REVISED NORTH AMERICAN DEGLACIATION MAPS

An updated deglaciation map sequence for North America (cf. Dyke and Prest, 1987; Geol Surv Can Map 1703A) is needed to support new postglacial rebound modelling, which will use a finer spatial scale than previous efforts and a 500 year time step. A chronological database, mainly radiocarbon dates with varve and tephra dates, was assembled. The dates constrain ice margin positions and shorelines of large glacial lakes. Dates on problematic materials such as marl, freshwater shells, lake sediment with low organic carbon content, marine sediment, bulk samples with probable blended ages, and most deposit feeding molluscs from calcareous substrates are excluded. These culled samples, many of which were used in previous reconstructions of deglaciation, yield ages that are too old. The residual data set contains ca 4000 dates. Marine shell dates, a major component, are adjusted for regionally variable marine reservoir effect based on a large new set of radiocarbon ages on live-collected, pre-bomb molluscs from Pacific, Arctic, and Atlantic shores. These corrections range from 800 years in the Pacific Ocean and Champlain Sea to 450 years in the SW Gulf of St Lawrence, and hence are larger than the previous conventional correction, a uniform 400 years. Deglaciation ages are correspondingly diminished.

The net effect is that deglaciation is delayed in most places by 1000-2000 years with respect to the Dyke and Prest (1987) reconstructions. However, the spatial pattern of ice recession resembles earlier reconstructions, being guided by the patterns on the Glacial Map of Canada. Major changes of interpretation of LGM ice extent in the Canadian Arctic Archipelago and the Atlantic Provinces are incorporated.

The maps were reviewed by numerous regional experts and particularly valuable comments were provided by John Clague for the Cordillera, by Jim Teller and Harvey Thorleifson for the Lake Agassiz region, by Mike Lewis for the Great Lakes region. Woodrow Thompson provided a radiocarbon database for Maine and selected ice margin positions. The maps are available as a GSC Open File.