First Subbottom Acoustic Reflector and Thickness of Recent Sediments in the Upper Estuary of the St. Lawrence River

Abstract

A hydroacoustic survey has outlined the extent of a shallow buried reflecting horizon in the upper estuary of the St. Lawrence River, between Quebec City and the Saguenay River. This surface is considered to represent the contact of postglacial estuarine alluvium with Pleistocene sediments. The depth of the reflector below the bottom changes from place to place, recording regional variations in the thicknesses of the estuarine deposits. Only at a few localities are these deposits more than 30 m thick. The reflector varies in characteristics throughout the estuary: It may represent Champlain Sea sediments where flat or gently undulating, and late Pleistocene glacial material, where rugged and contorted.