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Geological Society of America Bulletin

doi: 10.1130/0016-7606(1968)79[429:SOBINA]2.0.CO;2 1968;79;429-458Geological Society of America Bulletin

RICHARD LEE ARMSTRONG Sevier Orogenic Belt in Nevada and Utah

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RICHARD LEE ARMSTRONG Dept. Geology, Yale University, New Haven, Connecticut

Sevier Orogenic Belt in Nevada and Utah

Abstract: In Nevada and Utah, sedimentation in the Cordilleran miogeosyncline began beforethe appearance of Cambrian fossils and continued without erogenic interruption through theTriassic. During the Jurassic, deformation and regional metamorphism occurred in the westernpart of the miogeosyncline, and the area of sediment accumulation shifted onto the ColoradoPlateau.

A major source of clastic material appeared along the eastern margin of the Cordilleran miogeo-syncline in Early Cretaceous time; this source supplied the sediments that filled the Cretaceous toPaleocene Rocky Mountain geosyncline. Clasts in the Cretaceous conglomerates show an invertedstratigraphy, reflecting successive exposure of older and older rocks in an evolving orogenic beltalong the eastern side of the Cordilleran miogeosyncline. This source area was the Sevier orogenicbelt, which had a history of deformation through most of the Cretaceous (Sevier orogeny).Decollement thrusts with displacements of tens of miles are the characteristic structures of thebelt, but several large folds are also known. The largest thrusts are overlain unconformably byuppermost Cretaceous conglomerates.

Thrusting in the Sevier orogenic belt had virtually ceased by the time the Laramide orogenybegan east of the Sevier belt in latest Cretaceous time. Laramide mountains were the result ofuplift of great blocks of crystalline basement along nearly vertical, reverse, and steep thrust faults.The Uinta arch, which intersects the Sevier orogenic belt almost at a right angle, is the only one ofthese basement uplifts closely involved with the deformation of the Cordilleran miogeosyncline.

North-south-trending regional normal faulting of post-Oligocene age has broken up the orogenicbelt so that it is not immediately recognizable on geologic maps. Arch ranges, intrusive domes, andgravity slides are additional complications of the Tertiary geology, but widespread Tertiary de-posits, particularly Oligocene ignimbrites, make a paleogeologic reconstruction possible; thus, theSevier orogenic belt can be viewed as it existed before the normal faulting.

CONTENTS

Introduction 430 Tertiary structures 450Acknowledgments 430 Nevadan, Sevier, and Laramide orogenies . . . 451Geologic setting and stratigraphic history . . . 430 References Cited 453Pre-normal faulting paleogeology—Sevier orogenic

belt 432 FiSure

Paleogeologic map 432 1. Eastern Great Basin Tertiary correlation chart 433Foreland 434 2. Index map for Sevier orogenic belt, Nevada andSevier Orogenic Belt 435 Utah 436

General Statement 435 3. Relationship of Pole Canyon thrust to Sheep-Southern Nevada-southwestern Utah sector . 435 rock thrust, Sheeprock-West Tintic area,Wah Wah-Canyon Range sector 437 Utah 439Nebo-Charleston sector 437 4. Reinterpretation of Taylor and Ogden thrusts 441Northern Utah sector 438 5. Diagrammatic section across Rocky Mountain

Amount of shortening in Sevier orogenic belt . 440 geosyncline in central Utah 446Structural continuity of thrust belt 441 6. Jurassic to Paleocene correlation chart showingStyle and localization of thrusts 442 inverted stratigraphy of clasts in RockyHinterland 442 Mountain geosyncline 447

Stratigraphic evidence concerning age of deforma- 7. Geologic time scale 452tion in Sevier orogenic belt 444

Evidence for pre-Cretaceous Sevier arch . . . 444 Plate FacingCretaceous to Paleocene—Rocky Mountain geo- 1. Paleogeology of the Sevier Orogenic Belt . . 429

syncline 445Review of information provided by clast Table

provenance 445 1. Geologic Maps of the Eastern Great Basin andProblem of Canyon Range fanglomerate . . 448 Vicinity which were used for Construction

Summary 449 of the Paleogeologic Map 434

Geological Society of America Bulletin, v. 79, p. 429-458, 7 figs., 1 pi., April 1968

429



430 R. L. ARMSTRONG—SEVIER OROGENIC BELT IN NEVADA AND UTAH

INTRODUCTION

The eastern Great Basin in eastern Nevadaand western Utah is characterized by north-south-trending fault-block ranges composed ofcarbonate assemblage rocks of the Cordillerangeosyncline. The area under consideration isbounded on the west by the mid-PaleozoicAntler orogenic belt (Roberts and others, 1958),on the south by the Las Vegas shear zone(Longwell, I960), and on the east by theColorado Plateau. Although the Idaho bound-ary has been taken as an arbitrary northernlimit, it should be emphasized that structuresdescribed in the Great Basin persist withoutsignificant modification northward into, andeven past, central Idaho.

In order to understand the results of K-Ardating studies of the region, a review of avail-able knowledge of Great Basin geology was es-sential; the results of the K-Ar studies havebeen published elsewhere (Armstrong, 1963;1966; Armstrong and Hansen, 1966). The onlycomplete synthesis of Basin and Range geology(Nolan, 1943) has become a classic. Since theappearance of that report, a great amount ofwork has been done in the area, particularly asthesis projects. All Utah and approximately 80percent of eastern Nevada have been mappedin enough detail to show the most significantstructural features. Osmond (1960) discussedbriefly the tectonic history of the Basin andRange province in Utah and Nevada; Misch(1960) discussed certain structural features ofthe eastern Great Basin; and Gilluly (1963) hasreviewed the tectonic history of the westernUnited States. No up-to-date detailed synthesisof the geology of the eastern Great Basin exists,however, and this led King (1959, p. 142) tosay, after describing LongwelPs discoveries inthe Las Vegas region:

To pursue details of the structures in other partsof the eastern Great Basin would probably onlybewilder the reader without profit. Many folds andthrusts are known, but the larger pattern is for themost part undetermined. Not only have the funda-mental structures been obscured over wide areas byBasin and Range structure, but many of the rangeshave been little explored geologically.

It is the writer's opinion, however, thateventually we shall know more about theorogenic history of the Great Basin because ofthe faulting and volcanics, not in spite of them,for they provide exposures in the third dimen-sion and key horizons for reconstructing the de-

formed orogen now exposed at varied structurallevels.

The existence of thrust faults and folds ofMesozoic age along the eastern edge of theCordilleran geosyncline is common knowledge(Eardley, 1962, 1963); the same area has beenclearly recognized as a source of clastic materialduring the Cretaceous by Spieker (1946; 1949;1956) and his students and by Harris (1959)who proposed the name, Sevier arch, for theclastic source. This paper is a review and analysisof the geology of this fold and thrust belt.

ACKNOWLEDGMENTSI am indebted to the large number of

geologists who contributed directly and in-directly to this project through their studies inthe Great Basin. Without such previous work,this synthesis would be impossible. Kenneth F.Bick introduced me to the geology of the GreatBasin in 1956. Pierre Biscaye and, later, JuliaArmstrong assisted in the field studies during1961. C. R. Longwell, Paul Williams, RobertScott, T. B. Nolan, Keith Ketner, HaroldMasursky, L. I. P. Muffler, J. C. Taylor, andHoover Mackin provided hospitality and guid-ance in their respective field areas. Duringpreparation of the original manuscript, JohnRodgers, Edward Hansen, Keith Howard,Clark Burchfiel, D. H. Adair, Kenneth Pierce,and Pierre Biscaye provided helpful discussion.K. K. Turekian, John Rodgers, P. M. Orville,C. R. Longwell, R. J. Roberts, Peter Mischand D. H. Adair have read the present paperat various stages of preparation and providedhelpful comments. Much of the drafting wasdone by Gary Audette. Field work was sup-ported by National Science Foundation grantG14192. This research was done in major partwhile the writer was a National ScienceFoundation graduate fellow (1959-1962).

GEOLOGIC SETTING ANDSTRATIGRAPHIC HISTORY

Two principal parts of the Cordillerangeosyncline are recognized. The miogeosynclinein Nevada and Utah contains a thick section ofPaleozoic rocks of the carbonate assemblage1

(limestone, dolomite, clean sandstone, andlittle shale), and within, and west of the Antler

1 "Carbonate assemblage" and "siliceous assemblage"are used for the contrasting geosynclinal facies as sug-gested by Silberling and Roberts (1962) and R. J. Roberts(1964, written commun.).



GEOLOGIC SETTING AND STRATIGRAPHIC HISTORY 431

erogenic belt Paleozoic rocks of the siliceousassemblage (shale, dirty sandstone, chert, andvolcanic rocks) of the eugeosyncline occur.East of the geosyncline in the central WasatchRange and on the Colorado Plateau, rocks ofthe carbonate assemblage occur in a drasticallythinned and incomplete Paleozoic section.

The relationships between the Paleozoic sec-tions in the eugeosyncline, the miogeosyncline,and the adjacent shelf are obscured by majorthrust faults with displacements of tens ofmiles. Eugeosynclinal rocks have been thrustover miogeosynclmal rocks in western and cen-tral Nevada, and miogeosynclinal rocks havebeen thrust over thin shelf facies in southeasternNevada and western Utah. The present-daygeographic distribution of the various rockassemblages, therefore, does not represent theirdistribution at the time of deposition.

Older Precambrian crystalline rocks thatwere metamorphosed approximately 1.5 b.y.ago underlie the shelf sections in Utah andsouthern Nevada and the miogeosynclinal rocksin the Death Valley, California, region. Withinmost of the geosyncline, however, no provenolder Precambrian ( > 1 b.y.) rocks are exposed.In the Uinta Mountains, Cottonwood Uplift,and Death Valley areas, thick sections of young-er Precambrian sedimentary rocks uncon-formably overlie the older metamorphics andare in turn overlain unconformably by rocks ofthe Cordilleran geosyncline.

The Paleozoic history of areas west of a lineextending south by southwest from northeast-ern Nevada was complex because two majorPaleozoic orogenies occurred there (Robertsand others, 1958; Silberling and Roberts, 1962).In the miogeosyncline of eastern Nevada andwestern Utah, however, Eocambrian2 throughTriassic stratigraphic relations are relativelysimple. Gentle truncation of units occurs, par-ticularly along the eastern and western marginsof the miogeosyncline, but only one distinctlyangular unconformity is known (Stansburyanticline of Rigby, 1958), and this is only oflocal extent.

Deposition began in the Cordilleran miogeo-syncline before the oldest Cambrian fossils ap-peared. Approximately one third of the strati-

2 The term "Eocambrian" is used to emphasize thatno significant time gap separates the sediments referredto from overlying fossiliferous Cambrian strata. Usage ofthis term is the same as in the Caledonide area of Norwaywhere Eocambrian was first proposed by W. C. Broggerin 1900 (Holtedahl, 1960, p. 111-112).

graphic section of the miogeosyncline is com-posed of a basal clastic sequence which includesEocambrian, Lower, and Middle Cambrianquartzite and argillite and a widespread Eo-cambrian tillitic member. In southern Nevadaand at scattered localities elsewhere, dolomiteis present in this basal sequence. After EarlyCambrian time, carbonate deposition becamewidespread. Middle and Upper Cambrian de-posits are complexly intertonguing shale andcarbonate rocks, more dolomitic toward thetop. Lower Ordovician limestones with minorshale were succeeded in Middle Ordoviciantime by a distinctive, widespread, clean whitequartz sand, which is absent only locally overthe Tooele arch, a Cambrian and Ordovicianpositive element. Upper Ordovician, Silurian,and Lower Devonian deposits are almost ex-clusively dolomite, and Upper Devonian andlater Paleozoic carbonates are predominantlylimestone. Later Devonian sedimentation wasmore varied because of tectonic activity in andnear the miogeosyncline. A gentle arch formedin east-central Nevada between Middle andLate Devonian time; the Stansbury anticlinerose in north-central Utah during Late De-vonian time. From very late Devonian timeuntil later Pennsylvanian time, the Antlerorogeny affected sedimentation in Nevada andwestern Utah; a widespread uppermost De-vonian-Lower Mississippian shale was suc-ceeded by Lower Mississippian limestone,which was locally removed as a consequence ofEarly Mississippian warping and erosion. Dur-ing the rest of Mississippian and Pennsylvaniantime, a clastic wedge composed of material de-rived from the Antler orogenic belt extendedinto the miogeosyncline from the west. Sub-sidence of the Oquirrh basin in north-centralUtah began in Mississippian time and con-tinued into Permian time. From Late Mis-sissippian through Permian time, elastics shedfrom the rising basement uplifts of the An-cestral Rocky Mountains accumulated in theOquirrh basin which was bounded on the northby an east-west-trending monoclinal flexure.During Pennsylvanian time, most of easternNevada was the site of limestone deposition; inthe Oquirrh basin, more than 20,000 feet ofalternating limestone and quartzite accumu-lated. Thick lower and middle Permian de-posits—limestone with much quartz sand, sand-stone, siltstone, dolomite, and some evaporite—accumulated in the Arcturus basin in east-central Nevada, and thick limestone deposits




accumulated in southern Nevada; in Utah, cor-relative strata consist of alternating quartziteand limestone with minor dolomite. South-eastward from the miogeosyncline, Permianmarine strata intertongue with continental redbeds.

Upper Permian deposits (Park City Group)are a widespread blanket of relatively uniformthickness and lithology (limestone and dolomitewith minor chert and phosphate) over the en-tire region, including the Antler erogenic beltand much of the Colorado Plateau shelf. In themiogeosyncline, marine sedimentation con-tinued without erogenic interruption into theTriassic over much of Nevada and Utah.Triassic (or earliest Jurassic) rocks were the lastdeposits of the Cordilleran miogeosyncline. InMiddle Triassic time, marine waters withdrewfrom the eastern Great Basin in Nevada for thelast time, marking the beginning of the erogenicchapter in the history of the region. TheTriassic-Jurassic boundary probably lies withinthe widespread eolian sandstone (Navajo-Aztec-Nugget), which is the youngest pre-orogenic formation present in the eastern GreatBasin west of the Mesozoic fold and thrust belt.

During the Jurassic, the region of thickestsediment accumulation shifted to central andeastern Utah, and the western part of theCordilleran miogeosyncline became a sourcearea (Stokes, 1960) in response to erogenic de-formation taking place there. Continentalclastic deposits of Upper Jurassic and lowermostCretaceous age, derived from this westernsource, spread across the eastern edge of thePaleozoic miogeosyncline and the ColoradoPlateau. In Early Cretaceous time, the easternedge of the geosyncline became a source ofclastic material, which accumulated duringCretaceous and Paleocene time in the RockyMountain geosyncline still farther east. At a fewlocalities in eastern Nevada, continental LowerCretaceous deposits are present, but over mostof the region there is a great hiatus between de-posits of the Cordilleran geosyncline andTertiary deposits.

Tertiary strata of the eastern Great Basinand adjacent Colorado Plateau can be sub-divided into three major groups. The oldest iscomposed of nonvolcanic continental sediments—scattered Eocene lacustrine deposits and un-dated conglomerates in Nevada and westernUtah and Paleocene and Eocene fluviatile andlacustrine sediments that are well developed incentral Utah and northward into Wyoming.The middle group consists of widespread inter-

mediate to acidic volcanics, chiefly ignimbritesof latest Eocene, Oligocene, and early Mioceneage. The youngest group, Miocene to Recent,is a heterogeneous collection of discontinuousclastic units, volcanic-rich sediments, vol-canics (commonly basalts but also all othertypes), and lacustrine sediments, depositedduring the development of the Basin and Rangestructure. Figure 1 is a correlation chart il-lustrating Tertiary stratigraphic relationshipswithin, and adjacent to, the eastern GreatBasin.

A more complete review of the Precambrianthrough Tertiary stratigraphic history, to-gether with documentation not included in thispaper, may be found in Armstrong (1968).Palinspastic isopach maps for all Paleozoic sys-tems and three palinspastic paleostratigraphicprofiles across the region are included in the re-view.

PRE-NORMAL FAULTINGPALEOGEOLOGY—SEVIEROROGENIC BELT

Paleogeologic Map

The present-day structural pattern of theeastern Great Basin is dominated by the effectsof Tertiary normal faulting. Geologic maps ofthe region cannot clearly portray the generalfeatures of the pre-Tertiary structures that areexposed in separated ranges. Each individualexposure displays the older structures in adifferent attitude or aspect, and irregularly dis-tributed Tertiary volcanics and sediments donot make things clearer. If we remove theeffects of normal faulting and Tertiary sedi-mentation, we can view in a simple manner thebroad features of the pre-Tertiary structures.This can be done by a paleogeologic reconstruc-tion, as described by Levorsen (1960). All latereffects of sedimentation and deformation areerased and the resultant map portrays thegeology as it was at the time the unconformitywas buried. This technique is applied here tothe eastern Great Basin to display the pre-normal faulting paleogeology.

Plate 1, figure 1 shows the units which over-lie the unconformity used for the reconstruction(see also Fig. 1). The range in age of the uncon-formity is latest Cretaceous to earliest Miocene.This undoubtedly has a somewhat distortingeffect on the resultant paleogeology, but it doesnot alter the fundamental geologic pattern. Inall areas, the unconformity postdates the mainMesozoic deformation, although locally in cen-



PRE-NORMAL FAULTING PALEOGEOLOGY—SEVIER OROGENIC BELT 433

Figure 1. Eastern Great Basin Tertiary correlation chart.

tral Utah some minor folding and even thrust-ing may have occurred later than the uncon-formity. The unconformity, however, predatesthe major normal faulting. Early normal fault-ing may affect the pattern somewhat in easternand southern Nevada but only to a relativelyminor degree. The over-all structural pattern ofthe region was not significantly altered duringthe time spanned by the unconformity.

If the older structure is not enormously com-plex, the resultant paleogeologic map shoulddisplay the broad features of the regionalgeology during early Tertiary time. Plate 1,figure 2 shows the distribution of points (~900)where information on rocks underlying the un-conformity was recorded. The source maps con-sulted are given in Table 1. In addition tosimple rock age information supplied byTertiary—pre-Tertiary contacts, it is also oftenpossible to construct paleo-strikes and dips byrotating the oldest Tertiary deposits (com-monly ignimbrites) back to horizontal and

determining the effect of the same rotation onthe older rocks. In areas where a suitable un-conformity is lacking, it is possible to putlimits on the ages of rocks as those exposed inearly Tertiary time must have been as young as,or younger than, those now present.

The normal faulting responsible for thepresent topography and much of the geologiccomplexity of the Great Basin occurred mostlyduring Miocene and Pliocene time. Pre-Miocene normal faults are known in manyplaces in the region, but none approach themagnitude of displacement of the faults formedlater. No example can be cited where rocksdiffering in age by several geologic periods werejuxtaposed along pre-Miocene normal faults.Thrusting accounted for the major discon-tinuities present.

A significant structural feature of the regionis the widespread near-conformity of thePaleozoic sediments and Tertiary volcanics.Over large areas, the angularity of the uncon-



434 R. L. ARMSTRONG—SEVER OROGENIC BELT IN NEVADA AND UTAH

TABLE 1. GEOLOGIC MAPS OF THE EASTERN GREATBASIN AND VICINITY WHICH WERE USED FORCONSTRUCTION OF THE PALEOGEOLOCIC MAP* (PI. 1,fig. 3).

NevadaClark CountyLincoln County

White Pine County

Elko County

Bowyer and others (1958)Kellogg (1963)Tschanz (1960)Tschanzand Pampeyan (1961)Adair (1961)Bauer and others (1960)Douglass (1960)Drewes (1958; 1960)Fritz (1960)Langenheim and others (1960)Lloyd (1959)Nelson (1959)Playford (1962)Ward (1962)Whitebread and others (1962)Woodward (1964)Young, J. C. (1960)Harlow (1956)Nelson (1956)Schaeffer and Anderson (1960)Snelson (1955)

UtahState Geologic Map

Northeast QuarterNorthwest QuarterSouthwest Quarter

Other MapsWyoming

State Geologic MapOther Data from

Stokes and Madsen (1961)Stokes(1963)Hintze (1963)Hintze (1962)

Love and others (1955)Cochran (1959)Schick (1959)

* A geologic map of Nevada (Webb and Wilson, 1962)is available but was not used for construction of Plate 1,figure 3. It includes data from the references in the tablebut in a more generalized form.

formity is less than 5 degrees; only locally isit distinctly angular. Cook (1965, p. 54-55)states:

pre-volcanism deformation was sharply localizedalong axes that trend east of north, leaving betweenthe narrow belts of deformation broad areas of un-deformed Paleozoic rocks, the ignimbrites in manysections are essentially parallel to the underlyingsedimentary rocks . . . the attitude of the volcanicsin many places reflects the attitude of the subadja-cent sedimentary rocks; locally . . . the angularityof the unconformity is great.

Mackin (1961, oral commun.) made the sameobservation, and the writer also agrees. Therelationship is of importance in reconstructingthe pre-normal faulting paleogeology of the

region. The generalization does not apply tomore westerly areas near and in the Antlererogenic belt.

The observed near-parallelism of Tertiaryand pre-Tertiary rocks in the region establishesthat low dips were characteristic of pre-Tertiaryrocks during early Tertiary time. Extrapolationbetween scattered data, therefore, should besafe at least for distances of a few miles. Thenear-parallelism of units, however, is only ageneralization. In many areas, sharp angular un-conformities occur, and the paleogeologic re-construction must maintain consistency withthese relationships. The paleogeology becomesincreasingly complex westward into centralNevada. The reconstruction is limited to thearea from which suitable data are available andwhere the paleogeology appears to have beenfairly straightforward.

The pre-normal faulting paleogeology isshown on Plate 1, figures 3 and 4. The mapagrees with all the data collected, but in manyareas alternate interpretations of the data arepossible; this does not imply, however, that themajor structural features are in doubt. The mapportrays the geology as it would have beenmapped shortly after the end of the Creta-ceous. The degree of definition (resolving pow-er) is slightly better than that represented bymore detailed parts of the 1932 edition of theU. S. Geological Survey geologic map of theUnited States.

The general features of the paleogeology areimmediately evident. The structural trends aregenerally north to northeast. On the east sideof the map is a broad, virtually undeformed,foreland basin area filled by Cretaceous de-posits. Two broad arches which lie nearlyperpendicular to the regional trend occur inthis foreland. The middle of the map is oc-cupied by a fold-and-thrust belt with eastwardoverturning and thrusting. The region fartherwest displays a deceptively simple structuralpattern, in which, over broad areas, only gentlyfolded upper Paleozoic rocks are shown. Insouthern Nevada, the pattern is complicatedby an uplift exposing lower Paleozoic andEocambrian rocks and surrounded by klippenof Paleozoic rocks overlying middle and upperPaleozoic strata. This pattern is the summationof all the effects of Mesozoic and early Tertiarydeformation in the region.

Foreland

East of the belt of folds and thrusts is abroad area that was slightly deformed during




Mesozoic time; later in the era, it was a basin inwhich erogenic debris from the area now oc-cupied by the Great Basin accumulated. At theend of the Cretaceous and during early Tertiarytime, broad arches formed in the foreland.These can be seen on the paleogeologic map asthe Uinta arch in northern Utah and the CircleCliffs upwarp in the southern part of the state.

Sevier Orogenic BeltGeneral statement. The Sevier arch was

named by H. D. Harris (1959), who describedit as a late Mesozoic positive area in westernUtah and southeastern Nevada. The arch con-cept was based on paleogeology, and the archwas considered the source of the erogenicelastics shed to the east; thrusting was con-sidered to be the climax of arching during lateCretaceous time. Harris (1959, p. 2646) says:

There is no direct evidence of large-scale thrust-ing associated with the uplift of the Sevier arch.Deformation appears to have been generally limitedto upwarping and development of major folds, someof which undoubtedly developed into belts of struc-tural weakness that later became zones of thrusting.

The paleogeologic map of Plate 1, figure 3 isbased on more than ten times the amount ofdata presented by Harris and does not supportthe concept of a simple late Mesozoic arch, fornowhere is an eastern limb evident. It showsthat the exposure of old rocks in the area isindeed the result of thrusting, which was thedeformation responsible for the elastics shedinto the Rocky Mountain geosyncline duringthe latter half of the Mesozoic. It is suggestedthat "Sevier orogenic belt" is a better term forthe belt of thrusts and folds originally de-scribed as the Sevier arch.

In several recent studies, paleogeographicarches or geanticlines have been discovered tobe orogenic belts; for example, the Manhattangeanticline became the Antler orogenic belt(Roberts and others, 1958), and the Mesocordil-leran geanticline was the site of Jurassic de-formation (Misch, 1960; Armstrong andHansen, 1966). It does not appear reasonable toexpect enormous quantities of coarse elasticsfrom simple arching. Orogenic deformation,including faulting, is necessary to account forthe Mesozoic elastics.

In its earliest stages, the Sevier belt may wellhave been archlike. Permian isopachs offer thefirst faint suggestion of uplift along the locusof the belt. Further uplift in later Permian orearly Triassic time may be indicated by the pre-

Triassic unconformity in southern Nevada. Thefirst conclusive evidence of orogenic deforma-tion in the belt, however, is the appearance oflower Paleozoic clasts in early Colorado time inUtah and Nevada or as far back as the be-ginning of the Cretaceous in Idaho (F. C.Armstrong and Cressman, 1963, p. 10). Theonly possible source of these clasts is the Sevierbelt. Lower Cretaceous and Upper Jurassicsediments could have been derived from any-where in the eastern Great Basin, but they mayalso have come, at least in part, from the Sevierbelt during earlier stages of its development.The Eocambrian rocks of the belt occur almostexclusively in the sole of major thrusts. Theirappearance, in abundance, as clasts approxi-mately at the end of Colorado time is evidencethat thrust displacements of tens of miles exist-ed by then.

The relative ages of folds and thrusts in thebelt normally cannot be determined, but in theCanyon Range folding definitely postdates dis-placement on the major thrust (Christiansen,1952). In other mountain belts, such as theAppalachians, folding postdates movementalong regional bedding-plane thrusts (Rich,1934; Pierce and Armstrong, 1964), for afterfolding such thrusts are unable to develop.Probably, therefore, most of the Sevierthrusting represents an earlier stage of defor-mation than the folding. Some folding wasprobably also contemporaneous with, if notcaused by, thrusting (Rich, 1934; Cressman,1964). On the major thrusts of the belt, aminimum of 25 miles of total displacement hasoccurred; such a displacement is approximatelyequal to the thickness of the crust and cannotbe merely the climax of deformation in a tightfold. To summarize, thrusting was prolongedand of great magnitude in the Sevier belt. It wasnot merely the climax to earlier folding andarching. Figure 2 provides an index map for thefollowing detailed discussion of the Sevierorogenic belt.

Southern Nevada-Southwestern Utah Sector.The structural geology of Clark County,Nevada, has been discussed by Longwell (1949;1952; 1952a; 1960; 1962), and a county maphas been published (Bowyer and others, 1958).The paleogeology retains all features of Long-well's interpretation of the area. Modificationof the structural pattern by later Tertiary nor-mal faults is relatively minor.

The major thrust in terms of stratigraphicdisplacement is the Gass Peak thrust whichbrings the Lower Cambrian and Eocambrian




Figure 2. Index map for Sevier erogenic belt, Nevada and Utah.

quartzite over carbonates of the Pennsylvanianand Permian Bird Spring Formation. East ofthe major thrust is the Glendale thrust, whichoverrides the Muddy Mountain thrust; boththrusts apparently flatten out at depth inCambrian shales and both override Jurassicsediments.

The absence of lower or middle Tertiary sedi-ments prevents continuation of the paleo-geology south of the Las Vegas shear zone, butsimilar thrusts appear there, although they aredisplaced approximately 25 miles to the west.The Wheeler Pass and Keystone thrusts wouldcorrespond to the Gass Peak and Glendale-Muddy Mountain thrusts, respectively.

The structural geology of southeasternLincoln County has been discussed only in anabstract (Tschanz, 1960a), but a county maphas been published (Tschanz and Pampeyan,1961). In the Mormon Mountains in the south-east corner of the county, a thrust system,probably a continuation of the Glendale thrust,is present; the main thrust brings Cambrianover Mesozoic rocks which are locally im-bricate. The structure as illustrated on thepaleogeologic map undoubtedly is oversimpli-fied; it represents one plausible interpretationconsistent with the available data. The involutepattern of the thrust is probably due to topog-raphy.

Allochthonous blocks in the Beaver DamMountains have been interpreted as Tertiarygravity-slide blocks by Cook (1960) and Jones

(1963) and thus are not features of the paleo-geology.

To the east of the larger thrusts are two re-lated structures: the Iron Springs Gap struc-ture (Mackin, 1947; 1960a, p. 114-119) whichis a thrusted anticline formed at the end of adecollement in the Carmel Formation, and theVirgin-Kanarra fold (Gregory and Williams,1947; Threet, 1963, 1963a).

The westernmost thrust of the Sevier belt inLincoln County places Lower Cambrian overUpper Paleozoic rocks. It has a greater strati-graphic displacement than the other thrusts inthis sector and may have the greatest totaldisplacement. There is a problem as to exactlyhow it connects with the similar thrust in ClarkCounty. On the paleogeologic map, they areshown as the same thrust affected by later, butpre-middle Miocene, normal faulting. Ac-cording to an alternative interpretation sug-gested by D. H. Adair (1962, oral commun.),the thrusts are en echelon, the Gass Peak dyingout northward, the thrust in Lincoln Countygrowing in the same direction, each compensat-ing for the changing displacement on the other.In the first interpretation, certain UpperPaleozoic outcrops in Lincoln County areautochthonous relative to the major thrust; inthe second, they are allochthonous.

Approximately 15 miles west of the trace ofthe major thrust in Lincoln County is anelongate exposure of Lower Cambrian andEocambrian (C?) quartzite. The quartzite-



PRE-NORMAL FAULTING FALEOGEOLOGY—SEVIER OROGENIC BELT 437

Tertiary volcanic contact is well exposed nearDelamar, a ghost town. Except for this anti-cline, the structure above the thrust is a gentlehomocline from Cambrian to Mississippian andyounger rocks of the hinterland. The upperplate of the major thrust extends for mileswestward virtually undeformed in contrast tothe imbricate belt to the east. The stratigraphicsection of the upper plate must have beenrelatively competent. The Delmar anticlinemight be related to a step in the underlyingthrust surface in the same manner that surficialanticlines are related to steps in such thrusts asthe Cumberland Plateau thrust (Wilson andStearns, 1958).

Wah Wah-Canyon Range Sector. Betweenthe Wah Wah Mountains and the north endof the Canyon Range, there are many exposuresof the Sevier belt, and structural continuity ofthe various isolated pieces of the major thrustof the belt may be demonstrated with reason-able assurance.

The upper plate of the major thrust can betraced with structural continuity from LincolnCounty to the Wah Wah Range, but the actualthrust trace and the imbricate faults to the eastare obscured by volcanics. Miller (1958; 1963)described the geology of the southern end of theWah Wah Range and discussed the evidencefor continuity of the upper plate of the majorthrust which appears locally as the Wah Wahthrust, Frisco thrust (East, 1956; 1957),Mineral Range thrust (Liese, 1957), Pavantthrust (Maxey, 1946), and Canyon Rangethrust (Christiansen, 1952). These thrusts havethe Eocambrian-Lower Cambrian quartziteson their soles and override rocks ranging in agefrom Lower Cambrian to Jurassic, successively,toward the southeast. The southwest quarterof the geologic map of Utah (Hintze, 1963)shows that the Canyon Range allochthon over-rides Paleozoic carbonates of the Pavant Rangeallochthon, thus demonstrating that the majorthrust (the one which brings the Eocambrian-Lower Cambrian clastic sequence to the sur-face) bifurcates in this sector. A possible inter-pretation of this relationship is shown on thepaleogeologic map.

East of the major thrusts lies the imbricatebelt, but it is not well exposed and lacks thestructural continuity of the major thrusts.Southeast of the Wah Wah thrust is the BlueMountain thrust, a structural twin of the Glen-dale or Muddy Mountain thrusts to the south-west, on which Cambrian carbonates overrideJurassic (?) sandstone. The section between the

Blue Mountain and Wah Wah thrusts is com-plexly faulted, but all the faults are relativelyminor. The over-all pattern is highly gen-eralized on the paleogeologic map. In theFrisco area, the imbricate belt is complex and soconfused by later volcanics, intrusives, andfaults, that little generalization is possible atpresent. Southwest of the Pavant thrust,Crosby (1959) worked out an example of anoverturned sheared-offlimb of an anticline. Theentire section from Cambrian to Triassic dipsgently northwestward but is upside down! Themajor thrust overrides the overturned block,which is itself in thrust contact with underlyingstrata. The relationship of the overturned blockto structural units southwest of it is uncertain.

The Sanpete-Sevier anticline east of theCanyon Range has been described by Gil-liland (1963). Jurassic shale and other elasticsprotrude through Cretaceous sediments. Di-apiric phenomena are reported to be partiallyresponsible for the structure.

The Sevier Desert west and northwest of theCanyon Range is a large area where geologicdata are unavailable. It has furnished a name,but little supporting evidence, for the Sevierorogenic belt.

Nebo-Charleston Sector. A fault of uncertaincharacter separates the Canyon Range struc-tural block from the Gilson Mountains andother areas to the north at the only localitywhere the relationships are not covered. Costain(1960) considered this fault, the Leamingtonfault, to be a thrust with a dip of approximately30° N. It is also possible that the movement onthe fault may be normal or, in part, strike-slip.Morris and Shepard (1964) considered it to be astrike-slip fault and offered a slightly differentinterpretation than that shown on the paleo-geologic map for the area between it and theTintic district to the north.

Between Mount Nebo and the vicinity ofSalt Lake City is a distinctly different sector ofthe Sevier orogenic belt. The rocks involved indeformation are those of the Oquirrh basin.The differences in structural style and appear-ance of the map in this sector are believed to bedue to the difference in response to deforma-tion of the thick sediments of the Oquirrhbasin and not to differences in age of deforma-tion or applied stress.

The major thrust in this sector continues outof the Great Basin into the Southern WasatchMountains; at the southern end is the Nebothrust (Eardley, 1934). Overturned Triassicand Upper Paleozoic rocks are thrust over Mid-




die Jurassic shales at Mount Nebo. The Nebothrust is connected beneath a covered area withthe Charleston and Strawberry thrusts at thenorthern end of the Southern Wasatch block(Bissell, 1959; Crittenden, 1959). The Charles-ton thrust was recognized by Baker and others(1949) and has been described by Baker (1959)and Baker and Crittenden (1961). Along thisfault, the thick Oquirrh basin section has beenjuxtaposed against the much thinner shelf sec-tion. At the sole of the thrust, Cambrian andEocambrian quartzites are thrust over rocks asyoung as Middle Jurassic.

West of the thrust, there is a wide fold belt.In this belt are the Tintic and Stansbury foldswith Eocambrian rocks exposed in their coresand several smaller folds such as the Binghamand Pole Canyon synclines. Several of the foldsare overturned; a few have developed intothrusts. The Stansbury anticline (Rigby, 1958)is a reactivated older fold whose trend and lo-cation were predetermined by a buried De-vonian anticline. The geology of the Tinticanticlines has been discussed by Morris (1957).At the northern end of the Oquirrh Range isthe east-west-trending, north-dipping, NorthOquirrh thrust (Roberts and Tooker, 1961).

The large folds are confined to the Oquirrhbasin, but folding continues westward. Muchof the hinterland in Utah is folded, but thefolds are apparent!}' of smaller magnitude aspre-Pennsylvanian rocks are not exposed intheir cores.

The paleogeology becomes complex m theWest Tintic (Groff, 1959) and Sheeprock(Cohenour, 1959) areas. On the Sheeprockthrust, which was first described by Loughlin(Butler and others, 1920, p. 436), Eocambrianquartzites are thrust over lower and middlePaleozoic rocks. The thrust rises in the sectioneastward and has always been considered anormal part of the belt of thrusts in Utah.Cohenour (1959) recognized an additionalthrust dipping north and northeast, which hecalled the Pole Canyon thrust, the upper blockof which was considered to have moved southand southwestward in contrast to eastwardmovement on the Sheeprock thrust. Figure 3shows an alternative interpretation; the PoleCanyon and Sheeprock thrusts appear to be thesame thrust with eastward movement. This in-terpretation would explain eastward overturn-ing of the Eocambrian section below the PoleCanyon thrust. Total displacement on the PoleCanyon thrust is about 5 miles, which wouldeasily account for the 2-3 miles of stratigraphic

displacement on the Sheeprock thrust. TheGovernment Creek fault of Cohenour (1959)may be a separate thrust west of the Sheeprockthrust. South and west of the Sheeprock area isan area left bare on the paleogeologic map (PI. 1,fig. 3) because lack of exposure makes it impos-sible to determine what relationship the com-plex fold-and-thrust area has to the belt ofrelatively simple structure to the south.

Northern Utah Sector. No direct connectionbetween the thrust belt in northern Utah andthe Nebo-Charleston sector has been found.Autochthonous rocks of the Uinta arch extendto the edge of the Salt Lake basin; paleogeologicinformation in the basin and to the northwest isvery limited. It is possible, however, to tracethe Oquirrh Formation from central to north-ern Utah with only a few covered areas abouOmiles across; there is no evidence suggesting amajor structural discontinuity between the twoareas. Crittenden (1959; 1961) pointed out thatthe thrust sheets in the Nebo-Charleston andnorthern Utah sectors are parts of the samestructural block, a conclusion supported by thesimilarity in age, direction, and amount ofthrusting in both areas. Accordingly, the traceof the major thrust outlines a large re-entrantnear Salt Lake City, connecting the Charlestonwith the Willard thrust. The fault surface isprobably close to the base of the Eocambriansection. In all its present-day exposures, thePrecambriaii Farmington Canyon basementcomplex is autochthonous, or nearly so, whenit is contrasted with the Willard thrust sheet.

In the Northern Wasatch Mountains,paleogeologic and structural information areabundant, and the interpretation shown on themap can be considered reasonably certain. Ex-cept for the Farmington Canyon complex andthe immediately overlying Paleozoic stratabetween Salt Lake City and Ogden, all theNorthern Wasatch Mountains are an immenseallochthonous block.

The Willard and associated thrusts have beendescribed by Eardley (1944). East of Ogden, athick geosynclinal section, including 10,000 feetof Eocambrian, overlies (along the Willardthrust) a relatively thin shelf section with noEocambrian. The thrust dips east and at onetime was thought to have moved westward;this view is no longer generally accepted.

Drag features observable at Pineview Dam(including the gigantic Z fold) showing a down-dip (eastward) movement sense, and facies rela-tionships requiring structural continuity be-tween seosvnclinal areas and the Willard block



PRK-NORMAL FAULTING PALEOGEOLOGY—SEVIER OROGENIC BELT 439

POLE C A N Y O N.•-•.. THRUST

MAP MODIFIED FROMCOHENOUR, 1959;S T O K E S . 1963

S H E E P R O C KT H R U S T

Figure 3. Relationship of Pole Canyon thrust to Sheeprock thrust, Sheeprock-West Tintic area,Utah. Pre-Lower Cambrian stratigraphic units are indicated by A, B, C, and D from bottom to top,respectively; undifferentiated Paleozoic rocks, by PAL; Cambrian rocks, by C; Ordovician rocks, by O;Silurian rocks, by S; Devonian rocks, by De; Tertiary plutons, by Ti; Tertiary volcanic rocks, by TV;and Quaternary sediments, by Q.




prove eastward movement. The thrust risesin the stratigraphic section to the east, which isconsistent with eastward but not with west-ward movement. Indeed, no regional relation-ships support a western movement direction.Provable minimum displacement on the fault is30 miles.

Structural continuity of the Willard blockrequires connection of the Willard thrust withthe southern extension of the Paris thrust ofIdaho. F. C. Armstrong and Cressman (1963, p.18) did not accept this interpretation because ofa presumed difference in age of the thrusts, butthey were under the incorrect impression thatthe Willard is a Late Cretaceous or Paleocenethrust. They considered the Paris and Ogden-Taylor thrusts to be contemporaneous, andthey also argued that the thrust near WoodruffCreek (west-dipping 30 miles east of Ogden)might be too insignificant to project 24 milesnorthward. The west-dipping fault at WoodruffCreek must have a minimum displacement of 30miles. It could not die out before the Utah-Idaho boundary only 30 miles to the north. TheBrigham Quartzite occurs on the sole of thethrust continuously from Paris, Idaho, toWoodruff Creek, Utah. In view of the lack ofstructural complexities between the two areas,it is reasonable to accept structural continuitybetween the Paris and Woodruff Creek and,hence, the Willard thrusts. The major thrustbranches near the Idaho-Utah border, but thisaffects neither argument.

East of the major thrust is a series of thruststhat are part of the Idaho-Wyoming thrust belt.From west to east, these faults are the Crawford(Cambrian on Cretaceous), Absaroka (UpperPaleozoic on Cretaceous), and Darby (Juras-sic on Cretaceous) thrusts. These thrusts ap-pear to be successively younger eastward;each probably flattens out at depth into thesame or successively higher stratigraphic hori-zons eastward. Rubey and Hubbert (1959) andF. C. Armstrong and Cressman (1963) dis-cussed the evolution of the thrust belt north ofUtah.

The Ogden and Taylor thrusts were de-scribed by Eardley (1944) as east-dipping withwestward movement, in contrast to the east-ward movement on the overlying Willardthrust (for a while all three thrusts werethought to have moved west, but later theinterpretation of the Willard was changed).Westward movement requires a separateepisode of thrusting and a more complexstructural history for northern Utah. This in-

terpretation still is accepted by some geologists(Eardley, 1962; 1963; F. C. Armstrong andCressman, 1963, p. 18-19). From the map pat-tern, a reinterpretation appears possible (Fig.4). The Ogden and Taylor thrusts rise strati-graphically eastward relative to both upper andlower plates; this is awkward for a west-movingthrust and suggests that these thrusts movedeastward just as the overlying Willard thrustdid. No large difference in age is necessary, anda separate orogeny is not required. The smallerthrusts could be simply peel thrusts in es-sentially autochthonous rocks overridden bythe Willard block. It is noteworthy that base-ment crystallines are involved in these minorthrusts. The present dip of the faults must bethe result of folding and of later eastward tiltingof the Northern Wasatch Mountains; thistilting is already proven by the present dip ofthe Willard fault.

Amount of Shortening in Sevier Orogenic Belt

Several attempts have been made to estimatethe amounts of shortening associated with partsof the Sevier orogenic belt. Rubey (Rubey andHubbert, 1959, p. 190) gave an estimate ofshortening by thrusting of about 75 miles forthe Idaho-Wyoming belt north of Utah thatwas based on structural considerations. Crit-tenden (1961) estimated 40 miles for apparentdisplacement of isopachs by the Willard andCharleston thrusts. Along the Charlestonthrust, a minimum of 20 miles can be concludedfrom eastward displacement of the TinticQuartzite over Jurassic strata. Hintze (1960)gave an estimate of 12 miles for shortening byfolding and thrusting at Mount Nebo. In thedescription of the Nebo-Charleston sector, 5miles of displacement on the Sheeprock-PoleCanyon thrust was estimated. Hintze (1960)reported 8-10 miles of shortening in the NeedlesRange, which may be considered part of theSevier belt. A minimum displacement of 20miles is required for the Muddy Mountain andGlendale thrusts in southern Nevada in orderto explain observable structural relationships(Longwell, 1961, oral commun.), not to men-tion the larger Cass Peak thrust in the samearea.

In both northern and southern Utah, the soleof the major thrust is Eocambrian or lowerCambrian quartzite. From observable overrid-ing relations, a minimum displacement of 25miles in southern Utah and 30 miles in northernUtah is evident on this fault alone, and to thismust be added the displacement on the faults




GEOLOGY OF THEW A S A T C H MOUNTAINSE A S T O F O G D E N , UTAH

A F T E R E A R D L E Y , 1944

W - W I L L A R D T H R U S T0 - O G D E N T H R U S TT — TAYLOR T H R U S T

TO. — T E R T I A R Y AND Q U A T E R N A R YD-M- D E V O N I A N AND M I S S I S S 1PP I AN€ — C A M B R I A N C A R B O N A T E S€ t — C A M B R I A N Q U A R T Z I T Ep€(?)- P R E C A M B R I A N t ? ) C L A S T I C Sp€ — P R E C A M B R I A N M E T A M O R P H I C S

I N T E R P R E T A T I O N OFE f t R D L E Y , 1944

S U G G E S T E D I N T E R P R E T A T I O N

Figure 4. Reinterpretation of Taylor and Ogden thrusts.

farther east. The total displacement in thenorthern area must be greater than that esti-mated because the westernmost autochthonousand easternmost allochthonous sections arestrikingly dissimilar. Additional intervening,now eroded, strata are required.

To summarize, it would seem that totalshortening of 40 miles across the Sevier belt is aminimum value, and 60 miles is adequate toaccommodate comfortably any of the estimates.In the Nebo-Charleston sector much of theshortening is taken up by the folds and thrustswest of the major thrust so that the majorthrust in that area may have a relatively smallerdisplacement (as little as 10-20 miles).

Structural Continuity of Thrust BeltBetween Gass Peak and Canyon Range, the

evidence points to structural continuity of theupper plate of the major thrust, with the pos-sible exception of the area near the Lincoln-Clark County boundary. To the north, thereappears to be a single major thrust from Mount

Nebo to the Paris area in Idaho. The greatestuncertainty concerns the connection betweenthe Nebo and Canyon Range areas.

The Leamington fault appears to mark asignificant discontinuity in the upper plate ofthe major thrust of the Sevier belt; subsurfacedata may be required to understand the struc-tural relations in this area.

The folds and thrusts east of the major thrustdo not appear to be individually continuous. Inplaces, structural complexities in this imbricate,highly deformed zone are unresolvable in thepre-normal faulting paleogeology, but the zoneitself is continuous except where the thick sedi-mentary accumulation of the Oquirrh basin hasbeen pushed eastward over the foreland.

In over-all view, the Sevier belt is a con-tinuous entity from Nevada to Idaho; struc-tural style, age of deformation, magnitude ofshortening, and width of the highly deformedzone show no radical changes over a distanceexceeding 500 miles. Admittedly, there arevariations along strike, but these are explainable




by variations in the sediment accumulations in-volved in the deformation.

Style and Localization of Thrusts

The thrusts of the Sevier belt appear to bedecollement thrusts, like those of the easternCanadian Rocky Mountains (O'Brien, 1960).The major thrust invariably appears to flattenout at depth in the Eocambrian quartzite-shale sequence (PL 1, fig. 4). There is no otherway to explain the persistence along strike ofthe stratigraphic position of the thrust and thelack of apparent involvement of crystallinebasement rocks in the major thrusts. If thrustswith displacements of 20 miles steepened west-ward, great uplifts of Precambrian would beinevitable. This sort of reasoning has led tosimilar conclusions for thrusts in Wyoming(Rubey and Hubbert, 1959, p. 187) and insouthern Nevada (Longwell, 1950). The largerthrusts are restricted to the belt of rapid changeof thickness of the entire geosynclinal prism.Under regional compression, the thick geosyn-clinal section of 30,000-40,000 feet of quartziteand carbonate with minor shale, has behavedwith relative competence. The thinner sectionalong the edge of the geosyncline has failed inone or more thrusts along which the geosyn-clinal strata are piled up on the transition zone.The shelf section was too thin to transmit stressand undergo significant deformation. The rela-tively broad transition zone in northern Utahand adjacent Wyoming resulted in a broaderthrust-and-fold belt. A preexisting fold localizedthe Mesozoic Stansbury anticline. The Oquirrhbasin was evidently less competent than otherareas in the geosyncline, and, as a result, it ex-perienced more intense folding and perhaps lessshortening by thrusting than the rest of thebelt during erogenic deformation.

The shaly zones in the Eocambrian andCambrian clastic sequence provided the majorzones of bedding-plane movement in the re-gion; the major thrust rises from this level.Other important decollement zones are theMiddle Cambrian shales, into which a numberof the frontal thrusts apparently go downdip(Muddy Mountain, Glendale, Blue Mountain,Crawford), the various Mississippian shales,which are followed by important bedding-plane faults in the Southern Wasatch Moun-tains, and the Middle Jurassic shales andevaporites, into which several faults step up andflatten out (Blue Mountain, Pavant).

It is impossible to say definitely what hap-pens to the decollement in the Eocambrian sec-

tion as it continues westward from the Sevierthrust belt, for there is no direct evidence of itsexistence more than approximately 50 mileswest of the trace of the major thrust. In north-ern Utah, the Taylor and Ogden thrusts havewedges of basement in their upper plates, butthey are secondary thrusts about 30 miles westof the main fault trace. Along the Wasatchfront near Santaquin, Utah, crystalline Pre-cambrian rocks are shown on published maps instratigraphic continuity with the upper plateof the Nebo thrust. If this is correct, then, themajor thrust must be in crystalline rocks inthat area. An alternative possibility is that thethrust is concealed as a bedding-plane faultalong the mountain front.

In southern Nevada, the decollement can beobserved 30 miles behind the frontal WheelerPass thrust as the Johnnie thrust described byNolan (1929) and discussed by Burchfiel (1961,p. 129-131).

Eventually, the thrust must grade westwardinto mobile basement underlying the geo-synclinal area. This concept returns to some de-gree to the idea Nolan (1929, p. 469-471) ad-vanced to explain the Johnnie-Wheeler Passthrust. Armstrong and Hansen (1966) pro-posed that a mobile basement underlay areas tothe west of the thrust belt during the Mesozoic.During later stages of the deformation of theSevier belt, particularly during some of the latepostthrust folding, the basement within thefold and thrust belt may have become mobileenough to take part in the folding because oftectonic thickening of the overlying cover andthe consequent rise in temperature of the base-ment. In earlier stages, apparently, the base-ment was relatively rigid, and deformation wasconfined to the sedimentary cover.

Hinterland

West of the Sevier erogenic belt on thepaleogeologic map is a wide area of apparentlysimple structure that actually contains com-plex deformation of a different type than that inthe Sevier belt. Most of the complexity laydeep below the surface in early Tertiary time,but fortunately, normal faulting has deeplyexposed the region for geologic examination.

In the hinterland of southern Nevada, amajor domal uplift is apparent in the Groomarea. A number of exotic klippen occur near thedome; none has been mapped in detail, andlittle can be said about them. They containlower and middle Paleozoic rocks and overliemiddle and upper Paleozoic strata which are




gently folded. D. H. Adair (1962, oral commun.)pointed out the close association of the apparentklippen with pre-normal faulting structurallows. They might be remnants of a large over-thrust, but their chaotic character suggests thatthey are probably isolated blocks transportedby gravity, most logically from the Groomdome. Similar blocks may occur west of thedome on the Nevada test site. Several thrustplates of upper Paleozoic rocks in the SouthernEgan Range are shown by Tschanz (1960).These may be similar to the klippen in southernNevada.

More work is needed to evaluate these klip-pen and their relationships. It should bestressed that later work may show the klippen,or some of them, to be nonexistent. Enough ofthe region is covered to allow a number ofinterpretations. The largest klippe in southernLincoln County, the Pahranagat klippe, may beinterpreted as a thrust-bound wedge withoutgreat transport, but the writer feels the klippehypothesis is more likely on the present evi-dence.

East-central Nevada superficially appears tobe a region of broad gentle folds. The angularunconformity between Paleozoic and Tertiaryrocks in that region is small but increases west-ward. Cook (1965, p. 55) said, "the regionaldisconformity at the base of the volcanic se-quence changes into an angular unconformity. . . in the central Egan Range, the GrantRange, the White Pine Mountains and thePancake Range."

Many ranges in this area show a complexinternal structure characterized by bedding-plane or near bedding-plane "shearing off"faults, i.e., faults where beds are consistentlycut out instead of repeated (Misch, 1960).Misch (1960) discussed many examples of thistype of structure; in a number of cases, he con-sidered such faults to be parts of his regionaldecollement. In contrast to the complexity ob-served in many ranges, there are well-exposedranges in the same area where the entire Paleo-zoic section can be observed intact withoutsignificant faulting. This contrast between sim-plicity and complexity of structure in theranges was noted by Misch (1960) and is one ofthe remarkable features of the hinterland.

The near conformity of Tertiary volcanicstrata with Upper Paleozoic strata in the sameregion containing this complex faulting has ledsome to conclude that most of the deformationis Tertiary (J. C. Young, 1960, p. 169-170).Much of the deformation may well be Tertiary

and in some areas almost certainly is, but in theSnake Range "shearing off" faulting as well asfolding and high-angle faulting predate Cre-taceous plutons (Misch, 1960; Misch and Haz-zard, 1962). There appears, therefore, to havebeen fairly intense deformation, at least locally,before the time represented by the paleogeo-logic reconstruction. The paleogeologic methodwould be unable to detect bedding-plane faultsthat did not juxtapose rocks of contrasting ageor that did not reach the early Tertiary surface.Deformation actually appears to have beengreater at depth than near the surface. Most ofthe "shearing off" faults occur in lower Paleo-zoic rocks, often in the Cambrian-Eocambrianpart of the section (Misch, 1960).

From the available data, it seems necessaryto conclude that in the hinterland of the Sevierbelt, the surficial structure was fairly simple—mostly broad folds, with no complex tightlyfolded areas. Only upper Paleozoic and Meso-zoic rocks were exposed at the surface, for no-where do Oligocene volcanics lie on lowerPaleozoic rocks. Even in Sacramento Pass be-tween the central and southern Snake Ranges(in the blank area on PI. 1, fig. 3), Oligocenevolcanics lie on Pennsy Ivaman rocks disconform-ably. Much of the chaotic structure in thatarea is Tertiary (Misch, 1960). No major over-thrusting involving telescoping of the Paleo-zoic section occurred in this region. Totalshortening of the supercrustal rocks must havebeen relatively minor compared to that in theSevier belt. In early Tertiary time, the com-plex structures of this part of the eastern GreatBasin were deeply buried.

East of the Snake Range is the ConfusionRange fold belt which is a narrow, deeply down-folded zone containing strata as young as Meso-zoic. It is possible that this belt is the surfaceexposure of a zone sucked down between ad-jacent rising domal structures in a mobile base-ment. Alternatively, it could be a surficialstructure formed by gravity-propelled glidingof upper Paleozoic rocks eastward off the risingSnake Range dome, possibly even in Tertiarytime. In the Gold Hill area, the carefullyworked out structural history of Nolan (1935)is in agreement with a simple paleogeology. InEocene time, folded upper Paleozoic rocks werepresent at the surface. One fold was fairly tightand overturned to the east. The larger thrustsand most normal faults in the area are post-Eocene and, therefore, later than the timerepresented by the paleogeologic reconstruc-tion.




A model for the deep-seated structure of thehinterland has been proposed by Armstrongand Hansen (1966). The present-day fault-block ranges expose different tectonic levelswhich may be interpreted as an infrastructure,a mobile deeply buried zone affected by regionalmetamorphism, and a suprastructure, an unmeta-morphosed zone of broad open folds developedunder a relatively thin cover. These two tec-tonic levels may grade into one another, but inmost areas, they are separated by a zone oftectonic adjustment with disharmonic defor-mation and steep metamorphic gradients, anAbscherungszone.

The hinterland of the Sevier erogenic belt onthe paleogeologic map exposes only the supra-structure—unmetamorphosed upper Paleozoicrocks, broadly folded but with little over-allshortening. Deeply buried in early Tertiarytime but now exposed in areas such as the SnakeRange, Ruby Mountains, and Raft RiverRange is the infrastructure—metamorphicrocks, in places, migmatites, with complexminor structures and large-scale recumbentfolds. The transition between these two tectoniclevels occurs in most areas in the upper part ofthe Eocambrian clastic section, but elsewhere,it rises into lower, and, in the Ruby Mountainsarea, even to middle Paleozoic rocks. Thiszone, in which adjustments were made fordifferential deformation of infrastructure andsuprastructure, is the site of zones of intenseshearing. In many areas, faults have been recog-nized in or closely related to this zone; they arethe younger-on-older thrusts that characterizethe hinterland. A note of caution is needed,however, because other structures also havebeen categorized as younger-on-older thrustsso that some confusion on this point is inevi-table. Misch (1960) reviewed the occurrence ofthe structural complexities of the Abscherungs-zone, although in a somewhat different contextthan the tectonic framework proposed by Arm-strong and Hansen (1966).

Both Misch (1960) and Armstrong and Han-sen (1966) concluded that the orogeny whichaffected the hinterland was pre-Lower Creta-ceous, making the structures distinctly olderthan the Sevier orogenic belt. Lower Creta-ceous plutons which cut across both orogenicstructures and metamorphosed Eocambrian-Lower Cambrian rocks are unequivocal evi-dence of this. The regional decollement ofMisch (1960) cannot be related directly to theSevier orogenic belt as has been proposed byMiller (1963), for it is too high stratigraphically

and distinctly older. Moreover, it does not ap-pear possible to the writer or to Misch to con-sider that the Sevier belt is the result of gravitygliding eastward from central Nevada, the onlypossible area of "tectonic denudation." Rather,I feel compelled to seek the locus of shorteningdisplayed by the orogenic belt in crustal short-ening and deformation at depth within andwest of the Sevier belt. The Paleozoic blanketof the hinterland was already deformed and"nailed down" by cross-cutting Lower Creta-ceous plutons; only a very contrived explana-tion could specify a locus of gliding to bring theSevier belt out of Nevada by gravity.

An entirely analogous situation is providedby the Rocky Mountains in Canada. There,regional metamorphism, complex deformation,and emplacement of plutons of batholithic di-mensions in the hinterland occurred beforethrusting, with shortening exceeding 100 milesin the Rocky Mountains (White, 1959; Shaw,1963; Gabrielse and Reesor, 1964). It appearsnecessary to invoke some sort of Verschluckungto explain the development of these later Meso-zoic fold and thrust belts.

STRATIGRAPHIC EVIDENCECONCERNING AGE OFDEFORMATION IN SEVIEROROGENIC BELT

Evidence for a Pre-Cretaceous Sevier Arch

It was in later Permian time, perhaps, thatthe orogeny began in the Sevier belt. Permianisopachs show a southwest-trending arch insouthwestern Utah and adjacent Nevada thatcoincides with the belt of Mesozoic thrusting.Brill's maps (1963, p. 319) show that this archaffected the distribution of Leonardian stratabut not the distribution of Wolfcampian strata.Data for later times are lacking, but possiblythe arch is a precursor of later deformation inthe area; it would be an ancestral Sevier arch.

Throughout the region, the basal contact ofthe Triassic is a disconformity above Permianstrata. The time gap represented by the contactis somewhat greater in southern areas than innorthern ones. In southern Nevada, erosionalrelief of more than 100 feet and coarse con-glomerates are commonly found at the contact(Longwell, 1925; Secor, 1962). In the western-most exposures, in the Spring Mountains, atleast 1400 feet of Permian strata are missing be-low the disconformity, perhaps as a result of up-lift and erosion on the ancestral Sevier arch.

Jurassic deposits, now occurring only in the



STRATIGRAPHIC EVIDENCE CONCERNING AGE OF DEFORMATION 445

easternmost part of the Sevier erogenic belt andon the Colorado Plateau shelf, are significant assources of indirect information concerningevents in the Great Basin. The shift of sedimentsources during the Jurassic has been discussedby Wright and Dickey (1958, 1963, 1963a) andStokes (I960, 1963a, 1963b). During MiddleJurassic time, the hinterland of the Sevier beltbecame a source of clastic sediment. In thesouthern Wasatch Mountains, Upper Jurassicand lowermost Cretaceous conglomerates occurconformably on both upper and lower plates ofthe major thrust of the Sevier belt (Bissell,1959, p. 163; Stokes and Madsen, 1961), sug-gesting that uplift and deformation there beganafter the end of the Jurassic. In northern Utah,later Jurassic formations are truncated west-ward by erosion which preceded deposition ofLower Cretaceous Kelvin Conglomerate. Insouthern Idaho, movement on thrusts of theSevier erogenic belt may have begun as earlyas the end of the Jurassic (F. C. Armstrong andCressman, 1963, p. 8-16).

In summary, evidence for a pre-CretaceousSevier arch is suggestive but not conclusive.There is no evidence that extensive uplift orthrusting began in the belt before the end ofthe Jurassic; the story for the Cretaceous is anentirely different matter.

Cretaceous to Paleocene—Roc^yMountain Geosyncline

East of the Great Basin, an enormous ac-cumulation of Cretaceous sediments providesa detailed record of the advances and retreatsof the Cretaceous seas in response to eustaticchanges, deformation, and repeated great floodsof clastic materials from a westward source.Facies and thickness changes indicate a sourcewest of the present outcrop areas.

The Sevier erogenic belt was the source ofmaterial that accumulated in the CretaceousRocky Mountain geosyncline (Fig. 5). This isproven by contemporaneity of deformation inthe erogenic belt with sedimentation in theimmediately adjacent area to the east, by thecoarsening of elastics toward the orogenic belt,by the coarseness of elastics in the westernmostdeposits (some clasts being too large for trans-port of more than a few miles), and by con-clusive provenance studies. The Sevier orogenicbelt is the only possible source for the largequantity of carbonate and quartzite clasts de-rived from lower Paleozoic and Eocambrianrocks of the Cordilleran geosyncline. The pre-normal faulting paleogeology shows that rocks

of the same age and facies were not exposedelsewhere in the Great Basin during Cretaceoustime.

In central Utah, a relatively complete se-quence of Cretaceous strata have been studiedby Spieker (1946, 1949, 1956) and his students(Schoff, 1951; Hardy and Zeller, 1953). TheCretaceous of Utah recently was reviewed byBurger (1963). Early Cretaceous rocks areabsent in most westernmost exposures of Cre-taceous formations because of Early Cretaceousuplift and erosion; however, Early Cretaceousrocks are present only a few miles to the east.Numerous angular unconformities occur withinthe Cretaceous section along its western edge,indicating concurrent deformation and deposi-tion. In central Utah, middle and upper Mon-tana Price River Formation locally lies uncon-formably across major thrusts of the Sevierorogenic belt (Hintze, 1962). In southwesternWyoming, a similar situation prevails in thatupper Montana and Paleocene Evanston For-mation lies unconformably across major thrustfaults (Tracey and Oriel, 1959). In both areas,some deformation in the Sevier belt postdatesthe end of the Cretaceous.

The Cretaceous Rocky Mountain geosynclinedeposits thicken gradually from east to west inUtah, reach a maximum near the westernmostexposures, and thin drastically toward the adja-cent deformed belt. The greatest thickness ofLower Cretaceous deposits occurs in southernIdaho. The maximum thickness in any giveneast-west cross section decreases gradually from15,000 feet in Idaho to a few hundred feet insouthern Nevada. The Upper Cretaceous sec-tion is thick in central Utah, thins fairly rapidlysouthward toward southern Nevada, and thinsslightly, and then thickens again northwardinto Idaho and Wyoming.

Review of information -provided by clast prove-nance. The Sevier orogenic belt was the onlypossible source for the coarse lower Paleozoicclasts found in the Cretaceous deposits of theRocky Mountain geosyncline. The sequentialexposure of older and older units in the evolvingorogenic belt resulted in an inverted stratigra-phy of the lower Paleozoic clasts. The struc-tures in the belt are large thrusts with a fewfolds; this type of deformation must have pro-vided sources for the elastics.

The information available on clast composi-tion is summarized in Figure 6. By the begin-ning of the Cretaceous, upper Paleozoic clasticsources were present within the orogenic beltso that at least locally the Mesozoic cover had



446 R. I,. ARMSTRONG—SEVIER OROGENIC BF.I/F IN NEVADA AND UTAH




been removed. Ordovician quartzites first ap-pear in lower Colorado sediments in southernNevada and in the Lower Cretaceous sedimentsin Idaho. The distinctive Eocambnan quartz-ites appear first in the upper part of the Indian-ola Group (Colorado) of central Utah, and theybecome widespread and abundant in Montanaconglomerates from southern Nevada throughWyoming, at least as far north as the Tetonarea (Love, 1956). To a certain degree, the ap-pearance of abundant Eocambrian clasts isalmost coincident throughout the length of theSevier erogenic belt. This would suggest thatthe evolutionary stages of the orogenic beltwere similar over the entire region and thatstructures within the belt are of similar age andmay be extrapolated and correlated alongstrike.

Problem of Canyon Range fanglomerate. Inthe Canyon Range of central Utah (Christian-sen, 1952), more than 10,000 feet of red-matrixboulder conglomerates, conglomerates, andsandstones, with local lacustrine limestones,occur. These clastic rocks, situated 20 mileswest of the westernmost standard Cretaceoussections of the central Utah plateaus, will be re-lerred to as the Canyon Range fanglomerate.The fanglomerate lies with great angular uncon-formity across a major thrust, which places Eo-cambrian quartzites of the Cordilleran gco-synchnal section over Cambrian and youngercarbonates. The fanglomerate strata have beentilted to near vertical attitudes, and locallyminor movement along the major thrust hasplaced the Eocambrian quartzites in thrustcontact with them.

Many writers have considered this area sig-nificant for the dating of orogeny in centralUtah, but the quoted relationships here arediscordant with those elsewhere in the fold-and-thrust belt. This is due to Christiansen's (1952)assignment of the Canyon Range fanglomerateto the Indianola (?) conglomerate. Ele made theIndianola correlation strictly on the basis otlithologic similarity. No alternatives were dis-cussed, but at the time regional informationwas much more limited, and nothing about theage assignment seemed unreasonable. The timehas come for reassessment of the age of theCanyon Range fanglomerate.

Variegated siltstone, conglomerate, sand-stone, and light-colored limestone that overliethe fanglomerate with a slight angular (~10°)unconformity were considered North Horn(?)Formation, thus establishing a Cretaceous agefor the fanglomerate, which was assigned to theIndianola(?), although fossil evidence or tracing

of lateral continuity were lacking. The CanyonRange fanglomerate was also correlated withred conglomerates in the Snake Range of east-ern Nevada, which are now known to be Terti-ary (Armstrong, 1964, p. 73). This is typical ofhow misleading lithologic correlations amongelastics in this region may be. Spieker (1949,1956) emphasized that similar fades are re-peated in formations of different age.

The writer suggests that the Canyon Rangefanglomerate may be a lateral equivalent of thePaleocene and early Eocene Flagstaff Lime-stone of the Utah Plateaus. This correlation canbe supported by evidence as strong as that sup-porting an Indianola (?) age, although eithercorrelation may be proven to be correct whenfossil evidence is found. The important point isthat the age of the fanglomerate is not wellknown.

Nowhere do rocks known to be older thanupper Montana he unconformably across majorthrusts; hence, in terms of structural position,the fanglomerate should be Price River oryounger. The basal conglomerates of the Can-yon Range fanglomerate are composed mostlyof Eocambrian quartzite, some boulders ex-ceeding eight feet in diameter. An obvious localsource is present in the upper plate of the Can-yon Range thrust. In the Indianola strata of theGunnison Plateau (due east of the CanyonRange), the lower beds contain none of thedistinctive Eocambrian quartzite, but higher inthe section it appears in modest quantities (5-10 percent locally). Only the Price River andlater conglomerates contain the Eocambrianquartzites in abundance. Clast composition,therefore, would allow, at best, correlation ofthe Canyon Range fanglomerate with upperIndianola, and a Price River or later age wouldbe more likely.

Where proven, Indianola beds are as closelyassociated with major thrusts as the CanyonRange fanglomerate, the deformation is muchgreater. Northeast of Nephi, Indianola con-glomerates underlie an angular (almost 90 )unconformity below Price River conglomer-ates. The slight angular unconformity betweenChristiansen's Indianola (?) and North Horn(?)formations in an area much farther within theSevier orogenic belt is further evidence againstthe age he assigned to the older conglomerates.

On Long Ridge, northeast of the CanyonRange, Muessig (1951) recognized a fadesequivalent of the Flagstaff lacustrine depositsof the High Plateaus to the east that was com-posed of red matrix conglomerates and sands.This correlation is established by fossil evidence




and intertonguing with other Tertiary forma-tions. Flagstaff age conglomerates exposed nearMills, Utah, only 10 miles from the CanyonRange, are virtually identical to conglomeratesof the Canyon Range fanglomerate. Rock typesand clast compositions match so closely that onlithologic criteria correlation of Flagstaff For-mation with Canyon Range fanglomerate wouldbe stronger than the Indianola-Canyon Rangecorrelation. This is by no means proof, but itemphasizes the fact that facies can be matchedin rocks of widely different age. The Indianolaof the Gunnison Plateau certainly is similar tothe Canyon Range fanglomerate, but it is notas perfect a match as the Flagstaff of LongRidge. As to later deformation and structuralposition, the Flagstaff on Long Ridge is re-ported by Muessig (1951) to be locally tilted toan almost vertical attitude, just like the CanyonRange fanglomerate.

Thus, much doubt can be cast on the correla-tion of the Canyon Range fanglomerate withthe Indianola Group of central Utah, and it isat least equally possible to argue for a Flagstaffage although any age between Indianola andFlagstaff eventually may be established. Thepost-thrusting elastics of the Canyon Rangeshould not be considered evidence of a distinctepisode of pre-Upper Cretaceous thrusting inUtah, unless fossil proof of a Colorado age isfound.

SummaryThe evidence provided by the clastic rocks,

unconformities, and structural relationshipscan be used to arrive at a summary of the tim-ing of deformation in the eastern Great Basin.A faint suggestion of uplift along the Sevier beltis found in lessened Permian deposition and pre-Triassic erosion along a NE-SW-trending beltin southwestern Utah and southern Nevada. ByLate Jurassic time, deformation was well underway within the Great Basin, and the area wassufficiently elevated to be a clastic source. Bythe end of the Jurassic, fairly coarse conglom-erates were being shed.

In the southern Wasatch Mountains, faultingbegan just after the end of the Jurassic. In thissector of the Sevier erogenic belt, as elsewhere,deformation probably began earlier fartherwest (Sheeprock thrust, folds in the Oquirrhbasin). Movement on the major thrust mayhave begun as early as the end of the Jurassic innorthern parts of the Sevier orogenic belt(F. C. Armstrong and Cressman, 1963, p. 8-16). During most of Early Cretaceous time,thrusting must have been going on in the

Sevier orogenic belt. By early Colorado time,lower Paleozoic clasts were being shed from theentire belt. Later in Colorado time, Eocam-brian rocks became significant sediment sourcesin central Utah, and by Montana time clasts ofthese rocks were supplied in abundance fromthe entire belt. The Eocambrian rocks withinthe fold and thrust belt occur only in the solesof major thrusts. For the area to have been asource of these rocks, thrusting must have beenof large magnitude by the beginning of Mon-tana time. F. C. Armstrong and Cressman(1963, p. 8-16) consider that much of the move-ment on the Paris thrust in Idaho occurredduring Lower Cretaceous time. Movement onthe major thrusts of the Sevier belt was prob-ably approximately coincident with movementon the Paris thrust.

Major thrusting in the Sevier orogenic beltceased before the end of Montana time, for theupper beds of Price River of equivalent stratalie unconformably across the large thrusts.Later deformation consisted mostly of folding,at times with local thrusting. This period ofwaning deformation lasted into the Paleocene.During the Eocene, the orogenic belt was quiet,and erosion actively reduced the relief in-herited from the deformation.

Within the broad continuous pattern of de-formation of the eastern Great Basin, some finestructure can be recognized. In southernNevada, earlier deformation in the Sevierorogenic belt began in the west (Longwell,1952a; Secor, 1962). In the imbricate belt ofIdaho and Wyoming, a series of thrusts becomeyounger eastward (Rubey and Hubbert, 1959;F. C. Armstrong and Cressman, 1963, p. 8-16).In Utah, it is possible, but not provable, thatdeformation likewise began in the west alongthe major thrusts. The clastic source evidencedoes indicate that the major thrusts were activeduring much of the deformation of the belt.

The elastics of the geosyncline in the RockyMountains can be subdivided into three greatfloods or complex tongues. The first was UpperJurassic and Lower Cretaceous (Morrison,Dakota, Kelvin, Gannett). Diminished influxof elastics allowed the uppermost Lower Cre-taceous shales (Aspen and equivalents) tospread over a wide area. The second clasticflood corresponds to the Indianola Group andFrontier Formation of Colorado age. This wasfollowed by another time of widespread shaledeposition in late Colorado time (Milliard; partof Mancos). The flood of elastics in Montanatime came with the final major deformation ofthe orogenic belt (Price River, South Flat,



450 R. I.. ARMSTRONG—SEVIER OROGENIC HEET IN NEVADA AND UTAH

Echo Canyon, Mcsaverde, Adaville). The re-treat of the sea in latest Montana time afterdeposition of the "Lewis" Shale tongue abovethe Mesaverde Group in eastern Utah markedthe end of the Rocky Mountain geosyncline.

The total volume of Cretaceous sedimenteroded from the orogenic belt corresponds to astrip of material 25,000 feet thick and approxi-mately 100 miles wide (estimated from isopachsof Wcimer and Haun, 1960). More than half ofthis volume can be accounted for by thrustingof the geosynclinal section over the shelf withshortening corresponding to approximately 50miles. The remainder would have to be derivedfrom erosion of folds farther west and in theorogenic belt. Crudely, at least, the amount ofsediment in the Cretaceous geosyncline equalsthat estimated on structural grounds to havebeen removed from the orogenic belt. Of thethree major clastic floods, the second two areof greatest significance volumetrically. Ifamount of deformation and amount of debriscould be correlated, then presumably most ofthe evolution of the Sevicr belt took place inLate Cretaceous time. Such a correlation hasnot yet been established.

TERTIARY STRUCTURESThis discussion would be incomplete without

mention of some of the Tertiary complicationswhich have obscured the Sevier orogenic beltand ils possible relation to them. The mostevident structural development in the Tertiarywas the breakup of the region by normal faultswhich are discussed in detail by Nolan (1943),Mackin (1960a), and Moore (1960). The majorlate Tertiary faults have a generally north-south trend and displacements commonlygreater than 10,000 feet ; some displacementsexceed 30,000 feet. The origin of the normalfaulting is a matter of debate. Thompson (1960)supported the concept of crustal stretching asthe basic cause. Mackin (1960) has presentedan eruptive-tectonic hypothesis that explainsthe faults as collapse features due to withdrawalof lateral support by eruption of 60,000 cubicmiles of Tertiary volcanics. The time lag be-tween volcamsm and normal faulting presentsthis hypothesis with difficulties. It is too attrac-tive an explanation to be rejected, however,and is probably an important part of thephenomenon, especially in areas like the NevadaTest Site where volcamsm and faulting weresynchronous. The regional pattern of the fau l t -ing and the timing anomaly support the crustal-stretching hypothesis. The regional faulting

would have a deep tectonic control, and thevolcanic eruptions would simply modify thepattern.

One strong suggestion in favor of over-allthinning of the crust in the Great Basin regionis its present topographic altitude. It is, on theaverage, lower than surrounding areas. TheSierra Nevada on the west and the High Pla-teaus of Utah and the Wasatch Mountains onthe east project far above the Great Basin re-gion. Without crustal thinning, the regionwould probably now stand at altitudes of morethan 10,000 feet as an Alpine chain undergoingdeep dissection. Stretching has resulted in nor-mal faulting, crustal thinning, and resultinglowering of average altitude. Consequently,structures that formed at shallow depths in theorogen have been preserved, and there is anunusual opportunity to observe here all depthsof an orogenic belt simultaneously. The stretch-ing of the crust in the Great Basin may be onlypart of a much larger tectonic system, the EastPacific Rise, that was described by Menard(1960). The strain appears to have taken theform of distributed faulting in contrast to riftvalleys common elsewhere on oceanic rises oron the rise in Africa. Perhaps in the orogenicbelt, even some tens of millions of years af terdeformation had ended, the crust would sti l lbe warmer and thereby weaker than elsewhere.That the lower crust and upper mantle underthe Great Basin is relatively weak is shown byCrittcndcn's analysis (1963) of the isostaticrebound in the Bonneville basin which gaveviscosities an order of magnitude smaller underthe Great Basin than those found in Fen-noscandia. Under "tension" (when the verticalprincipal stress exceeds the horizontal principalstresses in deeper parts of the crust), the softer/.ones of the crust would stretch like taffy, butshallow rocks would fail by faulting of the Basinand Range type. The Colorado Plateau was un-affected because it lacked a softened basement;thus, the weak basement hypothesis providesan explanation for the coincidence of the east-ern margin of the Basin and Range provinceand the eastern edge of the Sevier orogenicbelt.

Normal faulting is not the whole story, how-ever. Several ranges, particularly those most ele-vated during the Tertiary, appear to have beenarched and may not have faulted margins(Misch, 1960, p. 19-20; Snelson, 1957; Wood-ward, 1964; Felix, 1956). These ranges occurin the hinterland of the Sevier belt and containrocks metamorphosed during the Mesozoic;



NEVADAN, SEVIER, AND LARAMIDE OROGENIES 451

these rocks would have been warm and moreplastic than the shallow rocks of the suprastruc-ture. As a consequence, these ranges could haveflexed during uplift, perhaps developing be-neath horsts formed at shallower tectonic levels.Brittle deformation would characterize thesuprastructural rocks; all suprastructure rangesappear to be simple fault blocks.

At a number of localities domal structures re-lated to Tertiary plutons have formed (Mackin,1960a; Blank, 1959; Cook, 1957; Wisser, 1960).Structural relief resulting from normal faulting,arching, and doming has resulted in gravityslides, some of large dimensions (Mackin, 1960a;Misch, 1960; Moores, 1963; Secor, 1962; Bis-sell, 1964; Cook, 1960 Jones, 1963; Armstrong,1964). The structural style characteristic ofthese slides is pervasive brecciation. In additionto newly initiated faults, many older faults mayhave been reactivated during the Tertiaryextension-gravity sliding regimen. The presentcomplex geology represents the summation ofJurassic, Cretaceous, and Tertiary effects. Withcareful work, as many as five episodes of struc-tural development have been worked out insome quadrangles (for example, Nolan, 1935).

NEVADAN, SEVIER, ANDLARAMIDE OROGENIES

According to Wilmarth (1938), the Nevadian(Nevadan) revolution was a term applied toearly Early Cretaceous and Late Jurassic di-astrophic movements; the Laramide revolutionwas a mountain-building period in the RockyMountain region that began in Late Cretaceoustime and ended in early Tertiary time. Theseterms, grounded in early conceptions of thenature of the erogenic record, are widely usedtoday in the writing of Cordilleran geologists,and they have been so well established that newterms or concepts have not received acceptance.Although most recent reviews of regional geol-ogy in the west (King, 1959; Clark and Stearn,1960) acknowledge an over-all continuity oforogeny in the Cordillera, the pictures pre-sented indicate that confusion has not beeneradicated. It is the universal practice to lumpstructures of the Sevier fold-and-thrust beltand the basement uplifts of the eastern RockyMountains under the term Laramide. Pre-Laramide orogenic activity in the Sevier belt isacknowledged, but the emphasis supplied bythe two discrete names Nevadan and Laramidenever allows a balanced picture to emerge. At-tempts to extend, broaden, or blur the twoconcepts, or to refine subdivisions within and

between Nevadan and Laramide have onlyperpetuated confusion.

Basically a revision of the nomenclature isrequired. At best, the most that can probablybe achieved is to add another term to the col-lections, something to allow at least a mentaldivorce of fundamentally different conceptsnow blurred into one.

In the Rocky Mountains, characterized bybasement uplifts, movement began close to theend of the Cretaceous. Keefer and Love (1963)reviewed the evidence in Wyoming. Upliftbegan in Maestrichtian time, was intense duringPaleocene and early Eocene time, and hadceased by middle Eocene time. The orogenicmovements here are Laramide in the classicsense, as the Laramie Range in southeasternWyoming was affected.

In the Sevier orogenic belt, orogenic de-formation began approximately at the begin-ning of the Cretaceous, and major thrustingended in Campanian time. Campanian andMaestrichtian Price River Formation uncon-formably overlies the thrusts in Utah, andMaestrichtian (?) Evanston Formation overliesthose in Wyoming. Relatively minor move-ments continued as late as Paleocene in bothareas. Thus the common implication that theCordilleran thrusts are Laramide structures iserroneous. The orogenic structures of the Sevierbelt are distinctly different in age from classicLaramide, and moreover, they represent adrastically different sort of deformation andtectonic regimen. General considerations on thenature of orogeny in the Cordillera are im-possible unless this fundamental distinction be-tween the Sevier belt and eastern RockyMountains is clearly recognized. If Nevadanand Laramide are terms that will continue tobe used by geologists, a new term, of equal rank,is necessary. The term Sevier orogeny is sug-gested for the deformation which produced thestructures of the Sevier orogenic belt, largelyduring Cretaceous time. This orogeny lies inthe middle of a period of geologic time (Fig.7). If it is used in the sense of the definition, itshould help clarify discussion of the regionalhistory. A proper summary then is that thecentral Cordillera of the western United Stateswas affected during the Mesozoic by at leastthe Nevadan, Sevier, and Laramide orogenies.These orogenies, along with the PaleozoicAntler and Sonoma orogenies, are the principalCordilleran orogenies which have been signifi-cant in the development of the central Cordil-lera of the western United States. Even thisstatement is an embarrassing oversimplification.



MILLIONS OF YEARS BEFORE PRESENT

TRIASSICUPPER

JURASSICLOW MIDDLE

AJOCIAN

ATHONIAN

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CRETACEOUSLOWER CRETACEOUS

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MESOZOICI NTRUSIONS IN ~i

A N T L E R BELT

- LIMITS ON TIME OF DEFORMATION ,AND METAMORPHISM IN EASTERNGREAT BASIN OF NEVADA ANDNORTHWEST UTAH

MESOZOIC•INTRUSIONS IN

E A S T E R N GREAT BASIN

PERIOD OF MAJOR THRUSTING' SEVIER OROGENIC BELT

LATEST MESOZOIC ANDTERTIARY INTRUSIONS .

OF EASTERN GREATBASIN

PERIOD OF ABUNDANT CLASTIC SUPPLY TO ROCKY MOUNTAIN .GEOSYNCLINE AND LATER INTERMONTANE BASINS

PERIOD OFIMMENSE. VOLCANICACTIVITY INEASTERN GREATBASIN

hIMMENSE iVOLCANIC _»JACTIVITY IN I

Figure 7. Geologic time scale. Tertiary time scale after Evernden and others (1964); 120 m.y. to Tertiary from work of Folinsbee and others (1960);and 230-120 m.y. modified from data given by Kulp (1961), Baadsgard and others (1961); and Armstrong (1964).



453

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454 K. ].. ARMSTRONG -SI-ATF.R OROGKNIC BKLT IN NKVADA AND UTAH

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Hook Chlls, Carbon, Kmcry . and Grand Counties Utah, and Garficld and Mesa Counties, Colorado:U. S. Geol. Survey Prof. Paper 332, 80 p.

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Gabrielse, Hubert, and Reesor, J. E., 1964. Gcochronology oi plutonic rocks in two areas ol the CanadianCordillera: Royal Soc. Canada, Spec. Pub. 8, p. 96-138.

Gilliland, W. N., 1963, Sanpetc-Scvier anticline of cent ra l Utah: Geol. Soc. America Bull. , v. 74, p.115 124.

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MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 9, 1965REVISED MANUSCRIPT RECEIVED JANUARY 4, 1966



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