PALYNOLOGY AND PALEOENVIRONMENTS OF LOTBI NIERE-SAND AND DESCHAILLONS-VARVES (UPPER PLEISTOCENE) OF THE ST-LAWRENCE VALLEY

Lotbiniere Sand and Deschaillons Varves are indicators of the drainage pattern of the St. Lawrence watershed, which presumably occurred duri ng the Late Sangamonian (5c and 5c-5b transition?). The pollen content of the Lotbiniere Sand suggests there was a Picea mariana - Betula - Pious boreal forest in the valley. in the lowermost zone of the Descha illons Varves, the pollen is very abundant: the boreal forest was prog ressively flooded by Lake Deschaillons. Above this zone, the vegetatio n seems to evolve toward a Picea and Betula open forest. The pollen of the upper contorted varves was presumably reworked from older materia l or transported from remote sources. The following paleoenvironmental sequence is reconstructed in the St. Lawrence Valley: (i) lowering of the relative drainage level, as indicated by the erosional disconform ity in lower rythmites; this phase, of unknown duration, is ascribed t o an eustatic lowering related to a global climatic cooling; (ii) an a ggradation phase, as indicated by the Lotbiniere Sand, before and at t he beginning of a glacial damming in the middle estuary; (iii) progres sive flooding of the valley and its boreal forest as ice caps are pres ent on adjacent Laurentian and Appalachian highlands; (iv) expansion o f Lake Deschaillons, limited by the Glens Falls sill, New York. The re lative elevation of the lake was at least 56 m. Great Lakes and upper St. Lawrence River drainage system was probably deflected to the Hudso n River. According to the number of varves, the regional ice caps prec eded the general invasion by Laurentide Ice Sheet of approximatively 3 800 years.