Introduction

The majority of ground ice research in Canada hasbeen focussed in the Mackenzie Delta and YukonCoastal Plain regions of the western Arctic (e.g.,Mackay, 1971; French and Harry, 1990; Mackay andDallimore, 1992), while there has been relatively littleground ice research in the Canadian High Arctic (e.g.,French et al., 1986). Massive ground ice has been notedlocally as an important component of the unconsolida-ted fine-grained sediments of the Fosheim Peninsula,Ellesmere Island, N.W.T., at elevations below marinelimit (Hodgson et al., 1991; Pollard, 1991; Robinson,1993). The recent discovery of massive ground ice sillswithin the bedrock of the Eureka Sound Group at HotWeather Creek has extended the scope of ground icestudies in this region.

Mapping the areal distribution and characteristics ofmassive ground ice is important to a better understan-ding of the permafrost, hydrological, and climatic con-ditions under which the ice formed. This paper presentsobservations from a natural exposure showing threedistinct ice types: 1) discrete ice sills within bedrock, 2)segregated massive ice, and 3) a large ice wedge. Thissite is of particular interest due to the paucity of reportsof ground ice occurring within bedrock in Canada.

Regional background and study site

The west-central Fosheim Peninsula, Ellesmere Island(Figure 1), is a broad, rolling, well-vegetated intermon-

taine lowland. The region is underlain by up to 2000 mof poorly consolidated clastic bedrock of the EurekaSound Formation (Ricketts, 1986). Unweathered out-crops in the incised valley of Hot Weather Creek consistof fining-upwards, poorly cemented, white quartzosesandstone and coal cycles, often with interbedded mud-stone and shale.

Abstract

This paper presents observations from a natural exposure on the bank of Hot Weather Creek, EllesmereIsland, showing three distinct massive ice types: 1) discrete ice sills within poorly consolidated bedrock, 2) seg-regated massive ice, and 3) a large ice wedge within overlying unconsolidated fluvial sediments. Stratigraphicobservations and hydrochemistry data indicate that the three ice types have different modes of origin andwater sources. Ice wedge formation followed marine regression, which exposed unfrozen sediments to cold airand promoted the aggradation of permafrost. The formation of segregated ice was promoted by the downwardprogression of permafrost, an ample water source and the presence of a fine-grained capping layer. The down-ward aggradation of permafrost combined with the presence of a lower confining boundary likely resulted inthe highly pressurized groundwater required to form ice sills in the already frozen bedrock.

Stephen D. Robinson, Wayne H. Pollard 949

MASSIVE GROUND ICE WITHIN EUREKA SOUND BEDROCK, ELLESMERE ISLAND, CANADA

Stephen D. Robinson, Wayne H. Pollard

Department of Geography and Centre for Climate and Global Change Research,McGill University, Montr�al, Qu�bec H3A 2K6 Canada

e-mail: srobin4@po-box.mcgill.ca e-mail: pollard@felix.geog.mcgill.ca

Figure 1. Location of the study site on the Fosheim Peninsula, EllesmereIsland, Canada.

At elevations below Holocene marine limit (approxi-mately 140-145 m a.s.l.), variable thicknesses of uncon-solidated marine silts and fluvial silty sand form aveneer over the bedrock. The marine sediments areoften ice-rich, with tabular ice bodies up to 8 - 10 mthick in exposure (Robinson, 1993), and numerous icewedges. Hot Weather Creek is a south-flowing tributaryof the Slidre River incised into the Eureka Soundbedrock in its lower reaches. Retrogressive thaw slumpsare common along its banks within the capping marinesediments (Pollard, 1991). Although no evidence ofextensive Late Wisconsinan ice cover has been found inthe area, Bell (1992) suggested the presence of uplandice caps and the extension of cirque glaciers in theSawtooth Mountains to the east. An emergence curvefrom Bell (1996) shows marine regression was initiatedabout 9 000 BP. The initial emergence was rapid, rea-ching the elevation of the study site (approximately 98m at the top of the section) by about 7 000 BP. Since thistime, the study site would have been subaeriallyexposed, allowing the deep penetration of cold, per-mafrost-forming temperatures. A permafrost thicknessof 500 m has been obtained from thermal measure-ments in Gemini Well, 7.5 km from the study site at anelevation of 126 m (Taylor et. al, 1982).

The study site is located on the western bank of HotWeather Creek, 2.5 km north of the confluence with theSlidre River (79¡58'N, 84¡28'W)(Figure 1). Massive icewas briefly exposed in July 1991 in an upper bluff sec-tion due to the failure of a sandy colluvium. This expo-sure was examined briefly before further failures co-vered the section. High water levels during 1992snowmelt resulted in the undercutting and removal ofsandy colluvium on the outside of a large meander,revealing a fresh section of Eureka Sound bedrock over-

lain by fluvial silty sand. This new exposure revealed aseries of ice sills within the bedrock, in addition to alarge tabular ice body and an ice wedge within theoverlying fluvial material (Figure 2).

The exposure was mapped and sampled betweenmid-July and mid-August 1992, before it was onceagain covered with sandy colluvium. In some sections,stratigraphic descriptions were not possible due to col-luvial cover and the unstable nature of the exposure. Icesamples were collected from the middle of the wedgeice, the segregated ice, and two of the ice sills (Figures 3and 4) for hydrochemical analyses. Concentrations ofmajor ions were determined in the geochemical labora-tories of Earth and Planetary Sciences at McGillUniversity.

Section descriptions

UNIT A: SEDIMENT, ICE WEDGE, AND SEGREGATED MASSIVE ICE

(FIGURE 3)The unconsolidated upper unit consists of 14 m of

interbedded fluvial sand and silt with some clay in afining-downwards sequence, with minor detrital coaland charcoal layers. Many of the silt layers contain vis-

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Figure 2. Bluff showing fluvial silty sand (Unit A) overlying Eureka Soundbedrock (Unit B) at the Hot Weather Creek study site. Three types of massiveground ice are found within this section, 1) ice sills within bedrock, 2) segre-gated massive ice, and 3) an ice wedge (covered by colluvium in photograph).

Figure 3. Stratigraphic section illustrating the relationship between the mas-sive ice bodies and enclosing unconsolidated sediments in Unit A. Depthsare in metres below the top of the section.

ible ice layers and the sands are cross-stratified. A layercontaining large rounded cobbles and gravel is foundnear the base of the unit.

An ice wedge up to 2 m wide extends from just belowthe top of the exposure to a depth of almost 7 m. Thevery soft wedge ice is bubble-rich and shows foliationsand banded sediment in planes parallel to the sedimentcontacts. Ten metres below the top of the section is a1.7 to 2.0 m thick tabular ice body conformably overlainby silt. The ice contains horizontal lenses of silt and coalvisually similar to the enclosing sediments, and is fairlybubble-rich. Sediment and bubble content decreases inthe lower sections of the ice. The upper sediment-ice

contact is slightly gradational but conformable, withsome ice lenses and reticulate ice in the overlying sedi-ment. A gradational contact between ice and icy-sandand gravel mixed with coal fragments is found at thebase of the ice. Where the ice is immediately underlainby a thin silt layer the contact is sharp. Diapiric uplift ofthe tabular ice body is noted in the zone immediatelybelow the ice wedge. All stratigraphic evidence pointsto a segregated origin for this ice body, with horizontalsediment banding, a conformable upper contact withfine-grained sediment, and coarser sediments at thelower boundary.

UNIT B: SEDIMENT AND MASSIVE ICE SILLS (LOWER UNIT)(FIGURES 4 AND 5)

Unit B is 12 m thick in exposure above the river level,and is composed of poorly consolidated sandstone,coal, shale and mudstone of the Eureka SoundFormation. Reworked Eureka Sound sands form thecontact with Unit A above, below which 1.5 m of thesection was covered by thick colluvium. A prominentblocky 48-55 cm thick coal horizon, with ice-filledcracks, occurs below the colluvium. The lower parts ofthe section are primarily alternating white sands anddarker silts, with occasional thin muddy coal layers.

A series of 8 horizontally-extensive ice sills, up to 57 cm thick, are found within these beds (Figures 4 and5). Several of the sill boundaries appear to be strati-graphically-controlled, while others cross-cut sedimentboundaries or bifurcate. Several sills pinch out horizon-tally within a 10 m distance, while others were tracedfor as much as 50 m along the exposure. In all cases, theice forms sharp contacts with the enclosing sediments,with occasional small suspended inclusions of hostmaterial. Where the sills are in contact with coarsesand, the sand often contains short ice-filled cracks upto 1.0 cm thick perpendicular to the sill. Sill ice is veryclear and hard and fractures conchoidially, indicative ofice formed under high pressure. Bubbles are noted onlyin some sills as a trail in the middle of the sill. A slightdiscolouration is occasionally noted in the middle of thesills. Ice crystals from 0.5 to 4 cm in diameter werenoted in the field, with the larger crystals foundtowards the centre of the sills.

Ice chemistry

The formation of ground ice involves either in situfreezing of groundwater or flow of groundwater from adistant source as part of the regional system. Since mas-sive ice usually consists of significant volumes of excessice, then a larger supply of water and a more efficienttransfer mechanism are needed than would be requiredto form pore ice or small ice lenses. Furthermore, sinceepigenetic massive ice forms as permafrost aggrades,then defining the water source and transfer processes

Stephen D. Robinson, Wayne H. Pollard 951

Figure 4. Stratigraphic section illustrating the relationship between ice sillsand the enclosing poorly-consolidated silts, coal, white, and dark sands of theEureka Sound bedrock. All sediment-ice contacts are sharp. Depths are inmetres below the top of the section. At least three additional sills were notedbelow 21 m, but were inaccessible.

provides useful information regarding permafrost for-mation. Ice chemistry reflects the water source and pro-vides a useful tool to differentiate water sources.

In general, dissolved ion concentrations are low withonly small variations between samples (Table 1). As isexpected, the ice wedge contains the lowest dissolvedion concentrations, which is consistent with its atmos-pheric origin and limited interaction with local geology.Ice with slightly elevated ion concentrations havechemical signatures that suggest a segregation origin,with water derived from the enclosing marine sedi-ments as groundwater infiltrated and migrated towardthe freezing front, or indirectly by diffusion. This obser-vation is consistent with both stratigraphic and struc-tural characteristics of the tabular ice body in Unit A.By comparison, the sill ice contains lower concentra-tions of Na, K, Mg, Cl, and SO4, suggesting a freshergroundwater source.

Since these data come from a collection of spot sam-ples rather than a continuous series, the trends withinany single ice body are unknown. The absence of a con-tinuous profile precludes identification of a chemicalunconformity which would provide stronger support

for the interpretation of separate water sources for theice wedge, segregated ice, and ice sills.

Interpretation

The three ice units differ markedly in their strati-graphic relations, chemistry, hardness, and bubble andsediment characteristics. Observations suggest that theice sills were formed through the freezing of waterinjected under high pressure into the already frozenEureka Sound sediments from depth. The sedimentsshowed no signs of disruption from water injection(e.g., scouring and inclusion of large amounts of sedi-ment), suggesting that the sediments were likely frozenat the time of water emplacement. In addition, the high-ly permeable Eureka Sound sediments would have like-ly drained or allowed water flow away from the sillshad they been unfrozen at the time of water emplace-ment. Ice hardness, conchoidal fracturing, and up to 24 m of heave of the overlying sediments all suggesthigh ice pressures at the time of formation. The ice len-ses are sometimes unconformable, yet always abrupt,with sedimentary boundaries, and enclosing material isoften too coarse to allow the formation of segregatedice. Two-sided freezing, as indicated by the increase inbubbles, last-to-freeze centre zone of discolouration,and increase in crystal size towards the centre of severalsills, is characteristic of ice sills and dykes (Mackay,1989) and necessarily excludes a segregated origin.

Observations indicate that the upper tabular ice bodyis of segregated ice origin, formed during the down-ward progression of permafrost into surficial sedimentsthat had been left thawed by marine inundation. Theseobservations include horizontal layering of suspendedsediment, the "weaker" nature of the ice, and heteroge-neous vertical stratification of bubbles and sedimentwithin the ice body. This interpretation is also support-ed by the marine-derived hydrochemical characteristicsof the ice. The formation of segregated ice requires thepresence of a low hydraulic conductivity capping layerand a high conductivity basal layer that is capable oftransmitting an adequate supply of water. Both of theserequirements are satisfied at the site, with the thin over-lying silt and the underlying gravels.

Model of ice formation

Marine regression resulted in the emergence of thissite at about 7 000 years B.P. The marine incursion leftboth deposits up to 30 m thick consisting of unfrozenmarine silts and deltaic sediments overlying bedrock inmany areas (Bell, 1992), and fluvial sands in the HotWeather Creek drainage channel. All of these sedimentswould have been unfrozen at the time of emergence.Depending on the duration of marine incursion, tens tohundred of metres of the underlying bedrock would

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Figure 5. Two ice sills with enclosing Eureka Sound sands and silts. The twosills join to the right of the photograph.

Table 1. Major ions in ice samples

have also been unfrozen. Bell (1992) suggests that earlyHolocene temperatures were comparable to the present,hence it is realistic to suggest mean air temperatureswere in the range of -15 to -20¡C. Bell also indicates thatbetween 9 000 and 6 000 years BP local upland ice capswere slowly degrading, providing the potential forgroundwater recharge from the meltwater.

The formation of segregated ice within Unit A waspromoted by the downward aggradation of permafrostthrough the saturated sands due to cold air tempera-tures, combined with an ample water source and thepresence of a fine-grained capping layer of silt foundimmediately above the tabular ice body. Ice wedge for-mation was also likely initiated shortly after marineregression. However, ice wedge growth to the point ofunloading and deformation of the underlying segrega-ted ice could not have occurred until after the formationof segregated ice. Permafrost continued to aggradethrough the Holocene sediments and into the under-lying bedrock following the initiation of the ice wedgeand the growth of the segregated ice body.

The injection of water under high pressure from depthinto the overlying frozen Eureka Sound bedrock waslikely caused by the high confining pressures intro-duced by the downward aggradation of permafrostcombined with the presence of a lower confiningboundary. The base of the unfrozen zone may havebeen a fine-grained lithologic layer, or possibly the sur-face of residual Pleistocene permafrost. There are noindications of a water conduit to the sills nor are thereany indications of the water continuing further uptowards the surface.

Discussion

Reports of massive ground ice in bedrock are rare,and have almost always been attributed to the freezingof bulk water in pre-existing fractures or voids (Lang,1966; Ford, 1984). Similarly, in his popular book Sibir,Farley Mowat (1970) described a dam project nearMirny, Siberia where...."frozen bedrock which hadseemed sturdy enough to support almost any weight,was found to be underlain by a cracked strata full of icelenses which could easily be deformed by the greatweight of the dam and so lead to its collapse". OnMelville Island, massive ice is found in soils derivedfrom the cryogenic weathering of weakly lithified sand-stones, siltstones, and shales (French et al., 1986). Anyice within the bedrock itself is confined to the highlyweathered or fissured zone directly beneath the residuum.

The only other case known by the authors where mas-sive ice likely formed within bedrock due to high injec-tion pressures is from the Huola Basin of northeastern

China (Wang, 1990). In a borehole the relatively pure icebody lies at a depth of 46-68 m, and is overlain by 6 mof coal and 40 m of fine- to coarse-grained sandstone.The base of the ice coincides with the base of per-mafrost, and is immediately underlain by unfrozenmudstone. The ice is interpreted to have formed by insitu freezing of water injected under pressure. Well-developed cracks in the bedrock may have acted as con-duits linking the ice body with the local groundwaterunder artesian pressures. The freezing of groundwaterat the freezing front may then have thrust the overbur-den upward. The Eureka Sound sediments did notshow any major fractures within the bedrock thatwould have allowed a link with groundwater at depth.However, these conduits may be present beneath theexposed section, or beneath parts of the section coveredby colluvium.

An examination of Eureka Sound bedrock exposureswithin a 5 km radius of this site failed to provide anyfurther ice exposures. This may suggest that the studysite is an isolated occurrence of massive ice withinpoorly consolidated bedrock. The paucity of previousreports may be partly due to the lack of fresh bedrockexposures.

Acknowledgments

Funding for this research was provided by theDepartment of Indian and Northern Affairs NorthernScience Training Program, Natural Science andEngineering Research Council, and the GeologicalSurvey of Canada. The authors also wish to acknow-ledge the generous support of the Polar ContinentalShelf Project (PCSP manuscript 02397). Financial assis-tance towards presentation of this paper at the SeventhInternational Conference on Permafrost was providedto the first author by the Royal Canadian GeographicalSociety. We would like to thank Dr. Sylvia Edlund forher hospitality at Hot Weather Creek basecamp, andRoger Edgecombe and Craig Forcese for their assistancein the field. Dr. Robert Gilbert supervised part of thiswork as SDR's M.Sc. thesis at Queen's University.

Stephen D. Robinson, Wayne H. Pollard 953

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