Scratching beneath the surface: Using Ground-Penetrating Radar to disentangle pronival ramparts, embryonic rock glaciers and moraines (Gardetta Plateau, Southwestern Alps)

Abstract

The distinction between debris ridges/ramparts that form at the base of mountain slopes in cold environments is important to understanding the (palaeo) environmental condition that prompted their formation. Despite the definition of diagnostic criteria, the identification of the formative (nival, permafrost creep, glacial) and post-depositional mechanisms remains uncertain as it is mostly reliant on surface observations. We present the results of a Ground-Penetrating Radar (GPR) survey carried out on landforms presenting similarities from a geomorphological perspective (i.e. pronival rampart, protalus rock glacier, moraine) in the same Alpine context. The 2D radar profiles provide imaging of the internal structure of these landforms, allowing us to formulate hypotheses on the depositional processes that led to their formation. In the pronival rampart, concave upward and mound-shaped radar reflections fit with the presence of a ridge/rampart at the base of a permanent snow patch. Mound-shaped, flat or counterslope reflectors reflect the existence of stratigraphic beds mantling the sides of the ridge and infill the depression between this and the talus upslope or the retreating snow patch front. Some sequences of GPR facies sequences are consistently referable to progradational/retrogradational depositional processes. Poor articulation of facies architecture characterizes frontal moraines and embryonic protalus rock glaciers. The succession of predominantly planar radar surfaces/facies is consistent with an important aggradational component that can be explained by a stratigraphic overlap of debris layers fed by the glacial front and/or of permafrost-affected sedimentary units.

The GPR imaging of the interior of these mountain landforms frequently found at the talus foot can offer crucial data for the correct interpretation of the formative mechanisms and, thereby of the environmental conditions that promoted their formation.