Oronto Group | |
---|---|
Stratigraphic range: Keweenawan | |
Type | Group |
Unit of | Keweenawan Supergroup |
Sub-units | Copper Harbor Conglomerate Freda Sandstone Nonesuch Shale |
Underlies | Bayfield 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]