Geologic units in Ashland county, Wisconsin

Additional scientific data in this geographic area

Migmatitic gneiss and amphibolite (Late to Early Archean) (Archean) at surface, covers 24 % 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.

Oronto Group; Freda Sandstone (Middle Proterozoic) at surface, covers 18 % of this area

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

Gneiss and amphibolite (Late Archean) at surface, covers 15 % 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).

Bayfield Group; Chequamegon Sandstone (Middle Proterozoic) at surface, covers 14 % of this area

Poorly sorted arkosic sandstone

Menominee Group; Blair Creek Formation (Early Proterozoic) at surface, covers 7 % of this area

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

Puritan Quartz Monzonite (Late Archean) at surface, covers 5 % 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)

Gabbroic rocks of Keweenawan affinity (Middle Proterozoic) at surface, covers 4 % of this area

gabbro, anorthosite, granite, peridotite

Oronto Group; Freda Sandstone; Conglomerate member (Middle Proterozoic) at surface, covers 3 % of this area

Oronto Group; Freda Sandstone; Conglomerate member

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

Metamorphosed graywacke in poorly exposed areas in northwestern Wisconsin. Sample at one locality (near Blockhouse Lake, 10 km northeast of Park Falls, T. 40 N., R. 1 E.) has a U-Pb zircon age of 1852 +/- 6 Ma (Sims and others, 1985b)

Mellen Intrusive Complex; Granite (Middle Proterozoic) at surface, covers 1 % of this area

Mellen Intrusive Complex; Granite (about 1000 Ma)

Metabasalt (Late Archean) at surface, covers 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.

Mellen Intrusive complex; Olivine gabbro (Middle Proterozoic) at surface, covers 0.8 % of this area

Mellen Intrusive complex; Olivine gabbro

Menominee Group; Ironwood Iron-formation (Early Proterozoic) at surface, covers 0.5 % 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

Granophyre (Middle Proterozoic) at surface, covers 0.2 % of this area

Granophyre

Powder Mill Group; Kallander Creek Volcanics; Upper rhyolite member (Middle Proterozoic) at surface, covers 0.2 % of this area

Powder Mill Group; Kallander Creek Volcanics; Upper rhyolite member

Copper Harbor Conglomerate (Middle Proterozoic) at surface, covers 0.1 % 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.

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; Nonesuch Shale (Middle Proterozoic) at surface, covers < 0.1 % of this area

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

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).

Bayfield Group; Devil's Island Sandstone (Middle Proterozoic) at surface, covers < 0.1 % of this area

Well-sorted quartzose sandstone