青海循化盆地中新世中期—上新世早期介形类组合及其地质意义

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

通过对青海省循化县积石镇羊圈贡拜村-西沟中新统-上新统实测剖面中介形类动物群的研究,在其中的35个层位样品中发现含介形类化石7属32种,并建立了两个组合:(1)Candona-Candoniella组合;(2)Ilyocypris-Candoniella-Candona组合。依据介形类组合带,将咸水河组上部时代厘定为中新世中期,而临夏组时代厘定为中新世晚期-上新世早期。根据介形类动物群在剖面上的分布规律,自下而上建立5个生态组合:(1)Candona-Can-doniella生态组合;(2)Ilyocypris errabundis-Ilyocypris dunschanensis生态组合;(3)Candona-Candoniella生态组合;(4)Ilyocypris biplicata-Candoniella生态组合;(5)Candonaoppressa-Candona exti ma生态组合。通过对介形类生态组合进行详细的生态特征分析,结合磁性年代学数据,将循化盆地14.6-5Ma的古气候划分为5个阶段:(1)14.6-9.5Ma为凉干期;(2)9.5-9.38Ma为热干期;(3)9.38-6.88Ma为冷干期;(4)6.88-6.83Ma为温干期;(5)6.83-5Ma为凉干期。

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.