Keewatin Ice Sheet—Re-evaluation of the
traditional concept of the Laurentide Ice Sheet
W. W. Shilts
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
C. M. Cunningham
4503 Prytania, A p t . E, New Orleans, Louisiana 70115
C. A. Kaszycki
A t o m i c Energy of Canada Ltd., c/o Geological Survey of Canada, 601 Booth Street
Ottawa, Ontario K1A 0E8, Canada
ABSTRACT
Patterns of dispersal of distinctive Proterozoic and Paleozoic erratics
across terrain formed on Archean and Aphebian crystalline rocks indicate
that (1) ice never f l o w e d from Hudson Bay into Keewatin in the region
from the Manitoba border (lat 6 0 ° N ) northward at least to lat 6 5 ° N ;
(2) westward-southwestward f l o w out of the bay, probably from a Labradorean dispersal center, interfaced with southward- and southeastwardflowing ice from the Keewatin dispersal center somewhere between
Nelson River and Churchill; and (3) at least 3 0 0 km of dispersal of distinctive erratics observed from the vicinity of the last position of the Keewatin
Ice Divide to the present coast of Hudson Bay required considerably more
time than the 1,000 to 3 , 0 0 0 yr that the divide has traditionally been
thought to have existed. In fact, the Keewatin Ice Divide and its precursors represent the centers of an independent, land-based ice sheet that
probably existed throughout the period of Wisconsin Glaciation.
INTRODUCTION
Preliminary results of a study of
glacial dispersal of distinctive bedrock
components west and southwest of Hudson
Bay indicate that popular modern models
of the dynamics and history of the
Keewatin sector of the Laurentide Ice
Sheet (henceforth also referred to as the
continental ice sheet) are in need of
revision.
The discussion and conclusions that
follow are based on study of several
thousand samples of till and other glacial
deposits collected in support of mapping,
prospecting, and geotechnical research
carried out in southern Keewatin (Fig. 1).
These samples, core samples from boreholes drilled to depths of 6 to 20 m along
a proposed gas pipeline route (Fig. 1), and
recently completed maps of surficial
geology offer a unique opportunity to
study glacial transport directions and till
composition.
Development of Concept of
Laurentide Ice Sheet
Tyrrell (1898) was the first to speculate on the history of the Keewatin section
of what Flint (1943, p. 329) definitively
GEOLOGY, v. 7, p. 537-541, NOVEMBER
1979
described as the Laurentide Ice Sheet.
Tyrrell felt that central Keewatin had
served as a major "gathering ground" and
dispersal center for glaciers and that it was
just one of several centers that were active
at various times throughout the last and
earlier glaciations.
Tyrrell's concepts were not seriously
challenged until 1943, when Flint proposed
a comprehensive model for the Laurentide
Ice Sheet. He suggested that it originated
in the highlands of northeastern North
America and migrated gradually westward
and southward to form a "vast, continuous Laurentide ice sheet" that was "thickest over Hudson Bay itself" (Flint, 1943,
p. 333). Flint stated (1943, p. 329) that
"the importance of the concept of
'Labradorean ice' and 'Keewatin ice' has
been exaggerated, and these units were
strictly limited both as to time and as to
areal importance." As late as 1975, Ives
and others stated that "the Keewatin and
Labrador-Ungava residual centers clearly
produced the striations and associated ice
movement forms that were incorrectly
interpreted by nineteenth-century geologists as evidence for the independent
Keewatin and Labradorean ice sheets"
(Ives and others, 1975, p. 123).
Using the aerial photographs that
became available in the late 1940s and
drawing upon limited field work north of
Baker Lake, both Bird (1953) and Taylor
(1956) concluded that the most important
glacial flow in Keewatin was radially
outward from Hudson Bay, westward and
northwestward. Although they recognized
other minor glacial movements, Taylor
and Bird concurred with Flint: Tyrrell's
"multidome" model was inadequate, and
the Laurentide Ice Sheet was probably a
single, massive dome centered on Hudson
Bay during the maximum Wisconsin
Glaciation.
Lee and others (1957) and Lee (1959)
defined the Keewatin Ice Divide as the
zone toward which the western remnant
of the Laurentide Ice Sheet shrank; it
marked the location(s) of the last vestiges
of the continental glacier on the mainland.
Although these authors did not challenge
the main conclusions of Bird and Taylor,
they demonstrated conclusively that drumlins, flutes, and striae east of Baker and
Yathkyed Lakes indicated eastward or
southeastward ice flow, and they modified
Bird's and Taylor's models to incorporate
the Keewatin Ice Divide as a late-glacial
feature, formed when rising marine waters
in Hudson Strait split an ice dome in
Hudson Bay, leaving major masses of
actively flowing remnant ice in Keewatin
and Labrador. Lee (1959) speculated
that the Keewatin Ice Divide came into
existence about 7,000 to 8,000 yr ago, on
the basis of 14C dates on marine shells in
raised beaches of the Tyrrell Sea south of
Hudson Bay (see also Craig, 1969; Lee,
1960). Similar evidence led Lee to conclude
that the last ice melted from the Keewatin
Ice Divide about 6,000 yr ago.
A model postulating a single dome
centered on Hudson Bay which split into
two late-glacial centers of outflow east and
west of the bay has been widely accepted
and has served as the basis for many
modern studies, such as (1) isostatic
uplift (Andrews and Peltier, 1976),
(2) paleoclimatic reconstructions (Hughes
537
Figure 1. Generalized ice-flow directions and dispersal of Paleozoic erratics west of Hudson Bay. Some data from Prest and others (1967). Inset
map shows location of southern Keewatin with respect to a recent published model of the Laurentide lee Sheet (modified from Hughes and others,
1977).
and others, 1977; Andrews and Barry,
1978), (3) glacial erosion and origin and
dynamics of large ice sheets (White, 1972;
Ives and others, 1975; Sugden, 1978), and
(4) volume calculations for the Laurentide
Ice Sheet (Paterson, 1972).
Doubt about the validity of a singledome model was expressed by Andrews
(1973, p. 186), who asked, "What was
the geometry of the Laurentide Ice Sheet
complex . . . was it a single-domed ice
sheet . . . or was it a complex of centers?"
GEOLOGY OF THE
KEEWATIN REGION
Bedrock Lithologies
For the purpose of this paper, the
geology of the Hudson Bay basin and of
the area west of Hudson Bay can be
simplified and summarized as follows:
flat-lying Paleozoic sedimentary rocks
rest on a crystalline Archean and Aphebian
basement beneath the surface of much of
Hudson Bay; these rocks occur to within
a few kilometres of the coast from
Southampton Island south to Churchill;
from Churchill southward they extend
more than 100 km inland as a fringe
around the southern part of Hudson Bay
(Fig. 1). Relatively flat-lying Proterozoic
sedimentary and felsic volcanic rocks with
characteristic red to purple hues lie on the
Archean-Aphebian crystalline basement
over a large area west and southwest of
Baker Lake (Fig. 1). The Proterozoic formations, collectively called the Dubawnt
Group (Donaldson, 1965) generally are not
metamorphosed and served as the source
of a vast train of red till and distinctive
erratics that blankets much of southern
Keewatin.
Keewatin Ice Divide
The Keewatin Ice Divide forms western,
northern, and southern boundaries of an
area of sustained ice flow into Hudson
Bay. It migrated to its last position from
the north and/or northwest (Cunningham
and Shilts, 1977) and came into existence
sometime after a southward to southeastward flow of ice in southern Keewatin
(Lee, 1959). The earlier southward flow
may be related to a center or centers of
outflow located somewhere in northern,
western, or northwestern Keewatin. The
eastward extension of the ice divide at
its south end is described here for the
first time.
Isostasy
Because isostatic data are often cited
to support the concept of a single ice
dome over Hudson Bay, a brief discussion
of alternative interpretations of marine
strandline elevations is presented here.
The highest strandlines of the Tyrrell Sea
are well marked in much of the region and
display a great deal of local variation,
GEOLOGY
suggesting that rebound took place under
ice cover (restrained rebound; Andrews,
1969) at the same time that it was occurring in ice-free areas. For example, southwest of Baker Lake, marine limit ranges
from 125 to 160 m above sea level over a
distance of about 20 km; the basin in
which the lower levels were recorded was
filled with an isolated ice mass, preventing
marine inundation while a significant
amount of uplift was taking place. Because
of the time-transgressive nature of marine
limit and the unknown magnitude of
rebound that took place while the land
was protected from wave action by glacial
ice, isostatic data derived from marine
limit in Keewatin are difficult to interpret
and at present should not be used to
speculate on former areas of greatest ice
thickness.
Dispersal of Interglacial Sedimentary
Rocks and Paleozoic Erratics from
Hudson Bay Basin
Till samples from boreholes drilled at
— 10 km intervals on the Canadian Shield
along the proposed gas pipeline route
south of Nelson River to Longlac,
Ontario, contain 20% to 40% Paleozoic
sedimentary rocks and fossils and interglacial?) marine shell fragments derived
from the vicinity of Hudson Bay, in locations as far as 120 km south or west of
the nearest known source of these components. Marine shell fragments and as much
as 40% Precambrian erratics were found
by us, by B. C. McDonald (1978, personal
commun.), and by R. G. Skinner (1979,
personal commun.) in all till units, preand post-Missanaibi ( = Sangamon) in
age, in sections located east of the boreholes, on Paleozoic substrate around the
southwest side of Hudson Bay. Our new
observations support those of McDonald
(1969) and Skinner (1973) in indicating
that except for late-glacial surges into
proglacial lakes or local deflections of
thin ice by topography, south of Nelson
River ice flow was consistently from
Nouveau Québec-Labrador, westward to
southward throughout several stadial
events. Figure 1 indicates that the ratio
of Paleozoic to Precambrian erratics
changes radically between the Nelson River
and Churchill, suggesting that the westward glacial flow south of Nelson River
is supplanted by eastward, southeastward,
or southward flow north of the river.
From the Manitoba border (lat 60°N)
northward to Baker Lake, Paleozoic
erratics have not been observed in boreholes, in stratigraphie sections, or in the
more than 7,000 samples of surface till
collected throughout the region. No
Paleozoic fragments were observed in
groups of 30 samples collected at Rankin
Inlet and Eskimo Point, located over
Precambrian bed rock only 5 km west of
the westernmost edge of continuous
Paleozoic substrate under Hudson Bay.
Dispersal of Erratics of
Dubawnt Group "Red Beds"
Distinctive red erratics and red till have
been dispersed eastward and southeastward
from the region of Dubawnt Group outcrop on which the central part of the
Keewatin Ice Divide is fortuitously located
(Fig. 2). A 150-km-wide train of red, clayrich till extends more than 300 km from
the Yathkyed Lake-Baker Lake area to
the shores of Hudson Bay, where it
disappears beneath the bay at Eskimo
Point. Where the train intersects the
present coast between Eskimo Point and
Rankin Inlet, Dubawnt erratics make up
an average of 6% to 10% of the coarse
fractions of till. The red till owes its color
mainly to the high percentages of finely
divided (glacially ground) hematite in the
fine-silt and clay-size fractions.
Whereas the red till train and the
general trends of Dubawnt isopleths clearly
indicate sustained ice flow in a southeasterly to easterly direction east of the
ice divide, the widespread Dubawnt
erratics southward at least to the Manitoba
border, across the major eastward extension of the ice divide (Fig. 1), support the
concept that prior to the southeastward
flow, a general southerly ice flow affected
most of the area, an idea first suggested
by Tyrrell (1898) and later confirmed by
Lee (1959).
DISCUSSION
The length and direction of glacial
dispersal of red (Dubawnt) components
can be translated into either a rate of flow,
assuming a certain time during which the
flow direction prevailed, or into duration
of flow, assuming typical flow rates.
Alternatively, the size of the dispersal
train can be attributed to episodes of
shorter transport during several glaciations, as proposed by Gillberg (1977,
p. 249). Because of (1) its location near
the center of the continental ice sheet,
and (2) the sharply defined northeastern
flank of the train, which parallels local
trends of ice flow determined from the
examination of abundant striated outcrops, drumlins, flutings, and so forth,
the red till train is not thought to be
related to several cycles of glacial erosion
and deposition, at least in its northernmost
parts. In any case, whether till components
underwent cyclic transportation is not
important, because east of the ice divide
all flow was toward the bay or southward.
Rates of Flow If Dispersal Was
Late Glacial
If southeastward glacier flow and
accompanying dispersal were related solely
to the Keewatin Ice Divide and if the ice
divide was formed in response to the
incursion of the sea into Hudson Bay,
points on the present coast could not have
539
Figure 2. Dispersal o f D u b a w n t Group erratics and distribution o f red till. N o t e that in southern part o f m a p , southeastward dispersal trends are
apparently little i n f l u e n c e d by f l o w from last position o f Keewatin Ice Divide ( D u b a w n t o u t c r o p s m o d i f i e d from Wright, 1 9 6 7 ; LeCheminant and
others, 1 9 7 7 ; D o n a l d s o n , 1 9 6 5 , 1 9 6 6 , 1 9 6 9 ) .
been influenced by southeastward flow
for longer than 2,000 yr (Lee, 1959) or,
according to Andrews and Peltier (1976,
p. 75), for longer than the 4,000 yr during
which drawdown in Hudson Strait affected
ice flow in the bay. The implications of
these constraints are that the available
time for formation of the northern flank
of the red till train and the time available
for transport of Dubawnt detritus to the
coast was 1,000 to 3,000 yr if it can be
assumed that about 1,000 yr was necessary
for retreat from the coast to the area of
the divide. This would require the basal
sediment load of the ice (boulders to clay
sizes) to be transported at average rates
of 60 to 175 m/yr for debris deposited at
540
Rankin Inlet (175 km from the nearest
outcrop of Dubawnt rocks), or 100 to 300
m/yr for debris deposited at Eskimo Point
(300 km southeast from the nearest
Dubawnt outcrops). The rates are minimum averages because no gradient is
assumed, and any part of the basal load
was likely to undergo several alternating
periods of lodging and renewed erosion
during its transport, as is evident from
its complete mixing with local debris at
any given site. We conclude that these
(basal) rates of flow are unreasonable,
particularly because such rates would have
had to be maintained throughout the ice
mass from the ice divide to the edge.
In addition to the difficulties cited
above, the concept of ice flow toward
Hudson Bay only in late-glacial time in no
way accounts for the time required for the
ea.rlier, more southerly dispersal of
Dubawnt detritus, which is related to a
center of outflow west or northwest of the
central and northern parts of the ice divide.
This earlier dispersal event occurred over
a distance of at least several hundred
kilometres as well. Furthermore, if rapid
flow was associated with drawdown into
the Tyrrell Sea, the ice divide would have
migrated westward or northward, opposite
to the southeastward direction of migration observed near Baker Lake (Cunningham and Shilts, 1977). This suggests that
some factor other than late-glacial events
N O V E M B E R 1979
controlled reorganization or migration of
centers of outflow. Finally, although the
ice front was in contact with the sea from
the modern coast to or near to the ice
divide, depositional features in that region
(integrated esker systems, particularly)
suggest massive stagnation of the ice
sheet rather than accelerated flow due to
drawdown.
Duration of Flow If Dispersal Was
Associated with Moderate Rates of
Basal Ice Flow
In light of the difficulties cited above
in accounting for the amount of glacial
dispersal in late-glacial time, an alternative
interpretation of the facts is presented
here. If lower rates of basal flow (actually
rates of basal transport of detritus) are
assumed, order of magnitude estimates
can be made for the duration of flow
from the Keewatin Ice Divide and/or
centers of outflow. Using an average flow
rate of 10 m/yr throughout the ice mass,
as Gillberg (1977) has done (realizing that
a gradient of basal flow velocities from
zero at the center of dispersal to some
higher rate near the periphery is the true
velocity profile), minimum times required
for transport of detritus to Rankin Inlet
(175 km) and Eskimo Point (300 km) are
17,500 and 30,000 yr, respectively. Boulton
and others (1977, p. 236) have estimated
that a particle starting at the center of
the British ice sheet would have required
19,000 yr to reach the eastern edge—
a distance of about 200 km.
These are minimum estimates, based
on observed dispersal only as far as the
coast at fairly high basal flow rates;
furthermore, when the time required for
earlier, southward dispersal is considered,
the estimates could probably be at least
doubled. Clearly, the amount of time
during which centers of outflow existed
in north-central Keewatin was tens of
thousands of years, an order of time
required for an entire glacial stage. It is,
therefore, our conclusion that the Keewatin Ice Sheet (Tyrrell, 1898) grew and
remained on land west of Hudson Bay
during the whole of the Wisconsin Glaciation. That ice flow during this glaciation
was always toward Hudson Bay in the
region east of the ice divide is further
supported by the lack of evidence of any
period of opposing flow from the Hudson
Bay basin; no striae, geomorphic features,
or, most importantly, westward dispersal
of Paleozoic erratics have been found
during extensive mapping and detailed
lithological investigations of surficial
deposits in Keewatin.
By analogy, Labradorean ice-flow
center(s) may have had a similar history,
and it is not improbable that several landbased centers of outflow may have
coalesced to form the central part of the
continental ice sheet. The clustering of
such centers close to Hudson Bay could
GEOLOGY
have produced ice-flow patterns in peripheral parts of the continental ice sheet that
would make it appear that a single center
existed over the bay.
CONCLUSIONS
The Keewatin Ice Divide, although
certainly the resting place of the last
vestiges of the Keewatin sector of the last
continental ice sheet, is either itself an
ancient feature or is located near an
ancient center or centers of outflow in
northern or central Keewatin. It and its
precursors appear to represent evidence
of the growth, maturation, and death of
an ice sheet on the low plateaus of
Keewatin. Thus, we have returned to
Tyrrell's (1898) concept of Keewatin as
one of several "gathering grounds" of the
last continental ice sheet. Recent studies
by Williams (1978) suggest that this conclusion has some climatological support.
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climatic i n f l u e n c e s of e x p a n d e d s n o w cover
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M e m o i r 3 5 0 , 9 1 p.
ACKNOWLEDGMENTS
Reviewed by J. T. Andrews, V. K. Prest, and
R. J. F u l t o n . B. G. Craig and R.N.W. DiLabio
made several suggestions that improved the
manuscript. We thank the Polar Gas consortium
and W. D. Roggensack o f EBA Engineering
Consultants, Ltd. ( E d m o n t o n ) for providing us
with all unused samples from their geotechnical
drilling program across the Canadian Shield.
T h e petrological study of till samples summarized in Figure 2 was f u n d e d jointly by
A t o m i c Energy o f Canada Ltd. and by the G e o logical Survey o f Canada as part o f a study o f
intensity o f glacial erosion.
MANUSCRIPT RECEIVED MAR.
12,
1979
MANUSCRIPT ACCEPTED AUG. 28,
1979
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