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 deposited 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.