SEDIMENT TRANSPORT CYCLES OF THE LAURENTIDE ICE SHEE

Unconsolidated sediment at the base of the LIS played a critical role ice dynamics. It has been suggested that the transition from the 41ka to the 100ka glacial cycles at ~0.9 Ma was related to removal of the regolith switching the Laurentide from soft-bed to hard-bed conditions, allowing the LIS to thicken substantially, changing its dynamic relation to climate. Analysis of remnant patches of drumlinized till across the Canadian Shield, along with boulder concentrations on the surface of till sheets in the Upper Midwestern United States, indicates that unconsolidated sediment continued to play an important role in LIS dynamics. These processes triggered a soft-to-hard transition over extensive areas of the LIS bed during the late Wisconsinan Stage.

Boulder concentrations in the upper portions of till sheets derived from igneous and metamorphic terranes are common. Although these features have been attributed to bio- or cryo-turbation, we suggest these concentrations are a natural consequence of glacier mechanics in this particular glacial geologic setting, and are primary features of till deposition. This study calculates mean transport length of glacigenic sediment from the lithologic composition of such boulder assemblages. The earliest late Wisconsin advances into central Minnesota have abundant carbonate from the Hudson Bay lowlands and greywacke from the Belcher Island Group, and calculated mean transport lengths exceed 1200 km. This early Late Wisconsin till is similar in texture and lithologic composition to the remnant patches of drumlinized till preserved across the Shield. Tills of successively younger ice advances have progressively shorter mean transport lengths, decreasing to <100km for the last advance.

We suggest that growth of the LIS is characterized by advection of sediment from the Hudson Bay basin across the Canadian Shield to its southern margin, constructing a soft bed over which the ice sheet advances. As the LIS approaches its maximum limit, this soft sediment is systematically removed, resulting in hard-bed conditions throughout much of the LIS interior. Therefore, LIS 100k cycles after 0.9 Ma are controlled by soft-bed conditions during ice buildup with a gradual change to hard-bed conditions just prior to the glacial maxima.