CHARACTERIZATION AND ANALYSIS OF MUDCRACKS OF THE MICHIGAN FORMATION, KENT COUNTY, MICHIGAN

Casts of shrinkage cracks are laterally continuous features exposed in underground tunnels in a former gypsum mine in western Michigan. Currently the mine is being used by the Michigan Natural Storage Company as a repository to store a variety of materials from various businesses, and to support educational geological investigations like this seminar project. The crack patterns are continuous and polygonal in shape and are identified as desiccation mudcracks rather than syneresis in origin. The mudcracks are commonly exposed in positive relief on the underside of the dolomite ceiling as are occasional wave ripples. The thin dolomite is underlain by approximately ½ meter of shale that overlies a thick interval of gypsum (~2.5 meters). This Mississippian aged sequence of gypsum-shale-dolomite is cyclical and repeats itself stratigraphically. The environment of deposition has been interpreted as a shallow marine basin that episodically developed hypersaline conditions that favored the precipitation of gypsum. Periodic influx of freshwater reduced salinities to more normal marine and resulted in deposition of mud. Continued shallowing and subaerial exposure of the mud is recorded by the mudcracks that are preserved as casts in the overlying dolomite. We are working to characterize the size, shape, and depth of mudcracks in the mine through samples we collected and brought back to our laboratory in order to better assess the depositional conditions and setting. We are also interested in better defining the role of the dolomite in preservation of the mudcracks through thin section and XRD analysis.