Impact of a Quaternary volcano on Holocene sedimentation in Lillooet River valley, British Columbia

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Impact of a Quaternary volcano on Holocene sedimentation in
Lillooet River valley,British Columbia
P.A.Friele
a,
T
,J.J.Clague
b
,K.Simpson
c
,M.Stasiuk
c
a
Cordilleran Geoscience,1021,Raven Drive,P.O.Box 612,Squamish,BC,Canada V0N 3G0
b
Department of Earth Sciences,Simon Fraser University,Burnaby,BC,Canada V5A 1S6;Emeritus Scientist,
Geological Survey of Canada,101-605 Robson Street,Vancouver,BC,Canada V6B 5J3
c
Geological Survey of Canada,101-605 Robson Street,Vancouver,BC,Canada V6B 5J3
Received 3 May 2004;received in revised form 15 December 2004;accepted 19 January 2005
Abstract
Lillooet River drains 3850 km
2
of the rugged Coast Mountains in southwestern British Columbia,including the slopes of a
dormant Quaternary volcano at Mount Meager.A drilling program was conducted 32–65 km downstream from the volcano to
search for evidence of anomalous sedimentation caused by volcanismor large landslides at Mount Meager.Drilling revealed an
alluvial sequence consisting of river channel,bar,and overbank sediments interlayered with volcaniclastic units deposited by
debris flows and hyperconcentrated flows.The sediments constitute the upper part of a prograded delta that filled a late
Pleistocene lake.Calibrated radiocarbon ages obtained from drill core at 13 sites show that the average long-term floodplain
aggradation rate is 4.4 mm a
￿1
and the average delta progradation rate is 6.0 m a
￿1
.Aggradation and progradation rates,
however,varied markedly over time.Large volumes of sediment were deposited in the valley following edifice collapse events
and the eruption of Mount Meager volcano about 2360 years ago,causing pulses in delta progradation,with estimated rates to
150 m a
￿1
over 50-yr intervals.Two of the volcaniclastic units identified in drill core correlate with previously documented
strong acoustic reflectors in Lillooet Lake at the downstreamend of the basin.The Mount Meager massif constitutes only 2%of
the Lillooet River drainage,but lithology counts of Lillooet River channel gravels indicate that a disproportionate percentage of
the sediment is derived from the volcano.The data indicate that deposits of large debris flows are important elements of the
sedimentary sequence and that Mount Meager dominates the sediment supply to Lillooet River.
D 2005 Elsevier B.V.All rights reserved.
Keywords:Valley fill;Stratigraphy;Debris flow;Hyperconcentrated flow;Fiord-lake;Holocene;Lillooet River;Mount Meager;British
Columbia
1.Introduction
Delta geomorphology,architecture,and sedimen-
tary processes are governed by a host of factors,
0037-0738/$ - see front matter D 2005 Elsevier B.V.All rights reserved.
doi:10.1016/j.sedgeo.2005.01.011
T Corresponding author.Fax:+1 604 898 4742.
E-mail address:friele@telus.net (P.A.Friele).
Sedimentary Geology 176 (2005) 305–322
www.elsevier.com/locate/sedgeo
including the character of the receiving basin (bathy-
metry,water circulation,wave and current regimes,
and thermal and density stratification) and changes in
base level,sediment supply,and climate (Kostaschuk,
1987;Smith,1991;Kazuaki et al.,2001).In this
paper,we examine the contribution of a Quaternary
volcano to a deltaic valley fill in the Lillooet River
valley in southwestern British Columbia.We demon-
strate that instability on the volcano during the
Holocene strongly influenced the evolution of the
delta.
The sediment fill in the Lillooet River valley
records postglacial infilling of a 75-km-long fjord lake
downstream from Mount Meager volcano (Fig.1).
Slaymaker (1972,1977) summarized historic sedi-
ment yield in the valley,whereas Jordan and Slay-
maker (1991) and Slaymaker (1993) estimated the
long-term yield using a sediment budget approach.
Jordan and Slaymaker (1991) noted a 50% discrep-
ancy between the budgeted and historic sediment
yields and surmised that it might be due to a change in
sediment delivery to Lillooet River following the
Little Ice Age or to underestimation of the frequency
or magnitude of landslides at Mount Meager.
Jordan and Slaymaker (1991) further proposed a
modification of the paraglacial sediment concept.
Paraglacial sedimentation involves large transfers of
sediment from uplands to river valleys during and
Fig.1.A) Map of southern British Columbia showing the location of the Mount Meager volcanic complex (MMVC).B) Southwestern British
Columbia,showing Highway 99 from Vancouver to Pemberton,and the Mount Meager volcanic complex.C) The study area showing the
Lillooet River valley,and drill sites DHPV01-12 and SH1.The white bars delineate the four river reaches:1-Meager Creek to Railroad Creek;2-
Railroad Creek to Ryan River;3-Ryan River to Green River;4 Green River to Lillooet Lake.Human settlement extends from the lower end of
reach 1 (at DHPV09) to Lillooet Lake.
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322306
immediately after deglaciation,causing rapid growth
of fans at tributary mouths (Ryder,1971) and
subsequent aggradation of floodplains (Church and
Slaymaker,1989).Jordan and Slaymaker (1991)
suggested that Holocene sediment yield in the
Lillooet River basin is episodic,with numerous
pulses induced by volcanism,landslides,Neoglacia-
tion,and land-use change.
Desloges and Gilbert (1994) studied sedimentation
rates in Lillooet Lake and suggested that fines (b63
Am),which make up the lacustrine sediment pile,are
produced most abundantly by glacial comminution of
rock debris and that landslides are a relatively minor
source of fine sediment.They thus stress the
dominance of the paraglacial sediment source to
Lillooet Lake.
Based on the history of other Cascade volcanoes,
episodic sedimentation should be recorded as a large
number of discrete thick beds within the Holocene
sediment fill in Lillooet River valley.Radiocarbon
ages from landslide debris exposed along Meager
Creek and upper Lillooet River (Friele and Clague,
2004;Jordan,1994;McNeely and McCuaig,1991)
suggest that at least 12 large prehistoric landslides
occurred in these valleys.In addition,an outburst
flood occurred shortly after the last eruption at
Mount Meager (Stasiuk et al.,1996).There probably
have been other large landslides,but they have not
been identified due to burial or erosion,while
smaller debris flows occur almost annually (Jakob,
1996).
By drilling the upper sediments of the Lillooet
valley fill,we test the episodic sediment yield model
proposed by Jordan and Slaymaker (1991).In this
paper,we describe the architecture of the upper valley
fill and calculate rates of floodplain aggradation and
delta-front progradation using radiocarbon ages on
fossil plant material recovered from drill cores.
Finally,we compile the evidence to counter the notion
that the pararglacial sediment supply dominates in the
Lillooet River basin.
2.Setting
Lillooet River flows in a glacially modified river
valley in the southern Coast Mountains of British
Columbia (Fig.1).The Lillooet River basin (3850
km
2
) is rugged,with up to 2800 m of local relief and
peaks up to 3000 m in elevation.About 15% of the
basin is glaciated.Most of the basin is underlain by
resistant plutonic rocks (Woodsworth,1977),but the
Mount Meager volcanic complex (Fig.2),a dormant
Quaternary volcano,underlies about 2% in the basin
headwaters.
Large parts of the Mount Meager volcanic complex
are hydrothermally altered (Read,1979).The alter-
ation,together with the steep slopes,result in high
rates of mass wasting and landslides.Four landslides
in excess of 1
￿
10
6
m
3
occurred in the last century
alone-in 1931,1947,1975,and 1998 (Bovis and
Jakob,2000;Carter,1932;Croft,1983;Evans,1987;
Mokievsky-Zubok,1977).The 1931 failure produced
a secondary debris flow that reached the mouth of
Meager Creek (Fig.2) and caused flood surges along
Lillooet River (Decker et al.,1977,p.161).Friele and
Clague (2004) documented flank collapses 8700 and
4400 years ago,involving at least 6
￿
10
8
m
3
of
material on the south side of the volcanic complex.
Debris flows from these flank collapses traveled the
length of Meager Creek into Lillooet River valley.In
addition,Plinth Peak (Fig.2) erupted explosively
about 2360 years ago (Clague et al.,1995;Nasmith et
al.,1967),producing volcanic and landslide deposits
that dammed upper Lillooet River (Stasiuk et al.,
1996;Stewart,2002).The dam failed,causing an
outburst flood that swept down the valley (Evans,
1992;Stasiuk et al.,1996).This instability is well
documented in the valleys adjacent to Mount Meager,
but its effects in the Lillooet valley to the south are
unknown because Lillooet River aggraded throughout
the Holocene (Jordan and Slaymaker,1991),burying
the evidence.
Lillooet River is divisible into four reaches below
the mouth of Meager Creek (Fig.1;after Jordan and
Slaymaker,1991).The first,most northerly reach
extends 25 km downvalley from Meager Creek to
Railroad Creek.The river in this reach is braided
(Fig.2),has a cobble bed (Fig.3),and a gradient of
0.006.Gravelly colluvial fans from tributary valleys
and rockslide deposits extend onto the active flood-
plain,which is up to 1 km wide.The second reach
extends 10 km downvalley from Railroad Creek to
the Ryan River fan (Fig.4).In this reach,the
channel is wandering (Desloges and Church,1987)
to meandering and has a pebble-cobble bed (Fig.3)
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322 307
Fig.2.View northwest up Lillooet River to the Mount Meager volcanic complex (Province of British Columbia airphoto BC563:113;taken
September 29,1949).In the foreground is a 12-km length of reach 1,showing the braided channel of Lillooet River and confining debris fans.
DGS indicates the fresh track of the 1931 debris flow that traveled the length of Meager Creek.The basin on the west flank of Pylon Peak is the
source of two large landslides mentioned in the text (8700 and 4400 years ago).The valley just west of Mount Meager is the source of large
landslides in 1932 and 1998.Plinth Peak is the source of the ca.2400-year-old eruption.
Fig.3.Bar-top texture and lithology trends in Lillooet River valley (after Kerr Wood Leidal,2002).Reach breaks and drill core locations are
also shown.
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322308
and a gradient of 0.0025.The active floodplain is
1.5–2 km wide and is bounded by steep bedrock
slopes.The third reach extends a further 15 km
downvalley to Green River.Prior to training,this
part of the river had an irregular meandering
planform (Fig.4).It has a sand to pebble gravel
bed (Fig.3) and a gradient of 0.0009.The active
floodplain in reach 3 is 1.5–2 km wide and extends
to the valley walls.The fourth reach,between Green
River and Lillooet Lake,is 15 km long and has a
straight to sinuous channel,a sand bed,and a
gradient of 0.0006.Reach 4 terminates at the modern
delta of Lillooet River.
Channel widths are 120–200 m,average channel
depths are 3–6 m,and thalwag depths are 7–9 m in
reaches 2,3,and 4.Width-to-depth ratios range from
20 to 60 (hydraulic geometry from Kerr Wood Leidal,
2002).Local relief on the floodplain ranges from 1 to
4 m(B.C.Ministry of Environment,1990).Vegetation
prior to settlement consisted of forest and wetlands
(Decker et al.,1977;Teversham and Slaymaker,
1977).
Lillooet Lake is 24 km long,1–2 km wide,and up
to 140 m deep (Desloges and Gilbert,1994).The lake
is impounded behind a paraglacial alluvial fan at the
mouth of Kakila Creek (Fig.1),and its level (196F4
m above sea-level) is controlled by bouldery fan
deposits at the outlet.The level was artificially
lowered 2.5 mby dredging the outlet in 1956 (Gilbert,
1972;Nesbitt-Porter,1985),thus its level prior to
settlement was 198.5 m asl.
The Lillooet River delta satisfies many of the
criteria of the fan-foreset delta of Smith (1991).This
type of delta forms where a stream deposits sandy and
gravelly bed load in a deep body of water.However,
the gradient of Lillooet River is too low (0.0006) and
the bed material is too fine (mainly sand) for this term
to be properly applied to the Lillooet delta.It is thus
more appropriately termed a lake-head foreset delta
(Kostaschuk,1987).
The delta is rapidly extending into Lillooet Lake.
Detailed ground surveys and air photo analysis
(Gilbert,1972,1975) indicate an average progradation
rate of 7–8 ma
￿1
between 1858 and 1948.In the early
1950s,the lowermost 30 km of the river was
straightened and dyked.These measures,along with
dredging,steepened the channel gradient,causing
incision and bank erosion.Thus,in the period 1948–
1969,progradation rates increased markedly to 20–30
m a
￿1
.From 1969 to the present,progradation rates
Fig.4.Aerial photograph of Lillooet River valley taken during the early stages of river training (Province of British Columbia airphoto BC396:
116;August 2,1947).The Ryan River fan forms the boundary between reaches 2 and 3.Cutoff ages are fromDecker et al.(1977).The locations
of drill sites DHPV06 and DHPV12 are shown.Note the meandering channel planform of Lillooet River,with distal wetlands,back channels,
and abandoned meander loops.
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322 309
have averaged 15 m a
￿1
(Jordan and Slaymaker,
1991;Kerr Wood Leidal,2002).
3.Valley fill sediments
Lillooet River is typical of the coarse-grained
streams described by Brierley (1989) and Miall
(1996).Common environments on the floodplains of
such rivers include channels,point bars,levees,
abandoned meander loops,oxbow lakes,and swamps
(Fig.4).A typical alluvial sequence consists of
upward-fining channel fills interlayered with overbank
sediments that fine away from channels (Table 1).
Alluvial fans extend into the Lillooet valley from
tributary streams underlain by plutonic rocks.Fans
from large basins (N15 km
2
) are mainly fluvial in
origin,whereas those from smaller basins have been
formed partly or largely by debris flow activity
(DeScally et al.,2001).
Volcanic debris flows triggered by eruptions
(Crandell,1971;Scott et al.,1992;Vallance and
Scott,1997) or flank collapse (Siebert,2002) may
leave valley-wide deposits many tens of kilometers
from their source (Iverson et al.,1998).The Lillooet
River valley fill sequence thus could include deposits
of debris flows and hyperconcentrated flows derived
from Mount Meager (Table 1).
4.Drilling rationale and methods
4.1.Rationale
Assuming a conservative,long-term progradation
rate of 5 m a
￿1
,the delta front was 50 km upstream
10000 years ago,soon after the valley was deglaci-
ated (Friele and Clague,2002).This puts the head of
Lillooet Lake somewhere within reach 1 (Fig.2) at the
beginning of the Holocene.Drilling sites within
reaches 1,2,and 3 (Figs.1 and Figs.3) are thus
within the area of the lake that was infilled during the
Holocene.Only the subaerial part of the valley fill was
targeted (i.e.materials above mean lake level,or 198.5
masl).It was not possible to drill the 100–140 mthick
subaqueous sequence because of the cost.Moreover,
we recognized that radiocarbon ages on proximal
deltaic sediments would be difficult to interpret due to
subaqueous landsliding (Gilbert,1972).
We chose target depths for drilling on the basis of
the estimated thickness of topset deposits,assuming
that mean lake level has not changed since the
middle Holocene.This assumption is justified on the
following grounds.First,recent studies of well-dated
alluvial fans in British Columbia indicate they
formed quickly during and immediately after degla-
ciation (Lian and Hickin,1996;Friele and Clague,
2002).Accordingly,the Kakila Creek fan was
Table 1
Criteria for interpretation of facies encountered in drill core
Facies Depositional
environment
Structure Lithology Texture Reference
Channel fill Active channel Clast-supported,
massive to stratified
(cross-bedded,rippled)
b50% volcanic Coarse silt to cobbles,
but dominantly very
fine sand to granule
gravel;clasts of mud
and peat due to bank
collapse.
Brierley,1989;
Gilbert,1972;Kerr
Wood Leidal,2002.
Overbank Floodplain Horizontally laminated
to bedded
Not studied Fine to very fine sand,
silt,and clay interbedded
with peat.
Brierley,1989;
Miall,1996.
Hyper-concentrated
flow
Channel and
overbank
Clast-supported,massive
to weakly bedded,
normally graded
N50% volcanic Poorly-sorted,sandy
granule gravel;may have
cap of wood debris
or pumice.
Pierson and Scott,1985;
Smith,1986
Debris flow Channel and
overbank
Massive to weakly
stratified,matrix
supported
N75% volcanic;
includes
hydrothermally
altered clasts.
Diamicton Pierson,1985;
Jordan,1994
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322310
probably built across Lillooet Lake during the early
Holocene,implying that mean lake level (ca.198.5
m asl) has changed little since then.Second,the
Lizzie Creek fan-delta today forms a sill that divides
Lillooet Lake into separate subaqueous basins
(Desloges and Gilbert,1994),and other,now-buried,
valley-side fan-deltas in Lillooet River valley may
have done the same.However,the surfaces of fans
emanating from large tributary basins,such as those
of Green,Birkenhead,and Ryan rivers,intersect
paleo-lake level before reaching the opposite valley
side.This observation indicates that the Holocene
lake was a continuous water body and not several
smaller lakes separated by fans.The pre-1950s lake
surface elevation (198.5 m asl) is thus taken as the
level of the paleo-lake over its entire length.
4.2.Field methods
Twelve sites along the axis of the valley were
drilled to depths of 10–40 m (Fig.1).About 260 m of
core were recovered,logged,and sampled.A conven-
tional dry auger fitted with a 10-cm split spoon
sampler was used at sites DHPV01,DHPV02,
DHPV03,DHPV04,DHPV05,and DHPV06 (Fig.
1).Drilling was done in 1.5-m (5-ft) flights.Most
retrieved cores occupied no more than about 60% of
the sampler.Due to poor core recovery,we switched
to sonic drilling technology for sites DHPV07,
DHPV08,DHPV09,DHPV10,DHPV11,and
DHPV12 (Fig.1).This system yielded continuous
core in 3-mflights.Core recovery was good,except in
cobble gravel,which was encountered only at the
most northerly site (DHPV09).
Cores were extruded into plastic sleeves and then
split and logged in the field.One half of each core was
archived at the Vancouver office of the Geological
Survey of Canada.Sediment texture,structures,clast
shape and lithology,plant material,and contacts were
described for strata as thin as 0.5 cm.Representative
cores and important features and contacts were
photographed.Samples were collected for radiocar-
bon dating,pebble lithology,and clay mineral and
grain size analyses.Surface elevations were estimated
from British Columbia Ministry of Environment
floodplain maps (B.C.Ministry of Environment,
1990),which have a 1-m contour interval and spot
elevations with decimeter accuracy.
Additional data were obtained during a geotech-
nical investigation for a new school at Pemberton
(Signal Hill;site SH1,Fig.1) in 2002.A rotary auger
was used to drill four 10–15-m holes,and a cable-tool
drilling system was used to drill one 110-m-deep
cased well.These drilling techniques yielded basic
lithological information and material for radiocarbon
dating.
5.Stratigraphy
Facies interpretations are based on the source
material summarized in Table 1.Drill cores were
grouped according to their location in the valley.Four
cores (DHPV09,DHPV11,DHPV06,and DHPV12)
are arranged in a downvalley transect 34–50 km from
Mount Meager (Fig.5).Three cores (DHPV03,
DHPV04,and DHPV05) form a cross-valley transect
(Fig.6) about 42 kmdownvalley fromMount Meager.
The stratigraphy at these seven sites differs in detail,
but all sites share a volcanic debris flow marker bed.
In contrast,volcaniclastic deposits were not encoun-
tered in three drill holes (DHPV02,DHPV08,and
DHPV01),53–57 km from Mount Meager (Fig.7).
However,a pumice-rich,sandy pebble gravel marker
bed occurs at or just above mean lake level at the
southernmost drill sites (DHPV07 and DHPV10),63–
65 km from Mount Meager (Fig.8).
5.1.Channel deposits
Fining-upward sequences with cobble or pebble
gravel at the base and medium to coarse sand at
the top are interpreted to be channel gravel and
point bar deposits.Cobble gravel beds up to 6 m
thick form the channel facies at site DHPV09.
Overbank facies are uncommon at this site,which
may reflect channel instability in reach 2 and the
upper part of reach 3.Channel units at other sites
consist of pebble gravel beds tens of centimeters
thick.Lags with mud ball clasts were noted at sites
DHPV03 and DHPV01.The lags occur at the base
of medium to coarse sand beds in sequences
ranging up to 6 m thick.The core at site DHPV08
contains three sequences that grade from channel
facies at the base to overbank facies at the top
(Fig.7).
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322 311
5.2.Overbank deposits
Laminated fines and peat are common at all drill
sites.Clastic strata range from millimeters to tens of
centimeters thick,and peat beds are up to several
metres thick.These deposits are commonly interbed-
ded with fine to very coarse sand and stringers of fine
gravel,which we interpret to be sand sheets.Most
overbank units are 2–5 m thick,suggesting consid-
erable channel stability during aggradation.The
maximum logged thickness is 8 m (sites DHPV11,
DHPV06,DHPV02,and DHPV07).
5.3.Hyperconcentrated flow deposits
Massive to weakly stratified beds of poorly sorted,
volcanic-rich coarse sand and fine gravel from a few
decimeters thick to about 4 m thick were encountered
at sites DHPV03,DHPV04,DHPV05,DHPV06,
DHPV11,and DHPV12.These sediments are inter-
preted to be hyperconcentrated flow deposits (Table
1).
Hyperconcentrated flow deposits in Lillooet valley
should contain a greater proportion of volcanic clasts
than normal river bedload.In the reaches of interest,
lithology counts of pebbles on bar surfaces of the
modern river yielded an average of 25% volcanic
clasts and 75% basement clasts (Fig.3).Lithology
counts of selected samples of very coarse sand and
pebbles (1–25 mm size class) from core (Table 2)
indicate that beds interpreted a-priori as normal
channel deposits contain 25–50% volcanic clasts,
whereas those interpreted as hyperconcentrated flow
deposits contain 50–100% volcanic clasts.The higher
percentage of volcanic clasts in some channel gravel
beds in cores,in comparison to modern channel
Fig.5.Core logs for sites DHPV09,DHPV11,DHPV06,and DHPV12 (see Fig.1 for locations).
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322312
gravel,may be due to reworking of prehistoric,
volcanic debris flow and hyperconcentrated flow
deposits.No historic volcanic debris flows have
traveled significantly far down Lillooet River valley,
but several have done so earlier in the Holocene
(Friele and Clague,2004).
Scattered,rounded pumice grains up to pebble-size
were noted throughout hyperconcentrated flow units
in cores DHPV05 and DHPV06.In addition,the
surface of the hyperconcentrated unit at DHPV05 is
capped by 30–50 cm of poorly sorted angular pumice
clasts ranging up to cobble size,interpreted to be float
(Pierson and Scott,1985).Awood fragment from the
hyperconcentrated flow unit at site DHPV06 yielded a
radiocarbon age of 2480F40
14
C yr BP (2406–2789
cal yr BP;Table 3).
A 2.5-m-thick hyperconcentrated flow unit at site
DHPV12 rests on channel gravel,which in turn
sharply overlies floodplain sediments dated at
2000F60
14
C yr BP (1923–2046 cal yr BP;Table
3).The dated floodplain sediments were eroded before
the overlying channel gravel was deposited and
provide only a poorly constrained maximum age for
the hyperconcentrated flow at this site.The hyper-
concentrated flow unit is thus appreciably younger
than 2000 years and is tentatively correlated with a
Fig.6.Core logs for sites DHPV03,DHPV04,and DHPV05 (see Fig.1 for locations).
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322 313
900-year-old,valley-filling debris-flow deposit in
upper Lillooet River valley (Jordan,1994).
Thick (2–6 m) hyperconcentrated flow units were
encountered at the base of the drill holes at sites
DHPV04 and DHPV11.An age of 3230F70
14
C yr
BP in sand 5 mabove the hyperconcentrated flow unit
at site DHPV04 and an age of 4550F90
14
C yr BP
from within the unit at site DHPV11 indicate that the
event occurred sometime between about 3300 and
5500 cal yr ago (Table 3).The flow may be associated
with a large flank collapse of Pylon Peak into Meager
Creek valley about 4400 cal yr ago (Friele and
Clague,2004).
5.4.Debris flow deposits
A layer of volcanic diamicton 2–4.5 m thick and at
least 1000 m wide was encountered in drill holes 34–
50 km downvalley from Mount Meager (Figs.5 and
Figs.6).Three samples of wood fragments collected
from the diamicton and one from peat on the surface
of the deposit provide a bracketed age between
2570F40
14
C yr BP and 2690F50
14
Cyr BP (Table
3).Based on these limiting dates,the deposit is 2540–
2970 cal yr old.Its age,volcanic provenance and
massive,matrix-supported structure indicate a debris
flow origin (Table 1).A strong acoustic reflector 13–
32 mbelow the floor of Lillooet Lake,which has been
estimated to be 2495F670 years old based on
sedimentation rates (Desloges and Gilbert,1994),
probably correlates with the debris flow.
The debris flow overrode overbank sediments at
four sites (DHPV03,DHPV04,DHPV11,and
DHPV12),indicating that it was not confined to the
river channel.The diamicton lies on channel gravel at
sites DHPV09,and DHPV05,suggesting that it was
Fig.7.Core logs for sites DHPV02,DHPV08,and DHPV01 (see Fig.1 for locations).
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322314
flowing down the old river channel at those sites.The
lower part of the diamicton includes gravel at
DHPV06 and rip-ups of silty floodplain sediments at
DHPV12 (Fig.9),suggesting significant shear stress
at the base of the debris flow,42–50 km downvalley
from the source.These observations indicate that the
debris flow likely traveled farther downvalley than we
have documented through drilling.
Fig.8.Core logs for sites DHPV07,DHPV10 and SH1 (see Fig.1 for locations).
Table 2
Lithology of selected gravel samples
Drill hole Depth (cm) Lithology (%) (1–25 mm size class) Facies interpretation
Volcanic Basement Miscellaneous
DHPV11 480–500 51 40 9 Hyperconcentrated flow
DHPV11 650–670 64 24 12 Hyperconcentrated flow
DHPV11 750–770 47 32 21 Hyperconcentrated flow
DHPV11 850–870 88 8 4 Hyperconcentrated flow
DHPV11 2415–2438 65 27 8 Hyperconcentrated flow
DHPV11 2600–2620 75 18 7 Hyperconcentrated flow
DHPV11 2980–3000 46 42 12 Hyperconcentrated flow
DHPV04 710–730 69 21 10 Hyperconcentrated flow
DHPV04 2081–2100 72 23 5 Hyperconcentrated flow
DHPV08 300–460 48 39 13 Channel
DHPV08 900–1000 37 55 8 Channel
DHPV08 1680–1830 49 42 9 Channel
DHPV02 760–910 35 58 7 Channel
DHPV02 1220–1370 23 71 6 Channel
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322 315
Table 3
Radiocarbon ages
Drill hole Latitude
(N)
Longitude
(W)
Depth
(cm)
Material Enclosing facies Lab number
a
Radiocarbon
age (
14
C yr BP)
b
Calendric age (yr BP)
c
Mode Range
DHPV09 508 31.314V 1238 04.359V 2920 Wood
fragment
Debris flow OS-36552 6370F35 7340 7297–7466
DHPV09 508 31.314V 1238 04.359V 3100–3200 Wood
fragment
Debris flow OS-36556 6250F30 7260 7076–7302
DHPV03 508 29.779V 1228 57.990V 490–500 Peat Overbank Beta-166050 1960F40 1960 1872–2043
DHPV03 508 29.779V 1228 57.990V 790 Wood
fragment
Debris flow Beta-166057 2690F50 2860 2794–2966
DHPV03 508 29.779V 1228 57.990V 2440 Peat Overbank Beta-166053 5130F60 5910 5785–6041
DHPV04 508 29.550V 1228 58.131V 1040 Twig Debris flow Beta-166058 3970F40 4460 4347–4575
DHPV04 508 29.550V 1228 58.131V 1500 Wood
fragment
Channel Beta-166051 3230F70 3480 3322–3684
DHPV05 508 30.048V 1228 58.053V 540 Peat Overbank Beta-166052 1780F60 1750 1590–1916
DHPV05 508 30.048V 1228 58.053V 680 Charcoal Paleosol Beta-166059 2570F40 2790 2544–2809
DHPV11 508 29.518V 1228 58.082V 1620 Wood
fragment
Overbank GSC-6647 2950F50 3150 3047–3285
DHPV11 508 29.518V 1228 58.082V 1810 Peat Overbank GSC-6646 3590F60 3930 3828–4028
DHPV11 508 29.518V 1228 58.082V 2410 Wood
fragment
Hyperconcentrated
flow
GSC-6645 4550F90 5240 5096–5490
DHPV06 508 27.674V 1228 56.155V 600 Peat Overbank Beta-166165 1250F60 1250 1045–1341
DHPV06 508 27.674V 1228 56.155V 886 Wood
fragment
Paleosol Beta-166166 2480F40 2630 2406–2789
DHPV06 508 27.674V 1228 56.155V 1960 Wood
fragment
Overbank Beta-166054 3870F40 4370 4200–4466
DHPV12 508 26.057V 1228 54.550V 800 Peat Overbank GSC-6654 2000F60 1990 1928–2050
DHPV12 508 26.057V 1228 54.550V 1110 Wood
fragment
Debris flow GSC-6648 2720F80 2870 2802–2968
DHPV02 508 24.239V 1228 53.147V 1510 Wood
fragment
Overbank Beta-166049 2840F80 3020 2826–3260
DHPV02 508 24.239V 1228 53.147V 1800 Wood
fragment
Overbank GSC-6651 3320F60 3610 3518–3686
DHPV02 508 24.239V 1228 53.147V 1840 Wood
fragment
Overbank GSC-6650 3700F60 4080 3978–4196
DHPV02 508 24.239V 1228 53.147V 1970 Wood
fragment
Overbank GSC-6649 3820F60 4270 4143–4399
DHPV02 508 24.239V 1228 53.147V 2260 Wood
fragment
Overbank Beta-166056 4410F40 5020 4914–5324
DHPV01 508 22.654V 1228 51.734V 1040 Peat Overbank GSC-6653 1220F50 1180 1111–1307
DHPV01 508 22.654V 1228 51.734V 1210 Peat Overbank GSC-6652 1990F60 1990 1923–2046
DHPV01 508 22.654V 1228 51.734V 1900 Wood
fragment
Channel Beta-166055 2510F40 2660 2411–2796
SH1 508 17.00V 1228 50.00V 680 Wood
fragment
Overbank Beta-139037 1860F70 1830 1661–2015
SH1 508 17.00V 1228 50.00V 1200 Wood
fragment
Channel GSC-6546 2680F60 2820 2799–2897
SH1 508 17.00V 1228 50.00V 1680 Wood
fragment
Foreset GSC-6575 3100F80 3370 3260–3433
a
Beta–Beta Analytic,GSC-Geological Survey of Canada,OS-.
b
Laboratory-reported uncertainties are 1j for Beta and OS ages and 2j for GSC ages.Ages are normalized to y
13
C=￿25.0xPDB.Datumis
AD 1950.
c
Determined from atmospheric decadal dataset of Stuiver et al.(1998) using the program CALIB 4.0.2.The range represents the 95%
confidence limit calculated with an error multiplier of 1.The mode is the average of the part of the calibration probability distribution that
explains more than 50% of the variance.Datum is AD 2000.
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322316
No evidence,however,has been found for the
debris flow at drill sites south of site DHPV12.Sites
DHPV02,DHPV08,and DHPV01 are located in a
relatively narrow section of the valley,occupied by
both Ryan and Lillooet rivers (Fig.4),where the
potential for erosion is higher.However,the debris
flow may have become confined to the old river
channel in this area,making it more difficult to
locate.
A second volcanic diamicton was encountered at
23–24 mdepth at site DHPV09.We interpret it to be a
debris flow deposit on the basis of the criteria
discussed above.The diamicton is capped by channel
gravel,indicating that its surface was eroded.Its
original thickness is thus unknown.We estimate the
age of the diamicton to be 4300–4530 cal yr based on
an extrapolation of an aggradation rate of 4.4 mm a
￿1
from underlying,radiocarbon-dated deposits (see
section 6.1).The debris flow that deposited the
diamicton probably resulted from a flank collapse at
Pylon Peak (Fig.3) about 4400 years ago (Friele and
Clague,2004).
A third volcanic diamicton,similar to those
described above,was encountered at 28.2–36.2 m
depth at site DHPV09.It was at least 8 m thick when
deposited and has a maximum age of 6250F30
14
C yr
BP (7076–7302 cal yr BP).No landslide or eruption
of this age has been documented in the valleys of
Lillooet River or Meager Creek.
6.Floodplain evolution
6.1.Aggradation rates
Average aggradation rates were estimated from
radiocarbon ages obtained from drill core (Table 4).
Each radiocarbon age was calibrated (Stuiver et al.,
1998),and the modal value of the probability
distribution was used as the most probable age of
the sample (Table 3).Average rates were calculated
fromdated depths to the surface at each site,as well as
between samples.In some cases,two or more dated
samples came froma single unit in a core,allowing an
average aggradation rate to be determined for a
specific facies,for example overbank fines.Uncer-
tainties in radiocarbon ages range from30 to 90 years,
thus aggradation values have possible errors of up to
20%.This error,however,is within the standard
deviation of the mean of all estimates (Table 4).No
attempt was made to correct for sediment compaction.
Average long-term (1000–7200 years) aggradation
rates range from 1.4 to 6.0 mm a
￿1
;the mean and
standard deviation are 4.4F1.3 mm a
￿1
(n=18) (Table
4).Average medium-term (100–1000 years) aggrada-
tion rates range from 1.0 to 6.5 mm a
￿1
,with a mean
and standard deviation of 3.4F2.3 mm a
￿1
(n=5).
Fig.9.Silt rip-up at the base of diamicton (Dmm) in core DHPV12.
The diamicton overlies floodplain silt (Fl) and sand (Ss).
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322 317
Medium-term rates are derived from peat and inter-
bedded peat and mud units.The higher long-term
average rate is explained by the fact that it incorpo-
rates debris and hyperconcentrated flow units.
Rapid aggradation occurs during extreme events
such as overbank floods,hyperconcentrated flows,
and debris flows.Debris flows can deposit valley-
wide sheets metres thick over valley lengths of many
tens of kilometers in minutes to hours.Recorded
thicknesses of debris flow deposits in the Lillooet
valley fill range from 2 to 8 m.Hyperconcentrated
flows may deposit sheets up to 1 m thick in overbank
settings and up to 6 m thick in river channels.In
contrast,rainfall and nival floods produce deposits up
to only about 1 m thick,and these deposits thin
rapidly away from river channels.
6.2.Progradation rates
Modal values of calibrated radiocarbon ages (Table
3) were used to estimate the position of the Lillooet
delta front at different times (Fig.10).It was assumed
that the elevation of the lake surface throughout the
Holocene was the same as that immediately prior to
dredging (198.5 m asl),as discussed previously.At a
few sites,radiocarbon samples were taken from mean
lake level and could be used directly to define delta
front positions.However,at most sites,the deepest
samples were above mean lake level.In these cases,it
was assumed that the sample represents the age of a
particular paleo-floodplain surface graded to mean
lake level.The corresponding delta front position was
determined by projecting the dated sample down-
valley to mean lake level along the slope of the
modern floodplain surface.An assumption implicit in
this analysis is that the slope of the paleo-floodplain is
similar to the modern one.The assumption appears to
be reasonable because the long-term aggradation rate
of 4.4 mma
￿1
results in a rate of delta progradation of
7.3 m a
￿1
,which is the historical rate prior to river
training (see Fig.6 in Jordan and Slaymaker,1991).
There are other possible sources of error in this
analysis.The largest amount of potential error stems
from the unevenness of the floodplain surface.Along
a line perpendicular to the valley axis,floodplain
relief can range up to 3 m,and the range from the
channel thalwag to the levee top is about 8 m.On a
surface with a slope of 0.0009 and with a difference of
4 min sample elevations,the possible error in a down-
valley projection is 4.5 km.This error can be reduced
by considering the depositional environment of the
dated samples and adjusting sample elevations
accordingly.Samples from overbank fines require no
adjustment in elevation,whereas those from channel
gravels must be adjusted upward.Additional error
may be introduced if the dated sample is not the same
age as the enclosing sediment.Radiocarbon ages on
Table 4
Long- and medium-term aggradation rates determined from
calibrated radiocarbon ages
Site Span
(cal yr)
Rate
(mm a
￿1
)
Comment
Long-term rates (1000￿7200 years)
DHPV09 7260 5.0 Base of diamict to surface
DHPV03 5910 4.1 Bottom sample to surface
DHPV03 3950 4.9 Between top and bottom
samples
DHPV04 3480 4.3 Bottom sample to surface
DHPV05 2790 2.5 Bottom sample to surface
DHPV05 1040 1.4 Between top and bottom
samples
DHPV11 5240 5.7 Base of flow to surface
DHPV11 2090 3.8 Between top and bottom
samples
DHPV06 4370 4.5 Bottom sample to surface
DHPV06 3120 4.4 Between top and bottom
samples
DHPV12 2825 4.6 Base of diamict to surface
DHPV02 5020 4.5 Bottom sample to surface
DHPV02 2000 3.8 Between top and bottom
samples
DHPV02 1250 3.7 First and fourth samples;
over-bank unit
DHPV01 2660 7.1 Bottom sample to surface
DHPV01 1440 6.0 Between top and bottom
samples
SH1 1830 3.7 Top sample to surface
SH1 2820 4.3 Second sample to surface
Average 4.4F1.3
Medium-term rates (100￿1000 years)
DHPV11 780 2.5 Upper two samples;
single peat unit
DHPV02 590 5.1 First and second samples;
over-bank unit
DHPV02 470 0.9 Second and third samples;
over-bank unit
DHPV02 190 6.5 Third and fourth samples;
over-bank unit
DHPV01 810 2.0 First and second samples;
single peat unit
Average 3.4F2.3
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322318
delicate plant remains in paleosols and peat probably
represent the true age of the sediments,whereas those
on wood fragments in gravel or diamicton must be
considered maxima.
With these caveats,we have calculated delta front
positions at each of the sites as follows.At site
DHPV09,we obtained radiocarbon ages on two
samples from the basal diamicton.The diamicton
rests on overbank fine sediments.We transferred the
maximum age of the diamicton vertically down to the
top of the overbank sediments and then along the
modern surface gradient downvalley to mean lake
level.Two deep samples were dated at sites DHPV03
and DHPV11,one froma hyperconcentrated flow unit
and another from overbank fine sediments.The
hyperconcentrated flow unit at DHPV11 rests con-
formably on overbank sediments,so we transferred
the age of that sample to the top of the overbank unit,
and then projected the mean depths and ages of that
sample and the sample from overbank sediments at
DHPV03 to lake level.A radiocarbon age was
obtained at the contact between gravel and laminated
sand at site DHPV06,probably on a bar surface
setting.This sample was not adjusted.Its trace,
projected downvalley,intersects a statistically equiv-
alent radiocarbon age on overbank fines at site
DHPV02.The mean of these two ages was projected
to mean lake level.A dated sample from a channel lag
deposit at site DHPV01 was adjusted 4 m upward and
projected downvalley to mean lake level.Lastly,the
upper sample from site SH1,collected from overbank
sediments,was projected directly to mean lake level.
This exercise yielded five delta front positions
(Fig.10) and four intervals over which progradation
rates could be calculated.Estimated average progra-
dation rates are 3 m a
￿1
from 7260 to 5575 cal yr BP,
7 m a
￿1
from 5575 to 4320 cal yr BP,4 m a
￿1
from
4320 to 2660 cal yr BP,and 13 m a
￿1
from 2660–
Fig.10.Longitudinal section of Lillooet River valley,showing radiocarbon ages used to reconstruct the position of the Lillooet Lake delta front
at different times during the Holocene.The radiocarbon ages allowed calculation of long-term,incremental,and pulse progradation rates.
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322 319
1830 cal yr BP.The long-term average for the period
7260–1830 cal yr BP is 6 m a
￿1
,similar to the
average historic rate prior to river training.
The pumice bed at DHPV07 (Fig.8),similar to that
at DHPV10,occurs at a depth of 11.2–11.5 m and
comprises 10 cm of openwork,rounded granule to
fine pebble pumice overlying 15 cm of pumice-rich,
very coarse sand to granule gravel.The rounded
grains indicate that the material was transported as
wash load rather than float,and the presence of
pumice at lake level within channel deposits places
the delta front at this site at the time of the eruption
about 2400 years ago.
Incremental progradation rates range from 3 to 13
m a
￿1
.The average rate during the interval 4840–
3850 cal yr BP is twice that during the interval 7260–
5575 cal yr BP.The period with the higher rate
includes a flank collapse of Pylon Peak into Meager
Creek valley about 4400 cal yr BP (Friele and Clague,
2004),and is likely recorded by the middle diamicton
at site DHPV09 and by the deepest hyperconcentrated
flow units at sites DHPV03 and DHPV11,based on
the estimated ages of these deposits.The average
progradation rate increases over threefold from 4320–
2660 cal yr BP to 2660–1830 cal yr BP.The latter
period includes a large debris flow and the eruption of
Plinth Peak 2360 years ago.The landslide is recorded
at sites DHPV09,DHPV03,DHPV04,DHPV05,
DHPV11,DHPV06,and DHPV12 by a valley-filling
sheet of diamicton that is 2–4 m thick in reach 2 and
the upstream part of reach 3.The eruption and
associated outburst flood are recorded by a hyper-
concentrated flow deposits at sites DHPV03,
DHPV04,DHPV05,DHPV11,and DHPV06.
Event,or pulse,progradation rates were much
larger than suggested by the averages cited above
(Fig.10).Much of the sediment delivered to the delta
front may have arrived during large debris flows and
hyperconcentrated flows (Vallance and Scott,1997),
or in a few decades following these events as the
deposits were reworked by Lillooet River (Major et
al.,2000).If the progradation rate preceding an event
is viewed as a background rate,akin to base flow in a
hydrograph,it can be subtracted from the incremental
rate encompassing the event.The resultant bevent
dischargeQ can then be adjusted to a more appropriate
delivery period,say 50 years.In the case of the 4400
year old landslide,the adjusted pulse rate over 50
years is 105 m a
￿1
.The pulse rate for the landslide
and eruption is about 150 m a
￿1
(Fig.10).
7.Sedimentation in Lillooet lake
The Mount Meager volcanic complex appears to be
the dominant source of sediment deposited in Lillooet
Lake.Both of the strong acoustic reflectors identified
by Desloges and Gilbert (1994) may stem from events
at Mount Meager.Desloges and Gilbert (1994)
correctly attributed the 2495F670-year-old reflector
to volcanism.They concluded,however,that the
upper acoustic reflector,at 6–13 m depth in the lake
sediments and estimated to be 890F240 years old,
was caused by an increase in the magnitude and
frequency of floods during the Little Ice Age.
Unbeknown to them,a 900-year-old,valley-filling
debris flow covering 1.9 km
2
of upper Lillooet River
valley had been documented by Jordan (1994).This
event is the more likely source of the upper reflector.
We argue that volcanic debris is the single most
important source of fine sediment in the Lillooet River
basin.Debris flow deposits at Mount Meager contain
25–50% silt and 3–10% clay by weight,and the clay-
size fraction is dominantly phyllosilicate minerals
(Friele and Clague,2004).Detailed examination of
the clay mineralogy of Lillooet Lake sediments could
confirm that Mount Meager is the dominant sediment
source in the basin.
8.Conclusions
Lillooet valley was occupied by a 75-km-long fiord
lake at the end of the Pleistocene and,over the last
10,000 years,has advanced its delta about 50 km
downvalley through the study area to the present head
of Lillooet Lake.Assuming an average yield of 400 B,
calculated using a sediment budget approach (Slay-
maker,1993),Lillooet River has deposited on the
order of 15 km
3
of sediment over this period.
Average rates of floodplain aggradation and pro-
gradation based on calibrated radiocarbon ages from
drill core are,respectively,4.4 mm a
￿1
and 6 m a
￿1
,
but large landslide events caused sediment pulsing.
The long-term average progradation rate is similar to
the pre-river training rate,but short-term rates,
P.A.Friele et al./Sedimentary Geology 176 (2005) 305–322320
calculated over 50-year intervals,include documented
sediment pulses at rates of 100–150 m a
￿1
.These
values are four to six times higher than the highest
recorded historical rates and are consistent with the
episodic sedimentation model presented by Jordan
and Slaymaker (1991).
Drilling and compilation of data from the literature
indicate that instability at Mount Meager has pro-
duced volcaniclastic deposits that can be traced from
the volcano’s flanks to the most distal portions of
Lillooet Lake basin.Floodplain sediments record at
least four sedimentation pulses associated with large
volcanic debris flows and hyperconcentrated flows.
The contributions of large and small landslides bias
the proportion of volcanic material in the bedload of
the river.Desloges and Church (1987),working in the
Bella Coola River valley in the central Coast
Mountains,found that the proportions of lithologies
in channel gravels are similar to the proportions of
bedrock types in the watershed.In contrast,volcanic
bedrock underlies only 2% of the Lillooet River
watershed,but volcanic lithologies make up 25–75%
of the gravel fraction of channel and hyperconcen-
trated flow deposits.
These results suggest that the Mount Meager
massif dominates the supply of sediment to Lillooet
River,and its presence is responsible for the rapid
rates of delta progradation and aggradation,both over
the historic period and the Holocene.Thus,the
paraglacial sediment supply model (Church and
Ryder,1972;Church and Slaymaker,1989),appli-
cable over much of the British Columbia landscape,
does not apply in valleys draining Quaternary
volcanoes.In these settings,extensive volcaniclastic
deposits must be considered characteristic elements
(c.f.Hewitt,1998) of the depositional environment,
and their occurrence must be factored into hazard
assessments.
Acknowledgements
We thank Stephen Black,John Vanloon,Neil
Vanloon,Bruce Miller,Jack Ronayne,Bob Menzel,
and John Beks for allowing us to drill on their
properties and for clearing snow for the drill rigs.
Drilling was done by Sonic Drilling Ltd.and Geotech
Drilling Services Ltd.Many of the radiocarbon ages
were provided by Roger McNeely (Geological Survey
of Canada Radiocarbon Laboratory).Kazuharu Shi-
mamura (GSC) supplied the digital elevation model
used in Fig.1.Critiques by Greg Brooks (GSC) and
anonymous journal reviewers greatly improved the
manuscript.
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