Mount Belknap and Red Hills 

Calderas and Associated Rocks, 

Marysvale Volcanic Field, 

West-Central Utah 

GEOLOGICAL SURVEY BULLETIN 1468 

Mount Belknap and Red Hills 

Calderas and Associated Rocks, 

Marysvale Volcanic Field, 

West-Central Utah 

By C. G. CUNNINGHAM and T.A. STEVEN 

GEOLOGICAL SURVEY BULLETIN 

Description of the rocks, eruptive history, and 

inferred subvolcanic environment associated 

with the formation of the J1ount Belknap 

and Red Hills calderas 

1468 

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

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

H. William Menard, Director 

Library of Congress Cataloging in Publication Data 

Cunningham, C. G. 

Mount Belknap and Red Hills calderas and associated rocks, Marysvale volcanic field, 

west-central Utah. 

(Geological Survey Bulletin 1468) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1468 

1. Calderas-Utah-Marysvale region. 2. Volcanic ash, tuff, etc.-Utah-Marysvale region. 

I. Steven, Thomas August, 191 7-joint author. II. Title. III. Series: United States Geological 

Survey Bulletin 1468 

QE75.B9 No. 1468 ( QE461] 557.3'08s 78-21191 [551.2'1'0979246) 

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

Washington, D.C. 20402 

Stock No. 024-001-03175-2 
CONTENTS 

Abstract_ 

Introduction _________ _ 

Acknowledgments_ 

Geologic setting __________ _ 

Mount Belknap Volcanics ____ _ Outflow facies ___________________ _ 

Lower heterogeneous member _ 

Joe Lott 'iliff Member 

Red Hills 'fuff Member 

Crystal-rich tuff member 

Upper red tuff member _ Porphyritic lava flows 

Gray Hills Rhyolite Member _ Upper gray tuff member __ _ Intracaldera facies __________________ _ 

Lower tuff member __ _ 

Blue Lake Rhyolite Member 

Middle tuff member ________ _ _______________ _ 

Mount Baldy Rhyolite Member and volcanic breccia member _ 

Young intrusive rocks _________ ------------------------------------------- __ Evolution of the Mount Belknap and Red Hills calderas 

Eastern source area Western source area _ Depth of the magma chambers _ 

References cited 

ILLUSTRATIONS 

FIGURE 1. Index map showing the location of the Mount Belknap and Red 

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Page 

Hills calderas _ 3 

TABLE l. 

2. 

2. Generalized stratigraphic column of the outflow facies of the 

Mount Belknap Volcanics _ _____________ _ 8 

3. Photograph of outflow facies of Mount Belknap and Red Hills 

calderas along U.S. 89 _ 11 

4. Geologic map of the Mount Belknap caldera 16 

5. Diagrammatic cross section showing stratigraphic relations of 

the Mount Belknap Volcanics 17 

6. Photograph of intracaldera facies of the Mount Belknap caldera 

seen looking west towards Mount Belknap 18 

7. Map of source areas for the Mount Belknap Volcanics _ 24 

8. Diagrammatic sketch showing hypothetical relations in the 

source areas for the Mount Belknap Volcanics __ 25 

TABLES 

Modal analyses of the rocks of the Mount Belknap Volcanics 

Chemical analyses of Mount Belknap Volcanics 

III 

Page 

9 

26 

MOUNT BELKNAP AND RED HILLS CALDERAS AND 

ASSOCIATED ROCKS, MARYSVALE VOLCANIC FIELD, 

WEST-CENTRAL UTAH 

By C. G. CUNNINGHAM and T. A. STEVEN 

Abstract 

The Mount Belknap caldera, in the 'fushar Mountains of west-central Utah, formed 19 

m.y. ago in Miocene time in response to the eruption of crystal-poor ash flows of the Joe 

Lott 'fuff Member of the Mount Belknap Volcanics. The Red Hills caldera, a smaller 

structure 14 km east of the Mount Belknap caldera, subsided during the same interval in 

response to eruption of similar tuffs of the Red Hills 'fuff Member. These rocks and 

associated rhyolitic ash-flow tuffs, lava flows, and volcanic breccias form the intracaldera and outflow facies of the Mount Belknap Volcanics, which constitutes a 

significant part of the Marysvale volcanic field. 

The Mount Belknap Volcanics was erupted from two source areas in a hilly terrain 

consisting of eroded intermediate-composition volcanoes of the 30-21-m.y.-old Bullion 

Canyon Volcanics. Quartz monzonitic stocks intruded vent-facies rocks in the cores of 

some of these intermediate- composition volcanoes about 23 m.y. ago, and ash-flow tuffs 

from local and distant sources interleave marginally with the Bullion Canyon volcanic 

pile. 

About 21 m.y. ago, alkali rhyolite of the Mount Belknap Volcanics became dominant, 

and viscous lava flows, lava domes, and ash-flow tuff sheets were emplaced widely. 

Domes and flows were erupted in the northeastern third of the eastern source area 21 

m.y. ago. Eruptions then migrated southwestward, and about 20 m.y. ago a fine-grained 

granite stock was emplaced in the central part of the eastern source area. About 19 m.y. 

ago, gas-rich ash flows were erupted nearby, and the Red Hills caldera subsided in 

consequence. Final activity was in the southwestern third of the eastern source area, 

where a group of viscous flows and domes was extruded 18 m.y. ago. 

Volcanic activity in the western source area was much more restricted in time. 

Eruptions did not begin until about 19 m.y. ago, and in the ensuing million years, large 

volumes of ash-flow tuff were erupted catastrophically, the Mount Belknap caldera 

subsided, and the depressed area was filled to overflowing with ash-flow tuffs, rhyolite 

lava flows, and volcanic breccias. Small-volume ash flows continued to be erupted from 

the western source area until 18.0 m.y. ago. The intracaldera fill is hydrothermally 

altered and cut by small rhyolitic stocks that tend to be localized above the buried 

ring-fracture zone of the caldera. 

Neither the Mount Belknap nor Red Hills calderas is resurgent, nor is there evidence 

for any significant late-stage igneous activity at either caldera. 

Both the Mount Belknap and Red Hills calderas were localized over high-level magma 

chambers above a common magma source. The differences in eruptive histories of the 

two areas are believed to reflect differences in shape, depth, and evolution of the 

individual magma chambers and cupolas. The more protracted and smaller volume 

eruptions from the eastern source area were derived from relatively small cupolas 

developed sequentially above an elongate high-level magma chamber, whereas the more 

1 
2 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

catastrophic ash-flow eruptions and large-scale caldera subsidence in the western source 

area reflect a broad, shallow, high-level magma chamber whose rooffailed suddenly and 

whose eruptive energy was spent quickly. 

When the normative mineral assemblage of the Mount Belknap Volcanics is plotted on 

the quaternary system Q-Or-Ab-H20, it falls close to the position of a magma with a 

water pressure of800 ±200 bars. When this pressure is modeled on a lithostatic system, a 

depth of 3 to 4 km to the high-level magma chamber prior to eruption is determined, 

depending on the assumed density of the overlying rocks. 

INTRODUCTION 

The Mount Belknap caldera is a major subsidence structure located 

in the central 'fushar Mountains, 260 km south of Salt Lake City and 

20 km west of Marysvale, Utah (fig. 1). The Red Hills caldera is a 

smaller coeval structure located 5 km north of Marysvale in the southern Antelope Range. Intracaldera and outflow facies volcanic rocks 

from these and associated centers constitute the complex accumulation of rhyolite ash-flow tuffs, lava flows, and volcanic breccias called 

the Mount Belknap Volcanics. These rocks were erupted 21-18 m.y. 

ago, and they constitute a significant part of the Marysvale volcanic 

field, which covers more than 2,000 km2 in west-central Utah. 

The town of Marysvale is in the north-trending Sevier River valley, 

at approximately 2,000 m altitude. The valley is bounded on the west 

by the 'fushar Mountains, which include Mount Belknap (3,700 m), 

Mount Baldy (3,683 m), Delano Peak (3,710 m), and Mount Brigham 

(3,586 m); on the north by the Antelope Range, a series oflow, rounded 

hills 6 km north-northeast of Marysvale; and on the east by the high 

(2,900 m), linear Sevier Plateau. The rocks and structures discussed in 

this report are largely in the 'fushar Mountains, Antelope Range, and 

Sevier River valley. 

The geology of the Marysvale area has been studied intermittently 

since the late 1800's, when Dutton (1880) published his report on the 

High Plateaus of Utah. Since then, numerous studies of the areal and 

economic geology have been published, but only some of the more 

important will be mentioned here. Callaghan (1938, 1939) published 

the first comprehensive studies of the igneous rocks and alunite deposits of the area and set up the volcanic stratigraphy. During World War 

II, the alunite deposits of the area were investigated and explored by 

the U.S. Bureau of Mines and U.S. Geological Survey; partial results of 

this work were published by Hild (1946) and Willard and Proctor 

(1946). The discovery of uranium in the Antelope Range in 1949led to a 

period of intense study of the area by Paul Kerr and his students P.M. 

Bethke, G. P. Brophy, H. M. Dahl, Jack Green, L. E. Woolard, and N. W 

Molloy, of Columbia University, under the auspices of the U.S. Atomic 

Energy Commission. Many of the results of this work appeared in Kerr 

and others (1957) and Kerr (1968). Molloy and Kerr (1962) studied a 
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INTRODUCTION 

U T A H 

o: Areaof 

f1gure 1 

Marysvale 

10 15 KILOMETERS 

"'-t;.,. 

~ 

~ -5> ~ 

C] Monroe 

"% 

~ c~~~~~L AHEA 

D HILLS CALDERA 

3 

FIGURE 1.-Index map showing the location of the Mount Belknap and Red Hills 

calderas. 

strip along Beaver Creek between Gold Mountain and the Antelope 

Range. 

Callaghan and coworkers from the U.S. Geological Survey mapped 

the geology of four 15-minute quadrangles (Callaghan and Parker, 

1961a, 1962a, 1962b; Willard and Callaghan, 1962), and the contiguous 

eastern part of another (Callaghan and Parker, 1961b). Callaghan 
4 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

(1973) assessed the mineral resource potential ofPiute County, and in 

that report he made first mention of the Mount Belknap caldera (p. 14) 

and recognized that the red Mount Belknap Rhyolite was a welded tuff 

erupted from the vicinity of Mount Belknap. 

The authors began investigating the geology and mineral deposits of 

the Marysvale area in 1975. Although much still-remains to be done, 

significant preliminary revisions have been made in the stratigraphy, 

structure, and petrologic evolution of the volcanic field; these ar~ being 

reported here and in Steven and others(l979). Emphasis here is on the 

evolution of the Mount Belknap and Red Hills calderas and associated 

rocks. 

ACKNOWLEDGMENTS 

We wish to thank many of our U.S. Geological Survey colleagues for 

assistance in this investigation. Gerald T. Cebula, Ezekiel Rivera, and 

Kenneth L. Gardner assisted in laboratory studies. Charles W. Naeser, 

Harald H. Mehnert, and Kenneth R. Ludwig contributed to our knowledge of the chronology and mineralogy of the rocks. 

John Carmony consulted with us in the field on aspects of the 

geology. He shared with us his interpretation that the Red Hills eruptive center was a caldera and clarified the relationship between the 

Red Hills Thff and Joe Lott Thff Members of the Mount Belknap 

Volcanics. Joan Anderson provided logistic support in the field. 

GEOLOGIC SETTING 

The Marysvale volcanic field is located at the western margin of the 

Colorado Plateaus province, in the High Plateaus subprovince that 

forms the transition between the Colorado Plateaus and the Basin and 

Range provinces. The volcanic rocks lie unconformably on Mesozoic 

and lower Cenozoic sedimentary rocks, which are widely exposed at 

the surface 30 km north of Marysvale near Richfield, and 64 km to the 

south near Panguitch. Within the main body of the Marysvale volcanic 

pile, sedimentary rocks are exposed only along the faulted eastern 

front of the Thshar Mountains (on Deer 'frail Mountain), and in a thin 

wedge along the northern side of a quartz monzonite stock on the 

western side of the Antelope Range, 1 km northeast of Big Rock Candy 

Mountain (fig. 1). Large-volume volcanic activity in the Marysvale 

area commenced about 30 m.y. ago and continued until 18 m.y. ago. 

Subsequent igneous activity has been episodic, and has resulted 

largely in extrusion of local basalt lava flows and rhyolitic ash-fall 

tuffs, and in local hydrothermal alteration and mineralization, related 

perhaps to buried intrusions. 

Most volcanic rocks in the Marysvale pile consist of intermediate-
GEOLOGIC SETTING 5 

composition lava flows, volcanic breccia, volcaniclastic deposits, and 

ash-flow tuffs that have been called Bullion Canyon Volcanics (Steven 

and others, 1979). These rocks accumulated around many scattered 

and in part clustered stratovolcanoes, and near-source lava flows and 

volcanic breccias commonly pass laterally into coalescing volcaniclastic aprons of volcanic breccia, conglomerate, and sandstone that extend 

outward in all directions. Volcanic eruptions began about 30 m.y. ago 

and lavas and breccias accumulated to a local thickness of100-200 m. 

About 29 m.y. ago, a great sheet of distinctive crystal-rich ash-flow tuff, 

the Needles Range Formation, spread eastward across the area (Shuey 

and others, 1976). The Needles Range has been identified in places 

near the base of the volcanic section around all sides of the Marysvale 

pile and extends unknown distances beneath Bullion Canyon rocks. 

After the Needles Range Formation was emplaced, a composite pile of 

Bullion Canyon Volcanics more than a kilometer thick accumulated in 

the Marysvale area. Concurrently, regional ash-flow sheets from many 

distant centers in the Great Basin area to the west extended into the 

High Plateaus area to intertongue marginally with the locally derived 

volcanic rocks (Anderson and others, 1975). 

About 27 m.y. ago, a series of distinctive crystal-rich ash flows was 

erupted from a local center a few kilometers north of Clear Creek (fig. 

1), and the resulting sheet, called Three Creeks Thff Member (Steven 

and others, 1979), has been recognized widely near the middle of the 

Bullion Canyon Volcanics. Another distinctive crystal-rich ash-flow 

tuff near the top of the Bullion Canyon Volcanics is called the Delano 

Peak Thff Member and is dated at 22 m.y. 

The Bullion Canyon Volcanics in the southern Antelope Range and 

adjacent areas was intruded by a series of intermediate-composition 

stocks about 23 m.y. ago (Steven and others, 1979). These intrude 

near-source lava flows and volcanic breccia and probably mark the 

cores of Bullion Canyon volcanoes. Adjacent volcanic rocks were 

widely altered about the same time, by hydrothermal activity probably 

associated with the intrusions. Later, unrelated uranium mineralization affected parts of the same area. 

About 21 m.y. ago, volcanic activity and rock compositions changed 

drastically in the Marysvale area, and silicic alkali rhyolite lava 

domes, lava flows, and ash-flow tuffs of the Mount Belknap Volcanics 

were erupted from two source areas, one in the central Thshar Mountains (western source area) and the other in the southern Antelope 

Range and adjacent areas (eastern source area). The first eruptions 

took place in the eastern source area, where a series oflava domes, lava 

flows, subordinate ash-flow tuffs, and a fine-grained granite intrusion 

were emplaced 21-20 m.y. ago. These were followed by major ash-flow 

eruptions in the western source area, with associated subsidence of the 
6 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

Mount Belknap caldera. The caldera in turn was filled with ash-flow 

tuff, viscous lava flows and lava domes, and volcanic breccias. Ash-flow 

eruptions from the eastern source area followed shortly after those 

from the western source, and the small Red Hills caldera subsided in 

response to these eruptions. Both calderas appear to have formed about 

19 m.y. ago. Continued eruptions from both sources resulted in local 

rhyolite lava domes and lava flows, and minor ash-flow tuffs. The 

youngest of these late units has been dated as 18.0 m.y. old. 

Basin-range faulting in the Marysvale area began after the Mount 

Belknap Volcanics accumulated. Fluviatile and subordinate lacustrine sediments were deposited in the developing basins to form the 

Miocene to lower Pleistocene Sevier River Formation. Basalt lava 

flows were erupted episodically during the same period, and are interbedded locally within the Sevier River Formation. The present topography of the central High Plateaus resulted from the late Cenozoic 

period of extensional faulting. 

MOUNT BELKNAP VOLCANICS 

The assemblage of rhyolite lava flows and tuffs that we call Miocene 

Mount Belknap Volcanics (Steven and others, 1979) has been described 

by different workers. The Mount Belknap Rhyolite and Joe Lott Thff 

were named by Callaghan (1939), who considered them to be sequential units. Kerr and others (1957) used ''Mount Belknap volcanic 

series" for the same assemblage of rocks described by Callaghan 

(1939), and also believed that the Joe Lott Thffwas a separate, younger 

unit. We have recognized the lateral equivalence of the Joe Lott Thff 

and certain ash-flow tuffs in the previously named Mount Belknap 

Rhyolite and have included all in the formation called Mount Belknap 

Volcanics (Steven and others, 1979). The Joe Lott was reduced in 

stratigraphic rank to Joe Lott Thff Member of the Mount Belknap 

Volcanics. 

The Mount Belknap Volcanics at one time covered more than 600 

km2 , and possibly more than 1,000 km2 , of west-central Utah. Most of 

this area was covered by ash-flow tuffs from the Mount Belknap caldera (western source area); the viscous rhyolite lava flows and smaller 

volume ash-flow tuffs from the Red Hills caldera (eastern source area) 

covered only a small area in and around the eruptive vents. The Mount 

Belknap caldera and its intracaldera fill cover an area of 130 km2

• 

The ash-flow tuffs and lava flows of the Mount Belknap Volcanics 

were deposited unconformably on the older Bullion Canyon Volcanics 

and related intrusive rocks. In the central mining area referred to by 

Kerr and others (1957), for example, 19-m.y.-old ash-flow tuff lies 

directly on a 23-m.y.-old quartz monzonite stock. This erosion surface 
MOUNT BELKNAP VOLCANICS 7 

had a minimum relief of150 min the central mining area and probably 

had several times more relief elsewhere (Gruner and others, 1951). 

Fluviatile arkosic sedimentary rocks deposited on the erosion surface 

beneath the Mount Belknap Volcanics have been described by Kerr 

and others (1957) and Molloy and Kerr (1962). The rough topography 

under the Mount Belknap Volcanics is most apparent where the underlying Bullion Canyon rocks consist either of vent-facies lava flows and 

breccias or of monzonite intrusions emplaced into vent-facies rocks. 

Thus the topography probably represents deeply eroded Bullion Canyon volcanoes near the center of the Marysvale volcanic pile. 

Remnants of the outflow sheet from the Mount Belknap caldera are 

now most abundant east and northeast of the caldera. Near the caldera 

they are partly localized in paleovalleys cut in the more elevated lava 

flows and volcanic breccias of Bullion Canyon Volcanics, but farther 

from the source the remnants form widespread sheets over flat-lying 

Bullion Canyon Volcanics. The largest remnants of outflow deposits 

occur in the Clear Creek drainage area and eastward nearly to Monroe. 

Near Monroe the unwelded distal end of the sheet is more than 25 km 

northeast of the caldera rim. Other major remnants are southeast of 

Marysvale, between the Sevier River and the Sevier Plateau, and in 

the upper Beaver River drainage area 5-11 km south of the Mount 

Belknap caldera. Small patches of ash-flow tuff and lava flows of the 

outflow facies occur locally in the western 'fushar Mountains west of 

the caldera. 

OUTFLOW FACIES 

The outflow facies of the Mount Belknap Volcanics consists of a 

complex sequence of units derived from both the eastern and western 

source areas. The intermixing of rocks from both source areas requires 

generally concurrent volcanic activity in the two areas. An upward 

progression occurs from basal local lava flows, lava domes, and minor 

pyroclastic rocks derived from the eastern source area, to voluminous 

ash-flow tuffs derived from both areas, to upper, local rhyolite lava 

flows and lava domes and minor ash-flow tuffs derived from both areas. 

LOWER HETEROGENEOUS MEMBER 

Eruption of the Mount Belknap Volcanics began about 21 m.y. ago 

(Steven and others, 1979) with the extrusion of a series of lava domes 

and lava flows from the eastern source area 8 km northeast of Marysvale. These rocks, along with associated stocks, subordinate tuffs, and 

volcanic sedimentary rocks, have been grouped into an informal lower 

heterogeneous member of the Mount Belknap Volcanics that lies between the Bullion Canyon Volcanics and the Joe Lott 'iliff Member of 
8 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

the Mount Belknap Volcanics (fig. 2). Kerr and others (1957) described 

several domes and flows of what we call the lower heterogeneous 

member, including the Teacup and Papsy's Hope domes. The Teacup is a 

nearly circular funnel-shaped structure with steeply inward dipping 

margins and a glassy rim; Kerr and others (1957) considered the 

Teacup to be a diatreme, but we interpret it to be the base and intrusive 

root of a lava dome. It consists of crystal-rich flow rock containing 

phenocrysts of sanidine, plagioclase, biotite, hornblende, quartz, and 

minor apatite, sphene, and magnetite in a devitrified or glassy matrix. 

Upper gray tuff member ( 18.0 m.y.) 

Gray Hills Rhyolite Member 

Upper red tuff member 

Red Hills Tuff Member 

~Rhyolite 

Joe Lott Tuff Member 

Porphyritic flows 

Volcanic sediments, minor tuffs, and flows 

FIGURE 2.-Generalized stratigraphic column of the outflow facies ofthe Mount Belknap 

Volcanics. Ages from Steven and others (1979). 

The early domes and plugs were followed by the intrusion of an 

epizonal stock, called the fine-grained granite by Kerr and others 
OUTFLOW FACIES 9 

(1957). The fine-grained granite intruded a 23-m.y.-old quartz monzonite stock in the central mining area about 20 m.y. ago (Steven and 

others, 1979). It is best exposed just west and northwest of the main 

uranium-producing area, and mine development and exploratory drilling indicate that its eastern flank widens at depth beneath the 

mineralized area where it is a host for some of the uranium-bearing 

veins. The granite contains crystals of quartz, orthoclase, plagioclase, 

and minor biotite in a groundmass commonly characterized by graphic 

intergrowths. 

A series of local crystal-rich ash-flow tuffs occurs at the base of the 

Mount Belknap Volcanics in lower Deer Creek and Beaver Creek 

canyons, where it underlies voluminous ash-flow tuffs derived from 

the Mount Belknap caldera: The local tuffs are 25 m thick near Beaver 

Creek and 20 m thick on the northern side of Deer Creek, 2.0 km west 

of U.S. 89. These crystal-rich rocks were mapped in part as rhyolite 

porphyry by Kerr and others (1957). A sample of the crystal-rich tuff 

was dated as about 19.6 m.y. old on zircon and 18.8 m.y. old on apatite, 

using the fission-track method (Steven and others, 1979), an age in 

excellent agreement with an average age of19.0 m.y. on glass from the 

basal vitrophyre of the overlying Joe Lott Thff Member (Bassett and 

others, 1963). These ages suggest that the local tuffs were erupted soon 

after the stock of fine-grained granite was emplaced. 

The crystal-rich tuff is notably more fernie than other members of 

the Mount Belknap Volcanics, as shown in its modal analysis (table 1). 

The tuff contains broken phenocrysts of locally glomeroporphyritic 

plagioclase ( An:1:l), sanidine, resorbed quartz, brown biotite, 

hornblende, and minor zircon and apatite. The groundmass consists of 

iron-oxide-stained devitrified glass containing tridymite in 

lithophysae. This crystal-rich tuff contains xenoliths of a flow rock 

characterized by large, rimmed sanidines in a felted groundmass of 

feldspar, biotite, and iron oxides. 

TABLE I.-Modal analyses, in percent, of the Mount Belknap Volcanics 

[ t, trace; leaders ( ---), none] 

Member (Sample No. l GRMASS QTZ ALK-FSPAR PL BlOT HNBLD OP AP + ZIR 

Upper gray tuff !M89l 97.4 1.4 0.8 0.4 t t t Upper red tuff CM19A) 98.4 .4 .8 .2 t 0.2 t t Crystal-rich tuff (M75) 70.0 2.8 23.6 1.6 0.8 t 0.8 0.4 Red Hills Thff (M93l 92.4 2.4 4.0 .6 .4 .2 t Joe Lott Thff (M94) 98.6 .8 .2 .4 t t t Ct:stal-rich tuff in ower heterogeneous member (M92) 78.6 6.0 6.8 4.0 1.4 1.4 1.2 .6 

JOE LOTT TUFF MEMBER 

The Joe Lott Thff Member of the Mount Belknap Volcanics is the 

largest ash-flow tuff erupted from the Mount Belknap caldera. It is a 
10 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

crystal-poor, multiple ash flow tuff showing compound cooling characteristics. It spread north, south, and east of the caldera. We have 

demonstrated (Steven and others, 1979) that part of the Mount 

Belknap red rhyolite as used by earlier workers is the more densely 

welded near-source lateral equivalent of the less welded peripheral Joe 

Lott Thff Member. The color is mostly a function of degree of welding; 

densely welded red rocks are most common near the caldera source, 

whereas the partially welded gray rocks predominate in the outer 

parts of the sheet. A complete lateral transition of color and degree of 

welding can be observed in the field in several places, but is especially 

well exposed in the vicinity of Mill Creek; here the most abrupt change 

takes place across a series of partly buried hills of Bullion Canyon 

andesite flows. The more densely welded ash-flow tuffs were partly 

ponded on the caldera side of the hills. The Joe Lott Thff Member 

comprises the voluminous, crystal-poor, outflow ash-flow tuff sheet 

derived from the Mount Belknap caldera (Steven and others, 1979). 

Another composite section of the Joe Lott Thff Member can be observed along Deer and Beaver Creeks in the eastern Thshar Mountains. The base of the Joe Lott Thff Member is exposed 3.2 km west of 

U.S. 89, along Deer Creek; here a vitrophyre 6 m thick rests on rhyolite 

lava flows and volcanic sedimentary rocks of the lower heterogeneous 

member. The vitrophyre is overlain by ash-flow tuff3 m thick, containing abundant lithic fragments of rhyolite flow rocks and darker Bullion Canyon flow rocks. The fragment-rich rock is in turn overlain by a 

vitrophyre 2m thick that grades up into a massive devitrified ash-flow 

tuff about 300 m thick, which forms most of the Joe Lott Member in this 

vicinity. 

Near the mouth of Deer Creek most of the Joe Lott Thff Member is 

light gray, but to the west it becomes more densely welded and red. The 

present surface eroded on densely welded red Joe Lott rocks is covered 

by a characteristic platy talus of red rock chips. The top 3m of the unit 

consists of a nonwelded zone containing prominent pumice fragments; 

this zone has been traced west along Deer Creek to the north-trending 

Thshar fault, where erosion has removed it from the western, upthrown side. The nonwelded top also has been mapped along Beaver 

Creek, and John Carmony (oral commun., 1976) observed it east of the 

Sevier River. The top ofthe Joe Lott ThffMember is best exposed at a 

rest area on U.S. 89, 6 km north of Marysvale (figs. 1, 3). 

The Joe Lott Thff Member appears to be a simple cooling unit near 

the Mount Belknap caldera, but it shows evidence of increasingly 

abundant partial cooling breaks-representing individual ash-flow 

boundaries-outward from the source. The compound cooling characteristics of the Joe Lott Thff Member are well displayed near the 

eastern end of Clear Creek Canyon, where the section was described by 
OUTFLOW FACIES 

FIGURE 3.-0utflow facies of Mount Belknap and Red Hills calderas at rest 

area along U.S. 89, north of Marysvale, Utah. 

11 

Molloy and Kerr (1962). Here the Joe Lott Thff Member rests unconformably on Three Creeks Thff Member of the Bullion Canyon Volcanics and is overlain by sands and gravels of the Sevier River Formation (Callaghan, 1939). 'IWo partial cooling units, each about 150 m 

thick, occur at the base of the section. Each of these units appears to 

consist of several individual ash flows. Both units consist oflight-gray, 

pumice-rich, partially welded ash-flow tuff showing prominent ironstained columnar joints. These two lower partial cooling units are 

capped by a distinctive 15-m pink tuff that probably formed from a 

single ash flow. This pink tuff is in part a stratified, crossbedded, 

base-surge deposit and in part a massive ash-flow tuff. Above the pink 

tuff is another partially welded ash-flow tuff identical in composition 

and lithology with the lower massive tuffs; this unit is more than 75 m 

thick and includes several partial cooling breaks. 

The Joe Lott Thff Member contains only about 1.4 percent phenocrysts (table 1). These consist of broken crystal fragments of quartz, 

sanidine, plagioclase, and minor biotite. The groundmass consists of 

collapsed and devitrified shards and pumice fragments. 

RED HILLS TUFF MEMBER 

The Red Hills Thff Member of the Mount Belknap Volcanics is an 

ash- flow tuff similar in gross appearance to the more densely welded 
12 MOUNT BELKNAP AND RED HILLS CALDERAS. UTAH 

parts of the underlying tuffs of the Joe Lott; it was erupted from the 

vicinity of the Red Hills (of Kerr and others, 1957, pl.12) in the eastern 

source area of the Mount Belknap Volcanics. The small Red Hills 

caldera, 1,200 min diameter, formed at the source, and the Red Hills, 5 

km north of Marysvale (fig. 1), mark the northeastern rim of the 

caldera. The Red Hills area was originally recognized as an eruptive 

source for the "red rhyolite" by Kerr and others (1957) and Gilbert 

(1957). Molloy and Kerr (1962) described the erupted material as a 

mosaic of rhyolitic flows and tuffs from a single magma chamber. John 

Carmony (oral commun., 1976) recognized the source as a caldera. 

The Red Hills Thff Member is a simple cooling unit of densely welded 

ash-flow tuff of the outflow facies. Near its source it rests unconformably on eroded 23-m.y.-old quartz monzonite in the central mining area, 

but west of the Sevier River, the tuff rests on the unwelded top of the 

Joe Lott Member. The Red Hills Member is thickest near its source, 

and thins westward in the direction of the Mount Belknap caldera. A 

vitrophyre 1 m thick occurs locally at the base. The Red Hills Thff 

Member is excellently exposed on the western side of U.S. 89, immediately south of the rest area 6 km north of Marysvale (fig. 3). Here 

the tuff is 300m thick; the upper few meters are unwelded and gray, 

but the remainder is densely welded, red, and spherulitically devitrified. The lower third contains prominent 0.5-m lenticular cavities, 

parallel to the compaction foliation, that represent weathered-out 

I i thophysae. 

The Red Hills Thff Member contains about 7.5 percent (table 1) 

phenocrysts, distinctly more than are in the underlying Joe Lott Thff 

Member. The most common phenocryst is anorthoclase, which commonly shows Carlsbad twinning, less commonly Baveno twinning. 

Other phenocrysts are quartz, plagioclase, and minor biotite. The 

groundmass is an aggregate of collapsed and devitrified shards and 

pumice fragments. 

In its caldera source area, the Red Hills ThffMember is characterized 

by strong secondary flow lineations, formed when the still hot, plastic 

welded tuff was drawn back into the subsiding caldera. Kerr and 

others (1957, p. 31) outlined the area where the lineations dip inward 

steeper than 60°. The dips of lineations flatten markedly outside the 

line, which thus probably approximates the position of the buried ring 

fracture of the caldera. The area called Barren Flat by Kerr and others 

(1957, pl. 12) is within the subsided center of the caldera. The location 

of the Red Hills caldera ring fracture is further marked by a series of 

vents containing pumiceous welded tuffs that were mapped as "white 

rhyolite" by Kerr and others (1957) and Gilbert (1957). 
OUTFLOW FACIES 13 

CRYSTAL-RICH TUFF MEMBER 

The crystal-rich tuff member is a distinctive widespread member of 

the Mount Belknap Volcanics that overlies the Red Hills Thff Member 

west of the Sevier River. It has not yet been recognized east of the river. 

It is about 20m thick near the top of the large cliff west ofU .S. 89, south 

of lower Deer Creek. It was mapped in part as rhyolite porphyry by 

Kerr and others (1957). 

The crystal-rich tuff member fills paleochannels that radiate from 

the Mount Belknap caldera. The member consists mostly of a major 

cooling unit about 15 m thick overlain by one or two thin cooling units; 

all cooling units are lithologically identical and locally have vitrophyres at their bases. A cross section of a paleochannel filled with 

crystal-rich tuff member is excellently exposed in a road cut along 

Beaver Creek, 4.6 km west of U.S. 89. The channel is cut into the Red 

Hills Thff Member and can be traced across the creek to the south. The 

channel here consists entirely of the major cooling unit of the member; 

the cooling unit is only partially welded and is salmon colored near the 

top, suggesting that this is also the original top of the ash-flow tuffs. 

A modal analysis (table 1) of the crystal-rich tuff member shows that 

it contains more crystals than any other tuff unit in the Mount 

Belknap Volcanics. The phenocryst assemblage is dominated by zoned 

anorthoclase crystals having 2Vs near 40°. The rest of the phenocrysts 

include quartz with beta-morphology, broken plagioclase crystals, and 

subordinate partly oxidized biotite and hornblende, magnetite, apatite, zircon, and sphene. The phenocrysts are in a red devitrified 

densely welded groundmass. 

The crystal-rich tuff member is recognized in the lower Beaver 

Creek-Deer Creek area 12.8 km east of the Mount Belknap caldera, 

near the Mount Holly Ski Area in the Beaver River basin 8.8 km south 

of the caldera, and along Rattlesnake Ridge just east of Fish Creek less 

than a kilometer north of the caldera. This distribution seems centered 

on the Mount Belknap caldera, and some of the paleochannels filled by 

the member seem to radiate outward from the same area. We therefore 

postulate that the member probably was derived from this source. 

UPPER RED TUFF MEMBER 

The upper red tuff member of the Mount Belknap Volcanics is a thin, 

widespread, crystal-poor, densely welded ash-flow tuff that was deposited on top of the crystal-rich tuff member. It averages 5 m thick and is 

best exposed along Beaver Creek, where it overlies one of the 

paleochannels containing the crystal-rich tuff member. A basal vitrophyre is present where the upper red tuff member crosses Indian 

Hollow (fig. 1). Molloy and Kerr (1962) mapped part of the tuff as their 
14 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

Crescent Hill Rhyolite and published a photograph of a hand sample 

(1962, plate 2). 

The upper red tuff member is a distinctive rock that is characterized 

by black fiamme as much as 3 em long. The fiamme consist of conchoidally fractured black glass enclosed in a densely welded red devitrified 

matrix. Phenocrysts consist of sparse broken crystals of quartz, alkalifeldspar, plagioclase, and oxidized hornblende. 

PORPHYRITIC LAVA FLOWS 

A series of locally derived porphyritic lava flows are exposed west of 

the Sevier River near the mouth of Beaver Creek, where they underlie 

the Gray Hills Rhyolite Member. Kerr and others (1957) mapped the 

flows as the ((purple porphyry." The source of these lava flows is probably just south of Beaver Creek where the flows are thickest. 

The porphyritic lava flows are petrographically classified as 

pyroxene-latites; they contain phenocrysts as much as 1 em across of 

andesine and pale-green diopsidic augite, as well as sparse oxidized 

crystals of hornblende in a felted groundmass ofmicrolites and hematite. 

GRAY HILLS RHYOLITE MEMBER 

The Gray Hills Rhyolite Member is a prominent member of the 

Mount Belknap Volcanics and comprises most of the unit Callaghan 

(1939) termed the gray facies ofthe Mount Belknap Rhyolite. It forms 

large flow-layered rhyolite lava flows with an aggregate thickness of 

500 min the area called Gray Hills by Kerr and others (1957, pl.12) on 

the north side of lower Beaver Creek. The name Gray Hills Rhyolite 

was first introduced by Molloy and Kerr (1962) for this rock unit. 

The Gray Hills Rhyolite Member consists of a light-gray rock containing sparse phenocrysts and characterized by contorted flow layering. Flow folds are generally overturned outward from the highest part 

of the Gray Hills, indicating that the rhyolite flows heaped up above 

their sources and spread outward by gravity flow. The rock is spherulitically devitrified, and it contains thin lenticular cavities lined with 

vapor-phase quartz and sanidine crystals. 

UPPER GRAY TUFF MEMBER 

The upper gray tuff member is an ash-flow tuff that has been dated 

at 18.0 m.y. (Steven and others, 1979) and is the youngest outflow unit 

in the Mount Belknap Volcanics. It caps the Gray Hills Rhyolite 

Member and is exposed along Indian Hollow, 1.8 km north of Beaver 
OUTFLOW FACIES 15 

Creek. Cobbles of the rock are found on erosional surfaces to the north, 

near Mill Creek. 

The upper gray tuff member is a massive light-gray pumice-rich 

rock that contains sparse phenocrysts of quartz and sanidine in a 

groundmass of collapsed and devitrified shards. 

INTRACALDERA FACIES 

The Mount Belknap caldera is filled with more than 900 m of interlayered ash-flow tuffs, rhyolite lava flows, and volcanic breccias. These 

units constitute the intracaldera facies of the Mount Belknap Volcanics and include the newly-named or informal (Steven and others, 

1979) lower and middle tuff members, the Blue Lake Rhyolite Member, 

the Mount Baldy Rhyolite Member, and volcanic breccia member. 

Talus, landslide, and mudflow deposits were deposited locally along 

the margin of the caldera, and in places in the interior of the caldera. 

The ash-flow tuffs exhibit various degrees of welding. The intracaldera 

facies has been hydrothermally altered and bleached in many places. 

The caldera fill is cut in several places by intrusive rocks that may 

represent feeders for some intracaldera rocks, or may be younger than 

intracaldera rocks. The distribution of the various members within the 

topographic margin of the caldera is shown in figure 4, and the stratigraphic relations are illustrated in figure 5. 

LOWER TUFF MEMBER 

The lowest stratigraphic unit exposed within the Mount Belknap 

caldera is the informal lower tuff member (Steven and others, 1979), 

which is exposed only in the eastern part of the caldera in deep canyons 

at the headwaters of Beaver Creek and an eastern tributary of Fish 

Creek. The unit consists of a densely welded crystal-poor ash-flow tuff 

similar to near-source tuffs in the outflow Joe Lott Thff Member. The 

color ranges from pink to gray, and sparse (1 percent or less) phenocrysts consist of quartz, sanidine, plagioclase, and biotite. A devitrified 

groundmass of collapsed shards and pumice fragments locally contains 

abundant rhyolite lithic fragments. The unit is locally silicified and in 

places contains disseminated pyrite. 

The lower tuff member is exposed over a vertical range of 450 m, but 

the base is nowhere exposed. Presumably the member exists throughout the caldera beneath the younger intracaldera facies, and possibly it 

forms a thick prism such as commonly forms where caldera subsidence 

and ash-flow eruptions are concurrent (Steven and Lipman, 1976). 

Without knowledge of these relations, it is not possible to reconstruct 

the early stages in the evolution of the Mount Belknap caldera. 

At the eastern margin of the caldera in the headwaters of Beaver 
16 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

FIGURE 4.-Geologic map of the Mount Belknap caldera (outline dashed where 

incompletely exposed). All patterned rock units belong to the intracaldera facies 

of the Mount Belknap Volcanics: Tmjl, lower tuff member; Tmbl, Blue Lake 

Rhyolite Member; Tmjm, middle tuff member, containing an included rhyolite 

flow (r); Tmb, Mount Baldy Rhyolite Member, containing local patches of overlying tuff(t); Tmv, volcanic breccia member; and Tmi, intrusive rock. Heavy lines, 

faults (dashed where inferred); U, upthrown side, D, downdropped side. 

Creek, the lower tuff member is overlain by similar tuffs of the middle 

tuff member to be described later. The contact is everywhere covered by 

scree talus or landslides, and its nature has not yet been determined. A 

short distance to the west, however, a wedge of rhyolite lava flows (the 

Blue Lake Rhyolite Member, fig. 6) separates the two tuff members. 
a_<( 

f-<(a:: 

zzw 

::::>~0 

Q.....I....J 

~w<( cnu 

Western source area Eastern source area 

FIGURE 5.-Diagrammatic cross section showing stratigraphic relations of the Mount Belknap Volcanics. Heavy lines, faults; arrows show 

direction of movement. Not to scale. 

z 

t-3 

:::0 

> (j 

> t"" 

t::J 

trl 

:::0 

> 

> 0 

ti:l 

w. 

1--' 

-.J 
18 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

FIGURE 6.-lntracaldera facies of the Mount Belknap caldera seen looking west 

towards Mount Belknap. Qls, Quaternary landslide. 

BLUE LAKE RHYOLITE MEMBER 

The Blue Lake Rhyolite Member is a wedge-shaped body of rhyolite 

lava flows that is as much as 560 m thick in the southern part of the 

caldera and thins northward and northeastward. It is absent near the 

eastern margin ofthe caldera in the headwaters of Beaver Creek and 

expands from a feather edge under Gold Mountain to a thickness of 

more than 500 m in the southeastern part of the caldera. The member 

forms prominent cliffs 100m high along the east side of Mount Belknap 

and thickens southward to 260m adjacent to Blue Lake. It is more than 

270m thick near Indian Creek near the western margin of the caldera, 

but the base is nowhere exposed in this vicinity. 

The Blue Lake Rhyolite Member is characterized by prominent, 

commonly contorted flow layering. These flow layers have been deformed into drag folds where the member is plastered against the 

western topographic wall of the caldera, adjacent to Indian Creek. The 

rock contains less than 1 percent phenocrysts, most of which are 

sanidine and quartz. 

In many places the Blue Lake Member is pervasively bleached and 

altered, and the degree of alteration appears generally to increase 

upwards. The most altered rock consists of a granular white silicified 

material in which nearly all primary textures have been destroyed. 

The general pattern of thickness and distribution, and the similarity 

in composition and phenocryst content between the . lava flows and 
INTRACALDERA FACIES 19 

overlying and underlying ash-flow tuffs suggest that the member 

represents continuing eruption of the same magma from the same 

magma chamber, except that the type of eruption changed from ash 

flow to lava flow. The member is thickest in the southern or southwestern parts of the caldera; this indicates that the ring fracture zone of the 

caldera in this area may have localized the actual vents through which 

the lavas were extruded. The Blue Lake Rhyolite Member lava flows 

were deposited against the flaring topographic wall of the Mount 

Belknap caldera, which requires that the main period of caldera subsidence preceded the Blue Lake eruptions. 

MIDDLE TUFF MEMBER 

The middle tuff member is a widespread prominent ash-flow tuff 

within the northern three-fourths of the Mount Belknap caldera. It is 

over 260m thick on the east side of the caldera, where it makes up Gold 

Mountain and Signal Peak, and it overtops the caldera rim to the east. 

An even greater thickness (500 m) occurs in the Line Canyon-Picnic 

Creek area in the northern part of the caldera. The middle tuff is only 

about 75 m thick on the west side of the caldera near Indian Creek (fig. 

4), and it appears to wedge out southward against lava flows of the 

Blue Lake Rhyolite Member in the southern part of the caldera. 

Most of the middle tuff member is densely welded, although local 

less welded partings, representing partial cooling breaks, can be seen 

in places. The only evidence for complete cooling breaks was seen 

adjacent to lenses of rhyolite lava flows that occur locally within the 

member. The basal contact of the middle tuff member on the underlying Blue Lake Rhyolite Member is well exposed in many places. Along 

the east side of the caldera, the base of the tuff consists of black glass 

fiamme in a devitrified matrix, but on the west side of the caldera 

where the tuffis thinner, the base consists of a black vitrophyre about 1 

m thick. Locally, volcanic mudflow deposits are interlayered between 

the Blue Lake Rhyolite Member and the middle tuff member. 

The middle tuff member is generally a homogeneous, crystal-poor, 

light-yellow to gray ash-flow tuff. At Gold Mountain and Signal Peak, 

the tuff has been pervasively hydrothermally altered, and in places it 

is a massive granular silicified rock with few vestiges of the original 

texture. The silicified, and in places ferruginous, tuff locally forms 

ledges that are more resistant than the adjacent rocks. Iron oxide from 

weathering of pyritic altered rocks has stained some of the rock to a 

golden hue, to give Gold Mountain its name. The tuff consists largely of 

compacted shards and pumice fragments in various stages of devitrification. The rock contains about 1 percent phenocrysts of quartz and 

minor sanidine and plagioclase. 
20 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

The middle tuff member represents a change in eruptive style, with 

a return to dominant ash flows similar to those that characterized the 

lower tuff member. No change in magma type or composition is indicated. Distribution and thickness relations show that the middle tuff 

member accumulated in the low areas in the northern and eastern 

parts of the caldera and wedged out southward against the underlying 

lava flows in the Blue Lake Rhyolite Member. Inasmuch as the middle 

tuff member is in an abutting depositional contact with the flaring 

topographic wall of the Mount Belknap caldera, its eruption was later 

than the main period of subsidence of the caldera. 

MOUNT BALDY RHYOLITE MEMBER AND VOLCANIC BRECCIA MEMBER 

The Mount Baldy Rhyolite Member consists of thick rhyolite lava 

flows and lava domes that form the upper part of the Mount Belknap 

caldera fill. It is compositionally similar to the underlying ash-flow 

tuffs and rhyolite lava flows in the caldera fill, and it appears to mark 

just another change in eruptive behavior of the same magma. The 

volcanic breccia member consists of fragments of rhyolite in a finegrained, muddy matrix that were deposited in low areas adjacent to 

some bulbous flows of the Mount Baldy Rhyolite Member. The volcanic 

breccia member is coeval with the Mount Baldy Rhyolite Member. 

The Mount Baldy Rhyolite Member is the most widely exposed unit 

within the caldera, and along with the associated volcanic breccias, it 

makes up virtually all the high mountains in the western two-thirds of 

the caldera. It abutted the western and southern topographic walls of 

the caldera and spilled out over the topographic rim onto the outlying 

terrain. Small patches of Mount Baldy Rhyolite Member flows in the 

outflow area are preserved at many places adjacent to the western 

caldera wall, and they may at one time have been parts of a nearly 

continuous sheet. Most of these patches rest on Bullion Canyon Volcanics, but locally they overlie outflow tuffs of the Joe Lott 'iliff 

Member, whose eruption led to the main caldera subsidence. 

Mount Baldy Rhyolite Member lava flows overlie nearly identical 

lava flows of the Blue Lake Rhyolite Member in the southern part of 

the Mount Belknap caldera, in an area where the Blue Lake flows are 

thickest and protrude above the level of accumulation of the middle 

tuff member. Farther north, the Mount Baldy flows extend across the 

feather edge of the middle tuff member onto a progressively thickening 

wedge of the same unit, and in the west-central part of the caldera, the 

flows abut and intertongue with the volcanic breccia member. These 

breccias in turn wedge northwestward and are overlain by more Mount 

Baldy lava flows that extend across the distal edge of the breccias onto 

the top of the middle tuff member in the northwestern part of the 

caldera. 
INTRACALDERA FACIES 21 

The southern mass of Mount Baldy Rhyolite Member lava flows is 

thickest in the same general area that the Blue Lake Rhyolite Member 

is thickest, and both units seem to have been erupted from the same 

source area above the southern ring fracture zone of the caldera. The 

thick mass of Mount Baldy flows in the northwestern part of the 

caldera possibly could have come from the same source, but thickness 

relations suggest that they were more probably erupted locally, possibly from the northwestern part of the ring fracture zone. Small patches 

of ash-flow tuff overlie Mount Baldy lavas near the head of Shingle 

Creek (fig. 4), indicating that these lavas were originally relatively 

thin in this area; they are much thicker farther north, where they 

probably stood as highlands above local source vents. 

The Mount Baldy Rhyolite Member consists of a light-gray to white 

fine-grained rhyolite. Contorted flow layers are locally prominent, but 

elsewhere the rock is nearly massive. Above timberline, the rhyolite 

exposures are commonly covered with flattened plates or chips derived 

by weathering of flow-layered rhyolite. The rhyolite consists of a 

granular mosaic of crystals of quartz, 50 J.Lm (micrometers) or less in 

length, as well as alkali-feldspar, minor plagioclase, and biotite, and 

magnetite converted to hematite. As in the lower part of the intracaldera facies, the Mount Baldy Rhyolite Member and especially 

the more permeable volcanic mudflow breccia member are widely 

bleached and altered. Dense granular silicified rock, in which the 

original textures were largely destroyed, is common. 

YOUNG INTRUSIVE ROCKS 

Several small stocks cut the ash-flow tuffs and rhyolite lava flows of 

the intracaldera fill. The largest stock (fig. 4), located in the headwaters of the North Fork of North Creek, consists of rhyolite, and at the 

levels now exposed, cuts the Mount Baldy Rhyolite Member. The stock 

is 1.3 km long and 0.6 km wide. An intrusive breccia zone marking part 

of the roof is exposed on the north side of the creek. The stock has a 

fine-grained border about 50 m thick that grades inward into a porphyritic core. The core contains scattered phenocrysts of plagioclase as 

much as 1 em across, and quartz and altered sanidine 0.4 em across. 

The groundmass is a finely granular mosaic of alkali- feldspar and 

quartz crystals. 

A small porphyritic stock cuts ash-flow tuff and rhyolite lava flows 

0.4 km south of the summit of Gold Mountain. This stock contains 

brecciated fragments of intrusive rock in a matrix of similar material. 

The phenocrysts are glomeroporphyritic plagioclase, alkali-feldspar, 

and oxidized brown biotite. The groundmass contains trachytic 

plagioclase microlites in an alkali-feldspar matrix. The stock has a 

distinct pink hue owing to the oxidation of enclosed pyrite. 
22 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

Another small plug cuts rhyolite lava flows at the U-Beva prospect 

along the North Fork of North Creek. It is exposed in the mine workings and consists of an altered porphyry containing prominent flowalined alkali-feldspars as much as 1 em across. Pyrite-bearing intrusive rock of intermediate composition is also found on a mine dump 1.1 

km west of the summit of Gold Mountain. The adit trends N. 72° E. 

under Gold Mountain; the intrusive is not exposed at the surface. 

All known stocks that cut the caldera fill are several kilometers 

inside the exposed topographic wall of the caldera, and they could well 

be located above a buried ring fracture zone. The largest stock, along 

the North Fork of North Creek, and the small plug at the U-Beva 

prospect are in the area believed to have been the source for rhyolite 

lava flows in both the Blue Lake and Mount Baldy Members; they may 

reflect continuing activity at this source. 

EVOLUTION OF THE MOUNT BELKNAP 

AND RED HILLS CALDERAS 

The approximately coeval Mount Belknap and Red Hills calderas 

illustrate some of the wide variety of geologic relations that can develop when even closely related magmas are erupted concurrently 

from nearby source areas in similar environments. Whereas the general model of ash-flow eruption and caldera development (Ratte and 

Steven, 1964; Smith and Bailey, 1968; Lipman, 1975; Steven and Lipman, 1976; Bailey and others, 1976) was applied to studies of both 

areas, the results were quite different. Minor differences in timing and 

conditions of eruption led to strongly contrasting volcanic accumulations and structures; depth and configuration of the source magma 

chambers may have had a major controlling influence. 

In the eastern source area (fig. 5), low-volume eruptions of predominantly viscous lava extended through a period of 3-4 m.y. Only once 

during this span did magmatic pressures build up enough to cause 

ash-flow eruptions, and these were of sufficiently small volume so that 

only a small cauldron about 1,200 m across subsided. In the western 

source area, on the other hand, premonitory eruptions were minor, 

magmatic pressures built high enough to cause catastrophic eruptions 

of many tens of cubic kilometers of ash-flow tuff, and a major caldera 

8-9 km across collapsed in consequence. Large-volume post-collapse 

eruptions followed immediately after subsidence. A span of about a 

million years embraced most activity in this source area; only minor 

episodic pyroclastic eruptions took place thereafter. 

The source areas for the Mount Belknap Volcanics were in a hilly 

terrain cut on vent-facies lava flows, volcanic breccias, and associated 

quartz monzonitic intrusive bodies that marked the eroded remnants 
EVOLUTION OF THE MOUNT BELKNAP AND RED HILLS CALDERAS 23 

of a cluster of Bullion Canyon central-vent volcanoes. The widespread 

sheet of outflow tuffs from the Mount Belknap caldera (Joe Lott Thff 

Member) in areas north and south of the caldera covers relatively flat 

lying volcaniclastic beds, lava flows, and ash-flow tuffs that formed 

aprons marginal to these volcanoes. The Mount Belknap sources thus 

were closely localized within the area of earlier Bullion Canyon 

sources. 

Eruption of the Mount Belknap Volcanics followed shortly after 

cessation of Bullion Canyon igneous activity. 'l.Wo quartz monzonite 

stocks of the Bullion Canyon, one (the central intrusion of Kerr and 

others, 1957) in the eastern source area and the other (Kimberly mine 

area) adjacent to the western source area, yielded radiometric ages 

near 23 m.y. (Steven and others, 1979). These stocks preceded the 

earliest Mount Belknap eruptions (21 m.y.) by only a few million years. 

Other intermediate-composition lava flows and tuffs in the Bullion 

Canyon Volcanics in nearby areas to the south (Delano Peak Thff 

Member and quartz latite lava flows near the Beaver River) have been 

dated as 22 m.y. old. All these data indicate that an even shorter time 

span separated Bullion Canyon and Mount Belknap eruptions. 

Despite the short interval separating eruptions of the two major 

volcanic assemblages of the Marysvale pile, there was a profound 

change in compositions of the rocks erupted. The Bullion Canyon 

Volcanics and associated units accumulated over a period of nearly 10 

m.y., during which time predominantly intermediate-composition 

calc-alkaline lava flows and tuffs were erupted. Compositions range 

from basaltic andesite to quartz latite, with the largest differences 

being between individual volcanic centers. No obvious widespread 

change with respect to time has been noted, although this observation 

is preliminary. Between 22 and 20 m.y. ago, and perhaps across an even 

shorter time span, magmatic compositions changed to highly siliceous 

alkali rhyolite and small volumes of quartz latite. 

The superposition of source areas for the contrasting magma types 

and the lack of transitional lithologies require that the change in 

composition be deep seated, probably reflecting changes in conditions 

of genesis. Rhyolites similar to those in the Mount Belknap Volcanics, 

along with widespread but locally distributed basalt lava flows, were 

erupted episodically throughout later Cenozoic time in southwestern 

Utah, generally concurrent with extensional basin-range faulting. 

The whole assemblage, including the Mount Belknap Volcanics, appears to be part of the bimodal basalt-rhyolite association described by 

Christiansen and Lipman (1972). 
24 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

EASTERN SOURCE AREA 

The eastern source area (fig. 7) for the Mount Belknap Volcanics is 

an elongate area about 15 km long and 5 km wide that extends from the 

low eastern flank of the Thshar Mountains northeastward nearly 

across the southern Antelope Range. Eruptions began in the eastern 

third of this area about 21 m.y. ago, and within a span of about a million 

years, several lava domes were extruded over an area about 5 km in 

diameter, and a pluton of fine-grained granite was emplaced at very 

shallow depths. In the succeeding million years-19 m.y. ago- activity 

shifted slightly to the southwest to the center of the eastern source 

area, where ash-flow eruptions emplaced the densely welded Red Hills 

ThffMember, and a small caldera slightly more than a kilometer across 

(Red Hills caldera) subsided at the source, drawing back downward 

some of the still hot and plastic overlying welded tuffs. The next 

eruptions in the eastern source area were in the vicinity of the Gray 

Hills in the western third of the area, where prominent rhyolite lava 

flows and lava domes were extruded about 18 m.y. ago. A later igneous 

episode may have taken place in the central part of the area, in 

connection with a period of mineralization that affected the central 

mining area just northeast of the Red Hills caldera. In this area several 

glassy rhyolite dikes were emplaced along fissures that also localized 

uranium-molybdenum-fluorine-bearing veins; the most concordant 

38° 

30' 

Topographic wall --r--~;:-, 

RED HILLS CALDE~~~i~ ~--~-~:J ..... - . w / / / 

/ ~· 1// 

I ~H / \. 1 Marysvale -... / 0 

-\~astern source area 

0 KILOMETERS 10 

I I I I I I I I I I 

FIGURE 7.-Source areas for the Mount Belknap Volcanics: TC, Teacup dome; fgg, finegrained granite; GH, Gray Hills. Bar and ball on downthrown side of fault. Eastern 

source area dashed, less well defined. 
EVOLUTION OF THE MOUNT BELKNAP AND RED HILLS CALDERAS 25 

radiometric ages on pitchblende from these veins range between 13 

and 10 m.y. (Steven and others, 1979). 

The southwestward progression of igneous activity probably reflects 

successive emplacement of shallow cupolas above a larger high-level 

magma chamber (fig. 8). Inasmuch as each successive area of activity 

w 

No details of surface 

volcanic rocks shown 

0 5 

I I I I I I 10 KILOMETERS 

I 

(/) 

::J 

0 

a>-

~~ 0>E 

e ..... Q>(\1 

a>-

~ .... 

Q) 

'-.0 

a>E 

3:Q> 

_3E 

E 

FIGURE B.-Diagrammatic sketch showing hypothetical relations in the source areas for 

the Mount Belknap Volcanics. Scale is approximate. 

appears to have been about 5 km across, the cupolas probably were 

similarly limited in lateral extent. This progression of sources in time 

and space was accompanied by a general change in texture and composition of the resultant rocks. The early lava domes and lava flows in the 

eastern part of the area contain abundant alkali and plagioclase 

feldspar, quartz, biotite, and hornblende phenocrysts; the general 

composition is rhyolite (table 2, sample A), but the silica content is 

somewhat lower than that of most other Mount Belknap Volcanics 

rocks. The fine-grained granite pluton from the central part of the 

eastern source area is porphyritic, but the phenocrysts are not abundant; the composition (table 2, sample B) is similar to that of the 

Thacup dome. The Red Hills ThffMember from the same area has about 

7.5 percent phenocrysts, but is slightly more silicic (table 2, sample F) 

than the fine-grained granite or the rock in the domes farther east. The 

lava flows and lava domes from the Gray Hills area (table 2, samples I, 
TABLE 2.- Chemical analyses of Mount Belknap Volcanics t...:> 

[D. 1., differentiation index; leaders(___.), not analyzed for J O':l 

~tNo: A B c D E F G H I J K M49B M92 M94 M91 M93 M75 M19A M87 M88 M89 

Lab. No. _ D177587W D178559W Dl78561W Dl78558W Dl78560W D177588W Dl78555W D178554W Dl78553W D178558W 

~ Major oxides [weight percent, recalculated without H20 and C02 (original values listed separately below) J 0 

c::: 

Al

SiO. 74.45 74.47 72.49 77.29 76.34 76.17 71.15 75.64 75.98 75.97 77.25 ~ 2

0 3 ____ 13.71 13.21 13.32 13.04 13.07 13.41 15.23 13.51 13.04 13.38 12.60 to Fe.o3 _ 1.14 1.51 2.10 .90 .92 1.14 1.64 .97 1.12 1.02 1.22 tz:j FeO __ .46 .78 .17 .04 .04 .04 .33 .29 .04 .04 .04 t'"' MgO 

CaO ______ 1.14 

.47 .33 .52 .05 .13 .17 .20 .16 .04 .05 .20 ~ .80 3.15 .50 .36 .69 .58 .53 .28 .04 .44 Na z 20 _ 3.84 2.69 3.46 3.18 3.72 3.01 4.50 4.19 4.18 3.98 2.64 K.o ___ 4.36 5.87 4.20 4.82 5.23 5.09 5.83 4.40 5.09 5.31 5.29 > no._ ~ .23 .21 .34 .06 .07 .16 .33 .12 .09 .10 .16 P.o. __ .15 .05 .19 .06 .06 .08 .14 .08 .06 .02 .05 > MnO .05 .04 .07 .04 .06 .04 .07 .09 .08 .08 .10 z BaO --- .04 --- --- --- --- --- --- --- --- --- t:::l 

~ 

'lbtals adjusted to 100 percent tz:j 

t:::l H2

0+ ____ 2.6 1.44 0.91 1.0 0.47 1.3 0.40 3.2 0.42 0.61 1.2 ::r: H.o-_ .38 .03 .81 .64 .33 1.1 .46 .77 .34 .22 1.1 ~ 

co. .02 .26 1.6 .06 .02 .02 .02 .04 .02 .02 .04 t'"' 

t'"' 

r::n 

Nonnative minerals (weight percent) 0 

> t'"' 

8_ 32.68 34.26 30.59 39.34 33.88 37.67 21.54 33.28 31.73 32.38 40.56 t:::l .94 1.07 --- 1.81 .78 1.89 1.81 1.09 .30 1.05 1.86 tz:j 

OR ___ 25.77 34.70 24.80 28.51 30.90 30.09 34.43 26.00 30.09 31.38 31.23 ~ 

AB- 32.51 22.77 29.29 26.93 31.49 25.50 38.06 35.46 35.34 33.70 22.36 > 

~r::n 

c::: 

~ ::r: 
AN ..... 4.72 3.70 8.42 2.09 1.40 2.86 1.96 2.10 0.97 WO .. 2.39 --- --- --- --- --- --- EN 1.16 .82 1.31 .13 .33 .41 .51 .39 .10 MT .98 2.02 --- .09 .12 --- .34 .92 .13 HM .. .47 .12 2.10 .84 .83 1.14 1.40 .34 1.03 IL .43 .41 .51 .12 .13 .18 .62 .22 .17 IN --- --- .16 --- --- --- --- --- --- RU. --- --- --- .06 --- --- --- AP .... .34 .12 .45 .15 .14 .20 .34 .20 .15 'lbtal .... 100.01 99.98 100.01 100.00 100.00 100.01 100.01 100.01 100.00 Salic 96.62 96.49 93.09 98.68 98.45 98.01 96.80 97.94 98.42 Fernie 3.39 3.48 6.92 1.32 1.56 1.99 3.21 2.07 1.58 D.l .. 90.97 91.72 84.67 94.78 96.27 93.26 94.03 94.75 97.16 

SAMPLE DESCRIPriONS 

A. Thacup dome in the lower heterogeneous member. Collected from the glassy margin on the south side of Thacup dome. 

Lat 38°30'41" N., long 112°10135" W. 

B. Fine-grained granite of the Central area (Kerr, 1957, p. 54, No.4). 

C. Crystal-rich welded tuff in the lower heterogeneous member. Collected 2.09 km west-northwest of the junction of Deer Creek and the Sevier River. 

Lat 38°30'00" N., long 112°16'46" W. 

D. Joe Lott 'IWT Member. Collected from road cut on U.S. highway 89, on south side of rest area alongside Deer Creek. 

Lat 38°30'08", long 112°15'25". 

E. Joe Lott Member. Collected from road cut along Clear Creek, 3.14 km west of the junction of Clear Creek and the Sevier River. 

Lat 38°34'50", long 112°17'31". 

F. Red Hills 'IWT Member. Collected from base of cooling unit, U.S. 89 on south side of the rest area alongside Deer Creek. 

Lat 38°30'08", long 112°15'25". 

G. Crystal-rich tuff member. Collected 1.45 km west of the junction of Beaver Creek and the Sevier River. 

Lat 38~8'37", long 112°14'57". 

H. Upper red tuff member. Collected from vitrophyre 1.03 km north of Beaver Creek, along Indian Hollow. 

Lat 38~8'18", long 112°17'34". 

I. Gray Hills Rhyolite Member. Collected 1.90 km south-southwest of the junction of Deer Creek and the Sevier River. 

Lat 38~9'21", long 112°15'41". 

J. Gray Hills Rhyolite Member. Collected 2.57 km west-northwest ofthejunction of Beaver Creek and the Sevier River. 

Lat 38~9'05", long 112°15'35". 

K. Upper gray tuff member. Collected 4.99 km west of the junction of Beaver Creek and the Sevier River. 

Lat 38~8'37", long 112°17'16". 

0.07 1.84 --- --- .13 .51 .10 --- .95 1.22 .19 .30 

-- --- --- .00 .05 .12 100.00 100.00 98.58 97.85 1.42 2.15 97.46 94.15 

t;l=j 

< 0 

t'-4 

s 

~ 

0 

z 

0 

"%j 

t;l=j 

~ 

~ 

0 

c:: 

z 

--3 

t:l:l 

t;l=j 

t'-4 

~ 

z 

> "t1 

> z 

t:1 

:;:o 

t;l=j 

t:1 

::c 

F 

t'-4 

00. 

0 

> t'-4 

t:1 

t;l=j 

~ 00. 

~ 

-.J 
28 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

J) in the western part of the eastern source area are of crystal-poor 

silicic rhyolite. These successive changes may reflect: (1) the source 

cupolas tapping progressively shallower levels of a compositionally 

zoned high-level magma chamber, or (2) the cupolas developing from 

progressively more silicic fractions at the top of an actively differentiating high-level magma chamber. Probably aspects of both alternatives were involved. 

The elongate area embracing the local vents within the eastern 

source area (fig. 7) probably reflects the general dimensions of the 

upper part of the underlying high-level magma chamber. The local 

volcanic accumulations that we have observed may also indicate the 

mechanism by which the eastern source area evolved. In the eastern 

source area, the top of this magma body apparently worked its way 

upward as a sequence of cupolas that in turn breached the surface to 

extrude local small-volume viscous lava flows and ash-flow tuffs. The 

concurrent eruption of closely similar volcanics from both the eastern 

and western source areas-less than 10 km apart-virtually requires a 

common magmatic source at depth (fig. 8). 

The shape of the high-level magma chamber underlying the eastern 

source area, as well as the limited dimensions of the protruding 

cupolas, and the length of time that the source area was an active 

center probably were major factors in controlling the type and volume 

of the volcanic eruptions. Magmatic pressures within a narrow, elongate chamber surmounted by cupolas oflimited lateral extent could be 

released more episodically than the same pressure within a broadtopped chamber at comparable depths. The fact that only one cupola 

seems to have been active at any given time gives added support to our 

hypothetical model (fig. 8). The gas content of the magmas increased 

with successive eruptions, inasmuch as the Red Hills Thff and Gray 

Hills Rhyolite Members, with their very abundant lithophysae and 

prominent spherulitic devitrification textures, appear to have been 

highly gas charged when erupted-probably more so than any other 

rocks in the Mount Belknap Volcanics. This increase in volatile content probably resulted from progressive crystallization of the underlying common parent magma chamber. The relatively narrow, high-level 

magma chamber underlying the eastern source area was tapped successively by smaller cupolas and permitted extrusion of only small 

volumes of viscous lava before clogging of the vents. Only once near the 

end of its eruptive history did the pressures in an extremely gas 

charged magma sufficiently exceed the strength of the containing 

rocks so that ash flows were erupted. The cupola, however, was so 

limited laterally that a cauldron only about a kilometer across could 

subside, and the pyroclastic eruptions were soon over. 
EVOLUTION OF THE MOUNT BELKNAP AND RED HILLS CALDERAS 29 

The depth to the top of the high-level magma chamber and protruding cupolas probably was also a strongly contributing factor to the 

style of eruption. The confining strength of wall rocks increases 

greatly with thickness, and when combined with the shape factor, 

slight variations in depth could have had major influence in inhibiting 

the explosive release of magmatic pressures required for pyroclastic 

eruptions. 

WESTERN SOURCE AREA 

The early stages in the evolution of the Mount Belknap caldera are 

obscure, as most relevant evidence is within the subsided block of the 

caldera (fig. 8) and is deeply covered by the intracaldera facies. The 

local abundance of lithic fragments of rhyolite flow rock within the 

lower part of the outflow Joe Lott Thff Member indicates that some 

premonitory lava eruptions had taken place, but the site of eruption, 

volume of lava, or form of accumulation can only be conjectured. As 

none of these early flow rocks have been found outside the area of the 

caldera, we postulate that they probably were of small volume and that 

the source vents were restricted to a relatively small area. 

Catastrophic pyroclastic eruptions took place suddenly and continued in a surging sequence of individual ash flows until the entire 

outflow sheet of Joe Lott Thffhad accumulated. A few minor pyroclastic eruptions preceded the main outbreak in the western source area, 

as one small ash-flow tuff no more than 9 m thick was noted beneath 

the main Joe Lott sheet in the Deer Creek area east of the caldera, and 

local crossbedded nonwelded base-surge deposits occur at the base of 

the Joe Lott sheet in the headwaters of Clear Creek north of the 

caldera. Elsewhere the Joe Lott Thff Member rests directly on older 

volcanic rocks. 

Near the caldera source, the Joe Lott ThffMember is a simple cooling 

unit, but compound cooling characteristics become increasingly apparent outward. Most of the sheetlike remnants of Joe Lott that accumulated on the plains surrounding the hilly source area show evidence for numerous partial to complete cooling breaks, and they obviously were deposited by many individual ash flows. A few rhyolite lava 

flows are interleaved with tuffs in the Joe Lott near the caldera source, 

also indicating the intermittent character of the eruptions. 

Eruption of the many tens ofkm3 of ash-flow tuff in the Joe Lott Thff 

Member caused the Mount Belknap caldera to collapse in the source 

area. Present dimensions of the very irregular shaped caldera (fig. 4) 

are about 13 km wide E-W, and 17 km long N-S. These margins all 

represent flaring topographic walls that caved inward from the structural margin of the caldera shortly after subsidence, and before the 
30 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

upper part of the caldera fill was deposited. The actual subsided block 

was probably 8-9 km across, and it may have gone down in several 

segments, which would account for some of the irregularity in the 

shape of the caldera margin. 

No criteria have been developed to permit close correlation between 

eruptive events within the caldera and the stratigraphy of the outflow 

sheet. Deductively it would seem probable that the outflow Joe Lott 

Thff Member is correlative with the largely buried lower tuff member 

within the caldera rather than with the younger intracaldera units. 

Caldera subsidence must have occurred only after sufficient ash had 

been erupted to remove support from the roof of the high -level magma 

chamber, and this ash would have spread over the surrounding terrain 

to form a significant part of the Joe Lott Thff. The lower tuff member 

probably accumulated when caldera subsidence began and before the 

Joe Lott eruptions ceased. The lower tuff member, however, is so poorly 

exposed that its thickness and distribution are not known, and its 

stratigraphic relations are largely conjectural. 

The overlying intracaldera facies-the Blue Lake Rhyolite Member, 

middle tuff member, Mount Baldy Rhyolite Member, and associated 

volcanic breccia member-were deposited against the flaring topographic wall of the caldera and thus after the main period of caldera 

subsidence. The virtual compositional identity of all intracaldera 

rocks, whether ash-flow tuff, lava flows, or volcanic mudflow breccia, 

argues for a succession of alternating lava and pyroclastic eruptions 

from the same magma chamber; these eruptions probably were in 

rapid succession, inasmuch as most took place before the crystal-rich 

member of the Mount Belknap Volcanics accumulated in the outflow 

area about 19 m.y. ago (Steven and others, 1979). In all, the major 

eruptions from the western source area of the Mount Belknap Volcanics seem to have taken place within about a million years. 

No late resurgence occurred in the Mount Belknap caldera. Instead, 

the core of the caldera subsided slightly again late in the period of its 

evolution-after eruption of the Mount Baldy Rhyolite Member. The 

subsidence occurred as a tilted trap-door block, hinged on the west and 

bounded by arcuate faults on the east (fig. 4). Maximum subsidence 

was no more than 150 m. The arcuate faults may mark the position of 

the main buried ring fracture zone on this side of the caldera, reactivated locally during this late episode of subsidence. 

In comparison with other calderas (Smith and Bailey, 1968; Steven 

and Lipman, 1976) the Mount Belknap caldera shows few if any ofthe 

later stages in the typical cauldron development cycle. The complex 

mixture of lava flows and ash-flow tuff filling the caldera is in contrast 

to more simple cauldron fills in other areas, where thick prisms of 

ash-flow tuff similar in composition to related tuffs in the outflow sheet 
EVOLUTION OF THE MOUNT BELKNAP AND RED HILLS CALDERAS 31 

filled the subsided area during or shortly after collapse. No resurgence 

is in evidence, nor are there any late ring-domes or other manifestations of late igneous activity. A few low-volume late ash-flow sheets 

(crystal-rich tuff member, upper red tuff member, and upper gray tuff 

member of Steven and others, 1979) appear to have been derived from 

the western source area, but the precise sources or intracaldera equivalents, if any, have not been identified. 

In reconstructing conditions at depth beneath the Mount Belknap 

caldera during its period of activity (19-18 m.y. ago), we must account 

for the sudden onset of pyroclastic eruptions, the brief period of violent 

activity, and the almost equally abrupt cessation oflarge-volume eruptions. A broad high-level magma chamber at least 10 km across must 

have underlain the area in order to account for the dimensions of the 

subsided block in the caldera. The lack of premonitory volcanic rocks 

indicates that the roof of the magma chamber confined most of the 

magmatic pressure until the time of violent failure. When magmatic 

pressure did cause the roof to fail, large volumes of ash were erupted 

rapidly; with removal of magmatic support the roof collapsed to form 

the Mount Belknap caldera. The caldera then filled with lavas and 

tuffs to complete the local cycle. The catastrophic eruptions evidently 

spent so much magmatic energy that volcanic activity here virtually 

ceased. Increased viscosity resulting from degassing is a process commonly appealed to in such circumstances, but this is probably an 

oversimplification. Adiabatic cooling from rapid vesiculation of the 

magma may have been another significant factor. 

DEPTH OF THE MAGMA CHAMBERS 

The reconstructions of magma sources (fig. 8) under lying the eastern 

and western source areas reflect our correlatio:g. of voluminous pyroclastic eruptions with broad, probably shallow chambers; more limited 

lava eruptions probably came from more restricted chambers. The 

compositional similarity of concurrently erupted products from both 

source areas, and the proximity of these areas, strongly suggest a 

common parent magma chamber-now represented by a batholithic 

body at depth; this common parent magma chamber fed shallower 

high-level magma chambers beneath each source area. Local shape 

and depth relations of these shallower high-level magma chambers 

seem to have exercised major control on type and volume of the volcanic eruptions. 

The Joe Lott and Red Hills ThffMembers are representatives of the 

magma present at depth beneath the respective source areas prior to 

eruption. The uniform lithology of these members (table 1) and their 

closely similar chemical analyses (table 2) suggest the same large and 

relatively uniform body of magma for both. The rapid vesiculation of 
32 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

this magma and explosive character of the ensuing ash-flow eruptions 

further suggest that the magma was saturated with volatiles. CIPW 

norms calculated from analyses of the Joe Lott and Red Hills Thff 

Members (table 2, samples D and F) consist mainly of quartz (Q), 

orthoclase (OR), and albite CAB). The high differentiation index 

(D.I=93.3 -94.8, table 2) (Thornton and Thttle, 1960), low anorthite 

content, normative composition of the magma forming these alkali 

rhyolite tuffs, and the evidence for volatile saturation, permit them to 

be treated as the quaternary system Q-Or-Ab-H20. Thttle and Bowen 

(1958) have shown that in this system the composition of the ternary 

minimum varies as a function of water pressure. When the normative 

mineral compositions (Q-Or-Ab) of these tuffs are plotted on the 

quaternary system Q-Or-Ab-H20, they fall close to the position of a 

magma with a water pressure of 800±200 bars. This is reasonably 

close to the 600±100 bars pressure determined by Lipman, Christiansen, and O'Connor (1966, p. F42) for the rhyolitic part of the 

Miocene Thpopah Spring Member of the Paintbrush Thff in Nevada. 

If the approximately 800 bars water pressure is modeled on a lithostatic system, an equivalent depth of about 3 to 4 km is obtained, 

depending on the density assumed for the overlying rocks. We consider 

this to be a realistic approximation of the depth at which the Mount 

Belknap magma last maintained any degree of equilibrium, and thus 

to be a realistic figure for the depth to the top of the magma prior to 

eruption. Because the structural margin of the Mount Belknap caldera 

is 8-9 km across, probably the roof of the high-level chamber that 

contained the magma was on the order of2 to 3 times as wide as it was 

thick (fig. 8). Catastrophic failure of such a weak roof, leading to 

violent pyroclastic eruptions and caldera subsidence, would seem inevitable. Roofs on the smaller chambers in the eastern source area, on 

the other hand, were probably about as thick as they were wide and 

were relatively much stronger. 

We can only speculate on the factors that caused the differences in 

size and shape of the source high-level magma chambers. The eastern 

source area is in an area of older (23 m.y.) quartz monzonite plutons 

that may have exerted considerable control on emplacement and 

therefore size, shape, and depth of the younger magma body. Some as 

yet undefined regional control may also be significant as the shape of 

the eastern source area and alinement of both calderas is parallel to 

the distribution of other intrusive rocks extending westward as far as 

Minersville (Kerr and others, 1957, p. 38). The western source area 

appears to have been in an area of older volcanoes, but evidence for 

preexisting large epizonal plutons is lacking. Conceivably the top of 

the high-level magma chambers could have been enclosed in Paleozoic 

and Mesozoic sedimentary rocks that underlie Tertiary volcanic rocks 
DEPTH OF THE MAGMA CHAMBERS 33 

throughout much of southwestern Utah. In addition, the element of 

chance in control of the configuration and depth of the magma bodies in 

each of the source areas cannot be discounted. 

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34 MOUNT BELKNAP AND RED HILLS CALDERAS, UTAH 

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;:, U.S. GOVERNMENT PRINTING OFFICE, 1979-677-026/39