4.  The Western Edge of the Colorado Plateau
                                   view southeast from Silver Reef
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Silver Reef, Utah, Geology Highlights
Geological Formations as viewed from the Museum at Silver Reef, Utah

By Robert F. Biek
Senior Scientist
Utah Geological Survey

Summary -
The horizon southeast of Silver Reef is dominated by brightly colored, flat-lying rocks that make up the western edge of the Colorado Plateau.   

The two prominent mesas are Hurricane Mesa, and to its north, Smith Mesa.  
Hurricane Mesa is capped by resistant sandstone and pebbly sandstone of the basal Chinle Formation, whereas Smith Mesa is capped by the younger Springdale Sandstone, the same rocks that host silver and uranium mineralization at Silver Reef. The slope between the two mesas is eroded into the Petrified Forest Member of Chinle Formation, the same rocks made famous by Petrified Forest National Park in Arizona.  Basal Chinle strata also form the resistant carapace of the whale-backed hill in the foreground, the northern end of the Virgin anticline, a large upwarp in the earth's crust that stretches southwest to St. George.

Atop Hurricane Mesa is the country's only privately owned supersonic test track. It is a 2.25-mile-long rocket-sled track used to test aircraft ejection seats and propulsion systems.  It was built following WWII and initially used in the 1950s to eject dummies off the cliff.  Just east of Hurricane Mesa lies Utah's oldest oil field, discovered in 1907 in an attempt to locate the source of nearby natural oil seeps. The peaks in the distance beyond Hurricane Mesa are in Zion National Park.


Detail -
The brightly colored layered rocks of Hurricane Mesa are part of the Moenkopi Formation, a sequence of mostly shallow-marine and tidal-flat strata deposited in the Early Triassic, nearly 250 million years ago at the beginning of the age of dinosaurs.  The alternating white gypsum and red siltstone and sandstone layers of the middle part of the formation are known as the Shnabkaib Member, named for beautiful exposures at Shinob Kibe butte near Washington.  The unusual name Shnabkaib may be a misspelling of the Piute Shinob (Great Spirit) and Kaib (mountain), loosely translated as Mountain of the Lord; less reverently, most geologists say that these rocks look like bacon.   At the time Shnabkaib strata were deposited, Utah lay at the flat, western margin of the great Supercontinent Pangea; minor changes in sea level created shoreline changes of many miles, thus leading to intricate, alternating layers of gypsum and mudstone.  These beds are wonderfully exposed at nearby Quail Creek State Park, home of Quail Creek reservoir and the site of a catastrophic dam collapse a few minutes after midnight on January 1, 1989.

Quail Creek State Park — located along the axis of the Virgin anticline with its well exposed, colorful strata and bright blue waters of Quail Creek Reservoir — is a geologist’s heaven, even though the northern part of the park is called Little Purgatory and the area to the south is known as Purgatory Flat.  

Interestingly, the anticline takes its name from the nearby Virgin River, originally named "La Virgen" by Spanish Catholic missionaries in honor of the Virgin Mary.  The juxtaposition of the life-giving river and harsh desert through which it flows was obviously on the minds of early European explorers and settlers. 

The Virgin anticline is a 30-mile-long, northeast-trending symmetrical fold made all the more visible by a resistant carapace of Shinarump Conglomerate, the same rock layer that caps Hurricane Mesa.  The fold marks the easternmost extent of significant Late Cretaceous compressional deformation in this part of the state, deformation associated with a mountain-building episode along the western margin of North America.

Quail Creek Reservoir is an offline reservoir for storage of Virgin River water, conveyed by pipeline from upstream of Pah Tempe hot springs.  The reservoir has two dams, the southern one of which is built on gypsiferous strata of the Shnabkaib Member.  The first southern dam, or dike, was a poorly designed earthen dam and began to experience severe leakage almost immediately following filling of the reservoir in 1985.  

Despite repeated efforts to fix the dike, it failed catastrophically a few minutes after midnight on January 1, 1989.  About 25,000 acre-feet (1/2 the reservoir capacity) flowed through a breach in the dike during a 12-hour period.   Peak discharge on the Virgin River at Bloomington was estimated to be about 60,000 cubic feet per second (csf).  Fortunately downstream evacuation prevented fatalities, but flooding caused millions of dollars of damage.  

The dike failed due to poor foundation design, inadequate understanding of the solubility of gypsum and the influence of joints, and because there was very limited participation of engineering geologists in the dike’s design and construction.  

A new dam was completed in 1990 as a roller-compacted concrete gravity dam and has a cutoff trench as much as 75 feet deep.

The Virgin oilfield, discovered in 1907 and thus Utah's oldest oilfield, produced about 200,000 barrels of oil over its lifetime; the last well was plugged in 1985.  The oil was produced from shallow wells about 500 to 800 feet deep that tapped limestone beds of the lower Moenkopi Formation.  Where this limestone interval is exposed in Timpoweap Canyon and its tributaries, tar seeps can still be found.


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