The Oronto Group is a thick

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

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. Components ...

sandstone and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

located beneath the Bayfield Group in northern

Wisconsin Wisconsin ( ) is a U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest of the United States. It borders Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michig ...

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

Keweenaw Peninsula The Keweenaw Peninsula (, ) is a peninsula of the U.S. state of Michigan. Part of the greater landmass of the Upper Peninsula of Michigan, Upper Peninsula, the Keweenaw Peninsula projects about northeasterly into Lake Superior, forming Keweena ...

and southern

Isle Royale Isle Royale (, ) is an Islands of the Great Lakes, island of the Great Lakes located in the northwest of Lake Superior and part of the U.S. state of Michigan. The island and the 450 surrounding smaller islands and waters make up Isle Royale Na ...

, and believed to extend into

Minnesota Minnesota ( ) is a U.S. state, state in the Upper Midwestern region of the United States. It is bordered by the Canadian provinces of Manitoba and Ontario to the north and east and by the U.S. states of Wisconsin to the east, Iowa to the so ...

. 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 the third of the four geologic eons of Earth's ...

, Freda Sandstone, and Nonesuch Shale.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).Bulletin, p. 105. The group is composed of conglomerate, 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 Feldspar ( ; sometimes spelled felspar) is 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 ''plagiocl ...

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

s, ferromagnesian compounds,

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

and

calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

than the Bayfield Group.

Notes

References

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