A Quantitative Assessment of the Sedimentology
and Geomorphology of Rock Avalanche Deposits
Dan H. Shugar, John J. Clague, and Marco Giardino
Abstract
We use digital photo-sieving and spatial statistics to quantify the debris of three landslides
on Black Rapids Glacier, Alaska, and the non-glacial Frank Slide, Alberta. The debris
sheets on Black Rapids Glacier have clusters of large blocks in parts of their distal rims;
small clusters of large blocks also occur elsewhere, including the proximal side of a high
medial moraine. Longitudinal flowbands formed by shearing within the debris and marked
by different block sizes characterize all three Black Rapids debris sheets. In contrast,
no flowbands are evident on the Frank Slide debris sheet. Especially large blocks form
a conspicuous cluster in the middle of the Frank Slide debris sheet. The distal edge is
composed of small blocks. The presence of many of the largest blocks at the peripheries of
the three Black Rapids Glacier debris sheets indicates that the landslides spread without
confinement. The lack of a coarse distal rim at Frank may indicate that the irregular
topography over which the debris traveled influenced the distribution of the largest blocks.
Patches of different types of carbonate rock within the Frank Slide debris sheet indicate
that source-zone stratigraphy is preserved within the debris sheet. Differences among the
studied debris sheets reflect different paths and substrates over which the landslides
traveled: unconfined spreading and continuous, progressive thinning of debris traversing
a relatively flat surface of snow and ice at Black Rapids Glacier; and topography-controlled
spreading over an irregular rising and vegetated surface at Frank.
Keywords
Rock avalanche Glacier Sedimentology Geomatics Frank Slide Black Rapids Glacier
Introduction
Many researchers have qualitatively described geomorphic
features of rock avalanche deposits, including raised rims,
flowbands, and lithologic zonation. Little quantitative work,
however, has been done to link debris sheet sedimentology
D.H. Shugar (*) J.J. Clague
Department of Earth Sciences, Centre for Natural Hazard Research,
Simon Fraser University, Burnaby, BC, Canada
e-mail: [email protected]
M. Giardino
Department of Earth Sciences, University of Torino, Torino, Italy
to observed large-scale geormorphic features. A thorough
characterization of rock avalanche debris is a necessary step
in understanding the flow mechanisms of large landslides.
Rock avalanche sedimentology is also important because
diamictons, previously thought to be glacial in origin, have
been reinterpreted as landslide debris with little or no
climatic significance (Porter and Orombelli 1980; Hewitt
1999; Larsen et al. 2005; Shulmeister et al. 2009).
In this paper we describe the sedimentology of four
rock avalanche debris sheets and evaluate whether runout
over glacier ice affects their characteristics. We studied
deposits of three rock avalanches on Black Rapids Glacier,
Alaska, and the deposit of the non-glacial Frank Slide,
Alberta.
C. Margottini et al. (eds.), Landslide Science and Practice, Vol. 4,
DOI 10.1007/978-3-642-31337-0_41, # Springer-Verlag Berlin Heidelberg 2013
321
322
D.H. Shugar et al.
Methods
Fig. 1 Locations of Black Rapids Glacier, AK, and Frank Slide, AB,
and photographs of the landslide debris sheets
Study Areas
Black Rapids Glacier is a 40-km-long, surge-type glacier in
the central Alaska Range of interior Alaska (Fig. 1). It has
a mean slope of 2 and an average width of 2.3 km. The
3 November 2002 Denali earthquake (M 7.9) triggered three
large rock avalanches that deposited ~25 106 m3 of
granitic debris over 11 km2 of the glacier’s ablation zone
to an average depth of 2–3 m (Shugar and Clague in press).
Slabs of rock 30–50 m thick broke along orthogonal joints
and rapidly fragmented as they moved down the 35–38
slopes and onto Black Rapids Glacier (Jibson et al. 2004).
We refer to the deposits of the three landslides as BRG-west,
BRG-middle, and BRG-east.
The Frank Slide is located on the east slope of Turtle
Mountain in the Crowsnest Pass region of southwest Alberta
(Cruden and Hungr 1986). The landslide occurred on the
morning of 29 April 1903 and destroyed part of the mining
town of Frank, killing about 70 people; it was Canada’s
worst landslide disaster. The debris sheet has a volume of
~37 106 m3 (Nicoletti and Sorriso-Valvo 1991), an area
of ~2.7 km2 (McConnell and Brock 1904), and an average
thickness of 14 m (Cruden and Hungr 1986). It consists mainly
of limestone of Paleozoic age (Langenberg et al. 2007).
We digitally photo-sieved blocks at the surface of the debris
sheets on orthorectified aerial photographs. Photo-sieving is
a method of grain-size analysis of coarse sediment, in which
the outline of each clast is manually traced on photographs
(Ibbeken and Schleyer 1986). Shugar and Clague (in press)
adapted the method for use with digital images in a GIS.
Field surveys were conducted (at Black Rapids Glacier in
2007; at Frank Slide in 2010) to verify the quantitative
photogrammetric assessments and to support sedimentological and geomorphological interpretations.
Vertical aerial photographs of the Black Rapids Glacier
landslides were flown on 7 September 2004. We produced
a 10-m DEM and a 0.25-m orthophoto mosaic from the
photographs. For Frank Slide, we used a 0.25-m orthophoto
flown in 2002 and provided by the Geological Survey of
Canada.
Individual blocks 1 m2 were manually digitized in
ArcGIS. More than 194,000 blocks were digitized at Black
Rapids Glacier, and more than 69,000 blocks were digitized
at Frank Slide. Block lengths (a-axis) and widths (b-axis)
were determined using the Bounding Containers ArcGIS
toolbox (Patterson 2008). We analysed results using
neighborhood analysis, which allows calculation of an
output value for each non-overlapping neighborhood.
Median (D50) and maximum (a-axis) block sizes and a-axis
standard deviation were calculated for 25 25 m
neighborhoods.
Results
Deposit Geometry and Physical Characteristics
The three Black Rapids Glacier landslides originated on
steep, north-facing rock slopes on the south side of the valley
occupied by the glacier. The landslides flowed across the
glacier, overtopping a medial moraine up to ~25 m high
(Table 1). Conspicuous longitudinal stripes, or flowbands
(sensu Dufresne and Davies 2009), reveal the flow direction
of the landslides (Fig. 2).
BRG-west had the simplest deposit geometry – the debris
traveled directly across the glacier and came to rest at its
northern margin, 3.4 km from the source. BRG-middle
initially traveled directly across the glacier, but on reaching
the distal side, changed course and traveled downglacier to
the east. The total runout distance is 5.6 km. Flowbands at
BRG-east indicate that some of the debris traveled directly
across the glacier and then, as at BRG-middle, turned to the
east. Much of the debris however, took a more direct route,
traveling directly 4.1 km to the northeast.
A Quantitative Assessment of the Sedimentology and Geomorphology of Rock. . .
323
Table 1 Characteristics of the four rock avalanches
Name
BRGwest
BRGmiddle
BRGeast
Frank
Slide
Height
(km)
0.73
Distance
(km)a
3.4
Length
(km)b
3.4
Width
(km)c
1.0
Deposit
thickness (m)
2–3
Area
(km2)
2.5
H/L
0.21
Fahr.
( )
12.1
Le
(km)d
2.3
0.80
3.0
5.6
1.5
2–3
3.9
0.14
8.1
4.4
0.98
3.6
4.1
1.4
2–3
4.7
0.24
13.4
2.6
0.80
3.3
3.3
1.3
14
2.7
0.24
13.6
2.1
Dimensionless
spreadinge
1,430/
300 ¼ 4.8
3,050/
400 ¼ 7.6
3,420/
400 ¼ 8.6
2,150/
700 ¼ 3.1
Volume
(m3)f
4.9–7.4
7.7–11.6
9.3–14.0
36.5
a
Distance measured from the highest point of the head scarp to the highest point of the deposit at the distal edge of the landslide
Total travel distance measured along the centreline of the rock avalanche
c
Width measured on orthophotos at half the runout distance
d
Excess travel distance, Le ¼ L H/tan(32 )
e
The quotient of maximum deposit width and initial width. Initial (scar) width measured on Google Earth
f
Volumetric calculations differ widely between studies. Volume is here calculated as the product of deposit area and average thickness
b
The Frank Slide, which became airborne as it crossed the
Old Man River, flowed 3.3 km across the floor of the valley,
spreading laterally to a maximum width of 2.1 km on a rising
topographic slope. The debris sheet covers 2.7 km2 and has
an average thickness of 14 m. Exposures along the railway
right-of-way were studied by Cruden and Hungr (1986). The
debris sheet lacks the longitudinal stripes that are common at
Black Rapids Glacier; instead it has an irregular hummocky
surface. The distal part of the debris sheet, termed the
“splash zone” by McConnell and Brock (1904), consists
mainly of mud and sand with scattered blocks (Cruden and
Hungr 1986).
Excess travel distance, defined by Hsü (1975) as the horizontal projection of the travel distance beyond what one
expects of a rigid mass sliding down an inclined plane with a
normal coefficient of friction (Le ¼ L H/tan(32 )), ranges
from 2.3 to 4.4 km for the Black Rapids Glacier landslides, and
2.1 km for Frank Slide. The three glacier landslides have
fahrböschungs ranging from 8.1 to 13.4 , whereas the Frank
Slide fahrböschung is 13.6 . Dimensionless spreading indices
(maximum slide width divided by initial width) for the Black
Rapids Glacier landslides range from 4.8 to 8.6; the index for
Frank Slide is only 2.1 (Table 1).
Fig. 2 Photographs of rock avalanche debris on Black Rapids Glacier.
(a) Aerial photograph of the northwest corner of BRG-middle, showing
longitudinal flowbands (arrows) turning to the east near the glacier
margin. (b) Large jigsaw brecciated block on BRG-west. (Modified
from Shugar and Clague in press)
The surface debris of the Black Rapids Glacier landslides
is composed of coarse angular blocks. So-called “jigsaw”
blocks of brecciated, but otherwise intact bedrock are
common within the middle of the debris sheets. The coarse
carapace of the debris sheet overlies a massive deposit of
finer, matrix-supported muddy sandy blocky debris.
Block Patterns
Block-size patterns of the four landslide debris sheets,
although not identical, have some similarities (Figs. 3, 4,
and 5). All three Black Rapids debris sheets have clusters
of large blocks in their distal rims, although clusters of
large blocks also exist elsewhere. Many 25 25 m
neighborhoods in the distal rim of BRG-west have blocks
ranging from 7 to 16 m in length (Fig. 3b). However, the
largest block on BRG-west, which is 26 m long, is located
near the middle of the debris sheet. And most of the >20
m-long blocks are found near the middle of BRG-middle.
324
D.H. Shugar et al.
Fig. 3 Maps of (a) maximum block size, (b) median block size, and
(c) standard deviation of block length per 25 25 m neighbourhood
for the Black Rapids landslides
Most neighborhoods on BRG-west have maximum block
sizes <5 m long. The neighborhood median block size
ranges from 2 to about 9 m. The area of relatively high
D50 in the proximal, southeast corner of the debris sheet
has relatively low block density compared with other areas.
Thus, the high median values here should be viewed with
caution.
Large blocks similarly characterize the distal parts of the
BRG-east debris sheet, although most are smaller than at
BRG-west – mainly 4–6 m long, with some up to 10 m in
length. The largest block at BRG-east is 23 m long and is
located within a train of very large blocks (>12 m) in the
distal half of the debris sheet. Elsewhere, maximum neighborhood block length is typically ~4 m. Median block size
on BRG-east is consistent across the debris sheet; most
neighborhoods have a D50 of ~2 m.
The typical maximum neighborhood block size on
BRG-middle is larger than at either BRG-west or BRGeast (e.g. Fig. 5). Blocks 6–10 m in length cover much of
the debris sheet. Narrow flowbands of finer debris separate
Fig. 4 Maps of (a) maximum block size, (b) median block size, and
(c) standard deviation of block length per 25 25 m neighbourhood
for the Frank Slide
areas of large blocks, and longitudinal stripes of very large
blocks (>10 m) exist locally. The largest block (37 m) is
located in the distal, northwest corner where much debris
stalled as the landslide decelerated and changed direction.
Maps of neighborhood median and maximum block
length at Frank Slide are shown in Fig. 4. The proximal
region of the debris sheet, south of Highway 3, is
characterized by neighborhood maximum block sizes ranging from 4 to 10 m, with isolated larger blocks. A large
cluster of very large blocks, >10 m in length, occurs in the
A Quantitative Assessment of the Sedimentology and Geomorphology of Rock. . .
Fig. 5 Block size distribution for the four debris sheets
middle of the debris sheet and is bisected by Highway 3 and
the railway; the largest block (24 m long) is located here.
The distal parts of the debris sheet are composed almost
entirely of smaller blocks, generally <5 m. The map of
25 25 m neighborhood median size (Fig. 4b) shows little
difference in d50 values over much of the debris sheet; the
typical value is ~2 m.
The Frank Slide debris sheet is also characterized by
lithological differences. South of the highway, most surface
blocks are massive to fractured fine- to coarse-grained
limestone ranging from grey to black in color. Dissolution
features along fractures on these blocks are especially
common on smaller blocks located in topographic lows.
Within the cluster of large blocks along the highway, blocks
are mainly massive, light-gray crystalline limestone, with
distinctive striped faces. Coarse-grained fossiliferous limestone blocks are also common. Decreasing block size and
darker limestones characterize a transition zone towards
the distal parts of the debris sheet. Elongated patches of
chert-banded limestone blocks are present along the western
edge of the debris sheet.
Discussion
The Frank Slide and BRG-west debris sheets are similar in
morphology, although Frank Slide debris sheet is much
thicker than the BRG debris (ca. 14 vs. 2 m). The Frank
Slide debris sheet also has a smaller spreading index
(Table 1), which suggests lower mobility.
The surfaces of the landslide debris sheets on Black
Rapids Glacier are characterized by conspicuous flowbands,
which Dufresne and Davies (2009) argue are fundamental
characteristics of granular flows on glaciers. Similar features
on other glaciers separate bands of different lithologies
325
(e.g. Shreve 1968). At Black Rapids Glacier, the flowbands
record differences in grain size rather than lithology. The
flowbands on BRG-middle and BRG-east match differences
in block size (e.g. compare Figs. 2a and 3a).
No flowbands are evident at Frank Slide, and there is not a
distal rim of large blocks, as at Black Rapids. McConnell
and Brock (1904) reported a prominent distal rim, 2–9 m
high at Frank Slide, but Cruden and Hungr (1986) found
only three sites where large blocks are exposed at the
perimeter. More commonly, they observed a digitate scarp,
bordered by a 100-m-wide hummocky deposit of fine
sediments and scattered blocks (“splash” area of McConnell
and Brock 1904). Similarly, our analysis does not reveal a
cluster of particularly large blocks at the periphery of the
Frank Slide. The rim described by McConnell and Brock
(1904) is composed, not of unusually large blocks, but of a
large number of average- or even smaller-than-average
blocks bulldozed into tall piles by the advancing rock
avalanche.
The cluster of large blocks near the centre of the Frank
Slide debris sheet is puzzling in the context of many other
rock avalanches that have concentrations of large blocks
near their margins (e.g. Locat et al. 2006; Hewitt 1999;
Porter and Orombelli 1980). One possible explanation is
that debris at the front of a rock avalanche flowing over
glacier ice fragments more slowly than debris traveling
over vegetated terrain, such as at Frank. It is also possible
that the irregular topography over which the Frank Slide
traveled influenced the distribution of the larger blocks.
The debris at Frank climbed 100 m over a distance of
1.5 km east of Oldman River; in contrast the Black Rapids
Glacier landslides traveled over nearly flat surfaces. Interestingly, at both Black Rapids Glacier and Frank, areas
of especially large blocks are also characterized by large
standard deviations (Figs. 3c and 4c), implying that large
blocks are mixed with much smaller blocks.
Conclusions
We use digital photo-sieving to quantify the block size
distribution and map large-scale geomorphic features in
the debris sheets of four historic rock avalanches.
Clusters of large blocks occur within all four debris
sheets, although those at Black Rapids Glacier are most
common in rims near the margins of the debris. Longitudinal flowbands in the Black Rapids Glacier debris sheets
mark differences in block size and are the result of differential shear with the streaming debris. In contrast, the
largest blocks in the Frank Slide debris occur in a cluster
in the middle of the debris sheet; no distal rim or
flowbands are present. Differences in block-size patterns
among the four debris sheets are likely due to differences
in the surfaces over which the rock avalanches traveled.
The lack of a coarse distal rim at Frank may indicate that
326
the irregular topography over which the debris traveled
influenced the distribution of the largest blocks. Patches
of different types of carbonate rock within the Frank Slide
debris sheet suggest that source-zone stratigraphy is preserved within the debris sheet.
Acknowledgments This work was funded through an NSERC
Discovery Grant to Clague and an NSERC-PGS doctoral scholarship,
a GSA Bruce ‘Biff’ Reed research grant, Northern Scientific Training
Program grants and an Arctic Institute of North America Grant-in-Aid to
Shugar. Giardino was supported by ICCS-FEP 2009. EACEA and
HRSDC provided funds through the EU-Canada cooperation project
“geoNatHaz”, whose participants are acknowledged for assistance in
the field in 2010. We thank Steve Sparks (Aero-Metric, Fairbanks,
Alaska) for providing aerial photographs of Black Rapids Glacier, and
the Geological Survey of Canada for providing the orthophotograph of
Frank Slide. Mark Hird-Rutter helped in producing the digital elevation
model of Black Rapids Glacier. Jon Pasher (Environment Canada,
Ottawa, Canada) and Dan Patterson (Carleton University, Ottawa,
Canada) provided assistance with digital photo-sieving.
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