Subaqueous morphology of lago Traful "Bosque Sumergido" area: active gravitational processes, new glacial features and tsunami hazard implications

GÓMEZ, EDUARDO ALBERTO; BEIGT, DÉBORA; VILLAROSA, GUSTAVO; ARIEL RANIOLO

Paraná

Congreso; XVII Reunión Argentina de Sedimentología; 2020

Asociación Argentina de Sedimentología

The recent public and scientific debate about a tsunami hazard related to apossible slope instability in the Bosque Sumergido area in Lago Traful (Carballo et al., 2019) led to a bathymetric and geophysical survey of the lakefloor around such area, requested by PN Nahuel Huapi authorities and carriedout by the authors of this contribution (IPATEC and IADO). The aim of thissurvey is to find evidence of the gravitational processes developing in thearea by studying the subaqueous morphology and substrate characteristics. Inorder to evaluate the possibility of an acceleration of the mass flows,subaqueous slopes are studied by detailed morphological surveys and subbottom profilesof the distal mass flow deposit. The geomorphological context is analyzed in across-section bathymetry from the northern to the southern coast, generating valuableinformation for the assessment of possible tsunami waves affecting VillaTraful. Fieldwork was carried out on December 2019. A phase measuring bathymetricsonar (GeoSwath Plus, GeoAcoustics) allowed the creation of high-resolution DEMsup to ~200m depth. For the geophysical survey of particular interest areas a StrataBoxTMprofiler (Marine Geophysical Instruments) and a PulSAR side-scansonar (Kongsberg Maritime) were used. Bathymetric profiles were performed in severalareas of the lake using a Garmin GPSMap 420 echosounder. The initial findings leadto the next observations: 1) The cross-section bathymetry shows a continuouslongitudinal slope change that separates two distinct sectors: the eastern areawith a flat bottom homogeneous topography, and depths of 130 ? 140 m; and the U-shapedwestern sector, with depths of 180 m and more. These two environments areseparated by a narrow elevated crest in the shape of an arch, which extends continuouslybetween the northern and southern laterals of the lake trough and presents 75m-deep top heights. 2) The Bosque Sumergido region presents two lobed morphologiesthat extend continuously from the coast up to ̴ 110 m depth, ending in the flatbottom area that characterizes the eastern environment. 3) The shape of thesubmerged trees can be identified, in life position, tilted or reclined onthe surface of these lobes. 4) The bathymetric and subbottom longitudinal profilesof the lobes show slope angles of 15 - 18° with topographic breaks, thicknessesand sedimentary structures that are consistent with typical subaqueous mass-wastedeposit morphologies. These observations lead to the following interpretations:A) Reported slow subaerial mass flows continue without substantial changes in underwaterenvironments. The observed slopes and subaqueous structures do not suggest topographicor sedimentary instability that may cause sudden acceleration of these mass flows.B) The study area shows clear presence of a subaqueous moraine with distinctivemorphology, oriented N-S in the lacustrine basin, which probably corresponds toa recessional moraine representing a relatively stable ice position during latePleistocene, allowing the development of a proglacial lake and a consequentlacustrine sedimentation in the eastern area of the present lake. C) The hazardsituation would be significantly different if these flows, instead of leaningover the shallower flat eastern bottom, had developed over the deeper lakefloor located west of the moraine, causing potential instability resulting inhigher tsunami hazard. D) This peculiar subaqueous morphology must be takeninto account when considering tsunami hazard hypotheses and, especially, whenmodelling wave propagation as this should be the main tool for tsunami hazard mapping.