辽西义县组叶肢介化石在冀北滦平盆地西瓜园组中的系统发现及其意义

冀北滦平县西瓜园南李营剖面是西瓜园组的命名剖面，自下而上可划分为西瓜园组一至四段，为一套单一岩相的沉积岩。在火斗山乡马圈子，西瓜园组以一层水下分流河道砾岩与其下伏大店子组四段为连续沉积，拉海沟马圈子剖面为西瓜园组与大店子组的界线层型。西瓜园组自下而上可连续划分为5个叶肢介化石带，与辽西义县组尖山沟层、老公沟层一金刚山层的叶肢介组合完全可以对比．义县阶的单位层型应以西瓜园组为层型剖面，原定义县阶的单位层型马神庙一宋八户标准地层剖面仅能作为义县阶的副层型。由于辽西义县组尖山沟层至金刚山层的叶肢介化石在冀北西瓜园组中的系统发现，冀北地区西瓜园组与辽西义县组完全可以对比，从而解决了30余年来冀北辽西晚期中生代地层的对比问题，具有重要的地层意义。

Discovery of conchostracans occurring in the Yixian Formation of western Liaoning in the Xiguayuan Formation of the Luanping basin,northern Hebei,China, and its significance.

The Liying section in the south of Xiguayuan, Luanping County, northern Hebei, is the naming section of the Xiguayuan Formation. This formation may be divided in ascending order into four members and consists of sedimentary rocks of monotonous rock facies. At Majuanzi, Huodoushan Township, a bed of subaqueous distributary channel conglomerate of the formation was de- posited continuously with its underlying Dadianzi Formation, and the Majuanzi section of Lahaigou is the boundary stratotype of the Xiguayuan Formation and Dadianzi Formation. Abundant conchostracan fossils were found in the Xiguayuan Formation of this section, including 5 genera and 7 species. From the base upward, the Xiguayuan Formation is divided into five conchostracan zones, which can be completely correlated with the conchostracan assemblages of the Jianshangou bed, Laogonggou bed and Jingangshan bed of the Yixian Formation in western Liaoning. The unit stratotype of the Yixian Stage takes the Xiguayuan Formation as its stratotype section, and the Mashenmiao-Songbahu standard stratigraphic section--the unit stratotype of the original Yixian Stage--can only serve as a parastratotype of the Yixian Stage. As the conchostracans occurring from the Jianshangou bed to Jingangshan bed of the Yixian Formation in western Liaoning have been systematically found in the Xiguayuan Formation of the Luanping basin, northern Hebei, the Xiguayuan Formation of northern Hebei can be completely correlated with the Yixian Formation of western Liaoning. Thus the problems of the correlation of Late Mesozoic strata between northern Hebei and western Liaoxi that have been existed for more than 30 years have been solved.