Quaternary Sedimentation in the St. Lawrence Estuary and Adjoining Areas, Eastern Canada: An Overview Based on High-Resolution Seismo-Stratigraphy

Abstract

The regional Quaternary seismo-stratigraphy of NW Gulf of St. Lawrence, based on 5700 line km of high resolution seismic reflection profiles, is described. The Quaternary sequence can be locally missing or can exceed 1.3 km in thickness. Five major stratigraphic units are recognized, which vary in their character and distribution so that at any location a variety of bedrock types may be overlain by a distinctive Quaternary sequence. These units relate to the advance and retreat of the Late Wisconsinan Ice Sheet. We interpret these units as: Unit 1, recording the presence of grounded glacial ice, including ice-loaded and ice-deposited sediments. Unit 2, ice-proximal coarse-grained sediment deposited either as a thin, conformable layer during the rapid retreat of an ice terminus, or as a wedge-shaped fan marking the position of an ice front still stand. Unit 3, ice-distal fine-grained sediment deposited from meltwater plumes at times of elevated sea levels and rapidly ablating sea ice. Unit 4, paraglacial deltaic sediment marking the melting of terrestrially-based ice caps, and the concommittant growth of deltas, rapidly prograding into a seaway undergoing rapidly falling sea levels. Unit 5, postglacial sediment reflecting the winnowing of shallow areas and deposition of organic-rich mud in deep basins, under modern sea level and océanographie conditions. A conceptual model dealing with the deposition of sediment associated with the withdrawal of a continental ice sheet is developed. The model includes the dynamics associated with the initial ice advance, terminal ice dynamics, retreat of the ice terminus, stable ice-fronts during the recessional phase, ice sheets ablating on land, and postglacial sedimentation under conditions of fluctuating sea levels.Le présent article se fonde sur des profils effectués par sismique reflection sur 5700 kilomètres dans le nord-ouest du golfe du Saint-Laurent. La séquence quaternaire, parfois absente, peut ailleurs avoir plus de 1,3 kilomètre d'épaisseur. On a relevé cinq grandes unités stratigraphiques dont la nature et la répartition varient. Liées à l'évolution de l'inlandsis du Wisconsinien supérieur, elles se présentent comme suit: unité 1 : sédiments de contact renfermant des sédiments comprimés par le glacier et d'autres déposés par lui; unité 2: sédiments proximaux grossiers, déposés soit en une couche mince et concordante durant le retrait rapide d'un front, soit sous forme de cône marquant la position d'un front stable; unité 3: sédiments distaux fins, déposés par des langues d'eau de fusion dans une mer à niveaux élevés et dont la glace fondait rapidement; unité 4: sédiments deltaïques paraglaciaires révélant la fusion de glaces continentales et la formation rapide et concomitante de deltas dans une mer accusant de fortes baisses de niveau : unité 5: sédiments postglaciaires révélant le triage de zones peu profondes et le dépôt, en profondeur, de boues riches en matières organiques, dans des conditions rappelant celles d'aujourd'hui. Un modèle, exposant les étapes de sé...

Ice sheet extension to the Celtic Sea shelf edge at the Last Glacial Maximum

“…Ice advance across a midlatitude Atlantic shelf implies glacimarine deposition not only by ice rafting, but also by suspension settling from turbid and buoyant meltwater plumes, at rates that decrease seaward from the ice margin (Syvitski and Praeg, 1989;Syvitski, 1991). In our interpretation, the overconsolidated stratified diamicts on outer Cockburn Bank consist of sediments originally deposited beyond the ice margin that were subsequently overridden and subglacially deformed during its advance (cf.…”

“…Scourse et al (1990Scourse et al ( , 1991 acknowledged that the presence of such deposits across an open Atlantic shelf was difficult to explain by iceberg rafting, especially given modelling evidence that glacially lowered sea levels resulted in significantly increased tide and wave energies in the Celtic Sea (Belderson et al, 1986;Uehara et al, 2006;see Scourse et al, 2009b). We note that along tidewater ice sheet margins low energy seabed conditions are favoured by water column stratification, a result of the summer input of turbid and buoyant meltwater plumes and winter sea ice cover, which together may limit the action of tidal-and wave-induced currents (Syvitski and Praeg, 1989;Syvitski, 1991).…”

Seismo-Stratigraphy and Sedimentology of Holocene Sediments off Grande Rivière de la Baleine, Southeastern Hudson Bay, Québec

“…Unit 5 is acoustically similar to the paraglacial deltaic unit described by Syvitski and Praeg (1989) in the St. Lawrence Gulf, but is of different origin as it postdates by a long period the retreat and disappearance of the ice sheet (see below). It occurs as a constructional deltaic wedge built outward from the mouth of Grande Rivière de la Baleine.…”

“…Hudson Bay (Josenhans et al, 1988;Josenhans and Zevenhuizen, 1990), Labrador Shelf (Josenhans et al, 1986). Scotian and mid-Norwegian Shelves (King et al, 1991), and Gulf of St. Lawrence (Syvitski and Praeg, 1989).They are designated as: ice contact sediments (unit 2), rythmically bedded clays and silts (unit 3), postglacial marine muds (unit 4) and, distal fluvio-deltaic sediments (unit 5). Each unit represents a particular stage in the déglaciation and the postglacial evolution of this region.…”

Les Dépôts lacustres d’obsturation de Saint-Placide, Charlevoix, Québec

“…La localisation exacte du barrage reste énigma-tique faute de connaissances sur les sédiments du fond de la vallée du Gouffre (Lajoie, 1981). Dans le moyen estuaire à l'aplomb de la vallée du Gouffre, l'épaisseur des sédiments quaternaires atteint plus de 290 m (Syvitski et Praeg, 1989). La langue glaciaire de la vallée du Gouffre devait avoir au minimum 2500 m de large et 300 m d'épaisseur, compte tenu de la taille de la vallée actuelle.…”