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TitleOrigin of lacustrine clays in the Great Slave Lowlands, Northwest Territories, Canada and implications for terrain stability
AuthorPercival, J B; Wolfe, S A; Grenier, A
SourceXV International Clay Conference, abstracts; 2013 p. 1
Alt SeriesEarth Sciences Sector, Contribution Series 20120425
MeetingXV International Clay Conference; Rio de Janeiro; BR; July 7-11, 2013
File formatpdf
ProvinceNorthwest Territories
Subjectssurficial geology/geomorphology; environmental geology; mineralogy; Nature and Environment; Science and Technology; Health and Safety; permafrost; ground ice; sensitive clays; climate effects; landslides; sediment stability; glacial history; glacial lakes; water levels; ice lenses; core samples; plasticity; plastic limit analysis; mineralogical analyses; depositional history; Holocene; Great Slave Lowlands; Glacial Lake McConnell; geological hazards; climate change; lacustrine sediments; permafrost thaw; cumulative effects; Phanerozoic; Cenozoic; Quaternary
ProgramClimate Change Geoscience, Land-based Infrastructure
AbstractClay-rich sediments are widespread in a discontinuous permafrost environment in the Great Slave Lowlands of the Northwest Territories, Canada. These sediments originate from glacial Lake McConnell (ca. 13.0 - 9.5 ka), a large glacial lake that covered much of northwestern Canada, and from Great Slave Lake, which has been receding during the Holocene due to differential isostatic uplift. These 10-15 m thick sediments occupy bedrock valleys and depressions at elevations within about 50 m of the present-day level of Great Slave Lake. They typically contain permafrost with excess ground ice in the form of ice lenses, which may account for 30% or more of the material by volume. The purpose of this study was to characterize the properties of these sediments leading to an improved understanding of potentially hazardous conditions in the Great Slave Lowlands. For example, ground thawing, either by disturbance or climatic change, has major implications for terrain stability.
Five boreholes were cored across a 400-m transect into fine-grained frozen sediments to depths ranging from 4.3 to 8.3 m beneath peatland and a 4 m-elevated ridge. Samples were logged in the field for sediment texture and visible ice content. Moisture content, grain size and Atterberg limits were measured on selected samples. Mineralogy was determined using X-Ray diffraction (XRD) of whole-rock (<2 mm) and clay-sized (<2µm) fractions with follow-up using scanning electron microscopy on the most clay-rich materials.
Field logs and grain-size analysis define three distinctive units at depth: a lower clayey unit (Group 1) with 35-75% clay, 25-55% silt and <10% sand; an intermediate silty unit (Group 2) with <35% clay 50-90% silt and <30% sand; and an upper sandy unit (Group 3) with <15% clay, 2-60% silt and 30-98% sand. Ice content increases with depth, with ice lenses in excess of 10 cm thick occurring within the lower clayey unit, Group 1. Atterberg limits reveal all units to behave as clays of high, intermediate and low plasticity for Groups 1 to 3, respectively. All samples plot parallel and above the 'A-line' on a traditional Liquid limit-Plasticity Index plot indicative of clays with a common origin. The mineralogy of the clay-sized fraction is about 30-45% clay minerals, 25-32% feldspars and amphiboles and 30-45% quartz. The constituent clay minerals are illite, chlorite and kaolinite of about 18%, 13% and 6%, respectively, with little variation between groups. Within the pulverized whole-sample fraction, the total non-clay vs. total clay mineral content ranges between 74-90% and 10-26%, respectively. Mineralogical variation is due mostly to an increase in quartz (ranges from 41-66%). Although there are only trace to minor amounts of mixed-layer clay minerals (i.e., illite-smectite) present, the proportion of these minerals increase with the quartz content up-profile within the clay-size fraction, suggesting an increased contribution of weathered sediments up-profile.
Based on XRD analysis, Group 1 is interpreted as glaciolacustrine clays of glacial Lake McConnell, derived from tills sourced from regional shield (plutonic granite, granodiorite and tonalite) bedrock. Sediment texture and trace to minor mixed-layer clay mineral contributions suggest that overlying silty and sandy units (Groups 2 and 3, respectively) originate from reworking of sediments during Great Slave Lake recession, and deposit ion in alluvial and nearshore environments. High ground ice contents, interpreted as originating from permafrost aggradation into these sediments following lake-level recession, suggests that these sediments represent hazardous permafrost terrain.
Summary(Plain Language Summary, not published)
Frozen ice-rich clay sediments are widespread in the sub-arctic environment of the Great Slave Lowlands, Northwest Territories, Canada. Thawing of these sediments, either by disturbance or climate change, has implications for terrin stability. We examined the origins and properties of these sediments to improve the understanding of this potentially hazardous terrain. Our findings show that these ice-rich sediments are related to the depositional environments from a former glacial lake (McConnell) and Great Slave Lake, and to the growth of permafrost after these lakes receded.