55

Chapter 4

GLACIATION OF THE HUDSON BAY REGION

W.W. SHILTS

Although the Hudson Bay Basin has been glaciated several times within the

past million years, most of its present day sedimentological characteristics are

inherited from the last glacial event. At the time of writing, the nature of

the last ice sheet that covered the basin is a topic o f considerable discussion

as is the history of glaciation of the region. to summarize current concepts of Hudson Bay's glacial history and to describe

the nature of its early post-Wisconsin history. The discussion will be based

largely on data from terrains flanking Hudson Bay on the west and south, where

recent mapping of isostatically uplifted terrain has yielded much sedimento-

logical information that is pertinent to the understanding of its modern

sedimentary environments (Fig. 4.1).

The objective of this chapter is

SUMMARY OF GLACIAL HISTORY

Although a great deal is known about the oscillations of the Laurentide Ice

Sheet around its unstable southern fringes, particularly in the United States,

much less is known about glacial oscillations across Hudson Bay, much nearer to

the more stable "heart" of the continental ice sheets (Prest, 1970). It has

only been during the past decade and a half that scientists have carried out

systematic studies o f the Pleistocene stratigraphy of the region that was

covered by this part of the Laurentide Ice Sheet.

have mapped surficial deposits in southern District of Keewatin and northern

Manitoba and have begun to examine exposures among the hundreds of kilometres of

stratigraphic sections found along the rivers and streams of the Hudson Bay

Lowland. The results of these studies have led to controversial and conflicting

conclusions about the history of glaciation of the Hudson Bay Basin.

A handful of earth scientists

Two possible interpretations may be made of the stratigraphic record of the

Hudson Bay Lowland, one suggesting a "maximum" number of glacial events and one

suggesting a "minimum" number. The stratigraphic interpretations are deduced

from the superposition of glacial and nonglacial sediments, from conventional

56

H U D S O N

e i r

Figure 4.1. Location map showing Paleozoic-Mesozoic sedimentary basin within heavy line.

carbon-14 dating of organic materials, and from a recently developed relative dating method based on amino acid racemization rates of marine mollusks

(Andrews et al., 1983).

erratics in glacial units (till) and are reworked or in situ components of Marine and fresh-water shell fragments occur as

57

interbedded nong lac ia l , w a t e r - l a i d u n i t s , such as t h e B e l l Sea sediments

(Skinner, 1973; Andrews e t al., 1983).

The phys i ca l s t r a t i g r a D h y o f t h e Hudson Bay Lowland suggests t h a t a t l e a s t

four g l a c i a l events preceded an i n t e r g l a c i a l episode.

pre-Wisconsin g l a c i a t i o n s i s p r e s e n t l y known a t o n l y one loca t i on , where f o u r

t i l l s u n d e r l i e t h e i n t e r g l a c i a l f o r e s t beds o f t h e M i s s i n a i b i Format ion on

M iss ina ib i R i v e r ( S h i l t s , 1985).

d i r e c t l y o v e r l a i n b y a maroon-coloured t i l l t h a t i s separated f rom two

ove r l y ing g ray t i l l s b y f l u v i a l depos i t s .

f l u v i a l depos i t s which suggests t h a t Hudson Bay was n o t b locked by g l a c i a l i c e

dur ing the n o n g l a c i a l i n t e r v a l . The uppermost gray t i l l i s d i r e c t l y o v e r l a i n

by the M i s s i n a i b i Format ion. The M i s s i n a i b i marine, f o r e s t , peat, f l u v i a l and

l a c u s t r i n e beds were deposi ted o r formed d u r i n g a c l i m a t i c i n t e r v a l s i m i l a r t o

t h a t which has a f f e c t e d t h e Hudson Bay Lowlands d u r i n g ou r present i n t e r g l a c i a l

(Figs. 4.2, 4.3; Skinner, 1973). The M i s s i n a i b i Format ion i s >75,000 "C years

o l d based on work b y S t u i v e r e t a l . (1978) and on r e c e n t l y acqui red amino a c i d

data (Rut ter , pers. corn. ) .

The evidence f o r f o u r

The l i g h t g ray lower t i l l a t t h i s s i t e i s

The g ray t i l l s a re a l s o separated b y

Since t h e M i s s i n a i h i n o n g l a c i a l i n t e r v a l , which i s assumed t o rep resen t t h e

Sangamon i n t e r g l a c i a l s tage (Skinner, 1973), as few as one, and as many as

three major expansions o f t h e Lauren t ide g l a c i e r complex have occurred i n the

reg ion surrounding Hudson Bay.

Skinner (1973) i d e n t i f i e d two g l a c i a l events represented by two t i l l sheets

separated by g l a c i o l a c u s t r i n e sediments i n and adjacent t o t h e Moose R ive r

basin.

throughout t h e western Hudson Bay Lowland, a t l e a s t as f a r n o r t h as C h u r c h i l l ,

Manitoba. N e t t e r v i l l e (1974), Andrews e t a l . (1983), and S h i l t s (1982, 1984b,

1985) have suggested t h a t some o r a l l p a r t s o f t h e Hudson Bay Lowland may have

been subjected t o t h r e e Wisconsin g l a c i a l events, based on p h y s i c a l

s t r a t i g r a p h i c r e l a t i o n s h i p s ( t h r e e t i l l s o v e r l y i n g i n t e r g l a c i a l beds).

add i t i on , am inos t ra t i g raph ic evidence based on i s o l e u c i n e racemiza t i on r a t i o s

( a I 1 e : I l e ) o f marine s h e l l s found as e r r a t i c s i n t i l l and i n s i t u i n mar ine

s i l t y c l a y s suggests t h a t more than one opening o f Hudson Bay occurred a f t e r

t he Sangamon stage.

nong lac ia l sediments throughout t h e Hudson Bay Lowland y i e l d a I 1 e : I l e r a t i o s

in termediate between r a t i o s c a l c u l a t e d f o r i n t e r g l a c i a l marine s h e l l s and those

ca l cu la ted f o r p o s t g l a c i a l mar ine s h e l l s (Andrews e t al., 1983).

South o f James Bay, McDonald (1969) and

McDonald (1969) recognized a b i p a r t i t e d i v i s i o n o f t h e Wisconsin

I n

S h e l l s c o l l e c t e d f rom p o s t - M i s s i n a i b i g l a c i a l and

I n assessing t h e i m p l i c a t i o n s o f t h e s t r a t i g r a p h y o f t h e Hudson-James Bay

Lowland f o r g l a c i a l events t h a t may have a f f e c t e d Hudson Bay, i t i s p o s s i b l e t o

propose "minimum" and "maximum" g l a c i a l models; t h e t r u e g l a c i a l h i s t o r y o f t h e

58

S E D I M E N T S

. - - 1 0 .: ;. 0- 0 I I*( . . .

INTERPRETATION

I

1

ROCK 1RATIGRAPHIl

UNllS

LOWER TILL

Figure 4.2. Skinner, 1973).

Composite sec t i on o f sediments o f t he M iss ina ib i Formation ( f rom

l a s t few hundred thousand years probably l i e s between these cons t ra in ing

models.

The "minimum" model, suggested by F l i n t (1943) and championed by Denton and

Hughes (1981), Dredge and Nielson (1985), and t o a lesser extent by

McDonald (1969, 1971) and Skinner (1973), impl ies t h a t du r ing the pre-

M iss ina ib i stage (presumably I l l i n o i a n ) and dur ing the post -Miss ina ib i stage

(Wisconsin), t h e core o f t he Laurent ide I c e Sheet was r e l a t i v e l y s t a b l e f o r

many tens o f thousands o f years over and around Hudson Bay (Fig. 4.4).

i c e i n Hudson Bay blocked the sea on the n o r t h and blocked o r covered

G lac ie r

59

~ ~~~

SEDIMENTS 1 INTERPRETATION ROCK UNITS

SL1

O f f - LAP

n c

51KKl CL4V WllW Kt-14lICD CL4S15

Figure 4.3. Composite section of late- and postglacial sediments (from Skinner, 1973).

northward- or eastward-draining river systems on the east, south, and west.

The "minimum" model further implies that the multiple till sequences observed

both beneath and above the Missinaibi Formation were deposited during glacial

surges into proglacial lakes or as a result of minor local oscillations of the

ice front as it was advancing or retreating through the Hudson Bay Lowland.

Thus, glacial erosion, glacio-isostatic adjustment, hydrologic perturbations

related to glaciation, and all the other phenomena associated with the passage

of continental glaciers would have been applied in two relatively long-lasting cycles over Hudson Bay. The complex events associated with the passage o f the

ice front across a particular site would have occurred twice (during advance

and retreat), over a period of a few hundred years at either end of two

prolonged periods of thick glacier cover spanning several tens of thousands

EVENTS

"I Tyrrell

M e glocio-

xcillotions I I IVorves I

Glocier Exponsion - 'Figure 4.4. "Minimum" Stratigraphic Model for Hudson Bay region. The multiple tills and nonglacial units are accommodated by proposing local oscillations of advancing and retreating ice fronts during two major glaciations (after Shilts, 1984a).

years (assuming that the presently exposed deposits of the Lowlands represent only the Illinoian and Wisconsin glacial stages).

The "maximum" glacial model, favoured by the author (Shilts, 1982, 1984b,

1985) and by Andrews et al. (1983), and supported to some degree by

Nettervi lle (1974), Skinner (1973), McDonald (1971), and Nielsen and Dredge

(1982), correlates most of the individual till sheets identified in the Hudson

Bay Lowland with major expansions of the Laurentide Ice Sheet (Fig. 4.5). The

intervening water-laid sediments or weathering zones are interpreted as marking

major shrinkage of the ice sheet, in many cases requiring evacuation of Hudson Bay itself with accompanying incursion of marine waters into all or part of the

Bay.

which are fluvial, occur near the present border of Hudson Bay (near the

geographical centre of the Laurentide Ice Sheet) at low altitudes (<lo0 m

a.s.l.), supports the idea that the Bay was not blocked by glaciers during

The very fact that weathering zones and water-laid deposits, many of

61

EVENTS

F igu re 4.5. "Maximum" S t r a t i g r a p h i c Model f o r t h e Hudson Bay reg ion . Most t i l l u n i t s a r e assigned t o g l a c i a l events mark ing as many as seven major d e t e r i o r a t i o n s and expansions o f t h e L a u r e n t i d e I c e Sheet - f o u r b e f o r e d e p o s i t i o n o f t h e i n t e r g l a c i a l M i s s i n a i b i Beds and t h r e e a f t e r t h e i r d e p o s i t i o n ( a f t e r S h i l t s , 1984b).

these i n t e r v a l s . L o w - a l t i t u d e f l u v i a l depos i t s , f o r ins tance, c o u l d n o t have

been l a i d down i f t h e sea was prevented f rom e n t e r i n g Hudson Bay b y an i c e mass

i n t h e bas in , s i n c e r i v e r s p r e s e n t l y d r a i n i n g i n t o t h e Bay would have been

dammed and f l o o d e d t o t h e l e v e l s o f t h e i r landward d i v i d e s . Thus, t h e

"maximum" model suggests t h a t ma jor c y c l e s o f expansion and sh r inkage o f t h e

Lauren t ide I c e Sheet may have occu r red as many as seven t imes d u r i n g t h e l a s t

two ( I l l i n o i a n - W i s c o n s i n ) g l a c i a t i o n s . I n t h i s model Hudson Bay would have

been sub jec ted t o a t l e a s t seven, r a t h e r than two, ma jor g l a c i a l events and,

t he re fo re , a l l o f eas te rn Nor th America would have exper ienced many more

e ros iona l , d e p o s i t i o n a l , g lac io -h ,yd ro log i ca l . load ing , and c l i m a t i c c y c l e s

than i t would have i f t h e "minimum" model i s c o r r e c t .

As shown b y McDonald (1969, 1972) and Sk inner (19731, t h e "minimum" model

can be accomodated w i t h i n t h e p h y s i c a l s t r a t i g r a p h i c framework o f t h e Hudson

Bay Lowland as l ong as (1) n o n g l a c i a l f l u v i a l g r a v e l s ( e x c l u s i v e o f t hose

beneath t h e M i s s i n a i b i a t i t s t y p e s e c t i o n ) a r e c o r r e l a t e d w i t h t h e M i s s i n a i b i

62

interglacial sediments, (2) other fluvial gravels are interpreted to be glacio-

fluvial, and (3) till sequences separated by glaciolacustrine or glaciofluvial sediments are considered to be deposited during surges or local oscillations of

the ice front during advance or retreat of the Laurentide Ice Sheet.

The "maximum" model can also be accomodated by the physical stratigraphy,

but also can explain (1) major provenance differences sometimes seen between

superposed tills (I.M. Kettles and P. Wyatt, personal comnunication),

(2) weathering zones seen on tills (Nielson and Dredge, 1982), and (3) most

importantly for the post-Missinaibi sequence, amino acid isoleucine

racemization ratios that fall between those of proven interglacial marine

shells and postglacial marine shells (Andrews et al., 1983).

DYNAMICS AND CONFIGURATION OF THE LAURENTIDE ICE SHEET

The last glaciation of the Hudson Bay region was effected by a continental

ice sheet consisting of several centres of outflow that shifted position and

interacted with each other in complex ways in space and time. As with the

history of the earlier continental ice sheets, the exact configuration of the

centres of ice dispersal are currently a matter of considerable discussion.

The model presented here is based on the author's geological data and is

discussed more fully elsewhere (Shilts et al., 1979; Shilts, 1980, 1982, 1985).

As emphasized in the introduction, a reasonably correct model of ice flow

configuration is a prerequisite for understanding the many characteristics of

modern Hudson Bay that are related directly to the effects of the last

glaciation (Adshead, 1983a, b, c).

Ideas concerning the dynamics and configuration of the last ice sheet to

cover Hudson Bay and, by inference, earlier ice sheets, are based on two types

of evidence:

1) The first type of evidence comprises geological and qeomorphological

observations, such as orientation of ice eroded or ice moulded landforms and

dispersal patterns o f distinctive erratics and mineral or chemical

components.

volume during the past two decades as geological work has been carried out

in conjunction with exploration for "frontier" mineral or energy resources

(Aylsworth, et al., .1981a, b, Arseneault, et al., 1981, 1982; Dredge, et

al., 1985; Thomas and Dyke, 1981a, b).

2) The second type of evidence is more hypothetical, being based on inferences

drawn from climatic data and theory, from data on isostatic uplift, and from

computer-generated mathematical models of the ice sheet based on the

physical properties of the ice-water system.

must, and increasingly do, conform to constraints imposed on them by

These observations have only been available in any significant

These hypothetical models

63

Figure 4.6. Major dispersal trains o f the central part of the Laurentide Ice Sheet. Note: 1) outcrop o f Paleozoic limestones and clastic rocks; 2) maximum dispersal area of Paleozoic erratics; Proterozoic erratics from eastern part of Hudson Bay are similarly distributed; 3 ) Dubawnt redbed outcrops; 4) dispersal train of Dubawnt erratics (from Shilts, 1982).

geologic and geomorphic data derived from glacial sediments (Denton and

Hughes, 1981; Boulton et al., 1985).

Study of the history and configuration of glacier flow in and around Hudson

Bay is facilitated by the fortuitous outcrop patterns of distinctive bedrock

lithologies beneath and adjacent to it (Fig. 4.6). Hudson Bay, itself, is

largely underlain by nearly flat lying, unmetamorphosed, easily eroded Paleozoic and Mesozoic ( ? ) sediments which comprise carbonate with minor

terrestrial and marine clastic facies (Sanford et al., 1979).

shore and offshore o f Hudson Bay is underlain by folded metasedimentary and

metavolcanic rocks of Proterozoic age, which also outcrop in the Sutton Ridge southwest o f Cape Henrietta Maria (Donaldson, 1986).

The eastern

These rocks form part of

64

the circum-Ungava geosyncline and are dominated by metagraywackes and i r o n

format ion (Dimroth e t al., 1970).

geosyncl ine prov ide a s u i t e o f e r r a t i c s t h a t are p a r t i c u l a r l y d i s t i n c t i v e i n

con t ras t t o t h e Paleozoic and c r y s t a l l i n e e r r a t i c s t h a t dominate t h e d r i f t

around Hudson Bay.

convenience o f reference.

West o f Hudson Bay, along the eastern edge o f t he Thelon sedimentary basin,

C o l l e c t i v e l y , the rocks o f t h e circum-Ungava

S h i l t s (1980) has termed these "dark e r r a t i c s " f o r

near Baker Lake, outcrops o f unmetamorphosed, red volcanic, v o l c a n i c l a s t i c , and

sedimentary rocks o f the Dubawnt Group have provided another source o f d i s t i n c -

t i v e e r r a t i c s (Donaldson, 1965).

e a s i l y eroded and produced a l a r g e amount o f debr is t h a t can be t raced f o r

hundreds o f k i lometers onshore and o f f sho re t o the mouth o f Hudson Bay.

southern p a r t o f the Bay, t h i s r e d d e t r i t u s i s v i r t u a l l y impossible t o d i s t i n -

guish from red c l a s t i c u n i t s w i t h i n the Paleozoic and Proterozoic sequence,

except i n c l a s t s l a rge r than the 2-6 mn s izes t h a t have been used t o map

d ispersa l .

As w i t h Paleozoic s t r a t a , these rocks were

I n t h e

Among and surrounding the d i s t i n c t i v e l i t h o l o g i c assemblages described above

are outcrops o f o lde r h i g h l y metamorphosed ! c r y s t a l l i n e ) Precambrian basement,

comprising most ly gneiss ic and g r a n i t o i d rocks w i t h b e l t s o f Archean metavol-

canic and metasedimentary rocks dominated by bas ic i n t r u s i v e and ex t rus i ve

u n i t s (greenstone b e l t s ) (Sanford e t al., 1979).

c r y s t a l l i n e ter ranes are c u t by a r e a l l y i n s i g n i f i c a n t b e l t s o f o r t h o q u a r t z i t e

and associated c ra tona l metasediments tha t , because o f t h e i r toughness and

d i s t i n c t i v e appearance, are found as e r r a t i c s throughout the no r the rn Hudson

Bay region.

West o f Hudson Bay the

F igure 4.6 sumnarizes the areal d i s t r i b u t i o n o f the groups o f e r r a t i c s

described above as t h e i r pa t te rns are p resen t l y known ( S h i l t s , 1982). Using

these d i spe ra l data and the trends o f i c e i nsc r ibed and i c e moulded features,

i t i s poss ib le t o hypothesize the con f igu ra t i on and l o c a t i o n o f various i c e

d i spe rsa l cent res and how they may have in te rac ted i n t ime and space

(Fig. 4.7). Dyke e t a l . (1982) have postu la ted an a d d i t i o n a l i c e f l o w cen t re

i n o r adjacent t o southern Hudson Bay a t some t ime dur ing the l a s t g l a c i a l

event, b u t s ince the re i s no d i r e c t evidence f o r such a cen t re a t t h i s time, i t

i s omi t ted from the recons t ruc t i on shown here. Other reconst ruct ions t h a t

accord w i t h the geologic evidence may be made (F ig. 4.8).

deg lac ia t i on o f t he reg ion began g l a c i e r s from a Keewatin cen t re may have been

vigorous enough t o d i sp lace i c e from a Labradorean-Nouveau Quebec cen t re from

a l l b u t t he eastern p a r t o f the Bay. Keewatin i c e would have provided a base

from which t h e l a t e g l a c i a l Cochrane readvance surged southward through the

reg ion south o f James Bay (Hughes, 1965).

Just before major

65

Figure 4.7. Possible ice flow configuration during maximum development Of Laurentide Ice Sheet. Note zones of confluence of Keewatin and Labrador ice sheets in Hudson Bay. Zones of confluence shifted depending on vigour and location of the t w o major ice dispersal centers. Arrows based on dispersal and geomorphic data. KID: Keewatin Ice Divide; NQ-LID: Nouveau Quebec-Labrador Ice Divide (after Shilts, 1980).

On the basis of striation and erratic dispersal data, both Bouchard and

Martineau (1985) and Klassen and Bolduc (1984) have recognized earlier ice flow

centres west and south of the one depicted in Quebec and Labrador on Figure 4.7.

The Keewatin Ice Divide also migrated a short distance southeastward to its

final location (Lee et al., 1957). Earlier ice flow centres with configurations

quite different from that depicted on Figure 4.7 are suspected for earlier glaciations, based on sparse striation and dispersal data.

relative timing and location of the various centres, however, the depositional

and erosional effects on Hudson Bay are for the most part related to configurations similar to those depicted by Figures 4.7 and 4.8.

Whatever the

66

Figure 4.8. Possible ice flow configuration early or late in development of Laurentide Ice Sheet. There is dispersal and striation evidence for this configuration i n Hudson Bay Lowland. Note how zones of confluence have shifted from Figure 4.7 (from Shilts, 1985).

In preparing the lithologic maps on which the deductions about ice flow

configuration are based, the author analyzed several till-like grab and short

gravity core samples collected by B.V. Sanford and C.F.M. Lewis from the bottom

o f Hudson Bay on a private cruise in 1971. Henderson (1983) also used these samples to carry out a study of the distribution of heavy minerals on the

bottom of the Bay.

composition of the till-like samples accords well with patterns mapped on land

contiguous to Hudson Bay. samples are slightly modified to unmodified glacial debris outcropping by

virtue of the low postglacial sedimentation rates over much of the presently

submerged Hudson Bay basin.

unconsolidated sediment that forms much of the bottom of Hudson Bay,

In both studies it was concluded that the pattern of

It was concluded further that many of the bottom

Thus, the composition and characteristics of the

67

particularly sediment that occurs on submerged hills and slopes, may be closely

related to glacial sedimentation patterns associated with the last glaciation.

Mapping of the isostatically uplifted floor of the Tyrrell Sea (ancestral Hudson Bay) in Keewatin generally confirms the thin and sporadic cover of marine sediment (Arsenault et al., 1981, 1982; Aylsworth et al., 1981a,b).

Some of the major constraints placed on interpretations of the glacial

history of Hudson Bay may be illustrated by referring to the map of glacial

dispersal from fossiliferous Paleozoic carbonate outcrops and from red bed outcrops of the Dubawnt Group (Fig. 4.6).

identified, even where present in very low concentrations, their dispersal

trains, which represent dispersal accomplished mainly during the last

glaciation, are fairly well known. Thus, it may be concluded from their

dispersal patterns that glaciers that traversed the Paleozoic outcrops that

underlie modern Hudson Bay never flowed onto land north of Seal River in Northern Manitoba, nor did they flow onto land east of a line approximating the

Ontario-Quebec border. This rules out the possibility that Hudson Bay could

ever have been covered by a thick dome of ice flowing radially onto land

surrounding it, a concept introduced by Flint (1943) and perpetuated in some modern literature (Denton and Hughes, 1981; Hughes et al., 1985).

Because both rock types are easily

LATE AND POSTGLACIAL HISTORY OF HUDSON BAY

Notwithstanding the combined influence of the various glacial events that have modified the surficial geology of Hudson Bay over the past several hundred

thousand years, far and away the most important glacial episode with respect to

the modern geological environment of the Bay was the last deglaciation.

The Laurentide Ice Sheet shrank toward centres that approximate the positions of its major gathering grounds in central District o f Keewatin, Foxe

Basin (Andrews, 1982), and Nouveau Quebec-Labrador.

displacement of the mantle and depression of the land surface to elevations of

at least 100 to 300 metres below the present surface.

Laurentide Ice Sheet had shrunk sufficiently to allow the sea to reenter Hudson

Bay through Hudson Strait, relative sea level was much higher than present, and

areas adjacent to the Bay were flooded (Fig. 4.9). This early configuration of

Hudson Bay has been termed the Tyrrell Sea by Lee (1960), and because isostatic

uplift is still going on, Tyrrell Sea and modern Hudson Bay merge at the

present shoreline. The discussion that follows will concentrate on the area

below the highest limit (marine limit) of the Tyrrell Sea and will summarize

both the retreat of glaciers inland and the marine regression.

Just prior to the opening of Hudson Bay to marine waters, large proglacial

lakes fronted the glacier margins south and west of the Bay.

The weight of ice caused

Thus, when the

These lakes

68

Figure 4.9. Maximum extent of postglacial marine inundation and spot elevations of marine limit (after Prest et al., 1967).

drained southward and westward through various routes into the St. Lawrence and

Mississippi River systems. As early as about 7800 "C years B.P. marine waters

penetrated the ice sheet as far south as the James Bay Lowlands

(Skinner, 1973).

Both Skinner (1973) and Hardy (1977) considered that the transition from

fresh water, proglacial lake conditions to marine conditions was very rapid.

They cited as evidence 1) the lack of raised nearshore features between the

lowest lacustrine and highest marine beaches, indicating precipitous lowering

of water levels, and 2) the presence of an intraformational conglomerate

69

separating laminated, unfossiliferous fresh-water clayey silts from overlying, massive, marine-fossil bearing silts in many sections in the James Bay region

(Fig. 4.3).

draining of proglacial lakes Barlow and Ojibway to sea level, accompanied by strong underflow of dense, oxygenated marine water through the basin and

beneath the fresh water wedge.

Both authors attributed the latter feature to catastrophic

The location of the original break in the ice sheet over Hudson Bay is not known precisely, but Skinner (1973) speculated that it probably occurred where

the ice was thinnest, over the bathymetric high that extends from Cape

Henrietta Maria to Manse1 Island.

Assuming that the initial marine incursion took place about 7800 “ C years ago and that marine deposition was occurring on or near the Keewatin Ice Divide

about 6000 14C years ago the rate of glacial retreat can be estimated to have averaged about 300 metres/year (Shilts, 1985). Average spacing of glacial

landforms thought to have been formed annually, DeGeer moraines and beads in beaded eskers, yield slightly lower rates of retreat of 217 to 290 metres/year

(Vincent, 1977).

Shilts (1985) has concluded that the ice sheets on either side of Hudson Bay

were largely stagnant after the marine waters separated them and that the ice front retreated by regu

around the Keewat i n and

subglacial drainage of

water, building ice con

systems, and because no

was deposited below mar

ar melting back toward the thicker ice and colder area

Nouveau Quebec-Labrador Ice Divides. Because the

he retreating glacier fronts debouched into standing

act fans and deltas periodically along major esker

subaerial outwash, such as is found above marine limit, ne limit, the ice front was almost certainly in contact

with the sea right up to marine limit.

the sea followed a period of subaerial exposure after retreat of the Keewatin

ice sheet, implying a rate of eustatic sea level rise exceeding the rate of

isostatic rebound at a time when the North American ice sheet had all but

disappeared. The sedimentological evidence from Keewatin and elsewhere and the probability that world wide sea levels would have stabilized at more or less

their present levels after disappearance of the continental ice sheets suggest

that her hypothesis is untenable.

Gilchrist (1982) inferred that onlap of

MARINE LIMIT

Nearshore features marking the highest level of marine inundation are nearly everywhere the most prominent of the marine strand lines developed between

marine limit and modern sea level (Fig. 4.10).

the ice front and marine limit at some time occupied the same place for a brief

period.

In most cases this is because

This juxtaposition of glacier ice, with its relatively heavy sediment

70

Figure 4.10. Marine limit and marine beaches surrounding a hill near Pitz Lake (GSC 203011-1).

load, and high-energy nearshore depositional environment, which provided condi-

tions for reworking this load as it was released by melting, provided ideal

conditions for development of a prominent strandline.

Because it is so well-developed, marine limit is probably the best mapped

postglacial feature around Hudson Bay.

Canada (Prest et al., 1967) and at more detailed recently compiled maps reveals

a considerable variation in the altitude of this feature around Hudson Bay,

ranging from over 300 m in the Richmond Gulf area to just over 120 m on Coats

and northeastern Southampton islands and near Baker Lake (Fig. 4.9). Because

the ice sheet had thinned considerably by the time marine waters cleaved it, isostatic rebound had already started, at a reduced rate, beneath the glacier

cover before marine waters inundated many regions. Thus, isostatic rebound was

taking place beneath glacier-covered and water-covered areas alike after

7900 years B.P.; Andrews (1969) termed the isostatic recovery beneath glacier

ice "restrained rebound", a very useful concept for evaluating early

postglacial events in the Hudson Bay basin.

A cursory glance at the Glacial Map of

Marine limit is a markedly time transgressive feature.

limit strandlines formed in the James Bay region as early as 7900 years B.P.

but strandlines marking marine limit formed over 2000 years later near Baker

Lake. Thus, this well developed and apparently continuous feature has an age

that can vary by over 2000 years around Hudson Bay.

limit, which has often been taken as a measure of magnitude o f isostatic

Earliest marine

The altitude of marine

71

depression and, t h e r e f o r e , o f l o c a t i o n o f maximum g l a c i a l l o a d i n g i s ,

consequently, open t o a range o f i n t e r p r e t a t i o n s . For ins tance, t h e l ow

(120 m) mar ine l i m i t on Coats I s land , near t h e i n i t i a l b reak- th rough o f mar ine

water, was p robab ly formed a t t h e onse t o f i s o s t a t i c recovery ; i t t h e r e f o r e

i n d i c a t e s t h a t g l a c i e r i c e was r e l a t i v e l y t h i n compared t o mar ine l i m i t s o f

s i m i l a r a l t i t u d e b u t younger age i n r e g i o n s t o t h e eas t and west o f Coats

Is land. Mar ine l i m i t a t about t h e same a l t i t u d e near Baker Lake, p robab ly as

much as 2000 yea rs younger than t h a t on Coats I s l a n d , most l i k e l y i n d i c a t e s

t h a t cons ide rab le r e s t r a i n e d rebound had taken p l a c e b e f o r e t h e i c e me l ted

away, exposing t h e l and t o nearshore mar ine processes.

g l a c i e r t h i ckness a t Baker Lake was p robab ly much g r e a t e r t han t h a t near Coats

Is land, i n s p i t e o f t h e s i m i l a r a l t i t u d e s o f mar ine l i m i t . A l t i t u d e o f mar ine

l i m i t can not be assumed t o be r e l a t e d t o magnitude o f g l a c i o - i s o s t a t i c

depression i n any c o n s i s t e n t way.

I n t h i s example,

Perhaps t h e most s t r i k i n g example o f t h e impor tance o f t h e concept o f

r e s t r a i n e d rebound was desc r ibed b y McDonald and Sk inner (1969) some years ago

i n t h e P i t z Lake area, west o f Baker Lake and d i r e c t l y on t h e l a s t p o s i t i o n of

t he Keewat in I c e D iv ide . There a sha l l ow b a s i n i s surrounded b y a horseshoe-

shaped r i d g e composed o f r e s i s t a n t v o l c a n i c f l o w s and v o l c a n i c l a s t i c rocks

(Fiq. 4.11).

p a r t s o f t h e r i d g e .

a re about 30 met res h i g h e r than those on t h e i n n e r s lopes, suggest ing t h a t a

b lock o f remnant i c e occupied t h e P i t z Lake basin, b l o c k i n g o u t t h e sea, u n t i l

over 30 m o f r e s t r a i n e d rebound had taken p lace .

Mar ine l i m i t i s w e l l marked w i t h i n t h e b a s i n and on t h e o u t e r

A l t i t u d e s o f mar ine l i m i t on t h e o u t e r s lopes o f t h e r i d g e

RETREAT OF I C E TO THE KEEWATIN I C E D I V I D E

The i c e sheet t h a t f i l l e d t h e nor thwest p a r t o f Hudson Bay a f t e r t h e i n i t i a l

i n c u r s i o n o f mar ine waters i n t o James Bay was r e l a t i v e l y s tagnant as i n d i c a t e d

by t h e ex tens i ve network o f eskers t h a t a r e now exposed on l and on Coats,

Manse1 (F ig . 4.12), and Southhampton i s l a n d s and on t h e Keewat in mainland.

f r o n t o f t h e i c e me l ted back i n c o n t a c t w i t h t h e sea t o t h e Keewat in I c e

D iv ide . The r e t r e a t o f t h e i c e f r o n t and concomi tan t on lap o f t h e T y r r e l l Sea

appears t o have taken p lace i n a f a i r l y r e g u l a r manner.

The

Two major slowdowns o r s t i l l s t a n d s i n t h e r e t r e a t o f t h e i c e f r o n t a r e

marked i n Keewat in by zones o f inc reased esker d e n s i t y (F ig . 4.13).

t e r r a i n between t h 6 w i d e l y spaced t r i b u t a r i e s o f t h e Maguse R i v e r esker system,

two bands o f sho r t , smal l eskers mark apparent slowdowns i n t h e r e t r e a t o f t h e

i c e f r o n t , p o s s i b l y i n response t o temporary c l i m a t i c d e t e r i o r a t i o n . S i m i l a r

concen t ra t i ons o f s h o r t eskers a re known a lonq bands assoc ia ted w i t h ma jor ou t -

wash t r a i n s and i c e marg ina l f e a t u r e s on t h e n o r t h and west s i d e o f t h e

Keewatin I c e D i v i d e (Ay l swor th and S h i l t s , 1985).

I n t h e

I n t h e mar ine environment

.oo .PS

, E M ,C 4 m.14

72

east of the Divide, these bands are associated with particularly thick seaward

blankets of fine-grained sediments and many o f the major eskers are interrupted

by massive gravel bulges that represent deposition in subaqueous outwash fans

(Rust and Romanelli, 1975). Several of the short, intervening eskers have

distributary channels reminiscent of "bird-foot" deltas in plan view

Figure 4.11. Elevations of marine limit in the Pitz Lake Basin (B.C. McDonald, unpublished data). Point elevations in metres, contours in feet (1 foot = 0.3048 m).

73

Figure 4.12. Esker ridge on Manse1 Island. Note subdued relief of meandering esker which was modified as it rose through wave base during offlap of Tyrrell Sea (GSC 203538-S).

(Fig. 4.14).

limit is hampered by the modification that took place as these deposits rose

through wave base during offlap of the Tyrrell Sea.

Study o f the glaciofluvial and glaciomarine deposits below marine

CONCLUSION

Several glaciations have occurred in the Hudson Bay region, but the overall

extent of the multiple glacial events is presently the subject of considerable

debate.

episodes accompanied by local oscillations of the advancing or retreating

glacier fronts that deposited additional glacial and proglacial sediments.

Recent advances in various dating techniques and renewed study of the hundreds o f kilometers of stratigraphic exposures in the Hudson-James Bay Lowland,

however, indicate that there may have been no fewer than seven major glacial

advances through the region with incursion of marine waters into Hudson Bay

after each glacial event.

The traditional view is that there were at least two principal glacial

It appears from geological evidence that the glaciers that covered Hudson

Bay flowed from gathering grounds in Keewatin and Nouveau Quebec-Labrador and impinged on each other in the region presently covered by marine waters.

Hudson Bay itself was probably never a centre of glacier outflow, with the

possible exception of its southwest corner, which is thought by some to have

been covered by an independent centre of outflow late in the last glacial

74

., 07. 90- 96.

em-

Figure 4.13. Detail of the Maguse River esker system, District of Keewatin. Note clusters of short eskers between trunk streams along belts where retreat of ice front, which was standing in >I00 m o f marine water of Tyrrell Sea, is inferred to have slowed temporarily (from Shilts, 1985).

episode.

within Hudson Bay are related to the provenance of glacial debris transported, in some cases for many hundreds of kilometres, along flow lines extending from

the glacier dispersal centres that flanked it.

Most of the compositional characteristics of sediments around and

Many of the modern characteristics of the Hudson Bay basin are derived from

the processes that accompanied the last ice retreat and accompanying marine

inundation of its isostatically depressed surface. After the glaciers were

split by marine waters about 7900 years ago, the edqes of the Keewatin and

Nouveau Quebec/Labrador glaciers melted back rapidly toward their dispersal

centres, the glaciers disappearing altogether within about 2000 years. The retreat of the ice front was generally regular, being interrupted occasionally I 1 . . L. _. . I ..IL.._ -z :__ Z._-L _ _ * _ _ - * ----:I-,.. 1 - ,-------- + ^ DY slowing o r temporary naitiny UT i c e TruriL reLredL, pub5 iu iy 1 1 1 r-rbpuii>r LU

climatic deterioration. In Keewatin, at least, the retreating ice was not very

75

Figure 4.14. Esker d e l t a south o f Kaminak Lake, D i s t r i c t o f Keewatin. Note b i rd - foo t shape o f d i s t r i b u t a r y channels and beaches and s p i t s formed as t h e complex was i s o s t a t i c a l l y ra i sed through wave base (EMR photo no. A21209-39).

act ive as deduced from the well-developed, i n teg ra ted drainage ne t o f eskers

tha t were l e f t behind on land from the mouth o f Hudson Bay t o the v i c i n i t y o f

the Keewatin I c e Divide.

most extensive marine sediment accumulations were l e f t i n reg ions adjacent t o

the major esker systems.

The esker systems debouched i n t o the sea, and the

ACKNOWLEDGMENTS

I n prepar ing t h i s chapter, Jan Aylsworth rendered inva luab le ass is tance

organiz ing f i gu res and references and e d i t i n g t h e t e x t .

k i n d l y l e n t me an unpublished manuscript summarizing the g l a c i a l h i s t o r y o f the

area east o f Hudson Bay from which I drew much va luable in format ion.

f u l l r e s p o n s i b i l i t y f o r conclusions drawn i n t h i s chapter.

Dr. J-S. Vincent

I assume

76

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Elsevier Oceanography Series, 44

CANADIAN INLAND SEAS Edited by

I.P. MARTINI

Department of Land Resource Science, University of Guelph, Guelph, Ont. N 1G 2W1, Canada

E LSEVl E R

Amsterdam - Oxford - New York - Tokyo 1986