Provenance of slackwater sediments in the Savanna Terrace, northwestern Illinois.

• Main Author: Johnson, Beth A.
• Other Authors: Philip J. Carpenter.
• Language: unknown
• Published: Northern Illinois University.Geology and Environmental Geosciences. 2009
• Subjects: Physical Geography; Geology; Green Bay; Ice Sheet
• Online Access: http://commons.lib.niu.edu/handle/10843/11901; http://hdl.handle.net/10843/11901

Sorry, the full text of this article is not available in Huskie Commons. Please click on the alternative location to access it. 321 p. The Savanna Terrace along the Upper Mississippi River was created during Late Wisconsin Episode deglaciation of the Laurentide Ice Sheet (LIS). Meltwater from the LIS periodically drained down the Mississippi River and caused tributaries to back up and deposit sediments in slackwater terraces. In the research area 8 km north of Savanna, Illinois, the slackwater clay terrace is composed of alternating red and gray glacial lacustrine materials of the Equality Formation. Coring in the field area showed slackwater materials up to ∼3.7 m thick deposited on top of fluvial channel bars, which contain sediments from previously deposited slackwater materials that were reworked by fluvial processes. Sediment cores as well as geophysical techniques such as electrical conductivity, ground penetrating radar, and resistivity aided in the development of cross-sections, which showed the slackwater materials thinning westward in the direction of the Mississippi and thickening to the southeast. Based on this information, it is likely that floodwaters backed up into a paleo-Rush Creek to create a slackwater lake, rather than being directly deposited by the active channel of the Mississippi. AMS Carbon-14 dates on channel bars ∼2.5 m below the slackwater deposits were obtained from spruce and larch needles, providing dates of 12,840 +/-80 14C yr B.P. at 6.4 m and 13,020 +/-45 14C yr B.P. at 7.1 m. X-ray Diffraction (XRD) and trace element analyses were used to determine the provenance of the red and gray materials. All sediments showed higher than expected contents of expandable clays; gray sediments also contained large amounts of Se and Cd whereas suspected red sediments were rich in kaolinite, chlorite, Cu, Fe, and Y. The red sediments probably originated in the ancestral Lake Superior Basin, whereas the bulk of the gray sediments probably came from the Des Moines glacial lobe. The presence of Sr in all sediments suggests a third source also contributed to these sediments, possibly from Green Bay Lobe drainage down the Wisconsin River.