ICE-FLOW CHRONOLOGY AND ICE RETREAT PATTERN IN THE OUTAOUAIS REGION, SOUTHWESTERN QUEBEC (CANADA)

Extensive Quaternary geology mapping (28 map sheets, 1:50K scale) in support to a large-scale groundwater survey program in the Outaouais region gave rise to new data sets that allowed a refinement of paleogeographic reconstructions in southwestern Quebec. Measurements of >700 ice-flow erosional features (striae, grooves, etc.) and their associated crosscutting relationships documented an ice-flow sequence comprising four main episodes: 1) a WNW-ESE movement of an unknown direction and age (possibly pre-last glaciation); 2) a SW movement associated with the onset of the last glaciation; 3) a S to SSE movement that represents the dominant (full-glacial) ice flow in the region, and finally 4) a SE to SSE movement that represents the deglacial ice flow.

Five ice-margin positions that preceded the build-up of the Saint-Narcisse Moraine were identified in the southern part of the Outaouais region based on the spatial distribution of ENE-WSW trending ice-contact deposits (moraines, ice-contact deltas, subaqueous outwash fans, eskers). Our work suggests that glacial Lake Candona, with an altitude of about 240 m at the end of this episode (~11,1 ka BP; 13 cal. ka), was in contact with the Pre-Saint-Narcisse position I, while positions II to V were formed in contact with the Champlain Sea. Using the same constraints, we proposed a correlation between the Saint-Narcisse Moraine (beginning of the Younger Dryas; 10,9 ka BP; 12,8 cal. ka) and the Saint-Louis-de-Masham Moraine to the west. From there, the ice-front margin changes to a more NE-SW orientation, up to the Ottawa Valley, where it joins the “A ice-margin” position suggested by Barnett (1988).

In the western part of the Ottawa Valley, ice-flow towards the SE is associated with the formation of a late-glacial lobe, which would have delayed the deglaciation of this sector. South of the Baskatong reservoir (in the north), and west of the study area, in the Lake Saint-Patrice region, a series of moraines orientated NE-SW represent the last well-defined ice-margin position in the region. These moraines are likely connected with the Mars-Batiscan morainic system (10 ka BP; 11,4 cal. ka) identified further east in the Mauricie region.