SURFICIAL KARST AND PSEUDOKARST FEATURES OF THE HIAWATHA NATIONAL FOREST, UPPER PENINSULA, MICHIGAN

Karstic features in Michigan’s Upper Peninsula are typically overlooked as small and insignificant; however, they are regionally extensive, have unexpected modes of formation, and are worthy of additional study. The regional stratigraphy of the southern half of the St. Ignace District of the Hiawatha National Forest contains the Manistique and Engadine Groups, middle Silurian dolostone units that comprise the northern edge of the Michigan Basin and outcrop as the Niagara Escarpment. Additionally, the Mackinac Breccia, a late Silurian unit, is exposed in the southernmost reaches of the forest. Surface karst in this area includes boulder fields, sinkholes, sinking streams, and pseudokarstic littoral caves.

Field data suggests that the regional surface karst of the Hiawatha National Forest is linked directly with the evolution of proglacial lakes Algonquin and Nippising. All mapped features correlate to these paleo-lake elevations. Relic littoral caves were identified and mapped along the Niagara Escarpment in the Manistique Group at the former water level of Lake Algonquin (250-260m amsl). Additionally, some littoral caves are found within the Mackinac Breccia that formed at Lake Nippising levels (185-195m amsl).

Boulder fields are found throughout the Hiawatha National Forest at both the Lake Algonquin and Lake Nippising levels. They are comprised of rocks and outcrops from the Engadine group. 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. Additionally, boulder fields are found at modern lake level within the Hiawatha National Forest along Lake Huron supporting this model of genesis.

Numerous sinkholes and sinking streams are also present within the Hiawatha National Forest. These features are all contained within the Engadine Group. Often times, many sinkholes are found in close proximity to each other. The sinks and springs are likely post glacial. Several sinks are draining deranged drainages, while others are swallowing entire streams. Only one spring is known in the area, however there are likely many more present along the waterways in the area.