Sedimentary structures deposited from dilute pyroclastic currents are largely dominated by the products of metre-scale dune bedforms containing backset lamination. Since the first seminal works in the 1970s, the vast majority of the literature called these structures antidunes and chutes-and-pools. This consensus led to a loose terminology, either as descriptive terms for structures containing backset beds or as an interpretation of hydrodynamic conditions related to Froude-supercritical flows. Here, the main characteristics of pyroclastic dune bedforms are summarized and classified under four categories. The literature is examined and a discussion is provided regarding a possible interpretation in terms of supercritical-flow bedforms, including antidunes, chutes-and-pools and cyclic steps. The interpretation of pyroclastic dune bedforms as related to supercritical conditions is a possibility among others, yet open questions remain, and under the present state of knowledge alternative interpretations are equally valid. There is consensus that an interpretation of pyroclastic dune bedforms as related to supercritical flows implies deposition from a basal underflow supporting an internal free-surface formed at a density interface within the overall current structure. Antidunes are likely to be limited to low-angle, incipient bedforms. Chutes-and-pools may occur in the form of series of steep truncations formed by an erosive supercritical ‘chute’, with a depositional signature occurring exclusively in the subcritical ‘pool’ of a Froude jump. Rare examples of cyclic steps may be found as large-scale undulations with superimposed metre-scale bedforms, or for short-scale, fully-aggrading structures in periodic trains. Additionally, some pyroclastic bedforms may be formed by granular flows passing granular Froude jumps or as frozen granular stationary waves. The dominance of chute-and-pool structures over bedforms characterized by relatively more stable morphodynamics (antidunes and cyclic steps) could be the likely result of pulsating flow conditions and abrupt high-rate deposition, impeding stable flow conditions. In a majority of cases, the growth of pyroclastic bedforms appears triggered by the influence of an inherited bed topography and by bed-flow feedback effects. Apart from a possible interpretation as supercritical bedforms, other specific dynamics of dilute pyroclastic currents may well explain the characteristics of pyroclastic dune bedform structures. In particular, the inherent turbulence fluctuations, internal organization and highly depositional dynamics may be key to the formation of pyroclastic dune bedforms. In this respect, rather than solely focusing on the Froude dimensionless number of the formative current, theoretical considerations accounting for the Reynolds, Richardson and Rouse dimensionless parameters may be also implemented for the understanding of pyroclastic bedforms.

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火山碎屑沙丘床形的超临界问题：概述

由稀有火山碎屑流沉积的沉积结构主要由包含反向叠置的米级沙丘床形产物主导。自1970年代第一批开创性著作以来，绝大多数文献都将这些结构称为反沙丘和溜槽池。这种共识导致了一个宽松的术语，要么是用于描述包含逆流床的结构的描述性术语，要么是对与弗洛德-超临界流有关的流体动力学条件的解释。在此，对火山碎屑沙丘床形的主要特征进行了归纳和归类为四类。检查了文献并讨论了关于超临界流动床形的可能解释，包括反沙丘，溜槽和水池以及循环步骤。与其他超临界条件有关的火山碎屑沙丘床形解释是一种可能性，但仍存在悬而未决的问题，在目前的知识状态下，替代解释也同样有效。人们一致认为，火山碎屑沙丘床形与超临界流有关，这意味着基底底流的沉积支持了内部自由表面，该自由表面形成于整个电流结构内的密度界面处。止痛药可能仅限于低角度的初始床形。溜槽和池可能以侵蚀性超临界“溜槽”形成的一系列陡峭截断的形式出现，而沉积特征仅出现在弗洛德跳变的次临界“池”中。循环阶梯的罕见示例可能是具有叠加的米级床形的大规模起伏，或者是周期性列车中的短规模，完全凝结的结构。另外，某些火山碎屑岩床形可能是由经过颗粒弗洛德跳变的颗粒流形成的，或者是冻结的颗粒固定波。以相对稳定的形态动力学（反沙丘和循环阶跃）为特征的床架上，斜槽和水池结构的优势可能是脉动流动条件和突然高速率沉积的可能结果，从而阻碍了稳定流动条件。在大多数情况下，火山碎屑床形的生长似乎是由继承的床形貌的影响和床流反馈效应引起的。除了可能解释为超临界床形外，稀火山碎屑流的其他特定动力学可能很好地解释了火山碎屑沙丘床状结构的特征。特别是，固有的湍流波动，内部组织和高度沉积动力学可能是形成火山碎屑沙丘床形的关键。在这方面，不仅可以仅关注形成电流的弗洛德无量纲数，还可以采用考虑雷诺，理查森和鲁斯无量纲参数的理论考虑来理解火成碎屑床形。