PYROCLASTIC LAYERS FROM LAKE MASCARDI, ARGENTINA: TOWARDS THE DEVELOPMENT OF A TEPHROCHRONOLOGY IN NORTHERN PATAGONIA

GUSTAVO VILLAROSA; VALERIA OUTES; DANIEL ARIZTEGUI; HECTOR OSTERA; M ARRIBERE; RIBEIRO GUEVARA SERGIO

COMUNICACIONES - DEPARTAMENTO DE GEOLOGIA. FACULTAD DE CIENCIAS FISICAS Y MATEMATICAS. UNIVERSIDAD DE CHILE

Departamento de geología de la Facultad de Cs Físicas y Matemáticas de la Universidad de Chile

Lugar: Santiago, Chile; Año: 2001 vol. 52 p. 122 - 122

0069-357X

Paleoenvironmental and paleoclimatic studies have been recently carried out on the Quaternary sequence of Lake Mascardi, Argentina, within the framework of an Argentinean-Swiss cooperative project, and mainly focused on the multidisciplinary study of lacustrine sedimentary cores from Lake Mascardi, a proglacial lake formed during the late Pleislocene retreat of the glaciers. Several ash falls originated in Andean volcanoes and derived from eastward wind driven plumes were ´´instantaneously" deposited as tephra layers in Lake Mascardi during post-glacial times, providing excellent stratigraphic and chronological markers. Volcanoes Calbuco. Osomo and perhaps Puyehue-Cordón Caulle are possible sources of plumes which, moving towards the east and southeast, originated ash-fall deposits in the studied area. These chronological markers allow to establish the synchronicity or diachrony of geological and climatic events along the dispersion area of a given pyroclastic deposit. Thus, tephra layers are a powerful tool to correlate and compare paleoclimatic records from both sides of the Andes. The sedimentary sequence in core PMAS93 provides a high-resolution chronology unique at this latitude. Ten tephra layers from Lake Mascardi have been characterized whereas three others are proposed as reworked deposits of previous ash falls. Trace element concentration ratios allowed further separation of the studied tephras into four groups. The range of Cs/La, Th/La and La/Yb is small and coincident with reported results for Quaternary lavas from SVZ volcanoes. The chondrite-normalized REE patterns further confirm trace element grouping and show a clear affinity among the different ash layers suggesting a common source that has been identified as the Osorno volcano. Some of the tephras, however, can also be associated to the Calbuco and Puyehue volcanoes. Our results show that these individual tephras can be recognized and separated combining petrography, shard morphology and geochemistry. Four tephras are considered to be potentially good markers. These preliminary data combined with a well-established chronology indicate that Lake Mascardi provides an excellent site to develop a detailed tephrochronology for this region.