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TitleComposition and origin of an ice-rich mound (lithalsa) in the Great Slave Lowland, Northwest Territories, Canada
AuthorGaanderse, A; Wolfe, S; Burn, C
SourceProceedings of GeoQuébec 2015, 68th Canadian Geotechnical Conference and 7th Canadian Permafrost Conference; 2015 p. 230
Year2015
Alt SeriesEarth Sciences Sector, Contribution Series 20150330
PublisherCanadian Geotechnical Society
MeetingGeoQuébec 2015, 68th Canadian Geotechnical Conference and 7th Canadian Permafrost Conference; Quebec City; CA; September 20-23, 2015
Documentbook
Lang.English
Mediapaper; on-line; digital
File formatpdf
ProvinceNorthwest Territories
NTS85J/10
AreaYellowknife; Boundary Creek; Great Slave Lake
Lat/Long WENS-115.0000 -114.9167 62.5500 62.5000
Subjectssurficial geology/geomorphology; ice; ice conditions; till mounds; deglaciation; glacial history; Great Slave Lowlands; Cenozoic; Quaternary
ProgramLand-based Infrastructure, Climate Change Geoscience
AbstractRecent recognition of raised mounds in the widespread discontinuous permafrost zone of the Great Slave Lowland, NWT, has prompted questions regarding their internal composition and development. To answer these, fieldwork was undertaken in the summers of 2011 and 2012 at the Boundary Creek Study Area (BCSA, 62° 32'N; 114° 58'W), 30 km west of Yellowknife. This region was deglaciated at about 13 cal kyr BP, after which fine-grained glaciolacustrine sediments were deposited across the lowlands by Glacial Lake McConnell, and subsequently washed into topographic lows between bedrock highs during lake level recession. At BCSA, a prominent mound 700 m long and 4 m high occurs adjacent to a peatland and, from the results of this investigation, is identified as a lithalsa.
Borehole drilling parallel to the short axis of the lithalsa revealed ice-poor conditions within the first 4 m of substrate, with substantial increases in ice content at greater depths within underlying clays. The lithalsa core is composed of layered ice lenses on the order of 0.1 m thick. Stratigraphic records indicate that the sequence of clays, silts and sands are sub-parallel but domed, in accordance with the surficial relief of the lithalsa. Estimated differential heave between the lithalsa and the adjacent peatland is approximately 2.8 m, of which 2.17 m may be directly accounted for by ice lenses within the first 8.4 m of sediment. The ?18O values between -18.4 and -16.6 ‰ from ice lenses at depth indicate that the layered ice core formed from modern meteoric waters. The groundwater source is likely from a talik associated with a pond flanking the north side of the lithalsa. A basal peat sample in the peatland indicates terrestrial exposure at approximately 1200 cal yr BP. Within the lithalsa, a detrital organic sample dating to 700 cal yr BP was collected below 3 m of sediment, whereas a 400 cal yr BP sample was recovered under a small peat pocket on top of the lithalsa. These results suggest terrestrial exposure and subsequent permafrost aggradation between 700 and 400 cal yr BP.
The mound at BCSA meets the main criteria for lithalsa formation: a warm ground thermal regime (approximately -1°C) to support water migration to ice lenses, an abundant water supply to feed the ice lenses via cryosuction, and the presence of fine-grained sediment promoting ice segregation and unfrozen water contents at depth.
Summary(Plain Language Summary, not published)
Recent recognition of lithalsas in the Great Slave Lowland, NT, prompted a detailed investigation to evaluate the internal composition and geomorphic origin of one lithalsa. Borehole drilling revealed ice-poor conditions within the first 4 m within the lithalsa, with substantial increases in ice content at greater depths. The lithalsa core is composed of layered ice lenses over 0.1 m thick, formed of isotopically-modern meteoric waters. The stratigraphic sequence of clays, silts and sands is sub-parallel, but domed in accordance with the surficial relief of the lithalsa. Estimated differential heave between the lithalsa and an adjacent peatland is approximately 2.8 m, of which 2.17 m is directly accounted for by ice lenses within the first 8.4 m of material. Carbon-14 dating indicates permafrost aggradation within the lithalsa occurred between 700 and 400 cal yr BP.
GEOSCAN ID297393