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• 1. [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