ABSTRACT: Has the Northern Gaspe Peninsula ever been invaded by an ice Sheet from Québec-Labrador?

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

Glacial models proposed for Gaspé Peninsula have evolved from simple to complex, over the last century, on the origin and extent of the glacial ice cover. Most research has beenconcentrated on the high plateaux and summits of northern Gaspé, where the absence of erratics and glacial striae and the presence of felsenmeer, have fed a long standing controversyconcerning the origin, thickness and basal dynamics of the local ice sheets. In the most recent syntheses on the question, David and Lebuis (1985), Chauvin and David (1987) andCharbonneau and David (1993) indicate the co-existence during the Last Glacial Maximum of local ice caps in the central and eastern parts of the peninsula and the Laurentide ice sheet in the western part and in the Baie-des-Chaleurs basin, but favour the extension of the continental ice sheet across the entire peninsula during an earlier glacial stage. This view is based on a long southeastward trending erratic train of erratics from a granitic intrusion in northern Gaspé and on extremely rare occurrences of precambrian erratics in eastern Gaspe (McGerrigle, 1952). As a counterbalance to this research in the interior of the peninsula, the evidence presented in this talkis derived from a sector along the north coast (Figure 1). This region is situated only 150 km from the edge of the Canadian Shield on the opposite shore of the Saint-Lawrence Estuary, and is characterised by sedimentary and volcanic lithologies, allowing easy recognition of Canadian Shield erratics, composed of granite gneiss and anorthosites. In this region a thin ice cover of local origin for this region is suggested by both erosional and depositional evidence. The trunk valleys descending to the north from the interior plateau display U-shaped cross sections in their lower reaches, and several possess low level cols, indicative of watershed breaching by ice flowing northward from the high plateaux (Hétu and Gray, 1985). However, glacial erosion appears to have been minimal on the coastal plateau at 300 - 400 m elevation - the back-walls of glacial cirques which incise the coastal plateau havenot been breached by glacial erosion, and other large and medium scale forms (flyggbergs, roches moutonnées and glacial grooves are totally missing from the plateau surface. Glacial striae, observed only rarely in the glacial troughs, and on the plateau surface, because of the friable substrate, demonstrate only one ice flow direction - from the interior toward the Saint Lawrence estuary. On a more regional scale, mapping of the numbers and surface area of lakesas indicators of contrasts in areal scouring by ice sheets reveal a much greater concentration in western Gaspé and on the high summit plateaux , than in the coastal plateau region of northern and eastern Gaspé. Glacial till cover is sporadic and thin on the coastal plateau, up to some tens of centimeters where present. In the coastal valleys, glacial deposits are also quite thin, except in locally developed lateral moraines. Eskers, as meltwater forms requiring a substantial ice thickness and abundant flows of subglacial meltwater for their formation, are missing. Stratigraphic exposures in glacial and proglacial deposits above the postglacial marine limit reveal only clasts of Gaspésie lithologies. Also there are no glaciolacustrine sediments which could have suggested progressive retreat of a Laurentide ice margin towards the north. The basal units, in exposures below the marine limits, are, on the contrary, fossiliferous marine clays of the earliest phase of the Goldthwait Sea, overlapped by diamictons and northward dipping proglacial deltaic beds as a result of Late Glacial recurrences of valley glaciers (Hétu and Gray, 2000).With regard to the question of glacial erratics, Canadian Shield derived lithologies have been found solely below the postglacial marine limit - usually in linear accumulations, withinLate Glacial and early postglacial marine facies. Their presence is explained by the transport of icebergs across the St Lawrence Estuary from the calving front of the Laurentide Ice Sheet during the deglaciation phase, and not by down-wasting of this ice sheet on the Gaspe Peninsula. There are, however, numerous erratics with a source in the igneous intrusive massifs on the high plateaux (Veillette and Cloutier, 1993). Their distribution supports the concept of flows northward from dispersion centers in the highlands of the Peninsula (Chauvin and David, 1987). Their rare occurrence on the coastal plateau indicates a thin ice cap centred further south on the higher level Chic-Choc and McGerrigle Mountain plateaux, which was canalised rapidly into outlet glaciers, which during the course of many glaciations, sculpted the coastal valleys, before merging with a tongue of the Laurentide ice sheet moving eastward down the St Lawrence estuary. Glacio-isostasy is an other element in favor of the existence, at least during the lastglacial episode, of a relatively thin ice cap in the northern Gaspé Peninsula. According to the morphological evidence from the highest proglacial delta in the coastal valleys, the maximum level attained by the Early Goldthwait Sea was ca 55 m at the moment the coast became deglaciated, towards 13,500 BP. In the Sept Iles sector on the north shore of the St Lawrence, this limit is more than 130 m. Westernmost Gaspé Peninsula, where the passage of the Laurentide Ice Sheet during the Late Wisconsin has left its traces on the landscape, also recorded higher total postglacial uplift (112 m). However the postglacial marine limit levels off in eastern Gaspé, and remains isometric on the north and south coasts of the peninsula. This suggests symmetrical glacio-isostatic loading of all of eastern Gaspé under a local ice dispersal centre, with more or less equal rebound around the entire coastline. The enigma of the rare occurrences of erratics from the eastern Gaspe Peninsula, interpreted as precambrian remains to be resolved, however. Since gneissic phases are occasionally present in the local igneous massifs of Lower Paleozoic age (McGerrigle, 1952), they cannot be considered for the moment as reliable evidence for overriding of the eastern Gaspé Peninsula by Laurentide ice during the penultimate glacial phase - the early Wisconsinan phase, as suggested by David and Lebuis (1985). If such erratics can be relocated, their Canadian Shield origin will have to be confirmed, on the basis of thin sections or radiometric ages.

Bibliography of Canadian Geomorphology