Armstrong (1968)

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Geological Society of America Bulletin

Sevier Orogenic Belt in Nevada and Utah

RICHARD LEE ARMSTRONG

Geological Society of America Bulletin 1968;79;429-458
doi: 10.1130/0016-7606(1968)79[429:SOBINA]2.0.CO;2
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Notes
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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 before
the appearance of Cambrian fossils and continued without erogenic interruption through the
Triassic. During the Jurassic, deformation and regional metamorphism occurred in the western
part of the miogeosyncline, and the area of sediment accumulation shifted onto the Colorado
Plateau.
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 to
Paleocene Rocky Mountain geosyncline. Clasts in the Cretaceous conglomerates show an inverted
stratigraphy, reflecting successive exposure of older and older rocks in an evolving orogenic belt
along the eastern side of the Cordilleran miogeosyncline. This source area was the Sevier orogenic
belt, 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 the
belt, but several large folds are also known. The largest thrusts are overlain unconformably by
uppermost Cretaceous conglomerates.
Thrusting in the Sevier orogenic belt had virtually ceased by the time the Laramide orogeny
began east of the Sevier belt in latest Cretaceous time. Laramide mountains were the result of
uplift 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 of
these 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 orogenic
belt so that it is not immediately recognizable on geologic maps. Arch ranges, intrusive domes, and
gravity slides are additional complications of the Tertiary geology, but widespread Tertiary de-
posits, particularly Oligocene ignimbrites, make a paleogeologic reconstruction possible; thus, the
Sevier orogenic belt can be viewed as it existed before the normal faulting.
CONTENTS
Introduction 430 Tertiary structures 450
Acknowledgments 430 Nevadan, Sevier, and Laramide orogenies . . . 451
Geologic setting and stratigraphic history . . . 430 References Cited 453
Pre-normal faulting paleogeology—Sevier orogenic
belt 432 FiSure
Paleogeologic map 432 1. Eastern Great Basin Tertiary correlation chart 433
Foreland 434 2. Index map for Sevier orogenic belt, Nevada and
Sevier 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 439
Nebo-Charleston sector 437 4. Reinterpretation of Taylor and Ogden thrusts 441
Northern Utah sector 438 5. Diagrammatic section across Rocky Mountain
Amount of shortening in Sevier orogenic belt . 440 geosyncline in central Utah 446
Structural continuity of thrust belt 441 6. Jurassic to Paleocene correlation chart showing
Style and localization of thrusts 442 inverted stratigraphy of clasts in Rocky
Hinterland 442 Mountain geosyncline 447
Stratigraphic evidence concerning age of deforma- 7. Geologic time scale 452
tion in Sevier orogenic belt 444
Evidence for pre-Cretaceous Sevier arch . . . 444 Plate Facing
Cretaceous to Paleocene—Rocky Mountain geo- 1. Paleogeology of the Sevier Orogenic Belt . . 429
syncline 445
Review of information provided by clast Table
provenance 445 1. Geologic Maps of the Eastern Great Basin and
Problem 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
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430 R. L. ARMSTRONG—SEVIER OROGENIC BELT IN NEVADA AND UTAH
INTRODUCTION formed orogen now exposed at varied structural
levels.
The eastern Great Basin in eastern Nevada The existence of thrust faults and folds of
and western Utah is characterized by north- Mesozoic age along the eastern edge of the
south-trending fault-block ranges composed of Cordilleran geosyncline is common knowledge
carbonate assemblage rocks of the Cordilleran (Eardley, 1962, 1963); the same area has been
geosyncline. The area under consideration is clearly recognized as a source of clastic material
bounded on the west by the mid-Paleozoic during the Cretaceous by Spieker (1946; 1949;
Antler orogenic belt (Roberts and others, 1958), 1956) and his students and by Harris (1959)
on the south by the Las Vegas shear zone who proposed the name, Sevier arch, for the
(Longwell, I960), and on the east by the clastic source. This paper is a review and analysis
Colorado Plateau. Although the Idaho bound- of the geology of this fold and thrust belt.
ary has been taken as an arbitrary northern
ACKNOWLEDGMENTS
limit, it should be emphasized that structures
described in the Great Basin persist without I am indebted to the large number of
significant modification northward into, and geologists who contributed directly and in-
even past, central Idaho. directly to this project through their studies in
In order to understand the results of K-Ar the Great Basin. Without such previous work,
dating studies of the region, a review of avail- this synthesis would be impossible. Kenneth F.
able knowledge of Great Basin geology was es- Bick introduced me to the geology of the Great
sential; the results of the K-Ar studies have Basin in 1956. Pierre Biscaye and, later, Julia
been published elsewhere (Armstrong, 1963; Armstrong assisted in the field studies during
1966; Armstrong and Hansen, 1966). The only 1961. C. R. Longwell, Paul Williams, Robert
complete synthesis of Basin and Range geology Scott, T. B. Nolan, Keith Ketner, Harold
(Nolan, 1943) has become a classic. Since the Masursky, L. I. P. Muffler, J. C. Taylor, and
appearance of that report, a great amount of Hoover Mackin provided hospitality and guid-
work has been done in the area, particularly as ance in their respective field areas. During
thesis projects. All Utah and approximately 80 preparation of the original manuscript, John
percent of eastern Nevada have been mapped Rodgers, Edward Hansen, Keith Howard,
in enough detail to show the most significant Clark Burchfiel, D. H. Adair, Kenneth Pierce,
structural features. Osmond (1960) discussed and Pierre Biscaye provided helpful discussion.
briefly the tectonic history of the Basin and K. K. Turekian, John Rodgers, P. M. Orville,
Range province in Utah and Nevada; Misch C. R. Longwell, R. J. Roberts, Peter Misch
(1960) discussed certain structural features of and D. H. Adair have read the present paper
the eastern Great Basin; and Gilluly (1963) has at various stages of preparation and provided
reviewed the tectonic history of the western helpful comments. Much of the drafting was
United States. No up-to-date detailed synthesis done by Gary Audette. Field work was sup-
of the geology of the eastern Great Basin exists, ported by National Science Foundation grant
however, and this led King (1959, p. 142) to G14192. This research was done in major part
say, after describing LongwelPs discoveries in while the writer was a National Science
the Las Vegas region: Foundation graduate fellow (1959-1962).
To pursue details of the structures in other parts
GEOLOGIC SETTING AND
of the eastern Great Basin would probably only
STRATIGRAPHIC HISTORY
bewilder the reader without profit. Many folds and
thrusts are known, but the larger pattern is for the Two principal parts of the Cordilleran
most part undetermined. Not only have the funda- geosyncline are recognized. The miogeosyncline
mental structures been obscured over wide areas by in Nevada and Utah contains a thick section of
Basin and Range structure, but many of the ranges Paleozoic rocks of the carbonate assemblage1
have been little explored geologically.
(limestone, dolomite, clean sandstone, and
It is the writer's opinion, however, that little shale), and within, and west of the Antler
eventually we shall know more about the
orogenic history of the Great Basin because of 1 "Carbonate assemblage" and "siliceous assemblage"
the faulting and volcanics, not in spite of them, are used for the contrasting geosynclinal facies as sug-
for they provide exposures in the third dimen- gested by Silberling and Roberts (1962) and R. J. Roberts
sion and key horizons for reconstructing the de- (1964, written commun.).
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GEOLOGIC SETTING AND STRATIGRAPHIC HISTORY 431
erogenic belt Paleozoic rocks of the siliceous graphic section of the miogeosyncline is com-
assemblage (shale, dirty sandstone, chert, and posed of a basal clastic sequence which includes
volcanic rocks) of the eugeosyncline occur. Eocambrian, Lower, and Middle Cambrian
East of the geosyncline in the central Wasatch quartzite and argillite and a widespread Eo-
Range and on the Colorado Plateau, rocks of cambrian tillitic member. In southern Nevada
the carbonate assemblage occur in a drastically and at scattered localities elsewhere, dolomite
thinned and incomplete Paleozoic section. is present in this basal sequence. After Early
The relationships between the Paleozoic sec- Cambrian time, carbonate deposition became
tions in the eugeosyncline, the miogeosyncline, widespread. Middle and Upper Cambrian de-
and the adjacent shelf are obscured by major posits are complexly intertonguing shale and
thrust faults with displacements of tens of carbonate rocks, more dolomitic toward the
miles. Eugeosynclinal rocks have been thrust top. Lower Ordovician limestones with minor
over miogeosynclmal rocks in western and cen- shale were succeeded in Middle Ordovician
tral Nevada, and miogeosynclinal rocks have time by a distinctive, widespread, clean white
been thrust over thin shelf facies in southeastern quartz sand, which is absent only locally over
Nevada and western Utah. The present-day the Tooele arch, a Cambrian and Ordovician
geographic distribution of the various rock positive element. Upper Ordovician, Silurian,
assemblages, therefore, does not represent their and Lower Devonian deposits are almost ex-
distribution at the time of deposition. clusively dolomite, and Upper Devonian and
Older Precambrian crystalline rocks that later Paleozoic carbonates are predominantly
were metamorphosed approximately 1.5 b.y. limestone. Later Devonian sedimentation was
ago underlie the shelf sections in Utah and more varied because of tectonic activity in and
southern Nevada and the miogeosynclinal rocks near the miogeosyncline. A gentle arch formed
in the Death Valley, California, region. Within in east-central Nevada between Middle and
most of the geosyncline, however, no proven Late Devonian time; the Stansbury anticline
older Precambrian ( > 1 b.y.) rocks are exposed. rose in north-central Utah during Late De-
In the Uinta Mountains, Cottonwood Uplift, vonian time. From very late Devonian time
and Death Valley areas, thick sections of young- until later Pennsylvanian time, the Antler
er Precambrian sedimentary rocks uncon- orogeny affected sedimentation in Nevada and
formably overlie the older metamorphics and western Utah; a widespread uppermost De-
are in turn overlain unconformably by rocks of vonian-Lower Mississippian shale was suc-
the Cordilleran geosyncline. ceeded by Lower Mississippian limestone,
The Paleozoic history of areas west of a line which was locally removed as a consequence of
extending south by southwest from northeast- Early Mississippian warping and erosion. Dur-
ern Nevada was complex because two major ing the rest of Mississippian and Pennsylvanian
Paleozoic orogenies occurred there (Roberts time, a clastic wedge composed of material de-
and others, 1958; Silberling and Roberts, 1962). rived from the Antler orogenic belt extended
In the miogeosyncline of eastern Nevada and into the miogeosyncline from the west. Sub-
western Utah, however, Eocambrian2 through sidence of the Oquirrh basin in north-central
Triassic stratigraphic relations are relatively Utah began in Mississippian time and con-
simple. Gentle truncation of units occurs, par- tinued into Permian time. From Late Mis-
ticularly along the eastern and western margins sissippian through Permian time, elastics shed
of the miogeosyncline, but only one distinctly from the rising basement uplifts of the An-
angular unconformity is known (Stansbury cestral Rocky Mountains accumulated in the
anticline of Rigby, 1958), and this is only of Oquirrh basin which was bounded on the north
local extent. by an east-west-trending monoclinal flexure.
Deposition began in the Cordilleran miogeo- During Pennsylvanian time, most of eastern
syncline before the oldest Cambrian fossils ap- Nevada was the site of limestone deposition; in
peared. Approximately one third of the strati- the Oquirrh basin, more than 20,000 feet of
alternating limestone and quartzite accumu-
2 The term "Eocambrian" is used to emphasize that
lated. Thick lower and middle Permian de-
no significant time gap separates the sediments referred
to from overlying fossiliferous Cambrian strata. Usage of posits—limestone with much quartz sand, sand-
this term is the same as in the Caledonide area of Norway stone, siltstone, dolomite, and some evaporite—
where Eocambrian was first proposed by W. C. Brogger accumulated in the Arcturus basin in east-
in 1900 (Holtedahl, 1960, p. 111-112). central Nevada, and thick limestone deposits
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432 R. L. ARMSTRONG—SEVIER OROGENIC BELT IN NEVADA AND UTAH
accumulated in southern Nevada; in Utah, cor- mediate to acidic volcanics, chiefly ignimbrites
relative strata consist of alternating quartzite of latest Eocene, Oligocene, and early Miocene
and limestone with minor dolomite. South- age. The youngest group, Miocene to Recent,
eastward from the miogeosyncline, Permian is a heterogeneous collection of discontinuous
marine strata intertongue with continental red clastic units, volcanic-rich sediments, vol-
beds. canics (commonly basalts but also all other
Upper Permian deposits (Park City Group) types), and lacustrine sediments, deposited
are a widespread blanket of relatively uniform during the development of the Basin and Range
thickness and lithology (limestone and dolomite structure. Figure 1 is a correlation chart il-
with minor chert and phosphate) over the en- lustrating Tertiary stratigraphic relationships
tire region, including the Antler erogenic belt within, and adjacent to, the eastern Great
and much of the Colorado Plateau shelf. In the Basin.
miogeosyncline, marine sedimentation con- A more complete review of the Precambrian
tinued without erogenic interruption into the through Tertiary stratigraphic history, to-
Triassic over much of Nevada and Utah. gether with documentation not included in this
Triassic (or earliest Jurassic) rocks were the last paper, may be found in Armstrong (1968).
deposits of the Cordilleran miogeosyncline. In Palinspastic isopach maps for all Paleozoic sys-
Middle Triassic time, marine waters withdrew tems and three palinspastic paleostratigraphic
from the eastern Great Basin in Nevada for the profiles across the region are included in the re-
last time, marking the beginning of the erogenic view.
chapter in the history of the region. The
Triassic-Jurassic boundary probably lies within PRE-NORMAL FAULTING
the widespread eolian sandstone (Navajo- PALEOGEOLOGY—SEVIER
Aztec-Nugget), which is the youngest pre- OROGENIC BELT
orogenic formation present in the eastern Great
Paleogeologic Map
Basin west of the Mesozoic fold and thrust belt.
During the Jurassic, the region of thickest The present-day structural pattern of the
sediment accumulation shifted to central and eastern Great Basin is dominated by the effects
eastern Utah, and the western part of the of Tertiary normal faulting. Geologic maps of
Cordilleran miogeosyncline became a source the region cannot clearly portray the general
area (Stokes, 1960) in response to erogenic de- features of the pre-Tertiary structures that are
formation taking place there. Continental exposed in separated ranges. Each individual
clastic deposits of Upper Jurassic and lowermost exposure displays the older structures in a
Cretaceous age, derived from this western different attitude or aspect, and irregularly dis-
source, spread across the eastern edge of the tributed Tertiary volcanics and sediments do
Paleozoic miogeosyncline and the Colorado not make things clearer. If we remove the
Plateau. In Early Cretaceous time, the eastern effects of normal faulting and Tertiary sedi-
edge of the geosyncline became a source of mentation, we can view in a simple manner the
clastic material, which accumulated during broad features of the pre-Tertiary structures.
Cretaceous and Paleocene time in the Rocky This can be done by a paleogeologic reconstruc-
Mountain geosyncline still farther east. At a few tion, as described by Levorsen (1960). All later
localities in eastern Nevada, continental Lower effects of sedimentation and deformation are
Cretaceous deposits are present, but over most erased and the resultant map portrays the
of the region there is a great hiatus between de- geology as it was at the time the unconformity
posits of the Cordilleran geosyncline and was buried. This technique is applied here to
Tertiary deposits. the eastern Great Basin to display the pre-
Tertiary strata of the eastern Great Basin normal faulting paleogeology.
and adjacent Colorado Plateau can be sub- Plate 1, figure 1 shows the units which over-
divided into three major groups. The oldest is lie the unconformity used for the reconstruction
composed of nonvolcanic continental sediments (see also Fig. 1). The range in age of the uncon-
—scattered Eocene lacustrine deposits and un- formity is latest Cretaceous to earliest Miocene.
dated conglomerates in Nevada and western This undoubtedly has a somewhat distorting
Utah and Paleocene and Eocene fluviatile and effect on the resultant paleogeology, but it does
lacustrine sediments that are well developed in not alter the fundamental geologic pattern. In
central Utah and northward into Wyoming. all areas, the unconformity postdates the main
The middle group consists of widespread inter- Mesozoic deformation, although locally in cen-
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PRE-NORMAL FAULTING PALEOGEOLOGY—SEVIER OROGENIC BELT 433
Figure 1. Eastern Great Basin Tertiary correlation chart.
tral Utah some minor folding and even thrust- determining the effect of the same rotation on
ing may have occurred later than the uncon- the older rocks. In areas where a suitable un-
formity. The unconformity, however, predates conformity is lacking, it is possible to put
the major normal faulting. Early normal fault- limits on the ages of rocks as those exposed in
ing may affect the pattern somewhat in eastern early Tertiary time must have been as young as,
and southern Nevada but only to a relatively or younger than, those now present.
minor degree. The over-all structural pattern of The normal faulting responsible for the
the region was not significantly altered during present topography and much of the geologic
the time spanned by the unconformity. complexity of the Great Basin occurred mostly
If the older structure is not enormously com- during Miocene and Pliocene time. Pre-
plex, the resultant paleogeologic map should Miocene normal faults are known in many
display the broad features of the regional places in the region, but none approach the
geology during early Tertiary time. Plate 1, magnitude of displacement of the faults formed
figure 2 shows the distribution of points (~900) later. No example can be cited where rocks
where information on rocks underlying the un- differing in age by several geologic periods were
conformity was recorded. The source maps con- juxtaposed along pre-Miocene normal faults.
sulted are given in Table 1. In addition to Thrusting accounted for the major discon-
simple rock age information supplied by tinuities present.
Tertiary—pre-Tertiary contacts, it is also often A significant structural feature of the region
possible to construct paleo-strikes and dips by is the widespread near-conformity of the
rotating the oldest Tertiary deposits (com- Paleozoic sediments and Tertiary volcanics.
monly ignimbrites) back to horizontal and Over large areas, the angularity of the uncon-
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434 R. L. ARMSTRONG—SEVER OROGENIC BELT IN NEVADA AND UTAH
TABLE 1. GEOLOGIC MAPS OF THE EASTERN GREAT region. The generalization does not apply to
BASIN AND VICINITY WHICH WERE USED FOR more westerly areas near and in the Antler
CONSTRUCTION OF THE PALEOGEOLOCIC MAP* (PI. 1, erogenic belt.
fig. 3). The observed near-parallelism of Tertiary
and pre-Tertiary rocks in the region establishes
Nevada
that low dips were characteristic of pre-Tertiary
Clark County Bowyer and others (1958)
rocks during early Tertiary time. Extrapolation
Lincoln County Kellogg (1963)
between scattered data, therefore, should be
Tschanz (1960)
Tschanzand Pampeyan (1961) safe at least for distances of a few miles. The
near-parallelism of units, however, is only a
White Pine County Adair (1961)
Bauer and others (1960) generalization. In many areas, sharp angular un-
Douglass (1960) conformities occur, and the paleogeologic re-
Drewes (1958; 1960) construction must maintain consistency with
Fritz (1960) these relationships. The paleogeology becomes
Langenheim and others (1960) increasingly complex westward into central
Lloyd (1959)
Nevada. The reconstruction is limited to the
Nelson (1959)
Playford (1962) area from which suitable data are available and
Ward (1962) where the paleogeology appears to have been
Whitebread and others (1962) fairly straightforward.
Woodward (1964)
The pre-normal faulting paleogeology is
Young, J. C. (1960)
shown on Plate 1, figures 3 and 4. The map
Elko County Harlow (1956)
agrees with all the data collected, but in many
Nelson (1956)
Schaeffer and Anderson (1960) areas alternate interpretations of the data are
Snelson (1955) possible; this does not imply, however, that the
Utah major structural features are in doubt. The map
State Geologic Map portrays the geology as it would have been
Northeast Quarter Stokes and Madsen (1961) mapped shortly after the end of the Creta-
Northwest Quarter Stokes(1963)
ceous. The degree of definition (resolving pow-
Southwest Quarter Hintze (1963)
er) is slightly better than that represented by
Other Maps Hintze (1962)
more detailed parts of the 1932 edition of the
Wyoming U. S. Geological Survey geologic map of the
State Geologic Map Love and others (1955)
United States.
Other Data from Cochran (1959) The general features of the paleogeology are
Schick (1959)
immediately evident. The structural trends are
generally north to northeast. On the east side
* A geologic map of Nevada (Webb and Wilson, 1962)
of the map is a broad, virtually undeformed,
is available but was not used for construction of Plate 1,
foreland basin area filled by Cretaceous de-
figure 3. It includes data from the references in the table
posits. Two broad arches which lie nearly
but in a more generalized form.
perpendicular to the regional trend occur in
this foreland. The middle of the map is oc-
formity is less than 5 degrees; only locally is
cupied by a fold-and-thrust belt with eastward
it distinctly angular. Cook (1965, p. 54-55)
overturning and thrusting. The region farther
states:
west displays a deceptively simple structural
pre-volcanism deformation was sharply localized pattern, in which, over broad areas, only gently
along axes that trend east of north, leaving between folded upper Paleozoic rocks are shown. In
the narrow belts of deformation broad areas of un- southern Nevada, the pattern is complicated
deformed Paleozoic rocks, the ignimbrites in many by an uplift exposing lower Paleozoic and
sections are essentially parallel to the underlying Eocambrian rocks and surrounded by klippen
sedimentary rocks . . . the attitude of the volcanics
of Paleozoic rocks overlying middle and upper
in many places reflects the attitude of the subadja-
Paleozoic strata. This pattern is the summation
cent sedimentary rocks; locally . . . the angularity
of all the effects of Mesozoic and early Tertiary
of the unconformity is great.
deformation in the region.
Mackin (1961, oral commun.) made the same
Foreland
observation, and the writer also agrees. The
relationship is of importance in reconstructing East of the belt of folds and thrusts is a
the pre-normal faulting paleogeology of the broad area that was slightly deformed during
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PRE-NORMAL FAULTING PALEOGEOLOGY—SEVIER OROGENIC BELT 435
Mesozoic time; later in the era, it was a basin in Triassic unconformity in southern Nevada. The
which erogenic debris from the area now oc- first conclusive evidence of orogenic deforma-
cupied by the Great Basin accumulated. At the tion in the belt, however, is the appearance of
end of the Cretaceous and during early Tertiary lower Paleozoic clasts in early Colorado time in
time, broad arches formed in the foreland. Utah and Nevada or as far back as the be-
These can be seen on the paleogeologic map as ginning of the Cretaceous in Idaho (F. C.
the Uinta arch in northern Utah and the Circle Armstrong and Cressman, 1963, p. 10). The
Cliffs upwarp in the southern part of the state. only possible source of these clasts is the Sevier
belt. Lower Cretaceous and Upper Jurassic
Sevier Orogenic Belt
sediments could have been derived from any-
General statement. The Sevier arch was where in the eastern Great Basin, but they may
named by H. D. Harris (1959), who described also have come, at least in part, from the Sevier
it as a late Mesozoic positive area in western belt during earlier stages of its development.
Utah and southeastern Nevada. The arch con- The Eocambrian rocks of the belt occur almost
cept was based on paleogeology, and the arch exclusively in the sole of major thrusts. Their
was considered the source of the erogenic appearance, in abundance, as clasts approxi-
elastics shed to the east; thrusting was con- mately at the end of Colorado time is evidence
sidered to be the climax of arching during late that thrust displacements of tens of miles exist-
Cretaceous time. Harris (1959, p. 2646) says: ed by then.
The relative ages of folds and thrusts in the
There is no direct evidence of large-scale thrust- belt normally cannot be determined, but in the
ing associated with the uplift of the Sevier arch.
Canyon Range folding definitely postdates dis-
Deformation appears to have been generally limited
placement on the major thrust (Christiansen,
to upwarping and development of major folds, some
1952). In other mountain belts, such as the
of which undoubtedly developed into belts of struc-
tural weakness that later became zones of thrusting. Appalachians, folding postdates movement
along regional bedding-plane thrusts (Rich,
The paleogeologic map of Plate 1, figure 3 is 1934; Pierce and Armstrong, 1964), for after
based on more than ten times the amount of folding such thrusts are unable to develop.
data presented by Harris and does not support Probably, therefore, most of the Sevier
the concept of a simple late Mesozoic arch, for thrusting represents an earlier stage of defor-
nowhere is an eastern limb evident. It shows mation than the folding. Some folding was
that the exposure of old rocks in the area is probably also contemporaneous with, if not
indeed the result of thrusting, which was the caused by, thrusting (Rich, 1934; Cressman,
deformation responsible for the elastics shed 1964). On the major thrusts of the belt, a
into the Rocky Mountain geosyncline during minimum of 25 miles of total displacement has
the latter half of the Mesozoic. It is suggested occurred; such a displacement is approximately
that "Sevier orogenic belt" is a better term for equal to the thickness of the crust and cannot
the belt of thrusts and folds originally de- be merely the climax of deformation in a tight
scribed as the Sevier arch. fold. To summarize, thrusting was prolonged
In several recent studies, paleogeographic and of great magnitude in the Sevier belt. It was
arches or geanticlines have been discovered to not merely the climax to earlier folding and
be orogenic belts; for example, the Manhattan arching. Figure 2 provides an index map for the
geanticline became the Antler orogenic belt following detailed discussion of the Sevier
(Roberts and others, 1958), and the Mesocordil- orogenic belt.
leran geanticline was the site of Jurassic de- Southern Nevada-Southwestern Utah Sector.
formation (Misch, 1960; Armstrong and The structural geology of Clark County,
Hansen, 1966). It does not appear reasonable to Nevada, has been discussed by Longwell (1949;
expect enormous quantities of coarse elastics 1952; 1952a; 1960; 1962), and a county map
from simple arching. Orogenic deformation, has been published (Bowyer and others, 1958).
including faulting, is necessary to account for The paleogeology retains all features of Long-
the Mesozoic elastics. well's interpretation of the area. Modification
In its earliest stages, the Sevier belt may well of the structural pattern by later Tertiary nor-
have been archlike. Permian isopachs offer the mal faults is relatively minor.
first faint suggestion of uplift along the locus The major thrust in terms of stratigraphic
of the belt. Further uplift in later Permian or displacement is the Gass Peak thrust which
early Triassic time may be indicated by the pre- brings the Lower Cambrian and Eocambrian
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436 R. L. ARMSTRONG—SEVIER OROGENIC BELT IN NEVADA AND UTAH
Figure 2. Index map for Sevier erogenic belt, Nevada and Utah.
quartzite over carbonates of the Pennsylvanian (1963) and thus are not features of the paleo-
and Permian Bird Spring Formation. East of geology.
the major thrust is the Glendale thrust, which To the east of the larger thrusts are two re-
overrides the Muddy Mountain thrust; both lated structures: the Iron Springs Gap struc-
thrusts apparently flatten out at depth in ture (Mackin, 1947; 1960a, p. 114-119) which
Cambrian shales and both override Jurassic is a thrusted anticline formed at the end of a
sediments. decollement in the Carmel Formation, and the
The absence of lower or middle Tertiary sedi- Virgin-Kanarra fold (Gregory and Williams,
ments prevents continuation of the paleo- 1947; Threet, 1963, 1963a).
geology south of the Las Vegas shear zone, but The westernmost thrust of the Sevier belt in
similar thrusts appear there, although they are Lincoln County places Lower Cambrian over
displaced approximately 25 miles to the west. Upper Paleozoic rocks. It has a greater strati-
The Wheeler Pass and Keystone thrusts would graphic displacement than the other thrusts in
correspond to the Gass Peak and Glendale- this sector and may have the greatest total
Muddy Mountain thrusts, respectively. displacement. There is a problem as to exactly
The structural geology of southeastern how it connects with the similar thrust in Clark
Lincoln County has been discussed only in an County. On the paleogeologic map, they are
abstract (Tschanz, 1960a), but a county map shown as the same thrust affected by later, but
has been published (Tschanz and Pampeyan, pre-middle Miocene, normal faulting. Ac-
1961). In the Mormon Mountains in the south- cording to an alternative interpretation sug-
east corner of the county, a thrust system, gested by D. H. Adair (1962, oral commun.),
probably a continuation of the Glendale thrust, the thrusts are en echelon, the Gass Peak dying
is present; the main thrust brings Cambrian out northward, the thrust in Lincoln County
over Mesozoic rocks which are locally im- growing in the same direction, each compensat-
bricate. The structure as illustrated on the ing for the changing displacement on the other.
paleogeologic map undoubtedly is oversimpli- In the first interpretation, certain Upper
fied; it represents one plausible interpretation Paleozoic outcrops in Lincoln County are
consistent with the available data. The involute autochthonous relative to the major thrust; in
pattern of the thrust is probably due to topog- the second, they are allochthonous.
raphy. Approximately 15 miles west of the trace of
Allochthonous blocks in the Beaver Dam the major thrust in Lincoln County is an
Mountains have been interpreted as Tertiary elongate exposure of Lower Cambrian and
gravity-slide blocks by Cook (1960) and Jones Eocambrian (C?) quartzite. The quartzite-