Glacial and environmental geology of northeastern Manitoba

Abstract

Quaternary deposits in northeastern Manitoba are related to pre-Wisconsin and Wisconsin glaciations, the Sangamon Interglaciation ,and postglaciallakes and seas. During the Wisconsin Glaciation, the map area was continually glaciated by Laurentide ice from two centres of outflow. Keewatin ice flowed southward and deposited a sandy to bouldery granitic till sheet, the surface of which is divided into belts of rib moraine and crevasse fillings. A second ice mass, centred over Hudson Bay, flowed westwards and deposited several silty calcareous till sheets, the uppermost of which is gently ridged by grounding-line moraines. The position of convergence of the two ice masses varied during the course of glaciation. A major segmented interlobate kame moraine, which was deposited largely during de glaciation, marks the final confluence. Lake Agassiz covered the area vacated by the receding glaciers and persisted as long as ice sheets blocked drainage into Hudson Bay. Turbidite sands and varved or massive clays deposited into this water body cover the southern part of the region. Hudson ice disintegrated suddenly about 7800 (or 8000) BP, and the glacioisostatic Tyrrell Sea flooded areas below 180 m elevation; the land continues to emerge at a rate of 40 cm per century. A major raised beach, formed about 7200 BP, constitutes a major aggregate deposit. Icy muskeg forms the terrain south of Seal River. Frost-riving of bedrock and aggradation or degradation of ground ice are current periglacial processes. Areas east of the Great Beach lie within the zone of continuous permafrost; those to the west lie in discontinuous permafrost. Because ground temperatures are near 0 C, the region is extremely sensitive to climatic or man-induced changes in thermal regime: a small change in ground temperature can produce major terrain disturbances. Permafrost has also limited the buffering capacity of the soils to counteract the effects of acid rain. The surface till sheet has anomalously high concentrations of uranium north of Seal River and interesting gold associations near Great Island.

A slope failure caused by drainage cutoff through the advancement of seasonal frost, Hudson Bay Lowland

“…Shallow flows and slides in ice-rich colluvium are common where forest fires have altered the insulative properties of the organic mat. Dredge and Nixon (1992) suggested that low ice contents at depth were responsible for the absence of massive permafrost-related slope failures along the rivers in this area.…”

A Survey of Lichens and Bryophytes in White Spruce, <em>Picea glauca</em>, Tree Islands on a Calcareous Beach Ridge in Northeastern Manitoba

“…The growing season ranges from 100 to 143 days, and prevailing winds are from the northwest. The area is underlain by continuous permafrost (Dredge and Nixon 1992). A more detailed description of the area is reported in Dredge (1992), Dredge andNixon (1992), andBrook (2001).…”

“…The region has a marine subarctic climate with a mean annual temperature of -7.3°C ranging from 12°C in July to -28°C in January. Precipitation averages 400 mm annually (rainfall equivalent) (Dredge and Nixon 1992). The growing season ranges from 100 to 143 days, and prevailing winds are from the northwest.…”

“…The highest beach ridge is 122 m above sea level located near Gillam and the lowest is the present day beach along the coast of Hudson Bay. Because of the deposition of seashells and silt from the marine water during regression of the Tyrell Sea, the majority of the sand and gravel on the Hudson Bay Lowlands is calcareous (Dredge 1992).…”

“…Krummholz islands of P. glauca trees are scattered on the subarctic tundra of the Hudson Bay Lowlands in Manitoba (Scott and Hansell 2002;Piercey-Normore 2005). The Hudson Bay Lowlands is a flat coastal plain that forms a broad transition be tween boreal forest and arctic tundra in Manitoba and Ontario, is a band of about 160 km wide along the southwest shore of Hudson Bay (Ahti 1964;Dredge 1992). During regression of the Tyrell Sea, 8000 years ago, beach ridges were formed along the coast of Hudson Bay due to isostatic rebound, tides, and wave action.…”