The Sea Retreat From The West Tarim Basin: Evidence From Late Eocene To Early Oligocene Ostracod Form The Bashibulake Section

A series of major geological events occurred in the Cenozoic era,including the uplift of the Plateau and disappearance of the Tethys sea.Tarim Basin is the largest Mesozoic and Cenozoic sedimentary basin in china,and the marine strata is widely developed in the western Tarim Basin.However,the age of sea retreat from west Tarim Basin is still in argue.Bashibulake section,which is located in the main channel,and consist of continuous sedimentary transition from marine to non-marine,can help us to better understand the regression of seawater from west Tarim Basin.This paper focus on the key strata of the Bashibulake section about last sea retreat-the Bashibulake Formation.The stratigraphic description,microfossil sampling and analysis have been taken.Relatively abundant ostracod,foraminifera(including planktonic foraminiferal),gastropod,and tooth of fish were identified from the Bashibulake Formation,and the ostracods were studied in detail.About 16 species of 12 genera have been identified.Through the study of ostracod biostraigraphic,Three ostracod assemblage zones have been established: 1)Cytherella evexa-Haplocytheridea unispinata-leguminocythereis vesca assemblage;2)Cytheretta circumspect assemblage;3)Haplocytheridea schirabadensis-Ruggieria rischtanensis-Haplocythereidea innae assemblage.Based on the biostratigraphy of the ostracod together with other evidence,it is inferred that the age of the Bashibulake Formation might be late Eocene,(but it can't exclud that the age of the member 4 and 5 of the Bashibulake Formation might be early Oligocene).The paleoecologly of ostracod shows that the ostracod funa is of typical marine species,indicating shallow marine environment.According to the ostracod,combined with sedimentology and other evidence,it is suggested that the top member 4 of Bashibulake Formation is the uppermost marine strata,for marinefossils have not yet been discovered above it,indicating that the tethys sea retreated from the study area during sedimentary of top Member 4 of the Bashibulake Formationthe,with ages of late Eocene(perhaps enter into the early Oligocene).