中文摘要：

滦平盆地榆树下剖面大北沟组出露完全、沉积连续、化石丰富,是研究热河生物群的起源、早期演化以及中国陆相晚侏罗世—早白垩世过渡地层的关键层位。论文对人工新揭露出的榆树下剖面进行了精细的实测;阐明了大北沟组与张家口组界线的标志,为凝灰质碎屑岩（沉积岩）的出现与晶屑岩屑熔凝灰岩（火山岩）的结束;大店子组与大北沟组界线的标志,为＂蜂窝梁砾岩＂或与之相当的粗碎屑岩的出现。剖面的主体大北沟组总厚度为224.51m,可分为3段46层。大北沟组二段与一段界线的标志,为正常沉积岩（不含或很少火山物质,开始出现丰富化石）的出现并取代凝灰质碎屑岩成为主要的岩层;大北沟组三段与二段界线的标志清晰,为一套青灰色粉砂凝灰岩的出现。依据新采集到的化石并结合前人资料,完成了榆树下剖面大北沟组综合生物地层的建立。首次在大北沟组二段中上部发现了介形类化石,并可延伸到三段顶部;初步鉴定出介形类化石9属17种,可划分为Luanpingella-Pseudoparacypridopsis-Eoparacypris组合带,并可进一步细分为Luanpingella postacuta和Pseudoparacypridopsis mountfieldensis 2个亚带。叶肢介化石的首现面在二段底部,可贯穿至三段顶部;叶肢介组合带的划分参考前人的结果,由下至上分为Nestoria pissovi-Jibeilimnadia ovata、Nestoria pissovi-Keratestheria gigantea和Yanshania xishunjingensis-Y.subovata3个化石带。昆虫化石主要分布在二段中部至三段中上部的页岩中,仅为三尾拟蜉蝣（Ephemeropsis trisetaris）一个种,据此建立昆虫类Ephemeropsis trisetaris化石带。此外,参考前人资料,尝试建立了大北沟组孢粉、双壳类和鱼类化石带。在大北沟组介形类、叶肢介、孢粉等生物的年代特征分析的基础上,结合大北沟组同位素测年结果、磁性年代特征和生物归属的认识,将大北沟组的地质时代归为早白?

英文摘要：

The Dabeigou Formation of the Yushuxia Section in Luanping Basin has well-exposed condition,continuous sedimentation and abundant fossils,which make it become the key layers in studying the origin and the early evolution of the Jehol Biota,as well as the Chinese terrestrial Late Jurassic-Early Cretaceous transition strata.Based on the newly artificial revealing Yushuxia Section,detailed stratigraphic discription and measurement are carried out by this study.The boundary between the Dabeigou and the Zhangjiakou Formation is represented by the appearing of tuffaceous clastic rocks（sedimentary rocks）and the ending of the tuffaceous rocks（volcanic rocks）.The boundary between the Dadianzi and the Dabeigou Formation is represented by the appearing of coarse clastic rocks（the so-called ＂Fengwoliang conglomerate＂）.The main strata of the Yushuxia Section is the Dabeigou Formation,which is 224.51 meters thick and can be divided into3 members and 46 layers in acending order.The boundary between the first member and the second member of the Dabeigou Formation is represented by the appearing of common sedimentary rocks（contains no or very little volcanic material,and has aboundant fossils）and the ending of tuffaceous clastic rocks.The boundary between the second member and the third member of the Dabeigou Formation is very clear,which is represented by the appearing of the cinerous silty tuff.According to the newly founding fossils and combining with previous studies,integrated biostratigraphy of the Dabeigou Formation has been established.Ostracod fossils firstly appear in the middle-upper part of the Dabeigou Formation,and they can extend to the upper part of the third member.Seventeen species belongs to 9genera of the ostracod fossils from this formations has been described,and they can divided into one ostracod assemblage zone：Luanpingella-PseudoparacypridopsisEoparacypris,and two subzone：Luanpingella postacuta and Pseudoparacypridopsis mountfieldensis.Conchostracan fossils can be found in the seco