Dating Of Sediments In The First-level River Terraces Of The Zoige Basin And Research On The Formation Process Of The Terraces

Geomorphology is a hot topic in the research of earth surface systems.It is of great scientific significance to comprehensively understand the surface morphology,development process and genetic mechanism.As an important branch of river geomorphology,it is also valued by scholars.Zoige Basin is located in the northeast of Tibet Plateau.It is characterized by active tectonic movement,complex and changeable climate,well developed valley landforms and exposed first to third grade river terraces.River terraces have natural advantages in response to regional paleoenvironmental changes.However,previous studies mainly focused on the terrace development time,terrace progression,and the basin drainage evolution revealed by the terrace.There is a lack of research on the response of terrace development to environmental change using first-order river terrace sediments.In this paper,a detailed field study on terraces of the main stream of the Yellow River in the Zoige Basin has been carried out,and the typical T1 terraces responding to the environmental evolution of the Zoige Basin have been obtained.Through sedimentological and chronological studies of terrace sediments,the chronological sequence of terrace sediments was constructed,and the formation process of first order terrace and the response of terrace sediments to environmental evolution were discussed.It is of great scientific significance to perfect the study of fluvial geomorphology in Zoige Basin.Based on a large number of field survey,select WDT and XCC two typical level of river terrace sections as the research object,and choose the first terrace section of the Yellow River Valley near the Maqu bridge as the reference object.Combined the OSL dating technology with physical and chemical experiments in laboratory,we studied stratigraphic chronology,sedimentology and sedimentary dynamics of two river terrace sections.And the process of terrace formation and its response to environmental evolution were deeply discussed.The main conclusions are as follows:（1）T1 terrace sediment dating and terrace chronological framework construction.The dating of sediments is mainly accomplished by SAR.260℃ was chosen as the preheat temperature and 220℃ was chosen as the cut heat temperature,which were determined by preheat plateau and dose recovery test.After experiment of preheat plateau,dose recovery,circulation ratio and recuperation,240℃ was chosen as the preheat temperature and 200℃ was chosen as the cut heat temperature.A total of 10 OSL samples from WDT and XCC section were tested for equivalent dose.Quartz particles with diameter of 90～125 μm were selected for all the sediments,which had passsd by IRSL.All equivalent dose values passed the reliability test.And then,determined the suitable age model for different types of sediments.Finally,AGE 2003 software was used to calculate the final AGE and the dating error was controlled within 10%,and the OSL ages of all kinds of sediments in the terrace section were obtained.Through the stratigraphic age correlation with the reference section,the chronological framework of the first river terraces in the main stream of the Yellow River in the Zoige Basin was finally constructed.（2）Sedimentological characteristics of different types of terraces on T1 terrace.By physical and chemical experiments in the laboratory and macroscopic sedimentary characteristics of field strata,the physical and chemical characteristics of various sediments were determined.By dividing the original data of grain-size into several End-member components,the dynamic assemblages of different types of sediments are determined.（3）According to the age and sedimentological characteristics of the section,the formation process of the first order fluvial terraces in the main stream of the Yellow River in the Zoige Basin is inversed.5～4 ka BP is the stage of terrace cutting.During this period,a paleoearthquake occurred near the Maqu active tectonic fault zone.Moreover,the climate environment of Zoige Basin is in the sub-warm and humid period,and the alpine ice and snow melt water flows into the river channel,which increases the flow rate and further strengthens the downward erosion force of the water,thus making the floodplain facies sediments at the original valley bottom of WDT and XCC sections relatively uplifted.4 ka BP to the present is the stage of sediment accumulation.The intensity of tectonic activity gradually weakened to stop,the subwarm and humid climate gradually turned to cold and dry climate,the sandstorm activity was frequent,and there was a second order climate fluctuation during the period,which caused the deposition of different types of sediments on the stable first river terrace of the Yellow River trunk stream in the Zoige Basin,and the uplift to the present height.During 4ka BP to 3.5 ka BP,a century-scale climate cooling event was recorded in the silt layer of XCC section,and then the Paleosol layer deposited in 3.5 ka BP to 3 ka BP recorded the climate warming phenomenon in the Zoige Basin.After 3 ka BP,aeolian sediments and modern subalpine meadow soil with weak soil formation were accumulated in WDT and XCC section.They recorded the decreasing period of climate temperature and humidity from 3 ka BP to 1.5 ka BP and the relative increasing period of climate temperature and humidity since 1.5 ka BP.