Study On Sedimentary Environment Of Holocene Slackwater Deposits In The Jiaozuo Section Of The Yellow River

The Yellow River has always suffered from frequent flooding,particularly under the background of the strengthening Asian summer monsoon,which caused devastating impacts on the human livelihood.Therefore,studying the causal mechanisms of the Yellow River floods is particularly important not only for hydraulic engineering design,but also for ecological conservation.In this thesis,the sedimentary and chronological characteristics of overbank slackwater deposits from the first terrace of the Jiayingguan section in the Jiaozuo Reach of the Yellow River were studied.The main results are as following:(1)Based on field investigation along the Jiaozuo section of the Yellow River,a sediment sequence with four units of typical overbank slackwater deposits were found on the first terrace of the northern bank of the Yellow River at Jiayingguan.Clear binary phase structure in each deposition unit.The lower part of this sediment sequence is lightyellow silt and the upper part is yellow silt and brown-red clay couplets.The compact thin bedding or wavy bedding conforms to the characteristics of laminar-flow sedimentation.The chronology of this sedimentary seduqnce was determined using the optically stimulated luminescence method(OSL)on the mixture of feldspar and quartz minerals and the final age value is obtained: The ages at 27 cm,64 cm,122 cm,and 265 cm are 1.0±0.2 ka,2.7±0.2 ka,4.4±0.3 ka,and 8.6±0.9 ka,of which the age of 1.0±0.2 ka and 2.7±0.2 ka are the spill flood sediments.The age range of the place is consistent with the corresponding flood period in Wuzhi County.(2)The chemical elements of the sediment samples was clustered and the sedimentary environment characteristics reflected by the geochemical elements of the sediments in the area were discussed.The assemblages of chemical elements of sediments in the Jiayingguan area can be roughly divided into three types.The first type consists of Si,K,Sr,Ca,Rb,and Y,which have peak values in the flooding sediments,reflecting the instability of the hydrological condition of the Yellow River;the second type is Al-affinity elements(including Al,Fe,and Mn),which are easy to transfer in the weathering process and thus reflect the intensity of climate and oxidation;and the third type is composed of Zr,Ba,Zn,Th and Pb,which are relatively stable in nature and are not easily affected by the changes of sedimentary environment.The Geochemical trace-elements of Rb and Sr and their radios indicate the intensity of overbank slackwater deposits.paleoclimate,and weathering intensity.The changes of Sr/Ca?Al and Fe content reflect the depth of the overbank slackwater deposits.The ratio of Al/(Al+K+Na+Ca)can be used as the chemical weathering strength.The indicators reflect the sedimentary environment characteristics of the overbank slackwater deposits.(3)Through grain-size analysis,the overbank slackwater deposits of the Jiayingguan section can be divided into two units: the river bed facies at the bottom,mainly comprising sand and silty sand,with strong sedimentary dynamics and the floodplain facies at the upper part,mainly consisting of silt.The floodplain sediments are mainly composed by transitional suspended matter and uniformly suspended matter of saltation,with grain size parameters consistent with fluvial sedimentary characteristics,especially in overbank slackwater deposits.The grain-size distribution is concentrated,well sorted,positively skewed with a medium peak,which shows the typical sedimentary characteristics of suspended sediment components in rivers in a high water-level detention environment.From the comparison of the color and magnetic susceptibility indicators,the redness(a*),the yellowness(b*)and the magnetic susceptibility both reach high values at 64 cm and 100 cm,and the enantiomorphic temperature and humidity and high-water sedimentary environment characteristics are relatively high.Obviously,the appearance of the fold line also indicates that the sedimentary environment has undergone a great change at this time,which is in accordance with the conditions of the overbank flooding deposits.(4)Combining the OSL dating and depth of the Jiayingguan section,an increasing trend of the aggradation rate of the floodplain can be observaed.The entire Jiayingguan section has been constantly changing in the historical period,and the river bed deposits to the floodplain deposits has a tendency to lower and then increase,which is consistent with the significant development of the river to a certain extent.By comparing the existing OSL ages of slackwater deposits in the Yellow River and the Hanjiang River Basins from literature of overbank slackwater deposits,it was found that the OSL ages of the Jiayingguan section is located in several stages of frequent paleoflooding,especially in the 2500-2900 a BP,4100-4700 a BP and 7900-9500 a BP stages,which reveals basinscale hydrological phenomina under the background of the strengthening Asian summer monsoon.