The origin of a tephra-like bed near
Mount Cayley volcano, southwestern
British Columbia, Canada1
Gregory R. Brooks and Edward J. Hickin
Abstract: A distinct white bed 1-2 cm thick is present within backwater deposits at the upstream end
of the Turbid Creek debris fan, southwestern British Columbia, Canada. This white bed and the
enclosing backwater deposits accumulated within an impoundment of Squamish River caused by a large
ca. 4800 BP debris avalanche originating from Mount Cayley, a Pliocene-Pleistocene volcano. The
white bed is composed of silt and clay detritus and resembles tephra. About 5 % of the grains exhibit
optical characteristics consistent with volcanic glass. The remainder, however, are mineral and lithic
particles (mainly cristobalite and Na-feldspar, with minor quartz, and trace amphibolite, mica group,
and chlorite). The white bed is interpreted to be dust derived from the debris avalanche and washed into
the impoundment, based upon (i) the high proportion of mineral and lithic grains within the bed, (ii) the
contemporaneity of the bed and the debris avalanche, (iii) the lack of Holocene volcanic activity at
Mount Cayley, and (iv) the significant age difference between the bed and known tephras in
southwestern British Columbia.
R&um6 : Une lit de couleur blanche ais6ment reconnaissable, d'une 6paisseur de 1-2 cm, est
interstratifik dans les d6p6ts du r6servoir d'eaues de retenue, juste en amont de l96ventai1de debris de
Turbid Creek. Ce lit de couleur blanche et les dCp6ts des eaux de retenue qui le confinent, se sont
accumults dans un r6servoir sur la riviBre Squamish crte par le barrage Mifi6 lors d'une grosse
avalanche de dkbris, survenue aux alentours de 4800 Av.P., qui a d6vale du mont Cayley, un volcan
d'gge Pliockne-Pl6istockne. Le matiriel de ce lit blanc est form6 de silt et d'argile et il ressemble h un
tephra. Environ 5 % des grains exhibent des propri6t6s optiques analogues i celles d'un verre
volcanique. Le reste est compos6 de petits grains de min6raux et de roches (principalement de la
cristobalite et du feldspath sodique, avec un peu de quartz et de trks petites quantit6s d'amphibolite, de
min6raux du groupe des micas et de la chlorite). Les observations suivantes : (i) la proportion 6lev6e de
grains de min6raux et de roches dans le lit; (ii) la contemporan6itt du lit et de l'avalanche de d6bris;
(iii) l'absence d'activite volcanique holockne au mont Cayley; et (iv) la diffkrence importante d'gge entre
ce lit et les tephra connus dans le sud-ouest de la Colombie-Britannique, suggbrent que ce lit de couleur
blanche est un d6p6t 6olien d6riv6 d'une avalanche de d6bris et d6lav6 dans un reservoir d'eau de
retenue.
[Traduit par la redaction]
Introduction
Tephra beds can provide information about the age, magnitude, and character of prehistoric volcanic eruptions (Fisher
and Schmincke 1984), which is important in assessing hazards
associated with future eruptions (e.g., Crandell et al. 1975).
Tephras are also useful stratigraphic marker beds for some
geological and archaeological studies. A considerable literature exists on the identification and correlation of tephra
Received February 15, 1995. Accepted August 14, 1995.
G.R. Brooks? Geological Survey of Canada, 601 Booth
Street, Ottawa, ON KIA OE8, Canada.
E.J.Hickin. Department of Geography and The Institute for
Quaternary Research, Simon Fraser University, Burnaby,
BC V5A lS6, Canada.
*
Geological Survey of Canada Contribution 11493.
Corresponding author (e-mail: gbrooks@gsc.emr. ca).
beds (see recent reviews by Sarna-Wojcicki and Davis 1991;
Knox 1993). The misidentification of a bed that exhibits
some characteristics resembling a "true" tephra can cause
erroneous interpretations of the local stratigraphy and, more
importantJy, the eruptive history of the volcano from which
the bed is attributed to have originated.
This paper reports a distinct white tephra-like bed within
backwater deposits that accumulated in an impoundment of
Squamish River caused by a large debris avalanche from
bunt Cayley volcano. The interpretation of this bed has
important implications for the Holocene history of Mount
Cayley, which is generally believed to have been dormant
since at least the Late Pleistocene. In this paper, we consider
eruptive and noneruptive origins for the white bed.
Background
Mount Cayley is a large v o l c a ~ ccomplex in the central
Garibaldi Volcanic Belt (Fig. 1) (Souther 1980). The com-
Can. J. Earth Sci. 32: 2040-2045 (1995). Printed in Canada I Imprim6 au Canada
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Brooks and Hickin
Fig. 1. Location map of the Mount Cayley volcanic complex.
Also shown are the Quaternaq volcanic centres of the
Garibaldi Volcanic Belt and distributions of Mazama and
Bridge River tephras (modified from Roddick et al. 1979;
Green et al. 1988; Mathewes and Westgate 1980; Westgate
and Naeser 1985).
Fig. 2. Map showing the location of exposures of the white
bed at the edge of the Turbid Creek debris fan (1 ft =
0.3048 m).
COLUMBIA
QUATERNARY VOLCANIC
CENTRES OF THE
'
DISTRIBUTION
OF MAZAMA TEPHRA
APPROXIMATE
DISTRIBUTIONOF
posite cone consists of andesite, dacite, and minor rhyodacite
flows, domes, and pyroclastic deposits that formed in three
distinct eruptive stages during the Pliocene and Pleistocene
(Souther 1980; Clague and Souther 1982). Rock from the last
eruptive stage has been K- Ar dated at 310 & 50 ka (Green
et al. 1988). The rugged edifice is deeply dissected with a
large amphitheatre-shaped basin carved into its southwestern
flank (Fig. 2). The steep slopes of this basin have a long
history of large landslides, several of which descended dlrectly
into Squamish Valley and impounded Squamish River (Brooks
and Hickin 1991; Evans and Brooks 1991, 1992; Brooks
1992).
The backwater deposits containing the white bed relate
directly to a massive collapse of the volcanic cone at
ca. 4800 BP (Evans and Brooks 1991, 1992; Brooks 1992).
The debris avalanche descended the slopes of Mount
Cayley, crossed Squamish Valley, and spread upstream and
downstream along Squamish River. The Turbid Creek debris
fan, produced by this landslide, extends about 6 km along
Squamish River and covers an area of about 8 km2 (Fig. 2)
(Evans and Brooks 1991). The landslide debris impounded
Squamish River and formed a reservoir that inundated the
valley bottom and extended upstream into the lower portions
of Elaho and upper Squamish valleys (Brooks and Hickin
1991). Sediment carried by Elaho and Squamish rivers
accumulated within this reservoir (Brooks and Hickin 1991).
Subsequent incision of the debris fan by Squamish River
breached the reservoir, restoring a riverine environment to
the valley bottom and exposing the debris avalanche and
backwater deposits.
The white bed
Distribution
We have found the white bed only in backwater deposits of
the ca. 4800 BP debris avalanche at the upstream end of the
Turbid Creek debris fan (Figs. 2, 3). Exposures containing
the white bed are located up to 8 km from the debris
avalanche source area. The bed was not found in cores collected from peat bogs in Elaho, upper Squamish, and Ashlu
valleys (Fig. 1).
Stratigraphic position
Brooks and Hickin (1991) described in detail debris avalanche
and backwater sediments (their "fan toe" deposit) exposed
at the upstream end of the Turbid Creek debris fan. A
representative lithostratigraphic section (Fig. 3) is briefly
described here. The basal unit (unit 1) consists of diamicton
at least 5 m thick that is part of a debris avdanche deposit
that impounded Squarnish River. This is overlain by up to
4.5 m of interbedded fluvial and debris flow deposits (unit 2)
reworked from the debris avalanche deposits immediately
following the landslide. Above this is 20 cm of sand and silt
lacustrine rhythmites (unit 3a); in some exposures, this unit
overlies directly the debris avalanche diamicton (unit 1).
Unit 3a is overlain successively by 6 m of silt and clay lacustrine rhythmites (unit 3b) and 2.2 m of sand and silt lacustrine rhythmites (unit 3 4 , both of which accumulated within
the lake impounded by the debris avalanche. Capping the
sequence is 2.6 m of fluvial sediments (unit 4) deposited by
Squamish River after the lake drained or was infilled. The
contact between the lacustrine and overlying fluvial sediments is gradational, and nowhere in the section is there
evidence of a major hiatus.
Following the deposition of unit 4, Squamish River incised
Can. J. Earth Sci. Vol. 32, 1995
Fig. 3. Stratigraphic section of deposits at the upstream end of the Turbid Creek debris fan,
showing the position of the white bed. The location of the section is shown in Fig. 2.
0
m
Legend
UNlT 4 - Fluvial
Lithofacies coding
Dmm
Gms
Sh
Sr
SP
St
FI
Fm
UNlT 3c - Lacustrine
-
5m
DIAMICTON, massive,matrix-supported
GRAVEL, stratified
SAND, horizontally stratified
SAND, ripple cross-laminations
SAND, planar cross-laminations
SAND, trough cross-laminations
SILTICLAY, laminations
SILTICLAY, massive
UNlT 3b - Lacustrine
Sediments
10 m
'White' bed
.
UNlT 3a - Lacustrine 1
Dmm
Sh
StISh
StISh
15m
UNlT 1 - Debris
Avalanche
.
ri
Dmm
,o,-.:
the debris fan, effectively terminating aggradation. The entire
sequence is attributed to the ca. 4800 BP debris avalanche
from Mount Cayley, based on (i) four radiocarbon ages
ranging from 4860 to 5310 BP on wood recovered from
debris avalanche diamicton of unit 1, and (ii) the lack of
major breaks within the sequence (Brooks and Hickin 1991;
Evans and Brooks 1991, 1992). Although Squamish River
was impounded by subsequent landslides from Mount Cayley,
none of the reservoirs was high enough to inundate the
4800 BP backwater deposits at the upstream end of the
debris fan.
The white bed is 1-2 cm thick and lies at the contact of
units 3a and 3b (Figs. 3, 4). It can be traced over a distance
of about 1100 m in exposures at the upstream end of the
Turbid Creek debris fan. The white bed contrasts markedly
with the darker coloured sediments immediately above and
below it (Fig. 4).
Grain characteristics
The white bed consists of silt and clay, and has a mean grain
AP.
. , .. '
SAND
~
13
~
DIAMICTON
-
Grain Size
SILT
CLAY
size of 15.6 pm and standard deviation of 13.6 pm. The grain
morphology includes angular shards and elongated and furrowed particles; some of the latter are slightly curved, with
a few containing spherical vesicles. The furrow and vesicle
characteristics are morphologic features common to tephra
shards (Heiken 1974). About 5% of the grains appear
isotropic under cross-polarized light, a characteristic of volcanic glass. X-ray diffraction analysis of a bulk sample of the
white bed indicates that the crystalline grains are mainly
cristobalite (polymorph of Si02 found in siliceous volcanic
rocks) and Na-feldspar; quartz and trace amounts of amphibolite, mica group, and chlorite are also present.
We experienced technical difficulties trying to analyze the
geochemistry of the isotropic grains using an electron microprobe because of the fine texture of the material. Specifically, problems concerned the grains "floating" and therefore
overlapping within the epoxy matrix of the polish thin section, which complicated both the identification and analysis
of the grains, and the generally small size of the isotropic
grains in relation to the microprobe beam width. Unreliable
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Brooks and Hickin
Fig. 4. Photographs of the white bed. (A) Oblique view of
an exposure showing the laterally continuous white bed
(arrows) at the base of the lacustrine unit 3b. (B) Close-up
view of the white bed, which is situated at the contact
between lacustrine units 3a and 3b.
Table 1. Chemical composition of grains in the white bed having
tephra-like morphologic features and glass composition of Mazarna
and Bridge River tephras.
SiO, (wt.%)
A1203
CaO
Na20
K2O
MgO
FeO
White bed
Mazama tephra
Bridge River
tephra
51.67f 5.34
(29.09-69.51)
18.58f3.98
(9.28-36.9)
3.54k1.90
(0.65-11.73)
1.17k0.85
(0.0-5.13)
0.95+0.36
(0.14-2.52)
3.56f 2.56
(0.0-15.83)
5.15k3.13
(0.11-18.97)
72.58f 0.31
74.35+0.73
14.40f0.16
13.72k0.27
1.66+0.10
1.40&0.18
5.18k0.20
4.61 k0.15
2.70+0.11
3.16k0.14
0.56k0.08
0.04+0.01
2.11 f0.08
1.62k0.18
Notes: Values are means & 1 SD, with the maximums and minimums
given in parentheses for the white bed. The analysis of the white bed
was performed by Mineral Resources Division, Geological Survey of
Canada on a Cameca SX50 electron microbe, which has four
wavelength-dispersive spectrometers. The microprobe was operated at
15 kV with a beam current of 10 nA and a beam width of approx. 3 pm.
Metals and simple compounds were used as standards. The raw counts were
converted to elemental concentrations using the Cameca PAP program.
Data for the Mazama and Bridge River tephras are from Westgate and
Gorton (1981).
results were obtained by this method. Analysis of grains contained upon a polished disk was done using tephra-like
morphologic features as a guide to grain identification. This
produced technically reliable data; however, 26 of the 46
grains analyzed were crystalline minerals (ferromagnesium
group, quartz, feldspar, and lithic fragments). While it is not
equivocal that the other 20 grains represent volcanic glass,
they collectively have a range of compositions that vary considerably from the volcanic glass of known tephras found in
southwestern British Columbia (Table 1).
Interpretations
Eruptive origin
The presence of isotropic grains within the white bed suggests that it could be tephra produced by a volcanic eruption.
The occurrence of the white bed within deposits relating to
the ca. 4800 BP impoundment of Squamish River has been
suggested to indicate that an eruption of Mount Cayley
accompanied, and even caused, the massive debris avalanche
that blocked the river (Evans 1990). Although a subsurface
heat source exists within the volcano, as indicated by the
presence of hot springs in the Turbid Creek basin (Souther
1980), we have found no evidence of recent eruptive products anywhere in the area. More significantly, the white bed
seems too fine to be the product of a local eruption.
No Holocene tephra deposits are known to occur in the
Turbid Creek debris fan area, although two are present
regionally in southwestern British Columbia. Mazama tephra
was produced by an eruption at Crater Lake (Mount Mazama),
Oregon, ca. 6800 BP (Bacon 19831, and occurs throughout
the Pacific Northwest. Mazama tephra has been identified in
Cheakamus Valley south-southeastof Mount Cayley (Mathews
1972), and in Ashlu Valley to the south-southwest (Brooks
1992), but it is not clear if the tephra covered the area now
buried by the Turbid Creek debris fan (Fig. 1). Bridge River
tephra is a product of the ca. 2350 BP eruption at Plinth
Peak, Mount Meager volcanic complex about 70 km northwest of Mount Cayley (Fig. 1) (Nasmith et al. 1967;
Mathewes and Westgate 1980). It probably does not occur
as far south as Mount Cayley (Fig. 1).
The white bed reported here is younger than Mazama
tephra and older than Bridge River tephra. Although it conceivably could consist of grains reworked from a deposit of
Mazama tephra upstream of the impoundment, there are no
known occurrences of Mazama tephra in Elaho and upper
Squamish valleys (Fig. 1). Even if Mazama tephra is present
in these areas, it likely is of limited extent and thus could not
produce such a concentrated reworked layer at the toe of the
Turbid Creek debris fan, especially since the tephra would
have to have been eroded and redeposited some 2000 radiocarbon years after the eruption. In addition, the composition
of the white bed with its high proportion of crystalline grains
differs significantly from Mazama and Bridge River tephras.
Although the white bed is not associated with either the
Mount Mazama or Mount Meager eruptions, it is conceiv-
Can. J . Earth Sci. Vol. 32, 1995
ably a product of a distant but unknown volcanic eruption.
Noneruptive origin
The fact that the white bed was deposited shortly after the
ca. 4800 BP debris avalanche suggests that the two are
related. The source rocks of the debris avalanche are the
product of the Mount Cayley and Vulcan's Thumb eruptive
stages (Evans and Brooks 1991) and consist of lava flows,
dykes, breccia, and pyroclastic deposits (Souther 1980;
Clague and Souther 1982). The isotropic grains within the
white bed thus could have been inherited from the source
rocks.
There are two reasonable noneruptive explanations for the
formation of the white bed. The bed may consist of silt and
clay grains that were transported from the debris avalanche
diamicton by subsurface runoff, and redeposited as a discrete
bed. However, the fact that the white bed is a single, welldefined layer (Fig. 4B) rather than an irregular, diffuse zone
does not support this hypothesis. Alternatively, the white bed
could be dust generated by the debris avalanche.
Large dust clouds commonly rise from and envelop rapidly
moving streams of fragmented rock and may be carried considerable distances by powerful air blasts (Eisbacher and
Clague 1984). Brief reference to dust clouds is contained in
the descriptions of many landslides (e.g . , Plafker and Ericksen
1978; Hewitt 1988). Some of this literature also mentions
dust deposits (e.g., McConnell and Brock 1904; Mathews
and McTaggart 1978), but these generally are not described
in detail. However, there are accounts of significant accumulations of landslide dust. Heim (1882) mentions a 2-3 cm
thick layer of black slate dust that was deposited upon the
rubble of the 1881 landslide at Elm, Switzerland, and Crandell and Fahnestock (1965) refer to "nearly an inch" of silt
and fine sand that settled on trees following a series of large
1963 rockfalls from the eastern flank of Mount Rainier volcano. In both cases, the sediment was later washed away
by rain.
The ca. 4800 BP debris avalanche transported volcanic
material from the slopes of Mount Cayley to the bottom of
Squamish Valley, pulverizing large amounts of rock into
sand and silt, which now constitute the matrix of the debris
avalanche deposit (Evans and Brooks 1991; Brooks 1992).
The mechanical disintegration of rock during movement of
the debris avalanche undoubtedly generated a large dust
cloud that would have settled on the valley floor. It is
unlikely, however, that the white bed is dust that settled out
directly from this cloud because the bed occurs within the
backwater deposits rather than lying directly on the debris
avalanche diamicton (Fig. 3). It is likely that the white bed
consists of sediment that was washed into the reservoir
shortly after it began to fill, and thus is a secondary dust
deposit. As mentioned above, the dust deposits described by
both Heim (1882) and Crandell and Fahnestock (1965) eventually were washed away by rainfall.
The dust hypothesis seems the most likely explanation for
the origin of the white bed, but this is not unequivocal
because of uncertainty concerning the interpretation of the
geochemical evidence. Although the high proportion of
crystalline grains within the white bed probably supports a
noneruptive origin, this could result from the incorporation
of lithic fragments into a tephra cloud, or perhaps from the
mixing of tephra and other silt and clay grains carried by
Squamish River into the lake impounded by the debris
avalanche. Similarly, if the 20 grains with tephra-like morphologic features are volcanic glass (Table I), their collective heterogeneous composition does not clearly support a
noneruptive hypothesis because lithic volcanic glass fragments also could have been incorporated into a tephra cloud.
Conclusions
A distinct, 1-2 cm thick, tephra-like bed containing a high
proportion of crystalline grains occurs within backwater
deposits that accumulated in response to the impoundment of
Squamish River by a large debris avalanche from Mount
Cayley ca. 4800 BP. This bed is interpreted to be landslidegenerated dust that settled in the valley and was later washed
into the lake impounded behind the debris avalanche deposit.
It does not constitute evidence of a volcanic eruption associated with the ca. 4800 BP debris avalanche from Mount
Cayley. We recommend caution when a new tephra bed is
encountered close to a dormant or extinct volcano that has
experienced large landslides; the bed is not necessarily the
product of a volcanic eruption.
Acknowledgments
The possible landslide-dust origin of the white bed was first
suggested to us by Peter Jordan of the British Columbia
Ministry of Forests. Drafts of this paper were read by Steve
Evans, John Westgate, Thomas Pierson, Jon Riedel, John
Clague, and an anonymous reviewer; their comments greatly
improved the paper and are appreciated. The research was
supported by the Natural Sciences and Engineering Research
Council of Canada and the Geological Survey of Canada
(GSC). We thank Mark Gawehns (Simon Fraser University)
for assistance in the field; David Walker (GSC) for scanning
electron microscope images; Michael Beaulne (Canadian
Centre for Mineral and Energy Technology) for microprobe
sample preparation; John Stirling (GSC) for the microprobe
analysis; Gordon Pringle (GSC) for discussion of the microprobe analysis; and Bob Delabio (GSC) for the X-ray diffraction analysis.
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