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Large Holocene landslides from Pylon Peak,
southwestern British Columbia
Pierre A. Friele and John J. Clague
Abstract: Mount Meager massif, the northernmost volcano of the Cascade volcanic belt, has been the site of very
large (>107 m3) landslides in the Holocene Epoch. We document two complex landslides at Pylon Peak, one of the
peaks of the Mount Meager massif, about 7900 14C and 3900 14C years ago (about 8700 and 4400 calendar years ago).
Together, the two landslides displaced - 6 × 108 m3 of volcanic rock from the south flank of Pylon Peak into nearby
Meager Creek valley. Each landslide consisted of at least two phases, an early debris flow resulting from failure of
hydrothermally altered pyroclastic rock at mid levels on the mountain and a later rock avalanche from a higher source.
Both debris flows likely traveled down Meager Creek, and preliminary evidence from drilling indicates the 4400-year-old
event traveled down Lillooet River into areas that are now settled and where population density is increasing rapidly.
The mobility of the debris flows was due to the high content of fine, weathered volcanic sediment and the availability
of sufficient water. The causes of the landslides are a wet climate and the presence of weak, hydrothermally altered
volcanic rock containing abundant phreatic water on glacially oversteepened slopes. The landslides may have been triggered
by earthquakes or by upwelling of magma to shallow depths within the volcano. However, they may also have occurred
without specific triggers following extended periods of progressive weakening of the volcanic rocks.
Résumé : Le massif du mont Meager, le volcan le plus septentrional de la ceinture volcanique de Cascade, a été le site
de très gros (>107 m3) glissements de terrain à l’Holocène. Nous documentons deux glissements complexes au sommet
Pylon, l’un des deux sommets du massif du mont Meager, il y a environ 7900 14C et 3900 14C années (soit environ
8700 et 4400 années civiles). Ensemble, les deux glissements de terrain ont déplacé environ 6 × 108 m3 de roches volcaniques
du flanc sud du sommet Pylon dans la vallée du ruisseau Meager avoisinant. Chaque glissement de terrain comprenait
au moins deux phases; en premier, une coulée de débris résultant de la défaillance, à mi-pente sur la montagne, de roches
pyroclastiques ayant subi une altération hydrothermale, puis une avalanche de pierre d’une source plus élevée. Les
deux coulées de débris ont probablement dévalé le ruisseau Meager; des évidences préliminaires de forages indiquent
que l’événement datant de 4400 ans a descendu la rivière Lillooet jusqu’à des endroits qui sont maintenant peuplés et
où la densité de la population s’accroît rapidement. La mobilité des coulées de débris était due à la haute teneur en sédiments
volcaniques, altérés et fins, et à la disponibilité d’une quantité d’eau suffisante. Les causes des glissements sont un climat
humide et la présence, sur les pentes surraidies par la glaciation, de roches volcaniques, incompétentes, qui ont subi une
altération hydrothermale et qui contenaient une grande quantité d’eau phréatique. Les glissements de terrain peuvent
avoir été déclenchés par des tremblements de terre ou par des remontées magmatiques jusqu’à de faibles profondeurs à
l’intérieur du volcan. Ils peuvent toutefois aussi avoir eu lieu sans déclencheurs spécifiques à la suite de longues périodes
d’affaiblissement progressif des roches volcaniques.
[Traduit par la Rédaction]
Friele and Clague
182
Introduction
A chain of active and dormant stratovolcanoes marks the
western fringe of the Cascadia subduction zone in the Pacific
Northwest of the United States and Canada. These volcanoes
represent a significant regional hazard because of their eruptive
potential and because they are underlain by weak, hydrothermally altered volcanic rocks that are susceptible to failure
(Finn et al. 2001). Many large, long-runout landslides originating
on Cascade volcanoes have inundated valleys tens of kilometres
from their sources, and some of the inundated areas now
support large populations (Pierson 1985; Pierson and Scott
1985; Vallance and Scott 1997).
The Cascade volcanic chain in southwestern British Columbia
includes three main volcanic centres: Mount Cayley, Mount
Garibaldi, and Mount Meager (Fig. 1a; Green et al. 1988).
Mountain valleys adjacent to these volcanoes are accessible
by roads and are intensively used for forestry and recreation.
Received 16 January 2003. Accepted 1 October 2003. Published on the NRC Research Press Web site at http://cjes.nrc.ca on
13 February 2004.
Paper handled by Associate Editor C.R. Burn.
Pierre A. Friele.1 Post Office Box 612, Squamish, BC V0N 3G0, Canada.
John J. Clague. Department of Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada and Geological Survey of
Canada, 101 - 605 Robson St., Vancouver, BC V6B 5J3, Canada.
1
Corresponding author (e-mail: friele@telus.net).
Can. J. Earth Sci. 41: 165–182 (2004)
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Fig. 1. (a) Volcanoes of the Cascade volcanic chain in British Columbia: M, Meager; C, Cayley; G, Garibaldi. (b) Geology and place
names in the Meager Creek study area. The contour interval is 100 m.
Several large landslides have occurred on the western flank
of Mount Cayley in historic time (Clague and Souther 1982;
Cruden and Lu 1992; Evans et al. 2001), and small debris
flows (<5 × 105 m3) occur there regularly, disrupting access
to the area. At least nine large landslides from Mount Cayley
blocked Squamish River during the Holocene (Brooks and
Hickin 1991; Evans and Brooks 1991). Floods caused by
failure of the landslide dams, and recurrent, late Holocene
landslides are responsible for channel instability downstream
(Hickin and Sichingabula 1988, 1989; Cruden and Lu 1989).
Mount Garibaldi volcano and nearby vents were active at
the end of the Pleistocene (Mathews 1952; Brooks and Friele
1992). The western flank of Mount Garibaldi erupted partly
on ice and collapsed about 10 600 14C years BP during glacial
retreat (Mathews 1958; Friele and Clague 2002a, 2002b).
Pyroclastic deposits and lavas exposed on the deeply eroded
flanks of the volcano periodically fail, generating large rock
avalanches and debris flows (Moore and Mathews 1978;
Clague et al. 2003).
Mount Meager volcano last erupted about 2400 years ago
(Clague et al. 1995), blanketing nearby areas in pumice and
sending ash as far east as Alberta (Nasmith et al. 1967).
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Table 1. Clay mineralogy of basal units of the two landslide deposits (M3 and M7) and Devastator Assemblage
rock (DA-4, DA-5).
Sample No.
Sample location
Quartz
Plagioclase
Illite
Chlorite
Kaolinite
Smectite
Mixed layer
M3
M7
DA-4
DA-5
Basal unit
Basal unit
Boundary Creek
Devastation Creek
m to tr
m to tr
m
a to m
m
a to m
a to m
a to m
a to m
a to m
m
m to tr
a
a to m
a
a
a to m
a
m
Note: a, abundant; m, minor, tr, trace.
Pyroclastic flows dammed upper Lillooet River, impounding
a lake that later drained catastrophically (Evans 1992; Stasiuk
et al. 1996). A large lahar associated with the eruption traveled
at least 50 km downvalley along Lillooet River (Friele 2002;
Simpson et al. 2003). Numerous large landslides have also
occurred at Mount Meager in the historic period (Carter
1932; Mokievsky-Zubok 1977; Croft 1983; Evans 1987; Jordan
1987, 1994; Jakob 1996; Bovis and Jakob 2000). The high
frequency of landslides in this area led Read (1981) to state
that the Mount Meager massif may be the most active landslide
area in the Canadian Cordillera.
Although much work has been done on modern debris
flow processes at Mount Meager (Jordan 1994; Jakob 1996),
no one has reconstructed the history of prehistoric landslide
activity in the area. This paper describes deposits of two
large postglacial landslides in Meager Creek valley (Fig. 1b)
on the south side of the Mount Meager massif. The deposits
were recognized by Read (1978, 1981) and Jordan (1994),
but they did not accurately map, date, differentiate, or describe
them. In this paper, we describe two distinct landslide deposits
and reconstruct the events that produced them. Our study
contributes to the growing body of literature on geologic
hazards associated with volcanoes in British Columbia and
elsewhere, and provides valuable data on geologic hazards
for land-use decision makers.
Study area
Mount Meager (2650 m above sea level (asl)) is a dissected
Quaternary stratovolcano with up to 2000 m of local relief
(Fig. 1b). It was covered by the Cordilleran ice sheet for several
thousand years at the end of the Pleistocene Epoch until the
ice sheet disappeared about 10 000 14C years ago (Friele and
Clague 2002a, 2002b).
The study area is characterized by a coastal montane climate
with warm summers and cool winters. Temperatures reach
25–35 °C during late July and early August when glacier
and snow melt is greatest. Fall storms produce heavy and often
prolonged rainfall. Precipitation in winter is dominated by
snow, with average snow depths in April of 2–3 m at
mid-elevation sites.
Intrusive rock of the Coast Plutonic Complex and pendants
of metasedimentary rock underlie Quaternary volcanic rocks
at Mount Meager (Woodsworth 1977; Read 1978, 1990).
The basement rocks are extensively jointed and altered (Read
1978), and locally show evidence of deep-seated creep (Bovis
and Evans 1996).
Read (1978, 1990) divided volcanic rocks at Mount Meager
into nine units, but here we describe only those units that are
present within the Meager Creek watershed (Fig. 1b). The
volcanic rocks overlie basement rocks mainly above 1500 m asl,
Fig. 2. Oblique view north to The Devastator and Pylon Peak,
showing the Devastator Assemblage (P1) and lava flows of the
Pylon Assemblage (P3).
but they extend down to 900 m asl south of Pylon Peak and
The Devastator in what may be an exhumed vent (Fig. 3;
Read 1978).
The oldest volcanic rocks include a basal breccia unit
(PL1, Fig. 1b) and a porphyritic dacite unit (PL2, Fig. 1b),
both of Pliocene age. The basal breccia unit is up to 300 m
thick and consists of blocks of granitic, volcanic, and metamorphic rocks up to 20 m long in a tuffaceous matrix (Read
1978). The porphyritic dacite unit has a maximum thickness
of 200 m and comprises subhorizontal lava flows.
The Pleistocene Devastator Assemblage (P1, Fig. 1b) overlies
the Pliocene rocks. It is a yellow-ochre-weathering unit up to
500 m thick comprising intensively hydrothermally altered
rhyodacite tuff, breccia, lava flows, and shallow intrusive
rocks (Read 1990). The Devastator Assemblage contains
abundant smectite (Table 1) and underlies most of the sites
where landslides have occurred and where potential instabilities
exist (Read 1978, 1990).
The Pylon Assemblage (P3, Fig. 1b) overlies the Devastator
Assemblage and consists of up to 850 m of porphyritic andesite
flows separated by thin reddened lenses of breccia and ash.
It forms the summits of Pylon Peak and The Devastator
(Fig. 2).
The exhumed vent near Pylon Peak is drained by Angel
Creek, which heads in a 1500-m-wide, hemispherical,
south-facing scarp (Fig. 2). The scarp crest forms an arête,
with ice-filled cirques to the north and unglaciated landslide
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Can. J. Earth Sci. Vol. 41, 2004
Fig. 3. Plan and stratigraphic cross-sections of the Angel Creek basin and valley-filling landslide units. (a) Geology and geologic features. (b) Cross-section from the headscarp to the toe of the 8700-year-old landslide. The ground water flow model is interpreted from
geothermal drilling results (GeothermEx Inc. 2001). (c) Cross-section of the run-up deposit.
basins to the east and west. Pylon Peak (2481 m asl) and
The Devastator (2327 m asl) form high points on the arête.
Methods
The extent and surface expression of the landslide deposits
in Meager Creek valley were determined through airphoto
analysis and ground traverses. Geologic units were identified
and then plotted on a 1 : 20 000-scale topographic base map
with 20-m contours. Most river and road-cut exposures in the
valley were examined and recorded. Texture, colour, lithology,
and clast angularity were logged for each geologic unit, and
unit contacts were described. Samples were collected and analyzed for matrix texture (sand:silt:clay ratio), clay mineralogy,
and age (14C and 36Cl). Clast lithology counts were made at
some sites.
Geology and geomorphology of the Angel
Creek basin
The landslide deposits described in this paper are derived
from the Angel Creek basin (Figs. 2, 3). The basin has an
area of 2.6 km2 and is underlain by the basal breccia unit
and the Devastator and Pylon assemblages. The volume of
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Friele and Clague
rock missing from the basin is about 4.5 × 108 m3, estimated
by projecting contours across the basin from smooth slopes
at its sides. About 70% (3.2 × 108 m3) of this volume are
Pylon Assemblage rocks, 20% (9 × 107 m3) are Devastator
Assemblage rocks, and 10% (4.5 × 107 m3) are the basal
breccia. The Angel Creek basin is unglaciated, but it supports
a small Neoglacial moraine, demonstrating that no significant
landslides have occurred at the head of the basin since the
end of the Little Ice Age in the late 1800s.
Hotsprings issuing from near 800 m asl in the Angel
Creek basin (Figs. 1b, 3a) indicate the presence of a very
high geothermal gradient in this area. Drilling results suggest
that a geothermal plume is situated directly beneath the vent
(Fig. 3b; GeothermEx Inc. 2001). This plume is probably
responsible for the intense hydrothermal alteration of the
Devastator Assemblage rocks.
Distribution and morphology of landslide
deposits
Landslide deposits derived from the Angel Creek basin
extend along Meager Creek valley from 2 km above the
confluence of Meager and South Meager creeks to at least
Lillooet River (Fig. 1b). The full downstream extent of the
deposits is unknown, as Lillooet River has aggraded its
floodplain during the Holocene (Jordan and Slaymaker 1991),
burying landslide materials derived from the Meager Creek
basin.
The Angel basin landslide materials comprise two morphologically distinct deposits. Each deposit appears to consist
of two distinct units, which we term the basal and surface
units. The basal units are similar in character, although different
in age. The surface units are lithologically similar but sedimentologically distinct.
The older landslide deposit (unit A8; Figs. 1b, 3) has an
undulating to hummocky, inclined surface sloping, on average,
7° to the northwest. Individual hummocks are steep-sided
with up to 20 m of local relief. At the southeast edge of the
deposit, a thin layer of volcanic landslide debris extends
75 m up the slope separating Barr and Hotsprings creeks to a
prominent, 300-m-long, 10-m-wide bench at 910 m asl (Fig. 3).
To the northwest, Meager Creek has deeply incised the deposit,
forming bluffs 100–140 m high. The deposit can be traced
upstream to near Angel Creek. It may have extended farther
upstream but was eroded or is covered by more recent fan
deposits of No Good and Boundary creeks. The deposit has
also been dissected by Hotsprings Creek, and only two remnant
ridges of landslide debris rise above the surface of the
Hotsprings Creek fan. No deposits of the older landslide
have been found east of Hotsprings Creek. The landslide
probably flowed into this area, but its deposits have been
eroded by Meager Creek.
The second large landslide deposit (unit A4; Figs. 1b, 3)
is inset in a stream-cut channel eroded into the older deposit.
It extends from the lower reach of South Meager Creek to
Lillooet River. At the confluence of Meager and South Meager
creeks, the deposit forms smooth, paired terraces at about
720 m asl, which slope 1°–3° downstream (Figs. 4, 5). The
terrace on the north side of Meager Creek grades into the
rising slope of the Angel Creek fan, whereas the terrace on
the south side of the creek abuts the higher, hummocky surface
169
Fig. 4. Longitudinal profile of Meager Creek showing distribution
of terrace surfaces. asl, above sea level.
of the older landslide deposit. The terraces extend 2 km
upstream along South Meager Creek from its mouth. They
stand about 10 m above South Meager Creek just upstream
of Barr Creek, but are 70 m above the creek at its confluence
with Meager Creek. The terraces drop in elevation downvalley
along Meager Creek (Fig. 4). Below Canyon Creek, they
delineate a smooth surface 10–20 m above Meager Creek
that can be traced downstream to Lillooet River. The surface
gradient of the terraces is slightly steeper than that of the
modern channel along this reach, resulting in a progressively
lower scarp downstream.
The older landslide deposit: instability 8700
years ago
Stratigraphy and sedimentology
A slump at location M3 (Fig. 6) provides the best exposure
of the older landslide deposit. Other river cuts (M5, M6, and
M9) and road cuts on the surface of the deposit provide
supporting detail.
Meager Creek section 3 (M3)
A slump 800 m downstream from the confluence of Meager
and South Meager creeks (Fig. 6) has created a 135-m-high
section through the older landslide deposit (Figs. 7, 8).
Seventeen metres of compact, matrix-supported, polymictic
diamicton (basal unit, Figs. 7b, 9a) are exposed at the base
of the section. Particle-size analysis of the matrix of the
diamicton yielded sand:silt:clay ratios of 53:45:2 and 63:25:12.
The clay-size fraction contains abundant chlorite, kaolinite,
and smectite, and minor illite (Table 1). Clasts range from
pebble to boulder size and are angular to rounded. About
70% of the stones are volcanic and 30% are derived from
basement rocks. The unit also contains abundant comminuted
wood. Three wood samples yielded radiocarbon ages of 7920 ±
100 (GSC-6511), 7940 ± 90 (GSC-6598), and 8480 ± 80 14C
years BP (TO-9559; Table 2).
The basal diamicton is overlain by 118 m of lithologically
banded (Figs. 7, 9b), matrix-supported, sandy diamicton
consisting entirely of volcanic detritus (source rock units P1
and P3). Clasts range from pebble to mega-gravel size (see
Blair and McPherson 1998 for expanded textural classification)
and are angular in shape. The lowest 27 m of the unit has a
yellow- to cream-coloured matrix and contains zones of finely
brecciated, granular to pebbly granitic debris and thinner
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Can. J. Earth Sci. Vol. 41, 2004
Fig. 5. View west to the confluence of South Meager and Meager creeks. The confluence terrace is visible in the center. At the left
are wetlands of South Meager Creek, which are underlain by landslide deposits graded to the terrace. In the distance, the fans of Angel, No Good, and Boundary creeks force Meager Creek hard against a steep valley wall underlain by granitic rock.
Fig. 6. Map showing locations of sections described in the paper. The contour interval is 100 m.
bands of red and grey, porphyritic andesite debris. A sample
of the diamicton matrix yielded a sand:silt:clay ratio of 74:21:5.
The upper 91 m of the unit consists of alternating bands of
red and grey, porphyritic andesite debris with a few bands of
yellowish debris. Samples of matrix yielded sand:silt:clay
ratios of 84:15:1 and 78:21:1. Two large, grey andesite blocks
on a hummock at the surface of the deposit gave 36Cl surface
exposure ages of 6 and 7.5 ka (Terry Swanson, personal
communication, 2002).
The contact between the two units is not well exposed at
section M3, but was observed in a gully 50 m upstream
(Fig. 10). At the gully site, the compact polymictic diamicton
is overlain successively by andesitic debris, yellow debris,
and a sliver of polymictic diamicton. The sequence is the reverse
of that at the base of section M3. The yellow debris has a
steeply inclined, sharp contact with the underlying andesitic
debris, which we interpret to be a thrust fault. Deformed
bands within the yellow material also suggest shear.
Other sections
The basal unit at section M3 is also exposed at section M5
at the confluence of Meager and South Meager creeks (Fig. 6).
At section M5, compact, matrix-supported polymictic diamicton
contains comminuted wood and a large log. Outer rings of
the log yielded a radiocarbon age of 8120 ± 70 14C years BP
(GSC-6474), and a wood fragment gave an age of 8020 ±
70 14C years BP (TO-8837; Table 2).
Sections M6, M8, and M9 (Fig. 6) provide exposures of
lithologically banded debris of the older landslide. Section
M6 is the largest of these exposures, with about 60 m of
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Fig. 7. Site M3. (a) Southerly view of the section showing the location of blocks sampled for 36Cl dating and the inset terrace at
720 m above sea level. (b) View to the west of the lower 50 m of the section, showing the basal unit at river level overlain by Devastator
Assemblage (P1) and Pylon Assemblage (P3) debris. The location of the exposed contact is also shown (Fig. 10).
cream-coloured volcanic debris unconformably overlain by
10 m of andesitic debris of the younger landslide deposit
(Fig. 11).
Landslide volume and surge velocity
We estimate that the older landslide deposit covered an
area of about 5.5 km2 prior to incision and erosion. We base
this estimate on the mapped distribution of the deposit, both
on surface and stream-cut exposures. The deposit is more
than 140 m thick along Meager Creek between South Meager
and Hotsprings creeks. It thins toward the valley sides, as
seen in exposures along Barr Creek. The average thickness
of the deposit is probably 50–80 m, which gives a volume of
3–4 × 108 m3.
As previously noted, a veneer of andesitic landslide debris
extends upslope 75 m from the main debris surface to a
prominent bench on the Barr Creek – Hotsprings Creek divide
(Fig. 3c). Above the bench, the hillslope is mantled by till.
The debris veneer and bench are a surge deposit (Hewitt
1999). Total run-up is about 100 m, measured from the base
of the debris exposed along Barr Creek. Application of the
velocity head equation, V = (2gh)0.5, where g is gravitational
acceleration and h is run-up (Chow 1959, p. 152), suggests a
surge velocity of 44 m s–1, consistent with velocities reported
for other rock avalanches in the region (e.g., Clague and
Souther 1982).
Interpretation
The clay mineralogy, matrix texture, ratio of non-volcanic
and volcanic lithologies, and presence of matrix-poor, fragmental
lenses suggest that the basal unit of the older landslide deposit
is a mixture of the basal breccia (PL1) and Devastator
Assemblage (P1). The overlying lithologically banded unit
comprises angular, sandy rhyodacitic and andesitic debris.
Altered, light green, cream, and yellow-banded debris at the
base of the lithologically banded unit is derived from the
basal breccia (PL1) and Devastator Assemblage (P1). This
debris is overlain by red- and grey-banded, porphyritic andesite
debris of the Pylon Assemblage (P3). The vertical sequence
of lithologies in the older landslide deposit thus is the same
as the sequence of rock units in the Angel Creek basin
(Fig. 3).
The basal unit is matrix-supported and contains mainly
subangular to subrounded clasts, some rounded clasts (Fig. 9a),
and abundant comminuted wood. These observations suggest
that the debris flowed, rounding clast edges, incorporating
and grinding vegetation, and entraining channel gravel. The
source of the unit is hydrothermally altered, vent-filling rocks
in the lower part of the Angel Creek basin.
Overlying the basal flow unit is a wedge of Devastator
Assemblage debris that thickens towards Angel Creek (it is
at least 60 m at sites M6, M8, and M9 versus 27 m at M3).
Between sites M3 and M6, the top of the debris wedge has
an apparent slope of 2.5°. This is a minimum estimate because
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Lab No.a
Radiocarbon age
(14C years BP) b
Calendric age range
(years before A.D. 2000) c
Location
Latitude (N)
Longitude (W)
Comments
McNeely and
McCuaig (1991)
Blake (1983)
Meager Creek, 6.75 km upstream from Capricorn
Creek. Maximum age for landslide debris.
No Good Creek. Debris flow deposit > 12m thick.
Source is Devastator Assemblage.
Meager Creek, 4.15 km from Pylon Peak.
Maximum age for landslide debris.
Meager Creek, 3.9 km from Pylon Peak. Maximum
age for landslide debris.
No Good Creek, confluence with Meager Creek.
Fluvial gravel underlying debris flow deposit.
Closest limiting age for event dated by GSC-4223
and GSC-4239.
No Good Creek, confluence with Meager Creek.
Lower part of debris flow deposit > 20 m thick.
Same section as GSC-4264.
No Good Creek, 0.6 km above its mouth. Lower of
two debris flow units in section about 30 m high.
May underlie unit dated by GSC-3509.
Meager Creek, 6.75 km upstream from Capricorn
Creek. Maximum age for landslide debris.
Devastation Creek near its mouth. From tree in
upright position, surrounded by debris flow material underlying 1975 landslide deposit.
Section H1. Laminated silt overlying younger landslide debris debris. Minimum age for younger
event.
Section M2. Surface unit. Maximum age for
younger event.
Section M9. Basal diamicton. Maximum age for
younger event.
Section M1. Basal diamicton. Maximum age for
younger event.
Section SM4. Diamicton beneath graded sand-silt
unit. Maximum age for younger event.
Section M2. Surface unit. Maximum age for
younger event.
Section M7. Basal diamicton. Maximum age for
younger event.
GSC-3811
210±50
50–470
50°33.5′
123°29.5′
GSC-3509
370±50
350–560
50°33.9′
123°31.9′
GSC-3736
460±50
390–600
50°33.5′
123°31.7′
GSC-3750
480±50
520–670
50°33.7′
123°30.8′
GSC-4264
800±70
620–960
50°33.7′
123°31.9′
GSC-4223
900±60
740–980
50°33.7′
123°31.9′
McNeely and
McCuaig (1991)
GSC-4239
990±70
790–1100
50°34.0′
123°30.5′
McNeely and
McCuaig (1991)
GSC-3733
1920±50
1770–2040
50°33.5′
123°29.5′
GSC-4302
2170±60
2040–2390
50°34.0′
123°33.7′
McNeely and
McCuaig (1991)
McNeely and
McCuaig (1991)
TO-8364
3440±70
3430–4140
50°34.3′
123°27.0′
This paper
GSC-4211
3930±70
4200–4620
50°34.7′
123°27.5′
TO-8955
3950±60
4050–4870
50°33.7′
123°28.8′
McNeely and
McCuaig (1991)
This paper
GSC-6467
4050±70
4410–4870
50°35.8′
123°26.2′
This paper
TO-9561
4060±50
4300–4890
50°33.5′
123°30.1′
This paper
GSC-4207
4080±70
4460–4880
50°34.7′
123°27.5′
GSC-6466
4080±60
4470–4880
50°33.6′
123°30.3′
McNeely and
McCuaig (1991)
This paper
McNeely and
McCuaig (1991)
McNeely and
McCuaig (1991)
McNeely and
McCuaig (1991)
Can. J. Earth Sci. Vol. 41, 2004
© 2004 NRC Canada
Reference
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Table 2. Radiocarbon ages from Meager Creek valley.
Radiocarbon age
(14C years BP) b
Location
Reference
Comments
Longitude (W)
123°30.3′
This paper
Section M7. Basal diamicton. Maximum age for
younger event.
Meager Creek above Hotsprings Creek. Debris flow
or rock avalanche deposit > 36 m thick. Dated
unit underlies another debris flow or rock
avalanche unit 17 m thick. Maximum age for
younger event.
Section SM3. Wood fragment in rip-up clast below
rock avalanche deposit. Maximum age for younger
event.
Section SM4. Top of graded sand–silt bed between
silt-rich diamicton and rock avalanche debris.
Sample is reworked, as date is older than TO-9561,
which is a maximum age for the silt-rich diamicton
below.
Section M2. Basal diamicton. Maximum age for
younger event.
Section M4. Basal diamicton. Maximum age for
younger event.
Section M2. Laminated silt overlying landslide
debris. Reworked, as date is older than GSC-4207,
GSC-4211, and TO-8836, which are stratigraphically
lower.
100 m upstream of section MC3. Basal diamicton.
Maximum age for younger event
Section M3. Basal diamicton. Maximum age for
older event.
Section M3. Basal diamicton. Maximum age for
older event.
Section M5. Basal diamicton. Maximum age for
older event.
Section M5. Basal diamicton. Maximum age for
older event.
Section M3. Basal diamicton. Maximum age for
older event
GSC-6476
4090±60
4470–4880
Latitude (N)
50°33.6′
GSC-3502
4100±70
4470–4880
50°34.1′
123°28.3′
Blake (1983)
GSC-6453
4120±80
4470–4890
50°33.5′
123°30.1′
This paper
GSC-6445
4160±50
4580–4880
50°33.5′
123°30.1′
This paper
TO-8836
4320±60
4580–5350
50°34.7′
123°27.5′
This paper
TO-9560
4390±120
4410–5650
50°33.7′
123°28.9′
This paper
TO-8365
4760±100
4920–5970
50°34.7′
123°27.5′
This paper
TO-9558
4860±70
5360–5960
50°33.7′
123°28.7′
This paper
GSC-6511
7920±100
8500–9080
50°33.7′
123°28.7′
This paper
GSC-6598
7940±90
8530–9080
50°33.7′
123°28.7′
This paper
TO-8837
8020±70
8510–9450
50°33.6′
123°30.0′
This paper
GSC-6474
8120±70
8830–9450
50°33.6′
123°30.0′
This paper
TO-9559
8480±80
9080–9940
50°33.7′
123°28.7′
This paper
a
Laboratories: GSC, Geological Survey of Canada, Ottawa, Ontario; TO, IsoTrace Laboratory, University of Toronto, Toronto, Ontario.
Laboratory-reported uncertainties are 1σ for TO ages and 2σ for GSC ages. Ages are normalized to δ13C = –25.0‰ PDB (Peedee Belemnite).
c
Determined from atmospheric decadal dataset of Stuiver et al. (1998) using the program CALIB 4.1. The range represents the 95% confidence limit calculated with an error multiplier of 2.
b
173
© 2004 NRC Canada
Calendric age range
(years before A.D. 2000) c
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Table 2 (concluded).
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Can. J. Earth Sci. Vol. 41, 2004
Fig. 8. Stratigraphic logs of selected sections. asl, above sea level.
the debris was eroded during formation of the Angel Creek
fan. Thickening of the debris towards Angel Creek suggests
that a block of Devastator Assemblage rocks slid into the
valley.
In contrast, overlying debris derived from Pylon Assemblage
rocks appears to thicken away from the source (0 m at M6,
90 m at M3). Distal thickening, lithologic banding, preservation
of source rock stratigraphy, and the angular shape of most of
the clasts suggest movement as a rock avalanche. This interpretation is supported by evidence of shear at the contact
between the basal and surface units at sites M3 and M5, the
hummocky surface morphology, and the run-up between
Hotsprings and Barr creeks.
The basal breccia and Devastator Assemblage are implicated
in the initial failure (Read 1978; Finn et al. 2001). Because
of the presence of the hydrothermal plume at shallow depth
beneath the Angel Creek basin, the rock mass probably had
a high phreatic water content. The saturated rock mass may
have liquefied at the narrow mouth of the basin, allowing it
to flow shortly after movement commenced (Iverson et al.
1997). Alternatively, it may have transformed into a flow after
it slid into the valley of Meager Creek. A portion of the
300-m-thick Devastator Assemblage slid en masse into the
valley on the heels of the flow. This material was then overridden by avalanching debris from the destabilized summit.
The rock avalanche achieved a velocity of up to 44 m s–1
and surged 100 m up the valley wall opposite the source
area. Thus, we infer a complex landslide event (terminology
after Turner and Schuster 1996, p. 53), involving flow of the
stratigraphically lowest rocks, followed by sliding of the
middle unit, and rock avalanching at the end.
Five radiocarbon ages on wood fragments collected from
the basal unit of the older landslide deposit range from 7920
to 8480 14C years BP (Table 2). Because all five ages are
wood fragments, they do not necessarily record the time of
death of the shrub or tree and should be viewed as maximum
ages for the landslide. Thus, the youngest of the ages,
7900 14C years BP (ca. 8700 calendar (cal.) years ago), is
closest to the time of the landslide.
The apparent surface exposure age of the deposit is
6000–7500 years old. We regard the 36Cl ages as minima because post-depositional weathering of the dated volcanic
blocks would render the ages younger than they should be.
The lack of evidence for pedogenic weathering or fluvial reworking of the top of the basal unit suggest that the basal
unit and the overlying lithologically banded debris are close
in age and probably were deposited during one large, multiphased
landslide.
The younger landslide deposit: instability
4400 years ago
Stratigraphy and sedimentology
Key exposures of the younger landslide deposit are located
near the confluence of South Meager and Meager creeks
(SM3, SM4, and M7) and in lower Meager Creek Valley
(M1 and M2) (Fig. 6). These sections and a section at
Hotsprings Creek are described in the following sub-sections.
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175
Fig. 9. Photographs of representative landslide deposits in Meager Creek valley. (a) Matrix-supported, silt-rich, polymictic diamicton at
site M3 deposited about 7900 14C years ago; note rounded clasts derived from the Meager Creek channel bed. (b) Lithologically banded
debris of the older landslide at site M9. (c) Angular, porphyritic andesitic debris derived from Pylon Assemblage rocks at site SM3. (d) Matrixand clast-supported andesitic debris derived from Pylon Assemblage rocks at site SM4. Shovel handle (in c and d) is 100 cm long.
South Meager Creek sections 3 and 4 (SM3 and SM4)
The section at site SM4 (Fig. 8), located 120 m upstream
of the confluence of Meager and South Meager creeks, is
about 60 m high. The lowest 3 m of exposed sediment is matrixsupported, polymictic diamicton containing comminuted wood
fragments and lenses of matrix-poor granitic fragments. Clasts
are dominantly subangular to subrounded and pebble to small
boulder in size. A wood fragment from this unit yielded a
radiocarbon age of 4060 ± 50 14C years BP (TO-9561;
Table 2). The basal unit is sharply overlain by 4.5 m of
normally graded sand and silt. Wood fragments from the upper
part of the sand–silt unit yielded an age of 4160 ± 50 14C
years BP (GSC-6445). The graded sand–silt unit is overlain
across an erosional contact by 1.2 m of normally graded,
medium to coarse sand containing rip-up clasts of silt. The
uppermost unit at SM4 is 40 m of matrix- to clast-supported,
angular to subangular debris. It extends to the terrace surface
at 718 m asl. Clasts in the debris are up to 10 m across and
are composed of about 80% andesite and 20% non-volcanic
lithologies.
At site SM3, which is on the opposite side of South Meager
Creek from SM4, wood from a silt rip-up within a normally
graded, medium to coarse sand bed yielded a radiocarbon
age of 4120±80 14C years BP (GSC-6453; Table 2). The
sand is overlain by about 50 m of rubbly and blocky debris
(Fig. 9c) consisting of about 90% andesite and 10% non-
volcanic clasts. Particle-size analysis of the matrix yielded a
sand:silt:clay proportion of 77:19:4.
Meager Creek section 7 (M7)
Site M7 is next to Meager Creek, 500–600 m upstream of
the mouth of South Meager Creek (Fig. 6). Three slumps at
this site provide additional exposure of the sediments underlying the terrace at the Meager – South Meager confluence
(Figs. 8, 12a).
The lowest unit at site M7 consists of 5 m of hydrothermally
altered, yellow debris. Its upper contact is sharp and irregular,
with up to 10 m of relief. This unit is overlain by 24 m of
compact, clast- to matrix-supported, polymictic diamicton
containing wood fragments, branches, and large logs. The
outer rings of a log yielded a radiocarbon age of 4090 ±
60 14C years BP (GSC-6476), and a wood fragment was
dated at 4080 ± 60 14C years BP (GSC-6466; Table 2). Clasts
are subangular to subrounded and are about 81% volcanic and
19% non-volcanic lithologies. A sample of the matrix had a
sand:silt:clay ratio of 57:39:4. Clay mineral analysis indicates the presence of abundant illite, chlorite, kaolinite, and
smectite (Table 1).
The diamicton is overlain by 1–2 m of gravel, consisting
of granitic pebbles, cobbles, and small boulders. This gravel
bed is sharply overlain by 5 m of fining-upward, horizontally
stratified, pebble–cobble–boulder gravel of dominantly volcanic
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Can. J. Earth Sci. Vol. 41, 2004
Fig. 10. Contact between the basal and surface units of the older landslide deposit at site M3. Structures indicate shear and thrusting
during emplacement. P1, Devastator Assemblage debris; P3, Pylon Assemblage debris. See Fig. 7a for location.
provenance (Fig. 12b). Above this is about 18 m of weakly
stratified, clast-supported, cobble–boulder gravel with sand
interbeds.
Hotsprings Creek section 1 (H1)
Section H1 is a road cut adjacent to the river bank at the
forestry bridge over Hotsprings Creek. Up to 2 m of laminated
and bedded silt and sand fill depressions on a surface underlain
by angular to subangular, andesitic landslide debris. A sample
of charcoal collected from a 1-cm-thick, charcoal-rich bed
20 cm above the base of the silt–sand unit yielded a radiocarbon age of 3440 ± 70 14C years BP (T0–8364; Table 2).
Meager Creek sections 1 and 2 (M1 and M2)
The sections at sites M1 and M2 expose sediments underlying
paired terraces that extend downstream from Canyon Creek
to Lillooet River. The lowest exposed unit at site M2, near
the mouth of Canyon Creek, consists of 13 m of compact,
matrix-supported, polymictic diamicton with a silt–sand matrix
and abundant wood (Fig. 8). A sample of the wood yielded a
radiocarbon age of 4320 ± 60 14C years BP (TO-8836;
Table 2). The basal diamicton is overlain by 6 m of massive,
poorly sorted, matrix-supported diamicton. This upper diamicton
has a higher clast content and a sandier matrix than the basal
diamicton. The outer rings of two logs in the upper diamicton
Fig. 11. View to the north of the section at site M6 showing older
landslide deposits derived from Devastator Assemblage rocks (P1).
These deposits are unconformably overlain by about 10 m of Pylon
Assemblage debris, which underlie the 720 m terrace.
were radiocarbon dated at 3930 ± 70 (GSC-4211) and 4080 ±
70 14C years BP (GSC-4207; Table 2). The upper diamicton
is conformably overlain by a 2-m-thick, upward-coarsening
unit of laminated silt, cross-stratified sand, and horizontally
bedded gravel. Charcoal from a silt bed at the base of this
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Fig. 12. Site M7. (a) View to the south, showing major stratigraphic
units. (b) Stratified gravel, which is overlain by the upper unit of
the younger landslide deposit.
upward-coarsening unit yielded a radiocarbon age of 4760 ±
100 14C years BP (TO-8365; Table 2).
The section at site M1, which is 1 km west of the mouth
of Capricorn Creek, has the same basal, polymictic diamicton
as the section at site M2. Wood from the basal diamicton at
site M1 yielded a radiocarbon age of 4050 ± 70 14C years
(GSC-6467; Table 2). The diamicton is overlain by 6.5 m of
sandy cobble–boulder gravel consisting mainly of non-volcanic
clasts and 1 m of boulder gravel consisting mainly of andesite
clasts. The andesite boulder bed is overlain by a 10-cm-thick
bed of granular to very coarse sand, which in turn is overlain
by 5 cm of laminated silt. The uppermost unit at site M1
comprises 2 m of upward-coarsening, cross-bedded sand and
horizontally bedded, polymictic, granule to pebble–cobble
gravel.
Interpretation
The younger landslide deposit consists of two distinct units.
The basal unit resembles the basal unit of the 8700-year-old
landslide deposit. Its clay mineralogy (Table 1), matrix particle
size, ratio of volcanic to basement lithologies, and long run-out
suggest that it is derived from the basal breccia and Devastator
Assemblage (Fig. 3) and that it was deposited by a debris
flow.
A second phase of failure is recorded by the subangular
andesitic debris that underlies the terrace at about 720 m asl
in the lower part of South Meager Creek valley. Deposits
associated with this failure are exposed along Meager Creek
as far downstream as Lillooet River. They are thickest and
coarsest at the Meager – South Meager confluence, where
they contain angular clasts up to mega-gravel size (Blair and
McPherson 1998). Farther up South Meager Creek and in
Meager Creek valley below Hotsprings Creek, the mega-gravel
fraction is lost and the gravel fraction becomes finer and
177
better rounded. We infer that the failure responsible for
these sediments originated as a rockslide or rock avalanche
and evolved into a debris flow as it moved away from the
source.
Eleven radiocarbon ages on wood from the younger unit
range from 3930 ± 70 to 4860 ± 60 14C years BP, although
most of the ages fall within the interval 4050–4150 14C
years BP (Table 2). Because detrital wood may be older than
the sediments in which it occurs, the youngest of the eleven
ages (3930 ± 70 14C years BP; GSC-4211) is closest to the
age of the younger landslide. The date from site H1 of 3440 ±
70 14C years BP (TO-8364) is a minimum age for the event.
Thus, the landslide happened sometime between 3400 and
3900 14C years BP, most likely about 3900 14C years BP
(4400 cal. years ago).
The closest radiocarbon ages from the two units of the
younger landslide are statistically the same, thus the two
units may have been deposited in quick succession. At several
sites, however, stratified sediments separate the two units. At
site SM4, for example, about 6 m of sand and silt separate
the basal and surface units, and at site M1, stratified gravel
separates the basal landslide unit from andesitic debris
interpreted to be the surface unit. Similarly, at site M7, sand
and well bedded, pebble–cobble gravel lie between the basal
unit and a weakly bedded, poorly sorted boulder gravel. The
gravels at sites M1 and M7 are interpreted to be fluvial
sediments deposited when the basal unit was reworked by
Meager Creek immediately after it was deposited. The stratigraphy implies that some time elapsed between deposition
of the two landslide units, although how much is unknown.
The interval, however, is too short to be resolved through
radiocarbon dating; it could be hours to decades.
The sediments just described were deposited by a complex
landslide about 4400 years ago. Like the landslide that occurred
4000 years earlier, it began as a failure of the basal, vent-filling
rocks in the Angel Creek basin. The initial failure rapidly
transformed into a large debris flow that reached the mouth
of Meager Creek and traveled down Lillooet River valley
(see Discussion). After the initial failure, silt and sand accumulated in local ponds on the debris, and the debris flow
deposits were reworked by Meager Creek. Within hours to
decades, part of Pylon Peak collapsed, producing a rock
avalanche that filled the area at the confluence of Meager
and South Meager creeks with up to 50 m of debris. The
rock avalanche transformed into a debris flow that moved up
South Meager Creek and down Meager Creek past Canyon
Creek.
The younger landslide deposit, prior to incision and erosion,
covered an area of at least 4.4 km2. The base of the deposit
is exposed at only a few sites, and its full downstream extent
is unknown, consequently it is difficult to obtain a reliable
estimate of the volume of the deposit. A minimum volume
of 2 × 108 m3 is obtained by multiplying a conservative average
thickness of 45 m by the minimum area of 4.4 km2.
Discussion
The rocks underlying The Devastator and Pylon Peak are the
source of at least two, very large, complex landslides during
the Holocene. The landslides produced the Angel Creek basin,
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178
a cirque-like amphitheatre extending from 1600 m asl to the
summit of Pylon Peak at 2286 m asl.
Using stratigraphic, sedimentologic, and morphologic evidence, we argue that each landslide was a multiphase event,
involving a huge debris flow followed by a rock avalanche
(Fig. 14). The debris flow phase of the first and larger landslide
occurred about 7900 14C years BP (8700 cal. years ago).
Exposure ages on boulders on the surface of the rock avalanche
provide a minimum age of 7500 36Cl years BP for that
phase. No evidence exists for a hiatus between the debris
flow and rock avalanche phases, and we tentatively conclude
that they occurred in rapid succession. The initial failure of
hydrothermally altered rock at the middle level of the mountain
likely destabilized the flow-layered rocks forming the summit
cone, inducing an immediate collapse.
The debris flow and rock avalanche phases of the younger
landslide occurred about 3900 14C years BP (ca. 4400 cal.
years ago). Stratified gravel and laminated silt locally separate
the deposits of the debris flow and rock avalanche phases,
indicating that some time elapsed between them. However,
the two phases are not resolved by radiocarbon dating,
suggesting that the hiatus was short, on the order of hours to
decades. Although we recognize that this event comprises
two discrete phases, we prefer to view the two phases as a
single complex event. Again, it is logical to infer that initial
failure of hydrothermally altered materials at mid-mountain
level debuttressed higher slopes, thus triggering a rock
avalanche. The Hope Slide (Mathews and McTaggart 1978)
is an historical example of a landslide involving two closely
spaced collapses, the second of which was triggered by the
first (Weichert et al. 1994).
The downstream extent of these two landslides is presently
poorly understood, but we have preliminary data that suggest
the 4400-year-old debris flow traveled at least 40 km downstream from the confluence of Meager Creek and Lillooet
River (Friele 2002; Simpson et al. 2003). Given the large
area of the Mount Meager massif underlain by hydrothermally
altered volcanic rock, similar large landslides may be expected
in the future. The causes, triggering mechanisms, volumes,
and rheology of landslides contribute to the severity of the
downstream hazard and are discussed in the following
sub-sections.
Potential causes and triggers
A number of factors may have contributed to the failures
at Pylon Peak: climate-mediated phenomena (Church and
Ryder 1972; Bovis and Jones 1992; Evans and Clague 1999);
earthquakes (Mathews 1979; Clague and Shilts 1996); volcanism
(Stasiuk et al. 1996); and gradual reduction in the strength of
the rocks forming the mountain slopes.
Many historic landslides at Meager Creek occurred during
warm weather when snow and ice melt is greatest (MokievskyZubok 1977; Bovis and Jakob 2000; see also Chleborad
1997). Increased soil pore-water pressures associated with
rainstorms or snowmelt may have triggered these failures,
but the cause of recent instability is a combination of weak
rocks (Read 1978, 1990; Finn et al. 2001) and glacial debuttressing (Evans and Clague 1999; Holm et al. 2004). Some
prehistoric landslides on the north side of the Mount Meager
massif were triggered by the 2400-year-old eruption (Evans
1992; Stasiuk et al. 1996). However, no landslide deposits of
Can. J. Earth Sci. Vol. 41, 2004
this age have been found in Meager Creek valley and no other
eruptions are known to have occurred during the Holocene.
However, magma may have moved to shallower depths beneath the Meager massif prior to the landslides ca. 4400 and
8700 years ago, causing increased circulation of hot fluids in
the vent area beneath Pylon Peak and triggering earthquakes.
Alternatively, the landslide ca. 4400 years ago may have
resulted from increased precipitation following the onset of
Neoglaciation (Bovis and Jones 1992). The landslides may
have occurred, however, without any specific trigger in response
to progressive weathering and gradual weakening of the altered
rock mass.
Volume estimates
The volume of missing rock from the Angel Creek basin
is estimated to be 4.5 × 108 m3. A small amount of the missing
rock may have been excavated by cirque glaciers and flushed
out of the basin by small debris flows, but most of the material
has been removed by large landslides. The missing volume
is a maximum for the combined volume of the two complex
landslides derived from the Angel Creek basin. Dilation of a
rock mass due to fragmentation during a landslide can increase
the volume of debris by 30%–60% (Turner and Schuster
1996, p. 42). Taking dilation into account, we estimate the
volume of the debris to be 6–7 × 108 m3. This estimate
compares favourably with our estimate of the volume of
landslide deposits in Meager Creek valley of about 6 × 108 m3.
This last estimate, however, may be a minimum, because the
base of the landslide deposits is not exposed and the downstream
extent of the deposits along Meager Creek is unknown. The
older landslide appears to have been approximately twice the
size of the younger one. Considering the proportions of basal
breccia and Devastator Assemblage rocks in the basin, it is
possible that the debris flow phase of each event involved
roughly 1 × 108 m3 of debris.
Rheology of the basal debris flow units
The mobility of debris flows is partly determined by their
clay content (Pierson 1985; Jordan 1994). Jordan (1994)
suggests that a matrix clay content of 5% by weight separates
coarse-grained debris flows from more mobile, fine-grained
ones. Particle-size analysis shows that the clay contents of
the basal debris flow units and the overlying rock avalanche
deposits are similar, typically 3%–10% (Fig. 13). However,
the proportion of silt and sand differs, with the basal debris
flow units containing 25%–45% silt and 50%–65% sand,
and the rock avalanche units containing 15%–25% silt and
70%–85% sand. The debris flow units, while generally having
sufficient clay for high mobility, are silt-rich rather than
clay-rich.
A water content of 10%–20% by weight or as little as
25% by volume is required for debris to maintain sufficiently
low viscosity to flow over long distances (Pierson 1985;
Costa 1988; Jordan 1994). Large debris flows may incorporate
their water from phreatic sources (Vallance and Scott 1997),
from snow and ice melt during eruptions (Pierson and Scott
1985), or by impounding streams (Carter 1932). The water
source for the basal debris flow units described in this paper
is unknown. Since each of the basal debris flows may have
had a volume of 1 × 108 m3 or more, at least 2.5 × 107 m3 of
water would be required for mobilization. Glacier ice is not
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Fig. 13. Matrix sand:silt:clay ratios of selected sediments and
source rocks. Basal landslide units and Devastator Assemblage
source rocks are silty (25%–45% silt), in contrast to surface units
and till, which are sandy (70%–85% sand, < 30% silt).
179
[1]
B = 200V 2 3
Applying eq. [1], to a lahar of volume 1 × 108 m3 yields
an inundation area of between 107 and 108 m2 at the 95%
confidence limits of the prediction (Iverson et al. 1998, Fig. 6).
If we assume the downstream limit of the 8700-year-old
rock avalanche deposit is the limit of the proximal hazard
zone, Lillooet River valley might have been inundated 12–65 km
downstream from Mount Meager by a 1 × 108 m3 debris
flow (Fig. 14) originating from near Pylon Peak. Preliminary
evidence from extensive drilling along Lillooet River valley
indicates that at least three lahars originating from somewhere
in the Mount Meager volcanic complex did reach the nowsettled portion of the valley during the Holocene (Friele 2002;
Simpson et al. 2003). One of these lahars has been tentatively
correlated with the 4400-year-old event (Simpson et al. 2003).
Conclusion
a probable source of the water because glaciers would have
been present only at high elevations 8700 and 4400 years
ago and would have been incorporated into the rock avalanche
debris, not the debris derived from altered volcanic rocks
lower on the slope. Although there is no direct evidence for
it, a reservoir impounded by a landslide dam may have provided
some of the required water. However, given the presence of
a hydrothermal plume, phreatic waters may have been the
source of much of the water that mobilized the debris flows
4400 and 8700 years ago.
Downstream hazards
Debris flows like those documented in this paper may
pose a hazard to people living in the Lillooet River valley as
far away as Pemberton. Examples from other areas illustrate
this hazard. The Osceola Mudflow flowed at least 120 km
from its source on Mount Rainier volcano in Washington
state about 5600 years ago (Vallance and Scott 1997), filling
valleys in Puget Lowland and greatly extending the deltas of
the Puyallup and Green rivers into Puget Sound. Numerous
lahars were triggered by the eruptions of Mount St. Helens
in May 18, 1980 (Pierson 1985) and March 19, 1982
(Pierson and Scott 1985). The Pine Creek and Muddy River
lahars (Pierson 1985) were unconfined flows at distances up
to 10 km from the crater; farther downstream they were
channelized and flowed 25 km to the Swift Creek Reservoir.
The Toutle River lahar, triggered by a breach of the crater
lake during the 1982 eruption, flowed 27 km before transforming into a hyperconcentrated flow that ran 83 km downstream from the crater (Pierson and Scott 1985).
The debris flows described in this paper, although not
triggered by eruptions, were sufficiently large and mobile to
travel many kilometres. Iverson et al. (1998) established a
relationship between deposit volume (V) and the planimetric
area (B) of the deposit outside the proximal hazard zone:
Volcanoes at the eastern margin of the Cascadia subduction
zone, which extends from northern California to southern
British Columbia, have been sites of numerous large landslides
during the recent geologic past. The Mount Meager massif
may be one of the least stable areas in British Columbia, the
site of at least three landslides larger than one million cubic
metres in the last century alone.
We have documented two landslides from the Mount Meager
massif that are two orders of magnitude larger than any of
the historic events, one about 4400 years ago and the other
about 8700 years ago. The landslides occurred in weak, hydrothermally altered, Pliocene–Pleistocene volcanic rocks on the
south side of Pylon Peak. Together, the landslides deposited
about 6 × 108 m3 of debris in the valley of Meager Creek
adjacent to, and downstream from, Angel Creek. The deposits
of each of the two landslides are similar, consisting of a
basal silt-rich diamicton deposited by a debris flow and an
overlying, more sandy, blocky diamicton deposited by a rockslide
or rock avalanche. Lithologic analysis suggests that the debris
flows originated in brecciated and hydrothermally altered
volcanic rocks within the vent of Pylon Peak. In contrast, the
source of the rock avalanche deposits is the less altered,
flow-layered rocks forming the mountain peak. The debris
flows may have traveled tens of kilometres from their source
due to their relatively high water content and fine matrix.
The source of the water is not certain, but may have been
phreatic in origin.
Neither landslide was triggered by a volcanic eruption, despite
an eruption about 2400 years ago. Perhaps movements of
magma at shallow depth, accompanied by earthquakes, caused
the failures. Alternatively, the landslides may have climatic
causes or may simply have occurred due to progressive deterioration of slopes under the influence of gravity without a
specific triggering event. Flank collapses like those 4400
and 8700 years ago, although rare, may pose a hazard to
people and property in the Lillooet River valley 32–74 km
downstream from Mount Meager.
Eruption-triggered lahars can be predicted by monitoring
the volcano, but lahars that are not caused by eruptions probably
cannot be forecast. Continued retreat of glaciers will further
debuttress altered volcanic rock slopes, increasing the potential
for mass failure. Given the rapid expansion of the town of
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180
Can. J. Earth Sci. Vol. 41, 2004
Fig. 14. Schematic depiction of the debris flow and rock avalanche phases of the two large Pylon Peak landslides. The extent of the
rock avalanche deposit defines the proximal hazard zone. The debris flows reached Lillooet Lake at times when the delta front was
much farther upvalley, and thus closer to the mouth of Meager Creek, than today (Friele 2002). A similar debris flow today would
likely extend into the settled portion of the Lillooet River valley.
Pemberton, due to its close proximity to the ski resort of
Whistler, decision makers should become informed as to the
potential for catastrophic lahars in this area.
Acknowledgments
Roger McNeely (Geological Survey of Canada Radiocarbon
Laboratory) and Rolf Beukins (IsoTrace Laboratory) provided
the radiocarbon ages reported in this paper. Terry Swanson
(University of Washington, Seattle, Wash.) supplied surface
exposure ages. Jeanne Percival (Geological Survey of Canada)
provided clay mineral analyses. Particle-size analysis was
done at Soilcon Laboratories, Richmond, B.C. The British
Columbia Ministry of Forests (Squamish Forest District) and
CRB Logging supported Friele’s fieldwork in Meager Creek
valley. Analytical work was funded through Clague’s Natural
Sciences and Engineering Research Council Operating Grant
(217051-01). Chris Burn, Matthias Jakob, and an anonymous
reviewer provided constructive reviews of a draft of the paper.
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