Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador)
Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador)
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DOI：
10.1111/sed.12542
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发表时间：
2019-08-01
期刊：
影响因子：
3.5
通讯作者：
Witting, Patrick
中科院分区：
文献类型：
--
作者：
Douillet, Guilhem Amin;Bernard, Benjamin;Witting, Patrick
火山碎屑流是由火山爆发引发的灾难性气体和颗粒流。对于它们的动力学，它们表现为颗粒密度流，并与浊流有相似之处。火山碎屑流偶尔沉积存款沙丘底形，具有特殊的层压模式，从什么被认为是代表稀释的低浓度和流体湍流支持的火山碎屑流的端部成员。本文介绍了一个高分辨率的沉积物板块（漆皮）的数据集与几个紧密间隔的横向剖面代表部分，通过单一的火山碎屑床型从2006年8月的通古拉瓦（厄瓜多尔）的爆发。大多数沉积特征包括逆冲层理和优先逆冲沉积。从涟漪尺度（几厘米）到最大的沙丘底形尺度（几米长），都证明了侵蚀性后退床的类似模式。在构造物的负侧，近垂直的截形的循环序列重塑和陡化了底形。相反，零星的粗粒透镜体和透镜体层通过充填槽而使底形变平。粗粒（碎屑高达10厘米），最不分选和块状结构仍然表现出线理模式，遵循一般的倒退层理趋势。地层结构表现出强烈的横向变化在几十厘米内，非常本地截断流垂直和流平行的方向。这项研究推断，床形的沉积模式是由四种形成机制造成的：（i）差异披覆;（ii）斜坡影响的跃移;（iii）截断爆发;和（iv）基于颗粒的事件。而大多数文献作出了一个简单的联系之间的倒退层理和弗劳德超临界流，这种解释是重新考虑在这里。事实上，亚临界沙丘、反向沙丘和“斜槽和水池”的特征可以在同一层位上和单一的床形中找到，只是横向上相隔很短的距离（几十厘米）。这些数据强调的脉动和高度湍流性质的电流和相干流结构，如Gortler涡的可能作用的影响。在这里，后退层理被解释为与预先存在的形态相互作用的微弱和减弱的电流的非常高的沉积环境的结果。通过与风洞实验的比较，近床流速的量化。据估计，约0中心点30 m·s（-1）的切变速度（相当于床面上方10 cm处6 - 8 m·s（-1）的纯风速）可使建设性床组就位，而截断相则是由冲击风速至少为30 - 40 m·s（-1）的突发造成的。
Pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. Pyroclastic currents occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute low concentration and fluid-turbulence supported end member of the pyroclastic currents. This article presents a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss-face deposition. From the ripple scale (a few centimetres) to the largest dune bedform scale (several metres in length), similar patterns of erosive-based backset beds are evidenced. Recurrent trains of sub-vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse-grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimetres, with very local truncations both in flow-perpendicular and flow-parallel directions. This study infers that the sedimentary patterns of bedforms result from four formation mechanisms: (i) differential draping; (ii) slope-influenced saltation; (iii) truncative bursts; and (iv) granular-based events. Whereas most of the literature makes a straightforward link between backset bedding and Froude-supercritical flows, this interpretation is reconsidered here. Indeed, features that would be diagnostic of subcritical dunes, antidunes and 'chute and pools' can be found on the same horizon and in a single bedform, only laterally separated by short distances (tens of centimetres). These data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Gortler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre-existing morphology. Quantification of near-bed flow velocities is made via comparison with wind tunnel experiments. It is estimated that shear velocities of ca 0 center dot 30 m.s(-1) (equivalent to pure wind velocity of 6 to 8 m.s(-1) at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 to 40 m.s(-1).