Forming Of The Three Gorges Of The Yangtze River And Sedimentary Response In The West Edge Of Jianghan Basin And Adjacent Area

The Yangtze River, with length is about 6380km, is the longest river in China and the third longest river in the world, which rises from Geladangdong Jokul, flows over Qinghai Province, Xizhang Province, Sichuan Province, Chongqing City, Yunna Province, Hubei Province, Hunan Province, Jiangxi Province, An'hui Province, Jingshu Province and Shanghai City and converge into the East China Sea and Huang Sea at last. The relief of Yangtze catchment is higher in the west (absolute elevation is about 6621m at the source) and lower in the east (the east edge of submarine delta is 55m below sea surface), and three relief stepladders can be observed notably. To the forming and evolution of such large river, it is the focus but difficult question in the science research area, although it was studied by B.Willis in 1903 firstly and has been discussed by many researchers for about one hundred years. Because the formers mainly used morphological methods, such as step number and elevation of terrace, canyon character, relationship of topographical units, there has no unified conclusion to origin of the Three Gorges.Because the Three Gorges lie in dividing position between the upstream and midstream of the Yangtze River, and is located in the transition position between the second and the third relief stepladder, so the evolution research of the Yangtze River at this reach is more important and valuable. At the same time, one of the largest hydraulic projects in the world is carrying through at the Three Gorges, so the research of origin of the Gorges has more practical value.Based on above reasons, this thesis chooses the former of Jianghan Plain桱ianghan Basin, especially west edge including Yichang and Changyang City, as researching area. By the theories and methods of sedimentology, the thesis mainly researches following questions: paleogeography evolution and sedimentary character from later stage of Mesozoic to Quaternary in Jianghan Basin; terrace character in Yichang and Three Gorges area; planation dividing and evolution relationship with Yangtze River; lithology character and evolution history of Huangling dome, which was thought as the drainage dividing between Chuan River and Xia River (they were believed as formers of the Yangtze River) by Li Siguang; sedimentary evolution of Sichuan Basin, Dongting Lake Basin, Poyang Lake Basin, estuary and delta of the Yangtze River; evolution history of the Huanghe River. By these researches, we can achieve forming process and time of the Three Gorges and provide foundation data for researching forming and evolution of whole Yangtze River, preventing and managing geological disaster in the Three Gorges, ensuring security of the Three Gorges reservoir, resolving question of Cenozoic relief invertion in China.By the research of stratigraphic contact relationship, stratigraphic spreading character, heavy mineral association character, trace elements and rare-earth elements character, gravel composition analysis, paleocurrent analysis and hydrodynamic condition analysis, the sixthlevel terrace named by formers is provided to be the sedimentary unit of alluvial fan, fan delta and lacustrine deposit. ESR date and sedimentary character of gavel layers in Luyanchong area show that they are sediments depositing in alluvial fan and fan delta, but not river channel deposits. ESR dating results to Yunchi Group and Shanxiyao Group indicate that forming time of Yunchi Group doesn't match whole lower Pleistocene, and Shanxiyao Group can not represent whole middle Pleistocene, so symbols of Qy, Qs, Qyd, Qp are used to represent Quaternary sediments in Jianghan Basin: Yunchi Group, Shaniyao Group, Yidu Group and Pingyuan Group, instead of using Qi, Q2, Q3 and Q4. According to contrasting ESR age of sandy interstitial matters in gravel layers with paleomagnetic age and TL age provided by formers, we can find that there is only small difference between these results. So it is creditable and accurate to use ESR signal to date age of Quaternary sediments, which coming from E' trap in aquartz...