The Early Cretaceous Ostracod Faunas And Its Biostratigraphic Significance In Hongliuxia Area,Yumen,Gansu Province

The Lower Cretaceous is well developed and the fossils are abundant in the HongLiuxia area,Yumen.Previous studies regarding to these strata are quite controversial,with different understanding of Late Jurassic,Late Jurassic to Early Cretaceous and Early Cretaceous.In this paper,a large number of well-preserved Ostracod fossils in the Xiagou Formation and Zhonggou Formation in the Hongliuxia area,Yumen,are studied.The purpose of this paper is to improve the relative dating of the strata,and to analyze the paleo-environment based on the fossils,the morphological features and the characteristics of the surrounding rocks the following results are obtained:1.Six Ostracod species belonging to 1 genera in the Xiagou Formation,including Cypridea(Cypridea)subyumenensis,Cypridea(Yumenia)subremota,Cypridea(Morinia)depressa,Cypridea sinensis,Cypridea zuoyunensis and Cypridea vitimensis were identified.Eleven species refering 7 genera are recovered from the Zhonggou Formation.They are Cypridea(Yumenia)paraoriformis,Cypridea(Yumenia)subremota,Cypridea(Cypridea)multispinosa,Cypridea postirotunda,Jiuquania paratrapezoidea,Timiriasevia sinensis,Candona dongliangensis,Limnocypridea diwopuensis,Limnocypridea symmetrica,Rhinocypris subzuoyunensis,Rhinocypris yumenensis and Yumenia suboriformis.2.TheCypridea-Limnocypridea-Rhinocypris-Yunmenia assemblage is established to represent the Ostracod fauna of the Xiagou and Zhonggou formations.3.The ostracods fossils in the Xiagou-Zhonggou Formations of the Hongliuxia area can be well compared with those of Yixian Formation and Jiufotang Formation in Liaoning Province,Shahai Formation in western Liaoning,Dabeigou Formation,Xiguayuan Formation and Qiucheng Formation in Hebei Province,Shouchang Formation and Guantou Formation in Zhejiang Province,Duhongmu Formation and Zhidan Group in Neimenggu Province,Hekou Formation in xining-minhe basin,Matoushan Formation in Yunnan Province,western Siberian Basin and Dong Wai Begalvitim Basin.Based on the research of these ostracods,the xiagou-zhonggou Formation belongs to early CretaceousBarremian-aptian period.4.It can be judged from the surface structures of the carapace value,features of these ostracods and the lithologic characteristics that the middle and lower part in xiagou Formation of the Hongliuxia belongs to the inland freshwater brackish braid delta sedimentary environment,and the upper part of xiagou Formation the fresh to brackish lacustrine environment.Zhonggou Formation might be the fresh to brackish a fan delta environment.