Refining the span and rates of deposition of the Glenwood Phase of glacial Lake Chicago

Building upon maps published by Bretz in the 1930's, we are remapping in GIS shoreline features of the Glenwood Phase of Glacial Lake Chicago in Illinois and Indiana using LiDAR DEMs. By obtaining sediment cores of Glenwood successions and determining age models of sediment accumulation rates using AMS C-14 ages of plant macrofossils, we seek to determine the span of early Glacial Lake Chicago, and indirectly determine the rate of downcutting of the Chicago Outlet. Our first site, Riggle Pond, is located near the fulcrum of the Griffith Spit, a lakeward-rotated spit (41.514, -87.379). Approximately 5 meters of sediment was sampled from rafts with a square-rod piston corer. The core bottomed out in fossiliferous, well-sorted fine sand. Spruce and larch needles were found throughout the pre-settlement sediment which is composed chiefly of silt. The lowermost 30 cm of core provided spruce needles and wood fragments that yielded ages of 13260 +/- 60 (ISGS-A3335) and 13450 +/-50 (ISGS-A3334) C-14 yr BP, respectively. Additional ages will be obtained from other parts of the core; we anticipate that SAR will decrease abruptly in response to the approximately 7 meter drop from the Glenwood to the Calumet levels that reflect erosion of the Chicago Outlet. Presently, it is not known if the drop from the Glenwood to the Calumet level was due to gradual regional changes in hydrology, or to rapid incision. We hypothesize that latter model would lead to an abrupt change in SAR in our core chronologies. To further test this hypothesis, we will be conducting similar analyses on sediment cores sampled adjacent to the Wilmette Spit (42.058, -87.761), and at Plum Creek Forest Preserve near Dyer, IN (41.490, -87.534). A revised history of the Glenwood Phase of Glacial Lake Chicago will help resolve issues of the nature of downstream contributions of Glacial Lake Chicago to the Illinois and Mississippi Rivers, and Gulf of Mexico.