Event stratigraphy along Las Piedritas watershed, northern Patagonian Andes, Argentina

LOSANO, NAHUEL; COTTET, JULIETA; VILLAROSA, GUSTAVO; BEIGT, DÉBORA; OUTES, VALERIA

Bariloche

Congreso; IAL IPA 2022; 2022

International Paleolimnology Association - International Association of Limnogeology

Northern Patagonia is frequently affected by abundant pyroclastic fallout as a consequenceof the explosive activity from the Southern Volcanic Zone. The remobilisation of thisunconsolidated material throughout watersheds may lead to secondary lahars. Since lakesediments usually record the processes affecting the catchment, the information preservedin them can be useful for the long-term analysis of these phenomena. Thus, in order toassess the spatial and temporal distribution of such events, this study attempts to traceremobilised tephra deposits along Las Piedritas watershed through riverside outcrop surveyand prodelta core sampling. A 46 cm-long core was taken at the mouth of Arroyo Las Piedritasinto Bahía Craft (Lago Nahuel Huapi), spanning the last ~300 years. Although distinct flowdeposits composed of volcaniclastic material are found at exposures along the creek, notephra layer of secondary (reworked) origin is recognised in the core. Instead, eight tephrainter‐beds of primary (airfall) origin are identified and characterised. As a result, accordingto their petrographic features (colour, glass shard morphology and mineral association),these deposits are correlated to different eruptive centres and, based on the estimatedsedimentation rate, matched to reported historical eruptions when possible. The oldesttephra, identified at the base of the core, is tentatively associated with the eruptive activityfrom Volcán Osorno. Several ashfall layers are attributed to eruptions of the Cordón Caulle,including the 1960 and 2011 eruptions. Also, three Calbuco‐sourced tephras are correlatedto the 1917, 1929 and 2015 eruptive events. The lack of reworked volcaniclastic deposits inthe core may be explained by the presence of a wet meadow in the delta plain, which wouldbe working as a sediment sink. Work is underway to retrieve cores from this wet meadowand other prodelta environments for reconstructing the magnitude and recurrence patternsof secondary lahars affecting the region.