Holocene tephrochronology of the lower RíoCisnesvalley, southern Chile

WELLER, DJ; DE PORRAS, M.E; MALDONADO, A.; MÉNDEZ, C.; STERN, C.R.

ANDEAN GEOLOGY

SERVICIO NACIONAL GEOLOGIA MINERVA

Lugar: Santiago de Chile; Año: 2017

0718-7092

Sediment cores from lakes and bogs in theRío Cisnesvalley contain tephra from explosive eruptions of volcanoes in the southern part of the Andean Southern Volcanic Zone (SSVZ). These tephra, which thicken and coarsen to the west, are attributed to eruptions from Melimoyu, Mentolat, Hudson, and potentially either Macá, Cay or one of the many minor eruptive centers (MEC) located both along the Liquiñe-Ofqui Fault Zone (LOFZ) and surrounding the major volcanoes. Correlation of the tephra betweentwo new cores in the lower Río Cisnes valley, and amongst other cores previously described from the region, and source volcano identification for the tephra, has been done using lithostratigraphic data (tephra layer thickness and grain size), petrography(tephra glass color, vesicle morphology, and type and abundance of phenocryst phases), and by comparison of bulk tephra trace-element characteristics with previously published whole-rock and bulk tephra chemical analysis. Four tephrasin these cores are attributed to eruptions of Mentolat, four to eruptions from Melimoyu, one possibly to Hudson,andsix cannot be assigned to a specific source volcano. Some of these tephra correspond to pyroclastic tephra fall deposits previously observed in outcrop,including the MEL2 eruption of Melimoyuand the MEN1 eruption of Mentolat. However, others have not been previously observed and represent the products of newly identified small to medium sized eruptions from volcanoes of the SSVZ. These results provide new information concerning the frequency andmagnitude of explosive eruption of SSVZ volcanoes and contribute to the evaluation of volcanic hazards in the region.