The exposed geology of the Capitol Reef area presents a record of mostly Mesozoic-aged sedimentation in an area of North America in and around Capitol Reef National Park, on the Colorado Plateau in southeastern Utah.
Nearly 10,000 feet (3,000 m) of sedimentary strata are found in the Capitol Reef area, representing nearly 200 million years of geologic history of the south-central part of the U.S. state of Utah. These rocks range in age from Permian (as old as 270 million years old) to Cretaceous (as young as 80 million years old.)NPS, "Capitol Reef" Rock layers in the area reveal ancient climates as varied as rivers and swamps (Chinle Formation), Sahara-like deserts (Navajo Sandstone), and shallow ocean (Mancos Shale).
The area's first known sediments were laid down as a shallow sea invaded the land in the Permian. At first sandstone was deposited but limestone followed as the sea deepened. After the sea retreated in the Triassic, streams deposited silt before the area was uplifted and underwent erosion. Conglomerate followed by logs, sand, mud and wind-transported volcanic ash were later added. Mid to Late Triassic time saw increasing aridity, during which vast amounts of sandstone were laid down along with some deposits from slow-moving streams. As another sea started to return, it periodically flooded the area and left evaporite deposits. Barrier islands, sand bars and later, tidal flats, contributed sand for sandstone, followed by cobbles for conglomerate, and mud for shale. The sea retreated, leaving streams, lakes and swampy plains to become the resting place for sediments. Another sea, the Western Interior Seaway, returned in the Cretaceous and left more sandstone and shale only to disappear in the early Cenozoic.
From 70 to 50 million years ago the Laramide orogeny, a major mountain building event in western North America, created the Rocky Mountains to the east. The uplift possibly acted on a buried fault to form the area's Waterpocket Fold. More recent uplift of the entire Colorado Plateau and the resulting erosion has exposed this fold at the surface only within the last 15 to 20 million years. Ice ages in the Pleistocene increased the rate of precipitation and erosion. The cracked upper parts of the Waterpocket Fold were especially affected and the fold itself was exposed and dissected.
Primary deposition of sediments Some important concepts: A formation is a formally named and defined geologic unit with unique characteristics. Those characteristics were created during a largely unbroken period of time and result from the specific depositional environment that the formation was laid down in. A member is a minor unit in a formation and a bed is a distinct subunit of a member. Groups are sets of formations that are related in significant ways such as, for example, all being deposited during a dry period that lasted millions of years or as the result of an ocean periodically flooding the same area over millions of years.
The various kinds of unconformities are gaps in the geologic record. Such gaps can be due to a prolonged absence of deposition or due to subsequent erosion that removes previously deposited rock units. The following sections are ordered from oldest to youngest rock units in order to create a geologic history of events. This is the opposite order one would see in an actual cross section of the sediments because newer rock units are deposited on top of older ones per the law of superposition.
Cutler and Kaibab formations (Permian)
In early Permian time, Utah was on a continental shelf that was occasionally covered by a shallow arm of the Panthalassa Ocean.Stokes, 1988, page 95, paragraph 3 That part of Laurasia was on a passive continental margin not unlike the current west coast of equatorial Africa. The resultant formations are part of the approximately 290- to 250-million-year-old Cutler FormationMorris et al., 2003, page 96, Tectonics and Geologic History" (called a group locally)'Cutler Formation' is the accepted usage by the USGS while 'Cutler Group' is preferred by the Utah Geological Survey, according to the GEOLEX database entry for 'Cutler'. Accessed March 18, 2006 Utah was nearly on the paleoequator while the first members of the Cutler Formation were deposited but it had migrated nearly to 10° north latitude by around 275 million years ago. The Cutler records sedimentation during this time and is composed of four members (youngest to oldest): White Rim Sandstone, (resistant caprock) Organ Rock Shale (locally missing), Cedar Mesa Sandstone, and the Elephant Canyon (locally buried).
Only the two sandstone members of the Cutler Formation, the Cedar Mesa and White Rim, are exposed in the park but they cannot be easily distinguished from each other and are thus often treated as a single stratigraphic unit there.Morris et al., 2003, page 86, "Culter Group", paragraph 1 The White Rim and Cedar Mesa are composed of fossilized cross-bedded sand dunes that were likely deposited in an arid coastal environment that periodically flooded with sea water. Sand in these formations are somewhat sorted by size, well-rounded (worn by abrasion), and range from very fine- to medium-grained.Morris et al., 2003, page 90
Good outcrops of the locally 800 foot (240 m) thick Cedar Mesa and 420 foot (128 m) thick White Rim can be found in the bottom of Sulphur Creek and at the bottom of the Circle Cliffs outside the park's western border.Harris et al., 1997, page 62 In other areas the Organ Rock Shale is between the Cedar Mesa and White Rim but it pinches out east of the park. Both the locally buried Elephant Canyon and missing Organ Rock are exposed in nearby Canyonlands National Park 60 miles (100 km) east (see geology of the Canyonlands area).
Later in Permian time, the Kaibab Sea invaded the land and laid down a limey ooze that later lithified to form the locally up to 200 foot (60 m) thick Kaibab Limestone. This is the same light gray to white formation that rims the Grand Canyon to the southwest (see Geology of the Grand Canyon area). The lower parts of the Kaibab were interbedded with sand and silt before its main component, limestone, was converted into chert-rich dolomite by the intrusion of magnesium. The formation contains fossils of invertebrates including brachiopods, bryozoans, crinoids, gastropods, and pelecypods. Outcrops of the cliff-forming Kaibab in Capitol Reef can only be seen in the deeper canyons located in the westernmost part of the park.Morris et al., 2003, page 90, "Kaibab Limestone" Retreat of the Kaibab Sea by the Mid Permian exposed its seabed to erosion, resulting in 100 foot (30 m) deep channels and the creation of a gap in the geologic record called an unconformity.
Moenkopi Formation (Triassic)
Local climatic conditions were wetter and more tropical in the Early Triassic than they were previously. In the Capitol Reef area the resulting Moenkopi Formation is divided into four members (from oldest to youngest): Black Dragon Member Sinbad Limestone Member, Torrey Member, and Moody Canyon Member. Distinctive exposures of the uppermost Moenkopi are along the lower slopes of the west-facing cliffs of the Waterpocket Fold. Uplift and subsequent partial erosion of the Moenkopi then created an approximately 6 million year long unconformity that lasted the entire Mid Triassic.
The 50 to 110 foot (15 to 34 m) thick slope-forming Black Dragon is composed of reddish conglomerate, sandstone, and siltstone that were likely deposited on a coastal plain followed by a tidal flat. Chert clasts from the underlying Kaibab Limestone make-up part of conglomerates at the member's base while ripple marks and mudcracks are common in its upper parts. Also common in the upper parts of the member are thin interbeds of carbonate rock with some fossils.
Later in the early Triassic, fossil-rich to muddy limey ooze with small amounts of silt and sand were laid down as a short-lived arm of the ocean covered the region. This created the locally 70 to 140 foot (21 to 43 m) thick yellowish-colored Sinbad Limestone member of the Moenkopi. Some of the fossils found in this layer include the brachiopod genus Lingula and the ammonite genus Meekoceras.
A tidal flat briefly returned to the area after the sea retreated. This created the locally 250 to 320 foot (76 to 98 m) thick reddish-brown to chocolate colored siltstone and fine-grained sandstone of the Moenkopi's Torrey Member.Morris et al., 2003, page 90 to 91, "Moenkopi Formation" Some of the finer-grained beds display ripple marks and mudcracks while the sandstone has horizontal and low-angle crossbedding. Small to large fossilized track-ways from amphibians and reptiles are found in this layer as well as casts of halite.
The youngest member of the Moenkopi is the 320 to 430 foot (98 to 130 m) thick Moody Canyon Member. Moody Canyon is informally sub-divided into two units: a lower slope-forming 200 to 300 foot (60 to 90 m) thick unit of reddish-brown siltstone and an upper cliff-forming 120 to 130 foot (37 to 40 m) thick unit of reddish-orange siltstone. About 30 to 40% of the stone in the upper unit is ripple-laminated while the lower unit is structureless to horizontally laminated.Morris et al., 2003, page 91 Good exposures of the ripple-laminated upper unit are found on the lower part of Egyptian Temple.
Chinle Formation (Triassic)
A complex, relatively high velocity and likely braided stream system covered most of southern Utah in the Late Triassic. Various members of the resulting Chinle Formation are found over much of the Colorado Plateaus. Logs, sand, mud and wind-transported volcanic ash from distant eruptions were mixed by streams as they migrated over a subsiding basin to form the Chinle. Uranium salts accumulated in this formation in economically extractable quantities and petrified wood was formed (petrification was probably aided by the presence of volcanic ash). Chinle members represented in the Capitol Reef area are (from oldest to youngest): Shinarump Member, Monitor Butte Member, Petrified Forest Member, and Owl Rock Member. Together they form the purple and orange rounded slopes and hills that are occasionally above white cliffs along the Waterpocket Fold's west face.
Discontinuous beds of sediment were deposited in broad channels eroded into the Moenkopi, creating the locally 0 to 90 foot (0 to 27 m) thick white to yellowish gray cliff-forming Shinarump Member of the Chinle Formation. The Shinarump is made of low to high-angle cross-stratified and friable fine to coarse-grained sandstone interbedded with conglomerate. Good outcrops of the Shinarump are found near the park's west entrance capping such features as the Egyptian Temple and Chimney Rock while all traces of this member are absent further east. Members above the Shinarump tend to be finer due to the slower speed of the streams that deposited them.
A river system then migrated northward and transitioned into either a large lake or marsh. As this occurred, bentonite-rich clay (formed in part from volcanic ash erupted from nearby volcanoes) and clayey cross-bedded sand with some interbeds and lenses of lime ooze were laid down. This eventually became the light purplish-gray claystone, sandstone, and carbonate rock of the Chinle's Monitor Butte Member. Burrows that measure 5 inches (13 cm) in diameter and 5 feet (1.5 m) long were excavated by lungfish into the Monitor Butte. This member easily stands out in the lower to middle slopes of the Waterpocket Fold's west face in such places as The Castle and along the cliff's just north of State Route 24 as it enters the park.