Structures at the Distal Limit of the Tschirgant Rock Avalanche, Tyrol, Austria
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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. i 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. ii 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 iv 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