Christoph Prager, Karl Krainer, Veronika Seidl & Werner Chwatal
Spatial features of Holocene Sturzstrom-deposits inferred from subsurface investigations (Fernpass rockslide, Tyrol, Austria)

Geo.Alp 3, 2006, p. 147–166

Abstract
A low frequency Ground Penetrating Radar (GPR) system was successfully applied for near subsurface explorations at different accumulation areas of the fossil Fernpass rockslide (Tyrol, Austria), which is one of the largest mass movements in the Alps. Based on detailed field studies and calibrated by drillings down to a depth of 14 m, the reflectors of the processed GPR-data could be well attributed to different depositional units. As a result, the distal rockslide deposits feature intensively varying accumulation geometries and are up to approximately 30 m thick. In addition, the topographically corrected GPR data show that the Toma, i.e. cone-shaped hills composed of rockslide debris, show deeper roots than the topographically less elevated rockslide successions between them. Compiled field-, drilling- and GPR-data indicate that the accumulation pattern and spread of the investigated rockslide deposits was obviously predisposed by the late-glacial valley morphology and that the Sturzstrom surged upon groundwater-saturated, fine-grained lacustrine sediments. Thus we assume that dynamic undrained loading, reducing the effective stresses between the rockslide and its incompetent substrate, enabled the extremely long run-out distance of the sliding mass measuring up to at least 15.5 km. Continuous gravitational spreading, which probably occurred subsequent to the rapid Sturzstrom flow, resulted in a further decomposition of the rockslide deposits and the generation of the present morphology, characterised by the well-known Toma hills and associated funnel- to basin-formed depressions which include several kettle-like lakes. After decomposition, the spreading rockslide deposits have locally been covered by on-lapping fluvial deposits. According to GPR data, these post-rockslide sediments can reach thicknesses of up to at least 20 m.


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