Large-scale subaerial and submarine Holocene and recent mass movements in the Betsiamites area, Quebec, Canada

At least three major landslide events formed the submarine and subaerial Betsiamites (Québec, Canada) landslide complex and mobilized an estimated total volume of 2000 million m3 (2 km3). Linkage between offshore and onshore geophysical investigations with borehole data and in situ testing allows reconstruction of the architecture of the Betsiamites River delta area and leads to the identification of the main failure events. The submarine scar of the Betsiamites landslide complex may have resulted from a first failure, dated at about 9250 cal BP, which mobilized a minimum volume of 200 million m3. A second landslide dated at 7250 cal BP mobilized a volume of 1300 million m3 over an area of 54 km2. The Betsiamites submarine landslide event dated at 7250 cal BP left the largest scar yet identified on the St. Lawrence estuary seafloor. Furthermore, this paper demonstrates that the subaerial scar of the Betsiamites landslide complex is a result of the Colombier landslide event, which was initiated by the 1663 earthquake and involved four successive failure phases: one submarine and three subaerial. The February 5th 1663 earthquake triggered a submarine landslide event, which reached the shoreline, and led in a short period of time successively to two subaerial flowslides in sensitive clayey material and a subaerial lateral spread. The four failure phases mobilized a possible total volume of about 530 million m3 over an area of 20 km2. The Colombier landslide event is among the largest documented historic landslides in Canada. The presence of submarine scars left by the early Holocene events acted as predisposition factors for the development of the failure while the earthquake of 1663 was the main triggering factor of the first submarine failure.