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

Abstract

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 km 3 ) 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. Keywords: Tephra, Tephrochronology, Volcanism, Hudson, Mentolat, Andes, Chile. RESUMEN. Tefrocronología holocénica cerca de Cochrane (~47° S), Chile Austral. Dos tefras holocenas reconocidas en af loramientos, testigos y trincheras en pantanos y lagos de la zona de Cochrane, Chile, han sido identificadas (sobre la base de su edad, color y morfología de las partículas vítreas, mineralogía y composición química de la roca total y del vidrio) como H1 y MEN1, provenientes de los volcanes Hudson y Mentolat, respectivamente. Nuevas edades radiocarbónicas AMS asociadas a estas tefras revelan diferencias sistemáticas entre los testigos lacustres (MEN1=7,689 y H1=8,440 años cal. AP) y los depósitos en superficie (MEN1=7,471 y H1=7,891 años cal. AP), en que las edades en los testigos lacustres son algo más antiguas. Los horizontes de la tefra H1 varían entre 8 y 18 cm de espesor, lo que indicaría que tanto el área de la isópaca de 10 cm como el volumen de esta erupción fue levemente mayor a lo previamente reconocido, y que la dirección de dispersión fue más hacia el sur. Los niveles de la tefra MEN1 varían entre 1 y 4 cm de espesor, lo que sugiere que esta fue una erupción relativamente grande (>5km 3 ). Algunos testigos lacustres también incluyen niveles más finos (<2 cm) de otra tefra (H2) del Holoceno tardío y de la tefra (H3) asociada a la erupción del año 1991, ambas derivadas del volcán Hudson. No se encontraron evidencias de tefras pleistocénicas tardías ni tampoco de la presencia del volcán Arenales, supuestamente ubicado al oeste de Cochrane y cuya existencia ha sido puesta en duda.

Tefrocronología en curso superior del valle del río Cisne (44°S), Chile Austral.

“…Although, there is still not enough spatial coverage to constrain isopach maps for the eruptions that produced all the tephra, some of these tephra do appear to correspond chronologically and petrochemically to some of the larger eruptions identified previously by Stern (1998, 2004), Mella et al (2012), Stern et al (2013) and Weller et al (2014), while others occurred at times when no such large eruption has been previously identified. Tephras 'a' in the two cores may correspond to Late-Holocene eruption MEL2 of Melimoyu (Table 1) previously documented by Naranjo and Stern (2004).…”

“…Tephra 'b' in Mallín El Embudo and 'f' in Lago Shaman do not result from the H2 eruption of Hudson as previously suggested (de Porras et al, 2012(de Porras et al, , 2014, but rather from a mid-Holocene eruption of the Melimoyu volcano not previously described. Tephras 'i' from the Lago Shaman core and 'h' from the Mallín El Embudo core may have been deposited during the mid-Holocene MEN1 eruption of Mentolat (Table 1; Naranjo and Stern, 2004;Stern et al, 2013). Tephra 'v' in Lago Shaman was produced by the Late-Glacial Ho eruption of Hudson (Table 1; Weller et al, 2014).…”

“…Tephra 'i' from Lago Shaman and 'h' from Mallín El Embudo are both similar in age and coarser grained than tephra 'h' from Lago Shaman, and we consider them to be deposited from the same eruption. This eruption may correspond to the MEN1 eruption, although the ~7,710±120 cal yrs BP age of the MEN1 eruption (Naranjo and Stern, 2004;Stern et al, 2013) is somewhat younger than the approximate ages (8,800 and 9,010 cal yrs BP, respectively; Table 1) of these two tephra (Fig. 7).…”

“…Age versus depth in the core of the different tephra analyzed from the Lago Shaman and Mallín El Embudo cores. Also shown are the ages for tephra MEL2 from Melimoyu (Naranjo and Stern, 2004) which may correlate with tephra 'a' in both cores, MEN1 from Mentolat (Naranjo and Stern, 2004;Stern et al, 2013) which may correlate with tephras 'i' in Lago Shaman and 'h' in Mallín El Embudo, and Ho from Hudson which correlates with tephra 'v' in Lago Shaman (Weller et al, 2014).…”

A Holocene record of environmental change from Río Zeballos, central Patagonia

“…The tephra layer at 268-274 cm is geochemically fingerprinted to the large eruption of Volcán Hudson (H1) Stern, 1998, 2004;Prieto et al, 2013;Stern et al, 2016). The maximum age for H1 of 7241 ± 23 14 C yr BP used here (Stern et al, 2016) is older than that of 6890 ± 100 14 C yr BP, given by Prieto et al (2013), but, as a similar exercise for the Volcán Reclus R1 layer has demonstrated, improved accuracy may be achieved by selecting ages from higher-resolution sediment and peat cores (McCulloch Table 2.…”