Sedimentary peels from pyroclastic density current deposits: revealing basal flow-boundary processes.
Final Report Abstract
Pyroclastic currents are flowing mixtures of gas and particles triggered by explosive volcanic eruption. They flow down the flanks of a volcano in a manner similar to snow avalanches. The sediment left after the flow record an eruptive history modulated by transport and depositional phases. Here, a field technique of outcrop impregnation was used to document pyroclastic sediments at the millimeter scale of individual laminae. The details so obtained have the potential to reveal new insights into the dynamics of transport and deposition of pyroclastic currents. Sediment plates (a type of lacquer peels) represent a sampling method whereby a thin plate of undisturbed sediments is obtained directly from the outcrop. A low-viscosity, hardening epoxy resin is applied to a freshly exposed cross-section of an unconsolidated deposit and impregnates a surface layer of the cross-section via capillary forces before solidifying. Upon hardening, a solid plate (0.5–5 cm thick and up to 2 m in length) of the sedimentary formation can be recovered and transported with full preservation of the initial organization of the particles. Sediment plates are capable of recording and highlighting details of stratification to a very fine scale and high degree of precision. This method represents a valuable tool for research, education, and exhibition purposes. A dataset of 50 m2 of sediment plates was created from the primary sedimentary structures emplaced during the August 2006 eruption of Tungurahua volcano (Ecuador). This dataset has been put to use in several contexts: 1) 3D full-scale imaging of pyroclastic dune bedforms, 2) Analysis of sedimentation processes at the boundary-layer scale, 3) Archiving and conservation, 4) As teaching material and for dissemination during exhibition in museums. 5) For data comparison with remote measurements (e.g., ground penetrating radar). A variety of sedimentary structures were evidenced for the first time and systematically analyzed in terms of fluid dynamics phenomenas and through analogue experiments. The results highlight a complex behavior close to the flow-substrate boundary. In particular, the existence of a basal granular flow, laminar sublayer, and occurrence of coherent flow structures is pointed from the natural deposits. The deposition of pyroclastic currents cannot be seen as a steady process but rather result from extreme processes of accumulation and erosion, that could occur within seconds of interval.
Publications
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Shark-fins: overturned patterns linked to shear instabilities at the flow-bed boundary. Examples from the deposits of the 2006 pyroclastic currents at Tungurahua volcano (Ecuador)
Douillet, G.A., Chaffaut, Q., Schlunegger, F., Kueppers, U. and Dingwell, D.B.
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"The supercritical question for pyroclastic bedforms". Workshop at the International Association of Sedimentologists Meeting in Québec, 2017
Douillet, G.A.
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2017. Dynamics and style transition of a moderate, Vulcanian-driven eruption at Tungurahua (Ecuador) in February 2014: pyroclastic deposits and hazard considerations
Romero, J.E., Amin Douillet, G., Vallejo Vargas, S., Bustillos Arequipa, J.E., Troncoso, L., Díaz Alvarado, J. and Ramón, P.
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2018. Revisiting the lacquer peels method with pyroclastic deposits: sediment plates, a precise, fine scale imaging method and powerful outreach tool. Journal of applied volcanology, 7(1), p. 11
Douillet, G.A., Kueppers, U., Mato, C., Chaffaut, Q., Bouysson, M., Reschetizka, R., Hoelscher, I., Witting, P., Hess, K.U., Cerwenka, A. and Dingwell, D.B.
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2019. Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador). Sedimentology, 66(5), pp.1531-1559
Douillet, G.A., Bernard, B., Bouysson, M., Chaffaut, Q., Dingwell, D.B., Gegg, L., Hoelscher, I., Kueppers, U., Mato, C., Ritz, V.A. and Schlunegger, F.
