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Thickness and structure of alpine slope sediments derived from geophysical investigations


Start date: 01.01.2003
Duration: 3 years
Funded by: DFG SA 985-1

Abstract

Understanding regional sediment budgets requires the quantification of stored material in sediment sinks. However, our knowledge of talus slope and basin volumes is still fragmentary. Geophysical investigations frequently offer the only opportunity of achieving significant results on debris volume and composition. The aims of the studies were (1) to gain insight into the inner structure of alpine sediment bodies using ground-penetrating radar (GPR) and further geophysical techniques, (2) to provide an estimate of long-term weathering and removal rates by measuring the total volume of debris accumulated on slopes and in basins and (3) to compare these values to short-term measurements of current processes and find reasons for discrepancies.

Description

Some main results can be summarized as follows:
  • Geophysical investigations, especially GPR, proved to be very potent for investigating loose debris in alpine environments. The bedrock surface and internal interfaces were detected down to a depth of 30-50 meters. GPR is the only geophysical method applied that allows for a detailed study of textural patterns which contributes critically to data interpretation, e.g. distinguishing talus debris from moraine and bedrock.
  • The cross-check between the different geophysical datasets yields excellent results when the setting is relatively straightforward (e.g. loose debris overlying bedrock). Under complex subsurface conditions, deviations between the methods occur because of lack of contrast between the sediment layers investigated. The combined application of GPR, 2D-resistivity and seismic refraction allows sophisticated subdivision of the sediment body.
  • Almost all of the talus deposits investigated are evidently stratified. Surficial debris flows and dry grain flows resulting from over-steepening provide the best explanations for the structures observed. A core of glacigenic debris is found under most of the talus deposits investigated.
  • Estimated rockwall retreat rates range from 200 to 850 mm/ka with a mean of 460 mm/ka. The process rates derived from talus volumes are significantly higher on average than those derived from direct measurements. The reason for the deviations lies in all probability in phases of higher sediment delivery in the past.



Fig. 1: Talus cones at the foot of the Dremelspitze, Lechtaler Alps



Fig. 2: Part of a 50-MHz-radargram of talus "Dremelspitze cone 2"



Fig. 3: Cross-section of "Dremelspitze cone 3" derived from multi-method geophysical investigation


Publications related to the subject (selected):

  • Otto, J.C. & Sass, O. (2006): Comparing geophysical methods for talus slope investigations in the Turtmann valley (Swiss Alps). Geomorphology 76: 257-272.
  • Sass, O. (2006): Determination of the internal structure of alpine talus deposits using different geophysical methods (Lechtaler Alps, Austria). Geomorphology 80: 45-58. doi:10.1016/j.geomorph.2005.09.006
  • Sass, O. (2006): Geophysical investigation of a relict talus slope in the Bavarian Alps, Germany. Zeitschrift für Geomorphologie 50 (4): 447-463.
  • Sass, O. & Krautblatter, M. (2007): Debris-flow-dominated and rockfall-dominated scree slopes: genetic models derived from GPR measurements. Geomorphology, doi:10.1016/j.geomorph.2006.08.012
  • Sass, O. (2007): Geophysical quantification of talus thickness and rockwall retreat in the eastern European Alps. Journal of Applied Geophysics, doi:10.1016/j.jappgeo.2006.12.003.