A Study On The Chronology Of The River And Lakes In The Western Part Of The Emei Terrace In The Yellow River

The western Yellow River Emei platform is in the Northeast of Weihe basin,and the basement is lacustrine sediment,shows that in the late Quaternary in the region has long been occupied by the lake.The platform is composed of a multi-stage sub platform.There are different thickness loess-paleosol on the platform.In front of the platform where near the Luo River,also developed a river terrace,and the loess paleosol laye covers the river terrace,indicating that the western Emei platform occurred much lake retreat movement.The study of the age of the river terraces in the western Yellow River Emei platform can infer the evolution of environment of rivers and lakes in the region,and from the whole understanding of Qinghai Tibet Plateau uplift and its influence on the environment.We can know the age of the terraces by determining the oldest overlying loess-paleosol sequence.The main method is the loess-paleosol sequence.The overlying loess-paleosol of the typical section of river terraces were measured the magnetic susptibility and paleomagnetism.Field investigation found that six terraces of rivers and lakes are widely distributed in the western Yellow River Emei platform,which we called T6,T5,T4,T3,T2,T1.T1 is a river terrace and others are lacustrine platform.Depenned on the analysis of the magnetic susptibility and paleomagnetism,the formation age of the terraces is 1.118 MaBP,850KaBP,620 KaBP,205KaBP,150 KaBP,and52KaBP,correspond S14,L9, L6,S2,L2, L1 SS accumulation period.According to the formation age of the terraces of lakes and rivers,we can infer the evolution of lakes and rivers in the region.There were five times lake regression corresponding to the S14,L9,L6,S2,L2 accumulation period,and the lake in the area should have completely disappeared because of the lake regression in the late L2.The Luohe River was under the cutting movement when the L1 SS started to accumulate.T5,T4,T3 platform in the lower part of the lake are mixed with the loess layer.The reason is that when the higher platform(T6,T5,T4)formated, the lake regression occured,so that the lower terrace was also exposed to the air,and began to accumulate loess layer.Below S2 on T4,there is lacustrine sediment.This is due to the lake transgression after L6 accumulated.L2 also contains a thin layer of lacustrine layer,we speculate that this is a part of the water depression,because there is no corresponding to the lake in this period of invasion.L2 on T3 contains thin lacustrinelayer,and this is due to the lake transgression after S2 accumulated. In short,after the rapid lake regression occurs,there will be a slow lake transgression.The formation of the lake terrace( T6,T5,T4,T3,T2) is the result of tectonic uplift;T1(Terrace of Luo River), mainly affected by the climate change.Through the comparative analysis with Shanglo profile,it is concluded that there should be a terrace dyring the formation of S11.At present,there is no discovery.In the future,the problem will be deeply studied.