A VIRTUAL FIELDTRIP OF LATE PLEISTOCENE AND HOLOCENE DEPOSITS OF LUDINGTON RIDGE, MASON COUNTY, MICHIGAN

The Ludington Ridge site, a Lake Michigan shoreline bluff located on state land 2.4 km north of Ludington Pumped Storage, contains late Pleistocene and Holocene deposits with a remarkable set of glaciotectonic deformation features previously described by Larson et al. (2003). Ludington Ridge is mapped as part of the Port Huron moraine (Leverett and Taylor 1915). The bluff face stands 55-75 m above the level of Lake Michigan (177 m). The uppermost 3 m is a deposit of Holocene eolian sand containing 2 paleosols. The eolian sand also buried another paleosol developed directly on the glacial deposits. A cross-section at the central exposure of the glacial deposits consists of about 27 m of gravelly fluvial beds, fining downward into about 2 m of sandy shoreface deposits with numerous heavy mineral (magnetite/garnet) laminations, thence into about 16 m of sandy/silty lacustrine deposits, and ending in 2-3 m of massive to laminated bottomset clay about 6 m above the present level of Lake Michigan. The lowest 6 m of deposits are under colluvium with a few glimpses of coarse to medium sand below the clay. The above deposits are interpreted to be glaciodeltaic sediments of the Port Huron phase deposited in glacial Lake Chicago (195 m).

The glaciodeltaic deposits are deformed by ~20 fine-grained diapirs along 1.7 km of shoreline. Most of the diapirs are cored with diamicton and pierce the entire glaciodeltaic stratigraphy. The source bed of the diapirs is below Lake Michigan. The diapirs likely formed when the Port Huron phase glacier advanced over its own glaciodeltaic deposits and destabilized the underlying Lake Border phase diamicton. Where the spacing between the diapirs is low (<~100 m), the glaciodeltaic deposits are deformed into a syncline with the bottomset clay layer becoming a low permeability aquiclude. The synclines created miniature ground-water basins that focus ground water into springs and seeps at the low point of the syncline.