Pleistocene geology of the Beausejour area, Manitoba

Abstract

The Pleistocene sediments exposed along the Winnipeg River from Lake Winnipeg to Seven Sisters Falls can be divided into four sedimentary units. These are, from oldest to youngest, a glacial till, a glacio-lacustrine clay, a glacio-lacustrine mud, and a glacio-fluvial and glacio-lacustrine sandy silt. During Wisconsin time, ice advanced from the Keewatin ice centre situated northwest of the study area depositing a calcareous till. Most of the till was derived from the Paleozoic carbonates of Manitoba. As the ice retreated, Lake Agassiz III, a large glacial lake, inundated the area. Sedimentation in the lake resulted in the deposition of a clay unit consisting mainly of illite and montmorillonite derived from Cretaceous and Jurassic shales. Further recession of the ice opened an eastern drainage outlet and the lake level dropped subjecting the clay to subareal erosion. An ice advance from the northeast Patrician ice centre blocked eastern drainage and Lake Agassiz IV came into existence. As the lake level rose, a mud unit consisting essentially of dolomite grains, quartz grains and clay minerals derived from Paleozoic carbonates, Precambrian granites and Cretaceous shales was deposited. Recession of the ice sheet opened eastern drainage systems and the lake level dropped gradually. A sandy silt unit was deposited in shallow water of Lake Agassiz IV as a result of delta construction and offshore processes. The sandy silt is composed mainly of quartz and dolomite grains derived from Precambrian granites and Paleozoic carbonates. Disintegration of the ice sheet in the Ne1son River basin opened northern drainage. Lake Agassiz was drained to the present level of Lake Winnipeg, the sandy silt was subjected to erosion, and a soil profile developed.