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TitleOrigins and implications of an ice-rich clay ridge (lithalsa) within the Boundary Creek watershed near Yellowknife, Northwest Territories, Canada
AuthorGaanderse, A J; Wolfe, S AORCID logo; Burn, C R; Kokelj, S V
SourceCANQUA-CGRG Biannual Meeting 2013, abstracts; 2013 p. 1
Alt SeriesEarth Sciences Sector, Contribution Series 20130141
PublisherUniversity of Alberta
MeetingCANQUA-CGRG Biannual Meeting 2013; Edmonton; CA; August 18-22, 2013
Mediapaper; digital
File formatpdf (Adobe® Reader®)
ProvinceNorthwest Territories
AreaYellowknife; Boundary Creek; Great Slave Lake
Lat/Long WENS-115.0000 -114.9167 62.5500 62.5000
Subjectssurficial geology/geomorphology; environmental geology; engineering geology; stratigraphy; geochemistry; Nature and Environment; Transport; Holocene; permafrost; ground ice; periglacial features; frost heaving; ice lenses; talik; sediments; clays; silts; sands; organic deposits; peatlands; ecosystems; drainage patterns; glacial lakes; depositional history; glacial history; deglaciation; ice retreat; emergence; surface waters; lakes; water levels; boreholes; stratigraphic analyses; water geochemistry; groundwater movement; isotopic studies; oxygen isotopes; meteoric waters; Boundary Creek Watershed; Glacial Lake McConnell; Laurentide Ice Sheet; lithalsas; Infrastructures; Road networks; glaciolacustrine sediments; lacustrine sediments; Phanerozoic; Cenozoic; Quaternary
ProgramClimate Change Geoscience
Released2013 08 01
AbstractThaw of ground ice may have a major influence on terrain conditions and ecosystems, particularly with respect to ground stability and drainage patterns. It may also be a significant consideration for routing and performance of transportation infrastructure. Newly discovered clay ridges, located within the widespread discontinuous permafrost zone in the region surrounding Yellowknife, Northwest Territories, may be lithalsas. Lithalsas are mineral-based permafrost mounds with high segregated ground-ice in their cores. Geotechnical reports have noted the presence of deep-seated ground ice in the Yellowknife region, but the observed ridges are not currently recognized as potentially ice-rich geomorphic landforms. There is presently no information about their stratigraphy, composition, and formation process.
The ridges are located within glaciolacustrine surficial deposits, originating from glacial Lake McConnell approximately 10,000 years ago, following post-glacial retreat of the Laurentide ice sheet, and from subsequent deposition as water levels receded during the Holocene to the present level of Great Slave Lake. Permafrost aggradation followed emergence of the terrain from the lake. The study examines one large ridge, over 700 m long, located adjacent to Highway 3 within the Boundary Creek watershed, 31 km west of Yellowknife. To examine ground ice conditions in these deposits, samples to depths of 8.4 m were obtained from 15 boreholes along a transect perpendicular to the long axis of the ridge.
Boreholes revealed distinct tilted layers of silts, sands, and dense clays, suggesting they have been heaved, with the same layers observed horizontally beneath the adjacent peatland. Ice lenses between 2 and 6 cm thick were common in samples below 4 m, and alternating layers of ice lenses up to 19 cm thick within clay-rich sediment below 6 m created a deep-seated ice-rich core beneath the ground surface. Ice segregation within the fine-grained sediments has heaved the ground, producing the observed lithalsa. Geochemical results indicate free water movement through the substrate, and variations in concentrations are consistent with defined soil stratigraphic layers. Delta-18O values between -16.6 and -18.4 permille from ice lenses at 4.0 to 8.4 m depth indicate that the ice-based core likely formed from modern meteoric water sources, eliminating buried glacial ice or glacial Lake McConnell water as possible sources.
Detrital organics from sediments below the peatland date to between 1570 and 1770 cal BP, indicating lacustrine deposition at that time, and an age of about 1240 cal BP from the base of the peat deposits indicates terrestrial emergence and probable permafrost aggradation at that time. Detrital organics from sediments below the lithalsa range in age from 3190 to 690 cal BP, implying that lacustrine deposition continued here until at least 700 years ago. Thus, permafrost aggradation accompanied by ice segregation within the lithalsa was initiated within the last 700 years. The primary groundwater source for ice segregation is likely from a talik associated with a pond flanking the north side of the lithalsa. Evidence of the lithalsa's incipient nature includes clays with high unfrozen water contents between ice-lenses at depths below 7 m, suggesting that potential movement of unfrozen water toward the ice-based core by cryosuction may still be occurring. Examination of historical air photos demonstrates that the recent Highway 3 realignment intersects the former edge of the lithalsa. Removal of the active layer and ice-poor near-surface permafrost has made ice-rich materials at depth vulnerable to thaw. As a result, the roadway has subsided nearly one metre from 2007 to 2011. Although the study examines a single lithalsa, there are nearly 1800 raised mounds mapped in the area, indicating the potential for a much larger amount of ice-rich terrain across the landscape than previously understood.
Summary(Plain Language Summary, not published)
Ground ice can have major influences on permafrost terrain and ecosystems in Northern Canada. Newly discovered features in the Yellowknife area appear to be ice-rich clay ridges, known as lithalsas. These features can affect ground stability and drainage patterns (i.e. by thermokarst), and can also have a significant impact on transportation infrastructure. The origins of the ice-rich nature of one clay ridge were determined from multiple lines of evidence stemming from borehole samples to 8.4 m depth. We conclude that permafrost aggradation and ice-segregation initiated within the ridge in the last 700 years. Highway construction over a portion of this ridge has caused the roadbed to subsided nearly one metre between 2007-2011.

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