Martine Clet-Pellerin

Pleistocene palynostratigraphy in the St. Lawrence Valley and middle Estuary

The litho-biostratigraphic framework of the upper middle and upper Quaternary units of the St. Lawrence basin results from the interplay of ice advance/retreat, isostatic depression/rebound, fluvial processes and climatic changes. Each main climatic cycle is characterized by a sedimentary sequence representative of the associated changing depositional environments. The climatic significance of the units is assessed with reference to the pollen content, 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) dominated assemblage, a pine (P. banksiana or P. divaricata) dominated assemblage, and a spruce dominated assemblage (Picea mariana), (3) the forest tundra (subarctic zone), (4) the shrub tundra with subarctic plants, and (5) the herb tundra (arctic zone). Three marine invasions, older than Late Wisconsinan, record a progressive vegetal recolonization: a pre-Illinoian marine episode, the post-Illinoian marine invasion of Guettard Sea (6–5 transition), and the Cartier Sea invasion (substage 5b?). Three types of lake inundated the lowlands: (1) lake systems associated with interstadial or interglacial fluvial drainage, (2) deglacial lakes, with a gradual vegetal recolonization, (3) pre-glacial lakes, whose pollen indicates boreal forest replacement by forest tundra and herb tundra.