Study On The Oldest Terrace And The Development Age Of The Qingyijiang River In Jingxian County

The fluvial function is an important external force to create the surface landforms.In the course of the development of the river,the river is constantly shaping the form of the surface,leaving a lot of traces and fluvial geomorphology.River terrace,as a former ancient river bed,is an important part of the river geomorphology.In general,the development of river terraces is influenced by factors such as climate change,tectonic movement,variations of base level of erosion,and changes in the internal system of rivers.Therefore,the formation process of river terraces recorded abundant information of palaeoenvironment and paleohydrology.It is an important information carrier for studying regional environmental changes and river development.On the northern fringe of Mt.Huangshan,many river terraces are developed at the junction between mountains and plains.The river terraces are most typical in the Jinxian Basin,Anhui Province,where the Qingyijiang River,one of the longest tributraies of the lower Yangtze River,developed three staircase terraces.In the field,there are two natural section of the oldest terrace(T3),namely CB-T3 section and SJ-T3 section.In this paper,these two natural profiles were selected as research objects.To determine the ages of the oldest terrace and oldest fluvial gravel bed of the Qingyijiang River in the Jingxian county the ESR dating and the paleomagnetic dating were applied.On this basis,the genesis of the oldest terrace of the Qingyijiang River in Yijiang county and the development time of the river were discussed.The main contents and results are as follows:(1)The age of the oldest terrace section of the QingYijiang River in the Jingxian County.The ESR dating results show that the bottom age of red clay on the CB-T3 terrace section of the Qingyijiang River is 884.3±274.9 Ka B.P..In addition,the depth of the B/M boundary is 8.4 m at this red clay layer,and then we can infer that the bottom age of red clay is about 0.93 Ma B.P.Because the development of the red clay is later than the terrace formation,the bottom age of the red clay can be seen as the latest age of the terrace.At the same time,the ESR age of the silt bottom boundary on the SJ-T3 terrace section is 905.4±272.4 Ka B.P..As the silt layer is developed earlier than the terrace,the age of its bottom boundary can be seen as the earliest age of terrace development.Therefore,it is concluded that the age of the oldest terrace of the Qingyijiang River is about 900 Ka B.P..(2)Analysis of the causes of the oldest terrace of the QingYijiang River in the Jingxian County.The literature shows that the intensity of East Asian summer monsoon is at a stage of weakening at 0.9 Ma B.P..Furthermore,based on the correlation analysis between East Asian summer monsoon intensity and regional precipitation and literature data during 1960~2012,we find that there is a negative correlation between the intensity of East Asian summer monsoon and regional precipitation,that is,the weakening of East Asian summer monsoon leads to the increase of regional precipitation.Meanwhile,there were several tectonic uplift movements at the study area and the surrounding area during the Quaternary period,and an important tectonic movement occurred in this region at about 1 Ma B.P..Therefore,it is deduced that the 900 Ka B.P.terrace may be a product of the interaction between regional tectonic uplift and East Asian summer monsoon.Regional tectonic uplift provides dynamic conditions,while the East Asian summer monsoon weakens the climate and controls the age.Furthermore,the regional tectonic uplift provides dynamic conditions,while the weakening of the East Asian summer monsoon provides climate conditions and controls the age of the terrace.(3)The development age of the Qingyijiang River.The oldest terrace and the oldest fluvial gravel layer are important evidence to indicate the development age of river.The oldest terrace of the river indicates the latest development age of Qingyijiang River,and the oldest fluvial gravel layer indicates the earliest development age of the Qingyijiang River.The T3 terrace is the oldest terrace of the Qingyijiang River in the Jingxian county,and the gravel layer of the T3 terrace is the oldest fluvial gravel layer.The dating results show that the age of the oldest terrace of the Qingyijiang River in Jingxian county is about 900 ka B.P.,while the age of the oldest fluvial gravel layer is about 1318 ka B.P..Consequently,we can conclude that the developmental age of Qingyijiang River should be between 1318~900 Ka B.P.,of which 1318 Ka B.P.is the earliest developmental age of the Qingyijiang River,and 900 Ka B.P.is the latest development age of Qingyijiang River.