Scholars' Mine Scholars' Mine

Masters Theses Student Theses and Dissertations

1969

Vertical and lateral variations in a welded tuff Vertical and lateral variations in a welded tuff

Ted Richard Dinkel

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VERTICAL AND LATERAL VARIATIONS

IN A WELDED TUFF

BY t.fl/lJ

TED RICHARD DINKEL/ ) Of'-/- If

A

THESIS

submitted to the faculty of

THE UNIVERSITY OF MISSOURI-ROLLA

in partial fulfillment of the requirements for the

Degree of

MASTER OF SCIENCE IN GEOLOGY

Rolla, Missouri

1969

ii

ABSTRACT

During the Tertiary period, nuee ardente type eruptions spread

large deposits of welded tuffs over southwestern Utah and southeastern

Nevada. The Wah Wah Springs Tuff is a member of one of the most exten-

sive of these units, the Needles Range Formation of Oligocene age. Its

known outcrop area is estimated at 13~000 square miles. Relief was

slight in the area during the time of eruption to enable the flows

to spread over so large an area.

In an effort to analyze the lateral and vertical continuity of

the member~ a petrographic study was undertaken. The average compo-

sition of the Wah Wah Springs Member was found to be 1. 7% quartz,

25.9% andesine (An44. 2), 4. 0% biotite, 6. 0% hornblende, 0. 3% diopside, /--- '1

1. 6% magnetite totaling 39.'8% crystals~ with no significant lateral

variations throughout its studied extent.

Vertical variations in mineral concentration were found. These

variations are due to a time differential between successive eruptions

forming a single cooling unit. The variations record changes occurring

in the magma chamber or mixing of magmas before or during eruption.

The density and relative porosity of the bulk rock samples were

also analyzed. The vertical variations found in these parameters

are indicators of the degree of welding. The changes in welding found

in the Wah Wah Springs Tuff correspond to the changes described in much

of the published literature. These changes are a moderately welded

tuff at the base grading into a densely welded zone approximately one-

third of the distance to the top of the member and gradual decrease

in welding upw,ards, to a degree lower than that at the base.

TABLES OF CONTENTS

ABSTRACT LIST OF FIGURES. LIST OF PLATES LIST OF TABLES

I. INTRODUCTION.

A. Purpose and Scope B. Location and Accessibility. C. Nomenclature and Definition of Terms. D. Acknowledgements.

II. PREVIOUS WORK •

A. Ignimbrites B. Area and Units .

III. GEOLOGIC SETTING.

A. General B. Sedimentary Rocks C. Intrusive Rocks D. Extrusive Rocks

l. Pre-Needles Range 2. Needles Range Formation . 3. Post-Needles Range.

E. Structure

IV. METHODS OF INVESTIGATION.

·A. General • B. Mineralogy. C. Density

V. PETROGRAPHY OF THE WAH WAH SPRINGS TUFF .

A. General B. Shingle Springs Section •

1. Field Characteristics 2. Petrography 3. Density

C. South Egan Section. l. Field Characteristics 2. Petrography 3. Density

D. Lund Section. l. Field Characteristics 2. Petrography 3. Density

··'

iii

Page ii

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10 10 11 11 11 12

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E. Indian Peak Section • • • l. Field Characteristics . . • • 2. Petrography ••.. 3. Density .•.......••••

F. Wah Wah Springs Section . . l. Field Characteristics . 2. Petrography 3. Density • . .

VI. VARIATIONS IN THE WAH WAH SPRINGS TUFF ••

VII. CONCLUSIONS •

REFERENCES CITED. •

PLATE SECTION .

APPENDIX.

TABLE I. TABLE II.

VITA. • • • •

Mineralogic data for all sections • • • • • • Density and percent weight gain data for all sections • • • .

iv

37

46

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LIST OF FIGURES

Figures

l. Location and general geologic map of the Needles Range ash-flow field and location of sections •••

2. East-west cross-section at the Shingle Springs Section showing

v

Page

5

the location of samples collected. 20

3. Data for the Sh~ngle Springs Section 22

4. East-west cross-section at the South Egan Section showing the location of samples collected. . • • 23

5. Data for the South Egan Section. • • • 26

6. East-west cross-section at the Lund Section showing the location of samples collected. • . 27

7. Data for the Lund Section. 30

8. East-west cross-section at the Indian Peak Section showing the location of samples collected. . • • • • 31

9. Data for the Indian Peak Section • • • .

10. North-west south-east cross-section at the Wah Wah Springs Section showing location of samples collected. •

11. Data for the Wah Wah Springs Section . . .

12. Best fit curves for the mineralogical data

33

34

36

42

13. Best fit curves for the density and percent weight gain data • 45

vi

LIST OF PLATES

Plates in Plate Section Page

I. Photomic~graph of embayed quartz crystal. • • • • • • • . • • 50

II. Photomicrograph of Thin Section No. 125 showing shard structure .•..•..•..•••••

III. Photomicrograph of Thin Section No. 126 showing shard structure. • • · • • • • • • . • • • • •

IV. Photomicrograph of embayed quartz crystal showing crystallographic control of resorption . • • • .

V. Photomicrograph of alteration in Thin Section No. 17

VI. Photomicrograph of perlitic cracks in the glassy

51

51

52

52

grO\mdmass of Thin Section No. 86. • • • • • • • • • • • • 53

LIST OF TABLES

Tab1es in Appendix Page

I. Mineralogical data for all sections. 54

II. Density and percent weight gain data for all sections. • 57

I. INTRODUCTION

A. Purpose and Scope

The purpose of the research leading to this thesis was to provide

a detailed petrographic study of the vertical and lateral variations

within an ignimbrite cooling unit. This thesis will not attempt to

answer completely the questions regarding the origin of magmas in­

volved or location of source areas and eruptive centers, but it is

hoped that it will be an initial study that will lead to elucidation

of these matters.

The area from which the samples studied were taken will, for the

purpose of this thesis, be called the Needles Range ash-flow field.

1

This area is one of little work to date. Several papers have been

p~~ished on locales generally surrounding this area and dealing with

many of the same units. The study of samples from the Wah Wah Springs -

Member of the Needles Range Formation was chosen with this lack of

previous work in mind. It is one of the most widespread members of

the formation. From gross aspects, the area involved appears to be

the center of deposition for this member.

The s·amples and thin-sections examined are part of. a larger suite

collected for the study of the stratigraphy of the entire Needles

Range Formation. These samples were collected by Dr. S. Kerry Grant,

the author's advisor, during several summers of work. The author

aided him in this during the summer of 1968. Thin sections were pre­

pared by the National Petrographic Service and by Edwin C. Ketten­

brink, Jr.

The primary method of study was by use of a petrographic micro-

scope, noting mineral percentage, texture, etc. The five-axis

universal stage was used for precise mineral determinations. Density

calculations were done on a Jolly balance. An I.B.M. 360 computer

system was used for some of the correlation of data.

B. Location and Accessibility

The Needles Range ash-flow field is located in Iron and Beaver

Counties in southwestern Utah and in Lincoln County in southeastern

Nevada. (see map, Fig. 1) It is approximately 160 air miles north-

west of Las Vegas, Nevada and 205 air miles southeast of Salt Lake

City, Utah. The ash-flow field, as limited for this thesis, is bound-

ed by the ll.3th meridian on the eastern edge; the 38th parallel. on

the south; the Millard-Beaver County line and the White Pine-Lincoln

County line on the north; and the Nye-Lincol.n County line on the

western edge. Many locations are accessible by Jeep roads only.

The Union Pacific Railroad crosses the area from Uvada to Mil.lford

through Lund, Utah. It is to be noted that the Needles Range For-

mation occurs outside this area, particularly on the High Plateau.

C. Nomenclature and Definition of Terms

Rocks of the kind studied in this thesis have been variously

called ignimbrites, welded tuffs, and ash-flow tuffs, among others.

These particular terms have been used interchangably at various times.

Ignimbrite is a generic name. Marshall (1935) visualized ignim-

2

]:;)rites as rocks formed by the fall from a cloud of hot viscous material. ' -

One type of eruption that could produce s~ch a cloud is the nuee

ardente. A nuee ardente is the type eruption that characterized the

volcanic activity at Pelee. It consists of a "glowingu cloud and a

lower very hot gas-solid mixture, with high mobility·. Ash-flow tuff

denotes the consolidated deposits of vo.lcanic ash remaining from an

ash flow. An ash flow may be the lower, geologically important

portion of a nuee ardente. This indicates the mechanism of dispersal,

size and state of the material. They may or may not be welded.

Welded tuff on the other hand is simply a rock in which the individual

glass fragments or shards remained plastic long enough to become

partly or wholly welded. The unwelded rock is called sillar.

In this thesis, when writing of the rock samples studied.from

the Wah Wah Springs Member, the term welded tuff will be used.

Mackin (1960) has stated that the silicic volcanic rocks of the

Great Basin are nearly all ignimbrites, that is depositional units

formed by eruption of the nu~e type. Thus when writing of the forma­

tion or rock-units of the area or of general principles, the term

ignimbrite will be used.

D. Aoknowledgements

I am indebted to Dr. S. Kerry Grant, my advisor, for his

assistance during my graduate study. He suggested the problem,

supplied the thin sections and samples studied and was always available

when help was needed.

Special thanks go to the National Science Foundation and the

V. H. McNutt Memorial Fund Conunittee for their support of the work

leading to this thesis. Also to Dr. Thomas R. Beveridge, Dr. Richard

D. Hagni, Dr. John D. Rockaway, John S. Trapp and Edwin C. Ketten­

brink, Jr. for their individual contributions to this work.

3

I am very grateful to my wife, Nancee, for her aid and under-

trs,tanding. ~

4

0 Alluvium

(W Sed1mentory Rocks

~ VI . R k W OCOniC OC ~

m1 . Rk &.:2 lntrus1ve oc s

0 m1les 30

eCedor C1tv

NEVADA

UTAH

Figure 1. Showing locat ion and general geologic map of the Needles Range ash-flow field and location of sections. The numbers on the map refer to the following sections: 1. Shingle Springs 2. South Egan 3. Lund 4. Indian Peak 5. Wah Wah Springs

~

01

6

II. PREVIOUS WORK

A. Ignirnbri tes

Ignimbrites have been the subject of many studies. A brief summary

of the major papers leading to the recognition of and development of

concepts regarding ignimbrites will be presented. It would be

beyond the scope of this thesis to list and outline all of the contri­

butions in this field. For a very complete bibliography and review

the reader is referred to "Ignimbrite Bibliography and Review" by

Earl F. Cook (1962) and "Paleovolcanology" in Earth-Science Review

also by E. F. Cook (1966).

Most rocks that were later to be called ignimbrites were initially

classified as flow rocks. The first geologist to describe a rock as

having been welded was Zirkel (1876). He was studying Nevada rhyolites

and thought that the pumice fragments appeared welded together. In

Iddings' work on Yellowstone rhyolites (1899), he set forth the first

hypothesis to explain the presence in a single rock, features of both

pyroclastic and liquid flow origin. He proposed that a gas-charged

lava might vesiculate at its surface forming glass and pumice fragments

and that these fragments would fall together while still hot enough

to become welded and then flow.

The observations of the eruptions of Mount Pelee and La Soufriere-

in the Carribean by Anderson and Flett (1903), Lacroix ( 1904) and a later

study at Pelee by Perret (1937) provided an understanding for a new

type of volcanic eruption, the nuee ardente. These descriptions formed

the basis f or present-day concept s of the origin of ash-flow materials

includi_ng welded tuffs. The pyroclastic deposits formed by eruptions

of the Pelean type are characterized by their complete lack of sorting

and wide range of dimensions of the constituents.

The next major step in recognition of the true nature of these

rocks came after the interpretation in Pelean terms of the 1 sand-flow'

eruption of Katmai in Alaska by Fenner (1920, 1923). Following

Fenner's works many geologists began to realize that large acid sheets

may have a nuee origin. One of these geologists was Patrick Marshall.

He applied the term ignimbrite to the sheet rhyolites in New Zealand

and assigned a Katmaian origin to them (1932, 1935).

In the 1930's much work was done in the western United States

on the study of ignimbrites. Mansfield and Ross (1935) described a

highly welded tuff in southeast Idaho. Fenner (1936) recognized

welded tuff in the Yellowstone rhyolites. Gilbert (1938) wrote a

very comprehensive and informative paper on the Bishop Tuff in

California.

More recent papers have defined and reviewed the subject. Smith

(l960a) wrote a comprehensive· review paper on ash flows and on zones

and zonal variations in welded ash flows (Smith l960b ). A comprehen­

sive review of the characteristics and recognition of ash-flow tuffs

was written by Ross and Smith (1961).

7

Since the 1940's ignimbrites have been described on every continent

and are believed to be abundant on the moon (Cook, 1962 p.S).

It should be noted that the eruptions described in several of the

papers listed above are extremely small in comparison to the ancient

eruptions that have produced the welded tuffs of the Great Basin. To

reconcile the two, the nuees must be increased several orders of

magnitude, or a new explanation for emplacement of ancient deposits

must be sought.

B. Area and Units

The Needles Range Formation was named by Mackin (1960) from

exposures in the Needles Range, Utah. Probably the earliest work done

on these rocks was by Dutton (1880) who described some "tufas" in the

High Plateaus. These units were ignimbrites or water-laid ash

deposits belonging to the Needles Range Formation (Mackin, 1963).

Blank (1959) in the Bull Valley District, McCarthy (l959) in

the Motoqua-Gunlock area and Wiley (l963) in the Jackson Mtn.-Tobin

Wash area found the Needles Range Formation represented by a few

tens of feet of non-welded crystal tuff interbedded with fluvial and

lacustrine sediments of the Oligocene-Eocene Claron Formation. All

of these areas are south and southeast of the Needles Range ash-

flow field as outlined in this thesis. Northeast of Cedar City in

the Red Hills, Threet (1952) recognized two Needles Range ignimbrites

each about 50 feet thick and interbedded with the same fluvial and

lacustrine sediments. Mackin (1963) in the Iron Springs District

8

found a single.Needles Range ignimbrite resting on the Claron. Cook

(1960) notes the Needles Range interbedded with the Claron in Washington

County , Utah.

Miller (1963) in a paper on the structural-stratigraphic units of

the Wah Wah Mountains, which lie within this area of study, does not

differentiate the Tertiary volcanics. East (1956, 1957) also treats

the volcanics as a single unit.

The Nevada portion of the area studied is included in a map by

Tschanz and Pompeyan (l96l).

Mackin (1963) studied the stratigraphy of the Needles Range

Formation from the Utah-Nevada line eastward approximately llO miles.

Cook (1965) discussed the stratigraphy of Tertiary volcanic rocks

of eastern Nevada. He shows the distribution of the Needles Range

Formation extending west from Utah-Nevada line covering some 9,000

square miles. In his paper there is a brief discussion of general

vertical and lateral variations. This discussion is limited to

variations in degree of welding. He incorporates some work of Martin

(1957, 1959) on vertical variations of several eastern Nevada

ignimbrites.

9

III. GEOLOGIC SETTING

A. General

The Wah Wah Springs Tuff is a member of the widespread Needles

Range Formation. Cook (1965) estimated the areal extent of the

formation to be at least 13,000 square miles. The outcrop area

occupies the east-central portion of the ignimbrite subprovince of

Eardley in his division of the igneous provinces of the western United

States (1951). The age of the Needles Range volcanics is Oligocene.

Armstrong (1963) has, by the potassium-argon method of dating,

determined the age of one member of the Needles Range Formation as 26

(+4,-2) million years.

B. Sedimentary Rocks

Sedimentary rocks, including Recent Alluvium, constitute the

major rock t~pe in the ash-flow field as shown on the generalized

geologic map, Fig. 1. The consolidated rocks range in age from

Cambrian to . Tertiary. They generally occur on the west side of the

mountain ranges where they are on the upthrown side of the basin

and range fault blocks.

C. Intrusive Rocks

There are only a few intrusive bodies cropping out within the

confines of the area of the ash-flow field. They are confined to

two belts trending northeast to south west. The northern-most belt

includes the Mineral Range, just east of the area, the Star Range,

the San Francisco District and a stock near Indian Peak. The rock

types are granites, granodiorite and quartz monzonites. The other

10

ll

belt south of the area includes Iron Springs and BuJ.l Valley Districts.

The rock types there are quartz monzonites.

D. Extrusive Rocks

1. Pre-Needles Range. Stone Cabin Formation. The Stone Cabin

Formation is the oldest volcanic outcropping in the ash-flow field.

The rock is crystal-vitric moderately welded tuff. The crystals

consist of 45% quartz, 30% sanidine, 20% plagioclase and 5% mafics;

together these crystals comprise 43% of the rock. Henceforth a

notation modified after Cook (1965) will be used. His notation gives

the composition of the crystals as 45/30/20/5//43 showing the percents

of quartz, sanidine, plagioclase and mafics. The last number is the

percent of crystals in the rock. The other notation used below

separates the mafics and the last two numbers before the double slash

are biotite and hornblende, in that order.

Windous Butte Formation. The Windous Butte Formation is a

quartz latite, crystal-vitric moderately welded tuff. Its composition

is 45/12/4l/2/0//37. The plagioclase composition is An0-30.

2. Needles Range Formation. The Needles Range Formation, as

originally described by Mackin (l960), consisted of two members.

These members were the Wah Wah Springs and Minersville Tuffs. Grant

(in press) has redefined the formation to include the Cougar Spar,

Escalante, and Shingle Spring Members, lying below the Wah Wah Springs,

and the Indian Peak and the Lund overlying the Wah Wah Springs.

Cougar Spar Tuff Member. The Cougar Spar Tuff is a thick, very

highly welded tuff found locally near Indian Peak. Its composition

is 9/0/60/7/20//36 plus a little diopside. The composition of the

plagioclase is An30-70. Lithic fragments make up approximately two

percent of the rock. The rock is a crystal-vitric dacite-andesite

highly welded tuff.

Escalante Tuff Member. The Escalante Tuff is a vitric-crystal

latite welded tuff. Its composition is 3/0/76/l9/2//l2. The com­

position of the plagioclase ~s An0-30. Quartzite and volcanic

lithic fragments form 3%-5% of the rock.

Shingle Springs Tuff Member. The Shingle Springs Tuff is a

crystal-vitric, dacite-rhyodacite welded tuff. The composition of

the rock is 17/2/58/lS/8//38. The plagioclase composition is

An30-50. It is characterized by large quartz and biotite grains.

Wah Wah Springs Tuff Member. The Wah Wah Springs Tuff, the unit

studied in this thesis, will be described in detail in a later

chapter. Its most conspicuous characteristic is its abundant visible

hornblende.

Indian Peak Tuff Member. The Indian P~ak Tuff Member is a

vitric-crystal to vitric-lithic dacite welded tuff. The composition

of this rock is 5/0/55/30/10//12. The plagioclase is An30-50.

Lund Tuff Member. The composition of the Lund Tuff is 28/ll/46/-

12/3//42. The composition of the plagioclase is An30-50. The rock is

a crystal-vi tric rhyodacite welded tuff. This uppermost unit has

large quartz without large biotite.

3. Post-Needles Range. Isom Formation. The Isom Formation is

a series of thin vitric ignimbrites. The mineralogy is simple:

plagioclase with a little biotite. It is characterized by lineated

tubular vesicles. This unit overlies the Needles Range Formation,

usually either directly on the Lund or Wah Wah Springs Members,

l2

13

throughout the entire area of Needles Range deposition, from Richfield,

Utah to Lund, Nevada.

E. Structure

The basic structural feature of the ash-flow field is the

north-south basin and range faulting. These faults are post-volcanic

and expose the large thicknesses of sedimentary rocks under the ignim­

brites on the western sides of the mountain ranges. The topography

of pre-volcanic time must have been essentially flat within basins,

to allow the ash-flows to spread over such great distances. Isopach

maps of the various ignimbrite units indicate a definite northeast­

erly to east-west trend for each basin. This shows that basins of

ample relief did exist prior to extrusion of each ignimbrite. Cook

(1965) gives the possible inference that subsidence occurred along

these trends either prior to or during volcanism or both, creating

topographic depressions in which the ignimbrites were deposited.

l4

IV. METHODS OF INVESTIGATION

A. General

The rocks and thin sections studied in this thesis are a

small part of a large suite of samples. These were collected for

a study of the stratigraphy of the entire Needles Range Formation.

They do not then constitute the ideal suite for a study of the

vertical variation of the Wah Wah Springs Tuff, as all were not

collected in vertical sequences. Relatively complete vertical sections

were obtained from the following locations: Cook's Shingle Spring

and South Egan sections (Cook, 1965), Wah Wah Springs, Lund and

Indian Peak sections. The location of these sections are shown in

Figure l. The study of these sections form the basis of the work

done.

B. Mineralogy

The majority of the research was a petrographic study of thin

sections using a Leitz SM-POL microscope. Of primary interest

were the minerals present and their abundance, including any textural

peculiarities accompanying their presence, resorbtion, alteration,

etc. Also noted was the texture and devitrification of the ground­

mass, whether shards were evident, how deformed they were and their

de vitrification.

The mineral percentages were calculated through the use of an

ocular grid. Random fields were viewed and the minerals occurring

at the intersections of the grid lines were counted. Three or four

fields were used (1.035-1.380 points) and the number of points for each

mineral was expressed as a percentage of the total number of grid

points from all the fields. From these percents, various ratios

were calculated, notably the hornblende-biotite ratio.

In determining the composition of the plagioclase feldspars and

the pyroxenes, a Leitz five axis universal stage was used. The

Federow-Nikitin method was employed for the plagioclase using

stereograrns from Muir (1955). The anorthite content was established

by measuring the angles between the principle optic directions and

the (010) pole. The transitional position between high and low

temperature plagioclase was also noted. A similar technique was used

for the pyroxenes, measuring the 2V angle and the angle between the

Z optic direction and the~ crystallogr~phic axis. Tables used for

these determinations were from Hess (19~9).

C. Density

The density of each welded tuff sample was determined through

several computations. The specimens were weighed after both drying

and soaking in water for approximately 24 hours, using a Sartorius

direct reading balance. The apparent (soaked) density was then

measured on a Jolly balance. The reason for soaking was to eliminate

errors introduced by absorbed water in the dry sample when weighed in

water. Then through the use of the following equations the true

density was calculated:

Volume = wet weight Ds

Dt = dry weight volume

15

dry weight Dt = wet weight

Ds = d£Y weight (Ds)

wet weight

where Ds = soaked density and Dt = true density. During these

calculations the percent weight gained on soaking was noted as an

indicator of relative porosity.

l6

17

V. PETROGRAPHY OF THE WAH WAH SPRINGS TUFF

A. General

The Wah Wah Springs is the most widespread member of the Needles

Range Formation. Its described localities extend some 200 miles east-

west, from the High Plateau in Utah to the Grant Range in Nevada and

at least 50 miles north-south. Within this area its thickness ranges

from 100 to 900 feet.

The rock is generally a grey to reddish color with a blotchy

lenticular pattern in places. It is a crystal-vitric dacite welded

tuff. The most common microscopic texture exhibited by these rocks

is vitroclastic. The mean composition of the Wah Wah Springs Member is

Quartz l.7% ± 2.5%, Andesine 25.9%· ± 8.6%, Biotite 4.0% ± ~.3%, ~

Hornblende 6. o% ± 2. 9%, Pyroxene 0. 3% ± 0. 4%, Magnetite l. 6·% ± 0. 6%.

The percent crystals is 39.8% ± 7.6%. The variability expression after

each mean is the standard deviation.

The average anorthite content for the member was An44. 2 ± 2. 3.

The average andesine formed at a temperature between that of high and

low temperature plagioclase, near 60% ± 20% of the distance from the

low to the high temperature lines. (Data from Slemmons (1962) gives ., this transition position as 85%.) The plagioclase is a transitional

type, being neither typical of strictly plutonic nor strictly extrusive

environments. It records the fact that the plagioclase formed at

depth and not after extrusion and that the magma temperature had

decreased some amount from the volcanic temperature.

From an average angle between Z and C of 4-1° and an average 2V

of 62°, the composition of the pyroxene was found to be diopside.

i

/ I

I \ \

I I.

l8

Two stages of fracturing of quartz and plagioclase have been noted

i

}n examining the thin sections. One type is manifested by single !

hngular fragments, with some rounded corners. This fracturing is

: believed to be caused by the explosive force of the eruption and

expansion of gases within the magma. This is the process which forms

the individual glass shards. The second type of fracturing is a

cracking of a crystal into several pieces all of which remain together.

This is thought to represent an in situ breakage, caused by pressures

acting on the crystals after emplacement. This is sometimes referred

to as mosaic breccia texture.

'\ An interesting aspect of these welded tuffs is their alteration.

The principal products of alteration are magnetite rimming hornblende

and biotite, and calcite on plagioclase. From the point of view of

a single thin section the alteration is quite selective. Only certain

minerals are affected and each alteration mineral selects a specific

host. In many thin sections the groundmass is unaltered (other than /

devitrification) around the altered mineral. This would point toward L/

deuteric alteration resulting from hot gases emanating from within

the ash flow. These emanations should then affect all locations .,,/

equally. However the alteration from a field point of view is also v"

selective, that is some localities are much more altered than others. -~

Some locations are unaltered. This type of selectivity should indicate -~~~

a post-depositional type of alteration, such as hydrothermal, controlled

by fractures and fluid conduits. Thus an impasse is reached over

which is the dominant type or whether the alteration is of but one kind.

Weathering is eliminated from consideration because it should be

pervasive especially in the porous tuffs. The total lack of alteration

19

in the porous tuffs at the base of

.. , ( ~,,~~-1- w .l ,_, '{> )

the member and the lack of typical

weathering minerals shows that weathering is not an important process

in the alteration of these rocks. !/

The density of the Wah Wah Springs Tuff has a mean of 2.31 ± 0.17

gm/cc .. It varies more than 0.17 within several of the vertical

sections. The extremes range from 1.94 to 2.57. The relative porosity

is indicated by the percent weight gained upon soaking. The mean of

the percent gain is 3.6% ± 2.9% and it ranges from 0.7% to 11.0%.

In the following descriptions of the vertical sections the data

will be shown for both thin section and mineral. The thin section

data will be in the form of a bar graph. A hypothetical sample is

described below.

Percent of rock 0.1 1.0 10 100

20

B. Shingle Springs Section

1. Field Characteristics. The outcrop of the Wah Wah Springs Tuff

at this locality is on the east slope of a ridge, one mile southwest of

Shingle Pass, Egan Range, Nevada. The base of the section is in Section

18, T8N, R63E. The tuffs have a northerly strike and dip to the · east

at 30°. The Shingle Springs Tuff underlies the Wah Wah Springs,

and the Windous Butte Formation underlies the Shingle Springs. The Wah

Wah Springs is overlain by the Isom Formation (Cook's Shingle Pass

ignimbrite). The thickness of the Wah Wah Springs Tuff is 375 feet

at this location. The tuff has no obvious vitrophyre but the lower

half is darker and more welded than is the upper half. The colors of

the rock are usually pinkish tan. Figure 2 shows a cross-section

at this locality and ·the position of the samples studied.

E

Fm.

Figure 2. East-west cross-section at the Shingle Springs Section showing the location of samples collected.

2. Petrography. The mineralogy of the Wah Wah Springs Tuff at the

Shingle Springs section is shown in Figure 3 and in Table I. The fol-

lowing information is a summary of those data in addition to other

mineralogic characteristics. Crystals make up from 34.7% to 47.9% of

the rock. Quartz is present in quantities from 0.0% to 1.2%, with the

greatest concentration near the middle of the unit. It occurs as both

21

anhedral and subhedral fractured grains. The mineral is frequently

effibayed. Plagioclase makes up 18.0% to 36.4% of the rock. The mineral

is often zoned but some crystals show no zoning at all. The anorthite

content varies from An40.3 to An48.0. The variation of both percentage

and composition of the plagioclase is cyclical with vertical position.

It is often fractured. Biotite makes up 3.0% to 6.3% and also varies

cyclically with vertical position. It is pleochroic from dark red

brown to reddish yellow brown. The crystals are often bent. Hornblende

constitutes 3.0% to 9.0% of the rock. The pleochroism is as follows:

X = light yellow brown, Y = reddish brown, and Z = reddish brown. The

Hb/Bio ratio is fairly constant ranging from 1.1 to 1.6. Diopside "makes

up 0.1% to 1.3% of the rock. It occurs as small colorless grains,

generally euhedral. Magnetite occurs as small discrete grains amounting

to 0.8% to 1.5% of the total. Generally there is little alteration of

the minerals. There is minor magnetite on hornblende and biotite. The

groundmass is glassy with but faint shard structure visible. It is

partially devitrified. Where the shards are visible the devitrification

has produced axiolitic structures. Several of the thin sections show

spherulites and in the upper 40% of the unit, cavity filling or

lithophysae are present.

3. Density. As is seen in Figure 3 and in Table II, the lower

half of the Wah Wah Springs Tuff is quite dense. The density decreases

in the upper half. The top is also more porous. The three samples in

the upper half average 7.8% increase in weight upon soaking, while the

two in the lower half average'l.S% weight gain.

~ .. -~""

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f/6/B io f:( r. & 'lllii'il -IB1 IIQI)

~f71

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lilf.\91 •f?'f

Figure 3. Data for the Shingle Springs Section. Mineral data for thin sections and individual minerals are in percent. Density data are in gm/cc.

/

.-.

94'ZL~J-"':-- ~ : -:-<7!-;._~,bt.\_ ... --,;;·~t .. , :~~- . :.,

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23

C. South Egan Section

1. Field Characteristics. At the South Egan Section the Wah Wah

Springs Tuff outcrops at the base of a prominent ledge. The base of the

section is in Section 2~ TSN» R62E, at the south end of the Egan Range,

Nevada. The ash flows strike northerly and dip 20° to the east. Below

the Wah Wah Springs is the Shingle Springs Tuff. Above the unit is the

Lund Tuff. The thickness at the South Egan Section is 465 feet. There

is a definite vi trophyre overlying a white loose tuff. The unit grades

upward to a moderately welded tuff and then to a lightly welded tuff.

at the top. The color of the rock in hand sample is generally tan.

Figure 4 shows a cross-section of the outcrop at this location and

the position of the samples collected.

w

126

Shingle Springs

Tuff

Wah Wah Springs Tuff

Figure 4. East-west cross-Eection at the South Egan Section showing the location of samples collected.

2. Petrography. Figure 5 and Table I show the complete mineralogy

of thin sections from the South Egan location. Crystals constitute

from 27.5% to 50.5% of the Wah Wah Springs Tuff. There is an occasional

lithic fragment of another volcanic. Quartz makes up from 0.5% to 4.1%

of the rock. The grains are generally rounde d and embayed. (See Plate I)

There is an occasional crystal face or angular fragment. Plagioclase is

present in quantities from 19.9% to 34.8%. The percentage increases from

the base to a maximum at 75 feet above the base and then decreases

upward. The crystals are subhedral as a rule with some fracturing.

Phantom crystals are evident with fine inclusions paralleling the

outlines of the crystals. Again there is a mixture of zoned and

unzoned crystals. The composition of the plagioclase varies from

An4l.O to An45.7. Biotite constitutes from l.O% to 5.3% with a

general decrease upward. Its pleochroism is from dark brown to light

brown. Hornblende makes up from 1.9% to 15.2%, also with general

decrease upward. Its pleochroism is: X = light green, Y = green,

24

and Z = dark green. There is an occasional inclusion of plagioclase and

magnetite. Many basal sections show twinning. The Hb/Bio ratio is

relatively constant varying from 1.9 to 2.9. Diopside occurs as 0.5%

to 0.7% of the rock. It is in subhedral grains often as inclusions

~n plagioclase, hornblende and biotite. Magnetite occurs as rounded

grains, present in percents from 0.6% to 1.9%. As a rule the rock

at this section appears very fresh, with little alteration. The

rimming of hornblende and biotite by magnetite is the most common type

of alteration. In several thin sections hornblende and biotite have

altered to talc-vermiculite. Groundmass. In the lowest two thin

sections the shard structure is very well preserved. In the lowest,

No. 125, the shards are not flattened while in No. 126 the shards are

compressed and bent around the crystals. (See Plates II and III)

In the remaining thin sections only faint shard structure is apparent

or the groundmass is completely glassy. There is some patchy crys­

tallization of the groundmass in the middle and upper portions of the

unit. Lithophysae and flattened pumice fragments are occasionally

present.

3. Density. As shown in Figure 5 and Table II, the density is

low in the stratigraphically lowest samples. A maximum is reached

while still in the lower one-third of the tuff and drops off sharply

in the upper two-thirds. The relative porosity varies inversely with

the density. The lowest sample gained 7.1% of its dry weight upon

soaking. The three samples with the highest density averaged 1.3%

gain. The topmost samples averaged 9.7% gain.

25

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Figure 5. Data for the South Egan Section. Mineral data for thih sections and individual. minerals are in percent. Density data are in gm/cc.

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27

D. Lund Section

1. Field Characteristics. At the Lund location the Wah Wah Springs

Tuff outcrop is on the eastern flank of a north-south ridge~ 2 miles

west of Lund~ Utah. The base of the section is in Section 18, T32S,

Rl4W. The tuff strikes N25°E and dips 20°W. Lying beneath the Wah Wah

Springs Tuff is the Escalante Tuff. A porphyritic andesite flow and

the Lund Tuff overlie the Wah Wah Springs at this locality. The. thick-

ness of the unit here is 500 feet. These relationships are shown

below~ in Figure 6, in addition to the positions of the samples col-

lected. The Wah Wah Springs Tuff has no vi trophyre, but the base is

more than moderately welded. The lower half of the unit is more

welded than is the upper half. The color of the rock at this section

is generally greyish red.

E

Escalante Tuff

w

Andesite flow

Tuff

Figure 6. East-west cross-section at the Lund Section showing the location of samples collected.

2. Petrography. The complete mineralogy of the thin sections from

the Lund section is shown in Figure 7 and in Table I. From 29.4% to

46.3% of the rock is made up of crystals. There is a general increase

upward in crystal content. In several of the thin section altered

lithic fragments are present in small quantities. Quartz makes up

from 0.0% to 3,8% of the rock. Its variation is sporadic. The grains

28

are usually rounded and emhayed by resorption. (See Plate IV) Many

grains however show a subhedral outline or angular fragments due to

fracturing in addition to cracked grains caused by apparent in situ

fracturing. Plagioclase constitutes from 19.6% to 37.5% of the rock.

There is a general increase upward in concentration. The crystals

are both zoned and unzoned. Phantoms are evident in some crystals.

The composition of the plagioclase varies from An41.7 to An46.3 in

an apparent cyclic variation with respect to vertical position of the

sample. The crystals are subhedral with both types of fracturing

as noted for quartz. Biotite content varies from 2.0% to 5.4%. It

is pleochroic from reddish brown to yellow green.. Hornblende makes

up from 1.4% at the top to 9.5% in a thin section near the bottom.

Its pleochroism is: X = yellow green, Y = red brown, Z = red brown.

The Hb/Bio ratio varies from 0.4 to 4.8. Diopside occurs as 0.0%

to 0.5% of the rock. Magnetite is present as 1.3% to 4.1% of the rock.

The most prevalent type of alteration is magnetite on hornblende and

biotite. In No. 18 and No. 19~ from the top of the tuff, magnetite

alteration has almost completely replaced both minerals making differ-

entiation difficult. It also rims diopside. Calcite alteration on

plagioclase is also common at this location. (See Plate V) Shards,

in the groundmass are not always apparent, and there appears to be no

relation with their presence and degree of welding. The groundmass

is often dark with devitrification in the upper thin sections. Litho-

physae and spherulites occur at most levels in the tuff.

3. Density. As shown in Figure 7 and Table II the density of the

stratigraphically lowest sample is low. The maximum density is reached

in the lower one-tenth of the unit and decreases upward in the tuff.

29

The relative porosity varies inversely with respect to density. The

lowest sample gained 2.2% of its dry weight upon soaking. The three

denser samples averaged 1.3% increase and the topmost sample gained 5.1%.

:;:.<j:,<::._.,·.;·

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.-ts-

Figure 7. Data for the Lund Section. Mineral data for thin sections and individual minerals are in percent. Density data are in grn/cc.

I. ":¥!_ .· :e, :6!..:1 ; ff ~- ~-·. · . ·' "" ·_ .. · ,_ .. : ·- ·' . . ;'""- I

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w 0

E. Indian Peak Section

l. Field Characteristics. The outcrop of the Wah Wah Springs

Tuff is on a prominent east-west ridge, 2 miles south of Indian Peak,

Needles Range, Utah. The base of the section is in Section 31, T29S,

Rl8W. The ash flows strike Nl8°E and dip 26°E. The base of the Wah

Wah Springs Tuff is in fault contact with the Shingle Springs Tuff and

older flows. Overlying the Wah Wah Springs is the Indian Peak Tuff

and the Lund Tuff. The total thickness of the outcrop is about 1300

feet but actual pre-faulting thickness is estimated to he about 900

feet. Figure 8 shows these relationships as well as the position of

the samples collected. The Wah Wah Springs is separated into a lower

darker tuff and an upper tuff of· light color. The colors of the tuff

are shades of red.

w

Older Flows 1 1

Shingle Springs Tuff

E

Lund Tuf.f Indian Peak

Tuff

Figure 8. East-west cross-section at the Indian Peak Section showing· the location of samples collected.

2. Petrography. Due to the faulting in the lower half of the

member all samples were not used for analysis of variations. The top

five were used for this purpose. No. 209 was used, even though it was

31

in the faulted zone, due to statistical requirements. Complete minera-

logical data is shown in Figure 9 and in Table I.

· The summaries below are for the five samples taken from the top

of the unit.

Crystals make up from 32.4% to 51.3% of the rock. Lithic frag­

ments of both volcanics and quartzites are occasionally present.

Quartz constitutes from 0.8% to 1.9% of the rock. Most grains are

quite rounded an embayed. Some grains have subhedral outlines and

a few are fractured. Plagioclase makes up from 18.7% to 36.2% of the

tuff. As usual some grains have oscillatory zoning. Some grains

have fractures occurring in situ. The composition varies from An39.7

to An45.0. Biotite occurs as 3.7% to 5.5% of the rock. It is pleo­

chroic from red brown to yellow. green. Hornblende makes up from 4.0%

to 6.9% of the rock. Its pleochroism is: X= yellow green, Y = red

brown and Z = red brown. The Hb/Bio ratio is fairly constant ranging

from 1.1 to 1.4 increasing upward. Diopside occurs as individual

rounded grains from 0.0% to 0.4% of the rock. Magnetite occurs as

individual grains from 1.5% to 2.0% of the rock. Apatite is present

32

in very small quantities. Alteration is rare in the upper half of the

tuff but takes the form of magnetite rimining the mafics. The groundmass

contains very few shards and pumice fragments. Only a small part of the

tuff is devitrified.

3. Density. The data for density are shown in Figure 9 and

Table II. The density does not vary greatly and it is obvious that it

does not follow the other sections. The relative porosity varies

little also. The percent weight gained upon soaking averaged 3.8%

for the five samples and it varied from 2.5% to 4.8%.

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F. Wah Wah Springs Section

1. Field Characteristics. The outcrop of the Wah Wah Springs

Tuff at its type locality is on low hills, one-half mile southeast

of Wah Wah Springs, Wah Wah Range, Utah. The base of the section is

in Section 11, T27S, Rl5W. The tuff strikes N50°E. and dips ll0 SE.

Underlying the Wah Wah Springs Tuff is a series of hornblende rich

flows and flqw breccia on Paleozoic rocks. Overlying the tuff is

a late Cenozoic basalt flow. The Wah Wah Springs Tuff is 450 feet

thick at this locality. Figure 10 shows these relationships and also

the position of the samples. The Wah Wah Springs Tuff consists of

two thin, dark grey basal vitrophyres separated by 40 feet of light

grey moderately welded tuff. Above · the vitrophyres is a thick light

grey tuff and a greyish pink zone at the top. The lower-most 100 feet

of the tuff has l-inch volcanic lithic fragments amounting to as

much as 4% of the rock.

NW SE

86 87 88 ~85 9~~Basalt

Paleozoics ---- ~Flow· s --- ............._ Wah Wah Springs-............ Tuff

Figure 10. Northwest-southeast cross-section at the Wah Wah Springs Section showing location of samples collected.

34

2. Petrography. Figure 11 and Table I show complete mineralogical

data for the thin sections from the Wah Wah Springs locality. From

28.2% to 60.9% of the rock is crystals. The lithic fragments are

not as evident in thin section as they are in hand sample. Quartz

35

makes up from 0.0% to 5.2% of the rock. The grains take their usual

form of being rounded and embayed. Many grains have angular outlines

due to fracturing. Plagioclase constitutes from 17.6% to 44.7% of the

rock. The crystals are typically both zoned and unzoned. Many are

fractured. The anorthite content varies from An38.0 to 45.3. Biotite

makes up 2. 6% to 5. 8%. It is pleochroic from dark red brown to light

greenish brown. · Some crystals are bent. Hornblende is present in

amounts from 3.2% to 7.6% of the rock. Both the green and brown

hornblendes are present in the various thin sections. The Hb/Bio ratio

varies from 0.7% to 2.9%. Diopside ranges in amount from 0.1% to 1.0%.

It is sometimes rimmed by hornblende. Magnetite occurs as rounded

grains, making up 0.8% to 2.0% of the rock. Alteration occurs only

in the top two thin sections. The lower thin sections look fresh

and are unaltered. The typical magnetite on hornblende and biotite,

and calcite on plagioclase constitute the alteration. The groundmass . .

in the lowest sample, a vitrophyre, is completely glassy with perlitic

cracks and crystallites. (See Plate VI) Shards are apparent only

in thin sections No. 87 and No. 88. There is some divitrification and

spherulitic development in the thin sections.

3. Density. As shown in Figure ll and Table II the double

vitrophyre observed in the field is evident from the density data. The

density decreases upward from the upper vitrophyre. The relative

porosity also shows this feature. Percent weight gained upon soaking for

the five samples is as follows: l. O%, 4. 7%, l.l%, 2. 4%, and 6. 2%, from

bottom to top.

~~*'"""''"'';,;

H o ,.,. /, /,, .J., b s 10

IBl IDIIfl IIIII 91

5 /ot/t &

0 5 10

~

1._ --~ I

D r-:-::1 ~

Cl

D~11st"ty I.e :~.o 2.:z. .z.¥" ~.~.

Pia. J /t>c./o..3 e

0 tP ,tO 30 """ SO

t*~~·., .. :J~~~: j)

u:;..: 'i"'"->t·,,;};':r?"'1i fi{ :1

r~....;;;;J

Tot41 c,.ysfQ Is o 10 .lD 30 lo/D $"0 'l)

~ ..... ~~1

Qi.la ,.t z o.z. f'~

r:-1 ~

[l\_.•'c.C ''~hi!

r¥·:~a

r.':"l ~

ll/,/6/o

~~

.-<Jo ~85"

.. a a

... e7

.. 86

... 90

--85" ... 88 -a7 ..-!3"

Figure 11. Data for the Wah Wah Springs Section. Mineral data for thin sections and individual minerals are in percent. Density data are in grn/cc.

- '"'""':"·' ... ,~ "':'~"J-'¥1"'· ·--"'~, ... ,,_._,.,,;-:::-::,_r~~:i'~~

.... 90 I~ L-~~- - """"-"- I .- :;;;;;-·""=.;"'' "' •( *;,,, ,_

qt O.Z 0-5" tO 1.0 $:0 fO ~0 ~o ICO I I I I I I I I

:#'BC ~~~-- ~~'~:~~' ~r:~~ ~-~· ~'-~.i.~ 5'0% ~ ·---· "rl•

1/lt!!Je!J

II%

<iiB 7 0 -:;::, _ _.,;;; =a!i;;.;:,.,.$ 1 1~'''._~ ,_.. -- . -- _I

9~

at o.z as- ~o xo s:o fO zo so too

S%~1~

w 0')

37

VI. VARIATIONS IN THE WAH WAH SPRINGS TUFF

The data were analyzed by using statistical tests including compu-

ter analysis. No primary lateral variations were detected. Most of the

parameters do exhibit a vertically controlled variation.

Absence of lateral variations will be discussed first. In analyzing

the lateral variations Student's t test was used to prove (or disprove)

the null hypothesis that there is no difference between the means from

the five locations. This was done for the four major minerals, quartz,

plagioclase, biotite and hornblende, the total crystal percent, anor-

thite content, density and percent weight gained upon soaking.

The equation fort is:

t = y -u

s/Yff in which Y is the mean of the

group being tested, u is the mean of the population, ~ is the standard

deviation of the population and n is the number of samples. The

applicable t values for the .10 significance level are:

t(4)= 2.132

t(S )= 2. 015

t ( 8) = l. 860.

The number in parenthesis is the degree of freedom which is equal to

the number of samples minus one. If the t test gives a value less than

the above, it is insignificant and the null hypothesis (Ho) is accepted.

The following is a summary of the results of the t tests for the

parameters tested.

~uartz u = 1.7 s = 2.5

t (5 samples) = 1. 09 (2.132) Ho accepted ss

t (6 samples) = 0.20 (2.015) Ho accepted se

tl (5 samples) = 0.91 (2.132) Ho accepted

t. lp (9 samples) = 0.00 (1.860) Ho accepted

t (5 WWS

samples) = 0.45 (2.132) Ho accepted

There is no di·fference ·between means of each sample group for

quartz.

Plagioclase u = 25.9 s = 8.6

t = 0.67 ace. ss

t = 0.16 ace. se

tl = 0.46 ace.

t. = 0.41 ace. lp

t = 0.87 ace. WWS

There is no difference between means of each sample group for

plagioclase.

Biotite u = 4.0 s = 1.3

t = 0.11 ace. ss

t = 0.09 ace. se

tl = 0.31 ace.

t. = 0.88 ace. lp

t = 0.66 ace. WWS

There is no difference between means of each sample group for

biotite.

38

Hornblende u = 6.0 s = 2.9

t = 0. 54 ace. ss

t = 1.50 ace. se

tl. = 0.3l. ~cc.

t. = 0.50 ace. ~p

t = 0.92 ace. wws

There is no difference between means of each sample group for

hornl:>lende.

Total. Crystals u = 39.8 s = 7.6

t = O.ll ace. ss

t = 0.09 ace. se

tl. = 0.31 ace.

t. = 0. 88 ace. ~p

t = 0.66 ace. wws

The!"e is no difference between means of each sample group for

total crystals.

Anorthite content u = 44.2 s = 2.3

t = o. 76 ace. ss

t = 0.11 ace. se

tl · = 0.57 ace.

' t. = 0.91 ace. ~p

t = 0.67 ace. wws

There is no difference between means of each sample group for

anorthite content.

39

40

)ensity u = 2.31 s = 0.17

t = 0.88 ace. ss

t = 0.43 ace. se

tl = 0.38 ace.

t. = 0.67 ace. ~p

t = l.OO ace. WWS

There is no difference between means of each sample group for

density.

Percent weight gain u = 3.6 s = 2.9

t = 1. 3l ace. ss

t = 1.17 ace. se

tl = l.08 ace.

t. = 0.70 ace. ~p

t = 0.38 ace. wws

There is no difference between means of each sample group for

percent weight gained.

All the above t tests show no significant lateral variations in

any of the parameters tested. If there are horizontal variations in

the Wah Wah Springs Tuffs they are either outside the sampled localities

or they are so small as to be mas.ked by sampling error.

Vertical variations are present. Since the t test has indicated

that all locations may belong to the same population the vertical

variations will be studied by combining all data from the five

locations. The data were analyzed by a computer program modified after

one written by I.B.M. {I.B.M., l968). This program applies the best

fitting curve to the data up to a tenth order polynomial equation.

41

The same eight parameters examined for lateral variations were

nalyzed for vertical variations. Seven of these parameters required

4th order equation to describe its variation. These best fit

urves are shown in Figures 12 and 13. ..

All of the mineralogic curves in Figure 12 show at least one and

our curves have two maxima. Quartz peaks at 48% of the member's

hickness above the base with 2.7%. Plagioclase has a major peak at

7% of the thickness with 30.8%. There is a lesser peak at 81% with

8.6% plagioclase. The highest percentage of biotite occurs at 100%

f the thickness and is 5.8%. It too has a lesser, very broad peak

t 35% thickness, the percent biotite being 4.3%. Hornblende follows

he form of biotite having its highest percentage, 8.2%, at 100% of

he thickness and its second peak at 17% with 6.8% hornblende. The

otal crystals also have two prominent peaks, the major one at 18%

hickness with 46.5% and the lesser peak at 84% thickness with 40.0% .

:rystals. The anorthite content increases rapidly from the base to

,eak at An45. 8 at 29% thickness. It decreases and then remains

•elatively constant at An43. 8.

The explanation of these variations can be only tentative in

lature. It is apparent that complex processes are operating in the

tagma chamber which are not completely understood.

The lack of lateral and presence of vertical variations would seem

:o point toward a multiple pulse eruption. Time plays a role here in

,roviding the different stages of crystallization or resorption shown

,y the vertical changes. Ti me b e tween eruptions must necessarily be

;hort to provide no break in the smooth welding curves discussed later.

42

I 1M

ID It>

0 0

Figure 12. Best fit curves for the mineralogical data. Absicissa is the vertical position by percent thickness.

/1/.

.,.

IJ<>,o .......

43

The cyclic nature of these curves indicates construction and de­

truction of the crystals. For each mineral the position of the first

aximum peak above the base correlates with the typical order of

rystallization as predicted by Bowen: hornblende and plagioclase being

arliest and concurrent, biotite and quartz following in that order.

'his sequence is confirmed by texture observed in the thin sections.

iliove each maximum the reduction of mineral concentration is possibly

~xplained by dissolvi.ng of the crystals. This may be accomplished by

Lncrease in temperature or reduction of pressure. The common embayment

)f quartz is rementioned here as evidence for one or the other. Vance

(1965) states that the melting point decreases with falling pressure

in a water deficient system. Lipman (1966 p. 823) suggests that the

water content of some Nevada rhyolitic ignimbrites might be 3% to 5%.

However the second peaks, and in two cases the highest peaks, occur

after a reduction in the amount of the mineral.

Contamination by a second magma may provide the additional crystals

of plagioclase, biotite and hornblende. If no quartz were added, this

would have had the effect of lowering its percentage. This mixing of

magmas could be concurrent with the tapping of the magma chamber. This

process might not allow sufficient time for mixing of liquids but the

eruptive shattering process in addition to the turbulent flow, in effect

fluidization, would certainly suffice. The contamination hypothesis

might be supported by the presence of two types of plagioclase, one

zoned and the other unzoned, if it were not f9r the constancy of the

anorthite content in the upper 40% of the member.

Assimilation of the country rock is also suggested as a cause of the

change in the magma. Migration of the magma up through the granites,

tartzites and limestones of the area in addition to the possible time

ttervals between eruptions, may have provided the additional elements

44

> enable a second generation of crystallization. Also to be considered

an upwelling of a second magma in the same chamber.

The density and relative porosity curves are shown in Figure 13.

1ey are nearly compliments of each other. Realizing that they are both

~ctions of the degree of welding this is not surprising. Where the

elding is most intense the density will be highest and porosity lowest.

he density curve can be used to show the degree of welding. The lower

ensi ty, more porous base corresponds to the lightly welded zone found

:enerally at the base of ash flows. The highest density and lowest

•orosity correlates with the vitrophyre, or in its absence, the densely

relded zone. The decreasing density and increasing porosity upward

:how a gradual lessening in degree of welding to below that at the

>ase. These zones are the same as noted by Smith (l960b). Ross and

)mith (1961, p. 47) state " .•. maximum welding occurs about one

third of the distance to the top. 11 Indeed the maximum density occurs

~t 28% of the distance to the top in the Wah Wah Springs Tuff. The only

departure from the norm is the slight increase in density at the very

top. This may be explained by the subsequent emplacement of a very hot

ash flow and partial secondary welding of the top of the Wah Wah Springs

Tuff.

45

I

ro

"'

/()

0 .,L...-.,.--...... -...-~'Y-.,..r-• ..--~7 Ar-cutt t«.~t ~ ... ;. :, ~

Figure 13. Best fit curves for density and percent weight gain. Absicissa is the vertical position by percent thickness.

VII. CONCLUSIONS

The Wah Wah Springs Tuff is an identifiable unit both strati­

tphically and petrographically over an area of several thousand

1are miles. No significant lateral variations in mineralogy,

1sity and relative porosity were detected.

Regular vertical variations in mineral concentration are present.

= vertical position of concentration maxima corresponds closely to

e sequence of crystallization in the magma chamber. The variations

e due to a time differential between eruption pulses. They record

anges occurring in the magma chamber or regular mixing from separate

ambers before or during eruption.

Vertical variations also occur in the density and relative

•rosity. These variables are controlled by the degree of welding.

le Wah Wah Springs Tuff has had a simple cooling history showing

tly slight deviation from the curves of previously J?ublished data.

46

47

REFERENCES CITED

Lderson, T. and Flett, J.S., 1903, Report on the eruptions of the Soufriere in St. Vincent in 1902~ and on a visit to Montagne Pelee in Martinique: Part: Royal Soc. [London] Philos. Trans., serA, vol. 200, p. 353-553.

'mstrong, R. L. ~ 1963, Geochronology and geology of the eastern Great Basin: Ph.D. Thesis, Yale Univ •

. ank, H.R., 1959, Geology of the Bull Valley District, Washington County, Utah: Ph.D. Thesis, Univ. of Washington.

>ok, E.F., 1960, Geologic atlas of Utah-Washington County: Utah Geol. and Mineralog. Survey Bull. 70.

1962~ Ignimbrite bibliography and review: Idaho Bureau of Mines -- and Geol. , Information Circular 13, 64p.

1965~ Stratigraphy of Tertiary volcanic rocks in eastern Nevada: Nevada Bur. Mines Rept. ll, 6lp.

__ 1966, Paleovolcanology: Earth-Science Reviews, vol. 1, nos. 2/3, p. 155-174.

1tton, C.E., 1880, Geology of the High Plateaus of Utah: U.S. Geog. -l

and Geol. Survey., Rocky Mtn. Region.

irdley~ A.J., 1951, Structural geology of North America: New York, Harper and Bros., p. 564.

ist, E.H., 1956, Geology of the San Francisco Mountains, western Utah: M.S. Thesis, Univ. of Washington.

1957, Evidence of overthrusting in the San Francisco Mountains, Beaver County, western Utah: (abstract) Geol. Soc. of Amer. Bull., vol. 68.

~nner, C.N., 1920, The Katmai Region, Alaska, and the great eruption of 1912: Jour. Geol., vol. 28, p. 569-606.

1923, The region and mode of empla cement of the grea t tuf f deposit in the Valley of Ten Thousand Smokes: Natl. Geog. Soc. Contra. Tech. Papers, Katmai, no. l, 74p.

1936, Bore-hole investigations in Yellowstone Park: Jour. Geol. vol. 44, p. 225-315.

ilbert, C.M., 1938, Welded t u f f in eastern Calif ornia: Geol. Soc. Amer., vol. 49, p. 1829-1862.

48

ss, H.H., 1949, Chemical composition and optical properties of common clinopyroxenes, Pt. 1: Amer. Mineral., vol. 34, p. 621-666.

tntsberger, D.V., 1961, Elements of statistical inference: Boston, Allyn and Bacon, Inc., 29lp.

B.M., 1968, System/360 Scientific subroutine package (360A-CM-03X), Version III Programer's Manual: In. Bus. Mach. Corp.

Ldings, J.P., Geology of the Yellowstone National Park: U.S. Geol. Sur., Monography 32 pt.2, 893p.

Lcroix, A. , 1904, La Montagne Pelee et sus eruptions: Paris, Masson et Cie, 622p. ·

!Carthy, W.R., 1959, Geology of the Motoqua-Gunlock area, Washington County, Utah: M.S. Thesis, Univ. of Washington.

tckin, J.H., Structural significance of Tertiary volcanic rocks in southwestern Utah: Amer. Jour. Sci., vol. 258, p. 81-131.

1963, Reconnaissance stratigraphy of the Needles Range Formation in southwestern Utah: Guidebook to the Geology of Southwestern Utah, 12th Ann. Conf., I.A.P.G. 1963, p. 71-78.

msfield, G.R. and Ross, C.S., 1935, Welded rhyolitic tuffs in south­eastern Idaho: Amer. Geophys. Union, Ann. Meeting, 16th, p. 308-321.

~shall, P., 1932, Notes of some volcanic rocks of ~he North Island of New Zealand: New Zealand Jour. Sci. Techonology, vil. 13, p. 198-200.

1935, Acid rocks of the Toupo-Rotorua volcanic district: Trans. Royal Soc. New Zealand, vol. 64, p. 323-366.

artin, R.C., 1957, Vertical variations within some eastern Nevada ignimbrites: M.S. Thesis, Univ. of Idaho.

__ 1959, Some field and petrographic features of New Zealand and American ignimbrites: New Zealand Jour: Geol. Geophys., vol. 2, p. 3.94-411.

iller, G.M., 1963, Outline of structural-stratigraphic units of the Wah Wah Mountains, southwest Utah: Guidebook to the Geology of South­western Utah, 12th Ann. Conf. I.A.P.G., 1963, p. 96-102.

uir, I.D., 1955, Transitional optics of some andesines and labradorites: Mineral. Mag., vol. 30, no. 228, p. 545-568.

erret, F.A., 1937, The eruption of Mt. Pelee 1929-1932: Carnegie Inst. Washington Pub. 458, l26p.

~s, C.S. and Smith, R.L., 1961, Ash-flow tuffs: Their origin, geologic relations and identification: U.S. Geol. Sur., Pro. Paper 366, 8lp.

49

:mmons, D.B., 1962, Determination of volcanic and plutonic plagioclases using a three- or four-axis universal stage: Geol. Soc. Amer., Special Paper 69, p. 41.

ith, R.L., l960a, Ash flows: Geol. Soc. Amer. Bull., vol. 71, p. 795-842.

l960b, Zones and zonal variations in welded ash flows: U.S. Geol. Sur., Pro. Paper 354-F, p. 149-159.

reet, R.L., 1952, Geology of the Red Hills area, Iron County, Utah: Ph.D. Thesis, Univ. of Washington, l07p.

chanz, C.M. and Pampeyan, E.H., 1961, Preliminary geologic map of Lincoln County, Nevada: U.S. Geol. Sur. Mineral Inv. Field Studies Map, MF-206, 1:200,000 .•

lnce, J. A. , 1965, Zoning in plagioclase: patchy zoning: Jour. Geol. , vol. 73, p. 636-651 •

. ley, M.A. , 1963, Stratigraphy and structure of the Jackson Mountain­Tobin Wash area, southwest Utah: M.A. Thesis, Univ. of Texas •

. rkel, F., 1876, Microscopical petrography: U.S. Geol. Explor. 40th Parallel Rept., vel. 6, 297p.

PLATE SECTION

.te I. Photomicrograph of embayed quartz crystal. Resorption has continued along cracks and edges in addition to large areas removed. The groundmas s around this grain . is altered. Crossed nicols. X75.

50

.. ..._;

\

I

e II. Photomicrog:r2aph of rzhin Section No. 125 showing shard structure. Note the tricuspate shape and lack of dis­tortion of the shards in the upper center of the photo and to the right of the large hornblende crystal. Plain light. X60.

te III. Photomicrograph of Thin Section No. 126 showing shard structure. In. contrast to No. 125 note the distortion and orientation of the shards in the upper half of the photo, and their being bent around the crystals. Plain light. xao.

51

II

III

ate IV. Photomicrograph of embayed quartz crystal showing crys­tallographic control of resorption. Plain light. X75 .

. ate V. Photomicrograph of alteration in Thin Section No. 17. The biotite and hornblende crystals are completely rimmed by magnetite. The grey areas in the white plagioclase grains are calcite. The groundmass is extensively devitrified. Some remnant shard structure is visible in the lower left hand corner of the photo. Plain light. X75.

52

IV

v

te VI. Pbli;!t;omicrograph: ef p:errtl:t:lc cracks in the glassy ground­ma~s of Thin .Seci:.ion No. 86. . '.f:be groundmass in this we.tdetd, tuff'; $~l?+~' ~-.'9~~J¥ei~~y -~~$~y witl) ':no-sh.~ ·· s-tlt'lt<:it\We·:···:· 'ibre 'light" ~~ ale~t'in the riglllt hand· ~ide of'·th,e photo are holes in the tbi'P section. Plain light. ?G:O;O •

~· :c'

53

VI

54

APPENDIX TABLE I

MINERALOGIC DATA FOR ALL SECTIONS

SHINGLE SPRINGS SECTION

Smp1 % % % % %· % % Hb/ Tot Ave No. Thk Qtz Pf Bio Hb Px Mt Bio Xt1 An%

176 94 0.3 18.0 6.3 9.0 0.1 1.0 1.4 34.7 48.0

179 81 0.8 31.4 3.'0 3.0 0.3 1.0 1.0 39.5 40.3

178 69 1.2 21.5 5.7 6.4 1.3 1.5 1.1 32.6 43.0

177 31 o:3 36.4 4.2 5.9 0.2 0.8 1."4 47.9 48.0

174 13 0.0 25.3 5.7 9.0 0.6 2.0 1.6 42.6 45.0

Mean 0.5 23.3 5.0 6.7 0.5 1.3 1.3 39.5 44.9

Stan. Dev. 0.4 5.1 1.3 2.5 O.l 0.2 0.2 6.1 3., 3

SOUTH EGAN SECTION

Smp1 % % % % % % % Hb/ Tot Ave No. Thk Qtz Pf Bio Hb Px Mt Bio Xt1 An%

128 78 0.5 19.9 1.4 4.0 0.6 1.1 2.8 27.5 44.3

130 61 4.1 23.8 1.0 1.9 0.7 0.6 1.9 32.1 41.0

129 33 1.8 27.7 4.7 8.9 0.5 1.4 1.9 44:.. 5 . 44.7

127 16 1. 8 34.8 2.3 5.8 0.5 1.2 2.5 46.4 45.3

126 12 2.1 25.5 :s. 3 15.2 0.5 1.9 2.9 50.5 43.0

125 6 0.9 20.6 4.4 10.9 0.6 1.5 2.5 38.9 45.7

Mean 1.9 25.4 3.3 7.8 0.6 1.3 2.4 40.0 44.0

Stan. Dev. 1.3 5.5 1.6 4 .. 9 0.3 0.4 0 •. 5 6.3 1.7

55

LUND SECTION

Smp1 % % % % % % % Hb/ Tot Ave No. Thk Qtz Pf Bio Hb Px Mt Bio Xtl An%

19 72 2.3 37.5 3.8 1.4 0.0 1.3 0.4 46.3 44.3

18 35 0.9 28.3 4.3 5.6 0.5 1.7 1.3· 41.3 46.3

17 10 3.8 25.7 5.4 6.7 0.0 4.1 1.2 45.7 44.7

16 7 0.0 27.6 2.0 9.5 0.2 2.7 4.8 42.0 46.3

15 1 0.3 19.6 2.5 4.6 0.1 2.2 1.8 29.4 41.7

· Mean 2.7 27.7 3.6 5.6 0.2 2.4 1.9 40.9 . 4:4~ 7

Stan. Dev. 2.8 6.5 1.4 2.9 0.2 0.3 1.8 6.8 1.9

INDIAN PEAK SECTION

Smp1 % % % % % % % Hb/ Tot Ave No. Thk Qtz Pf Bio Hb Px Mt Bio Xt1 An%

203 98 1.5 25.6 4.8 6.9 0.4 2.0 1.4 41.2 44.0

202 96 0.8 18.7 4.8 6~2 0.0 1.9 1.3 32.4 39.7

201 84 1.0 24.5 4.0 5.4 o.o 1.8 1.3 36.7 44.3

200 68 1.6 36.2 5.5 6.3 0.0 1.7 1.2 51.3 45.0

209 50 1.9 26.4 3.7 4.0 0.2 1.5 1.1 37.7 44.0

206 43 4.4 18.9 4.1 3.9 1.2 1.3 0.9 33.8

207 38 1.4 23.2 3.8 7.1 0.1 1.6 1.9 37.2

210 20 0.0 22.9 2.1 6.2 0.0 1.5 2.9 32.7

211 15 3.0 25.7 3.9 3.4 o.o 1.6 0.9 37.6

Mean 1.7 24.7 4.1 5.5 0.2 1.7 1.5 37.8 43.4

Stan. Dev. 1.3 5.2 1.0 1.4 0.4 0.2 0.4 5.8 2.1

56

WAH WAH SPRINGS SECTION

Smpl % % % % % % % Hb/ Tot Ave No. Thk Qtz Pf Bio Hb Px Mt Bio Xtl An%

90 85 0.8 29.2 4.2 . 4.3 0.6 l.O 1.0 40.1 45.3

85 50 5.2 17.6 5.8 4.2 0.3 2.0 0.7 35.1 45 . 3

88 11 3.9 44.7 2.6 7.6 l.O l.1 2.9 60.9 43.3

87 9 l.l 34.1 4.5 4.8 0.7 1.2 1.1 46.4 45.3

86 5 0.0 20.9 3.2 3.2 0.1 0.8 1.0 28.2 38.0

Mean 2.2 29.3 4.1 4.8 0.5 l.2 1.3 42.1 43 .4.

Stan. Dev. 2.2 10.8 1.2 1.6 0.4 0.1 0.9 12.4 3.2

)hing1e Springs

No. J.76 J.79 J.78 J.77 J.74

TABLE II

DENSITY AND PERCENT WEIGHT GAIN DATA FOR ALL SECTIONS

Density

(average) 2.24 ± 0.6 gm/cc

2.(JO 2.16 2.17 2.42 2.47

South Egan (average) 2.28 ± ·a. 9 gm/cc

No. J.28 1.94 J.30 2.06 J.29 2.54 J.27 2.55 J.26 2.49 J.25 2.09

Lund (average) 2.28 ± 0.1 gm/cc

No. J.9 2.06 18 2.30 J.7 2.37 16 2.40 J.S 2.28

Indian Peak (average)· 2.35 + 0.5 gm/cc

No. 203 2.2J. 202 2.17 201 2.19 200 2.32 209 2.21 206 2.57 207 2.55 210 2.43 21J. 2.52

Wah Wah Springs (average) 2.39 :t 0.1 gm/cc

No. 90 2.22 85 2.46 88 2.55 87 2.29 86 2 . 45

57

Percent weight gain

5.3% ± 3.7%

10.0% 6. 7% 6.8% 0.9% 2.1%

5.0% ± 4.3%

11.0% .8.3% 2.J.% 0.9% 0.8% 7 .1.%

2. 2% + 1.. 7% ' -

5.J.% 1.9% J..2% 0.7% 2.2%

2.9% ± 1.4%

4.4% ~ .. a% ~.8% 2.5% 4.3% 1.3% 1.2% 3.1% l. 7%

3.1% ± 2.3%

6.2% 2.4% 1.1% If.. 7% LO%

58

VITA

Ted Richard Dinkel was born in St. Louis, Missouri on September

, 1944. He received his primary education in St. Louis and Jennings,

ssouri. His secondary education was completed at Fairview High

hool in Jennings, Missouri. The author received his college education

the University of Missouri-Rolla, earning a Bachelor of Science

gree in G_eology on May 28, 1967. In September 1967 he enrolled

a graduate student in the University of Missouri-Rolla. During his

'urse of graduate study he has held a N~tional Science Foundation

'aineeship and has been supported in his research by a V. H. McNutt

!rnorial Foundation grant. He has also been on appointment as a

~aduate assistant in the Geology Department.

In May, 1967 the author was appointed a 2nd Lieutenant in the

1ited States Army Reserve, receiving his commission in the Army

~telligence Service.

On September 3, 1967 the author was married to the former

ancee Denise Sisson of Springfield, Missouri.

He gained industrial experience working for the Eagle-Picher

ompany and Kerr-McGee Corporation during the summers of 1965 .and 1967

espectively.

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