Calcareous Nanno-fossils Of The Wulagen Formation In The Aertashi Section Of The Western Tarim Basin And Tethys Retreat Time

Western Tarim basin is present in the vicinity of the Kunlun Mountains,western Tethys that holds a one key stratigraphic Aertashi section of the Paleogene marine sediments.Wulagen formation which is the part of this section is rich in calcareous fossils which is important for studying Tethys retreat system in western Tarim basin.16 species among the 11 genera of calcareous nannofossils were defined :Reticulofenestra dictyoda,Reticulofenestra umbilicus,Pontosphaera pulchra,Pontosphaera multipora,Pontosphaera plana,Zygrhablithus bijugatus,Blackites tenuis,Blackites inversus,Neococcolithes dubius,Discoaster nodifer,Cyclicargolithus luminis,Chiasmolithus solitus,Chiasmolithus modestus,Micrantholithus crenulatus,Braarudosphaera bigelowii,Coccolithus pelagicus and Reticulofenestra umbilicus-Chiasmolithus solitus calcareous biozonation scheme was adopted.Late Lutetian to early Bartonian age is given based on calcareous nannofossils.The sea level change intrusion timing was not earlier than 42.87 Ma and diminishing time was no later than 40.4 Ma which mark the exact boundary time of the deposition of marine Wulagen Formation in the Western Tarim Basin.Braarudosphaera bigelowii is a typical species among the 16 species obtained in this study.Because the proportion of Braarudosphaera bigelowii is increasing in the lower and upper parts of the Wulagen Formation,the species of fossils in the middle part are obviously higher than those in the lower and upper parts.Therefore,it is considered that the sea level change in the Wulagen Formation is a process from shallow to deep and from deep to shallow.