Walker J. Facies models, Canada, 1992

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TRACE FOSSILS 67

ravinement surface lack

Glossifungites

suites (see Table 2) and suggest the

fill may have been

fluvial.

The Turonian Cardium Formation of

the Pembina field, central Alberta (Fig.

16 in Chapter

12), contains a series of

shorefaces associated with transgres-

sive surfaces of erosion (Walker and

Eyles, 1991). Below erosion surface

E5 (Chapter

12), silty shales contain a

diverse trace assemblage, including

Planolites,

Chondrites, Helminthopsis,

Terebellina, Asterosoma, and rare

Zoophycos. This suite reflects a distal

Cruziana ichnofacies, suggesting off-

shore to shelfal accumulation. The

erosion surface (E5) cut into this facies

is marked by a Glossifungites assem-

blage consisting of robust

Thalas-

sinoides that penetrate into the un-

derlying silty shales. The burrows are

passively filled with pebbles and sand

from overlying structureless conglomer-

ates (Vossler and Pemberton,

1988b).

The conglomerates grade upward into

overlying marine shelfal shales that

contain Helminthopsis,

Planolites,

Figure

Robust, unlined, coarse sand-

filled Arenicolites shaft excavated into a

silty to sandy shale. The firmground suite

cross cuts a Planolites, Helminthopsis,

Asterosoma, Chondrites, and

Terebellina

softground assemblage. The Glossifungites

suite corresponds to a transgressive

ravinement surface followed by stillstand

progradation of coarser

clastics. Albian

Viking Formation, Kaybob

Field, Alberta.

Chondrites, Terebellina, and Zoo-

phycos. The erosional discontinuity is

interpreted to have initially formed

during

lowstand of relative sea level,

but was extensively modified during

transgression. In places, the conglom-

erates rest on a surface of initial trans-

gression (E5) and are erosively trun-

cated during a resumption of trans-

gression (T5; see Figs. 15 and 17 in

Chapter 12). In other places, the T5

surface cuts down to become coplanar

with

(Fig. 17 in Chapter 12). Col-

onization of the surface by the omis-

sion suite tracemakers corresponds to

a hiatus in deposition between initial

transgression (forming E5) and

shore-

face progradation associated with a

period of stillstand. The stillstand

shoreface was ultimately drowned

during resumed transgression

(T5),

marked by the capping marine shelfal

shales. The

transgression/stillstand

shoreface model has also been

applied to the Viking Formation of the

Joffre field (Table 2; Downing and

Walker, 1988).

Another example of bioturbation at

an amalgamated

lowstand/transgres-

sive surface is in the Bertie and Bois

Blanc formations of southern Ontario.

The contact between them corre-

sponds to the

SilurianIDevonian un-

conformity (Pemberton

a/., 1980).

Dolomites of the Bertie Formation con-

tain softground Thalassinoides sys-

tems and reflect the final stages of

transgression in the Silurian. The

SilurianIDevonian unconformity was

initiated during a period of extreme

shallowing and subaerial exposure,

which produced a karsted surface.

Subsequent transgression is indicated

by colonization of the karsted hard-

ground by boring organisms and de-

velopment of a

Trypanites ichnofacies

(Fig.

17C), manifest by abundant Try-

panites weisei and Gastrochaenolites.

Sediment of the overlying unburrowed

Oriskany sandstone of the Bois Blanc

Formation was deposited as a wide-

spread sheet which subsequently suf-

fered localised removal. The surface

corresponds to an amalgamated

low-

stand erosion and transgressive ravine-

ment

(EIT)

surface.

Nondepositional hiatuses

(diastems or nonsequences)

Nondepositional hiatuses are com-

monly formed in carbonate settings

where cessation of deposition results in

synsedimentary lithification of the

sea-

floor and development of hardgrounds.

These hardground surfaces are colo-

nized rapidly by marine organisms. The

absence of significant deposition of

sediment related to hardground forma-

tion strongly favours the colonization of

such surfaces, and unbored hard-

grounds are more the exception than

the rule.

In Upper Cretaceous chalks of north-

western Europe, nonerosional

(nonde-

positional) omission surfaces are

abundant through much of the succes-

sion (Bromley,

1975), particularly in

some of the Cenomanian grey chalks

of southern England (Kennedy, 1967).

In the Upper Cretaceous

(Campanian-

Maastrichtian boundary) white chalks

of Kronsmoor, Schlesswig-Holstein,

Germany, many of the more obvious

omission surfaces have been termed

"pseudo-hardgrounds" (Bromley, 1975).

Three main trace fossil associations

typically occur in such settings;

pre-

omission, omission, and post-omission

suites. The pre-omission suite consists

Thalassinoides, Chondrites, and

Zoophycos; the traces have a pale

blue fill because of disseminated pyrite.

The omission suite corresponds to the

Glossifungites ichnofacies, and con-

sists of a monospecific assemblage of

Thalassinoides. These are distinguished

by a grey, clayey chalk fill from the

overlying unit and a faintly ferruginous

coating on the burrow walls because of

exposure to seawater during the period

of nondeposition. The post-omission

suite occupies the overlying unit and

consists of Thalassinoides,

Chondrites,

and Zoophycos, which are virtually

identical to the pre-omission suite. This

last suite penetrates the discontinuity

surface through re-excavation of the

omission suite burrow fill. Bromley

(1 975) interpreted this reburrowing to

be a function of low nutrient content of

the underlying chalk rather than of sub-

strate hardness. The lack of new

ichno-

fossil forms introduced in the omission

suite and post-omission suite strongly

suggests that negligible change oc-

curred in the depositional setting before

or after development of the omission

surface (Bromley, 1975).

Where synsedimentary lithification

occurs at the seafloor, omission suites

are separated into prelithification

burrows and postlithification

borings

68 PEMBERTON, MACEACHERN, FREY

(Bromley, 1975). Prelithification suites

may include both softground suites and

firmground suites. Recognition of a

bored (rather than burrowed) sedimen-

tary surface is unequivocal evidence of

sediment omission (erosional or non-

depositional) and the presence of a

hardground (Bromley, 1975).

Depositional Discontinuities

Condensed Sections

Condensed sections were deposited

over a long span of time, but are thin

because of slow rates of hemipelagic

or pelagic sedimentation. They are

typically most extensive during periods

of maximum transgression

(Loutit ef

a/,, 1988), when the basin is starved of

terrigenous material (Van Wagoner et

a/., 1990), and form maximum flooding

surfaces. Recognition of condensed

sections is important, both from a strat-

igraphic and facies analysis point of

view. Such thin deposits may easily be

missed during biostratigraphic ana-

lysis, if sampling is not conducted care-

fully. Missing the condensed section

would result in the interpretation of an

apparent time gap in the

biostrati-

graphic record. This in turn would imply

a major unconformity even though sed-

imentation was actually continuous

(Van Wagoner

eta/., 1990).

Condensed sections tend to contain

abundant and diverse deep water

faunas, whereas over- and

underlying

deposits, particularly those accumulated

fluvial, estuarine, and shallow-marine

settings, tend to be nonfossiliferous or

only slightly fossiliferous. If faunas are

collected from successive condensed

sections, in several discrete sequences

(in the sequence stratigraphy sense),

the sedimentologist might erroneously

conclude that continuous deep water

conditions prevailed across that interval,

and could misinterpret the intervening

nonfossiliferous sandstones as being

deep water in origin (Van Wagoner et

a/., 1990).

Several condensed sections have

been described from the rock record

(Legget, 1980; Jenkyns, 1980; Leckie et

a/.,

1990), and Loutit et a/. (1988) sum-

content and overall stressful condi-

tions for benthic organisms. Six se-

lected condensed sections from

epeiric settings show a general adher-

ence to conditions of higher total

organic carbon, reduced oxygen val-

ues, low concentrations of benthic

foraminifera, and minimal or no bur-

rowing (Fig. 24). The relationships of

low oxygen, preservation of organic

carbon, and biologically lethal

seafloor

conditions have been discussed by

numerous authors (Byers and Larson,

1979; Jenkyns, 1980;

Legget, 1980;

Savrda and Bottjer, 1987). Savrda and

Bottjer (1987) noted that ichnofaunas

are generally more indicative of both

magnitudes of, and rates of change in,

oxygen levels than are macrobenthic

body fossil suites. Bromley and Ekdale

(1984) found that with decreasing oxy-

genation at the seafloor,

Planolites,

Thalassinoides, and Zoophycos pro-

gressively disappear before Chon-

drites does, suggesting that the Chon-

drites tracemaker may have been

capable of surviving conditions of

anoxia. Savrda and Bottjer

987) also

found that burrow sizes decrease with

increasing depth and decreasing

oxygen levels.

In a study of the Monterey Formation

(Miocene) of California and the

Nio-

brara Formation (Cretaceous) of Colo-

rado, Savrda and Bottjer (1987) indi-

cated that oxygen-related

ichno-

coenoses could be defined, and used

to distinguish units of more uniform or

less uniform bottom-water oxygena-

tion. One possible means of

recog-

nising condensed sections char-

acterised by dysaerobic or anaerobic

conditions may be by the presence of a

suspiciously unburrowed or only

slightly burrowed dark carbonaceous

shale lying between more intensely

burrowed marine deposits.

Condensed sections overlying trans-

gressive lags, and coinciding with

maximum flooding, are shown in

Figure 25. This core is from the Lower

Cretaceous (Albian)

Bluesky Form-

ation of the Knopcik Field, west-central

Alberta. Near the base of the core,

foreshore and upper shoreface sand-

stones containing Macaronichnus, are

erosionally truncated and overlain by a

conglomerate and pebbly shale, inter-

preted to reflect transgressive

ravine-

ment. The overlying shale is black,

slightly fissile, pyritic, and

glauconitic,

and is conspicuously unburrowed. It is

interpreted as a condensed section,

deposited during maximum flooding.

The condensed section grades up-

ward into offshore silty shales and

sandy shales burrowed with a diverse

Cruziana ichnofacies, consisting of

COMMON CHARACTERISTICS OF CONDENSED SECTIONS

Legend:

present;

not addressed; Blank

not present

most

their

common

charac-

Figure

Six condensed sections selected from epeiric settings are summarized. Data were

teristics (Fig.

24).

collected as follows: Cambro-Ordovician (Baltic Shield)

Lindstrom (1963) and Jenkyns

The

ichnological signatures

'On-

(1986); Jet Rock Shales (Toarcian; U.K.)

Morris (1979); Shaftsbury Formation (Albian;

densed

have

yet

Alberta, Canada)

Leckie eta/. (1990); Mowry Shale (Late Albian; Wyoming, U.S.A.)

Byers

mented adequately.

In general, the

and Lawon (1979) and Jenkyns (1980); Awgu Shale (Turonian; Benue Trough, Nigeria)

units tend to be unburrowed, which is

Petters (1978) and Jenkyns (1980); Eocene-Oligocene Boundary (Alabama, U.S.A.)

Loutit et

..ll-.-s!~~..s.@lmen!a!.I~n.ra!es

..........................................................................................................

-*:--.Fl@u~@..oryge?_~a!uesss

..?

.,.....

H!~~.e~(~ar!k..mat!e~.~~"ten!~_T.OC~

..L

,.....

~~~~.~~ent~a~'o_!!..~.!..II.!a!!num.e!~!.mer!!s~!~d.iu.m~

.~:-.-.II.?~~.~e.o!.~!!~.~I!n~~.m.i!!e~@s.~~:~:..~!au.~nI!e!..~?s~~a!~!..s~~~r.Cel...

..li_,

!!.Q!!.Gamma:ra~.-cwn!s

.......................................................................

.........................

commonly attributed to low oxygen

a/. (1988).

.___.-

_----

...X...

Abundant and diverse plankton

.......................................................

.....

.a.

Ab~.$~.?!.and.d_ive'se.m.i~:r~!!a!na

..a.aaaaaa.a.aaaaa...aaaaaaaaaa.aaaaaa..aaaaaa..aaaaaaaaaa.

...........................

...........

E~.%an!-~~!.~~~".~!~~!!!!!".C.!o~a~.I?i!e3

XXXXXX

1.0:

....

~~~.e!~en!ra?~n~?f.E"!~s.f~ra.~!n.!era

......................

..................

11. Abruptly overlies nonmarine or shallow marine sediments

x x

.................................

12. Section overlain by downlap surface

.................................................................................................................................................

........................

13. Associated with burrowed, bored or slightly lithified tops x7

...........

___..!

.............................................

14. Generally unburrowed or sparsely burrowed interval xxxxx?

XXXXXX

......................

....

X...

XXX

.................................

?XX

...

X...

......-

...

X...

...

X...

...........

....?i

...........

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X...

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X...

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X...

........

X...

............................

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...

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...IS

X...

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......................

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.................................

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Dome Total Knopcik

06-32-74-12~6

GRAIN SIZE

Transgressive

Sand

Sheet

i..

ggg

s-'

Offshore

Figure

Strip log of a cored interval of the Bluesky Formation (Lower Albian) in the

Knopcik Field in west-central Alberta (symbols explained in Fig.

19).

Facies interpretations

are given on the right-hand side of the log; key surfaces are marked on the left. The basal

section shows an upper shoreface sandstone transgressed with ravinement, overlain by a

condensed section. A second offshore to shoreface progradational cycle is developed (near

top of section) and is also erosively transgressed, capped by a transgressive sand sheet and

followed by a second condensed section.

Terebellina, Planolites, Chondrites,

Helminthopsis, Zoophycos, small

Rosselia, Palaeophycus,

Teichichnus,

Asterosoma, and rare Skolithos. This

passes into storm-dominated lower- to

middle-shoreface sandstones, re-

flecting shoreface progradation. To-

wards the top of the core (Fig.

25),

ravinement surface is overlain by in-

tensely but indistinctly burrowed,

pebbly medium to coarse sandstone

interpreted as a transgressive sand

sheet. It is abruptly overlain by dark,

slightly fissile, pyritic silty shale con-

taining sparse Planolites and

Chon-

drites, interpreted as another con-

densed section.

SUMMARY

In the second edition of Facies Models

(Frey and Pemberton,

ISM),

we showed

that recurrent ichnofacies are not only

valuable environmental indicators but

also archetypal facies models. The

same holds true today (Frey et al.,

1990).

However, in

1984,

trace fossils

had little or no place in refined genetic

stratigraphic applications. This is cer-

tainly not true today; trace fossils are

very important both in defining

bounding discontinuities of various

types, and in contributing to their inter-

pretation. The particular tracemakers

colonizing the discontinuities, and the

intensity of burrowing or boring,

are in-

dependent of the genesis and regional

extent of the surface itself, except in sit-

uations where there is an environmental

gradient

(e.g., freshwaterlbrackish-

waterlmarine). The allostratigraphic sig-

nificance of any given discontinuity is a

problem that can be resolved only

through the integration of

sedimentolog-

ical, stratigraphic, ichnological, and pa-

leontological techniques.

ACKNOWLEDGEMENTS

Mike Ranger kindly permitted use of his

computer program to draft the strip logs

presented in the paper. Don

McPhee

and Howard Brekke spent numerous

hours scanning and constructing the

figures, and Ms. Cathy Gendron typed

numerous drafts of the text. Roger

Walker made many editorial sugges-

tions that helped to improve the

manuscript. Funding was provided by

the Natural Sciences and Engineering

Research Council of Canada and is

gratefully acknowledged.

After helping to complete this paper,

PEMBERTON, MACEACHERN, FREY

Bob Frey passed away on January

1992. He will be long remembered for

his

classic work that helped lay the

foundation for modern ichnology. We

dedicate this paper in

his

honour.

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MI C

Carolyn H. Eyles, Department of Geography,

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Hamilton, Ontario L8S 4K1

INTRODUCTION

An understanding of glacial deposi-

tional systems and models has many

important applications. First, glacial de-

posits contain data that can be used to

unravel the long, but intermittent, re-

cord of global change and glaciation

during geological time (Fig. 1, and see

Chapter 2). Second, glacial deposits

are fundamental to studies of environ-

mental geology in heavily urbanized

mid-latitude areas of North America and

Europe. The investigation of

geo-engi-

neering characteristics, hydrogeological

regimes and contaminant migration in

glacial sediments is currently a major

stimulus of glacial sedimentological re-

search. It is akin to the stimulus of hy-

drocarbon reservoir studies on aspects

clastic and carbonate sedimentology

in the 1950s. A third and closely related

aspect of glacial studies is their applica-

tion to economic geology. This involves

exploration for mineral deposits hosted

in glacial sediments and rocks (placers;

Eyles,

1990), and the use of glacial sed-

iments in the search for mineral de-

posits on glaciated shields (drift

prospecting;

DiLabio and Coker, ,1989).

Glacial strata have traditionally been

ignored by petroleum geologists, but

Late Paleozoic glaciated basins contain

significant reserves of oil and gas in

Australia, Argentina, Brazil, Bolivia,

northern Saudi Arabia, Jordan and

Oman

(Levell et a/., 1988; Franca and

Potter, 1991).

In many people's minds, glacial

sedimentary facies are still uniquely

characterized by tills (or if lithified,

tillites). These are unsorted mixtures

of all available sizes, from boulders to

clay, deposited directly by glaciers

and ice sheets. However, sediments

having the same outcrop appearance

(hence the same aspect, or facies)

can be deposited in a wide range of

glacial and nonglacial environments

(Fig. 2). The term

diamict (if lithified,

diamictite) should therefore be used as

a descriptive, nongenetic term for such

poorly sorted facies regardless of

origin. Only those

diamict(ite)s known

to be deposited directly by glacier ice

can be identified as true

till(ite)s. A

central problem in stratigraphic studies

is therefore to determine whether spe-

cific

diamict(ite) facies are glacial or

nonglacial in origin. There are many

examples in the literature where sedi-

ments originally described as tills and

tillites have been shown to have a

nonglacial origin.

Diamict(ite)s are

Figure 1

Earth's glacial record. Black bars depict glacial strata where ages are constrained by radiometric dating. Cretaceous ice caps may

have existed in the interior of Antarctica and northern Siberia. Data compiled principally from Hambrey and Harland (1981). Representative

references:

(1) Miall (1985),

(2)

Eisbacher (1985), Eyles, C.H. (1988), Nystuen (1985), (3) Vaslet (1990),

(4)

Crowell and Frakes (1975), Visser

(1 983,

1989), Levell

et a/.

988), Franca and Potter (1 991), (5) Frakes and Francis (1 988),

(6)

Martinson

eta/.

(1 987), Andrews (1 987).

EYLES, EYLES

merely a common facies type and are

produced in a range of sedimentary en-

vironments and climates. It is com-

monly impossible to deduce the origin

of a

diamict(ite) from a single small out-

crop. It is always important to examine

the related facies in conformable strati-

graphic context above and below the

diamict (Chapter

I),

in order to assess

the evidence for its depositional envi-

ronment by reference to associated

$nn;-r Thie nnnrn-nh ic hncarl

thn

lablrja.

IIJ

upp~

VLIUI

UUDGU

premise

that

diamict

prOducedr

Figure

Diamict(ite) facies.

Clast-rich massive diarnict produced by ice-rafting and

sediment gravity

settling of suspended fines in a glaciomarine environment. Late Cenozoic Yakataga Fm.,

will tend to be intimately associated Alaska. B) clast-poor massive diamict produced by ice-rafting and settling of suspended

with other gravity-emplaced diagnostic

fines in an ice-contact glaciolacustrine setting. Note thin silt wisps above ice-axe. Late

facies such

turbidites (see later in

Wisconsin Sunnybrook diamict, Scarborough Bluffs, Ontario.

OUCIATED VALLEY

~U~U~CUCIAL

OUCDFLUVUL

Figure

Depositional environments and typical vertical profiles of facies deposited during

single phase of glacier advance and retreat in

various glacioterrestrial and glaciomarine environments.

this chapter, and in Chapter 13).

Facies associations vary in different

depositional environments, as does the

geometry or three-dimensional shape

of the deposit. The best interpretations

therefore require detailed vertical pro-

files of the deposits, along with infor-

mation on lateral variability and

depositional geometry (architecture)

beyond the local outcrop.

Commonly, glacial facies are inti-

mately associated with the deposits of

many other environments, particularly

rivers, lakes, continental shelves, and

slopes. In most cases, the role of the

glacier has been to deliver large vol-

umes of sediment to a marine or

lacus-

trine basin, where primary glacial

sediments are then reworked by

nonglacial sedimentary processes.

However, sedimentary products

(fac-

5. GLACIAL MODELS 75

ies) found within different glacial and

glacially influenced environments can

be identified, and occur in distinct

facies associations (families of related

facies). Glacial

depositional systems

can be recognized as distinct three-di-

mensional assemblages of process-

related facies that record major paleo-

geomorphic basin elements. The spa-

tial and chronological linkage of these

depositional systems identifies two dis-

tinctive systems tracts,

glacioterrestrial

and glaciomarine

(Fig. 3).

THE GLACIOTERRESTRIAL

SYSTEMS TRACT

Glaciation of continental interiors

creates a variety of glacioterrestrial

depositional environments which can

be grouped into four depositional sys-

tems;

subglacial, supraglacial, glacio-

ARCTIC

OCEAN

PACIFIC OCEAN

SEDIMENT

CWER

lacustrine

and

glaciofluvial

(Fig. 3).

Substrate relief and tectonic setting

play a fundamental role in the deposi-

tion and preservation of terrestrial gla-

cial sediments in these environments.

Extensive Quaternary glacial deposits

of cratonic continental interiors (Figs.

and

record the growth of large ice

sheets but are relatively poorly pre-

served in the Earth's glacial record.

On the other hand, reworked conti-

nental glacial deposits are a very

common stratigraphic component of

orogenic belts. They record the growth

of cordilleran ice masses and preser-

vation of glacial sediments in tectoni-

cally active basins. The bulk of gla-

cially derived sediment is preserved in

the ocean basins (Fig.

Bell and

Laine, 1985). Cold climate (periglacial)

processes

also'affect terrestrial glacial

deposits beyond the ice margin,

causing extensive deformation of sedi-

ments, and influencing the deposition

fluvial and eolian facies.

LOW-RELIEF

GLACIOTERRESTRIAL

SElTlNGS

Glaciation of low-relief, continental in-

teriors results in crudely concentric

belts of different glaciated terrains that

reflect the growth and shrinkage of large

ice sheets. This is well shown by the

distribution of Pleistocene glacial de-

posits in North America (Figs.

5).

Several depositional systems can be

recognized (Fig. 3).

Subglacial depositional systems

Conditions at the base of large ice

sheets vary widely in response to dif-

ferent ice temperatures and velocities.

Wet-based ice (Figs.

6A, B) occurs

where conditions are close to the pres-

sure melting point, and the ice slides

over the substrate. In dry-based condi-

tions (Fig. 6A) the ice remains frozen

to the bed and moves mostly by in-

ternal deformation. Subglacially de-

posited diamict facies occur principally

below wet-based ice (Figs. 6C, D). En-

glacial debris is transported within a

thin

m) basal layer that consists of

irregular layers of ice and debris. In-

tense abrasion occurs both between

particles in the base of the layer, and

between these particles and the sub-

Figure

Schematic distribution of sediments resulting from Quaternary glaciation of northern

strate.

Clasts or masses of debris that

North America. Note widespread distribution of thick, glacially derived marine sediment in

collide with the bed may become

ocean basins; these sediments are preferentially presewed in the Earth's glacial record.

lodged against the substrate as a result

EYLES. EYLES

of pressure-melting and release from

the ice base. The process is akin to

smearing peanut butter across a piece

of toast.

characteristic glacially

faceted clast shape (bullets, Fig.

evolves as a result of overlying dirty

ice moving over the lodged clast. Con-

tinued lodgement produces lenticular

beds of dense, overconsolidated

dia-

mict termed lodgement till (Figs. 8, 9,

10). Long axes of the clasts have a

strongly preferred alignment parallel to

ice flow. Measurement of long axis ori-

entation for a few bullet clasts gives a

rapid indication of ice flow direction.

Lodgement tills lie on marked local

and regional unconformities, and tend

to have regional sheet-like geometries

(Figs. 8,

9).

Total thicknesses rarely

exceed 50

m. Strongly lenticular till

units occur locally within the sheet-like

forms. They have cross cutting and

overlapping relationships as a result of

intermittent erosion of the substrate in

response to change in ice velocities.

Changes in ice flow direction and the in-

troduction of differing debris lithologies

can

result.in a stack of several lodge-

ment tills, superimposed during a

single glaciation (Fig. 8). Each till unit

contains clasts and matrix from con-

trasting bedrock sources. This empha-

sizes the need for care in interpreting

ice

advancelretreat cycles from mul-

tiple-till stratigraphies.

Gravel-filled channels resulting from

subglacial stream drainage are also

integral components of the subglacial

stratigraphy (Fig. 8; Eyles et al., 1982).

Channels have a planar upper surface

truncated by overlying diamict. They

are oriented subparallel to ice flow di-

rection and are genetically related to

esker ridges (Fig.

8).

Therefore, the

presence of glaciofluvial facies within

lodgement-till complexes does not

necessarily indicate glacier retreat.

Sediments and poorly lithified rocks

below lodgement-till successions are

commonly observed to be extensively

deformed as a result of subglacial

tec-

tonism (Fig.

11;

Croot, 1988). This is

akin to spreading peanut butter on very

soft bread. These

glaciotectonized sub-

strates are particularly common over

large areas of the Prairie Provinces in

western Canada, and consist of weak

mud rocks and poorly lithified sand-

stones (Sauer et al., 1990).

Eskers record transient water flow

below the ice sheet. They

infill tunnel

networks that may be cut either into

the bed or up into the overlying ice

Glaciolacustrine

Figure

Schematic distribution of Quaternaly glacioterrestrial sediments in northern North America. Glaciolacustrine sediments obscure

large areas.