北京西山下苇甸中上寒武统风暴沉积特征及模式

Storm deposition characteristics and models for the Middle and Upper Cambrian in Xiaweidian,Xishan area,Beijing

通过对北京西山下苇甸剖面中典型风暴成因竹叶状灰岩的野外实测和镜下薄片分析,明确了59套风暴沉积特征,确定了风暴沉积序列并建立了陆表海风暴作用沉积模式,主要取得如下认识:(1)北京西山下苇甸寒武系共有泥晶灰岩相、含砾屑泥晶灰岩相、颗粒泥晶灰岩相和泥晶颗粒灰岩相4种岩相类型。(2)鲕粒灰岩-砾屑灰岩型序列发育原地型风暴岩,泥岩和粉砂岩-砾屑灰岩型序列发育近源型风暴岩,泥晶灰岩-砾屑灰岩型序列发育远源型风暴岩。(3)原地型风暴岩发育侵蚀面,砾屑含量高,长短轴比大;近源型风暴岩砾屑定向性好,可见递变层理;远源型风暴岩基质含量高。(4)竹叶状灰岩沉积在陆表海背景下的潮下带,原地型、近源型和远源型风暴岩沉积范围以正常浪基面和风暴浪基面为界,距风暴中心距离和搬运距离依次增加。本次研究不仅补充了下苇甸寒武系的风暴沉积记录,而且为该时期的古环境研究提供了证据。

Based on field survey and thin section analysis of tempestites in the Xiaweidian section,western Beijing,59 sets of storm deposits are characterized.The storm sedimentary sequences are determined,and an epicontinental sea deposition model is established.There are four sedimentary facies types in the Cambrian,Xiaweidian section:micritic limestone,pebbled micritic limestone,wackstone and packston,where in-situ,proximal and distal tempestites are developed in sequences of oolitic-gravel limestone,mudstone/siltstone-gravel limestone and micritic-gravel limestone,respectively.In-situ tempestites develop erosion surfaces and have high grain content and large axial ratio;proximal tempestites contain flat peebles with good orientation and show graded bedding;and distal tempestites have high matrix content.The three tempestites types are deposited in the subtidal zone under epicontinental sea environment,with the in-situ,proximal and distal types deposited from top to bottom and separated by normal-and storm-wave bases,respectively,in the order of increasing distance from the storm center and thus increasing transport distance.Results of this study not only complement the storm deposition records of the Cambrian in Xiaweidian,but also provide evidence for the study of palaeoenvironment in this period.