The geology of the Zion and Kolob canyons area includes nine known exposed formations, all visible in Zion National Park in the U.S. state of Utah. Together, these formations represent about 150 million years of mostly Mesozoic-aged sedimentation in that part of North America. Part of a super-sequence of rock units called the Grand Staircase, the formations exposed in the Zion and Kolob area were deposited in several different environments that range from the warm shallow seas of the Kaibab and Moenkopi formations, streams and lakes of the Chinle, Moenave, and Kayenta formations to the large deserts of the Navajo and Temple Cap formations and dry near shore environments of the Carmel Formation.
Subsequent uplift of the Colorado Plateau slowly raised these formations much higher than where they were deposited. This steepened the stream gradient of the ancestral rivers and other streams on the plateau. The faster-moving streams took advantage of uplift-created joints in the rocks to remove all Cenozoic-aged formations and cut gorges into the plateaus. Zion Canyon was cut by the North Fork of the Virgin River in this way. Lava flows and cinder cones covered parts of the area during the later part of this process.
Zion National Park includes an elevated plateau that consists of sedimentary formations that dip very gently to the east. This means that the oldest strata are exposed along the Virgin River in the Zion Canyon part of the park, and the youngest are exposed in the Kolob Canyons section. The plateau is bounded on the east by the Sevier Fault Zone, and on the west by the Hurricane Fault Zone. Weathering and erosion along north-trending faults and fractures influence the formation of landscape features, such as canyons, in this region.
Grand Staircase and basement rocks
The Grand Staircase is an immense sequence of sedimentary rock layers that stretch south from Bryce Canyon National Park through Zion National Park and into the Grand Canyon. Within this sequence, the oldest exposed formation in the Zion and Kolob canyons area is the youngest exposed formation in the Grand Canyon—the Kaibab limestone. Bryce Canyon to the northeast continues where the Zion and Kolob areas end by presenting Cenozoic-aged rocks. In fact, the youngest formation seen in the Zion and Kolob area is the oldest exposed formation in Bryce Canyon—the Dakota Sandstone.
In the Permian period, the Zion and Kolob area was a relatively flat basin near sea level on the western margin of the supercontinent Pangaea. Sediments from surrounding mountains added weight to the basin, keeping it at relatively the same elevation. These sediments later lithified (turned to rock) to form the Toroweap Formation, now exposed in the Grand Canyon to the south but not in the Zion and Kolob area. This formation is not exposed in the park, though it does form its basement rock.
Deposition of sediments
Kaibab Limestone (Upper Permian) In later Permian time, the Toroweap Basin was invaded by the warm, shallow edge of the vast Panthalassa ocean in what local geologists call the Kaibab Sea. At that time, Utah and Wyoming were near the equator on the western margin of the supercontinent Pangaea.
Starting 260 million years ago, the yellowish-gray limestone of the fossil-rich Kaibab Limestone was laid down as a limy ooze in a tropical climate. During this time, sponges, such as Actinocoelia meandrina, proliferated, only to be buried in lime mud and their internal silica needles (spicules) dissolved and recrystallized to form discontinuous layers of light-colored chert. In the park, this formation can be found in the Hurricane Cliffs above the Kolob Canyons Visitor Center and in an escarpment along Interstate 15 as it skirts the park. This is the same formation that rims the Grand Canyon to the south.
Farther to the west, a complex island arc assemblage formed above a subduction zone. To the east, in western Colorado, a mountain range similar to today's Himalayas called the Uncompahgre Mountains bordered the Utah lowland. The interfingering of the Kaibab with the White Rim Sandstone, now exposed in Capitol Reef National Park area, to the east suggests that the marine facies of the Kaibab migrated eastward in response to a relative sea-level rise, or transgression (the White Rim is not exposed in the Zion area). The sea moved back and forth across Utah, but by the Middle Permian, the sea had withdrawn and the Kaibab Limestone was exposed to erosion, creating karst topography and channels reaching 30 m (100 ft) in depth.
Moenkopi Formation (Lower Triassic)
Volcanoes continued to erupt through the Early Triassic on the north–south trending island arc to the west, which was located along what is now the border between California and Nevada. Shallow, marine water stretched from eastern Utah to eastern Nevada over a beveled continental shelf. As the sea withdrew around 230 million years ago, fluvial, mudflat, sabkha, and shallow marine environments developed, depositing gypsum (from lagoon evaporites), mudstones, limestones, sandstones, shales, and siltstones.
It took many thousands of thin layers of these sediments to form the thick Moenkopi Formation. A prograding shoreline laid down muddy delta sediments which mixed with limy marine deposits. The fossilized plants and animals in the Moenkopi are evidence of a climate shift to a warm tropical setting that may have experienced monsoonal, wet-dry conditions.
The Red Canyon Conglomerate, the basal member of the Moenkopi, fills broad east-flowing paleochannels carved into the Kaibab Limestone. Some of these channels are up to several tens of feet deep and may reach 200 ft (61 m) deep in the St. George area. A thin, poorly developed soil, or regolith, formed over the paleotopographic high areas between the channels.
The depositional environment was a nearshore one where the seashore alternated between advance (transgression) and retreat (regression). At Zion, the limestones and fossils of the Timpoweap, Virgin Limestone, and Shnabkaib members of the Moenkopi Formation document transgressive episodes. Unlike the Timpoweap and Virgin Limestone members, the Shnabkaib contains abundant gypsum and interbedded mudstone resulting from deposition in a restricted marine environment with complex watertable fluctuations. Regressive, red bed layers separate the transgressive strata. Ripple marks, mud cracks, and thinly laminated bedding suggest that these intervening red shale and siltstone units were deposited in tidal flat and coastalplain environments.
Outcrops of this brightly colored red, brown, and pink banded formation can be seen in the Kolob Canyons section of the park and in buttes on either side of State Route 9 between Rockville, Utah, to the south and Virgin, Utah, to the southwest of the park borders. Progressively higher beds are exposed until the top of the formation is reached at the mouth of Parunweap Canyon (when traveling to the park on Route 9).
Chinle Formation (Upper Triassic) Later, uplift exposed the Moenkopi Formation to erosion and Utah became part of a large interior basin drained by north and northwest-flowing rivers in the Upper Triassic. Shallow river deposition along with volcanic ash eventually became the mineral-rich Chinle Formation. The irregular contact zone, or unconformity, between the Chinle and the underlying Moenkopi can be seen between Rockville and Grafton in southwestern Utah.
Petrified wood and fossils of animals adapted to swampy environments, such as phytosaurs, lungfish, and lacustrine bivalves, have been found in this formation as well as conifer trees, cycads, ferns, and horsetails. Relatively plentiful uranium ore, such as carnotite and other uranium-bearing minerals, has also been found. The purple, pink, blue, white, yellow, gray, and red colored Chinle also contains shale, gypsum, limestone, sandstone, and quartz. Iron, manganese oxides and copper sulfide are often found filling gaps between pebbles. Purplish slopes made of the Chinle can be seen above the town of Rockville.
The sand, gravel, and petrified wood which made up these deposits were later strongly cemented by dissolved silica (probably from volcanic ash from the west) in groundwater. Much of the bright coloration of the Chinle is due to soil formation during the Late Triassic. The lowermost member of the Chinle, the Shinarump, consists of a white, gray, and brown conglomerate made of coarse sandstone, and thin lenses of sandy mudstone, along with plentiful petrified wood. The Shinarump was laid down in braided streams that flowed through valleys eroded into the underlying Moenkopi Formation. This member of the Chinle forms prominent cliffs with thickness up to , and its name comes from a Native American word meaning "wolf's rump" (a reference to the way this member erodes into gray, rounded hills).
A succession of volcanic-ash-rich mudstone and sandstone with a thickness of make up the Petrified Forest Member of the Chinle, which was deposited by lakes, highly sinuous rivers and on the surrounding floodplains. This is the same bright, multicolored part of the Chinle that is exposed in Petrified Forest National Park and the Painted Desert. Petrified wood is, of course, also common in this member.
Moenave and Kayenta formations (Lower Jurassic) Early Jurassic uplift created an unconformity above the Chinle Formation that represents about ten million years of missing sedimentation between it and the next formation, the Moenave. Periodic incursions of shallow seas from the north during the Jurassic flooded parts of Wyoming, Montana, and a northeast–southwest trending trough on the Utah/Idaho border. The Moenave was deposited in a variety of river, lake, and flood-plain environments.
The oldest beds of this formation belong to the Dinosaur Canyon Member, a reddish, slope-forming rock layer with thin beds of siltstone that are interbedded with mudstone and fine sandstone. The Dinosaur Canyon, with a local thickness of , was probably laid down in slow-moving streams, ponds and large lakes. Evidence for this is in cross-bedding of the sediments and large numbers of fish fossils.
The upper member of the Moenave is the pale reddish-brown with a thickness of and cliff-forming Springdale Sandstone. It was deposited in swifter, larger, and more voluminous streams than the older Dinosaur Canyon Member. Fossils of large sturgeon-like freshwater fish have been found in the beds of the Springdale Sandstone. The next member in the Moenave Formation is the thin-bedded Whitmore Point, which is made of mudstone and shale. The lower red cliffs visible from the Zion Human History Museum (until 2000 the Zion Canyon Visitor Center) are accessible examples of this formation.
At thick, the Kayenta Formation's sand and silt were laid down in early Jurassic time in slower-moving, intermittent streambeds in a semiarid to tropical environment. Interbedded sandstone, basal conglomerates, siltstones, mudstones, and thin cross-beds are typical channel and floodplain deposits found in the Kayenta. Paleocurrent studies show that the Kayenta rivers flowed in a general westward to southwestward direction.
Fossilized dinosaur footprints from sauropods have been found in this formation near the Left Fork of North Creek. Mountains in Nevada and California continued to rise in the Lower Jurassic as plate motions forced North America northward. Eventually, this created a rain shadow and brought widespread desertification. Today the Kayenta is a red and mauve rocky slope-former that can be seen throughout Zion Canyon.
Navajo Sandstone (Lower to Mid Jurassic)