| The Parlung Zangbo River is located on the northeast side of the East Himalayan syntaxis that has strong regional tectonic activity.Under the influence of the humid monsoon of the Indian Ocean,the rainfall is large.Marine glaciers are widely developed,and debris flows under the influence of rainfall and glacial activity.The Previous studies mainly focused on the time and scope of the ancient glaciers activities,and the causes and hazards of debris flow,landslides and landslides along G318.But the investigation of Quaternary basic geology was not sufficient.Based on a large number of field surveys,geophysical prospecting,geological drilling,Uav aerial photography technology.,and remote sensing interpretation,this article combined previous Quaternary survey data to investigate and analysis sedimentary characteristics and distribution characteristics of Quaternary sediments,and the role of Quaternary sediments in the evolution of the rivers in Bomi-Suotong section of the Palongzangbu.and at last the main Quaternary sediments in the area is discussed with the example of the Xuewaka terrace,the Suotong glacial platform and the debris flow in Tian mo valley.This will enrich the Quaternary basic geological contents of the region and provide geological data for the prevention and control of geological disasters and human production and engineering activities,especially the Sichuan-Tibet Highway and the Sichuan-Tibet Railway,which will soon begin construction.It is found in the field investigation that the Quaternary sediments of Parlung Zangbo distributed along the river valley with various genetic types,including three main types as follows: moraines,alluvial deposits,and debris flow deposits.According to the distribution characteristics of the Quaternary sediments,the distribution map of the Quaternary sediments in the research area was drawn.Xuewaka developed four terraces,which once developed a barrier lake.Presumably,it was formed in the lower reaches of Qiuzhu Longba by blocking the Parlung Zangbo.The results of the OSL dating of the two stage terrace show that the results of the sample are 34.5 + 4.5ka BP and 39.9 + 5.2ka BP,corresponding to the late3 stage of the deep-sea oxygen isotope(MIS)(3a),representing the glacial period in the last ice period,and the accumulation of the terrace indicates that the terrace formation is related to the climate and local datum changes.In the lower reaches of the terrace of Qiuzhu Longba,two stages of debris flow accumulation fan were developed.The early debris flow was covered on the terrace.The two-stage debris flow accumulation fan caused the river to bend to the opposite bank.Meanwhile,the drilling showed that a layer of angle-shaped gravel was developed in the gravel layer,which proved that the formation of the terrace is related to the change of the local datum level caused by the debris flow activity in Qiuzhu Longba and Nadui Longba as well as the river blocking in the lower reaches of the glacier.The Suotong platform is the platform of the moraine transformation,developing five terraces.The drilling reveals that the four terraces developed dualistic structure,and the gravel grinding circle was poor,which is proved that the terrace was mainly controlled by climate and it experienced the climate events for at least two times.The "9.4" huge debris flow of the Tianmo Gou was formed by the direct initiation of the massive melting of glaciers caused by strong rainfall and the persistent high temperatures under the condition of the steep gully terrain and abundant provenance,and debris flow blocked the Parlung Zangbo.The glacier activity and debris flow are frequent in the basin,and the local reference surface changes have important control effect on the formation of river terrace.Meanwhile,the river erosion,transport and deposition of the Quaternary sediments in the valley have a profound influence on the formation of the Quaternary accumulation geomorphology.The formation of the Quaternary sediments in the research area is obviously functioned by the rapid uplift and compressional strike-slip caused by the subduction of the Indian Plate into the Eurasian Plate since 3Ma,which is also controlled by the southwest monsoon climate developed synchronously with the Qinghai-Xizang Plateau.It is mainly manifested in the following aspects:(1)There are many moraine ridges developed in the valley.According to the age dating of Chidan Longba's moraine ridge moraine and the opposite shore moraine,they are 91.3 ±4.7 ka BP and 27.2 ±4.7 ka BP respectively,which is later than the last two glacial periods(300-130 ka)and may be formed in the last glacial period.The conclusion is consistent with moraine ridges in the Parlung Zangbo Basin were developed since the last two glacial periods.(2)The debris flow in the study area is concentrated in 6-9 months.It has a good correlation with the rainy season(3-10 months)and high temperature(6-9 months)of Bomi,showing obvious seasonal characteristics.(3)The river terraces are distributed discontinuously along the river,whose formation may be synchronized with the glacier activity and the debris flow's activity.When using river terraces to study regional tectonics and climate change,we should take full account of the control effect of local base level changes on terrace formation.The moraine deposits and the debris flow deposits enter into the main river,which leads to the following phenomena: on the one hand,it changes the shape of the main river bed,causing the river channel to migrate,bending to the other bank,even blocking the river to form a barrier lake with the result of forming a crack point in the longitudinal section of the river;on the other hand,it transports a large amount of debris materials with different particle sizes,such as silts,rocks,which increases the sediment content of the main river,and forms the gravel banks in the lower reaches of the river(mainly debris flow);the stable blocking dam changes the shape of the river valley,and the river channel's choked sedimentary area gradually transforms the mountain canyons into broad valleys.Debris flow accumulation fans indicate the scope of the historical mudslide threat and have a certain guiding role in determining the debris flow prone warning zone.The stable accumulation bodies,such as the moraine ridge,the debris flow accumulation fan,and the river terrace,have the potential to explore and utilize;the longitudinal section of the river channel forms an obvious crack point in the upper reaches of the Guxiang Gully.In addition,the wide valley area in the upper reaches of Guxiang Gully is seriously silted.In the process of future river development,large terraces can be formed,which can become another large human settlement in the Bomi region. |
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