REVISED CHRONOLOGY OF ICE RETREAT AND CHAMPLAIN SEA INVASION IN SOUTHERN QUÉBEC, CANADA

AMS radiocarbon cross-dating of terrestrial plant debris and marine shells sampled in a lake basin on Mount St. Hilaire (Québec, Canada) provides a direct assessment of a reservoir effect of circa 1800 ¹⁴C years on shells of the early stage of Champlain Sea (1400 ¹⁴C years for marine reservoir corrected ages). From the age of 10,850 ± 40 ¹⁴C yr B.P. on lowermost terrestrial plant debris and pollen-based extrapolation of bottommost ages of this lake and of another lake nearby, the age for ice retreat near the top of Mount St.Hilaire is estimated at 11,250 ± 150 ¹⁴C yr B.P. Since Mount St. Hilaire was located 30-40 km north of the position of the ice front when Champlain Sea invaded the St. Lawrence Lowlands, and applying an estimated rate of ice retreat of 250 m/¹⁴C yr, the age of the invasion of Champlain Sea in central St. Lawrence Valley is estimated at 11,100 ± 100 ¹⁴C yr B.P., i.e. close to the end of the Allerød stage. This age also dates the end of proglacial Lake Candona, the ultimate glaciolacustrine phase resulting from the coalescence of Glacial Lakes Iroquois, Vermont and Memphremagog. These results indicate a 500 - 1000 years younger regional chronology of ice retreat, now congruent with the results of other authors, e.g. a palynological study of the lacustrine to marine transition correlated with radiocarbon-dated lake sediments in eastern Ontario, and the New England varve chronology. The age of 11,900 ¹⁴C yr B.P for the Littleton-Bethlehem Moraine in New Hampshire is acknowledged. Roughly south to north, the main halts of the retreating front of the Laurentide Ice Sheet are tentatively dated in southern Québec: 11,550 ± 150 ¹⁴C yr B.P. for the Frontier Moraine, 11,200 ± 150 ¹⁴C yr B.P for the Ulverton-Tingwick Moraine on the Appalachian piedmont, 10,800-10,500 ± 100 ¹⁴C yr B.P for the Saint-Narcisse Moraine, and circa 10,000 ¹⁴C yr B.P. for the Mars-Batiscan Moraine, 70 km further north on the Laurentian Uplands.