Paleogene Bivalves Biostratigraphy And Paleoecological Environment In Western Tarim Basin

The Tarim Basin is located in Xinjiang Uygur Autonomous Region,western China.It is the largest Mesozoic-Cenozoic sedimentary basin in China.It is located in the southern Tianshan Mountains,the Altun Mountains and the Western Kunlun Mountains.A complete set of Late Cretaceous-Paleogene marine strata is widely developed in the western part of the basin.The Smhana and Bashbulak sections studied in this paper are located in the piedmont area of the Tianshan Mountains.The Paleogene strata are well developed and continuous in there and they are rich in fossils.The two sections are the necessary places for the Tethys Sea to enter and leave the western Tarim Basin.They are of great scientific significance and research value to the stratigraphic division of the study area,the restoration of the paleo-bay ecological environment and the final regression event of the Tethys Sea.In this paper,the bivalve fossils in the Smhana section and Bashbulak section in the piedmont area of the South Tianshan Mountains are identified and analyzed,and the bivalve biostratigraphy is established based on the regional geological data.There are 12 genus and 21 species of bivalve fossils were identified,including Ostrea(Turkostrea)strictiplicata Roulin et Delbos;Ostrea(Turkostrea)strictiplicata borgalensis Vyalov;Ostrea(Turkostrea)cizancourti Cox;Sokolowia buhsii(Grewingk);Sokolowia buhsii alpha;Sokolowia buhsii gamma;Sokolowia orientalis;Kokanostrea kokanensis;Flemingostrea yengisarica(Wei);Platygena asiatica;Panopea heberti Bosquet;Chlamys(Hilberia)30-radiatus Sowerby;Cubitostrea tianschanensis;Cubitostrea prona Wood;Flemingostrea hemiglobosa(Romanovskiy);Pycnodonte(Pycnodonte)camlus;Panopea elogata Leymerie;Astarte rugata Sowerby;Anomia girondica;Ferganea bashibulakeensis Wei;Ferganea ferganensis;Panopea heberti Bosquet;It is suggested that the last regression in the piedmont area of the South Tianshan Mountains was not earlier than the late Eocene.According to the ecological characteristics of bivalves and the sedimentary characteristics of rocks in the field,the bivalves collected from the Paleogene strata in the piedmont area of Tianshan Mountains are divided into 6 ecological communities:(1)Pycnodonte community;(2)Flemingostrea-Panopea community;(3)Ostrea(Turkostres)community;(4)Sokolowia-Kokanostrea community;(5)Platygena community;(6)Ferganostrea-Cubitosta community.Based on the analysis of the characteristics and succession of bivalves and regional sedimentary characteristics,four transgressions in the Early Paleogene in the western Tarim Basin consist of four cycles,namely,Altash,Zimgen-Gejitag,Late GejitagQatar-Uragen-Early Bashbulak and Middle-Late Bashbulak.It can be inferred that the Tethys Sea is under study.The study area withdrew in the late Eocene and the highest marine layer was located in the fourth member of the Paleogene Bashbulak Formation...