## 2 Study Area and Data
### Geological and Hydrogeological Setting  
Due to the complex structure of nappes and the large variety of different sedimentary rocks of the Northern Calcareous Alps, only the lithological units, directly influencing the formation of the Ludoialm slide, were examined. In the landslide area, Cretaceous sediments of the Gosau Group were mapped, characterised by flat bedding planes. The sequence of the Cretaceous layers at the toe of the landslides has already been described in detail by [PERSON] [52], showing a stratification from the bottom to the top based on a layer of (1) coal, (2) blue-grey sandstone, (3) gray marl with gastropods, (4) sand with corals and gastropods, (5) grey marl with gastropods, (6) breccia with components of rudist limestone, (7) grey clay with components of white shells, (8) solid limestone with sandstone, (9) grey clay, partly petrified, and (10) lime- and sandstone. A layer of glacial deposits with gravel to boulders showing scratch marks, e.g., lodgement till, were deposited on an unconformity formed by erosional processes on top of the Gosau sediments. The bulk mineralogical composition of the uppermost Gosau sediments, i.e., a clay-rich layer which  
Figure 1: The location and surroundings of the Ludoialm landslide (Latitude: 47\({}^{\circ}\)27\({}^{\prime}\)10.86\({}^{\prime\prime}\) N; Longitude: 11\({}^{\circ}\)49\({}^{\prime}\)47.12\({}^{\prime\prime}\) E) (OpenTopoMap). represents the boundary with the glacial deposits, was determined by X-ray diffractometer (XRD) measurements. The high content of calcite, dolomite, and sheet-silicates indicated a mary composition, with illite, chlorite, kaolinite, and vermiculite as main clay minerals. In general, the uppermost layer of the Gosau sediments and its contact with the glacial deposits is essential for an understanding of landslide processes, since the extraordinary flat dipping basal shear zone is located within this zone. Decomposed, weathered, and heavily fractured marks were mapped at least where the contact is outcropping at the toe of the slide. The marks showed a wide range of states depending on the degree of weathering and water content, characterised by a range of moderately weathered rock to a soft and clay-rich residual soil with high plasticity. Figure 2 shows different weathering grades of Cretaceous marl by the field investigation. Given that water is present in the entire landslide area, these residual soils are mainly encountered in the uppermost layer which was later on overlaid by the glacial deposits. Sandstone layers interstratified within the marls ranging from decimetres to metres in thickness were found during the geological field survey within and outside of the landslide area. Due to the lack of drillings, the spatial extent of the sandstone layers is unclear but significant to understand the hydrogeological situation, for controlling the landslide failure, and the deformation behaviour. It is assumed that the interstratifications of sandstone provide permeable zones within the clay-rich Gosau sediments and thus affect the pore pressure distribution at the basal shear zone. Glacial deposits, which essentially form the landslide mass, represent the product of local glaciation characterised by low transport distances. Therefore, evidence was found in the degree of rounding and shape from gravel to boulder as well as the occurrence of rocks that could be stratigographically assigned to the surrounding area. The fine-grained matrix of the glacial deposits consists of silty-clayey materials with a varying content of sand. Figure 3 shows different outcrops of glacial deposits at the landslide area. Stratification within the glacial deposits was locally observed in the scrap area, suggesting that debris flow activity may have caused to some extent the relocation and deposition of talus and reworked glacial deposits. Numerous ponds and waterlogging zones were mapped on the landslide surface, indicating a low permeability of the area.