A reconnaissance of mesozoic strata in nothern Yuma County, southwestern Arizona
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Authors Marshak, R. Stephen
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Link to Item http://hdl.handle.net/10150/566541 A RECONNAISSANCE OF MESOZOIC STRATA
IN NORTHERN YUMA COUNTY, SOUTHWESTERN ARIZONA
by
R. Stephen Marshak
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements For the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
1 9 7 9 STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of re quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg ment the proposed use of the material is in the interests of scholar ship. In all other instances, however, permission must be obtained from the author.
SIGNED:
APPROVAL BY THESIS DIRECTOR
This theses has/b^en approved on the date shown below:
?.J. CONEY/ Professor of Geology ACKNOWLEDGMENTS
I am grateful to the members of my committee, Drs. Peter Coney,
George Davis, and Wesley Peirce, for their assistance in this project.
Support for this research was generously provided by the State of
Arizona Bureau of Geology and Mineral Technology, and I am grateful to
- the members of the Bureau staff, especially Wesley Peirce, Stanley
Keith, and Joseph Lavoie, for their advice and encouragement throughout
this project. Gregory McNew, George Sanders, and Robert Schafer, of
the Department of Geosciences, assisted with the petrographic studies.
Support for the preparation of thin sections was provided from the
Bert S. Butler scholarship fund of the Department of Geosciences. I
appreciate the information provided by Stephen Reynolds, Ed DeWitt,
W. J. Crowell, and Brad Robison, who are or were students of the
Department of Geosciences. My sincere thanks are extended to my
friends at The University of Arizona for their moral support when it was
needed most. Stanley Keith and Paula Trever helped in the field on my
last trip there, when the thermometer read 117°F. My appreciation also
goes to my parents, Robert and Ruth Marshak, who have encouraged me in
all stages of my education.’. Finally, I would like to express my deepest
gratitude to my fiancee, Kathryn Collin, who edited and typed four drafts
of the manuscript, and without whose love and support this project would
never have been completed.
iii TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS...... vi
LIST OF T A B L E S ...... x
A B S T R A C T ...... xi
1. INTRODUCTION ...... 1
Statement of the Problem ......
History of Geologic Study in Yuma County . H UJ Methodology and Terminology......
2. THE DOME ROCK MOUNTAINS...... 7
Lithologies...... 7 Mesozoic Strata...... 7 Bounding Terranes...... 23 Contact Relations...... 33 Structural Features...... 40 F a u l t s ...... 40 Folds...... 41 Other Structural Features...... 41 Discussion...... 48 Stratigraphy...... 48 Structure...... 50 Age Constraints...... 52
3. THE LITTLE HARQUAHALA MOUNTAINS...... 54
Lithologies...... 54 Mesozoic Strata...... 54 Bounding Terranes...... 60 Contact Relations...... 62 Structural Features...... 62 Folds and F a u l t s ...... 62 Discussion...... 63 Stratigraphy...... 63 Age Constraints...... 63
iv V
TABLE OF CONTENTS— Continued
Page
4. THE GRANITE WASH MOUNTAINS...... 65
Lithologies...... 65 Mesozoic Strata...... 65 Bounding Terranes...... 72 Contact Relations...... 74 Structural Features...... 74 Folds...... 74 Faults ...... 76 Other Structural Features...... 76 Discussion...... 76 Stratigraphy ...... 76 Age Constraints...... 80
5. THE PLOMOSA, BUCKSKIN, AND NORTHERN DOME ROCK MOUNTAINS. . . . 82
Plomosa Mountains...... 82 Lithologies...... 82 Contact Relations...... 88 Discussion ...... 90 Buckskin Mountains...... 91 Lithologies...... 91 Field Relations...... 94 Discussion...... 94 Northern Dome Rock Mountains...... 95
6. SUMMARY, CORRELATION, AND CONCLUSION...... 96
Summary...... 96 Correlation...... 103 Conclusion...... 106
LIST OF REFERENCES 108 LIST OF ILLUSTRATIONS
Figure Page
1. Location Map of Areas in Yuma County Mapped by Wilson (1960) and referred to on the Geologic Map of Arizona as "Mesozoic Sedimentary Rocks"...... 2
2. Newspaper Report Concerning the Discovery of a New Mountain Range in Yuma County...... 5
3. Location Map Showing Mapped Areas and Major Contacts Studied in the Dome Rock Mountains...... 8
4. Distance Photograph of the Central Dome Rock Mountains, Showing Outcrops of Mesozoic Strata...... 16
5. Schematic Stratigraphic Column and Strip Map of the Eastern Flank of the Dome Rock Mountains...... (pocket)
6. Schematic Chart Showing the Range of Individual Lithologies in the Section of Mesozoic Strata of the Dome Rock Mountains...... 17
7. Photomicrograph of Calcareous Quartz Sandstone From the Dome Rock Mountains...... 18
8. Photograph of Conglomerate Outcrop From Unit A of the Dome Rock Mountains...... 19
9. Photomicrograph of Greywacke With Possible Tuffaceous Component...... 21
10. Photograph of Granule Conglomerate Outcrop in the Dome Rock Mountains...... 22
11. Photomicrograph of the Feldspathic Greywacke of the Dome Rock Mountains...... 24
12. Distance Photograph of the Meta-Volcanic Terrane of the Dome Rock Mountains...... 26
13. Photograph and Tracing Showing Contact of Mesozoic Strata With Meta-Basalt on La Cholla Mountain...... 27
vi vii
LIST OF ILLUSTRATIONS— Continued
Figure Page
i 14. Reconnaissance Geologic Map of the Copper Bottom Pass Area, Dome Rock Mountains, Arizona ...... (pocket)
15. Photomicrograph of Meta-Crystalline Tuff From the • Dome Rock Mountains...... 28
16. Photograph of Coarse Meta-Crystalline Tuff Outcrop in the Copper Bottom Pass Area of the Dome Rock Mountains ...... 29
17. Photomicrograph of Dark Grey Meta-Volcanic Macke of Cunningham Mountain, in the Dome Rock Mountains. . . . 31
18. (A) Photomicropgrah of Mylonite Derived From Meta- Crystalline Tuff (B) Photomicrograph of Mylonite Derived From Dark Grey Meta-Volcanic Macke ...... 32
19. Reconnaissance Geologic Map of the Southern End of the Dome Rock Mountains...... (pocket)
20. Distance Photograph of Meta-Crystalline Tuff Outcrop at the Southern End of the Dome Rock Mountains .... 34
21. Photographs and Tracing of Dark Grey Meta-Volcanic Macke in Contact Mith Mesozoic Strata...... 36
22. Photograph of Calcareous Quartz Sandstone — Mylonite Contact...... 37
23. Map Showing Bending of Foliation of the Meta-Volcanic Terrane Into Parallelism Mith the Cunningham Mountain F a u l t ...... 39
24. Photograph and Tracing of Intrafolial Folds at the Copper Bottom Mine, Dome Rock Mountains...... 42
25. Photograph Showing High Angle Between Bedding and Foliation Planes in a Layer of Calcareous Quartz Sandstone on La Cholla Mountain...... 44
26. Equal Angle Lower Hemisphere Projections of Poles to Bedding and Foliation Planes in (A) the Mesozoic Strata and (B) the Meta Volcanic T e r r a n e ...... 45
27. Photograph of Conjugate Joint Set in the Meta-Volcanic Terrane of the Copper Bottom Pass A r e a ...... 46 viii
LIST OF ILLUSTRATIONS— Continued
Figure Page
28. Equal Angle Lower Hemisphere Projection of Lineations, Kink-Fold Axes, and Poles to Fractures, Measured in the Meta-Volcanic Terrane of the Copper Bottom Pass A r e a ...... 47
29. Topographic Map of the Little Harquahala Mountains...... 55
30. Schematic Geologic Strip Map in the Little Harquahala Mountains...... 59
31. (A) Photomicrograph of Fine-Grained Maroon Sandstone, of the Little Harquahala Mountains (B) Photo- Micrograph of Coarse-Grained Maroon Sandstone From the Same A r e a ...... 61
32. Topographic Map of the Granite Wash Mountains...... 66
33. Schematic Geologic Strip Map in the Granite Wash Mountains...... 67
34. (A) Photomicrograph of Fine-Grained Greywacke of the Granite Wash Mountains (B) Photomicrograph of Coarser-Grained Greywacke From the Same Location .... 71
35. Photograph of a Hand Sample of Biotite Gneiss From Unit D of the Granite Wash Mountains...... 73
36. Distance Photograph and Tracing of the Contact Between Mesozoic Strata and Laramide Granite in the Granite Wash Mountains ...... 75
37. Photograph of Class 2 Fold Refolding Class 1 Isoclinal Fold in Unit B of the Granite Wash Mountains...... 77
38. Equal Angle Lower Hemisphere Projection of Fold Hinges Measured Near the Gardlock Claims, Granite Wash Mountains...... 78
39. Photograph and Tracing of Block Faulting in Unit C of the Granite Wash Mountains...... 79
40. Topographic Map of the Plomosa Mountains...... 83
41. Schematic Geologic Strip Map in the Plomosa Mountains. . . . 86 ix
LIST OF ILLUSTRATIONS— Continued
Figure Page
42. Photograph of Hand Samples of Biotite Gneiss From . the Plomosa Mountains...... 87
43. Equal Angle Lower Hemisphere Projection of Fold Axes in the Marble Outcrops Near the Bouse-Quartzsite Road, Plomosa Mountains...... 89
44. Schematic Chart of Units Bounding or Cutting Mesozoic Sedimentary Rocks in Mountain Ranges of Yuma County, Arizona...... 98
45. Schematic Map Indicating the Variations of Lithology in the Areas Mapped as Mesozoic Strata on the Geologic Map of Arizona...... 99 LIST OF TABLES
Table Page
1. Lithologies, Dome Rock Mountains...... 9
2. Lithologies, Little Harquahala Mountains ...... 56
3. Lithologies, Granite Wash Mountains...... 68
4. Lithologies, Plomosa Mountains ...... 84
5. Lithologies, Buckskin Mountains...... 92
6. Summary of Principal Characteristics of Mesozoic Strata in the Mountain Ranges of Northern Yuma C o u n t y ...... 97
x ABSTRACT
This study reports on the geology of Mesozoic sedimentary rocks in five mountain ranges of northern Yuma County. Outcrop features of these deposits are diverse> a consequence of variations in lithologic composition and metamorphism (which locally reaches amphibolite grade).
The most complete exposure of these deposits occurs in the Dome Rock
Mountains, where the sedimentary section is about 5000 m thick and is only mildly metamorphosed. This section structurally overlies mildly metamorphosed volcanic and volcanoclastic terranes which are themselves
faulted together. The composition of detritus indicates that the nature of source terranes changed with time as the section was depos
ited. Portions of the Dome Rock Mountains section correlate litholog
ically with deposits of other ranges. Contact relations suggest that
these rocks were deposited during post-mid-Jurassic and pre-Laramide
time. Foliation planes through much of the Dome Rock Mountains out
crop area are oriented at high angles to the bedding. This foliation,
the distribution of metamorphism, and the presence of folding and
faulting throughout the ranges studied show that regional deformation
has affected these rocks. Although a structural pattern is evident
within each range, the relation of structures among the different
ranges is unclear.
xi CHAPTER 1
INTRODUCTION
Statement of the Problem and Purpose of this Research
The geology of Yuma County, in southwest Arizona, has not been intensely studied. In fact, the 1:375,000 scale Geologic Map of Yuma
County, Arizona (Wilson, 1960) is the only source of geological in formation for much of the region, and is the basis for the geologic representation of the region on the Geologic Map of Arizona (Wilson,
Moore, and Cooper, 1969). This lack of data is an obstacle to the full comprehension of Cordilleran tectonic history, for Yuma County is stra tegically located in or close to many important Cordilleran tectonic belts or provinces, both ancient and modem. The need for basic infor mation on the geology of the County was the motivation of the present study.
The purpose of the present study is to evaluate the outcrops in northern Yuma County (Figure 1) which are labeled on the Geologic Map of Arizona (Wilson et al., 1969) as "Mesozoic Sedimentary Rocks" (more briefly referred to here as Mesozoic Strata) with the aim of answering the following questions: 1) what types of lithologies occur in these outcrops; 2) what is the distribution of these lithologies; 3) how do
the outcrops of different ranges compare or contrast; 4) how have the
rocks of these outcrops been altered or metamorphosed; 5) what is the nature of the contact relations between Mesozoic Strata and adjacent
1 2
Figure 1. Location Map of Areas in Yuma County Mapped by Wilson (1960) and referred to on the Geologic Map of Arizona as "Mesozoic Sed imentary Rocks" BS = Buckskin Mountains; P = Plomosa Mountains; GW = Granite Wash Mountains; NDR = Northern Dome Rock Mountains; LH = Little Harquahala Mountains; NW = New Water Mountains; DR = Dome Rock Mountains; LHI = Livingston Hills; KO = Kofa Mountains; MI = Middle Mountains; CD = Castle Dome Mountains; CA = California. 3 lithologies; 6) how have the Mesozoic Strata been deformed; and 7) what geologic age constraints are available for the Mesozoic Strata?
The total area to be covered in the available time required that this study be of a reconnaissance nature. The present study is limited to the outcrops of Mesozoic Strata in the Dome Rock, Little
Harquahala, Granite Wash, Plomosa, and Buckskin Mountains. Of these, the outcrops in the Dome Rock Mountains were studied in the most detail.
The Mesozoic Strata proved to be much more complex than believed prior
to this study, and the observations made during the course of this
study were often surprising. The Mesozoic outcrops of the region
contain important clues to the unsolved mysteries of Cordilleran tec
tonics .
History of Geologic Study in Yuma County
Exploration of the Yuma County desert began in the second half
of the 19th century, when prospectors began to search for valuable
minerals in the region (Keith, 1978). Signs of these early explorers,
in the form of abandoned addits, rotting claimstakes, bulldozed tracks,
and piles of rusted cans, scar the desert. Geologic surveys, which
commenced after the turn of the century (Lee, 1908; Bancroft, 1911;
Jones, 1916a, b; Ross, 1922; Darton, 1925) were responsible for mapping
physiography, documenting mineral resources, and describing some of
the lithologic units of the region. Eldred Wilson, of the Arizona Bu
reau of Mines, systematically mapped southern Yuma County during the
late 1920*8 and early 1930’s, and published this work in 1933 (Wilson,
1933). Interestingly, when he conducted this study, some mountain 4 ranges in the county were still undiscovered (Figure 2). Wilson examined the northern part of the county in the 1950’s, and produced a geologic map of the whole county in 1960 (Wilson, 1960).
During the late 1960’s, F.K. Miller mapped the New Water Moun tains and the Livingston Hills (Miller, 1966, 1970; Miller and McKee,
1971). Completed theses are available concerning the geology of the,
Plomosa Mountains (Jemmett, 1966), the Granite Wash Mountains (Cian- canelli, 1965), the Harquahala Mountains (Varga, 1976), and the Living ston Hills (Harding, 1978).
Methodology and Terminology
The information presented in this paper is based on field and laboratory studies conducted in 1978. Walking traverses were made of the Mesozoic Strata outcrops in the Dome Rock, Little Harquahala,
Granite Wash, Plomosa, and Buckskin Mountains (Figure 1). In the first four ranges, the traverses were made approximately perpendicular to the strike of the contacts (bedding planes or foliation planes) between lithologic types. Annotated strip maps portray the geology of the tra versed areas. The geologic complexity of the study area in the Buck
skins did not permit a straight line traverse or strip map to be made. Critical areas off the traverse lines were also examined in the
ranges, and in the Dome Rock Mountains, geologic maps of critical con
tact areas between the Mesozoic Strata and boundary units were prepared.
Eighty unstained thin sections of representative lithologies were exam
ined with a petrographic microscope. 5
New Mountains i^ound in Arizona1
• - — -• - U. A. Geologists Are Be4 sponsible for Map - - . Changes" / - j r v. ———-*• ' : ; ► The discovery of & xew range ol _ xnotintalns in ' T n m a m tm .ty vvaj jannoTnc^d yesierday Oiroztfx 'th i • hurea-u of mines at the Unlvenilt*
j The discovery of^lhe-runge wtlciu h a s n o t b eea nam ed os "vet. ,r..aSe erversJ^aye ago hj ^3d--ed TTHson. geologist; of. 'the bureau iTaff. and ziis assistant. "Robert K. S. Belnenum. assistant geologist at ‘the unlversfty. The range was mot I shown on any map* cf that regtamv ■-acoordmg to Dr. (1/M. Datler, dean -of the 'mines ooliege. vrho an- 'nounced Wilson's discovery. y'j: ‘ ? Wilson ami Helneman are doing herearch work on the geology and minorai resources of tiie county-1 Their conclusions and findings will *ba published a t a.later date In bul letin form. .At the present there is no published data araUr-Me about I the geology and mineral resources I of parts of the county. Wilson and Helneman are still in ! the -field, and are expected to re- j turn from Yuma soon. Dean Butler 3 said- " . •--
Figure 2. Newspaper Report Concerning the Discovery of a New Mountain Range in Yuma County. — This clipping was found in the files of Eldred Wilson at the State of Arizona Bureau of Geology and Mineral Technology in Tucson. It was printed circa 1929, but the exact reference is unknown. 6
Because formation names have not been assigned to most of the outcrops discussed in this paper, it is necessary to refer to sequences by the sometimes awkward use of lithologic names. Lithologic names with accompanying descriptions are tabulated, and number key used in the tables is employed in accompanying stratigraphic columns and anno tated strip maps. The lithologic names given are field names, and when possible, they follow the classification scheme of Travis (1955). If
the protolith of a low-grade metamorphic rock is recognizable, the name used for the rock is that of the protolith prefixed by meta-, as suggested by Miyashiro (1973). CHAPTER 2
THE DOME ROCK MOUNTAINS
The Dome Rock Mountains are a 35 km long north-south trending range located between the town of Quartzsite and the Colorado River
(Figure 1). The highest peak in the range is Cunningham Mountain, which rises 3316 feet above sea level. Most of the 90 tan of Mesozoic
Strata in the Dome Rock Mountains crop out in the central and southern portions of the range. The small outcrops of Mesozoic Strata at the northern tip of the range are described in Chapter 5. On the Geologic
Map of Arizona, the Mesozoic Strata are shown to be in contact with
Mesozoic Schist, Mesozoic Gneiss, and Mesozoic Granite. An index to mapped areas and important contacts is presented as Figure 3.
Lithologies
Mesozoic Strata
Ten lithologies were distinguished in the Mesozoic Strata
traversed in this range (lithologies 1-10, Table 1). The protoliths
of these ten lithologies are sedimentary, but all ten have been mildly
metamorphosed, as manifested by: 1) formation of well—developed non
bedding plane foliation, 2) noticeable deformation of conglomerate
clasts, 3) transformation of shales into phyllites, 4) recrystalliza
tion of quartzites, and 5) mineralogic changes such as sericitization
of feldspars and growth of secondary chlorite. The choice of
7 8
LOCATION MAP OF MAPPED AREAS AND MAJOR CONTACTS DOME ROCK MOUNTAINS, ARIZONA EXPLANATION (£) Contoa bNhm&n Mesozoic sedimentary rock (to southeast) and Mesozotc Volcanic ferrane (to northw est) (J) Fault, offsets sedimentary-vote ante terrane contact (£) Contact between Mesozoic sedimentary terrane (to north) and Mesozoic metovolconic terrane (to south)
0 =
SOUTHERN CNO, OOUE ROCK NOUNTAINS
Figure 3. Location Map Showing Mapped Areas and Major Contacts Studied in the Done Rock Mountains. — Contact 1 is referred to in the text as the "Cunningham Mountain Fault." Contact 2 is referred to in the text as the "Copper Bottom Pass Fault." TABLE 1: LITHOLOGIES, DOME ROCK MOUNTAINS
# NAME DESCRIPTION
1 calcareous quartz light buff to green or maroon, weathering to dark brown or black . sandstone (desert varnish); grains are fine to medium sand sized, moderately well sorted, angular with high sphericity, composed of quartz (60- 90%), chert (20-30%), calcite (10-40%), feldspar (5-10%), clay (< 10%), with accessory zircon, magnetite, epidote, and biotite (altered to chlorite); locally there are pebbles of pink quartzite; cement is calcite; calcite which constitutes tip to 50% of the rock is partly detrital in origin; layers are 1 - 5 m thick, with bed ding plane partings at intervals of 0.1 - 1 m, foliation planes cross-cut bedding planes, layers are continuous on a scale of 0.5 km, but pinch and swell along strike; locally, dark, heavy minerals are concentrated in streaks, which may define large-scale cross beds, resistant ledge former.
2 quartzite vitreous, maroon to green or grey, weathering to dark brown or black; grains are fine sand sized or not visible, vitreous, well- sorted, angular, with ragged intergrown edges, composed of quartz (80-95%), chert (0-10%), feldspar (0-10%), with accessory mag netite and epidote and minor clay; layers are 2-10 m with bedding plane partings at intervals of 0.1 - 1 m, layers are continuous on the scale of 0.5 km, foliation planes cross-cut bedding planes; resistant ledge former. Alternative name, quartz arenite.
3 maroon phyllite lustrous maroon, locally green or grey, weathering to similar color with good phyllitic luster; grains are fine silt sized or not visible, composed of clay (30-60%) and quartz (40-70%); widespread abundant 0.1-0.5 mm grains of pyrite or limonite after pyrite, lo cally pyrite crystals exceed 10 cm in length, locally, abundant elongate 1-2 cm long lenses of chlorite; layers are 1-4 m thick 10 TABLE 1, cont.: Lithologies, Dome Rock Mountains
// NAME DESCRIPTION
with bedding plane partings at intervals of 1-10 cm, foliation planes cross-cut bedding planes; non-resistant trough former.
4 quartzite conglomerate maroon to green or grey, weathers to dark brown or black; large clasts/matrix = 80/20; large clasts are pebble and small cobble sized, composed of pink quartzite (80-100%) and limestone (10-20%), matrix is medium to coarse grained sand sized, composed predomi nantly of quartz and chert; occurs as restricted lenses within lithologies 1 and 2, and as massive layers up to 30 m thick, folia tion planes cross-cut bedding planes; resistant ledge former.
4' complex conglomerate grey, weathering to dark brown; large clasts are composed of quartzite, greywacke(?), phyllite, and possibly volcanic rock; matrix composed of phyllite or greywacke; occurs as restricted lenses or as beds. Alternative name, polycomponent conglomerate.
5 greywacke light bluish or greenish grey, weathering to black or brown (desert varnish), large grains/matrix 70/30; large grains are medium sand sized (0.05-0.5 mm), poorly sorted, angular composed of quartz (60-80%), chert (15-25%), feldspar (5-15%); matrix composed of sericite (70-90%), quartz (5-15%), chlorite, with accessory magnetite; layers are 0.5-5 m thick, bedding plane partings more closely spaced; both resistant ledge former and non- resistant trough former; pebbly; lithology is highly variable; ma trix in many samples is probably derived from feldspar and devi- trified glass, and some matrix may actually be aphanitic lithic fragments, if so, greywacke field name is inappropriate, and a preferable name is tuffaceous litho-feldspathic quartz arenite. TABLE 1, cont.: Lithologies, Dome Rock Mountains
# NAME DESCRIPTION
6 granule conglomerate very light greenish grey to white, with phyllitic lustre, weathering to light tan; grains/matrix 80/20; grains are coarse sand to granule sized (0.5-3 mm), poorly sorted, angular to subangular, composed of quartz (50-70%), chert (10-20%, and phyllite (20-30%), embedded in a matrix of phyllite (sericite-clay(?)) and chert; matrix is possibly tuffaceous; cement is composed of silica and, locally, calcite; layers are 2-20 m thick with bedding plane partings at 0.5-2 cm; foliation planes not parallel to bedding, forms rubbly non-resistant outcrops; widely interbedded with phyllite.
7 grey phyllite light to dark grey, weathering to same color with good phyllitic lustre; grains are fine silt-sized or not visible, composed of clay, sericite, quartz, and corbonaceous material(?), possibly tuffaceous; layers 2-10 m, bedding plane parting is very close, locally has pencil cleavage; very soft, erodes into shimmery troughs.
8 igneous clast conglomerate dark greenish grey, weathering to dark brown or black (desert var nish); large clasts/matrix = 40/60, large clasts pebble to cobble sized, well rounded, composed of medium grained quartz monzoriite (feldspar 50%, quartz 40%, muscovite 4%, biotite 3%, chlorite 3%) (30-40%), quartzite (40-50%), sandstone or volcanic (?) (5-10%), phyllite (5-10%); matrix coarse sand sized, composition similar to lithology 9; layers are 2-10 m thick, bedding parting is poorly developed; moderately resistant ledge former.
9 feldspathic greywacke light greenish grey, weathering to dark brown or black (desert H varnish); grains/matrix ■ 60/40; large grains are medium to very H TABLE 1, cont.: Lithologies, Dome Rock Mountains
0 NAME DESCRIPTION
coarse sand sized (0.02-1 mm), poorly sorted, angular, composed of quartz (30-40%), chert (20-30%), feldspar (30-40%); matrix composed of sericite (80%), quartz (10%), chlorite (10%); possibly tuffaceous; layers are 0.5-2 m thick, bedding plane parting more closely spaced; rubbly moderately resistant ledges.
10 siltatone greywaake dark greyish tan weathering to black or brown (desert varnish); com posed of silt sized grains with aphanitic matrix; grains/matrix = 80/20; grains composed of quartz (50-70%), feldspar (30-50%); matrix composed of sericite (70%), chlorite (20%), and quartz (10%); pos sibly tuffaceous; beds 0.5-3 m, layers 1-3 m thick, bedding plane parting is closely spaced; crops out as -very tough resistant ledges.
11 meta-basalt black to dark maroon; aphanitic, locally with 0.5-2 cm long amyg- dules filled with chert; composed of outcrops in thick layers, parts on irregular foliation surfaces spaced at 1-2 cm intervals.
12 meta-crystalline tuff very light greenish-grey to white with phyllitic luster, weathering to similar color with dark brown and green crystals standing out of the weathered surfaces; large crystals/matrix = 20/80; large grains are coarse sand sized, angular, composed of quartz (60-70%), feld spar (30-40%); matrix is aphanitic, composed of chert (60-70%), sericite (30-40%), chlorite (5-10%); locally there are brown pumice(?) fragments; beds are defined by contrast in grain coarseness, beds 0.5-3 m(?) thick. H 13 meta-andeaite(?) light to dark olive green, weathering to similar color or dark black TABLE 1, cont.: Lithologies, Dome Rock Mountains
if NAME DESCRIPTION
or brown; porphyritic aphanitic (phenocrysts account for 10-20%); phenocrysts composed of sericitized feldspar, matrix composed of chlorite (10-60%), sericitized feldspar (30-50%), quartz (10-30%), calcite (0-20%), magnetite (5-15%); original texture largely altered; occurs in layers 0.5-2 m thick with blocky splitting; moderately resistant ledge former.
14 light tan meta-volcanic light pinkish tan, weathering to dark brown or black (desert var- wadke nish); grains/matrix = 60/40, grains are coarse sand sized (0.2-1 mm), composed of angular (sub-euhedral) feldspar (predominantly plagioclase) (50-60%), quartz (40-50%); matrix is aphanitic, com posed of sericite (60-70%), quartz (20-30%), chlorite (10-20%); locally there are pebbles and cobbles of hypabyssal intrusive rock; bedding not apparent; resistant cliff former.
15 dxxvk. grey meta-volcanic dark greenish grey, weathering to dark brown or black (desert vxzcke varnish) with prominent white grains; grains/matrix = 60/40; grains are 0.5-3 mm (locally up to 1.5 cm), angular, composed of feldspar (mainly plagioclase) (60-80%) (feldspar has well- developed cleavage, is locally euhedral, and widely sericitized), quartz (15-25%); matrix composed of fine-grained sericite (60- 80%), quartz (10-30%), chlorite (10-20%); bedding not apparent, outcrops are moderately resistant; actinocelia fossil associated with this unit.
my Ionite very light maroonish grey, weathers to similar color with phyl litic luster; aphanitic, locally with quartz eyes (0.02 mm in diameter); composed of sericite (50-60%), fractured quartz TABLE 1, cont.: Lithologies, Dome Rock Mountains
# NAME DESCRIPTION
and feldspar grains (40-50%), clear quartz eyes (0-10%); has dis tinctive texture— foliation defined by anastimosing bands of seri- cite (with parallel extinction) which surround the fractured quartz and feldspar grains; locally there are strain shadows adjacent to the broken grains; there is some evidence of fluxion structure; lithology occurs as a 1-10 m thick layer at the boundary between the sedimentary and meta-volcanic terranes; validity of term myIonite is questionable, alternative is sheared rock.
17 quartz monzonite dark greenish grey and pink, weathering to dark brown and black; medium-grained equigranular; composed of feldspar (mostly plagio- clase) (50-60%), quartz (30-40%), chlorite (10-20%).
18 aplite light tan, weathering to dark black or brown; fine-grained, equi granular, sugary appearance; composed of quartz (60-70%), alkali feldspar (30-40%), biotite (0-10%); occurs as narrow dikes cutting the quartz monzonite (lithology 17). 15 metamorphic or sedimentary rock field names was based on qualitative judgment of the metamorphic appearance.
Throughout the outcrop area of Mesozoic Strata, stratification
is characterized by the regular interlayering of resistant and non-
resistant lithologies. The layers, which are between 1 and 20 m thick,
dip to the southeast, so selective erosion gives ridge crests a serrate
appearance (Figure 4). The section of Mesozoic Strata which is assumed
to be upright and which, as measured on this traverse, is approximately
5000 m thick, can be divided into seven units. Descriptions and a
schematic stratigraphic column of these units, as well as a map showing
their location on the ground, are presented as Figure 5 (in pocket).
There are no distinctive marker horizons in this section, so units
were recognized by their component group of lithologies. A chart
showing the range in the section of individual lithologies is presented
as Figure 6. This chart also shows an estimate of the relative propor
tion of feldspar in the lithologies.
The base of the section is Unit A. In the lower part of this
unit, the resistant rock layers are composed predominantly of calcareous
quartz sandstone (lithology 1)(Figure 7), which locally has streaks of
dark or heavy minerals (e.g., magnetite, apatite, and zircon). Other
resistant layers are composed of quartzite (lithology 2) and quartzite
clast conglomerate (lithology 4). The conglomerate is composed almost
entirely of grey or pink quartzite clasts with minor proportions of
limestone clasts (Figure 8). In the upper part of Unit A, a higher
proportion of the resistant layers are composed of quartzite. 16
Figure 4. Distance Photograph of the Central Dome Rock Moun tains, Showing Outcrops of Mesozoic Strata 17
turn ‘5600
8 - 4 0 0 8
F -3000
2688
•1888
4— 4— I— h 4-t •6 n ' I® t • 7 • S 41 4 3 2 1 H M L L i t h l i | y i i . prep. fild sp ir
Figure 6. Schematic Chart Showing the Range of Individual Lithologies in the Section of Mesozoic Strata of the Dome Rock Moun tains. — Numbers refer to lithologies of Table 1; letters refer to units described in Figure 5. H = high; M - medium; L * low. Scale in meters indicated at the right of the chart. 18
Figure 7. Photomicrograph of Calcareous Quartz Sandstone From the Dome Rock Mountains Figure 8. Photograph of Conglomerate Outcrop From Unit A of the Dome Rock Mountains . — Hammer point rests next to two lime stone clasts. 20
Throughout Unit A, the non-resistant layers are composed of maroon phyllite (lithology 3), much of which is speckled with grains of pyrite or limonite after pyrite. Elongate green reduction spots occur widely in the phyllite. Unit A is distinguished by the lack of feldspathic lithologies (Figure 6) and by the abundance of limonite-speckled phyllite.
The resistant layers of Unit B (Figure 5) are composed of
(tuffaceous?) greywacke (lithology 5)(Figure 9) and complex (polycom ponent) conglomerate (lithology 4'), with minor amounts of calcareous quartz sandstone. The non-resistant layers are composed of phyllite
(lithology 3). Unit B is distinguished from Unit A by the presence of greywacke layers, and by the presence of complex conglomerate.
The resistant layers of Unit C are composed of greywacke
(lithology 5), interlayered with thick, moderately resistant layers of granule conglomerate (lithology 6)(Figure 10). Non-resistant layers of Unit B are composed of maroon phyllite (lithology 3) and grey phyl
lite (lithology 7). The occurrence of granule conglomerate and grey phyllite distinguish Unit C from Unit B.
The resistant layers of Unit D are composed of greywacke
(lithology 5) and granule conglomerate (lithology 6). The non-
resistant layers are composed of carbonaceous grey phyllite (lithology
7). The lack of maroon phyllite distinguishes Unit D from the units
beneath it. Unit E is similar to Unit D, but has a higher proportion
of grey phyllite and almost no granule conglomerate. It is character
ized by very evenly bedded outcrops. 21
Figure 9. Photomicrograph of Greywacke With Possible Tuffa- ceous Component 22
Figure 10. Photograph of Granule Conglomerate Outcrop in the Dome Rock Mountains. — Note the phyllite layer below the hammer and the clasts of phyllite next to the hammer head. 23
Unit F is composed of igneous clast conglomerate (lithology 8) interbedded with feldspathic greywacke (lithology 9)(Figure 11) and grey phyllite (lithology 7). Unit F is markedly different from lower units because granitic clasts appear in its conglomerate, and because the proportion of feldspar in greywacke layers is higher relative to the units below. Both greywackes (lithologies 5 and 9) of Units B through E and F through G are feldspathic, but the proportion of feldspar is higher in the greywackes of Units F through G (Figure 6).
The resistant layers of Unit G are composed of siltstone grey wacke (lithology 10). The less resistant layers are composed of feld spathic greywacke with minor proportions of igneous clast conglomerate.
Unit G is distinguished from lower units by the presence of the dis tinctive very hard siltstone greywacke.
There are two important lithological transitions in the Dome
Rock Mountains section. The first is at the top of Unit A, where lithologies begin to record the influx of feldspathic and possibly tuffaceous material, and conglomerates indicate a more complex source terrane. The second transition is at the top of Unit E, where another increase in the proportion of feldspar occurs, and granitic clasts appear in conglomerate layers.
Bounding Terranes
The Geologic Map of Arizona indicates that the Mesozoic Strata of the Dome Rock Mountains contact terranes of Mesozoic Gneiss and
Mesozoic Schist (Figure 1). In the Copper Bottom Pass area and at
the southern end of the range, the two areas examined in the present 24
Figure 11. Photomicrograph of the Feldspathic Greywacke of the Dome Rock Mountains 25
study, the metamorphic grade of these two terranes is so low (lowermost
greenschist facies) that protoliths are readily recognizable. Thus,
the field names assigned to lithologies in the bounding terranes (Table
1) are protolith names prefixed by meta-. This terminology is in
general agreement with that proposed by Crowell, who mapped an extensive
portion of the central Dome Rock Mountains in 1975 (unpublished map).
Crowell did not distinguish any volcanoclastic units.
Eight lithologies were distinguished in the portions of the
bounding terranes mapped in the present study (lithologies 11-18,
Table 1). These terranes have a rugged, generally non-stratified out
crop appearance (Figure 12). In the La Cholla Mountain area (Figure
13), the Mesozoic Strata are underlain by meta-basalt (lithology 11).
The dark color of the meta-basalt contrasts strikingly with the light
color of the overlying Mesozoic Strata to the east (Figure 13).
Closely spaced foliation planes give this aphanitic lithology a slate
like appearance, but the presence of abundant stretched amygdules
throughout the rock confirms that its protolith was volcanic.
Figure 14 (in pocket) portrays the geology of the Copper Bottom
Pass area. Unit V-1T of this map is composed of meta-crystalline tuff
(lithology 12)(Figure 15), locally interlayered with meta-andesite(?)
(lithology 13). Where the interbeds of meta-andesite(?) occur, the
stratification of the unit is well defined. Elsewhere, stratification
is more cryptic, and must be discerned on the basis of the contrast
between coarser and finer layers of the tuff. Locally, the tuff is so
. coarse (Figure 16) that an alternative name could be meta-volcanic 26
Figure 12. Distance Photograph of the Meta-Volcanic Terrane of the Dome Rock Mountains . — Taken looking northwest from the top of Cunningham Mountain. 27
Figure 13. Photograph and Tracing Showing Contact of Mesozoic Strata With Meta-Basalt on La Cholla Mountain. — Mmv = Meta basalt; Ms = Mesozoic Strata; barbed line = Cunningham Mountain Fault. 28
Figure 15. Photomicrograph of Meta-Crystalline Tuff From the Dome Rock Mountains 29
Figure 16. Photograph of Coarse Meta-Crystalline Tuff Out crop in the Copper Bottom Pass Area of the Dome Rock Mountains 30 breccia or agglomerate. The meta-crystalline tuff is a distinctive lithology; the high sericite content makes its outcrops very shiny and poorly resistant to erosion.
Structurally overlying Unit V-1T is Unit V-1W, which is com posed of highly variable, fairly resistant light tan meta-volcanic wacke (lithology 14). This lithology locally includes cobble-sized fragments of andesite or dacite porphyry.
Unit V-l* in the map area of Figure 14 is composed exclusively of dark grey meta-volcanic wacke (lithology 15). Except for local variations in grain size. Unit V-l* is monotonously uniform through its mapped extent (Figure 17). The dark grey meta-volcanic wacke has well developed foliation with no visible stratification, but it is not a compositionally banded gneiss as indicated on the Geologic Map of
Arizona. Unit V-l’ is quite distinct from Unit V-1W, not only in color, but also by the fact that it is not associated with any volcanic flows or tuffs. As will be discussed later in this paper. Unit V-l* may be of a different age than the meta-volcanic terrane to the north.
The contact between Unit A and the underlying terranes in
the central Dome Rock Mountains is marked almost everywhere by a
distinctive 1 to 10 m thick layer of light grey, shiny mylonite
(lithology 10, Table 1; Unit M on Figure 14). This lithology is
composed primarily of sericite grains which are aligned in undulating,
anastomosing bands (Figure 18). The bands undergo simultaneous
extinction under crossed nicols. Grains of quartz, surrounded by
sericite, are fractured, irregular, and sometimes bordered on one 31
Figure 17. Photomicrograph of Dark Grey Meta-Volcanic Wacke of Cunningham Mountain, in the Dome Rock Mountains 32
Figure 18. (A) Photomicrograph of Mylonite Derived From Meta- Crystalline Tuff (B) Photomicrograph of Mylonite Derived From Dark Grey Meta—Volcanic Wacke. — Both samples are from the Copper Bottom Pass area of the Dome Rock Mountains. 33 side by strain shadows. The myIonite grades into lithologies beneath it by gradual coarsening.
The Mesozoic Strata at the southern end of the Dome Rock Moun
tains contact a terrane composed of interlayered meta-basalt and meta andesite (lithologies 11 and 13, Table 1; Unit V-2 on Figure 19 in pocket). These meta-volcanic rocks, which are quite resistant to
erosion, have a well-stratified appearance. The contact between the meta-volcanic terrane and the Mesozoic Strata is about 2.5 km north
of the location indicated on the Geologic Map of Arizona. The Laramide
Intrusive which is indicated as cutting the Mesozoic Strata on the
earlier map is actually a layer of meta-crystalline tuff (lithology 12) which is interlayered with the volcanic flows. The light color of
this rock makes it stand out in stark contrast to the dark meta-basalt
and meta-andesite(?) flows (Figure 20).
In contact with Unit V-2 at the junction between the North
Trigo Peaks and the Dome Rock Mountains are outcrops of quartz mon-
zonite (lithology 17; Unit AG on Figure 19) cut by narrow dikes of
aplite (lithology 18). Overlying the meta-volcanic terrane in this
area is a thick sequence of silicic to intermediate well-stratified
mid-Tertiary volcanic flows and tuffs, and volcanoclastic sandstones
and conglomerates. The color of these rocks varies from cream to
bright red.
Contact Relations
The northern contact between the Mesozoic Strata and the adja
cent terrenes (Figures 3 and 14) is a fault, henceforth named the 34
Figure 20. Distance Photograph of Meta-Crystalline Tuff Outcrop at the Southern End of the Dome Rock Mountains 35
"Cunningham Mountain Fault." Its location near the Copper Bottom Mine is about 1 km east of the location indicated on the Geologic Map of
Arizona. Elsewhere, its location is similar to that indicated for the contact between the Mesozoic Strata and adjacent metamorphic units on the Geologic Map of Arizona.
This fault juxtaposes rocks of quite contrasting appearance
(Figures 13 and 21) and generally occurs in topographic shelves or
saddles. It dips about 30° to the southeast, and is generally sub parallel to the bedding attitude of the overlying Mesozoic Strata.
However, on the two small knolls 0.3 km west of the Copper Bottom
Mine (Figure 14), the fault cross-cuts the bedding of Unit A.
The composition of the footwall is not constant along strike.
On La Cholla Mountain (Figure 17), the fault juxtaposes Unit A (above)
with meta-basalt (below). North of the Copper Bottom Pass Fault, in
the map area of Figure 14, the Cunningham Mountain Fault juxtaposes
Unit A with meta-crystalline tuff, and south of the Copper Bottom Pass
Fault it juxtaposes Unit A with dark grey meta-volcanic wackes. How
ever, the structural features of the contact are fairly constant along
strike, regardless of the composition of the underlying rock. Every
where, the contact is marked by a band of myIonite which ranges in
thickness from 1 to 5 m. In good exposures, the contact between the
mylonite and the overlying calcareous sandstone of Unit A is sharp
(Figure 22). It is not clear if the mylonite zone involves phyllite
below the calcareous sandstone, but the mylonite clearly grades into
the underlying lithologies. The foliation of the underlying terrenes 36
Figure 21. Photograph and Tracing of Dark Grey Meta-Volcanic Wacke in Contact With Mesozoic Strata. — Taken looking south from Cunningham Mountain. Ms = Mesozoic Strata; mv = dark grey meta- volcanic wacke; barbed line = Cunningham Mountain Fault. 37
Figure 22. Photograph of Calcareous Quartz Sandstone - Mylonite Contact. — Taken in the Copper Bottom Pass area of the Dome Rock Mountains. 38 is disrupted by the fault; foliation planes are bent toward the contact
(Figure 23). The axis of the flexure indicated in Figure 23 (the pole
P in the inset diagram) trends northeast. Locally, within the mylonite band, there are small kink folds with hinges bearing northeast, and for much of its length, the mylonite band is injected with thick veins of milky white quartz. These veins have been prospected, so a series of addits follows the trace of the fault.
The contact between the Mesozoic Strata and the meta-volcanic
terrane at the southern end of the Dome Rock Mountains (Figure 19) is,
as stated earlier, 2.5 km north of the position indicated on the
Geologic Map of Arizona, and as a consequence, is much longer than
previously shown. The contact is not well exposed, but three small
exploratory shafts in the contact area expose hematite-stained breccia,
so it is here concluded that the contact is a fault. The Laramide
Intrusive indicated on the Geologic Map of Arizona, as stated earlier,
is a meta-crystalline tuff layer within the meta-volcanic terrane.
Overlying the meta-volcanic terrane in the map area of Figure
19 is a thick section of unmetamorphosed mid-Tertiary volcanic and
clastic rocks. The contact of these rocks with the meta-volcanic
terrane is an erosiohal unconformity.
Contact relations between adjacent lithologic layers or between
units within the section of Mesozoic Strata are unclear. Bedding
plane structures cannot readily be seen, because rock layers break
preferentially on foliation planes, and there are no marker beds.
Outcrop patterns suggest the presence of unconformities or fault 39
Figure 23. Map Showing Bending of Foliation of the Meta- Volcanic Terrane Into Parallelism With the Cunningham Mountain Fault . Contour interval is 40'. Msa = Mesozoic Strata; Mmi = meta- volcanic terrane. Inset shows equal angle projection of the poles to the foliation planes. P = pole to the great circle containing these poles. 40 contacts between Units A and B, and between Units F and G. The change in thickness of layers along strike is perhaps indicative of local unconformities or diastems.
Structural Features
Faults
The Cunningham Mountain Fault, which marks the northern bound ary of the Mesozoic Strata, was discussed earlier in this chapter.
Another fault cuts northwest across the Dome Rock Mountains near
Copper Bottom Pass (Figures 3 and 14). It has been referred to in
this paper as the Copper Bottom Pass Fault, and its trace is marked
by a deep erosional valley through which the El Paso Natural Gas
Pipeline runs. Where the fault zone crops out, in a wash at the base
of Cunningham Mountain, it appears as a zone of intense shearing in
which the foliation of the rocks involved has been rotated into par
allelism with the fault.
The Copper Bottom Pass Fault offsets the Cunningham Mountain
Fault by about 1 km in a left-lateral sense, indicating that it has
a separation of at least 1 km. This movement, however, cannot account
for the juxtaposition of the dark grey meta-volcanic wacke (Unit V-l’)
to the south with the meta-volcanic rocks (Units V1T and V1W) to the
north. This fact suggests that the Copper Bottom Pass Fault must
have been active both prior to and after the motion of the Cunningham
Mountain Fault. 41
Folds
Two intrafolial folds occur at the Copper Bottom Mine (Figures
14 and 24). The mine addits penetrate the hinge areas of the folds, where there are large veins of copper-stained white coarse-grained quartz. The folds involve, principally, three resistant layers of calcareous-cemented quartz sandstone, and have hinges oriented approx imately 35°, N60-80°E.
Isoclinal folding within Mesozoic Strata is suggested by the monotonous repetition of lithologies and by the occurrence of shearing and phyllitization within the section. But though outcrop patterns locally suggest the shape of isoclinal folds, no credible hinges were found, and the existence of such folds remains speculative.
Kink folds, as mentioned earlier, occur within the myIonite layer. They also occur locally within the meta-volcanic terrane of the Copper Bottom Pass area, where they have hinge attitudes of 10-
15°, N75°E.
Other Structural Features
Through much of the Dome Bock Mountains, the Mesozoic Strata dip homoclinally to the southeast. Dips appear to steepen at higher elevations in the range, and are steeper and more variable in the southern part of the range. Bedding attitudes of the meta-volcanic terrane, where measured in the Copper Bottom Pass area, are subparallel
to those of the Mesozoic Strata.
Penetrative metamorphic foliation is well developed in the
Dome Rock Mountains. In the central portion of the range, foliation 42
Figure 24. Photograph and Tracing of Intrafolial Folds at the Copper Bottom Mine, Dome Rock Mountains . — Ms = Mesozoic Strata; Mmv = meta-volcanic terrane; barbed line = Cunningham Mountain Fault. 43 planes dip 30-35° to the north, except where refracted at lithologic boundaries, and are thus at a high angle to bedding planes (Figure 25).
At the southern end of the range, foliation planes dip southward. The significance of this change is not yet understood, but it appears that, overall, rocks of the southern end of the range are more highly de
formed than those of the central part of the range.
The bedding— foliation relation of rocks in the La Cholla
Mountain— Copper Bottom Pass areas is displayed on the equal area projections of Figure 26. As indicated by this figure, foliation planes have similar attitudes in both the Mesozoic Strata and in the
underlying meta-volcanic terrane.
A penetrative set of conjugate joints (Figure 27) cuts the
meta-volcanic terrane of the Copper Bottom Pass area. One set is
oriented N80-90°E, 30-40°N; the other set is near horizontal. The
angle between the two joint sets is bisected by the average pole to
foliation planes. A summary diagram of joint attitudes, lineations,
and kink fold axes is presented as Figure 28.
Veins of milky white quartz cut the Mesozoic Strata and the
meta-volcanic terrane in several localities. These veins appear to
be concentrated in areas where other forms of deformation, such as
faulting or intrafolial folding, are also evident. The orientation
pattern of these veins, if any, was not studied, except at one loca
tion in the meta-volcanic terrane of Copper Bottom Pass. Here, the
quartz filled large vertical tension gashes oriented N55-60°E, within
an envelope oriented N85°W, 85°N. 44
Figure 25. Photograph Showing High Angle Between Bedding and Foliation Planes in a Layer of Calcareous Quartz Sandstone on La Cholla Mountain N 45 A
Pole to Foliation Plane Pole to Bedding Plane
.*♦ •
Figure 26. Equal Angle Lower Hemisphere Projections of Poles to Bedding and Foliation Planes in (A) the Mesozoic Strata and (B) the Meta-Volcanic Terrane. >— Data obtained in the Copper Bottom Pass area. Figure 27. Photograph of Conjugate Joint Set in the Meta- Volcanic Terrane of the Copper Bottom Pass Area . — Note bat hanging from cactus for scale. 47
• Lineation o Kink Fold Axis
□ Pole to Fracture
O Average Pole to Foliation Planes N
Figure 28. Equal Angle Lower Hemisphere Projection of Linea- tions, Kink-Fold Axes, and Poles to Fractures, Measured in the Meta- Volcanic Terrane of the Copper Bottom Pass Area. 48
Discussion
Stratigraphy
The Mesozoic Strata which crop out in the central and southern
Dome Rock Mountains and which are composed of mildly metamophosed sedi mentary rocks are very thick. Unless repeated by thrust faults or iso
clinal folds, the section is about 5 km thick.
The nature of the depositional environment in which the sedi ments comprising this section were deposited must be inferred from
textural features of the rocks, for bedding surface features are obscure
and no environmental index fossils were found. Textural features such
as the abundance of conglomerate, the current stratification of sand- •
stone, the maroon hue of the phyHites, and the abundance of pyrite in
the phyllites suggest that the sediments forming the rocks of Unit A
were deposited in a continental environment or a nearshore environment
(Blatt, Middleton, and Murray, 1972). The textural features of Units
B-G are less diagnostic. The abundance of poorly sorted greywacke
suggests that submarine debris flows must also be considered as a
possible depositional mechanism.
Contrasts in gross mineralogy among the different units in the
section (Figure 6) are indicative of changes in the composition of the
source terrane that occurred during deposition. The coarser lithologies
of Unit A are composed of quartz, chert, and calcite, with only minor
amounts of feldspar, and the conglomerate layers are composed primarily
of quartzite clasts, with minor proportions of limestone clasts. This
mineralogy suggests that Unit A is composed of sedimentary rocks 49 derived from a source composed primarily of quartzite, limestone, and quartz-rich sandstone. Paleozoic strata of western Arizona contain such lithologies (Varga, 1977; Miller, 1970), so it is a reasonable suggestion that the detritus which composes the rocks of Unit A was derived from Paleozoic sedimentary rocks. The feldspar component of the rocks could have been derived from pre-Paleozoic basement rocks, or from volcanic rocks.
The increase in the feldspar proportion of the Mesozoic Strata and the presence of possible tuffaceous material in units above the
Unit A - Unit B boundary (Figure 6) implies that a different source terrane was supplying detritus to the depositional basin of Yuma
County while these units were deposited. This terrane must have in
cluded crystalline and volcanic rocks. Above the Unit E - Unit F boundary, cobbles of granite or quartz monzonite appear, and there is
an increase in the feldspar proportion in the detritus. The presence
of such components requires that the source terrane include plutonic
rocks.
The meta-volcanic terrane which contacts the Mesozoic Strata is
composed of igneous rocks that are characteristic of marginal volcanic
arcs formed as a consequence of the subduction of oceanic lithosphere
beneath a continent (Dickinson, 1970). The meta-volcanic lithologies
of the Dome Rock Mountains thus probably represent remnants of a vol
canic arc.
The dark grey meta-volcanic wacke of Cunningham Mountain (Unit
V-l*) is composed of detritus derived from a volcanic terrane. But, 50 as no volcanic flows were found intercalated with this wacke, it is not clear whether or not this rock was deposited within a volcanic arc terrane.
Structure
Widespread penetrative foliation in low-grade metamorphic rocks is often considered to be an axial plane cleavage related to regional folding (Hobbs, Means, and Williams, 1976). It is possible to calcu late the axis attitude of the regional fold from orientation data on the foliation and the bedding planes of one limb. Poles to bedding and foliation planes in the Copper Bottom Pass area are presented in Figure
26. The pole to the great circle containing these poles is the presumed fold axis, and in this case is oriented approximately 20*, N70*E. A - working hypothesis made here is that the central Dome Rock Mountains are part of the lower limb of a large, south-facing recumbent regional syncline with an axis oriented 20*, N70*E. Foliation usually forms perpendicular to the axis of maximum finite shortening (Hobbs et al.,
1976). Thus, the inferred direction of maximum finite shortening for this regional fold is NNW-SSE. Whether this fold affects rocks at the southern end of the range is not clear from available data.
The hinges of the intrafolial folds at the Copper Bottom Mine are oriented 35*, N60-80*E, fairly close to the attitude of the pro posed major fold axis, suggesting that they may be parasitic folds on a limb of the major fold. Their vergence is in the correct sense.
The hinge attitudes of kink folds in the meta-volcanic terranes, 10-
15*, N75“E, are also fairly close to the proposed axis of the major 51 fold, suggesting that these folds formed by intra-limb shearing. By possible analogy, Hamilton (1964) maps a major south-facing recumbent syncline in the Big Maria Mountains, 15 km to the northwest of the
Dome Rock Mountains. The strike of the axis of this fold is similar to that of the fold that is here proposed for the Dome Rock Mountains.
The nature of the Cunningham Mountain Fault is enigmatic.
Attitudes of lineations, flexures, and kink folds in the myIonite band that defines the fault suggest that the motion of the hanging wall rocks was toward the southeast with respect to the footwall rocks.
There are three possible interpretations of this fault: 1) it is a sheared unconformity; 2) it is a low-angle thrust fault which tilted to its present orientation after activity (the dip of mid-Tertiary strata at the southern end of the range suggests that the whole range may have tilted by as much as 40® to the east); or 3) it is a normal fault that was active in its present attitude. A combination of the above interpretations is also possible. A strong case cannot be argued for any of these hypotheses with available data. The orienta tion of motion suggests that activity of the fault might have been a late stage manifestation of the deformational event which formed the
foliation of the central Dome Rock Mountains. The fact that the
foliation of units adjacent to the fault is only locally affected by
the fault suggests that movement on the fault has not been large.
Miller and McKee (1971) discuss low-angle faulting that occurred in the
Livingston Hills, 10 km to the east. 52
The NW-trending Copper Bottom Pass Fault, which cuts completely
across the range, probably was active both prior to and after the move ment on the Cunningham Mountain Fault. Separation is left-lateral,
though the exact sense of slip is not known. Miller (1970) and Miller
and McKee (1971) mapped several NW-trending strike-slip faults in the
New Water Mountains and the Livingston Hills. It is possible that
the Copper Bottom Pass Fault is part of the same set.
Age Constraints
The meta-volcanic rocks of the Dome Rock Mountains have been
radiometrically dated as "Late Triassic or Early Jurassic" (L.T.
Silver, pers. commun., referenced in Miller, 1970), suggesting that
these rocks formed as part of the early to mid-Mesozoic volcanic arc
of North America. This arc has been documented in adjacent areas of
southeastern Arizona, northern Sonora, and throughout California
(see Davis, Monger, and Burchfiel, 1978; Schweickert, 1978; Rangin,
1978; Cooper, 1971), where it has been shown to have been active
until mid-Jurassic time.
The Mesozoic age assignment for the sedimentary rocks of the
Dome Rock Mountains is reasonable, and was probably originally assigned
because of the dissimilarity between these rocks and documented
Paleozoic and Cenozoic rocks. For descriptions of Paleozoic and
Cenozoic strata, see Miller (1970), Varga (1977), and Eberly and
Stanley (1978). The fact that certain units within the Mesozoic
section of the Dome Rock Mountains contain clasts that could have been 53 derived from Paleozoic units suggests that the section is post-
Paleozoic. As yet, however, no fossils have been found which could serve to constrain the age range more tightly. If the Cunningham
Mountain Fault has had little movement, then it is possible that the
Mesozoic Strata stratigraphically overlie the meta-volcanic terrane and are thus post-mid-Jurassic.
The dark grey meta-volcanic wacke of Cunningham Mountain
(Unit V-l*) has been considered separately from the meta-volcanic terrane in this report, although it was probably derived from a volcanic terrane, because it is not associated with volcanic flows, and because of the possibility that it is of a different age. This possibility is based on the discovery (by B. Robison and A. French) of the Permian guide fossil Actinoeetia in a cherty layer of a sub- angular boulder found at the divide in the range. However, no fossils have yet been found in place in the wacke, thereby supporting the proposal that the Actinoaetia represents the age of the unit. CHAPTER 3
THE LITTLE HARQUAHALA MOUNTAINS
The Little Harquahala Mountains are a small range south of 9 the town of Salome (Figures 1 and 29). Approximately 8 km of
Mesozoic Strata crop out in the Little Harquahala Mountains, according to the Geologic Map of Yuma .County, Arizona (Wilson, 1960). The map also indicates that the Mesozoic Strata contact units labeled "Meso zoic Schist" and "Quaternary Basalt."
Lithologies
Mesozoic Strata
The rocks within the area mapped as Mesozoic Strata are rela tively unmetamorphosed (Table 2). Primary sedimentary structures such as ripple marks, graded beds, cross-beds, mudcracks, and raindrop impressions are visible. The locations of lithologic types found within the terrane of Mesozoic Strata and adjacent terranes are represented on the schematic geologic strip map (Figure 30).
In the southwest c o m e r of the map area (Figure 30), there are outcrops composed of interbedded maroon quartzite (lithology 1, Table
2), maroon silty shale (lithology 2), and limestone (lithology 3). The total thickness of the sequence is at least 300 m. To the northeast, this sequence contacts an equally thick sequence composed predominantly of maroon conglomerate (lithology 4) with minor interbeds of sandstone
54 55
Figure 29. Topographic Map of the Little Harquahala Mountains — Location of Figure 30 is indicated by the heavy line. Source: Hope, Arizona 7.5’ U.S. Geological Survey Topographic Quad rangle Map. TABLE 2: LITHOLOGIES, LITTLE HARQUAHALA MOUNTAINS
// NAME DESCRIPTION
1 maroon quartzite maroon to green weathering to dark black or brown (desert varnish); grains are medium sand sized (0.3 mm), well sorted, composed of sub- angular elongate quartz (60-70%), chert (10-20%), feldspar (10-20%), calcite ( 10%), hematite ( 10%), with minor epidote, muscovite, biotite(?), and clay; firmly cemented with predominantly silica ce ment; beds are 0.5-1 m thick, with blocky splitting; resistant ledge former. Lithology is locally conglomeratic with distinctive pink quartzite pebbles and cobbles.
2 maroon eitty shale maroon to green, weathering dark brown or black (desert varnish); grains are silt sized, moderately well sorted, well rounded, composed of quartz and chert (40-50%?), clay (40-50%), with minor feldspar and muscovite; firmly cemented with predominantly silica cement; beds are 0.5-1 m thick, splitting on irregular closely spaced (10-20 cm) surfaces; non-resistant trough former.
3 limestone grey, weathering to similar color; composed of recrystallized car bonate; occurs in beds 0.5-1 m thick; moderately resistant ledge former with very rough weathered surface.
4 maroon conglomerate maroon with pink clasts weathering to dark black or brown (desert varnish); poorly sorted; clasts/matrix = 80/20; large clasts are cobble to small boulder sized, composed of quartzite; matrix is coarse sand sized, composed of quartz and chert; occurs as massive, beds 1-3 m thick; resistant ledge former; locally includes interbeds of greenish sandstone, siltstone, and shale. TABLE 2, cont.
# NAME DESCRIPTION
5 green-grey conglomerate •light greenish grey, weathering to dark brown or black (desert var nish) ; poorly sorted; large clasts/matrix = 30/70; large clasts are pebble to cobble sized, composed of quartzite (80%), with limestone, conglomerate, arkose, and sandstone combined making 20%; matrix is medium to coarse sand sized, composed of quartz and chert, with local calcareous matrix, occurs as thick beds (1-3 ra) with local interbeds of sandstone and shale; non-resistant trough former.
6 maroon feldepathic maroon, weathering to black or brown (desert varnish); grains are quartz sandstone fine to medium, poorly sorted, composed of angular to subangular quartz (20-30%), feldspar (15-30%), chert (20-30%), lithic frag ments (10-30%), with minor amounts of hematite, magnetite, calclte, and epidote; occurs in beds 0.2-1.5 m thick, crops out as resistant ledges; locally conglomeratic with lenses of quartzite, limestone, and sandstone pebbles and cobbles; alternative name, arkosic lithic arenite.
7 maroon feldspathic maroon, weathering to black or brown (desert varnish); grains are quartz siltstone fine silt or smaller, composed of quartz, chert, feldspar, and and mudstone clay, with minor amounts of hematite and calclte; occurs as 0.2-1 m thick layers, but splits on closely spaced planes; non-resistant trough former.
8 Tertiary volcanic rocks includes: basalt or andesite (dark grey, aphanitic, locally vesi cular), tuff (white, fine to medium grained, glassy), pumice and scoria (brick red), and tuffaceous sandstone (olive green, poorly sorted, composed of ash and volcanic lithic fragments). TABLE 2, cont.
If NAME DESCRIPTION
9 schistose volcanic .greenish grey or pink (varicolored), large clasts/ matrix = 60/40; agglomerate large clasts composed of pebbles and small cobbles (flattened) of sandstone, pink quartzite, basalt or andesite, basalt or andesite porphyry; matrix composed of quartz sand, lithic fragments and ash(?); rock is schistose and breaks on closely spaced foliation planes; resistant ledge former.
10 granite (?) greenish grey, weathering to brown or black, phaneritic (medium to coarse grained), equigranular, composed of quartz, feldspar, and chlorite (exact mineralogy not determined); non-resistant.
Ui oo 59
Figure 30. Schematic Geologic Strip Map in the Little Harqua- hala Mountains . — Numbers in parentheses refer to lithologic descriptions of Table 2. M = Martin Peak; H = Harquar Peak. 60 and shale. This conglomerate is distinctive, for it is composed en tirely of pinkish quartzite clasts.
To the northeast, the maroon conglomerate sequence is covered by Tertiary volcanic rocks (lithology 8), which crop out on hill 2081, and on the two symmetrical hills immediately to the south (Figure 29).
The principal lithology in the low-lying land northeast of the volcanic hills is green-grey conglomerate (lithology 5). This conglomerate contrasts with the maroon conglomerate in that it contains a much higher proportion of matrix, and includes more diverse rock types as clasts. The spurs which rise from the lowlands to the base of Harquar
Peak (Figures 29 and 30) are composed of maroon feldspathic sandstone, siltstone, and mudstone (lithologies 6 and 7)(Figure 31). The maroon feldspathic sandstone includes abundant lithic fragments, which are probably of volcanic origin. Conglomerate lenses in the sandstone include clasts of pink quartzite.
Bounding Terranes
At the northern border of the map area (Figure 30), at the base of Harquar Peak, the Mesozoic Strata contact schistose volcanic agglom erate (lithology 9). This rock forms much of the southeast flank of the peak. This area was labeled "Mesozoic Schist" on the Geologic Map of Arizona.
To the east, the Mesozoic Strata contact granite (lithology 10).
At Martin Peak, the granite contacts an overturned section of Paleo zoic rocks. From west to east, this Paleozoic section consists of
Bolsa Quartzite (arkosic pebble conglomerate and maroon vitreous 61
Figure 31. (A) Photomicrograph of Fine-Grained Maroon Sand stone, of the Little Harquahala Mountains (B) Photomicrograph of Coarse-Grained Maroon Sandstone From the Same Area 62 quartzite), Martin(?) Formation (grey-tan dolomite interbedded with quartzite), Escabrosa Limestone (light grey, massive recrystallized limestone), and Supai Formation (dark brown-purple layers of quartzite• interbedded with lighter tan or buff layers of limestone and shale).
Contact Relations
The contact between Mesozoic Strata and schistose volcanic agglomerate at the base of Harquar Peak is a fault which dips steeply to the northeast. The contact between the Tertiary volcanic rocks and the Mesozoic Strata is a depositional unconformity; the volcanic rocks were deposited over the Mesozoic Strata.
The granite occurs in low-lying areas surrounding the outcrops of Mesozoic Strata, and was never observed to intrude these Mesozoic
Strata. Either: 1) the sedimentary rocks were deposited over the granite, or. 2) the sedimentary rocks were faulted over the granite.
The applicability of either of these hypotheses cannot be determined with available data.
\ Structural Features
Folds and Faults
Hinges of folds in the outcrop area of Mesozoic Strata were not located on the ground, but the existence of folds is suggested by
the variability of dips measured.
The only faults that were directly observed in this reconnais
sance were those which occur near the base of Harquar Peak, marking
the contact between the Mesozoic Strata and the schistose volcanic 63 agglomerate. Where measured, they dip steeply. It is expected-that more detailed mapping will document additional faults.
Discussion
Stratigraphy
The abundance of conglomerate, the presence of mudcracks, rain drop impressions, and crossbeds, and the ubiquitous red color of these rocks suggests that much of the sediment composing the Mesozoic Strata in the Little Harquahala Mountains was deposited in the alluvial fans and mudflats of a subaerial environment (Blatt et al., 1972). The poor sorting and angularity of grains in the sandstones suggests the nearness of the source terrene.
The presence of pink quartzite clasts of the Mesozoic Strata
that are lithologically similar to quartzites within the units of
Paleozoic section of Arizona (e.g., the BoIsa Quartzite and the Supai
Formation) suggests that Paleozoic rocks were in the source terrane of
the Mesozoic Strata. The presence of feldspar and probable volcanic
lithic fragments suggests that the source terrane also included vol
canic and perhaps other crystalline rocks.
Age Constraints
No fossils were found in the Mesozoic Strata, and no radio-
metric ages have been published for the igneous rocks of the range,
so precise age constraints are not known. The maroon quartzite —
silty shale— limestone sequence, which crops out at the southwestern
corner of the map area (Figure 30) lithologically resembles the Supai 64
Formation of Martin Peak. This sequence also crops out near Black
Rock Mountain, 10 km to the south. The Black Rock Mountain sequence is labeled "Mesozoic Schist" on the Geologic Map of Arizona, but it is not schistose.
- Lack of lithologic affinity of other sedimentary rocks within the Mesozoic Strata terrane with nearby Paleozoic sections confirms that these sedimentary rocks are not Paleozoic. The presence within
the Mesozoic Strata of clasts that appear to have been derived from
Paleozoic units indicates that these sedimentary rocks are post-
Paleozoic in age. The deposition of mid to late Tertiary volcanic
rocks over the Mesozoic Strata suggests that the sedimentary rocks
are pre-mid to late Tertiary. The best estimate based on available
data is that the Mesozoic Strata in the Little Harquahala Mountains
are Mesozoic or early Tertiary in age. CHAPTER 4
THE GRANITE WASH MOUNTAINS
The Granite Wash Mountains, west of Salome, are continuous with the Little Harquahala Mountains and the Harcuvar Mountains.
The Geologic Map of Arizona indicates that Mesozoic Strata cover an
area of about 75 in the Granite Wash Mountains, and that these
sedimentary rocks contact Mesozoic Granite, Laramide Granite, and
Paleozoic-Mesozoic (undivided) shale, quartzite, and limestone.
Lithologies
Mesozoic Strata
The Mesozoic Strata undergo progressive metamorphism from
southwest to northeast (Figures 32 and 33). For purposes of dis
cussion, the mapped area of Mesozoic Strata was divided into four
units, each characterized by a distinctive sequence of lithologies
and by distinctive metamorphic features. The boundaries between
Units A, B, C, and D are labeled on the strip map (Figure 33).
Unit A is composed of unmetamorphosed locally conglomeratic
greywacke and silty shale (lithologies 1 and 2, Table 3)(Figure 34).
Clasts of these lithologies include quartz, feldspar, and lithic
fragments. The pebbles and cobbles of conglomeratic lenses in this
unit are composed primarily of quartzite. Primary sedimentary struc
tures, such as load casts, graded beds, edgewise conglomerate, and
65 66
Figure 32. Topographic Map of the Granite Wash Mountains . — Location of Figure 33 is shown by the heavy line. Sources: Hope, Utting, Salome, and Vicksburg, Arizona 7.5' U.S. Geological Survey Topographic Quadrangle Maps. 67
SCHEMATIC GEOLOGIC STRIP MAP plugs of amphibole- in the biotite quart! diorite GRANITE WASH MOUNTAINS (10) biocite gneiss, and augen gneiss (8,9)
kUofn#t«rs white marble (6)'
fault - southern block relatively down
aaphibole-chlorite-quartz- / ^ plagioclase schist (meta- / :%:%{{l%arcz-sericiCe basic igneous (5) { -• (7) . A B
outcrops of siliceous limestone (6) good exposure# of two rhvolite d ike (1IH XT phases of folding - chaotic terrene
aophibole-chlorite-quartz- plagioclase schist (meta Card lock Claims basic igneous) (5; ^ (True Blue Mine)
biotite-quartz-feldspar schist and p h y llite (aeta-graywacke and s i lt y shale) (3,4)
access route
good exposures of primary sedimentary structures- relatively unmetaoorphosed erayvacke" and silty shale, locally conglomeratic(1,2)
Figure 33 Schematic Geologic Strip Map in the Granite Wash Mountains, Numbers in parentheses refer to lithologic descrip- tions in Table 3. Letters A, B, C, and D refer to units discussed in the text. TABLE 3: LITHOLOGIES, GRANITE WASH MOUNTAINS
# NAME DESCRIPTION
1 gveywadke greenish grey to orangeish tan, weathering to dark brown or black (desert varnish); grains/matrix = 60-90/10-40; grains are silt to very coarse sand sized (0.05-1 mm), poorly sorted, subangular to angular, elongate, composed of quartz (40-50%), chert (10-30%), feldspar (10-30%), and lithic fragments (derived from volcanic, clastic, and metamorphic rocks(?)) (0-20%), with accessory chlorite, muscovite, epidote, and magnetite; matrix is composed of very fine grained quartz (60%) and clay or sericite (40%); cement is composed of silica and locally calcite; locally, lithology is composed of up to 30% limonite; beds are 0.2-2 m thick, are locally graded, and are continuous on the scale of 0.2 km; locally the lithology contains well rounded granules to small cobbles of greenish grey quartzite, and locally it contains rip-up clasts of shale; lithology has blocky parting, current marks, and load casts on bedding surfaces, and is a resistant ledge former.
2 silty shale light grey, weathering to orangeish brown; grains are clay to silt; composed of quartz (20-60%) and clay (30-80%); beds are 0.2-1 m thick; lithology parts on closely spaced planes (1-2 cm apart) and is a non- resistant trough former.
3 biotite-quaxtz feldspar white, bluish and greenish grey, or reddish tan, weathering to schist (meta-grey- orangeish tan or dark brown or black (desert varnish); grains are waake) silt to very coarse sand sized, locally conglomeratic, poorly sorted, angular (modified by recrystallization), composed of quartz (50-70%), feldspar (10-30%), chert (10-20%), sericite- muscovite (10-20%), biotite (5-40%), with accessory magnetite and locally chlorite and hematite; lithologic layers are 0.2-2 m $ thick with schistose parting at 1-2 cm intervals; lithology occurs
I TABLE 3, cont.
# NAME DESCRIPTION
both as a teslstant ledge former and a non-resistant trough former.
4 phyViite light greyish blue, weathers to similar color; grains are clay to very fine silt sized, composed of clay or sericite, with lesser proportions of quartz and biotite; lithology occurs as non-resistant trough former.
5 amphibole-chtorite- light to dark greyish green with spots and streaks of dark bluish quartz-plagioclase green, weathering to tan or dark brown or black (desert varnish), eohiet with dark bluish green spots weathering into pits; grains are medium to coarse, have indistinct boundaries and are highly altered and intergrown, composed, of plagioclase (40-60%), quartz-chert (10-30%), amphibole (20-30%), chlorite (10-30%); lithologic layers crop out parallel to foliation of adjacent layers, are 0.5-50 m thick and have bloth blocky and platy parting; lithology occurs as resistant ledges.
6 aitioeoua limestone limestone is dark grey, weathers to similar color; grains are medium and marble sand sized, composed of calcite (50-70%), quartz and clay (30-50%); marble is white with tan chert bands; layers are 0.5-2 m thick, and occur as resistant layers.
7 quartz-eericite schist white to very light grey, weathers to similar color, has shiny luster; grains are medium to coarse, and are composed of subrounded clear quartz (40-50%), sericite (30-40%), feldspar (10-20%?), with accessory magnetite and chlorite, locally there are flattened pebbles of quartz- S ite; lithologic layers are 1-10 m thick, have platy parting on closely TABLE 3, coat.
# NAME DESCRIPTION
spaced foliation planes; and occur as moderately resistant ledges and non-resistant troughs; lithology has non-penetrative llneation defined by sheared and aligned mineral grains.
8 biotite gneiss light bands are white, dark bands are greenish to greyish black, weathering to brown; grains fine to medium, dark layers composed of biotite (50-70%), and plagioclase (30-50%); light layers are composed of plagioclase (50-70%), and quartz (30-50%); lithologic layers are 2-15 m thick, compositional bands are 1-2 cm thick; lithology occurs as resistant ledges.
9 augen gneiss grey with white augens, weathering to orangeish tan; augens com posed of clusters of fine-grained quartz and plagioclase, matrix composed of biotite (20-40%), quartz (30-50%), and plagioclase (20-30%); lithologic layers are 2-5 m thick, parting is on irreg ular foliation surface; augens are 2-3 mm long; lithology occurs as moderately resistant ledge former.
10 hornblende-biotite dark green and white, porphyritic phaneritic, composed of plagio quartz diorite clase (20-30%), quartz (10-20%), hornblende (30-40%), biotite (20-30%); hornblende phenocrysts are up to 3 cm long; occurs as small plugs intruding the gneiss and augen gneiss.
11 rhyolite white to pink, weathering orangeish tan; aphanitic to porphyritic; lithology occurs as dikes. 71
Figure 34. (A) Photomicrograph of Fine-Grained Greywacke of the Granite Wash Mountains (B) Photomicrograph of Coarser-Grained Greywacke From the Same Location 72 flute casts occur locally in this sequence. A fossil shell fragment was found in a silty shale layer by Robert Powell in 1978. At the northern boundary of Unit A, the lithologies have a bluish hue, and are cut by regularly spaced (10-20 cm apart) calcite veins.
Unit B is composed of interlayered biotite-quartz-feldspar
schist (lithology 3), phyllite (lithology 4), siliceous limestone
(lithology 6), and amphibole-chlorite-quartz-plagioclase schist
(lithology 5). A rhyolite dike cuts this unit just south of the
Gardlock Claims.
Unit C is composed of quartz-sericite schist (lithology 7)
and fault-bounded outcrops of amphibole-chlorite-quartz-plagioclase
schist. Unit D is composed of fine-grained biotite gneiss (lithology
8) with well-developed compositional banding (Figure 35) and coarse
grained augen gneiss (lithology 9). Intruded into the gneiss are
small plugs of amphibole-biotite-quartz diorite (lithology 10). A
1 m thick layer of highly deformed marble crops out near the top of
Unit D.
Bounding Terranes
To the south, the Mesozoic Strata are covered by alluvium.
At the location labeled P in the northwest quarter of Figure 32, the
Mesozoic Strata contact a metamorphosed and deformed marble-quartzite
sequence of probable Paleozoic age, and a small granitic body of un
known age (Ciancanelli, 1965). To the north, the Mesozoic Strata
contact the plutonic rocks which form Salome Peak. Discussion of the
petrology of this intrusive complex is beyond the scope of this paper. 73
Figure 35. Photograph of a Hand Sample of Biotite Gniess From Unit D of the Granite Wash Mountains 74
Contact Relations
The contact between the Mesozoic Strata and the granitic rock of Salome Peak is intrusive. The contact location is shown by the dashed line at the base of Salome Peak on Figure 32. The granitic rocks appear to intrude the metamorphosed Mesozoic Strata as a laco- lith, as shown in the photograph of the contact taken from the town of
Salome (Figure 36).
Contact relations between lithologic layers of the metamorphosed
Mesozoic Strata are cryptic, because the contacts have been obscured by metamorphism. The contact between the amphibole-chlorite-quartz- plagioclase schist and other lithologies is locally irregular, suggest ing that the schist protolith intruded the Mesozoic Strata. This con clusion is also suggested by the coarse texture of this schist.
Structural Features
Folds
Folding of the Mesozoic Strata occurs throughout the traversed area. Three principal classes of folds can be defined: 1) folds which deform original bedding. These are fairly open and have amplitudes ranging from 1 to 10 m in the unmetamorphosed lithologies of Unit A, but are tight and isoclinal in the more metamorphosed rocks of Unit
B. These folds are difficult to recognize in Units C and D. The axial planes of these folds are subparallel to metamorphic foliation in Unit
B, suggesting that these folds cause transposition of bedding into
foliation; 2) folds which refold the class 1 folds, and thus deform \ f ■ t f " 1 ( U' -I'.; v'v. # 6
Figure 36. Distance Photograph and Tracing of the Contact Between Mesozoic Strata and Laramide Granite in the Granite Wash Mountains. — Photograph taken looking west from the town of Salome. Ms = Mesozoic Strata; Lgr = Laramide granite. 76 the metamorphic foliation (Figure 37); and 3) small-scale kink folds, observable throughout the traversed area.
Fold attitudes were measured near the Gardlock Claims (True
Blue Mine, on Figure 32), which are in Unit B of Figure 33. The axes of these folds are plotted on a stereographic projection (Figure 38), with F^ representing axes of class 1 folds, Fg representing axes of class 2 folds, and K representing axes of class 3 kink folds.
Faults
High-angle faults cut the traversed area at many locations.
For simplicity, most of these have not been shown on the strip map
(Figure 33). At least four faults can be recognized in the photo
graph of Unit C (Figure 39).
Other Structural Features
Metamorphic foliation is developed in Units B, C, and D.
The principal foliation of Unit C is a schistosity defined by paral
lelism of platy minerals, whereas in Unit D, the principal foliation
is compositional banding. Lineations locally appear on foliation planes.
Discussion
Stratigraphy
The depositional environment of the sedimentary rocks in Unit
A is equivocal. Primary structures, such as flute casts, graded beds,
and edgewise conglomerate, suggest that the sediments were deposited
by turbidity currents and/or debris flows. The mineralogy of the
sedimentary rocks suggests that they were derived from lithologically 77
Figure 37. Photograph of Class 2 Fold Refolding Class 1 Isoclinal Fold in Unit B of the Granite Wash Mountains 78
* Fg Fold Axis
o Kink Fold Axis
■ F1 Fold Axis
N
Figure 38. Equal Angle Lower Hemisphere Projection of Fold Hinges Measured Near the Gardlock Claims, Granite Wash Mountains 79
Figure 39. Photograph and Tracing of Block Faulting in Unit C of the Granite Wash Mountains . — Photograph taken looking south east. 5 = amphibole-chlorite-quartz-plagioclase schist; 7 = quartz- sericite schist. 80 complex source areas composed of quartzite and limestone as well as volcanic lithologies.
The gross mineralogy of the metamorphosed sedimentary rocks encountered on the traverse is compatible with their derivation from protoliths similar in composition to the rocks of Unit A. The biotite- quartz-feldspar schist and the biotite gneiss are probably the meta morphosed equivalent of the greywacke of Unit A. Coarser layers of schist contain partially dissolved quartzite pebbles. The phyllite and the quartz-sericite schist is probably the metamorphic equivalent of the silty shale in Unit A.
The texture and mineralogy of the amphibole-chlorite-quartz- plagioclase schist suggests.that the schist is the metamorphic equivalent of a mafic igneous rock (Winkler, 1974). No evidence of detrital grains in this rock is visible in hand sample or in thin section.
Because folds and faults occur repeatedly, the thickness of
the original sedimentary section of the Granite Wash Mountains could not be determined in this reconnaissance. Based on adjusting the hori
zontal length of the outcrops for dip and for perhaps 80% shortening by deformation, my estimate is that the original sedimentary section was about 1500 m thick.
Age Constraints
The sedimentary rocks of the Granite Wash Mountains described
in this chapter must be post—Precambrian, because they contain fossil
shell fragments. Lack of lithologic affinity with known Paleozoic 81 sections of the area suggests that these sedimentary rocks are post-
Paleozoic. They must be pre-Tertiary, because they are intruded by granitic rocks which are latest Cretaceous in age; Eberly and Stanley
(1978) report an age of 69 m.y. (K-Ar biotite) from granodiorite near
Granite Wash Pass. Unfortunately, no radiometric dates are available which indicate the age of metamorphism of the sedimentary rocks, so the time relation between metamorphism and intrusion is not known. CHAPTER 5
THE PLOMOSA, BUCKSKIN, AND NORTHERN DOME ROCK MOUNTAINS
The rocks within the areas in the Plomosa, Buckskin, and northern Dome Rock Mountains mapped on the Geologic Map of Arizona as
Mesozoic Strata have been intensely metamorphosed.
Plomosa Mountains
The Plomosa Mountains are a 30 km long, narrow north-south trending range extending south from the village of Bouse to Interstate
Highway 10 (Figures 1 and 40). The Geologic Map of Arizona indicates that the narrow ridge forming the northern two-thirds of the range is composed of Mesozoic Strata. Jemmett (1966) mapped the northern half of the range.
Lithologies
Descriptions of the lithologies encountered in the Plomosa
Mountains are listed in Table 4. As indicated on the map. Figure 41, the most widespread lithology within the terrane of Mesozoic Strata is biotite gneiss (lithology 1, Table 4)(Figure 42). Mineralogical and textural characteristics of this lithology vary greatly from layer to layer. Interlayered with the gneiss are layers of quartzite and phyllite (lithology 2). A wide band of maroon phyllite can be
traced for the width of the range (Figure 41). As a result of the varying resistance to erosion of different layers of gneiss, large
82 83
r t-v--
IIU' 27 X
^__
mlifis *-
Figure 40. Topographic Map of the Plomosa Mountains . Location of Figure 41 is indicated by heavy black line. Source: Bouse, Arizona 7.5’ U.S. Geological Survey Topographic Quadrangle Map. TABLE 4: LITHOLOGIES, PLOMOSA MOUNTAINS
// NAME DESCRIPTION
1 biotite-gneiee alternating grey and white layers, weathering to brown or black (des ert varnish); grains are fine to medium, composed of quartz(65%), feldspar(predominantly sericitized microcline)(20%), chlorite(with intergrowths of biotite)(10-15%), withaccesspry magnatite; locally there are feldspar augens and detrital quartzite clasts(?); litho logic layers are l-2tn thick, compositional bands are l-2cra thick: crops out both as resistant ledges and non-resistant troughs; this lithologylis highly variable in outcrop appearance.
2 phyllite dark maroon to grey, weathers to similar color; has phyllitic listre; composed of altered clay and silt sized quartz grains; lithologic layers are 2-15m thick, and are very thinnly laminated; non-resis tant trough former. (
3 meta-quartzite dark grey, white, green, and maroon; grains are aphanitic to medium sand sized; composed of quartz (90%) with minor feldspar and sericite; local coloration caused by hematite and diopside(?); lithologic lay • ers are l-5m thick; resistant ledge former.
4 sitioeoue marble white with dark tan bands on weathered surfaces; individual grains are not visible; white layers are recrystallized carbonate, tan bands are chertiferbus; outcrops as reistantledges up to 5m thick.
5 granite grey and white weathering to dark rust brown; grains are medium sized; composed of microcline (40-50%), quartz (30-40%),sericite (20-30%), chlorite (10%), with accessory magnetite and biotite; outcrops as resistant plugs, locally with roof pendants of gneiss. TABLE 4, cont.
If NAME DESCRIPTION
6 rhyolite pink to white, fine to medium grained, locally with 0.5 cm phenocrysts of plagiodase; outcrops as 1-2 m wide dikes.
7 Mid-Tertiary sed intermediate to silicic flows, tuffs, welded tuffs, and volcanoclastic imentary and sedimentary rocks. volcanic rocks
8 Chapin Wash(?) buff to bright red and black unmetamorphosed conglomerates (with Formation gneiss clasts), volcanoclastic sedimentary rocks, and intermediate to silicic volcanic flows and tuffs. 86
EXPLANATION
\ ; : * l Tertiary sedismitary and kirJ volcanic rocks(7)
rhyolite dike(6)
Tertiary sedimentary and volcanic rocks with,^^ gneiss conglomerate(8)
granite (5)
#%yllite(2) in a distinctive 15m wide band within the biotite gneiss
biotite gneiss(1) "sns5slss3bi.s?
approximate contact
probable fault contact
N
oof pendants of gneiss
.'^/SCHEMATIC GEOLOGIC STRIP MAP - .-r __;v' *: 7 In min c t h e PLOMOSA MOUNTAINS
Figure 41. Schematic Geologic Strip Map in the Plomosa Moun tains. — Numbers in parentheses refer to lithologic descriptions in Table 4. 87
Figure 42. Photograph of Hand Samples of Biotite Gneiss From the Plomosa Mountains 88 gneiss outcrops, from a distance, have a stratified appearance, but the boundaries between layers are foliation planes, not bedding planes.
On the south side of the Bouse-Quartzsite Road, biotite gneiss occurs as roof pendants within an extensive body of unmetamorphosed granite (lithology 5). Dikes of rhyolite cut the outcrops of gneiss and granite near the road, and narrow dikes of aplite are locally abundant. Near the Bouse-Quartzsite Road, there is a sequence of interlayered quartzite and marble (lithologies 3 and 4). The cherty bands within the marble of this sequence outline tight z-fold pairs which verge southeast and have axes oriented about 30°, S30°E (Figure
43).
To the east, the Mesozoic Strata contact a sequence of buff and red volcanic rocks (flows and welded tuffs), and red continental de posits (conglomerates, sandstones)(lithology 8) which may correlate with the Chapin Wash Formation (Lasky and Webber, 1949). Conglomerates in this sequence contain abundant angular clasts of gneiss. To the south, the Mesozoic Strata contact a sequence of interlayered volcanic flows and tuffs (lithology 7). The bounding units at the north end of the Plomosas were not studied in this reconnaissance. They are described by Jemmett (1966).
Contact Relations
The western contact between the marble-quartzite sequence and the gneiss and granite terrane is a fault whose trace is followed by a rhyolite dike. Movement on this fault has caused cataclastic deforma
tion of the adjacent granite. The pass through the range in which the 89
• Fold Axis N
Figure 43. Equal Angle Lower Hemisphere Projection of Fold Axes in the Marble Outcrops Near the Bouse-Quartzsite Road, Plomosa Mountains 90
Bouse-Quartzsite Road was built also follows a fault. This fault jux taposes the marble-quartzite sequence with the granite and biotite gneiss terrane (Figure 41). The contacts of the Mesozoic Strata with the Tertiary sedimentary-volcanic sequences locally appear to be deposi- tional unconformities. Jemmett (1966) mapped portions of the contact between the Tertiary sedimentary-volcanic sequences and the gneiss as ' a fault.
Discussion
The textural and mineralogical variability of the biotite gneiss-phyllite-quartzite sequence suggests that there was variability in the protoliths from which these lithologies were derived. Quartzite and phyllite were probably derived from sandstone and shale, respective ly. The biotite gneiss was probably derived from a sedimentary rock which had a significant component of feldspar and mafic minerals.
Greywacke or tuffaceous sandstone could be protoliths. The very bio- tite-rich layers of gneiss could have been derived from sills or volcanic flows, as suggested by Jemmett (1966). The original strati graphic thicknesses of the sedimentary and igneous (?) rocks from which this sequence was derived is not known. No clear age constraints are known for the biotite gneiss-phyllite-quartzite sequence in the Plomosa
Mountains, except that the rocks must be older than the 21-25 m.y. volcanics (age dates described by Eberly and Stanley, 1978) that overlie
them.
The marble—quartzite sequence adjacent to the Bouse-Quartzsite
Road closely resembles marble-quartzite sequences in nearby ranges 91
(e.g., New Water Mountains, Miller, 1970) that are considered to have originated as Paleozoic sedimentary rocks, and it is proposed that the marble-quartzite sequence of the Plomosas was derived from Paleozoic strata.
Buckskin Mountains
The Buckskin Mountains are near the northern border of Yuma
County, 25 km northeast of the village of Bouse. The Geologic Map of
Arizona indicates that there is a 35 km^ area of Meosozoic Strata at the southern end of the easternmost ridge of this range (Figure 1).
This outcrop is the northernmost outcrop of Mesozoic Strata indicated on the Geologic Map of Arizona. The lithologies encountered on my traverse of the Buckskin Mountains are listed in Table 5. The struc tural complexity of this area precluded the preparation of a strip map.
Lithologies
All but one of the lithologies encountered are intensely meta morphosed. The principal metamorphosed lithologies include: h o m - blende-biotite schist, gneiss, augen gneiss, siliceous marble, quartzite, and dolomitic marble (lithologies 1-5, Table 4). These lithologies are cut by plugs, dikes, and lit-par-lit intrusions of pegmatite gneiss
(lithology 6), and locally by unmetamorphosed sills of andesite or dacite porphyry (lithology 7). The metamorphosed lithologies exhibit well-developed lineation which trends N50-60°E. Cherty bands in the siliceous marble outline recumbent isoclinal folds. The above-listed
field names are generalizations. Each lithologic type exhibits wide TABLE 5: LITHOLOGIES, BUCKSKIN MOUNTAINS
// NAME • DESCRIPTION
1 homblende-biotite dark greenish grey to light grey, weathering to black or brown (des schist ert varnish); grains are fine to medium, composed of biotite and hornblende (together, 20-50%), plagioclase and quartz (together, 20-50%), with minor muscovite, epidote, sphene, magnatite, chlorite, calcite and limonite; foliation variable - most layers are finely laminated, but some break on widely spaced partings and appear horn- felsic on the scale of a handsample; occurs both as resistant ledges and non-resistant troughs, depending on local composition and foliation.
2 siliceous marble grey, white, of buff: grains not visible in handsamble, composed of recrystallized carbonate with 2-6 cm thick irregular bands of chert- iferous carbonate spaced at 10-30 cm intervals; tremolite is visible on fracture surfaces; lithologic layers are from 1-5 m thick, and occur as ledges with very rough weathered surfaces.
3 meta-quartsite dark maroonish grey weathering to dark black or brown (desert varnish); grains are fine sand sized, equigranular, and locally vitreous; com posed of recrystallized quarts with minor muscovite; irregular fol iation planes are spaced at 10-20 cm intervals; resistant ledge former.
4 cblomitia marble buff, weathering to similar color; grains are coarse sand sized (0.4 cm in diameter), equigranular, angular, and are composed of recrys tallized dolomite; foliation planes are widely spaced and irregular; moderately resistant ledge former.
5 gneissj and dark layers are greenish grey, and light layers are white, weather augen gneiss ing to tan; grains are medium to coarse, most layers have augens (0.1-0.5 cm) in diameter; principal dark minerals (40%) are biotite and hornblende, principal light minerals (60%) are quartz and feld- TABLE 5: cont.
# NAME DESCRIPTION
spar; minor pyrite, sphene, chlorite, and garnet; foliation planes are 1 mm - 1 cm apart; locally, rock includes elongate augens; outcrops are rubbly troughs.
6 pegmatite gneiss white weathering to same color; grains are fine to coarse, with grain boundaries locally not clearly defined; composed of quartz layers (40-50%) and potassium feldspar layers (40-50%) with minor amounts of muscovite: texture is complex - monomineralic layers pinch and swell; crops out as dikes, plugs, and lit-par-lit injec tions; lit-par-lit injections exhibit boudinage both on the scale of the outcrop and on the scale of the handsample; foliation surfaces are lineated; resistant ledge former.
7 daaitei?) porphry greenish grey with white phenocrysts weathering tan; porphyritic aphanitic with 1-6 mm long phenocrysts that are composed of plagioclase (50%), quartz (30%), biotite (altered to chlorite)(20%), with minor muscovite and magnatite; crops out as thin, unmeta morphosed sills.
vo w 94 variability in texture, proportions of minerals, and outcrop charac teristics. For example, the homblende-biotite schist occurs both in well-foliated .coarse crystalline and in fine-grained homsfelsic varieties.
Field Relations
The lithologies of the Buckskin Mountains are intimately inter- layered throughout the study area. This interlayering may reflect original stratigraphy, but it may also be a consequence of tight iso clinal folding or interlayer shearing (Reynolds and DeWitt, pers. commun., 1978). The interlayering of lithologies with varying resis tance to erosion gives outcrops of these metamorphosed sedimentary rocks a stratified appearance, but the planes between contrasting lithologies are foliation planes, not bedding planes. Foliation attitudes in the study area grossly define a large-scale arch with a northeast-southwest trending hinge line. Locally, the homblende- biotite gneiss and the augen gneiss appear to be a basement underlying
the marble and quartzite sequences.
Discussion
The structural complexity of the Buckskin Mountains necessi
tates that they be mapped in detail before complete interpretation
can be made. Rehrig and Reynolds (1977) proposed that the Buckskin
Mountains formed part of a northwest trending belt of "metamorphic
core complexes" that extends across Arizona. Structural features
recorded in the metamorphosed sedimentary rocks discussed here, such 95 as lineations and recumbent isoclinal folds, indicate that the rocks in the area labeled Mesozoic sedimentary rocks on the Geologic Map of
Arizona are part of this "metamorphic core complex," as it was defined by Rehrig and Reynolds.
The marble-quartzite sequence of the Buckskin Mountains re sembles documented Paleozoic marble-quartzite sequences in nearby ranges (Miller, 1970). Thus, it is proposed that some of the sedi mentary rocks in the area indicated on the Geologic Map of Arizona as Mesozoic sedimentary rocks were deposited during Paleozoic time.
The exact nature of the protolith of the schist and gneiss in this range is not yet known. Greywacke with a mafic volcanic(?) component is a possibility, and as such a lithology does not occur in the
Paleozoic section of Arizona, the Mesozoic age assignment for these lithologies is reasonable.
Northern Dome Rock Mountains o The Geologic Map of Arizona shows a 4 km area of Mesozoic
Strata at the northern tip of the Dome Rock Mountains (Figure 2).
These rocks are metamorphosed. Lithologies from this area include biotite schist, augen gneiss, quartzite, and, locally, thin layers of marble. Foliation planes of these strata dip 40° to the north.
At their southern limit, these rocks contact an extensive body of augen gneiss, with golf ball sized augens. CHAPTER 6
SUMMARY, CORRELATION, AND CONCLUSION
This chapter summarizes the data presented in this paper and uses this data to attempt stratigraphic correlation of Mesozoic Strata among ranges within northern Yuma County and with rocks in ranges of adjacent regions. It also includes speculations on the timing and possible tectonic significance of the deposition and deformation of these rocks.
Summary
Table 6 summarizes important lithologic and structural features of the six deposits of Mesozoic Strata examined in this study. Figure
44 schematically represents the contact relations of Mesozoic Strata in the various ranges studied with adjacent units. Suggested modifi cations to the Geologic Map of Arizona are indicated on Figure 45.
From Table 6 and Figure 44, the following points can be inferred:
1) Most of the lithologies within the areas labeled Mesozoic sedimentary rocks on the Geologic Map of Arizona are sedimentary rocks or metamorphosed sedimentary rocks. However, non-sedimentary or meta sedimentary lithologies crop out in these areas in the Granite Wash,
Buckskin, Plomosa, and Dome Rock Mountains. In the Granite Wash Moun tains, probable mafic sills are extensively interlayered with the meta sedimentary rocks. In the Buckskin Mountains, the area mapped as
96 TABLE 6: SUMMARY OF PRINCIPAL CHARACTERISTICS OF MESOZOIC STRATA IN THE MOUNTAIN RANGES OF NORTHERN YUMA COUNTY
MOUNTAIN RANGES PRINCIPAL LITHOLOGIES DEGREE OF HETAMORfTIlSM IMI'ORTANr STRUCTURAL ELEMENTS
Buckakin Mountains hornblende-blotlte schist, siliceous marble, amphibolite(7) grade lithologies are complexly interlayered meta-quartzite, dolomltlc marble, banded gneiss and folded; extension and c a tn c la sis and augen gneiss, pegmatite gneiss, dacite por perhaps indicated by bond Inage, augen phyry. Marble quartzite sequence probably formation, and llncatlon (N50°E). meta-Paleozoic strata.
Dome Rock calcareous quartz sandstone, q u a rtz ite , maroon lower grconschist angular discordance between bedding and Mountains phylllte, quartzite conglomerate, greywacke, grade foliation planes; low- and high-angle granule conglomerate, grey phylllte, igneous faulting; parasitic folding. clast conglomerate, feldspathic greywacke, slltstone greywarke.
GranLte Wash greywacke, silty shale, blotlte-quartz-feld progressively meta multi-phase folding throughout; normal Mountains spar schist, phylllte, amphlbole-chlorite- morphosed from sttb- faulting; cntaclastlc deformation quartz-plagioclase schist, siliceous marble, greenschist to amphi affects the northern portion of the quartz-sericite schist, blotite gneiss, augen b o lite area. gneiss.
Little llarquattain marotn q u a rtz ite , maroon s i l t y shale, lime unmetamorphosed sequence may overlie basement on a low- M: :m tain s stone, raroon conglomerate, green-grey con angle fa u lt; setiqenc.e is folded and glomerate, maroon feld spathic sandstone, fatil ted. sf)tstone and mudstone.
Piomosa Mountains biotlte gneiss, phylllte, meta-quartzite, amphibolite grade variation in the foliation attitude of siliceous raatbic, granite. Marble-quartzite the gneiss may indicate late folding; sequence probrHe meta-Paleozolc strata. area is cut by faults; tight z-fold pairs in the siliceous marble layers.
Northern End biotite-plaglociase-quartz schist, quartzite, amphibolite(?) or north-dipping foliation; folding of Dome Rock augen schist, marble. upper grcettsrhisL marble. 1 Mountains Figure 44. Schematic Chart of Units Bounding of Cutting Mesozoic Sedimentary Rocks in Mountain Ranges of Yuma County, Arizona. —r This chart indicates the nature of contacts of Mesozoic Strata with adjacent units. 99
NORTHERN YUMA COUNTY GEOLOGY x Contact 'x Approximate Contact X. Gradational Contact 0 Granite m Mesozoic Strata-unmet. Mesozoic Stroto-met. a Paleozoic Strata \ Mt. Range Boundary 0 km 20
Figure 45. Schematic Map Indicating the Variations of Lithology in the Areas Mapped as Mesozoic Strata on the Geologic Map of Arizona . — Abbreviations are the same as those used in Figure 1 with the following additions: B = Bouse; Q = Quartzsite; V = Vicksburg; S = Salome. 100
Mesozoic sedimentary rocks includes bodies of pegmatite gneiss and andesite or dacite prophyry. In the Plomosa Mountains, an extensive body of granite drops out within the area, and there are possibly flows or sills of mafic igneous rock interlayered with the Mesozoic
Strata. In the Dome Rock Mountains, a portion of the meta-volcanic terrane, which underlies the Mesozoic Strata, was included in the area labeled Mesozoic Strata.
2) The degree of metamorphism of the sedimentary rocks in northern Yuma County is variable. In the Dome Rock Mountains, the
Mesozoic Strata are mildly metamorphosed (to lower greenschist grade).
In the Granite Wash Mountains, the Mesozoic Strata have undergone progressive metamorphism through amphibolite(?) grade, and in the
Buckskin, northern Dome Rock, and Plomosa Mountains, the Mesozoic Strata are metamorphosed throughout (to amphibolite(?) grade). Figure 45 dis tinguishes the highly metamorphosed sedimentary rocks from the mildly metamorphosed or unmetamorphosed sedimentary rocks.
3) Direct evidence for the age of the Mesozoic Strata in northern Yuma County is scarce, for no identifiable fossils have yet been found, but the available data is consistent with the Mesozoic age assignment of Wilson (1933, 1960). Fossil shell fragments have been obtained in the Livingston Hills (Harding, 1978), and in the Granite
Wash Mountains, implying that the deposits in these ranges are post-
Precambrian. Lack of lithologic affinity of most of the rocks within areas labeled Mesozoic sedimentary rocks (exceptions can be found in the Plomosa, Little Harquahala, and Buckskin Mountains, where there 101 are marble-quartzite sequences that are more readily correlatable with the Paleozoic section) with nearby documented Paleozoic strata
(Hiller, 1970; Varga, 1977) precludes correlation of the Mesozoic
Strata with the Paleozoic section. The lack of lithologic affinity of the Mesozoic Strata with nearby documented mid to late Tertiary strata (Eberly and Stanley, 1978) precludes correlation of the Meso zoic sedimentary rocks with the mid to late Tertiary strata. By this reasoning, the Mesozoic sedimentary rocks are post-Paleozoic and pre- mid to late Tertiary.
In Chapter 2, the proposal was made that the Mesozoic Strata of the Dome Rock Mountains post-date underlying Triassic-Jurassic volcanic rocks. The volcanic rocks are associated with a marginal volcanic arc, which ceased activity in mid-Jurassic time. In Chapter
4, the suggestion was made that the Mesozoic Strata of the Granite
Wash Mountains predate a dated Laramide pluton. If the Mesozoic Strata in all the ranges of northern Yuma County correlate, then the above suggestions imply that the Mesozoic Strata are post-mid-Jurassic and pre-Laramide in age.
4) A complete section of Mesozoic Strata from northern Yuma
County probably exceeds 5000 m in thickness.
5) The detritus composing the sedimentary lithologies of the
Mesozoic Strata came from a variety of sources, including limestone- quartzite, volcanic, granitic, and metamorphic terranes. The nature of the source terranes clearly changed with time, as indicated by the succession of lithologies in the Dome Rock Mountains. This succession 102 records early input from a limestone-quartzite and metamorphic(?)
terrane, and later input from a volcanic and granitic terrane.
Harding (1978) noted a similar succession in the nearby Livingston
Hills.
6) From textural and bedding plane features, it is inferred
that portions of the Mesozoic Strata of the Little Harquahala Mountains were deposited in a shallow water or subaerial environment, and that portions of Mesozoic Strata in the Granite Wash Mountains were deposited by turbidity currents and submarine debris flows. Textural features of
the Mesozoic Strata in the Dome Rock Mountains are enigmatic. It ap pears that portions of the section were deposited in a shallow water or
continental environment, while other portions could have been deposited by submarine debris flows in deeper water. Metamorphism has obscured
textural features of other ranges studied, and the depositions! envi
ronment of Mesozoic Strata in those ranges is not known.
7) Structural features of the Mesozoic Strata in northern Yuma
County are highly variable, and record a complex sequence of post-depo-
sitional deformational events. There is as yet insufficient data to de
termine the relationship between the structures in the various ranges.
8) The contacts of Mesozoic Strata with adjacent units in
the Dome Rock Mountains are slightly mislocated on the Geologic Map
of Arizona (compare Figure 1 with Figures 3 and 45). The contact of
Mesozoic Strata with "Precambrian Gneiss" (recent evidence indicates
that the gneiss may be much younger (Rehrig and Reynolds, 1977) in the
Buckskin Mountains is very complex. 103
Correlation
At first impression, it is the contrasts between the Mesozoic
Strata of the ranges discussed rather than the similarities that stand out. Structural style, metamorphic grade, gross mineralogy, and texture vary widely among the Mesozoic Strata of the ranges. The purpose of this discussion is to explore possible correlations among the Mesozoic
Strata of the various ranges, and to consider possible correlations of the Mesozoic Strata with units outside of northern Yuma County.
The most direct correlation can be made between the Mesozoic
Strata of the Dome Rock Mountains and that of the Livingston Hills, as described by Miller (1970) and Harding'(1978). In the Livingston
Hills, Miller divided the Mesozoic Strata into two sequences: 1) "Meso zoic (?) continental redbed deposits" composed of "maroon mudstone with interbedded sandstone and pebble conglomerate," and 2) the "Livingston
Hills Formation" which is 12,000 feet thick and is composed of four members which are, from bottom to top: 4700' of massively bedded boul der conglomerate with interbeds of arkose or greywacke, 5500' of well- lithified medium- to coarse-grained greywacke and conglomerate grey wacke, with siltstone interbeds near the top, and 2200' of greywacke.
In the Livingston Hills, the Livingston Hills Formation overlies the continental redbed deposits.
Unit A of the Dome Rock Mountains section is composed primarily of maroon phyllite, maroon quartzite, calcareous-cemented sandstone, and quartzite with minor conglomerate, and could be the mildly meta morphosed equivalent of the redbed deposits of the Livingston Hills. 104
Units B-G in the Dome Rock Mountains, which include lithologies such as
greywacke, conglomeratic greywacke, and siltstone-greywacke, could cor
relate with the Livingston Hills Formation. Both sections have simi
lar outcrop appearance (characterized by monotonous alternation of
resistant and non-resistant lithologies) and both sections are of com
parable thickness. Direct correlations of the members of the Livingston
Hills Formation to units in the Dome Rock Mountains are not possible
with available data.
Lithologic correlation of other sequences of Mesozoic Strata
in northern Yuma County with the Mesozoic Strata of the Dome Rock
Mountains and the Livingston Hills remains tentative. The maroon
sandstone and shale sequence of the Little Harquahalas perhaps corre
lates with the continental redbed deposits of the Livingston Hills and
Unit A of the Dome Rock Mountains, but the abundance of volcanic ma
terial in the rocks of the Little Harquahala sequence is not consistent
with this suggestion. The greywacke-siltstone sequence in the Granite
Wash Mountains perhaps correlates with the Livingston Hills Formation
and with Units B-G of the Dome Rock Mountains. Lithologies in the
Buckskin and Plomosa Mountains are too intensely metamorphosed for
direct lithologic correlation to be possible.
The Geologic Map of Arizona indicates that there are extensive
outcrops of Mesozoic Strata in the Castle Dome, Kofa, and Middle Moun
tains of Yuma County. According to Wilson (1933), the Mesozoic Strata
of the Castle Dome Mountains are composed of greenish-grey thick-
bedded shales, impure cherty limestones, maroon shales (described as 105 being "wackyish"), arkosic sandstones, quartzites, and conglomerates.
Jones (1916b) and Wilson (1933) noted that the Mesozoic Strata of the
Kofa Mountains are composed of grey-buff shales, sandstones, conglom erate, and limestone. G. Haxel (pers. commun., 1978) noted lithologic
similarity between the Mesozoic Strata of the Middle Mountains and a
section of sedimentary rocks in nearby California to which he gave the name "Winterhaven Formation" (Haxel, 1977). The base of this section
is a unit composed of 80 meters of very altered andesite or dacite.
Overlying the basal unit is a 60 meter thick "quartz arenite unit"
that is composed of slightly feldspathic quartz arenite interlayered with flaggy semi-schist, and locally, conglomerate. The top of the
section is an "argillitic siltstone unit" composed of slightly cal
careous argillitic siltstone or micro-greywacke, medium to coarse
pebbly greywacke, argillite, and granule to pebble conglomerate. The
lithologies of the Kofa, Castle Dome, and Middle Mountains, as de-..
scribed in this paragraph, are similar to lithologies of the Mesozoic
Strata in the Dome Rock Mountains and the Livingston Hills, and thus
are tentatively correlated lithologically with the Dome Rock Mountains
Mesozoic Strata and with the Livingston Hills Formation.
Pelka (1973) described a 23,000* thick south-dipping homoclinal
section of sandstone, conglomerate, lithic wacke, and mudstone in the
McCoy Mountains about 10 km to the west of the Dome Rock Mountains.
This section, to which Pelka assigned the name "McCoy Mountains Forma
tion," unconformably overlies a Lower Jurassic(?) granodiorite porphyry,
and exhibits penetrative metamorphic foliation. From this description, 106 it appears likely that the McCoy Mountains Formation correlates with the Mesozoic Strata of the Dome Rock Mountains and with the Livingston
Hills Formation.
Conclusion
Yuma County, during part of the Mesozoic, was a locus of deposition receiving detritus from a complex and changing source terrane. The depositional basin possibly extended throughout the county and into southeastern California. Contact relations discussed in this report suggest that the sediments composing the Mesozoic Strata of Yuma County were deposited in post-mid-Jurassic and pre-Laramide time.
The data available are not yet sufficient to pinpoint source terranes. One possibility is the Triassic to Cretaceous (?) Mogollon
Highlands (Harshbarger, Repenning, and Irwin, 1957; Cooley and David son, 1963) which have been suggested as a source for Mesozoic deposits of the Colorado Plateau. The Highlands could also have shed detritus to the south, into Yuma County. The Paleozoic sedimentary rocks and
Precambrian crystalline rocks exposed in the Highlands (Cooley and
Davidson, 1963) could have provided the detritus characteristic of
Unit A in the Dome Rock Mountains. The extensive volcanic arc that developed in the western North American Cordillera during late Jurassic through Cretaceous time (see Armstrong and Suppe, 1973) perhaps supplied the detritus that now composes the greywacke—rich sequences of Mesozoic
Strata in northern Yuma County. 107
If the Yuma County Mesozoic Strata are of post-mid-Jurassic to pre-Laramide age, then deformation and metamorphism of these rocks
could have occurred in Upper Cretaceous and Lower Tertiary time. This period of geologic history has been recognized as a time of intense
tectonism in the North American Cordillera (see Coney, 1978).
This paper has presented descriptive data on lithologic and
structural characteristics of the Mesozoic Strata of northern Yuma
County. Unfortunately, this reconnaissance raises more questions
than it answers, questions that can be answered only with the combined
information of dating and detailed mapping. A further investment of much time and effort is necessary before the full geologic history of
this portion of the Cordillera is.known. This report hopes to serve
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, R.T. Moore, and J.R. Cooper, 1969, Geologic map of Arizona: Arizona Bureau of Mines and U.S. Geol. Survey.
Winkler, H.G.F., 1974, Petrogenesis of Metamorphic Rocks: Springer- Verlag, New York, 320 p. I ?A 3 7r. 5 p0 5 x. A. UNIT DESCRIPTIONS { 0 i; %F w'A'''Wr&JFtfl r f YO// w 4 NOTE: Numbers refer to lithologies listed in Table 1. —- - — /Z __ ^ vV-YV' - { ^ X Z ,:' X' UNIT T: Tertiary volcanic flows,' ignimbrites, tuffs, and volcanoclastic sediments (conglomerates and sandstones). Unconformably over- lies Unit V-2.
^r- UNIT G: Feldspathic greywacke(10) with minor conglomerate(8) interbedded with siItstone-greywacke(10) and grey phyllite(7) (locally tuf- faceous?). Unit G is distinguished from older units by the ap ""' '-^ ^ a A x ' W pearance of the distinctive tough siltstone layers. s) Z •5000 .. UNIT F: Igneous clast conglomerate(8) interbedded with feldspathic grey wacke (9) and grey phyllite(7). Unit F is distinguished from older units by the appearance of phaneritic igneous clasts in .//* the conglomerate and by an increase in the proportion of feld spar in the greywacke. ______4(1179/ UNIT E: Grey phyllite(7) interbedded with conglomerate(41) and grey wacke (5) . Unit E is distinguished from older units by the ^~T = abundance of grey phyllite and by very evenly bedded outcrops. It is similar to Unit D.
UNIT D: Grey phyllite(7) interbedded with greywacke(5), and granule cong lomerate (6) . Unit D is distinguished from older units by the lack z- of maroon conglomerate and by the presence of very soft carbona- k- ceous(?) grey phyllite. r a ? T UNIT C: Granule conglomerate(6) interbedded with maroon and grey phyllite (3,7) and greywacke(5). Unit C is distinguished from older units by the appearance of the distinctive granule conglomerate.
\ „< UNIT B: Greywacke(5) interbedded with minor calcareous sandstone(1?), maroon phyllite(3), and massive conglomerate(41). Unit B is dis ' ' ..»**' tinguished from older units by the appearance of greywacke and z by the appearance of diverse clasts in the conglomerate. — 4 0 0 0 CB - -: UNIT A: Lower half - calcareous sandstone(l), locally with small lenses Q a of quartzite pebble-cobble conglomerate(4), interbedded with ;V maroon phyllite(3), and conglomerate(4). Upper half - maroon quartzite(2) interbedded with maroon phyllite(3). .Unit A is distinguished from younger units by the abundance of sandstone and quartzite and by the lack of feldspathic lithologies. It is distinguished from older units by the lack of volcanic component. Thickness is measured from the contact with the volcanic terrane , (V-l) near Cunningham Mountain.
UNIT M: Mylonite(16). Age of deformation uncertain, protolith was Unit V-l. True thickness (exaggerated in column) is 1 - 12 meters.
UNIT V-2: Meta-andesite (?)(13), meta-basalt(11), and meta-crystalline tuff (12). Unit V-2 has been faulted into position at the top of the sedimentary section. True thickness is not implied.
UNIT V-l: North of the pipeline - meta-crystalline tuff(12), meta-basalt(11) and light tan meta-volcanic wacke( 1 4 ) south of the pipeline - dark grey meta-voleanic wacke(15).
EXPLANATION — 3000
contact, dashed where strike and dip of n<^25 approximate bedding 'Oo0 * * 0 o'*® ® o 0 0 0 o o0 ° 0- e *
fault, dashed where strike and dip of approxiamte foliation
calcareous sandstone(l) §||j quartzite clast conglomerate(4) Q quartzite(2) diverse clast conglomerate(41) maroon phyllite(3) (Bg greywacke(5)
g ranule c o n g l o m e r a t e (6) b grey phyllite(7)
igneous clast •2000 conglomerate(8)
feldspathic greywacke(9) 1X1
siltstone-greywacke(lO) llZl
/ () V; meta-basalt, meta-andesite, ___ meta- crystalline tuff, V ' 0 and meta-volcanic wacke, L-±J ZTiWZ.: (11,12,13,14,15)
Tertiary volcanics and L£\| sediments
mylonite(16) |l|| = 1 ^
-C-1289 m m ...
__ m______. m » »•. m o«. , m M M . 1000
m &
x
SCHEMATIC STRATIGRAPHIC COLUMN AND J _ f Omefers y / (5 STRIP MAP / \ OF % m m U m EASTERN DOME ROCK MOUNTAINS /C, ’ 1 T ^-»J < ' * V » L r 4 ^ ^ >'»<*„ v-l /» A «-V V< « * r * *• *. C W . 1 . . ^ J (BASE MAP FROM U.S.G.S. CUNNINGHAM MOUNTAIN(7.5') AND TRIGO PEAKS (15') TOPOGRAPHIC QUADRANGLE MAPS) . a 4 y r . k . » V v ^ V A v ^ & V r » << •» V A .
miles The contact of unit G with units V-2 and T is shown in a separate figure, Thicknesses are approximate.
kilometers
Figure 5. Schematic Stratigraphic Column and Strip Map of the Eastern R. Stephen Marshak, M.S. Thesis, Department of Geosciences 1979 Flank of the Dome Rock Mountains hr i / ; '1 ! C\1 ' r. U— 1 7 RECONNAISSANCE GEOLOGIC MAP X V-/W - COPPER BOTTOM PASS AREA DOME ROCK MOUNTAINS, ARIZONA
EXPLANATION W s i m 7 Quaternary sediment V -'T s I# L. 40 Mesozoic calcareous sandstone,quartzite, -1 < / ? conglomerate, and maroon phyllite M My Ionite - / - Jk. , V-IT Mesozoic meta-crystalline tuff
- V-/W Mesozoic tan volcanic wacke r V-l' dark grey volcanic wocke
o 1 N 7 7 ) V-IT
4 Depositional contact . v T ilhxp--- ~ 7 ^ s \ Gradational contact High angle fault-, dashed where inferred, dotted where covered. Where known, ^ 7 . . •V • U, upthrown side; D, downthrown side Tectonic contact V Strike and dip of foliation - with trend of lineation j; ,# Strike of vertical foliation Strike and dip of bedding . < Macroscopic fold - strike and dip of s axial plane-, trend and plunge of fold c
• . axis shown with down fold profile 1 V-/' 7 •iV*’l Mesoscopic fold - trend and plunge of 47 ; 7 fold axis shown with down fold profile %?"" 25 V 47 «• „ - < ^ 5 / = 0 ^ " "A "xv^—>■. X, Km # 3# v —<" <; / s. Miles
\_ (Base from U S.(3 3. Cunningham Mountain 7.5' topographic quadrangle map)
Figure /4. Reconnaissance Geologic Map of the Copper R. Stephen Marshak, M.S. Thesis Bottom Pass Area, Dome Rock Mountains, Department of Geosciences I979 Arizona
0
2 ^
RECONNAISSANCE GEOLOGIC MAP OF THE SOUTHERN END OF THE DOME ROCK MOUNTAINS, ARIZONA EXPLANATION Q Quaternary unconsolidated sediment V-2 Mesozoic meta-andesite-i-meta-basalt
T Mid-Tertiary vole. + sedim. rocks V-2' Mesozoic meta-tuff
G Mesozoic siltstone +graywacke A Mesozoic quartz monzonite + aphte N ^ Contact Strike + dip of foliation ^ , High angle fault, dashed where ^ Strike + dip of bedding ) n \ / ' approximately located. U, upthrown V-2 k 0 VX side-, D, downthrown side. ( (Base from U.S.G.S. Trigo Peaks 15' 0 MILE ( 0 Km / ! AX # \ V ° topographic quadrangle map) \ I Figure 19. Reconnaissance Geologic Map of the Southern End R. Stephen Marshak, M. S. Thesis of the Dome Rock Mountains, Arizona Department of Geosciences 1979 0.
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