This dissertation has been 63—2551 microfilmed exactly as received
SHULTZ, Charles High, 1936- PETROLOGY OF MT. WASHBURN, YELLOWSTONE NATIONAL PARK, WYOMING.
The Ohio State University, Ph.D., 1962 Geology
University Microfilms, Inc., Ann Arbor, Michigan PETROLOGY OF MT. WASHBURN, YELLCWSTONE NATIONAL PARK, WYOMING
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University
By
aiARLES HIGH SHULTZ, B.S
The Ohio State University 196 2
Approved by
Adviser Department of Geology CONTENTS Page Chapter 1. Introduction ...... 1 General Statement ...... I Location ...... I Geomorphology and Climate ...... 2 Topography ...... 2 Drainage ...... 4 Glacial Geology ...... 3 Climate ...... 6 Source of D a t a ...... 6 Field Mapping ...... 6 Laboratory ...... 7 Acknowledgments...... 7 Chapter II, Regional Geology and Summary of Previous W o r k ...... 10 Regional Tectonics ...... 10 Yellowstone National Park ...... 10 Beartooth Mountains ...... 12 Gallatin R a n g e ...... 14 Snake River Basin ...... 14 Teton Mountains and V i c i n i t y ...... 15 Absaroka R a n g e ...... 17 Mt, Washburn and the Washburn R a n g e ...... 21 Recent Geologic Activity in the Mt, Washburn A r e a ...... 24 Hot Springs ...... 24 Seismic Activity ...... 28 Terminology and Classification of Fragmental Volcanic Rocks ...... 29
Chapter III. Early Acid Breccia ...... 33 Introduction ...... 33 Pétrographie mineralogy of the Volcanic Rocks , 33 Plagioclase ...... 33 S a n i d i n e ...... 33 Hornblende...... 37 Orthopyroxene ...... 41 Clinopyroxene ...... 42 Other Primary Minerals ...... 42 Secondary Minerals ...... 43 Sedimentary Rocks and Associated Breccias . . . 43 General Statement ...... 43 Exposures in Carnelian Creek Valley .... 44 ' Exposures Southwest of Mt. Washburn .... 47 Interpretation...... 56 Welded Tuff ...... 57
13. Page Chapter III. Early Acid Breccia - continued Lava Flows and Associated B r e c c i a s ...... 60 Field Occurrence...... 60 Megascopic Description and Petrography . . . 63 Lava F l o w s ...... 63 B r e c c i a ...... 69 Breccia Dikes ...... 70 Minéralogie and Textural Associations . . . 74 Modal Analysis ...... 76 Source of the Volcanic R o c k s ...... 77 C o n c l u s i o n ...... 79 Chapter IV. Early Basic Breccia - Nonfragmental R o c k s ...... 80 Introduction ...... 80 Pétrographie Mineralogy ...... 80 Plagioclase ...... 80 Olivine ...... 84 Orthopyroxene ...... 84 Clinopyroxene ...... 88 H o r n b l e n d e...... 88 Other Primary Minerals ...... 91 G round ma s s ...... 91 Secondary Minerals ...... 92 Basalt Dikes ...... 96 Composite Dike ...... 103 Field Occurrence and Megascopic Description. 103 Petrography of the Border Phase ...... 105 Petrography of the Central Phase ...... 108 Comparison of the Border and Central Phase . 110 Petrogenesis ...... 113 Quartz-Diorite Intrusive ...... 114 F l o w s ...... 117 Introduction ...... 117 Normal Channel Flows ...... 118 Irregular Bodies of Nonfragmental Rocks . . 127 Mixed Flow-Fragmental R o c k s ...... 130 Fragments Without Nonfragmental Equivalents 132
Chapter V, Early Basic Breccia - Fragmental Rocks 137 Introduction ...... 137 Field Occurrence ...... 138 Megascopic Description and Petrography .... 147 Monolithologic Breccia ...... 147 Megascopic Description ...... 147 Petrography...... 148 Heterolithologic Breccia - Poor Bedding and S o r t i n g ...... 149 Megascopic Description ...... 149
111 Page Chapter V. Early Basic Breccia - Fragmental Rocks continued Megascopic Description and Petrography - continued Heterolithologic Breccia - Some Bedding and S o r t i n g ...... 152 Megascopic Description ...... 152 Petrography...... 153 Tuffaceous Rocks ...... 156 Megascopic Description ...... 156 Petrography...... 157 Origin of Breccia ...... 160 Epigene Processes Modifying Primary Breccias , 167
Chapter VI, Volcanic Rocks of Uncertain Age . . . 169 Introduction ...... 169 Hornblende Andésite D i k e s ...... 169 Lava Flows on Observation P e a k ...... 174
Chapter VII. Chemical Analyses ...... 177 Introduction ...... 177 Normative Analyses ...... 177 Discussion of the New Chemical Analyses .... 179
Chapter VIII. Petrogenesis ...... 182 Character and Origin of the M a g m a ...... 182 Cyclic Behavior of the Eruptions ...... 187 Mineral Petrogenesis and the Nature of the Magma 188 Introduction...... 188 Plagioclase ...... 188 Hornblende...... 189 Nucléation and Paragenesis ...... 190 C o n c l u s i o n ...... 194
Chapter IX. Plateau Rocks ...... 196 Introduction ...... 196 Exposures North of Mt, Washburn ...... 197 Field Occurrence...... 197 Megascopic Description ...... 199 Petrography ...... 199 Exposures South of Mt, Washburn ...... 201 Field Occurrence...... 201 Megascopic Description ...... 202 Petrography ...... 202 O r i g i n ...... 203
Chapter X. Structure ...... 206
Chapter XI, Source of the Early Basic Volcanic R o c k s ...... 214
IV Page
Chapter XII. Tertiary Geologic History of the Mt. Washburn Region ...... 218
References Cited ...... 221 Autobiography ...... 226 ILLUSTRATIONS
Figure Page
1 Location m a p ...... 3
2 A spatter cone developed around a thermal s p r i n g ...... 27
3 Mud pots containing thick pitch-black mud , 27
4 A general view of Washburn Hot Springs . . 27
5 Typical euhedral plagioclase phenocryst . . 36
6 Slightly oxidized hornblende phenocryst showing typical “pyroxenic” opacité .... 36
7 Hornblende phenocryst with a partially oxidized border ...... 40
8 Oxyhornblende largely replaced by black type o p a c i t é ...... 40
9 Hornblende entirely replaced by black type o p a c i t é ...... 40
10 Typical Early acid sedimentary rocks • . . 48 11 Photograph of a semi-polished slab of Early acid conglomeratic sandstone ...... 49
12 Examples of carbonized plant remains and a coalified l o g ...... 50
13 Photomicrograph of a typical Early acid sandstone...... 51
14 Representative samples of fossil plant materials...... 54
15 Photomicrograph of Early acid welded tuff . 59
16 Typical felty-microporphyritic texture of Early acid andésite la v a s ...... 65
17 Extreme alteration of Early acid andésite l a v a ...... 67
VI Figure Pag© 18 Same as Figure 17, but taken with crossed n i ç o i s ...... 19 Highly oxidized Early acid andésite breccia fragment ...... 67 20 Photograph of a semi-polished specimen of an Early acid heterolithologic breccia dike . 72
21 Photomicrograph of the central portion of a breccia d i k e ...... 72 22 Photomicrograph of the fine-grained border of a breccia dike ...... 72 23 A photomicrograph of an Early acid breccia d i k e ...... 75
24 Same as Figure 23, but with crossed nicols 75 25 Plagioclase phenocryst showing an unusual type of twinning characteristic of square g r a i n s ...... 82 26 Common type of complex plagioclase p h e n o c r y s t ...... 82
27 Large euhedral plagioclase phenocryst showing alteration ...... 83
28 The same plagioclase phenocryst as above (fig. 27) but with crossed nicols ...... 83
29 Small euhedral phenocryst of olivine partially altered to bowlingite ...... 85
30 Somewhat irregular, rounded olivine phenocrysts showing minor alteration to b o w l i n g i t e ...... 85
31 Partly resorbed olivine, nearly at extinction 86
32 Nearly euhedral olivine phenocryst entirely altered to fibrous bowlingite ...... 86
33 Orthopyroxene, nearly at extinction, show ing complete mantling by polysynthetically twinned diopside ...... 87
Vll Figure Page
34 Euhedral diopside phenocryst showing 4 or 3 prominent compositional zones ...... 89
35 Typical round, compact, g1omeroporphyritic aggregate of diopsidic augite ...... 89 36 An example of extreme resorption of an oxyhornblende phenocryst with a black type opacité border ...... 90
37 Incipient clinopyroxene needles closely associated with tiny grains of magnetite. . 92 38 A typical cavity coating of botryoidal zeolites and poorly crystallized mont- morillonite...... 94
39 Same as above (fig. 38) but with crossed n i c o l s ...... 94
40 A typical cavity filling of fibrous bowling ite with separate aggregates of zeolites and a center of opal ...... 95
41 Same as above (fig, 40) but with crossed n i c o l s ...... 95
42 Epidote developed from oxyhornblende opacité and plagioclase crystals ...... 97 43 Basalt dike standing out in relief shown cutting a lava f l o w ...... 98
44 A typical basalt dike showing horizontal columnar jointing ...... 98
45 A typical microporphyritic texture with an intersertal groundmass...... 100
46 A typical microporphyritic texture with an intergranular groundmass ...... 100 47 Photomicrograph of the autobrecciated portion of the composite d i k e ...... 104
48 Photomicrograph of the fine-grained, micro porphyritic border phase of the composite d i k e ...... 106
Vlll Figure Page
49 An example of the peculiar minéralogie relationships found in the border phase of the composite d i k e ...... 107 30 Central phase of the composite dike showing felty microporphyritic texture of the main rock and endogenic i n c l u s i o n ...... 109
51 Photomicrograph of the quartz diorite intrusive...... 116
52 Photomicrograph of diopside showing inversion to a green amphibole ...... 116
53 Series of field photographs depicting the common variations in the form and occurrence of lava f l o w ...... 120
54 Photomicrograph showing the micropor phyritic -felty texture of flows containing h o r n b l e n d e ...... 123
55 Photomicrograph of the unusual nearly trachytic texture of the light-red lava S o w s ...... 123
56 Photograph of a semi-polished hand specimen showing black flow bands ...... 124 57 Photomicrograph of the specimen pictured above (fig. 5 6 ) ...... 124
58 Outcrop of a contorted and discolored mixed lava and fragmental f l o w ...... 131
59 Photograph of a semi-polished hand specimen of the same type of flow rock pictured above (fig. 5 8 ) ...... 131
60 Photomicrograph of the strongly banded lava f l o w s ...... 133
61 Photomicrograph showing distinct lithic fragments that occur in flow bands .... 133
62 Photomicrograph of a typical vitrophyric fragment ...... 135
63 Same as above (fig, 62) but with crossed n i c o l s ...... 135
IX Figure Page
64 View along the west cliffs of Mt, Washburn 139
65 An outstanding example of bedding that results from two thin lava f l o w s ...... 139
66 Bedding in heterolithologic volcanic b r e c c i a s ...... 140
67 Well-developed stratification in volcanic b r e c c i a ...... 140
68 Well-bedded coarse tuff and volcanic breccia cut by a vertical basalt dike . . . 141 69 View along the west cliffs of M t , Washburn, looking south ...... 142
70 View of the first peak east of Mt. Washburn summit, looking north ...... 142 71 A light-colored alluvial cut-and-fill structure within a brown, massive, unsorted heterolithologic breccia ...... 143
72 Closer view of the channel filling above (fig. 7 1 ) ...... 143 73 Channel-shaped body of water-laid volcanic breccia ...... 144
74 A coarse volcanic breccia that probably originated as a lahar ...... , , 144
75 A chalcedony vein filling showing layers of botryoidal chalcedony along the border , 146
76 Typical massive brown heterolithologic b r e c c i a ...... 150 77 Extremely coarse, unsorted, chaotic breccia completely lacking any indication of stratification ...... 150
78 Photograph of a semi-polished slab of heterolithologic volcanic breccia ...... 154 79 Heterolithologic breccia showing fine grained tuff concentrations...... 154 Figure Page 80 Well-stratified lapilli tuff containing accretionary lapilli balls ...... 158 81 Photomicrograph of a single accretionary lapilli b a l l ...... 158 82 Photomicrograph of tuff showing graded b e d d i n g ...... 159
83 Photomicrograph showing an excellent example of heterolithologic lapilli tuff. . 159
84 An excellent example of autobrecciation of a flow-like b o d y ...... 165
85 Photograph showing the typical porphyritic texture of the hornblende andésite dikes, . 171 86 Photograph showing the porphyritic texture of a dike containing a hornblende- plagioclase segregation ...... 172
87 Photomicrograph of the hornblende andésite of figure 86 showing porphyritic-felty te x t u r e ...... 172
88 Epidotized trachybasalt from Observation P e a k ...... 175 89 A view from the east wall of Carnelian Creek Canyon looking northwest ...... 198
90 Photomicrograph of nonwelded vitric rhyolitic t u f f ...... 201
91 A flow composed of devitrified rhyolitic g l a s s ...... 204
XI Plate Page
I Topographic map of Mount Washburn and the Washburn R a n g e ...... (pocket)
II Geologic map of Mount W a s h b u r n ...... (pocket)
III Generalized structural geologic map, Mount Washburn and vicinity ...... (pocket)
IV Hypothetical cross section of Mount Washburn and the Washburn Range ...... (pocket)
Table 1 Classification of volcanic breccias .... 32
2 Mineral suites of the Early acid lava flows 76
3 Typical modal analyses of Early acid lavas 77
4 Representative modal analysis of Early basic breccia basalt dikes ...... 102 Comparison of the modal analyses of the border and central phases of the composite d i k e ...... Ill
Comparison of various pétrographie properties of the border and central phases of the composite d i k e ...... 112
Typical modal analysis of the quartz diorite intrusive ...... 117
8 Representative modal analysis of the normal channel flows ...... 125
Common mineral associations of Early basic breccia channel flows 126
10 Representative modal analysis of the hornblende andésite dikes ...... 173
11 Chemical and normative analyses of Early acid breccia and Early basic breccia . . . (pocket)
12 Total chemical analyses, and modal analyses of phenocrysts recalculated to 100 percent of samples No,3 and No.4, Table 11 .... 180
Xll CHAPTER I. INTRODUCTION
General Statement
Mt, Washburn is a very prominent peak in the north central part of Yellowstone National Park and is a com ponent of a small horseshoe-shaped range of mountains known as the Washburn Range. From the summit there is a spectacular view, which includes the entire park and the surrounding mountain ranges to a distance of 120 miles.
The mountain was named for Henry Dana Washburn who was a
Civil War general, Surveyor General of Montana, and the man who gave Old Faithful geyser its name in 1870, It is composed of Middle Eocene volcanic breccias and tuffs with minor dikes and lava flows and is part of the Absaroka volcanic field; but the mountain is almost entirely isolated from this field by rhyolitic welded tuffs of the Yellowstone
Plateau.
The purpose of this investigation was to determine the mode of origin of the breccias, the source of the breccias and lavas, the evolution of the magmas, and the cause of the prominence and isolation of Mt. Washburn and the Wash burn Range,
Location
Yellowstone National Park is located in the northwest corner of Wyoming and includes small parts of Montana œd
Idaho (fig. 1). Mt. Washburn is located approximately in 2 the north-central part of the park at 44° 48* north latitude and 110° 26* west longitude. The area included in the geo logic field map lies within 44° 45* to 44° 51* north latitude and 110° 23* to 110° 30* west longitude, and is on the Tower
Junction, Wyoming topographic quadrangle (U. S. Geol. Survey,
1959). The physiographic boundaries of the map area are the
Grand Canyon of the Yellowstone River on the east, Tower
Creek Canyon on the north and northwest, and the plains of the Yellowstone Plateau on the south. No natural physio graphic boundary exists on the west, so longitude 110° 30* west was arbitrarily chosen. The Washburn Range, in which only reconnaissance work was done, lies directly to the west and northwest of Mt. Washburn (Plate I).
Geomorphology and Climate Topography
The Washburn Range, rising to elevations between 9,000 and 10,000 feet, is a horseshoe-shaped series of rugged, well-dissected peaks and ridges with heavily forested slopes. The horseshoe is open to the northeast and is bounded by two northeast-oriented arms which are connected in the west by a narrow north-south ridge. The arms have fairly steep southern faces and more gentle northern slopes characterized by broad, nearly parallel ridges. The north- south connecting ridge has a steep eastern face and a gentle, ridgeless western slope. The most important part of the southern arm and the highest mountain in the range is Mt. M O
IDAHO WYOMIN9
UTAH COLOm ADO
FIGURE I. LOCATION MAP
Area of Geologic Map MONTANA T WYOMING
f0IHr. WASHBURN
o
Y«Ho»»ton* Lek#
/ NEC / MOUNTAINS
PINYON PEAK HIOHLAMO w Washbunif elevation 10,243 feet. The mountains are sur rounded by a very youthful plateau with an average elevation of 8,000 to 8,200 feet. The plateau is dissected by deep and narrow canyons, the most famous of which is the Grand
Canyon of the Yellowstone, The maximum relief in the map area is approximately
3,800 feet between the base of the Grand Canyon and the summit of Mt, Washburn, a distance of 4 miles. More usual relief is about 1,800 to 2,000 feet.
Drainage
The master stream in the area is the Yellowstone River.
It flows north, skirting the eastern edge of the Washburn
Range in a great concave arc. The interior of the Washburn
Range, commonly known as the Washburn Amphitheater, is drained by Tower Creek and its tributaries, which joins
Yellowstone in the north at Tower Falls, The northern and western exterior of the range is drained by tributaries of the Yellowstone and Gardiner Rivers, such as Lava Creek and
Black Tail Deer Creek, The southern section is drained by tributaries of both the Madison and the Yellowstone Rivers, In the southwestern corner of the range are three small lakes, the largest of which is Grebe Lake with a width slightly larger than one-half mile. 5
Glacial Geology
The Washbum Range has had an Involved and interesting glacial history that has been very well summarized by Howard (1937). The major Ice stream, the Lamar Glacier, which originated In the Absaroka Range to the east of the park and partially In the Beartooth Range to the north of the park, Inundated the Washburn Range but probably never completely burled It.
Evidence of glaciation Is abundant everywhere. The plain north of Mt. Washburn Is covered with glacial drift consisting predominantly of volcanic rock, but with abundant granite gneiss and limestone erratics Indicating that some of the Ice originated In the Beartooths. This plain shows many other evidences of glaciation such as abandoned stream channels, hummocky topography, and topographic depressions, which often contain small lakes or marshes. The largest depression Is about one mile long with a closure of nearly
60 feet, and contains three small lakes surrounded by grassy meadows. Other bodies of glacial drift, along with glacial scratches, grooves, and polish, occur throughout the range, and glacial erratics are fairly common everywhere except at the highest elevations, where they are lacking. Mt. Wash b u m itself was a local source of Ice as evidenced by four well-developed cirques on the north face of Mt, Washburn and along the eastern spur of the mountain. 6
Climate Yellowstone National Park is characterized by long severe winters and short cool summers, which considerably restrict the field season. Snow may be expected to fall any month of the year. As an example of the difficulties sometimes encountered, on August 16-17, 1960, Mt. Washburn was buried under 13 inches of snow with drifts up to 36 inches. Maximum summer temperatures are generally in the seventies and winter temperatures sometimes fall to 30 to
40 degrees below zero. Thirty feet of snow is not unusual in winter.
Source of Data
Field Mapping
The writer spent two summer field seasons (approxi mately 4% months) mapping and cdlecting specimens of rocks, fossils, and minerals. The geology was plotted on enlarged portions of U. S. Geological Survey topographic map proofs supplied to the writer by the Yellowstone Park staff. The Survey was in the process of a complete remapping program in the park at the time of this investigation and the completed topographic maps at a scale of 1:62,500 are now available.
Portions of the Tower Junction and Mammouth, Wyo. -Mont. sheets, which include the entire Washburn Range, are re produced in Plate I for the convenience of the reader.
The precise location of field stations was accomplished by the use of standard aerial photographs. 7
Laboratory
Approximately 600 hours were spent in the laboratory examining materials collected in the field. The materials studied included 120 thin sections, 14 semi-polished rock slabs, and hundreds of rock and mineral specimens, includ ing plant fossils. Examinations were made by means of a pétrographie microscope supplied with a mechanical stage, a 5-axis universal stage, and a binocular microscope. Al though the microscopic work chiefly involved thin sections, a few minerals were investigated with the use of index of refraction oils. Modal analyses of thin sections were made according to Chayes* technique, i.e., using a mechan ical click stage to count 1400 points on a standard grid covering a 22 x 22 mm, thin section.
Pétrographie examinations were supplemented by two chemical analyses made at the rock analysis laboratory.
College of Mineral Industries, Pennsylvania State Uni versity, through a grant from the Friends of Orton Hall,
Department of Geology, The Ohio State University. In addition, several alteration minerals were investigated, using the X-ray powder diffraction technique. Special thanks are extended to Dr. R. T. Tettenhorst of the De partment of Mineralogy, The Ohio State University, for his help in this work. 8
Acknowledgments This study Is part of a larger project that has as its goal a better understanding of the origin and evolu tion of the Absaroka IRange. The project is under the direction of Professor Willard H. Parsons, Chairman of the Department of Geology, Wayne State University, and is being financed by National Science Foundation grants.
Professor Parsons in turn helped to finance the present study by supplying field expense money and aerial photo graphs. Other financial assistance for field and labora tory work was obtained through a fellowship from The Ohio
State University, the Texaco Fund administered by the
Department of Geology, and the Friends of Orton Hall Fund, which is supported by alumni of the Department of Geology.
Professor Carl A. Lamey was the writer's adviser at The Ohio State University, He has eai-ned unlimited grati tude and deserves extraordinary recognition for the great number of patient hours spent with the writer in all aspects of the work, including a field check. Professors
E, G, Ehlers and R. T. Tettenhorst of the Department of
Mineralogy spent many hours assisting the writer with minéralogie problems and with X-ray analyses. The writer would like to thank the Yellowstone-Bighorn Research
Association for the many courtesies extended to him while in their camp at Red Lodge, Montana, and also Dr, Richard
D. Krushensky who originally suggested the problem and who 9 counseled the writer in many problems of volcanic geology.
Dr. Erling Dorf of Princeton University kindly identified the fossil plant collection, and Mr. Jack Henderson served as a very able field assistant during the 1939 field season.
The writer is especially grateful to the staff of Yellow stone National Park who made the two summers spent in the park very enjoyable indeed. CHAPTER II. REGIONAL GEOLOGY AND SUMMARY OF PREVIOUS WORK
Regional Tectonics
The Yellowstone-Absaroka volcanic region is located
In the Central Rocky Mountains. It Is situated on the
continental platform east of the main Laramide fold and
thrust belt. The fundamental sti-ucture of the area Is
characterized by vertical basement uplifts with inter
vening basins containing thick accumulations of sediments.
Paleozoic strata are typically thin continental shelf
sediments dominated by carbonates, but Mesozoic and
Cenozoic rocks are predominantly clastic and commonly quite thick. Tertiary volcanics were deposited upon
this complex structure during the waning stages of the
Laramide Orogeny and "... have been localized by three
types of tectonic features: (1) axes of depressions, (2) axes of old uplifts, and (3) major structural linea ments or basement rifts*' (Paisons, 1958, p. 36).
Yellowstone National Park
Yellowstone National Park, an area of about 3,000
square miles, is a rugged, heavily forested volcanic plateau approximately 8,000 feet in elevation with local relief up to 2,000 feet. The park is world famous for its thermal springs and geysers, and the abundance of wildlife. It is surrounded on three sides by a ring of
10 11 high mountainB which attain elevations of 10,000 to
13,000 feet. On the west, the plateau drops 1,000 feet to the Snake Hiver plain.
This large plateau is divided into a series of smaller plateaus and basins by deep, narrow canyons that are controlled by geologic units, faults, mountain masses, and thermal spring areas. The most important plateaus and their location within the park are the following!
Mirror Plateau, northeast; Madison Plateau, west cen tral ; Pitchstone Plateau, southwest; Central Plateau, central; and Two Ocean Plateau, southeast. The largest basin lies between the Absaroka Mountains on the east and the Pitchstone and Central Plateaus on the west. This basin contains the waters of Yellowstone Lake. The
Yellowstone River valley south of Yellowstone Lake, the
Grand Canyon of the Yellowstone, the Madison Canyon in the west, the Snake River Canyon in the south, and the
Lamar Valley in the northeast comprise the most prominent canyons.
Two mountain masses within the park project abruptly above the general level of the plateau, Mt. Washbum
(the object of the present study) is in the north central section of the park, and the Red Mountains are in the south central portion. The latter have been elevated along fault planes and are considered to be a possible vent for some of the rhyolitic rocks that underlie the plateau. 12
The most recent work concerning the geology of Yellowstone Park is an excellent report by Boyd (1961).
He has found that the rhyolite rocks, which form the
plateau, are composed of flows and welded tuffs with
subsidiary rhyolite domes, basalt, and rhyolite-basalt
mix-lavas. The structural history, chiefly block fault
ing, has been worked out in detail and the volcanic rocks have been divided into units (see Chapter IX in this report).
The entire plateau may be a huge volcano-tectonic depression
(caldera) and comparisons have been made between Yellow
stone Park and the Lake Toba area in Sumatra (Boyd, 1961,
p. 412). Hamilton (1959) considers that the rhyolites may actually represent the upper crust (salic differentiate)
of an extrusive lopolith and has compared the park rhyolite
to the "red rocks" of the Bushveld, Sudbury, and Wichita lopoliths.
Beartooth Mountains
The Beartooth Mountains, which rise to elevations of
12,000 feet and more, border Yellowstone Park on the north and northeast. This range is a structural block of the earth*s crust about 80 miles long and 35 to 40 miles wide,
trending northwest. It is bounded on all sides except the
southeast by high-angle thrust or normal faults, which
occasionally are broken by tear faults (Foose, et al,,
1961, p. 1149). The southeast border is obscured by
Absaroka volcanic rocks that overlap the block and partially 13 fill the Cooke City zone. This zone is a faulted and depressed region near the middle of the Beartooth block, and is oriented northwest-southeast parallel to the elong ation of the range. The block is topographically highest and has the greatest structural relief along the east and northeast edges, and it slopes southwest toward the park. The mountains are composed of a Frecambrian granitic gneiss core flanked by migmatites, amphibolites, and metasediments such as quartzites, iron formation, and biotite schist representing an Archean sedimentary sequence
(Eckelman and Poldervaart, 1957), Intruding the Precam- brian core are igneous rocks of various ages such as metagabbro, ultramafics, pegmatite, metabasalt, and felsic porphyries. Along the north border of the block is the famous Stillwater complex, interpreted as a tilted remnant of a differentiated Precambrian lopolith (Hess,
1960), Originally this block was covered with approxi mately two miles of Paleozoic and Mesozoic sediments.
These now occur draped around the borders and in a few scattered remnants on the uplifted mass (Foose eT, 1961),
Although the internal structures of the Beartooths were created in Precambrian time, the present deformation began in the late Cretaceous and was in full force by the early Eocene,
Detailed geologic information on the Beartooth
Mountains is abundant. Two works in particular should be 14 referred to for detailed etratigraphic and tectonic infor mation: a three-part work entitled Geologic Evolution of the Beartooth Mountains. Montana and Wyoming by Eckelman and Poldervaart (1957), Spencer (1959), and Harris (1959); and Structural Geology of the Beartooth Mountains, Montana and Wyoming by Foose, Wise, and Garbarini (1961).
Gallatin Range
West of the Beartooth Mountains across the valley of the north flowing Yellowstone River and in the northwest corner of the park is the Gallatin Range. The general elevation of this range is 10,000 to 10,500 feet, the highest peaks being Electric Peak in the north and Mt,
Holmes in the south. This wedge-shaped block is elevated along north-south normal faults, and the major structure within the range is a synclinal trough with flanks of Pre cambrian gneiss and a core of Cretaceous shale and sand stone (Iddings and Weed, 1899). These sedimentary rocks are intruded by sheet-like and laccolithic masses of basic igneous rocks and arc partially buried by Tertiary volcanic breccias similar to the Absaroka volcanic rocks. The flanks of the range are buried beneath rhyolitic welded tuff of the Yellowstone Plateau.
Snake River Basin
The Snake River Basin is south of the Gallatin Range across the Madison River valley and represents the only 15 section of the park not surrounded by high mountains. The basin slopes southwestward from the Yellowstone region into south-central Idaho and cuts across the Laramide fold and thrust belt almost at right angles. The basin is approximately 40 to 70 miles wide and about 1,000 feet below the general level of Yellowstone Plateau. It is composed of rhyolitic lavas and welded tuff, and olivine basalt, all of which are the same as the rocks in the park.
Within the basin along the western edge of Yellowstone Park is Island Park Caldera. This structure is approximately
18 miles in diamter with a rim of rhyolitic welded tuffs and an interior filled with basalt flows (Hamilton, 1960, p. 102). The eastern half of the basin is considered to be a syncline or downwarp, but the western half has been shown to be a graben bounded by major normal faults (p. 104),
Teton Mountains and Vicinity
The Teton Mountains, considered to be the most spec tacular mountains in the United States, are south of the
Snake River basin and form the southwestern portion of the mountainous wall around Yellowstone Park, They are pri marily a fault block mountain that slopes west, with a precipitous eastern face bounded by a high angle normal fault. The exposed core Is a complex of Precambrian metasediments and Igneous intrusions that are overlain by
8,000 to 10,000 feet of predominantly marine Paleozoic and 16 Mesozoic sedimentary rocks (Edmund, 1956, p, LSI). Over- lying the sediments are Absaroka volcanic rocks such as volcanic breccia, tuff, and andésite flows that are in turn overlain by Yellowstone rhyolitic flows and olivine basalts. To the north the range merges with the Pitch- stone Plateau, a portion of the Yellowstone Plateau.
Some writers (Hamilton, 1960, p. 92) believe that there is a structural connection between the Teton Mountains to the south and the Madison Range (the next range west of the Gallatin Range) to the north, which is buried by the rocks of the Snake River Basin. This buried structural ridge may have acted as a dam that was overflowed from the east toward the west by the viscous rhyolites extruded in the park. East of the Teton Mountains lies Jackson Hole, the
Pinyon Peak Highlands that are the northern foothills of the Wind River Range, and finally the Absaroka Mountains enclosing the entire eastern half of the park. Jackson
Hole, with its surface cover of Pleistocene sediments, is the tectonic basin on the down-thrown east side of the
Teton fault. The northwest-oriented Wind River Range is another uplifted crustal block with a Precambrian core flanked by Paleozoic and Mesozoic sedimentary rocks. It is represented in the north (Pinyon Peak Highlands) pri marily by faulted Mesozoic strata which protrude above the lavas of the plateau (Hague, 1912, p. 4), Chicken Ridge, 17 located approximately 10 miles east of the Hed Mountains within the park boundaries, is the northernmost repre sentative of the Wind River Range,
Absaroka Range The final link in the mountainous rim around Yellow stone Park is the Absaroka Range^ which encloses the entire eastern border with peaks up to 13,000 feet. This range is quite unlike any other range in the Central Rocky
Mountains in that it is a rugged, dissected Tertiary volcanic plateau composed of nearly horizontal andesltic and basaltic breccia, tuff, and flows. These volcanic rocks, oriented in a northwesterly direction, stretch from the Owl Creek and Washakie Ranges in the south to the Bear tooth Mountains in the north, Absaroka volcanics merge into the Big Horn Basin, which lies to the east. The range is approximately 140 miles long and 60 miles wide at its greatest width.
A great deal of geologic research has been done in the past and is currently being accomplished in the Absaroka
Range, a classic area of Tertiaiy volcanic geology, Hayden and his contemporaries did much extensive reconnaissance work in the Absarokas and Yellowstone Park early in the
1870*s , which was published in U. S. Geological & Geo graphical Survey Annual Reports (1872, 1873, 1883).
Early surveys culminated in the classic U. S. Geological
Survey Monograph 32 by Arnold Hague et a^, (1899), Modern 18 work has been done by House (1936, 1937, 1940), Love (1939), and Parsons (1939), Recently (1953 to the present). Parsons has been directing an extensive research program concerning
Absaroka volcanics, especially in the northern part of the range, which proposes to achieve a better understanding of the origin of breccias, sources of the volcanics, and his tory of the range. Some papers have already resulted from this ambitious program (Parsons, 1958, 1960), and a few masters and doctoral theses (e.g., Krushensky, 1960;
Gliozzi, 1959). This report concerns one of the six critical areas chosen by Parsons for detailed study. The extrusive volcanic materials have been divided into six major stratigraphie units (Hague, 1899a) as followsi
Late basalt sheets
Late basic breccia
Late acid breccia
Early basalt sheets
Early basic breccia
Early acid breccia
The Early acid breccias consist of silts, muds, tuffs, agglomerate, and volcanic breccias that vary in composition from hornblende andésite to hornblende-mica andésite trend ing toward dacite. In places, angular fragments of gneisses and schists are included within the formation. Recent work has demonstrated the presence of clastic sedimentary rocks such as volcanic sandstones interbedded with conglomerates 19 near the base of the formation (Krushensky, I960, p. 42). The Early basic breccias are composed of dark-coloved volcanic breccias with occasionally Interbedded flows of basalt. The formation Is very thick and extensive, In marked contrast to the underlying Early acid breccia.
The composition of the volcanic material Is hornblende- pyroxene andeslte or pyroxene andeslte along with minor basalt.
The Early basalt sheets lie upon the Early basic breccia and attain a thickness of 1,500 feet near vents.
Locally the flows have vesicular tops and commonly are
Interbedded with thin layers of tuff and fine breccias. The rocks are very fine grained, but usually contain phenocrysts such as auglte, olivine, or plagloclase.
The basalt or sometimes andeslte-basalt tends to be alkali rich and may contain leuclte or orthoclase in the groundmass. Iddings (1899, p. 328) gave the name absaroklte to basaltic rocks containing auglte and olivine phenocrysts with abundant orthoclase and leuclte In the groundmass.
The Late series of volcanics Is very similar to the
Early series (I.e., lower acid Intermediate breccias such as dacite and andeslte trending upward toward andesltlc- basalt breccias and culminating In alkali basalt flows) and since it Is concentrated In the southern part of the
Absaroka Eange, will not be reviewed here. The whole sequence of volcanic rocks Is 6,500 feet thick In the 20 center of the range and several hundred feet near the periphery (House, 1937, p. 1262). Thickness of individual units varies considerably due to the erosional relief of the surface on which the rocks were deposited. The relief of the pre-volcanic topography was 1,000 to 2,000 feet (Parsons, 1958, p. 39),
The volcanic rocks were extruded from many widely scattered vents and fissures. Most of these probably have not been recognized but a few of the larger centers have been defined and studied. These include Grande11 Volcano in the Grande11 Basin, the White Mountain Volcano located in the Sunlight Basin, the Sunlight Volcano located near the headwaters of Sunlight Greek, and the Washakie Needles near the southeast end of the Absaroka Hange. In addition to extrusive centers there are many intrusive centers such as the Ishowooa-Deer Greek area in the southern part of the range and Electric Peak in the northern part of
Yellowstone Park. The mines located near Gooke Gity,
Montana, and in the Sunlight Basin, Wyoming, are areas of minor sulfide mineralization.
The Absaroka volcanic cycle occurred during the Eocene
Epoch. This dating is based on a study of vertebrate fossils by Glenn L, Jepson and of plant fossils by Erling Dorf (1960) and F, H, Knowlton (1899). Dating is also made possible by correlating the volcanics with sedimentary rocks of known age in adjacent Tertiary basins such as the 21
Bighorn Basin. Correlations are made on the basis of the appearance of heavy minerals of volcanic origin and tuff beds in the sedimentary rocks. This work has been done by Love (1939) and Hay (1954),
The stratigraphie terms Early acid, Early basic, and mo forth, are not completely satisfactory because the names themselves are misleading, i.e., the rocks are never acid and often are neither basic nor breccia. Krushensky
(1960) has attempted to remedy these difficulties by pro posing and defining formational names to replace Hague's inadequate nomenclature. However, the older stratigraphie terminology will be adopted in this report because it is so deeply entrenched in the literature. In addition, the correlation of Krushensky*s formations with the Mt.
Washburn area, which lies 25 miles to the west of the type sections, would be difficult and probably impossible since it is unlikely that physical continuity ever ex isted between stratigraphie units in the two areas.
Mt. Washburn and the Washburn Range
Detailed geologic studies of Mt. Washburn have never been made even though it is generally considered to be a major volcanic source. Cursory observations were made by members of the Hayden Survey and some time later by Hague.
The published conclusions of these early workers have been quoted to the present time, propagating many misconceptions about the range. 22
The first report on the geology and origin of Mt. Washburn was by Holmes (1883, pp. 32-35) who spent a few days examining the mountain near the end of the 1882 field season. Holmes said that the range was composed of stratified eruptive rocks that dip at various angles and have a generally north-south strike. Beds of compact hornblendic trachyte alternate with basalts, compact con glomerates, and other rocks of poorly defined species” (p. 33). Near Washburn summit the upturned edges of hornblende trachyte, which forms the core of the mountain, could be observed surrounded by massive layers of hori zontal breccias and conglomerates composed largely of dark, compact fragments of varied sizes, that seemed to be basalt.
"They (the breccias) appear to be the stratified ejecta- menta formed about the cones of ancient volcanoes*' (p. 34),
Holmes apparently thought that the peaks of the Washburn Hange gave the impression of being a connected series of cones built up after the plateau was formed by an accumu lation of volcanic ejecta, but stated that the range was older than the plateau (p. 35), However, he considered the breccias to be subsequent to both the rhyolite plateau and the hornblendic rocks making the core of the mountain
(p. 35). Hague (1899b, p. 439) considered Mt, Washburn to be the culminating point of a huge volcano formed entirely of fragmental Early basic breccia. He thought that the 23
Washburn Range was the rim of the crater surrounding the central discharging vent (Washburn Amphitheater) now partially filled with rhyolite. In a later publication
Hague (1912, p. 7) stated that "The eruptive origin of
Mount Washburn has long been recognized, and it is fre quently referred to as a volcano. It is however simply the highest peak among several others, and represents a later outburst which destroyed in a measure the original rim and firm of the older crater." The entire volcano is called the Sherman Volcano, and was considered to be a source for the basic andésites composing the cone and also the rhyolites of the plateau, which partially bury the range (p. 8).
A few other comments on the origin of Mt. Washburn and the associated range may be found in the literature.
De Martonne (1915, p. 238) was the first to publish the fault-block theory of the mountain. The form of the range as well as the northward dip of the southern arm of the horseshoe were opposed to the concept that the range was a volcano, according to de Martonne, Jones and Field (1929) in their study of the Grand Canyon implied that Mt, Washburn was an erosional remnant (or monadnock) on a landscape buried by rhyolite. Howard, as part of his History of the Grand Canyon ... (1937, pp. 9-13), made some detailed ob- sejTvations in the range and presents evidence that "... seems to support a fault origin for the Washburn Range, 24 with the greater part of the uplift preceding the extru
sion of the rhyolite." All other references to Mt, Wash
burn usually quote Hague's statements or simply refer to
the range as a volcano.
Some very recent literature is available concerning the andesitic breccias in close proximity to Mt. Washburn.
Erling Dorf (1960) published a paper on the Tertiary fossil forests of Yellowstone Park, especially in the Specimen Ridge area, which is only eight miles northeast of Mt.
Washburn on the eastern side of the Grand Canyon. An even more recent paper by Charles W, Brown (1961) concerns
the stratigraphy and structure of the area immediately north
of the Washburn Range. This paper even includes the northernmost portion of the Washburn Range (Prospect Peak), but the only reference to Mt. Washburn is that the bulk
of the mountain is formed of coarse, chaotic pyroclastic debris (Brown, 1961, p. 1179).
Recent Geologic Activity in the Mt. Washburn Area
Hot Springs
Yellowstone National Park is world famous for its hot
springs and geysers. While no geysers occur in the Wash burn area, several very interesting springs and fumaroles are present. These are two or three miles south of Mt.
Washburn and three miles east of the Grand Loop Road on
the plateau near the contact with the older breccias
(Plate II). Active and extinct springs and fumaroles are 25 scattered throughout the forest of this area, but two not able concentrations have been namedi Inkpot Springs and
Washburn Hot Springs. These springs were first observed and described by Lieutenant Doane of the Washburn Party in 1870, and later described in detail by A. C. Peale
(1883, pp. 86-88).
The springs produce relatively little water, although gases, especially steam, are abundant. The waters are al ways clouded and many springs are simply mud pots activated by bubbles of escaping steam. Commonly mud pots build spatter cones around the spring, occasionally to a height of four feet (fig. 2). Inkpot Springs are mud springs, named for the pitch-black color of their mud (fig. 3).
Washburn Hot Springs, situated on a barren hillside, are composed of hundreds of hissing steam vents, fumaroles, spatter cones, and springs (fig. 4). The ground surface at this locality is very spongy and a strong odor of SO2 and H 2 S fills the air. In certain places, if the grayish- white surface crust is gently broken and turned over, beautiful masses of skeletal sulfur crystals along with alum may be disclosed. The crystals coat steam-filled vents and passageways which honeycomb the hillside. The most common spring deposit is pure white siliceous sinter, which is usually chalky but may contain solid masses of light-gray opal. Along one of the streams in the area, the sinter has been eroded and redeposited as an angular 26
Figure 2. A spatter cone developed around a thermal spring. Washburn Hot Springs.
Figure 3, Mud pots containing thick pitch- black mud. Inkpot Hot Springs.
Figure 4. A general view of Washburn Hot Springs, 27
Figure 2
Figure 3
Figure 4 28 sedimentary breccia composed of particles 2 to 10 mm In diameter that have subsequently been cemented by yellow crystalline sulfur.
The springs are classified as the sulfate type (Allen,
1935) because they produce very little groundwater and arc characterized by direct emanations from an Igneous source.
Seismic Activity From the time of the earliest explorers until the present day, seismic shocks have been a common phenomena in the park area. F. V. Hayden reported several severe shocks on the 20th of July, 1871, while camped near Steam boat Point on Yellowstone Lake and predicted that ” ... if this part of the country should ever be settled and careful observations made, it will be found that earthquake shocks are of \ a r y common occurrence" (Hayden, 1872, p. 82), A nearly complete list of the earthquakes felt in the park during modern times may be found in Yellowstone * s Living
Geology by William A. Fischer (1960).
The most recent major shock occurred on August 17, 1959, causing loss of life and extensive damage. The earthquake was centered in the Madison Canyon area near Hebgen dam, approximately 60 miles west of Mt, Washbum. Several fault scarps came into existence during the quake and a disastrous landslide closed Madison Canyon, damming the river. The shock was probably near 10 on the Modified Mercalli scale
(Fischer, 1960, p. 24)and was distinctly felt in the 29
Washbum area. No damage resulted, however, although
several very recent rock falls were noted in the Grand
Canyon, During the early part of the 1960 field season, seismic shocks were quite frequent and at least one was
strong enough to wake the writer from a sound sleep.
Fault scarps, which indicate prehistoric but very
recent tectonic activity, are quite common in the Wash
burn Range and especially on the surrounding plateau.
Further discussion of these features will be deferred
to the chapter on structural geology.
Terminology and Classification of Fragmental Volcanic Rocks
Fragmental volcanic rocks are breccia, tuff, and
related volcanic sediments. There are numerous classifi
cations of these rock types, and the terminology of frag
mental volcanic rocks may seem quite confusing. Recently,
however, new classifications and definitions have been
proposed to eliminate the confusion.
The classification and terminology of Wentworth and
Williams (1932) is the standard for volcanic geology.
It is a useful classification, but limited in scope be
cause it deals only with pyroclastic materials, i.e.,
matter ejected from a volcanic vent in either a liquid, 30 plastic, or solid state. The primary characteristic of this classification is sizet
> 32 mm volcanic breccia 4-32 mm lapilli-tuff 0,25-4 mm coarse tuff < 0.25 mm fine tuff
Secondary factors in the classification are the shape of the fragments, the origin of the material, and the internal structure, and the crystallinity of the fragments. Fisher (1958) pointed out that volcanic breccia is commonly used in a more general sense, Wentworth and
Williams use the term to mean material of pyroclastic origin but Fisher (1958, p. 1072) suggests that volcanic breccia be used to designate all material composed of large ( ^ 2 mm) angular volcanic fragments regardless of origin. In this report, the term will be used according to Fisher’s definition. The adjectives used to modify volcanic breccias are meant to be purely descriptive but often have genetic implications. In any case, the modifying term may be interpreted to indicate a certain origin for the breccia.
In a breccia the fragments may be described as essential (juvenile, belonging to the magma in eruption), accessory (previously consolidated volcanic rocks of consanguinous origin), or accidental (igneous, metamorphic, or sedimentary 31 rocks of nonconsanguinous origin) (Wentworth and Williams,
1932). In addition, fragments may be described according to color, size, degree of roundness, and internal features such as vesicles, phenocrysts, and crystallinity. Also a volcanic breccia may be described according to the nature of the matrix, the percentage of matrix and fragments, the nature of the bedding if any, the size variation of the fragments, the sorting, and the general color of the whole rock. Volcanic breccias are often described as monolithologic if one, type of rock fragment predominates
(>75 percent) or heterolithogic if several types of rock fragments are present.
Volcanic tuffs (indurated ash) may be described ac cording to the predominant general composition, A tuff may be vitric, crystal, or lithic if glass, crystals
(intratelluric, not accidental), or rock fragments
(essential, accessory, or accidental) predominate
(>75 percent) respectively. Compound terms such as
lithic-crystal or vitric-crystal are commonly used, the
first term indicating the most common constituent.
A genetic classification of volcanic breccias pro posed by Fisher (1960) has been adopted for use in this report. The classification along with a partial explana
tion may be found in Table 1. 32 Table I. Classification of Volcanic Breccia
I. Autoclastic volcanic breccia — self-fragmentation of semi-solid or solid lava — non-explosive A, Flow breccia -- fragmentation of an advancing and congealing lava flow (aa, block breccia) B, Volcanic intrusion breccia — movement of liquid semi-solid or solid lava under con finement (peperite breccia, friction breccia, vent breccia, breccia pipes, autobreccia) II- Pyroclastic breccia -- formed by explosions and ejection of liquid or solid fragments from volcanic sources A. Vulcanian breccia -- vulcanian or ultra- vulcanian explosion B. Pyroclastic flow breccia (nuees ardentes) -- explosive extrusions of s o 1 id -"liquid - gas mixtures (welded tuff, ignimbrite) 1. Pelean type — extrusions through flanks of a dome or collapse of a dome without attendant explosion 2. Krakatoan type -- vertical low- pressure explosions through craters 3. Fissure type -- low-pressure upwelling of effervescing magma through fissures C. Hydrovolcanic breccia -- steam explosions as a result of hot lava in contact with water or ice D- Phreatovocanic breccia - - hot lava or magma in contact with ground water resulting in explosions
III- Epiclastic volcanic breccia — volcanic fragments of any origin moved by epigene geomorphic agents or gravity A- Laharic breccia — mudflow containing angular blocks of chiefly volcanic origin B- Water-laid volcanic breccia — angular volcanic rocks that originate from any volcanic region undergoing rapid erosion. Gradational to volcanic conglomerates and sandstones C. Volcanic talus breccia CHAPTER 111. EARLY ACID BRECCIA
Introduction Hague’s Early acid breccia in the Mt. W a s h b u m region consists of a sequence of older welded tuffs, breccias, and fossil-bearing volcanic sandstones and conglomerates several hundred feet thick, and a younger series of hornblende andeslte flows and minor associated pyroclastic breccias.
The Early acid classification is based on the following distinctive features: 1. The predominance of hornblende among the mafic constituents of the igneous rocks. 2. The presence of volcanic sediments containing abundant nonvolcanlc material. 3. The occunrence of certain fossils restricted to the Early acid breccia. 4. The fact that these rocks stratigraphically underlie other strata considered to be Early basic breccia.
5. Characteristic light color, alteration, and general appearance of the Early acid rocks, which are distinctive in relation to Early basic rocks.
Pétrographie Mineralogy of the Volcanic Rocks
Plagioclase
Plagloclase occurs as large irregular and conmonly glomeroporphyritic phenocrysts (maximum length - 3 mm), as
33 34 euhedral lath-shaped mlcrophenocrysts (0.5 to 1.5 mm), and as microlites (0.1 to 0.2 mm) in the groundmass of porphy- ritic rocks. The composition of the feldspar varies from rock to rock, but is generally calcic andesine or sodic labradorite. Specific compositions in terms of the percent of anorthite will be discussed in other sections of this and following chapters, which concern the various rock types. Zoning is always present and varies from continuous-normal to faintly oscillatory-normal to high-frequency oscillatory- normal (see Homma, 1936), Occasionally, minor reverse zon ing is present. One or two reaction zones within the crystal are not unusual, especially in the larger phenocrysts
(fig. 5). Commonly, masses of tiny, nearly opaque, glass like inclusions are associated with the reaction zones and are situated away from the reaction interface toward the core of the crystal. Twinning according to the albite-
Carlsbad twin law is abundant, almost to the exclusion of all other laws, but minor acline twinning is present. The Carls bad surface in a few specimens is multiple (2 or 3 surfaces), especially in more calcic plagioclases, and it is not unusual
^ The anorthite content of plagioclase was determined with a universal stage using the Rittman zone method (Emmons, 1943, p. 115) and the extinction angle curves for plagio clase with high-temperature optics (van der Kaaden, 1951, pp. 51-52). The plagioclase of volcanic rocks always shows high-temperature optics with only rare exceptions (ibid., p. 17), and so the plagioclase of Washbum volcanic rocks is assumed to show high-temperature optics. An error of 5 to 10 percent anorthite is involved with andesine and labradorite if the wrong curves are used. 35 for the Carlsbad plane to be irregular. Plagloclase may be perfectly fresh or entirely altered, with all grada tions in between. The most common alteration is zeoliti- zation, which generally is associated with a small quanti ty of material resembling sericite. The zeolite may replace certain zones in the plagioclase, or it may be disseminated along crystallographic planes throughout the grain, produc ing a sort of sieve structure. More commonly, the zeolite occurs as irregular patches that destroy such features as twinning, zoning, and optical relief. In rocks that have undergone extensive hydrothermal alteration, plagioclase may be replaced by carbonate or pyrite, and may have green montmorillonite developed along fractures and cleavage.
Sanidine
This mineral is rather restricted in occurrence and has been identified only in the welded tuffs. The optic angle ranges from 0° to 20°, which is the normal optic angle variation for sanidine. Crystals are euhedral but are almost always broken, producing angular fragments. Carlsbad twinning is always present and is represented by a straight twin plane. The sanidine is unaltered al though in some specimens is partially resorbed, As grains approach extinction under the microscope, they present a reticulated pattern indicating that the sanidine is cryptoperthitic. 36
0,1 mm
Figure 5. Typical euhedral plagioclase phenocryst showing high-frequency oscillatory-normal zoning and two reaction zones. Note the concentration of nearly opaque grains away from the reaction interface toward the core of the crystal.
0.6 mm j Figure 6. Slightly oxidized hoxmblende phenocxyst showing typical "pyroxenic” opacité consisting of orthopyroxene, magnetite and plagioclase. Note the thin "black" type opacité at the edge of the horn blende crystal. 37
Hornblende Hornblende is the predominant mafic mineral in the
Early acid volcanic rocks and always occurs as phenocrysts.
It varies from normal green hornblende to oxyhomblende in which the ferrous iron has been entirely oxidized to ferric iron, and almost all grains are characterized by an opacité border.
Optically, the properties of hornblende are quite vari able due to variable oxidation, but the 2V of unoxidized grains is 84° to 86° and are optically negative. Pale-green to dark greenish-brown pleochroism is common for fresh crystals,
Hornblende usually occurs as individual euhedral crystals, but small glomeroporphyritic aggregates do occur, which com monly contain orthopyroxene. Maximum size of the phenocrysts is about 3.3 mm, but grains approximately 0.5 to 2,0 mm in
2/ , , Opacité is a general pétrographie term applied to the groundmass of volcanic rocks and refers to the high con centration of opaque minerals, especially magnetite dust, in the groundmass. The term •opacité^ in regard to horn blende refers to a concentration of opaque minerals around the borders of hornblende grains. Taneda (1941, p. 58) defined two types of opacitizationi 1, "Black" type -- hornblende replaced in part by a fine aggregate of magnetite and pyroxene, producing a black, nearly opaque (i.e., slightly biréfringent) border. 2, "Pyroxenic" type -- hornblende replaced in part by a corona of pyroxene with highly variable optical properties, magnetite, and some plagioclase. This type of opacité is usually fairly coarse and individual grains are easily discernible. Taneda’s terminology will be used throughout this paper. Experimental evidence bearing on the origin of hornblende opacité will be discussed in the chapter on petrogenesis. 38 length are far more common. Crystals are generally rounded due to resorption, and frequently the larger grains have a cavernous interior, which is interpreted to indicate incom plete crystal growth. Compositional zoning is prevalent and also twinning on the (100) face. Inclusions are rather uncommon although some grains are poikilitic and contain plagioclase, orthopyroxene, or chunks of groundmass.
Opacitization is the most common type of alteration. The black type of opacitization is the most common and varies in degree from a thin outer shell of opacité to the entire replacement of hornblende by opacité (figs. 7-9).
Pyroxenic type of opacitization is less common, and again varies in degree from a simple corona (fig. 6) to the en tire replacement of the mineral by granular orthopyroxene, magnetite, and plagioclase. In addition to opacitization, hornblende may alter to bowlingite, which resembles chlorite or serpentine in the initial stages. Hornblende is also replaced by opal, chalcedony, carbonate, and an unidentified zeolite. 39
Figure 7, Hornblende phenocryst with a partially oxidized border and a thin black type opacité rim.
Figure 8. Oxyhornblende largely replacedby black type opacité.
Figure 9, Hornblende entirely replaced by black type opacité and partially resorbed. 40
Figure 7
• 0 fl&A
Figure 8
Figure 9 41
Orthopyroxene The orthorhombic pyroxene in the Early acid volcanic rocks shows little variation in composition. On the basis of the optic axis angle (2V = 68°-75®, negative) measured with a universal stage, the composition is Enyg_yg, or bronzite according to Poldervaart’s classification
(WahlStrom, 1955, p. 160), Compositional zoning usually consisting of only two zones is almost always apparent and represents a slight increase in the magnesium content of the mineral. There is a fair abundance of grains that show uniform, highly pleochroic (in pink and green) cores with a 2V = 68° (Enyg) bounded rather sharply by a uniform, nearly non-pleochroic exterior shell with a 2V = 75° (Enyç).
Orthopyroxene commonly occurs as individual, subhedral or occasionally euhedral crystals, but at a few places
loosely grouped aggregates of small phenocrysts closely associated with magnetite are present. Individual pheno crysts are 0,5 to 1.0 mm in length and tiny grains within the groundmass are 0.1 mm in diameter. Orthopyroxene with composition the same as the phenocrysts also occurs in the reaction coronas (pyroxenic type opacité) around hornblende phenocrysts. Fibrous, pale-green bowlingite is the most common type of alteration, sometimes producing bowlingite pseudomorphs, but usually leaving remnants of the ortho pyroxene. In one case, perfect euhedral pseudomorphs of
opal are present. 42
Clinopyroxene Monoclinic pyroxene is a rather minor constituent of these volcanic rocks and again, as in the case of ortho pyroxene, has a uniform composition regardless of rock
type. The optic axis angle is 53® to 55® and the bire fringence is very high, indicating a diopsidic augite
(WahlStrom, 1955, p. 163), This mineral occurs as sub hedral to euhedral individuals 0.5 mm in length, but
occurs more commonly as loosely united glomeroporphyritic aggregates up to 2.5 mm in diameter, or as 3 to 5 grain aggregates 0.4 to 1.0 mm in diameter. Simple and multiple
twinning on the (100) face is usually present. The clino- pyroxene alters to pale-green bowlingite, which occasionally
is bright orange due to iron staining, or it may be replaced by carbonate. Unaltered grains predominate.
Other Primary Minerals
Apatite occurs as euhedral to rounded grains, or as elongate needles. Usually only a veiry few tiny grains
occur in each thin section, so it is a rare accessory min eral. It is always associated with or included in horn blende phenocrysts, but occasionally occurs as inclusions
in the largest plagioclase phenocrysts.
Magnetite is ubiquitous, and is present as euhedral
to rounded grains, or as "dust" throughout the groundmass. 43
It also occurs as Inclusions within orthopyroxene and In hornblende opacité. Almost always, magnetite Is partially to completely altered to hematite.
Secondary Minerals
Quartz, pyrite, and carbonate have been identified as replacements of primary minerals or of groundmass. Chalcedony-quartz, thompsonlte, analclte, natrollte, chabazlte, opal, carbonate, and a brilliant emerald-green mineral structurally resembling montmorlllonlte occur as cavity or vein fillings. Epldote Is fairly common In the groundmass of some strongly altered rocks In addition to the alteration minerals mentioned In previous sections.
The common occurrence of hematite and llmonlte Is at tributed to weathering.
Sedimentary Rocks and Associated Breccias
General Statement
The Early acid sedimentary rocks are the oldest rocks exposed In the Mt. Washburn area. Even though the volume of these strata Is small, they are very Interesting and shed much light on the early history of the region. The rocks are predominantly sandstones and conglomerates com posed of andesltlc volcanic rocks. Paleozoic sedimentary rocks, and Frecambrlan metasedlmentary and Igneous rocks.
The proportion of andesltlc volcanic rocks varies consider ably, ranging from a mere trace to nearly 100 percent of 44 the total rock. Along with these sedimentary rocks are lesser amounts of volcanic breccias and tuffs that are contaminated by nonvoLcanic constituents. Exposures occur in the valley of the upper reaches of Carnelian Creek, in various stream valleys, and as man- made outcrops along the Grand Loop Hoad southwest of Mt. Washburn. The large body of Early acid rocks southwest of Mt. Washburn (Plate II) are newly described and have not been recognized previously. All exposures are fairly poor, usually occurring as separate outcrops showing a small Stratigraphie range between long covered zones. Because of this limitation, no stratigraphie section was measured, although the sedimentary rocks are estimated to be at least 200 to 300 feet thick. The lower contact of these rocks with the underlying ones is not observable and the upper contact with the overlying lavas and asso ciated breccias is gradational.
Exposures in Carnelian Creek Valley Outcrops in Carnelian Creek Valley are best seen in the bed of the main stream and along the banks. In the lower part of the section, the beds are quite massive, commonly 10 to 20 feet thick. Bedding is indicated by changes in color or grain size, but bedding planes are not well developed. Much of the material has a distinct tuffaceous aspect and some is clearly breccia. Deeply weathex*ed conglomerates that contain large quantities of 45
Precambrian metamorphic and Igneous rocks, have pebbles ranging in size from one inch to 1.5 feet in diameter although 3 to 6 inches is quite common, and also interbeds of medium-grained sandstones and grits. A 12-foot thick section of brownish, micaceous, shaly siltatone with beds l/l6 to 2 inches thick was observed between rusty beds of coarse conglomerate.
The rocks are usually some shade of green, such as light olive gray, pale greenish yellow, or just pale green, although there is a great deal of brown staining due to weathering. Tuffaceous sediments and breccias tend to be compact and contain large quantities of angular ande- sitic fragments showing hornblende, biotite, plagioclase, and quartz phenocrysts. Drab-green sandstones contain abundant quartz grains, and associated siltstones are usually micaceous. The well-rounded boulders of the conglomerates are composed of greenish-black hornblende basalt, granite gneiss, amphibolite, quartzite, and other
Precambrian rocks in a sandy matrix that appears to be rich in clay.
Petrographically, the conglomerates and sandstones always contain some volcanic constituents, the amount ranging from just a few fragments to nearly 90 percent of the rock. The volcanic rocks are microporphyritic hornblende andésite with a groundmass texture ranging from felty to pilotaxitlc, although there is great variability in the details of the textures and the 46 percentage of each mineral that forma phenocrysts. In addition to andésite, fragments in the conglomerate in clude chlorite schist, mica schist, and quartz-garnet schist.
The most abundant sand-size grains are angular quartz and oligoclase-andesine that is strongly zeolitized. Since
some of the quartz grains show well developed wavy ex
tinction and other grains show uniform extinction, more
than one origin is postulated for this mineral. Other minerals present in varying amounts include microcline and other potash feldspars, both commonly sericitized, blue-green amphibole, common green hornblende, green biotite, colorless diopside, muscovite, pale-pink garnet, sphene, and magnetite.
Sand grains and lithic fragments are imbedded in a
fine-grained, cryptocrystalline matrix that resembles
chalcedony, along with abundant pale-green montmorillonite- type clay. Secondai*y minerals include carbonate and pyrite.
Tuffaceous or breccia-like rocks are composed of angular andesitic volcanic fragments; the nonvolcanic component is considerably reduced although always present.
The andésites are similar to those found in the sandstones and conglomerates, but important differences are the occur
rence of a considerable number of grains with trachytic
and hyalopilitic ^ textures, and fairly common glassy
—^Hyalopilitic is a texture formed by microphenocrysts and microlites of plagioclase with small amounts of interstitial glass between mineral grains. 47
fragments. The glass in these fragments Is partially devitrified, and in those places pale-green montmoril-
lonite-type clay is present. Microporphyritic andésites may contain phenocrysts
of plagioclase, hornblende, biotite, andminor Beta-quartz
in various proportions, but they are not all present in
all fragments. Plagioclase, which shows a considerable variety in the degree of zeolitization, ranges in composi tion from An^Q to An^^, but most grains are between An^^
and An,„. Hornblende and biotite occur as euhedral 40 phenocrysts in colors from green to brown, and show all degrees of black type opacitization.
Exposures Southwest of Mt, Washburn
The outcrops of Early acid strata southwest of Mt, Washburn, which are newly discovered, show a stratigraphie
sequence similar to that in Carnelian Creek valley. The
most interesting outcrops occur in the valley of a stream
that begins in the pass between Dunraven and Hedges Peaks
and flows southeast.
At the point where this stream crosses the Grand
Loop Road, a recent excavation for a small water reservoir
dam exposed a series of interbedded olive-gireen conglomer
ates and sandstones with conglomerate lenses and bands,
all of which are composed of nonvolcanic materials (fig. 10).
The sandstones, which dip gently west, occur in beds 1/2 to
2 inches thick or in massive beds 4 feet thick, and show 48
Figure 10. Typical Early acid aadiaantary rocks showing conglomerate at the base overlain by massive coarse sand stone and slltstone containing coallfled logs and plant fossils near the conglomerate contact. The uppermost unit is thinly bedded sandstone showing crude cross-bedding. crude cross-bedding. The sand is usually fairly coarse but minor silt beds do occur. The conglomerate, on the other hand, is massive and contains pebbles up to 8 cm
in length although 1 to 2 cm is more conmon. Near the top of one conglomerate bed, where silt is predominant, numerous coalified logs and plant fossils are present. In the vicinity of coalified material, the slltstone that shows fine laminations is bleached white except for spotty brown limonite stains. The well-rounded pebbles in the conglomerates are composed of a great variety of lithologie types such as light-colored plutonic rocks, schists, sub
ordinate porphyritic andésites, ironstone concretions, white oolitic limestone, metaquartzite, and quartz-clay 49
Figure II. Photograph of a aami-pollmhad alab of Early acid conglomaratic aandatone ahowlng the nature of the bedding and well-rounded pebbles composed of metaquartzite (M), ironstone concretions (I). limestone (L), porphyritic andésite (A), chert (C), and slltstone (S). siltstones (fig. 11). The pebbles are imbedded in a coarse, sandy, light olive-green matrix similar to the sandstones, in which quartz, muscovite, feldspar, garnet, and pyrite are megascopically identifiable. None of the fossil leaves or carbonized matter (fig. 12) observed in these sediments could be identified by genus and species. The coalified material is mostly bright, shiny vitrain with brown limonite stains, but dull masses of fusain are also present. Some of the carbonaceous material has the appearance and texture of charcoal.
Microscopic examination of the sandstones disclosed abundant feldspar, very angular quartz, and other minerals in a matrix of pale-green, cryptocrystalline montmorillonite, which causes the green color of the rocks, and secondary 50
Figure 12m Examples of carbonized plant remains and a coallfled log that occur within Interbedded Early acid sandstones and conglomerates along the Grand Loop Eoad aouthwest of Mt «Washburn.
Figure 12a
Figure 12b
P
Figure 12c 51
Figure 13. Ftiotomlcrograph of a typical Early acid sandstone with abundant feldspar, angular quartz, and muscovite in a matrix of green montmorillonite and secondary carbonate. Crossed nicols. carbonate (fig. 13). The feldspars are andesine (An, ), which is generally altered to zeolites and replaced by carbonate, microcline, and sericitized orthoclase. Quartz occurs as three distinct types that show (1) wavy ex tinction, (2) nonwavy extinction with bubble trains, and (3) nonwavy extinction lacking bubble trains. Other minerals include brown biotite partially altered to blue chlorite, muscovite, garnet, zircon, tourmaline, magne tite, apatite, green hornblende, monazite, rutile, myr- mekitic quartz and feldspar, and an unusual abundance of sphene. In addition to the secondary or alteration min erals already mentioned, pyrite, limonite, hematite, and clinozoisite also occur. The mineral grains of the conglomerates have approxi mately the same composition as those of the sandstone. 52
Lithic fragments and pebbles include chert, granitic rocks, several kinds of schist, limestone, quartz-clay slltstone, and moderately abundant felty to pilotaxitlc microporphyritic andésites. Conglomerates differ from sandstones mainly by containing a greater concentration of andesitic volcanic materials.
Approximately 100 yards upstream from the outcrops just described is a series of massive, arkosic conglomerates, grits, and sandstones. The rocks are pale greenish-yellow to very pale orange and differ from the previously described rocks in the greater abundance of coarse angular fragments, although well-rounded pebbles of granite and mica-schist are present. Microscopically, strongly altered micropor phyritic volcanic rocks are predominant, although nonvolcanic materials similar to those described previously are not rare. The color of the rock is attributable to an abundance of very fine-grained, low-birefringent material resembling kaolin, which forms the groundmass of the volcanic frag ments and the matrix of the conglomerates.
About 100 yards west of the last location and along the old Tower Falls-Canyon Junction road in a small quarry, a series of volcanic sandstones and conglomerates may be seen. These sedimentary rocks differ from the others in that they are composed entirely of andesitic volcanic pebbles and mineral grains. The beds are massive and structureless, although the contact between conglomerate 53 and sandstone is distinct. Poorly developed bedding planes dip gently westward and are cut by a fault zone along the south end of the quarxy. Slickensided fault planes are coated by zeolites, especially heulandite, which occurs in crystals up to 8 mm in length. The rocks are fine- to medium-grained tuffaceous sandstones with interbedded conglomerates containing well- rounded pebbles (1 to 4 cm) and smaller angular fragments
(5 mm or less). The sandy material is light olive gray, but pebbles arc often grayish purple to pale olive. Mega scopically, the only identifiable constituents are white plagioclase and black hornblende phenocrysts in andésite pebbles, and a few carbonate veins. Rusty-brown or black fossil leaves and stems occur in certain sandstone layers (fig. 14), and some minor coalified material is present. All fossil specimens were sent to Dr. Erling Dorf (Princeton University) who kindly identified the following speciest Aralia notata Lesquereux Carya culveri Knowlton (?) Juglans crescentia Knowlton Quercus weedi Knowlton
Quercus yanceyi Knowlton
This group includes such familiar deciduous trees as hickory, walnut, and oak, which no longer grow in this region. 54 Flgux*e 14. Representative samples of fossil plant materials collected from an Early acid tuffaceous sandstone that crop# out in a quarry on the old Canyon-Tower Falls Eoad southwest of Mt. Washburn.
Figure 14a
Figure 14b
Figure 14c 55
Microscopic examination showed that these rocks are composed entirely of felty microporphyritic andesitic fragments, pebbles, or associated minerals. This common lithology, which was observed at numerous other locations, again shows many variations in the details of the texture, although the overall rock classification is the same. In addition, microvitrophyres are fairly common. Phenocrysts include green hornblende and plagioclase (An^2 _4 g)' The rock is greatly altered and contains crisscross carbonate veins, pale-green bowlingite, and secondary quartz.
Other occurrences of water-laid clastic sedimentary rocks with occasional, light-colored interbeds of breccia are common in stream valleys throughout this area.
The true nature of the outcrops along the Grand Loop Road may be easily overlooked at first glance because of intensive alteration and weathering, which cause the yel lowish gray to dusky yellow color of the rock. Close examination may be required to differentiate pebbles from their tuffaceous matrix, and the pebbles themselves may actually be andésite, metaquartzite, granite gneiss, chert, or mica-schist even though their outward appearance may be quite similar. Alteration minerals in these rocks include pistacite, bowlingite, aggregates of quartz, and secondary iron oxides, all of which tend to obliterate the internal differences of the rocks. 56
Interpretation
The source of the nonvolcanic component of the Early acid sedimentary rocks must be from a complex plutonic igneous and metamorphic terrane containing associated minor sedimentary rocks. Apparently the only nearby region that contains these materials is the Beartooth
Mountains north of the Washburn Range. This hypothesis is strengthened by the report of coarse conglomerates composed of Precambrian metamorphic rocks, which appear to fill channels cut into the Beartooth block along its southern margin (Brown, 1961, p. 1177). These channel deposits are considered to be contemporaneous, with
Pierce’s Grande11 conglomerate (p. 1177), which is thought to be slightly older than true Early acid breccias, but the relation between the Beartooths as a source area and the Washburn area to the south as a site of deposi tion is quite clear.
Volcanic materials came from multiple centers, in dicated by the variety of textures and variability of phenocrysts of andesitic pebbles. These centers were close to the Washburn area, but at a distance great enough to allow the rounding of pebbles through the process of transportation. Some centers were very close, since ash apparently was contributed to the sediments by aerial transport, and minor volcanic breccias are interbedded with the sedimentary rocks. It is suggested 57 that many of these centers are Located along the Cook City zone of the Beartooth block.
The sediments were deposited under subaerial condi tions , probably on an alluvial plain, and were transported
short distances only. This Is Indicated by the poor sort ing of grain size, occurrence of Interbedded conglomerate and sandstone, crude cross-bedding, well-rounded pebbles and angular sands, the Immaturity of the sediments (glass fragments, potash feldspars), and the presence of coallfled
log accumulations and fossil broad-leaf plants. The fossils
Indicate warm-temperate to subtropical forests with 50 to
60 Inches of rain per year In a region of relatively low mountains (less than 3000 feet) and broad flat lowlands
(Dorf, 1960, p. 257).
Welded Tuff Until recently the occurrence of welded tuffs In
Early acid rocks had never been described. Brown (1961,
p. 1177) refers to "... a thick sequence of daclte crystal
and vltrlc tuffs, lapilli tuffs, and sparse breccia beds" that overlie basal daclte breccia of the Early acid sec
tion. It Is quite possible that the daclte crystal and vltrlc tuffs described by Brown are the same as the welded
tuffs described In this report, since the general strati graphie sequence In the two areas or at least the asso
ciated rock types are similar. 5 8
Welded tuff is exposed near the headwaters of Car nelian Creek north of the northwest-trending dike plotted
on the geologic map (Plate 11) and also may be present in
the main portion of Early acid rocks southwest of Mt. Washburn as a poorly exposed outcrop along the Grand Loop Road. The Carnelian Creek exposures are very thinly lay
ered and break into slabby or flaggy pieces. The outcrops are cut by an intersecting set of nearly vertical joints
that cause numerous small waterfalls and steep banks along the stream. The rocks are generally grayish yellow green but are pale greenish yellow near an andésite dike, and
contain clear, glassy feldspar crystals, small black sub
rounded pebbles, and flattened lapilli-size pumice frag
ments that appear as short apple-green streaks.
The outcrop on the Grand Loop Road is so badly altered
that the original properties are entirely destroyed. The rock is light grayish yellow and quite fine grained. The
only notable features in an otherwise homogenous rock are
limonite spots that appear to have been pyrite crystals and a few structures that may have been pumice fragments.
The tuffs show excellent vitroclastic texture caused
predominantly by glass shards and a few collapsed pumice
fragments. Approximately 20 percent of the rock is com posed of mineral grains and lithic fragments. The maximum
grain size is between 2.5 and 3.0 mm, but grains between 1.0 and 1.5 mm are much more common. Many of the grains 59 are broken, producing angular specks 0.05 to 0.3 mm in diameter. Sanidine and plagioclase (An^^ are the most abundant minerals, but minor quantities of quai*tz, green hornblende, brown biotite, magnetite, diopside, apatite, zircon, and olivine are present (fig. 15). Most of these minerals occur as euhedral crystals or as angular broken fragments. Lithic fragments are quite variable and often show opacitized groundmass, presumably due to mag- matic reaction. Felty-microporphyritic andésites with plagioclase phenociysts, and opacitized microporphyritic rocks with plagioclase and biotite phenocrysts are present.
Figure 15, Photomicrograph of Early acid welded tuff showing vitroclastic texture and containing a lithic fragment, quartz, sanidine, magnetite, olivine, and zircon. Plane light. The glass shards and pumice fragments are generally devitrified and changed to pale-green, pleochroic mont morillonite that varies considerably in the degree of crystallinity. The only other alteration is minor black
type opacitization of hornblende and biotite.
The highly altered material that outcrops on the Grand
Loop Road contains abundant pseudomorphs composed of a ma
terial that resembles talc or muscovite. Beta-quartz is
apparently the only surviving primary mineral, and the
vitroclastic texture, if it ever existed, has been entirely destroyed by abundant montmorillonitic clays and iron oxides.
On the basis of their general form, large masses of hema
tite surrounded by an iron-stained zone are probably pyrite
pseudomorphs.
The welded tuffs are classified as latites because of their lack of appreciable quartz and the abundance of
andesine. On account of the limited areal extent of these
rocks, no vent area could be defined. For a brief dis
cussion of the origin of welded tuff, see the section in
this report concerning Plateau Rocks.
Lava Flows and Associated Breccias
Field Occurrence
Volcanic rocks occur in the upper portion of the Early
acid stratigraphie section and comprise the bulk of this
group. S(xne flows are present in the Carnelian Creek area,
but most are located in the main body of Early acid rocks 61 southwest of Mt. Washburn. Fairly good exposures may be
seen along the Grand Loop Road and in the stream valleys flowing south and southeast from the Hedges Peak area. The prominent mountain front north of Canyon Junction, where rise the tributaries of Cascade Creek, Is composed almost entirely of lava flows. Outcrops are rather poor for the most part owing to weathering alteration, and the nature of the rock Itself. Natural exposures are usually elongate, humpy, light brown to reddish rocky masses with hackly fracture, which pro duces gruss-covered slopes. The lavas tend to be rather massive with obscure structure, although banded rocks are common In a few places, especially just south of Hedges Peak along the mountain front. A few cases of spheroidal weathering occur and most of the flows show an abundance of Irregular joints. Slickensided surfaces occur within the volcanic rocks and commonly are coated with zeolites or silica.
The structure of the lavas was difficult to determine. Apparently, most of the flows are sheet-like In form and are presently oriented with an approximate north-south strike and a gentle western dip (10-30®) except In the area south of Hedges Peak, where strikes are erratic and dips are as high as 50®. In the area just mentioned, an area of ex
tensive hydrothermal alteration, the well-banded flows are cut by a dozen breccia dikes that ai*e oriented nearly at 62 right angles to the dip. The dikes are 1-1/2 feet to 1/2
inch wide and weather into relief, producing walls two to
three feet high. They are not very long (maximum 100 feet), commonly bifurcate, and may change direction sharply along
joint surfaces. The lava flows are also cut by a few light- colored, porphyritic hornblende andésite dikes, which will be described in a later section.
In addition to the sheet-like flows, channelized flows occur near the contact with Early basic breccia rocks and
probably represent one of the latest events during the
Early acid volcanic cycle. These flows overlie extensively altered sheet lavas and breccias, and are considerably frac
tured by joints that are commonly filled with chalcedony.
Closely associated with the lavas are fairly minor quantities of chaotic volcanic breccia that seems to be monolithologic and quite angular except the partially- rounded larger fragments (>6 inches). Barring a few instances where there has been extensive late hydrothermal action, fractures and weathered surfaces do not show dif ferences between fragments and matrix in the breccias. In many cases, this causes great difficulty in distinguish ing breccias from strongly jointed and weathered lava flows.
It is thought that the compact nature of these volcanic breccias is partially caused by alteration processes. Fragments in the breccia are apparently of the same com position as the lava flows. 63 In the vicinity of Hedges Peak the contact between
Early acid and Early basic rocks is not sharp and is generally marked by a small topographic bench, which is sometimes marshy owing to the presence of springs. The contact may actually be gradational, because the horn blende andésite lavas seem to intertongue with the over- lying diopsidic augite-bearing breccias that are typical of Early basic rocks, through a stratigraphie interval of
10 to 20 feet. The lower contact of the Early acid vol canic rocks is gradational and quite indefinite. The underlying sandstones and conglomerates become richer in angular volcanic fragments and tuff, and eventually are replaced by volcanic rocks entirely. The actual thickness of the flows and breccias was never measured because of insufficient outcrop due to forest cover, but it is esti mated that the Early acid volcanic rocks are at least 500 feet thick in this area.
Megascopic Description and Petrography
lava Flows — The massive flows are all aphanitic rocks that show a few phenocrysts, usually black horn blende needles up to one centimeter in length but also small white plagioclase grains. The rocks are medium dark gray if fresh, but are commonly light brownish gray, gray ish red, or grayish yellow, owing to weathering and altera tion. Hornblende needles show little or no orientation. 44
These lavas are very hard and compact, and very difficult
to break. Badly altered specimens generally show consid
erable porosity. The banded flows differ from the massive flows in two
ways. First, the hornblende needles are more abundant and
are situated in parallel planes or layers, but are randomly oriented within a single layer. Second, alteration has
selectively taken place in certain layers, strongly ac
centuating the flow structure.
The channel flows do not differ significantly from the others except that the aphanitic texture seems to be finer
than average, almost glassy, hornblende needles are quite
large (up to one inch), and plagioclase phenocrysts are
not visible. Also, the weathered surfaces are a distinc
tive light brick red. Stretched vesicles occur in the
channel flows, but were not observed in other types of flows.
The texture of the flows is always microporphyritic 4/ with a felty to pilotaxitic —^ groundmass. Phenocrysts, which consist of plagioclase (generally An^^ maximum
variation An., ), hornblende, bronzite, and minor 46-74 ' * glomeroporphyritic diopsidic augite, comprise 30 to 40
percent of the rock (fig. 16), They range in length from
3 mm to 0.1 mm, although most grains are 0.5 mm to 1.3 mm.
4 / In this report, pilotaxitic will be used to designate a texture that shows good alignment of abundant plagioclase microlites. A felty texture is also composed of abundant plagioclase microlites, but flow alignment is lacking. 6^
Figure 16, Typical felty-microporphyritic texture of Early acid andésite lavas with plagioclase and opacitlzed (black and pyroxenic type) hornblende microphenocrysts. Note the cavernous interior of one hornblende phenocryst, and the two distinct sizes of plagioclase. Microlites of plagioclase are abundant in the groundmass, but are too small to be resolved by the microscope at this low magnification.
Flow structure, where present, is indicated by the align ment of hornblende and plagioclase phenocrysts, and
plagioclase microlites. This structure is commonly ac centuated by selective alteration of the primary minerals
in certain layers. The alteration consists of oxidation
of the mafic minerals, and the filling of veins and ovoid cavities with opal and zeolites. In rocks that do not show
flow structure, veins and cavity fillings are randomly distributed and are composed of a variety of zeolites, clays,
carbonate, and silica (fig, 17-18). Pyrite, carbonate,
and quartz are prominent replacement minerals in some rocks. 66
Figure 17. Extreme alteration of Early acid andésite lava showing oxyhomblende and plagioclase phenocrysts. Note the cavities and veins filled with opal, zeolites, and quartz. Plane light.
Figure 18, Same as Figure 17, but taken with crossed nicols
Figure 19. Highly oxidized Early acid andésite breccia fragment showing pyroxene pseudomorph (Px), opacitized hornblende (Hb), and zeolitized plagioclase (PI). Plane light. 67
Figure 17
Figure 18
Figure 19 68 The groundmass in all flow rocks is very fine grained and nearly cryptocrystalline (fig. 16), The most abundant distinguishable grains are plagioclase and orthopyroxene microlites, and magnetite. There is, however, always an abundance of poorly defined material, especially in altered rocks. These materials include abundant low-birefringent,
feathery minerals resembling feldspar (possibly potash feld
spar, or quartz, or both); abundant, irregular tiny needles assumed to be incipient pyroxene; magnetite "dust" and non descript, nearly opaque grains; and unevenly distributed
secondary minerals such as interlocking quartz, zeolites, and montmori11oni te.
One important feature of these flows is that three distinct grain sizes are generally discernible (fig. 16), The largest grains (1,5 to 3.0 mm) consist of hornblende, glomeroporphyritic aggregates of diopsidic augite, plagio clase microphenocrysts, and huge, blob-like plagioclase crystals (possibly xenocrysts). The grains of intermediate
size (0.2 to 0.6 mm) are chiefly plagioclase laths, but orthopyroxene and many magnetite grains fall into this size group. It is thought that pyroxenic opacité, so common in these rocks, belongs to this crystallization cycle. Finally,
the smallest grains (.01 to O.I mm) comprise the groundmass described above.
Since all the flow rocks are composed of labrador!te
and hornblende, along with a little pyroxene, and since the 69 textures are always felty to pilotaxitic, these rocks may be called hornblende andésites or, in some cases, horn- blende-pyroxene andésites. All the andésites are micro porphyritic and a few phenocrysts (about 5 percent) are always present.
Breccia. — The breccias, which are always strongly altered, consist of fragments similar in appearance to the flow rocks described above. As pointed out previously, the fragmental nature of many of the breccias is difficult to detect in hand specimen as well as in outcrop, and sawing and polishing commonly are necessary to disclose the true texture. The fragments are quite angular, and although they are not perfectly monolithologic the minéralogie and
textural variations are quite small.
The fragments are microporphyritic with a groundmass that is suspected to have been felty before extensive
alterations took place. Phenocrysts are as large as 2 mm
but dimensions approaching 0.5 mm are far more common. The rocks are extensively altered to minerals such as
zeolites, carbonate, quartz, sericite, magnetite, hematite, and montmorilIonite. Phenocrysts include plagioclase (An^^
that is usually replaced by zeolites and a little sericite
(fig. 19), green hornblende that is opacitized (black type)
and altered to bowlingite, and pyroxene that is entirely altered to hematite and bowlingite (fig. 19). The ground mass consists of plagioclase microlites, a little apatite
and magnetite, and a great host of alteration minerals. 70 Carbonate is notable in that it forms large replacement masses in the groundmass, some of which contain crystals up to 1 mm in diameter.
Breccia Dikes. The breccia dikes are composed of heterolithologic aphanitic fragments containing a few horn blende and feldspar phenocrysts. Small fragments tend to be quite angular but larger ones show some rounding. The coarsest material Cup to 2 cm) is located in the middle of
the dike, grading outward to very fine-grained material along the dike borders, which have a chert-like aspect.
The thicker dikes may have a curtain of fine-grained ma terial between two coarser portions. The gradation in grain size from coarse to fine is not continuous, as indi cated by coarser streaks within the fine-grained borders
(fig. 20), Fresh rock is medium brownish gray and weathered surfaces are light grayish tan to light brick red. Al
though the lithic fragments vary in color, the texture and mineral content of all fragments seem quite similar mega-
scopically and closely resemble those of the lava flows
into which the dike has been intruded. Irregular joints, which are approximately normal to the outer surface of the dike and which appear to cut only the fine-grained border material, are usually present.
Microscopic examination disclosed considerable varia tion in texture, but mineral composition is fairly constant.
Textures are usually felty-microporphyritic, but excellent 71
Figure 20. Photograph of a aemi-poliahed specimen of an Early acid heterolithologic breccia dike composed of angular fragments of hornblende andésite. Note the coarse material in the center of the dike grading to fine-grained borders, and the mineralized joints normal to the dike wall.
Figure 21. Photomicrograph of the central portion of a breccia dike showing variable fragment texture, veins, and cavity fillings. Plane light.
Figure 22. Photomicrograph of the fine-grained border of a breccia dike, showing pseudo-crossbedding and lenses of coarser material. Note the zeolite vein normal to the banding. Plane light. 72
Figure 20
Figure 21
Figure 22 73 pilotaxitic and trachytic textures are common (fig. 21). Phenocrysts, consisting of hornblende and plagioclase only, are usually very small (0.23 to 0.5 mm with 1.0 mm as a maximum). The composition of plagioclase is quite vari able and on the basis of compositional groupings, the presence of two kinds of plagioclase is indicated (An^2-41 and An^g_gg). Hornblende also shows considerable varia tion from common green hornblende to oxyhomblende. It is usually opacitized (black type) and may be replaced by opal, chalcedony, and zeolites. In addition to hornblende and plagioclase, minor quantities of apatite and quartz and plentiful magnetite are present. Peculiarly rounded plagioclase grains quite unlike most of the plagioclase, and small quantities of strongly sericitized potash feld spars, are probably foreign.
The borders of the dikes show a distinct "mylonitized" appearance owing to the presence of parallel bands of crushed fine-grained material (fig. 22). These bands are separated by lenses and wedges of angular, coarser ma terial that in some cases produces a kind of pseudo crossbedding. The fragments range in size from 0.04 to
0,2 mm, but much of the material is biréfringent "dust".
There is considerable alteration within the dikes, resulting in numerous pseudomorphs, and secondary mineral ization is an outstanding feature. Angular interstices 74 between lithic fragments are filled with zeolites, es pecially chabazite, chalcedony, quartz, and minor opal
(fig. 23-24). All the cavity fillings seem to be connected to one another by a network of veins, which extend from the cavi ties along irregular joints to the border of the dike (fig. 20).
Mineralogic and Textural Associations
Table 2 lists commonly associated minerals in Early acid volcanic rocks. The texture of all suites is basically felty-microporphyritic, but there is some minor variation in the degree of development of the felty groundmass texture.
All suites contain magnetite and apatite,and secondary or alteration minerals are not considered. The minerals are arranged from top to bottom within each suite in order of decreasing abundance, and the suites themselves are ar ranged from top to bottom in order of decreasing incidence.
The order of the suites also defines the order of decreas ing development of felty groundmass textures. 75
Figure 23. A photomicrograph of an Early acid breccia dike showing a cavity filling lined with zeolite (light gray), and filled with chalcedony and quartz (white). Note the variation in texture of the enclosing angular breccia fragments. Plane light.
Figure 24. Same as figure 23, but with crossed nicols. Note the gradation from fibrous chalcedony near the margin of the cavity to equant quartz grains at the center. 76
Table 2. Mineral Suites of the Early Acid Lava Flows 1. labradorite (An-, hornblende bronzite (locally equally as abundant as hornblende, but usually less plentiful) 2. labradorite (An-, --) hornblende bronzite (always less abundant than hornblende) diopsidic augite (minor)
3. andesine-labradorite (An^2_6o^ diopsi%^^((approximately equal amounts)
4. andesine-labradorite (discontinuous, possibly two distinct plagioclases, or possibly foreign crystals) diopside (percentage of diopside equals combined bronzite and hornblende) bronzite hornblende
Modal Analysis
The following modal analysis (Table 3) is considered to be representative of the Early acid lavas, and is the most abundant suite of minerals shown in Table 2. The analysis was made by counting 1400 points in a grid approxi mately 22 X 22 mm by using a mechanical click-stage. The figure for each primary mineral includes alteration min erals that may be present, but secondary minerals such as quartz, zeolites, clays and carbonate, which occur in cavity fillings or as replacements of primary minerals not related to the composition of the primary minerals
(e.g., pyrite after plagioclase), are recorded separately. 77
Table 3. Typical Modal Analysis of Early Acid Lavas
Component Percent
Groundmass (plagioclase, pyroxene, magnetite, secondary minerals) 58.9 Labradorite CAn^g_gQ) P, M 28.3 Hornblende P, M 3.4 Bronzite (En^g^y^) M, G 3.2 Magnetite M, G 1.7 Secondary Minerals 4,3 Total 100.3
Total P 8i M (approximately) 36.0
P = phenocryst; M = microphenocryst; G = small grains.
Source of the Volcanic Eocks
A definite vent or vents from which the breccias and the lavas erupted was never located in the field, but cer tain lines of evidence permit speculations concerning a possible vent area. This hypothetical vent lies approxi mately one mile south to southwest from Hedges Peak along the mountain front north of Canyon Junction, and approxi mately the same distance northwest of the Grand Loop Road from the intersection of the road with the Howard Eaton
Trail. Evidence which indicates the close proximity of a vent in this area is the following:
1. Strong flow banding and steep dip of the lavas
2. Breccia dikes
3. Coarsest breccia 4. Center of alteration 78
The first point is nearly self-explanatory. Lavas as they c(xne from the vent are in a state of maximum fluidity and therefore would be more likely to develop flow structure than at some distance from the vent. The dip of the flows is likely to be steeper near a vent since small extrusions, which occur more frequently, would accumu late near the vent causing a cone to form, and only the larger extrusions would send lava to areas remote from the vent, where the dip would be less steep.
The occurrence of breccia dikes is generally consid ered a good indication of the proximity of a vent. Curtis
(1954, p. 464) noted the normal association of breccia in trusions (dikes and domes) in the vicinity of eruptive centers. The possible mechanisms of brecciation will be discussed in the chapter of this report on fragmental
Early basic rocks.
The angular nature of the rock fragments and the slight variation in composition and texture, indicate that the breccia is pyroclastic in origin. If this is true, the coarsest fragments would be deposited near the vent as a general rule.
When a volcanic cycle subsides and extrusion comes to a halt, volcanic activity is usually represented by the emission of gases and hydrothermal solutions, which follow the old vent conduit and may possibly reach the surface as hot springs. Since the proposed vent area is 79 strongly mineralized with opal, chalcedony, carbonate, and zeolites, it is likely that the vent is nearby.
Conclusion
The time period represented by the Early acid breccia began with alluvial sedimentation occasionally Interrupted by the deposition of wind-born ash and pyroclastic volcanic breccia.
The volcanic component that was deposited intermittent- ly with alluvial sediments, was derived from vents along the Cooke City zone of the Beartooth block. The quantity of volcanic materials increased until they dominated the alluvial sediments. At this stage a local source of vol canic rocks came into existence and buried the alluvial plain beneath sheets of andesitic lava and pyroclastic breccia. The local volcanic cycle was probably initiated with the occurrence of a nuees ardente. The petrogenesis of the magmas that produced the andesitic lavas and vol canic breccias will be discussed in Chapter VIII of this report. CHAPTER IV. EARLY BASIC BRECCIA - NCNFRAGEMENTAL ROCKS
Introduction
Included under nonfragmental rocka are dikes of vari ous types, lava flows. Irregular crosscutting Igneous bodies, several distinctive types of fragments that occur In breccias but that are not represented as dikes or flows, and hybrid or mixed flow-fragmental rocks. The nonf rag- mental rocks comprise 10 percent or less of Mt, Washburn volcanic rocks, but any discussion of the petrography of the nonfragmental rocks Is also a discussion of the petrog raphy of the volcanic breccias, since the components of the breccias are the same as or similar to the nonfrag mental rocks. Therefore the section on pétrographie min eralogy, which follows, applies to nearly all rocks com posing Mt, Washburn. The outstanding feature of all min erals In these rocks Is their nearly constant properties and composition regardless of the origin of the rock In which they occur. Apparently, the most Important variant from rock type to rock type Is the kind of mafic mineral present and Its percentage of the total rock.
Pétrographie Mineralogy
Plagioclase
Plagioclase, usually labradorite or less commonly calcic andesine. Is the most abundant mineral In all the volcanic rocks and occurs as phenocrysts, microphenocrysts,
80 81 and microlites. Crystals are usually euhedral to subhedral laths or less commonly nearly square rectangles (fig. 25). Intergrown grains on the (010) surface and small glomero porphyritic aggregates are quite common (fig. 26). Zoning is always present, and is usually high-frequency oscilla tory zoning. Reverse zoning is common and most larger grains show one or two reaction zones within the crystal (fig. 27-28)#
Small, beady, nearly opaque, glass-like inclusions are com monly associated with reaction zones or are concentrated in the center of the crystal. Twinning according to the albite-
Carlsbad law is by far the most common type. The twin lamellae are very fine and quite numerous but are unevenly distributed. The Carlsbad plane is commonly irregular and multiple, producing a very complex crystal (fig. 26). Al- bite twinning alone is the next most common type of twinning, followed by minor acline and rare Manebach twinning. Nearly square grains generally show a very complex twin composition consisting of albite-Carlsbad, Manebach, acline, and Baveno
(fig. 25).
Plagioclase is usually unaltered, but in certain cases it alters to zeolites such as analcite, which destroys the twinning, and also to a pale-green, cryptocrystalline ma terial resembling clay. Both alterations commonly follow certain compositional zones of a crystal or may be concen trated in the core, which is usually more calcic. Incipient development of sericite and epidote may be present in grains that occur in strongly altered rocks. 82
Figure 25, Plagioclase phenocryst showing an unusual type of twinning characteristic of square grains. The twins are albite-Carlsbad, acline, Manebach and Baveno, Note oscillatory zoning. Crossed nicols.
Figure 26. Common type of complex plagioclase phenocryst with oscillatory zoning and albite-Carlsbad twinning. Intergrowths, largely along the (010) face, result in glomeroporphritic texture. Some intergrowths occur along the (001) surface and Carlsbad composition planes are commonly multiple. Note the inclusion of a pyroxene crystal. Crossed nicols. 83
Figure 27. Large euhedral plagioclase phenocryst show ing alteration of the core to analcite. The plane of the photograph is nearly equivalent to the (010) surface of the plagioclase crystal. Plane light.
Figure 28. The same plagioclase phenocryst as above showing two reaction zones and oscillatory zoning. Crossed nicols. 84
Olivine Olivine is a rare mineral in these rocks and is usual ly represented by only 2 or 3 bowlingite pseudomorphs in thin sections of the more basic rocks. It occurs as indi vidual euhedral to rounded phenocrysts or microphenocrysts, but never exists as a groundmass constituent. Fresh olivine that is always high in magnesium and shows little variation in composition from rock to rock is unusual and alters to bowlingite (fig. 29-30). In some cases the olivine, or possibly the bowlingite, has been replaced by carbonate or opal. The larger phenocrysts may show a corona of diopside, or of orthopyroxene and plagioclase (fig. 31-32), probably caused by magmatic reaction.
Orthopyroxene
Orthopyroxene in these rocks is always bronzite En^2„82' although the most common composition of bronzite is Enyg_gQ.
The determination was made by measuring the optic angle and the sign with the universal stage (2V = 67® - 80®, sign neg ative; see Wahlstrom, 1955, p. 160). The composition is constant regardless of rock type. Compositional zoning is common but shows a maximum variation of only 5 percent En.
Crystals are mostly euhedral microphenocrysts although some monomineralic glomeroporphyritic aggregates usually are pres ent. There is a close association between bronzite and mag netite that occurs as vermicular inclusions in bronzite or as grains attached along the border of the pyroxene. It is 85
Figure 29. Small euhedral phenocryst of olivine partially altered to bowlingite that is in turn partially replaced by carbonate. Note zoning in orthopyroxene grains in the upper right-hand corner of the photograph. Crossed nicols.
Figure 30. Somewhat irregular, rounded olivine phenocryst showing minor alteration to bowlingite. Plane light. 86
Figure 31. Partly resorbed olivine, nearly at ex tinction, surrounded by a corona of twinned diopside grains. Crossed nicols.
Figure 32, Nearly euhedral olivine phenocryst entirely altered to fibrous bowlingite and surrounded by sub hedral grains of orthopyroxene and plagioclase. Note the curving fractures in bowlingite, preserved from the original olivine. Plane light. 87 not unusual for bronzite to occur with thin crystals of diopside attached to the prism faces and, in some cases, the orthopyroxene is entirely mantled by polysynthetically twinned diopside (fig. 33). The relationships seem to in dicate that the diopside developed from the bronzite through magmatic reaction or by some type of inversion. It is pos sible that bronzite acted as a seed crystal or nucleus for later diopside. The composition of the diopside, determined with a universal stage, is the same as normal diopside phenocrysts in other parts of the rock, Bronzite is never twinned and almost never altered except in a few strongly oxidized rocks.
Figure 33. Orthopyroxene, nearly at extinction, showing complete mantling by polysynthetically twinned diopside. Crossed nicols. 88
Clinopyroxene The optic angle (2V = 52° - 58°), the high-birefring ence (.027 - .030), and (1.685 - 1.680) indicate that clinopyroxene in these rocks is near diopside in composi tion (see Wahlstrom, 1955, p. 163), The maximum variation of optical properties extends the composition to diopsidic augite. Compositional zoning with two or four prominent zones is almost always present (fig, 34) and in the largest phenocrysts (10 mm) as many as 25 oscillations occur in some cases. Reverse zoning is rarely present. The exterior zones and the smallest grains are usually diopsidic augite,
2V = 47°. Diopside may be euhedral but subhedral grains are more common, and it shows a distinctly less well- developed crystal form than bronzite. Twinning is always present. It occurs as simple twins with the twin surface, which is multiple in some cases, parallel to (100), and less commonly as polysynthetic twins parallel to (001) especially in grains inverted from bronzite. The clino pyroxene is almost never altered and shows a pronounced tendency to occur in nearly round, loose to compact glomero- porphyritic aggregates (fig. 35).
Hornblende
Hornblende is always somewhat oxidized and commonly quite close to oxyhornblende or basaltic hornblende in composition. The color is brownish green to dark reddish brown depending on the state of oxidation. The intensity 89
Figure 34. Euhedral diopside phenocryst showing 4 or 5 prominent compositional zones and two twin bands parallel to (100). The view is approximately per pendicular to (010). Crossed nicols.
Figure 35. Typical round, compact, glomeroporphyritic aggregate of diopsidic augite composed of anhedral individual grains, some of which show zoning and twinning. Crossed nicols. 90
Figure 36. An example of extreme resorption of an oxyhornblende phenocryst with a black type opacité border.
of pleochroism also increases in proportion to the degree
of oxidation. Crystals occur only as phenocrysts with a
euhedral shape that is commonly rounded due to resorption
(fig, 36). Hesorption and advanced oxidation are usually accompanied by a black type opacité border. Hornblende
is commonly zoned and larger grains are usually poikilitic,
Cavernous or defective crystals are common, and pinacoidal
twins, some of which are multiple, occur in larger pheno crysts. In a few cases, hornblende contains a core of bronzite, a relationship that seems to indicate hornblende developed from bronzite by magmatic reaction. In addition
to opacitization, hornblende is altered to bowlingite and replaced by carbonate in a few places. 91
Other Primary Minerals Other primary minerals noted are omnipresent magne tite and a little biotite that occurs only in a few special lava flows. Generally, magnetite occurs as tiny equant grains and dust-sized particles in the groundmass, or as microphenocrysts and grain aggregates. Magnetite may also occur as vermicular inclusions in orthopyroxene and as an alteration product around olivine and oxyhornblende.
Biotite, usually highly pleochroic from pale greenish tan to dark brown, is usually opacitized in a manner similar to oxyhornblende. It occurs as irregular to ragged plates, and is altered to bowlingite in a few cases.
Groundmass
The groundmass of all rocks is predominantly plagio- clase microlites and grains, or nondescript felsic minerals with low gray birefringence. Some grains of orthopyroxene, clinopyroxene, and magnetite, along with abundant secondary minerals such as montmorillonite, zeolites, and opal, and are also often present. The specific components of the groundmass vary from rock to rock and will be discussed in greater detail for each rock type considered. However, one mineral deserves special attention. It occurs as tiny needles with an index of refraction distinctly higher than that of plagioclase (fig. 37). These needles seem to be monoclinic in form, extinguish at nearly 45°, and are closely associated with tiny grains of magnetite. 92
Needles sometimes occur in such abundance that the ground mass of certain rocks is nearly opaque. Although all optical
properties could not be determined because the crystals are
8 0 extremely small, the available information indicates that the needles are probably a clinopyroxene.
Figure 37. Incipient clinopyroxene needles closely as sociated with tiny grains of magnetite. The needles are very abundant in certain rocks and are almost always present in the groundmass of all rocks. In the above photomicrograph, they appear as a smear on a large plagioclase phenocryst whose surface is slightly below the surface of the slide.
Secondary Minderals Secondary minerals show two major forms of occurrence
that are not entirely distinct and therefore grade from one
to the other. They are cavity fillings or coatings, and
replacements or alterations of primary minerals. The most 93 important single secondary mineral is montmorillonite and other compositionally related species. Montmorillonite in the form of bowlingite is best crystallized in olivine paeudomorphs (fig. 32). Here it occurs as feathery or fibrous aggregates in which each fiber is length positive. The mineral shows moderate posi tive relief, very high birefringence (greater than .030), distinct double refraction, slight pleochroism, and a pale green color. On the basis of optical properties, the min eral is called bowlingite. Outside of olivine pseudomorphs, in cavities (fig. 38-41) and as disseminations throughout the groundmass, the properties of the montmorillonite vary considerably. The material that is most poorly crystalline is usually in a fibrous or granular cryptocrystalline state, changes in relief from moderately positive to strongly neg ative, in color from pale green to medium brown, and usual ly shows contraction cracks indicating that it probably originated as a gel (fig. 38-39). This material is nearly indistinguishable from many zeolites and biréfringent opal. X-ray diffraction analyses were made on material care fully extracted from olivine pseud omorphs and various types of cavities. The material in pseudomorphs is very soft and easily cut with a knife, presents a greasy luster, and feels slippery to the touch. The analyses proved that the material in the pseudomorphs is trioctohedral (Mg in the octohedral lattice sites) montmorillonite, i.e., saponite that is called 94
0.1 mm
Figure 38. A typical cavity coating of botryoidal zeolites and poorly crystallized montmorillonite showing contraction cracks. The center of the cavity is empty. Plane light.
0.1 mm
Figure 39. Same as above, but crossed nicols. Note the banded and botryoidal nature of the coating and the fibrous structure, normal to the border of the cavity, producing spherulitic extinction. 95
Figure 40. A typical cavity filling of fibrous bowlingite (light gray) with separate aggregates of zeolites (white) and a center of opal (dark border). Plane light.
Figure 41. Same as above, but crossed nicols. Note the fibrous structure of the bowlingite. 96 bowlingite where it is in a fibrous form. This confirmed the bowlingite determination made on the basis of optical properties. Mineral matter from the cavities, which varied considerably, some of it resembling opal and zeolites, proved to be extremely poorly crystalline saponite on the basis of X-ray analysis. Many other secondary minerals do occur in Early basic volcanic rocks, but they are always minor. True zeolites occur as roundish aggregates of wedge-shaped crystals es pecially associated with bowlingite (fig. 40-41), and colorless opal is sparingly present. Secondary quartz associated with tiny rosettes of bowlingite is conmon in a few dikes, and epidote occurs in a few xenolithic in clusions (fig. 42).
Basalt Dikes The basalt dikes are remarkably similar throughout the map area, both in form and composition. They occur most abundantly in the spurs to the south and east of Mt.
Washburn and on Dunraven Peak. South of Mt. Washburn the dikes are all oriented approximately N. 20° W. and are so abundant that they might be considered as a small dike
swarm. North of these areas the dikes rapidly disappear.
Most of them are very short and usually only 10 to 20 feet wide. They may bevel to a feather edge that passes into a joint both upward and along strike, bifurcate, disappear
into the bottom of channelized lava flows, and in some cases 97
Figure 42. Epidote (dark mottled gray) developed from oxyhornblende opacité (ri^ht side) and plagioclase crystals of a xenolithic inclusion in a lava flow. Also, note the alteration of plagioclase to zeolite (light gray), especially in the core of the crystals. disappear along strike only to reappear again trending in
the same direction but offset 10 feet or so into an ad
jacent joint. The dikes always are nearly vertical,
show well-developed horizontal columnar jointing, and
sheet jointing parallel to the dike wall (fig. 43).
Commonly but not always they stand out in relief and
contain vesicles in the center stretched out parallel to the strike of the dike (fig. 44). The contacts of
the dikes with the country rock, generally volcanic
breccia, show no evidences of contact metamorphism and
the texture and mineral content of the dike apparently
does not change. The only visible change is an incon
spicuous gradation from dark gray to brick red on 98
Figure 43. Basalt dike standing out in relief shown cutting a lava flow. Note the horizontal columnar jointing and we11-developed sheet jointing parallel to the dike wall. West cliffs of Mt. Washburn.
Figure 44. A typical basalt dike showing horizontal columnar jointing and concentration of vesicles in the center. South spur of Mt. Washburn. 99 weathered surfaces of a dike within 2-4 inches of the con tact , and a slightly finer grained texture. A similar discoloration occurs around breccia xenoliths, which are rare. One example of multiple intrusion was found, in which a later porphyritlc hornblende andésite dike was injected parallel to a basalt dike; and one very inter esting composite dike was discovered, which will be dis cussed separately in detail. In hand specimen, the basalt dike rocks are black or dark gray to dark yellowish gray depending on the degree of alteration. All are aphanitic but usually a few pheno crysts of pyroxene or olivine are visible, and in some cases fine plagioclase laths may be visible. A pale yel low to resinous yellowish brown alteration mineral that feels slippery to the touch and shows a greasy luster is usually present. Vesicles, or at least tiny irregular cavities, are commonly present and are usually coated or filled with the resinous yellowish brown mineral described above.
In thin section, the dikes are holocrystalline with a microporphyritic texture. The texture of the groundmass is generally intersertal ^ but may grade to intergranular
•^Intersertal (fig. 45) is used to describe a texture in which glass, secondary minerals, fine opaque grains or cryptocrystalline material occurs between small unoriented laths of plagioclase and pyroxene grains.
^Intergranular (fig. 46) is a texture created by unor iented plagioclase laths that enclose small grains of pyroxene or magnetite. 100
Figure 45. A typical microporphyritic texture with an intersertal groundmass. Microphenocrysts include complexly twinned labradorite, bronzite associated with magnetite, and diopsidic augite. Crossed nicols
Figure 46. A typical microporphyritic texture with an intergranular groundmass. Note the zoned and twinned diopside phenocryst and spherical amygdules of an opal-like substance. Crossed nicols. LOI
or felty depending on the size and number of plagioclase microlites. Microphenocrysts occur up to 1.7 mm long, but are seldom over 0.5 mm. These include lath-shaped
and well-zoned labradorite (An^o Tnt average about An__ , /u o u ^ o 5 olivine entirely altered to bowlingite, bronzite (En^g * diopsidic augite (2V = 50® - 54®), and magnetite. Pyroxene,
especially clinopyroxene, tends to occur in loose, ragged,
glomeroporphyritic aggregates whereas the other minerals
occur as individual phenocrysts. Flow orientation of
plagioclase laths is generally lacking, but local orienta
tion parallel to the borders of phenocrysts does occur.
Vesicles or irregular cavities are almost always present
and comprise nearly 9 percent of the rock in a few cases.
These cavities are coated or filled with bowlingite, opal,
and zeolites. The groundmass consists of tiny plates and microlites
of plagioclase 0,1 mm or less in length, granular pyroxenes,
and magnetite, but no olivine. Commonly the groundmass is
heavily contaminated with bowlingite and zeolites, and may
contain tiny pyroxene needles in such abundance as to cause it to be nearly opaque. Abundant anhedral, low biréfringent
material between recognizable minerals may be potash feld
spars or quartz.
Modal analyses (Table 4) of basalt dikes vary some
what in detail, but the general features are usually quite
similar. Unidentifiable and indistinguishable groundmass 102 materials generally account for 45 percent of the total rock, and phenocrysts usually range between 30 and 50 per cent with 35 percent as an average.
Table 4, Representative Modal Analysis of Early Basic Breccia Basalt Dikes
C omponent Percent
Labradorite 34 Diopsidic augite 6 Bronzite T-5 Olivine pseudomorphs T-9 Magnetite 1 Secondary (excluding pseudomorphs) 6 Groundmass 45
Labradorite is always the most abundant mineral by far, and diopsidic ai%ite is usually second and always greater in abundance than bronzite. There seems to be a sympathetic relationship between olivine and bronzite, i.e., where one is abundant, the other occurs only in traces.
In conclusion, the basalt dikes seem to be fairly typical volcanic rocks, i.e., they are slightly porphyritic aphanites showing many features resulting from magmatic disequilibrium, such as prominent mineral zonation, lack of exsolution in pyroxenes, and poorly crystallized ground mass material. There is a tendency for the minerals, es pecially plagioclase, to occur in three distinct size groups (phenocrysts, microphenocrysts, groundmass). The dikes may be considered basalts on the basis of the pres ence of labradorite, olivine, and magnesium pyroxenes. 103
Composite Dike
Field Occurrence and Megascopic Description
The composite dike will be discussed in detail, not because it is one of the longest and widest dikes in the area, but primarily on account of the rare and unusual petrologic phenomena it contains. The dike is somewhat longer than 1/2 mile, a giant among Mt, Washburn dikes, and extends N . 20° W. immediately west of Washburn summit and along the east flank of the south spur of Mt, Washburn.
Its maximum width is 75 feet and it shows nearly vertical contacts. The most striking feature of the dike is that the border phase is quite distinct from the interior, as shown by variable reactions to weathering and erosion.
The border phase stands as a low wall and the interior is a topographic depression except for a central, rounded ridge producing the broad, symmetrical, ’*W**-shaped cross profile of the dike. At some places there are subsidiary ridges lower than the central one, symmetrically displaced with regard to the center of the dike. The contact of the two phases is distinct but not sharp, because there seems to be a gradational zone several inches thick between them.
In a few places this contact is marked by sheets of auto breccia in which angular fragments of the black border phase seem to be included in a later igneous rock that is similar to the central phase of the main dike (fig, 47). 104
Figure 47. Photomicrograph of the autobrecciated portion of the composite dike showing dark angular fragments of the border phase Imbedded In Igneous rock similar to the central phase, clearly showing age relationships. Plane light.
The border is composed of aphanitic dense, black andésite with a few olivine phenocrysts now altered to dark yellowish-orange montmorillonite, and an occasional small xenolith. The central phase is a light olive-gray andésite with abundant tiny irregular cavities and a few
(5 percent) hornblende phenocrysts, some of which are
8 mm in length. In addition to hornblende phenocrysts, this phase contains an occasional, peculiar, egg-shaped aggregate of hornblende crystals from one to three centi meters in diameter. The central phase weathers into a rounded form of a light brick-red color, with a gruss- covered surface due to the hackly fracture of the rock. 105
Petrography of the Border Phase The texture of the border phase is microporphyritic with a felty to subpilotaxitic groundmass (fig. 48), which was unexpected considering the megascopic appearance of the rock. Microphenocrysts and phenocrysts comprise 25 percent of the rock and include plagioclase, orthopyroxene, olivine, clinopyroxene, hornblende-biotite, and magnetite, listed in order of decreasing abundance. All grains are small, usually less than one millimeter, but some mafic glomeroporphyritic aggregates and olivine phenocrysts are 2 to 3 mm in diameter. Most phenocrysts vary from
0.40 to 0,75 mm, and the groundmass minerals are 0.1 mm or less. Alteration of the rock is not general except for a few minerals, although discrete cavity fillings of bowlingite, zeolites, and some opal arc fairly com mon (3 percent). Plagioclase ranges in composition from An^]^_^o, with an optically determined composite average of An^^, and so may be called andesine. The feldspar shows very strong oscillatory zoning and commonly contains one or two reaction surfaces. Orthopyroxene occurs as three apparently distinct compositional types that are all either enstatite or mag- nesian bronzite: small, euhedral microphenocrysts 2V = 83°, negative (Eng^); small anhedral microphenocrysts 2V = 74°, positive (EHç^); large phenocrysts and most other grains
2V = 83° - 90°, positive (Engy 3 _çq 3 ), (see Wahlstrom, 1955, p. 160). It is usually zoned, as shown by strongly 106
Figure 48. PhotomicrographcC the fine-grained, micro porphyritic border phase of the composite dike with phenocrysts of hornblende (H) partly converted to bio tite (B), partially altered olivine (01), andesine (A), and diopside-mantled orthopyroxene (O) imbedded In a felty groundmass. Crossed nicols. variable pleochroism, and may grade toward the border into common green hornblende (magmatic reaction?) or be mantled along prism faces by thin diopside crystals (seed-crystal effect?). Perfectly euhedral olivine is nearly pure forsterite Fa^ (2V = 85®, positive) (see Wahlstrom, 1955, p. 204) and 50 percent is altered to bowlingite, which is partially replaced by carbonate. Clinopyroxene occurs as diopside (2V = 55° - 60°) in poorly developed crystals that are often a mass of tiny grains in crysta11ographic con tinuity. Ordinary grains show a thin outer shell closer to diopside than the interior, and commonly occur with wavy extinction (strained?). Common green hornblende occurs 107 as slightly opacitized (black type) microphenocrysts (2V = 68®) that are commonly broken where elongate, as grains (2V = 70®) containing or developed from orthopyroxene, and as peculiar crystals (2V = 74°) with cores of biotite (fig. 49). The variation in the measured optic angle is probably not signif icant due to experimental error, and therefore all hornblende is probably the same composition. The groundmass consists of plagioclase microlites, and grains of both kinds of pyroxene, imbedded in a matrix of feldspathic material, magnetite dust, and pyroxene needles. On the basis of mineral content and chemical analyses, which will be discussed later, the border phase will be called pyroxene-olivine andésite.
Figure 49. An example of the peculiar minéralogie re lationships that occur in the border phase of the com posite dike. The photomicrograph shows opacitized (black type) common hornblende (Hb) with a core of biotite (B) and small patches of orthopyroxene (0), Apatite (A), and plagioclase (F) occur as inclusions. Plane light. 108
Petrography of the Central Phase The texture of this phase is again microporphyritic with a felty to subpilotaxitic groundmass, and the grain size is approximately the same as that of the border phase
(fig. 50), but alteration is much more extensive. The groundmass is strongly zeolitized and infiltrated with olive-drab saponite, imparting a greenish gray color to the rock, Microphenocrysts, which comprise 25 percent of the rock, are listed as follows in order of decreasing abundance: plagioclase, hornblende, and clinopyroxene.
Plagioclase is quite similar to that contained in the border phase except that it is altered to a colorless clay like mineral. Composition ranges from An^Q_^Q with an average of An^^. The 2V of hornblende is 76°. This min eral shows strong pleochroism from very pale green to dark brownish-green. It also shows distinct zoning, and is unaltered and unopacitized. Traces of biotite occur rarely in the center of large hornblende grains. Granu lar and skeletal crystals of clinopyroxene are common, and small euhedral crystals show wavy extinction (strained?).
The optic axis angle is 48° to 53° indicating diopsidic augite. The groundmass is predominantly a fine-grained mosaic of feldspathic material with abundant saponite, zeolites, secondary quartz, and magnetite dust. Imbedded in this matrix are distinct plagioclase microlites, and grains of pyroxene and magnetite. The rock may be called hornblende andésite. 109
Figure 50. Central phase of the composite dike showing felty microporphyritic texture of the main rock (left) and endogenic inclusion (right). Minerals include hornblende, quartz, plagioclase, and diopside. Plane light.
The hornblende ciystal aggregates contain up to 30 percent of plagioclase and quartz (fig. 50). The aggre gate is usually a fairly spongy network commonly showing foliation. Hornblende shows optical properties identical to those of the hornblende in the main rock, except that the grains are much larger and show a more nearly equant crystal form. These crystals are quite cavernous and oc casionally arc composed of separate parts in crystallo- graphic continuity. Plagioclase, which shows strain fea tures such as wavy extinction and bent lamellae, appears to have crystallized later than hornblende, and quartz, which shows wavy extinction and a we11-developed parting, 110
partially fills the interstices between hornblende and plagioclase. The aggregates are interpreted to be endogenic,
being part of an early segregated layer of crystals in a small, partially isolated magma chamber that was disrupted during an intrusive phase of a neighboring magma body.
This interpretation would account for the strain features developed in quartz and plagioclase, and explain the simi larity of minerals in the aggregates to minerals in the main rock.
Comparison of the Border and Central Phases
Both phases contain approximately 25 percent pheno crysts, but the central phase contains considerably more
secondary minerals. The border phase is finer grained, resulting in a greater percentage of groundmass material.
Table 5 compares modal analyses of both phases, and Table 6 compares various pétrographie properties. Ill
Table 5. Comparison of the Modal Analyses of the Border and Central Phases of the Composite Dike
Percent Percent Central Border Component Phase Phase
Groundmass 47.0 69.2 Secondary minerals (mostly saponite) 15.0 2.5 Enstatite-bronzite not present 5.5 Diopside-diopsidic augite 2.1 4.5 Olivine (50 percent altered to bowlingite) not present 3.5 Andesine (An^Q.gg) phenocrysts 19.4 13.5 microlites 10.8 inc.in gm. Hornblende 4.4 1.1 Biotite trace trace Apatite trace not present
Total 100.0 100.3 112
Table 6. Comparison of Various Pétrographie Properties of the Border and Central Phases of the Composite Dike
Property Central Phase Border Phase
Texture microporphyrit ic identical but with felty to groundmass subpi1otoxitic finer-grained groundmass
Visible minerals (phenocrysts) hornblende olivine
Inclusions endogenic horn- few foreign blende-plagio- xenoliths clase aggregates
Zoning and reaction 1. moderate zoning 1. strong zoning minerals in plagioclase all minerals, thin outer shells on mafic minerals 2. biotite from 2. biotite from hornblende hornblende 3. hornblende from ortho pyroxene
Alteration and strong alteration, slight alteration, secondary min many cavity few cavity eralization fillings fillings
Rock name hornblende olivine-pyroxene andésite andésite 115
Fetrogenesls
The composite nature of this dike was brought abcut
by the intrusion of two different magmas along the same
channelway. In this case, the original more basic magma
did not have sufficient time to crystallize before the
second more acidic magma was injected, and thus a good deal of commingling of the magmas resulted, producing
hybridization in some cases and brecciation in others.
The occurrence of a large number of mineral phases in disequilibrium, such as occur in the border phase of the composite dike, is believed to indicate the mixing of
partially crystalline magmas (Turner and Verhoogen, 1951,
p. 76). Hybridization of the border phase is shown by the
abundance of peculiar reaction phenomena, the many oscil
latory zones and resorption surfaces in plagioclase, and
the occurrence of thin shells on many mafic phenocrysts different in composition from the main portion of the crystals. The hornblende and hornblende with biotite cores in the border phase were probably introduced dur
ing the second cycle of injection. The composite nature
of the dike is also indicated by autobrecciation of the border phase producing breccia sheets between the border and central portions of the dike, and also by the common
occurrence of broken and strained crystals in the border phase. 114 The first cycle of intrusion probably began with the forceful injection of a normal olivine-pyroxene andésite magma along a prominent joint. This intrusive action triggered a second magma into motion. The second magma was probably situated in a somewhat isolated chamber and was a more acid differentiate of a larger basaltic magma body that yielded both magmas in question. In the second chamber, quiet crystallization of hornblende andésite ac companied by crystal settling was taking place, bq.t this was disrupted by intrusion. The intrusive material of
the second cycle of injection included hornblende andésite magma and aggregates of segregated crystals, which partial ly mixed with the smaller quantity of partially crystallized
olivine-pyroxene andésite magma intruded slightly earlier.
Quartz Diorite Intrusive
The largest intrusive mass in the M t . Washburn area is a dike-like body of quartz diorite about two miles south of
Mt, Washburn (Plate II). The diorite is not well exposed, but does underlie small hills and ridges. It consists of two connected bodies, one elongated in a north-south direc tion and a second stretching to the northwest.
The diorite shows a fine-grained phaneritic texture and is the coarsest igneous rock in the map area. Plagio clase and biotite are the only megascopic minerals and there are no phenocrysts except in one phase of the intrusive that contains round phenocrysts of pyroxene. The last men tioned phase is much finer grained than most of the intrusive 115 and contains abundant pyrite, which causes brown limonite coatings on weathered surfaces. All samples of the intru sive rock are dark gray and usually have a greenish tint.
Microscopic examination shows that the texture of the diorite is nonporphyritic intergranular except the finer- grained phase mentioned above that contains diopside phero- crysts. The texture is actually somewhat peculiar because ophitic plates of quartz seem to enclose all other minerals, producing a kind of subophitic texture with respect to quartz (fig. 51).
The diorite is composed of plagioclase (An^^_^^), diopside (2V = 57°), hypersthene (2V = 56° - 62°, negative), biotite, quartz, and magnetite. The plagioclase is rather unusual in that apparently it originally occurred as euhedral laths, but now almost always is rounded due to resorption, especially where it is in contact with quartz. Also, grains usually show two distinct nonoscillatory zones: a core
(Ang2 _5 4 ) atid a shell (Ani^^_^y) in which twinning is poorly developed and disappears near the periphery. Diopside oc curs as tiny euhedral grains, as rounded compact glomero- porphyritic aggregates, but most commonly as irregular grains and aggregates. The periphery of glomeroporphyritic aggregates is strongly replaced by biotite grains, produc ing a kind of sieve stincture, and epidote is common through out the aggregates. The larger irregular plates of diopside commonly are changed into a pale-green pleochroic amphibole 116
Figure 51. Photomicrograph of the quai-tz diorite in trusive showing ophitic plates of quartz (Q), biotite (B), diopside (D), hypersthene (H),and plagioclase CP). Texture is intergranular to subophitic with respect to quartz. Crossed nicols.
Figure 32. Photomicrograph of diopside CD) showing inversion to a green amphibole CA), and finally to biotite CB), presumably through magmatic reaction. Plain light. 117
(2V = 71°, negative) along the border and along cleavage cracks, and the amphibole, in turn, is changed into normal brown biotite (fig. 52) at the very edge. Quartz shows evidence of straining, such as wavy extinction and well- developed parting. Generally, alterations are very minor, usually just bowlingite from biotite and hypersthene, but the fine-grained prophyritic phase is fairly strongly al tered. Clinozoisite and some pistacite are scattered as tiny grains throughout the porphyritic phase, and irregu lar patches and grains of pyrite are associated with epidote concentrations. The pyrite, however, might not be secondary, since rocks high in pyrite lack any traces of magnetite.
Table 7, Typical Modal Analysis of the Quartz Diorite Intrusive
Component Percent Plagioclase 39 Quartz 27 Diopside 11 Amphibole from diopside 1 Hypersthene 10 Biotite 9 Magnetite 3 Pyrite T Total 100
Flows
Introduction
Lava flows in the Mt, Washburn area are quite diverse in composition and appearance compared with the simplicity of the basalt dikes. They grade from normal flows into 118 volcanic breccia, producing various hybrid rocks, and also take on many Irregular forms so that It Is difficult In some cases to differentiate Intrusive sills, extrusive flows, and "squeeze-ups". In view of this diversity, the discussion of the Early basic breccia lava flows Is divided somewhat artificially Into three units; normal channel flows, Irregular Igneous bodies, and mixed flow-fragmental rocks. As In the case of the basalt dikes, all the lava flows and related rocks are localized on Mt. Washburn Itself, on the east and south spurs of Mt. Washburn, on Dunraven Peak, and on Hedges Peak. To the north and west of these areas, the flows rapidly die out and only a rare channelized flow extends for any distance In these directions.
Normal Channel Flows
The channel flows Include the largest bodies and the greatest volume of nonfragmenta1 Igneous rocks In the map area. These flows usually have a restricted lateral ex tent, a channel-like form, and are commonly Interbedded with the volcanic breccia, but In some cases sheet-like bodies occur (fig. 33). Channel flows are especially prominent on the west cliffs of Mt, Washburn and on the first and second peaks east of Mt. Washburn summit on the east spur. Flows cause local steepening of slopes and In places cap peaks with steep, blocky forms. Channel flows may split, forming distributaries, and In some cases have 119
Figure 53. Series of field photographs depicting the common variations in the form and occurrence of lava flows.
Figure 33A. A thick channelized lava flow that moved from right (south) to left (north). Second peak east of Mt. Washburn summit on the east spur,
Figure 53B. Thin, banded flows (hybrid rocks) interbedded with volcanic breccias. Southern end of the south spur of Mt. Washburn.
Figure 53C. Nearly sheet-like flows interbedded with volcanic breccia. First peak east of Mt. Washburn summit on the east spur. 120
Figure 53A,
Figure 55B,
Figure 530. 121 apparently been fed by dikes from below (minor fissure eruptions). Sheet jointing parallel to the lower contact is normally well developed and in places, curves upward and merges into a less well developed vertical columnar jointing. The bottom contact of flow with breccia is commonly gradational and along this contact the flow con tains stretched vesicles in some cases. Usually there is much discoloration caused by alteration to silica minerals and clays. The concentration of secondary minerals near the base of flows is probably due to the damming effect of the dense flow rocks against hydrothermal solutions permeating the underlying porous breccias. The tops of flows are generally vesicular to frothy, and characterized by hematitic alteration. True autobreccia within the channel flows is not uncommon, and strong flow structure is not usually associated with channel flows. This type of flow may reach a thickness of 60 feet or more, but most are considerably less thick. The rocks are usually black or dark gray basalts or andésites, but the color changes to grayish red or pale red depending on the degree of alteration. The texture of most flows is fine-grained aphanitic or occasionally glassy, and commonly slightly porphyritic with not more than 10 percent phenocrysts. Hornblende and pyroxene pre dominate as phenocrysts, but olivine and tiny laths of plagioclase may be present, especially in vitrophyres. 122
A striking and characteristic channel flow is the massive lavas that cap the second peak east of Mt. Washburn
(fig. 53A). The lower portion of the flow is a pale-red oxyhornblende andésite containing bronze-colored biotite and a few foreign xenoliths, and the upper portion is a dark-gray andésite of the same composition. Some of the more unusual channel flows, especially those that are gradational into the mixed flow-fragmental rocks, show flow structure indicated by maroon and black, or black and dark-gray layers. Weathered surfaces on all channel flows are usually light brown or light brick red.
Microscopically, the channel flows vary from holo- crystalline to hypocrystalline, and are always micro porphyri tic (microphenocrysts 24-50 percent, average
35 percent). The groundmass texture varies from inter- sertal where orthopyroxene and olivine are common, to felty-pilotaxitic or subtrachytic where hornblende is common (fig. 54, 55, 57). This textural difference is probably indicative of the higher silica content of horn- blendic rocks. Phenocrysts include plagioclase (An^^_^2)* olivine (Fa^^^) ï diopside-diopsidic augite, bronzite
(En<7 «7 _Q2 ), oxyhornblende, and very rare biotite, Plagio clase laths quite commonly show alignment parallel to the flow banding or at least are arranged concentrically around the larger phenocrysts. The dark-gray to black flow banding, which is quite common in certain rocks, is 123
Figure 54. Photomicrograph showing the microporphyritic- felty texture of flows containing hornblende. Note the small glomeroporphyritic aggregates of diopside, magne tite , and strongly resorbed and opacltized (black type) oxyhornblende in a hypocrystalline felty groundmass. Plane light.
Figure 55, Photomicrograph of the unusual nearly trachytic texture of the light-red lava flows which cap the second peak east of Mt. Washburn summit, with microphenocrysts of entirely opacltized hornblende, partially opacltized biotite (B), and labradorite. Plane light. 124
Figure 56. Photograph of a aeml-pollshed hand specimen showing black flow bands, rounded pyroxene glomero- porphritic aggregates, and light-color mineralized fractures.
Figure 57. Photomicrograph of the specimen pictured above. The black banding is due to the concentration of magnetite dust in the matrix of a rock with microporphyritic inter- sertal texture. Note diopsidic augite phenocrysts and partial alignment of plagioclase laths parallel to the black banding. Plane light. 125 due to the concentration of magnetite dust in certain lay ers (fig. 56-57). The cause of this concentration could not be determined from the writer’s examinations and no explanation was found in the literature.
Alterations in the lava flows are not extensive ex cept for olivine (to bowlingite) and oxyhornblende (re sorbed and opacitized), and occasionally plagioclase (to zeolite). Mineralized joints and cavities filled with saponite and related minerals, zeolites, and opal are present. Secondary quartz is an unusual alteration min eral, and epidote is present in some places, especially in xenoliths in the more acidic rocks. Devitrification of glassy rocks, producing a low biréfringent aggregate resembling quartz and feldspar, is a common alteration.
Table 8. Representative Modal Analysis of the Normal Channel Flows Component Percent
Labradorite (An^/^_@2) 14.0 Diopside 6.1 Bronzite (Enyy_g2) 6.3 Olivine (Fa]^2^ (altered) 0,3 Oxyhornblende 3.3 Magnetite 1.5 Secondary minerals 0.8 Groundmass 67.2 Total 99.5
The groundmass of the rock used for the modal analysis of Table 8 is composed of plagioclase microlites and grains, 126
pyroxene grains and needles, magnetite dust, and minor
secondary minerals (bowlingite and zeolites). The modal analysis is somewhat atypical in that plagioclase usually
occurs in greater abundance and diopside in greater quan
tity than bronzite. Table 9 lists the common mineral associations that
occur in the Early basic channel flows. All associa tions contain labradorite (An^^^g) as the most abundant mineral, and all contain magnetite. In the table, all components are listed in order of decreasing abundance, and minerals that are distinctly minor or rare are so designated.
Table 9. Common Mineral Associations of Early Basic Breccia Channel Flows
1. diopside 5. diopside olivine oxyhornblende bronzite (minor) 6, oxyhornblende 2. diopside diopside bronzite bronzite (rare) olivine 7, oxyhornblende 3. diopside diopside bronzite 8, oxyhornblende 4. diopside diopside bronzite oxybiotite (rare) oxyhornblende (minor) olivine (rare)
The texture of all associations in Table 9 is micro porphyritic, but the groundmass texture changes from inter- sertal in association (1) to pilotaxitic in association (8) with all gradations between. The series from (1) to (8) is 127 believed to show a relative Increase in silica content of the rock and a more advanced stage of differentiation.
This hypothesis is questionable, however, because all rocks contain 50 to 70 percent groundmass, the composi tion of which is very difficult to evaluate, because of the extremely fine-grained nature of most components.
Indeed, chemical analyses of these rocks, which will be discussed later, seem to indicate that an estimate of the bulk chemical composition of the rock on the basis of the composition of the phenocrysts can be very misleading.
Irregular Bodies of Nonfragmental Rocks
Within the area of occurrence of dikes and lava flows are various irregular bodies of igneous rock. The origin of these bodies is very difficult or impossible to deter mine in many cases. For instance, many thin lava flows grade into volcanic breccia at both upper and lower sur faces but the same body could be interpreted to be a shallow injection of magma (sill) between breccia layers. Small pipe-like bodies occur and also masses of igneous rock with vertical flow banding, which is autobrecciated in part, laterally surrounded by volcanic breccia. These vertical masses in many cases appear to be "squeeze ups".
Lava flows may be covered with fragmental rocks before the flows are solid, and due to the superincumbent load may actually be injected upward through the younger fragmental volcanics. The same effect may be achieved by the injection 128 of a eill-like body between poorly consolidated older vol canic breccias with subsequent upward intrusion through a zone of weakness in the overlying breccias. Three inter esting examples of irregular bodies will be discussed.
At the very summit of M t . Washburn there is a chaotic mixture of aphanitic basalt and a variety of fresh to se verely altered volcanic breccias and lapilli tuffs. These rocks show a characteristic light brown weathering surface that shows tiny aggregates of feldspar crystals, and is strongly jointed producing curved, slabby blocks. Mega- scopically, no internal structure is visible, but some excellent examples of partially autobrecciated intrusive relationships occur in a road cut immediately south of the summit.
Exposed in the cliffs at the south end of the south spur of Mt. Washburn are a series of very interesting partially crosscutting sill-like bodies, which arc usually less than five feet thick. These rocks are very dense, hard, black basalts with large olivine and pyroxene phenocrysts and abundant small laths of plagioclase. Microscopically, these sill-like bodies are some of the most unusual rocks in the Mt. Washburn area, because they show a very strik ing intergranular-porphyritic texture and because they contain an abundance of almost perfectly fresh olivine.
The phenocrysts are huge (1,5 to 3.0 mm) laths of labrador ite (An^Q); glomeroporphyritic, zoned diopsidic augite
(2V = 52° in the core; 2V = 47° for thin outer shells); 129 bronzite (En^y) In part Inverted to polysynthetically twinned diopsidic augite (2V = 47° - note the correspondence
to the thin outer shells of diopsidic augite phenocrysts); and rounded to euhedral olivine (^*20-25^ that is slightly altered to bowlingite. The groundmass is composed of granular clinopyroxene, magnetite, and a little olivine enclosed between a mass of unoriented laths of plagioclase.
Near the intersection of Sulfur Creek with the con tact between Early basic and Early acid rocks two miles
southwest of Mt. Washburn is a massive body of basaltic rock.
Megascopically, the rock is a black basalt with large (2-5 mm) pyroxene glomeroporphyritic aggregates. Closely associated with this basalt are rocks showing much hematitic alteration, which appear to be breccias in some cases and lava flows in
others. These properly belong to the group of mixed flow- fragmental rocks. The basalt, which contains labradorite,
clinopyroxene, orthopyroxene, and olivine phenocrysts in a we11-developed intergranular matrix, is strongly altered to bowlingite and carbonate. It is interesting to note
that these massive basalts and extensively altered rocks are in close proximity to the proposed location of the main Washburn vent, which will be discussed in a later chapter. 130
Mixed Flow-Fragmental Kecks Interbedded with volcanic breccia in the area of flows and dikes are spectacular, thin lava flows with well- developed and highly distorted flow banding. These flows commonly grade into breccias, both above and below. They contain peculiar discontinuous clots and lenses that appear to have been fragments, which are now smeared and stretched due to flowage (fig. 58-59), Distinct, undistorted lithic fragments also occur, as well as irregular ’’frothy" areas.
The flow-banding is usually accentuated on weathered sur faces by etching and variable color changes. Maroon or red discolorations are the most common, producing strik ing banded red and black rocks.
The rocks are always aphanitic basalt and rarely con tain phenocrysts. They are usually black to dark gray where fresh, but alteration and weathering produce green ish black, yellowish gray, purple, and maroon discolora tions, Hematitic alteration, which produces reddish colors, is highly characteristic of these rocks, and seems to be concentrated in frothy or fragment-rich portions of the flows. Yellowish or greenish alteration colors are largely due to montmor il Ionite or chalcedony. Elxtensive hematitic alteration commonly obscures the contact with overlying volcanic breccias. Microscopically, the rocks are normal microporphyritic- intersertal in texture and contain the usual minerals. The red and black bands are distinctly shown with the microscope. 13 L
Figure 38. Outcrop of a contorted and discolored mixed lava and fragmental flow showing what appears to be inclusion of fragments partially smeared out parallel to the flow bands. Six inch scale.
Figure 59. Photograph of a semi-polished hand specimen of the same type of flow rock pictured above. Note the distinct contrast in individual flow bands and the knot on the right-hand side of the photograph, which may have been an included fragment. 132
The black bands are essentially unaltered normal basalt
showing excellent alignment of plagioclase laths. The red bands, on the other hand, show an almost total lack
of orientation of minerals, and even more striking, all
mafic minerals, including pyroxenes, are strongly opaci- tized by hematite (fig. 60). These layers also contain
a dlstincly greater abundance of zeolites and opal. A
closer examination of the red bands discloses a distinct tuffaceous appearance and individual lithic fragments
(fig. 61), This observation confirms the suspicion that
these flows are mixed flow-fragmental rocks, which at some places occur as regularly banded, dense igneous rocks.
A discussion of the origin of such mixed rocks will be
included with the section concerning the origin of breccias.
Fragments Without Nonfragmental Equivalents
As a matter of convenience, two highly distinctive fragment-types will be discussed here, even though they
apparently haven’t any nonfragmental equivalents.
The first of these is a black vitrophyre that contaire
abundant white plagioclase laths approximately two milli
meters long and less abundant green pyroxene phenocrysts. Total phenocryst content is approximately 15 percent,
Vitrophyres are not uncommonly a portion of many lava
flows, but none so fresh nor with such coarse phenocrysts occur in outcrop. The vitrophyric fragments are very
abundant, almost predominant, in certain local breccia units, 133
Figure 60, Photomicrograph of the strongly banded lava flows that apparently contain smeared fragments. The clear bands that show good alignment of plagioclase laths and contain fresh mafic minerals, correspond to the black bands of hand specimens. The bands that con tain abundant opaque grains caused by hematitic altera tion of mafic minerals and showing no alignment of plagioclase laths, correspond to the red bands. Plane light.
Figure 61. Photomicrograph showing distinct lithic fragments that occur in flow bands showing much hematitic alteration, which correspond to the reddish bands of hand specimens. Plane light. 134 especially in one that is well exposed along the Grand Loop Road at Garnelian Point Lookout (west cliffs of Mt. Washburn). The rock is composed of labradorite, bronzite with as sociated magnetite, and rare diopside, all imbedded in light brown glass (N = 1.504) containing tiny grains of the same minerals. All phenocrysts are perfectly euhedral, and the rock is entirely unaltered. The plagioclase, which is char acterized by highly oscillatory zoning, displays many rare twin types such as fcurlings (fig. 62-63), and frequently occurs as multi-grained intergrowths.
A second type of distinctive fragment is a very coarsely porphritic basalt that is probably the most strik ing lithologie type in the Mt. Washburn area. It occurs as the predominant component of an extremely coarse and chaotic breccia unit that outcrops between Dunraven and Hedges Peaks, and as an important breccia unit that out crops on the ridge extending north into the Washburn
Amphitheatre between the headwaters of Camelian and
Tower Creeks. The fresh rock is dark gray and contains approximately 40 percent phenocrysts composed of huge
(5-10 mm) pyroxene crystals or aggregates and abundant laths of plagioclase (1-2 mm). The weathered surface is light tan on which phenocrysts stand out in relief pro ducing a striking, spotted appearance. In thin section, the rock is hypocrystalline with a porphyritic to microporphyritic texture and a very fine- 135
Figure 62. Photomicrograph of a typical vitrophyric fragment with microporphyritic texture. Phenocrysts are labradorite and bronzite with associated magnetite, imbedded in a brown glass matrix containing tiny grains of the same minerals. Plane light.
Figure 63, Same photograph as above but with crossed nicols. Note the plagioclase twin fcurling and well- developed oscillatory zoning (lower right corner). 136 grained pilotaxitic groundmass, which was unexpected con sidering the megascopic appearance and the lack of horn blende. Flow structure, as shown by streaks of magnetite dust and partial alignment of plagioclase laths, occurs in a few cases. Phenocrysts include labradorite (20 percent), orthopyroxene (7 percent), clinopyroxene (4 percent), and magnetite (1 percent). The groundmass, which is composed
of plagioclase laths, pyroxene grains and needles, magne tite, and some brown glass, constitutes 62 percent of the rock, and secondary minerals (montmori11onite and zeolites) make up the remaining 7 percent.
Plagioclase CAn^Q_3 3 ) occurs as intergrown aggregates
on the (010) surface showing high-frequency oscillatory
zoning, and commonly two or three reaction surfaces. Ortho- pyroxene is present as euhedral individuals with a composi tion of approximately EUqq, Clinopyroxene is characterized by high-frequency oscillatory zoning, and tends to be cavernous. The optic angle varies from 52° to 60° and
Ny is approximately 1.690, indicating a composition of
Fsj^3 -Wo^3 -En^Q (diopsidic augite). MontmorilIonite oc curs as botryoidal coatings in irregular cavities that are sometimes stretched parallel to flow structure. CHAPTER V. EARLY BASIC BRECCIA - FRAOŒNTAL ROCKS
Introduction
Approximately 93 percent of the Early basic breccia volcanic rocks in the Mt. Washburn area is volcanic breccia that originated in many different ways. Some of the breccia beds are distinctly pyroclastic, some autoclastic, others laharic, or water laid, but since all gradations exist be tween these types, the specific origin of a particular breccia bed is commonly impossible to determine. In this discussion of the Early basic breccia each category of breccia is considered to be only a small section of a continuous spectrum of breccias. Theirefore, the divi sion of breccia into certain classes is somewhat arti ficial, but is necessary for purposes of discussion.
The thickness of the Early basic breccia varies con siderably. It is estimated to be approximately 100 feet near Hedges Peak, but is at least 2000 feet along the east spur of Mt. Washburn, No stratigraphie section was measured because there is no exposed top or bottom to the formation in areas suitable for measurement. Also, the lack of appreciable lateral extent of units within the formation, and the common occurrence of cut-and-fill structures, would have rendered any particular section unimportant in relation to the whole.
137 L38 Field Occurrence
The Early basic breccia fragmental rocks occur as poorly stratified volcanic breccias with interbedded tuffs and lava flows. Bedding is apparent from across a valley, but at an outcrop it is very difficult to determine in most cases. Stratification is shown by tuff beds, lava flows, material reworked by minning water at the top of massive breccia beds, changes in the concentration, size, or com position of coarse fragments, or differences in weathering and alteration that produce color contrasts (fig. 64-68,
71-74). Near Mt. Washburn and Dunraven Peaks, the breccias are massive and poorly sorted, and in places occur in beds greater than 100 feet thick, although 10 to 30 feet thick
is more common. North and northwest from these peaks stratification becomes progressively better developed.
In the Washburn Range near Cook Peak the breccias are well bedded and show the appearance of alluvial conglomer ates in beds 4 to 6 feet thick.
Individual breccia beds are limited in lateral extent, because of extensive paleotopgraphy (fig, 69-70) and num erous cut-and-fill structures (fig, 71-72), and probably were not extensive initially. To the north and northwest
of Mt. Washburn, where bedding is much better developed, channel-shaped bodies of volcanic breccia or conglomerate are common (fig. 73). In the same area, the tops of thick
breccia beds apparently have been reworked by running water
(fig. 74), In short, there appears to be #very gradation 139
Figure 64. View along the west cliffs of Mt. Washburn showing massive beds of heterolithologic volcanic breccias nearly lacking stratification. Notice that the fragments stand out in relief and that weathering causes prominent vertical forms to develop in these rocks (background). Big Horn sheep (encircled) serve as scale.
Figure 65. An outstanding example of bedding that results from two thin lava flows with an interbed of scoriacéous Vulcanian breccia (under the hammer). South spur of Mt. Washburn. 140
Figure 66. Bedding in heterolithologic volcanic breccias shown by coarser fragments in the upper bed than in the lower one. Each layer probably represents one extrusion or flow of breccia. Note the vertical weathering form and the fact that fragments stand out in relief on weathered surfaces.
Figure 67. We11-developed stratification in volcanic breccia probably owing to reworking by running water. The strata dip to the west and are steeper than normal due to block faulting. Ridge between Tower and Gamelian Creeks. 141
Figure 68. Well-bedded coarse tuff and volcanic breccia cut by a vertical basalt dike (right). The breccia bed is four feet thick. West cliffs of M t . Washburn.
and modification from the poorly stratified, massive, un sorted volcanic breccias near Mt, Washburn, to the moder
ately well stratified, less thickly bedded, better sorted
volcanic breccias and conglomerates northwest in the
Washburn Range.
Massive volcanic breccias tend to produce steep rocky
slopes or cliffs. Weathering causes the dense breccia
fragments to stand out in relief (fig. 66-67, 74) and
creates prominent vertical forms (fig. 64, 66) in massive,
nearly horizontal beds, which in advanced stages become
hoodoos. 142
Figure 69. View along the west cliffs of Mt. Washburn, looking south, showing an excellent example of paleo- topography. The light-gray massive breccia in the lower right corner was apparently eroded in part, and then buried by a later, nearly white breccia. Note the well- developed bedding plane and the small cliff associated with a thin layer of tuff in the left-central portion of the photograph.
Figure 70. View of the first peak east of Mt. Washburn summit, looking north, showing a series of lava flows and volcanic breccias (right) that were apparently partially eroded and then buried by a more recent volcanic breccia (upper left). 143
Figure 71, A light-colored alluvial cut-and-fill etruc- ture within a brown, massive, unsorted heterolithologic breccia. Note the lens shape of the channel filling, and the irregular lower contact. West cliffs of Mt. Washburn.
Figure 72. Closer view of the channel filling above, showing the nature of the stratification, the angular fragments, and prominent joints. Six-inch scale. 144
Figure 73, Channel-shaped body of water-laid volcanic breccia wit% a 12-Inch tuff bed separating two coarser units. Underlying the breccia from the outcrop to the roadway (lower right) is a thick unit of bentonitic tuff and lapilli. Note the recesses in the outcrop caused by the more rapid weathering of tuff.
Figure 74, A coarse volcanic breccia that probably orig inated as a lahar. The upper portion was reworked by running water that caused the stratification shown in the middle of the photograph. Note the large size of several fragments and the apparent graded bedding. 145
Interbedded tuff beds weather more rapidly than the volcanic breccias. Tuff beds commonly are represented by recesses or cave-like features along cliff faces (fig. 69, 73) and are associated with local increases in slope, caused by the greater resistance to weathering of the overlying breccia. Tuff beds are commonly only 4 to 5 feet thick but beds greater than 20 feet thick are known. The thicker beds that show good stratification are probably remnants of material deposited in small ponds or lakes. Some tuff beds are quite variable in thickness and transgress the breccia stratification planes. These beds appear to be composed of wind-drifted ash that smoothed out irregular ities on old erosion surfaces.
Fossils are rare on or near Mt, Washburn, but a few whitish fossil leaves occur in a tuff lens on the west cliffs of Mt, Washburn. The leaves were identified as Gyperacites giganteus Knowlton by Dr. Erling Dorf (Prince ton University). The species was considered to be Early acid, but it is a very generalized form and probably has a greater range than previously recognized (Dorf, personal communication). In the Washburn Range many varieties of petrified wood are quite common but none of the specimens collected could be identified as to genus and species.
Petrified tree stumps and trunks with attached root sys tems are well exposed along the eastern face of Cook Peak, and one log 50 feet long and 3 feet in diameter imbedded in breccia was found near the summit of Folsom Peak. 146
Within the volcanic breccias are many occurrences of low-temperature secondary minerals. Chalcedony is by far the most common mineral, followed by opal, but amethystine quartz crystals, masses of carbonate, and barite crystals also occur, especially on the northern slopes of Mt. Wash burn. All minerals occur as fillings in cavities such as veins, fault breccia, lava pipes, vesicles, and irregular openings in coarse porous breccia. Chalcedony is usually botryoidal, translucent, and white to light gray (fig. 75).
Figure 75. A chalcedony vein filling showing layers of botryoidal chalcedony along the border, and banded horizontal layers in the center. Note the empty cavity in the center with a fracture leading to the border.
These minerals were probably emplaced during the waning stages of the Early basic volcanic cycle by low-temperature hydrothermal solutions, or by recirculated hydrothermal so lutions mixed with mete prie waters from surface hot springs and geysers, or both. 147
The lower contact of the Early basic breccias may
Intertongue with the Early acid breccia near Hedges Peak, but In most cases the contact Is probably eroslonal. No unit overlies the Early basic breccia In the Washburn area so no upper contact exists. Presumably, eroslonal forces have worked on the Early basic breccia since It came Into existence.
Megascopic Description and Petrography
Monollthologlc Breccia Megascopic Description. -- Most monollthologlc breccias are not composed entirely of one rock type, ex cept true autobreccias, but one llthology always predom inates (>75 percent). The breccias are usually very dark gray but are commonly dark red due to hematltic alteration.
Monollthologlc breccias are generally well lithlfied, and are only very slightly porous. These breccias are com monly associated with lava flows or mixed flow-fragmental rocks and therefore are concentrated on the south and east spurs of Mt. Washburn. They are usually very dense and are composed of very angular fragments of black aphanitic basalts containing abundant tiny vesicles filled with a resinous yellowish-brown material. No phenocrysts are visible except in one case, In which the fragments are all vitrophyres containing plagioclase and pyroxene phenocrysts. One unusual monollthologlc breccia, which occurs on the south spur of Mt. Washburn, Is composed of clndery scorlaceous 148 fragments Imbedded in a finely bedded tuffaceous matrix (fig. 65). This breccia is Interpreted to be a Vulcanian breccia, and represents the only unquestionably aerially transported breccia observed in the Washburn area. Although monolithologic breccias are very interesting and greatly help to shed light on the possible origin of volcanic breccia, they are a very minor constituent and perhaps amount to only 1 percent of the total mass of vol canic breccia around Mt. Washburn.
Petrography. -- Fragments in monolithologic breccias all show hyalopilitic to intersertal microporphyritic tex tures, Microphenocrysts consist of labradorite (An^g 62^* diopside, bronzite, and a few pseudomorphs of olivine. Alterations are not important, but oxidation to hematite is commonly encountered, as well as some poorly crystal lized montmorilIonite, Microlites of plagioclase show a surprisingly well-developed flow alignment in some cases.
The scoriaceous fragments in the Vulcanian breccia show completely opacitized groundmasses, and contain abun dant irregular vesicles that are filled or coated with several poorly defined zeolites. The tuffaceous matrix fills the exterior vesicles of the fragments. 149
Heterolithologic Breccia - Poor Bedding and Sorting
Megascopic Description. -- Heterolithologic breccias of this type tend to be dark gray or medium brown and con tain a high concentration of angular fragments in a tuff aceous matrix. All heterolithologic breccias are moderately well lithified and fairly porous. Sorting is generally very
poor, but there commonly seems to be a noticeable concentra tion of a certain fragment size, such as 2 to 4 inches or 3 to 6 inches. Commonly there are a few larger blocks from
1 to 10 feet in diameter (fig. 76) and in a few cases large blocks are quite common, especially in thick chaotic breccias (fig. 77). It is interesting to note that the larger frag ments commonly show flow banding, often of the mixed flow- fragmental variety, and are somewhat rounded. Most frag ments are less than 10 inches in diameter, and are dense
basalts, microvesicular basalts, flow-banded basalts, and andésites. All are aphanitic with only an occasional large pyroxene phenocryst, and may be light gray, dark gray, black, maroon, or brick red. Vesicular rocks and even a little scoria are much more common than would be predicted to be
present on the basis of the nonfragmental rocks. A few
breadcrust bombs do occur but they are extremely rare. The matrix of the breccias shows a distinct tuffaceous
aspect. There seems to be a well-defined gap between the
size of fragments and the size of grains in the matrix, i.e., there is no continuous gradation from one to the other. 150
Figure 76, Typical massive brown heterolithologic breccia showing roughly uniform fragment size except for the large block in the center. Normally the fragments stand out in relief, but this photograph shows a joint surface in a road excavation that weathering has not yet affected.
Figure 77, Extremely coarse, unsorted, chaotic breccia completely lacking any indication of stratification. Nearly all fragments in the photograph are solidly im bedded in tuffaceous matrix. Notice that the fragments weather into relief. 151
The percent of fragments in the breccia varies consider ably, but is usually 40 to 70 percent. In some cases, fragments comprise nearly 90 percent of the breccia, and on the other end of the scale, a complete gradation ex ists to 100 percent tuff (matrix), although intermediate rocks (60 to 90 percent matrix) are not common. Fragments appear to be suspended in the tuffaceous matrix, especially in breccia with a high percentage of matrix (fig. 78), The composition of the matrix seems to be lithic tuff of com position similar to the breccia fragments. If black ba salt fragments dominate, the matrix is dark gray; if there are numerous maroon or brick-red fragments, the matrix is brownish or reddish gray.
Alterations are not very common, although discolora tion of scoriaceous fragments occurs at many places. Some scorias have become amygdaloids because chalcedony has filled the vesicles, discoloring the fresh rock around them, Hematitic alteration, especially of scoriaceous fragments, is commonly present, and a resinous yellowish or greenish-brown mineral with a greasy feel (probably montmorilIonite) is widely distributed. There is no sharp distinction between monolithologic and heterolithologic breccias, but the latter are much more common and account for nearly 60 percent of all rocks in the vicinity of Mt. Washburn, 152
Heterolithologic Breccia - Some Bedding and Sorting
Megascopic Description. -- Many of the statements made about poorly sorted and bedded heterolithologic breccias also apply to these breccias, but colors are generally lighter, such as medium gray, light brownish gray, and whitish light gray. Stratification and sorting are com monly visible in hand specimens. Gravelly concentrations and partial rounding of the fragments are common. As a rule, the size of fragments seems to be less than in un sorted breccias, but some of the largest blocks in the volcanic breccias of Mt. Washburn occur in these breccias.
One block measured 15 x 5 x 6 feet. The tuffaceous matrix
seems to be finer grained and more abundant than in un sorted breccias. This featui*e causes the breccia to resem ble a mass of mud with imbedded subangular rock fragments.
Alterations and secondary mineralizations are similar to those described previously except for a striking emerald- green mineral that permeates the matrix of the breccias
in a few places.
The partially bedded and sorted heterolithologic breccias comprise approximately 40 percent of the rock mass in the vicinity of Mt. Washburn. However, north and northwest of Mt. Washburn they rapidly increase in import ance, only to decrease again in the same direction. This decrease in volume is due to the grading of heterolithologic breccias into well bedded and sorted alluvial breccias and conglomerates in the Washburn Range. L53
Petrography. -- The petrography of both types of heterolithologic breccias, as well as many of the alluvial breccias and conglomerates, is so similar that all will be discussed here under one heading.
The breccias are composed of rock fragments of many lithologies in a distinctly finer matrix that is a lithic- crystal tuff of the same composition as the rock fragments.
The most abundant fragment textural types, along with any distinctive features, are listed below in order of decreas ing abundance: 1. Microporphyritic intersertal with abundant ir regular vesicle-like cavities and some flow alignment of plagioclase laths,
2. Microporphyritic intersertal (average variety).
3. Microporphyritic intersertal with strongly opacitized groundmass. 4. Microporphyritic intersertal with large quantities of magnetite and tiny pyroxene needles in the groundmass (probably unoxidized phase of 3.).
5. Vitrophyres with hyalopilitic groundmass con taining light to medium brown glass.
6. Large glomeroporphyritic aggregates of pyroxenes.
More than 50 percent of all rock fragments show micro porphyritic texture with intersertal groundmass and abun dant irregular cavities (microvesicles). There is a notable lack of fragments with felty groundmass textures.
The most abundant mineral is labradorite (An^]^_^^), but zoning causes wider variations (An^g_gy maximum).
Diopside or diopsidic augite is the next most abundant 154
Figure 78, Photograph of a aeml-polished slab of hetero lithologic volcanic brt^cia showing variable composition of the fragments, angularity of the fragments, tuffaceous matrix, and lack of sorting.
Figure 79. Heterolithologic breccia showing fine-grained tuff concentrations around lithic fragments, defining the stratification and indicating that the breccia has been reworked by running water. 155 mineral, but this is followed closely by orthopyroxene that is usually bronzite. A very small quantity of olivine
is almost always present, but it is usually altered to bowlingite. Magnetite is the only other primary mineral.
It is of significance to note that oxyhornblende is en
tirely absent except for a rare grain or two. Variations in lithologie characteristics within one
textural type are caused by differences in 1 ) the percent
of amygdules or microvesicles; 2 ) percent of phenocrysts;
3) relative percent of mafic minerals and plagioclase; 4) the degree of orientation (flow structure) of plagio
clase laths; and 5) the extent of oxidation and alteration. In most cases, in thin section, it is very difficult
to separate large lithic fragments from the matrix that is
composed of smaller crystals and fragments of similar com position except for strongly opacitized fragments and
vitrophyres. The microvesicles or irregular cavities arc usually
filled or at least coated with a variety of secondary min
erals. Most of these minerals are fibrous and occur in layers parallel to the cavity wall. Optical properties vary strongly from highly biréfringent material resembling
bowlingite to isotropic or nearly isotropic minerals re
sembling opal or zeolites. Small bodies of zeolites such
as natrolite and analcite do occur. In some cases, there
is an abundance of a fibrous, bright emerald-green mineral 156 that strongly resembles bowlingite. However, X-ray powder diffraction analyses of this material demonstrated that the lattice is not expandable, and this fact along with other properties suggests a mica-like structure.
Tuffaceous Rocks
Megascopic Description. -- The properties of tuffaceous rocks are quite variable, indicating multiple origins for these materials. Grain size varies from silt to coarse angular sand with occasional admixtures of lapilli (4-32 mm) and scoriaceous breccia fragments. Beds may be massive, but more commonly show stratification caused by lenses and streaks of silt-sized material in coarse sandy or lapilli tuff. Colors range from light brownish gray to yellowish gray to light olive gray. The finer-grained tuffs occasion ally are silicified producing a chert-like rock, and many tuffs show bentonitic properties. Moist tuffs tend to splatter when struck with a hammer, and specimens sawed in water swell and thin slabs warp. Two unusual types of tuff occur along the west cliffs of Mt. Washburn. One appears to be a secondary breccia composed of fragments of silty tuff in a coarse sandy matrix. It was probably formed by running water that broke up a previously deposited layer of silt, and de posited silt fragments along with coarse sand carried by the water, producing a breccia-like texture. Another un usual tuff occurs in small channel-like deposits. It 157 contains flattened spherical forms that are Interpreted as accretionary lapilli formed as rain fell through a dust-filled atmosphere shortly after an aerial eruption or as rain fell into powder-dry dust accumulations on the ground (fig. 80-81), Each body exhibits a fine-grained shell and a coarser interior approximately the same size as most particles in the tuff.
Petrography. -- The textures, mineralogy, and alter ation of the tuffs are quite similar to those of the breccias, and so a discussion of these features will not be repeated.
However, the tuffs afforded excellent opportunities to ob serve the nature of the bedding, and the manner in which the fragments are cemented to one another forming a solid rock. Microscopically, bedding is frequently marked by the occurrence of a very fine dust layer. Below this layer, the particles usually become coarser and coarser producing graded bedding (fig. 82). In a few cases, a high concen tration (about 50 percent) of heavy minerals, especially pyroxene, occurs near the top of the bed (fig. 83). This is probably due to the winnowing effect of air or water currents that carried away lighter minerals.
Careful microscopic examination disclosed the presence of a nearly isotropic mineral with an index of refraction slightly less than balsam. This mineral thinly coats all fragments and permeates the whole rock. The mineral is 158
Figure 80. WeII-stratified lapilli tuff containing accretionary lapilli balls (flattened spherical objects associated with the finer-grained central layer).
Figure 81, Photomicrograph of a single accretionary lapilli ball showing the fine-grained border of the object and slightly flattened shape. Notice that the grain size within the body is about the same as that enclosing the body. 159
Figure 82. Photomicrograph of tuff showing graded bedding. The Lower bed is sharply separated from the coarser upper bed, which contains two opacitized lithic fragments, by a fine-grained dust layer.
Figure 83. Photomicrograph showing an excellent example of heterolithologic lapilli tuff. The Lower bed (left) shows a remarkable concentration of pyroxene that is probably a kind of lag concentrate owing to the winnowing action of air currents. 160
pale tan, occasionally shows faint streaks of birefringence, and usually small contraction cryks. It is probably a
zeolitic mineral, but might also be a clay. It is believed that this mineral acts as the binding agent for nearly all fragmental rocks in the Washburn area, except for a few cases where montmorillonite seems to be the binding agent.
Origin of Breccia
With the exception of some tuffs and breccias that may be credited to Vulcanian eruptions (forceful ejection into the atmosphere), the great majority of the massive,
poorly sorted heterolithologic volcanic breccias must have
originated by some other process. Admittedly, many of the
breccias were modified by epigene processes after they were
erupted, especially north and northwest from Mt. Washburn.
The epigene processes that produced these "secondary" vol
canic breccias will be discussed in the next section, but what process or processes originally created these frag
mental rocks? It is postulated that brecciation took place
in a subterranean environment before extrusion, through an autobrecciating process caused by vésiculation and accom
panying fracturing of the magma. A brief ire view of pertin
ent and modern theories of brecciation will be considered
before details of this theory are examined.
Durre11 (1944, p. 269), during the course of his ex
amination of certain breccia dikes, noted the close associa
tion of breccia and propylitic alterations. He concluded 161 that both resulted from the activities of escaping gas constituents from a rapidly cooling magma, supplemented by water in the cool country rock. Essentially, breccia tion was the result of the expulsion of volatiles by crystallization of the magma when it was too viscous to vesiculate, but could still decrepitate. Gates (1959) studied the breccia pipes in the Sho shone Range, Nevada, and concluded that brecciation took place before extrusion and was caused by the rapid evolu tion of gases from rising volatile-rich magmas. Breccia tion also took place during times of magma subsidence by collapse stoping, subsidence, and rock-bursting.
Parsons (1960, p. 144) postulated underground breccia tion in vents or calderas for the origin of the unsorted heterolithologic breccias that account for 50 percent of the volcanic breccias in the Absaroka volcanic field.
The mechanisms of brecciation are phreatic explosions due to infiltrated surface waters, resulting in hot steam ing mudflow-like outpourings, and gas explosions at depth with no surface outbreak until later. Krushensky (1960, p. 187) postulates subterranean brecciation, but proposes a different mode of brecciation. Brecciation is by rock burst, the expulsion of rock fragments into an open space, initiated by continued magmatic pressure from below. Perhaps the most impelling theory of brecciation, and one that is quite similar to the writer’s concept of the process of brecciation in the Washburn volcano, was proposed 162
by Curtis (1954, p. 469), He studied the volcanic debris
of the Mehrten formation, which is quite similar in composi
tion, texture, and structure to Washburn volcanic breccias,
and concluded that the volcanic rocks were brecciated be
fore extrusion by a mechanism not unlike Durrell*s theo retical process. The intiuding magma is believed to have
been quite viscous, almost half crystallized, and very low
in volatile content. With these properties, very low con
fining pressure (superincumbent load) would be necessary
to keep gases in solution. Nevertheless, vésiculation would begin at or near the surface as a result of drastic
reduction of confining pressure. This loss of volatiles
and cooling by expansion would further increase the vis cosity of the magma, so that the magma could no longer
adjust internally to the upward motion fast enough, caus ing the magma to fracture. The developing fractures, dilating joints with accompanying microspalling, and
further vésiculation would fragmentize the magma rather
rapidly. Further upward motion would partly pulverize
fractured blocks, causing partial rounding by attrition.
During periods of eruption of the Washburn volcano,
the magma as it approached the surface is thought to have been rather viscous, very low in volatile content, in an advanced stage of crystallization, and crystallizing rapidly.
High viscosity would tend to retard the diffusion of dis
solved gases in the melt to form bubbles, and bubbles that do form would tend to remain in place, i.e., they would not 163 migrate upward (Vcrhoogen, 195I, p, 736). However, as the confining pressure is lessened with approach to the surface and as crystallization proceeds, the remaining magma will become less and less capable of holding its dissolved gases. At this stage, the point of retrograde boiling or the "second boiling point" is rapidly ap proached, i.e., the point at which volatiles will sud denly come out of solution due to the great internal pressures of these gases at magmatic temperatures. If this take place, a paroxysmal eruption is likely to occur
(Morey, 1922, p. 224). However, it is clear that most Washburn heterolithologic breccias are not pyroclastic
(evidence to be presented later), and therefore it is assumed that retrograde boiling did not occur and no volcanic explosion took place. The possible reasons for the failure of Washburn magmas to explode are many, but the chief reason is related to the kinetics of gas evolution.
The most important factors that control the kinetics of gas evolution and the degree of vésiculation are the initial volatile content of the magma, the degree of over saturation of gas in the molten magma, the rate at which oversaturation takes place, the confining pressure, the nature and amount of suspended crystals, and viscosity
(Verhoogen, 1951, p. 737), Viscosity in turn depends on temperature, chemical composition of the magma, degree of crystallization, and the quantity of volatiles in solution. 164
Note that the last two factors were also mentioned above, clearly showing the interdependence of the variables. For
Washburn magmas, then, the combination of variables was such that the magma had vesiculated only to a small degree
and had become quite viscous just prior to extrusion.
At this stage the magma began to fragmentize in a manner similar to Curtis* theory, Rounding of the frag ments by attrition and mixing with previously brecciated
rock took place as the molten magma and fragmentized ma
terial moved toward extrusion. Magma does not move irre
sistibly and continuously onward toward extrusion but may
pause, or actually retreat. In this event, brecciation
may take place by subsidence, stoping, and rock-bursting
of vent walls and of the overlying cover of volcanic rocks
in the vent. The evidence which indicates that the un sorted heterolithologic breccias of Mt, Washburn originated
through subterranean autobrecciation caused by vésiculation
and fracturing is the following:
1. Most fragments are microvesicular, vesicular, or scoriaceous, although the latter may be due to Vulcanian explosions.
2. No fluoidal texture nor bomb-like structures, and angular shape of most fragments - solid when extruded,
3. Borders of most fragments arc marked with irregu lar holes (vesicles) - vésiculation occurred before cre ation of the fragment. 4. No hydrous minerals such as hornblende in breccia fragments - low volatile content. 165
5. Roughly uniform fragment size in most breccias with an occasional large block usually showing flow structure - stoping and subsidence of previously ex truded lava flows thoroughly mixed with autobreccia.
6 . Flagioclase microlites parallel to borders of the vesicles - advanced stage of crystallization.
7. Heterolithologic composition of the breccias, but variation of texture and composition within a rather restricted range, possibly no greater than that expected within a single magma body - autobrecciation.
8 . Monolithologic breccias, mixed flow-fragmental rocks, and internally autobrecciated lava flows (fig. 84) - autobrecciation.
9. Intrusion of breccia dikes, brecciated before intrusion (Early acid breccia, south of Hedges Peak) - autobrecciation and mixing.
Figure 84. An excellent example of autobrecciation of a flow-like body located on the Grand Loop.Road at the first curve north of Dunraven Pass. Notice that the brecciation begins in the center and grades outward to massive lava. 166
The nature of the extrusion is visualized as upwell- ing masses of large and small blocks in a fine-grained material of the same composition, producing a bulging dome. The mixture includes autobrecciated crystallized magma, blocks from the vent walls, from the throat of the vent, and talus from the cone at the surface. This mass of material would be hot, moderately mobile, and fairly well saturated with water. In such a state, an eruption might develop into a "primary" lahar. If the magma moved up the vent fast enough and the vesiculating-fracturing process broke down, mixed lavas and fragmental material or pure molten lava might be extruded. In some eruptions,
if vésiculation proceeded rapidly, or abundant volatiles were present, small Vulcanian explosions might occur,
throwing ash and molten lava into the air, producing the few Vulcanian breccias and tuffs that were observed in the field. The exsolved volatiles might pass into the atmosphere, or they might permeate the porous breccias, depositing clays, silica, and zeolites, lithifying loose fragmental aggregates, or causing alterations such as
oxidation and replacement of primary minerals. All Washburn breccias then are probably autoclastic volcanic breccias in their initial stage of formation.
Each individual bed of breccia represents one extrusion. 167
Epigene Processes Modifying Primary Breccias
After the breccias had been extruded, possibly as
"primary" lahars, the normal geomorphic processes that operate on the Earth * s surface began to act on these fragmental volcanic rocks. Gravity, wind, and running water all had their affect, but a source of water is the most important question. Dorf (1960, p. 257) after his study of fossil plants of Eocene volcanic rocks in
Yellowstone Park, postulated that the climate during this interval was warm-temperate to subtropical with
50 to 60 inches of rain each year. If this hypothesis is correct, there would be adequate water to develop « all the features observed in Washburn fragmental rocks.
Rain falling on loose volcanic debris would rework the tops of breccia beds causing the sandy and gravelly stratification so often observed. Heavy rains or waters from a crater lake expelled during an eruptive stage, might soak porous breccias, mobilizing them into lahars.
Trees growing on the slopes of a primitive Mt. Washburn might be buried in an upright position with alluvial con glomerates and breccias derived from the rapid stream erosion taking place on the cone. This phenomena was observed many times in the Washburn Range, and paleo- topography and channel-and-fill structures have pre viously been described from Mt. Washburn. Along the steeper valley walls, various kinds of landsliding. 168 along with alluvial fans and talus cones, were probably common. During dryer seasons, winds probably winnowed the fines from between larger fragments and drifted them across the surface of the volcano, filling Irregularities.
Small ponds and lakes were probably abundant In this area of heavy rainfall, and acted as catchment basins for wind- driven dust, sheet-washed material, and alluvial sands. CHAPTER VI. VOLCANIC ROCKS OF UNCERTAIN AGE
Introduction
There are two groups of nonfragmental volcanic rocks of uncertain age in the Washburn region. One of these is a group of light-colored hornblende andésite dikes that occur southwest of Mt. Washburn, and the other is a series of lava flows that cap Observation Peak in the southwest corner of the Washburn Range. Although the latter are not in the map area, they are included because they may be the only representatives of the Early basalt sheets in the Washburn region.
Hornblende Andésite Dikes
The hornblende andésite dikes are a group of 6 to 8 intrusives, which occur southwest of Mt, Washburn in an area that is predominantly Early acid volcanic sedimen tary rocks. They are not identical by any means, but their similarities are so striking that they are grouped together.
These dikes tend to be much thicker than the basalt dikes of Mt, Washburn and in places attain a thickness of 100 feet, although most are much thinner. One dike is over one mile long and cuts northwesterly across Hedges
Peak and the upper reaches of Carnelian Creek. The dike rock usually breaks into blocky chunks, possibly as a re sult of a poorly developed horizontal columnar jointing.
169 170
Weathered surfaces are normally almost white with a tan or brick-red tint, but the fresh rocks are medium light gray, olive gray, or yellowish gray. The rocks show a striking porphyritic texture with 20 to 40 phenocrysts consisting predominantly of hornblende and usually some plagioclase (fig, 85). One dike, which properly should be called a dacite, shows biotite and a few quartz pheno crysts in addition to hornblende and plagioclase. Horn blende occurs as black needles up to 7 mm in length, which
in a few cases are aligned parallel to flow structure.
Plagioclase occurs as large white grains up to 5 mm in diameter, and shows zoning to the naked eye. A few of
these dikes contain hornblende-plagioclase clots or
segregates up to 4 cm in diameter (fig. 8 6 ). Segrega
tions of this sort were noted previously in the com
posite dike near the summit of M t . Washburn. Microscopically, the texture of the dike is por
phyritic to microporphyritic with a felty to sub-
pilotaxitic groundmass (fig, 87). Phenocrysts comprise
24 percent of the rock and the remaining 76 percent is
largely groundmass with a few percent of secondary min
erals, Labrador!te (Ang^_yQ), green hornblende, magne
tite, and apatite are always present, but a small amount
of diopsidic augite or bronzite may be present or, more
rarely, spinel, zircon, and sphene. The dacite contains
abundant biotite, Beta-quartz, and andesine (An3 g-4 4 ). 171
Figure 85. Photograph showing the typical porphyritic texture of the hornblende andésite dikes. Phenocrysts are predominantly black hornblende needles, but a few plagioclase crystals are visible.
Flow structure is occasionally present as shown by the alignment of hornblende phenocrysts and plagio clase microlites. The hornblende-plagioclase segrega tions are composed of an open network of very long plagioclase laths (up to 1 0 mm) and interlocking needle like hornblende grains (2 to 3 mm), often arranged in a star-shaped pattern. Both show the same composition as the corresponding minerals in the main rock. Quartz partially fills the cavities in this network, in places producing a micropegmatitic texture with feldspar. 172
Figure 8 6 . Photograph showing the porphyritic texture of a dike containing a hornblende-plagioclase segregation. Note the long spoke-like feldspar crystals in the segrega tion and the fine-grained border. Also, notice the zoning visible in the plagioclase phenocrysts.
9-
Figure 87, Photomicrograph of the hornblende andésite of figure 8 6 showing porphyritic-felty texture, resorption and black type opacitization of hornblende, and analcite alteration in the core of the plagioclase phenocryst that causes the apparent zoning visible in the photograph above. Notice the orthopyroxene (very light gray) in the center of the hornblende phenocryst near the left edge of the photograph. Plane light. 173
The apparent zoning in plagioclase, visible to the naked eye, is actually due to analcite alteration in the core of many of the larger phenocrysts (fig. 87). Horn blende phenocrysts are usually partially resorbed, causing rounding of euhedral crystals, and usually show a thin black type opacité border. In one case, the larger hornblende phenocrysts contain bronzite (Eng^) cores, with gradational contacts between the two phases (fig. 87), This hornblende has optical properties identical to those of the hornblende in other parts of the rock.
The groundmass is predominantly plagioclase microlites and tiny magnetite grains along with tiny pyroxene (?) needles. Alteration minerals such as zeolites and bowling- ite are common in the groundmass, and some potash feldspar may be present. Other secondary minerals include quartz, carbonate, sericite, and epidote.
Table 10. Representative Modal Analysis of the Hornblende Andésite Dikes Component Percent Groundmass (plagioclase microlites, magnetite, pyroxene needles, secondary minerals) 73.4
Labradorite (Ang^_yQ) 9 . 8 Hornblende (and a little bronzite) 8,4 Bowlingite pseudomorphs after clinopyroxene (?) 4,4 Carbonate 2.4 Quartz 0.8 Magnetite 0.4 Apatite T Zircon T Total 99.6 174
The age of the hornblende andésite dikes is uncertain,
but some estimate of their relative age is possible. These dikes cut a Washburn type basaltic dike 200 yards southeast
of Dunraven Pass. They also cut Early acid volcanic sedi ments and flows, and the lower part of the Early basic
breccia southwest of Mt. Washburn and Hedges Peak. How
ever, no representatives of this lithology occur on Mt. Washburn itself, cutting the main portion of Early basic
breccia. Therefore the hornblende andésite dikes are con
temporaneous with the Early basic breccia or slightly later.
They cannot be much later, though, because they belong to
the same volcanic cycle. This assumption is based on the
similarity in form, composition, and occurrence of minerals
in the Early basic rocks and the dikes in question.
Lava Flows on Obseirvation Peak
Observation Peak is located on the very southwest corner of the Washburn Range just north of Grebe Lake. It
is capped by at least 1 0 0 feet of medium-gray trachybasalt,
Each flow is 15 to 20 feet thick and is vesiculated at the top. The flows break up into large blocks and the whole mass dips gently (15®) to the southwest.
The rock is aphanitic except for a few (3-5 percent)
large (2-4 mm) phenocrysts of plagioclase and tiny grains
of biotite. It is a rather porous rock and shows fairly
abundant green epidote, especially associated with cavities
(irregular vesicles). 175
Microscopically, the rock appears to be strongly epi- dotized. Epidote occurs as "phenocrysts" (possibly as an alteration product after pyroxene) and as abundant tiny grains in the groundmass. The optic angle of the epidote is 87° - 89° (negative) and therefore it is probably clinozoisite. A few grains of zoisite are also present.
The texture of the rock is microporphyritic tending to ward intersertal and shows good alignment of plagioclase laths (fig. 8 8 ). Numerous irregular holes, presumably microvesicles, are present. Plagioclase commonly shows reverse zoning - an andesine core (An^g) and a labradorite shell CAn^2 _5 y) with resorption between. Biotite is rare, but occurs as pale tan to dark reddish brown irregular plates. Minor diopside (2V = 58°) is also present.
Figure 8 8 . Epidotized trachybasalt from Observation Peak showing a labradorite phenocryst and microphenocrysts biotite (B), and clinozoisite (C), The groundmass is largely epidotized. Note the numerous irregular micro vesicles (black areas). 176
These flows are distinctly different from any rocks observed near Mt, Washburn, and stratigraphically lie upon the Early basic breccia. The presence of significant bio tite indicates a higher content of potash than most Wash- b u m volcanic rocks. Therefore, it is postulated that these flows are contemporaneous with the alkali-rich Early basalt sheets. CHAPTER VII. CHEMICAL ANALYSES
Introduction
Two new chemical analyses of Washburn rocks were made. One sample was chosen to represent Early acid rocks and the other to represent Early basic. The Early acid sample (No. 3 of Table 11) is a channelized lava flow of hornblende andésite that crops out just south east of Hedges Peak. The Early basic sample (No. 4 of
Table 11) is an olivine-pyroxene andésite that was col
lected from the border phase of the composite dike just south of the summit of Mt. Wbshburn. In addition to the new analyses, six other analyses of rocks in the immedi ate Washburn area were found in the literature. All are recorded in Table 11 (in pocket) along with normative analyses. The norms for the two new chemical analyses were calculated by the C.I.P.W. technique (see Cross et al., 1903).
Normative Analyses The following generalizations may be made about the position of the small group of Early acid and Early basic rocks from the Washburn region in the C.I.P.W. rock classification (Table 11):
1. All belong to Class II (salic minerals dominate over femic) except No, 8 , which belongs to Class III (salic and femic minerals about equal),
177 178
2. Alt Early acid rocks belong in Order 4 (feldspar dominant over quartz).
3. Most Early basic rocks belong in Order 5 (feldspar alone or extremely abundant).
4. All belong in Rang 3 (potash and soda in feldspar about equal to lime in feldspar) except the most silica- rich Early acid rock (No. 1 of Table 11), which belongs in Rang 2 (potash and soda in feldspar dominate over lime in feldspar).
5. All belong in Subrang 4 (soda in feldspar dominates over potash in feldspar).
It is very interesting to note that these generaliza tions were all predictable on the basis of qualitative pétrographie examinations of Washburn volcanic rocks.
In addition to hundreds of analyses from the Absaroka volcanic field, rocks from a number of other volcanic regions in the world belong to the same C.I.P.W. classes as the Washburn rocks. These are Mont Pelee and many other localities in the West Indies; the 1888 eruption of Vulcano, Lipari Islands ; Japan; the San Juan Moun tains area, Colorado; and the Cascade Range in Washing ton and Oregon, including Crater Lake (Washington, 1917).
In addition, the chemical and normative analyses of the Washburn rocks are reasonably close to Nocholds* (1954, p. 1019) typical calc-alkali andésite. 179
Discussion of the New Chemical Analyses
For purposes of comparison, the chemical analyses of sample No, 3 and No. 4 of Table 11 are reproduced in Table
1 2 , along with modal analyses of the phenocrysts recalcu lated to 100 percent. It is obvious from the table that the chemical analysis would not be predicted from the modal analyses of the phenocrysts, except in a general way. The silica content is much higher than expected, and no potash- bearing phenocrysts are present. It is believed that these components are largely concentrated in the groundmass, prob ably as the "unidentified felsic material". However, on the basis of phenocryst composition, the higher magnesia content of sample No. 4 could be predicted, as well as the higher alumina of sample No. 3. The high water content of both analyses is in part from hornblende, which occurs in both samples, but it largely belongs to secondary minerals such as bowlingite, zeolites, and opal.
It has been said that the chemical composition of rocks is their most fundamental characteristic, and that the chemical composition is represented by the minerals (Gross et al., 1903, p. 89). But as seen above, the min erals only crudely express the chemical composition. Many times undue importance is ascribed to feldspars for classi fication, resulting in many misinterpretations. For in stance, if feldspars were used for classification, all rocks in Table 11 should be called basalts because labradorite is 180
Table 12. Total Chemical Analyses, and Modal Analyses of Phenocrysts Recalculated to 100 Percent of Samples No. 3 and No. 4, Table 11.
Sample No.3 Sample No.4 Component Percent Percent SiO^ 60.81 39.17 Al^O, 15.46 13.65 TiOg .59 .45
2.42 2 . 2 1 F^2°3 FeO 3.84 3.62
MnO .08 . 1 1 MgO 3.16 7.38 CaO 5.11 4.81 Na^O 3.26 3.34 KgO 1.93 1.67 H 2 O . 2.49 3.14
P2 O 3 . 2 2 . 2 0
CO2 .35 . 1 1 F .04 . 0 2 Total 99,76 99.78
Plagioclase 77.3(An^3_g3) 47.2(An^]^_3Q) Hornblende 9.0 3.8 Orthopyroxene 8.7(Enyy) 19.2(Enoo) Diopside none 17.7
Olivine none 1 2 .2 (Fa2 ) Magnetite 4.8 1.7 Total 99.8 99.8
Explanation of Table 12
Sample No.3, Early acid lava flow near Hedges Peak; original modal analysis shows 38.9 percent groundmass (mostly plagioclase microlites and other unidentified felsic materials, and much less abundant pyroxene(?) needles and magnetite) and 4.3 percent secondary minerals (analcite, opal, montmorilIonite, carbonate). Phenocrysts represent only 36,6 percent of the rock. LSI
Sample No.4. Early basic dike (border phase of the composite dike) just south of the summit of Mt, Washburn; original modal analysis shows 69.2 percent groundmass (mostly plagioclase microlites and other unidentified felsic materials, a little magnetite and pyroxene(7) needles, and some secondary minerals) and 2.5 percent secondary minerals (almost entirely bowlingite, with a trace of zeolites and carbonate). Phenocrysts represent only 28,6 percent of the rock. the only feldspar present. This classification would be confirmed in most cases by the presence of olivine near forsterite, magnesian bronzite, or diopside. It is con cluded, therefore, that classifying a volcanic rock on the basis of the phenocrysts alone, where 60 to 80 percent of the rock is unidentifiable groundmass, is a very risky business at best. CHAPTER VIII. PETR06ENESIS
Character and Origin of the Magma Washburn volcanic rocks are calc-alkalic and members of the basalt-andesite-rhyolite association, which com monly occurs in orogenic regions. VoLcanism in the Wash- buzTL area and the Absaroka Range was probably initiated by the culmination of the Beartooth uplift (Laramide) during the beginning of the Eocene (Foose et al., 1961, p. 1165), and so Washburn volcanic rocks properly belong in an orogenic suite. Pétrographie and chemical evidence already presented indicates that these volcanic rocks are similar to rocks in other volcanic regions, such as the
San Juan Mountains in Colorado and the Cascade Range.
Washburn rocks are included within the Yellowstone sub province of Larsen’s (1940) central Montana pétrographie province. Volcanic rocks from the Crandell subprovince in the Absaroka Range are chemically and petrographically very similar to Washburn rocks, but the Absaroka Range as a whole shows slightly aIkalic-calcic characteristics (Larsen, 1940). The basalt-andesite-rhyolite association in the Washburn region is not fully developed, especially the acid end of the association. Basaltic andésite and andésite are in great abundance, but the most acid rocks present are dacite and latite, and these are only minor.
In this respect, Washburn rocks are very similar to the Cascadian suite.
182 183
The rhyolitic rocks of the Yellowstone Plateau are not comagmatlc with Washburn rocks and are associated only by coincidence. The time lapse between the two groups of rocks is too great (Middle Eocene to Pliocene) for them to belong to the same volcanic cycle. All rocks of the Washburn region clearly belong to the same cycle, because each mineral species such as plagioclase, whether it oc curs in Early acid lava flows, in Early basic volcanic breccias, or in hornblende andésite dikes, is very similar in form, character, and composition. Cross states that,
experience shows that in a magma of given character the augite or hornblende formed will in general be of a more or less constant character" (Cross ^ , 1903, p.171).
The basalt-andesite-rhyolite association is too com plex to be explained by any simple scheme of magmatic evo lution, but presumably some sort of differentiation is involved from a parental tholeiitic basalt magma. Most explanations involve fractional crystallization of a basaltic magma and differentiation, but in addition, mixing of magmas and assimilation of country rock are commonly postulated. Turner and Verhoogen (1951, p.223) suggest that many of the features of this association may be the result of differential fusion of basaltic rocks beneath or within the sial. A more conventional view is that of Larsen (1940, p.943), who believes that two periods of differentiation are involved to yield the varying sub province characteristics within the central Montana petro- 184
graphie province, First there was a period of deep-seated
fractionation that involved the removal of calcic plagio
clase and hypersthene from basaltic magma. Then, after
upward migration and segregation into parent magmas for
each subprovince, there was further differentiation under
local conditions, which sometimes involved assimilation
of granitic rocks. Recently, some very interesting experimental evidence
has been presented that has an important bearing on the
origin of the basalt-andesite-rhyolite association and the Washburn volcanic rocks. Osborn (1959, p. 617) has found
that the course of crystallization and the final products
of a basaltic magma are radically different depending on whether the total composition is held constant during
crystallization or whether the partial pressure of oxygen
(Po^) is held constant. In the latter case, oxygen must
continually be added to the system, usually from a gaseous
phase such as water or carbon dioxide. Crystallization
may take place at equilibrium or accompanied by perfect
fractional crystallization and differentiation. In nature, crystallization usually follows some course between these
two extremes, but since Washburn rocks show so many features
of disequilibrium (well developed zoning in all minerals,
especially plagioclase, lack of exeolution in pyroxenes, incomplete reaction of olivine with magma to form pyroxene),
the course of crystallization was probably closer to per
fect fractional crystallization. 185
If extreme fractional crystallization takes place under conditions of constant total composition, the final differentiate will be high in iron and silica, but the quantity of differentiate will be relatively small (Osborn,
1959, p. 618), On the other hand, if fractional crystalli zation takes place under conditions of constant partial
pressure of oxygen, olivine, pyroxene, and magnetite will be extracted from solution, but the ratio of iron to iron
plus magnesium in the magma will change only little and
there will be a large quantity of siliceous material as
a final differentiate. By the use of chemical analyses,
Osborn (1959, p. 633) has shown that "the tholeiitic ba salts and the Skaergaard liquids have trends like liquids
formed from fractional crystallization in the MgO-FeO- at constant total composition, ... and the trends of the basalt-andesite-dacite association of the Cascades are like those liquids crystallizing at cm-
stant Po^."
If the above hypothesis is correct, it would certainly help to explain the high silica content of Washburn volcanic rocks, the presence of magnesium-rich mafic minerals that
are invariably present, and the fact that most of the iron
oxide is apparently in the groundmass. Other factors, how
ever, may exert a control on these important pétrographie characteristics of Washburn volcanic rocks. Hornblende, which is commonly present in these rocks, is a very basic 186 mineral, and by its crystallization the silica content of the magma may increase considerably. The high magnesium content of olivine and pyroxene may be partially the re sult of rapid crystallization at high temperature, pre venting reaction of these minerals with the magma to form ones with higher iron content in the same solid solution series.
The presence of the continuous water phase needed to maintain Po^ in Washburn magmas might be questioned, but the common occurrence of hornblende in the rocks and brecciation by microvesiculation described in Chapter V attest to the presence of water. The amount of water necessary to maintain pressure need not be large and Wu.s probably small in Washburn magmas. Osborn (1959, p. 643) postulates that magmas diffemtiating in orogenic belts probably have a higher water content, as suggested by the explosive nature of many magmas in the calc-alkalic series and widespread hydrothermal activity.
Some evidence indicating mixing of magmas is present in Washburn rocks. Resorption zones that frequently occur in larger plagioclase phenocrysts indicate radical changes in the magma, such as the mixing of two magmas. In some cases there seems to be two plagioclases or three bronzites present, which differ in morphology and composition, possi bly indicating magmatic mixing. Mixing of small, shallow segregated magma bodies probably took place to form the 187 composite dike (Chapter IV). Evidence of assimilation, however, is lacking. The basement underlying Mt. Washburn is almost certainly granitic, possibly with a thin cover of Paleozoic sedimentary rocks, but no foreign xenoliths of any type occur in Washburn volcanic rocks. In addition, the rocks are very low in potash, which would not be the case if granite had been assimilated.
Cyclic Behavior of the Eruptions
The sequence of eruptions in the Absaroka Range from
Early acid, to Early basic, to Early basalt sheet, to Late acid and so forth, generally represents a change from dacite, to andésite, to andesitic basalt and basalt, to potash ba salts, followed by a repeat of the series. Brown (1961, p. 1184) believes that each of these eruptive cycles
"... was initiated by periodic tapping and draining off of the upper, volatile-rich, relatively silicic fraction of the source magma." Individual eruptions from the Wash burn volcano probably represent the more volatile-rich portion of the local magma body. It is likely that each eruptive cycle came to a halt when the volatile content of the magma was reduced below a certain threshold level necessary to cause eruption. The time between eruptions is controlled by the rate of diffusion of volatiles toward the surface in a crystallizing magma. 188
Mineral Petrogenesis and the Nature of the Magma
Introduction
Detailed examination of minerals in Washburn volcanic
rocks indicates that each individual crystal has its own history. Although the general composition of two adjacent plagioclase crystals may be quite similar, one may show faint oscillatory-normal zoning whereas the other may show one or two internal resorption zones and strongly developed
oscillatory-normal and reverse zoning, indicating that magmas are very complex physio-chemical systems. In a
stationary magma, there are horizontal and vertical gradi ents of temperature, of volatile content, of the partial pressure of the volatiles, and of hydrostatic pressure.
In addition there are horizontal and vertical variations
in the size, number, and composition of crystals that have formed, and in the rate at which new crystals are develop
ing. This picture is complicated even more because these
internal variables are continuously changing as crystalli
zation and cooling proceed, and because the magmas are constantly stirred by convection currents. Therefore, it
is not surprising that each crystal has its own individual history. Possible specific causes for such crystal indi viduality will be suggested in the following sections.
Plagioclase The oscillatory zoning in plagioclase is probably due
to rhythmical intermittent changes in physical and chemical 189
conditions within a magma, and to convection currents carrying crystals into different environments (Kaaden, 1951,
p. 67). These changes include the movement of a crystal within a magma (convection, gravity settling), the move
ment of the liquid and crystal as a body, the mixing of
two segregated magmas, and the loss of volatiles and re lease of pressure (eruption). Changes in temperature and
pressure strongly control the composition of the plagio
clase that is forming. At higher temperatures, only the more calcic plagioclases crystallize, and therefore if a
fairly sodic crystal is thrust into an environment of
higher temperature, resorption will take place. A similar
effect is obtained with sudden changes in pressure, which
causes a shift in the melting and freezing point of
plagioclase.
Hornblende
Hornblende is a very useful mineral for study in vol
canic rocks, because it is a sensitive indicator of temp
erature and volatile content of the magma. In a dry melt,
it has been found that common green hornblende changes to oxyhornblende at approximately 800°C (Barnes, 1930, p. 402),
This change in the properties of hornblende is entirely caused by the change from ferrous to ferric iron within the crystal lattice, and may be prevented if hydrogen (from the dissociation of water) is present in the magmatic atmosphere. In addition to oxidation, if the temperature of the magma 190
becomes high enough (mixing with hotter magma, oxidation
near the surface), the lattice of hornblende will break
down. Black type opacité borders around hornblende crystals
represent the breakdown of the lattice, forming tiny grains
of pyroxene and magnetite. Sometimes the opacité Is re sorbed Into the magma, causing the rounded shape of many
hornblende crystals, and In some cases It combines with materials from the magma to form pyroxene type opacité
(Early acid lavas). It Is believed that the oxidation of
hornblende, caused by the loss of volatiles and the break down of the lattice, takes place just prior to extrusion. Resorption of hornblende during extrusion Is Inevitable
because of the loss of volatiles, unless cooling takes
place rapidly (Larsen, 1937, p. 893). Whether hornblende
or pyroxene will form In a magma depends not so much on
the composition, as on the presence or absence of water
in the magma and Its partial pressure. If the temperature
Is high and the water content Is very low, only pyroxenes are likely to form, but If there Is an appreciable quanti
ty of volatiles present at relatively high pressure, some hornblende Is likely to form, even at a high temperature.
Nucléation and Paragenesls
Washburn volcanic rocks show pétrographie evidence of
three distinct stages of crystallization that probably repre sent three major changes In the environment of the magma.
The stages are very clear In Early acid lavas, but are not 191 always as distinct in Early basic rocks. The clearness of the stages is probably a measure of the rapidity of change from one magmatic environment to another.
The first stage is represented by the huge phenocrysts or g1orneroporphyritic aggregates of olivine, pyroxenes, or plagioclase (especially the large complexly twinned and intergrown crystals). Even though these crystals are not very numerous, they are distinctly larger than the other crystals in the rock and are commonly the only minerals visible to the naked eye. This condition of only a few large crystals in a finer matrix is a function of the rate of nucléation during the initial stages of crystallization.
If the free energy (Gibbs) difference between magma and potential crystal nuclei is small, the number of nuclei to form will be small, but crystals will grow rapidly and possibly to a large size (Fyfe, Turner, and Verhoogen,
1958, p. 73), The free energy difference depends on en tropy differences of the two phases (large decrease favors slow rate of nucléation).
The rate of growth of crystals may be rapid under the above conditions, but it is restricted by the rate of dif fusion of material to the site of crystallization. The rate of diffusion is controlled by the amount and distri bution of materials forming the crystal in the magma, the temperature, viscosity, quantity of volatiles, pressure, and physical motion of the magma and crystal. The alternate 192 oversaturation and depletion of materials forming a crystal, in the vicinity of the crystal, which are controlled by the rate of diffusion, may produce some of the oscillatory zoning that occurs in plagioclase and pyroxene. Continuous or normal zoning is an expression of the increase in the
Na:Ca ratio of plagioclase or the Fe:Mg ratio of olivine and pyroxene under conditions of disequilibrium during fractional crystallization, resulting in incomplete mag matic reaction.
The cause of agglomeration of individual crystals, especially pyroxenes, to form larger aggregates is not known, but is probably related to energy conditions of nucléation, and turbulence or mixing in the magma.
Since the number of nuclei is small in the first stage of crystallization, indicating a small difference in free energy between magma and crystal nuclei, then the decrease in entropy must be large or the temperature dif ferences between actual and equilibrium conditions must be small. If this is correct, it may be assumed that the first stage of crystallization took place under conditions approximating equilibrium.
The second stage of crystallization is represented by microphenocrysts of minerals such as plagioclase laths, clinopyroxene, orthopyroxene, and hornblende, and is caused by a sudden change in the environment of the magma. In this stage, crystals are much more numerous, but considerably 193
smaller than in the first stage. Therefore, the free energy difference between the magma and potential crystal nuclei was large, resulting in more numerous nuclei than the first stage, but the nuclei grew much more slowly. The greater free energy differential was probably caused by large dif ferences between the actual and equilibrium temperatures, due to rapid cooling. The stage presumably represents the upward motion of the magma toward eruption from a relatively quiet chamber, and represents a shorter time interval than the first stage.
Some question may be raised concerning the proper placement of hornblende in this stage, and indeed, the large size of some crystals and occasional zoning suggests that it belongs in stage one. Crystallization in the first stage took place at a relative high temperature, but as pointed out in the preceding section, temperature is not the only controlling factor in the crystallization of hornblende. The water content and its pressure in the magma strongly control and affect the crystallization of this mineral. Therefore, it is possible and probable that hornblende formed in stage nne as well as stage two.
The third and final stage of crystallization is repre sented by the fine-grained groundmass that is so abundant in all Washburn rocks. It is composed predominantly of plagioclase and pyroxene microlites. This stage probably began just prior to extrusion and continued after extrusion 194 in the case of lava flows. Cooling and crystallization took place so rapidly that there was considerable differ ence between actual and equilibrium temperatures, causing the nucléation of an infinitely greater number of very tiny crystals than in the other two stages.
Features such as the oxidation of hornblende, the breakdown of the lattice of hornblende producing pyroxene and magnetite, and the thin outer zones (shells) of pyrox enes and plagioclase are probably associated with the loss of volatiles (largely water) between stage two and stage three. The loss of volatiles is caused by the release of confining pressure just prior to extrusion, in some cases resulting in microvesiculation, which caused fragmentation of the partially crystalline and rapidly freezing magma.
Conclusion
The local parental magma for Washburn volcanic rocks was derived from a deep seated tholeiitic basalt by frac tional crystallization under the condition of constant partial pressure of oxygen. The resulting calc-alkalie differentiate, which was more siliceous than the original magma, was injected into the Absaroka-Yellowstone region during the culmination of the Laramide orogeny. It erupted periodically during the Middle Eocene, producing fragmental volcanic rocks, lavas, and dikes, all of the basalt-andésite- dacite association. 195
The Washburn rocks began to crystallize from the local parental magma in a relatively shallow chamber during a period of temporary stability under conditions approaching magmatic equilibrium at constant partial pressure of oxygen.
The oxygen pressure was maintained by a small percentage of partially dissociated water throughout the initial period of crystallization that proceeded more rapidly when segre gated portions of the magma moved upward toward extrusion.
During this second stage of crystallization, the magma was well stirred by convection currents and the upward motion of the magma. Just prior to extrusion, or shortly after ward, the volatile content of the partially crystallized magma was lost to the atmosphere or to the adjacent rocks.
The loss of volatiles was accompanied or succeeded by the third or final stage of crystallization in which the magma became completely crystalline (solid in the case of glassy rocks). CHAPTER IX. PLATEAU ROCKS
Introduction
M t . Washburn and the Washburn Range are surrounded on all sides by the rhyolitic rocks of the Yellowstone Plateau, and even the Washburn Amphitheater is filled with these rocks.
F. R. Boyd has studied the rocks of the Yellowstone Plateau for several years and recently (1961) published an outstand ing paper on the subject. He found that the plateau is com
posed of rhyolitic tuffs, welded tuffs, and flows, along with minor olivine basalts. Boyd worked out the time re lationships of the many units, but only two units are of
major importance. The older unit is the Yellowstone tuff, which was probably emplaced in a single rapid series of
eruptions (p. 397), and the younger unit is the Plateau
flows. These flows were deposited in a basin partially bounded by fault scarps and bordered by highlands of Yel lowstone tuff (p. 412). The basin is probably a volcano- tectonic depression caused by the rapid eruption of the
Yellowstone tuff (p. 412). The rhyolitic rocks are thought to be Pliocene, but more recent work seems to indicate that
they may be all or in part Pleistocene (p. 410),
The rhyolitic rocks that lie west of the Washburn
Range, in the Washburn Amphitheater, and north and north
east of Mt. Washburn belong to Boyd's Yellowstone tuff (1961, Plate I), and those south of Mt. Washburn along the Grand
196 197
Canyon belong to the Canyon flow of the Plateau flows (1961, Plate I), Since the writer's observations conform closely
to those of Boyd and since the Plateau rocks are of second
ary importance in this report, the rhyolitic rocks will be
described only briefly. On the geologic map (Plate II) the
Plateau rocks are plotted as one unit.
Exposures North of Mt. Washburn Field Occurrence
The rhyolitic rocks in the area north of Mt. Washburn
are all nonwelded or welded tuffs. They are generally al most horizontal, but dip away from the older volcanic
breccias in places as much as 20®. The dip of the tuffs away from the old land form of Mt. Washburn is believed
to be caused by the greater compaction of tuff in thicker
sections of ancient valleys developed in Washburn rocks.
In other words, the pre-rhyolite valleys controlled the
location of existing valleys. The contact with Eocene
volcanic breccias is sinuous and many isolated small patches
of tuff occur on the breccia surface. These features are
caused by the maturely dissected topography developed on
Washburn breccias before they were buried by the tuffs
(fig, 89). The Plateau surface is heavily forested, so
outcrops are generally confined to stream valleys and can
yons such as Gamelian Creek Canyon, The tuffs occur in layers 2 to 6 feet thick and are cut at right angles to
the layering by a crude columnar jointing, especially in 198 dense, welded tuffs. Jointing, and the fact that the layers dip away from Mt. Washburn, causes the steep canyon walls, cut into the tuffs, to be unstable, and this has resulted in several landslides. One such landslide dammed Carnelian
Creek, causing a lake to form. The lake is now represented by a small lacustzâne deposit (Plate II), The rock shows a tendency to weather spheroidally, but blocky chunks bounded by joints and layering planes are com mon, It decrepitates or flakes quite readily, producing gruss,
Figure 89. A view from the east wall of Gamelian Creek Canyon looking northwest across the heavily forested Wash- b u m Amphitheater toward Prospect Peak. The wall of the canyon has collapsed, producing a erescent-shaped escarp ment behind the photographer, and a jumble of landslide blocks in the foreground. Notice the crude columnar jointing developed-in the welded tuff of the canyon wall. Also, notice the contrast in topography between the Early basic volcanic breccias of Prospect Peak and the welded tuff of the Washburn Amphitheater, 199
Megascopic Description
The welded tuffs are usually black (mostly glass), pink, or lavender, and contain glassy phenocrysts of quartz and feldspar. They usually show pseudo-flow structure and may contain flattened pumice fragments that leave holes in the rock after weathering. These rocks are noi-mally quite dense, and some produce a metallic ring when struck with a hammer.
The nonwelded tuffs, which occur along Antelope Creek, are of low density and very porous. They contain dark red dish-brown scoriaceous fragments, chunks of pumice up to 5 inches in diameter, black glass, and small rounded pebbles. The matrix is usually light yellowish or grayish brown, and contains glassy quartz and feldspar crystals in addition to the fragments listed above.
Petrography
The welded vitric tuffs show excellent vitroclastic texture and contain approximately 25 percent mineral grains.
Pumice and foreign lithic fragments are present but are volumetrically unimportant. Many examples of welding such as flattened shards and pumice fragments, appressed shards between mineral grains, and shards draped over the edges of mineral fragments are present. The most abundant min erals are Beta-quartz and high-temperature K-Na feldspars such as sanidine or anorthoclase. Oligoclase (An^^) is also present along with minor hornblende, clinopyroxene, 200 magnetite, and fayalitic olivine. Many samples (lavender color) are devitrified, probably by pneumatolytic processes, producing microspherulitic and axiolitic Z/ structures of tridymite and K-Na feldspars.
The nonwelded tuffs are composed predominantly (65-70 percent) of angular shards of light-brown glass (fig. 90). The index of refraction of the glass varies between 1.494 and 1.498 Indicating a composition in the range of rhyolite
(Wahlstrom, 1955, p. 286). Pumice and lithic fragments ac count for 20 percent of the rock and mineral grains approxi mately 10-15 percent. The glass of the pumice fragments is generally colorless, but the exterior of all fragments is coated with light-brown glass identical to the shards.
Vesicles of the pumice are usually stretched out or flat tened. The most common minerals are again Beta-quartz,
K-Na feldspar, and oligoclase-andesine CAn2 ç_3 g). Mafic minerals, none of which is very common, include diopside
(2V = 56°), enstatite (2V = 70°, positive), fayalitic olivine, green hornblende, and magnetite. Lithic frag ments include pilotaxitic andésite, quartz siltstone and sandstone with clay-biotite matrix, and micropegmatite or graphic granite.
7 / —^ An axiolite is an elongate body or flattened spherulite composed of feldspar fibers that are grouped normally to the outline of the body. The central part commonly is more highly crystallized and may contain distinct crystals (Ross and Smith, 1961, p. 4). 201
Figure 90. Fhotomlcrogreph of nonwelded vitric rhyolitic tuff containing light-brown glass shards, enstatite (En), inverted to or coated with diopside (D), magnetite, Beta- quartz (Q), K-Na feldspar (F), and plagioclase (P).
Exposures South of Mt. Washburn
Field Occurrence
Many of the remarks concerning the tuffs north of Mt,
Washburn hold true for the rhyolite flows south of Mt. Wash burn. These rocks belong to Boyd's Canyon flows because they are best exposed along the Grand Canyon. A few out crops also occur along the Grand Loop Road. One such ex posure that is composed of spherulitic glass shows remark able distortion of the layering. Flow structure appears to be "folded" in disharmonie fashion; some flow banding is vertically oriented. Since this exposure is very near
the Early acid breccia contact, it is postulated that the apparent folding is due to turbi^lence as rhyolitic lavas flowed onto the older rocks. The layering exposed in the 202
Grand Canyon section is again nearly horizontal and each layer varies from 6 inches to 6 feet thick. Landsliding features and vertical cliffs showing poorly developed col umnar jointing are again present.
Megascopic Description The rocks of this group of flows are distinctly more glassy and show better flow structure than the welded tuffs.
The glasses are usually black, but brown-black mottled glasses are fairly common. They usually contain a very small percent (1-2 percent) of phenocrysts, and abundant spherulites or layers of spherulites. Lithophysae up to
10 inches in diameter occur in some cases. Hydrothermal alteration related to hot spring activity causes discolor ation, devitrification, or decrepitation of the glasses into a chalk-like substance. The many hues of the Grand
Canyon are caused by this type of alteration. Less com monly, the flows are dense, light-gray to lavender rhyo- lites, similar in appearance to the welded tuffs.
Petrography Most of the glass is colorless and contains hair-like crystallites. A kind of flow-structure results from abun dant streaks of orangish-yellow to medium-brown glass con taining no crystallites. This glass forms a network that encloses the clear glass. The network, in many cases, ap pears to be the remnants of extremely welded shards and 203 pumice fragments. The brown glass streaks flatten around phenocrysts, and so it Is possible that the Canyon flows are partially or entirely welded tuffs. Index of refrac tion of the glass is 1,484 and is within the range of rhyolitic glass.
Phenocrysts are a very minor (1-2 percent) constituent,
Plagioclase (An^^ ^^) containing a few zircon crystals, and magnetite that is partially altered to hematite, are the only minerals present. Spherulitic alteration is the most abundant type, and rocks that are strongly attacked by residual gases are characterized by axiolitic stinctures.
Axiolites are composed of wedge-shaped or bladed tridymite
(75 percent), high temperature Na-K feldspar (15 percent), and quartz (10 percent) (fig. 91). Some isolated spheru lites contain a small aggregate of plagioclase grains as a nucleus, and individual spherulites in glasses that are entirely divitrified by a mass of spherulites show a rim of nearly opague grains (magnetite, amphibole, biotite), possibly as a result of the force of crystallization
(fig. 91).
Origin
The rocks of the Yellowstone Plateau are a bimodal suite (rhyolite and olivine basalt), and do not belong to the orogenic basalt-andésite-rhyolite association (Hamil ton, 1959, p. 227), They are not related to the andésites and basalts of the Absaroka volcanic field, and it is only 204
Figure 91. A flow composed of devltrlfled rhyolitic glass that is now a mass of spherulites with axiolitic structures The axiolites contain coarse blades of tridymite and some high-temperature Na-K feldspars. a coincidence that they overlie the andésites (p. 228), such as in the Mt. Washburn area.
The welded and nonwelded tuffs are very interesting rocks that probably originated as ash-flows. Recently there has been a tremendous surge of work concerning these rocks, beginning with Gilbert (1938) and continuing with
Enlows (1955), McTaggert (I960), Smith (1960), Ross and Smith (1961), and Boyd (1961). An ash-flow is a mobile suspension of ash and volcanic gas that is emplaced by a mechanism that conserves heat and some of the volatile constituent (Ross and Smith, 1961, p. 38). The flow re mains near the surface of the earth and retains enough heat to cause welding of individual pyroclastic particles when the flow comes to rest. The flow is initiated by an 205 explosive, pyroclastic eruption (nuée ardente) of the later
products of differentiation that contain a high concentra tion of volatiles (ibid. , p. 38). Through a thermodynamic analysis of this type of eruption and some experimental evidence, Boyd (1961, p. 387) concluded that **welding in the Yellowstone t^ff could be explained if the tuff was emplaced as a pyroclastic flows and if the magma contained
less than 4 percent water at the start of the eruption” . CHAPTER X. STRUCTURE
It is believed that the present location and configura tion of the Washburn Range and Mt. Washburn is the result of faulting related to the Laramide orogeny and volcano- tectonic collapse. De Martonne (1915, p. 238) originally postulated a fault origin for Mt, Washburn. More recently, Howard (1937, p. 9-12) interpreted the range as a fault- block mountain. He believed that the range attained almost its present altitude by faulting before extrusion of the rhyolite. However, evidence near Observation Peak shows that there was renewed movement along the same fault after the rhyolite was deposited, displacing the rhyolite approxi mately 500 feet. Most of the faults that produced the pres ent Washburn Range are inferred on the basis of the config uration of the range, because the rhyolite flows and welded tuffs obscure the older structures. The fundamental structural framework of the Washburn region is the northwest-southeast oriented structural pat tern of the Laramide orogeny, Washburn is located between the Beartooth uplift on the northeast and the main Laramide fold and thrust belt on the southwest. Secondary structural control is exerted by tensional features that came into ex istence during the waning stages of the Laramide orogeny and by volcano-tectonic collapse features caused by the extrusion of large volumes of subterranean magma onto the surface.
206 207
Numerous bedding measurements were made in the field in spite of and also because of the great difficulty in making such measurements accurately. The dip or plunge of bedded or channel-shaped bodies in the Early basic breccia is gently to the north from the summit area of
Mt, Washburn. The observed values are approximately 20°-25° on the east and south spurs and around the sum mit of Mt. Washburn, but these flatten to 5°-10° in very short distances, north and northwest of Mt. Washburn.
The dip changes slightly to northeast, east of the summit, and to northwest, west of the summit. However, there is a rapid change of the strike to approximately north-south and of the dip to west, and an increase in dip to 25°-35°, from a line west of Mt. Washburn north-south through Dun- raven Pass approximately parallel to the Grand Loop Eoad,
This increase in dip and change of strike is probably due to block faulting and tilting to the west along approxi mately north-south faults. A similar change takes place in the Washburn Hange and is probably caused by the same mechanism. Dips of 6°-10° north on Prospect and Folsom
Peaks change in short distances to 25° west on Cook Peak and on the north-south limb of the Washburn Range. North west striking normal faults near Cook Peak permitted the section of the Washburn cone between Cook and Observation Peaks to be tilted westward. 208
Strikes and dips of the Early acid breccia in Carnelian Creek valley show considerable discordance with the struc ture of adjacent Early basic breccia outcrops, and there fore some tilting or folding probably occurred between the
Early acid and the Early basic breccias. Brown (1961, p. 1187) noted similar deformation in the area just north of the Washburn Range.
Slickensided surfaces are fairly uncommon in the
Washburn area, but they clearly show that faults are pres ent. No exposed faults of any significant displacement were observed in the field, largely due to the nature of the volcanic breccias and the lack of recognizable stra tigraphy. It is interesting to note, however, that most of the slickensided surfaces arc nearly vertical and or iented approximately north-south. Faults are also indi cated by northwest lineated topographic depressions in the Washburn Range between Cook and Folsom Peaks, and north-south depressions on the ridge between Gamelian and Tower Creeks,
Joints are fairly common in Washburn volcanic breccias.
They stand out clearly because hydrothermal solutions ap parently used joints as channelways, causing them to be discolored, and because weathering frequently follows joints producing the prominent vertical weathering forms of volcanic breccia. No systematic study was made of the 209 joints, but most are nearly vertical and oriented approxi mately north-south in the area of Mt. Washburn,
The relative shape of the Washburn Range, with its steep western and southern slopes, and narrow north-south connecting ridge, was discussed previously, Mt, Washburn is attached to the Washburn Range by a narrow belt of breccia near Observation Peak, but otherwise is separated from the range by the expanse of the Washburn Amphitheater, which is filled with rhyolitic tuffs.
The steep mountain front along the southeastern side of the Washburn Range from the Grand Canyon on the east, where it cuts the east spur of Mt. Washburn, to Observation
Peak in the west, is believed to be the result of movement along a major northeast-southwest transverse normal fault zone that cuts Laramide structures nearly at right angles.
The fault has lowered the southeastern portion of the Wash burn volcano, which straddled the fault zone, relative to the northwestern portion (Mt, Washburn). The southeastern portion is now buried beneath the Plateau flows. This fault probably came into existence during the waning stages of
Laramide deformation when compressional stresses were being relaxed, and is probably an extension of the northern seg ment of the Snake River graben or downwarp (Hamilton, 1960, p. 104). Faulting along this zone has been mapped by Boyd
(1961, Plage 6) in the western part of Yellowstone Park near Madison Junction and Secret Valley, and by Brown (1961, 210 p. 119I) in the eastern part of the park along the valleys of Soda Butte and Chalcedony Creeks (see Plate III, in pocket). Other evidence that indicates the presence of this fault zone is the following: 1. The north-flowing Yellowstone River suddenly makes a right angle turn to the east as it approaches the zone.
The river then flows parallel to the zone toward the north east. Along this section, the river has carved the Grand
Canyon, Finally it crosses the zone, cutting the east spur of Mt. Washburn, and continues flowing northwestward.
2. Rhyolitic rocks of the Grand Canyon are strongly altered by hydrothermal solutions and the Canyon itself contains many hot springs and fumaroles.
3. Several clusters of hot springs areas such as
Washburn Hot Springs, Inkpot Hot Springs, and Joseph’s
Coat Hot Springs arc located along this zone.
4. This fault zone defines the boundary between the older Yellowstone tuffs and the younger Plateau flows.
Boyd (1961, p. 412) postulates volcano-tectonic collapse resulting in a huge depression following the eruption of the Yellowstone tuff. The collapse followed the trans verse fault zone along this section of the depression.
5. Three small lakes (Wolf, Grebe, Cascade) are lo cated along this zone at the southwest corner of the
Washburn Range. No other explanation for the existence of the lakes is apparent. 211
Henceforth, this transverse fault zone will be referred
to as the Madison-Soda Butte Creek fault zone.
It is believed that an arcuate, normal fault extending
from Tower Falls approximately along the valley of Tower
Creek to a col just east of Observation Peak, separates Mt. Washburn proper from the Washburn Range with a rela
tive downwaixi movement of the Mt. Washburn block (Plate III,
in pocket). Evidence of the fault is visible at Tower Falls (see Howard, 1937, p. 13), and it would explain the steep
southern and eastern slopes (fault scarps) of the Washburn
Range from Prospect Peak to Observation Peak. The stra tigraphy and petrology of the Early basic volcanic breccias
strongly suggest that M t , Washburn and the Washburn Range
were once part of a continuous sheet of fragmental rocks. The arc-shaped normal fault would explain the discontinuity
that now exists. This faulting is probably related to the
depression of the southwestern part of the Beartooth block,
which is obscured by volcanic rocks. The depression is
probably due to "... the massive transfer of subcrustal
material to the surface" (Foose e^ al., 1961, p. 1165)
forming the Absaroka Range. It is interesting to note
that the northwestern limit of the depressed portion of
the Beartooth block is approximately defined by the
Madison-Soda Butte Creek zone. De Martonne (1915, p. 238) expressed the opinion that
the relation between the Beartooths and Yellowstone Park is 212 a series of step faults. This relationship is probably correct, at least in part, since Brown (1961, Plate I) shows normal faults parallel to the elongation of the
Beartooth block along the Lamar River valley and on the
Buffalo Plateau, and believes that the valley of the Yel lowstone River directly north of the Washburn Range is a downfaulted block (Brown, 1961, p. 1187). These normal faults are shown in Plates III and IV in the pocket of the writer’s report. The Gardiner thrust that defines the southern boundary of the Beartooth block northwest of Mt.
Washburn is parallel to the strike of the normal faults. The intersection of these northwest striking faults re lated to the Beartooth block and the Madison-Soda Butte
Creek zone probably localized the vent of the Washburn volcano.
The following is a chronological list of the struc tural events in the M t . Washburn region:
1. Major uplift of the Beartooth block during Middle and Late Fort Union (Paleocene) time and culmination dur ing Early Wasatch (Eocene) time (Foose et , 1961, p.1165).
2. Creation of the Madison-Soda Butte Creek fault zone transverse to Laramide structures, and normal faults parallel to the elongation of the Beartooth block during Middle Eocene time.
5. Minor block faulting or folding of Early acid vol canic rocks and sediments. 213
4. Creation of the arcuate, normal fault separating
Mt. Washburn and the Washburn Range, immediately follow ing renewed activity along the Madison-Soda Butte Creek fault that split the Washburn volcano at the end of Early basic breccia volcanism.
5. Westward tilting of sections of the Washburn cone along north-south hinge lines, north of the Madison-Soda
Butte Creek zone shortly after the arcuate fault. 6. Regional uplift during Miocene or Pliocene time
in the order of 4000 to 6000 feet (Foose e^ aT,, 1961, p. 1165). Possibly this uplift triggered the Plateau cycle of rhyolitic volcanism.
7. More activity along the Madison-Soda Butte Creek zone owing to caldera collapse caused by the rapid extru
sion of the Yellowstone tuff. The renewed activity re
sulted in further downfauIting of the southeastern half
of the Washburn volcano. CHAPTER XI. SOURCE OF THE EARLY BASIC VOLCANIC ROCKS
It is proposed that the vent or vents for the Early
basic breccia volcanic rocks in the Mt, Washburn region
is located two to three miles southeast to south-southeast
of Mt, Washburn. The vent is now buried beneath the pla
teau formed by the Canyon (Plateau) flows and probably is
situated on the northwest side of the Grand Canyon. It was localized along the transverse Madison-Soda Butte Creek fault zone at the intersection with northwest or
iented faults that approximate the southwest margin of
the Beartooth block. The orifice was probably elongate
or fissure-like parallel to the transverse fault zone, or may have been a series of several aligned openings.
Renewed movement on the Madison-Soda Butte Creek fault
split the volcanic cone around the vent following the
cessation of volcanic activity, and caused the southeast ern half of the cone to move down at least 3,000 feet,
permitting its complete burial by later rhyolitic lavas.
Other local vents related to the main vent probably
existed near the summit of the cone or on the upper flanks. In Chapter IV, the possibility of fissure erup
tions from dikes was discussed,and the existence of small pipe-like bodies that may be volcanic necks was also pointed out. The vent for Early acid lava flows and breccias, which was discussed in Chapter III, is located
214 215
just south of Hedges Peak, four or five miles west of the
proposed Early basic vent.
The cone associated with the vent of the Washburn
volcano was broad and erosion-scarred, with dips up to
25° near the vent. Generally, slopes varied between 6° and 12° away from the vent area, but were only 3° to 4°
in outlying areas. The cone was approximately 25 to 30
miles in diameter at the base and stood 3,000 feet above
the general terrain. In relation to the present general
elevation of the region and the height of Mt. Washburn
(10,243), the maximum elevation of the cone was probably nearly 12,000 feet. Eruptions from the vent
were spasmodic, and were separated by relatively long
periods of quiescence. During these periods, rapid
erosion attacked the cone, causing the paleotopography and the cut-and-fill structures that occur in breccia
outcrops of Mt. Washburn. The site of the vent for the Washburn volcano was
deduced from the following:
1, Quaquaversal dips around the vent area from north east along the east spur of M t . Washburn, to north and
northwest near Mt, Washburn, to west on Dunraven and
Hedges Peaks, to southwest on Observation Peak in the
west. Dips generally increase toward the vent area.
2, Concentration of all types of lava flows and
related nonfragmental rocks near the vent area. 216
3. Concentration of all dikes, which show a crude radial pattern with respect to the vent, in the vicinity of the vent.
4. Very thickly bedded, unsorted, heterolithologic volcanic breccias are concentrated in the vent area. North and northwest of the vent, beds are thinner and better stratified, sorting is better, and fragments show some rounding. In other words, epigene processes exerted much more control in this area than near the vent, where
igneous processes were predominant. *
5. Monolithologic breccias occur only in the vent area.
6. In a general way, the coarsest fragments are localized in the vent area. No large blocks occur in
the Washburn Range.
7. Plant fossils are absent or extremely rare in the vent area, but northward, especially in the Washburn
Range, the occurrence of plant fossils and petrified wood increases. Near Cook Peak, large trees with root systems attached occur in upright positions where they grew.
8. Cut-and-fill structures and paleotopography are common near the vent where slopes were steepest and erosion would have been most vigorous. North and west from the vent area these features no longer occur. 217
9. The coarsest and largest masses of intznislve
Igneous rock are located in the vent area.
10. The vent area is characterized by abundant
hematitic and montmorillonitic alterations. Northward,
alterations are generally represented by zeolites and
silica minerals. Montmorillonite is not a hydrothermal
mineral, but may attain some mobility and travel short
distances (k. T. Tettenhorst, personal communication).
The magnesium and other materials that form montmorillon
ite were derived from magnesium-rich primary minerals
such as olivine and pyroxene. These minerals are badly altered in the vent area, and so it is assumed that magnesium for montmorillonite came from the vent area.
11. The volcanic breccias are better lithified close to the vent than to the north because of the abundance
of secondary minerals, which bind the fragments together.
12. Hot springs such as Washburn Hot Springs and
Inkpot Hot Springs are concentrated in the vent area.
13. Minor sulfide mineralization (pyrite dissemina
tions) is associated with the vent area. CHAPTER XII. TERTIARY GEOLOGIC HISTORY OF THE MT. WASHBURN REGION
The Tertiary geologic history of the Mt. Washburn region is summarized as follows:
1. Uplift and deformation of the Beartooth block during Paleocene time, culminating in the beginning of the Eocene,
2. Alluvial sedimentation of coarse detrital ma terials derived from the rising Beartooth block on a broad alluvial plain just prior to and during the be ginning of Early acid time (Middle Eocene).
3. Initiation of volcanic eruptions in the airea of the Absaroka Range and along the Cooke City zone of the
Beartooth block, contaminating the sands and gravels derived from the Beartooths with andesitic volcanic detritus.
4. Transverse faulting along the Madison-Soda Butte Creek zone approximately normal to the elongation of the
Beartooth block as a result of the relaxation of east- west Laramide compressional forces. Beginning of local volcanic activity in the M t . Washburn region, probably with a pyroclastic ash-flow or nuée ardente, and fol lowed by extrusion of Early acid lavas and minor Vul- canian breccias.
218 219
5. A brief period of erosion between the Early acid
and Early basic breccias in most portions of the Washburn
region, with the possible exception of the area along the
transverse fault zone where volcanic activity may have
been continuous. Minor block faulting or even some folding may have taken place during this period of erosion.
6. Early basic breccia volcanic cycle resulting in
the extrusion of huge quantities of andesitic volcanic breccia and minor lava flows along with the intrusion
of basalt dikes. Possible intrusion of hornblende andé
site dikes near the end of the cycle.
7. Extrusion of trachybasalt flows exposed on Ob
servation Peak.
8. With the cessation of the volcanism near the end
of the Middle Eocene, activity was renewed along the
Madison-Soda Butte Creek transverse fault zone causing
Mt. Washburn to split into two halves. The southeastern
half moved downward, and the northwestern half was broken
by an arcuate block fault, creating the separation of the
Washburn Range and Mt. Washburn proper. The renewed
faulting was caused by the extrusion of great quantities
of breccia and lava in the Absaroka Range and at Mt. Wash
burn. Smaller sections of the Washburn volcanic cone
were tilted westwaixi following the arcuate faulting. 9. A long and extensive period of erosion from Late Eocene to the Pliocene, during which time a well dissected 220 topography was developed in the Middle Eocene sequence of volcanic rocks in the Washburn region.
10, ScHnetime during the Miocene or Pliocene, the whole region, including Mt, Washburn and the Beartooth block, was uplifted 4000 to 6000 feet. 11, Extrusion of a great volume of welded tuff (Yel lowstone tuff) emplaced by pyroclastic ash-flow during the Pliocene, partially burying both halves of the Wash burn volcano. This ei-uption might have been triggered by regional uplift. It caused a great volcano-tectonic depression to form (now the major portion of the Yellow stone Plateau) that was in part controlled by the
Madison-Soda Butte Creek transverse fault zone. Thus the two halves of the Washburn volcano were separated farther.
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Iddings, J, P. and Weed, W. H ., 1899, Descriptive geology of the Gallatin Mountains, U, S. Geol. Survey Mono graph 32, pt. 2, Geology of the Yellowstone National Park: p. 1-59,
Jones, O. T. and Field, R. M . , 1929, The resurrection of the Grand Canyon of the Yellowstone: Amer. Jour. Sci., ser. 5, V. 17, p. 260-278,
Kaaden, Gerrit van der, 1951, Optical studies on natural plagioclase feldspars with high- and low-temperature optics: diss., Utrecht, Rijksuniv., 105 p. 224
Knowlton, F. H . , 1899, Fossil flora, in U. S. Geol. Survey Monograph 32, pt. 2, Geology of tïïe Yellowstone National Paries p. 651-882.
Krushensky, Richard D.. 1960, Geology of the volcanic fea tures of the Hurricane Mesa area, Park County, Wyoming: The Ohio State Univ., unpublished Doctoral disserta tion, 217 p.
Larsen, Esper S., 1940, Pétrographie province of central Montanas Geol. Soc. Amer. Bull., v. 51, p. 887-948.
Larsen, Esper S., et al., 1937, Petrologic results of a study of the mTnerals from the Tertiary volcanic rocks of the San Juan Region, Colorado: Amer. Min., v. 22, p. 889-905.
, 1938, Petrologic results of a study of the minerals from the Tertiary volcanic rocks of the San Juan Region, Colorado: Amer. Min., v. 23, p. 227-257,
Love, J. D . , 1939, Geology of the southern margin of the Absaroka Range, Wyoming: Geol. Soc. Amer. Special Paper 20, 134 p.
Martonne, E. de, 1915, Le Parc National du Yellowstone: Esquisse Morphologique, Amer. Geog. Soc. Mem. Volume Transcontinental Excursion of 1912: p. 231-250.
McTaggert, K. C., 1960, The mobility of nuees ardentes: Amer. Jour. Sci., v, 258, p. 369-387%
Morey, G. W . , 1922, The development of pressure in magmas as a result of crystallization: Wash. Acad. Sci. Jour,, V. 12, p. 219-230.
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Osborn, E. F., 1959, Role of oxygen pressure in the crystal lization and differentiation of basaltic magma: Amer. Jour. Sci., v. 257, p. 609-647.
Parsons, W. H . , 1939, Volcanic centers of the Sunlight area. Park County, Wyoming: Jour. Geol., v. 47, p. 1-26,
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Parsons, W. H., I960, Origin of Tertiary volcanic breccias, Wyoming: Internat. Geol. Gong., 21st, Copenhagen 1960, Proc, sec. 13, pt, 13, p. 139-146.
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Turner, Francis J. and Verhoogen, Jean, 1951, Igneous and metamorphic petrology, 1st Éd.: New York, McGraw- Hill Book Co., 602 p.
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Wahlstrom, Ernest E., 1955, Pétrographie mineralogy: New York, John Wiley and Sons, 408 p.
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Wentworth, C. K. and Williams, Howell, 1932, The classifi cation and terminology of the pyroclastic rocks : Nat. Res. Coun. Bull., No. 89, p. 19-53. AUTOBIOGRAHÎY
I, Charles High Shultz, was born in Lancaster,
Pennsylvania, May 29, 1936, I received my secondary school education in the public schools of Lancaster and Manheim Township, Lancaster County, Pennsylvania, and my undergraduate training at Franklin and Marshall
College, which granted me the Bachelor of Science de gree in June, 1958, I began my doctoral studies at The Ohio State University in September of 1958, and
since then have held the following appointments: Graduate Assistant, 1958-1961; University Fellow,
Summer Quarter, 1960; National Science Foundation
Fellow, Summer Quarter 1961; and National Science Foundation Cooperative Fellow, September 1961 to
June 1962. I am a member of the Society of the
Sigma Xi and of the Geological Society of America.
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Quartz diorite intrusion « w z w u 0 w Early basic breccia
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/ Plunge of channel*shaped lava flows and lahars
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1
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WNWMItilM MilMTM.Mt * \ 44*45' 1 1 0 * 2 5 miles MOLMY #V OHAM.lt H. IHULTI KA UVIL I f ti -44*Stf m m ■lilt !'iv!v]v. I I I I * » ' « * I I•>)> '.vi'i Iff* **«• >>■>>•> I , I , * * a » , , I « j • ■■■■> I I,,,,.,.,,,. . , # « I I I ' I » * » I I I ' I I * » » I I l l» «| l ««■«■' 4l(4f»*«l»««»4«4l4«»t X Washburn; 'X'ivivXv/.-:* ■ ■ I 4 * « / '' \ %v\ v: v,:X\\vX<:'>::AvX:ÿ^ % *« •*♦*! ,*4«lt4«*«»«««' s i i i m m r n •Il wn m niR « wun M M M ANION S31IN 2 31V0S ,«.011 I V •Il !.. m 111 * * » • '//.'.'I V.'.v*• * * ■ • • •.# • • • V>: . 1:: t • 'f!: l*V* * • V- Av:; % : y i '.I:: i .••V v-:( '.v:% •■/j X i• • • • ••% ,.,,111, •■441 I, 1 • I > I • I Li'i ' w . . • • I < ■ • I • I > • H < I I t • ■ • < • • • • • • ■ • I • • I < I • > > 4 , < t a I I • i i • • <4 • * b a a I r * ■ * a n IH» #* ■■■ ■■■■•■■■1 • • I > • <4 I I b ■ > • • ■# d « «bibiiaiiabaaiv a Va d aaaaaaaa ia .VA.V m A t I I u I I \ M I t « M t 44-45' tl N. IMULTZ Quartz diorite intrusion 8 i 0 ID Early basic breccia A Early acid breccia ! y Strike and dip m A / Plunge of ctiannei-stiaped lava flows and labors / Approximate contact Inferred contact IHE FEW FAULTS THAT OCCUR IN IHE WASHBURN AREA ARE PLOTTED ON PLATE III, GENERALIZED STRUCTURAL GEOLOGIC MAP, MOUNT WASHBURN m VICINITY. FOR THE MOST PART, IHESE FAULTS lÉ •liKlîk * ...... »...... I é * é _ & ÈÆ ■.y.'. .1 • I . 1 • • ; i u ...... à W t * * I t * * • “ •*1011 Ri*‘» t.'f • ’.Foul»'.*.*-. sssæ! • /».»* « • * » * I *♦. m :*:y ÊÈ .» ». I ■ t f c 4llu VIUM 1 8 . • • • J 1 * '/* «** /* • •: Il • I • I » • 4 I I i M» m m Ê i i æ Im s * '- iii 44 30 üiii M â mm IlO'iO 10 15 MILES GEOLOGY ANO STRUCTURE I SCALE AND COMPILED FROM FOOS GARSARINEdSGI), 8R0WN(I96II (I960. PLATE I (BIERALIZEO STRUCTURAL t MOUNT WASHBURN AND HOMO 45*10' ÏSÏ « A Tvlïÿ;" m m m m ■ifew k m m m ^ m 110*50' EXPLANATION w m Plateau flows and reloted rocks Yellowstone tuff and related rocks SISK Early acid breccia, Early basic breccia, iiAiwmiiiiia Early basalt sheets Precambrian, Paleozoic, Mesozoic and ISii minor Tertiory rocks i :,' k % , / Thrust fouh I y ■ I Fault (published) I■ - 44*50' 110*00' A ' Inferred fault (published) ODIFIED ANO ^ inferred foulf (this report) , W I S E A N D / A N O B O Y D M* > Strike end dip 10*, Plunge of chonnel*shoped body ' (breccio or lava) ## Inferred Washburn volcanic vent OLOGIC MAP Î.V'AV* m ■'.'•0.7.VS 44*50' HYPOTHETICAL CROSS SECTIO (lOUTH) MOUNT WASHBURN >Jp243 FEET I VERTICAL IXAaCINATION I M IL E PLATE E )N OF MOUNT WASHBURN AND THE WASHBURN RANGE PRMKCT PEAK ^ W E S F E E T W A L E E » MILES |:6ES00 E WASHBURN AND THE WASHBURN RANGE ^PKOmOT PEAK fStSFEr •UPPAL (UANTOO 6 2 9 0 0 (MONTH) EXPLANATION W Z '.'J ! > i*«'i *1 *r*i'i PLATEAU FLOWS -J a * YELLOWSTONE TUFF VENT COMPLEX AND FAULT BRECCIA iLO PLATIAU OTM MOUNTAIN#) EARLY BASIC LAVA FLOWS PÜ 'mm, 4 ■ ' i 4 /* EARLY BASIC VOLCANIC BRECCIA TO CONGpMERATE m m EARLY ACID LAVA FLOWS EARLY ACID CONGLOMERATE AND SANDSTONE m BASEMENT COMPLEX Tabic 11. Chemical and Nonatlva Analyaca e C o m p o n e n t 1 ______2 3 S l O j 6 1 c 5 6 6 1 . 4 5 6 0 . S AI 2O 3 1 4 . 7 3 1 5 . 0 7 1 5 . 4 T i O g . 8 7 2 . 8 0 . 5 F e ^ O j 4 . 4 7 4 . 4 6 2 . 4 F e O 1 . 2 3 1 . 1 8 3 . 8 F t f m e t a l - - M n O . 3 4 n o n e .0 M g O 3.57 3.02 3.1 G a O 4 . 8 7 5 . 3 7 5 . 1 N a . O 5 . 1 0 4 . 0 0 5 . 2 K , 0 2 . 2 4 1.22 1 . 9 P j O s . 0 4 T .2 L l ^ O - . 0 5 - C O j - . 3 F - . 0 - S - - SO 3 - . 2 9 - H ^ C 1 . 4 2 1 . 2 3 2 . 4 ( T o t a l 1 0 0 . 4 4 1 0 0 . 1 4 9 9 . 7 ! Q u a r t z 1 0 . 5 6 1 8 . 9 0 1 7 . 5 : O r t h o c l a s e 1 3 . 3 4 7 . 2 3 U. 6( A l b l t e 4 2 . 9 7 3 4 . 0 6 27.71 A n o r t h i t e 1 0 . 5 6 1 9 . 1 8 2 1 . 6 ( D l o p a i d a 1 0 . 5 8 1 . 9 4 2 . 2C Hyperathene 4 . 0 0 6 . 7 0 1 1 . 0 : n o n e llmenite ' 1.67 2.58 1 Magnetite 2.55 none 3 Titanite none 3.53 n Hematite 2.72 4.48 n Apatite none none 1 Volatiles 1.42 1.52 2 Total 100.41 100.12 99 Name of rock hornblende pyroxene hombl analyzed andésite andésite andes] Common rocks in andésite, andésite andes: the same C.I.P.W. quartz diorite dioril — class _ _ 1I 1. Hornblende andésite (Early acid), Tower Creek, chemical alyais by Gooch; reported by Iddlngs (1899, p. 272) as contai plagioclaae, hornblende, and a little augite; normative anal by Washington (1917, p. 351). 2. Pyroxene andésite (Early acid?). Agate Creek; chemical alyais by Whitfield; reported by Iddings (Clarke and Hlllebr 1897, p. 134) aa containing autite, hyperathene, labradorite, magnetite in a glaasy, microlitic groundmass; normative anal by Washington (1917, p. 373). 3. Hornblende andésite (Early acid) lava flow southeas Hedges Peak; chemical analysis by Ingamells, The Pennsylvania S University; for modal analysis see Table 12 in this report ; no tive analysis by the writer. 4. Olivine-pyroxene andésite (Early basic), dike 200 y south of Mt. Waahbum summit; chemical analysis by Ingamells, Pennsylvania State University; for modal analysis see Table 1 this report; normative analysis by the writer. 06 .91 1.82 none none 2.13 48 3.25 3.48 none 6.50 3.02 ne none none 2.74 none nonè ne none none 7.24 none none )6 .34 1.34 1.01 .34 .34 34 3.15 1.65 1.55 1.55 none 56 99.78 100.51 99.19 100.69 99.56 olivine-pyroxene pyroxene »nde basalt ;e andeaite andeaite basalt basalt aniesite, :e. andeaite baaalt, basalt. basalt, basalt. I quartz diorite quartz basalt andeaite basalt andeaite basalt diabase Explanation of Table 11 5. Pyroxene andeaite (Early baaic), west of Dunraven Peak; chemical analyaii by Gooch; reported by Iddlnga (Clarke and Hille- brand, 1897, p, 135) aa containing labradorite, augite, hyperathene, and magnetite in a microlitic groundmaaa; normative analyaia by Waahington (1917, p. 485). 6. Baaalt (Early baaic), Yellowstone Canyon; chemical analyaia by Whitfield; reported by Iddinga (Clarke and Hlllebrand, 1897, p. 135) aa containing labradorite-bytownite, augite, olivine, mag netite, and a little brown glaaa; normative analyaia by Washington (1817, p. 485). 7. Basalt approachir^ pyroxene andeaite (Early baaic), dike on the north spur of Mt, Waahbum; chemical analyaia by Whitfield; reported by Iddinga (Clarke and Hlllebrand, 1897, p. 136) aa con taining labradorite, augite, aerpentinized olivine, and magnetite, in a groundmaaa of globulltic and microlitic brown glaaa; normative analyaia by Waahington (1917, p. 485). 8. Baaalt (Early baaic), aouthveat of Dunraven Peak; chemical analyaia by Gooch; reported by Iddinga (Clarke and Hlllebrand, 1897 p. 135) aa containing augite, olivine, labradorite-bytovnite. and negnatlte in a globulltic glasay groundmaaa; normative analyaia by Waahington (1917, p. 609). lârly Acid Bnoclt and Early Baaic Bnccia in tha Mt. Waahbum Kaflon 4 5 6 7 8 5 9 . 1 7 5 6 . 4 7 5 1 . 7 0 5 3 . 7 5 5 1 . 7 0 1 5 . 6 5 1 5 . 3 3 1 7 . 9 0 2 0 . 7 5 1 5 . 1 8 . 4 5 . 9 9 3 . 1 7 n o n a 1 . 2 4 2.21 2 . 5 4 7 . 2 4 4 . 5 0 2 . 0 9 3 . 6 2 4 . 5 3 1.00 3 . 5 3 8 . 5 4 - - 1 . 8 1 . ,11 . 1 8 T T T 7 , 3 8 5 . 0 8 2 . 7 7 3 . 7 6 8 . 1 8 4 . 6 1 6 . 9 3 6 . 9 4 7 . 1 8 8 . 7 3 3 . 3 4 3 . 8 1 4 . 1 7 4 . 1 6 2 . 3 1 1 , 6 7 1.66 1 . 6 2 1 . 3 7 1 . 8 1 .20 . 5 4 . 4 1 . 1 5 .21 m . 0 3 .11 - - n o n a . .02 ------. 0 9 - . 3 2 T 3 . 0 4 1 . 6 5 1 . 1 5 1 , 5 5 . 1 6 9 9 . 7 7 9 9 . 7 1 9 9 . 2 3 1 0 0 . 7 0 1 0 0 . 2 4 1 1 . 6 4 6.66 4 . 1 4 2 . 5 8 n o n a 10.01 10.10 9 . 4 5 8 . 3 4 11.12 2 8 . 3 0 3 1 . 9 6 3 5 . 6 3 3 5 . 1 1 1 9 . 3 9 1 7 . 2 4 2 2 . 5 2 2 7 . 8 0 3 3 . 6 4 2 5 . 5 8 4 . 6 4 6 . 4 8 ^ . 8 9 1 5 . 5 7 - 2 0 . 3 0 1 4 . 5 9 1 1 . 7 4 20.12 4 . .4,#''