YOUNGER DRYAS GLACIAL READVANCE OVER THE NORTHERN MARGIN OF GOLDTHWAIT SEA AND MASS MOVEMENTS IMPLICATIONS ON THE COAST DOWNSTREAM THE SAGUENAY RIVER, ST. LAWRENCE ESTUARY, QUÉBEC

The study of the Quaternary deposits of the Upper North Shore of the lower St. Lawrence Estuary is a part of the Landslide-prone areas mapping Program, leaded by the Ministère des Transports du Québec. Using vertical air photographs, hillshaded LiDAR maps and oblique photographs by helicopter, natural sections and cliffs were located along 50 km of the coast downstream the mouth of the Saguenay River. The coast comprises exposures of Precambrian rock masses and long segments of a marine terrace. The terrace is sloping gently from the foothills of the Canadian Shield, at 120 m a.s.l., to erosional cliffs. The cliffs, eroded in various deposits, are 30 to 40 m high; large strandflats are exposed at low tide beside their foot. The regional succession comprises: - compact till, with underlying striations (120 ±10º); - a lower reddish clay unit, with scarce Portlandia arctica shells, overlain by a grey silty unit; - a group of deposits laid down at a glacier margin, including till (135º), compact unsorted silty sand and stratified deposits; - an upper marine unit, grading from lower stratified clay to stratified silt and upper massive silt, or to upper prodelta silt and sand. The cliffs are capped by sand or gravel beds. The lower reddish clay and silt, with ¹⁴C ages from 11,950 to 11,610 cal y., represent the early phase of the post-glacial Goldthwait Sea. The overlying glacial complex and a local moraine, with ¹⁴C ages from 11,300 to 10,850 cal y., correspond to an Early Younger Dryas glacial readvance, correlated to the 700 km-long Saint-Narcisse Moraine.

Erosion along coastal and river banks in these emerged marine deposits has contributed to the development of very large retrogressive landslides. In that region, the lateral spread of Grandes-Bergeronnes in 1896, which affected almost 1.7 km of coastline, and the two flowslides of Longue-Rive, in 1986 and 2006, are good examples of such a threat. Geotechnical investigations of landslide scars in that region highlighted the fact that the lower reddish clay and stratified silt could have played an important role as predisposing factor to very large landslides. Thereby, understanding the lateral and vertical variations and extension of the Quaternary units on a regional basis along the coastal cliffs and river banks provides a basis for the evaluation and delimitation of landslide-prone areas.