| River terrace,which is one of the most important information carriers of climatic change,tectonic movement and river evolution,is of great significance in understanding the mechanism in which the fluvial system responses to climate change and tectonic movement,and in revealing the evolutionary history of a river.Recently,the researches on the fluvial terraces have gained many remarkable achievements in the development model,chronology and river evolution,etc.however,domestically,the related studies have been mainly focused on the upper and middle reaches of the main rivers,for instance the Yellow River and the Yangtze River,whereas the lower reaches of the rivers and its tributaries have not been taken enough attentions.Meanwhile,in the Quaternary red clay accumulation zone affected by the East Asian monsoon,the accurate dating of the river terraces is a difficult and urgent problem because of the thinner overlying deposit on the fluival terraces.The lower Yangtze River is located in both the lower reaches and the red clay accumulation area.If some studies of the river terraces are carried out in the lower Yangtze River,these studies will not only provide a typical case of the fluvial geomorphology researches,but also expand the study content of the fluvial geomorphology,and finally provide some basic data for the studies of the regional palaeoenvironment changes in the lower reaches of the Yangtze River.The Qingyijiang River,which issues northward from Mt.Huangshan,Anhui Province,is one of the longest tributaries of the lower Yangtze River.The river terraces are typically developed in the upper and middle reaches of the Qingyijiang River,which are good information carries of the palaeoenvironment changes in the northern piedmont of Mt.Huangshan and the lower reaches of the Yangtze River.Therefore,the Qingyijiang River was chosen as the river case,and the Jingxian Basin was selected as the study area.Based on ESR dating,paleomagnetic dating,OSL dating,and gravel fabric analysis,grain size analysis,sporopollen analysis,the development mechanism of the river terraces(including proluvial platform)and the evolution history of the Qingyijiang River were discussed.The main contents and results are as follows:(1)Chronology sequence of river terraces of the Qingyijiang River? Based on field survey,GPS measurement and image contrast,a well-preserved sequence of one proluvial platform(P)and three fluvial terraces(i.e.,T3,T2 and T1)was presented.A corresponding sequence of the gravel beds was also found,according to three natural sections,that is,Chengbei cross section,Xikou cross section and Xige cross section.The heights of the platform and the terraces above river level are?65 m??40 m??20 m and?7m,respectively.? Based on ESR dating,paleomagnetic dating,OSL dating and red clay sequence matching,the dating results showed that one proluvial fan platform and three fluvial terraces were formed at?1.32 Ma??0.9 Ma??0.6 Ma and?1.5 ka,respectively,which were therefore corresponding to four significant incised events of the Qingyijiang River at?1.32 Ma,?0.9 Ma,?0.6 Ma and?1.5 ka.(2)Development mechanism of river terraces of the Qingyijiang River?According to the relevant literatures,the East Asian summer monsoon showed three significant stepwise decreases at?1.3 Ma,?0.9 Ma and?0.6 Ma,and significantly reduced at?1.5 ka.Based on the relationship between the intensity of the East Asian summer monsoon and the regional precipitation during 1960-2012,it was concluded that the study area experienced four apparent increased periods of precipitation at?1.3 Ma,?0.9?0.6 Ma and?1.5 ka.?The climate wet stage ? and the climate wet stage ?,which were reflected by the grain-size index of the red clay,were roughly corresponding to the two apparently increased periods of precipitation at?0.6 Ma and?1.5 ka,respectively.In addition,the climate wet stage ?,which lasted nearly 100 ka,were probably the wettest palaeoclimatic period in the study area.? Based on matching the chronology sequences of river terraces with the tectonic movements and the East Asian summer monsoon changes,the river terraces of the Qingyijiang River were combined archives of the tectonic uplift and the significant weak periods of the East Asian summer monsoon,that is,the so-called development mechanism driven by the tectonic uplift and controlled by the East Asian summer monsoon.Moreover,the tectonic uplift provided the force and space for the development of the river terraces,whereas the significant weak periods of the East Asian summer monsoon climatically triggered the formation of the river terraces and determined the time of incision.(3)Development process and age of the Qingyijiang River? According to the analyses of diameter,orientation,shape and lithology of the gravel beds,the results showed that the gravels of P platform were typical proluvial fan deposits,and the gravels of T2 terrace and T1 terrace were typical fluvial sediments,whereas the T3 terrace gravels might be resulted from the superimposition of the proluvial fan deposit and the fluvial deposits,with the proluvial fan deposits to the bottom and fluvial deposits to the top.Moreover,the drainage perimeter and palaeocurrent of the Qingyijiang River have been similar to the present flow since the formation period of the T3 terrace gravels.? Based on the results of the gravel fabric analysis,it was concluded that the Qingyijiang River originated from one of the braided rivers on the proluvial fan at?1.32 Ma,and successively experienced three stages of the proluvial fan,the braided river and the palaeo-Qingyijiang River,that is,the so-called development process of the Qingyijiang River from the braided river on the proluvial fan.? The dating results of the oldest terrace and the oldest fluvial gravels showed that the oldest timing of the Qingyijiang River was?1.32 Ma,whereas the most recent timing of the Qingyijiang River was?0.9 Ma.Therefore,the formation timing of the Qingyijiang River was limited to the period from?1.3 Ma to?0.9 Ma. |
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