滇西道街盆地上新世怒江埋藏砾石层、堰塞湖沉积 的发现及其磁性地层学

在对怒江道街—惠通桥段的考察研究中，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，厚近100m的堰塞湖成因的砂层及其下伏厚约100m的砾石层。冲积层的同位素年龄测定结果表明，6级低阶地形成于中更新世以来。望江楼剖面的磁性地层研究结果显示，组成相当于第九级阶地上部的湖相砂层，上部以正向极性为主，含2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的中—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。据此在进一步的区域地层对比后认为：怒江至少在中新世的中晚期已开始形成并下切，至上新世早期已下切至现今河床附近。上述研究结果为更科学地认识青藏高原东缘的强烈隆升过程提供了非常有价值的新线索。

Buried gravels and dammed-lake sediments of the Pliocene Nujiang River discovered in the Daojie basin, western Yunnan Province and their magneto-stratigraphic evidence

During the field work, the authors discovered that along the Daojie-Huitongqiao segment of the Nujiang River there are not only multiple low terraces less than 150m above the river level but also multiple high terraces as well as dammed-lake sediments and their underlying gravels between 180 and 380m above the river level. These high terraces consist of thick bedded alluvial gravels, which are rounded and have complex composition, the dammed-lake sediments consist of sand and are nearly 100m thick, and the underlying sediments consist of alluvial gravels and are about 100m thick. The isotope dating shows that 6 low terraces should be formed since the Middle Pleistocene. The magneto-stratigraphic study of the Wangjianglou section shows that the upper lacustrine sand beds corresponding in composition to the upper part of the 9th terrace are dominated by positive polarity but include two short periods of negative polarity, whereas the lower lacustrine sand beds are of negative polarity. According to their correlation with the standard magneto-stratigraphic column, these lacustrine beds should have been formed during the Middle-Late Pliocene, i.e., 4.2-2.6MaBP, whereas their underlying buried terraces and higher terraces might have been formed during the Early Pliocene-Late Miocene. The regional stratigraphic correlation made on the basis of this deduction suggests that, while the Tibetan Plateau began rising in the Middle-Late Miocene, the Nujiang River probably began its life as a river. It has long been in a downcutting state. By the Early Pliocene, it had downcut into a place below its present river bed.