SEDIMENTOLOGICAL AND GEOMORPHOLOGICAL INVESTIGATION OF VOIDS IN THE MT. BALDY DUNE OF THE INDIANA DUNES NATIONAL LAKESHORE

Mt. Baldy, in the eastern part of the Indiana Dunes National Lakeshore, is the largest active dune along the southern Lake Michigan shoreline and is the most popular attraction in the park. In July 2013, a 6-year-old boy was trapped 3 m or more below surface in a hole within the dune. Because neither the formation processes nor the extent and significance of the hazard represented by this hole were known, Mt. Baldy was closed to foot traffic and remains so today. Similar holes have been discovered since 2013. To understand the development of the voids, we undertook a study of the architecture of the dune using ground-penetrating radar (GPR) and solid-earth (GeoProbe) cores. Three ~250-m and 33 80-m transects oriented north-northwest to south-southeast, respectively, were established perpendicular to the shoreline and longitudinal to the direction of sediment transport of the dune. Elevation-corrected, multispectral GPR (50, 100, 250 MHz) was selectively collected along these transects. Additionally, 24 GeoProbe cores up to 24 m deep were collected across the Mt. Baldy landscape, particularly along some of the GPR transects. Five light-shielded core samples obtained at select intervals and locales in the subsurface provided optically stimulated luminescence dating; organic samples retrieved from several other cores yielded radiocarbon dates.

We used the data from the cores and GPR transects to define four major stratigraphic horizons. These include an upper and lower dune that were separated by a time-transgressive paleosol developed on the lower dune. The upper dune formed during the 20^th century and corresponds to present-day Mt. Baldy; the lower dune formed at 3-4 ka, after ancestral Lake Michigan fell from the peak mid-Holocene (Nipissing phase) water level. The dunes are underlain by basal back-barrier lacustrine and wetland deposits that accumulated during the rise to the peak Nipissing. This study shows that the voids that closed Mt. Baldy occur on the stoss side of Mt. Baldy, where trees that were rooted into the paleosol and buried during the landward migration of the dune in the early 20^th century are now being unearthed as the dune continues to migrate. At this locale, some of the trees decay in place, producing voids of widely varying size, depth, and orientation related to the positon of the buried tree trunks and limbs.