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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Glaciogenic Lithofacies N Eyles and M Lazorek, University of Toronto, Toronto, ON, Canada ã 2013 Elsevier B.V. All rights reserved.
The Meaning of Facies The term glaciogenic refers to any sediment or landform that results from glacial action. In the main, glaciogenic sediment is produced directly by erosion and comminution at the base of glaciers and ice sheets and redistributed by other processes. The word facies means ‘appearance of’ (hence ‘face’). Lithofacies are simply different types of clastic or chemical sediments (and their lithified rock equivalents) produced by gravity, water, ice, or wind in sedimentary environments. Sediments are readily classified informally into facies based simply on color, texture, lithology of clastic particles, internal structure, thickness and geometry, presence or absence of body and trace fossils, and any postdepositional structures (e.g., reddish, fossiliferous, pebbly quartz-rich sandstone with burrows). There are several formal classifications of sedimentary lithofacies (Eyles et al., 1983; Ghibaudo, 1992; Walker, 1992). A widely used practice is to study lithofacies accumulating in the present day and use these as keys to interpret ancient deposits and their formative processes. Unfortunately, few sedimentary environments are typified by a single diagnostic lithofacies because most environments are spatially complex, involving the interaction of many different processes. An exception is the environment where large storm waves create distinct hummocky bedforms in shallow water (producing a distinctive hummocky and swaley cross-stratified sand facies). The latter is referred to as a so-called universal facies type because its origin is unique to a particular setting (i.e., wave-dominated shallow water). Another example is that of a graded turbidite bed deposited by turbidity currents flowing downslope under water. So-called common facies types, on the other hand, are produced in many environments (eolian, deep marine, fluvial, etc.) and are not uniquely diagnostic of any one depositional setting (e.g., rippled cross-laminated sands).
The Complex Glacial Environment Consider the wide range of environments found around glaciers (Figures 1(a), 1(b), and 2). Meltwater emerges from under the ice as large rivers that are often ponded in front of (or even under) glaciers to form lakes in which large fans and deltas are built. Seasonal and long-term variations in the position of the ice front give rise to complex diachronous (time-transgressive) patterns of sedimentation and partial destruction, and reworking of glacial facies either by melt streams or bulldozing by the ice front. In the absence of vegetation and the presence of strong downglacier (katabatic) winds, eolian transport and deposition of fine sediment are important. Steep slopes and melting ice result in mass movement processes. In marine environments where glacial sediment was deposited under water, crustal rebound may lift sedimentary successions either above water or into energetic shallow waters where they will be reworked and redeposited.
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Typical glaciogenic sediment is poorly sorted and uniquely fingerprinted by the presence of striated and glacially shaped pebbles or boulders. The greatest yields of such sediment (mostly in the form of silt, so-called rock flour) are from temperate wet-based valley glaciers that are able to slide rapidly across their relatively steep beds (e.g., Hooke and Elverhoi, 1996). These glaciers are capable of eroding their beds to form deep basins often enclosed by rock bars. This creates a typical up-and-down and locally overdeepened long profile (thalweg) in glaciated valleys; glaciated basins often exhibit parts that have been eroded well below sea level (e.g., Great Lake basins in North America). The lower seaward end of glaciated valleys forms narrow, steep-walled, and very deep troughs or fjords. Glaciers in areas of permafrost (dry-based polar glaciers) are frozen to their beds and move slowly by internal creep and so do not generate a thick depositional record. Ice streams form where parts of polar ice sheets can slip on wet sediment (Bennett, 2003). Subpolar glaciers share characteristics of both wet- and dry-based glaciers. While useful in terms of understanding modern glacial processes and deposits, a thermal regime cannot easily be inferred from ancient glacial lithofacies. A polar thermal regime is transitory; at the end of any one glaciation, dry-based glaciers eventually warm and melt. Polar conditions predominate today in Antarctica but ancient deposits preserved along the continental margin record sediment delivered by wet-based glaciers (e.g., Eyles et al., 2001).
Diamict Versus Till Diamict (diamictite when a rock) is a nongenetic umbrella term for any poorly sorted deposit regardless of depositional environment (Figures 1(c), 1(d), and 3(a)). Man-made concrete is a diamictite. Sliding glaciers produce a wide range of diamicts that contain admixtures of most textural classes ranging from clay to large boulders. These diamict facies are specifically identified as till because they were deposited directly by ice. A till (tillite: rock) is a specific genetic term for a diamict deposited directly from ice either terrestrially on land or in the glaciomarine realm. Diamicts form in a wide range of glacial and nonglacial environments such as on the slopes of volcanoes, under water, or on land where debris moves downslope as debris flows or in addition as a consequence of meteorite impact (Figure 3(a) and 3(b)). They are said to be a common facies (not universal facies; see above) because they have no unique environmental interpretation. They look very similar regardless of their origin. It is by the study of other facies with which they are conformably interbedded that valuable information regarding depositional setting is gained. Walther’s law states that facies found in a continuous vertical sequence must have been spatially related at the time. If, on the other hand, facies are separated
GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
(a)
(b)
(c)
(d)
(e)
(f)
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Figure 1 (a) Many processes in areas of wet-based glaciation are dominated not by ‘glacial’ processes per se but by the activity of meltwater. Here is an extensive braided outwash plain in southern Alaska where meltwaters leaving the ice front have reworked almost all primary glacial sediment and landforms. (b) A calving tidewater ice margin (see Figure 9). (c) Diamict deposited by debris flow processes. (d) Subglacially deposited deformation till (Figures 4 and 6). (e) Diamict deposited underwater by downslope flow; note systematic coarsening of pebble sizes upward (known as inverse grading developed where debris flows become transitional to turbidites; Figure 3(b)). (f) Classic varves composed of couplets of summer deposited silt (white laminae) and dark winter mud. Numerous white-colored granules of silt (silt clasts) were brought in during summer underflows (Figure 7).
by disconformities or unconformity surfaces, then the facies above or below a diamict can provide little information. The application of Walther’s law yields some indication of the overall depositional environment (whether it was deposited in deep or shallow water; or on a slope; or near a volcano or an impact crater; or onland, in a terrestrial setting, under a glacier perhaps?). A diamict conformably interbedded within undisturbed turbidites cannot be a till but is more reasonably interpreted as a debris flow since these are genetically related to turbidity currents. This sort of contextual analysis greatly constrains the possible origin of the diamict(ite).
Conceptually, till is deposited in three ways. Till accumulates by the passive meltout of englacial debris under stagnant dead ice (meltout till), by meltout of debris from flowing ice (akin to smearing peanut butter on toast: lodgement till) or by subglacial mixing of preexisting sediment (akin to smearing peanut butter on soft bread: deformation till; Figure 4). Most tills found in mid-latitude areas affected by Pleistocene ice sheets are probably deformation tills; subglacial shearing and mixing of preexisting sediment is the most efficient method of making, transporting, and depositing large quantities of till. Deformation till is commonly massive, rests on deformed
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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Glaciated valley
Glaciolacustrine Massive sands and associated deltaic facies Turbidites
(b)
Braided fluvial, glaciolacustrine Debris flow
(f) Sh
Debris flow Turbidites with dropstones Debris flow
c
s
(i) Slu
c
s
s
Mo
rain a
mp
Sl op e
“Rainout” and debris flow diamictites s g
(a)
(e)
Braided fluvial Gravel debris flow Till
Ice
(d)
elf
l ba
s
(g)
Ice
(c)
be
(h)
nks
rg
sco
urs
g
(b)
Subglacial
Glaciofluvial
Supraglacial (j)
Sandy channel system
Cross-cuting till sheets
Chaotic slumped facies
Color legend
X-stratified sands
Massive gravels
Boulder pavement Till
(c)
c
s
s
g
Glacitectonized bedrock/sediment
Debris flow c s
(d)
Fiord
Subglacial till
Superimposed gravel bars
Glacitectonized bedrock/sediment s g
Submarine channel
(e)
Debris flow
c
s
s
c
Shelf Mud
Large slump blocks (olistostromes) Debris flow Turbidites
(f)
c
s
s g
Chaotic, dumped facies Scoured bedrock surface
(g)
Channelized massive-graded gravels
Debris flow
Bioturbated muds c s
s g
(h)
c s
s
Base of slope
Turbidites
Channelized massive, graded sands
s
Diamict/ till (glacial) Bedrock g
g
Slope
Turbidites
Turbidites
Mud Sands Gravels
Thick, rainout, diamictites and debris flows with sediments rafts s g
(i)
c
s
Iceberg scour Glacially-shaped and striated clasts Thick “rainout” and resedimented diamicts Coquinas Turbidites Boulder pavement s g
Distal turbidites Laminated muds with rare dropstones Contourites
Massive muds with dropstone horizons
(j)
c s s g
Figure 2 Principal glacial depositional systems associated with large ice sheets that reach sea level. Quaternary glaciations have left a very prominent glacial record on land but most sediment (perhaps as much as 90%) is deposited offshore on continental slopes. Such deposits dominate the record of older glaciations in Earth history because the terrestrial record is easily eroded.
substrate sediments, and usually includes rafts of undigested substrate sediment in its basal parts recording incomplete mixing. Some are held to exhibit lamination and banding similar to gneissic textures in high-grade metamorphic rocks subject to strong shearing. The same facies are produced by debris flows under water, however requiring care in interpretation and consideration of the wider depositional context. At modern glaciers flowing over stiff rock, lodgement till is more important. Meltout will be of local significance where evidence indicates that ice has stagnated and melted in situ. There have been several techniques confidently proposed in the past as a Holy Grail for identifying subvarieties of till. Such methods center on samples examined in the laboratory and include geochemical and grain size characteristics, small-scale structures seen in thin sections, microsurface textures of quartz sand grains, or the orientation of clasts (clast fabric analysis). All of them ultimately proved simplistic because of overlapping properties. In reality, till is deposited by interactions of all the three processes identified above (indeed, processes may change through time at any one site) but one process will be dominant. Volumetrically, most till is
produced by subglacial deformation of other sediments as ice sheets grow and expand outward and cannibalize their own outwash deposits.
Glacial Depositional Systems: Facies Models and Facies Associations Geologists recognize a wide range of different environments around and under the margins of glaciers, each one creating a distinct depositional system. These systems do not have sharp boundaries but are intimately linked as a consequence of sediment moving from one system to another (Walker, 1992). The system forms a characteristic deposit with a distinct morphology (e.g., a deltaic depositional system) and several recurring systems occur in glacial environments (Figure 2). Terms such as landsystem and landform/sediment association have also been used. One advantage of this approach is that in areas of modern-day and Pleistocene glaciation, it is possible to predict the types of facies found in the subsurface by simply identifying landform types typical of different systems.
GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Processes
A. Volcanic
Characteristics
Deposits
Rock fall Creep
Olistostrome avalanche deposit
Slide
Creep deposit
B. Rock falls (olistostromes)
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Slide
Slump
Slump Debris flow
D. Fault gouge
Density-modified grain flow Fluidized flow Liquefied flow Hyperconcentrated flow
Slump
Debrite
Turbidite (coarse, medium, fine grained)
Coarse-grained Medium-grained Fine-grained turbidite turbidite turbidite (classical) (Bouma)
Deposit
C. Mixing of coarse and fine grained sediment downslope (mixtites)
Turbidity flow
Debrite
E. Regolith
E D
Facies
F. Meteorite impact breccias
C B
Sediment support
A
(a)
Coherent slumping
Cohesive matrix strength and fluid buoyancy
Turbulence high concentration low-concentration
(b)
Figure 3 (a) Diamicts form in a wide range of environments. Identifying diamicts deposited directly by glacier ice (till; e.g., Figure 6) requires analysis of associated facies. (b) Gravity-driven processes and their deposits.
In pre-Pleistocene rocks, however, this cannot be done, and interpretation is reliant on consideration of facies associations. Facies associations are defined as distinct vertical successions of genetically related lithofacies (Miall, 1997; Walker, 1992). As emphasized above, it is by recognizing these packages (rather than any one unique facies type) that ancient glacial settings can be recognized. A three-dimensional (3D) block diagram called a facies model is a useful tool for representing depositional systems and their constituent facies associations. These models provide a mental image that can embrace different data derived from studies of outcrops, subsurface drill core, or geophysical data (such as 3D seismic or ground-penetrating radar), and the study of processes and deposits in a controlled laboratory experiment. The model will vary from one particular case to another (such as tectonic setting), but it is a useful tool for distilling large amounts of data, for predicting subsurface conditions and for comparison with stratigraphic successions of unknown origin. Drawing a facies model forces consideration of the 3D distribution of facies. For each depositional system (and thus facies model), a representative vertical profile can be made showing typical facies and the sedimentary evolution of the system through time (Figure 5). These profiles suffer from the obvious limitation of depicting 4D conditions in two dimensions, but the detailed assessment of facies in measured outcrops or drill core (called facies logging) is the essential starting point in any analysis. Simple lithofacies codes are available to aid labeling and identification and serve as a reminder of what to look for
in the field. Two-dimensional panels (say of cliff exposures) are very useful in showing lateral variability and 2D geometry of facies (such as those found in channels) and, increasingly, there is a move to consider the 3D architecture of deposits. Much progress has been made in this respect in fluvial, eolian, and deep marine environments (prompted by the search for hydrocarbons and reservoir analysis by petroleum geologists, and aided by geophysical techniques such as 3D seismic). The need to model the movement of groundwater through glacial sediments is a major stimulus (Boyce and Eyles, 2000).
Terrestrial Glacial Depositional Systems Subglacial Depositional System The terrestrial subglacial depositional system is composed of sediments deposited under the ice, and is typically dominated by sheet-like deposits of deformation till (Figure 6). These tills rest on eroded and deformed remnants of older preexisting sediment. These were glaciotectonized when ice advanced over the sediments and they were remobilized as till. A lowrelief landform called a till plain (ground moraine of the older literature) is created. The most distinctive feature of a till plain are bedforms called flutes (long and thin) and drumlins (long and wider) elongated in the direction of ice flow. The origin of these is still contentious but they are increasingly being recognized as a product of either erosion of preexisting sediment (e.g., where they show cores of nontill sediment) or deformation and remobilization of deformation till (Benn and
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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Supraglacial melt
Moulins Englacial melt
2
1B 1A 1B 2 3 4 5 6
Direct loss of water
Ice flow
5
3
No basal sliding; internal deformation only Sliding on basal meltwater film and areal abrasion Selective erosion of bed by pressurized meltwater and slurries; subglacial lakes Conduit flow of meltwater over impermeable crystalline bedrock (eskers) Subglacial recharge of meltwaters into aquifers Overpressured sediment undergoing pervasive shear (deformation till/drumlins/no eskers) “Excessive” meltwater that cannot be discharged as (4) or (5) flow in channels cut into bed and leaves ice sheet as melt streams (7)
Unconsolidated sediment
6 4 Crystaline bedrock
Frozen bed movement versus wet-based temperate movement
1
7
Sedimentary rock
Low profile margin of ice sheet resting on soft sediment
A - Polar dry based (no meltwater)
Total surface movement T1
T0 5
Sediment Internal deformation deformation
Frozen Glacier bed Internal deformation
Ice Meltwater channel
B - Temparate wet based
Deformation till
Sliding Glacier bed Basal slip
Preexisting sediments
Figure 4 Summary diagram of processes operating below ice of contrasting thermal regime. The generation and movement of till as a deforming layer (5) occurs below the outer margins of a wet-based ice sheet resting on sediment. Most modern glaciers are sliding on rock and are unrepresentative of Quaternary ice sheets.
Evans, 1998; Boulton, 1996; Hart, 1999). Fundamentally, a streamlined till plain is the ‘soft bed’ of the former glacier, in effect a giant slickenside surface such as along a fault plane veneered by ground-up rock (gouge). End moraines are ridges that form perpendicular to ice-flow direction and mark pauses in ice retreat, creating a ribbed appearance to the till plain. Moraines are built either by the extrusion of soft deforming till from under the ice margin or by bulldozing of sediment by a
readvancing ice front. The gravelly deposits of subglacial rivers are left as sinuous esker ridges (Brennand, 1994). Till plains may be overlain by extensive spreads of gravelly outwash facies (e.g., Kjaer et al., 2004) and silty glaciolacustrine deposits left from ephemeral lakes ponded against the retreating ice margin. Glacial outwash deposits are dominated by poorly sorted, crudely bedded, sheet-like gravel facies deposited in shallow braided rivers. There is an absence of
GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Facies association
Pebbly beach
IRD
Depositional energy High
Low Sandstone facies St Trough cross-stratified Scs Swaley cross-stratified Hcs Hummocky cross-stratified Sh Horizontally laminated Sr Rippled Sg Graded Sm Massive Bioturbated Sb
60
St
Gr FA 3
Gr
CL,Q Sb 56
Conglomerate facies
Q,CL
?
W
?
Sb
PBiii Gr 52
FA 5
FI,Sr St
FA 5
48
FI
Sr W FA 3
W 44
FA 5
?
Sb CL
Gr
Gs Gg Gr B
Fl Fm Fb
Laminated or thin-bedded Massive Bioturbated
CL Q V
Clast layer Coquina Volcanic ash Clasts
FL
Trough cross-stratification
St FA 2
St
Hcs
Swaley/hummocky cross-stratification
Sb
Bioturbation
40
Q,CL
FA 3
Stratified Graded Rippled Breccia
Fine-grained facies
Sr FA 2
23
Fossils
? w
Coalified logs/wood
Sb,Fb
Deformation FA 5
36
C
FI HCS
FA 3
C
PBii
Bounding discontinuity Erosion surface
St FA 5
Paleocurrent direction
FI,Sr
32
fl
r
,fl
fl,s
s
st
sc
sc s
ds Mu
Sa
nd
ton e sto ne Me Fin Co d e ng Co ium lom ars era e te
FI
Facies
Figure 5 Typical vertical profile showing facies types, paleocurrents (small circles with arrows), and amount of ice-rafted debris (IRD) in a glacially influenced shelf succession (Eyles et al., 1998). Vertical scale is in meters.
large-scale cross-stratified facies because of a lack of deep channels and an abundance of coarse debris (Miall, 1992). Glacial lake facies include distinctive rhythmically laminated silts and clays that record seasonal patterns of sedimentation (varves).
Varved facies dominate glacier-fed lakes (Figure 7). Varve thickness typically decreases upward in measured profiles recording ice retreat. So-called proximal varves may be many meters thick and composed of ripple cross-laminated sands.
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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Esker
X
Push ridge
Ice
Drumlin
Vertical joint sets oriented subparallel to ice flow direction Erosive base to diamict units
Drumlin
Basal debris dropped into subglacial channel Boulder pavement Shear surfaces Sediment raft
Channel fill
Outwash stream
Drumlins
Esker
Variable sediments in channel fill
X
Sheared out bedrock (‘smudges’) Shaped oriented clast Bedrock rafts
X
Boulder pavement
Channel fill
1m
Glaciotectonized bedrock/sediment X
Figure 6 Facies model for subglacial depositional system (Figure 2) involving deposition of successive sheet-like units of deformation till.
Till-like sediments may form in ice-contact glacial lakes where icebergs drop coarse sediment into bottom muds (rainout diamict). These are discriminated from tills proper by consideration of their geometry (typically blanket-like, resting conformably on underlying deposits) and the associated glaciolacustrine facies with which they are conformably interbedded. Local deformation and incision result from the scouring of the lake floor by drifting ice masses such as icebergs (Eyles et al., 2005).
Supraglacial Depositional System The subglacial depositional system is often partially covered by sediments that were transported on the ice surface (Figure 8). These supraglacial sediments were lowered onto the underlying till plain as the underlying glacier melted. A distinctive hummocky topography marked by craters (kettle holes) forms when buried ice melts and overlying sediment collapses. This topography is common at the margins of valley glaciers that transport substantial quantities of rubbly rock fall debris derived from adjacent rock walls. Bouldery supraglacial debris can blanket the immediate ice margin, insulating it from melt and thereby creating a belt of hummocky ice-cored sediments underlain by subglacial sediments and landforms. This is the hallmark of the glaciated valley depositional system. In addition, large volumes of sediment accumulate between the glacier and the valley sidewalls. With ice retreat, lateral moraine ridges left perched on the valley sides are quickly destroyed by mass wasting. Mass flow and meltwater processes rapidly convert the glacial fill of a confined valley into thick accumulations of outwash gravel and sand. The term paraglacial has been used in reference to a
short-lived phase of intense reworking and sediment transport by rivers immediately after deglaciation. There is still much uncertainty regarding the origin of the hummocky terrain left behind in flat mid-continent regions by Pleistocene ice sheets. This distinctive topography extends across huge tracts of the glaciated portion of the mid-continent North American plains underlain by soft Mesozoic rocks. This terrain was initially regarded as a supraglacial deposit akin to that left by valley glaciers, but the process of how thick sediment could accumulate on top of the ice sheet was never clear. The composition of hummocky moraine offered clues; it is composed of the same clay-rich till found in nearby drumlins into which it passes. Indeed, transitional landforms can be recognized where the formerly flat till plain was pressed into hummocky streamlined shapes (humdrums). It is now recognized as the product of the pressing of a soft till substrate, under portions of the ice margin that stagnated (Boone and Eyles, 2001). Here, the extensive development of a soft bed in mid-continent reflects the presence of weak, fine-grained rock. Such topography may identify relatively faster flowing ice streams within the ice sheet (Hart, 1999) moving across a soft bed of overpressured shales. Large rafts of glaciotectonized soft rock were moved below the ice.
Marine Glacial Depositional Systems The term glaciomarine identifies processes or deposits that occur within a narrow zone in direct contact with the margin of a glacier (or ice sheet) that reaches sea level. The term
GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
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Glacial lakes Icebergs Ice
Overflows
Slumping
Interflow Underflows
Subglacial facies
Diamicts Laminated silts and clays
1. Ice contact (a)
Ice
Meltstreams
Seasonal ice Overflows
Subglacial facies Braided river Rippled facies sands 2. Nonice-contact
Interflows Underflows
Mud
Rhythmically laminated silts and clays
(b)
‘Classical varves’ 1 Winter
2
3
Summer
Clay
Draped lamination
Winter
Draped lamination and climbing ripples
I
II
III
IV
V
Winter
Climbing ripples Clay
Suspension Grain flow Turbidity current
I
Fan delta
II
1
III IV 2
Bed
rock 3
a on
er
ov
ec
l ic
V
as
Se
(c)
Figure 7 Facies models for glaciolacustrine depositional system (Figure 2) with (a) ice-contact and (b) non-ice-contact types. Typically, the deposits of non-ice-contact lakes are strongly influenced by seasonal variations in the volume of meltwater entering the basin, reflected in varved sedimentation. (c) Slumping on oversteepened delta fronts (I–V) in winter may complicate this.
ice-contact marine or proglacial marine are synonyms for marine settings where glacial influences predominate. Outside this zone and away from direct contact with the ice margin is the glacially influenced marine environment where marine
processes, not glacial, dominate. In areas affected by Quaternary glaciations, glacioisostatic depression and subsequent postglacial uplift of coastal margins have left widespread marine glacial deposits exposed above sea level. Knowledge of
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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
C S S G Sediment gravity flows moving into troughs occupied by meltstreams or lakes
1 3 1 1m
1
3 ICE
Ice core Deformed sediments
1
3
Trough filling
3
1
1
(a)
3 2
3
Final hummock form 2
4 5
5m 4 3
1
(c)
2 4 (b) Figure 8 (a) Facies model for supraglacial deposition where ice surface is covered by debris. Melt of buried ice results in widespread slumping and flow of sediment as it is lowered onto subglacial sediments below. Final ice melt produces a resulting in a chaotic hummocky terrain. (b and c) Typical hummock stratigraphy showing: (1) debris flow diamict with rafts of other sediments, (2) diamict melted out from dead ice (meltout till), (3) outwash gravels that accumulated in troughs, (4) glaciotectonically deformed subglacial sediment or rock. Faults and slump structures are common throughout.
marine glacial depositional systems is also key to interpreting the pre-Pleistocene record of glaciation given selective preservation of offshore basinal facies (see below).
Glaciomarine Depositional System The glaciomarine depositional system occurs where an ice margin terminates in water (Figure 9). When advancing, ice will commonly bulldoze an underwater ridge (morainal bank) in front. This deposit is subsequently overridden to produce thrust sediment and deformation tills (Boulton et al., 1996). A retreating glacier will leave morainal banks on basement highs where ice could ground and stabilize temporarily (e.g., Powell and Domack, 1995). This setting is highly dynamic because the ice margin is partially floating and thus unstable, losing mass by calving icebergs. This environment is dominated by strong jets of meltwaters issuing below water at the ice front, delivering coarse gravel and sand facies deposited on subaqueous meltwater fans (e.g., Hunter et al., 1996). Deposition by turbidity currents results in a wide variety of massive and graded gravel and sand facies found in multistorey crosscutting channels. Plumes of suspended sediment are also released. These produce blankets of massive mud where mud rains out in quiet water areas; pebbly muds form where
ice-rafted debris is dropped to the seafloor. Tides interact with plumes to produce rhythmically laminated mud facies; seasonal varved deposits may also occur. Ice proximal submarine outwash facies rest unconformably on lowermost subglacially deposited and glaciotectonically deformed facies such as till. In general, glaciomarine deposits deposited along ice margins in water have a low preservation potential. These accumulations are usually destroyed by shallow marine erosion as a result of glacioisostatic uplift, by iceberg scour, and by downslope mass wasting. Typically, shallow water facies (beach gravels) truncate underlying glaciomarine facies.
Glacially Influenced Marine Depositional System ‘Glacially influenced’ is an umbrella term for deposits accumulating in the more remote parts of a glaciated basin beyond the direct reach of glaciers (i.e., distal to the glaciomarine realm; Figure 2). The depositional system is fed by glaciclastic sediment that has been extensively reworked by currents or gravity. Areas affected by glacioisostatic movements of the crust created by the weight of the ice sheet, or glacioeustatic sealevel changes (or floating icebergs), are also said to be glacially influenced. The setting necessarily encompasses a much
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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
8 7 2 5
4 5 2
6
3
6 1
Figure 9 Facies model for glaciomarine depositional system near a tidewater glacier margin (see Figure 1(b)). (1) Glaciotectonized marine sediments or bedrock, (2) till, (3) stratified accumulation of diamict deposited by slumping of till and other debris, (4) mud with ice-rafted debris, (5) channeled gravel and sand of submarine fan, (6) slumps and sediment gravity flow facies (Figure 3(b)), (7) iceberg scour and plume of suspended sediment, (8) push ridge. The entire sediment package commonly takes the form of large ridges (morainal banks) reflecting short-lived pauses in the ice margin as it retreats.
broader range of environments across continental shelves and slopes (e.g., Anderson, 2002; Eyles et al., 1998). Here, a glacial influence is expressed in the form of rapid changes in water depths brought about by glacioeustasy (the removal of water from the oceans to build ice sheets) and glacioisostasy (the unloading and loading of the lithosphere by removing and returning water from the oceans during glaciations, or direct loading by ice sheets). These two processes act together to create complex spatial and temporal variations in water depths and depositional conditions in different areas of the same sedimentary basin. Detailed examination of glacial facies and disconformities in vertical successions will provide data as to whether water depths were increasing or decreasing (Miall, 1997). It is dangerous however to interpret these as necessarily reflecting changes in sea level per se; sea level may have remained unchanged with the basin floor itself responding to different loads. For this reason, changes in water depths evident from the study of stratigraphic successions are best described as relative sea-level changes, not in absolute (eustatic) sea level. Floating ice masses and the introduction of coarse ice-rafted debris, and ‘rainout’ of mud from plumes of suspended sediment are other significant distal glacial influences on otherwise normal marine sedimentation processes. Several times during the last 100 ka, huge armadas of icebergs were released from rapidly calving margins of Northern Hemisphere ice sheets and are recorded as distinct ice-rafted horizons in mud (Andrews, 1998); coarser layers of ice-rafted facies are known from the ancient record (Eyles et al., 1998). Blankets of stony mud
subject to repeated winnowing and scour by iceberg keels (ice keel turbation) cover the mid and outer parts of glacially influenced shelves below storm wave base (WoodworthLynas and Dowdeswell, 1994). These substrates host a wide range of infaunal and epifaunal organisms reflected in diverse ichnofacies types (Eyles and Vossler, 1992). Deep-water muds pass shoreward into inner shelf areas, and interfinger as fairweather deposits within thick storm-influenced and shoreface sands. Most glacially influenced continental shelves are not major repositories of glacially influenced marine facies. Water depths are too shallow, so their floors are current-swept and most shelves are undergoing uplift or only slow subsidence. This results in the inability to create space for (accommodate) substantial thicknesses of sediment. Large areas were exposed at times of glacioeustatic sea-level lows and scoured by ice sheets or large melt rivers. Typically, shelves are characterized by condensed successions where deposits of only the last glaciation are found resting on major erosion surfaces. Some glacially influenced continental margins (e.g., Antarctica) are slowly subsiding. These show a well-defined subsurface structure (evident on seismic records) of horizontal ‘topsets’ composed of relict shelf sediments that have aggraded vertically through time and which may include subglacial sediments (Anderson, 2002; Dowdeswell and O’Cofaigh, 2002). Along glaciated continental margins, the bulk of glacial sediment accumulates in deep-water basins on the continental slope or at its base. These experience enhanced sediment
28
GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
supply at times of lowered sea level when rivers or glaciers discharge sediment directly to the shelf edge (e.g., Hooke and Elverhoi, 1996). Sediments pushed across the shelf under glaciers are transported downslope at the shelf edge by slumping and sediment gravity flows and promote progradation of the slope (Anderson, 1999). The outward growth of the slope during successive glaciations produces large foresets of downslope-thickening wedges of sediment clearly evident on deep seismic records.
Using Glacial Lithofacies to Unravel Pre-Quaternary Glacial Climates Study of modern Quaternary glacial facies has been instrumental in understanding ancient cold climates in Earth history. The oldest sedimentary rock so far known is of 3.8 Ga and provides the first record of running water. In the sedimentary record of ancient cold climates, there are seven episodes of extended glaciation (glacioeras). There are two Precambrian glacioeras (the late Archean–early Proterozoic ca. 2.8–2.3 Ga and the Neoproterozoic ca. 750–600 Ma). These are followed by the three Phanerozoic glacioeras of the end Ordovician (ca. 440 Ma), the late Paleozoic (the longest: ca. 350–250 Ma), and the late Cenozoic glacioera, which is the best documented (but where major uncertainties still remain as to causal mechanisms). This last glacioera was the culmination of global cooling after 55 Ma and commences with the growth of the Antarctic Ice Sheet at about 40 Ma. Large continental ice sheets only appear much later in the Northern Hemisphere (mostly after 3 Ma). Their extent was controlled by Milankovitch astronomical variables creating Pleistocene glaciations and interglacials. Mountain glaciers probably existed for much of Earth’s history but left no record, and the length of apparent nonglacial intervals without significant ice covers (such as the midProterozoic) awaits explanation. The removal of water from the oceans during Ordovician glaciations is considered by some to be the main driver behind extinction events in marine biota (e.g., Brenchley et al., 2003). The pre-Pleistocene glacial record is fertile ground for research because much of it remains to be investigated using detailed facies analysis. In general, glacial terrestrial depositional models are of limited use in examining this record because glacially influenced marine facies dominate Earth’s ancient (pre-Pleistocene) glacial record (e.g., Allen et al., 2004; Eyles, 1993). This simply reflects the selective preservation of offshore basinal facies. The origins of ancient glaciations result from the interplay of tectonics creating high ground (tectonotopography), principally by rifting of large plates, the changing paleogeography and oceanography of the planet arising from rifting and plate migrations, variations in the solar flux, astronomical Milankovitch variables, and reductions in atmospheric CO2 created by weathering of sediment. One school of thought argues that Precambrian and Phanerozoic glacioeras were radically different, with older glaciations being of global extent (e.g., Hoffman and Schrag, 2002). This model is the subject of much debate and opposition from those in favor of a uniformitarian approach applying knowledge of modern glacial depositional systems and facies to the reconstruction of ancient conditions (Eyles and Januszczak, 2004).
See also: Varved Lake Sediments. Glacial Landforms, Sediments: Glaciomarine Sediments and Ice-Rafted Debris; Tills. Quaternary Stratigraphy: Morphostratigraphy/Allostratigraphy.
References Allen PA, Leather J, and Brasier MD (2004) The Neoproterozoic Fiq glaciation and its aftermath: Huqf Supergroup of Oman. Basin Research 16: 507–535. Anderson JB (2002) Antarctic Marine Geology, p. 330. Cambridge: Cambridge University Press. Andrews JT (1998) Abrupt changes (Heinrich events) in late Quaternary North Atlantic marine environments: A history and review of data and concepts. Journal of Quaternary Science 13: 3016. Benn DI and Evans DJA (1998) Glaciers and Glaciation, p. 734. London: Arnold. Bennett MR (2003) Ice streams as arteries of an ice sheet: Their mechanics, stability and significance. Earth-Science Reviews 61: 309–359. Boone S and Eyles N (2001) A geotechnical model for the formation of hummocky moraine by till deformation below stagnant ice. Geomorphology 38: 109–124. Boulton GS (1996) Theory of glacial erosion, transport and deposition as a consequence of subglacial sediment deformation. Journal of Glaciology 42: 43–62. Boulton GS, Van der Meer JJ, Hart J, et al. (1996) Moraine emplacement in a deforming bed surge – An example from a marine environment. Quaternary Science Reviews 15: 961–987. Boyce J and Eyles N (2000) Architectural element analysis applied to glacial deposits; anatomy of a till sheet near Toronto, Canada. Geological Society of America Bulletin 112: 98–118. Brenchley PJ, Carden GA, Hints L, et al. (2003) High-resolution isotope stratigraphy of Late Ordovician sequences: constraints on the timing of bio-events and environmental changes associated with mass extinction and glaciation. Geological Society of America Bulletin 115: 89–104. Brennand TA (1994) Macroforms, large landforms and rhythmic sedimentary sequences in subglacial eskers. Sedimentary Geology 91: 9–55. Dowdeswell JA and O’Cofaigh C (eds.) (2002) Glacier-Influenced Sedimentation on High-Latitude Continental Margins, p. 310. London: Geological Society of London. Special Volume. Eyles N (1993) Earth’s glacial record and its tectonic setting. Earth-Science Reviews 35: 1–248. Eyles N, Daniels J, Osterman L, and Januszczak N (2001) Ocean Drilling Program Leg 178 (Antarctic Peninsula): Insights in to the sedimentology of continental shelf topsets and foresets. Marine Geology 178: 135–156. Eyles CH, Eyles N, and Gostin V (1998) Facies and allostratigraphy of high latitude glacially influenced marine strata of the Early Permian southern Sydney Basin, Australia. Sedimentology 45: 121–163. Eyles N, Eyles CH, and Miall AD (1983) Lithofacies types and vertical profile models; an alternative approach to the description and environmental interpretation of glacial diamict sequences. Sedimentology 30: 393–410. Eyles N, Eyles CH, Woodsworth-Lynas C, and Randall T (2005) The sedimentary record of drifting ice (early Wisconsin Sunnybrook deposit) in an ancestral ice-dammed Lake Ontario. Quaternary Research 63: 171–181. Eyles N and Januszczak N (2004) Zipper Rift: Neoproterozoic glaciations and the diachronous break up of Rodinia between 740 and 620 Ma. Earth-Science Reviews 65: 1–73. Eyles N and Vossler S (1992) Ichnology of a glacially-influenced continental shelf and slope: The Late Cenozoic Gulf of Alaska (Yakataga Formation). Palaeogeography, Palaeoecology, Palaeoclimatology 94: 193–221. Ghibaudo A (1992) Subaqueous sediment gravity flow deposits: Practical criteria for their field description and classification. Sedimentology 39: 423–454. Hart JK (1999) Identifying fast ice flow from landform assemblages in the geologic record: A discussion. Annals of Glaciology 28: 59–66. Hoffman PF and Schrag DP (2002) The Snowball Earth hypothesis: Testing the limits of global change. Terra Nova 14: 129–155. Hooke RL and Elverhoi A (1996) Sediment flux from a fjord during glacial periods, Isfjorden, Spitsbergen. Global and Planetary Change 12: 237–249. Hunter LE, Powell RD, and Smith GW (1996) Facies architecture and grounding-line fan processes of morainal banks during the deglaciation of coastal Maine. Geological Society of America Bulletin 108: 1022–1038. Kjaer KH, Sultan L, Kruger J, and Schomacker A (2004) Architecture and sedimentation of outwash fans in front of Mydarsjokull ice cap, Iceland. Sedimentary Geology 172: 139–163.
GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Miall AD (1992) Alluvial models. In: Walker RJ and James NP (eds.) Facies Models: Response to Sea Level Change, pp. 119–142. Canada: Geological Association of Canada. Miall AD (1997) The Geology of Stratigraphic Sequences, p. 437. Berlin: Springer. Powell RD and Domack EW (1995) Glaciomarine environments. In: Menzies J (ed.) Glacial Environments – Processes, Sediments and Landforms, pp. 445–486. Boston: Butterworth.
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Walker RG (1992) Facies, facies models and modern stratigraphic concepts. In: Walker RG and James NP (eds.) Facies Models: Response to Sea level Change, pp. 1–14. Canada: Geological Association of Canada. Woodworth-Lynas CMT and Dowdeswell JA (1994) Soft sediment striated surfaces and massive diamicton facies produced by floating ice. In: Deynoux M, Miller JMG, Domack EW, Eyles N, Fairchild IJ, and Young GM (eds.) Earth’s Glacial Record, pp. 241–259. Cambridge: Cambridge University Press.
The Meaning of Facies The term glaciogenic refers to any sediment or landform that results from glacial action. In the main, glaciogenic sediment is produced directly by erosion and comminution at the base of glaciers and ice sheets and redistributed by other processes. The word facies means ‘appearance of’ (hence ‘face’). Lithofacies are simply different types of clastic or chemical sediments (and their lithified rock equivalents) produced by gravity, water, ice, or wind in sedimentary environments. Sediments are readily classified informally into facies based simply on color, texture, lithology of clastic particles, internal structure, thickness and geometry, presence or absence of body and trace fossils, and any postdepositional structures (e.g., reddish, fossiliferous, pebbly quartz-rich sandstone with burrows). There are several formal classifications of sedimentary lithofacies (Eyles et al., 1983; Ghibaudo, 1992; Walker, 1992). A widely used practice is to study lithofacies accumulating in the present day and use these as keys to interpret ancient deposits and their formative processes. Unfortunately, few sedimentary environments are typified by a single diagnostic lithofacies because most environments are spatially complex, involving the interaction of many different processes. An exception is the environment where large storm waves create distinct hummocky bedforms in shallow water (producing a distinctive hummocky and swaley cross-stratified sand facies). The latter is referred to as a so-called universal facies type because its origin is unique to a particular setting (i.e., wave-dominated shallow water). Another example is that of a graded turbidite bed deposited by turbidity currents flowing downslope under water. So-called common facies types, on the other hand, are produced in many environments (eolian, deep marine, fluvial, etc.) and are not uniquely diagnostic of any one depositional setting (e.g., rippled cross-laminated sands).
The Complex Glacial Environment Consider the wide range of environments found around glaciers (Figures 1(a), 1(b), and 2). Meltwater emerges from under the ice as large rivers that are often ponded in front of (or even under) glaciers to form lakes in which large fans and deltas are built. Seasonal and long-term variations in the position of the ice front give rise to complex diachronous (time-transgressive) patterns of sedimentation and partial destruction, and reworking of glacial facies either by melt streams or bulldozing by the ice front. In the absence of vegetation and the presence of strong downglacier (katabatic) winds, eolian transport and deposition of fine sediment are important. Steep slopes and melting ice result in mass movement processes. In marine environments where glacial sediment was deposited under water, crustal rebound may lift sedimentary successions either above water or into energetic shallow waters where they will be reworked and redeposited.
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Typical glaciogenic sediment is poorly sorted and uniquely fingerprinted by the presence of striated and glacially shaped pebbles or boulders. The greatest yields of such sediment (mostly in the form of silt, so-called rock flour) are from temperate wet-based valley glaciers that are able to slide rapidly across their relatively steep beds (e.g., Hooke and Elverhoi, 1996). These glaciers are capable of eroding their beds to form deep basins often enclosed by rock bars. This creates a typical up-and-down and locally overdeepened long profile (thalweg) in glaciated valleys; glaciated basins often exhibit parts that have been eroded well below sea level (e.g., Great Lake basins in North America). The lower seaward end of glaciated valleys forms narrow, steep-walled, and very deep troughs or fjords. Glaciers in areas of permafrost (dry-based polar glaciers) are frozen to their beds and move slowly by internal creep and so do not generate a thick depositional record. Ice streams form where parts of polar ice sheets can slip on wet sediment (Bennett, 2003). Subpolar glaciers share characteristics of both wet- and dry-based glaciers. While useful in terms of understanding modern glacial processes and deposits, a thermal regime cannot easily be inferred from ancient glacial lithofacies. A polar thermal regime is transitory; at the end of any one glaciation, dry-based glaciers eventually warm and melt. Polar conditions predominate today in Antarctica but ancient deposits preserved along the continental margin record sediment delivered by wet-based glaciers (e.g., Eyles et al., 2001).
Diamict Versus Till Diamict (diamictite when a rock) is a nongenetic umbrella term for any poorly sorted deposit regardless of depositional environment (Figures 1(c), 1(d), and 3(a)). Man-made concrete is a diamictite. Sliding glaciers produce a wide range of diamicts that contain admixtures of most textural classes ranging from clay to large boulders. These diamict facies are specifically identified as till because they were deposited directly by ice. A till (tillite: rock) is a specific genetic term for a diamict deposited directly from ice either terrestrially on land or in the glaciomarine realm. Diamicts form in a wide range of glacial and nonglacial environments such as on the slopes of volcanoes, under water, or on land where debris moves downslope as debris flows or in addition as a consequence of meteorite impact (Figure 3(a) and 3(b)). They are said to be a common facies (not universal facies; see above) because they have no unique environmental interpretation. They look very similar regardless of their origin. It is by the study of other facies with which they are conformably interbedded that valuable information regarding depositional setting is gained. Walther’s law states that facies found in a continuous vertical sequence must have been spatially related at the time. If, on the other hand, facies are separated
GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
(a)
(b)
(c)
(d)
(e)
(f)
19
Figure 1 (a) Many processes in areas of wet-based glaciation are dominated not by ‘glacial’ processes per se but by the activity of meltwater. Here is an extensive braided outwash plain in southern Alaska where meltwaters leaving the ice front have reworked almost all primary glacial sediment and landforms. (b) A calving tidewater ice margin (see Figure 9). (c) Diamict deposited by debris flow processes. (d) Subglacially deposited deformation till (Figures 4 and 6). (e) Diamict deposited underwater by downslope flow; note systematic coarsening of pebble sizes upward (known as inverse grading developed where debris flows become transitional to turbidites; Figure 3(b)). (f) Classic varves composed of couplets of summer deposited silt (white laminae) and dark winter mud. Numerous white-colored granules of silt (silt clasts) were brought in during summer underflows (Figure 7).
by disconformities or unconformity surfaces, then the facies above or below a diamict can provide little information. The application of Walther’s law yields some indication of the overall depositional environment (whether it was deposited in deep or shallow water; or on a slope; or near a volcano or an impact crater; or onland, in a terrestrial setting, under a glacier perhaps?). A diamict conformably interbedded within undisturbed turbidites cannot be a till but is more reasonably interpreted as a debris flow since these are genetically related to turbidity currents. This sort of contextual analysis greatly constrains the possible origin of the diamict(ite).
Conceptually, till is deposited in three ways. Till accumulates by the passive meltout of englacial debris under stagnant dead ice (meltout till), by meltout of debris from flowing ice (akin to smearing peanut butter on toast: lodgement till) or by subglacial mixing of preexisting sediment (akin to smearing peanut butter on soft bread: deformation till; Figure 4). Most tills found in mid-latitude areas affected by Pleistocene ice sheets are probably deformation tills; subglacial shearing and mixing of preexisting sediment is the most efficient method of making, transporting, and depositing large quantities of till. Deformation till is commonly massive, rests on deformed
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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Glaciated valley
Glaciolacustrine Massive sands and associated deltaic facies Turbidites
(b)
Braided fluvial, glaciolacustrine Debris flow
(f) Sh
Debris flow Turbidites with dropstones Debris flow
c
s
(i) Slu
c
s
s
Mo
rain a
mp
Sl op e
“Rainout” and debris flow diamictites s g
(a)
(e)
Braided fluvial Gravel debris flow Till
Ice
(d)
elf
l ba
s
(g)
Ice
(c)
be
(h)
nks
rg
sco
urs
g
(b)
Subglacial
Glaciofluvial
Supraglacial (j)
Sandy channel system
Cross-cuting till sheets
Chaotic slumped facies
Color legend
X-stratified sands
Massive gravels
Boulder pavement Till
(c)
c
s
s
g
Glacitectonized bedrock/sediment
Debris flow c s
(d)
Fiord
Subglacial till
Superimposed gravel bars
Glacitectonized bedrock/sediment s g
Submarine channel
(e)
Debris flow
c
s
s
c
Shelf Mud
Large slump blocks (olistostromes) Debris flow Turbidites
(f)
c
s
s g
Chaotic, dumped facies Scoured bedrock surface
(g)
Channelized massive-graded gravels
Debris flow
Bioturbated muds c s
s g
(h)
c s
s
Base of slope
Turbidites
Channelized massive, graded sands
s
Diamict/ till (glacial) Bedrock g
g
Slope
Turbidites
Turbidites
Mud Sands Gravels
Thick, rainout, diamictites and debris flows with sediments rafts s g
(i)
c
s
Iceberg scour Glacially-shaped and striated clasts Thick “rainout” and resedimented diamicts Coquinas Turbidites Boulder pavement s g
Distal turbidites Laminated muds with rare dropstones Contourites
Massive muds with dropstone horizons
(j)
c s s g
Figure 2 Principal glacial depositional systems associated with large ice sheets that reach sea level. Quaternary glaciations have left a very prominent glacial record on land but most sediment (perhaps as much as 90%) is deposited offshore on continental slopes. Such deposits dominate the record of older glaciations in Earth history because the terrestrial record is easily eroded.
substrate sediments, and usually includes rafts of undigested substrate sediment in its basal parts recording incomplete mixing. Some are held to exhibit lamination and banding similar to gneissic textures in high-grade metamorphic rocks subject to strong shearing. The same facies are produced by debris flows under water, however requiring care in interpretation and consideration of the wider depositional context. At modern glaciers flowing over stiff rock, lodgement till is more important. Meltout will be of local significance where evidence indicates that ice has stagnated and melted in situ. There have been several techniques confidently proposed in the past as a Holy Grail for identifying subvarieties of till. Such methods center on samples examined in the laboratory and include geochemical and grain size characteristics, small-scale structures seen in thin sections, microsurface textures of quartz sand grains, or the orientation of clasts (clast fabric analysis). All of them ultimately proved simplistic because of overlapping properties. In reality, till is deposited by interactions of all the three processes identified above (indeed, processes may change through time at any one site) but one process will be dominant. Volumetrically, most till is
produced by subglacial deformation of other sediments as ice sheets grow and expand outward and cannibalize their own outwash deposits.
Glacial Depositional Systems: Facies Models and Facies Associations Geologists recognize a wide range of different environments around and under the margins of glaciers, each one creating a distinct depositional system. These systems do not have sharp boundaries but are intimately linked as a consequence of sediment moving from one system to another (Walker, 1992). The system forms a characteristic deposit with a distinct morphology (e.g., a deltaic depositional system) and several recurring systems occur in glacial environments (Figure 2). Terms such as landsystem and landform/sediment association have also been used. One advantage of this approach is that in areas of modern-day and Pleistocene glaciation, it is possible to predict the types of facies found in the subsurface by simply identifying landform types typical of different systems.
GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Processes
A. Volcanic
Characteristics
Deposits
Rock fall Creep
Olistostrome avalanche deposit
Slide
Creep deposit
B. Rock falls (olistostromes)
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Slide
Slump
Slump Debris flow
D. Fault gouge
Density-modified grain flow Fluidized flow Liquefied flow Hyperconcentrated flow
Slump
Debrite
Turbidite (coarse, medium, fine grained)
Coarse-grained Medium-grained Fine-grained turbidite turbidite turbidite (classical) (Bouma)
Deposit
C. Mixing of coarse and fine grained sediment downslope (mixtites)
Turbidity flow
Debrite
E. Regolith
E D
Facies
F. Meteorite impact breccias
C B
Sediment support
A
(a)
Coherent slumping
Cohesive matrix strength and fluid buoyancy
Turbulence high concentration low-concentration
(b)
Figure 3 (a) Diamicts form in a wide range of environments. Identifying diamicts deposited directly by glacier ice (till; e.g., Figure 6) requires analysis of associated facies. (b) Gravity-driven processes and their deposits.
In pre-Pleistocene rocks, however, this cannot be done, and interpretation is reliant on consideration of facies associations. Facies associations are defined as distinct vertical successions of genetically related lithofacies (Miall, 1997; Walker, 1992). As emphasized above, it is by recognizing these packages (rather than any one unique facies type) that ancient glacial settings can be recognized. A three-dimensional (3D) block diagram called a facies model is a useful tool for representing depositional systems and their constituent facies associations. These models provide a mental image that can embrace different data derived from studies of outcrops, subsurface drill core, or geophysical data (such as 3D seismic or ground-penetrating radar), and the study of processes and deposits in a controlled laboratory experiment. The model will vary from one particular case to another (such as tectonic setting), but it is a useful tool for distilling large amounts of data, for predicting subsurface conditions and for comparison with stratigraphic successions of unknown origin. Drawing a facies model forces consideration of the 3D distribution of facies. For each depositional system (and thus facies model), a representative vertical profile can be made showing typical facies and the sedimentary evolution of the system through time (Figure 5). These profiles suffer from the obvious limitation of depicting 4D conditions in two dimensions, but the detailed assessment of facies in measured outcrops or drill core (called facies logging) is the essential starting point in any analysis. Simple lithofacies codes are available to aid labeling and identification and serve as a reminder of what to look for
in the field. Two-dimensional panels (say of cliff exposures) are very useful in showing lateral variability and 2D geometry of facies (such as those found in channels) and, increasingly, there is a move to consider the 3D architecture of deposits. Much progress has been made in this respect in fluvial, eolian, and deep marine environments (prompted by the search for hydrocarbons and reservoir analysis by petroleum geologists, and aided by geophysical techniques such as 3D seismic). The need to model the movement of groundwater through glacial sediments is a major stimulus (Boyce and Eyles, 2000).
Terrestrial Glacial Depositional Systems Subglacial Depositional System The terrestrial subglacial depositional system is composed of sediments deposited under the ice, and is typically dominated by sheet-like deposits of deformation till (Figure 6). These tills rest on eroded and deformed remnants of older preexisting sediment. These were glaciotectonized when ice advanced over the sediments and they were remobilized as till. A lowrelief landform called a till plain (ground moraine of the older literature) is created. The most distinctive feature of a till plain are bedforms called flutes (long and thin) and drumlins (long and wider) elongated in the direction of ice flow. The origin of these is still contentious but they are increasingly being recognized as a product of either erosion of preexisting sediment (e.g., where they show cores of nontill sediment) or deformation and remobilization of deformation till (Benn and
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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Supraglacial melt
Moulins Englacial melt
2
1B 1A 1B 2 3 4 5 6
Direct loss of water
Ice flow
5
3
No basal sliding; internal deformation only Sliding on basal meltwater film and areal abrasion Selective erosion of bed by pressurized meltwater and slurries; subglacial lakes Conduit flow of meltwater over impermeable crystalline bedrock (eskers) Subglacial recharge of meltwaters into aquifers Overpressured sediment undergoing pervasive shear (deformation till/drumlins/no eskers) “Excessive” meltwater that cannot be discharged as (4) or (5) flow in channels cut into bed and leaves ice sheet as melt streams (7)
Unconsolidated sediment
6 4 Crystaline bedrock
Frozen bed movement versus wet-based temperate movement
1
7
Sedimentary rock
Low profile margin of ice sheet resting on soft sediment
A - Polar dry based (no meltwater)
Total surface movement T1
T0 5
Sediment Internal deformation deformation
Frozen Glacier bed Internal deformation
Ice Meltwater channel
B - Temparate wet based
Deformation till
Sliding Glacier bed Basal slip
Preexisting sediments
Figure 4 Summary diagram of processes operating below ice of contrasting thermal regime. The generation and movement of till as a deforming layer (5) occurs below the outer margins of a wet-based ice sheet resting on sediment. Most modern glaciers are sliding on rock and are unrepresentative of Quaternary ice sheets.
Evans, 1998; Boulton, 1996; Hart, 1999). Fundamentally, a streamlined till plain is the ‘soft bed’ of the former glacier, in effect a giant slickenside surface such as along a fault plane veneered by ground-up rock (gouge). End moraines are ridges that form perpendicular to ice-flow direction and mark pauses in ice retreat, creating a ribbed appearance to the till plain. Moraines are built either by the extrusion of soft deforming till from under the ice margin or by bulldozing of sediment by a
readvancing ice front. The gravelly deposits of subglacial rivers are left as sinuous esker ridges (Brennand, 1994). Till plains may be overlain by extensive spreads of gravelly outwash facies (e.g., Kjaer et al., 2004) and silty glaciolacustrine deposits left from ephemeral lakes ponded against the retreating ice margin. Glacial outwash deposits are dominated by poorly sorted, crudely bedded, sheet-like gravel facies deposited in shallow braided rivers. There is an absence of
GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Facies association
Pebbly beach
IRD
Depositional energy High
Low Sandstone facies St Trough cross-stratified Scs Swaley cross-stratified Hcs Hummocky cross-stratified Sh Horizontally laminated Sr Rippled Sg Graded Sm Massive Bioturbated Sb
60
St
Gr FA 3
Gr
CL,Q Sb 56
Conglomerate facies
Q,CL
?
W
?
Sb
PBiii Gr 52
FA 5
FI,Sr St
FA 5
48
FI
Sr W FA 3
W 44
FA 5
?
Sb CL
Gr
Gs Gg Gr B
Fl Fm Fb
Laminated or thin-bedded Massive Bioturbated
CL Q V
Clast layer Coquina Volcanic ash Clasts
FL
Trough cross-stratification
St FA 2
St
Hcs
Swaley/hummocky cross-stratification
Sb
Bioturbation
40
Q,CL
FA 3
Stratified Graded Rippled Breccia
Fine-grained facies
Sr FA 2
23
Fossils
? w
Coalified logs/wood
Sb,Fb
Deformation FA 5
36
C
FI HCS
FA 3
C
PBii
Bounding discontinuity Erosion surface
St FA 5
Paleocurrent direction
FI,Sr
32
fl
r
,fl
fl,s
s
st
sc
sc s
ds Mu
Sa
nd
ton e sto ne Me Fin Co d e ng Co ium lom ars era e te
FI
Facies
Figure 5 Typical vertical profile showing facies types, paleocurrents (small circles with arrows), and amount of ice-rafted debris (IRD) in a glacially influenced shelf succession (Eyles et al., 1998). Vertical scale is in meters.
large-scale cross-stratified facies because of a lack of deep channels and an abundance of coarse debris (Miall, 1992). Glacial lake facies include distinctive rhythmically laminated silts and clays that record seasonal patterns of sedimentation (varves).
Varved facies dominate glacier-fed lakes (Figure 7). Varve thickness typically decreases upward in measured profiles recording ice retreat. So-called proximal varves may be many meters thick and composed of ripple cross-laminated sands.
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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
Esker
X
Push ridge
Ice
Drumlin
Vertical joint sets oriented subparallel to ice flow direction Erosive base to diamict units
Drumlin
Basal debris dropped into subglacial channel Boulder pavement Shear surfaces Sediment raft
Channel fill
Outwash stream
Drumlins
Esker
Variable sediments in channel fill
X
Sheared out bedrock (‘smudges’) Shaped oriented clast Bedrock rafts
X
Boulder pavement
Channel fill
1m
Glaciotectonized bedrock/sediment X
Figure 6 Facies model for subglacial depositional system (Figure 2) involving deposition of successive sheet-like units of deformation till.
Till-like sediments may form in ice-contact glacial lakes where icebergs drop coarse sediment into bottom muds (rainout diamict). These are discriminated from tills proper by consideration of their geometry (typically blanket-like, resting conformably on underlying deposits) and the associated glaciolacustrine facies with which they are conformably interbedded. Local deformation and incision result from the scouring of the lake floor by drifting ice masses such as icebergs (Eyles et al., 2005).
Supraglacial Depositional System The subglacial depositional system is often partially covered by sediments that were transported on the ice surface (Figure 8). These supraglacial sediments were lowered onto the underlying till plain as the underlying glacier melted. A distinctive hummocky topography marked by craters (kettle holes) forms when buried ice melts and overlying sediment collapses. This topography is common at the margins of valley glaciers that transport substantial quantities of rubbly rock fall debris derived from adjacent rock walls. Bouldery supraglacial debris can blanket the immediate ice margin, insulating it from melt and thereby creating a belt of hummocky ice-cored sediments underlain by subglacial sediments and landforms. This is the hallmark of the glaciated valley depositional system. In addition, large volumes of sediment accumulate between the glacier and the valley sidewalls. With ice retreat, lateral moraine ridges left perched on the valley sides are quickly destroyed by mass wasting. Mass flow and meltwater processes rapidly convert the glacial fill of a confined valley into thick accumulations of outwash gravel and sand. The term paraglacial has been used in reference to a
short-lived phase of intense reworking and sediment transport by rivers immediately after deglaciation. There is still much uncertainty regarding the origin of the hummocky terrain left behind in flat mid-continent regions by Pleistocene ice sheets. This distinctive topography extends across huge tracts of the glaciated portion of the mid-continent North American plains underlain by soft Mesozoic rocks. This terrain was initially regarded as a supraglacial deposit akin to that left by valley glaciers, but the process of how thick sediment could accumulate on top of the ice sheet was never clear. The composition of hummocky moraine offered clues; it is composed of the same clay-rich till found in nearby drumlins into which it passes. Indeed, transitional landforms can be recognized where the formerly flat till plain was pressed into hummocky streamlined shapes (humdrums). It is now recognized as the product of the pressing of a soft till substrate, under portions of the ice margin that stagnated (Boone and Eyles, 2001). Here, the extensive development of a soft bed in mid-continent reflects the presence of weak, fine-grained rock. Such topography may identify relatively faster flowing ice streams within the ice sheet (Hart, 1999) moving across a soft bed of overpressured shales. Large rafts of glaciotectonized soft rock were moved below the ice.
Marine Glacial Depositional Systems The term glaciomarine identifies processes or deposits that occur within a narrow zone in direct contact with the margin of a glacier (or ice sheet) that reaches sea level. The term
GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
25
Glacial lakes Icebergs Ice
Overflows
Slumping
Interflow Underflows
Subglacial facies
Diamicts Laminated silts and clays
1. Ice contact (a)
Ice
Meltstreams
Seasonal ice Overflows
Subglacial facies Braided river Rippled facies sands 2. Nonice-contact
Interflows Underflows
Mud
Rhythmically laminated silts and clays
(b)
‘Classical varves’ 1 Winter
2
3
Summer
Clay
Draped lamination
Winter
Draped lamination and climbing ripples
I
II
III
IV
V
Winter
Climbing ripples Clay
Suspension Grain flow Turbidity current
I
Fan delta
II
1
III IV 2
Bed
rock 3
a on
er
ov
ec
l ic
V
as
Se
(c)
Figure 7 Facies models for glaciolacustrine depositional system (Figure 2) with (a) ice-contact and (b) non-ice-contact types. Typically, the deposits of non-ice-contact lakes are strongly influenced by seasonal variations in the volume of meltwater entering the basin, reflected in varved sedimentation. (c) Slumping on oversteepened delta fronts (I–V) in winter may complicate this.
ice-contact marine or proglacial marine are synonyms for marine settings where glacial influences predominate. Outside this zone and away from direct contact with the ice margin is the glacially influenced marine environment where marine
processes, not glacial, dominate. In areas affected by Quaternary glaciations, glacioisostatic depression and subsequent postglacial uplift of coastal margins have left widespread marine glacial deposits exposed above sea level. Knowledge of
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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
C S S G Sediment gravity flows moving into troughs occupied by meltstreams or lakes
1 3 1 1m
1
3 ICE
Ice core Deformed sediments
1
3
Trough filling
3
1
1
(a)
3 2
3
Final hummock form 2
4 5
5m 4 3
1
(c)
2 4 (b) Figure 8 (a) Facies model for supraglacial deposition where ice surface is covered by debris. Melt of buried ice results in widespread slumping and flow of sediment as it is lowered onto subglacial sediments below. Final ice melt produces a resulting in a chaotic hummocky terrain. (b and c) Typical hummock stratigraphy showing: (1) debris flow diamict with rafts of other sediments, (2) diamict melted out from dead ice (meltout till), (3) outwash gravels that accumulated in troughs, (4) glaciotectonically deformed subglacial sediment or rock. Faults and slump structures are common throughout.
marine glacial depositional systems is also key to interpreting the pre-Pleistocene record of glaciation given selective preservation of offshore basinal facies (see below).
Glaciomarine Depositional System The glaciomarine depositional system occurs where an ice margin terminates in water (Figure 9). When advancing, ice will commonly bulldoze an underwater ridge (morainal bank) in front. This deposit is subsequently overridden to produce thrust sediment and deformation tills (Boulton et al., 1996). A retreating glacier will leave morainal banks on basement highs where ice could ground and stabilize temporarily (e.g., Powell and Domack, 1995). This setting is highly dynamic because the ice margin is partially floating and thus unstable, losing mass by calving icebergs. This environment is dominated by strong jets of meltwaters issuing below water at the ice front, delivering coarse gravel and sand facies deposited on subaqueous meltwater fans (e.g., Hunter et al., 1996). Deposition by turbidity currents results in a wide variety of massive and graded gravel and sand facies found in multistorey crosscutting channels. Plumes of suspended sediment are also released. These produce blankets of massive mud where mud rains out in quiet water areas; pebbly muds form where
ice-rafted debris is dropped to the seafloor. Tides interact with plumes to produce rhythmically laminated mud facies; seasonal varved deposits may also occur. Ice proximal submarine outwash facies rest unconformably on lowermost subglacially deposited and glaciotectonically deformed facies such as till. In general, glaciomarine deposits deposited along ice margins in water have a low preservation potential. These accumulations are usually destroyed by shallow marine erosion as a result of glacioisostatic uplift, by iceberg scour, and by downslope mass wasting. Typically, shallow water facies (beach gravels) truncate underlying glaciomarine facies.
Glacially Influenced Marine Depositional System ‘Glacially influenced’ is an umbrella term for deposits accumulating in the more remote parts of a glaciated basin beyond the direct reach of glaciers (i.e., distal to the glaciomarine realm; Figure 2). The depositional system is fed by glaciclastic sediment that has been extensively reworked by currents or gravity. Areas affected by glacioisostatic movements of the crust created by the weight of the ice sheet, or glacioeustatic sealevel changes (or floating icebergs), are also said to be glacially influenced. The setting necessarily encompasses a much
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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
8 7 2 5
4 5 2
6
3
6 1
Figure 9 Facies model for glaciomarine depositional system near a tidewater glacier margin (see Figure 1(b)). (1) Glaciotectonized marine sediments or bedrock, (2) till, (3) stratified accumulation of diamict deposited by slumping of till and other debris, (4) mud with ice-rafted debris, (5) channeled gravel and sand of submarine fan, (6) slumps and sediment gravity flow facies (Figure 3(b)), (7) iceberg scour and plume of suspended sediment, (8) push ridge. The entire sediment package commonly takes the form of large ridges (morainal banks) reflecting short-lived pauses in the ice margin as it retreats.
broader range of environments across continental shelves and slopes (e.g., Anderson, 2002; Eyles et al., 1998). Here, a glacial influence is expressed in the form of rapid changes in water depths brought about by glacioeustasy (the removal of water from the oceans to build ice sheets) and glacioisostasy (the unloading and loading of the lithosphere by removing and returning water from the oceans during glaciations, or direct loading by ice sheets). These two processes act together to create complex spatial and temporal variations in water depths and depositional conditions in different areas of the same sedimentary basin. Detailed examination of glacial facies and disconformities in vertical successions will provide data as to whether water depths were increasing or decreasing (Miall, 1997). It is dangerous however to interpret these as necessarily reflecting changes in sea level per se; sea level may have remained unchanged with the basin floor itself responding to different loads. For this reason, changes in water depths evident from the study of stratigraphic successions are best described as relative sea-level changes, not in absolute (eustatic) sea level. Floating ice masses and the introduction of coarse ice-rafted debris, and ‘rainout’ of mud from plumes of suspended sediment are other significant distal glacial influences on otherwise normal marine sedimentation processes. Several times during the last 100 ka, huge armadas of icebergs were released from rapidly calving margins of Northern Hemisphere ice sheets and are recorded as distinct ice-rafted horizons in mud (Andrews, 1998); coarser layers of ice-rafted facies are known from the ancient record (Eyles et al., 1998). Blankets of stony mud
subject to repeated winnowing and scour by iceberg keels (ice keel turbation) cover the mid and outer parts of glacially influenced shelves below storm wave base (WoodworthLynas and Dowdeswell, 1994). These substrates host a wide range of infaunal and epifaunal organisms reflected in diverse ichnofacies types (Eyles and Vossler, 1992). Deep-water muds pass shoreward into inner shelf areas, and interfinger as fairweather deposits within thick storm-influenced and shoreface sands. Most glacially influenced continental shelves are not major repositories of glacially influenced marine facies. Water depths are too shallow, so their floors are current-swept and most shelves are undergoing uplift or only slow subsidence. This results in the inability to create space for (accommodate) substantial thicknesses of sediment. Large areas were exposed at times of glacioeustatic sea-level lows and scoured by ice sheets or large melt rivers. Typically, shelves are characterized by condensed successions where deposits of only the last glaciation are found resting on major erosion surfaces. Some glacially influenced continental margins (e.g., Antarctica) are slowly subsiding. These show a well-defined subsurface structure (evident on seismic records) of horizontal ‘topsets’ composed of relict shelf sediments that have aggraded vertically through time and which may include subglacial sediments (Anderson, 2002; Dowdeswell and O’Cofaigh, 2002). Along glaciated continental margins, the bulk of glacial sediment accumulates in deep-water basins on the continental slope or at its base. These experience enhanced sediment
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GLACIAL LANDFORMS, SEDIMENTS | Glaciogenic Lithofacies
supply at times of lowered sea level when rivers or glaciers discharge sediment directly to the shelf edge (e.g., Hooke and Elverhoi, 1996). Sediments pushed across the shelf under glaciers are transported downslope at the shelf edge by slumping and sediment gravity flows and promote progradation of the slope (Anderson, 1999). The outward growth of the slope during successive glaciations produces large foresets of downslope-thickening wedges of sediment clearly evident on deep seismic records.
Using Glacial Lithofacies to Unravel Pre-Quaternary Glacial Climates Study of modern Quaternary glacial facies has been instrumental in understanding ancient cold climates in Earth history. The oldest sedimentary rock so far known is of 3.8 Ga and provides the first record of running water. In the sedimentary record of ancient cold climates, there are seven episodes of extended glaciation (glacioeras). There are two Precambrian glacioeras (the late Archean–early Proterozoic ca. 2.8–2.3 Ga and the Neoproterozoic ca. 750–600 Ma). These are followed by the three Phanerozoic glacioeras of the end Ordovician (ca. 440 Ma), the late Paleozoic (the longest: ca. 350–250 Ma), and the late Cenozoic glacioera, which is the best documented (but where major uncertainties still remain as to causal mechanisms). This last glacioera was the culmination of global cooling after 55 Ma and commences with the growth of the Antarctic Ice Sheet at about 40 Ma. Large continental ice sheets only appear much later in the Northern Hemisphere (mostly after 3 Ma). Their extent was controlled by Milankovitch astronomical variables creating Pleistocene glaciations and interglacials. Mountain glaciers probably existed for much of Earth’s history but left no record, and the length of apparent nonglacial intervals without significant ice covers (such as the midProterozoic) awaits explanation. The removal of water from the oceans during Ordovician glaciations is considered by some to be the main driver behind extinction events in marine biota (e.g., Brenchley et al., 2003). The pre-Pleistocene glacial record is fertile ground for research because much of it remains to be investigated using detailed facies analysis. In general, glacial terrestrial depositional models are of limited use in examining this record because glacially influenced marine facies dominate Earth’s ancient (pre-Pleistocene) glacial record (e.g., Allen et al., 2004; Eyles, 1993). This simply reflects the selective preservation of offshore basinal facies. The origins of ancient glaciations result from the interplay of tectonics creating high ground (tectonotopography), principally by rifting of large plates, the changing paleogeography and oceanography of the planet arising from rifting and plate migrations, variations in the solar flux, astronomical Milankovitch variables, and reductions in atmospheric CO2 created by weathering of sediment. One school of thought argues that Precambrian and Phanerozoic glacioeras were radically different, with older glaciations being of global extent (e.g., Hoffman and Schrag, 2002). This model is the subject of much debate and opposition from those in favor of a uniformitarian approach applying knowledge of modern glacial depositional systems and facies to the reconstruction of ancient conditions (Eyles and Januszczak, 2004).
See also: Varved Lake Sediments. Glacial Landforms, Sediments: Glaciomarine Sediments and Ice-Rafted Debris; Tills. Quaternary Stratigraphy: Morphostratigraphy/Allostratigraphy.
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Walker RG (1992) Facies, facies models and modern stratigraphic concepts. In: Walker RG and James NP (eds.) Facies Models: Response to Sea level Change, pp. 1–14. Canada: Geological Association of Canada. Woodworth-Lynas CMT and Dowdeswell JA (1994) Soft sediment striated surfaces and massive diamicton facies produced by floating ice. In: Deynoux M, Miller JMG, Domack EW, Eyles N, Fairchild IJ, and Young GM (eds.) Earth’s Glacial Record, pp. 241–259. Cambridge: Cambridge University Press.