AN ABSTRACT OF THE THESIS OF Carolyn Pugh Peterson for the degree of Master of Science in Geology presented on December 12, 1983 Title: Geology of the Green Mountain-Young's River Area, Clatsop County, Northwest Oregon Recicted for Privacy Abstract Approved: (Alan R. Niem) The upper Eocene to lower Oligocene Oswald West mudstone is the oldest formation (informal) in the Green Mountain-Young's River area. This 1,663 meter thick hemipelagic sequence was depos- ited in a low-energy lower to upper slope environment in the Coast Range forearc basin. The formation ranges from the late Narizian to the early Zemorrian(?) in age and consists of thick-bedded bio- turbated foraminiferal claystone and tuffaceous siltstone. Rare glauconitic sandstone beds also occur. In the eastern part of the study area, the upper part of the Oswald West mudstone is inter- bedded with the upper Refugian Klaskanine siltstone tongue. This informal unit consists of thick bioturbated sandy siltstone and silty sandstone that is a lateral deep-marine correlative of the deltaic to shallow-marine Pittsburg Bluff Formation in the north- eastern Coast Range. Discontinuous underthrusting of the Juan de Fuca oceanic plate at the base of the continental slope of the North American plate caused extensive uplift and subsidence along the Oregon continental margin throughout the Cenozoic (Snavely et al., 1980). Initiation of Oregon Coast Range uplift and accompanying erosion in the early Miocene, coupled with a global low stand of sea level (Vail and Mitchum, 1979), stripped most of the Oligocene (Zemor- rian) Oswald West strata and in places much of the uppermost Eocene (upper Refugian) Oswald West strata in the field area, cre- ating an unconformity. Deformation accompanying uplift included a system of east-west-trending, oblique-slip faults. The Pillarian-to-Newportian AstoriaFormation unconformably overlies the Oswald West mudstone andreflects deposition off- shore from an open, storm-dominated coast during anearly-to- middle Miocene transgression. Deposition of the Big Creek sand- stone and Silver Point mudstone membersof the Astoria Formation was controlled in part by submarinepaleotopography that developed as a result of early Miocenedeformation of the Oswald West strata. The up to 200 meter thick Big Creek membervaries from storm- deposited laminated sandstone to bioturbatedmollusk-bearing silty sandstone that accumulated during fair weatherconditions on the inner to middle shelf. Overlying and perhaps in part laterally equivalent to the Big Creek member is the up to200 meter thick, deeper marine Silver Point member which consistsof two litholo- gies: 1) interbedded, micaceous, turbidite sandstones andlami- nated mudstone; and 2) laminated bathyal mudstone that inter- tongues with and caps the turbidite sequences. The turbidite lithology is composed of two facies: 1) an underlying sand-rich facies, transitional between the shallow-marine BigCreek member and bathyal Silver Point strata, that was deposited onthe outer shelf by storm-induced turbidity currents; and2) an overlying sand-poor facies that was deposited at bathyaldepths. The turbi- dite facies channelized, and at some places removedthe underlying Big Creek member and were deposited directly overOswald West mud- stone. The Astoria depositional sequence ranges, from inner to outer neritic to bathyal facies and reflectscontinued deepening and anoxic depositional conditions of the Astoriabasin through the middle Miocene. Big Creek and Silver Point sandstone petrology reflects volcanic sources from an ancestral western Cascadesvol- canic arc and metamorphic and granitic basement rocksfarther east via an ancestral Columbia River drainage system. Diagenetic ef- fects include: (a) formation of local calcite concretionary cements; and (b) formation of pore-filling smectitefrom altera- tion of volcanic rock fragments. At least six middle Miocene Columbia River Basalt intrusive episodes affected the Green Mountain-Young's River area soonafter deposition of the Astoria Formation. These basalt sills and dikes include normally polarized and reversely polarizedlow Mg0 high TiO low Mg0 low TiO and high Mg0 Grande Ronde basalt chemical 2' 2' subtypes and two porphyritic Frenchman SpringsMember basalts (Ginkgo and Kelly Hollow(?) petrologic types). These basalt in- trusions are virtually indistinguishable, based onchemistry, from subaerial flows of the plateau-derived Columbia RiverBasalt Group subtypes at nearby Nicolai Mountain and Porter Ridge. This corre- lation supports the Beeson et al. (1979) hypothesisthat the intru- sions are not of local origin but formed by theinvasion of the flows into the Miocene shoreline sediments to form"invasive" sills and dikes. Many dikes were emplaced along northeast- and northwest-trending faults, and some (i.e., Ginkgo) cut older sills (Grande Ronde). A laterally extensive Frenchman Springs sill oc- curs under an older widespread GrandeRonde sill. From this older over younger intrusive relationship, amechanism of "invasion" of sediment from overlying lava flows is difficult to envision. A pulse of rapid subduction starting in the middle Miocene (Snavely et al., 1980) was accompanied by renewed uplift, inten- sive block faulting, and continued development of the earlier formed Coast Range uplift. Left-oblique northeast-trending faults and conjugate northwest-trending right-oblique faults offset Grande Ronde and Frenchman Springs dikes and sills. This conju- gate fault pattern may reflect oblique east-west convergence between the North American and Juan de Fuca plates. The Silver Point mudstones and Oswald West mudstones have high total organic carbon contents, up to 5.5%, but are thermally immature and may act only as a source for biogenic gas(?) in the subsurface. Suitable reservoir rocks, such as the gas-producing upper Eocene Cowlitz Formation C & Wsandstone, may pinch out be- fore reaching the Green Mountain-Young's River area and are yet to be penetrated by exploration drilling. Post-middle Miocene fault traps abound in the area, although these faults might alsobreach subsurface natural gas reservoirs in the Green Mountain-Young's River area. GEOLOGY OF THE GREEN MOUNTAINYOUNG'S RIVERAREA, CLATSOP COUNTY, NORTHWEST OREGON by Carolyn Pugh Peterson A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed December 12, 1983 Commencement June 1984 APPROVED: Redacted for Privacy Professor of Geology in charge of major Redacted for Privacy Chairman of Department o eology Redacted for Privacy Dean of GraduatlIchool (f Date thesis is presented December 12, 1983 Typed by Therese Belden for Carolyn Pugh Peterson Acknowledgments Many people helped this study reach itscompletion, and along the way offered input, criticisms and laughterthat encouraged me to keep on working. Biostratigraphic control and paleoecologic indicators are difficult to retrieve from the rain forests of northwestOregon. The following people managed to decipher theflora, fauna, and trace fossils preserved, sometimes all toopoorly, in the rocks I sent to them: John Barron, Kristin McDougall, C. Kent Chamberlain, David Bukry, J. Platt Bradbury, Ellen Moore, andLaurel Bybell. Equipment, geochemical analyses, and data were cheerfully loaned or provided. Shaul Levi of O.S.U. gave instruction on the use of the fluxgate magnetometer aswell as loaning it for use in the field. Dr. Peter Hooper of W.S.U. offered to do XRFanalysis of basalt samples at very reasonable graduate student rates. Terry Mitchell made Amoco's facilities for hydrocarbon sourcerock geochemistry available and offered welcome encouragement. Alan Seeling of Diamond Shamrock Co. provided well cuttingsof a wild- cat well drilled in the study area as well asadditional chrono- stratigraphic data. Amoco and Arco oil companies provided generous fieldand laboratory support. Friends and fellow students encouraged me, and offered their ideas to help improve this thesis. Dave Nelson, Jim Olbinski, and Jeff Goalen were my companions in the fieldand the office and helped iron out problems and developgeologic hypotheses. Dr. A. R. Niem field checked the thesis area, ac- quired financial aid, and helped this thesis develop to its present form. This manuscript underwent a metamorphosis following its juvenile period. The following people offered constructive sug- gestions that helped improve this thesis: Curt Peterson, Kristine McElwee, Phil Rarey, and Drs. K. F. Oles and H. Enlows. To those that provided friendship and companionship, and put up with me, thank you. TABLE OF CONTENTS Page INTRODUCTION 1 Purpose 1 Location and Accessibility 1 Previous Work 3 Methods of Investigation 4 Field Work 4 Laboratory Methods 5 REGIONAL STRATIGRAPHY 7 A NORTHWESTERN-NORTHEASTERN OREGON COAST RANGE STRATIGRAPHIC PROBLEM 12 STUDY AREA STRATIGRAPHY 14 OSWALD WEST MUDSTONE 16 Nomenclature and Regional Distribution 16 Distribution 17 Lithology 17 Contact Relations 20 Age 21 Correlation 22 Petrology 25 Environment of Deposition 25 PITTSBURG BLUFF FORMATION 28 Nomenclature and Regional Distribution 28 Distribution 29 Lithology 29 Contact Relations 30 Age 32 Correlation 33 Petrology 34 Environment of Deposition 36 ASTORIA FORMATION 39 Introduction 39 Big Creek Sandstone Member 40 Nomenclature and Regional Distribution 40 Distribution 43 Lithology 43 Contact Relations 45 Age 47 Correlation 48 Petrology 50 Page Silver Point Mudstone Member 66 Nomenclature and Regional Distribution 66 Distribution 67 Lower Silver