GEOLOGICAL SURVEY P ROFESSION A L PAPER 1149
Cenozoic Stratigraphic and 

Structural Framework of 

Southwestern Utah

By PETER D. ROWLEY, THOMAS A. STEVEN,

JOHN J. ANDERSON,am/ CHARLES G. CUNNINGHAM

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1149

A summary of the Cenozoic sedimentary and 

volcanic stratigraphy of southwestern Utah, and 

the structures that controlled deposition of 

the strata and that displaced these strata

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Main entry under title:

Cenozoic Stratigraphic and Structural Framework of Southwestern Utah

(Geological Survey Professional Paper 1149)

Bibliography: p. 17

1. Geology, Stratigraphic Cenozoic. 2. Geology-Utah. I. Rowley, Peter D. II. Series:

United States Geological Survey Professional Paper 1149 

QE690.C438 551.7'8f 09792 79-17228

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 

Stock Number 024-001-03227-9
CONTENTS

Page

Abstract -- ——— — - —— - ——————— —— ———— ——— -- ———— ——— ———— -- 1

Introduction - —————— ———— - ———— —— — ——— —— - ———— ——— ———— —— 1

Acknowledgments — —— ————————— — ——— ———— ———— —————————————— 2

Stratigraphy - —————— ——— ———— —— - ————————— - ———— ——— ————————— 3

Lower Tertiary (and Upper Cretaceous?) sedimentary sequence ———————————————— 4 

Middle Tertiary volcanic sequence —— —————— ——— ———— —————————————— 5

Early (pre-Needles Range Formation) calc-alkalic 

volcanic rocks —————————— — — ———— —————————— ————————————— 5

Needles Range Formation ———————————————————————————————————— 6 

Late (post-Needles Range Formation) calc-alkalic 

volcanic rocks ————— ————————— ———————————— ——— ——————————— 7

Marysvale volcanic field ————————————————————————————————— 7 

Other volcanic fields in southwestern Utah ——————— ——— ——— ——————— 9

Ash-flow tuff province of southwestern Utah — ———— — —————————— ——— 10

Upper Cenozoic sedimentary and volcanic sequence ———————— —————————— ——— 11 

Basin-fill sedimentary rocks ——— —— —————————— —————————— —————— 12

Bimodal volcanic rocks ———————— ———— —————————— —————— ——————— 12

Structural Geology ——————— ————————— —————————— —————————————————— 14

Older structures ——————————————————————————————— ———— ———————— 14 

East-trending features ——— —— ——— ———— ——— ———————————— ——————— 15

Basin-range faults ————————————— —————————— —————— —————————— — 15

Low-angle Tertiary faults —————————————— ———— —————————— ———————— 17

The Colorado Plateaus ———————————————————————————————————————— 17 

References cited - ———— —————————————— ——— —————— - —— —————— ———— 17

ILLUSTRATIONS

FIGURES 1. Map of physiographic and major structural features of southwestern Utah— —— — — — —— — — ———— ———— — ———— — 2

2. Generalized lithology map of southwestern Utah— — — — — ——— — — — — — ———— — — — — —————— ——————— ———— 3

3. Correlation chart for Cenozoic rocks of southwestern Utah — — — — — —— — —————— ———— — ——— —— —— — ——————— 4

4. North-south cross section across Marysvale volcanic field — ———— __ — — _ ———— — —— — —— — ——————— ——— ___ 9

METRIC-INCH-POUND SYSTEM EQUIVALENTS

Metricunit Multiplied by To give inch-pound

Meter(m) ————————————— 3.28 ——————————————Feet(ft) 

Kilometer (km)—————————— .62 —————————————Miles (mi)

III

CENOZOIC STRATIGRAPHIC AND STRUCTURAL 

FRAMEWORK OF SOUTHWESTERN UTAH

By PETER D. ROWLEY, THOMAS A. STEVEN, JOHN J. ANDERSON,1 

and CHARLES G. CUNNINGHAM

ABSTRACT

Cenozoic rocks of southwestern Utah chronicle a complex history of 

early Tertiary sedimentation followed by middle and late Tertiary 

and Quaternary volcanism and basin-fill sedimentation. Sedimentation in latest Cretaceous(?) to Oligocene time took place largely in two 

broad basins that developed east of a highland formed during the 

Sevier orogeny in Late Cretaceous time. Calc-alkalic volcanism began 

in early Oligocene time and continued into the early Miocene; many 

widely scattered, partly clustered, intermediate-composition 

stratovolcano complexes formed in two east-northeast-trending belts. 

The intervening lowlands were covered by thin but widespread ashflow tuff sheets. Little deformation took place during most of early 

sedimentation and calc-alkalic volcanism, but minor faulting can be 

documented locally, especially along several east-trending lineaments. The Basin and Range and Colorado Plateaus provinces appear 

to have begun to separate into identifiable structural entities about 26 

m.y. ago.

About 20 m.y. ago, east-west extension started to imprint a northerly striking pattern of basin-range faults onto the older easttrending igneous belts and underlying sedimentary rocks the present 

structural and topographic grain of southwestern Utah formed. 

Uplifted blocks were eroded and sediments were deposited in the 

adjacent developing basins. Concurrently a bimodal assemblage of 

mostly small volumes of basaltic and rhyolitic rocks was erupted. 

Basalt flows are very widely distributed, but the rhyolitic rocks were 

erupted from local centers, many of which are located along the 

east-trending lineaments within the older igneous belts.

Mineralization took place at many volcanic centers during the 

waning stages of the calc-alkalic volcanism, and around some of the 

younger rhyolitic centers episodically throughout the span of bimodal 

volcanism.

INTRODUCTION

Most of southwestern Utah is covered by continental 

sedimentary and volcanic rocks of Cenozoic age. The 

volcanic rocks are especially important because of their 

associated metallic mineral resources, and exploration 

interest is high. Southwestern Utah also has potential 

for important petroleum and geothermal resources. 

This paper reviews the lithology of the Cenozoic rocks,

1 Department of Geology, Kent State University, Kent, Ohio 44242.

their mode of occurrence, and the structures that affect 

them, as an aid to the various exploration programs.

The area described extends from the eastern part of 

the Basin and Range province across the adjacent High 

Plateaus subprovince of the Colorado Plateaus province 

(fig. 1). This area encompasses the igneous province of 

southwestern Utah; this province extends westward 

into southeastern Nevada but is separated from the 

igneous province of west-central Utah by mountain 

ranges underlain almost entirely by pre-Cenozoic rocks.

The work leading to this report was begun in the 

1960's by Anderson and Rowley as a regional study of 

the volcanic terrane in the southwestern High Plateaus 

and adjacent part of the Basin and Range province. 

Results of this study were published by J. J. Anderson 

(1971), Anderson and others (1975), Rowley, Anderson, 

and Williams (1975), and Rowley (1978). Rowley later 

completed some unfinished work of the late J. Hoover 

Mackin in the Iron Springs mining district and published a series of geologic maps (Mackin and Rowley, 

1975, 1976; Mackin and others, 1976; Rowley, 1975, 

1976; Rowley and Threet, 1976). In 1975, Steven, Cunningham, and others began a detailed investigation of 

the Marysvale mining district. Preliminary reports on 

several aspects of this work have been published by 

Steven and others (1977, 1979); Steven, Cunningham, 

and Rowley (1978); Steven, Rowley, and Cunningham 

(1978); Steven and Cunningham (1979a); Cunningham 

and Steven (1977, 1978a, 1979a, 1979b); and Cunningham, Steven, and Naeser (1978). A brief study of young 

rhyolites in southwestern Utah and their geothermal 

potential resulted in reports by Lipman and others 

(1978); Mehnert, Rowley, and Lipman (1978); and Rowley, Lipman, and others (1978). More recently, these 

separate studies have been consolidated into a comprehensive investigation of the geology and mineral 

resource potential of the Richfield 2° quadrangle, in 

which all four of the present authors are participating. 

A series of geologic maps also is being published (Steven, 1978, 1979a, 1979b; Cunningham and Steven,
CENOZOIC STRATIGRAPHIC AND STRUCTURAL FRAMEWORK OF SOUTHWESTERN UTAH

39

114 113 112" 111'

FIGURE 1.—Physiographic features and major structural features of southwestern Utah. Basin and Range — Colorado Plateaus boundary shown 

by heavy short-dashed line; Pavant Range, Tushar Mountains, and named plateaus belong to the High Plateaus subprovince. Central part 

of Pioche-Marysvale igneous belt on north, and of Delamar-Iron Springs igneous belt on south, patterned. Dot-dash line, east edge of major 

Sevier thrust faults, modified from Crosby (1973). Heavy long-dashed lines, axes of lineaments, consisting from north to south of the Black 

Rock, Blue Ribbon, and Timpahute lineaments. Fine dashed line on the north, approximate minimum boundaries of depositional basins of 

the North Horn to Crazy Hollow Formations; to the south, the Claron Formation and related rocks. Tushar highland is between them. A, 

Antimony; B, Boulder; Be, Beaver; C, Circleville; CC, Cedar City; CF, Cove Fort; E, Enterprise; Es, Escalante; F, Fillmore; H, Hurricane; K, 

Kanab; L, Loa; M, Minersville; Ma, Marysvale; Mi, Milford; P, Panguitch; Pa, Parowan; R, Richfield; S, Stateline mining district; Sa, 

Salina; SG, St. George.

1978b; 1979c, 1979d; Rowley, 1979; Rowley and others, 

1979). A preliminary "state of the art" compilation of 

the geology of the Richfield quadrangle (Steven, Rowley, and others, 1978) was prepared early in this investigation and is being significantly modified by continuing field studies. The present report summarizes data 

from all these studies.

ACKNOWLEDGMENTS

Collaboration and conversations with P.L.Williams, H. H. Mehnert, R. E. Anderson, E. H. McKee, C. 

W. Naeser, R. J. Fleck, G. P. Eaton, M. G. Best, M. G. 

Nelson, G. A. Wadsworth, K. E. Budding, J. M. Edwards, W. K. Hamblin, D. S. Barker, G. A. Izett, M. M.
STRATIGRAPHY

114° 113° 112° 111'

Calc-alkaline

volcanic rocks 

Lower Tertiary 

sedimentary rocks

FIGURE; 2.—Generalized lithology map of southwestern Utah, modified from Hintze (1963). Dots mark locations referred to in figure 1.

Machette, D. A. Lindsey, D. R. Shawe, J. T. Abbott, G. L. 

Galyardt, T. D. Fouch, Eugene Callaghan, A. G. 

Everett, H. H. Sturr, and the late J. H. Mackin helped 

formulate many concepts given in this paper. Thesis 

work by J. M. Sigmund and S. H. Decatur of Kent State 

University also contributed significant data. Suggestions by R. E. Anderson and K. A. Sargent materially 

improved the manuscript.

STRATIGRAPHY

The Cenozoic rocks of southwestern Utah (fig. 2) 

comprise three broad sequences that differ in age and

rock type and that formed in different tectonic environments. The oldest of these is made up of uppermost 

Cretaceous(?) to Oligocene fluvial and lacustrine rocks 

that unconformably overlie Mesozoic and Paleozoic 

rocks. These rocks were deposited in marginal basins 

east of highlands formed during eastward-directed 

thrust-faulting and folding of the Sevier orogeny and 

warping of the Laramide orogeny. A middle Tertiary 

volcanic sequence consisting mostly of upper Oligocene 

and lower Miocene calc-alkalic volcanic rocks overlies 

the lower Cenozoic sedimentary sequence. The volcanic 

rocks, underlain by related plutons, formed many scattered, partly clustered, intermediate-composition 

stratovolcanoes separated by broad areas covered by
CENOZOIC STRATIGRAPHIC AND STRUCTURAL FRAMEWORK OF SOUTHWESTERN UTAH

thin, regionally distributed silicic ash-flow tuffs. The 

youngest sequence consists of upper Cenozoic fluvial 

and lacustrine deposits that filled developing basins 

during basin-range tectonism, and of a bimodal volcanic 

assemblage of alkali basalt and high-silica alkali rhyolite that was erupted episodically during the same time 

span. Basin-range tectonism and related volcanism 

both began 21-20 m.y. ago in southwestern Utah and 

have continued to the present.

LOWER TERTIARY (AND UPPER CRETACEOUS?) 

SEDIMENTARY SEQUENCE

Lower Tertiary sedimentary rocks in southwestern 

Utah were deposited in at least two separate basins and 

as thin patches farther west. The most widespread basin 

contains a poorly studied succession, named the Claron

Formation, which consists of more than 300 m of fluvial 

and lacustrine strata. These rocks were loosely referred 

to as Wasatch Formation in early reports; the name 

Claron was applied by Leith and Harder (1908) to rocks 

of this assemblage in the Iron Springs district; and 

Mackin (1968) described the Iron Springs rocks in more 

detail. The formation is well exposed in the Red Hills 

(Threet, 1963b), and nearly continuous outcrops occur 

over large areas in the High Plateaus (Anderson and 

Rowley, 1975). The formation also is found in widely 

separated exposures in other parts of the Basin and 

Range province, such as the Bull Valley Mountains 

(Blank, 1959), Pine Valley Mountains (Cook, 1957), and 

Black Mountains (fig. 3). Relations in all these areas 

suggest deposition in a broad, flat basin that extended 

across the present southern High Plateaus and adjacent 

parts of the Basin and Range province as far west as the 

eastern or central Bull Valley Mountains.

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1 Of Wiley (1963).

FIGURE 3. Generalized correlation chart for Cenozoic rocks, excluding Holocene deposits, of southwestern Utah. Vertical lines, strata absent. 

Local unconformities not shown. Numbers refer to K-Ar ages, rounded to the nearest million years. Heavy lines, boundaries of 

stratigraphic sequences. Lines dashed where approximately located.
STRATIGRAPHY

In the eastern Bull Valley Mountains, the thick 

fanglomerate in Grapevine Wash (Wiley, 1963), was 

deposited in front of, and was locally overridden by, an 

eastward-advancing thrust sheet during the Sevier 

orogeny (Cook, 1957, 1960a; Wiley, 1963; Rowley, Anderson, and others, 1978b). This unit, presumably of 

latest Cretaceous and earliest Tertiary age, underlies 

and is gradational into basal rocks of the Claron Formation (Wiley, 1963).

The Claron Formation was deposited through a broad 

span of time, from at least early Eocene through middle 

Oligocene. Locally, deposition may have begun as early 

as Paleocene or latest Cretaceous. Most of the age data 

comes from the upper part of the formation, which locally is interbedded with (Blank, 1959; Mackin, 1960; 

Wiley, 1963), or more commonly overlain by, ash-flow 

tuffs of the 30- to 29-m.y.-old Needles Range Formation, 

discussed later. R. E. Anderson (oral commun., 1976) 

found volcanic clasts in basal conglomerate of the Claron east of the Pine Valley Mountains, indicating that 

here the formation may be all of middle to late 

Oligocene age. The lower part of the Claron Formation 

is much more poorly known. On the basis of a detailed 

stratigraphic and paleontologic study of the basal Tertiary sedimentary rocks of the southeastern High 

Plateaus, Bowers (1972) named two new formations 

(Upper Cretaceous and Paleocene(?) Canaan Peak and 

Paleocene (?) Pine Hollow Formations) for rocks that are 

generally correlative with the lower part of the Claron 

and retained informal names for units that are correlated with the upper part of the Claron. Much more 

detailed work is needed to establish how widely these 

units of Bowers extend and how they fit with the Claron 

assemblage.

A different succession of lower Tertiary sedimentary 

rocks is exposed in the High Plateaus and adjacent 

Colorado Plateaus north of the Marysvale volcanic 

field. Most of these rocks are outside the area of interest 

to this report and will be mentioned only briefly. This 

northern succession consists of fluvial and lacustrine 

sedimentary rocks of the Upper Cretaceous and 

Paleocene North Horn Formation, the Paleocene and 

lower Eocene Flagstaff Limestone, the Eocene Colton 

and Green River Formations, and the EoceneC?) Crazy 

Hollow Formation. These formations, many bounded by 

unconformities, have been studied in detail by Spieker 

(1946, 1949) and his students and colleagues, principally from Ohio State University (for example, Stanley 

and Collinson, 1979). The lithologies of this northern 

succession differ somewhat from those of the Claron 

Formation, and separate systems of nomenclature seem 

justified.

The northern and southern successions of lower Tertiary sedimentary strata are separated by the younger 

Marysvale volcanic field, which seems to have accumulated above an older highland that divided the area east 

of the Sevier orogenic belt into two basins. This highland, which we call the Tushar highland, trended east 

across the area now occupied by the Mineral Mountains, 

Tushar Mountains, and central Sevier Plateau (Callaghan, 1973; Anderson and Rowley, 1975). The only 

lower Tertiary sedimentary rocks that occur on the 

highland are patches of conglomerate generally less 

than 30 m thick (Earll, 1957; Liese, 1957; Kennedy, 

1960, 1963; Callaghan and Parker, 1962a; Cunningham and others, 1978). The margins of the apparent 

basins are covered by volcanic rocks, but drill log data 

(Ritzma, 1972) show the Claron to be only 180 m thick 

west of Antimony. The Claron in Antimony Canyon, to 

the southeast, is about 450 m thick (Williams and 

Hackman, 1971); here Smith (1957) found conglomerate 

thickening northward. Stanley and Collinson (1979) 

noted that in the northern basin south of Salina, carbonate rocks give way southward to clastic sedimentary 

rocks, some of which are derived from the south.

The third succession of lower Tertiary sedimentary 

rocks in southwestern Utah consists of local patches of 

mostly thin, unnamed conglomerate that have been 

mapped in the San Francisco Mountains (East, 1966), 

southern House and Confusion Ranges (Hintze, 1974a, 

b), and Burbank Hills and Cricket Mountains (Hintze, 

1963). The rocks pinch out laterally against preexisting 

hills and are missing in many areas. The hills are especially abundant near the Nevada border and appear to 

be parts of a highland formed by the Sevier orogeny. The 

patches of sedimentary rocks probably represent deposition in many local low-lying areas. In adjacent parts of 

eastern Nevada, however, larger basins formed in 

which thick sediments of the Sheep Pass and other 

formations were deposited (Fouch, 1979).

MIDDLE TERTIARY VOLCANIC SEQUENCE

The middle Tertiary volcanic sequence consists almost entirely of calc-alkalic igneous rocks that range in 

age from about 35 m.y. to about 19 m.y. They can be 

divided into three successive assemblages in which the 

middle assemblage consists of the Needles Range Formation, a distinctive ash-flow tuff marker unit that 

occurs throughout most of southwestern Utah. Volcanic 

rocks below the Needles Range Formation formed local 

silicic ash-flow sheets and scattered intermediatecomposition stratovolcanoes. These volcanic rocks are 

lithologically similar to those above the Needles Range 

Formation.

EARLY (PRE-NEEDLES RANGE FORMATION) 

CALC-ALKALIC VOLCANIC ROCKS

Calc-alkalic volcanism in southwestern Utah 

began in early Oligocene time, about 35 m.y. ago, and
CENOZOIC STRATIGRAPHIC AND STRUCTURAL FRAMEWORK OF SOUTHWESTERN UTAH

between then and eruption of the Needles Range Formation 30—29 m.y. ago, many scattered intermediatecomposition stratovolcanoes formed, and a few regional 

ash-flow sheets accumulated. No change in composition 

or eruptive behavior took place during or after eruption 

of the Needles Range tuffs, and the early rocks are 

merely the precursors of the more voluminous postNeedles Range volcanic rocks in the same area. They 

are separated solely because the Needles Range Formation provides such an excellent stratigraphic and structural marker horizon over much of southwestern Utah. 

Inasmuch as the Needles Range Formation rests directly on the Claron Formation over much of the area, 

the early volcanoes do not appear to have formed a 

continuous volcanic field.

Early volcanism in the Marysvale volcanic field 

formed at least three major pre-Needles Range igneous 

complexes. North and northwest of Marysvale, one 

major stratovolcano and several minor flank volcanoes 

underlie the Needles Range Formation. These volcanoes are made up of intermediate-composition 

(rhyodacitic?) porphyritic lava flows, flow breccia, and 

mudflow breccia that in the Marysvale Canyon area, 

north of Marysvale, are more than 400 m thick (Steven 

and Cunningham, 1979b). These rocks are assigned to 

the Bullion Canyon Volcanics, discussed later. A second 

early stratovolcano, at least 300 m thick, is exposed on 

the scarp of the southern Sevier Plateau, east of Circleville. Mudflow breccia from this volcano resembles 

the rocks of the overlying Mount Button Formation, 

discussed later. The third complex is represented by the 

30.4-m.y.-old (Damon, 1968) Spry laccolith south of 

Circleville. This intrusion was emplaced into and 

domed the Claron Formation, thus creating a topographic high against which the Needles Range Formation and younger rocks pinched out (Anderson and Rowley, 1975; Grant, 1979). Mercury prospects occur near 

the pluton contacts (Doelling, 1975).

Farther south in the High Plateaus, a pre-Needles 

Range mudflow breccia and a local 31.1-m.y. (Fleck and 

others, 1975) silicic crystal-poor ash-flow tuff, totalling 

as much as 50 m in thickness, occur in the northern 

Markagunt Plateau and southernmost Sevier Plateau. 

No source for these rocks has been identified.

In the southern Mineral Mountains, dacitic to andesitic volcanic mudflow breecia and lava flows at least 100 

m thick define a local pre- Needles Range volcanic complex, which pinches out southward in the northern 

Black Mountains. Elsewhere in the Black Mountains, 

Needles Range Formation rests on Claron Formation 

except in a few scattered places where Rowley (1976, 

1978, and unpub. data, 1978) mapped a regional ashflow tuff generally less than 15 m thick; this tuff tentatively is correlated with the Indian Peak unit 2 of Conrad (1969).

Farther west, Stringham (1963) noted pre-Needles 

Range volcanic rocks in the hills south of the San Francisco Mountains, and Lemmon, Silberman, and Kistler 

(1973, analyses 11-13) mapped 32.5- to 33.6-m.y.-old 

rhyodacite to andesite lava flows in the central Wah 

Wah Mountains that probably represent part of an old 

stratovolcano. These rocks reach thicknesses greater 

than 400 m (Hintze, 1974c). In the central and northern 

Wah Wah Mountains, Burbank Hills, southern Confusion Range, and southern House Range, Bushman 

(1973) and Hintze (1974a, b) mapped the Tunnel Spring 

Tuff, a widespread pre-Needles Range silicic ash-flow 

tuff. The Tunnel Spring Tuff, herein adopted, was defined by Bushman (1973), who described a type section 

in the Crystal Peak area east of the Tunnel Spring 

Mountains, which in turn are just east of the Burbank 

Hills. The tuff is as much as 570 m thick and locally is 

overlain by conglomerate as much as 100 m thick. 

Elsewhere in the southern Wah Wah Mountains, preNeedles Range volcanic rocks are locally more than 700 

m thick in paleovalleys (Campbell, 1979; M. G. Best, 

written commun., 1979). In the southern Mountain 

Home Range, volcanic rocks under the Needles Range 

Formation are as much as 150 m thick (Campbell, 

1979), whereas in the northern part of the range they 

are 60-90 m thick (Best, 1976).

NEEDLES RANGE FORMATION

The Needles Range Formation was first recognized and 

defined by Mackin (1960). It is an extremely widespread 

series of ash-flow sheets covering at least 39,000 

km2 according to Shuey, Caskey, and Best (1976). The 

formation consists of lithologically similar crystal-rich 

ash-flow tuffs that individually may be more than 500 

m thick near their cauldron sources, but in most places 

the tuffs are less than 100 m thick. Mackin (1960) recognized two regional members, a lower Wah Wah 

Springs Tuff Member and an upper Minersville Tuff 

Member. On the basis of much more work, Best and 

others (1973) defined four regional members, which 

from bottom to top are the Cotton wood Wash Tuff 

Member, the Wah Wah Springs Tuff Member of Mackin, the Lund Tuff Member (generally equivalent to the 

abandoned Minersville Tuff Member of Mackin), and 

the Wallaces Peak Tuff Member. The formation was 

derived 30-29 m.y. ago (Armstrong, 1970; Fleck and 

others, 1975) from sources believed to be in the southern 

Mountain Home Range (M. G. Best, oral commun., 

1978) and from the Mount Wilson volcanic center in the 

Wilson Creek Range of eastern Nevada (Ekren and 

others, 1977).

Remnants of the formation have been found from the 

Fish Lake and Awapa Plateaus on the east to as far as
STRATIGRAPHY

east-central Nevada on the west (Mackin, 1963; Cook, 

1965; Anderson and Rowley, 1975; Shuey and others, 

1976). Isolated exposures of Needles Range rocks occur 

in the Confusion Range about 15 km north of the area of 

figure 2 and possibly also in the Little Drum Mountains 

more than 40 km north of the area of figure 2 (Best and 

others, 1973; Pierce, 1974; Shuey and others, 1976). The 

wide distribution of this formation indicates that most 

of southwestern Utah was a broad plain of low relief 

that was studded by scattered earlier stratovolcanoes at 

the time of deposition of Needles Range Formation. 

Locally, however, the tuffs filled paleovalleys and 

pinched out against highlands. In the Mountain Home 

Range and nearby areas, these highlands consist of 

Paleozoic rocks (Conrad, 1969; Bushman, 1973), 

perhaps relics from the Sevier orogeny. To the southeast, the formation was not deposited in the Iron 

Springs area, perhaps due to a preexisting highland; 

and on a more local scale, it pinched out against the 

dome created by the Spry intrusion and against local 

preexisting stratovolcanoes in the Marysvale volcanic 

field and elsewhere.

LATE (POST-NEEDLES RANGE FORMATION) CALC-ALKALIC 

VOLCANIC ROCKS

The post-Needles Range Formation calc-alkalic 

volcanic rocks in southwestern Utah comprise two general facies that differ markedly in form and distribution. Central-vent volcanoes (stratovolcanoes, shield 

volcanoes, and volcanic domes) were concentrated in 

two generally east trending igneous belts, the PiocheMarysvale belt to the north and the Delamar-Iron 

Springs belt to the south (fig. 1). Where clustered, these 

volcanoes were surrounded by broad coalescing aprons 

of volcaniclastic debris. The low areas between the 

belts, and between scattered volcanoes within the belts, 

were covered by the second facies, which consists of thin 

but widespread regional sheets of ash-flow tuff. The two 

facies accumulated concurrently and are complexly intertongued in places.

The distribution of major central-vent volcanic fields 

within the Pioche-Marysvale and Delamar-Iron 

Springs igneous belts is known in a general way, but 

internal details within many of these fields are known 

only locally; and even here the coverage is quite uneven. 

The Marysvale volanic field is the best known local 

accumulation, but major problems remain to be solved. 

The discussion on central-vent volcanic fields will first 

be on the Marysvale field and then on other volcanic 

fields and will concentrate in those areas where our 

information is most complete and will give scant attention elsewhere. This treatment is not intended to indicate the relative importance of the different areas.

MARYSVALE VOLCANIC FIELD

The largest and best known volcanic accumulation 

in southwestern Utah is the Marysvale volcanic field, 

which is more than 100 km in diameter and covers much 

of the southern High Plateaus and parts of the adjacent 

Black Mountains and southern Mineral Mountains of 

the Basin and Range province. The pioneer regional 

work in the Marysvale area was done by Dutton (1880) 

and by Callaghan and his associates (Callaghan, 1938, 

1939, 1973; Callaghan and Parker, 1961a, 1961b, 

1962a, 1962b; Willard and Callaghan, 1962). More recent work has modified many of the conclusions from 

these and other earlier studies and has provided new 

information on the eastern (Williams and Hackman, 

1971), southern (Anderson and others, 1975), and central and northern parts (Steven and others, 1977,1979; 

Steven, Cunningham, and Rowley, 1978; Steven, Rowley, and Cunningham, 1978; Cunningham and Steven, 

1977, 1979a) of the field. Wender and Nash (1979) recently conducted a petrologic study of some samples 

from the Marysvale area.

The Marysvale field is dominated by voluminous 

rhyodacitic to andesitic stratovolcanoes and surrounding volcaniclastic aprons. These volcanoes can be divided into a vent facies, consisting of lava flows and flow 

breccia and subordinate mudflow breccia, which merges 

radially to an alluvial facies, consisting of mudflow 

breccia and subordinate conglomerate, sandstone, flow 

breccia, and lava flows. Some individual stratovolcanoes can be distinguished by distinctive lithology, but 

elsewhere adjacent volcanoes consist of identical rock 

types. Locally derived ash-flow tuffs interbed with the 

central-vent volcanic deposits in many places.

The northern part of the Marysvale volcanic field is 

dominated by a large pre-Needles Range Formation 

stratovolcano centered in the northern Tushar Mountains. The eastern flank of this volcano is excellently 

exposed in Marysvale Canyon, north of Marysvale. The 

sequences of younger volcanic rocks on the northern 

and southern flanks of this volcano are strikingly different, and precise correlations are difficult. All these 

rocks, at least 1,000 m thick, were included in the Bullion Canyon Volcanics or Dry Hollow Formation of Callaghan (1938). Steven and others (1977,1979), however, 

abandoned the name Dry Hollow; the rocks of the volcano are herein placed in the Bullion Canyon Volcanics.

The post-Needles Range Formation volcanic rocks on 

the northern flank of the older stratovolcano consist of a 

series of ash-flow tuff and tufflava units and viscous 

intermediate-composition lava flows that intertongue 

with a small, basaltic andesite shield volcano. One of 

the ash-flow tuff units, the 27-m.y.-old Three Creeks 

Tuff Member of the Bullion Canyon Volcanics, extends 

from its cauldron source in the southern Pavant Range
8 CENOZOIC STRATIGRAPHIC AND STRUCTURAL FRAMEWORK OF SOUTHWESTERN UTAH

around the western and southern flanks of the older 

stratovolcano and provides a stratigraphic tie between 

the northern and central parts of the Marysvale field. 

The whole northern flank assemblage is capped by the 

Osiris Tuff, a regional ash-flow sheet that was erupted 

about 22 m.y. ago. The Osiris Tuff will be discussed 

separately.

The northeastern and eastern flanks of the older 

stratovolcano are covered by a thick sequence of lava 

flows, volcanic breccia, and ash-flow tuff that forms 

much of the northern Sevier Plateau. These rocks are 

too poorly known to be discussed in detail. Farther 

south in the central Sevier Plateau, the post-Needles 

Range Formation volcanic rocks comprise two 

stratovolcano complexes, each at least 300 m thick; the 

older of these is andesitic (Little Table area) and the 

younger is dacitic (Langdon Mountain area) (Rowley, 

1979). These rocks intertongue southward with and 

overlie the Mount Button Formation, to be discussed 

later.

The southern flank of the older stratovolcano in the 

northern Tushar Mountains is covered by a mixed assemblage of porphyritic intermediate-composition volcanic mudflow breccia and lava flows of the Bullion 

Canyon Volcanics, which includes a wedge of the Three 

Creeks Tuff Member within it. The mudflow breccia 

appears to have been derived in part from the older 

stratovolcano and in part, along with the interlayered 

lava flows, from contemporaneous volcanic activity at 

several centers. These flank accumulations intertongue 

southward with the distinctive rocks of the Mount Button Formation.

The Mount Button Formation (Anderson and Rowley, 

1975) is an assemblage of crystal-poor dacitic to andesitic rocks erupted from many widespread stratovolcanoes in the southern Sevier Plateau, northern Markagunt Plateau, Black Mountains, and southern Mineral Mountains. Vent-facies lava flows and flow breccia 

in the centers of the volcanoes give way laterally to a 

great composite apron of volcaniclastic debris formed by 

coalescing alluvial facies from the many centers. The 

Mount Button Formation intertongues with the postNeedles Range Formation volcanics in the central 

Tushar Mountains (fig. 4). The Mount Button volcanic 

centers are concentrated along an east-trending belt 

(Blue Ribbon lineament, discussed later) that extends 

from the Circleville Canyon area westward between the 

Tushar Mountains and the Markagunt Plateau and 

along the northern margin of the Black Mountains. 

Most of the rest of the formation belongs to the alluvial 

facies, which forms aprons that are especially broad to 

the south, east, and west, where few older or concurrent 

volcanoes existed to interfere with the outflow. The 

Mount Button Formation was deposited after ash flows 

of the Needles Range Formation extended into the region; the upper part of the formation is as young as 21 

m.y. old.

In the western Black Mountains, a local rhyodacitic 

stratovolcano complex formed concurrently with some 

of the Mount Button eruptions, and its products, named 

the Horse Valley Formation (Anderson and Rowley, 

1975), intertongue with the upper part of the Mount 

Button Formation (Rowley, 1978). Elsewhere, other 

distinctive rock types are interlayered with the Mount 

Button Formation. These include the 25-m.y.-old 

Beaver Member (Anderson and Rowley, 1975; S. H. 

Becatur, unpub. data, 1978), a flow dome in the southwestern Tushar Mountains and eastern Black Mountains; the 25-m.y.-old Kingston Canyon Tuff Member, 

an ash-flow tuff in the central and southern Sevier 

Plateau; and the Antimony Tuff Member, an ash-flow 

tuff near the top of the formation in the central Sevier 

Plateau.

Southward, the spreading Mount Button alluvial 

apron encountered other locally derived accumulations 

in the northern Markagunt Plateau and southern 

Sevier Plateau. Continued eruptions from the Spry laccolith area and concurrent erosion produced the Buckskin Breccia and related conglomerate and mudflow 

breccia, which are overlain by the 24-m.y.-old Bear Valley Formation (J. J. Anderson, 1971), an eolian sandstone derived from the west and deposited in easttrending grabens in the northern Markagunt Plateau. 

Bepending on locality, the Mount Button alluvial facies 

underlies, is interlayered between, or overlies these 

local units.

About 22 m.y. ago, during the period of calc-alkalic 

volcanic eruptions, the widespread distinctive Osiris 

Tuff was erupted from a probable caldera source in the 

central Sevier Plateau. This ash-flow sheet forms an 

excellent marker horizon that has been mapped over 

most of the Marysvale volcanic field. Some of the older 

stratovolcanoes stood above the level of Osiris deposition, but all the lower flanks of these volcanoes and 

much of the outflow alluvial apron were covered by it. 

The tuff is very extensive to the east (Williams and 

Hackman, 1971), where it drained down off the slopes of 

the volcanoes and spread out on the eastern plain. 

Another, more local ash-flow tuff, the Belano Peak Tuff 

Member of the Bullion Canyon Volcanics, was erupted 

from a caldera source in the central Tushar Mountains; 

this unit also has been dated as 22 m.y. old, but it 

stratigraphically overlies the Osiris Tuff.

A flow dome complex in the Lousy Jim Creek area in 

the southwestern Tushar Mountains (J. M. Sigmund, 

unpub. data, 1978) also was erupted about 22 m.y. ago, 

but it overlies both the Osiris Tuff and the Belano Peak 

Tuff Member.

Quartz monzonite plutons were intruded into the 

northern part of the Marysvale volcanic field, largely in 

the area of the pre-Needles Range stratovolcano and its 

flanking younger volcanics. These stocks probably represent high-level cupolas from a major batholith that 

was emplaced about 23 m.y. ago, late during the period
STRATIGRAPHY

NORTH 

Pavant Range9

SOUTH

-Tushar Mountains Markagunt Plateau

50 KILOMETERS

FIGURE 4.—Schematic north-south cross section across the Marysvale volcanic field, from the Pavant Range to the northern Markagunt 

Plateau, showing the geology at about 21 m.y. ago. Vertical scale exaggerated; thickness of regional ash-flow tuffs (black) is especially 

exaggerated. Tl, rocks of lower Tertiary sedimentary sequence; TV, pre-Needles Range volcanic rocks; Tn, Needles Range Formation; Tbv, 

Bear Valley Formation; Tb, Bullion Canyon Volcanics, including Tbt, Three Creeks Tuff Member and its source cauldron, and Tbd, Delano 

Peak Tuff Member and its source caldera; Tm, Mount Button Formation; To, Osiris Tuff. The named rocks unconformably overlie Mesozoic 

and Paleozoic sedimentary rocks (not labeled in the figure).

of calc-alkalic volcanism. Although probably representing the source magma for the volcanic rocks, the plutons 

only locally were emplaced in the centers of older volcanoes. In several places the stocks are unconformably 

overlain by the Osiris Tuff, indicating local erosion 

prior to Osiris eruption. Except for the early Spry laccolith, no plutons are exposed in the southern part of the 

Marysvale volcanic field, where the Mount Button 

Formation is the predominant rock unit. The magma 

chambers for these little-differentiated volcanic rocks 

apparently were much deeper and did not aggregate 

into a rising batholithic body as did those to the north 

(T. A. Steven, unpub. data, 1978).

The northern part of the Marysvale field contains 

significant amounts of hydrothermally altered and 

mineralized rock. Mineralization involved introduction 

largely of chalcophile elements during the plutonic 

episode 23 m.y. ago. Base and precious metals have been 

mined in the area for years. Because discussion of the 

deposits is outside the scope of this report, the reader is 

referred to the reports of Callaghan (1973); Steven and 

others (1977, 1979); Steven, Cunningham, and Rowley 

(1978); Steven, Rowley, and Cunningham (1978); Steven and Cunningham (1979a), Cunningham and Steven (1978a, 1979b), and Cunningham, Steven, and 

Naeser (1978).

OTHER VOLCANIC FIELDS IN SOUTHWESTERN UTAH

Post-Needles Range Formation calc-alkalic volcanic rocks occur in many other places in southwestern 

Utah. Most of these accumulations are poorly known. 

Some may have been large, complex fields, but moft 

appear to have been small, and all seem to have been 

more deeply eroded than the Marysvale field.

In the Pioche-Marysvale igneous belt just west of the 

Marysvale field, the central and northern Mineral 

Mountains contain the largest batholith (nearly 250 

km2) exposed in Utah. This batholith is mostly mediumto coarse-grained quartz monzonite and granite (Condie, 1960). It has yielded K-Ar ages of 15 to 9 m.y., but 

Rb-Sr data from 11 analyses of whole-rock samples by 

C. E. Hedge, U.S. Geological Survey (written commun., 

1976), show wide scatter, and a poorly controlled isochron suggests an age for the main batholith of about 

35 m.y. The data indicate that sizeable chemical modification, especially Sr loss, occurred at 15 to 7 m.y. ago 

(Lipman and others, 1978). Recent geologic mapping by 

Nielson and others (1978) reveals that the batholith is a 

composite of numerous intrusive phases; the discordant 

age data thus could have resulted from older plutons 

being significantly modified by the emplacement of 

younger ones. The young ages suggest that at least 

some of the plutons in the Mineral Mountains batholith 

were intruded during bimodal igneous activity in late 

Cenozoic time, to be discussed later. Rhyodacitic lava 

flows and mudflow breccia, possibly related to phases of 

the batholith, are exposed in scattered places in the 

southern Mineral Mountains and northern Black 

Mountains; these rocks occur in areas that are structurally low with respect to the highly uplifted main part of 

the range. Any other volcanic rocks that may have been 

related to the batholith have been eroded or are buried 

in the grabens east and west of the range. Several small 

mining districts in the southern and eastern Mineral 

Mountains have produced lead, silver, zinc, tungsten, 

and gold from or above the contact zone of the batholith 

and related stocks.

A major volcanic field, having a diameter of more 

than 40 km, is exposed in the southern San Francisco
10 CENOZOIC STRATIGRAPHIC AND STRUCTURAL FRAMEWORK OF SOUTHWESTERN UTAH

Mountains and adjacent areas to the south. East (1966) 

estimated that the volcanic rocks in the San Francisco 

Mountains are about 1,500 m thick and include tuff as 

well as andesitic to rhyolitic lava flows. Although 

Lemmon, Silberman, and Kistler (1973) have 

documented pre-Needles Range Formation volcanic 

rocks in this area, most of the rocks appear to overlie the 

Needles Range Formation (Erickson, 1973). Stringham 

(1963,1964) noted basaltic to latitic lava flows of undetermined thickness that apparently overlie the Leach 

Canyon Formation (24 m.y. old) in the area south of the 

San Francisco Mountains. Dacitic lava flows, ash-flow 

tuff, and volcanic mudflow breccia still farther south, in 

the southern Shauntie Hills, were estimated by Rowley 

(1978) to be at least 500 m thick and to represent the 

southern part of a more extensive stratovolcano complex. The San Francisco Mountains and adjacent area 

have been deeply eroded, and quartz monzonite stocks 

are widely exposed. K-Ar ages of these stocks and of 

ash-flow tuffs in this area (Lemmon and others, 1973) 

range from about 28 m.y. to 21 m.y. The San Francisco 

mining district and other smaller districts in the volcanic field have produced major amounts of silver, copper, gold, lead, zinc, and subordinate values in other 

metals (Butler, 1913; Hintze and Whelan, 1973; Shawe 

and others, 1978). Copper deposits of probable porphyry 

type, which deserve more detailed exploration, include 

those at the O.K. and Cactus mines and perhaps those at 

the Comet and Hickory mines. The San Francisco volcanic field, like the Marysvale field, began forming 

prior to Needles Range eruptions, and igneous activity 

continued at least until 21 m.y. ago.

The southern Wah Wah Mountains are underlain 

by volcanic rocks (Taylor and Powers, 1953; Miller, 

1966) that lie adjacent to, and perhaps are an extension 

of the San Francisco field. The map pattern (Steven, 

Rowley, and others, 1978) indicates that most of these 

deposits postdate the Needles Range Formation. The 

rocks here and just to the north occur in the Pine Grove 

mining district. This area has been extensively altered 

to alunite (NG deposit of Parkinson, 1974; Hall, 1978) 

and mineralized with fluorspar and uranium (Whelan, 

1965; Lindsey and Osmonson, 1978). A pluton containing a molybdenum porphyry ore deposit underlies the 

district (Wall Street Journal, January 8, 1978; Shawe 

and others, 1978).

The southern Mountain Home Range to the west 

not only contains abundant pre-Needles Range volcanic 

rocks as described above, but also a thick younger, apparently post-Needles Range stratovolcano complex 

north of Modena. The Indian Peak mining district in the 

southern Mountain Home Range has produced fluorspar from Needles Range Formation and other volcanic 

rocks that were intruded by quartz diorite and 

granodiorite stocks (Thurston and others, 1954; Bullock, 1976). Thick volcanic rocks in the Stateline district, which formerly produced gold and silver from 

veins, include rhyolite and overlie a unit whose description resembles that of Needles Range Formation (Butler and others, 1920; Thomson and Perry, 1975).

Volcanic accumulations and related plutons also 

occur along the Delamar-Iron Springs igneous belt (fig. 

1). At the eastern end of the belt, 20-m.y.-old quartz 

monzonite porphyry laccoliths in the Iron Springs district (Mackin, 1960, 1968; Rowley and Barker, 1978) 

and Pine Valley Mountains (Cook, 1957) were emplaced 

in Mesozoic and Cenozoic sedimentary strata. Three of 

the laccoliths in the Iron Springs district were the 

source of major hematite and magnetite replacement 

orebodies in the adjacent Carmel Formation (Jurassic). 

Thin layers of dacitic to andesitic mudflow breccia, 

either derived locally or coming from centers in the Pine 

Valley Mountains to the south, are interlayered at several horizons within an overlying sequence of ash-flow 

tuffs. In the Pine Valley Mountains, locally derived 

sequences of latitic and andesitic lava flows, ash-flow 

tuff, and volcanic mudflow breccia, at least 1,400 m 

thick, have been placed in several named formations by 

Cook (1957).

The geology of the eastern Bull Valley Mountains is 

known through the mapping of Blank (1959), Blank and 

McKee (1969), Cook (1960a), McCarthy (1959), and 

Wiley (1963); but the western part, extending to the 

Nevada border, is poorly known and is shown by Cook 

(1960a, maps 1, 2, and 8) only as undifferentiated Tertiary volcanic rocks. The eastern Bull Valley Mountains contain numerous eruptive centers, the oldest of 

which produced andesitic lava flows and flow breccia, as 

much as 400 m thick, that lie between ash-flow tuffs 

dated as 22 to 21 m.y. old (Blank, 1959). Blank showed 

that ash-flow tuffs and lava flows of the overlying Rencher Formation (discussed in next section) came from an 

eruptive complex whose core now is marked by several 

quartz monzonite porphyry plutons, including the 

Bull Valley intrusion. These intrusions have been prospected for iron ore, and in many aspects they resemble 

intrusions of the Iron Springs district (Wells, 1938; 

Tobey, 1976). The Mineral Mountain pluton, farther 

west in the Bull Valley Mountains, may be related to 

this same pluton belt (Cook, 1960a). Above the Rencher 

Formation in the Bull Valley Mountains, locally derived andesitic volcanic breccia and lava flows, as much 

as 180 m thick, are overlain by regional and locally 

derived ash-flow tuff, airfall tuff, basaltic lava flows, 

and volcanic sedimentary rocks of the 700-m-thick Cove 

Mountain Formation of Cook (1960a) and Blank (1959).

ASH-FLOW TUFF PROVINCE OF SOUTHWESTERN UTAH

From about 26 m.y. ago to about 19 m.y. ago or less, 

much of southwestern Utah was a broad plain covered
STRATIGRAPHY 11

by thin regional silicic calc-alkalic ash-flow sheets derived from sources in the Basin and Range province. 

This plain was surmounted by contemporaneous 

intermediate-composition stratovolcanoes that were 

concentrated along the east-northeast-trending 

Pioche-Marysvale and the Delamar-Iron Springs igneous belts. The sheets intertongued with and pinched out 

against thick lava flows and volcanic breccia of the 

stratovolcanoes. None of the regional sheets was as 

extensive as the Needles Range Formation, which 

formed a flat base over which most of the younger sheets 

spread. Mackin (1960) was the first to recognize the 

pyroclastic origin of these sheets, and he applied most of 

the original names to them, used later in this section. 

Their outcrop areas are shown by Cook (1965), Williams 

(1967), and Rowley, Anderson, and others (1978a). 

Mackin (1960), Anderson and Rowley (1975), and Rowley and Barker (1978) described the tuffs; and Armstrong (1970), Fleck, Anderson, and Rowley (1975), and 

E. H. McKee (in Hausel and Nash, 1977, fig. 2) determined their radiometric ages. The following discussion 

summarizes data from all these sources.

The Isom Formation, about 26—25 m.y. old, consists 

of densely welded crystal-poor ash-flow tuff, lava flows, 

and tuff lava that commonly show secondary flow 

characteristics. The formation generally does not exceed 20 m in thickness except in the western Iron 

Springs district, where the formation locally is more 

than 250 m thick. The source of the formation may be 

located in the western part of the district or the adjacent 

Escalante Desert. In most places the Isom Formation 

rests on the Needles Range Formation. From base to top 

the Isom consists of the Blue Meadows, Baldhills, and 

Hole-In-The-Wall Tuff Members; the Baldhills Member 

is the most widespread.

The Quichapa Group (Williams, 1967; Anderson and 

Rowley, 1975) overlies the Isom Formation and consists, from base to top, of the Leach Canyon Formation, 

Condor Canyon Formation, and Harmony Hills Tuff. 

The Leach Canyon Formation consists mostly of 

crystal-poor ash-flow tuff rich in lithic and cognate 

fragments that thickens westward to more than 250 m 

in its possible source area near Caliente, Nev. (Williams, 1967). It consists of the widespread Narrows Tuff 

Member, about 24 m.y. old, and the overlying less widespread Table Butte Tuff Member. The Condor Canyon 

Formation consists of two crystal-poor ash-flow tuffs, 

the Swett Tuff Member (about 23 m.y. old) and the 

overlying Bauers Tuff Member (about 22 m.y. old). The 

Condor Canyon Formation units thicken westward to a 

combined total of more than 150 m near Caliente, their 

possible source area. The Harmony Hills Tuff, about 21 

m.y. old, consists of crystal-rich ash-flow tuff that thickens to about 150 m in the eastern Bull Valley Mountains. Blank (1959) suggested that its source area was 

in the eastern Bull Valley Mountains, but more recent

work indicates that it is of similar or greater thickness 

in the southern Clover Mountains of Nevada and may 

have its source in the southern part of the Caliente 

cauldron complex (Noble and others, 1968; Noble and 

McKee, 1972; Ekren and others, 1977).

Two other regional ash-flow tuffs occur stratigraphically above the Harmony Hills Tuff in the Iron Springs 

district, Pine Valley Mountains, Bull Valley Mountains, and southeastern Nevada. The Rencher Formation, herein adopted, was named by Cook (1957) for an 

ash-flow tuff and related lava flows about 180 m thick in 

the northwestern Pine Valley Mountains; Blank (1959) 

found that the formation is as much as 300 m thick in 

the eastern Bull Valley Mountains and, as noted above, 

that this was the source area. The overlying Page Ranch 

Formation was defined by Cook (1957) for exposures 

northwest of Page Ranch, just south of the Iron Springs 

district. Cook included within the formation a silicic, 

crystal-poor ash-flow tuff of significant areal extent, 

which Mackin (1960) called the Kane Point Tuff 

Member. This tuff, with an isotopic age of 19 m.y. and a 

thickness of as much as 180 m, overlies fanglomerate in 

its type area (Mackin, 1960) and intertongues with locally derived volcanic rocks farther to the south (Cook, 

1957). Blank (1959) and Cook (1960a) correlated the 

Kane Point Member with Blank's Racer Canyon Tuff 

Member of the Cove Mountain Formation. The type 

area of the latter member is Racer Canyon in the northeastern Bull Valley Mountains. In later reports the 

name Racer Canyon Tuff has been used (Noble and 

others, 1968; Noble and McKee, 1972; Ekren and 

others, 1977) at the formation rank, and it has received 

K-Ar ages of 18.2 and 20.3 m.y. (Noble and McKee, 

1972). It is herein adopted at the formation rank as 

Racer Canyon Tuff. Kane Point Tuff Member is herein 

abandoned in favor of the Racer Canyon Tuff, and Page 

Ranch Formation is herein adopted but restricted to the 

rocks below the Racer Canyon Tuff in the Page Ranch 

type area. The source of the Racer Canyon Tuff probably 

is the Caliente cauldron complex (Noble and others, 

1968; Noble and McKee, 1972; Ekren and others, 1977). 

Other unnamed local ash-flow sheets exist in southwestern Utah, but their distributions and sources are 

poorly known.

UPPER CENOZOIC SEDIMENTARY 

AND VOLCANIC SEQUENCE

Extensional tectonism beginning about 20 m.y. 

ago, affected the Basin and Range and High Plateaus 

areas throughout later Cenozoic time. Warping and 

normal faulting divided the areas into upthrown and 

downthrown blocks, such that the higher areas were 

subject to erosion and the lower areas became sites of 

sedimentation. Early-formed sedimentary rocks were 

deformed by the continuing tectonism.
12 CENOZOIC STRATIGRAPHIC AND STRUCTURAL FRAMEWORK OF SOUTHWESTERN UTAH

Episodic volcanic activity accompanied the deformation and sedimentation, and a bimodal assemblage of 

basalt lava flows and high-silica alkali rhyolite lava 

flows, domes, and pyroclastic rocks was erupted from 

about 21 m.y. ago to Quaternary time. The basalt flows 

are very widespread, although generally of low volume. 

The rhyolitic rocks formed around more restricted centers and range in volume from minor flows to major 

composite accumulations.

BASIN-FILL SEDIMENTARY ROCKS

The grabens and other structural valleys in most 

parts of southwestern Utah contain well-consolidated to 

poorly consolidated continental sediments of late 

Cenozoic age. The sediments are largely coarse- to 

fine-grained clastic fluvial rocks, but lacustrine rocks 

and airfall tuffs occur locally and are especially prominent in some valleys.

The High Plateaus subprovince has had similar 

geomorphic history throughout its extent. Drainage 

within this area is fairly well integrated, and erosion 

has cut deeply into the basin-fill sediments in many 

places; thus a composite outline of late Cenozoic history 

can be pieced together. The sediments, at least 350 m 

thick in some basins, were named Sevier River Formation by Callaghan (1938), who recognized that these 

rocks were derived from erosion of upthrown blocks and 

deposited in downthrown areas during late Cenozoic 

block faulting. Anderson and Rowley (1975) described 

the lithology and distribution of these rocks in the 

southern High Plateaus and suggested that the name 

Sevier River Formation be used throughout the province in preference to local terms such as Parunuweap 

Formation, a name that was abandoned by Anderson 

and Rowley (1975).

The age of the Sevier River Formation varies from 

basin to basin depending on the local structural history. 

The Sevier River Formation overlies the Joe Lott Tuff 

Member (19 m.y.) of the Mount Belknap Volcanics 

along the northern side of the Tushar Mountains, where 

Steven and others (1977, 1979) obtained fission-track 

ages on two tuff beds in the formation: 13.8 m.y. for one 

bed near the base and 6.9 m.y. for one near the eroded 

top of the formation. South of Marysvale the formation 

is overlain by a basalt lava flow that was dated by the 

K-Ar method (Damon, 1969) as 12.6 m.y. old. We believe that elsewhere the Sevier River Formation may be 

locally as old as 20 m.y. and as young as early Pleistocene.

In the Basin and Range province, the grabens are 

much more extensive than the horsts, and each appears 

to have had a different structural and depositional history. The present drainage system appears to have 

formed very recently, probably in Pleistocene time.

Downcutting is much less than in the High Plateaus, 

and the deposits, which locally are 1,000 m or more 

thick, are largely covered by veneers of upper Quaternary sediments. The scattered exposures of basin-fill 

sediments in southwestern Utah now are given numerous informal names and locally have been correlated 

with the Sevier River Formation or Muddy Creek Formation of Nevada. Although they commonly resemble 

those of the Sevier River Formation, the different histories of each basin combined with the presently fragmentary character of our data make it inadvisable to 

extend the term Sevier River Formation throughout 

southwestern Utah. We prefer to treat all these deposits 

in the Basin and Range province informally as basin-fill 

deposits.

In the Basin and Range province, the lower parts of 

the basin-fill deposits are exposed only where their lateral edges lap up against the ranges. Along one such 

outcrop in the southern Black Mountains (Rowley, 

1975, 1976), basaltic lava flows that intertongue with 

the lower part of a basin-fill deposit have been dated 

(K-Ar) as 8.8 and 9.6 m.y. old (Anderson and Mehnert, 

1979). Near Cove Fort, west of the southern Pavant 

Range, Best, McKee, and Damon (1979) have given ages 

of 7.4 and 9.2 m.y. for basalts interbedded with lower 

parts of the basin-fill deposits. The base of the deposits 

in deeper basins cannot be dated, but in some places 

they probably are much older, possibly as much as 20 

m.y.

The upper parts of the basin-fill deposits in the Basin 

and Range province can locally be demonstrated to be 

quite young. In the basin north of Beaver, where the 

upper part of the basin fill is well exposed (Anderson 

and others, 1978), numerous tuff beds have been demonstrated to be latest Pliocene and Pleistocene in age 

(unpub. data supplied by R. E. Anderson, M. M. 

Machette, G. A. Izett, and H. H. Sturr, 1978, and used 

with permission).

BIMODAL VOLCANIC ROCKS

Upper Cenozonic basalt and alkali rhyolite cones, 

lava flows, domes, and tuff beds are more widespread in 

southwestern Utah then elsewhere in the State. They 

do not, however, approach the volumes of the older 

calc-alkalic volcanic rocks previously discussed. The 

bimodal rocks were erupted concurrently with deposition of the Sevier River Formation and other basin-fill 

deposits, and the two assemblages are interlayered in 

many places. The basalt commonly forms low-volume 

flows that are widely distributed. Only in the Black 

Rock Desert area west of Cove Fort do they compose 

sizeable volcanoes. The rhyolitic accumulations, on the 

other hand, tend to be more localized around restricted 

source areas, where they range from small local piles to
STRATIGRAPHY 13

large composite accumulations of flow and pyroclastic 

material. Bimodal rocks are important economically, 

for they localize geothermal resources and sulfur deposits. Metallic mineral deposits, generally of lithophile 

elements such as uranium, molybdenum, beryllium, 

fluorine, and tungsten, are commonly found in or near 

the rhyolites.

Data pertaining to lithology, chemistry, ages, and 

distribution of basalt in southwestern Utah have been 

discussed by numerous workers, including Leeman and 

Rogers (1970), Best and Brimhall (1970, 1974), Condie 

and Barsky (1972), Hoover (1974), Anderson and Rowley (1975), Stewart, Moore, and Zietz (1977), Clark 

(1977), and Best, McKee, and Damon (1979). Many, but 

far from all, basalts occur along the Basin and 

Range—Colorado Plateaus province boundary (Best and 

others, 1979). Most basalts in southwestern Utah are 

less than 7 m.y. old, but in addition to older ages already 

mentioned, Best, McKee, and Damon (1979) have dated 

10 more basalts in Utah, the oldest of which is a 

13.3-m.y.-old flow west of Milford. Basalts less than 2 

m.y. old are concentrated in the Black Rock Desert and 

in a belt lying between St. George, Cedar City, and 

Panguitch.

Olivine-bearing lava flows that in some places have 

identical field appearances to basalts have been dated 

stratigraphically at between 22 and 19 m.y. and by 

K-Ar methods at 21.1 m.y. (Best and others, 1979) in the 

Piute Reservoir area of the central Sevier Plateau (Rowley and others, 1979). Chemical analyses of the flow 

dated by K-Ar methods indicates, however, that it is not 

a true basalt (Wender and Nash, 1979; Best and others, 

1979). These flows, at least some of which exhibit calcalkalic affinities, may mark the earliest products of the 

late Cenozoic basaltic volcanism.

Alkali rhyolite centers are most abundant along the 

Pioche-Marysvale igneous belt, where they occur discontinuously from near the Nevada-Utah line on the 

west to the vicinity of Marysvale on the east. The 

largest accumulation along this trend is the Mount 

Belknap Volcanics on the top of the Marysvale volcanic 

field (Steven and others, 1977, 1979; Cunningham and 

Steven, 1977, 1979a). The Mount Belknap Volcanics 

consists of a complex assemblage of lava flows, volcanic 

domes, and associated breccias and ash-flow tuffs that 

were erupted from two concurrently active source areas 

21-16 m.y. ago. Activity began near the northeastern 

end of the eastern source area, in the Antelope Range, 

about 21 m.y. ago with extrusion of a series of flow 

domes. Eruptive centers migrated southwestward over 

the next 4 m.y., leaving behind a complex of flows, 

domes, and ash-flow tuffs underlain by intrusive feeders. The largest ash-flow tuff unit was erupted about 19 

m.y. ago from the middle of the eastern source area, and

the small Red Hills caldera subsided at the vent area 

(Cunningham and Steven, 1977, 1979a). A stock of 

fine-grained granite, related to the Mount Belknap Volcanics, was intruded about 20 m.y. ago into an older 

calc-alkalinc quartz monzonite pluton in the southern 

Antelope Range (Cunningham and Steven, 1978a). At 

the western source area, on the crest of the Tushar 

Mountains, the major Mount Belknap caldera subsided 

about 19 m.y. ago in response to eruption of the Joe Lott 

Tuff Member of the Mount Belknap Volcanics. The 

caldera was filled almost immediately by comagmatic 

lava flows and ash-flow tuffs. The products of the eastern and western source areas complexly intertongue in 

the area between the sources.

Uranium-bearing veins, formed sometime between 18 

and 10 m.y. ago (Cunningham and Steven, 

1978a, 1979a), have been mined periodically in the Central Mining Area of the Antelope Range since 1949 

(Kerr and others, 1957; Kerr, 1968). A local stock, possibly mineralized with molybdenum, has been postulated to underlie the district (Cunningham and Steven, 1978a). Alunite of similar age also has been mined 

in the Marysvale area. Deposits of uranium, molybdenum, and related elements may be related to the 

Mount Belknap caldera (Cunningham and Steven, 

1979b). A 14-m.y.-old stock, possibly mineralized, is 

believed to underlie Alunite Ridge in the east-central 

Tushar Mountains (Cunningham and others, 1978). 

The Deer Trail mine along the eastern base of the 

Tushar Mountains has long produced lead-zinc-copper 

ore from a zone marginal to the postulated stock (Cunningham and others, 1978).

Elsewhere in the Pioche-Marysvale igneous belt, a 

string of rhyolite centers occurs along the east-trending 

Blue Ribbon lineament (Rowley, Lipman, and others, 

1978) from the southern Mountain Home Range eastward to the Sevier Plateau. Five isotopic ages (Mehnert 

and others, 1978) indicate that the centers are younger 

toward the east, from a 20.2-m.y.-old rhyolite at the 

Staats mine, southern Wah Wah Mountains (Lindsey 

and Osmonson, 1978), to a 4.7-m.y.-old center at Phonolite Hill, southern Sevier Plateau (H. H. Mehnert, written commun., 1978). Rhyolite also occurs on the lineament in the southern Mountain Home Range (Thurston 

and others, 1954), not far from fluorspar deposits of the 

Indian Peak mining district, as well as in the Shauntie 

Hills to the east and at several places in the northern 

Black Mountains (Rowley, Lipman, and others, 1978).

In other parts of the Pioche-Marysvale belt, the 0.8- to 

0.5-m.y.-old rhyolite of the Mineral Mountains (Lipman 

and others, 1978) unconformably overlies the Mineral 

Mountains batholith and seems genetically related to 

the nearby Roosevelt Hot Springs area. Upper Cenozoic
14 CENOZOIC STRATIGRAPfflC AND STRUCTURAL FRAMEWORK OF SOUTHWESTERN UTAH

rhyolite centers also occur west and north of the Mineral Mountains (Mehnert and others, 1978) and east of 

the Mineral Mountains (Haugh, 1978). Rhyolite also is 

exposed at several places in the hills south of the San 

Francisco Mountains (P. L. Williams, oral commun., 

1976), in the Stateline mining district (Butler and 

others, 1920), and in parts of the southern Mountain 

Home Range (D. A. Lindsey and D. R. Shawe, oral 

commun., 1975).

Rhyolitic rocks also occur in the Delamar-Iron 

Springs igneous belt. Cook (1957) described a small 

rhyodacitic lava flow of Pleistocene(?) age, which he 

called the Eight Mile Dacite, in the northeastern Pine 

Valley Mountains; Hausel and Nash (1977) gave a 

chemical analysis of this rock. Blank (1959) mapped a 

rhyolitic ash- flow sheet as much as 120 m thick, the Ox 

Valley Tuff, throughout the Bull Valley Mountains. It 

was named for exposures overlooking Ox Valley in the 

northeastern Bull Valley Mountains, and the name was 

formalized by Cook (1960a) and Noble and McKee 

(1972). This tuff, also herein adopted, has received K-Ar 

ages of 12.3 and 15.1 m.y., and probably was derived 

from the Caliente cauldron complex (Noble and McKee, 

1972; Ekren and others, 1977). The tuff is overlain by 

rhyolite and rhyodacite lava flows, as much as 370 m 

thick, that may be derived from the Flattop Mountain 

area of the northeastern Bull Valley Mountains (Blank, 

1959). Obsidian clasts occur in stream gravels adjacent 

to Modena (Umshler, 1975), but bedrock sources, presumably not far away, have not been discovered. Just 

west, in Nevada, abundant young rhyolites have been 

mapped by Stewart and Carlson (1974) and Ekren and 

others (1977).

STRUCTURAL GEOLOGY

Cenozoic rocks in southwestern Utah accumulated 

during three successive structural regimes that generally correspond to the three rock sequences. Thrust 

faulting and folding during the Sevier orogeny in Late 

Cretaceous time and warping during the Laramide 

orogeny in latest Cretaceous and early Tertiary time 

left a topographic legacy of mountains to the west bordered by basins to the east. Calc-alkalic igneous activity 

in middle Tertiary time was concentrated along eastnortheast-trending belts, probably structurally controlled, within which more local east-trending lineaments 

have been recognized; some early high-angle faulting 

took place concurrently and may have marked the beginning of Basin and Range-Colorado Plateaus differentiation. The late Cenozoic was the time of regional 

block faulting, with continued igneous activity, partly 

along the east-trending lineaments.

OLDER STRUCTURES

Eastward-directed thrust faults and related folds of 

the Sevier orogeny (Armstrong, 1968) have affected the 

pre-Cenozoic rocks of the Basin and Range province and 

the western High Plateaus area of southwestern Utah 

(Burchfiel and Hickcox, 1972; Crosby, 1973). The 

thrusts form a sinuous belt whose eastern edge (fig. 1) 

extends northwest across the Basin and Range province 

of southwestern Utah to intersect the High Plateaus 

north of Cove Fort. Northeast- or north-northeaststriking overturned and open folds, locally thrust 

faulted, form a parallel belt to the southeast, where they 

mark the leading edge of the Sevier disturbed belt. One 

such fold, extending from the eastern Bull Valley 

Mountains through Iron Springs and the Red Hills, 

later localized at least a half dozen iron-bearing plutons 

(Mackin, 1960; Threet, 1963b; Tobey, 1976). Other related folds include the Virgin anticline—Kanarra fold, 

which extends from east of St. George north past Cedar 

City (Threet, 1963a; Averitt, 1962; Kurie, 1966), and 

minor warps in the High Plateaus farther east (Stokes 

and Heylmun, 1963, fig. 2).

Armstrong (1968) considered the Sevier orogeny to be 

mostly Cretaceous, but in the eastern Bull Valley 

Mountains (Wiley, 1963), related thrusting and folding 

may have extended into early Tertiary time, and in the 

Pavant Range and areas to the north, folding and local 

thrusting continued into Paleocene time (Stanley and 

Collinson, 1979). The highlands resulting from the 

Sevier orogeny shed much clastic material into adjacent 

basins to form the uppermost Cretaceous(?) and lower 

Tertiary sedimentary sequence of southwestern Utah. 

Highlands were most pronounced along the western 

edge of the State, and here they apparently have never 

been covered by a significant thickness of lower Tertiary sedimentary rocks.

The Colorado Plateaus area of southwestern Utah 

was locally warped in Late Cretaceous and early Tertiary time, and we ascribe this deformation to the 

Laramide orogeny. The highlands that formed shed a 

significant amount of the clastic sediment of the lower 

Tertiary sedimentary sequence; some of this sediment 

may have come from the Arizona part of the Colorado 

Plateaus province (Young and McKee, 1978). The Circle 

Cliffs upwarp, southeast of the High Plateaus, may 

have remained a highland until eruption of the Needles 

Range Formation (Rowley, 1968). The Laramide 

orogeny may also have created the two basins that contain the Claron Formation and the North Horn through 

Crazy Hollow Formations, along with the intervening 

Tushar highland, discussed earlier.
STRUCTURAL GEOLOGY 15

EAST-TRENDING FEATURES

The Pioche-Marysvale and Delamar-Iron Springs 

igneous belts extend east-northeast across southern 

Utah and probably are structurally controlled. The 

coincidence of mineral deposits within these belts has 

been recognized since the time of Butler and others 

(1920), and it has been described in greater detail by 

Hilpert and Roberts (1964), Stacey, Zartman, and 

NKomo (1968), Shawe and Stewart (1976), and Rowley 

and others (1978a). These belts contain most of the 

known volcanic centers, plutons, hot springs, igneousrelated mineralized rocks, and hydrothermally altered 

rocks in southwestern Utah; they also contain virtually 

all the known east-striking faults. Stewart, Moore, and 

Zietz (1977) noted that the calc-alkalic rocks in igneous 

belts of western Utah are younger from north to south. 

Igneous rocks of the Pioche-Marysvale igneous belt 

seem to be generally older than those of the DelamarIron Springs belt, tending further to support this 

hypothesis.

Several east-trending lineaments (fig. 1), defined by 

an alinement of topographic features, faults, magnetic 

anomalies, volcanic centers, plutons, hot springs, and 

mineralized and hydrothermally altered rocks, have 

been identified within the igneous belts. Ekren and 

others (1976) defined the Timpahute lineament from 

features that extend from west of the Tempiute mining 

district in southeastern Nevada eastward to just east of 

the State line. It may extend along the northern side of 

the Delamar-Iron Springs igneous belt as far east as 

Cedar City.

Rowley, Lipman, and others (1978) defined the Blue 

Ribbon lineament as a feature about 25 km wide and 

360 km long that extends from the central Sevier 

Plateau westward into Nevada, where it connects with 

the 230-km-long Warm Springs lineament of Ekren and 

others (1976). The Blue Ribbon lineament crosses the 

Pioche-Marysvale igneous belt at a low angle; it begins 

on the east along the southern side of the igneous belt, is 

about in the middle of the belt at the Nevada-Utah State 

line, and is along the northern side of the belt farther 

west in Nevada. Some faults along the Warm Springs 

lineament in Nevada have had strike-slip displacement, but predominant movement along the eaststriking faults on the Blue Ribbon lineament seems to 

have been dip slip. Within southwestern Utah, eaststriking faults are progressively less common eastward 

along the lineament, and alkalic rhyolite centers tend 

to be younger eastward. Igneous activity and faulting 

along the lineament range in age from at least 26 m.y. to 

the present, concurrent with both calc-alkalic magmatism and basin-range faulting.

The northern side of the Pioche-Marysvale igneous 

belt is partly defined by an obscure lineament that 

extends from the Sevier Plateau at least as far west as 

the northern Mineral Mountains and, based on 

aeromagnetic anomalies, perhaps 50 km or more 

farther west (fig. 1). Crosby (1973) called the lineament 

the Black Rock offset zone and suggested that strikeslip faulting was important along it; this suggestion has 

yet to be substantiated, however. G. L. Galyardt (oral 

commun., 1976), T. A. Steven (unpub. data, 1977), and 

Rowley, Lipman, and others (1978) also noted various 

geologic features along the trend of the Black Rock 

lineament, which is defined by a downwarp along Clear 

Creek on the northern flank of the Tushar highland, by 

aeromagnetic and other geophysical anomalies, by dipslip and strike-slip (?) faults, by volcanic centers, by 

sulfur occurrences and hot springs, and by the abrupt 

northward termination of the Marysvale volcanic pile. 

It spans an age from at least 27 m.y. to the present.

BASIN-RANGE FAULTS

Most of the present topography of the Basin and 

Range province and the High Plateaus of southwestern 

Utah is due to basin-range block faulting. Topographic 

scarps of 1,000 m or more are common, and in order to 

produce some of the deep grabens (Cook and Hardman, 

1967), relative movement on some faults must have 

been at least several times this amount. The Mineral 

Mountains seems to be the highest upthrown block in 

southwestern Utah; it stands higher structurally than 

the Colorado Plateaus to the east of it. Basin-range 

deformation included some warping as well as faulting, 

as evidenced by the Wasatch monocline in the northeastern High Plateaus and a monocline north of Cedar 

City (Threet, 1963a; Averitt and Threet, 1973).

Faults are abundant in the Basin and Range province, where the basins occupy more area than the 

ranges. In the High Plateaus, on the other hand, faults 

appear to be less abundant and to have less displacement, and the upthrown blocks seem to have greater 

area than the downthrown blocks. Range-margin faults 

predominate in the High Plateaus, and the main parts 

of the upthrown blocks are cut by relatively few faults. 

The abundance of faults and the amount of displacement decrease eastward across the High Plateaus to the 

edge of the nearly unfaulted main body of the Colorado 

Plateaus province.

Most basin-range faults fall into two sets, one of 

which strikes north-northeast and the other northnorthwest, and the trends of the basins and ranges 

depend on which set predominates in any given area. 

Generally speaking, the north-northeast-trending set
16 CENOZOIC STRATIGRAPfflC AND STRUCTURAL FRAMEWORK OF SOUTHWESTERN UTAH

predominates in southwestern Utah. The faults in most 

places occur in zones rather than as single isolated 

planes, and within these zones the fractures form four 

main patterns: parallel, en echelon, rhombic, and zigzag. Parallel faults characterize most zones. Commonly 

the faults in such a zone all have the same sense of 

displacement, producing a step pattern of offset; most of 

the offset across such a zone is across one or a very few 

predominant faults within the zone. The blocks between 

the faults are commonly tilted in the direction opposite 

that of the fault, forming antithetic tilt blocks that 

reduce the apparent overall displacement on the faults. 

In some places, as in the Markagunt Plateau (Anderson, 

1965), faults in a parallel zone dip in opposite directions, 

giving rise to horsts and grabens.

Zones of en echelon faults, most of whose individual 

faults dip in the same direction, are common in some 

places; an example is the Sevier fault zone, which defines the western edge of the Sevier Plateau (Rowley, 

1968). The origin of some en enchelon fault zones has 

been ascribed to wrench faulting in basement rocks, but 

in southwestern Utah no evidence for a wrench-fault 

origin has been found.

Fault patterns characterized by rhombic-shaped 

blocks are generally produced by north-northeast and 

north-northwest-trending fault zones intersecting at 

angles of about 60° and 120°. Such patterns are common 

in southwestern Utah. They resemble in form the pattern described by Donath (1962) in southern Oregon, 

but in southwestern Utah they are well developed only 

in small scattered areas and are poorly explained by 

wrench-fault stress fields. These patterns appear to 

occur in exceptionally stressed areas and to have resulted from torsional deformation between major fault 

blocks or in zones within major blocks that have been 

twisted in different directions. Several rhombic sets lie 

along the Blue Ribbon lineament, including a major set 

northeast of Circleville on the western side of the central Sevier Plateau, and other sets lie along the easttrending scarp between the northern Markagunt 

Plateau and southern Tushar Mountains. A rhombic set 

in the Antelope Range-Sevier area lies at least partly on 

the Black Rock lineament. Rhombic patterns, which 

indicate areas that have been subject to greater tectonic 

stress, may localize igneous activity and mineralization; the Central Mining Area of the Marysvale district 

occurs in such an area (Cunningham and Steven, 

1978a).

A zigzag fault pattern reflects an interplay of the 

same north-northeast- and north-northwest-striking 

fault sets responsible for the trends of mountains and 

basins, for rhombic fault patterns, and for minor features along individual faults. Hamblin (1970) and Best 

and Hamblin (1970, 1978) have observed zigzag patterns at all scales along the Hurricane fault zone. The 

Sevier, Tushar, and Indian Hollow fault zones (Molloy 

and Kerr, 1962), bounding respectively the western 

Sevier Plateau, eastern Tushar Mountains, and eastern 

Pavant Range, have an overall zigzag map pattern.

Basin-range faulting can be dated by determining the 

radiometric age of the rocks involved. By this method, 

faulting is known to have begun in earliest Miocene 

time and to have extended to the Holocene. The largest 

displacements, however, seem to have taken place after 

7 m.y. ago.

The oldest faulting possibly related to basin-range 

tectonism took place somewhat before eruption of the 

Osiris Tuff, 22 m.y. ago. Pre- Osiris high-angle faulting 

in the southern High Plateaus has been noted by J. J. 

Anderson (1965,1971), Rowley (1968), and Rowley, Anderson, and others (1978a), and in the Cove Fort area by 

T. A. Steven (unpub. data, 1978). Near Cove Fort, this 

faulting clearly followed eruption of the Three Creeks 

Tuff Member, 27 m.y. ago. Whereas some of this faulting may be ancestral to the main basin-range breakup 

later, some or all may have resulted from deformation 

along the east-northeast-trending igneous belts. 

Clearly, however, the widespread presence of the Osiris 

Tuff throughout the Marysvale field (fig. 4) indicates 

that true basin-range topography had not started to 

form by this time. Similarly, there is no evidence that 

the tuffs, as young as 19 m.y. old, of the ash-flow tuff 

field of southwestern Utah erupted onto basin-range 

topography.

From about 20 m.y. ago to 7 m.y. ago, the High 

Plateaus and perhaps other parts of southwestern Utah 

as well may have been characterized by broad warping 

as much as or more than by faulting. The developing 

basins received continental sediments of the Sevier 

River Formation in the High Plateaus and basin-fill 

sediments in the Basin and Range province. The 

character and age of the deformation, unfortunately, is 

difficult to document conclusively.

Major faulting in the Clear Creek area between the 

Pavant Range and Tushar Mountains took place after 7 

m.y. ago, the fission-track age obtained from a tuff bed 

near the top of the Sevier River Formation near Sevier, 

Utah (Steven and others, 1977). The basin itself, however, had been in existence at least by 14 m.y. ago, the 

fission-track age from a tuff bed near the base of the 

Sevier River Formation west of Sevier (Steven and 

others, 1977, 1979). A basin west of the Pine Valley 

Mountains appears to have started to subside about 8 

m.y. ago (Anderson and Mehnert, 1979). Elsewhere in 

southwestern Utah, the age of major faulting seems to 

have been younger than 10 m.y. based on offsets of dated 

basalt flows. Anderson and Mehnert (1979) believed
REFERENCES CITED 17

that most faulting along the Hurricane fault zone is 

Quaternary, and Pleistocene faults are abundant elsewhere in southwestern Utah.

Holocene faults occur locally (Anderson, 1978), and 

Anderson and Bucknam (1979) discovered a 800 km2 

area near Enterprise in the southern Escalante Desert 

that has experienced uplift of at least 30 m, probably in 

Holocene time. Faulting is still active in the area. The 

axis of the intermountain seismic belt (Smith and Sbar, 

1974) runs south along the Wasatch front, apparently 

into Arizona (Anderson, 1978). The southern Nevada 

seismic belt, bearing east across southern Nevada, enters Utah and swings northeast to intersect the intermountain seismic belt near Cedar City.

LOW-ANGLE TERTIARY FAULTS

Low-angle Tertiary faults of two types are known 

in southwestern Utah. The first consists of large slide 

blocks derived from oversteepened flanks of domes 

lifted by rapidly emplaced shallow intrusions. The spectacular slides on the eastern flank of Iron Mountain 

laccolith in the Iron Springs district (Mackin, 1960; 

Blank and Mackin, 1967; J. H. Mackin and P. D. Rowley, unpub. data, 1976) involved rocks as young as the 

Harmony Hills Tuff. Individual slide blocks, more than 

4 km long, moved eastward as much as 2 km on subhorizontal, concave-upward, spoon-shaped surfaces. 

Radiometric ages on the laccolith, and thus the age of 

the slide blocks, are about 20 m.y. (Armstrong, 1970). 

Blank (1959) and Cook (1960a) discussed similar structures of the same general age in the Pine Valley and 

Bull Valley Mountains.

The second type of low-angle Tertiary structure is 

denudation faults (or distension or attenuation faults) 

that moved on low-angle normal fault planes (Anderson, 1971; Armstrong, 1972). The denudation faults 

represent extension off the shoulders of uplifted basin 

ranges, or represent the low parts of listric faults now 

exposed by deep erosion following major uplift of the 

range. The Tertiary gravity slide blocks studied by 

Dobbin (1939), Cook (1960a, b), and Jones (1963) in the 

Beaver Dam Mountains may be denudation faults. 

Nielson and others (1978) mapped low-angle normal 

faults cutting the batholith of the Mineral Mountains, 

and these too may be denudation faults.

THE COLORADO PLATEAUS

The Basin and Range-Colorado Plateaus province 

boundary follows an important structural hingeline 

that has been active since early Paleozoic or even late

Precambrian time (Hunt, 1956; Gilluly, 1963; Hintze, 

1973). This hinge line has had major influence on eastwest sedimentary facies changes throughout this time. 

The present Basin and Range and Colorado Plateaus 

provinces separated structurally in middle to late 

Cenozoic time, after deposition of the Needles Range 

Formation 30-29 m.y. ago (Rowley, Anderson, and 

others, 1978a). In late Cenozoic time, after 10 m.y. ago, 

both provinces underwent major uplift with respect to 

sea level, and they assumed their present high structural level (Best and Hamblin, 1978).

The beginning of structural separation of the Basin 

and Range and Colorado Plateaus provinces is believed 

to be chronicled by the distribution of ash-flow tuffs 

belonging to the calc-alkalic volcanic assemblage (Rowley, Anderson, and others, 1978a). The Isom Formation 

(26—25 m.y.) and the different formations in the 

Quichapa Group (24—21 m.y.) are widespread throughout much of the Basin and Range province of southwestern Utah, but then terminate near the present western 

edge of the High Plateaus. The termination of the tuffs 

is interpreted to be due to topographic scarps that existed at different times and different places within a 10- 

to 20-km-wide ancestral boundary zone. Anderson and 

Mehnert (1979) have criticized this idea of an ancestral 

boundary largely because of their observation of similar 

thicknesses of tuffs across a small area now occupied by 

the Cedar City—Parowan monocline and because of the 

fact that no alluvial sediments have not been found 

between the volcanic units near the boundary zone. 

However, the Cedar City—Parowan monocline probably 

formed largely west of, and much later than, the ancestral boundary. The specific cause of the inferred scarps 

in the ancestral boundary zone is not known, but either 

high-angle faulting or gentle warping would account for 

uplift to the east. The difference in relief between the 

two provinces at this old age could have been as low as 

30—100 m, or it could have been much higher. Most 

likely the ancestral uplift was accomplished in many 

episodes throughout early Miocene time, but erosion 

periodically lowered and modified the front. Anderson 

and Mehnert (1979) documented a case for Miocene or 

Pliocene deposition on the Kolob Terrace of coarse 

gravels that were derived from the Pine Valley Mountains; they probably were deposited during a period 

when this part of the Colorado Plateaus province was 

lower than the source area in the Basin and Range 

province.

REFERENCES CITED

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*U.S. GOVERNMENT PRINTING OFFICE: 1979-677-026/6