Tonto Group
The Tonto Group is a name for an assemblage of related sedimentary strata, collectively known by geologists as a Group, that comprises the basal sequence Paleozoic strata exposed in the sides of the Grand Canyon. As currently defined, the Tonto groups consists of the Sixtymile Formation, Tapeats Sandstone, Bright Angel Shale (or Formation), Muav Limestone (or Formation), and Frenchman Mountain Dolostone. Historically, it included only the Tapeats Sandstone, Bright Angel Shale, and Muav Limestone. Because these units are defined by lithology and three of them interfinger and intergrade laterally, they lack the simple layer cake geology as they are typically portrayed as having and geological mapping of them is complicated.
| Tonto Group | |
|---|---|
| Stratigraphic range: Cambrian, | |
Muav Limestone-(broader-based gray cliff supporting tall-reddish Redwall Limestone cliff) and Bright Angel Shale-(greenish & extensive slope-former), resting on Tapeats Sandstone-(short, dark vertical cliff on gorge rim) and the Tonto Platform, inner canyon, Granite Gorge (the two units are easily seen below the red-stained Redwall Limestone (~550 ft thick)) | |
| Type | Geologic group |
| Unit of | Sauk sequence |
| Sub-units | Frenchman Mountain Dolostone, Muav Limestone, Bright Angel Shale, Tapeats Sandstone, and Sixtymile Formation |
| Underlies | Redwall Limestone. (Locally underlies Temple Butte Formation that fills paleovalleys cut into unconformity separating Redwall Limestone from either Frenchman Mountain Dolostone or Muav Limestone. |
| Overlies | Vishnu Basement Rocks and Grand Canyon Supergroup |
| Thickness | 1,250 feet (380 m) |
| Lithology | |
| Primary | sandstone, conglomerate, siltstone, shale, limestone, and dolomite |
| Other | calcareous mudstone and glauconitic sandstone |
| Location | |
| Region | Arizona-(northern) and, Nevada-(southern) |
| Country | United States-(Southwestern United States) |
| Type section | |
| Named for | Tonto Creek or Tonto Basin, |
| Named by | G. K. Gilbert |
| Location | Backtail Canyon |
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