Paléophytogéographie de la formation de Scarborough : nouvelles données et implications

Abstract

Palynological study of the upper part of the Don Formation, in the Toronto area, confirms the progressive change from a mixed forest with numerous thermophilous trees (Quercus, Ulmus, Carya) to a cold balsam fir forest (Abies balsamea, Picea mariana, Pinus banksiana) towards the end of the Sangamonian climatic optimum. The Scarborough Formation pollen content indicates a forest-tundra or treed tundra environment similar to modern subarctic conditions. Macrofossil content also points to harsh climatic conditions. Mean annual temperature could then have been 10°C lower than the present in the Toronto area. Our data refute the reconstruction of a climatic cycle (cold-warmer-cold) within the Scarborough Formation. To explain the simultaneous presence of floristic elements typical of presently contrasted climatic regions (cold boreal, warm temperate), we propose that Lake Scarborough acted as a climatic buffer for riverine plants: thermophilous species survived in an englaciation context in remote regions. The upper part of the Don Formation corresponds with the end of the Sangamonian climatic optimum (end of isotopic substage 5e). After a sedimentary gap of unknown duration, the Scarborough Formation is apparently correlative to the Lévrard Till in the Middle St. Lawrence River Valley (isotopic stage 5b).