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Pumice and Ash Falls, Lava Flows and Epiclastic Deposits. An

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Pumice and Ash Falls, Lava Flows and Epiclastic Deposits. An AN, ABSTRACT OF THE THESIS OF Jeremy B. Jay for the degree of Master of Science in the Department of Geologypresented on June 2, 1982. Title: The Geology and Stratigraphy of the Tertiary Volcanic and Volcaniclastic Rocks With Special Etrphasis on the Deschutes Formation, From Lake Simtustus to Madras in Central Oregon. Abstractapproved:a-001 E.M. Taylor A sequence of volcanic, volcaniclastic, and epiclastic deposits from Oligocene to Recent age are exposed in the region from Lake Simtustus to Madras in central Oregon.The epiclastic sediments of the Oligocene John Day Formation are unconformably overlain by two flows of the middle Miocene Columbia River Basalt Group. The upper Miocene and lower Pliocene Deschutes Formation generally overlies the Columbia River Basalt. The Deschutes Formation includes ash flows, pumice and ash falls, lava flows and epiclastic deposits. An ash- flow tuff with a composition similar to other Deschutes ash flows occurs between flows of Columbia River basalt suggesting that the formations interfinger. The two flows of Columbia River basalt have compositions typical of the Prineville Chemical Type. The flows contain an average of 1.24 weight percent P2051 and K20 is high compared to other types of Columbia River basalt. The Columbia Riverbasaltand underlyingrockshavebeen moderatelydeformed,producing broad anticlines and synclines and normalfaultswith smalldisplacement. Northwest of Madras,the Columbia River basalt dips three or four degrees to the southeast. The Deschutes Formation and overlying rocks are generallyflat lying and undeformed. No faults were found extendingintothe Deschutes deposits. The PeltonBasalt member is an 80to150 foot thick unit of tholeiitic basalt thatoccurs near the base of the Deschutes Forma- tion. It containsnumerous discontinuousflow units whichrepresent separate lobes of a compound lava flow. No interbeds or paleosols between flow units were found.The basaltis probably the result of a single prolonged eruption. Six intraformational lava flows occur interstratified with volcaniclastic and epiclasticdeposits. Five of these flows arebasalt;one flow is basaltic andesite. The Deschutes volcaniclastic rocks generally occurina four hundred foot thick intervalabove the Pelton Member and below rim- forming basalt lavas. The best exposures of these deposits are at the Vanora Cliff and at the cliff on the northside of themouth of Willow Creek. Rocks in bothof these areas are exposed as the result of largelandslides. Theash-flowtuffs range in composition from dacite to rhyoliteand range from threeto sixty feet thick. The deposits generally have restricted areas of exposure.The strikes of paleochannelsfilled by ash flowsand the orientationof the long axes of clastsin underlyingfluvial conglomerates indicate a west or southwest provenance for these deposits. Seven lithic-richlaharic-breccia deposits occur interstratified with the ash flows and epiclastic deposits. The laharic breccias are the most resistant and extensive volcaniclastic rocks inthe mapped area. In certain outcrops, the breccias consist of as muchas seven- ty percent angular to subrounded lithic clasts suspended in afine- grained matrix of glass shards. The deposits are more numerous, thicker, and more poorly sorted to the north and northwest indicating a provenance to the northwest. Many features of the deposits are similar to volcanic lahars but other characteristics indicatethat the materials were subjected to temperatures above the Curie pointof the clasts. Some of these units might have been produced by acom- bination of fluvial materials and a hot pyroclastic flow. A widespread sheet of tholeiitic basalt caps the Deschutes Form- ation. Similarity between eight chemical analyses, constant pheno- cryst mineralogy, and uniformnormal paleomagnetic polarity of samples from Binder's Canyon to Round Butte Dam suggest that therim- forming flow is a single sheet of lava that originally covered the mapped area. Round Butte cinder cone and lava flow are the youngest Deschutes Formation deposits in the mapped area. The Round Butte deposits have a composition similar to other intraformationallavas and therefore represent a continuation of the volcanism that has occurred through- out the late Miocene and early Pliocene in the Deschutes Basin. THE GEOLOGY AND STRATIGRAPHY OF THE TERTIARY VOLCANIC AND VOLCANICLASTIC ROCKS, WITH SPECIAL EMPHASIS ON THE DESCHUTES FORMATION, FROM LAKE SIMTUSTUS TO MADRAS IN CENTRAL OREGON by Jeremy Barth Jay A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Masters of Science Completed June 01 1982 Commencement June 1983 APPROVED: Date thesis rs presented Typed by Bonnie Jay for JeremyBarth Jay ACKNOWLEDGEMENTS I would like to thank Dr. Paul Ragland for encouraging my pur- suit of a graduate degree in geology. His concern and effort on my behalf was the single most important step leading to my graduate degree. A grant for some of the field and laboratory expenses ofthe thesis was provided by the Oregon Department of Geology andMineral industries. Camping facilities were provided by Portland General Electric Company. I would liketo thank Debbie Cannon, Rick Conrey, Tom Dill, Glenn Hayman, Neil Bingert, Robert Thomason, and Dave Thormalen for the friendship and support throughout the field season, and during preparation of the thesis. Scott Hughes and Karl Wozniak have both been friends and advisors throughout my graduatecareer. Dr. Ed Taylorhas earnedmy highestesteemas an educator, advi- sor, andgeologist. His concern,patience,and assistance were responsible for the completion of this thesis. Ed has had more of an influence on me than he will ever know. Special thanks go to my wife,Bonnie,who contributed many hours toward the completion of this thesis. Graduate school would not have been possible and would surely have been less enjoyable without shar- ing it with her. Finally, this thesis is dedicated to my parents, Jane and Danny Jay. The most valuable inheritance one might receive is an excellent education. That inheritance has produced an open and inquisitive mind with which to explore and enjoy life. TABLE OF CONTENTS INTRODUCTION Location and Physical Characteristics 1 Previous Work 3 General Geology 4 Purpose 5 Methods and Procedures 6 JOHN DAY FORMATION Introduction 8 Provenance 9 Physical Characteristics 10 COLUMBIA RIVER BASALT GROUP Introduction and Stratigraphic Relationships 11 Distribution, Lithology, and Petrography 15 DESCHUTES FORMATION Introduction 19 Pre-Pelton Basalt Epiclastic and Volcaniclastic Rocks 20 Pelton Basalt Member 22 Intraformational Lava Flows 28 Deschutes River Canyon Intraformational Lavas 28 Willow Creek Canyon Intraformational Lavas 32 Dry Canyon Intraformational Lava Flow 34 Deschutes Volcaniclastic Rocks 36 Deschutes Ash-flow Tuff Deposits 39 Earliest Deschutes Ash-flow Tuff 39 Vanora Cliff Exposures 40 Willow Creek Exposures 41 Jeep Trail Intracanyon Ash-flow Tuff 52 Indian Campground Ash-flow Tuff 53 Access Road Ash-flow Tuff 53 Deschutes Lithic-rich Deposits 55 Description and Distribution of Individual Lithic-rich Volcaniclastic Deposits 57 Pumice-fall Deposits 79 Correlation of Units 81 Individual Exposures Vanora Cliff 84 Campbell Canyon 84 Willow Creek Exposures 84 Deschutes Formation Epiclastic Rocks 89 Mud and Debris Flows 89 Rim-forming Basalt Lava General Characteristics 93 Lithology, Petrography, and Chemistry 94 Round Butte General Characteristics 97 Lithology, Petrography, and Chemistry 98 QUATERNARY INTRACANYON BASALT FLOW 101 SUMMARY AND CONCLUSIONS 103 BIBLIOGRAPHY 107 APPENDIX 111 LIST OF FIGURES Figure Page 1. Index map of thesis area in central Oregon 2 2. Photomicrographs of sample of Columbia River basalt. 13 3. Steep-walled canyon eroded into Pelton basalt in Willow Creek Canyon. 23 4.Exposuresat theaccess road grade near Round Butte Dam. 29 5.Landslide deposits at the base of the Vanora Cliff. 37 6. Silicic ash-flow tuff (Tdaw1) at the base of the Willow Creek exposures. 44 7.Large xenolith of ash-flow material incorporated into the Willow Creek ash-flow tuff number 2 (Tdaw2). 47 8.Large, dark, glassy pumices in the intracanyon ash flow at the Willow Creek exposures (Tdaw4). 49 9.Base of the lithic-rich deposit number 2 (Td12) showing lag deposit. 60 10. Contact between the bottom ash flow (Tdaw1) and the second lithic-rich deposit(Td12)at Willow Creek. 60 11. Close up of clasts five feet from the baseof lithic- rich deposit number 2 (Tdl2). 64 12. Exposures of the second and thirdlithic-rich deposits at the Willow Creek cliff. 65 13. Photomicrograph of a thin section from near the bottom of lithic-rich deposit number 2 (Tdl2). 66 14. View of lithic-rich deposit number 3 (Td13) dipping eastward into a paleochannel erodedinto the second laharic breccia(Td12)and the bottom ash flow (Tdaw1)at the Willow Creek exposures. 71 15. Picture of pumice-fall deposit and overlyinglithic- rich deposit number 4 (Td14). 71 16. Elliptical vug and radially jointed clastin lithic- richdeposit number4 (Td14). 73 17. Correlation diagram forexposures. 83 18. Picture and schematic drawing of the exposures at the Vanora Cliff(S.W. 1/4sec.6, T.10S., R.13E.). 86 19. Picture and schematic drawing of the exposures near the mouth of Campbell Canyon (S.W.1/4sec.8,T.10S., R.13E.). 87 20. Picture and schematic drawing of the Willow Creek exposures (S.W. 1/4 sec. 19, T.10S., R.13E.). 88 21. Cast of tree trunk and limbs in mudflow deposit (S.E. 1/4 sec. 10, T.11S., R.12E.). 92 22. Photomicrograph of Round Butte basalt showing glomeroporphyritic clots of pyroxene and plagioclase. 100 Plate 1. Geologic Map of a portion of the Deschutes Basin from Lake Simtustus to Madras, Jefferson County, in map Oregon. pocket LIST OF TABLES Table Page 1.Chemical analyses of Columbia River basalt. 14 2. Chemical analyses of ash-flow tuffs from the John Day and Deschutes Formations. 17 3. Chemical analysis of pre-Pelton Basalt pumice-fall deposit. 21 4.Chemical analyses of Pelton Basalt. 27 5.Chemical analyses of Deschutes intraformational lava flows. 35 6.Chemical analyses of Willow Creek ash-flow tuff number 1. 45 7.Chemical analyses of Willow Creek ash-flow tuffs 2, 3, and 4.
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