Sedimentological Studies On Holocene Paleoflood Slack Water Deposits In The Drainage Basin Of Yiluo River

The Holocene was the quick development and prosperity period of the mankind's civilization, and also the great change period of the global environment. The drainage basin of Yiluo River, which lies in the middle reach of the Yellow River, is one of the prehistoric culture centers in China, and there are a great many of prehistoric sites in it. Existing studies reveal that floods and droughts were so frequent in this area in Holocene that brought significant impact onthe evolution of Chinese culture. Deep studies on the Holocene loess-palaeosol sequences and the slack water deposits profiles, identifying and obtaining the information of paleoflood recorded in them, ensuring the amount, frequency, time and scale of the extreme Holocene paleofloods, are very helpful in revealing the characters and rules of the environment change during Holocene in this drainage basin and the middle reach of the Yellow River.Based on detailed field investigation, five typical profiles, including one high-resolution Holocene loess-palaeosol profile GDZ and four Holocene slack water deposits profiles such as HWC, LHT, YWC and YYC, were sampled and studied by the proxy climatic data of grain size and magnetic susceptibility in laboratory. Information about extreme Holocene paleofloods in those slack water deposits profiles can be identified on the basis of the results of experiment. It also provides evidences for further research on the environment change and human activities and the interaction between them in Holocene in the middle reach of the Yellow River. The main results are as following:1. GDZ profile is a representative Holocene loess-palaeosol profile, which recorded the information about the climate and environment change in and around Yiluo Basin: Late Last Glacial(14700～11500aB.P.) dry-cold climate and frequent abrupt change; Early Holocene(11500～8500aB.P.) temperature rised back but still dry-cold, the summer-wind enhanced while the winter-wind weakened; Mid-Holocene (8500～3100aB.P.) the warmest and humidest period in Holocene, strong summer-wind and pedogenesis, similar climate with the Middle and Lower reach of Yangtze River currently, weakened sand storm and aeolian dust accumulation, strong leaching and argillic processes, the most suitable period was 6760～6520aB.P.; Late Holocene (3100～1500aB.P.) the climate exacerbated to dry-cold, strong winter-wind and weaker pedogenesis, vegetation deteriorated, frequent and strong aeolian dust accumulation, drier than the Early Holocene.The forming process of L₀ is that a large number of silt and clay were deposited in the plain by the frequent breaching of dike and route-changing, overflowing of the Yellow River since Yellow River formed. The dry and loose silt was blown by strong wind along NE direct in winter and spring, forming sand storm, and the thinner grain was carried up in the suspending state and deposited in the place about 20-150km distance beside the region of the silt source. At last, with the physical, chemical and biological ped0genic process, the aeolian sediment became the sandy loess.2. HWC, LHT, YWC and YYC are all Holocene slack water deposits profiles. Based on the cultural relics, the slack water layers of the 4 profiles are identified as the results of Late Holocene paleofloods. The power of flood corresponding to the upper layers is stronger than the lower layers. In LHT, YWC and YYC profiles, the law of magnetic susceptibility is the same, lower in slack water deposits and higher in aeolian find sand layers. The difference lies on the material source of slack deposits and sand. The source of slack deposits is the loess of the upper reach where the soil erosion is very heavy, while the source of sand is the airslaked rocks of mountainous area in the upper reach.3. There were several "interbedded slack deposit-aeolian fine sand layers" in both LHT and YWC profiles, the silt layers are slack water deposits and the aeolian fine sand layers were carried by the NE direct wind. When the aeolian dust carried by the NE direct wind formed L₀ in GDZ in Late Holocene, the strong NE wind in the river valley and frequent paleofloods cooperated the"interbedded slack deposit-aeolian fine sand layers" in LHT and YWC at the same time. Based on the analysis of grain size, the fine sand layers are identified as aeolian deposits carried by the NE wind from the riverbed, while the clayey silt slack layers and silty sand slack layers have totally recorded 12 Holocene paleoflood events in LHT profile. The top 3 layers of LHT profile seem to be the deposits of 3 recent and modern floods. The layers in YWC profile is the most complex and various among these profiles. The experimental data show that more than 10 Holocene paleoflood events were recorded in the following layers of this profile, such as silty sand slack layers, slity sand clay slack layers, silt slack layers, clayey silt slack layers and clay slack layers. The "interbedded slack deposit-aeolian fine sand layers" indicate that the abrupt deterioration and fluctuation of climate and environment system, stronger NE direct wind and frequent paleoflood events in this region in Late Holocene.4. Experimental data show that at least 3 extreme, Late Holocene paleoflood events were recorded in the HWC profile, corresponding to the 3 silt layers. Analysis of experimental data reveals that 8 Holocene paleoflood events were recorded in YYC profile. Because of the gravel riverbed and the lack of sand in riverbed, the "interbedded slack deposit-aeolian fine sand layers" in LHT and YWC profiles were replaced by the slack deposits and medium sand layers. Each layer of this profile represents one paleoflood event of the Late Holocene.