DEGLACIATION PATTERN OF THE WESTERN ST. LAWRENCE LOWLANDS (PETITE-NATION RIVER VALLEY, QUEBEC, CANADA) USING SURFACE AND SUBSURFACE LITHOSTRATIGRAPHY

The Petite-Nation River Valley is located at the junction of the St-Lawrence lowlands and the highlands of the Canadian Shield, north of the Ottawa River. The sediments present in this narrow valley recorded several events during the last deglaciation, notably regarding the pattern of ice retreat and the subsequent invasion by the Champlain Sea. The area is also characterized by the presence of the St-Narcisse Moraine (Younger Dryas). In spite of this, the understanding of the regional deglaciation remain tenuous, mainly due to the thick sediment cover related to the Champlain Sea that masks most of the deglacial landforms. The objective of this study is to produce a framework for the deglaciation by combining detailed mapping of surficial deposits and borehole stratigraphy. Surficial geology mapping (1:50K scale) was achieved using 3D panchromatic air-photo analysis combined with field investigations to validate the nature of sediments and landforms during the summers of 2011 and 2012. Information regarding the subsurface stratigraphy was obtained through the compilation of more than 1600 boreholes, from which the different sediments present were classified into 6 types according to their sedimentological origin. A total of 20 geological cross-sections were subsequently assembled using a GIS approach. Finally, a complete conceptual topo-stratigraphic model representing the subsurface sediment assemblages for the whole study area was created. Our results on the surficial and buried ice-margin features indicate that the ice front has retreated parallel to an axis oriented ENE-WSW in the region. The presence of a succession subaqueous outwash fans and ice-contacts deposits suggest the occurrence of at least three pre-St-Narcisse morainic fronts, which are located south of the main St-Narcisse Moraine. Directly south of the St-Narcisse Moraine, near the Simon Lake, boreholes revealed the presence of fine-grained marine sediments trapped within coarse-grained glaciofluvial deposits. The upper glaciofluvial unit is part of an outwash plain associated with the main St-Narcisse Moraine at 220 m, around 12 m below the maximum limit of the Champlain Sea in the area (232 m). These results suggest a rather active ice retreat, which was apparently punctuated by an ice readvance during the St-Narcisse cooling episode.