Document generated on 09/26/2021 5:34 a.m. Géographie physique et Quaternaire Late Wisconsinan Sedimentation in the Québec City Region: Evidence for Energetic Subaqueous Fan Deposition During Initial Deglaciation La sédimentation dans la région de Québec au Wisconsinien suspérieur : mise en place d’un cône sableux de turbidité au cours de la phase initiale de la déglaciation Sedimentación del wisconsiniano tardío en la región de la ciudad de Quebec, Canadá : evidencia de un depósito subacuático en cono de alta energia durante el deshielo inicial Don Cummings and Serge Occhietti Volume 55, Number 3, 2001 Article abstract In the St. Lawrence Valley just west of Québec City, initial deglaciation was URI: https://id.erudit.org/iderudit/006854ar accompanied by an energetic northward discharge of meltwater into a body of DOI: https://doi.org/10.7202/006854ar water, possibly subglacial, that was present in the axis of the valley. At Pointe Saint-Nicolas, a thick (> 35 m) ice-proximal sandy subaqueous fan was See table of contents deposited during this event. The subaqueous fan is composed primarily of northwest-dipping turbidite sheets, with minor hyperconcentrated underflow channel fills and gravel-outwash deposits. By 11 200 14C BP, subaqueous fan Publisher(s) sedimentation had stopped, and massive to rhythmically interbedded glaciomarine muds were being deposited. Introduction of muds into the basin Les Presses de l'Université de Montréal promoted subaqueous slope failure and debris flowing locally. Between 10 950 and 10 800 14C BP, the Laurentide Ice Sheet readvanced southward onto the ISSN south shore, and deposited the subglacial St. Nicolas Till. With continued isostatic rebound and associated forced regression, tidal currents began to 0705-7199 (print) rework emergent coastal zones, and fluvial systems started to incise their 1492-143X (digital) valleys. Explore this journal Cite this article Cummings, D. & Occhietti, S. (2001). Late Wisconsinan Sedimentation in the Québec City Region: Evidence for Energetic Subaqueous Fan Deposition During Initial Deglaciation. Géographie physique et Quaternaire, 55(3), 257–273. https://doi.org/10.7202/006854ar Tous droits réservés © Les Presses de l'Université de Montréal, 2001 This document is protected by copyright law. Use of the services of Érudit (including reproduction) is subject to its terms and conditions, which can be viewed online. https://apropos.erudit.org/en/users/policy-on-use/ This article is disseminated and preserved by Érudit. Érudit is a non-profit inter-university consortium of the Université de Montréal, Université Laval, and the Université du Québec à Montréal. Its mission is to promote and disseminate research. https://www.erudit.org/en/ GPQ_55-3.qxd 21/05/03 16:00 Page 257 Géographie physique et Quaternaire, 2001, vol. 55, no 3, p. 257-273, 10 fig., 2 tabl. LATE WISCONSINAN SEDIMENTATION IN THE QUÉBEC CITY REGION: EVIDENCE FOR ENERGETIC SUBAQUEOUS FAN DEPOSITION DURING INITIAL DEGLACIATION Don CUMMINGS* and Serge OCCHIETTI, respectively, Département des Sciences de la Terre, Université du Québec à Montréal, C.P.8888, succusale Centre-ville, Montréal, Québec H3C 3P8, and Centre de recherche en géochimie isotopique et en géo- chronologie (GÉOTOP), Université du Québec à Montréal, C.P.8888, succursale Centre-ville, Montréal, Québec H3C 3P8. ABSTRACT In the St. Lawrence Valley just RÉSUMÉ La sédimentation dans la région RESUMEN Sedimentación del wisconsiniano west of Québec City, initial deglaciation was de Québec au Wisconsinien suspérieur : tardío en la región de la ciudad de Quebec, accompanied by an energetic northward dis- mise en place d’un cône sableux de turbidi- Canadá : evidencia de un depósito subacuá- charge of meltwater into a body of water, pos- té au cours de la phase initiale de la dégla- tico en cono de alta energia durante el sibly subglacial, that was present in the axis of ciation. Pendant la phase initiale de la dégla- deshielo inicial. En el valle del San Lorenzo the valley. At Pointe Saint-Nicolas, a thick ciation de la vallée du Saint-Laurent, dans la justo al oeste de la ciudad de Quebec un (> 35 m) ice-proximal sandy subaqueous fan région à l’ouest de la ville de Québec, des deshielo precoz fue acompañado de una des- was deposited during this event. The sub- eaux de fonte à haute énergie se sont déver- carga enérgica de agua de fusión en direc- aqueous fan is composed primarily of north- sées vers le nord dans une masse d’eau très ción norte hacia un cuerpo de agua proba- west-dipping turbidite sheets, with minor probablement sous-glaciaire localisée dans blemente subglacial ya presente en el eje del hyperconcentrated underflow channel fills and l’axe de la vallée. Ce déversement est à l’ori- valle. En Punta San Nicolas ese derrame dio gravel-outwash deposits. By 11 200 14CBP, gine d’un cône sableux de turbidité à penda- origen a un depósito arenoso próximo al hielo subaqueous fan sedimentation had stopped, ge nord-ouest, associé à des dépôts de cou- de un espesor de 35 m en forma de cono. and massive to rhythmically interbedded rants de fond hyperconcentrés et à des Dicho cono subacuático esta compuesto en glaciomarine muds were being deposited. graviers et galets d’épandage proglaciaire. primer lugar por capas de turbit del noroeste Introduction of muds into the basin promoted Vers 11 200 14C BP, des boues glaciomarines asociados en lo alto con corrientes de fondo subaqueous slope failure and debris flowing massives et des lits intercalés de rythmites hiperconcentradas y aluvión de grava. Hace locally. Between 10 950 and 10 800 14CBP, sont déposés dans l’axe de la vallée enfon- aproximadamente 11 200 14C BP, la sedi- the Laurentide Ice Sheet readvanced south- cée par glacioisostasie. Cet apport de boues mentación del cono subacuático se detuvo y ward onto the south shore, and deposited the marines engendre localement des glisse- simultáneamente cantidades masivas de subglacial St. Nicolas Till. With continued iso- ments sous-aquatiques et des coulées de lodos glaciomarinos fueron depositados de static rebound and associated forced regres- débris sur les pentes immergées. Entre manera rítmica. La introducción de lodo en la sion, tidal currents began to rework emergent 10 950 et 10 800 14CBP, l’Inlandsis laurenti- cuenca provocó la fractura de la pendiente coastal zones, and fluvial systems started to dien a progressé de nouveau jusque sur la subacuática y conllevo la acumulación incise their valleys. rive sud de l’actuel Saint-Laurent et déposé le de desechos localmente. Entre 10 950 y till sous-glaciaire de Saint-Nicolas. Les cou- 10 800 14C BP, La Capa glacial Lorenzana rants de marée ont ensuite remanié les litto- avanzó nuevamente en dirección sur hasta la raux progressivement émergés par relève- rivera originando el depósito subglacial de las ment glacioisostatique. Les réseaux fluviatiles tillitas de San Nicolas. Posteriormente, las cor- ont commencé à inciser des vallées, en éro- rientes de mareas fueron definiendo el litoral dant et redéposant les dépôts antérieurs. emergente mientras que el sistema fluvial fue provocando la ablación de los valles. Manuscrit reçu le 25 octobre 2001 ; manuscrit révisé accepté le 22 septembre 2002 *Current address: Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5. Email: [email protected] GPQ_55-3.qxd 21/05/03 16:00 Page 258 258 D. CUMMINGS and S. OCCHIETTI INTRODUCTION Shilts, 1993). All of the units outlined in this stratigraphic framework have been previously identified at the Pointe Saint- The deglacial history of the Québec City region is poorly Nicolas site, with the notable exception of the Québec City Till understood, and has proven difficult to integrate into larger (Fig. 3). basin-scale deglacial models of the St. Lawrence Valley (LaSalle et al., 1977; Chauvin et al., 1985; Parent and LaSalle (1984, 1989) proposed a correlation between the Occhietti, 1988, 1999; Occhietti et al., 2001b). During deglacia- Quaternary stratigraphy of the Québec City region and the tion, the greater Québec City region acted as a gateway for more extensively studied Quaternary stratigraphy of the Central major meltwater discharges into the Atlantic Ocean (Parent St. Lawrence Valley established by Gadd (1971). Specifically, and Occhietti, 1988, 1999; Rodrigues and Vilks, 1994), expe- he correlated the Pointe Saint-Nicolas and Bécancour tills, the rienced complex ice margin fluctuations (Blais and Shilts, Anse-aux-Hirondelles Formation and the St. Pierre Sediments, 1989; LaSalle and Shilts, 1993; Fournier, 1998; Paradis and the Beaupré and Deschaillons Varves, and the Québec City Bolduc, 1999; Bhiry et al., 2001), was transgressed by the and Gentilly tills (Fig. 2). Recent revisions to the stratigraphic Champlain Sea (Occhietti et al., 2001a), and underwent a framework in the Central St. Lawrence Valley (Lamothe, 1989; subsequent episode of isostatic rebound and base level fall Occhietti et al.,1996; see Fig. 2) were not available to LaSalle that forced shorelines to regress to their present day position. when he proposed his basin-scale correlations (1984, 1989). With limited bedrock striation and glacial landform data in These revisions have been extended in part to the Québec regions below the Champlain Sea maximum (~200 m asl), City region; for example, the Beaupré Formation is now corre- these events are poorly constrained in time and space. In an lated with the St. Maurice Rhythmites (Occhietti, 1990). The attempt to provide insight into the deglacial history of the applicability of the revised framework of Occhietti et al. (1996) Québec City region, this paper investigates the sedimentol- to the Québec City region will be commented on briefly in the ogy of a well-exposed outcrop in a sand pit at Pointe Saint- discussion section. Nicolas that has served as the de facto type section for the Québec City region. DEGLACIATION OF THE ST. LAWRENCE VALLEY The Pointe Saint-Nicolas site is located in the axis of the St.