Boletín de la RSEHN. Sección Geológica

La realización de una cartografía geomorfológica detallada en una superficie de 220 km2 en torno al Lago de Sanabria (Noroeste de España), junto con la caracterización sedimentológica de los depósitos glaciares, han permitido reconstruir la extensión alcanzada por el hielo durante el máximo glaciar local. De acuerdo con este modelo, el aparato glaciar instalado sobre la cuenca del Lago de Sanabria alcanzó una extensión de al menos 155 km2, situando su frente más avanzado en el valle Tera (940 m). Sus límites septentrional y oriental habrían sobrepasado los límites de estudio, de modo que este aparato glaciar constituiría una fracción de un casquete montañoso más grande instalado sobre el Macizo de Trevinca, al oeste de la zona de estudio. Se ha aplicado un modelo matemático para establecer la altitud alcanzada por el hielo a lo largo de 29 perfiles longitudinales siguiendo las paleo-direcciones de flujo del hielo indicadas por las evidencias geomorfológicas. La integración de todos los datos mediante un SIG ha permitido elaborar un modelo digital de la topografía del hielo a partir del cual se ha podido determinar tanto la distribución de potencias del glaciar (entre 0 y 454 m), como su volumen total (estimado en 22,9 km3) durante el máximo glaciar local.

The Sanabria Lake (1,000 m a.s.l) is the biggest natural freshwater lake in Spain, located in NW Iberia. Both the Sanabria Lake and the regional landscape show features which indicate the presence of Quaternary glacial activity. In this study geomorphological mapping and sediment characterization were carried out on an area of 220 km2 centered on the Sanabria Lake, including its hydrographic basin (127 km2). The geomorphologic evidence were compiled on a Geographic Information System (GIS) in order to delineate the maximum extent ever reached by glaciers. According with this 2D reconstruction model, the glacial ice would have extended on 155 km2 of the study area, exceeding its limits to the North and to the West. A 3D modeling of the ice surface was conducted through the application of the Profiler v.2 spreadsheet designed by Benn & Hulton (2010) on 29 longitudinal profiles, along the palaeo ice-flow directions inferred from glacial evidence (glacial valleys and cirques, sheepback rocks, glacial striations or moraine deposits). In each profile, iterative calculations were done in steps spaced 100 m from the glacial front to the head of the valley. A 3D reconstruction model including the digital ice model and ice contours of the glacier surface was obtained by combining the results obtained from ice profiles with the glacier boundaries of the previous 2D reconstruction model. As shown by the 3D model, the glacier surface would have ranged between 2,098 and 940 m a.s.l. The curvature change between ice contours 1,500 and 1,400 m fits to the elevation of the lateral moraines pointing the location of the glacier´s paleo-ELA (Equilibrium Line Altitude). An ice thickness model has been obtained by combining the digital terrain model with the digital ice model. This model shows thickness values ranging from 0 to 454 m, with maxima located in the confluence between the Tera and Segundera-Cárdena valleys (400-454 m). The high ice thickness and the paleo-ELA proximity would have made the confluence point between both glacial valleys a place with important rates of glacial erosion, as evidenced by the presence of sheepback rocks scattered on an area of 7.5 km2. One of the main limitations of the model is the use of a current digital terrain model, including glacial features (like glacial depressions filled by sediments or moraine ridges) younger than the glacial phase reconstructed in this study, which would lead to thickness estimations smaller than real values. On the other hand, the digital terrain model also presents erosive features younger than the glacial episode which is being reconstructed. This is the case of fluvial incisions superimposed to the glacial valleys, where thickness estimations must be bigger than the real values. Despite these limitations, the 3D model offers a good opportunity to reconstruct the glacier surface morphology and thickness variations, as well to estimate the ice volume that covered the study area during the local glacial maximum (22.9 km3).