Complex Interactions of Rock Avalanche Emplacement with Fluvial Sediments: Field Structures at the Tschirgant Deposit, Austria

Abstract

The Tschirgant rock avalanche in Tyrol, Austria, produced a complex deposit 200–250 × 106 m3 in volume and 9.8 km2 in area. The landslide resulted from deep-seated failure of an intensely deformed carbonate rock mass on the southeast face of a 2370 m-high ridge. The rock mass rapidly fragmented as it moved towards the floor of the Inn River valley. Part of the debris collided with and moved around an opposing bedrock ridge and flowed into the Oetz valley, reaching up to 6.3 km from source. A large volume of mobilized sedimentary gravels and sands occurs beneath, within, and atop the rock avalanche deposit. Within proximal inter-hummock depressions, entrained gravels and sands extend to the deposit surface; elsewhere they are intercalated in narrow bands that dip towards and into flow direction. Some of these sediments were liquefied and mobilized en route, whereas others were most likely inherited from the source area. None of them shows signs of fragmentation, suggesting different mechanical behaviour or low fragmentation pressures at the time of entrainment. Generally, exposures of the basal contact of large rock avalanche deposits are rare, but at Tschirgant they are well exposed and reveal substrate injection features (some > 10 m across), thrust and normal faults, entrained sand and gravel rip-up clasts, corrugated basal shear contacts, and disturbed underlying material. Mixing of rock avalanche and substrate material is only observed at the distal margin, suggesting longer travel distances or particular material properties to allow mixing to take place. Ongoing research focuses on these substrate interaction features to reveal details of rock avalanche movement, flow paths, and emplacement.