GRIKE (KLUFTKARREN) DEVELOPMENT IN THE HIAWATHA NATIONAL FOREST, UPPER PENINSULA, MICHIGAN

The St. Ignace District of the Hiawatha National Forest is host to extensive grike fields, the formational history and timing of which has been little researched and is, as such, not well understood. These grikes are present throughout the region where the Engadine Group, a middle Silurian dolostone composed of the Rockview, Rapson Creek, and Bush Bay Formations, outcrops. Grikes (Kluftkarren) are linear dissolutionally enlarged joints, usually forming within exposed carbonate bedrock. The mechanisms and timing of the formation of the grike fields and other local karst features (primarily relict littoral caves and boulder fields) are all bound tightly by the Upper Peninsulas glacial history, and as such, understanding these grike fields will play a major role in understanding the area as a whole.

Grike morphology and orientation data was collected during both the summer of 2015 and the summer of 2016. This data included width, depth, length, and azimuth measurements. This data was used to calculate the apparent dissolution rate of the grikes, assuming a 0 mm starting width and that development began after glacial retreat and emergence from glacial lakes Algonquin and Nipissing at 11ka and 4ka, respectively. The results, when compared with the theoretical dissolution rates for the given lithology and climate, showed a large disconnect between the two values. Observed dissolution rates were calculated to be between 3 and 25 mm / 1000 years, whereas theoretical rates were determined to be just 0.0177 mm / 1000 years. Grikes that were contained within the Rockview Formation appear to have a slower dissolution rate (3-12 mm / 1000 years) than the grikes contained within the Bush Bay Formation (12-25 mm / 1000 years).

Several factors have been determined as possible explanations for these discrepancies; 1) carbon loading from organic debris increasing aggressiveness, 2) freshwater-freshwater mixing zones affecting dissolution efficiency, 3) non-zero initial grike widths due to the effects of glacial loading/unloading on regional joint sets or grike development before/during glaciation, and 4) lithological heterogeneity modifying dissolution rates.