Pyroclastic Layers from Northern Nahuel Huapi Area: New Tephrochronological Data

GUSTAVO VILLAROSA; VALERIA OUTES; R. CORDERO; A. RAMOS; H. OSTERA; E. CRIVELLI

Pucón, Chile

Congreso; IAVCEI General Assembly; 2004

Servicio Nacional de geología y Minería de Chile

Along the northern area of the Nahuel Huapi Lake a sequence of pyroclastic layers representingpostglacial ash falls and paleosoils is observed. These eruptive units are identified in various outcropsand correlated to the Puyehue Volcano and Cordón Caulle Volcanic Complex as possible source vents. 14C dates for the paleosoils and tephras are presented: the bottom of the sequence is represented by a tephra that lies on a glacial deposit –bottom layer of Member Lago Mascardi – and presents a thin carbon layer intercalation which was dated in 7632±228 years BP; two of the five paleosoils that follow upwards were also dated: 3100±180 years BP for the oldest and 2750±250 years BP for the youngest -Puyehue paleosoils-; on top of them a conspicuous white layer -Capa de Arroz, Member Río Blanco- follows, the date obtained is 2215 +- 85 years BP. Characterization of the most conspicuous layers is presented: Major elements composition and trace elements by X-ray fluorescence, EDS analyses on single grains of selected glass shards, SEM observations of glass shards to relate their morphology and size to the mechanism of fragmentation that produced the ashes. These tephra layers provide chronological markers for the area. Another interesting observation is that in lacustrine records from the area some tephra layers that are not identified in the outcrops are present, they are actually under study in order to develop acomplete Tephrochronology that will become a powerful tool to correlate and compare paleoclimatic records from both sides of the Andean Cordillera.14C dates for the paleosoils and tephras are presented: the bottom of the sequence is represented by a tephra that lies on a glacial deposit –bottom layer of Member Lago Mascardi – and presents a thin carbon layer intercalation which was dated in 7632±228 years BP; two of the five paleosoils that follow upwards were also dated: 3100±180 years BP for the oldest and 2750±250 years BP for the youngest -Puyehue paleosoils-; on top of them a conspicuous white layer -Capa de Arroz, Member Río Blanco- follows, the date obtained is 2215 +- 85 years BP. Characterization of the most conspicuous layers is presented: Major elements composition and trace elements by X-ray fluorescence, EDS analyses on single grains of selected glass shards, SEM observations of glass shards to relate their morphology and size to the mechanism of fragmentation that produced the ashes. These tephra layers provide chronological markers for the area. Another interesting observation is that in lacustrine records from the area some tephra layers that are not identified in the outcrops are present, they are actually under study in order to develop acomplete Tephrochronology that will become a powerful tool to correlate and compare paleoclimatic records from both sides of the Andean Cordillera.