冀北晚侏罗世张家口组段的划分

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

4冀北晚侏罗世张家口组的岩石地层单位长期以来一直没有一个较统一的划分方案,划分较为混乱,影响了其合理的地层层序的建立及区域地层对比。根据《冀北中生代陆相火山-沉积盆地1∶25万填图方法研究》项目的工作成果,提出张家口组以相序的岩石组合为基本单元将其依次划分为3个岩性段的方案。一段主要为弱喷发相序的岩石组合;二段主要为爆发相序的岩石组合;三段主要为喷发沉积相序的岩石组合。三者分别代表该组某破火山的初始爆发阶段、大规模爆发阶段和破火口塌陷沉积阶段。由此划分的岩性段可以较好地反映火山构造特征,建立合理的地层层序,有助于火山演化历史的研究,使岩石地层单位的划分与火山-沉积盆地的研究有机地结合在一起。

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 radonal stratigraphic sequence and help study the volcanic evolution history, thus integrating the lithostratigraphic division with the study of volcanic-sedimentary basins organically.