TEPHRA STUDIES IN NAHUEL HUAPI AREA: TOWARDS THE DEVELOPMENT OF A TEPHROCHRONOLOGICAL FRAMEWORK FOR NORTHWESTERN PATAGONIA, ARGENTINA

GUSTAVO VILLAROSA; VALERIA OUTES; DANIEL SELLES; ERNESTO CRIVELLI; H. OSTERA; R. CORDERO

Dawson City, Yukon Territory, Canada

Workshop; International Field Conferenceand Workshop on Tephrochronology & Volcanism; 2005

Iternational Union for Quaternary Research (INQUA) Sub-comission for Tephrochronology & Volcanism

Several paleoenvironmental and paleoclimatic studies have been carried out at Nahuel Huapi National Park area (41°S, 71° 40’W) based on lacustrine records from several lakes formed during the late Pleistocene retreat of glaciers. Sediment cores as well as archaeological excavations in the area show several pyroclastic layers, most of them corresponding to ash fall deposits from wind driven plumes originated by explosive eruptions from Andean volcanoes providing excellent stratigraphic and chronological markers that become a very powerful tool for multidisciplinary research. The purpose of this study is to develop a tephrochronology for the area based on the fingerprinting of tephras from very well dated cores from lacustrine sediments covering post glacial times, surface exposures and several auxiliary cores from other small lakes and peat. Records from two main sites have been selected as reference because of their continuity, time span and good chronological control: lake Mascardi and lake Trebol cores. Lake Mascardi record consists of a 12-m long core with more than 60 pyroclastic layers while 33 ash fall layers were identified in the 7-m long Trebol core, both covering the last 15,000 years. Tephra characterization was achieved using petrography, shard morphology and major, trace and REE geochemistry. Chronological models for both cores were obtained based on AMS calibrated dates that allow precise interpolation in order to calculate ages for the tephra layers. Additionally, valuable information was obtained from exposures located on the northern Nahuel Huapi lake area, where several ash fall layers interbedded with paleosols, constitute a thick sequence. One of these tephras, which is an excellent marker, identified in several archaeological sites, has been fingerprinted, dated, correlated with the corresponding layer in the cores and its dispersion presented in an isopach map. So far fifteen tephras have been considered as potentially good markers for the area. Puyehue-Cordón Caulle, Osorno, Calbuco and Antillanca group volcanoes were identified as their possible source