Ostermann et al 183-189.pub Available online http://amq.aiqua.it ISSN (print): 2279-7327, ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, 33 (2), 2020, 183-189 CHARACTERISTICS AND DATING OF THE ROCK AVALANCHE AT PRAGSER WILDSEE/LAGO DI BRAIES (DOLOMITES, ITALY). Marc Ostermann 1, Susan Ivy�Ochs 2, Frowin Ruegenberg 3, Christof Vockenhuber 2 1 Geological Survey of Austria, Vienna, Austria. 2 Ion Beam Physics, ETH, Zurich, Switzerland. 3 Department of Geology, University of Innsbruck, Innsbruck, Austria. Corresponding author: M. Ostermann ABSTRACT: Major gravitational slope deformations are widely disseminated in the Dolomite Mountains (NE Italy), one of the world's most conspicuous landscapes and part of the UNESCO world heritage list. In the Prags valley a rock avalanche still dams a backwater lake - the Pragser Wildsee/Lago di Braies. The volume of rock debris accumulations comprise approximately 30-40 106 m³ of mainly dolomitic rock and the area covered by rock debris is about 3.5 km². The run-out distance of about 8.5 km and a maximum vertical drop of 1150 m (H/L-ratio: 0.13) yield a run-out travel angle (Fahrböschungswinkel) of 8°. Especially in the surrounding of Schmieden/Ferrara a hummocky landscape with numerous hills and ridges is developed. Here some classical Toma hills are encountered, i.e. isolated cone- to pyramidal- or roof-shaped hills composed of rock avalanche de- bris. Mainly because assumptions about the origin of this hummocky landscape has been related to glacial processes, the forma- tion of Pragser Wildsee was previously thought to be of Lateglacial age. We applied cosmogenic 36Cl surface exposure dating of four boulders within the debris accumulations and obtained an early Holo- cene age of 8.3±0.7 ka for the event. Our findings go along with the results of published radiocarbon dates from sediment-drill- cores within the backwater lake, which indicate a recalibrated minimum age for the slope failure of 7410±100 cal years BP (Irmler, 2003). The accumulations at Pragser Wildsee show strong similarities to the Obernberg rock avalanche in geomorphology as well as in age, making them the first examples of early Holocene rock avalanches forming Toma hill landscapes. Keywords: Rock avalanche, early Holocene, cosmogenic nuclide dating, Toma hills, Pragser Wildsee/Lago di Braies, Dolomites. � 1. INTRODUCTION In the past, several hypotheses have been formu- lated regarding the formation of the Pragser Wildsee/ Lago di Braies. Although it was clear that the lake was dammed by debris deposits, their origin was controver- sial. It was especially the hilly valley bottom morphology downriver of the lake itself that caused speculations. According to Damian (1899), Hantke (1983) and Heiss (1992), a huge debris flow fan, spreaded from the Riedl valley (Figs. 1, 2), together with smaller landslides and rock falls gave rise to the damming of the lake. Another hypothesis considered the wide deposits as glacial (Dal Piaz 1930; Pia, 1937) or composed of landslide masses subsequently transported and rearranged by a glacier (Klebelsberg, 1927, 1935, 1956; Castiglioni, 1964; Engelen, 1972). Abele (1974) instead assumed that Pragser Wildsee/Lago di Braies was dammed by a mas- sive rock slope failure event. Recently, the deposits have been described more in detail by Furlanis (2013) and in the comments to the recently published geologi- cal CARG map sheet 016 Toblach/Dobbiaco (Gianolla et al., 2018). The reclassification of deposits previously misidentified as having origins other than landsliding has been done commonly in the last decades (e.g. Spreafico et al., 2018). Here, we describe the results of an extensive field- work, including sedimentological, morphological anal- yses and dating of the rock avalanche in the Prags Val- ley. 2. METHODS Field mapping was conducted on a scale of 1:5000 using topographic maps and airborne laserscan images in 10 m resolution provided by the Autonomous Province of Bolzano - South Tyrol. Geometric calculations includ- ing volume estimations were made with AutoCAD 2014. The estimations of the deposit thickness is based on the information from a 20 m long drill core conducted by the Geological Survey of South Tyrol (Fig. 3). For exposure dating with 36Cl in the proximal sector of the rock avalanche, the surfaces of four boulders were sampled (Tab.1). Sample preparation is described in Ivy-Ochs et al. (2009). 36Cl and natural Cl (isotope dilution; Ivy-Ochs et al., 2004) were determined with https://doi.org/10.26382/AMQ.2020.07 184 Ostermann M. et al. accelerator mass spectrometry (Synal et al., 1997). Major element and minor concentrations (Tab. 2) were used to determine 36Cl production rates (Alfimov & Ivy- Ochs, 2009 and references therein). The radiocarbon age published by Irmler (2003) were re-calibrated with OXCAL 4.3 (Bronk Ramsey, 2009). 3. GEOLOGICAL FRAMEWORK The Prags Valley/Braies Valley is located in the northeastern part of the Dolomites, which are part of the Southern Alps. The Dolomites consist mainly of the marine sediments of the Upper Permian and Triassic periods, which were deposited in tropical to subtropical climate on Permian volcanic rocks and, as in the vicinity of the Puster Valley, on metamorphic crystalline base- ment rocks (Brandner et al., 2007b) (Fig. 2). Regarding to the Pragser Wildsee rock avalanche there are three lithologies within the Permo-Mesozoic sediment succes- sion (Fig. 2) which are important to describe more in detail, because these are the formations involved in the phenomenon. The Buchenstein Formation: pelagic basin sedi- ments that interlock with the clinoforms of the prograding Ladinian carbonate platforms (Brandner et al., 2007b). The limestones of the Buchenstein Fm. is well-bedded (cm-dm), partly bituminous and laminated (Keim, 2008). Thin volcanogenic clastics “Pietra Verde“-layers (cm-dm) are interbedded - decreasing the mechanical properties of the whole succession. The Schlern Formation consists of pre- and post- volcanic carbonate rocks, which formed mighty reefs or their clinoforms in the Ladinian. The reef slope interlocks with the basin sediments of the Buchenstein Formation. The rocks were formed in a tropical subtropical climate (Muttoni et al., 2003) in marine shallow waters and later dolomitized, whereby the coarse banking is relatively unclearly recognizable (Pia, 1937). The Fernazza Formation corresponds essentially to the basin-deposits of the proximal, alkaline volcanic activity in the Dolomites. The most common lithotypes are volcanoclastic, turbiditic, massive sandstones, ho- mogeneous or graded, dark green to blackish colour, greenish tuffites of the "Pietra Verde" type, siliceous tuffites, hyaloclastites and laminated calcareous siltites. Laminated, siliceous calcareous lutites, hemipelagites and marls are also found in between. At the top of the Fig. 1 - Distribution of major mass movements in the western Dolomites, Italy, indicated on a hill-shade image (acc. to Ostermann & Gruber, 2014, based on Brandner et al., 2007a). DSGSD stands for Deep-seated gravitational slope deformation. The insert-map shows the assumed maximum extent of the rock avalanche with removed post-failure accumulations (mainly debris flow fans). The red line repre- sents the scarp. 185 Characteristics and dating of the rock avalanche at Pragser Wildsee/Lago di Braies (Dolomites, Italy) Fig. 2 - Geological map of Prags valley (modified detail of the Carta Geologica d'Italia alla scala 1:50.000, map sheet 016 Toblach/ Dobiaco). In magenta the nowadays outcropped debris accumulations are highlighted. A-C indicate the three different accumulation areas mentioned in the text. SCS: St. Kassian Fm.; SCIa: Schlern Fm. in reef facies; IMF: Fernazza Fm.; DCS: Kassianer Fm.; BUC: Buchenstein Fm.; MTB: Ambata Fm.; DON: Dont Fm.; WEN: Wengen Fm.; The full legend can be found in Gianolla et al., 2018. The black line indicates the thrust fault separating the Permo-mesozoic sediments succession in the South from the metamorphic basement in the North. The green line indicates the orientation of the schematic cross-section (insert figure, 2 x super-elevated). Fig. 3 - Hillshade image of the Prags valley with indicated rock avalanche accumulations in magenta: light magenta for the maximum extent of the accumulations, dark magenta indicates the accumulations as drawn in the Geological map. Red line: main scarp. The yellow circle indicates the location of a 20m long drilling, conducted by the South-Tyrolian Geological Survey. The insert photo, view towards north, shows the Pragser Wildsee in the foreground and the Riedl valley and the Prags valley with indicated debris accumulations in the back- ground. The insert-map shows a detail of the hillshade with 10 m contour lines and the sampling points for surface exposure dating with obtained 36Cl ages. 186 Ostermann M. et al. Fernazza Fm. fine-grained calcarenites and marly lime- stones are present. 4. CHARACTERISATION OF THE ROCK AVALANCHE SCARP AND THE SLOPE FAILURE ACCUMULATIONS 4.1. Characteristics of the scarp area The scarp area of the rock avalanche is located in the upper part of the Riedl valley at elevations between about 2000 and 2350 m a.s.l. (Fig. 3). Schlern Fm. is the prevalent formation in the crown area which is also reflected in the lithological composition of the debris accumulations. Along the southern and south-western flank of the Riedl valley, here a well-banked Schlern Fm. is exposed. The banks dip with � 60° towards S-SW (more or less parallel to the northern slope) represented a predisposing factor for a dip slope slide (e.g. Abele, 1974; Hoek & Bray, 1981). The Schlern Fm. also shows pronounced jointing and partly karst-widened cracks, which were already mentioned by Pia (1937). There are several tectonic lineaments, in different orientations, nearby the scarp area (Fig. 2) and many rock slope failures show these tectonic constrains (e.g. Stead & Wolter, 2015), but we could not find convincing evi- dence for tectonic lineaments that delimited the failed rock mass here. According to the map of Van Husen (1987) the LGM-ice surface in the Prags valley was between 2300 and 2400 meters a.s.l., which hints that the scarp area at least was partly covered with ice dur- ing this period. 4.2. Characteristics of the rock avalanche accumula- tion area The accumulation area of the rock avalanche at Pragser Wildsee can be divided into three distinguisha- ble areas (A-C) (Fig. 2 & 3). A) Area A refers to the proximal deposits in the Riedl valley. Here the deposits are mainly (>95%) com- posed of Schlern Fm. blocks, which are strongly weathered and can reach several tens of cubic me- ters in volume lying on top of finer material (carapace facies). The accumulations are partly overlaid by deposits of secondary landslides coming from the north (shallow landslides, slope debris). In the south- ern part of the Riedl valley, debris flows accumulation overlay the rock avalanche deposit, mainly consisting of reworked and relocated landslide material. B) Accumulation at Area B (Fig. 2 & 3) is located on the west side of Herrstein and forms a proximal surge barrier with a maximum total thickness of about 150 m, which was created under high-energy conditions on the opposite slope. The sediments here consist of a massive, matrix-supported diamict with a loose and Fig. 4 - Hillshade image of the outer Prags valley with highlighted Toma hills and debris ridges (magenta). The arrows indicate the travel path of the rock avalanche. On the insert photo, view towards southwest, the scarp is indicated with a dashed line in magenta and the accumulations with a dashed with line. In the foreground the Toma hills and ridges are accentuated. partially open framework structure. Blocks and mega- blocks of massive dolomites are found, the matrix is gravelly and slightly sandy. All components are from angular to sub-rounded. It represents the main accu- mulation zone of the rock avalanche and it is primari- ly responsible for the damming of the lake. The mor- phology can be described as very unsettled. On the surface, there are dark weathered blocks of Schlern Fm., up to several 10 cubic metres in size, in non- exposed areas the blocks are angular. Landslide material is also found on the NW shore of the lake below the debris cone from the Riedltal valley, which was confirmed by sediment-echographic investiga- tions (Irmler, 2003). C) Area C represents the distal part of the rock ava- lanche deposits in the Prags valley and it is primarily characterized by the presence of undulating relief known as Toma hills. Their genesis is directly con- nected to the rock slope failure event. The Prags valley therefore is a very beautiful example of a so- called Toma landscape (Penck & Brückner, 1901; Abele, 1974). In many places the landslide material is overlain by debris flow and alluvial material from the side valleys, but can be traced back up to the sports 187 Characteristics and dating of the rock avalanche at Pragser Wildsee/Lago di Braies (Dolomites, Italy) field north of the village of Schmieden/Ferrara (Fig. 4). Within the accumulation area C the thickness of the deposit was estimated to be only 5 m. Beside some blocks of Schlern Fm. in meter scale, there are mostly poorly sorted, angular components in cm-dm size. The average size is therefore finer than the material at accumulation area B. Contrary to older assumptions (e.g. Klebelsberg, 1935; Pia, 1937; Castiglioni, 1964; Abele, 1974) the rock avalanche material in the Prags valley was not transported by a glacier out of the valley. There is no evidence of gla- cial transport, nor of glacial overlay. 4.3. Geometrical characteristics of the Pragser Wildsee rock avalanche Rock avalanche deposits exposed at the surface cover an area of ~1.7 km2. Taking into account those parts of the avalanche that are buried by alluvial fans, talus slopes and colluvium, an area of ~3.5 km2 was estimated. The distance from the upper limit of the de- tachment scarp (2330 m a.s.l.) to the most distal Toma hill (1180 m a.s.l.) is 8.5 km, yielding a Fahrböschung angle of 8°. According to our calculation based on a digital ele- vation model, the total volume of rock avalanche depos- its is 36 million m³, so a volume between 30 and 40 mil- lion m³ can be given. 5. DATING THE PRAGSER WILDSEE ROCK AVA- LANCHE For exposure dating with 36Cl, in the proximal sec- tor of the rock avalanche (accumulation area B), the surfaces of four boulders of Schlern Dolomite were sam- pled (locations in insert map in Fig. 3). Unfortunately, the distal sector of the avalanche deposit (accumulation area C) with transversal ridges and Toma hills is devoid of suitable boulders (those lying well above the enclos- ing sediment). We obtained the following exposure ag- es: 7.82 ±0.45 ka (PRAG1), 8.18 ±0.55 ka (PRAG2), 8.89 ±0.76 ka (PRAG3), and 8.28 ±0.64 ka (PRAG4). Sample site and dating information is given in Tab. 1 and the chemical composition of the samples in Tab. 2. The average age of 8.3 ±0.7 ka indicates an early Holo- cene age for the rock avalanche event. Tab. 1 - Sample information, AMS data and calculated exposure ages. Exposure ages were calculated using Stone et al.(1996, 1998) production rates (see also Alfimov & Ivy-Ochs, 2009) and Lal /Stone scaling (Stone, 2000). No erosion correction was made. Tab. 2 - Chemical composition of analysed rock samples, major oxides in wt %, trace elements in ppm. Tab. 3 - Major parameters of the rock avalanches at Pragser Wildsee and Obernberg. At the base of a 12.1 m long drill core a tree needle was found during the investigations of the lake deposits by Irmler (2003). Its age was determined by 14C dating. The age was determined to 6504 ±47 uncal. years BP which corresponds to a calibrated 14C age of 7410 ±100 cal. years BP. This age can be regarded as the mini- mum age of the lake and therefore as a minimum age of the landslide. 6. DISCUSSION The Pragser Wildsee/Lago di Braies rock ava- lanche shows conspicuous similarities to the Obernberg rock avalanche (Ostermann et al., 2012) situated about 60 km in the NW of the Prags valley within the Brenner Mesozoic (Eastern Alps, Austria). Both rock avalanches show runout paths with a runup/impact on the opposite slope and a further propagation within a narrow valley that changes its orientation several times. The concep- tual model of the kinematics is discussed in Ostermann et al. (2012) and in Tab. 3 we summarize the basic pa- rameters of both rock avalanches. The age of 8.3 ±0.7 ka of the Pragser Wildsee rock avalanche and the 8.6 ±0.6 ka Obernberg rock ava- lanche may suggest a relation with the “8.2 ka cooling event” (cf. Rohling & Pälike, 2005), but for more detailed discussion about this potential relationship, refer to Os- termann et al. (2012). Nevertheless, we underline the importance of strong tectonic events as triggers for Hol- ocene rock avalanches (e.g. Ambrosi & Crosta, 2006; Prager et al., 2008) and there are recent (e.g. 2008 Wenchuan earthquake, Guo et al., 2015) and historic (e.g. 1348 Villach earthquake, Brandt, 1981) examples for earthquake triggered landslides. 7. CONCLUSIONS 1 According to the results of surface exposure dating of boulders we think that the Pragser Wildsee rock ava- lanche occurred 8300 ±700 years ago; this age is supported by a 14C minimum age of 7410 ±100 a cal BP of organic remnants from a drill core out of the backwater sediments within the Pragser Wildsee (Irmler, 2003). 2 The Pragser Wildsee rock avalanche is one of the few dated mass-wasting event in the Alps that potentially was associated with the 8.2 ka climatic cooling. The precise nature of the 8.2 ka event in the Southern Alps, however, is insufficiently documented to sustain speculations on a triggering cause of the rock ava- lanche under a particular climatic condition. 3 The distal 3.5 km of rock-avalanche deposits show an array of transverse ridges and Toma hills. These were previously interpreted as moraines and kames. The internal fabric and nature of the sediment of the ridges are, however, incompatible with glacial moraines, but consistent with an origin from a rock avalanche. 4 Several factors can be named as predisposing for the rock avalanche at Pragser Wildsee: the tectonic/ structural framework of a dip-slope layered rigid cal- careous sediment succession, dissected by faults and joints, the destabilization in the upper part of the Riedl valley due to pronounced facies interlocking and the karstification, especially in the Schlern Fm. ACKNOWLEDGEMENTS This research was supported by Autonome Provinz Bozen-Südtirol - Provincia autonoma di Bolzano - Alto Adige, Amt für Hochschulförderung, Universität und Forschung, Project: Age Inventory and Analysis of Cata- strophic Rock Slope Failures in South Tyrol. The Labor- atory of Ion Beam Physics, ETH Zurich, supported labor- atory work, and AMS measurements. Special thanks goes to the team of the Department of Geology at the University of Innsbruck. 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