长江武汉段4.5～2.5 ka沉积地层与古洪水标志识别

Sedimentary strata and paleoflood identification indexes of Wuhan section, Yangtze River, during 4.5～2.5 ka BP

长江中游近年洪灾频发,武汉江段更是其防洪重点,开展武汉地区的古洪水研究,从而延长古洪水记录,对洪水预测具有重要意义。本文选取武汉地区河谷沉溺湖区SK10钻孔沉积作为研究对象,运用AMS 14C测年技术厘定地层年代,通过粒度和地球化学元素的测定与分析,与文献中记录的古洪水频发期进行对比,建立一套适用于长江武汉段古洪水沉积的识别指标,并对长江武汉段距今4.5~2.5 ka的古洪水期进行研究。结果表明:①基于后湖与长江的分布关系,在钻孔沉积中选取粒度指标(P95、砂含量、平均粒径、黏土/粉砂、(粗粉砂+砂)/黏土)和地球化学指标(Na2O、Al2O3、Fe2O3、Rb/Sr、Zr/Rb)作为古洪水频发期识别标志,对武汉地区的洪水频发期沉积有较好的指示作用。②通过指标识别出的古洪水频发期为:2608±30 a BP、2814±35 a BP、3094±35 a BP、3577±35 a BP、3830±35 a BP、3939±35 a BP、4084±35 a BP、4193±35 a BP、4391±35 a BP左右,与文献记录中距今4.5~2.5 ka的几次洪水频发期基本一致。

Objectives:Frequent floods have occurred in the middle reaches of the Yangtze River in recent years,and the Wuhan Section is also the focus of flood protection in this area.Thus,it is of great significance to carry out the study on the paleoflood of the Yangtze River in Wuhan area so as to prolong the record of the paleoflood and forecast the flood.Methods:In this paper,the borehole SK10 of sink river valley lake in Wuhan area is selected as the research object.The stratigraphic age is determined by using AMS 14C dating technique.Through the determination and analysis of grain size and geochemical elements,a set of identification indexes for the paleoflood frequent occurrence period in Wuhan Section of the Yangtze River is established to study the paleoflood period in this section during 4.5 ~ 2.5 ka BP Results:Research shows:(1)Based on the distribution relationship between the Houhu Lake and the Yangtze River,grain size indexes(P95,sand content,average grain size,clay/silt,(coarse silt + sand)/clay) and geochemical indexes(Na2O,Al2O3,Fe2O3,Zr/Rb,Rb/Sr) were selected as the identification indices for the paleoflood frequent occurrence in the borehole sediments,which have better indication for the paleoflood frequent occurrence period in Wuhan area.(2)Frequent occurrence of paleofloods identified by indicators are about 2608±30 a BP,2814±35 a BP,3094±35 a BP,3577±35 a BP,3830±35 a BP,3939±35 a BP,4084±35 a BP,4193±35 a BP,4391±35 a BP,which are basically consistent with the records of several floods during 4.5 ~ 2.5 ka BP in the literature.Conclusions:The method of multi-indexes analysis has been successfully applied to Wuhan Area for paleoflood sediments identification,providing some new evidences for paleoflood frequent period in Wuhan Section of the Yangtze River.