Débordement du Courant glaciaire du Saint-Laurent sur la bordure nord du moyen estuaire pendant le Bølling-Allerød et réponse de la marge de l'Inlandsis laurentidien à la Période froide intra-Allerød au sud du Québec, Canada
Résumé
The middle and upper Estuary regions of the St. Lawrence River include the Laurentian highlands to the north and the Appalachian piedmont and uplands to the south. From a synthesis of the literature, the glacial dynamics of these regions are the result of: 1) the changing position of an ice dispersion centre in the Québec-Labrador Dome; 2) the upstream retreat of a calving bay from the Gulf of St. Lawrence to the middle Estuary at the origin of the rapid inland extension of the St. Lawrence Ice Stream (StLIS) of the Laurentide Ice Sheet (LIS); 3) the progressive separation of appalachian ices from the LIS; and 4) an increasing topographic
control on ice flows. The LIS ice flowed southeastward over the area independently of structural topography during the Late Glacial Maximum or closely thereafter. From published and original data, the StLIS eventually reached the lower part of the middle estuary as a
moving-inland head of ice flow convergence. The ice diversion accelerated the regional-scale
ablation on both sides of the estuary likely under the warmer Bølling climate. A generalized
thinning in the middle estuary is inferred to cause the northward ice-flow reversal of the
Appalachian ice divide to expand into the studied region, consequently initiating the Northward
Ice Overflow (NIO) on the Laurentian margin, up to elevations of 750 to 950 m in the Beaupré
Basin. This likely took place between the end of the Bølling and the early Allerød.
Based on this study, the northern part of the region records a detailed Late Glacial sequence of
ice flows and retreat events. After the initiation of the NIO, the overflow ice in the Beaupré
Basin thinned by at least 100 m during part of the Allerød. The Beaupré Group of Moraines and
the Savane Moraine in the Laurentians provide evidence of a significant subsequent slowdown
in the glacial retreat. These moraines are attributed to the Intra-Allerød Cold Period (IACP)
between approximately 13.250 and 13.050 cal ka BP, based on their older age position relative
to: 1) the Saint-Narcisse Moraine of early Younger Dryas; 2) the deglaciation of a lake basin
whose basal sediments are dated at 12.750 cal ka BP; and 3) the local marine invasion
(Goldthwait Sea) prior to 13-12.9 ± 0.1 cal ka BP. Laterally to these moraines, the glacial
megalineations of the Montmorency massif and Gouffre Valley represent a major glacial flow
toward the SSE from the Québec-Labrador Dome, which did not override the Beaupré Basin.
This LIS flow is closely followed by a reactivated StLIS which flowed into small valleys and
lowlands of the north shore of the middle and upper estuary from the Québec City region to the
Malbaie Valley (glacial striations and the Escarpment and South-Éboulements moraines). The
LIS flow episode toward the SSE probably began at the start of the IACP. At least two moraine
belts (Cherry River–Mégantic and Mount Ham) are tentatively associated with the IACP in
southern Québec and thus would have a climatic origin.
These new findings confirm a much shorter duration for the sequence of late glacial events in
the region than was proposed in previous models as well as continuity between the early StLIS
in the Gulf (the Laurentian Channel Ice Stream) and the late StLIS of the middle estuary. The
IACP slowdown of the glacial retreat opens the possibility of correlation at the scale of the
southeastern margin of the LIS.
RÉSUMÉ
La région de l'estuaire moyen et supérieur du Saint-Laurent comprend les reliefs des Laurentides au nord, et le piémont et les hautes terres des Appalaches au sud. En synthétisant la littérature, la dynamique glaciaire dans cette région résulte des facteurs suivants, dans le contexte de réchauffement climatique : 1) la position changeante d'un centre de dispersion des glaces du dôme du Québec-Labrador ; 2) le retrait vers l’amont d'une baie de vêlage, du golfe du Saint-Laurent vers l'estuaire moyen, à l'origine de l'extension rapide vers l'intérieur du continent du Courant glaciaire du Saint-Laurent (CGStL) de l'Inlandsis laurentidien (LIS); 3) la séparation progressive des glaces des Appalaches du LIS ; et 4) un contrôle topographique croissant lié à l’amincissement des glaces. Pendant le Dernier Maximum Glaciaire ou peu après, la glace de l’Inlandsis laurentidien s'est écoulée sur la région vers le sud-est indépendamment de la topographie structurale. Ensuite, en tenant compte des nouvelles données, le CGStL a atteint la partie inférieure de l'estuaire moyen en tant que tête de convergence du courant glaciaire remontant vers l'intérieur des terres. Cette déviation de l’écoulement de la glace a accéléré l'ablation régionale des deux côtés de l'estuaire pendant l’optimum du Bølling. Un amincissement généralisé dans l'estuaire moyen a favorisé l'inversion de l'écoulement vers le nord, à partir de la ligne de partage des glaces des Appalaches. D’après cette étude, cette inversion a entraîné un flux vers le nord (Débordement sud-nord) sur la marge des Laurentides, jusqu’à plus de 750 à 950 m d’altitude dans le Bassin de Beaupré, probablement entre la fin du Bølling et le début de l'Allerød. La marge nord du moyen estuaire enregistre une séquence détaillée d'écoulements et de retraits glaciaires pendant le Tardiglaciaire. Dans le Bassin de Beaupré, le Débordement sud-nord est suivi d'un amincissement d’au moins 100 m de la glace de débordement (phase plus chaude de l'Allerød). Les moraines du Groupe de Beaupré et la Moraine Savane dans les Laurentides témoignent ensuite d'un ralentissement important du retrait glaciaire. Ces moraines sont attribuées à la Période Froide Intra-Allerød (Intra-Allerød Cold Period, IACP) entre environ 13,250 et 13,050 cal ka BP, sur la base de leur position plus ancienne par rapport à 1) la Moraine de Saint-Narcisse du début du Dryas récent, 2) la déglaciation d'un bassin lacustre dont les sédiments basaux sont datés à 12,750 cal ka BP et 3) l'invasion marine locale (Mer de Goldthwait) avant 13-12,9 ± 0,1 cal ka BP. Latéralement à ces moraines, les mégalinéations glaciaires du massif de Montmorency et de la vallée du Gouffre représentent un écoulement glaciaire majeur vers le SSE du dôme du Québec-Labrador. Cet écoulement de l’inlandsis est rapidement suivi d’une réactivation du Courant glaciaire du Saint-Laurent qui a débordé dans les petites vallées et les basses terres de la rive nord de l'estuaire moyen et supérieur, de la région de Québec jusqu'à la vallée de la Malbaie (stries glaciaires et moraines de l'Escarpement et des Éboulements-Sud). L'épisode d'écoulement majeur vers le SSE du LIS a probablement débuté au début de l’IACP. Au moins deux ceintures de moraines (Cherry River-Mégantic et Mont Ham) sont probablement associées à l'IACP au sud du Québec et auraient ainsi une origine climatique.
Ces nouveaux résultats confirment que la séquence des événements glaciaires tardifs dans la région a une durée plus courte que ce qui était proposé dans les modèles antérieurs, et qu'il y a une continuité entre le CGStL précoce dans le Golfe et le CGStL tardif de l'Estuaire moyen. Le ralentissement du retrait glaciaire pendant l'IACP ouvre la possibilité de corrélations à l'échelle de la marge sud-est du LIS.
Mots-clés : Courant glaciaire du Saint-Laurent, Bølling-Allerød, IACP, flux glaciaires
ABSTRACT
St. Lawrence Ice Stream overflow on the northern edge of the middle Estuary during the Bølling-Allerød and response of the Laurentide Ice Sheet margin to the Intra Allerød Cold Period in southern Québec, Canada
The middle and upper Estuary regions of the St. Lawrence River include the Laurentian highlands to the north and the Appalachian piedmont and uplands to the south. From a synthesis of the literature, the glacial dynamics of these regions are the result of: 1) the changing position of an ice dispersion centre in the Québec-Labrador Dome; 2) the upstream retreat of a calving bay from the Gulf of St. Lawrence to the middle Estuary at the origin of the rapid inland extension of the St. Lawrence Ice Stream (StLIS) of the Laurentide Ice Sheet (LIS); 3) the progressive separation of Appalachian ices from the LIS; and 4) an increasing topographic control on ice flows. The LIS ice flowed southeastward over the area independently of structural topography during the Late Glacial Maximum or closely thereafter. From published and original data, the StLIS eventually reached the lower part of the middle estuary as a moving-inland head of ice flow convergence. The ice diversion accelerated the regional-scale ablation on both sides of the estuary likely under the warmer Bølling climate. A generalized thinning in the middle estuary is inferred to cause the northward ice-flow reversal of the Appalachian ice divide to expand into the studied region, consequently initiating the Northward Ice Overflow (NIO) on the Laurentian margin, up to elevations of 750 to 950 m in the Beaupré Basin. This likely took place between the end of the Bølling and the early Allerød.
Based on this study, the northern part of the region records a detailed Late Glacial sequence of ice flows and retreat events. After the initiation of the NIO, the overflow ice in the Beaupré Basin thinned by at least 100 m during part of the Allerød. The Beaupré Group of Moraines and the Savane Moraine in the Laurentians provide evidence of a significant subsequent slowdown in the glacial retreat. These moraines are attributed to the Intra-Allerød Cold Period (IACP) between approximately 13.250 and 13.050 cal ka BP, based on their older age position relative to: 1) the Saint-Narcisse Moraine of early Younger Dryas; 2) the deglaciation of a lake basin whose basal sediments are dated at 12.750 cal ka BP; and 3) the local marine invasion (Goldthwait Sea) prior to 13-12.9 ± 0.1 cal ka BP. Laterally to these moraines, the glacial megalineations of the Montmorency massif and Gouffre Valley represent a major glacial flow toward the SSE from the Québec-Labrador Dome, which did not override the Beaupré Basin. This LIS flow is closely followed by a reactivated StLIS which flowed into small valleys and lowlands of the north shore of the middle and upper estuary from the Québec City region to the Malbaie Valley (glacial striations and the Escarpment and South-Éboulements moraines). The LIS flow episode toward the SSE probably began at the start of the IACP. At least two moraine belts (Cherry River–Mégantic and Mount Ham) are tentatively associated with the IACP in southern Québec and thus would have a climatic origin.
These new findings confirm a much shorter duration for the sequence of late glacial events in the region than was proposed in previous models as well as continuity between the early StLIS in the Gulf (the Laurentian Channel Ice Stream) and the late StLIS of the middle estuary. The IACP slowdown of the glacial retreat opens the possibility of correlation at the scale of the southeastern margin of the LIS.
Keywords: St. Lawrence Ice Stream, Bølling-Allerød, IACP moraines, ice flows
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