PROVENANCE OF SLACKWATER SEDIMENTS IN THE SAVANNA TERRACE, NORTHWESTERN ILLINOIS

The Savanna Terrace along the Upper Mississippi River was created during Late Wisconsin Episode deglaciation of the Laurentide Ice Sheet (LIS). Melting of the LIS created several proglacial lakes that periodically drained down the Mississippi River and caused 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. Terrace remnants can be found along Rush Creek and extend as far as 9 km from the main channel of the Mississippi. Based on thickening slackwater deposits in the southeast, it is likely that the outburst floodwaters spilled over into a paleo-Rush Creek to create a slackwater lake rather than being deposited by the active channel of the Mississippi. AMS Carbon-14 dates on channel bars in the fluvial materials 2 m below the overbank deposits were obtained from spruce and larch needles, providing dates of 12,840 ±80 ¹⁴C yr B.P. at 5.2 m and 13,020 ±45 ¹⁴C yr B.P. at 6.1 m. XRD and trace element analysis was used to distinguish between the two materials as well as to confirm provenance from the hypothesized source areas of Lake Superior for the red sediments and Lake Agassiz for the gray sediments. Suspected Lake Agassiz materials contained large amounts of smectite and Se whereas suspected Lake Superior materials were rich in kaolinite, Cu, and Fe. Lake Superior materials also contained feldspars and carbonate minerals, though in small quantities.