ABSTRACT: Ice-dammed lake deposits of St Placide, Charlevoix, Quebec. [Les dépôts lacustres d'obturation de Saint-Placide, Charlevoix, Québec.]

CGRG Bibliography of Canadian Geomorphology
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Abstract

Ice-dammed Lake Deposits of St. Placide, Charlevoix, Quebec. One hundred and twenty metres of Wisconsinan sediments were deposited in the valley of the Bras du Nord-Quest, west of Baie-Saint-Paul (Charlevoix). Several stratigraphic exposures show thick sub-aquatic deposits overlain by till. The sub-aquatic sediments include fine sands topped by 60 m of rhythmites. These rhythmites contain dropstones, diamictic lenses and syngenetic involutions. All of these features are indicative of a proglacial environment. Despite their thickness, the rhythmites belong to one single palynozone: a shrub tundra dominated by Betula glandulosa, Alnus crispa and Gramineae. Due to the presence of a few fresh water diatoms and the high elevation (300 m) of the top of the rhythmites, it is suggested that the rhythmites were deposited in a lake rather than in a marine environment. Given its physiographic location in an open valley in connection with the St. Lawrence Estuary, the paleolake had to be an ice-dammed lake. A melt-out till about 20 m thick overlies the Ice-dammed Lake Deposits of St. Placide. Locally the till - a diamicton with some fine sandy layers - thickens into a moraine of up to 52 m. Glacial flow was from the north. The Ice-dammed Lake Deposits of St. Placide and the till are two members belonging to one single formation related to the Trois-Rivieres Stadial. The Ice-dammed Lake Deposits of St. Placide date from the beginning of that stadial whereas the melt-out till and the moraine may represent its end. In the studied section, the maximum phase of the Trois-Rivieres Stadial may possibly be correlated to either a sedimentary gap or to an erosional event at the top of the rhythmites.

Bibliography of Canadian Geomorphology