Origin and paleoecology of post-Lake Agassiz raised beaches in Manitoba

Abstract

Post-Lake Agassiz raised beaches have been identified 6 m above the present level of Lake Winnipegosis. The level, termed the Dawson level, is radiocarbon dated at 4870 ± 80 years BP (GSC-4138) and 4900 ± 70 years BP (GSC-4139) at Dawson Bay and at 5050 ± 100 years BP (BGS-1126) at Denbeigh Point. The dates are estimated to be 350 years too old due to the hard-water effect and the Dawson level is believed to have formed about 4550 years ago. The 6 m drop in the level of the northern part of Lake Winnipegosis is attributed to isostatic tilting. The fossil assemblages of the beaches are the same and are dominated by the extinct gastropod Marstonia gelida. Two marine invertebrates, the foraminifer Elphidium gunteri and the ostracod Cytheromorpha fuscata, are present in the samples. These marine animals are believed to have been brought into the area by migrating waterfowl and to have survived because of the high salinity of the lake.