DEVELOPMENT AND GROWTH OF PARABOLIC DUNES ALONG THE SOUTHEASTERN SHORE OF LAKE MICHIGAN

Examples of all stages in the progression from trough blowouts to fully developed parabolic dunes are present along the southeastern coast of Lake Michigan. Topographic profiles along the central axes show that in early stages gentler slopes (6 - 21 degrees) at the bottom and top of windward faces are separated by steeper (21 - 33 degrees) central segments. As the parabolic form develops, the gentler slope at the top expands at the expense of the central steep segment. A depositional lobe with a slope < 5 degrees forms at the crest and varies in size depending on the net sand supply. Paleosols, representing portions of the earlier lee face, are sometimes exposed by erosion along the windward face. Amounts of forward migration (derived from paleosol positions) and average rates of migration (derived from radiocarbon ages) over the last 200 - 500 years vary from 30 to 240 m and 0.2 to 0.6 m/yr. A rate of lateral expansion of 0.5 m/yr calculated from the spacing of thin, annual, carbon-rich layers exposed along the flank of one parabola are only slightly less than the calculated rate of forward migration (0.6 m/yr). Reconstructions of the former positions of one parabolic dune indicates several episodes of stabilization and remobilization with the position and orientation of the central axis changing during this process. As they migrate inland the crests of parabolic dunes become higher while the arms grow longer and further apart. Mass balance calculations for one large parabolic dune shows that it contains 2 to 3 times more sand than the original dune ridge from which it formed. As it migrated inland this dune moved over relatively low back dunes and a sandy lake plain both of which represent significant sources of sand, while the beach represents another potential source.