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

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

San Carlos de Bariloche

Congreso; IAL IPA 2022. Lakes, Memories of the Landscape; 2022

International Association of Limnogeology- International Paleolimnology A ssociation

Northern Patagonia is frequently affected by abundant pyroclastic fallout as a consequence of the explosive activity from the Southern Volcanic Zone. The remobilisation of this unconsolidated material throughout watersheds may lead to secondary lahars. Since lake sediments usually record the processes affecting the catchment, the information preserved in them can be useful for the long-term analysis of these phenomena. Thus, in order to assess the spatial and temporal distribution of such events, this study attempts to trace remobilised tephra deposits along Las Piedritas watershed through riverside outcrop survey and prodelta core sampling. A 46 cm-long core was taken at the mouth of Arroyo Las Piedritas into Bahía Craft (Lago Nahuel Huapi), spanning the last ~300 years. Although distinct flow deposits composed of volcaniclastic material are found at exposures along the creek, no tephra layer of secondary (reworked) origin is recognised in the core. Instead, eight tephra inter beds of primary (airfall) origin are identified and characterised. As a result, according to their petrographic features (colour, glass shard morphology and mineral association), these deposits are correlated to different eruptive centres and, based on the estimated sedimentation rate, matched to reported historical eruptions when possible. The oldest tephra, identified at the base of the core, is tentatively associated with the eruptive activity from 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 correlated to the 1917, 1929 and 2015 eruptive events. The lack of reworked volcaniclastic deposits in the core may be explained by the presence of a wet meadow in the delta plain, which would be working as a sediment sink. Work is underway to retrieve cores from this wet meadow and other prodelta environments for reconstructing the magnitude and recurrence patterns of secondary lahars affecting the region.