Richardson et al. (2010) analyzed apatite fission-track (AFT) and U-Th/He low-temperature thermochronology of the Three Gorges area of the Yangtze River, indentified an enhanced cooling event at 40–45 Ma, and therefore proposed that the incision of the gorges began in the Eocene. This time inference is comparable with the age of widespread denudation of the Sichuan Basin, just upstream of the Three Gorges, as suggested by Richardson et al. (2008). The immediate consequence of the incision is the outflow of the Sichuan Basin to the east, which would have resulted in changes in the sedimentary environments and associated facies of the basin infill in the Jinaghan Basin, just downstream of the Three Gorges.
Jianghan Basin began rifting in the late Cretaceous, and became a graben-type basin during the Paleogene, with vast deposition of lacustrine and evaporitic sequences (Fig. 1). The lithology of the Yuyang Formation (late Cretaceous) includes redbed sandstone and mudstone with minor gypseous mudstone. Shashi Formation, of the Paleocene, is similar in lithology to the Yuyang Formation. Eocene strata is the major hydrocarbon-bearing strata basin-wide, and is divided into three formations that contain various proportions of oil shale and evaporate. Late Eocene Qianjiang Formation is ∼3000 m thick, and is dominated with hundreds of halite beds with various thicknesses, ranging from a few centimeters to 2 m. The Jinghezhen Formation of the Oligocene is composed of gray mudstone, with minor hydrocarbon–bearing shale in the lower part. Facies analysis of Paleogene sediments does not show any major trunk streams with a comparable size of the Yangtze River running through the Jianghan Basin.
At the end of the Paleogene, the basin went through a period of adjustment, characterized by uplift, folding, and erosion, and eventually turned into a generally subsiding system (Dai, 1997). Guanghuasi Formation (Miocene and Pliocene) and Pingyuan Formation (Quaternary), characterized by gray mudstone, sandstone, and conglomerate, unconformably overlie the Jinghezhen Formation, and cap up the entire basin. Facies analysis indicates that Neogene sediments are mainly of fluvial nature, formed under the influence of large river systems.
Sediments, from the erosion of, or bypassing through, the Sichuan Basin, are the proximal provenance for the Jianghan infill, provided that the gorges were incised. Our observation of the landscape reveals that most of the topographic relief in Sichuan Basin is lower than 500 m, and is involved with Jurassic and Cretaceous layers that have been folded parallel with the present-day topography. This close correlation of magnitudes and gradients in structural relief with topography implies that the present-day topography is produced by uplift along faults and folds, without strong erosion (Hubbard and Shaw, 2009). Had the Sichuan Basin been denuded in the Eocene, the first place to collect the sediments would be the Jianghan Basin, which does not seem to be the case in the stratigraphic record.
In summary, our studies of stratigraphy and sedimentology of Sichuan and Jianghan Basins do not support claims that the Three Gorges were incised during the Pleistocene (e.g., Li et al., 2001), nor the Eocene. Most likely, the incision occurred as part of the process of regional tectonic adjustment around the Oligocene–Miocene boundary, during which basin development over eastern China went through an important evolution, from graben-type to general subsiding, and associated drainage networks would also be adapted to a new tectonic regime.
This research was supported by National Natural Science Foundation of China (NSFC 40830107) and UNESCO IGCP581.