THE IMPACT OF GLACIAL LAKE MADAWASKA ON THE OPENING OF APPALACHIAN ICE COMPLEX

The Bas-Saint-Laurent, an area between La Pocatière and Matane along the St. Lawrence estuary in Québec, is made up of plateau landscapes. The clear contrast between the coastal plain to the north and the Appalachian Highlands to the south is divided by a steep piedmont. These landscapes generally follow the regional southwest–northeast Appalachian bedrock structure with the exception of the Témiscouata and Matapédia valleys. The BSL was occupied by the Laurentide Ice Sheet (LIS) during the Wisconsinan glaciation while the eastern Gaspé peninsula was occupied by regional Gaspé Ice Cap (GIC) belonging to Appalachian Ice Complex (AIC). After the Last Glacial Maximum (LGM), the LIS margin retreated toward the north and the St. Lawrence valley opened, isolating the AIC from the LIS in the BSL that form moraines at the piedmont of the Appalachians. These moraines are the only ones associated with the northern margin of the AGC in the BSL, with Saint-Antonin and St-Jean-Port-Joli moraines to the west and the Luceville and Neigette moraines to the east. This geomorphological continuum is interrupted by the Trois-Pistoles delta that opens onto the Témiscouata–Madawaska Valley, which was occupied then by the glacial Lake Madawaska.

Glacial Lake Madawaska contributed to the early opening of this valley and to the rapid separation of the AGC into two local caps—the Eastern Appalachian Ice Cap (EAIC) and the Western Appalachian Ice cap (WAIC). The valleys perpendicular to the Appalachian structure aided in the initiation and acceleration of deglaciation processes. Two associated complexes summarize the Quaternary paleogeography. First, the Appalachian piedmont moraine formed by the AIC, which includes landforms of the EAIC and WAIC. Then, the Trois-Pistoles delta, which is associated with the glacial Lake Madawaska varve complex.