Biostratigraphy And Paleoecology And Palaeoenvironmen Studies On The Cretaceous Bivalves From Southwestern Tarim Basin, China

The marin Cretaceous in Southwestern Tarim Basin contains abundant bivalve fossils, which are the important basis for the study of the biostratigraphy and paleoecology and palaeoenvironmen. Based on identification, statistics and analysis of bivalve fossils from Simuhana, Kuzigongsu in the southern Tianshan Mountains and Akecheyi, Aertashi in the Western Kunlun Mountains, three bivalve biostratigraphyhic zones are recognized in Simuhana profile: Flaventia ovalisCardita sp. assemblage zone, Ostrea oxiana- Ostrea vatonnoides- Ostrea nunmus assemblage zone, Rhynchostreon suborbiculatum- Lima marrotiana- Pycnodonte(Costeina) costei assemblage zone; five bivalve biostratigraphyhic zones are recognized in Akecheyi profile:(1)Ichthyosarcolites tricarinatus- Ostrea oxiana assemblage zone,(2)Rhynchostreon plicatulum- Plicatula auressensis- Trigonarca Passyana assemblage zone,( 3) Rhynchostreon suborbiculatum- Pycnodonte(Costeina) costei- Cyprimeria? paba assemblage zone,( 4) Anisocardia(Antiquicyprina) obtuse- Cardita ulugqatesis- Trigonarca Passyana assemblage zone,(5)Biradiolites boldjuanensis zone; two bivalve biostratigraphyhic zones are recognized in Kuzigongsu profile:(1)Rhynchostreon suborbiculatum- Plicatula pyriforma assemblage zone,(2)Anisocardia(Antiquicyprina) obtuse- Trigonarca Passyana- Cardita upwarensis assemblage zone. Based on the study of the bivalve fossils from Kukebai formation of the above profiles, the results show that the deposition in the upper member and middle member of the Kukebai formation in the study area are not equal to the time. Moreover, based on the occurrence and ecology of bivalves and association as well as the sedimentary facies, tree communities are retrieved in Simuhana profile:(1)Flaventia – Cardita community,(2)Ostrea community,(3)Rhynchostreon- Lima – Pycnodontei community;six communities are retrieved in Akecheyi profile:( 1) Ichthyosarcolites community,( 2)Rhynchostreon- Plicatula- Trigonarca community,(3)Rhynchostreon- Pycnodonte community,(4)Ostrea community,(5)Anisocardia- Cardita- Trigonarca community,(6)Biradiolites boldjuanensis community. According to the succession of the biotic communities, analyzes the ancient ecology of Cretaceous period in Southwestern Tarim Basin. Moreover, the difference of ancient ecology between southern Tianshan Mountains and Western Kunlun Mountains are compared. The result shows that the differences between bivalve’s communities from the two areas in late Cretaceous period are caused by the difference of foreland basin’s flexure degree in the Kunlun Mountain and Tianshan Mountain which formed different ecological environment conditions.