江汉平原沉积环境演化及对三峡贯通的指示

选取江汉平原孔深400.59m的新沟钻孔，通过对其40Ma以来的沉积物进行岩性描述及粒度特征的研究，探讨了此岩芯剖面沉积环境的演变过程。结果表明，该岩芯垂直剖面沉积物颗粒基本构成是以砂、粉砂、粘土为主，沉积环境变化频繁。根据粒度参数特征所反映的沉积环境意义，划分出4个沉积阶段: 3.93～3.66MaB.P.(深度400.59～375.03m)为深湖相沉积环境，   3.66～2.17MaB.P.(深度 375.03～.195.90m)为河湖相沉积环境，   2.17～1.77MaB.P.(深度195.90～147.08m)为浅湖相沉积环境，   1.77MaB.P.～0.11kaB.P.(深度147.08～0m)为河湖相交替频繁的沉积环境；  从而揭示了江汉平原新沟钻孔4.0Ma以来的沉积环境的演变过程。对比江汉平原新沟孔和周老孔的磁学参数及磁性矿物特征的研究结果，发现在新沟孔深度109.15～98.82m间江汉平原水系曾发生过重要调整，江汉平原的沉积环境和物质成分均发生了重大改变。该层位可能就是长江三峡贯通的层位，其贯通时间新沟孔古地磁年龄约为1.0MaB.P.左右。

SEDIMENTARY ENVIRONMENT EVOLUTION OF THE JIANGHAN PLAIN AND ITS SIGNIFICANCE TO THE FORMATION OF THREE GORGES

The Jianghan Plain，situated at the middle reaches of the Yangtze River，is the first large basin after the Three Gorges. Its sediments must have recorded the origin and evolutionary history of the Yangtze River and the Three Gorges. Depositional environment research can be chased back to as early as the beginning of the 20th Century by the particle analysis method. The deposit particle is related to transporting force，mode and distance of clastics，and hydrodynamic conditions，thus can reflect the depositional environment and provenance well. Grain size characteristics of the Xin′gou Core(40059m deep)sediments from the Jianghan Plain which have a record of ca4.0Ma B.P.continuous deposition are studied to analyze the depositional environment. The basic composition of sediment particles is dominated by clay，sand，and siltsand，with the ratio changing along the changes of the depositional environment. According to the lithologic characteristics of the drilling section，combined with the paleomagnetic dating result of the Xin′gou and Zhoulao cores and with the international standard polar column(S.C.Cande,1995)，the Xin′gou Core depositional environment from bottom up can be divided into the following four depositional phases: 3.93～3.66MaB.P.(400.59～375.03m deep)，the lacustrine depositional environment. 3.66～2.17MaB.P.(375.03～195.90m deep)，the alternation of fluvial and lacustrine depositional environment. 2.17～1.77MaB.P.(195.90～147.08m deep)，the shallow lacustrine depositional environment. 1.77Ma B.P.～0.11kaB.P.(147.08～0m deep)，the fluvial facies and lacustrine facies which was mainly represented by fluvial facies. The comparison of magnetic parameters and magnetism mineral characteristics between the Xin′gou and the Zhoulao cores reveals that the section of 109.15～98.82m in the Xin′gou Core recorded a significant change of depositional environment and material composition，which was likely caused by the running through of the Three Gorges at about 1.0MaB.P.