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Thegeology of Utah, in the western United States, includes rocks formed at the edge of the proto-North American continent during thePrecambrian. A shallow marine sedimentary environment covered the region for much of thePaleozoic andMesozoic, followed by dryland conditions, volcanism, and the formation of the basin and range terrain in theCenozoic.[1]
The easternUinta Mountains near the Colorado line and theRaft River-Dove Creek Mountains contain the oldest rocks in Utah from more than two billion years ago.Rubidium-strontium dating of the Red Creek quartzite in 1965 indicated an age of 2.3 billion years. Other geologists found 2.4 billion-year-oldschist andgneiss in theAlbion Range Green Creek Complex. Prior to the 1960s, geologists inferred the Vishnu and Farmington Canyon gneiss and schist asArchean in age, but subsequent research indicated a formation between 1.6 and 1.5 billion years ago in theProterozoic. From 1.75 to 1.65 billion years ago, Utah was impacted by theMazatzal orogeny. Thick sequences of sedimentary rock from theNeoproterozoic, includingtillite, underlie much of the state.[2]
Utah has numerous rocks deposited during thePaleozoic, as multicellular life diversified for the first time. BasalCambrianquartzite, up to 4,000 feet (1,200 m) thick, is sometimes difficult to distinguish from Precambrian rocks. A shallow marine environment produced mottledlimestone andstromatalite beds, deepening to the west. Thin shale and siltstone units of the Whirlwind Formation likely originated from sediment sources in the east. ThePioche Shale contains some of the oldest Cambrian fossils, followed by a continuous sequence oftrilobite fossils in theHouse Range.
In eastern Utah, which rests on thecraton of the proto-North American continentLaurentia, there are noOrdovician deposits, but Early Ordovician clastic limestone, Middle Ordovician quartz sandstone, and Late Ordoviciandolomite is common in the west. Geologists have found intraformational limestone from the Early Ordovician, originating as limestone forming in a tidal bed is redeposited as pebbles in situ.Brachiopod, trilobite, andechinoderm fossils are common in these rocks. Middle Ordovician quartzites form distinctive orange, brown, and pink cliffs above gray limestone slopes, set beneath black dolomite cliffs in the north and west.Chert-rich dolomite cliffs form in the most widespread Ordovician unit: theFish Haven-Ely Springs Dolomite.
Only a single stratigraphic unit—theLaketown Dolomite—formed during theSilurian, far from sediment sources in a carbonate platform environment, with few distinct fossil assemblages. Although thickest in the west,Devonian strata are present in the east, unlike Ordovician and Silurian rocks. A local orogeny produced the Stansbury Uplift, leading to downward erosion into older rock units dating to the Precambrian and the deposition of the coarse conglomerate Stansbury Formation. Shale and quartz sandstone are often interbedded with carbonates, as in the Hanauer Formation in theThomas-Dugway Range.
In theMississippian, up to 6,000 feet (1,800 m) of sediment accumulated in the Oquirrh Basin. Marine deposition was continuous, with numerous fossils in an unbroken record, such as thecrinoid stems, brachiopods, and corals in theChainman-Manning Canyon Shale. Silicified brachiopod and coral fossils mark theRedwall,Leadville,Madison, Gardison,Joana, andLodgepole limestones, and only the northwest lacks extensive limestone formations.
Geologists focused significant attention on thePennsylvanian rocks of theParadox Formation in theParadox Basin after the discovery of oil, potash, and rock salt. The basin took shape as part of the Uncompahgre Uplift, produced by the uplift of the ancestralRocky Mountains. Several thousand feet of salt accumulated beneatharkose shed off the mountains. Up to 13,000 feet of sedimentary rocks accumulated in the Oquirrh Basin.
A smallunconformity separates these rocks from those formed in thePermian. These rocks tend to be difficult to correlate, with significant lateral variation, cross-bedded sandstones indicating wing transportation, and a lack of late-Permian rocks. A major marine transgression generated theKaibab Limestone (the rimrock of the Grand Canyon), represented by thePark City andPhosphoria groups in Utah.[3]
In the earlyMesozoic, one final marine deposit formed in western Utah during theTriassic—theThaynes Limestone—before the region was uplifted. The early-TriassicMoenkopi Formation is a mudstone with thin layers of limestone formed as the sea spread out across mud flats and hosts theCanyonlands in southern Utah. From east to west, mudstone grades into limestone.
TheNavajo,Wingate, andKayenta sandstones are distinguished by the cross-bedding of the Kayenta Sandstone across theColorado Plateau, which tends to be more characteristic of a stream environment than the sand dune deposits that make up the Wingate Sandstone.The Navajo Sandstone began to form in the late Triassic but is mainly aJurassic formation, including the formations inZion National Park. Other important Jurassic units include theEntrada Sandstone, which hostsArches National Park, and theMorrison Formation, with notable fossils preserved atDinosaur National Monument. The thickest Jurassic sediments are betweenDevil's Slide, east of Ogden and Marysvale, which includegypsum and other evaporites, mined from the Arapien Shale. TheSevier orogeny to the west shed gravel and other sediments into the Morrison Formation. The formation was entirely continental in origin, recording shallow lakes, shifting streams, and volcanic ash. A shallow seaway formed in theMiddle Jurassic, leaving behind the fossil-bearingCarmel andTwin Creek formations.
For the last time, Utah was covered by shallow seas in theCretaceous, and with the continued uplift of theSevier orogeny leading to erosion, deposits from the time are double the thickness of Jurassic units. TheKelvin and Lower Indianola formations indicate continuous sand and gravel deposition in streams along the foothills of the mountains, while both theBurro Canyon andCedar Mountain formations (which are rich in fossils) point to floodplain deposits similar to those in the underlying Morrison Formation. Near the Wyoming line, theDakota Sandstone is overlain by theAspen-Mowry Shale. In theWasatch Plateau, theFerron Sandstone represents a brief marine regression to the east and serves as an important natural gas horizon. The Emery andMesaverde sandstones indicate similar regressions (the Mesaverde marks the last one in the late Cretaceous). Coals formed in units, such as theStraight Cliffs Formation.[4]
In theCenozoic, theLaramide orogeny uplifted the Rocky Mountains. NearFlaming Gorge, thePaleoceneFort Union Formation angularly overlies the Cretaceous Ericson Sandstone, while on the south slope of the Uinta Mountains, theWasatch Formation overlies the Cretaceous Mesaverde Sandstone. The Wasatch Conglomerate formed as the coarsest debris accumulated in basins near the uplifted areas, with sediments growing finer further east and reaching up to 13,000 feet thick through theEocene in theUinta Basin.Large lakes played an important role as well. Paleocene Lake Flagstaff left algal limestone in the Southern Wasatch Mountains, along with marl-mudstone inBryce Canyon,Cedar Breaks, andRichfield. Eocene Lake Green River deposits do not extend as far south but include an important oil shale resource. Erosion wore down the Uinta Mountains enough that theDuchesne River Formation covered over earlier Cenozoic basin fill.
An abrupt shift to volcanic activity in theNeedle Range and at Marysvale, Crystal Peak, Tintic, and Bingham took place in theOligocene, erupting with thick ash flowtuff and producing weldedignimbrite. The Needle Range ash flow tuff is the most extensive, covering 13,000 square miles in southwest Utah and eastern Nevada. While most tuffs arerhyolite, the Needle Range tuff includesamphibolite,biotite, andplagioclase phenocrysts. Volcanism was related to the subductingFarallon Plate and took place from 35 to 19 million years ago, predating the block faulting of theBasin and Range Province, making tuff useful for stratigraphic comparisons. Several unusuallaccolith intrusions formed, including theHenry,La Sal,Abajo, and Three Peaks mountains.Volcanic activity lessened from 19 to 16 million years ago, followed by basalt flows, cinder cones, and theTopaz-Spor Mountain rhyolite into theMiocene,Pliocene, andQuaternary. During this time and continuing to the present, block faulting created the basin and range terrain in western Utah and Nevada.
Lake Bonneville formed during the Quaternary, covering 20,000 square miles. TheGreat Salt Lake is the remnant of the deepest part of the lake, where the depth was once as much as 1,000 feet (300 m). During thePleistocene, glaciers formed in the mountains, and in the case of the Temple Lake Stade and Gannett Peak Stade, advanced 4,000 years ago in theHolocene.[5]
Utah is divided into several distinct regions based ontectonic andstratigraphic trends. These regions include: theColorado Plateau,Uinta Mountains,Uinta Basin,Basin and Range Province, and the Central Utah Thrust Belt, among others.
The Central Utah Thrust Belt runs north to south across Utah and is a remnant segment of the continent-scaleSevier orogeny. This mountain belt formed from 50–160– million years ago due to subduction of theFarallon oceanic crust under theNorth American Plate. The Central Utah Thrust Belt can be divided into smaller thrust sheet segments, including the Charleston-Nebo Thrust Salient, the Emery Uplift, and the Gunnison-Paxton Segment.[6]
This region has been the focus of 21st-century petroleum exploration. In 2004, a well struck oil in what is now the Covenant Oil Field, 3 km south ofSigurd.[7] As of 2020, 27 million barrels had been produced from this field, from 34 production wells, averaging 3,400 barrels per day.[8]
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