The Oronto Group is a thick group of

arkose Arkose () or arkosic sandstone is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose. Quartz is c ...

sandstone and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

located beneath the Bayfield Group in northern

Wisconsin Wisconsin () is a state in the upper Midwestern United States. Wisconsin is the 25th-largest state by total area and the 20th-most populous. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake M ...

,Bulletin, p. 48. along the northwestern side of the

Keweenaw Peninsula The Keweenaw Peninsula ( , sometimes locally ) is the northernmost part of Michigan's Upper Peninsula. It projects into Lake Superior and was the site of the first copper boom in the United States, leading to its moniker of "Copper Country." As o ...

and southern Isle Royale, and believed to extend into

Minnesota Minnesota () is a state in the upper midwestern region of the United States. It is the 12th largest U.S. state in area and the 22nd most populous, with over 5.75 million residents. Minnesota is home to western prairies, now given over to ...

. Because it is almost entirely red in color and highly tilted, it is considered separate from the Bayfield Group. The Oronto Group is divided into the

Copper Harbor Conglomerate The Copper Harbor Conglomerate is a geologic formation in Michigan. It is part of the larger Oronto Group and its formation dates to the Stenian period of the Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2 ...

, Freda Sandstone, and

Nonesuch Shale The Nonesuch Shale is a Proterozoic geologic formation that outcrops in Michigan and Wisconsin, United States, but has been found by drill holes to extend in the subsurface as far southwest as Iowa. The Nonesuch is a lacustrine sequence of sha ...

.Scott W. Imbus and others, "Organic geochemistry and sedimentology of middle Proterozoic Nonesuch Formation - hydrocarbon source rock assessment of a lacustrine rift deposit," in ''Lacustrine Basin Exploration'', Tulsa, Okla.: American Association of Petroleum Geologists, p.197-208 As the group is devoid of fossils, its age is difficult to ascertain,Bulletin, p. 100. though it may be of Keweenawan Age (about 1100

Mya Mya may refer to: Brands and product names * Mya (program), an intelligent personal assistant created by Motorola * Mya (TV channel), an Italian Television channel * Midwest Young Artists, a comprehensive youth music program Codes * Burmese ...

).Bulletin, p. 105. The group is composed of

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** Co ...

, sandstone and shale. It is typically red, with bands, streaks, and spots of greenish-white no more than a few inches thick.Bulletin, p. 49. The total thickness of the group is unknown, but may be as much as deep.Bulletin, p. 50. The group is a member of the Keweenawan Supergroup. The Oronto Group has a higher proportion of undecomposed minerals,

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...

s, ferromagnesian compounds,

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...

and calcium carbonate than the Bayfield Group.

Notes

References

* Geologic groups of Michigan Geologic groups of Minnesota Geologic groups of Wisconsin {{US-geologic-formation-stub