Surficial Geology of the Gulf of St Lawrence

The Gulf of St. Lawrence is an inland sea of triangular shape, which occupies an area of approximately 250, 000 km 2. It has an irregular submarine topography composed of long trough-shaped valleys and shelves of varying widths and relief. Acoustical and sampling data indicate that the submarine topography is controlled by the lithology and structure of the underlying bedrock and an uneven cover of glacial and postglacial sediments . Glacial deposits form the oldest unconsolidated sediment units and are composed of tills and glaciomarine pelites as well as some glaciolacustrine clays. These units are relatively thin except along the southern edge of the Laurentian Trough where tills and glaciomarine pelites form a thick wedge composed of coalescing fans. Postglacial marine sediments (pelites , sands, and gravels) cover the glacial deposits in most parts of the gulf: the pelites occupy the floors of the major troughs and shelf valleys whereas sediments of the coarser grain sizes (sands and gravels) form the cover on the trough slopes and the adjacent shelves. Mineralogical and chemical data indicate that the sediments have been derived from underlying sedimentary rocks (calcareous and non- calcareous) as well as from crystalline rocks of the Canadian Shield to the north and west and that Wisconsin glaciations have been the most important single factor in controlling the dispersal pattern of terrigenous material in the gulf. The geological, morphological, and sedimentological data indicate that the main stages i n the development of the present morphology and sediments in the gulf are : (1) the establishment of a preglacial valley system, ( 2) the modification of the landscape by repeated glaciations during the Pleistocene Epoch, ( 3 ) the lateglacial ( late- Wisconsin) ice readvances and retreats, and (4) the modification of the relict glacial morphology and sediments by postglacial changes in sea level and by present depositional conditions. At the present, specific, sedimentary environments of deposition, active redistribution, and non-deposition of terrigenous material occur in the gulf.