Cutler, P. M.1, Clayton, L.2, Mickelson, D.M.1, Colgan, P.M.3, and Attig, J.W.2
Department of Geology and Geophysics, University of Wisconsin-Madison, Wisconsin 53706
2Wisconsin Geological and Natural History Survey, Madison, Wisconsin 53705, USA
3Department of Geology, Northeastern University, Boston, MA 02115, USA
Suitably located gravel pits within the outwash fans are rare, but three such examples have been found.Each pit extends to within 100 to 200 m of the glacial maximum ice margin. Exposures are typically 5-7 m high, and consist of laterally extensive (up to 150 m wide), clast-supported cobble and boulder gravel units, each capped with planar and trough cross-bedded sand.At all three fans, up to four such discrete glaciofluvial packages exist below a strikingly coarse clast-supported massive boulder gravel that lies within 2 m of the fan surface. Due to its lateral continuity and extent, and the size of clasts, this boulder unit is interpreted as a jökulhlaup deposit from a sediment-rich proglacial sheet flood emanating from the associated tunnel channel. One puzzle is the apparent lack of evidence for pitted outwash on the fans, as is often observed in jökulhlaup deposits.
Paleodischarge estimates from boulders within this unit are tentative because of uncertainties in the original fluid density, channel width, and the undoubtedly unsteady nature of the ‘event’. Nonetheless, peak discharge was probably between 700 and 5,000 m3/s at all three pits. At the lower end of this discharge range, three water sources are viable: an extreme precipitation event, drainage of a supraglacial lake, or drainage of a subglacial lake. For most of the discharge range, however, the large volume of stored water required implies that the only reasonable source is a subglacial-lake outburst.
Environmental factors were favorable for formation of subglacial lakes behind the southern margin of the Laurentide Ice Sheet in Wisconsin. For example, proglacial permafrost was present as ice advanced to its glacial-maximum extent, and the hydrogeologic setting of the ice margin resulted in inconsequential drainage through the groundwater aquifer. Numerical ice-sheet modelling of the Green Bay Lobe from 55,000 to 21,000 yr B.P. reveals that the frozen-bed zone, formed as ice overrode the permafrost, probably extended on the order of 100 km upstream from the ice margin at the glacial maximum. Permafrost was up to 200 m deep, and the bed gradually thawed from the upstream end until trapped subglacial meltwater was able to drain to the margin. There was ample stored water to move the biggest boulders.
The challenge remains to
determine the significance of the jökulhlaup deposit to overall tunnel-channel
genesis. Hundreds to a few thousand years probably elapsed before this
final large drainage event, based on the existence of multiple units of
glaciofluvial gravels beneath the coarse boulder facies, and the fact that
at least 30 m and possibly more than 70 m of sand and gravel lies in two
of the fans. During this time the majority of the fan was constructed,
possibly with additional coarse boulder units at depth, and apparently
with the fan apex in a stable position. Proglacial permafrost was concurrently
degrading, and supraglacial meltwater was probably able to penetrate to
the bed within a few km of the terminus. The final jökulhlaup most
likely took advantage of a pre-existing drainage route, enlarging the channel
and flushing out the subglacial boulder lag that accumulated during the
preceding less competent flow regime. Evidence for jökulhlaups at
stratigraphically similar locations in all three tunnel-channel fans raises
the possibility that a major release of water occurred just before ice
retreated from its glacial-maximum position.