长江三峡深槽沉积的研究

STUDIES ON THE SEDIMENTS OF THE DEEP TROUGHS IN THE THREE GORGES OF THE CHANGJIANG RIVER

文章研究长江三峡两个基岩深槽沉积岩砾的岩性组成与粒度特征 ,深槽底部的砾石成分以深槽基岩崩落蚀余物质为主 ,深槽的中上部则以长江底流搬运沉积物为主。14 C测年数据显示 ,深槽的发育在 3.5万年以前 ,以侵蚀深切为主 ,没有留下蚀余堆积 ;3.5万年以后 ,仍以侵蚀为主 ,但已有蚀余为主的岩块堆积 ;大约 2 .4万年以来 ,逐渐有缓慢的冲积砾石堆积 ;至葛洲坝建成以后 ,该地发育了比较稳定的冲积砾石层和砂层。看来大约 35kaB .P .前后 ,该地水动力强度发生过重大变化 ,与施雅风等关于 3～ 4万年前长江上游多雨期研究成果相吻合。

Two of rocky deep troughs at the site of the Three Gorges Dam are studied in this paper. The analysis of lithology and particle size, the prevailing sediments are the residue of rockfall from bedrock at the bottom of the deep troughs. The alluvial gravels are found at the middle and top parts of the deep troughs. The particle size of the sediments became finer from the bottom to the top, which suggests the processes of formation of the rocky deep troughs from downcutting erosion to aggregation. 14 C dating shows that the development of the troughs occurred at about 35kaB.P. No corrosive and remained sediments can be found due to stronger downcutting erosion. A large amounts of corrosive and remained sediments is found only after 35kaB.P. Diluvial sediments are found since 24kaB.P. Accumulation of alluvial gravel and sand layer in the deep troughs have taken place after the Gezhou Dam were built up. The studies show that the hydraulic conditions in the upper reaches of the Changjiang River have been obviously changed since 35 000 years, which is coincided with the research results carried out by Shi Yafeng (excessive rainy period in the upper reaches of the Changjiang River during the periods of 40～30kaB.P.