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      Sedimentology and geomorphology of the deposits from the August 2006 pyroclastic density currents at Tungurahua volcano, Ecuador

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          Abstract

          The deposits of the pyroclastic density currents from the August 2006 eruption of Tungurahua show three facies associations depending on the topographic setting: the massive, proximal cross-stratified, and distal cross-stratified facies. (1) The massive facies is confined to valleys on the slopes of the volcano. It contains clasts of >1 m diameter to fine ash material, is massive, and interpreted as deposited from dense pyroclastic flows. Its surface can exhibit lobes and levees covered with disk-shaped and vesicular large clasts. These fragile large clasts must have rafted at the surface of the flows all along the path in order to be preserved, and thus imply a sharp density boundary near the surface of these flows. (2) The proximal cross-stratified facies is exposed on valley overbanks on the upper part of the volcano and contains both massive coarse-grained layers and cross-stratified ash and lapilli bedsets. It is interpreted as deposited from (a) dense pyroclastic flows that overflowed the gentle ridges of valleys of the upper part of the volcano and (b) dilute pyroclastic density currents created from the dense flows by the entrainment of air on the steep upper flanks. (3) The distal cross-stratified facies outcrops as spatially limited, isolated, and wedge-shaped bodies of cross-stratified ash deposits located downstream of cliffs on valleys overbanks. It contains numerous aggrading dune bedforms, whose crest orientations reveal parental flow directions. A downstream decrease in the size of the dune bedforms, together with a downstream fining trend in the grain size distribution are observed on a 100-m scale. This facies is interpreted to have been deposited from dilute pyroclastic density currents with basal tractional boundary layers. We suggest that the parental flows were produced from the dense flows by entrainment of air at cliffs, and that these diluted currents might rapidly deposit through “pneumatic jumps”. Three modes are present in the grain size distribution of all samples independently of the facies, which further supports the interpretation that all three facies derive from the same initial flows. This study emphasizes the influence of topography on small volume pyroclastic density currents, and the importance of flow transformation and flow-stripping processes.

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          Author and article information

          Contributors
          +49-89-21804271 , +49-89-21804176 , g.douillet@min.uni-muenchen.de
          Journal
          Bull Volcanol
          Bull Volcanol
          Bulletin of Volcanology
          Springer Berlin Heidelberg (Berlin/Heidelberg )
          0258-8900
          1432-0819
          24 October 2013
          24 October 2013
          2013
          : 75
          : 11
          : 765
          Affiliations
          [ ]Earth & Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany
          [ ]Ecole et Observatoire des Sciences de la Terre, Université de Strasbourg, Strasbourg, France
          [ ]Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador
          [ ]Institut de Recherche pour le Développement, UMR volcan, Quito, Ecuador
          [ ]Laboratoire de Géosciences Marines, Université de Brest, Plouzané, France
          [ ]Laboratoire de Géophysique Interne et Techtonophysique (LGIT), Grenoble, France
          Author notes

          Editorial responsibility: V. Manville

          Article
          765
          10.1007/s00445-013-0765-7
          4456066
          26069386
          cd97f63f-d379-4fcc-b4aa-86474f00cccb
          © The Author(s) 2013

          Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.

          History
          : 21 January 2013
          : 20 September 2013
          Categories
          Research Article
          Custom metadata
          © Springer-Verlag Berlin Heidelberg 2013

          tungurahua,pyroclastic density currents,flow stripping,sedimentary wedge,hydraulic jump

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