Evidence
for Rapid Drumlin Formation through Intense Sediment Deformation resulting from
Increased Pore Water Pressure during a Surge of the Huron Lobe.
Kevin P. Blake, Department of Geography and Geology, Eastern
Michigan University, Ypsilanti, MI 48197 USA
Nell Orscheln (EMU ’02), Department of Geography and Geology, Eastern
Michigan University, Ypsilanti, MI 48197 USA
Michael Iacoboni (EMU ’02), Department of Geography and Geology, Eastern
Michigan University, Ypsilanti, MI 48197 USA
Michael Hoenerhoff (EMU ’02), Department of Geography and Geology, Eastern
Michigan University, Ypsilanti, MI 48197 USA
Drumlins composed primarily of sorted material were
studied in the Metz drumlin field, south of Rogers City, MI, where the surface
relief between drumlins and intervening low areas is between 3-10 m. An exposure along the central long-axis of
one drumlin extends 130 m and is over 15 m deep, revealing sediments to a greater
depth than the surface relief. The
lower package of drumlin sediments are distinct, sorted layers of sand, silt,
and clay that are mildly deformed to a depth of over 12 m below the
surface. The clay/silt layers have been
sheared, hardened, and thrust over overlying sediments, creating a stack of
sheared clay/silt layers with intervening very fine sand and coarser sand
layers. The upper 2-9 m of sediment
consists of a contorted mix of intensely deformed gravel, sand, silt, and
diamicton. Truncated beds at the lower
contact with the layered sorted sediments indicate that the contact is
erosional, with thrust faults on the ice proximal side and normal faults on the
ice distal side of the drumlin. Blocks
of sediment, locally quarried from the underlying sediments, are less deformed
than the intensely deformed sediments they are contained within. These structures indicate that drumlin
formation was initiated by an increase in pore water pressure within the
sediments underlying the ice sheet. An
increase in pore water pressures would lower the effective normal stresses of
the overlying ice and change the deformation style from shear deformation of
deep clay/silt layers to contorted deformation of sand, silt, and diamicton
layers at the surface to a lower effective depth. The high pore water pressures responsible for this deformation
must have initiated a surge of the Huron Lobe during the Greatlakean stage of
glaciation. Local ice stagnation
features, such as enormous kames and eskers stretching for tens of miles,
indicate that stagnation followed the surge.
This new evidence may show that entire drumlin fields form by rapid
deformation of subglacial sediments related to surges of ice lobes.