Remobilized Cordón Caulle 2011 tephra deposits in north-Patagonian watersheds: Resedimentation at deltaic environments and its implications

BEIGT, DÉBORA; GÓMEZ, EDUARDO A.; VILLAROSA, GUSTAVO; TOYOS, GUILLERMO; OUTES, VALERIA; BEIGT, DÉBORA; GÓMEZ, EDUARDO A.; VILLAROSA, GUSTAVO; TOYOS, GUILLERMO; OUTES, VALERIA

GEOMORPHOLOGY

ELSEVIER SCIENCE BV

Año: 2019 vol. 341 p. 140 - 152

0169-555X

The Argentinean side of the AndeanNorth Patagonia is drained by several fluvio-lacustrine systems flowing eastwardsfrom their headwaters at the Andes mountains towards large glacial lakes. These watersheds are recurrentlyaffected by the deposition of tephra erupted from the many active Andean volcanos located in theSouthern Volcanic Zone. This research focusses on two fluvial systems (Pireco and Totoral, PT) located in NahuelHuapi National Park, which were severely impacted by the Cordón Caulle 2011 eruption (CC2011). We investigateparticularly the resedimentation of pyroclasticmaterials on their lacustrine deltas and its potential implicationsconsidering 1) the deltas morphology and 2) mass-wasting processes developing at the delta fronts.Original tephra fall deposits were registered at Nahuel Huapi National Park in 2011. Based on the isopach map,we performed a GIS-based volume estimation of the deposit for the studiedwatersheds. Delta plain progradationfrom 2011 to 2017 was analyzed from satellite images. We studied the temporal evolution of the subaqueousmorphology and mass-wasting processes at the delta fronts using high resolution bathymetric grids registeredin 2011, 2014 and 2017. During June 2011 the studied watersheds received a tephra input of ~72 x 106m3. Thesignificant progradation experienced by both deltas especially during the firstmonth of the eruptionwas favoredby 1) abundant precipitation at the watersheds concomitant to heavy tephra deposition and 2) the pre-existingsubaqueous morphology characterized by a wide, shallow ?terrace? bordering PT delta plains. About six yearsafter the beginning of CC2011, 6% of the tephra fallen over the entire basins had been resedimented at thedelta environments (terrace and delta front), totalizing ~3 x 106t. Evidence of pre-eruption and/or co-eruptivemass-wasting processes occurring at PT delta fronts was found on November 2011. Up to 30-m thick pyroclasticdeposits were accumulated at PT delta fronts since November 2011, buryingmass-wasting features at the upperdelta fronts. A suspended-sediment collector installed at the prodelta in 2012 detected hyperpycnal currentstransporting pyroclastic materials. These currents probably explain the development of mass-movements athigh depths, such as the one that displaced and filled up the bottom sediment trap. This frontally-confined landslideinitiated at a depth of N60 m and involved tephra and organic materials deposited at the lower part ofTotoral delta front. Because of their recent deposition and loose consolidation, we propose that fresh tephra depositsat PT delta fronts could become particularly unstable and represent a potential hazard to failure, especiallyif a strong earthquake affects the study area in the next years. A landslide-induced tsunami wave generated atthese deltas would impact rural settlements located along the coasts of Brazo Rincón and probably reach thecoastal city of Villa La Angostura.