Fault
Orientations as Evidence for Subglacial Deformation in Drumlin Formation,
Northeastern Michigan
Michael Iacoboni (EMU ’02), Department of Geography and Geology, Eastern
Michigan University, Ypsilanti, MI 48197 USA
Kevin P. Blake, Department of Geography and Geology, Eastern
Michigan University, Ypsilanti, MI 48197 USA
Countless hypotheses have been proposed to attempt to
explain the origin of drumlins, including direct ice erosion, erosion by
subglacial flooding, and differential deformation of subglacial sediments. A few miles south of Rogers City, Michigan,
a gravel pit exposes the sediments of a drumlin along the entire length of its
cross-section. The drumlin is composed
of layers of sorted sands, silts, clays, and diamicton. The upper several meters are extremely
deformed. Below this, the sediments are
less deformed to a depth of over 12 m.
Here, the clays have deformed plastically, whereas the sands have
deformed in a brittle fashion. Using a
Brunton compass, fault orientations were measured throughout the sand layers,
then plotted using Stereonet 4.9.5a.
Analysis of the plots showed most fault planes occurred roughly
perpendicular to the drumlin orientation, with a few nearly vertical planes
roughly aligned with the gravel pit face.
The alignment of the faults perpendicular to the drumlin axis seems to
reflect direct basal ice deformation, presenting strong evidence that the
process that caused the deformation at depth is directly related to the process
that formed the drumlin itself.