Walker J. Facies models, Canada, 1992

Подождите немного. Документ загружается.

---

5. GLACIAL MODELS 77

base.

Such tunnel fill eskers are ex-

-reat as steep-sided

ridqes that record the internal plumbing

system of the ice sheet (Fig. 12):

Internally, the eskers contain cross-

bedded sand and gravel facies with re-

stricted variance in paleocurrent

directions. A beaded form is typical of

eskers deposited as overlapping sub-

aqueous fans in water ponded against

the retreating ice front. Beaded eskers

show rapid downstream~~ons

info

fine-grained glaciolacustrine facies.

Other

bedforms exposed by the

retreat of wet-based, sliding glaciers

are streamlined

drumlins,

which form

part of extensive swarms (Fig. 13).

Their formation is not well understood,

but most inferred mechanisms invoke

Internal deformation

either incremental lodgement around

subglacial obstacles such as rock

highs, or erosion of sediment over-run

by the ice sheet. Studies of drumlins at

modern ice margins, and geophysical

investigations in Antarctica (Alley

al.,

1987), show that subglacial sedi-

ments can be pervasively deformed

below rapidly flowing ice (Fig.

6D).

this case, sliding along the

icelsub-

strate interface is less important than

movement resulting from internal de-

formation of the substrate. These beds

are composed of diamicts (deforma-

tion till; Fig. 14) and formed by the me-

chanical mixing and homogenization

of eroded sediments, as in a concrete

mixer. Indeed, the glacier base can be

regarded as a giant shear zone, with

deformation till being equivalent to fault

gouge and drumlins as large-scale

slickensides. The thickness of the de-

forming layer below large Quaternary

ice sheets is not yet established, but

could have reached several tens of

metres. The deformability of sediments

overridden by the ice sheet is dictated

by their geotechnical and hydrogeo-

logical properties. Spatial variation in

the shear-strength of subglacial sub-

strate sediments gives rise to streams

of deforming

diamict separated by

areas of the bed that are resistant to

deformation (Fig.

15).

Erosion around

these more resistant portions of the

bed results in streamlined

residual

drumlins. Detailed subsurface investi-

gations show that many drumlins origi-

nate by erosion of pre-existing sedi-

ment, with deformation till resting

unconformably on a resistant core of

overridden proglacial sediments (Fig.

15; Boyce and Eyles, 1991). Other

models of drumlin genesis involve the

release of catastrophic subglacial sheet

floods (Shaw and Sharpe, 1987) but

this hypothesis has yet to be ade-

quately tested by studies of the internal

Figure

Glacially-shaped clasts. A)

Figure 6

A) Top, movement of dry-based (polar) glacier by internal deformation. Glacier is

exposed in front of a modern

glacier

frozen to the bed; bottom, in contrast wet-based glaciers move by internal deformation and

(Athabasca Glacier, Alberta). B) weathering

basal sliding. Horizontal arrows indicate relative amounts of ice movement.

Movement of

out from a late Proterozoic

tillite (Tauodeni

wet-based glacier on bedrock substrate.

"Stiff-bed" model for accretion of till sheets below

Basin, Mauretania).

wet-based ice (see Fig.

8).

Accretion occurs by incremental smearing of englacial debris

clasts (to left.bothflhah,.,

against substrate (lodgement till).

"Soft-bed" model where till is produced below wet-

point up-qlacier: also

nate-truncated-auk

based ice by subglacial shearing of overridden sediments (deformation till; see Fig.

14).

and striae.

78 EYLES, EYLES

structure and facies of the drumlins.

The important criteria for identifying

continental glacial facies are

the

presence of underlying regionally ex-

tensive unconformities and 2) the re-

stricted regional thickness of individual

subglacially deposited

diamict(ite)

units. This thickness is generally less

than 50 m, and is commonly much less.

By contrast with the subglacial environ-

ment,

diamict(ite) thicknesses in glacio-

lacustrine and glaciomarine environ-

ments are much greater (see below).

The second criterion of restricted

regional thickness applies regardless of

the relative importance of lodgement

(stiff-bed) and deformation (deforming

bed) below the ice sheets (Fig. 6) and

regardless of the precise origin of

sub-

glacial bedforms. The presence or

absence of subglacial

bedforms (drum-

lins, eskers and glaciotectonic struc-

tures), subaerial

outwash and other

cold climate facies, provides further

contextual criteria for the identification

of subglacially deposited

till(ite)s.

Supraglacial depositional system

The thin, outer margin of an ice sheet

commonly stagnates, leaving broad

zones of hummocky topography un-

derlain by highly variable sediments

with complex geometries. During the

last glaciation, stagnant ice extended

over many thousands of square

kilo-

metres in North America (Figs.

4, 5).

Strong compression between active ice

upglacier and the dam of stagnant

marginal ice results in complex folding

and thickening of the basal debris layer

(Fig. 16). This debris may be slowly

melted out in situ to form basal melt-out

till. The melt-out till, together with other

diamict facies released on the ice

surface, is associated with a distinct

hummocky topography that records

complex disintegration of the ice mar-

gin (Fig.

17D, E; Shaw, 1979; Lawson,

1982; Kasizcki, 1987; Paul and Eyles,

1990). If the thin outer margin of the ice

sheet becomes frozen to the substrate,

large bedrock slabs may be detached

and glaciotectonized. This is because

ice movement is accomplished not by

basal sliding, but by deformation of un-

derlying substrate sediments.

An inactive or retreating margin of

this type becomes buried under a

drape of diamict formed by the melt-out

of englacial debris on the ice surface

(Figs. 16,

17A,

C).

This drape is

very unstable and is moved by sedi-

ment gravity flows into kettle basins

generated by the local melt of buried

ice (Boulton, 1972; Fig.

17b, c). Kettle

fills are subsequently preserved as

hummocks when adjacent ice cores,

under a thinner sediment cover, melt-

down more rapidly to produce topo-

graphic lows (relief-inversion). Typical

vertical profiles through hummocks

Vertical joint sets oriented

sub-parallel to ice flow

Erosive base to diarnict units

Outwash stream

THIN, SHEET-LIKE REGIONAL GEOMETRY OVER MAJOR

UNCONFORMITIES. LENTICULAR LOCAL GEOMETRY

Shear surfaces

WITH CROSS-CUTTING DIAMICT UNITS AND LINEAR

CHANNEL FILLS.

Variable sediments in

Sheared-out bedrock

Shaped, oriented clast

Figure

Depositional environment, stratigraphy andxn_eralized

___

---

-_-

-----

GLACIAL MODELS

Figure

Low relief till plain being exposed by retreat of an Icelandic glacier; lines parallel to

ice margin are annual push ridges. Section in centre of photograph is

m high.

Day,

luurillullluallallu, cllylallu.

Figure

(left)

Folded rafts of Creta-

ceous Chalk in glaciotectonized glacioma-

rine deposits of N.E. Norfolk, England. Ice

movement was from right to left. Sea wall

in front of section is

m high.

Figure

(lower left)

Sinuous esker

ridge

(8m

high) exposed by retreat of

Icelandic ice margin

km distant.

Figure

Drumlin swarm and flutes in

Saskatchewan produced by ice sheet

flowing toward top right. Note

en echelon

ar-

rangement of drumlins suggesting erosional

streamlining of pre-existing sediment (see

Fig.

15).

Scale bar (centre-right) is

km.

show uppermost successions of resed-

imented massive, graded and stratified

diamicts (Fig. 16). Supraglacial dia-

micts resedimented as debris flows are

generally conformable, have clast im-

brication parallel to flow, clasts that

project from the tops of beds, rafts and

fragments of pre-existing sediments,

channelized cross sectional form,

Figure

Deformation till. A)

Deformation till recently exposed at the

margin of Saskatchewan Glacier, Alberta.

Note "chaotic" appearance and lack of any

clast fabric. Largest clast shown is approxi-

mately 15 cm diameter. B) 10 m thick ex-

posure of

Halton Till at Toronto, Ontario,

deposited during the Late Wisconsin

glaciation (about 25-1 5,000 years ago). Till

rests erosively on sands.

Sheared-out

lens of chalk in clay-rich deformation till;

Late Wisconsin deposits of the east

Yorkshire coast, England.

EYLES. EYLES

lenticular downslope-thickening geo-

metry, an absence of relief on upper

bed surfaces and a tendency to fill to-

pographic lows. Massive and crudely

bedded diamict facies predominate;

the crudely bedded facies result from

successive superimposition of thin

massive flows. These diamict facies

are interbedded with glaciofluvial and

lacustrine facies. They overlie basal

melt-out till (Fig.

16), which may drape

over bedrock rafts that were present in

the former ice base. Glaciotectonized

(ice thrust) bedrock and incorporated

substrate sediments can be an impor-

tant component of the sedimentary

succession (Fig.

16).

Under certain

conditions, englacial structures such as

folded basal debris successions

survive basal melt-out in the form of

ridge-like

bedforms oriented transverse

to former ice flow direction (Shaw,

1979).

Glaciolacustrine

depositional system

Glaciolacustrine ponding results from

overdeepening by glacial erosion,

glacial disruption of former drainage

systems, and the release of large vol-

umes of meltwater. Basins vary from

narrow alpine types in areas of high

relief (see below), to large continental-

scale lakes akin to interior seaways.

These large lakes are ponded in iso-

statically depressed continental inte-

riors evacuated by ice sheets (Fig.

5).

Lake Agassiz is the most famous ex-

ample, extending over an area of

about 1,000,000 km2 of North America

(Teller and Clayton, 1983). A simple

distinction between ice-contact and

non ice-contact lake bodies can be

made (Fig. 18). One of the most cha-

racteristic

glaciolacustrine facies con-

sists of

varves,

which are annually

produced couplets of contrasting grain

sizes resulting from different condi-

tions of sedimentation in winter and

summer. A varve typically comprises a

couplet of a coarser summer layer

(sand

and/or silt) and a finer winter

layer (clay). Proximal equivalents may

include varved gravels, but these are

difficult to recognize. The classic mod-

el of varve formation in non ice-contact

glacial lakes emphasizes a strongly

seasonal regime driven by summer

supraglacial melting across the ice

margin. Significant supraglacial melt is

Figure

Erosional model of drumlin for-

mation; drumlins are cut by erosional

streams of subglacially-deforming till (see

Fig.

6D).

Top diagram shows Y-shaped

area of coarse-grained proglacial

outwash

associated with finer-grained sediments in

front of an advancing ice margin. When ice

overrides the area, the coarser-grained

sediments, which are relatively resistant to

deformation, become streamlined and

broken up into drumlin forms (middle dia-

grams). Arrowed lines are sediment flow

lines. Bottom diagram shows cross section

through resultant drumlins which have a

sandy core (stippled), overlie fine-grained

sediments (dashes) and are draped with a

thin veneer of deformation till (triangles).

After Boulton

(1987), and Boyce and Eyles

(1991).

suppressed in winter. Under this

regime sedimentation is dominated by

the growth of fan-delta lobes. In

summer, sediment laden, higher den-

sity meltwaters move down these

lobes as density underflows. A distinct

succession of sandy lithofacies is de-

posited in each summer melt season

(Fig. 19) and records the start, increase

and ultimate decline of density under-

flow activity (Ashley, 1975). The suc-

cession is bounded top and bottom by

a winter clay layer deposited from sus-

pension (Figs. 19, 20A, B). The

summer sandy layers thin distally into

silts with isolated starved ripples and

rare ripple-drift cross lamination. The

base of the coarser layer is commonly

sharp, and the layer may be graded

(Figs. 19,

200). It may contain multiple

laminations representing deposition by

a single pulsating or intermittent den-

sity underflow. There may also be a

minor contribution of pelagic material

from interflows or overflow plumes of

5. GLACIAL MODELS 81

suspended sediment. The dark clay

unit (winter layer) may show normal

grading indicating deposition of sus-

pended sediment under the ice cover

of a closed lake. Clay layer thick-

nesses are generally uniform across

the basin but may contain massive or

cross-stratified sands and laminated

silts which record the winter

drawdown

of lake levels and delta foreslope

slumping (Fig. 19; Shaw, 1977). Bur-

rowing and trace fossils are commonly

present in the summer layer but not the

winter clay.

Unfortunately, the term

varve

(or

varvite) continues to be used as a

routine descriptive term for sediments

that are better termed rhythmically

laminated (rhythmites), or laminated

sediments (laminites). The real signifi-

cance of many varved successions as

indicators of seasonally controlled sed-

imentation remains to be assessed rig-

orously. Many of the layers described

as varves comprise single or

multiple-

graded units of silt and clay with divi-

sions

D and

of the Bouma se-

quence for turbidites (Fig. 3 in Chapter

13). These may have been deposited

by discrete-event turbidity currents with

no evident seasonal control. In

pre-

Quaternary successions varvites are

used to infer glaciolacustrine

(i.e., con-

tinental) environments and seasonality

of climate. These inferences may be

incorrect if the layers are

discrete-

event turbidites of marine origin (or

even tidally formed laminites, see

Chapter 11). However, thin-bedded

turbidites may also be interbedded

with seasonally controlled layers so

the identification of varved succes-

sions and varvites requires detailed

field study

(e.g., Ashley, 1975).

A seasonal, varved, depositional

regime is characteristic of non

ice-

contact lakes in low-relief basins where

sedimentation is driven solely by

sea-

.sonal surface melting of the ice sheet.

In high-relief lake basins in mountain

X-SECTION THROUGH A TYPICAL TROUGH-FILL

PRESERVED

A HUMMOCK

Figure

Depositional environment, stratigraphy and generalized vertical profile for supraglacial deposits at margin of stagnant, debris-rich,

ice margin (see Fig. 17).

(1)

debris flows,

(2)

melt-out till,

(3)

outwash, (4) deformed substrate,

(5)

lodgement till (see

Fig.

or deformation till

(see Fig. 14).

'EpEUeD UJalSaM

u! Aqde~6odol uo!l€?J6alu!s!p Ayaowwnq

adeospuel 6u!llo~

'(9 '64 aas) epeue3

su!eld ulalsam u! Kqde~6odol uo!le~6alu!s!p 'pal!sod

-ap-Kllepel6e~dns 'Ayooururnq 6u!~oqs qde~601oqd ~!y

.ueap Ala~!lela~ s! puno~6yoeq u! ao! !s!~qep la!3elBe~dns q3nur su!e)uoo qdw6

-oloqd lo ued le~luao u! aol .eysely 'JapelE) eysnueleyy uo aqol moll s!Jqap lepel6e~dns

'w~!J 01 say (umoqs IOU) u!6~ew a31 .MO~) s!Jqap

Aq padd!~ls 6u!aq spqap

JaAoo a3epns 411~ ua6~aqzl!dg u! (46!4

91) ysowwnq pa~oo-ao! a6~e-l

.Aqde~60dol uo!le~6aiu!s!p 'Ky3owurnq

~U!MO~S ua6~aqzl!ds u! U!~JRU a3! ~U!ISEMUMOP iueu6els

yde~6oloqd J!e Ie3!uaA

'sape4 pue sluauruoJ!Aua 1e!oe16e~dns

eln6lj

GLACIAL MODELS 83

Ico

contact

GLACIAL

LAKES

zones, seasonality is easily suppressed

because resedimentation processes

operate year round (see below).

Sedi-

mentological modelling of ice-contact

glacial lakes is frustrated by the logistic

difficulty of working on modern

proglacial lakes, and the small size of

modern lake basins compared to

Pleistocene and older examples.

Extensive Pleistocene

glaciolacus-

trine deposits exposed around the

modern Great Lakes in North America

are of considerable significance to the

study of sedimentation in large,

ice-

contact lakes in low-relief settings

(Figs. 21, 22). The deposits consist

predominantly of stacked successions

of diamicts and variably bedded sands

(Eyles and Eyles, 1983). Diamicts are

fine grained and have a blanket-like

Figure

Contrasting depositional conditions in ice-contact and non ice-contact lakes.

geometry, thickening in topographic

lows and thinning over highs. Inter-

nally, they are complex assemblages

of massive and stratified facies (Figs.

CLAY

20E, 21, 22). The massive diamict

facies result from the rapid rain out of

suspended sediment and ice-rafted

DRAP~D

debris onto the basin floor. Strati-

LAMINATION

111

fication may subsequently be devel-

CLIMB~NG

oped by reworking of this sediment by

traction currents or downslope

resedi-

lL'

mentation. _qia.victs are commonly

DRAPED

~~~~;t)r~.o~iIC1by..~tja~

of lami-

LAMINATION

AND

CLIMBING

-SUSPENSION

ifl'"

~n~d~ays,.p~tur~

RIPPLES

GRAIN

FLOW

+Gorigin, containing dropstones (Fig.

TURBIDITY

CURRENT

20C). These are ~n~o~er~~

-!-

coarsening-upwards succession of

ripple-la$nated, phnai-an-da60ugh

cross-bedded sands which record delta

---

progradat.ion_ over-sites of diamictac-

cumulation. The most common deltaic-

--.-

facie~ isa crudely bedded silty sand

with abundant liquefaction structures

indicating rapid subaqueous deposi-

tion. Sands are commonly loaded into

the upper surfaces of diamict as a

result of rapid deposition of sand onto

a water-saturated diamict substrate.

Littoral and shallow water sediments of

such large lakes are commonly storm

influenced (Eyles and Clark, 1988) and

subject to scouring by floating ice

masses (Teller and Clayton, 1983;

Woodworth-Lynas and

Guignk, 1990).

Extremely rapid changes in lake levels,

caused by the creation or removal of

ice or sediment dams, allows the close

Figure

Deposition of seasonally-controlled (valved) sediments in non ice-contact lake.

juxtaposition of shallow and deep water

Note proximal to distal

(1,

trend from thick (cm to m) variably-rippled sands to thin (cm)

(sands

and

diamicts

respectively;

silts bounded top and bottom, by a winter clay layer. Slope failure of the delta front in winter

Figs.

21, 22) and may trigger large-

may result in more complex successions

(1- see

~haw,.1977). scale slumping events.

EYLES, EYLES

Glaciofluvial

deposits

The depositional systems described

above contain significant volumes of

sediment deposited by glacial melt-

water rivers (Fig.

23).

At the ice

margin, aggradation at the head of out-

wash fans is commonly rapid enough

to bury portions of the ice margin. This

leads to deformation structures in

coarse-grained, crudely bedded or

massive proximal

outwash gravels

when the buried ice subsequently

melts.

Outwash surfaces pitted (with

kettles

craters)

by the melt of buried

ice, may extend over many square kilo-

metres. They are flanked by eskers or

complex ice-contact diamict succes-

sions (Fig. 16).

The

outwash plain or

sandur

(plural:

sandar

in Icelandic) lies beyond the im-

mediate ice terminus. Glacial

outwash

rivers are typically of multiple-channel

(braided) type (Chapter

7).

Deposition

is commonly dominated by large floods

(jokulhlaups), to the extent that a single

flood event may accomplish most of the

sediment transport each year. Wind

action, in the absence of vegetation,

results in deposition of wind-blown

silts (loess) and sand (see below).

During early postglacial conditions,

swamps may become established in

the proximity of water bodies and

anastomosed river systems may

develop in areas experiencing glacio-

isostatic rebound and rising base level

(Chapter

7).

Thick peat accumulations

may ultimately generate coals, such

as those of the Permo-Carboniferous

Figure

Glaciolacustrine facies. A) Proximal varved sands (e.g., 1 in Fig.

19)

with winter clay layers arrowed. These facies were deposited

in a non ice-contact lake (Fraser River valley, British Columbia). B) Lowermost rippled sands with overlying draped lamination (silt) with winter

clay layers arrowed

(e.g.,

in Fig.

19).

Deposited in a non ice-contact lake (Glacial Lake Hitchcock, Massachussetts). C) Laminated silts and

clays containing abundant ice-rafted material. Some laminae show normal grading from silt to clay. Note variable thickness of laminae.

Deposited in an ice-contact lake; Don Valley Brickyard, Toronto, Ontario.

Va~ed silts and clays deposited in Lake Agassiz, northern Ontario.

Note relatively constant thickness of laminations. Scale in cm; photograph courtesy T.

Warman.

crudely stratified muddy diamict formed by

ice-rafting and settling of suspended fines in an ice-contact glaciolacustrine environment; photo shows about

m of outcrop. Late Wisconsin

deposits, Scarborough Bluffs, Ontario.

.sa!fiauoa6 dololno

JOJ

.6!

eas .o!JeluO 'sunla y6no~oqeog la pasodxa

xalduoo au!~lsn3elo!oel6 u!suoos!~ elel y6noJyl al!)oJd lequah paz!leepl

am619

.o!leiuo 'sunla

qfinoroq~eos ie pewdxa (shelo 'sq!s 'spues 'su!ue!p) xalduoo au!~lsnoelo!oelB (u!suoos!~) uo!leps16 lsel

hiauoa6 doJan0

ernly

SL~D

yBnoiwms....I

SpUBS

llBnoiwms"'Z

~uII)!~ yoaqAuuns

""E

spues-sLe/3 &I!S e#purocU

""P

gm!a kwu!weg"".s

qwqa

eU!I3MapU~ly""9

1/11 UWH""2

spues

qonbal/

ew."P

86 EYLES, EYLES

coal-bearing glaciated basins of the

southern continents (Martini and

Glooschenko, 1985).

Glaciofluvial processes are important

because they may completely rework

sediment deposited by the glacier (Fig.

23).

The sedimentary record indicating

the presence of ice may therefore be

destroyed. This is a problem in the inter-

pretation of ancient deposits, because

braided river deposits occur in many de-

positional settings (Chapter

7).

glacial

connection may be very difficult, if not

impossible to identify. Evidence must be

sought from the presence or absence of

cold climate (see below) periglacial

structures, or from the occurrence of

glacial clast shapes (Fig.

and stria-

tions. This is a particular problem in

high-relief settings.

HIGH-RELIEF

GLACIOTERRESTRIAL SETTINGS

In mountainous areas typical of active

tectonic settings, steep substrate

slopes and valley floor rivers are the

dominant influence on sedimentation.

Late glacial resedimentation pro-

cesses rework primary glacial sedi-

ment downslope into braided river

systems or fan-deltas on the margins

of lakes. Deposits are dominated by

interbedded debris flow diamicts,

braided river gravels and

glaciolacus-

trine facies (Fig.

24)

and comprise a

relatively distinct

glaciated valley

depositional system. The term

para-

glacial

has been used to identify a

short-lived transitional period from

glacial to

fluvial environments (Ryder,

1971). Downslope transport of large

volumes of poorly sorted debris is

common, particularly in glaciated vol-

canic areas where large volumes of

coarse-grained poorly sorted volcanic

sediments are remobilized downslope

lahars

(

Eyles and Eyles, 1989; see

Chapter

6).

The presence of glacially

streamlined bullet

boulciers within a

diamict(ite) may be the only indication

of a glacial contribution to the sediment

Figure

High relief valley infill facies.

A) Late Wisconsin crudely bedded debris

flow diamictites cropping out along the Bow

Valley, Alberta. These deposits are prefer-

entially cemented and eroded into

steep-

sided pillars (hoodoos). Cliff approximately

Figure 23 Glaciofluvial environments and facies. A) Scott outwash fan, Alaska. B) Typical

m high. B) Valley infill stratigraphy near

braided river gravel facies showing planar tabular sheets of massive to poorly-stratified

Banff, Alberta, comprising crudely bedded

gravels deposited on longitudinal bars. Note thin

(30

cm) wedges of sand deposited along

debris flow diamictites overlying

glacioflu-

trailing edge of gravel bars (see Chapter

7).

Section is approximately

m high.

vial

outwash deposits.