Eocene Micropaleontology In Tuna,Yadong,Southern Tibet

The Yadong area was once a residual basin during the demise period of the Neo-Tethys,with continuous marine Paleocene to Eocene strata,which can provide good information on the late evolution and the final closure time of the Tethys in Tibet.A systematic study on micropaleobiostratigraphy of the Sand-Shale Member of the Zhepure Formation(i.e.the youngest marine deposits in the region)in the Gulupu section in Tuna,Yadong was carried out.Abundant planktic and benthic foraminiferas,pteropods,microgastropods,echinoids,microbivalves,radiolarians,bryozoans,and a small number micobrachiopods,scaphopoda,crinoids,fish teeth were recovered for the first time.Combined with ostracodes,calcareous nannofossils,calcareous algaes,charophytes,dinoflagellates,and sporopollen,which have been reported by other members of our team,the group was named "Tuna biota".The planktic gastropods(pteropods,4 species of 2 genera,including a new species:Limacina yadongensis n.sp.)were yielded from the Sand-Shale Member,which is the first report in Tibet,and the "coastal currents hypothesis" was proposed to explain the occurrence of the deep water pteropod fauna in shallower water environment.11 species from 8 microgastropod genera(including a new species:Benthonella yadongensis n.sp.)were identified and some predator traces left on the surface of benthic gastropod shells were found.By analyzing the predation relationship and the types of predation traces,it is speculated that fish may be predators.And the reason for the miniaturization of the shell was analyzed,it is suggested that the shell miniaturization should be the result of the interaction of higher paleosalinity and paleotemperature.At the same time,the Eocene radiolarians in the Sand-Shale Member of the Zhepure Formation were firstly reported,12 species from 11 radiolarian genera were identified,and two assemblage zones were divided:Amphibrachium paleogenicum-Buryella clinata zone and Heliodiscus pentasteriscus-Pseudostaurosphaera perelegans zone,spanning the interval from Ypresian to late Lutetian.Simultaneously,24 species of 11 redeposited Cretaceous radiolarians genera were identified from the Sand-Shale Member of the Zhepure Formation,and the mechanism of the redeposited Cretaceous radiolarians in Eocene stratum was analyzed.In addition,based on the characteristics and distribution of the biota in Tuna,four kinds of biofacies were described,two transgression-regressive cycles of sea water were identified,and the paleogeographic evolution of Neo-Tethys were analyzed.It is considered that there was a short-team semi-closed gulf in the Tiina area of Yadong during early Eocene.Afterwards,it was an open residual basin,during which seawater desalination process occurred twice.At the end of Eocene,the water depth in this area was less than 30 m.Among them,planktic foraminiferas are distributed in the whole Sand-Shale Member.Totally 119 species of 24 genera were identified and 7 planktic foraminiferal biozones were recognized(Morozovella formosa formosa,Morozovella aragonensis,Acarinina pentacamera,Hantkenia nuttalli,Globigerinatheka subconglobata subconglobata,Morozovella spinulosa-Acarinina bullrooki and Globigerinatheka semiinvoluta zones),spanning the interval from middle Ypresian to middle Priabonian(P7-P16).Based on the planktic foraminiferal data,the age of the top of the Sand-Shale Member(i.e.the youngest marine deposits)can be referred to middle Priabonian.By comparing with other areas in southern Tibet,it is considered that the youngest marine strata in Tuna should be the youngest in Tibet,therefore,the age of which should represent the time when the New-Tethys finally disappeared in Tibet.