Capitol Reef N.P. UT | GeologyVirtualTrips

CAPITOL REEF NATIONAL PARK

FIELD TRIP STOP -- CAPITOL REEF NATIONAL PARK, UTAH

(Monocline exposing Mesozoic Strata)

LOCATION: In south-central Utah near the intersection of State Highways 24 and 12 between Torrey and Caineville, UT.

GEOLOGIC FEATURES: Monocline; Inclined Strata; Differential Weathering; Desert Varnish; Tafoni or Honeycomb Weathering

DESCRIPTION: Capitol Reef National Park lies along the strike of the Waterpocket Monocline that trends Northwest-Southeast for nearly 100 miles. The name "Capitol" is derived from the domal weathering pattern similar to the U.S. and other Capitol buildings found within the Navajo Sandstone, and "Reef" referring to the steep escarpments of resistant rock found within the fold of the monocline.

Strata in the park are predominantly Mesozoic sedimentary rocks (Triassic to Cretaceous) with some minor Permian strata that make-up the downward flexure of a Monoclinal Fold. Rocks outside the park’s western boundary (above the flexure) lie several thousand feet higher than the easterly-dipping strata located within the park boundary. Several disconformities exist in the park, in particular between the Moenkopi Fm. and Chinle Fm, and between the Chinle Fm. and Wingate SS among younger unconformities.

The fold is thought to have been created during the Laramide Orogeny (40 or 50 to 70 mya) and was subsequently raised 15-20 mya during uplift of the Colorado Plateau. Subsequent erosion breached the flexure in the monocline creating an escarpment where differential erosion of inclined resistant Mesozoic sandstone and softer rock created the distinct topography that forms the eastern portion of the park.

Environments of deposition of Mesozoic strata are mainly non-marine (sand dunes, streams, channels, point bars, lakes, rivers, flood plains and tidal flats, with only a small amount of marine strata.

Rocks younger than the Jurassic Navajo Sandstone are not readily seen from the main scenic road of the park due to the height of the escarpment, but are exposed on Route 24 east of the visitor center.

Sedimentary Column as viewed from Scenic Highway:

Navajo SS - Age: Jurassic;-Triassic Rock type: Sandstone; Thickness: 950-1400 ft, Color: White. Environment: Desert Dune. Contents: Large Scale Crossbeds, Common Tafoni (Honeycomb Weathering)

Kayenta Fm - Age: Triassic; Rock Type: Conglomerate, Sandstone, Siltstone; Thickness: 350 ft; Color: Red to Brown; Environment: River (Fluvial); Contents: Small-scale Crossbeds

Wingate SS - Age: Triassic; Rock type: Sandstone; Thickness: 350 ft; Color: Pale orange to red; Environment: Desert Sand Dune; Contents: Large-scale Crossbeds; Smooth Vertical Cliffs; Blocky (vertical joints), Common Desert Varnish.

Chinle Fm - Age: Triassic; Rock type: Mudstone, Siltstone, Sandstone; Basal Conglomerate (Shinarump Mbr);Thickness: 550-640 ft; Color: Purple, Orange and Brown in upper parts, White to Yellow-Gray in lower portion; Environment: Streams (Fluvial), Floodplain; Contents: Petrified Wood, Uranium in Shinarump Member.

Moenkopi Fm - Age: Triassic; Rock Type: Sandstone, Siltstone, Mudstone; Thickness: 690-1000 ft; Color: Reddish Brown; Environment: Tidal Flat with minor nearshore marine.

STUDENT QUESTIONS:

(1) Define: Monocline

(2) Draw a Cross-Section of a Monocline

(3) What is responsible for the varied colors of the rocks?

(4) How is Tafoni (Honeycombed Weathered rock) formed?

(5) Explain the process of Differential Weathering and how it accounts for the topography of Capitol Reef.

(6) Define: Disconformity.

(7) CHALLENGE: How might a fault be responsible for the formation of a monocline? Show the position of the fault as well as the upthrown and downthrown sides on the cross-section noted above.

(8) CHALLENGE: Describe a brief geologic history (stratigraphic, tectonic, erosional) of the Capitol Reef area from the beginning of the Triassic to the present day.

SELETED REFERENCES:

-The University of Utah. 2018. Capitol Reef National Park and Surrounding Area: A Geological Tour Guide. Accessed April 19, 2021.https://sed.utah.edu/CapReef.htm

-National Park Service, Capitol Reef - Geology. U.S. Department of the Interior. Accessed April 19, 2021 from https://nps.gov/care/learn/nature/geology.htm

-US Parks.com, Inc. (2000-2001). Capitol Reef National Park Geology. Accessed April 19, 2021 from https://www.us-parks.com/capitol-reef-national-park/geology.html

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

PHOTOS:

Figure 1 - Chimney Rock. The outcrop shows the reddish-brown Moenkopi Fm. overlain by the lowermost Member (Shinarump) of the Chinle Fm. forming the rectangular block at the very top of the vertical wall. Strata of the Moenkopi represent mainly tidal flat and floodplain deposits.

Figure 2 - The Castle. The steep cliff north of the Visitor Center and having vertical joints is the Triassic Wingate Sandstone. Directly below this rock is the Chinle Fm. The Chinle is grayish in color near its base followed stratigraphically by rubbly silts and mud shaving an interspersed resistant sandstone layer. Below the gray part of the Chinle is the red-brown Moenkopi Fm. The reddish horizontal beds of the Kayenta Fm. can be seen to overly the Wingate to the far left of the photo and to the right of The Castle.

Figure 3 - Smooth vertical wall of the Wingate Sandstone. The Wingate represents aeolian sand dunes having cross-bedding which is difficult to see compared to crossbeds of the Navajo Sandstone. Walls characteristically have a vertically jointed appearance

Figure 4 -Near the bottom of the photo we see the Moenkopi Fm. (Red-Brown). This is followed strtaigraphiically by the basal Shinarump Member of the Chinle Fm (thin yellowish conglomeratic sandstone), the grayish layer of the lower Chinle, and the reddish slope-forming sediment of the upper Chinle with an interbedded resistant sandstone. Above the Chinle Fm. is the massive, blocky wall of the Wingate SS. Strata dip towards the east as part of the monoclinal flexure.

Figure 5 - Oyler Mine. The yellowish-gray layer is the Shinarump Member of the Chinle Fm. This fluvial conglomeratic sandstone was mined for Uranium beginning in 1904. People can be seen at the abandoned opening to the mine.

Figure 6 - Rocks are seen to dip to the east as part of the flexure in the Waterpocket Monocline. Stratigraphically from bottom to top, rock formations are the Triassic Moenkopi, Chinle, and Wingate Formations as described in the Figures above. In this photo we also see the horizontally layered reddish strata of the Kayenta Fm which is overlain by the light-colored Triassic to Jurassic Navajo Sandstone.

Figure 7 - Domal weathering pattern of the Navajo Sandstone caps the outcropping. The Kayenta SS separates the Navajo from the smooth vertical cliff of the Wingate SS.

Figure 8 - Ripple Marks in tidal sediments of the Moenkopi Fm

Figure 9 - Tidal Flat sediments of the Moenkopi Fm

Figure 10- Crossbeds of the Navajo Fm exposed in Capitol Gorge.

Figure 11 - Aeolian crossbeds of the Navajo Fm as exposed in Capitol Gorge

Figure 12 - Honeycomb weathering within the crossbeds of the Navajo Fm.

Figure 13 - Close-up of Honeycomb weathering within the crossbedded Navajo Fm. in Capitol Gorge. These are not tubes but spherical pockets likely caused by weathering of less well-cemented sands. They are slso referred to as Tafoni or Swiss-Cheese weathering.

Figure 14 - Desert Varnish draped across the Wingate Fm. as exposed in Capitol Gorge. Dark vertical lines are the result of iron and manganese oxides.

Figure 15 - Domal weathering pattern of the Navajo Sandstone as exposed in Capitol Gorge.

Figure 16 - Capitol Dome. as viewed from Route 24 east of the Visitor's Center.

Figure 17 - Looking westward from a road east of the park boundary, beds of the monoclinal flexure younger than those viewed from the Park's Scenic Drive dip eastward.