Paleophytogeography of the Scarborough formation: New data and implications

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