ABSTRACT: Holocene bedload transport and mass wasting sedimentation in a steep coastal area, St. Lawrence Estuary, Canada.

CGRG Bibliography of Canadian Geomorphology
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Abstract

The integration of multibeam surveys (including backscatter index) with high-resolution (seismic and sidescan sonar) geophysical surveys and seafloor sampling is used to build a comprehensive geological framework of the Quaternary succession of the St. Lawrence Estuary (eastern Canada). The sedimentary succession locally reaches up to 500 m in thickness and likely records at least the last 2 major continental glacial and interglacial sequences. Rapid sedimentation rates in the Estuary (up to 7m/kyrs) have blanketed any glacial morphology that may have been present. Superficial coring (<1–10 m) only allows sampling of the last few hundred to thousand years, so a relative chronology was established from seismostratigraphy. The multibeam survey in the upper Estuary, and mostly within the Saguenay – St. Lawrence Marine Park, shows spectacular bedforms such as mega-sandwave trains and multiple slope failures. This suggests a very dynamic sedimentary environment as well as slope instability. Seismic profiles upstream from the mouth of a major river (Saguenay River) show that the very well developed bedforms may be relict from a time when the drainage regime in the Estuary was different, likely with stronger discharge downriver and stronger tidal currents. Surface sandwave trains, up to 10 m in amplitude and with an average 200 m wavelength, imagedby the multibeam and sidescan sonar, are observed on seismic profiles below thick landslide deposits which partially cover them. Camera pictures show that the landslide material is heterogeneous with fine grained sediments mixed with large cobbles; sand is currently migrating over the slide. This sand migration represents the modern sediment transport at the origin of smaller scale ripples superposed on the mega-bedforms. Some of the landslide deposits are well colonized by the benthic community, while others are almost devoid of benthic biota. This suggests that the triggering of the landslides is episodic and might correspond to recurring large earthquakes. Indeed, the upper Estuary lies within the most active seismic area in eastern Canada (the Charlevoix Seismic Zone) and is still affected today by the glacio-isostaticadjustments that followed the last deglaciation. Timing of these slide events is crucial and is the focus of current research as these data will be play a key role in the preparation of integrated ocean management plans, the ultimate goal of this project.

Bibliography of Canadian Geomorphology