Geology of the Bryce Canyon area

The exposed geology of the Bryce Canyon area in

The formations exposed in the area of the park are part of the Grand Staircase. The oldest members of this super sequence of rock units are exposed in the Grand Canyon, the intermediate ones in Zion National Park, and its youngest parts are laid bare in Bryce Canyon area. A small amount of overlap occurs in and around each park.

Grand Staircase

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The rocks exposed in Bryce Canyon are about 100 million years younger than those in nearby Zion National Park, and the rocks exposed in Zion are younger than those in the Grand Canyon to the south.

There are, however, shared rock units between all three, creating a supersequence of

A

Uplift from the Laramide paused briefly in the Eocene.^[2] Meandering streams flowed slowly over the resulting broad and almost featureless plains. Periodic but extensive floods inundated large areas perhaps once every 1,000 years; spreading mud, cobbles and fine silt over the plains.^[4] Erosion incised these deposits between flood events and plant growth was abundant. Oxidation of the iron in the mud and silt turned the soil into hematite, giving it a pink and red hue. These sediments were later lithified into the up-to- 700 feet (200 m)-thick Pink Member of the locally Eocene-aged Claron Formation (previously called the Wasatch Formation).^[4] Channelized conglomerates of this member can be easily seen in Red Canyon along State Route 12 while its silt and mudstones compose most of the park's fragile and colorful spires called hoodoos. Geologist Clarence Dutton called the iron oxide-rich lower member of the Claron the Pink Cliffs series due to its colorful appearance.^[5]

A large system of shallow but expansive

• various sand and cobble deposits near shore,
• calcium-poor muds further from shore,
• calcium-rich mud in deeper water, and
• pure limey oozes were deposited in the deepest waters.

The limey oozes and mud were later lithified into the limestone and interbedded siltstone of the up-to-300 foot (90 m)-thick White Member of the Claron.^[4] This member erodes into white-colored monoliths that are found only at the highest elevations of the Paunsaugunt Plateau. Fossils are rare in the White Member and consist mainly of freshwater snails and clams, indicating that the lakes supported little life.^[4] Most arches and natural bridges in the park, including the famous Natural Bridge, were carved from sandstone beds in the Claron.

Marysvale volcanics

A gap in the geologic record following deposition of the Claron Formation was interrupted 34 to 31 mya by eruptions from the nearby

Most of the activity at the Marysvale volcanic field occurred during three separate intervals; ~34–22 mya, 22–14 mya and 9–5 mya.[9] Dacites and andesites were erupted above an evolving batholith in the first interval. Crystal-rich ash from the 27-million-year-old (myo) Three Creeks caldera, 24 myo Big John caldera and 23 myo Monroe Peak caldera hardened into tuff.^[9] The second and third intervals saw eruptions of great quantities of rhyolites. Alkali-rich rhyolite erupted in the second interval from the 19 myo Mount Belknap caldera.^[9]

The Marysvale volcanic field collapsed under its own weight around 20 mya; likely due to weaknesses in the evaporites of the Carmel Formation some 5,000 feet (2,000 m) below.^[2] Folding and faulting of the Claron Formation from the collapse created Ruby's Inn thrust. A slight downwarp running east–west and perpendicular to the thrust motion called the Bryce syncline was also created.^[2] Volcanic activity in the Marysvale and Bryce area ceased some 500,000 years ago.^[8] Basaltic rocks of the same age can be seen along the Sevier Fault near the entrance to Red Canyon; the dark colored volcanic rocks have been displaced by the fault some 900 feet (300 m) and are now in direct contact with the much older Claron Formation.^[10]

Late Cenozoic tectonics

Formation of the High Plateaus

Younger rock units were laid down but were mostly removed by subsequent uplift-accelerated erosion. Outcrops of these formations can be found in the northern part of the park and in a few places on the plateau rim. Among these are the 50-to-100-foot (20 to 30 m) thick Oligocene or Miocene-aged Boat Mesa Conglomerate and the Pliocene to early Pleistocene-aged Sevier River Formation.^[11] The Boat Mesa is made mostly of conglomerates with minor amounts of sandstone and some limestone from lakes, representing stream and overbank flood deposits.^[4] The brownish-gray sandstone and gravels of the Sevier River Formation were laid down in valleys that were part of the ancestral Sevier River drainage system.^[4][11]

Around 15 mya in Miocene time, tensional forces in Nevada to the west were so great that the crust was spread thin, creating the Basin and Range Province.^[2] These same forces segmented what is now the western part of the Colorado Plateau into nine different smaller plateaus, including the Paunsaugunt that the park sits astride.^[12][13] Long, north–south-trending normal faults were either newly created or reactivated from older pre-existing faults; a plateau rose on one side of each fault while valleys subsided on the other as the crust was extended in an east–west direction.^[2] Two of these faults bound the Paunsaugunt Plateau; the Sevier on the west and the Paunsaugunt on the east. Movement along these two faults has displaced the Claron Formation by 2,000 feet (600 m) relative to the Paria and Sevier valleys.^[13]

The Hurricane Fault marks the west edge of the Markagunt Plateau and is the topographic boundary between the Basin and Range and the Colorado Plateaus provinces.^[13] Cedar Breaks National Monument, which shares almost the same exposed geology and erosional features as Bryce Canyon National Park, sits on the west edge of the Markagunt.

The entire Colorado Plateau then started to uplift from near sea level to several thousand feet (over a kilometer) in elevation.^[11] An alternate theory is that the Laramide orogeny uplifted what is today the Basin and Range and the Colorado Plateau and that the tensional forces that formed the Basin and Range caused that region to subside relative to the Colorado Plateau.^[4] Whatever the order of events were, the High Plateaus region of the Colorado Plateau were by now nearly complete.

Modern drainage and erosion

Drainage of the Colorado Plateau was significantly altered by the opening of the

Formation of approximately the current depth of the Colorado River's Grand Canyon by 1.2 mya further caused its tributaries to cut down even deeper.^[14] Headward erosion of one of those tributaries, the Paria River, eroded north-northwestward toward what is now Paria Amphitheater.^[15] The river took a route roughly parallel to and east of the Paunsaugunt Fault. Erosion from snow and rain that fall directly on the east-facing rim of the Paunsaugunt Plateau forms gullies that widen into alcoves and amphitheaters while differential erosion and frost wedging create the hoodoos. Streams on the plateau do not contribute to the formation of alcoves or amphitheaters because they flow away from the rim.^[15] Erosion continues in this fashion today.

Hoodoo formation in Bryce Canyon

The Pink Member of the Claron Formation is largely composed of easily eroded and relatively soft limestone. When rain combines with

In the winter, melting snow seeps into cracks and joints and freezes at night. The force of the expanding ice helps to erode the rock of the Claron Formation. Over 200 of these freeze/thaw cycles occur each year in Bryce Canyon.^[16] Frost wedging exploits and widens the nearly vertical joint planes that divide the Pink Member of the Claron Formation.

Internal layers of mudstone, conglomerate and siltstone interrupt the limestone horizontally. These layers are more resistant to attack by carbonic acid and they can therefore act as protective capstones of fins, windows and hoodoos. Many of the more durable hoodoos are capped with a type of magnesium-rich limestone called dolomite.^[16] Dolomite dissolves at a much slower rate, and consequently protects the weaker limestone underneath.

However, the same processes that create hoodoos will also eventually destroy them. In the case of Bryce Canyon, the hoodoos' rate of erosion is 2–4 feet (0.6–1.3 m) every 100 years.^[16] As the canyon continues to erode to the west it will eventually capture (in perhaps 3 million years) the watershed of the East Fork of the Sevier River. Once this river flows through Bryce Amphitheater it will dominate the erosional pattern; replacing hoodoos with a V-shaped canyon and steep cliff walls typical of the weathering and erosional patterns created by rivers. A foreshadowing of this can be observed in Water Canyon while hiking the Mossy Cave Trail. A diversion canal has been taking a portion of the East Fork of the Sevier River through this section of the park for over 100 years.^[16]

Notes

References

• Cunningham, Charles G. (2002). "Volcanic rocks and ore deposits of the Marysvale Volcanic Field, west-central Utah". The Geological Society of America. Archived from the original on 2011-06-08. Retrieved 2007-08-12.
• Davis, George H.; Pollock, Gayle L. (August 2003). "Geology of Bryce Canyon National Park". In Paul B. Anderson (ed.). Geology of Utah's Parks and Monuments. Bryce Canyon Natural History Association and Utah Geological Association.
  ISBN 1-882054-10-5
  .
• Harris, Ann G; Tuttle, Esther (1997). Geology of National Parks (Fifth ed.). Iowa: Kendall Hunt Publishing Co.
  ISBN 0-7872-5353-7
  .
• Kiver, Eugene P; Harris, David V (1999). "Bryce Canyon National Park and Cedar Breaks National Monument (Utah)". Geology of U.S. Parklands (5th ed.). New York: John Wiley & Sons, Inc. pp. 522–30.
  ISBN 0-471-33218-6
  .
• Rowley, Peter D.; Cunningham, Charles G.; Steven, Thomas A.; Workman, Jeremiah B.; Anderson, John J.; Theissen, Kevin M. (2002). Geologic Map of the Central Marysvale Volcanic Field, Southwestern Utah. Denver, Colorado: United States Geological Survey. Retrieved 2007-08-12. {{cite book}}: |work= ignored (help)
•  This article incorporates public domain material from Hoodoos. National Park Service. Retrieved 2007-07-24.
• "Geology field notes: Bryce Canyon National Park, Utah". National Park Service. 2005-01-04. Archived from the original on 2007-07-08. Retrieved 2007-08-02.

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Geology of Bryce Canyon National Park

Further reading

• DeCourten, Frank (1994). Shadows of Time; the Geology of Bryce Canyon National Park. Bryce Canyon Natural History Association.
• Tufts, Lorraine Salem (1998). Secrets in The Grand Canyon, Zion and Bryce Canyon National Parks (Third ed.). North Palm Beach, Florida: National Photographic Collections.
  ISBN 0-9620255-3-4
  .