The exposed geology of the Bryce Canyon area in Utah shows a record of deposition that covers the last part of the Cretaceous Period and the first half of the Cenozoic era in that part of North America. The ancient depositional environment of the region around what is now Bryce Canyon National Park varied from the warm shallow sea (called the Cretaceous Seaway) in which the Dakota Sandstone and the Tropic Shale were deposited to the cool streams and lakes that contributed sediment to the colorful Claron Formation that dominates the park's amphitheaters. Other formations were also formed but were mostly eroded following uplift from the Laramide orogeny which started around 70 million years ago (Mya). This event raised the Rocky Mountains far to the east and caused the retreat of the sea that covered the Bryce Canyon area. After Laramide mountain building came to an end, about 15 mya, a large part of western North America began to be stretched into the nearby Basin and Range topography. The greater Bryce area was uplifted as part of the High Plateaus by the same forces. Uplift of the Colorado Plateaus and the opening of the Gulf of California by 5 mya changed the drainage of the Colorado River and its tributaries, including the Paria River, which is eroding headward between two plateaus adjacent to the park. The uplift caused the formation of vertical joints which were later preferentially eroded to form the free-standing pinnacles called hoodoos, badlands, and monoliths we see today. 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
The Grand Staircase is a sequence of sedimentary rock layers, first defined in the 1870s, that stretch south for 100 miles (160 km) 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; formed from shallow thrust faulting caused by the Sevier orogeny. 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 300 feet (90 m) thick mark the arrival of the Cretaceous Seaway. Called the Dakota Sandstone, it is the oldest formation exposed in the Bryce Canyon area but the youngest one exposed in the Zion and Kolob canyons area to the southwest. Abundant amounts of petrified wood, oyster beds containing millions of fossils, and coal are all found in the Dakota. In the Bryce Canyon area, this formation can be seen in the Paria Valley where it is found as a lithified blanket sand that probably accumulated on beaches, in lagoons, and vast coal-producing swamps as the Cretaceous Seaway transgressed (advanced inland) over the region and later retreated. 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. The resulting dark gray to black Tropic Shale records the maximum transgression of the seaway into Utah and is 1,000 feet (300 m) thick in the Paria Amphitheater. It forms the unvegetated badlands seen in the Tropic Valley and is perhaps the most fossil-rich formation in the area; containing numerous straight-shelled and coiled ammonites.
Retreat The Cretaceous Seaway was already retreating east and to the south by the time the locally 1,700 feet (500 m) thick Straight Cliffs Formation was deposited. 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 700 feet (200 m) 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
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