HOLOCENE PALEOFLOOD SLACK WATER DEPOSITS AT THE YIJUN REACH OF THE BEILUOHE RIVER

Three layers of paleoflood slack water deposits (SWD) have been found intercalated in the Holocene loess-soil sequence on the riverbank at the Yijun reach of the Beiluohe River, the middle reach of the Yellow River. Each of the SWD represents an individual flooding event. The data of grain-size distribution, magnetic susceptibility, loss-on-ignition, calcium carbonate and chemical analysis indicate that these deposits were deposited as the suspended load from the floodwater. The slack water deposits consist of sandy silt in contrast to the loess and palaeosol which are made of silt and clayey silt. They are well sorted as the results of long-distance transportation in floodwater. The SWD are coarser than surrounding deposits because they were sourced from the loess region at the upper and middle reaches of the Beiluohe River. The magnetic susceptibility, Fe content and loss-on-ignition of the SWD are very low because they were well mixed during rainstorm erosion and floodwater transportation. According to climatic and stratigraphic correlation with the dated Holocene profiles in the region, we conclude that these palaeoflood events happened in 8 000~7 800 aBP (SWD1), 6 000~5 000 aBP (SWD2) and 4 200~4 000 aBP (SWD3) respectively, corresponding to the abrupt climatic events during the mid-Holocene Climatic Optimum.