 | Geology of the Capitol Reef area: Encyclopedia II - Geology of the Capitol Reef area - Triassic events
Geology of the Capitol Reef area - Triassic events
Geology of the Capitol Reef area - Moenkopi Formation
Local climatic conditions were wetter and more tropical in the Early Triassic than they were previously. In the Capitol Reef area the resulting Moenkopi Formation is divided into four members (from oldest to youngest):
- Black Dragon Member
- Sinbad Limestone Member,
- Torrey Member, and
- Moody Canyon Member.
Distinctive exposures of the uppermost Moenkopi are along the lower slopes of the west-facing cliffs of the Waterpocket Fold. Uplift and subsequent partial erosion of the Moenkopi then created an approximately 6 million year long unconformity that lasted the entire Mid Triassic.
The 50 to 110 foot (15 to 34 m) thick slope-forming Black Dragon is composed of reddish conglomerate, sandstone, and siltstone that were likely deposited on a coastal plain followed by a tidal flat. Chert clasts from the underlying Kaibab Limestone make-up part of conglomerates at the member's base while ripple marks and mudcracks are common in its upper parts. Also common in the upper parts of the member are thin interbeds of cardonate rock with some fossils.
Later in the early Triassic, fossil-rich to muddy limey ooze with small amounts of silt and sand were laid down as a short-lived arm of the ocean covered the region. This created the locally 70 to 140 foot (21 to 43 m) thick yellowish-colored Sinbad Limestone member of the Moenkopi. Some of the fossils found in this layer include the brachiopod genus lingula and the ammonite genus Meekoceras.
A tidal flat briefly returned to the area after the sea retreated. This created the locally 250 to 320 foot (76 to 98 m) thick reddish-brown to chocolate colored siltstone and fine-grained sandstone of the Moenkopi's Torrey Member. 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:
- a lower slope-forming 200 to 300 foot (60 to 90 m) thick unit of reddish-brown siltstone and
- an upper cliff-forming 120 to 130 foot (37 to 40 m) thick unit of reddish-orange silstone.
About 30 to 40% of the stone in the upper unit is ripple-laminated while the lower unit is structureless to horizontally laminated. Good exposures of the ripple-laminated upper unit are found on the lower part of Egyptian Temple.
Geology of the Capitol Reef area - Chinle Formation
A complex, relatively high velocity and likey 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). Chinle members represented in the Capitol Reef area are (from oldest to youngest):
- Shinarump Member,
- Monitor Butte Member,
- Petrified Forest Member, and
- Owl Rock Member.
Together they form the purple and orange rounded slopes and hills that are occasionaly above white cliffs along the Waterpocket Fold's west face.
Discontinuous beds of sediment were deposited in broad channels eroded into the Moenkopi, creating the locally 0 to 90 foot (0 to 27 m) thick white to yellowish gray cliff-forming Shinarump Member of the Chinle Formation. The Shinarump is made of low to high-angle cross-stratified and friable fine to coarse-grained sandstone interbedded with conglomerate. Good outcrops of the Shinarump are found near the park's west entrance capping such features as the Egyptian Temple and Chimney Rock while all traces of this member are absent further east. Members above the Shinarump tend to be finer due to the slower speed of the streams that deposited them.
A river system then migrated northward and transitioned into either a large lake or marsh. As this occurred, bentonite-rich clay (formed in part from volcanic ash erupted from nearby volcanoes) and clayey cross-bedded sand with some interbeds and lenses of limey ooze were laid down. This eventually became the light purplish-gray claystone, sandstone, and carbonate rock of the Chinle's Monitor Butte Member. Burrows that measure 5 inches (13 cm) in diameter and 5 feet (1.5 m) long were excavated by lungfish into the Monitor Butte. This member easily stands out in the lower to middle slopes of the Waterpocket Fold's west face in such places as The Castle and along the cliff's just north of Utah State Route 24 as it enters the park.
Very sinuous rivers covered the area in later Triassic time, forming the sediments of the Chinle's Petrified Forrest Member. The Petrified Forrest is composed of reddish-orange bentonite-rich cross-bedded siltstones and clayey fine-grained quartz sandstones. The lower part of this member forms gullied slopes and the upper part forms a resistant cliff called the 'Capitol Reef bed.' Carbonate nodules along with fossil bivalves, coprolites, marine snails, lungfish toothplates, and tetrapods are found in the Petrified Forrest Member.
Deposition from a series of lakes then dominate the geologic record, resulting in the locally 150 to 200 foot (45 to 60 m) thick Owl Rock Member of the Chinle. The Owl Rock is made of purple and orange mudstones, fine-grained sandstones and siltstones with interbedded 1 to 10 foot (30 to 300 cm) thick green to mottled pink micritic and knobby limestone. Fossilized desiccation mudcracks that are up to 4 inches (10 cm) wide and 3 feet (1 m) deep are found at the very top of the Owl Rock (they were later filled with sand from the overlying Windgate Sandstone). Also found in the Owl Rock are trace fossils of cylindrical burrows and fossils of ostracodes. The Owl Rock erodes into littered slopes and is found directly below the Windgate Sandstone along the Waterpocket Fold's west face.
Other related archives1950s, 1979, Africa, Arches National Park, Barrier islands, Boulder, Canyonlands National Park, Capitol Reef National Park, Carbonate, Chert, Colorado, Colorado Plateaus, Cretaceous, Dikes, Faralon Plate, Geology of the Canyonlands area, Geology of the Grand Canyon area, Goblin Valley State Park, Grand Canyon, Green River Formation, Holocene, Jurassic, Laramide orogeny, Laurasia, Mesozoic, Morrison Formation, Nevada, North America, North American Plate, Panthalassa Ocean, Permian, Petrified wood, Pleistocene, Rocky Mountains, Sahara, Sandstone, Sevier orogeny, Sundance Sea, Tertiary, Thousand Lake Mountains, Triassic, Uranium, Utah, Utah State Route 24, Waterpocket Fold, Western Interior Seaway, Zion National Park, ammonite, ammonites, amphibians, arches, arid, badlands, basaltic, beaches, bentonite, bivalves, brachiopod, brachiopods, bryozoans, buttes, canyons, carbon, carbonate rock, cephalopods, chert, clay, clays, climate, coal, coastal plain, composite volcanos, conglomerate, continental shelf, coprolites, crinoids, crocodile, crocodiles, delta, desert, dinosaur, dinosaurs, dolomite, earthquakes, erosion, evaporation, evaporite, fault, flood plains, flooding, foraminifera, formations, fossil, fossilized, fossils, gastropods, geology of the Zion and Kolob canyons area, glaciers, glauconite, graben, gullied, gypsum, halite, hoodoos, ice ages, invertebrates, iron, lagoons, lake, lakes, limestone, limey, lingula, lithified, little ice ages, lungfish, magma, magnesium, marsh, members, mesas, mollusks, monoliths, mountain range, mudstone, mudstones, orogeny, paleoequator, pelecypods, petrified wood, petroleum, pluvial, pollen, popcorn, potassium, precipitation, quartz, reptiles, river deltas, salts, sand, sand dunes, sandstone, sea level, sea water, shale, silica, silicate, sills, silt, siltstone, siltstones, skeletons, slot canyons, smectite, snails, streams, swampy, tetrapods, tidal flat, tidal flats, tide, trace fossils, turtles, uplifted, uranium, volcanic ash, volcanoes, winds
 Adapted from the Wikipedia article "Triassic events", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki |
