Tonto Group

Tonto Group
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))
Type Geologic group
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)
Lithology
Primary sandstone, conglomerate, siltstone, shale, limestone, and dolomite
Other calcareous mudstone and glauconitic sandstone
Location
Region northern Arizona and southern Nevada
Country United States of America
Type section
Named for Tonto Creek
Named by Gilbert (1875),[1] (Walcott, 1883),[2] and (Noble, 1914)[3]
Grand Canyon stratigraphy.

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.[4] 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.

Geologic sequence

The units of the Tonto Group: (~544-505 Mya)

See also

References

Wikimedia Commons has media related to Tonto Group.
  1. ↑ 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.
  2. ↑ 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.
  3. ↑ Noble, LF (1914) The Shinumo quadrangle, Grand Canyon district, Arizona. Bulletin no. 549, US Geological Survey, Reston, Virginia.
  4. 1 2 Chronic, H (2001) Roadside Geology of Arizona. The Mountaineers Books, Seattle, Washington. (softcover, ISBN 978-0-87842-147-3) p. 179.
  5. ↑ Lucchitta, I. (2001) Hiking Arizona's Geology, Mountaineers's Books, Seattle, Washington. (softcover, ISBN 0-89886-730-4) pp. 62-68, Kaibab Trail, pp. 66-67, photo and Fig. 10, View to the northwest from the top of the Redwall Limestone along the (S.) Kaibab Trail. The sketch identifies the geologic units visible from here.
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