DISSOLUTIONAL FEATURES OF THE HIAWATHA NATIONAL FOREST, UPPER PENINSULA, MICHIGAN

Michigan’s Upper Peninsula contains the Hiawatha National Forest (HNF) in which extensive dissolutional features exist; a subject of study for the last several years. The St. Ignace District of the HNF is home to vast grike fields, sink holes, springs and sinking streams. These features form in the Engadine group (Rockview, Rapson Creek, and Bush Bay Formations) and Manistique group (Schoolcraft and Cordell Formations), which are Silurian-aged dolostones present in outcrops of the area and form the Niagara Escarpment.

Several grike fields, found in the Bush Bay and Rockview Formations, have been catalogued. Data collected in previous years along with 2018 focused on the width and orientation of the grikes. These dissolutionally enlarged vertical joints ranged from a few cm’s across to over half a meter, and vary in depth from cm’s to m’s deep. The grikes are approximately oriented to the regional joint sets. The timing of formation of these grikes and the orientation of the regional joint sets is poorly understood. If they are pre-glacial a dissolution rate cannot be readily calculated, however, if they are post-glacial a dissolution rate of ~10-30 mm / 1000 yrs is expected.

The mapping of sink holes, springs and sinking streams provides a more detailed understanding of the hydrogeology of the area. Several sinkholes and sinking streams were located not far from grike fields and the sinkholes follow the grike field orientations; this suggests that the sinkholes and sinking streams are exploiting the underlying joint sets. Some of the sink holes drain glacially formed deranged drainage. Most of the known springs in the area discharge to the Carp River as base-level springs; there is one known alluviated spring in the area. These springs are contained within the Bush Bay, Rockview, and Cordell Formations.

One known exposure of solution pans was discovered in the Rapson Creek Formation. These dome shaped indentations in the rock occur from long periods of standing water dissolving the rock. This karren is post glacial, otherwise it would have been removed during glaciation. Detailed measuring of the length, width and depth of several solution pans allowed the volume of rock lost due to dissolution since glacial retreat to be calculated; a denudation rate of ~1.5mm / 1000yrs, assuming the only loss of rock is in the pans themselves.