Oronto Group
Stratigraphic range: Keweenawan
TypeGroup
Unit ofKeweenawan Supergroup
Sub-unitsCopper Harbor Conglomerate
Freda Sandstone
Nonesuch Shale
UnderliesBayfield Group
Location
Region Michigan
 Wisconsin
Country United States

The Oronto Group is a thick group of arkose sandstone and shale located beneath the Bayfield Group in northern Wisconsin,[1] along the northwestern side of the Keweenaw Peninsula and southern Isle Royale,[2] and believed to extend into Minnesota.[3] Because it is almost entirely red in color and highly tilted, it is considered separate from the Bayfield Group.[1] The Oronto Group is divided into the Copper Harbor Conglomerate, Freda Sandstone, and Nonesuch Shale.[4] As the group is devoid of fossils, its age is difficult to ascertain,[5] though it may be of Keweenawan Age (about 1100 Mya).[6]

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.[3] The total thickness of the group is unknown, but may be as much as 21,000 ft (6,400 m) deep.[7] The group is a member of the Keweenawan Supergroup.[8]

The Oronto Group has a higher proportion of undecomposed minerals, feldspars, micas, ferromagnesian compounds, magnetite and calcium carbonate than the Bayfield Group.[3]

Notes

  1. 1 2 Bulletin, p. 48.
  2. ↑ Geologic Map of the Lake Superior Region: Minnesota, Wisconsin, and Northern Michigan (Map). 1 : 1,000,000. S-13. Minnesota Geological Survey. 1982. Retrieved June 20, 2012 – via WikiMedia Commons.
  3. 1 2 3 Bulletin, p. 49.
  4. ↑ 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
  5. ↑ Bulletin, p. 100.
  6. ↑ Bulletin, p. 105.
  7. ↑ Bulletin, p. 50.
  8. ↑ "Geologic Unit: Keweenawan". GEOLEX Database. United States Geological Survey. Retrieved June 20, 2012.

References


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