堰塞湖溃坝堆积物的粒度特征分析:以岷江上游叠溪古滑坡堰塞湖为例

Analysis of the Grain-size Characteristics of Dammed Lake's Outburst Deposits: An Example from Diexi Paleolandslide-dammed Lake in the Upper Minjiang River

岷江上游叠溪古滑坡堰塞湖在地质历史时期(20~30 ka BP之间)发生了多次溃决,并在其下游岷江两岸形成一套规模罕见(长约5 km)的溃坝堆积物。通过现场调查、测量取样和室内筛分试验,分析了溃坝堆积物的物质组成、粒度分布及粒度参数等特征。结果表明:(1)堆积物主要由砾石、卵石、砂及少量粉/黏土组成;(2)从上段至下段,溃坝堆积物的平均粒径和分选系数逐渐变小,偏度值逐渐变大,且包含了所有的峰态类型;(3)溃坝堆积物各段的粒度频率曲线均为双峰型,概率累积曲线均表现为上凸型,且从上段至下段逐渐变紧凑。这些特征反映了溃坝堆积物物源区以粗砾石/块石为主,且从上段至下段,粗砾石逐渐减少,细粒成分逐渐增加,沉积物的分选性逐渐变好,溃决洪水的水动力强度逐渐减弱。本研究结果对于认识堰塞湖溃坝堆积物沉积特征及沉积环境具有重要指导意义。

Several burst events of Diexi paleo-lanslide-dammed lake occurred during a prehistory period （be- tween 20 and 30 ka BP） in the upper Minjiang River, which formed an abundance of outburst deposits （appro- priately 5 km in length）. Based on field investigation and measurement and sampling, and laboratory sieving test, the material composition, and the grain-size distribution and parameters were analyzed. The results showed the following aspects ： （ 1 ） the deposits were mainly composed of gravel, cobble, sand, and a small amount of silt and clay; （2） from the upper segment to the lower segment, the mean particle size and sorting of the depos- its gradually decreased ; the skewness became larger, and the kurtosis of the sediment was distributed in all the types; and （3） the grain-size frequency curves were all found to be bimodal; the granularity accumulated curves were all convex, and gradually became compact from the upper to the lower segments. These characteris- tics indicated that the subpopulations reflected the detrital characteristics of the source area, which was charac-terized by gravels as the main component. Moreover, from the upper segment to the lower segment, the gravel sediment gradually became lessened while the fine-grained sediment was found to be increased. Also, the hy- drodynamic intensity of outburst floods displayed a gradually weakening, and the sorting of sediments was im- proved. The study results had important guiding significance for the understanding of the sedimentary character- istics and sedimentary environment of outburst deposits induced by dammed lake.