Facies Succession of Rock Avalanches Triggered by Wenchuan Earthquake, Sichuan, China

Study on rock avalanche kinematics is still a hotspot in the world, especially after the Wenchuan earthquake. To examine this mechanism further, some rock avalanches triggered by the Wenchuan earthquake are taken as objects in this paper. After a detailed field investigation of the vertical profiles revealed by natural longitudinal trenches formed in the rock avalanche deposits, an obvious facies succession along their sliding paths is observed, and proposed to be divided into five zones from the top to the bottom and named as follows: the air-blast zone, the bouncing and jumping zone, the sliding and deposit zone, the basal travel zone, and the original accumulation zone. Based on the detailed field investigations of these zones, the main characteristics of each zone are presented in detail: (1) Besides the appearance of uprooting, snapping, and scraping of trees in the air blast zone, the most typical feature of this zone is the distribution of the branches on trees, with the branches on the upstream sides broken off in sharp contrast to the well-preserved branches on the downstream sides. (2) In the bouncing and jumping zone, the display of accumulated fine grains and isolated blocks is common. (3) The sliding and deposit zone is the main deposit zone of rock avalanche along its sliding path, which accounts for more than 80 % of the total debris volume. In this zone, the most outstanding feature is the presence of inverse grading with coarse clasts accumulated on the upper part and fine grains in its lower. (4) In the basal travel zone, an obvious alignment feature of particles is observed along sliding path, indicating the shearing direction of this zone.