摘要： 北京周口店地区中元古界雾迷山组以含硅质条带的白云岩为主,发育不规则硅质砾屑。这些硅质砾屑形态各异,具有强烈扰动的迹象,目前对其成因解释一直存在争议,如泄水构造、冲刷构造、震积岩、风暴岩等。文中选择北京周口店地区黄山店村恒顺厂剖面为研究对象,通过野外精细的沉积学分析,系统描述了岩性及沉积构造特征,确定了该套沉积地层发育典型的碳酸盐岩风暴沉积序列。该风暴沉积序列自下往上可分为5段: A段,为风暴前正常沉积的中厚层状泥晶白云岩;B段,具口袋构造的侵蚀冲刷面及硅质砾屑,为风暴高潮期产物;C段,灰色硅质条带白云岩,普遍发育平行层理和波状、丘状、洼状交错层理,是风暴衰减期产物;D段,灰白色薄层状泥晶白云岩,代表了风暴间歇期的正常沉积;E段,含硅质团块白云岩,为正常天气下海水局部扰动形成。该剖面上递变层理几乎不发育,结合侵蚀冲刷面特征、砾屑高含量以及杂乱堆积方式,认为该套沉积为典型的近原地风暴沉积。结合古地理资料,推测研究区在雾迷山组沉积时期可能处于热带海洋环境,热带气旋引发的频繁风暴潮导致了近原地碳酸盐岩风暴沉积的形成。上述研究成果不仅补充了燕山地区雾迷山组的风暴沉积记录,而且为该时期华北板块的古地理、古纬度和古气候研究提供了重要的沉积学证据。

关键词: 雾迷山组, 原地风暴沉积, 口袋构造, 丘状交错层理, 周口店地区

Abstract: The Mesoproterozoic Wumishan Formation is widely exposed in the Zhoukoudian area,Beijing. It mainly comprises of dolostones with abundant siliceous bands and intraclasts. The irregular shape of these intraclasts suggests that they were strongly deformed before deposition. Its genesis remains controversial,though they were interpreted as dewatering or scouring structures,seismites,or storm-related deposits(tempestites). Here we present results of detailed sedimentological studies on the Wumishan Formation in the Hengshunchang section located in the Zhoukoudian area. Based on systematic analysis of lithology and sedimentary structures,we interpret the studied unit as a typical carbonate tempestite sequence. Sedimentary cycles in the sequence can be divided into five units(A to E)from bottom to top. Unit A consists of a medium- to thick-layered dolo mudstone,as fair-weather deposits before the storm. Unit B is characterized by scouring surface with gutter casts and siliceous intraclasts,representing the product of storm surge. Unit C is composed of gray-banded siliceous dolostones,with continuous siliceous strips and well-developed parallel and cross beddings(including wavy cross bedding,hummocky and swaley cross stratification)that imply deposition during the storm attenuation period. Unit D is a gray-white,thin-layered dolo mudstone,interpreted as post-storm deposits. Unit E consists of dolostones with siliceous nodules,likely formed by local disturbance of seawater in fair-weather conditions. This distinct sedimentary sequence,combined with conspicuous scouring bottom surface,high content and extremely disorganized intraclasts,suggests that the Hengshunchang section represent an in situ tempestite and the study area was located near the tropics in the Mesoproterozoic period. At that time,the surface seawater temperature was likely relatively high. Tropical cyclones were prone to occur,which may have triggered frequent storm surges and eventually typical storm deposits formed at the Hengshunchang section. The research results not only supply the storm sedimentary records of the Wumishan Formation in Yanshan area,but also provide important sedimentological evidence for the paleogeography,paleolatitude and paleoclimate of North China plate in this period.

Key words: Wumishan Formation, in situ tempestite, gutter casts, hummocky cross stratification, Zhoukoudian area

中图分类号: 

• P588.24+5