长江三峡阶地的年代对比法及其意义

Terrace age correlation and its significance in research of Yangtze Three Gorges, China

长江三峡的成因是一悬而未决的难题,而阶地的划分对比又是解决三峡贯通问题的切入点.作者在详细研究的基础上提出在阶地对比中的年代对比法.利用对比结果,结合在三峡出口处记录水系演化历史的宜昌地区第四纪早期沉积特征、物源分析及ESR测年等方面的研究结果,认为贯通三峡的长江在宜昌地区出现的时间晚于湖相沉积结束的时间(0.75±0.08) Ma,而长江三峡段的阶地最多可以划分为5级,其中最老一级阶地的年龄为0.7～0.73 Ma.由此证实了长江三峡是形成于中更新世早期的相对年轻的河谷.

Terrace correlation is very important in analyzing the evolution of the Three Gorges of the Yangtze River. It is a difficult but significative problem. However, there is not a unitized division scheme to the terraces of the Yangtze River in the Three Gorges. At the same time, the traditionary terrace correlation on the basis of step number and relative height is limited in application. On the basis of our research on the origin of the Yangtze Three Gorges, the authors think that age correlation must be considered in terrace correlation. Using the result of age correlation of terraces of the Yangtze River in Three Gorges and on the basis of our research result of sedimentary character, provenance analysis and ESR dating of early Quaternary sediments in the Yichang area, the authors believe that the time of the Yangtze River cutting through the Three Gorgess was not earlier than the end time ((0.75)±0.08) Ma of lacustrine sediments in the Yichang City. There are only five terraces correlating well with each other in the reach of the Three Gorges, the oldest of which formed at 0.7~0.73 Ma. All of these results prove that the Three Gorges of the Yangtze River are young valleyes and formed at the early stage of middle Pleistocene.