Geology of the Bryce Canyon area

The Grand Staircase is a sequence of sedimentary rock layers, first defined in the 1870s, that stretch south for from Bryce Canyon National Park through Zion National Park and into the Grand Canyon. Bryce Canyon is located within the Pink Cliffs, the highest and youngest rise within the Grand Staircase. ==Cretaceous Seaway==

Advance In the Cretaceous, a shallow seaway spread into the interior of North America from the Gulf of Mexico in the south into Utah and later to the Arctic Ocean in the far north. Geologists call this shallow sea the Cretaceous Seaway or Western Interior Seaway. The seaway divided North America into two halves: an eastern portion dominated by the already ancient Appalachian Mountains and a western part composed primarily of the still growing Sevier Mountains; As the shoreline moved back and forth, the Bryce area alternated from being part of the Sevier landmass to being under the Cretaceous Seaway. Alternating layers of nonmarine, intertidal, and marine sediments lay on top of each other as a result. Conglomerate, siltstone, and fossil-rich sandstone that together are up to thick mark the arrival of the Cretaceous Seaway. It sits unconformably on much older Jurassic formations that are not exposed in the immediate area (see geology of the Zion and Kolob canyons area for a discussion about these older sediments). Mud and silt were deposited on top of the Dakota Formation as the seaway became deeper and calmer in the area. Its members represent various stages in this process. The cliff-forming sandstone of the Tibbet Canyon Member was conformably deposited on top of the Tropic Shale in shallow marine and later near shore environments. Shale and sandstone from the Smoky Hollow Member were deposited on top of its basal layer of coal-rich mudstone in coastal swamps and lagoons on the shore of the seaway. While the alternating layers of shale and sandstone mixed with massive coal deposits of the John Henry Member were laid down in swamps, lagoons and fluvial environments, one member, the Drip Tank, is not found in the Bryce Canyon area. This formation erodes into almost unclimbable cliffs and escarpments of whitish to yellow-gray sandstones with comparatively thin interbedded layers of shale and mudstone. Shark teeth are found in the lower parts of the formation. Lakes and east-flowing rivers became the dominant resting place for sediment following the retreat of the Cretaceous Seaway. The shales and sandstones of the locally thick Wahweap Formation were deposited in moving water (fluvial setting). This formation is part of the Grey Cliffs of the Grand Staircase mentioned previously. It contains abundant fossils of vertebrates, including dinosaurs such as the hadrosaurs. ==Continental deposition==

Flood plains uplift and erode in Claron Formation, Bryce Canyon A flood plain crossed by rivers and lakes developed in the area. Mud and sand accumulated in this setting to become the gray sandstones and mudstones of the Kaiparowits Formation. This formation is up to thick in the Bryce Canyon area, but other parts of the Kaiparowits in the region are several hundreds of feet (tens of meters) thick. Compression from the Laramide event deformed the land in the area to form the up to 5° dipping Bryce Canyon Anticline. All of the Canaan Peak, Pine Hollow, Kaiparowits, and Waheap formations, along with part of the underlying Straight Cliffs, were removed from the anticline's crest by erosion before the Claron Formation was deposited. An angular unconformity therefore exists along the anticline's crest. The park also sits on the western gently dipping flank of the much larger Kaibab uplift, which was also formed as a result of the Laramide. Climate change and cycles caused the lakes in the system to expand and shrink through time. As they did so, they left beds of differing thickness and composition stacked atop one another; Volcanic ash and lava from these flows are found less than from Bryce Canyon but at least some volcanic material was likely deposited directly in the park area only to be later removed by erosion. ==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 thick Oligocene or Miocene-aged Boat Mesa Conglomerate and the Pliocene to early Pleistocene-aged Sevier River Formation. The Boat Mesa is made mostly of conglomerates with minor amounts of sandstone and some limestone from lakes, representing stream and overbank flood deposits. 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. Headward erosion of one of those tributaries, the Paria River, eroded north-northwestward toward what is now Paria Amphitheater. 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. 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 carbon dioxide it forms a weak solution of carbonic acid. This acid helps to slowly dissolve the limestone in the Claron Formation grain by grain. It is this process of chemical weathering that rounds the edges of hoodoos and gives them their lumpy and bulging profiles. 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. 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. 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. 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 called the Tropic Ditch has been taking some or all of water from the East Fork of the Sevier River through this section of the park for over 100 years. ==Notes==