Geomorphic evolution of the Yangtze Gorges and the time of their formation

The Three Gorges of the Yangtze link its upper rocky valley to downstream a lluvial sections. A series of stepped landforms exist in the Three Gorges a rea, the Sichuan Basin upstream and the Jianghan Basin downstream. These la ndforms are characterized by two planation surfaces, one erosional surface, and up to seven terraces. The higher planation surface (named the Exi Surf ace, similar to 1800-2000 m a.s.l.) and the lower planation surface (1200-1 500 m) were probably formed in the Tertiary (prior to 3.4-3.6 Ma B.P.). No correlation seems to exist between the drainage networks of earlier period with the present Yangtze system. It is suggested that the erosional surface found in the study area at 800-1200 m in elevation above sea level was for med in Late Pliocene of Early Pleistocene. During this period following the last planation event, the ancestral Yangtze might have begun to adjust its drainage network. The seven terraces, well developed along the eastern Sic huan Basin and the Three Gorges, date from Early to Late Pleistocene (1.16- 0.01 Ma B.P). The chronostratigraphic evidence and diagnostic sediments are generally comparable in time and space from the Sichuan Basin, through the Three Gorges, to the Jianghan Basin. Therefore, the present study proposes that the Three Gorges were cut not later than the initiation of the earlie st terrace. The fluvial landforms in the Yangtze Gorges are characterized a lso by valley-in-valley cross-sections, i.e. an older U-shaped valley cut b y a younger V-shaped valley. A tectonic rise accompanied by a rapid downcut ting of the river channels predominated in the region. (C) 2001 Elsevier Sc ience B.V. All rights reserved.