STRUCTURAL ANALYSIS OF SUBGLACIALLY DEFORMED SEDIMENT IN SOUTHWEST MICHIGAN

Along the traditionally mapped Valparaiso Morainic system in portions of Berrien, Van Buren, and Allegan Counties, Michigan, USA the glacigenic sediments have been deformed. The Lake Michigan lobe of the Laurentide Ice Sheet has most recently influenced this area of southwest Michigan. Fine-grained lacustrine sediments overlain by a reddish-brown diamicton characterize this upland. The deformation is observed not only in the diamicton but in the underlying lacustrine sediments as well. Eleven exposures were studied along the upland and at some locations, the deformation occurs as deep as 10 meters. Deformation includes faults, low angle shear planes, and a wide range of folds including upright, recumbent, drag, and sheath. The presence of reverse faults and style of folding indicates the deformation being compressional in nature. The diamicton also exhibits strong clast fabric alignments. Bedding of the fold limbs, fold axes, fault surfaces, as well as the long axis orientations of clasts within diamictons were measured. The structural data were collected using a Brunton compass and processed using StereoWin 1.2.0. Analysis of the deformation yields the principal stress directions involved in producing the strain. Data analysis from deformation found within a drumlin in the study area also shows a positive correlation between principal stresses of deformation and ice flow direction. Results of the structural analysis show that there is a strong correlation between the deformation observed in the lacustrine sediment and the fabric alignment in the diamicton. This leads to the conclusion that the deformation of the subglacial sediments is the result of stresses applied by the active ice.