ABSTRACT
A very strange phenomenon has attracted a lot of attentions in Qiaojia County, southwest China. Thick sedimentary sequences have been observed in local area of Qiaojia County. It is supposed to be a basin that controlled by the Xiaojiang fault. Two ancient landslides were found nearby Qiaojia basin. The relationship between the landslides and the basin is the key point that we studied in this paper. Drill cores revealed the internal structure of the basin that consisted of lacustrine sediments, fluvial deposits and diluvial deposits. The ancient landslide 1# occurred on the hill behind Qiaojia basin, and it is easy to identify because of the apparent boundary characteristics, especially the cambered head scarp. It did not influence the revolution of Qiaojia basin directly, but provided a large number of source for debris flow which deposited on the basin. The ancient landslide 2# occurred on the left bank of Jinsha River, 4 km away from the north edge of Qiaojia basin. The residual body of the landslide elongates nearly 3 km with a height of 230 m. And the residual volume was estimated about 50 × 106 m3. The boundaries of the landslide are very easy to recognize because of the typical terrain characteristics and the suddenly changed bed rock attitude. The original volume of the landslide is calculated about 0.1 billion m3. Such a large rock landslide rushed into the Jinsha River, must dam up the stream for a long time. Therefore, we speculated there used to be a large dammed lake which was caused by the ancient landslide 2#. All of the Qiaojia basin was submerged by the dammed lake. The fine material deposited in the lake and formed the lacustrine sediments. The existence of the two ancient landslides have influenced the evolution of the Qiaojia basin.
Keywords: Ancient landslide; dam; basin; earthquake
