Geologic units in Sawyer county, Wisconsin

Additional scientific data in this geographic area

Mafic metavolcanic rocks (Early Proterozoic) at surface, covers 19 % 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.

Migmatitic gneiss and amphibolite (Late to Early Archean) (Archean) at surface, covers 18 % 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.

Granite-tonalite (Early Proterozoic) at surface, covers 16 % of this area

Gray to pinkish-gray, medium-grained, generally equigranular granite to tonalite and granitoid gneiss; locally includes diorite. Intrudes older metavolcanic rocks. Zircon ages range from 1852 +/- 15 to 1862 +/- 5 Ma

Cambrian, undivided (Cambrian) at surface, covers 15 % of this area

Sandstone with some dolomite and shale, undivided; includes Trempealeau, Tunnel City, and Elk Mound Formations

Biotite schist (Early Proterozoic) at surface, covers 15 % 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)

Barron Quartzite (Early Proterozoic) at surface, covers 8 % of this area

Pink to maroon to light-gray, medium-grained, moderately sorted quartzite. Red argillite (pipestone) interbedded locally. At least 213 m thick

Chengwatana Volcanic Group (Middle Proterozoic) at surface, covers 5 % of this area

Gray basalt flows, interflow breccia, tuff, and minor sedimentary rocks

Granitic rocks of 1760-Ma age group (Early Proterozoic) at surface, covers 3 % of this area

In northern Wisconsin, granite to granodiorite of varied texture. In southern Wisconsin (not mapped separately), red to pink alkali-feldspar granophyric granite associated with rhyolites of both peraluminous and metaluminous affinities (unit Xr of Marshfield terrane).

Gneiss and amphibolite (Late Archean) at surface, covers 1 % 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).

Gabbroic rocks of Keweenawan affinity (Middle Proterozoic) at surface, covers 0.4 % of this area

gabbro, anorthosite, granite, peridotite