北京市平原区下伏泥砂砾石层特征及成因探讨

Geological Characteristics and Origin of the Mud Gravel Stratum in Beijing Plain Area

北京市平原区下伏的泥砂砾石层发现较早,但对其成因一直存在着争议。本文通过对最新钻探资料的分析,经系统的古地磁样品测试,发现泥砂砾石层最早形成于上新世,距今约三百万年,这一时期的砾石层分布最广,厚度很大。此后更新世、全新世也有砾石层形成,但分布范围和厚度都显著减小。资料表明,上新世北京地区气候比较温暖湿润,不存在形成冰川沉积物的条件,此外第四纪的泥砾石层的分布和层位也与第四纪冰期的划分不能对应。北京市平原区下伏泥砂砾石层具有泥石流成因的基本特点,综合各种资料分析,应主要是洪水泥石流成因。

The mud gravel stratum underlying Beijing plain area has been found long ago,but its origin is still in dispute.In this paper,by analyzing the newest drilling data and testing the paleomagnetism samples,the results indicated that（1） the mud gravel stratum was formed in the Pliocene,about 3 million years ago.（2） The mud gravel stratum in this time was very thick and distributed most widely in this area,and the later gravel strata were also formed in Pleistocene and Holocene,but the distribution range and thickness were obviously reduced.The existing geologic documents show that,the Pilocene climate in the Beijing area was quite warm and humid,so no condition to form glacial sediment.In addition that the stratigraphic horizon and the distribution of the mud gravel stratum can not correlate to the Quaternary glaciation.Therefore,based upon the basic characteristics of the debris flow and the comprehensive analysis of different kinds of documents,the author concluded that the aggradation of flood and debris flow is the main origin of mud gravel stratum.