Geology of the Zion and Kolob canyons area
The geology of the Zion and Kolob canyons area includes nine known exposed
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
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.[2] 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
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.[4]
Starting 260 million years ago, the yellowish-gray limestone of the
Farther to the west, a complex
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.[6]
It took many thousands of thin layers of these sediments to form the 1,800-foot (550 m) thick Moenkopi Formation.[7] 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.[8]
The Red Canyon Conglomerate, the basal member of the Moenkopi, fills broad east-flowing paleochannels carved into the Kaibab Limestone.[9] 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.[8] A thin, poorly developed soil, or regolith, formed over the paleotopographic high areas between the channels.[9]
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.
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.[9] 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.[6]
The sand,
A succession of volcanic-ash-rich mudstone and sandstone with a thickness of 350-foot (110 m) make up the Petrified Forest Member of the Chinle, which was deposited by lakes, highly sinuous rivers and on the surrounding floodplains.
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.[10] 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.[8] The Moenave was deposited in a variety of river, lake, and flood-plain environments.[10]
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.[11] The Dinosaur Canyon, with a local thickness of 140 to 375 feet (43 to 114 m), was probably laid down in slow-moving streams, ponds and large lakes.[7] 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 75 to 150 feet (23 to 46 m) and cliff-forming Springdale Sandstone.[7] It was deposited in swifter, larger, and more voluminous streams than the older Dinosaur Canyon Member.[11] Fossils of large sturgeon-like freshwater fish have been found in the beds of the Springdale Sandstone.[11] The next member in the Moenave Formation is the thin-bedded Whitmore Point, which is made of mudstone and shale.[11] The lower red cliffs visible from the Zion Human History Museum (until 2000 the Zion Canyon Visitor Center) are accessible examples of this formation.[3]
At 200 to 600 feet (61 to 183 m) thick, the
Fossilized
Approximately 190 to 136 million years ago
Most of the sand, made of 98% translucent, rounded-grain quartz, was transported from coastal sand dunes to the west, in what is now central Nevada.[7] Today the Navajo Sandstone is a geographically widespread, pale tan to red cliff and monolith former with very obvious sand dune cross-bedding patterns (photo). Typically the lower part of this remarkably homogeneous formation is reddish from iron oxide that percolated from the overlaying iron-rich Temple Cap formation while the upper part of the formation is a pale tan to nearly white color.[7] The other component of the Navajo's weak cement matrix is calcium carbonate, but the resulting sandstone is friable (crumbles easily) and very porous. Cross-bedding is especially evident in the eastern part of the park where Jurassic wind directions changed often. The crosshatched appearance of Checkerboard Mesa is a good example (photo).
Springs, such as Weeping Rock (photo), form in canyon walls made of the porous Navajo Sandstone when water hits and is channeled by the underlying non-porous Kayenta Formation.[14] The principal aquifer in the region is contained in Navajo Sandstone.[15] Navajo is the most prominent formation exposed in Zion Canyon with the highest exposures being West Temple and Checkerboard Mesa.[3] The monoliths in the sides of Zion Canyon are among the tallest sandstone cliffs in the world.
Temple Cap and Carmel formations (Middle Jurassic)
Utah and western Colorado were deformed as the rate of subduction off the west coast increased in the Middle Jurassic
Desert conditions returned briefly, creating the White Throne member, but encroaching seas again beveled the coastline, forming a regional
A warm, shallow inland sea started to advance into the region (transgress) 150 million years ago, finishing the job of flattening the sand dunes.
Many unique environments were created by the migrating Sevier thrust system, and the four members of the Carmel Formation in southwest Utah capture these changing environments. Both open marine (
Outcrops of the Carmel Formation are most notably exposed on Horse Ranch Mountain[16] (photo) in the Kolob Canyons section of the park and near Mt. Carmel Junction east of the park.[3] Other formations totaling 2,800 feet (850 m) thick may have been deposited in the region during Late Jurassic and Early Cretaceous only to be uplifted and entirely removed by erosion.
Dakota Sandstone (Lower Cretaceous)
Mountains continued to rise in the Sevier orogenic belt to the west during the Cretaceous while the roughly north-south trending Western Interior Basin expanded.[17] 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.[17] The western shoreline of the seaway was in the vicinity of Cedar City, Utah, while the eastern margin was part of the low-lying, stable platform ramp in Nebraska and Kansas.[17]
The pebble to cobble
Tectonic activity and erosion
Regional forces
East–west-directed compression from subduction off the west coast affected the area in later Mesozoic and early Tertiary time by folding and thrust faulting strata. Evidence for the Sevier Orogeny part of this period can be seen in the Taylor Creek area in the Kolob section of the park.[18] Chunks of Moenave strata have been compressed to the point of thrusting themselves over the same formation in the Taylor Creek Thrust Fault Zone, located on the east flank of the Kanarra anticline.[20]
Tensional forces forming the
Subsequent
Volcanic activity
Explosive
Then from at least 1.4 million to 250,000 years ago in Pleistocene time basaltic lava flowed intermittently in the area, taking advantage of uplift-created weaknesses in the Earth's crust.[22] Volcanic activity was concentrated along the Hurricane Fault west of the park that today parallels Interstate 15. Evidence of the oldest flows can be seen at Lava Point and rocks from the youngest are found at the lower end of Cave Valley.[22]> Some cinder cones were constructed much later in the southwest corner of the park.[22]
Some of these lava flows blocked rivers and streams, impounding small lakes and ephemeral ponds in the process. About 100,000 years ago, basalt from the largest cinder cone in the park, Crater Hill, flowed over the area.[23] 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.[24] 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).[17] Thirteen lava flows are mapped in and near Zion dating from 1.5 million to 100,000 years ago.[25] 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
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.[22] 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.[17]
In all about 6,000 feet (1,800 m) of sediment were removed from atop the youngest exposed formation in the park (the Late Cretaceous-aged Dakota Sandstone).[3] The Virgin River carved out 1,300 feet (400 m) of sediment in about 1 million years.[a][27] This is a very high rate of downcutting, about the same rate as occurred in Grand Canyon during its most rapid period of erosion.[27] About 1 million years ago, Zion Canyon was only about half as deep as it is today in the vicinity of Zion Lodge.[26] Assuming that erosion was fairly constant over the past 2 million years, then the upper half of Zion Canyon was carved between about 1 and 2 million years ago and only the upper half of the Great White Throne was exposed 1 million years ago and The Narrows were yet to form.[27]
Downcutting and canyon widening continue today as the process of erosion continues to try to reduce the topography to sea level. In 1998 a flash flood temporarily increased the Virgin River's flow rate from 200 to 4,500 ft³/s (6 to 125 m³/s).
Landslides and earthquakes
Landslides more than once dammed the Virgin River and created lakes where sediment accumulated. Every time the river eventually breached the slide and drained the lake, leaving a flat-bottomed
The area is periodically rocked by mild to moderate
Notes
References
- ^ Harris, Tuttle & Tuttle 1997, p. 34.
- ^ a b NPS
- ^ a b c d e f g NPS and ZNHA 2004
- ^ a b c d Graham 2006, p. 28
- ^ a b c Leach 2000, p. 16
- ^ a b c d Harris, Tuttle & Tuttle 1997, p. 35.
- ^ a b c d e f g h i GORP
- ^ a b c d e f Graham 2006, p. 29
- ^ a b c Biek et al. 2003, p. 114
- ^ a b c Biek et al. 2003, p. 115
- ^ a b c d e f g Harris, Tuttle & Tuttle 1997, p. 38.
- ^ a b c d e f g Graham 2006, p. 30
- ^ a b Tufts 1998, p. 43
- ^ Harris, Tuttle & Tuttle 1997, p. 33.
- ^ Graham 2006, p. 12
- ^ a b c Harris, Tuttle & Tuttle 1997, p. 40.
- ^ a b c d e f g Graham 2006, p. 31
- ^ a b c Harris, Tuttle & Tuttle 1997, p. 41.
- ^ Biek et al. 2003, p. 118
- ^ a b Graham 2006, p. 19
- ^ Biek et al. 2003, p. 119
- ^ a b c d e Harris, Tuttle & Tuttle 1997, p. 42.
- ^ Biek et al. 2003, p. 121
- ^ Graham 2006, p. 18
- ^ Graham 2006, p. 10
- ^ a b Biek et al. 2003, p. 126
- ^ a b c Graham 2006, p. 32
- ^ a b Graham 2006, p. 17
- ^ Harris, Tuttle & Tuttle 1997, p. 31.
- ^ Biek et al. 2003, p. 128
- ^ Graham 2006, p. 9
Bibliography
This article incorporates public domain material from websites or documents of the National Park Service.
- Biek, Robert F.; Grant C. Willis; Micheal D. Hylland; Hellmut H. Doelling (August 2003). "Geology of Zion National Park, Utah". In Paul B. Anderson (ed.). Geology of Utah's Parks and Monuments. Bryce Canyon Natural History Association and Utah Geological Association. ISBN 1-882054-10-5.
- GORP. "Zion National Park Geology". GORP / Orbitz Away LLC. Archived from the original on 2010-01-25. Retrieved 2008-08-11.
- Graham, J. (2006). Zion National Park Geologic Resource Evaluation Report (PDF). Denver, Colorado: National Park Service. pp. 27–35. Natural Resource Report NPS/NRPC/GRD/NRR—2006/014. Retrieved 2008-08-13. (public domain text)
- Harris, Ann G.; Tuttle, Esther; Tuttle, Sherwood D. (1997). Geology of National Parks (5th ed.). Iowa: Kendall/Hunt Publishing. pp. 30–42. ISBN 0-7872-5353-7.
- Leach, Nicky (2000). Zion National Park: Sanctuary in the Desert. Mariposa, California: Sierra Press.
- Tufts, Lorraine Salem (1998). Secrets in The Grand Canyon, Zion and Bryce Canyon National Parks (3rd ed.). North Palm Beach, Florida: National Photographic Collections. p. 43. ISBN 0-9620255-3-4.
- NPS. "Zion: Geologic Formations". National Park Service. Retrieved 2013-11-12. (public domain text)
- NPS and ZNHA (Summer 2004). "The Geology of Zion". Zion Map and Guide. National Park Service and Zion Natural History Association.