Avalon Subglacial
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Cold deglaciation, pressurized groundwater and the origin of ice-disrupted bedrock features on Newfoundland’s Avalon Peninsula
During the Pleistocene epoch, the Avalon Peninsula of Newfoundland was extensively and repeatedly glaciated. Ice caps and glaciers, typically 100's of meters thick, shaped, smoothed and fractured bedrock outcrops while transporting and depositing glacial erratics and till. Much of the erosion was caused by warm-based glacial ice, moving in basal-sliding mode. The resulting Holocene landscape incorporates zones of bare glacially-contoured bedrock interspersed with areas of glacial drift that are often vegetated (sometimes forested) or covered with peat. Intact bedrock surfaces, exposed and smoothed by basal-sliding glacial erosion, provide a background against which disruptive follow-on erosion processes, specifically linked to deglaciation, are made distinct and recognizable.
In several parts of the Avalon, Holocene-epoch deglaciation has left behind ice-disrupted bedrock outcrops where the extent and abundance of bedrock disruption stands out as both extreme in degree and unusual in character. Such atypical ice-disrupted bedrock outcrops are found in widespread lightly-vegetated zones (barrens) extending over at least 220 square kilometers of the inland Avalon Peninsula. They occur in a variety of regionally-metamorphosed bedrock types, including well-indurated ancient (Ediacaran) siltstones and sandstones along with well-indurated ancient volcanic rocks (flows, tuffs and ignimbrites).
At the affected sites, the erosion patterns observed on exposed bedrock are atypical in that they indicate extensive frost wedging, a process that is unexpected in a region sculpted mainly by the action of warm-based Pleistocene-epoch glaciers. Frost wedging is thermodynamically prohibited beneath a warm-based glacier and relict frost-wedged features that roughen bedrock are vulnerable to overprinting (transporting of loosened rock or smoothing) by basal-sliding glacial erosion.
The survival of heavily roughened or brecciated bedrock erosion features implies that the features were formed after basal-sliding glacial erosion had largely ended. Thus, the distinctive frost-wedged features seen on the Avalon must have been formed under late or post-Pleistocene climatic conditions, that is, near the end of the Younger Dryas cold period or during the early portion of the Holocene. The particular features of interest include:
-- Bedrock fragments (often joint blocks) that have been displaced upward, resembling instances of subaerial bedrock frost heave.
-- Bedrock (regionally metamorphosed, hence foliated) that has been widely and strongly grooved or delaminated along planes of foliation.
-- Networks of bedrock fissures.
-- Bedrock that has been intensely brecciated on scales ranging from centimeters to meters, but not subsequently glacially transported.
-- Accumulations of large, angular, rock fragments at the base of glacially-carved cliffs. These rock accumulations sometimes resemble scree, but show evidence of cold subglacial origin.
The features listed above can occur on slopes and near cliff edges, but they often occur on ground that is approximately level.
A cursory analysis of the atypical ice-induced erosion features found on the Avalon could lead to the conclusion that two sequential processes were at work. First, bedrock was stressed, fractured and shifted mechanically by moving warm-based glacial ice during the late stages of the Wisconsin glaciation of the Avalon. Then, bedrock was further disrupted by subaerial bedrock frost heave, matching the subaerial process observed in present-day non-glaciated polar areas. This two-phase line of interpretation implies that the Avalon region experienced a significant interval of dry periglacial climate conditions (polar desert) following deglaciation.
Definitive observations indicate that what superficially resembles freeze-thaw weathering or subaerial bedrock frost heave on the Avalon Peninsula is actually the consequence of a subglacial process. Frost-wedged and frost-jacked bedrock features, unexpected in the context of warm-based glacial ice cover, were nevertheless formed subglacially. In light of this conclusion, the assumption of a late-Wisconsin interval of polar desert conditions on the Avalon Peninsula is no longer warranted. Instead, the deduction that brecciation of bedrock found on certain portions of the Avalon Peninsula occurred primarily beneath substantial glacial ice focuses attention on the mechanical and thermal energy conversions that can occur beneath cold glacial ice cover.
When bedrock fragments are shifted by ice in a direction tangential to the surface of the ground, they can be interpreted as being shifted solely by glacial ice loading. Within this interpretation, the energy needed to break and move the rock was mechanical energy directly or indirectly derived from glacial ice moving downhill. Since this process does not entail conversion of heat into mechanical energy, a temperature gradient is not required and the energy conversion can be highly efficient.
Tangentially-directed bedrock shifts are not always the result of glacial mechanical action. Shifts tangential to the ground (for example, frost-wedged fissures) can also result from thermal action in a process analogous to that described below for normally-directed bedrock shifts.
Rock fragments that are shifted in a direction incorporating a significant component normal to the ground surface (resembling bedrock frost heave) can be interpreted as rock that was shifted, at least in part, by the conversion of heat flow energy to mechanical energy via ice crystal growth pressure (analogous to ice segregation in frozen soils). This subglacial bedrock erosion process, driven by heat transfer and requiring liquid-phase water, implies a subfreezing thermal regime (hence cold-based glaciation). Additionally, since it involves a water-to-ice phase transition, the process requires an ambient temperature gradient sufficient to remove the heat of crystallization. The efficiency with which ice can shift rock in a thermally-driven process is limited by the second law of thermodynamics and is correspondingly very low. In most instances a small fraction of one percent of heat flow energy can be converted to mechanical energy during freezing.
Subglacial pressurized groundwater can fracture bedrock and displace fragments normal to the ground through purely hydraulic action. This potentially efficient process, like the brecciation of bedrock by direct glacial ice loading, requires no temperature gradient and can occur beneath temperate glacial ice. In practice, most observations of non-tangential rock displacements seen on the Avalon appear to preclude interpretation as solely hydraulically-driven occurrences (see Upward-displacement).
When tangentially-directed and normally-directed bedrock shifts are observed in close association, it suggests that mechanical and thermal processes were operating in tandem. Cold glacial ice does not typically slide on the ground and is commonly assumed to cause little erosion. However, when cold ice under tangential load cannot slide, it necessarily transfers shear stress to the ground. If the basal shear stress exceeds the strength of a frozen substrate (bedrock), the substrate will yield. The process is accelerated when bedrock substrate is simultaneously stressed or fractured by a thermally-driven process. In this case, the mechanical action of cold glacial ice can cause erosion matching the severity of lee-side "plucking" commonly associated with warm-based glacial erosion.
A thermally-driven subglacial bedrock-disruption process requires a source of mobile groundwater. During a protracted period of thick, warm-based glaciation, permeable bedrock will be penetrated and saturated by subglacial water. During deglaciation, ambient pressure at the ice-rock interface diminishes and compressed groundwater rebounds elastically toward the surface. Rock also rebounds elastically, but water rebounds more than rock because of the lower bulk modulus of water relative to common rock-forming minerals.
If deglaciation occurs while the ice-rock interface remains at a subfreezing temperature (cold deglaciation), groundwater becomes confined below surface at elevated relative pressure. Trapped, pressurized groundwater thus results when deep, warm-based glaciation transitions to deep cold-based glaciation and then to shallow cold-based glaciation and then to deglaciation. The resultant hydraulic overpressure, combined with an adequate thermal gradient can lead to normally-directed bedrock disruption occurring independently or in conjunction with tangential (basal shear) loading by cold glacial ice. The frequent operation of this type of process on the Avalon Peninsula can be inferred from the abundance of pronounced subglacial upward-displaced (frost-heave-like) bedrock features observed over wide areas.
The scale and extent of thermal-origin bedrock disruption seen in close association with mechanical-origin bedrock disruption, suggests a dynamic deglaciation environment on parts of the Avalon Peninsula. This dynamic environment was characterized by strong temperature gradients and high rates of ice creep. Observations in several areas reveal intense and highly localized (down to meter-scale) glaciofluvial erosion of bedrock features, including erosion of upward-displaced bedrock features. Such tightly-channeled glaciofluvial erosion suggests the sudden drainage of melt ponds, even while adjacent glacial ice and bedrock remained at a subfreezing temperature. Deviatoric stress associated with the irregular loading around subglacial cavities would lead to high rates of ice creep and intense and variable shear-loading of bedrock.
Bedrock erosion features seen on the Avalon Peninsula imply a sequence of extreme fluctuations of climate. During the Younger Dryas cold period, originally temperate glaciers and ice caps were converted to cold-based ice cover and a freezing front was driven one to three meters deep into underlying bedrock. Heat conduction through thick glacial ice, acting alone, could not account for the lowering of temperature at the ice-rock boundary. Rather, advection driven by the creep of steeply sloping ice must have aided significantly in moving heat from the base of the ice to the surface. At the end of the Younger Dryas period, a sudden onset of extreme climate change (warming) caused rapid deglaciation. Time was insufficient for ice at depth to warm, even as surface ice was quickly stripped off by a combination of melting and creep.
As ice thickness diminished, groundwater confined beneath frozen bedrock moved toward the surface, primarily following permeable boundary layers between tectonically tilted beds, but also following foliation and cross jointing (all of the bedrock in the affected areas has been regionally metamorphosed). Upon reaching the freezing front, groundwater was forced to move through frozen rock as pore water, kept in a mobile (quasi-liquid) state by capillary effects. When bedrock was insufficiently confined, pore water crystallized to ice in cavities and the resultant crystal growth pressure shifted bedrock fragments (often as joint blocks). Ongoing advection in overlying glacial ice worked alongside thermal conduction to remove the heat of crystallization.
Rapid climate change and accompanying rapid deglaciation appear to have caused an intense process of bedrock erosion on the Avalon Peninsula. In severity, the erosion resembles glacial "plucking" directed upward from horizontal surfaces or from gently or moderately sloping surfaces. Ice created from crystallizing groundwater augmented existing glacial ice, yielding the effect of a glacier seeming to emerge from bedrock and flow partially or totally normal to the ground (non-tangential ice flow). This occurrence might be unique to the Avalon Peninsula, but might also have contributed to unusual bedrock erosion features seen in Wales. The Welsh features, as for example those seen on Glyder Fawr and Glyder Fach, are presently attributed to late Pleistocene periglacial activity or to Holocene freeze-thaw weathering.
The implications of rebounding groundwater beneath diminishing cold glaciers could be significant in the context of modern-day climate change. If elastically rebounding groundwater reaches the base of a cold glacier without freezing, a cold-based glacier resting on bedrock can suddenly convert to a warm-based glacier, accelerating its flow. Accordingly, accelerating climate warming could cause bedrock areas of Greenland or Antarctica to deglaciate more rapidly than would be expected if glaciers remained cold-based.
The subglacial groundwater-rebound model has significant implications for the bedrock geomorphology of parts of the Avalon Peninsula. The process is intensely disruptive to jointed or foliated bedrock, causing a macro-scale bedrock volume expansion (density decrease) as joints are widened by accumulating ice. The process in some respects resembles the brecciation of rock by intruding magma, where ice takes on the role of magma. A matrix-supported breccia results, which then partially collapses as the matrix (ice) departs in a warming climate. The remaining fragmented bedrock would be highly vulnerable to displacement in a subsequent episode of basal-sliding glaciation.
The rugged inland topography of parts of the Avalon Peninsula could reflect a repeating cycle of bedrock fragmentation and removal as episodes of warm-based glaciation were followed by transition to cold-based glaciation and then deglaciation. If the depth of bedrock disruption presently observed (1 to 3 meters) was repeated in past episodes of cold deglaciation, then each glacial-interglacial cycle would efficiently strip off 1 to 3 meters of bedrock. During extended intervals of warm-based basal-sliding glaciation, this fragmented bedrock would be deposited on lowlands as coarse till or else carried out to sea. Such a process could account for some of the substantial inland glacially-formed cliffs seen in specific areas of the Avalon. This interpretation implies that the Younger Dryas cold period was not unique, but rather was matched by similar intervals accompanying previous major glacial cycles.
In summary, observations of bedrock geomorphology on parts of the Avalon can be linked to paleoglacial thermal regimes and to paleoglacial hydrogeology. This linkage can then be used to make inferences regarding climate changes at the end of the Wisconsin glaciation and potentially at the ends of preceding glacial periods.
