Stratigraphic division of the Late Jurassic Zhangjiakou Formation in northern Hebei, China

For a long time there has been no a relatively unifying division scheme for the lithostratigraphic units of the Late Jurassic Zhangjiakou Formation in northern Hebei; as a result, its division has been confused, thus affecting the establishment of a rational stratigraphic sequence and regional stratigraphic correlation. According to the outcomes of the project Study of the 1∶250000 Mapping Method for Mesozoic Continental Volcanic-Sedimentary Basin in Northern Hebei, the authors propose a stratigraphic division scheme for the Zhangjiakou Formation, in which the formation is divided into three lithologic members with the rock associations of the facies sequence as basic units. The First Member is mainly a rock association of weakly eruptive facies sequence, the Second Member is dominated by a rock association of eruptive facies sequence, and the Third Member is mainly marked by a rock association of eruptive-sedimentary facies sequence. The three members represent the initial eruption stage, large-scale eruption stage and collapse stage of a certain caldera in this formation respectively. The lithologic members thus distinguished may better reflect the characteristics of volcanic architecture, favor the establishment of a rational stratigraphic sequence and help study the volcanic evolution history, thus integrating the lithostratigraphic division with the study of volcanic-sedimentary basins organically.