Key words: basal till,
basal sliding, deformation, clast fabric, fast ice
flow
BETTY
J. SOCHA AND DAVID M. MICKELSON, Department of Geology and Geophysics,
University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin, USA,
53706; bjsocha@geology.wisc.edu;
davem@geology.wisc.edu.
Re-advancing post-LGM ice of the Green Bay Lobe of
the Laurentide Ice Sheet overrode sequences of soft
sediment including saturated clay-rich lake sediment, wind-blown silt, and
clayey and sandy older diamictons. We examined sedimentary sequences for
indications of the soft sediments response to ice overriding. At several locations near the former ice
margin, clay-rich diamicton with a homogeneous
appearance, uniform grain-size distribution, and relatively strong clast-fabric, overlie or grades into sediment that has
pre-deformation sedimentary features.
These include textural laminations, color and texture contrast, included
organic material, silt stringers, and crenulated beds of fine sand, silt and
clay. We suggest that the basal till
features, including relatively strong clast-fabric
that is parallel to ice flow direction, developed in thick layers of clay-rich
lake sediment due to the shear stress of the
overriding ice. Lateral movement of
sediment appears to have been minimal.
Sediment was transported only a short distance and retains some primary
structures and characteristics of the parent material. Laboratory analysis of the matrix of the
clayey diamicton indicates a uniform grain-size
distribution at individual locations, but fairly wide variation in matrix
composition on a regional basis.
Preserved clast-fabric parallel to ice-flow
direction indicates the presence of little interstitial ice in the sediment as
it was deposited by the glacier.
Fast ice flow is indicated by ice margin positions, and chronology
based on stratigraphy and radiocarbon dates on
organic material. Advance and retreat rates were on the order of a half a
kilometer per year from 15,000 to 13,800 calendar years B.P. Because the fast-flowing ice did not move
sediment long distances and produced local deformation of sediment, we infer
that the ice coupled to the bed only locally and temporarily. At other times high water pressure produced
decoupling and sliding at the glacier sole.
Widespread, pervasive deformation of the whole thickness of the glacier
bed is contraindicated.