Morpho-sédimentologie et mouvements de masse au large de la rivière Betsiamites, estuaire du Saint-Laurent

A complex submarine geomorphology was revealed from multibeam bathymetry and seismic reflection surveys conducted between 2001 and 2006 in the Lower St. Lawrence Estuary between the Betsiamites River and Rimouski, Quebec. In this paper, we describe the submarine morpho-sedimentology of an area of about 500 km with focus on the various mass movement features observed. A spatio-temporal sequence for the occurrence of the mass movements and a chronology suggesting ages for the failures are established. Four main geomorphological features were mapped within the three physiographic regions of the area: bedrock lineaments, mass movement morphologies, paleochannels, and pockmarks. Four of the observed mass movement deposits were dated. The Paraglacial event is dated at 9 kyr BP. The Landslide Channels event left a major landslide scar characterized by two large channels on the shelf and a large sediments fan in the Laurentian Channel. These two features are related to one another as sediments source and sink. The Landslide Channels event is dated around 7.25 kyr cal BP. Morphological observations and sediment core analyses allow us to identify a least two different recent (i.e. less than 1 kyr BP) debris flow deposits on the shelf and in the Laurentian Channel. Two different Pb-dated debris flow deposits were identified and associated to two recent earthquake episodes: (1) the AD 1663 (M~7) earthquake and (2) AD 1860 (M~6) or AD 1870 (M-6.5) earthquakes. The 1663 debris flow deposit is associated with a subaerial landslide observed on shore. In addition to a complex geomorphology influenced by mass movements, we have identified several regions on the shelf and on the Laurentian Channel with evidences of pockmarks which could potentially influence submarine slope stability in the Estuary.