Vestige of a subaerial rock avalanche deposit in an upper Cretaceous synorogenic wedge-top succession (Gosau Group, Eastern Alps)

Abstract

Modern subaerial catastrophic mass-wasting (rockslides, rock avalanches) are identified as being huge bulky masses with a surface littered with boulders to blocks. The identification of partly preserved deposits of ‘fossil’ subaerial catastrophic mass-wasting, in contrast, requires assessment of deposit fabrics. In the Alps, within the synorogenic Gosau Group (Turonian to Ypresian) that accumulated in an oblique-convergent setting, we had identified a ‘megabed’ (Wäschkogel megabed) up to 95 m thick that is interpreted as a vestige of a subaerial rock avalanche deposit. This megabed is an extremely poorly sorted, disordered, clast-supported deposit of sand- to boulder-sized dolostone fragments from an upper Triassic succession that comprises the local bedrock. In the megabed, numerous clasts display crackle, jigsaw and mosaic fabrics with fitted subclast boundaries. The matrix is a lithified gouge of fragmented dolomite crystals. In its upper part, the megabed displays very thick planar parallel stratification. Across strata, no change in sediment fabric is discerned.The distinctive features of the megabed, i.e., the sharp intercalation between fluvial deposits, large bed thickness with a uniform fabric, abundant clasts fractured in situ, and the gouge matrix all indicate that the megabed and lateral correlatives accumulated from a rock avalanche. The planar stratification in the upper part of the megabed is interpreted as a dynamic feature produced by internal simple shear during a terminal phase of rock avalanche propagation. Bedrock geology and the distribution of other Gosau Group deposits in the area suggest that the rock avalanche propagated along a SSW–NNE striking incised valley. This is the first identification of a subaerial rock avalanche deposit in the Gosau Group. Our case study may help to consolidate the awareness of deposits of fossil subaerial catastrophic mass-wasting.