The Distribution,Age And Cause Analysis Of River Terraces,in Pu River Basin Of Xifeng

As the result of the response of river system to the ground uplift and climate change,river terraces record the information of the ground uplift and climate change during the development of River Valley.Therefore,the study of river terraces can not only reconstruct the river evolution history,but also extract the information of land uplift and climate change during the formation of terraces.Xifeng is located in the middle of the Loess Plateau.In the region,the runoff of Pu River and other rivers is small,but the valley is wide and asymmetric,and multi-level river terraces are developed,which are the result of multiple erosion-accumulation of rivers,and contain rich structural climate information.At present,there are few researches on it.Through field survey and indoor chronological analysis of river terraces in the Puhe River Basin of Xifeng,this paper obtained the characteristics,distribution,order and chronological framework of river terraces in the study area.At the same time,it discussed river development model and dynamic source,and the following results and understanding were:1.Field investigation and field measurement showed that Seven-level river terraces were developed in the Puhe River Basin of Xifeng,and the overlying loess strata of T7~T2 terraces are: S0~L15,S0~L9,S0~S5-1,S0~S1,S0~L1,and S0~L1,respectively.2.The comparison of the astronomical tuning ages of the loess-paleosol overlying the terraces and the analysis of the OSL dating of the terraces sediments showed that the formation ages of the seven-level river terraces were: 1.26 Ma,0.94 Ma,0.53 Ma,0.13 Ma,70~60ka,11 ka and 4ka.3.In the Puhe River Basin of Xifeng,the formation of the river terraces were jointly controlled by the ground uplift and climate change.The ground uplift provided the power and space for the river to undercut,and the climate change controlled the transformation of river accumulation-undercut.When the ground uplift rate was slow(T7~T4),the river was less sensitive to climate change,and terraces formed and preserved only during periods of strong climate change;The river was sensitive to climate change when the local surface uplift reaches a certain level(T4~T1),and corresponding terraces formed and preserved in each period of interglacial climate change.4.By comparing the formation ages of river terraces with the uplifting episode of the Tibet Plateau and analyzing the characteristics of regional rivers,it was concluded that the uplifting of the Tibet Plateau and the northeastward expansion were the driving forces for the formation of river terraces in Pu River Basin of Xifeng.