Boggs S. Principles of Sedimentology and Stratigraphy
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276
Chapter 8 I Continental (Terrestrial) Environments
8.5 GLACIAL SYSTEMS
Introduction
I have placed glacial systems last in this discussion of continental environments
because the glacial environment, in a broad sense, is a composite environment
that includes fluvial, eolian, and lacustrine environments. It may also include
parts of the shallow-marine environment. Glacial deposits make up only a rela
tively minor part of the rock record as a whole, although glaciation was locally im
portant at several times in the geologic past, particularly during the late
Precambrian, late Ordovician, Carboniferous/Permian, and Pleistocene (Eyles
and Eyles, 1992). Glaciers presently cover about 10 percent of Earth's surface,
mainly at high latitudes. They exist primarily as large ice masses on Antarctica
( �86 percent of the world's glaciated area) and Greenland ( � 11 percent of the
world's glaciated area) and as smaller masses on Iceland, Baffin Island, and Spits
bergen. Small mountain glaciers occur at high elevations in all latitudes of the
world. About SO percent of the world's fresh water is tied up in glacial ice, of
which most is in Antarctica (Hambrey, 1994, p. 31). By contrast to their present dis
tribution, ice sheets covered about 30 percent of Earth during maximum expan
sion of glaciers in the Pleistocene and extended into much lower latitudes and
elevations than those currently affected by continental glaciation.
The glacial environment is confined specifically to those areas where
more or less permanent accumulations of snow and ice exist. Such environ
ments are present in high latitudes at all elevations (continental glaciers) and
at low latitudes (mountain or valley glaciers) above the snowline-the eleva
tion above which snow does not melt in summer. Mountain glaciers form
above the snowline by accumulation of snow. They move downslope below
the snowline only if rates of accumulation of snow above the snowline exceed
rates of melting of ice below. The factors affecting glacier movement and the
mechanisms of ice flow (e.g., Martini et a!., 2001; Menzies, 1995) are not of pri
mary interest here. Our concerns are the sediment transport and depositional
processes associated with glacial movement and melting and the sediments
deposited by glaciers.
Environmental Setting
The glacial environment proper is defined as all those areas in direct contact
with glacial ice. It is divided into the following zones: (1) the basal or subglacial
zone, influenced by contact with the bed, (2) the supraglacial zone, which is
the upper surface of the glacier, (3) the ice-contact zone around the margin of
the glacier, and (4) the englacial zone within the glacier interior. Depositional
environments around the margins of the glacier are influenced by melting ice
but are not in direct contact with the ice. These environments make up the
proglacial environment, which includes glaciofluvial, glaciolacustrine, and
glaciomarine (where glaciers extend into the ocean) settings (Fig. 8.27). The
area extending beyond and overlapping the proglacial environment is the
periglacial environment.
The basal zone of a glacier is characterized by erosion and plucking of the
underlying bed. Debris removed by erosion is incorporated into the bed of the
glacier. This debris causes increased friction with the bed as the glacier moves and
thus aids in abrasion and erosion of the bed. The supraglacial and ice-contact
zones are zones of melting or ablation where englacial debris carried by the glaci
er accumulates as the glacier melts. The glaciofluvial environment is situated
downslope from the glacier front and is characterized by fluctuating meltwater
8.5 Glacial Systems
277
Subueous line
outwash
Giomari
Bedrock
ow and abundant coarse engladal debris that is available for fluvial transport.
e glaciofluvial environment is one of the characteristic environments in which
braided streams develop. Extensive outwash plains or aprons may also be present
along the margins of outwash glaciers. Lakes are very common proglacial fea
tures, created by ice damming or damming by glacially deposited sediments.
Meltwater streams draining into these lakes may create large coarse-grained
deltas along the lake edge, while finer sediment is carried outward in the lake by
sussion or as a density underow (Fig. 8.23). Glaciers that extend
out to sea
create an important environment of glaciomarine sedimentation where sediments
are
deposited close to shore by melting of the glacier in contact with the ocean or
farther out on the shelf or slope by melting of ice blocks, or icebergs.
The glacial environment may range in size from very small to very large.
Valley glaciers are relatively small ice masses confined whin valley walls of a
mountain. Piedmont glaciers are larger masses or sheets of ice formed at the base
of a mounta front where mountain glaciers have debouched from several val
leys and coalesced. Ice sheets, or continental glaciers, are huge sheets of ice that
spread over large continental areas or plateaus.
ansport and Deposition in Glacial Environments
Transport of sediment by ice is a kind of fluid-flow transport, although ice flows
very slowly as a high-viscosity, non-Newtonian pseudoplastic. Glaciers can ow
at
rates as high as 80 m per day during sporadic surges; however, typical flow
rates are on the order of centimeters per day (Martini et al., 2001, p. 50). In A Tramp
Abroad, Mark Tw ain describes his (fictitious) disappointment, after pitching camp
on an alpine glacier in expectation of a free ride down the valley, to find that the
view from his camp remained the same day after day. Glaciers advance if the rate
of
accumulation of snow in the upper reaches (head) of the glacier exceeds the rate
of ablation (melting) of ice in the lower reaches (snout). The balance behveen ac
cumulation and melting is illustrated in Figure 8.28. Ice must !low internally
om the head of the glacier to replace that lost by melting at the snout. Flow of
e is laminar, and !low velocity is greatest near the top and center of the glacier.
Ve locity decreases toward the walls and tloor, alough not necessarily to zero.
Figure 8.27
Glacial and associated
proglacial environments.
[After Edwards, M. B., 1986,
Glacial environments, in
Reading, H. G. (ed.), Sedi
mentary environments and
facies, 2nd ., Fig. 13.2, p.
448, reproduced by permis
sion of Elsevier Science Pub
lishers, Amsterdam.]
278
Chapter 8 I Continental e estrial) Environments
Fire 8.28
Diagrammatic two-dimensional illus
tration of the balance between glac
ier accumulation and melting and
the movement of ice within a glaci
er. A. Valley glacier. B. Ice sheet. [(A)
after Sharp, R. P. , 1988, living ice
Understanding glaciers and glacia
tion: Cambridge University Press,
Fig. 3.5, p. 58, and Fig. 3.6, p. 59,
reproduced by permission; (B) based
on
Sugden and john, 1976.]
Glaciers retreat if the rate of melting exceeds the rate of accumulation. They reach
a state of equilibrium, neither retreating nor advancing, when rates of melting and
accumulation are equal, although inteal movement of ice continues.
Sediment is entrained by glaciers by quarrying and abrasion by ice as the
glacier erodes its bed and by falling or sliding of material from the valley
walls. Some of this sediment is transported in contact with the valley walls
and floors and is responsible for much of the abrasion. Part of the remaining
load is carried on the upper surface of the glacier and part is carried within.
The internal load is derived either from the joining of ice streams from two or
more valleys or by the washing or falling of material from the surface into
crevasses (Fig. 8.29A). Much of the sediment transported by glaciers is carried
along the bottom and sides, as illustrated in Figure 8.298. The entrained sedi
ment load includes large and small blocks of rock as well as extremely fine
sediment, called rock flour, produced by grinding of the rock-studded glacier
base over bedrock. Thus, the glacier sediment load typically consists of an ex
tremely heterogeneous assortment of particles ranging from day-size grains to
meter-size boulders. Glaciers never become overloaded with debris to the
point that they become immobilized. As glacial ice melts, however, the sedi
ment load is dropped to form various kinds of glacial moraines. See Kirkbride
(1995) for details of sediment transport by glaciers.
As glaciers move downslope below the snowline, they eventually reach
an elevation where the rate of melting at the front of the glacier equals or ex
ceeds the rate of new snow accumulation above the snowline. If the rate of
melting approximately equals the rate of accumulation, the glacier achieves a
state of equilibrium in which it neither advances nor retreats. Within such an
equilibrium glacier, internal movement of ice continues to carry the rock load
along and supply rock debris to the melting snout of the glacier. This process
causes a ridge of unsorted sediment, called an end moraine or terminal
moraine, to accumulate in front of e glacier. Lateral moraines, or marginal
moraines, can accumulate from concentrations of debris carried along the
edges of the glacier where ice is in contact with the valley wall. Medial moraines
may form where the lateral moraines of two glaciers join (Fig. 8.29). When the
rate of melting at the snout of a glacier exceeds the rate of new snow accumula
tion above the snow line, the glacier retreats back up the valley. If a glacier re
treats steadily, it drops its load of rock debris as lateral moraines, medial
Lateral moraine
Ground moraine
8.5 Glacial Systems
279
Figure 8.29
A. Susitna Glacier, eastern Alaska
Range, Alaska. The dark stripes are
sediments acquired by joining of ice
streams from the various valleys. [A.
Photograph by Austin Post, Ameri
can Geographic Society Collection
archived at the National Snow and
Ice Data Center, University of Col
orado, Boulder, and obtained from
the Internet at http:/ /-nsidc.col
orado.edu, downloaded Dec. 9,
1998]. B. Schematic representation
of sediment transport paths within
a glacier and the various kinds of
glacial moraines; compare with Fig.
A. [After Sharp, R. P., 1988, Living
ice-Understanding glaciers and
glaciation: Cambridge University
Press, Fig. 2.5, p. 30, reproduced
by permission.)
moraines, and a more or less evenly distributed sheet of ground moraine. If
the glacier retreats in pulses, it leaves a succession of end moraines, called
recessional moraines.
As glaciers melt on land, large quantities of water run along the margins, be
neath, and out from the front of the glacier to create a meltwater stream. Such
stams flow with high but variable discharge in response to seasonal and daily
temperature variations. Near the glacier front, the meltwater quickly becomes
choked with suspended sediment and loose bedload sand and gravel, leading to
for mation of branching and anastomosing braided-stream channels. Streams that
discharge into glacial lakes tend to build prograding delta systems into the lakes
with steeply inclined foresets that grade downward to gently inclined bottomset
beds (Edwards, 1986). Ve ry fine sediment discharged into the lake from streams
may be dispersed basin ward in suspension by wd-driven waves or currents. If a
large enough concentration f sediment is present in suspension to create a densi
ty difference in the water, a density underflow or turbidity current will result that
c carry sediment along the lake bottom into the middle of the basin. Strong
winds blowing over a glacier or an ice sheet pick up fine sand from exposed, dry
outwash plains and deposit the sand downwind in nearby areas as sand dunes.
Fine dust picked up by Wind can be kept in suspension and transported long dis
nces before being deposited as loess in the periglacial environment or as pelagic
sediment the ocean.
280
Chapter 8 I Continental errestrial) Environments
Where glaciers extend beyond the mouths of river valleys to enter the sea,
their sediment load is dumped into the ocean to form glacial-marine sedi
ments. Sedimentation under these circumstances may take place in four differ
ent ways:
1.
Melting beneath the terminus of the glacier allows large quantities of glacial
debris to be released onto the seaoor with little reworking (Fig. 8.30).
2. Large blocks of ice calve off from the front of the glacier and float away as ice
bergs. ese icebergs gradually melt, allowg their sediment load to drop
onto the seafloor, either on the shelf or in deeper water.
3. Fresh glacial meltwater charged with fine sediment can rise to the surface to
form a low-density overflow above denser saline water. Silt and flocculated
clays then gradually settle out of suspension from this freshwater plume.
4. Mixing of fresh meltwater and seawater may produce a high-density under
flow that can carry sand-size sediment seaward.
Glacial Fades
Because the broad glacial environment encompasses the proglacial and periglacial
environments as well as the glacial environment proper, it is necessary, to avoid
confusion, to distinguish between glacial facies deposited directly from the glacier
and facies transported and reworked by processes operating beyond the margins
of glaciers. Furthermore, it is desirable to distinguish between glacial facies de
posited on land and those deposited on the seafloor. Ta ble 8.1 illustrates the range
of sedimentary facies that are affected some way by glacial processes. Many of
these facies are subtypes of facies deposited in fluvial, lacustrine, eolian, shallow
marine, and deep-marine environmen, which are treated elsewhere in this book
Therefore, we focus our discussion here primarily on grounded-ice facies and
proximal marine-glacial facies.
Figure 8.30
Model for glaciomarine sedimentation in front of a wet-based tidewater glacier. Rapid
mixing of fresh water and seawater adjacent to tunnel mouths may produce a high-densi
ty underflow capable of transporting sand-grade sediment and possibly coarser material.
Much
of the fresh glacial meltwater rises to the surface of the sea as a low-density over
flow layer; as this layer mixes with seawater, silt and flocculated clay gradually settle from
suspension. Note also the settling of dropstones from melting ice blocks. [After Edwards,
M. B., 1986, Glacial environments, in Reading, H. G. (ed.), Sedimentary environments and
facies, 2nd ed., Fig. 13.5, p. 453, reproduced by permission of Elsevier Science Publishers,
Amsterdam.]
Facies of continental glacial environments
Gunded ice facies
Glaciouvial facies
Glaciolacusine facies
Facies of proglacial lakes
Facies of periglacial lakes
Cold-climate periglacial facies
Facies of marine glacial environment
Proximal facies
Continental shelf facies
Deep-water facies
8.5 Glacial Systems
281
ur: Eyles, N,, and A. D. Mia I!, 19, Glacial facies, in R G. Walker (ed.), Facies models, 2nd ed : Gtsdence Cana
Reprint r. 1, p. 1-38.
Sediment deposited directly from glaciers on land is called till. Several kinds
of till are recognized, including basal melt-out till, ablation till (supraglacial melt
out
till), and lodent till, deposited under a sliding glacier (Martini et al., 2001 ).
Glacial sediment melted from glaciers in lakes or e ocean is called waterlaid
till. If direct deposition from a glacier cannot be proved, the term diamict is used
for poorly sorted, unconsolidated glacial deposits; the term diamicton is used for
their consolidated equivalents.
Continental lee Fa cies
Grounded Ice Fa cies
Unstratified Diami. Till deposited directly from ice on land in various kinds
of moraines consists of unstratified, unsorted pebbles, cobbles, and boulders
(Fig. 8.31) with an interstitial matrix of sand, silt, and clay. They are thus charac
terized by a bimodal particle-size distribution in which pebbles predominate in
the coarser fraction, with cobbles and boulders scattered throughout (Easter
brook, 1982). Some pebbles are rounded, indicating that they are probably
stream pebbles entrained by the ice. Others may be faceted, striated, or polished
owing to glacial abrasion. Elongated pebbles and cobbles tend to show some
preferred orientation, commonly with their long dimensions parallel to the di
rection of glacial advance. They may also be crudely imbricated, with long axes
dipping upstream. Pebble composition can be highly diverse and may include
rock types derived from bedrock located hundreds of kilometers distant. Sands
d silts are commonly angular or subangular. Much of the silt in glacial de
posits is produced by glacial abrasion and grinding.
Stratified Diami. In addition to deposition directly from melting ice, glacial
debris can be deposited from meltwaters flowing upon (supraglacial), within
(glacial), underneath (subglacial), or marginal to the glacier. The deposits of
these meltwaters form on, against, or beneath the ice and thus are commonly
own as ice-contact sediments. They are reworked to some degree by meltwater
and thus exhibit some stratication. They are also better sorted than sediments
282
Chapter 8 I Continental (Terrestrial) Environments
Figure 8.31
Thiok, poorly stratified, poorly sorted glacial diamict (Quater
nary), Alaska. [Photograph courtesy of Gail M. Ashley.]
deposited directly from ice, commonly lack the characteristic bimodal size distri
bution of direct deposits, and may contain pebbles rounded by meltwater trans
port. These stratified deposits can accumulate in channels or as mounds or ridges
known as kames, kame terraces, or eskers. Kames are small mound-shaped accu
mula
tions of sand or gravel that form in pockets or crevasses in the ice. Kame ter
races are similar accumulations deposited as terraces along the margins of valley
glaciers. Eskers are narrow, sinuous ridges of sediment oriented parallel to the
direction of glacial advance. They are the deposits of meltwater streams that
probably flowed through tunnels within the glacier. The deposits were then let
down onto the subglacial surface after the ice melted. Stratified diamicts are
commonly characterized by slump or ice collapse features, including contorted
bedding and small gravity faults. Stratified glacial facies can include gravels,
sands, and silts, some of which may be extremely well stratified, as illustrated
in Figure 8.32.
Facies of Proglacial and Periglacial Environments
As discussed, meltvvaters issuing from glaciers transport large quantities of glacial
debris downslope and deposit it as glaciofluvial sediment in braided streams or
as glaciolacustrine sediment in glacial lakes, formed by ice damming or moraine
damming. These transported and reworked deposits take on the typical charaoter
istics of the environment which they are deposited; however, they may retain
some characteristics that identify them as glacially derived materials. For exam
ple, the large daily to seasonal fluctuations in meltwater discharge may be reect
ed in abrupt changes in particle size of sediments deposited in meltwater streams
or lacustrine deltas. Sediments deposited in streams or lakes very close to the glac
ier front may also display various slump deformation structures caused by me1t
ing of supporting ice. As mentioned in the discussion of lakes, one of the most
10m
8m
6m
2m
A
Gardner Lake
Stion 10
Flgure 8.32
8.5 Glacial Systems
283
A. Ve rtical stratigraphic profile of glacial sediments deposited in an esker system. The envi
ronment is interpreted as ice-tunnel gravel overlain by laminated, marine fan sand and di
amictite.
Lithoacies code: G, gr.ael; S, sand; Si, si
lt
; Dsi, silty diamictite; Cl, clay. Grain
size: c, coarse; f, fine. Other: Bl and 82, bedding surfaces .upon whic dip of surface fan
measured. B. Photograph showing laminated sand overlain by fine and deformed into
ball and pillow structures, In turn overlain by a assive diami(tite {5.S to 9 m interval in
A), interpreted as a debris now on tile fan surface. Person on left gives cale. C. Close-up
vi of laminated sand within a thin diamictite interbed. Scae increments are 5 em. [After
Ashley, C. M., et al., 1991 , Sedimentology of ate Pleistocene (Laurentide) deglacial-phase
deposits, eastem Maine: An example of a temperate marioe grounded ice-sheet margin:
Gl, S. America Spec. paper l61 , Fig. 5, p. 113.]
characteristic properties of glaiat lakes is the pesence of varves, which form in
response to seasonal variations in meltwater flow. Additional details on the har
acteristics of glaciofluvial and glaciolacustrine sediments are given by
Brzikowski and van Loon (11), Maizels (1995), and Ashley (1995).
Sand can ransponted by wind from outwash plains and deposited as
dunes periglacial areas adjacent to glaciers (e.g., Derbyshire and Owen, 1996);
however, the primary wind deposits in many periglacial environments are silts.
Deflation of rk flour and other fine sediment from outwash plains and alluvial
plains provies enormous quantities f silt-size sediment that is transported by
wind and deposited as widespread sheets of fine, well-sorted loess. Because of the
even size of its gras, lss typically lacks well-defined stratification. It is com
sed dominantly of angular grains of quartz but may also contain some clays.
284
C
hapter 8 I Continental (Terrestrial) Environments
Marine Glacial Facies
Proximal Fa cies
In environments where marine water i in direct contact wih the glacier margin
(e.g., Fig 8.33), substantial quantities f sediment are deposited directly from
meltwater conduits or tunnels into subaqueous ans, with addHional sediment
suppied by melting of rafted ice (Fig. 8.34; Eyles and Miall, 1984; Powell and
mack, 1995). Coarse cobbfes and gravels accllmlllate at the tops of fans,
and sands and gravels accumulate within channels owing to sediment gravity
underflows. Mud and sand are contributed by ice melting and "rain-out" of sus
pended sediment. Some reworking vf sediment curs by downslope sediment
gravity flows and episodic traction-current activity. Proximal glacial-marine
sediments may thus range from poorly sorted, prly stratified diamicts that re
semble those deposited on land to coar-grained stratified diamicts, with a
muddy sandy matrix, that may display current-produced stmctures. Melting of
buried ice masses can cause surface subsidence and assodated deformation and
fa ulting of sediment.
Distal Fa cies
Away from the proximal environment in which glaciers are in direct contact
with marine water, glacial sediment is supplied by floating ice masses, and
deposition is dominated by marine processes. Melting of icebergs supplies both
fine sediment and coarse debris to the ocean floor by "rain-out," or faout (Fig.
8.30 and 8.34). Ice-rafted debris deposited on the continental shelves may be re
worked to some degree by marine waves and currents (and possibly turbidity cur
rents) and may be affected by iceberg gunding (where icebergs touch bottom).
In deeper water on the continental shelf, the debris fa llout from floating ice may or
may not be retransported by turbidity currents to deeper water. Ice-rafted debris
Figure 8.33
Tidewater glaciers flowing into the ocean. Note the nearshore plume of sediment derived
from the melting glaciers and the many small blocks of floating ice. College Fiord Glacier,
Chugach Mountains, southeastern Alaska. [U.S. Navy photog raph, American Geographic
Society Collection archived at the National Snow and Ice Data Center, University of Col
orado, Boulder, and obtained from the Internet at http:/ /-nsidc.colorado.edu, down
loaded Dec. 9, 1998.]
Flgure8.34
8.5 Glacial Systems
285
®
®
Proximal subaqueous sedimentation from glaciers: (1) glacially tectonized marine sedi
ment, (2) lensate lodged diamict units, (3) coarse-grained stratified diamicts, (4) pelagic
muds and diamicts, (5) coarse-grained proximal outwash, (6) interchannel cross-st,atified
sands with channel gravels, (7) resedimented facies (debris flows, slides, turbidites), and
(8) supraglacial debris. Sediment deformation results from ice advances, melt of buried
ice, and ice-turbation. [From Eyles, N., and A. D. Miall, 1984, Glacial facies, in R. G. Walk
er (ed .), Facies models: Geoscience Canada Reprint Ser. 1, Fig. 8, p. 20, reprinted by per
mission of Geological Association of Canada.]
that settles to the deep ocean floor is probably little modified by further deposi
tional processes, except for deposition of a mantle of hemipelagic or pelagic
sediment. In general, glacial-marine sediments are distinguished from ground
ed glacial diamicts by the presence of some stratification and from all on-land
glacial
diamicts by the presence of marine fossils and dropstones. Dropstones
are scattered cobbles or boulders that drop to the seafloor from melting ice
blocks or iebeFgs. FossH evidence that particularly suggests a glacial-marine
origin includes fossils preseFved in growth position as whole shells (i.e., fossils
enfombed by fallout sediment); marine molluscs or barnacles attached to
glacially faceted pebbles; preservation of delicate ornamentation on shells; and
the presence of forams and diatoms in the matri material (Easterbrook, 1982).
Veical Facies Successions
Successive advances and retreats of valley glaciers and ice sheets produce com
plex vertical successions of facies as ice progressivety overrides proglacial envi
nments during glacial advance, and conversely as direct ice deposits and
ice-contact deposits are reworked in the proglacial environment as a glacier re
treats. These facies are much too varied and complex to attempt description here;
however, Figure 8.35 illustrates some typical vertical profiles that might develop
different parts of a glacial environment during a single phase of glacier advance
d retreat. e also Brodzikowski and van Loon (1991, Chapter 9) and Edwards
(1986, p. 46566).