ZION CANYON N.P.

 

FIELD TRIP STOP – SAND DUNES IN A JURASSIC DESERT

 

LOCATION:  Zion National Park is found in southwest Utah, near Springdale.

 

GEOLOGIC FEATURES: Large-scale crossbeds; Desert environment; Joints

 

DESCRIPTION: Sedimentary rocks of Zion represent about 150 my of Mesozoic strata. Minor Tertiary and Quaternary lava and pyroclastics are scattered. Rocks of Zion N. P. basically begin where the geologic history of Grand Canyon ends.  Rocks at Bryce Canyon to the north represent strata deposited after those at Zion.

     The dominant rock in the park is the Navajo Sandstone. Deposited between 190 and 136 mya, this Jurassic Formation  is mainly responsible for the white to tan and red steep cliffs throughout the park.  Most characteristic of this formation is the large-scale crossbedding representative of wind deposited sand dunes of desert origin.  Sand grains are clear and round. Reddish hues are due to the leaching of iron oxide from the overlying rock formations.  The huge crossbed sets, at about 34 degrees to the horizontal, represent the leeward face of dunes as they marched across the desert floor.

     Subsequent to Cenozoic sedimentation in the area, 10,000 ft. of uplift of the Colorado Plateau along with accompanying fracturing (jointing) and faulting caused rivers and streams to rapidly cut near-vertical cliffs particularly in the thick (2000 ft.) and relatively soft rocks of the Navajo Sandstone.

 

STUDENT QUESTIONS:

(1) Horizontal beds are composed of inclined strata that dip in the direction of dune advance. Using a carefully drawn series of drawings, illustrate how a sand dune can create this large-scale horizontal bed of inclined strata.

(2) How does the Leeward side of a dune differ from the Windwaerd side?

(3) Do the inclined stratra of the dunes violate the Law of Original Horizontality?

(4) Why are deset dunes composed of sand-sized particles rather than clay, silt, or pebble-sized particles?

(5) Sand particles of a desert can be identified not only bt their size, but also by their sorting and rounding. Define "sorting" and "rounding" and then explain why desert sandstones are typically well-sorted and well-rounded.

(6) CHALLENGE: How might the orientation of sand dune movement be used to supplement Continental Drift reconstructions and paleoclimate interpretaions?

SELECTED  REFERENCES:

-Harris, A. and E. Tuttle. 1983 (3rd ed).Geology of the National Parks. Kendall/Hunt Publishing Co., 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.

-National Park Service. Zion -The Geology of Zion. Accessed on 2/08/20: https://www.nps.gov/zion/learn/nature/upload/Geology07.pdf

SHORT VIDEO: 

Through the park, mainly Navajo Sandstone with large Crossbeds (Ancient Sand Dunes): https://www.youtube.com/watch?v=Oko8-PM9hvs&feature=youtu.be (Video by Scott Marintsch)

PHOTOS:

Figure 1 - Near the Zion Canyon Visitor Center.  The steep verical cliffs are the aeolian Jurassic Navajo Sandstone while the slopes and ledges below are composed of Triassic non-marine river deposits.

Figure 2 - Crossbeds of the Navajo Sandstone.

Figure 3 - Navajo Sandstone. Inclined strata represent the leeward slopes of sand dunes migrating from left to right.

Figure 4 - Crossbedding in the Navajo Sandstone

Figure 5 - Checkerboard Mesa. Notice the iron oxide staining on the lower half of the outcrop.

Figure 6 - Checkerboard Mesa

Figure 7 - Checkerboard Mesa. Ancient sand dunes move from left to right.

Figure 8 - Checkerboard Mesa. Note the distict separation of iron stained strata below.

Figure 9 - Crossbedding in the Navajo Sandstone

Figure 10 - Crossbedding of ancient sand dunes. Tectonic activity has tilted the layer of crossbeds to the left.

Figure 11 - Joint-controlled steep vertical cliffs abound in the Park.

Figure 12 -  Navajo Sandstone forms steep vertical cliffs.

Contact or COMMENTS