| Tonto Group | |
|---|---|
| Stratigraphic range: Cambrian, [1] | |
 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[1][2] |
| 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[1][2] |
| Thickness | 380 m (1,250 ft) |
| 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,[3] |
| Named by | G. K. Gilbert[3] |
| Location | Backtail Canyon[4] |
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.[1][2] 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.[6][7]
- ^ a b c d Karlstrom, K.E., Mohr, M.T., Schmitz, M.D., Sundberg, F.A., Rowland, S.M., Blakey, R., Foster, J.R., Crossey, L.J., Dehler, C.M. and Hagadorn, J.W., 2020. Redefining the Tonto Group of Grand Canyon and recalibrating the Cambrian time scale. Geology, 48(5), pp. 425–430.
- ^ a b c Connors, T.B., Tweet, J.S., and Santucci, V.L., 2020. Stratigraphy of Grand Canyon National Park. In: Santucci, V.L., Tweet, J.S., ed., pp. 54–74, Grand Canyon National Park: Centennial Paleontological Resource Inventory (Non-sensitive Version) . Natural Resource Report NPS/GRCA/NRR—2020/2103. National Park Service, Fort Collins, Colorado, 603 pp.
- ^ a b Cite error: The named reference
Gilbert1875awas invoked but never defined (see the help page). - ^ Cite error: The named reference
Rose2011awas invoked but never defined (see the help page). - ^ Karlstrom, K., Crossey, L., Mathis, A., and Bowman, C., 2021. Telling time at Grand Canyon National Park: 2020 update. Natural Resource Report NPS/GRCA/NRR—2021/2246. National Park Service, Fort Collins, Colorado. 36 pp.
- ^ Beus, S.S. and Billingsley, G.H., 1989. Paleozoic strata of the Grand Canyon, Arizona. in Elston, D.P., Billingsley, G.H., and Young, R.A., ed., pp. 122–127, Geology of Grand Canyon, northern Arizona (with Colorado River guides): Lees Ferry to Pierce Ferry, Arizona. P. M. Hanshaw. Field trips for the 28th International Geological Congress. American Geophysical Union, Washington, D.C.
- ^ Huntoon, P.W., 1989. Cambrian Stratigraphic Nomenclature, Grand Canyon, Arizona‐Mappers Nightmare. in Elston, D.P., Billingsley, G.H., and Young, R.A., ed., pp. 128–130, Geology of Grand Canyon, northern Arizona (with Colorado River guides): Lees Ferry to Pierce Ferry, Arizona. P. M. Hanshaw. Field trips for the 28th International Geological Congress. American Geophysical Union, Washington, D.C.