Valley Development Of The Lower Niyang River In Late Quaternary

River terraces,dammed lake sediments,alluvial fans and other river valley landforms and their related sediments record the history of river valley development,and are also geological data to study the response process of river system to regional tectonic and climate change.There are multi-stage terraces and dammed lake platforms in the section from Milin to Grand Canyon of Yarlung Zangbo river and the lower reaches of Niyang river.The study used chronology and environmental indicators to explore the valley development in this area.Based on the field investigation of river terraces,alluvial fans and dammed lake sediments of the lower Niyang river and the section from Milin to the Grand Canyon of the Yarlung Zangbo river,this paper selects the Lamaling lacustrine accumulation platforms and the loess platform in Langou village as the research objects,and discusses the development process of the lower Niyang river in late Quaternary by analyzing the chronology and environmental indicators of sedimentary profiles.The main conclusions are as follows:(1)There are at least two stages of lacustrine accumulation platforms of the lower Niyang River and the section from Milin to Grand Canyon of Yarlung Zangbo River.L1 is mainly distributed in Milin,Mirui,Luxia,Molang,Lamaling,Guoruo and Yongjiu,the platform surface is 30-50 m above river level.L2 is mainly distributed in Lilong,Yusong,Niding,Mirui,Lamaling and Geersa.The distribution of platform surface is discontinuous,and the thickness of section is 3-10 m.There are two terraces of Niyang river in Gongbujiangda and Nixi in the upper reaches,and only one in the lower reaches.The alluvial fan is well developed at the entrance of tributary intel,and often depends on the main stream terrace.Based on the available age data,it is found that the formation age of each section is different,which indicates that many river dammed events have occurred since the late Quaternary.(2)The grain size,magnetic susceptibility and geochemical elements indicate that the L2 of Lamaling experienced the fluvial facie to deep lacustrine facie to shallow lacustrine facie,and the valley has experienced undercuted to dammed to aggradation to undercuted in 45.8 to 44.6 ka.During the MIS3 b,the lacustrine sedimentation rate in Lamaling was 7.53-9.43 mm/a,which was higher than that of lakes in Tibet Plateau.According to the data of lacustrine sediments and sedimentation rate,it is found that the accumulation age of dammed lake sediments mainly corresponds to the glacial period.The lacustrine sediments recorded the occurrence of damming events in the early ice stage in the last glacial,MIS3 b,LGM,the mid-early Holocene glacial fluctuation and Neoglacial period.The development process of the lower Niyang River Valley in the late Quaternary can be divided into two models: undercuted to dammed to undercuted to dammed and undercuted to dammed to graded.(3)Loess deposits are distributed in Langou village and Birishen mountain,with a thickness of 2-5 m.Langou village loess deposit was formed during the last glacial,with a deposition rate of about 0.147 mm/a.The grain size characteristics of loess in langou village are similar to those of Loess Plateau and Western Sichuan.The geochemical elements of Langou loess are similar to the UCC normalized curves of river and lake sediments,which indicates that the loess is formed by wind transport and accumulation of river and lake materials in the near river valley.Compared with Guliya ice core,Dongge cave stalagmite and Chen lake paleoprecipitation and paleotemperature curves,the average grain size of loess is smaller,the magnetization value increases and the value of Rb/Sr is larger since the period from ?25 ka,11.7 ka to neoglacial period and ?2 ka,indicating that the southwest monsoon is enhanced in this period.The average grain size of loess is larger,the magnetization value decreases and the value of element Rb/Sr is smaller in the early glacial period,from-25 ka to Holocene and neoglacial period,which may correspond to the weakening period of three southwest monsoon...