Zion geological history

C http://www.nature.nps.gov/geology/parks/zion/geol_history.cfm
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
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.
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.