白洋淀漕河全新世早期古洪水事件的沉积特征及气候背景

古洪水沉积物是研究地史时期极端降水事件和古环境演化的重要载体。基于地质记录开展古洪水重建研究,可以为揭示区域洪水历史过程及机制提供依据,对水资源利用和工程建设等具有重要的现实意义。本文以漕河古河道北岸出露较好的李迪城村剖面为研究对象,综合地层序列、野外沉积特征、沉积构造及沉积物粒度分析,对白洋淀地区漕河古河道沉积记录的古洪水事件进行了识别,发现该剖面具有以下主要特征:古洪水沉积层与下伏地层存在明显的冲刷侵蚀面,沉积体形态呈透镜状,由泥砾、炭屑、双壳类及灰黑色粉细砂混杂组成,发育中小型交错层理;洪水沉积物粒度指标(中值粒径、砂含量、黏土/粉砂值及Q90)表现为高值,以细砂为优势组分,含混杂的粉砂及少量黏土,分选较差;古洪水沉积层中古树与双壳类集中埋藏,呈定向排列,其指示的方向与遥感影像揭示的古河道方向一致。通过该剖面沉积特征及地层AMS ¹⁴C年代分析,确定漕河在全新世早期约10.8~9.6 ka BP发生了4期古洪水事件,这4期古洪水事件可以与该时期华北地区其它古洪水事件进行对比。当时正值全新世早期,东亚夏季风增强,为气温和降水频繁波动上升时期,据此推断白洋淀地区全新世早期的古洪水事件是当时气候背景下的产物。

Sedimentary characteristics and climatic background of early Holocene paleoflood events in Caohe,Baiyangdian

Paleoflood sediment is an important carrier to study extreme precipitation events and paleoenvironmental evolution in geological history.The study of paleoflood reconstruction based on geological records can provide basis for revealing the historical process and mechanism of regional flood,and has important practical significance for water resources utilization and engineering construction.In this paper,the Lidicheng village profile,which is well exposed on the north bank of the ancient channel of Caohe River,is taken as the research object.Based on the analysis of stratigraphic sequence,field sedimentary characteristics,sedimentary structure and sediment grain size,the ancient flood events recorded by the ancient channel of Caohe River in Baiyangdian area are identified.It is found that the profile has the following main characteristics:there are obvious scouring and erosion surfaces between the paleoflood sedimentary layers and the underlying strata,and the sedimentary body is lenticular in shape,which is composed of muddy gravel,charcoal fragments,bivalves and grayish black silt-fine sand,and medium-and small-sized cross-bedding is developed;The grain size indexes of flood sediment (median grain size,sand content,clay/silt and Q90) have high values,with fine sand as the dominant component,mixed silt and a small amount of clay,and poorly sorted;Ancient trees and bivalves are intensively buried in the paleoflood sediments in a directional arrangement,and the direction is consistent with that of ancient river channels revealed by remote sensing images.Based on the sedimentary characteristics of the profile and AMS ¹⁴C age analysis,it is determined that four paleoflood events occurred in Caohe at about 10.8~9.6 ka BP in the early Holocene,which can be compared with other paleoflood events in North China during this period.It is inferred that the paleoflood events in Baiyangdian area were the product of the climate in the early Holocene.At that time,the East Asian summer monsoon was strengthened,with frequent fluctuations in temperature and precipitation.