Heart of Avalonia |
Cold-based Glacial Geomorphology 2:
Subglacial Bedrock Erosion
in a
Basal Shear Stress Environment
Subglacial Bedrock Erosion
in a
Basal Shear Stress Environment
Introduction
A cold-based glacier remains anchored to frozen substrate by the strong adhesive bond formed between cold ice and frozen rock. When a glacier does not slide relative to the surface beneath it, one of the most effective mechanisms of bedrock erosion by glacial ice is suppressed or eliminated. Specifically, cold-based ice does not tend to drag abrasive entrained debris along the ground. Glaciotectonic erosion (the fracturing and shifting of rock by direct, mechanical ice loading) remains a viable process under cold glacial conditions. It might reasonably be assumed that glaciotectonic erosion is the predominant, if not the sole, mode of bedrock erosion expected beneath a cold-based glacier. However, observations from the bedrock frost-heave affected areas of the Avalon Peninsula indicate that additional important mechanisms of erosion characterize the interaction of cold glacial ice with permeable foliated bedrock and groundwater.
Although ice at the bottom surface of a cold glacier does not move relative to the ground it contacts, basal ice will transmit shear stress (basal shear stress) to the ground. Shear stress in the rocks abutting the base of a cold glacier can be expected to equal or exceed in magnitude the shear stress experienced in any other part of the glacier. Bedrock subjected to a combination of hydrostatic stress and basal shear stress is exposed to a fundamentally different environment than is bedrock subjected solely to hydrostatic stress. A key difference relates to the symmetry of the applied stress. Hydrostatic stress acts symmetrically, while basal shear stress breaks the symmetry of the bedrock stress environment and creates the possibility that asymmetric erosion processes will occur.
Rock is often heterogeneous and anisotropic. Thus, when loaded symmetrically, bedrock may respond asymmetrically. Accordingly, hydrostatic stress can, in principle, lead to asymmetric deformation or erosion of bedrock. This raises the question: Can the specific effects of basal shear stress be isolated when observing erosion patterns imparted on heterogeneous or anisotropic bedrock by glacial loading? It will be argued below that when bedrock is locally homogeneous on the scale of centimeters to meters, the answer is "yes" and that bedrock erosion patterns reflecting conditions of asymmetric stress are sometimes clearly observable. Furthermore, the resulting erosion patterns can be used to infer the existence and character of relict subglacial shear stress. The diagram below shows a hypothetical example.
Although ice at the bottom surface of a cold glacier does not move relative to the ground it contacts, basal ice will transmit shear stress (basal shear stress) to the ground. Shear stress in the rocks abutting the base of a cold glacier can be expected to equal or exceed in magnitude the shear stress experienced in any other part of the glacier. Bedrock subjected to a combination of hydrostatic stress and basal shear stress is exposed to a fundamentally different environment than is bedrock subjected solely to hydrostatic stress. A key difference relates to the symmetry of the applied stress. Hydrostatic stress acts symmetrically, while basal shear stress breaks the symmetry of the bedrock stress environment and creates the possibility that asymmetric erosion processes will occur.
Rock is often heterogeneous and anisotropic. Thus, when loaded symmetrically, bedrock may respond asymmetrically. Accordingly, hydrostatic stress can, in principle, lead to asymmetric deformation or erosion of bedrock. This raises the question: Can the specific effects of basal shear stress be isolated when observing erosion patterns imparted on heterogeneous or anisotropic bedrock by glacial loading? It will be argued below that when bedrock is locally homogeneous on the scale of centimeters to meters, the answer is "yes" and that bedrock erosion patterns reflecting conditions of asymmetric stress are sometimes clearly observable. Furthermore, the resulting erosion patterns can be used to infer the existence and character of relict subglacial shear stress. The diagram below shows a hypothetical example.
The above diagram illustrates a situation where the effects of basal shear stress could theoretically leave behind an observable mark. Assume that the bedrock substrate is foliated, with the planes of foliation dipping vertically and striking north-south ("north" defined as pointing to the top of the diagram). Furthermore, assume that a process of bedrock erosion by subglacial ice extrusion is operating. Groundwater, freezing between layers of foliated rock, is delaminating the rock, either by ejecting slivers of rock upward or by wedging rock layers further apart in the horizontal plane. The delamination action is indicated by the east-west arrow marked "Ice crystal growth pressure" on the diagram.
The off-axis (pointing north-east) red arrow shows the direction of a theoretical application of basal shear stress imposed by the overlying glacier. Can the off-axis shear loading influence the end result of the bedrock delamination process? If a mechanism existed (see below) for off-axis shear stress to modify bedrock delamination, then a change in the strike of the apparent planes of foliation would comprise a possible outcome. The modified strike might be intermediate in angle between the actual north-south strike of the foliation planes and the perpendicular to the direction of the shear stress. Preliminary observations suggest that the strike of delamination planes in foliated bedrock can sometimes be modified relative to the strike of pre-glacial, tectonically-induced foliation ("tectonically-induced" in this context refers to ancient plate tectonics, not glaciotectonics). Tentative examples from the field will be presented in a subsequent section. In this section, some preliminary observations of changes in the apparent dip of pre-existing foliation planes will be presented. Asymmetry in surface patterns of bedrock erosion by subglacial ice extrusion, deemed to result from basal shear stress, will also be presented in this section.
The off-axis (pointing north-east) red arrow shows the direction of a theoretical application of basal shear stress imposed by the overlying glacier. Can the off-axis shear loading influence the end result of the bedrock delamination process? If a mechanism existed (see below) for off-axis shear stress to modify bedrock delamination, then a change in the strike of the apparent planes of foliation would comprise a possible outcome. The modified strike might be intermediate in angle between the actual north-south strike of the foliation planes and the perpendicular to the direction of the shear stress. Preliminary observations suggest that the strike of delamination planes in foliated bedrock can sometimes be modified relative to the strike of pre-glacial, tectonically-induced foliation ("tectonically-induced" in this context refers to ancient plate tectonics, not glaciotectonics). Tentative examples from the field will be presented in a subsequent section. In this section, some preliminary observations of changes in the apparent dip of pre-existing foliation planes will be presented. Asymmetry in surface patterns of bedrock erosion by subglacial ice extrusion, deemed to result from basal shear stress, will also be presented in this section.
Mechanisms of bedrock erosion by cold ice
A sample of macro-crystalline, weathered muscovite (obtained just west of the Dover Fault, outside of the Heart of Avalonia region) is shown below.
The above-illustrated sample of the phyllosilicate mineral, muscovite, has been partially delaminated along (001) crystal planes by the action of intruding water and vegetation. A question arises: What is the relative strength of inter-(001)-plane bonding in muscovite, when compared with the ice-muscovite bond as might be formed when the (001) external surface of a macro-crystalline specimen was frozen down onto a hard substrate? Alternatively: Can muscovite be cleaved along (001) by freezing a macro-crystalline specimen to a block of ice or a wetted hard surface and applying appropriate shear stress or tensile stress?
The phyllosilicate-rich rocks commonly found in frost-heave-affected areas of the Avalon Peninsula contain an indeterminate percentage of muscovite and are polycrystalline in composition, with grain sizes ranging from micron-scale to millimeter-scale. However, the phyllosilicate crystal grains have been aligned with generally parallel (001) planes by regional metamorphism, yielding slatey or schistose rock textures in many cases. These regionally-metamorphosed rocks could potentially, in analogy with the process postulated above for macro-crystalline muscovite, be delaminated by the application of shear stress or tensile stress imposed by abutting cold ice. This action is illustrated in the diagram below.
The phyllosilicate-rich rocks commonly found in frost-heave-affected areas of the Avalon Peninsula contain an indeterminate percentage of muscovite and are polycrystalline in composition, with grain sizes ranging from micron-scale to millimeter-scale. However, the phyllosilicate crystal grains have been aligned with generally parallel (001) planes by regional metamorphism, yielding slatey or schistose rock textures in many cases. These regionally-metamorphosed rocks could potentially, in analogy with the process postulated above for macro-crystalline muscovite, be delaminated by the application of shear stress or tensile stress imposed by abutting cold ice. This action is illustrated in the diagram below.
The above diagram postulates a mechanism by which cold glacial ice could directly erode foliated bedrock. The process may or may not operate in nature as a stand-alone process, but, when amplified by the effects of groundwater intrusion, freezing, and ice crystal growth pressure, the mechanism becomes more functional and more probable. Specifically, the process appears to play a key role in bedrock erosion by subglacial ice extrusion. If stress applied by ice to crystal grains of foliated rock can dislodge and remove the grains, then the resultant erosion could show a pattern indicative of the stress configuration. An example is illustrated in the following diagram.
The above diagram shows an occurrence of bedrock erosion by subglacial ice extrusion. Pressurized groundwater migrates through pores in frozen bedrock, freezing when the pores become wide enough to thermodynamically favor crystalline water over liquid water (pore water) at an ambient sub-freezing temperature. A combination of ice crystal growth pressure and hydraulic pressure forces ice and rock fragments upward into the overlying cold glacier.
The numbered rectangles, 1 through 6, represent crystal grains that are sequentially (in numerical order) removed from the rock by shear stress accompanying the ice extrusion process. In the absence of horizontal ice flow and corresponding horizontally-directed shear stress, symmetry would dictate that grains on either side of the central cavity would dislodge with equivalent energy expenditure or probability. Basal shear stress imparted from the overlying glacier would break the symmetry, potentially leading to the situation shown in the diagram. On both sides of the cavity, rock fails along (001) planes of weakness, but the failure on the right is step-wise, leading to a sloped macroscopic failure surface. Grains of rock separate from substrate in a sequence that minimizes the mechanical energy expended in a combination of ice deformation and rock deformation.
It is hypothesized that a process of the type described above has occurred at many different locations on the Avalon Peninsula, yielding asymmetric erosion patterns that cannot otherwise be readily explained. When an asymmetric erosion pattern is observed, either the substrate must be asymmetric or the erosion process must be asymmetric, or both.
The numbered rectangles, 1 through 6, represent crystal grains that are sequentially (in numerical order) removed from the rock by shear stress accompanying the ice extrusion process. In the absence of horizontal ice flow and corresponding horizontally-directed shear stress, symmetry would dictate that grains on either side of the central cavity would dislodge with equivalent energy expenditure or probability. Basal shear stress imparted from the overlying glacier would break the symmetry, potentially leading to the situation shown in the diagram. On both sides of the cavity, rock fails along (001) planes of weakness, but the failure on the right is step-wise, leading to a sloped macroscopic failure surface. Grains of rock separate from substrate in a sequence that minimizes the mechanical energy expended in a combination of ice deformation and rock deformation.
It is hypothesized that a process of the type described above has occurred at many different locations on the Avalon Peninsula, yielding asymmetric erosion patterns that cannot otherwise be readily explained. When an asymmetric erosion pattern is observed, either the substrate must be asymmetric or the erosion process must be asymmetric, or both.
The above diagram illustrates the symmetric pattern expected following erosion by ice extrusion in the absence of any symmetry-breaking basal shear stress imparted by overlying glacial ice. This type of erosion pattern has been widely observed at several bedrock-frost-heave localities on the Avalon Peninsula. The erosion has the effect of sharply enhancing the visibility of the foliation in the bedrock. The affected bedrock takes on an easily recognized delaminated appearance that can be used to identify sections of bedrock that have undergone erosion by subglacial ice extrusion.
When basal shear stress is present, modification of the symmetric erosion pattern shown in the previous diagram to the asymmetric pattern shown above, becomes a valid possibility based solely on symmetry considerations. The mechanism by which the asymmetric pattern could form might involve preferential step-wise separation of small crystal grains as described previously. The asymmetric erosion pattern diagrammed above has been observed at several localities on the Avalon Peninsula.
Because a symmetry argument forms an important part of the attempt to interpret erosion patterns as having resulted from basal shear stress, other factors that could lead to breaking symmetry must be considered. The most significant complicating factor is the angle between the local plane of the glacier base and the plane of foliation. For example, if the glacier base is horizontal and the dip of the plane of foliation deviates from 90 degrees, then an asymmetric condition is introduced. Similarly, asymmetry arises when an ice sheet intersects sloping bedrock possessing vertically dipping foliation. In both these similar instances, erosion by ice extrusion could theoretically yield an asymmetric outcome without requiring the presence of basal shear stress.
Because a symmetry argument forms an important part of the attempt to interpret erosion patterns as having resulted from basal shear stress, other factors that could lead to breaking symmetry must be considered. The most significant complicating factor is the angle between the local plane of the glacier base and the plane of foliation. For example, if the glacier base is horizontal and the dip of the plane of foliation deviates from 90 degrees, then an asymmetric condition is introduced. Similarly, asymmetry arises when an ice sheet intersects sloping bedrock possessing vertically dipping foliation. In both these similar instances, erosion by ice extrusion could theoretically yield an asymmetric outcome without requiring the presence of basal shear stress.
Rotation of the plane of foliation
Bedrock in frost-heave-affected areas of the Avalon Peninsula is typically strongly foliated. The foliation results from an abundance of pyhllosilicate minerals in the affected bedrock and extensive regional metamorphism. Foliation in Avalon bedrock has been observed to dip steeply, if not vertically, at most locations. Prominent exceptions are found in local areas where the foliation dips at low angles or is horizontal. The plane of foliation in these areas appears to have been rotated about a horizontal axis by glaciotectonic action. The following diagram illustrates the process.
The above diagram illustrates the mechanical deformation of a section of foliated bedrock by ice loading. Planes of foliation, originally dipping vertically, were rotated about a horizontal axis by torque applied at ground level after bedrock became decoupled from substrate at depth. This type of deformation can occur when basal shear stress is sufficient to overcome the resistance of the bedrock to failure in shear along multiple repeating planes paralleling foliation planes. Additional ice-induced torque is required to cause rock to fail in tension at the base of the sheared zone, and to overcome the energy barrier to cavitation when bedrock layers are detached and tilted while subjected to ambient hydrostatic pressure imparted by the overlying glacier.
It is unclear to what extent the above-described process could function without the aid of intruding pressurized groundwater and subsequent ice segregation taking place between bedrock layers. Rotation of sections of bedrock is commonly observed in the field, but occurrences are generally accompanied by other types of bedrock disruption, specifically erosion by ice extrusion and bedrock frost heave. A problem of causation thus arises. Did hydraulic pressure and crystal growth pressure facilitate detachment of the bedrock from substrate and failure of bedrock in shear? Alternatively: Did glaciotectonic loading cause bedrock detachment and shear failure, providing enhanced pathways (repeating faults) for groundwater to approach the glacier base.
The most prominent occurrences of bedrock rotation are found when lack of confinement on the sloping side of a hill or ridge permits bedrock to rotate without obstruction. Rotation is also promoted when ice bears against the side of a ridge such that normal stress is applied to the bedrock in addition to shear stress. In this instance the entire upper portion of a ridge can be rotated. In principle, warm-based ice could rotate bedrock through the application of normal stress to the side of a ridge. This has undoubtedly occurred at many locations on the Avalon Peninsula, but the result would likely entail catastrophic failure of the ridge and removal and transport of the resultant broken rock. When a ridge remains largely intact, albeit deformed in rotation, and shows subglacial bedrock-frost-heave features, cold-based glacial loading becomes the preferred interpretation.
When a section of bedrock is forced to yield in multiple planes of shear failure, a question arises as to how densely the planes of failure (faults) will be spaced. Clearly, a larger number of faults produced in a section of rock entails a larger input of energy into the rock shearing process. However, the cavitation energy associated with creating voids beneath rotated blocks must also be considered. The diagrams below show the considerations.
It is unclear to what extent the above-described process could function without the aid of intruding pressurized groundwater and subsequent ice segregation taking place between bedrock layers. Rotation of sections of bedrock is commonly observed in the field, but occurrences are generally accompanied by other types of bedrock disruption, specifically erosion by ice extrusion and bedrock frost heave. A problem of causation thus arises. Did hydraulic pressure and crystal growth pressure facilitate detachment of the bedrock from substrate and failure of bedrock in shear? Alternatively: Did glaciotectonic loading cause bedrock detachment and shear failure, providing enhanced pathways (repeating faults) for groundwater to approach the glacier base.
The most prominent occurrences of bedrock rotation are found when lack of confinement on the sloping side of a hill or ridge permits bedrock to rotate without obstruction. Rotation is also promoted when ice bears against the side of a ridge such that normal stress is applied to the bedrock in addition to shear stress. In this instance the entire upper portion of a ridge can be rotated. In principle, warm-based ice could rotate bedrock through the application of normal stress to the side of a ridge. This has undoubtedly occurred at many locations on the Avalon Peninsula, but the result would likely entail catastrophic failure of the ridge and removal and transport of the resultant broken rock. When a ridge remains largely intact, albeit deformed in rotation, and shows subglacial bedrock-frost-heave features, cold-based glacial loading becomes the preferred interpretation.
When a section of bedrock is forced to yield in multiple planes of shear failure, a question arises as to how densely the planes of failure (faults) will be spaced. Clearly, a larger number of faults produced in a section of rock entails a larger input of energy into the rock shearing process. However, the cavitation energy associated with creating voids beneath rotated blocks must also be considered. The diagrams below show the considerations.
The above diagram shows a vertically-foliated section of bedrock having width W and divided into N blocks by faults created when torque applied at the surface exceeds the strength of the rock.
When the blocks rotate through an angle, A, about a horizontal axis, voids are created beneath the blocks as shown in magenta on the above diagram. The total area (volume) is calculated as indicated at the top of the diagram, and shows a linear dependence on the reciprocal of N where N is the number of faults. Thus, as the density of faults rises by a given factor, the cavitation energy associated with rotating the blocks falls by the same factor. However, the energy expended to shear the rock and create the faults rises in direct proportion to N.
An immediate consequence of the above argument is that faults should be evenly spaced so as to minimize the cavitation energy and consequently the total energy needed to drive the deformation. A second consequence is that rock that is difficult to shear will have a low fault density and rock that is subjected to large hydrostatic pressure (hence high cavitation energy) will have an elevated fault density. The intrusion of extruded ice or groundwater into cavities formed when blocks rotate would reduce the barrier to cavitation and thus affect the outcome of the above analysis.
Repeating faults seen in areas of suspected glaciotectonic rotation of bedrock are often evenly spaced. This even fault spacing is potentially a useful indicator of glaciotectonically rotated bedrock. Fault density can vary widely depending on bedrock type and location. The range of variation runs from millimeter spacing to meter spacing.
In some areas, bedrock foliation might show non-vertical dip intrinsically, that is, not as a result of geologically recent glaciotectonic action. These areas appear to be somewhat rare and are difficult to identify unambiguously. Ancient tectonic action has folded rock structures on the Avalon Peninsula, yielding substantially tilted bedding planes covering wide areas at most locations. However, large-scale tectonically-induced foliation is not commonly observed to dip at angles deviating significantly from vertical. The process of crystal grain realignment that characterizes regional metamorphism appears to have continued after folding ceased, likely at depths where topographic features were of negligible influence. Nevertheless, the possibility of intrinsic non-vertical dip in planes of foliation must be considered, and, where possible, separated from glaciotectonic overprinting.
An immediate consequence of the above argument is that faults should be evenly spaced so as to minimize the cavitation energy and consequently the total energy needed to drive the deformation. A second consequence is that rock that is difficult to shear will have a low fault density and rock that is subjected to large hydrostatic pressure (hence high cavitation energy) will have an elevated fault density. The intrusion of extruded ice or groundwater into cavities formed when blocks rotate would reduce the barrier to cavitation and thus affect the outcome of the above analysis.
Repeating faults seen in areas of suspected glaciotectonic rotation of bedrock are often evenly spaced. This even fault spacing is potentially a useful indicator of glaciotectonically rotated bedrock. Fault density can vary widely depending on bedrock type and location. The range of variation runs from millimeter spacing to meter spacing.
In some areas, bedrock foliation might show non-vertical dip intrinsically, that is, not as a result of geologically recent glaciotectonic action. These areas appear to be somewhat rare and are difficult to identify unambiguously. Ancient tectonic action has folded rock structures on the Avalon Peninsula, yielding substantially tilted bedding planes covering wide areas at most locations. However, large-scale tectonically-induced foliation is not commonly observed to dip at angles deviating significantly from vertical. The process of crystal grain realignment that characterizes regional metamorphism appears to have continued after folding ceased, likely at depths where topographic features were of negligible influence. Nevertheless, the possibility of intrinsic non-vertical dip in planes of foliation must be considered, and, where possible, separated from glaciotectonic overprinting.
Preliminary Observations 1: Rotation
The following preliminary observations are included in this section to exemplify the main concepts presented above relating to the effects of basal shear stress on subglacial bedrock erosion. Rotation is considered first, then erosion by ice extrusion, because asymmetric patterns of erosion by ice extrusion are difficult to identify unambiguously when the substrate has been rotated.
Discussions of related topics, to follow under the headings "Erosion by Subglacial Ice Extrusion" and "Cold-based Glacial Geomorphology" (pending), will incorporate additional field observations demonstrating the effects of basal shear stress on aspects of bedrock erosion under cold glacial ice.
Discussions of related topics, to follow under the headings "Erosion by Subglacial Ice Extrusion" and "Cold-based Glacial Geomorphology" (pending), will incorporate additional field observations demonstrating the effects of basal shear stress on aspects of bedrock erosion under cold glacial ice.
The above aerial photo shows a section of bedrock (lower left in frame) that has been sheared and rotated by glaciotectonic action. The orange strap (visible in low contrast) seen just above the area of rotated bedrock is 1 meter long. A side-on view of the same occurrence is shown below.
This occurrence appears to have resulted from an increase in the speed of ice deformation at the top lee edge of a slope. Within this interpretation, rapid ice deformation occurring near the ground was correlated with an increase in the local basal shear stress. The enhanced shear stress, combined with a lack of rock confinement, led to rotation of a near-surface (top 1-2 m) zone of bedrock. Unmodified foliation at this locality dips vertically. Repeated failure of foliated rock in shear along originally-vertical planes of weakness resulted in delamination of the rock. The nearest coast lies to the left (~3.5 km distant) in the side-on view shown above. Other ice-modified features in the area suggest that ice movement was directed mainly toward the nearest coast. Frost-heaved bedrock monoliths are common in the area surrounding the above-illustrated feature, pointing to a cold-based conclusion to glacial activity in this area.
The repeating faults following planes of foliation in the feature shown above dip at a non-vertical angle. This bedrock feature might reasonably be interpreted as providing an example of intrinsically tilted foliation (foliation that developed with non-vertical dip during ancient plate tectonic activity). However, nearby (within tens of meters) bedrock shows vertically dipping foliation. Folding of bedrock structures in the surrounding region has yielded only gradual shifts in the dip of bedding planes, with significant variations occurring on the scale of hundreds of meters or kilometers. It therefore appears probable that the non-vertical dip of the planes of foliation shown above are of recent glaciotectonic origin.
The nearest coast (~3.5 km distant) lies to the right in the scene shown above and nearby indicators of ice flow direction suggest that ice was generally moving toward the nearest coast. The bedrock was apparently forced to yield along multiple parallel planes of shear failure by a combination of ice-induced normal stress and basal shear stress. The above feature might have been deformed in whole or in part by warm-based glacial flow, the warm ice applying solely normal stress. The sharp edge of the feature facing the camera appears to have been the result of gross rock failure, followed by the glacial transport of rock fragments out of the area. This observation suggests that sliding, warm-based glacial activity was a significant factor in shaping the feature. However, numerous frost-heaved monoliths and evidence of erosion by ice extrusion are found nearby and suggest that glacial activity concluded with a cold-based phase. An example of a frost-heaved bedrock occurrence about 25 m to the right of the rock face shown above is provided in the following photo.
The nearest coast (~3.5 km distant) lies to the right in the scene shown above and nearby indicators of ice flow direction suggest that ice was generally moving toward the nearest coast. The bedrock was apparently forced to yield along multiple parallel planes of shear failure by a combination of ice-induced normal stress and basal shear stress. The above feature might have been deformed in whole or in part by warm-based glacial flow, the warm ice applying solely normal stress. The sharp edge of the feature facing the camera appears to have been the result of gross rock failure, followed by the glacial transport of rock fragments out of the area. This observation suggests that sliding, warm-based glacial activity was a significant factor in shaping the feature. However, numerous frost-heaved monoliths and evidence of erosion by ice extrusion are found nearby and suggest that glacial activity concluded with a cold-based phase. An example of a frost-heaved bedrock occurrence about 25 m to the right of the rock face shown above is provided in the following photo.
The above-illustrated feature comprises delaminated bedrock that was subjected to vertical frost heave. The vertical orientation of the feature implies that foliation of the underlying bedrock remained vertically dipping (not rotated) at this location. The frost-heave feature occurs on a slight indent in the local topography where normal stress from ice loading (excluding hydrostatic pressure) would be absent and basal shear stress would be subdued. Delamination of the feature was probably caused by erosion by ice extrusion. The survival of the fragile feature precludes the possibility of ice movement by basal sliding subsequent to the frost heave event.
The above photo shows deformation of bedrock in shear and rotation, apparently the result of glaciotectonic loading augmented by subglacial ice extrusion. Note that angles of rotation vary and are greatest near the top and on the right-hand side of the feature where rock confinement is diminished. The glacial loading of the rock was solely via basal shear stress. The feature incorporates numerous internal voids and upward-shifted blocks, suggesting that subglacial bedrock frost heave was occurring in conjunction with glaciotectonic loading.
An example of bedrock failure in shear and rotation is shown above. The deformation of the bedrock resulted from a combination of glaciotectonic loading and subglacial bedrock frost heave. Several slabs of delaminated rock have been shifted upward and outward, extending past the original topographic surface that existed prior to deformation of the feature. Unmodified bedrock foliation dips vertically at this location.
An area of bedrock where the planes of foliation dip at a shallow angle is shown above. The bedrock has apparently been sheared and rotated by glaciotectonic action. Note the fan-shaped row of slabs of delaminated rock extending into the foreground. The slabs were likely ejected from substrate by a combination of frost wedging (bedrock frost heave) and shear loading by cold glacial ice deforming in creep. The slabs were then transported a short distance within deforming glacial ice.
The hillside feature shown above incorporates areas of bedrock that have been rotated by as much as 90 degrees by glaciotectonic action. The greatest rotation occurs in the least confined rock (nearest the camera). The bedrock behind the strongly-deformed edge of the hill has also undergone shear failure and rotation, but to a lesser degree. About 50 m back from the edge, planes of foliation dip vertically. Numerous large frost heave monoliths are accompanied by occurrences of erosion by ice extrusion at several locations behind the eroded edge shown in the photo.
Repeating planes of shear failure are evidenced by erosion-widened cracks visible in the foliated bedrock shown above. The non-vertical dip of the failure planes is likely a result of glaciotectonic loading. The direction of ice motion was presumed to be toward the nearby coast (toward the left in the photo). Abundant bedrock frost heave features found in the surrounding area suggest that glacial activity in this area culminated in a cold-based phase.
Preliminary Observations 2: Erosion by ice extrusion
A typical pattern of grooves in foliated bedrock that has been delaminated and eroded by subglacial ice extrusion is shown in the photo below.
When bedrock foliation dips vertically, the local topography is level, and significant basal shear stress is absent, a symmetric pattern of erosion by ice extrusion is expected. Symmetry, in this context, refers to local mirror symmetry of ice-eroded grooves about planes of foliation.
The above closeup photo of bedrock with grooves formed by erosion by ice extrusion illustrates approximate symmetry in the shape of the grooves. These symmetric grooves provide contrast with asymmetric grooves (as will be illustrated below) formed in similar rock.
Another example of symmetric grooves formed by subglacial ice extrusion is shown in the above photo. Note the low-relief frost-heaved slabs (above and to the right of center in the frame). Erosion by ice extrusion is closely associated with subglacial frost heave in most instances.
Roughly symmetric grooves formed by subglacial ice extrusion are shown in profile in the above photo.
In principle, all of the patterns shown above, and attributed to subglacial ice extrusion, could have resulted instead from Holocene erosion processes (for example, freeze-thaw weathering) attacking rock that incorporates repeating planes of weakness. Repeating planes of vulnerability to Holocene erosion could be a consequence of glaciotectonic loading and resultant failure of foliated rock in shear (see Preliminary Observations 1: Rotation, above). Although efforts to exclude this possibility (at least for some occurrences) have been presented in Bedrock Erosion by Subglacial Ice Extrusion - Part 01, Bedrock Erosion by Subglacial Ice Extrusion - Part 02 and Illustrative Feature 09, these previous discussions have focused on rock that had been dislodged from substrate and shifted by glacial action. Symmetric erosion patterns formed on in situ bedrock and similar to the patterns shown above are more vulnerable to interpretation as Holocene weathering than are asymmetric patterns. Therefore, observations of asymmetric erosion patterns can help support the interpretation that both the symmetric and asymmetric in situ patterns were formed in a subglacial environment.
In principle, all of the patterns shown above, and attributed to subglacial ice extrusion, could have resulted instead from Holocene erosion processes (for example, freeze-thaw weathering) attacking rock that incorporates repeating planes of weakness. Repeating planes of vulnerability to Holocene erosion could be a consequence of glaciotectonic loading and resultant failure of foliated rock in shear (see Preliminary Observations 1: Rotation, above). Although efforts to exclude this possibility (at least for some occurrences) have been presented in Bedrock Erosion by Subglacial Ice Extrusion - Part 01, Bedrock Erosion by Subglacial Ice Extrusion - Part 02 and Illustrative Feature 09, these previous discussions have focused on rock that had been dislodged from substrate and shifted by glacial action. Symmetric erosion patterns formed on in situ bedrock and similar to the patterns shown above are more vulnerable to interpretation as Holocene weathering than are asymmetric patterns. Therefore, observations of asymmetric erosion patterns can help support the interpretation that both the symmetric and asymmetric in situ patterns were formed in a subglacial environment.
The area of bedrock shown above was smoothed by basal-sliding glacial erosion (warm-based), before being eroded by subglacial ice extrusion in a final stage of cold-based glacial activity. The bedrock is a homogeneous, regionally metamorphosed siltstone with a slatey texture. The foliation at this roughly level site dips vertically. Nevertheless, the grooves formed by erosion of the bedrock are distinctly asymmetric in appearance. The two photos below show closeups of the eroded surface.
This site potentially reflects a process of erosion by subglacial ice extrusion where basal shear stress (corresponding ice deformational flow was directed toward left in photos) affected the symmetry of the eroded grooves. If this interpretation is valid, the surface provides a direct record of basal shear loading by a cold glacier. Note that dragging of ice across rock is not needed to produce the pattern. Rather, the pattern reflects removal of rock grain-by-grain in a high-stress environment. Other locations featuring similar asymmetric grooved erosion patterns have been observed. Additional examples are shown below.
An occurrence of bedrock frost heave that appears to have formed in a high shear-stress environment is illustrated in the above photo. The rock is streamlined in a direction pointed toward the nearby coast. Streamlined frost-heaved bedrock will be discussed as a later topic (pending) but the phenomenon might reasonably be interpreted as an indicator of deformational flow of cold-based ice. Erosion of the frost-heaved blocks could potentially reflect step-wise stripping of individual grains of foliated phyllosilicate-rich rock by ice-induced shear stress.
Closeups of the patch of bedrock marked by the white rectangle labeled "3" in the above photo are shown below.
Closeups of the patch of bedrock marked by the white rectangle labeled "3" in the above photo are shown below.
The patch of bedrock shown above appears to show foliation dipping at a non-vertical angle. Asymmetric erosion by ice extrusion has created the imbricated appearance. A profile photo of the bottom, right corner of the feature is shown below.
Asymmetric grooves interpreted as characterizing erosion by ice extrusion in a basal shear stress environment are visible in the above photo. The fine-grained sedimentary bedrock at this location possesses vertically-dipping foliation.
The photo below shows another example in the same rock type, but from a different location.
The photo below shows another example in the same rock type, but from a different location.
In the above photo, grooves appearing to follow foliation in the bedrock suggest a non-vertical dip in the planes of foliation. The tape defines a level orientation. Foliation in bedrock at this location dips vertically and the sloping grooves appear to be the result of erosion by ice extrusion, modified by basal shear stress in overlying ice cover.
Summary
The analyses and preliminary observations presented above suggest that erosion of bedrock by cold-based glaciers has occurred on the Avalon Peninsula and can be recognized. Erosion of smooth bedrock by glaciotectonic shear stress requires that ice be bonded to the bedrock and is thus indicative of cold-based glacial action. Erosion processes involving subglacial bedrock frost heave and erosion by ice extrusion are unique to a cold-based glacial environment, and include the added requirement of the participation of subglacial pressurized groundwater.
heartofavalonia.org Exploring Geologic History