Geologic units in Gogebic county, Michigan

Additional scientific data in this geographic area

Puritan Quartz Monzonite (Late Archean) at surface, covers 15 % of this area

Pink to pinkish-gray, medium- to coarse-grained, equigranular to inequigranular granite to granodiorite in Puritan batholith south of Gogebic Range. Age 2710 +/- 140 Ma (Sims and others, 1977)

Jacobsville Sandstone (Middle Proterozoic) at surface, covers 12 % of this area

Red, brown, and white quartzose sandstone, and minor siltstone, shale and conglomerate

Migmatitic gneiss and amphibolite (Late to Early Archean) (Archean) at surface, covers 11 % 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, undivided (Early Proterozoic) at surface, covers 11 % 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

Portage Lake Volcanics (Middle Proterozoic) at surface, covers 8 % of this area

Lava flows, mostly basalt, andesite and felsite flows and subordinate interflow sedimentary rocks.

Porcupine Volcanics (Middle Proterozoic) at surface, covers 6 % of this area

Generally dark-gray basalt, andesite, and felsite flows and subordinate interflow sedimentary rocks

Menominee Group; Blair Creek Formation (Early Proterozoic) at surface, covers 6 % of this area

Dominantly dark-gray, massive, porphyritic tholeiitic basalt. Includes a basal conglomerate and a lean iron-formation in middle of formation

Gneiss and amphibolite (Late Archean) at surface, covers 5 % of this area

Interlayered quartzofeldspathic gneiss and amphibolite in Marenisco, Mich., area and northwestern Wisconsin. Protoliths are bimodal intermediate and mafic volcanic rocks (Sims and others, 1984).

Copper Harbor Conglomerate (Middle Proterozoic) at surface, covers 5 % of this area

Red lithic conglomerate and sandstone; mafic to felsic volcanic flows similar to those of the unnamed formation (unit Yu) are interlayered with the sedimentary rocks.

Menominee Group; unexposed magnetic unit (Early Proterozoic) at surface, covers 4 % of this area

Surrounds gneiss domes in Gogebic County, Mich. and Vilas County, Wisc., and occurs in north-central Dickinson County. Probably correlative with part of Blair Creek Formation (unit Xbc). Previously correlated with uppermost unit of Blair Creek Formation (Sims and others, 1984).

Metabasalt (Late Archean) at surface, covers 3 % of this area

Derived from mafic to intermediate pyroclastic rocks and massive to pillowed lava flows. Unit mapped as Ramsey Formation by Prinz (1981) south of Gogebic Range; mapped as Mona Schist and Kitchi Schist in northern complex of Marquette District.

Powder Mill Group; Siemens Creek Volcanics (Middle Proterozoic) at surface, covers 2 % of this area

Dark-gray basalt and minor porphyritic andesite. Generally strongly magnetic with reversed remanent magnetism. Underlain by a thin unit of quartzose sandstone (Bessemer Quartzite).

Baraga Group; Copps Formation (Early Proterozoic) at surface, covers 2 % of this area

Fine- to medium-grained graywacke and less abundant gray to black slate. Thin basal conglomerate west of Lake Gogebic contains clasts of underlying Archean rocks in a quartzitic and argillaceous matrix.

Oronto Group; Freda Sandstone (Middle Proterozoic) at surface, covers 2 % of this area

Mainly reddish-brown, medium- to fine-grained lithic arkosic sandstone, siltstone, and micaceous silty shale.

Menominee Group; Ironwood Iron-formation (Early Proterozoic) at surface, covers 2 % of this area

Interbedded cherty and slaty iron-formation. Locally, secondary enrichment has formed "soft ore" bodies. In eastern part of Gogebic Range, Ironwood Iron-formation is intercalated with pyroclastic rocks of Emperor Volcanic Complex, and in eastern half of T. 47 N., R. 43 W. (where mapped within unit Xip) consists entirely of black slate

Metagraywacke (Late Archean) at surface, covers 2 % of this area

Gray, fine-grained, thin- to thick-bedded, commonly graded graywacke and slate in Marenisco, Mich. area

Powder Mill Group; Kallander Creek Volcanics (Middle Proterozoic) at surface, covers 1 % of this area

Basalt, andesite and lesser rhyolite flows. Basalt flows near base of the formation contain plagioclase phenocrysts, some in radiating clusters

Biotite schist (Late Archean) at surface, covers 0.9 % of this area

Quartz-plagioclase-biotite schist of probably intermediate volcanic protolith in area south of Gogebic Range

Baraga Group; Tyler Formation (Early Proterozoic) at surface, covers 0.9 % of this area

Light- to dark-gray, feldspathic, fine-grained sandstone, argillaceous siltstone, and argillite. Near base, ferruginous argillite contains beds of cherty sideritic and pyritic iron-formation

Oronto Group; Nonesuch Shale (Middle Proterozoic) at surface, covers 0.8 % of this area

Gray, green, and brown lithic siltstone, shale, and sandstone. Copper sulfides and native copper occur locally near base

Menominee Group; Emperor Volcanic Complex (Early Proterozoic) at surface, covers 0.3 % of this area

Metamorphosed intermediate to mafic pyroclastic and flow rocks in easternmost part of Gogebic Range

Porcupine Volcanics; Rhyolite (Middle Proterozoic) at surface, covers 0.2 % of this area

Porcupine Volcanics; Rhyolite

Baraga Group; Michigamme Formation; Mafic to intermediate flows and pyroclastic rocks, including Clarksburg Volcanics member (Early Proterozoic) at surface, covers 0.2 % 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

Powder Mill Group; Kallander Creek Volcanics; Upper rhyolite member (Middle Proterozoic) at surface, covers < 0.1 % of this area

Powder Mill Group; Kallander Creek Volcanics; Upper rhyolite member

Puritan Quartz Monzonite (Late Archean) at surface, covers < 0.1 % of this area

Pink to pinkish-gray, medium- to coarse-grained, equigranular to inequigranular granite to granodiorite in Puritan batholith south of Gogebic Range. Age 2710 +/- 140 Ma (Sims and others, 1977)

Copper Harbor Conglomerate; Volcanic member (Early Proterozoic) at surface, covers < 0.1 % of this area

Basaltic to andesitic volcanic flows interbedded within the conglomerate and sandstone

Jacobsville Sandstone (Middle Proterozoic) at surface, covers < 0.1 % of this area

Red, brown, and white quartzose sandstone, and minor siltstone, shale and conglomerate

Portage Lake Volcanics (Middle Proterozoic) at surface, covers < 0.1 % of this area

Lava flows, mostly basalt, andesite and felsite flows and subordinate interflow sedimentary rocks.

Oronto Group; Freda Sandstone; Conglomerate member (Middle Proterozoic) at surface, covers < 0.1 % of this area

Oronto Group; Freda Sandstone; Conglomerate member

Porcupine Volcanics (Middle Proterozoic) at surface, covers < 0.1 % of this area

Generally dark-gray basalt, andesite, and felsite flows and subordinate interflow sedimentary rocks

Powder Mill Group; Kallander Creek Volcanics (Middle Proterozoic) at surface, covers < 0.1 % of this area

Basalt, andesite and lesser rhyolite flows. Basalt flows near base of the formation contain plagioclase phenocrysts, some in radiating clusters

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

Metabasalt (Late Archean) at surface, covers < 0.1 % of this area

Derived from mafic to intermediate pyroclastic rocks and massive to pillowed lava flows. Unit mapped as Ramsey Formation by Prinz (1981) south of Gogebic Range; mapped as Mona Schist and Kitchi Schist in northern complex of Marquette District.

Oronto Group; Freda Sandstone (Middle Proterozoic) at surface, covers < 0.1 % of this area

Mainly reddish-brown, medium- to fine-grained lithic arkosic sandstone, siltstone, and micaceous silty shale.

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

Light- to dark-gray, feldspathic, fine-grained sandstone, argillaceous siltstone, and argillite. Near base, ferruginous argillite contains beds of cherty sideritic and pyritic iron-formation

Powder Mill Group; Kallander Creek Volcanics; Upper rhyolite member (Middle Proterozoic) at surface, covers < 0.1 % of this area

Powder Mill Group; Kallander Creek Volcanics; Upper rhyolite member

Menominee Group; Ironwood Iron-formation (Early Proterozoic) at surface, covers < 0.1 % of this area

Interbedded cherty and slaty iron-formation. Locally, secondary enrichment has formed "soft ore" bodies. In eastern part of Gogebic Range, Ironwood Iron-formation is intercalated with pyroclastic rocks of Emperor Volcanic Complex, and in eastern half of T. 47 N., R. 43 W. (where mapped within unit Xip) consists entirely of black slate

Powder Mill Group; Siemens Creek Volcanics (Middle Proterozoic) at surface, covers < 0.1 % of this area

Dark-gray basalt and minor porphyritic andesite. Generally strongly magnetic with reversed remanent magnetism. Underlain by a thin unit of quartzose sandstone (Bessemer Quartzite).

Menominee Group; unexposed magnetic unit (Early Proterozoic) at surface, covers < 0.1 % of this area

Surrounds gneiss domes in Gogebic County, Mich. and Vilas County, Wisc., and occurs in north-central Dickinson County. Probably correlative with part of Blair Creek Formation (unit Xbc). Previously correlated with uppermost unit of Blair Creek Formation (Sims and others, 1984).