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

Geology of the Capitol Reef area - Jurassic events

Geology of the Capitol Reef area - Glen Canyon Group

All three formations of the Glen Canyon Group were laid down in the Mid to Late Triassic during a time of increasing aridity. Slightly clockwise and northward movement of the North American Plate was bringing the area into a dryer climatic belt. The direction of cross-bedding in Glen Canyon Group sand dunes suggests that prevailing winds from the north transported the sand into the region. Outcrops of the Glen Canyon Group's three formations are the most prominently exposed rock layers in the spine of the Waterpocket Fold. Together they reach a thickness of 1500 to 2700 feet (460 to 820 m) in the area and their sandstones are seen in many of the arches, domes and slot canyons in Capitol Reef. 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 orange-colored cross bedded fossilized sand dunes made of fine-grained and well-rounded quartz sand. Wingate outcrops are found capping the Waterpocket Fold's western escarpment. Prominent examples are easy to see near the Visitor Center in the Fruita Cliffs and in The Castle.

Later in Triassic time, slow-moving, southwestward flowing streams laid down thin-bedded layers of sand in channels and across low flood plains. Fossilized footprints of dinosaurs and the crocodile-like tritylodonts can be found in this 350 foot (107 m) thick ledgy-slope forming formation, called the Kayenta. The Kayenta weathers into three units; a lower ledge and middle cliff that are dominated by sandstone and an upper slope that has relatively more siltstone. It is often difficult to spot the contact between the Windgate and Kayenta due to their similar color and grain size. One place where the contact is easiest to discern is west of Utah State Route 24 along the Fremont River at mile marker 82.

A massive Sahara-like desert then invaded the area, covering it with 800 to 1100 feet (240 to 335 m) of accumulated white to tan-colored fossilized sand dunes. The resulting formation, called the Navajo Sandstone, is composed of cross-bedded and very clean sandstone with well-rounded, generally very fine-grained and frosted sand. 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 such as Capitol Dome and Navajo Dome. In other places it forms massive cliffs and monoliths. Subsequent erosion leveled the tops of the sand dunes and left them in up to 60 foot (18 m) thick, easy to discern, layers.

Geology of the Capitol Reef area - San Rafael Group

Frequent but short-lived changes in sea level during the Mid to Late Jurassic periodically flooded the area with shallow extensions of the ocean. The resulting San Rafael Group is composed of four formations that were deposited on top of the Glen Canyon Group's regionally-tracable erosion surface. San Rafael Group formations are (from oldest to youngest);

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

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

Climatic conditions were still arid when the locally 0 to 100 feet (0 to 30 m) thick Page Sandstone was deposited above the reach of high tide yet near the shore of an advancing sea (sabkha-like conditions). 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.

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 1000 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.

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. Distinctive jointing systems in the Entrada lead to the formation of cathedrals and monoliths in Capitol Reef's Cathedral Valley, arches in Arches National Park (see Geology of the Arches area) and 'goblins' (the local name for hoodoos) in nearby Goblin Valley State Park. Entrada exposures in the southern part of the park are mostly made of flat-bedded siltstones and erode into slopes. Moving north, Entrada exposures increasingly become rich in cross-bedded sandstone and erode into cliffs with fewer and fewer slopes.

Fine-grained sand and silt mixed with sandy lime were laid down as sediments on top of the Entrada Sandstone, forming the locally 0 to 175 foot (53 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 light grayish-green Curtis can be seen as a capstone in the northern section of the park while it is locally absent in the southern part.

Tidal flat conditions returned as the shallow sea that created the Curtis Formation retreated from land. Thin beds of reddish-brown mudstone alternating with less frequent beds of greenish-gray sandstone and limestone were deposited as sediments, forming the locally 50 to 250 foot (15 to 75 m) thick Summerville Formation. This formation erodes into ledgy cliffs and slopes and can be seen above Curtis caprock in Cathedral Valley. Fossilized mudcraks and ripplemarks are found in the Curtis along with up to 28 foot (8.5 m) thick gypsum-rich beds.

Geology of the Capitol Reef area - Morrison Formation

Again above sea level, streams laid down mud and sand in their channels, on lakebeds, and in swampy plains during the Upper Jurassic. This became the Morrison Formation, which is locally divided into three members (from oldest to youngest);

• Tidwell Member,
• Salt Wash, and
• Brushy Basin.

The 50 to 100 foot (15 to 30 m) thick Tidwell Member is locally difficult to recognize and therefore may or may not be in the area's rocks. Easier to identify and study outcrops elsewhere in southeastern Utah indicate they were deposited in hypersaline lagoons.

Clay, mud, silt, cross-bedded sand and pebbles were later deposited by meandering streams and in flood plains, forming the locally 100 to 500 foot (30 to 150 m) thick Salt Wash Member. Claystone and mudstone beds of this member erode into gray slopes that can exhibit brown, red, yellow, and green colors. Salt Wash sandstone is moderately sorted and fine to medium grained while pebble conglomerate beds are made of chert with small amounts of silica-rich limestone. Both bed types erode into ledges and small cliffs. The Salt Wash was locally mined in the 1950s to extract uranium ore.

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. Clays in this member are rich in smectite and thus tend to swell when wet and dry to a crumbled surface that looks a bit like popcorn. Fossilized dinosaur bones are abundant in this member in several places located in Utah and western Colorado. The bones are usually scattered and are thus hard to identify but nearly complete skeletons have been found in lake floor and flood-plain clays. Good exposures of the Brushy Basin Member can be seen in the Bentonite Hills.

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