Medieval age determined for the hitherto undescribed/undated rock avalanche of Münster (Inn valley, Eastern Alps)

Abstract

Over the entire Alps, ~75% of ‘catastrophic’ mass-wastings (CMW; rockslides and rock avalanches >105 m3 in volume) still are undated. We have studied and radiocarbon-dated a rock avalanche deposit (RAD) that was not investigated in any detail before. The RAD is located near the village of Münster in the Inn valley, a seismogenically active belt decorated with CMWs of pre- to post-Glacial age. The rock avalanche detached from a succession of Triassic carbonate rocks pre-conditioned for failure by halokinesis from subjacent evaporites. The detached volume of rock was determined at 6 Mm3, the fahrböschung of the rock avalanche is 16°, the runout is 4.76 km and the 3D-area of the deposit is 1.7 km2. A conservative estimate of the final volume of the deposit amounts to 10.5 Mm3, corresponding to a minimum of ~20% of clastic material entrained en route. After detachment, the rock mass bypassed a steep proximal slope and was funneled into a short valley. During its distal runout over a wide alluvial fan, the avalanche spread laterally and –because of presence of a steep-flanked whaleback transverse to propagation direction – digitated into three fingers. Several lines of independent evidence suggest that the rock avalanche descended in association with snow. Combined probability ranges of radiocarbon dates of three samples excavated from the outermost fringe of a conifer log found within the RAD indicates an age range of 1022–1049 cal CE (1022–1151 cal CE 95.4% probability range) as the most probable proxy age of catastrophic mass-wasting. Four radiocarbon ages (6648–6249 cal BCE, 95.4% probability range of all samples) of charcoal and wood from the underlying alluvial fan record active aggradation of the fan until at least the terminal early Holocene. The Münster rock avalanche is the youngest one of the array of pre- to post-Glacial catastrophic mass-wastings along the Inn valley.