Geologic units in Dickinson county, Michigan

Baraga Group; Michigamme Formation, undivided (Early Proterozoic) at surface, covers 31 % 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

Migmatitic gneiss and amphibolite (Late to Early Archean) (Archean) at surface, covers 29 % 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.

Munising Formation (Late Cambrian) at surface, covers 13 % of this area

Munising Formation

Dickinson Group, undivided (Late Archean) at surface, covers 8 % 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.

Trempealeau Formation (Late Cambrian) at surface, covers 7 % of this area

Trempealeau Formation

Menominee and Chocolay Groups, undivided (Early Proterozoic) at surface, covers 5 % of this area

Mapped where units are too thin to show separately on map

Chocolay Group, undivided (Early Proterozoic) at surface, covers 4 % 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

Menominee Group; Hemlock Formation (Early Proterozoic) at surface, covers 1.0 % of this area

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

Badwater Greenstone (Early Proterozoic) at surface, covers 1.0 % 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.

Mafic metavolcanic rocks (Early Proterozoic) at surface, covers 0.9 % 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.

Granitic rocks (Early Proterozoic) at surface, covers 0.6 % 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).

Metagabbro (Early Proterozoic) at surface, covers 0.2 % of this area

Mafic to ultramafic bodies defined and delineated principally by positive magnetic and gravity anomalies; includes diorite bodies. Comagmatic with unit Xmi

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.

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

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.

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.

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

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

Mafic to ultramafic bodies defined and delineated principally by positive magnetic and gravity anomalies; includes diorite bodies. Comagmatic with unit Xmi