North-Central Section - 42nd Annual Meeting (24–25 April 2008)

Paper No. 15
Presentation Time: 8:00 AM-12:00 PM


JOHNSON, Beth A. and STRAVERS, Jay, Department of Geology and Environmental Geosciences, Northern Illinois University, Davis Hall 312, Normal Rd, DeKalb, IL 60115,

The Savanna Terraces are part of an extensive late Wisconsin Episode glaciofluvial and glacial lacustrine terrace system that consists of slackwater clay deposits of alternating red and gray sediments believed to be the result of drainage from glacial lakes Agassiz and Superior. These are the oldest and highest terraces along the Mississippi River that do not have a loess cover. Noted by Flock (1983) as one of the most prominent and continuous terraces along the Upper Mississippi, the terraces can be traced from Pepin County, Wisconsin to Jackson County, Illinois. The Savanna Terraces north of Savanna, Illinois, are actually a complex system that includes terraces of differing ages and lithologies. Lithologic analysis was completed using hand auger samples as well as a Geoprobe 6600 direct push rig to take continuous core samples via the Dual Tube 22 method. No fewer than five different levels of terraces have been defined based on morphostratigraphy and lithology: one clay terrace of the Equality Formation (lacustrine), two glaciofluvial sand terraces of the Henry Formation (Henry I and Henry II) of the Savanna Terrace System, and two Holocene or post glacial alluvial terraces of the Cahokia Formation (Cahokia I and Cahokia II). Adding another level of complexity are the Parkland Sands, which originated as fluvial sands and were later transported and deposited by eolian processes, forming dunes on the surface of Henry and Equality terraces. Radiocarbon dating performed by Flock (1983) indicates that sediments in these terraces were deposited between 13,100 and 9,500 14C yr B.P. However, these dates appear to relate to a single terrace level (Deer Plain Terrace). Coring in the Equality terrace has revealed a complex stratigraphy of slackwater sediments alternating with fluvial sands, indicating multiple megaflood events at this site. Further OSL dating of the terrace sediments will be completed in hopes of determining the exact sequence of drainage events represented at this site as well as their correlation to specific drainage outbursts.