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Chemistry and age of late Cenozoic air- fall ashes in southeastern Arizona Item Type text; Thesis-Reproduction (electronic) Authors Scarborough, Robert B. 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 27/09/2021 15:25:53 Link to Item http://hdl.handle.net/10150/566605 CHEMISTRY AND AGE OF LATE CENOZOIC AIR-FALL ASHES IN SOUTHEASTERN ARIZONA by Robert Bryan Scarborough A Thesis Submitted to the Faculty of DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE WITH A MAJOR IN GEOCHRONOLOGY In the Graduate College THE UNIVERSITY OF ARIZONA 19 7 5 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of re­ quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg­ ment the proposed use of the material is in the interests of scholar­ ship. In all other instances, however, permission must be obtained from the author. SIGNED: APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: fX\c P. E. DAMON Date Professor of Geosciences PREFACE The production of this thesis would have not been possible without the help and instruction of many people and the continued financial aid given to the Laboratory of Isotope Geochemistry by an AEC contract A T (11-1)-689, "Correlation and Chronology of Ore Deposits and Volcanic Rocks", and NSF grant Ga-31270, with the same title. The idea was suggested to the author by Dr. Paul E. Damon in the fall of 1967. Early discussions with him and Dr. C. Vance Haynes, then in charge of the C-14 dating laboratory at this University, on the problem and general ideas concerning methodology laid the groundwork for this thesis. Preliminary field work was aided by George Lammers (for the area around St. David) and Larry Agenbroad (for the Lower San Pedro Valley). Jon Powell, then an undergraduate in the Geology Department, cooperated with the author in a detailed stratigraphic study around Camel Canyon. Dr. Everett H. Lindsay has cooperated continually with the author on relating conclusions of geochemical analyses with what is known of the vertebrate paleontology of the areas, and has pointed out the location of several ash beds. Recent discussions with Dr. Noye Johnson of Dartmouth College, Hanover, New Hampshire, have pointed out the importance and value of obtaining paleomagnetic data in valley-fill sediments, as he has done iii iv for parts of the San Pedro Valley. It is clear that as details of the paleomagnetic stratigraphy of the San Pedro Valley become known, a much more complete history will be told. Permission to collect samples on the Post Ranch, near St. David, was graciously given by Mr. Elmer Post. The author also wishes to thank the Navajo Nation for granting of a collection permit to the Laboratory of Vertebrate Paleontology under the leadership of Dr. Everett Lindsay, for the collection of the sample at White Cone Maar. Many people assisted the author in aspects of laboratory analy­ sis. The various facets of potassium-argon dating were introduced by Dennis Coleman, A. William Laughlin, Darwin Marjaniemi, Judith Percious,- and Robert A. Palmer. Most of the chart reductions for argon analyses, a most tedious task, were graciously done by Kathleen Roe. The potas­ sium analyses were performed by Anne Sigleo and the author. The ultra­ sonic probe technique was taught by Peter Kuck. X-ray fluorescence technique was taught by Raymond L. Eastwood. Larry D. Arnold provided ideas on hydration rates. Ideas and equipment for the production of illustrations were provided by Donald B. Sayner and Peter Kresan. Reading and criticism of an early draft of the thesis was pro­ vided by Kathleen Roe and Dr. Damon. Their criticism helped immensely to make the test more readable. Finally, I wish to thank my parents, Mr. and Mrs. William B. Scarborough, who very unselfishly made my work and course load so much more pleasant by easing the burden at home. TABLE OF CONTENTS Page LIST OF TABLES ............ ........ vii LIST OF ILLUSTRATIONS....................................... viii ABSTRACT.................................... ix 1. INTRODUCTION ................................................. 1 1.1 The Problem and the Thesis Goal . ................... 1 1.2 Previous Work in Ash C o r r e l a t i o n ............ .. 6 1.3 Previous Geologic Work in the Study Areas .......... 8 2. GEOLOGY OF THE A S H E S ...................... 21 2.1 Geologic Occurrence of the A s h e s ..................' 21 2.2 Sampling Localities ................................. 27 2.3 Boundary Conditions for Source Areas of the Ashes . 36 3. CHEMICAL A N A L Y S E S .............. 39 3.1 Sampling Technique and Sample Preparation .......... 39 3.2 Chemical Analysis Techniques ....................... 41 3.3 Atomic Absorption Methodology ....................... 41 3.4 X-Ray Fluorescence Technique ....................... 56 3.5 Silica and Water Determinations ..................... 56 3.6 Interpretation of Chemical Analysis Data .......... 57 4. POTASSIUM-ARGON DATING ....................... 67 4.1 Previous Work in K-Ar Dating of G l a s s .............. 67 4.2 K-Ar Methodology and D a t a ........................... 70 4.3 Discussion of Inconsistent Samples ................. 72 4.4 Argon Retention Characteristics of Silicic Glass . 80 5. RESULTS AND C O N C L U S I O N S ..................................... 82 5.1 Interpretation of Chemical Analyses .............. 82 5.2 Conclusions of K-Ar Analyses ....................... 86 v vi TABLE OF CONTENTS— Continued Page 5.3 Conclusions of the S t u d y .......................... 89 5.3,1 Conclusions of Methodology . ............. 89 5.3.2. Conclusions Concerning Ash Bed Ages and Correlations........................ 90 5.4 Conclusions Concerning Post-Miocene Valley History . 94 5.5 Possible Source Areas of the A s h e s ................ 98 LIST OF REFERENCES............................ 100 \ LIST OF TABLES Table Page 1. Sample locations ........................................... 28 2. Makeup of individual atomic absorption stock solutions and the stock mixture 43 3. Makeup of working atomic absorption standards ............ 44 4. Approximate parameters used for analysis on the Perkin- Elmer model 403 atomic absorption spectrophotometer . 45 5. Chemical analysis of samples, in weight % of the element except where noted ...................................... 46 6. Confidence levels of F values................ 58 7. F values for member samples of the SPVA ash . ............. 59 8. F values comparing average SPVA ash chemistry with member ash c h e m i s t r i e s ......................................... 59 9. F values for member samples of the SPVB a s h .............. 60 10. F values comparing average SPVB ash chemistry with member ash c h e m i s t r i e s ......................................... 60 11. F values of Camel Canyon ashes and spatially related ashes . 61 12. F values of samples from the El Paso site. Lower San Pedro V a l l e y ................ 61 13. F values of younger ashes from the Upper San Pedro Valley and 111 Ranch compared with SPVA and SPVB a s h .......... 62 14. F values of older ashes from the Lower San Pedro Valley, Verde Valley, and the Bouse F o r m a t i o n ................... 62 15. Average chemistry of the main ashes of this study, corrected for analytical biases noted in Table 5 .................. 66 16. K-Ar ages on glass samples .................................. 73 17. Experiment with RBS-32-68, sample with anomalous age .... 79 vii LIST OF ILLUSTRATIONS Figure Page 1. Cenozoic History of the Lower San Pedro Valley for an Area 15 Kilometers North of Mammoth, Arizona ............ 9 2. Arizona Drainage Map Showing Areas of Investigation .... 16 3. Ash Locations in the San Pedro V a l l e y ..................... 17 4. Cross-Section along the San Pedro Valley Showing Elevation of Ash Bed Outcrops and Correlative Ash H o rizons........ 31 5. Stratigraphy at California Wash .................. .... 33 6. Ash Samples for the Verde Valley .......... ........ 33 viii ABSTRACT Whole rock chemical analyses and potassium-argon age dates have been obtained for a series of vitric air-fall ashes which are contained within Pliocene valley-fill sediments in parts of Southern Arizona, with the majority of analyses coming from the Upper and Lower San Pedro Valley. Other select samples have come from the Verde Valley, the San Simon Valley, the Bidahochi Formation in the northeastern part of the state, and the Bouse Formation along the lower Colorado River. Chemical analyses have shown the presence of two distinct but chemically related ash chemistries, each of which encompasses several ash layers. A third ash chemistry is observed at only one sampling location. Potassium- argon ages have defined two periods of volcanic activity in the areas studied. These are an event between 5 and 6 million years ago and another between 2 and 3 million years ago. One possible source area for some of the older ashes of the Lower San Pedro Valley is the Hopi Buttes volcanic field of northeastern Arizona. These chronological data are used to hypothesize that the San Pedro Valley drainage has been integrated with the Gila River drainage since most of Quiburis time, and that the San Pedro River has maintained its present gradient for the entire time encompassing the deposition of the Quiburis Formation.

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