金沙江巧家-新市镇河段的水系变迁

DRAINAGE EVOLUTION OF QIAOJIA-XINSHIZHEN SECTION OF JINSHA RIVER

水系结构特征指示金沙江巧家-新市镇河段原本存在两条相向的互不连通的古水系,往南流的西溪河-白鹤滩-巧家古水系和往北流的牛栏江河口段-永善-新市镇古水系。地貌、沉积学和阶地堆积物重矿物成分等分析也表明巧家-白鹤滩河段古水系曾是向南流的,现今巧家-白鹤滩河段的北流是被袭夺而发生倒流的结果,金沙江是后来才贯通的。第四纪以来云南高原的拱升使西溪河-白鹤滩-巧家古水系南流受阻,并在牛栏江河口段-永善-新市镇古水系的支流对坪河的溯源侵蚀下被袭夺,造成巧家-白鹤滩河段的倒流和金沙江巧家-新市镇河段的连通。阶地堆积物的ESR测年数据显示,巧家-白鹤滩河段的倒流、金沙江-川江水系的贯通发生在1Ma左右。

Drainage evolution history of the upper Yangtze River, which contains information about the uplift of Tibetan Plateau, has drawn many scholars＇ attention. In this study,, analysis of drainage structure, geomorphology, sedimentology,and mineral assemblage of terraces＇ sediments were utilized to investigate drainage evolution history of the Qiaojia - Xinshizhen section of the Jinsha River, the section from Yibin to Zhimenda of the upper Yangtze River. Analysis of drainage structure of the study area indicates that there were two opposite paleo-drainages in the Qiaojia - Xinshizhen river section： the south-flow Xixihe - Baihetan - Qiaojia paleo-drainage and the north-flow Niulanjiang estuarine section - Yongshan - Xinshizhen paleo-drainage. The Qiaojia abandoned river channel and the ab plane orientations of gravels of old terrace show that the Qiaojia - Baihetan section once should have flowed southward. We also find sedimentological remnants of the south-flow paleo-drainage,such as melange aggregation of sand and gravels in Dongehuan and Gongshan. Moreover there are very thick fluvial or lacustrine sediments in Songming region,south of Gongshan,suggesting the existence of large-scale water systems at one time. Diversity of heavy mineral assemblages between the sediments of ＂paleo-terrace＂ and modern/low terrace indicates that they belong to different drainage systems, and confirms the existence of south-flow paleo-drainage, showing that the Jinsha River only run-through later. We suggest that the Late Quaternary uplift of the Yunnan Plateau blocked the south flowing of the Xixihe - Baihetan - Qiaojia paleo-drainage, and made it captured by the upstream-erosion process of the ancient Duiping River （one of the branches of the Niulanjiang - Yongshan - Xinshizhen paleo-drainage） , and therefore, triggered the reversal-flow of the Qiaojia- Baihetan section and the connection of Qiaojia- Xinshizhen section of the Jinsha River. In addition,ESR datings of terrace sediments of the Qiaojia - Yibin river section show that the reversal-flow of the Qiaojia- Baihetan river section and the connection of Jinshajiang-Chuanjiang river system occurred at around 1Ma.