Mapping the Phases of Glacial Lake Algonquin in the upper Great Lakes region, Canada and USA, using a Geostatistical Isostatic Rebound Model

Scott A. Drzyzga, Ashton M. Shortridge and Randall J. Schaetzl

This study reviews Glacial Lake Algonquin, examines the Main and two ‘‘Upper Group’’ phases in northern Michigan and nearby Ontario, reports their spatial extents, and reassesses the lake history in light of isostatic rebound. Our paper presents the most accurate and detailed maps of Glacial Lake Algonquin in this region that have yet been published. Fieldwork was conducted at 243 ancient shoreline sites, which yielded position data that support geostatistical models that represent differentially upwarped water planes. Model parameters that describe water plane tilt are reported for the Main, Ardtrea and Upper Orillia phases. Geostatistical water plane models were used to adjust a digital contemporary elevation model, thereby creating a digital proglacial elevation model for each phase. Maps of these phases and the data that support them suggest (1) proto-Cockburn Island, Ontario existed as an islet in the lake that was deglaciated before the outlet at North Bay, Ontario was opened, (2) the Main and Ardtrea phases of the lake extended into the northern Lake Michigan basin, and (3) the Main and Ardtrea water planes intersect at places near Little Traverse Bay (by Lake Michigan) and Thunder Bay (by Lake Huron). Mapped isobases generally conform to those published in other works and suggest the regional pattern of isostatic adjustment has not changed substantially during the last 13,000 years.