风暴沉积的旋回性及其控制因素分析—以北京西山下苇甸剖面中寒武统张夏组砾屑灰岩为例

Cyclicity and Controlling Factor Analysis of Storm Deposition—A Case Study from Flat-Pebble Conglomerates in Middle Cambrian Zhangxia Formation at Xiaweidian Profile in West Hill, Beijing

摘要: 风暴沉积为典型的事件沉积，尽管其时空分布具有随机性，但是垂向发育强度与频率具有周期性演变。本文以北京西山下苇甸剖面中寒武统张夏组风暴沉积为例，通过野外剖面观察、实测结合前人研究成果，在研究剖面识别出8套(S1~S8)以竹叶状砾屑灰岩为代表的两类风暴沉积序列：I类风暴沉积序列，形成于水体相对较深的深潮下带下部，代表的风暴强度大，高峰期时间短；II类风暴沉积序列，沉积环境能量较I类风暴层序高，可能形成于深潮下带中–上部，代表的风暴规模不大，作用时间较短。风暴沉积事件主要发生在剖面下部，中部频率降低，向上又逐渐增加，从剖面下部到上部风暴强度呈增强到逐渐减弱的趋势。风暴沉积序列类型与海平面变化直接相关，海侵时期主要发育I型风暴沉积序列，而海退时则主要发育II型风暴沉积序列，且风暴事件主要发生在海侵过程中。研究剖面在沉积时期发生了两次较大规模的海退–海侵旋回，第一次海侵过程中，风暴强度大，风暴高峰期时间较短，第二次海侵过程中，风暴规模不大，作用时间很短。

Abstract: As a typical type of event deposition, storm deposition has random spatial and temporal distribu-tion, however, presents periodic evolution of intensity and frequency on vertical development. In this study, we have carried on a case study from flat-pebble conglomerates in Middle Cambrian Zhangxia formation at Xiaweidian profile in West Hill. Based on field observations and measure-ment, combined with previous research results, we have recognized 8 sets of flat-pebble conglo-merates including two types of storm deposition sequence. Type I storm sedimentary sequence formed in the lower part of the relatively deep subtidal zone, representing a strong storm with short peak time. While Type II storm sedimentary sequence, which developed in higher energy sedimentary environment than Type I, may form in the middle and upper part of the deep subtidal zone, representing small storm scale and short duration of action. The storm deposition events mainly occurred in the lower part of the profile, the frequency decreased in the central section, and gradually increase upwards. In general, the storm intensity weakened from the lower part to the upper part of the profile. The type of storm sequence is directly related to sea level change. During transgression period, type I storm sequence was well developed, while during the regression process, type II storm sequence was prevailed, and the storm events mainly occur in the transgression. There were 2 large-scale regression and transgression cycles in the stratigraphic succession of the studied area. The first transgression process was characterized by strong storm intensity and shorter peak time. Otherwise, the second transgression process was characterized by small storm scale and short duration of action.