Recent disturbance events recorded in the sedimentary infill of lago guillelmo (argentina): tephra falls and hydrogeomorphic processes

In this work we focus on two natural disturbances that affect the Parque Nacional Nahuel Huapi area: Tephra fall events from Andean volcanoes and hydrogeomorphic processes within catchments related to heavy rains. In order to look for evidence of these disturbance events in the lacustrine sedimentary record, we perform a multi-approach analysis in La Cantera stream watershed, including the subaqueous environment of the distal prodelta area, northwest portion of Lago Guillelmo (Northern Patagonia). We carried out a surface analysis, describing the main morphometric features of the selected watershed, informally called La Cantera stream watershed, and we also analyzed satellite and historical images, looking for evidence of processes related to these disturbance events in the watershed and morphological changes in the distal zone and in the alluvial fan generated over the delta. The subaqueous environment was analyzed by means of expeditive bathymetries, lacustrine cores and Ground Penetrating Radar (GPR) profiles, taken in the distal deltaic environment of the watershed. In the sedimentary record of the lake we identified 10 tephra layers attributed to Cordón Caulle, Calbuco and Osorno Volcanoes for the last 200 years. These were chronologically correlated with previous documented eruptions of these volcanic centers, by means of vitroclast morphology, mineral association and stratigraphic position. Also we identified 10 layers corresponding to sediments with terrigenous components and terrestrial organic matter, interpreted as hyperpycnites. We attribute two hyperpycnite deposits to a debris flow that occurred in the area in February, 2015, based on the magnitude of that event that caused several damages and road cuts and the stratigraphic position of these deposits in the cores. After the correlation between the GPR radargrams with the lacustrine cores we associate one major reflector with a (1) tephra layer and (2) lobe-shaped morphologies, interpreted as a hyperpycnite deposit, located in front of the mouth of an abandoned river course. The anthropic activity in the region associated with the construction of the Ruta Nacional 40 and the gravel mining after 1969, affected the drainage of La Cantera watershed, hindering the possibility that the different types of flows could plunge into the lake. This work also shows the importance of the integration between the processes that occur throughout a basin with the underwater processes that take place in the deltaic environment, to understand the extension and characteristics of the disturbances studied.