Subglacial Bedrock Brecciation by Upward Displacement
In the photo of ice-brecciated bedrock shown above, the brecciation is dominated by upward displacement of joint blocks. The displaced joint blocks are, for the most part, anchored in bedrock substrate and underlain by a cavity. This mode of bedrock brecciation mimics the appearance of subaerial bedrock frost heave, a process that typically occurs under periglacial climate conditions, either ancient or modern.
Convincing evidence from various locations on the Avalon Peninsula demonstrates that instances of apparent subaerial bedrock frost heave (like that shown above) are, in reality, instances of subglacial bedrock displacement. The illustrated displaced-bedrock features are thus more than 11000 years old, dating back at least to the final stage of Pleistocene glaciation on the Avalon.
Because of the prevalence of upward-displaced bedrock in glacially brecciated areas on the Avalon Peninsula, understanding how these features were formed is critical to understanding the overall subglacial bedrock brecciation process. Moving glacial ice can load obstructing bedrock, induce fracture, and scatter the resulting fragments. However, upward displacement is not an expected mode of bedrock brecciation to derive solely from the mechanical action of glacial ice. Even in cases where local ice pressure forces glacial ice to move uphill, associated rock displacements would be expected to remain roughly parallel to the ground.
There are two potential explanations for the upward displacement of bedrock blocks occurring beneath glacial ice.
Firstly, it could be argued that pressurized subglacial groundwater penetrated bedrock along joints or permeable bedding layers, these joints or bedding layers dipping at a shallow angle. Hydraulic action could then dislodge overlying blocks from bedrock substrate and push the dislodged blocks upward. This explanation can apply to cases where overlying glacial ice is either warm-based or cold-based.
Alternatively it could be argued that the accumulation of ice in pores, joints or voids beneath blocks of bedrock generated ice overpressure, causing upward block displacement. This explanation can apply only to cases where overlying glacial ice is cold-based, with the ice resting on bedrock that is frozen to a depth of at least a few 10's of centimeters.
Each explanation has its own concerns. The main issue with the hydraulic hypothesis is that the water pressure needed to overcome the friction of rock dragging against rock would rapidly be relieved once leaks formed. The copious inflow needed to maintain high pressure in a poorly sealed cavity would tend to scatter bedrock blocks or fragments and generate other recognizable artifacts of glaciofluvial erosion. The hydraulic hypothesis would have the best chance in explaining the upward displacement of blocks possessing a wide cross-section parallel to the ground relative to their thickness. Commonly, upward-displaced blocks seen on the Avalon are long in vertical extent and relatively narrow in horizontal cross-section, making these blocks unlikely candidates for displacement by hydraulic action.
The main concern with the ice-accumulation hypothesis is that it requires the movement of significant quantities of water or ice through rock, while the rock is maintained at a subfreezing temperature. Moving water through frozen permeable bedrock implies capillary-like channels in the rock that are sufficiently fine to inhibit crystallization. It is difficult to transport large volumes of liquid over substantial distances through very fine channels. The motion of liquid under these conditions more closely resembles wicking or diffusion than bulk flow. The alternative, moving crystallized water (ice) through larger channels (joints, fissures) in permeable rock, would be intensely disruptive to the rock. Ice deforming by creep in narrow channels would impart intense shear stress to the channel walls and, when approaching exit surfaces, would load bedrock in tension leading to failure.
A hybrid water transport model can alleviate many of the concerns associated with the ice accumulation hypothesis. In a hybrid model, groundwater travels long distances along bulk pathways through unfrozen permeable bedrock and then migrates a short distance through fine channels upon reaching the freezing front. Fine channels can be formed along the ice-rock boundary when wider channels become filled with ice.
To aid in understanding the processes generating upward displacement of subglacial bedrock blocks, a detailed example is presented below. This example, in many respects, typifies bedrock brecciation by upward displacement as is seen over wide areas of the Avalon Peninsula. The example supports the hypothesis that ice accumulation within frozen subglacial bedrock was the key process driving bedrock brecciation by upward block displacement.
Detailed examination of an upward-displaced block
The large tilted block shown above has been displaced upward from bedrock substrate. Gaps on two sides of the block allow access to the underlying cavity. For purposes of the following discussion, this block has been photographed in aerial view, ground-level view and underground view.
The block shown above is comprised of regionally metamorphosed Ediacaran-age sedimentary rock from the Maturin Ponds Formation, Musgravetown Group, as described in geologic mapping of the Avalon Peninsula. The rock consists of irregular, often thin, alternating layers of grey sandstone and purplish-red mudstone. Both the sandstone and mudstone components are rich in phyllosilicate minerals and all parts of the rock were subjected to crystal alignment during regional metamorphism yielding a homogeneous slaty or schistose cleavage.
An example of rocks from the Maturin Ponds Formation is shown above. This example is from a road cut (~3 m top to bottom) and the visible fracturing was caused by blasting rather than by glacial action or freeze-thaw weathering. The Maturin Ponds rock is well indurated and strongly resistant to erosion (including freeze-thaw weathering) under Holocene conditions. When broken by blasting or with a hammer, the rock does not tend to cleave along planes of foliation. When eroded under cold subglacial conditions, rock failure along planes of foliation becomes commonplace.
An aerial view of the block is shown above. The orange strap is 1 meter long.
The four main faces of the roughly rectangular major block are labeled in the above photo. The face designated "E" (East) strikes parallel to the strike of tectonically-induced foliation in the surrounding bedrock. The tectonic foliation strikes approximately 25 degrees true at this location.
For convenience, references to compass directions given in the discussion of the underground views (appearing later in this section) refer to the labelled faces as indicated above. These face-referenced directions are rotated from actual compass directions by about 25 degrees clockwise, looking down.
The white rectangular box surrounds a nearby minor occurrence of upward-displaced joint blocks.
Locator map:
The magenta dot near the bottom of the topographically-shaded area in the above map of the Isthmus of Avalon indicates the location of the large upward-displaced block. The block lies about 3 km from the nearest coast in a valley between small ridges. No clear evidence of sliding, warm-based glacial erosion (for example, striations, lee-side brecciation) can be seen in the nearby area. However, nearby occurrences of upward-displaced bedrock have been deflected top-westward, suggesting creep directed west or southwest toward the nearest coast. This creep occurred in glacial ice that was not sliding.
In the above view, looking north, the large upward-displaced block is visible inside the white rectangle. Glacial ice moving west toward the coast at this location would initially gain elevation crossing the ridge seen on the left side of the photo. Indications of shear-loading of bedrock by ascending ice can be found on the ridge.
The upward-displaced block is shown in the above view, looking north. The block is tilted, top-westward. The block is also tilted in the north-south direction, with the north end of the block displaced further upward than the south end of the block. This suggests glacial ice loading directed with a southwesterly component.
The above view, looking north, shows the top surface of the block (original ground surface, before displacement), dipping downward toward the south. The grooves visible on the top surface follow planes of foliation. These grooves are an artifact of subglacial erosion that occurred before the block was displaced. Similar grooves are not seen on the opposing (north-facing) side of the block (an underground surface before displacement). Grooves, matching those on the displaced block, can be seen on bedrock lying east of the block in the above photo. It is probable that these grooves all share a common subglacial origin.
Another view of the top of the main block is shown above, looking southeast. Note the similar appearance of the eroded surface on the top of the block and on surrounding bedrock. In contrast, the north-facing side of the main block (just visible in the above photo) lacks indented erosion along planes of foliation.
Grooves in bedrock to the east of the main block appear to indicate non-vertical dip in the planes of foliation. Other evidence at this site suggests that the intrinsic (tectonically-induced) foliation of the local bedrock dips approximately vertically. The apparent sloping dip seen above is most likely an artifact of subglacial erosion influenced by basal shear stress (this is discussed in more detail later in this section).
A closeup of subglacially-eroded bedrock just east of the main block is shown above. The apparently tilted foliation and the streamlined shape of the bedrock outcrop combine to suggest ice loading/motion in a westerly direction.
The above view looks approximately south and shows a north-facing side surface of the block that was underground before displacement. The original bedrock (ground-level) surface was displaced upward by 2 m on the northeast corner of the block. The block is about 80 cm thick.
In the above photo, the upward-displaced block (hammer stands just in front) is viewed from the east. This photo looks west toward the coast, about 3 km distant, and shows the view heading in the presumed down-ice direction.
In the above photo, looking south, the main block is visible in silhouette near the top center of the frame. The rocks in the foreground show evidence of erosion by glacial ice moving toward the right (west). The bedrock outcrop has been roughened by the upward displacement of joint blocks.
The shapes of the displaced blocks have subsequently been modified by glacial erosion. Basal sliding at this location, if it occurred after roughening, would likely have dislodged the loosened, obstructing fragments and removed them from the site. Accordingly, it can be inferred that ice motion following roughening was by creep only.
Bedrock to the east of the main block appears to have been loaded in compression by ice moving westward. Fragments then rebounded (exfoliation) after ice pressure was relieved.
The relatively flat east face of the main block is tilted approximately 30 degrees from the vertical. A question arises as to how much of this tilt is due to glaciotectonic loading and how much is due to non-vertical pre-glacial dip in the intrinsic (induced by regional metamorphism) foliation of the bedrock.
The second photo above shows the top portion of the block pictured in the first photo, but rotated 30 degrees counterclockwise. Note that the top surface of the block in the rotated image roughly parallels the slope of the adjacent bedrock seen in the foreground. This suggests that the full 30 degrees of tilt is due to glaciotectonic loading of the block preceding, during or following upward displacement.
The dip of the planes of foliation in bedrock near the main block was estimated to be 72 degrees (18 degrees off vertical). If the intrinsic (pre-glacial) foliation in the bedrock dipped at 72 degrees, then the main block was pushed upward along a sloping trajectory. During or after uplift, the block was tilted an additional 12 degrees by glaciotectonic loading.
Analysis of the foliation of bedrock in glaciated areas can be ambiguous. When observing planes of foliation in bedrock, what is actually being observed? Visible planes of foliation are more correctly viewed as planes of repeating rock failure. Rock will preferentially fail along macro planes of crystallographic weakness (actual planes of tectonic foliation) when shear stress is coincidentally aligned with the intrinsic planes of weakness.
When shear stress is misaligned, then step-wise failure can occur where sliding takes place in small increments along an escalating succession of parallel crystallographic planes. The macro failure of the rock then follows a step-wise (staircase-like) pseudo-plane that deviates from the optimal (crystallographic, ancient tectonic) plane of weakness.
Observations from several locations on the Avalon Peninsula suggest that bedrock that has undergone glacial loading can show glaciotectonically-modified foliation, that is, foliation that is not aligned with preferential planes of weakness induced by regional metamorphism. Evidence supporting this conclusion can be found in areas where bedrock is sedimentary, fine-grained, thickly bedded and homogeneous. In such instances, ancient tectonically-induced foliation would not be expected to vary significantly over scales of meters or tens of meters given that the scales associated with folding in most areas of the central Avalon are hundreds of meters or kilometers.
Local meter-scale variations in apparent dip-angles of foliation are frequently observed in specific bedrock areas that also show multiple independent indications of disruptive glacial loading. In a few places, planes of apparent foliation are curved, with a radius of curvature of the order of a meter or less. This suggests that observed meter-scale variations in the dip of planes of foliation are likely of glaciotectonic origin.
In some instances, blocks that have been torn loose by glacial action lack visible foliation, whereas nearby intact bedrock of the same type appears distinctly foliated. This implies that, absent glaciotectonic loading, foliation induced by regional metamorphism may not result in readily observable parallel planes of rock failure. When foliation is not visible, rock will typically break in an irregular manner when hammered, even though the rock has undergone regional metamorphism.
It can be concluded that the visible foliation in bedrock near the main block is likely a result of foliation from regional metamorphism that was overprinted following shear loading by the basal ice of a cold glacier. The pre-glacial foliation in the bedrock presumably dipped approximately vertically. The tilted planes of visible bedrock failure are indicative of glaciotectonic shear stress applied by cold ice frozen to the bedrock. Similarly, the main block has been tilted top-westward entirely or mostly by glaciotectonic loading.
The conditions within which the main block was displaced upward consisted of a cold subglacial environment with ice applying a significant component of westward or southwestward directed basal shear stress to the surrounding bedrock. Ice deforming in creep as dictated by the near-ground stress configuration further loaded the displaced block, increasing its angle of tilt.
Below-ground Observations
Index photos
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To aid in orienting photos taken below ground level, the main block (1) and other associated displaced blocks have been given reference numbers in the above eight photos. Note that the last two photos show below-ground views of the cavity beneath block 1, first looking downward toward the south, then looking upward toward the west.
Block 1: Main block
Block 2: An upward-displaced joint block adjacent to block 1 that was displaced a short distance along a path similar to that of block 1.
Block 3: The topmost block in a stack of three blocks on the west side of the main block. The two blocks behind and beneath block 3 are not visible in the underground photos.
Block 4: This block appears to be an extension of block 5 that was detached and then displaced upward and tilted top-westward by ice pressure. As ice melted the block became jammed in this position.
Block 5: This block forms the east wall of the cavity underlying block 1. Block 5 is completely detached from bedrock and is underlain by brecciated rock. Block 5 appears to be resting at a height below the level of adjacent bedrock. This indicates that the block was displaced upward and that underlying bedrock was brecciated and displaced westward, leaving a cavity below block 5. Block 5 then settled back into the cavity when ice melted.
Block 6: This block is large and of indeterminate size, blending into adjacent bedrock. Block 6 has been shifted very little and is separated from adjacent bedrock by joints rather than faults.
Block 7: This block lies on the bottom of the cavity beneath block 1 and is visible in below-ground views only. The upward-facing surface of block 7 exceeds 1 square meter in area and the block obstructs the bottom view of the west side of the cavity beneath block 1. The foliation in block 7 appears to run parallel to the ground, suggesting that the block rotated 90 degrees while settling.
Block 8: This below-ground block, elongated in a westerly direction and possessing an almost square cross-section, has been indexed to provide an added reference point to help establish orientation in the below-ground photos.
The above photo shows a view looking down at the northwest corner of block 1. The bottom surface of block 1 is 65 cm below ground level at this corner. The hole just east of block 3 was one of two access points for below-ground photography.
The fissure shown above runs along the east side of block 1, reaching the northeast corner of the block. This fissure provided a second camera access point. The tape measure extends down to brecciated rock at the bottom of the cavity underlying block 1.
The fissure ending at the northeast corner of block 1 is shown above. The asterisk marks an upward-displaced joint block that terminates in a fracture before reaching the surface. A closeup of the area enclosed in the white rectangle is shown in the first photo below. The second photo below shows an upward-looking view of the same area.
The bottom of the upward-displaced block (*) is shown alongside block 2 in the above photos. The missing rock directly beneath the block (*) has apparently been displaced southward (there is no other open direction) and then fallen into the large cavity that underlies block 1.
Note the ledge on block 2 visible in the first photo just below the top index number "2". The missing rock above this ledge must also have been pushed southward and fallen to the base of the cavity. Block 2 is about 2 meters in length, top to bottom.
Scale views
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The photos above show views of the cavity beneath block 1. North is on the left and the views are directed downward and toward the east. The tape measure seen in the second frame of the bottom pair of photos reaches down to the deepest point achievable. The overall dimensions of the cavity below block 1 (length, width, height) are all of the order of 1-2 meters.
North-directed views
The above photo shows the bedrock wall forming the north boundary of the cavity beneath block 1. Although block 1 presumably dragged against this wall during upward displacement, no abrasion marks were left. Block 1 is now jammed against the top parts of blocks 2 and 6 (not visible in photo) with sufficient force to prevent block 1 from settling back into the underlying cavity.
Note the fault in the bedrock wall extending top to bottom of the frame just to the right of center. This fault was apparently formed when a portion of the wall (indexed as block 6) was displaced southward. The southward displacement became possible only after block 1 was displaced upward.
This picture shows a straight-on closeup of the central portion of the fault described in the preceding photo. An additional irregular fault with lesser displacement is visible running through block 6 to the right. Both faults reflect southward movement of sections of the north wall. This movement was presumably driven by horizontal ice pressure and lacked any upward component. Block 2, visible on the far right, is separated from the adjacent bedrock wall by a fault incorporating both horizontal and vertical displacement.
The above photo shows a portion of the north wall lying just to the left (west) of the section shown in the preceding photo. An indentation is visible where pieces of bedrock have been spalled from the wall, potentially by groundwater entering through a crack visible as a white line running through the spalled area. The spalling presumably occurred while the cavity beneath block 1 was filled with ice. No corresponding broken fragments could be seen at the bottom of the cavity below the indented area. Another view of the indented area shown above can be seen at roughly the 35 cm mark on the tape measure shown in the first photo under the heading "Scale" above.
The picture above shows the bottom end of the fault separating block 2 (an upward-displaced block) from the adjacent block 6. Rock along the fault has been brecciated by the frictional stress associated with the displacement of block 2.
The bottom of block 2 is visible in the above photo. It could not be determined if block 2 was displaced upward before block 1 underwent its much larger upward displacement or if block 1 helped drag block 2 upward. In either case, it is instructive to look at block 2 as a stand-alone example of upward displacement of bedrock.
Note that the rock directly below block 2 is brecciated. This is a common feature in all subsurface observations of upward-displaced bedrock. There is never a clean detachment surface beneath an upward-displaced block. This supports the hypothesis that groundwater freezes in bedrock before augmenting the ice mass underlying an upward-displaced block. The process of crystallization and the associated upward movement of ice through bedrock is intensely disruptive.
The above photo, taken looking north at the base of block 2 about 170 cm below ground level, shows the extensive brecciation of the bedrock underlying block 2.
Two views of the detachment surface at the base of block 2 are shown above. The apparent curvature of the ribbing on the detachment surface is primarily or entirely a fisheye lens effect. The ribbing follows foliation in the rock. Not all upward-displaced blocks show such pronounced ribbing on the upper detachment surface. However, the ribbing seen above is not unique, and similar observations have been made at other locations on the Avalon in different types of rock.
Clearly, the rock at the base of block 2 did not fail along a joint. Rather, the rock failed repeatedly along multiple planes of weakness associated with the foliation. This "tearing" failure mode implies a higher level of stress than would be required to initiate block displacement above a pre-existing joint. This observation favors the hypothesis that ice crystal growth pressure, rather than hydraulic pressure, initiated the rock failure.
Zoomed-out views of the base of block 2 and the brecciated bedrock on the floor of the main cavity are shown above, looking north. The snow seen on the floor of the cavity was blown down through entrance holes from the surface during a wind-driven snowfall. Other below-ground photos, not showing snow, were recorded earlier in the season.
The middle portion of block 2, about a meter below ground level, is shown above. A joint running from top to bottom of the block divides the block into two sections. Although the joint appears to have been penetrated by groundwater or ice, no relative rock motion has occurred. A ledge about 5 cm wide extends across both sections of block 2. The ledge, running across the middle of the frame, is hard to see in the above straight-on view. Some rock has been pushed out from the joint just below the ledge. Part of the apparent non-vertical angle of the side surfaces of block 2 is due to camera tilt.
Two more views of the central portion of block 2 show the ledge as described above along with the joint separating the block into two sections.
Both photos above show the upper northeast corner of the cavity underlying block 1. The second photo shows rock just above that shown in the first photo. Adjacent rock situated immediately east of block 2 has been brecciated by a combination of frictional interaction with block 2 and ice load applied from below. Block 5, lying to the east of the brecciated zone, appears relatively undisturbed. Separation from block 5 by rock comprising blocks 1 and 2 (along with the smaller intermediate blocks) probably occurred mainly in tension deriving from westward-directed glacial ice load.
Points of contact between block 1 (main block) and block 2 (forming part of the north wall) can be seen in the above two photos looking upward toward the north. Although block 1 weighs several tonnes, the frictional load borne by the contact against block 2 might be small since block 1 also bears substantially against block 6 at points to the west of block 2.
The multiple points of contact between block 1 and the north wall suggest that block 1 was eroded by dragging against the north wall as block 1 was forced into its final position. The dragging under high load abraded rock at the edge of block 1, removing high points and producing a conforming fit. The dragging may have occurred downward, during settling, rather than upward, during initial displacement.
East-directed views
Two views near the bottom of the cavity beneath block 1 are shown above. Typically, the rock at the bottom of a cavity beneath an upward-displaced joint block is strongly brecciated. However, in this instance, block 7 and other smaller blocks with a similar aspect ratio (wide in horizontal extent, low in height) obscure most of the bottom of the cavity.
These blocks are intermediate blocks that were dislodged by ice before settling into their present position after ice melted. The planes of foliation in these intermediate blocks are approximately parallel to the ground, indicating that the blocks have been rotated by 90 degrees. Brecciated rock at the bottom of the cavity extends at least 60 cm below the top surface of block 7.
Brecciated bedrock on the east side of the floor of the cavity is visible in the above photo. Larger blocks that were displaced upward without being intensely brecciated are also visible.
Based on below-surface observations at this site and at several other sites of upward bedrock displacement, it appears that the finest brecciation occurs in rock that remains unshifted at the bottom of the cavity. Presumably, ice either crystallizes from the melt within this bottom rock, or else ice that has crystallized at greater depth is forced through the bottom rock under high pressure. In either case, brecciation down to centimeter or millimeter scales commonly occurs. Sections of rock that detach from the bottom and move upward with intruding ice are subjected to less of the disruptive stress and are thus not brecciated as finely.
The above closeups of the floor of the cavity beneath block 1 show additional views of coarser fragments of brecciated bedrock overlying finer fragments.
The closeup above shows the bottom northeast corner of the cavity beneath block 1. The depth at this point is about 2 m below ground level. Block 5 overlies an indented slab(s) which then overlies more finely brecciated rock. Further brecciated rock covers the bottom of the indented area seen in the bottom left two thirds of the frame.
The parallel alignment of the fragments at the rear of the indented space indicates that these back fragments, although detached from bedrock, have shifted little in position. The indented area may indicate a point of entry for pressurized water/ice that contributed to filling the large cavity beneath block 1.
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The four pictures above show different views of the base of block 5, starting at the northeast corner and then aiming further toward the southeast with each successive photo.
Two more views looking up toward the northeast of the cavity beneath block 1 are shown above. Faults in the north wall of the main cavity are plainly visible.
The top northeast corner of the cavity beneath block 1 is shown in the above photo. Further pictures showing the load-bearing contact between block 1 and the north wall are provided under the heading "Upward views" below.
South-directed views
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The above three photos (second photo has camera tilted), looking south southeast, show the south end of block 1 where it makes contact with brecciated rock at the base of the main cavity. Block 5, forming the major portion of the east boundary of the main cavity terminates in coarsely brecciated bedrock about a meter before reaching the south end. At the south end of the cavity, the end of block 1 rests on a bed of coarsely brecciated rock fragments. The face marked with a yellow (#) for reference is the same face in all photos.
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Two more views of the south end of the main cavity, directed slightly west of south, are shown above.
Since there is no indication of conformance between the base of block 1 and any of the underlying broken rock, it appears likely that the south end of block 1 was initially displaced upward and then settled back after ice melted. The present elevated position of block 1 is the result of upward displacement and tilting, followed by debris obstruction at the south end and jamming against the wall at the north end as the block tried to settle back.
The two above photos, looking south southwest, provide side views of block 7. The visible side surface of block 7 resembles the bottom surface of the west side of block 1. This bottom surface is a detachment surface where block 1 was torn from bedrock. Block 7, lying with planes of foliation roughly parallel to the ground, possibly separated from the west side of block 1 and tilted 90 degrees as it settled. The top surface of block 7 does not follow a plane of foliation and looks like an above-ground eroded surface (see "Downward-directed views"). This somewhat weakens the argument that block 7 was once part of the below-ground west side of block 1.
Broken rock visible in the right background of the photos lacks indication of having failed along planes of foliation. When foliated bedrock is brecciated directly by intruding ice, the rock typically delaminates (at least partially) along planes of foliation. Completely irregular fracture surfaces imply rock failure induced by rock-rock interaction rather than by rock-ice interaction. It is likely that the fractured rock visible in the background of the photos was crushed against bedrock by block 1 while block 1 was being shifted by ice pressure.
West-directed views
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The above photos, directed westward and upward, show block 3 and block 8. Block 3 was likely detached from the west side of block 1 by shear stress induced by differential ice loading and/or friction with bedrock at the west side of the main cavity. The rock failure leading to the separation of block 3 occurred on a plane of foliation.
Block 8 shows little evidence of foliation and was likely formed as a result of rock failure along perpendicular cross joints. The block appears to have settled into its present position although it is unclear where it might have originated. Note that block 8 lies on a bed of irregularly brecciated rock. In contrast, the rock fragments alongside block 3 are fractured along planes of foliation and remain aligned. These fragments were probably peeled from block 3 by shear stress as block 3 was forced to slide against block 1 on the east and against bedrock or other large blocks lying to the west of block 3. This sliding potentially occurred under high compressional load while block 1 was being pushed westward by overlying glacial ice.
The above photo shows a zoomed-out view of the northwest corner of the main cavity beneath block 1.
Two closeups of the side surface and bottom end of block 3 are shown above. Although block 3 is jammed in position and did not settle, rock originally mating with the irregularly-indented north-facing surface of block 3 is no longer sufficiently nearby to be recognized. This rock might have been broken into small fragments or it might have settled to the bottom of the cavity in larger pieces. Either case emphasizes the dynamic high-stress underground ice environment that existed during the time that the upward displacement of block 1 was proceeding.
The above view looks southwest, showing what may be a detachment surface where block 3 was torn from bedrock..
Downward-directed views:
The above photo shows the view looking straight down along the north wall of the cavity beneath block 1. Note that block 2 is suspended over brecciated bedrock, while block 6 extends downward, presumably to the base of the main cavity. Differential motion between block 2 and block 6 has torn fragments from the sides of one or both blocks. These fragments are now jammed in the channel between the two larger blocks.
A view of block 7, along with other rock fragments covering the north half of the base of the cavity, is shown above. Some of the fragments (excluding block 7) have flat, parallel surfaces and are thin relative to their horizontal dimensions. These fragments were likely cleaved off block 1 or the walls of the cavity and then rotated while settling. The flat surfaces of the blocks generally follow planes of foliation. The foliation dips, as discussed earlier, at a near-vertical angle in surrounding bedrock. Thicker blocks that are partially rotated can be seen near the center right of the frame.
A view of the central area of the cavity floor is shown above. The top of the photo is south.
A view looking south along the east wall of the main cavity (block 5) is shown above. Note the brecciated rock extending beneath the base of block 5. These fragments apparently settled to the base of the cavity while block 5 was still in an elevated position. Block 5 then settled, dropping straight down into its present location.
Two views toward the south end of the cavity bottom are shown above. The large block seen in the upper left of the photos could be broken off from the south end of block 5. Block 5 terminates at the top of the above frames, ending at a gap extending southward.
A view of the northwest bottom corner of the cavity beneath block 1 is shown above. The origin of the very large block 7 is unclear, as is the process by which it was shifted to its present position. The shape of the top surface of block 7 is relatively smooth ,but does not conform to a plane of foliation or cross joint. The top surface of block 7 most closely resembles an above-ground rock surface that was eroded by sliding glacial ice. A closeup view of the surface of block 7 is shown below.
Upward-directed views:
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The three photos above show the underside of the central portion of block 1 starting at the north end and extending to the south end. The underside appears to be divided into three sections, a central section that shows a relatively flat smooth surface, and two side sections where rock has been torn to expose planes of foliation. These divisions are not apparent on the above-ground sides or top of block 1. Since the borders between the sections run roughly parallel to planes of foliation (bedding in the sedimentary rock is steeply inclined relative to foliation), it is unlikely that the bulk characteristics of the rock would change when crossing the borders. This implies that the bottom appearance of block 1 reflects variations in the mode of action of intruding ice rather than variations in characteristics of the rock comprising block 1.
The north wall of the main cavity, extending from block 3 (west) to block 5 (east), is shown in the above upward-looking view. The north end of the bottom surface of block 1 is also shown. This bottom surface was shaped when accumulating pressure of intruding ice caused the block to detach from underlying rock. The surface does not follow a single joint or plane of weakness, but rather shows a complex topography indicating tearing of bedrock, presumably under high stress. No conforming separation surface is seen at the bottom of the cavity because of the intense brecciation that accompanied the ice-driven upward displacement of block 1. Brecciation likely commenced before block 1 began to move upward, and fragmented rock undoubtedly became incorporated into the volume of ice accumulating beneath block 1.
When the ice beneath block 1 eventually melted, all blocks and rock fragments settled and only those that jammed remained elevated. The essential characteristics of the cavity beneath block 1 match observations recorded beneath many other upward-displaced blocks on the Avalon Peninsula. These characteristics include a jammed block with a well-defined under-surface overlying a cavity and then a deep accumulation of loose rock fragments of various sizes. A clean break, with conforming surfaces on bottom bedrock and on the underside of an upward-displaced block has not been seen. From observations made to date, it appears that brecciation of bedrock at depth is intrinsic to the process of upward block displacement.
A closeup of the east side of the bottom of block 1 is shown above. This view illustrates the complexity of the detachment surface formed when block 1 was torn from underlying bedrock. Much of the rock failure occurred along planes of foliation even though the orientation of these planes is roughly parallel to the displacement. This type of tearing breaks vastly more molecular bonds than does failure along a single plane perpendicular to the displacement. However, the broken bonds are the weak inter-layer bonds in aligned phyllosilicate crystals. These bonds might have been further weakened by the penetration of groundwater and the subsequent crystallization of ice between layers.
It should be noted that the slaty cleavage apparent on the torn rock is not usually well presented in the regionally metamorphosed sedimentary rock comprising block 1. Although the rock is comprised of aligned phyllosilicate crystals the intrinsic tectonic cleavage is generally not obvious unless the rock has been brecciated by ice. The highly visible cleavage (as seen above) appears to be a consequence of groundwater penetration, ice crystallization and ice-induced shear loading.
Repeating joints, following planes of foliation in the east side of block 1, are evident in the above photo. The white lines suggest that water may have infiltrated the joints in the past, or may be infiltrating the joints at present. Such infiltration provides a possible pathway for pressurized groundwater to have moved through the rock before block 1 became separated from underlying bedrock.
A widened joint can be seen intersecting the bottom of block 1 in the above closeup. This joint divides the central section of the underside of block 1 from the west section. The joint might have been widened by frost wedging, or it might have been widened by stress applied to block 1 as the block was torn from bedrock, displaced upward and tilted.
The above photo highlights the difference in surface texture of the central section of the bottom of block 1, when compared to the the east section or the west section. Little evidence of rock failure exploiting weaknesses along planes of foliation can be seen in the central section.
A view of the north end of the bottom of block 1 is shown above. This view shows visible evidence of tearing that exposed foliation in the west side of block 1. The north end of block 1 is jammed against block 6 (visible above) and against block 2 (out of view).
Tearing of rock that exposed planes of foliation on the west section of the underside of block 1 is evident in the above photo. Note the contrast between the central section (lower left, just above the patch of snow) and the west section.
The above photo shows the registration of block 1 against blocks 2 and 6. Note the indent in the upper right of block 6. Block 1 registers against this indent and against the adjacent unindented surface of block 6. This indicates that block 1 abraded itself and block 6 so as to produce the conforming fit. The fit against block 2 could have been tightened when block 2 was displaced southward by ice pressure. The upward displacement of block 2 (see "North-directed views", above) was probably not significantly hindered by the contact with block 1.
Block 2 shows a pronounced ledge below its point of contact with block 1. The rock that originally lay above this ledge became detached and was shifted away before block 1 and block 2 became jammed together.
A closeup of the load-bearing contact between block 1 and blocks 2 and 6 is shown above. About 50% of the weight of block 1 (estimated weight of block 1 ~ 10000 kg) is borne by this contact.
A good fit between the edge of block 1 and the north wall of the main cavity can be seen in the above photo. This fit is a likely result of abrasion that occurred when block 1 settled into its present position following the departure of underlying ice.
The above photo shows block 1 contacting block 2 above a ledge in block 2. The non-abraded sharp corner on the ledge implies that block 1 descended, rather than ascended to reach its present jammed position. This suggests that ice pressure forced block 1 upward past its present position and that block 1 then settled back until it became jammed.
Summary
The detailed example presented above shows an instance where a large block of well indurated rock was detached from bedrock and displaced upward. The block shows evidence of side-loading by glacial ice during or following upward displacement. This implies that the upward displacement occurred prior to the Holocene, while the area was still under glacial ice cover.
The upward-displaced block lies in a small valley and the implied ice-flow direction at the site requires glacial ice thickness of at least several tens of meters to clear a nearby ridge before ice could move toward the coast. Seasonal temperature variations are insufficient to drive a frost-heave process beneath more than 20 m of ice. The block was thus displaced upward by hydraulic pressure or by ice crystal growth pressure arising from a temperature gradient that was not seasonally induced.
Intense brecciation of rock in the cavity below the upward-displaced block, a complex pattern of rock tearing at the base of the block and wide leak paths for pressurized water all suggest that displacement by hydraulic pressure was unlikely. Pressure of accumulating ice in a cold subglacial environment thus stands out as the preferred explanation for the observed block displacement.
Thermodynamic considerations demand a cold subglacial environment to remove heat of crystallization and allow conversion of heat flow energy to mechanical energy through a freezing process. A source of mobile water in the liquid state is also needed. During the upward-displacement event, more than 1 cubic meter of groundwater needed to enter the cavity below the block and freeze.
The subglacial bedrock brecciation and upward displacement event illustrated by the above example is not unique. It has been repeated in thousands of similar occurrences found across substantial areas of the Avalon Peninsula.
AvalonSubglacial