A GEOPHYSICAL INVESTIGATION OF LARGE-SCALE GLACIOTECTONIC DEFORMATION, LUDINGTON RIDGE, MICHIGAN

Late Wisconsin glaciotectonic processes have resulted in large deformation structures, which are visible in a 1.5km long section of cliff face along the eastern shore of Lake Michigan south of Ludington, Michigan. Several apparent clay diapirs rise from below beach level to near the top of the ~50m high cliff. The clay was deposited in a proglacial lake environment, while overlying sands and gravels were deposited as glacial outwash. The sequence is topped by areas of eolian sand. Groundwater springs throughout the area highlight complicated groundwater flow patterns, while gullies are an indication of past landslides. Currently, little is known about the local stratigraphy near the deformation structures and whether the deformation structures are isolated or continuous features. Characterization of this system will improve understanding of the glacial history of west Michigan and the effects of this deformation on the local hydrology. In order to study the inland extent and orientation of these structures we used a suite of geophysical methods and stratigraphic interpretations. The primary method of study was electrical resistivity, which utilizes the electrical contrast between the clay and the overlying sands and gravel. Laboratory measurements show that for volumetric water contents between 10 and 30%, the resistivity of sand and clay units ranges from 430 to 140 Ohmm and from 225 to 20 Ohmm, respectively. Six Wenner-array constant spread traverses (CST) were performed along parallel transects perpendicular to the expected orientation of the clay deformation structures. These CSTs, with a-spacings of 30 and 90m and measurement intervals ranging from 15 to 90m, were designed to characterize both vertical and lateral variability. To improve interpretation of these data, 1D soundings and forward modeling of stratigraphic sections from well logs and cliff outcrops were used. Higher resolution data was gathered on areas of interest using ground-penetrating radar and multi-electrode roll-along Wenner surveys with a-spacings ranging from 6 to 162m. Results from field surveys indicate that one of the deformation structures, which is seen in the cliff face, extends inland for a distance of at least 250m. The SW-NE orientation of this structure coincides with the dominant surface topography for the Ludington Ridge.