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Zion National Park

Geologic History

This section highlights the map units (i.e., rocks and unconsolidated deposits) that occur in Zion National Park and puts them in a geologic context in terms of the environment in which they were deposited and the timing of geologic events that created the present landscape.

This section summarizes the tectonic and depositional history found in Zion Natural Park. The tectonic history includes two major orogenic (mountain- building) events: one that occurred in the Mesozoic and one that began in the Mesozoic and ended in the Cenozoic with uplift of the Colorado Plateau. The depositional history includes a wide variety of depositional environments.

Tectonic History
Folds and Faults
Folds and faults are not abundant in Zion; however, fault locations are important because faults are zones of weakness where earthquakes and mass- movements tend to reoccur. The Hurricane fault, created by Tertiary- age (Miocene) Basin- and- Range faulting, coincides with part of the older Sevier thrust fault. This coincidence suggests that the Sevier thrust fault created a zone of weakness that was reactivated by the Hurricane fault.

The folds and thrust faults in Zion are primarily associated with two Mesozoic to Tertiary orogenic events: the Sevier Orogeny and the Laramide Orogeny. Both orogenies are the result of lithospheric plate collisions and subsequent subduction along the western margin of North America. Compressive forces during the Sevier Orogeny initiated thrust faulting and mountain building to the west during the Cretaceous. The Rocky Mountains were built during the Laramide Orogeny that extended from Late Cretaceous to Eocene. Figure 8 lists some of the important North American tectonic events and life forms that occurred throughout geologic time.

Extending for nearly 64 km (40 mi) from near Toquerville to near Cedar City, the Kanarra anticline has its east limb exposed within Zion (Appendix A). Parts of the crest of the fold are also exposed at the mouths of Taylor Creek and Camp Creek. Strata on the east limb dip from 20 degrees to 35 degrees east (Biek et al. 2000). The dip of the beds flattens abruptly and is nearly horizontal under the great cliffs of Navajo Sandstone. The Hurricane fault zone has sheared off the western limb of the fold as well as parts of the crest and eastern limb along a line roughly parallel with the fold axis.

In the Kolob Canyons area, the Taylor Creek thrust fault zone, which has pushed older strata on top of younger, replicates Jurassic strata on the east limb of the Kanarra anticline (Biek et al. 2000). Taylor Creek thrust faults are back thrusts generated by the regional west to east compression during the Sevier Orogeny. The backthrusts are subparallel to bedding with fault planes dipping to the east (Appendix A) (Hamilton 1987; Biek et al. 2000). In Zion repetition of the resistant, cliffforming Springdale Sandstone member of the Moenave Formation best illustrates the Taylor Creek fault zone. The Kayenta, Chinle, Moenkopi, and Kaibab strata are also displaced by smaller back thrusts associated with the Taylor Creek fault zone. The strata were displaced about 610 m (2,000 ft) vertically and about 762 m (2,500 ft) horizontally.

Figure 4. Sketch map modified from Gregory (1950) showing the location of Zion NP
Figure 4: Sketch map modified from Gregory (1950) showing the location of Zion NP and the position of the major plateaus, terraces, cliff lines, and faults in the Zion region. ZION: Zion National Park; BRCA: Bryce Canyon National Park; CEBR: Cedar Breaks National Monument; PISP: Pipe Spring National Monument.

Cenozoic- age normal faulting has further disrupted the sedimentary rocks at Zion. While most of the Colorado Plateau was not greatly affected by Basin- and- Range normal faulting, extensional forces broke the western margin of the Colorado Plateau into a series of large blocks bounded by the north- south trending Hurricane, Sevier, Paunsaugunt, and other faults (figure 4) (Gregory 1950; Biek et al. 2000). These large fault systems that parallel the western margin of the plateau demonstrate that Zion, CEBR, and BRCA are in a transition zone between the Colorado Plateau Province and the Basin and Range Province. Zion lies on an intermediate fault block bounded by the Hurricane fault zone to the west and the Sevier fault zone to the east.

Faults of lesser linear extent are also present in Zion (Appendix A). A graben (fault bound valley) is formed by the offset along the East and West Cougar Mountain faults located in the southwest part of Zion. These faults are parallel, northwest- trending, steeply dipping normal faults probably related to Basin- and- Range extension although the timing of the faulting is poorly defined. The faults do not offset the 250,000 year- old Grapevine Wash basalt flows and the youngest rock displaced by the fault is Jurassic (Biek et al. 2000).

Another northwest- trending normal fault is the Wildcat Canyon fault that parallels the Cougar Mountain faults (Appendix A). Temple Cap and Carmel strata in Wildcat Canyon have been displaced about 55 m (180 ft) (Biek et al. 2000). Biek et al. (2000) in their text and Hamilton (1987) on his map interpret the movement along the fault to be down- to- the- east, but Biek et al. (2000) have the offset drawn as down- to- the- west. Determining the correct orientation of this fault may be important in order to predict direction of movement in the future.

The 1.0 million year old Lava Point flow to the north is not offset by the Wildcat Canyon fault. This fault is probably contemporaneous with the East and West Cougar Mountain faults.

The Bear Trap Canyon fault, a northeast- trending, highangle normal fault, with down- to- the- west movement, and more than 274 m (900 ft) of displacement (Hamilton 1992; Biek et al. 2000) merges with the East Cougar Mountain fault (Appendix A).

Minor folds of limited extent have been mapped in the Kolob area. One fold that is about 151- 172 m (495- 564 ft) long has the Jurassic Kayenta at the surface. Another fold 72 m (236 ft) long is located in Jurassic Moenave Formation through Quaternary deposits. A third fold also affects Moenave Formation strata and extends for about 106- 116 m (348- 380 ft) on the surface.

Joints
In contrast to the limited number of folds and faults, joints are ubiquitous throughout Zion. The joints are exceptionally well developed, and are instrumental in orienting today's canyon network by channeling runoff (Biek et al. 2000). Joints are simply cracks in the bedrock without any significant offset. The most prominent joints in Zion trend north- northwest and are found in the Navajo Sandstone. These joints are nearly vertical and are spaced widely apart with some uniformity. Crushed or sheared zones associated with the joints indicate two diametrically opposite types of crustal stresses: one set related to compression and one set related to tension (Biek et al. 2000). Rogers (2002) suggests that joints were initiated with tension related to Basin- and- Range extension, but that the joints did not propagate until surface erosion began cutting into the rock and preferentially following and facilitation joint development. The result is near parallel, regularly spaced, joint- controlled canyons.

Some joints near rock surfaces formed because of erosion. These joints are termed exfoliation joints and form roughly parallel to the rock face as overlying bedrock and sediments are eroded. Other joints, such as those at Checkerboard Mesa, are thought to form due to local expansion and contraction near the surface of the rock as it is subjected to constant, persistent temperature and moisture changes.

Depositional History
The strata of Zion represent layer upon layer of overlapping and interfingering marine and non- marine depositional environments (figure 9).

Permian Period
About 275 million years ago the Permian equator passed through what is now eastern Utah and Wyoming along the western margin of Pangaea, the supercontinent forming as the globe's landmasses sutured together (Biek et al. 2000; Morris et al. 2000). A dry, high atmospheric pressure climatic belt prevailed in this western part of Pangaea and resulted in restricted marine evaporitic conditions over much of the cratonic shelf seaway (Peterson 1980). Warm, shallow seas and sabkhas (broad, very flat surfaces near sea level) covered the area. Farther to the west, a complex island arc assemblage formed above a subduction zone as lithospheric plates collided (Silberling and Roberts 1962). To the east, in western Colorado, the majestic, jagged peaks (similar to today's Himalayas) of the Uncompahgre Mountains bordered the Utah lowland.

The Toroweap Formation contains evidence of four environments of deposition created by the advance and retreat of the shoreline across northern Arizona and southwestern Utah (Rawson et al. 1980). From west to east, these four environments include an open marine environment, restricted marine, sabkha, and eolian dune environments. As sea level continued to rise during the initial Toroweap transgression, normal marine organisms such as brachiopods, crinoids, corals, and bryozoans entered the Zion area, and their shell material is incorporated in the Toroweap limestones. The fossiliferous limestone, dolomite, and limy sandstone environments record three transgressive pulses (Rawson et al. 1980).

Following the last transgression, the sea withdrew to Nevada and coastal sabkha environments spread over Zion. Eolian dune fields formed east of the sabkhas. One last Toroweap transgressive pulse swept marine environments back into the area from the west.

The Kaibab Limestone records the last in a long series of shallow seas that transgressed over the Zion region throughout the Paleozoic Era. Oolites, disarticulated and broken marine fossil fragments, dolomite, siliceous sponge spicules, and gypsum, all found in the Kaibab, formed under shallow, near- shore, warm and arid climatic conditions (Hamilton 1992). Spherical, modern oolites, similar to those found in the Kaibab Limestone, are currently being formed in warm, shallow marine water where they slowly accrete carbonate mud to their round surfaces as waves gently roll them back and forth over the sea bottom.

The interfingering of the Kaibab with the White Rim Sandstone in the Capital 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 (Dubiel et al. 1996). The sea moved back and forth across Utah, but by the Middle Permian, the sea had withdrawn and the Kaibab Limestone was exposed to subaerial erosion (Morris et al. 2000). Dissolution of the Kaibab created karst topography and channels reaching 30 m (100 ft) in depth cut into the limestone surface (Morris et al. 2000).

The close of the Permian brought the third, and most severe, mass extinction of geologic time. Although not as famous as the extinction event that exterminated the Dinosaurs at the end of the Mesozoic, the Permian extinction was much more extensive.

Almost 96% of all species were extinct by the end of the Permian (Raup 1991). The most recent hypothesis regarding the Permian event suggests that a comet, about 6- 13 km (4- 8 mi) in diameter, slammed into Earth (Becker et al. 2001), triggering vast volcanic eruptions that spread lava over an area two- thirds the size of the United States.

Triassic Period
During the Triassic (250 to 206 million years ago), the supercontinent Pangaea reached its greatest size. All the continents had come together to form a single landmass that was located symmetrically about the equator (Dubiel 1994). To the west, explosive volcanoes arose from the sea and formed a north- south trending arc of islands along the border of what is now California and Nevada (Christiansen et al. 1994; Dubiel 1994; Lawton 1994).

Shallow, marine water stretched from eastern Utah to eastern Nevada over a beveled continental shelf. As the sea withdrew, fluvial, mudflat, sabkha, and shallow marine environments developed (Lower Triassic, Moenkopi Formation) (Stewart et al. 1972A; Christiansen et al. 1994; Doelling 2000; Huntoon et al. 2000). The Red Canyon Conglomerate, the basal member of the Moenkopi, fills broad east- flowing paleochannels carved into the Kaibab Limestone (Biek et al. 2000). Some of these channels are up to several tens of feet deep and may reach 61 m (200 ft) deep in the St. George area. A thin poorly developed soil or regolith formed over the paleotopographic high areas between the channels (Biek et al. 2000).

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 (Stewart et al. 1972A; Dubiel 1994; Huntoon et al. 2000; Morris et al. 2000).

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 (Biek et al. 2000).

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 (Stewart et al. 1972A; Hamilton 1992; Biek et al. 2000).

Figure 5. Stratigraphic column for Zion National Park.
Figure 5. Stratigraphic column for Zion National Park.

The Early Triassic is separated from the Late Triassic by a regional unconformity (figure 5). This unconformity marks a change from the shallow marine environments of the Lower Triassic Moenkopi Formation to mostly continental sedimentation in the Upper Triassic Chinle Formation. The Middle Triassic remains a mystery. No rocks that span this time (from 242- 227 Ma) have been preserved in Utah. By the Late Triassic, Utah was part of a large interior basin drained by north- and northwestflowing rivers (Biek et al. 2000). Braided streams deposited coarse sediments (Shinarump Conglomerate member) in paleovalleys eroded into the underlying Moenkopi Formation (Dubiel 1994; Biek et al. 2000).

High- sinuosity stream, flood plain, and lake sediments (Petrified Forest member) overly the braided stream deposits in the Zion region (Stewart et al. 1972B; Dubiel 1994; Biek et al. 2000). Aquatic crocodile- like Phytosaurs, lungfish, and lacustrine bivalves inhabited a Utah that looked vastly different in the Upper Triassic than it does today. Rather than a semi- arid desert environment, the Zion area was a coastal lowland supporting amphibians, reptiles, freshwater clams, snails, ostracodes, and fish. The moist climate supported conifer trees, cycads, ferns, and horsetails (Stewart et al. 1972B; Dubiel 1994; Biek et al. 2000). Periodically, volcanic ash from the volcanic arc off the continental margin to the west drifted into the area and was subsequently altered to bentonitic clay that today is notoriously susceptible to landslides and for causing foundation problems in southwest Utah.

About ten million years is missing between the Chinle Formation and the Early Jurassic Moenave Formation. This basal Jurassic unconformity extends from central and western Wyoming, through Utah and the Four Corners area, and into northwest New Mexico and the San Juan Basin (Pipiringos and O'Sullivan 1978; Peterson 1994).

Jurassic Period
Throughout the Jurassic's 100 million years, periodic incursions from the north brought shallow seas flooding into Wyoming, Montana, and a northeast- southwest trending trough on the Utah/Idaho border. The Jurassic western margin of North America was associated with an Andean- type margin where the eastward subduction of the seafloor gave rise to volcanism similar to that found in today's Andes of South America. Volcanoes formed an arcuate north- south chain of mountains off the coast of western Pangaea in what is now central Nevada. To the south, the landmass that would become South America was splitting away from the Texas coast just as Africa and Great Britain were rifting away from the present East Coast and opening up the Atlantic Ocean. The Ouachita Mountains, formed when South America collided with North America, remained a significant highland, and rivers from the highland flowed to the northwest, towards the Plateau. The Ancestral Rocky Mountains and the Monument Upwarp also remained topographically high during the Jurassic.

Bordered by these highlands, the Western Interior Basin was a broad, shallow depression on the southwest side of the North American craton. The basin stretched northward from its southern margin in Arizona and New Mexico across the Canadian border. The basin was asymmetric, rapidly subsiding along the west side and more gently dipping farther east.

The Moenave Formation was deposited in a variety of river, lake, and flood- plain environments (Biek et al. 2000). Ripple marks, cross- bedding, reddish and gray siltstone and shale, fossil fish scales, and bones of Semionotus kanabensis suggest low energy streams and ponded drainages (Dinosaur Canyon and Whitmore Point members) (Hamilton 1992). The thin, discontinuous lenses of intraformational conglomerate, fine- grained rip- up clasts (mud clasts "ripped- up" by currents and transported elsewhere), and fossil plant fragments found in the Springdale member record deposition in river channels (Biek et al. 2000).

Fluvial processes continued to affect southwestern Utah by the deposition of the Kayenta Formation. 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 (Morris et al. 2000). Mountains in Nevada and California continued to rise in the Early Jurassic as plate motions forced North America northward. Eventually, this created a rain shadow. Gradually, sand dune deposits reaching 240 to 340 m (800 to 1100 ft) overtook the fluvial systems of the Kayenta. These dune fields became the Navajo Sandstone, part of the world's largest coastal and inland paleodune field (Blakey 1994; Peterson 1994; Biek et al. 2000). The large- scale (18 m, 60 ft), high- angle, crossbeds of the Navajo attest to the presence of Sahara- like sand dunes during the Early Jurassic (Biek et al. 2000; Morris et al. 2000).

Extensive eolian sand seas, called ergs, developed in the Western Interior Basin mainly because the region was located about 18 degrees north latitude at the beginning of the Jurassic and about 30- 35 degrees north latitude at the end of the Jurassic (Parrish and Petersen 1988; Chan and Archer 2000; Kocurek and Dott 1983; Peterson 1994). This latitude marks today's trade wind belt where hot, dry air descends from the upper atmosphere and sweeps back to the equator in a southwesterly direction, picking up any moisture as it goes - the latitude of intense evaporation. Most modern hot deserts of the world occur within the trade wind belt and during the Jurassic, the climate of the Colorado Plateau appears to have been similar to the modern Western Sahara. In the Sahara, the world's largest desert, only 10% of the surface is sand- covered. The Arabian Desert, Earth's sandiest desert, is only 30 percent sand- covered. The Jurassic deserts that occurred across the Colorado Plateau for roughly 40 million years (not counting the time represented by erosion) contained sand dunes that may be the largest recorded in the rock record (Kocurek and Dott 1983). These ergs formed on a coastal and inland dune field affecting southern Montana, eastern Utah, westernmost Colorado, southwest Colorado, northeastern Arizona, and northwestern New Mexico (Kocurek and Dott 1983; Peterson 1994). The volume of sand in these systems was enormous. Ergs may have covered 106 km2 (41 mi2) with as much as 1.5x105 km3 (3.6 x104 mi3) of sand being deposited (Saleeby et al. 1992). Two types of cyclicity have been observed in Navajo sandstone. First there are layers of annual deposition where 1 to several meters of sand accumulates on the dune face during strong winds, separated by thinner wedges of sand deposited during light and variable winds. These have been interpreted as deposition during seasonal monsoon winds from the north (Loope et al. 2001). Secondly, studies of cyclicity in the annual dune sets suggest that the region experienced contrasts of wetter and drier periods on a decade scale in the Early Jurassic (Chan and Archer 2000).

Great, sweeping Navajo cross- beds are wonderfully preserved at Zion. As in modern deserts, where ground water reached close to the surface, oases formed. Planar sandstone and limestone beds found in the middle and upper parts of the Navajo represent oasis deposits formed in these active dunefields. One good example of fossil oasis deposits can be seen along the Canyon Overlook Trail (Biek et al. 2000). The top of the Navajo Formation and the end of the Early Jurassic is marked by another regional unconformity.

As the pace of west coast collision increased in the Middle Jurassic (about 160 to 180 Ma) to about as fast as fingernails grow, the rock layers on the continental side of the collision, in Utah and western Colorado, deformed in response to the collision to the west (Sevier Orogeny). The sea began to encroach on the continent from the north. Broad tidal flats and streams carrying red mud (Sinawava member of the Temple Cap Formation) formed on the margins of a shallow sea that lay to the west, and flat- bedded sandstones, siltstones, and limestones filled depressions left in the underlying eroded strata (Wright et al. 1962; Hamilton 1992; Biek et al. 2000; Doelling 2000). Streams eroded the poorly cemented Navajo Sandstone, and water caused the sand to slump. Desert conditions returned briefly (White Throne member), but encroaching seas again beveled the coastline, forming a regional unconformity.

Figure 9. Depositional environments represented by the strata in Zion National Park.
Figure 9. Depositional environments represented by the strata in Zion National Park.

Crinoid, pectin, clam, and oyster fossils of the Carmel Formation were deposited in a shallow inland sea (Biek et al. 2000). 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 (figure 9). Both open marine (crinoids) and restricted marine (pelecypods, gastropods) environments are represented in the Co- op Creek member. Sandstone and gypsum in the Crystal Creek and Paria River members signal a return to desert conditions in a coastal setting (Biek et al. 2000; Morris et al. 2000).

Cretaceous Period
As mountains rose in the west and the roughly northsouth trending Western Interior Basin expanded in the Cretaceous, the Gulf of Mexico separating North and South America continued to rift open in the south, and marine water began to advance northward into the basin. At the same time, marine water advanced onto the continent from the Arctic region.

The seas advanced and retreated many times during the Cretaceous until the most extensive interior seaway ever recorded drowned much of western North America (figure 10). The Western Interior Seaway was an elongate basin that extended from today's Gulf of Mexico to the Arctic Ocean, a distance of about 4827 km (3,000 mi) (Kauffman 1977). The western margin of the seaway coincided with the active Cretaceous Sevier orogenic belt with the westernmost extension of the shoreline in the vicinity of Cedar City, Utah. The eastern margin was part of the low- lying, stable platform ramp in Nebraska and Kansas.

Figure 10. Location of the Cretaceous Period, Western Interior Seaway. Shaded areas indicate land above sea level. North arrow indicates
Figure 10. Location of the Cretaceous Period, Western Interior Seaway. Shaded areas indicate land above sea level. North arrow indicates
the Cretaceous north. Modified from Rice and Shurr (1983).

The pebble to cobble conglomerate and tan sandstone that compose the Cretaceous rocks exposed at the top of Horse Ranch Mountain include alluvial- fan and alluvial- plain sediments that grade laterally into coastal plain, marginal marine, and marine deposits (Biek et al. 2000). For the first time in the history of the Mesozoic, the source area for these terrestrial clastic sediments is from the west, a result of the Sevier Orogeny.

Tertiary Period
Explosive andesitic volcanism dominated the area to the west of Zion during Oligocene and early Miocene time and probably inundated the region with hundreds of feet of welded tuff that has since eroded away (Biek et al. 2000). Three of these tuff layers are preserved on top of Brainhead Peak. Some of these enormous cascadia- type volcanoes produced eruptions that exceeded the largest Yellowstone eruptions (Dave Sharrow, Zion National Park, personal communication 2005). About 21 million years ago the Pine Valley laccolith formed. This typical mushroom- shaped laccolith is one of the largest intrusions of this type in the world. Debris- flows carried boulders of this intrusion onto the Upper Kolob Plateau indicating that the Hurricane Cliffs could not have been present at the time.

Quaternary Period
Synthesizing geologic maps for the quadrangles that cover Zion, 100 Quaternary units are mapped on the NPS- GIS digital map. These units are summarized in Biek et al. (2000) who organized the surficial deposits into six main types of surficial sedimentary deposits common in the park:
. alluvium,
. colluvium and residuum,
. talus,
. eolian deposits, mass- movement deposits (including landslides and debris flows), and
. lacustrine or basin- fill deposits.

Unlike the consolidated bedrock units, these surficial units are classified according to their interpreted mode of deposition, or genesis. In addition to these (but too small to map), are the rare tufa deposits associated with springs and the basalt flows and cinder cones that stand in stark contrast to the surrounding red- rock strata.

The surficial deposits of Zion speak to an active recent history of the park. Older debris- flow deposits contain subrounded basalt boulders brought in from a western source before the Hurricane fault zone was a significant topographic barrier to deposition. Analyses of the basalt flows and cinder cones reveal an eruptive cycle that may have lasted less than 100 years before going extinct (Biek et al. 2000). The volcanic vents appear to be located along faults and joints, structurally weak zones in the rock.

Quaternary basalt flowed down canyons and drainages onto valley floors, just as magma does today. Because basalt is more resistant to erosion than sedimentary rocks, however, erosion has removed the surrounding sedimentary rock that once stood at higher elevations so that the basalt now caps ridges that separate adjacent drainages. Thus, they form an "inverted topography" in which the valleys that were once flooded with basalt are now ridges and plateaus.

Impounded behind landslides and lava flows, small lakes and ephemeral ponds filled the canyons of Zion. About 100,000 years ago, the Crater Hill basalt flow blocked the Virgin River near the present- day ghost town of Grafton. Behind this barrier, Lake Grafton grew to become the largest of at least 14 lakes that have periodically formed in the park.

Zion National Park is a monument to erosion and the impact that water has in a dry, sparsely vegetated landscape. Runoff from precipitation and snowmelt has eroded thousands of feet of strata from the Zion block in the Quaternary. Canyon cutting could only begin in earnest when the Colorado River began flowing through Grand Canyon and on to the sea about 4.5 million years ago. The Virgin River could then link with the Colorado and begin expanding its watershed into the Colorado Plateau. It does this at the expense of the Sevier River drainage, which has less erosive energy because it has a gentle gradient draining to the Great Basin about 4,000 feet in elevation, rather than sea level.

Normally a small, placid stream, easy to wade across, the Virgin River does not seem capable of eroding such an immense canyon as Zion. However, the Virgin River carries away more than 1 million tons of rock waste each year due its steep gradient of about 13 meters per kilometer (69 ft/mi) (Biek et al. 2000). Nearly all of the sediment transport occurs during floods because the capacity of the river to move sediment increases exponentially as the streamflow increases. A ten- fold increase in flow, a common occurrence, results in a 1,000- fold increase in sediment transport. Peak flows, however, are quite variable with a range from 0.6- 256 m3/sec (21- 9,150 cfs) near Springdale to 0.6- 638 m3/sec (21- 22,800 cfs) downstream near Virgin. During the wetter Pleistocene past, average sediment transport was probably even greater than it is today.

Downcutting and canyon widening are the two dominant erosional processes forming the canyons at Zion (Biek et al. 2000). Downcutting is represented at The Narrows at the head of Zion Canyon where the North Fork of the Virgin River flows through a spectacular gorge cut into the Navajo Sandstone. Acting like a ribbon of moving sandpaper through The Narrows, the Virgin River has carved a 305 meter- deep (1,000 ft) gorge that, in places, is only 5 m (16 ft) wide at the bottom.

The second dominant erosional process, canyon widening, makes use of the different erosional properties between the Kayenta Formation and the overlying Navajo Sandstone. The thin- bedded siltstone, sandstone, and shale of the Kayenta Formation are softer and more easily eroded than the massive sandstone of the Navajo. Consequently, as the Kayenta is eroded and slips away in landslides, the Navajo cliffs are undercut. Seeps and springs at the contact of the permeable Navajo and relatively impermeable Kayenta further undermine the Navajo cliffs until they collapse in rockfalls and landslides. Failure of the Navajo is facilitated by the vertical joints in the sandstone, as well. During canyon widening the Virgin River acts primarily as a conveyor that transports the material washed off the slopes downstream.

Carved in the Jurassic- age Navajo Sandstone, the sheer walls of Zion Canyon rise 610 m (2,000 ft) from the canyon floor. A narrow slot in its upper reaches, the canyon widens below The Narrows where the North Fork of the Virgin River has cut a wider flood plain in the less resistant beds of the Jurassic Period Kayenta and Moenave Formations (Biek et al. 2000).

The Virgin River has cut down about 396 m (1,300 ft) in about 1 million years. This rate of canyon cutting is about 40 centimenters/1,000 years (1.3 ft/1,000 yr). This is a very rapid rate of downcutting, about the same rate as occurred in Grand Canyon during its period of most rapid erosion. About 1 million years ago, Zion Canyon was only about half as deep as it is today in the vicinity of Zion Lodge (Biek et al. 2000). Definitive evidence is sparse for determining long- term erosion rates of Zion Canyon, but if the assumption is made 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. Downcutting and canyon widening continue today as the relentless process of erosion continues to bevel the landscape to sea level.


References:

Biek, R.F., Willis, G.C., Hylland, M.D., and Doelling, H.H., 2000, Geology of Zion National Park, Utah, in D.A. Sprinkel, T.C. Chidsey, Jr., and P.B. Anderson, eds., Geology of Utah's Parks and Monuments: Utah Geological Association Publication 28, p. 107- 138.

Becker, L., Poreda, R. J., Hunt, A. G., Bunch, T. E., and Rampino, M., 2001, Impact event at the Permian- Triassic boundary: Evidence from extraterrestrial noble gases in fullerenes: Science, Feb. 23, p. 1530- 1533.

Blakey, R. C., 1994, Paleogeographic and tectonic controls on some Lower and Middle Jurassic erg deposits, Colorado Plateau, in Mario V. Caputo, James A. Peterson, and Karen J. Franczyk, eds., Mesozoic Systems of the Rocky Mountain Region, USA: Rocky Mountain Section, SEPM (Society for Sedimentary Geology), Denver, CO., p. 273- 298.

Chan, M. A. and Archer, A. W., 2000, Cyclic eolian stratification on the Jurassic Navajo Sandstone, Zion National Park: Periodicities and implications for paleoclimate, in D.A. Sprinkel, T.C. Chidsey, Jr., and P.B. Anderson, eds., Geology of Utah's Parks and Monuments: Utah Geological Association Publication 28, p. 607- 618.

Christiansen, E. II, Kowallis, B. J., and Barton, M. D., 1994, Temporal and spatial distribution of volcanic ash in Mesozoic sedimentary rocks of the Western Interior: an alternative record of Mesozoic magmatism, in M. V. Caputo, J. A. Peterson, and K. J. Franczyk, eds., Mesozoic Systems of the Rocky Mountain Region, USA: Rocky Mountain Section, SEPM (Society for Sedimentary Geology), Denver, CO., p. 73- 94.

Doelling, H. H., 2000, Geology of Arches National Park, Grand County, Utah, in D.A. Sprinkel, T.C. Chidsey, Jr., and P.B. Anderson, eds., Geology of Utah's Parks and Monuments: Utah Geological Association Publication 28, p. 11- 36.

Dubiel, R. F., 1994, Triassic deposystems, paleogeography, and paleoclimate of the Western Interior, in M. V. Caputo, J. A. Peterson, and K. J. Franczyk, eds., Mesozoic Systems of the Rocky Mountain Region, USA: Rocky Mountain Section, SEPM (Society for Sedimentary Geology), Denver, CO., p. 133- 168.

Dubiel, R. F., Huntoon, J. E., Condon, S. M., Stanesco, J. D., 1996, Permian deposystems, paleogeography, and paleoclimate of the Paradox Basin and vicinity, in M.W. Longman and M.D. Sonnenfeld, eds., Paleozoic Systems of the Rocky Mountain Region: Rocky Mountain Section, SEPM (Society for Sedimentary Geology), p. 427- 444.

Gregory, H.E., 1950, Geology and Geography of the Zion Park Region Utah and Arizona: U.S.G.S. Professional Paper 220, 200 p.

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