The Studies On Early Pleistocene Terraces Of Weihe In Longxi Basin Northwest China

Fluvial geomorphology is an important target in geoscience. In different time and space scales we do the research of formation and evolution about river bed through fluvial geomorphology. The terrace is the abandoned riverbed which can be used to recover the history of fluvial geomorphological evolution.We selected three early pleistocene terraces of Longxi as the study object in our work. Based on palaeomagnetic data and loess-paleosol sequence, the palsosol overlying on three terraces preserved along Weihe in Longxi basin were S12, S11 and S10 and the three river terraces were formed about 1.12 Ma,1.08 Ma and 1.03 Ma. Generally, the highest terrace has been considered as an important sign to reflect the appearance of river, so the research of highest terrace reveals that the Weihe appears no later than 1.12 Ma in Longxi basin.Refering the research results of past fluvial geomorphology, there has been a total of 10 Weihe terraces in Longxi basin since 1.2 Ma and the bottom of the loess covering on all terraces are paleosol, respectively, S12, S11, S10, S8, S7, S6, S4, S1, Sm, and S0, which indicate that the incision of river and the formation of terraces are corresponding to the transition. of glacial-interglacial. But not all the transition of glacial-interglacial can lead to the formation of Weihe terraces. So the climate change is an important factor to form terrace, but it does not mean that climate change is the only factor to control the development of terrace.Through linear analysis of the age of 10 terraces and the elevation of terraces, we found that the uplift history Longxi basin are mainly distributed in five periods:1.1-1.0 Ma,1.0-0.86 Ma, 0.86-0.71 Ma,0.71-0.13 Ma and 0.13-0 Ma。It is when the uplift reaches a certain rate (1.12-1.03 Ma,0.86-0.71 Ma和0.13-0 Ma), the climatic changes can result in river incision and the terraces are preserved. If the uplift is very slow, it is very difficult to form terrace in Lonxi basin. Climate change may control the age of terrace formation, but the development of terrace still requires a certain degree of ground uplift. In conclusion, the formation of Weihe river terrace is attributed to coupling effect of climatic changes and uplift. And this "climate-uplift coupling model" not only can explain the formation of Longxi Weihe terraces, Lanzhou Yellow River terraces and eastern Qilian mountain river terrace but also can explain the terrace formation of other area in the earth.The Longxi basin has been strong uplift since 0.13 Ma which can be reflected by the Weihe terraces. And the analysis of stream length-gradient SL Index, Hack Profile and gradient index (SL/K) shows that the tectonic uplift is very strong in the Longxi Basin of Qinghai-Tibet Plateau northeastern margin.