Geologic units in Ontonagon county, Michigan

Additional scientific data in this geographic area

Jacobsville Sandstone (Middle Proterozoic) at surface, covers 39 % of this area

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

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.

Portage Lake Volcanics (Middle Proterozoic) at surface, covers 9 % 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

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.

Baraga Group; Michigamme Formation, undivided (Early Proterozoic) at surface, covers 5 % 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

Porcupine Volcanics; Rhyolite (Middle Proterozoic) at surface, covers 4 % of this area

Porcupine Volcanics; Rhyolite

Oronto Group; Nonesuch Shale (Middle Proterozoic) at surface, covers 1 % of this area

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

Menominee Group; Blair Creek Formation (Early Proterozoic) at surface, covers 1 % of this area

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

Powder Mill Group; Siemens Creek Volcanics (Middle Proterozoic) at surface, covers 0.6 % 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).

Copper Harbor Conglomerate; Volcanic member (Early Proterozoic) at surface, covers 0.6 % of this area

Basaltic to andesitic volcanic flows interbedded within the conglomerate and sandstone

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

Portage Lake Volcanics; Rhyolite at Bergland

Gabbroic rocks of Keweenawan affinity (Middle Proterozoic) at surface, covers < 0.1 % of this area

Gabbro, anorthosite, granite, peridotite

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

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