SEDIMENT TRANSPORT CYCLES OF THE LAURENTIDE ICE SHEET II: LODGEMENT TILL AND GLACIAL BEDFORMS

Lodgement till, associated overlying meltout tills, and distinct landform assemblages deposited by the Rainy lobe of the Laurentide ice sheet (LIS) are exposed across central and northern Minnesota. These features range in age from prior to the Last Glacial Maximum (LGM) ca.27 cal ka BP to the final ice retreat from Minnesota about 13.5 cal ka BP. Sedimentology of lodgement tills reflect systematic evolution in sediment provenance, expressed as decreasing mean transport length (MTL), indicating evolving erosional patterns beneath the LIS. Till associated with the Late Wisconsin maximum ice limit (MIL) has a MTL >800 km reflecting provenance in the eastern part of the Hudson Bay Basin, while tills of progressively younger ice margins are characterized by MTL as low as a few 10s of km. At the MIL, a meltout facies overlies lodgement till and is composed of a boulder layer generally a 1-clast-thick with little associated fine sediment. Meltout facies associated with the youngest ice margins increases to up to 5 m thickness, with abundant >1 m diameter boulders and coarser texture.

Despite changes in lithology, the diameter of the largest clasts in lodgement tills is remarkably uniform at <40-60 mm diameter; clasts of larger size are absent. The homogeneity of the lodgement till, its geotechnical properties, and uniformity of largest clast size, suggest that clasts greater than about 40-60 mm in diameter were ‘unlodgeable’ and remained in traction at the ice-bed interface and were ultimately deposited by meltout.

Along much of the flowline, lodgment till is associated with drumlins, however, near the transition from shield bedrock to drumlinized till, transverse ribbed moraine is common. This setting represents a boundary between an interior region with high basal shear stress over bedrock and a down-ice region with low basal shear stress characterized by relatively high sliding velocity over thick subglacial till. This transition from hard to soft bed favors formation of transverse ridges in an overall erosional regime.