| 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]
