BEDROCK CONTROL ON SUBGLACIAL DRAINAGE STYLE, LATE WISCONSIN SUPERIOR LOBE, MINNESOTA

The late Wisconsin Superior lobe in Pine County, Minnesota, had a bed of four different bedrock types. Distinct subglacial drainage systems developed over each substrate. Although other factors (e.g. geothermal heat flux; ice thickness) may also have influenced the drainage style, the bedrock exerted primary control. Bedrock variables that may be important include resistance to erosion, permeability, porosity and structural heterogeneities.

Glaciated crystalline substrates are characterized by drumlins with indistinct channels in interdrumlin swales. They are larger than hypothesized linked-cavity systems modeled for hard-bed valley glaciers, but resemble them in form. Evenly indurated sandstone substrates are characterized by broad, closely spaced valleys in which prominent eskers are preserved. The valleys are especially deep (30 m) where aligned with joints. "Tunnel valleys" of this size are not predicted in subglacial drainage models. Sandstone of variable induration shows no distinct channel system at the surface but uncemented beds and joints were excavated or widened at some time resulting in sinkhole development. Subglacial drainage may have entered these weak areas and removed material. Terrain underlain by rift-setting basalt flows folded into a plunging syncline displays closely spaced, joint- and bedding-controlled narrow and shallow valleys with small eskers. An anomalously broad, high, straight esker-type ridge of sand and gravel south of Pine County in Wisconsin exposes flat-bedded gravel and sand and is capped with a ventifacted stone pavement and loess. It may be a result of focusing of subglacial flow from the basalt terrain and eventual unroofing of a subglacial tunnel.

The drumlins, large tunnel valleys with eskers, and small valleys with eskers evolved at the same time. The sinkholes may have developed during a later phase of the lobe given their alignment with a younger ice margin. Existing models of subglacial hydrology do not explain these drainage patterns or differences in style formed by the Superior lobe in Pine County, Minnesota.