| Summary: | The Glenwood phase of proglacial Lake Chicago is its oldest phase, but is the least studied. By developing the and ideas previously published maps by Bretz in the 1930s and studies by Hansel, Mickelson, Thompson and others I will be redefining the span and role of the Glenwood Phase of proglacial Lake Chicago. This task will be completed by using LiDAR DEMs and by obtaining sediment cores proximal to Glenwood phase features and using age models created by AMS C-14 ages. Using this information, in addition to grain size analyses and clay mineral analyses, 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 its successive Calumet phase. 22 new radiocarbon ages of needles, charcoal and plant marcrofossils were obtained from cores sampled near Glenwood phase features that can still be easily identified. The age models created from the new ages indicate a disturbance in the record at about 15.1 ka. Presently, it is yet debated if the drop from the Glenwood to the Calumet level was due to gradual changes in hydrology or to rapid incision of the Chicago Outlet. If the sediment accumulation rate abruptly decreases simultaneously with the end of the Glenwood phase and subsequently the ~7meter drop from the Glenwood to the Calumet lake levels, that reflects sudden erosion of the Chicago Outlet. The produced age models do present evidence for an abrupt incision near the end of the Glenwood, but a more in depth look at LiDAR evidence proves the Glenwood/Calumet transition to be a bit more complex. 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 as well as upstream contributions from the Laurentide Ice Sheet into Glacial Lake Chicago.
|
|---|