Morphostratigraphy of nearshore shallow-water submarine mass-movements in the Charlevoix area, Middle St. Lawrence Estuary (Eastern Canada)
Presented by Gabriel JOYAL on 23 Sep 2013 from 17:15 to 19:15
Type: Poster presentation
Session: Poster session
Track: Poster presentations
In the past few years, a number of terrestrial and submarine surveys undertaken along the north shore of the St. Lawrence Estuary (eastern Canada) have led to a better understanding of underwater mass movements failure mechanisms, their terrestrial extent, their probability of occurrence and their chronology (e.g., Cauchon-Voyer et al., 2008, 2011, Locat et al., 2012). Here we present a compilation of multibeam and seismic data collected during cruises on board R/V Coriolis II in 2011 and R/V Louis-Edmond-Hamelin in 2012 in the Charlevoix region (Middle St. Lawrence Estuary) in order to describe the geomorphology and extent of nearshore submarine mass movements. These results are of great significance in this region because of transport infrastructures and communities built in the coastal zone (Locat et al., 2012). The mapped sectors, Cap-au-Corbeau and St-Irénée, are located in Charlevoix-Kamouraska seismic zone (CKSZ), the most active seismic zone in eastern Canada (Lamontagne, 1987). This region is also locally covered with a thick Quaternary sedimentary sequence (Praeg et al., 1992) that can potentially fail during important seismic events. Here, we present results from a swath bathymetry survey using a multibeam echosounder (Reson Seabat 8101, 250 kHz) and an interferometric bathymetric sonar (GeoAcoustic GeoSwath, 250kHz) as well as results from a seismic survey using sparkers (Applied Acoustics Squid 2000; Locat et al., 2011). These data allow the mapping of the surface morphology and internal structures of a shallow water (<70 m water depth) landslide complex (Joyal and Lajeunesse, 2013; Turmel and Locat, 2013). The Cap-au-Corbeau sector consists of three coalescent mass wasting systems that cover an area of <1 km2 and has a 1 km long crown, with a maximal offshore extent of <750 m. Translated block up to 240 000 m3 induced compression ridges in the outer part of the landslides. Above the easternmost headwall scarp (<1-8 m WD), we mapped a 180-m wide and 80-m long oval-shaped bathymetric depression that could be representative of an active landslide. This depression is up to 6 m deep and is located at less than 80 m from the coastline. Terrestrial field observations have shown evidences soil lowering in the sedimentary units located directly onshore the bathymetric depression. In the St-Irénée sector, four distinct mass movement lobes ranging from 0.8 km2 to 5.2 km2 were identified. The surface and subsurface morphology of these landslides indicate that the westernmost lobes were formed by retrogressive landslides and the easternmost lobes were formed by translated outrunner blocks that have spread downslope along a 300 to 500 m path on a well-defined and almost horizontal basal shear surface. The translated block also produced in this sector a thick pressure ridges zone characterized by fold and thrust. The distance between the headwall scar and the coastline varies between 80 and 460 m. In this sector we did not identified evidence for interactions between terrestrial and submarine sections of mass movements since the coastline is essentially rocky. These results demonstrate that, in the Cap-au-Corbeau sector, the upper depression could be a second phase of mass movement that occurred in a larger and older mass movement complex or an evidence for recent landslide activity. In the St-Irénée sector, larger submarine mass movement lobes are either 1) located near the coastline, but in area of rocky coastal environment, or 2) located relatively far enough offshore to reduce potential hazards onshore. Nevertheless, as we know that the farthest headwall scars are located near the sedimentary coastline, constant monitoring and surveying need to be undertaken in order to detect eventual signs of retrogression. The identification of potential links between the onshore and offshore part of a landslide is of great importance in hazards management for coastal transportation facilities. Subsidence in the nearshore zone might be a key in understanding the post-failure behavior of landslides in this type of environment. Future work is needed to assess the internal morphology of the shallowest part of the landslide using high resolution seismic data. These data are the key to interpreting the processes that drive coastal lowering in this sector and could allow deciphering whether the formation of the nearshore bathymetric depression is due to sediment settlement, rotational slumping or landslide retrogression. Finally, we consider that better integrated knowledge from terrestrial and submarine morphology is needed in this specific research field to precisely assess natural hazards associated to the onshore extent of submarine mass movements. Combined LiDAR and multibeam bathymetry mounted on a very shallow water (<1 m) research vessel would provide valuable data for identifying areas of either slow or rapid coastal lowering and sediment transport dynamics along the coast. References cited Cauchon-Voyer, G., Locat, J., St-Onge, G., 2008. Late-Quaternary morpho-sedimentology and submarine mass movements of the Betsiamites area, Lower St. Lawrence Estuary, Quebec, Canada. Marine Geology 251, 233–252. Cauchon-Voyer, G., Locat, J., Leroueil, S., St-Onge, G., Demers, D., 2011. Large-scale subaerial and submarine Holocene and recent mass movements in the Betsiamites area, Quebec, Canada. Engineering Geology 121, 28–45. Joyal, G., Lajeunesse, P. 2013. Projet Charlevoix : Glissements côtiers sous-marins de Charlevoix ouest : Cap-au-Corbeau, St-Joseph-de-la-Rive et St-Irénée, Rapport LERN-Charlevoix-13-01, 26 p. Lamontagne, M., 1987. Seismic activity and structural features in the Charlevoix region, Quebec. Canadian Journal of Earth Sciences 24, 2118–2129. Locat, J., Terhzaz, L., Turmel, D., Lajeunesse, P., Mucci, A., Pelletier, É., St-Onge, G. 2011. Rapport de mission COR1103 – Fjord du Saguenay et estuaire du Saint-Laurent, May 24th to June 2nd 2011. Locat, J., Turmel, D., Noël, F., Amiguet, C., Lajeunesse, P., St-Onge, G., Terhzaz, L., 2012. Exploring the contrasting signatures of submarine landslides along the North Channel of the St. Lawrence Middle Estuary (Charlevoix), Québec, Canada. In Eberhardt et al. (eds.) Landslides and Engineered Slopes : Protecting Society through Improved Understanding, Taylor & Francis Group, London, 1077–1083. Praeg, D., D’Anglejan, B., Syvitski, J.P.M., 1992. Seismostratigraphy of the Middle St. Lawrence Esturary: A Late Quaternary Glacial Marine to Estuarine Depositional/Erosional Record. Géographie physique et Quaternaire 46, 133–150. Turmel, D., Locat, J., 2013, Projet Charlevoix : Rapport sur les données de sismique marine, Rapport LERN-Charlevoix-13-02, 28p.
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