Recent disturbance events recorded in the sedimentary infill of Lago Guillelmo (Argentina): tephra falls and hydrogeomorphic processes

Abstract

We studied two natural disturbances that affect the Parque Nacional NahuelHuapi area: tephra fall events from Andean volcanoes and hydrogeomorphicprocesses within catchments related to heavy rains. We carried out a surfaceanalysis, describing the main morphometric features of the selected watershed,informally called La Cantera watershed; analyzing satellite images and historicalaerial photographs; looking for evidence of processes related to these disturbanceevents in the watershed and morphological changes in the alluvial fan generatedover the delta. The subaqueous environment was analyzed through bathymetries,sediment cores, and Ground Penetrating Radar (GPR) profiles, taken in the distaldeltaic environment of the watershed. In the sedimentary record of the lake, weidentified 12 tephra layers deposited in the last 200 years, attributed to CordónCaulle, Calbuco, and Osorno volcanoes. These were chronologically correlatedwith previously documented eruptions of these volcanic centers, using vitroclastmorphology, mineral association, and stratigraphic position. We also identified10 epiclastic layers corresponding to sediments with terrigenous componentsand terrestrial organic matter, interpreted as hyperpycnites. We assign twohyperpycnite deposits to a debris flow that occurred in the area in February 2015that caused several damages and roadblocks, based on the stratigraphic positionof these deposits in the cores, and the identification of sediments associated withthis event on the lake surface in satellite images. After the correlation betweenthe GPR radargram units with the lacustrine cores, we associated one major radarunit with a (1) tephra layer and (2) lobe-shaped morphologies, interpreted asa hyperpycnite deposits, located in front of the outflow of an abandoned rivercourse. The results allow inferring a significant anthropic impact in the lacustrinerecord, as a result of the construction of Route 40 and gravel mining activitiesafter 1969 that affected the distal drainage network of the La Cantera watershed,generating topographic lows that make it difficult for subaerial flows to reachthe lake. This work shows the importance of the integration between subaerialprocesses that occur throughout a basin with the underwater processes that takeplace in the deltaic environment, to understand the extension and characteristicsof these natural disturbances.