| Oronto Group | |
| Type: | Group |
| Age: | Keweenawan |
| Region: | |
| Unitof: | Keweenawan Supergroup |
| Subunits: | Copper Harbor Conglomerate Freda Sandstone Nonesuch Shale |
| Underlies: | Bayfield Group |
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. 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.[3] As the group is devoid of fossils, its age is difficult to ascertain,[4] though it may be of Keweenawan Age (about 1100 Mya).[5]
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.[6] The total thickness of the group is unknown, but may be as much as 21000feet 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.[6]
