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• 1. [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

• 2. [Article] Geology of the Buster Creek-Nehalem Valley area, Clatsop County, northwest Oregon

  Six Tertiary rock units are exposed in the Buster Creek-Nehalem Valley area. They are, from oldest to youngest: upper Eocene Tillamook Volcanics; upper Eocene Cowlitz Formation; upper Eocene Keasey Formation; ...

  Citation × Citation Citation Abstract Copy Title: Geology of the Buster Creek-Nehalem Valley area, Clatsop County, northwest Oregon Author: Olbinski, James Steven Six Tertiary rock units are exposed in the Buster Creek-Nehalem Valley area. They are, from oldest to youngest: upper Eocene Tillamook Volcanics; upper Eocene Cowlitz Formation; upper Eocene Keasey Formation; upper Eocene Vesper Church formation (informal); upper Eocene to Oligocene Pittsburg Bluff Formation; and middle Miocene Depoe Bay Basalt. The Tillamook Volcanics is composed of basaltic andesite subaerial flows (SiO2 54.19% to 55.67%) and volcanic debris flows. The platy jointed subaerial flows are generally porphyritic with a pilotaxitic groundmass; a few aphyric flows with well defined flow banding are present. Highly vesicular flow tops typically show alteration of ilmenite to leucoxene producing a diagnostic bleached white appearance. Debris flows are comprised of very poorly sorted, angular, volcanic and minor sedimentary rock clasts in a mud matrix. Commonly this lithology is intruded by porphyritic and vesicular dikes. Major oxides of the Tillamook basaltic andesites suggest that these volcanic rocks may have erupted in an oceanic island "spreading center" environment. The Cowlitz Formation unconformably overlies the Tillamook Volcanics in the study area. This formation is divided into five informal members: a basal basaltic andesite conglomerate; a unit of interbedded micaceous arkese, volcanic lithic arenite and mudstone; a thick structureless mudstone; rhythmically laminated turbidite sandstone and siltstone; and a thick upper arkosic sandstone. The basal conglomerate represents a fluvial braided stream to high energy marine shoreline environment. The finer grained middle members indicate a deepening up sequence. The upper sandstone member is a micaceous, well-sorted, hummocky bedded, porous, very friable sandstone deposited in a high energy storm-dominated shelf to nearshore environment. The upper sandstone member of the Cowlitz Formation ("Clark and Wilson sand" of the Mist gas field) is the current target of active drilling in adjacent areas by several petroleum companies. This upper sandstone and lower arkosic sandstones represent favorable targets in contact with organic-rich, but immature, source rocks in potential fault bounded structural highs and erosional pinchouts in the northern part of the study area. The Narizian to Refugian Keasey Formation is represented by the Jewell member (informal), in the study area. The Jewell member, a well-bedded to laminated, tuffaceous, indurated mudstone, unconformably overlies the more extensively faulted Cowlitz Formation. Upper slope water depths of 200-600 m are indicated by foraminiferal and molluscan assemblages. A lithologically distinct and mappable turbidite unit crops out in the study area and is informally named in this thesis the Vesper Church formation. The Vesper Church formation represents west- to northwest-trending turbidite-filled "sea gullies" deposited at bathyal depths (1,000-1,500 m) on the lower slope. Thin-bedded channelized turbidites with Bouma c, d and e sequences and local thick amalgamated sandstones characterize this unit. The upper Eocene (Refugian) Pittsburg Bluff Formation conformably overlies the Vesper Church formation. Molluscan assemblages from the basal part of this formation indicate water depths of 20-200 m; thus, a significant shallowing episode between the Vesper Church and Pittsburg Bluff formations is indicated. Thick, bioturbated, glauconitic to fine-grained tuffaceous sandstone and sandy mudstone suggest an inner to middle (possibly outer) continental shelf depositional environment for the Pittsburg Bluff strata. Following a period of high-angle northeast trending faulting, two middle Miocene Depoe Bay Basalt dikes intruded the sedimentary formations exposed in the area. The reversely polarized Northrup Creek dike can be traced over 8.5 km and has a paleomagnetic declination of 170° and a steep inclination of -74°, possibly a result of secular variation of the geomagnetic pole during cooling of the dike through its Cuire temperature (Nelson, 1983). These aphanitic to sparsely micro-porhyritic tholeiitic basalts are chemically identical to the Grande Ronde Basalt of the Columbia Plateau. A recent hypothesis by Beeson et al. (1979) suggests that all Miocene coastal basalts represent the distal ends of subaerial Columbia River basalts which reached the Miocene shoreline and intruded or "invaded" soft marine sediments. The reversed stratigraphic order of high and low MgO Depoe Bay (Grande Ronde) basaltic sills intruding Keasey and Cowlitz strata, as much as 1600 m below the surface in the petroleum exploration well Quintan_a Watzek 30-1 Watzek 30-1 in this study area, illustrates a complicating factor in the emplacement mechanism if these are "invasive" intrusions. In late middle Miocene the northern Oregon Coast Range anticline was formed, possibly in response to a period of underthrusting (Snavely et al., 1980b). Contemporaneously, a second set of left-lateral and right-lateral conjugate faults (N55°E and N55°W) cut the middle Miocene basalt dikes. This faulting may be related to north-south compression and clockwise rotation of western Oregon and southwest Washington associated with the oblique subduction of the Juan De Fuca Plate beneath the North American Plate as suggested by Magill et al. (1981) and Coe and Wells (1982). Title: Geology of the Buster Creek-Nehalem Valley area, Clatsop County, northwest Oregon Author: Olbinski, James Steven Six Tertiary rock units are exposed in the Buster Creek-Nehalem Valley area. They are, from oldest to youngest: upper Eocene Tillamook Volcanics; upper Eocene Cowlitz Formation; upper Eocene Keasey Formation; upper Eocene Vesper Church formation (informal); upper Eocene to Oligocene Pittsburg Bluff Formation; and middle Miocene Depoe Bay Basalt. The Tillamook Volcanics is composed of basaltic andesite subaerial flows (SiO2 54.19% to 55.67%) and volcanic debris flows. The platy jointed subaerial flows are generally porphyritic with a pilotaxitic groundmass; a few aphyric flows with well defined flow banding are present. Highly vesicular flow tops typically show alteration of ilmenite to leucoxene producing a diagnostic bleached white appearance. Debris flows are comprised of very poorly sorted, angular, volcanic and minor sedimentary rock clasts in a mud matrix. Commonly this lithology is intruded by porphyritic and vesicular dikes. Major oxides of the Tillamook basaltic andesites suggest that these volcanic rocks may have erupted in an oceanic island "spreading center" environment. The Cowlitz Formation unconformably overlies the Tillamook Volcanics in the study area. This formation is divided into five informal members: a basal basaltic andesite conglomerate; a unit of interbedded micaceous arkese, volcanic lithic arenite and mudstone; a thick structureless mudstone; rhythmically laminated turbidite sandstone and siltstone; and a thick upper arkosic sandstone. The basal conglomerate represents a fluvial braided stream to high energy marine shoreline environment. The finer grained middle members indicate a deepening up sequence. The upper sandstone member is a micaceous, well-sorted, hummocky bedded, porous, very friable sandstone deposited in a high energy storm-dominated shelf to nearshore environment. The upper sandstone member of the Cowlitz Formation ("Clark and Wilson sand" of the Mist gas field) is the current target of active drilling in adjacent areas by several petroleum companies. This upper sandstone and lower arkosic sandstones represent favorable targets in contact with organic-rich, but immature, source rocks in potential fault bounded structural highs and erosional pinchouts in the northern part of the study area. The Narizian to Refugian Keasey Formation is represented by the Jewell member (informal), in the study area. The Jewell member, a well-bedded to laminated, tuffaceous, indurated mudstone, unconformably overlies the more extensively faulted Cowlitz Formation. Upper slope water depths of 200-600 m are indicated by foraminiferal and molluscan assemblages. A lithologically distinct and mappable turbidite unit crops out in the study area and is informally named in this thesis the Vesper Church formation. The Vesper Church formation represents west- to northwest-trending turbidite-filled "sea gullies" deposited at bathyal depths (1,000-1,500 m) on the lower slope. Thin-bedded channelized turbidites with Bouma c, d and e sequences and local thick amalgamated sandstones characterize this unit. The upper Eocene (Refugian) Pittsburg Bluff Formation conformably overlies the Vesper Church formation. Molluscan assemblages from the basal part of this formation indicate water depths of 20-200 m; thus, a significant shallowing episode between the Vesper Church and Pittsburg Bluff formations is indicated. Thick, bioturbated, glauconitic to fine-grained tuffaceous sandstone and sandy mudstone suggest an inner to middle (possibly outer) continental shelf depositional environment for the Pittsburg Bluff strata. Following a period of high-angle northeast trending faulting, two middle Miocene Depoe Bay Basalt dikes intruded the sedimentary formations exposed in the area. The reversely polarized Northrup Creek dike can be traced over 8.5 km and has a paleomagnetic declination of 170° and a steep inclination of -74°, possibly a result of secular variation of the geomagnetic pole during cooling of the dike through its Cuire temperature (Nelson, 1983). These aphanitic to sparsely micro-porhyritic tholeiitic basalts are chemically identical to the Grande Ronde Basalt of the Columbia Plateau. A recent hypothesis by Beeson et al. (1979) suggests that all Miocene coastal basalts represent the distal ends of subaerial Columbia River basalts which reached the Miocene shoreline and intruded or "invaded" soft marine sediments. The reversed stratigraphic order of high and low MgO Depoe Bay (Grande Ronde) basaltic sills intruding Keasey and Cowlitz strata, as much as 1600 m below the surface in the petroleum exploration well Quintan_a Watzek 30-1 Watzek 30-1 in this study area, illustrates a complicating factor in the emplacement mechanism if these are "invasive" intrusions. In late middle Miocene the northern Oregon Coast Range anticline was formed, possibly in response to a period of underthrusting (Snavely et al., 1980b). Contemporaneously, a second set of left-lateral and right-lateral conjugate faults (N55°E and N55°W) cut the middle Miocene basalt dikes. This faulting may be related to north-south compression and clockwise rotation of western Oregon and southwest Washington associated with the oblique subduction of the Juan De Fuca Plate beneath the North American Plate as suggested by Magill et al. (1981) and Coe and Wells (1982). Close