重庆万州长江岸坡陡崖后退速率的估计

重庆万州的长江岸坡发育剥蚀基岩平台与陡崖,我们采用三角坡面法估计了陡崖的后退速率。采用该方法的依据是,铺垭口的陡崖下存在形成时代不同的坡面,同时该地点的地层近水平,所以我们假定陡崖的后退过程类似干旱-半干旱环境中的平顶岩屑堆地貌的形成过程。通过对前一阶段残留坡面的函数拟合,可以估计陡崖的后退距离。如将残留坡面上T3阶地的年龄近似为崩坡积的形成年龄,则计算出的陡崖后退速率大约是0.51 m/ka。这个数据表明,该地区最高的450 m的剥蚀基岩平台大约在2.3 M a前左右被长江切穿,推测2.3 M a可能是三峡贯通年龄的上限。

Rate Estimation of Scarp Retreat on Bank Slope at Wanzhou of Chongqing

Three denuded bedrock platforms at elevations of 450 m,275 m and 235 m respectively develop on the bank slope of Changjiang River in Wanzhou of Chongqing.A scarp of height of about 40 m occurs on the edge of the 450 m platform.The process of the scarp retreat is a key to understand the evolution of the Changjiang River.But quantitatively estimating the retreat rate remains as an unsolved issue.We found the geomorphic characters at Puyakou are comparable to that of the talus flatiron formed in arid and semi-arid environments,e.g.almost horizontal strata,colluvia.Another evidence to assume the forming process of the scarp at Puyakou is similar to that of the talus flatironis that we can distinguish different slope stages below the scarp,and the position of the apex of residual slope can be figured out.Different slope stages are mainly determined by applying ~(14)C and TL dating of deposits.Based onthis viewpoint,we estimate the retreat rate of the scarp by analyzing triangular slope facets.We calculate the retreat distance of the scarp by using the function fitting of the residual slope.The retreat distance is 73 m.Since the age of deposits cannot be accurately determined,we put forward an indirect approach,i.e.the age of T3 terrace is used as an approximation of the formingage of the colluvium.Thus,we obtain a retreat rate of about 0.51 m/ka for the past 143 ka.This result implies that the highest denuded bedrock platform was possibly cut off 2.3 Ma ago, and remarkable changes of the water system took place at that time.This age is close to the estimated time of the first appearance of Three Gorges.