GEOCHRONOLOGY OF LATE WISCONSIN ICE MARGIN FLUCTUATIONS OF THE SOUTHERN LAURENTIDE ICE SHEET

Despite over a century’s research, the absolute geochronology of ice margin fluctuations along the southern Laurentide Ice Sheet is poorly constrained. Historically, this largely reflects the limitations of radiocarbon dating imposed by the scarce amounts of organic material found in direct association with late Wisconsin glacial sediments because of prevailing tundra conditions at that time and the difficulty of interpreting the few available radiocarbon ages (relating the age of an organic sample to the age of the glacial landform on which it is sampled, unequivocally relating radiocarbon ages to distinct ice margin positions, etc…).

One promising avenue of inquiry is evaluating the ages of sediments from pro-glacial lakes that existed only at discrete times associated with well-defined ice margin positions. Recently, Attig et al. (2011) successfully applied optical dating to lacustrine sediments from the Baraboo Hills of south-central Wisconsin to constrain the onset of the Green Bay Lobe’s thinning and retreat from its maximum position. Additional pro-glacial lake sites in south-central Wisconsin have the potential to constrain both the end of the last ice advance and the onset of the last ice retreat using a combination of optical and radiocarbon dating.

Several small tributaries in south-central Wisconsin lie in a unique position to constrain the timing of the Green Bay Lobe’s advance and retreat in this fashion. The mouths of these tributaries were blocked by aggrading outwash in the mainstem valleys, and ice advanced into the headwaters of these tributaries only at the maximum ice extent. As a result, the tributaries hold a sedimentary archive of outwash-dammed lacustrine sediment prior to and following the late Wisconsin maximum and outwash during the late Wisconsin maximum. On one such tributary near Cross Plains, WI, a radiocarbon date from ~30 cm above the top of the late Wisconsin outwash indicates ice had retreated out of the headwaters of the valley by 16,900 cal yr BP and the resulting lake existed until at least 7,700 cal yr BP. Optical dating of sediment at this site will constrain both the end of the last ice advance and the onset of the terminal retreat. Because the stratigraphy of the site is well understood, dates from this site provide easily interpretable dates for advance and retreat of the Green Bay Lobe.