1 INTRODUCTION

Massive ice takes several forms; pingos, ice wedges,ice lenses, and tabular massive ice. The developmentof many types of subsurface ice is fairly well under-stood; however, the origin and formational processesof tabular massive ice bodies are still matters of somedebate. Tabular massive ice bodies occur throughoutthe Canadian Arctic and are distinctive in that theytend to have much larger lateral dimensions than thick-ness. Some tabular massive ice bodies have been inter-preted to be of segregated origin (Mackay 1971,Moorman et al. 1998), while others are thought to beburied glacier ice (Michel 1985, Fujino 1986, Vaikmaeet al. 1993, Robinson et al. 1992). Tabular massive icebodies can have complex histories and some bodiesmay contain ice representing more than one forma-tional type. A complicating factor is that some of theprocesses of ice formation at the base of a glacier (e.g.regelation) are similar to ice segregation processesoccurring in permafrost (Souchez & Lorrain 1991).

Landforms resulting from the burial of glacier iceare readily observable in many proglacial and peri-glacial settings (e.g. kettle lakes). However, the processof ice burial has not been thoroughly examined. Thispaper presents the findings of our investigations at asite, where the burial of glacial ice by deltaic deposi-tion in a small lake was documented.

2 STUDY SITE

The study site is in the southern portion of Bylot Islandin Arctic Canada (79°30�N, 73°08�W). The area is wellwithin the zone of continuous permafrost (Heginbottom1995), and an equilibrium permafrost thickness of upto 400 m has been estimated from shallow ground

temperature measurements (Moorman & Michel 2000).The glacial history of the island is complex; however,the extent of modern glaciers is thought to have beenrelatively stable over the last several tens of thousandsof years, with a minor glacial maximum occurringwithin the last 100 years (Klassen 1993). Some of theglaciers on southern Bylot Island are currently retreat-ing while other glaciers show little or no sign of retreat.

Near the snout of Glacier C93 (Fig. 1), a tongue of ice flows northwest from the main glacier follow-ing a major fault line and the junction between theArchean-Proterozoic crystalline basement rock andthe Cretaceous-Tertiary sedimentary platform. Thestudy site is located approximately half way along thistongue, where an indentation in the valley side hasenabled a small lake to be dammed against the side ofthe glacier, into which a delta grew and partially cov-ered the glacier.

3 METHODS

Aerial photographs were used to study the historicalrecord of lake levels and delta growth. The strati-graphic relationships within the delta were examinedin exposures at stream cut-banks. The subsurface geo-metry of the delta architecture was imaged with ground-penetrating radar (GPR). Ice physical properties werealso recorded to differentiate the origins of various iceunits.

4 RESULTS AND DISCUSSION

4.1 Sedimentation history

Aerial photographs and repeated site investigationsfrom 1993 to 1999 revealed considerable temporal

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Burial of glacier ice by deltaic deposition, Bylot Island, Arctic Canada

B.J. MoormanEarth Sciences Program, University of Calgary, Calgary, Alberta, Canada

F.A. MichelDepartment of Earth Sciences, Carleton University, Ottawa, Ontario, Canada

ABSTRACT: Burial of glacier ice and a lateral moraine by deltaic deposition is documented for a small lake thatis periodically dammed against the side of a glacier on southern Bylot Island, Arctic Canada. Two very differentstreams flow into the lake. One of the streams carries a very heavy load of sand resulting in the development of amulti-level deltaic complex that extended over and buried the lateral moraine and adjacent glacier ice. Radio-carbon dating of an organic-rich horizon within one of the youngest deltas indicates that this ongoing process haspreserved ice for likely hundreds of years. The ice-poor nature of the sand blanketing the ice enhances the poten-tial for long-term preservation, except where the area is undercut by fluvial erosion.

Permafrost, Phillips, Springman & Arenson (eds)© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

variability in lake levels (Figs 2, 3). The lake intowhich the delta formed consists of two main basins.The basin to the east is fed by an ice-marginal streamrunning along the south side of the glacier, while a“terrestrial” stream flows over the ice-free sedimen-tary terrain to the south of the glacier into the westernbasin.

The general history of the site, as outlined inMoorman and Michel (2000), consists of four major

stages: (1) initial glacial retreat resulting in the devel-opment of the ice-cored lateral moraine and allowingthe initial lake to form, into which delta complex D1grew (Fig. 4), (2) expansion of the glacier that blockedthe outlet and caused the water levels to rise resultingin the deposition of delta complex D2, (3) glacialretreat enabling partial drainage of the lake, leading to incision of the D1 and D2 complexes and the formation of lower delta complex D3, and currently

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Figure 1. A false colour 1989 Landsat image of the study area on Bylot Island, Arctic, Canada. Note that the eastern basinstill has a residual lake (black), while the western basin is almost completely drained (grey).

Figure 2. Aerial photograph from 1948 showing the icedammed lake at just below the D1 stage. The ice-marginalstream enters the eastern basin at the top right. The terrestrialstream enters the western basin from the lower portion of thephotograph. The outlet stream is to the left. The light colouredwater reveals the high suspended sediment content.

Figure 3. Aerial photograph from 1982 showing theresidual ponds after lake drainage. The eastern basin is stillice covered as it is fed by cold glacier runoff, while thewestern basin is fed from the warmer terrestrial stream andis already ice free.

(4) continuing retreat of the glacier resulting in thefurther lowering of the base level and down cuttingthrough the D3 delta complex.

The ice-marginal stream carries a wide range of sed-iment, dominated by silt and clay. A bedrock ridge (cen-ter in Fig. 2) provides a stable base level for the easternbasin resulting in relatively constant lake levels in thebasin. The base level for the western basin is controlledto a greater extent by the glacier position as the outletstream is ice marginal (at left in Fig. 2). The terrestrialstream entering the western basin originates in thepoorly consolidated sandstone to the south and is ladenwith sand and almost completely devoid of silt and clay.

The silt and clay carried by the ice marginal streamentering the lake from the east blanket the entire lakebed. When water levels are high, there is considerablemixing and the suspended sediment from the ice marginal stream is dispersed throughout both basins(Fig. 2). When lake levels are low, the basins are iso-lated and the ice marginal stream does not contributeto sediment deposition in the western basin (Fig. 3).

The thickness of the units containing foreset bedsrange from less than 0.5 m in D2 to over 4 m in D3,indicating that lake stands were highly variable duringformation of the delta.

The differences in sand volume and lateral extent ofthe three deltaic complexes indicate that the durationof lake stands resulting in the deposition of each of thedelta complexes was variable as well (Fig. 4).

This is supported by the many strand lines on thevalley sides. Direct measurements were also made ofwater levels dropping by 5 m in 7 days in the late summer of 1995.

4.2 Ice burial

Ice burial at this site has occurred in three ways; con-tainment within the lateral moraine as the glacierretreated, burial beneath deltaic sediments as the deltaexpanded over the lateral moraine and then onto theexposed glacier ice, and by the redeposition of sedi-ment derived from retrogressive thaw slumps ontoexposed ice.

Compared to other glaciers in the region, GlacierC93 is currently retreating slowly and has veryinsignificant lateral moraines. Where visible, the crestof the lateral moraine rises less than 2 m above the sur-rounding terrain. Exposures and GPR profiles revealedthat a 4–10 m thick ice core is present beneath approx-imately 1 m of ice-poor till and a lacustrine mud cap(Fig. 5).

As a result of the delta extending out onto the gla-cier, up to 20 m of ice has been preserved beneath thedeltaic sand. As the delta first built out over the lateralmoraine, it encased the ice-cored moraine as well asdirectly covering the glacier ice.

Deltaic sediment at lower elevations (e.g. the lowerreaches of the D3 complex) are typically well sortedfine to medium size sand (Fig. 6). Deltaic deposits athigher elevations (e.g. D1) are frequently interbedded

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Figure 4. Map of the western basin showing the distribu-tion of deltaic sands as of 1995.

Figure 5. Stratigraphic section through an unburied portion of the lateral moraine to the east of the delta withinthe western basin.

with thin stringers of lacustrine mud (Fig. 7). This isthe result of the two basins being hydrologically con-nected during high stands, and the fine grained sediment from the ice-marginal stream entering theeastern basin being dispersed into the western basin.The layers of lacustrine mud ranged 1–20 mm inthickness. In comparison to the grey till found in themoraine, the lacustrine mud was brown in colour.When the lake levels are low, the two basins are not

connected and the only sediment entering the westernbasin is the sand from the terrestrial stream. Theinterbedded nature of the sediment higher in the deltasuggest that there was considerable fluctuation in lakelevels in the past as there is today.

The ice buried at the study site varies greatly inappearance and physical properties. Ice encased bythe moraine ranged from being laden with very poorlysorted glacial debris, to having a fine grained sedi-ment content of less than 5%. This is typical of thebasal ice observed in the region.

White bubble-rich, sediment-free ice, similar to theexposed ice of the neighbouring glacier, was discov-ered at higher elevations and positions closer to thecentre-line of the glacier. The ice exposed immediatelybeneath the D1 and D3 deltaic sands was clear, bubbleand sediment free, roughly 1-cm diameter equidimen-sional crystals with no preferred C-axis orientation.

4.3 Ice segregation

Segregated ice was present within the interbedded sandand mud units. The shallow lacustrine setting, wherefrost susceptible sediments are interbedded with highpermeability sediments offered an ideal environmentfor lake bottom ice segregation, similar to that reportedby Burn (1990). The ice content ranged from isolatedlenses less than 1 cm in size to tabular layers of ice up to1 m thick, with laterally continuous ice layers less than5 cm thick being the norm. The heave demonstrated bythe thicker layers of segregated ice represent extraordi-nary heave rates considering the unstable thermal con-ditions associated with dramatically changing surfacehydrological conditions. Overall, segregated ice makesup a very small portion of the ice covered by the deltaicsands. It is estimated to be less than 0.1% of the total byvolume. Segregated ice was not present in the exposedmoraine or surficial lacustrine sediment cap. The upperice unit in Figure 6 was interpreted to be segregated ice,while the lower ice unit is likely glacier ice. Below 4 m,the exposure was covered with colluvium.

4.4 Temporal context

Down cutting within the younger D3 complex exposeda section containing two organic-bearing silty-finesand units overlying clear massive ice (Fig. 6). Thelowermost silty fine sand unit rested directly over themassive ice and was subsequently covered by 45 cm ofmedium to coarse deltaic sand.

The upper organic-rich silty sand unit, radiocarbondated at 1300�/�60 years B.P. (WAT# 6335), under-lies a thin (approximately 5 cm) continuous horizon ofcoarse equigranular ice. The remainder of the sequenceabove the ice layer consisted of deltaic sands.

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Figure 6. Stratigraphic section through delta complex D3,showing multiple layers of sand with appreciable amountsof organics contained with two thin layers.

Figure 7. Stratigraphic section through the delta complexD1 showing considerable lacustrine deposits accumulatingbefore deltaic sedimentation began.

The radiocarbon date of 1300 years on organicswithin this D3 complex provides a time frame for theformation of this delta and the older D1 and D2 com-plexes. The organic material used for dating clearly didnot grow in situ, but was carried into the lake from thesurrounding catchment area. Since it is likely that theorganics represent material accumulated over a periodof time, the 1300 year age should be considered as amaximum age for deposition within the D3 sequence.

Since the D3 complex is the youngest sedimentarydeposit at the site, it is likely that no extended highlevel lake has occupied the valley since. The 1948 aer-ial photograph in Figure 2 confirms that lakes do peri-odically flood the study site; however, the lack ofassociated sediments indicates that they are of shortduration. As noted earlier, rapid fluctuations in lakelevel were observed during our investigations.

4.5 Ice stability

Preservation of segregated ice and the ice buriedbeneath the lateral moraine and delta complexes at theside of Glacier C93 depends greatly on the erosionalpotential of the stream and the hydrological base levelcreated by the glacier ice dam (or lack thereof). In gen-eral, the deltaic sand remains stable when thawed thusproviding a durable cover for the buried ice. Unlike inthe deltaic sands, when the ice encased by fine-grainsediments (e.g. segregated ice and moraine ice core)melts, a liquid slurry forms. As a result, retrogressivethaw slumps are common in areas of exposed moraine.The thaw slumps are characterized by sloping head-walls 2–10 m in height with near vertical faces near thesurface of the ice-poor glacial or lacustrine sediment

capping the massive ice. In the current configuration,retrogressive thawing of the ice-rich lateral morainecontinues unabated, although debris is accumulating atthe toe of the slumps. On the other hand, the ice-poornature of the deltaic sands make the areas covered bysand more stable, and they are not subject to erosionexcept when undercut by a stream. Thus the majorthreat to preservation of buried glacier ice within thedeltaic complexes is by down cutting and exposure dueto stream migration.

5 CONCLUSIONS

Sandy deltaic sediments that were deposited withintemporary lakes dammed along the ice margin havecovered glacier ice and the adjacent lateral moraine.The average lake stand has since dropped and thestream is depositing its sediment load at lower levels.Burial of the glacier ice by deltaic sand in this per-mafrost environment has resulted in its preservation forhundreds of years. The ice-poor nature of the deltaicsands makes them less susceptible to thermal erosionand thus enhances the probability of long-term glacierice preservation. The greatest threat to maintaining astable ice-cored deltaic complex is down cutting by thestream during periods of low lake level, and the erosionof buried ice-cored moraines.

ACKNOWLEDGEMENTS

The authors would like to thank the Polar ContinentalShelf Project for its logistical support, and NSERC for

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Figure 8. A schematic cross section through the delta in the western lake basin, with the location of the sections shown.Note that the legend is the same as in Figure 4.

its financial support to F.A. Michel. The field assis-tance from Mark Elver, Deb Kliza, Lynn Moorman,Amy Lyttle, and Carolyn Deacock is greatly appreci-ated. Renard Emmanual and an anonymous reviewerare acknowledged for their very useful comments inimproving the manuscript.

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