Determination of the internal structure of alpine talus deposits using different geophysical methods (Lechtaler Alps, Austria)

Abstract

The total thickness and structure of alpine talus accumulations were investigated in a study area in the Northern Alps using ground-penetrating radar (GPR) completed by 2D-geoelectric (ERT) and seismic refraction soundings. The loose, steep talus cones of the Parzinn cirque are situated at an elevation of 2100–2300 m a.s.l. and end downslope at late-Glacial protalus ramparts. The surrounding rockwalls consist of “Hauptdolomit”, a brittle dolostone. In most of the GPR profiles, the bedrock surface was clearly recognizable from the radargrams. Total talus thicknesses of up to 45 m were estimated. The uppermost talus layer is characterized by surface-parallel radar reflection patterns, intermediate resistivities (4–10 kΩ m) and low seismic velocities (400–500 m/s). These deposits are interpreted as stratified debris, built up during the Holocene by rockfall deposits occasionally resorted by debris flows. Below this unit, a zone of more irregular radar reflection patterns and enhanced electrical resistivities follows. The combined results lead to the assumption of very coarse grained, paraglacial rockfall deposits, or to a core of basal moraine beneath the talus. Some zones of especially high resistivity (> 25 kΩ m) may be related to sporadic permafrost occurrence in the depressions between the talus and the moraine ridges. Striped radar structures are also recognizable in the moraine bodies. The stratified sediments give evidence for glaciofluvial transport or debris flows as an agent of protalus rampart formation. However, these features probably result from a combination of different processes including periglacial creep. GPR turned out to be a powerful tool for the determination of debris volumes and internal structures. However, supporting geoelectric and seismic investigations are advisable to validate the results and to facilitate and improve the interpretation. The results point to an average Holocene rockwall retreat of 500 to 800 mm/ka, which is roughly three times higher than recent backweathering rates.