LAKES HISTORY: A KEY TO UNDERSTANDING THE DEGLACIATION PATTERN OF THE LAURENTIDE AND LANAUDIERE REGION (SOUTHERN QUEBEC)

During the 1960s and 1970s, several studies indicated that proglacial lakes were formed by blockage of natural drainage pathways by the retreating ice margin in the Laurentides and Lanaudière region. Results from a new quaternary mapping program (2017-2020) using aerial photos and LiDAR analysis, fieldwork and GIS modelling are used here to suggest the extension of Lake Candona, the glacial lake that preceded the marine invasion of the area. In the Laurentian foothills, many deltas and shorelines marked the maximum altitude reached by the Champlain Sea: (247 m above sea level (m a.s.l.) at Saint Adèle; 230 m a.s.l. at Dalesville and 225 m a.s.l. farther north on the Saint-Narcisse Moraine). However, along the north side of the Saint-Lawrence valley, between Pine Hill and Saint-Calixte, and in the valley of Rivière du Nord, ice-contact deltas, subaqueous fans, numerous washing limits indicate the presence of a body of water at an altitude varying between 280 and 298 m a.s.l., or approximately 50 m above marine limit. These landforms extend over a strip approximately 60 km long. It is believed that they constitute the first evidences of the extension of Lake Candona onto part of the Canadian Shield. This interpretation is compatible with regional data reported by Rayburn (2004) in New York and Vermont, and Parent and Occhietti (1988, 1999) in the foothills of the Appalachian Mountains, in southern Quebec. Farther north, on the Canadian Shield, deltas, shorelines, and terraces are linked to the development of several ice-dammed lakes. These lakes developed in the valleys of the Rivière Rouge (max: 524 – min: 225 m a.s.l.), the Rivière Ouareau (639-342 m a.s.l.), the Rivière Du Nord (457-296m a.s.l.), mainly north of the Saint-Narcisse Morainic complex. The reconstruction of the evolution of these glacial lakes made it possible to identify several glaciolacustrine levels and to trace more than an hundred successive positions of the ice margin related to those levels. The proposed ice-marginal positions indicate a mode of retreat where the thinning of the ice sheet first lead to the deglaciation of the summits while the depressions were still occupied by ice lobes which blocked the drainage and allowed for the development of proglacial lakes.