Depoe Bay Basalt, and Intrusive Cape Foulweather Basalt. Beach Sands, Pleistocene Marine Terraces, and Stream Alluvium Unconform- Ably Overlie These Tertiary Units
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AN ABSTRACT OF THE THESIS OF Thomas Neil Smith for the degree Master of Science (Name) (Degree) in Geology presented on /97 (Major Department) (Dte) Title: STRATIGRAPHY AND SEDIMENTATION OF THE ONION PEAK AREA, CLJATSOP COUNTY, OREGON / Abstract approved: Dr. .R. Nie Six Tertiary units are exposed in the Onion Peak area near the town of Cannon Beach., Oregon.The units consist of:late Eocene to early Miocene Oswald West mudstones (informal), middle Miocene Astoria Formation (Angora Peak sandstone and Silver Point mudstone members.-informal), middle Miocene basalts of intrusive and extrusive Depoe Bay Basalt, and intrusive Cape Foulweather Basalt. Beach sands, Pleistocene marine terraces, and stream alluvium unconform- ably overlie these Tertiary units. The Oswald West rnudstones consist of more than 1600 feet of well-bedded, burrowed, tuffaceousiltstones and mudstones with sub ordinate tuff aiid glauconitic sandstone beds.ForaminiLeral and trace fossil evidence suggest that these mudstones were deposited in marine waters of upper bathyal depths. The overlying 1700-foot thick Astoria Formation is divided into two informal mappable members: the 1100-foot thick Angora Peak sandstone and the 600-foot thick Silver Point mudstone (proposed informally in this study).The Angora Peak sandstone member is composed of several hundred feet of thickly laminated feldspathic sandstones with local cross-bedded lithic conglomerates and carbon- aceoiis siltstones.The overlying Silver Point mudstone member consists of rhythmically interbedded mudstones and graded turbidite sandstones, overlain by bedded mudstones, thin siltstones, and local conglomerate lenses.Conglomerate clast lithologies and heavy mineral suites of the Silver Point and Angora. Peak members are similar to the sediment carried by the Columbia River today and may be an ancient deposit of that river system.The provenances for these strata were pre-Miocene andesitic and dacitic rocks of the western Cascades, the Coast Range Eocene Tillamook basalts, and plutonic, metamorphic, and sedimentary contributions from eastern Oregon and Washington, Idaho, Montana, and Canada,Channel fluvial sandstones, conglomerates, and well-bedded shallow marine sandstones of the Angora Pea.k member, probably interfingering with interbeaded, deeper marine (600 feet-outer shelf) mudstones and turbidite snd- stones of the Silver Point member, are interpreted tq have been deposited near the mouth of a river, adjacent to the oceans a delta. Dikes, sills, peperites, and irregular intrusive bodies of middle Miocene Depoe Bay Basalt intruded Oswald West mudstones and the Astoria Formation.These aphanitic to finely crystalline equigranular basaltic intrusives locally fed over 2000 feet of palagonitized pillow lavas and basaltic breccias which now form the highest peaks in the area.The intrusives locally penecontemporaneously deformed the Astoria strata into a series of large-scale soft sediment deformation folds and sedimentary breccias. Cape Foulweather Basalt intrudes all sedimentary units and cuts the Depoe Bay Basalts.The basalt is recognized by sparse, large plagioclase phenocrysts. The area is cut by several east-west and north-south trending high angle faults with ip to 500 feet displacementA large north-south syncline in the central part of the thesis area delineates a structural and depositional basin. A smaller north-plunging anticline is present near the coast. Recent landslides in the Silver Point member have been particu- larly destructive along the coast and inland; they have been caused by wave, stream action, and by man over-steepening unstable slopes.Crushed basalt quarry rock and potential petroleum reser- voirs in the Angora Peak sandstones in stratigraphic and structural tra traps, particularly in nearby offshore areas, are the main geological economic resources of the aiea. Stratigraphy and Sedimentation of the Onion Peak Area, ClatsQp County, Oregon by Thomas Neil Smith A THESIS submitted to Oregon State Tjniversity in partial fulfillment of the requirements for the degree of Master of Science June 1975 APPROVED: Assistant Professor of Geology in charge of major hairman, Department of Geology 9 Dean of Graduate School Date thesis is presented C-"cX Typed by Opal Nichols for Thomas Neil Smith ACKNOWLEDGEMENTS The writer wishes to thank the American Associationof Petrole- urn Geologists for a Grand-in.Aidwhich helped defray field and laboratory expenses. I also wish to thank Dr. A. R. Niern, my majorprofessor, for his advice in the field and laboratory and carefulediting of the manu- script.Drs. K, F, Oles and E. M. Taylor criticallyread the manu- script.Microfossil identification and paloecologcal data were pro- vided by Weldon W. Rau of the Washington Departmentof Natural Resources; W. 0. Addicott of the U. S. GeologicalSurvey identified the megafossils; and C. K. Chamberlain of Ohioniversity identified the trace fossils.Dr. E. M. Taylor of Oregon State University pro- vided chemical analyses of rock samples. I would also like to thank my parents and closefriends for their aid and understanding while writing this thesis. TABLE OF CONTENTS Pag INTRODUCTION 1 Location and Accessibility 1, Climate and Vegetation 4 Purposes of Investigation 5 Previous Work 5 Methods of Investigation 7 Field Methods Analytical Methods 9 REGIONAL STRATIGRAPHY 12 DESCRIPTIVE GEOLOGY OF THE THESIS AREA 18 Late Eocene to Early Mioceie Mudstones 18 Oswald West Mudstones 18 Lithologies and Structures 19 Petrology 23 Contact Relations 25 Age and Correlation Depositional Environment 28 Astori3 Formatior 32 Angora Peak Sandstone Member 33 Lithologies and Structures 35 Petrology 45 Contact Relations 56 Age and Correlation 57 Depositional Environment 58 Silver Point Mudstone Member 60 Lithologies and Structures 62 Petrology 71 Contact Relatiois 77 Age and Correlation 78 Depositional Environment 79 Angora Peak Sandstone and Silver Point Mudstone Members 81 Conglomerate Pebble Lithologie s 81 Grain Size Analysis 86 A Depositional Model 88 TABLE OF CONTENTS (Cont.) Page The Basalts 91 Depoe Bay Intrusive Basalt 92 Distribution 92 Lithologies and Contact Characteristics 95 Petrology 100 Depoe Bay Extrusive Rocks 101 Distribution 101 Lithologicz Characteristics 103 Petrology 107 Origii 107 Contact Relatior).s 110 Age and Correlation 110 Cape Foulweather Intrusive Rock 111 Distribution and Lithologic Characteristics 111 Petrology 112 Age and Correlation 112 Other Intrusive Rock 113 Distribution and Lithologic Characteristics 113 Petrology 114 Chemical Composition and Correlations 115 Quaternary Deposits 11 STRUCTURAL GEOLOGY 124 Regional Structure 124 Structure of the Thesis Area 124 Soft Sediment Deformation Structures 129 ECONOMIC AND ENGINEERING GEOLQGY 135 Coal 135 Petroleum 135 Crushed Rock 139 Landsliding 140 GEOLOGIC HISTORY 144 Transport Directions 144 Provenance 147 Geologic Summary and Conclusions 151 BIBLIOGRAPHY 155 TABLE OF CONTENTS (Gout.) Page APPENDICES 161 Appendix I.Principal Reference Section of the Angora Peak Sandstone Member. 161 Appendic II.Principal Reference Section of the Silver Point Mudstone Member 169 Appendix Eli.Principal Reference Section of the Angora Peak and Silver Point Members. 176 Appendix IV.Pebble Lithologies of Gonglomera.tes in the Astoria Formation. 182 Appendix V.Size Analyses of Selected Sandstone Samples. 183 Appendix VI.Heavy Mineralogy of Selected Samples. 184 Appendix VII.Modal Analyses of Selected Sandstone Samples. 185 Appendix VIII.X-ray Analyses of Selected Samples. 186 Appendix IX.Chemical Analyses of Selected Igneous Samples. 187 Appendix X.Modal Analyses of Selected Basalt Samples. 188 Appendix XI.Location of Samples. 189 LIST OF FIGURES Figure Page 1 ,Index map showing location of Onion Peak area. 2 z View looking south from Ecola State Park across thesis area. 3 3 Correlation chart of the Tertiary formations of northern-central Oregon Coast Range. 13 4 Typical crumbly outcrop of Oswald West mudstone 5 Large-scale planar cross-bedingin AngoPe4 sandstones. 38 6 Calcareous concentrations in laminated Angora Peak sandstone. 38 7 Vertical and contorted .Angora Peak strata in the 100-foot sea cliff north of Hug Point. 41 Thoroughly mixed sandstone with cohesive mud' stone clasts and large rounded pebbles in the ngora Peak member. 41 9 Microfaulted, typical laminated upper Angora Peak sandstones. 44 10 Broken chaotic angular blocks of laminated upper Angora Peak sandstones. 44 11 Sandstone lens in poorly sorted boulder con- glomerate; Angora Peak sandstone member. 46 12 Classification of Angora Peak and Silver Point sandstones. 48 13 Quartz sc1ist fragment in Angora Peak sandstone. 49 14 Well-bedded sandstones and mudstones of the Silver Point meiber. 63 LIST OF FIGURES (Cont.) Figure Page 1 Close.'.up of bedded sandstones and mudstones of the Silver Point member. 63 16 Graded conglomerate lenses in the Silver Point member. 68 17 Clean, carbonate cemented, channelized sandstone from the Silver Point member. 74 18 Relative abundance of pebble lithologies in Angora Peak and Silver Point zonglomerates 19 Sea stack of Depoe Bay Basalt at Silver Point. 93 20 Small, highly irregular fractured intrusive of Depoe Bay Basalt intruding Angora Peak sandstones. 93 21 Horizontal, columnar jointing in a ZO-foot thick Depoe Bay Basalt dike. 96 Z2 Close-up of peperite dike of Depoe Bay Basalt. 99 23 Basaltic glass fragment with pilotaxitic texture from peperite dike. 99 24 Depoe Bay Basalt dike rock showing intersertal texture of brown alteration products. 102 25 Pillow fragments of Depoe