ABSTRACT: Lithological nature and origin of pebbles from Hamilton Cove, St Romauld, Quebec. [Nature lithologique et origine des cailloux de l'anse Hamilton, Saint-Romauld, Québec.]

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Abstract

Lithology and nature of boulders and pebbles in Anse Hamilton, Saint-Romuald, Quebec. The unconsolidatedsubstrate of the Anse Hamilton, at Saint-Romuald, on the south shore of the St. Lawrence Estuary in front of Quebec City, is a mixture of silt, sand and coarse debris similar to a diamicton or a till. There is a great variety in the size, shape and roundness of clasts as well as their lithological composition. The tidal zone, approximately 300 m wide, is composed of two units: a tidal marsh dominated by Scirpus americanus in the upper half, and a bare sandy tidal flat in the lower half. The lithology of clasts was determined for boulders at the surface of the tidal flat, for the coarse debris of the diamicton, and for the pebbles of a small beach at the base of a micro-cliff cut into the tidal marsh. Of the 2725 boulders examined, 49,9% were Precambrian, 47,6% sandstone and 1,3% limestone. Of the2976 clasts from the silty and sandy diamicton, the following proportions were obtained: 16,9% Precambrian, 36,1% sandstone, 25,5% limestone, 12,4% shale, 7,7% quartz, and 1,3% quartzite. Based on 1201 pebbles, the lithological nature of the clasts from a pocket beach was 25,8% Precambrian, 26% sandstone, 37,4% limestone, 7,5% shale and 2,4% quartz. The coarse debris from the tidal flat at Saint-Romuald originated from three major geological units occurring in the Quebec City area: the Laurentidian Shield about 20 km to the north, the St. Lawrence Lowlands about 30 km to the southwest, and the Appalachians in the vicinity of the studied site. Most coarse debris was transported to the cove by ice rafting during the Holocene, particularly during the last millenium. The silty-sandy and gravelly deposit of the Anse Hamilton is thus a recent drift-ice diamicton rather than a reworked till.

Bibliography of Canadian Geomorphology