Post-glacial record of significant ashfall events in extra Andean Northern Patagonia, Argentina

VALERIA OUTES; VIRGINIA IGLESIAS; GUSTAVO VILLAROSA; PATRICIO I. MORENO; BRENT ALLOWAY; WHITLOCK, CATHY

PUERTO VARAS

Congreso; CITIES ON VOLCANOES 9 Understanding volcanoes and society: the key for risk mitigation?; 2016

SERNAGEOMIN

A necessary firststep to evaluate volcanic hazards in Patagonia is to reconstruct the history ofexplosive volcanic events that have affected this region. Significant ashfallevents are well preserved in lake sediments of the arid environments of the Patagoniansteppe, east of the Andes. We studied four lake sediment cores and soil recordsin sites around 43° S. Bayesian chronological models were developed forcalibrated tephra dating. A 10cm-thick tephra was deposited soon after 17,900yr BP in Cóndor lake (Con) area. This tephra is present in Theobald lake (Th)and exposed near Laguna La Zeta (LZ). Petrographical and geochemical datasuggests it may be derived from a significant eruption of Michinmahuidavolcano. Six rhyolitic tephras from Chaitén volcano are identified in Mosquitolake (Mos), LZ , Th and LC. Two tephras were dated at 10,500 yr BP and 9700 yrBP. The second one, reaching up to 16 cm thick, represents the most importantChaitén event in the area.   Two thick(>2cm) tephras identified in Mos, Con and Th cores were dated at 7760 yr BP,and 5450 yr BP, representing another two major Chaitén explosive events. Twoeruptions previous to the 2008 event were dated at 420 cal yr BP, 530 to 478 yrB.P.  Tephra derived from the 2008Chaitén eruption in all these lakes shows a maximum thickness of ~2cm. Boththickness and dispersion range of the identified major tephras indicate thatthe 2008 eruption cannot be considered the largest Holocene eruptive event fromChaitén volcano.