Structural geology of the Cat Mountain rhyolite in the northern Tucson Mountains, Pima County, Arizona Item Type text; Thesis-Reproduction (electronic) Authors Knight, Louis Harold, 1943- 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 26/09/2021 12:31:24 Link to Item http://hdl.handle.net/10150/551931 STRUCTURAL GEOLOGY OF THE CAT MOUNTAIN RHYOLITE IN THE NORTHERN TUCSON MOUNTAINS, PIMA COUNTY, ARIZONA Ly Louis H. Knight, Jr. 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 THE UNIVERSITY OF ARIZONA 196? STATEMENT BY AUTHOR This thesis has been submitted in partial fulfill­ ment of the 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 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: EVANS 6. MAY® Professor of Geology ACKNOWLEDGEMENTS I wish to express ay gratitude to Dr. Evans B. Mayo of the Geology Department of the University of Arizona for his suggestion of the problem, his assistance in the prepar­ ation of this manuscript, and for spending a day in the thesis area along with Dr. Edgar J. McCullough, Jr., and the author. I would also like to thank Dr. John S. Sumner and . and Dr. Edgar J. McCullough, Jr. of the University of Ari­ zona Geology Department for reviewing the manuscript. Special thanks also go to Mr. Glen Henderson of the Sahuaro National Monument, Western Division for permission to con­ duct the study, and to my wife, Carol, for assisting in the preparation of the manuscript. iii TABLE OF CONTENTS Page LIST OF FIGURES . .................................. vi LIST OF PLATES ................................... vii A B S T R A C T ............................................viii INTRODUCTION ....................................... 1 Purpose and Scope of the Investigation........ 1 Location and Accessibility ..................... 1 T o p o g r a p h y ................ 3 Methods of S t u d y ........................... • • • 3 Previous Investigations ......................... 4 GEOLOGY .............................................. 5 General ........... ................... 5 Sedimentary and Effusive R o c k s ................ 3 Introduction ............................... 5 Precambrian and Paleozoic Xenoliths ........ 7 Cretaceous R o c k s .......... 7 Recreation Red Beds ..................... 7 Amole Arkose ....................... 8 Tucson Mountain C h a o s ................... 8 Cat Mountain Rhyolite .......... 11 Basal Tuff M e m b e r ..................... 11 Welded Tuff M e m b e r ................... 16 Origin of the Cat Mountain Rhyolite • 19 Anklam Formation ...................• • • 21 Cam Boh Rhyolite ..........................23 Flow Breccia M e m b e r ............... 23 Flow Member .................••... 25 Old Yuma Andesite......................... 28 Intrusive Igneous R o c k s ...................... 29 Cretaceous Rocks ........................... 29 Older Andesite Porphyry................... 29 Amole Quartz Monzonite ................... 30 Amole Latite ...................... 32 Younger Andesite Porphyry................. 34 Structure •••• ..................... ...•• 35 F o l d s .......................................... 35 F a u l t s .......... 35 iv V TABLE OF CONTENTS (continued) Page North-Striking Faults ................ • 38 Northwest-Striking Faults .............. 38 Northeast-Striking Faults ............... 39 Age of the F a u l t s ......................... 41 J o i n t s ........................................41 Foliation and Bedding ....................... 42 INTERPRETATION OF THE STRUCTURE....................... 44 Structural Evolution .............................. 44 S U M M A R Y ................................................ 46 RECOMMENDATIONS FOR FURTHER S TUDY ..................... 4? APPENDIX I ..................... Petrographic Descriptions ♦ . General ................. Cat Mountain Rhyolite . £ £ £ £ Basal Tuff M e m b e r ......................... 48 Welded Tuff M e m b e r ....................... 51 Anklam Formation ............................. 58 Cam Boh Rhyolite ............................. 59 Flow Breccia Member ....................... 59 Flow Member ............................... 60 Old Yuma Andesite.......................... 63 Older Andesite Porphyry....................... 63 Amole Quartz Monzonite ..................... 64 Amole L a t i t e ..................................65 Younger Andesite Porphyry ................... 66 LIST OF R E F E R E N C E S ....................................6? LIST OF FIGURES Figure Page 1. Location of Thesis A r e a ....................... 2 2. Generalized Stratigraphic Column ............... 6 3. Structure Map of the Northern Tucson Mountains • 9 4. Basal Tuff Member of Cat Mountain Rhyolite, Variety # 3 ......................................13 5« Basal Tuff Member of Cat Mountain Rhyolite, Variety # 4 ......................................13 6. Welded Tuff Member of Cat Mountain Rhyolite . 20 7* Graded Bedding in the Anklam Formation...........20 8. Flow Breccia Member of Cam Boh Rhyolite .... 24 9. Flow Member of Cam Boh Rhyolite, Agglomerate Near Base of Member............ 24 10. Flow Member of Cam Boh Rhyolite ................. 27 11. Flank of Gentle Anticline in the Anklam Formation 36 12. Fold Cut by Younger Andesite Porphyry Dike . 36 13* Pace and Compass Map of Northwest Part of Thesis A r e a ....................................37 LIST OF ELATES Plate Page 1. Geologic Map of a Part of the Northern Tucson Mountains, Pima County, Arizona . in pocket 2. Interpretive Cross Sections ............ in pocket vii ABSTRACT Detailed study of the Cretaceous Cat Mountain Rhyolite in the northern Tucson Mountains shows that the rhyolite was not emplaced by downfsuiting along west-strik­ ing faults as was previously supposed. Instead, the Cat Mountain Rhyolite seems to be an ash flow sequence and part of a normal volcanic rock sequence that was tilted steeply northward and was cut by northeast-striking normal faults during the Laramide orogeny. Two new Upper Cretaceous formations are proposed, the Cam Boh rhyolite and the Old Yuma andesite, which have a combined thickness of 4000 feet or more. viii INTRODUCTION Purpose and Scope of the Investigation The purpose of this investigation is to determine the mode of emplacement of the Cat Mountain Rhyolite in the northern Tucson Mountains. Brown (1939) interpreted sever­ al areas of Cat Mountain Rhyolite in the northern Tucson Mountains as Tertiary rhyolite blocks which had been down- faulted into undifferentiated Cretaceous volcanic rocks• More recent field investigations have cast considerable doubt on the validity of Brown*s interpretation. Instead, the Cat Mountain Rhyolite of the northern Tucson Mountains is now considered to be part of a normal depositions! sequence of Cretaceous volcanic rocks that has subsequently been steeply tilted. This conclusion is based on detailed study of approximately 2 square miles and on regional geologic reconnaissance. location and Accessibility The thesis area is located in sections 7, 8, 9$ 16, 17, and 18, Township 13 South, Range 12 East, 12 miles northwest of Tucson, Arizona, in the Tucson Mountains (Figure 1). The area may be reached on foot from Picture Rocks Road, a secondary road serving the northern part of 1 2 Figure I. Location of Thesis Area 5 the Sahuaro National Monument, Western Division, or "by a jeep trail that leads from Picture Rocks Road to the Old Yuma Mine. In addition, several old mining trails enter the area, but most of these are impassable. Topography The thesis area lies at elevations between 2500 and 5250 feet. It consists of several rounded hills having a maximum local relief of 500 feet. In the southern part many of the higher hills are west-trending hogbacks developed on steeply dipping Cat Mountain Rhyolite. The topography in the northwestern part of the area is very subdued, expressing the shattered nature of the underlying rocks. The area is drained in the East by northeast and east flowing intermittent streams, and in the West by north­ ward flowing intermittent streams. Methods of Study A base map at a scale of 1:6000 was acquired by enlarging the U. S. Geological Survey, Cortaro, Arizona quadrangle topographic map. Aerial photographs at a scale of 1:21,000 were used to correct the enlarged map by means of semicontrolled radial triangulation. Other methods in­ clude photogeologic interpretation, which proved to be a valuable aid in the initial stages of mapping, and a limited amount of pace and compass mapping at scales of 1:2400 and 1:5000. The field