CANYON DE CHELLY NATIONAL MONUMENT

FIELD TRIP STOP – CANYON DE CHELLY NATIONAL MONUMENT

 

LOCATION:  Canyon de Chelly (pronounced “de-SHAY”) is located in Arizona approximately 2 miles east of the city of Chinle.  Sites are accessible from the South Rim Drive (Route 7) and North Rim Drive (Route 64).

 

GEOLOGIC FEATURES:  Cross-bedding (Aeolian Sand Dunes); de Chelly Sandstone (Lower Permian); Unconformity;  Shinarump Conglomerate (Upper Triassic); Pinnacles (Spires, Towers); Mesas and Buttes.

 

DESCRIPTION: Canyon de Chelly is etched into the Defiance Plateau by a river whose headwaters are located in the Chuska  Mountains to the east.  It is also considered part of the Colorado Plateau.  Canyon walls rise up to 1000 ft above the canyon floor.

 

The most prominent rock exposed in the vertical walls of the canyon is the Permian de Chelly Sandstone which is capped by a relatively thin layer of the Upper Triassic Shinarump Conglomerate.  The de Chelly Sandstone is mainly composed of well-defined large-scale cross-bedded sandstones but can also be massive and seemingly unstratified. (This is the same cross-stratified rock seen in Monument Valley to the north.) Rocks represent dune deposits that that were laid down on top of reddish-brown coastal floodplain deposits of the Lower Permian Organ Rock Fm. which is not prominently exposed in the canyon. PreCambrian rock underlies the Permian strata.  The dark maroon Shinarump Conglomerate is the basal member of the Chinle Fm. that unconformably overlies the Permian strata and represents stream-deposited sand and gravel largely eroded from highlands to the southeast. Pebbles consist of PreCambrian igneous and metamorphic rock as well as Paleozoic sedimentary rock.

 

The paucity of Paleozoic rocks in this part of the Colorado Plateau suggests that this might have been a structural high at that time.  The Defiance Plateau is thought to have been uplifted from the end of the Cretaceous during the Laramide Orogeny to the lower Tertiary. Subsequent erosion forms today’s canyon.

 

From a historical perspective, the canyon is known for its occupation by Native Americans for nearly 5000 years beginning with the Ancestral Pueblos (Anasazi) and followed by the Pueblo, Hopi, and Navajo peoples.

STUDENT QUESTIONS:

(1) How can sand dunes help to determine the direction of the prevailing winds existing at the time of their formation?

(2) Define the windward and the leeward sides of a sand dune and explain how each dune can literally move.

(3) Describe the geologic formation of the unconformity separating the de Chelly Sandstione and the Shinarump Conglomerate.

(4) CHALLENGE – Compare the age of rocks within and around Canyon de Chelly with those exposed in the Grand Canyon to the southwest. Which time periods are missing in Canyon de Chelly and why might this be so?

SELECTED REFERENCES:

-Canyon de Chelly. Accessed on July 8, 2022 from: www.hanksville.org/voyage/geology/CdC.html

-de Chelly Sandstone. Accessed on July 8, 2022 from: www.hanksville.org/voyage/geology/deChelly.html

-Godsey, Holly S. Interpreting the Geologic History of Canyon de Chelly. Integrate.  Accessed on July 8, 2022 from: https://serc.carleton.edu/integrate/workshops/methods2012/activities/godsey.html. (Especially see the links to “References and Resources” and “Description and Teaching Materials.”)

-National Park Service. Canyon de Chelly National Monument.  U.S. Department of the Interior. Brochure: GPO:2020—411-224/82523.

 

PHOTOS:

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Figure 1 - Spider Rock is a pinnacle that rises about 800 ft. above the canyon floor. Pinnacles (pillars, spires) are erosional remnants that are created as running water cuts through a plateau (or mesa, or butte) that is associated with a hard caprock. Subsequent wind and rain continue the erosional process. (From Spider Rock Overlook)

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Figure 2 - Spider Rock is an erosional remnant of the de Chelly Sandstone seen forming the nearly 1000 ft walls of the surrounding plateau (From Spider Rock Overlook)

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Figure 3 - Cliff dwelling in Mummy Cave constructed by the Ancestral Puebloan people (Anasazi) and occupied between 300 and 1300 AD.  Exfoliation of the sandstone can be seen on the nearby walls. (From Mummy Cave Overlook)

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Figure 4 - The cliff dwelling at Mummy Cave is built into a cliff of the de Shelly Sandstone. Note the massive cross-bedding as well as black desert varnish.  (From Mummy Cave Overlook)

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Figure 5 - Small butte stands alone carved away from the adjacent mesa.(From Sliding House Overlook)

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Figure 6 - Note the massive amount of cross-bedding that is inclined to the right of the photos denoting a prevailing wind direction from the left.  The relatively large size of the cross-beds is also indicative of an aeolian origin. Maroon-colored cap rock of the Shinarump Conglomerate is seen in the upper right. (From Sliding House Overlook)

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Figure 7 - Massive amounts of unidirectional crossbedding is typical of the Permian deChelly Sandstone.  The non-crossbedded maroon-colored Shinarump Conglomerate (Triassic) forms the rim rock of the canyon, unconformably overlying the de Chelly Sandstone. (From Massacre Cave Overlook)

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Figure 8 - Desert Varnish covers the wall of de Chelly Sandstone.  This coating is composed primarily of manganese oxide. (From Sliding House Outlook)