Paleoflood Events Recorded By Holocene Fluvial Sediments In Baiyangdian

Holocene climate change has been the focus of attention of many scholars and an important period for the study of global environmental change.Holocene paleoflood events are an expression of extreme climatic conditions and environmental evolution.Paleoflood sediments are an important vehicle for studying extreme precipitation events and palaeoenvironmental evolution during the geohistoric period.Paleoflood reconstruction studies based on geological records provide a basis for revealing the historical processes and mechanisms of regional floods,and are of great practical significance for water resources utilization and engineering construction.It also provides a scientific basis for understanding the climatic background of the occurrence of flood events on long time scales.In this paper,based on the field Quaternary geological and geomorphological survey in Baiyangdian area,three profiles were selected for the study:LDC profile in the lower Caohe River,DBC profile in the upper Dabucun of the Daqing River,and XH profile in the lower South Rejects River.The paleoflood events recorded in the late Quaternary rivers(lower Caohe,lower South Reject River and upper Daqing River)in Baiyangdian area were identified.Using AMS¹⁴C chronology combined with the stratigraphic sequence of river deposits,a preliminary time series of Holocene paleoflood events in the Baiyangdian area was established and compared with other areas to explore the climatic background of Holocene paleoflood events in Baiyangdian.The main conclusions were obtained as follows.(1)The paleoflood deposits in Baiyangdian area since the Holocene can be divided into two categories:riverine paleoflood deposits and fluvial flood deposits.The main identification marks of riverine paleoflood deposits are:there are obvious scouring and erosion surfaces between the paleoflood deposits and the underlying strata,and the morphology of the sediment is lenticular,consisting of mud and gravel,wood chips,bivalves and gray-black powder and fine sand mixed,with small and medium-sized interlayer.The particle size of the sediment is characterized by fine sand as the dominant component,containing mixed powder sand and a small amount of clay,which is poorly sorted.The particle size indexes(median particle size,sand content,clay/pulverized sand and Q90)of the flood sediments show high values.The sediments of the river floodplain are mainly composed of clayey silt,clayey silt,silt and fine sand;in the vertical direction,the color,particle size,structure and structure of the sediments have changed abruptly,and they are dark brown in color,with a thickness of 2-20 cm,which is darker than the sediments of the river floodplain;the top of the flood unit has a thin clayey cover layer;there is an organic layer in the paleoflood layer;there is a sedimentation interval between two paleoflood units.There is an interval between two paleoflood sedimentary units.The high value of magnetization rate in the paleoflood sedimentation unit indicates the enrichment of magnetic sediments carried by the flood.(2)Based on the chronological data and regional comparisons,15 major stages of Baiyangdian paleoflood events were identified in two phases:early Holocene(10.8-8.4 ka)with 5 stages,and the dated events were 10.8 ka BP,9.6 ka BP,9.4 ka BP,and 8.4 ka BP,respectively;middle and late Holocene(4.6-1.3 ka)with 10 stages The events dated to 3.9 ka BP,1.9 ka BP,and 1.3 ka BP,respectively.(3)Regional comparison shows that the five paleoflood events recorded in the early Holocene sedimentary record in the Baiyangdian area can be compared with those in other areas of North China during the same period.It is inferred that the early Holocene paleoflood events in the Baiyangdian area were a product of the climate at that time,as the transition from the last ice age to the early Holocene was taking place during a period of enhanced East Asian monsoon climate with frequent fluctuations in temperature and precipitation.The 10 paleoflood events(4.6-1.3 ka BP)that occurred in the middle and late Holocene are more consistent with the late Holocene paleoflood events in the middle and lower reaches of the Yellow River,indicating that the paleoflood events in this period were widespread in the region.The identification and dating of paleoflood events in the sedimentary records of the Caohe,Daqinghe and South Reject River in the Baiyangdian area enriches the study of paleoflood sedimentology in the floodplain area,and provides an important basis for ecological management,water resource regulation and flood mitigation in the Xiongan New Area.