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• 1. [Article] Geology of the Rock Creek-Green Mountain area, southeast Clatsop and northernmost Tillamook counties, northwest Oregon

  The middle to upper Eocene Tillamook Volcanics form the volcanic core or basement rocks in the Rock Creek-Green Mountain area. They consist of a thick sequence of high TiO2 tholeiitic basalt, basaltic ...

  Citation × Citation Citation Abstract Copy Title: Geology of the Rock Creek-Green Mountain area, southeast Clatsop and northernmost Tillamook counties, northwest Oregon Author: Safley, L. Eugene The middle to upper Eocene Tillamook Volcanics form the volcanic core or basement rocks in the Rock Creek-Green Mountain area. They consist of a thick sequence of high TiO2 tholeiitic basalt, basaltic andesite, and andesite subaerial flows and breccias. Detailed mapping, petrography, major oxide plots, magnetic polarity, and radiometric age dating prove that the volcanic rocks that form the Green Mountain outlier are an upthrown, more fractionally differentiated basement block of Tillamook Volcanics. Geochemical plots, regional geology, and field relationships indicate that the Tillamook Volcanics erupted in a forearc tectonic setting and formed an extensive, moderate relief, rugged oceanic island of overlapping shield volcanoes. Subsequent transgression and subsidence of the Tillamook Volcanics, perhaps due to thermal cooling of the crust, resulted in deposition of upper Narizian (upper Eocene) beach to high energy inner shelf, mollusc-bearing, basaltic boulder conglomerate and basaltic sandstone of the Roy Creek member of the Hamlet formation (informal). Petrography, geochemistry, pebble count data, and stratigraphic relationships show that the coarse- to fine-grained basaltic sandstone and conglomerates were derived from the underlying Tillamook Volcanics and were probably deposited along a rocky coastline composed of sea stacks and headlands muck like Oregon's coastline today. Micaceous lithic arkosic sandstone and siltstone and minor basaltic sandstone and volcanic debris flow interbeds of the overlying Sunset Highway member of the Hamlet formation (informal) were deposited in an inner to middle shelf environment. The extrabasinal micaeous arkosic sands were derived from granitic/metamorphic source areas in eastern Oregon, eastern Washington, and Idaho. They were transported via an ancestral Columbia River drainage system to the shoreline where they interfinger with basaltic debris flows and basaltic sandstone that were locally eroded from rocky Tillamook basalt headlands. Broken and abraded molluscan fossils, parallel lamination and hummocky cross-bedding, moderately well sorted character of the sands, and sheet-like sandstone geometry indicate a high energy, storm-dominated, shallow marine depositional environment. Extensive diagenesis of the highly chemically reactive Roy Creek basaltic sandstone has resulted in significant porosity and permeability reduction after burial due to formation of chloritic and smectite clays and locally zeolite and calcite filling intergranular pore spaces. Thin section and scanning electron microscopy, however, show only minor thin smectite clay coats and scattered micaceous arkosic sandstones of the Sunset Highway member. This has resulted in the preservation of much primary intergranular porosity. Further transgression and/or subsidence of the upper Eocene shelf sands resulted in deposition of micaceous and carbonaceous silty mudstones and minor thin-bedded turbidite sandstone unit of the Sweet Home Creek member of the Hamlet formation in a middle to upper slope depositional environment. Bathyal benthic foraminiferal assemblages collected are assigned to an late Narizian age. Thin interbeds of graded micaceous arkosic sandstone with Bouma B, C, and D divisions were derived from adjacent Sunset Highway shelf deposits resuspended by storm waves and transported by turbidity flows into upper slope environments. Thin dikes and thick irregular-shaped sill-like bodies of calc-alkaline Cole Mountain basalt intrude the Sweet Home Creek member. The coarse-crystalline (microgabbroic to gabbroic) to finely crystalline basalt is composed of phenocrysts of calcic plagioclase and augite in a clay altered intersertal to hyalopilitic groundmass. The Cole Mountain basalt is petrographically and chemically distinct from the high Ti02 and low Si02 basalt flows with pilotaxitic texture that form the older Tillamook Volcanics. They are more similar to the arc-derived caic-alkaline Goble Volcanics of southwest Washington. Regression and/or progradation resulted in deposition of thick sheet sandstones of the Cowlitz Formation. Trough cross-laminations indicate paleocurrent transport directions were to the west and northwest prior to clockwise tectonic rotation. Parallel to trough cross-laminations, the high degree of rounding and sorting, and the low degree of bioturbation indicate deposition occurred in a high energy, storm-dominated, inner and middle shelf environment. Uplift of the Oregon Coast Range forearc region that was initiated in the late middle Miocene resulted in subaerial weathering and differential erosion of the softer sedimentary rocks in contrast to the resistant basaltic rock units. Minor alluvial basaltic stream gravels and sands formed along major, low gradient streams. Recent rock fall/landslide debris occurs along the south side of Quartz Creek. The regional structure consists of a northward-plunging gravity high or structural upwarp of Tillamook Volcanics that transects the middle of the thesis area. This structural high is cut by a series of major east-west striking high-angle normal faults that are offset by a younger set of northwest-trending right-lateral and northeast-trending left-lateral oblique-slip faults. The conjugate oblique-slip pattern of faulting is characteristic of wrench faulting perhaps due to a north-south shear couple created by oblique subduction of the Juan de Fuca oceanic plate beneath the North American continental plate. The downwarp of upper Eocene Hamlet sedimentary rocks preserved between the isolated upthrown block of Tillamook Volcanics at Green Mountain on the north from the main Tillamook highland on the south may be a pull-apart depression formed by oblique-slip motion on these fault-bounded upthrown volcanic basement blocks. This pattern is much like the structure of the Mist Gas Field in the subsurface to the northeast. Continued commercial production from the upper Eocene Cowlitz Formation (Clark and Wilson sandstone) since 1978 in the Mist Gas Field, has resulted in renewed interest in the hydrocarbon potential of the Northwest Oregon. The micaceous sandstones of the Sunset Highway member of the Hamlet formation could also serve as an attractive deeper reservoir target based on porosity and permeability analysis and may equate to the "Clatskanie" sandstone in the subsurface at Mist. Total organic carbon values and woody kerogen from carbonaceous mudstone from the Sweet Home Creek member suggest it as a potential source rock for wet gas in the subsurface. Local thermal heating of the Hamlet mudstone by upper Eocene Cole Mountain basalt sills has created some high vitrinite reflectance values and represent a potential "quickflash" maturation mechanism for gas. Title: Geology of the Rock Creek-Green Mountain area, southeast Clatsop and northernmost Tillamook counties, northwest Oregon Author: Safley, L. Eugene The middle to upper Eocene Tillamook Volcanics form the volcanic core or basement rocks in the Rock Creek-Green Mountain area. They consist of a thick sequence of high TiO2 tholeiitic basalt, basaltic andesite, and andesite subaerial flows and breccias. Detailed mapping, petrography, major oxide plots, magnetic polarity, and radiometric age dating prove that the volcanic rocks that form the Green Mountain outlier are an upthrown, more fractionally differentiated basement block of Tillamook Volcanics. Geochemical plots, regional geology, and field relationships indicate that the Tillamook Volcanics erupted in a forearc tectonic setting and formed an extensive, moderate relief, rugged oceanic island of overlapping shield volcanoes. Subsequent transgression and subsidence of the Tillamook Volcanics, perhaps due to thermal cooling of the crust, resulted in deposition of upper Narizian (upper Eocene) beach to high energy inner shelf, mollusc-bearing, basaltic boulder conglomerate and basaltic sandstone of the Roy Creek member of the Hamlet formation (informal). Petrography, geochemistry, pebble count data, and stratigraphic relationships show that the coarse- to fine-grained basaltic sandstone and conglomerates were derived from the underlying Tillamook Volcanics and were probably deposited along a rocky coastline composed of sea stacks and headlands muck like Oregon's coastline today. Micaceous lithic arkosic sandstone and siltstone and minor basaltic sandstone and volcanic debris flow interbeds of the overlying Sunset Highway member of the Hamlet formation (informal) were deposited in an inner to middle shelf environment. The extrabasinal micaeous arkosic sands were derived from granitic/metamorphic source areas in eastern Oregon, eastern Washington, and Idaho. They were transported via an ancestral Columbia River drainage system to the shoreline where they interfinger with basaltic debris flows and basaltic sandstone that were locally eroded from rocky Tillamook basalt headlands. Broken and abraded molluscan fossils, parallel lamination and hummocky cross-bedding, moderately well sorted character of the sands, and sheet-like sandstone geometry indicate a high energy, storm-dominated, shallow marine depositional environment. Extensive diagenesis of the highly chemically reactive Roy Creek basaltic sandstone has resulted in significant porosity and permeability reduction after burial due to formation of chloritic and smectite clays and locally zeolite and calcite filling intergranular pore spaces. Thin section and scanning electron microscopy, however, show only minor thin smectite clay coats and scattered micaceous arkosic sandstones of the Sunset Highway member. This has resulted in the preservation of much primary intergranular porosity. Further transgression and/or subsidence of the upper Eocene shelf sands resulted in deposition of micaceous and carbonaceous silty mudstones and minor thin-bedded turbidite sandstone unit of the Sweet Home Creek member of the Hamlet formation in a middle to upper slope depositional environment. Bathyal benthic foraminiferal assemblages collected are assigned to an late Narizian age. Thin interbeds of graded micaceous arkosic sandstone with Bouma B, C, and D divisions were derived from adjacent Sunset Highway shelf deposits resuspended by storm waves and transported by turbidity flows into upper slope environments. Thin dikes and thick irregular-shaped sill-like bodies of calc-alkaline Cole Mountain basalt intrude the Sweet Home Creek member. The coarse-crystalline (microgabbroic to gabbroic) to finely crystalline basalt is composed of phenocrysts of calcic plagioclase and augite in a clay altered intersertal to hyalopilitic groundmass. The Cole Mountain basalt is petrographically and chemically distinct from the high Ti02 and low Si02 basalt flows with pilotaxitic texture that form the older Tillamook Volcanics. They are more similar to the arc-derived caic-alkaline Goble Volcanics of southwest Washington. Regression and/or progradation resulted in deposition of thick sheet sandstones of the Cowlitz Formation. Trough cross-laminations indicate paleocurrent transport directions were to the west and northwest prior to clockwise tectonic rotation. Parallel to trough cross-laminations, the high degree of rounding and sorting, and the low degree of bioturbation indicate deposition occurred in a high energy, storm-dominated, inner and middle shelf environment. Uplift of the Oregon Coast Range forearc region that was initiated in the late middle Miocene resulted in subaerial weathering and differential erosion of the softer sedimentary rocks in contrast to the resistant basaltic rock units. Minor alluvial basaltic stream gravels and sands formed along major, low gradient streams. Recent rock fall/landslide debris occurs along the south side of Quartz Creek. The regional structure consists of a northward-plunging gravity high or structural upwarp of Tillamook Volcanics that transects the middle of the thesis area. This structural high is cut by a series of major east-west striking high-angle normal faults that are offset by a younger set of northwest-trending right-lateral and northeast-trending left-lateral oblique-slip faults. The conjugate oblique-slip pattern of faulting is characteristic of wrench faulting perhaps due to a north-south shear couple created by oblique subduction of the Juan de Fuca oceanic plate beneath the North American continental plate. The downwarp of upper Eocene Hamlet sedimentary rocks preserved between the isolated upthrown block of Tillamook Volcanics at Green Mountain on the north from the main Tillamook highland on the south may be a pull-apart depression formed by oblique-slip motion on these fault-bounded upthrown volcanic basement blocks. This pattern is much like the structure of the Mist Gas Field in the subsurface to the northeast. Continued commercial production from the upper Eocene Cowlitz Formation (Clark and Wilson sandstone) since 1978 in the Mist Gas Field, has resulted in renewed interest in the hydrocarbon potential of the Northwest Oregon. The micaceous sandstones of the Sunset Highway member of the Hamlet formation could also serve as an attractive deeper reservoir target based on porosity and permeability analysis and may equate to the "Clatskanie" sandstone in the subsurface at Mist. Total organic carbon values and woody kerogen from carbonaceous mudstone from the Sweet Home Creek member suggest it as a potential source rock for wet gas in the subsurface. Local thermal heating of the Hamlet mudstone by upper Eocene Cole Mountain basalt sills has created some high vitrinite reflectance values and represent a potential "quickflash" maturation mechanism for gas. Close

• 2. [Article] Geology of the Hamlet-North Fork of the Nehalem River area, southern Clatsop and northernmost Tillamook Counties, northwest Oregon

  The middle to late Eocene tholeiitic Tillamook Volcanics compose the oldest rock unit in the Hamlet-North Fork of the Nehalem River area. Geochemical plots and field relationships indicate that these rocks ...

  Citation × Citation Citation Abstract Copy Title: Geology of the Hamlet-North Fork of the Nehalem River area, southern Clatsop and northernmost Tillamook Counties, northwest Oregon Author: Rarey, Phillip Jay The middle to late Eocene tholeiitic Tillamook Volcanics compose the oldest rock unit in the Hamlet-North Fork of the Nehalem River area. Geochemical plots and field relationships indicate that these rocks were produced in an extensional tectonic setting in the developing forearc and formed an extensive tholeiltic oceanic island. The volcanics consi5t of a thick sequence of normally and reversely polarized subaerial basalt and basaltic andesite flows in the Hamlet-North Fork of the Nehalem River area. The "Gray's River area" Goble Volcanics in southwest Washington are chemically and stratigraphically correlative to the Tillamook Volcanics. Cessation of Tillamook volcanism resulted in thermal subsidence and transgression of the overlying Hamlet formation. Upper Narizian (middle to upper Eccene) nearshore fossiliferous basaltic boulder-pebble conglomerates and basaltic sandstones of the Roy Creek member of the Hamlet formation (informal) were deposited along a rocky basaltic coastline over the subsiding volcanic "island". Scanning electron microscopy shows that radial pore-filling chloritic cement has significantly reduced porosity in Roy Creek member sandstones. Micaceous and carbonaceous silty mudstones and rare thin basaltic turbidite sandstones of the Sweet Home Creek member of the Hamlet formation (informal> were deposited on the outer shelf to upper slope above the Roy Creek member as the basin continued to deepen. The Sweet Home Creek member contains abundant bathyal benthtc foraminifera assignable to the upper Narizian stage. Calcareous nannofossils collected from the unit have been assigned to subzone CP-14a which is in agreement with foraminifera data. The upper part of the Sweet Home Creek member is in part a deep marine correlative to shelf arkosic sandstones of the Cowltiz Formation which pinches out into the Sweet Home Creek member in eastern Clatsop County. Much of the detritus in the Sweet Home Creek member was derived from plutonic and metamorphic sources in contrast to the locally derived Roy Creek member. Calc-alkaline Cole Mountain basalt (informal) intrudes and overlies the Sweet Home Creek member. Cole Mountain basalt was formed in a compressional tectonic environment and emplaced on the outer continental shelf as shallow intrusions and submarine flow. The unit is chemically and petrographically distinct from the Tillamook Volcanics and chemically similar to and stratigraphically correlative to the type Goble Volcanics (e.g. low Ti02 and low P205). Unconformably overlying the Cole Mountain basalt and the Sweet Home Creek member is the bathyal, Refugian (upper Eocene), Jewell member of the Keasey Formation. It consists of three parts a basal glauconitic sandstone-siltstone, a laminated tuffaceous sandstone unit with rare small arkosic sandstone channels and occasional clastic dikes, and an upper laminated to bioturbated tuffaceous silt-mudstone. trkosic sandstones were derived from an ancesteral Columbia River system whereas abundant tuffaceous detritus was derived locally from the Cascade arc. The Refugian lower Smuggler Cove formation (informal) gradationally overlies the Jewell member and consists of bioturbated, tuffaceous, bathyal mudstones. Outer shelf, very fine-grained tuffaceous sandstones of the David Douglas tongue (informal) of the Pittsburg Bluff Formation and deeper marine correlative outer shelf to upper slope glauconitic sanstones of the middle Smuggler Cove formation overlie the lower Smuggler Cove formation. The upper Smuggler Cove formation consists of uppermost Refugian to Zemmorian bathyal, bioturbated, fossiliferous, well-indurated tuffaceous siltstone. Laminated carbonaceous mudetones and thin (<1/2 m) arkosic sandstone beds of the ball park unit in the Smuggler Cove formation overlie and interfinger with (7) the upper Smuggler Cove formation. The ball park unit is late Zemorrian (Oligocene) or Saucesian (Early Miocene) in age. Fluvial-deltaic to shallow marine sandstones and conglomerates of the lower to middle Miocene angora Peek member of the astoria Formation unconformably overlies the Smuggler Cove formation. Numerous middle to upper Miocene basalts and gabbros intrude the sedimentary rocks in the thesis area. The intrusive rocks are chemically, magnetically, petrographically, and chronologically correlative to the Grande Ronde Basalt, Frenchman Springs Member, and Pomona Member of the Columbia River Basalt Group on the Columbia Plateau. The Grande Ronde Basalt intrusives have been divided into three chemical-magnetostratigraphic units in the thesis area and correlated to subaerial Columbia River Basalt flows located approximately 35 km to the northeast. The intrusive rocks are thought to have formed by invasion of voluminous subaerial flows into soft, semiconsolidated marine sediments as first envisioned by Beeson et. al. (1979). Uplift of the Coast Range forearc ridge from late Miocene to present has resulted in subaerial erosion and exposure of rock units. Thin alluvial gravels and sands were deposited in the southeastern corner of the thesis area during the Quaternary. Structure in the thesis area is dominated by a series of east-west trending high angle faults and a younger series of conjugate northeast-and northwest-trending high angle oblique slip faults. Proton precession magnetometer traverses confirm the presence of the faults. The structure may have been produced by partial coupling of the forearc region with the subducting Farallon plate. The thesis area has been actively explored for hydrocarbons. Geologic mapping, however, shows that significant sandstone reservoirs are not present in the subsurface and, therefore, the area has low potential of hydrocarbon production. Mudstones in the thesis area average approximately 0.9-1.1% total organic carbon with vitrinite reflectance values ranging from 0.53% Ro (unbaked) to 0.72% Ro (baked). Therefore, the mudstones are a marginal to poor source of thermogenic gas but a possible source of methane gas. Title: Geology of the Hamlet-North Fork of the Nehalem River area, southern Clatsop and northernmost Tillamook Counties, northwest Oregon Author: Rarey, Phillip Jay The middle to late Eocene tholeiitic Tillamook Volcanics compose the oldest rock unit in the Hamlet-North Fork of the Nehalem River area. Geochemical plots and field relationships indicate that these rocks were produced in an extensional tectonic setting in the developing forearc and formed an extensive tholeiltic oceanic island. The volcanics consi5t of a thick sequence of normally and reversely polarized subaerial basalt and basaltic andesite flows in the Hamlet-North Fork of the Nehalem River area. The "Gray's River area" Goble Volcanics in southwest Washington are chemically and stratigraphically correlative to the Tillamook Volcanics. Cessation of Tillamook volcanism resulted in thermal subsidence and transgression of the overlying Hamlet formation. Upper Narizian (middle to upper Eccene) nearshore fossiliferous basaltic boulder-pebble conglomerates and basaltic sandstones of the Roy Creek member of the Hamlet formation (informal) were deposited along a rocky basaltic coastline over the subsiding volcanic "island". Scanning electron microscopy shows that radial pore-filling chloritic cement has significantly reduced porosity in Roy Creek member sandstones. Micaceous and carbonaceous silty mudstones and rare thin basaltic turbidite sandstones of the Sweet Home Creek member of the Hamlet formation (informal> were deposited on the outer shelf to upper slope above the Roy Creek member as the basin continued to deepen. The Sweet Home Creek member contains abundant bathyal benthtc foraminifera assignable to the upper Narizian stage. Calcareous nannofossils collected from the unit have been assigned to subzone CP-14a which is in agreement with foraminifera data. The upper part of the Sweet Home Creek member is in part a deep marine correlative to shelf arkosic sandstones of the Cowltiz Formation which pinches out into the Sweet Home Creek member in eastern Clatsop County. Much of the detritus in the Sweet Home Creek member was derived from plutonic and metamorphic sources in contrast to the locally derived Roy Creek member. Calc-alkaline Cole Mountain basalt (informal) intrudes and overlies the Sweet Home Creek member. Cole Mountain basalt was formed in a compressional tectonic environment and emplaced on the outer continental shelf as shallow intrusions and submarine flow. The unit is chemically and petrographically distinct from the Tillamook Volcanics and chemically similar to and stratigraphically correlative to the type Goble Volcanics (e.g. low Ti02 and low P205). Unconformably overlying the Cole Mountain basalt and the Sweet Home Creek member is the bathyal, Refugian (upper Eocene), Jewell member of the Keasey Formation. It consists of three parts a basal glauconitic sandstone-siltstone, a laminated tuffaceous sandstone unit with rare small arkosic sandstone channels and occasional clastic dikes, and an upper laminated to bioturbated tuffaceous silt-mudstone. trkosic sandstones were derived from an ancesteral Columbia River system whereas abundant tuffaceous detritus was derived locally from the Cascade arc. The Refugian lower Smuggler Cove formation (informal) gradationally overlies the Jewell member and consists of bioturbated, tuffaceous, bathyal mudstones. Outer shelf, very fine-grained tuffaceous sandstones of the David Douglas tongue (informal) of the Pittsburg Bluff Formation and deeper marine correlative outer shelf to upper slope glauconitic sanstones of the middle Smuggler Cove formation overlie the lower Smuggler Cove formation. The upper Smuggler Cove formation consists of uppermost Refugian to Zemmorian bathyal, bioturbated, fossiliferous, well-indurated tuffaceous siltstone. Laminated carbonaceous mudetones and thin (<1/2 m) arkosic sandstone beds of the ball park unit in the Smuggler Cove formation overlie and interfinger with (7) the upper Smuggler Cove formation. The ball park unit is late Zemorrian (Oligocene) or Saucesian (Early Miocene) in age. Fluvial-deltaic to shallow marine sandstones and conglomerates of the lower to middle Miocene angora Peek member of the astoria Formation unconformably overlies the Smuggler Cove formation. Numerous middle to upper Miocene basalts and gabbros intrude the sedimentary rocks in the thesis area. The intrusive rocks are chemically, magnetically, petrographically, and chronologically correlative to the Grande Ronde Basalt, Frenchman Springs Member, and Pomona Member of the Columbia River Basalt Group on the Columbia Plateau. The Grande Ronde Basalt intrusives have been divided into three chemical-magnetostratigraphic units in the thesis area and correlated to subaerial Columbia River Basalt flows located approximately 35 km to the northeast. The intrusive rocks are thought to have formed by invasion of voluminous subaerial flows into soft, semiconsolidated marine sediments as first envisioned by Beeson et. al. (1979). Uplift of the Coast Range forearc ridge from late Miocene to present has resulted in subaerial erosion and exposure of rock units. Thin alluvial gravels and sands were deposited in the southeastern corner of the thesis area during the Quaternary. Structure in the thesis area is dominated by a series of east-west trending high angle faults and a younger series of conjugate northeast-and northwest-trending high angle oblique slip faults. Proton precession magnetometer traverses confirm the presence of the faults. The structure may have been produced by partial coupling of the forearc region with the subducting Farallon plate. The thesis area has been actively explored for hydrocarbons. Geologic mapping, however, shows that significant sandstone reservoirs are not present in the subsurface and, therefore, the area has low potential of hydrocarbon production. Mudstones in the thesis area average approximately 0.9-1.1% total organic carbon with vitrinite reflectance values ranging from 0.53% Ro (unbaked) to 0.72% Ro (baked). Therefore, the mudstones are a marginal to poor source of thermogenic gas but a possible source of methane gas. 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

• 4. [Article] Stratigraphy and sedimentation of the Neahkahnie Mountain - Angora Peak area, Tillamook and Clatsop Counties, Oregon

  Four distinct lithologic units compose the Tertiary rocks of the Neahkahnie Mountain - Angora Peak area, located along the northwest Oregon coast near the town of Nehalem. The Tertiary units are the late ...

  Citation × Citation Citation Abstract Copy Title: Stratigraphy and sedimentation of the Neahkahnie Mountain - Angora Peak area, Tillamook and Clatsop Counties, Oregon Author: Cressy, Frank Beecher Four distinct lithologic units compose the Tertiary rocks of the Neahkahnie Mountain - Angora Peak area, located along the northwest Oregon coast near the town of Nehalem. The Tertiary units are the late Oligocene to early Miocene Oswald West mudstones, the middle Miocene Angora Peak sandstone member of the Astoria Formation, and middle Miocene intrusive and extrusive rocks of the Depoe Bay Basalt. These units are unconformably overlain by Pleistocene and Recent beach and dune sands, alluvium, and tidal flat muds. The Oswald West mudstones and the Angora Peak sandstone member are informal stratigraphic units proposed in this study. The Oswald West mudstones consist of over 1600 feet of well-bedded, highly burrowed, tuffaceous siltstones and silty mudstones interbedded with minor amounts of graded turbidite sandstones and submarine slump deposits. Foraminifera and trace fossils suggest deposition occurred in marine waters of upper bathyal depths. The Angora Peak sandstone consists of over 1800 feet of thin- to thick-bedded, locally cross-bedded, fine- to coarse-grained arkosic sandstones, pumiceous and basaltic conglomerates, carbonaceous and micaceous siltstones, and local coal beds. The inter-fingering shallow marine and fluvial sandstones are interpreted to have been deposited in a high-energy, wave dominated, deltaic environment which reworked the sediments into extensive delta-front sheet sands similar to those observed in the modern Niger and Rhone deltas. Mineralogy, heavy minerals, and conglomerate clast lithologies indicate that most of the sediments were derived from local uplifted areas of Eocene basalts and early Tertiary sediments and from the Oligocene Little Butte Volcanics in the western Cascades. Rare metamorphic and plutonic clasts, sedimentary quartzite, heavy minerals, and sandstone mineralogy suggest that metamorphic, igneous, and Paleozoic sedimentary terranes in eastern Oregon and Washington, British Columbia, Idaho, and Montana supplied some of the sediments, possibly via an ancestral Columbia River. Dikes, sills, and plugs of aphanitic to finely crystalline Depoe Bay Basalt intrude the older sedimentary rocks and locally are feeders for palagonitized pillow breccias which unconformably overlie the Angora Peak sandstone. The major intrusive body is a 1200-foot thick diabasic sill referred to as the Neahkahnie sill. The extrusive basalts formed in a subsiding marine basin in which over 1600 feet of pillow lavas, pillow breccias, and minor basalt flows were locally extruded. The area is cut by a series of west-northwest and north-trending faults. Two synclines and an anticline strike subparallel to the west-northwest trending faults. Title: Stratigraphy and sedimentation of the Neahkahnie Mountain - Angora Peak area, Tillamook and Clatsop Counties, Oregon Author: Cressy, Frank Beecher Four distinct lithologic units compose the Tertiary rocks of the Neahkahnie Mountain - Angora Peak area, located along the northwest Oregon coast near the town of Nehalem. The Tertiary units are the late Oligocene to early Miocene Oswald West mudstones, the middle Miocene Angora Peak sandstone member of the Astoria Formation, and middle Miocene intrusive and extrusive rocks of the Depoe Bay Basalt. These units are unconformably overlain by Pleistocene and Recent beach and dune sands, alluvium, and tidal flat muds. The Oswald West mudstones and the Angora Peak sandstone member are informal stratigraphic units proposed in this study. The Oswald West mudstones consist of over 1600 feet of well-bedded, highly burrowed, tuffaceous siltstones and silty mudstones interbedded with minor amounts of graded turbidite sandstones and submarine slump deposits. Foraminifera and trace fossils suggest deposition occurred in marine waters of upper bathyal depths. The Angora Peak sandstone consists of over 1800 feet of thin- to thick-bedded, locally cross-bedded, fine- to coarse-grained arkosic sandstones, pumiceous and basaltic conglomerates, carbonaceous and micaceous siltstones, and local coal beds. The inter-fingering shallow marine and fluvial sandstones are interpreted to have been deposited in a high-energy, wave dominated, deltaic environment which reworked the sediments into extensive delta-front sheet sands similar to those observed in the modern Niger and Rhone deltas. Mineralogy, heavy minerals, and conglomerate clast lithologies indicate that most of the sediments were derived from local uplifted areas of Eocene basalts and early Tertiary sediments and from the Oligocene Little Butte Volcanics in the western Cascades. Rare metamorphic and plutonic clasts, sedimentary quartzite, heavy minerals, and sandstone mineralogy suggest that metamorphic, igneous, and Paleozoic sedimentary terranes in eastern Oregon and Washington, British Columbia, Idaho, and Montana supplied some of the sediments, possibly via an ancestral Columbia River. Dikes, sills, and plugs of aphanitic to finely crystalline Depoe Bay Basalt intrude the older sedimentary rocks and locally are feeders for palagonitized pillow breccias which unconformably overlie the Angora Peak sandstone. The major intrusive body is a 1200-foot thick diabasic sill referred to as the Neahkahnie sill. The extrusive basalts formed in a subsiding marine basin in which over 1600 feet of pillow lavas, pillow breccias, and minor basalt flows were locally extruded. The area is cut by a series of west-northwest and north-trending faults. Two synclines and an anticline strike subparallel to the west-northwest trending faults. Close

• 5. [Article] Geology of the Tillamook Head--Necanicum Junction area, Clatsop County, northwest Oregon

  Five distinct lithologic units compose the Tertiary rocks in the Tillamook Head - Necanicum Junction area of the northern Oregon Coast Range. They are: the late Eocene to early Miocene Oswald West mudstones, ...

  Citation × Citation Citation Abstract Copy Title: Geology of the Tillamook Head--Necanicum Junction area, Clatsop County, northwest Oregon Author: Neel, Robert Holmes Five distinct lithologic units compose the Tertiary rocks in the Tillamook Head - Necanicum Junction area of the northern Oregon Coast Range. They are: the late Eocene to early Miocene Oswald West mudstones, the middle Miocene Angora Peak sandstone and Silver Point mudstone members of the Astoria Formation, and the middle Miocene intrusive and extrusive Depoe Bay and Cape Foulweather basalts. These units are locally overlain by Pleistocene marine terraces and Holocene beach and dune sands, stream alluvium, and landslide deposits. The Oswald West mudstones, Angora Peak sandstones, and Silver Point mudstones are informal stratigraphic names used in this study. The Oswald West mudstones consist of over 2, 000 feet of well-bedded, intensely burrowed, silty mudstones and siltstones interstratified with minor glauconitic sandstones, tuff beds, and thick bedded tuffaceous siltstones Foraminifera, trace fossils, glauconite, and the general fine-grained character of these rocks suggest that deposition occurred in an open marine deep-water environment, possibly as part of a prodelta or on the outer continental shelf. The Angora Peak member is composed of several hundred feet of moderately sorted, medium- to coarse-grained quartzose-feldspathic and lithic sandstones. The sandstones unconformably overlie the Oswald West mudstones and are thick bedded, laminated, and less commonly cross-laminated. Sedimentary structures, sorting, and stratigraphic relationships suggest that the sandstones were deposited in a high energy, wave-dominated environment, possibly as delta sheet sands. The sands were, in part, redistributed by waves and longshore drift to form offshore coastal barrier bars and.linear clastic shoreline deposits. The overlying 650-foot thick Silver Point member consists dominantly of dark gray, micaceous, laminated mudstones and very thin siltstones. Rhythmically interbedded mudstones and fine-grained turbidite sandstones occur in the lower part of the unit. Deposition occurred in cool, low energy, open marine conditions, probably of sublittoral to upper bathyal depths. Oversteepening of Angora Peak sheet sands on the delta front may have resulted in periodic slumping of sands which were transported by turbidity currents into a shelf basin of the deeper water Silver Point delta-slope environment. An east to west paleocurrent pattern in the Salver Point turbidite sandstonesis consistent with this model. Sandstone petrography and heavy mineral suites of the Astoria Formation suggests a dominantly volcanic and sedimentary provenance for these strata, probably the pre-Miocene volcanic rocks of the western Cascades and local Coast Range Eocene basalts and sandstones. Mineralogy and rare rock fragments in the sandstones also suggest that some detritus was derived from the granitic, metamorphic, and Paleozoic sedimentary terrains of eastern Oregon and Washington, and from British Columbia and western Idaho, possibly transported via an "ancestral Columbia River drainage system. Numerous dikes, sills, and irregular-shaped plutons of Depoe Bay Basalt and Cape Foulweather Basalt intruded the Oswald West mudstones and Astoria Formation. The major intrusive body in the area is the 900-foot thick sill that forms Tillamook Head. At Ecola State Park, the forceful intrusion of this sill into the semi-consolidated Silver Point strata at very shallow depths produced local synsedimentary folds. Over 1, 000 feet of extrusive Depoe Bay Basalt palagonitized pillow lavas and breccias and 2.00 feet of very local Cape Foulweather pillow lavas lie with angular unconformity over the Silver Point member. Deformation which accompanied the general Coast Range uplift during the late Miocene to Pleistocene formed four northeast-trending folds in the thesis area and two sets of high angle faults which strike north-south and northwest- southeast through the area. In addition, active landslides are abundant throughout the area, particularly along the coast. Economic resources of the area include basalt quarry rock for rip-rap and road base, basalt stream gravels for road aggregate, and potential petroleum resources may occur in the adjacent offshore area. Title: Geology of the Tillamook Head--Necanicum Junction area, Clatsop County, northwest Oregon Author: Neel, Robert Holmes Five distinct lithologic units compose the Tertiary rocks in the Tillamook Head - Necanicum Junction area of the northern Oregon Coast Range. They are: the late Eocene to early Miocene Oswald West mudstones, the middle Miocene Angora Peak sandstone and Silver Point mudstone members of the Astoria Formation, and the middle Miocene intrusive and extrusive Depoe Bay and Cape Foulweather basalts. These units are locally overlain by Pleistocene marine terraces and Holocene beach and dune sands, stream alluvium, and landslide deposits. The Oswald West mudstones, Angora Peak sandstones, and Silver Point mudstones are informal stratigraphic names used in this study. The Oswald West mudstones consist of over 2, 000 feet of well-bedded, intensely burrowed, silty mudstones and siltstones interstratified with minor glauconitic sandstones, tuff beds, and thick bedded tuffaceous siltstones Foraminifera, trace fossils, glauconite, and the general fine-grained character of these rocks suggest that deposition occurred in an open marine deep-water environment, possibly as part of a prodelta or on the outer continental shelf. The Angora Peak member is composed of several hundred feet of moderately sorted, medium- to coarse-grained quartzose-feldspathic and lithic sandstones. The sandstones unconformably overlie the Oswald West mudstones and are thick bedded, laminated, and less commonly cross-laminated. Sedimentary structures, sorting, and stratigraphic relationships suggest that the sandstones were deposited in a high energy, wave-dominated environment, possibly as delta sheet sands. The sands were, in part, redistributed by waves and longshore drift to form offshore coastal barrier bars and.linear clastic shoreline deposits. The overlying 650-foot thick Silver Point member consists dominantly of dark gray, micaceous, laminated mudstones and very thin siltstones. Rhythmically interbedded mudstones and fine-grained turbidite sandstones occur in the lower part of the unit. Deposition occurred in cool, low energy, open marine conditions, probably of sublittoral to upper bathyal depths. Oversteepening of Angora Peak sheet sands on the delta front may have resulted in periodic slumping of sands which were transported by turbidity currents into a shelf basin of the deeper water Silver Point delta-slope environment. An east to west paleocurrent pattern in the Salver Point turbidite sandstonesis consistent with this model. Sandstone petrography and heavy mineral suites of the Astoria Formation suggests a dominantly volcanic and sedimentary provenance for these strata, probably the pre-Miocene volcanic rocks of the western Cascades and local Coast Range Eocene basalts and sandstones. Mineralogy and rare rock fragments in the sandstones also suggest that some detritus was derived from the granitic, metamorphic, and Paleozoic sedimentary terrains of eastern Oregon and Washington, and from British Columbia and western Idaho, possibly transported via an "ancestral Columbia River drainage system. Numerous dikes, sills, and irregular-shaped plutons of Depoe Bay Basalt and Cape Foulweather Basalt intruded the Oswald West mudstones and Astoria Formation. The major intrusive body in the area is the 900-foot thick sill that forms Tillamook Head. At Ecola State Park, the forceful intrusion of this sill into the semi-consolidated Silver Point strata at very shallow depths produced local synsedimentary folds. Over 1, 000 feet of extrusive Depoe Bay Basalt palagonitized pillow lavas and breccias and 2.00 feet of very local Cape Foulweather pillow lavas lie with angular unconformity over the Silver Point member. Deformation which accompanied the general Coast Range uplift during the late Miocene to Pleistocene formed four northeast-trending folds in the thesis area and two sets of high angle faults which strike north-south and northwest- southeast through the area. In addition, active landslides are abundant throughout the area, particularly along the coast. Economic resources of the area include basalt quarry rock for rip-rap and road base, basalt stream gravels for road aggregate, and potential petroleum resources may occur in the adjacent offshore area. Close

• 6. [Article] Stratigraphy and sedimentation of the Onion Peak area, Clatsop County, Oregon

  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 ...

  Citation × Citation Citation Abstract Copy Title: Stratigraphy and sedimentation of the Onion Peak area, Clatsop County, Oregon Author: Smith, Thomas N. 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 unconformably overlie these Tertiary units. The Oswald West mudstones consist of more than 1600 feet of well-bedded, burrowed, tuffaceou siltstones and mudstones with sub ordinate tuff and glauconitic sandstone beds. Foraminiferal 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 carbonaceous 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 Peak member, probably interfingering with interbedded, deeper marine (600 feet-outer shelf) mudstones and turbidite sndstones of the Silver Point member, are interpreted to have been deposited near the mouth of a river, adjacent to the ocean as 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 up to 500 feet displacement A 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 particularly 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 reservoirs in the Angora Peak sandstones in stratigraphic and structural tra traps, particularly in nearby offshore areas, are the main geological economic resources of the area. Title: Stratigraphy and sedimentation of the Onion Peak area, Clatsop County, Oregon Author: Smith, Thomas N. 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 unconformably overlie these Tertiary units. The Oswald West mudstones consist of more than 1600 feet of well-bedded, burrowed, tuffaceou siltstones and mudstones with sub ordinate tuff and glauconitic sandstone beds. Foraminiferal 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 carbonaceous 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 Peak member, probably interfingering with interbedded, deeper marine (600 feet-outer shelf) mudstones and turbidite sndstones of the Silver Point member, are interpreted to have been deposited near the mouth of a river, adjacent to the ocean as 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 up to 500 feet displacement A 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 particularly 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 reservoirs in the Angora Peak sandstones in stratigraphic and structural tra traps, particularly in nearby offshore areas, are the main geological economic resources of the area. Close