The Early Cretaceous Bivalves Fauna And Its Biostratigraphic Significance In Hongliuxia Area,Yumen,Gansu Province

The nonmarine Mesozoic Erathem in Hong Liuxia area,Yumen,Western Gansu Province are well developed.From bottom to top,it’s divided into Chijinpu Formation,Xiagou Formation and Zhonggou Formation.Considerable controversy have been aroused about the age of the Mesozoic Erathem,which has fallen into three categories,including late Jurassic,late Jurassic to Cretaceous and early Cretaceous.In this study,a large number of well-preserved bivalve fossils is obtained from Xiagou and Zhonggou Formation,which lays a foundation for the further determination of stratum age.The bivalves fauna of the Xiagou Formation includes 23 species belonging to 7 genera.They are Musculiopsis fuxinensis,M.yixianensis,M.yumenensis,M.subrotundus,M.trigonicus,M.gansuensis,Sphaerium anderssoni,S.coreanicum,S.pujiangense,S.selenginensis,S.chientaoense,S.ellipticoides,Gansuella angulate,G.xinminpuensis,G.subrotunda,Nakamuranaia chingshanensis,N.subrotunda,N.aff.chingshanensis,Arguniella lingyuanensis,A.yanshanensis,A.liaoxiensis,Inversidens wuerheensis and Unio yanbianensis.Based on the fauna the Nakamuranaia subrotunda-Musculiopsis fuxinensis assemblage is established.It only consists of 4 species bivalves belonging to 2 genera in Zhonggou Formation,which comprise M.subrotundus,M.gansuensis,S.pujiangense and S.anderssoni.Bivalves from Xiagou and Zhonggou formation offer a striking comparison to bivalve fauna of early Cretaceous from middle Aptian to early Albian in Western Zhejiang,Western Liaoning,Eastern Heilongjiang,Southeastern Korea,Eastern Transbaikal and Southwestern Japan.The comparison of biostratigraphic combined with the result of U-Pb isotopic age(109 + 1.6Ma)of volcanic neck facies zircon from upper Xiagou Formation,broadly proves that the age of Xiagou-Zhonggou Formation is Early Cretaceous from middle Aptian to Albian.