岷江上游叠溪混杂堆积体的沉积特征及其成因分析

青藏高原东缘岷江上游叠溪河谷段在地质历史时期发生了一次大规模滑坡堵江事件,形成一个特大型堰塞湖。堰塞湖形成后在晚更新世晚期(约27 ka B.P.)发生了溃决,并在坝体下游形成长约5 km的天然混杂堆积体,判断其为叠溪古滑坡堰塞湖溃决后形成的溃坝堆积。该套溃坝堆积体具有叠瓦构造、孔洞构造、块状构造、杂基构造、支撑—叠置构造及韵律互层构造。从上游至下游,溃坝堆积体的出露厚度逐渐变薄,砾石碎屑成分表现出由粗变细的变化趋势。溃坝堆积体是由高流态灾难性洪流及常态流和河流态两种机制形成,相应地具有两大类沉积相:巨砾层相及砾石层相和砂层相,依据溃坝堆积的地貌结构和沉积相特征可以推断叠溪古滑坡堰塞湖至少发生过一次极其罕见的灾难性溃决洪水事件。

Sedimentary Characteristics and Causes of Formation of Diamicts in Diexi,Upper Minjiang River,China

A distinct landslide damming incident occurred during a prehistoric period in the upper Minjiang River at the southeastern margin of the Tibetan Plateau. This formed a very large dammed lake which breached and caused a large outburst flood at about 27 ka. Between the section of the ancient dam and its downstream areas (5 km), an abundance of diamicts have been observed for the first time. We determine that they are outburst deposits induced by breaching of the Diexi paleolandslide-dammed lake. The sedimentary structures of the outburst deposits include imbricates, cavitation, matrix and massive structures, gravel support-stacked, as well as rhythmitic interbedded structures, among others. From upstream to downstream, the thickness of the outburst deposits gradually thins, and the gravel shows a tendency to change from coarse to fine. Hyperconcentrated flood flows and normal stream or fluctuating flows are recorded, indicating contrasting regimes of sediment transport. Also, two kinds of sedimentary facies occur in the outburst deposits: boulder facies, and gravel and sand facies. From the way in which the sedimentary facies developed, at least one catastrophic outburst flood event can be assumed to have occurred in the Diexi paleodammed lake.