Sea-Floor Conditions Around the Magdalen Islands in the Southern Gulf of St. Lawrence

Abstract

Acoustical, underwater photographic, and bottom sediment sampling data indicated that the major geomorphological features of the shelf throughout the southern Gulf of St. Lawrence are not related to the present environmental conditions. Submarine troughs and tunnel valleys give ample morphological evidence of extensive Pleistocene glaciation. Bottom samples and acoustical records revealed that sandstone bedrock with an intermittent veneer of sediments occurs on the sea floor between Prince Edward Island and the Magdalen Islands, as well as to the north of the Magdalen Islands. In contrast, the shelf areas west and east of the Magdalen Islands have an almost continuous sediment cover with only small exposures of bedrock. These sediments are considered to be mainly the erosional products of the local bedrock. Areas of elevated bottom relief such as Bradelle Bank are bedrock elevations on top of which there is a nearly continuous but relatively thin sediment cover. A submarine terrace between approximately 28 and 34 fath, well developed around the Magdalen Islands, is considered to mark a former sea level.