INTERNAL ARCHITECTURE OF THE MT. BALDY DUNE AT THE INDIANA DUNES NATIONAL LAKESHORE

Mt. Baldy Dune is an active coastal parabolic dune in the Indiana Dunes National Lakeshore near Michigan City, Indiana. To understand the sediment distribution within and below the dune, 11 km of multi-spectral ground penetrating radar (GPR) and 19 sediment cores were collected along transects oriented longitudinal to the direction of dune movement. The data show that the landform consists of three facies: modern dune, relict dune, and back-barrier lacustrine.

The modern dune facies consists of southeast-dipping, 20- to 30-m-long parallel foresets (27- 31°) of fine- to medium-grained sand. Higher frequency GPR data shows that foreset reflections are interrupted by chaotic reflections and scattered hyperbolas. The modern dune facies is separated from the underlying relict dune facies by a prominent reflection on a paleosol (6-12 m relief) that intersects the stoss side of Mt. Baldy.

The relict dune facies is composed of fine- to medium-grained sand, but its GPR reflections are more variable. Some dip similar to the modern dune, but many are divergent and convergent, with lower dips and low-angle truncations. Beds also include landward dipping parallel-subparallel, landward-dipping convex, and subparallel upper-boundary concordant reflections.

The back-barrier lacustrine facies occurs at the base of the dune and consists of moderately continuous, parallel-and subparallel reflections that dip <3° landward. The uppermost reflection (<1° landward dip) becomes the modern ground surface landward of Mt. Baldy. Higher angle internal reflections downlap lower angle reflections and are truncated or toplap overlying reflections.

The back-barrier lacustrine facies is traceable lakeward to an uppermost elevation of ~183 m IGLD. This elevation is the top of a Nipissing phase depositional transgression sequence, consisting of washover, eolian, and back-barrier lacustrine deposits. At Mt. Baldy’s current position, only the back-barrier lacustrine deposits remain along the lakeshore. From bottom to top, the three facies record the lake-level rise to the Nipissing phase, eastward parabolic dune migration after the lake-level fall from the peak Nipissing, and lastly modern dune formation in the late 1800s to early 1900s that has become parabolic in the late 1900s.