thumb|300px|The [[Waterpocket Fold is the major geographic feature in the area of the park. This view is from above Capitol Reef Scenic Drive looking back at the west face of the broken and eroded fold.]]
thumb|300px|The [[Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah that makes up much of the famous prominent rock formations in protected areas such as Capitol Reef National Park and Canyonlands National Park. From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically-jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone. Picture from Glen Canyon National Recreation Area, Utah.]]
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.) 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.
250px|thumb|[[Stratigraphic section (USGS),
Geologic cross section (NPS)]]
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)
thumb|The ocean surrounding the supercontinent [[Pangaea is Panthalassa]]
In early Permian time, Utah was on a continental shelf that was occasionally covered by a shallow arm of the Panthalassa Ocean. 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 Formation (called a group locally) 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 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.
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. 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. 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. 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: Good exposures of the ripple-laminated upper unit are found on the lower part of Egyptian Temple.
Chinle Formation (Triassic)
[[File:Chinle Formation showing each member on a cliff above Capitol Reef Scenic Drive.jpeg|thumb|Chinle Formation section showing each member represented in the Capitol Reef area: Monitor Butte Member (m), the two units of the Petrified Forest Member (p) and the Owl Rock Member (o - partially obscured by overlaying Wingate rubble) (cropped image). Non-cropped version ]]
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). They are, from oldest (lowest) to youngest (highest);
- Wingate Sandstone
- Kayenta Formation
- Navajo Sandstone
Sand dunes migrated back and forth on the shore of the Sundance Sea, creating the 350 foot (107 m) thick cliff-forming Wingate Sandstone. This formation is composed of three members; the
- Harris Wash,
- Judd Hollow, and the
- Thousand Pockets.
Together they were laid on top of the Navajo sand dunes as the sea slowly flooded the vast desert. An outcrop of the Judd Hollow Member can be seen from mile marker 86.5 as a red cliff above the Fremont River falls. The cross-bedded sandstone just above the red cliff is an example of the Thousand Pockets Member.
thumb|left|The Fremont River crosses the Waterpocket Fold in the upper half of this satellite picture, while the white line of Capitol Reef bisects the lower half.
In Mid Jurassic time gypsum, sand, and limey silt were deposited in what may have been a graben that was periodically covered by sea water and thus a place where repeated flooding was followed by evaporation. The resulting Carmel Formation is composed of 200 to 1,000 feet (60 to 300 m) of reddish-brown siltstone, mudstone and sandstone that alternates with whitish-gray gypsum and fossil-rich limestone in a banded pattern. Fossils include marine bivalves and ammonites. Most of the Carmel has been removed from the Waterpocket Fold's crest but outcrops can be seen capping the Golden Throne and atop various domes in the area. It can also be seen as reddish-brown triangular-shaped spurs called 'flatirons' that form the Waterpocket Fold's eastern rampart.
thumb|[[Cathedral Mountain (Capitol Reef)|Cathedral Mountain in Cathedral Valley is composed of Entrada Sandstone capped by Curtis Formation]]
A near-shore environment dominated by barrier islands, sand bars and tidal flats later returned to the region. The sand and silt deposited created the 400 to 900 foot (120 to 275 m) thick reddish orange Entrada Sandstone.
The 200 to 350 foot (60 to 105 m) thick Brushy Basin Member is composed of claystone, mudstone, and siltstone with small amounts of conglomerate and sandstone.
The passive continental margin became active when the Farallon Plate started to dive below the North American Plate. Geologists call the resulting mountain-building event the Sevier orogeny. Compressive forces detached sedimentary units across western Utah and Nevada from their Precambrian basement rocks and pushed them eastward. The up to 150 foot (45 m) thick formation consists of fine-grained tan to brownish-gray colored quartz-rich sandstone that is interbedded with thin layers of carbon-rich shale, coal, and conglomerate.
Mancos Shale and Mesaverde Formation (Cretaceous)
Approximately 94 to 85 million years ago, the seaway advanced onto and retreated from land as it laid down the Mancos Shale.
Open marine conditions created the locally 40 to 720 feet (12 to 220 m) thick gullied slope-forming Tununk Shale Member. It is made of bluish-gray shale with interbedded mudstone, fine-grained sandstone and siltstone. The Tununk erodes into a slope and is locally fossil-rich. It is most prominently exposed in the Blue Desert immediately southeast of Cathedral Valley and contains fossilized examples of cephalopods, bivalves, and fish scales.
A wave-dominated delta and river system then spread over the area, creating the locally 205 to 385 feet (62 to 117 m) thick cliff-forming Ferron Sandstone. It is composed of brown fine-grained sandstone along with white cross-bedded sandstone with interbedded carbonate-rich gray shale.
The Western Interior Seaway was shrinking due to infilling and uplift while the high mountains to the east were being reduced by erosion. Barrier beaches and river deltas migrated eastward into the seaway. The resulting 300 to 400 foot (90 to 120 m) thick Mesaverde Formation consists of light-brown to dark-gray thick-bedded and cross-stratified sandstone with interbedded dark gray shale and intertongues with the Masuk Member of the overlying Mancos Shale. Only small remnants are found capping a few mesas in the park's eastern section<!-- red link (see geology of the Mesa Verde area)-->.
Uplift and Cenozoic events
Waterpocket Fold, Lake Uinta and volcanism
frame|Waterpocket Fold cross section showing eroded part (NPS)<br/>[[media:Waterpocket Fold - Looking south from the Strike Valley Overlook.jpg|Photo of the fold looking south from the Strike Valley Overlook (USGS)]]
The Laramide orogeny compacted the region from about 70 million to 50 million years ago and in the process created the Rocky Mountains. Many monoclines (a type of gentle upward fold in rock strata) were also formed by the deep compressive forces of the Laramide. One of those monoclines, called the Waterpocket Fold, is the major geographic feature of the park. The 100 mile (160 km) long fold has a north–south alignment with a steeply east-dipping side. The rock layers on the west side of the Waterpocket Fold have been lifted more than 7,000 feet (2,100 m) higher than the layers on the east. Thus older rocks are exposed on the western part of the fold and younger rocks on the eastern part. This particular fold may have been created due to movement along a fault in the Precambrian basement rocks hidden well below any exposed formations. Small earthquakes centered below the fold in 1979 may be from such a fault.
Contemporary with the Waterpocket Fold's formation was the development of an intermontane (between mountains) basin in the area. Lake Uinta filled this basin with stream water derived from the north and south. This large lake existed from about 58 million until 35 million years ago and is responsible for creating the Flagstaff Limestone and Green River Formation, which locally reach a thickness of around 200 feet (60 m). Elsewhere these formations have a combined thickness of over 9,000 feet (2,740 m). Examples can be seen in South Desert and Cathedral Valley at the northern end of the fold.
Erosion
thumb|The Fremont River has been able to keep up with the uplift of the Waterpocket Fold.
Ten to fifteen million years ago the entire region was uplifted several thousand feet (well over a kilometer) by the creation of the Colorado Plateaus. This time the uplift was more even, leaving the overall orientation of the formations mostly intact. Most of the erosion that carved today's landscape occurred after the uplifting of the Colorado Plateau with much of the major canyon cutting probably occurring between 1 and 6 million years ago. Even in this desert climate, water is the erosional agent most responsible for the carving of the landscape. The pull of gravity, in the form of rock falls or rock creep, plays a major role in the shaping of the cliff lines. Wind is a minor agent of erosion here.
The drainage system in the area was rearranged and steepened as the Waterpocket Fold was uplifted. Larger streams, such as the Fremont River, were more likely to keep up with the uplift by downcutting into the Waterpocket Fold faster. Other streams, such as Sand Creek, changed their course by flowing parallel to the fold and cutting into less resistant formations. Yet other streams tried to keep up with the uplift by carving slot canyons only to later change course, leaving their canyons literally high and dry. A total of 7,000 feet (2,100 m) of overlying Mesozoic and Cenozoic sediment has been removed by erosion in the area.
thumb|Big Thomson Mesa seen from space
Wetter and cooler conditions developed during the Pleistocene epoch and briefly returned via at least two neoglacial episodes (little ice ages) in the current epoch, the Holocene. The various rivers and streams in the area were engorged by increased precipitation and with melt-water from mountain glaciers on the Henry Mountains to the east and the Aquarius Plateau to the west of the park. Flash floods, mass wasting of hillsides, frost wedging, and landslides all contributed to a significantly faster rate of erosion. Glaciers plucked 20- to 30-million-year-old black basaltic boulders from atop Boulder and Thousand Lake Mountains that were subsequently deposited over the park area by meltwater streams from the glaciers, rockslides and floods.
References
Works cited
- Billingsley, G.H., Breed, W.J. and Huntoon, P.W.; 1987; Geologic Map of capitol Reef National Park and vicinity, Utah; Utah Geologic Survey (viewed March 25, 2006)
- Halka Chronic, Roadside Geology of Utah (Mountain Press; 1990)
- Ann G. Harris, Esther Tuttle, Sherwood D., Tuttle, Geology of National Parks: Fifth Edition (Iowa, Kendall/Hunt Publishing; 1997)
- L. F. Hintze, Geologic History of Utah, Brigham Young University Geology Studies, v. 20, Part 3. Provo UT, page 181
- Thomas H. Morris, Vicky Wood Manning, and Scott M. Ritter, "Geology of Capitol Reef National Park, Utah" in Geology of Utah's Parks and Monuments, Douglas A. Sprinkel, Thomas C. Chindsey, Jr., and Paul B. Anderson, Editors (Salt Lake City; Utah Geological Association; 2003)
- National Park Service, "Capitol Reef: Geology" (viewed March 25, 2006)
- William Lee Stokes, Geology of Utah (Salt Lake City; Utah Museum of Natural History; 1988)