ABSTRACT: New glacial lake episodes during the deglaciation of the southern part of the Canadian Shield : the Taureau Glacial Lake of the Saint-Michel-des Saints area (Quebec).

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Abstract

Recent mapping of surficial deposits in the Saint-Michel-des-Saints region (Quebec) have permitted to identify glaciolacustrine sediments in several valleys of this sector of the southern part of the Canadian shield, a vast territory where the deglaciation pattern of the Laurentide Ice Sheet is poorly known. The glaciolacustrine deposits found in the Saint-Michel-des-Saints area (laminated deep water sediments, deltaic sands, and sandy beach ridges) are associated with the lowering levels of successive glacial lakes in the Saint-Maurice river watershed. These glacial lakes were trapped between the ice margin, which was retreating northward, and the highlands to the south, which form the limit with three others watersheds (Rouge river, l'Assomption river and Maskinongé river watersheds). According to Dyke et al. (2003), the deglaciation of those highlands started about 10 250 years 14C BP. Three lacustrine phases have been identified in the studied area. The first phase, which was the highest (500 m) but the less extensive, occupied the Saint-Zénon area. This glacial lake was drained southward via the Noire river, from an outlet in the highlands south of Saint- Zénon. The second phase (460 m) has occupied the Sauvage and Pin Rouge rivers valleys, north of Saint- Zénon. The drainage of this glacial lake was eastward, through a pass leading to the Mastigouche river. At the same time, farther west, the Matawin river depression was flooded by another body of water in contact with the Laurentide Ice Sheet, initiating the third glacial lake phase which has been the longest and most extensive in the studied area. We suggest to name that glacial lake episode "glacial lake Taureau". As the ice margin retreated, the glacial lake Taureau gradually expended northward. At its maximum extent, a distance of 20 km was separating the glacial lake's outlet (Anodin river; 400 m) to the south and the ice margin to the north. Deltas and beaches associated with the glacial Lake Taureau (phase 3) have been correlated by their altitude and two isobases are proposed at 410 m and 420 m. The distance between these isobases revealed an isostatic warping of 0.9 m/km toward the NNW (345º). A 1.3 m section of laminated glaciolacustrine sediments in the Kaiagamac lake area, identified as varves by their pollen content (85 couplets), combine with drill holes data in the vicinity, allowed an estimate of 140 years for the duration of glacial lake Taureau. From this estimate, we have evaluated that the ice margin was retreating northward at a rate of around 142 m/year. By 10 100 years 14C BP, the retreat of the Laurentide Ice Sheet north of the Matawin river (outlet at 350 m) and the isostatic rebound have put an end to glacial lake Taureau.

Bibliography of Canadian Geomorphology