EOLIAN PROCESSES AND CHANGES IN COASTAL DUNE MORPHOLOGY DURING COLD AND WET WINTER SEASONS ALONG THE SOUTHERN LAKE MICHIGAN

Three years of monitoring the inland migration of Mt. Baldy dune at the southern coast of Lake Michigan reveal several times higher (up to 5.9 m/year) rates than other Lake Michigan coastal dunes. About 70% of annual dune migration occurs during autumn (Sep. - Nov.) and spring (March - May). During cold and wet winters (Dec. - Feb.) freezing rain, snow, and lake ice inhibit and modify sand transport from the stoss to the lee slope of this dune. The stoss (blowout) slope is eroded and devoid of loose sand, exposing landward dipping strata of the former dune lee slope. Northerly winds blowing over the frozen beach and dune surface release gravel and granule size particles by sublimation. The gravel and granules then erode the blowout and dislodge finer sand from the frozen dune surface. Granule ripples form near the dune crest and persist at the dune surface well into late spring. Strong winds keep the blowout free of snow and near the dune crest they create low angle translatent strata made of sand and snow. Wind erosion of these strata creates a series of temporary yardangs that reach up to 1.5 m in height. The upper slip face of Mt. Baldy dune receives most of the winter sedimentation and, in respect to its autumn position, shifts inland up to 4 m, rises up to 1.5 m, and achieves >60 degrees slope, which in places has vertical or overhanging cliffs. Additional loading of the upper slip face by sand and snow causes development of fractures along the dune crest and sliding of frozen sand/snow blocks down the slip face. By the end of the winter the toe of the slip face receives a minor amount of sand and remains at its autumn position. During spring reversing, southerly winds rework some of the sand from the upper slip face into small secondary dunes that develop along the Mt. Baldy crest. Thawing of buried snow and frozen sand layers from the upper slip face continues until late spring, when the toe of the slip face eventually receives all of the winter sand accumulation in addition to its spring accumulation. Preservation of granule layers within unconsolidated eolian dunes or ancient eolianites thus can be used to infer cold winter climates and deposits over the frozen dune surface. Distorted and broken pinstripe laminations found in eolian strata may represent frozen sand blocks sliding, or melting of entrapped snow patches along the dune slip face.