Anja Dufresne

Process dependence of grain size distributions in rock avalanche deposits

Rock avalanches are a form of hazardous long-runout landslide and leave fragmented deposits of complex sedimentology that, if studied in detail, can provide insight into their emplacement processes. Complexity arises due to the myriad overlapping factors known to contribute to the final deposit fabric, such as source structures, lithology (i.e. material properties), topographic feedback, substrate interaction and emplacement processes (i.e. internal factors), as well as our reliance on (un)suitable exposures. Herein, we present sedimentological data from two carbonate rock avalanche deposits (Tschirgant in Austria and Flims in Switzerland), where changes in lithology can be eliminated from the causal equation due to their largely mono-mineralic composition. We further eliminated the effects of external influences such as topography or substrate interactions by detailed facies mapping of the deposit interior. Since sedimentary properties locally vary within less than 1-m2 outcrop area, emplacement processes are the only causes that remain to explain the different fabrics. Characteristic (fractal) grain size distributions of three distinctive sub-facies in the interior of these, and other, rock avalanche deposits—jigsaw-fractured, fragmented, and shear zone facies—can be linked to specific processes acting during emplacement. We suggest that a heterogeneously distributed and progressively increasing particle breakage in the moving granular mass best explains the ranges of fractal dimensions and associated features for the respective sub-facies, from simple breakage along pre-existing planes, through dynamic fragmentation which locally minimises coordination number, to zones of shear concentration. No exotic emplacement mechanisms (such as air-layer lubrication or fluidised substrates) are required to produce these features; continued, heterogeneous degrees of fragmentation of an initially intact source rock best explains the sedimentary record of rock avalanches.

Rock Avalanche Sedimentology—Recent Progress

Since Yarnold and Lombard (Field trip guidebook—Pacific section, 9–31, 1989) presented a systematic facies model for ancient rock avalanche deposits in dry climates, more landslide researchers have organized observations from one or more case studies into general sedimentological descriptions and facies models (references are provided in the main text). These recent advances show that rock avalanches are multi-facies deposits. Retention of source stratigraphy and a general three-part division of a coarse-grained, largely unfragmented upper part or carapace, a finer-grained body of diverse sedimentology, and a basal facies influenced by interactions with runout path materials are the most common observations. The greatest variation in the grain size distribution and comminution intensity occurs between the bouldery carapace and the matrix-supported interior, i.e. the body facies which constitutes the largest deposit volume. Most striking, but not surprising, is the highly heterogeneous nature of the body facies with a number of sub-facies and discontinuity layers, which must reflect highly heterogeneous states of stress within the deforming granular mass. These features within the body facies are the most important for studying those emplacement dynamics that are not affected by boundary conditions, such as runout path sediments. Where the base is exposed, a characteristic basal facies with substrate injections and/or a basal mixed zone and/or deformation features can be found, usually above a very sharp contact to the underlying, disrupted sediments. The overall commonalities of internal rock avalanche features indicate that some basic processes must act universally during their emplacement. The value of these sedimentological models and descriptions lies in contrasting universally valid features with those that are a function of unique geological, topographic, or structural settings, or which might suggest different/additional emplacement dynamics of a specific deposit.

Complex Interactions of Rock Avalanche Emplacement with Fluvial Sediments: Field Structures at the Tschirgant Deposit, Austria

The Tschirgant rock avalanche in Tyrol, Austria, produced a complex deposit 200–250 × 106 m3 in volume and 9.8 km2 in area. The landslide resulted from deep-seated failure of an intensely deformed carbonate rock mass on the southeast face of a 2370 m-high ridge. The rock mass rapidly fragmented as it moved towards the floor of the Inn River valley. Part of the debris collided with and moved around an opposing bedrock ridge and flowed into the Oetz valley, reaching up to 6.3 km from source. A large volume of mobilized sedimentary gravels and sands occurs beneath, within, and atop the rock avalanche deposit. Within proximal inter-hummock depressions, entrained gravels and sands extend to the deposit surface; elsewhere they are intercalated in narrow bands that dip towards and into flow direction. Some of these sediments were liquefied and mobilized en route, whereas others were most likely inherited from the source area. None of them shows signs of fragmentation, suggesting different mechanical behaviour or low fragmentation pressures at the time of entrainment. Generally, exposures of the basal contact of large rock avalanche deposits are rare, but at Tschirgant they are well exposed and reveal substrate injection features (some > 10 m across), thrust and normal faults, entrained sand and gravel rip-up clasts, corrugated basal shear contacts, and disturbed underlying material. Mixing of rock avalanche and substrate material is only observed at the distal margin, suggesting longer travel distances or particular material properties to allow mixing to take place. Ongoing research focuses on these substrate interaction features to reveal details of rock avalanche movement, flow paths, and emplacement.

An overview of rock avalanche-substrate interactions

Large rock or debris avalanches inevitably encounter and interact with a variety of earth materials along their paths. These substrate materials influence rock and debris avalanche emplacement in one or several of the following ways (1) longer runout due to an increase in volume by entrainment on the steep failure slope, (2) higher mobility by reduction in basal frictional resistance (e.g. emplacement over glacier ice), or (3) a larger area of deposition due to transformation into debris flows, contrasted by (4) runout impediment due to interactions along the flatter runout path (e.g. bulldozing of substrate material or entrainment of high-friction debris), and introducing (5) flow complexities resulting from changes in basal mechanical properties and other localized interactions. Additionally, the total area affected by a rock avalanche may extend beyond the deposit margin itself when sediments in front of the rock avalanche are bulldozed or are mobilized and flow independent of the rock avalanche for some further distance.

The 2015 landslide and tsunami in Taan Fiord, Alaska

Glacial retreat in recent decades has exposed unstable slopes and allowed deep water to extend beneath some of those slopes. Slope failure at the terminus of Tyndall Glacier on 17 October 2015 sent 180 million tons of rock into Taan Fiord, Alaska. The resulting tsunami reached elevations as high as 193 m, one of the highest tsunami runups ever documented worldwide. Precursory deformation began decades before failure, and the event left a distinct sedimentary record, showing that geologic evidence can help understand past occurrences of similar events, and might provide forewarning. The event was detected within hours through automated seismological techniques, which also estimated the mass and direction of the slide - all of which were later confirmed by remote sensing. Our field observations provide a benchmark for modeling landslide and tsunami hazards. Inverse and forward modeling can provide the framework of a detailed understanding of the geologic and hazards implications of similar events. Our results call attention to an indirect effect of climate change that is increasing the frequency and magnitude of natural hazards near glaciated mountains.