The depositional environment and petrographic analysis of the Lower Cretaceous Morita Formation, Bisbee Group, southeastern Arizona and northern Sonora, Mexico
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Authors Jamison, Kermit
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Link to Item http://hdl.handle.net/10150/557979 THE DEPOSITIONAL ENVIRONMENT AND PETROGRAPHIC ANALYSIS
OF THE LOWER CRETACEOUS MORITA FORMATION, BISBEE GROUP,
SOUTHEASTERN ARIZONA AND NORTHERN SONORA, MEXICO
by HERMIT JAMISON
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements For the Degree of
MASTER OF SCIENCE WITH A MAJOR IN GEOLOGY
In the Graudate College
THE UNIVERSITY OF ARIZONA
1 9 8 3 Call N o . BINDING INSTRUCTIONS INTERLIBRARY INSTRUCTIONS
Dept. i *9 7 9 1 Author: J ttm ilO Il, K e 1983 548 Title: RUSH______PERMABIND- PAMPHLET GIFT______COLOR: M.S. POCKET FOR MAP COVERS Front Both Special Instructions - Bindery or Repair PFFFPFKirF 3 /2 2 /8 5 Other------— .
r- STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfill ment of requirements 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 acknow ledgement 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 judgment the proposed use of the material is in the inter ests of scholarship. In all other instances, however, permission must be obtained from the author.
SIGNED:
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
WILLIAM R. DICKINSON 7 Date Professor of Geology ACKNOWLEDGEMENTS
I would like to.thank William R. Dickinson and
Joseph F. Schreiber, Jr., for their guidance and patience
in preparation of this thesis, Harald Drewes of the United
States Geological Survey for his advice and comments f and
Keith P. Young for his assistance in identification of
fossils. Special thanks go to Robert E. Warzeski for his
invaluable assistance for that portion of the study which was conducted "south of the border."
Grants from the Geological Society of America, the
University of Arizona Graduate School, and the Cities Service
Company helped defray research expenses.
iii TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS...... vi
LIST OF T A B L E S ...... xii
ABSTRACT...... xiii INTRODUCTION ...... 1
GEOLOGIC SETTING OF THE MORITA FORMATION ...... 3
Pre-Cretaceous Rocks ...... 8 Lower Cretaceous Rocks ...... 11 Post-Lower Cretaceous Rocks...... 16 Area of Study...... 21
THE CENTRAL MULE MOUNTAINS OF THE MORITA FORMATION...... 23
Sandstone Petrography...... 27 Key Horizons...... 30 Faulting...... 41
THE SOUTHERN MULE MOUNTAINS SECTION OF THE MORITA FORMATION ...... 45
Gold Hill Section...... 51 Black Knob Section...... 55
THE SIERRA ANIBACACHI SECTION OF THE MORITA F O R M A T I O N ...... 62
Lithology and Paleontology ...... 65 Faulting...... 79
NORTHERN EXPOSURES OF THE MORITA FORMATION ...... 89
Gadwell Canyon ...... 30 Dragoon Mountains...... 93 Steele Hills ...... 97
iv V
TABLE OF CONTENTS— Continued
Page
PETROGRAPHY OF THE MORITA FORMATION...... 99
Petrographic Analysis of Morita S a n d s t o n e s ...... 104 Diagenesis of Morita Sandstones ...... 121
DEPOSITIONAL ENVIRONMENT OF THE MORITA FORMATION. . . . 130
CONCLUSIONS...... 137
APPENDIX A: MODAL POINT COUNTS OF SAMPLED MORITA FORMATION SANDSTONES...... 140
APPENDIX B: MODAL POINT COUNTS OF SELECTED MORITA:ANDBISBEE FORMATION . SANDSTONES ...... _ ...... 142
APPENDIX C: POINT COUNTING PROCEDURES...... 144
REFERENCES...... 150 LIST OF ILLUSTRATIONS
Figure Page
1. Location of Chihuahua trough with respect to the Jurassic-Cretaceous domains in north western Mexico and southwestern U.S...... 4
2. Location of area of study in southeastern Arizona and northern Sonora...... 6
3. Major geographic features of southeastern Arizona and northern Sonora...... 7
4. Stratigraphic correlation of Lower Cretaceous rocks, Chihuahua trough...... 14
5. Paleogeographic diagram of rock units in northern Chihuahua trough...... 17
6. Geologic cross-section, Sierra Anibacachi, Sonora ...... 18
7. Columnar representation of Bisbee Group/ Formation rocks examined this report ..... 22
8. Lithologic description of Morita Formation, Mural Hill section, central Mule Mountains, Arizona ...... in pocket
9. Lithologic description of lower Morita Formation, Gold Hill section, southern Mule Mountains, Arizona...... in pocket
10. Litologic description of upper Morita Formation, Black Knob Ridge section, southern Mule Mountains, Arizona . . . .in pocket
11. Mural Hill, central Mule Mountains, near Bisbee, Arizona ...... 24
12. Cleavage development in a mudstone member of the lower Morita Formation, Mural Hill traverse, central Mule Mountains. . . . . 28
vi V i l
LIST OF ILLUSTRATIONS— Continued
Figure Page
13. Photomicrograph of typical subarkose, Morita Formation, Mural Hill Traverse, central Mule Mountains, A r i z o n a ...... 31
... ' ’ 14. Calcareous algal (?) structures from the - lower Morita Formation, Mural Hill section. Mule Mountains, Arizona ...... 33
15. Mollusca and echinoidea; fossils from the Lower Cretaceous Morita Formation ...... 35
16. Trigonid fossils of the Lower Cretaceous Morita Formation ...... 36
17. Septarian structures in concretions from the Morita Formation, Mural Hill section, Mule Mountains, Arizona ...... 38
18. "Pitted" texture on massive quartzarenite bed in the Morita Formation, Mural Hill section. Mule Mountains, Arizona...... 39
19. "Pitted" texture on quartzarenite b e d ...... 40
20. Diagrammatic cross-section of Bisbee Group in central Mule Mountains, Arizona...... 42
21. Geologic cross-section. Mural Hill, Mule Mountains, Arizona ...... 43
22. Eastward dipping Bisbee strata in central Mule Mountains, Arizona ...... 44
23. Structural divisions in Bisbee Quadrangle, Arizona ...... 46
24. Gold Hill from the northwest showing overthrust fault ...... 47
25. Gold Hill from the northwest, southern Mule Mountains, Arizona ...... 48
26. Geologic cross-sections through Gold Hill, southern Mule Mountains, Arizona...... 50 viii
LIST OF ILLUSTRATIONS— -Continued
Figure Page
27. Black Knob, southern Mule Mountains, Arizona, as viewed from the northwest ...... 52
28. Black Knob, southern Mule Mountains, Arizona, as viewed from the north ...... 53
29. "Pitted" texture on massive quartzarenite member of the Morita Formation, southern Mule Mountains, A r i z o n a ...... 56
30. Upper portion of "pitted" member in southern Mule - Mountains, Arizona...... 57
31. Geologic cross-section through Black Knob, southern Mule Mountains, Arizona ...... 58
32. Cerro la Morita, Sonora, as viewed from the starting point of the Sierra Anibacachi traverse...... 63
33. Unnamed peak in Sierra Anibacachi from the west ...... 64
34. Morita Formation strata as exposed in the Sierra Anibacachi, Sonora ...... 67
35. View to the south from a point high in the section in the Sierra Anibacachi, Sonora ...... •...... 68
36. Burrows in a sandstone member near the base of the Morita Formation, Sierra Anibacachi, Sonora ...... 69
37. Petrified wood from the Morita Formation, Sierra Anibacachi, Sonora...... 71
38. Oyster-laden biostrome, near Morita/lower Mural horizon. Sierra Anibacachi, Sonora ...... 72
39. Northern slope of "unnamed peak" in the Sierra Anibacachi, Sonora, where traverse terminated...... 73 ix
LIST OF ILLUSTRATIONS— Continued
Figure Page
40. Biostrome located in Sierra Anibacachi, Sonora, correlated with Perilla Member of Stoyanow1s Lowell Formation, now classified as lower Mural Limestone...... 76
41. View to the northeast in the Sierra Anibacachi, S o n o r a ...... 78
42. Discontinuous bedding attributed to low angle thrust faulting, in sandstone and mudstone sequence, Morita Formation, Sierra Anibacachi, Sonora...... 80
43. Subhorizontal sandstone member resting on more disturbed lower sandstone member, apparently as the result of low angle thrust faulting, Morita Formation, Sierra Anibacachi, Sonora ...... 81
44. Slickensides on thinly bedded sandstone member within the Glance Conglomerate near the stratigraphic contact between the Morita Formation and the Glance Conglomerate, Sierra Anibacachi, Sonora. . . . 83
45. Bedding-plane fault within massive limestone member of. upper Mural Limestone near the contact with the lower Mural Limestone, Sierra Anibacachi, Sonora...... 84
46. Brecciated zone along bedding-plane fault within massive limestone member of upper Mural Limestone near the lower Mural Limestone contact. Sierra Anibacachi, Sonora ...... 85
47. Montes Canova as viewed to the northeast from atop the Sierra Anibacachi, looking across the Caldaron Valley, Sonora ...... 86
48. Geologic map of area immediately east of Sierra Anibacachi, Sonora, Mexico...... 87
49. Gadwell Canyon area of northern Mule Mountains, Arizona, view to the southeast ...... 91 X
LIST OF ILLUSTRATIONS— Continued
Figure Page
50. Gadwell Canyon area of northern Mule Mountains, Arizona, view to the s o u t h ...... 92
51. Abandoned adit in Morita strata, Gadwell Canyon, northern Mule Mountains, Arizona...... 94
52. Geologic cross-section, Dragoon Mountains, Arizona...... 95
53. Q-F-L plot with provenance and maturity regimes for sandstones analyzed on basis of constituent grain framework ...... 102
54. Qm-F-Lt plot with provenance and maturity regimes for sandstones analyzed on basis of constituent grain framework...... 103
55. Calcite replacement of silica cement ...... 106
56. Calcite replacement of silica cement with incipient replacement of constituent quartz grains ...... 107
57. "Dust rims" on quartz grains ...... 108
58. Q-F-L plot, all study sites less Mural Hill. .. . 110
59. Q-F-L plot. Mural H i l l ...... Ill
60. Qm-F-Lt plot, all study sites less Mural Hill. . . 112
61. Qm-F-Lt plot-. Mural H i l l ...... 113
62. Photomicrograph of feldspathic graywacke ...... 115
63. Photomicrograph of volcanic lithic clast ...... 116
64. Generalized mineralogical constituents of igneous rocks ...... 119
65. Variation in mineralogic composition of sandstone as a function of grain size...... 122
66. Photomicrograph of chalcedonic quartz-lined vug...... 124 xi
LIST OF ILLUSTRATIONS— Continued
Figure Page
67. Calcite replacement of quartz and chert ...... 126
68. Dolomite rhombohedrons as diagenetic replacement of silica in a quartzarenite. . . . 127
69. Paleocurrent directions, Dragoon and Mule Mountains...... 131
70. Lithologic description of Morita Formation, Sierra Anibacachi section, Sonora, Mexico...... in - pocket
71. Photomicrograph of microcline (K-feldspar) grains in a subarkose...... 146 LIST OF TABLES
Table Page
1. Grain Parameters ...... 100
2.. Approximate Average Mineral Composition of Common Intrusive Rocks ...... 118
xii ABSTRACT
The Morita Formation of the Bisbee Group of south--
eastern Arizona is part of a thick clastic and limestone
sequence deposited during Lower Cretaceous Aptian time at
the northern end of the Chihuahua trough. The Morita
sediments were deposited conformably upon the Glance
Conglomerate and grade upward into the Mural Limestone.
The Morita Formation is a deltaic deposit that accumulated
.in marginal-marine and shallow marine environments. These
sediments were derived from the cratonic platform to the
north and constitute distal-fan and basin fill sediments.
Petrographic analyses of sandstones cropping out in
a north-south section from the Steele Hills to the Sierra
Anibacachi indicate that the sandstone beds generally had
a recycled orogenic provenance. A 200+ foot segment in the
upper Morita Formation at Mural Hill in the Mule Mountains
apparently had a magmatic arc provenance. Cross-bedding is
indicative of deltaic deposits of a southeasterly flowing
river.
x i n INTRODUCTION
The Morita Formation, along with the other consti tuent formations of the Lower Cretaceous Bisbee Group, was named by F . L. Ransome in 1904 for localities in and around the Mule Mountains of southeastern Arizona. According to
Ransome, the Morita Formation was named for the Morita
Hills, "lying just south of the international boundary, between longitude 109*45' and 109*50The Morita Hills have limited exposures of the Morita Formation as compared to more extensive exposures in the Mule Mountains to the north and the Sierra Anibacachi to the south. In a descrip tion of Upper Cretaceous sediments in the Cabullona basin farther south, Taliaferro (1933) reported a section of the
Morita Formation in excess of 5000 feet thick, considerably thicker than any section known in southern Arizona. Subse quent researchers (Hayes 1970a, 1970b? de Cserna 1970?
Rangin 1977, 1978a? Rangin and Cordoba 1977) have reported on the development of the Chihuahua trough and its sediments which have been dated on the basis of faunal assemblages as
Neocomian(?) to Albian in age.
The present work is an examination of the litholo
gies of the various outcrops of the Morita Formation and
equivalent facies in the Bisbee Formation, extending in a
1 line from the Steele Hills on the north, southward through the Dragoon Mountains, the Mule Mountains, and into the
Sierra Anibacachi in Sonora, Mexico. GEOLOGIC SETTING OF THE MORITA FORMATION
The Bisbee Group and its lithologic equivalents have long been recognized as a transgressional and re- gressional sequence of the Early Cretaceous sea, which extended northward from what now is geographically repre sented by the province of Chihuahua, Mexico (Figure 1).
The Chihuahua trough, itself a northern extension of the
Late Jurassic and Early Cretaceous Mexican Geosyncline, has been regarded as the westernmost extension of the
"proto-Gulf of Mexico," and represents a major sedimentary basin of Early Cretaceous time. The lowest member of the group, the Glance Conglomerate, has been correlated with an ill-defined Middle Mesozoic tectonic event which pro duced a widespread terrigenous clastic sequence of varying thickness and lithology (Cordoba et al. 1971; Titley 1976;
Bilodeau 1979). Conformably overlying this basal conglomer ate, variegated beds of mudstone with intercalated lenses of sandstone and limestone have been assigned to the
Morita Formation. The uppermost members of the Morita
Formation grade imperceptibly into similar lithologies of
the lower Mural Limestone. The lower Mural Limestone
consists of increasingly more calcareous members, and
the boundary between the lower Mural Limestone and the
3 4
□ PALEO-OCEANIC TERRANE I------1 LOWER AND MIDDLE CRETACEOUS VOLCANIC-PLUTONIC I------1 COMPLEX I I UPPER JURASSIC VOLCANIC-PLUTONIC COMPLEX I------, LOWER (?) MIDDLE JURASSIC VOLCANIC PLUTONIC ■ 3 COMPLEX | 1 APTIAN-ALBIAN EXTERNAL SEDIMENTARY BASIN
| | COASTAL BASEMENT
n EXTERNAL PLATFORM JURASSIC-CRETACEOUS DOMAINS IN NORTHWESTERN MEXICO AND SOUTHWESTERN U.S. after Rangin, 1978b
Figure 1. Location of Chihuahua trough with respect to the Jurassic-Cretaceous domains in northwestern Mexico and southwestern U.S. 5 upper Mural Limestone is marked by a distinct faunal change
(Ransome 1904a, 1904b; Scott 1979). The upper Mural Lime stone is a highly resistant, ridge-forming member of a massive micritic limestone that locally is represented by patch reefs characterized as coral-algal-rudist reefs
(Scott and Brenckle 1977; Scott 1979) . The upper Mural
Limestone represents the maximum transgressive phase of the Early Cretaceous sea; it is overlain by the Cintura
Formation, a regressive sequence of mudstone with inter calated sandstone and limestone lenses. The rocks of the
Cintura Formation are lithologically indistinguishable from those of the Morita Formation, and represent the transition from marine to terrigenous deposition. However, the deposits superjacent to the Cintura Formation have everywhere been removed by subsequent erosion, and the next chronological sequence of rocks found in the study area (Figure 2) in cludes the Upper Cretaceous Cabullona Group in Sonora,
Mexico, of Turonian(?)-Senonian age, as well as the
Sugarloaf Quartz Latite of the Dragoon Mountains, which has been dated isotopically as Campanian (Marvin et al.
1973). Thus, following the deposition of the Bisbee Group,
there exists a significant hiatus which spans, at a minimum,
all of the Cenomanian. In localities where the Mural Limestone is not
present, such as the Steele Hills, the Dragoon Mountains,
and the nearby Tombstone Hills (Figure 3), the Bisbee Group ARIZONA
PHOENIX
TUCSON
ISBEE
SONORA
HERMOSILLO
Figure 2. Location of area of study in southeastern Arizona and northern Sonora. m ° w 110°45'W > |Z OUTCROP LOCALITIES: 1. Foothills of Steele Hills 2. Foothills of Dragoon Mts 3. Gadswell Canyon 4. Mural Hill 5. Gold Hill 6. Black Knob 7. Sierra Anibacachl h32°N
c Chlrlcahua Mts. CH Canelo Hills D Dragoon Mts. • DC Dos Cabezas Mts. E Empire Mts GA Galliuro Mts. GH Gunnison Hills GU Guadalupe Mts. H Huachuca Mts. LD Little Dragoon Mts. JLH Johnny Lyon Hills ML Mule Mts. MU Mustang Mts. PA Patagonia Mts. PE Pedregosa Mts. R ' Rincon Mts. RBH Red Bird Hills S Swisshelm Mts. SH Steele Hills SR Santa Rita Mts. TH Tombstone Hills W Winchester Mts. WH Whetstone Mts. WP Willcox Playa CC Cerro Cabullona SC Sra. Cenlza SM Sra. Magallanes CM Cerro la Morita SDC Sra. de Cananea SMO Sra. Madre Occidental CP Cerro Prieto SDM Sra. del Manzanal SSA Sra. San Antonio SA Sra. Anibacachl SOP Sra. de Pintos SSJ Sra. San Jose Figure 3. Major geographic features of southeastern Arizona and northern Sonora. 8 has been referred to as the Bisbee Formation. However, a few marine fossils were identified by Gilluly (1956, p. 78) in the lower exposures of the Dragoon Mountain section, suggesting that the Bisbee Formation in the Dragoon Mountains is equivalent to the Morita Formation of the Bisbee Group.
The geologic evidence in the Steele Hills is less conclusive, and the strata at that locality are assumed to be equivalents of the Morita Formation based upon regional characteristics of strike and orientation, and the relative paucity of
Cintura Formation rocks within southeastern Arizona.
The following lithologic descriptions of the geologic units in the area are taken in part from Taliaferro (1933),
Hayes (1970a, 1970b), and Bilodeau (1979).
Pre-Cretaceous Rocks
The oldest rocks in southeastern Arizona and northern
Sonora are the metasedimentary and metavolcanic rocks of the
Early Precambrian Pinal Schist. The Pinal Schist is widely
distributed in southern Arizona and northern Sonora, and is
locally intruded by plutonic rocks also of Precambrian age
(Wilson 1962). Silver (1978) places the metamorphism and
major orogenic deformation of the Pinal Schist at 1625-
1680 m.y.b.p. Depositionally overlying this crystalline
basement are the Late Precambrian clastic sedimentary and
volcanic rocks of the Apache Group. In the study area, the
Apache Group rocks are restricted to the Little Dragoon
Mountains, though they occur more widely to the north 9
(Cooper and Silver 1964) . Silver (1978) dates the Apache
Group strata as 1100-1420 m.y. old.
The Paleozoic section is composed of 1500-2100 m
(5000-7000 feet) of sedimentary rock deposited primarily in a late Paleozoic structural basin variously called the
Sonoran Geosyncline (Wilson 1962), the Sonoran Embayment
(Titley 1976), or the Pedregosa Basin (Peirce 1976;
Greenwood, Kottlowski, and Thompson 1977) . Epeirogenic fluctuations caused repeated transgressions and withdrawals of marine waters with widespread deposition of Cambrian,
Devonian, Mississippian, Pennsylvanian, and Permian strata throughout the region. These rocks are primarily shallow-
shelf marine carbonates with some shale and sandstone.
Silurian strata are not exposed anywhere in southeastern
Arizona or northern Sonora, whereas Ordovician rocks are
rare and restricted to the mountain ranges to the east of
the study area (Pye 1959; Wilson 1962; Hayes and Landis
1964; Peirce 1976). Several disconformities, caused by the
epeirogenic movements, occur within the stratigraphic sec
tion.
Upper (?) Triassic and Lower Jurassic strata of
southeastern Arizona and northeastern Sonora rest uncon-
formably on the Paleozoic marine rocks and largely consist
of volcanic tuffs, flows, and breccias with subordinate
amounts of sedimentary redbeds; they are generally restric
ted to the region west of the study area. These strata have 10 a combined thickness of over 3000 m with a distribution that suggests deposition in linear fault-controlled basins (Hayes,
Simons, and Raup 1965; Hayes and Drewes 1968; Drewes 1971) .
In the area under study, the Walnut Gap Volcanics crop out in the Gunnison Hills, which lie between the Steele Hills to the north and the Dragoon Mountains to the south. No isotopic age dates have been obtained from these volcanic rocks to date, and their age can only be approximated stratigraphically. The volcanic rocks lie disconformably upon the Lower Permian Naco Group and are overlain discon formably by the Lower Cretaceous Glance Conglomerate. In northern Sonora no equivalent Triassic and Jurassic rocks are found, but farther south in central Sonora the Triassic is represented by the Barranca Group, a thick sequence of red sandstones with intercalated siltstones and coal (Rangin 1978a).
Within the study area, the Jurassic Period is repre sented by relatively large plutons of the Juniper Flat
Granite in the Mule Mountains and the Gleeson Quartz
Monzonite in the Dragoon Mountains. More restricted out crops of quartz monzonite and granite occur in the Dragoon
Mountains and are thought to be genetically related to the larger Jurassic plutons (Gilluly 1956). In Sonora Jurassic volcanic and sedimentary sequences crop out over a broad area from Sierra Azul (west of Nogales, Sonora) southeast ward to southern Sonora (Figure 1). The deposits lie 11 outside the immediate study area, and consist of andesitic and rhyolitic graywackes with intercalated limestones which contain ammonites of Liassic to Oxfordian age (Rangin and
Cordoba 1976; Rangin 1978a). These volcanic rocks are part of the continental margin magmatic arc that trended west- northwest across southern Arizona in Late Triassic through
Early Jurassic time (Coney 1978).
Lower Cretaceous Rocks
The Bisbee Group consists of both marine and non marine rocks which were first described by Durable (1902), who divided the sequence at Bisbee into four stratigraphic units without giving them formal names. In 1904 Ransome formally identified these rocks as the Bisbee Group, and redefined Durable's stratigraphic units as four separate formations: Glance Conglomerate, Morita Formation, Mural
Limestone and Cintura Formation. Three of the formations, the Glance Conglomerate, the Mural Limestone, and the Cintura
Formation, were named for localities in the Mule Mountains, whereas the Morita Formation was named for the Morita Hills just south of the international border. Ransome failed to designate type sections for these formations; however, sub sequent workers have defined principal reference sections for the various formations, with Gilluly1s (1956) Mural Hill section and Stoyanow's (1949) Black Knob section being two.
In 1949 Stoyanow defined another unit, the Lowell Formation, 12 primarily based upon a faunal assemblage which was well represented in the Ninety One Hills area a short distance to the south of the Mule Mountains. However, this nomen clature is not in current use, and the names assigned by
Ransome stand today as the accepted nomenclature for the
Bisbee Group and its formations.
Outcrops of the Morita Formation are far less exten sive in the Morita Hills than in the Mule Mountains to the north, in the Sierra Anibacachi to the south, and in the
Huachuca Mountains to the west, where continuous sections have been reported (Gilluly 1956; Taliaferro 1933? Hayes
1970a). Fossil evidence within the Morita Formation and the overlying Mural Limestone indicates a Comanchean age, corre lative with the upper part of the Trinity of Texas. With the exception of the Glance Conglomerate, the various sub divisions of the Bisbee Group are not recognized outside of the Bisbee area, principally because the Mural Limestone wedges out rapidly to the north, and the previously stated
similarity of the Morita and Cintura formations make them
indistinguishable with the absence of the Mural Limestone.
The east-west facies contrasts within the Bisbee Group are
even more marked than those in the north-south dimension.
To the west in south-central Arizona the massive and rela
tively pure limestones of marine origin grade rapidly into
thinly bedded silty or argillaceous limestone, and the
sandstones become increasingly more arkosic. In the Whetstone 13
Mountains to the west of the study area, Tyrrell (1957) has divided the Bisbee Group into five formations whose lithologies can be traced westward into the Empire and Santa
Rita Mountains. To the east in the Chiricahua, Peloncillo,
Little Hatchet and Big Hatchet Mountains of southeastern
Arizona and southwestern New Mexico, the Bisbee Group retains a middle calcareous member, but the basal Glance Conglomerate disappears. In New Mexico and south of the border in Sonora, the Lower Cretaceous facies of the Bisbee Group is generally more calcareous and frequently lacks a basal conglomerate.
Zeller (1965, 1970) divided the Lower Cretaceous rocks of
New Mexico into three formations: Hell-to-Finish, U-Bar, and
Mojado formations (from older to younger). In Sonora and
Chihuahua a variety of names have been assigned to the Lower
Cretaceous rocks (Figure 4). However, in northeastern
Sonora in the area immediately south of the Mule Mountains, the four lithologic divisions of the Bisbee Group identified by Ransome are recognizable, and throughout this area
(generally referred to as the Cabullona basin) Taliaferro
(1933) and subsequent workers have retained the same nomen clature as those used in the Mule Mountains to the north.
The thickness of the Bisbee Group/Formation is extremely variable owing to the great local relief of the underlying surface and to the variable amounts of subsequent erosion. In Arizona, it attains a maximum thickness of about 15,000 feet (4500 m) in the Empire and the Dragoon Figure 4.Figure SERIES STAGE LOWER CRETACEOUS #. 3 .7 *# no? - - 105? - - - 105? © Chihuahua trough. Chihuahua Stratigraphic correlation of lower Cretaceous rocks, Cretaceous lower of correlation Stratigraphic MYBP - F L RNOE 1*04 0 * 1 RANSOME, F. L. - © . . 1*37 3 * 1 , L L E R Y T W. W. - © .. ODB, . ORQE TR , N J #UROSRI, l»7l UlRRO-SARCIA, # AND J. E S, TORR - RODRIQUEZ R. CORDOBA, # 7 * 1 D.A. - Z, LE A Z 0N 6 M. C. © - © - VN NT! TNAD l # 7 l# STANDARD, ! T IN H VAN - © AIMTI SAE O TE ETR ITRO O NRH MRC,- E# KAUFFMAN, l#77 E.#. AMERICA,- NORTH OF INTERIOR WESTERN THE FOR SCALE RADIOMETRIC - Q
H 15
Mountains, but rarely exceeds 10,000 feet (3000 m) in thick
ness in other areas (Hayes 1970b). The section thickens
southward into Sonora where the Cintura Formation is not
present. In the Cabullona basin it attains a maximum thick
ness of about 8200 feet (Taliaferro 1933, p. 19).
The basal Glance Conglomerate is extremely variable
in thickness as well as in composition, with clasts varying
in size and lithology. Clast lithology has been demonstrated
(Hayes 1970a; Bilodeau 1979) to vary with the composition of
the underlying Precambrian or Paleozoic rocks in the imme
diate area of deposition, thus implying short transportation
distances. The Morita Formation represents a transition
from terrestrial to marine deposition, and is composed of
fluvial and deltaic sandstone and siltstone with an increas
ing amount of minor limestone near its upper contact. The
Mural Limestone is a relatively massively-bedded, fossili-
ferous, marine shelf limestone unit which weathers to promi
nent topographic ridges in areas where it is particularly
thick and massive. Conformably above the Mural, the marine
siltstone and mudstone of the Cintura Formation grades
upward into non-marine deltaic and fluvial sandstone nearly
impossible to distinguish from those in the Morita Formation.
The top of the Cintura Formation was deemed a "surface of
Quaternary erosion" by Ransome (1904b), and the original
thickness of the regressive phase cannot be ascertained.
This total sequence of strata represents the transgression 16 and regression of a shallow marine sea during Aptian-Albian time. This sea entered Arizona from the southeast (Figures
1 and 5) where it had existed in Mexico as a northwest trending linear trough since the Late Jurassic (Cordoba et al. 1971; Hayes 1970b; Rangin 1978a, 1978b).
Post-Lower Cretaceous Rocks
Upper Cretaceous rocks are found within the study area only in the Cabullona basin in Sonora, Mexico, and in the vicinity of the Dragoon Mountains. In Sonora, Taliaferro
\ (1933) described a conformable sequence of detrital sediments with minor limestones and volcanic ash measuring in excess of 8500 feet in thickness. This Cabullona Group, as he called it, comprised five lithologically distinct formations which, in ascending order, were called: the Snake Ridge
Formation, the Camas Sandstone, the Packard Shales, the upper red beds, and an unnamed rhyolite tuff. These Upper
Cretaceous rocks are not observed in direct contact with the
Lower Cretaceous Bisbee Group, but Taliaferro (1933) and
Rangin (1977) (Figure 6) have depicted this contact to be an angular unconformity with thin fault slivers of Precambrian and Lower Cretaceous rocks separating these two major sedi mentary sequences. On the basis of some dinosaur remains found in the Snake Ridge Formation, Taliaferro estimated the age to be Senonian or possibly Turonian. Hayes (1970b) correlated the Cabullona Group with the Upper Cretaceous AREA OF STUDY
Figure 5. Paleogeographic diagram of rock units in northern Chihuahua trough, after Rangin, (1978b).
Color coding same as Figure 1. 18
SIERRA ANIBACACHI, SONORA
—AFTER C. RANGIN, 1977
A \ uKc ^ Kg _Kmu NE
Figure 6. 19
Fort Crittenden Formation in the Santa Rita and Huachuca
Mountains in Arizona, which is presumed to be of early
Campanian age. On the basis of palynological studies on the basal part and the middle of the lower part of the
Cabullona Group, Almeida (Almeida and Martinez 1978) ob
tained a Late Cretaceous (pre-Maastrichtian) age.
In the Dragoon Mountains, outcrops of highly altered
rocks were mapped by Gilluly as the Sugarloaf Quartz Latite.
Marvin et al. (1973) obtained a K-Ar isotopic age of 7.45 ±
2.9 m.y. on biotite from a welded tuff within the Sugarloaf
Quartz Latite.
To the west of the study area, in the mountain ranges
of south-central Arizona, the Upper Cretaceous is represented
by the conglomeratic and volcanic rocks of the Fort
Crittenden, Fort Buchanan, and Salero Formations. On the
western margin of the study area in the Tombstone Hills, the
Schieffelin Granodiorite and the Uncle Sam quartz latite
tuff have been isotopically dated at, respectively, 72 m.y.
(Creasy and Kistler 1962) and 7.19 ± 2.4 m.y. (Newell 1974).
These two rock units were originally mapped as lithologically
different entities; however, recent investigation now
supports the possibility that they are differentiates of the
same magma (Devere 1978) . Gilluly (1956) tentatively
assigned the nearby Bronco Volcanics to the Upper Cretaceous
on the basis of their stratigraphic relationship to the
underlying Bisbee Formation. ' 20
The Tertiary rocks within the study area consist principally of a number of plutonic and volcanic rocks and
extensive continental deposits, predominantly coarse clastic
basin fills. The major basins in the area everywhere con
tain a Quaternary alluvium cover; Quaternary units generally
rest directly upon Tertiary basin fill of Miocene and
Pliocene age. In the north, the Texas Canyon Quartz
Monzonite, which crops out extensively in the Little Dragoon
Mountains immediately to the west of the study area, has
produced numerous Eocene isotopic dates (Livingston et al.
1967; Marvin et al. 1973). In the Steele Hills, the Galiuro
Volcanics and the associated Three Links Conglomerate, both
mid-Tertiary in age, disconformably overlie the Bisbee
Formation. In the northern part of the Dragoon Mountains,
the Stronghold Granite has been dated as Oligocene (Marvin
et al. 1973; Damon and Bikerman 1964). In the southern
portion of the Dragoon Mountains, the SO Volcanics have been
dated as Eocene (Marvin et al. 1973). To the east of the
Dragoon Mountains, a group of isolated outcrops, collectively
known as the Pearce Volcanics, protrudes through the allu
vium of the Sulphur Spring Valley. These rhyolitic and
andesitic rocks are thought to be Tertiary, but no isotopic
dates have been obtained to date. The Pat Hills, a few
miles to the northeast of the Pearce Volcanics, are mainly
comprised of an andesite which has been dated as Oligocene
(Drewes 1980). 21
South of the Dragoon Mountains in the study area.
Tertiary rocks are restricted to alluvial basin fill. Wide
spread Tertiary volcanics are encountered farther south in
Mexico. The Baucarit Formation occurring in the Arizpe area
and elsewhere consists of Miocene (?) intercalated basalt
flows and conglomerate (Gonzalez 1978) . At Cananea (northern
Sonora) 5000 meters of Eocene volcanics have been assigned
to the Elenita, Henrietta, and la Mesa formations by
Valentine (1936).
Area of Study
Field work was conducted in seven outcrop localities
in southeastern Arizona and northeastern Sonora (Figures 3
and 7). These localities represent a north-south section
across the northern end of the Chihuahua trough (Figure 5).
These localities are, from north to south: 1) Steele Hills,
2) Dragoon Mountains, 3) Gadwell Canyon (northern Mule
Mountains), 4) Mural Hill (central Mule Mountains), 5) Gold
Hill (southern Mule Mountains), 6) Black Knob (southern
Mule Mountains), and 7) Sierra Anibacachi (Figure 3).
Detailed lithological descriptions of the sections from
Mural Hill, Gold Hill, and Black Knob are found in Figures
8, 9, and 10 (in pocket). STEELE DRAGOON GADWELL MURAL GOLD BLACK SIERRA HILLS MTS CANYON HILL HILL KNOB ANIBACACHI s
CINTURA FM
4000 MURAL LS 1000" 3000 ? BVV ^ — VOVJ 2000 t BISBEE 500- FM MORITA FM —— 1000 - — METERS -1 FEET = : 4 * » < GLANCE CONGLOMERATE
Figure 7. Columnar representation of Bisbee Group/Formation rocks examined this report
N) fo THE CENTRAL MULE MOUNTAINS SECTION OF THE MORITA FORMATION
The best known and first described section of the
Morita Formation is the section exposed along a ridgeline extending from the Winwood Addition of the town of Bisbee northeastward towards the crest of Mural Hill near the eastern margin of the Mule Mountains (Figure 11). It was here that Ransome (1904b, p. 65) defined the upper boundary
of the Morita Formation as being "the upper surface of a bed
of hard buff sandstone or quartzite, which outcrops...in
such a manner as to find topographic expression in a series
of little bench-like spurs...." This "hard buff sandstone"
is actually a massive quartzarenite (Figure 8) 293 feet
thick at Mural Hill. Although prominent at this locality,
it is evidently of limited geographic extent and topographic
expression (Stoyanow 1949, p. 25; Gilluly 1956, p. 73; this
report). Subsequent workers have agreed that the contact
between the Morita Formation and the overlying lower Mural
Limestone as defined by Ransome is not an abrupt lithologic
break, but is rather a transitional zone in which limestone
beds become more abundant and sandstone beds become less
abundant, more calcareous, and less resistant. Ransome also
recognized this boundary to be transitional ("The divisional
plane marks practically the upper limit of the sandstones
23 Figure 11. Mural Hill, central Mule Mountains, near Bisbee, Arizona 25 and red shales, although...it does not. define the lowest appearance of fossiliferous limestones."' (Ransome 1904b, p. 65). Using this relative abundance criterion in the
Huachuca Mountains, Hayes (1970a) was able to define a
"close approximation" of the boundary between the two formations.
Ransome (1904a, p. 5; 1904b, p. 64) reported the
Morita Formation to be 1800 feet thick north and east of
Bisbee, with the beds thickening to the south. Hayes
(1970a) designated the Mural Hill section as the principal reference section for the Morita Formation, and he obtained a measurement of 2605 feet (855 meters) at this locality.
A thickness of 2708 feet was obtained for this section during this study, which correlates reasonably well with that obtained by Hayes. The difference in the measured thickness obtained by Hayes and by the author may be attri buted in part to the irregular surface of the basal Glance
Conglomerate and to the lack of a precise starting point for the measured section. The ridge on which the section was measured flattens south of the Winwood Addition, and a
starting point for this study was selected to produce a maximum measured thickness. This point occurs in a
"paleotrough" of the underlying Glance Conglomerate.
In 1949, Stoyanow described a type section for his
newly designated Lowell Formation (equivalent to the
fossiliferous upper Morita Formation and the lower Mural 26
Limestone) in the Ninety One Hills Area, south of the Mule
Mountains near the international border. Though his desig nation of the Lowell Formation is no longer accepted as a constituent member of the Bisbee Group, his description of the Mural Hill Ridge section was of considerable help in establishing the facies relationships among the various sections studied.
The lithology of the Morita Formation as described by Hayes (1970a, p. A15-A17) is consistent in most respects with that observed at Mural Hill in this study:
The Morita formation is made up almost entirely of pinkish-gray to pale-red [occasionally] felds- pathic sandstones and grayish-red siltstones and mudstones? pebble conglomerate, greenish-gray clay- stone, and impure limestone are minor constituents. Typically, beds of the coarsest sandstone rest with sharp scour-and-fill contact on underlying units of siltstone or mudstone and grade upward into finer sandstone, thence into siltstone and mudstone. Al though this pattern varies, it seems to reflect a crudely cyclic or, at least, repetitious sequence of deposition. Beds of fine siltstone to mudstone make up slightly more than half of the formation in the Mule Mountains..., but sandstone is dominant in the upper several hundred feet....Thin units of greenish-gray claystone are found throughout the formation in the Mule Mountains but seem to be most common in the middle part....The few thin beds of impure limestone occur only in the top 600 feet (180 m) ....Thin beds of pebble conglomerate are locally present in the basal few hundred feet....
The coarser sandstone beds in the formation are also the most resistant and the lightest colored. On fresh fracture, they are mostly pinkish gray but range to pale red and grayish red purple; weathered outcrops range from moderate yellowish brown to yellowish gray. Beds are generally a foot to several feet thick, but very thin or massive beds are also found. Most sandstone is thinly laminated and much is cross-laminated. Texturally, the coarser sandstone 27
is, in general, moderately well sorted and medium grained....The grains are mostly subrounded. Petrified wood is locally present in abundance in some of the coarser sandstone beds.
The very fine grained sandstone, which typically grades downward into the coarser sandstone and usually grades upward into siltstone, is less resis tant and darker in color than the coarser sandstone. Most of it is grayish red, but some beds are grayish red purple and greenish gray. Bedding is obscure, and laminations are not evident. Textural sorting does not differ systematically from that in - the coarser sandstone, but the grains tend to be sub- angular to angular.
The dominant fine siltstone and mudstone of the Morita are characterized by their distinctively and uniformly grayish red color on fresh and weathered surfaces and by their lack of well-defined bedding or conspicuous sedimentary structures. Closely spaced joint planes that have no relation to the faint bedding are so common that determination of bedding attitudes is very difficult where interbeds of other lithologies are absent (Figure 12). These rocks were only studied megascopically, but they appear to range from muddy siltstone to silty mud stone. Clay minerals are probably dominant, but quartz and other minerals apparently are important constituents. Irregular subspherical white calcitic concretionary nodules are conspicuous in some silt stone and mudstone units.
Beds of greenish-gray calcareous or dolomitic claystone make up only a small fraction of the Morita but may be of significance in regional correlations. This claystone, like the grayish-red mudstone, lacks conspicuous bedding or other sedimentary structures....
Thin beds of medium gray sandy and argillaceous oyster-bearing limestone occur in the upper part of the Morita...[this limestone] typically weathers to a mottled light gray and medium gray....
Sandstone Petrography
Whereas there is a remarkable degree of similarity between the petrographic grain analyses of sandstone members 28
Figure 12. Cleavage development in a mudstone member of the lower Morita Formation, Mural Hill traverse, central Mule Mountains. 29 conducted by Hayes and those reported in this study, there is one striking difference: the ratio of plagioclase to total feldspar.
Hayes conducted microscopic analyses of 22 samples of sandstone from the Morita Formation at Mural Hill by thin- section point counting. His results showed that quartz constitutes 57 to 91 percent of 24 of 27 samples, and 5 to
13 percent of the remaining three samples; feldspar consti tutes l.*5 to 40 percent of the samples studied and averages about 11 percent. A similar microscopic analysis of 20
sandstone samples collected from Mural Hill in this study
shows that quartz constitutes 63 to 94 percent of 17 of the
20 samples, and 5 to 11 percent of the remaining three
samples; feldspar constitutes from 1.5 to 30 percent of the
samples studied and averages about 13 percent. The close
similarity of these two independent analyses is corrobora
tive of the results, particularly since the two suites of
sample sandstones were collected independently of each other.
However, whereas Hayes reported an average of about
20 percent for the ratio of plagioclase to total feldspar,
this study arrived at a much higher percentage of plagio
clase, specifically, 97.5 percent. This is an anomalously
large percentage; however, the thin-sections used in this
study were stained for plagioclase and K-feldspar by two
independent laboratories with similar results. 30
The average sandstone (Figure 13) and the dominant sandstone type is subarkose or feldspathic sandstone, as defined by Pettijohn (1957), and subarkose as defined by
McBride (1963) and Folk (1968) . A small percentage is orthoquartzite (Pettijohn 1957) or quartzarenite (McBride
1963; Folk 1968). Another small percentage of the sand stone is feldspathic graywacke (Pettijohn 1957) or feldspathic litharenite (McBride 1963; Folk 1968).
Key Horizons
Fossils were found at two separate horizons within the section. At the 210 foot horizon (Figure 8) a biostrome of calcareous algae (?) was noted near the base of a non- calcareous mudstone bed (Figure 14). The algal (?) clusters were devoid of internal structure and exhibited few external features. The bulbous morphological features and the anas tomosing structures by which the colonies attached to the substrate are indicative of algal structures. Similar structures were reported by Schafroth (1965) in the Apache
Canyon and Shelleburg Canyon formations in the Empire
Mountains.
At the 2400 foot horizon, a micritic limestone member contains a biostrome of fragmental oysters (Ostrea sp. and Exogyra sp.), with lesser amounts of pseudo quadrate Trigonids (Figures 15 and 16) and gastropods. a. unpolarized light
b. polarized light
Figure 13. Photomicrograph of typical subarkose, Morita Formation, Mural Hill Traverse, central Mule Mountains, Arizona (x 32). 32
c. polarized light with gypsum plate
Figure 13— Continued 33
a. In situ occurrence of algal (?) structures
b . Detailed view of algal (?) structure
Figure 14. Calcareous algal (?) structures from the lower Morita Formation, Mural Hill section. Mule Mountains, Arizona. 34
Figure 14— Continued Figure 15. Mullusca and echinoidea; fossils from the Lower Cretaceous Morita Formation. a - c , Crassostrea sp., (X 0.3); d, sp. cf. Chlamys thompsoni Stoyanow, (X 0.4); e, Pecten (Hinnites?) sp (X 0.4); f, Idonearca stephensoni Stoyanow, (X 0.4); g, Hemiaster sp., (X 0.5), h, i, Acanthoplites sp. cf berkeyi Stoyanow, (X 1) 36
Figure 16. Trigonid fossils of the Lower Cretaceous Morita Formation
a - d , Trigonia mearnsi, (X .25); e - i , Trigonia stolleyi, (X 0.5); j, Trigonia (?) sp. (X 0.4) 37 At the 2305 foot horizon,'a dense sequence of cal careous concretions occurs in a variegated red and green calcareous mudstone (Figure 17). Although similar concre tions exist elsewhere in the formation, these are distinc tive in that they attain a maximum diameter of one foot and
frequently exhibit septarian structures.
Still another unusual characteristic of the Mural
Hill section is a distinctive weathering texture of two of
the massive sandstone beds. These beds, encountered at
the 936 foot and 1595 foot horizons, consist of a relatively t pure quartzarenite, or othoquartzite, which exhibits an
unusual pock-marked or "pitted" texture on the weathered
surfaces (Figures 18 and 19). These pits are of a rather
uniform size (0.5 - 1.0 cm) and of variable density, the
density being relatively constant throughout a stratigraphic
horizon. Microscopic examination of thin-sections of these
members indicates systematic disaggregation of the quartz
grains due to a selective dissociation of the silica cement.
This dissociation is apparently a weathering effect, inas
much as the pitted texture of these strata seldom exceeds
more than a centimeter in depth from the surface of the
exposed face. These structures are extremely persistent
laterally, and are useful in the correlation of beds from
one locality to another. 38
Figure 17. Septarian structures in concretions from the Morita Formation, Mural Hill section, Mule Mountains, Arizona (X 2). 39
« -f
■s 4 '
> Figure 18. "Pitted" texture on massive quartzarenite bed in the Morita Formation, Mural Hill section, Mule Mountains, Arizona. :--:
40 41 Faulting
Ransome (1904a, 1904b) has made reference to the nearly vertical normal faulting (Figures 20, 21 and 22) in this portion of the Mule Mountains. This faulting, which was recognized by Ransome as post-Cretaceous, is now deemed to be Late Tertiary (early Pliocene; Gilluly 1956) normal faulting due to regional extension within the Basin and
Range province. However, no mention has been made in the literature to low angle thrust faulting, or bedding plane faulting. Such apparent faulting was evident within the
Mural Hill section at a recent road cut, and may exist elsewhere in those segments which are partially covered.
Slickensides were observed at the outcrop, but their orientation was not recorded. Mr. Richard Graham, pre viously a geologist for Phelps Dodge Corporation in the
Bisbee area, stated (personal communication) that the entire
.. sequence of Paleozoic, as well as Mesozoic sediments in the
Mule Mountains, is rife with low angle reverse and bedding plane faults. Such faulting in the central Mule.Mountains is consistent with observations of similar low angle/bedding plane faulting observed in the northern and southern Mule
Mountains as well as the Sierra Anibacachi. f Clnturo Formation
Upper Mural Limestone Lower Lower Mural Limestone Cret. Diagrammatic Rendering ~r - Mori*a Formation Geologic Section
Glance Conglomerate Mule Mountains Bisbee/Arizona Precambrionl Pinal Schist
Figure 20. Diagrammatic cross-section of Bisbee Group in central Mule 4* Mountains, Arizona ro MURAL HILL
-AFTER HAYES AND LANDIS, 1964
Figure 21. Geologic cross-section, Mural Hill, Mule Mountains, Arizona 44 f
Figure 22. Eastward dipping Bisbee strata in central Mule Mountains, Arizona.
Mural limestone separates overlying Cintura Formation from lithologically identical Morita Formation. Note displace ment along near-vertical normal fault near center of photograph. THE SOUTHERN MULE MOUNTAINS SECTION OF THE MORITA FORMATION
The Lower Cretaceous sediments exposed on the eastern flank of the Mule Mountains comprise the northeast limb of a broad anticlinal structure centered near the town of Bisbee.
Proceeding southeastward along the axis of this anticline, the Cretaceous tract, which forms the northeast limb of the anticline, becomes restricted by the Gold Hill block (Ransome
1904a, 1904b) on the southwest and the alluvium of the Sulphur
Spring Valley on the northeast (Figure 23). This southern extension of the Cretaceous tract essentially constitutes a continuation of the stratigraphic exposures of the Bisbee
Group as they exist in the central Mule Mountains. However, the contact with the Gold Hill block constitutes a reverse fault, which was designated by Ransome (1904a, 1904b) as the
Gold Hill overthrust fault. Locally the Bisbee strata are upended and, in places, overturned along the fault (Figures
24 and 25), and, at the surface, displaced Paleozoic strata overlie Cretaceous strata.
The nature of the Gold Hill reverse fault has been debated by various researchers throughout the years. Ransome
(1904a, 1904b) described the Gold Hill block as an overthrust to the northeast, and portrayed the thrust fault as probably
45 B1SBEE Mural Hill
GLANCE BLOCK
GOLD HILL ESPINAL PLAIN BLOCK
NACO ARIZONA SONORA Figure 23. Sturctural divisions in Bisbee Quadrangle Arizona, after Ransome (1904b) 47
A GOLD MILL FROM THE NORTHWEST SHOWING OVERTHRUST FAULT.
Figure 24. Gold Hill from the northwest showing over thrust fault, Ransome, 1904a, b .
Paleozoic limestones (Gold Hill) have been thrust east ward over lower Cretaceous Glance Conglomerate which has been locally overturned. Morita strata (on left) conformably overlie the Glance Conglomerate and dip to the east. Figure 25. Gold Hill from the northwest, southern Mule Mountains, Arizona.
Unmapped low angle thrust fault/bedding plane fault on left
oo 49 flattening to the southwest (Figure 26a). Hayes and Landis
(1964) agreed with Ransome's general interpretation of the fault, but depicted it (Figure 26b) as maintaining a constant angle to depth. Jones (1963, 1966) argued for a steepening of the Gold Hill fault at depth, and attributed its presence to differential vertical uplift, vice horizontal compression.
Bilodeau and Keith (1978) and Bilodeau (1979) suggested that
the western section of the Gold Hill fault is linked with
the Abrigo and Bisbee West faults (Figure 23), that Laramide compression to the east-northeast reactivated movement along
these latter two preexisting faults, displacing the western
terrain between one and two miles eastward, and that this
displacement variously manifested itself along a sinuous
front as. a strike-slip fault, an overthrust fault, or an
intermediate "transpressional" fault. They assume that these
faults maintain a constant moderately high angle of dip to
depth.
Existing maps of the Morita Formation within the
southern Mule Mountains show it relatively devoid of faults,
and one is tempted to presume that the strata are relatively
undeformed with no apparent structural discontinuities across
the range. Unfortunately such is not the case. Large un
mapped steeply dipping faults have segmented the strata of
the Morita Formation in the southern Mule Mountains into
large disconformable blocks. One such fault lies a short
distance to the east of the exposures as depicted in Figure 25, GOLD HILL
- AFTER RANSOME, 1904 sw
-A FTER HAYES AND LANDIS, 1964
Figure 26 Geologic cross-sections through Gold Hill, southern Mule Mountains, Arizona 51 and another truncates the strata at the base of Black Knob on its southwestern flank (Figures 27 and 28). For the pur poses of this report, the Morita Formation in the southern
Mule Mountains can be considered to occupy three terrains:
1) a basal section that rests conformably upon steeply dip ping and overturned Glance Conglomerate, 2) an intermediate section that is bounded on both top and bottom by faults that strike at a relatively high angle to the bedding planes, and
3) an upper section that dips moderately to the northeast and is overlain conformably by the beds of the lower Mural
Limestone. The Gold Hill section of this report is repre sentative of the first of the three terrains, and the Black
Knob section, the last of the three (Figure 7).
Gold Hill Section
The Gold Hill stratigraphic section of this report was measured along the topographic ridge extending north eastward from Gold Hill (normal to the strike of the strata).
There the underlying Glance Conglomerate is overturned to
the northeast at its base and the dip of the bedding de
creases up-section (Figures 24 and 25). The dip of the
Morita Formation varies from vertical at its basal contact
with the Glance Conglomerate to less than ten degrees a
short distance to the east (Figure 24).
The Gold Hill section as depicted by Ransome (Figure
24) was found to be slightly in error in that a small fault 52
■e,',
Figure 27. Black Knob, southern Mule Mountains, Arizona, as viewed from the northwest.
Sandstone member in foreground continues on strike to the right center of photograph. Unmapped fault separates these steeply dipping middle Morita strata from shallow dipping upper Morita strata which comprise Black Knob. 53
• W ' - V
•V»
Figure 28. Black Knob, southern Mule Mountains, Arizona, as viewed from the north.
Fault separates middle Morita strata (on right) from upper Morita strata (on left). 54 was found near the saddle of the col and is evidenced by a change in dip of the bedding from 55 degrees on the south west to 20 degrees on the northeast (Figure 25). This fault is somewhat typical of the high angle faults within the southern Mule Mountains in that the gouge has been highly silicified with secondary silica, and, locally," euhedral crystals of quartz occupy the interstices along the fault zone. The existence of such faults obviously limits the value of the section for descriptive purposes; however, it is believed that such faults, herein described as bedding plane fault or low angle thrust faults, generally have not
' - - . - . disturbed the stratigraphic sequence within the section except along planes of major dislocation where the section
is essentially repeated across strike.
Twelve hundred and ninety feet of stratigraphic
section were described at Gold Hill, and this description is
set forth in Figure 9. Five samples of sandstone beds were
collected for petrographic analysis within this section.
Lithologically the section is not unlike the basal portion
of the Mural Hill section. At the 1246 foot horizon there -
is a 44 foot thick "pitted" sandstone bed similar in appear
ance to those described at the Mural Hill section. It is
not known for certain that this bed correlates with the lower
Mural Hill bed (936 foot horizon), but such a correlation is
not inconsistent with the gradual southward thickening of
the formation. Another more massive "pitted" sandstone bed 55
(Figures 29 and 30) occurs high in the intermediate section midway between Gold Hill and Black Knob, and this bed is tentatively correlated with the "pitted" sandstone bed en countered at the 1595 foot horizon at Mural Hill.
Black Knob Section
Whereas both the Mural Hill and the Gold Hill sec tions were measured from the basal contact with the under lying Glance Conglomerate, the Black Knob section, which is representative of the upper section of the Morita Formation in the southern Mule Mountains, was measured along a ridge on the southwest flank of Black Knob, across the top of Black
Knob at its summit, and thence northeasterly along the gentler retreating slope of the mountain into the foothills where the Mural Limestone is encountered (Figure 31). The reference point from which this section is measured is the intersection of the southwesterly oriented ridge with a plane of faulting that can be readily ascertained when viewed from a more distant position atop the main ridge line of the southern Mule Mountains (Figures 28 and 29). This traverse was utilized by Stoyanow (1949) in describing the facies of his Lowell Formation, and his paleontologic descriptions were useful in establishing the stratigraphic horizons within the upper Morita Formation and within the lower Mural
Limestone at this locality. Figure 10 contains the detailed
lithologic description of this section. Lithologically the
section is similar to the upper portion of the Mural Hill 56
Figure 29. "Pitted" texture on massive quartzarenite member of the Morita Formation, southern Mule Mountains, Arizona. 57
Figure 30. Upper portion of "pitted" member in southern Mule Mountains, Arizona.
Major fault lies structurally above this member, and rocks become increasingly altered with pronounced cleavage in vicinity of fault. BLACK KNOB
—AFTER HAYES AND LANDIS, 1964
Figure 31. Geologic cross-section through Black Knob, southern Mule Mountains, Arizona 59 section. However, limestone beds are more prevalent in the
Black Knob section, and the sandstone beds are less prevalent and less massive. The prominent massive sandstone bed that
Ransome used to define the top of the Morita Formation at
Mural Hill does not exist at Black Knob. One can only approximate the stratigraphic marker horizon by utilizing faunal assemblages within the fossiliferous members.
Stoyanow was able to subdivide the Lowell Formation into seven members on the basis of faunal horizons of am monites and Trigonids species. At Mural Hill he tentatively placed the top of the sandstone member that Ransome used to define the upper limit of the Morita Formation in the third
(counting upward) member, i.e., the Saavedra Member. This member coincidentally represented the faunal horizons of two types of Trigonids (Stoyanow 1949, p. 18). Within the
Saavedra Member occur the upper horizon of Trigonia reesidei and the lower horizon of pseudoquadrate, or scabroid,
Trigonids. Numerous specimens of T. mearnsi and T. stolleyi were found high in the section near the base of Black Knob, where they are characteristic of the Perilla Member (Stoyanow
1949, p. 13), well above the Saavedra Member and, thus, high
in the lower Mural Limestone. However, no Trigonids or ammonites were found on this traverse below this horizon.
Stoyanow places the base of the Lowell Formation at a
fossiliferous limestone bed at the summit of Black Knob. \ From this horizon Stoyanow collected specimens of Trigonia 60 reesidi and Kazanskyella arizonica. Much time was spent in scouring the crest of the mountain and along the strike of this fossiliferous member, but none of these latter speci mens were to be found. The member did contain numerous specimens of high spired gastropods and much fossil hash.
Farther downslope (upsection) a specimen of Pecten (Chlamys) sp. (Figure 15d) and a specimen of Anatina sp. were found
in the float. Several specimens of Trigonia stolleyi were noted stratigraphically below the fossiliferous Perilla
Member. However, stratigraphically above the fossiliferous member situated at the summit of Black Knob, the lithologic descriptions of Stoyanow are not correlatable to those
determined by this study until one reaches the Perilla
Member at the 1950 foot horizon, well into the lower Mural
Limestone. Due to the uncertainty of stratigraphic level
at the upper boundary of the Morita Formation, the Black
Knob section was described (Figure 10) to the 2232 foot
horizon where the talus from the upper Mural Limestone was
encountered.
Seven samples of sandstone beds were collected for
petrographic analysis within this upper section with the
stratigraphically highest bed sampled being located at the
1679 foot level. This bed is 170 feet stratigraphically
below the Trigonia mearnsi/T. stolleyi biostrome, which is
correlated with the Perilla Member of the Lowell Formation, and 945 feet stratigraphically above the base of the Lowell
Formation at the summit of Black Knob. THE SIERRA ANIBACACHI SECTION OF THE MORITA FORMATION
The location for the traverse in the Sierra
Anibacachi was selected on the basis of the regional mapping of Rangin (1977) and the earlier field studies by Taliaferro
(1933). Taliaferro reported in excess of 5000 feet of Morita
Formation on the northeastern side of Snake Ridge in the
Cabullona basin. Since the Morita Formation crops out so
extensively in this area, it is not at all certain that the
traverse was conducted along a line which represents the maximum stratigraphic thickness of the formation at this
locality. The actual thickness determined in this study
for the Morita Formation is approximately 4200 feet, the
upper horizon being somewhat indefinite due to insufficient
lithologic criteria for a precise boundary. Similarly, the
basal contact of the Morita Formation with the Glance
Conglomerate is somewhat subjective at this locality. The
traverse was begun at a point approximately 2.5 miles (4 km)
to the southeast of Cerro la Morita (Figure 32) and was
terminated at an unnamed peak (Figure 33) approximately 2
miles (3 km) to the northeast of the starting point.
Although it was the intention of the author to obtain
a stratigraphic lithologic description of the Morita Formation
at this locality, this facet of the study was abandoned for
62 Figure 32. Cerro la Morita, Sonora, as viewed from the starting point of the Sierra Anibacachi traverse (looking northwest).
Cerro la Morita is interpreted to be allochthonous Paleo zoic strata that rest discordantly on cretaceous strata. 64
Figure 33. Unnamed peak in Sierra Anibacachi from the west.
This peak marks the termination of the Sierra Anibacachi traverse. It is capped with Mural Limestone which con formably overlies Morita strata in the foreground. 65
two reasons: 1) extensive areas of the outcrop are covered with alluvium, and 2) numerous bedding plane/low angle thrust
faults were evident, some of which presented unresolvable quandaries as to the correct stratigraphic relationships of
the truncated members. .
Lithology and Paleontology
The Glance Conglomerate underlying the Morita For mation at this locality differs from that noted in the Mule
Mountains and elsewhere in Arizona in that it contains
numerous members of reddish-gray micritic limestone through
out the upper part of the formation. The Glance Conglomerate
was not traced to its basal contact at this locality, but a
traverse of one half mile down section, towards Snake Ridge,
from the Glance/Morita contact indicated that the inter
calated gray micritic limestone and reddish-brown sandstone
members were not confined to a transitional zone between the
Glance Conglomerate and the Morita Formation. These conglom
eratic, arenaceous, and micritic alternations were deemed to
be part of the Glance Conglomerate, and a boundary was
selected at the last appearance of the reddish-brown conglom
eratic member with angular clasts. Near the starting point
of this traverse, an enigmatic white conglomerate with well-
rounded clasts crops out at the crest of a small hill. It
is seemingly truncated by a fault that separates it from the
measured section of the Morita Formation. 66
The Morita Formation in the Sierra Anibacachi
(Figures 34 and 35) differs lithologically from the Arizona sections in a number of respects. 1) The limestone lenses are confined to the lower and upper boundaries, with the intervening members consisting of a rhythmical alternation between reddish-brown mudstone/siltstone and fine grained buff quartzitic sandstone; very few of these alternating members are calcareous, and when they are, they are generally less calcareous than those noted in Arizona; 2) The sand stones are uniformly fine grained, with few beds coarse enough to be classified as medium grained; consequently, only six sandstone samples were collected from the Sierra
Anibacachi for petrographic analysis; 3) Variations in lithology (i.e., dolomitic, fossiliferous, cross-bedded, conglomeratic, concretionary, and algal horizons) are far less prevalent than were noted in the Arizona sections; 4)
Sandstone beds, though as numerous as those to the north, tend to be thinner than in Arizona; and 5) No "pitted" sandstone beds were encountered in the Sierra Anibacachi traverse. With these exceptions, the members of the Morita
Formation are typical of the members in the Mule Mountains.
No fossils were noted in the limestone members at
the base of the section; however, 1 cm wide burrows were
noted (Figure 36) in a sandstone bed in the lower portion
of the section. At several horizons higher in the section Figure 34. Morita Formation strata as exposed in the Sierra Anibacachi, Sonora. 68
Figure 35. View to the south from a point high in the section in the Sierra Anibacachi, Sonora.
Mural Limestone crops out in the middle ground on the left. 69
Figure 36. Burrows in a sandstone member near the base of the Morita Formation, Sierra Anibacachi, Sonora. 70 fossilized wood was found, though none of it was noted to be in a normal growth position (Figure 37).
Higher in the section, close to the lower Mural boundary, gray micritic limestone beds begin to appear with increasing frequency. The first such member was encountered at the 3059 foot horizon. It is four feet thick and unfos- siliferous. The first fossiliferous bed, at the 3140 foot horizon, is approximately twelve feet in thickness, and contains undiagnostic fossil hash. At the 3490 foot horizon a 28 foot thick limestone biostrome contains large oysters
(Figure 38) including specimens of Exogyra quitmanensis
Cragin and Crassostrea sp. (Figures 15a, b, c). From this point in the section, the traverse proceeded up the moder-. . ately steep southwesterly slope of the unnamed peak (Figure
39), which is capped by a 100+ foot section of upper Mural
Limestone. Although numerous fossils appeared in the float, it is probable that the majority of them were displaced from higher stratigraphic horizons. Specimens of Trigonia mearnsi,
T. stolleyi, Idonearca stephensoni, Hemiaster sp., Meretrix
sp., Pholadomy sp., Pecten sp., Tapes (?) sp., and
Acanthoplites sp. were collected on this southwestern exposure of the unnamed peak. However, the next fossils to
appear in place on this slope were found at the 4281 foot
horizon where a five foot thick zone of Trigonia mearnsi was
noted. Referencing the biostratigraphy that Stoyanow so
systematically provided, this T. mearnsi zone is found in Figure 37. Petrified wood from the Morita Formation, Sierra Anibacachi, Sonora (X 0.4). 72
Figure 38. Oyster-laden biostrome, near Morita/lower Mural horizon, Sierra Anibacachi, Sonora. Figure 39. Northern slope of "unnamed peak" in the Sierra Anibacachi, Sonora, where traverse terminated.
Fossils found midway up slope correlated with Perilla member of Lowell Formation, equivalent to lower Mural Limestone. 74 the Perilla Member of the Lowell Formation, well above the
Saavedra Member wherein the Morita/lower Mural boundary lies (Ransome 1904b, p. 65; Stoyanow 1949, p. 25). The base of the massive upper Mural Limestone was measured at the 4515 foot horizon.
A 40 foot thick badly fractured quartzarenite bed was encountered near the base of the unnamed peak at the 3450 foot horizon. This sandstone bed locally is the last major sandstone stratum to be found in the section; however, as noted in the Arizona sections, this bed does not persist stratigraphically throughout the Sierra Anibacachi and, thus, would be of limited value in establishing a formational boundary.
With regard to the upper boundary of the Morita
Formation, Taliaferro (1933, p. 22) stated:
There is no sharp break between the Morita and the Mural; the Morita becomes increasingly calcareous toward the top, and thin lenses and layers of lime stone appear, forming an almost insensible gradation into beds that are predominantly limestone. The lower part of the Mural consists of about 300 feet of thin-bedded, shaly limestone, interbedded with buff sandstones and reddish-brown calcareous shales.
Both Taliaferro (1933) and Viveros (1965) described occasional beds of well-rounded limestone pebble conglomerates within the Morita in the vicinity of the Cabullona basin, a
short distance to the south. Viveros divided the Morita
Formation into two members. The lower member consists of
an alternating series of brown lutites and thin-bedded buff-
colored, calcareous, fine to medium grained, angular 75 quartz-rich sandstones. The upper member is characterized by a rhythmical series of reddish-brown lutites and reddish- gray sandstones, with intercalations of beds of conglomerate and of limestone. The sandstones are occasionally coarse grained, even conglomeratic, exhibit upward fining, and are occasionally cross-bedded. This latter description is shared by Taliaferro (1933) who stated:
There are occasional beds of well-rounded lime stone pebble conglomerates, and an occasional impure shaly limestone. The sandstones are mostly red to red-brown, but light gray to white sandstones occur; they are frequently cross-bedded and ripple marked.... The sandstones are occasionally conglomeratic, con taining small well rounded pebbles, and thin lenses of light conglomerate are not rare.
Whereas no such conglomerates and conglomeratic
sandstones were noted in this study, except as previously mentioned near the start of the traverse, occasional cross
bedding was noted in the sandstones, but was not sufficient
ly developed to obtain measurements of its orientations.
Approximately one mile northwest of the Sierra
Anibacachi traverse a highly fossiliferous biostrome was
exhumed by a through-going stream (Figure 40). Within this
biostrome were found specimens of Trigonia mearnsi, T.
stolleyi, Exogyra quitmanensis, Crassostrea sp., Acantho-
plites sp., Hemiaster sp., Pholadomya sp., and aggregated
calcareous worm tubes. On the basis of this rich fossil
assemblage, this biostrome is equated to the Perilla Member
of Stoyanow's Lowell Formation. The base of this fossil 76
Figure 40. Biostrome located in Sierra Anibacachi, Sonora, correlated with Perilla Member of Stoyanow's Lowell Formation, now classified as lower Mural Limestone.
Trigonia stolleya and T. mearnsi specimens are readily identifiable. 77 zone was measured to be stratigraphically 470 feet below the contact between the lower and upper members of the Mural
Limestone which was well exposed to the northeast (Figure
41). The base of the T. mearnsi zone on the Sierra
Anibacachi traverse only one mile southeast was noted to be 234 feet below the contact between the lower and upper members of the Mural Limestone, or exactly one half the separation to the north. Whereas the gradual thickening of the Morita Formation southward has been substantiated, the apparent rapid thickening of the lower Mural Formation southward is inconsistent with the probable depositional interpretations that can be attributed to this sequence.
One is left with the conclusion that the fossil assemblages or biostromes do not necessarily represent time correlatives.
Rather it seems more probable that these biostromes repre
sent environmental correlatives to a degree that was not
recognized by Stoyanow. A similar faunal inconsistency
exists with regard to Stoyanow1s assignment of Acanthoplites as younger than Kazanskyella (E. Warzeski, K. Young, per
sonal communication).
In the central Sierra Anibacachi a few highly
altered dikes were encountered in the Glance Conglomerate
and the Morita Formation. The dikes noted in this study
were approximately two feet in thickness, and cut dis
cordantly across the bedding. Farther south in the
Cabullona basin Taliaferro (1933) described a "large number 78
Figure 41. View to the northeast in the Sierra Anibacachi, Sonora.
Lower Mural Limestone strata on right (overlain by upper Mural Limestone) are lithologically similar to strata of the Morita Formation, extreme left. 79 of small, vertical dikes which cut practically all the formations, but which are most numerous in the Upper
Cretaceous." He noted that the dikes were later than the folding of the Upper Cretaceous strata, and surmised that they were intruded along lines of tensional weakness developed during the Late Tertiary regional uplift.
Faulting
There is a pervasive system of bedding plane/low angle faults which was encountered in the entire Lower
Cretaceous section as it is exposed in the Sierra
Anibacachi. The majority of these faults are confined to the less competent siltstones and mudstones, and are identifiable in arroyos by the light colored gouge which stands out in contrast to the reddish brown color of the undisturbed sediments. Where there has been no rapid down cutting through these faulted members, the faults are not evident. These low angle faults are not entirely restric ted to the less competent members? when the sandstone members are involved, the structure becomes more complex.
Excellent exposures of such faulting were encountered in
the sides of an arroyo crossed at the 425 foot horizon of
the traverse (Figures 42 and 43).
Rangin (1977) depicted a major thrust fault exten
ding the length of the Sierra Anibacachi at the Glance/
Morita contact. This contact was carefully examined in the 80
Figure 42. Discontinuous bedding attributed to low angle thrust faulting, in sandstone and mudstone sequence, Morita Formation, Sierra Anibacachi, Sonora.
Light colored gouge on left is diagnostic of mudstone/ siltstone beds which have accomodated faulting. 81
Figure 43. Subhorizontal sandstone member resting on more disturbed lower sandstone member, apparently as the result of low angle thrust faulting, Morita Formation, Sierra Anibacachi, Sonora. 82 region of the central Sierra Anibacachi, and no evidence
for an intraformational fault was found. Slickensides
(Figure 44) were found within the Glance Conglomerate near
the contact, but the contact itself appears conformable
at this location. A thrust fault was also evident within
the basal portion of the upper Mural Limestone (Figures 45
and 46). This fault can be traced laterally at the same
stratigraphic level for some distance. The location of
this fault appears somewhat peculiar in that it is situated
only a few feet above the base of the upper Mural Limestone.
The less competent siltstones of the lower Mural Limestone
would seemingly provide a more favorable plane of disloca
tion .
Continuing to the northeast beyond the ridge of
Mural Limestone that constitutes the eastern margin of the
Sierra Anibacachi, one descends into the Caldaron Valley
separating the Sierra Anibacachi from the neighboring . t Montes Canova (Figure 47). Although this region lies
beyond the immediate area of study, it should be noted
that the Montes Canova strike parallel to the Sierra
Anibacachi and extend the length of the latter range,
merging together at their southern extremities in a U-
shaped manner (Figure 48). The crest of the Monte Canova
is another ridge of northeasterly dipping Mural Limestone,
and the southwesterly flank of this range is composed
entirely of the Morita Formation. A hidden fault extends 83
Figure 44. Slickensides on thinly bedded sandstone member within the Glance Conglomerate near the stratigraphic contact between the Morita Formation and the Glance Conglomerate, Sierra Anibacachi, Sonora. 84
Figure 45. Bedding-plane fault within massive limestone member of upper Mural Limestone near the contact with the lower Mural Limestone, Sierra Anibacachi, Sonora. 85
Figure 46. Brecciated zone along bedding-plane fault within massive limestone member of upper Mural Limestone near the lower Mural Lime stone contact, Sierra Anibacachi, Sonora. 86
Figure 47. Montes Canova as viewed to the northeast from atop the Sierra Anibacachi, looking across the Caldaron Valley, Sonora.
In background are the Swisshelm and Pedregosa Mountains of southeastern Arizona; Pehlps Dodge Douglas smelter, on the right. Figure 48. Geologic map of area immediately east of Sierra Anibacachi, Sonora, Mexico the length of the Caldaron Valley, buried under alluvium
(Warzeski, personal communication). Recent detailed mapping in this area by Warzeski has disclosed that Glance
Conglomerate lies at the base of the Morita Formation at one locality in the Caldaron Valley, thus forming a repeated section of the Bisbee Group within the Sierra
Anibacachi and the Montes Canova.
Throughout the length of the Sierra Anibacachi and the Montes Canova, steeply dipping normal faults are oriented normal to the strike of the strata and the ranges. This faulting is most evident along the ridges of Mural Limestone; the faulting is generally down to the southeast, consistent with the normal faulting noted in the Mule Mountains (Figures 20 and 22). NORTHERN EXPOSURES OF THE MORITA FORMATION EQUIVALENT
To the north of the central Mule Mountains, the
outcrops of the Morita Formation tend to become discon
tinuous with no sections amenable to a complete formational
description. The northernmost exposure of the Mural
Limestone is located in Abbot Canyon in the northern Mule
Mountains. North of Abbot Canyon there exists a consider
able thickness of Cretaceous strata which is lithologi
cally similar to the Morita Formation. There is little
doubt that the section in the northern Mule Mountains is,
in fact, the Morita Formation. However, as one proceeds
farther northward, one becomes less confident that he is
observing strata of Morita facies, since the lithology
of the Cintura Formation is essentially identical to that
of the Morita Formation. For this reason, Gilluly (1956)
mapped the areas north (and west) of the Mule Mountains
utilizing the Bisbee Formation as the undifferentiated
unit equivalent to the whole of the Bisbee Group in its
southern exposures.
For purposes of this study, samples of sandstone
members of the Bisbee/Morita Formation were collected
from three of these northerly exposures: 1) Gadwell
Canyon, in the northern Mule Mountains; 2) Eastern flank/ 89 90 foothills of the Dragoon Mountains; and 3) Northern flank/ foothills of the Steele Hills.
Gadwell Canyon
At Gadwell Canyon near the eastern end of the
Government Butte fault (Figures 49 and 50), the Bisbee strata are subhorizontal with increasing dip to the east.
The strata are little deformed in the northern Mule Moun tains, and it is relatively easy to ascertain stratigraphi- cally that one is dealing with a facies of the Morita
Formation, and not the Cintura Formation. Lithologically the section differs from that at Mural Hill in that the siltstone/mudstone members are not so prevalent, and the sandstone members tend to be fine grained to very fine grained. A few gray unfossiliferous micritic limestone members are found intercalated within the strata; however, the section tends to be more uniform in appearance than that at Mural Hill, with light reddish-gray, very fine grained, subarkosic arenites being the predominant litholo gy. Channel and scour structures are occasionally evident, and the strata exhibit the same lateral facies variations that exist to the south. The walls of the canyon were examined for medium grained sandstone members from which
samples might be attained; however, nothing suitable was
found. Three sandstone samples were obtained from isolated
outcrops near the mouth of the canyon where they jut 91
Figure 49. Gadwell Canyon area of northern Mule Mountains, Arizona, view to the southeast. 92
Figure 50. Gadwell Canyon area of northern Mule Mountains, Arizona, view to the south.
Morita Formation strata are subhorizontal in this portion of the Mule Mountains. 93 through a thin mantle of alluvium. Strikes and dips of these outcrops were taken to insure that they were consis tent with the undisturbed strata, and that they were not outliers of allochthonous strata protruding from the nearby Government Butte fault.
No fossils or trace fossils were noted in this area, and, with the exception of the Government Butte fault to the northwest, the area is generally devoid of structure. However, there is some evidence of bedding plane faulting with minor dislocations. An abandoned adit was noted (Figure 51) in the side of the canyon, situated so as to exploit any mineralization that might have favored the zone of faulting.
Dragoon Mountains
Approximately 15 miles (25 km) to the north of
Gadwell Canyon one encounters subdued outcrops of the
Bisbee Formation on the eastern flank of the central
Dragoon Mountains. There the Bisbee Formation apparently has been folded to the east, probably in response to east- west compressional forces that have locally overturned the Bisbee strata (Figure 52). The alluvium from the adja cent Sulphur Spring Valley covers the upper portion of the
Bisbee Formation, and, in the foothills of the Dragoon
Mountains, has relegated the exposures of the Bisbee
Formation to a region of "islands" that protrude through 94
Figure 51. Abandoned adit in Morita strata, Gadwell Canyon, northern Mule Mountains, Arizona. DRAGOON MTS. —AFTER J. GILLULY, 1956 i
Figure 52 Geologic cross-section. Dragoon Mountains, Arizona 96 the onlappxng blanket of alluvium. On the west the strata of the Bisbee Formation are truncated by a series of,steeply dipping subparallel faults that occupy the central axial portion of the Dragoon Mountains. One such fault separates the main exposure of the Bisbee Formation from a narrow band of conglomerate on the west that is
lithologically and structurally correlatable to the Glance
Conglomerate. Although there is no depositional contact
exposed between this Glance Conglomerate equivalent and
the Bisbee strata to the east, the Bisbee sandstone mem bers become increasingly coarser downsection and are
locally conglomeratic just east of the fault (Gilluly
1956). Thus, one is led to believe that not much of the
section is missing along the fault, and that the Bisbee
strata to the east are relatively low in the section,
correlative to the Morita Formation. This hypothesis is
substantiated by Gilluly's discovery (1956) of poorly
preserved marine fossils in limestone members of the
Bisbee Formation two miles east of Dragoon Camp, a mining
encampment in the central Dragoon Mountains. This loca
tion is very close to, and immediately upsection from,
the locality from which four samples of sandstone members
were collected within the Bisbee Formation for this study.
Thus, these sandstone members are, with a reasonable
degree of certainty, assumed to be facies equivalents of
the Morita Formation. 97
The lithologic description of the Bisbee Formation as it occurs at this locality is not unlike that of the lower Morita Formation at Mural Hill. The sandstone mem bers are cross-bedded to a greater degree, thereby allowing the attainment of a number of measurements on their orien tations. The Bisbee Formation at this locality has been intruded by a number of sills of rhyolite, and the lithology of the Bisbee strata has been considerably altered by high temperature metamorphism along the margins of the intrusions.
Steele Hills
The most northerly sandstone samples were obtained from the foothills of the Steele Hills, situated at the northern end of the Sulphur Spring Valley, midway between the Little Dragoon Mountains and the Winchester Mountains
(Figure 3). There the Bisbee is separated from a rhyolitic member of the Tertiary Galiuro Volcanics, which constitute the main portion of the Steele Hills, by a steeply dipping fault which strikes NW-SE across the northern end of the
Steele Hills. As with the exposures in the Dragoon
Mountains, the Bisbee strata are discontinuous in exposure, for the most part, with alluvium obscuring the major por tion on the formation in varying degrees. Cooper and
Silver (1964) have described the structure of the Bisbee
Formation (which they refer to as the "Morita and Cintura formations") as closely folded outcropping along the 98 upthrown northeasterly side of the previously mentioned fault. Lithologically the Bisbee strata in the Steele
Hills resemble that of the Dragoon Mountains, except that no conglomeratic members were noted. Six samples were collected from sandstone members that protruded from the alluvium along a traverse normal to the strike of the bedding and the fault. Because no conglomeratic or fossiliferous members have been noted in the outcrops, and because the exposed strata represent so little of the
Bisbee section as it probably had existed in that area, it remains unknown whether the Bisbee Formation there is correlative to the Morita or Cintura Formation. South of the Steele Hills and continuing into the adjacent
Gunnison Hills (Figure 3), numerous outcrops of Glance
Conglomerate are found in depositional and in fault contact with the Paleozoic strata. However, Cooper and Silver postulated an extension of the Antelope Tank fault between these two terrains, thus negating any predilection of equating the section to the Morita Formation on the basis of proximity of outcrop to that of the Glance. However, as was noted earlier, there are no known occurrences of
the Cintura Formation north of the central Mule Mountains,
and, if the exposures in the Dragoon Mountains are indeed
equivalent to strata of the Morita Formation, it would
seem likely that the strata in the Steele Hills shall
eventually be proven to be equivalent facies of the Morita
Formation. PETROGRAPHY OF THE MORITA FORMATION
Petrographic analysis of sandstone beds in the
Morita Formation (and its equivalents) was conducted by pointcounting grains in thin-sections; these were prepared from the samples collected from each of the seven previously X described areas. The results were tabulated utilizing grain parameters as set forth in Table 1, and are presented here in Appendices A and B. Using the lithologic and grain parameters described by Dickinson (1970) , and the subsequent modal analysis techniques developed by Graham,
Ingersoll, and Dickinson (1976), Dickinson and Suczek
(1979), and Dickinson and Valloni (1980), ternary plots of the various data were obtained, which then allowed inferences to be drawn as to the provenance and maturity of the sandstone beds and their lithologic assemblages.
The utility of plotting sandstone compositions on a ternary diagram is that specific fields within the various diagrams have been demonstrated to systematically correlate with distinct provenances (Dickinson and Suczek
1979). Two basic ternary plots, quartz-feldspar-lithic fragments (Q-F-L) and moncrystalline quartz-feldspar- total lithic fragments (Qm-F-Lt) (Table 1) have been interpreted in the same manner as was done in the 99 Table 1. Grain Parameters
Q = Qm+ Qp Q = TOTAL QUARTZOSE GRAINS Qm= MONOCRYSTALLINE QUARTZ GRAINS Qp = POLYCRYSTALLINE QUARTZOSE GRAINS
Lt = L + Qp Lt = TOTAL APHANITIC LITHIC FRAGMENTS L = Lv + Ls Lv = VOLCANIC-METAVOLCANIC-HYPABYSSAL LITHIC FRAGMENTS Ls = APHANITIC SEDIMENTARY-METASEDIMENTARY LITHIC FRAGMENTS 101 above-mentioned studies, ascribing three fields within each of these plots to three distinct tectonic regimes (Figures
53 and 54). These three regimes were defined by Dickinson and Suczek (1979) as follows: 1) continental block, for which sediment sources are on shields and platforms or in faulted basin blocks; 2) magmatic arc, for which the sources are within active arc orogens of island arcs or active continental margins; and 3) recycled orogen, for which sources are deformed and uplifted stratal sequences in subduction zones, along collision orogens, or within foreland fold-thrust belts.
Whereas both the Q-F-L and Qm-F-Lt plots can be assessed as to provenance and maturity, the Q-F-L plot is a better indicator of maturity, and the Qm-F-Lt plot, of provenance. Polycrystalline quartz-volcanic lithic fragments-sedimentary lithic fragments (Qp-Lv-Ls) and monocrystalline quartz-plagioclase-potash feldspar (Qm-P-K) plots have proven instructive in the analysis of arc terranes (Graham et al. 1976; Dickinson and Suczek 1979;
Dickinson, Helmold, and Stein 1979); however, these plots are less useful in the analysis of non-arc provenances.
The relative sparseness of lithic fragments within the samples analyzed in this study provided an insufficient base from which to derive statistically meaningful
Qp-Lv-Ls plots. 102
RECYCLED CONTINENTAL OROGEN BLOCK PROVENANCES PROVENANCES
Increasing Ratio Decreasing Oceanic/Continental Maturity or Components Stability
Increasing Ratio Plutonic/Volcanic Components . N \ \ > /
Figure 53. Q-F-L plot with provenance and maturity regimes for sandstones analyzed on basis of constituent grain framework (after Dickinson and Suzcek 1979) 103
Qm
CONTINENTAL BLOCK PROVENANCES RECYCLED OROGEN // PROVENANCES
^'(Plutonic) V.
MAGMATIC ARC PROVENANCES
Figure 54. Qm-F-Lt plot with provenance and maturity regimes for sandstones analyzed on basis of constituent grain framework (after Dickinson and Suzcek 1979) I 104
Petrographic Analyses of x Morita Sandstones
Following the analytical procedures of these earlier workers, a common compositional framework class ification for sandstone analysis was adopted in this study. Interstitial cement and matrix, as well as extraneous framework constituents (i.e., heavy minerals and calcareous grains) were disregarded, and, volumetric estimates of the relative proportions of the pertinent constituents were calculated. Modal compositions were categorized utilizing the following constituents (Graham et al. 1976): 1) stable quartzose grains, Q, including both monocrystalline quartz grains, Qm, and polycrystal line quartzose lithic fragments, Qp, which are princi pally chert grains; 2) monocrystalline feldspar grains,
F, including plagioclase, P, and potassium feldspar, K, and 3) unstable polycrystalline lithic fragments, L, of two kinds— a) Lv, volcanic and metavolcanic types, and b) Ls, sedimentary and metasedimentary types. The total lithic fragments, Lt, then equal the sum of unstable lithic fragments, L, plus stable quartzose lithic frag ments, Qp (Dickinson and Suczek 1979).
Whereas previous workers have excluded from their analyses sandstones that contain significantly high pro-
. - > portions of matrix and/or cement, no such exclusion was made in this study for the following reason. 105
Because modern depositional environments seldom
i produce sediments containing more than 5 to 10 percent fine grained material interstitial to the framework grains, ancient sedimentary rocks containing significantly greater percentages can be considered to have experienced diagen- etic processes whereby certain unstable framework grains have been altered to interstitial material. The percen tages of interstitial material in the 51 samples of this study varies between 14.5 percent and 81.8 percent. The vast majority of this interstitial material is calcite and silica. As such, it is not representative of material derived from diagenetic alteration of the framework grains. When calcite appeared as diagenetic replacement of silica cement (Figures 55 and 56), it was treated as interstitial calcareous cement. Commonly calcite is of primary origin in the Morita Formation, and where it was a framework constituent, it was so tabulated. The quartz grains commonly exhibit silica overgrowths (Figure 57) with a commensurate reduction in sandstone porosity.
Since the silica overgrowths on the detrital quartz grains are not part of the compositional framework upon which
this study is predicated, this silica was discounted in
calculating the framework composition. Instead, the
silica overgrowths were treated statistically as non-
calcareous interstitial material (Inc) (See Appendix C) . 106
a. polarized light
b . polarized light with gypsum plate Figure 55. Calcite replacement of silica cement. Borders of individual quartz grains show effect of diagene- tic replacement of quartz by calcite. Photomicrograph of subarkose, Morita Formation, from Mural Hill traverse, central Mule Mountains, Arizona (X 32). 107
Figure 56. Calcite replacement of silica cement with incipient replacement of constitutent quartz grains (X 32).
Photomicrograph of subarkose, Morita Formation, from Mural Hill traverse, central Mule Mountains, Arizona. 108
Figure 57. Dust rims" on quartz grains (X 32). 109 The ternary plots for the seven localities are depicted in Figures 58 through 61. The petrographic analyses for all seven locations consistently depict a continental block provenance.
However, the twenty samples from Mural Hill exhibit a variation from this basic pattern which can be better interpreted by subdividing the section into five segments: 1) The first two samples, KMM-13 and KMM-35, were obtained from the basal portion of the section where reworked clasts of the Glance Conglomerate are present.
These samples plot within the continental block provenance field in the relatively mature segment of the field. 2)
The next six samples, KMM-153 to KMM-1139, were obtained from that portion of the section that was devoid of reworked clasts of Glance Conglomerate. These samples again plot within the continental block provenance field, but the stratigraphically lower samples plot at the less mature end of the field, with succeeding samples gravi tating towards the Q parameter, or the mature end of the field. 3) The next three samples, KMM-1140 to KMM-1345, plot unmistakably in the magmatic arc provenance field.
4) The next three samples, KMM-1600 to KMM-1768, jump back again close to the Q parameter, indicative of very mature sediments of a continental block provenance.
5) The last six samples, KMM-2006 to KMM-2618, plot in a somewhat random manner within both the recycled orogen 110
ALL STUDY SITES - LESS MURAL HILL
Figure 58. Q-F-L plot, all study sites less Mural Hill Ill
Figure 59. Q-F-L plot. Mural Hill 112
ALL STUDY SITES - LESS MURAL HILL
Figure 60. Qm-F-Lt plot, all study sites less Mural Hill MURAL HILL Qm KMM no*.
13 - 35
153 - 1139
1140 - 1345
1600 - 1768
Figure 61. Qm-F-Lt plot. Mural Hill 114 provenance field and the continental block provenance field. The plots of these five divisions have been annotated separately in Figures 59 and 61. It is of interest that Ransome (1904b, p. 64) stated:
About 1200 feet above the base of the Morita formation occurs a bed of reddish-brown grit from 10 to 15 feet in thickness, but otherwise there is no marked interruption in the monotonous repe tition of sandstones and shales extending from the top of the Glance conglomerate to the bottom of the Mural limestone.
It would appear that Ransome*s observation of a
"grit" corresponds closely to what is compositionally and texturally classifiable as a feldspathic graywacke
(Pettijohn 1957), feldspathic litharenite (McBride 1963;
Folk 1968), or lithic wacke (Dott 1964); (Figures 62 and
63) .
The modal•results of the Mural Hill section might appear incongruent, on first analysis, with those of the other localities. However, this is not necessarily so.
First, the anomalous "gritty" member, to use
Ransome*s terminology, is located near the middle of the
stratigraphic section of the Morita Formation. The
sections sampled in the southern Mule Mountains (Figure
7) were representative of the top and bottom of the
section. If the grit member persists to the south, it
will undoubtedly be found in the unsampled middle section. Also, the samples obtained from the northern Mule Mountains and the Dragoon Mountains may represent a small 115
b . polarized light with gypsum plate Figure 62. Photomicrograph of feldspathic graywacke (X 32). Sample from the "grit" member of the Morita Formation, Mural Hill traverse, central Mule Mountains, Arizona. 116
Figure 63. Photomicrograph of volcanic lithic clast (X 100) . 117 portion of the total section, which might easily correspond to a continental block provenance segment of the Mural
Hill section. The same might be said of the Steele Hills samples, but it isn’t even certain that these samples are of a Morita facies. The "grit" member at Mural Hill was reported to be ten feet in thickness (Ransome’ 1904b, p. 64).
The same member was measured to be 200 feet thick (this report). Regardless of this discrepancy, the thickness of the "grit" member is relatively thin when compared to the total thickness of the Morita Formation at Mural Hill.
With only six sandstone samples retrieved from the 4200 foot thick section in the Sierra Anibacachi, it is conceiv able that the "grit" member, or its facies equivalent, extends into Mexico, and was straddled in the sampling of the Sierra Anibacachi section.
The same exceedingly high ratio of plagioclase to
K-feldspar that was noted in the Mural Hill section, was noted in the other six study areas as well, with the follow ing exception. Two of the three samples from the Gadwell
Canyon location exhibited a significantly higher K-feldspar content, producing plagioclase to K-feldspar ratios of 55 and 46 percent. These latter values are typical for sands derived from a plutonic source (Feniak 1944 ? Suttner 1974, p. 82). Table 2 and Figure 64 illustrate the extreme variability of plagioclase to K-feldspar ratios in various types of intrusive rocks. Normal sedimentary processes Table 2. Approximate average mineral composition of common intrusive rocks (after Wedepohl, 1969) .
In vol. percent of: Granite Grano- Quartz Diorite Gabbro Upper Crust diorite diorite (intrusive rocks)
Plagioclase 30 46 53 63 56 41
Quartz 27 21 22 2 —— 21
Potassium feldspar 35 15 6 3 — — 21 (incl. microperthite)
Amphibole 1 13 12 12 1 6
Biotite 5 3 5 5 — — 4
Orthopyroxene — —— — 3 16 2
Clinopyroxene —— —— — — 8 16 2
Olivine — — — — — — — 5 0.6
Magnetite, ilmenite 2 2 2 3 4 2 VO o o in o in in o Apatite o in 0.8
M 119
As." # / & / ^£eff& L7 0* m m / /& 9 *#■ < ^ ^ ^ M A F J G MINERALS _____
Figure 64. Generalized mineralogical constituents of igneous rocks (after Larsen 1966) 120 tend to produce proportionately lower ratios of plagio- clase to K-feldspar (Folk 1968; Parfena and Yarilova 1965;
Blatt 1967b). This has been attributed to the differential bonding strengths and the silicon/oxygen ratios of the constituent mineral components (Blatt 1967b, p. 1036;
Blatt, Middleton, and Murray 1980, p. 296). High plagio- clase to K-feldspar ratios have been variously attributed to the predominance of plagioclase feldspar over K-feldspar in igneous rocks (Baturin 1947; Wedepohl 1969, p. 248) , the leaching of potassium due to a persistently high water table in a subtropical paleoclimate (Todd 1968), and meta- morphic terranes (Suttner 1974) . The existence of anomalously low K-feldspar content of the majority of the samples in this study cannot be attributed to any specific factor.
Whereas the value of the petrographic techniques developed by Dickinson and his colleagues and utilized herein have merit for enabling researchers to identify specific mineralogic and sedimentary petrologic provinces, the limitations of these techniques must be borne in mind so as not to extrapolate inferences of a sedimentary or tectonic nature which cannot be substantiated by the analyzed data. Precise characterization of mineralogic provinces must include more than tabulation of percent feldspar, quartz, and rock fragments (Suttner 1974, p. 81).
Blatt et al. (1980) have noted: 121
The abundance in a clastic sediment of any type of rock fragment depends on three factors: (a) the abundance of the source rock upstream in the paleo-drainage basin; (b) mechanical and chemical durability of the fragment during weathering, transportation, and diagenesis; and (c) the grain size of the sediment being examined.
With respect to this latter point (i.e., the grain size), Figure 65 illustrates the variance that can result for petrographic studies of the type reported herein as a function of grain size. Whereas the provenance studies were based ideally on the use of medium grained sandstones
(W. R. Dickinson, personal communication), one frequently must utilize coarse or fine grained rocks when medium grained rocks are not available. These variations in grain size can have a dramatic effect on the relative percentages of clastic mineral and lithic components of samples under study (Blatt 1967a, 1967b; Pittman 1970) .
Diagenesis of Morita Sandstones
The diagenetic effects that have been noted in this study consist of: 1) deposition of chloritic dust rims on quartz grains, 2) syntaxial silica overgrowths and euhedral quartz crystal lining of vugs and fractures,
3) coalescent neomorphism of chert grains, 4) formation of concretions, 5) calcite replacement of silica cement, 6) dolomitization, 7) development of authigenic kaolinite and
illite, and 8) mechanical disaggregation of sand grains.
Although this sequence of diagenetic effects cannot be 122
1 [
ui H
Grovel Sand I Cloy
Figure 65. Variation in mineralogic composition of sandstone as a function of grain size (after Blatt, Middleton and Murray 1980) 123 interpreted as a strict ordering of earliest through latest diagenesis, this sequence does represent a rough ordering of early, or locomorphic, diagenetic events through later, or phyllomorphic, diagenetic events.
The formation of chloritic dust rims on detrital quartz grains (Figure 57) represents an early phase of diagenesis. The sandstone members presently are uniformly non-porous principally due to syntaxial silica overgrowths on the monocrystalline quartz grains (Figures 13 and 57).
The syntaxial silica overgrowths occasionally represent incomplete welded boundaries. However, the more common occurrence is that of a complete welded boundary, resulting in discordant lattice configurations between contiguous quartz grains (Figure 13c). (Such,textures have been referred to as "composite grains" by Hubert 1960, p. 135).
A few of the sandstones contain some chalcedony (Figure
66) which has partially, and in some cases totally, filled interstitial pores.
Chert grains exhibited a varied textural appearance, ranging both in size and in degree of coalescent neomorph ism. Textural extremes often were noted to exist side- by-side within a thin-section, and frequently quartz grains appeared to comprise a continuum from chert to aggregate quartz to monocrystalline quartz. 124
Figure 66. Photomicrograph of chalcedonic quartz-lined vug, polarized light (X 100).
Subarkose, Morita Formation, from Mural Hill traverse, central Mule Mountains, Arizona. 125
Formation of calcareous concretions (Figure 17) resulted from redistribution of chemicals within the layers of mudstone.
The replacement of silica by calcite is generally confined to interstitial matrix and cement (Figures 55 and
56). Infrequently it was noted that this replacement was not confined to the matrix or cement, but extended to the detrital quartz grains (Figure 67). In all cases the calcite replacement demonstrated an initial preference for the interstitial matrix/cement, followed by replacement along the rims (surfaces) of the quartz grains, and eventual flooding, or total replacement, of the entire quartz grain (Figures 55, 56, and 67). A few samples from the Black Knob section exhibit minor amounts of dolomite.
These occurrences commonly take the form of individual rhombs of good crystal outline within masses of calcite
(Figures 67 and 68). Such replacement has been interpreted by Goldsmith and Graf (1955) and Goldsmith, Graf, and Witters
(1962) as representing an exsolution phenomenon rather than the result of introduction of magnesium from an outside source.
Commonly quartzarenites will produce thin kaolini- tic coatings on the individual quartz grains, while arkoses diagenetically produce thin coatings of illite.
The weathering or mechanical disaggregation of quartz grains in forming the "pitted" surfaces on some 126
Figure 67. Calcite replacement of quartz and chert, polarized light (X 100).
Diagenesis has proceeded to the stage where grains have been altered to a large degree; they start to lose their primary mineral characteristics and texture. Diagenesis includes formation of dolomite as well as calcite. Photomicrograph of quartzarenite, Morita Formation, from Black Knob traverse, southern Mule Mountains, Arizona. 127
Figure 68. Dolomite rhombohedrons as diagenetic replace ment of silica in a quartzarenite, unpolarized light (X 100).
Morita Formation, Black Knob traverse, southern Mule Mountains, Arizona. 128 sandstone members in the central and southern Mule Mountains has been discussed earlier in this study. The cause of this disaggregation is assumed to be a preferential dis solution of the silica cement. Why such dissolution should be confined to certain zones of the sandstone members is not well understood, but the extended lateral distribution of these structures is indicative of a syngenetic causal factor.
A few of the more calcareous samples can be classified as matrix supported sandstones. However, the vast majority are grain supported, and, as such, probably possessed a relatively high primary porosity.
Considerable data have been collected from a large number of cores pertaining to reduced intergranular porosity with increase in burial depth. It has been found that porosity can decrease either linearly or nonlinearly with depth at greatly different rates (Blatt et al. 1980, p. 418). Syntaxial, or authigenic overgrowth of silica on quartz grains is abetted by increasing compaction, temperature, and salinity, all of which accompany increased depth of burial (Blatt et al. 1980, p. 421).
Blatt (1979, p. 148) has defined parameters for deposition of syntaxial silica overgrowths within a quartz sand, and has concluded that the bulk of quartz cement in orthoquartzites is precipitated at very shallow depths. In support of a shallow depth of precipitation, no 129 evidence for pressure solution of quartz grains was noted in this study.
A moderate depth of burial of Morita strata is supported by other observations, specifically: 1) a local cleavage at angles to the bedding (Figure 12), 2) a morphological distortion of some fossil specimens (Figure
15i), 3) a compaction of chert grains so as to assume intergranular conformations within the primary porosity, and 4) the ductile deformation of Bisbee strata along zones of apparent overthrusting (Figures 24, 26, 31, and
52) . DEPOSITIONAL ENVIRONMENTS OF THE MORITA FORMATION
Paleocurrent directions were calculated for cross- bedded sandstone members of the Morita and Bisbee Formations in the central Mule Mountains and the Dragoon Mountains. A total of 14 directional current structures (cross-bedded sandstones) were measured (seven at each locality), and their interval percentages plotted on rose diagrams (Figure
69) after correcting for tectonic tilt. (A steronet was used to rotate the data, thereby correcting for bedding plane dip.) The data for the central Mule Mountains is
indicative of a general north to south directed sediment
transport, and, for the Dragoon Mountains, a general north west to southeast directed transport. Though these results must be evaluated statistically as tenuous due to the
limited number of observations upon which they are based,
observations for the central Mule Mountains correlate well
with the paleocurrent directions reported by Bilodeau
(1979, p. 28) for the subjacent Glance Conglomerate. The
greater degree of scatter in the plot for the Dragoon
Mountains may be attributed to the fact that the strata
from which they were derived was tectonically overturned
(contemporaneous rotation of the strata during tectonic
130 131
PALEOCURRENT DIRECTIONS
AS DETERMINED FROM CROSS-BEDDING --
DRAGOON MTS MURAL HILL
Figure 69. Paleocurrent directions. Dragoon and Mule Mountains 132
tilting cannot be detected or compensated), vagaries in
current direction that correspond to divergent deltaic channel orientations (e.g., "bird-foot" deltas), or mean
dering river or tidal channels.
:In general, the Morita Formation and its equivalent
facies to the north correspond to estuarine and peritidal
conditions that existed at the margin of the Comanchean
sea at the northern end of the Chihuahua trough (Figure 5).
The Morita and equivalent Bisbee strata represent a classic
transgressive sequence that bridges a period of time be
tween the deposition of what has been interpreted as a
proximal alluvial fan sequence (i.e., Glance Conglomerate)
shed into a subsiding basin from an actively rising mountain
front (Hayes 1970a? Bilodeau 1979), and the deposition of
marine shallow water deposits of the advancing Comanchean
sea (i.e., lower Mural Limestone) (Hayes 1970a).
The Morita Formation probably represents depo sition on a slowly subsiding deltaic plain. The sandstone beds that rest in a cut-and-fill relation on underlying siltstone and mudstone units are believed to represent channel deposits of meandering streams that flowed across the plain, and the inter vening siltstone and mudstone units are believed to represent interfluvial flood deposits. The thin oyster-bearing limestone beds in the upper part of the Morita undoubtedly were deposited in brackish waters that intermittently flooded the delta from an advancing sea to the southeast. (Hayes 1970a, p. A17)
Certainly the rugged relief that existed in pre-
Morita times from which the Glance Conglomerate was derived
was sufficiently eroded to fill the preexisting declivities 133 and provide a relatively graded surface upon which the lower strata of the Morita Formation were laid. This interpretation is supported by the dramatic local changes in thickness and lithofacies,' and the lateral and vertical interfingering of Glance Conglomerate with the sandstones and siltstones of the Morita Formation (Bilodeau 1979, p.
30). This same interfingering of basal Morita Formation with the Glance Conglomerate in the Sierra Anibacachi
(Figure 70— in pocket) indicates the regional extent of the early Cretaceous diastrophism that preceded the deposition of the Morita strata. Certainly this tectonism had ceased by the initiation of Morita deposition since the Morita deposits are primarily fine to very fine grained sediments.
The source of the Morita sediments was assuredly more distal than the underlying Glance Conglomerate as demon
strated by the contrasting lithologies and textures of the
two clastic sequences.
The relative abundance of fossilized wood, some
of which has been reported in situ (Tyrrell 1957? Schafroth
1965), attests to Morita facies to the west of the study
area have been evaluated as brackish and fresh water
environments (Gilluly 1956? Schafroth 1965? Tyrrell 1957?
Hayes 1970b)? to the east the Morita facies become more
calcareous and less clastic (Epis 1956? Kottlowski 1963?
Zeller 1965). Subsidence of the local basin must have
kept pace with the rate of sedimentation as the fluvial 134 sandstone deposits typically fine upwards into thicker sequences of finer clastic deposits. The generalized pattern of sedimentation consisted of irregular cycles of
coarser channel deposits and finer overbank, estuarine, and tidal flat deposits. Channel and scour structures, uni directional and bidirectional crossbedding, algal struc tures and trophic burrows in the lower portion and a rich neritic fauna in the upper portion of the formation attest to a shallow water environment. Throughout the current area of study (Figures 2 and 5), the Morita Formation can be regarded as a large deltaic complex whose proximal area lay at some distance to the north; the distal facies lay to the south and east, as measured from the north end of the Chihuahua trough. The northward increase in grain
size, the absence of a massive Mural Limestone member north of the Mule Mountains, and the occurrence of brackish water fauna in the Bisbee facies in the adjacent ranges
suggests that the apex of the delta must have been situated not too far north of the present exposures in the Dragoon
Mountains and the Steele Hills. The Morita strata consti
tute an essentially continuous deltaic deposit across some
200 miles (320 kilometers) stretching from the Dragoon
Mountains (Steele Hills?) in the north to the Sierra
Anibacachi and beyond to the Cabullona basin in the south.
A delta of this size is consistent with a major river
system whose drainage basin comprised a large portion of 135 the western interior. The sandstone members, typically fining upward, would then represent the meandering courses of a large number of splays that fanned across the delta, bringing well-sorted quartz-rich sands from deep within the craton, and depositing them ultimately on the pro grading and aggrading surface of the delta. The grain size diminishes southward concommitantly with a decrease in frequency of cross-bedding, as might be expected on the distal portions of the delta. Smaller tributaries drained
the coastal regions to the west of the basin, occasionally providing influxes of conglomerates as noted in the
Cabullona basin.
Midway through this depositional cycle, a pulse of
clastic detritus from a dissimilar lithologic province
provided a sudden influx of immature sediments from an
area not previously noted in the sedimentary record.
Explanations for such an occurrence might include the
erosion of youthful canyons in regions close to the area of
deposition, or the damming of the river valley to the north
by a volcanic flow. This latter interpretation is consis
tent with the occurrence of volcanic flows within the
Glance Conglomerate of the Huachuca Mountains to the west
of the study area (Weber 1950; Hayes 1970a; Bilodeau 1979) .
Interestingly, Gonzalez (1978) has identified a lithologi
cally similar sequence of texturally immature rocks high
in feldspar and volcanic rock fragments within the 136
Temperales Formation of the Ceja Group , a sequence of Lower
Cretaceous rocks lithologically and temporally equivalent to the Bisbee Group. Gonzalez was likewise unable to account for the provenance of these texturally immature rocks, and suggested that they may have been derived from the Jurassic volcanics farther to the west, de Cserna
(1970, p. 109) cites an occurrence of interbedded volcanics in Lower Cretaceous (Albian) limestones in an area north of Hermosillo. CONCLUSIONS
1. The principal topographic constraints which were directly responsible for the accumulation of the
Morita sediments were the uplifted blocks to the north and west from which the Glance Conglomerate was derived, and a deepening linear trough extending northwestward from the Mexican geosyncline, or proto-Gulf of Mexico.
2. The deposits of the Morita Formation and its equivalent facies within the Bisbee Formation to the north
and west represent a deltaic accumulation of sediments which was discharged from a major river system draining
a portion of the Western Interior Cretaceous basin.
3. The deltaic deposits of the Morita Formation
fluctuated between terrigenous and marine deposits as a
result of uneven rates of subsidence and deposition; how
ever, all sedimentation occurred very close to sea level,
as evidenced by peritidal and shallow water neritic sedi mentary structures and fossils.
4. The general conformation of the Morita Formation
is a prism that thickens southward attaining a thickness
of 4200 feet in the Sierra Anibacachi and 5000+ feet in the
Cabullona basin. Thicknesses in the Mule Mountains of
southeastern Arizona range from 2700 feet in the central
section to over 3000 feet in the southern section.
137 138 5. The Morita Formation represents a transgression of the Comanchean sea during late Aptian time. Culmination of this transgression occurred early in Albian time, with
the deposition of the Mural Limestone, a deeper water marine
formation that conformably overlies the Morita Formation.
6. Whereas the majority of the sandstone members
of the Morita Formation are texturally mature subarkoses and
quartzarenites of a continental block provenance, one
texturally submature member exposed in the central Mule
Mountains is of a distinct magmatic arc provenance.
7. Both low angle reverse and high angle normal
faulting has occurred within the Morita Formation to an
extent not previously reported. The net displacement along
many of these faults appears to be minimal; however, major
displacement may account for duplication of the section,
which is so evident in the northern Sonora portion of the
study area.
8. Some inconclusive evidence, as discerned from
petrographic and structural observations within the Morita
Formation, suggest a moderate depth of burial prior to
Tertiary exhumation.
9. The boundary between the Morita Formation and
the superjacent Mural Limestone is a transitional boundary,
wherein the more clastic sediments, which constitute the
lower Morita Formation, interfinger with lenses of micritic
limestone, typical of the upper Mural Limestone. The 139
delineation of the boundary between these two formations,
as established in the current literature, appears to have validity only within restricted areas of exposure.
Stoyanow's (1949) designation of an intermediary formation
(i.e., Lowell Formation) encompassing the transitional
members of the two formations appears to have merit. It
has regional application throughout the geographical extent
of the two formations, but it is not in accord with accepted
usage. APPENDIX A
MODAL POINT COUNTS OF SELECTED MORITA FORMATION SANDSTONES
I = interstitial cement and matrix, Ic = calcareous interstitial cement and matrix, Inc = non-calcareous inter stitial cement and matrix, M = mica, H = heavy mineral, 0 = opaque mineral, Ca = calcareous allochem, Cf = calcite-filled fracture, Cu = calcite replacement of an unknown mineral component, U = unknown mineral component; see Table 1 for other symbols. 140 141
(N in o c\ o CN in o rH 00 VO H
KMM-2 7.3 54.8 — 54.8 — KMB-0 Inc 1 1 i i KMB-2 29.7 31.5 30.3 22.0 3.8 20.5 0.8 55.0 9.3 12.8 5.3 22.8 27.3 43.5 14.3 33.0 23^3 31.8 29.5 3 1 ^ 0 30.0 42.3 40.3 56.6 27.8 55.6 81.8 I (Inc + Ic) Ic -- — 2.5 18.5 - 16.8 13.8 14.3 - - - 20.0 - - - - 2.0 — 6.8 I (Inc + Ic) 2.5 2.5 29.7 31.5 30.3 24.5 22.3 20.5 17.6 57.5 23.1 15.3 19.6 22.8 27.3 43.5 34.3 33.0 23.3 31.8 29.5 33.0 30.0 49.1 % total framework M % total framework ' 0:4 6.3 0.7 - 1.4 H M - — 0.3 0.3 0.9 - - -- 0.4 — — — 1.7 0.3 — 6.8 0 H — ‘ — - 0.4 1.1 - - - 0.3 0.6 0.6 0.3 - 0.4 6.4 - - 0.7 - 0.7 2.0 — 3.4 - 17.4 Ca 0 — 2.6 1.3 1.1 1.3 - -- 2.9 0.6 6.5 - - 1.8 0.4 -- 0.7 —. - 1.5 — 1.7 3.9 Cf Ca — _ __ — 57.0 — 1.6 0.3 0.9 — — ------— — ■ 13.0 20.1 - 19.7 Cu Cf — — —— - - - -- — — 0.8 1.7 2.1 U Cu - - - 5.6 10.9 — _ 13.3 — - —- 3.4 u 3.5 5.8 2.1 7.1 0.4 0.4 0.9 0.7 0.3 0.6 0.3 0.6 1.3 0.9 1.9 0.3 1.7 4.9 0.4 0.7 0.3 1.5 0.4 0.7 1.5 % Q-F-L 96.1 43.4 88.6 83.8 77.6 Q % Q-F-L 3.9 54.9 10.7 16.2 22.4 F Q 82.2 89.6 77.1 76.4 81.5 82.0 74.2 98.5 90.2 95.1 90.7 87.9 85.4 89.3 - 1.8 0.7 L F 90.7 93.9 17.0 7.4 24.0 96.7 93.7 91.8 17.8 10.4 21.0 23.2 17.7 15.5 9.3 | 6.1 30.3 28.5 25.5 3.3 4.9 5.8 11.6 1.5 4.9 3.4 6.8 1 1 .4 14.6 10.7 L — 1.8 0.4 0.7 2.5 52.8 64.1 50.6 1.4 2.4 14.2 — 4.9 1.5 2.5 0.8 % Lt i - 3.8 . 1. volcanic-hypabyssal % Lt - — — ” 2. quartz-mica tectonite 1. volcanic-hypabyssal 2.6 i _ 3.4 - 16.3 15.8 14.7 - - - 3.8 6.3 - - 3. argillite-shale 2. —— quartz-mica tectonite 5.1 2.5 3.3 . — —— — — ' * — — — 88.0 92.3 78.1 92.3 91.7 4. chert 3. argillite-shale — — 5.1 3.4 23! 3 —— __ 61.5 - 31.9 16.7 25.9 13.3 — — 12.0 — 15.6 7.7 8.3 5. aggregate quartz 4. chert 69.9 80.0 64.7 16.7 35.7 66.7 63.2 74.4 90.0 79.3 66.7 60.0 84.6 12.7 3.7 3.7 77.8 37.5 64.3 36.5 90.4 59.6 79.2 63.0 73.3 42.9 300.0 5. aggregate quartz 33.3 36.8 12.8 7.5 13.8 6.7 40.0 15.4 1.2 0.5 0.5 22.2 45.8 — 1.9 9.6 8.5 4.2 1 1 .1 13.3 57.1 DO. 0 92.3 93.7 DO.O DO . 0 Qp (4+5) 3.8 6.3 — — Ls (2+3) Qp (4+5) 100.0 100.0 87.2 — — — — 97.5 93.1 73.4 DO . 0 DO . 0 13.9 4.2 20.6 DO .0 83.3 64.3 38.4 DO. 0 68.1 83.4 74.1 86.6 DO .0 100.0 3.8 Lv (1) Ls (2+3) -— 10.2 2.5 3.4 26.6 - - 69.9 80.0 64.7 - 16.7 35.7 61.5 - 31.9 16.7 25.9 13.3 — — Lv (1) - - 2.6 3.4 16.3 15.8 14.7 — % Qm—F—Lt | " 83.9 22.1 77.9 71.4 59.7 Qm % Qm—F—Lt 3.9 54.9 10.7 16.2 22.4 F Qm 68.9 62.3 64.6 62.7 71.6 75.0 65.3 87.5 12.2 23.0 11.4 12.5 17.9 Lt F 87.1 86.0 8.5 4.6 10.9 93.8 86.7 87.4 71.1 79.7 83.6 83.0 82.9 87.7 17.8 8.0 21.0 23.2 17.7 15.5 3.3 1 1 .6 1 .5 4 .9 3 .4 6 .8 1 1 .4 14.6 10.7 Lt 9.3 6.1 30.3 28.8 25.5 4.9 5.8 13.3 29.8 14.4 14.1 10.7 9.5 3.6 7.9 61.3 66.6 63.7 2.9 8.4 6.8 23.1 27.4 15.4 9.1 9.6 5.7 2.5 1.6 % Qm-P-K •95 .6 28.7 87.9 81.5 72.7 Qm % Qm-P-K 4 .4 71.3 11.7 18.5 27.3 P Qm 79.5 88.7 75.4 73.0 80.2 82.9 90.4 85.0 97.9 94.2 96.3 92.5 88.0 85.0 89.1 - — 0.4 K P j 93.4 21.9 13.9 29.9 96.6 94.7 93.8 20.5 11.3 24.6 25.0 19.8 17.1 8.9 6.6 78.1 86.1 70.1 3.4 5.3 6.2 15.0 2.1 5.4 3.8 7.5 8.0 15.0 10.9 K - - - — 2.0 — 0.7 —-—-—— -— 0.4 - — _ 4.0 — —- i
\ APPENDIX B
MODAL POINT COUNTS OF SELECTED MORITA AND BISBEE FORMATION SANDSTONES
I = interstitial cement and matrix, Ic = calcareous inter stitial cement and matrix, Inc = non-calcareous interstitial cement and matrix, M = mica, H = heavy mineral, O = opaque mineral, Ca = calcareous allochem, Cf = calcite-filled fracture, Cu = calcite replacement of an unknown mineral component, U = unknown mineral component; see Table 1 for other symbols. 142 143
CO 00 o in id vo o in VD H CN o in in P— in rH ^ CO i 1 1 1 1 CN CO in CN in 00 co in o h H CN CO 53 53 u u U in rH r- co T—t rH rH CN CN 00 CO rH CN CO TP 1 I I II I l I I I I I 53 53 53 g | 1 i O o B m % total rock
% total rock 8 ! E E 5 i 29.5 29.8 19.0 14.5 0.3 Inc --- - I 25.0 Ic Inc 37.5 28.8 30.8 26.5 32.8 19.3 35.0 22.0 16.3 27.3 21.8 18.5 21.5 22.8 23.0 27.0 39.5 22.0 31.0 29.5 29.8 19.0 14.5 25.3 I (Inc + Ic) Ic 6.3 2.8 2.5 2.0 1.3 3.3 1.3 - — 2.3 — — 4.3 — — 12.3 8.0 3.0 I (Inc + Ic) 43.8 31.6 33.3 28.5 34.1 22.6 36.3 22.0 16.3 29.8 21.8 18.5 25.8 22.8 23.0 39.3 39.5 30.0 34.0 % total framework S _ 0.3 % total framework M ! I - 0.4 - - - H M ■ — — — — - - 0.3 ■ —» 0.3 — 0.3 _ o 00 —— 0.6 0.6 0.3 0 H 0.4 0.7 0.3 -- — — — 1 - 0.4 0.3 — 0.4 — - - Ca 0 0.4 1.8 - - - 0.4 5.9 0.3 — .— - 0.3 0.6 ! — — - - _ Cf Ca ” ~ — - - . ———— — - - Cf — — — — - - -— — • — — — Cu 0.4 1.0 0.4 0.9 2.0 2.7 U Cu 1.3 5.8 2.6 1.4 0.4 5.5 j 3.9 — — 5.0 — — 4.7 — — ; 6.2 - 2.5 1.1
U 1 0.4 0.7 0.4 0.6 • - M ro s - 1.2 - 0.9 3.0 6.1 - 0.4 0.8 % Q-F-L % Q—F—L 300.0 300.0 92.2 81.9 74.5 Q 7.8 Q 79.6 71.0 89.5 88.5 88.9 73.2 18.1 25.5 F ! 89.9 92.9 83.3 95.9 87.8 100.0 90.5 99.0 99.7 99.1 99.6 300.0 300.0 — — L ; • . F 20.4 29.0 10.5 11.5 10.7 26.8 ! 9.6 7.1 16.7 4.1 12.2 — 6.9 0.9 0.4 L 0.4 - - - - i 0.4 - 2.6 1.0 0.3 % Lt ! % Lt 1. volcanic-hypabyssal — — — — 2. quartz-mica tectonite 1. volcanic-hypabyssal 9.1 - 50.0 _ 21.1 2.5 2. quartz-mica tectonite — — — — - - 3. argillite-shale 90.0 98.2 80.0 70.0 75.0 4. chert 3. argillite-shale — — — *— - -— — mm mm mm 15.8 5.0 1.4 10.0 1.8 20.0 30.0 25.0 5. aggregate quarts 4. chert 72.7 300.0 300.0 90.9 90.9 89.9 50.0 300.0 300.0 60.0 300.0 100.0 52.6 87.5 80.8 88.2 300.0 84.0 91.7 5. aggregate quartz 27.3 - - 9.1 11.1 — - — 40.0 — - 10.5 5.0 17.8 11.8 - 16.0 8.3 300.0 300.0 100.0 300.0 300.0 Op (4+5) Ls (2+3) Qp (4+5) 100.0 300.0 300.0 300.0 90.9 100.0 50.0 300.0 300.0 300.0 300.0 100.0 63.1 92.5 98.6 100.0 300.0 300.0 300.0 Lv (1) Ls (2+3) — — — — — — - ~ — — — — 15.8 5.0 1.4 LV (1) 9.1 — - 50.0 — — — - 21.1 2.5 - % Qm—F—Lt % Qm-F-Lt 35.8 80.3 89.0 78.9 69.0 Qm — — 7.8 18.1 25.5 F Qm 74.7 60.7 86.4 80.6 85.1 63.9 89.5 91.6 79.4 94.0 82.7 95.0 86.1 86.2 76.1 91.6 96.7 90.8 86.1 14.2 19.7 3.1 3.0 5.5 Lt F 20.4 29.0 10.5 11.5 10.7 26.8 9.6 7.1 16.7 4.1 12.2 — 6.9 — — 0.9 0.4 Lt 5.0 10.3 3.1 7.9 4.2 9.3 0.9 1.3 3.9 1.9 5.1 5.0 6.9 13.8 23.9 7.5 2.9 9.2 13.9 % Qm—P—K % Qm-P-K 300.0 300.0 91.9 81.4 73.0 Qm — — 8.1 10.2 12.4 P Qm 78.6 67.7 89.2 87.5 88.8 70.5 90.3 92.8 82.6 95.8 87.2 100.0 92.5 300.0 300.0 99.0 99.6 300.0 300.0 — ' — - 8.4 14.6 K P 21.4 32.3 10.8 12.5 11.2 29.5 9.7 7.2 17.4 4.2 12.8 - 7.5 - - 1.0 0.4 K APPENDIX C. POINT COUNTING PROCEDURES
1. Four hundred points were counted per thin-
section at 1 nun spacing, giving an approximate 75 percent coverage of the thin-section. Standard deviation was determined to be 5 percent or less (Van der Plas and Tobi
1965) for the grain parameters selected.
2. The vast majority of the calcite is replace ment or secondary calcite; whenever it could be determined what the primary material (i.e., cement, matrix, overgrowth,
grain) was, the point was counted as the primary material/
substance. Whenever the diagenesis of calcite replacement
had progressed to a stage where it was no longer apparent
as to the nature of the primary material/substance, the
point was counted as interstitial calcite (Ic) or calcite
replacement of an unknown grain (Cu). Thus, the calcite
content of the rock, when present, is always understated.
In other words, the summation of the various calcite
categories gives a value less than the total calcite
content of the rock. (In most cases, the rock samples
taken were from sandstone lenses which were bounded by
mudstone and calcareous rocks. The calcite content is of
no significance to the calculations and plots, and the
144 145 subsequent percentile calculations were recomputed disre garding calcite, heavy minerals, and opaque minerals.)
3. In most instances where silica overgrowths were observed, a "dust ring" was evident denoting the boundary between the original grain and the silica overgrowth. The
Q-F-L plots by definition require that only detrital quartz be included in the analysis. However, it was not always possible to discriminate whether the point being counted was original quartz or overgrowth. Therefore, the following procedure was adopted:
a. If the overgrowth was in optical continuity with the quartz grain, the point was counted as monocrystal line quartz (Qm) regardless of the existence or position of the dust ring.
b. If the overgrowth was not in optical continuity with any of the contiguous quartz grains, the point was counted as non-calcareous interstitial cement and matrix (Inc).
c. Eight random thin-sections were reexamined to determine the percentages of quartz in overgrowth grains where dust rings were clearly evident. One hundred points were counted to determine the percentages that fell inside
and outside the dust rings. In all eight cases the per
centages of overgrowth quartz was between 18 and 22 percent, with an average of 19.4 percent. In those eight samples
the observed percentage of overgrowth was applied to the 146 count of monocrystalline quartz (Qm) and the calculated number of points was transferred to the non-calcareous interstitial cement and matrix count (Inc). In the remaining samples, quartz overgrowths were clearly present to the same degree (i.e., tight grain contacts with no porosity, or "welded grains"), or there were no overgrowths, the cement/matrix being calcite. Where quartz overgrowths were present, the overgrowths were counted as per (a), above, and the total Qm and Ic counts were adjusted using the 19.4 percent calculated average value.
4. Only a relatively few thin-sections showed stained evidence of K-feldspar (Figure 71). However, the potassium staining was clearly evident on these few thin- sections, which included a thin-section of rhyolite obtained from the Dragoon Mountain section. Fifty thin-sections were duplicated from the collected samples in the event that a chemical anomaly prevented the first batch from being properly stained for K-feldspar. However, the results were the same the second time. Additionally, fifteen stained thin-sections were duplicated by a dif ferent laboratory, but again the results showed anomalously low percentages of potassium feldspar.
5. Chert grains exhibited varying degrees of coalescent neomorphism. In most instances, the grain was clearly of a chert origin, and was so counted. However, the apparent end-product of this coalescent neomorphism 147
Figure 71. Photomicrograph of microcline (K-feldspar) grains in a subarkose (X 32). Morita Formation, Gadwell Canyon, northern Mule Mountains, Arizona. produces a grain which approximates that of aggregate quartz. Where the recrystallization had progressed to such an extent that the, grain was no longer clearly of a chert origin, it was counted as aggregate quartz. There fore, the percentages of aggregate quartz, though of little statistical significance in any case, are probably somewhat inflated at the expense of quartz. Similarly, some thin-sections exhibited partial dolomitic replacement of chert. Wherever the dolomite was clearly of a cherty origin, it was counted as chert.
6 . In certain instances, chert or silica gel was encountered as a matrix between grains, and was tabulated as "non-calcareous interstitial cement and matrix" (Inc) whenever it was clearly an amorphous texture. Some thin- sections showed evidence of compaction; the contact points between adjacent quartz grains clearly depicting the stress by undulatory extinction in otherwise straight-extinction grains. These same thin-sections frequently showed what appeared to be compacted chert grains (i.e., "pseudo matrix," see Dickinson 1970) where the chert had apparently been forced into the interstices of adjacent grains. Such chert was counted as chert (Qc), in spite of its accomo dation to adjacent grains.
7. When sericitization or accumulation of hema tite, clay, or other alteration products obscured a grain to the extent that its primary features were no longer 149 discernible, the grain was tabulated as unknown (U), even though a logical inference might lead one to classify a sericitized grain as plagioclase feldspar.
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UNIVERSITY OF ARIZONA LIBRARY
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