IRSL-DATED GLACIOLACUSTRINE LAURENTIDE ICE MARGINAL POSITIONS IN THE CANADIAN SOUTHEASTERN APPALACHIANS AND THE ESTIMATION OF REGIONAL ICE VOLUME AT THE OIS3/OIS4 TRANSITION

The Quaternary geology of southeastern Quebec is characterized by extensive Late Pleistocene sedimentary sequences. The OIS4 to OIS3 transition is marked by an important episode of regional ice volume reduction along the southeastern margin of the Laurentide ice sheet (LIS), until topographically-controlled standstill at the edge of the Appalachian front, south of the St-Lawrence Valley. This setting led to the development of two large glaciolacustrine water bodies in northward-flowing Chaudière and St-Francois valleys, resulting in the deposition of locally thick glaciolacustrine and deltaic sediments of Glacial Lake Gayhurst. The stratigraphic architecture of the Gayhurst episode is, however, complex, and its subsurface extent discontinuously documented. Here we define the physical lateral extension of the Gayhurst Formation based on the study of stratigraphic sections as well as surficial and borehole data acquired during an extensive groundwater and 3D Quaternary mapping project. Our results indicate that Gayhurst sedimentary sequences are characterized by outwash fans, deltaic, and deepwater sediments. Our data also provide elevation estimates of some possible outlets that controlled the lake evolution. The low-level stage of the lake was at an elevation of about 370 ± 5 m a.s.l. The outlets associated with this event are the valleys of Famine and Daaquam rivers and/or through Island Pound (Vermont/New Hampshire). Thinly bedded varves record the initiation of the LIS readvance into the Appalachians highlands, which subsequently covered the low level outlets and caused the lake level to rise at about 430 m. The depositional age for the fine-grained sediments are assessed using a new approach in IRSL dating, that analyses unflocculated mud. Luminescence chronology constrains the time frame for this interstadial event to between ca. 60 to 35 ka. This study thus proposes meltwater routing and regional glacier ice distribution and volume scenarios in the Northeastern sector of the LIS, mainly in and around Québec, New-Brunswick, Maine, and Vermont, from the dawn of OIS4 to the climax of OIS3.