Structure and stratigraphy of the Helmet Peak area, Pima County, Arizona

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Authors Studebaker, Irving Glen, 1931-

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/553924 STRUCTURE AND STRATIGRAPHY OP THE HELMET PEAK AREA, PIMA COUNTY, ARIZONA

by ' Irving G. Studebaker

A Thesis submitted to the faculty of the DEPARTMENT OF GEOLOGY In Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE ' In the Graduate College UNIVERSITY OF ARIZONA

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STATEMENT BY AUTHOR

This thesis has been submitted in partial fu lfill­ 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 Geology Department or the Dean of the Graduate College when in their judgement the proposed use of the material is in the interests 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:

w. c v Date P ro fe ss o r TABLE OF CONTENTS Page Introduction 1 Acknowledgements 2 Stratigraphy 2 . Perm ian 3 Epitaph dolomite 3 Scherrer formation 3 Concha limestone 3 Rainvalley formation J+ Cretaceous (?) !{. Arkose member 4 Lim estone and s il ts to n e member 5 Sandstone and s il ts to n e member 6 Depositional conditions 7 Correlations 9 Igneous rocks 11 Rocks of unknown relations 12 Structure 13 Discussion 1)| Summary 17 Summary 18

Conclusions 20 Appendix 22 References cited 26 LIST OF ILLUSTRATIONS

Page P la te 1. Pediment surface extending eastward Follow ing from the Slerrita Mountains as viewed page 1 from the north* : ' ' . ; P la te 2. Geologic columns of the Tucson In p ocket Mountains and Helmet Peak area* P la te 3. Outcrop gradation of the limestone Follow ing and siltstone member in the southern page 12 part of the area* P la te 4* Geologic map of the Helmet Peak area* In pocket ( P la te 5* Tectonic map of the Helmet Peak area. In pocket

Figure l e Sketch location map of the Helmet Follow ing Peak a re a . page 1 Figure 2. Generalized east-west cross section Follow ing of the Helmet Peak area* page 34 INTRODUCTION

The Helmet Peak area lies twenty-two miles south- southwest of Tucson, Arizona in the Pima mining d istrict (Figure l.)» The area under investigation includes most of section 11, and the western part of section 12, Township 17 South, Range 12 East, Gila and Salt River Meridian. Elevations in the area range from 3450 feet in section 12 east of Helmet Peak to 4015> feet at the top of Helmet Peak. The entire area lies on the Slerrita Mountain pediment which is a classic example of a pediment formed by parallel slope retreat. The pediment has a gentle eastward slope with Helmet Peak and other hills appearing as erosional remnants. (Plate !•)• The rocks of the region are uniformally beveled and maximum relief, to the top of Helmet Peak, is 500 to 600 f e e t .

This investigation was conducted to determine the geologic history, structure and stratigraphy in the area adjacent to Helmet Peak. Permian and Cretaceous (?) rocks crop out and have been intruded by andesites and granite. Faults mark the north and south boundary of the area and alluvial deposits cover the rocks on the east and west. 1 PLATE lo PEDIMENT SURFACE EXTENDING EASTWARD FROM THE SIERRITA MOUNTAINS AS VIEWED FROM THE NORTH. FIGURE I. SKETCH LOCATION MAP OF THE HELMET PEAK AREA.

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PAPAGO INDIAN RESERVATION

SAN XAVIER MINE HELMET PEAK V jr i * PIMA MINE AREA

SIERR4TA TWIN ^ BUTTES 2

Limestone and quartzite are generally well exposed but siltstone, sandstone, arkose and igneous rocks form topographic lows and bed rock is generally obscured by alluvium.'. Bed rock is exposed in approximately thirty per­ cent of the area with the best exposures in road and stream cuts and along the flanks and crest of Helmet Peak* Previous work in the area consists of a recon­ naissance thesis by Mayuga in 19^2, a thesis by Park in 1929 and a district survey by Ransome in 1922, Park’s thesis area lies north of the area under consideration in this paper* Mines in this district have heen Intermittently active since the late 1800*s , ACKNOWLEDGEMENTS Invaluable assistance was received from msuiy persons in the completion of this study. The assistance of Dr. W. C. Lacy, in both discussion of principles and the field investigation, is greatly appreciated. Dr. H. W. Miller examined some rock material for microfossils and Dr. W. D. Pye discussed depositional conditions with the author* STRATIGRAPHY The Permian sediments are limestones, dolomites and quartzites about 1,200 feet thick* The earliest and 3 latest Permian time Is not represented In this area.

The Cretaceous (?) beds are arkose, lim estone, siltstone and sandstone with some volcanic flows or in­ trusions near the top of the section. The Cretaceous (?) section is approximately 2,000 feet thick and may extend beyond the boundaries of Cretaceous tim e.

Perm ian Epitaph dolomite (Gilluly; Cooper and Williams 192W»

The Epitaph dolomite is a dolomite with knots of silica and may contain sandy lim estones and red shales.

This formation is approximately 100 feet thick (Bryant

1925) and crops out in the central and northern part of

Helmet Peak.

Scherrer formation (G illuly, Cooper and W illiams 192^)*

The Scherrer formation consists of two sandstone members separated by a dolom itic lim estone member. The sandstones are commonly quartzitic probably due to hardening at the surface. Gradation between the sandstone and quartzite is rapid and may occur in short distances.

This formation is 372 feet thick (Bryant 1925) and crops out in the north central part of Helmet Peak.

Concha lim estone (Bryant 1955)*

The Concha lim estone is a prominent c liff former. li­ lt is a light to purplish gray 'limestone with - varying chert content that approaches 75 percent in certain zones.

The chert varies from light gray to light brown weather­ ing nodes and stringers. The base of the Concha lime­ stone is above the last sandstone member of the Schemer formation and the top occurs at the top of the massive gray lim estone immediately below the lowest black lim e­ stone beds at the base of the Ralnvalley formation. This formation is 3^6 feet thick (Bryant 1955) and crops out in the central part of Helmet Peak.

Rainvalley formation (Bryant 1 9 5 5 )# The Rainvalley formation has more lithologic variation than the Concha limestone and contains sandstone beds, sandy or shaly limestone and dolomites. This for­ mation contains less chert than the Concha limestone. The base of the Rainvalley is marked by the first thin bedded black limestone above the highest massive gray limestone of the Concha and the top is not exposed at Helmet Peak.

This formation is 4 7 1 feet thick (Bryant 1 9 5 5 ) and crops out in the central and southern part of Helmet Peak.

Cretaceous (?) Arkose member.

The arkose member is a massive, silicified , poorly sorted clastic rock consisting of quartz and feldspar (plate 2j. The quartz grains sire sub angular to rounded and vary In size from 1 to 10 millimeters. The feldspars are decomposed. Weathered surfaces have a black to reddish brown color while fresh surfaces are white to light gray with the quartz gray and the feldspars a chalky gray white color. Bedding is poorly exposed and can be distinguished only in recent stream cuts. This member is approximately 2,2f>0 feet thick and crops out in the southwestern part of the area. Limestone and siltstone member. The lim esto n e and s i l t s t o n e member i s an in terb ed d ed sequence of limestone and siltstone beds (plate 2j * Lime­ stone beds are 4 inches to 12 inches in thickness and form projections up from the surface. The limestone is sandy, gray in color and brown weathering. Limestone beds are re­ crystallized and have a sandy appearance on weathered surfaces Limestone beds have lithologic and fossil variations with several horizons containing pelecypods (Mactra) up to l /2 inch and circular algal masses up to 1- 1 /2 in c h e s. The siltstone or fine grained sandstone is brown to reddish brown in color and poorly exposed, beds are from 1 inch to 4 inches in thickness. Siltstone beds do not crop out on the surface and are observed only where exposed in s e v e ra l p ro sp ec t p i t s . This member crops out in th e so u th - central part of the area west of the sandstone and siltstone member and is approxim ately 100 feet thick* 6

Sandstone and siltstone member* The sandstone and s il ts to n e member i s an i n t e r — bedded sequence of sandstones and siltstones with extreme variations in both thickness and lithology (Plate , Sand­ stone beds are thicker than associated siltstone beds and the sandstone beds become thinner toward the top of this member* The sandstones are gray to red, brown or green in color and are in part arkosic* The beds are from 2 inches to 18 inches in thickness, coarse to fine grained and may exhibit cross or graded bedding. The sandstones are hardened and probably would be classified as orthoquartzites. This hardening is probably intensified by the action of weathering at the surface. The siltstones are soft and poorly exposed, they are red or green in color and are from 1 inch to 6 inches in thickness. They are broken and several fracture directions are present in any exposure producing a fine blocky character. Conglomerate lenses present near the top of the se­ quence consist of a coarse grained sandstone matrix with rounded quartz fragments up to 2 inches in diameter. Small red volcanic fragments in the conglomerate matrix reflect the presence of volcanic units in this part of the section. Red volcanic flows or intrusions, southeast of Hel­ met Peak, are concordant with the sandstone and siltstone' 7 member. The sandstone and s il ts to n e member i s approxim ate­ ly 2,k$0 feet thick and crops out adjacent to Helmet Peak on the east and west*

Depositional Conditions Regressive and transgressive seas deposited marine Permian sediments over much of southeastern Arizona. Regres­ sive sea deposits are represented by the Scherrer and Rain- valley formations and transgressive sea deposits are repre­ sented by the Concha limestone* The youngest Permian sedi­ ments were formed by regressive seas* . Neither Triassic nor Jurassic rocks are recognized in southeastern Arizona* The Bisbee group was deposited in southeastern Arizona during Cretaceous time* Correlateable Cretaceous marine deposits are not known north of the wedging out of the marine limestone members of the Bisbee group. Brennan in 1957# at Cienega Gap, records a sequence which contains no marine limestone members as probably equivalents to the Bis­ bee group* Depositional conditions for the continental Cretace­ ous (?) beds are not readily apparent. Wide spread fossil horizons are absent and individual beds are not traceable for more than several hundreds of feet. Paleogeography for the 8

Triassic, Jurassic, Cretaceous and Cenozoic appears to fall into a pattern that extends to the present time. An axiom of geology states “ that the present is the key to the past". The following features are apparent from an examination of the present geologic conditions in southeastern Arizona* ■ 1. The area is positive at the present time* 2. Typical marine sediments are not being deposited near the area except in the Gulf of lower California. 3* Marine sediments are not being deposited over the area. 4* Deposition is of.the continental type in basins or river valleys and the river valleys are probably tectonic depressions similar to the basins in origin. . The Cretaceous (?) section at Helmet Peak is slightly le s s th an 5,000 feet thick consisting of the sandstone and s i l t s t o n e member 2,450 feet thick, the limestone and siltstone member 100 f e e t th ic k , and th e arkose member 2,250 feet thick* A basinal deposit is - indicated by deposition of this magnitude and any type of shelf deposit is precluded. The Cretaceous sediments character is probably best explained by a delta or shoreline type deposit with the varied types of sediments representing different gradients and distance to the source area* 9 The paleogeography of the region probably consisted of an embayntont, similar to the Gulf of lower California,: extending to the Bisbee area. Positive areas to the north and west were supplying sediments. The positive area has been tectonically active at least two times since the deposi­ tion of the Cretaceous (?) beds and it is reasonable to assume that at the time of deposition it was active.

Correlations Permian correlations are well established by Bryant in 1 9 # . Cretaceous (?) correlations are difficult because no extensive marker beds are present and because the beds are unfossiliferous. W. H. Brown (1939) named a series of formations in the Tucson Mountains and tentatively dated one of these formations, the Amole arkose, as upper Cretaceous but McKee in 19f>l had the fossil evidence reexamined and dated these beds as probably lower Cretaceous. Mayuga in 19l|2 transferred two of Brown’s formation names to Helmet Peak. These formations are the Amole arkose and Recreation red beds. Mayuga cited lithologic sim ilarity as reason for this. He considered the outcrops east of Helmet Peak to be Recreation red beds and those west of Helmet Peak to be Amole ark o se. The fossil record, around Helmet Peak, is not impres­ sive consisting of only small pelecypods and small concentric 10 algal forms in the limestone and silt stone member. The pelecypods are poorly preserved, probably due to recrystal­ lization of the limestone, and are difficult to identify, however, according to Mayuga (1942) these were identified as Mactra. The limestone therefore is no older than Cretaceous. East of Helmet Peak are outcrops that consist of thin bedded, arkosic sandstones which may contain local resistant fine grained red sandstone beds. The reddish outcrops ap­ pear more extensive than they actually are due to the flat dip and the resistant nature of the sandstone. Volcanic rocks are exposed southeast along the strike of the sandstone and siltstone member. The volcanic rocks are contemporaneous with the sandstones and siltstones and actually are inter- bedded with them because fragments of the red volcanic rock is incorporated into the top conglomeratic lenses of the sandstone and siltstone member west of Helmet Peak. The vol­ canic rocks are either flows or intrusions but they are con­ temporaneous with the sandstone and siltstone member. The outcrops west of Helmet Peak consist of a repe­ tition of the sandstone and siltstone member with the struct­ ures in the Cretaceous (?) rocks continuing under Helmet Peak. The lim esto n e and s il ts to n e member and arkose member l i e con­ formably below the sandstone and siltstone member. The Helmet Peak arkose member is p ro b ab ly e q u iv a le n t with the Amole arkose of the Tucson Mountains area because 11 the arkose seems to represent a wide spread depositional condition (Plate 2.). The beds east of Helmet Peak are not correlated with the Recreation red beds because the Re­ creation red bed of the Tucson Mountains are older than the Amole arkose and the beds east of Helmet Peak are younger than the arkose (Plate 2,)• Kinnison (1958) in a thesis raised Brown’s Amole arkose to a group rank and described four formations in this group (Plate 2,) in the Amole mining district south of th e a re a o f Brown’s work. He b e lie v e s th a t th e Amole Group crosses the boundary between the Cretaceous and Tertiary* This maybe true also in the Helmet Peak area* Kinnison's Mouse House formation maybe equivalent to the limestone and siltstone member of the Helmet Peak area but this is un­ likely because the limestones pinch out rather abruptly, with in the Helmet Peak area, into a coarse sandstone and this indicates the depositional area of the limestone was small, maybe hundreds of feet or, at the most, a mile or so in maximum dimension. In the field I have observed the sandy limestone grade into a coarse sandstone in an outcrop length of 3 feet (Plate 3*).

IGNEOUS ROCKS Four different intrusive igneous rocks occur in the area! the first is an andesite intrusion west of Helmet 12 Peak which has forced, up Cretaceous (?) sediments and forms an autobreccia along its edge; the second is a granite in the southwestern part of the area which is emplaced in arkose and has an intrusive relationship; the third is a question­ able andesite intrusion that does not display good intrusive relationships; and the fourth consists of andesite intrusions along the south boundary fault (Plate )• Small andesite intrusions occur along the south boundary fault and in this respect are similar to alteration features that are associated with all the faults in the area. These small andesite intrusions seem to cross bedding but the relationship between the bedding and intrusions are in­ d i s t i n c t . The andesites and granites show intrusive relation­ ships into the Cretaceous (?) sediments and were intruded along fault zones. The andesite intrusion in the north- central portion of the area disrupts the strike of sandstone beds in the sandstone and siltstone member. The granite, which is normally coarse grained, becomes fine grained as the arkose contact is approached indicating a chilled con­ tact and intrusive relationship,

ROCKS OF UNKNOWN RELATIONS Quartzite is present following fault zones and pro­ bably represents silicified country rock adjacent to PLATE 3 . OUTCROP GRADATION OF THE LIMESTONE AND SILTSTONE MEMBER IN THE SOUTHERN PART OF THE AREA. 13 fractures* Almost all major faults have this quartzite or s 11idification as a narrow band paralleling the trace of the fault. The silicification appears to be caused by hydrothermal solutions that have come up along the broken fault zone.

STRUCTURE The Helmet Peak area reveals both faulting and fo ld in g . . The folding is probably in response to one episode of tectonic activity and consists of a broad syncline in Cretaceous (?) sediments and a tight anticline in Permian sediments. The syncline and anticline plunge steeply to the southeast. Fault movement is present in three directions; these directions are a north-south direction represented by the Democrat tear, a north-east direction represented by the Bobby tear, and an east-west direction represented by the Central and South boundary faults. The faulting occurred at several times because the Democrat tear and Bobby tear have large intrusions along their fault zones while the Central and South boundary faults contain only minor in- trusions or no intrusions at all* A major episode of fault- ing occurred later than the folding of the Cretaceous (?) beds and offset the limestone and siltstone member along ■ " ' 34 the Central fault about 2,£00 f e e t In a l e f t l a t e r a l move­ m ent. Andesites and granites were forcefully Intruded In­ to the folded Cretaceous (f) sediments*

D iscu ssio n The Helmet Peak anticline appears to be on top of the folded Cretaceous (?) sediments and the forces that folded the Cretaceous (?) sediments probably placed the Permian beds on top the Cretaceous (?) beds (Figure 2.)* Helmet Peak is probably a klippe as indicated by the follow­ ing evidence! 1 . The Permian limestone outliers do not have roots and Cretaceous (?) beds crop out on all sides. , 2. Conglomerate lenses are found on the south­ west side of Helmet Peak and are also found along strike on the southeast side. These lenses are not disturbed and seem to be continuous. 3« Similar sandstone beds crop out on the northeast and northwest sides of Helmet Peak and appear to strike under the Peak* These beds are structurally confused but ggagijglB ______3 4 0 0 FEET

HORIZONTAL

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FIGURE 2 . GENERALIZED EAST-WEST CROSS SECTION OF THE HELMET PEAK AREA. 1* this may be near the sole of the thrust and as might be expected the beds are broken* lj.e Permian limestone outliers have been faulted off the south end of Helmet Peak and these have no roots but sit upon Cretaceous (?) rocks (Plate !).•)• Two mechanisms are possible to explain the position of the Helmet Peak anticline on top of the folded Cretaceous (?) beds* The f i r s t e x p la n a tio n is that thrusting put th e - Helmet Peak anticline in its present position. This thrust­ ing may have closed the period of Cretaceous (?) folding or happened during a later tectonic disturbance* The second explanation is that the Helmet Peak anticline was thrust up through fractures in Cretaceous (?) rocks somewhat in the manner of a diapyre fold. The second method has two advantages; the first advantage is that it explains the sharp­ ness of the anticlinal fold which was caused by the dragging of the sides of the anticline as it rose; and the second ad­ vantage provides the mechanism for implacement of Helmet Peak by the forces applied to the Permian formations during the deformation of the Cretaceous (?) beds* The Helmet Peak area has been tectonically active since Permian time with proof of this fact as follows: 16 1. Part of the Concha limestone has been faulted out (Bryant 1955)• 2. Cretaceous (?) beds are folded. Thrust or thrust like mechanisms placed Permian over Cretaceous (?) formations* 3# A number of Intrusions are present which forcefully intrude folded Cretaceous (?) beds* 4* Miocene beds are disturbed in the region and later sediments are resting upon them in an angular unconformity (Brennan 1957)o The disturbance is probably represented, in the Helmet Peak area, by faults which offset Cretaceous (?) beds and by a late

. ' ■ : - . . . . Tertiary conglomerate. These faults, which offset folded Cretaceous (?) beds, are of large magnitude and the limestone and s i l t s t o n e member i s o f f s e t about 2,500 feet along the Central fault with

a left lateral movement (Plate 5*)* Further proof of tectonic activity is that the dips of beds south of the Central fault are west while those north of the

fault are east (Plate 5.)» 17 f?« Faults, with left lateral movement, have offset portions of the Ralnvalley formation In an en echelon manner at the south end of Helmet Peak and the south end of the peak Is elongated eastward along these faults.

Summary The structural events of the Helmet Peak area probably occurred In the following order: 1. Permian sediments were deposited under marine conditions. 2. Non-deposition or erosion occurred with part of the Concha limestone being faulted out. 3. The Cretaceous (?) beds were deposited in a basin upon an unstable tectonic area which may have sloped toward the south. These deposits may represent a delta or shore line type of deposit. !}.• The Cretaceous (?) beds were folded, faulted and mineralized. 5» Intrusions occurred along existing f a u l t s . 6. Helmet Peak was forced.into its present 18 p o s itio n . 7. Faults, with left lateral movement, offset the limestone and siltstone member 2,500 feet and the block between the.Central fault and South boundary fault was tilted to the east. 8. Tertiary gravels were deposited.

SUMMARY The Helmet Peak area Cretaceous (?) beds are not so structurally complex that they cannot be fit into a comprehensible pattern. These beds are almost 5,000 feet thick and were deposited in a basin. The Cretaceous (?) beds are in a syncline which plunges steeply to the southeast. Helmet Peak, which is made up of Permian sediments, is an anticline plunging 70 degrees to the southeast and appears to be a klippe. The Permian sediments consist of limestones, dolomites and quartzites and were deposited from marine se a s. The Cretaceous (?) sediments, which consist of the arkose member, limestone and siltstone member, and the sandstone and siltstone member which contains interbedded volcanic material, were deposited in a probable delta or shoreline type conditions in a basin with the different 19 sedimentary types representing different gradients and varied distances to the source area. Several periods of faulting are represented and intrusions have been localized along north-south and northeast trending fault zones. Fault offset is difficult to interpret but a period of left lateral faulting in an east-west direction, later than the folding of the Cretaceous (?) sediments, has offset the limestone and s i l t s t o n e member 2,500 f e e t along th e C e n tral f a u l t and tilted the southern part of the area enough to change bed dips from east to west. Blocks of the Rainvalley formation at the south end of Helmet Peak have been offset eastward by left lateral fault movement along the Central f a u l t . Fault movement into the Tertiary is indicated by a conglomerate.

Univ. o' Anynnfl i ihrary CONCLUSIONS Two types of conclusions may be drawn from this study. These are conclusions that have been proven and those which need more investigation. The following conclusions have been proven: 1. The Cretaceous (?) sediments were d ep o site d in some type of su b sid in g area, local in extent. The type of sediments which are present suggest a delta or shoreline deposit with the difference between members probably representing changing gradients and transportation distance. 2. Cretaceous (?) sediments were folded. This was accompanied by shattering and fracturing. 3. Helmet Peak was thrust over Cretaceous (?) beds or what is more probable Permian formations were forced over the Cretaceous (?) beds in a diapyre like a c tio n . Ij.. Granite and andesites were intruded along the north-south Democrat tear and northeast Bobby tear. 20 21 5 . The lim esto n e and s il ts to n e member was o f f s e t by 2 , 5)00 feet of left lateral movement along the east west Central fault and tilted in probable Miocene or later time. The south end of Helmet Peak is offset in en echelon blocks along the Central fault. The following problems need more work to reach a conclusion: 1. A method or process is needed to find the top and bottom of the Cretaceous (?) beds. 2 . A method is needed to date accurately red bed type deposits. . The possibility exists that the Cretaceous (?) beds may be in part Tertiary, Jurassic or even Triassic as well as Cretaceous. APPENDIX The following measured Permian section of Helmet Peak is after Bryant 1955» Bryant's thesis contains detailed lithologic and fossil descriptions. The formation thicknesses are as follow: F eet Snyder H ill Group Rainvalley formation j+Tl Concha limestone 346 Schemer formation 372 1,189 Cretaceous (?) beds as determined from measured surface exposures in the Helmet Peak area. Conglomerate lenses measured at the top of th e sandstone and s il ts to n e member in a small wash 1,000 feet southwest of Helmet Peak. Stratigraphic top of the exposure F eet Coarse gray sandstone with graded bedding with a conglomeratic sandstone near the top, gray, reddish brown, white and black quartz fragments to 1 1 /2 in c h es. 32 Cover. 29 Coarse gray quartz sandstone not conglomeratic. - 3 22 23 F eet

Cover, 25 Coarse gray sandstone with some red and gray rounded fragments to 1/2 inch with graded bedding. Ij.

C o v e r . 1 3 5

Coarse gray sandstone with conglomeratic fragments to 1 inch. The sandstone is spotted with red rounded fragments. 15

C o v e r . 1 8

Fine grained black sandstone. 5

C o v e r . 1

Fine grained black sandstone. 1 Purple gray cross bedded fine grained sandstone. 16

C o v e r . 8 Coarse grained to conglomeratic sandstone with grad&di bedding. Small red volcanic fragments in the matrix. Conglomerate fragments reddish brown up to 1 1 /2 in c h e s. 6 349 Sandstone and s il ts to n e member as exposed 1,700 feet south along the main road from the bridge in section 11. Stratigraphic top of the exposure ■ F eet Interbedded red and green siltstone. 5 Thin bedded sandstone. 9 24

F e e t

Red siltstone. 3 / 4

Gray brown thin bedded sandstone., 7

Interbedded red siltstones and fine grained gray s a n d s t o n e . 8

Blocky thin bedded s a n d s t o n e . 1 0

Red fine grained sandstone. 1 / 4

Blocky brown gray sandstone with no apparent bedding. 6

Broken sandstone with fine grained light green material in the matrix. 2 1

Blocky gray white fine grained sandstone. 4

Interbedded thin beds of green siltstone and gray sandstone. 9

Red siltstone. l

Fine grained gray sandstone with red siltstone inter beds. Jfc2

1 1 7

Limestone and siltston e member as exposed in the southeastern part of section 11.

Stratigraphic top of the exposure

F e e t

Sandy, gray brown weathering lim estone, recrystallized. 2

C o v e r . 9

Brown weathering gray sandy limestone with pelecypods and algae that do not weather out also contains some c h e rt. Cover. . Brown w eathering gray sandy limestone with pelecypods and algae that do not weather out. Cover. Fine grained sandy limestone with pelecypods and algae not to aboundant, weathered surface brown and sandy ap p earin g . Cover. Gray black crystaline lim esto n e w ith many ,pelecypod shells (1/2 inch) which weathers gray. This limestone contains aboundant algae, forms ridges and looks recrystallized. Cover. Crystalline gray sandy, brown weathering limestone with traces of pelecypod shells 1/2 inch long and circular algal accumulations to 1 1/2 inches. Cover. Same as above limestone. References cited

Brennan, D. J ., 1957# Geological reconnaissance of Gienega Gap, Pima County, Arizona: Univ. Arizona, Ph. D. Thesis, 53 p.

Brown, W. H., 1939, Tucson Mountains, an Arizona basin range type: Geol. Soc. America Bull. v . 50, no. 5# p. 697-760.

Bryant, D. L ., 1955# Stratigraphy of the Permian system in southern Arizona: Univ. Arizona, Ph. D. Thesis, 2 9 9 p .

G i l l u l y , J. , C o o p e r , J . R. and W illiams, J . S ., 1954# I»ate Paleozoic stratigraphy of central Cochise County, Arizona: U. S. Geol. Survey, Prof. Paper 266, 1+9 p

K in n i s o n , J . E ., 1958, Geology and ore deposits of the southern section of the Amole mining d istrict, Tucson Mountains, Pima County, Arizona: Univ. Arizona, M. S. Thesis, 118 p.

Mayuga, M. N., 191+2, the geology and ore deposits of the Helmet Peak area, Pima County, Arizona: Univ. Arizona, Ph. D. Thesis, 12l+ p.

McKee, E. D ., 1951# Sedimentary basins of Arizona and adjoining areas: Geol. Soc. America Bull. v. 62, no. 5# P• 481-505.

P ark , C. F ., 1929# Geology of the San Xavier district: Univ. Arizona# M. S. Thesis, 30 p. Ransoms, F. L., 1922, Ore deposits of the Sierrita Mountains, Pima County, Arizona: U. S. Geol. Survey, Contributions to economic geology pt. 1, p. 407-428. PLATE 4 .

GEOLOGIC MAP OF TH E HELMET PEAK AREA PIMA COUNTY, ARIZONA IRVING G STUDEBAKER 1959

LEGEND

QUATERNARY

SAND AND GRAVEL

CRETACEOUS (?)

SANDSTONE AND SILTS TONE MEMBER WITH VOLCANICS PROSPECT PIT, ADIT LIMESTONE AND SlLTSTONE

ta ARKOSE MEMBER MINE SHAFT PERMIAN FORMATION CONTACT

rainvalley formation FAULT CONCHA LIMESTONE SOUTH BOUNDARY FAULT THRUST TYPE FAULT SCHERRER FORMATION ROAD / EPITAPH DOLOMITE 500 1000 INTRUSIVE ROCKS FEET

ANDESITE I A

ANDESITE BRECCA

GRANITE I G

ROCKS OF UNKNOWN RELATIONS ^ f £ ? 7

TUCSON MOUNTAINS HELMET PEAK AREA AMOLE MINING DISTRICT MODIFIED FROM BROWN 1939 STUDEBAKER 1959 MODIFIED FROM KINNISON 1958

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MASSIVE AND FLOWS DIOPSIDE ANDESITE BRECCIA AND STRATIFIED TUFF SAFFORD TUFF BREAK

CAT MTN RHYOLITE

- UNCONFORMITY —

ARKOSE INTERBEDDED WITH I SHALES AND THIN-BEDDED LIMESTONES i AMOLE ARKOSE

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BEDDED SANDSTONES AND ASSIVE FINE-GRAINED SHALES, ARKOSE, OCCASIONAL ' SILTSTONES. GRADED AND LJ ME STONE S BRICK-RED SANDSTONES CROSS BEDDED- VOLCANICS RECREATION RED BEDS AND CONGLOMERATES NEAR ECHO VALLEY FM. THE TOP SANDSTON E SILTSTON E BREAK AND VOLCANICS MOUSE HOUSE F M o- THIN LIMESTONE, SHALE BEDS

ARKOSE, QUARTZITE, SILT­ PRINCIPALLY ANDESITE STONE, SOME CONG. FLOWS AND TUFFS

DEAD COW FMo -ALTERNATING THIN BEDS OF LIMESTONE SILTSTONE LIMESTONE AND SILTSTONE

FINE AND COARSE GRAINED ARKOSE. IN PLACES SILICIFIED prr-.v. CRETACEOUS VOLCANiCS IRON AND MANGANESE STAINED

ARKOSE i LIKE ECHO VALLEY FM. I BRAUN FM. I

BREAK BREAK BREAK ------1 PERMIAN LIMESTONE PERMIAN LIMESTONE PERMIAN LIMESTONE MASSIVE BLACK LIMESTONE

VERTICAL SCALE 0 1000 2000 i- i t------1 l FEET PLATE 5 . TECTONIC MAP OF THE HELMET PEAK AREA

LEGEND

PERMIAN INTRUSIVE ROCKS

ANDESITE SOUTH BOUNDARY FAULT LIMESTONE, DOLOMITE, QUARTZITE ANDESITE BRECCIA CRETACEOUS (?) 5 7 ? GRANITE

D o O o ARKOSE 1000 2000 LIMESTONE SILTSTONE CONTACTS FEET SANDSTONE SILTSTONE AND VOLCANICS FAULT ROCKS OF UNKNOWN RELATIONS THRUST TYPE FORMATION QUARTZITE STUDEBAKER 1959 E 9 7 9 /

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