Abstract:

Taking the Waka debris-flow gully in the dry-hot river valley area of the upper Jinsha River as an example, the concept of sedimentary facies was introduced and the debris-flow sedimentary types were divided according to the characteristics of the sedimentary facies. The debris-flow sedimentary facies of Waka profile were divided into three types, Facies A (coarse-gravel layer), Facies A1 (fine-gravel layer) and Facies B (surface mud layer), i.e. Three or four samples were collected from each unit and 61 samples were collected in all and their grain sizes were analyzed. The characteristics of grain-size distribution and fractal structure of these samples were analyzed based on grain-size data. The results show the following aspects: (1)The curves of the grain-size distribution of the three sedimentary facies are all of bimodal type, reflecting bad sorting of sedimentary materials for debris-flow deposits. (2)Facies A and Facies A₁ are of coarse-fine mixed gravel layers, and the grain-size distribution is not uniform. However, Facies B is of clay layers which include few or no gravel, and the grain-size distribution is more homogeneous. (3)The fractal features of Facies A and Facies A₁ are more apparent than that of Facies B, and the grain-size fractal dimensions of Facies A and Facies A₁ are 2.692,1 and 2.697,1 with relative coefficient of 0.868,2 and 0.832,2, while the grain-size fractal dimension of Facies B is 2.9007 with relative coefficient of 0.43.The differences of grain-size distribution and fractal feature among Facies A, Facies A₁ and Facies B reveal that these three debris-flow sedimentary facies formed in different sedimental environments.

Key words: debris flow, sedimentary sub-facies, grain size distribution, fractal structure