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Description: Enigmatic structures are documented in deposits of dilute pyroclastic currents and grouped under the term "shark-fins". They consist of an overturning of a few laminae on a decimeter scale, forming overbent "flames" or convolute laminae, which occur in successive, periodic patterns. More than 200 shark-fins were investigated and measured in the cross-laminated deposits from the 2006 pyroclastic currents of Tungurahua volcano (Ecuador). These shark-fins are interpreted in terms of syndepositional soft sediment deformation pattern where- by waves form at the interface of a shear horizon at the flow-bed boundary and rework the bed. The shark-fins are not related to Kelvin-Helmholtz instabilities. Instead, a theoretical framework based on two layers separated by a shear horizon is developed. The calculated growth rate of the waves is com- pared to sedimentation rates in order to infer aspects of the stability and preservation of such sheared interfaces. The process-based interpretation is supported by the results from the physical model. Various other incidental patterns are presented and discussed, which likely result from collisions of flows with deposits, intraflow events and syn-flow slumping. We thus identify the necessary key ob- servations for the interpretation of shark-fin structures for different types of triggers. Such observa- tions on flow-bed interactions contribute to the understanding of a flow rheology, shear partitioning, and the transmission of shear stress out of the flow and into the substrate.