Walker J. Facies models, Canada, 1992

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. .

._-.

.._

----

5. GLACIAL MODELS

(Eyles and Eyles, 1989).

Ice contact glaciolacustrine sedi-

mentation in high-relief settings is com-

parable to glaciomarine sedimentation

in fiords (see below). The presence of

structural lineaments in the bedrock

allows glaciers to erode their valleys

well below sea level. Alpine areas in

western Canada and Europe contain

elongate overdeepened lakes,

com-

monly termed fiord-lakes because of

similarity to their marine counterparts.

?!&dominant lacustrine

---

deposits

a~-massiye-and

deformed

---

silts de:

posited during deglaczm (Hsu and

~~eXSmple, seismic-re-

flection investigations of the Quater-

nary fill of Okanagan Lake in British

Columbia suggest that up to 800 m of

silt accumulated during a single phase

of late Wisconsin glaciolacustrine sedi-

mentation (Eyles

etal.,

1991; Figs.

25,

26). Under such conditions, a varved

depositional

reg,ime is suppressed by

---I--__

____

t-cG%g of larae volumes of, sedL

ment into the basln. This reaima

also supxessed by downsIop_e resedi-

mentatio~o_~es~s~.taperate

ye=-roundjIlx~as~o~t~ml

~WpewgJhe burial of stagnant

ice under supraglacial water bodies is

also important in promoting mass

movement

(Postma et al., 1983).

Varved silts and clays are deposited.

o?Iy dmcj the v-ery latest starved

stage of

lake infilling, when the glacier

has retreated from the basin and melt-

water inputs are driven by summer

supraglacial melting.

Cold climate modification

Cold climate weathering processes

greatly affect continental glacial sedi-

ment and ground surfaces beyond the

ice margin. The term

periglacial is used

as a broad umbrella term to identify

glacial and nonglacial cold climates-

However, definitions and environments

encompassed by this term vary greatly

(Washburn, 1980; Dionne, 1985).

Cold climate environments are

chat

~terized. by

l).eoli@~deflatianand.de-

pmn, 2) downslope mass-wasting-

.proces~

mechanical disturbance

nearxaceXa6- baeAr6wth "pf

ice lenses, and

a_$grad~atio~

braided rivers. Cold climate structures

-5_C_-

and facies are most commonly associ-

ated with major unconformities sepa-

rating stratigraphic successions. Where

mean annual temperatures are less

than

-4OC

the ground is perennially

frozen (permafrost). Permaf rost

reaches a maximum reported depth of

1450

m in Siberia (Williams and Smith,

1989); much of this formed during

WEST

(5) (6)

EAST

krn

UNIT

V.E.

3.@

750

Figure

Seismic cross section and interpretation of thick, rapidly-deposited ice-contact

glaciolacustrine silts (unit II) along a glacial lake trough. Okanagan Lake, British Columbia

(from Eyles

etal.,

1991).

Quaternary ice ages and is now ac-

tively degrading. However, during the

summer the surface melts to a depth of

only

m (the active layer).

Permafrozen substrates are charac-

terized by the growth of subsurface ice

layers. Near the surface (less than

50 m), ice occurs as segregated

masses in the form of layers, lenses,

dikes, or massive tabular masses many

metres thick. Considerable mechanical

disturbance of the frozen strata occurs.

At depth, ice occurs as a pore filling.

Large areas of submarine permafrost

occur in the arctic as a result of deep

freezing of strata first exposed and

then flooded by glacio-eustatic sea

level change (see below).

Despite the widespread extent and

depth of present-day permafrost, sedi-

mentary structures produced by

permafrost have not been widely des-

cribed from the geological record. This

is due to lack of preservation or recog-

nition. Structures most diagnostic of

former frozen ground are ice wedge

casts formed by severe ground

contraction, cracking, and the freezing

of melt-waters in contraction cracks

(Black, 1976). Host sediments experi-

ence mechanical disturbance caused

by wedge growth. The resultant ice

wedge (Fig. 27A) comprises part of

extensive poly-gonal networks. With

climatic amelioration, ice wedges melt

to leave ice wedge casts filled with

debris derived from surface slumping

(Fig.

278; Williams, 1986). The recog-

nition of Quaternary ice wedge casts is

not straightfotward (see Black, 1976);

many Late Proterozoic examples may

be sand dikes produced

seismic

Figure

Outcrop of Late Wisconsin de-

formed silts exposed along the margins of

Okanagan Lake, British Columbia. These

silts are probably the same as those repre-

sented by seismic unit

I1 on Fig.

25.

EYLES, EYLES

shock (Fig. 28a; Eyles and Clark, 1985).

Complexly involuted cryoturbation

structures are alleged to form when late

summer refreezing generates high pore

water pressures in the surface active

layer. Pore water movement in the

active layer sets up a series of slow-

moving convection cells which textu-

rally sort sediment. This process of soil

churning produces sorted stone poly-

gons on the ground surface (Fig.

28B).

Collapse structures and faulting may

accompany the melt of larger ice lenses

Figure 27

Periglacial ice wedges. A)

Modern ice wedge in silts, Alaska. B) Ice

wedge cast filled with sand and gravel pen-

etrating Late Wisconsin glaciolacustrine

sands and silts; Newfoundland, Canada.

Hammer (circled) for scale.

in the permafrost (thermokarst). Cryo-

turbation structures used to infer peri-

glacial climates are not unique to cold

climates. Similar structures occur as a

result of subaqueous loading, liquefac-

tion, and seismic shock. Their common

presence in glaciated areas may simply

record the widespread occurrence of

thawed, saturated sediments.

Windblown silt (loess) and sand

derived from deflation of large braided

stream networks are distinctive peri-

glacial features (Deynoux et

a/., 1989).

Extensive loess blankets and dune

fields occur in Quaternary continental in-

teriors, such as the Nebraska Sand Hills

where a periglacial sand sea covers

over

50,000

km2 (see Wright et al.,

1984). Loess has a thickness of more

than

300

m in parts of China. However,

there is only one report of a pre-Quater-

nary loessite (Edwards, 1979).

The predominant periglacial facies

in areas of Pleistocene glaciation

are coarse-grained

fluvial sedi-

ments. These record seasonal braided

Figure

Periglacial structures.

Bedding plain view of polygonal sandstone dykes in-

terpreted as periglacial in origin; Late Proterozoic Port Askaig Formation, Scotland. These

features have also been interpreted as soft sediment deformation structures (Eyles and

Clarke,

1985).

Hammer (circled) for scale. B) Modern polygonal structures (sorted stone

circles) in Spitzbergen, produced by periglacial churning of saturated sediments.

stream activity and indicate stream

access to large volumes of coarse,

mechanically weathered debris.

Stratified diamicts are also important

periglacial facies. They result from a

wide variety of downslope mass move-

ment processes

(solifluction or gelifluc-

tion; French, 1976) when seasonally

thawed surface layers become water

saturated. These facies commonly

infill

the lower points of the topography and

interfinger with braided river facies.

GLACIOMARINE SYSTEMS TRACT

A simple division of marine glacial de-

positional systems distinguishes conti-

nental shelf settings from continental

slopes and fiords. Proximity to an ice

margin determines whether the envi-

ronment is dominated by glacial pro-

cesses (ice proximal) or marine pro-

cesses (ice distal; Fig. 29). Regional

climate is another important control

because it dictates the volume of melt-

water reaching the marine environ-

ment. Temperate oceanic environ-

ments, for example, receive large

volumes of meltwater and mud which

are supplied directly to the shelf (Fig.

3). These sediment-nourished environ-

ments can be contrasted with sedi-

ment-starved settings that are typical

of deeply frozen polar areas. Melt-

water inputs are severely restricted

and chemical and biogenic deposition

become relatively important, as in

Antarctica (Fig. 30; Domack, 1988).

Clearly, the thicker glaciomarine

de-

GLACIAL MODELS 89

posits of temperate oceanic areas are

more likely to be preserved in the rock

record (Anderson and Ashley, 1991).

LOW-RELIEF GLACIOMARINE

SElTINGS

Continental shelf: ice proximal

Given sufficient i_ce_vplume, ice mar-

---

gins extend onto contlne~tal shelves

where they construct large submarine

rii~i%-do7ipljxes (mor@naT6anks]I

---

Figure 31 summarizes the morainal

bank environment as identified from

modern and Quaternary tidewater set-

tings (Powell and Elverhoi, 1989;

Dowdeswell and Scourse, 1990;

Anderson and Ashley, 1991). There is

little information from pre-Quaternary

settings. Morainal banks are character-

ized by the presence of strong melt-

water flows from the ice margin,

structural complexity caused by bull-

dozing (due to ice margin advances)

and iceberg scouring, and the release

of large volumes of mud from

melt-

waters. Meltwaters are released as

,effJux jets,(Fig. 31) that deposit prox-

~mal gravels and sands by strong un-

derflow~ moving across subaqueous

fan systems. As the jet moves sea-

ward, lower density meltwaters con-

taining fine-grained suspended sedi-

ment rise to the surface, or to interme-

diate depths in the water column.

These suspended sediment plumes

can extend many tens of kilometres

from the ice front, and result in distal

mud belts

(Syvitski et a/., 1987). Tidal

current interaction with the rain-out of

fine-grained suspended sediment

results in laminated muds that may be

difficult to distinguish from varves in

glaciolacustrine settings.

Massive

and/or variably graded dia-

mict facies are deposited by sediment

gravity flows originating from the col-

lapse of structurally or depositionally

oversteepened slopes. Sediment is re-

worked and mixed during downslope

movement. Poorly sorted basal debris

issuing from the ice front may be spilled

downslope as debris flows. This occurs

where grounded ice begins to float (at

the grounding line) and subglacial slur-

ries are dumped to form grounding-line

fans (the till deltas of Alley etal., 1987).

The combination of ice-rafting, depo-

sition of fines from efflux jets and asso-

ciated plumes, and size sorting by

currents either on the

seafloor or in the

water column, gives rise to a wide

range of rain-out diamict facies pro-

duced in situ on the

seafloor (Fig. 31).

The finer-grained facies include lami-

nated and massive muds with variable

clast contents. Coarser, commonly

stratified diamict facies accumulate in

areas of episodic traction currents (Fig.

32B). They contain ice-rafted drop-

stones, abundant traction-current

structures, and rounded clasts rolled

along as bed load. The sandy facies

occur as part of a lithofacies conti-

nuum with pebbly sands and poorly

sorted gravels. Where present,

micro-

fauna and macrofauna provide evi-

PROXIMAL: (ice margin

several km)

VS.

DISTAL: (several km

'000

km)

"DIRECT GLACIAL SEDIMENTATION

INTO THE MARINE ENVIRONMENT"

"GLACIALLY

INFLUENCED

CONTINENTAL]

MARGIN SEDIMENTATION"

glacial processes dominate

marine processes dominate

rapid deposition and resedimentation of

glacial influences limited to ice-rafting,

meltwater deposits and ice 'dumped'

'berg scouring', release of suspended

material on morainal banks and

ice-

sediment, sea-level change and

contact subaqueous fans; ice-push

sediment starvation

features

complex facies associations and

geometries

'?A

Ausw&AM

Figure

29 Differentiation in glaciomarine environments.

EYLES, EYLES

dence of marine influence in ancient

sions, with the upper parts of the

suc-

above wave base, resulting in erosion

successions. However, brackish water

cessions dominated by laminated and

and deposition of a cap of gravelly

may inhibit biological activity in ice-

massive muds and fine-grained 'rain-

nearshore facies.

proximal glaciomarine settings.

oul diamicts. Later, isostatic emergence

Morainal banks consist of suba-

and rapid sediment accumulation may

Continental shelf:

ice

distal

queously deposited ice-contact facies

bring these retreat successions up

Ice distal environments are influenced

which are characterized by heteroge-

neous facies types, rapid lateral and

vertical facies variability and irregular

bed geometries (Fig. 31). These land-

forms commonly record the outer limit

of grounded ice on continental

shelves, and are the marine equiva-

lents of the moraine systems de-

posited onshore (Fig. 3). Eyles and

McCabe

(1989)

showed that morainal

banks on a Late Pleistocene glaciated

shelf (Irish Sea Basin) are systemati-

cally related to drumlin swarms

onshore. Drumlins record deforming

bed conditions (Fig. 15) and the off-

shore morainal banks were con-

structed when large volumes of basal

debris were flushed to the ice sheet

Figure

Glaciomarine deposition along high relief Antarctic continental margin. Subglacial

margin

deposited subaqueously.

deposits accumulate when ice extends across shelf. Postglacial reworking and resedimentation

Glacier retreat from morainal banks

of these deposits is coeval with deposition of siliceous and organic oozes under conditions of

results in fining-upward facies succes-

clastic starvation (see Domack, 1988). Numbers refer to sedimentation rates (crn/1000 yr).

Figure

Proximal subaqueous sedimentation. 1) glaciotectonized marine sediments; 2) lenticular lodgment or deformation till units (see

Figs. 10 and 14); 3) coarse-grained stratified diamictc (Fig. 32

B);

4) pelagic muds, sandy diamicts; 5) coarse-grained proximal outwash; 6) in

terchannel cross stratified sands with channel gravels;

resedimented facies (debris flow, slides and turbidites); and

supraglacial debris.

Deformation results from ice advances, melt of buried ice and iceberg

turbation. Suspended sediment plumes shown by shading. Same model

may apply, with modifications, to sedimentation adjacent to grounding lines below large ice shelves.

GLACIAL MODELS 91

by 1) deposition of fine-grained sus-

pended sediment from plumes;

downslope resedimentation of sedi-

ments; 3) water depth changes resulting

from both variation in the volume of con-

tinental ice (glacio-eustatic sea level

changes) and crustal response to ice-

sheet loading (glacio-isostatic sea level

change);

scour by icebergs, storms

and currents; and

bioturbation.3

dominant sedimentary process on-many

temperate glacially influenced

conti-

rien&l_sh9bes

is-the production

of-e&

te~v~,blanket-like

rain-outdiamict-

faciees_byy4he~ettlingof

mudf rom sus-

pended

sedi~tpIummes-andmc~a~~

debris-tcorn-iceher_gs (Figs. 32a, C).

Rain-out d-[amicts can reach thick-

nesses of many tens of metres and-

.---

pend~ng on the textural characteristics

of the incoming sediment plumes. The

diamicts commonly contain abundant

microfauna (foraminifera) and macro-

hrna. Rates

deposition can be as

high as 1 m per thousand years. Most

ice-rafted debris is carried into the

marine environment by icebergs re-

leased from tidewater ice margins.

Sediment laden bergs can travel many

hundred kilometres from their source

areas. Modern rain-out diamict facies

are largely absent from the Antarctic

continental shelf, because insignificant

volumes of meltwater and fine-grained

sediment are released to the marine en-

vironment by the cold-based Antarctic

ice sheet. The ice sheet is grounded

below sea level, and is fringed by exten-

sive floating ice margins

(ice

shelves),

which make up about 80 per cent of the

Antarctic coastline. Icebergs released

from the front of ice shelves contain no

debris. Any englacial debris is melted

out close to the point where the ice

shelf begins to float, and is not trans-

ported to the ice shelf edge. Con-

sequently, sedimentation rates are less

than

cm per thousand years (Domack,

1988; Fig.

30).

Rese-dimentation processes operate

read~ly on glacially influenced conti-

nental_shelves as a result of high pore-

water-p~ssures_senera!ed-in

rapidly

deposited

finergrained sediment. Seis-

mic shock and grounding icebergs can

also cause resedimentation.

RezxdL

mented

diamicts may be eith~massive

---

crudely stratified,_and contain folded

interbeds of sand and/or mud. They

are com~mcYinly^associated with tur-

bidites. In manv

casesitmay-bediffi-

cult to differentiate massive rain-out

diamicG%thosfb?mTdbT reFdi-

--L-

mentation proc-esses unless a

dis-

placed faunacanBgJdentitied.

Sea level changes on glacially-influ-

enced conjinental_~helves may be

Figure

Glaciomarine facie~ in low relief settings. A) Massive rain-out diamictite containing abundant micro and macrofauna; Late

Cenozoic Yakataga Fm., Alaska (see Lagoe et

a/.,

1989).

Stratified, sandy diamictite recording interplay of rain-out and traction-currents in

ice-proximal environment. Late Wisconsin deposits of the

Isle of Man,

U.K.

C) lnterbedded diamictites (dark coloured) and shelf-sandstones

(light coloured) in 1000 m thick outcrop of the Yakataga Fm., Alaska. Note planar tabular geometry of units.

Coquina bed consisting of

pecten shells in a coarse sand matrix. Yakataga Fm., Middleton Island, Alaska.

92 EYLES, EYLES

erosion surfaces, and by the deposition

-?----

of extensive muiTbiSannkets

TFF~T

Boulder laa horizons result from win-

nowing of diamicts on the

seafloor

under high~elgy_ regimes (lowered

sea levels). These lags may be subse-

quently striated by grounding glacier

ice to form marine

bgulder pavements

---

(Fig. 33; Eyles, 1987). Starvation of

clastic sediment under high-energy

conditions may also allow the formation

of biogenic deposits such as

caauinaa

(Figs: 32D, 33). FEina of

the

Wis

recorded by e-io-

turbated muds. These horizons may

represent relatively deep waterm-

/Fig. 33). However,

mipretation of discon-

formities, erosion surfaces and mud

blankets in shelf successions is ex-

tremely difficult, because of the com-

plex interactions between

glacio-

eustatlc and glacio-isostatic sea level

changes, discussed below.

Linear or curved furrows,

prodxed

by the scouraqf iceberg and sea-

so~c~els, are abundant_

oa*-

ernhigh-*latitude shelves (and some

lak~nl;i-uncertainty still surrounds

their identification in the ancient record

(Woodworth-Lynas and Guigne, 1990).

Berg scours can be as wide as 50 m

and as deep as 2 m, and may extend

for several tens of kilometres.

S3s

are easily destrxecJ-_by wave pro-

cesses in shallow water or by

down-

--.----"----

s~mmentation, ancEE7not

likely to be preserved in

r&ord. Seafloor sediments subject to

repeated ice scouring (iceberg tur-

bates) are not well understood and

may resemble diamicts produced

sub-

glacially. It is likely that iceberg tur-

bates have been described in the

literature as tills or

tillites.

Eurrowing is zmmon, in glacsG-

fluenced distal marine environments

------2___11-

_---

but the

tracem~k,e~~th_m_selves~-

seldom preserved. lchnofacies distri-

GGfions differ in several ways from the

classic

shoreline-nearshore-shelf-

slope model for nonglacial continen-

tal margins (Chapter

4).

Rapid deposi-

tion rates and poorly consolidated

muddy sediments exclude suspension

feeders. This limits the development of

the

Skolithos ichnofacies characteristic

of clean, shifting sand substrates in

nonglacial settings. The

Cruziana ichno-

facies assemblage predominates on

muddy glaciated shelves (Eyles et

a/., in

press,b). Evidence for rapid sedimenta-

tion is provided by common escape

structures (fugichnia) and the mass

mortality of bivalves. Firm ground

burrows typical of the Glossifungites

ichnofacies are restricted to sediment

surfaces undergoing erosion in shallow

water. These surfaces may be ve-

neered by coquinas or boulder lags, de-

scribed above. Changes in salinity and

the development of brackish water con-

ditions resulting from meltwater inputs,

may be recorded by dwarf feeding

traces. Elsewhere, the role of turbidity

currents and debris flows is to promote

downslope transport of food. This ex-

plains the development of a diverse

Cruziana ichnofacies in slope areas

where, under nonglacial conditions, the

Nereites ichnofacies characterized by

grazing traces is found.

The

km-thick Late Cenozoic Yaka-

taga Formation of Alaska provides an

excellent example of an ancient gla-

cially influenced shelf deposit. It con-

tains stratiform successions of

fos-

siliferous rainout and resedimented

diamicts interbedded with marine muds,

coquinas, boulder pavements, and

storm-deposited sandstones (Figs. 32,

33). Another example of a preserved

shelf succession is the Late Precam-

brian Port Askaig Formation. Its shelf

origin is suggested by the presence of

tidally influenced sandstones

inter-

bedded with diamictites (Eyles, 1988).

HIGH-RELIEF

GLACIOMARINE SETTINGS

Fiords

F--Ugh-latitude, glacially over-

dee~ened coastalvalleyswith floors cut

sauxal-h&d

mefers below-sea

!cxeL

Many are still occupTki by active

tidewater ice margins and associated

ice-contact depositional systems (Fig.

3). Others receive meltwaters from

inland ice margins, and the transported

sediment is deposited a fiord-head fan

deltas. Comprehensive reviews o iord

oceanography and sedimentation (both

in the presence and absence of ice

margins) have been presented by

Syvitski et

a/. (1987). When ice ad-

vances

downfio~d,any

glacial or nonglacial sediments are

Ia~~&-rqnov!.

GkciaI se-tatjon

is-restricted-io

fhwnd-~f the glacial

cycksuhen

ice retreats upfiord.

retreat occurs in relatively shallow

w_a_ter_and

----

marked.

deposition of

morainal banks. By contrast, in deep

watem glac4ers unGgo raijjd.

c&ng

~ia%3t&i:

7blmKs-and pebbly muds are an impor-

tant facies type in fiords. Variation in

meltwater discharge, tidal flows and

MIDDLETON ISLAND

lithology paleobathymetry

9999'

ftttt

Conglomerate

Diarnictite

'...

..;

,,.,

:,..

Sandstone

...

Turbidites

99999

Coquina

AAAA

Boulder Pavement

Figure

Sedimentological log and paleo-

bathymetric curve (based on foraminifera1

data), for the Yakataga Fm. exposed on

Middleton Island, Alaska. Note relatively

deep water conditions during deposition of

the bioturbated mudstones near the top of

the log and relatively shallow conditions

during formation of coquinas and boulder

pavements.

GLACIAL MODELS

waves produce finely laminated cou-

plets of coarser and finer sediment

derived from suspended sediment

plumes. Fiord-head sediment accumu-

lations are very prone to downslope

resedimentation. Slumping and sedi-

ment gravity flows down active chan-

nels are ubiquitous and generate

turbidity currents.

Simja~sedimentat-

ion

patterns--occur

on glacially influ-

enced continental slopes (see above).

Continental slope and abyssal plain

Glacially influenced slopes are major

repositories of glacial sediment and are

preferentially preserved in the ancient

record. Most Late Proterozoic

"tillite"

successions probably consist~o~

ma_'ine deb?isfiowFintima~ely associ-

ated

w-ith-turbidites recording the

.-.-.-A

downslope reworking of glacial sedi-

ment by mass flow into a deep water

base-of-slope setting. Late Cenozoic

slope deposits in the Gulf of Alaska and

eastern Canada (Laurentian Fan, Piper

a/.,

1985; Chapter

13)

formed under

very different tectonic regimes but re-

veal similar slope morphologies and de-

positional processes. The Surveyor Fan

lies adjacent to the heavily glaciated

coast of southern Alaska and records

marine glacial sedimentation since the

Late Miocene; the fan contains about

175,000

km3 of glacially derived sedi-

ment. Slopes show channel systems

separated by smooth interchannel slope

Figure

Glaciornarine facies in high relief settings. A) Stratified diarnictite of the Yakataga Frn. exposed at Yakataga Reef, Alaska.

Stratification results from incomplete mixing of sand, gravel and diarnict facies during downslope resedimentation. B) Large,

400 rn deep, sub-

marine channel (profile

in Fig.

containing interbedded debris flow diarnictites and turbidites; Yakataga Frn., Icy Bay, Alaska.

Stacked

graded gravel beds deposited within submarine channel. Stratigraphic top to right; person for scale. Yakataga Fm., Middleton Island, Alaska.

Stratified diarnictite consisting of folded interbeds of sandstone and diarnictite, formed as subaqueous debris flows. Late Paleozoic ltarar6

Group, Parana Basin, Brazil. Scale in crn.

EYLES, EYLES

VARIATDN ONLY

ZONE

AFFECTED

BOTH

GLACO

BOSTATK: MOVEMENT

AND

QUCO

EUZITATK:

SEMEVEL

VARIATON

EUSTATK: SEA LEVEL

(m)

Figure

Schematic model to show relationship between glacio-eustatic and glacio-isostatic sea level change around a glaciated conti-

nental margin. Top left; hypothetical glaciated continent showing spatial extent of glacio-isostatic crustal depression (Zone

and limit of fore-

bulging (dashed lines). Zone outside of forebulge (A) experiences glacio-eustatic sea-level change only. Pattern of sea level variation in Zone

B results from interaction of glacio-eustasy and glacio-isostasy. Top right; progressive crustal deformation accompanying growth of ice sheet.

Ice sheet grows from right to left on diagram. Note development of peripheral zone of relatively deep water between ice margin and

fore-

bulged crust. Bottom; spatial and temporal (0-80 ka) variation in relative sea level and associated sedimentary environments at the margin of

a continental ice sheet. Zones A and B are defined in diagram top right. Adapted from Boulton (1990). Note that ice retreat is in direction of in-

creasing glacio-isostatic downwarping; ice margin may float and calve back to coastal margin catastrophically. Also note relative slow fall of

7d---Chanter2L.

-----

___

...

.- -.~~

-. .

.--

--.-

.-..-

. .

5. GLACIAL MODELS 95

areas. Sediment gravity flows and other

mass movements are the dominant pro-

cesses in both Alaska and eastern

Canada. Active channels typically tend

to fill with graded gravels (Fig.

34C).

Abandoned channels are plugged by

muds and rain-out diamict facies (see

base of log shown in Fig. 33; Eyles,

1987), recording the release of coarse

glacial

outwash from glaciers that have

advanced to the shelf break (Fig. 3).

Interchannel areas receive large vol-

umes of suspended sediment, together

with debris bulldozed over the shelf

break by the ice sheet. Interpretation of

seismic profiles suggests the presence

of massive debris flow facies (reworked

muds with scattered clasts; Figs.

34A,

B, D) that are broadly channelled in

cross section and which thin downslope

and interfinger with laminated muddy

turbidites. The term till tongue is unfortu-

nately still used to describe such debris

flow units. Large-scale slumping of the

upper and mid slope areas generates

large rafts of slumped sediment, and

creates an irregular slope relief subse-

quently

infilled and smoothed by sedi-

ment ponding.

The formation of ice sheets results in

global lowstands of sea level, with shore-

lines close to the shelf-slope break. The

sediment delivered directly to the

shelf-

slope break is largely resedimented

downslope as turbidity currents. Conse-

quently, base of slope and basin plain

settings during periods of global

lowstand

are dominated by thick, sandy turbidites,

as discussed in Chapter 13.

FACIES SUCCESSIONS AND

BOUNDING DISCONTINUITIES

IN GLACIATED BASINS

The concepts of sequence stratig-

raphy allow the subdivision of the sedi-

mentary record into depositional se-

quences bounded by unconformities

(Chapters 1, 2). The unconformities

are thought to have formed as a result

of globally synchronous (eustatic) sea

level changes and have been used to

construct worldwide stratigraphic corre-

lations. However, the recognition of

eu-

static sea level changes in glaciated

basins is very difficult, given the num-

ber of other processes which create

relative changes in sea level. The de-

velopment of large continental ice

sheets produces global changes in sea

level (glacio-eustacy) because large

volumes of water are locked up as ice

(Chapter

2).

In addition to glacio-eu-

static changes,

glacio-isostatic sea lev-

el changes are caused by ice loading

and unloading of the Earth's crust. This

results in elevation or depression of

the

seafloor and thus creates relative

changes in sea level and water depths

local to the glaciated basin. Variation

in the magnitude of crustal loading ac-

ross a glaciated basin can cause one

part of the basin to experience a fall in

relative sea level at the same time as

water depths are increasing elsewhere

in the basin. The combination of these

effects in glaciated basins makes as-

sessment of the stratigraphic signifi-

cance of individual unconformity sur-

faces extremely difficult.

A simple model for the deposition of

sequences on glaciated shelves is

based on the movement of ice out onto

the shelf at times of glacio-eustatically

lowered sea level. This erosional event

creates a sequence boundary.

Ice-

contact depositional systems (Fig.

deposited against the advancing ice

margin will not survive glacial advance

but will be reworked subglacially as de-

formation till and/or be recycled to the

ice front as a morainal bank. The shelf

break and deep water will ultimately

stop the advance of the ice margin.

Much of the sediment pushed across

the continental shelf by advancing ice

will be discharged down the slope.

The position of ice margins that have

moved onto continental shelves is con-

trolled not so much by climate but by

crustal downwarping below and

around the ice sheet (Fig. 35). Down-

ward flexing of the crust, and displace-

ment of mantle material beneath and

immediately adjacent to the ice sheet,

is reflected in the formation of a pe-

ripheral forebulge (Fig. 35, top right

and left). The crust can be depressed

by as much as 600 m below the ice

sheet and for some distance beyond

the ice margin. Such crustal depres-

sion far exceeds the magnitude of

glacio-eustatic sea level drop (approxi-

mately 150 m; see Chapter 2) and so

creates the situation where high rela-

tive sea levels occur around the ice

margin at times of global glacio-eu-

static sea level low stand. The

fore-

bulge migrates away from a growing

ice sheet to a distance of several hun-

dred kilometres from the ice margin,

and collapses as the ice sheet re-

treats. This gives rise to a very complex

succession of sea level changes (re-

sulting from both glacio-eustatic and

glacio-isostatic adjustments) through-

out the glacial cycle and across

glaciated continental shelves.

In the simple model of an ice sheet

margin that has moved out to the conti-

nental shelf edge, the increase in water

depths caused by glacio-isostatic

downwarping

(i.e. a deepening of rela-

tive sea level) may be sufficient to ini-

tiate deglaciation by extensive calving

along the ice margin. This situation pro-

duces high ice flow rates (Hughes,

1987) which cause large volumes of

subglacial debris and meltwater to be

flushed to the ice margin (Eyles et al.,

1991). Poorly integrated networks of

tunnel valleys composed of

steep-

sided, flat-floored channels, and cut by

meltwater or fluidized sediment, may

result from this process. These valleys

are a common feature of unconformity

surfaces on glaciated shelves (Boulton

and Hindmarsh, 1987).

Most glaciomarine sedimentation

occurs during glacier retreat when large

volumes of meltwater are available.

Thick fining-upward successions, con-

sisting of relatively coarse-grained

ice-

contact deposits overlain by finely la-

minated proglacial silts, result from

glacier retreat. At the end of the glacial

cycle when ice has retreated from the

marine environment, crustal rebound

results in rapid shallowing of the coastal

margin (Fig. 35, bottom). Deeper water

muds will be uplifted and eroded,

thereby creating sequence bounding

un-

conformities in shallower parts of the

basin. Initial rates of rebound are ex-

tremely rapid and may approach several

metres per year. Thereafter, rates de-

crease exponentially and about 10,000

years after the end of the glacial cycle,

isostatic rebound is essentially complete

(Fig. 35, bottom). The coastal margin

may subsequently be flooded by

continuing postglacial eustatic sea

level rise (Fig. 35, bottom). In offshore

areas that escaped rebound into

shallow water, postglacial flooding of

the shelf results in mud deposition.

Thus, erosion in one part of the basin

can be coeval with flooding and depo-

sition elsewhere. Boulton (1990) pre-

sents a very useful summary of com-

plex sea level variations across gla-

ciated basins and describes the re-

sulting architecture of glaciomarine

sedimentary sequences and their

EYLES, EYLES

bounding unconformities. Even where

detailed biofacies and

paleobathy-

metric data are available in relatively

young Late Cenozoic successions, the

effects of glacio-eustacy and

glacio-

isostasy cannot be readily discrimi-

nated

(e.g., Fig. 33).

Recognition of glacio-eustatic cycles

in the

rock record

Analysis of Quaternary deep ocean sedi-

ments shows that glaciations are

strongly cyclic in nature, alternating with

interglacial conditions in a steady rhythm

driven by Milankovitch astronomical vari-

ables (Chapter

2).

At first sight, repeated

glacio-eustatic sea level cycles provide

an obvious mechanism for the formation

of stratigraphic sequences in pre-Qua-

ternary glacial deposits. However, com-

plex changes in water depths and

relative sea level

within

glaciated basins

are generated by crustal loading by the

ice sheet and other processes. These

tend to mask any change in water depth

caused by glacio-eustacy, as has been

clearly demonstrated for many Quater-

nary basins (see above).

Tectonics have been the overriding

control on the deposition of stratigraphic

successions in most pre-Quaternary

glaciated basins, negating any simple

identification of glacio-eustatic sea level

changes. For example, many Late

Proterozoic glacial successions com-

prise lowermost deep water mass-flow

diamictites overlain by a thick,

shoaling-

upward tutbidite succession. These suc-

cessions, up to 1 km thick, are defined

by bounding unconformities and have

been attributed to glacio-eustacy

(Eisbacher, 1985). A more likely expla-

nation is that they record deposition in

rapidly subsiding rift basins. Other Late

Proterozoic shelf successions also

show repeated diamictite and sand-

stone strata

(e.g., Eyles,

1988)

that

could be interpreted as a record of

glacially controlled cycles of sea level

change.

However, short-term glacio-eustatic

oscillation of one hundred metres or

less cannot, by itself, account for the

preservation of unconformity-bounded

stratigraphic sequences. Without tec-

tonic subsidence or a sufficient supply

of sediment, repeated

glacio-eustati-

cally driven oscillations of sea level

produce erosion surfaces. This

clear-

ly demonstrated on tectonically stable

continental shelves that were subject to

repeated glaciation during the Quater-

nary. These shelves commonly show

DISTRIBUTION

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