CGRG Bibliography of Canadian Geomorphology
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Author : Bolduc, A.M.; and Ruest, C.
Date : 2003.
Title : Lithologic variability of tills on the Appalachian Piedmont, Québec.
Publication : Joint Annual Meeting of the Canadian Quaternary Association and the Canadian Geomorphology Research Group. Halifax, Nova Scotia, June 8-12, 2003.
Issue :
Page(s) :
Abstract
On the south shore of the St. Lawrence River, near Québec City, most of the landscape is covered by marine sediments of the Champlain Sea. The main rivers, Chaudière and Etchemin, are cut into bedrock and stratigraphic sections rarely show more than fluvial and/or marine sediments on bedrock. The Beaurivage River, a tributary of the Chaudière River, has cut through a sequence of tills, about 8 m thick, that clearly displays a colour difference (grey at the base and red above) at about 2.75 m down from the top. The colour difference is assumed to be due to a change in the lithologic composition of the till reflecting a major shift in ice flow direction. The area was affected by at least 4 main ice flow events, all four of them recognized from the striae record on both the south and the north shore of the St. Lawrence. The older flows towards the southwest and southeast are the inception and regional flow from the Laurentide Ice Sheet, while the northeast flow represents the St. Lawrence Ice Stream (SLIS) and the latest northward flow is a re-adjustment of the ice sheet after the SLIS ceased to operate. The hypothesis is that at least 2 of these ice flows can be identified on the basis of the lithologic composition of the till at the Beaurivage River section. The results show that the grey till is dominated by crystalline and calcareous debris. The regional ice flow from the Laurentide Ice Sheet is clearly registered in this part of the section, bringing material from the Laurentians (crystalline) and the St. Lawrence Lowlands (limestone). The red till shows a small but significant increase in the proportion of red mudstones. Red mudstones and shales are locally derived (Nappe de la Chaudière). Another source, the Bécancour Formation, can only be brought in by a northeastern flow. Until ice flow shifted, the contribution from the harder crystalline and calcareous material outweighed the percentage of fragile red mudstones. When ice flow shifted to the northeast, during the St. Lawrence Ice Stream episode, the contribution to the site from the harder debris was shut off. The soft red mudstones, on the other hand were readily integrated in the ice. The similar increase noted for the grey mudstones / siltstones and sandstones supports this interpretation. Some volcanic lithologies were identified in the uppermost portion of the section, and those could only have been brought in by a northeast flow. The northward flow could not be identified in the section, perhaps due to the short duration of that event or because there are no diagnostic lithologic markers in the vicinity of this site.
Bibliography of Canadian Geomorphology