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

TypeGeological formation
Unit ofTonto Group
UnderliesTemple Butte Formation in Frenchmen Mountain to western Grand Canyon. Redwall Limestone and locally Temple Butte Formation in central Grand Canyon.
OverliesMuav Limestone
Thicknessup to 1,217 feet (371 m)
Lithology
Primarydolomite
Othershale
Location
RegionNorthern Arizona (Grand Canyon) and southern Nevada.
CountryUnited States of America
Type section
Named forFrenchman Mountain, Nevada
Named byV. S. Korolev and K. E. Karlstrom, S. M. Rowland and others
LocationFrenchman Mountain, Nevada
Year defined2023
Coordinates36.1915°N 115.0076°W / 36.1915; -115.0076
Regionsouthern Nevada
CountryUnited States of America
Thickness at type section1,217 feet (371 m)
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