Pleistocene palynostratigraphy in the St. Lawrence Valley and middle Estuary

The litho-biostratigraphic framework of the upper middle and upper Quaterna ry units of the St. Lawrence basin results from the interplay of ice advanc e/retreat, isostatic depression/rebound, fluvial processes and climatic cha nges. Each main climatic cycle is characterized by a sedimentary sequence r epresentative of the associated changing depositional environments. The cli matic significance of the units is assessed with reference to the pollen co ntent, and the evolving trend of the inferred vegetation. Five assemblages are defined: (1) the mixed woods assemblage related to temperate deciduous forests, (2) the boreal forest subdivided into a balsam fir (A. balsamea) d ominated assemblage, a pine (P. banksiana or P. divaricata) dominated assem blage, and a spruce dominated assemblage (Picea mariana), (3) the forest tu ndra (subarctic zone), (3) the shrub tundra with subarctic plants, and (5) the herb tundra (arctic zone). Three marine invasions, older than Late Wisc onsinan, record a progressive vegetal recolonization: a pre-Illinoian marin e episode, the post-Illinoian marine invasion of Guettard Sea (6-5 transiti on), and the Cartier Sea invasion (substage 5b?). Three types of lake inund ated the lowlands: (1) lake systems associated with interstadial or intergl acial fluvial drainage, (2) deglacial lakes, with a gradual vegetal recolon ization. (3) pre-glacial lakes, whose pollen indicates boreal forest replac ement by forest tundra and herb tundra. (C) 2000 Elsevier Science Ltd and I NQUA. All rights reserved.