AN ABSTRACT OF THE THESIS OF AngelaK.McDannel for the degree of Master of Science in Geology presented onJanuary 12, 1989 Title: Geology of the Southernmost Deschutes Basin, Tumalo Quadrangle,Deschutes County, Oregon Abstract approved: Dr. EdwardM. Taylor The Tumalo quadrangle lies approximately 30 kilometers behind the Cascade volcanic arc and marks the southernmost extent of continuously exposed Deschutes Formationrocks. Deschutes Formation rocks in the Tumalo quadrangle include late Miocene volcaniclastic sedimentary rocks,ignimbrites and lapillistones, and late Miocene to early Pliocene basalts and basalticandesites. Volcaniclastic sediment and pyroclastic flows originated from sources west to southwest of the Tumalo area and were the products of Early High Cascade arc volcanism. Most pyroclastic flows and falls had silicic compositions and silicic volcanic material dominated the sedimentarydeposits. Intermittent and apparentlywidespread,high discharge events onto a back-arc alluvial plain deposited tuffaceoussediments,much of which became incipient soils during long periods of subaerial exposure. The close of sedimentation was coincident with the initiation of local volcanism at approximately 5.4Ma. Several small monogenetic shield volcanoes and cinder cones in the Tumalo area erupted the basalt and basaltic andesite lavas which cap the sedimentary section. The basalts are typically porphyritic and high in A1203. Basaltic andesites are aphyric to sparsely porphyritic, are commonly high in FeO and Ti02, appear younger than the basalts, and are not related to the basalts by simple fractionation. Both lava types have relatively evolved compo- sitions based on their Fe' values. An inlier of older Tertiary rhyodacite lavas known as Cline Buttes lies near the northern boundary of the study and is the only locally exposed pre-Deschutes Formation unit. The southern part of the Tumalo quadrangle marks the boundary between continuously exposed Deschutes Formation rocks to the north and Late High Cascade volcanic units to the south. After a hiatus of approximately 4 million years, deposition in the Tumalo area resumed at approximately 0.6 Ma when pyroclastic flows from the High Cascade Range flowed east and north- east into the Tumalo area to rest disconformably above the Deschutes Formation. A diktytaxitic basalt from Newberry volcano locally overlies the pyroclastic units. Widespread, thick alluvium obscures some 16 km2 of bedrock in the study area and appears to be the result of deposition during high discharge events associated with late Pleistocene to Holocene age glaciation of the High Cascade Range. Some thirty en echelon normal faults of the Tumalo fault zone trend between N15°-35°W and cut Pleistocene and older units in the Tumalo quadrangle. Deschutes Formation basalt and basaltic andesite vents are localized in the fault zone. This suggests volcanism was structurally controlled and that an tensional stress regime with a northeast-southwest least compressive stress orientation prevailed in the Tumalo area between the late Miocene to late Pleistocene. Geology of the Southernmost Deschutes Basin, Tumalo Quadrangle, Deschutes County, Oregon by Angela K. McDannel A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed January 12, 1989 Commencement June 1989 logy in charge of major n Dean of Gradu e School Date thesis is presented January 12, 1989 ACKNOWLEDGEMENTS Few projects are completed single handedly and this one has certainly been no exception. I thank Ed Taylor for introducing me to the Deschutes basin, for the numerous, tireless treks he took with his students into the area, and for his enduring patience in seeing this project to completion. I also greatly benefited from discussions with other students who worked in the basin and owe the Deschutes-for- lunch-bunch much gratitude. Britt Hill was invaluable for his 11th hour discussions and support.My friends Clare and Robin assisted in last minute drafting and assembly of the text. Dave, faithful partner that he is, assisted me in numerous calm to panicky endeavors over the course of this project. My greatest debt is to the people of the Tumalo area who, by and large, responded kindly to my requests to trespass upon their proper- ty. Sharon and Dan Gambrel assisted me in numerous ways during field work. Without the generous hospitality of Darla Robson and her family my work in the field would have been more difficult and less comfort- able. Their friendship has supported me in my efforts to complete this project. I thank Larry Snee for obtaining an age date on a Deschutes Formation lava in the Tumalo area. Rich Conrey is appreciated for his help with major and trace element analyses at WSU and for sharing his ideas and friendship with me over the years. Financial assistance for field work came from the Sohio field research fund at OSU. TABLE OF CONTENTS Introduction.... ......... .... ............................... 1 Location,Access, and Physiography......................... 3 Previous Work.............................................. 3 Regional Geology. .... .......................... 6 Rock Classification and Terminology........................ 11 Stratigraphy and Lithology of Map Units..........................14 Pre-Deschutes Formation Rocks.............................. 14 Cline Buttes.........................................14 Discussion...........................................18 Deschutes Formation Rocks..................................23 Sedimentary Rocks .................................... 27 Lithology......................................29 Facies.. .............................31 Depositional Environment ....................... 46 TetherowDiscussion Debris-Flow ..................................... Deposit....................... 5149 Ignimbrites .... ............................. 53 White Rock Ignimbrite..........................55 Other Ignimbrites .............................. 60 Discussion.....................................65 Lavas.......s .....................................68 Source Vents ................................... 71 Age of Lavas..... ........................73 Unit Descriptions of lavas..................... 74 Geochemistry .................................... 97 Stratigraphic Correlation ........................... 108 Quaternary Units..........................................111 Pyroclastic Units..s ........................... 111 Desert Spring Tuff ............................ 112 Bend Pumice...................................115 Tumalo Tuff...................................116 Lavas......... ................................118 High Cascade sLavas............................118 .................................119 AlluvialNewberry Deposits Lava ................................... 121 Discussion..........................................124 Structure and Tectonics ......................................... 126 Tumalo FaultZone..... ..........................126 Regional Structural Relationships.........................131 Discussion.. ...........................................137 Summary and Conclusions.........................................139 Bibliography...................................................143 Appendix I:Deschutes Formation Measured Sections..............154 Appendix II:Chemical Analyses of Volcanic Rocks............... 157 IIa:Cline Buttes Silicic lavas...................... 158 IIb:Deschutes Formation Basalts and BasalticAndesites....159 IIc:Deschutes Formation Ignimbrites and Debris Flow Deposits .............................................. 165 IId:Pleistocene lavas and ignimbrites ..................... 166 LIST OF FIGURES Figure Page 1. Stratigraphy of the Tumalo quadrangle 2 2. Location map of the Deschutes Basin and Tumalo 4 quadrangle 3. Oblique aerial photo of Cline Buttes 15 4. Outcrop of Cline Buttes rhyodacite lava 15 5. Spherulites in Cline Buttes rhyodacite lava 17 6. Photomicrograph of spherulites in Cline Buttes rhyo- 17 dacite lava 7. Map of silicic volcanic centers in the Deschutes basin 19 8. Outcrop of Deschutes Formation near Lake Chinook 24 9. Map of geology of the Deschutes basin and adjoining 25 areas 10. Deschutes Formation volcaniclastic rocks exposed in 28 the wall of the Deschutes River canyon lla. Photomicrograph of massive tuffaceous sandstone in 30 plane light llb. Phototmicrograph of massive tuffaceous sandstone, 30 polarized light 12. Outcrop of Facies A, massive sandstone 34 13. Outcrop of bedded sandstone of Facies B 36 14. Outcrop of laminated sandstone of Facies C 38 15. Outcrop of interlaminated sandstone, siltstone and 40 mudstone of Facies D 16. Outcrop of Facies E, pebble conglomerate 42 17. Outcrop of massive conglomerate of Facies E 45 18. Primary lapillistone of Facies F 47 19. White Rock ignimbrite 56 Figure Page 20. Vertical sections representative of the standard 57 ignimbrite flow unit and the White Rock ignimbrite 21. Fine ash layer at base of White Rock ignimbrite 59 22. Unnamed pink Deschutes Formation ignimbrite 62 23. Location map of Tumalo Volcanic Center and Bull 67 Springs inlier 24. Outcrop of Deschutes Formation basaltic andesite lava 69 25. Outcrop of platy basaltic andesite lava 69 26. Location map of Deschutes Formation source vents in 72 the Tumalo Quadrangle 27. Oblique aerial photo of Long Butte 77 28. Photomicrograph of lava Tdbl 78 29. Photomicrograph of altered lava Tdbl 80 30. Photomicrograph of lava Tdb2 81 31. Photomicrograph of lava Tdb3 84 32. Photomicrograph