Holocene tephrochronology around Cochrane (~47° S), southern Chile

Abstract

Two Holocene tephras encountered in outcrops, cores and trenches in bogs, and lake cores in the area around Cochrane, southern Chile, are identified (based on their age, tephra glass color and morphology, mineralogy, and both bulk and glass chemistry) as H1 derived from Hudson volcano, and MEN1 derived from Mentolat volcano. New AMS radiocarbon ages indicate systematic differences between those determined in lake cores (MEN1=7,689 and H1=8,440 cal yrs BP) and surface deposits (MEN1=7,471 and H1=7,891 cal yrs BP), with the lake cores being somewhat older. H1 tephra layers range from 8 to 18 cm thick, suggesting that both the area of the 10 cm isopach and the volume of this eruption were larger than previously suggested, but not greatly, and that the direction of maximum dispersion was more to the south. MEN1 tephra layers range from 1-4 cm in thickness, indicating that this was probably a reasonably large (>5 km3) eruption. Some of the lake cores also contain thin layers (<2 cm) of late Holocene H2 tephra and the recent H3 (1991 AD) tephra, both derived from the Hudson volcano. No tephra evidence has been observed for any late Pleistocene tephra, nor for the existence of the supposed Arenales volcano, proposed to be located west of Cochrane.