MIDDLE MIOCENE—EARLY PLIOCENE OSTRACOD ASSEMBLAGES IN THE XUNHUA BASIN,QINGHAI AND THEIR GEOLOGICAL SIGNIFICANCE

Through the study on the section of Miocene—Pliocene ostracod assemblages at Yangquangongbai Village-Xigou,Xunhua County,Qinghai,7 genera and 32 species ostracod fossils in 35 beds,were found,and two assemblages were recognized:(1)Candona-Candoniella assemblage,and(2)Ilyocypris-Candoniella-Candona assemblage.According to the ostracod assemblage zones,the age of the upper Xianshuihe Formation was determined as the Middle Miocene,and the age of the Linxia Formation was the Late Miocene—Early Pliocene.According to the distribution of ostracod in the section,5 ecological assemblages were recognized ascendingly:(1)Candona-Candoniella ecological assemblage,(2)Ilyocypris errabundis-Ilyocypris dunschanensis ecological assemblage,(3)Candona-Candoniella ecological assemblage,(4)Ilyocypris biplicata-Candoniella ecological assemblage,and(5)Candona oppressa-Candona extima ecological assemblage.Through detailed ecological analyses of the ostracod assemblages,combined with magnetic chronological data,the paleoclimate of the Xunhua Basin at 14.6—5 Ma is divided into 5 stages:(1)14.6—9.5 Ma is a cool-dry stage,(2)9.5—9.38 Ma is a hot-dry stage,(3)9.38—6.88 Ma is a cold-dry stage,(4)6.88—6.83 Ma is a warm-dry stage,and(5)6.83—5 Ma is a cool-dry stage.