Central Utah Thrust Belt:

Tectonics of a New Exploration Province

G E O S C I E N C E E X P L A I N E D

The Middle-Jurassic Arapien Shale in central Utah as major cap rock for petroleum traps.

Geologic divisions of Utah showing the distribution of producing oil and gasfields, and the location of the recently discovered Covenant field in centralUtah. Numbers 1 through 8 are described in the table. The majority of Utah’s oiland gas fields are concentrated in two large basins – the Uinta basin in north-east, and the Paradox Basin in southeast.

The Sevier orogenic belt formed as a retro-arc,fold-and-thrust belt from Late Jurassic-Eocene.

The basins in Alberta (Northern Rockies, Canada)and western Wyoming (Middle Rockies, USA) are

classic petroleum provinces. Central Utah hasrecently joined its petroliferous sister basins.

The recent discovery and development of the Covenant oil field by Wolverine Oil & Gas Corporation in the Central Utah fold-and-thrust belt opens a new exploration province in NorthAmerica, and gives new impetus for research on the geologic structure and evolution of the area.

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Rasoul Sorkhabi and Greg Nash, Energy &Geoscience Institute (EGI), University ofUtah, Salt Lake City

Utah is divided into four geologic regi-ons:

Central (Middle) Rocky Mountains innortheast which includes a fold-and-thrustbelt extended from western Wyoming aswell as two mountain ranges – the north-south running Wasatch Range and theeast-west running Uinta Mountains.

The Colorado Plateau in eastern Utahwhich also includes parts of Colorado, NewMexico, and Arizona. Several major riversnotably the Green, Colorado, San Juan, andVirgin Rivers have carved out deep andvast canyons on the Colorado Plateau insoutheast Utah. This river incision coupledwith the arid climate has created a colorfuland unique landscape on Earth - the RedRock Country dominated by red and whitesandstones and shales of Mesozoic age.These canyons provide a valuable oppor-tunity for geologists to examine sedimen-tary formations which are largely buried incentral Utah.

The Basin-and-Range which is a vastregion of stretched and thinned crust, nor-mal faults, grabens, igneous intrusions, anddeserts occupying western Utah as well asparts of California, Arizona, Nevada, Ore-gon, and Idaho. The Basin-and-Range pro-vince formed by the extensional tectonicactivities during Oligocene to Recenttimes.

The High Plateaus of Utah which topo-graphically separates the lower Canyon-lands on the Colorado Plateau on the eastfrom the Basin-and-Range on the west.Structurally, the High Plateaus are a transi-tion zone from the compressional tecto-nics of Colorado Plateau to the extensionaltectonics of the Basin-and-Range. ThisTransition Zone includes the Central Utahfold-and-thrust belt which is part of theCordilleran-Sevier orogenic belt.

The Orogenic BeltsThe Cordilleran-Sevier orogenic belt

runs from 6,000 kilometers from Canadathrough western USA (Montana, Wyoming,Utah, and Nevada) down to Mexico. It for-med as a result of the subduction of theancient (now consumed) Kula and Farallonoceanic plates of Pacific side underneaththe North American continental plate fromthe Late Jurassic through the Eocene. Lar-ge batholiths (Coast Mountain Batholith,Idaho Batholith, and Sierra Batholith) are

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Stratigraphic column forcentral Utah (modified

after Hintze, 1988, Geo-logic History of Utah).

An outcrop Jurassic Navajo Sandstone displaying cross-bedding structure in Zion National Park, southwestUtah (photo by authors).

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products of this subduction history andmark the extent of the Cordilleran-Sevierorogenic belt.

The Sevier orogeny should not be con-fused with the Laramide orogeny whichalso affected parts of the western U.S.A.The former was older (Late Jurassic-Eoce-ne) thin-skin tectonics; the latter wasyounger (latest Cretaceous-Eocene) thick-skinned tectonics. Basement uplifts such as the Uinta Mountains and Colorado Pla-teau are largely products of the Laramideorogeny. Although both Sevier and Larami-de orogenies are probably related to thecompressional forces of ocean plate sub-ductions, the exact mechanism and timingdifferentiating these two events has longbeen a matter of debate among geologists.

The Central Utah fold-and-thrust belt islargely included in the High Plateaus ofUtah but also extends to the west compri-sing both exposed and concealed foldsand thrust faults.This overthrust belt is tec-tonically an extension of the fold-thrustbelt in western Wyoming and northeastUtah. Indeed, there are similarities betwe-en the Covenant field in central Utah andthe Pineview and the Anshutz oil fields innortheast Utah; they all produce from theNavajo sandstone and are trapped inthrust folds.

StratigraphyA thick succession of sedimentary rocks

spanning late Proterozoic to Recent timesand of various depositional environmentsare found in central Utah. Of particularimportance for petroleum generation are

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the Mississippian formations. These sedi-ments were deposited in seas that coveredmost of Utah at that time, and reached amaximum thickness in the subsidingOquirrh Basin in northwestern Utah. Theyprimarily consist of organic rich carbonatesinterbedded with shale layers.

Three significant organic rich Mississip-pian (Lower Carboniferous) shale beds,which crop out, include the Chiulos ShaleMember of the Great Blue Limestone (witha maximum thickness of ~550 m), the Man-ning Canyon Shale (~600 m) and the DellePhosphatic Member.

The Colorado Plateau has an abundanceof Mesozoic sandstone units that can andsometimes do act as petroleum reservoirrock. Some of these extend westward intothe Basin and Range and the TransitionZone, most notably Lower Jurassic NavajoSandstone, which is desert eolian depositforming major petroleum reservoirs inWyoming, and outcropping in the famousnational parks in southern Utah.This unit isoverlain unconformably by Jurassic TwinCreek Limestone, which exhibits strongtectonic fracturing in folded and faultedoutcrops.Together, these two units form anexcellent column of reservoir rock whichmay reach a thickness of 800 m in the area.

The Twin Creek Limestone is overlainwith Middle-Jurassic Arapien Shale, a mari-ne-evaporitic facies consisting of calcare-ous shale, with minor calcareous sandsto-nes being found locally (especially at thedistal margins), abundant gypsum at shal-low levels, and halite (salt rock) at the base.Salt diapirism has deformed the Arapien

Utah’s petroleum exploration and production date back to the late nineteenth century.

No. Year Drilling or Discovery1 1891 First oil well in Utah (Bamberger and Millis Well #1),

south of Green River town, ~300 m deep, dry hole2 1892 Utah’s first gas field (Farmington field) discovered3 1907 First oil field (Virgin field), abandoned in 1970s4 1948 First commercial production (Equity #1 Well) from

an oil field in Uinta Basin5 1975 Pineview oil field by Amoco Production Co.

from Jurassic sandstone reservoir, Absaroka thrust sheet6 1983 Anshutz Ranch East field by Amoco

Utah’s most prolific oil and gas field7 1983 Deepest producing payzone (Brotherson 2-14B4 Well)

Altamont field, ~6,242 m feet deep8 2003 Covenant oil field in the central Utah overthurst belt (17-1 Kings Meadow

Ranches Well, ~3,258 m deep)

Numbers refer to locations on the map on page 51.

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Rasoul Sorkhabi (rsorkhabi@egi.utah.edu), Ph.D. ingeology (1991) and Greg Nash (gnash@egi.utah.edu), Ph.D. in geography (1996) are research pro-fessors at the University of Utah’s Energy & Geosci-ence Institute, Salt Lake City. Recently, they havebeen involved in research concerning the tectonicand stratigraphic framework of central Utah.

An outcrop of fractured Twin Creek limestone(Lower-Middle Jurassic) within a major fold struc-ture (Pigeon Creek, central Utah) (Photo by Aut-hors).

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Shale; moreover, salt has also moved alongfaults and fractures formed during theSevier orogeny, which were reactivated bymid- to upper-Cenozoic extensional tecto-nics. The composition of evaporitic mine-rals and fine-grained mud, coupled withconsiderable thickness, makes the ArapienShale an efficient cap rock for petroleumaccumulation.

In Central Utah, Arapien Shale crops outalong two major linear strips, while thereservoir rocks are buried except for anoutcrop of Navaho sandstone in the PavantRange.

Structural EvolutionThe Central Utah fold-and-thrust belt

consists of a series of thrust faults whichbecome younger from west to east asdeformation migrated in that direction.These thrust faults are described below:

(1) The Canyon Range Thrust (formedduring Nocombain-late Albian, ca. 145-110Ma) is exposed as a klippe (a thrust sheetisolated from the root thrust zone) in theCanyon Range. Neoproterozoic-Cambrian

rocks, with a thickness of about 4 km, lie onthe hangingwall while the footwall consistof over 3.5 km thick Neoproterozoic toDevonian rocks. Some authors considerthat the Canyon Range Thrust was reacti-vated during Campanian-Maestrichtianepochs.

(2) The Pavant Thrust system (Late Albi-an-Coniacian, ca. 110-86 Ma), best exposedin the Pavant Range, displays structuralcomplexity with both hangingwall duplexand footwall imbricate thrust faults. Over-all, the Pavant thrust system brings theCambrian quartzites (Tintic Formation)atop Lower Jurassic Navajo sandstone.

(3) The Paxton Thrust system (Santoni-an-Early Campanian, ca. 86-75 Ma) is main-ly defined from subsurface (seismic andwell) data; it is therefore a less known fault.One well (Placid Paxton #1, 1982) penetra-ting the Paxton thrust found Cambrianrocks faulted over Jurassic formations.Another well (the Amaco Sevier BridgeWell, 1981) discovered hangingwall duplexthrusts with displacements at the Triassic-Middle Jurassic levels.

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Thrust faults, major towns, highways, gas pipeline, Covenant field, andoutcrops of Navajo and Arapien formations.

A chart for the structural evolution of thrust faults in central Utah including synoro-genic clastic deposits shed from the thrust uplifts (data sources: P. DeCelles, 2006,GSA Bulletin, v. 118, pp. 841-846; A. Villien and R.M. Kligford, 1986, AAPG Memoir 64,pp. 281-307.; G. Willis, 1999, Utah Geological Association Publication 27, pp. 1-9.)

(4) The Gunnison Thrust (Late Campani-an-Maastrichtian, ca. 75-65 Ma) bringsJurassic rocks atop Cretaceous formations.The Covenant field was discovered on thehangingwall of the Gunnison Thrust. Wol-verine drilling showed that the Gunnison isa duplex thrust consisting of the mainthrust plane and a "back-limb thrust" onthe hangingwall so that the Lower Jurassicreservoir rocks (Navajo Sandstone andTwin Creek Limestone) are repeated.

(5) The Wasatch Thrust is not exposedbut is hypothesized by some geologists asthe easternmost thrust fault in central Utahand beneath the Wasatch Plateau. Thisthrust possibly formed at 60-50 Ma and hasdisplaced Jurassic formations.

The development of these thrust faultshave been constrained from the juxtapo-sed sedimentary formations on the faulthangingwall and footwall as well as theage of clastic sediments shed from thethrust uplift onto the foreland basin. Thestratigraphic data for thrust dating comesfrom outcrops as well as drilled wells.

Summarizing the thrust evolution of

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The Sevier OrogenicBelt

The Sevier belt is mainly a thin-skinnedtectonic regime, meaning that the Pre-cambrian basement has been detachedfrom the Phanerozoic sedimentary coveralong a deep decollement (detachment)fault. Moreover, this orogenic belt formedas a retro-arc ("behind arc") fold-and-thrust belt with thrust faults generally dip-ping west in contrast to the eastward sub-duction of the oceanic plates. Field obser-vations and sandbox analog experimentshave shown that a region undergoing pla-te-scale compressional stress often pro-duces a doubly-verging wedge boundedby thrust belts in opposing directions. Inthe case of the Sevier belt, this structuralsetup may be explained by accounts ofthe juxtaposition of hot-young batholith(on the west) against the old, cold, anddense Precambrian basement of NorthAmerica on the east. As a result, the Pre-cambrian crust was detached along thesoft layers at the basement-cover interfa-ce and underthrust the sedimentary cover.As compressional stress as well as fluidoverpressure accumulated at particularhorizons within the sedimentary cover,thrust planes were generated and thrustsheets were pushed for tens of kilome-ters, piling older sedimentary rock on topof the younger ones.

The various processes involved in thedevelopment of the Sevier fold-and-

thrust belt.

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central Utah, it seems that as the thrustfaults have become younger from west toeast, both duration of thrusting and theamount of fault displacement has decrea-sed. Older thrusts have displaced deeperrocks, and as younger thrust developedeastwardly, they displaced younger forma-tions. The Central fold-and-thrust belt hasbeen also affected by normal faults duringthe Basin-and-Range extension.

A New Exploration FocusAccording to the U.S. Energy Informati-

on Administration, Utah with crude oil pro-ven reserves of 215 million barrels, andwith oil production of 40,000 barrels a day,ranked 12th and 13th, respectively, in the

United States for onshore production in2004. In the aftermath of the Covenantfield discovery in central Utah, these statis-tics are already changing. Some geologistsestimate that there are one billion barrelsof oil hidden in central Utah. Land leasingprices have hiked from $10 an acre a fewyears ago to over $1000 an acre in the areapresently. The central Utah fold-and-thrustbelt has become a scene of oil rush withmore than twelve companies already pre-sent. Almost all of these companies areindependent oil firms. Only time and otheroil field discoveries will tell whether andwhen major petroleum companies willmove in (come back to) central Utah fold-and-thrust belt.

A view of the Pavant thrust fault near the town of Fillmore, central Utah, one of the earliest thrust faultsdeveloped during the Sevier orogeny in Utah. The Pavant thrust strikes north-south and dips gently (about10º) eastward (photo by authors).