Conchostracan biostratigraphy of the Dadianzi Formation in the Luanping basin, northern Hebei, China and the boundary of continental Jurassic and Cretaceous strata

The Donggou section of Dadianzi Village in Luanping County, northern Hebei is the naming section of the Dadianzi Formation by Mr. Wang Si-en in 1990 year. This formation may be divided in ascending order into four members in the Zhangjiagou section. At Xiaodonggou, Huodoushan Town, Luanping County, Northern Hebei a bed of subaqueos distributary channel conglomerate of the formation was deposited continuously with its underlying Dabeigou Formation,and it is the boundary stratotype of the Dabeigou Formation and the Dadianzi Formation. Abundant conchostracan fossils were found in the Dadianzi Formation of this section, including 7 genera and 47 species. From the base upward, the Conchostracan biostratigraphy of the Dadianzi Formation is divided into five conchostracan zones. Because of the event of drop in temperature and the event of vanish and newborn of conchostracan fauna between the Dabeigou Formation and Dadianzi Formation, the boundary of continental Jurassic-Cretaceous System is determined in the point between the Dabeigou Formation and Dadianzi Formation.