Guilhem Amin Douillet, Quentin Chaffaut, Fritz Schlunegger, Ulrich Kueppers, Donald B. Dingwell

Enigmatic structures grouped under the term "shark-fins" are documented in laminated deposits of pyroclastic currents. 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 cross-laminated deposits from the 2006 pyroclastic currents of Tungurahua volcano (Ecuador). Shark-fins often underlie erosive lines. They occur in periodic repetitions or amalgamated in swarms and can show signs of migration as well as concurrent growth during sedimentation. These shark-fins are interpreted in terms of syn-depositional soft sediment deformation. The patterns are interpreted as due to the reworking of the bed by wave instabilities, which form around a shear horizon between a traction carpet and the bed. A theoretical framework based on two fluid layers separated by a shear horizon is adapted from the Holmboe instability problem. The calculated growth rates of the waves are compared to sedimentation rates in order to infer aspects of the stability and preservation of the sheared interfaces. The good agreement of the physical model with field data supports the process-based interpretation. In particular, such modified Holmboe instabilities seem more adequate than Kelvin-Helmholtz instabilities to explain overturned shark- fins structures. Other incidental overturned patterns are presented and discussed, which likely result from an intraflow event as well as from syn-flow slumping. Such observations 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.