TRACKING THE EVOLUTION OF GLACIAL LAKES YAHARA AND SCUPPERNONG, SOUTH-CENTRAL WISCONSIN, USA

As the MIS 2 Green Bay Lobe retreated to the northeast from its maximum position in south-central WI, glacial Lakes Yahara (gLY) and Scuppernong (gLS) formed as proglacial lakes fed by meltwater from the retreating ice sheet. The lakes persisted until the sills that controlled drainage to the south downcut sufficiently for lake level to drop, forming modern lakes in the Madison area within the footprint of gLY and completing draining gLS. While the general geologic history of the lakes has been documented, little chronologic control exists to constrain either lake and the precise extent of gLS is poorly understood.

Cores from four locations within the gLY basin capture the stratigraphic shift from lacustrine to marsh sediment as lake level fell, and the laminated lake sediment has been found to contain abundant plant macrofossils of riparian species as well as gastropods and bivalves. Initial AMS radiocarbon ages indicate that gLY formed sometime prior to ~18.2 ka (Beta-563548), and that the lake ceased to exist in the timeframe of ~11.7 to ~11.2 ka (Beta-563547 and Beta-563550, respectively). These age constraints will continue to be refined by additional coring and radiocarbon dating specific to gLY.

Mapping of gLS and associated coring suggests that the lake was likely less extensive than previously hypothesized. The proglacial landscape was likely very dynamic, evidenced by different elevations of ice-contact deltas associated recessional-phase moraines. 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. The lowest areas of this landscape contain lacustrine sediments overlain by marsh sediments. These areas had active lacustrine deposition until at least ~9.1 ka (Beta- 563553), and transitioned to marsh by ~6.8 ka (Beta-563552). Ice-wedge polygons are common on the gLS lake plain in areas that are higher on the landscape and not covered by marsh sediments. Prior studies indicate that permafrost conditions were pervasive in southern Wisconsin only up until ~15.0 ka. Therefore, most of the sediments were likely deposited and exposed subaerially prior to that time, which is consistent with deposition in gLS beginning prior to ~17.9 ka (Beta- 548564).