of Ichnofossils And Sedimentary Environments In The Third Member Of The Paleogene Shahejie Formation In Dongpu Sag

Through the observation of 21 drilling cores in Dongpu Sag,including Sanchunji district in southern area,Pucheng and Weicheng district in northern area,Qianliyuan and Baimiao district in eastern area.Based on the identification of trace fossils in the third member of Paleogene Shahejie Formation in Dongpu Sag,10 ichnogenus and 16 ichnospecies are discovered,including Domichnia of 2 ichnogenus and 5 ichnospecies: Skolithos vertivalis,Skolithos linearis,Skolithos isp.,Skolithos magnus,Thalassinoides isp.;Fodinichnia of 5 ichnogenus and 8 ichnospecies: Palaeophycus tubularis,Palaeophycus isp.,Planolites montanus,Planolites beverlegensis,Planolites isp.,Teichichnus isp.,Beaconites isp.,Taenidium isp.;Pascichnia of 3 ichnogenus and 3 ichnospecies: Mermoides isp.,Helminthoidichnites tenuis,Helminthopsis isp..According to the lithology,sedimentary structures,composition and distribution characteristics of trace fossils,delta,fan delta,lacustrine facies were developed.Northern area WuCheng and Guyunji have the delta front and prodelta subfacies;Pucheng district have the delta front and prodelta subfacies;Southern area Sanchunji and Machang have the delta front,prodelta subfacies and shore-shallow lake subfacies..Eastern area Qianliyuan and baimiao have the fan delta front and fan prodelta subfacies,shore-shallow lacustrine facies and semi-deep or deep lacustrine facies.Four ichnoassemblages are divided,which reflect different environments and water depth.Including Palaeophycus—Planolites ichnoassemblage,mainly composed of Domichnia and Fodinichnia,and Pascichnia,which reflectes the lakeshore or normal delta front environment;Planolites—Helminthoidichnites ichnoassemblage,mainly consists of Fodinichnia,Pascichnia and Domichnia,which generated in the shallow lake or prodelta sedimentary environment;Planolites-Teichichnus ichnoassemblage,is commonly made up of Domichnia and Fodinichnia,reflecting shallow lake environment or distal area of delta front;Mermoides-Helminthopsis ichnoassemblage,mainly composed of Domichnia and Pascichnia,which formed in the semi-deep lake or deep lake environment.