The Michigamme Formation is the uppermost unit of the Marquette Range Supergroup, a thick succession of Paleoproterozoic metamorphosed sedimentary rocks, ranging from metaconglomerates to metaquartzites to metadolostones to slates to iron-formation. The Michigamme outcrops throughout a large synclinal structure in the vicinities of the towns of Marquette, Negaunee, Ishpeming, and Michigamme. This large syncline is called the Marquette Trough.
The Michigamme Fm. is moderately heterolithic, consisting of up to six members of varying lithologies in places. The upper slate member and lower slate member make up the bulk of the formation. They contain slates, phyllites, and finely crystalline schists. They represent metamorphosed shales and graywackes (dirty sandstones), generally interpreted as deep-water marine turbidites (Cannon & Klasner, 1977; Klasner & Cannon, 1978). The unit also has some iron-formation in three of its members. In places, the Michigamme has a member with metavolcanic rocks (Clarksburg Volcanics Member), including metatuffs, volcanic metabreccias, and lava flows altered to amphibolite.
Stratigraphy & Age: Michigamme Formation, upper Baraga Group, uppermost Marquette Range Supergroup, upper Paleoproterozoic, ~1.85-1.86 billion years.
Staurolite schist (5.6 cm across at its widest) from the Michigamme Formation at a roadcut along Cardinal Road, just S of the western end of Lake Michigamme, western UP of Michigan, USA (see map). This rock has crystals a bit too small to really qualify as a schist, but they're a bit too large to really qualify as a phyllite. The indentations at the top and a couple of the bumps in the lower part of the rock are where staurolite crystals have popped out.
Some info. from:
Cannon & Klasner (1977) - Bedrock geologic map of the Diorite and Champion 7½-minute quadrangles, Marquette County, Michigan. United States Geological Survey Miscellaneous Investigations Series Map I-1058.
Klasner & Cannon (1978) - Bedrock geologic map of the southern part of the Michigamme and Three Lakes quadrangles, Marquette and Baraga Counties, Michigan. United States Geological Survey Miscellaneous Investigations Series Map I-1078.