Late-Quaternary morpho-sedimentology and submarine mass movements of the Betsiamites area, Lower St. Lawrence Estuary, Quebec, Canada

Abstract A complex submarine geomorphology was revealed from multibeam bathymetry and seismic reflection surveys conducted between 2001 and 2007 in the Lower St. Lawrence Estuary offshore Betsiamites River, Quebec, Canada. In this paper, we describe the submarine morpho-sedimentology of an area of ∼ 500 km 2 with focus on the consequences of four mass movement events. The general morpho-sedimentology of the area and submarine features resulting from mass movement processes, channel erosion and gas seepage are described. A spatio-temporal sequence for the occurrence of the mass movements and a chronology for the failures are established. We propose dates for four of the observed mass movement deposits. A buried paraglacial debris flow deposit is dated as older than 9280 cal BP, whereas a major landslide scar characterized by two topographic depressions on the shelf and a sediment lobe in the Laurentian Channel were dated around 7250 cal BP. Morphological observations and sediment core analyses allow us to identify a least two different recent (i.e., less than 500 yr old) debris flow accumulations associated with two recent earthquakes: (1) the AD 1663 (M ∼ 7) and (2) AD 1860 (M ∼ 6) or AD 1870 (M ∼ 6.5) earthquakes. In addition to a complex geomorphology influenced by mass movements, we have identified several regions on the shelf and on the Laurentian Channel with evidence of pockmarks, which could potentially influence submarine slope stability in the Estuary.

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