Paper No. 17
Presentation Time: 9:00 AM-6:30 PM


KRYZER, Robert, Geological Sciences, University of Minnesota Duluth, 230 Heller Hall, 1114 Kirby Dr, Duluth, MN 55812, MOOERS, Howard D., Earth and Environmental Sciences, University of Minnesota Duluth, 230 Heller Hall, 1114 Kirby Drive, Duluth, MN 55812 and LARSON, Phillip, Duluth Metals Ltd, 306 W Superior Street, Suite 610, Duluth, MN 55802,

Ribbed (Rögen) moraine in northeastern Minnesota associated with the Rainy lobe of the Laurentide Ice Sheet is extensively distributed at transition of scoured Canadian Shield bedrock and thick drumlinized till and is associated with eskers. This setting suggests a strongly extensional glaciological setting as the ice transitions from bedrock to thick unconsolidated sediment. As part of our investigation of ribbed moraine formation simple glaciological modeling is used to conceptualize the ice flow regime and ground-penetrating radar (GPR) is used to study internal structure of the forms.

Our numerical flow model used a specified accumulation pattern and basal shear stress distribution to calculate the balance velocity and assess increasing ice velocity across the bedrock-till transition where ribbed moraines occur. GPR profiles were collected parallel to ice flow and multiple profiles on one form were used for 3-dimensional analysis.

Modeling suggests that the velocity increase across the transition is likely no greater than 100%. GPR reveals that about half of the ribbed moraine forms have an imbricated structure on the stoss side that we interpret as depositional.

We conclude that ribbed moraine in NE Minnesota formed in a net erosional environment. As the drumlinized till thinned, ribbed moraine became the stable bedform. We interpret the imbricate structures on the stoss side of the forms as local deposition of till suggesting that the forms were migrating upglacier. The association with eskers suggests that effective subglacial drainage enhances ribbed moraine formation.