These backset bedforms have commonly been referred to as regressive, for example by Allen 18, who interpreted them as sandwaves deposited by wet and cool pyroclastic surges. Early work on such structures in PDC deposits interpreted them similarly as the result of supercritical flows 31, 32, 33, 34. 1b, d), which are generally thought to be formed under supercritical flow conditions 16, 19, 29, 30. Denser, granular fluid-based PDCs are usually thought to be responsible for the creation of massive deposits, lacking in sedimentary structures 6, 9, 27, 28.īedform-related sedimentary structures in PDC deposits include backset features (i.e., upstream-dipping beds) formed by stoss-side aggradation, similar to chute-and-pool structures and antidunes found in fluvial systems (Fig. Various types of cross-stratified bedforms occur in PDC strata and are assumed to be formed by dilute, high-velocity (surge) PDCs 8, 18, 20, 21, 22, 23, 24, where tractional processes dominate in the flow-boundary zone due to the predominance of fluid turbulence as a particle support mechanism 9, 11, 25, 26. The presence and morphology of sedimentary structures, such as bedforms, in a deposit can be interpreted to tell us about the internal behaviour of the density current that formed them 15, 16, 17, 18, 19. Understanding of PDC behaviour, therefore, is primarily based on interpretation of the geological record preserved in sedimentary deposits 6, 7, 8, 9, 10, complemented by analogue and numerical modelling 11, 12, 13, 14. The dynamics and depositional processes of PDCs are difficult to analyse due to their destructiveness, and the concealment of the internal dynamics by an accompanying ash cloud. Understanding the behaviour of these particle-laden, fast-moving currents is fundamental to decreasing the risks they pose to society. These flows also pose a major geohazard, with deep-sea turbidity currents threatening seafloor infrastructure and PDCs being responsible for over 90,000 deaths since 1600 CE 4, 5. Deep-sea turbidity currents deposit the largest sediment accumulations on the Earth 1, density currents emplace ejecta blankets around bolide impact craters 2 and pyroclastic density currents (PDCs) can transport thousands of cubic kilometres of volcanic material during a single event 3. Particulate density currents are the largest mass transporters of sediment on the Earth’s surface. We examine how our findings impact the understanding of bedform features in outcrop, using the example of the Pozzolane Rosse ignimbrite of the Colli Albani volcano, Italy, and thus highlight that interpretations of the formative mechanisms of these features observed in the field must be reconsidered. We are able to, for the first time, define phase fields for the formation of bedforms in PDC deposits. Here we show, through analogue experiments, that a variety of bedforms can be produced by denser, aerated, granular currents, including backset bedforms that are formed in waning flows by an upstream-propagating granular bore. The occurrence of stratified layers, cross-stratification, and bedforms in these deposits has been assumed as indicative of dilute, turbulent, supercritical flows causing traction-dominated deposition. Our understanding and hazard assessments of these flows rely on interpretations of their deposits. Pyroclastic density currents (PDCs) are a life-threatening volcanic hazard.
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