The Neotectonic Movement From The Quaternary Period And The Water System Evolvement Of The Qingyijang River Valley In Sichuan

The Qingyijiang River is situated in the eastern margin of theQinghai-Tibet, where exists multilevel river terraces in the Yaan area in the south of the Chengdu basin.There are two major gravel beds,which are Mingshan-Qionglai Gravel Bed and Danleng-Simeng Gravel Bed,deposited on the base of these terraces.The river terrace and the gravel beds are not only the record of the hydrodynamic force condition and the origin of deposit,but also marking the local tectonic activity and the climate changes.After the survey of the river terraces,we get the information of the layout,the height and the age of the terraces.After the survey of gravel beds,we know the area and the age of the two gravel beds,the ingredient,granularity,orientation of the gravels.And according this,we discuss the evolution of the water system of Qingyijiang river.The results are giving just as following:1)The river terracesExcept for the terraceâ… ,which is accumulational terrace,all other terraces are bottom terraces.The number of terraces differs from one drainage area to another.An type is the Caoba-Wangu terraces which has 4 terraces,and in Yangping,on the south of Hongya,there are 3 terraces.We have survey the different terraces and got the data of their height and age,based on which we have calculated the incision rate of Qingyijing river and uplift rate of the terraces in different period.2)The gravel bedsThey are fluvial gravel beds,with the charcter of dual structure.The ingredients are quartzite,granite,diorite,dolerite,sandstone and lava.The diamater of the gravels is in the range of 1-40cm,mostly in 5-25cm.The psephicity is good but the sorting coefficient is low.The degree of weathering relates to the number of terraces.Usually, there is no weathering for terracesâ… ,weak weathering for teracesâ…¡,weak to semi weathering for terraceâ…¢and semi to strong weathering for terraceâ…£.The gap filling of the gravels is sand or clunch when the weathering of the gravel bed is strong.3)The history of the evolution of water systemWe partition the history of evolution into 5 phases:1 In the early mid-Pleistocene Epoch(Q₂¹),the Qingyijiang river flowed from southwest to north east and deposited the terraceâ…£of the Mingshan-Qionglai gravel bed.2 From the mid mid-Pleistocene Epoch(Q₂²)to the late mid-Pleistocene Epoch (Q₂³),the Xiongpo anticline uplifted which forced the river changed its way to southeast.After that,the river flow through Shunlong,Hongya and Zhongxing area and joins the Minjiang river.During which,the terraceâ…¢of the tow gravel beds was deposited.3 From the early late-Pleistocene Epoch to(Q₃¹)to the mid late-Pleistocene Epoch(Q₃²),the river had little changes with the weader tectonics movement and deposited the terraceâ…¡.4 From the mid late-Pleistocene Epoch(Q₃²)to the Holocene Epoch(Q₄),the river had to changed its way to southeast in Yaan area due to the uplift of the Xiongpo anticline and the Mingshan-Qionglai mesa.It cut off the south part of the anticline on the west of Luoba,and met the terraceâ…¡of Danleng-Simeng gravel bed.It made a detour by the north of the terraceâ…¡. Now the terraceâ… started to desposited.5 In the Holocene Epoch(Q₄),Due to the uplift of the Shansuchang anticline and the Danleng-Simeng basin,the Qingyijiang river was interdicted again, which made it to cut off the south part of the anticline and flow to southeast.