Sedimentology of rock avalanche deposits - Case study and review

Detailed sedimentological studies concerning different facies types and the factors influencing their development (such as lithology, topography, substrates, etc.) were carried out at the carbonate Tschirgant rockslide-rock avalanche deposit in the Eastern Alps of Austria. Several depositional fades and sub-fades have been identified at this and other sites by many researchers. These fades are found throughout the debris, regardless of depth or travel distance, and result from highly heterogeneous stress distributions in time and space during emplacement. One of the most important recent observations in terms of emplacement dynamics is that shear is not confined to the base, but is distributed throughout the debris (except the coarse carapace). Due to the preservation of source stratigraphy in these non-turbulent mass movements, any horizontal facies or grain size variations need to be regarded in context since they strongly depend on source stratigraphy and hence the spatial distribution of lithologies (i.e. geological units of different mechanical properties). For example, some lithologies do not form a boulder carapace due to absence of large in-situ blocks in the source rock mass. However, at Tschirgant, slight fades maturation with distance (within the same lithology) is observed, i.e. clast fragmentation progresses to create the distinct, fades-characteristic grain size distribution only after some distance travelled. These details are revealed through fades-based sampling and analysis. Studies based on bulk sampling, in some cases, observed progressively reduced grain sizes with distance and depth which might be a function of added variations within different fades sampled together. In this paper, the wealth of observations on rockslide and rock avalanche sedimentology of the past decades is combined with a detailed case study of a (lithologically) relatively simple deposit, to outline global and site-specific depositional structures and fades with the aim to summarize, review, and refine the current state of knowledge on rock avalanche and rockslide emplacement processes. (C) 2016 Elsevier B.V. All rights reserved.