Geologic units in Iron county, Michigan

Baraga Group; Michigamme Formation, undivided (Early Proterozoic) at surface, covers 53 % of this area

Thick and stratigraphically varied formation of sedimentary and less abundant volcanic rocks. Metamorphosed graywacke is predominant rock of Michigamme Formation. Calcareous concretions common. Metamorphosed from cholorite to sillimanite grade (Cannon, 1986). Mapped undivided in poorly exposed areas

Menominee Group; Hemlock Formation (Early Proterozoic) at surface, covers 14 % of this area

Predominantly mafic to intermediate volcanic flows and pyroclastic rocks with interlayered slate and tuff beds

Paint River Group; Upper part (Early Proterozoic) at surface, covers 7 % of this area

Dominantly Fortune Lakes Slate, but includes other rocks in poorly exposed areas. Correlated with upper part of Baraga Group.

Paint River Group; Lower part (Early Proterozoic) at surface, covers 7 % of this area

Includes Stambaugh Formation, Hiawatha Graywacke, Riverton Iron-formation, and Dunn Creek Slate (James and others, 1968). Correlated with lower part of Baraga Group

Badwater Greenstone (Early Proterozoic) at surface, covers 5 % of this area

Dark-greenish-gray, pillowed to massive tholeiitic basalt and pyroclastic rocks. Correlated with the Hemlock Formation on basis of geology and similarity in chemical composition.

Migmatitic gneiss and amphibolite (Late to Early Archean) (Archean) at surface, covers 4 % of this area

Varied gneisses of mostly unknown age in cores of gneiss domes and fault-bounded uplifts (Archean gneiss terranes). Except for the Watersmeet dome (Late to Early Archean), all dated rocks are Late Archean. Includes granite of Late Archean age that transgresses gneisses and amphibolite.

Baraga Group; Michigamme Formation; Mafic to intermediate flows and pyroclastic rocks, including Clarksburg Volcanics member (Early Proterozoic) at surface, covers 3 % of this area

Poorly exposed. Includes Clarksburg Volcanics member which is mostly mafic to intermediate pyroclastic rocks in Marquette trough, between Marquette Bay and Champion

Chocolay Group, undivided (Early Proterozoic) at surface, covers 2 % of this area

In Dickinson County, Mich., includes Randville Dolomite, Sturgeon Quartzite, and Fern Creek Formation. In Marquette range, between Marquette Bay and Champion, includes Wewe Slate, Kona Dolomite, Mesnard Quartzite, and Enchantment Lake Formation

Metadiabase (Early Proterozoic) at surface, covers 2 % of this area

Variably metamorphosed dikes, sills and subconcordant sheets of diabasic rocks. Probably coeval (in part) with volcanic rocks of Hemlock Formation (unit Xh)

Dickinson Group, undivided (Late Archean) at surface, covers 1 % of this area

Metavolcanic and metasedimentary rocks in Dickinson County, Mich. From youngest to oldest, consists of Six-Mile Lake Amphibolite, Solberg Schist, and East Branch Arkose.

Peavy Pond complex (Early Proterozoic) at surface, covers 0.5 % of this area

Chiefly hornblende metagabbro, but includes metanorite, metatonalite, granodiorite, and granite (Bayley, 1959).

Mafic metavolcanic rocks (Early Proterozoic) at surface, covers 0.4 % of this area

Dominantly tholeiitic basalt and basaltic andesite flows and tuffs; associated with sheet dikes, massive and layered metagabbro, and ultramafic rocks. In northeastern Wisconsin, rocks have been named the Quinnesec Formation.

Baraga Group; Volcanic-sedimentary unit (Early Proterozoic) at surface, covers 0.2 % of this area

Interlayered fine- to medium-grained metasedimentary rocks and metavolcanic rocks in poorly exposed areas in southern Baraga County, Mich.

Menominee and Chocolay Groups, undivided (Early Proterozoic) at surface, covers < 0.1 % of this area

Mapped where units are too thin to show separately on map

Menominee Group, undivided (Early Proterozoic) at surface, covers < 0.1 % of this area

Includes Negaunee Iron-formation, Siamo Slate, and Ajibik Quartzite in areas where units are too thin to map individually.

Granitic rocks (Early Proterozoic) at surface, covers < 0.1 % of this area

Gray to pinkish-gray, mottled, medium-grained syenite, granite, and granodiorite. Moderately strong propylitic alteration and weak cataclasis. Forms small bodies south of Crystal Falls, MI (James and others, 1968). A body near Tobin Location has a U-Pb concordia intercept age of 1840 +/-5 Ma (Z.E. Peterman, written communication, 1988). Includes porphyritic red granite that intrudes Lake Archean Dickinson Group - Red mylonitic gneissic granite (about 1970 Ma).

Mafic metavolcanic rocks (Early Proterozoic) at surface, covers < 0.1 % of this area

Dominantly tholeiitic basalt and basaltic andesite flows and tuffs; associated with sheet dikes, massive and layered metagabbro, and ultramafic rocks. In northeastern Wisconsin, rocks have been named the Quinnesec Formation.

Paint River Group; Upper part (Early Proterozoic) at surface, covers < 0.1 % of this area

Dominantly Fortune Lakes Slate, but includes other rocks in poorly exposed areas. Correlated with upper part of Baraga Group.

Chocolay Group, undivided (Early Proterozoic) at surface, covers < 0.1 % of this area

In Dickinson County, Mich., includes Randville Dolomite, Sturgeon Quartzite, and Fern Creek Formation. In Marquette range, between Marquette Bay and Champion, includes Wewe Slate, Kona Dolomite, Mesnard Quartzite, and Enchantment Lake Formation

Badwater Greenstone (Early Proterozoic) at surface, covers < 0.1 % of this area

Dark-greenish-gray, pillowed to massive tholeiitic basalt and pyroclastic rocks. Correlated with the Hemlock Formation on basis of geology and similarity in chemical composition.

Baraga Group; Michigamme Formation, undivided (Early Proterozoic) at surface, covers < 0.1 % of this area

Thick and stratigraphically varied formation of sedimentary and less abundant volcanic rocks. Metamorphosed graywacke is predominant rock of Michigamme Formation. Calcareous concretions common. Metamorphosed from cholorite to sillimanite grade (Cannon, 1986). Mapped undivided in poorly exposed areas

Paint River Group; Lower part (Early Proterozoic) at surface, covers < 0.1 % of this area

Includes Stambaugh Formation, Hiawatha Graywacke, Riverton Iron-formation, and Dunn Creek Slate (James and others, 1968). Correlated with lower part of Baraga Group