Mark Scott Cranswick for the degree of Masterof Science December 7, 1979 in Geology presented on Title:THE STRATIGRAPHY,STRUCTURE AND PETROGRAPHY OF KEYES MOUNTAIN,TERTIARY UPPER CLARNO FORMATION, WHEELERCOUNTY, OREGON Redacted for Privacy Abstract approved: Dr.eith F. Oles The Keyes Mountain area,located approximately six kilome- ters northeast of Mitchell,Oregon, consists of a volcanicpile of interstratified Clarno andesiteflows and mudflows with verylocal tuff and vent agglomeratedeposits.Intrusive andesites are also common. The Upper Clarno Formationof the Keyes Mountain area unconformably overlies the Permianmetasediments and the Greta- ceous Gable Creekand Hudspeth Formations.It unconformably underlies the John Day andColumbia River Basalt Formations. The Clarno Group isradiometrically dated between 29.4 0. 6 and 48. 9 ± 5. 2 m. y. ; fromearly Eocene to middle Oligocenetime. The andesite flows contain two tofive phenocryst phases. These phenocrysts are plagioclase,clinopyroxene, hypersthene, hornblende and lamprobolite inorder of decreasing abundance. Intrusive andesites are very similar to aridesite flows except that they commonly contain hornblende. Indirect evidence indicates Keyes Mountain was a volcano and the source of some Upper Clarno strata in the Mitchell area.This conclusion is inferred from features that are characteristic of vol- canic loci: quaquaversal dips, extremely large clasts withinthe muclllows, mudflow paleocurrent directions from the Keyes Mountain vicinity, the intracanyon nature of mudilows and andesite flows with respect to one another, the noncorrelative nature of sequences of andesite flows exposed in different ridges, and a possible vent agglomerate. In post-Clarno time, Keyes Mountain stood as a paleotopo- graphic high that was marginally onlapped by the John Day and Columbia River Basalt Formations. The Strati.graphy, Structure, and Petrography of Keyes Mountain, Tertiary Upper Clarno Formation, Wheeler County, Oregon by Mark Scott Cranswick A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed December 1979 Commencement June 1980 APPROVED: Redacted for Privacy Profes- of Geoi'ogy in charge of major Redacted for Privacy co- advisor Redacted for Privacy Chairman of Dep nt of Geology Redacted for Privacy Dean of 3rad.uate S Date thesis is presented December 7, 1979 Typed by Opal Grossnicklaus for MarkScott Crariswick ACKNOWLEDGEMENTS I extend my appreciation for thefriendliness and helpfulness of the townspeople of Mitchell andranchers of my project area. I thank Dr. Keith F. Oles for allhis guidance and help, both in the field and in preparation of the manuscript.Also, I wish to thank Dr. Edward. M. Taylor, Dr. HaroldE. Enlows and Dr. Robert Lawrence for their help with the technicalparts of this study.Very much appreciated was the $800 thesisassistance award from Union Oil Company of California for partial supportof this study. Foremost, I extend my gratitude forever to myfather who has guided me with admirable knowledgeand wisdom.To him I attribute my success in this project, in the pursuitof my profes- sion, and in this world. My mother, in her example of self-disciplineand integrity of character, has unfailingly supported andhelped me mold my future.To her I will always be deeply thankful andindebted. Above all,I dedicate this work and all future endeavors to my wife, Barbara. Forall her help, encouragement, confidence and inspiration I express my profoundthankfulness. TABLE OF CONTENTS Page INTRODUCTION 1 Location 1 Geography and Climate 1 Accessibility and Exposure 4 Purpose and Methods of Investigation 5 Geologic Setting 8 Previous Work 10 STRATIGRAPHY 14 Permian Metasediments 14 General Characteristics 14 Cretaceous Gable Creek and HudspethFormations 15 General Characteristics 15 Clarno Formation 16 Introduction 16 Radiometric Ages of Clarno Andesites 17 I.Andesite Flows 19 Geomorphology 19 General Characteristics 19 Mineralogy 25 Flow Breccias 26 Petrography 28 II. Mudflow Breccias 40 Geomorphology 40 General Characteristics 42 Lithology 44 Petrography 49 III.Volcanic Agglomerate 51 Geomorphology 51 General Characteristics 52 Discussion 55 Lithology 56 Petrography 59 IV.Tuffs 61 Geomorphology 61 General Characteristics 61 Lithology 62 V.Intrusive Andesites 63 Geomorphology 63 General Characteristics 63 Lithology 70 Petrography 71 John Day Formation 82 General Characteristics 82 Columbia River Basalt 84 General Characteristics 84 STRUCTURE 86 CONCLUSIONS 89 GEOLOGIC HISTORY 91 REGIONAL TECTONIC HISTORY AND THE CLARNO FORMATION 96 Generation of Caic-alkaline Magma 96 Hypotheses of the Tectonic Regime of Early Tertiary North-Central Oregon 101 Introduction 101 Plate Tectonics 103 Cenozoic Tectonic Rotation of the Columbia Arc 107 Other Hypotheses of Origin 111 REFERENCES CITED 114 Appendix A.Chemical Analyses 122 LIST OF FIGURES Figure Page 1 Index map of the thesis area 2 2 View to northeast of Keyes Mountain 3 3 View to south of Monroe Roughs 3 4 View west-southwest to west peak of Keyes Mountain showing andesite flows and dipslopes 6 5 Rock units of the Mitchell quadrangle (from Oles and Enlows, 1971) 9 6 Geochronology of the Clarno Group (modified from Enlows and. Parker, 1972) 12 7 View to north of west and south peaks of Keyes Mountain, showing Clarno andesite flows 21 8 View north-northeast of Baldy Mountain with ridge of Clarno andesite flows below 21 9 View northwest of south slope of south peak of Keyes Mountain showing Clarno andesite flows 22 10 View northeast of a ridge of Cretaceous Hudspeth Formation capped by Clarno andesite flows 22 Li AFM diagram for Clarno andesite flows 23 12 Ande site flow breccia 27 13 Photomicrograph of hornblende-bearing, hype rsthene, clinopyroxene labradorite andesite 27 14a Photomicrograph of hype rsthene- and hornblende-bearing, clinopyroxene labradorite andesite 32 14b Same as Fig. lqa but with crossed nicols 32 15a Photomicrograph of hypersthene, clinopyroxene andesine andesite 33 Page Same as Fig. 15:a but withcrossed nicols 33 Photomicrograph of lamprobolite-bearing, clino- pyroxene plagioclaseandesite 34 Same as Fig. 16a but withcrossed. nicols 34 17 Photomicrograph of hypersthene,clinopyroxene labradorite andesite 37 41 18 Outcrop of CLarno mudflow 43 19 Closeup of a Clarno mudflowbreccia 43 20 Ridge composed entirely ofClarno mudflow 45 21 CLarno mudflow channel 22 Large boulders of Cretaceousrocks in a mudflow channel 45 47 23 Cia mo mudflow brecciacomposed of tuff and andesite 24 Outcrop of bedded sedimentarystrata within a Clarno mudllow 50 50 25 Photomicrograph of Clarno mudflowmatrix 54 26 Outcrop of Clarno volcanic agglomerate 54 27 View of the Harrison Creekparasitic-vent agglomerate 58 28 Closeup of the same as Fig. 27 58 29 Closeup of the same as Fig. 27 of 30 Photomicrograph of the volcanic agglomerate Marshall Creek 60 31 Dike of Clarno ande site 64 32 Closeup of the same as Fig. 31 65 67 33 Large Cia mo andesite dike in MonroeRoughs Figure Page 34 Clarno andesite dike 69 35 Intrusion breccia of the same dike as in Fig. 3 69 36a Photomicrograph of biotite- and clinopyroxene- bearing labradorite andesite 75 36b Photomicrograph of the same as Fig. 36a but with crossed nicols 75 37 Photomicrograph of hypersthene.- and hornblende- bearing clinopyroxene andesine-labradorite andesite 76 38a Photomicrograph of a hornblende labradorite andesite from a plug 77 38b Photomicrograph of the same as Fig. 3a but with crossed nicols 77 39 Photomicrograph of a hornblende-bearing, clinopyroxene andesine andesite 79 40 Photomicrograph of andesine-, clinopyroxene- and hornblende-bearing andesite 80 LIST OF TABLES Table Pag I Volumetric modes of selected Clarno andesite flows 30 II Volumetric modes of selected Clarno andesite intrusions 73 LIST OF PLATES Plate Page 1 Geologic map of the Keyes Mountain area pocket THE STRATIGRAPHY, STRUCTURE, AND PETROGRAPHY OF KEYES MOUNTAIN, TERTIARY UPPER CLARNO FORMATION, WHEELER COUNTY, OREGON INTRODUC TION Location The study area is located in north-central Oregon, in the south-central part of Wheeler County (Fig.1), east of Mitchell, and north of U. S. Highway 26.It encompasses approximately 28 square miles within T. 11 and 12 S., R. 22 and 23 E. Geography and Climate The study area lies within the Blue Mountains and is charac- terized by a dissected mature landscape composed of mountain ranges and plateaus (Thornbury, 1965) (Figs. 2 and 3).Keyes Mountain, at 1, 739 meters elevation (5, 704 feet), is the highest point.The lowest point is at 1, 036 meters (3, 400 feet) along U. S. Highway 26.The maximum topographic relief is 701 meters (2,300 feet). The countryside around the town of Mitchell is characterized by extremes in climatic conditions.The average summer tempera- ture is 21°C (70°F), reaching an average maximum of 29°C (85°F). The average winter temperature is -1°C (30°F), reaching an average minimum of -8°C (18°F).The average fluctuation in daily SALEMPORTLANDWasco Co. CORVALLISPRINEVELLJeffersonH(L/E4ii-1MONDneeeO Crook Co. I OREGON II Figure 1.Index- map - of the Keyes- Mountain- - .______ study area (shown- in------ solid black) - - I. 3 KeyesMountain Peggy Butte west\ peak south peak - 2 ' - S - r- S '- - ..-y - -" .- - : , ti _* - Figure 2.View of Keyes Mountainlooking northeast. -,. KeyesMountain south peak west peak the north side of the west and south Figure 3.View to south of Monroe Roughs, located on peaks of Keyes Mountain, in sections 13,14, 23, and 24, T. 11 S. ,R. 22 E. 4 temperatures is 10°C (22°F)