Isotopic Ages and Stratigraphy of 

Cenozoic Rocks of the Marysvale 

Volcanic Field and Adjacent Areas, 

West-Central Utah 
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Isotopic Ages and Stratigraphy of 

Cenozoic Rocks of the Marysvale 

Volcanic Field and Adjacent Areas, 

West-Central Utah 

By Peter D. Rowley, Harald H. Mehnert, Charles W. Naeser, Lawrence W. Snee, 

Charles G. Cunningham, Thomas A. Steven, John J. Anderson, 

Edward G. Sable, and R. Ernest Anderson 

U.S. GEOLOGICAL SURVEY BULLETIN 2071 

An update of the Cenozoic stratigraphy 

of the Marysvale volcanic field, based on 

new and published K-Ar, fission-track, 

and 40 ArP9 Ar ages on igneous rocks 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1994 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

For Sale by U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government 

Library of Congress Cataloging-in-Publication Data 

Isotopic ages and stratigraphy of Cenozoic rocks of the Marysvale volcanic field and adjacent areas, west-central Utah I by Peter D. Rowley ... [et al.]. 

p. em.- (U.S. Geological Survey Bulletin ; 2071) 

"An update of the Cenozoic stratigraphy of the Marysvale volcanic field, based on new 

and published K-Ar, fission-track, and 40 Arf39 Ar ages on igneous rocks." 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:2071 

1. Geology, Stratigraphic-Cenozoic. 2. Rocks, Igneous-Utah-Marysvale 

Region. 3. Radioactive dating-Utah-Marysvale Region. I. Rowley, Peter D. 

II. Series. 

QE75.B9 no. 2071 

[QE690] 

557.3 s-dc20 

[551.7'8'09792] 93-41469 

CIP 
CONTENTS 

Abstract........................................................................................................................... 1 

Introduction..................................................................................................................... 2 

Geologic Setting.............................................................................................................. 4 

Lower Cenozoic Sedimentary Sequence . .. .. . . . . . . .. ... . . . . . . .. .. . . . . . . . . . .......... .. .. .. . . . . . . . .. .. . . . . . . .. .. 9 

Claron Formation and Related Units ... .. . . ....... .. .......... .. ......... .. ............. ........ .. ...... 9 

Middle Cenozoic Calc-alkaline Igneous Sequence......................................................... 11 

Bullion Canyon Volcanics . . . . . . . . . . . . . . . .. .. . . . . . . . .. . . . . . . . . .. .. . . . . . . . .. .. . . . . . . . . .. .. .. . . . . . . . .. . . . . . . . . . . 11 

Mount Dutton Formation...................................................................................... 12 

Intertongued Great Basin Ash-flow Sheets........................................................... 13 

Local Volcanic Centers......................................................................................... 14 

Monroe Peak Caldera............................................................................................ 17 

Calc-alkaline Intrusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

Upper Cenozoic Sedimentary and Bimodal Igneous Sequence ...................................... 21 

Potassium-rich Mafic Volcanic Rocks.................................................................. 21 

Mount Belknap Volcanics..................................................................................... 23 

Small Rhyolite Volcanic Centers.......................................................................... 24 

Silicic Intrusions . . . . . .. . . . . . . . . .. .. .. . . . . . .. .. . . . . . . . . .. .. . . . . . . . .. . . . . . . . . . . .. . . . . . . . .. .. . . . . . . . . . .. .. . . . . . . .. .. . . 24 

Basalt Lava Flows................................................................................................. 26 

Basin-fill Sedimentary Rocks............................................................................... 27 

Conclusions..................................................................................................................... 28 

References Cited . . . . . . . ... . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . ... . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. .. .. . . . . . . . .. . . . . . . . . .. 29 

FIGURES 

1. Index map showing location of the Marysvale volcanic field, Utah............................................................................. 3 

2. Composite stratigraphic column of Cenozoic rocks in and near the Marysvale volcanic field ....... .. .. ......... ................ 5 

3. Generalized geologic map, showing locality of samples whose ages and analytical data are reported here................ 7 

4. Graphs of 40 ArP9 Ar age spectrum for biotite and sanidine of sample 83-69X............................................................ 21 

TABLES 

1. Data for potassium-argon dates, Marysvale volcanic field and adjacent areas, Utah................................................... 10 

2. Data for fission-track dates, Marysvale volcanic field and adjacent areas, Utah.......................................................... 18 

3. Data for 40 ArJ39 Ar dates for biotite and sanidine from sample 83-69X, eastern Sevier Plateau, Utah........................ 20 

III 

Isotopic Ages and Stratigraphy of Cenozoic Rocks of the 

Marysvale Volcanic Field and Adjacent Areas, 

West-Central Utah 

By Peter D. Rowley, Harald H. Mehnert, Charles W. Naeser, Lawrence W. Snee, 

Charles G. Cunningham, Thomas A. Steven, John J. Anderson,1 

Edward G. Sable, and R. Ernest Anderson 

ABSTRACT 

The Marysvale volcanic field lies at the east end of the 

east-northeast-trending Pioche-Marysvale igneous belt, the 

site of extensive Cenozoic volcanism and mineralization 

above a major batholith complex. The field is mostly in the 

High Plateaus transition zone between the Great Basin and 

the main part of the Colorado Plateau. Cenozoic rocks of the 

Marysvale field can be divided into three main sequences: 

(1) lower Cenozoic (Paleocene through Oligocene) fluviallacustrine sedimentary rocks (mostly the Claron Formation) 

that overlie Paleozoic and Mesozoic rocks with angular 

unconformity; (2) thick, middle Cenozoic (Oligocene and 

lower Miocene, 34? to 22 Ma), intermediate-composition, 

predominantly calc-alkaline volcanic rocks and related 

source intrusions; and (3) upper Cenozoic (23 Ma to Quaternary), compositionally bimodal (basalt and high-silica rhyolite) volcanic rocks, local source intrusions associated with 

these bimodal rocks, and overlying and intertongued clastic 

basin-fill sedimentary rocks. The overall volume of volcanic 

rocks in the field is at least 12,000 km3, of which about 95 

percent is calc-alkaline and 5 percent is part of a bimodal 

assemblage. More than 90 percent of the volume of Marysvale volcanic rocks consists of lava flows, volcanic mudflow 

breccia, and flow breccia, and less than 10 percent consists 

of tuff. 

The lower Cenozoic sequence consists of continental 

sedimentary rocks that are locally tuffaceous. Most of the 

sedimentary rocks in the Marysvale volcanic field have been 

mapped as the upper Paleocene to Oligocene Claron Formation, but new stratigraphic studies are currently redefining 

the sequence. Airfall tuffs in the upper part of the sedimentary sequence have isotopic ages as old as 34 Ma, and most 

of these tuffs probably represent early eruptions from the 

Marysvale field. 

1 John J. Anderson, Dept. of Geology, Kent State University, Kent, 

Ohio 44242. 

The middle Cenozoic calc-alkaline rocks centered in 

the northern and central Tushar Mountains and northern 

Sevier Plateau are mostly relatively highly evolved dacite 

and subordinate andesite called the Bullion Canyon Volcanics, deposited between about 34? and 22 Ma. Deposits from 

volcanic centers that intertongue on the north include the 

andesitic to dacitic volcanic rocks of Dog Valley and the 

overlying andesitic to dacitic volcanic rocks of Wales Canyon. Deposits from volcanic centers that intertongue on the 

east include the andesitic volcanic rocks of Signal Peak, the 

andesitic volcanic rocks of Little Table, the dacitic volcanic 

rocks of Willow Spring, and the dacitic volcanic rocks of 

Langdon Mountain. Large, shallow, intermediate-composition consanguineous intrusions were emplaced into the 

flanks and cores of the major volcano complexes late during 

Bullion Canyon volcano evolution. Failure of the cover 

rocks above some of these shallow intrusions resulted in 

eruption of ash-flow tuffs and subsidence of the source areas 

to form calderas. Two tuff members in the Bullion Canyon 

Volcanics, the 27 -Ma Three Creeks Tuff Member and the 

24-Ma Delano Peak Tuff Member, came from calderas in 

the southern Pavant Range and southern Tushar Mountains, 

respectively. Bullion Canyon volcanism was largely terminated by eruption of the Osiris Tuff at about 23 Ma in the 

Marysvale area and northern Sevier Plateau, resulting in the 

large Monroe Peak caldera. Hydrothermally altered rocks 

and deposits of base and precious metals formed during Bullion Canyon magmatism in the Tushar Mountains and 

Sevier Valley. 

In contrast to the moderately evolved rocks in the center 

of the Marysvale field, the middle Cenozoic rocks in the 

southern part of the field are less evolved andesite and dacite, 

containing small sparse phenocrysts. Most rocks of the 

southern part of the field belong to the 34 ?-to 21-Ma Mount 

Dutton Formation, a thick sequence that intertongues to its 

north with the Bullion Canyon Volcanics and other units. 

The Mount Dutton Formation contains several tuff members 
2 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST-CENTRAL UTAH 

whose sources probably were in the northern Marysvale 

field. Shallow, 26-20?-Ma calc-alkaline laccoliths unrelated 

to Mount Dutton magmatism were emplaced into lower parts 

of the Claron Formation in the northern Markagunt Plateau, 

mostly south of the Mount Dutton source volcanoes. Some 

of these laccoliths erupted to form dacitic lava flows and ashflow tuff. The laccoliths were fed from a batholith complex 

that underlay the southern flank of the Marysvale volcanic 

field; this complex is at the east end of the Delamar-Iron 

Springs igneous belt, an east-northeast-trending belt south of 

and parallel to the Pioche-Marysvale igneous belt. 

The beginning of late Cenozoic bimodal (basalt and 

high-silica rhyolite) magmatism overlapped in time with the 

termination of calc-alkaline magmatism. The switch to 

bimodal magmatism corresponded with an increase in eastwest extension and the progressive decrease and final cessation of subduction under much of the western United States. 

Bimodal magmatism generally began with potassium-rich 

mafic volcanic rocks that began to be erupted 23-21 Ma 

from small local centers scattered throughout the Marysvale 

field. The main mass of bimodal rocks, however, is high-silica rhyolite of the Mount Belknap Volcanics, which formed 

from 22 to 14 Ma. The Mount Belknap Volcanics consist 

mostly of ash-flow tuff, lava flows, flow breccia, and intrusions derived from an eastern source area in the Antelope 

Range and southeastern Tushar Mountains and a western 

source area in the central Tushar Mountains. The western 

source area includes the Mount Belknap caldera that erupted 

the Joe Lott Tuff Member (19 Ma) of the Mount Belknap 

Volcanics. Hydrothermally altered rocks and associated vein 

alunite deposits and lithophile-element mineral deposits 

such as uranium and molybdenum formed during Mount 

Belknap magmatism. Small centers of high-silica rhyolite 

domes and lava flows as young as 0.5 Ma are scattered elsewhere in the volcanic field. Basalt lava flows and cones with 

normal potassium contents were erupted from 16 Ma to Quaternary time from widely scattered sources in and near the 

Marysvale field; these are interbedded with basin-fill sedimentary rocks derived largely from erosion of nearby higher 

areas. The oldest of these sedimentary rocks is about 14 Ma, 

but the main grabens of the Great Basin and transition zone 

probably did not start to form until about 10 Ma, when the 

episode of basin-range extension began. In the area of the 

Marysvale volcanic field, the main part of this extensional 

episode took place between 8 and 5 Ma. 

INTRODUCTION 

The Marysvale volcanic field is one of the major 

Cenozoic volcanic fields in the western United States (fig. 

1 ). Most of the field is in the High Plateaus subprovince of 

the Colorado Plateaus province, a transition zone between 

the Great Basin and the stable, main part of the Colorado 

Plateau. Part of the field extends into the Great Basin as in 

the Mineral Mountains and the east half of the Black 

Mountains. The Marysvale field lies at the east end of the 

east-northeast-trending Pioche-Marysvale igneous belt, 

the site of extensive Tertiary volcanism and mineralization above a major batholith complex; thus igneous rocks 

of the same general types and ages as in the Marysvale 

field extend west and west -southwest along the belt from 

west of the Mineral Mountains into Nevada. East of the 

Marysvale field, new dates from laccoliths of the Henry, 

La Sal, and Abajo Mountains of central and southeastern 

Utah indicate that most intrusive activity there is coeval 

with that in the Marysvale field (Sullivan and others, 1991; 

Nelson and others, 1992). 

Although investigations of the geology of the Marysvale field began in the late 19th century (for example, Dutton, 1880), detailed studies did not begin until the 1930's and 

1940's, when Callaghan (1938, 1939) described the basic 

stratigraphy during geologic mapping and studies of mineral 

deposits centered around Marysvale (Callaghan and Parker, 

1961a, b, 1962a, b; Willard and Callaghan, 1962). In the 

1950's, P.F. Kerr and his students at Columbia University 

(Kerr and others, 1957, among others) undertook detailed 

studies of the uranium deposits in the Marysvale mining district. The reports of these studies included the first isotopic 

ages of the volcanic rocks and ore deposits (Bassett and others, 1963). A significant development that would prove useful in the Marysvale field was the realization by Mackin 

(1960), based on studies in the Iron Springs mining district 

15 km west of Cedar City (fig. 1), that many volcanic units 

in southwestern Utah are regional ash-flow sheets that provide key marker horizons in working out volcanic stratigraphy. Mackin (1960), with help from his students at the 

Universities of Washington and Texas, set up the basic 

stratigraphy of these tuff units in the eastern Great Basin, and 

Armstrong (1970) was the first to determine their K-Ar ages. 

Two of these students, J.J. Anderson and P.D. Rowley, 

applied Mackin's concepts to the northern Markagunt Plateau and southern Sevier Plateau beginning in 1963, and 

developed the first modem stratigraphic framework of the 

southern part of the Marysvale volcanic field (for example, 

Anderson and Rowley, 1975); K-Ar ages of numerous units 

in the same area were determined by Fleck and others 

(1975). Williams and Hackman (1971) described the basic 

stratigraphy of the eastern Marysvale field. In 1975, T.A. 

Steven, C.G. Cunningham, P.D. Rowley, J.J. Anderson, 

M.G. Best, and others began a major USGS mapping initiative in the heart of the Marysvale field, with special attention 

directed toward establishing the geologic framework of the 

widespread mineral resource occurrences. Isotopic dating by 

H.H. Mehnert and C.W. Naeser established the chronology 

of the detailed stratigraphy, and many of the ages obtained 

are given in earlier reports (such as Cunningham and Steven, 

1979a; Rowley, Steven, and others, 1979; Steven and others, 

1979). The USGS project resulted in more than 200 open-file 
INTRODUCTION 

UTAH 

... 

I 

8 

~ 0 

0' u 

:t '\AXIS OF PIOCHE-MARYSVALE 

IGNEOUS BELT 

AXIS OF DELAMAR-IRON 

SPRINGS IGNEOUS BELT 

.....-·<_OUTLINE OF MARYSVALE 

VOLCANIC FIELD 

20 40 60 KILOMETERS 

3 

Figure 1. Index map showing location of the Marysvale volcanic field, plateaus and ranges of the High Plateaus, and adjacent areas 

in Utah, modified from Anderson and Rowley (1975, fig. 1) and Steven and others (1979, fig. 1). A, Annabella; AR, Antelope Range; 

B, Beaver; BC, Bryce Canyon National Park; BH, Brian Head; C, Cedar City; CB, Cedar Breaks National Monument; Ci, Circleville; 

E, Elsinore; F, Fillmore; H, Hatch; IS, Iron Springs; J, Joseph; Ju, Junction; K, Kingston Canyon; M, Marysvale; Mi, Milford; Mo, 

Modena; Mv, Minersville; P, Panguitch; Pa, Parowan; R, Richfield; S, Salina; Se, Sevier; Sp, Spry. Great Basin-Colorado Plateau 

boundary is just west of the towns of Fillmore, Beaver, Parowan, and Cedar City. Patterned areas are underlain largely by volcanic 

rocks. 
4 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST -CENTRAL UTAH 

and published maps and reports, many of which are listed by 

Steven and Morris (1987). Many of these are detailed 

(1:24,000 and 1:50,000) geologic maps of the volcanic field. 

The geology, geophysics, and mineral resources of the main 

eruptive center were compiled at 1:50,000 scale as USGS 1-

Maps 1430-A through 1430-G (Cunningham and others, 

1983; Cunningham, Steven, Row ley, and others, 1984; Cook 

and others, 1984; Campbell and others, 1984; Miller and others, 1984 a, b, c). Calderas within the volcanic field were discussed by Cunningham and Steven (1979a), Steven (1981), 

Steven, Rowley, and Cunningham (1984), Steven, Cunningham, and Anderson (1984 ), and Budding and others (1987). 

Reports of specific mineral deposits, including their isotopic 

ages, are given by Cunningham and Steven ( 1979b, c), 

Steven and others (1981), Cunningham and others (1982), 

Cunningham, Rye, and others (1984), Cunningham, Steven, 

Campbell, and others (1984), Podwysocki and Segal (1983), 

Beaty and others (1986), and Steven and Morris (1987). In 

1990, Steven and others published a 1 :250,000-scale geologic map showing most of the Marysvale field and much of 

the rest of the Pioche-Marysvale igneous belt. In a report in 

progress, Rowley and others discuss the regional tectonic 

and magmatic setting of the Marysvale field. 

This report summarizes what is presently known about 

the Cenozoic stratigraphy of the Marysvale field, with an 

emphasis on our estimate of the best ages of the various 

rock units. We discuss previously published isotopic ages 

and we publish new ages and analytical data. Most ages are 

by the K-Ar technique, but fission-track and two 40 Arf39 Ar 

ages also are reported. The K-Ar method is described by 

Evernden and Curtis (1965). 

The fission-track age of apatite was determined with the 

population technique, whereas the external detector method 

was used to date zircons (Naeser, 1976, 1979). Apatite separates were mounted in epoxy, polished, then etched in 7 percent HN03 at 23 oc for 40 seconds. Zircon grains were 

mounted in Teflon and etched in a eutectic melt of KOHNaOH (Gleadow and others, 1976) at 215 oc for 30-50 

hours. The Teflon mounts were covered with a muscovite 

detector and irradiated along with neutron dose monitors (Udoped glasses SRM 962 also covered with muscovite detectors) in the USGS reactor at Denver, Colo. The neutron dose 

was determined using the track density in the muscovite 

detectors and the Cu calibration for SRM 962 (Carpenter and 

Reimer, 1974). 

Two new 40 Arf39 Ar mineral ages from a single sample 

help constrain temporal relationships in the Marysvale volcanic field. The 40 Arf39 Ar isotopic dating method is a variant 

of the conventional K-Ar method. For 40 Arf39 Ar analysis, 

samples are irradiated in a nuclear reactor to produce 39 Ar 

from 39K. In this study, 100-mg samples of sanidine and 63-

mg samples of biotite were irradiated in the USGS Denver 

TRIG A reactor for 30 hours at 1 megawatt. Each sample was 

irradiated adjacent to an aliquant of standard MMhb-1 hornblende for which K=l.555 percent, 40 ArK=l.624x1o-9 

mollg, and K-Ar date=520.4 Ma (Samson and Alexander, 

1987). Samples were experimentally degassed in a doublevacuum resistance furnace via step heating in 10-13 steps 

under ultra-high vacuum for about 20 minutes at each temperature step. The mass 40, 39, 38, 37, and 36 isotopes of 

argon were analyzed using a Mass Analyser Products series 

215 rare-gas mass spectrometer. Raw isotopic data were corrected for volume, mass discrimination, trap current, radioactive decay of 37 Ar and 39 Ar, and interfering isotopes of 

argon. 

Where necessary, previously determined isotopic ages 

compiled here have been recalculated using the lUGS decay 

constants (Steiger and Jager, 1977). Classifications of volcanic and plutonic rocks are those of lUGS (Le Bas and others, 

1986, and Streckeisen, 1976, respectively). We have not 

attempted in this report to review comprehensively the voluminous literature on the stratigraphy of the area, but in the 

discussion of individual units, we cite the most pertinent 

references, to which the reader may refer for additional 

information. 

Acknowledgments.-We thank D.M. Cheney, P.L. FitzMaurice, G.T. Cebula, and Ezekiel Rivera for their care in 

separating the minerals dated in this study. We are grateful 

to J.H. Mackin, P.L. Williams, D.S. Barker, M.G. Best, H.R. 

Blank, Florian Maldonado, A.L. Deino, R.B. Scott, M.E. 

MacLachlan, C.S. Gromme, and L.F. Hintze for many comments over many years that helped formulate concepts given 

in this report. Significant improvements to the manuscript 

were kindly provided by K.S. Kellogg, J.M. O'Neill, Florian 

Maldonado, M.G. Best, W.A. Bryant, and L.M. Carter. 

GEOLOGIC SETTING 

Cenozoic rocks of the Marysvale volcanic field can be 

divided into three main sequences: (1) lower Cenozoic (Paleocene through Oligocene) fluvial-lacustrine sedimentary 

rocks that overlie Paleozoic and Mesozoic rocks in angular 

unconformity; (2) thick, middle Cenozoic (Oligocene and 

lower Miocene, 34? to 22 Ma), intermediate-composition, 

generally calc-alkaline volcanic rocks deposited mostly from 

Figure 2 (facing page). Composite stratigraphic column of Cenozoic rocks in and near the Marysvale volcanic field. Numbers refer 

to isotopic ages of volcanic rocks rounded to the nearest half million years. Boundaries of the three stratigraphic sequences shown 

by heavy lines. Most rock units intertongue with units shown to 

their left or right; units that are restricted in age (thin boxes) are 

shown within the unit with which they intertongue the most, but 

many of the units shown by thin boxes intertongue with other units 

farther left or right. See text for discussion of "related units" that 

cannot be adequately shown in the figure. The age of the Brian 

Head-Claron boundary is poorly known and is shown by a queried 

line. For emphasis, patterns shown for Mount Belknap Volcanics, 

Mount Dutton Formation, and Bullion Canyon Volcanics. 
AGE 

QUATERNARY 

10 Ma 

MIOCENE 

20 Ma 

24 Ma 

OLIGOCENE 

30 Ma 

38 Ma 

EOCENE 

SEQUENCE 

UPPER 

CENOZOIC 

SEDIMENTARY 

AND 

BIMODAL 

IGNEOUS 

SEQUENCE 

MIDDLE 

CENOZOIC 

CALC-ALKALINE 

IGNEOUS 

SEQUENCE 

LOWER 

CENOZOIC 

SEDIMENTARY 

SEQUENCE 

GREAT BASIN 

Not treated 

Not treated 

GEOLOGIC SETTING 

MARYSVALE VOLCANIC FIELD AND VICINITY 

Local basalt 

Sevier River Formation 

and other basin-fill 

sedimentary rocks 

Local 

high-silica 

rhyolite 

Claron Formation (part) 

5 
6 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST-CENTRAL UTAH 

stratovolcanoes and calderas, and related source intrusions; 

and (3) upper Cenozoic (23 Ma to Quaternary), compositionally bimodal (basalt and high-silica rhyolite) volcanic rocks, 

deposited from varied rhyolite sources such as calderas and 

volcanic domes and from basalt sources such as cinder cones 

and shield volcanoes, local source intrusions associated with 

these bimodal rocks, and overlying and intertongued clastic 

basin-fill sedimentary rocks (fig. 2). 

The basal, mostly prevolcanic sedimentary sequence 

consists in large part of the Claron Formation in and near the 

Marysvale field; the age of this formation is poorly constrained but probably is late Paleocene through Oligocene. 

The overlying volcanic rocks in the field have a volume of at 

least 12,000 km3, of which about 95 percent is generally 

calc-alkaline and 5 percent is part of a bimodal assemblage 

(basalt and high-silica rhyolite) (Rowley and others, report 

in progress). More than 90 percent of the volume of Marysvale volcanic rocks consists of lava flows, volcanic mudflow 

breccia, and flow breccia, and less than 10 percent consists 

of tuff (Rowley and others, report in progress). Calc-alkaline 

magmatism began in the field by at least 32 Ma, and probably earlier (34 Ma?), and continued until about 21 Ma. Most 

bimodal magmatism took place between 23 Ma and 16 Ma, 

but some small rhyolitic and basalt centers erupted after this, 

to as recently as the Quaternary. Basin-fill sedimentation in 

the northern Tushar Mountains began between 19 and 14 Ma 

and continues today. 

The majority of the magmatism in the Marysvale volcanic field took place in middle Cenozoic (Oligocene and 

Miocene) time, and resulted in intermediate-composition, 

calc-alkaline lava flows, flow breccia, volcanic mudflow 

breccia, and tuff derived from central volcanoes and calderas 

clustered near Marysvale, Utah. The sequence corresponds 

to the voluminous calc-alkaline magmatism common 

throughout the western United States that probably formed 

during subduction beneath the western United States. (See 

for example Lipman and others, 1972; Lipman, 1992.) Most 

volcanic edifices were small to large stratovolcanoes, but 

shield volcanoes and volcanic domes also were common. 

Near Marysvale, especially in the northern and central 

Tushar Mountains and northern Sevier Plateau, most rocks 

are dacite and andesite that contain abundant phenocrysts. 

Most of these relatively highly evolved rocks were called the 

Bullion Canyon Volcanics, a name first used by Callaghan 

(1939). This thick sequence was deposited between 34? and 

22 Ma (Steven and others, 1979) from vents centered in the 

Tushar Mountains (fig. 3). Volcanic rocks from centers on 

the north (southern Pavant Range) that intertongue with the 

lower part of the Bullion Canyon Volcanics include the 

informally named volcanic rocks of Dog Valley, the overlying volcanic rocks of Wales Canyon, and an unnamed dome 

(Cunningham and others, 1983). Other volcanic rocks that 

intertongue on the northeast (Pavant Range) and east (Sevier 

Plateau) with rocks of the Bullion Canyon Volcanics 

include, from north to south (Cunningham and others, 1983), 

(1) the volcanic rocks of Signal Peak, an andesitic shield volcano sequence in the northern Sevier Plateau and southern 

Pavant Range; (2) the volcanic rocks of Little Table, an 

andesitic shield volcano sequence in the central Sevier Plateau; (3) the volcanic rocks of Willow Spring, a dacitic stratovolcano sequence in the central Sevier Plateau; and ( 4) the 

volcanic rocks of Langdon Mountain, a dacitic stratovolcano 

sequence in the central Sevier Plateau. These units are more 

extensive east of the areas we have mapped (Williams and 

Hackman, 1971; Mattox, 199la, 1992). The units are not 

specifically shown in figure 2. 

Shallow consanguineous intrusions of monzonite and 

quartz monzonite were emplaced into the flanks and cores of 

the major volcanoes of the Bullion Canyon Volcanics and 

other units late during volcano evolution. Failure of roof 

rocks above some of these shallow intrusions resulted in 

eruption of ash-flow tuff units, which covered much of the 

field and beyond, and in subsidence of source areas to form 

calderas (Steven, Rowley, and Cunningham, 1984). The 

Three Creeks Tuff Member and the Delano Peak Tuff Member of the Bullion Canyon Volcanics came from the Three 

Creeks caldera in the southern Pavant Range and the Big 

John caldera in the southern Tushar Mountains, respectively. 

Most Bullion Canyon volcanism terminated with eruptions 

of the Osiris Tuff at about 23 Ma from the northern Sevier 

Plateau; the source area subsided as the Monroe Peak 

caldera, which occupies an area in the Marysvale and northern Sevier Plateau area. Hydrothermally altered rocks and 

mineral deposits of base and precious metals (mostly of chalcophile elements) formed during Bullion Canyon magmatism in the Tushar Mountains and Sevier Valley. 

In contrast to the shallow, more evolved rocks in the 

center of the Marysvale field, the stratovolcano rocks in the 

southern part of the field are more mafic (andesite and subordinate dacite) and most contain small sparse phenocrysts. 

These belong to the Mount Dutton Formation (Anderson and 

Rowley, 1975), a thick sequence of volcanic mudflow breccia, lava flows, and flow breccia that formed from about 34 

to 21 Ma and intertongued on the north with Bullion Canyon 

Volcanics and the products of other volcanoes (Fleck and 

others, 1975). The Mount Dutton is derived from numerous 

centers that are aligned roughly east-west through the northern Markagunt Plateau, southern Tushar Mountains, and 

Figure 3 (facing page). Generalized geologic map of the Marysvale volcanic field and vicinity, Utah, showing locality of samples 

whose ages and analytical data are reported here. Sample numbers 

are those given in tables 1, 2, and 3. A, Annabella; B, Beaver; BC, 

Bryce Canyon National Park; BH, Brian Head; C, Cedar City; CB, 

Cedar Breaks National Monument; Ci, Circleville; E, Elsinore; F, 

Fillmore; H, Hatch; J, Joseph; Ju, Junction; K, Kingston Canyon; M, 

Marysvale, Mi, Milford; Mv, Minersville; P, Panguitch; Pa, Parowan; R, Richfield; S, Salina; Se, Sevier; Sp, Spry. Hachures show the 

following calderas: BJ, Big John caldera; MB, Mount Belknap 

caldera; MP, Monroe Peak caldera; TC, Three Creeks caldera. 
GEOLOGIC SETTING 

25 50 KILOMETERS 

Upper Cenozoic sedimentary and 

bimodal igneous sequence 

Basin-fill rocks 

Basalt, rhyolite I M~ddle Cenozoic calc-alkaline 

Igneous sequence 

Lower Cenozoic sedimentary 

sequence 

Pre-Cenozoic rocks 

Intrusive 

rocks 

7 
8 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST -CENTRAL UTAH 

southern Sevier Plateau. These centers occur along a broad, 

long-lived, east-striking fault zone named the Blue Ribbon 

lineament, which extends westward into Nevada (Rowley, 

Lipman, and others, 1978; Bartley and others, 1992) and is 

one of numerous lineaments in the Great Basin and transition 

zone (Ekren and others, 1976; Rowley and others, report in 

progress). Mount Dutton source intrusions are rarely 

exposed, probably because phenocryst-poor Mount Dutton 

rocks were erupted from deep magma chambers. As a consequence, hydrothermally altered surface rocks and mineral 

deposits are sparse in areas underlain by the Mount Dutton 

Formation. The Mount Dutton Formation also contains 

several tuff members whose eruptive sources probably were 

in the northern Marysvale field. 

Shallow, calc-alkaline laccoliths unrelated to Mount 

Dutton magmatism were emplaced into lower parts of the 

Claron Formation in the northern Markagunt Plateau south 

of the Mount Dutton sources. The largest of these laccoliths 

is the Spry intrusion (Anderson and Rowley, 1975), which 

erupted to form a dacitic vent complex, lava flows, and 

tuffs (Anderson, Rowley, Blackman, and others, 1990). 

Borrowing on an idea of H.R. Blank (oral commun., 1991) 

and developed in greater detail by Nickelsen and Merle 

(1991), Merle and others (1993), Anderson (1993), and 

Maldonado (in press), we conclude that this and other laccoliths in the area were fed from a batholith complex that 

underlies the southern flank of the Marysvale volcanic field 

(Rowley and others, report in progress). This batholith 

complex is part of the Delamar-Iron Springs igneous belt, 

which is south of and parallel to the Pioche-Marysvale 

igneous belt, and it may extend westward to laccoliths of 

the "Iron Axis" (Blank and others, 1992). 

The middle Cenozoic calc-alkaline volcanic rocks of 

the Marysvale volcanic field are overlain by upper Cenozoic bimodal (basalt and high-silica rhyolite) volcanic 

rocks. The changeover from calc-alkaline to bimodal magmatism was abrupt in places and marks a dramatic change 

in petrologic regime, as in other parts of the western United 

States (Christiansen and Lipman, 1972), as subduction 

along the west side of North America waned. For example, 

in the western part of the Monroe Peak caldera, latest calcalkaline intracaldera lava flows that mark the termination of 

caldera activity were deposited at 22-21 Ma, the same time 

as nearby high-silica rhyolite volcanic domes of the bimodal sequence were being extruded. In general, the first 

products of bimodal magmatism in the Marysvale field 

consist of potassium-rich mafic volcanic rocks from local 

centers (Best and others, 1980). Some of these have been 

named, such as the mafic lava flows of Birch Creek Mountain (22.5 Ma) and the mafic lava flows of Circleville 

Mountain (23 Ma) (Anderson and Rowley, 1975; Anderson, Rowley, Blackman, and others, 1990). Other smaller 

centers of similar-age potassium-rich mafic volcanic rocks 

have been mapped elsewhere in the Marysvale field 

(Steven and others, 1990). 

The oldest bimodal rocks, defined by Callaghan (1939) 

as the Mount Belknap Volcanics, were deposited in part 

within but mostly west of the Monroe Peak caldera between 

22 and 14 Ma (Cunningham and Steven, 1979a). Mount 

Belknap Volcanics consist mostly of high-silica rhyolitic 

ash-flow tuff, lava flows, flow breccia, and intrusions 

exposed in the central and northern Tushar Mountains and 

Antelope Range. The tuff units and most flows are derived 

from two main sources, an eastern source area mostly in the 

Antelope Range that includes the small Red Hills caldera, 

and a western source area in the central Tushar Mountains 

that includes the large Mount Belknap caldera and its main 

ash-flow tuff, the Joe Lott Tuff Member (19 Ma) of the 

Mount Belknap Volcanics (Steven, Rowley, and Cunningham, 1984; Budding and others, 1987). Hydrothermally 

altered rocks, including vein alunite deposits, and lithophileelement mineral deposits such as uranium and molybdenum 

formed during Mount Belknap volcanic activity. 

Small high-silica rhyolite domes and related lava flows 

of the bimodal suite, as young as Pleistocene, are scattered 

throughout the volcanic field. Basalt lava flows derived from 

small local cones, as young as Pleistocene, are also widely 

scattered in and near the Marysvale field. 

The Marysvale field, which is mostly in the Colorado 

Plateau-Great Basin transition zone (fig. 1), is broken by 

numerous high-angle normal faults of several middle Tertiary to Quaternary episodes of east-west extension (Rowley 

and others, report in progress). The faults of the oldest of 

these episodes of extensional deformation, which formed 

during emplacement of fundamentally calc-alkaline rocks, 

are not well known because they have been covered by 

younger volcanic rocks and have been deformed by later 

faults. Some faults of this older episode bound a series of 

west-northwest-striking horsts and grabens in the northern 

Markagunt Plateau, perhaps related to the Blue Ribbon lineament, that are well constrained at about 27-25 Ma (J.J. 

Anderson, 1971, 1985, 1988). Older faults also include those 

formed above calc-alkaline batholiths (Steven, 1988). The 

change to bimodal magmatism at about 22 Ma was coeval 

with a regional increase in east-west extension; faults of the 

interval from 22 to 10 Ma, however, are poorly known, in 

part because of a lack of regional stratigraphic units of this 

age. Certainly basins started to form, perhaps largely by 

warping, soon after 22 Ma, and basin-fill sediments derived 

from erosion of upwarped areas started to be deposited then; 

these sediments make up the lower part of the Sevier River 

Formation (Callaghan, 1938; Steven and others, 1979). The 

main extensional deformation in the Marysvale field probably took place later, during the basin-range episode, which 

began at about 10 Ma and created the present fault-block 

topography. The age of most deformation of this episode is 

constrained by relationships in the Kingston Canyon area of 

the southern Sevier Plateau (see section, "Small Rhyolite 

Volcanic Centers") at between 8 and 5 Ma (Rowley and others, 1981 ), with lesser offset continuing into the Pliocene and 
LOWER CENOZOIC SEDIMENTARY SEQUENCE 9 

Quaternary. Most faults are normal faults, but strike-slip 

zones occur locally as accommodation zones or in areas 

where the maximum principal stress axis changed locally 

from vertical to horizontal and oriented north-south (Anderson and Barnhard, 1992). Grabens formed during basinrange faulting, and basin-fill sedimentary rocks of the upper 

part of the Sevier River Formation (Steven and others, 1979; 

Anderson, 1987) were deposited in these basins. 

LOWER CENOZOIC SEDIMENTARY 

SEQUENCE 

CLARON FORMATION AND RELATED UNITS 

The oldest Tertiary unit in the Marysvale volcanic field 

is a sequence of fluvial and lacustrine sedimentary rocks that 

we have mapped as the Claron Formation (Anderson and 

Rowley, 1975), after correlating them with rocks in the Iron 

Springs mining district (for example, Mackin, 1947, 1960; 

Mackin and Rowley, 1976). The lower "red" Claron, as 

much as about 400 m of red and orange limestone, shale, 

sandstone, and conglomerate, is spectacularly exposed as the 

beautiful hoodoos in Bryce Canyon National Park and Cedar 

Breaks National Monument south of the Marysvale field 

(fig. 1). The rocks record erosion of highlands formed during 

Laramide deformation and deposition in the basin that may 

have started to form in the west and south and transgressed 

north and northeast (Goldstrand, 1990, 1992). These rocks 

are overlain by a sequence that we previously mapped as 

"white" Claron (Anderson and Rowley, 1975), also as much 

as about 400 m thick, of white and light-gray limestone succeeded upward by mostly gray tuffaceous sandstone, claystone, conglomerate, and minor white limestone and 

chalcedony. In places, some beds in the "white" sequence 

are unconformable on the "red" Claron. In the heart of the 

Marysvale field, the Claron is thin or missing, suggesting 

either that the area was a topographic high and few sediments were deposited or that uplift that preceded deposition 

of the volcanic rocks led to erosion of Claron rocks (Anderson and Rowley, 1975). 

We have not studied the Claron Formation in detail. 

With additional work, the rocks now lumped as the Claron 

could probably be partly or completely reassigned as other 

formations and members, as was done locally for the lower 

Claron by Bowers (1972) in Table Cliff Plateau southeast of 

the Marysvale volcanic field (fig. 1), Bowers (1990) in the 

Bryce Canyon Park area, and Anderson and Kurlich ( 1989) 

and Kurlich and Anderson ( 1991, in press) in the Hatch area 

of the southeastern Markagunt Plateau (fig. 1). In a regional 

study of southwest Utah, Goldstrand (1990, 1992) studied 

and extended Bowers' units and proposed a new unit, all of 

rocks that we formerly may have mapped as the lower part 

of the red Claron. In addition, mapping and stratigraphic 

studies in progress by E.G. Sable, J.J. Anderson, Florian 

Maldonado, and others in the Brian Head area of the southwestern Markagunt Plateau (fig. 1) indicate that the upper 

white part of the sequence that here we previously called the 

Claron is better broken into the Claron (the lower white and 

light-gray limestone, about 60 m thick) and an overlying 

clastic unit (about 300m near Brian Head). The overlying 

clastic unit was mapped in the Red Hills west of the Markagunt Plateau (fig. 1) as the sedimentary and volcanic rocks of 

the Red Hills (Maldonado and Williams, 1993a, b; Maldonado and others, 1992). Based on later work in the Brian 

Head area, this clastic sequence now is identified as the 

Brian Head Formation (restricted) by Maldonado and Moore 

(1993), Anderson (1993), and Maldonado (in press). Presently Claron rocks are the subject of a major study on their 

continental depositional environments and overall origin 

(Mullett and others, 1988a, b; Mullett, 1989). Preliminary 

results of this study indicate that the original strata in the formation were modified and extensively bioturbated during 

development of B soil horizons that formed after deposition. 

The age of the Claron is poorly constrained, and 

because of the incidence of rare invertebrate fossils (Gregory, 1951) that suggested that lower parts are Eocene, we 

previously considered the formation to be Eocene and Oligocene (Anderson and Rowley, 1975). Goldstrand (1990), 

however, noted that palynomorph fossils in samples from 

basal Claron rocks from the eastern Pine Valley Mountains 

indicate a late Paleocene age for these rocks, whereas a fission-track date of 50±6 Ma that he determined from rocks 

collected just below the Claron contact in the Table Cliff Plateau indicates that there the basal Claron is Eocene. He also 

found gastropod fossils in the lower part of the Claron that 

resemble those collected from the Paleocene to Eocene Flagstaff Formation of central Utah by LaRocque (1960). Thus 

we agree with Goldstrand (1990, 1992) that, as currently 

used, the Claron Formation ranges from late Paleocene to 

Oligocene. The current stratigraphic studies of the Brian 

Head Formation, however, may restrict the upper age of the 

Claron, and the long overall age range of the Claron suggests 

its further future subdivision. 

Regionally, the age of the upper parts of the Claron 

and Brian Head Formations is constrained by a widespread 

ash-flow tuff that commonly overlies them, the Oligocene 

Wah Wah Springs Formation of the Needles Range Group 

(seep. 13), whose K-Ar ages average 29.5 Ma (Best, Christiansen, and Blank, 1989) and whose unpublished 

40 Ar/39 Ar ages are somewhat older than 29.5 Ma (M.G. 

Best, written commun., 1993). In the southeastern Bull Valley Mountains (fig. 1), tuff beds of the Wah Wah Springs 

Formation and overlying Isom Formation (described in section, "Intertongued Great Basin Ash-flow Sheets") occur 

interbedded within limestone near the top of basal Tertiary 

units that may correlate with either the Claron Formation or 

the Brian Head Formation (Blank, 1959; Wiley, 1963; 

Hintze, 1986; Anderson and Hintze, 1993). Similarly, 
10 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST-CENTRAL UTAH 

Wah Wah Springs Formation in the top part of the Brian 

Head Formation just east-southeast of Brian Head received 

four new K-Ar dates (see table 1). 

In an effort to better constrain the age of the upper part 

of the Claron and the age of the Brian Head Formation, we 

dated tuffs in the units. Schneider (1967) reported "mica 

zones" in the red Claron rocks at Cedar Breaks National 

Monument but, despite a careful search, we were unable to 

find any such beds. In the northern Markagunt Plateau and 

southern Sevier Plateau (Anderson, Rowley, Machette, and 

Table 1. Data for potassium-argon dates, Marysvale volcanic 

field and adjacent areas, Utah. 

Rock unit, sample No. 2K20 *40Ar(10-1o *40Ar Age in 

(fig. 3); mineral analyzed! (percent) moles/gram) (percent) m.y.±2cr 

Sevier River Formation 

80-331B; s 5.62, 5.58 0.448 34.1 5.6±0.4 

78-609; s 3.48, 3.48 0.684 39.2 13.6±0.8 

Rhyolite ofTeddys Valley 

78-1741; w 5.01, 4.96 0.570 31.6 7.9±0.5 

Rhyolite of Beaver airport 

78-1695A; W 5.13, 5.09 0.615 83.5 8.3±0.3 

Basalt 

M673;W 2.76, 2.80 0.436 55.5 10.9±0.4 

AC-PANG;W 1.60, 1.62 0.1231 21.8 5.3±0.5 

Basaltic andesite 

79-S-3; W 2.51, 2.61 0.912 70.3 21.6±1.0 

Potassium-rich mafic volcanic 

rocks 

79-R-A; W 3.74, 3.79 1.182 78.4 21.7±0.8 

79-R-B. W 3.72, 3.80 1.201 76.4 22.1±0.8 

Mineral Mountains batholith 

78-1835; s 7.75, 7.72 2.496 78.3 22.3±0.8 

78-1835; B 8.33, 8.34 2.719 70.3 22.5±0.9 

Lava flows, Monroe Peak 

caldera 

M678; S 6.05, 6.45 1.835 45.2 20.3±0.8 

M678; B 8.38, 8.39 2.649 78.5 21.8±0.8 

M718; S 7.88, 8.17 2.480 87.4 21.3±0.7 

83-602; s 8.69, 8.70 2.803 70.0 22.3±0.8 

83-602; B 7.19, 7.34 2.377 67.0 22.6±1.0 

Haycock Mountain Tuff 

89USA2A; P 1.002, 0.995 0.218 58.7 22.3±1.1 

89USA1A; B 7.54, 7.64 2.627 48.6 22.8±1.1 

89USAlA; S 11.54, 11.27 0.323 91.8 24.8±1.0 

Pluton, Monroe Peak caldera 

M932;A 6.81, 6.95 2.293 40.8 23.2±1.2 

Antimony Tuff Member 

78-1288; s 8.47, 8.49 3.127 89.9 25.4±0.9 

Tuff of Albinus Canyon 

M739; P 0.73, 0.75 0.273 71.2 25.3±1.3 

Spry intrusion 

84-257; p 0.59, 0.58 0.2214 57.9 26.1±1.8 

Volcanic rocks of Bull Rush 

Creek 

84-259; p 0.47, 0.48 0.1820 18.7 26.4±3.2 

84-259; H 0.65, 0.65 0.2771 40.3 29.4±2.2 

Wah Wah Springs Formation 

USA60A; B 5.76, 6.25 2.6557 58.2 30.4±3.1 USA60A;H 0.95, 0.94 0.4172 68.8 30.4±1.1 USA60B; B 5.93, 5.57 2.7101 54.1 32.4±3.4 USA60B;H 0.83, 0.84 0.3564 85.9 29.1±1.0 

. 1Constants: 40KA.6=0.581x1o-10yr-1, A.p=4.962x1o-10yr-l, 40KfK=l.67xl0-4; 

rmneral analyzed: A, alunite; B, biotite; H, hornblende; P, plagioclase; S, sanidine; 

W, whole rock. 

2J)etermined by flame photometer. 

others, 1990), a local, densely welded, crystal-poor ash-flow 

tuff (sample R-4 of Fleck and others, 1975) in beds apparently equivalent to the Brian Head Formation has a K-Ar 

date on biotite of 31.9±0.5 Ma (Fleck and others, 1975; 

Anderson and Kurlich, 1989). Maldonado (in press) reported 

a K-Ar date of about 34 Ma from an ash-flow tuff bed in the 

Brian Head Formation from the Red Hills. Probably some of 

the airfall and ash-flow tuffs in the Claron and Brian Head 

Formations represent early eruptions from the Marysvale 

volcanic field. 

SAMPLE DESCRIPTIONS 

80-331B. Airfall tuff from upper part of Sevier River Formation, from 3.9 km east of 

Annabella; lat 38°42'38" N ., long 112°00'42" W. 

78--609. Airfall tuff from Sevier River Formation, from 11 km south-southeast of 

Marysvale; lat 38°21'45" N., long 112°10'35" W. 

78-1741. Apache tears from rhyolite lava flow of the rhyolite of Teddys Valley, from 

about 17 km south-southwest of Beaver (shown by Anderson, Rowley, Machette, 

and others, 1990); lat 38°08'55" N., long 112°43'25" W. Preliminary data published by Anderson, Rowley, Machette, and others, 1990. 

78-1695A. Apache tears from rhyolite lava flow of the rhyolite of Beaver airport, from 

about 10 km south-southwest of Beaver (shown by Anderson, Rowley, Machette, 

and others, 1990); lat 38°11'40" N., long 112°41'10" W. Preliminary data first 

published by Anderson, Rowley, Machette, and others, 1990. 

M673. Basalt lava flow from ridge just north of City Creek, west of Junction; lat 

38°16'12" N., long 112°16'22" W. 

AC-P ANG. Basalt lava flow from 1.5 km south-southeast of Panguitch; lat 4 7°48' N ., 

long l12°25'45"W. 

79-S-3. Basaltic andesite lava flow that overlies tuff of Albinus Canyon, from Dry 

Gulch, west of Joseph; lat 38°37'40" N., long 112°15'30" W. 

79-R-A. Lava flow in fault block poking through basin-fill sediments, from 1.5 km 

east-northeast of the racetrack east of Beaver; lat 38°17'10" N., long 

112°35'20" w. 

79-R-B. Lava flow capping Black Mountain, from 3 km southeast of the racetrack east 

of Beaver; lat 38°15'50" N., long 112°34'30" W. 

78-1835. Glassy margin of a hypabyssal plug in southern Mineral Mountains; lat 

38°14'50" N., long 112°51'20" W. 

M678. Lava flows of Monroe Peak, within Monroe Peak caldera, from roadcut 0.35 km 

northeast of Hunts Lake and 16.5 km northeast of Marysvale; lat 38°32'54" N., 

long 112°05' W. 

M718. Lava flows of Monroe Peak, within Monroe Peak caldera, from 9.6 km southsoutheastofJoseph (shown by Rowley and others, 1988b); lat 38°32'30"N., long 

112°10'40" W. Preliminary data published by Rowley and others (1988b). See 

also fission-track date of this sample in table 2. 

83--602. Lava flows from volcanic dome of Lower Box Creek Reservoir, within Monroe Peak caldera; lat 38"28'15" N., long Ill 0

58' W. 

89USA2A. Basal vitrophyre of Haycock Mountain Tuff from the southwest side of the 

top of Haycock Mountain; lat 37°43'03" N., long 112°34'50" W. 

89USA1A. Haycock Mountain Tuff from Bunker Creek; 0.2 km north of dirt road and 

3.4 km west-southwest of Panguitch Lake, lat 37°42'01" N., long 112°41'40" W. 

M932. Replacement alunite due to emplacement of intracaldera pluton of Monroe Peak 

caldera, from summit of Monroe Peak; lat 38°28'42" N., long 112°06'16" W. 

78-1288. Basal vitrophyre of Antimony Tuff Member of Mount Dutton Formation, 

from 22 km southeast of Marysvale; lat 38°20'45" N., long 112°04'35" W. 

M739. Ash-flow tuff collected in Albinus Canyon, from 3 km west-northwest of Elsinore; lat 38°41'46" N., long 112°11'30" W. 

84-257. Intrusive rock from roadcut along U.S. Highway 89 about 17 km south-southwest of Circleville (shown by Anderson, Rowley, Blackman, and others, 1990); 

lat 38°02'10" N., long 112°21'20" W. Preliminary data published by Anderson, 

Rowley, Blackman, and others, 1990. 

84-259. Dike of the volcanic rocks of Bull Rush Creek, from about 16.5 km southsouthwest of Circleville (shown by Anderson, Rowley, Blackman, and others, 

1990); lat 38001'55" N., long 112°19'25" W. Preliminary data published by 

Anderson, Rowley, Blackman, and others, 1990. 

USA60. Ash-flow tuff about 3m thick and located 3m below Isom Formation (sample 

USA60A is from higher in the bed than B) in Brian Head Formation from Lowder 

Creek, from 3.7 km east-southeast of Brian Head; lat 37°40'52" N., long 

112°4 7'48" w. 
MIDDLE CENOZOIC CALC-ALKALINE IGNEOUS SEQUENCE 11 

MIDDLE CENOZOIC CALC-ALKALINE 

IGNEOUS SEQUENCE 

BULLION CANYON VOLCANICS 

The Oligocene and Miocene Bullion Canyon Volcanics 

(Callaghan, 1939; Steven and others, 1979) is a complex 

assemblage of stratovolcano rocks in the central and northem parts of the Marysvale volcanic field. The rocks range in 

composition from andesite to low-silica rhyolite, but most 

are dacitic and commonly contain abundant phenocrysts of 

mostly plagioclase and subordinate amounts of biotite, hornblende, and pyroxene. These relatively highly evolved rocks 

were derived from shallow source cupolas of an underlying 

batholith; numerous intrusions are currently exposed, 

emplaced into Bullion Canyon and other volcanic units. 

Using terminology adapted from Parsons (1965, 1969) and 

Smedes and Prostka (1973), Anderson and Rowley (1975) 

applied "vent facies" to rocks deposited in and near sources 

made up of clustered stratovolcanoes and ''alluvial facies" 

for more distal flank asemblages. Vent-facies rocks consist 

mainly of lava flows, autoclastic flow breccia, and volcanic 

mudflow breccia that were deposited on the slope of a volcanic edifice (stratovolcano), whereas alluvial facies rocks 

consist of volcanic mudflow breccia, conglomerate, and 

sandstone deposited farther away on the edifice and in valleys draining the edifice. The Bullion Canyon Volcanics fits 

these criteria, although intertonguing vent facies and alluvial 

facies were not separated during mapping. Bullion Canyon 

vent-facies rocks are concentrated in the area between the 

central Tushar Mountains and the western Sevier Plateau; 

alluvial-facies rocks occur in other parts of the Tushar 

Mountains as well as in the southern Pavant Range and 

northern Sevier Plateau. The thickness of the formation is 

probably at least 1,500 m, although no complete sections are 

known in the heart of the vent complex, where deposits are 

thickest. Basal parts of the Bullion Canyon Volcanics underlie the >29.5-Ma Wah Wah Springs Formation. A clast from 

a bed of apparent volcanic mudflow breccia near the base of 

the unit in the southwestern Tushar Mountains was dated 

directly using fission-track methods on zircon at 29.9±2.6 

Ma (Kowallis and Best, 1990); the bed underlies the 27-Ma 

Three Creeks Tuff Member (fig. 2; see paragraph following), 

but its stratigraphic relationship to the Wah Wah Springs 

Formation is unknown. Airfall and waterlaid tuff near Salina 

that is believed to have been derived from the Marysvale volcanic field was dated by K-Ar methods at 34.2 Ma by Willis 

(1985). Along with the tuffs in the Brian Head Formation, 

this is the oldest volcanic activity yet reported in the Marysvale field, and it may represent early eruptions of the Bullion 

Canyon Volcanics. 

Subdivision of the Bullion Canyon Volcanics is partly 

based upon mapping distinctive ash-flow tuff members in 

the unit. One of these is a thick, crystal-rich, ash-flow sheet 

that earlier (for example, Rowley, 1968) was correlated with 

one of the ash-flow tuffs of the Needles Range Group (see 

section, "Intertongued Great Basin Ash-flow Sheets"). Best 

and others (1973), during a regional stratigraphic study of 

Needles Range tuffs, traced the same unit over large parts of 

the Great Basin and proposed for it the name W allaces Peak 

Tuff Member of the Needles Range Formation (later raised 

to group rank; fig. 2); they interpreted it to be the youngest 

Needles Range unit. Later mapping in the Marysvale field 

showed that the unit was derived from the Marysvale field, 

and Steven and others (1979) defined it as the Three Creeks 

Tuff Member of the Bullion Canyon Volcanics. Best and 

Grant (1987) later correlated the Wallaces Peak with the 

Three Creeks, then abandoned the name Wallaces Peak in 

favor of Three Creeks; they also elevated the rank of Needles 

Range to a group. Some exposures of Three Creeks Tuff 

Member miscorrelated with the Needles Range Group (for 

example, Cunningham and others, 1983) are now recognized 

as the Three Creeks (in for example, Rowley and others, 

1987; Steven and others, 1990). The Three Creeks Tuff 

Member is a light-gray and tan, moderately welded, crystalrich, dacitic ash-flow tuff that occurs in the lower part of the 

Bullion Canyon Volcanics and is spread over much of the 

southern High Plateaus and adjacent Great Basin. It probably 

was the largest ash-flow tuff from the Marysvale field and 

was derived from the Three Creeks caldera in the southern 

Pavant Range (Steven, 1981). A thin sequence of mostly volcanic mudflow breccia and conglomerate containing abundant clasts of the Three Creeks Tuff Member was mapped in 

a few places in the northern Sevier Plateau (Cunningham and 

others, 1983) and called the volcanic rocks of Cliff Canyon; 

it represents erosion of near-source Three Creeks rocks and 

should be included with the Bullion Canyon Volcanics. Similar clasts in mudflow and conglomerate beds at the same 

stratigraphic level have been seen in the southern Sevier Plateau (Rowley, 1968) and in the Brian Head Formation in the 

Red Hills (Florian Maldonado, written commun., 1993). 

Steven and others (1979) reported a series of K-Ar and fission-track mineral ages on the Three Creeks Tuff Member, 

two K-Ar dates (32.1±1.4 and 30.6±1.3 Ma, both on biotite) 

of which were rejected because they predate the underlying 

Wah Wah Springs Formation; K-Ar dates on plagioclase of 

26.9±2.2 and 28.9±1.3 Ma and fission-track dates on zircon 

of 27.4±1.3 and 27.0±1.2 Ma, however, are reasonable. In 

addition, Caskey and Shuey (1975) reported a K-Ar date on 

biotite of 27.5±0.4 Ma. Kowallis and Best (1990) published 

a zircon fission-track date of 25.1±2.2 Ma on a sample of the 

Three Creeks collected from western Utah. The average age 

of the six dates that we consider reliable is 27.1 Ma, our best 

estimate of the age of the member. 

The Delano Peak Tuff Member was redefined by 

Steven and others (1979) from the Delano Peak Latite Member initially named by Callaghan (1939) that occurs in the 

upper part of the Bullion Canyon Volcanics. It is a dark-reddish-brown, densely welded, crystal-rich, dacitic ash-flow 
12 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST-CENTRAL UTAH 

tuff whose distribution is largely confined to the Tushar 

Mountains. It was derived from the Big John caldera of the 

central Tushar Mountains (Steven, Cunningham, and Anderson, 1984). Steven and others (1979) determined three discordant dates on a single sample of the unit. One, a K-Ar date 

on biotite of 28.9±1.4 Ma, is suspect because it is older than 

the Three Creeks Tuff Member; another, a fission-track date 

on zircon of 21.8±1.0 Ma, is suspect because it is younger 

than the Osiris Tuff (see section, "Monroe Peak Caldera"), 

which stratigraphically overlies it. The third, a fission-track 

date on apatite of 23.7±3.6 Ma, has a larger error bar than the 

other two but is the most reasonable based on stratigraphic 

relationships and ages of underlying and overlying units. 

Our best estimate of the age of the Delano Peak is thus 24 

Ma, although it could be slightly younger. Steven, Cunningham, and Anderson (1984) published a fission-track date on 

zircon of 22.6±1.0 Ma, but it is suspect because it is numerically younger than the age of the overlying Osiris Tuff. 

Thick sequences of flows and breccia of the Bullion Canyon 

Volcanics overlie the Delano Peak Tuff Member. 

MOUNT DUTTON FORMATION 

The Oligocene and Miocene Mount Dutton Formation 

(Anderson and Rowley, 1975) makes up another complex 

assemblage of stratovolcano deposits that is comparable in 

thickness (about 1,500 m) to, although more widespread and 

voluminous than, the Bullion Canyon Volcanics. The formation intertongues with the south edges of the Bullion Canyon 

Volcanics, and makes up the southern flank of the Marysvale 

field. The rocks are mostly andesite and subordinate dacite 

(Mattox and Walker, 1989; Mattox, 1992) and are in general 

more mafic than those of the Bullion Canyon. The Mount 

Dutton also differs from the Bullion Canyon in that in large 

part it contains sparse, small phenocrysts, mostly of pyroxene and plagioclase. These less evolved rocks apparently 

came from much deeper source magma chambers than the 

numerous consanguineous intrusions that fed and were 

emplaced into the Bullion Canyon Volcanics. Thus only 

dikes and sparse small plugs of the Mount Dutton Formation 

are presently exposed, as opposed to the large and abundant 

plutons associated with the Bullion Canyon Volcanics. Most 

of the clustered stratovolcano sources of the Mount Dutton 

Formation make up an east-trending belt along the Blue Ribbon lineament (Rowley, Lipman, and others, 1978), which 

extends from the northern Black Mountains eastward along 

the boundary between the southern Tushar Mountains and 

northern Markagunt Plateau and into parts of the southern 

Sevier Plateau. Most vents that have been mapped are spectacularly exposed locally (Blackman, 1985; Anderson, Rowley, Machette, and others, 1990; Anderson, Rowley, 

Blackman, and others, 1990) so that Anderson and coworkers were able in most places to separate vent-facies rocks 

from more distal alluvial-facies rocks. The formation can be 

subdivided in many places by thin ash-flow tuff members 

that serve as markers in the thick stratovolcano sequence. 

Lower parts of the formation underlie the Wah Wah Springs 

Formation, and upper parts overlie the Osiris Tuff (see p. 

17). Fleck and others (1975) determined the following K-Ar 

ages on flows and dikes from within the Mount Dutton Formation: (1) 26.7±0.8 Ma on hornblende from a lava flow 

(their sample R-19) from low in a section of vent-facies 

rocks in the southeastern Tushar Mountains; (2) 25.8±0.7 

Ma on whole rock from a lava flow (sample R-5) from low 

in a section of alluvial-facies rocks in the northern Markagunt Plateau; (3) 23.5±0.4 Ma on whole rock from a dike 

(sample R-23) of upper vent-facies rocks in the southern 

Tushar Mountains; (4) 23.0±0.4 Ma on biotite from a lava 

flow (sample R-24; incorrectly reported as the Osiris Tuff in 

Fleck and others, 1975) from high in a section of vent-facies 

rocks in the southwestern Tushar Mountains; and (5) 

21.2±0.5 Ma on whole rock from a lava flow (sample R-27) 

in alluvial-facies rocks that may represent an extrusive product of the Iron Peak intrusion (see section, "Calc-alkaline 

Intrusions") in the northern Markagunt Plateau but is 

mapped within the top of the Mount Dutton Formation. Mattox (1992) reported one K-Ar whole-rock date of 24.9±1.1 

Ma on a basalt lava flow of the Mount Dutton Formation 

from Little Creek Peak in the northern Markagunt Plateau. 

The Beaver Member of the Mount Dutton Formation 

(Anderson and Rowley, 1975) is a sequence of crystal-rich 

dacitic volcanic domes and lava flows occurring low in the 

Mount Dutton Formation, above the Wah Wah Springs Formation. It is exposed in the northwestern Markagunt Plateau 

and eastern Black Mountains (Anderson, Rowley, Machette, 

and others, 1990). Fleck and others (1975) determined two 

K-Ar dates on plagioclase from the member, 26.2±0.5 (sample R-13) and 25.0±0.8 (sample R-15) Ma. Our best estimate 

of the age of the member is the average ofthe dates, 25.6 Ma. 

The Kingston Canyon Tuff Member of the Mount Dutton Formation (Anderson and Rowley, 1975) is a thin, purple, densely welded, crystal-poor, trachytic ash-flow tuff 

occurring within alluvial-facies mudflow breccia in the 

Sevier Plateau and the southwestern Tushar Mountains; it is 

rarely exposed within vent-facies rocks. It overlies the Wah 

Wah Springs Formation and Three Creeks Member of the 

Bullion Canyon Volcanics in the lower part of the Mount 

Dutton sequence. Its source has not been discovered, but it is 

similar in petrology to the tuff of Albinus Canyon (see section, "Local Volcanic Centers") that is exposed in the general area of the town of Sevier. Although the source of that 

unit is also unknown, textural features in the Albinus Canyon 

rocks near Sevier suggest proximity here to its source, perhaps under the basin-fill sedimentary rocks of nearby Sevier 

Valley. Fleck and others ( 197 5) determined a K-Ar date of 

25.8±0.4 Ma on biotite from the Kingston Canyon Tuff 

Member (sample R-1). 

The Antimony Tuff Member of the Mount Dutton Formation (Anderson and Rowley, 1975) occurs above the 
MIDDLE CENOZOIC CALC-ALKALINE IGNEOUS SEQUENCE 13 

Kingston Canyon Tuff Member, and volcanic mudflow breccia locally more than 300 m thick generally separates them. 

The Antimony Tuff Member is a thin, red, densely welded, 

crystal-poor trachytic ash-flow tuff similar to the Kingston 

Canyon Tuff Member, and it likewise is exposed mostly 

within alluvial-facies rocks in the Sevier Plateau. It is chemically and petrologically almost identical to the tuff of Albinus Canyon, on which it rests near Sevier (Rowley and 

others, 1988b ), and thus it probably was derived from the 

same source as the tuff of Albinus Canyon. A new K-Ar date 

on sanidine from a sample (78-1288) of the Antimony Tuff 

Member collected from the central Sevier Plateau is 

25.4±0.9 Ma (table 1). In addition to the Kingston Canyon 

and Antimony Tuff Members, a petrologically similar informal unit, the tuff member of Piute Reservoir, was mapped 

between Kingston and Antimony in the western part of the 

central Sevier Plateau (Rowley, Cunningham, and others, 

1979). 

INTERTONGUED GREAT BASIN ASH-FLOW 

SHEETS 

Regional ash-flow tuff units derived from caldera 

sources in the Great Basin, generally southwest of the 

Marysvale field, are locally intertongued marginally with 

rocks of the Marysvale field, where they provide thin, welldated, distinctive stratigraphic markers. The tuff units are 

widespread southwest of the Marysvale field in southwestem Utah and southeastern Nevada (Mackin, 1960; Anderson 

and Rowley, 1975; Siders and Shubat, 1986; Best, Christiansen, and others, 1989; Best and others, 1993; Rowley, 

Nealey, and others, in press; Scott and others, in press). 

Many are intertongued with Marysvale rocks only in the 

southwestern part of the Marysvale field (in the southern 

Black Mountains and Markagunt Plateau); an exception is 

the Wah Wah Springs Formation, which is distributed over 

most of the Marysvale field (Row ley, Anderson, and others, 

1978; Best, Christiansen, and Blank, 1989). Rowley, Anderson, and others ( 1978) suggested that the distribution pattern 

of the post-Wah Wah Springs tuffs was influenced by an 

early (26-24 Ma) episode of structural uplift of the Colorado 

Plateau with respect to the Great Basin. This suggestion was 

disputed by Anderson and Mehnert (1979), who provided 

evidence for east-sloping physiography across the Great 

Basin-Colorado Plateau boundary south of Cedar City, 

Utah, sometime following emplacement of plutons in the 

Pine Valley Mountains (21.5 Ma). Maldonado (in press) 

favored an easterly sloping northern Markagunt Plateau at 

about 21-20 Ma. Nonetheless, Great Basin tuffs pinch out 

along the southwest edges of the Marysvale field, so a 

regional paleoslope in this area to the west or southwest 

remains possible; such a west or southwesterly paleoslope 

could be due to structural control or to highlands caused by 

the locally derived Marysvale volcanic rocks. Of these 

alternatives, J.J. Anderson (1985, 1988, 1993) favored structural control by west-northwest-striking, southwest-facing 

fault scarps that formed after 26 Ma. 

The oldest of the Great Basin tuff sheets in the Marysvale volcanic field is the widespread Wah Wah Springs Formation of the Needles Range Group (Mackin, 1960; Best and 

Grant, 1987). This is a light-gray, pink, and tan, moderately 

welded, crystal-rich, dacitic ash-flow tuff derived from the 

huge Indian Peak caldera straddling the Nevada-Utah State 

line and north of Modena (fig. 1) (Best, Christiansen, and 

Blank, 1989). The tuff generally rests on the Claron and 

Brian Head Formations and intertongues with lower parts of 

the Bullion Canyon Volcanics and Mount Dutton Formation. 

It extends even farther to the northeast into and north of the 

Marysvale field than the distribution shown by Best, Christiansen, and Blank (1989, fig. 5b; Best, Christiansen, and 

others, 1989, fig. R29) according to recent field and paleomagnetic studies by C.S. Gramme and M.G. Best (M.G. 

Best, written commun., 1993). The average K-Ar age of the 

Wah Wah Springs Formation is 29.5 Ma (Best, Christiansen, 

and Blank, 1989), but unpublished 40 ArJ39 Ar dates indicate 

that the actual age is somewhat older than this (M.G. Best, 

written commun., 1993). E.G. Sable collected a 3-m-thick 

ash-flow tuff of the Wah Wah Springs Formation from the 

top part of what we now call the Brian Head Formation and 

a few meters below the Isom Formation, from Lowder 

Creek, about 3 km east-southeast of Brian Head (fig. 3). Four 

new dates on biotite and hornblende, respectively, on each of 

two samples from the same bed are: 30.4±3.1 and 30.4±1.1 

Ma (sample USA60A) and 32.4±3.4 and 29.1±1.0 Ma (sample USA60B) (table 1). 

Above the Wah Wah Springs Formation, but exposed 

only in a few places in the Black Mountains and northern 

Markagunt Plateau (Best, Christiansen, and Blank, 1989; 

Best, Christiansen, and others, 1989), is the Lund Formation 

of the Needles Range Group. It resembles the Wah Wah 

Springs Formation but was derived from the White Rock 

caldera, which overlaps the west side of the Indian Peak 

caldera. The Lund Formation has an average isotopic age of 

27.9 Ma (Best, Christiansen, and Blank, 1989), which 

accords with the average age of 27.1 Ma that we determined 

for the overlying Three Creeks Tuff Member of the Bullion 

Canyon Volcanics. 

The IsomFormation (Mackin, 1960) consists of numerous densely welded, crystal-poor, trachytic ash-flow tuff, 

tufflava, and lava flows. The source of the formation is 

unknown and probably buried by younger rocks, but based 

on isopach data it seems to be on the southeast side of the 

Indian Peak caldera (Best, Christiansen, and Blank, 1989). 

The formation is more than 200 m thick in much of southwestern Utah, including the western part of the northern 

Markagunt Plateau, but farther east and north it is thin; it is 

not present in the Tushar Mountains. The lowest member of 

the Isom Formation in the Markagunt Plateau is the Blue 

Meadows Tuff Member, a simple ash-flow cooling unit 
14 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST-CENTRAL UTAH 

apparently restricted to the plateau. We have yet to find the 

Blue Meadows west of the Markagunt Plateau, in the direction of the source of the Isom Formation, so it is possible that 

it is a small local tuff derived from the Marysvale field and 

related to the lithologically similar Kingston Canyon and 

Antimony Tuff Members of the Mount Dutton Formation 

(J.J. Anderson, unpub. data, 1992). The Blue Meadows has a 

K-Ar date on plagioclase of 25.9±0.4 Ma (sample R-7; Fleck 

and others, 1975). 

Most Isom cooling units in the southwestern Marysvale 

field and to the west are contained within the Baldhills Tuff 

Member, and they are mostly dark reddish brown and dark 

gray. The average K-Ar age of tuffs correlated with the 

Baldhills Member is 25.7 Ma based on two dates, one by 

Armstrong (1970) on whole rock of 25.7±0.5 Ma (sample 

160) and one by Fleck and others (1975) on plagioclase of 

25.7±0.4 Ma (sample R-8). Scott and others (in press) 

recently published a plagioclase K-Ar date of 25.9±0.8 Ma 

determined by H.H. Mehnert on a Baldhills sample from the 

Delamar Mountains of Nevada. These ages, however, may 

have been superseded by a 40 Arf39 Ar date of about 27 Ma on 

the Baldhills published by Best, Christiansen, and others 

(1989; fig. R1; no analytical data yet published). This date is 

similar to a 40 Arf39 Ar date on plagioclase of 27 .4±2.5 Ma for 

the "tuff of Hamilton Spring" (Taylor and others, 1989), a 

unit correlated on the basis of paleomagnetic data and petrology with the Baldhills Tuff Member (C.S. Gromme, written 

commun., 1989). The uppermost member of the Isom Formation is the Hole-in-the-Wall Tuff Member, an undated, 

red, simple cooling unit that, unlike the Baldhills, was recognized by C.S. Gromme to overlie the Shingle Pass Tuff of 

Nevada (Scott and others, in press); the upper cooling unit of 

the Shingle Pass Tuff has a 40 Ar/39 Ar date of 26.0 Ma (Best, 

Christiansen, and others, 1989, table R3; analytical data not 

published). The discrepancy of the Ar ages with the concordant K-Ar ages suggests that the samples of the ''Baldhills" 

dated by the K-Ar method are from upper parts of the 

Baldhills Tuff Member or are from the Hole-in-the-Wall 

Tuff Member. The ages on the Blue Meadows suggest that it 

is mapped in the upper part of the Baldhills west of the Markagunt Plateau. 

A series of ash-flow tuff units belonging to the 

Quichapa Group (Williams, 1967; Anderson and Rowley, 

1975) are exposed along the southwestern margin of the 

Marysvale field. The oldest of these is the Leach Canyon 

Formation, a tan, poorly welded, crystal-poor, rhyolite ashflow tuff whose source is not known but which, on the basis 

of isopach data (Williams, 1967), may be near the north side 

of the Caliente caldera complex of Nevada-Utah, about 180 

km southwest of Marysvale. The average age of the Leach 

Canyon Formation is 24.0 Ma, on the basis of three K-Ar 

.dates (26.7±1.0 Ma on plagioclase from a sample from 

Nevada and 24.6±0.5 Ma on biotite and 22.9±0.5 Ma on 

sanidine from a sample from Utah) by Armstrong (1970) and 

two zircon fission-track dates (24.2±2.0 Ma from a sample 

from Utah and 21.6±2.0 Ma from a sample from Nevada) by 

Kowallis and Best (1990). These discordant ages make it 

impossible for us to determine the actual age of the tuff at 

this time. Preliminary Ar dates obtained by A.L. Deino and 

M.G. Best (oral commun., 1991), however, suggest an age of 

23.8 Ma, Miocene (Best and others, 1993). 

The Condor Canyon Formation of the Quichapa Group 

consists of two dark-reddish-brown, densely welded, crystalpoor, rhyolite ash-flow tuff units, from older to younger the 

Swett Tuff Member and the Bauers Tuff Member. The 

Bauers Tuff Member and probably the Swett Tuff Member 

were derived from the Clover Creek caldera, near Caliente in 

the Caliente caldera complex (Rowley and Siders, 1988; 

Rowley, Anderson, and others, 1990; Rowley and others, 

1992). The average isotopic age of the Swett is 23.7 Ma, representing six K-Ar dates on biotite and plagioclase by Armstrong (1970). The average K-Ar age of the Bauers is 22.3 

Ma from two determinations on biotite and sanidine by Armstrong (1970) and one on plagioclase by Fleck and others 

(1975), but these ages have been supplanted by more precise 

40 ArJ39 Ar determinations that average 22.78 Ma (Best, 

Christiansen, and others, 1989, table R3; no analytical data 

yet published). A similar unpublished 40 Ar/39 Ar date on 

sanidine of 22.8±0.1 Ma by L.W. Snee was obtained from a 

sample from the intracaldera pluton in the Clover Creek 

caldera (Rowley, Shroba, and others, in press). 

The Harmony Hills Tuff, the youngest unit of the 

Quichapa Group, is a light-gray and pink, moderately 

welded, crystal-rich, high-alkali andesite ash-flow tuff. The 

caldera source has not been found, but isopach and distribution data suggest that it is in the Bull Valley Mountains of 

Utah (Blank, 1959; Williams, 1967; H.R. Blank, oral commun., 1990). The average K-Ar age of the Harmony Hills 

Tuff is 21.6 Ma from five dates on biotite and plagioclase by 

Armstrong (1970) and one on biotite by Noble and McKee 

(1972); the actual age, however, is probably 22.5 to 22 Ma 

on the basis of isotopic ages of plutons and ash-flow tuff 

units in the Iron Springs mining district (Rowley and others, 

1989) that postdate the Harmony Hills. 

LOCAL VOLCANIC CENTERS 

The Marysvale volcanic field contains numerous local 

calc-alkaline volcanic centers whose products were mapped 

separately from the Bullion Canyon Volcanics and Mount 

Dutton Formation. Centers include stratovolcanoes, volcanic 

domes, shield volcanoes, and tuff vents, and they consist of 

lava flows, flow breccia, volcanic mudflow breccia, and tuff. 

The oldest rocks belong to an andesitic to dacitic vent- and 

alluvial-facies sequence of probable stratovolcano origin 

that underlies the Three Creeks Tuff Member of the Bullion 

Canyon Volcanics in the southern Pavant Range. These were 

mapped as informal units at 1:50,000 by Cunningham and 

others ( 1983) as the volcanic rocks of Dog Valley and the 
MIDDLE CENOZOIC CALC-ALKALINE IGNEOUS SEQUENCE 15 

overlying volcanic rocks of Wales Canyon; during compilation at 1:250,000, Steven and others (1990) lumped them 

with the Bullion Canyon Volcanics. The volcanic rocks of 

Dog Valley are at least 400 m thick; the Wah Wah Springs 

Formation is shown intertongued in the middle of the section 

by Cunningham and others (1983), but we now correlate this 

tuff with the Three Creeks Tuff Member. A lava flow in the 

lower part of the volcanic rocks of Dog Valley below the 

Three Creeks Tuff Member yielded a fission-track date on 

zircon of 33.6±2.6 Ma (Kowallis and Best, 1990). The volcanic rocks of Wales Canyon, with a thickness of at least 200 

m, occur between the Wah Wah Springs Formation and the 

Three Creeks Tuff Member. About 7 km west of Joseph, also 

in the southern Pavant Range, a dacitic center at least 6 km 

in diameter, probably a volcanic dome complex, formed 

after deposition of the volcanic rocks of Wales Canyon and 

before deposition of the Three Creeks Tuff Member. This 

complex was mapped informally as a quartz latite and rhyodacite volcanic dome and flow (unit T qd) by Cunningham 

and others (1983). The complex remained a topographic 

highland during later deposition of the Three Creeks Tuff 

Member, which pinched out against it. 

A thick sequence of andesite and basaltic andesite lava 

flows, flow breccia, and volcanic mudflow breccia that overlies the Three Creeks Tuff Member makes up a volcanic plateau and shield volcano in the northern Sevier Plateau. This 

sequence has been called informally the volcanic rocks of 

Signal Peak and is at least 1,000 m thick. The Antimony Tuff 

Member is intertongued with lava flows in the middle and 

upper part of the sequence. To the west, in the Antelope 

Range, the Osiris Tuff (23 Ma) overlies similar basaltic 

andesite lava flows, as much as 50 m thick, that in most 

places rest on the Antimony Tuff Member, which in turn 

rests on the tuff of Albinus Canyon (Rowley and others, 

1988b ). In the southeastern Pavant Range, the basaltic 

andesite is thicker and overlies the tuff of Albinus Canyon, 

although some of the basaltic andesite may intertongue with 

the tuff of Albinus Canyon, which in turn overlies the Three 

Creeks Tuff Member. The volcanic rocks of Signal Peak are 

extensive east of the area we have mapped (Rowley and others, 1986b) and were previously mapped as the latite and 

basaltic andesite flows undifferentiated unit of Williams and 

Hackman (1971). 

A new whole-rock K-Ar date of 21.6±1.0 Ma (table 1) 

on basaltic andesite (sample 79-S-3) that overlies the tuff of 

Albin us Canyon in the southeastern Pavant Range is surprisingly young because in the northern Antelope Range 5 km to 

the southeast, lithologically similar basaltic andesite underlies the Osiris Tuff. This calc-alkaline basaltic andesite may 

represent late volcanism that formed the volcanic rocks of 

Signal Peak. The date indicates that at least some of the 

basaltic andesite is the same age as the potassium-rich mafic 

volcanic rocks (see section of this name). However, the 

chemistry of the basaltic andesite is distinctly different from 

that of both the potassium-rich mafic volcanic rocks and the 

tuff of Albinus Canyon; thus they are mapped separately 

(Cunningham and others, 1983). 

The tuff of Albinus Canyon consists of numerous thin 

cooling units, as much as 200 m thick, of red and gray, vesicular, densely welded, crystal-poor, trachytic ash-flow tuff, 

tufflava, and basaltic andesite lava flows. The tuffs and 

tufflavas resemble those of the Isom Formation as well as the 

Kingston Canyon and Antimony Tuff Members of the 

Mount Dutton Formation. A new K-Ar date on plagioclase 

(sample M739) from the tuff of Albinus Canyon in the 

Pavant Range is 25.3±1.3 Ma (table 1); this is concordant, 

within analytical error, with dates on the Kingston Canyon 

and Antimony. The tuff of Albinus Canyon is considered a 

proximal accumulation from a nearby source that also 

erupted the Kingston Canyon and Antimony Tuff Members. 

The similarity in composition, appearance, and age of the 

tuff of Albinus Canyon and the Kingston Canyon and Antimony Tuff Members with the units of the Isom Formation 

and other units from sources in Nevada is a regional petrologic problem about which we and others currently are perplexed. Best, Christiansen, and others (1989) have discussed 

the Isom compositional type of tuff and other compositional 

types and have suggested that they represent magmas in different areas that had similar origins and crystallization 

histories. 

In the central and northern Sevier Plateau, products of 

several stratovolcanoes and shield volcanoes were mapped 

(Cunningham and others, 1983) between the Bullion Canyon 

Volcanics to the north and west and the Mount Dutton Formation to the south. The oldest of these, the volcanic rocks 

of Little Table, consists of andesitic and trachyandesitic lava 

flows, flow breccia, and volcanic mudflow breccia making 

up a volcanic plateau derived from either a stratovolcano or 

a shield volcano. Both vent- and alluvial-facies rocks were 

mapped, with a total thickness at any one place of at least 700 

m. The unit overlies the Three Creeks Tuff Member of the 

Bullion Canyon Volcanics and intertongues with the Mount 

Dutton Formation, including the Kingston Canyon and Antimony Tuff Members. The upper part of the volcanic rocks of 

Little Table intertongues with and is overlain by two local, 

informally named stratovolcano sequences, one a dacitic to 

andesitic alluvial-facies sequence named the volcanic rocks 

of Willow Spring, at least 150 m thick, and the other a 

trachyandesitic to trachydacitic alluvial- and vent-facies 

sequence named the volcanic rocks of Langdon Mountain, at 

least 400 m thick. These two units postdate the Antimony 

Tuff Member of the Mount Dutton Formation and predate 

the Osiris Tuff. The volcanic rocks of Little Table, Willow 

Spring, and Langdon Mountain are probably more extensive 

east of our area of detailed mapping (Rowley and others, 

1986a, b; Mattox, 1991a), where only reconnaissance maps 

are now available (Williams and Hackman, 1971 ). 

A series of shallow calc-alkaline laccoliths unrelated to 

Mount Dutton magmatism were emplaced into the Claron 

Formation in the northern Markagunt Plateau and adjacent 
16 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST -CENTRAL UTAH 

areas (Anderson and Rowley, 1975). These laccoliths are 

south of the area of Mount Dutton sources, and they were 

intruded in a region where alluvial-facies Mount Dutton volcanic rocks are relatively thin. The largest of these laccoliths 

is the Spry intrusion (described in detail in the section, 

"Calc-alkaline Intrusions"), which is consanguineous with 

an overlying dacitic central vent complex consisting of lava 

flows and tuff; the deposits of the vent complex intertongue 

with the lower part of the Mount Dutton Formation (Anderson, Rowley, Blackman, and others, 1990). During and after 

volcanic eruptions, the Spry intrusion invaded the lower part 

of the related volcanic edifice. The central vent complex is 

exposed in unnamed hills between the northern Markagunt 

Plateau on the west and the southern Sevier Plateau on the 

east, about 15 km south-southwest of Circleville. The central 

vent complex is on the northeast side of the Spry intrusion 

and consists of as much as 250 m of lava flows, dikes, volcanic mudflow breccia, sandstone, and tuff, probably including 

the Buckskin Breccia (see next paragraph). This vent complex is informally named the volcanic rocks of Bull Rush 

Creek (Anderson, Rowley, Blackman, and others, 1990). 

Two K-Ar dates determined on a sample from a glassy dike 

of the unit were first reported by Anderson, Rowley, Blackman, and others (1990) and are repeated here (table 1, sample 84-259). One of these dates, 29 .4±2.2 Ma on hornblende, 

is rejected because it is considered too old due to excess 

argon; the other, 26.4±3.2 Ma on plagioclase from the same 

sample, is geologically reasonable and is similar to a new 

date on the Spry intrusion (see following). 

The most widespread product of the Spry intrusion vent 

complex is the Buckskin Breccia (Anderson and Rowley, 

197 5), which consists of at least five cooling units of a tan and 

light-gray, poorly welded, crystal-poor, dacitic ash-flow tuff 

that spread over much of the northern Markagunt Plateau and 

southern Sevier Plateau (Yannacci, 1986). Eruption of the 

Buckskin Breccia may have produced a caldera, but none has 

been found. The Buckskin Breccia is distinctive in that it is 

characterized by as much as 50 percent by volume of cognate 

lithic fragments of white, crystal-rich flow rocks that are petrologically similar to quartz monzonite porphyry of the Spry 

intrusion and to flows and dikes of the volcanic rocks of Bull 

Rush Creek. The Buckskin Breccia, which overlies and intertongues with rocks of the Isom Formation, exhibits large 

thickness variations, from zero to 250m, because it fills westnorthwest-striking grabens that formed after emplacement of 

the Isom (Anderson, 1965, 1971, 1993). The Buckskin Breccia probably dates at 26 Ma because it interfingers with some 

cooling units of the Isom Formation (Anderson, Rowley, 

Machette, and others, 1990) and because of dates of this age 

on the Spry intrusion (described in a subsequent section) and 

volcanic rocks of Bull Rush Creek. The youngest age of the 

Buckskin Breccia is constrained by the overlying Bear Valley Formation, an eolian sandstone locally more than 300 m 

thick that filled the remainder of some of the grabens (Anderson, 1971). Fleck and others (1975) determined two K-Ar 

dates on tuff beds in the upper part of the Bear Valley Formation, one of 24.6±0.4 Ma from biotite from an airfall tuff 

(sample R-10) and one of24.5±0.5 Ma from plagioclase from 

a crystal-poor ash-flow tuff (sample R-9). Armstrong (1970) 

determined two K-Ar dates on separates of hornblende-bearing biotite from the "Rodger (sic) Park [Basaltic] Breccia" (a 

stratigraphic name proposed by Callaghan and Parker, 

1962a, but abandoned by Anderson and Rowley, 1975) from 

the northern Markagunt Plateau of 26.5±0.5 and 27.9±0.6 

Ma. The coordinates Armstrong gave for these dates, when 

plotted on geologic maps of the area (Anderson and Rowley, 

1987, and Anderson and others, 1987, respectively), leave 

unclear exactly what unit he dated, but the presence of biotite 

and hornblende that he reported in the sample suggests that 

the unit is the Buckskin Breccia. 

The relative ages of the Bear Valley and Leach Canyon 

Formations are not known because, where exposed together 

in the Red Hills west of the Markagunt Plateau, they are in 

fault contact along bedding-parallel low-angle faults (Red 

Hills shear zone of Maldonado and others, 1992; Maldonado, in press) and in the Markagunt megabreccia (Anderson, 1993) that are partly within soft beds of the Bear Valley 

Formation. Isotopic ages so far determined suggest, however, that the Leach Canyon is younger than the Bear Valley. 

Yet deposition of eolian sand apparently continued after deposition of the type Bear Valley Formation. In the southern 

Black Mountains (Rowley, 1978), thin Bear Valley-type 

sandstone interfingers between two ash-flow cooling units of 

the Leach Canyon Formation, and eolian sandstone lenses 

lithologically similar to the Bear Valley Formation have 

been mapped from levels above the Isom Formation to well 

up in the Mount Dutton Formation (for example, Anderson, 

Rowley, Blackman, and others, 1990). 

Local volcanic centers are exposed about 8 km east of 

Beaver, in the southwestern Tushar Mountains. The oldest 

center erupted the tuff of Lion Flat, a light-gray, unwelded, 

crystal-poor, rhyolite ash-flow tuff (Cunningham and others, 

1983) that overlies the Osiris Tuff (see p. 17). The center 

(Wickstrom, 1982) was suggested to be a small caldera 

(Steven, Rowley, and Cunningham, 1984), but more likely 

the tuff of Lion Flat represents tuff-ring deposits associated 

with an overlying trachydacite volcanic dome because the 

tuff contains clasts of dome rock (C. G. Cunningham, unpub. 

data, 1993). This dome is about 8 km in diameter and exhibits spectacularly exposed vertical flow foliation. Originally 

mapped within the Dry Hollow Formation (Callaghan and 

Parker, 1962a), the dome was briefly described under this 

name by Anderson and Rowley (1975), but later detailed 

study indicated that a local name was more appropriate (Sigmund, 1979). The name Dry Hollow Formation was abandoned by Steven and others ( 1979), who found that rocks 

previously mapped as that unit were correlated with several 

other units. Following the suggestions of Sigmund, the rocks 

of the dome are called informally the formation of Lousy Jim 

(Cunningham and others, 1983). Fleck and others (1975) 
MIDDLE CENOZOIC CALC-ALKALINE IGNEOUS SEQUENCE 17 

determined two K-Ar dates on the unit, one of 22.4±0.4 Ma 

on sanidine (sample R-16) and the other of 22.3±0.4 Ma on 

biotite (sample R-32). 

The youngest of the local accumulations of calc-alkaline rocks in the southern Tushar Mountains is called the lava 

flows of Kents Lake (Anderson, Rowley, Blackman, and 

others, 1990). These flows overlie the Mount Dutton Formation, uppermost parts of which contain the Osiris Tuff. The 

main rock type is strongly flow layered, porphyritic andesite, 

accumulating to thicknesses of as much as 250 m. 

A local, recently recognized, densely welded ash-flow 

tuff, whose source is unknown but which is confined to the 

central Markagunt Plateau, was defined by Anderson (1993) 

as the Haycock Mountain Tuff. The tuff is significant 

because it is reported by Anderson to overlie the Markagunt 

megabreccia, a major gravity-slide deposit that is widely 

exposed in the central and northern Markagunt Plateau 

(Sable and Anderson, 1985; Anderson, 1993). The Markagunt megabreccia is one of several large Tertiary deformational features in the southern High Plateaus and Red Hills; 

others of these are the 22.5-20 Ma shallow, low-angle, 

extensional Red Hills shear zone (Maldonado and others, 

1989, 1990, 1992; Maldonado, in press) and 30-20 Ma thinskinned thrust faults (Davis and Krantz, 1986; Lundin and 

Davis, 1987; Lundin, 1989; Bowers, 1990; Nickelsen and 

Merle, 1991; Merle and others, 1993). All or some of these 

structural features may have a common denominator, 

namely the emplacement of a major shallow batholith (H.R. 

Blank, oral commun., 1991) centered under the area west of 

Spry and Panguitch (see section, "Calc-alkaline Intrusions"). 

A new K-Ar date (sample 89USA2A, collected by E.G. 

Sable) on biotite from the Haycock Mountain Tuff is 

22.3±1.1 Ma (table 1), but new K-Ar dates on another sample (89USA1A) are discordant, at 22.8±1.1 Ma on biotite 

and 24.8±1.0 Ma on sanidine (table 1). The discordant sanidine date is here rejected, and we currently are making 

40 Arf39 Ar analyses of the separates as well as chemical studies of the tuff itself, which petrographically is similar to the 

Leach Canyon Formation. 

A local sequence of "latite" lava flows was mapped by 

Williams and Hackman ( 1971) in the eastern A wapa Plateau 

at the east edge of the Marysvale field. Rocks of these flows 

were chemically analyzed by Mattox (1991a), who showed 

them to be trachyte. Mattox (1991a) reported a K-Ar date on 

sanidine (his sample 119) from the flows of 23.1±1.0 Ma. 

Because these rocks are similar in chemistry (Mattox, 

1991a) and age to the Osiris Tuff, they were visited by P.D. 

Rowley in October 1992, who also concluded that the flows 

were from a small local volcanic center. 

MONROE PEAK CALDERA 

The Monroe Peak caldera in the northern Sevier Plateau is the largest caldera ( 17x26 km) in the Marysvale field 

(fig. 3 ). The caldera formed from eruption of the Osiris Tuff 

on the site of a high volcanic plateau built during terminal 

stages of eruption of the Bullion Canyon Volcanics, volcanic rocks of Little Table, and volcanic rocks of Willow 

Spring; and the caldera formed against volcano edifices 

then being built on the east side of the plateau during deposition of the volcanic rocks of Signal Peak and of Langdon 

Mountain. The map of the western and central part of the 

caldera was compiled by Cunningham and others (1983), 

and the eastern part was mapped by Rowley and others 

(1986a, b), after which we realized that the western part 

underlies the Marysvale mining district. Consequently 

Rowley and others (1988a, b) remapped the western part of 

the caldera. Steven, Rowley, and Cunningham (1984) summarized the geology of the caldera. 

The caldera subsided in response to its eruption of the 

trachytic Osiris Tuff, one of the most widespread ash-flow 

units in the volcanic field. The Osiris was informally recognized by Williams and Hackman ( 1971) and defined by 

Anderson and Rowley (1975). Fleck and others (1975) determined three concordant K-Ar dates on biotite from the Osiris 

Tuff, 23.4±0.4 (sample R-3), 22.7±0.4 (sample R-12), and 

22.9±0.4 (sample ES-1) Ma; our best estimate of the age of 

the Osiris is their average, 23.0 Ma. The isotopic ages of the 

Osiris Tuff and the Delano Peak Tuff Member of the Bullion 

Canyon Volcanics are similar, and Cunningham and others 

(1983) interpreted the Osiris Tuff to be older. Later field 

work by Rowley and J.J. Anderson in the Puffer Lake area of 

the Tushar Mountains, however, discovered a stratigraphic 

section containing both units that indicates that the Osiris is 

the younger. 

The Osiris Tuff extends southwestward to the southeastern Black Mountains (Anderson, Rowley, Machette, and 

others, 1990), where its southwestern known occurrence 

overlaps the northeasternmost known occurrence of the 

Bauers Tuff Member of the Condor Canyon Formation 

(Rowley, 1978). The Bauers is virtually identical in lithology, petrology, and isotopic age to the Osiris Tuff, but their 

relative ages are unknown; they are another example of a 

petrologic type of tuff erupted from different places at the 

same time. Two ash-flow cooling units confined to the area 

near Beaver and formerly called the "tuff of Black Mountain" and "tuff of Beaver River" (Machette and others, 1984) 

were later correlated on the basis of detailed petrography 

with the Osiris Tuff (Lanigan and Anderson, 1987; Anderson, Rowley, Machette, and others, 1990a); the terms "tuff 

of Black Mountain" and "tuff of Beaver River" are abandoned. 

Most of the fill in the Monroe Peak caldera consists of 

thick, deeply hydrothermally altered Osiris ash-flow tuff, 

locally interbedded with breccia and megabreccia beds as 

thick as 250 m that resulted from collapse of caldera walls. 

Alteration was caused by emplacement of abundant 

intracaldera intrusions, discussed in the following section. 

The uppermost part of the intracaldera fill, however, consists 
18 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST-CENTRAL UTAH 

of a series of mostly fresh dacitic and andesitic lava flows 

and volcanic domes as much as 400 m thick. Several new 

isotopic determinations constrain the age of these rocks. 

Somewhat discordant K-Ar dates of a sample (M678) of a 

plagioclase- and sanidine-rich, dacitic lava flow sequence 

from 17 km northeast of Marysvale, near the top of the fault 

scarp that defines the west edge of the Sevier Plateau, are 

21.8±0.8 Ma on biotite and 20.3±0.8 Ma on sanidine (table 

1). Another sample (M718) from the same map unit but 

exposed in the western part of the caldera in Sevier Valley 

yielded a K-Ar date on sanidine of 21.3±0.7 and a fissiontrack date on zircon of 15.3±1.4 Ma (modified from preliminary dates published by Rowley and others, 1988b); the new 

and published analytical data are given in tables 1 and 2. The 

fission-track age probably represents resetting by a buried 

rhyolitic or granitic intrusion of the Mount Belknap Volcanics near the sample site; the best age of the unit thus is the 

average of the three K-Ar dates, 21.1 Ma. New K-Ar dates 

were also determined from a sample (83-602) of another 

map unit, a sanidine-rich, dacitic, lava flow sequence near 

the top of the caldera fill in the eastern part of the caldera but 

whose relative age with respect to the plagioclase- and sanidine-rich flows is not known. These dates are 22.6±1.0 Ma 

on biotite and 22.3±0.8 Ma on sanidine (table 1), for an average age of 22.5 Ma. These ages suggest that volcanism in the 

caldera was quite long lived, extending perhaps as long as 

almost 2 m. y. after the initial eruption of the Osiris Tuff, with 

apparently the youngest volcanism in the western part of the 

caldera. 

Although some of the uppermost dacitic and andesitic 

flows and domes were locally intruded and resurgently 

domed by late consanguineous intracaldera intrusions; most 

intracaldera flows and domes were deformed into a gentle 

saucer-shaped structural basin that extends over the full 

width of the caldera. Some of the intracaldera intrusions 

came to within several hundred meters of the paleosurface; 

other intrusive masses probably were continuous upward 

with lava flows and domes in the upper part of the caldera 

fill. 

CALC-ALKALINE INTRUSIONS 

A shallow Cenozoic composite batholith underlies the 

central part of the Marysvale volcanic field and extends 

westward under other parts of the Pioche-Marysvale igneous 

belt (Steven, Rowley, and Cunningham, 1984, fig. 2; Steven 

and others, 1990; Steven and Morris, 1987). Cupolas of various ages, generally progressively younger toward the east, 

extended upward to shallow depths; some of these are partly 

exposed. Most of the exposed intrusive rocks are crystal-rich 

porphyries ranging in composition from granite through predominant quartz monzonite to diorite. All calderas in the 

Marysvale field occur above the batholith complex; the 

calderas probably represent foundered roofs of cupolas. 

Table 2. Data for fission-track dates, Marysvale volcanic field 

and adjacent areas, Utah. 

Rock unit, sample No. 

(fig. 3), mineral 2ps 3pi 4<jlX10ts 

analyzed1 

Intrusions of Mount 

Belknap Volcanics 

M684A:Z 2.42 7.73 1.03 

(347) (555) 

M693;Z 5.62 17.21 1.02 

(781) (1,195) 

Calc-alkaline stock 

M290;Z 2.08 3.53 0.61 

(529) (449) 

Lava flows, Monroe 

Peak caldera 

M718; Z 5.78 23.07 1.02 

(1,017) (2,029) 

Intrusion. Monroe 

Peak caldera 

M569; A .092 .272 1.00 

(189) (566) 

M569;Z 3.58 9.47 .954 

(811) (1.074) 

1 Z, zircon, A, apatite. 

2ps. tracks/cm2 fossil, x106 ; (number of tracks counted). 

3pi, tracks/cm2 induced, x106; (number of tracks counted). 

4<jl, neutrons!cm2. 

5Ap=7.03x10·17/yr. 

SAMPLE DESCRIPTIONS 

u Ageinm.y.5 

(ppm) ±2cr 

220 19.2±2.8 

490 19.9±2.0 

80 21.5±2.9 

650 15.3±1.4 

7.8 20.0±4.1 

290 21.4±2.2 

M684A. Small rhyolite stock near the Trinity mine, east of Big Rock Candy Mountain; 

lat 38°30'58" N.,long 112°15'18" W. 

M693. East-west glassy rhyolite dike that cuts the fine-grained granite in the Central 

Mining Area (shown by Rowley and others, 1988b); lat 38°30'15" N., long 

112°13'05" W. Preliminary data published by Rowley and others (1988b). 

M290. Small porphyritic rhyodacite stock north of Deer Trail Mountain, from about 

0.5 km south of Pine Creek at the east entrance of Bullion Canyon: lat 38°25' 4 7" 

N.,long 112°17'53" W. 

M718. Lava flows of Monroe Peak, within Monroe Peak caldera, reset by an 

underlying pluton of bimodal magmatism, from 9.6 km south-southeast of Joseph 

(shown by Rowley and others 1988b); lat 38°32'30" N., long 112°10'40''W. Preliminary data published by Rowley and others (1988b). See also K-Ar date of this 

sample in table 1. 

M569. Holocrystalline quartz monzonite porphyry, an intracaldera intrusion of the 

Monroe Peak caldera, from Dry Canyon, Sevier Plateau; lat 38°31'58" N., long 

112°07'35" w. 

Small, shallow, intermediate-composition, porphyritic 

intrusions are exposed in scattered places throughout the 

Tushar Mountains. Most cut the Bullion Canyon Volcanics 

and probably represent upward extensions of magma chambers that were the source of the Bullion Canyon. South of 

Cove Fort, in the northwestern Tushar Mountains (Steven 

and Morris, 1983), the intrusions cut rocks of the Bullion 

Canyon Volcanics, including the Three Creeks Tuff Member 

(27 Ma) and rocks as young as the tuff of Albinus Canyon 

(25.5 Ma); the intrusive rocks are unconformably overlain 

by the Osiris Tuff. Similar stocks cutting the Bullion Canyon 

Volcanics were mapped by Machette and others (1984) farther south, along the western front of the Tushar Mountains, 

and by Cunningham and others (1983) in other parts of the 

range. Some stocks altered and mineralized their adjacent 

wallrocks, resulting in precious-metal deposits (Cunningham, Steven, Campbell, and others, 1984; Steven and Morris, 1987). One such stock of propylitized quartz monzonite 
MIDDLE CENOZOIC CALC-ALKALINE IGNEOUS SEQUENCE 19 

in the northern Tushar Mountains contains the major epithermal gold-quartz-pyrite-carbonate veins of the Kimberly 

district (Cunningham, Steven, Campbell, and others, 1984); 

the stock yielded a fission-track date on zircon of 24.1±1.2 

Ma (Steven and others, 1979). The Indian Creek quartz 

monzonite stock in the western Tushar Mountains, with epithermal gold-quartz-pyrite veins developed at the Rob Roy 

mine, is of unknown age; fission-track dates from samples 

from the stock were reset by intrusions of younger bimodal 

rocks (Cunningham, Rye, and others, 1984) (see section, 

"Silicic Intrusions"). One intrusion, called the Hoover pluton 

by Rowley and others (1988b), located about 7 km north of 

Marysvale, may have localized minor amounts of copper in 

marginal contact-metamorphosed sedimentary rocks at the 

Trinity mine (Kerr and others, 1957) and developed spectacularly colorful hydrothermally altered rocks at a local tourist 

attraction named Big Rock Candy Mountain. The intrusion 

is truncated by the margin of the Monroe Peak caldera. Uranium mineralization was related to emplacement of a small 

stock of the Mount Belknap Volcanics near the Trinity mine, 

as discussed on page 25. 

The most extensive exposures of intrusive rocks in the 

Marysvale field are associated with the youngest calc-alkaline caldera, the Monroe Peak caldera. Several intrusions 

were separated in the mapping of the Marysvale mining district in the western part of the caldera, whereas elsewhere in 

the caldera the rocks were not divided into individual plutons 

(Cunningham and Steven, 1979b; Cunningham and others, 

1983; Steven, Rowley, and Cunningham, 1984; Rowley and 

others, 1986a, b, 1988a, b). Most rocks are monzonite and 

quartz monzonite porphyry and have textures that range 

from coarse grained in pluton interiors to fine grained near 

intrusive contacts; at their contacts, they are difficult to distinguish from hydrothermally altered, cogenetic Osiris Tuff 

intracaldera fill into which the intrusions are emplaced. Fission-track dates from sample M569 of holocrystalline rock 

from an interior zone of an intracaldera intrusion in Dry Canyon, at the west edge of the northern Sevier Plateau, are modified from preliminary dates on this sample first reported by 

Rowley and others (1988b) and are given in table 2: 21.4±2.2 

Ma from zircon and 20.0±4.1 Ma from apatite. The zircon 

age is considered to represent the true age of the intrusive 

rock. It compares favorably with the ages of the intracaldera 

lava flows and domes that occur at the top of the caldera fill 

that are cogenetic with the undivided intrusive rocks. The 

zircon age, in conjunction with these volcanic ages, demonstrates that intracaldera volcanism and intrusion continued 

for 1-2 m. y. after eruption of the Osiris Tuff and initial collapse of the caldera. The apatite age from the Dry Canyon 

intrusive rocks is within analytical error of the zircon date 

but is believed to have been reset by plugs of the younger 

Mount Belknap Volcanics (see section, "Silicic Intrusions") 

that are abundant in the area. Replacement alunite was a significant alteration mineral produced by hydrothermal activity related to intracaldera intrusions emplaced into the 

Monroe Peak caldera. A new K-Ar date on replacement 

alunite from Marysvale Peak (sample M932) is 23.2±1.2 Ma 

(table 1) and is thus numerically older than other dates of 

intracaldera intrusions and volcanic rocks, but it is statistically concordant. 

Among several individually mapped intrusions of the 

Monroe Peak intracaldera sequence, the most studied is the 

Central intrusion (Central "intrusive" of Kerr and others, 

1957) because it is host to the largest uranium deposits of the 

Marysvale mining district (Kerr and others, 1957; Callaghan, 1973; Cunningham and Steven, 1979a, b; Steven and 

others, 1981; Cunningham and others, 1982; Cunningham, 

Rye, and others, 1984). The intrusion is ringed by spectacularly well exposed areas of hydrothermally altered country 

rocks and hot-spring quartz masses, interpreted to be the tops 

of a series of hydrothermal convection cells of circulating 

ground water (Podwysocki and Segal, 1983; Cunningham, 

Rye, and others, 1984 ). The Central intrusion has been abundantly dated by various isotopic methods, although some 

previous isotopic determinations did not give it a satisfactory 

age (summarized in Rowley and others, 1988a, b). We consider the stock to be about 22 Ma on the basis of (1) discordant fission-track dates of 23.3±2.6 Ma (apatite) and 

19.0±0.8 Ma (zircon) on a sample (M27A) of intrusive rock 

(Steven and others, 1979), and (2) more consistent K-Ar 

dates (Steven and others, 1979; Cunningham, Rye, and 

others, 1984) on replacement alunite from the surrounding 

advanced argillically altered rocks, namely 23.2±1.2, 

22.7±1.1, 22.5±1.0, and 21.8±4.3 Ma. The stock was 

emplaced at a shallow level, then eroded and unroofed by 

about 19 Ma, when it was unconformably overlain by the 

Red Hills Tuff Member of the Mount Belknap Volcanics 

(see section, "Mount Belknap Volcanics"). 

A small porphyritic rhyodacite stock (unit Trd of Cunningham and others, 1983) cuts redbeds of the Upper 

Triassic Chinle Formation near the base of the fault scarp 

preserved on the east side of the Tushar Mountains just south 

of the east entrance to Bullion Canyon. Sample M290 from 

this stock yielded a new fission-track date on zircon of 

21.5±2.9 Ma (table 2). We consider this stock to belong to 

the calc-alkaline sequence on the basis of a chemical analysis of the stock. 

Minerals such as nepheline and corundum that are 

characteristic of rocks relatively rich in alkalies occur in one 

place in the Marysvale volcanic field, about 20 km southeast of Marysvale (Agrell and others, 1986). Rowley and 

S.O. Agrell (unpub. data, 1985) mapped the area and concluded that the rocks formed in a fine-grained, altered, 

nepheline syenite breccia pipe that is 1.3 km in maximum 

diameter and was emplaced into intertonguing alluvialfacies volcanic-mudflow breccia beds of the volcanic rocks 

of Little Table and the volcanic rocks of Langdon Mountain. New 40 Arf39 Ar dating (table 3; fig. 4) of biotite and 

sanidine from a syenite vein within the breccia pipe produced disturbed spectra with the following weight-average 
20 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST-CENTRAL UTAH 

Table 3. Data for 40 Arf39 Ar dates 1 for biotite and sanidine from sample 83--69X, eastern 

Sevier Plateau, Utah. 

Temp 2Radiogenic 2K-derived 40 Ar Rj39 Ar K3 Radiogenic Pet. 39Ar Apparent 4Error (Ma) 

co 40Ar 39Ar yield, pet. of total age (Ma) 

Biotite; 63.4 mg; 1=0.007743±.25 percent 

500 0.0121 0.0078 1.55 5.9 0.1 22 ±12 

600 .0017 .0040 .42 2.3 0 6 ±18 

650 .0335 .0291 1.145 13.7 .4 15.9 ± 0.4 

700+ .29285 .17699 1.655 34.8 2.1 23.0 ± .2 

750+ .50494 .30275 1.668 61.2 3.7 23.2 ± .2 

800+ 1.3649 .81734 1.670 71.9 9.9 23.18 ± .07 

850+ 2.0251 1.1939 1.696 79.0 14.4 23.54 ± .11 

900+ 2.7645 1.6648 1.661 87.6 20.1 23.05 ± .07 

950+ 1.8314 1.1085 1.652 84.6 13.4 22.93 ± .06 

1000+ 1.3704 .83794 1.635 71.4 10.1 22.70 ± .06 

1050 1.9421 1.1054 1.757 55.8 13.3 24.38 ± .07 

1150 1.8224 .9799 1.860 36.8 11.8 25.8 ± .1 

1250 .0897 .0525 1.707 36.8 .6 23.7 ± .8 

Total-gas date 1.697 23.55 ± .13 

Weight-average date 73.7 23.08 ± .09 

Isochron date [(40Art36Ar)i=304±5] 99.9 23.00 ± .05 

Sanidine; 100.8 mg; 1=0.007637±.25 percent 

700 0.7884 0.4541 1.74 32.6 5.6 23.8 ± 2 

800+ 2.4733 1.4030 1.763 79.2 17.3 24.13 ± 0.07 

900+ 2.7986 1.5742 1.778 86.4 19.4 24.33 ± .07 

950+ .97773 .55155 1.773 56.6 6.8 24.26 ± .08 

1000+ .65416 .36485 1.793 48.7 4.5 24.53 ± .18 

1050+ .72399 .39925 1.813 54.4 4.9 24.81 ± .20 

1100+ .96859 .54654 1.772 65.1 6.7 24.25 ± .07 

1150+ 2.7432 1.5364 1.785 80.1 18.9 24.43 ± .07 

1250+ 1.9108 1.0552 1.811 69.7 13.0 24.78 ± .10 

1450 .44391 .24143 1.839 46.8 3.0 25.16 ± .07 

Total-gas date 1.782 24.39 ± .09 

Weight-average date 91.4 24.40 ± .10 

Isochron date[(40 Ar/36 Ar)i=294±3] 100 24.51 ± .05 

1 Analytical data for "Radiogenic 40 Ar" and "K-derived 39" are calculated to five places right of the 

decimal; "40 Ar Rf39 ArK" is calculated to three places right of the decimal. "Radiogenic 40 Ar" and "K -derived 39 Ar" 

are rounded to significant figures using calculated analytical precisions. Apparent ages and associated errors were 

calculated from unrounded analytical data and then each rounded using associated errors. All analyses were done 

in the Argon Laboratory, USGS, Denver, Colo. Decay constants are those recommended by Steiger and Jager 

(1977): A.e=0.581xl0-10/yr, A-p_ =4.962x10-10fyr, and A.=A.e+Ap-=5.543x1010fyr. The standard for this experiment is 

hornblende MMhb-1 with percent K=1.555, 40ArR=l.624x10-9 mole/g, and K/Ar age=520.4 Ma (Samson and 

Alexander, 1987). 

2 Abundances of "Radiogenic 40 Ar" and "K -derived 39 Ar" are reported in volts. Conversion to moles can 

be made using 2.078x1o-12 moles ArN for sensitivity of the mass spectrometer at the time of the experiment. 

Detection limit at the time of this experiment was 2x10-17 moles Ar measured on a Faraday detector. 

3"40 ArRf39 ArK" has been corrected for all interfering isotopes including atmospheric argon. Abundances 

of interfering isotopes of argon from K and Ca were calculated from reactor production ratios determined by 

irradiating and analyzing pure CaF 2 and K SO 4 simultaneously with these samples. The measured production 

ratios are (40 Arf39 Ar)K=8. 78x10-3, (l8 Arf3~ Ar)K= 1.31x10-2, (l7 Arf39 Ar)K= 1.11x1 04 , (l6 Arf37 Ar)ca =2.69x10-4, 

(l9 Arf37 Ar)ca=6.55x1o-4, and (l8Arf37 Ar)ca=3.59x1o-5. Corrections were also made for additional interfering 

isotopes of argon produced from irradiation of chlorine using the method described by Roddick (1983). Irradiation 

was for 30 hours at 1 megawatt in the TRlGA reactor at the USGS, Denver, Colo. 

41cr error. 

+step is used in weight-average date. 

SAMPLE DESCRIPTION 

83-69X. Syenite vein within breccia pipe, 10 km south-southwest of Greenwich, eastern Sevier Plateau: lat 38°21'37" N., 

long 111°59'48" W. 

dates: (1) 23.1±0.1 Ma on biotite, and (2) 24.4±0.1 Ma on 

sanidine. Both age spectra show evidence of excess 40 Ar, 

but because sanidine generally is a better argon geochronometer, we interpret 24.4±0.1 Ma to be the better estimate 

for the emplacement age of the vein. 

The Spry intrusion, defined by Anderson and Rowley 

( 197 5) and studied in somewhat greater detail by Grant and 

Anderson (1979), is a batholith-sized exposure of monzonitic porphyry along U.S. Highway 89 about 15 km south of 

Circleville (Anderson, Rowley, Blackman, and others, 
UPPER CENOZOIC SEDIMENTARY AND BIMODAL IGNEOUS SEQUENCE 21 

1990). Low-amplitude aeromagnetic anomalies (Zietz and 

others, 1976; Blank and Kucks, 1989) and gravity lows 

(Blank and Kucks, 1989; Cook and others, 1989; Viki Bankey, unpub. data, 1992) indicate that the intrusion extends 

50 km south of its exposed part, mostly under valley-fill 

deposits. The body intruded into volcanics, including the 

volcanic rocks of Bull Rush Creek, that are believed to be 

its own ejecta. The intrusion arched the Claron Formation, 

Buckskin Breccia, and Bear Valley Formation, and it is considered to be a laccolith emplaced into the lower part of the 

Claron Formation, similar to other intrusions in the northern 

Markagunt Plateau (Anderson and Rowley, 1975; Anderson, Rowley, Machette, and others, 1990; Anderson, Rowley, Blackman, and others, 1990). A K-Ar date from 

plagioclase (sample 84-257) from the intrusion is 26.1±1.8 

Ma, modified from a preliminary date on the sample published by Anderson, Rowley, Blackman, and others (1990) 

and given in table 1. This date is concordant with a plagioclase date of 26.4±3.2 Ma (sample 84-259; table 1) on a 

cogenetic dike cutting the volcanic rocks of Bull Rush 

Creek (discussed on p. 16). The actual age of some parts of 

the Spry intrusion may be somewhat younger than indicated 

by these two concordant ages because the intrusion appears 

to postdate at least lower parts of the Bear Valley Formation; as noted on page 16, tuff beds in the upper part of the 

Bear Valley have been dated at about 24.6 Ma. 

The youngest known of several small laccoliths in the 

northern Markagunt Plateau is the Iron Peak intrusion, 

whose base and main body are exposed several kilometers 

east of Parowan. The geology of the body was summarized 

by Anderson and Rowley (1975), who called it the Iron 

Point intrusion. A K-Ar whole-rock date (sample R-28) by 

Fleck and others (1975) is 20.2±0.5 Ma. Dikes probably 

related to the intrusion have been dated by H.H. Mehnert at 

about 20 Ma (Maldonado, in press). Lava flows exposed 

near the body and probably derived from it, although not 

separated in mapping from Mount Dutton Formation 

(Anderson and Rowley, 1975), have an age of 21.2 Ma (see 

p. 12). The age of the pluton and that of its possible ejecta, 

however, are in some doubt because mapping in the 

Parowan quadrangle (Maldonado and Moore, 1993) suggests that flow breccia probably derived from the Iron Peak 

intrusion here is overlain by the Bauers Tuff Member (22.8 

Ma) of the Condor Canyon Formation (Florian Maldonado, 

written commun., 1993). The laccolith may have been 

localized by the Red Hills shear zone (Maldonado, in 

press). The laccolith, which was studied in detail by Spurney (1984), is gabbro and diorite porphyry. The intrusion is 

more mafic than other plutons in the Marysvale field and 

contains noncommercial iron veinlets (Bullock, 1970). We 

consider the Iron Peak intrusion to mark the northeast end 

of the "'Iron Axis" (fig. 1), a northeast-trending string of 

mostly quartz monzonite porphyry laccoliths that also 

underlie the Iron Springs and Bull Valley iron mining districts (Mackin, 1947, 1960; Blank and Mackin, 1967; 

co 28 

::2: 

z 

w' 26 (.9 

<! 

1-

z 

w 24 

AGE SPECTRUM FOR BIOTITE 83-69X/58/DD14 

~ I .-....___j===l 

~ ~====~====~_j 

<! 22 

2QL-~L-~--~---L---L--~--~--~--L-~ 

0 10 20 30 40 50 60 70 80 90 100 

A PERCENT 39 ArK RELEASED 

AGE SPECTRUM FOR SANIDINE 83-69X/50/DD14 

B PERCENT 39 ArK RELEASED 

Figure 4. 40 Ar/39 Ar age spectrum for sample 83-69X, Marysvale 

volcanic field. A, biotite; B, sanidine. 

Rowley and Barker, 1978; Van Kooten, 1988; Barker, 

1991; Blank and others, 1992). The age of the plutons at 

Iron Springs appears to be about 22 Ma (Rowley and others, 1989). 

UPPER CENOZOIC SEDIMENTARY 

AND BIMODAL IGNEOUS SEQUENCE 

POTASSIUM-RICH MAFIC VOLCANIC ROCKS 

Early mapping in the southern part of the Marysvale 

volcanic field (Callaghan, 1939; Williams and Hackman, 

1971; Anderson and Rowley, 1975) recognized basaltic 

rocks at two stratigraphic levels. These consist of an older 

sequence of "basaltic andesite" or "older basalt" lava flows, 

flow breccia, and conglomerate generally overlying the 
22 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST -CENTRAL UTAH 

Osiris Tuff and locally intertongued with rhyolite lava flows; 

and a younger sequence of lithologically similar basalt lava 

flows, scoria, and cinder cones locally intertongued with 

basin-fill sedimentary rocks but generally occurring at the 

top of the stratigraphic column at any given location. Best 

and others (1980) also distinguished an older mafic rock 

sequence of about 25-21 Ma and younger basalt sequence of 

14 Ma to Pleistocene age. They found that despite the general lithologic similarity between the two groups of rocks, 

the two could be distinguished geochemically: rocks of the 

older sequence contain K20 values generally exceeding 3 

weight percent and K20/Na20 greater than 1, whereas most 

rocks of the younger sequence are true basalt, containing significantly lower potassium values. They referred to rocks of 

the older sequence as potassium-rich mafic lavas, and since 

then, we have called them potassium-rich mafic volcanic 

rocks (see for example, Cunningham and others, 1983; 

Steven and others, 1990) except where we apply stratigraphic names to separate mappable accumulations. We conclude that the older sequence represents the first product of 

bimodal volcanism, which formed because of increased 

extensional deformation in the area. Based on the geochemistry of these potassium-rich mafic volcanic rocks (Mattox 

and Walker, 1989, 1990; Walker and Mattox, 1989; Mattox, 

1991a, b, 1992), Mattox and Walker (1989) related the rocks 

to the demise of the subduction zone under western North 

America. Mattox (1992) disagrees somewhat with us, however, in that he finds the potassium-rich mafic rocks to be 

related to the termination of Mount Dutton volcanism and 

thus might be better included in the middle Cenozoic volcanic assemblage. 

The thickest and most extensive accumulations of the 

potassium-rich mafic volcanic rocks are in the southern part 

of the Marysvale field, where some have been mapped as 

informally named units derived from separate local volcanic 

centers. The oldest are the mafic lava flows of Birch Creek 

Mountain (Anderson and Rowley, 1975; Anderson, Rowley, 

Blackman, and others, 1990), in the southern Tushar Mountains. The unit overlies the lava flows of Kents Lake, which 

in tum rests on the Mount Dutton Formation that contains the 

Osiris Tuff in its uppermost part. The mafic lava flows of 

Birch Creek Mountain consist of crystal-poor, olivine-bearing trachybasalt lava flows (Wickstrom, 1982) with a maximum thickness of 150 m. Fleck and others (1975) 

determined two K-Ar whole-rock dates on lava flows from 

the unit, 22.9±0.4 (sample R-20) and 22.4±0.4 (sample 

R-17) Ma (Anderson, Rowley, Machette, and others, 1990). 

Similar potassium-rich mafic flows northwest of Birch 

Creek Mountain (Machette and others, 1984) may correlate 

with the mafic lava flows of Birch Creek Mountain. One of 

these flows has a K-Ar whole-rock date of 23.2±0.2 Ma 

(Best and others, 1980) from a sample collected on Black 

Mountain, 5 km east-southeast of Beaver. This age, however, is considered somewhat too old because the flow here 

overlies not only the 24-Ma Delano Peak Tuff Member of 

the Bullion Canyon Volcanics but also two cooling units of 

the 23-Ma Osiris Tuff ("tuff of Black Mountain" and "tuff 

of Beaver River" of Machette and others, 1984, names now 

abandoned) as well. The mafic flow also laps up against, 

and is younger than, the 22.5-Ma formation of Lousy Jim in 

the Beaver River drainage area. Thus, we determined a KAr whole-rock age from another sample (79-R-B) from 

Black Mountain; this date, first mentioned by Machette and 

others (1984), is 22.1±0.8 Ma (table 1). Another new K-Ar 

whole-rock date of 21.7±0.8 Ma (table 1) is from another 

potassium-rich mafic volcanic rock (sample 79-R-A) collected from a lava flow poking through younger basin-fill 

sedimentary rocks 4 km east of Beaver (Machette and 

others, 1984 ). This last sample may correlate with the 

mafic lava flows of Birch Creek Mountain. 

The mafic lava flows of Birch Creek Mountain are 

overlain by, and perhaps locally intertongued with, the 

mafic lava flows of Circleville Mountain (Anderson and 

Rowley, 1975; Anderson, Rowley, Blackman, and others, 

1990). The Circleville Mountain unit consists of lava flows 

and flow breccia distinguished by numerous phenocrysts of 

plagioclase and pyroxene, which form sequences in the 

southeastern Tushar Mountains as thick as 350 m. Fleck 

and others (1975) determined a K-Ar whole-rock date 

(sample R-22) of 22.8±0.4 Ma on the unit, indicating that it 

is essentially contemporaneous with the mafic lava flows of 

Birch Creek Mountain. In the southern Tushar Mountains, 

another unit, which Anderson, Rowley, Machette, and 

others (1990) called the mafic gravels of Gunsight Flat, 

consists of clasts derived from the mafic lava flows of 

Birch Creek Mountain and Circleville Mountain and is 

locally intertongued with the mafic lava flows of Birch 

Creek Mountain. These fanglomerates and fluvial conglomerates, interpreted to be mostly shed northward from paleofault scarps, are considered to be contemporaneous with the 

mafic lava flows. Cunningham, Steven, Campbell, and others (1984) published a whole-rock K-Ar date of 22.8±0.7 

Ma for a sample of the mafic lava flows of Birch Creek 

Mountain collected where it is interbedded with the mafic 

gravels of Gunsight Flat (Cunningham and others, 1983). 

In the northern A wapa Plateau and Fish Lake Plateau, 

Smith and others (1963) and Williams and Hackman 

(1971) mapped basaltic andesite lava flows. Some of these 

may be correlative with the calc-alkaline volcanic rocks of 

Signal Peak, but others may correlate with the potassiumrich mafic volcanic rocks. Best and others ( 1980), in fact, 

so designated a sample south of Fish Lake on which they 

determined a K-Ar whole-rock date of 24.5±0.5 Ma. Fleck 

and others (1975) determined a K-Ar date on biotite of 

22.4±0.4 Ma for a sample (EM-1) of basaltic rock overlying Osiris Tuff in the southern Wasatch Plateau; it probably also belongs to the potassium-rich mafic volcanic 

rocks. Mattox (1991a) published three K-Ar plagioclase 

dates from "basaltic andesite" flows on the A wapa Plateau 

that he determined by geochemical analyses to correlate 
UPPER CENOZOIC SEDIMENTARY AND BIMODAL IGNEOUS SEQUENCE 23 

with the potassium-rich mafic volcanic rocks; these dates 

are 25.8±1.4, 25.2±1.6, and 23.2±1.5 Ma. 

Elsewhere in the Marysvale volcanic field, potassiumrich mafic volcanic rocks form volumetrically smaller 

sequences of lava flows, reminiscent in size and scattered 

distribution to the younger, local sequences of basalt lava 

flows. (See section, "Basalt Lava Flows.") For example, in 

Sevier Valley near the town of Junction, potassium-rich 

mafic volcanic rocks as thick as 125 m overlie the Osiris 

Tuff. Wender and Nash (1979), on the basis of chemical 

analyses of these rocks, called them absarokite and noted 

that they are distinct from younger basalts. Best and others 

(1980) determined a K-Ar whole-rock date of 21.1±0.2 Ma 

from one of these olivine-bearing flows and assigned it on 

the basis of chemistry to the suite of potassium-rich mafic 

lava flows. 

MOUNT BELKNAP VOLCANICS 

Increased extension in the Marysvale volcanic field 

led to a rise in isotherms and development of eutectic partial melts with the composition of high-silica rhyolite (C. G. 

Cunningham, unpub. data, 1988). The largest accumulation 

of volcanic rocks of the bimodal sequence, in fact, consists 

of high-silica rhyolite of the Mount Belknap Volcanics 

(Callaghan, 1939; Cunningham and Steven, 1979a; Steven 

and others, 1979). The formation embraces a complex 

assemblage of lava flows, flow breccia, volcanic mudflow 

breccia, volcanic domes, ash-flow tuff, and hypabyssal 

intrusions that were emplaced between 22 and 14 Ma from 

two concurrently active eastern and western source areas 

that cross the central part of the volcanic field. Mount 

Belknap volcanism began with development of a series of 

crystal-rich volcanic domes and flows in the northeastern 

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

Sevier Valley in the western part of the older Monroe Peak 

caldera. Magmatism in the eastern source area resulted in 

hydrothermal alteration and uranium mineralization of the 

Central Mining Area of the Marysvale mining district. 

Numerous K-Ar and fission-track ages were determined on 

these rocks, as summarized by Rowley and others (1988a, 

b). Many K-Ar ages on rocks in the eastern source area 

determined by Bassett and others ( 1963) are discordant, 

internally and with those of other workers, and many are 

herein discounted. Among more reliable ages on rocks 

from the eastern source area are the following from a single 

sample of the glassy margin of the Teacup dome (Steven 

and others, 1979): (1) discordant fission-track dates on apatite and zircon of, respectively, 27.8±3.9 and 20.9±1.1 Ma; 

and (2) K-Ar dates on biotite of 21.6±1.1 Ma and on sanidine of 21.6±0.9 and 21.4±1.0 Ma. The apatite date is 

rejected, leaving an average age for the others of 21.4 Ma. 

Centers that erupted flows, domes, and tuff migrated southwestward over the next 4 m.y. The largest ash-flow tuff 

from the eastern source area is the Red Hills Tuff Member 

of the Mount Belknap Volcanics (Steven and others, 1979). 

It was erupted from the small Red Hills caldera near 

Marysvale (Cunningham and Steven, 1979a, b). The age of 

the Red Hills Tuff Member is 18.9±0.7 Ma on the basis of a 

K-Ar determination on sanidine (Cunningham and others, 

1982). It is overlain by a series of domes and flows as 

much as 500 m thick named the Gray Hills Rhyolite Member (Steven and others, 1979). Bassett and others (1963) 

determined K-Ar dates of 20.5, 20.5, and 19.5 Ma on 

the unit. 

In the western source area, centered on the crest of the 

Tushar Mountains, the large Mount Belknap caldera 

subsided in response to eruption of the largest ash-flow 

tuff of the episode of bimodal magmatism, the Joe Lott 

Tuff Member (Steven and others, 1979; Budding and 

others, 1987) of the Mount Belknap Volcanics. Budding 

and others (1987) reported two K-Ar dates on the Joe Lott: 

18.3±1.1 Ma on sanidine from the upper cooling unit and 

17.2±0.7 Ma on hydrated basal vitrophyre of the lower 

cooling unit. They rejected these dates, however, and concluded that the age of the member is about 19 Ma on the 

basis of isotopic ages from underlying and overlying volcanic rocks. The underlying rocks include a series of heterogeneous flows and tuff units making up the oldest rocks 

derived from the Mount Belknap caldera. Fission-track 

dates on zircon and apatite from a sample of tuff are 

19.6±0.8 and 18.8±1.7 Ma, respectively (Steven and 

others, 1979). The overlying rocks include the Red Hills 

Tuff Member and the informally named (Steven and 

others, 1979) crystal-rich tuff member of the Mount 

Belknap Volcanics, which overlies both the Joe Lott and 

Red Hills Tuff Members. Two samples from the crystalrich tuff member yielded the following discordant dates 

(Steven and others, 1979): (1) a K-Ar date on sanidine of 

18.5±0.8 Ma and a fission-track date on zircon from the 

same sample of 15.3±0.8 Ma; and (2) a K-Ar date on sanidine of 19.4±0.8 Ma and fission-track dates on zircon from 

the same sample of 17.1±0.8 and 16.1±0.7 Ma; the fissiontrack dates are rejected because they are less reasonable 

geologically. The caldera was filled quickly by comagmatic rhyolite lava flows, breccia, ash-flow tuff, and 

hypabyssal rocks that are subdivided into a series of named 

members (Steven and others, 1979), some very thick 

(Cunningham and Steven, 1979b). The products of the 

eastern and western source areas complexly intertongue in 

areas between the sources. For example, the upper gray 

tuff member of the Mount Belknap Volcanics, considered 

to be the youngest identified product derived from the 

Mount Belknap caldera (western source area) overlies the 

Gray Hills Rhyolite Member (eastern source area) of the 

Mount Belknap Volcanics, about 5 km west of Marysvale; 

the upper gray tuff has a fission-track date on zircon of 

18.0±0.8 Ma (Steven and others, 1979). 
24 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST-CENTRAL UTAH 

SMALL RHYOLITE VOLCANIC CENTERS 

Small high-silica rhyolite centers synchronous with the 

basin-range episode of maximum extensional deformation 

are scattered throughout the Marysvale volcanic field. Several of these are along the east-striking Blue Ribbon lineament (fig. 1). The largest volcanic center on the lineament 

and the most important in terms of its tectonic implications 

is in the Kingston Canyon area of the southern Sevier Plateau, as first noted by Rowley (1968) and dated and discussed in detail by Rowley and others ( 1981 ). Kingston 

Canyon is a deep, west-flowing antecedent canyon that cuts 

through the Sevier Plateau, which is an east-tilted fault block 

uplifted on its west side more than 1,500 m along north-striking basin-range faults. Rhyolite flows that predate canyon 

cutting yielded K-Ar dates on sanidine of7.6±0.6 Ma and on 

whole-rock obsidian of 7 .6±0.4 Ma. A rhyolite volcanic 

dome deposited in the base of the canyon yielded K-Ar dates 

on sanidine of 5.4±0.3 Ma and on whole-rock obsidian of 

4.8±0.2 Ma. A basalt clast of country rock in a premonitory 

rhyolite tuff cone into which the dome was emplaced yielded 

a K-Ar whole-rock date of 7 .8±0.5 Ma, indicating that basalt 

was emplaced at least at depth at the same time as the rhyolite flows that predated canyon cutting. Rowley and others 

(1981) interpreted that the cutting of Kingston Canyon indicated that the main episode of basin-range faulting in the 

area was between about 8 and 5 Ma. 

In the Black Mountains, on the west side of the Marysvale volcanic field and farther west along the Blue Ribbon 

lineament, several small rhyolite centers have been dated. 

Two small centers, the rhyolite of Beaver airport (sample 

78-1695) and the rhyolite of Teddys Valley (sample 

78-1741), yieldedK-Arwhole-rockobsidiandates of8.3±0.3 

and 7.9±0.5 Ma, respectively (Anderson, Rowley, Machette, 

and others, 1990); these are published again in table 1. Two 

other centers farther west, on the edge of the Marysvale field, 

are represented by the rhyolite of Blue Ribbon Summit with 

a K-Ar sanidine date of 7.9±0.5 Ma (Mehnert and others, 

1978; Rowley, Lipman, and others, 1978) and the rhyolite of 

Minersville Reservoir with an unpublished K-Ar date by S .H. 

Evans, Jr. of 7.5±0.3 Ma (T.A. Steven, written commun., 

1984; mentioned by Evans and Steven, 1982, and Anderson, 

Rowley, Blackman, and others, 1990). Other rhyolites west 

of the Marysvale field and along the Blue Ribbon lineament 

were dated by Mehnert and others (1978) and Rowley, Lipman, and others (1978) and mapped by Rowley (1978). 

The rhyolite of Gillies Hill is a sequence of rhyolite volcanic domes and tuff in the Great Basin (Steven and Morris, 

1983; Machette and others, 1984) but north of the Blue Ribbon lineament. Evans and Steven (1982) published four 

biotite K-Ar dates from the volcanic center of 9.2±0.5, 

9.1±0.6, 8.0±0.3, and 7.0±0.4 Ma, and one sanidine K-Ar 

date of 9.1±0.3 Ma. The rhyolite was emplaced along and 

postdates a major north-striking basin-range fault zone. Cunningham, Steven, Campbell, and others (1984) published a 

fission-track date on apatite of 8.4±0.6 Ma from a flow in the 

rhyolite of Gillies Hill farther southeast, along the west scarp 

of the Tushar Mountains. 

The rhyolite of the Mineral Mountains, at the west edge 

of the Marysvale field, consists of little-eroded rhyolitic tuff, 

flows, and volcanic domes ranging in age between 0.8 and 

0.5 Ma according to K-Ar, fission-track, and obsidian-hydration dating (Lipman and others, 1978; Mehnert and others, 

1978). This Pleistocene rhyolite is a product of magmatism 

that today is still active under the Mineral Mountains and has 

been developed for commercial electrical energy in the 

Roosevelt geothermal area. 

SILICIC INTRUSIONS 

Intrusive rocks related to the eastern source area of the 

Mount Belknap Volcanics and to satellitic rhyolite centers of 

the Marysvale volcanic field are abundant. Most intrusions 

consist of hypabyssal rhyolite dikes and plugs that for the 

most part are discussed with the Mount Belknap Volcanics, 

but the geochronology of the stocks is summarized here. One 

of the smaller stocks is composed of fine-grained granodioritic to granitic rock that intruded the west side of the Central 

intrusion (discussed in the section, "Calc-alkaline Intrusions"), as mapped by most workers in the Central Mining 

Area. The stock was mapped and discussed by Rowley and 

others ( 1988a, b) as belonging to the fine-grained silicic pluton (unit Tif), and it occurs just inside the margin of the Monroe Peak caldera. Cunningham and Steven ( 1979a, b) 

interpreted the pluton to belong to the bimodal sequence and 

to be perhaps a source for some of the rhyolitic volcanic 

rocks in the eastern source area of the Mount Belknap Volcanics. Kerr and others (1957) and Rowley and others 

( 1988a, b), however, suggested that the pluton resembles the 

more silicic parts of the Central intrusion and thus instead 

might be interpreted as a late-stage, more silicic, differentiated phase of the Central intrusion. The fine-grained silicic 

pluton is a host for many of the uranium ore bodies in the 

Central Mining Area north of Marysvale (Cunningham and 

Steven, 1979a). The following concordant K-Ar and fissiontrack dates were determined on samples of intrusive rocks 

and alteration minerals of the pluton by Steven and others 

(1979) and Cunningham and others (1982): (1) a fissiontrack date on zircon from the pluton of 20.2±0. 9 Ma; (2) a KAr date on biotite of a dike of the fine-grained silicic pluton 

of 21.4±0.9 Ma and two fission-track dates on zircon from 

the same sample of 21.3±0.9 and 19.5±0.7 Ma; (3) a K-Ar 

date on biotite from the Central intrusion adjacent to the finegrained silicic pluton of 21.6±0.9 Ma and a fission-track date 

on zircon from the same sample of 20.4±0.9 Ma (both interpreted to be dates reset by the fine-grained silicic pluton); 

and (4) a fission-track date on apatite from a sample of the 

fine-grained silicic pluton, collected at depth in the Freedom 

mine, of 22.1±4.4 Ma. The average of these seven analyses 
UPPER CENOZOIC SEDIMENTARY AND BIMODAL IGNEOUS SEQUENCE 25 

is 20.9 Ma, our best estimate of the age of the fine-grained 

silicic pluton. Its age is younger than ages of even the latest 

volcanism in the Monroe Peak caldera, suggesting that the 

fine-grained silicic pluton might better be included within 

the bimodal sequence, as originally proposed by Cunningham and Steven (1979a, b). 

A series of glassy rhyolite dikes cut the Central intrusion and fine-grained silicic pluton. These hypabyssal intrusions contain uranium- and molybdenum-bearing veins, 

which are interpreted to be genetically related to the intrusive activity. Cunningham and others (1982) suggested that 

the dikes pass downward into a hidden stock that was the primary source of the uranium in the Central Mining Area. Cunningham and Steven (1979b) also proposed, based on values 

of molybdenum that increase downward in mines in the Central Mining Area, that the apex of this stock may contain a 

porphyry-type molybdenum deposit. The following K-Ar 

and fission-track determinations give the age of the glassy 

rhyolite dikes: (1) discordant fission-track dates on apatite 

and zircon from a dike sample of, respectively, 23.3±4.7 and 

18.1±0.8 Ma (Cunningham and others, 1982); (2) a fissiontrack date on zircon of 19.0±1.0 Ma (Cunningham and others, 1982); and (3) a fission-track date on zircon from an 

apparent glassy dike (sample M693) of 19.9±2.0 Ma, modified from a preliminary date first published by Rowley and 

others (1988b) and given in table 2. Rejecting the apatite date 

that is numerically too old (although within analytical error 

ofthe others) yields an average age of 19.0 Ma. Cunningham 

and others (1982) independently arrived at a similar age by 

dating the effects of uranium mineralization in the Central 

Mining Area: (1) a U-Pb isochron date on uranium vein 

material of 19.0±3.7 Ma, (2) a fission-track date on quartz 

adjacent to pitchblende of 16.5±4.3 Ma, and (3) K-Ar dates 

on two samples of sericite adjacent to veins of 21.1±0.6 and 

20.5±0.7 Ma. 

A small stock of rhyolite of Mount Belknap Volcanics 

cuts the metamorphosed Lower Permian Toroweap Formation (Cunningham and others, 1983) near the Trinity mine, 

northeast of Big Rock Candy Mountain and 8 km north of 

Marysvale. A sample (M684A) yielded a new fission-track 

date on zircon of 19.2±2.8 Ma (table 2). 

Hypabyssal intrusions that postdate the Mount Belknap 

Volcanics occur locally in the Marysvale area. A small rhyolite stock named the Beaver Creek stock that intrudes the 

Gray Hills Rhyolite Member at the east edge of the Tushar 

Mountains has a fission-track date on zircon of 15.7±0.8 Ma 

(Cunningham and others, 1982). About 9 km west of Marysvale, a rhyolite dike yielded a fission-track date on zircon of 

16.6±0.7 Ma (Cunningham and others, 1982); it is associated 

with ore at the Copper Belt mine there. 

The youngest igneous activity in the eastern source area 

of the Mount Belknap Volcanics occurred at about 14 Ma 

under Alunite Ridge, about 12 km southwest of Marysvale in 

the eastern Tushar Mountains. Here a structural dome was 

interpreted by Cunningham and Steven ( 1979c) to be caused 

by a buried rhyolitic or granitic stock. K-Ar dates on coarsegrained vein alunite from the ridge making up the exposed 

top of the dome and the result of degassing of the underlying 

stock are 14.3±0.6 and 13.8±0.6 Ma (Steven and others, 

1979). In addition, K-Ar dates on sericite from an underlying 

base-metal manto in the Deer Trail mine (Beaty and others, 

1986) are 14.2 and 12.8 Ma (Bassett and others, 1963). 

Igneous activity of the 22- to 14-Ma Mount Belknap 

magmatic episode also took place about 9 km north of 

Marysvale, where the youngest economic mineral deposit in 

the Central Mining Area, replacement natroalunite, formed 

above inferred intrusions. The main evidence for this age of 

magmatism comes from K-Ar dates of natroalunite at the AI 

Kee Mee prospect of 17.7±1.0 (Cunningham, Rye, and 

others, 1984) and 16.6±0.8 Ma (Steven and others, 1979), at 

the Alum King mine of 13.9±0.9 Ma (Cunningham, Rye, and 

others, 1984), and at the Big Star mine of 12.6±2.5 Ma (Cunningham, Rye, and others, 1984). Additional evidence 

comes from the new fission-track date on zircon from sample M718 of 15.3±1.4 Ma (table 2; discussed in the section, 

"Monroe Peak Caldera"), which indicates a resetting of the 

emplacement age of the lava flows of the Monroe Peak 

caldera, probably related to emplacement of nearby rhyolite 

plugs; this date from the lava flow is modified from a preliminary one published by Rowley and others (1988b ). In addition, samples from a basal glass of the rhyolite of Big Star 

(Cunningham and others, 1983) near the Alum King and Big 

Star mines gave K-Ar dates of 15.2, 14.0, and 12.0 Ma (Bassett and others, 1963); these dates indicate that volcanism 

also resulted from this igneous pulse. 

Many features of the plutons of the Marysvale field are 

analogous to those exhibited by the 25- to 9-Ma Mineral 

Mountains batholith, which is the largest exposed batholith 

in Utah (Nielson and others, 1986; Steven and others, 1990; 

Coleman and Walker, 1992). This batholith occurs on the 

west side of the Marysvale volcanic field and is better 

exposed than most intrusive rocks elsewhere in the field. 

This batholith, in fact, provides a unique north-south cross 

section through the batholithic complex that underlies much 

of the Pioche-Marysvale igneous belt because the Mineral 

Mountains underwent huge amounts of uplift due to later 

(basin-range) extension associated with its formation as a 

core complex (Price and Bartley, 1992). Minor early phases 

(about 25 Ma) of coarse-grained quartz monzonite and granodiorite represent calc-alkaline intrusion. Later intrusive 

rocks became more silicic with time (Coleman and Walker, 

1992). The main phase of the batholith consists of mediumgrained plutonic rocks of variable compositon, from diorite 

to high-silica granite, that are more alkaline than the 25-Ma 

plutons (Nielson and others, 1986). These rocks formed from 

about 22 to 11 Ma according to dating by Nielson and others 

(1986) and Aleinikoff and others (1986), or between 18 and 

16 Ma according to petrology and dating by Coleman and 

Walker ( 1992 ). These rocks formed during the bimodal igneous episode, although Coleman and Walker (1992) have 
26 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST-CENTRAL UTAH 

considered all rocks except high-silica rhyolite of 9 Ma to 

have a calc-alkaline signature, perhaps reflecting calc-alkaline source rocks that were partially melted. Rapid and 

extensive uplift of the range during formation as a core complex coincided with silicic intrusions of 9 Ma that fed rhyolite hypabyssal rocks and the rhyolite of Gillies Hill (Nielson 

and others, 1986). Pleistocene rhyolite domes, flows, and 

tuff (Lipman and others, 1978; Mehnert and others, 1978) 

are the result of later magmatism in the same area, and the 

west side of the range now contains geothermal energy 

resources. Two new K-Ar dates published here from an 

intrusive rhyolite mass in the southeastern Mineral Mountains indicate that this mass is satellitic to the main mass of 

the Mineral Mountains batholith. The dates (sample 

78-1835) are 22.5±0.9 Ma on biotite and 22.3±0.8 Ma on 

sanidine (table 1). 

Rhyolitic or granitic intrusions related to the western 

source area of the Mount Belknap Volcanics and underlying 

areas west of this source area are exposed in the western 

Tushar Mountains, as mapped by Machette and others 

( 1984) and described by Cunningham, Steven, Campbell, 

and others (1984). One such intrusion is the U-Beva rhyolite 

stock, in the southwestern part of the Mount Belknap 

caldera, which yielded a fission-track date on zircon of 

19.8±1.2 Ma (Cunningham, Steven, Campbell, and others, 

1984 ). A rhyolite cone-sheet dike nearby but just outside the 

wall of the caldera gave a fission-track date on zircon of 

20.3±0.9 Ma (Cunningham, Steven, Campbell, and others, 

1984 ). These two dates represent magmatism associated 

with the Mount Belknap caldera. These rocks in tum are 

intruded by similar silicic rocks that postdate the Mount 

Belknap Volcanics. For example, a fission-track date on apatite from the same sample of the U-Beva stock yielded a date 

of 13.4±2.7 Ma, and another nearby rhyolite cone-sheet dike 

yielded a fission-track date on apatite of 9.5±1.9 Ma (Cunningham, Steven, Campbell, and others, 1984 ). These ages 

indicate partial (the 13.4-Ma date) to complete (the 9.5-Ma 

date) resetting by a younger buried, approximately 9.5-Ma 

intrusion related to the rhyolite of Gillies Hill to the west 

(Cunningham, Steven, Campbell, and others, 1984). Coleman and Walker (1992) noted that the youngest intrusions of 

the Mineral Mountains batholith date at 9 Ma, the same age 

as the rhyolite of Gillies Hill, and thus the latter is the extrusive equivalent of the former. The intrusive sources under 

the western Tushar Mountains may correlate with late phases 

of the Mineral Mountains batholith, and all these magmatic 

events are believed by Nielson and others (1986) to be synchronous with the Cove Canyon detachment fault of the 

southwestern Mineral Mountains and to basin-range development of the graben underlying the Beaver basin. 

The Sheep Rock alunite deposit west of the U-Beva 

rhyolite stock, along the basin-range fault that defines the 

west edge of the Tushar Mountains, appears to be underlain 

by an intrusion, because alunite replaces rhyolite flows of 

the 19-Ma Mount Baldy Rhyolite Member of the Mount 

Belknap Volcanics that poured over the margin of the 

Mount Belknap caldera. Replacement alunite from the 

deposit yielded K-Ar dates of 9.4±0.5 Ma (Steven and 

others, 1979) and 9.2±0.3 Ma (Cunningham, Steven, 

Campbell, and others, 1984). A gold deposit just north of 

the alunite deposit was likely localized by the same presumed hidden rhyolite or granite stock, which in tum may 

have been emplaced along the range-front fault, thereby 

suggesting that it, like the one across the Beaver basin at 

Gillies Hill, was active at about 9 Ma. 

Inferred resetting by underlying, hidden intrusions also 

is exhibited by samples of the calc-alkaline Indian Creek 

stock, located 4 km west of the rim of the Mount Belknap 

caldera and 7 km northwest of the U-Beva stock. Although 

presumably this intrusion has an age of 24-23 Ma based on 

its composition and relative age, a fission-track date on zircon from the stock is 19.3±1.0 Ma, reset by an underlying 

intrusion probably related to Mount Belknap magmatism 

(Cunningham, Steven, Campbell, and others, 1984 ). A 

fission-track date on apatite from the stock is 12.8±2.1 Ma, 

which is interpreted to be partially reset by a still younger 

intrusion probably related to Gillies Hill magmatism (Cunningham, Steven, Campbell, and others, 1984). A third 

inferred episode of emplacement of underlying intrusions of 

rhyolite or granite is indicated by two fission-track zircon 

dates of rhyolite dikes at the Mystery Sniffer uranium mine, 

which is east of the Indian Creek stock and about 1 km west 

of the margin of the Mount Belknap caldera (Cunningham, 

Steven, Campbell, and others, 1984). The dates are 16.1±0.8 

and 15.6±1.1 Ma and suggest another hidden intrusion, with 

attendant hydrothermal activity (Cunningham, Steven, 

Campbell, and others, 1984 ). 

BASALT LAVA FLOWS 

Small fields of olivine basalt lava flows, flow breccia, 

and cinder cones are scattered through and around the 

Marysvale volcanic field (Cunningham and others, 1983; 

Steven and others, 1990). Vents rarely occur near either 

older sources of middle Cenozoic calc-alkaline igneous 

rocks or sources of the first (23-16 Ma) of the upper Cenozoic high-silica rhyolites; instead, they are much more common on the flanks of or beyond the Marysvale field. Some 

basalt vents, however, do coincide with source areas of the 

small fields of younger (9 Ma to Quaternary) high-silica rhyolite, suggesting that emplacement of basalt may have supplied the heat that led to partial melting of crust to form the 

rhyolites. Basalts overlie and intertongue with basin-fill sedimentary rocks (see next section) and generally coincide 

with basin-range extensional faulting. Although erosion has 

removed evidence of many basaltic vents, it seems that many 

were partly controlled by northerly-striking joints or zones 

of weakness related to adjacent major north-striking rangefront faults and that the basalts were deposited on the edges 
UPPER CENOZOIC SEDIMENTARY AND BIMODAL IGNEOUS SEQUENCE 27 

of upthrown (footwall) fault blocks, in the manner explained 

theoretically by Ellis and King (1991) and noted by the field 

data of R.E. Anderson ( 1988). 

The sou them flank of the Marysvale volcanic field and 

areas just south of the field contain some of the largest 

basalt fields in the eastem Great Basin (Anderson and 

Rowley, 1975) and some of the best examples of basalt 

louderbacks on upthrown (footwall) tilt fault blocks. These 

examples include basalt flows on the southem Sevier Plateau and Markagunt Plateau (Steven and others, 1990; 

Hintze, 1980; Moore and Nealey, 1993). The eastem flank 

of the Marysvale field also contains scattered basalt cinder 

cones and flows that cap all tilted blocks of the eastern 

High Plateaus (\Villiams and Hackman, 1971). Basalt flows 

also are abundant along and west of the Colorado 

Plateau-Great Basin boundary near the western part of the 

Marysvale field (Steven and Morris, 1983; Machette and 

others, 1984; Steven and others, 1990) and in the Black 

Rock Desert northwest of the Marysvale field (Steven and 

others, 1990). 

Best and others (1980) studied the chemistry and isotopic ages of ma:1y of the true basaltic rocks throughout a 

broad area that ir eludes the Marysvale field. Overall, dates 

from the basalts they analyzed from the Marysvale field 

range between 14 Ma and Holocene. Histograms of isotopic ages that they published suggest three pulses of basalt 

magmatism in the area, at 14 to 11 Ma, 9 to 5 Ma, and 3 

Ma through Quaternary. A significant basalt flow at the 

west end of Red Canyon, 12 km southeast of Panguitch, 

dated by Best and others (1980) at 0.6±0.1 Ma, was vertically offset between 60 and 180 m along the Sevier fault 

zone (Rowley and others, 1981). A new K-Ar whole-rock 

date of 5.3±0.5 Ma (table 1) was determined on a faulted 

and tilted basalt lava flow (sample AC-PANG) from 1.5 km 

south-southeast of Panguitch. Anderson and Rowley (1975) 

compiled data on basalt occurrences from and south of the 

southem Marysvale field. Basalt ages between 12.6 and 0.8 

Ma were published by Anderson and Mehnert (1979) from 

the Markagunt Plateau and areas to the southwest. Anderson and Christenson (1989) listed ages of Quaternary basalt 

flows in the same area. Mattox (1991a) reported two new 

K-Ar whole-rock dates from the Awapa Plateau, including 

the oldest (16.4 Ma) so far determined for a basalt from the 

Marysvale field. On the north side of the Marysvale field, 

two basalt flows (9.2 and 7.4 Ma) dated by Best and others 

(1980) are mentioned in the next section because they intertongue with the Sevier River Formation. 

Basalt is rare in the central part of the Marysvale field. 

A new K-Ar whole-rock date was determined on sample 

M673 from the southeastern Tushar Mountains at 10.9±0.4 

Ma (table 1). This date is similar to those from a basalt on 

the edge of the western Sevier Plateau to the east that was 

dated by Damon (1969) at 12.9±0.6 Ma and by Best and 

others (1980) at 12.7±0.3 Ma. 

BASIN-FILL SEDIMENTARY ROCKS 

Grabens and related structural valleys that formed as 

closed basins during extension in the Great Basin and Great 

Basin-Colorado Plateau transition zone contain upper Cenozoic, poorly to moderately consolidated, basin-fill sedimentary rocks. Most of these sedimentary rocks are coarse- to 

fine-grained clastic fluvial rocks, but lacustrine rocks and 

airfall tuff occur locally, and in places the fluvial rocks are 

interbedded with basalt flows. The basins formed in large 

part after 10 Ma, during basin-range faulting (Zoback and 

others, 1981; Anderson and others, 1983; Anderson, 1989), 

although some clearly formed earlier as grabens or downwarps. Ages of basins differ from place to place depending 

on when the bounding faults or structural sags were active. 

Most basins in the Great Basin evolved independently, and 

their strata cannot be correlated from basin to basin (Anderson and others, 1983). 

The Beaver basin near the west edge of the Marysvale 

volcanic field is a typical basin in the Great Basin that was 

studied in detail. The upper part of its fill is well exposed 

and the stratigraphy consists of local informal names 

(Anderson and others, 1978; Machette and others, 1984; 

Machette, 1985; Anderson, Rowley, Machette, and others, 

1990). Dates of soil horizons, airfall tuff beds, basalt flows, 

and fossils constrain the stratigraphic units to the Pliocene 

and Quaternary (Machette, 1985). The age of beds in the 

lower part of the basin is not known, although the approximate 9-Ma date on the rhyolite of Gillies Hill on the north 

side of the basin, and the rhyolite's eruptive source interpreted to be along a basin-range fault (Evans and Steven, 

1982) indicate that the Beaver basin dates at least to this 

time. Furthermore, in the Cedar City area south of the Beaver basin, Anderson and Mehnert (1979) determined K-Ar 

whole-rock ages of basalts that suggested that this basin 

began to form about 10 Ma. 

In contrast to the many closed basins in the Great Basin, 

the drainages in the High Plateaus transition zone are well 

integrated. The name Sevier River Formation was applied by 

Callaghan (1938) to the sedimentary rocks of the various 

integrated basins throughout the area. Exposed basin-fill 

sedimentary rocks are at least 350 m thick in some basins and 

are well exposed in many places because rivers have cut 

down through the poorly consolidated sediments (Anderson 

and Rowley, 1975). Anderson (1987) and Rowley and others 

( 1988a, b) summarized the geomorphic evolution of the 

basins and concluded that most formed independently, as in 

the Great Basin; these basins currently have structural relief 

of as much as 2,000 m, and were formerly largely filled with 

sedimentary deposits, much of which have since eroded 

away. Most drainage systems superposed on the thick fill cut 

vertically through the structural margins to form deep canyons in bedrock, then integrated with streams in other basins, 

eroding away as much as 1,000 m of the fill. Some bedrock 

canyons, such as Kingston Canyon, are antecedent due to 
28 CENOZOIC ROCKS, MARYSVALE VOLCANIC FIELD, WEST-CENTRAL UTAH 

uplift of fault blocks from 8 to 5 Ma as the river kept pace. 

Drainage integration continued during the Pliocene and 

Pleistocene. 

The Sevier River Formation overlies the Joe Lott Tuff 

Member (19 Ma) and other units of the Mount Belknap Volcanics. Fission-track zircon dates on two tuff beds in the 

Sevier River Formation from near the town of Sevier, one 

from near the base of the formation and the other from near 

the eroded top, are 14.2±0.9 and 7.1±0.7 Ma, respectively 

(Steven and others, 1979). West of Sevier, basalt flows interbedded with the Sevier River Formation yielded K-Ar 

whole-rock dates of 9.2±0.4 and 7.4±0.7 Ma (Best and 

others, 1980). The basalt flow south-southeast of Panguitch, 

newly dated at 5.3±0.5 Ma and discussed in the preceding 

section, is probably intertongued in the upper part of the 

Sevier River Formation (R.E. Anderson, unpub. data, 1992). 

A new K-Ar date on sanidine from an airfall tuff (sample 78-609) near the top of exposures of the Sevier River 

Formation on the edge of a pediment in the Sevier Valley, 11 

km south-southeast of Marysvale, is 13.6±0.8 Ma (table 1). 

This dated tuff is low in the formation and much of the overlying section has been removed by erosion, but this overlying section may have formerly filled much of the valley. 

Gravity data suggest that the basin fill of Sevier River Formation under the pediment is hundreds of meters thick. 

About 16 km south of Marysvale, the Sevier River Formation is similarly eroded and its exposed lower part is overlain 

by a basalt flow, mentioned in the preceding section, that has 

K-Ar whole-rock dates of 12.9±0.6 Ma (Damon, 1969) and 

12.7±0.3 Ma (Best and others, 1980). 

Another new K-Ar date of 5.6±0.4 Ma (table 1) was 

determined on sanidine from an airfall tuff (sample 

80-331B) in the Sevier River Formation. This sample was 

collected from the base of the western scarp of the northern 

Sevier Plateau near Annabella, where the Sevier River Formation consists of coarse clastic sediments that represent an 

old fan deposit derived from the scarp. The sample thus 

dates part of the movement on the Sevier fault zone that 

formed the scarp. The date is in agreement with relationships in the Kingston Canyon area, where the main offset 

of the southern Sevier Plateau is documented at between 8 

and 5 Ma. Anderson and Barnhard (1992) first mentioned 

the date but interpreted instead that the Sevier River Formation here predates fault uplift of the high Sevier Plateau 

along the Sevier fault zone. 

CONCLUSIONS 

Stratigraphic studies and compilation of new and published K-Ar, fission-track, and 40 Arf39 Ar ages of Cenozoic 

rocks of the Marysvale volcanic field and adjacent areas of 

Utah show that the rocks can be placed in three stratigraphic 

sequences. These are the lower Cenozoic sedimentary 

sequence, the middle Cenozoic calc-alkaline igneous 

sequence, and the upper Cenozoic sedimentary and bimodal 

igneous sequence (fig. 2). The lower Cenozoic sedimentary 

sequence consists of continental sedimentary rocks whose 

stratigraphy is poorly known and currently under revision by 

other workers and by some of us. Units of this sequence 

include the Claron Formation and overlying Brian Head Formation. The Brian Head contains isotopically dated airfall 

tuff beds as old as 34 Ma that probably represent the earliest 

known eruptions of the Marysvale field. 

About 95 percent of the eruptive volume of the 

Marysvale volcanic field is represented by the middle Cenozoic calc-alkaline igneous sequence and consists of 

volcanic mudflow breccia and subordinate amounts of lava 

flows, flow breccia, and ash-flow tuff. The rocks range 

from basaltic andesite to low-silica rhyolite in composition 

and from 34(?) to 22 Main age. The sequence formed during subduction beneath the western United States. Two 

great intertonguing units dominate the middle Cenozoic 

sequence: the more differentiated Bullion Canyon 

Volcanics (at least 32 to 22 Ma) in the central and northern 

part of the volcanic field, and the generally more mafic 

Mount Dutton Formation (at least 32 to 21 Ma) in the 

southern part of the field. Mapped units from smaller centers also are intertongued with these two major rock bodies. 

Rocks of the Bullion Canyon Volcanics and related units 

were derived from and overlie a major batholith complex 

that also extends west of the field. Intrusive rocks from this 

complex are exposed locally in the northern and central 

parts of the field, and hydrothermally altered rocks and 

deposits of base and precious metals resulted from them. 

The cover rocks above some higher intruding cupolas that 

developed on the batholith complex failed, and then the 

resulting fractures tapped major ash-flow sheets that have a 

cumulative volume about 10 percent of that of the massive 

deposits of volcanic mudflow breccia and lava flows. Tuff 

includes the Three Creeks Tuff Member (27 Ma) of the 

Bullion Canyon Volcanics that erupted from the Three 

Creeks caldera, the Delano Peak Tuff Member (24 Ma) of 

the Bullion Canyon Volcanics that erupted from the Big 

John caldera, and the Osiris Tuff (23 Ma) that erupted from 

the Monroe Peak caldera. The magma sources for the 

Mount Dutton Formation and related units were deeper 

than those of the Bullion Canyon Volcanics, and intrusive 

rocks related to these units are rarely exposed. In addition 

to regional ash-flow tuff derived from the Marysvale field, 

regional ash-flow units from sources in the Great Basin 

west of the Marysvale field are intertongued with rocks of 

the field. These tuff units include the Wah Wah Springs 

Formation (>29.5 Ma), Isom Formation (27-26 Ma), Leach 

Canyon Formation (24 Ma), Condor Canyon Formation 

(23.5-23 Ma), and Harmony Hills Tuff (22 Ma). The tuffs 

from within and outside the field provide regional marker 

horizons that assist stratigraphic subdivision of the 

volcanic field. 
REFERENCES CITED 29 

The upper Cenozoic sedimentary and bimodal igneous 

sequence consists mostly of lava flows and ash-flow tuff 

ranging in age from 23 Ma through Quaternary. About 5 

percent of the eruptive volume of the Marysvale volcanic 

field is represented by the upper Cenozoic bimodal igneous 

rocks, which consist of basaltic andesite, basalt, and rhyolite. The sequence is interpreted to represent cessation of 

subduction and increased extension in the western United 

States; the main phase of faulting is the basin-range episode 

of about 10 Ma to Quaternary, which climaxed in the area 

at about 8 to 5 Ma. The sedimentary rocks are largely 

basin-fill clastic fluvial rocks deposited in basins formed 

probably by warping before the basin-range episode and in 

relatively downfaulted areas during the basin-range episode. Lava flows of potassium-rich mafic volcanic rocks, 

ranging in age from at least 23 to 21 Ma, generally were the 

first of the bimodal volcanic rocks and were derived from 

small centers scattered in and near the Marysvale volcanic 

field. The largest bimodal volcanic unit is the Mount 

Belknap Volcanics (23-14 Ma), which formed above a 

western and eastern source area spanning the central part of 

the Marysvale volcanic field. Intrusive rocks of the source 

area are exposed locally, and their intrusion resulted in 

hydrothermally altered rocks and deposits mostly of uranium and molybdenum. A major ash-flow sheet, the Joe 

Lott Tuff Member (19 Ma) of the Mount Belknap Volcanics, was erupted from the Mount Belknap caldera above 

the western source area. Other small rhyolite centers, as 

young as 0.5 Ma, are scattered throughout the Marysvale 

field. Basalt fields of 16 Ma to Quaternary are likewise 

small and scattered throughout the Marysvale field, though 

basalt fields of the same age are larger outside the field. 

The present mountains and valleys were formed by 

basin-range block faulting, especially between 8 and 5 Ma. 

Downthrown areas accumulated clastic sediment of the 

upper part of the Sevier River Formation from erosion of 

upthrown areas. Drainage integration, largely by rivers 

superposed on the basin fill, led to erosion of the soft basinfill sediments and created the present topography. 

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Utah and eastern Nevada: U.S. Geological Survey Journal of 

Research, v. 6, no. 2, p. 175-192. 

Rowley, P.D., McKee, E.H., and Blank, H.R., Jr., 1989, Miocene 

gravity slides resulting from emplacement of the Iron Mountain 

pluton, southern Iron Springs mining district, Iron County, Utah 

[abs.]: Eos, v. 70, no. 43, p. 1309. 

Rowley, P.D., Nealey, L.D., Unruh, D.M., Snee, L.W., Mehnert, 

H.H., Anderson, R.E., and Gromme, C.S., in press, Stratigraphy 

of Oligocene and Miocene ash-flow tuffs in and near the Caliente caldera complex, southeastern Nevada and southwestern 

Utah, Chapter B in Scott, R.B., and Swadley, W C, eds., Geologic studies in the Basin and Range-Colorado Plateau transition in southeastern Nevada, southwestern Utah, and 

northwestern Arizona, 1992: U.S. Geological Survey Bulletin 

2056. 

Rowley, P.D., Shroba, R.R., Simonds, F.W., Burke, K.J., Axen, 

G.J., and Olmore, S.D., in press, Geologic map of the Chief 

Mountain quadrangle, Lincoln County, Nevada: U.S. Geological Survey Geologic Quadrangle Map GQ-1731, scale 

1:24,000. 

Rowley, P.D., and Siders, M.A., 1988, Miocene calderas of the Caliente caldera complex, Nevada-Utah [abs.]: Eos, v. 69, no. 44, 

p. 1508. 

Rowley, P.O., Snee, L.W., Mehnert, H.H., Anderson, R.E., Axen, 

G.J., Burke, K.J., Simonds, F.W., Shroba, R.R., and Olmore, 

S.D., 1992, Structural setting of the Chief mining district, eastern Chief Range, Lincoln County, Nevada, Chapter H in Thorman, C.H., ed., Application of structural geology to mineral and 

energy resources of the central and western United States: U.S. 

Geological Survey Bulletin 2012, 17 p. 

Rowley, P.D., Steven, T.A., and Mehnert, H.H., 1981, Origin and 

structural implications of upper Miocene rhyolites in Kingston 

Canyon, Piute County, Utah: Geological Society of America 

Bulletin, pt. I, v. 92, p. 590-602. 

Rowley, P.D., Steven, T.A., Anderson, J.J., and Cunningham, C. G., 

1979, Cenozoic stratigraphic and structural framework of 

southwestern Utah: U.S. Geological Survey Professional Paper 

1149,22 p. 

Rowley, P.D., Williams, P.L., and Kaplan, A.M., 1986a, Geologic 

map of the Greenwich quadrangle, Piute County, Utah: U.S. 

Geological Survey Geologic Quadrangle Map GQ-1589, scale 

1:24,000. 

---1986b, Geologic map of the Koosharem quadrangle, Sevier 

and Piute Counties, Utah: U.S. Geological Survey Geologic 

Quadrangle Map GQ-1590, scale 1:24,000. 

Sable, E.G., and Anderson, J.J., 1985, Tertiary tectonic slide megabreccia, Markagunt Plateau, southwestern Utah: Geological 

Society of America Abstracts with Programs, v. 17, no. 4, p. 

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Samson, S.D., and Alexander, E.C., Jr., 1987, Calibration of the 

interlaboratory 40 Arf39 Ar dating standard, MMhb-1: Chemical 

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Schneider, M.C., 1967, Early Tertiary continental sediments of central and south-central Utah: Brigham Young University Geology Studies, v. 14, p. 143-194. 

Scott, R.B., Gromme, C.S., Best, M.G., Rosenbaum, J.G., and Hudson, M.R., in press, Stratigraphic relationships of Tertiary volcanic rocks in central Lincoln County, southeastern Nevada, 

Chapter A in Scott, R.B., and Swadley, W C, eds., Geologic 

studies in the Basin and Range-Colorado Plateau transition in 

southeastern Nevada, southwestern Utah, and northwestern 

Arizona, 1992: U.S. Geological Survey Bulletin 2056. 

Siders, M.A., and Shubat, M.A., 1986, Stratigraphy and structure of 

the northern Bull Valley Mountains and Antelope Range, Iron 

County, Utah, in Griffen, D.T., and Phillips, W.R., eds., Thrusting and extensional structures and mineralization in the Beaver 

Dam Mountains, southwestern Utah: Utah Geological Association Publication 15, p. 87-102. 

Sigmund, J.M., 1979, Geology of a Miocene rhyodacite lava flow, 

southern Tushar Mountains, Utah: Kent, Ohio, Kent State University M.S. thesis, 35 p. 

Smedes, H.W., and Prostka, H.J., 1973, Stratigraphic framework of 

the Absaroka Volcanic Supergroup in theY ellowstone National 

Park region: U.S. Geological Survey Professional Paper 729-C, 

33 p. 

Smith, J.F., Jr., Huff, L.C., Hinrichs, E.N., and Luedke, R.G., 1963, 

Geology of the Capitol Reef area, Wayne and Garfield Counties, Utah: U.S. Geological Survey Professional Paper 363, 

102p. 

Spurney, J.C., 1984, Geology of the Iron Peak intrusion, Iron 

County, Utah: Kent, Ohio, Kent State University M.S. thesis, 

84p. 

Steiger, R.H., and Jager, Emile, 1977, Subcommission on geochronology-Convention on the use of decay constants in geo- and 

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p. 1-7. 

---1988, Mosaic faulting as a guide to mineral exploration in 

the Richfield 1 o x 2° quadrangle, western Utah, in Schindler, 

K.S., ed., USGS research on mineral resources-1989 program 

and abstracts: U.S. Geological Survey Circular 1035, p. 71. 

Steven, T.A., Cunningham, C. G., and Anderson, J.J., 1984, Geologic history and uranium potential of the Big John caldera, southern Tushar Mountains, Utah, Chapter B in Steven, T.A., ed., 

Igneous activity a9-d related ore deposits in the western and 

southern Tushar Mountains, Marysvale volcanic field, westcentral Utah: U.S. Geological Survey Professional Paper 1299, 

p. 25-33. 

Steven, T.A., Cunningham, C.G., and Machette, M.N., 1981, Integrated uranium systems in the Marysvale volcanic field, westcentral Utah, in Goodell, P.C., and Waters, A.C., eds., Uranium 

in volcanic and volcaniclastic rocks: American Association of 

Petroleum Geologists Studies in Geology 13, p. 111-122. 
REFERENCES CITED 35 

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1979, Revised stratigraphy and radiometric ages of volcanic 

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Published in the Central Region, Denver, Colorado 

Manuscript approved for publication September 24, 1993 

Edited by Lorna Carter 

Graphics prepared by Mari L. Kauffmann 

Photocomposition by Carol A. Quesenberry 

*U.S. CDVElUo1ENl' PlUl'll'IR; OFFICE: 1994-573-038/86031 

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407-434. 

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