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.