RESUMO
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.
RESUMO
El complejo volcánico Cayambe esta constituido por tres edificios: a) un antiguo edificio efusivo, el "Viejo Cayambe"; b) el volcán joven "Nevado Cayambe" caracterizado por una historia mucho mas compleja y dinamismos eruptivos más explosivos; c) un cono satélite construido en el extremo oriental del complejo, el "Cono de la Virgen". La actividad reciente del Nevado Cayambe (últimos 4000, aBP) ha presentado trees ciclos de actividad de 300, 600 y 900 años de duración, separados por periodos de calma de 600 y 1000 años respectivamente. El último periodo eruptivo, iniciado hace 100 años, se ha caracterizado por el crecimiento de domos de lava, la generación de flujos piroclásticos de colapso de domo, la producción de lahares asociados a la fusión del casquete glaciar y una limitada distribución de piroclastos. Una síntesis de estos de estos datos permite proponer un primer mapa de peligros volcánicos asociados a este volcán. (AU)