Sedimentary evidence of outburst deposits induced by the Diexi paleolandslide-dammed lake of the upper Minjiang River in China

This study reveals that a distinct damming incident occurred during a prehistory period (approximately 30 ka BP) in the upper Minjiang River, at the southeastern margin of the Tibetan Plateau, which formed a giant dammed lake. It can be inferred that paleoseismic induced major landslides were the direct cause of the river damming. The dammed lake breached and caused a large outburst flood at about 27 ka BP. Between the section of the prehistoric ancient dam-body and its downstream areas (5 km), an abundance of diamicts consisting of a poor-sorted wide range of particle sizes, including gravel, sand, and minor silt and clay, have been observed. The outburst deposits in the Diexi area maybe divided into two periods: the thick outburst deposits represented the earlier and largest dammed-break event, and the deposit terrace was due to the subsequent and smaller dammed-break event. The sedimentary structures of the outburst deposits included imbrication, cavitation, matrix, and massive structures, as well as gravel support-stacked structures. Furthermore, a special sedimentary structure became evident, consisting of a rhythmite interbedded structure composed of a coarser gravel layer, and a finer sand and gravel layer, with no sand lens formed within the layers. This special structure can potentially provide an important original criterion for the outburst deposits in this area. The two types of facies associations have been recorded which indicate contrasting sediment transport regimes. The characteristics of boulder facies were most likely associated with hyperconcentrated flood flows. However, the finer-grained Gm/Gfm/GRm facies, which overlaid the Bcm facies, were determined to represent the waning stage of the flooding events. The delicate structures, as well as the absence of coarse components in the Sh facies, indicated an episode of accumulations, which was subsequent to the deposition of the underlying Bcm facies. The results of this study potentially provide important implications for the interpretation of the diamicts in the Diexi area. The grain-size frequency curves of the outburst deposits in the Diexi area were found to all be bimodal (a main peak and an inconspicuous sub-peak). These findings indicated a poorly sorted sedimentary structure. It was found that, from the upstream segment to the downstream segment, the gravel sediment became gradually lessened, while the fine-grained sediment increased. Additionally, the outburst flood hydrodynamic intensity was found to have gradually become weakened.