Geologic units in Bayfield county, Wisconsin

Bayfield Group; Chequamegon Sandstone (Middle Proterozoic) at surface, covers 31 % of this area

Poorly sorted arkosic sandstone

Oronto Group; Freda Sandstone (Middle Proterozoic) at surface, covers 27 % of this area

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

Chengwatana Volcanic Group (Middle Proterozoic) at surface, covers 15 % of this area

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

Gneiss and amphibolite (Late Archean) at surface, covers 6 % 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; Orienta Sandstone (Middle Proterozoic) at surface, covers 6 % of this area

Poorly sorted arkosic sandstone

Copper Harbor Conglomerate (Middle Proterozoic) at surface, covers 6 % 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.

Bayfield Group; Devil's Island Sandstone (Middle Proterozoic) at surface, covers 2 % of this area

Well-sorted quartzose sandstone

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

Gabbroic rocks of Keweenawan affinity (Middle Proterozoic) at surface, covers 1 % of this area

gabbro, anorthosite, granite, peridotite

Migmatitic gneiss and amphibolite (Late to Early Archean) (Archean) at surface, covers 1 % 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.

Ironwood Iron-formation of Menominee Group and Palms Formation of Chocolay Group (Early Proterozoic) at surface, covers 0.6 % of this area

Mapped where units are too thin to show separately on map (Gogebic Range). Palms Formation includes Bad River Dolomite and Sunday Quartzite of Chocolay Group

Oronto Group; Nonesuch Shale (Middle Proterozoic) at surface, covers 0.6 % of this area

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

Granophyre (Middle Proterozoic) at surface, covers 0.5 % of this area

Granophyre

Powder Mill Group; Kallander Creek Volcanics; Upper rhyolite member (Middle Proterozoic) at surface, covers 0.4 % of this area

Powder Mill Group; Kallander Creek Volcanics; Upper rhyolite member

Metadiabase (Early Proterozoic) at surface, covers 0.2 % 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).

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

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

Mellen Intrusive complex; Olivine gabbro (Middle Proterozoic) at surface, covers < 0.1 % of this area

Mellen Intrusive complex; Olivine gabbro

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

Oronto Group; Freda Sandstone; Conglomerate member