GEOPHYSICAL ANALYSIS OF WISCONSIN EPISODE DEPOSITS IN THE SAVANNA TERRACE NEAR SAVANNA, ILLINOIS

The Savanna Terrace can be traced for approximately 1,000 km along the Mississippi River from Pepin County, Wisconsin to Jackson County, Illinois. The terrace is composed of Wisconsin Episode Glacial sediment deposited during deglaciation of the Laurentide Ice Sheet (LIS). Meltwater from the LIS flowed south through the ancient Mississippi flooding its tributaries creating slackwater environments. The study site 8 km north of Savanna, Illinois is near the southern boundary of the Driftless Section in northwestern Illinois and is exposed by Rush Creek. Core analysis describes the area of interest as approximately 2.97 m of slackwater materials of the Equality Formation above fluvial sands of the Henry Formation. Geophysical analysis at this site was performed to determine the boundary between the fluvial sands and slackwater sediments, and also to evaluate which geophysical method would yield the most accurate results in high clay regions. Supplementing previously conducted core and electrical conductivity (EC) analysis, this project utilized resistivity and ground-penetrating radar (GPR). Two walk-away GPR surveys at 50 MHz and 100 MHz were conducted along a 20 m transect oriented north and south. The CMP profiles were conducted along the same transect with 2 m spacing between antennae. The best GPR data was obtained with 50 MHz antennae because of the high concentration of clay-sized (<2 μm) particles (40%-50%). For resistivity, a Schlumberger Array was used along the same transect extended 40 meters each end. The geophysical data illustrated that the contact between the slackwater sediments and the fluvial sands is relatively flat and is at ~3 m depth, which is within ~.03 m described in the core data.