Ichnofossils And Their Sedimentary Environments In The Middle Part Of Third Member Of The Paleogene Shahejie Formation In Dongpu Sag

Based on the observation of 11 drilling cores,the large number of ichnofossils are discovered in the middle part of third member of the Paleogene Shahejie Formation in Dongpu Sag,including 23 ichnogenus and 31 ichnospecies.According to the ethological analysis,these ichnofossils can be divided into 4 ecological types,including(1)Domichnia: Cylindricum isp.,Skolithos linearis,Skolithos vertivalis,Skolithos isp.,Psilonichnusisp.,Thalassinoides isp.,Trichichnus linearis,Trichichnus isp.;(2)Fodinichnia: Beaconites antarctiaus,Palaeophycus tubularis,Planolites beverleyensis,Planolites montanus,Steinichnus isp.,Taenidium serpentinum,Taenidium isp.,Teichichnus isp.;(3)Pascichnia: Diplopodichnus isp.,Helminthoidichnites tenuis,Helminthopsis isp.,Megagrapton isp.,Mermia carichensis,Mermoides isp.,Nereites irregularis,Pilichnus lacustris,Semirotundichnus dongyingensis,Vagorichnus isp.;(4)Agrichnia: Paleodictyon isp.1,Paleodictyon isp.2,Paleoddityon isp.3,Protopaleodictyon isp.1.According to the composition,distribution and surrounding rock characteristics of ichnofossils,four ichnoassemblage are suggested:(1)Palaeophycus-Planolites ichnoassemblage,mainly composed of Domichnia and Fodinichnia,which reflectes the shallow lake and delta-the outer fan environment;(2)Planolites-Helminthopsis ichnoassemblage,mainly consists of Fodinichnia,Pascichnia and Domichnia,which generated in the deeper shallow lake or delta front and distal environments;(3)Planolites-Vagorichnus ichnoassemblage,is commonly made up of Domichnia and Fodinichnia,which occurs in the former delta or the slump turbidite deposits,reflecting deep water environment;(4)Protopaleodictyon-Semiroundichnus ichnoassemblage,mainly composed of Agrichnia and Pascichnia,which formed in the semi-deep lake-deep lake environment.