Pseudo-fissures
A pseudo-fissure is an elongated cavity formed in bedrock when ice intrudes between two parallel joints, brecciates the rock between the joints, and then pushes the brecciated rock outward into glacial ice. Little or no lateral motion of bedrock blocks occurs during the formation of a pseudo-fissure. This contrasts with a normal fissure which forms when ice enters a joint and causes lateral displacement of adjacent bedrock, thereby widening the joint.
Pseudo-fissures provide strong evidence of the intrusion of groundwater into subglacial bedrock, the freezing of the intruding water, the brecciation and displacement of bedrock via ice crystal growth pressure and the glacial transport of displaced rock fragments. A bedrock landscape showing pseudo-fissures can be interpreted as an area where the process of subglacial bedrock brecciation has operated in the past.
A pseudo-fissure often closely resembles a normal fissure formed by frost wedging or glaciotectonic loading. Distinguishing a pseudo-fissure from a normal fissure requires an examination of the fissure to determine whether lateral motion of bedrock has occurred. The best indication of lack of lateral motion is provided by noting the presence of undisplaced bedrock remaining within the fissure. Since bedrock is brittle and incompressible, a normal fissure cannot remain partially occupied by sections of undisplaced bedrock.
Pseudo-fissures are abundant in the glacially brecciated bedrock areas of the Avalon Peninsula. Several examples are presented below. These examples support the theory that pressurized and crystallizing groundwater was instrumental in brecciating bedrock beneath retreating cold glaciers on the Avalon Peninsula at the end of the Younger Dryas cold period.
Example 00:
The row of blocks marked "A" above lies between two parallel joints. The blocks have been displaced upward slightly by subglacial bedrock frost heave, but they have not been brecciated and expelled from substrate. This feature is not a pseudo-fissure but rather is a precursor to a pseudo-fissure. In this instance, groundwater intrusion and associated ice accumulation was insufficient to complete the process of forming a pseudo-fissure.
Example 01:
A pseudo-fissure can be seen to the left of the hammer in the background of the above photo. The undisplaced blocks seen in the foreground preclude the possibility that the cavity appearing to the left of the hammer was formed by the widening of a single joint through lateral displacement of the adjacent bedrock.
A closeup of the pseudo-fissure is shown on the left. A tape measure shown in the right-hand photo reads 68 cm from the bedrock surface to the brecciated bedrock debris at the bottom of the pseudo-fissure.
Example 02:
Ferns can be seen growing amongst brecciated bedrock at the bottom of a pseudo-fissure in the above photo. Photos of the same feature, taken from the opposite end, are shown below.
Fragments of undisplaced brecciated bedrock can be seen occupying one end of the pseudo-fissure. The severity of brecciation increases in the direction approaching the open portion of the pseudo-fissure.
A closeup of the brecciated bedrock debris leading toward the open section of the pseudo-fissure is shown in the above photo. The bedrock fragments become more irregular and more deeply recessed as they approach the open section.
A view straight down into the pseudo-fissure is shown above. Although joints are apparent in the rock wall to the left, close inspection indicated that none of the joints were faults. The rock wall experienced little or no lateral shift during formation of the pseudo-fissure. The lack of lateral shift indicates an approximate balance of ice pressure on either side of the wall.
Example 03:
An incomplete pseudo-fissure is shown above. Ice accumulating below joint blocks in the central portion of the feature apparently pushed the blocks up into overlying glacial ice. Creep in the overlying ice then shifted the blocks to their present position at the end of the pseudo-fissure.
A closeup of blocks displaced from the pseudo-fissure is shown above. Note the abundant occurrence of bedrock blocks affected by subglacial upward displacement seen in the backgrounds of the above photo and the previous photo.
Two additional closeups of the pseudo-fissure are shown above.
Example 04:
A pseudo-fissure can be seen in the background of the above photo. The foreground shows an area of more disorganized bedrock brecciation.
A closeup of the pseudo-fissure is shown above. Several undisplaced joint blocks remain lodged in the pseudo-fissure.
Example 05:
The bedrock outcrop shown in the above aerial view contains a small pseudo-fissure, marked "A". The yellow strap is 1 meter long. The bedrock at this location is regionally-metamorphosed well-indurated Ediacaran sedimentary rock. The rock has likely been altered by metasomatism. Bedding dips at about 30 degrees, but the rock does not separate along bedding planes. Although regional metamorphism has imparted a slatey (sometimes schistose) texture to the rock, the rock generally does not preferentially fracture along planes of foliation unless delaminated by Pleistocene subglacial ice/groundwater action.
Two views of the pseudo-fissure are shown above. The tape measure seen on the left reads 60 cm from the bedrock surface down to brecciated rock at the bottom of the pseudo-fissure.
Two views of brecciated bedrock at the top of the pseudo-fissure are shown above. The section of brecciated bedrock has been delaminated along planes of tectonic foliation by the crystallization of water within the rock or by ice that was extruded through the rock. The pseudo-fissure remains open beneath the brecciated area, with the section of brecciated bedrock extending downward from the surface by about 15 cm. A fiber-optic digital inspection camera was used to view the brecciated rock from below.
A closeup view looking upward at the bottom of the section of brecciated bedrock was obtained by inserting a digital inspection camera into the pdeudo-fissure. Although the delamination of the rock extends from top to bottom of the brecciated area, close examination indicated that the rock was not macro-shifted by ice pressure. Rather, the rock was delaminated in situ.
Example 06:
The above aerial photo shows a bedrock outcrop with a small pseudo-fissure located just to the left of "A". This feature is nearby to the outcrop shown as Example 05 and is composed of the same type of regionally metamorphosed sedimentary rock.
A ground-level view of the outcrop and pseudo-fissure is shown above. Both the near end of the pseudo-fissure and the far end are terminated with a section of bedrock that has been delaminated in situ. Closeup views of the near end (first photo) and far end (second photo) are shown below.
The pseudo-fissure extends beneath both of the delaminated areas shown above. Although not plainly visible in the perspective of the second photo, a small slab of upward-displaced bedrock extends out from the pseudo-fissure just to the left of a patch of vegetation. This is shown more clearly in the photo below.
A slab of upward-displaced bedrock can be seen extending out from the pseudo-fissure to the right of "A" in the above photo. The open central portion of the pseudo-fissure was presumably formed when fragments similar to "A" were expelled completely from substrate by ice pressure and then carried away by overlying cold glacial ice deforming in creep.
Example 07:
Since the bedrock outcrop seen above is unconstrained on the left, ice accumulating in a joint or, alternatively, glacial ice moving from right to left, could be expected to laterally displace rock and form a normal fissure. However, debris remaining in the large fissure seen on the left suggests that this feature is primarily a pseudo-fissure.
The two photos above show debris in the gap that, along with the varying width of the gap, imply that the gap in the bedrock is a pseudo-fissure. There are no faults evident among the repeating joints seen on either side of the pseudo-fissure. In the absence of glacial ice cover, the ice pressure needed to expel rock from the pseudo-fissure would be expected to displace rock to the left, creating a normal fissure. However, under thick glacial cover, the bedrock outcrop is constrained on the left by ice. A balance of pressure then determines how the rock will move. If the constraint from above is less than the constraint from the left, rock will preferentially be displaced upward. A small amount of leftward rock displacement might have helped initiate pseudo-fissure formation by reducing rock-on-rock friction that would tend to hinder upward rock displacement.
Example 08:
The above aerial photo shows an inland cliff edge formed from thickly-bedded fine-grained sedimentary rock. The flat surface where the 1 meter long yellow strap lies is a bedding plane surface, dipping downward away from the cliff edge at about 20 degrees. The large, irregular gap to the right of "A" is a normal fissure formed when block "A" was pushed outward from the cliff edge by ice pressure. The much smaller and straighter gap below and to the right of "B" is a pseudo-fissure.
A ground-level photo of the fissure "A" is shown above. It is unlikely that this feature was formed by glaciotectonic action. The feature is located at a presumed ice divide midway between two bays. While ice flow directions undoubtedly varied at this location over the course of the Pleistocene, only the final direction of ice flow at the end of the Younger Dryas could account for the opening of the observed fissure. Evidence from nearby subglacial frost-heaved features that were deflected by ice movement shows late-stage ice motion was minimal, ambiguous or in the opposite direction to that needed to open the fissure. The fissure is apparently a result of pressurized groundwater (and associated ice formation in a cold subglacial environment) emanating from the cliff face during deglaciation.
Four top views of pseudo-fissure "B" are shown above. The pseudo-fissure measured 84 cm deep at the point where the tape is located.
A side view of the cliff face and the rock containing the pseudo-fissure is shown above, along with a closeup of brecciated bedrock below the pseudo-fissure.
Views of the cliff face just below the pseudo-fissure, taken from opposite ends, are shown above. Pressurized ice appears to have displaced rock sideways, out from both ends of the pseudo-fissure, as well as displacing rock upward.
Example 09:
A cliff-edge pseudo-fissure is shown above. The pseudo-fissure commences near the lower end of the 1 meter yellow strap and extends toward the left of the frame.
The two above views taken from opposite directions show rock debris inside the pseudo-fissure.
Two more views of the feature are shown above. The bedrock in the immediate area has been severely brecciated by subglacial groundwater/ice intrusion. It can be inferred that little movement of overlying glacial ice occurred after the pseudo-fissure was formed. The slender and highly jointed outer wall of the feature would be unlikely to survive significant glaciotectonic loading/displacement without disintegrating.
Delamination of the outer wall of the pseudo-fissure can be seen in the above photos. The delamination generally follows planes of tectonic foliation in the regionally metamorphosed sedimentary rock. The delamination might have been caused by groundwater penetrating the rock and freezing in a cold subglacial environment. Alternatively, glaciotectonic loading acting parallel to the cliff face could have caused the rock to fail repeatedly in shear.
Example 10:
A large pseudo-fissure at the edge of a cliff is shown in the above aerial photo. The yellow strap seen center-right in the frame is 1 meter long. While this feature is clearly not a normal fissure, a question arises as to whether rock was ejected upward from the gap (the usual case when a pseudo-fissure is formed) or whether rock fragments were pushed sideways out of the gap toward the open end of the feature. The same question arises in considering the previous Example 09.
It is unlikely that glacial ice movement alone, even if appropriately directed, could account for the formation of a long narrow slot without simultaneously displacing the rock on the external wall of the pseudo-fissure. It is more reasonable to suggest that pressurized and crystallizing groundwater emanating from the inside rock face might push rock fragments out of the end of the slot if the pressure gradient in the overlying glacier was appropriately directed. However, the friction associated with displacement along the slot is higher than the friction associated with upward displacement. Furthermore, foliation in the bedrock makes the external rock wall more likely to fail (hence completely disintegrate) under loading along the slot than under upward-directed loading.
Four more aerial views of the pseudo-fissure are shown above.
A view of the front (open) end of the pseudo-fissure is shown above.
The above photo shows a view along the pseudo-fissure from the back (closed) end. Note the accumulation of dislodged joint blocks at both the front and back ends of the feature. In its central portion, the pseudo-fissure is about twice as deep as it is at the front (open) end.
Two views from inside the pseudo-fissure are shown above. The first view shows brecciated rock at the front (open) end, while the second view is directed toward the back end of the pseudo-fissure.
A closeup of brecciated bedrock at the back end of the pseudo-fissure is shown above.
The outward-facing surface of the outer wall of the pseudo-fissure is shown in the above two photos. The front (open) end of the pseudo-fissure is toward the right. A large cavity is visible in the left section of the rock wall. This cavity aligns approximately with the back end of the pseudo-fissure. The cavity appears to have resulted from the displacement of blocks directly into glacial ice abutting the cliff face. Some partially displaced blocks are still lodged in the cavity. The same ice pressure that formed the pseudo-fissure likely caused the cavity to form when ice penetrated the outer rock wall.
Brecciated bedrock behind the back end of the pseudo-fissure is shown in the above photo. Several narrow fissures or pseudo-fissures are visible, along with bedrock blocks affected by subglacial upward displacement.
The entire ridge containing the pseudo-fissure has been affected by subglacial bedrock brecciation. The disrupted rock includes several examples of large individual joint blocks that were displaced vertically by ice (subglacial upward displacement) along with mounds of joint blocks displaced upward chaotically in areas of intense local brecciation.
Example 11:
Two aerial photos of a bedrock ridge that has been disrupted by intruding subglacial groundwater and consequent ice pressure buildup are shown above. The long gap crossing the center of the bedrock ridge is a pseudo-fissure. Other smaller fissures are also visible on the ridge. Some of these may be normal fissures. Note the presence of a series of joint blocks remaining in the gap at one end of the presumed pseudo-fissure. These undisplaced blocks, along with others in the middle section of the gap provide evidence that the long gap is a pseudo-fissure rather than a normal fissure.
The above two views, looking in opposite directions, show the central section of the pseudo-fissure. Brecciated bedrock fragments and vegetation make it difficult to determine the depth of the feature, but most portions appeared to measure about 50 cm deep before encountering debris.
A chain of undisplaced joint blocks at the end of the pseudo-fissure can be seen in the above photo. This terminal portion of the bedrock ridge has been more severely brecciated than the central area.
Pressurized groundwater penetrated the bedrock ridge through a pair of long parallel joints beneath what is now the pseudo-fissure. As groundwater approached the cold glacier/ bedrock interface, ice crystallization generated sufficient overpressure to drive joint blocks up into overlying glacial ice. These blocks were subsequently removed from the area by ice creep occurring in the overlying cold glacier. Since the velocity of glacial ice creep approaches zero at ground level, the blocks needed to rise substantially above ground level before they could be transported laterally.
At the end of the ridge (seen above), lateral confinement by bedrock diminished and local ice overpressure was relieved by a more generalized brecciation. In this terminal region, joint blocks displaced within the pseudo-fissure zone were not shifted upward sufficiently to be cleared away by glacial ice creep.
Example 12:
In the above photo, a tape measure is shown extending 38 cm down into a slot in an outcrop of sedimentary bedrock. If the slot was a normal fissure, its depth would be expected to remain constant along its length.
In the above photo, the tape has been moved to a different point along the same slot as shown in the previous photo. The depth of the slot is 18 cm at the point indicated.
The above photo shows an overexposed closeup of the bedrock outlined with the magenta rectangle in the previous photo. The slot is 18 cm deep at the left of the frame, diminishing in depth toward the right. Bedrock that has been brecciated, but not displaced, is visible at the bottom of the slot. The tapering of the slot, combined with its diminishing depth suggests that the slot is a pseudo-fissure. Examination of the other slots seen in the preceding photos of this bedrock outcrop indicated that the other slots are also pseudo-fissures.
Example 13:
The above photo shows a terrace near the top of a high glacially-carved cliff. The regionally metamorphosed sedimentary rock comprising the cliff seems highly resistant to freeze-thaw weathering and associated mass wasting. Rather, the rock debris seen along the base of the terrace appears to have been dislodged by a subglacial brecciation process. The block marked "A" in the photo has not been displaced outward from the cliff edge. A pseudo-fissure is evident behind the block.
The cliff edge seen in the above photo was formed when joint blocks were separated from bedrock by pressurized groundwater converting to ice beneath a cold glacier. Many of the blocks entered the glacier and were carried off the terrace and over the edge of a high cliff seen to the right. The block at the center of the photo ("A" in the previous photo) has a gap behind it that is partially filled with brecciated bedrock. This gap is a pseudo-fissure where rock fragments were expelled sideways rather than upward.
A closeup view (overexposed to show recessed area) of the pseudo-fissure behind block "A" is shown above. The bedrock behind the block was brecciated by the intrusion of groundwater and the resulting production of over-pressurized ice. A portion of the brecciated rock was expelled into ambient cold glacial ice and transported, while additional brecciated rock that was not removed can be seen in the photo.
A closeup view of brecciated rock in the pseudo-fissure behind block "A" can be seen in the above photo.
More examples:
Three more examples of pseudo-fissures are shown above.
Summary:
Pseudo-fissures demonstrate several key aspects of the hypothesis that the freezing of pressurized groundwater beneath a cold glacier brecciated bedrock on the Avalon Peninsula.
It can be deduced that pseudo-fissures are subglacial in origin because some or all of the rock originally present in most pseudo-fissures has been removed completely from the site by moving glacial ice. Pseudo-fissures are not solely glaciotectonic features because glaciotectonic action cannot displace rock upward from within a deep narrow slot.
Pressurized groundwater, acting alone, cannot displace rock fragments from a pseudo-fissure. Leakage around dislodged fragments would relieve hydraulic pressure unless groundwater flow was very high. High groundwater flow rates would require substantial (hence obvious) inlet channels.
Pseudo-fissures are a subglacial upward-displacement phenomenon where rock is brecciated and displaced by ice crystal growth pressure. The requisite groundwater enters through pores and joints in bedrock. Thermodynamic considerations reduce ice crystal growth pressure to zero under temperate (warm-based) ice conditions. Thus, the formation of pseudo-fissures requires cold-based glaciation.
AvalonSubglacial