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.
KeywordsRock avalanche Facies Fragmentation Emplacement processes
Rock avalanches are large (>106 m3) highly mobile landslides that result from the sudden failure of rockslopes. The initially relatively intact rock mass is broken during fall and runout, and highly fragmented debris is created and spread over square kilometres within minutes with travel velocities of several 10 m/s. Much effort has focused on elucidating their initiation and emplacement mechanisms (see e.g. Legros 2002) as they often travel much farther than expected. The last decade has seen an increase in sedimentological studies of their deposits (e.g. Crosta et al. 2007; Dunning and Armitage 2011; Weidinger et al. 2014; Dufresne et al. 2016a, 2016b; Zhang et al. 2016). These studies show that there is no average grain size distribution (GSD) that could characterise any given deposit meaningfully as a whole (i.e. for inter-deposit comparison) due to sampling limitations and deposit heterogeneity. Instead, even within small outcrops, a wide span of GSDs and clast arrangements are observed, and differences in the amount of fines from different lithologies seem apparent. What is common and generally scale-independent is that the main interior of deposits with significant runout are composed of fragmented debris (i.e. broken into fragments), and fragmented but relatively undisaggregated clasts—the oft-cited jigsaw puzzle clasts—with the component parts usually angular to very angular (see Weidinger et al. 2014 and references therein). Source stratigraphy is retained despite long runout distances (e.g. Heim 1932; Yarnold and Lombard 1989; Weidinger et al. 2014), and shear bands, faults and block-in-matrix fabrics are common features (e.g. Friedmann 1997; Crosta et al. 2007; Davies and McSaveney 2009; Weidinger et al. 2014; Dufresne et al. 2016b). Three main depositional facies are commonly recognised (see below). However, sedimentological analyses are often based on “bulk” sampling of particular locations regardless of the local, sometimes subtle at outcrop-scale, variations in sedimentology. The aims of those studies were to either characterise a specific deposit GSD for comparison with other deposits or to find trends of e.g. grain size reduction with depth and distance (e.g. Val Pola, Italy, Crosta et al. 2007; Daguangbao, China, Huang et al. 2012; Taranaki, New Zealand, Roverato et al. 2015). During bulk sampling, materials from different facies are collected in one sample, and the detailed information sought in this study are lost as a consequence. Hence, for the purpose of finding information on specific processes responsible for differences in GSD, an alternative approach is proposed herein.
“Facies (Latin ‘aspect’ or ‘appearance’ of something) refers to a body of sediment with a distinctive combination of properties that distinguish it from a neighbouring sediment” (Walker 1992 as cited in Evans and Benn 2004; see also Reading 2009). Variations in GSD across the different depositional facies of rock avalanches and rockslides should reflect specific emplacement processes, such as (1) simple fracturing along pre-existing zones of weakness, (2) distributed dynamic fragmentation throughout the moving mass beneath some threshold overburden thickness (McSaveney and Davies 2007), or (3) strain localisation in narrow shear zones, either at the base or in ephemeral layers throughout the body, which could be “inconspicuous” (Davies and McSaveney 2009) in the field. Herein, we apply detailed mapping of rock avalanche sedimentology on the outcrop-scale to identify the different depositional facies before sampling for GSD analyses. It is these details that are needed in order to understand the variations in grain size reduction and hence the processes that are acting within the fragmenting granular mass during rock avalanche emplacement. Therefore, we suggest that sampling for process understanding should be based on prior facies mapping.
The Tschirgant and Flims deposits are amongst the largest rockslide-rock avalanche deposits in the European Alps (Fig. 1). Both originated from carbonate rockslopes, which is the most common source lithology of large, rapid landslides in this mountain belt (Abele 1974). Slope collapse with a drop height of 1400 m of the Tschirgant ridge 3 ka ago (Ostermann et al. 2016) deposited between 180 and 250 mill/m3 (Abele 1974; Pagliarini 2008; Patzelt 2012) of highly comminuted debris over 9.8 km2, with a runout of at least 6.8 km (Patzelt 2012). Its deposit contains both linear rocksliding and radial rock avalanche spreading components (Dufresne et al. 2016a). Flims, with a deposit volume of 8–12 km3 (Heim 1932; von Poschinger et al. 2006), is substantially larger than Tschirgant and dominated by rocksliding emplacement. Its total drop height is 1100 m, covering an area of ∼52 km2 (Heim 1932; Dunning 2004; von Poschinger et al. 2006). Sedimentological investigations of both deposits show that they are multi-facies deposits. We present the facies below.
Sampling was based on detailed facies mapping, adapting procedures of Glicken (1996): 1-m2 outcrop areas (mapping windows) were cleaned of slopewash, talus and weathered material, made as planar as possible, then sprayed with water to enhance contrasts and documented by sketches and photographs. Locations of mapping windows were chosen in the carbonate deposit interior to focus on rock avalanche processes only (e.g. avoiding basal zones where mixing with substrates substantially alters debris composition and properties). Each sample was taken from within a specific facies avoiding boundaries with other facies so as to not “contaminate” the sample with material from another facies. A standard phi sieve (16-mm to 63-μm diameter plus receiver pan) tower of woven wire mesh sieves (Retsch) was used and dry-sieving performed using a vibrating table for 10 min. Manual end-point tests of each size fraction were performed: Each sieve was held, still above the next smaller one, at a slightly inclined position and tapped with a metal rod, turned through 90°, tapped, etc. until less than 0.1% of the charge passes through the sieve. Without these endpoint tests, as much as 45% of the material, <63 μm will remain (predominantly) within the 63–250-μm fractions. Sieving was followed by laser diffractometry of material below 250 μm for complete grain size distribution analyses. The laser-sizer results were binned following the phi-scale and integrated with the sieve results. The discrepancy between the two methods lies within only 2–3% (e.g. Beuselinck et al. 1998). Using GSD plots of the equivalent number of grains calculated from the sieve aperture and a density estimate against mean grain size in the Phi interval (after Hooke and Iverson 1995), samples with a heavy-tailed distribution were analysed for self-similar (power-law) behaviour. The statistical methods of Clauset et al. (2009) and Gillespie (2015) were used to assess the significance of the power-law fit, and the size range a fit was valid over. Maximum likelihood estimators were used to determine the values over which behaviour was determined to be self-similar, whilst the goodness of fit was estimated via a Kolmogorov-Smirnov statistic. The result of interest from this procedure here is the fractal dimension, D, the scaling exponent in the power-law relationship.
Rock avalanche facies and grain size distribution
Final rock avalanche deposit fabric and GSD are expected to be heterogeneous since “the state of stress in a deforming granular medium in which grain bridges are continuously forming and breaking is clearly heterogeneous” (Hooke and Iverson 1995, p. 57); yet some common features were found between deposits. Observations of rock avalanche exposures have led to the general consensus on three main deposit facies (Crosta et al. 2007; Dunning and Armitage 2011; Weidinger et al. 2014; Dufresne et al. 2016b): (1) the carapace—an open network of large blocks armouring the deposit surface; (2) the body facies, which makes up the main interior and shows diverse fabrics; and (3) a basal facies in contact, and often mixed or interleaved, with runout path material. The boundary between the basal and the body facies is usually sharp, but between the carapace and the body, there is a transitional sub-facies that we have mapped, with clasts still surviving at sizes in excess of the spacing of natural fracture planes or defects—perhaps an explanation for observations of crude inverse grading at outcrop scale exposures (Hungr and Evans 2004).
The coarsest sub-facies is the “jigsaw-fractured facies” (Fig. 2a) consisting of angular to very angular clasts up to several cm in size with jigsaw-fit arrangement of individual fragments. Isolated jigsaw-fractured clasts are common in rock and debris avalanche deposits (e.g. Brideau and Procter 2015, and references therein) and are sometimes considered as characteristic and discriminatory for identification purposes. Breaking is usually restricted to failure along pre-existing, inherent, rock-type-specific planes, and their GSD is hence strongly coarse-skewed, with a minor tail of fines that likely originates in the matrix infilling between fracture boundaries (Fig. 2b).
A finer, fragmented facies (Fig. 2c) makes up most of the deposit interior and can be addressed as the “typical” rock avalanche fabric. Comminution transgressing inherent failure planes of the intact rock creates additional surfaces. This facies contains isolated remnant jigsaw-fractured clasts, intact survivor clasts of much larger size than the surrounding majority of fragmented clasts, and clasts with distinctive radial fracturing suggestive of failure originating at point contacts. Grains in the fragmented facies, evidenced by their increasingly more irregular shapes and reduced grain sizes, have experienced more deformation than the jigsaw-fractured clasts. The grain size curve is closer to a bell-shape, as finer materials are relatively more abundant than in the jigsaw-fractured facies, and there are relatively fewer coarser clasts (Fig. 2d).
Comparing the GSD curves of Tschirgant and Flims (Fig. 4a, b) with data from granular shear experiments (Fig. 4c, d) supports the idea of increased grain crushing across our three observed sub-facies. The jigsaw-fractured facies is closest to a virtual “initial distribution” (i.e. closer to joint spacing at the source), the fragmented facies results from higher shear strain and/or repeated crushing, and finally, shear bands experience the highest degree of fragmentation. After prolonged shearing, i.e. larger shear strain, or at increasing confining pressure, an “ultimate grain size distribution” (Einav 2007) or “practical maximum density curve” (Lade et al. 1996) might be approximated (Fig. 4c, d). What this ultimate distribution may look like for a rock avalanche is unknown, if it exists at all. At Tschirgant and Flims, there is still evidence of “incomplete” fragmentation at all sample localities, with fractal dimensions remaining well below the theoretical computation by Crosta et al. (2007) of an ultimate distribution with a fractal dimension (D) of 3.0 (Fig. 4e).
In all cases, our data do fit a power-law distribution when using a linear fit to log-transformed data (Fig. 5a) with R2 values in excess of 0.9 and with D-values always higher than those estimated using maximum-likelihood analyses—where the fit was not deemed significant. Despite this being perhaps less rigorous, we report them here as comparative values of increasing comminution intensity (Storti et al. 2007) or damage (Nakata et al. 2001), and to compare our values with those previously reported in the literature for rock avalanche deposits, experiments, and theoretical computations. This shift from rejection of a power-law fit to these data, to acceptance for all samples with a high goodness-of-fit value, questions the validity of the fractal nature, specific D-values or fractal ranges for many rock avalanche GSDs published, and some of the interpretations linked to this where D-values have theoretical links, e.g. to fragmentation probabilities.
A D-value of 2.58 theoretically approximates an in situ packing arrangement that maximises coordination number and so minimises the probabilities of fragmentation, since clasts of similar size are effectively cushioned from each other by a range of particle sizes surrounding them (Sammis et al. 1987)—hence the longevity of survivor clasts. The D-values in our deposits approximate those of Storti et al. (2007) for brecciated fault rocks (dashed vertical grey lines in Fig. 6), and our shear band values overlap with those of shear zones in fault rocks (solid vertical grey lines in Fig. 6); there is a close similarity of processes between tectonic fault zones, rock avalanches and experimental shearing/crushing that crosses several orders of magnitude.
Since the three sub-facies identified herein can be found in all locations confirms that fragmentation is on-going throughout runout (previously alluded to by Davies et al. 1999; Hewitt 2002); this has emplacement implications. The moving mass is not passively travelling over a basal shear plane, but rather, shear is distributed throughout the mass for the duration of motion, often localising in some narrow zones—our shear facies. Therefore, pure basal sliding (on e.g. melt, nanoparticles, air, or fluidised substrate) alone does not explain long runout. Any emplacement hypothesis that aims at not only predicting runout distances based on stress/friction parameter variation, but which also attempts to explain the underlying mechanical processes, must include heterogeneous stress distributions and continuing fragmentation throughout most of the flow length, thickness and emplacement duration. A number of hypotheses to explain long runout have been proposed and include (1) dynamic fragmentation (Davies 1982; Davies and McSaveney 2009), (2) acoustic fluidization (Melosh 1979), (3) pressure variations (Johnson et al. 2016), (4) multi-slide plug flow (Roverato et al. 2015), (5) plug or viscous flow (Voight et al. 1983; Kelfoun and Druitt 2005), (6) a lubricating basal layer (Campbell 1989) or (7) undrained loading of the underlying saturated sediments (Hutchinson and Bhandari 1971). These hypotheses all agree with preservation of stratigraphy in the deposits since none evokes turbulence within the granular flow. Formation of a carapace above a deforming granular body is also agreeable with all of them. Some of the hypotheses, however, require reduction of frictional resistance in a basal layer (5, 6, 7); hence, all observed basal deformation, including substrate erosion, entrainment and mixing, is accounted for. But, also those hypotheses without focus on basal layer deformation do not oppose explanation of these phenomena, simply based on the boundary dynamics of granular flows over deformable substrates.
Explanations of the internal granular facies as described in this paper might limit some of the hypotheses. Jigsaw-fractured clasts require differential stresses and pressures that are low enough and not in disagreement with most models of a dynamically deforming granular body. Processes to produce the fragmented facies, however, require higher stresses and pressures best explained by the dynamic fragmentation model. Acoustic fluidization alone does neither require nor cause dynamic particle breakage, nor would basal layer processes necessarily induce stresses within the overlying granular mass that are high enough for particles to break along newly formed surfaces. The most important observation in our facies model is that shear is not restricted to a basal layer but distributed throughout the entire rock avalanche body (beneath sufficient overburden, thereby exempting the carapace, naturally). This is supported by the dynamic fragmentation hypothesis (Davies 1982; Davies and McSaveney 2009), the pressure variations modelled by Johnson et al. (2016), acoustic fluidisation (Melosh 1979), and is observed in the more empirical multi-slide plug flow model (Roverato et al. 2015; an extension of the plug-flow model).
We can conclusively exclude exotic processes (such as air-layer lubrication) since they do not explain the observed sedimentological features. Likewise, sliding on a thin film of melt is not a viable candidate to explain long runout since only few RS/RAs contain frictionites (e.g. Erismann 1979) and the films are too thin and viscous to support rapid motion of a large overburden. Furthermore, observations at the Köfels rockslide in Austria suggest that frictionites may simply be an expression of shear concentration since they are found in extension of fine-grained shear zones within the deposit.
The surface of thick rock avalanches, on the other hand, tells us nothing of the interior arrangement of facies. It rather indicates the degree and geometry of spreading of the debris (e.g. Dufresne et al. 2016a).
This present study systematically addressed uncertainties regarding sampling and GSD analyses of rock avalanche deposits. Our results particularly emphasise a need for improved sampling strategies if information about fragmentation processes and spatial distribution of grain size changes within the RA is sought. Therefore, sampling strategies should be based on prior facies mapping if we truly want to understand the processes underlying rock avalanche emplacement. Each facies produces a unique grain size distribution, and their histograms can serve as a tool for facies/process identification and for sensible comparison between deposits and lithologies. Our results support a small number of emplacement theories and rule out any exotic explanation for the long runout of large rock avalanches; continued, heterogeneous degrees of fragmentation of an initially relatively intact source rock best explains the sedimentary record of rock avalanches. They furthermore serve to refine numerical models (e.g. heterogeneous stress distribution as input parameter) and, as Hutchinson (2006) generally stated about the value of sedimentological studies of rock avalanches, to “counteract any tendency for modelling to run ahead of field observations”.
This research was partially funded by the German Research Foundation grant DU1294/2-1 to AD. We gratefully acknowledge thorough review by the editor Mauri McSaveney and by two anonymous reviewers. AD is indebted to Christoph Prager for abseiling during sample collection at Tschirgant.
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