POLLEN CONTENT OF SANGAMONIAN INTERGLACIA L DEPOSITS, ILE-AUX-COUDRES, MIDDLE ST-LAWRENCE-ESTUARY, QUEBEC

Prodelta sandy silt, delta sand, stratified sand and silt were excavat ed at low tide or at the base of the cliffs on the NW coast of lie Aux Coudres, middle St. Lawrence Estuary. A detailed pollen analysis of t hese units has revealed a Sangamonian vegetation sequence which is obs erved for the first time in the St. Lawrence Valley. The lower prodelt a unit was deposited during a Picea mariana and Pines banksiana region al boreal forest phase during which Abies is decreasing. This phase is followed by a delta episode, then by a regional mixed forest with Pic ea and Quercus (17%) related to an interglacial climatic optimum. The following vegetation sequence shows the transition between an intergla cial climatic optimum and a glacial episode. The mixed forest changes progressively to fir forest (Picea, Abies, Betula); boreal forest (Pic ea, Pines); then shrub tundra (Betula, Alnus crispa). Previously descr ibed overlying sands and varves are completing this cooling sequence. The Pleistocene units of lie Aux Coudres are a significant marker betw een the sequences of Lake Ontario, of the St. Lawrence fluvial valley and of the Gulf of St. Lawrence. The thin bed with a high percentage o f Qeurcus is most likely related to the Sangamonian climatic optimum ( end of isotopic substage 5e). It confirms the pre and early Sangamonia n age ascribed to the Pleistocene deposits extending up to 160 m below the water level of the St. Lawrence Estuary as recorded by high resol ution seismic data. The climatic cooling (end of 5e and 5d) which foll ows the optimum phase is characterized by a continuous vegetation chan ge similar to those observed in Nova Scotia.