Stratigraphic and sedimentologic investigation of collapse features in the Mount Baldy Dune in the Indiana Dunes National Park

Abstract

Mount Baldy dune is one of the largest active eolian dunes along the southern shore of Lake Michigan and a famous tourist attraction within the Indiana Dunes National Park. It is an anthropogenically disturbed landform where restoration efforts include native beach-grass plantings and restricted foot traffic. In July 2013, a 6-year-old boy was buried more than 3 m below the dune stoss surface after falling into a hole, or “collapse feature.” While previous work indicated that holes formed through the decay of buried oak trees (Quercus spp.), neither the origin of this hole in relation to the dune’s stratigraphy nor its significance as a potential geological hazard were understood. Groundpenetrating radar, sediment cores, and radiocarbon and optically stimulated luminescence dates were collected to define sedimentary packages and facies, and interpret the dune’s depositional history. The data show that an alongshore migrating coastal dune field stabilized ∼3000 ya atop backbarrier lagoonal deposits. Modern Mount Baldy evolved as sands were mobilized and transported onshore over the relict dune field, triggered by ongoing downdrift coastal erosion related to the development of the Michigan City Harbor. The internal architecture of the dune reveals how the topography of the forested landscape, now buried beneath the modern dune form, contributes to variations in the thickness of the upper dune package, controlling the relation of buried and decayed trees to the surface of the stoss slope. The data on sand thicknesses, were used with maps and aerial photographs to create a geohazard map of the dune.