INTERPRETING PALEOHYDROGRAPHIC DATA RECONSTRUCTED FROM STRANDPLAINS OF BEACH RIDGES IN THE LAURENTIAN GREAT LAKES

The most detailed Laurentian upper Great Lakes paleohydrographs have been reconstructed by studying strandplains of beach ridges or lateral chronosequences of many well-preserved ancient shorelines that have filled embayments just prior to and following the peak Nipissing high water-level stand at 4,500 years ago. Systematic and detailed study of subsurface sedimentary contacts in individual beach ridges yield the most accurate past water-level elevation and optically stimulated luminescence dating of foreshore deposits within beach ridges yield the most accurate age model for the strandplain sequence. Measured elevation and modeled age data are combined to create relative paleohydrographs. They are called “relative” paleohydrographs because they are in reference to a specific strandplain site from which paleohydrographs were reconstructed. Also within these relative paleohydrographs are clues to past lake-level variations due to (1) glacial isostatic adjustment (GIA), observed as long-term trends in strandplain relative paleohydrographs and (2) climate, shown as the short-term oscillations about the long-term trend. An iterative process of adjusting each paleohydrograph to remove GIA helps to find oscillations common to many paleohydrographs and is used to reconstruct outlet paleohydrographs for each lake basin. This research investigates methods to improve age models and estimates of GIA, to substantiate the assignment of climate to relatively short-term oscillations in paleohydrographs, and to isolate the potential for past changes in outlet conveyance (erosion and sedimentation) that would be reflected in all paleohydrographs within common lake basins.