THE FORMATION OF DUNE DECOMPOSITION CHIMNEYS IN A MIGRATING COASTAL DUNE, INDIANA DUNES NATIONAL LAKESHORE

Dune decomposition chimneys are collapse features that form in migrating dunes, when portions of buried trees progressively decompose. These features were only recently discovered and researched at the Mount Baldy Dune, a rapidly migrating parabolic coastal dune in the Indiana Dunes National Lakeshore. Early research indicates that saprotrophic wood decay fungi (genus Lepiota) continue to actively decompose oak (Quercus spp.) trees after burial and facilitate the biomineralization of a calcium carbonate-rich cement at the contact between the organic material and surrounding siliciclastic sands. The carbonate-rich cement offers temporary stability allowing for open voids to persist for hours to days after the organic debris collapses. Geophysical analysis of Mount Baldy was conducted to determine the relation between the distribution of observed decomposition chimneys and an Inceptisol, now exposed on the stoss slope due to ongoing dune migration. Analyses included 11 km of multi-spectral ground penetrating radar (GPR) collected along transects oriented longitudinal to the direction of dune movement. Numerous hyperbolas are visible in the GPR reflections from the foreset beds and are interpreted as reflections off buried tree limbs and trunks. The collapse features are occurring at the modern surface in an area directly above a roughly east-west oriented limb of a relict, low-relief parabolic dune. Sands overlying the paleosol associated with the buried dune, and encasing modern trees rooted in that surface, are generally less than 5-6 m in thickness. Aerial photographs constrain the timing of dune migration and indicate that the entire process of burial, cementation, organic decomposition, and collapse occurred within 70-100 years at Mount Baldy. While tree casts and rhizocretions may be considered common in calcareous eolianites, the sands of Mount Baldy and coastal dunes of Indiana are dominated by quartz (~70%) that have lesser amounts of dolomite (<15%) and K-feldspar (~10%) as estimated by SEM analysis. Ongoing research is evaluating evidence that infiltrating carbonate-rich precipitation is contributing to the development of decomposition chimneys in dunes dominated by siliciclastic sediments.