Structure and stratigraphy of the Helmet Peak area, Pima County, Arizona Item Type text; Thesis-Reproduction (electronic); maps Authors Studebaker, Irving Glen, 1931- Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 28/09/2021 12:11:00 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 1 9 5 9 : : r:v V/i'.AHi'.l1 . i'/M -v. A: - o *: i » . - •- «■ - .■ ■. v. ' ■ i: '.'fr ‘i C •' 'i ao o..o (■:' o.C.f--0 fi Ac; yO-'Y0-OVJVfO ?0 :X E m i l fs-l I Q o 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. SANTA CATALINA MTNS wjgjM ' ‘""V-m'fr'ir* f s % ^ * : TUCSON RO^O t SAN ' XAVIER MISSION 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 .