A MECHANISTIC UNDERSTANDING OF GREEN BAY LOBE TILL DEFORMATION THROUGH EXPERIMENTAL ANALYSIS

Subglacial tills are often preserved in the Quaternary geologic record. However, directly inferring kinematics and dynamics of glacier motion from these deposits is difficult because we lack fundamental knowledge about how these properties relate to till fabrics. To better constrain the information garnered from the glacial geologic record, we simulated subglacial slip with UW-Madison’s state-of-the-art ring shear device and then analyzed the resulting till deposits with anisotropy of magnetic susceptibility (AMS) and microstructural analysis for comparison with field data.

The device used to simulate subglacial slip is a large-diameter ring shear that spins a ring of temperate ice (60 cm outer diameter, 16 cm width and 25 cm height) over a 12 cm layer of Green Bay Lobe (GBL) till. The ice ring is pressed into the till bed with vertical effective stresses characteristic of modern-day glaciers and ice streams, while meltwater is allowed to drain from the system. The ice ring is gripped at its upper surface by a toothed platen and spun over a stationary till bed, initiating slip. Importantly, the sample chamber walls are clear acrylic, which facilitates direct observation of the ice-bed interface. Photos of the till bed are captured at regular intervals and then analyzed with particle tracking software to continuously map deformation and shear within the till during the experiment. This experimental setup produces till deformation under a range of realistic effective stresses and driving velocities.

Upon completion of the experiment, we melt the ice and sample the bed directly for AMS and microstructure analysis. Several depths are targeted for AMS to compare till fabrics with camera observations, and vertical sections of till are collected and prepared for use in microstructural analysis. From these observations, we link the dynamics and kinematics of till deformation recorded during the experiment to the properties of the till that would be preserved in the geologic record. These results are compared to AMS fabrics observed in basal till members from GBL drumlins to better understand how the GBL may have responded to changes in driving stress and why it was capable of building such a broad range of subglacial landforms.