Sedimentary facies and depositional processes of the Diexi Ancient Dammed Lake, Upper Minjiang River, China

Abstract

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.