Analysis of Slackwater Sediments Found In the Savanna Terraces along the Mississippi River, Northwestern Illinois

The Savanna Terraces along the Upper Mississippi were created during Late Wisconsin Episode deglaciation of the Laurentide Ice Sheet (LIS). Melting of the LIS created several proglacial lakes, which would periodically drain down the Mississippi River and cause tributaries feeding into that river to back up and deposit sediments in slackwater terraces. In the research area 8 km north of Savanna, Illinois, the slackwater terraces are composed of alternating red and gray glacial lacustrine materials from the Equality Formation. The red slackwater materials fall under the color values of 5 YR and 10 YR, and are dominantly composed of silt-sized particles (~70%) which fine upward in individual layers. Most of these materials have an average clay content of 10%, although some layers have as much as 22% clay. The gray slackwater materials (when present) tend to be more clay rich, with an average clay content of 27%, and fall under the color values of 2.5 Y and 5 Y. Magnetic susceptibility measurements indicate the gray materials have a higher susceptibility than the red materials (94 x 10^-5 versus 20 x 10^-5 [SI units], respectively), a fact that can help correlate these layers over long distances. Geochemical analysis is also underway to attempt to link the slackwater materials in the Savanna Terraces to the upriver source area. Geophysical surveys suggest ground-penetrating radar (GPR) signals penetrate overlying sands to reveal terrace deposits as deep as 10 m. GPR wave velocity is much higher in the sands (0.11 m/ns-0.15 m/ns) than the clayey terrace deposits (0.07 m/ns), which strongly attenuate GPR signals. Resistivity soundings also suggest terrace deposits may be identified beneath sands since the resistivity of terrace clays is typically very low, about 1-10 ohm-m. Thus, both GPR and resistivity surveys may be useful in correlating terrace deposits.