GRAND CANYON - Viewed from the South Rim
FIELD TRIP STOP – VIEWS INTO THE CANYON FROM THE SOUTH RIM
LOCATION: The South Rim of the Grand Canyon in North Central Arizona.
GEOLOGIC FEATURES: Law of Original Horizontality; Angular Unconformity; Non-conformity; Disconformity.
DESCRIPTION: Rocks of the Grand Canyon span those of the PreCambrian Vishnu Schist (1.7 bya) to the Permian Kaibab Limestone (245 mya). The detailed description and close-up photos of these rocks as well as their environments and sequence of formation are presented in the Field Trip locality described herein as Grand Canyon – Viewed from the River Level.
STUDENT QUESTIONS:
(1) Vertically list the Formations present in the Grand Canyon with oldest on the bottom and youngest on top. Include the Periods and Eras of geologic time represented by each.
(2) Describe the sequence of geologic events that took place in order to form the angular unconformity separating the PreCambrian sediments of the Grand Canyon Series from the Cambrian Tapeats Sandstone.
(3) Name two other unconformities viewed in the Canyon and state, in millions of years, the gaps in time that are missing between rock formations.
(4) Walls of the Grand Canyon have a characteristic shape of steep cliffs and gentle slopes. Explain why.
SELECTED REFERENCES:
-Harris, A. and E. Tuttle. 1983 (3rd ed.). Geology of the National Parks. Kendall/Hunt Publishing Company. Dubuque, IO. 554 pp.
-Harris, D.V. and E.P. Kiver. 1985 (4th ed.). The Geologic Story of the National Parks and Monuments, John Wiley and Sons, New York, 464 pp.
PHOTOS:
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Figure 1 - The Bright Angel Trail winds through the relatively gentle slope of the Bright Angel Shale and Muav Limestone (Cambrian) and continues above the steep vertical cliff of the Redwall Limestone (Mississippian) before continuing into the more gentle slopes of the Supai Fm. Note the absence of Ordovician, Silurian, and Devonian sediments in a Disconformity.
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Figure 2 - Viewed from the west end of the park (Between Mohave and Pima Points). On either side of the Colorado at river level is the PreCambrian Vishnu Schist. Directly above the Vishnu is The Cambrian Tapeats Sandstone ("The Great Unconformity," a Non-conformity). The top of the Tapeats forms a platform ("The Tonto Platform") upon which lies the gently sloping Cambrian Bright Angel and Muav Fms.
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Figure 3 - Viewed from Navajo Point at the east end of the park. Note the distinctive Angular Unconformity between the PreCambrian Grand Canyon Series below and the Cambrian Tapeats above ("The Great Unconformity").
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Figure 4 - Close-up of Angular Unconformity
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Figure 5 - Viewed between Grandview and Yaki Points. Note the distinctive white band near the Canyon rim representative of the Permian Coconino Sandstone, representing a desert sand dune environment. The Toroweap and Kaibab lie above the Coconino. In the foreground, the prominent cliff-former is the Mississippian Redwall Limestone. The base of the Redwall meets the gentle-sloping Cambrian sediments. The Supai lies above the Redwall, however, the younger Permian strata seen in the distance have been completely eroded away in the lower two-thirds of the photo. Note, too, the characteristic erosion of the Redwall in the shape of amphitheaters throughout.
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Figure 6 - View from Mather Point. The partial white slab balanced in the upper left appears to be a piece of the Coconino Sandstone (also seen as the white band in the background) resting on the reddish Hermit Shale.
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Figure 7 - Viewed from Mather Point. In the foreground we see the Supai resting atop the large vertical cliff of the Redwall Limestone. The greenish-gray gentle slopes are the Cambrian shales. Steep cliffs above the Cambrian are Redwall. Above the Redwall are the Supai, Hermit Shale, Coconino (white band), Toroweap (dark vegetative covering) and Kaibab Limestone (light-colored limestone capping the Canyon).
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Figure 8 - The Tapeats overlies the dark Vishnu Schist. The Bright Angel Shale has weatherd back exposing the Tonto Platform. The Muav, Redwall, and Supai are exposed in much of the Canyon. The younger Permian strata (including the lighter-colored Coconono and Kaibab) are exposed closer to the northern rim in the distance.