Structures at the Distal Limit of the Tschirgant Rock Avalanche, Tyrol, Austria

Master Thesis M. Sc. Geology

Structures at the distal limit of the Tschirgant rock avalanche, Tyrol, Austria

Submitted by Annette Sophie Bösmeier

Supervisors Dr. Anja Dufresne Dr. Christoph Prager

Freiburg, 29th May 2014

Contents

Abstract i Kurzfassung ii Acknowledgement iii

1. Introduction 1

1.1. Topic 1 1.2. Overview of catastrophic rock slope failures 1.3. Investigations at the Tschirgant rock avalanche 3

2. Geological overview 6

2.1. Geographical setting 6 2.2. Regional geology 7 2.3. Structure and lithologies of the scarp area 8

3. Geomorphological mapping of the distal limit 17

3.1. Overview 17 3.2. Methods 18 3.3. Results 19

4. Detailed mapping 35

4.1. Methods 35 4.2. Results 37 4.3. Results and discussion of large-scale structures 45

5. Particle size analysis 51

5.1. Theoretical background and motivations 51 5.2. Methods 52 5.3. Results and discussion 56 5.4. Overview and outlook 65

6. Discussion 67

6.1. Geomorphology 67 6.2. Connection between geomorphology and detailed mapping 68 6.3. Connection and additional evidences by determination of grain size distribution 71

7. Conclusion 73

References 74 Appendix: List of samples and outcrops 79 Declaration

Abstract

Rock avalanches are extreme mobile, large (>106 m3) mass movements that can develop huge velocities, a high damaging potential and feature particularly long runouts and special morphological and sedimentological features. The process of dynamic rock fragmentation is suggested to play a major role in the development of unusual long runouts, nevertheless, the processes are not yet fully understood and further studies of rock avalanche deposit features are required to obtain a deeper understanding of movement and emplacement dynamics. The Tschirgant rock avalanche deposit, dating back to about 3500 BP, is the deposit of one of the major catastrophic rock slope failures in western Austria with a volume of more than 200 *106 m3 and a runout of over 6 km. It features a multifaceted appearance with its heterogeneous, hummocky terrain and preserved, complex internal structures. Failure of the south-east facing Weißwand scarp, consisting of Middle- to Upper-Triassic limestones and dolomites of the Tschirgant ridge was structurally predisposed and moreover, facilitated by the existence of weak Raibl beds at the slope toe. Geomorphological features of the distal area of the Tschirgant rock avalanche deposit were investigated during 6 days of field work and resulted in the creation of a lithological and a geomorphological map. These maps reflect on the one hand the various shapes, sizes and preferred alignment of hummocks, and on the other hand the predomination of Raibl dolomite and rauhwacken debris within the mapping area together with the location of few unusual polymict gravels. Furthermore, detailed mapping of the debris was possible along a river escarpment that cuts through the deposit. Thereby, internal deposit features like the characteristics of its basal contact to underlying sediments including brittle, as well as ductile deformation features, the existence and properties of a basal mixing zone, and entrainment of diverse sediments could be recognized. Finally, several samples of different zones were analyzed with a sieving and weighing method and selected samples also by lasersizer, to obtain grain size frequency distributions. As a result, a major influence on rock fragmentation of both, lithological properties, as well as localities of the samples within the debris were revealed. For instance, concentration of shear and fragmentation within the basal zone of the debris could be recognized. Altogether, the results from geomorphological mapping, from investigation of small-scale internal structures and from gsd measurements show extensive and complex interactions between the descended rock mass and both topography and runout path materials, which led to complex flow dynamics and emplacement processes.

Kurzfassung

Bergstürze sind äußerst mobile, große (>106 m3) Massenbewegungen, die enorm hohe Geschwindigkeiten erreichen können, ein hohes Gefährdungspotential besitzen und besonders große Reichweiten, sowie morphologische und sedimentologische Besonderheiten aufweisen. Vermutlich spielt dabei der Prozess der „dynamic rock fragmentation“ eine große Rolle, dennoch werden die Vorgänge noch nicht völlig verstanden und für ein tieferes Verständnis der Bewegungs- und Ablagerungsprozesse ist weiterführende Foschung nötig. Die Ablagerung des Tschirgant-Bergsturzes ist etwa 3500 Jahre alt und gehört mit ihrer Masse von über 200 *106 m3 und ihrer Reichweite von mehr als 6 km zu einem der größten katastrophalen Hangversagen im Westen Österreichs; mit ihrem verschiedenartigen, hügeligen Gelände und den komplexen inneren Strukturen, die erhalten sind, kennzeichnet sich die Ablagerung durch ein vielfältiges Erscheinungsbild aus. Das Versagen der südöstlichen „Weißwand“ am Hang des Tschirgant Rückens, welcher aus Kalksteinen und Dolomiten des Mittleren bis Oberen Trias besteht, war strukturell prädisponiert und darüberhinaus von inkompententen Raibler Schichten am Hangfuß begünstigt. Die Geomorphologie des distalen Ablagerungsbereichs wurde während 6-tägiger Feldarbeit untersucht, was zur Erstellung einer lithologischen und einer geomorphologischen Karte führte. Diese Karten geben einerseits die unterschiedlich geformten und bevorzugt ausgerichteten Bergsturzhügel verschiedener Größe wieder, andererseits wird das Vorherrschen von Raibler Dolomit- und Rauhwacken-Schutt im Kartiergebiet veranschaulicht, sowie das ungewöhnliche Auftreten polimikter Gerölle. Desweiteren war eine detaillierte Kartierung der Bergsturzschutts entlang einer Flußböschung möglich, welche die Ablagerung anschneidet. Dabei wurden innere Merkmale der Ablagerung erkannt, wie die Eigenschaften ihres basalen, durch spröde und duktile Deformation gekennzeichneten Kontaktes zu darunterliegenden Sedimenten, das Auftreten und die Eigenschaften einer basalen, durchmischten Zone, sowie Einschaltung verschiedenartiger Sedimente. Schließlich wurden einige Proben verschiedener Zonen durch Siebung und Abwiegen analysiert und ausgewählte Proben zusätzlich durch Lasersizer-Messung, um Korngrößenverteilungen zu erhalten. Diese ergaben, dass die Gesteinsfragmentierung sowohl durch Lithologie, als auch durch die Lage der Probe innerhalb der Ablagerung wesentlich beeinflusst wurde. So konnte beispielsweise eine Konzentration von Scherung und Fragmentierung im basalen Bereich der Ablagerung erkannt werden. Insgesamt zeigen die Ergebnisse aus geomorphologischer Kartierung, aus Untersuchung kleinmaßstäblicher interner Strukturen und aus der Bestimmung von Korngrößenverteilungen ausgiebige und komplexe Wechselwirkungen zwischen dem heruntergekommenen Gestein und sowohl der Topographie, als auch den Talflursedimenten, was zu komplexen Prozessen bezüglich Fließdynamik und Ablagerung geführt hat.

Acknowledgement

For the creation of this thesis I am thankful to several persons, amongst them especially

Dr. Anja Dufresne, my supervisor, who did not only provide me an interesting access to the topic during joint field work, but was also greatly supporting me in my field work as well as in theoretical questions at any time,

Dr. Christoph Prager, my co-supervisor, who promptly supported me concerning all upcoming questions,

Angela Thiemann, Marie-Luise Bühler and Dr. Frank Sommer, who were always helpful and giving me first instructions to laboratory work,

Dr. Hannes Kleindienst, who provided great pictures of my field work area, my father, Hubert Bösmeier, and my grandmother Viktoria Danninger, who were not only financially enabling me my studies and all associated excursions and field work, but are also keeping me grounded at any time.

iii

1. Introduction

1.1. Topic

This work focuses on the distal section of the Tschirgant rock avalanche deposit in Tyrol, Austria. Based on already existing research (Hauser, 1993; Stötter et al., 2007; Pagliarini, 2008; Prager et al. 2008; Prager, 2010; Patzelt, 2012; Prager et al., 2012; Dufresne et al., in press), geomorphologic mapping and notably, detailed outcrop mapping was undertaken. A closer look at a particular outcrop and the structures it reveals, as well as an investigation of grain size distribution and the degree of fragmentation of the rocks were intended to make a contribution to a better understanding of the emplacement process.

1.2. Overview of catastrophic rock slope failures

Landslides, in their various appearances, are among the most destructive natural hazards, being responsible for over thousand deaths each year and incredible costly damages. The attention on this widespread threat has increased over the last decades, not only due to expanding infrastructural facilities and the steady growth of population which is often associated with settlement in potentially endangered areas (Clague & Stead, 2012). It is necessary to better understand the interaction of natural events triggering landslides, like heavy rainfall or earthquakes, and anthropogenic influences on slope stability, such as constructional or agricultural alteration of the environment. The increasing need for risk management is connected with an intensification of scientific research in the domain of landslide hazard, to better estimate the possible exposure to a hazardous event. So for example, hazards that are related to landslides, like the formation of landslide dams and their possibly catastrophic breaching have to be considered. Primarily however, the area that can possibly be threatened by a landslide event needs to be estimated, which implies the necessity of research with respect to the mobility of landslides, i.e. the processes governing and ambient conditions influencing the flow dynamics.

Among mass movement processes, there exists a huge variety. The downslope movements exhibit very different appearances according to the geological, topographical or climatic environments they take place in. Furthermore, the material type of the mass – whether it consists of rock or of coarse or fine soil – makes some sort of categorization possible, as well as the failure or movement mechanism. Also, scale certainly plays a crucial role with respect to the behavior of a rock mass. It is differentiated between rock-falls and –topples, rock slides, spreads, flows and rock avalanches. However, the major part of landslides cannot be fitted to these categories as while moving downslope, the masses change their style of movement and therefore exhibit a ‘complex’ behavior (Clague & Stead, 2012).

Especially large mass movements (>106 m3) are complex, they can easily develop from rock slides to rock avalanches, so called “Sturzströme”. Starting as a catastrophic, that is, suddenly occurring rock slope failure, the huge rock masses reach velocities of more than 100 km h-1, sometimes even more than 250 km h-1 according to Hewitt et al. (2008) and just take a short time, i.e. minutes, to travel down from the scarp area. These rock avalanches are extremely mobile and rapid flows with an enormous and uncontrollable damaging potential. They feature particularly long runouts and special morphological and sedimentological features. This allows the assumption that the simple model of frictional grain flow is not adequate anymore for those large masses of rock and that different processes act during the rock avalanche detachment and emplacement. However, the discussion about the processes enabling the unexpected long runout distances is still ongoing, hypotheses include mechanical processes like acoustic fluidization (Collins & Melosh, 2003), processes affecting the basal part of the rock avalanche (Abele, 1997), like undrained loading (Dufresne & Davies, 2009; Hungr & Evans, 2004), development of a basal plane of reduced friction through melted rock and further models. Davies and McSaveny (2009) recently introduced a model to explain the remarkably decreased frictional resistance by dynamic rock fragmentation, a process that results in reduction of the effektive intergranular stresses and thereby also a reduction of the resistance to shear (Davies & McSaveney, 2009). Considering the various environments in which long runout rock avalanches occur, as well as their peculiar features and materials, the process proposed by Davies and McSaveny seems to be the most convincing theory. Nevertheless, it cannot easily be determined whether a single mechanism is sufficient, or if long runout rather includes a mix of different processes (Clague & Stead, 2012). No ultimate explication for the runout mechanisms of large rock avalanches is found yet and fundamental questions are still open (Prager et al., 2012). For a more reliable prediction of future events it is necessary to develop appropriate models of rock avalanche movement and emplacement, based on significant observations in field and laboratory. To confirm any model, it will be helpful to observe the fragmentation and comminution of a deposit, features that appear throughout in excess runout rock avalanches (Davies & McSaveney, 2009). Moreover, as those huge debris avalanches stopp very abruptly when their velocity decreases until a certain threshold value (Hewitt, 2008), it can be assumed that mechanisms determining the emplacement may still be well reflected in internal structures and the morphology of the deposits. Thus the examination of those deposit features may deliver precious hints to infer mechanical processes. Besides, topography and substrates of the area buried by a rock avalanche certainly have an influence on its behavior and need to be examined. Entrainment of substrates, water, ice or snow change the properties of the rock mass, it may evolve into an extremly mobile debris flow generating a widespread and thin deposit (Dufresne et al., 2009). Furthermore, the interaction of a rock avalanche with substrate may be reflected in morphological features such as ridges and large scale flow structures, and even if substrate is not entrained, the transmission of forces can lead to its deformation and induce a change of flow behavior (Dufresne et al., 2009). 2

1.3. Investigations at the Tschirgant rock avalanche

Fig. 1. Geological map showing major landslide deposits in western Austria. White: Quaternary rocks, light grey: Calcareous Alps and Ötztal basement complex, grey: rock avalanche deposits, black lines show fault systems (Prager et al., 2012, modified).

The Tschirgant rock avalanche is one of the major rock landslide deposits that can be found in western Tyrol. They are oriented in a N-S direction and extend from regions north of the Inn valley to the Ötztal (fig.1, Stötter et al., 2007). All those catastrophic rock slope failures have an obvious structural predisposition, huge volumes up to 1000*106 m3 and long run-out distances as far as 15.5 km in common, thus can be classified as “Sturzströme”, on the basis of their sedimentological and morphological properties (Prager et al., 2012). Recent investigation have assigned Holocene age for most of the major landslides in the western Tyrol area and have therefore corrected the former assumption of late-glacial age with deglaciation and subsequent relief-steepening as trigger for the slope failures (Stötter et al., 2007; Prager et al., 2008). Ongoing seismic activity since approximately the last 14.000 years can be inferred from dating of various mass movements in Tyrol and circumjacent regions. The earthquake activity, quaternary fluvial valley incision, and the complex folded and faulted rocks have promoted the deep-seated failures of bedrock in the region (Prager et al., 2012).

Two rock avalanches came down from the isolated, steep Tschirgant ridge, the smaller, about 25 – 34 *106 m3 Haiming rock avalanche, and just few kilometers in south-westerly direction the considerably larger Tschirgant event (Prager et al., 2012). Both failures are deep-seated and structurally predisposed by the complex structure of the Northern Calcareous Alps. The area is situated within the Inntal thrust unit and affected by cataclasis and brittle deformation, also the fault-planes and beddings of the Middle- to Upper-Triassic limestones and dolomites are dipping desk-like out of the steep Tschirgant slope which favored a translational slide of the rocks. The catastrophic slope failure was moreover facilitated by the more competent Wetterstein carbonate formation overlying weak Raibl strata (Prager et al., 2012).

Recently, 14C dating was carried out at the Tschirgant deposit by Patzelt (2012). For example, relicts of wood found in the southern area within the deposit resulted in roughly 2885 +/- 25 BP and examination of a buried fireplace in the south-eastern margin showed an age of 3465 +/- 45 BP. Altogether, eight different places within the Tschirgant and the Haiming deposit allowed radiocarbon dating, the results revealed a temporal congruence of the two deposits, which suggests seismic events as trigger for the rock slope failures. It is now assumed, that both deposits derived from at least two different events, one between 3700 and 3500 BP and another one between 3200 and 3000 BP (Patzelt, 2012).

The Tschirgant rock avalanche detached from the so called ‘Weißwand’ with a total fall height of 1400 m, the top of the scarp is situated at 2160 m a.s.l., and the rock mass went down in a south-east direction towards the opening of the Ötztal with a travel angle of 12.7°. An area of 9.8 km2 (Patzelt, 2012) is now covered by the deposit, with a thickness of 10 to more than 65 m, run-out length is at least 6.2 km (Prager, 2010). The total volume of the rock mass was estimated to be about 180 – 240 *106 m3 according to Pagliarini (2008) and 200 -250 *106 m3 according to Patzelt (2012). It could spread in the Inn valley without lateral confinement, however, at the beginning of the Ötz valley, a bedrock ridge formed an obstacle to the runout, diverting the spreading rock mass into two areas.

Stratigraphy of the source rock is largely preserved within the deposit despite vigorous disintegration of the rocks (Prager et al., 2012). The morphology of the deposit, however, exhibits an enormous multifaceted appearance. This includes, amongst others, differently large, megablock-covered hummocks in proximal and medial sections, areas with a transverse-ridge-like relief on the eastern lateral margin and a distal hummocky, less megablock-rich area. Especially due to the incision of the deposit by the Ötztaler Ache throughout the whole length of the deposit, outcrop situation is superb and allows in places great sight of vertical profiles of the rock avalanche deposit and underlying substrates. This enables detailed studies of structures within the range of the basal rock avalanche contact. So, accessibility, outcrop availability and condition, as well as the excellent preservation of internal structures make this rock avalanche deposit an outstanding example for research. Prospective associated advancements in modeling might eventually also contribute to a better evaluation of future potential risks, after all the Tschirgant scarp bears the risk of another rockslide, as Prager (2007) pointed out. A rock mass of up to 12 *106 m3 might detach and could again dam both the Inn and the Ötztaler Ache, causing flooding and extreme damage (Stötter et al., 2007). And just as a result of the ongoing accumulation of extreme rainfall events in alpine regions, future hazards should be considered in addition, which again emphasizes the necessity for a closer look at rock avalanche emplacement mechanisms.

In this work, focus is laid on detailed field studies of deposit sedimentology and associated morphology within the distal part of the Tschirgant rock avalanche deposit. The aim is to observe how runout and emplacement processes might have been affected by the terrain and the properties of the substrates, on which the rock avalanche was going down: which interactions of the debris with runout path material can be recognized by observing geomorphology and internal deposit structures, and how could this have influenced the runout? Also, grain size analyses of selected lithologies should contribute to investigate the rock mass disintegration during catastrophic slope failure and runout. The connected challenges during field and laboratory work shall be addressed as well, to provide reliable data for subsequent research.

2. Geological overview

2.1. Geographical setting

Fig. 2. Google Earth map (modified) showing the Tschirgant ridge (middle) with the prominent ‘Weißwand’ scarp and the location of the rock avalanche deposit below (big red circle), as well as the position within Austria (small map).

The free-standing, up to 2370 m a.s.l. high Tschirgant mountain ridge is situated in north- eastern Tyrol, about 50 km east of Innsbruck. It extends over more than 10 km in ENE-WSW direction. The deposit of the rock avalanche, detached from the south-east facing Weißwand scarp area (see fig. 3), covers about 6 km2 of the Inn and the Ötz valleys. This study focuses on the distal, south-east margin of the rock avalanche deposit with an area of about 0.7 km2. The most distal point of this area is about 6 km away from the scarp toe. The mapping area is bordered by the villages of Ambach in the east and Sautens in the west. Erosion by the meandering Ötztaler Ache created exposures of the rock avalanche deposit and fluvial terraces in roughly NW-SE, i.e. parallel to avalanche motion direction. The remaining unmodified deposit surface is mainly covered by forest.

Fig. 3. Photography illustrating the Tschirgant with Weißwand and Haiming scarps (marked in red), as well as their lithologies. Deposit area is outlined in white, the working area is marked in blue (Prager et al., 2008, modified). 6

2.2. Regional geology

Fig. 4. Map showing overall structures that includes basement units and the Austroalpine cover. The red dot marks the approximate location of the Tschirgant rock avalanche. (modified from Eisbacher & Brandner, 1995)

The Tschirgant ridge is situated on the north side of the Inn valley, in the region of the NE directed Inn valley fault system, and at the southern border between the western part of the Northern Calcareous Alps and the Ötztal-Stubai polymetamorphic complex (fig. 4). Alpine tectonics detached the Middle- to Upper-Triassic sediments of the Inntal sheet and formed cover nappes. The structure of the lithological extremely multilayered nappes has been further complicated as they underwent heteroaxial, polyphase folding and brittle shearings (Eisbacher & Brandner, 1995). The internal shortening produced steep to tilted strata, and also the NW striking dextral transverse faults that cut through the Inntal sheet illustrate the displacement caused by the north-west directed overthrust of the Ötztal basement complex (Eisbacher & Brandner, 1995). This intense deformation led to deep-seated cataclasis, which in turn facilitated the enormous glacial and fluvial erosion of the Inn valley (Prager, 2010).

2.3. Structure and lithologies of the scarp area

Structure

Fig. 5. NW-SE profile through the Tschirgant massive close to the scarp area (modified from Eisbacher & Brandner, 1995).

The south-east facing slope of the Tschirgant is part of the Tarrenz syncline (fig. 5) and owes its subvertically oriented beds to thrusting, steepening and overturning by the Ötztal complex. Brittle deformation of rock including the large-scale structures of overturned strata and differently directed and variably steep fault systems (fig. 5, 6) mainly contributed to the development of potential failure planes, but also determined jointing of the source rock (Prager, 2010). A complex fracture system comprising listric bedding- and fault planes that “dip desk-like out of the slope” (Prager et al., 2012) facilitated translational sliding.

Fig. 6. A: view to the Weißwand scarp area in NW direction. B: profile, according to scale, through the scarp, including the major faults, the assumed failure plane and sketch of the pre-rock avalanche slope (yellow line), as well as the main lithologies (red: Raibl strata, pink and blue: Wetterstein Formation, green: Muschelkalk, yellow: Reichenhaller beds, violet: Reifling and Partnach Formation grey: Hauptdolomit Formation) (modified from Pagliarini, 2008).

Lithology

Not only structural predisposition, but also lithological conditions enabled the catastrophic rock slope failure. The inverse stratigraphy, karst formation and weak, Upper-Triassic Raibl beds forming the toe of the slope mechanically favored slope instability. Moreover, strongly mineralized springs appear in the area of the Raibl beds, which points to hydrochemical dissolution and again might account for reduced stability (Prager et al., 2012).

A detailed look on source lithology and structures is important to estimate the transformation of rocks during emplacement. To obtain the necessary information, especially the works of Pagliarini (2008) and Prager (2010) have been used. Fig. 7 shows a profile of the permo-mesozoic sediments of the Northern Calcareous Alps that developed in the Tethys ocean. They formed in shallow marine and to some extent in terrestrial environment, exhibit an enormous spectrum of bio- and lithofacies and various thicknesses due to subsidence and ascension of the crust and connected diverse sedimentation conditions. Therefore, the Northern Calcareous Alps are additionally classified into three different sections, among them the ‘Tyrolean Facies’ which prevails in the region of Tschirgant (Pagliarini, 2008).

Fig. 7. Stratigraphic profile of the western Northern Calcareous Alps, lithologies of the Tschirgant scarp area are colored (modified from Pagliarini, 2008).

The almost heart-shaped niche of the Weißwand (see fig. 6, left) has a hollow with a depth of 15 to 250 m. The niche exhibits a maximum width of about 990 m in E-W direction. It narrows upwards and downwards, with a minimum of 550 m at its bottom (Pagliarini, 2008). As indicated in fig. 6, the scarp area mainly consists of Wetterstein Formation, Muschelkalk and Raibl strata (Pagliarini, 2008). However, the detachment zone has not yet been explored thoroughly, due to its difficult accessibility, and there is still a lack of the exact thicknesses of the different lithologies. According to Prager (2010), the lithologies occurring at the Weißwand scarp are thick-bedded to massive dolomites of the Wetterstein reef formation in the upper part of the scarp and the diverse Raibl beds at the lower part. They include alternating sequences of siliciclastic sedimentary rocks, dolomites and rauhwacken and are separated from the massive Wetterstein dolomites by thin-bedded limestones and dolomites from the Alpine Muschelkalk and Reichenhaller beds at the thrust fault (compare fig. 8)(Prager, 2010). In the following, a short description of the main lithologies of the scarp area and the respective occurrence in the mapped rock avalanche debris is given.

Fig. 8. Sketch of the scarp area with major faults and appearing lithologies (on the basis of Pagliarini, 2008, p. 68).

The Reichenhaller beds represent the most basal layer of Triassic carbonates. As a major detachment horizon, the layer is tectonically strongly stressed and folded, and in consequence of this fact, sparsely exposed at the Tschirgant with hardly determinable thickness. Lithologically, this formation shows a high variability, including dm-bedded, bright to middle grey dolomites, scattered sandy and marly layers, and some brownish-ochre Rauwacken (Pagliarini, 2008). Rocks of the Reichenhaller beds are assumed to appear thinly at the Weißwand between the Raibl beds and Muschelkalk, however, in the mapped area of the deposit they were not found.

Muschelkalk is supposed to exist in the lower part of the scarp area, its thickness fluctuates due to the hinge structure of the folded sediment layers. In general, the Muschelkalk Formation it is composed of medium to thin bedded limestone and dolomite, and is subdivided into three different formations, the Virgloria, Steinalm and Reifling Formation. The Virgloria Formation contains middle- to thick-bedded limestone and dolomite, among them poorly fossiliferous, distinctly developed bioturbated ‘Wurstelkalke’. They are middle to dark grey in color, blended in a marly, ochery matrix and alternate with 20 – 60 cm thick, dark grey limestone beds. Dolomitisation partly results in characteristic ‘Zebradolomite’. The Steinalm Formation, emanating from shallow waters, consists of predominantly dolomitised, clearly bedded to massive beds of carbonates with a blotchy, slightly pinkish color and dolomite crusts. The Reifling Formation, finally, includes nodular limestone (‚Knollenkalk‘) and bedded limestone with intercalations of tuff and tuffite. The characteristic Knollenkalk has a bright to brownish grey, in parts even reddish color, it is dolomitised to some extent and exhibits a wavy surface texture. The bedded limestone is up to 50 cm thick and consists of reef detritus; voids are just partly filled with sparry limestone (Pagliarini, 2008). This lithology has not been found within the distal, mapped area, which might imply that the Muschelkalk is sparsely appearing in the scarp area, or possibly this formation can be found in different, for instance more northwestern parts of the deposit.

The Partnach Formation is marked by thin-bedded, weak sediments of deep waters. They include very dark colored, slaty and negligibly fossiliferous marls, mudstones and siltstones, as well as scattered, thin intercalations of limestone (reef detritus) that is strongly folded. The Partnach beds form the transition between the upper part of the Steinalm Formation within the Muschelkalk and the Wetterstein Formation (see fig. 7) (Pagliarini, 2008).

The Wetterstein Formation, generated during Ladin to lower Karn (238 – 225 mya), is one of the ‘Hauptfelsbildner’ of the Northern Calcareous Alps. Tollmann (1976) describes a vertical partition into three sections: (1) lower, massive and vaguely bedded, dark limestone including a basal, 50 to 110 m thick section of ‘Partnachübergangskalke´; (2) in the middle well bedded and brighter dasycladaceae containing limestone, and (3) on top very fine bedded, bright limestone together with red, grey and black layers of breccias and marls. A partly dolomitisation of particularly the lower beds of the formation, the reef facies, may occur, it is then called Wetterstein dolomite (Tollmann, 1976).

This reef facies of the Wetterstein Formation comprises the upper and major part of the scarp (fig. 8) due to inverse stratification. It exhibits a thickness of 600 to 700 m. The highly porous, medium- to thick-bedded limestones and dolomites of this facies have middle to dark grey color and contain typical oolitic structures in dm-scale, mostly dolomitic and filled with coarse collapse breccias. Dedolomitization partly transformed the rocks to calcite again, weathering contributed to accumulation of grus and intensified the porous character of the material (Pagliarini, 2008). In the mapped area of the rock avalanche deposit, very few bigger Wetterstein dolomite boulders are found on top of the eastern hummocky area. They have sizes > 50 cm (fig. 9, left).

Fig. 9. Wetterstein block, outcrop 17 (left). Hand samples (right) of Wetterstein dolomite (a, outcrop 17) and Wetterstein or Raibl dolomite (b, outcrop 14).

Northalpine Raibl beds is a collective term describing the middle to upper Carnian sediments. They originated from a shallow marine environment (Gruber et al., 2009) and are composed of alternating successions of limestones and dolomites, claystone and sandstone, as well as rauhwacken. Basically, the Raibl beds can be divided into six different distinguishable horizons. This order is however modified by the strong folding and stacking of beds in the Tschirgant region, therefore, the composition is better characterized in the following way: adjacent to the Wetterstein Formation, a siliciclastic series including sandstone and brown to black slate is present, followed by the characteristic ochery-yellow band of limestones and dolomites, marly and affected by bioturbation. The upper two thirds of the Raibl beds - in the area of the Tschirgant roughly 150 m thick - are dominated by a thick and highly variable series of carbonates. In addition to limestone, dolomites, evaporates, a major layer of rauhwacken and lithified dolomite breccias, further sandstone and few slate can be found (Tollmann, 1976). Dolomites are frequently bituminous and middle grey or slightly brownish in color, and mostly darker than Wetterstein dolomite (C. Prager, pers. com.). The rauhwacken are porous, brownish-grey to ochery-grey collapse breccias, and contain gypsum and anhydrite; they border to the Hauptdolomit formation (Pagliarini, 2008). At Tschirgant, the Raibl beds altogether may have a thickness of about 200 m (Tollmann, 1976). Owing to the preservation of source stratigraphy in the rock avalanche deposit, Raibl beds are the dominant lithology of the distal deposit area.

Fig. 10. A, B: two dolomite collapse breccias, probably from the Raibl strata (outcrop 21). C: Raibl dolomite (outcrop 24).

In fig. 10, the ochre veins surrounding angular dolomite fragments show the characteristic color of the Raibl beds. The dolomite fragments are quite bright in color, they could be assigned to the Wetterstein Formation. The rock of outcrop 24 is a massive, middle-grey to brownish dolomite of the Raibl beds. Both sample places are situated within the southeastern and therefore the most distal part of the mapped area.

Hauptdolomit consists of relatively homogeneous, middle to dark grey-brownish and sometimes bituminous dolomite. It is mostly microcrystalline, laminated and dm-thick bedded and carries mm-layered rhythmit. This lithology is occurring at the Tschirgant at the very bottom of the scarp area and is found at the deposit margins. Its presence in the distal part of the rock avalanche debris, however, is uncertain, as the differentiation to Raibl dolomite is complicated (Pagliarini, 2008). Thus, Hauptdolomit cannot be excluded from also appearing in the mapped deposit region.

Quaternary geology

During the Quaternary, particularly the Pleistocene, the alpine landscape has been modified by glaciations. The huge Inn Valley Glacier advanced from Switzerland through Tyrol in NE direction and carved deeply into the Inn valley. Thereby, it significantly contributed to material erosion, entrainment and sedimentation along its path, which highly influenced the geological setting. After regression of the glaciations in Holocene age, huge masses of transported rocks have been left behind. Fluvial erosion, sedimentation and rock mass movements still have been continuing to shape the landscape. So for instance in the region around the village of Imst, southwest of the Tschirgant, a late-glacial, widespread lakeland area is supposed, that left behind lacustrine sediments (Hauser, 1993). Also, shortly after glaciation, the Ötztaler Ache carried huge masses of water, the whole valley contained a broad river system and after decline of the discharge, sedimentation of Ötztal gravels took place. However, knowledge about the sediments deriving from fluvial terraces within the Inn and Ache valleys is limited as few borehole data is available, especially for depths of more than 80 m. At least, the few borehole profiles close to Ötztal Bahnhof show that the substrates covered by the Tschirgant rock avalanche are made up of tens of meters thick fluvial gravels and ground moraine at elevations between 650 and 700 m a.s.l. (Hauser, 1993).

Fig. 11. Clip of a map showing the quaternary geology of the distal, SE area of the Tschirgant deposit, including the mapped area between Kandlschrofen, the Sautens alluvial fans and the crystalline ridge of Rammelstein (modified from Patzelt, 2012).

Fig. 11 shows part of the map by Patzelt (2012), which was used as a guidance to assess the substrates underlying the rock avalanche deposit and also helped to characterize the morphological features of the area. Concomitantly, it illustrates different features of quaternary geologic evolution both before and after the rock avalanche emplacement.

The roughly north-directed, by black arrows indicated traces of glacial abrasion on Rammelstein originate from the Ötztal glacier that moved in NE direction and thus was connected with the Inn valley glacier (Patzelt, 2012). Later, after glaciation, considerable 15

post-glacial rock mass movement by erosive and gravitational forces took place. In the area of the Ache river bed, sediments have been deposited. Those fluvial sediments mostly predate the wide alluvial fans that extend from the SW towards the valley, and also form the substrates underlying the avalanche deposits at least in the domains of the river embankments. It can be inferred from the map furthermore, that the alluvial fans also partly predate the rock avalanche, as its deposit locally covers the northwestern edge of an alluvial fan in the region of the village Sautens. On the other side of the Ötztaler Ache however, an alluvial fan, probably more recently formed by the Stuibenfall, is overlapping the rock avalanche deposit.

Post-depositional modification of topography and geology also includes extensive erosion of material by the Ache, this becomes apparent within the map (fig. 11) by the steep embankments along the riverbed. Moreover, it suggests the assumption that the most distal part of the rock avalanche deposit has already been eroded by the river.

Furthermore, especially at the southeastern rims of the deposit and in the area bordering to the east end of the Rammelstein ridge, medium to large blocks of gneiss and other metamorphic material overlap the rock avalanche deposit and merge with the carbonate blocks of the carapace. This post-depositional rock fall in places even complicates the determination of the rock avalanche margins.

3. Geomorphological mapping of the distal limit

3.1. Overview

Geomorphological studies deal with the landforms of a region and their development. Therefore, also the processes connected to characteristic features are observed, they are in general governed by the interrelation of weathering, erosion and deposition, as well as structural and volcanic activity (Griffiths & Whitworth, 2012). The huge gravitational mass movements of landslides certainly have an immense impact on landforms. Based on an activity classification for landslides (Cruden and Varnes, 1996, in Griffiths & Whitworth, 2012), the Tschirgant Rock avalanche is grouped among young landslides with its age of around 3500 BP, its still quite sharp and sparsely vegetated scarp and the hummocky and relatively well discernable deposit margins (Clague & Stead, 2012). The rock masses with volume of about 200*106 – 250*106 m3 (Patzelt, 2012) detached from the Weißwand scarp and descended a total height difference of 1480 m from the top with an elevation of 2160 m a.s.l. to the 680 m a.s.l. slope toe. The masses moved downwards with a travel angle of 12°. They were emplaced on an originally circa 9.8km2 wide area within the Inn valley, south of the Inn river and at the beginning of the Ötz valley. The debris spread fan-shaped, covering the Inn – Ötztaler Ache river junction (675 m a.s.l.), and travelling up opposing slopes of Amberg, Holzberg and distal Kandlschrofen to heights of around 850 m to 920 m. In places, the deposit thickness reaches 65 m (Patzelt, 2012).

Fig. 12. Digital terrain model showing the travel direction of the avalanche (red arrow), the extent of the deposit (black boundary), as well as the position of the villages Roppen, Sautens and Ambach that surround the mapping area. 17

The deposit landscape is not only intersected by the Ache and the Inn rivers, but also by infrastructure, and bordered and partly covered by urban construction. Nevertheless, the major part of the deposit, especially the core, is well-preserved. However, the broad NE-SW directed Inn valley and the meandering, roughly NW-SE directed Ache created deep incisions into the debris due to fluvial erosion. Associated steep erosion slopes facilitated secondary gravitational movements within the deposit (Pagliarini, 2008). It can be seen in fig. 12 that the morphology is characterized by a high variability. It can be sub-divided into several areas, depending on the composition of hummocks, ridges and depressions, as well as features of the carapace. So for example, the biggest hummocks are located within the medial deposit, additionally the most and biggest megablocks appear in that region. An area that is clearly distinguishable from this medial zone is the distal area situated in the SE with a highly different deposit morphology. As already mentioned, the entire rock avalanche deposit probably derives from at least two different events with a few hundred years between the two. However, lithological and geomorphological mapping of the area has not recognized chronological evidence or even an accurate determination of the age of different deposit areas yet, apart from the few sample sites that permitted age determination and could therefore be clearly dated.

3.2. Methods

The field work for this project was realised within 6 days in the fall of 2013. During this period, initially an overview of the Tschirgant rock avalanche deposit was attained by visiting several outcrops throughout the deposit under the direction of my adviser Dr. Anja Dufresne and thereby getting a first impression of the morphological characteristics, lithologies and internal structures of the different localities. This first survey of the deposit facilitated an orientation within the mostly forest-covered and by numerous paths crossed area, also a feeling for the border between rock avalanche deposit and surrounding geology could be established. Subsequently, the distal part of the deposit was investigated, outcrop availability and accessibility were evaluated and were found to be good. Especially the well-preserved and reasonably well accessible erosion bank of the Ötztaler Ache (orographic right side) was recognized as an outcrop offering a promising and highly interesting profile of the deposit. Those considerations led to the definition of the area to be mapped (see fig. 12, 13). Then, strategically, the whole area was perambulated by bike and on foot, on the few streets, the little paths in the woods and cross-country through the hummocky terrain. High resolution airborne laserscan-data and a digital elevation model (DEM) were obtained from TIRIS (www.tirol.gv.at). Those maps were used for orientation and as mapping base. Field notes included the description of characteristic and striking morphological features like depressions, steepness of slopes, shape of hummocks, texture of carapace and vegetation cover. Lithologies have been described and samples taken at various localities (see map of sample localities). The rock samples originate from construction sites, from embankments

18 next to roads or paths, from river bluffs, or just as “Lesesteine” from the forested floor. Furthermore, samples were taken from megablocks or by digging into the ground. Procedures after field work included detailed description, comparison and categorisation of the rock samples to generate a lithological map. Moreover, a 3D model of the topography using the tiris data was produced in ArcScene 10. Together with the field notes it helped to locate size, shape and orientation of hummocks and other morphologic features. Finally, the results were commented regarding to emerging challenges in connection with mapping and to what extent possible formation history might be inferred.

3.3. Results

The overall mapping area can be roughly divided into 2 areas - an eastern and a western one - of hummocky, mostly forest-covered rock avalanche deposit (fig. 13). They are separated from each other by the 200 m to 500 m wide and approximately NNW oriented, relatively flat, low terrain which is formed by ancient river terraces from the Ötztaler Ache, and where the broad meandering Ache is flowing through. The difference in elevation between the riverbed and the highest hummocks is about 40 m. Also, the terrain is slightly increasing with at least 1.5° in the direction of the Ötz valley. Outcrop availability and preservation of original morphology is limited by development of the village of Sautens in the SW of the mapping area, which is mostly constructed on top of alluvial fans (compare fig. 11), but also covers the most southern deposit expanse. Likewise, the little village of Ambach in the east has been constructed on top of both rock avalanche deposit and an alluvial fan that is associated with the Stuibenfall.

Fig. 13. Distal deposit area bordered by villages and mountain-ridges and crossed by the Ache.

Lithology

Examination of the lithological setting has been accomplished within about two days of field work. Whereas the former rock avalanche deposit was completely removed from the flat area of river bed and terraces, the western and eastern parts of the mapping area provided already sufficient insights. The list of samples can be found in the appendix, localities are marked in the lithological map, fig. 18. Although most of the hummocky terrain of the western zone is partially even very densely covered by predominantly a vegetation of Scots Pine, some small bushes and a mossy or grassy forest floor, abundant rock avalanche fragments could be found. Especially the few bigger blocks of more than 50 cm edge length could be easily recognized. These dolomite rocks were mostly assigned to the Wetterstein Formation because of their massive appearance and slightly brighter color (compare fig. 9). However, also smaller blocks of Raibl dolomite have been spotted, distinguished by its darker, slightly brownish and bituminous rock type and smaller sizes compared to the more massive Wetterstein boulders.

Fig. 14. Dolomite blocks from the NW corner of the mapped area. Sample localities 14 (left) and 11 (right.)

Outcrop 12 in the NW of the mapping area is a road cut in the steep forested hummock slope. It contains relatively bright-colored, fine fragmented gritty debris with marly appearance. Among bigger hand samples from the slope bottom are few brecciated reef limestones, bright mm-thin bedded limestones, ochre-colored Raibl rauhwacken, darker grey carbonates that represent Raibl dolomites and lots of very crumbly, weak marly-mellow limestone, possibly bioturbidite, from the lower horizons of the Raibl beds (compare 2.3.). Especially the upper part of the outcrop is formed by the characteristic ochre-yellow and fine material from the Raibl beds. 20

Fig. 15. Carapace of outcrop 12 with blocks in fine ochre and bright grey matrix of Raibl material.

As far as it could be recognized despite dense growth of moss, the carapace zone also contains bigger rock fragments (>10 cm), however, in general no grading of grain size was seen, mainly because of the difficult accessibility and steepness of the slope that eased toppling of rocks and slipping of fine material which covered original stratification (fig. 15).

Small fragments of the ochre-colored Raibl rauhwacken and middle-grey dolomite have been found at various places in the NW area. Those findings together with the exposure at outcrop 12 and the discovery of ochre-brown colored soils by digging (outcrop 19) lead to the assumption that this part of the rock avalanche deposit mainly originates from the Raibl beds at the slope toe of the Weißwand scarp. Few dolomite megablocks are notably located in the SW below and on top of steep hummocks (sample localities 16 - 18); the smooth undulating terrain in between is free of boulders. The megablocks seem to originate from the Wetterstein Formation and might have fallen down on top of the Raibl beds already at the scarp area, so that they might have been transported and deposited on top of the Raibl material. The same might explain the appearance of a dolomite megablock at the NE margin of the mapping area (sample loc. 28).

The area east of the Ötztaler Ache valley is in parts easier to access because of a greater number of forest paths and small trails. Also, the vegetation cover of pines, few larch trees, bushes, moss and grass on top of the hummocks is a bit less dense and enables the discovery of a high number of sample rocks. Especially the more than 300 m long river bluff on the orographic right side of the Ache shows perfect profiles through the hummocks (outcrops 1, 26, 27), even though the slope is partly too steep for access. The very eastern part of this hummocky terrain is, however, covered by the few houses of the village Ambach and thus, both lithology and original morphology can only be estimated. Nevertheless, a recent construction site (outcrop 21) provided a fantastic, nearly 10 m long profile, cutting about 4 m deep into the slope of the hummock (fig. 16). The lithology could clearly be identified as fragmented Raibl dolomites and rauhwacken. The dolomites have middle grey color, they are sparitic, contain lots of calcite veins and are very brittle. They often form jigsaw-fractured clasts in this outcrop, have a clast-supported stratification and in general a coarser grain size than the weaker, ochre-greyish rauhwacken. Those supposed collapse-breccias are smaller fragmented, though some boulders are available as well (see fig. 10). They form marly, ochre schlieren without exact boundaries to the grey dolomite fragments. 21

Fig. 16. Outcrop 21 in the SE, composed of fragmented, ochre Raibl rauhwacken alternating with Raibl dolomites.

Fig. 17. View towards the Weißwand, above a river bend of the Ache. Sample loc. 26. 22

Within outcrop 21, grading is hard to recognize, as zones of smaller and larger grain size continually mix; boulders appear throughout the deposit. No typical coarsening upwards can be detected at this locality. However, the upper 1 m to 10 cm contain less boulders or coarse cobbles, therefore the carapace seems absent. In comparison to outcrop 21, the river bluff at sample locality 26 exhibits much more of the ochre-colored rauhwacken (fig. 17). Even before closer analysis by sieving it is obvious that those rauhwacken are much finer grained. They also do not seem to be so marly and have a more intense color, therefore, the rauhwacken at outcrop 26 may have a slightly different composition than those of outcrop 21. Nevertheless, they are both comprised by the Northalpine Raibl beds. Fig. 17 again shows an upper part formed by grey dolomite which is coarser grained than the rauhwacken, but no coarsening upwards for the profile is recognized. Outcrop 1 at a south-east facing slope of the river bluff provided such good insights in internal rock avalanche structures, that it was mapped in detail (see chapter 4). It contains fragmented Raibl rauhwacken and Raibl dolomite on top of a basal mixing zone and fine to coarse fluvial sediments at the base. Traces of Raibl rauhwacke in form of ochre-colored rock fragments were recognized on several hummock ridges in the eastern mapping area (see lithological map, fig. 18), it might be prevalent more frequently, but due to vegetation cover it cannot always be detected. Nevertheless, some long leaved grass often occurs on top of the Raibl rauhwacke material and was used as an additional indicator for this lithology. However, as the grass also occurs on rivers gravels, it is not a conclusive evidence. Wetterstein megablocks don’t appear in the eastern region, except from the already mentioned NE deposit margin, but some probably Raibl dolomite boulders were found in a depression between hummocks and on the SE slope of a hummock (sample loc. 2). The overall lithology of the eastern mapping area is, similarly to the western zone, assigned to originate from the Raibl beds from the lower scarp area.

A significant amount of slightly rounded to angular metamorphic rock fragments, particularly granite gneiss, paragneiss and micaschists, are found frequently in the whole area, especially along the road embankments towards Sautens (sample localities 12 to 16, compare fig. 18) and near the margins of the deposit close to the Rammelstein. These polymict gravels appear in grain sizes from fine gravel to boulders of more than 1 m and is supposed to be scree, dating from post-avalanche rock-fall of slopes at the mouth of the Ötz valley. The appearance of polymict, metamorphic angular to rounded gravel along the road embankments, the ancient river bluffs (sample loc. 15,16, 22 – 25), may also be a point for short-term transportation of rocks from gravel banks by the river during times of elevated water level. The post-depositional landslide dam has elevated the level of the Ache nearly 25m compared to today´s level (Patzelt, 2012) and explains the high elevation of the gravels (at around 740 m a.s.l. close to sample loc. 16) in comparison to the present river bed at around 720 m a.s.l.. Furthermore, well-rounded polymict fluvio-glacial gravels from the Ötztaler Ache occur dispersed within the area, preferentially in depressions between hummocks and along the 23 river bluffs close to the river terraces. This gives rise to fundamental questions about the emplacement processes that promoted such a distribution of different rock types. For example, in a depression at sample loc. 32 rounded, polymict gravels appear at more than 760 m a.s.l., thus it is highly unlikely that they derive from the dammed Ötztaler Ache. It may be assumed that the gravels originate from the Stuibenfall stream between Amberg and Kandlschrofen, however, their well rounded shape implies a longer way of transportation and is evidence for their origin from the Ache river. So it could be supposed that the gravels are river sediments that have risen post-depositional between hummocks. It shall be responded to this mechanism in more detail in chapter 4. However, these polymict gravels may also derive from the source area. This is not unlikely as well, as few variable, rounded fluvial pebbles of metamorphic rock type have been found on an excursion to the summit of Tschirgant on the ridge directly above the scarp area, as already discovered by Prager (Prager, 2010). This shows that although the collapsed rock may already have contained such pebbles, it is nevertheless questionable if they had appeared in higher quantities at the source area of the mapped deposit – the undermost part of the scarp. Moreover, interestingly, at few places even carbonate fluvial pebbles occur on top of the rock avalanche deposit (sample loc. 9, 32). Because of their lithology, the Ötz valley cannot be their source – they should be Inn gravels, eventually incorporated and transported by the landslide or also deriving from the scarp area.

In summary, the results of these investigations are presented in the lithological map (fig. 18), the extension of the alluvial fans was gathered from Patzelt (2012). Nevertheless, despite the several outcrops and high number of rock samples, still uncertainties exist concerning the lithologies of the distal deposit. As indicated in fig. 6 (B), Pagliarini (2008) mentions the occurrence of Muschelkalk next to the Raibl beds, also cropping up of Reichenhaller beds and the Reifling Formation cannot be excluded, however they could not have been clearly identified. The Northalpine Raibl beds are a variable formation, so that the differentiation between Raibl, Wetterstein, Reichenhaller dolomite or Hauptdolomit is not an easy thing to do, given the limited data.

Fig. 18. Lithological map that includes sample locations and area of detailed mapping, mapped rock avalanche debris and other quaternary geological features.

Deposit limits

The Tschirgant rock avalanche deposit is nearly completely covered with Scots pine (Pinus sylvestris), unless tree growth is anthropogenically or morphologically limited - most probably because of the difference in soil properties, the chalky soil loving pine has advantages over other native trees. Thus, the so-called “Forchet” already gives hints about the extent of the deposit. This was used during investigation as a first indication, as pine trees were found all over the hummocky terrain of the mapping area. At the margins of the mapped area, limits of the deposit are difficult to determine, because the southwestern and southeastern part are extensively covered by alluvial fans and also by parts of villages. The middle and southern distal limit is moreover nearly completely eroded by the Ötztaler Ache or also covered by an alluvial fan. Thanks to the Ache valley, the rock avalanche could travel in SE direction up the Ötz valley without confinement. Original southern debris margins are not to find, due to erosion by the Ache. However, according to Patzelt (2012), the most distal outcrop of rock avalanche material is situated at 760 m a.s.l. in a former river embankment nearly 1k m south of the Kandlschrofen. This is still nearly 700 m south of the most distal, SE deposit corner of the mapped area. There (sample loc. 23, 24), carbonate deposit material is gradually transitioning to polymict Ötztal rubble along the south and southeast ancient river bluff, cutting E-W directed through a shallow hummock. West of the Ache valley, the deposit is controlled by the elongate outcrop of the ‘Rammelstein’, a crystalline basement ridge. It is supposed to have split the detached rock mass into two parts, the northern one forming now the deposits of the working area. The transition from deposit to the high elevated Rammelstein is clearly confined by topography. Lithologically, polymict debris including rock avalanche material and especially big boulders of gneiss occur in this zone (sample loc. 4, 5) and may imply a transportation of substrate by the rock avalanche as it swept over the extension of the Rammelstein (“Prallhang”). In the east, the rock avalanche was confined by the steep slopes of Amberg and Kandlschrofen. An alluvial fan and road construction along the slope have prevented an unmodified preservation of the deposit morphology. The most distal carbonates were found close to the level of the road (sample loc. 24) and on Kandlschrofen some occasional sample pieces also at higher elevation, representing a “spray zone” of the avalanche. However the terrain was hard to access due to the steepness of the slope, so no clear determination of the extension and margins of the spray zone was undertaken. Patzelt (2012) reports evidence that the avalanche reached an elevation of 848 m a.s.l. at Kandlschrofen.

Hummocks and other topographic features

The map of hummocks and other morphological features (fig. 20) is the basis of the following explanations. For the creation of this map, a combination of field work data and estimation of hummock extent by the use of contour lines and 3d-view of the general shape of the terrain was applied. The use of ArcScene was enormously helpful for an all-embracing examination of surface features, as in the field it was not possible to view the 0.7km² large area with its dense forest and steep hummocky terrain as a whole. Nevertheless, the differentiation of several hummocks and the definition of their extent was not seldomly subject to interpretation. On the one hand, just a few outcrops within the whole area, notably outcrop 1, made it possible to clearly define the border between rock avalanche debris and underlying substrate and therefore determine the thickness of the hummock. On the other hand, the investigated area does not feature the typical “toma” landscape of hummocks that occur isolated and are thus easily to demarcate from one another. The topography rather exhibits a majority of connected and merging hummocks, sometimes forming elongated lines of hills and just few isolated and nearly conical-shaped hummocks.

In the map, the recognized hummocks are grouped into different sizes of their estimated height with 5 m-intervals, from hills with an elevation of less than 5 m to prominent hummocks with more than 20 m height. Particularly in the western part of the mapping area however, a subdivision into individual hummocks was locally not possible, for example at the continuously hilly, ascending landscape at the SW margin of the deposit (compare fig. 20). Moreover, the anthropogenic modifications of the deposit morphology in the SW make it impossible to detect original hummock shape. Also at the eastern margin close to Amberg and Kandlschrofen a dashed line marks estimated positions of hummocks that are nowadays covered by roads and houses. It becomes apparent, that most of the features are between 5 m and 15 m high and are distributed relatively evenly throughout the mapped area. Especially the smaller hummocks below 10 m are more conically shaped with nearly round to oval base. The larger and higher structures are less oval and less clear. While most of the higher hummocks also have a larger base area, a few of the more than 10 m high ones form elongate ridges with a narrow base (compare map: NW edge of the western area and middle part of the eastern). With regard to the steepness of slopes it appears as if the steepest sides - apart from the slopes formed by river erosion – were facing roughly the scarp area. More gentle slopes, in general, are on the side that is averted from the scarp area, respectively oblique to it. This is however just the case for some sectors of the mapped area and it is more pronounced in the eastern mapping area.

At first sight, it might appear that the terrain and arrangement of hummocks do not have an obvious pattern, and that hummocks are very different in elevation, size and shape. However, the eastern and western parts of the mapped area exhibit some different, interesting features. 27

It turns out, that the average elevation of the highest hummock tops in the eastern area is approximately constant or just slightly decreasing from NW to SE with values of around 777 m to 769 m (profile B, fig. 19). This reflects a decrease of hummock size with distance to the source scarp and ascending terrain. In the western area, however, hummock top elevations are increasing from NW to SE with values of around 765 m to around 780 m (profile A, fig.19). Besides, a clearly visible elevated margin of the deposit proceeds crescent- shaped from the east end of the Rammelstein to the ENE edge of the village Sautens (see fig. 20). These two facts suggest that the original terrain that was covered by the rock avalanche might already have been higher on the SW part close to Rammelstein, than on the east part close to Kandlschrofen.

Fig. 19. Profiles (not to scale) through the deposit area (compare fig. 20): western (A-A’) and eastern (B-B’) parts.

Fig. 20. Map of hummocks and further terrain features.

Whereas hummocks are in general characterized by length-to-height ratios (L/H) < 10 and length-to-width ratios (L/W) between 1 and 2, ridges are defined by L/H > 10 and L/W >2 (Dufresne & Davies, 2009). Based on those definitions, it becomes apparent that a few of the marked hummocks can already be labeled as ridges. Whereas the L/H > 10 criterion is not fulfilled by most of the structures, about 40% of the marked “hummocks” are more than twice as long as they are wide. Those ridge-like elevations are often formed by merging of several hummocks to a `compound hummock` (Clavero, Sparks, Huppert, & Dade, 2002) in approximate NE-SW direction (fig. 21, major hummock). To keep it simple, in the map of surface features all structures are denominated as “hummock” and it was satisfactory to categorize them according to their elevation.

Fig. 21. Clip from map of surface features, eastern area. Notable features include their elongation and preferred NE- SW orientation.

Concerning shape and direction of hummocks which are outlined by the sketched ridges or long axes of hummocks, it is clearly visible in the map that the eastern area comprises more structures with a preferred SW-NE to WSW-NNE direction (fig. 21). This corresponds approximately to a right angle in relation to the rock avalanche travel direction from the NW to the SE. Those obviously not flow-parallel, but oblique to right-angled arranged ridges are called “transverse ridges”. They are concentrated in the eastern part, in the western part of the mapping area however, this alignment is less pronounced.

Also a few depressions and lowered planes are marked in the map of hummocks. The grassy plane in the middle of the eastern hummocky area can represent an area exposed to recent fluvial deepening by discharge from the gully between Amberg and Kandlschrofen that may also be associated with the alluvial fan (see fig. 20). A feature that seemed very conspicuous during field work is an elongate flattening to shallow deepening extending along the most distinctive NE-SW directed, more than 100 m long ridge in the NW end of the western mapping area (fig. 22). It is outstanding in comparison to other ridges of the mapping area. That this very steep and prominent ridge possesses a depression on top can be explained by secondary slumping of the debris and may eventually be linked to a high gradient of the underlying, original terrain consisting of river bluffs.

Fig.22. Flattened to deepened top of ridge. 30

Hints on pre-depositional topography and original deposit morphology

The alignment of hummocks and especially the profiles through eastern and western parts suggest that the rock avalanche was emplaced on a SE ascending terrain that included the ancient, wide valley of the Ache – but to what extent can pre-depositional landscape forms still be traced through the rock avalanche deposit? The elevated margin of the deposit in the SW where pronounced hummocks can hardly be found may have resulted from the burial of an ancient alluvial fan by the rock avalanche. This alluvial fan derived from the Holzberg in the SW of the mapping area.

Fig. 23. Curved lines in western deposit (white dashed lines). Cut from map of surface features.

Very interesting features can be examined in the western hummocky terrain (Fig. 23). The curved, right- bended lines between the hummocks, also elongation and alignment of the ridge in the NW are striking. Observation of contour lines in this area suggests that these surface features don’t derive from erosional processes by water runoff. Those lines probably reflect pre-avalanche topography, the prominent curves point to former river embankments. Several ancient embankments could have existed, like nowadays still visible by the various river terraces, and have been buried by the landslide. This would explain the two nearly parallel, then converging curves appearing in the relief and it would imply, that the rock avalanche has covered the original landscape sheet-like and still offers a glimpse of the morphology before rock avalanche emplacement.

The preservation of several river terraces in the middle of the mapping area between the western and eastern hummock areas enables a review of the former river bed positions of the Ache. The river cut deeper and deeper into the valley after it was dammed by the landslide and probably quickly found its way to the Inn valley again. The flow path the river has taken after it had been buried by the rock avalanche is in general governed by the lowest and steepest decreasing topography, as well as by erodibility of the material. Apparently, the Ötztaler Aches has chosen a northward directed flow path along the slope toe of Amberg. This may be an indication for a lower elevation of the distal and lateral areas in comparison to the central part of the Tschirgant rock avalanche debris. Concerning the wide gap in between the eastern and western hummocks, original deposit morphology can´t be determined. Whether the area now covered with fluvial terraces and the river bed has formerly been hummocky throughout, or whether there already existed some sort of gap or lower topography directly after emplacement cannot be clarified. The 31 latter is, however, unlikely as there is no compelling reason for a discontinuous deposit morphology. It is rather logical that the Ache broke through the landslide dam close to its original river bed and thus eroded the hummocks in this area.

The flat area in the very NW including the few very small and low hummocks (dashed lines in map) however lacks an interpretation. Does it eventually represent a “natural” gap in between the large, medial hummocks in the NW and the distal deposit, originated during deposition by subsurface processes somehow allowing the rock avalanche masses to slide over the land and to leave just a small volume of material behind (A. Dufresne, pers. com.)? Or had parts of the deposit in this area simply been eroded as well? As the plane with the little hummocks is under active agricultural cultivation, also a considerable anthropogenic alteration of the topography in this area plays a role and needs to be kept in mind. Besides, the identity of the hummocks as rock avalanche material cannot be verified.

Furthermore, the degree of influence of river erosion on the original deposit morphology is not clear. The vague position of the ancient river beds make it difficult to determine whether all of the slopes bordering to the river bed have been formed by erosion. Whereas the SW margin of the eastern hummocky terrain is obviously cut off by the river, the slopes of the hummocks in the NW eastern area may be more or less preserved. When interpreting direction and shape of hummocks, this fact should be kept in mind to avoid false conclusions. For example, the orientation of the long axis of the tallest hummock in the western part (marked by a dashed line) may be interpreted carefully: the large hummock might as well have had an original NE-SW direction and part of it was just cut off so that it is not visible anymore.

Inferences about emplacement mechanisms

The examination of surface features already raises some interesting questions and allows, to some extent, certain interpretations concerning the movement and emplacement dynamics of the rock avalanche. To start with, it turns out that the initial hypothesis of a laminar flow of material from the scarp area can be verified by the mapped lithology, as the Raibl beds, the rock type that appears in the distal deposit area, derives from the lowermost part of the Weißwand and at least at some outcrops the original stratification of different beddings can still be recognized. For example at outcrop 21, fragmented Raibl rauhwacken and dolomite layers are barely mingled. Certainly of special interest is the pattern of hummocks and ridges. For instance, in the western hummocky zone (NW) it is outstanding that relatively steep slopes alternate with terrace-like areas in between them. A possible explanation for these planar areas could be local extensional spreading during emplacement or subsidence of debris directly after rock avalanche emplacement. This might also explain the flattening and depression of the steep northwestern chain of hummocks. Most striking feature concerning the alignment of hummocks is their preferential orientation which is more or less right-angled to the direction of propagation of the rock avalanche. Those transverse ridges are usually associated with a compressional and decelerating flow regime (Dufresne & Davies, 2009) – in this case, deceleration may definitely have played a role as we talk about the most distal zone of the whole deposit. The upward movement towards the Ötz valley certainly has contributed to a slowdown of the rock mass. Sudden deceleration may as well have been caused by increased frictional resistance, possibly by bulldozing of weak, deformable valley fill sediments. Unfortunately, the deposit area directly bordering Amberg and Kandlschrofen is modified. It might have given more insights in the avalanche motion when colliding with an obstacle. The sudden stopping because of the mountain slopes probably also influenced surrounding areas and may be the reason for the formation of transverse features in the mapped area.

Fig. 24. Offset of hummock ridges, arrow indicates shear sense. Clip from map of surface features.

When taking a closer look at the eastern hummock area, a change in topography can be recognized towards the SW (fig. 24). Big hummocks appear to merge into smaller and narrower structures and locally, some offset of hummock ridges in SE direction is visible. It seems as if some sinistral shearing of the transverse ridges took place. How can these structures be explained?

The shearing suggests a difference in mobility, like a later stopping of the more western, inner part of the debris flow, respectively a lower velocity in the lateral parts. Eventually, the NE part of the debris decelerated more suddenly due to collision with the mountain slopes than the part of the debris flowing over the river bed. This might point to a better, more unimpeded spreading of the debris at some distance to the bordering Amberg and Kandlschrofen which may have initiated sinistral shearing. Also the pre-depositional topography might have affected movement. Possibly a stronger increase in elevation towards Amberg and Kandlschrofen led to differential movements within the debris. Can we observe a transition from compressional to extensional regime? And to what extend may the water provided by the Ache river and substrate material have influenced debris motion? In the region of the Ache valley, the debris was emplaced on saturated river gravels. Undrained loading of the substrate might have contributed to an enhancement of the flow by decreasing frictional resistance. This might have lead to locally different velocities that resulted in shearing. Moreover, a fluidization of debris by mixing with water can result in characteristic longitudinal structures (Dufresne & Davies, 2009), however, this can initially not be assumed as such structures are not found.

To address those questions, a closer investigation of internal deposit structures is necessary. Especially the well exhibited outcrop at the orographic right bank of the Ötztaler Ache provides the possibility to have examine - inter alia - interactions between debris and substrate material and study, whether subsurface structures are somehow reflected in morphology to achieve a better understanding of the emplacement scenario. Also the possible origin of river gravels on top of the deposit shall be discussed further on.

4. Detailed mapping

4.1. Methods

At the Tschirgant rock avalanche deposit, a few localities along the river bluffs offer great possibilities to examine internal deposit structures, in particular features that reflect interactions between the rock avalanche and the substrates (valley-fill sediments) it was emplaced on. Within the mapped area, the best opportunity to observe these features is offered by the orographic right escarpment of the Ache in the eastern mapping area (fig. 25). There, the deposit is exposed along the river over more than 100 m. Locally, it extends over a height of more than 30 m from its carapace down to the base, the transition to underlying sediments and finally to the recent river bed. The slopes are very steep with an inclination of mostly more than 50°, also vertical to slightly overhanging slopes are present.

Fig. 25. NW-SE directed escarpment of the Ötztaler Ache (detailed mapped outcrop 1 in the red box, compare fig. 26).

For on-site mapping, the NW corner of this slope was chosen, as it provides an easily accessible, vertical profile through the debris including underlying in-situ sediments. Besides, the site is very well positioned as the outcrop stretches in NW-SE direction, thus more or less parallel to avalanche motion direction. This enabled the creation of longitudinal profiles which is optimal for observing structures that formed in the course of the emplacement. Furthermore, the large size of the outcrop and its complete exposure of the deposit from top to base provide ideal conditions for detailed mapping of structures with reference to

35 larger-scale features. Stability of the debris is remarkable high, which might be due to the strong interlocking of the fragmented, often jigsaw-fractured rocks, and locally due to meteoric lithification (Sanders et al., 2010). The structures are well preserved because of the steepness of the slope that leads to continuous erosion of material and uncovering of fresh sections through the deposit. Outcrop 1 was well accessible by a little path on the forested, gently dipping NW facing side of the hummock. Due to the steep and partly overhanging slopes, the area on the bluff that could be accessed for the field work was, unfortunately, limited to around 25 m of slope in the very northwestern part of the escarpment. First and foremost, safety needed to be considered carefully when working beneath a 5 - 10 m high, vertical scarp of debris. Thus a climbing helmet was worn for being protected against rock fall. Furthermore, pictures of the slope were taken at each day of work, to control the next day, whether alterations of the slope may be visible, that could have pointed out to current destabilization processes. Weather conditions that might have an influence on stability were considered, also temperature changes during the day that heated up the SW facing slope were kept in mind. Finally, work was always carried out carefully, it was avoided to spend more time than necessary underneath the overhanging scarp and most of the work was not carried out alone, but in the presence of Dr. Anja Dufresne. After gaining an overview of the features exposed at the site, 1 m x 1 m mapping “windows” following the method described by Glicken (1996) were set up to observe rock avalanche debris fabric. Three places were chosen which display different, significant structures and represent suitable places for bulk sampling. Preferably vertical areas were chosen for the windows, otherwise the surface was straightened. It was cleared from slope wash and colluvium with tools, and the area was thinly sprayed with water for a stronger contrast of different materials and colors. A wooden 1 m x 1 m frame was used to set the section, and as a scale in connection with a folding meter stick. Subsequently, pictures were taken of the window area as a whole, as well as several detailed pictures covering approximately 20 cm x 30 cm, for the observation of small scale structures and grains. Windows were furthermore sketched. The drawings included prominent structures, the type of boundaries between different materials and also coarser gravels with more than 5 cm, which were equally used as marks for drawing true to scale. The texture was described in detail in the field, supposed formation processes like shear movements were noted, and offset and dip of few material boundaries were measured locally. Rock samples of the different units for grain size analyses were taken where appropriate and possible (compare chapter 5). Sub-horizontal aerial photographs were taken by a quadcopter by Dr. Hannes Kleindienst and his team (GRID-IT GmbH), it enabled a great overview of the exposed slope in high resolution. Thus, large-scale drafts of the outcrop could be produced. The localities of the different windows were noted in those drafts, to be able to arrange the windows into the general view and obtain a better understanding of the large-scale dynamics that produced the internal structures. Therefore, not only a description of materials within the windows was performed, but also surrounding facies, like for example the stratification of the basal sediments, were taken into account. 36

4.2. Results

In the following, after a rough draft of the complete outcrop, a detailed description of the windows one by one is given, to respond best to individual features. Descriptions include lithological and grain size description, appearance of boundaries between different units and characteristic structures. Finally, an interpretation and discussion of derived emplacement processes is attempted by combination with overall outcrop sketches.

Fig. 26. Overview of outcrop 1 and locations of mapping windows. Flow direction from left to right.

The overview sketch in fig. 26 depicts the rock avalanche debris consisting of a major, 8 - 10 m thick layer of thoroughly fragmented Raibl rauhwacke and interbedded, also fragmented Raibl dolomites at the topmost area and close to the base of the deposit which is at an elevation of about 750 m a.s.l.. The transition from rock avalanche material to in-situ underlying sediments is marked by an around 1 - 2 m thick basal mixing zone, this mixed zone is a typical feature of large rock avalanches (Yarnold & Lombard, 1989). The deposit forms the SE part of an elongated hummock. Different interbedded layers of fragmented rock avalanche material are slightly dipping in the direction of the rock avalanche movement. Within the area of the mapping windows, sediments are injected into the debris in narrow dykes, also dipping in flow direction. Further inclusions and rip-up clasts of mobilized sediments within the basal mixing zone and lowermost part of the fragmented Raibl dolomites and rauhwacken will become obvious in smaller-scale pictures.

Fig. 27. Picture and interpretation of first, most NW located window.

The first mapped window (fig. 27) mainly includes four different units – basal sediments (I), a basal mixing zone (II), pure rock avalanche debris (III) and intercalations of different sediments (IV-VI). Lithology of the lowermost part (I) consists of orange to rust-colored, polymict gravels and sands, probably fluvial deposits that derive from the Ötztal. They are well to moderately sorted, sub-angular to rounded and exhibit a sub-parallel stratified normal grading with an upper, about 20 - 40 cm thick finer layer (coarse sand) transitioning to an altogether coarser grain size. Alluvial stratification is represented by vaguely visible, nearly horizontal alignment of elongated grains. The basal mixing zone (II) is a matrix-supported, diamictic unit, including angular, as well as well-rounded gravels of up to 20 cm grain size, embedded in a much finer, bright beige matrix. Gravels are very diverse in shape and lithology. They comprise mostly low spherical, angular fragments of darker blue- to brownish-grey dolomites; they probably derive from the Raibl beds and are of the same lithology as unit III. Also some higher spherical, sub- angular bright grey limestone clasts, possibly from the Wetterstein Formation, as well as few rounded to well-rounded carbonate and metamorphic pebbles, probably river sediments, are present. Some more angular, metamorphic gravels can be found as well. They often have a more elongate form and contain mostly Qtz, Bt and Ms. Lithological composition of the mixing zone is depending on grain size (see chapter 5.3), but in general composed by a

38 major, 60 – 80 % part of dolomites, 10 – 20 % limestone and 5 – 20 % metamorphic rock fragments. Percentage of the latter is decreasing with decreasing grain size. The pure rock avalanche material within the mapping window (III) is a clast-supported unit made up of densely fragmented, dark blue-greyish Raibl dolomite clasts. Locally, jigsaw- fractured clasts are still recognizable as they are little to not disaggregated. Between the fragments, some fine whitish powder appears. At places, more of this matrix is available, however, it is not as abundant as the matrix in II and also brighter in color. The metamorphic, medium to coarse sediments (IV) have a different composition than the sediments at the base – they are more ochre-colored and in general coarser: they comprise few rounded, fluvial cobbles and coarse gravel within a mass of fine gravel. Unit V is again a slightly different sediment unit that contains large, well-rounded fluvial gravels in a fine, bright ochre to beige matrix. The very small sectors (VI) in unit III seem brighter colored and fine grained, but are hard to differentiate from III. Possibly, they represent some minor flame injections of fine matrix from one of the surrounding units. The boundaries of the different materials are mostly quite well visible, especially the contact between mixed rock avalanche debris and the underlying sediments is very sharp. Also the contacts between mixing zone and intercalated sediments or pure rock avalanche debris are well distinguishable. However, the basal mixing zone comprises some areas which contain more carbonates. They can be designated as clastclusters or even stone lines (fig. 24, lower third, topmost III). Partly, they appear similar to jigsaw-fractured clasts from the IIIrd unit, which is why some more or less well visible parts within unit II are counted to III.

Concerning structures within this window, the most obvious feature is certainly the loaded to faulted contact between I and II that exposes some normal faults in direction of debris flow and dextral shear sense. Dipping of the contact between basal sediments and mixing zone could be measured at two points and resulted in 142/33 and 128/34, which shows a gentle dip towards SE, the direction of flow. No mixing of basal sediments and rock avalanche debris can be recognized and the more or less preserved stratification of the sediments suggest little disturbance of substrate material by avalanche emplacement. Units IV and V are rip-up clasts; their orientation and provenance will be discussed later. Fig. 28 is a close-up of the transition between fragmented Raibl dolomites (III) and the basal mixing zone (II). Orientations of the differently sized rock fragments in the debris suggest that a dextral shearing in flow direction occurred, and the upper dolomite fragment mass intruded the mixed debris.

Fig. 28. III – II boundary with inferred shear. 39

The difference between clast-supported debris and matrix-supported basal mixing zone is nicely visible in fig. 28. From the apparent shearing movements that can only be recognized in the upper unit, it may be inferred that the debris of the mixing zone might have behaved more ductile during or shortly after emplacement than the intruding, pure dolomite fragments.

Fig. 29. Lithologies and structures in mapping window F2.

The units of F2 (fig. 29) are quite similar to F1. Basal sediments (I) are composed of the same material as in F1, even though they are in general a bit coarser, being composed of mostly fine to medium gravel. Just the upper 10 cm are coarse sands as in F1. Moreover, grains tend to have low sphericity and often angular shape. Again, the base of the rock avalanche debris is formed by the matrix-supported, about half a meter thick mixing zone with the same composition of F1 II. On top of this unit, strongly fragmented Raibl dolomites (III) are present. In comparison to F1 they seem to be more comminuted and have a higher matrix content, which is reflected in the very fine and bright powder in which the jigsaw-fractured clasts are embedded. Besides, it is striking that the biggest gravels with grain sizes in the range of 5 – 20 cm are mostly comprised within the mixing zone, just like in F1.

The contact from basal sediments to the mixing zone is again very clear. Boundaries between the mixing zone and overlying dolomite debris, however, are gradational or unclear; it seems as if the mixing zone and the unit of dolomite fragments are gently merging into one another, forming sections with different degree of mixing. The upper third, center, seems to be a unit still connected to the basal mixing zone, but with a higher content of dolomites and inclusions of jigsaw-fractured clasts. The upper two areas marked with (III) may just consist of Raibl dolomites, but they also contain a high amount of fine matrix that very much resembles the matrix from unit II. Dolomite jigsaw-fractured clasts are also appearing in the lower part of the mixing zone. One elongate dolomite clast (fig. 30) is fragmented by numerous jigsaw cracks, but is not disaggregated. Other parts (IIa) of the basal mixing zone seem to somehow differ from the IInd unit because of a bit darker color. Those areas are more marly and slightly porous, but no clear boundaries to the surrounding debris can be found.

As in window F1, the contact between underlying sediments and the basal mixing zones is again characterized by loading and shearing. In F2, those structures are even larger and more obvious, thanks to the wavy appearance in the middle of unit I (fig. 30) and the two following, parallel shear planes that dip with 124/50 in flow direction. Orientation of shear is thus similar to the shear contacts observed in F1 (fig. 28). Dip in rock avalanche travel direction is in F2 also reflected by the orientation of the mixing-zone layer and the direction of the long axis of the few bigger gravels within II. Fig. 30 clearly displays ductile deformation of the substrate under the loading of the avalanche debris. Whereas the sediments internally don’t display any obvious comminution towards the boundary, the jigsaw-fractured and still more or less aggregated clast is obviously deformed crescent-shaped following the contact to the deformed substrate. Furthermore, the fragmented clasts are surrounded by very fine comminuted, whitish material. Thus comminution seems to be variably pronounced in the different units and materials, which will be discussed further in chapter 5.

Fig. 30. Close-up of the contact between substrate and overlying avalanche debris including a jigsaw-fractured clast of Raibl dolomite.

Fig. 31. Lithologies and structures in mapping window F3.

Like the other two mapping windows, F3 (fig. 31) contains basal sediments (I) covered by rock avalanche material which consists of a basal mixing zone (II) and fragmented Raibl dolomites (IVa). The interesting features of F3 are different units of sediments (IIIa-c) that are intercalated in the rock avalanche debris, and a thin band of very fine-grained rock avalanche carbonates (IVb).

Unit I is comparable to F1 I and F2 I and consists of ochre- to rust-colored, relatively fine- grained and well sorted gravels and sands. They locally show clearly preserved stratigraphy. Again, a sharp boundary can be observed between basal sediments and the mixing zone. The mixing zone (II) is lithologically very similar to F1 II and F2 II; polymict, variably sized, angular to rounded rock fragments are embedded in ochre-colored, fine matrix. The mixing zone comprises a narrow, 10 – 30 cm thick band that tapers and dips in rock avalanche travel direction. A straight and clear boundary that also dips in flow direction separates the mixing zone from a 15 - 30 cm thick band of poorly sorted, intercalated sediments (IIIa). This layer is matrix- supported and contains predominantly metamorphic fluvial pebbles with well-rounded shape, but low sphericity. Some of the gravels have a coarse grain size of more than 7 cm. Well-rounded carbonate gravels are present, too. The smaller grain sizes exhibit a higher percentage of angular rock fragments. The matrix of IIIa seems similar to the matrix of the 42 basal mixing zone, but appears slightly darker in color. It is noticeable that the long axes of most of the biggest gravels have a preferred orientation (sub-)parallel to the dip of the sediment band. A thin, upper zone (IIIb) that is apparently part of the sediment intercalation is made up of brighter and slightly greenish sediments with in general smaller grain sizes. Gravels with more than 5 cm are not present in this unit. The boundaries between IIIa and IIIb are partly unclear. Especially in the lower half of the mapping window, the materials are mixing up. Boundaries between IIIb and the Raibl dolomite fragments are clearly visible, the entrained sediments form a tongue shaped intrusion into the rock avalanche debris. IIIc is a very narrow band of intercalated micaceous sands that predominantly contain Qtz, Ms, Bt and Fsp. The intercalation has very sharp boundaries to the surrounding debris concerning upper and lower contacts and doesn’t seem to be mixed with it. However, the intercalation is tapering and thinning in and against direction of flow and these margins are hard to recognize. Along this elongated lens of micaceous sands extends a thin and more elongated band of fine-grained, dark grey-blue, angular fragments in a very fine, white matrix (IVb). This material is made up of pure carbonates, it is supposed to be Raibl dolomite. The boundaries to the mixing zone and the entrained sediments are not very distinct because the material IVb has very little volume. However, mixing with other units is unlikely, as the bulk sample contained only carbonate rocks. Finally, the upper third of F3 is formed by clast-supported carbonate rock avalanche debris. Big, jigsaw-fractured, dark greyish-blue dolomite clasts are enclosed in a mass of finer grained clasts and abundant, very fine, white powder. The upper right edge of fig. 31 shows an orange color due to slope wash from Raibl rauhwacken debris, as it was out of reach during cleaning. Although horizontally not more than 4 m away from F2, the unit of dolomite fragments in F3 slightly differs from F2 III. The debris in F3 seems to be finer grained, the dolomites are slightly different colored and matrix material has an exceptional white color. This illustrates a locally high variation within the rock avalanche debris concerning lithology and probably also fragmentation.

The structures in F3 exhibit features of both shear movement and ductile deformation. Intercalated sediments and the fine-grained dolomites (IVb) show a prominent dip towards SE, i.e. the rock avalanche travel direction. The dip of those units was measured at the boundary of IIIc and IVb and resulted in 144/22, thus shows a dip roughly in flow direction. Comminution and stretching of IVb and elongation of the micaceous sands along the dip point to an extensive regime or movement along this dip. Also, unit IVb seems like a fractured dolomite clast, which was sheared along the straight boundary between mixing zone and entrained sediments, and was strongly comminuted and stretched over more than half a meter. The surrounding materials don’t appear to be that fine grounded (see chapter 5). This might point to a concentration of shear within a very narrow zone of a particular lithology. Also, the entrained sediments, at least as noticed during on-site investigations, don’t show features of fragmentation or comminution. To clarify those points, grain size 43 distributions of the various units were investigated (bulk sample points see fig. 31) and will be discussed in the following chapter. Moreover, the different appearance of the boundaries between entrained sediments and dolomite debris, respectively mixing zone is striking. Whereas a very sharp and straight contact exists between II and IIIa, the upper margin of entrained sediments IIIa and IIIb shows ductile behavior and a loaded and intruding contact towards the overlying rock avalanche debris. Also, boundaries between sediments and rock avalanche are mostly very clear and no mixing of entrained sediments with rock avalanche debris can be discovered during observations on-site. The preferred orientation of big, elongate gravels in the entrained sediments is an interesting feature. The gravels seem to have been oriented during their injection into the rock avalanche debris, their long axes are mostly oriented parallel to the margins of the injection.

4.3. Results and discussion of large-scale structures

Outcrop overview

Fig. 32. Picture (by GRID-IT GmbH) and interpretation of outcrop 1: mapped structures including the results of the detailed mapping of windows, locations and shapes of various entrained sediments (see legend), as well as inferred shear movement and positions of faults. 45

The results from the detailed mapping of the three windows have been combined and used to produce a more detailed sketch and interpretation of the structures that are exhibited by outcrop 1 (fig. 32, compare fig. 26). The base is formed by graded and (especially on the right side of the picture well visible) stratified sediments, of which the upper 50 cm to 1 m consist of the rust-colored, gravels and sands that are unit I in all mapping windows. The lower part of the sediments is marked by a more greyish color and coarser grain size, transition to the rust-colored gravels is gradual. Although still stratified, the substrate is disrupted. This is very well visible due to prominent normal faults (compare fig. 32) which can be traced easily by the different colors of the sediments. The amount of offset varies, it is in the range of about 40 cm to 1 m. The normal faults are mostly relatively steep dipping in flow direction; close to F3, a graben structure can be recognized. Entrained sediments of various compositions are included in the sketch. They are intercalated in the basal mixing zone in narrow bands, partly even with intact stratigraphy (coarse fluvial gravels), or as rip-up clasts (medium to coarse sediments and micaceous sands) without visible preserved stratigraphy. They are not mixed with rock avalanche material. The major band of coarse sediments (compare F3 IIIa) dips in rock avalanche travel direction. Another small package of coarse sediments (F1 V) shows an opposite dip. In principal, however, the entrainment of sediments into the rock avalanche debris, as well as the normal faulted substrate strongly point to an overall extensional regime within this outcrop. Moreover, the different sediments are probably associated with each other, which can be recognized in their local arrangement; for example, micaceous sands repeatedly appear close underneath coarse sediments. This means, original spatial arrangement of differently composed sediments is possibly still revealed by the location of entrained sediments.

Fig. 33. Close-up of the left side of outcrop 1.

Boundaries between the basal mixing zone and the more extensive zone of fragmented Raibl dolomites are only locally clearly visible. At the left side of the outcrop, transition from mixing zone to pure rock avalanche debris is unclear, this is marked by the broken line (fig. 32). When taking a closer look (fig. 33), a nearly swirled mixing can be noticed. Between F1 and F2, the mixing zone is diapir-like intruding the overlying debris, this might have occurred due to buoyancy of the overpressured and liquefied mixing zone, either during or shortly after rock avalanche emplacement. The substrate was probably water-saturated by the Ötztaler Ache and therefore, a substantial amount of water also may have been provided to 46 the base of the rock avalanche debris. Undrained loading might have influenced the basal flow behavior of the rock avalanche, as high pore pressure can reduced frictional resistance and fluidize parts of the debris (Shugar & Clague, 2011; Dufresne & Davies, 2009; Hungr & Evans, 2004). Fluidization of the mixing zone then might also explain the nearly turbulent intermixing of basal mixing zone and dolomite fragments. This is clearly in contrast to the intercalated sediment rip-up clasts or narrow bands that are not swirled and mixed although unconsolidated, but entrained as whole clasts.

To summarize again, mapping of outcrop 1 revealed features that preserved different stages of interaction between the rock avalanche debris and substrate material (see also Dufresne et al., in press) and allow to draw some conclusions on rock avalanche motion and emplacement.

1) A shallow basal mixed zone (Yarnold & Lombard, 1989; Hewitt, 2008) exists. It obviously already had incorporated and transported substrate material on its way. Furthermore, it contains rock avalanche material that otherwise doesn’t exist in the distal area (e.g. Wetterstein limestone), which also indicates long transportation. It is a thoroughly mixed zone with variable transition to overlying pure rock avalanche debris. It is mainly transitional and locally clearer, showing a diapir-like intrusion. Properties of this mixing zone must have an influence on flow behavior and emplacement of the rock avalanche, as it did not form just shortly before or during stopping of the avalanche, but existed for a while.

2) Sediments were mobilized and entrained into the rock avalanche without mixing as rip-up clasts. They might have been intercalated very shortly before stopping of the rock avalanche flow, which is suggested by absent mixing and possibly absent fragmentation. The intercalations dip in flow direction which suggests an extensional flow regime.

3) The on-site sediments of outcrop 1 display various faults with offset of less than 1 m. They have been faulted, but also deformed ductilly by the rock avalanche, however, no mixing can be seen. The predominating normal faults also allow to assume that the rock avalanche motion was influenced by an extensional regime.

4) Entrained sediments might have an influence on, or may be associated with local shear concentration within the rock avalanche debris. This is suggested by the structures within F3 (IIIa – IVb, see above).

5) The intercalation of sediments into the rock avalanche debris, as displayed in fig. 32, also shows that differential motions occurred within the rock avalanche debris, or at least within the mixing zone (Dufresne et al., in press).

6) Way of intermixing between mixing zone and overlying Raibl dolomite debris (fig. 33) may indicate a transition from laminar to turbulent flow regime at the rock avalanche base, probably in advance to sediment entrainment.

Overview of the river escarpment

Fig. 34. Picture (by GRID-IT GmbH) and interpretation of the NW-SE directed escarpment of the Ötztaler Ache. 48

To conclude, a sketch of the overall exposed slope at the orographic right side of the Ötztaler Ache was produced (fig. 34, compare fig. 25). The exposed debris extends over nearly 100 m in rough NW – SE direction, outcrop 1 is located in its very left section. The vegetation- covered low part between the two hummocks in the middle of the escarpment includes Raibl dolomite and rauhwacken debris. Debris layers seem to dip against the flow direction, but are not well recognizable. This part might eventually have been formed by secondary slumping as vegetation cover is going much further down than on the bordering scarps. Thus, this middle part was left out. Due to the different colors of Raibl rauhwacke and Raibl dolomite debris, location and orientation of the various sections of the debris was well recognizable. It is conspicuous that the debris sections (mixing zone, Raibl dolomites, Raibl rauhwacke and a differentiated upper, brighter and more dolomitic part) of the left, more northwestern hummock gently dip in rock avalanche travel direction, whereas the zones of the right hummock show opposite dip. Not only the right hummock itself is elevated in comparison to the left one, but also the different zones have an upward offset. Besides, thickness of the rauhwacke debris within the southeastern hummock is decreased, whereas the layer of Raibl dolomite fragments is much thicker. As already interpreted through detailed mapping of outcrop 1, the normal faulted in-situ sediments, as well as the intercalations of sediments that also dip towards flow direction point to an extensional regime in this area. This assumption is further reinforced by the overall arrangement of both hummocks.

Although a sharp contact exists between rock avalanche debris and in situ substrates, parts of them could have been eroded and incorporated into the moving debris, also the intact stratigraphy of in situ sediments does not exclude bulldozing (A. Dufresne, in press). The substrates are deformable and probably water saturated, which could have made them mechanically weak and therefore, easily mobilizable (Dufresne et al., 2009).

A Fig. 35. Model for substrate material with reduced frictional resistance: ploughing and entrainment of substrate, followed by mobilization of substrate. B The arrow in (C) marks the displacement of eroded substrate (modified from Dufresne, 2012).

A. Dufresne (2012) worked on exploring the interactions between flow and runout path material by developing experimental models for rock avalanches. Fig. 35 shows the model for a low friction material that is eroded and mobilized, and finally forms an active layer at the base of the rock avalanche debris. Debris material subsequently mimics the surface form of the substrate (Dufresne, 2012).

Eventually, this model (fig. 35) may help to explain the recognized structures (fig. 34). Erosion of substrate and redeposition farther away in flow direction might explain subsidence of the left hummock (fig. 34) associated with normal faulting, and upward offset of the right hummock. Especially the NW dipping areas of sediments and overlying rock avalanche debris (fig. 34, left hummock) which has roughly the same dip, resembles structures in the model. However, the preserved nearly horizontal alluvial stratification in the NW of the outcrop seems to point to little disturbance of the substrate material at least within this area. So possibly, significant and profound mobilization of substrate material only took place farther ahead of this hummock, for example, within the area of the boundary to the next, southeastern hummock.

5. Particle size analysis

5.1. Theoretical background and motivations

Longtime research on rock avalanches in various environments has revealed that both unusual mobile flow behavior and pervasive fragmentation of the source rock are characteristic features of very large (> 106 m3) landslides (Davies & McSaveney, 2009). Furthermore, Davies & McSaveney (2009) suggest the existence of a relation between those two characteristics: High velocities and long runouts of large rock avalanches can be explained by a very low frictional resistance to flow. This, in turn, can be traced back to a decrease of effective stresses by pressures that result from continuous fragmentation of grains. This intense, ongoing comminution is supposed to be associated with enormous stresses that occur during the descent of huge rock masses (Davies & McSaveney, 2009).

In addition to this theoretical model, which explains processes that lead to long runouts, the movement dynamics of landslides are supposed to be highly influenced by the local topography and by substrates that were encountered by the rock avalanche on its travel path (e.g. Dufresne et al., 2009; Hewitt et al., 2008; Nicoletti & Sorriso-Valvo, 1991; Abele, 1997).

Thus, for a better understanding of the movement dynamics, a closer observation of field evidence of fragmentation in combination with local substrate characteristics is necessary. Both, degree of debris fragmentation, as well as nature, extent and possible influence of encountered substrate materials are of interest. During field work, different questions came up:

• Concerning the degree of fragmentation, can any dependence on material or location within the deposit be observed? • Which kind of variations in grain size distribution can be observed within the different zones that were mapped? • Are the entrained sediments also affected by comminution? • Can the assumption of local shear concentration, which was made during observation of small scale structures (compare 4.2), be verified? • Generally, how to carry out appropriate sampling of rock avalanche debris and which method is most suitable for determination of its grain size distribution?

This is why a part of this work is dedicated to the analysis of grain size distributions (‘gsd’) of different parts of the rock avalanche debris.

5.2. Methods

Sampling strategy

Addressing the issues mentioned in 5.1., initially implies the search for a reasonable choice of sampling strategy. Bulk samples were taken from the particular areas within the mapping windows, so that grain size distributions of different materials could be directly related to existing structures. Substrate material was sampled at different places and at different distances from basal contact. Also material from the basal mixing zone, from the pure Raibl dolomite debris and from different areas of entrained sediments (compare sample localities at mapped windows, 4.2.) was sampled. Finally, bulk samples of small volume were taken from the supposably strongly sheared zones. Besides, a bulk sample of fragmented Raibl rauhwacke from outcrop 27 has been taken, as well as samples from the more distal outcrops 21 and 24 for reasons of comparability. Sample volume was, as far as possible, customized on the biggest grain size (Evans & Benn, 2004). However, the entire range of clast sizes could not be covered for all of the samples. This is why detailed photography of the sample environment was undertaken as well to record biggest grain size and to provide data for an additional use of a photographic method. Sample volumes for sieving of between 600 g and 1.4 kg were considered to be sufficiently large for most of the samples, whereas less than 200 g of sample mass was taken from the few very fine grained zones. There, sample size was anyhow restricted by the small scale of the features (compare e.g. fig. 31, F3 IIIc, IVb). Also, little volume was gathered from zones that should just be analyzed concerning their fine fraction, e.g. for investigating potential comminution in entrained sediments (F2 IIId, F3 IIIa). Certainly, attention was paid to avoid zones of meteoric lithification which would have modified original grain size of the debris. Unintentional mechanical disruption of the sample by further breakage of rocks could have happened during the taking of the sample out of the compacted debris, as well as by transportation and sieving. Apart from careful handling, this could hardly be prevented. Yet, modifications are supposed to be neglectable in comparison to the fragmentation process during rock avalanche emplacement. Also, secondary agglomerations of fine grains, which might hinder the observation of original grain size, possibly might have been broken up again during sieving.

The grain size analysis was undertaken with a standard sieving and weighting method. The analyses were performed with manual recording of grains > 32 mm, standard sieves with grid from 16 mm to 36 µm, and for selected samples, with lasersizer measurement of the fine fraction (36 – 0.058 µm).

Standard sieving

In order to find the most appropriate procedure for sieving with standard sieves, at first, one selected sample was measured several times with different methods and the results were compared. Therefore, the sample with the maximum weight (F3 IVa, 2595g) was divided into two equivalent parts. The first part was sieved by four different methods that should examine the effects of wet or dry sieving. Also, the influence of the number of sieves on the final grain size distribution in between 36 µm and 16 mm was assessed:

1) dry sieving with all available sieves (grid size [µm]: 8000, 6300, 4000, 2000, 1000, 630, 500, 450, 400, 250, 200, 160, 125, 63, 36), manual shaking and brushing of the very fine particles from bigger grains 2) using the same technique of manual, dry sieving with a smaller number of sieves (grid size [µm]: 8000, 4000, 2000, 1000, 500, 250, 125, 63, 36) 3) dry sieving with the smaller number of sieves and with the help of a vibrating table (“Retsch AS 200 basic”) 4) wet sieving using the vibrating table and a continuous flow-through of 8.5 l of water for around 10 to 15 min, using the smaller number of sieves

For method 1 and 2, manual brushing of fine particles into the sieves has already been recognized as a major source of error; fine dust remained on the rough surface of the big rock fragments or went into the air, although the work was undertaken carefully. Thus, the particles were missing for the measurement. With method 3, dust remained on bigger rock fragments as well, despite using the vibration table. In method 4, the water that percolated through the grids picked up the very fine material, so that it was not lost, it was subsequently collected in large bins. Slowly, over a period of several days, after sedimentation of the material, the upper, clear layer of water was decanted. Thereby, a smaller amount of water including fine material could be further processed by drying. Together with the oversize fractions from the sieves, it was put in a drying furnace at 40°C for at least 24h.

Weighing

After drying, the different grain size fractions have been weighed with accuracies of 1 (above 200 g), respectively 4 digits (below 200 g). The use of the more accurate scale was preferred; but, as it could only be used for masses up to 200 g including the container, grain sizes with higher mass could just be weighed with the less accurate, big scale. It was intended to adjust the accuracy of the big scale by error estimation by weighing all masses with both scales and comparing the results. However, the deviations were quite small, with mostly less than 0.5 % difference. Therefore, it was abstained from correcting the measured values of the big scale. Moreover, the sum of the weight of all grain sizes after sieving and weighing was compared to the initial weight of the sample, to determine material loss during the treatment.

Fig. 36. Cumulative grain size distribution for F3 IVa, for comparison of different sieving strategies.

After plotting the results (fig. 36), it was clear that there is almost no noticeable difference between the particle size curves for manual dry sieving with the high and low number of sieves (curves 1 and 2). Furthermore, it was striking that especially below 0.25 mm, the wet method (4) shows a remarkable higher yield in comparison to all other methods. Especially when using the vibrating table without water (3), a high portion of the finer material seems to stick to coarser grains and not to go through the sieves. This implies a considerable impairment of the measurement. Besides, the second part of the bulk sample F3 IVa was also treated according to method 4, which provided very close and thus, satisfying results. In general, however, lasersizer measurements replace the measurements from sieving below 250 µm. But as only some samples were measured by lasersizer, the decision was made to examine all samples by using the method 4, that means the wet sieving method with 9 sieves and the use of the vibrating table. Results of the sieving method are presented as frequency distributions per weight (fig. 37 and following).

Additional remarks

All in all, grain size distributions have been determined for 15 bulk samples. Among them, eight samples were treated by wet sieving with method 4 as described above. For two samples (21d, 27), wet sieving was not considered to be suitable, as the samples contained a major amount of fragile Raibl rauhwacke material (i.e. clay particles) which would have been affected by the method, e.g. by dissolution processes. So they have been dry sieved and manually, according to method 2. The remaining five samples were very small in volume. This was why they have been sieved dry as well until a grid size of 250 µm, and the remaining fine fraction was measured by the lasersizer. Besides, for wet sieving with the vibrating table, different amounts of water between 4 l and 8.5 l have been used. This was decided individually, depending on how much water it took for washing out all of the fine particles and finally obtaining a clear outflow. A problem during the sieving of several bulk samples was hindering of water outflow due to clogging of sieves with a very small grid. Possibly, this happened because of an exceptional high amount of grains below a size of 250 µm. The clogging particles could be remobilized by a temporary increase of the vibration frequency. A further remark is dedicated to decantation: several days have been planned for the sinking of the fine particles prior to decantation. However, as the finest particles might take tens of days for settling down, a minor part of the fine fraction may have been lost.

Lasersizer measurements

Lasersizer measurements have been performed with a Malver Mastersizer Microplus of the Mineralogy department (Albert-Ludwigs-Universität Freiburg). It covers a range of grain sizes from 58 nm to 556 µm, thus allows to examine the content of particles between medium sand and mud. For the measurements, 0.08 g to 0.1 g of the grain size fraction below 250 µm was taken from selected samples. Samples were not pre-treated with ultrasonic sound to dissolve possible grain agglomerations, as comparative measurements by A. Dufresne with and without ultrasonic treatment have resulted in negligible differences within the final grain size distribution graphs. So, the sample were inserted directly. Finally, only four of the 15 samples could have been chosen for laser diffraction measurement. This is mainly due to specific material parameters that are prerequisite for the measurement process. For example, the relative refraction index can only be determined exactly for samples that mainly consist of one mineral. In the case of pure rock avalanche debris samples, this is dolomite, (CaMg)(CO3)2. Most of the other samples contain a considerable amount - or are fully consisting - of metamorphic rocks with various constituent minerals like Qtz, Bt, Ms, Pgk and others in variable percentage, which made it too complex within the scope of this work to adjust the measurement settings.

5.3. Results and Discussion

All in all, 15 different bulk samples have been analyzed by the standard sieving method. The location of the particular samples can be seen within the mapping windows (for outcrop 1, chapter 4.2), unless otherwise stated, and the locations of the outcrops in the lithological map (fig. 18).

• F1: F1 II (basal mixing zone)

• F2: F2 I a (basal sediments, 30 cm below basal contact) F2 I b (basal sediments, 5 cm below basal contact) F2 III d (entrained sands, located between F1 and F2, small sample)

• F3: F3 I (basal sediments, 15 cm below basal contact) F3 II (basal mixing zone, just matrix, very small sample) F3 III a (entrained sediments, just matrix, small sample) F3 III c (entrained micaceous sands, very small sample) F3 IV a1 (Raibl dolomite debris) F3 IV a2 (Raibl dolomite debris) F3 IV b (Raibl dolomite debris, small sample)

• Other outcrops: 21 c (Raibl dolomite debris) 21 d (Raibl rauhwacke debris) 24 (basal mixing zone) 27 (Raibl rauhwacke)

Due to the high variability in materials, their internal structures and localities, none of the samples can be considered as representative for the whole Tschirgant rock avalanche deposit, but only reflect very local properties. In the following, the frequency distributions of the different grain sizes of the analyzed bulk samples are compiled and discussed in groups of similar materials.

Basal, in-situ sediments

A B

C Fig. 37. Gsd of basal, in-situ sediments.

Basal, in-situ sediments have been wet sieved, according to method 4. The corresponding samples from the windows F2 and F3 (fig. 37 B and C) that originate both from about 10 to 15 cm below the basal contact of rock avalanche debris and substrate are quite similar in gsd. They consist predominantly of very coarse to coarse sand and have a pronounced peak for grain sizes between 0.5 and 2 mm, which reflects their well sorting. Less than 5% of the mass are grains with a size of 36 µm or less. Sediments from just about 20 cm deeper in stratification (fig. 37 A) show an even smaller percentage of silt and a clear shift to coarser grain sizes of fine to medium gravel, apparently without clear peak. This reflects the normal grading of the sediments. Even though the percentage of particles below 36 µm is higher with smaller distance to the contact to rock avalanche debris (fig. 37 B and C in comparison to fig. 37 A), it is supposed that those gsd don´t reflect a considerable fragmentation of in-situ sediments by rock avalanche emplacement. The particles below 500 µm might have been more in this case.

Entrained sediments

A B

C Fig. 38. Gsd for various entrained sediments.

From entrained sediments that appear within window F3 (F3 IIIc, F3 IIIa, fig. 31) and between the windows F1 and F2 (compare fig. 32), only small bulk samples have been taken. Due to little volume, it has been decided to dry sieve them. Lasersizer measurement was planned, but finally could not have been used because of the composition of various minerals. Two of the samples (fig. 38 A, C) are qualitatively comparable, they are both micaceous sands. The gsd of the sample F3 IIIc (fig. 38 A) shows an even finer composition. However, just a very small sample could have been taken (5 g) from this thin and elongate band of entrained sand (fig. 31). Mixing with fine fragmented material from the mixing zone cannot be excluded. So, the question remains open, whether the high content of fine sand exists due to possible mixing of the sediments with fine rock avalanche debris, whether the sand might have been entrained and comminuted by shearing or whether it has already been that fine grained in advance of the injection. Photographic record of the mapping window F3 revealed that the cobbles and pebbles within F3 IIIa and F3 IIIc are well-rounded and not fragmented. So, at least the coarse grain size fraction of these entrained sediments has obviously not been affected by fragmentation. Fig. 38 B displays the gsd of a sample from the matrix of intercalated sediments (F3 III a). Biggest grain size of the corresponding zone is about 10 cm, but has not been sampled - a little sample was taken to investigate the fine fraction. The analyzed material is obviously poorly sorted without a prominent peak. The percentage of grains below 250 µm is smaller than within the micaceous sands, which could possibly be an argument against fragmentation during rock avalanche emplacement, or just reflects the original difference of the sediments.

Basal mixing zone debris

A B

C Fig. 39. Gsd for basal mixing zone debris.

Gsd of mixing zone debris are represented by three different samples. Fig. 39 A shows a bulk sample from the basal mixing zone of F1. The sample had a large mass, however, it did not contain the biggest grain sizes (15 – 20 cm), as these few rocks were too big and too strongly attached to the wall to be sampled. Fig. 39 C shows a sample of the matrix material of the corresponding basal mixing zone within F3. In this case, biggest grains were below 10 cm. Fig. 39 B finally refers to a bulk sample from a more distal outcrop that also contains both carbonate fragments and polymict rounded to angular grains. It is supposed to also be part of the basal mixing zone, although, within the outcrop, the distance to the basal contact cannot be seen. In comparison to the in-situ sediments (fig. 37), an obviously higher content of fine material < 36 µm (fig. 39 A, B) respectively < 250 µm (fig. 39 C) can be recognized in these three gsd. Furthermore, the sample F1 II has a bimodal frequency distribution with peaks < 36 µm and at around 8 mm. The finest particles dominate in the distribution, which points to pervasive fragmentation of the debris and reflects its matrix-supported character. Nevertheless, the other grain sizes are relatively evenly distributed, especially between 36 µm and 1 mm. All in all, the gsd reflects the poorly sorted, diamictic character of the basal mixing zone, that was already recognized during detailed mapping of F1 (compare chapter 4.2.). Moreover, a lithological observation of the different grain size fractions of F1 II between 1.6 and 2 mm reveals a decreasing content of limestone from about 20 % to 10 % and an even stronger decrease of incorporated metamorphic (mostly gneissic) grains from 20 % to 5 % with decreasing grain size. The (probably Raibl) dolomite content is continuously increasing from around 60 % to 85 %. This fact of abruptly decreasing content of metamorphic rocks with decreasing grain size may on the one hand just reflect the original gsd of the rocks that have been incorporated by the rock avalanche on its way. On the other hand, it might be

59 explained by a much smaller degree of fragmentation of incorporated grains in comparison to rock avalanche debris from the source scarp. This, in turn, might indicate a stronger fragmentation of source material in advance of incorporation of material, that means, a less strong fragmentation within the distal range of the travel path. The material from outcrop 24 (fig. 39 B) also includes a wide range of grain sizes with a substantial amount of fine grains < 36 µm in comparison to e.g. the basal sediments (fig. 37). The overall gsd shows poor sorting of the debris. Fig. 39 C finally is the gds for the matrix of the basal mixing zone within outcrop 1, it belongs to the zone F3 II, and is closely associated with F1 II (see 4.2.). Even though the fine particles are again dominating like in F1 II, the percentage of material < 250 µm is proportionally smaller than in fig. 39 A. Because of the increasing dolomite content with decreasing grain size in F1 II, the assumption was made that the finest fraction of F3 II is predominantly dolomitic and lasersizer measurements (fig. 42) were undertaken.

Raibl dolomite debris

A B

C D

Fig. 40. Gsd for Raibl dolomite debris.

Grain size frequency distributions of purely dolomitic rock avalanche material from the Weißwand scarp (fig. 40 A-D) include the Raibl dolomite bulk sample from F3 IVa, that has been divided in half and used to determine the sieving method (5.2.), a very small sample from the elongated, possibly sheared zone F3 IV b and a Raibl dolomite bulk sample from outcrop 21 (compare fig. 16). Outcrop 21 is approximately 300 m in southeastern direction of F3 at outcrop 1 and thus, more distal. It is obvious that the two frequency distributions from the halved bulk sample F3 IVa (fig. 40, A, B) are very similar and only have slight variations, which gives credit to the accuracy of the sieving method. In comparison to the gsd from the basal mixing zone and the entrained and in-situ sediments, those distributions are unimodal and show a prominent peak at around 4 mm. The slightly elevated percentage of grains < 36 is indicative of fine comminution of the material and reflects the abundance of fine matrix in between the clasts (compare 4.2.). Very few clasts of around 5 -7 cm represent the biggest grain size within this zone; they are not included in the bulk sample. The gsd for the sample from outcrop 21 (fig. 40 D) resembles distributions of F3 IVa, however, fragmentation seems to be less strong. The peak of the unimodal distribution is at 8 mm and thus, at bigger grain size than for the samples F3 IVa 1/2. For fine sand and mud particles, percentage is comparatively lower. In general, fragmentation is supposed to increase with travel distance; this is, however, not the case here. These differences may possibly come from a slightly different source material (see 3.3.). Also, the sample from outcrop 21 was sampled just about 3 – 4 m below the top of the hummock, whereas F3 is overlaid by a more than 12 m thick layer of debris and is

61 situated directly above the basal rock avalanche debris – substrate contact. So possibly, a reasonable explication for stronger fragmentation of the dolomites in F3 (fig. 40 A, B) may be their location at greater depth, closer to the basal contact. Fig. 40 C belongs to a sample of a thin elongated, probably sheared zone of dolomite. It shows a quite equal distribution of different grain sizes without pronounced peak, which represents poor sorting. The fine fraction has been analyzed in detail by the mastersizer (fig. 42).

Raibl rauhwacke debris

A B

Fig. 41. Gsd for Raibl rauhwacke debris from outcrops 21 (A) and 27 (B).

The two gsd of rauhwacke debris are quite different, reflecting the different appearance of the material in the field. Whereas the debris at outcrop 21 is coarser in grain size and possibly includes some contamination with dolomite, the material from outcrop 27 is very fine and silty. The different peaks of the gsd show this: whereas the rauhwacke fragments from outcrop 21 (fig. 41 A) have a majority (referring to mass portion) of grains between 4 and 8 mm, the rauhwacke debris in outcrop 27 has a bimodal distribution with a prominent peak at 63 µm and a little peak at 0.5 mm. The overall percentage of fine sand and silt is much higher than in outcrop 21. A comparison of Raibl rauhwacke and dolomite debris from outcrop 21 (fig. 41 A and fig. 40 D) enables a direct comparison of spatially very closely related, but lithologically differing samples. Both lithologies have a peak between 4 and 8 mm, the rauhwacke grains, however, have a higher percentage of grains below 500 µm. Then again, the two samples have the same amount of material below 36 µm. Fig. 41 B shows probably the comparatively highest values for grain sizes between 36 µm and 250 µm with reference to all analyzed samples. Percentage below 36 µm is, however, very low. Therefore, those two gsd (fig. 41) may indicate a stronger fragmentation of rauhwacke in comparison to dolomite, but only within a certain grain size range (36 - 250 µm). For grain sizes < 36 µm, frequency is comparatively higher in samples of dolomite debris (fig. 40) and especially in the basal mixing zone debris (fig. 39). This fact suggests a strong dependence of fragmentation mechanics on involved material properties.

Lasersizer results

Lasersizer measurements cover the grain size range between 58 nm to 556 µm. They were carried out for the grain fraction <250 µm of the pure dolomite debris samples of window F3 (F3 IVa 2, F3 IV b), 21c, and the basal mixing zone in F3 (F3 II). Fig. 42 shows only the size range of the lasersizer measurements, as differences between the samples are better visible than when included in plots of the whole clast size range. All of the samples show a prominent peak between 20 – 40 µm and another increase towards coarser grain sizes from 200 µm on. For grain sizes between 5 and 50 µm, the basal mixing zone F3 II has the highest values, whereas it has by far the lowest clay content; below particle sizes of 1.0 µm, the sample F3 IVb has the highest percentage, followed by F3 IVa 2 and 21c. Values for the smallest measured particle size, 58.2 nm, were in the range of 0.037 % (pure dolomite debris, F3 IV a 2) to 0.008 % (mixing zone, F3 II). A lack of material at 2 - 3 µm and around 50 – 100 µm can be recognized by the appearance of minima in the curves and might possibly be related to fragmentation mechanisms. All in all, these measurements prove considerable fragmentation of the material to grain sizes between fine sand and mud.

Fig. 42. Grain size distributions for the fraction <250 µm of the samples consisting purely or predominantly of Raibl dolomite.

For obtaining cumulative gsd curves (fig. 43), results from the lasersizer measurements (58 nm – 250 µm) were combined with the results from the standard sieving and weighting method (0.250 – 32 mm). As the lasersizer analyzed probes below 250 µm from sieving, but it got also results above this grain size, those values were included in the “finer than 500 µm” fraction of the sieving analyses for reasons of consistency.

The sample F3 II contains a much higher part of grains below about 0.25 mm (mud to medium sand), as shown by the steep rise of its curve (fig. 43). However, that F3 II seems comparatively much finer that the other samples, is most probably explained by the choice of sample material: just a small sample of fine matrix of the basal mixed zone F3 II has been taken, the coarse clasts have been avoided. Therefore, a direct comparison of the whole grain size range is in this case misleading. Instead, the results of fig. 42, the comparison of the fine grain fraction below 250 µm, are more significant. They suggest a much lower comminution of the basal mixed zone in comparison to the pure dolomite debris at least below 5 µm. Although, it becomes clear in fig. 43 that both Raibl dolomite samples from F3 are finer than the debris from outcrop 21c as already indicated before, and that both in mass and grain size range comparable samples F3 IVa 2 and 21 c show overall coarser grain sizes for 21 c.

Fig. 43. Cumulative grain size distribution curves, combining the results from standard sieving and weighting (0.250 – 32 mm) with the lasersizer measurements (58 nm – 250 µm, see fig. 42).

5.4. Overview and outlook

Altogether, these analyses reflect very well the highly heterogeneous character of the Tschirgant rock avalanche debris. The various frequency distributions not only show a strong influence of source material on final grain size, also sample locality plays an important role.

• Lithology obviously has a major influence on fragmentation. This is not only pointed out by strong similarities of frequency distribution curves among groups of similar material, e.g. pure dolomite debris in comparison to basal mixing zone debris (fig. 41 A, B, D vs. fig. 39 A, B), but also by variations in gsd of samples with different lithologies at the same location (compare Raibl dolomite and Raibl rauhwacke debris of outcrop 21).

• Essentially, Raibl rauhwacke debris has the highest content of very fine to fine sand of all measured samples (fig. 41 B), whereas the polymict basal mixing zone contains the highest silt content (fig. 39). So possibly, degree of fragmentation is not only strongly influenced by lithology, but also by the location of the debris within the flow. Pervasive comminution of the basal mixed debris may indicate a concentration of shear and fragmentation of the debris close to the basal contact, as already observed at other landslides (Davies & McSaveney, 2009).

• Lasersizer measurements reveal that content of clay (< 5 µm) is highest for the probe F3 IVb in comparison to other Raibl dolomite samples (fig. 42). Thus, the initial assumption of local shear concentration (compare 4.2.) may be verified.

• The analyzed substrates (fig. 37) are coarse sands to medium gravels, they are normal graded and stratified. They contain little mud, which may indicate that they have not been considerably fragmented by the rock avalanche, at least at several cm below the basal contact.

• Analyzed entrained sediments (fig. 38) are micaceous sands and sand to fine gravel, they have a considerable amount of fine sand to mud. Whether this is, however, result of comminution during injection or mixing with rock avalanche debris matrix, or original material characteristics, remains unclear.

The examination of possible fragmentation of entrained sediments requires more extensive and elaborate studies, possibly an advancement of lasersizer measurements for polymict materials or observations by scanning electron microscopy. This could be combined with an evaluation of micro-scale grain shapes which might prove fragmentation. Besides, more detailed studies considering the lithology of different grain size ranges within samples of comparable composition, but at different distances from source, might possibly help to examine the fragmentation processes along the travel path of the rock avalanche (compare gsd of the basal mixing zone). Especially for the basal mixing zone, a closer observation of the lithological composition of its mud and sand particles will be needed to verify the assumption of increasing dolomite content with decreasing grain size, and to examine fragmentation processes concerning incorporated substrate material in a more profound way. Finally, further investigations, especially evaluation of the detailed photographs of the different zones within the mapping windows are needed: a complete description of gsd including the coarser fraction can only be achieved by combining the volumetric sieving method with a grid by number technique, e.g. by a photographic technique or use of a survey tape, as described and performed by Casagli et al. (2003). However, these last analyses are beyond the scope of this study.

6. Discussion

In summary, the main results from the topics on which this work is focused - geomorphological mapping of the distal area, detailed mapping, and finally bulk sampling and determination of grain size frequency distributions – will be recapitulated shortly, as well as analyzed with regard to consistencies and derived conclusions concerning emplacement processes.

6.1. Geomorphology

Mapping of geomorphological features shows that the rock avalanche debris in the mapped area forms two main, hummocky and mostly forest-covered areas east and west of the Ötztaler Ache, cut through by the broad riverbed. The debris is mainly consisting of Raibl dolomite and rauhwacke, whereas additionally few Wetterstein dolomite boulders appear in the western area. In the eastern hummocky area comparatively more ochre-colored rauhwacke debris than in the western zone has been found. All in all, this distribution shows a preservation of the lithological sequence of the scarp area, and therefore indicates laminar flow processes. Interestingly, at several places, rounded to angular polymict gravels have been found, and an exceptional feature is particularly the frequent appearance of well-rounded fluvio-glacial gravels on top of the rock avalanche debris (chapter 3.3.,fig. 18).

Morphology at the distal area is characterized by a variety of features:

• Hummocks appear in different shapes and sizes. In addition to a few conical shaped hummocks, compound hummocks are common, and in places, hummocks merge to elongated ridge structures.

• The eastern part of the mapping area borders to the slopes of Amberg and Kandlschrofen, thus, the avalanche was topographically confined at its southeastern margin. The restriction of flow at this zone may be associated with a compressive regime, and might be reflected by a preferred orientation of the long axes of hummocks and elongated ridges perpendicular to the rock avalanche travel direction.

• Whereas elevation of hummocks in the eastern area is constant to slightly decreasing, the western hummocky zone shows an ascending terrain (see fig. 19) and an elevated margin, which may drive from a general rise of topography and/or from flow processes. The presence of an elevated margin, as well as the indication of transverse ridges in the east, may result from bulldozing of substrate materials, which could have had a considerable effect on debris emplacement.

• Then again, the deposit seems to trace parts of the pre-avalanche topography, like possible former river embankments in the western area (fig. 20) which indicates a major influence of topography on deposit morphology.

• The hummock ridges along the orographical right river escarpment indicate offset of hummocks in direction of avalanche flow (fig. 21). This is suggested to be a sign of local flow enhancement in the area of the river bluffs in comparison to the eastern zone, where rock avalanche debris might have been slowed down faster by lateral confinement.

6.2. Connection between geomorphology and detailed mapping

It seems as if the rock avalanche flow has behaved in different ways according to the localities, which may be traced back to the diverse nature of the terrain it was emplaced on. This fact might indicate a fast response of flow mechanics to topography and substrate characteristics. Whether and in which way some of the just mentioned features could be related to an interaction with substrates, was further investigated by detailed mapping of internal deposit characteristics.

As already discussed in detail (chapter 4.3.), the mapping of the facies of three mapping windows, as well as mapping of structures at outcrop 1 along several tens of meters, allowed insights into flow movement processes, especially concerning

• existence of a basal zone characterized by strong fragmentation and pervasive mixing with incorporated and transported runout path material, • existence of rip-up clasts and intercalated, in flow direction dipping sediments, • ductile deformation and predominantly normal faulting of in-situ sediments close to the basal contact to the rock avalanche debris, • indication of shear concentration within the debris close to the basal contact.

Mapping of lithologies and structures within outcrop 1 was furthermore extended by an overall sketch of the river escarpment (4.3., fig. 34) which shows a profile through two hummocks separated by a low, possibly slumped zone. In the sketch, normal faulting of substrates and a dip in direction of rock avalanche flow can be observed at the left hummock, whereas the right hummock shows an upward offset and an opposite directed dip. Besides, surface morphology roughly reflects elevation and partly also dipping of the debris – substrate contact. These structures are suggested to be associated with an extensional regime, this is obvious at least for the left hummock (fig. 34) where normal faulting can be seen. Furthermorethe recognized structures might be explained by a model of interactions between rock avalanche debris and runout path material (Dufresne, 2012; chapter 4.3., fig. 35). It demonstrates

68 ploughing of the debris in weak, erodible substrates, mobilization and bulldozing of substrates, and associated mimicking of the deformed substrates by the debris morphology. This corresponds to the subsidence of the right hummock, which includes normal faulting and dip in flow direction of the basal contact (fig. 34), as well as to the opposite dip and upward offset of the different zones of the left hummock.

Such bulldozing processes might be an explication as well for the elevated margin of the western hummocky zone, assuming that the rock avalanche debris, moving in southeastern direction and reaching the ancient river bed of the Ötztaler Ache, encountered water- saturated (therefore mechanically weak) sediments there and bulldozed them in SE direction up the valley.

Thus, debris morphology is probably strongly influenced by local properties of the substrates, e.g. where they can be eroded easily and where they get bulldozed and accumulate seems essential for debris flow behavior. However, the linkage between the mapped structures at the river escarpment and the transverse ridge-like morphology of the eastern zone bordering to Amberg and Kandlschrofen is not yet entirely clear. Possibly, a transition exists between areas of extension (mapped normal faults) and a compressional regime towards the adjacent mountain flanks. This could also be indicated by the offset of hummock ridges in flow direction (compare 3.3., fig. 21).

The question remains open, to what extent flow dynamics of the Tschirgant rock avalanche in the distal area have been influenced by the runout path material concerning long runout. Was the effect of bulldozing of sediments and subsequent deceleration of flow (Dufresne & Davies, 2009) higher than the influence of encounter and incorporation of saturated substrates, which may have contributed to an enhancement of the flow (Davies & McSaveney, 2009)? Evidences of mapping, at least at outcrop 1, let assume that an enhancement of the debris flow took place: not only normal faulting of the substrates (4.3., fig. 32), also nature of sediment intercalations point to extensive conditions during emplacement. The intercalations of sediments (fig. 31, 32), dipping in direction of flow, imply the formation of temporary zones of weakness and extension within the rock avalanche debris and the opening of faults in which sediments under overpressure – assumably water-saturated and fluidized – could intrude (Dufresne et al., in press). Dip of the intercalations might be explained by local deceleration of the debris, while debris closer to or at the flow front was decelerated comparably less strongly. Moreover, the opening of faults and nature of the rip- up clasts indicates differential movement within the debris (Dufresne et al., in press).

The entrained sediments (fig. 32) don’t have the same composition as the in-situ sediments within the mapping windows at the basal contact to the debris, so their origin is not obvious at first. They could derive

• from entrainment of sediments at an earlier point - from a more northwestern area – and transportation of the gravels within the debris, above a basal shear plane. This is less likely because of the lack of mixing between entrained sediments and rock avalanche debris.

• from bulldozing and intercalation of differently composed in-situ sediments very shortly before or during stopping of the debris flow. This more likely, as it is indicated by the preserved connection of intercalated sediments to underlying sediments without being cut off by the contact between debris and in-situ sediments (compare fig. 32, close to F3).

Finally, the observations during detailed outcrop mapping enable as well the establishment of a connection between observed lithologies (chapter 3.3., fig. 18) and emplacement dynamics. The appearance of rounded to angular polymict gravels, especially close to margins of the deposit and along road embankments, may be related to post-avalanche rock-fall and/or fluvial transportation of material. However, for the well-rounded, polymict fluvio-glacial gravels that exist on several places on top of the rock avalanche debris (e.g. outcrop 32 and 22, fig. 18), those explanations are unlikely – even though these gravels are predominantly present in depressions between hummocks, they are too elevated to have been transported there by the Ache river. The abundance of these gravels is an argument against their origin from the Weißwand scarp (compare chapter 3.3.). So it seems very likely that these gravels on top of the rock avalanche debris are associated with emplacement dynamics, they could form part of intercalated sedimentsas mapped in the debris profile at outcrop 1. Also, debris of the basal mixing zone may locally extend until the deposit surface, it contains polymict gravels, as well. This could even provide an explanation for the appearance of rounded carbonate gravels within the debris, assuming that they derive from the Inn valley and have been incorporated and transported until the distal deposit area.

6.3. Connection and additional evidences by determination of grain size distributions

Determination of gsd of several samples from the mapping windows at outcrop 1 and a few other locations within the eastern hummocky zone reveal that both, lithology and sample location obviously had a strong influence on fragmentation of the debris (see chapter 5). Furthermore, some hints on flow dynamics that have been recognized during detailed mapping are reflected in grain size frequency distributions.

• Whereas the analyzed in-situ sediments don’t seem to be considerably fragmented even close to the basal contact, debris of the basal mixed zone exhibits pervasive fragmentation, which was already assumed when observing the coarse sands and gravels of the substrates in comparison to the clasts and fine matrix of the basal mixing zone during field work (compare 4.2.).

• The in-situ substrates have been faulted and ductilly deformed; gsd show that they lack comminution, which may imply that they exhibited a different mechanical behavior than the fragmented debris mass (Dufresne et al., in press). This might be connected with water saturation of the substrates (Dufresne et al., 2009).

• Lasersizer measurement of a Raibl dolomite sample from F3 could furthermore reveal exceptional high clay content which points to a very local concentration of shear close to the debris base (compare 5.3., fig. 42), as suggested during facies mapping. The questions still remain open, whether the shear focusing in this case is related to a difference in lithological properties, and how it could be associated with the equally oriented injection of sediments.

• Entrained sediments contain a considerable amount of fine sand to mud, this can have different reasons, and fragmentation during injection into the rock avalanche debris could not yet been proved. At least the coarse grain size fraction is mostly well rounded and therefore, it obviously had not been affected. A lack of fragmentation might be related to possible water-saturation of the sediments and associated high pore fluid pressures that decreased the stresses on the material (Dufresne, in press).

Subsequently to their concepts about fragmentation dynamics within the motion of large rock masses, Davies & McSaveney (2009) introduced a model of ‘basally weak’ conditions during rock avalanche emplacement. It refers to a concentration of shear and fragmentation within a basal zone of the rock avalanche flow, probably related to an initial zone of reduced effective stresses by water saturation of the base and thereby decreased shear resistance. Observed structures and gsd measurements in the course of this work could fit to these conditions. For instance, the shallow Raibl dolomite sample of outcrop 21c is in general coarser than comparable dolomite samples from the mapping windows at the base of the deposit (5.3., fig. 40). Also, the observed concentration of shear close to the basal contact 71

(5.3., fig. 42), as well as the comparatively high content of fine material within samples of the basal mixing zone (5.3., fig. 39) suggest, that the emplacement conditions at the distal area of the Tschirgant rock avalanche could have been basally weak. Subsequent decreased resistance to shear might be associated with an enhancement of the rock avalanche flow, and thus, could also be compatible with local zones of extension, as recognized by detailed mapping of the profile along the river escarpment (chapter 4).

To sum up, it becomes apparent that the results for gsd, just like the various geomorphological and small-scale structural features, illustrate the highly heterogeneous nature of the distal area of the Tschirgant rock avalanche deposit. This reflects complex flow dynamics and emplacement processes, which can hardly be explained by one single model, but may require the linkage of different approaches, and further, more detailed studies e.g. concerning potential fragmentation of entrained sediments (see 5.4.).

7. Conclusion

In the course of investigating a part of the distal deposit of the Tschirgant rock avalanche within this work, a set of valuable observations were made. Highly heterogeneous features with regard to both, geomorphological large-scale structures and small-scale internal structures, as well as particle size distribution could have been recognized.

The hummocky deposit surface reflects interactions of the debris flow with the topography, indicated by shape and alignment of hummocks and ridges, and gives hints on an influence of the runout path material on the flow. This might have contributed, among other things, to a local transition of compressive to extensive flow regime, and to existence of an elevated margin in the southwest. Detailed mapping of a vertical profile through the deposit along a river bluff enables a closer investigation of flow and emplacement processes and demonstrates connections between deposit morphology and internal structures. It revealed different stages of interaction between the rock avalanche debris and substrates, for instance it indicates processes of bulldozing of substrates and locally, an extensive regime. Observations point to enhancement of the flow, possibly associated with the interaction with water saturated sediments. Determination of grain size distributions of several specific samples not only allows precious hints on concentration of shear close to the basal contact, but also proves strong dependence of degree of fragmentation on particular lithologies and location within the flow. Furthermore, the high variability of grain size frequency distributions also point out the necessity of extensive sampling. No single sample can be considered as representative for the whole rock avalanche due to its variability in materials and structures. Finally, the results from gsd measurements and mapping of internal structures might point to basally weak conditions during debris flow and emplacement which means concentration of shear and fragmentation within a basal zone, possibly enhanced by water saturation.

Altogether, features at the distal limit of the Tschirgant rock avalanche are an impressive example for extensive and complex interactions between a descending rock mass and both topography and substrate materials. Quantification of the effects of rock avalanche – substrate interactionson the emplacement process, especially concerning enhancement or deceleration of the debris flow, still needs to be acquired by various further studies, for example by observing more closely the degree of fragmentation of incorporated material. In general, further detailed studies at the Tschirgant rock avalanche debris can contribute to a future more profound comprehension of processes and the related conditions that lead to long runouts of rock avalanches.

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Appendix: List of samples and outcrops

sample type, location lithology

1 'Lesesteine' and bulk samples for Raibl rauhwacke (ochery), Raibl dolomites, mixed gsd analyses; long slope along debris; non-RA polymict sands and gravels Ötztaler Ache, position of mapping windows

2 valley between hummocks few bigger carbonate blocks (>30cm)

3 few lesesteine; Kandlschrofen, possibly Wetterstein surge/spray zone of RA

4 Lesesteine, western margin of dolomite: not Raibl, probably Wetterstein. mapping area, close to polymict pebbles (not RA), carbonates and gneiss Rammelstein, possibly surge of RA (not RA) megablocks

5 Lesesteine; strongly vegetation gneiss, no RA covered

6 megablocks > 1m3 possibly Wetterstein

7 Lesesteine at a construction site mix of material from an alluvial fan and RA carbonates (no ochery Raibl rauhwacken)

8 road embankment carbonates and fluvial pebbles

9 Lesesteine various rounded pebbles

10 few sample pieces on steep, (angular) carbonates elongated ridge

11 various Lesesteine, very steep slope small pieces of carbonate, Raibl dolomite; fluvial pebbles

12 outcrop next to the road Raibl beds (ochery rauhwacken, dolomites, clayey matrix, light to ochre-colored marly bioturbidite limestone)

13 little sample pieces at the road lots of fluvial pebbles and carbonates embankment

14 megablock Wetterstein dolomite

15 various rocks at the embankment few carbonates; slightly rounded to angular metamorphic rocks 79

16 megablock carbonate (Wetterstein)

17 some megablocks, Lesesteine Wetterstein dolomite

18 megablocks Wetterstein dolomite, gneiss

19 ochery soil, grass covered Raibl rauhwacken (ochery), carbonates; some fluvial pebbles

20 ochery, clayey soil similar to 12 probably Raibl rauhwacken; gneiss megablock

21 Lesesteine, bulk samples for gsd porous, brecciated Raible rauwacken, dolomites analyses; construction site

22 bulk sample polymict, carbonate-fragments (probably from Raibl beds), quartz, micaceous rocks

23 Lesesteine; steep slope, former carbonates (dolomites); metamorphic rocks river embankment. deposit margin

24 Lesesteine dolomite; fluvial pebbles

25 former river embankment polymict, carbonates, predominantly angular metamorphic rocks, micaceous sands

26 Lesestein on RA hummock Raibl dolomites on top of ochery Raibl rauhwacken

27 bulk sample ockery, fine Raibl rauhwacken debris from the slope

28 megablock at deposit margin Wetterstein

29 Lesesteine limestone (Wetterstein?)

30 Lesestein from the top of grass ochery Raibl rauhwacken and Raibl dolomite covered hummock

31 Lesesteine ochery Raibl rauhwacken, Raibl dolomite, brecciated limestone (Wetterstein)

32 Lesesteine rounded fluvial pebbles including carbonates (possibly from Inn valley)

Declaration

I hereby declare that I am the sole author and composer of my thesis and that no other sources or learning aids than those listed have been used. Furthermore, I declare that I have acknowledged the work of others by providing detailed references of said work. I hereby also declare that my thesis has not been prepared for another examination or assignment, either wholly or excerpts thereof.

______Place, date Signed