The relative influence of Laurentide and local ice sheets during the last glacial maximum in the eastern Chic-Chocs Range, northern Gaspé Peninsula, Quebec

Abstract

Geochemical signatures, erratic dispersal, and striae indicate glacial flows in the north-central Gaspé Peninsula exclusively from Appalachian sources, except for the coastal fringe. During the last glacial maximum (LGM), ice from the Monts McGerrigle flowed northward and northwestward over the summits of the eastern Chic-Chocs Range and the coastal plateau to the Gulf of St. Lawrence. Lesser flows also occurred to the north from Mont Albert and the western Chic-Chocs Range. Where intersecting striae were noted, the older flows are associated with a locally developed ice cap. The rare occurrences of striae and erratics, and morphometric and lithological characteristics of summit diamictons, imply limited basal erosion by both local and regional ice caps. This is postulated to result from early protection of rock surfaces by the initial buildup of thin, frozen-based ice in the eastern Chic-Chocs Range. A compact till mantle in the cols and on the coastal plateau and striae parallel to col directions indicate a downslope transition to wet-based ice. Occasional gneiss boulders above the marine limit between Cap-Chat and Sainte-Anne-des-Monts indicate a slight onlap of the Laurentide Ice Sheet (LIS), but the latter was excluded from the interior of the peninsula east of Cap-Chat. Whole-rock geochemistry from three granitoid erratics on Chic-Chocs summits and one in the York River basin indicates a local Devonian rather than a Precambrian Canadian Shield source. The absence of southward downwarping of synchronous postglacial marine limits across the Gaspé Peninsula corroborates this view for the LGM.