GLACIAL LAKE SCUPPERNONG IN THE SOUTHERN FOOTPRINT OF MIS 2 GREEN BAY LOBE

The MIS 2 Green Bay Lobe (GBL) of the Laurentide Ice Sheet produced hundreds of modern and relict glacial lakes in eastern Wisconsin. The lobe reached its maximum extent to form the Johnstown moraine ~24.6 ka and retreated ~18.5 ka. Several lakes formed parallel to the retreating ice margin: glacial Lakes Middleton (gLM), Yahara (gLY), and Scuppernong (gLS). Using a combination of accelerator mass spectrometry (AMS) radiocarbon dating of plant macrofossils, optically stimulated luminescence (OSL) age estimates from ice-wedge polygons, and stratigraphic data from geoprobe and rotosonic cores drilled into GBL footprint lakes, further constraints on the timing, geographic extent, and dynamics of the GBL advance and retreat are obtained. Mapping and coring of these southern GBL footprint lakes has suggested the following sequence of events in the late Milton and early Lake Mills phases: formation of interspersed lacustrine-dominated environments beginning at ~17.9 ka, drainage and development of permafrost conditions dominated by ice-wedge polygon landforms by ~15 ka, and transition to marsh and wetlands by ~9.8 ka. The elevation and drainage of these lakes are thought to be controlled by isostatic rebound response, and sill down-cutting at the outlet of the Rock River.

Buried bedrock valleys in the gLS basin preserve ~100 m of lacustrine sediment. A new rotosonic core drilled in southeast Jefferson County north of Palmyra retrieved laminated fine-grained sediment from gLS, ending in refusal within the underlying glacial sediment of the Horicon Formation. Detailed description, particle size analysis of fine-grained sediment and laminations, and further chronostratigraphic sampling throughout the core, provides insight to the nature of glacial Lake Scuppernong, sedimentation rate, and the sequence of events that filled the bedrock valley.