Sedimentary Characteristics And Records Of The Late Holocene Paleoflood Along Abandoned Channels Of The Yellow River In North Henan

Due to the unique geological and geomorphic conditions in the Yellow River Basin,there is a long-term greater risk of flood disasters.The study of paleo-floods can greatly extend the time-space sequence of flood records,and understand the laws and mechanisms of floods more scientifically and rationally,which has important scientific significance and practical value.the abandoned of the Yellow River in northern Henan is one of the core areas where the lower reaches of the Yellow River was breached and rerouted in the historical period.It contains rich paleohydrological information and is an ideal area for carrying out paleo-flood research in plain areas.Based on a large number of field surveys,this paper chooses the Longwangmian section(LWM)of Daming,Hebei and the Gaocheng section(GC)of Henan Puyang as the typical sections of the study area.Sedimentary facies studies were carried out through observation of field sedimentary characteristics and thin slice identification.It was found that flood sediment-paleosoil/lake sedimentary sequences were widely developed in the strata in the study area.The flood deposits mainly include paleo-flood retention deposits and paleo-flood floodplain facies sandy deposits,which are quite different from non-fluvial deposits in color and sedimentary characteristics.Combining the characteristics of grain size composition,grain size parameters,grain size frequency distribution curve and grain size probability cumulative curve,a total of 7 paleoflood sedimentary units were identified in the study area.Comprehensive comparison of optical luminescence and radiocarbon dating results and historical records of major floods,we believe that the flood events should correspond to: The prehistoric flood at about 4000 a BP,the major flood in the Eastern Zhou period at about 2100?2800 a BP,the seasonal flood at about 2000 a BP in the Western Han Dynasty,the major flood at about 1800 a BP in the Eastern Han Dynasty,and the flood at about 1000 a BP in the Tang and Song Dynasties.The Great Flood,the Great Flood of the Ming and Qing Dynasties around 120?230 a BP,and the 1st period of Modern Great Flood.On this basis,combined with relevant research results at home and abroad,the typical flood deposition sequence and deposition characteristics of the old channel area of the Yellow River in northern Henan are revealed:(1)In the middle of the flood deposition unit,the flood floodplain facies sandy deposits with sedimentary characteristics such as cross bedding and ascending ripple bedding are deposited,and due to changes in hydrodynamic conditions,they have a binary structure of lower thickness and upper thickness.The lithology is mainly gray-yellow or gray-white silt-fine sand,with an average particle size between 20.5 and 69.17 ?m.The frequency distribution curve is nearly normal,and the main peak is single and sharp.The probability cumulative curve is a three-stage typical fluvial facies,with obvious rolling components and a large transition mass slope.(2)The top and bottom of the flood unit often form layered,massive and uniformly composed stagnant deposits,which have good field identification.The lithology is dominated by red-brown silty clay,with an average particle size between 10 and 20 ?m.The frequency distribution curve is normally distributed,and the main peak is single and sharp.The probability cumulative curve is two-stage,and the suspended component is greater than 30% of the general fluvial deposits,indicating the characteristics of its rapid deposition.Compared with the typical bedrock canyon area of the Yellow River Basin,the paleo-flood sediments in the old course area of the Yellow River in northern Henan have a finer grain size and a more compact structure.However,the shape of the frequency distribution curve is similar,with single peaks and sharp kurtosis,indicating the low-energy and stable sedimentary environment of the stagnant deposits.It can still be used as one of the important signs of the paleo-flood research in the old channel area of the Yellow River in northern Henan.The establishment of typical flood sequences and sedimentary characteristics in the abandoned of the Yellow River in northern Henan Province provides reference and demonstration for subsequent investigation and research of paleo-floods in the North China Plain.