云南怒江河谷新近纪砾石层与堰塞湖相沉积的发现及其意义

Discovery of Neogene Gravels and Dammed-Lake Sediments in Nujiang(Salween) River Valleys,Yunnan Province,China,and Their Implications

作者在怒江道街—惠通桥段考察研究中,填绘了道街盆地的晚新生代地质图,测量了多条河谷横剖面,不仅重新厘定了河谷中拔河150m以下的多级低阶地,而且在拔河180～380m的高度上发现了多级由磨圆良好、成分复杂的厚层冲积砾石层所组成的高阶地,及厚近100m的堰塞湖成因的砂层及其下伏厚约100m的砾石层。本文采集并测试了低阶地冲积物的OSL、U系与ESR年龄,并对湖相砂层进行了磁性地层学研究。研究表明,6级低阶地形成于中更新世以来;在组成相当于第九级阶地上部的湖相砂层剖面的磁性地层柱中,上部以正极性为主含两次短期的负极性,下部则为负极性。与标准磁性地层柱对比表明,该湖相地层形成于4.2～2.6MaBP中—晚上新世,湖相地层之下的埋藏阶地与高阶地则应形成于上新世早期至中新世晚期。据此,本文初步进行了区域地层对比,并认为:伴随青藏高原在中新世中晚期开始阶段性隆升,怒江已开始形成并下切,至上新世早期已下切至现今河床之下。

Field survey in the Nuiiangjie-Huitongqiao section of Daoiie Basin focused on geological mapping for the Neogene Daojie basin and measurement of several crossections in river valley. This not only redefines multiple-level low terraces at ca 150 m below river level but found higher terraces at the river level of 180 m to 380 m, which consist of multiple well-rounded thick fluvial gravel layers with complicated compositions, and about 100-m-thick sandstone originating from dammed lake and underlying 100-m-thick gravel layers. This study tested the ages of sediments collected from the lower terrace using OSL, U-series and ESR methods, and also analyzed their paleomagnetic stratigraphy. The results show that six lower terraces may have formed during Middle Pleistocene and that the upper part of the dammed-lake sediments is dominated by normal polarity with two short negative polarities, whereas the lower part is of negative polarity. Contrasting with the standard magnetic polarity time scale indicates that the dammed-lake sediments formed during Mid-Late Pliocene at around 4. 2~2. 6 Ma, and underlying terrace and higher terrace formed from Early Pliocene to Late Miocene. Based on our comparative study of regional sequences, it is concluded that, in company with intermittent uplift of the Tibetan Plateau since the Middle and Late Miocene, the Nujiang started to take a shape and cut down to today＇s river beds by the Early Pliocene.