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

Geology of the Capitol Reef area - Cretaceous events

Geology of the Capitol Reef area - Cedar Mountain and Dakota Sandstone

Early Cretaceous time brought continental deposition that was dominated by rivers to the area. Sandstones and mudstones accumulated to form the 0 to 166 foot (50.5 m) thick slope-forming Cedar Mountain Formation. The 73 foot (22 m) thick Buckhorn Conglomerate Member thins out north and east of the park and is nearly absent in it, making it difficult to distinguish the underlying Morrison from the somewhat more pastel-colored Cedar Mountain. Fossilized freshwater animals such as mollusks and ostrapods along with dinosaurs, fish scales, pollen and a genus of fern called Tempskya have been found in this formation.

The passive continental margin went active when the Faralon 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 weight of the resulting high mountain range that formed to the west, lowered much of Utah and allowed the sea to invade. This grew into a vast sea that periodically divided North America in the Cretaceous called the Western Interior Seaway.

Non-marine sediments of the approximately 100 to 94 million year old Dakota Sandstone were deposited on the shore of this seaway early in the Cretaceous. The 0 to 150 foot (0 to 45 m) thick formation consists of fine-grained tan to brownish-gray colored quartz-rich sandstone interbedded with thin layers of carbon-rich shale, coal, and conglomerate.

Petrified wood is found in the lower part of the formation while fossilized marine bivalves such as Corbula and Pycnodonte newberryi are in the upper layers. This fossil progression shows a record of flooding that created the seaway. Dakota erode into small cliffs and hogbacks that can be seen in the southern section of the park.

Geology of the Capitol Reef area - Mancos Shale

Approximately 94 to 85 million years ago, the seaway advanced onto and retreated from land as it laid down the Mancos Shale. The Mancos is composed mostly of shale but two of its members, the Ferron and Muley Canyon, are sandstone that were laid down when relative sea level temporarily dropped. The five Mancos members from oldest to youngest are:

1. Tununk Shale,
2. Ferron Sandstone,
3. Blue Gate Shale,
4. Muley Canyon, and
5. Masuk.

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.

Open marine conditions created the 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. 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 205 to 385 feet (62 to 117 m) thick cliff-forming Ferron Sandstone. The marine bivalve Inoceramus and trace fossils of Ophiomorpha are found in the lower part of this member. Ferron Sandstone north of the area contains seams of coal and petroleum geologists have studied this member to model oil-bearing regions.

Open marine conditions returned in the Late Cretaceous, forming the 1200 to 1500 foot (365 to 460 m) thick slope-forming Blue Gate Shale. This member is composed of bentonite rich clays, siltstone and some sandstone. It erodes into gullied slopes similar in appearance to the Tununk Shale. The presence of two species of planktonic foraminifera in the upper Blue Gate, Clioscaphites vermiformis and Clioscaphites choteauenis, was used to date this member.

An ancient shoreline once again approached the area, resulting in the formation of the 300 to 400 feet (90 to 120 m) thick Muley Canyon Member. It is composed of evenly-bedded, fine-grained sandstone and carbon-rich shales. Coal beds are found in the upper parts of this member, indicating continental coastal plain conditions at that time.

Alternating layers of shallow marine and non-marine sediments were deposited as the shoreline fluctuated back and forth over the area. These sediments became the 650 to 750 foot (200 to 230 m) thick Masuk Member. The Masuk consists of cliff-forming cross-bedded sandstones and slope-forming yellowish-gray to bluish-gray mudstones. Fossils of bivalves, ceratopsian dinosaurs, crocodiles, gastropods, and turtles have been collected in this member.

Geology of the Capitol Reef area - Mesaverde Formation

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 shale. Only small remnants are found capping a few mesas in the park's eastern section (see Geology of the Mesa Verde area).

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