Uplift of Tibet Plateau and formation of Asian Monsoon greatly affect East Asian geomorphological evolution, climate change and environment systems. Thus, those phenomena also control the origin, size and direction of river systems. The Yangtze River, as the most important linkage between Tibet Plateau and the East Asian marginal seas, delivers large volumes of water, sediment, and associated chemicals from its headwater regions and tributaries to the East China Sea, significantly influencing sedimentary system evolution in its drainage basin. Therefore, the formation of the modern Yangtze River and its geological-time evolution history have been paid more and more attention to since the beginning of the last century. After debated for more than a century, the First Bend in Shigu area and the Three Gorges have been known as the key capture point of the Yangtze River’s evolution history. In particularly, the Three Gorges incision period remains greatly controversial, which mainly focuses on Cretaceous period-Neogene period, early Pleistocene period, and late Quaternary period.
The Yichang Gravel, just located downstream and outlet of the Three Gorges with an inverted triangle shape, is mainly distributed in western Jianghan Basin with over 1 000km2. Because of its wide distribution and key geographical location, many typical profiles of Yichang Gravel have been the critical materials for studies on stratigraphic division, geomorphic evolution, and paleoenvironment change in middle Yangtze River Basin, especially on the Three Gorges incision history. Based on the previous field investigation, the Yichang gravel unconformably overlies the Cretaceous bedrocks and underlies the mid-Pleistocene vermicular red earth. In addition, studies on heavy mineral assemblages, Pb isotopic compositions of detrital K-feldspar grains, magnetic characteristics as well as pollen assemblage characteristics have showed that sediments in Yichang Gravel are mainly derived from upper Yangtze River Basin, such as Jinshangjiang drainage, Minjiang drainage, Jialingjiang drainage and Wujiang drainage. Based on the above comprehensive analysis, researchers demonstrated that the depositing time of Yichang Gravel can best constrain the Three Gorges incising time.
The absolute altitude of Yichang Gravel exceeds 110m, and many thick sand lens are developed from top to bottom of the profiles. In this study, we applied the quartz Ti-Li center ESR dating method in Yichang Gravel to determine its absolute formation age, and then to constrain the minimum cutting-through time of Three Gorges. Eight samples(SXY-1, SXY-2, YC-1—4, LJY-1, LJY-2)were collected from the sand lens at depths of 4m, 19m, 40m, 51m, 63m, 75m, 83m and 99m respectively from the top of the profile. At the same time, in order to evaluate the residual dose of Ti-Li center after sunlight bleaching, we also sampled four modern surface Yangtze River sediments near Yichang Gravel for ESR measurement.
The result shows that the quartz Ti-Li center ESR signal intensity of the 4 modern fluvial sediments samples are zero, which implies that the Ti-Li center ESR signal intensity of quartz in Yichang Gravel sand lens could be bleached to zero before the last burial. Thus, the above results indicate that the ESR dating results of this paper are reliable. The ESR absolute age from top to bottom of the profile is 0.73Ma BP,0.87Ma BP,0.98Ma BP,1.04Ma BP,1.05Ma BP,1.10Ma BP, 1.11Ma BP, 1.12Ma BP, respectively. The ESR dating results show that the Yichang Gravel began to deposit at about 1.12Ma BP until 0.73Ma BP, and the Ti-Li center ESR age indicates that the Yangtze River cut through Three Gorges area no later than 1.12Ma BP.
