PROGLACIAL LAKE GAYHURST: MIS3 DEGLACIATION OF THE APPALACHIAN MOUNTAINS

The Wisconsin glaciation of Southeastern Quebec is represented by the surface, Lenoxville Till, which is separated by nearshore and deep-water facies of the Gayhurst Formation from the lower Chaudiere Till. The southeastern sector of the Laurentide lce Sheet experienced an important episode of ice volume reduction during transition from marine isotope stage 4 to 3, which led to the development of a large glacial lake in the northward-draining Chaudiere and St-Francois River valleys (Quebec). Interpretations of the paleogeography and environment of deposition of the Gayhurst Formation are derived mainly from many stratigraphic sections and boreholes to bedrock that expose or penetrate nearshore and deltaic sediments that provide elevation estimates of possible outlets that controlled lake evolution. In Glacial Lake Gayhurst as elsewhere in Appalachians, glacio-lacustrine sediments deposited by retreating glaciers are generally thick and irregularly bedded in couplets of variable thickness. Where accommodation space existed, density underflow deposits of laminated silt and clay may exceed one hundred meters or more in thickness. In contrast, fine-grained sediments deposited in lake Gayhurst by the advancing Lennoxville glacier consist of draped, and evenly laminated sediments. In the case of well-bleached sediments of near shore or deltaic facies within Gayhurst Formation, IRSL can provide unique information about the depositional age and physical correlation of units with similar luminescence characteristics. In this study, we apply new IRSL dating approaches developed in the UQAM Lux laboratory. At the type section of the Gayhurst Formation, near Drolet, Quebec, a preliminary IRSL age of 45+/- 5ka was determined for the deltaic facies that is draped by evenly laminated silt and clay typically deposited as a distal facies of an advancing (in this case, Lennoxville) glacier. This suggests that the last major expansion of the southeastern sector of the Laurentide Ice Sheet began late in or at the end of MIS 3.