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. It started when a shallow invading sea in the Permian laid down sandstone then, as it deepened, limestone. After the sea retreated in the Triassic streams deposited silt before the area was uplifted and underwent erosion. After that conglomerate then logs, sand, mud and wind-transported volcanic ash were added.

The Mid to Late Triassic brought a time of increasing aridity, during which vast amounts of sandstone were created 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 left again, leaving streams, lakes and swampy plains to become the resting place for sediments. Another sea returned in the Cretaceous leaving more sandstone and shale.

Uplift associated with the building of the Rocky Mountains and later the Colorado Plateaus exposed all the above layers far above sea level. Pleistocene ice ages increased the rate of erosion. The 10,000 feet (3000 m) of differentially eroded Mesozoic-aged sedimentary rock in the area dominates the landscape.

Deposition of sediments

Permian

In Permian time the area of the park was on the margin of a shallow and transgressing (advancing onto land) sea which created the Cutler Formation. Only the sandstone of the youngest (and thus topmost) member of the Cutler Formation, the White Rim, is exposed in the park. Outcrops can be found in the bottom of Sulphur Creek and at the bottom of Circle Cliffs outside the park's western border. Lower (older) members of the Cutler are exposed in nearby Canyonlands National Park (see Geology of the Canyonlands area ). The White Rim was deposited as cross-bedded sand dunes.

After the sea invaded the land it laid down a limey ooze that later lithified to form the Kaibab Formation (the same formation that rims the Grand Canyon to the southwest). The limestone of the Kaibab was interbedded with sandstone and was eventually converted into dolomite by the intrusion of magnesium.

Early to Mid Triassic

A marine embayment opened to the north in the Triassic and streams deposited reddish-brown silt, which later became the siltstone of the Moenkopi Formation, much of which has fossilized ripple marks. Uplift and subsequent partial erosion of the Moenkopi then created an unconformity, a gap in the geologic record.

Discontinuous beds of conglomerate were deposited on top of the eroded surface of the Moenkopi, creating the Shinarump Member of the Chinle Formation. 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 soft red and pink shales of the Chinle. Members above the Shinarump tend to be finer due to the slower speed of the streams that deposited them. Uranium salts accumulated in this formation in economically extractable quantities and petrified wood was created (petrification was probably aided by the presence of volcanic ash). The formation tends to erode into rounded hills and slopes.

Glen Canyon Group

Outcrops of the Glen Canyon Group's three formations are most prominently exposed in the spine of the Waterpocket Fold. They are, from oldest (lowest) to youngest (highest);

• Wingate Sandstone
• Kayenta Formation
• Navajo Sandstone

All three were laid down in the Mid to Late Triassic during a time of increasing aridity.

Sand dunes migrated back and forth on the shore of an ancient sea, creating the 350 foot (107 m) thick cliff-forming Wingate Sandstone. This formation is composed of orange-colored cross bedded fossilized sand dunes.

Later in Triassic time, slow-moving streams laid down thin-bedded layers of sand in channels and across low plains. Fossilized footprints of dinosaurs and the crocodile-like tritylodonts can be found in this ledgy-slope forming formation, called the Kayenta.

A massive Sahara-like desert then invaded the area, covering it with 1000 feet (300 m) of accumulated white to tan-colored sand dunes. The resulting formation, called the Navajo Sandstone, is composed of cross-bedded and very clean sandstone. It reached its greatest thickness, 2000 feet (610 m) in what is now Zion National Park (see geology of the Zion and Kolob canyons area). The cross-bedded make-up of the Navajo leads to the formation of curvilinear canyons and rounded domes. Subsequent erosion leveled the tops of the sand dunes.

San Rafael Group

The San Rafael Group is composed of four Triassic-aged formations (from oldest to youngest);

• Carmel Formation,
• Entrada Sandstone,
• Curtis Formation, and
• Summerville Formation.

San Rafael formations can be seen on the east-dipping part of the Waterpocket Fold.

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. A part of the resulting Carmel Formation can be seen capping the Golden Throne.

A near-shore environment dominated by barrier islands/sand bars and tidal flats later returned to the area. The sand deposited created the reddish brown Entrada Sandstone. Distinctive jointing systems in the Entrada lead to the formation of arches in nearby Arches National Park (see Geology of the Arches area ) and 'goblins' (the local name for hoodoos) in the Cathedral Valley part of the Capital Reef area.

Conglomerate, sandstone, and shale (in that order) were laid down as sediments on top of the Entrada Sandstone forming the 50 foot (15 m) thick erosion resistant Curtis Formation. A green iron potassium silicate called glauconite in the Curtis indicates it was deposited in a shallow sea. Outcrops of the Curtis can be seen as a capstone in the northern section of the park.

Reddish-brown mud and white sand were deposited in tidal flats as the shallow sea that created the Curtis Formation regressed (retreated from the land). This formed the mudstone and sandstone of the Summerville Formation, which often erodes into ledgy cliffs.

Late Jurassic to Cretaceous

Again above sea level, streams laid down mud and sand in their channels, on lakebeds, and in swampy plains. This became the Morrison Formation. Its lower (older) member, the Salt Wash, was locally mined in the 1950s to extract uranium ore while its upper (younger) member contains abundant dinosaur bones. The bones are scattered and are thus hard to identify.

Similar non-marine sediments were deposited early in the Cretaceous and become the Dakota Sandstone. Outcrops of the Dakota can be seen in the southern section of the park. This formation was later covered by a vast sea that periodically divided North America in the Cretaceous that geologists call the Cretaceous Seaway.

As the Cretaceous Seaway transgressed (advanced) and regressed (retreated) from land it laid down alternating layers of the Mancos Shale. Parts of this formation are found in some mesas and buttes in the southernmost part of the park and in badlands east of the park.

The youngest formations exposed in the park are part of the Mesaverde Group. Only small remnants are found capping a few mesas in the park's eastern section (see Geology of the Mesa Verde area ).

Uplift

The Laramide orogeny (mountain-building episode) started to compact the region near the end of the Cretaceous period, forming the ancestral Rocky Mountains. Many monoclines were formed by compressive forces associated with the Laramide during this time. One of those monoclines, called the Waterpocket Fold, is the major feature of the park. It has a north-south alignment and a steep east-facing side. It is possible that this particular fold may have been created where it is due to the existence of a fault well below any exposed formations (some small earthquakes centered below the fold in 1979 may be from such a fault).

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.

Compressive forces caused by the Laramide orogeny were followed by some minor stretching as the crust tried to find a new equilibrium. This created weaknesses in the crust that magma took advantage of by squeezing toward the surface, probably starting in the Oligocene epoch. Dikes and sills respectively intruded through and between formations now exposed in South Desert and Cathedral Valley at the northern end of the fold. Small basaltic lava flows then erupted through fissures at the surface. Igneous activity continued sporadically afterwards. Subsequent erosion preferentially removed the softer sedimentary rock that initially entombed the dikes, sills, and volcanic plugs, often leaving them standing in relief.

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 pluvial climate ( anarid climate, watered and made cooler by distant large glaciers) developed during the Pleistocene epoch and briefly returned via at least two neoglacial episodes (little ice ages) in the Holocene. These times brought increased precipitation and consequently a faster rate of erosion. Glaciers plucked black basaltic boulders from atop Boulder and Thousand Lakes mountains that were subsequently deposited over the park area by floods of glacial melt water.

Reference

• Geology of National Parks: Fifth Edition, Ann G. Harris, Esther Tuttle, Sherwood D., Tuttle (Iowa, Kendall/Hunt Publishing; 1997) ISBN 0-7872-5353-7

External links

• National Park Service: Capitol Reef Geology
• American Park Network: The Creation of Capitol Reef