title: Sedimentology and geomorphology of the deposits from the August 2006 pyroclastic density currents at Tungurahua volcano, Ecuador.
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 thesedilutedcurrents 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.
authors.affiliation: 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.
authors.forename: Guilhem Amin
authors.initials: GA
authors.lastname: Douillet
authors.affiliation: Ecole et Observatoire des Sciences de la Terre, Université de Strasbourg, Strasbourg, France ; Laboratoire de Géosciences Marines, Université de Brest, Plouzané, France.
authors.forename: Ève
authors.initials: È
authors.lastname: Tsang-Hin-Sun
authors.affiliation: Earth & Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Kueppers
authors.affiliation: Ecole et Observatoire des Sciences de la Terre, Université de Strasbourg, Strasbourg, France ; Laboratoire de Géophysique Interne et Techtonophysique (LGIT), Grenoble, France.
authors.forename: Jean
authors.initials: J
authors.lastname: Letort
authors.affiliation: Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador.
authors.forename: Daniel Alejandro
authors.initials: DA
authors.lastname: Pacheco
authors.affiliation: Earth & Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany.
authors.forename: Fabian
authors.initials: F
authors.lastname: Goldstein
authors.affiliation: Earth & Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany.
authors.forename: Felix
authors.initials: F
authors.lastname: Von Aulock
authors.affiliation: Earth & Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany.
authors.forename: Yan
authors.initials: Y
authors.lastname: Lavallée
authors.affiliation: Earth & Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany.
authors.forename: Jonathan Bruce
authors.initials: JB
authors.lastname: Hanson
authors.affiliation: Ecole et Observatoire des Sciences de la Terre, Université de Strasbourg, Strasbourg, France.
authors.forename: Jorge
authors.initials: J
authors.lastname: Bustillos
authors.affiliation: Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador ; Institut de Recherche pour le Développement, UMR volcan, Quito, Ecuador.
authors.forename: Claude
authors.initials: C
authors.lastname: Robin
authors.affiliation: Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador.
authors.forename: Patricio
authors.initials: P
authors.lastname: Ramón
authors.affiliation: Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador.
authors.forename: Minard
authors.initials: M
authors.lastname: Hall
authors.affiliation: Earth & Environmental Sciences, Ludwig-Maximilians-Universität, Munich, Germany.
authors.forename: Donald B
authors.initials: DB
authors.lastname: Dingwell
keywords: Flow stripping; Hydraulic jump; Pyroclastic density currents; Sedimentary wedge; Tungurahua