GLACIAL LAKES IN THE FOOTPRINT OF THE SOUTHERN GREEN BAY LOBE OF THE LAURENTIDE ICE SHEET (Invited Presentation)

The MIS 2 Green Bay Lobe (GBL) of the Laurentide Ice Sheet formed large glacial lakes while at/near its maximum position (glacial Lake Wisconsin) and during its retreat from east-central Wisconsin (glacial Lake Oshkosh). The southern portion of the lobe is also associated with several smaller, though still significant, glacial lakes that existed on the glacio-isostatically depressed footprint of the lobe during and after its retreat: glacial Lakes Middleton (gLM), Yahara (gLY), and Scuppernong (gLS). Previous mapping and research have documented these lakes’ histories; current research reported here is providing new AMS radiocarbon chronologic control for the existence of these lakes and improved geographic constraint for gLS.

gLM existed as a meltwater outflow basin between the terminal and first recessional moraines of the GBL. Cores from two locations in this basin constrain retreat of the GBL and initial exposure of the gLY and then gLS basins. Cores from 10 locations within the gLY basin contain laminated fine-grained sediment with abundant riparian and aquatic plant macrofossils and gastropod and bivalve shells. Age estimates from the riparian/terrestrial plant macrofossils document lacustrine deposition occurring by ~18.2 ka and persisting for at least 6,500 years following GBL retreat. Stratigraphy of the gLY cores also document the shift from lacustrine to marsh sediment as lake level ultimately fell in response to down-cutting of the sill on the Rock River to the south.

Mapping of gLS and data from 51 associated cores suggest that the lake was less extensive than previously hypothesized (e.g., Clayton, 2001). Lake levels likely fluctuated widely during retreat of the Green Bay Lobe due to complex interactions between meltwater discharge, complex lake basin shape in the low-relief drumlinized landscape, and shifting outlets. Ages from two cores indicate lacustrine deposition had started by ~17.9 ka; the lake drained progressively over thousands of years in response to isostatic rebound and the same sill down-cutting that affected gLY. Ice-wedge polygons that are common on higher areas of the gLS lake plain indicate that drainage had begun prior to the end of permafrost conditions at ~15.0 ka. The lowest areas of this landscape experienced lacustrine deposition until at least ~9.1 ka, and had transitioned to marsh by ~6.8 ka.