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Second Interim Essay : Bedrock Erosion by Subglacial Ice Extrusion
This second interim essay updates previous interpretations regarding the origin of ice-disrupted bedrock features found on Newfoundland’s Avalon Peninsula. The updated interpretations reflect observations and analysis completed as of the middle of July, 2018.
Recent observations have been aimed at reducing uncertainty in the prior deduction that most or all of the major episodes of bedrock disruption/erosion presently being investigated were formed under ice, before deglaciation of the Avalon Peninsula was completed. This conclusion is critical because bedrock erosion features can be subject to completely different interpretations, depending on whether the features were formed under ice or in a subaerial environment.
Additional recent observations and analysis have focused on identifying characteristic bedrock erosion patterns that potentially indicate the movement of groundwater through bedrock in a cold subglacial environment. Since bedrock is weakened and eroded when groundwater crystallizes in small confined spaces, observations of small-scale erosion features can provide a link between hydrogeology and larger-scale bedrock disruption processes.
Convincing evidence has been accumulated to support the theory that wide areas of exposed bedrock on the Isthmus of Avalon and on the Carbonear sub-peninsula have been eroded by the action of ice extruded from the ground. The erosion by ice extrusion took place beneath thick (exceeding 100 m) cold-based glacial ice cover. This ice extrusion model covers several different modes of subglacial bedrock disruption/erosion, with features ranging in scale from tenths of millimeters to tens of meters.
Erosion by Ice Extrusion vs. Alternative Erosion Processes
Bedrock erosion by subglacial ice extrusion is a process that has not been widely observed and documented. This is probably because the process is rare, but an element of lack of recognition might also arise since the process can easily be confused with other more common modes of glacial and cold-climate bedrock erosion. All of the instances of bedrock disruption and erosion found on the Avalon Peninsula and attributed in the present discussion to subglacial ice extrusion, can be superficially explained by selectively invoking alternative better-known glacial and periglacial erosion processes.
Following is a list of potential erosion features formed by ice extrusion from bedrock, along with possible alternative interpretations.
1) Large-scale disruption of bedrock by subglacial ice extrusion on level or low-sloping surfaces can resemble instances of subaerial bedrock frost heave. Subaerial bedrock frost heave is commonly observed in polar regions affected by past or present periglacial climate conditions where temperature fluctuations are unmoderated by the presence of glacial ice cover.
2) Large-scale disruption of bedrock by ice extrusion on the sides of hills and ridges can resemble plucking by warm-based glaciers.
3) Disruption of bedrock by ice extrusion at the top edges of slopes and on sloping surfaces can resemble the disruption of bedrock by mechanical loading (shear stress) associated with ground-level tangential glacial ice movement.
4) Smaller-scale features of erosion by ice extrusion including delamination of foliated bedrock, widened joints and patterns of eroded grooves can resemble freeze-thaw weathering or other weathering processes driven by meteoric water as might occur under Holocene climate conditions.
Each of the above-mentioned alternative explanations for the bedrock disruption and erosion seen over selected areas of the Avalon Peninsula can be excluded by observations specifically targeted at identifying the presence and action of concurrent overlying cold-based glacial ice. Rebuttals to the more commonly-recognized alternative modes of origin are given below for the four cases listed above.
Subaerial bedrock frost heave: This process can be excluded in situations where thick glacial ice has clearly interacted with an occurrence of disrupted bedrock. For example, when several individual monoliths protruding from bedrock have been tilted and crushed, always leaning in the same direction, the action of moving glacial ice is implied. When a glacial erratic lies on top of crushed protruding rock fragments, the uplift of the rock fragments presumably preceded the arrival of the erratic. Thickness of overlying glacial ice can be inferred from observations of ice flow direction taken in the context of local topography. Ice moving toward the coast from an inland area must be thick in comparison to the heights of topographic obstructions. Overlying thick glacial ice blunts seasonal temperature variations and the accompanying thermal gradients, eliminating a possible role for meteoric water or glacial meltwater in a bedrock disruption process. In a deep subglacial environment, groundwater drives any bedrock disruption that cannot be directly attributed to the mechanical action of moving glacial ice.
Plucking by warm-based glacial ice flow: This process cannot readily account for bedrock disruption when rock shifts are directed vertically upward from a surface that is roughly horizontal. Similarly, plucking cannot generally account for bedrock fragments shifted outward from the sides of small hills where shifted rock is observed on two opposite sides of the same hill, with the shifts directed outward from the hill on both sides of the hill.
Mechanical loading by moving glacial ice: This process cannot generally account for rock shifted vertically upward from a horizontal surface, except in cases where the vertical shift is limited to a few centimeters (exfoliation). Glacial shear loading applied at ground level would favor rotation over pure translation for sections of underlying bedrock that were tall relative to their thickness. Hence, deep narrow fissures with approximately parallel side walls are unlikely to originate solely from mechanical loading by moving glacial ice. Delicate bedrock erosion structures, particularly tall slender free-standing monoliths, would probably be obliterated by glacial ice sliding across a bedrock surface. Fragile and unstable hillside occurrences of disrupted bedrock would not likely withstand erosion by glacial ice moving in basal-sliding mode. Some fragile or precarious disrupted-bedrock features could probably withstand the limited near-ground ice deformation associated with the creep of a cold-based glacier.
Holocene erosion: Subglacial ice extrusion generates distinctive patterns of joint widening and surface erosion in affected bedrock. These erosion patterns can be confused with the results of Holocene weathering processes such as frost wedging, freeze-thaw weathering or the erosion of rock by the action of glacial meltwater or meteoric water. Close correlation of distinctive patterns of bedrock erosion with nearby instances of larger-scale bedrock disruption implies that the distinctive erosion patterns are unlikely to be solely the result of attack by surface water/ice. This deduction can be supported by noting the degree of erosion of similar rock types in a local area. When nearby similar rock is eroded very little, it implies that the particular rock type is generally resistant to erosive attack occurring solely under Holocene weather conditions.
Bedrock erosion by subglacial ice extrusion generates several types of characteristic disruption to exposed bedrock surfaces. This characteristic disruption/erosion is most clearly recognizable on bedrock surfaces that have been previously smoothed by basal-slip glacial erosion. Principal characteristic modes of erosion by ice extrusion are listed below.
1) Monoliths, groups of monoliths and mounds of blocks that have been shifted outward from a bedrock surface. Evidence of subsequent glacial action affecting a disrupted bedrock area must be used to dismiss the possibility of a subaerial origin.
2) Individual blocks and chains of blocks that have been rotated outward from sloping bedrock surfaces. The geometry of the occurrence, observed in the context of the local topography and inferred ice flow direction, must be considered so as to exclude the possibility of plucking.
3) Individual fissures or systems of parallel fissures or linked fissures. Fissures often occur in poorly confined bedrock near the top edges of slopes. The possibility of fissures originating from the mechanical action of moving glacial ice must be considered. Fissures formed by ice loading directed tangentially to the ground surface would tend to be wedge shaped, wide at the top and narrow at the bottom. Deep fissures with roughly parallel side walls are more likely the result of an accumulation of ice extruded from bedrock.
4) Joint widening, delamination of foliated bedrock and formation of grooves. These indications of erosion by ice extrusion are widely observed in select areas of the Avalon Peninsula that additionally show independent evidence of larger-scale bedrock disruption by ice extrusion. The smaller-scale subglacial erosion artifacts can mimic the results of various Holocene erosion processes. A repeating pattern of deep V-shaped groves is commonly formed on surfaces affected by subglacial ice extrusion. This erosion pattern, potentially resembling the action of Holocene freeze-thaw weathering, typically leaves previously smoothed bedrock surfaces with the appearance of coarse tree bark. In some cases, repeating grooves eroded by ice extrusion can reach 20 cm in depth. This distinctive erosion pattern can be clearly separated from Holocene erosion when it is seen affecting only the original top side of a block that was expelled from bedrock and then had tipped over and been shifted by glacial action. Delamination of foliated bedrock by ice extrusion can be recognized when rock that strongly resists delamination by hammering is found to be conspicuously delaminated only in an environment where other forms of obvious disruption by extruded ice are also observed.
Subglacial Ice Extrusion and Hydrogeology
Observations of instances of relict bedrock erosion by subglacial ice extrusion can provide information about subglacial hydrogeology as it existed during the final stage of deglaciation preceding the Holocene. Within the context of the subglacial ice extrusion model, groundwater moved toward the glacial bed in affected areas and froze before exiting bedrock. The hydraulic gradient beneath depleting cold-based glacial ice cover was sufficient to drive groundwater toward the glacial base. This hydraulic gradient originated in the unloading of water-saturated bedrock as deglaciation proceeded. Given a bedrock aquifer of thickness approximating the thickness of ice removed during deglaciation, the magnitude of the hydraulic gradient could have been of the order of unity.
The hydraulic conductivity of foliated bedrock in several areas of the Avalon Peninsula was sufficient to allow groundwater to migrate at a rate that was significant over the time interval during which subglacial bedrock disruption occurred. In conformance with typical observations of vertical shifts of disrupted bedrock, an amount of ice of the order of 1 meter thickness was extruded from bedrock during deglaciation of affected areas on the Avalon Peninsula. Thus, as deglaciation proceeded, glacial ice thickness in specific areas was augmented by of the order of 1 meter due to ascending ice flow. Given that deglaciation occurred over a 1000 year interval, the magnitude of the hydraulic conductivity of foliated bedrock, parallel to the plane of foliation, could have been in the range of 10^(-11) m/s, although this value might vary by orders of magnitude over short distances.
Ice extrusion from bedrock was irregular, occurring in plumes as evidenced by strong local spatial variations in bedrock disruption intensity. During intervals of ascending ice flow, complex patterns of tangential near-ground glacial ice creep interacted with the ascending flow. Net ice creep patterns were sometimes preserved in patterns of ice-shifted bedrock fragments.
Bedrock was subjected to tensile stress and shear stress by a combination of hydraulic pressure and ice crystal growth pressure. The mechanical strength of well-indurated bedrock was compromised by ice segregation at both the micro (crystal grain) level and the macro (joint block) level. The micro-level disruption of rock consisted of delamination of aligned grains of phyllosilicate minerals. The macro-level disruption of bedrock, although driven by the formation of lenses of segregated ice, arose as a consequence of initial micro-level disruption.
Direct observational evidence for the micro-level delamination of grains within macro-disrupted bedrock potentially exists. All of the bedrock where severe disruption by ice has been observed belongs to one of four formations described in geological mapping of the Avalon Peninsula. These are: Bull Arm Formation (volcanic), Big Head Formation (sedimentary), Maturin Ponds Formation (sedimentary) and Gibbett Hill Formation (sedimentary). All of these formations are of Ediacaran age and all are comprised of significant proportions of phyllosilicate crystals at the millimeter scale that have been aligned by tectonic compression. The rocks from three of the four formations (Maturin Ponds excepted) are out of equilibrium under present conditions and form visible rinds at exposed surfaces. Rocks from all four formations incorporate some active divalent iron which will react with water to form easily recognized trivalent iron. By hammering rock specimens until fresh, irregular internal surfaces are exposed, hints of the paths of water moving through the rock can be discerned by noting the iron (III) coloration visible on foliation-aligned crystal-grain surfaces.
Joints are created in ice-disrupted bedrock when micro-level delamination of grains permits the development of crystals of segregated ice. Ice crystal growth pressure can increase tensile stress within rock by one or more orders of magnitude, depending on rock temperature and the initial hydraulic pressure of in-flowing groundwater. Before ice segregation begins, groundwater transport within intact bedrock presumably occurs between weakly bonded layers of aligned phyllosilicate crystals. The specific phyllosilicate minerals comprising significant portions of ice-disrupted rock on the Avalon Peninsula have not been determined. It might be guessed that the rocks would contain mixed layers including montmorillonite along with the more likely illite or muscovite formed from illite during regional metamorphism. The presence of a water-absorbing mineral like montmorillonite might help account for the significant hydraulic conductivity of seemingly intact, non-porous and well-indurated bedrock.
It is probable that the hydraulic conductivity of foliated Avalon bedrock is highly directional, with flow strongly channeled in a direction parallel to the plane of foliation. In most locations on the Avalon Peninsula, the plane of foliation dips approximately vertically. Anisotropic hydraulic conductivity, along with water channels formed by systems of cross joints, could help account for the concentration of local groundwater flow, forming plumes of extruded ice, as indicated by strong local variations in bedrock disruption intensity.
Ice segregation occurring between crystal grains and within joints weakens rock, making the rock more readily subject to erosion by other causes. The shear stress generated by ice creep within a widened joint can augment erosion, as can a variety of Holocene weathering processes. Such secondary erosion likely accounts for the visibly widened joints and grooves seen extending toward exposed surfaces of ice-disrupted bedrock. Visibly widened joints frequently taper at depth, suggesting a transition from pore water moving between crystal grains to segregated ice accumulating in a widened joint. Clearly, a widened joint in bedrock can become a site for frost wedging by meteoric water during Holocene winters. Surprisingly, an abundance of such widened joints can be observed where Holocene freeze-thaw weathering has failed to fracture (into separate pieces) affected bedrock fragments, leaving widened joints that taper to zero width at one end. This common observation suggests that freeze-thaw weathering is often ineffective in causing bedrock failure over a 10000-year interval, even when a pre-existing widened joint is present.
Summary
In summary, a combination of independent field observations points toward a theory that bedrock on certain portions of Newfoundland’s Avalon Peninsula was eroded by ice extruded beneath cold-based glacial ice cover. The extrusion of ice was driven by a pressurized aquifer existing within unfrozen or frozen bedrock that was rich in crystal-aligned phyllosilicate minerals. When groundwater approached the rock-ice boundary, reduced rock confinement and diminishing temperature led to ice segregation and consequent delamination of phyllosilicate grains at the micro level. The resultant erosion-weakened rock then failed at the macro level generating large-scale mechanical disruption of bedrock along with widened joints and characteristic patterns of grooved erosion on rock surfaces. Analysis of bedrock areas eroded by subglacial ice extrusion can provide information on the hydrogeology of frozen bedrock in the affected areas.
This second interim essay updates previous interpretations regarding the origin of ice-disrupted bedrock features found on Newfoundland’s Avalon Peninsula. The updated interpretations reflect observations and analysis completed as of the middle of July, 2018.
Recent observations have been aimed at reducing uncertainty in the prior deduction that most or all of the major episodes of bedrock disruption/erosion presently being investigated were formed under ice, before deglaciation of the Avalon Peninsula was completed. This conclusion is critical because bedrock erosion features can be subject to completely different interpretations, depending on whether the features were formed under ice or in a subaerial environment.
Additional recent observations and analysis have focused on identifying characteristic bedrock erosion patterns that potentially indicate the movement of groundwater through bedrock in a cold subglacial environment. Since bedrock is weakened and eroded when groundwater crystallizes in small confined spaces, observations of small-scale erosion features can provide a link between hydrogeology and larger-scale bedrock disruption processes.
Convincing evidence has been accumulated to support the theory that wide areas of exposed bedrock on the Isthmus of Avalon and on the Carbonear sub-peninsula have been eroded by the action of ice extruded from the ground. The erosion by ice extrusion took place beneath thick (exceeding 100 m) cold-based glacial ice cover. This ice extrusion model covers several different modes of subglacial bedrock disruption/erosion, with features ranging in scale from tenths of millimeters to tens of meters.
Erosion by Ice Extrusion vs. Alternative Erosion Processes
Bedrock erosion by subglacial ice extrusion is a process that has not been widely observed and documented. This is probably because the process is rare, but an element of lack of recognition might also arise since the process can easily be confused with other more common modes of glacial and cold-climate bedrock erosion. All of the instances of bedrock disruption and erosion found on the Avalon Peninsula and attributed in the present discussion to subglacial ice extrusion, can be superficially explained by selectively invoking alternative better-known glacial and periglacial erosion processes.
Following is a list of potential erosion features formed by ice extrusion from bedrock, along with possible alternative interpretations.
1) Large-scale disruption of bedrock by subglacial ice extrusion on level or low-sloping surfaces can resemble instances of subaerial bedrock frost heave. Subaerial bedrock frost heave is commonly observed in polar regions affected by past or present periglacial climate conditions where temperature fluctuations are unmoderated by the presence of glacial ice cover.
2) Large-scale disruption of bedrock by ice extrusion on the sides of hills and ridges can resemble plucking by warm-based glaciers.
3) Disruption of bedrock by ice extrusion at the top edges of slopes and on sloping surfaces can resemble the disruption of bedrock by mechanical loading (shear stress) associated with ground-level tangential glacial ice movement.
4) Smaller-scale features of erosion by ice extrusion including delamination of foliated bedrock, widened joints and patterns of eroded grooves can resemble freeze-thaw weathering or other weathering processes driven by meteoric water as might occur under Holocene climate conditions.
Each of the above-mentioned alternative explanations for the bedrock disruption and erosion seen over selected areas of the Avalon Peninsula can be excluded by observations specifically targeted at identifying the presence and action of concurrent overlying cold-based glacial ice. Rebuttals to the more commonly-recognized alternative modes of origin are given below for the four cases listed above.
Subaerial bedrock frost heave: This process can be excluded in situations where thick glacial ice has clearly interacted with an occurrence of disrupted bedrock. For example, when several individual monoliths protruding from bedrock have been tilted and crushed, always leaning in the same direction, the action of moving glacial ice is implied. When a glacial erratic lies on top of crushed protruding rock fragments, the uplift of the rock fragments presumably preceded the arrival of the erratic. Thickness of overlying glacial ice can be inferred from observations of ice flow direction taken in the context of local topography. Ice moving toward the coast from an inland area must be thick in comparison to the heights of topographic obstructions. Overlying thick glacial ice blunts seasonal temperature variations and the accompanying thermal gradients, eliminating a possible role for meteoric water or glacial meltwater in a bedrock disruption process. In a deep subglacial environment, groundwater drives any bedrock disruption that cannot be directly attributed to the mechanical action of moving glacial ice.
Plucking by warm-based glacial ice flow: This process cannot readily account for bedrock disruption when rock shifts are directed vertically upward from a surface that is roughly horizontal. Similarly, plucking cannot generally account for bedrock fragments shifted outward from the sides of small hills where shifted rock is observed on two opposite sides of the same hill, with the shifts directed outward from the hill on both sides of the hill.
Mechanical loading by moving glacial ice: This process cannot generally account for rock shifted vertically upward from a horizontal surface, except in cases where the vertical shift is limited to a few centimeters (exfoliation). Glacial shear loading applied at ground level would favor rotation over pure translation for sections of underlying bedrock that were tall relative to their thickness. Hence, deep narrow fissures with approximately parallel side walls are unlikely to originate solely from mechanical loading by moving glacial ice. Delicate bedrock erosion structures, particularly tall slender free-standing monoliths, would probably be obliterated by glacial ice sliding across a bedrock surface. Fragile and unstable hillside occurrences of disrupted bedrock would not likely withstand erosion by glacial ice moving in basal-sliding mode. Some fragile or precarious disrupted-bedrock features could probably withstand the limited near-ground ice deformation associated with the creep of a cold-based glacier.
Holocene erosion: Subglacial ice extrusion generates distinctive patterns of joint widening and surface erosion in affected bedrock. These erosion patterns can be confused with the results of Holocene weathering processes such as frost wedging, freeze-thaw weathering or the erosion of rock by the action of glacial meltwater or meteoric water. Close correlation of distinctive patterns of bedrock erosion with nearby instances of larger-scale bedrock disruption implies that the distinctive erosion patterns are unlikely to be solely the result of attack by surface water/ice. This deduction can be supported by noting the degree of erosion of similar rock types in a local area. When nearby similar rock is eroded very little, it implies that the particular rock type is generally resistant to erosive attack occurring solely under Holocene weather conditions.
Bedrock erosion by subglacial ice extrusion generates several types of characteristic disruption to exposed bedrock surfaces. This characteristic disruption/erosion is most clearly recognizable on bedrock surfaces that have been previously smoothed by basal-slip glacial erosion. Principal characteristic modes of erosion by ice extrusion are listed below.
1) Monoliths, groups of monoliths and mounds of blocks that have been shifted outward from a bedrock surface. Evidence of subsequent glacial action affecting a disrupted bedrock area must be used to dismiss the possibility of a subaerial origin.
2) Individual blocks and chains of blocks that have been rotated outward from sloping bedrock surfaces. The geometry of the occurrence, observed in the context of the local topography and inferred ice flow direction, must be considered so as to exclude the possibility of plucking.
3) Individual fissures or systems of parallel fissures or linked fissures. Fissures often occur in poorly confined bedrock near the top edges of slopes. The possibility of fissures originating from the mechanical action of moving glacial ice must be considered. Fissures formed by ice loading directed tangentially to the ground surface would tend to be wedge shaped, wide at the top and narrow at the bottom. Deep fissures with roughly parallel side walls are more likely the result of an accumulation of ice extruded from bedrock.
4) Joint widening, delamination of foliated bedrock and formation of grooves. These indications of erosion by ice extrusion are widely observed in select areas of the Avalon Peninsula that additionally show independent evidence of larger-scale bedrock disruption by ice extrusion. The smaller-scale subglacial erosion artifacts can mimic the results of various Holocene erosion processes. A repeating pattern of deep V-shaped groves is commonly formed on surfaces affected by subglacial ice extrusion. This erosion pattern, potentially resembling the action of Holocene freeze-thaw weathering, typically leaves previously smoothed bedrock surfaces with the appearance of coarse tree bark. In some cases, repeating grooves eroded by ice extrusion can reach 20 cm in depth. This distinctive erosion pattern can be clearly separated from Holocene erosion when it is seen affecting only the original top side of a block that was expelled from bedrock and then had tipped over and been shifted by glacial action. Delamination of foliated bedrock by ice extrusion can be recognized when rock that strongly resists delamination by hammering is found to be conspicuously delaminated only in an environment where other forms of obvious disruption by extruded ice are also observed.
Subglacial Ice Extrusion and Hydrogeology
Observations of instances of relict bedrock erosion by subglacial ice extrusion can provide information about subglacial hydrogeology as it existed during the final stage of deglaciation preceding the Holocene. Within the context of the subglacial ice extrusion model, groundwater moved toward the glacial bed in affected areas and froze before exiting bedrock. The hydraulic gradient beneath depleting cold-based glacial ice cover was sufficient to drive groundwater toward the glacial base. This hydraulic gradient originated in the unloading of water-saturated bedrock as deglaciation proceeded. Given a bedrock aquifer of thickness approximating the thickness of ice removed during deglaciation, the magnitude of the hydraulic gradient could have been of the order of unity.
The hydraulic conductivity of foliated bedrock in several areas of the Avalon Peninsula was sufficient to allow groundwater to migrate at a rate that was significant over the time interval during which subglacial bedrock disruption occurred. In conformance with typical observations of vertical shifts of disrupted bedrock, an amount of ice of the order of 1 meter thickness was extruded from bedrock during deglaciation of affected areas on the Avalon Peninsula. Thus, as deglaciation proceeded, glacial ice thickness in specific areas was augmented by of the order of 1 meter due to ascending ice flow. Given that deglaciation occurred over a 1000 year interval, the magnitude of the hydraulic conductivity of foliated bedrock, parallel to the plane of foliation, could have been in the range of 10^(-11) m/s, although this value might vary by orders of magnitude over short distances.
Ice extrusion from bedrock was irregular, occurring in plumes as evidenced by strong local spatial variations in bedrock disruption intensity. During intervals of ascending ice flow, complex patterns of tangential near-ground glacial ice creep interacted with the ascending flow. Net ice creep patterns were sometimes preserved in patterns of ice-shifted bedrock fragments.
Bedrock was subjected to tensile stress and shear stress by a combination of hydraulic pressure and ice crystal growth pressure. The mechanical strength of well-indurated bedrock was compromised by ice segregation at both the micro (crystal grain) level and the macro (joint block) level. The micro-level disruption of rock consisted of delamination of aligned grains of phyllosilicate minerals. The macro-level disruption of bedrock, although driven by the formation of lenses of segregated ice, arose as a consequence of initial micro-level disruption.
Direct observational evidence for the micro-level delamination of grains within macro-disrupted bedrock potentially exists. All of the bedrock where severe disruption by ice has been observed belongs to one of four formations described in geological mapping of the Avalon Peninsula. These are: Bull Arm Formation (volcanic), Big Head Formation (sedimentary), Maturin Ponds Formation (sedimentary) and Gibbett Hill Formation (sedimentary). All of these formations are of Ediacaran age and all are comprised of significant proportions of phyllosilicate crystals at the millimeter scale that have been aligned by tectonic compression. The rocks from three of the four formations (Maturin Ponds excepted) are out of equilibrium under present conditions and form visible rinds at exposed surfaces. Rocks from all four formations incorporate some active divalent iron which will react with water to form easily recognized trivalent iron. By hammering rock specimens until fresh, irregular internal surfaces are exposed, hints of the paths of water moving through the rock can be discerned by noting the iron (III) coloration visible on foliation-aligned crystal-grain surfaces.
Joints are created in ice-disrupted bedrock when micro-level delamination of grains permits the development of crystals of segregated ice. Ice crystal growth pressure can increase tensile stress within rock by one or more orders of magnitude, depending on rock temperature and the initial hydraulic pressure of in-flowing groundwater. Before ice segregation begins, groundwater transport within intact bedrock presumably occurs between weakly bonded layers of aligned phyllosilicate crystals. The specific phyllosilicate minerals comprising significant portions of ice-disrupted rock on the Avalon Peninsula have not been determined. It might be guessed that the rocks would contain mixed layers including montmorillonite along with the more likely illite or muscovite formed from illite during regional metamorphism. The presence of a water-absorbing mineral like montmorillonite might help account for the significant hydraulic conductivity of seemingly intact, non-porous and well-indurated bedrock.
It is probable that the hydraulic conductivity of foliated Avalon bedrock is highly directional, with flow strongly channeled in a direction parallel to the plane of foliation. In most locations on the Avalon Peninsula, the plane of foliation dips approximately vertically. Anisotropic hydraulic conductivity, along with water channels formed by systems of cross joints, could help account for the concentration of local groundwater flow, forming plumes of extruded ice, as indicated by strong local variations in bedrock disruption intensity.
Ice segregation occurring between crystal grains and within joints weakens rock, making the rock more readily subject to erosion by other causes. The shear stress generated by ice creep within a widened joint can augment erosion, as can a variety of Holocene weathering processes. Such secondary erosion likely accounts for the visibly widened joints and grooves seen extending toward exposed surfaces of ice-disrupted bedrock. Visibly widened joints frequently taper at depth, suggesting a transition from pore water moving between crystal grains to segregated ice accumulating in a widened joint. Clearly, a widened joint in bedrock can become a site for frost wedging by meteoric water during Holocene winters. Surprisingly, an abundance of such widened joints can be observed where Holocene freeze-thaw weathering has failed to fracture (into separate pieces) affected bedrock fragments, leaving widened joints that taper to zero width at one end. This common observation suggests that freeze-thaw weathering is often ineffective in causing bedrock failure over a 10000-year interval, even when a pre-existing widened joint is present.
Summary
In summary, a combination of independent field observations points toward a theory that bedrock on certain portions of Newfoundland’s Avalon Peninsula was eroded by ice extruded beneath cold-based glacial ice cover. The extrusion of ice was driven by a pressurized aquifer existing within unfrozen or frozen bedrock that was rich in crystal-aligned phyllosilicate minerals. When groundwater approached the rock-ice boundary, reduced rock confinement and diminishing temperature led to ice segregation and consequent delamination of phyllosilicate grains at the micro level. The resultant erosion-weakened rock then failed at the macro level generating large-scale mechanical disruption of bedrock along with widened joints and characteristic patterns of grooved erosion on rock surfaces. Analysis of bedrock areas eroded by subglacial ice extrusion can provide information on the hydrogeology of frozen bedrock in the affected areas.
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