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• 1. [Article] The geology of the Elk Mountain-Porter Ridge area, Clatsop County, northwest Oregon

  Eight Tertiary sedimentary and volcanic units crop out in the thesis area. From oldest to youngest they are the: Sager Creek formation (informal); Pittsburg Bluff Formation; Northrup Creek formation (informal); ...

  Citation × Citation Citation Abstract Copy Title: The geology of the Elk Mountain-Porter Ridge area, Clatsop County, northwest Oregon Author: Goalen, Jeffrey Scott Eight Tertiary sedimentary and volcanic units crop out in the thesis area. From oldest to youngest they are the: Sager Creek formation (informal); Pittsburg Bluff Formation; Northrup Creek formation (informal); Smuggler Cove formation (informal); Wickiup Mountain and Cannon Beach members (both informal) of the Astoria Formation; the Grande Ronde Basalt, and Frenchman Springs Member of the Wanapum Basalt, both of the Columbia River Basalt Group. Also, areally limited, unnamed sedimentary strata interbedded between flows of the Columbia River Basalt Group crop out in the study area. Quaternary deposits consist of alluvium, colluvium, and landslide debris. Rhythmically-bedded, foram-bearing, carbonaceous to micaceous mudstone and graded, fine-grained, feldspathic turbidite sandstone are the dominant lithologies of the upper Eocene (Refugian) Sager Creek formation. Plane-laminae and climbing ripple-laminae typical of Bouma b,c, and d intervals are common in the thin turbidite sandstone beds. Contemporaneous, rare, thick, sandstones may represent submarine feeder channels that supplied the more widespread, thinly-bedded overbank turbidite sandstones. Foraminiferal paleobathymetry indicates that deposition was in bathyal water depths. Sager Creek deposition was followed by a regression or offlap of the late Eocene to early Oligocene (Refugian) sea as indicated by the molluscan fossils and thick, bioturbated sandstone of the predominantly shallow-marine Pittsburg Bluff Formation. The lower part of the formation consists of outer shelf, glauconitic, fossiliferous sandstone and subordinate mudstone. Higher in the section are middle-shelf, fine-grained, bioturbated, tuffaceous, arkosic sandstones. These sandstones contain minor glauconite, wave- and storm-generated molluscan shell hash beds, carbonized wood fragments, and calcareous concretions. Deposition occurred in 20 - 50 m open-marine shelf water depths; however, the depositional environment may have shallowed to a bay-like setting (Moore, 1982, written communication). The upper part of the unit consists of thin- to medium-bedded, carbonaceous siltstone and mudstone with minor ashfall tuffs. Therefore, deposition of the Pittsburg Bluff Formation occurred as a shallowing-upward, then deepening depositional episode, punctuated by contemporaneous, intermittent eruption of nearby calc-alkaline western Cascade volcanoes. The deep-marine Oligocene to lower Miocene (Zemorrian to Saucesian) Northrup Creek formation is predominantly composed of thinly-laminated mudstone interbedded with thin, very fine-grained, graded, micaceous arkosic sandstone. Bouma c-d-e and a-b-e sequences are common in the turbidite sandstone; Bouma a-e intervals occur locally in sandstone/mudstone couplets. Paleocurrent indicators suggest that the predominant transport direction of these turbidite deposits was northeast to southwest. Abraded, carbonaceous plant debris and mica are abundant in the sandstone laminations. The upper part of the formation consists of thick, mollusk-bearing, moderately- to well-sorted, arkosic sandstone and minor polymict grit beds. A shallowing-upward, high-energy, shallow-marine shelf environment of deposition is indicated for the upper part of the formation. Contemporaneous with shelf and slope deposition of the Sager Creek, Pittsburg Bluff and Northrup Creek formations, the deep-water late Eocene to early Miocene Smuggler Cove formation was deposited as a distal or lateral correlative in a low-energy, outer shelf to slope setting. This foram-bearing unit consists of thick, bioturbated, bathyal, tuffaceous mudstone and siltstone with minor thin- to thick-bedded ashfall and current-reworked tuff. A marine onlap is indicated by the conformable relationship between the upper sandstone unit of the Northrup Creek formation and the overlying Smuggler Cove formation in the eastern part of the thesis area. The overlying lower to middle Miocene Astoria Formation contains two members in the thesis area: a high-energy, shallow shelf, fine- to medium-grained, fossiliferous, micaceous arkosic sandstone (Wickiup Mountain member), and an overlying, thinly-laminated, deepmarine mudstone (Cannon Beach member). Diatom floras indicate that a thermal "oceanographic irregularity" (water warmer than normal) may have occurred during deposition of the Cannon Beach member. At least six, and possibly eight, flows of the middle Miocene Grande Ronde Basalt (Columbia River Basalt Group) are present in the thesis area. Individual flows have been ascribed to the (N1?), R2, and N2 magnetozones. Geochemically, the flows consist of low MgO high Ti02, low MgO low Ti02, and high MgO subtypes. These subaerial to submarine flows are correlated to Mangan and others (1986) chemical subtypes 2D, 5C, 5A, and 4A of the Columbia Plateau-derived Grande Ronde Basalt of eastern Washington and eastern Oregon. The correlations are based on similarity of age, major element chemistry, stratigraphic position, and magnetic polarity. Field evidence suggests that thick, submarine pillow and breccia complexes generated sufficient pressure to autoinvasively inject into the Eocene to middle Miocene sedimentary strata of the Plympton/Porter ridge and Elk Mountain areas. This process apparently formed many randomly-oriented dike- and sill-like intrusions in the western part of the thesis area. However, in the eastern and central parts of the thesis area, three sub-parallel dikes (the Northrup, Beneke, and Fishhawk Falls dikes) extend along linear trends for tens of kilometers. This suggests that their emplacement was, in part, influenced by earlier or contemporaneous regional tectonism. At least two, and as many as five, flows of the Frenchman Springs Member of the Wanapum Basalt occur in the thesis area: one to two abundantly plagioclase-phyric Basalt of Ginkgo flow(s), and one to three Basalt of Sand Hollow flows (terminology after Beeson and Tolan, 1985). These flows consist of subaqueous pillow palagonite breccia and vesicular, columnar-jointed, subaerial basalt. No Frenchman Springs flows are invasive in the thesis area. Local middle Miocene sedimentary interbeds between flows of Grande Ronde and Frenchman Springs Basalt are lithologically and sedimentologically diverse. Common lithologies are fine- to mediumgrained, arkosic sandstone, coarse-grained basaltic sandstone, and structureless mudstone. Depositional environments represented by these strata are fluvial, marginal-marine, and shallow-marine. Thicknesses of individual interbeds range from 0.2 to 50 m. The thesis area is located on the northwest flank of the Oregon Coast Range anticline, adjacent to the Nehalem Arch. Large-scale northeast-trending oblique to strike-slip left-lateral faults coupled with northwest-trending oblique to strike-slip right-lateral faults dominate the structure of the area. These faults may have formed as conjugate shears (Riedel shear) caused by north-south compressive stress related to the oblique subduction of the Juan de Fuca Plate beneath the North American Plate. Five episodes of deformation are suggested by faults, dike orientations, unconformities, and other geological relationships within the thesis area: a late Eocene north-south compressional episode, an early Oligocene to early Miocene uplift, a middle Miocene northwest-southeast extensional episode, a post-middle Miocene to Pliocene (?) north-south compressional episode, and a north-south extensional event that occurred between the post-middle Miocene and Recent. Although crushed rock (for road and revetment construction) is currently the primary mineral resource within the thesis area, several potential fault traps on this northwest flank of the Nehalem Arch may contain significant reserves of natural gas. This conclusion is based upon field, laboratory, and subsurface (well) data. The Clark and Wilson sandstone of the middle to late Eocene Cowlitz Formation, the producing unit at the nearby Mist gas field, represents the most attractive target horizon. Additionally, the porous and permeable upper sandstone unit of the Northrup Creek formation could contain shallow hydrocarbon reserves beneath the northern part of the area. Title: The geology of the Elk Mountain-Porter Ridge area, Clatsop County, northwest Oregon Author: Goalen, Jeffrey Scott Eight Tertiary sedimentary and volcanic units crop out in the thesis area. From oldest to youngest they are the: Sager Creek formation (informal); Pittsburg Bluff Formation; Northrup Creek formation (informal); Smuggler Cove formation (informal); Wickiup Mountain and Cannon Beach members (both informal) of the Astoria Formation; the Grande Ronde Basalt, and Frenchman Springs Member of the Wanapum Basalt, both of the Columbia River Basalt Group. Also, areally limited, unnamed sedimentary strata interbedded between flows of the Columbia River Basalt Group crop out in the study area. Quaternary deposits consist of alluvium, colluvium, and landslide debris. Rhythmically-bedded, foram-bearing, carbonaceous to micaceous mudstone and graded, fine-grained, feldspathic turbidite sandstone are the dominant lithologies of the upper Eocene (Refugian) Sager Creek formation. Plane-laminae and climbing ripple-laminae typical of Bouma b,c, and d intervals are common in the thin turbidite sandstone beds. Contemporaneous, rare, thick, sandstones may represent submarine feeder channels that supplied the more widespread, thinly-bedded overbank turbidite sandstones. Foraminiferal paleobathymetry indicates that deposition was in bathyal water depths. Sager Creek deposition was followed by a regression or offlap of the late Eocene to early Oligocene (Refugian) sea as indicated by the molluscan fossils and thick, bioturbated sandstone of the predominantly shallow-marine Pittsburg Bluff Formation. The lower part of the formation consists of outer shelf, glauconitic, fossiliferous sandstone and subordinate mudstone. Higher in the section are middle-shelf, fine-grained, bioturbated, tuffaceous, arkosic sandstones. These sandstones contain minor glauconite, wave- and storm-generated molluscan shell hash beds, carbonized wood fragments, and calcareous concretions. Deposition occurred in 20 - 50 m open-marine shelf water depths; however, the depositional environment may have shallowed to a bay-like setting (Moore, 1982, written communication). The upper part of the unit consists of thin- to medium-bedded, carbonaceous siltstone and mudstone with minor ashfall tuffs. Therefore, deposition of the Pittsburg Bluff Formation occurred as a shallowing-upward, then deepening depositional episode, punctuated by contemporaneous, intermittent eruption of nearby calc-alkaline western Cascade volcanoes. The deep-marine Oligocene to lower Miocene (Zemorrian to Saucesian) Northrup Creek formation is predominantly composed of thinly-laminated mudstone interbedded with thin, very fine-grained, graded, micaceous arkosic sandstone. Bouma c-d-e and a-b-e sequences are common in the turbidite sandstone; Bouma a-e intervals occur locally in sandstone/mudstone couplets. Paleocurrent indicators suggest that the predominant transport direction of these turbidite deposits was northeast to southwest. Abraded, carbonaceous plant debris and mica are abundant in the sandstone laminations. The upper part of the formation consists of thick, mollusk-bearing, moderately- to well-sorted, arkosic sandstone and minor polymict grit beds. A shallowing-upward, high-energy, shallow-marine shelf environment of deposition is indicated for the upper part of the formation. Contemporaneous with shelf and slope deposition of the Sager Creek, Pittsburg Bluff and Northrup Creek formations, the deep-water late Eocene to early Miocene Smuggler Cove formation was deposited as a distal or lateral correlative in a low-energy, outer shelf to slope setting. This foram-bearing unit consists of thick, bioturbated, bathyal, tuffaceous mudstone and siltstone with minor thin- to thick-bedded ashfall and current-reworked tuff. A marine onlap is indicated by the conformable relationship between the upper sandstone unit of the Northrup Creek formation and the overlying Smuggler Cove formation in the eastern part of the thesis area. The overlying lower to middle Miocene Astoria Formation contains two members in the thesis area: a high-energy, shallow shelf, fine- to medium-grained, fossiliferous, micaceous arkosic sandstone (Wickiup Mountain member), and an overlying, thinly-laminated, deepmarine mudstone (Cannon Beach member). Diatom floras indicate that a thermal "oceanographic irregularity" (water warmer than normal) may have occurred during deposition of the Cannon Beach member. At least six, and possibly eight, flows of the middle Miocene Grande Ronde Basalt (Columbia River Basalt Group) are present in the thesis area. Individual flows have been ascribed to the (N1?), R2, and N2 magnetozones. Geochemically, the flows consist of low MgO high Ti02, low MgO low Ti02, and high MgO subtypes. These subaerial to submarine flows are correlated to Mangan and others (1986) chemical subtypes 2D, 5C, 5A, and 4A of the Columbia Plateau-derived Grande Ronde Basalt of eastern Washington and eastern Oregon. The correlations are based on similarity of age, major element chemistry, stratigraphic position, and magnetic polarity. Field evidence suggests that thick, submarine pillow and breccia complexes generated sufficient pressure to autoinvasively inject into the Eocene to middle Miocene sedimentary strata of the Plympton/Porter ridge and Elk Mountain areas. This process apparently formed many randomly-oriented dike- and sill-like intrusions in the western part of the thesis area. However, in the eastern and central parts of the thesis area, three sub-parallel dikes (the Northrup, Beneke, and Fishhawk Falls dikes) extend along linear trends for tens of kilometers. This suggests that their emplacement was, in part, influenced by earlier or contemporaneous regional tectonism. At least two, and as many as five, flows of the Frenchman Springs Member of the Wanapum Basalt occur in the thesis area: one to two abundantly plagioclase-phyric Basalt of Ginkgo flow(s), and one to three Basalt of Sand Hollow flows (terminology after Beeson and Tolan, 1985). These flows consist of subaqueous pillow palagonite breccia and vesicular, columnar-jointed, subaerial basalt. No Frenchman Springs flows are invasive in the thesis area. Local middle Miocene sedimentary interbeds between flows of Grande Ronde and Frenchman Springs Basalt are lithologically and sedimentologically diverse. Common lithologies are fine- to mediumgrained, arkosic sandstone, coarse-grained basaltic sandstone, and structureless mudstone. Depositional environments represented by these strata are fluvial, marginal-marine, and shallow-marine. Thicknesses of individual interbeds range from 0.2 to 50 m. The thesis area is located on the northwest flank of the Oregon Coast Range anticline, adjacent to the Nehalem Arch. Large-scale northeast-trending oblique to strike-slip left-lateral faults coupled with northwest-trending oblique to strike-slip right-lateral faults dominate the structure of the area. These faults may have formed as conjugate shears (Riedel shear) caused by north-south compressive stress related to the oblique subduction of the Juan de Fuca Plate beneath the North American Plate. Five episodes of deformation are suggested by faults, dike orientations, unconformities, and other geological relationships within the thesis area: a late Eocene north-south compressional episode, an early Oligocene to early Miocene uplift, a middle Miocene northwest-southeast extensional episode, a post-middle Miocene to Pliocene (?) north-south compressional episode, and a north-south extensional event that occurred between the post-middle Miocene and Recent. Although crushed rock (for road and revetment construction) is currently the primary mineral resource within the thesis area, several potential fault traps on this northwest flank of the Nehalem Arch may contain significant reserves of natural gas. This conclusion is based upon field, laboratory, and subsurface (well) data. The Clark and Wilson sandstone of the middle to late Eocene Cowlitz Formation, the producing unit at the nearby Mist gas field, represents the most attractive target horizon. Additionally, the porous and permeable upper sandstone unit of the Northrup Creek formation could contain shallow hydrocarbon reserves beneath the northern part of the area. Close

• 2. [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

• 3. [Article] Geology of the Green Mountain-Young's River area, Clatsop County, northwest Oregon

  The upper Eocene to lower Oligocene Oswald West mudstone is the oldest formation (informal) in the Green Mountain-Young's River area. This 1,663 meter thick hemipelagic sequence was deposited in a low-energy ...

  Citation × Citation Citation Abstract Copy Title: Geology of the Green Mountain-Young's River area, Clatsop County, northwest Oregon Author: Peterson, Carolyn Pugh The upper Eocene to lower Oligocene Oswald West mudstone is the oldest formation (informal) in the Green Mountain-Young's River area. This 1,663 meter thick hemipelagic sequence was deposited in a low-energy lower to upper slope environment in the Coast Range forearc basin. The formation ranges from the late Narizian to the early Zemorrian(?) in age and consists of thick-bedded bioturbated foraminiferal claystone and tuffaceous siltstone. Rare glauconitic sandstone beds also occur. In the eastern part of the study area, the upper part of the Oswald West mudstone is interbedded with the upper Refugian Klaskanine siltstone tongue. This informal unit consists of thick bioturbated sandy siltstone and silty sandstone that is a lateral deep-marine correlative of the deltaic to shallow-marine Pittsburg Bluff Formation in the northeastern Coast Range. Discontinuous underthrusting of the Juan de Fuca oceanic plate at the base of the continental slope of the North American plate caused extensive uplift and subsidence along the Oregon continental margin throughout the Cenozoic (Snavely et al., 1980). Initiation of Oregon Coast Range uplift and accompanying erosion in the early Miocene, coupled with a global low stand of sea level (Vail and Mitchum, 1979), stripped most of the Oligocene (Zemorrian) Oswald West strata and in places much of the uppermost Eocene (upper Refugian) Oswald West strata in the field area, creating an unconformity. Deformation accompanying uplift included a system of east-west-trending, oblique-slip faults. The Pillarian-to-Newportian Astoria Formation unconformably overlies the Oswald West mudstone and reflects deposition offshore from an open, storm-dominated coast during an early-to-middle Miocene transgression. Deposition of the Big Creek sandstone and Silver Point mudstone members of the Astoria Formation was controlled in part by submarine paleotopography that developed as a result of early Miocene deformation of the Oswald West strata. The up to 200 meter thick Big Creek member varies from storm-deposited laminated sandstone to bioturbated mollusk-bearing silty sandstone that accumulated during fair weather conditions on the inner to middle shelf. Overlying and perhaps in part laterally equivalent to the Big Creek member is the up to 200 meter thick, deeper marine Silver Point member which consists of two lithologies: 1) interbedded, micaceous, turbidite sandstones and laminated mudstone; and 2) laminated bathyal mudstone that intertongues with and caps the turbidite sequences. The turbidite lithology is composed of two facies: 1) an underlying sand-rich facies, transitional between the shallow-marine Big Creek member and bathyal Silver Point strata, that was deposited on the outer shelf by storm-induced turbidity currents; and 2) an overlying sand-poor facies that was deposited at bathyal depths. The turbidite facies channelized, and at some places removed the underlying Big Creek member and were deposited directly over Oswald West mudstone. The Astoria depositional sequence ranges, from inner to outer neritic to bathyal facies and reflects continued deepening and anoxic depositional conditions of the Astoria basin through the middle Miocene. Big Creek and Silver Point sandstone petrology reflects volcanic sources from an ancestral western Cascades volcanic arc and metamorphic and granitic basement rocks farther east via an ancestral Columbia River drainage system. Diagenetic effects include: (a) formation of local calcite concretionary cements; and (b) formation of pore-filling smectite from alteration of volcanic rock fragments. At least six middle Miocene Columbia River Basalt intrusive episodes affected the Green Mountain-Young's River area soon after deposition of the Astoria Formation. These basalt sills and dikes include normally polarized and reversely polarized low Mg0 high TiO₂, low Mg0 low TiO₂, and high Mg0 Grande Ronde basalt chemical subtypes and two porphyritic Frenchman Springs Member basalts (Ginkgo and Kelly Hollow(?) petrologic types). These basalt intrusions are virtually indistinguishable, based on chemistry, from subaerial flows of the plateau-derived Columbia River Basalt Group subtypes at nearby Nicolai Mountain and Porter Ridge. This correlation supports the Beeson et al. (1979) hypothesis that the intrusions are not of local origin but formed by the invasion of the flows into the Miocene shoreline sediments to form "invasive" sills and dikes. Many dikes were emplaced along northeast- and northwest-trending faults, and some (i.e., Ginkgo) cut older sills (Grande Ronde). A laterally extensive Frenchman Springs sill occurs under an older widespread Grande Ronde sill. From this older over younger intrusive relationship, a mechanism of "invasion" of sediment from overlying lava flows is difficult to envision. A pulse of rapid subduction starting in the middle Miocene (Snavely et al., 1980) was accompanied by renewed uplift, intensive block faulting, and continued development of the earlier formed Coast Range uplift. Left-oblique northeast-trending faults and conjugate northwest-trending right-oblique faults offset Grande Ronde and Frenchman Springs dikes and sills. This conjugate fault pattern may reflect oblique east-west convergence between the North American and Juan de Fuca plates. The Silver Point mudstones and Oswald West mudstones have high total organic carbon contents, up to 5.5%, but are thermally immature and may act only as a source for biogenic gas(?) in the subsurface. Suitable reservoir rocks, such as the gas-producing upper Eocene Cowlitz Formation C & W sandstone, may pinch out before reaching the Green Mountain-Young's River area and are yet to be penetrated by exploration drilling. Post-middle Miocene fault traps abound in the area, although these faults might also breach subsurface natural gas reservoirs in the Green Mountain-Young's River area. Title: Geology of the Green Mountain-Young's River area, Clatsop County, northwest Oregon Author: Peterson, Carolyn Pugh The upper Eocene to lower Oligocene Oswald West mudstone is the oldest formation (informal) in the Green Mountain-Young's River area. This 1,663 meter thick hemipelagic sequence was deposited in a low-energy lower to upper slope environment in the Coast Range forearc basin. The formation ranges from the late Narizian to the early Zemorrian(?) in age and consists of thick-bedded bioturbated foraminiferal claystone and tuffaceous siltstone. Rare glauconitic sandstone beds also occur. In the eastern part of the study area, the upper part of the Oswald West mudstone is interbedded with the upper Refugian Klaskanine siltstone tongue. This informal unit consists of thick bioturbated sandy siltstone and silty sandstone that is a lateral deep-marine correlative of the deltaic to shallow-marine Pittsburg Bluff Formation in the northeastern Coast Range. Discontinuous underthrusting of the Juan de Fuca oceanic plate at the base of the continental slope of the North American plate caused extensive uplift and subsidence along the Oregon continental margin throughout the Cenozoic (Snavely et al., 1980). Initiation of Oregon Coast Range uplift and accompanying erosion in the early Miocene, coupled with a global low stand of sea level (Vail and Mitchum, 1979), stripped most of the Oligocene (Zemorrian) Oswald West strata and in places much of the uppermost Eocene (upper Refugian) Oswald West strata in the field area, creating an unconformity. Deformation accompanying uplift included a system of east-west-trending, oblique-slip faults. The Pillarian-to-Newportian Astoria Formation unconformably overlies the Oswald West mudstone and reflects deposition offshore from an open, storm-dominated coast during an early-to-middle Miocene transgression. Deposition of the Big Creek sandstone and Silver Point mudstone members of the Astoria Formation was controlled in part by submarine paleotopography that developed as a result of early Miocene deformation of the Oswald West strata. The up to 200 meter thick Big Creek member varies from storm-deposited laminated sandstone to bioturbated mollusk-bearing silty sandstone that accumulated during fair weather conditions on the inner to middle shelf. Overlying and perhaps in part laterally equivalent to the Big Creek member is the up to 200 meter thick, deeper marine Silver Point member which consists of two lithologies: 1) interbedded, micaceous, turbidite sandstones and laminated mudstone; and 2) laminated bathyal mudstone that intertongues with and caps the turbidite sequences. The turbidite lithology is composed of two facies: 1) an underlying sand-rich facies, transitional between the shallow-marine Big Creek member and bathyal Silver Point strata, that was deposited on the outer shelf by storm-induced turbidity currents; and 2) an overlying sand-poor facies that was deposited at bathyal depths. The turbidite facies channelized, and at some places removed the underlying Big Creek member and were deposited directly over Oswald West mudstone. The Astoria depositional sequence ranges, from inner to outer neritic to bathyal facies and reflects continued deepening and anoxic depositional conditions of the Astoria basin through the middle Miocene. Big Creek and Silver Point sandstone petrology reflects volcanic sources from an ancestral western Cascades volcanic arc and metamorphic and granitic basement rocks farther east via an ancestral Columbia River drainage system. Diagenetic effects include: (a) formation of local calcite concretionary cements; and (b) formation of pore-filling smectite from alteration of volcanic rock fragments. At least six middle Miocene Columbia River Basalt intrusive episodes affected the Green Mountain-Young's River area soon after deposition of the Astoria Formation. These basalt sills and dikes include normally polarized and reversely polarized low Mg0 high TiO₂, low Mg0 low TiO₂, and high Mg0 Grande Ronde basalt chemical subtypes and two porphyritic Frenchman Springs Member basalts (Ginkgo and Kelly Hollow(?) petrologic types). These basalt intrusions are virtually indistinguishable, based on chemistry, from subaerial flows of the plateau-derived Columbia River Basalt Group subtypes at nearby Nicolai Mountain and Porter Ridge. This correlation supports the Beeson et al. (1979) hypothesis that the intrusions are not of local origin but formed by the invasion of the flows into the Miocene shoreline sediments to form "invasive" sills and dikes. Many dikes were emplaced along northeast- and northwest-trending faults, and some (i.e., Ginkgo) cut older sills (Grande Ronde). A laterally extensive Frenchman Springs sill occurs under an older widespread Grande Ronde sill. From this older over younger intrusive relationship, a mechanism of "invasion" of sediment from overlying lava flows is difficult to envision. A pulse of rapid subduction starting in the middle Miocene (Snavely et al., 1980) was accompanied by renewed uplift, intensive block faulting, and continued development of the earlier formed Coast Range uplift. Left-oblique northeast-trending faults and conjugate northwest-trending right-oblique faults offset Grande Ronde and Frenchman Springs dikes and sills. This conjugate fault pattern may reflect oblique east-west convergence between the North American and Juan de Fuca plates. The Silver Point mudstones and Oswald West mudstones have high total organic carbon contents, up to 5.5%, but are thermally immature and may act only as a source for biogenic gas(?) in the subsurface. Suitable reservoir rocks, such as the gas-producing upper Eocene Cowlitz Formation C & W sandstone, may pinch out before reaching the Green Mountain-Young's River area and are yet to be penetrated by exploration drilling. Post-middle Miocene fault traps abound in the area, although these faults might also breach subsurface natural gas reservoirs in the Green Mountain-Young's River area. Close

• 4. [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

• 5. [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