Arches N.P. | GeologyVirtualTrips

ARCHES NATIONAL PARK

FIELD TRIP STOP – EROSION ALONG JOINT PLANES AS A CONSEQUENCE OF SALT TECTONICS

LOCATION: In eastern Utah, about four miles north of Moab along Route 191.

GEOLOGIC FEATURES: Mesozoic sediments; Evaporite (Salt) flow; Joint development; Differential erosion; Arches; Fins; Spires; Graben; Frost Wedging; Mechanical Weathering.

DESCRIPTION: Episode 1 - Deposition of thousands of feet of salt (an evaporite) about 300 mya (Pennsylvanian Period) when seawater became restricted and subsequently evaporated.

Episode 2 - In the Permian and Triassic, sediments began to cover the salt. In the early Jurassic Period (210 mya) the buff-colored desert sandstones of the Navajo Fm. were laid down. On top of the Navajo, the red-orange Entrada Sandstone (140 mya) was deposited representing stream and sand dune environments. (Most of the arches in the National Park are formed in this formation.) Thousands of feet of sediment accumulated above the Entrada, most of which was eventually eroded away.

Episode 3 - Due to the weight of the overlying sediments and the Laramide Orogeny (mainly Lower Tertiary), the underlying salt began to flow, creating upwarps (folds) in the overlying sediment. This buckling resulted in parallel joints (cracks) in the rocks overlying the salt.

Episode 4 - The infiltration of water down the newly created cracks began to dissolve the underlying salt. Rocks above the salt eventually collapse into the void vacated by the salt, creating a graben.

Episode 5 - Vertical cracks widened as weathering dissolved the cement between sand grains and moved the sand away. Today, the thin rock walls left behind as a result of this erosion are called “Fins.” Differential weathering of more easily eroded layers below more resistant layers resulted in the formation of arches within the Fins

STUDENT QUESTIONS:

(1) Draw a series of cross-sections (vertical slices) visually depicting the five Episodes noted above

(2) Provide two reasons why one layer of rock might weather more quickly than an adjacent layer.

(3) Many rock layers in the American southwest, including the Entrada Fm., are reddish in color. Why might this be so?

(4) What other rock types are considered to be Evaporites besides common salt (Halite)?

(5) How does Frost Wedging operate to wear away at the Arches and Spires of the National Park?

(6) How does Mechanical weathering differ from Chemical Weathering? Which type mainly operated at Arches N.P. and why?

(7) CHALLENGE: How is the salt under the Park thought to be responsible for the huge vertical displacement along the Moab Fault?

(8) CHALLENGE: Domes of salt play a critical role in the accumulation of oil and natural gas, especially in the Gulf of Mexico. Why is that so?

(9) CHALLENGE: It has been shown that the Mediterranean Sea once dried up due a blockage at the Straits of Gibraltar. What evidence exists to prove this?

SELECTED REFERENCES:

-Admin. U.S. National Parks & Monuments (2011, Sept, 28). National Parks Blog, Moab Fault Arches National Park. Accessed on May, 7, 2021: https://www.nationalparksblog.com/moab-fault-arches-national-park

-U.S. Parks. Arches National Park Geology. Accessed on Jan 12, 2020: https://www.us-parks.com/arches-national-park/geology.html

PHOTOS:

Figure 1 - Reddish sandstones of the Jurassic Entrada Formation. Rocks are exposed as linear "fins" due to erosion along vertical joints that leave the rocks exposed as upright tabular sheets,

Figure 2 - One of the nearly 2000 arches found in the Arches National Park. Note the spire to the left and the more rapid erosion below the cap rock. This differential erosion undoubtedly led to the formation of arches in more continuous tabular fins.

Figure 3 - An inside-out view of an arch.

Figure 4 - "Balanced Rock" in the center of the photo.The elements involved in Frost Wedging and other types of Mechanical Weathering will undoubtedly cause the rock to tumble in the near future.

Figure 5 - Reddish Jurassic sedimentary strata suggestive of deposition in a non-marine environment

Figure 6 - Horizontal layers of the Sandstone contrast with the vertical cliffs created by vertical jointing

Figure 7 - The snow-covered La Sal Mountains form the backdrop to the reddish Jurassic sandstones.

Figure 8- Other types of 'Balanced Rocks' caused by differential erosion of softer shales under more resistant sandstone.

Figure 9 - North Window Arch.

Figure 10- Linear "fins" due to erosion along vertical joints leaving the rocks exposed as upright tabular sheets,

Figure 11 - A series of mainly Normal Faults exposed in the Pennsylvanian Honaker Trail Formation found just outside the western boundary of Arches N.P. Hwy 191, seen at the base of this formation, marks the position of the North-South trending Moab Fault. Rocks east of the fault have been downdropped over 2600 ft relative to the rocks seen in the photo. Normal Faulting along the Moab Fault is believed to have been the result of the dissolution and/or motion of the salt within the Paradox Formation that underlies the strata beneath Arches N.P. east of the fault plane.