Frenchman Mountain Dolostone
The Frenchman Mountain Dolostone is the uppermost and youngest of five Cambrian geologic formations that comprise the Tonto Group. It consists of beds of mottled white to gray dolomite often separated by thin seams of shale, especially in its lower part. In the Grand Canyon, this formation forms vertical cliffs that thicken westward between the top of the Muav Limestone and the base of either the Devonian Temple Butte Formation or Mississippian Redwall Limestone. Because of unidentified trace fossils and lack of datable body fossils, the Frenchman Mountain Dolostone exact age is uncertain. Within the Grand Canyon, its thickness varies between 200 and 450 feet (61 and 137 m). West into the Lake Mead region, it thickens considerably and is 1,217 feet (371 m) thick at Frenchman Mountain near Las Vegas, Nevada.
| Frenchman Mountain Dolostone | |
|---|---|
| Stratigraphic range: Middle Cambrian | |
| Type | Geological formation |
| Unit of | Tonto Group |
| Underlies | Temple Butte Formation in Frenchmen Mountain to western Grand Canyon. Redwall Limestone and locally Temple Butte Formation in central Grand Canyon. |
| Overlies | Muav Limestone |
| Thickness | up to 1,217 feet (371 m) |
| Lithology | |
| Primary | dolomite |
| Other | shale |
| Location | |
| Region | Northern Arizona (Grand Canyon) and southern Nevada. |
| Country | United States of America |
| Type section | |
| Named for | Frenchman Mountain, Nevada |
| Named by | V. S. Korolev and K. E. Karlstrom, S. M. Rowland and others |
| Location | Frenchman Mountain, Nevada |
| Year defined | 2023 |
| Coordinates | 36.1915°N 115.0076°W |
| Region | southern Nevada |
| Country | United States of America |
| Thickness at type section | 1,217 feet (371 m) |