FORMATION OF SURFICIAL KARST FEATURES IN THE HIAWATHA NATIONAL FOREST, UPPER PENINSULA, MICHIGAN

The Hiawatha National Forest of Michigan’s Upper Peninsula contains the Engadine Group, a middle Silurian dolostone unit which comprises the northern edge of the Michigan Basin and outcrops in places as the Niagara Escarpment. The Engadine Group is in part characterized by an assortment of surficial karst features, which have been scarcely researched and are poorly understood in regard to both timing and formational mechanism.

Field data collected during the summer of 2015 suggests that the regional surface karst of the Hiawatha National Forest is linked directly with the evolution of proglacial Lakes Algonquin and Nippising. Relic littoral caves were identified and mapped along the Niagara escarpment at the former water level of Lake Algonquin (~250-260m amsl). The littoral caves are found oriented along joints and would have formed within a few thousand years as Lake Algonquin was a short lived proglacial lake.

Boulder fields are found throughout the Hiawatha National Forest at both the Lake Algonquin and Lake Nippising levels. Boulder fields are formed as a result of coastal processes exposing, destroying and reworking regional outcrops of the Engadine Group, implying that boulder fields and their associated outcroppings represent some of the former rocky coasts of the proglacial lakes.

Finally, extensive grike fields are found around the Hiawatha National Forest. Grikes follow the regional joint sets and their formation most likely occurred after the retreat of the glacial lakes as any preexisting grikes should have been plucked away during the glaciation. Field data for three different grike fields suggests that these grikes are expanding at between 7 and 25 mm / 1000 years, which exceeds the theoretical maximum denudation rates of 0.0177 mm / 1000 years for this environment and lithology. Possible explanations for this difference could be due to the enlarged joints existing pre-deglaciation (unlikely); increased pCO₂ loading from accumulation of organic matter in the grike (likely); and finally, possible freshwater-freshwater mixing during the higher pro-glacial lake levels allowing for more accentuated dissolution (possible).