SUBAQUEOUS MORPHOLOGY OF LAGO TRAFUL “BOSQUE SUMERGIDO”: ACTIVE GRAVITATIONAL PROCESSES, NEW GLACIAL FEATURES AND TSUNAMI HAZARD IMPLICATIONS

BEIGT DEBORA; EDUARDO A. GOMEZ; GUSTAVO VILLAROSA; LUIS ARIEL RANIOLO

Paraná

Congreso; XV RAS -VIII OCTAVO CONGRESO LATINOAMERICANO DE SEDIMENTOLOGIA; 2021

Asociación Argentina de Sedimentología

The recent public and scientific debate about a tsunami hazard related to a possible slope instability in the ―Bosque Sumergido‖ area in Lago Traful (Carballo et al., 2019) led to a bathymetric and geophysical survey of the lake floor around such area, requested by PN Nahuel Huapi authorities and carried out by the authors of this contribution (IPATEC and IADO). The aim of this survey was to find evidence of the gravitational processes developing in the area by studying the subaqueous morphology and substrate characteristics. In order to evaluate the possibility of an acceleration of the mass flows, subaqueous slopes are studied by detailed morphological surveys and sub-bottom profiles of the distal mass flow deposit. The geomorphological context was analyzed in a cross-section bathymetry from the north to the south coast, generating valuable information for the assessment of possible tsunami waves affecting Villa Traful. Fieldwork was carried out on December 2019. A phase measuring bathymetric sonar (GeoSwath Plus, GeoAcoustics) allowed the creation of high-resolution DEMs up to ~200m depth. For the geophysical survey of particular interest areas a StrataBoxTM profiler (Marine Geophysical Instruments) and a PulSAR side-scan sonar (Kongsberg Maritime) were used. Bathymetric profiles were performed in several areas of the lake using a Garmin GPSMap 420 echosounder. The initial findings lead to the next observations: 1) The cross-section bathymetry shows a continuous longitudinal slope change that separates two distinct sectors: the eastern area with a flat bottom homogeneous topography, and depths of 130 – 140 m; and the U-shaped western sector, with depths of 180 m and more. These two subaqueous morphological environments are separated by a narrow, elevated crest in the shape of an arch, which extends continuously between the north and south laterals of the lake trough. 2) The ―Bosque Sumergido‖ region presents two lobed morphologies that extend continuously from the coast up to 110 m depth, ending in the flat bottom area that characterizes the eastern environment. 3) The shape of the submerged trees can be identified, in ―life position‖, tilted or reclined on the surface of these lobes. 4) The bathymetric and sub-bottom longitudinal profiles of the lobesshow slope angles of 15 - 18° with topographic breaks, thicknesses and sedimentary structures that are consistent with typical morphologies of a subaqueous mass-waste deposit. These observations lead to the following interpretations: A) Reported slow subaerial mass flows continue without substantial changes in underwater environments. The observed slopes and subaqueous structures do not suggest topographic or sedimentary instability that may cause sudden acceleration of these mass flows. B) The study area shows clear presence of a subaqueous moraine with distinctive morphology, oriented N-S in the lacustrine basin, which probably corresponds to a recessional moraine representing a relatively stable ice position during late Pleistocene, allowing the development of a proglacial lake and a consequent lacustrine sedimentation in the eastern area of the present lake. C) The hazard situation would be significantly different if these flows, instead of leaning over the shallower flat eastern bottom, had developed over the deeper lake floor located west of the moraine, causing potential instability resulting in higher tsunami hazard. D) This peculiar subaqueous morphology must be taken into account when considering tsunami hazard hypotheses and, especially, when modelling wave propagation as this should be the main tool for tsunami hazard mapping.