AN ABSTRACT OF THE THESIS OF THOMAS M. MURPHY for the degree of MASTER OF SCIENCE in Geologypresented on May 4, 1981 Title: GEOLOGY OF THE NICOLAI MOUNTAIN-GNAT CREEK AREA, CLATSOP COUNTY, NORTHWESTERN OREGON Abstract Approved: Alan R. Niem Twelve rockunits,from upper Eocene to middle Miocene are exposed in the Nicolai Mountain-Gnat Creekarea. They are, from oldest toyoungest:Pittsburg BluffFormation;Oswald West mudstone; Big Creek sandstone, upper Silver Point mudstone, and Pipeline mudstone members of the AstoriaFormation;Depoe Bay Basalt;Grande Ronde Basalt; Cape Foulweather Basalt; and sandstone correlative to the Vantage Member of the Ellensburg Formation;Frenchman Springs Member of the Wanapum Basalt; Clifton,Formation (defined byauthor);and Pomona Member of the Saddle Mountains Basalt. Upper Eocene Pittsburg Bluff Formation is a fine-grained sandstone and laminated siltstone deposited in an inner to middle continental shelfenvironment. Concordant onlapping by the upperOligocene,middle shelf to upper slope glauconitic Oswald Westmudstone,establishes a tie between the northeastern and northwestern Oregon Coast Range stratigraphies. The lower to middle Miocene Astoria Formationrepresents another marine onlapsequence,beginning with the fine-grained cross-bedded inner shelf Big Creek sandstone member. The overlying laminted upper Silver Point and Pipelinemudstone members were deposited ina deepening middle shelf to upper slope environment. The middle Miocene tholeiitic basalts extrudedfrom two sources. GrandeRonde,FrenchmanSprings,and Pomona basalts of theColumbia RiverGroup erupted from fissures east of the Cascades and flowed down an ancestral ColumbiaRiver valley entering the sea inthe study area. Simultaneously, petrologically similarbut less voluminous Depoe Bay and Cape Foulweather are correlativeto thelow MgO Grande Ronde and Frenchman Springs inthe study area.Abundantdikesand sills and thebathyalmudstone interbeds suggest that the coastal pillowbasalts extrudedlocally ontothe seafloor. Subaerial plateau-derived flowsare associatedwith cross-bedded, fluvial to shallowmarine arkosic sandstoneinterbeds. Some of these subaerial lavas flowed into the sea, forming "lava deltas" of foresetpillowpalagoniteand possibly "invasive" sills. The basalt stratigraphyallows a nearly flow by flow correlation of Grande Ronde and Frenchman Springs units from the Clackamas River area (WesternCascades) into thisstudy area. The presence of Pomona Memberin thisareais the first substantiated recognition of thispetrologically and chemically distinctive subaerial flow in northwestern Oregon. The 200-meter thick Clifton formation (previously called the Pliocene(?)sandstone atClifton)is now dated by diatom assemblages and stratigraphic position as middle Miocene in age.Three lithofacies arerecognized. Facies 1, at the base and top of theunit,is anarkosic,fine- tocoarse-grained sandstone withcross-bedding,vertical Rosselia burrows, lignitic coalbeds,and rare molluscan shells.It represents a river mouth and shallow marine offshore bardeposit. Facies 2, in the middle of theformation,consitst of well-laminated diatom-bearing carbonaceous and micaceous shelf/slope siltstones with thin fine-grained turbiditesandstones. Facies consists of channelized siltstonebreccias,chaotic debris flows, thick amalgamated arkosic grainflows,structureless sandstones, and minor volcanic pebble conglomerates. These lithologies suggest a canyon head and slump deposit formed as a submarine channel cut into the shelf/slope siltstones of Facies 2.Sandstone petrography and grain size analyses indicate that the detritus of the Clifton and other sandstones in the area was derived from acidigneous, metamorphic,and intermediate volcanic provenances similar to those drained by the Columbia River today. Despite high measured permeabilities, breaching limits the reservoir potential of the sandstone.Mudstone units are immature to mature hydrocarbon source rocks.A potential for gas exists in the Cowlitz or its equivalent in the subsurface and in the Clifton channel sandstones projected into the offshore area. Frontispiece:Nicolai Ridge rocks dipping to the northwest. Bend in the Columbia River is in part the result of normal faulting along the ridge. View is from the east. THE GEOLOGY OF THE NICOLAI MOUNTAIN-GNAT CREEK AREA,CLATSOPCOUNTY,NORTHWESTERN OREGON by Thomas M. Murphy A Thesis submitted to Oregon State University 1981 in partial fulfillment of the requirements for the degree of Master of Science Completed May 4, 1981 Commencement June, 1981 APPROVED BY: Associate Professorof Geology in chargeof major Chairman,Departmentof Deanof Gra uate School Date thesis is presented May 4, 1981 Typed by NenaM. Germany for ThomasM. Murphy ACKNOWLEDGEMENTS As with any work of thismagnitude,there are numerous people who aid in itscompletion, all ofwhich cannot be mentionedhere. But there are a few whosehelp I could not have donewithout. These deserve special thanks. Al Niem, my major professor, and especially Wendy Niem, both of whose countless hours and direction not only helped with this thesis but taught me the fundamentals of scientific investigation. Laurie Hanson who spent the better part of three years chasing my ideas instead of her own and typed the initial draft. Pat and BethJackson,truefriends,who helped with preparation of the manuscript but more importantly were there during the low points. Len Hazelof Gulf Oil Company,who awarded me a Technical Fellowshipin 1978 and ParkeSnavely ofthe U.S. Geologic Survey,who helped me with summerfield support. Without their financial commitment this thesis could not have been. Many thank to the people who supported me with their technical expertiseincluding: Weldon Rau, Ellen Moore, Warren Addicott,and John Barron of the U.S. Geologic Survey; Kent Chamberlain,RomanSchmitt,Monty Smith, Hans Schrader, Ed Taylor,Keith Oles and Scott Hughes of Oregon State University; and Wes Franklin and Cristie Stewart of Tenneco Oil Company. Finally, to Betty Murphy who nutured the inquisitivenature and developed the character in me to complete such a project. TABLE OF CONTENTS INTRODUCTION 1 Purpose of Investigation 1 Location and Accessibility 2 Methods of Investigation 4 Field Methods 4 Laboratory Methods 5 Previous Work 8 BIOSTRATIGRAPHIC FRAMEWORK 11 REGIONAL GEOLOGY 14 DESCRIPTIVE GEOLOGY 22 Introduction 22 Pittsburg Bluff Formation 24 Nomenclature and Distribution 24 Lithology and Sedimentary Structures 25 Contact Relations 28 Age and Correlation 29 Depositional Environment 30 Oswald West Mudstone 34 Nomenclature and Distribution 34 Lithologyand Sedimentary Structures 35 Contact Relations 37 Age and Correlation 39 Depositional Environment 40 Astoria Formation 42 Big Creek Sandstone Member 44 Nomenclatureand Distribution 44 Lithology and Sedimentary Structures 45 Contact Relations 48 Age and Correlation 49 Depositional Environment 51 Upper Silver Point Mudstone Member 54 .Nomenclature and Distribution 54 Lithology and Sedimentary Structures 55 Contact Relations 56 Age and Correlation 57 Pipeline Member 58 Nomenclature and Distribution 58 Lithology and Sedimentary Structures 59 Contact Relations 61 Age and Correlation 62 Depositional Environment of Pipeline and Upper Silver Point Mudstone 62 BASALTS 65 Columbia River Basalt Group 65 Coastal Basalts 71 Stratigraphy 76 Magnetostratigraphy 78 Grande Ronde Basalt 80 Chemistry 81 Physical Characteristics 88 Subaqueous Flows 94 Petrography of Grande Ronde and Depoe Bay Basalts 99 Frenchman Springs Member of the Wanapum Basalt 101 Chemistry 102 Physical Characteristics 105 Subaqueous Flows 107 Petrography of Frenchman Springs and Cape Foulweather Basalts 109 Depoe Bay Petrologic Type 110 Chemistry 117 Magnetic Polarity 112 Physical Characteristics 113 Cape Foulweather Petrologic Type 116 Chemistry 117 Magnetic Polarity 118 Physical Characteristics and Distribution 118 Pomona Member of the Saddle Mountains Basalt 121 Chemistry 121 Petrography 124 Sedimentary Interbeds 127 Vantage Member 129 Minor Sandstone Interbeds 133 Interbeds in the Coastal Basalts 135 Correlation of the Basalts within the Nicolai Mountain-Gnat Creek Area 140 Correlation of the Yakima Subgroup from the Clackamas River Area to the NicolaiMountain-Gnat Creek Area 148 Commentson the Origin of the Coastal Basalts 153 Clifton Formation 167 Nomenclature 167 Distribution 169 Lithologyand Sedimentary Structures 169 Facies 1 171 Facies 2 178 Facies 3 180 Contact Relations 184 Lower Contact 184 Upper Contact 187 Intraformational Contacts 187 Age and Correlation 190 PaleocurrentAnalysis 195 Depositional Environment 198 Depositional Model 214 Quaternary Deposits 219 Sedimentary Rock Petrography and Provenance 223 Clifton Formation 226 Pittsburg Bluff Formation 243 Oswald West Mudstone 248 Astoria Formation 251 Sedimentary Rock Grain Size Analysis 256 Comparison of StatisticalSize Parameters of the Clifton Formation and the Pipeline Sandstone Member 70 STRUCTURAL GEOLOGY 274 Regional 274 Local 275 GEOLOGIC HISTORY 281 ECONOMIC GEOLOGY 293 Crushed Rock and Gravel 293 Aggregate for Cement 294 Coal Potential 295 Petroleum Potential 295 CONCLUSIONS 300 REFERENCES CITED 304 APPENDICES 318 I Measured Reference Sections 318 a-a' - Pittsburg Bluff Formation 318 b-b'-Contact-Pipeline Mudstone and Clifton Formation 319 c-c' - CliftonFormation,Facies 2 & 3 321 d-d'-CliftonFormation,Facies 1 & 3 324 e-e' - Columbia River Basalt Flows at Bradley State Park 325 II Major OxideValues from Basalts 3 28 III Trace Element Values from Basalts 331 IV Fossil Checklist