Dynamic Deglaciation
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Bedrock Brecciation During Rapid Deglaciation of the Avalon Peninsula

​March, 2024

Synopsis

Glacially-sculpted bedrock outcrops are abundant on Newfoundland’s Avalon Peninsula.  Many of these outcrops show zones of smoothed bedrock, sometimes accompanied by lee-side plucking, that typify landforms shaped by basal sliding glacial erosion.  Other zones of exposed bedrock have been extensively roughened or severely disrupted by ice in an atypical manner, seemingly incompatible with freeze-thaw weathering or with erosion by a sliding warm-based glacier.  These atypical ice-disrupted bedrock outcrops are of special interest because observations of patterns of rock displacement can provide evidence regarding unusual late-stage glacial/hydrogeologic activity.  These observations of displaced bedrock are then potentially indicative of unusually severe climate change immediately preceding the onset of the Holocene. 

Two substantial areas, the Isthmus of Avalon area and the Heart’s Content Barrens area, show atypical patterns of ice-induced breakage and displacement of bedrock.  Specifically, ice-disrupted bedrock outcrops in both regions show instances of subglacial brecciation that, in many respects, resemble (and hence could be misinterpreted as) occurrences of modern freeze-thaw weathering or of subaerial (periglacial) bedrock frost heave.  This atypical mode of subglacial bedrock fracture and displacement was apparently caused by two separate processes working in concert:

 1)  Stress transmitted to bedrock by cold glacial ice deforming in creep.
 
 2) Ice crystallization pressure and hydraulic loading accompanying the depressurization of groundwater confined beneath frozen subglacial bedrock.

The glacial conditions leading to the occurrence of the combined processes (items 1 & 2 above) can be termed dynamic deglaciation.  Dynamic deglaciation is a consequence of the sudden melting of a cold glacier.  The record of dynamic deglaciation on the Avalon is distinct and recognizable because bedrock in the affected areas is well exposed, highly resistant to Holocene weathering and abundantly disrupted by intense late-stage glacial/hydrogeologic action.


Climate and glaciation 

At the time of the last glacial maximum (LGM), approximately 25000 years before present (BP), the entire Avalon Peninsula and adjacent offshore areas lay under glacial ice that was 100’s of meters thick.  At LGM and through most of the Pleistocene, the glacial ice cover was temperate at depth.  Temperate conditions resulted because the blanket of glacial ice was sufficiently thick to insulate bedrock and allow geothermal heat flux to bring the basal ice to the melting point.   Frictional heating from ice movement would augment the geothermal heating.   Evidence for warm-based glacial activity is found in bedrock features that have been smoothed, streamlined and striated by basal-sliding glacial erosion.  Such features are common on the Avalon Peninsula.

About 14700 years BP, a sharp warming trend (the Bolling Allerod interstadial) caused rapid thinning of North American ice sheets.   About 12900 years BP, glacial-period climate conditions suddenly returned to Newfoundland with the onset of the Younger Dryas stadial.   In one of the leading theories developed to explain a rapid descent back into glacial conditions, meltwater resulting from thinning of the Laurentide ice sheet interrupted the Atlantic meridional overturning circulation (AMOC).  This slowed ocean heat transport from the tropics to the surface waters of the North Atlantic.  The Younger Dryas cold period lasted until 11700 years BP and was followed by very rapid climate warming (presumably caused by a restart of the AMOC), concluding in conditions approximating those of the Holocene today.

It can be reasonably postulated, although without direct evidence, that ice sheets overlying the Avalon Peninsula thinned substantially during the Bolling-Allerod and then remained static or began to rebuild during the Younger Dryas.  With the thinner ice cover and the drop in average surface air temperatures accompanying the Younger Dryas, basal glacial ice converted from a temperate condition (water/ice equilibrium) to a cold condition (ice below the melting temperature).

The conversion to cold ice might have occurred primarily via thermal conduction, although advective heat transfer resulting from ice movement would accelerate the process.  Based only on thermal conduction and with an average annual air temperature of -10 degrees, less than 50 years is needed to cool a uniform temperate ice column to -2 degrees at 100 meters depth.  This calculation assumes that the initially temperate ice had minimal liquid water content.   With a water content of 1%,  the 100 m ice column would take about twice as long to reach -2 degrees at the base.

Direct evidence for the transition to cold-based glacial conditions in the Isthmus of Avalon area and the Heart’s Content Barrens area is provided by patterns of bedrock erosion.  Upward displacement of bedrock fragments (usually joint blocks) is common in both areas.  Displacement of joint blocks upward (often vertically) from substrate is a critical indicator of action that cannot be attributed solely to the movement of glacial ice.  The upward displacements resemble periglacial frost heave occurrences, except that in many instances the upward-displaced fragments have been further rotated or translated by side-loading.  The side loading implies that glacial ice remained present while bedrock fragments were being displaced upward. 

Upward displacement of bedrock, either by hydraulic action or by ice crystal growth pressure, requires a cold-ice environment to seal leaks and/or remove heat of crystallization.   Evidence from large upward-displaced blocks found on the Avalon Peninsula suggests that cold conditions penetrated as much as 3 meters deep into subglacial bedrock during the Younger Dryas.

At the end of the Younger Dryas, an abrupt change to a warmer climate occurred on the Avalon.  This climate change led to rapid and complete deglaciation even as the depleting ice cover remained cold-based.  Rapid warming at the surface of a cold-based glacier will cause accelerated ice loss but will not quickly produce a warm-based glacier.  Surface ice temperatures cannot exceed zero degrees, slowing conductive heat flow from above even when air temperatures are many degrees above zero.

Advective heat transfer is hindered near the base of a cold glacier because basal glacial ice deforms slowly when frozen onto bedrock.  Downward advective heat transfer across the rock-ice boundary and into frozen bedrock cannot occur.  However, rising groundwater can transfer heat advectively into frozen bedrock from below.    

While abrupt climate warming will not tend to create temperate glaciers from cold glaciers, the thinning of ice caused by fast warming will cause a rapid decrease of hydrostatic pressure at the ice-rock interface.  This pressure decrease creates an overpressure condition in an aquifer confined beneath frozen bedrock.

The combination of groundwater overpressure and frozen permeable bedrock is a principal cause of rapid and intense bedrock brecciation during deglaciation.  The term dynamic deglaciation emphasizes the high thermal gradients and high pressure gradients that accompany the rapid removal of glacial ice cover from frozen bedrock. 

Pressurized groundwater can fracture and shift bedrock directly by hydraulic stress.  Alternatively, with an adequate thermal gradient, water can freeze in porous bedrock and convert heat to mechanical energy through crystal growth pressure.  This latter mode, resembling periglacial bedrock frost heave, was an important mode of subglacial bedrock brecciation on the Avalon.

The rapid melting of cold glacial ice cover in a fast-warming climate produced glacial karst on the Avalon  at the end of the Younger Dryas.  The karst topography was created when warm water accumulated in melt-ponds and then drained suddenly to the glacial base.  Torrents of meltwater flowed through subglacial channels across bedrock creating a complex glacial topography of channels, ice caves, ice pillars, ice bridges and funnel-shaped depressions.  Evidence of glaciofluvial bedrock erosion along multiple irregular pathways is abundant on the Avalon in areas where atypical ice-disrupted bedrock is prevalent.

Further evidence for the past existence of glacial karst is found in the inconsistent direction of deflection often seen in nearby upward-displaced sections of bedrock.  Sometimes a group of vertically-displaced bedrock slabs will be distinctly tipped, all leaning to the north (for example) while another group a few meters away will be tipped southward.  This inconsistent deflection implies glacial ice moving in multiple directions over a small region.  The high and variable basal stresses imparted by deforming cold glacial ice in a karst topography would explain these observations.  Glacial karst is unlikely to persist in glacial ice cover that is not stagnant.  An assumption of stagnant ice in the latter stages of deglaciation is reasonable given the presence of cold (non-sliding) ice and the generally low relief of bedrock features on the Avalon Peninsula.    

 
Host bedrock

Percolating and freezing pressurized groundwater generates characteristic and recognizable features of subglacial brecciation in frozen bedrock.  The characteristic features are found only at locations where deep underlying host bedrock is both porous and permeable.  The present-day recognizability of pre-Holocene rock breakage is enhanced  when exposed bedrock is highly resistant to Holocene weathering that could otherwise obscure or obliterate 11000 year-old brecciation features.

The bedrock exposed on the Isthmus of Avalon and on the Heart’s Content barrens has been folded and regionally metamorphosed.  All of the bedrock on the Heart’s Content barrens is of sedimentary origin, while bedrock on the Isthmus is a mixture of volcanic rock and sedimentary rock.  The volcanic rock found on the Isthmus is largely extrusive and occurs in thick beds corresponding to recurrent episodes of volcanic activity.  Both the sedimentary and volcanic rocks tend to be comprised primarily of phyllosilicate minerals.

Most of the rock on the Isthmus and on the Heart’s Content Barrens is of Ediacaran age.  The bedrock  in both areas is well indurated and appears highly resistant to erosion under Holocene conditions.  The rock has typically been strongly foliated by tectonic compression or tectonic shear and has been widely penetrated by hydrothermal fluids (hence metasomatized) in many places.  The rock shows little susceptibility to freeze-thaw weathering and typically erodes by dissolution in rainwater at a rate comparable to the erosion rate of exposed hydrothermal quartz.  Soil formation from the rocks is poor, as evidenced by the general lack of vegetation in many areas.

Folding of the bedrock has introduced joint systems that help to render the rock water-permeable.  Groundwater can migrate along joints and also along the boundaries between bedding planes in thickly bedded sloping structures.  The permeability of Avalon bedrock has likely been enhanced by repeated episodes of loading and unloading over hundreds of thousands of years of glacial cycling.  Bedding plane boundaries and large-scale longitudinal joints provide the principal pathways for deep groundwater penetration.

Foliation and cross joints generate additional groundwater pathways.  Cross joints intersecting longitudinal joints distribute groundwater pressure throughout large volumes of bedrock.  Foliated bedrock  provides capillary channels comparable in scale to those found between grains in frozen soil.  A multiplicity of narrow channels hinders freezing and allows capillary water to migrate through frozen bedrock.  Capillary water crystallizes upon contacting ice in cavities (ice segregation).  This process, occurring subglacially, generates crystal growth pressure that fractures and shifts bedrock in a manner resembling subaerial bedrock frost heave. 


Dynamic deglaciation: characteristic features

Features formed during dynamic deglaciation include:

 1)  Upward-displaced joint blocks (blocks having dimensions ranging from centimeters to meters).  Upward-displaced blocks are sometimes completely ejected from bedrock substrate.  Blocks often show evidence of side loading by glacial ice (tilting or rotation), lateral shifting by glacial ice (translation) or glaciofluvial erosion following upward displacement.

 2)  Local areas of intensely brecciated bedrock incorporating mounds of loose blocks.  Mounds typically remain partially anchored in bedrock and overlie the cavities from which they were ejected.  Blocks show correlations between adjacent angular fragments. These correlations differentiate them from glacially-transported till where fragments are distributed more randomly.

 3)  Fissures where joints have been widened by ice crystal growth pressure or by ice-confined hydraulic pressure.

 4)  Remnant cavities (cavities left behind when bedrock fragments are ejected from substrate and then glacially transported away).  Often, remnant cavities are elongated and resemble fissures.

 5)  Inland cliffs and associated talus.  Inland cliffs are often formed on the Avalon by subglacial erosion in rocks that are highly resistant to freeze-thaw weathering or other types of sub-aerial weathering.  Cliff-edge breakage patterns and adjacent talus show distinct evidence of past glacial loading, subglacial brecciation and glacial rock transport.

 6)  Erosion by ice extrusion.  Subglacial ice extruded from foliated bedrock delaminates the rock and forms distinctive closely-spaced repeating surface grooves following planes of foliation.

 7) Localized glaciofluvial erosion.  Rounded protruding bedrock features coexist alongside angular features indicating tightly channeled erosion by turbulent debris-laden glacial meltwater.

​ 8) Inconsistent directions of rock deflection over local areas.  These inconsistencies imply variable directions of ice-induced stress as would be expected beneath glacial karst. 

All of the above-listed characteristic features can be observed over wide areas of the Isthmus of Avalon and the Heart’s Content Barrens regions on Newfoundland’s Avalon Peninsula.  Observation of all of these features, frequently seen in combination, supports the theory that rapid deglaciation at the end of the Younger Dryas stadial was a significant factor affecting the bedrock geomorphology of parts of the Avalon.

Further evidence suggests that bedrock brecciation by rapid deglaciation was not a one-time occurrence on the Avalon Peninsula.   Recurrent instances of subglacial bedrock brecciation, separated by intervals of basal-sliding glacial erosion (implying re-glaciation), are indicated at several locations.  The Younger Dryas does not appear to have been a unique event.


Occurrences elsewhere

Occurrences of ice-disrupted bedrock similar to those found on the Avalon are seemingly rare.  Where they are seen, they are likely to have been interpreted as resulting from freeze-thaw weathering or from glacial (warm-based) plucking.  Scandinavia and the UK and Ireland share similarities in climate history with Newfoundland and a few sites in these East-Atlantic localities show patterns of ice-induced rock disruption that might indicate dynamic deglaciation at the end of the Younger Dryas.   However, the scale and extent of the features observed on the Avalon appear to be unmatched globally.

Glacial ripping:  Recent (2020 and ongoing) investigations into the origin of boulder spreads along the former edge of the Fennoscandian Ice Sheet in Sweden have implied a mode of subglacial bedrock brecciation termed "glacial ripping".  This process involves the subglacial displacement and transport of bedrock blocks, first by hydraulic fracture and jacking and then by glacial traction.  In some ways, glacial ripping appears to resemble processes seen on the Avalon and discussed above.  Key differences relate to the presumed origin of pressurized subglacial water (drainage of meltwater lakes, Sweden vs. trapped artesian groundwater, Avalon) and the subglacial thermal environment (warm-based, Sweden vs. cold-based, Avalon).    ​​

Dynamic deglaciation on the Avalon has left behind a shattered bedrock landscape.