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Sedimentology and geomorphology of the deposits from the August 2006 pyroclastic density currents at Tungurahua volcano, Ecuador
Bulletin of Volcanology ( IF 3.5 ) Pub Date : 2013-10-24 , DOI: 10.1007/s00445-013-0765-7
Guilhem Amin Douillet 1 , Ève Tsang-Hin-Sun 2 , Ulrich Kueppers 3 , Jean Letort 4 , Daniel Alejandro Pacheco 5 , Fabian Goldstein 3 , Felix Von Aulock 3 , Yan Lavallée 3 , Jonathan Bruce Hanson 3 , Jorge Bustillos 6 , Claude Robin 7 , Patricio Ramón 5 , Minard Hall 5 , Donald B Dingwell 3
Affiliation  

The deposits of the pyroclastic density currents from the August 2006 eruption of Tungurahua show three facies associations depending on the topographic setting: the massive, proximal cross-stratified, and distal cross-stratified facies. (1) The massive facies is confined to valleys on the slopes of the volcano. It contains clasts of >1 m diameter to fine ash material, is massive, and interpreted as deposited from dense pyroclastic flows. Its surface can exhibit lobes and levees covered with disk-shaped and vesicular large clasts. These fragile large clasts must have rafted at the surface of the flows all along the path in order to be preserved, and thus imply a sharp density boundary near the surface of these flows. (2) The proximal cross-stratified facies is exposed on valley overbanks on the upper part of the volcano and contains both massive coarse-grained layers and cross-stratified ash and lapilli bedsets. It is interpreted as deposited from (a) dense pyroclastic flows that overflowed the gentle ridges of valleys of the upper part of the volcano and (b) dilute pyroclastic density currents created from the dense flows by the entrainment of air on the steep upper flanks. (3) The distal cross-stratified facies outcrops as spatially limited, isolated, and wedge-shaped bodies of cross-stratified ash deposits located downstream of cliffs on valleys overbanks. It contains numerous aggrading dune bedforms, whose crest orientations reveal parental flow directions. A downstream decrease in the size of the dune bedforms, together with a downstream fining trend in the grain size distribution are observed on a 100-m scale. This facies is interpreted to have been deposited from dilute pyroclastic density currents with basal tractional boundary layers. We suggest that the parental flows were produced from the dense flows by entrainment of air at cliffs, and that these diluted currents might rapidly deposit through “pneumatic jumps”. Three modes are present in the grain size distribution of all samples independently of the facies, which further supports the interpretation that all three facies derive from the same initial flows. This study emphasizes the influence of topography on small volume pyroclastic density currents, and the importance of flow transformation and flow-stripping processes.

中文翻译:

厄瓜多尔通古拉瓦火山 2006 年 8 月火山碎屑密度流沉积物的沉积学和地貌学

2006 年 8 月通古拉瓦火山喷发的火山碎屑密度流沉积物根据地形环境显示出三种相关联:块状、近端交叉层状和远端交叉层状相。(1) 块状相仅限于火山斜坡上的山谷。它包含直径大于 1 m 的碎屑到细灰物质,是块状的,被解释为由致密的火山碎屑流沉积而成。它的表面可以呈现出覆盖有圆盘状和水泡状大碎屑的裂片和堤坝。这些易碎的大碎屑必须沿着路径漂流在流的表面才能保存下来,因此意味着这些流的表面附近有一个尖锐的密度边界。(2) 近端交错层理相出露在火山上部的河谷堤岸上,既有块状粗粒层,也有交错层理的灰、火山灰层。它被解释为沉积于(a)从火山上部山谷的平缓山脊溢出的稠密火山碎屑流和(b)由陡峭的上侧翼夹带空气而产生的稀火山碎屑密度流。(3) 远端交错层状相出露为空间有限、孤立、楔形的交错层状灰分沉积体,位于河谷河岸悬崖下游。它包含许多不断升级的沙丘床型,其波峰方向揭示了父母的流动方向。下游沙丘床型尺寸减小,在 100 米尺度上观察到晶粒尺寸分布的下游细化趋势。该相被解释为由具有基底牵引边界层的稀火山碎屑密度流沉积而成。我们认为母流是由悬崖夹带空气的密集流产生的,这些稀释的流可能通过“气动跳跃”迅速沉积。三种模式存在于所有样品的晶粒尺寸分布中,与相无关,这进一步支持了所有三种相都来自相同的初始流动的解释。本研究强调了地形对小体积火山碎屑密度流的影响,以及流动转变和流动剥离过程的重要性。该相被解释为由具有基底牵引边界层的稀火山碎屑密度流沉积而成。我们认为母流是由悬崖夹带空气的密集流产生的,并且这些稀释的流可能通过“气动跳跃”迅速沉积。三种模式存在于所有样品的晶粒尺寸分布中,与相无关,这进一步支持了所有三种相都来自相同的初始流动的解释。这项研究强调了地形对小体积火山碎屑密度流的影响,以及流动转变和流动剥离过程的重要性。该相被解释为由具有基底牵引边界层的稀火山碎屑密度流沉积而成。我们认为母流是由悬崖夹带空气的密集流产生的,这些稀释的流可能通过“气动跳跃”迅速沉积。三种模式存在于所有样品的晶粒尺寸分布中,与相无关,这进一步支持了所有三种相都来自相同的初始流动的解释。这项研究强调了地形对小体积火山碎屑密度流的影响,以及流动转变和流动剥离过程的重要性。我们认为母流是由悬崖夹带空气的密集流产生的,并且这些稀释的流可能通过“气动跳跃”迅速沉积。三种模式存在于所有样品的晶粒尺寸分布中,与相无关,这进一步支持了所有三种相都来自相同的初始流动的解释。这项研究强调了地形对小体积火山碎屑密度流的影响,以及流动转变和流动剥离过程的重要性。我们认为母流是由悬崖夹带空气的密集流产生的,这些稀释的流可能通过“气动跳跃”迅速沉积。三种模式存在于所有样品的晶粒尺寸分布中,与相无关,这进一步支持了所有三种相都来自相同的初始流动的解释。这项研究强调了地形对小体积火山碎屑密度流的影响,以及流动转变和流动剥离过程的重要性。这进一步支持了所有三个相都来自相同的初始流动的解释。这项研究强调了地形对小体积火山碎屑密度流的影响,以及流动转变和流动剥离过程的重要性。这进一步支持了所有三个相都来自相同的初始流动的解释。这项研究强调了地形对小体积火山碎屑密度流的影响,以及流动转变和流动剥离过程的重要性。
更新日期:2013-10-24
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