The Distribution,Age And Its Geomorphological Significance Of Tao River Terrace,Lintao Basin

River terraces is one of the most prominent phenomenon of mountain valley geomorphology,The research of the morphology,process and chronology of the river terrace-the typical stratiform geomorphic unit can not only rebuild the developing process of the valley,but extract some regional ancient environmental information such as tectonic activities and climate changes.Lintao Basin is a small intermontane basin located on the western Loess Plateau,where,according to field investigation,the Tao River is developing and reserved 6-8 grades of terraces,with 6 on the east,4 on the west and 1 grades of loess mesa without exposure of the layer of the river gravel.Each grade of terraces is covered with eolian loess of different thickness,serving as a good carrier for the study of chronology of the terrace.The research of senior terrace chronology adopts the method combining loess magnetostratigraphy and climatic stratigraphy while of the lower terrace adopts the one combining OSL dating and climatic stratigraphy.As for those without obtaining the accurate dating,we estimate by the terrace elevation-age linear relationship.The result shows that all the overlying aeolian loess on T1-T6 Terraces in Lintao Basin of Tao River is paleosol,respectively S0,Sm,S1,S4,S5,S8 Paleosol Layer.According to the orbital tuning age,we infer that the time of the formation of the terraces is respectively 14ka,57ka,0.13Ma,0.42 Ma,0.62 Ma,0.87Ma.The analysis of the sediment grain size parameters also shows that the bottom of overlying aeolian loess of all grades of the terraces is paleosol layer,indicating that the river undercutting terraces were formed during glacial-interglacial transition.Since 0.87 Ma,the area has experienced many orbital scale climatic transitions,but not every time it brought the terrace into formation,indicating that climate changes is not the only factor for the formation of the terrace in the region.With a comprehensive study on the spatial distributions of the terraces and chronology,we obtained the history of the undercutting of the Tao River in Lintao Basin since 0.87 Ma:the river has totally been undercut for about 155meters,with an average incision rate of 0.18 m/ka.During 0.87-0.13 Ma,it has been undercut for 116 meters,with an average incision rate of 0.16m/a.And since 0.13 Ma,it has been undercut for about 39 meters,with an average incision rate of 0.3m/a.The undercutting of the passage of the Tao River during 0.87-0.13 Ma and in 0.13 Ma,manifested two stages-slow and fast.The Loess Climatic Stratigraphy shows that there are in total 7 climatic transitions during 0.87-0.13 Ma,among which only 4 corresponds the formation of the terrace.However,since 0.13 Ma with every climatic transition,terraces came into being.The differences may manifest the control of the uplift rate of the ground to the formation of the terraces,that is to say,slow rate of uplift is not conductive to the formation of the terrace.Above all,we come to a conclusion that the formation of the Tao Terrace in the Lintao Basin results from the undercutting of the river driven by the climate changes against the background of the uplift of the ground.