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Outburst sediments are widely distributed in the Upper Jinsha River in the Southeastern (SE) Tibetan Plateau. In order to understand the sedimentary characteristics of these sediments, gravel fabric, particle size distribution and quartz sand surface textures were used to analyze the mesotextures and microtextures of the Xuelongnang outburst sediments. It was determined that these sediments usually have a short transport distance, and are distributed over a distance of approximately 3.5 km. The gravel fabric and particle size distribution analysis represent the different mesotextures with different transport distances and suggest gradually changing hydrodynamic conditions. The statistical data of quartz surface textures exhibit the transformation process of quartz sands by outburst flood. The variation from abundant sub-angular shapes (> 75%); to common V-shaped percussion cracks, solution crevasse, medium relief, and chatter marks (50% to 75%); to rare meandering ridge, underwater polished surface, large conchoidal fracture (> 100 μm), directional etch pits, and crystalline overgrowths (< 5%), present the features in different combinations to other sedimentary environments. The microtexture characteristics of landslide-dam outburst deposits are also consistent with those of the mesotextures. As the distance from the residual dam increased, the quartz grain microtextures showed gradual or abrupt changes, such as increased frequencies of V-shaped percussion cracks, decreased frequencies of the adhering particles, and growth of the solution crevasses. These findings could potentially be used as a discriminant mark to distinguish outburst sediment from other types of sediments, e.g., subaqueous, eolian, glacial, etc.

A landslide-dam break flood can cause great damage to the downstream area in a mountainous watershed. The analysis of quartz sand surface textures is a valid microscale method by which to study the genetic mechanisms of flood deposits. However, previous studies have not considered the microstructures of dam-break outburst deposits. In this research, samples from 24 sites along the Diexi dam-break flood deposits in southwestern China were examined and analyzed using scanning electron microscopy (SEM) photomicrographs. In addition, the frequency statistical results of quartz sand microtextures were also compared in different sedimentary environments. On one hand, the upstream end of dam-break flood deposits contained quartz grains with breakage blocks and inherited microtextural characteristics, while the downstream quartz sands are characterized by v-shaped and small conchoidal fractures. The different combinations of quartz sand surface textures indicate the transformation process changing from high energy collisions to low energy collisions over a short distance. On the other hand, the results exhibit the difference between dam-break flood deposits and other sediments such as debris-flow deposits or normal flood deposits, which reveals the potential of this microscopy method for recognition of dam-break flood deposits. The features of quartz sand microtextures also indicate that channel topography, sediment concentration and other factors may play important roles in routing of dam-break flood events, thus we suggest that the quartz sand microtextures can be used as a complementary method rather than the only identification criterion of dam-break flood deposits.

Landslide events causing rivers to become dammed are common natural disasters in mountainous area. Based on the sedimentary facies analysis of 44 sections between proximal and distal parts of a dammed lake, this study reveals the sedimentary facies distribution of the Diexi Ancient Dammed Lake located in the upper Minjiang River, on the southeastern margin of the Tibet Plateau. Fluvial gravels and sand were deposited in the proximal part of the dammed lake. Rhythmically bedded silt and horizontally laminated silt to clay were deposited in the distal part of lake due to the deep and steady water conditions. After progradation of the fluvial system, flood sediments were deposited in the middle part of the lake. The flood carried a large amount of sediments into the lake, and the decelerating flow generated successively alternating layers of gravels and sand, followed by alternating layers of silt with low-angle cross-stratification, parallel bedding, and climbing ripples downstream. Gravel layers formed by four fluvial progradations were found based on the distribution of sedimentary facies in 44 sections. After the first fluvial progradation, lacustrine sediments at the highest point emerged from the water surface. The interval between the second and third fluvial progradation is relatively short, and only a terrace was formed, namely terrace VI. After the fourth fluvial progradation, terrace IV was formed. Although gravel layers formed by fluvial progradations corresponding to terraces II, III and V were not found, it can be determined that a terrace level does not necessarily correspond to a period of fluvial progradation. Therefore, the analysis of sedimentary facies upstream of landslide dams can be used as a supplementary method for inferring the evolution of the dammed lake.