A series of pyroclastic density currents were generated at Tungurahua volcano (Ecuador) during a period of heightened activity in August 2006. Dense pyroclastic flows were confined to valleys of the drainage network, while dilute pyroclastic density currents overflowed on interfluves where they deposited isolated bodies comprising dune bedforms of cross-stratified ash exposed on the surface. Here, the description, measurement, and classification of more than 300 dune bedforms are presented. Four types of dune bedforms are identified with respect to their shape, internal structure, and geometry (length, width, thickness, stoss and lee face angles, and stoss face length). (1) “Elongate dune bedforms” have smooth shapes and are longer (in the flow direction) than wide or thick. Internal stratification consists of stoss-constructional, thick lensoidal layers of massive and coarse-grained material, alternating with bedsets of fine laminae that deposit continuously on both stoss and lee sides forming aggrading structures with upstream migration of the crests. (2) “Transverse dune bedforms” show linear crests perpendicular to the flow direction, with equivalent lengths and widths. Internally, these bedforms exhibit finely stratified bedsets of aggrading ash laminae with upstream crest migration. Steep truncations of the bedsets are visible on the stoss side only. (3) “Lunate dune bedforms” display a barchanoidal shape and have stratification patterns similar to those of the transverse ones. Finally, (4) “two-dimensional dune bedforms” are much wider than long, exhibit linear crests and are organized into trains. Elongate dune bedforms are found exclusively in proximal deposition zones. Transverse, lunate, and two-dimensional dune bedforms are found in distal ash bodies. The type of dune bedform developed varies spatially within an ash body, transverse dune bedforms occurring primarily at the onset of deposition zones, transitioning to lunate dune bedforms in intermediate zones, and two-dimensional dune bedforms exclusively on the lateral and distal edges of the deposits. The latter are also found where flows moved upslope. Elongate dune bedforms were deposited from flows with both granular-based and tractional flow boundaries that possessed high capacity and competence. They may have formed in a subcritical context by the blocking of material on the stoss side. We do not interpret them as antidune or “chute-and-pool” structures. The dimensions and cross-stratification patterns of transverse dune bedforms are interpreted as resulting from low competence currents with a significant deposition rate, but we rule out their interpretation as “antidunes”. A similar conclusion holds for lunate dune bedforms, whose curved shape results from a sedimentation rate dependent on the thickness of the bedform. Finally, two-dimensional dune bedforms were formed where lateral transport exceeds longitudinal transport; i.e., in areas where currents were able to spread laterally in low velocity zones. We suggest that the aggrading ash bedsets with upstream crest migration were formed under subcritical flow conditions where the tractional bedload transport was less important than the simultaneous fallout from suspension. This produced differential draping with no further reworking. We propose the name “regressive climbing dunes” for structures produced by this process. A rapid decrease in current velocity, possibly triggered by hydraulic jumps affecting the entire parent flows, is inferred to explain their deposition. This process can in principle hold for any kind of particulate density current.

中文翻译：

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2006 年 8 月厄瓜多尔通古拉瓦火山喷发的稀释火山碎屑密度流产生的沙丘床形态

2006 年 8 月，在通古拉瓦火山（厄瓜多尔）的活动加剧期间，产生了一系列火山碎屑密度流。密集的火山碎屑流被限制在排水网络的山谷中，而稀薄的火山碎屑密度流则在间流溢出，在那里它们沉积了孤立的物体，其中包括暴露在地表的交叉层状灰烬沙丘床。在这里，介绍了 300 多个沙丘床型的描述、测量和分类。根据其形状、内部结构和几何形状（长度、宽度、厚度、stoss 和 lee face 角以及 stoss 面长度）确定了四种类型的沙丘床型。(1) “细长的沙丘床型”形状光滑，长（在流动方向）比宽或厚。内部分层包括stoss-constructional，厚的块状和粗粒物质的透镜体层，与连续沉积在背风侧和背风侧的细纹层交替，形成了随着顶部向上游迁移而形成的加积结构。(2)“横向沙丘床型”显示垂直于流向的线性波峰，长度和宽度相等。在内部，这些床型表现出精细分层的沉积灰层与上游波峰迁移。床具的陡峭截断仅在 stoss 侧可见。(3)“月状沙丘床型”呈新月形沙丘状，层理形式与横向相似。最后，（4）“二维沙丘床型”比长更宽，呈现线性波峰并组织成列。细长的沙丘床仅在近端沉积区中发现。在远端灰体中发现了横向、月形和二维沙丘床形态。灰体内发育的沙丘床型在空间上存在差异，横向沙丘床型主要发生在沉积带的起始处，过渡到中间带的月状沙丘床型，二维沙丘床型仅出现在沉积物的侧缘和远侧边缘. 在流动向上移动的地方也发现了后者。细长的沙丘床由具有高容量和能力的颗粒流和牵引流边界的流沉积而成。它们可能是在亚临界环境中通过在抛物侧阻塞材料而形成的。我们不将它们解释为反沙丘或“斜槽和池”结构。横向沙丘床型的尺寸和交叉分层模式被解释为由具有显着沉积速率的低能力流产生，但我们排除了它们作为“反沙丘”的解释。类似的结论适用于月状沙丘床型，其弯曲形状是由依赖于床型厚度的沉积速率产生的。最后，在横向输运超过纵向输运的地方形成二维沙丘床；即，在电流能够在低速区横向扩散的区域。我们建议在亚临界流动条件下形成具有上游波峰迁移的加积灰层，在这种条件下，牵引床荷载传输不如悬浮的同时沉降重要。这产生了差异悬垂，无需进一步返工。我们建议将此过程产生的结构命名为“回归攀爬沙丘”。水流速度的快速下降可能是由影响整个母流的水跃引发的，被推断可以解释它们的沉积。该过程原则上适用于任何种类的颗粒密度电流。