DEPOSITIONAL PROCESSES
Depositional processes of a glacier system is a large and broad topic.
Deposition within the glacier system can occur in numerous ways. Rock fall
can deposit boulders or sediment on top of the glacier, which then are
transported and redeposited in a different glacial environment. Deposition
can also occur in a proglacial environment through the process of glaciofluvial
actions, or even marine environments. The main focus of our study is depositional
processes within the glacier margin, or more specifically within the subglacial
system. Deposition in this subglacial environment is strongly tied to erosion
and the theory of a deformable
bed. The deformable bed issue is covered in the Glacier Systems page
and the basic ideas controlling a deformable bed are not covered in this
page, however we will talk about deposition and to some extent erosion
in the deforming bed.
Deposition in the Deformable Bed
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Figure1. Modified from Boulton 1996.
Ice flow in this figure is from right to left. D.L.is the deforming
layer in the sediment.U.D.L. is the undeformable layer in the sediment.
Deposition from a deforming bed layer will occur if the driving stresses
from glacial movement fall, or the shear strength of the deforming layer
increases so that the driving stresses are no longer high enough to sustain
deformation. An important factor in controlling shear strength is a change
in pore water pressure. If pore water pressure drops, the effective
normal pressure pushing the sediment grains together increases,
this results in the frictional strength of the sediment to rise (Benn
and Evans 1998). Basically shear deformation occurs when sub glacial
drainage is so poor that high pore water pressures exist, and therefore
low effective normal pressure develops in the sediment beneath the glacier
sole (Boulton
1996). Deposition within the subglacial deforming layer occurs
from the base up, this is because frictional strength within the sediment
increases with depth. Essentially, the deepest sediments come to rest (are
deposited), first when shear stresses fall or when the frictional strength
of the sediment rises (Benn
and Evans 1998). Deformation till and glacitectonite are common
till products of this process.
Two zones are "recognized" to exist within the deformable bed. One zone
is the layer that is being actively deformed. This is the layer in which
shear deformation tends to cause dilation
of the grain skeleton producing a much lower density than the underlying
consolidated, undeformed layer. The interface between these two layers
is assumed by Boulton
1996, to form a rigid surface above which sediment is being actively
deformed and acts as a viscous fluid. Erosion in this environment is said
to occur when the material form the consolidated layer is added to the
deforming layer, thus lowering the rigid interface between the two layers.
Deposition occurs under the opposite situation; when material from the
unconsolidated, deforming layer is added to the consolidated, undeforming
layer (Boulton
1996).
The role of sediment grain size plays an important role in the processes
of the deforming bed. The role of sediment was modeled by Geoffrey Boulton
in 1996 in a Journal of Glaciology article titled the Theory of glacial
erosion, transport, and deposition as a consequence of subglacial sediment
deformation. What is found in the model is that the deforming layer
thickness needs to be the greatest in sandy
till where the strain rate is relatively small, and the deforming
layer is the least thick in clay where the strain
rate for the same stresses is very large. This is due largely because
the effective normal pressure at the top of the sandy till needs to be
very low so that the till is soft enough to deform to relatively great
depths. Also effective
pressures need to be weak in the clay so to offset the large cohesion
of clay. However sand drains relatively well which leads to high effective
pressures which will inhibit deformation unless the sand is saturated.
Clay and silt tend to impede drainage so that low effective pressures can
be maintained (Boulton
1996). Boulton also found that silty sediments are most susceptible
to erosion and that silty tills tend to dominate where there is an appropriate
sediment supply. This finding is backed up by the fact that tills produced
by Pleistocene
ice sheets are predominately silt rich.
Three basic properties of debris release:
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Lodgment, or deposition from the sliding ice induced by frictional processes.
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Melt out, or deposition of debris from the melting of stagnant, or slowly
moving ice.
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Deposition due to gravity within the glacier, at the glaciers bed.
Deposition by Lodgment:
Lodgment till is the glacial debris that has been smeared onto the deformable
bed from the movement of the glacier. This process occurs where the frictional
drag between the bed and debris is more than the shear stress implied by
the moving ice. This stress is then great enough to inhibit further movement
of the till. The lodgment process can occur for small minute particles
or for large areas of debris rich basal ice. One of the most distinctive
properties of lodgment tills are the abundance of asymmetrical stoss and
lee clasts. These formations resemble miniature roches mountinees with
smooth upglacier (stoss) sides and fractured down glacier (lee) sides These
characteristics are formed after clasts have become lodged and continue
to be overridden by active ice (Benn
and Evans 1998).
A couple of situations where Lodgment can occur.
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| Figure 2 |
Figure 3 |
Figure 4 |
Figures 2,3,4 modified from Boulton, 1982
Frictional Retardation see Figure 2
Where the glaciers bed is most rigid, the friction imposed by an overriding
particle and the bed is due mainly to the interlocking of asperities beneath
the particle and the small roughness elements on the bed. The resisting
forces that produce these tills are derived from the friction between these
rigid elements.
Ploughing of the Bed see Figure 3
When the deformable bed consists of erodable materials such as gravel,
sand, or previously laid till, particles in the basal ice can plough up
a trail of debris. When this debris is plowed up and compacted due to the
flow of the glacial ice, a large enough resistance will inhibit further
movement of the debris and thus becomes lodged as till into the deformable
bed.
Lodgment of Debris rich Ice Mass see Figure 4
Debris can also be lodged against other particles that are protruding
from the bed. This process forms clusters of debris that are smeared against
the glaciers bed. Slabs of debris rich ice will lodge against the bed of
the glacier if the frictional drag at the base of the slab exceeds the
strength of the ice overlying the slab (Benn
and Evans 1998). The differences between debris rich, and clean
ice could bring about the evolvement of a decollement
plane directly above the debris rich ice. This subsequent plane
can develop into a new sliding base of the glacier and the ice below this
plane develops into part of the substratum.
Deposition by Melt out:
Deposition by Melt out refers to the deposition of sediment due to
the process of melting slowly moving or stagnant, debris rich ice. This
process can occur both in supraglacial and subglacial environments. In
the supraglacial environment deposition by meltout is a common phenomenon
and can deposit a significant amount of debris. The heat involved in subglacial
process can be derived from either geothermal sources, sensible heat from
incoming melt water, or if the ice is thin enough from heat in the atmosphere.
Heat from these sources is generally quite small which relates to the very
slow, and low amounts of basal melt out. Typical rates for basal melt out
are in the range of .5-1.2mm. per year. Rates may be higher when the ice
is in volcanically active areas.
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Figure 5 modified from Boulton 1982
Thaw Consolidation:
During melt out, debris rich ice undergoes a volume reduction called
thaw consolidation. This action is attributable to ice melt and the subsequent
drainage of the melt water. The amount of this consolidation is a function
of the original debris content of the ice. Thaw consolidation will be great
if debris content is low and consequently the consolidation will be little
if the debris content in the ice is high. The characteristics of released
debris during thaw consolidation also depends on the equilibrium between
the production of melt water and the subsequent drainage of this melt water
from the specific site.
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If the melt water is draining at a speed equal to the rate of its production,
the debris will be deposited with almost no disturbance, other than that
of the resulting thaw consolidation.
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If the melt water cannot drain easily, pore water pressures will rise during
melt out, disintegrating the cohesive strength of the material and increasing
the likelihood of remobilization. This failure resulting in remobilization
of the melt out debris can occur if the glacier bed is sloping any more
than eight degrees.
Melt out till fabrics show a lowering in dip values, and a growth relative
to englacial fabrics. The compaction during the melt out process lowers
the extent of dip values while it also reduces the vertical height of the
fabric. These conditions lower uniformity of the till.
Deposition by gravity:
Basal debris can also be deposited by the influence of gravity into
sub glacial cavities. Obstructions under the overriding ice can lead to
cavities where material can be deposited, or deposition due to gravity
can occur below the possible ice overhangs at the glacial margin. These
accumulations of debris into cavities may eventually recombine into the
glaciers base as a sub glacial deforming layer, or frozen into the basal
ice.
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Figure 6 modified from Boulton,1982
Primary Glaciogenic Till:
Sublimation till:
Sublimation till is defined as the sediment expelled by the sublimation
of glacier ice and is immediately deposited without any transport. This
till is closely related to melt out till except that the ice is lost due
to the process of sublimation. Sublimation is defined as the transition
of a substance in the solid state, to a vapor state without any passage
through the liquid stage. This till if derived from a form of freeze drying
and requires extremely cold and arid conditions. Sublimation till could
be found in such places as Antarctica, or the Arctic where the climate
is cold and arid. The process needed to form this till requires contact
of the atmosphere with the glaciers bed possibly through pore spaces in
the glacial ice.
Due to the large amount of time involved in producing this type of
till, it is extremely volatile and can generate delicate englacial structures
such as augen, laminae bent round clasts, foliation, and attenuated folds.
The till does have strong clast orientations parallel to former ice flow
directions. Since this process takes such a long time to produce the till,
it is extremely loosely packed and liable to collapse due to the slow loss
of its parent ice. Consequently this till has very little possibility of
being preserved for any extended period of time.
Glacitectonite till:
Glacitectonite till is sediment or rock that has been deformed by the
process of sub glacial shearing but maintains some of the structural features
of the parent material. As a response to the glaciers own interior stresses,
the ice has to either deform plastically by bending and folding, or in
extreme cases by brittle fracture. Glacitectonite till is the product of
these actions. These tills have a wide range of source types and can consist
of any rock type. This till is most abundant in glaciers that are frozen
to their bed in the terminal zone, where the glaciers movement cannot be
facilitated by thrusting or internal deformation. Glacitectonite till is
extremely common in present day glaciers.
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Figure 7 drawing of a hypothetical till sequence.
1.sandur 2. melt out till 3. deformation till (from the deformable
bed) 4. Glacitectonite
5.lodgement till 6.fine grained basal till (slurry) 7.dead buried glacial
ice
8.stagnant glacier margin 9.bedrock 10. former ice surface
Tills and Magnetism:
The reconstruction of ice age periods and their resulting extents can
be derived from the deposited till and the resulting magnetism. As in any
other sediment, the particular silt sized grains that make up the till
may be aligned with the earths magnetic field as they were deposited. Some
sediments such as sub aqueous deposits have high degrees of natural remnant
magnetism. Glaciolacustrine and glaciomarine sediments are deposited in
low stress environments and their preference to align themselves with the
earths magnetic field is unrestrained. These magnetic particles in the
sub glacial deposits are most likely to be in alignment parallel to the
orientation of shear, and usually show similar conformity with clast fabric
orientations. This ability of a till to become magnetized has been used
to distinguish between different chronostratigraphic sequences, which are
the resulting sediment facies of these tills. This ability of magnetization
in the till can also determine relative ice flow directions.
Sub glacial Deposition over Time and
Space
During the advance phase of a glacier the processes of erosion and deposition
advance further from the source in a wave like fashion. The glacier erodes
in one zone and deposits in another zone in front of the erosional area.
As the glacier continues to advance the zone of erosion advances as does
the zone of deposition. When the glacier retreats erosion is essentially
reduced to a minimum and deposition is at a maximum stage. Basically the
glacier covers its tracks so to speak, with a blanket of till that covers
not only the erosional surface but also the advance phase till as well
(figure 8 below), (Boulton
1996).
In figure 8 four zones of erosion/deposition can be identified. First,
is an ice divide zone that experiences slight erosion, and may have a thin
layer of till that has been deposited during retreat. Second, is a zone
of strong erosion where the till that is derived is from retreat only.
Third, is a zone of no erosion of pre glacial beds that is overlain by
a thick sequence of advance and retreat phase till, but may have an intervening
erosional surface between the advance and retreat phase till. Fourth, is
a zone of no erosion that is overlain by a thick till layer that was continuously
deposited during the glacial cycle (Boulton
1996).
However we know that more than one advance/ retreat cycle can exist
within a glacial episode. This scenario is described in figure 9. The glacier
advances to point A where it reaches a standstill. Later in the cycle the
glacier advances once again, this time to point B. Notice at both points
A and B advance phase tills are deposited. When the glacier advances to
point B it does not completely eradicate the advance phase till at point
A. Many times during a glacial advance the ice will ride over the till
layer rather than destroy it. Finally, the glacier recedes leaving behind
a veneer of retreat phase till. Later during the glacial cycle, or even
during a subsequent glaciation the glacier may readvance over the same
territory to point C. Here at point C the glacial till is layered. This
layering corresponds with the previous deposits of advance and retreat
phase till that has been stacked by a later glaciation or surge in the
glacial cycle, and then is buried by retreat phase till (Boulton
1996).
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Fig.8 & 9 Modified from Boulton 1996
These two figures show the advance and retreat phase tills during a
glacial cycle.
The red color is associated with advance. Yellow is associated with
retreat.
Dashed line indicates the former ground surface before glaciation.
The composition of glacial till in relation to source lithology is
the final issue for this page. Since till is a sediment rather than a land
form the sediment must have a source. Glaciers, as has been discussed in
previous pages are an excellent transporter
of debris. So the question remains, were does the sediment come from
in relation to where it has been placed. This is not an easy question since
till is basically a poorly sorted sediment derived from a glacial system.
It then becomes difficult to determine where various units of till originate.
Only by knowing exact locations where specific debris could be extracted
is again, difficult to determine. However by knowing something about the
transporting capabilities of the sub glacial system, it then becomes possible
to theorize where certain till sequences originate. In figure 10, below,
we see that in areas of little erosion, near an ice divide, the till in
that location has not been transported very far. In locations farther from
the ice divide we see that the composition of the till is much more varied.
Again this is due to the transporting capabilities of the glacier. The
figure below can be read in terms of percentage of till composition in
distance from the final ice divide.
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Figure 10.Modified from Boulton 1996
The composition of retreat phase till in relation to source lithology.
The composition is shown in terms of percentage of source lithology.