RELATIONSHIP OF BEDROCK GEOLOGY AND KARST FEATURES IN THE ST. IGNACE DISTRICT OF THE HIAWATHA NATIONAL FOREST, UP MICHIGAN

The investigation of karst features in the St. Ignace District of the Hiawatha National Forest (HNF) has been the focus of several seasons of fieldwork. Once major karst features were identified, the question of relationship to bedrock geology was addressed. Detailed stratigraphic work revealed the nature of the carbonate bedrock. Two major groups of Middle Silurian strata are present in the study area: the Manistique Group and the Engadine Group. The Cordell Formation of the Manistique Group, composed of cherty dolostone, is the oldest exposed carbonate unit in the study area. Above the Manistique Group are the Rockview, Rapson Creek, and Bush Bay Formations, respectively of the Engadine Group. The formations of the Engadine Group are generally chert-limited or chert-lacking dolostones. Jointing associated with isostatic unloading appears to be the major structural feature in the study area, and dips of the bedrock geology average around 1-2 degrees towards the south. The average topographic gradient of the area is approximately 100 feet/mile which results in deranged drainage for the majority of the area. Local deviations in gradient from the average represent major topographic features, primarily the Niagara Escarpment.

All bedrock units host various karst features (e.g. sinks, springs, sinkholes, grikes, alvar, boulder fields); however, major sinks, springs, and sinkholes appear to be restricted to transition from the Cordell to Rockview Formations and in the Bush Bay Formation. Major grike fields, representing enlargement of the jointing in the bedrock, are present in all formations in the study area. Boulder fields, which in the study area are interpreted as being paleolake shores, are also found in all formations and are suspected to have a dissolutional component to their formation. The most impressive karst features in the study area are Biscuit Sink and Biscuit Spring. These two hydrologic features appear to be the result of chert beds in the Cordell Formation behaving as a hydrologic barrier, thus promoting water movement up-dip (north) along grikes. Overall, differences in bedrock geology has little effect on surifical karst formation in the study area, with the notable exception being the chert-bedded portions of the Cordell Formation acting as a hydrologic barrier.