Rampton (1987) recognized Late Wisconsinan Laurentide glaciation in the Fort Nelson area, and Klassen (1978, 1987) documented four Cordilleran advances ...
~
Quaternarylnternational,Vol.32, pp. 21-32, 1996.
}Pergamon
Copyright© 1996INQUA/ElsevierScienceLtd. Printed in GreatBritain. All rightsreserved. 1040-6182/96 $32.00
1040-6182(95)00061-5
LAURENTIDE, CORDILLERAN, AND MONTANE GLACIATION IN THE WESTERN PEACE RIVER m GRANDE PRAIRIE REGION, ALBERTA AND BRITISH COLUMBIA, CANADA N o r m Catto,* D a v i d G.E. Liverman,'~ Peter T. Bobrowsky:~ and Nat Rutter§
*Department of Geography, Memorial University of Newfoundland, St John's, Newfoundland A1B 3X9, Canada JcGeological Survey Branch, Newfoundland Department of Mines and Energy, St John's, Newfoundland, Canada 5;B.C. Geological Survey Branch, Ministry of Energy, Mines and Petroleum Resources, Victoria, British Columbia, Canada §Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Geological investigations over the last decade in the western Peace River-Grande Prairie district of northwestern Alberta and northeastern British Columbia have provided important chronostratigraphic information regarding Laurentide, Cordilleran and Montane Quaternary glacial events. Numerous 14C age determinations, as well as stratigraphic and sedimentological analyses of numerous exposures, indicate that this area was first glaciated by Cordilleran ice sometime before the Late Wisconsinan. During the Late Wisconsinan, ca. 22,000-14,000 BP, Laurentide glacier ice covered much of the area. Between approximately 15,000 and 10,000 BP, Montane ice affected only the western part of the Peace region. There is no evidence for synchroneity of the ice advances and, therefore, no evidence for ice coalescence in this region. The resultant 'ice-free-corridor' was most likely a laterally fluctuating zone which existed during a several thousand year period in the Late Pleistocene. Climatic conditions along the 'corridor' during this period were probably unfavorable for human migration, but adequate for some plants and small animals. Numerous isolated areas along the western foothills of the northern Rocky Mountains, west of the Laurentide glaciation limit, were never glaciated during the Late Wisconsinan. Copyright © 1996 1NQUA/Elsevier Science Ltd
have occurred locally at some time during the Quaternary (Roed, 1975), the extent, chronology, and synchroneity of coalescence remain uncertain (Rutter, 1984; Dyke and Prest, 1987; Clague, 1989; Bobrowsky et al., 1990; Catto and Mandryk, 1990; Bobrowsky and Rutter, 1992). Relationships between Montane glaciers and Cordilleran ice are also uncertain, and differ considerably from region to region along the Rocky Mountains. In Jasper, Cordilleran ice advanced through Yellowhead Pass and along the Miette River, and coalesced with Montane ice from the Sunwapta and Athabasca valleys. The combined glacial mass then flowed eastward along the Athabasca River, to the eastern border of Jasper National Park (Levson and Rutter, 1989). Other local Montane glaciers in Jasper advanced independently of the Cordilleran ice. South of Jasper, Montane ice acted alone, without a contribution from Cordilleran glaciation based in interior British Columbia, in the Brazeau River valley (Boydell, 1978; Kvill, 1984), in the Bow River valley of Banff National Park (Rutter, 1972), in the upper Highwood River valley (Jackson, 1977), and in Waterton Lakes National Park (Stalker and Harrison, 1977; Karlstrom, 1987). In the Liard River basin, north of Peace River, Cordilleran ice acted independently of Montane glaciation, and was the dominant factor in forming the landscape (Klassen, 1978, 1987; Clague, 1989; Ryder and Maynard, 1991). Thus, the relationships between the glaciers originating from the three sources vary consider-
INTRODUCTION Several major investigations during the past decade have indicated that the regional Quaternary geology of western ALberta and adjacent British Columbia is complex (e.g. Clague, 1981, 1989; Bobrowsky et al., 1990; Catto and Mandryk, 1990; Bobrowsky and Rutter, 1992). Three major sources of glaciation have influenced the region. Laurentide ice advanced west-southwest from the Canadian Shield. Montane ice advanced generally eastward and northeastward from numerous trunk valleys, cirques, and ice fields in the Rocky Mountains. Outlet glaciers from the Cordilleran ice sheet complex flowed eastward from the interior of British Columbia located west of the Rocky Mountain Trench, passing through the Rocky Mountains along low passes, such as Yellowhead Pass in the Jasper area, and along the Peace River valley west of Hudson Hope. The relationship between these three glacial systems has been a matter of debate. One of the principal issues centres around the suggestion of an 'ice-free corridor', a zone between the Laurentide and combined Cordilleran and Montane glacial margins that remained unglaciated during the last glacial maximum and could have served as a migration route for humans, plants and animals moving either southeastward from Alaska or northwestward from the central plains (e.g. Johnston, 1933; Martin, 1973; Rutter, 1984; Mandryk, 1990). Although coalescence of these two ice fronts appears to 21
N. Catto et al.
22 57 ° N
26°w
--It::
FortSt. John
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•-~-:-'~- : ~.?:..'-=~,~.~-:.-' - ~ . . . . . . .:.::~--:.---. :'-'-::'-'-::'-'-":'-.'---:::::::...
~ PeaceRiver
Dawson Creek
L
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......
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k.'~=?FZ££%
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-
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-
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-
-
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i54
o
N
116°W ~
Land above 3000' (950 m)
r-0
100 kilometres
200
FIG. 1. Map of study area.
ably throughout the regions of the Canadian Rocky Mountains. The western Peace River - Grande Prairie region is located in west-central Alberta and northeastern British Columbia, encompassing the area from Grande Prairie in the east to Williston Lake in the west (Fig. 1). The region lies within the Peace River drainage basin, and the topography slopes dominantly northeastward. The southwestern border of the area is delimited by the Rocky Mountains. The northeastern and eastern margins consist of dissected peneplains, plateaus and rolling uplands. Prior to the construction of the W.A.C. Bennett Dam, the Peace River formed at the confluence of the Finlay and Parsnip Rivers, and debouched from the Rocky Mountain Trench. It traverses the basin from west to east. The region thus lies centrally amidst the three sources of glaciation. Laurentide ice was able to advance southwest from the Lake Athabasca area, following the Peace River valley (Dyke and Prest, 1987; Liverman, 1989). Areas of Montane glaciation were located to the southwest and west, in the Rocky Mountains. Cordilleran ice flowing from the Omineca Ranges to the southwest of the Rocky Mountain Trench could penetrate the region, either via the Parsnip and Peace River valleys, or by overriding the relatively low summits flanking the Parsnip River and those in the Pine Pass area. During all Quaternary glacial events, therefore, the Peace River - Grande Prairie region was potentially subject to glacial influence from all three distinct sources. Hence, investigation of this region affords an excellent opportunity to further refine the understanding of the relationships between the Laurentide, Cordilleran, and Montane glaciers. This paper is designed to define and reconstruct the course of late glacial events throughout the region, utilizing newly-obtained data.
PREVIOUS
WORK
Following the pioneering work of Dawson (1881), and Johnston (1933), among others, recent investigations have been centred on mapping and stratigraphic analysis of the region. Mathews (1963, 1972, 1978, 1980) mapped the Charlie Lake area, in the vicinity of Fort St John, British Columbia, and extended this work on a reconnaissance basis to northeasternmost British Columbia and into the Grande Prairie area of Alberta. Mathews (1978) described the stratigraphy in the Charlie Lake area as consisting of two similar Laurentide tills, separated by undated silts at a single locality. The age of the upper till was suggested by Mathews (1978, 1980) to be Late Wisconsinan. In exposures along the Peace River southwest of Fort St John, he noted a gradational eastward increase in the proportion of typical Laurentide clast types. On this basis, he suggested that Laurentide and Cordilleran ice coalesced in the Late Wisconsinan. The lower till was thought to predate the Late Wisconsinan. A still earlier Lanrentide advance was postulated from the presence of gravels underlying the lower till, containing clasts of Laurentide provenance. Only one Cordilleran advance was recognized, and was thought to be Late Wisconsinan (Mathews, 1978, 1980). A similar stratigraphy was postulated for the Dawson Creek area of British Columbia by Reimchen (1980). 14C age determinations from the Saddle Hills (e.g. >30,000 BP, WAT-361) were initially thought to indicate the absence of ice cover in the Late Wisconsinan (White et al., 1979). Subsequently, White et al. (1985) re-interpreted these determinations, and rejected the numerical ages as contaminated. White et al. (1985) suggested that the presence of ice over the Saddle Hills until ca. 12,000 BP indicated that coales-
Glaciation in the Western Peace River - Grande Prairie Region
23
TABLE 1. lac age determinations referred to in text Locality ~
Lat. (°N)
Long. (°W)
Laboratory Determination No 2
Age (BP)
Reference
Finlay R.
57 ° 10'
125020 '
AECV-383C
>40400
Bobrowsky, 1989
Finlay R. Finlay R.
57010 , 57010 ,
125020 , 125o20 ,
AECV-381C AECV-385C
>40330 >40180
Bobrowsky, 1989 Bobrowsky, 1989
Finlay R.
57 ° 18'
125027 ,
AECV-386C
>40130
Bobrowsky, 1989
Finlay R.
57 ° 18'
125027 ,
AECV-348C
>40000
Bobrowsky, 1989
Finlay R.
57°16 ,
125028 ,
AECV-353C
37,190+2870
Bobrowsky, 1989
Finlay R.
57010 '
125°20 ,
AECV-350C
36,5105:2570
Bobrowsky, 1989
Finlay R.
57o09 ,
125o15 ,
AECV-380C
33,490+1780
Bobrowsky, 1989
Finlay R.
57 ° 10'
125°20 ,
AECV-382C
32,7505:3180
Bobrowsky, 1989
Finlay R.
57009 ,
125 ° 15'
AECV-349C
29,880± 1680
Bobrowsky, 1989
Finlay R.
57 ° 11'
125o17 ,
AECV-352C
29,280+1230
Bobrowsky, 1989
Finlay R.
57°18 ,
125027 ,
AECV-379C
26,8004-1450
Bobrowsky, 1989
Finlay R. Finlay R.
57 ° 16' 57°22 '
125°28 , 125034 '
AECV-351C TO-709
23,2804-750 18,750±120
Bobrowsky, 1989 Bobrowsky, 1989
Finlay R.
57017 ,
125028 '
TO-708
15,180±100
Bobrowsky, 1989
Finlay R.
55047 '
125005 ,
GSC-2036
10,100±90
Alley and Young, 1978
Finlay R.
55°47 '
125°05 '
GSC-2036-2
10,000±140
Blake, 1986
Portage Mtn.
56°01 '
122007 ,
GSC-2859
25,800±320
Mathews, 1978
Ft St John
56°10 ,
120o44 ,
TO-2742
13,970±170
this paper
Ft St John
56°17 '
120°56 ,
SFU-454
10,770±120
Driver, 1988
Ft St John
56°17 '
120056 ,
SFU-300
10,450±150
Driver, 1988
Ft St John
55059 ,
120016 ,
GSC-1654
10,400±170
Rutter, 1977
Ft St John
56o17 ,
120o56 '
SFU-378
10,380±160
Driver, 1988
Ft St John
56009 '
120042 ,
AECV-1206C
10,240±160
Bobrowsky et al., 1991
Ft St John
56 ° 17'
120056 ,
RIDDL-392
10,100±210
Driver, 1988
Watino
55o43 ,
117038 ,
AECV-414C
>40,170
Liverman et al., 1989
Watino
55043 '
117038 ,
GX-1207
>38,000
Lowdon and Blake, 1970
Watino
55°43 ,
117038 ,
GSC- 1020
43,500±620
Lowdon and Blake, 1970
Watino
55°43 '
117°38 ,
1-2615
35,500+3300
55043 ,
117038 ,
1-2516
- 2300 35,500+2300
Lowdon and Blake, 1970
Watino
-1800
Lowdon and Blake, 1970
Watino
55043 '
117°38 ,
1-2626
34,900+3000
Watino
55043 ,
117038 ,
AECV-416C
-2000 31,530±1440
Lowdon and Blake, 1970 Liverman et al., 1989
Watino
55o43 ,
117038 ,
1-4878
27,400±850
Westgate et al., 1972
Watino
55043 '
117038 ,
GSC-2895
10,200±100
Churcher, 1984
Watino
55043 ,
117°38 ,
GSC-2902
10,200±100
Churcher, 1984
Watino Simonette
55°43 , 55008 ,
117°38 , 118012 ,
AECV-272C AECV-428C
9, 920±220 37,010±2690
Bobrowsky et al., 1990 Liverman et al., 1989
Simonette
55008 '
118 ° 12'
GSC-4623HP
>51,000
this paper
High Level
58 ° 10'
117020 ,
AECV-719C
22,020±450
Bobrowsky and Rutter, 1992
Boone Lk
55035 ,
119026 '
WAT-361
>30,000
White et al., 1979
JLocality designations correspond to localities shown on Fig. 5. See references cited for further details.2Laboratory identifications: AECV - - Alberta Environmental Centre, Isotope Laboratory; GSC - - Geological Survey of Canada; GX - - Geochron Laboratories; I - - Teledyne Isotopes; RIDDL - RIDDL Laboratory; SFU - - Simon Fraser University; TO - - Isotrace Laboratory; WAT - - University of Waterloo.
c e n c e b e t w e e n L a u r e n t i d e and C o r d i l l e r a n ice m a s s e s in the Late W i s c o n s i n a n w a s p r o b a b l e . C h r o n o l o g i c a l control for the last L a u r e n t i d e g l a c i a t i o n in w e s t e r n A l b e r t a is c u r r e n t l y p r o v i d e d b y several t a c age
determinations
(Table
1) o b t a i n e d
from
wood
r e t r i e v e d in the fluvial b e d s at W a t i n o . T h e ages, r a n g i n g in a s c e n d i n g s t r a t i g r a p h i c s e q u e n c e f r o m > 4 0 , 1 7 0 (AECV-414C)
to 27,4004-850
(I-4878),
indicate
that
t h e s e fluvial s e d i m e n t s are m i d - W i s c o n s i n a n ( W e s t g a t e e t al., 1971, 1972; L i v e r m a n e t al., 1989). A t H i g h Level, n o r t h o f W a t i n o , a 14C age d e t e r m i n a t i o n o f 2 2 , 0 2 0 + 4 5 0 B P ( A E C V - 7 1 9 C ) o n a m a m m o t h tusk s u g g e s t s that the L a t e W i s c o n s i n a n a d v a n c e m a y p o s t - d a t e 22,020 B P ( B o b r o w s k y a n d Rutter, 1992). A t W a t i n o , the f o s s i l i f e r o u s b e d s overlie p r e - g l a c i a l g r a v e l s w h i c h lack clasts o f L a u r e n t i d e p r o v e n a n c e .
24
N. Catto etal.
Laurentide till is not exposed in this section, but regionally overlies these pre-glacial fluvial sediments. Previous faunal investigations (Reimchen, 1968; Churcher and Wilson, 1979) suggested a Sangamonian or preLate Wisconsinan age for the underlying gravel strata. Liverman et al. (1989) demonstrated that the basal gravel forms a broadly conformable sequence with the overlying finer sediments. Examination of the provenance of the sediments indicates that the entire sequence was deposited prior to Lanrentide glaciation in the area. The regional surface till, therefore, is of Late Wisconsinan age. No indications of pre-Late Wisconsinan Laurentide glaciations in the region were observed by Liverman (1989). In contrast to the stratigraphy evident southeast and east of Grande Prairie (e.g. St-Onge, 1972; Andriashek, 1985; Andriashek and Fenton, 1989), the stratigraphy exposed at the Watino and Simonette River outcrops (Liverman et al., 1989) indicates that there is no evidence for either multiple Laurentide glaciation or pre-Late Wisconsinan Laurentide glaciation in the Grande Prairie region. The Late Wisconsinan glaciation, therefore, was the only Laurentide glaciation to affect the Grande Prairie region. All of the units in the sequence of two diamictons separated by silt, described by Mathews (1978), are thus considered to be of Late Wisconsinan age in the absence of numerical chronological data. Bobrowsky (1989) and Bobrowsky and Rutter (1992) investigated the Finlay River area north of Williston Lake. Two glacial events were recognized: an early, preLate Wisconsinan glaciation, corresponding to the 'Early Cordilleran' glaciation of Rutter (1976, 1977) and a single Late Wisconsinan event, correlative to both of Rutter's Portage Mountain glaciations. The older event was the most extensive, argued to be part of a panprovincial true Cordilleran Ice Sheet, whereas the younger event was confined to the Rocky Mountain Trench and Peace River valley. Tills deposited during these glaciations are separated by wood-bearing sediments. 14C age determinations (Table 1) on 15 samples in ascending stratigraphic order range from >40,400 (AECV-383C) to 15,180-t-100 BP (TO-708) (Bobrowsky, 1989; Bobrowsky and Rutter, 1992). The Finlay River valley was deglaciated ca. 10,1005:90 BP (GSC-2036; Alley and Young, 1978), indicating a relatively short duration of Late Wisconsinan glaciation. Bobrowsky (1989) did not recognize a distinct glacial event correlative to the Latest Wisconsinan/Early Holocene Deserters' Canyon episode (Rutter, 1976, 1977) in the northern Finlay River valley.
STRATIGRAPHY AND GLACIAL HISTORY Laurentide glaciation
The history of Laurentide glaciation in the Grande Prairie area can be deduced from the chronological and lithologic evidence from the Watino and Simonette sections (Liverman et al., 1989). A new high pressure ~4C age determination of >51,000 BP (GSC 4623-HP) has
been obtained from the Simonette section, from wood collected in the same location as that previously dated at 37,010±2690 BP (AECV-428C) (Liverman et al., 1989). The sample was composed of wood fragments, and may represent a mixture of ages, with older reworked wood forming the bulk of the sample. D/L ratios of amino acids from the wood, however, show little difference to those obtained from sediments at Watino which have finite 14C ages (Liverman et al., 1989). The sediments were thus likely deposited during the Middle Wisconsinan, a conclusion supported by paleontological analyses at Watino (Reimchen, 1968; Westgate et al., 1972). These sedimentary successions indicate that Laurentide ice did not reach the Grande Prairie area, and therefore could not reach the Peace River Valley, prior to the Late Wisconsinan. The termination of the Laurentide glaciation in the Fort St John area is chronologically controlled by a 14C age determination of 13,970:L170 (TO-2742) on wood recovered at the contact between Laurentide provenance till and overlying glaciolacustrine sediments. Investigations were made in the areas west of Fort St John and Dawson Creek in an attempt to define the limit of Laurentide glaciation. Throughout the Dawson Creek and Fort St John areas, the pebbles and cobbles found in glacial diamictons are dominantly locally derived from the Cretaceous Shaftesbury and Kaskapau shales and from the Cretaceous Dunvegan sandstone (Catto, 1991, 1993). Clasts derived from the montane region to the southwest, including orthoquartzite, chert, limestone and dolomite from Palaeozoic and Mesozoic strata, together with granitic clasts from the Omineca Range to the west, are also commonly present. Granitic, gneissic, gabbroic and basaltic clasts derived from the Canadian Shield, and clasts of Athabasca sandstone and rare carbonate clasts containing fragments of Devonian corals derived from northeastern Alberta, are present in diamictons in the eastern and northern parts of the region, but are absent from the southwestern part. The maximum Laurentide glacial limit (as marked by the distribution of erratics derived from the Canadian Shield) is of Late Wisconsinan age, as indicated by the stratigraphy and 14C determinations at Watino to the east. Laurentide erratics have been reported in the foothills of the Rocky Mountains by Mathews (1978, 1980), Reimchen and Rutter (1972), Liverman (1989), Bobrowsky and Smith (1992), and Catto (1993). Glaciolacustrine sediments associated with the stages of Glacial Lake Peace (Mathews, 1980; Liverman, 1989, 1991) occur throughout the central and eastern parts of the region at elevations below 850 m a.s.1. Erratics found below the maximum known elevation of glaciolacustrine deposits thus may be ice rafted rather than emplaced by the Laurentide ice sheet, and are therefore not necessarily indicative of glacial extent. Glaciolacustrine diamictons commonly contain clasts derived from the Canadian Shield (Catto, 1991, 1993). The approximate boundary appears to be controlled by altitude, with erratics being found up to an elevation of nearly 950 m a.s.l. (Fig. 2). The western limit probably represents a minimum for glacial extent, especially in the foothills areas southwest
Glaciation in the Western Peace River - Grande Prairie Region
25
57 ° N 126°W ,-.
i ,-I~< -==1
Peace River
Pea~----R-~Ner
[]
Creek
&:
Gr,,nde Prairie -
--
..'.
M a t h e w s "('~§8"6): o..
,,, -.
.'.-
This study ":.'.'--Z :3".-::: 54 ° N 116°W
t
0
100 kilometres
200
FIG. 2. Distribution of Lanrentide erratics, and inferred Laurentide limit (of Late Wisconsinan age). Grey dots mark locations where Laurentide provenance erratics were observed. The Laurentide limit recognized by Mathews (1980) is also shown for comparison.
of Dawson Creek. This boundary is west of the zone of coalescence suggested by Mathews (1980). Diamictons deposited by Laurentide ice contain a varied suite of minerals. Minerals derived from the local bedrock units and those which crop out in the montane areas to the southwest, including quartz, feldspar, calcite, dolomite, biotite and vermiculite, occur in all of the diamictons examined in the Peace River area (Catto, 1991, 1993). Minerals derived from igneous and metamorphic rocks of the Canadian Shield, including magnetite, hornblende, garnet, pyroxene, tourmaline, corundum, apatite, titanite, kyanite and others, in addition to some quartz, feldspar and biotite, are found in diamictons throughout the areas to the northeast and east of the limit of large Canadian Shield erratics. Diamictons from the southwestern part of the Dawson Creek area lack minerals characteristic of high-grade metamorphic rocks (Catto, 1993). The boundary between Laurentide, and Cordilleran and Montane deposits is not lithologically or mineralogically distinct, as considerable reworking and transport of material has occurred subsequent to deglaciation. The presence of Laurentide granites and gneisses, Athabasca sandstone and fossiliferous limestone clasts derived from the Devonian strata of northeastern Alberta, indicate that these diamicton units were deposited at some time after the initial Laurentide advance. The absence of these clasts from the southwestern part of the area indicates that Laurentide ice did not extend to the southwestern margin of the region. The distribution of erratics indicates that Laurentide ice moved westward into the foothills of the Rocky Mountains to its Quaternary maximum with no apparent
obstruction from Montane or Cordilleran ice. Glaciers require gravitational potential to move up-slope, the magnitude of which is determined by the gradient of the ice surface, the ice thickness at the source of the glacier, and the distance from the centre of the ice sheet (Paterson, 1981). The Laurentide glacier in northeastern British Columbia had a low gradient (2-5 m/km), due to the susceptibility of the underlying Cretaceous shales and sandstones to deformation (Mathews, 1974; Boulton et al., 1985; Fisher et al., 1985). Laurentide ice advancing into the Peace River valley would be thin, gently sloping, and moving against the topographic slope. The flow directions and extent of the Laurentide advance were thus highly susceptible to topographic influence. Clast fabric patterns and geomorphic flow directional indicators in the areas covered by the Laurentide glacier generally show a gradual change of orientation westward from southwesterly to south-southeasterly at the Laurentide margin (Mathews, 1980; Catto, 1991, 1993), although individual basal till exposures with fabrics recording western, southern, and northern flow are present in the Fort St John area, including the Kiskatinaw and Halfway rivers. In some exposures, progressive vertical variations in fabric orientation, with the fabrics in the upper parts of the till units corresponding more closely to the regional pattern, indicate that the initial ice flow was strongly influenced by topography. The ice over-topped the Saddle Hills in the eastern part of the region (White et al., 1985; Liverman, 1989) which have a maximum elevation of 940 m, but was unable to reach elevations higher than 950 m a.s.l, in the foothills and Rocky Mountain front.
26
N. Catto et al.
Cordilleran and Montane Glaciation
The relationship between Montane and Cordilleran ice in the Finlay River - Williston Lake area has been documented by Bobrowsky and Rutter (1992). The earliest advance, pre-dating the Late Wisconsinan, was extensive. Clast lithological analyses of diamictons indicates that material had been transported from the Omineca Ranges and other locations west of the Rocky Mountain Trench to the Williston Lake and Hudson Hope areas (Bobrowsky et al., 1991). Ice flowed eastward and northeastward across the Finlay River valley, and unconfined glaciers extended east beyond the foothills into the Halfway River area, Weathered Cordilleran erratics perched high on mountain peaks observed by Mathews (1978, 1980) and Rutter (1976) are considered to be correlative with this extensive early glaciation. Multiple till sections in the Halfway River area provide insight into the timing and relationship of Cordilleran and Laurentide glaciations (Fig. 3). At several localities, a lower till containing materials of Cordilleran provenance, but lacking Canadian Shield erratics, is overlain by fluvial and lacustrine sediments which in turn are covered by a till containing both Laurentide and Cordilleran lithologies. Pebble fabrics in the diamictons suggest ice flow from the west for the lower till and ice flow from the northeast for the upper till. This stratigraphy indicates that the initial Cordilleran/Montane glaciation advanced to the east and had receded prior to the Late Wisconsinan
Laurentide advance. Cordilleran and Montane provenance clasts initially transported by the eastward-flowing ice were deposited in the Halfway River area, and were thus available for re-incorporation and transport during the later Laurentide advance. The youngest advance in the region, correlative to the Late Portage Mountain event of Rutter (1977), was restricted to the Finlay, Parsnip, and Peace River valleys. Ice flowed as confined glaciers in the major valleys, and terminated at the Portage Mountain moraine. Clast provenance studies indicate that this ice was derived from local Montane sources, as distally-derived clasts are extremely rare. The terrain to the east of the Rocky Mountain Front, therefore, remained uncovered by Montane or Cordilleran Ice throughout the Late Wisconsinan. The chronology of these events is constrained by stratigraphic relationships and 14C age determinations. The pre-Late Wisconsinan and Late Wisconsinan tills are separated by lacustrine and fluvial sediments containing numerous organic deposits. 14C analysis of these materials (Table 1) has resulted in ages ranging from beyond the range of 14C determination to 15,180i100 BP (TO-708) for the non-glacial interval (Bobrowsky and Rutter, 1992). Similarly, a suite of post-glacial age determinations exist for the region, with the oldest at 13,970-4-170 BP (TO-2742). Thus, the early advance was a pre-Late Wisconsinan event, representing the combined efforts of Cordilleran and Montane sources, and extended
FIG. 3. Stratigraphic succession exposed along the Halfway River. The stratigraphy consists of a lower till containing materials of Cordilleran provenance (LC), overlain by fluvial and lacustrine sediments (MFL), which in turn are capped by a till containing both Laurentide and Cordilleran provenanceclasts (UL).
Glaciation in the Western Peace River - Grande Prairie Region east of the Rocky Mountain Front. The later advance was of Late Wisconsinan age, and was a short-lived event involving topographically-confined local Montane glaciers. The strongest evidence for the passage of eastwardflowing Montane ice in the Grande Prairie area is the well developed fluting field fanning out from the Redwillow Valley. The flutings are erosional, developed in the Cretaceous shale and siltstone bedrock, and till of Montane provenance has not been recognized. Flow directions indicated by the flutings range from east to almost due north. The fluting pattern is also evident in the Dawson Creek map area (Reimchen, 1980; Catto, 1993), and originates from the gap cut by the Redwillow River in the mountain front. The fluting field forms a well-defined fan. There is no evidence of coalescence between Montane and Laurentide ice in the form of deflection of the flutings, in contrast to the pattern observed in the Athabasca Valley (Roed, 1975; Rains et al., 1993) and the northward diversion of the Moose River glacier by Laurentide ice northeast of the Mackenzie Mountains (Duk-Rodkin and Hughes, 1992). The flutings are draped by glaciolacustrine silt and clay below 850 m elevation, but can be traced as surface lineations as far east as Hythe and Beaverlodge, and north to the base of the Saddle Hills (Liverman, 1989). Their limit is east of the boundary between Montane and Laurentide glaciation suggested by Mathews (1980), and 40-80 krn east of the Laurentide limit defined here. Above 850 m, the flutings are covered by diamicton containing Laurentide provenance clasts within the limits of Lanrentide glaciation. Throughout most of the region, the flutings are oriented parallel to local slopes (as well as to the regional slope), and minor streams developed between the flute crests following deglaciation and lake drainage have accentuated local relief. The genesis of flutings is controversial. They have been interpreted to form through deposition initiated by the moulding action of flowing ice (Boulton, 1976). Alternatively, flutings have been interpreted as erosional features carved by sub-glacial meltwater flow beneath either active or stagnating ice (Shaw and Ashley, 1988; Rains et al., 1993). The symmetrical fan form, not marked by deflection to the south, suggests that the development of the Redwillow flutings involved Montane ice alone, regardless of the mode of formation. The mode of genesis of the flutings is thus not directly relevant for interpretation of the relationship between Montane and Lanrentide ice. In the Kamisak Lake area, a cross cutting relationship is seen between the fluting field and subsequently-formed north-south oriented channels (Fig. 4). The surface sediment in this area consists mainly of gravel containing granitic clasts, likely derived as outwash from Laurentide ice. The series of north-south oriented channels are interpreted as meltwater fed channels, which drained ice lying to the north of Kamisak Lake. The area of channel development is at present a topographic high, bounded by valleys to north and south. Meltwater could thus have been supplied to the area only if ice in the direct vicinity
27
formed a temporary topographic high with relief of at least 100 m.
Relationship Between Montane~Cordilleran and Laurentide Ice
The stratigraphic and chronologic relationships indicate that Laurentide and Montane/Cordilleran ice did not coalesce in the Peace River area. The major preCordilleran advance was of pre-Late Wisconsinan age. The Montane advance indicated by the Redwillow fluting field may be correlative with this Cordilleran event, and appears to pre-date the deposition of Laurentide diamicton. Laurentide ice did not reach this area prior to the Late Wisconsinan, and so the most extensive Cordilleran advance could not coalesce with Laurentide ice. In the Late Wisconsinan, Laurentide ice advanced to its maximum position in the foothills at some time between 27,400 and 13,970 BP. The maximum Laurentide advance may have occurred after 22,020 BP, if the ~4C age determination on the mammoth tusk found at High Level (Bobrowsky and Rutter, 1992) is accepted as valid. Ice advanced from the west to form the Portage Mountain moraine after 15,180 BP. Although the western limit of Laurentide ice was located east of Portage Mountain, it is more likely that Laurentide ice had already retreated further east by 15,180 BP. Coalescence thus did not occur in the Hudson Hope region during the Late Wisconsinan. The Portage Mountain moraine was interpreted by Rutter (1977) as a glaciofluvial feature. Recent investigations have confirmed its formation as a glaciofluvial deltaic deposit formed by discharge from Montane ice into standing water to the east of Portage Mountain. Thus, the moraine appears to represent the Late Wisconsinan limit of Montane ice, which was restricted from further eastward expansion by the impoundment of a lake in the western Peace River valley by retreating Laurentide ice bordering the lake to the east and northeast. In the Grande Prairie area, the Laurentide and Montane glaciers were able to advance, constrained only by topography, to their respective maximum positions. The extents of the glacial advances overlap, and thus the maximum advances could not have been synchronous. The maximum advance of Laurentide ice is known to be of Late Wisconsinan age (Liverman et al., 1989). If Montane glaciers did advance along the Redwillow Valley prior to the Laurentide glaciation, the preservation of the flutings must be explained. The origin of these landforms is enigmatic. Mathews (1980) suggested that the Redwillow flutings indicate flow of Montane ice unobstructed by Laurentide ice and therefore suggested that they represented a late Montane advance, following the retreat of Laurentide ice. The draping of the landforms by undeformed Laurentide glacigenic and glaciolacustrine sediment, however, indicates that the landforms formed prior to the single Laurentide advance noted in the Grande Prairie region. This stratigraphy also precludes the possibility of development of the flutings by Montane ice overriding stagnant Laurentide ice, as has been
28
N. Catto et al.
FIG. 4. Stereopairof the KamisakLake area, RedwillowValley, showingflutings associatedwith Montaneice truncated by subsequentlydeveloped Laurentide meltwaterchannels. (AlbertaGovernmentaerial photographs,prints AS-2313 10 and 11, copyrightGovernmentof Alberta, Her Majesty the Queen in right of Alberta, reproduced from the collection of the Ministry of Forestry, Lands, and Wildlife by permission.)
documented in the Arctic Red area of the Mackenzie Mountains by Duk-Rodkin and Hughes (1992). The thin and gently sloping Laurentide ice sheet (Mathews, 1974) likely had little erosive capability, as indicated by the presence of scattered regoliths developed on Cretaceous and Tertiary strata throughout western and central Alberta (e.g. Catto, 1984). Preservation of preexisting landforms which have undergone glaciation has been documented in several regions of the prairies (Clayton and Moran, 1982; Klassen, 1989). Laurentide ice could thus have had little effect on the underlying Montane flutings until the ice moved up-slope into the
foothills. It would then be in a state of compressive flow, with decreasing forward velocity. Such flow conditions enhance basal erosion, with ice flow decoupling from the base (Paterson, 1981; Aber, 1982; Ruszczynska-Szenajch, 1985). The proportion of Laurentide provenance clasts drops sharply in the southwestern part of the Grande Prairie area (Mathews, 1980; Liverman, 1989). The drop in proportion of Shield clasts can be accounted for by dilution of Laurentide provenance material through incorporation of large amounts of bedrock and re-worked Montane derived sediments as the glacier moved upslope into the mountain
Glaciation in the Western Peace River - Grande Prairie Region
0 +~:~t
+
,. . . . . . . . . . . . . . , .........,,, ,,,,,,,
.......
t
29
......
m,4oo-2~7o(Gsc-t654) toaoo~2~o(RIDDL-392)k I
10 --
~o,?ro~12o( s ~
I D• 6
~o,a m ~
13,970"1:170 t T O - 2 7 ~ + - ~
20--
• ~,s00~320(Gsc-2~9) 2a,2~
o
- ] -j'~-'0
Io
I
( c s c - ~ !I+ ! + v . : V '~?
~ - 20 22,020~4~(AECV-719C)O
(AECV-351¢)
D 2 6 , 80~.750 (AECV-379C)
29,Z80~1230(~CV-3S2C)
27,~o,x~o(mrs) @
: 3 2 , ~ 1 8 0 (AECV-382C) 33,490:1:1780 (AECV-380C)
31,530:t1440 ( A E C V 4 1 6 C ) •
30,-- 1 2 9 ~ 1 ~ o {~cv-349c) [~
~
36,510:t.2570 (AECV-350C) r i p 37,19~2870 (AECV-353C)
34,900+3000/-~ (I-2626) O
3s.~o÷3z~o/-xaoo(P2SlOO
~
37,010#.2690 ( A F ~ e W ~
@
>38~00 (cxq2ff/)O
40~ ,4o,000(~cv-a~c) _ ~ > 4 o , ooo(nEcv-aaac3
~' Wnliston Lake
--30
43~
Western Peace R. Valley
Cordmeran & Montane ~ IGlaciation
O14C age determination (see table 1)
-40
(c.sc.lo2o)@
Alberta
Plateau
Laurenflde ~aciation
O14C age > 38,000BP
FIG. 5. Time--distance diagram, Western Peace River-Grande Prairie region, summarizing the stratigraphic relationships outlined in this study, and the 14C age determinations constraining it. See text for further discussion.
30
N. Catto et al.
front. There is no evidence of coalescence between Montane and Laurentide ice in the form of deflection of the flutings, in contrast to the pattern observed in the Athabasca Valley (Roed, 1975; Rains et al., 1993). Further research is required in the upper Redwillow Valley to firmly resolve the stratigraphy in this area.
DISCUSSION Evidence from the western Peace River valley between Williston Lake and Fort St John and in the adjacent area of westernmost Alberta suggests that coalescence between Cordilleran/Montane and Laurentide glaciers did not occur at any time during the Quaternary (Fig. 5). The Portage Mountain moraine, representing the maximum eastward extent of Late Wisconsinan Montane ice, was built as a glaciofluvial feature into waters impounded by the retreating Laurentide glacier to the east and northeast. In contrast, the evidence relating to Montane glaciation in the Redwillow valley, in the southwestern part of the Grande Prairie region, cannot be unequivocably interpreted. The interpretation favored here is that Montane/ Cordilleran ice had retreated prior to the time Laurentide ice reached the Redwillow area during the Late Wisconsinan. The corridor between the Montane/Cordilleran and Laurentide glaciers was likely at its most restricted during the Late Wisconsinan, when Laurentide ice attained its maximum western limit. The area between the ice masses would be highly unsuitable as a migration route, marked by a harsh climate (Catto and Mandryk, 1990; Mandryk, 1990) and partially flooded beneath impounded glacial lakes. The development of suitable climatic and ecological conditions for human transit and occupation, as occurred ca. 11,000-10,500 BP (Driver, 1988; Bobrowsky et al., 1990) required the retreat of the Laurentide glacier from the region. Montane glaciation in the Late Wisconsinan was largely local in nature in the relatively low-lying Northern Rocky Mountains, and its influence was restricted to major outlet valleys such as the Peace River. It is possible that the Lanrentide ice sheet was able to expand to its maximum extent in the northwestern prairies during the earlier part of the Late Wisconsinan because relatively restricted Cordilleran and Montane ice allowed more precipitation to pass over the mountains, thus feeding the margin of the Lanrentide ice sheet. As Cordilleran and Montane ice cover increased during the later part of the Late Wisconsinan, the Laurentide ice sheet correspondingly retreated. The results here indicate that recent depictions of the configuration and chronological evolution of both the Laurentide and Cordilleran ice sheets in this area (Dyke and Prest, 1987; Clague, 1989; Klassen, 1989) should be re-evaluated. Reliable extrapolation of this pattern of glacial interaction to the regions directly to the north and south of the western Peace River-Grande Prairie area is difficult, and may not be possible. The Athabasca valley served as a major conduit for Cordilleran ice (Levson and Rutter, 1989), and thus the Late Wisconsinan history in
this region would differ significantly from that of the Peace River basin. Differences have been documented in the glacial histories of individual valleys in the Mackenzie Mountains (Duk-Rodkin and Hughes, 1992) and throughout the foothills adjacent to the southern Canadian Rocky Mountains (e.g. Rutter, 1972; Boydell, 1978; Kvill, 1984; Karlstrom, 1987; Catto and Mandryk, 1990; Bobrowsky et al., 1990; Bobrowsky and Rutter, 1992). The differences in climatological and topographic parameters throughout the eastern part of the Cordillera would suggest that Quaternary glacial advances and retreats should not be synchronous at all localities. In northern British Columbia and southeasternmost Yukon, relatively little research has been undertaken. Rampton (1987) recognized Late Wisconsinan Laurentide glaciation in the Fort Nelson area, and Klassen (1978, 1987) documented four Cordilleran advances in the Watson Lake, Yukon, area of the Liard River basin. The broad, low-lying Liard basin would have permitted unconfined northeastward flow of Cordilleran ice, and hence the patterns of glaciation might differ from those of western Peace RiverGrande Prairie. Denny (1952) proposed that a similar relationship to that suggested here exists in the Fort Nelson area, although he did not cite any chronological control data. Future research is necessary in northeasternmost British Columbia. Although an ice-free region thus may have existed in the western Peace River-Grande Prairie region throughout the Late Wisconsinan, it was relatively narrow, and climatic conditions were unquestionably harsh at the height of glaciation. Paleoclimatic reconstructions for other postulated ice-free regions of western Alberta (e.g. White et al., 1985; Mandryk, 1990) suggest the presence of a sparse herb-tundra vegetation. The area would not be suitable for human migration during the height of the Late Wisconsinan, although some species of plants and animals could have survived. The presence of wood dated to approximately 14,000 BP indicates that conditions had improved by this time, possibly allowing migration as proposed by Mandryk (1990), although definitive evidence of human occupation post-dates 11,000 BP (Driver, 1988; Bobrowsky et al., 1990). The postulated 'ice-free corridor' in the western Peace RiverGrande Prairie region should best be viewed as a laterally shifting ice-free zone rather than a static feature, given the asynchroneity of the discordant ice masses. The geological data outlined here suggests that areas in the foothills of the northern Rocky Mountains, located away from the influence of major outlet valleys and thus postulated to be located in the zone between the westernmost limit of the Laurentide glaciation and the eastern limit of Montane glaciation, may preserve a paleoenvironmental/geological record spanning the Late Wisconsinan glacial episode, and perhaps the entire Wisconsinan. ACKNOWLEDGEMENTS All of us acknowledgediscussionswith numerouscolleaguesthroughout western Canada, especially C. Mandryk, C. Schweger, and W. H. Mathews. Fieldassistancewas providedby G. Bradbury,R. Gardiner,L.
Glaciation in the Western Peace River - Grande Prairie Region Halsey, C. Squair, G. Thistle and V. Levson. Funding was provided in part by the Geological Survey of British Columbia (Bobrowsky, Catto); the Northern Student Training Program, DIAND (Catto); the Boreal Institute (Catto, Rutter); Memorial University of Newfoundland (Catto); the Killam Foundation (Liverman); the University of Alberta (Liverman, Rutter); and through NSERC research grants (Catto, Rutter). The manuscript was improved through the comments of Dr M.M. Fenton (Alberta Geological Survey) and Dr R.J. Fulton (Geological Survey of Canada). This is contribution # 026 of the British Columbia Geological Survey.
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