Description

Unconsolidated surficial deposits (Maps - GeoServ) consist exclusively of glacial and related materials from the Late Wisconsinan glaciation, and sediments reworked by subsequent geomorphic processes. All surficial materials predating the Wisconsinan glaciation were eroded by continental glaciers during the last glaciation. The following description of Quaternary deposits is presented in stratigraphic order, from oldest to youngest. All the deposits are present in the study area but each of them is not necessarily present at one location.

Till: Glacial till unconformably overlies the bedrock. Till forms a persistent and generally thin cover, especially over the hummocky Precambrian rocks, but can be thicker ( 1 to >20 m) in depressions, in valleys, and up-ice side of bedrock ridges. Over the Canadian Shield, till is mainly stony and sandy with a small percentage of clay (< 7%). Clay content increases towards the south of the region, which is probably related to the erosion, by the glacier, of the softer, carbonate-rich Paleozoic rocks. Till structure varies from relatively loose near the surface, to very compact with depth. This is related to the origin of the till: the more compact and fine matrix till was deposited as lodgment till under the glacier, whereas the less compact and coarser till was deposited by melting of stagnant ice. The finer particles of the upper till may have been washed away by meltwater from the glacier or reworked by lacustrine and marine wave action. Standard penetration resistance varies considerably throughout the region and within one site, and is likely to be more controlled by the presence of gravel and boulders than the compactness of the till.

Glaciofluvial sediments: Glaciofluvial deposits consist of coarse to medium grained sand and gravel, poorly to well sorted and bedded, with numerous cobbles, boulders, and lenses of till. They were deposited in ice contact or near-ice positions by glacial meltwater during the last phase of glaciation, when glaciers were stagnant or retreating. North of the Ottawa River, these deposits lie at the bottom and sides of valleys, occurring as eskers, kame terraces, and subaqueous outwash trains and fans deposited at the retreating ice margin in valleys and in low-lying areas in the rugged Precambrian terrain. Numerous outwash fans occur at the edge of the Ottawa Valley. These sediments were deposited at the edge of stagnant ice occupying the lowlands south and east of the Precambrian highlands. South of the Ottawa River, glaciofluvial sediments were deposited at the margin of the retreating ice lobes, as interlobate deposits, or as englacial or subglacial deposits. Most glaciofluvial deposits in the lowlands are buried by marine deposits or have been reworked by wave action of the Champlain Sea.

Glaciolacustrine sediments: As glaciers were retreating, fine grained sediments, derived from the nearby glacial source, were deposited in quiet freshwater. The extent and duration of the freshwater body is not known precisely, but the relatively small amount of lacustrine fine grained sediments deposited suggests that the lake was short-lived and was soon replaced by a marine invasion (Champlain Sea).

Marine offshore sediments: Marine sediments, composed of red-and-grey banded and massive grey clays and clayey silts, are present throughout the lowlands, but are most abundant in bedrock depressions along the Ottawa River valley, with thicknesses exceeding 100m. Unlike typical clays composed of clay minerals, these sediments consist of finely ground residue of rocks (rock flour) over which the glaciers have passed. This characteristic, combined with open structure typical of rapid sedimentation and flocculation in saline or brackish water, accounts for the potential instability of these sensitive marine clays.

Marine nearshore sediments: Nearshore sediments are found near marine limit, on topographic highs, and at lower levels as the sea was retreating. They result from the reworking of existing sediments; these include beaches, deltaic, and estuarine sediments. Beach sediments reflect the nature of underlying materials that have been reworked; gravel, sand, and boulders were eroded from till and glaciofluvial deposits; slabs and shingles result from erosion of sedimentary bedrock. Sheets and bars of fine to medium grained sand are associated with reworked glaciofluvial deposits. Deltaic and estuarine deposits consist of medium to fine grained sand plains that were deposited at the mouth of rivers entering the sea and generally overlie fine grained marine sediments. These deposits are regionally extensive as sedimentation occurred at an advancing delta front as water levels fell and the sea retreated from the valleys. These deposits are generally well sorted and commonly fossiliferous.

Post-Champlain Sea deposits: As the Champlain Sea retreated, marine and glacial deposits were eroded and reworked by freshwater streams and lakes. Older deposits consist of medium grained stratified sand with some silt in the form of fluvial terraces and bars and spits within abandoned channels cut into marine clays. Younger deposits correspond to silty sand, silt, sand and clay on present floodplains and alluvial fans in area of low relief. These deposits are stratified and include minor gravel, disseminated organic matter, marl, and peat bogs within the abandoned channels.

Sources: Fulton and Richard, 1987, Gadd, 1987, Richard, 1990, 1984a, b, c, 1982a, b, c, d, St-Onge 1987.