USING GROUND PENETRATING RADAR TO BENEFIT INTERPRETATIONS OF THE GLACIAL HISTORY OF PICTURED ROCKS NATIONAL LAKESHORE, MICHIGAN

Pictured Rocks National Lakeshore (PIRO) is one of Michigan’s most scenic and popular parks, yet it is the only U.S. National Park without a detailed surficial map. Although PIRO is best known for its Paleozoic sandstone cliffs along the southern shores of Lake Superior, Quaternary sediments cover the Paleozoic bedrock, highly contributing to the park’s geomorphic appeal. The lack of detailed characterization of surficial deposits and limited subsurface data in this area prevents adequate interpretation of the processes and history responsible for the creation of this landscape.

More than 35 km of Ground Penetrating Radar (GPR) data were collected as part of an ongoing effort to examine the complex glacial history of PIRO and surrounding area. Transects mostly followed dirt trails and logging roads in an effort to help characterize the glacial deposits. Information about subsurface characteristics, such as drift thickness, depositional structures, and bedrock lithology and structures, helps reveal the influence of ice behavior in the area responsible for the resulting geomorphology.

The extremely low conductivity (high resistivity) and sandy nature of the glacial drift and outwash in this area provided optimal conditions for deep penetration into the subsurface using GPR range settings of 400 nS with 100MHz antennae. Vertical Electrical Sounding work confirmed the high resistivities, yielding 8,000 to more than 20,000 Ohm-m in two areas. Common Midpoint surveys revealed radar velocities that range between 0.09 and 0.13 m/nS. Using a dielectric constant of 9 (a value based on velocity determinations), radar depths conservatively reached 18 meters.

The GPR data needed minimal post-processing, though elevation corrections were necessary for accurate interpretations of depositional structures and outwash transport directions inferred from reflection patterns. Information revealed from the GPR data will be used to supplement the ongoing project of mapping the Quaternary geology of PIRO and surrounding area in detail (1:24000 scale). The data will be used in conjunction with topographic and sedimentological information to help classify glacial landforms, as well as enhance the understanding of the ice sheet dynamics and glacial history of this area.