Feasible task? The application of the Schmidt-hammer method for dating rock glaciers made of conglomerate rock

Abstract

The Schmidt-hammer (SH) is a well-established method in studying glacial and periglacial landforms in alpine regions. Schmidt-hammer rebound values (or R-values) allow the relative-age dating of landforms by quantifying the degree of weathering and therefore length of surface exposure. R-values are controlled by lithological variations impacting weathering rates. SH sampling in a specific study should therefore focus on one lithology. Earlier studies found out that some rock types are less suitable than others because of their specific weathering rind development over time. Some rock types are even unsuitable for the application of the SH. In the past, conglomerates for instance were rarely used for SH measurements, which may be related to problems in SH discrimination between the matrix and the clasts of conglomerates. In this study, we applied the SH method at several relict rock glacier systems and lateral moraine ridges which consist of conglomerate and sandstone material of Upper Carboniferous age. The landforms studied are in two cirques (Rosaninalm, Hinteralm; 46.9°N, 13.8°E) in the Gurktal Alps, Austria. We accomplished SH sampling at altogether 21 sites with 100 individual SH measurements per site. The 21 SH measurement sites are located along five longitudinal profiles that were placed over the different landscape forms, the longest one over a horizontal distance of 1.3 km. Measurements focussed on the matrix material of the conglomerates. Mean R-values vary between 29.6 and 39.0. Using these results and assuming a reasonable mean decrease in R-value of 1.5 per ka (based on nearby data from gneiss material), one can assume that the landforms studied were formed over a total period of approximately 6000 years. Individual landform units, in our case mostly rock glaciers, seem to have formed over periods of between 1.1 and 4.9 ka. By using these age estimates and present permafrost conditions, the onset of moraine and rock glacier formation was presumably during the Oldest Dryas and landform stabilisation occurred during the early Holocene. Several uncertainties remain, however, which will be addressed at the poster.