REFINING THE SPAN AND RATES OF DEPOSITION OF THE GLENWOOD PHASE OF GLACIAL LAKE CHICAGO

By reexamining landforms and building upon ideas published in maps and reports by Bretz in the 1930s and later studies by others, I will be redefining the span and role of the Glenwood phase of Glacial Lake Chicago in Illinois and Indiana. These tasks will be completed by interpreting LiDAR DEMs, obtaining sediment cores of nearshore features, interpreting analyses of particle-size distribution and clay-size mineralogy, and producing age models using new AMS C-14 ages. I will determine the span of the Glenwood phase of Glacial Lake Chicago and indirectly determine the timing of any large meltwater floods and the rate of downcutting of the Chicago Outlet during the transition from the Glenwood phase to the successive Calumet phase.

Three sites were chosen to complete this study: Riggle Pond, Plum Creek and Harms Woods. Our first site, Riggle Pond, is located near the fulcrum of the Griffith Spit, a lakeward-rotated spit (41.514 N, -87.379 W). Plum Creek, previously studied by Bretz, is within a forest preserve near Dyer, Indiana (41.490 N, -87.534 W). Harms Woods is located west of the Wilmette Spit (42.058 N, -87.761 W). Among the three sites, 21 new AMS C-14 ages were determined.

Hypotheses to explain the ca. 7-m drop from the Glenwood to the Calumet level includes changes in watershed area, or to rapid incision of the Chicago Outlet by catastrophic overflow. I hypothesize that the abrupt decrease in the sediment accumulation rate at Riggle Pond at the end of the Glenwood phase (about 15,200 cal yr BP) indicates a relatively abrupt lowering of lake level consistent with catastrophic erosion of the Chicago Outlet. Prior to the downcutting event, the progradation of the spit ridges of the Wilmette and Griffith spits indicates some incision, but interpretation of the DEM is complicated by urbanization and aeolian activity.

A revised history of the Glenwood Phase of Glacial Lake Chicago will help resolve issues of the timing and magnitude of overflow events of Glacial Lake Chicago via the Chicago Outlet that ponded in slackwater lakes in the Illinois River valley, and thence to the Mississippi River and Gulf of Mexico. Moreover, the new dates of the Glenwood phase may inform events to the north, such as meltwater contributions from Lake Michigan, in addition to meltwater of the Erie lobe during waning of the Port Huron Phase.