AMQ SR06 Reitner et al 165-181.pub Available online http://amq.aiqua.it ISSN (print): 2279-7327, ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, 33 (2), 2020, 165-181 THE EARLY HOLOCENE BUCHWIESE ROCK AVALANCHE (EASTERN ALPS, AUSTRIA): GEOLOGICAL CONDITIONS, KINEMATICS, MORPHOLOGICAL AND SEDIMENTARY LEGACY. Jürgen M. Reitner 1, Susan Ivy-Ochs 2, Olivia Steinemann 2, Daniela Lattner 3, Alexander Römer 1 1 Geologische Bundesanstalt / Geological Survey of Austria, Vienna, Austria. 2 Laboratory of Ion Beam Physics, ETH Zurich, Zurich, Switzerland. 3 Former member of the Geological Survey of Austria, Vienna, Austria. Corresponding author: J.M. Reitner ABSTRACT: In this study we reconstructed the Buchwiese rock avalanche in the Lienz Dolomites in Eastern Tyrol (Austria). We used a multi-method approach combining geological field mapping, the analysis of digital elevation model (DEM) data, cosmogen- ic 36Cl exposure dating, and a geoelectrical survey to unravel the detachment mechanisms, emplacement processes and timing of the Buchwiese rock avalanche. According to the results of the 36Cl exposure dating, the event took place at 10.8±0.9 ka during the Early Holocene. The failure of a rock mass with a volume of 27x106 m3 was enabled by a dip-slope in strata of the Kössen For- mation (limestone, marls, claystone), Oberrhätkalk (massive to thickly bedded limestone), Allgäu Formation (mottled limestone and marl) and Rotkalk (red nodular limestone and marl), in combination with N-S to NNE-SSW trending brittle faults. We regard fatigue of the fine-grained rocks of the Kössen Fm. (claystone, marl), typical slaking rocks, since the Last Glacial Maximum as the major cause for this catastrophic rock slope failure. With the reconstructed drop height (H) of 1200 m and runout length (L) of 3.5 km, the Buchwiese rock avalanche has a Fahrböschung angle α of 19° and H/L ratio of 0.36. Due to the geological conditions, the initial failure occurred as a plane slide. In the deposition area, we observe strong control of lithological properties, topographic conditions, and substrate materials along the pathway on the morphology and sedimentology of the rock avalanche deposit. Longi- tudinal ridges, indicating spreading of an unconfined flow, are comprised mostly of massive limestone (Oberrhätkalk). The cara- pace facies consisting of clast-supported boulders is only developed in areas with limestone (mostly Oberrhätkalk but also lime- stone of the Kössen Fm.). The body facies in the upper parts are dominated by jigsaw subfacies with a subordinate occurrence of fragmented subfacies in the outcrops. Even in the middle part of the deposition area, we observe the prevalence of the moderately fragmented jigsaw subfacies within large areas of Kössen Fm. debris, which consists of alternating claystone, limestone beds. Such a finding may indicate preferential deformation within the claystone beds. After partial collision with a bedrock ridge and a small jump, the fragmented subfacies dominates. This collision led to the formation of a fan-like megaboulder cluster consisting of detached and fragmented Oberrhätkalk boulders with volumes up to 1000 m3 and a fan-like distribution. The results of geoelectri- cal surveys reflect different amounts of fragmentation with the carapace facies, showing high resistivity, while the body facies re- veals low resistivity. Preserved source stratigraphy within the dilated rock mass indicates predominantly laminar rock avalanche movement. All the morphological and sedimentary evidence supports a dynamic fragmentation model as the best mechanical explanation for the Buchwiese rock avalanche. Keywords: Rock avalanche, fragmentation, cosmogenic 36Cl exposure dating, Holocene, Eastern Alps. 1. INTRODUCTION Large catastrophic rock slope failures (rock slides, rock avalanches) with volumes of >106 m3 are rare but are nevertheless perhaps the most spectacular forms of mass displacement in high-relief mountains (Abele, 1974). They suddenly change landscapes and have been perceived in human history as catastrophes. Due to the usually surprising occurrence of such rapid land- slides, the enormous amount of energy released as a result and corresponding dust formation, there are no direct observations of the processes like internal and basal facies fragmentation (Hewitt et al., 2008). Our understanding of catastrophic rock slope failures, their mechanics and kinematics, mostly relies on the study of paleo-events and thus on the morphological and geolog- ical legacy of such processes (Heim, 1932; Abele, 1974; Erismann & Abele, 2001). Among these catastrophic rock slope failures, rock avalanches are the most fascinating features due to high velocities of more than 200 km/h and due to an extreme- ly long runout (Heim, 1932; Abele, 1974; Hewitt et al., 2008). The comprehension of how rock is transformed to a highly fragmented material that acts like a laminar fluid with reduced friction is one of the great challenges in landslide research (Hungr et al., 2005; Davies & McSaveney, 2009; Imre et al., 2010; Aaron & McDougall, 2019). Thus, the description of lithological properties of original and crushed bedrock may, together with that of morphological features, provide a sound basis for a better understanding of the processes in- volved (Dunning, 2004; Crosta et al., 2007; Weidinger et al., 2014; Dufresne et al.; 2016a, b; Dufresne & Dunning, 2017) and for modelling approaches (e.g. https://doi.org/10.26382/AMQ.2020.12 166 Reitner J.M. et al. 167 Aaron et al., 2020; Singeisen et al., 2020). In addition, there is great societal interest, especially in countries with increasingly populated Alpine valleys, in the poten- tial danger related to such rapid landslides. Hence, a better perception of causes and triggers of such hazards ranging e.g. from material fatigue, over earthquakes to climate change and, finally, of the chronology of pro- cesses before the failure are of eminent importance (e.g. Eisbacher & Clague, 1984; Zerathe et al., 2014; Wood et al., 2015). Recent progress in geochronology, especially the application of surface exposure dating with terrestrial cosmogenic nuclides like 10Be and 36Cl (Ivy-Ochs & Kober, 2008) together with 14C and U/Th dating (Prager et al., 2008; Ostermann et al., 2007, 2017) is enabling a tremendous extension of the dated paleolandslide record in the Alps (Prager et al., 2008; Ivy-Ochs et al., 2017; Pánek, 2019). Bauer (1990) first mentioned a rock avalanche deposit in the study area. Reitner (2003a) provided a first detailed map and description of the Buchwiese rock avalanche with an estimated volume of c. 22x106 m3. In this paper, we present new geological, geochronological and geophysical data of the reconstructed pre-historical Buchwiese rock avalanche, as a contribution to research on catastrophic landslides in the Alps. The aim of this study is to show the influence of different lithologies and thus rock properties together with that of topographic conditions and substrate materials along the pathway on the morphology and sedimentology of the rock ava- lanche deposit. 2. GEOLOGICAL SETTING AND PREVIOUS RE- SEARCH The study area is located in Eastern Tyrol on the northern flank of the Lienz Dolomites mountain group within the Eastern Alps, c. 5 km SE of the city of Lienz in the Drau Valley (Fig. 1, 2). Tectonically, the Lienz Dolo- mites mountain group is part of the Austroalpine Superu- nit and, more specifically, of the Drauzug-Gurktal Nappe System (Schmid et al., 2004; Schuster et al., 2014). The latter is subdivided in the study area into the Lienz- Dolomites Nappe and the Kreuzeck-Gailtaler-Alpen Nappe (Linner et al., 2013). <<< ---------- ---------- Fig. 1 - Maps showing (a) the location of the study area and (b) the geology (modified after Linner et al., 2013). Contour interval is 100 m. The bold steepled black line shows the traces of the geological profile in Fig. 3. The early Holocene Buchwiese rock avalanche Fig. 2 - View from the opposite Drau Valley flank on the study area and surrounding. The yellow steepled line shows the run of the scarp of the Buchwiese rock avalanche followed by the deposition area surrounded by the white line. Note the subglacially shaped peaks like Rauchkofel and Auerlingköpl in contrast to the rugged peaks above like Spitzkofel indicating nunataks during the LGM. Numbers indicate following locations: 1 - Weissstein, 2 - Rossboden, 3 - longitudinal ridge with boulders Lienz 5 and Lienz 6, 4 - Kreithof, 5 - Kohlstatt creek, 6 - Buchwiese. The source area and the upper and middle part of the deposition area of the Buchwiese rock avalanche are made up of rocks of the tectonically higher Lienz- Dolomites Nappe (Figs. 1, 3), which occur in the Am- lacher Wiese Syncline (Schmidt, 1995; Blau & Grün, 1995). The lowermost part of the deposition area is underlain by mica schists of the Kreuzeck-Gailtaler- Alpen Nappe. Descriptions of fault systems and their chronology are provided by Schmidt (1995), Brandner et al. (2001) and, in combination with a hydrogeological characterisation of faults and rocks, by Probst et al. (2003). The scarp area is part of the southern limb of the Am- lacher Wiesen Syncline with a general dip towards the north. In the following, the lithology of the rocks in the scarp area are characterised according Bauer (1990): • The Kössen Formation (Rhätium) in the study area consists of an interbedded up to 350 m thick se- quence of black pelites (claystone, marls) and dark limestone with an ochre colour due to weathering (Fig. 4a). Geotechnically, the fine-grained lithologies (claystone and marl) are classified as slaking rocks showing disintegration upon exposure to air or mois- ture in combination with an irreversible loss of strength (Fig. 4b). A large number of mass move- ments, mostly slides and mass flows, in the Northern Calcareous Alps in Tyrol and Bavaria are linked with the occurrence of this lithology (Nickmann, 2009; Nickmann & Thuro, 2013; Lotter & Gruber 2020). Even moderate weathering is sufficient for reaching 168 the critical state for failure (Nickmann & Thuro, 2013), and in a dip-slope setting the saturated Kössen Fm. tends to fail at slope angles of 10-15° (Lotter & Gruber, 2020). • The Oberrhätkalk on top of the Kössen Fm. consists mostly of massive but sometimes thickly bedded limestone with a thickness of up to 25 m (Fig. 4c). This brittle lithology makes up cliffs, like Weissstein (Figs. 1, 2) or bedrock ridges, like north of Kreithof (Fig. 5). • The Allgäu Formation (Lower Jurassic) in the scarp area consists of greenish-grey mottled limestone and marls, partly chert-bearing. The marls are also classi- fied as slaking rocks. Rotkalk (red nodular limestone and marl; Pliensbachium - Tithonium) has a thickness of 7 to 17 m (Blau & Grün, 1995). During the Last Glacial Maximum (LGM; = Würmi- an Pleniglacial) the area was covered by the Drau Glac- ier, which was part of the large Alpine transection glaci- er complex (Reitner et al., 2016). The ice surface was in the range of 2200 m a.s.l. (Reitner, 2003b) with only the highest sharp peaks (Fig. 2) as nunataks above. Sub- glacial till consisting of a matrix-supported and massive diamicton showing a high consolidation is typical of temperate glaciers of the LGM (Reitner & Menzies, 2020). Glacial erosion is evident in the overdeepened Lienz Basin with a maximum bedrock depth of c. 600 m below the valley floor just north of the study area (Burschil et al., 2019). Additional documents of subgla- cial shaping are the mica schist areas within the small ---------- ----->>>>> Fig. 4 - Images from the field in the scarp (S) area and in the uppermost deposition area (A) : 4a) Outcrop S-1 (for location see Fig. 1) with the typical appearance of the nortward dipping Kössen Fm. as an alternation of limestone, marl and claystone beds (1m scale). 4b) Close up of claystone showing a disintegration typical for slaking rock (1m scale). 4c) view from east towards the Weissstein consisting of mas- sive to thickly bedded Oberrhätkalk limestone. 4d) Subordinate scarp at S-2 with the northward dipping Kössen Fm (dip 360/56) in a typical dip-slope situation. Note encircled person for scale. 4e) The boulder Lienz 5 consisting of massive Oberrhätkalk limestone sampled for 36Cl exposure dating at the outer flank of uppermost longitudinal ridge. Note the rounded edges. 4f) Outcrop A-1 at the same ridge as Lienz-5 (in Fig. 4e) showing clast supported diamicton (lithofacies SCc) with very angular Oberrhätkalk clasts. Note the whitish colour of the sur- face due a thin, very bright, floury, calcareous precipitation (Image: B. Imre). 4g) Outcrop A-2 with jigsaw facies within Oberrhätkalk (1m scale). 4h) Outcrop A-2 with fragmented facies consisting of a diamicton with very angular clasts of the Kössen Fm. limestone (1m scale) (Image: B. Imre). Fig. 3 - Geological profile of the rock avalanche area and the sequence of processes during rock avalanche formation and emplacement (for location see Fig. 1). Reitner J.M. et al. 169 The early Holocene Buchwiese rock avalanche basin of Lake Tristach (Fig.1) as well as rounded peaks like the Rauchkofel (Fig. 2) and the abraded bedrock ridge north of Kreithof (Fig. 5). Following the climax of the LGM (27-19 ka; Monegato et al., 2007; Ivy-Ochs, 2015), remnants of delta deposits document the brief early Lateglacial phase of ice-decay (~19 ka) in the study area (Reitner et al., 2016). By 18.5 ka the glacier tongue area (Schmidt et al., 2002) and all major Alpine val- leys (van Husen, 2000), like the Drau Valley at Lienz, were free of ice. Afterwards no Lateglacial advances affected the study area. The following phase was characterised by infill of the overdeepened basin (Burschill et al., 2019) and by fluvial erosion. On the surrounding slopes, different types of mass move- ments occurred ranging from deep-seated slope de- formations to rock slides and rock avalanches (Bauer, 1990; Reitner, 2003 a, b; Reitner et al., 2014; Reitner & Linner, 2009). The area of Lienz has low seismicity (cf. Reiter et al., 2018) with no faults known to be tectonically active. 3. METHODS Geological mapping at the 1:10,000 scale was performed in the year 2000 (Reitner, 2003a) for the Geological map sheet Lienz (Linner et al., 2013). Further morphological analysis of the mostly forested terrain benefitted from available high-resolution air- borne laser scanning data and a digital elevation model (DEM) with 1-m resolution, which were provid- ed by TIRIS (www.tirol.gv.at/) since 2010. For lithofa- cies descriptions, we applied the coding of Keller (1996). We performed landslide volume estimations by reconstructing a pre-failure surface of the scarp area and calculating the differential volume of this surface and the present surface (DEM) in ARCMAP. 2.5-dimensional models were produced using ARCSCENE. We took four samples from the tops of the larg- est boulders in the Buchwiese rock avalanche depos- it for cosmogenic 36Cl surface exposure dating in November 2007. All sampled boulders are of Ober- rhätkalk limestone. Sample information is given in Tab. 1. Sample preparation followed the method of iso- tope dilution (35Cl) described in Ivy-Ochs et al. (2004). Total Cl and 36Cl were measured in the same target at the ETH accelerator mass spectrometry (AMS) facility of the Laboratory for Ion Beam Physics (LIP) with the 6 MV tandem. Measured 36Cl/Cl ratios were normalized to 170 the ETH internal standard K382/4N with a value of 36Cl/ Cl =17.36×10-12, which is calibrated against the primary 36Cl standard KNSTD5000 (Vockenhuber et al., 2019). Measured ratios were corrected for a full process chem- istry blank value of 4.2±3.6×10-15. We calculated 36Cl exposure ages with the LIP ETH in-house MATLAB Tab. 1 - Sample information and results of 36Cl surface exposure dating. Fig. 5 - DEM showing characteristic morphological elements in the lower part of the deposition areas B and C. The most striking features are the bedrock ridge north of Kreithof, the fan-like morphology SW of Kreithof and the megaboulder cluster surrounded by a black steepled line. The rock avalanche deposit is indicated in orange. Pink triangles show the cluster of Rotkalk boulder. The red line shows the run of the geoelectrical profile P1. Reitner J.M. et al. code based on the parameters presented in Alfimov & Ivy-Ochs (2009 and references therein) and the ele- mental composition of every rock sample (measured with ICP-MS at Act labs S.A., Ontario, Canada) (Tab. 2). Ages have been corrected for topographic shielding, which was measured in the field using compass and clinometer. Shielding corrections were calculated with the topographic shielding calculator in the online calcu- lator at https://hess.ess.washington.edu. Ages were corrected using an erosion rate of 5 mm/ka (André, 2002). No correction was made for snow cover; in gen- eral, such corrections are made only on bedrock sam- ples (cf. Bichler et al., 2016). Final errors on the ages (Tab.1) include both analytical and production rate un- certainties. We performed a geoelectrical survey to foster a 2D -image of the rock avalanche structure. Landslide inves- tigations with the integration of geoelectrical outcomes for landslide characterization, like description of land- slide geometry body, can be found in Perrone et al. (2004) and a comparable case study about rock ava- lanche deposit in Ostermann et al. (2012). A good over- view about theory and field design and the limitations of the resistivity method is given in Aizebeokhai (2010) and Loke et al. (2013). Three electrical resistivity tomogra- phy (ERT) profiles (P1, P2, P3; location indicated in Fig. 1) were carried out in August 2012 with a GEOMON4D resistivity meter, an in-house development of the Geo- logical Survey of Austria. 93 electrodes were used with the specific layout parameters listed in Tab. 3. For better resolution, we used a gradient electrode array configu- ration (Dahlin & Bing, 2006) with 4300 measurements. Power input of electrical current was adequate (100- 400 mA) and data quality was quite good. Sample anal- ysis showed, that electrical noise was sufficiently low and occasional 50 Hz crosstalk could be eliminated with sufficient time window length. Geoelectrical processing started with raw data filtering, like elimination of outliers, analysis of signal to noise ratio, etc. Geoelectrical inver- sion was carried out with the Res2DInv software (©Geotomo Software, Malaysia). Following standard procedures, we inverted geoelectrical data with a smooth inversion based on an L2-norm criterion, with an average resistivity starting model and topographic cor- rection incorporated into the inversion (Loke, 2019). 4. RESULTS 4.1 Field description In order to facilitate the field description, the area is subdivided into four domains: Scarp area and the depo- sition areas A-C (from proximal to distal, see Fig. 1). Scarp area The maximum 800 m wide and 0.7 km2 large de- tachment niche lies between the limestone bedrock ridge of Weissstein (Fig. 4c) in the west and the Rossboden in the east (Fig. 2). In the uppermost de- tachment area between 1700 m and 1900 m a.s.l., the niche diminishes successively in the upward direction indicating in total a missing rock mass (Fig. 3). The ma- jortiy of the detachment niche is made up of Kössen Fm. which shows a general dip towards north. Dip angles are in the range of 30 to 50° but may vary due to small scale folding between 30° towards south and 75° to- wards north. Given the original slope at Weissstein with angle of 37° (Fig. 3), a dip-slope situation is evident as indicated by large outcrops (S-1, S-2; Figs. 1, 4a, 4d). In addition, still ongoing creep movement in the average 25° steep terrain results in an undulating surface (Fig. 1), proof of the weak rock mechanical properties of the Kössen Fm. (claystone, marls). This is also visible in tension gaps and small scarps at the eastern margin of the scarp area. The Weissstein is made up of up to 25 m thick Oberrhätkalk (Figs. 2, 4c), which is overlain on the northern flank by folded dark Allgäu Fm. and up to 15 m of Rotkalk (Figs. 1, 3). The southern margin of the detachment area north of Auerlingköpfl peak (2026 m a.s.l.) consists of Hauptdolomit (thick-bedded, bench- forming dolostone). The presence of large dolostone boulders indicates loosening of Hauptdolomit after the detachment of the rock mass consisting of Kössen Fm. at this location. N-S to NNE-SSW oriented segments of the west- ern scarp mirror rare minor near vertical brittle faults (at S-1in Fig. 1). Such a direction is also evident in the joint distribution of the outcrops and as well in lineaments (Fig. 1) in the areas consisting of brittle Hauptdolomit. Deposition area A (Schrettiswiese) Just 600 m downslope of the limestone cliff of Weissstein, a prominent SW-NE trending ridge marks the lateral limit of the rock avalanche deposit. This 350 m long ridge consists on the surface of large limestone boulders of Oberrhätkalk (Fig. 4e). The height on its outer side is around 5 m. Based on the incised creek which did not reach bedrock, the thickness of the rock avalanche deposit is most likely in the range between 20 and 60 m. 171 Tab. 2 - Elemental composition of leached samples. Values below detection limit are marked with “<”. Cl values are from AMS measure- ments. Tab. 3 - Key parameters of the electrical tomography profiles. The early Holocene Buchwiese rock avalanche 172 Reitner J.M. et al. Boulders like the one of the sample sites Lienz 5 (Fig. 4e) and Lienz 6 (Fig. 1) with a volume of up to 50 m3 can be classified as subangular to subrounded on the surface. In contrast, rare outcrops, like A-1, exhibit just 0.5 m below the surface a clast-supported scree without matrix (SCc) consisting of very angular to angu- lar clasts of gravel to boulder size (Fig. 4f). The surface of the clasts is coated by a � 1 mm thick very bright, floury, calcareous precipitation. This observation indi- cates that the less angular clast boulder shapes on the surface are the result of weathering. Outcrops in the deeper part (e.g. A-2) show materi- al of the Kössen Fm. where the limestone strata are characterised by irregular fractures not in accordance with the joint patterns of the original bedrock (Fig. 4g). The amount of fragmentation changes without a sharp boundary from relatively compact limestone outcrops with an extent of some metres to very angular clasts of decimetre-size. Thus, this facies shows the characteris- tics of the “jigsaw-fractured (sub-)facies” described by Dufresne et al. (2016a) as part of the body facies. At the same outcrop A-2, a matrix-supported, brownish diamic- ton with very angular clasts in a silty-sandy matrix dis- plays a more intensive comminution of the “fragmented (sub-)facies” (Fig. 4h). Again, very angular clasts are fragmented. Deposition area B (Kreithof ) The middle part of the deposition area is character- ised by a change of slope from around 15° above 1100 m a.s.l. to 20-25° between 1100 and 900 m a.s.l. This step occurs in the western prolongation of the bedrock ridge north of Kreithof consisting predominantly of Ober- rhätkalk (Figs. 4, 5). Due to faulting (Fig. 3), this mas- sive white limestone disappears towards the west. How- ever, the occurrence of Hauptdolomit seems to be re- sponsible for the increase in steepness. The facies of the outcrops (B-1, B-2; Figs. 1, 4) is com- parable to that of A-2. Large outcrops of Kössen Fm. (marly limestone to claystone; B-1 in Fig. 6a) show the lithological succession of the release area. However, the dip of the strata differs considerably from the scarp area with e.g. 315/45 to 350/52 for B-1 and 220/65 for B-2. Again, mechanically competent limestone components show an internal fragmentation with irregular fractures (B-1 in Fig. 6b). The occurrence of the fragmented sub- facies (Fig. 6c) is very restricted. In contrast, the small B-3 outcrop shows a very dense and consolidated matrix-supported diamicton (Dmm) with a greyish-green to reddish silty-clayey ma- trix. Based on the lithological characteristics this me- lange-like deposit represents most likely strongly frag- mented Allgäu Fm. maybe in contact with Rotkalk marls. The morphology of the deposition area shows in the central parts S-N-oriented longitudinal ridges with a height of less than 5 m, indicating the flow direction of the rock avalanche (Figs. 1, 5). At the eastern limit near Kreithof, a fan shaped landform with small channel-like features is evident on the DEM (Fig. 5). Drainage trenches at site B-4 show a maximum 0.5 m thick layer of a clast-rich diamicton with gravel to cobble size clasts overlying strongly commuted dark claystone of the Kössen Fm., which due to weathering, appears to be a plastic silty clay. Hence, this fan-shaped area has been mapped as a rock avalanche area which was finally shaped by a precursor of the Kohlstatt creek after the rock avalanche event. Deposition area C (Buchwiese) with megaboulder clus- ter NW of the Oberrhätkalk bedrock ridge at Kreithof, the deposition area is characterised by a cluster of out- standingly large boulders (Figs. 6d, 6e) consisting of white limestone (Fig. 5). The areal distribution of these megaboulders with a volume between 10 and c. 1000 m3 is best described as a narrow fan with an apex on the southern end near Kreithof. No ridge-like structures are evident in this area. The only ridge (max. height 5 m, c. 200 m length) with large but in general smaller boulders of the same lithology in this area is just outside the megaboulder cluster (C4 in Figs. 5, 6g). Rare outcrops like C-3 along the creek at the northwestern rim show that the rock avalanche deposit is underlain by polymictic delta de- posits with subanglular to rounded crystalline and lime- stone clasts, typical for deposits of the phase of ice- decay (Reitner et al., 2016). The rock avalanche depos- its in the distal part (C-2) comprise diamictons with an- gular Allgäu Fm. clasts in a silty-sandy matrix. In the northeastern part a small cluster of 1-2 m3 of subangular to subrounded Rotkalk boulders (Fig. 6f) can be mapped (C-5 in Figs. 1, 5). Outcrops close to the limit of the rock avalanche deposit show a < 2 m thick deposit consisting of a matrix-supported diamicton with a silty-sandy matrix on top of mica schists. The clasts consist of lithologies of “Rotkalk” and Allgäu Fm, both originally occurring at the northern (distal) part of the scarp area. The most distal parts of the rock avalanche depos- its reach down to the alluvial deposits of the River Drau. However, no indications of such deposits further to the north, or of fluvially reworked deposits, have been found so far in this area which is nowadays intensively used for agriculture and a golf resort. 4.2. Geophysics Profile P1 (Fig. 7a) shows a contrast between the bedrock, in this case made of mica schists with an elec- trical resistivity of > 900 Ωm and the rock avalanche <<<<< ---------- ---------- Fig. 6 - Images from the field in middle (B) and lower part (C) of the deposition area. 6a) Outcrop B1 showing a fragmented alternation of limestone and claystone beds of the Kössen Fm. (dip 350/52; 0.6 m long yellow pick for scale) 6b) detail from 6a with a limestone in jigsaw facies. 6c) outcrop B1 with the rare occurrence of the fragmented facies (matrix-supported diamicton with very angular clasts) (hammer length 28 cm).6d) the megaboulder cluster at site C-1. 6e) sample site Lienz 7 within the megaboulder cluster. Encircled head of a person for scale. Note the giant boulder in the background (red X). 6f) The typical maximum size of the Rotkalk (red limestone) at site C-5 (yellow hammer for scale) Note the subangular shape. 6g) Longitudinal ridge at site C-4. Direction of view towards NW. 173 The early Holocene Buchwiese rock avalanche deposit. The latter with a thickness of up to 40 m reveals in general a low resistivity of 50-320 Ωm. Only the uppermost 10 metres can display resis- tivities of 900 Ωm and higher, especially in the first c. 200 m where the profile crosses the mega- boulder cluster (Fig. 5). At the SE end of the pro- file the bedrock crops out in accordance with the mapping results (Fig. 1).The profile P2 (Fig. 7b) reveals a less pronounced contrast between bed- rock made of Lavant Fm. (siliciclactic limestone, marl, siltstone; resistivity 85-200 Ωm) and the rock avalanche deposit (30-80 Ωm). The interpreted thickness of the latter shows a strong variance of 5 to 30 m. Profile P3 (Fig. 7c) displays rock ava- lanche material with a maximal thickness of 10 m on top of two different bedrock lithologies (Lavant Fm. and Allgäu Fm.). Like at P1, an uppermost high resistivity layer (200-550 Ωm) occurs be- tween profile metres 75 and 100 covering thicker rock avalanches deposits with (50-70 Ωm). 4.3 Area, volume and Fahrböschung The mapped rock avalanche deposits cover an area of 2.7 km2. When considering the already eroded parts, an original area of c. 2.8 km2 seems to be reasonable, which is four times larger than the scarp area. The results of geological mapping and geoelectrical surveying (Fig. 7) show varying rock avalanche deposit thicknesses of 2-60 m, respectively 5-40 m, and indicate that a precise esti- mate of the volume of the deposit is challenging. The most reliable approach is the estimate of volume based on the missing volume in the detachment niche. The scenario with a planar surface (Fig. 8a) reveals 20x106 m3 whereas the more realistic one with a slightly convex surface (Fig. 8b) results in 27x106 m3. This would corre- spond to an average 10 m thick rock avalanche deposit. However, an increase in volume of the deposit com- pared to the detached carbonate bedrock of 25-30% (Abele, 1974; Hungr & Evans, 2004) due to the commi- nution process has to be considered, resulting in a cal- culated average thickness of c. 13 m. Taking into ac- count the modification of the scarp area due to small mass movements after the event, the most reliable esti- mate for the original maximum scarp altitude is c. 1850 m a.s.l. With the lowermost parts of the deposit at 650 m a.s.l., we have a drop height (H) of 1200 m. In combi- nation with the runout length of the rock avalanche (L) of 3.5 km, the Buchwiese rock avalanche has a Fahrböschung α (after Heim, 1932) of 19° and a H/L ratio of 0.36. In comparison, the average slope angle of the pathway is 16°. 4.4. Age The results of the surface exposure dating with 36Cl are shown in Tab. 1. Ages range from 9.8±0.5 ka (Lienz 7) to 11.9±0.5 (Lienz 5) (Figs. 1, 5c, 6b). All four ages overlap within the given uncertainties. There is no spatial pattern with respect to the ages. The spread in ages may be attributable to processes that lead to too old ages, like inheritance, and too young ages, such as post-depositional movement of the boulder or spalling of the surface. As there is no independent means to estab- 174 lish which ages are the true ages, we take an average of all four ages. The average age is 10.8±0.9 ka. The uncer- tainty on the mean reflects both the uncertainty on the individual ages and the spread in the ages. 5. DISCUSSION The legacy of the reconstructed Buchwiese rock avalanche with an age of 10.8±0.9 ka reveals numerous morphological and sedimentary features that indicate control by both the lithological distribution in the scarp area and the pathway morphology and geology. 5.1 Geometry When comparing the geometry of the detachment area with that of the deposition area the fourfold areal extension (0.7 km2 vs. ~2.8 km2) of the latter is evident. This is overwhelmingly the result of an extension in the longitudinal (runout) direction (0.9 m vs. c. 3 km) and not that of a widening (0.8 km vs 1.2 km). In general, such a geometry of the rock avalanche displays the morphologi- cal conditions along the pathway with a slope allowing unconfined flow and the absence of a major obstacle with the exception of the bedrock ridge at Kreithof in the mid- dle part (Figs. 1, 3). The Early Holocene age provides a reasonable explanation of the sharp limit between the most distal part of the rock avalanche and the alluvial deposits of the River Drau. Considering the reconstructed progradation phases of an alluvial fan just 4 km down-valley (Patzelt & Poscher, 1995), the modern valley floor of the River Drau seems to have been in an aggradational phase since the Younger Dryas (12.8-11.7 ka) with only lateral erosion of the originally meandering River Drau (Burschil et al., Fig. 7 - The results of the geoelectrical survey at profile P1, P2 and P3 (for location see Figs. 1, 5). Note the colour bar for restivity of P1 is different from P2 and P3. Reitner J.M. et al. 2019). Thus, any remnants of the rock avalanche north of the current limit, if they ever existed, must have been reworked. The presence of a thick layer of Buchwiese rock avalanche deposits in the centre of the valley can be ruled out (Burschil et al., 2019). A limited amount of uncertainty exists regarding the extension in two cases. The fan-like structure of the partly reworked deposit south of Kreithof could potential- ly indicate a branch towards the east. However, no fur- ther traces of the rock avalanche have been found in the catchment of the eastward draining creek. The other case concerns the relation between the rock avalanche deposit on the northwestern limits and the Lake Tristach basin. The latter is the result of subglacial shaping. Allu- vial fans nowadays constrain the eastern limit of the lake. However, a further extension of this small basin and maybe the lake towards the east, in the range of few hundred metres, into the area now covered by the alluvial fans and parts of the rock avalanche seems to be likely. Thus, a subordinate branch of the Buchwiese rock avalanche flowing towards the west into the area now covered by the modern Lake Tristach cannot be ruled out without limnogeological investigations. 5.2. Morphology, lithological distribution and facies Ridges within the rock avalanche deposit are the most evident morphological indicators of fluid-like runout motion. All mapped features in the study area are best interpreted as longitudinal ridges (Dufresne & Davies, 2008). The most prominent one in terms of length and height occurs in deposition area A with the Lienz 5 boul- der on top (Fig. 1). According to the lithology of the boul- ders (massive limestone from Oberrhätkalk) its for- mation started already after around 600 m. The same lithology is evident at ridge at C-4 in the lowermost dep- osition area C. Smaller ridges consist of Kössen lime- stone. Considering the topographic conditions enabling unhindered spreading, the presence of ridges in the case of the Buchwiese rock avalanche seems to be linked to the availability of sufficient mechanically com- petent material (Dufresne & Davies, 2009) such as lime- stone. In contrast, the less competent materials of the Kössen Fm. (marl, claystone) were not able to form such ridges. This lithological control is also evident in the facies of the deposits. The occurrence of a blocky, clast- supported carapace facies overlying a body facies (Dunning, 2004; Weidinger et al., 2014; Dufresne et al., 2016a) is a common feature in rock avalanche deposits (e.g. Dufresne et al., 2016b; Reitner et al., 2018; Singeisen et al., 2020). Such a carapace facies is only present at the ridges and in other limited areas where limestone especially the massive Oberrhätkalk occurs, e.g. in the megaboulder cluster. Despite the absence of large outcrops, which would allow tracing the changes of facies and fragmentation with depth, the available outcrop and geoelectrical data show in general the typi- cal succession of a rock avalanche: Below a carapace facies, if present, clast-supported diamictons (A-1 in Fig. 4e) occur which resemble the blocky facies of Dufresne et al. (2016a). Outcrops of the interior in the deposition areas A and B consist mostly of strongly fragmented rock (outcrops A-2 and B-1 in Figs. 4e, g, h) which represent the jigsaw subfacies of Dufresne et al (2016a). In comparison, the occurrence of matrix- supported diamicton, the fragmented subfacies appears to be limited (e.g. lower part of outcrop A-2; Fig. 4f). The rare outcrops of the deeper part of deposition area C only show the fragmented subfacies. Geophysical data (in Fig.7) also display the differ- ent amounts of fragmentation. The carapace facies in P1 (megaboulder cluster) and P3 is easy to identify. However, an absolute quantification of the degree of fragmentation via the resistivity values (in Ωm) seems to be problematic due to the lithological variations of the fragmented source rocks within the profiles. The megaboulder cluster represents within the distal deposition area C, an outstanding and rather iso- lated feature. It is the largest coherent area with cara- pace facies comprising some Oberrhätkalk limestone boulders of enormous size of up to 1000 m3. The fan- shaped distribution with an apex just below the western edge of the Kreithof Oberrhätkalk bedrock ridge pro- vides a clue for the provenance of the megaboulders. According to the most likely scenario, the western part of the glacially shaped bedrock ridge was impacted and eventually detached by the rock avalanche. The fan-like distribution and limited fragmentation as shown by the carapace facies support this suspected style of for- mation. Excluding the megaboulder cluster, the distribution of lithologies within the rock avalanche deposit mirrors the sedimentary succession in the detachment area, with the clasts of the Rotkalk and Allgäu Fm. in the most 175 Fig. 8 - Reconstructed paleo-topographies of the scarp area for volume calculations of the detached mass. A) The scenario with a planar surface reveals 20x106 m3 B) The scenario with a slightly convex surface results in 27x106 m3. The early Holocene Buchwiese rock avalanche distal part. As in other cases (Heim, 1932; Strom, 2006; Hewitt et al., 2008; Dufresne et al., 2016a, b; Rossato et al., 2020a), the preserved source stratigraphy within the dilated rock mass is a clear argument for a predomi- nantly laminar style of rock avalanche movement. 5.3 The age, chronology of failure, kinematics, and its implications The age of the Buchwiese rock avalanche of 10.8±0.9 ka, based on averaging of all four boulder ages, is in accordance with the superposition of the deposit on top of Lateglacial delta deposits (phase of ice-decay, Reitner et al., 2016) and the evident round- ing of the boulders due to carbonate dissolution since deposition. The age also shows that the rock slope fail- ure occurred 8-9 ka after the last glacial shaping and oversteepening of the slope during the LGM. The weakest lithology in the source area are the marls and claystones, i.e. the slaking rocks of the Kössen Fm. Fatigue of this material over some thou- sand years together with the general dip-slope situation are regarded as the main cause for the slope failure and the detachment of a rock mass as a translational slide (Cruden & Varnes, 1996) or rock planar slide (Hungr et al., 2014). NNE-SSW trending faults facilitated the for- mation of the niche which finally developed with a most- ly plane sliding surface (Fig. 3), which also cut through the competent Oberrhätkalk as well as the Allgäu Fm. Tension gaps, which are still present in the stable mar- gin, most likely were the precursors of the slope failure. In such a setting with alternations of fine-grained aqui- cludes and fractured aquifer within brittle limestone (Probst et al., 2003), variations in the hydrostatic pres- sure over time were likely an additional driver for the slope failure. Currently there is no indication of a trigger like a paleo-earthquake documented in other archives nearby. In addition, the regional seismicity is low (Reiter et al., 2018). The slope failure occurred during the Preboreal, a stage known for a considerable warming trend and its dryness (Schmidt et al., 2006; Magny et al., 2007; Ilyashuk et al., 2011), which makes in general a higher wetness associated with high hydrostatic pressure in the jointed rocks as a cause / trigger unlikely. Neverthe- less, Magny (2004) reported evidence of a distinctly wetter phase with higher lake-levels in Central Europe at 10.3-10.0 ka, which does overlap with the Buchwiese failure event within the given error range. When com- paring the altitude of the source area with the known spatial development in the region (Steinemann et al., 2020), permafrost could have occurred for the last time during the Younger Dryas. Permafrost degradation as- sociated with the marked warming around 10.5 ka (Ivy- Ochs et al., 2009; Protin et al., 2019) may have contrib- uted to the weakening of the slope (Krautblatter et al., 2013) as well. In such a geological setting, the detachment oc- curred in the initial phase as a translational slide. The onset of the first ridge after a travel distance of approx. 600 m shows a nearly instantaneous fragmentation leading to the typical fluid-like behaviour of a rock ava- lanche. Such a strong fragmentation is especially evi- dent in the brittle limestone. In comparison, large out- 176 crops of Kössen Fm. debris upstream of the Kreithof ridge with a comparable moderate fragmentation in the middle part of the flow, may indicate a longer sliding phase within this lithology due to the preferential defor- mation of the weak claystone layers. The pathway shows a slight bending NNE to NE at the south-eastern flank of the Rauchkofel peak. Such a flowline could have been predetermined to some extent by pre- existing paleo-valleys. After this, the strongest interfer- ence with topography occurred by the collision with at least parts of the western limestone ridge extension at Kreithof (Fig. 5). On the one hand, this development led to the formation of the megaboulder cluster. Due to the break in slope a jump of less than 200 m could have occurred. On the other hand, the rock avalanche smoothed the glacially shaped topography south of the glacially moulded bedrock ridge which explains the greatest thickness in this area (Fig. 3). After the assumed short-distance jump, the rock- avalanche moved with more or less the same width leading to further fragmentation. The patchy coverage in the areas close to the alluvial deposits of the River Drau together with a thinning of the rock avalanche cover indicate the final run-out. The facies and the morpholo- gy of the deposits are in accordance with the hypothe- ses of dynamic fragmentation (McSaveney & Davies, 2006; Davies & McSaveney, 2009) as the main me- chanical reason for reducing the internal friction of the moving mass and, eventually, the long run-out. 5.4 Comparison with other catastrophic slope fail- ures The Buchwiese is one of the rare cases of an un- confined rock avalanche which remained on the slope and did not reach the valley floor. Most of the rock ava- lanches discussed in recent years are characterised either by events which hit the valley floor or had interac- tions with valley flanks (e.g. Hewitt et al., 2008; Ostermann et al., 2012; Grämiger et al., 2016; Ivy-Ochs et al., 2017). Hence comparison with respect to the kinematics and the influence of topographic obstacles and constraints is not an easy task. Based on available data under dip-slope condi- tions, the Kössen Fm. tends to fail as slides or relatively slow flows or a combination of these (Nickmann, 2009; Nickmann & Thuro, 2013; Lotter & Gruber, 2020). In cases with a dip much steeper than the slope, cata- strophic rock slides are known, like the two cases (Mordbichl, Lienzer Klause) from the northwestern flank of Lienz Dolomites in the upper Drau Valley (Puster Valley; Reitner, 2003a, 2016; Linner et al., 2013). Such a tectonic setting, different from the Buchwiese scarp area, may explain the formation of a listric sliding plane, independent of the bedding in the cases of Puster Val- ley. In general, the Lienz Dolomites, despite their high relief, do not have an abundance of catastrophic rock slope failures. Beside the Buchwiese and the two rock- slides (Mordbichl, Lienzer Klause) with a Late Holocene age, only the Laserz rock avalanche consisting of frag- mented dolostone occurred during a Younger Dryas glacier advance (Reitner et al., 2014). Within a distance of 25 km there are only two additional cases within crys- Reitner J.M. et al. talline tectonic units, Feld near Matrei (Reuther et al., 2006) and Kals (Veit, 1988), which are altogether not linked to one tectonic system as has been noted at oth- er sites (Brenner cluster - Ostermann & Sanders, 2017; Piave Valley - Rossato et al., 2020b). There has been some discussion of a temporal cluster of large landslides in the Alps during the Early Holocene, which comprised the Kandersteg, Flims and Koefels events (Ivy-Ochs et al., 2017 and references therein). Indeed, the recently presented 36Cl exposure age, 3.2±0.2 ka, for the Kandersteg event (1.1 km3) weaken this conjecture as the Kandersteg event oc- curred during the Late and not the Early Holocene (Singeisen et al., 2020). By comparing the Buchwiese age of 10.8±0.9 ka with other dated events in the Alps it is evident that dated Early Holocene catastrophic rock slope failure are rare when comparing with the available records (Prager et al., 2008; Ivy-Ochs et al., 2017). The two largest events in the Alps, Flims (10-12 km3) and Köfels (3 km3) with ages of 9.4 ka (von Poschinger & Haas, 1997) and 9.5 ka (Nicolussi et al., 2015), overlap with the Buchwiese event within the age uncertainties. Nevertheless, the fact that the sites are distant from each other makes implicating a seismic trigger difficult. The strongest age overlap is with the rather small Len- zanger rock avalanche (10.8±1.1 ka) in the Hohen Tau- ern mountain range. Those deposits overlie both a Younger Dryas till and the Durchgangwald rockslide deposits (12.6±1.0 ka; Bichler et al., 2016). The marked warming around 10.5 ka in the Alps, expressed as re- treat of glaciers back to as small as or smaller than their Little Ice Age extents (Schimmelpfennig et al., 2012; Protin et al., 2019) and especially as permafrost degra- dation, may have been the critical factor that led to slope failure at Buchwiese. Nevertheless, the release area itself is not in an area occupied by a glacier during the Younger Dryas (Reitner et al., 2016). 6. CONCLUSIONS We present reconstruction of the Buchwiese rock avalanche based on geological field mapping, analysis of DEM data, 36Cl exposure dating, and a geoelectrical survey. Based on the results of the 36Cl exposure da- ting, the event took place at 10.8±0.9 ka during the Ear- ly Holocene and, thus, 8-9 ka after the last glacial shap- ing and oversteepening of the slope during the LGM. The age is the average of all four obtained boulder ag- es. The failure of a rock mass with a volume of 27x106 m3 was enabled by dip-slope sliding within strata of the Kössen Fm. (limestone, marls, claystone), Oberrhätkalk (massive to thickly bedded limestone), and Rotkalk (red nodular limestone and marl) dipping to the north in com- bination with N-S to NNE-SSW running brittle faults. Weak layers in the fine-grained rocks of the Kössen Fm. (claystone, marl) are the major lithological precondition for the mass movement. We consider fatigue of such slaking rocks since the LGM as the major cause for this catastrophic rock slope failure. According to the geological conditions, the initial failure occurred as a translational slide with a mostly plane sliding surface. In the deposition area, strong control of lithology and topography on the development of different facies and morphological features is evident: 1. Longitudinal ridges indicate spreading of an uncon- fined flow. The onset of the first longitudinal ridges shows a strong fragmentation resulting in fluid-like movement after only 600 m of transport. The most prominent ridges with respect to height and length consist of the massive Oberrhätkalk. Smaller ridges are developed in Kössen limestone whereas the other lithologies were not able to form ridges. 2. The carapace facies consisting of clast-supported boulders is only developed in areas with limestone (mostly Oberrhätkalk but also limestone of the Kössen Fm.). 3. The body facies in the upper parts (deposition area A) are dominated by the jigsaw subfacies with a subordinate occurrence of fragmented subfacies in the outcrops. Even in the middle part (area B) the prevalence of the moderately fragmented jigsaw subfacies within large soutcrops of Kössen Fm. con- sisting of alternating claystone, limestone beds is evident. This may indicate preferential deformation within the claystone beds. 4. After partial collision with a bedrock ridge consisting of massive Oberrhätkalk limestone and a small jump, the fragmented facies dominates. 5. This collision led to the formation of megaboulder cluster consisting of detached and fragmented Ober- rhätkalk boulders with volumes of up to 1000 m3 and a fan-like distribution. The preserved source stratigraphy within the dilat- ed rock mass is a clear argument for a predominantly laminar style of rock avalanche movement. This finding, together with the presence of longitudinal ridges indicat- ing fluidisation, as well as the facies distribution, support the dynamic fragmentation model (McSaveney & Davies, 2006; Davies & McSaveney, 2009) as the best mechanical explanation for the Buchwiese rock ava- lanche. ACKNOWLEDGEMENTS Comments provided by Jeff Moore and an anony- mous reviewer helped us to improve the paper. We are grateful for the help of Hanns Kerschner during sam- pling in November 2007. 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