Stationary surface waves and antidunes in dense pyroclastic density currents

Stationary antidunes are a product of critical flow in open channel systems, but with poor preservation potential. They are related to the existence of stationary surface waves in the overriding current, but their existence in the dense pyroclastic density current regime has been unrecognized to date. Experiments presented here demonstrate that surface waves in simulated dense pyroclastic density currents show both supercritical downstream-migrating and critical stationary wave behaviour. Deposits from the Pozzolane Rosse ignimbrite (Italy) demonstrate the presence of stationary wave antidunes in deposits from dense pyroclastic currents which imply progressive aggradation from long-lived quasi-stable critical flow conditions during their emplacement. The narrow stability fields for the formation of these deposits reinforces that they are unlikely to be widely preserved in the geological record, but highlights that dense pyroclastic density currents cannot be assumed to be simply supercritical flows, and they may be substantially slower than over-riding dilute currents.