Stratigraphic range: Early to Middle Cambrian 544–505 Ma
Muav Limestone-(broader-base 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))
|Unit of||Sauk sequence|
|Sub-units||3)-Muav Limestone, 2)-Bright Angel Shale, 1)-Tapeats Sandstone|
|Underlies||Redwall Limestone. Locally overlain by Temple Butte Limestone that fills paleovalleys cut into unconformity separating Redwall Limestone from Muav Limestone.|
|Overlies||Vishnu Basement Rocks, and 8)-Sixtymile Formation, 7)-Chuar Group, 6)-Nankoweap Formation, 5–1)-Unkar Group,
(or Vishnu Basement Rocks and members of the Grand Canyon Supergroup, units 8-1)
|Thickness||1,250 feet (380 m)|
|Primary||sandstone, conglomerate, siltstone, shale, limestone, and dolomite|
|Other||calcareous mudstone and glauconitic sandstone|
|Region||northern Arizona and southern Nevada|
|Country||United States of America|
|Named for||Tonto Creek|
|Named by||Gilbert (1875), (Walcott, 1883), and (Noble, 1914)|
The Cambrian Tonto Group is the three-member sequence of geologic formations that represent the basal section of Paleozoic rocks in the Grand Canyon. The group is about 1,250 feet (381 m) thick. The base unit, the Cambrian Tapeats Sandstone was deposited upon the erosion surface of the Vishnu Basement Rocks, which is found in Granite Gorge (the Inner Gorge). The erosion resistant Tapeats Sandstone forms the platform, called Tonto Platform, that the two less erosion resistant upper layers, the Bright Angel Shale and Muav Limestone, rest on.
The Tonto Trail is a mostly horizontal trail on the south side of Granite Gorge. The horizontal Tonto Group units are laid upon the Vishnu Basement Rocks above an angular unconformity as the Vishnu Basement Rocks have a dip of about 15 degrees. This erosion unconformity prior to the deposition of the Tapeats upon the tilted Vishnu Basement Rocks is about 1,000 million years (1.0 billion years), and is called the Great Unconformity.
The units of the Tonto Group: (~544-505 Mya)
View of Tower of Set peak-(and sub-unit cliff section, attached at SE), from Tonto Trail, Granite Gorge, north of Mohave Point, Grand Canyon Village, South Rim.
The peak is behind and separated from a cliff unit (with small prominence), in front-(photo center, right, Tower of Set (peak) to its left).
Vertical erosion in cliff of Redwall Limestone, upon horizontal-layered Muav Limestne cliff, (bottom 25 percent of cliff). The Tapeats Sandstone sits in foreground on Granite Gorge, (75 ft thick?), and is seen as thinly-bedded, in a multi-layered sequence of small beds. The slope-former above is the Bright Angel Shale, and is easily seen with thin, inter-bedding, as well as one resistant cliff unit.
The Redwall Limestone cliff section in Grand Canyon is about 450 feet (137 m) thick.
(high resolution photo)
|Wikimedia Commons has media related to Tonto Group.|
- Gilbert, GK (1875) Report upon the geology of portions of Nevada, Utah, California, and Arizona, Chapter 6. In GM Wheeler, ed., pp. 17-187, Report on the Geographical and Geological Explorations and Surveys West of the One Hundredth Meridian, vol. 3. U.S. Geological and Geographical Survey, Publication of the Wheeler Survey, Washington, D.C., 681 pp.
- Walcott, CD (1883) Pre-Carboniferous strata in the Grand Canyon of the Colorado, Arizona. American Journal of Science, 3d ser., vol. 26, pp. 437-442,484.
- Noble, LF (1914) The Shinumo quadrangle, Grand Canyon district, Arizona. Bulletin no. 549, US Geological Survey, Reston, Virginia.
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- Arizona Road & Recreation Atlas, Benchmark Maps, 2nd Edition, c. 1998, 112 pages, pp. 28–29, 62-63.
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