Submarine Sand Dunes in the St. Lawrence Estuary

Abstract

Areas of sand dunes and ripples have been observed in the estuary of the St. Lawrence River below Quebec City. A train of dunes 18 km long is present in the channel of Ile-aux-Coudres. Symmetrical dunes as high as 12 m and reaching 120 m in wave-length are abundant. Symmetrical subsurface crests are also revealed by seismic reflection profiling. The mode of origin of these dunes is not yet established. However, because of the high degree of stratification present in the water column, the possible role played by internal water waves in their formation should be considered.

Seismostratigraphy of the Middle St. Lawrence Esturary: A Late Quaternary Glacial Marine to Estuarine Depositional/Erosional Record

“…D 'Anglejan (1971 ) noted that local asymmetry of large-scale bedforms in the île aux Coudres basin was contrary to the direction which would be expected from present tidal current strengths. Monahan (1976) repeatedly surveyed smaller scale bedforms in one part of the area east of île aux Coudres, in depths of 13-19 m, at least twice a day for two weeks, and found no detectable migration or change of shape on sidescan sonar images although the survey covered "virtually every stage of the tidal cycle".…”

“…In the middle estuary unit 5 includes estuarine muds (d 'Anglejan and Brisebois, 1974), sand with bedforms (d 'Anglejan, 1971), and sandy/gravelly surficial veneers (d 'Anglejan and Brisebois, 1978;Loring and Nota, 1973), which unconformably overlie the surfaces of units 3 and 4 (Fig. 7).…”

“…7); it may also be present on part of the sill at the entrance to the Saguenay. Sand bedforms were recognized by d 'Anglejan (1971 ) on the floor of the île aux Coudres basin between Cap aux Oies and Baie-Saint-Paul, as well as upstream in the North Channel between île aux Coudres and île d'Orléans (Fig. 1 a).…”

“…A 4.65 m long piston core from this lens, collected in 1986 at 64 m depth (Praeg et al, 1987), consists of massive, wellsorted, medium to coarse grained sand with occasional pebbles, composed of subangular to subrounded quartz, feldspar, mafic and rock grains. Seabed samples of similar texture and composition have been described from this area by d 'Anglejan (1971) and Monahan (1976).…”

“…6e; Syvitski and Praeg, 1990), and the underlying glacial marine clays of unit 3 represent a poor source of sand (d 'Anglejan and Brisebois, 1978). The sand could be derived from fluvial erosion of glacial deposits on the shores (d 'Anglejan, 1971), in particular during postglacial emergence and fluvial downcutting; this is recognized to have been significant in the Baie-Saint-Paul and La Malbaie valleys (Rondot, 1974;Poulin, 1976;Dionne, 1977). Enhanced early postglacial erosion and transport corresponds to the paraglacial cycle (Church and Ryder, 1972;Jackson era/., 1982), augmented here by lowered base levels.…”