AN ABSTRACT OF THE THESIS OF Linda J. Baker for the degree of Master of Science in Geology presented on March 3, 1988. Title: The Stratigraphy and Depositional Setting of the Spencer Formation, West-central Willamette Valley, Oregon; a Surface-subsurface Analysis Abstract approved: Redacted for Privacy Dr. Kieth F. Oles The late Eocene Spencer Formation crops out in the low hills on the western edge of the central Willamette Valley, Oregon.Surface exposures in eastern Benton and southeastern Polk Counties and oil/gas well records and cuttings in Polk, Marion and Linn Counties were studied to determine Spencer stratigraphy, regional lithologic variations, and depositional environment. Methods used include: study of outcrops, petrography, texture, and well cuttings, as well as the correlation of well logs and microfossil data of McKeel (1984, 1985). The distribution of the underlying early-late Eocene Yamhill Formation is also briefly considered. The Yamhill Formation consists of the Miller sandstone member enclosed between mudstones. The lower and middle Yamhill record shoaling from bathyal to marginal marine depths, and they are overlain by bathyal upper Yamhill mudstones. The Miller sandstone is lens-shaped, trends parallel to the Corvallis fault, and reaches a maximum thickness of approximately 2,000 feet on the east side of the fault. The Miller sandstone grades westward into bathyal mudstones, and eastward into volcanic tuffs and flows. Thinning of the Miller sandstone and upper Yamhill mudstone along the Corvallis fault suggests movement during early late Eocene. The absence of Yamhill strata along the outcrop belt to the southwest may be related to this tectonic activity. Alternatively, Yamhill strata may have been misidentified as Tyee Formation or Spencer Formation. The Spencer Formation was deposited in a tectonically active forearc basin during a transgression which was interrupted by several short-term regressional/progradational events. The Spencer is stratigraphically divided (informally) into a lower sandstone-rich member and an upper mudstone member; it is also divided geographically (informally) into northwestern, east-central, and southern provinces. The lower member is 700 feet thick in the northwestern and southern areas, and thickens to 1400 feet in the east-central area. As compared to the north and south areas, sandstones in the east-central area are coarser (fine to medium versus very fine to fine), the sandstone to siltstone ratio is higher, and volcanic interbeds are more common. Deposition is thought to have been at inner shelf and shoreface depths, grading eastward into nonmarine. In the northwestern area, abundant hummocky cross-bedding of arkosic to arkosic-lithic lower Spencer sandstones suggests deposition on a storm wave-dominated shelf. Periods of shoaling to shoreface depths are indicated. In the south, sandstones are markedly more volcanic-rich (dominantly arkosic litharenites), contain more fossils, and are more highly bioturbated. Shelf-storm deposits in the south are normally graded with a basal lag of coarse volcanic grains and fossils. Besides a more proximal volcanic source, a shoal/barrier within-the southern part of the basin may have caused the different sediment character. Deposition was probably at middle to inner shelf depths at the outcrop belt. It may have deepened slightly eastward before shoaling to nonmarine in the easternmost part of the study area.Volcanism was active nearby on the eastern and southeastern margins of the basin. Small volcanic centers within the basin may have created highs and acted as localized volcanic sources. As transgression continued, upper Spencer mudstones were deposited at middle to upper bathyal depths. Volcanic activity increased on the eastern edge of the basin. Mudstones grade eastward and upward into tuffs and flows of the eastern Willamette volcanic facies. The Stratigraphy and Depositional Setting of the Spencer Formation, West-central Willamette Valley, Oregon; a Surface-subsurface Analysis by Linda J. Baker A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed March 3, 1988 Commencement June 1988 APPROVED: Redacted for Privacy ProfeLqr of Geol6iy in charge of major Redacted for Privacy gad of department 6f ology Redacted for Privacy Dean of Gradua School Date thesis is presented March 3, 1988 ACKNOWLEDGEMENTS First, I would like to thank Dr. Enlows for the time and effort he put forth to start me on a thesis project and find funding, despite his failing health. I also wish to thank Dr. Oles for his help with funding, and for his thorough reviews of my thesis drafts. I am indebted to Dr. Van Atta for his willingness to be a long-distance advisor and for his field and laboratory expertise. My thanks also go to Dr. Lawrence for serving on my thesis committee. For their financial support, I would like to thank Union Oil of California and the Oregon Department of Geology and Mineral Industries. I would also like to thank the many people who helped me with field work and/or technical expertise including Tim Fleming, Dr. and Wendy Niem, Bill King, and Susan Ivy. Special thanks go to my parents and my husband Dan for their moral support, patience, and help through the ups and downs as I tried to complete my master's degree. TABLE OF CONTENTS ?age INTRODUCTION 1 Location and Accessibility 3 Previous Work 4 Methods 6 Field Methods 6 Petrology 6 Grain Size Analysis 7 Subsurface Data 9 Time Scale Definition 9 REGIONAL GEOLOGY 12 Cenozoic History 12 Eocene Stratigraphy 18 The Siletz River Volcanic Series 18 Early and Middle Eocene Stratigraphic Problems 20 The Umpqua Formation/Group 25 The Tyee and Elkton Formations 25 The Yamhill Formation 28 The Coaledo, Spencer, and Cowlitz Formations 30 The Nestucca and Toledo Formations 33 Oligocene Stratigraphy 34 Intrusive Rocks 35 Structure of the study area 35 SUBSURFACE ANALYSIS 39 Yamhill Stratigraphy 41 Spencer Stratigraphy 44 Volcanic Units 53 Subsurface Constraints on Structural Movements 54 PETROLOGY AND STRATIGRAPHY OF SURFACE EXPOSURES 58 Lithology/Stratigraphy 59 Monmouth-Buena Vista Area 59 Starr Creek-Belfountain-Monroe Area 74 Albany-Corvallis Area 85 Petrography 88 Textural Analysis 99 DISCUSSION 109 Late Eocene Tectonic Activity 109 Stratigraphic Problems 115 Yamhill Formation: Depositional Environments 116 Spencer Formation: Depositional Environments 116 Monmouth-Buena Vista Area 118 Subsurface Geology: Northern Area 125 Albany-Corvallis Area 126 Subsurface Geology: Central Area 128 Starr Creek-Belfountain-Monroe Area 130 Subsurface Geology: Southern Area 133 Depositional History 134 Provenance 138 Diagenesis 141 SUMMARY AND CONCLUSIONS 144 FUTURE WORK AND UNRESOLVED PROBLEMS 150 REFERENCES 152 APPENDICES 164 (I) Formulas for Calculation of Grain Size Parameters 164 (II) Grain Size Analysis Data 165 (III) Fraction Percentage Plots of Grain Size Data 167 (IV) Hummocky Cross-stratification and Hummocky Sequences 170 LIST OF FIGURES Figure Page 1. Location map of study area 2 2. Time scales: traditional versus recent 10 3. Tectonic setting of western Oregon 13 4. Cenozoic geologic history of western Oregon 15 5. Correlation chart of Tertiary strata of western Oregon 21 6. Geologic map of Eocene strata, Oregon Coast Range 23 7. Comparison of terminologies used for lower and middle Eocene strata, southern Oregon Coast Range 24 8. Structural geology of Philomath area 38 9. Reference map showing well and cross-section locations 40 10. Isopach map of Miller sandstone member, Yamhill Formation43 11. Isopach map of upper Yamhill mudstone 45 12. Isopach map of lower member of Spencer Formation 48 13. Fence diagram: subsurface geology of the central Willamette Valley 52 14. Structure contour map: base of the Spencer Formation 55 15. Outcrop location map 60 16. Stratigraphic section: "clay" pit 63 17. Lithologies and sedimentary structures at "clay" pit 64 18. Stratigraphic section: Helmick Hill 65 19. Lithologies and sedimentary structures at Helmick Hill 66 20. Sedimentary structures at Monmouth Road summit 69 21. Stratigraphic section: Buena Vista bluffs 70 22. Lithologies and sedimentary structures at Buena Vista 71 23. Stratigraphic section: Dawson beds 76 24. Lithologies at Dawson and Reese Creek 78 25. Idealized composite stratigraphic section of lower member of the Spencer in the southern area 80 26. Stratigraphic section: Belfountain Road outcrop 82 27. Lithologies of upper part of lower member of Spencer in the southern area 84 28. Stratigraphic section: volcanic-rich beds, west Albany 89 29. Lithologies of Spring Hill and upper member of the Spencer, central area 90 30. Mineral classification of Spencer sandstones 92 31. Photomicrograph of basal Spencer, southern area 96 32. Correlation chart of Spencer terminology 98 33. Binary plot of grain size versus skewness 102 34. Binary plot comparing grain size analysis results with those of the western Tualatin Valley 104 35. C-M binary plot of Spencer grain size data 106 36. Schematic cross-section showing interpreted movement on the Corvallis Fault 110 37. Possible paleogeographic setting during deposition of the lower member of the Spencer 113 38. Hummocky bedding and hummocky sequences 119 39. High energy shoreface facies 121 40. Normally graded shelf sequences 129 41. Block diagrams across the central Willamette Valley during Spencer deposition 135a 42. Ternary diagram comparing Spencer sandstone composition and tectonic setting/provenance 139 43. Bedform phase