Estructuras glaciotectónicas en cercanías a Laguna Grande, valle del río Santa Cruz, Argentina

Abstract

A lo largo de las barrancas que limitan el cauce actual del río Santa Cruz, al este del Lago Argentino, afloran depósitos glaciolacustres, glaciofluviales y morrénicos. Sobre el piso y laterales del valle de dicho río se encuentran geoformas glaciares vinculadas al avance y estabilización de la masa de hielo, estrechamente relacionadas con estos depósitos. Al presentar un alto grado de conservación permitieron posicionar geográficamente al paleo-glaciar Argentino. Los afloramientos glaciolacustres correspondientes al paleolago Argentino, muestran deformaciones frágiles y dúctiles que se han analizado en el contexto de las diferentes glaciaciones ocurridas en esta comarca andino-patagónica austral y de la impronta dejada, principalmente, por los avances glaciares correspondientes a la Última Glaciación. Las orientaciones de estas estructuras glaciotectónicas, como así también el potente espesor de sedimento deformado, las menores dimensiones de los cabalgamientos y pliegues, y su comparación y diferenciación con las estructuras tectónicas observadas hacia la posición actual del Lago Argentino, permiten separar a estos episodios de una actividad neotectónica y por el contrario, pueden ser relacionados a un origen local asociado con los rápidos avances glaciares ocurridos en el área.

Along the upper valley of Santa Cruz river, eastern of Argentino Lake, glacial lake, fluvioglacial and moraine deposits crop out, in the nearby cliff along the current channel of the Santa Cruz River. In the valley there are glacial landforms as laterals moraines, frontal moraines and drumlins related with advances-retreats and stabilization of the ice mass. The first ones show an arcuate shape in plant view and together with the radial distribution of drumlins suggests a lobular form of the ice front. The positions of these landforms also evidence the palaeo-position of the glacier front. These outcrops show deformations that have been analyzed in the context of glaciations that happened in the Southern Patagonia-Andean region and of the imprint that was mainly left by the Pleistocene glacial advances. The deformations considered here are independent of the regional tectonic structure and they are mostly localized and bounded in stratigraphic terms. The style of deformation also allows it to be separated from regional episodes of Quaternary tectonic activity. On the contrary, these structures can be related to local causes, associated with the effect produced by glacial advances on previous glacigenic deposits. Evidences of glacier presence and movement are suggested by the thrusts and overturned folds that formed an arcuate pattern, perpendicular to the orientation of the drumlins axes. Due to the thickness of deformed sediments and progressive lengthening of drumlins axes towards the east, it is possible that the origin of deformation was related with a glacial surge episode. This assumption also shows the importance of a detailed structural analysis, to help out for evidences of paleoclimate and glacial dynamic in the past.