Geomorphology of seismically-induced mass-movements in lakes of south-central Québec

English Abstract:

Although Late-Quaternary sediments in lakes located within or near the Charlevoix-Kamouraska and Western-Québec seismic zones are known to have been intensely remobilized by seismic event since deglaciation, postglacial mass movements in lakes of southern Québec inundated by the Champlain Sea and located more than 70 km from these seismic zones remain poorly documented. Recent high-resolution swath bathymetry and acoustic stratigraphy surveys undertaken in four lacustrine basins of south-central Québec (lakes St-Joseph, Aux-Sables, Mékinac and Maskinongé) revealed the widespread occurrence of mass movement deposits and morphologies associated with postglacial seismic activity. Swath bathymetry data at a resolution of 0.5 to 1 m allows the observation in great detail of various morphologies associated with mass movements, such as failure scarps, lobe-shaped deposits, hummocky surfaces, compression ridges and gullies. Subbottom profiles and sediment core data show many rapidly deposited layers which have been dated to ~1000 years BP in Lake Maskinongé, ~915 years BP in Lake-aux-Sables and ~760 years BP in Lake Saint-Joseph. Our results indicate that at least one seismic event triggered mass movement in these four lakes during the last ca. 1000 years. Further analysis and dating of sediment cores will allow determining whether these mass movement deposits recorded the same regional seismic event or different events related to different seismic zones.