The supercritical question for pyroclastic dune bedforms: An overview

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