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TitleLithalsa distribution, morphology and landscape associations in the Great Slave Lowlands, Northwest Territories
AuthorStevens, C W; Wolfe, S A
Source40th Annual Yellowknife Geoscience Forum, abstracts of talks and posters; by Watson, D M (ed.); Northwest Territories Geoscience Office, Yellowknife Geoscience Forum Abstracts Volume 2012, 2012 p. 48
Alt SeriesEarth Sciences Sector, Contribution Series 20120249
Meeting40th annual Yellowknife Geoscience Forum; Yellowknife; CA; November 13-15, 2012
File formatpdf
ProvinceNorthwest Territories
AreaGreat Slave Lowlands
Subjectssurficial geology/geomorphology; ice; ground ice; permafrost; glacial features; landscape types; lithalsa
ProgramLand-based Infrastructure, Climate Change Geoscience
LinksOnline - En Ligne
AbstractThe distribution of ice-rich terrain is an important geotechnical consideration for the engineering of northern infrastructure. Within the Great Slave Lowlands, Northwest Territories, fine-grained silts and clays deposited by Glacial Lake McConnell (ca. 13 to 9.5 cal ka BP) and ancestral Great Slave Lake (post 9.5 cal ka BP) are widely distributed across discontinuous permafrost terrain. Whereas these sediments are known to contain excess ice attributable to permafrost aggredation and ice segregation, little is known about the distribution of ice-rich terrain in this region.
Mineral lithalsas are permafrost mounds, caused by the formation of segregate ice when the permafrost aggrades into the ground, and are most commonly encountered within fine-grained (lacustrine, glaciolacustrine or marine) sediments in discontinuous permafrost terrain. Lithalsas, like ice-wedges and pingos, represent a form of ice-rich permafrost terrain that can be readily identified on the basis of surface geomorphology. A total of 1,777 ice-rich mineral lithalsas have been mapped over 3,680 km2 using monochromatic stereo-pair aerial photographs, across the Great Slave Lowlands and Uplands. Drill cores indicate lithalsas in this region consist of ice-rich silt and clay, with segregated ice lenses up to 10 cm thick. Three distinct morphologies are recognized including: i) circular, ii) linear and (iii) crescentic shapes, exhibiting conical and ride-like forms up to 8 m in height and more than 100 m in width. A linear correlation between lithalsa height and width suggests that 1 cm of vertical growth is accompanied by about 15 cm of lateral growth at the peripheral edge. Lithalsa distribution is skewed towards lower-elevation terrain, with 97.7% located within the Great Slave Lowlands, and most of these within 10-15 m above present-day (~156 m asl) lake level. This proximity to present-day lake levels suggests that many lithalsas in the region are late Holocene in age. These features predominately occur adjacent to water bodies and follow the regional distribution of frost susceptible silt and clay, particularly within former streams, embayments and fault-controlled valleys. Landscape associations suggest lithalsa formation is controlled by sedimentological, thermal and hydrological conditions.
The thermal and physical disturbance of lithalsa features in the Great Slave Lowlands can result in substantial terrain subsidence. Analysis of historical aerial photographs has identified lake expansion caused by the recent degradation of lithalsa. In addition, road surface subsidence on the order of 85 cm in the last decade along a section of Highway 3 at Boundary Creek can be attributed to degradation of permafrost within a mineral lithalsa following road construction.