AN ABSTRACT OF THE THESIS OF David Ezra Nelson for the degree of Master of Science in Geology presented on March 19, 1985 Title: Geology of the Fishhawk Falls Jewell Area, Clatsop County, Northwest Oregon // / Abstract approved: Redacted for privacy Dr. Alan R. Niem, Thesis advisor The study area is locatedon the NW flank of the northern Oregon Coast Range. Seven Tertiary formations compose the bedrock units: the Tillamook Volcanics, the Cowlitz, Keasey and Pittsburg Bluff formations, OswaldWest mudstone (iniormal), Astoria Formation (Silver Point member), and Grande Ronde (Depoe Bay) Basalt of the Columbia River Group. Major element geochemistry, petrology, and radiometric ages of volcanic rocks near Green Mountain suggest that this is an upthrown tectonic block of middle toupper Eocene Tillamook Volcanics. This outlier is composed of subaerial basaltic andesite flows and epiclastic debris flows which are intruded by complexes of basaltic andesite dikes. The volcanics typically contain 58-627 Si02 and reflecta rugged stratovolcano or mid-ocean island thatrose above sea level. Radiometric dates from the uppermost part of theupper Tillamook Volcanics in andnear this study area are from 37.1 0.4 m.y. (late middle Eocene) to 42.4+0.5 m.y. (early late Eocene). Preliminary paleornagnetic investigation of three. sites in the Tillamook Volcanics and overlying Cougar Mountainintrusions indicates that 48.4 26° of clockwise rotation has occurred since the late middle Eocene. The middle to upper Eocene (Narizian) Cowlitz Formation overlies the Tillamook Volcanicsw th angular unconformity in the study area. The Cowlitz is an onlapping marine sequence of basaltic and arkosic sandstone and deep-marine turbidite sandstone overlain bya regressive seq.ence of arkosic sandstone. Four lithofacies are considered informal members of the 465-rn--thick Cowlitz Formation. These are: a basal fluvial-littoral basaltic andesite conglomerate (Tc1), a lower shallow-marine sandstone composed of intrabasinal basaltic sandstone and extrabasinal micaceous arkosic sandstone and mudstone (Tc2),a thin turbidite sandstone and siltstone unit (Tc3), and anupper shallow-marine arkosic sandstone (Tc4). The lower sandstones are volcanic arenites derived locally from the Tillamook Volcanics. They are interbedded with micaceous plagi clase-bearing arkosic sandstone redistributed from the Cowlitz delta of southwestern Washington via longshore drift. The friable arkosic upper sandstone membermay be correlative to the "Clark and Wilson" reservoir sandstone in the Mistgas field. The Cowlitz Formation is overlain by the 1,640-2,050-rn- thick upper Narizian to RefugianKeasey Formation. Three mappable lithofacies of the Keaseyare defined as informal members based on lithology and biostratigraphy. The tuffaceous Jewell rnudstone forms the lower member ofthe Keasey Formation. It contains foram and coccolith assemblages that correlate to the Valvulineriatumeyensis Zone of California of Donnelly (1976). The middle member is the Vesper Church member which is late Refugian inage. This well-bedded to laminated graded sandstone andmudstone unit was deposited inested slope channels by turbidites as micaceous arkosic Cowlitz sands and mudswere redistributed offshore. The thick-bedded, structureless tuffaceousupper mudstone member is upper Refugian andcontains Foraminifera equivalent to the Uvigerina vicksburgensis Zone of California. Foraminifera paleoecology suggests that the three members were deposited inupper bathyal depths. The upper Eocene to Oligocene (upper Refugianto Zemorrian) 200-500-rn-thick Pittsburg Bluff Formation disconformably overlies the Keasey Formation. The thick-bedded tuffaceous to arkosic Pittsburg Bluff sandstones were deposited ina shallow-marine inner to middle shelf environment. Local uplift and/or eustatic sea level fall (corresponding to the end of cycle TE3of Vail and Hardenbol, 1979) combined tocause a late Eocene marine regression and disconformity at the base of the Pittsburg Bluff Formation. The rnollusc-rich bioturbated Pittsburg Bluff sandstone and the overlying tuffaceous OswaldWest deep-marine mudstone are, in part, coeval faciesthat represent inner to middle shelf sands andan adjacent deep-water outer shelf-upper slope depositionalenvironment, respectively. An Oligocene marine transgressioncorresponding to Vail and Hardenbol's cycle T01 eventuallycaused onlap of Oswald West slope muds over inner shelfPittsburg Bluff sands.The Oswald West is Oligocene (Zemorrian)in age in the thesis area but is as young as lower Miocene (Saucesian)in western Clatsop County.The deep-marine lower Mioceneto middle Miocene Silver Point mudstoneand the turbidite sandstones of the Astoria Formationoverlie the Oswald West mudstones with minor unconformity. The sedimentary strata of thethesis area were intruded by several dikes and sillsof middle Miocene tholeiitic basalt.Principal among the intrusionsare three extensive dikes (Beneke, Fishhawk Falls,and Northrup Creek) composed of low-MgO-high-Ti02 andlow-MgO-low-Ti02 basalt.These subparallel NE-trending dikesare 10 to 20 km long and are geochemically and magneticallycorrelative to low-MgO--high-Ti02 and low-MgO-low-Ti02subaerial flows of the Grande Ronde Basaltthat are exposed 6.5 km NE of the thesis area near Nicolai Mountainand Porter Ridge.This correlation supports the hypothesisof Beeson and others (1979) that these intrusionswere formed by ttinvasivehl flows of Columbia River Basalt.The dikes and flows represent the R2 and N2 magnetozones.Paleomagnetic results and detailed mapping along the three majordikes indicate that the dikes are cut into many small blocks by NE-trendingdextral and NW-trending sinistral faults.Oblique slip along these faults may have rotatedthese small blocks clockwise from the expected middleMiocerie declination.One such fault-bounded block in Benekequarry has been rotated 110 clockwise relative to otherblocks in the quarry. The area has apparently undergone fourperiods of structural deformation. Late Eocene deformation produced normal faults and a highly faultedanticline in the Cowlitz, Keasey and Pittsburg Bluff formations. Uplift of the Oregon Coast Range commenced during theOligocene. NW-SE extension in the middle Miocenepermitted intrusion of the NE-trending dikes, perhaps alongpre-existing faults or joints. Post-middle Miocene N-Scompression produced a conjugate shear couplet of dextralNW-trending and sinistral NE-trending oblique-slip faultsand associated E-W thrust faults. These may composea 20-km-wide zone of distributed shear between the Oregon Coast Rangetectonic block and the smaller tectonic blocks of SWWashington (Wells, 1981). These post-middle Miocene faultsmay be related to wrench tectonics and NW-trending dextralslip faults observed on the Columbia Plateau. They may have formed sympathetically to regional right-lateral wrenching ofthe North American plate by oblique subduction ofthe Juan de Fuca plate and/or extensional opening of the Basin andRange province (Magill and others, 1982). The north-central andeastern parts of the study area hold the greatest potentialfor natural gas accumulation in the permeable upper Cowlitzsandstones (Tc4) that may be correlative to the "Clark andWilson" sand. Mapping of the limited outcrops of thispotential reservoir suggests that the unit may pinchout to the west. It is overlain by Keasey mudstones that couldform cap rock. A highly faulted NW-trending antiform in thesubsurface, postulated from surface mapping, represents the beststructural trap in the area if the reservoir sands are not breached by faulting. GEOLOGY OF THE FISHHAWK FALLS JEWELL AREA, CLATSOP COUNTY, NORTHWESTOREGON by David Ezra Nelson A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Thesis presentedon March 19, 1985 Commencement June 1985 APPROVED: Redacted for privacy Associate Professor of Geology in charge ofmajor Redacted for privacy Chairman, Department of($eology Redacted for privacy Dean of Gradiat School Date thesis is presented March 19, 1985 Typed by David E. Nelson andCarolyn F. Knapp-Nelson Printed in Prestige Pica typestyle 1 ACKNOWLEDGEMENTS To the many people who have aided and supportedme in this study, my sincere thanks. This work was funded jointly by Diamond-Shamrock Corporation and Houston Oil & Minerals (now TennecoOil Exploration and Production Company). Special thanks for the monetary support are due to Chuck Wheeler, WesternDistrict Geologist, and geologist Alan Seeling, of Diamond-Shamrock Corporation. The project was suggested by Dr. AlanR. Niem, my major professor, who generated theinterest and support of Diamond-Shamrock and Houston Oil & Minerals. I extend my gratitude to Alan and Wendy Niem, both of whomhelped me to improve the manuscript significantlyover the first draft. Special thanks to Wendy for retypingsome of their suggested revisions; your effort made my revisions a lot easier. The paleomagnetic study wouldnot have been possible without the guiding interest of Dr. Shaul Levi,my committee member. Shaul provided knowledge, equipment and theuse of the magnetics laboratory in the College of Oceanographyat Oregon State University. Dr. J. G. Johnson instilled inme the importance of good writing, though the length of thisreport far exceeds his preferences. Thank you, Jess, for your excellent teaching; and sorry about the length. Several individuals deserve credit for their help by providing special diagnoses free of charge. Drs. Ellen Moore, Kristin McDougall,