GEOMORPHIC HISTORY OF LAKE MICHIGAN COASTAL DUNES AT P.J. HOFFMASTER STATE PARK, MICHIGAN

The geometry of coastal dunes at Hoffmaster State Park (11 km south of Muskegon) is among the most complicated along southeastern Lake Michigan. Young (<5 yrs) foredunes (relief <5 m) occur along the beach in front of a linear coastal dune ridge (up to 12 m high) with many blowouts. Three sets of large (up to 60 m high and 450 m long) parabolic dunes occur further inland. The set closest to the coast (Parabolic Set 3) partially overrides Parabolic Set 2, which partially overrides the set furthest inland (Parabolic Set 1). At their trailing edges Sets 2 and 3 are connected to fragments of linear dune ridges inland of the contemporary continuous ridge. At its leading edge Parabolic Set 1 partially buries a group of lower (relief of 10 -15 m) interconnected backdunes (Backdune Set 2) that overrides the still lower (relief <10m) Backdune Set 1. Samples were collected from approximately 1.3 m below the surface at 7 sites with a view to dating the sand deposited just prior to dune stabilization. Samples were sent to the Sheffield Centre for International Drylands Research for optical luminescence dating. Quartz was extracted from these samples and their palaeodose estimated using a single aliquot regeneration protocol. Despite a weak naturally acquired luminescence signal and replicates probably indicating some incomplete bleaching or post-depositional mixing of sediment, absolute ages were obtained for all 7 samples which are consistent with the overall site stratigraphy. Backdune Set 1 gives an age of 7.17 +/- 0.48 (1σ) Ka corresponding to the Nipissing I transgression. Backdune Set 2 gives an age of 4.32 +/- 0.28 Ka in the north and 2.25 +/- 0.25 Ka in the south. Parabolic Set 1 gives ages of 2.83 +/- 0.19 Ka (south) and 2.20 +/- 0.16 Ka (north) while Parabolic Set 2 gives ages of 2.79 +/- 0.26 Ka (south) and 1.9 +/1 0.13 Ka (north). These ages suggest that parabolic dunes may have stabilized in the south a few hundred years earlier than in the north. Parabolic Set 3 is currently active but a paleo-Inceptisol exposed in their interiors indicates an earlier period of stability. The large parabolic dunes appear to have developed from blowouts in coastal dune ridges. The presence of fragments of older dune ridges inland of the contemporary ridge suggests that there may have been a net progradation of the coast in this area since the Nipissing transgression.