三峡库区长江阶地冲积物的年代测定

Dating of River Terraces in the Three Gorges Reservoir Area

三峡库区的长江阶地是研究川江、峡江水系演变的重要证据。发育冲积物的阶地主要出现在库区西半部的川东和重庆的中低山、丘陵区,最多有6级。阶地研究中遇到的主要问题是难以确定阶地的形成时代。我们采用TL法和ESR法测量了三峡库区6个地点阶地的堆积年龄。测量数据显示,ESR法基本不适合测定三峡库区冲积物的年龄;尽管TL法本身存在缺陷,但测得的T1￣T4的堆积年龄得到其他独立的测年结果的支持。阶地T1￣T4的TL年龄依次为7￣17ka、28￣46ka、62￣81ka和103￣105ka。此外,根据年代地层对比的方法以及前人的古地磁测量结果,推测三峡库区第四级阶地的年龄大于101ka,第五级阶地的年龄大于392ka,而第六级阶地的年龄小于780ka。从而初步建立了三峡库区阶地堆积的时间序列。

The river terraces of the Changjiang River in the Three Gorges Reservoir area are key to the study on the evolution of the water system of the Chuanjiang River and the Xiajiang River. These terraces mainly develop in the western part of the reservoir area, i.e., the hill and low mountain region in the eastern part of Sichuan. Up to 6 levels of terraces are identified. The main problem encountered in terrace related studies is the lack of reliable temporal controls on the terrace formation. We measured the terrace ages by using the TL and ESR methods. The results show that the ESR method is likely unsuitable for dating fluvial sediments in the Three Gorges Reservoir area. Although the TL method has intrinsic flaws, the ages of terrace T1-T4 given by this method are supported by independent dating results. The TL ages of terrace T1-T4 are estimated to be 7-17 ka, 28-46 ka, 62-81 ka and 103-105 ka respectively. In addition, we infer that terrace T4 is older than 101 ka, terrace T5 older than 392 ka and terrace T6 younger than 780 ka based on the chronostratigraphic and the reported paleomagnetic study. Consequently, an approximate time series of terrace accumulation is established.