The Origin And Palaeoclimatic Implications Of The Loess-like Sediment In The Valley Of Jinsha Jiang River

The loess-like sediment in Jinsha River provides an ideal geological materials for researching the palaeoclimate of southwest China, because it is a thick and successively aeolian deposition. It also provides an important information for studying the relationship between the mosoon and the uplift of Tibetan Plateau, the formation and evolution of "dry-hot vally" in Jinsha River, as well as the interaction between the mode of atmosphric circulation of westsouth Tibetan Plateau and the climate change of surrounding regions.In this study, the grain-size features of the Loess-like sediment which is widely distributed on the terraces and high lands of the Jinsha River valley in Yunnan Province are compared with the typical Quaternary loess(Longxi section and Shagou section) and representative Jinsha River fluvial samples. The results indicate that the grain-size distribution pattern of the Loess-like sediment display a bimodal or polymodal distribution, with the silty fraction being dominant, and is essentially similar to that of Quaternary Loess. In grain-size distribution plots, C-M plots, triangular plots, or grain-size parameters and vector analysis,the Loess-like sediment is also similar to that of Quaternary Loess, but greatly differs from the fluvial sediments. So we can conclude that the Loess-like sediment is of aeolian origin. The geochemical characterization of Loess-like sediment is similar to that of dammed lake sediment and fluvial sediment in the same area, which is significantly different from the typical loess in northwest China, it shows that the Loess-like sediment originated from near material. The valley wind provids the force for tansportion of the loess-like sediment.Result of paleomagnetic stratigraphy research indicates that B/M boundary is not appeared in Huatan profile, so the age of the HuaTan loess is younger than0.78MaBP. The age of loess-like sediment at5m and9m in Huatan profile by OSL are81.23±8.24ka BP、122.71±8.12ka BP. Based on average sedimentation the age of the base of Huatan loess-like sediment is373.47±8.18ka BP. It indicates that the time of grassland and forest steppe appeared at recently400ka BP and the "dry-valley" formed at the same time.From comparing the main oxide content, the main element comparision of loess-like sediment and dammed lake sediment, the main element comparision of loess-like sediment and Shagou loess, CIA,A-CN-K, we can conclude that Ca,Mg,Na is lost during the chemical weather. The curves of grain-size and geochemical characterization indicate that the process of loess-like sediment deposition is controlled by glacial-interglacial cycle, and the loess-like sediment has special feature in smaller time scale. The valley wind of Jinsha jiang River became stronger at0.15Ma BP, and the environment bcame warm and wet at0.12Ma BP.