Ta. Thompson et Sj. Baedke, STRAND-PLAIN EVIDENCE FOR LATE HOLOCENE LAKE-LEVEL VARIATIONS IN LAKE-MICHIGAN, Geological Society of America bulletin, 109(6), 1997, pp. 666-682
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.