| Titre | Origin of lacustrine clays in the Great Slave Lowlands, Northwest Territories, Canada and implications for terrain stability |
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| Auteur | Percival, J B ;
Wolfe, S A; Grenier, A |
| Source | XV International Clay Conference, abstracts; 2013 p. 1 |
| Image |  |
| Année | 2013 |
| Séries alt. | Secteur des sciences de la Terre, Contribution externe 20120425 |
| Réunion | XV International Clay Conference; Rio de Janeiro; BR; juillet 7-11, 2013 |
| Document | livre |
| Lang. | anglais |
| Media | numérique |
| Formats | pdf |
| Province | Territoires du Nord-Ouest |
| Sujets | pergélisol; 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 |
| Programme | Géosciences de changements climatiques |
| 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 ID | 292276 |
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