Abstract The Blanco River drains the southern flank of the Chilean Chaiten volcano toward the Pacific Ocean. During 2008, much of the rhyolitic tephra erupted by the volcano was remobilized by this (and others) fluvial system. In the Blanco River, the sediments were temporarily stored upstream of the valley dammed by logs, epiclastic and pyroclastic detritus. The logjam generation was controlled by a complex combination of several factors including abundant tephra influx, rains, steep topographic gradients, forest vegetation, and widening of the fluvial course. Sediment-laden flows generated by logjam breakup or overflow severely affected the downstream-located Chaiten village. The resulting sedimentary record is remarkable by the important proximal-distal changes in sediment composition, grain-size and types of flow within specific depositional episodes. Exposures of fluvial deposits underlying those of the 2008 eruption in the upland sector of the Blanco River display a similar sedimentary record suggesting a comparable hydro-sedimentary dynamics during deposition. This contribution analyzed the sedimentary succession that underlies the 2008 eruption deposits, in order to: i) document in detail the sedimentary processes occurred in the Blanco River as consequence of the interaction with the Chaiten volcano tephra and analyze if this hydro-sedimentary behavior departs from the expected response in similar situations; and ii) contribute to the dating of generation and rupture of logjams and exploration of the associated volcanic hazard. Basic methodology included facies analysis, recognition and correlation of depositional episodes, and 14C dating of wood and charcoal in the deposits. Four depositional episodes (named A to D) were identified within the proximal and distal fluvial segments. The proximal sector is located along the current fluvial course and the distal sector is a lowland floodplain area. In the proximal fluvial segment, the depositional episodes representing syn-eruptive conditions are composed by volcaniclastic gravels and sands, formed by stream flows (with different sediment concentration) and debris flows; and reworked ash and lapilli strata generated by the dilute flow-debris flow continuum. In this fluvial segment, particularly in the depositional episodes B and C, there are downstream facies and compositional changes that suggest recurrent generation and rupture of logjams. Depositional episode B probably occurred between 1216 and 1294 cal AD and 1391–1438 cal AD as suggested by the radiocarbon dates, and is related with the widespread “Vilcun Tephra” from the Chaiten volcano. In the distal fluvial segment the depositional episodes record sedimentation of volcaniclastic mud in ponded zones or temporally flooded areas, with incorporation of abundant organic matter, associated with sediment (sandy)-laden dilute flows and scarce preservation of ash-fall beds. It is further proposed that downstream compositional changes in syn-eruptive fluvial deposits (volcaniclastic sediments that change downstream to pyroclastic sediments) can be indicative of the presence of former logjams in similar fossil fluvial volcaniclastic sequences.

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火山碎屑沉积受僵局破裂影响？智利布兰科河的一个例子

摘要 布兰科河从智利柴腾火山的南翼流向太平洋。2008 年，火山喷发的大部分流纹质火山灰都被这个（和其他）河流系统重新激活。在布兰科河，沉积物被临时储存在被原木、外碎屑和火山碎屑围成的河谷上游。僵局的产生是由多种因素的复杂组合控制的，包括大量的火山灰流入、降雨、陡峭的地形梯度、森林植被和河道的扩大。由堵塞破裂或溢流产生的含泥流严重影响了位于下游的柴腾村。通过沉积物成分的重要近端-远端变化，由此产生的沉积记录是显着的，特定沉积事件中的颗粒大小和流动类型。2008 年布兰科河高地部分喷发的河流沉积物的暴露显示出类似的沉积记录，表明沉积期间的水沉积动力学具有可比性。这篇文章分析了 2008 年喷发沉积物背后的沉积序列，以便：i) 详细记录布兰科河与柴腾火山 tephra 相互作用导致的沉积过程，并分析这种水沉积行为是否与类似情况下的预期反应；ii) 有助于确定僵局产生和破裂的年代以及相关火山灾害的勘探。基本方法包括相分析，沉积事件的识别和关联，以及沉积物中木材和木炭的 14C 年代测定。在近端和远端河流段内确定了四个沉积事件（命名为 A 到 D）。近端扇区位于当前河流流向，远端扇区是低地洪泛区。在近河段，代表协同喷发条件的沉积事件由火山碎屑砾石和砂岩组成，由溪流（不同含沙量）和泥石流形成；以及由稀流-碎屑流连续体产生的重新加工的灰烬和火山岩层。在这个河流段，特别是在沉积事件 B 和 C 中，存在下游相和成分变化，表明堵塞的反复产生和破裂。沉积事件 B 可能发生在公元 1216 年至 1294 年和公元 1391 年至 1438 年之间，如放射性碳日期所示，并且与柴腾火山广泛分布的“维尔昆火山灰”有关。在远端河流段，沉积事件记录了积水区或暂时淹没区火山碎屑泥的沉积，并掺入了丰富的有机物质，与含沉积物（沙质）的稀流和灰落床的稀缺保存有关。进一步提出，同喷发河流沉积物（火山碎屑沉积物在下游变为火山碎屑沉积物）的下游成分变化可以表明在类似的河流火山碎屑岩序列中存在以前的僵局。