【作者】 向芳；

【导师】 王成善；

【作者基本信息】 成都理工大学 ， 古生物学与地层学， 2004， 博士

【摘要】 长江是我国第一、世界第三大河，绵延6380公里，从喀拉丹东雪山南麓冰川出发，干流先后流经青海、西藏、四川、云南、重庆、湖北、湖南、江西、安徽、江苏和上海等省市，在东海与黄海的交界处入海。长江流域的总地势为西高东低，横跨中国东西三大地势台阶，西部源头海拔高达6621m，东部水下三角洲的东缘位于黄海平均海平面以下55米，江源与水下三角洲相对高差约6676米。作为这样一条大江，其形成演化的历史一直备受关注，同时也是学术界长期争论的焦点和解决的难点。关于长江流域三峡段的演化问题，最早由B.Willis在1903年提及，迄今已有近100年的历史，但是由于前人主要依据阶地的级数、高程等特征，三峡峡谷特征，地貌间相互关系等地貌学方面的研究，造成三峡成因上的次成谷和先成谷之争、三峡形成时间早晚之争一直难有定论。 由于三峡位于长江上游和中游的交界处、中国地势第二台阶和第三台阶的转折部位，因此对于该段河流演化的研究具有重要的理论价值。同时，中国，甚至世界上最大之一的水利工程在此建设，三峡地区的长江演化研究也就具有重要的现实意义。 正是居于这些原因，本论文从沉积学的理论和方法入手、以长江三峡山口处江汉平原的前身江汉盆地、特别是包括宜昌—枝江一带在内的江汉盆地西缘为主要工作区域，通过对江汉盆地中生代晚期—第四纪的沉积特征和古地理演化过程、宜昌地区及三峡地区的阶地特征、夷平面分期及其与河流演化的关系、被李四光先生认为是峡江和川江分水岭的黄陵穹窿的岩性和演化特征等方面的探讨，并结合对长江三峡上游四川盆地，下游洞庭湖盆地、鄱阳湖盆地、长江河口三角洲沉积区的研究，以及黄河形成演化研究资料的对比，来对长江三峡的形成时间及形成过程进行研究和探讨。从而为最终解决长江流域的形成演化、长江流域地质灾害的预防和治理、三峡水库的安全性问题以及中国新生代地形倒转研究提供基础数据和参考。 论文通过对宜昌地区云池组、善溪窑组命名剖面附近的沉积物进行地层接触特征、展布关系、重矿物组合特征、砾石成分分析、古流向和水动力条件分析等方面的研究，认为前人划分的第6级阶地并不存在，云池组为冲积扇—扇三角洲—内陆湖泊沉积，善溪窑组下部为扇三角洲一湖泊沉积，上部主要为湖相沉积，并不是具有二元结构的河流阶地。同时通过对卢演冲一带的沉积物进行年代测定、沉积物特征对比，发现卢演冲地区的砾石层实际上为云池组下部和善溪窑组下部沉积，同样为冲积扇—扇三角洲成因，而不是河流二元结构的下部沉积。通过对云池组、善溪窑组进行ESR测年分析，发现云池组的形成时代并不能代表整个早更新世、善溪窑组的形成时代分属于早更新世和中更新世，因此不能用Q₁、Q₂来代表云池组和善溪窑组。鉴于沉积体这种岩石地层单位与年代地层单位的不一致，在论文中采用了用Qy、Qs、Qyd、Qp来分别代表江汉盆地第四纪沉积物：云池组、善溪窑组、宜都组、平原组。其中云池组的ESR年龄为1.15-1.08Ma，善溪窑组的年龄为0.87-0.75Ma。通过将本次对砾石层中砂质填隙物ESR年龄数据与前人所作的古地磁年龄、热释光年龄进行对比，发现数据差别不大，说明利用α石英受α、β射线产生的E’陷阱中心的ESR信号进行第四纪早期沉积物的年龄测定具有一定的可信性和准确度。 对于江汉盆地西缘宜昌地区来说，其沉积及古地理演化表现为:早自坐世为受天阳坪断裂控制的内陆湖泊沉积环境，沉积中心在天阳坪断裂的北侧，此时儿乎不存在来白黄陵地区的水系;晚白全世同样为受断裂控制的内陆湖盆环境，但湖盆面积扩大、断裂活动减弱，且后期出现咸化湖相沉积;第三纪早期盆地由断陷盆地转变为坳陷盆地，同时石膏的出现代表了局限咸化湖盆的存在;渐新世一中新世，受构造影响，缺失该时期的沉积;上新世时在鸦鹊岭一带出现的泥灰岩沉积反映了陆源物质供应不足的局限沉积环境的存在;早更新世一早中期为冲积扇一扇三角洲一内陆湖泊沉积，一甲一更新世晚期一中更新世早期为扇三角洲一湖相沉积，后期出现沉积结束以后的网纹红十化作用。这种沉积及古地理演化特征说明，在中更新世早期以前，不存在大河体系在该区的切割和疏淘作用。同时，对云池组和善溪窑组的主要重矿物分析发现，重矿物组合中错石、金红石、磷灰石的含量具有正相关性，而不存在有角闪石、辉石，这与现代长江河床沉积物以及长江第4级阶地沉积物重矿物特征、前人所做的长江三峡沿岸各级阶地重矿物组合特征明显不同。同时，在砾石成分上，云池组及善溪窑组中可以见到花岗质砾石，但是缺乏在现代长江沉积物和长江阶地沉积物中可以找到的黄陵穿窿西侧出露的辉氏岩、闪长岩和在四川西部大面积分布的峨眉山玄武岩，因此说明云池组、善溪窑组沉积时，并没有切过黄陵弯窿的大河体系存在。通过对云池组和善溪窑组中玄武质砾石的岩石学特征、稀土元素和微量元素特征的分析结果，结合三峡地区、特别是湖北省境内宜吕地区附近的玄武质岩体的分布特征，同样发现此沉积阶段没有来自峨眉山玄武岩分布区的物质，同时，该分析结果及砾石古水流分析均反映出云池组和善溪窑组沉积时，有来更多还原

【Abstract】 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更多还原