GEOSCAN, résultats de la recherche

Menu GEOSCAN


TitreOrigin of lacustrine clays in the Great Slave Lowlands, Northwest Territories, Canada and implications for terrain stability
AuteurPercival, J B; Wolfe, S A; Grenier, A
SourceXV International Clay Conference, abstracts; 2013 p. 1
Année2013
Séries alt.Secteur des sciences de la Terre, Contribution externe 20120425
RéunionXV International Clay Conference; Rio de Janeiro; BR; juillet 7-11, 2013
Documentlivre
Lang.anglais
Medianumérique
Formatspdf
ProvinceTerritoires du Nord-Ouest
Sujetspergélisol; glace fossile; argiles sensibles; effets climatiques; glissements de terrain; stabilité du sédiment; antecedents glaciaires; lacs glaciaires; niveaux d'eau; lentilles de glace; échantillons carrotés; plasticité; analyse de limites de plasticité; analyses minéralogiques; antecedents de sedimentation; Holocène; Lac glaciaire de McConnell; changement climatique; sédiments lacustres; dégel du pergélisol; effets cumulatifs; géologie des dépôts meubles/géomorphologie; géologie de l'environnement; minéralogie; Nature et environnement; Sciences et technologie; Santé et sécurité; Phanérozoïque; Cénozoïque; Quaternaire
ProgrammeGéosciences de changements climatiques, Infrastructures terrestres
Résumé(disponible en anglais seulement)
Clay-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.
Sommaire(Résumé en langage clair et simple, non publié)
Les sédiments argileux gelés et riches en glace sont très répandus dans l'environnement subarctique des basses terres des Grands Esclaves, dans les Territoires du Nord-Ouest, au Canada. Le dégel de ces sédiments, que ce soit par suite de perturbations ou de changements climatiques, a des conséquences sur la stabilité des terrines. Nous avons examiné les origines et les propriétés de ces sédiments afin d'améliorer la compréhension de ce terrain potentiellement dangereux. Nos résultats montrent que ces sédiments riches en glace sont liés aux environnements de dépôt d'un ancien lac glaciaire (McConnell) et du Grand lac des Esclaves, et à la croissance du permafrost après le retrait de ces lacs.
GEOSCAN ID292276