COLORADO NAT'L MON | GeologyVirtualTrips

COLORADO NATIONAL MONUMENT

FIELD TRIP STOP - Horizontal Strata and Unconformities

LOCATION: Colorado National Monument is located in northeastern Colorado. The West Entrance is found off Highway 340 near Fruita. The Park’s East Entrance is found off Monument Road in Grand Junction. Entrances are connected by the 23 mile long Rim Rock Drive that contains about a dozen views or overlooks. The Drive itself largely follows a flat bench formed in the Kayenta Formation.

GEOLOGIC FEATURES: Horizontal Bedding; Unconformities (Non-Conformity and Disconformity); Red-Colored bedding; Eolian Crossbedding; Monocline

DESCRIPTION: Colorado National Monument is found along the northeast flank of the 25 x 100 mile Uncompahgre Uplift (Plateau). Rocks are mainly flat Mesozoic sedimentary rocks that dip steeply along the northeast border, thereby forming a Monocline. Mesozoic rocks are underlain by PreCambrian crystalline rock. Uplift of the Plateau and formation of the monocline and the Redlands Fault in the flexure occurred in the Upper Tertiary. Subsequent erosion removed any Tertiary formations and continued to produce the present topography.

Within the Monument, Mesozoic rocks of the red Upper Triassic rocks of the Chinle Fm. lie on basement PreCambrian metamorphic and igneous rocks. (These are the same PreCambrian rocks exposed just east at the Black Canyon of the Gunnison N.P.) The boundary between these two rock types is called “The Great Unconformity.”

Rocks of the Chinle represent stream and floodplain deposits. Immediately above the Chinle is the Upper Triassic to Lower Jurassic Wingate Sandstone that indicates deposition of windblown desert sand dunes. The Kayenta Fm. (Lower Jurassic) suggests the existence of shallow streams.

An unconformity separates the Kayenta from the overlying Middle Jurassic Entrada Sandstone. (The Navajo Sandstone and Carmel Formations are missing here, having been eroded away in the Lower Jurassic.) Sediments of the Entrada indicate the presence of Sand Dunes. Above the Entrada are the Middle Jurassic Wanakah, Upper Jurassic Morrison, and Lower Cretaceous Burro Canyon Formations having rocks suggesting Lake, Stream, and Floodplain environments.

(Following another, yet brief, unconformity, the coastal plain deposits (Lagoon and Beaches) of the Upper Cretaceous Dakota Sandstone were laid down, followed by the Offshore deeper water muds of the Mancos Shale. These two Formations, however, do not occur within the confines of the Colorado National Monument.)

Stratigraphic Column of Rock Formations in the Colorado National Monument Area

MANCOS SHALE (Does not occur within the Monument) – Age: U. Cretaceous; Color: Gray & Black; Rock Type: Shale; Thickness: 3800-4500’; Environment: Offshore marine; Content: Underlies most of Grand Valley

DAKOTA SANDSTONE(Does not occur within the Monument) – Age: U. Cretaceous; Color: Coaly; Rock Type: Shale, Sandstone, Coal, Conglomerate; Thickness: 103-150’; Environment: Coastal Plain (Lagoon, Beaches) along a large inland sea.

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BURRO CANYON FM – Age: L. Cretaceous; Color: Green; Rock Type: Mudstone, Sandstone & Conglomerate; Thickness: 60-100’; Environment: Stream and Floodplain. Petrified wood and dinosaur remains.

MORRISON FM – Age: U. Jurassic; Color: Variable (Red, Green, Purple, Gray); Rock Type: Mudstone with Sandstone lenses; Thickness: 532-600’; Environment: Lake, Streams, Floodplains; Content: The Morrison is subdivided into 3 Members. Dinosaurs remains present. Volcanic ash beds.

WANAKAH (= SUMMERVILLE) FM – Age: M. Jurassic; Color: Variable (Red & Green); Rock Type: Mudstone with Sandstone; Thickness: 31-54’; Environment: Lake & Stream

The major formations viewed from Rim Rock Drive are as follows:

ENTRADA FM – Age: M. Jurassic; Color: White above Salmon-red; Rock Type: Sandstone; Thickness: 150’; Environment: Sand Dunes near Inland Sea; Content: Top: White horizontal stair-like layers. Bottom: Salmon crossbed-filled cliffs.

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KAYENTA FM – Age: L. Jurassic; Color: Red & Purple; Rock Type: Sandstone & Conglomerate. Siltstone & Shale; Thickness: 45-80’; Environment: Streams and Floodplains; Content: Channels. Forms a bench between Wingate below and Entrada above. The upper part of the Kayenta Fm. is particularly well-cemented and forms a resistant cap to the cliff-forming Wingate below.

WINGATE SANDSTONE – Age: U. Triassic to L. Jurassic; Color: Buff and light Red; Rock Type: Sandstone; Thickness: 329-350’; Environment: Eolian Desert Dunes; Contents: Crossbeds and Level Beds. Highest cliffs.

CHINLE FM – Age: Upper Triassic (227 – 209 mya); Color: Red; Rock Type: Mudstone, Shale, Siltstone; Thickness: 80-100’; Environment: Stream and Floodplain

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UNNAMED FM - Age: PreCambrian (1.72-1.80 byo); Color: Black; Rock Type: Gneiss, Schist, Granite

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STUDENT QUESTIONS:

(1) How much geologic time is missing between the PreCambrian and Chinle Fm.?

(2) What is a Monocline?

(3) How can a fault be related to the formation of a monocline?

(4) What part(s) of a sand dune is represented by large-scale crossbeds?

(5) Describe the geologic process(es) that created Independence Monument.

(6) What causes red coloration in sedimentary rocks?

(7) CHALLENGE: How can aeolian sand dune deposits be located above stream and floodplain deposits at any single geographic locality?

(8) CHALLENGE: Why are some rocks more resistant to weathering than others?

(9) CHALLENGE: Some publications assign a Jurassic age to the Wingate Sandstone while others assign an age of Upper Triassic to Lower Jurassic. Why might this be so?

SELECTED REFERENCES:

-Harris, David V. and Eugene P. Kiver. 1985 (4th ed.). The Geologic Story of the National Parks and Monuments. John Wiley and Sons, New York, 464 pp.

-Lohman, S.W. 1981. The Geologic Story of Colorado National Monument. Geological Survey Bulletin 1508. U.S. Department of the Interior. Accessed June 7, 2023 from http://pubs.usgs.gov/bul/1508/report.pdf

-National Park Service. Colorado National Monument-Historic Rim Rock Drive. U.S. Department of the Interior. (updated October 24, 2022). Accessed June 7, 2023 from https://www.nps.gov/colm/planyourvisit/historic-rim-rock-drive.htm

-National Park Service. Colorado National Monument-Rim Rock Drive Geology. U.S. Department of the Interior. Accessed on June 7, 2023 from https://home.nps.gov/colm/planyourvisit/upload/01294%20Rim%20Rock%20Geo.PDF

-National Park Service. Colorado National Monument-Rock Layers of the Monument. U.S. Department of the Interior. Accessed June 7, 2023 from https://www.nps.gov/colm/planyourvisit/upload/01294%20Rock%20Layers.PDF

-Scott, Robert B. et al. 2001. USGS Geologic map of Colorado National Monument and adjacent areas, Mesa County, Colorado. U.S. Geologic Survey Report. Accessed June 7, 2023 from https://www.usgs.gov/maps/geologic-map-colorado-national-monument-and-adjacent-areas-mesa-county-colorado

-Williams, Felicie and Halka Chronic. 2014 (3rd Ed). Roadside Geology of Colorado. Mountain Press Pub. Co., Missoula, Montana

PHOTOS:

Figure B - From National Park Service. Colorado National Monument-Rock Layers of the Monument.

Figure A - Map of Colorado National Monument from the National Park Service handout.

CHINLE FM.

WINGATE SS

KAYENTA FM.

Figure 1 - Cold Shivers Point Overlook – The first Overlook upon entering the Monument from the East side, Cold Shivers Point shows the dark PreCambrian rock (1.72 – 1.8 byo) at the very base of Columbia Canyon. Resting unconformably on the PreCambrian and forming a gentle slope due to its soft nature are the Upper Triassic floodplain deposits of the Chinle Fm. The large and steep buff to reddish-colored cliffs represents the Upper Triassic to Lower Jurassic Wingate Sandstone which contains numerous sets of crossbeds suggestive of a desert environment. Capping the Wingate is a thin layer of the Kayenta Fm., a relatively resistant well-cemented lighter-colored sandstone. It can commonly weather to form toadstool-shaped structures as seen atop the cliff to the right of the photo.

PreCAMBRIAN

CHINLE FM.

WINGATE SS

KAYENTA FM.

Figure 2 - Red Canyon Overlook – This long and narrow canyon gives us an excellent view of the cliff-forming Wingate Sandstone. Close examination shows the large-scale crossbedding indicative of sand dunes in a deseet environment. The gentle slope at the base of the Wingate represents deposits of the easily weathered red-colored Chinle Fm. which, in turn, lies above the black PreCambrian rocks at the valley bottom. The lighter-colored Kayenta Fm caps the Canyon walls. The Entrada Sandston is visible in the upper left corner of the photo.

Figure 3 - Fallen Rock Overlook – At this Outlook, a large block of the Wingate Sandstone capped by the Kayenta Fm. has broken from the canyon wall and moved vertically downwards over 100 ft. to finally rest on the debris-covered and gently sloping Chinle Fm. It is thought that the expansion of ice within a vertical joint (frost wedging) pried the block from the cliff face. Rather than tumbling forward the block fell straight downward into a void created by the weathering and erosion of the soft Chinle sediments below.

KAYENTA FM.

ENTRADA SS

MORRISON FM.

WINGATE SS

Figure 4 - Upper Ute Canyon Overlook - The infamous Mummy is an attraction at this overlook. Weathered into the shape of an Egyptian burial figure. The “body” is composed of the Entrada Sandstone and lies directly on the Kayenta Fm. Upon close examination, the salmon-colored Entrada can be seen to contain crossbedding suggesting the presence of sand dunes. The uppermost portion of the Wingate can be seen directly below the Kayenta Fm. The uppermost Entrada contains whitish distinctly horizontal beds. The brush-covered hills at the top of the photo are largely of the Morrison Fm.

WINGATE SS

KAYENTA FM.

CHINLE FM.

ENTRADA SS

MORRISON FM.

Figure 5 - Highland View – The view here spans the layer cake topography above the PreCambrian crystalline rock from the Triassic Chinle Fm through the Jurassic Morrison Fm. that caps the Monument Mesa. The roadside pullover from which the photo was taken lies on top of the Morrison Fm. The main cliff-former is the Wingate Sandstone. Immediately below the Wingate is the gently sloping outcropping of the Chinle Fm. The Kayena Fm. caps the Wingate. The salmon-colored Entrada Sandstone lies in unconformably atop the Kayenta. The brush-covered outcroppings at the mesa top are formed of the Morrison Fm.

Figure 6 - Artists Point – This view is named for the variety in topography and color of the rocks. As for the nearby Highland View, the PreCambrian underlies the Valley bottom, the reddish Chinle slopes gently till it meets the vertical cliffs of the Wingate which also forms the buttes of the valley. The well cemented Kayenta forms a clearly defined bench. The Entrada appears as a salmon colored cliff above which are the Wanakah and thicker Morrison Fms.

Figure 7 - Coke Ovens Overlook – The Coke Ovens is a row of beehive shaped domes composed of the Wingate Sandstone. Their name is derived from their shapes which resemble old coal or coke ovens. These domes are the result of increased erosion along north-south trending joints.

ENTRADA SS

KAYENTA FM.

Figure 8 - Coke Oven Overlook – Photographed from the Overlook we see the smooth salmon-colored walls of the Middle Jurassic Entrada Sandstone that unconformably overlies the Lower Jurassic Kayenta Fm. Note the cross beds in the Entrada suggesting the presence of sand dunes.

Figure 9 - Grand View Overlook – Here we see pinnacles formed by the weathering and erosion of the Wingate Sandstone within Monument Valley.

Figure 10 - Grand View Overlook – This overlook contains a broadside view of Independence Monument. The gently sloping red Chinle Fm clearly underlies the steeply sloping Wingate Sandstone which is capped by a thin layer of the Kayenta Fm.

Independence Monument

Figure 11- In dependence Monument View – Independence Monument (far right) is all that remains of a wall that connected to the mesa to its left. The Monument is a 450 ft tall monolith which is capped by the resistant rock of the Kayenta Fm. This cap rock protects the rock of the Wingate Sandstone below from higher rates of erosion. Once the Kayenta is removed, the Wingate’s erosion will be accelerated forming a rounded dome (similar to the Coke Ovens) before it disappears for good. Each year on July 4th climbers ascend the Monument to celebrate the U.S. Independence Day.

Moonoclinal Bend

Figure 12 - Fruita Canyon – Here, near the northern border of the Monument, we see the upper flexure of the monocline as the strata begin to dip towards the northeast. Rocks of the Chinle Fm. are clearly visible by their red coloration and gentle slope leading up the cliffs of the Wingate Sandstone.

CHINLE FM.

WINGATE SS

KAYENTA FM.

Figure 13 - Near Balanced Rock View - The sharp bend in the road near Balance Rock illustrates the gently sloping, red, soft Chinle sediment underlying the buff-colored Wingate Sandstone. A thinner layer of the well-cemented and resistant Kayenta Fm. caps the Wingate.

Figure 14 - Balanced Rock View – Balanced Rock is found within Fruita Canyon near the northwestern entrance to the Monument. Here, a 600 ton boulder of the Wingate Sandstone lies perched on top of a column of sandstone. The Rock and column represent the remains of the Wingate resulting from the action of weathering within and around vertical joints, soft sediment, and bedding planes.