Evidence of the Occurrence and Magnitude of Terrestrial Landslides in Recent Saguenay Fjord Sediments

Abstract

The 1971 St. Jean Vianney landslide resulted in the deposition of a distinctive grey-coloured layer of late glacial to early Holocene-age Laflamme Sea clay throughout the upper arm of the Saguenay Fjord. An older comparatively ubiquitous clay layer observed in gravity cores collected from the upper half of the fjord appears to be associated with an exceptionally large 17th century landslide of Laflamme Sea clay and littoral sand deposits that also occurred near St. Jean Vianney. The ancient slide is believed to have been triggered by a major earthquake that occurred in the Saguenay region on February 5, 1663. A cohesive debris flow underlies a sandy clay layer of presumed 1663 slide-related sediments. The debris flow is attributed to local slumping of terrestrial and submarine deposits of Laflamme Sea clay, and of Holocene estuarine sediments, as a result of the initial magnitude 7 earthquake shock. The sandy clay layer of 1663(?) slide sediment appears to have been deposited as a cohesionless mass flow. The flow was probably initiated by the failure of the Saguenay River delta foreslope on which large volumes of reworked landslide sediments would have been deposited by the spring freshet of 1663.