Quaternary geology of the Ottawa region, Ontario and Quebec
Authors: Fulton R J 
Published: September 2013

The Ottawa region is a lowland area, underlain largely by flat-lying bedrock consisting of Paleozoic carbonates and shales, enclosed on the north, west, and south by uplands underlain by Precambrian igneous and metamorphic rocks. The contrasting sedimentary and
crystalline bedrock have produced till facies which can be readily characterized by carbonate content. In addition, occurrence of areas of Precambrian rocks which contain distinctive trace element components has produced chemically distinct trains of till. Till, which occurs as a thin and
discontinuous caver, is the main Quaternary sediment in the uplands. Quaternary sediments in the lowlands generally consist of fine grained marine sediments and fluvial and offlap marine sands. Eskers, kames, and outwash trains are common features in valleys of upland areas. In the lowlands
glaciofluvial deposits do not occur as distinct, easily traced features because they were buried by later sediments and were reworked by marine erosion during regression of the Champlain Sea. Most glaciofluvial deposits in the lowlands contain features which suggest that they were deposited largely
as subaqueous outwash. Glacial deposits and most marine sediments are Late Wisconsinan and fluvial deposits are mainly Holocene. During the single recognized Late Wisconsinan advance, ice moved in a general north to south direction but three major lobes, separated by thin belts of glaciofluvial
sediments, developed during deglaciation. Early during the retreat of ice from the lowlands, ice marginal lakes entered the area from the Lake Ontario and Lake Champlain basins. At about 12 ka (according to marine shell dates) marine waters of the Champlain Sea replaced the freshwater in the
depressed lowlands. Sediments deposited in the Champlain Sea consist of clays, silty clays, and sands which can in general be related to deposition in a shoaling basin. The deposits contain a variety of assemblages of macrofossils, foraminifers, and ostracodes which indicate that the Champlain Sea
was a salinity stratified water body. By at least 11.3 ka the basin was invaded by water with salinities as high as 34°/00 which formed a high salinity wedge. Later, during recession, salinities gradually decreased. Tundra-woodland vegetation was present around the basin at the time of
deglaciation, but a more closed forest developed soon after. Spruce was the first tree species to dominate the forest in southern parts of the area but poplar was the first to colonize areas immediately west and north of the Champlain Sea basin. In the south the spruce dominated forests were
replaced by pine dominated forests, whereas in the north the poplar dominated forest was invaded first by spruce and then by birch before pine became the dominant species. Many of the other species that make up the modern forest invaded the area shortly after 8 ka. A chronology based on pollen
stratigraphy suggests that the Champlain Sea invaded the area between about 11 and 11.5 ka. This is several hundred years later than the date of invasion indicated by marine shells. One reason suggested for this discrepancy is a dilution of the sea water carbon by old carbon derived from the melting
ice. Limit of marine submergence varies from about 125 m in upper St. Lawrence River valley ta 200 m near Ottawa and about 165 m near the western limit of the Champlain Sea. Uplift apparently occurred at a rate as great as 10 m per century immediately following deglaciation but insufficient data are
available to produce good uplift curves. By about 10 ka uplift had caused the Champlain Sea to drain from the area and a major fluvial system had developed in the approximate position of the modern Ottawa River.