STRAND-PLAIN EVIDENCE FOR LATE HOLOCENE LAKE-LEVEL VARIATIONS IN LAKE-MICHIGAN

Lake level is a primary control on shoreline behavior in Lake Michigan . The historical record from lake-level gauges is the most accurate so urce of information on past lake levels, but the short duration of the record does not permit the recognition of long-term patterns of lake- level change (longer than a decade or two). To extend the record of la ke-level change, the internal architecture and timing of development o f five strand plains of late Holocene beach ridges along the Lake Mich igan coastline were studied. Relative lake-level curves for each site were constructed by determining the elevation of foreshore (swash zone ) sediments in the beach ridges and by dating basal wetland sediments in the swales between ridges. These curves detect long-term (30+ yr) l ake-level variations and differential isostatic adjustments over the p ast 4700 yr at a greater resolution than achieved by other studies. Th e average timing of beach-ridge development for all sites is between 2 9 and 38 yr/ridge. This correspondence occurs in spite of the embaymen ts containing the strand plains being different in size, orientation, hydrographic regime, and available sediment type and caliber. If not c oincidental, all sites responded to a lake-level fluctuation of a litt le more than three decades in duration and a range of 0.5 to 0.6 m. Mo st pronounced in the relative lake-level curves is a fluctuation of 12 0-180 yr in duration. This approximate to 150 yr variation is defined by groups of four to six ridges that show a rise and fall in foreshore elevations of 0.5 to 1.5 m within the group. The 150 yr variation can be correlated between sites in the Lake Michigan basin. The approxima te to 30 and 150 yr fluctuations are superimposed on a long-term loss of water to the Lake Michigan basin and differential rates of isostati c adjustment.