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• 1. [Article] The geology of the Nicolai Mountain-Gnat Creek area, Clatsop County, northwestern Oregon

  Twelve rock units, from upper Eocene to middle Miocene are exposed in the Nicolai Mountain-Gnat Creek area. They are, from oldest to youngest: Pittsburg Bluff Formation; Oswald West mudstone; Big Creek ...

  Citation × Citation Citation Abstract Copy Title: The geology of the Nicolai Mountain-Gnat Creek area, Clatsop County, northwestern Oregon Author: Murphy, Thomas M. Twelve rock units, from upper Eocene to middle Miocene are exposed in the Nicolai Mountain-Gnat Creek area. They are, from oldest to youngest: Pittsburg Bluff Formation; Oswald West mudstone; Big Creek sandstone, upper Silver Point mudstone, and Pipeline mudstone members of the Astoria Formation; 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 by author); 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 shelf environment. Concordant onlapping by the upper Oligocene, middle shelf to upper slope glauconitic Oswald West mudstone, establishes a tie between the northeastern and northwestern Oregon Coast Range stratigraphies. The lower to middle Miocene Astoria Formation represents another marine onlap sequence, beginning with the fine-grained cross-bedded inner shelf Big Creek sandstone member. The overlying laminted upper Silver Point and Pipeline mudstone members were deposited in a deepening middle shelf to upper slope environment. The middle Miocene tholeiitic basalts extruded from two sources. Grande Ronde, Frenchman Springs, and Pomona basalts of the Columbia River Group erupted from fissures east of the Cascades and flowed down an ancestral Columbia River valley entering the sea in the study area. Simultaneously, petrologically similar but less voluminous Depoe Bay and Cape Foulweather are correlative to the low MgO Grande Ronde and Frenchman Springs in the study area. Abundant dikes and sills and the bathyal mudstone interbeds suggest that the coastal pillow basalts extruded locally onto the sea floor. Subaerial plateau-derived flows are associated with cross-bedded, fluvial to shallow marine arkosic sandstone interbeds. Some of these subaerial lavas flowed into the sea, forming "lava deltas" of foreset pillow palagonite and possibly "invasive" sills. The basalt stratigraphy allows a nearly flow by flow correlation of Grande Ronde and Frenchman Springs units from the Clackamas River area (Western Cascades) into this study area. The presence of Pomona Member in this area is the first substantiated recognition of this petrologically and chemically distinctive subaerial flow in northwestern Oregon. The 200-meter thick Clifton formation (previously called the Pliocene (?) sandstone at Clifton) is now dated by diatom assemblages and stratigraphic position as middle Miocene in age. Three lithofacies are recognized. Facies 1, at the base and top of the unit, is an arkosic, fine- to coarse- grained sandstone with cross-bedding, vertical Rosselia burrows, lignitic coal beds, and rare molluscan shells. It represents a river mouth and shallow marine offshore bar deposit. Facies 2, in the middle of the formation, consitst of well-laminated diatom-bearing carbonaceous and micaceous shelf/slope siltstones with thin fine-grained turbidite sandstones. Facies consists of channelized siltstone breccias, chaotic debris flows, thick amalgamated arkosic grain flows, 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 acid igneous, 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. Title: The geology of the Nicolai Mountain-Gnat Creek area, Clatsop County, northwestern Oregon Author: Murphy, Thomas M. Twelve rock units, from upper Eocene to middle Miocene are exposed in the Nicolai Mountain-Gnat Creek area. They are, from oldest to youngest: Pittsburg Bluff Formation; Oswald West mudstone; Big Creek sandstone, upper Silver Point mudstone, and Pipeline mudstone members of the Astoria Formation; 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 by author); 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 shelf environment. Concordant onlapping by the upper Oligocene, middle shelf to upper slope glauconitic Oswald West mudstone, establishes a tie between the northeastern and northwestern Oregon Coast Range stratigraphies. The lower to middle Miocene Astoria Formation represents another marine onlap sequence, beginning with the fine-grained cross-bedded inner shelf Big Creek sandstone member. The overlying laminted upper Silver Point and Pipeline mudstone members were deposited in a deepening middle shelf to upper slope environment. The middle Miocene tholeiitic basalts extruded from two sources. Grande Ronde, Frenchman Springs, and Pomona basalts of the Columbia River Group erupted from fissures east of the Cascades and flowed down an ancestral Columbia River valley entering the sea in the study area. Simultaneously, petrologically similar but less voluminous Depoe Bay and Cape Foulweather are correlative to the low MgO Grande Ronde and Frenchman Springs in the study area. Abundant dikes and sills and the bathyal mudstone interbeds suggest that the coastal pillow basalts extruded locally onto the sea floor. Subaerial plateau-derived flows are associated with cross-bedded, fluvial to shallow marine arkosic sandstone interbeds. Some of these subaerial lavas flowed into the sea, forming "lava deltas" of foreset pillow palagonite and possibly "invasive" sills. The basalt stratigraphy allows a nearly flow by flow correlation of Grande Ronde and Frenchman Springs units from the Clackamas River area (Western Cascades) into this study area. The presence of Pomona Member in this area is the first substantiated recognition of this petrologically and chemically distinctive subaerial flow in northwestern Oregon. The 200-meter thick Clifton formation (previously called the Pliocene (?) sandstone at Clifton) is now dated by diatom assemblages and stratigraphic position as middle Miocene in age. Three lithofacies are recognized. Facies 1, at the base and top of the unit, is an arkosic, fine- to coarse- grained sandstone with cross-bedding, vertical Rosselia burrows, lignitic coal beds, and rare molluscan shells. It represents a river mouth and shallow marine offshore bar deposit. Facies 2, in the middle of the formation, consitst of well-laminated diatom-bearing carbonaceous and micaceous shelf/slope siltstones with thin fine-grained turbidite sandstones. Facies consists of channelized siltstone breccias, chaotic debris flows, thick amalgamated arkosic grain flows, 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 acid igneous, 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. Close

• 2. [Article] Geology of the Saddle and Humbug Mountain area, Clatsop County, northwestern Oregon

  Six early to middle Tertiary geologic units crop out in the Saddle and Humbug Mountain area. They include the late Eocene to early Miocene Oswald West muds tones, the lower Silver Point and the upper Silver ...

  Citation × Citation Citation Abstract Copy Title: Geology of the Saddle and Humbug Mountain area, Clatsop County, northwestern Oregon Author: Penoyer, Peter E. Six early to middle Tertiary geologic units crop out in the Saddle and Humbug Mountain area. They include the late Eocene to early Miocene Oswald West muds tones, the lower Silver Point and the upper Silver Point tongues of the middle Miocene Astoria Formation, and the middle Miocene Depoe Bay and Cape Foulweather Basalts. One new lithologically distinct unit, the Falls Creek member of the Oswald West muds tones, is mapped, described, and named informally herein. These Tertiary units are locally overlain by Quaternary stream alluvium and landslide deposits. The Oswald West mudstones consist of more than 500 meters of thickly bedded grayish to yellowish orange bioturbated tuffaceous siltstones and mudstones interstratified with minor glauconitic sandstones and tuff beds. An open-marine, deep-water, slightly reducing depositional environment (outer shelf-continental slope) is indicated by Foraminifera, trace fossils, glauconite, and the predominantly fine-grained character of the unit. Deposition of the Falls Creek silts tone member which contains a very shallow water molluscan fossil assemblage probably occurred during gradual shallowing near the end of the late Oligocene, possibly due to adjacent deltaic progradation. An angular unconformity separates the Oswald West muds tones from the overlying lower Silver Point tongue and suggests that a broad uplift occurred during an early Miocene hiatus. Thin feldspathic sandstone lenses occur locally above the unconformity and mark the beginning of a middle Miocene marine transgression during lower Silver Point time. The approximately 300-meter thick Silver Point member is composed of two intertonguing lithosomes. The lower tongue consists of rhythmically bedded, light gray, laminated, micaceous and carbonaceous sandstones and dark gray siltstones. Graded bedding and partial Bouma sequences suggest that these sandstones were deposited by turbidity currents, possibly on a delta slope or outer delta platform. Paleocurrent measurements and facies patterns indicate that these turbidity flows originated at a south to southeasterly source, probably by slumping off the nearby Angora Peak delta front. Thick-bedded, structureless, arkosic sandstones which locally interfinger with the basal part of the lower Silver Point tongue probably also reflect redeposition of "clean" delta front sheet sands into deeper water delta slope environments. The lower Silver Point tongue grades upward into the 200-meter thick upper Silver Point tongue which consists of dark gray structureless to finely laminated mudstones and rare thin sandstone beds. Foraminifera and the overall fine-grained lithology indicate that this unit was deposited under deep marine (upper bathyal), low energy conditions and reflects continuing marine transgression over the region. Postulated underthrusting caused by convergence of the Juan de Fuca oceanic plate and the North American continental plate along the middle Miocene Oregon continental margin (Ku lm and Fowler, 1974a, 1974b), may have caused the uplift and high-angle faulting and development of an unconformity which followed upper Silver Point marine deposition. Rapid subsidence and marine transgression ended this short-lived erosion period as evidenced by the local eruption of over 600 meters of palagonitized Depoe Bay submarine basaltic breccias and pillow lavas and more than 200 meters of sparsely porphyritic Cape Foulweather submarine pillow lavas. The Depoe Bay Basalt lies with angular unconformity over the faulted Silver Point member and Oswald West mudstones. A local basaltic conglomerate interbed within the Depoe Bay breccias (near the base) suggests that some early Depoe Bay volcanic buildups developed above wave base. Chemical analyses show that these basalts are tholeiites and are comparable to the type Cape Foulweather and Depoe Bay petrologic-types along the central Oregon Coast. All Tertiary sedimentary units are intruded by numerous dikes, sills, and irregular-shaped plutons. Regional dike swarms and local feeder dikes to Depoe Bay Basalt eruptive centers such as Saddle Mountain, commonly, either parallel or coincide with northeasttrending or northwest-trending lineaments and high-angle faults, suggesting a structural control for the emplacement of the intrusives and location of the volcanic centers. The largest fault in the study area (Humbug Mountain fault) is co-linear with the northwest-trending Gales Creek Fault and another unnamed fault southeast of the study area. In total, these three faults form an almost continuous line of faults across the northern Oregon Coast Range for a distance of 100 km. Title: Geology of the Saddle and Humbug Mountain area, Clatsop County, northwestern Oregon Author: Penoyer, Peter E. Six early to middle Tertiary geologic units crop out in the Saddle and Humbug Mountain area. They include the late Eocene to early Miocene Oswald West muds tones, the lower Silver Point and the upper Silver Point tongues of the middle Miocene Astoria Formation, and the middle Miocene Depoe Bay and Cape Foulweather Basalts. One new lithologically distinct unit, the Falls Creek member of the Oswald West muds tones, is mapped, described, and named informally herein. These Tertiary units are locally overlain by Quaternary stream alluvium and landslide deposits. The Oswald West mudstones consist of more than 500 meters of thickly bedded grayish to yellowish orange bioturbated tuffaceous siltstones and mudstones interstratified with minor glauconitic sandstones and tuff beds. An open-marine, deep-water, slightly reducing depositional environment (outer shelf-continental slope) is indicated by Foraminifera, trace fossils, glauconite, and the predominantly fine-grained character of the unit. Deposition of the Falls Creek silts tone member which contains a very shallow water molluscan fossil assemblage probably occurred during gradual shallowing near the end of the late Oligocene, possibly due to adjacent deltaic progradation. An angular unconformity separates the Oswald West muds tones from the overlying lower Silver Point tongue and suggests that a broad uplift occurred during an early Miocene hiatus. Thin feldspathic sandstone lenses occur locally above the unconformity and mark the beginning of a middle Miocene marine transgression during lower Silver Point time. The approximately 300-meter thick Silver Point member is composed of two intertonguing lithosomes. The lower tongue consists of rhythmically bedded, light gray, laminated, micaceous and carbonaceous sandstones and dark gray siltstones. Graded bedding and partial Bouma sequences suggest that these sandstones were deposited by turbidity currents, possibly on a delta slope or outer delta platform. Paleocurrent measurements and facies patterns indicate that these turbidity flows originated at a south to southeasterly source, probably by slumping off the nearby Angora Peak delta front. Thick-bedded, structureless, arkosic sandstones which locally interfinger with the basal part of the lower Silver Point tongue probably also reflect redeposition of "clean" delta front sheet sands into deeper water delta slope environments. The lower Silver Point tongue grades upward into the 200-meter thick upper Silver Point tongue which consists of dark gray structureless to finely laminated mudstones and rare thin sandstone beds. Foraminifera and the overall fine-grained lithology indicate that this unit was deposited under deep marine (upper bathyal), low energy conditions and reflects continuing marine transgression over the region. Postulated underthrusting caused by convergence of the Juan de Fuca oceanic plate and the North American continental plate along the middle Miocene Oregon continental margin (Ku lm and Fowler, 1974a, 1974b), may have caused the uplift and high-angle faulting and development of an unconformity which followed upper Silver Point marine deposition. Rapid subsidence and marine transgression ended this short-lived erosion period as evidenced by the local eruption of over 600 meters of palagonitized Depoe Bay submarine basaltic breccias and pillow lavas and more than 200 meters of sparsely porphyritic Cape Foulweather submarine pillow lavas. The Depoe Bay Basalt lies with angular unconformity over the faulted Silver Point member and Oswald West mudstones. A local basaltic conglomerate interbed within the Depoe Bay breccias (near the base) suggests that some early Depoe Bay volcanic buildups developed above wave base. Chemical analyses show that these basalts are tholeiites and are comparable to the type Cape Foulweather and Depoe Bay petrologic-types along the central Oregon Coast. All Tertiary sedimentary units are intruded by numerous dikes, sills, and irregular-shaped plutons. Regional dike swarms and local feeder dikes to Depoe Bay Basalt eruptive centers such as Saddle Mountain, commonly, either parallel or coincide with northeasttrending or northwest-trending lineaments and high-angle faults, suggesting a structural control for the emplacement of the intrusives and location of the volcanic centers. The largest fault in the study area (Humbug Mountain fault) is co-linear with the northwest-trending Gales Creek Fault and another unnamed fault southeast of the study area. In total, these three faults form an almost continuous line of faults across the northern Oregon Coast Range for a distance of 100 km. Close

• 3. [Article] Geology of the Elsie-lower Nehalem River area, south-central Clatsop and northern Tillamook counties, northwestern Oregon

  The middle Eocene Tillamook Volcanics form the oldest rock unit in the Elsie-lower Nehalem River area. K-Ar age determinations and age constraints imposed by foraminiferal and calcareous nannofossil assemblages ...

  Citation × Citation Citation Abstract Copy Title: Geology of the Elsie-lower Nehalem River area, south-central Clatsop and northern Tillamook counties, northwestern Oregon Author: Mumford, Daniel Franklin The middle Eocene Tillamook Volcanics form the oldest rock unit in the Elsie-lower Nehalem River area. K-Ar age determinations and age constraints imposed by foraminiferal and calcareous nannofossil assemblages of overlying sedimentary strata indicate an absolute age of about 42 Ma for the uppermost Tillamook Volcanics. Major oxide values indicate that the upper Tillamook Volcanics are highly fractionated high Fe-Ti tholeiitic basalts and basaltic andesites. These volcanics were erupted in a developing forearc under an extensional plate tectonic setting and formed a moderately large oceanic island. These subaerial flows are predominantly aphyric to plagioclase-augite porphyritic and have a pilotaxitic flow texture. Epochs of both normal and reverse magnetic polarity are recorded. Thermal subsidence related to the end of the volcanism resulted in deposition of the transgressive late Eocene Hamlet formation over the "Tillamook island". The informal Hamlet formation consists of three members. From oldest to youngest they are: the Roy Creek member, the Sunset Highway member, and the Sweet Home Creek member. Three lithofacies are present in the Roy Creek member. The stratigraphically lowest of these consists of basaltic boulder-pebble conglomerate and locally fossiliferous pebbly basaltic sandstones which were deposited in a high energy nearshore environment around rocky basaltic headlands and sea stacks of the Tillamook Volcanics. Molluscan fossils in this lithofacies are correlative to the middle to late Eocene "CowlitzCoaledo" fauna. Successively overlying lithofacies are a very coarseto coarse-grained shallow marine basaltic sandstone lithofacies and a medium- to fine-grained basaltic sandstone lithofacies. This fining upward sequenced documents progressive deepening of the depositional basin. Framework clasts in all three Roy Creek member lithofacies were predominantly derived from the Tillamook Volcanics. Pore-filling diagenetic chlorite, smectite (nontronite), calcite, and zeolite (clinoptilolite and heulandite) cements severely reduce the porosity of Roy Creek member sandstones. The Sunset Highway member of the Hamlet formation conformably overlies the Roy Creek member in eastern Clatsop and western Columbia counties and pinches out to the west at about the longitude of the Nehalem River in T. 4 N., R. 8 W.. The Sunset Highway member is predominantly composed of interbedded micaceous arkosic sandstone, lithic arkose, and muddy micaceous arkosic siltstone with a few beds of basaltic sandstone and basaltic debris flow breccias. The dominant micaceous arkosic composition of the Sunset Highway member reflects a distant extrabasinal granitic-metamorphic provenance and contrasts with that of the locally derived underlying basaltic Roy Creek member. Low angle trough cross-bedding, hummocky bedding, and microcross-laminations in fine to medium-grained arkosic sandstones are interpreted to have been produced by large storm-generated waves and on a high energy inner shelf. Thin interbeds of bioturbated mudstone and mollusc-bearing bioturbated sandstones formed during periods of fairweather conditions and during lower sedimentation rates. Rare matrix supported, basaltic debris-flow breccias and basaltic sandstones were derived from nearby basaltic headlands and by rivers draining the Tillamook Volcanics. Minor secondary intraparticle porosity occurs with some primary intergranular porosity in relatively matrix-free Sunset Highway member arkosic sandstones. However, much of the porosity and permeability of these potential sandstones has been reduced by diagenetic smectite coatings on framework grains and potassium feldspar overgrowths of feldspars. The mudstone-dominated Sweet Home Creek member was conformably deposited on the Sunset Highway member in eastern Clatsop and western Columbia counties. In western Clatsop County the Sweet Home Creek member directly and conformably overlies the Roy Creek member due to pinch out of the Sunset Highway member. Upper Narizian to lowermost Refugian benthic foraminiferal assemblages from this unit indicate outer shelf to upper slope sedimentation and continued subsidence of the depositional (Astoria) basin. Micromicaceous and carbonaceous silty mudatone dominates this unit but thin-bedded micaceous arkosic turbidite sandstones are present in the lower part, and rare, thin basaltic turbidites are present in the upper half. X-ray diffraction analysis shows that the dominant clay minerals in the Sweet Home Creek member niudstone are smectite (montmorillonite), kaolinite, and illite. The Cole Mountain basalt (informal) intrudes and locally overlies the Sweet Home Creek member. This caic-alkaline basaltic andesite is thought to have formed in a compressional plate tectonic regime and been emplaced on the outer shelf and upper slope as shallow irregular sills and dikes and minor submarine pillow basalt-hyaloclastite complexes. Siliceous nodules associated with pillowed units locally contains a few per cent pyrite and are associated with small areas of high-grade supergene copper-silver mineralization. The normally polarized Cole Mountain basalt is chemically, petrographically, and lithologically distinct from the Tillamook Volcanics and Grande Ronde Basalt of the Columbia River Basalt Group. The uppermost Narizian and Refugian (late Eocene) Jewell member of the Keasey Formation disconformably overlies the Cole Mountain basalt and Sweet Home Creek member. A thin basal glauconitic sandstone-siltstone reflects a period of reduced sedimentation under slightly reducing conditions and marks the disconformity. The unit primarily consists of laminated to thin bedded tuffaceous mudstone with a few thin tuff beds, small micaceous arkosic sandstone channels and clastic dikes. Clay minerals in the Jewell member are dominated by smectite (montmorillonite), with minor kaolinite and illite (degraded mica) in the lower part of the unit. Benthic foraminiferal assemblages in the unit indicate bathyal or slope depths and have been assigned to the lower Refugian to upper Narizian stages. In the middle Miocene, irregular dikes and sills of the Grande Ronde Basalt of the Columbia River Basalt Group intruded the late Eocene sedimentary strata in the thesis area. Two magneto-chemical types of Grande Ronde Basalt, N2/low MgO-low Ti02 and N2/ high MgO, were identified in the thesis area. These were geochemically and magnetically correlated to subaerial flows of magneto-chemical types IA and 5A of Mangan and others (1986) on the Columbia Plateau. The intrusions or invasive flows are interpreted to have been derived from voluminous plateau eruptions by invasion into soft, unconsolidated Neogene sediments at the marine/coast interface and then into the more brittle but ductile Paleogene strata of the area as first proposed by Beeson and others (1979). Uplift of the Coast Range was initiated in the late Miocene as a result of rapid offshore underthrusting in the subduction zone (Snavely and others, 1983). This has resulted in subaerial erosion and exposure of the faulted and gently folded forearc ridge and deposition of Quaternary alluvial gravels and sands along major rivers and creek in the thesis area. The dominant structural features of the Elsie-lower Nehalem River area are generally down-to-the-north, east-west-trending high angle faults with oblique offset and a conjugate set of oblique slip northwest-trending right-lateral and northeast-trending left-lateral faults. Folds are broad and relatively minor. The major east-west-trending fault pattern may have been initially produced by extensional stresses related to subsidence of the "Tillamook island". The conjugate strike-slip fault pattern may have been created by partial coupling of the forearc basin with oblique subduction of the Farallon plate. Other than timber, locally used rock aggregate from small quarries is the only resource that has been realized in the thesis area. Most quarries are developed in dikes and sills of Grande Ronde Basalt and the aggregate is used to macadamize logging roads. Diagenetic events have resulted in significant loss of porosity and permeability of potential reservoir sandstones in the area. The most favorable targets are relatively matrix-free micaceous arkosic sandstones in the Sunset Highway member, but these have been breached by erosion in the eastern part of the thesis area and pinch out in the western part of the area where potential mudstone cap rocks (e.g., Hamlet and Keasey formations) are present. Mudstones in the area contain woody-structured kerogen and average about 1% total organic carbon. These potential source rocks are generally thermally immature but have locally been baked by basaltic intrusions. This results in elevated vitrinite reflectance values (in the oil window and beyond) and, therefore, the mudstones may be potential source rocks for methane generation. Mineralized fault zones have substantial width and length but do not appear to carry anomalous concentrations of any metals other than arsenic. High-grade supergene copper-silver mineralization associated with Cole Mountain basalt intrusions has been documented but appears to be very localized and is not thought to be a viable exploration target. Title: Geology of the Elsie-lower Nehalem River area, south-central Clatsop and northern Tillamook counties, northwestern Oregon Author: Mumford, Daniel Franklin The middle Eocene Tillamook Volcanics form the oldest rock unit in the Elsie-lower Nehalem River area. K-Ar age determinations and age constraints imposed by foraminiferal and calcareous nannofossil assemblages of overlying sedimentary strata indicate an absolute age of about 42 Ma for the uppermost Tillamook Volcanics. Major oxide values indicate that the upper Tillamook Volcanics are highly fractionated high Fe-Ti tholeiitic basalts and basaltic andesites. These volcanics were erupted in a developing forearc under an extensional plate tectonic setting and formed a moderately large oceanic island. These subaerial flows are predominantly aphyric to plagioclase-augite porphyritic and have a pilotaxitic flow texture. Epochs of both normal and reverse magnetic polarity are recorded. Thermal subsidence related to the end of the volcanism resulted in deposition of the transgressive late Eocene Hamlet formation over the "Tillamook island". The informal Hamlet formation consists of three members. From oldest to youngest they are: the Roy Creek member, the Sunset Highway member, and the Sweet Home Creek member. Three lithofacies are present in the Roy Creek member. The stratigraphically lowest of these consists of basaltic boulder-pebble conglomerate and locally fossiliferous pebbly basaltic sandstones which were deposited in a high energy nearshore environment around rocky basaltic headlands and sea stacks of the Tillamook Volcanics. Molluscan fossils in this lithofacies are correlative to the middle to late Eocene "CowlitzCoaledo" fauna. Successively overlying lithofacies are a very coarseto coarse-grained shallow marine basaltic sandstone lithofacies and a medium- to fine-grained basaltic sandstone lithofacies. This fining upward sequenced documents progressive deepening of the depositional basin. Framework clasts in all three Roy Creek member lithofacies were predominantly derived from the Tillamook Volcanics. Pore-filling diagenetic chlorite, smectite (nontronite), calcite, and zeolite (clinoptilolite and heulandite) cements severely reduce the porosity of Roy Creek member sandstones. The Sunset Highway member of the Hamlet formation conformably overlies the Roy Creek member in eastern Clatsop and western Columbia counties and pinches out to the west at about the longitude of the Nehalem River in T. 4 N., R. 8 W.. The Sunset Highway member is predominantly composed of interbedded micaceous arkosic sandstone, lithic arkose, and muddy micaceous arkosic siltstone with a few beds of basaltic sandstone and basaltic debris flow breccias. The dominant micaceous arkosic composition of the Sunset Highway member reflects a distant extrabasinal granitic-metamorphic provenance and contrasts with that of the locally derived underlying basaltic Roy Creek member. Low angle trough cross-bedding, hummocky bedding, and microcross-laminations in fine to medium-grained arkosic sandstones are interpreted to have been produced by large storm-generated waves and on a high energy inner shelf. Thin interbeds of bioturbated mudstone and mollusc-bearing bioturbated sandstones formed during periods of fairweather conditions and during lower sedimentation rates. Rare matrix supported, basaltic debris-flow breccias and basaltic sandstones were derived from nearby basaltic headlands and by rivers draining the Tillamook Volcanics. Minor secondary intraparticle porosity occurs with some primary intergranular porosity in relatively matrix-free Sunset Highway member arkosic sandstones. However, much of the porosity and permeability of these potential sandstones has been reduced by diagenetic smectite coatings on framework grains and potassium feldspar overgrowths of feldspars. The mudstone-dominated Sweet Home Creek member was conformably deposited on the Sunset Highway member in eastern Clatsop and western Columbia counties. In western Clatsop County the Sweet Home Creek member directly and conformably overlies the Roy Creek member due to pinch out of the Sunset Highway member. Upper Narizian to lowermost Refugian benthic foraminiferal assemblages from this unit indicate outer shelf to upper slope sedimentation and continued subsidence of the depositional (Astoria) basin. Micromicaceous and carbonaceous silty mudatone dominates this unit but thin-bedded micaceous arkosic turbidite sandstones are present in the lower part, and rare, thin basaltic turbidites are present in the upper half. X-ray diffraction analysis shows that the dominant clay minerals in the Sweet Home Creek member niudstone are smectite (montmorillonite), kaolinite, and illite. The Cole Mountain basalt (informal) intrudes and locally overlies the Sweet Home Creek member. This caic-alkaline basaltic andesite is thought to have formed in a compressional plate tectonic regime and been emplaced on the outer shelf and upper slope as shallow irregular sills and dikes and minor submarine pillow basalt-hyaloclastite complexes. Siliceous nodules associated with pillowed units locally contains a few per cent pyrite and are associated with small areas of high-grade supergene copper-silver mineralization. The normally polarized Cole Mountain basalt is chemically, petrographically, and lithologically distinct from the Tillamook Volcanics and Grande Ronde Basalt of the Columbia River Basalt Group. The uppermost Narizian and Refugian (late Eocene) Jewell member of the Keasey Formation disconformably overlies the Cole Mountain basalt and Sweet Home Creek member. A thin basal glauconitic sandstone-siltstone reflects a period of reduced sedimentation under slightly reducing conditions and marks the disconformity. The unit primarily consists of laminated to thin bedded tuffaceous mudstone with a few thin tuff beds, small micaceous arkosic sandstone channels and clastic dikes. Clay minerals in the Jewell member are dominated by smectite (montmorillonite), with minor kaolinite and illite (degraded mica) in the lower part of the unit. Benthic foraminiferal assemblages in the unit indicate bathyal or slope depths and have been assigned to the lower Refugian to upper Narizian stages. In the middle Miocene, irregular dikes and sills of the Grande Ronde Basalt of the Columbia River Basalt Group intruded the late Eocene sedimentary strata in the thesis area. Two magneto-chemical types of Grande Ronde Basalt, N2/low MgO-low Ti02 and N2/ high MgO, were identified in the thesis area. These were geochemically and magnetically correlated to subaerial flows of magneto-chemical types IA and 5A of Mangan and others (1986) on the Columbia Plateau. The intrusions or invasive flows are interpreted to have been derived from voluminous plateau eruptions by invasion into soft, unconsolidated Neogene sediments at the marine/coast interface and then into the more brittle but ductile Paleogene strata of the area as first proposed by Beeson and others (1979). Uplift of the Coast Range was initiated in the late Miocene as a result of rapid offshore underthrusting in the subduction zone (Snavely and others, 1983). This has resulted in subaerial erosion and exposure of the faulted and gently folded forearc ridge and deposition of Quaternary alluvial gravels and sands along major rivers and creek in the thesis area. The dominant structural features of the Elsie-lower Nehalem River area are generally down-to-the-north, east-west-trending high angle faults with oblique offset and a conjugate set of oblique slip northwest-trending right-lateral and northeast-trending left-lateral faults. Folds are broad and relatively minor. The major east-west-trending fault pattern may have been initially produced by extensional stresses related to subsidence of the "Tillamook island". The conjugate strike-slip fault pattern may have been created by partial coupling of the forearc basin with oblique subduction of the Farallon plate. Other than timber, locally used rock aggregate from small quarries is the only resource that has been realized in the thesis area. Most quarries are developed in dikes and sills of Grande Ronde Basalt and the aggregate is used to macadamize logging roads. Diagenetic events have resulted in significant loss of porosity and permeability of potential reservoir sandstones in the area. The most favorable targets are relatively matrix-free micaceous arkosic sandstones in the Sunset Highway member, but these have been breached by erosion in the eastern part of the thesis area and pinch out in the western part of the area where potential mudstone cap rocks (e.g., Hamlet and Keasey formations) are present. Mudstones in the area contain woody-structured kerogen and average about 1% total organic carbon. These potential source rocks are generally thermally immature but have locally been baked by basaltic intrusions. This results in elevated vitrinite reflectance values (in the oil window and beyond) and, therefore, the mudstones may be potential source rocks for methane generation. Mineralized fault zones have substantial width and length but do not appear to carry anomalous concentrations of any metals other than arsenic. High-grade supergene copper-silver mineralization associated with Cole Mountain basalt intrusions has been documented but appears to be very localized and is not thought to be a viable exploration target. Close