Geology of the Capitol Reef area: Encyclopedia II - Geology of the Capitol Reef area - Permian events

Geology of the Capitol Reef area - Permian events

Geology of the Capitol Reef area - Cutler Group

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 Group. Utah was nearly on the paleoequator while the first formations of the Cutler Group were deposited but it had migrated nearly to 10° north latitude by around 275 million years ago. The Cutler Group records sedimentation during this time and is composed of four formations (oldest to youngest):

• Elephant Canyon Formation (locally buried),
• Cedar Mesa Sandstone,
• Organ Rock Shale (locally missing), and the
• White Rim Sandstone.

Only the two sandstone formations of the Cutler Group, the Cedar Mesa and White Rim, are exposed in the park but they can't be easily distinguished from each other and are thus often treated as a single stratigraphic unit there. 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 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).

Geology of the Capitol Reef area - Kaibab Limestone

Later in Permian time, the Kaibab Sea invaded the land and laid down a limey ooze that later lithified to form the locally 0 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. 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 uncomformity.

Adapted from the Wikipedia article "Permian events", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki