Ice flow and glacial transport in Lower St. Lawrence, Quebec

Abstract

The Late Wisconsinan history of ice movements is reconstructed for the region of Lower St. Lawrence and western Gaspésie based on an inventory of glacial striations and an analysis of the glacial dispersal of Appalachian indicator rocks and Precambrian erratics. The invasion by laurentide ice fom across St. Lawrence Valley appears to have been the first major glacial event. During this phase, ice moved easterly and southeasterly over the study area. Restricted glaciation may have preceded this event in higher are as; local ice caps likely covered Gaspésie and Miramichi Highlands. The development of a marine ice stream in St. Lawrence Valley radically changed the ice flow pattern. Ice now moved down-valley towards Gulf of St. Lawrence: in a northeastern direction along the valley axis and towards north or north-northeast over the study area. Decreasing discharge in the ice stream due to drawdown and accelerated calving as a result of late glacial rise of sea level caused the progression of a calving bay up St. Lawrence Valley, passing the study area between 14 and 13 ka BP. During passage of the calving bay, ice movement over the study area shifted from directions towards north-northeast and north to northwest and west. During these events, an Appalachian ice divide developed, which stretchedfrom the highlands of central Gaspésie, through northwestern New Brunswick and northern Maine, into the Eastern Townships of southern Quebec. Fossiliferous tills, observed at several places, indicate that Laurentide as well as Appalachian ice readvanced at least once after passage of the calving bay. Laurentide ice covered the coastal zone of the Rivière-du-Loup area around 11.8 ka BP and in the Rimouski area, Appalachian ice readvanced in marine waters after 12.3 ka BP.

Late Wisconsinan deglaciation and glacial lake development in the Appalachians of southeastern Québec

“…Judging from the width of the terrain in which strong northeastward ice-flow reorientation has been observed, the shear zones bordering the ice stream were probably several kilometers wide. At 12100 years BP, a residual ice cap in the Appalachian uplands had become detached from the ice sheet and, as suggested by several radiocarbon dates from the region between Québec City and Rivière-du-Loup (see Table III in Parent and Occhietti, 1988; also TO-948: 12450 ± 160 years BP in Rappol, 1993), an arm of the Goldthwait Sea had invaded the estuary. The Saint-Jean-Port-Joli…”

“…These observations of crosscutting glacial striae were recorded in a region extending for about 100 km both upstream and downstream from Québec City and at elevations up to about 300 m. It is reasonable to assume that these late re-orientations of ice flow are coeval with those observed south of the St. Lawrence River. The late-glacial ice-flow patterns shown in Figure 10 are based on the known distribution and trend of these sets of glacial striae in the region extending from the Laurentian Highlands to the White Mountains (Chauvin et al, 1985;Lortie and Martineau, 1987;Parent, 1987;Lowell, 1985;Rappol, 1993;Lanoie, 1995). These patterns clearly define a major late-glacial ice stream, which we will simply call the St. Lawrence Ice Stream (SLIS).…”

Lithologie des cailloux de la baie de Mitis, rive sud de l’estuaire maritime du Saint-Laurent (Québec) : un exemple de transport glaciaire et glaciel complexe

“…Les anorthosites, par exemple, ne peuvent provenir d'ailleurs. Concernant les apports possibles provenant des monts Notre-Dame et des Chic-Chocs, les études faites sur la dispersion glaciaire en Gaspésie (Lebuis et David, 1977 ;Chauvin, 1984 ;David et Lebuis, 1985 ;Prichonnet et Desmarais, 1985 ;Chauvin et David, 1987 ;Charbonneau et David, 1993 ;Rappol, 1993 ;Prichonnet, 1995) indiquent clairement que les granites et les roches métamorphiques associées ont rarement atteint la rive nord de la Gaspésie et n'ont pas été déplacés vers le sud-ouest en direction de la baie de Mitis. D'après B. Hétu (comm.…”

“…Ils ont sans doute été relâchés ensuite par des icebergs issus de la plate-forme de glace reculant vers Québec (Thomas, 1977), mais aussi à partir de l'Inlandsis laurentidien dont le front se trouvait en bordure de la rive nord de l'estuaire (Chauvin et al, 1985). Les écoulements glaciaires connus (Lortie et Martineau, 1987 ;Rappol, 1993 ;Prichonnet, 1995 ;Dionne et Occhietti, 1996) concordent avec cette interprétation. Si c'est réellement le cas, le rôle des icebergs dans la sédimentation au début de la Mer de Goldthwait prend une importance capitale.…”

“…Si c'est réellement le cas, le rôle des icebergs dans la sédimentation au début de la Mer de Goldthwait prend une importance capitale. N'étant pas dans du till, qui en contient effectivement peu (Rappol, 1993 ;Prichonnet, 1995), mais plutôt dans des dépôts argileux, les cailloux pré-cambriens n'ont pu être mis en place directement par la glace laurentidienne même s'ils en sont issus. Il faut alors évoquer un relais par les icebergs.…”