Geology of the Zion and Kolob canyons area

alt=Red rock mountains|thumb|right|300px|Kolob Canyons from the end of Kolob Canyons Road. Stream erosion has incised the Kolob Plateau to form canyons that expose the red-orange colored Navajo Sandstone and other formations.

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.

thumb|alt=Diagram with different colored layers|This geologic [[cross section (geology)|cross section shows the layering of the formations mentioned below]]

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.

thumb|left|alt=Hill with some plants | [[Hurricane Cliffs/Kaibab Formation]]

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. 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. 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.

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. in the Jurassic the Colorado Plateau area's climate increasingly became arid until 150,000 square miles (388,000 km<sup>2</sup>) of western North America became a huge desert, not unlike the modern Sahara. Navajo is the most prominent formation exposed in Zion Canyon with the highest exposures being West Temple and Checkerboard Mesa. Rifting in the Gulf of Mexico helped the southern end of the basin to subside, which allowed marine water to advance northward. At the same time, the shoreline advanced inland from the Arctic region. The seas advanced and retreated many times during the Cretaceous until one of the most extensive interior seaways ever, called the Western Interior Seaway, drowned much of western North America from the Gulf of Mexico to the Arctic Ocean. A small remnant of the Dakota is exposed on top of the -high Horse Ranch Mountain (photo).

Tensional forces forming the Basin and Range physiogeographic province to the west about 20 to 25 million years ago in Tertiary time created the two faults that bound the Markagunt Plateau (which underlies the park): the Sevier Fault on the east and the Hurricane Fault on the west. The Hurricane fault zone is a major, active, steeply west-dipping normal fault that stretches at least 155 miles (250 km) from south of the Grand Canyon northward to Cedar City, Utah. Along the southern boundary of the park, tectonic displacement along this fault is about 3,600 ft (1,098 m). The lava traveled into Coalpits and Scoggins Washes to the south and accumulated to a depth of over 400 ft (122 m) in the ancestral Virgin River valley near the present-day ghost town of Grafton, Utah. Water impounded behind the two blockages, forming Coalpits Lake and Lake Grafton respectively.

Lake Grafton was the largest of at least 14 lakes that have periodically formed in the park (most were from landslides; see below). More recent flows of less than 10,000 years in age occurred north of Zion and east of Cedar Breaks National Monument.

Erosion and canyon formation

thumb|alt=Red rocks|Rockslide debris in Kolob Canyons

Stream downcutting continued along with canyon-forming processes such as mass wasting; sediment-rich and abrasive flood stage waters would undermine cliffs until vertical slabs of rock sheared away. This process continues to be especially efficient with the vertically jointed Navajo Sandstone.

All erosion types took advantage of preexisting weaknesses in the rock such as rock type, amount of lithification, and the presence of cracks or joints in the rock. Basalt flows concentrated in valleys but subsequent erosion removed sedimentary rock that once stood at higher elevations. The resulting inverted relief consists of ridges capped by basalt which are separated by adjacent drainages. This is a very high rate of downcutting, about the same rate as occurred in Grand Canyon during its most rapid period of erosion.

The area is periodically rocked by mild to moderate earthquakes, which often trigger landslides. For example, on September 2, 1992, a Richter Magnitude 5.8 earthquake caused 14 million cubic meters (18 million cubic yards) of mostly Moenave Formation to slide downslope atop the weak claystone of the Petrified Forest member of the Chinle Formation. The quake was centered on the Washington Fault, about southwest. Three houses and two water tanks were destroyed when the slope they were built on dropped and extended laterally a similar distance over a period of several hours. The landslide is visible just outside the park's entrance in Springdale, Utah.

Notes

References

Bibliography

(public domain text)
(public domain text)

External links

National Park Service.gov: "Geologic History of Zion National Park"
National Park Service.gov: "Geologic Resource Evaluation Report of Zion National Park"