1V3I9O1O3O § NI I 

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M, A I, » 

Revised Stratigraphy and 

Radiometric Ages of 

Volcanic Rocks and 

Mineral Deposits in the 

Marysvale Area, 

West-Central Utah

By T. A. STEVEN, C. G. CUNNINGHAM, C. W. NAESER, and 

H. H. MEHNERT

Mineralization took place mam times during:

. » i . r i . .

a complex history oj volcanic activity 

in late Cenozoic time

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

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Main entry under title:

Revised Stratigraphy and Radiometric Ages of Volcanic Rocks and Mineral Deposits in the

Marysvale Area, West-Central Utah 

(Geological Survey Bulletin 1469) 

Bibliography: p. 37 

Supt. of Docs, no.: I 19.3:1469

1. Geology, Stratigraphic-Oligocene. 2. Geology, Stratigrapliic-Miocene. 3. Volcanic ash, 

tuff, etc. Utah Marysvale region. 4. Mineralogy Utah Marysvale region. 5. GeologyUtah Marysvale region.

I. Steven, Thomas August, 1917- II. Series: United States Geological Survey Bulletin 1469 

QE7S.B9 No. 1469 IQE693) 557.3'08s (551.7*8) 78-26434

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

Washington, D.C. 20402 

Stock No. 024-001-03187-6
CONTENTS

	Page 

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

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

Geologic setting ............................................................ 3

Previous stratigraphy ....................................................... 6

Present investigations ....................................................... 8

Needles Range Formation and older volcanic rocks ............................ 9

Bullion Canyon Volcanics .................................................... 12

Lower part............................................................. 12

Three Creeks Tuff Member ............................................. 13

The Dry Hollow Formation problem ..................................... 17

Delano Peak Tuff Member .............................................. 21

Volcanic centers and intrusive rocks of the Marysvale

Canyon-Antelope Range area ............................................... 22

Mount Belknap Volcanics ................................................... 25

Outflow facies .......................................................... 25

Intracaldera facies ...................................................... 28

Intrusive rocks ......................................................... 30

Sevier River Formation ...................................................... 31

Alteration and mineralization ................................................ 32

References cited ............................................................ 36

ILLUSTRATIONS

Page 

FIGURE 1. Map of southwestern Utah showing High Plateaus and

adjacent areas .............................................. 3

2. Map of the Marysvale area, Utah, showing ash-flow-related

cauldrons and points of reference ............................ 4

3. Diagrammatic cross section showing stratigraphic relations of

the Mount Belknap Volcanics ................................ 24

TABLES

TABLE 1. Fission-track and K-Ar analytical data, sample descriptions, and

sample localities ............................................ 18

ill

.82 35IOSU9

REVISED STRATIGRAPHY AND RADIOMETRIC AGES OF

VOLCANIC ROCKS AND MINERAL DEPOSITS IN THE

MARYSVALE AREA, WEST-CENTRAL UTAH

By T. A. STEVEN, C. G. CUNNINGHAM, C. W. NAESER, and 

H. H. MEHNERT

ABSTRACT

The Marysvale area, Utah, is near the northeast end of a broad belt of Tertiary igneous 

rocks that extends east-northeastward from southern Nevada to central Utah. In late 

Oligocene and early Miocene time (30-21 m.y. (million years) ago), a composite volcanic 

center consisting of numerous local volcanoes was active in the general Marysvale area. 

The volcanoes consisted of near-source lava flows, volcanic breccias, and minor pyroclastic deposits of generally intermediate composition flanked by coalescing aprons of 

volcaniclastic debris. Locally derived ash-flow tuffs form important marker beds within 

the assemblage. Concurrently with local volcanism, ash flows from distant sources in the 

Great Basin extended into the Marysvale area to interleave marginally with locally 

derived volcanic rocks.

Local volcanic activity in the Marysvale area probably began about 30 m.y. ago with 

eruption of mafic intermediate-composition lava flows and volcanic breccias. About 29 

m.y. ago, a widespread sheet of crystal-rich ash-flow tuff (Oligocene Needles Range 

Formation) was spread across the region from distant, as yet unidentified centers to the 

west or southwest. The Needles Range Formation is exposed all around the Marysvale 

area, but may have been excluded in places within it by local volcanoes. Accumulation of 

intermediate-composition volcanic rocks from local centers continued after deposition of 

the Needles Range Formation, and by 21 m.y. ago, a major pile of volcanic rocks had been 

built in the Marysvale area. The name Bullion Canyon Volcanics is applied to this 

assemblage of Oligocene and Miocene volcanic rocks.

About 27 m.y. ago a sequence of distinctive crystal-rich ash flows was erupted from a 

center in the northern part of the Marysvale pile, and a small trapdoor type block 

subsided at the source. The resulting ash flow tuff sheet, here named Three Creeks Tuff 

Member of the Bullion Canyon Volcanics, made up a significant part of what was 

formerly considered part of the Dry Hollow Formation. The Dry Hollow previously was 

believed to be younger than the Bullion Canyon Volcanics. However, we have found the 

Three Creeks Tuff Member in Bullion Canyon, in the type locality of the Bullion Canyon 

Volcanics, thereby demonstrating the lateral equivalence of the Dry Hollow and part of 

the Bullion Canyon. We abandon the name Dry Hollow Formation.

Bullion Canyon rocks overlying the Three Creeks Tuff Member consist of a poorly 

understood assemblage that includes a basaltic andesite shield volcano, ash-flow tuffs of 

diverse lithologies and sources, and intermediate-composition lava flows and volcanic 

breccia. The Delano Peak Tuff Member (formerly the Delano Peak Latite Member) is one 

of the more distinctive of the ash-flow units in the upper part.

Between 21 and 18 m.y. ago, repeated rhyolitic eruptions from two source areas in the 

Marysvale area produced a heterogeneous assemblage of ash-flow tuffs, lava flows
2 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

and domes, and associated pyroclastic and mudflow breccias, and emplaced local rhyolite 

and granite intrusive bodies. Two cauldrons, the major Mount Belknap caldera and the 

minor Red Hills caldera, subsided in response to ash-flow eruptions. Intracaldera and 

near-source densely welded tuffs of the Mount Belknap Volcanics (formerly Mount 

Belknap Rhyolite) are generally equivalent to the less welded outflow facies of the Joe 

Lott Tuff Member (formerly formation rank), and the whole rhyolitic assemblage is here 

called the Miocene Mount Belknap Volcanics. The outflow is designated the Joe Lott 

Tuff Member of the Mount Belknap. The intracaldera and outflow facies from both 

source areas are divided into several new formal and informal members with complex 

interrelationships.

Basin-range faulting began after eruption of the Mount Belknap Volcanics, and 

continued through much of the remainder of Cenozoic time. The Sevier River Formation 

consists of fluviatile and minor lacustrine sediments deposited in basins that developed 

concurrently with faulting. Basalt flows were erupted widely but in low volume during 

the period of Sevier River sedimentation.

Alteration and mineralization in the Marysvale area took place at several times and 

at many different locations during middle and late Cenozoic time. Volcanic rocks 

adjacent to local monzonitic intrusions in the southern Antelope Range were widely 

altered about 22 m.y. ago during terminal stages of eruption of the Bullion Canyon 

Volcanics. Alunite of potential economic value was formed at many of the centers of 

alteration, and sparse copper is known at a few. The gold- and silver-bearing veins in the 

Kimberly area on the northern slope of the Tushar Mountains appear also to have 

formed at about this time.

The uranium-molybdenum-fluorine mineralization in the main uranium producing 

district of the Marysvale area (central mining area) took place later, after accumulation 

of the Mount Belknap Volcanics. This mineralization was most intense in the southern 

Antelope Range in the area that had previously been widely altered and mineralized 

during the latter part of Bullion Canyon volcanic activity. The major mineralized area in 

the eastern Tushar Mountains, characterized by zonally arranged alunite deposits and 

base-and-precious-metal veins and mantos, formed about 14 m.y. ago, after Mount 

Belknap volcanism. Widespread occurrences of uranium in the fill and in adjacent walls 

of the Mount Belknap caldera also formed after the Mount Belknap Volcanics had 

erupted.

A much younger, probably Pleistocene, period of mineralization is represented by 

native sulfur deposits along the northwest side of the Marysvale volcanic pile. These 

deposits probably are related genetically to geothermal systems generated by nearby 

basaltic volcanoes of Quaternary age.

INTRODUCTION

Restudy of the regional geology and mineral resources of the Marysvale area, Utah, begun in 1975, has resulted in significant revisions in the 

local volcanic stratigraphy. More such revisions will undoubtedly be 

required as the study progresses. Concurrently with the field studies, 

numerous radiometric age measurements of volcanic rocks and 

mineralized materials have been made. These, in conjunction with ages 

already published by earlier workers, establish a time frame for volcanic 

activity and mineralization in the area. The revised stratigraphy and age 

data have specific, application to several mineralized districts near 

Marysvale, and are intended to aid in exploring for and in developing 

any mineral resources that may exist.
GEOLOGIC SETTING

113 112°

FIGURE 1. High Plateaus and adjacent areas of southwestern Utah. Stippled areas are 

underlain largely by volcanic rocks. G, Gunnison; F, Fillmore, R, Richfield, Mv, Marysvale; B, Beaver; M, Milford; P, Panguitch; CC, Cedar City; S, Spry intrusion. Modified 

from Anderson and Rowley (1975, fig. 1).

GEOLOGIC SETTING

The Marysvale area (figs. 1 and 2) is near the northeastern end of a 

broad belt of Tertiary igneous rocks and associated mineral deposits as 

much as 100 km (kilometers) wide that extends for more than 350 km 

east-northeastward from southeastern Nevada into central Utah. The 

northeastern terminus is along the eastern margin of the Awapa Plateau 

and Fish Lake Mountains (Williams and Hackman, 1971) at the border 

between the High Plateaus subprovince and the Colorado Plateaus 

province proper (fig. 1). Marysvale is in the Sevier River valley about 65 

km west-southwest of this terminus. The geologic relations described in 

this report are exposed largely in the Tushar Mountains and southern 

Pavant Mountains west and northwest of Marysvale.
STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

V> Kimberty ^ 2 \\\ ^Red Hi||S A [ \ >

l-^^-^K'6» Caldera / > ^

FIGURE 2. Ash-flow-related cauldrons and points of reference in the Marysvale area, 

Utah. 1, The Narrows; 2, Winkleman alunite deposit; 3, Yellowjacket alunite deposit, 4, 

The Teacup, 5, Red Hills; 6, Whitehorse alunite deposit; 7, Gray Hills; 8, Deertrail mine; 

9, Mineral Products mine; 10, Sheeprock mine; 11, Blue Lake.

The broad belt of igneous rocks, or various parts of it, has been called 

by many names, including the Wah Wah-Tushar belt (Hilpert and 

Roberts, 1969, p. 29-3 l),Pioche belt (Roberts, 1966; Shawe and Stewart, 

1976, p. 226), Pioche-Beaver-Tushar belt (Callaghan, 1973) and informally the Pioche-Marysvale belt. Subsidiary trends within the belt have 

been called the Delamar-Iron Springs subbelt (Shawe and Stewart, 1976) 

and the Blue Ribbon lineament (Rowley and others, 1978). These refinements have little impact on this report other than to indicate
GEOLOGIC SETTING 5

existence of the complex belt, and the geographic position of the Marysvale area within it.

The Marysvale area has long been recognized as being a composite 

volcanic center that shed volcaniclastic debris and possibly pyroclastic 

deposits laterally in all directions to intertongue with volcanic units from 

other centers. The Marysvale accumulation consists of numerous local 

volcanoes made up of complex assemblages of near-source lava flows 

and volcanic breccias of generally intermediate composition (largely 

andesitic and rhyodacitic) that were erupted between 30 and 21 m.y. 

ago. The local vent facies rocks pass laterally into reworked deposits that 

coalesce into a widespread composite apron surrounding the clustered 

volcanoes. Local and regional ash-flow sheets are interleaved with the 

Marysvale volcanic assemblage, and provide much-needed stratigraphic 

markers.

During the same interval that the generally intermediate-composition 

volcanic rocks were erupted near Marysvale (30-21 m.y.. ago), widespread ash-flow tuff sheets were accumulating in adjacent parts of the 

Great Basin area from the Tushar Mountains westward into Nevada. 

This great ash-flow tuff field has been studied by many geologists, 

chiefly by J. H. Mackin (1960, 1963) and many of his students (Mackin 

and others, 1954; Nelson, 1950; Mackin and Nelson, 1950; Blank, 1959; 

Blank and Mackin, 1967; Threet, 1952, 1963a, 1963b; Cook, 1957, 

1960, 1965; Williams, 1967; Anderson, 1965, 1971; Anderson and 

Rowley, 1975; Rowley, 1968). Other workers who have studied aspects 

of the volcanic geology of southwestern Utah are Armstrong (1970); 

Best and others (1973); Bushman (1973); Caskey and Shuey (1975); 

Earll (1957); Erickson (1973); Erickson and Dasch (1963); Gromme, 

McKee, and Blake (1972); Lemmon, Silberman, and Kistler (1973); 

Liese (1957); McGookey (1960); and Shuey, Caskey, and Best (1976). As 

a result, the complex stratigraphy of the ash-flow tuff field is fairly well 

known, and the distribution (although not the sources) of many of the 

individual sheets has been established. This well-known sequence in the 

Great Basin area, however, is only partly coordinated with coeval events 

in the Marysvale area, largely because many details of the volcanic 

sequence in the latter area have been poorly known.

Beginning about 21 m.y. ago, the volcanic materials erupted in the 

Marysvale area changed markedly to silicic alkali rhyolite flows, domes, 

and ash-flow tuffs. The largest volume of these silicic rocks was erupted 

between then and 18 m.y. ago, and a major caldera1 in the central 

Tushar Mountains subsided 19 m.y. ago in response to catastrophic

'We use the terminology of Smith and Bailey (1968, p. 616) in which cauldron is the general term describing all 

volcanic subsidence structures regardless of size or shape, and caldera is limited to more or less circular depressions 

generally formed by volcanic subsidence.
6 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

eruption of many tens of km3 of ash-flow tuffs. Similar rocks were 

erupted episodically throughout the remainder of Cenozoic time in 

west-central Utah, but only locally and in relatively small volume (Lipman and others, 1975; Mehnert and others, 1977; Erickson, 1973, p. 

20_21; Earll, 1957, p. 60.62; Petersen, (1975). Scattered basalt flows 

were erupted from many centers during the same interval.

PREVIOUS STRATIGRAPHY

The basic stratigraphy of the Marysvale area as previously understood 

was largely established by Callaghan (1939). The bulk of intermediatecomposition volcanic rocks was called Bullion Canyon Volcanics of 

Miocene(?) age. As described by Callaghan, the lower 2,500 feet2 of the 

unit consist predominantly of latitic and andesitic tuffs and breccias with 

10 percent or less flows. The Delano Peak Latite Member near the 

middle of the formation was considered to be a single thick lenticular 

lava flow over 800 feet thick. The upper part of the formation was 

described as a mixture of dark lava flows and volcanic breccias in which 

the flows constitute 50 percent or more of the assemblage; this part of 

the unit was believed to be 2,500 feet or more thick.

The sequence of rocks overlying the Bullion Canyon Volcanics of 

Callaghan (1939) was rearranged somewhat by Calwaghan and Parker 

(1961a, 1961b, 1962a, 1962b) and Willard and Callaghan (1962) to 

include the Roger Park Basaltic Breccia to the south, and a younger unit 

of thick widespread latite flows they called Dry Hollow Formation, both 

interpreted by them to be Pliocene(P) in age. Some tuffs and basaltic 

andesite lava flows were included in the Dry Hollow Formation. A major 

criterion Callaghan and associates used to distinguish the units was 

degree of alteration; the Bullion Canyon was described as widely altered, 

whereas the Roger Park and Dry Hollow are characteristically fresh. The 

age of the Roger Park was changed to Miocene by Anderson and Rowley 

(1975).

The young rhyolites of the Marysvale area were split into two presumably sequential units of Pliocene(P) age. Those in the central and 

eastern Tushar Mountains were called Mount Belknap Rhyolite, and 

were separated into the informal red Mount Belknap welded tuffs and 

the gray Mount Belknap lava flows. These were believed to be somewhat 

older than, although closely related to, the softer, gray, partly welded 

rhyolite tuffs of the Joe Lott Tuff that crop out in places all around the 

periphery of the high Tushar Mountains.

Fluviatile and minor lacustrine deposits of the Sevier River Formation 

filled structural basins that formed in late Cenozoic time, during widespread normal faulting that followed the major rhyolitic eruptions. 

Local basalt lava flows were erupted during the period of late faulting 

and Sevier River sedimentation.

This sequence established by Callaghan and associates has been used 

with only slight modification by most geologists who have worked in the 

Marysvale area in the last 25 years. A group from Columbia University,

'Thicknesses from Callaghan (1939) are given in the units used in the original report (1 foot=0.3 m.).
PREVIOUS STRATIGRAPHY 7

led by P. F. Kerr, studied the main uranium-producing district and some 

surrounding areas during the 1950's (summarized by Kerr and others, 

1957). They used the basic framework with litde change, but gave formal 

member names to numerous individual local lava flow and volcanic 

breccia units in the Bullion Canyon Volcanics of Callaghan (1939). 

Kennedy (1960, 1963a, 1963b) accepted without significant modification the main elements of the early stratigraphy, as did Callaghan (1973). 

The geology of the Marysvale area is depicted on the geologic map of 

Utah (Hintze, 1963) according to this same scheme.

The stratigraphic sequence used by Molloy and Kerr (1962) disagrees 

in important aspects with that used previously in the area, and does not 

fit with many of our own observations.

Geologists working into the Marysvale area from elsewhere, largely 

from the Great Basin to the west and southwest, have questioned aspects 

of the locally established sequence. Mackin (1963) believed that he 

recognized ash-flow units of his Needles Range Formation within the 

Dry Hollow Formation in the Clear Creek area between the Tushar 

Mountains and Pavant Range. The Needles Range Formation is a distinctive and exceptionally widespread ash-flow tuff unit at or near the 

base of the Tertiary volcanic succession throughout much of southwestern Utah and adjacent parts of Nevada, and is an excellent marker unit. 

Followup work by Caskey and Shuey (1975) demonstrated that the 

Needles Range Formation is indeed present in this area, but at a lower 

horizon near the base of the volcanic pile, and that the unit observed by 

Mackin was a lithologically similar but probably local ash-flow tuff 

higher in the volcanic succession. This younger tuff was named the Clear 

Creek Tuff Member of the Bullion Canyon Volcanics by Caskey and 

Shuey (1975), an appropriate name from local considerations, but one 

that has been preempted by prior use elsewhere and is thus unacceptable. Both Mackin (1963) and Caskey and Shuey (1975) recognized that 

the welded tuff along Clear Creek was equivalent in part to a local tuff 

member and in part to a latite lava flow member of the previously 

described Dry Hollow Formation.

A most important contribution in the volcanic stratigraphy of the 

High Plateaus area of Utah was made by Anderson and Rowley (1975), 

who mapped large areas in the southern Sevier Plateau, Markagunt 

Plateau, southern Tushar Mountains, and Black Mountains (fig. 1) 

south of the Marysvale area. These workers summarized all available 

data on the Cenozoic volcanic sequence to the south and southwest, and 

attempted to make preliminary correlations with the sequence described 

by Callaghan and associates in the Marysvale area. They noted certain 

discrepancies between the established Marysvale stratigraphy (largely 

within the Bullion Canyon Volcanics) and observations made by them. 

Being unable to coordinate the described sequences in the two areas,
8 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

Anderson and Rowley (1975) named their assemblage of intermediatecomposition lava flows and volcanic mudflow breccia the Mount Button 

Formation, and suggested that the term Bullion Canyon Volcanics 

might be a more appropriate term in the northern part of the Marysvale 

volcanic pile. This introduction of duplicate nomenclature for what is 

probably the same conceptual assemblage of rocks may have had advantages as a temporary palliative, but in the long term it cannot do other 

than confuse future stratigraphic discussions.

Anderson and Rowley (1975) clearly showed that the regional Needles 

Range Formation is at or near the base of the volcanic sequence along the 

southern margin of the Marysvale pile, that the overlying ash-flow sheets 

of the Great Basin sequence intertongue with peripheral deposits from 

the Marysvale centers along the southwestern side of the pile, and that 

other ash-flow tuff sheets of different provenance intertongue with 

similar marginal deposits to the south and southeast.

Williams and Hackman (1971) mapped the east end of the broad 

igneous belt in the northern Sevier and Awapa Plateaus and Fish Lake 

Mountains. Most of these deposits are marginal correlatives of the Bullion Canyon Volcanics or of the Mount Button Formation, but Williams 

and Hackman also distinguished at least one regional ash-flow tuff sheet 

(Osiris Tuff of Miocene age) also described by Anderson and Rowley 

(1975).

These peripheral studies thus have built a skeletal geologic framework 

nearly all the way around the local Marysvale pile. The framework of 

overlapping regional ash-flow tuff sheets provides a basis for deciphering the local complexities and placing them in a regional context.

PRESENT INVESTIGATIONS

Our own field investigations (by Steven and Cunningham) in the 

Marysvale area, made during the summers of 1975 and 1976, have 

indicated that major revisions are required in the previously used 

stratigraphy. Corroboratory evidence has come from radiometric dating 

of volcanic and mineralized rock samples (by Naeser and Mehnert). 

Rather than being sequential units, the formerly described Bullion Canyon Volcanics and Bry Hollow Formation are largely lateral equivalents. 

The Clear Creek Tuff Member of Caskey and Shuey (1975), herein 

renamed the Three Creeks Tuff Member of the Bullion Canyon, is a 

distinctive locally derived ashflow unit that has been recognized within 

both the Bullion Canyon and Bry Hollow; lithologic correlation is supported by radiometric ages. The Belano Peak Latite Member (Callaghan, 1939) is a thick simple cooling unit of densely welded ash-flow 

tuff of unknown source within the Bullion Canyon Volcanics in the 

central Tushar Mountains.
NEEDLES RANGE FORMATION AND OLDER VOLCANIC ROCKS 9

Ash-flow tuffs in the Mount Belknap Volcanics (the red member of 

the Mount Belknap) are densely welded near-source equivalents of the 

less welded peripheral Joe Lott Tuff; the two facies intergrade laterally 

and comprise a single outflow sheet derived from the Mount Belknap 

caldera source in the central Tushar Mountains. Rhyolite lava flows and 

domes of comparable composition (the gray member of the Mount 

Belknap) were erupted from a separate source area north of Marysvale 

over a span of 3-4 m.y., a span that also included the pyroclasttstic 

eruptions from the Mount Belknap caldera source. The caldera was 

filled by a complex mixture of rhyolite lava flows, volcanic breccias, and 

ash flow tuffs lithologically identical with comparable rock types in the 

Mount Belknap Volcanics outside the caldera.

Hydrothermal alteration and mineralization took place around local 

centers during both Bullion Canyon and Mount Belknap periods of 

eruption. In places the effects of different periods of hydrothermal 

activity are superimposed.

These relations are only fragments of the total architecture of the 

Marysvale volcanic pile, but they do provide some limits within which 

other aspects of the geology must abide. They have direct bearing on 

mineral exploration now being conducted in the area, and this factor 

alone provides a major incentive to publish our partial conclusions now. 

The distribution of near-source (vent) facies rocks versus outflow volcaniclastic facies rocks within the Bullion Canyon Volcanics has not been 

determined yet, nor has the attempt been made to separate out mappable units within the Bullion Canyon other than a few selected ash-flow 

tuffs. Subtle variations have been observed within the Mount Belknap 

Volcanics, but the geologic significance of these remains to be established in more than a cursory manner. In only one very limited area has 

our work extended far enough out toward the margins of the local 

volcanic pile to include any of the regional ash-flow tuff sheets from 

distant sources in the Great Basin or elsewhere. These and other matters 

are for future study and discussion.

NEEDLES RANGE FORMATION 

AND OLDER VOLCANIC ROCKS

Inasmuch as most of our effort to date (1976) has been directed 

elsewhere, only the broad aspects of the lower part of the Bullion 

Canyon Volcanics of Callaghan (1939) can be discussed in this report. 

Work by others, corroborated by scattered observations by us, indicate 

that this part of the volcanic succession consists of locally derived 

intermediate-composition lava flows and volcanic breccias interleaved 

with ash-flow tuff sheets from various sources. The quantities of the 

different constituents vary widely from place to place, and little now can 

be said about relative distributions.
10 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

A major breakthrough (Mackin, 1960, 1963) in understanding the 

stratigraphy of the lower part of the volcanic succession has come from 

recognition of ash-flow tuffs of the Needles Range Formation near the 

base of the pile at many scattered localities around the periphery of the 

Marysvale accumulation. As the Needles Range rocks in this vicinity are 

commonly underlain by a few meters to as much as several hundred 

meters of locally derived lava flows and volcanic breccias, volcanism in 

this vicinity obviously started somewhat earlier. Fleck, Anderson, and 

Rowley (1975, p. 56-57; with additional details by Anderson and 

Rowley, 1975, p. 12-13) obtained an age of 31. 1±0.5 m.y. for a local 

ash-flow tuff below the Needles Range in the northern Markagunt 

Plateau, south of the Marysvale pile (fig. 1). The position of this tuff with 

respect to the pre-Needles Range volcanic rocks from centers near 

Marysvale is not known, but the age at least gives evidence for the 

beginning of volcanic activity in the general region. In the San Francisco 

and Wah Wah Mountains 90-115 km west of Marysvale (fig. 1), Lemmon, Silberman, and Kistler (1973) dated local pre-Needles Range volcanic rocks as 34-31 m.y. old; whereas these ages have regional significance on the beginning of middle Tertiary volcanism in southwestern 

Utah, they may have litde bearing on the time that volcanism began near 

Marysvale. The relatively small volume of pre-Needles Range volcanic 

rocks known near Marysvale suggests that local volcanic activity here 

probably was not significant before about 30 m.y. ago, but additional 

data from radiometric dating are obviously needed.

Caskey and Shuey (1975, p. 18) described a local andesitic lava flow, 

which they called the Sulphur Peak Member of the Bullion Canyon 

Volcanics, several hundred meters thick, at the base of the volcanic 

succession along the southwestern margin of the Pavant Range northeast of Cove Fort (fig. 2). They did not specify the stratigraphic position 

of this lava flow with respect to the Needles Range Formation in their 

text discussion, but on their table 1 it is shown as older. At Site 200 of 

Caskey and Shuey (1975, fig. 3), 4.5 km north of Cove Fort on Highway 

15, where they made magnetic measurements on the Needles Range 

Formation, we estimate that dark volcanic mudflow breccia 50-70 m 

thick underlies the Needles Range. When traced laterally southeastward 

for a kilometer or so, the Needles Range sheet overlies a thick porphyry 

lava flow of intermediate composition (possibly the Sulphur Peak 

Member of Caskey and Shuey, 1975) that must have been derived from a 

nearby source. Red and white sedimentary rocks of the lower Tertiary 

Claron Formation underlie the volcanic succession north of Site 200.

The base of the volcanic sequence was mapped in reconnaissance by us 

along Second Creek, 10 km southeast to south of Kanosh (fig. 2) to 

establish the local succession within our own area of concern. Here the 

Claron Formation is overlain by dark volcanic mudflow breccia a few 

tens of meters thick, which in turn is overlain by a succession several
NEEDLES RANGE FORMATION AND OLDER VOLCANIC ROCKS 11

hundred meters thick of complexly intertonguing ash-flow tuffs of 

Needles Range Formation and local dark volcanic breccia and minor 

lava flows. Even the basal volcanic breccia just above the Claron Formation contains fragments of Needles Range ash-flow tuff. Clearly ash 

flows of distant derivation accumulated concurrently with locally derived intermediate-composition volcanic breccia and lava flows.

South and southeast of the Marysvale volcanic pile, Anderson and 

Rowley (1975, p. 14) described thin local deposits of ash-flow tuff, lava 

flows, volcanic mudflow breccia, and volcanic arenite that separate the 

sedimentary Claron Formation from the Needles Range Formation. A 

local laccolithic body called the Spry intrusion (Anderson and Rowley, 

1975, p. 16) 45 km south of Marysvale (fig. 1) is believed to have been 

emplaced during the same interval; related volcanic eruptions spread 

autoclastic volcanic breccia and ash-flow tuffs widely over adjacent parts 

of the Markagunt Plateau and southern Tushar Mountains.

The Needles Range Formation is a distinctive crystal-rich ash-flow 

tuff that extends over an area of more than 50,000 km2 in southwestern 

Utah and adjacent parts of Nevada. At least three separate ash-flow 

sheets, the Cottonwood Wash, Wah Wah Springs, and Lund Tuff Members, with different distributions and probably different local sources 

have been recognized (Shuey and others, 1976). All members contain 

abundant phenocrysts of plagioclase, quartz, biotite, and hornblende, in 

different proportions, set in a variably welded matrix. Numerous 

radiometric ages measured on samples of the three members from 

widely scattered areas, collected at different times by different geologists, and analyzed in different laboratories, indicate that the Needles 

Range eruptions took place in late Oligocene time about 29 m.y. ago; a 

"best estimate" age of 28.9± 1.2 m.y. has been given by Fleck, Anderson, 

and Rowley (1975, p. 58).

Rocks of the Needles Range Formation have been recognized as far 

east as the Fish Lake Mountains plateau northeast and east of the 

Marysvale area (fig. 1) (McGookey, 1960; Williams and Hackman, 1971), 

in the Sevier and Markagunt Plateaus southeast and south of Marysvale 

(Anderson and Rowley, 1975) and on the west side of the Pavant Range 

northwest of Marysvale (Caskey and Shuey, 1975). Magnetic measurements made by Caskey and Shuey (1975) and distribution maps published by Shuey, Caskey, and Best (1976) indicate that the Needles 

Range rocks near Marysvale belong to the Wah Wah Springs Tuff 

Member of the formation.

Whereas known exposures of Needles Range Formation virtually 

surround the Marysvale area, no evidence is known to indicate that the 

formation ever extended unbroken across the area. The base of the 

volcanic pile near Marysvale can be seen along the eastern face of the 

Tushar Mountains between Bullion Canyon and Cottonwood Creek 

(fig. 2), where an anticline of pre-Tertiary sedimentary rocks is exposed
12 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

in cross section. Cursory examination of the lower part of the volcanic 

succession above the sedimentary rocks in local areas did not disclose any 

Needles Range ash-flow tuffs here; this may indicate that the sedimentary rocks are part of a prevolcanic hill that stood above the level of 

accumulation of the Needles Range Formation and earlier volcanic 

rocks. Elsewhere the Needles Range may have been excluded by local 

volcanoes built by earlier or concurrent eruptions, as for example the 

Spry intrusion (Anderson and Rowley, 1975), or by lower Tertiary 

upwarps that predate the volcanic rocks (Rowley, 1968).

BULLION CANYON VOLCANICS

The Oligocene and Miocene Bullion Canyon Volcanics is a complex 

assemblage of near source and outflow facies lava flows, volcanic breccias, and volcanic mudflow breccias that accumulated around a cluster of 

generally intermediate composition volcanoes centered in the Tushar 

Mountains, Antelope Range, and northern Sevier Plateau. Ash-flow 

tuffs of local and distant derivation interleave marginally with the 

heterogeneous volcanic rocks of the clustered volcanoes.

The resulting pile of volcanic rocks is obviously so complex that it will 

eventually have to be split into numerous units to be mapped separately. 

Our present state of knowledge is too rudimentary in many aspects to do 

this now in more than a preliminary manner. Certain ash-flow tuff units 

are distinctive and can be recognized as individual members, but the 

whole assemblage must now be treated as a single formational unit, 

which, following Callaghan (1939), we call the Bullion Canyon Volcanics.

LOWER PART

Bullion Canyon Volcanics overlying the Needles Range Formation 

have been accorded even less attention than those below the formation 

during these early stages of our investigation. In the type locality (Callaghan, 1939), along the southern slope of Bullion Canyon, the lower 

volcanic units of the Bullion Canyon seem largely to be mudflow breccias. This is the area where Callaghan (1939) described the lower 2,500 

feet of the formation as consisting largely of volcanic tuffs and breccia, 

and less than 10 percent of lava flows. The thickness cited by Callaghan 

for the lower part probably includes an ash-flow tuff near the top to be 

described by us as part of the Three Creeks Tuff Member (new name).

We have studied the Bullion Canyon Volcanics overlying the Needles 

Range Formation in local detail only along the southwestern side of the 

Pavant Range north of Interstate Highwa^70 (fig. 2), in an area mapped 

previously by Caskey and Shuey (1975). Thick local lava flows and 

associated volcanic breccia of intermediate composition are interlayered
BULLION CANYON VOLCANICS 13

with ash-flow tuffs from unknown sources. Two of the tuffs form welldefined ledges that can be traced easily across the mountain front; these 

were named Wales Canyon Tuff Member by Caskey and Shuey (1975) 

for exposures in Wales Canyon north of Interstate Highway 70 and 8 km 

east of Cove Fort. Our observations in the type area of the Wales Canyon 

indicate that two ash-flow sheets of somewhat contrasting phenocryst 

mineralogy are included in this member; both contain 30-40 percent 

phenocrysts, largely plagioclase, but the lower sheet contains biotite and 

hornblende as the mafic crystals, whereas the upper sheet contains 

biotite and pyroxene. These ledges are broadly lenticular and wedge out 

laterally against topographic irregularities on the underlying lava flows. 

Caskey and Shuey (1975, p. 20) reported a maximum thickness of both 

units of about 200 meters in the type area. The Wales Canyon ledges are 

overlain in part by thick local lava flows, and in part by our Three Creeks 

Tuff Member. Present evidence is inadequate to indicate whether the 

known exposures of the Wales Canyon Member of Caskey and Shuey 

(1975) are parts of local sheets of limited distribution, or are parts of 

regional sheets.

Other ash-flow units, generally only partially welded and poorly exposed, are reported by Caskey and Shuey (1975, p. 18) from the postNeedles Range Formation volcanic section in this area, but at this time 

we have no idea of their distribution or lithology.

THREE CREEKS TUFF MEMBER

A thick crystal-rich ash-flow tuff forms the walls of the canyon of 

central Clear Creek for about 8 km (fig. 2), and is widespread in adjacent 

parts of the northern Tushar Mountains and Pavant Range where it 

overlies the intermediate-composition lava flows and interlayered ashflow tuffs of the lower part of the Bullion Canyon Volcanics. The 

crystal-rich ash-flow tuff ranges from densely to slightly welded and 

shows striking compound cooling characteristics. The less welded parts 

were called the tuff of Dry Hollow Formation by Callaghan and Parker 

(1962b), whereas the more densely welded parts were generally included with adjacent lava flows into an informal latite flow member. 

Mackin (1963) correlated the entire ash-flow unit with his Needles 

Range Formation, which it closely resembles in gross aspect and phenocryst mineralogy. Caskey and Shuey (1975) recognized that the unit was 

distinct from the Needles Range Formation and probably was of local 

derivation. They named it the Clear Creek Tuff Member of the Dry 

Hollow Formation, and separated it into three informal divisions that 

differ in degree of welding and purportedly in phenocryst content. We 

follow Caskey and Shuey in recognizing this crystal-rich ash-flow tuff as 

a separate unit that can be divided into three parts. By recognizing it
14 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

within .both the Bullion Canyon Volcanics and the Dry Hollow Formation, we demonstrate the lateral equivalence of these two assemblages of 

rocks that heretofore have been considered to be sequential.

Whereas the name Clear Creek Tuff Member applied by Caskey and 

Shuey (1975) to this ash-flow unit is an excellent choice from local 

considerations, the name has been preempted by prior usage as a geologic name in other states. Thus there seems no alternative to renaming 

the unit, and for this purpose we have chosen the name Three Creeks 

Tuff Member; this name is taken from Three Creeks, a northern tributary of Clear Creek that traverses the main area of exposure of the unit in 

the southern Pavant Range (fig. 2). The type area of the Three Creeks is 

in the cliffs along Three Creeks just upstream from the mouth of Pole 

Creek where the upper two parts recognized both by Caskey and Shuey 

(1975) and by us are exposed within the trapdoor cauldron that formed 

in the source area of the Three Creeks. Inasmuch as the Three Creeks is 

distinct in distribution and source from all other volcanic units in the 

Marysvale pile, occurs within both the Bullion Canyon and Dry Hollow 

Formations, and provides a distinctive marker unit within the volcanic 

succession, we assign it member rank.

Nearly half of the Three Creeks Tuff Member consists of phenocrysts 

in a matrix of devitrified glass shards and collapsed pumice fragments. 

The phenocrysts are about 30 percent andesine, 12 percent amphibole, 

and 2 percent quartz with beta morphology, together with less than one 

percent each of biotite, Fe-Ti oxides, sanidine, apatite, sphene, zircon, 

and scattered grains of calcium-rich monocline pyroxene. The amphibole is kaersutite and is commonly oxidized to an assemblage of Fe-Ti 

oxides. The amphibole:biotite ratio is variable, but exceeds 5:1 in all 

samples examined.

The Three Creeks Tuff Member is a multiple ash flow compound 

cooling unit over much of its extent. To the northeast, near its distal 

margin in the eastern Pavant Range, volcanic materials from other 

sources intertongue between some of the individual ash flows, demonstrating that the Three Creeks is indeed a composite sheet in the sense 

defined by Smith (1960a, p. 801; 1960b, p. 158). The lower part of our 

Three Creeks Member is generally hard and densely welded, although 

the multiple ash flow compound cooling characteristics can still be recognized in the ledgy outcrops that display minor variations in degree of 

welding and local partings. The lower part is best displayed in the 

reference section of the Three Creeks in the Narrows area (fig. 2) of 

Clear Creek Canyon, where it forms vertical cliffs of densely welded tuff 

100 m high. These cliffs break back into a stepped area of benches and 

cliffs reflecting compound cooling as the transition toward the softer 

middle part of the Three Creeks is approached. The total thickness of
BULLION CANYON VOLCANICS 15

the lower part exposed along Clear Creek is about 200 m, and the base of 

the member is not exposed.

The lower part of the Three Creeks Member is also widely exposed in 

the headwaters area of Clear Creek, Three Creeks, and Pole Creek, 

where it rests on intermediate-composition lava flows and welded ashflow tuffs in the lower part of the Bullion Canyon Volcanics. The densely 

welded ledges of the lower part of the Three Creeks in these areas form 

broad, slightly inclined surfaces where erosion has stripped off the 

overlying softer tuffs. The joint pattern in the densely welded tuffs 

exposed on these surfaces has been etched out by weathering and 

erosion to give a rough texture to the topography that appears on aerial 

photographs as a distinctive reticulate pattern. An arbitrary contact 

between the lower and middle parts was taken at the highest horizon on 

which this pattern can be recognized. This subjective position can be 

used only where the lower layers are widely stripped, as in the headwaters of Pole and Three Creeks. To the south along the canyon of Clear 

Creek, a local parting marked by a meter or so of bedded volcaniclastic 

sediments was used to separate the members. This parting is well exposed along the southern side of the ridge between Clear Creek and 

Three Creeks about 400 m west of their confluence, where it separates 

soft, partially welded tuffs about 10m above the highest densely welded 

tuff of the lower member. This parting marks the first clear break above 

the lowest rocks of the Three Creeks Tuff Member exposed along Clear 

Creek. Although the parting appears to be at about the same place in the 

transition from generally hard lower part to soft middle part of the 

member as the arbitrary position used to separate the parts farther 

north, we have no assurance that the positions in the transition we have 

used in different places correspond more closely than within a few tens 

of meters of each other.

The lower part grades up into the middle part of the Three Creeks 

Member by a general decrease in degree of welding. Most of the middle 

part consists of soft, partially welded tuff containing some ledges of 

harder, more densely welded tuffs. The middle part crops out as prominent ledgy cliffs and benches along Clear Creek and along the middle 

courses of Three Creeks and Pole Creek. Most of the middle part is white 

to light-gray, variably welded tuff identical in mineralogy with the lower 

part. Depending on the local hardness, its topographic expression 

ranges from smooth slopes or rounded outcrops with local areas of 

badland topography, to strongly ledgy slopes, where numerous more 

densely welded layers are interspersed in the softer tuffs. The middle 

part is 200-250 m thick along Clear Creek, where it is part of the outflow 

sheet; within the cauldron source area as exposed along Pole Creek, the 

middle part is more than 500 m thick, and the base is not exposed. This 

relationship is interpreted to indicate subsidence of the cauldron while
16 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

the middle part was accumulating. The middle part is generally softer in 

the outflow facies, and more ledgy in the intracaldera facies.

The upper part of the Three Creeks Tuff Member is restricted to a 

moat area along the southern margin of the source cauldron, and to the 

outflow sheet north of the cauldron. It is as hard and densely welded as 

the lower part, and it typically forms hard caps on hills whose lower 

slopes are of soft tuffs of the middle part. The phenocryst mineralogy is 

closely similar to that of the lower and middle parts, and it differs from 

them only in stratigraphic position and degree of welding.

The base of the upper part was nowhere seen in the outflow area in the 

headwaters of Three Creeks and Pole Creek because talus from the 

capping hard upper part covers the smooth slopes on the softer underlying middle part. Along the southern margin of the source cauldron, however, the contact is a clear unconformity; the densely welded 

upper part abuts and wedges out against the topographic wall of the 

cauldron. This relation is exceptionally well shown on the west side of 

Pole Creek canyon, but is also apparent on both sides of Three Creeks 

canyon to the west. A tongue of the hard upper part filling a canyon 

tributary to the cauldron extends southwest across Clear Creek about 2.5 

km west of the mouth of Three Creeks.

The belt of hard upper part of Three Creeks Tuff Member along the 

southern side of the source cauldron is an erosional remnant left in the 

moat of the cauldron after resurgence had uplifted the core and erosion 

had removed that part of the sheet that once connected these exposures 

with those capping the hills in the outflow area to the north.

The strongly propylitized succession of volcanic rocks flanking Bullion Canyon in the central Tushar Mountains southwest of Marysvale 

(fig. 2), and forming the type locality of the Bullion Canyon Volcanics 

(Callaghan, 1939), contains within it crystal-rich welded ash-flow tuffs, 

about 100 m thick, that appear once to have been identical to the Three 

Creeks Tuff Member in its type area. Alteration has modified the original matrix and phenocryst compositions so that only pseudomorphs of 

the original plagioclase (now albite), biotite, and hornblende (both now 

chlorite and (or) clay) have survived; the quartz phenocrysts are unchanged. The phenocryst types and abundances are the same as those in 

the type Three Creeks Tuff Member, however, and the relic texture is 

identical. The stratigraphic position of this tuff within a section of 

generally intermediate composition lava flows and volcanic breccia also 

is comparable.

Caskey and Shuey (1975) reported a K-Ar age of 27.5±0.4 m.y. for the 

biotite in the lower part of the Three Creeks Tuff Member from the 

upper Clear Creek drainage area. Our samples (table 1, no. 15) from the 

same part of the section but from the Narrows along Clear Creek gave
BULLION CANYON VOLCANICS 17

K-Ar ages of 26.9±2.2 m.y. and 28.9±1.3 m.y. for plagioclase, and 

30.6±1.3 m.y. and 32.1 ±1.4 m.y. for biotite. A fission-track age of 

27.4±1.3m.y. (table 1, no. 15) for zircon was obtained from the same 

sample. No reason is known for the discordance between the cluster of 

ages at 26.9-27.5 m.y. and the cluster at 28.9-32 m.y. However, the 

average age of the older cluster (30.5 m.y.) is older than the stratigraphically underlying Needles Range Formation (28.9 m.y.) and is thus geologically unreasonable. A fission-track age on zircon from a sample 

(table 1, no. 14) of the propylitized welded tuff from the southern side of 

Bullion Canyon is 27.0± 1.2 m.y. This age is in close agreement with the 

younger and more geologically reasonable ages on the Oligocene Three 

Creeks from the Clear Creek area, and to the degree that any of the ages 

can be trusted it supports the correlation made above.

Recognition of the Three Creeks Tuff Member within the type locality 

of the Bullion Canyon Volcanics establishes the partial equivalence of 

the Dry Hollow Formation (Callaghan, 1939; Callaghan and Parker, 

1961a, 1961b) and the Bullion Canyon Volcanics of Callaghan (1939). 

This indicates that degree of alteration, which was a major criterion used 

by Callaghan and associates to distinguish the relatively fresh Dry Hollow from the commonly altered Bullion Canyon, has little stratigraphic 

significance. This conclusion is supported by radiometric ages on sericite 

(12-14 m.y., Bassett and others, 1963) and alunite (14 m.y., this report, 

table 1, nos. 16 and 17) from the altered and mineralized area south of 

Bullion Canyon, to be discussed later. Evidently the alteration and 

mineralization were much later than emplacement of the host rocks.

THE DRY HOLLOW FORMATION PROBLEM

The rocks immediately overlying the Three Creeks Tuff, constituting 

much of the Dry Hollow Formation of Callaghan and Parker (196la, 

1961b; 1962a, 1962b), comprise a mixture of intermediate-composition 

lava flows and volcanic breccia with interlayered welded ash-flow tuffs; 

this mixture varies widely from place to place, and at present our 

knowledge of the assemblage is incomplete. In the Clear Creek area (fig. 

2), Caskey and Shuey (1975) distinguished three ash-flow tuff units and 

one intermediate-composition lava flow unit in the Dry Hollow above 

the rocks we have called Three Creeks Tuff Member. Our reconnaissance suggests that elsewhere lava flows are locally much more 

abundant than the tuffs.

Along the eastern side of the Pavant Range the assemblage of lava 

flows and tuffs typical of the Clear Creek area intertongue laterally with 

a sequence of mafic lava flows that Callaghan and Parker (196 la; 1962b) 

called the basaltic andesite flows of the Dry Hollow Formation. Callaghan and Parker (196la; 1962b) noted that these mafic flows thicken
18 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

TABLE \.-Fission-track and K-Ar analytical data and sample descriptions, Marysvale area, Utah

[For ps and pi, ( ) indicates number of tracks counted. Constants: 

A F = 7. 03x10" ^yr' 1 ; *4 °KA e 

atomic abundance, t*°K = 1.167X1Q-1*]

0. 581x10" J °yr" ! = 4. 962x10" l °yr" l

Fission-track analytical data

Sample No. ; 

mineral analyzed 1 2 ps

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

M153

(DF-1336) ;

M152

(DF-1335);

N89

(DF-1093) ;

M75

(DF-1083) ;

M74

(DF-1080) ;

M74

(DF-1352);

M92

(DF-973) ;

M92

(DF-972);

M28

(DF-1106) ;

M28

(DF-1106) ;

M29B

(DF-1092) ;

M50

(DF-1078) ;

M49B

(DF-1122) ;

M49B

(DF-1124) ;

M83

(DF-1114);

M83

(DF-1115) ;

M27A

(DF-1130) ;

M27A

(DF-1129(;

M98

(DF-1334);

75-S-20

(DF-1123) ;

M90

(DF-1107) ;

Z

Z

Z

Z

Z

Z

Z

A

Z

Z

Z

Z

Z

A

Z

A

A

Z

Z

Z

Z

0.972

(144)

2.57

(369)

4.29

(755)

4.44

(534)

4.72

(918)

3.90

(632)

4.09

(927)

.129

(537)

8.19

(1137)

8.22

(1484)

6.44

(865)

5.17

(957)

3.54

(607)

.136

(283)

4.03

(840)

.114

(237)

.206

(429)

8.32

(1464)

5.34

(668)

5.74

(903)

4.11

(762)

3 pi

10.45

(774)

13.89

(999)

15.22

(1338)

14.51

(873)

14.80

(1439)

16.91

(1370)

18.20

(2064)

5.93

(2469)

26.84

(1864)

9.65

(871)

20.38

(1368)

13.00

(1204)

11.05

(946)

.271

(565)

12.72

(1325)

.259

(540)

.468

(976)

10.95

(963)

17.12

(1070)

14.47

(1139)

9.49

(879)

Vio15

1.28

1.29

1.07

.835

.846

1.24

1.46

1.45

1.07

.418

1.08

.851

1.09

.930

1.15

.904

.888

.418

1.29

1.14

1.06

5 T

7.1+0.7

14.2+0.9

18.0+0.8

15.3+0.8

16.1+0.7

17.1+0.8

19.6+0.8

18.8+1.7

19.510.7

21.3+0.9

20.410.9

20.2+0.9

20.911.1

27.813.9

21.811.0

23.713.6

23.312.6

19.010.8

24.111.2

27.011.2

27.4+1.3

U 

(ppm)

260

350

460

560

560

440

400

13

810

740

610

490

330

9.4

350

9.2

17

840

430

410

290
BULLION CANYON VOLCANICS 19

TABLE 1 . Fission-track and K-Ar analytical data and sample descriptions, Marysvale area, UtahContinued

K-Ar analytical data

Sample No. ;

mineral analysed6

16.

17.

18.

5.

4.

7.

8.

19.

20.

21.

10.

11.

15.

22.

M51A;

M51;

M48;

M74;

M75;

M28;

M29;

M53;

M47B;

M30;

M49B;

M49B;

M49B;

M83;

M90;

M90B;

M90;

M90B;

M256;

Al

Al

N

S

S

B

B

Al

Al

Al

S

S

B

B

P

P

B

B

Al

*Z, zircon; A,

2 PS,

3pi,

"*,

tracks/cm

tracks/cm2

K2 0

(percent)

6.73,

9.10,

4.79,

5.62,

7.38,

7 5.81

7 8.53,

7.65,

8.18,

6.94,

6.33,

9.70,

8 . 94.,

8.04,

.74,

.82,

5.47,

7.38,

9.00,

apatite .

fossil,

induced

6.

9.

4.

5.

7.

8.

7.

7.

6.

6.

9.

8.

8.

.

.

5.

7.

9.

*i^°Ar (10 -1 0 *kOR>r

moles/gram) (percent)

73

17

84

67

32

58

66

99

93 

35

55

95

11

75

75

45

19

11

1.

1.

1.

1.

1.

1.

2.

2.

2.

2.

1.

3.

2.

3.

.

.

2.

' 3.

1.

xlO6 .

, xlO6 .

N 

S

neutrons/cm2 .

349

893

155

581

977

806

678

424

639

284

967

053

805

385

292

329

542

242

234

Om

6A1

66.

62.

59.

81.

75.

77.

59.

13.

63.

52.

83.

89.

61.

67.

39.

77.

83.

86.

49.

age in m.y

, alunite ;

, natroalunite ; 

, sanidine .

7 By

2

1

0

1

6

0

6

4

0

0

2

9

5

6

3

4

0

0

2

B,

P,

13.

14.

16.

19.

18.

21.

21.

21.

22.

22.

21.

21.

21.

28.

26.

28.

32.

30.

9.

120-

biotite

5 T

810.6

310.6

610.8

410.8

510.8

410.9

610.9

814.3

511.0

711.1

411.0

610.9

611.1

911.4

912.2

911.5

111.4

611.3

410.5

f

plagioclase;

isotope dilution.

SAMPLE DESCRIPTIONS

1. Glassy welded ash from the upper pan of the Sevier River Formation. Located 49 m S. 35° E. from the intersection of 

U.S. highways 89 and 4; lat 38°35'20" N., long 112°15'21" W.

2. Glassy welded ash from the lower pan of the Sevier River Formation. Collected 50 m above the Joe Lott Tuff Member 

upper contact, in the Castle Creek campground on Mill Creek; lat 38°32'55" N., long 112°2r26" W.

3. Upper gray tuff member of Mount Belknap Volcanics. Collected 4.99 km west of the junction of Beaver Creek and 

the Sevier River; lat 38°28'37", long 112°17'16"

4. Crystal-rich tuff member of Mount Belknap Volcanics. Collected 1.45 km west of the junction of Beaver Creek and 

the Sevier River; lat 38°28'37", long 112°14'57°.

5. Crystal-rich tuff member of Mount Belknap Volcanics. Collected on Grizzly Ridge, 2.4 km west of Puffer Lake; lat 

38°18'53", long 112°22'35".

6. Crystal-rich welded tuff in the lower heterogeneous member of Mount Belknap Volcanics. Collected 2.09 km 

west-northwest of the junction of Deer Creek and the Sevier River; lat 38°30'00", long 112°16'46".

7. Fine-grained granite dike cutting quartz monzonite in the central uranium area. Located 61 m west of the Freedom 

No. 2 mine; lat 38"29'58". long ll^'SS".

8. Altered quartz monzonite (M29B) cut by fine-grained granite dike (M28), in the central uranium area. Samples 

picked from the dump of the V.C.A. mine; lat 38°29'55", long 112°12'48°.

9. Freshest sample obtainable of the fine-grained granite in the central uranium area. Located at the portal of the Blue 

Eagle mine; lat S8°30'24", long 112°13'10".

10. Teacup dome in the lower heterogeneous member of Mount Belknap Volcanics; collected from the glassy margin on 

the south side of Teacup dome; lat 38°30'41", long 112°10'35".

11. Delano Peak Tuff Member of Bullion Canyon Volcanics. Located near the southeast margin of the Mount Belknap 

caldera, 1.04 km northeast of Mud Lake; lat 38°22'22", long 112°23'52".
20 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

TABLE 1. Sample Descriptions Continued

12. Quartz monzonite from the central uranium area. Fresh sample from the dump of the Prospector 1 mine; lat 

38°29'45", long 112° 12'47".

13. Quartz monzonite from the Gold Mountain mining district. Collected on Tip Top, near the Sevier mine; lat. 

38°29'20", long 112°24'31".

14. Propylitized Three CreeksTuff Member of Bullion Canyon Volcanics; collected on the south side of Bullion Canyon, 

1.84 km northeast of Mount Brigham; lat 38°23'23", long 112°20'00".

15. Three Creeks Tuff Member of Bullion Canyon Volcanics; collected along Clear Creek at the Narrows; lat 38°34'23", 

long 112°22'44".

16. Vein alunite from Mineral Products mine on Alunite Ridge, lat 38°22'25", long 112°19'20".

17. Vein alunite from Mineral Products mine on Alunite Ridge, lat 38°22'25", long 112°19'20".

18. Vein natroalunite from Al Kee Mee mine, northeast of the central uranium area, lat 38°31 '39", long 112°10'12".

19.Replacement alunite from the Winkleman alunite deposit in Deer Creek; lat 38°29'38", long 112°17'32".

20. Replacement alunite from the Yellow Jacket mine, north of the central uranium area; lat 38°32'04", long 112° 12'30".

21. Replacement alunite from the Whitehorse alunite mine, southeast of the central uranium area; lat 38°28'25", long 

112°H'27".

22. Replacement alunite from the Sheeprock mine, 13 km north-northeast of Beaver, Utah; lat 38°22'50", long 

112°34'05".

eastward and are as much as 600 m thick in places in the northern Sevier 

Plateau (fig. 1). Our own observations suggest that this assemblage of 

flows probably constituted a broad shield volcano that extended eastward into and perhaps across the area of the present northern Sevier 

Plateau.

In the eastern part of the northern Sevier Plateau just east of the area 

shown in figure 2, and adjoining the area mapped by Callaghan and 

Parker (1961 a), Williams and Hackman (1971) showed the basaltic andesite flows equivalent to the Dry Hollow to underlie the Osiris Tuff, a 

regional sheet of densely welded ash-flow tuff that extends throughout 

wide areas east and south of the Marysvale volcanic pile (Anderson and 

Rowley, 1975, p. 31-32). Rowley and Anderson (1975, p. 30) reported 

Osiris Tuff on the eastern flank of the Pavant Range west of Elsinor (fig. 

2) but the stratigraphic position of this occurrence is not known. The age

of the Osiris Tuff was determined by Fleck, Anderson, and Rowley 

(1975) to be 22-23 m.y., on the basis of four concordant K-Ar determinations on biotite.

In the southern part of the Marysvale volcanic pile, thick quartz latitic 

lava flows flanking the Beaver river west of the mouth of Merchant 

Creek (fig. 2) were mapped by Callaghan and Parker (1962a) as Dry 

Hollow Formation. Anderson and Rowley (1975) redefined the Dry 

Hollow Formation as limited to these flows and equivalents, and noted 

that in the southern Tushar Mountains their redefined Dry Hollow is 

younger than the Osiris.Tuff. Fleck, Anderson, and Rowley (1975) 

obtained closely concordant K-Ar ages of 21.7±0.4 and 21.8±0.4 m.y. 

from two of these flows, in conformity with their stratigraphic position 

above the 22-23-m.y.-old Osiris Tuff.

Thus we find that similarity in lithologic descriptions (only partly 

matched by similarity in actual rock appearance) between some 

phenocryst-rich rocks in the northern part of the Marysvale volcanic pile 

and some thick local quartz latite lava flows in the southern part of the
BULLION CANYON VOLCANICS 21

field has led to erroneous correlations. The rocks called Dry Hollow by 

Callaghan and Parker (1962b) in the Clear Creek area, where descriptions most closely fit those of the flows near Beaver River, are actually 

densely welded ash-flow tuffs that we now include in the Three Creeks 

Tuff Member.

The confusion arising from a succession of erroneously interpreted 

relations has nearly eliminated any utility the name Dry Hollow Formation might have had. Recognizing the Three Creeks Tuff Member as a 

separate unit within the Bullion Canyon Volcanics, and recognizing the 

lateral equivalence of the remainder of the type Dry Hollow with parts of 

the Bullion Canyon Volcanics, takes care of the nomenclature requirements of the assemblage of rocks originally defined as Dry Hollow 

Formation. The post-Osiris quartz latitic lava flows adjacent to Beaver 

River appear distinct enough to warrant separate designation. For reasons herein detailed, however, the name Dry Hollow for these flows 

seems totally inappropriate, and we recommend that they be treated as 

an informal unit until more is known about their stratigraphic relations. 

We therefore abandon the name Dry Hollow Formation.

DELANO PEAK TUFF MEMBER

Our observations to date (1976) on those units of the Bullion Canyon 

Volcanics above the Three Creeks Tuff Member in the central Tushar 

Mountains have centered on a thick lens of ash-flow tuff that Callaghan 

(1939) called the Delano Peak Latite Member and that he interpreted to 

be a single thick lava flow in the middle of the formation. We identify the 

Delano Peak as a largely propylitized simple cooling unit of densely 

welded crystal-rich ash-flow tuff and not a latite flow. The rock contains 

46 percent phenocrysts including 32 percent andesine, 9 percent amphibole converted to Fe-Ti oxides and calcite, 4 percent Fe-Ti oxides, 

and less than 1 percent each of quartz, biotite, and apatite in a 

groundmass of devitrified, altered glass and pumice. The Delano Peak is 

as much as 250 m thick near the center of the lens. It forms a series of 

prominent cliffs along the east side of the crest of the Tushar Mountains 

from the head of Bullion Canyon southeastward for nearly 9 km, and 

irregularly caps some of the ridges that extend northeastward down 

from the crest of the range. We have determined three radiometric ages 

on a sample (table 1, no. 11) of the unit: a K-Ar age on biotite is 

28.9± 1.4 m.y., and fission-track ages on apatite and zircon are 23.7 ±3.6 

m.y. and 21.8 ± 1.0 m.y. respectively. The biotite age is obviously too old, 

inasmuch as the Delano Peak unit is stratigraphically younger than the 

27-m.y.-old Three Creeks unit. The more nearly concordant apatite and 

zircon ages are geologically reasonable, and suggest that the unit was 

emplaced about 22 m.y. ago or in Miocene time.

The Delano Peak unit is distinctive in rock type, origin, and topographic expression, and forms a much-needed stratigraphic marker
22 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

within the Marysvale volcanic pile. We herein call it the Delano Peak 

Tuff Member of the Bullion Canyon Volcanics. The type locality is at 

Delano Peak the same locality used by Callaghan (1939) when he 

originally defined the unit.

We have mapped none of the rocks either directly below or,above the 

Delano Peak. The section of rocks between the Three Creeks and Delano Peak is within the informal lower sequence of the Bullion Canyon 

that Callaghan (1939) described as being predominantly volcanic tuff 

and breccia. We have identified intermediate-composition lava flows at 

places in this section, and volcanic mudflow breccia elsewhere, but have 

no idea of their relative abundances or distributions. Callaghan and 

Parker (1962a) described the rocks overlying the Delano Peak Tuff 

Member as consisting mostly of dark calcic latite lava flows.

VOLCANIC CENTERS AND INTRUSIVE ROCKS OF THE 

MARYSVALE CANYON-ANTELOPE RANGE AREA

Near-source lava flows and volcanic breccia (vent-facies of Parsons, 

1969) of the Bullion Canyon Volcanics are excellently exposed for the 

length of Marysvale Canyon (fig. 2), from 4 km north of Marysvale to the 

confluence of Sevier River and Clear Creek, and make up most of the 

Antelope Range to the east. Our own observations in this area have been 

limited to preliminary reconnaissance traverses and to observations 

along Highwaay 89, which follows the bottom of Marysvale Canyon. 

Kerr and others (1957, pi. 12) mapped the southern two-thirds of 

Marysvale Canyon and Antelope Range at a scale of 1:12,000, and 

separated out numerous individual lava flow and volcanic breccia units 

within the Bullion Canyon Volcanics. Several monzonitic to quartz monzonitic intrusive bodies cut the vent-facies volcanic rocks (Kerr and 

others, 1957, pi. 12; Callaghan and Parker, 1961a; Willard and Callaghan, 1962); the largest of these are in the southern Antelope Range, 

where two plutons, 1-3 and 3-4 km across, are widely exposed. Smaller 

bodies of fine-grained granite cut the larger of the quartz monzonitic 

plutons (referred to locally as the "central intrusive") in the vicinity of the 

main uranium-producing area (central mining area of Kerr and others, 

1957). The central mining area is located just east of reference point 5, 

fig. 2, and the central intrusive body occupies the western two-thirds of 

the area between points 4 and 5.

This assemblage of near-source volcanic rocks and associated intrusive bodies represents part of a cluster of Bullion Canyon central vent 

volcanoes. The stratigraphic position of these volcanic rocks with respect 

to Bullion Canyon Volcanics elsewhere in the Marysvale area, particularly those places where the Bullion Canyon is segmented by recognizable ash-flow tuffs, is still problematic. The walls of Marysvale Canyon 

expose vent-facies rocks over a vertical range of nearly 900 m, yet 

neither the Needles Range Formation nor Three Creeks Tuff Member
VOLCANIC CENTERS AND INTRUSIVE ROCKS 23

have yet been recognized in this area. On the basis of abundant inclusions of quartzite within some of the smaller quartz monzonitic intrusions, Kerr and others (1957, p. 42) postulated that the base of the 

volcanics lies at shallow depths beneath present exposures in this area. 

This interpretation is supported by the presence of an uplifted wedge of 

contact-metamorphosed sedimentary rocks along the northern margin 

of one of the larger intrusions (Kerr and others, 1957, pi. 12). This 

admittedly tenuous suggestion needs more definitive supporting evidence, but if it is true, the thick assemblage of vent-facies volcanic rocks 

exposed in the Marysvale Canyon-Antelope Range area could be 

time equivalent to much of the Bullion Canyon sequence and included 

ash-flow tuffs exposed elsewhere in the region. Under this interpretation, local volcanoes in this area stood high and excluded the ash-flow 

sheets that periodically flooded lower lying surrounding terrain. This 

suggestion will obviously be the target of future investigations.

An upper limit to the age of the Bullion Canyon Volcanics in the 

southern Antelope Range is give by radiometric age determinations on 

the large quartz monzonitic intrusion (the "central intrusive") in that 

area. Bassett and others (1963) analyzed seven biotite concentrates from 

three rock samples; four analyses ranged between 21.3 and 25.3 m.y., 

whereas three were between 25.1 and 27.2 m.y. Two of the more discordant determination (25.1 and 27 m.y.) were of altered biotite from 

sample KA 11, and can be disregarded. The third discordant determination (27.2 m.y.) was one of three determinations on biotite from 

sample. KA 2; the other two determinations were more closely concordant at 21.3 and 22.3 m.y. An average of the four most nearly 

concordant determinations reported by Bassett and others (1963) is 23 

m.y. McDowell (1971, p. 15) determined a K-Ar age of 21.8±0.7 m.y. on 

biotite from the same pluton. We have obtained a fission-track age of 

23.3±2.6 m.y. (table 1, no. 12) on apatite from the same quartz monzonitic intrusion; this corresponds closely to the average of most concordant ages obtained by Bassett and others and to the age by McDowell, 

leading us to conclude that this intrusive body was emplaced about 23 

m.y. ago. This age is older than the youngest Bullion Canyon rocks in the 

central Tushar Mountains, which overlie the 22-m.y.-old Delano Peak 

Tuff Member.

The fine-grained granite bodies that cut the 23-m.y.-old "central 

intrusive" were interpreted by Kerr and others (1957) to be closely 

related to the enclosing quartz monzonite and to be derivatives of the 

same magma. Our radiometric age determinations on the fine-grained 

granite, to be discussed more fully in later sections, indicate that this 

granite was emplaced significantly later, about 20 m.y., during the 

period of rhyolitic eruptions responsible for depositing the Mount 

Belknap Volcanics. Different genesis and petrologic association are indicated.
W Upper gray

Upper red '"" member 

tuM member / ^<<.'' > .'.^v» 

fVS=^aj.^g-4»r^VGray H\\\'s**

Lower he

Western source area Eastern source area

FIGURE 3. Diagrammatic cross section showing stratigraphic relations of the Mount Belknap Volcanics. Heavy l

of movement. Not to scale.
MOUNT BELKNAP VOLCANICS 25

MOUNT BELKNAP VOLCANICS

Between 21 and 18 m.y. ago in Miocene time, two source areas, one in 

the central Tushar Mountains and the other in and adjacent to the 

southern Antelope Range, were the sites of repeated rhyolitic eruptions 

that produced a heterogeneous assemblage of ash-flow tuffs, lava flows 

and domes, and associated volcanic breccias, and also emplaced local 

rhyolitic to granitic intrusive bodies (fig. 3). Two calderas, the major 

Mount Belknap caldera and the minor Red Hills caldera (fig. 2) subsided 

in response to the ash-flow eruptions. With minor exceptions, the assemblage consists of highly silicic, phenocryst-poor alkali rhyolite. The 

heterogeneous rocks that accumulated near their source in the central 

Tushar Mountains and Antelope Range were called Mount Belknap 

Rhyolite by Callaghan (1939), Callaghan and Parker (1961a; 1961b; 

1962a, 1962b), and Willard and Callaghan (1962), and the "Mount 

Belknap volcanic series" by Kerr and others (1957, p. 24). Generally less 

welded outflow tuffs belonging to the same assemblage were interpreted 

to be younger than the Mount Belknap and were called Joe Lott Tuff by 

these workers.

We have demonstrated the general equivalency of part of the Mount 

Belknap with the Joe Lott rocks and call the whole assemblage the Mount 

Belknap Volcanics, a formational name comparable in rank and 

lithologic ending to the underlying Bullion Canyon Volcanics (Callaghan, 1939). The Mount Belknap Volcanics is divided into a complex 

outflow facies (that includes the Joe Lott) comprising many individual 

units of lava flows and ash-flow tuffs from both source areas; an intracaldera facies of ash-flow tuffs, lava flows, and volcanic mudflow 

breccia filling the Mount Belknap caldera; and associated intrusive rocks 

that cut both facies. The Joe Lott is reduced in stratigraphic rank to a 

member of the Mount Belknap.

The exposures of rhyolite lava flows and densely welded tuffs on the 

steep slopes of Mount Belknap constitute the type locality of the Mount 

Belknap Volcanics. However, these exposures are limited to the lower 

part of the caldera fill, and most of the heterogeneous intracaldera and 

outflow facies members are not represented. Each formal member will 

be accorded a type area as it is described below.

A detailed account of the Mount Belknap Volcanics and of the evolution of the associated calderas is given in another report (Cunningham 

and Steven, 1979).

OUTFLOW FACIES

Mount Belknap eruptions began in the southern Antelope Range 

21-20 m.y. ago with the extrusion of a number of viscous rhyolite flows
26 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

and domes. Teacup rhyolite dome (table 1, no. 10) in the southeastern 

Antelope Range (fig. 2) which yielded a fission-track age of 20.9±1.1 

m.y. on zircon and K-Ar ages of 21.6±1.1 m.y. onbiotiteand 21.4±1.0 

and 21.6±0.9 m.y. on sanidine, is typical of these; but numerous other 

domes formed nearby. The rock contains about 45 percent phenocrysts 

in a matrix that is black glass near the margin of the dome and de vitrified 

elsewhere.The phenocrysts include about equal proportions of sanidine 

and oligoclase, with lesser amounts of brown biotite, green hornblende, 

apatite, sphene, and zircon. Heterogeneous minor lava flows and tuffs at 

the base of the Mount Belknap Volcanics are exposed in lower Deer 

Creek canyon just west of southern Marysvale Canyon (fig. 2); fissiontrack age determinations (table 1, no. 6) on zircon (19.6±0.8 m.y.) and 

apatite (18.8±1.7 m.y.) from a local ash-flow tuff at the top of this local 

accumulation indicate that these eruptions continued at least until 19 

m.y. ago. All these early rocks are here included in an informal lower 

tuff member of the Mount Belknap Volcanics.

About 19 m.y. ago the Mount Belknap caldera source area became 

active, and major pyroclastic eruptions spread rhyolitic ash flows outward in all directions. The resulting tuffs are densely welded near their 

source, but they become progressively less welded toward the periphery, 

where they have been called the Joe Lott Tuff. All but the most densely 

welded phases of this sheet show obvious multiple-ash-flow compound 

cooling, characteristics. The tuff contains 1.4 percent phenocrysts including quartz, plagioclase, alkali feldspar, and traces of biotite and 

accessory minerals in a groundmass of shards and pumice.The Mount 

Belknap caldera subsided in response to these ash-flow eruptions. The 

outflow sheet deposited by these major pyroclastic eruptions is here 

reduced in stratigraphic rank from formation to the Joe Lott Tuff 

Member of the Mount Belknap Volcanics. The type locality of the Joe 

Lott (Callaghan, 1939), along Joe Lott Creek 2.5^4.0 km south of Clear 

Creek, has excellent exposures of the outflow member. The member is 

about 460 m thick at this locality and shows a wide range in degree of 

welding as well as numerous partial to complete cooling breaks, representing individual ash-flow boundaries.

Pyroclastic eruptions in the eastern source area in the southern Antelope Range followed those from the Mount Belknap caldera source in 

rapid succession, and a local, densely welded ash-flow tuff as much as 

300 m thick accumulated on both sides of the southern Marysvale 

Canyon, in the vicinity of the source. The rock contains 7.6 percent 

phenocrysts of alkali-feldspar, quartz, plagioclase, Fe-Ti oxides, and 

traces of apatite and zircon. The vent area was in the Red Hills-Dome 

Hill area (Kerr and others, 1957, pi. 12) 3-5 km north of Marysvale; 

subsidence accompanied eruption, and the still soft welded tuffs were 

drawn back downward by the subsiding block of a small caldera about a 

kilometer across. Small rhyolite lava flows are interleaved with the ashflow tuffs near their source. The rocks derived from the Red Hills
MOUNT BELKNAP VOLCANICS 27

caldera are here called the Red Hills Tuff Member of the Mount 

Belknap Volcanics. The type area is in the Red Hills about 4.8 km N. 10° 

E. of the center of Marysvale (fig. 2, loc. 5).

The Red Hills area was 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 mixture of rhyolite flows and 

tuffs from a single magma chamber. John Carmony (oral commun., 

1976) suggested to us that the source was in fact a caldera, and we have 

accepted and enlarged upon his suggestion.

Shallow paleochannels were cut into the tops of both the Joe Lott and 

Red Hills Tuff Members, and were in turn filled with a distinctive 

ash-flow tuff containing about 30 percent phenocrysts of quartz, 

sanidine, plagioclase, and biotite. Remnants of this tuff have been found 

south, east, and north of the Mount Belknap caldera and are believed to 

have been derived from that source. K-Ar age determinations are 

18.5±0.8 m.y. (table 1, no. 4) on sanidine from this unit near the 

southern end of Marysvale Canyon 12 km east of the caldera (fig. 2) and 

19.4±0.9 m.y. (table 1, no. 5) on sanidine from an outcrop near the 

mouth of Merchant Creek 10.5 km south of the caldera (fig. 2). Fissiontrack ages (table 1) on zircon from these same samples were respectively 

15.3±0.8 m.y. and 17.1 ±0.8 m.y.; not only do these numbers disagree 

with the K-Ar ages, but they also contradict evidence from other 

radiometric ages that Mount Belknap volcanism was largely over by 

18 m.y. ago. This distinctive ash-flow tuff is here informally called the 

crystal-rich member of the Mount Belknap Volcanics.

A thin, lower, densely welded ash-flow tuff about 5 m thick has been 

mapped between lower Beaver Creek and lower Deer Creek, where in 

places it overlies the upper crystal-rich member and in places overlies the 

Red Hills Tuff Member. It appears to have been deposited by a single 

ash flow from an unknown source. This unit is a distinctive crystal-poor 

rock characterized by black, partly collapsed pumice lapilli (fiamme) as 

much as 3 cm long, enclosed in red devitrified matrix. A basal vitrophyre 

is exposed locally. It is sufficiently distinctive to serve as a local marker 

horizon in the area where it occurs. We informally call this unit the 

upper red tuff member of the Mount Belknap Volcanics.

Locally derived small-volume porphyritic latite lava flows containing 

phenocrysts of plagioclase, clinopyroxene, and hornblende underlie 

parts of Beaver Hill (Kerr and others, 1957, pi. 12), 1.6 km northwest of 

Marysvale and south of Beaver Creek (fig. 2). Kerr and others (1957, p. 

32) called these flows "purple porphyry." Whereas these particular lava 

flows occupy a stratigraphic position between the upper red tuff 

member and the Gray Hills Rhyolite Member (to be described next), 

they probably are only one of many local flow units that exist at one place 

or another within the Mount Belknap Volcanics.The flows contain 

phenocrysts as much as one cm across of andesine and pale-green
28 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

diopsidic augite, as well as sparse oxidized crystals of hornblende in a 

felted groundmass of microcline and hematite. They are noteworthy 

mostly because their intermediate composition and porphyritic texture 

contrast with the more common crystal-poor rhyolite that constitutes 

most of the formation.

The Gray Hills (Kerr and others, 1957, pi. 12) north of lower Beaver 

Creek (fig. 2, loc. 7) are made up largely of thick rhyolite lava flows with 

prominent contorted flow layers. Total thickness of the unit is 700 m or 

more. Phenocrysts are typically sparse, and the flows are characterized 

by devitrification spherulites and by thin lenticular gas cavities lined with 

euhedral vapor-phase crystals of quartz and sariidine. The thick local 

flows, containing contorted flow-folds overturned generally outward 

from the highest parts of the Gray Hills, argue for accumulation of 

viscous lava above local vents. These rocks constitute most of the "gray 

fades" of the Mount Belknap Rhyolite of Callaghan (1939), and were 

called the Gray Hills Rhyolite by Molloy and Kerr (1962). We call these 

rocks the Gray Hills Rhyolite Member of the Mount Belknap Volcanics. 

The area of distribution and the type area (the Gray Hills) of the 

member are virtually coextensive.

The youngest unit in the composite outflow facies of the Mount 

Belknap Volcanics that we have identified to date (1976) is a local layer of 

massive, light-gray, pumice-rich, welded ash-flow tuff that caps the Gray 

Hills Rhyolite Member near Indian Hollow, 1.8 km north of lower 

Beaver Creek on the west side of the Gray Hills. It contains a few 

scattered phenocrysts of quartz and sanidine in a devitrified welded ash 

matrix. A fission-track age on zircon from this youngest Mount Belknap 

unit (table 1, no. 3) is 18.0±0.8 m.y. We informally call this unit the 

upper gray tuff member of the Mount Belknap Volcanics.

INTRACALDERA FACIES

Rocks filling the Mount Belknap caldera are a mixture of densely 

welded crystal-poor ash-flow tuffs identical with near-source parts of the 

Joe Lott Tuff Member, thick crystal-poor rhyolite lava flows containing 

contorted flow layers, and volcanic mud flow breccia and associated 

finer grained volcanic sediments. The lower part of the caldera fill, 

exposed best in the eastern and northern parts of the caldera, consists 

largely of massive densely welded ash-flow tuff that forms layers as 

much as 500 m thick and is petrographically similar to the outflow Joe 

Lott Tuff Member. A wedge of rhyolite flow rock splits the welded tuffs 

into two parts. Only the top of the lower part is exposed in the deeper 

canyons, and we have no idea how thick this tuff is in the lower, buried 

parts of the caldera; we informally call this lower unit the lower tuff 

member of the intracaldera facies of the Mount Belknap Volcanics.

The overlying wedge of lava flows is here called the Blue Lake Rhyolite Member of the intracaldera Mount Belknap Volcanics. The cliff 

exposures of this member on the flanks of the glaciated valley adjacent to
MOUNT BELKNAP VOLCANICS 29

Blue Lake comprise the type area; it is about 260 m thick in this vicinity. 

The Blue Lake Member is exposed locally along the west side of the 

caldera, and prominently just east of Mount Belknap and Mount Baldy 

in the eastern third of the caldera, and it thins northward and eastward. 

It is absent near the eastern margin of the caldera. Its stratigraphic 

position is everywhere covered in the northern part of the caldera, but 

the great thickness of the overlying welded ash-flow tuff in this area 

suggests that it may wedge out in this direction as well. The rock contains 

less than one percent phenocrysts, most of which are sanidine and 

quartz.

The ash-flow tuffs above the Blue Lake lava flows are identical with 

those in the lower member; these thin southward against the underlying 

lava flows and are locally absent along the south and perhaps southwest 

sides of the caldera. To the north and northeast, however, they thicken 

to as much as 500 m, and contain local small rhyolite lava flows. These 

tuffs are called informally the middle tuff member of the intracaldera 

Mount Belknap Volcanics.

The upper part of the Mount Belknap caldera fill, now exposed 

largely in the western and southern parts of the caldera, consists of thick, 

crystal-poor rhyolite lava flows, domes, and marginal talus and mudflow 

breccias, with local lenses of densely welded tuff. These flows consist of a 

granular mosaic of quartz, alkali feldspar, minor plagioclase, biotite, and 

hematite. Some of the flows and domes are as much as 500-700 m thick 

and 1-2 km across; they commonly thin abruptly toward their margins 

and are flanked by wedging masses of rudely layered volcanic mudflow 

breccia and minor finer sediments deposited in low areas between flows. 

Small lenses of ash-flow tuff also accumulated in the low areas.

The lava flows in the upper part of the caldera are herein called the 

Mount Baldy Rhyolite Member of the intracaldera Mount Belknap 

Volcanics. The excellent exposures on the precipitous cliffs forming the 

upper slopes of Mount Baldy (fig. 2) are designated as the type area, 

where the member is about 300 m thick. The associated volcaniclastic 

rocks are called informally the sedimentary breccia member of the 

intracaldera Mount Belknap Volcanics.

Much of the intracaldera facies has been altered to dense white siliceous rock, probably by hydrothermal activity associated with the many 

vents that fed the intracaldera flows. The more permeable lava flow 

margins and flanking breccias were particularly susceptible to alteration. 

These altered rocks are widely distributed, and their striking white color 

makes them seem even more abundant than they actually are; this is one 

natural cauldron that obviously was used for cooking.

To date it has not been possible to establish precise chronology between intracaldera and outflow facies of the Mount Belknap Volcanics. 

By analogy with calderas in the San Juan Mountains of Colorado
30 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

(Steven and Lipman, 1976), in southern Nevada (Byers and others, 

1976), and elsewhere, the thick layers of densely welded tuff (lower and 

middle tuff members) in the lower part of the caldera fill may be 

generally correlative with the outflow Joe Lott Tuff Member. Caldera 

subsidence commonly results from voluminous ash-flow eruptions that 

deposit the outflow sheet; the thick intracaldera tuffs then accumulate in 

the subsided or concurrently subsiding basin. The flows and domes in 

the upper part of the fill (Mount Baldy Member) abut the topographic 

wall around much of the western and southern parts of the caldera 

margin, but locally on the northwest side of the caldera some of the 

youngest flows extend out over the edge of the caldera and lie on top of 

outflow Joe Lott Tuff Member. At one place a lens of distinctive crystalrich ash flow tuff 5-10 m thick is interleaved with these young lava flows 

above the Joe Lott; this lens is believed to be correlative with the informal 

crystal-rich member of the outflow Mount Belknap Volcanics described 

above. If this correlation is correct, a younger age limit is placed on most 

of the intracaldera facies. All outflow facies units younger than the 

crystal-rich member so far identified are of small volume and areal 

extent, and the intracaldera facies seems largely to have been deposited 

either concurrently with or shortly after accumulation of the outflow Joe 

Lott Tuff Member.

INTRUSIVE ROCKS

Rhyolite or granitic intrusive bodies were emplaced locally in the two 

main source areas during eruption of the Mount Belknap Volcanics. 

Some of those bodies now exposed were probably feeders for overlying 

volcanic rocks, and many more of these would undoubtedly be exposed 

were the level of erosion somewhat deeper beneath areas that currently 

expose numerous rhyolite flows and domes. We mapped several such 

pluglike intrusions within the Mount Belknap caldera, and Kerr and 

others (1957, pi. 12) mapped others in the southern Antelope Range 

near The Teacup and adjacent volcanic domes. A few rhyolite dikes 

were mapped by Kerr and associates in the southern Antelope Range 

and were described by them as being exposed underground in some of 

the uranium mines in the central mining district.

The largest intrusive body emplaced during the Mount Belknap 

period of igneous activity is the small stock of fine-grained granite that 

cuts the older (23 m.y. old) quartz monzonitic central intrusive in the 

southern Antelope Range, described previously. Kerr and others (1957) 

believed this granite to be comagmatic with the enclosing quartz monzonite. However, concordant fission-track ages on zircon (19.5±0.7, 

21.3±0.9, and 20.2±0.9 m.y.) from two samples (table 1, nos. 7 and 9) 

are supported by K-Ar ages (table 1) on biotite (21.4±0.9 m.y., no. 7, and 

21.6±0.9, no. 8) from the same area. A fission-track age of 20.4±0.9 

m.y. on zircon from a sample (table 1, no. 8) of altered quartz monzonite 

from the same area apparently was reset during emplacement of the
SEVIER RIVER FORMATION 31

nearby fine-grained granite. An overall average of all these generally 

concordant ages is 20.7 m.y. The fine-grained granite thus appears to be 

a more coarsely crystalline representative of the same magma that 

erupted the nearby rhyolite ash-flow tuffs, domes, and flows from the 

eastern source areas of the Mount Belknap Volcanics.

The complex relations that exist between the fine-grained granite 

(21-20 m.y. old), the adjacent Red Hills caldera and associated ash-flow 

tuffs (19 m.y. old), the glassy rhyolite dikes described by Kerr and others 

(1957), and the uranium-bearing veins that cut all these rocks in the 

^central mining district are an obvious target for future detailed investigation.

SEVIER RIVER FORMATION

The Sevier River formation (Callaghan, 1938, p. 100-101) consists of 

predominantly fluviatile and minor lacustrine sediments deposited in 

the developing basins of the High Plateaus in late Cenozoic (Miocene to 

early Pleistocene) time. Sedimentation was generally concurrent with 

the widespread basin-range faulting that disrupted the area and led to 

the present mountainous terrain. Many local ash-fall tuffs within the 

Sevier River Formation represent episodic volcanic activity in the region 

during the same period. Basalt flows are locally interleaved with the 

Sevier River sediments, also indicating episodic volcanism. As each local 

basin of deposition had its own unique history of structural development 

and sedimentation, it is difficult to establish any widespread chronology 

within the Sevier River Formation. Furthermore the formation is generally soft and easily eroded, and outcrops are commonly sparse. Locally, 

however, it forms badland topography with virtually total exposure.

Our mapping to date (1976) has touched only slightly on the Sevier 

River Formation and associated basalt flows, and we can add little to 

descriptions already published. We collected a sample of volcanic ash 

from a tuff bed about 50 m above the base of the Sevier River Formation 

along Joe Lott Creek 2.4 km south of the confluence of Clear Creek and 

Mill Creek, where the formation was deposited on the Joe Lott Tuff 

Member of the Mount Belknap Volcanics. The two formations appear 

conformable and no sign of channeling was noted. Zircons separated 

from this sample (table 1, no. 2) yielded a fission-track age of 14.2±0.9 

m.y. We have no explanation yet for the seeming lack of erosion during 

the several millions of years between emplacement of the Joe Lott and 

deposition of the Sevier River, as deformation related to basin-range 

faulting has been documented elsewhere in the region during this same 

interval.

Another sample of volcanic ash was collected from a tuff bed near the 

eroded top of the Sevier River Formation, at the type locality near the 

confluence of Clear Creek and the Sevier River (fig. 2). Zircons from this 

sample (table 1, no. 1) yielded a fission-track age of 7.1±0.7 m.y. This
32 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

age certainly does not represent the end of Sevier River sedimentation; it 

merely indicates that this sedimentation continued well into the late 

Cenozoic, and that in the depositional basin now drained by Clear Creek, 

it spanned more than 7 m.y.

ALTERATION AND MINERALIZATION

Altered and mineralized rocks are widespread in the Marysvale area, 

and geographically they can be separated into many local mining districts and mineralized areas. These have been cataloged in detail by 

Callaghan (1973), who gave excellent descriptions of the main mines and 

prospects in each district or area. For our purposes, a broader classification based on geologic occurrence is more convenient. We will discuss 

age relationships of mineralization in several of the larger areas of 

occurrence, and some economic implications of these relationships.

The monzonitic intrusive rocks in the southern Antelope Range and 

adjacent parts of the eastern Tushar Mountains are surrounded by a belt 

as much as 4 km wide that contains numerous local areas of intensely 

altered and mineralized rocks. These areas include many of the wellknown alunite deposits of the Marysvale area, and some of the altered 

rocks contain local occurrences of copper minerals. Altered and 

mineralized rock is exposed within this belt for at least 24 km around the 

western, northern, eastern, and southeastern periphery of the intrusive 

center, and may be present under the remainder of the periphery 

beneath a cover of younger volcanic rock and alluvium. Whereas some 

of the altered rocks are along the margins of the exposed intrusive 

bodies, more commonly they are well out into the surrounding volcanic 

rocks, where they tend to form clusters of altered areas marking centers 

of hydrothermal activity. These more isolated centers may be localized 

above hidden satellitic intrusions, as is further suggested by the presence 

of small plugs of quartz monzonite within some of the outlying altered 

areas (Kerr and others, 1957, pi. 12; Willard and Callaghan, 1962).

Except for small low-grade occurrences of copper at the Copper 

Butte-Winkleman mine area (fig. 2, loc. 2) and the Trinity mine (just east 

of the Sevier River and north of the mouth of Deer Creek) on the west 

and northwest sides of the intrusive center, published descriptions of the 

alunitic altered areas (summarized in Kerr and others, 1957; and Callaghan, 1973) do not indicate significant associated base-metal or 

precious-metal contents. The Trinity deposit is in a wedge of contactmetamorphosed sedimentary rock uplifted along the northern flank of 

one of the larger monzonitic bodies. Sparse chalcopyrite in this wedge 

may indicate a potential for contact-metamorphic copper ores in the 

buried sedimentary rocks beneath other altered areas. Small quantities 

of uranium were noted by Kerr and others (1957) in many altered areas, 

but, as will be discussed later, the uranium is demonstrably younger than 

the alunite and probably was introduced during a later, unrelated 

mineralizing episode.
ALTERATION AND MINERALIZATION 33

Geologic evidence in many places indicates that most alunitic alteration in the southern Antelope Range.predated eruption of the Mount 

Belknap Volcanics. The larger areas of intensely altered rocks are limited to vent-facies Bullion Canyon Volcanics peripheral to a cluster of 

monzonitic plutons (one of which was dated as 23 m.y. old). In many 

places unaltered Mount Belknap Volcanics rest unconformably on intensely altered Bullion Canyon Volcanics. This geologic evidence is 

strongly supported by K-Ar ages on alunite from the Winkleman mine 

(fig. 2; table 1, no. 19) west of the intrusive center (21.8±4.3 m.y.), from 

the Yellow Jacket mine (table 1, no. 20) north of the intrusive center 

(22.5± 1.0 m.y.), and from the White Horse mine (table 1, no. 21) south 

of the center (22.7± 1.1 m.y.). Apparently emplacement of the monzonite intrusive rocks was followed shortly by intense hydrothermal alteration of the adjacent volcanic rocks. Future investigations will focus on 

whether any metallic mineralization took place at lower levels during this 

period of hydrothermal activity.

Renewed igneous activity during the Mount Belknap period of volcanism erupted rhyolitic lavas and ash flows, and it also emplaced rhyolite and fine-grained granite intrusions in the same general area occupied by the 23-m.y.-old monzonitic intrusions and associated altered 

rocks in the southern Antelope Range and adjacent areas (the eastern 

source area of the Mount Belknap Volcanics). Associated hydrothermal 

activity altered local bodies of rock (largely to clay and silica) and deposited veins containing significant quantities of uranium, molybdenum, 

and fluorine. In places this newly mineralized material is located separately from the older altered and mineralized rock, but in other places 

mineralized material of both ages are superimposed. In areas where the 

hydrothermal episodes were superimposed, paragenetic relations are 

complex.

The young veins and altered zones cut the fine-grained 21-20-m.y.-old 

granite of the central mining district, as well as the 19-m.y.-old Red Hills 

Tuff Member of the Mount Belknap Volcanics. Glassy rhyolite dikes 

associated with uranium-bearing veins have been reported by Kerr and 

others (1957); a general association in time between the uranium 

mineralization and Mount Belknap volcanic eruptions thus is indicated.

Radiometric age determinations (lead/uranium) on pitchblende from 

the central mining district have ranged widely. J. L. Kulp and associates 

(in Kerr and others, 1957, p. 61) determined the age of sooty pitchblende to be 9.8±1.2 m.y. from the 2 «6pb/238U ratio. This age was 

corrected by B. J. Giletti (in Kerr and others, 1957, p. 61) to 10.5±1.2 

m.y. Kulp and associates (reported by Walker, 1963, p. 31) also obtained 

a 207Pb/235U age of 24.0±10 m.y. from the same pitchblende a geologically unreasonable age for material deposited after intrusion of the 

pre-mineralization fine-grained granite about 21-20 m.y. ago. Kerr 

(1968, p. 1030) reported a lead/uranium age of about 13 m.y. from
34 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

uraninite from the same area, analyzed by D. S. Miller. The wide spread 

in lead/uranium ages that have been obtained from pitchblende (uraninite) from the central mining district casts doubt on the accuracy of any 

of them. This doubt seems justified as well by the fact that sooty pitchblende seems to have been the material most commonly analyzed. We 

have attempted to obtain better material for analysis, but so far without 

success. All that can be said at this time is that the uraniummolybdenum-fluorine veins are younger than the Mount Belknap host 

rocks, and possibly are several millions of years or more younger.

Vein natroalunite from the Al Kee Mee prospect, 1.6 km north of the 

Teacup (fig. 2, no. 4), has a K-Ar age of 16.6±0.8 m.y. (table 1, no. 18), 

indicating that alunitic alteration in the southern Antelope Range also 

took place during two widely separated episodes. The Al Kee Mee 

natroalunite and the uranium-molybdenum-fluorine deposits of the 

central mining district could possibly have been formed during the same 

general period of mineralization.

Another important mineralized area containing zonally arranged altered rocks and mineral deposits is in the eastern Tushar Mountains in 

the drainage basins of Bullion Canyon and Cotton wood Creek (fig. 2). 

Vein deposits of alunite in the Alunite Ridge area on the north side of 

Cottonwood Creek canyon are flanked on the north by base- and 

precious-metal deposits in veins in volcanic and sedimentary rocks on 

both sides of Bullion Canyon and in vein and manto deposits in 

sedimentary rocks on the eastern front of the Tushar Mountains between Bullion Canyon and Cottonwood Creek. The volcanic rocks in this 

mineralized area were pervasively propylitized, probably as a result of 

the same period of hydrothermal activity that formed the veins and 

mantos. Radon is present in many of the old mine workings, indicating 

the nearby presence of uranium.

Whereas it was tacitly assumed by most earlier workers that the alunite 

deposits in the eastern Tushar Mountains and Antelope Range were 

formed at approximately the same time, evidence now indicates that 

they formed during different and probably unrelated periods of 

hydrothermal activity. The deposits in the eastern Tushar Mountains 

are in largely volcaniclastic facies of the Bullion Canyon Volcanics, and 

do not appear to be associated with any local volcanic center of that age. 

The ash-flow sheet of Three Creeks Tuff Member extended across the 

area, indicating that no local volcanic edifice existed at that time. Bassett 

and others (1963) reported a K-Ar age of 14-13 m.y. for sericite associated with the base- and precious-metal manto deposit in sedimentary 

rocks at the Deertrail mine (fig. 2) at the eastern front of the Tushar 

Mountains, and we have obtained K-Ar ages of 13.8±0.6 and 14.3±0.6 

m.y. from two samples (table 1, nos. 16 and 17) of alunite from the 

Mineral Products mine (fig. 2) on Alunite Ridge. Thus the altered and 

mineralized rocks in the eastern Tushar Mountains appear to have
ALTERATION AND MINERALIZATION 35

formed 7-8 m.y. after development of the main alunite deposits in the 

southern Antelope Range, and are more nearly coeval with the younger 

natroalunite of the Al Kee Mee prospect and the uraniummolybdenum-fluorine mineralization of the central mining area.

Another relatively young alunite deposit in the Marysvale area is at the 

Sheeprock mine at the western base of the Tushar Mountains 13 km 

north-northeast of Beaver, Utah (fig. 2). As described by Callaghan 

(1973, p. 117-119), the alunite here formed in flow-banded rhyolite 

typical of the Mount Belknap Volcanics. A K-Ar age (table 1, no. 22) on 

alunite from the Sheeprock mine is 9.4±0,5 m.y., marking a third 

episode of alunitic alteration in the greater Marysvale area.

Ore deposits in the Kimberly mineralized area (Gold Mountain district) on the northern flank of the Tushar Mountains (fig. 2) consist of 

gold- and silver-bearing quartz-carbonate veins cutting Bullion Canyon 

Volcanics. The volcanic rocks are cut by small monzonitic intrusions and 

are widely propylitized; on casual inspection the Bullion Canyon rocks 

seem to consist of near-source volcanics typical of those that accumulate 

around volcanic centers. Zircon from one of the small monzonitic intrusions (table 1, no. 13) at the Sevier mine yielded a fission track age of 

24.1 ±1.2 m.y., well within the period of Bullion Canyon eruptions.

The altered and mineralized rocks of the Kimberly area- form the 

north wall of the Mount Belknap caldera, and mineralization appears to 

have predated eruption of the Mount Belknap Volcanics that fill the 

caldera. The northern margin of the mineralized area is a northwesttrending normal fault that drops unaltered Joe Lott Tuff Member of the 

Mount Belknap Volcanics down against the altered and mineralized 

Bullion Canyon Volcanics. The fault is one of many basin-range faults 

that fragmented the High Plateaus in late Cenozoic time, and thus does 

not provide a close bracket on the age of mineralization. The lack of 

evidence for hydrothermal activity in welded tuffs of the Joe Lott in the 

hanging wall, however, suggests that mineralization at the Kimberly area 

took place before the Mount Belknap Volcanics were erupted. Several 

kilometers north of this major fault, two clay pits along Mill Creek 

exploit kaolinite deposits that formed by intense alteration of Joe Lott 

Tuff Member adjacent to a minor east-northeast trending fault (Callaghan, 1973, p. 119-120). Evidently two periods of hydrothermal 

activity affected rocks on the north side of the Tushar Mountains, and 

the possibility of superimposed mineralization in the Kimberly area 

cannot be discounted.

Scattered uranium occurrences are known throughout the Mount 

Belknap caldera fill as well as in the immediately adjacent wall rocks. 

Some of these seem to be secondary concentrations related to the widespread but patchy alteration that affected the more permeable parts of 

the caldera fill. In other places, particularly in the precaldera rocks 

forming the west wall of the caldera, as well as locally within the caldera
36 STRATIGRAPHY AND RADIOMETRIC AGES, MARYSVALE AREA, UTAH

fill, the uranium occurs in veins and seems to have been introduced 

(Callaghan, 1973, p. 51-52). Both types of occurrences, however, are 

either in or associated with rocks or structures related to the Mount 

Belknap caldera, and are of that age or younger; thus they are broadly 

comparable in age and geologic association with the major uranium 

deposits of the central mining district in the southern Antelope Range to 

the east.

Some of the vein-type uranium deposits west of the caldera are in or 

near a quartz monzonite intrusion that cuts Bullion Canyon Volcanics on 

either side of Indian Creek 1.5-4.8 km west of the caldera. The volcanics 

adjacent to the pluton are locally altered and are cut by gold-bearing 

quartz veins (Callaghan, 1973, p. 30). Assuming that this intrusive body 

and associated altered and mineralized rocks are related to Bullion 

Canyon igneous activity, there seems a strong probability that here, too, 

at least two episodes of mineralization are superimposed.

An even younger mineralizing event in the Marysvale area is represented by the native sulfur deposits at Sulphurdale and Sulphur Peak 

near Cove Fort (fig. 2) (Callaghan, 1973, p. 121). The sulfur occurs in 

siliceous sinter in alluvium of the present surface, and probably is Pleistocene in age. It may be due to hydrothermal activity caused by the same 

period of volcanism that formed the nearby Quaternary basaltic volcanoes west of Cove Fort.

The mineralized areas we have discussed represent only selected parts 

of the total assemblage of mineralized occurrences in the Marysvale 

area. For a comprehheeensive discussion of all known occurrences, the 

reader is referred to Callaghan's (1973) excellent review. The areas 

discussed here were chosen to illustrate the multiple ages of mineralization, extending from early Miocene time to Pleistocene, which can now 

be demonstrated or reasonably inferred. Not only did mineralization 

take place during widely separated periods of hydrothermal activity, but 

in many places products of the different periods of activity are superimposed. Some mineral commodities were deposited during more than 

one mineralizing episode, but others apparendy only during one.Proper 

appraisal of the economic potential of any given mineralized area thus 

involves consideration not only of "what," "where," and "how much," 

but also of "when," and "in association with which."

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a U.S. GOVERNMENT PRINTING OFFICE: 1979-677-026/49
Steven, Cunningham, Naeser, and Mehnert STRATIGRAPHY AND RADIOMETRIC AGES OF VOLC