Lintong Period, Wei River And The Yangtze River Yidu Reach Period Of Holocene Ancient Flood Sedimentology

We have done some field survey on the Weihe River and Changjiang river, and we got fifty nine sedimental samples from two profiles which contain several slackwater deposits and paleostage indicators. They were analyzed in the sedimentologic laboratory in Shaanxi Normal University, including Grain-size distribution, magnetic susceptibility, loss-on-ignition, CaCO3, hydroscopic water and chemical elements content. We contrast the characters of Holocene paleoflood Deposits in two profiles by sedimentologic analysis. The main results are as following:(1) After detailed observation, we found that the Holocene paleoflood Slackwater Deposits were kept at the ancient river water in the bay. In the CDC profile, the color of the Holocene paleoflood Slackwater Deposits is turbid orange, which is close to the upper eolian loess and the lower paleosol. But the texture and bedding are both markedly different. The Holocene paleoflood Slackwater Deposits are clay quality of a material and have the level or wavy bedding, dense hard and the fragments with shell-liked fracture. In the WBC profile, The Holocene paleoflood Slackwater Deposits are pure uniform loose silty sand and have the color of turbid yellow orange. These features are different from loess and paleosol.(2) Through particle size experimental analysis, the results show that the Holocene paleoflood Slackwater Deposits in CDC profile are silty clay, size of a fine, sorting better than other sediments; magnetic susceptibility, loss-on-ignition and hydroscopic water content are both lower than paleosol, higher than malan loess; CaCO3content is opposite. These features show that the Holocene paleoflood Slackwater Deposits in CDC profile have experienced the flood of transportation and redeposition. There are three sections paleoflood Slackwater Deposits in WBC profile. SWD2is silty clay, SWD1and SWD3are both sand clay. Three layers of paleoflood Slackwater Deposits are coarse granularity especially SWD2, Whose sorting is better than SWD1and SWD3. Three layers of paleoflood Slackwater Deposits’ loss-on-ignition and hydroscopic water content are lower than paleosol and eolian loess especially SWD2. The CaCO3content of paleoflood Slackwater Deposits is larger than paleosol and eolian loess especially SWD2. The results show that hydrodynamic force is very strong and transports a lot of sand when paleoflood happened.(3) In CDC and WBC profiles, the chemical elements content of paleoflood Slackwater Deposits is different from loess and paleosol. The result shows that the material source of the Holocene paleoflood Slackwater Deposits is different from loess. In CDC profile, the Curves of major element and trace element standardized by UCC are similar to paleosol. The composition of the local flood’s element is mainly decied by local soil parent material. In WBC profile, the Curves of major element and trace element standardized by UCC are different from paleosol and loess. These features show that the source material of the Changjiang river flood is more complex and tributaries into flood in the area of the elements had great influence. The sediments of two profiles are both typical paleoflood Slackwater Deposits.(4) The sedimentation characteristics of paleosol and loess are significant different in two profiles. In CDC profile, paleosol contains more clay and fine silt, less coarse silt and sand. Median grain of level and average level is lower. Magnetic susceptibility, loss-on-ignition and hydroscopic water content is higher. In WBC profile, because the middle and lower reaches of Changjiang river are low-lying, staying in reduction environment in a long time, soil gleying, FeO forming, so magnetic susceptibility is lower. The CaCO3content of paleoflood Slackwater Deposits is also lower. These features are opposite to loess. These results shows that the paleosol formation period summer monsoon prevail, warm and humid, stronger biological function, more soil clay. The loess formation period winter monsoon prevail, cold and dry, dust accumulation.(5) In CDC profile, OSL dating shows that these palaeoflood events occurred between3200to3000a BP, exactly the same time as that documented in the upper reaches of the Weihe River. This time was the ending of the mid-Holocene climatic optimum, and also the ending of the Shang Dynasty in Chinese history. In WBC profile, OSL dating and archeology shows that three palaeoflood events occurred in Song, Yuan, Qing dynasty.These research results and methods have important theoretical significance; Through OSL dating and archeology, we determine the time of these palaeoflood events and contrast the sedimentology characteristics of paleoflood Slackwater Deposits in Changjiang river and Weihe river. The contrastive results for these regions hydraulic engineering construction, traffic engineering construction, water regulation, flood control and watershed ecology, and so provide an important basis, and enrich the comparison study of paleoflood.