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• 151. [Article] Reconstructing the Physical Record of a Four-Million-Year Volcanic System : Geochemistry, Thermobarometry, and Geologic Map of the Mount Jefferson Area, Oregon

  Volcanic and sedimentary deposits of the Mount Jefferson area (MJA) record a fourmillion-year history of arc-related volcanism related to the subduction of the Juan de Fuca plate beneath North America. ...

  Citation × Citation Citation Abstract Copy Title: Reconstructing the Physical Record of a Four-Million-Year Volcanic System : Geochemistry, Thermobarometry, and Geologic Map of the Mount Jefferson Area, Oregon Author: DiGiulio, Jennifer Volcanic and sedimentary deposits of the Mount Jefferson area (MJA) record a fourmillion-year history of arc-related volcanism related to the subduction of the Juan de Fuca plate beneath North America. 171 mapped stratigraphic units over an area of 150 km² reveal four periods of volcanic activity resulting in diverse composition lavas ranging from ~48 to 72 wt% SiO₂. Eruptive periods are divided into (1) ~4.0 - 0.78 Ma; (2) 0.78 - 0.3 Ma; (3) 0.3 - 0.02 Ma; and (4) eruptions of the past 13,000 years. Repeated glaciations over the past 800,000 years have sculpted the landscape of the MJA and include the Pleistocene glaciations of Jack Creek (early Wisconsin) and Cabot Creek (late Wisconsin), and the Holocene glaciation of Jefferson Park. Anomalously glassy lava flows, columnar jointing, and streamlined shapes are lithologic evidence of intraglacial eruptions in numerous units, including the andesite of Whitewater Creek (Qawc), andesite and dacite of Park Butte (Qapb; Qdpb), and the basaltic andesite of Whiskey Creek (Qbawh). Mineral compositions and textures are highly variable among the four units. Patchy to oscillatory zoning in plagioclase and dissolution cores and partially resorbed rims in amphiboles indicate pervasive disequilibrium conditions. Feldspar (~An₃₅₋₉₈) and amphibole (~1.1-1.5 ͥ ͮ Al) compositions are relatively continuous across a broad range, and pyroxene compositions are typically ~En₄₂₋₄₉ and En₆₅₋₇₄. Phenocryst assemblages of units Qawc, Qapb, Qdpb, and Qbawh were probed to assess pressure and temperature conditions of pre-eruptive magmas in the MJA. Estimates from amphiboles, feldspars, and pyroxenes indicate temperatures ranging from ~650 to 1100 °C. Pressure estimates indicate crystallization depths of ~3 to 25 km, with the majority of crystallization occurring between ~15 and 25 km. Pyroxene temperatures (Putirka model) are always the highest, (~950-1150 °C) and plagioclase-amphibole pair (HBAS model) temperatures are the lowest (~650-875 °C), with amphibole temperatures (Ridolfi model) falling in between those ranges. Calculated partition coefficients of Sr and Ba from this study range from 1.5 to 6.75 and 0.12 to 1.00, respectively, in close agreement with calculated partition coefficients of Bindeman et al. (1998) and Dohmen and Blundy (2014). Reconstructed Sr concentrations range from 227 to 799 µg/g, which is inconsistent with the melting of a Sr-rich andesite end-member produced at 30-40 km depth as proposed by Conrey et al. (2001). The diverse spread of data reported here suggests complex petrologic mixing processes predominantly occurring in the midto upper crust beneath the MJA and contributes to the understanding of pre-eruptive magmatic conditions in the Cascade volcanic arc Title: Reconstructing the Physical Record of a Four-Million-Year Volcanic System : Geochemistry, Thermobarometry, and Geologic Map of the Mount Jefferson Area, Oregon Author: DiGiulio, Jennifer Volcanic and sedimentary deposits of the Mount Jefferson area (MJA) record a fourmillion-year history of arc-related volcanism related to the subduction of the Juan de Fuca plate beneath North America. 171 mapped stratigraphic units over an area of 150 km² reveal four periods of volcanic activity resulting in diverse composition lavas ranging from ~48 to 72 wt% SiO₂. Eruptive periods are divided into (1) ~4.0 - 0.78 Ma; (2) 0.78 - 0.3 Ma; (3) 0.3 - 0.02 Ma; and (4) eruptions of the past 13,000 years. Repeated glaciations over the past 800,000 years have sculpted the landscape of the MJA and include the Pleistocene glaciations of Jack Creek (early Wisconsin) and Cabot Creek (late Wisconsin), and the Holocene glaciation of Jefferson Park. Anomalously glassy lava flows, columnar jointing, and streamlined shapes are lithologic evidence of intraglacial eruptions in numerous units, including the andesite of Whitewater Creek (Qawc), andesite and dacite of Park Butte (Qapb; Qdpb), and the basaltic andesite of Whiskey Creek (Qbawh). Mineral compositions and textures are highly variable among the four units. Patchy to oscillatory zoning in plagioclase and dissolution cores and partially resorbed rims in amphiboles indicate pervasive disequilibrium conditions. Feldspar (~An₃₅₋₉₈) and amphibole (~1.1-1.5 ͥ ͮ Al) compositions are relatively continuous across a broad range, and pyroxene compositions are typically ~En₄₂₋₄₉ and En₆₅₋₇₄. Phenocryst assemblages of units Qawc, Qapb, Qdpb, and Qbawh were probed to assess pressure and temperature conditions of pre-eruptive magmas in the MJA. Estimates from amphiboles, feldspars, and pyroxenes indicate temperatures ranging from ~650 to 1100 °C. Pressure estimates indicate crystallization depths of ~3 to 25 km, with the majority of crystallization occurring between ~15 and 25 km. Pyroxene temperatures (Putirka model) are always the highest, (~950-1150 °C) and plagioclase-amphibole pair (HBAS model) temperatures are the lowest (~650-875 °C), with amphibole temperatures (Ridolfi model) falling in between those ranges. Calculated partition coefficients of Sr and Ba from this study range from 1.5 to 6.75 and 0.12 to 1.00, respectively, in close agreement with calculated partition coefficients of Bindeman et al. (1998) and Dohmen and Blundy (2014). Reconstructed Sr concentrations range from 227 to 799 µg/g, which is inconsistent with the melting of a Sr-rich andesite end-member produced at 30-40 km depth as proposed by Conrey et al. (2001). The diverse spread of data reported here suggests complex petrologic mixing processes predominantly occurring in the midto upper crust beneath the MJA and contributes to the understanding of pre-eruptive magmatic conditions in the Cascade volcanic arc Close

• 152. [Article] Geology of pluvial Lake Chewaucan, Lake County, Oregon

  Pluvial Lake Chewaucan was a late Pleistocene lake, as much as 375 feet deep, covering 480 square miles in the northwestern part of the Great Basin in southern Oregon. The lake basin, now occupied by Summer ...

  Citation × Citation Citation Abstract Copy Title: Geology of pluvial Lake Chewaucan, Lake County, Oregon Author: Allison, Ira Shimmin, 1895- Pluvial Lake Chewaucan was a late Pleistocene lake, as much as 375 feet deep, covering 480 square miles in the northwestern part of the Great Basin in southern Oregon. The lake basin, now occupied by Summer Lake, Upper and Lower Chewaucan Marshes, and Lake Abert, was formed by down-dropped fault blocks bounded by imposing fault scarps, notably Winter Ridge and Abert Rim. Several large landslides occurred along the east side of Winter Ridge. Lake Chewaucan shore features include wave-cut cliffs and caves, beaches, terraces, bay bars, spits (as at The Narrows), and a huge alluvial fan built by Chewaucan River at Paisley. Later, at lower lake stages, part of the fan deposit of sand and gravel was distributed across four-mile-wide Paisley Flat, which subsequently became a divide between Winter Lake in the Summer Lake basin and ZX Lake (new name) in the Chewaucan Marshes-Lake Abert part of the Lake Chewaucan basin. Overflow from ZX Lake later cut a shallow channel across the divide enroute to Winter Lake. The bottom sediments of Lake Chewaucan are exposed mainly in the bluffs of Ana River, the main source of Summer Lake water. The stratigraphic section there is about 54 feet thick and composed mainly of silt, with numerous seams of sand, oolites, occasional pebbles, and many layers of volcanic ash, especially near the top. Fossils found in the area include 1) mammals and birds obtained from man-occupied caves near Paisley, 2) ostracods, diatoms, and small mollusks in the Ana River section, 3) similar tiny snail shells in a gravel pit north of the Ana Springs Reservoir, and 4) additional shells from the 4425-foot level near Ten Mile Butte east of Summer Lake. The snail shells have radiocarbon ages of >25,900, 22,000, and 17,500 years - all within the span of the Tioga-Pinedale glacial stage of the Sierra Nevada and the Rocky Mountains. The top 4520-foot shoreline, the 4485-foot beach and Paisley Caves, and the bulk of the Paisley fan may possibly be Tahoe in age, but the wave erosion of the Paisley fan, development of Paisley Flat, overflow from ZX Lake, and later formation of ZX Red House beach are assigned to Tioga-Pinedale time. The history of Lake Chewaucan is thought to be analogous to those of Lake Bonneville, Lake Lahontan, and Searles Lake, and correlative with climatic changes recorded in marine deposits. The post-Lake Chewaucan history of the basin includes Anathermal, Altithermal, and Medithermal climatic changes, as shown by a pollen profile in Upper Chewaucan Marsh. Mount Mazama pumice sand fell in the area about 6,600-6,700 years ago. Desiccation and wind work were strong in Altithermal time. In the Neopluvial (new term), corresponding to Neoglaciation in the mountains (perhaps 4,000-2,000 years ago), new lakes many tens of feet deep developed in the Summer Lake and Chewaucan Marshes-Lake Abert basins. Later, Summer Lake and Lake Abert were reduced to the very shallow, alkaline bodies of water of the present day. Title: Geology of pluvial Lake Chewaucan, Lake County, Oregon Author: Allison, Ira Shimmin, 1895- Pluvial Lake Chewaucan was a late Pleistocene lake, as much as 375 feet deep, covering 480 square miles in the northwestern part of the Great Basin in southern Oregon. The lake basin, now occupied by Summer Lake, Upper and Lower Chewaucan Marshes, and Lake Abert, was formed by down-dropped fault blocks bounded by imposing fault scarps, notably Winter Ridge and Abert Rim. Several large landslides occurred along the east side of Winter Ridge. Lake Chewaucan shore features include wave-cut cliffs and caves, beaches, terraces, bay bars, spits (as at The Narrows), and a huge alluvial fan built by Chewaucan River at Paisley. Later, at lower lake stages, part of the fan deposit of sand and gravel was distributed across four-mile-wide Paisley Flat, which subsequently became a divide between Winter Lake in the Summer Lake basin and ZX Lake (new name) in the Chewaucan Marshes-Lake Abert part of the Lake Chewaucan basin. Overflow from ZX Lake later cut a shallow channel across the divide enroute to Winter Lake. The bottom sediments of Lake Chewaucan are exposed mainly in the bluffs of Ana River, the main source of Summer Lake water. The stratigraphic section there is about 54 feet thick and composed mainly of silt, with numerous seams of sand, oolites, occasional pebbles, and many layers of volcanic ash, especially near the top. Fossils found in the area include 1) mammals and birds obtained from man-occupied caves near Paisley, 2) ostracods, diatoms, and small mollusks in the Ana River section, 3) similar tiny snail shells in a gravel pit north of the Ana Springs Reservoir, and 4) additional shells from the 4425-foot level near Ten Mile Butte east of Summer Lake. The snail shells have radiocarbon ages of >25,900, 22,000, and 17,500 years - all within the span of the Tioga-Pinedale glacial stage of the Sierra Nevada and the Rocky Mountains. The top 4520-foot shoreline, the 4485-foot beach and Paisley Caves, and the bulk of the Paisley fan may possibly be Tahoe in age, but the wave erosion of the Paisley fan, development of Paisley Flat, overflow from ZX Lake, and later formation of ZX Red House beach are assigned to Tioga-Pinedale time. The history of Lake Chewaucan is thought to be analogous to those of Lake Bonneville, Lake Lahontan, and Searles Lake, and correlative with climatic changes recorded in marine deposits. The post-Lake Chewaucan history of the basin includes Anathermal, Altithermal, and Medithermal climatic changes, as shown by a pollen profile in Upper Chewaucan Marsh. Mount Mazama pumice sand fell in the area about 6,600-6,700 years ago. Desiccation and wind work were strong in Altithermal time. In the Neopluvial (new term), corresponding to Neoglaciation in the mountains (perhaps 4,000-2,000 years ago), new lakes many tens of feet deep developed in the Summer Lake and Chewaucan Marshes-Lake Abert basins. Later, Summer Lake and Lake Abert were reduced to the very shallow, alkaline bodies of water of the present day. Close

• 153. [Article] Spring temperature responses of oaks are synchronous with North Atlantic conditions during the last deglaciation

  To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Ecological ...

  Citation × Citation Citation Abstract Copy Title: Spring temperature responses of oaks are synchronous with North Atlantic conditions during the last deglaciation Author: Noirot-Cosson, Paul-Emile, Stambaugh, Michael C., McMurry, Erin R., Lachenbruch, Barbara, Voelker, Steven L., Meinzer, Frederick C., Guyette, Richard P. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Ecological Society of America and can be found at: http://www.esa.org/. Title: Spring temperature responses of oaks are synchronous with North Atlantic conditions during the last deglaciation Author: Noirot-Cosson, Paul-Emile, Stambaugh, Michael C., McMurry, Erin R., Lachenbruch, Barbara, Voelker, Steven L., Meinzer, Frederick C., Guyette, Richard P. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by Ecological Society of America and can be found at: http://www.esa.org/. Close

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

  Six Tertiary units are exposed in the Onion Peak area near the town of Cannon Beach., Oregon. The units consist of: late Eocene to early Miocene Oswald West mudstones (informal), middle Miocene Astoria ...

  Citation × Citation Citation Abstract Copy Title: Stratigraphy and sedimentation of the Onion Peak area, Clatsop County, Oregon Author: Smith, Thomas N. Six Tertiary units are exposed in the Onion Peak area near the town of Cannon Beach., Oregon. The units consist of: late Eocene to early Miocene Oswald West mudstones (informal), middle Miocene Astoria Formation (Angora Peak sandstone and Silver Point mudstone members.-informal), middle Miocene basalts of intrusive and extrusive Depoe Bay Basalt, and intrusive Cape Foulweather Basalt. Beach sands, Pleistocene marine terraces, and stream alluvium unconformably overlie these Tertiary units. The Oswald West mudstones consist of more than 1600 feet of well-bedded, burrowed, tuffaceou siltstones and mudstones with sub ordinate tuff and glauconitic sandstone beds. Foraminiferal and trace fossil evidence suggest that these mudstones were deposited in marine waters of upper bathyal depths. The overlying 1700-foot thick Astoria Formation is divided into two informal mappable members: the 1100-foot thick Angora Peak sandstone and the 600-foot thick Silver Point mudstone (proposed informally in this study). The Angora Peak sandstone member is composed of several hundred feet of thickly laminated feldspathic sandstones with local cross-bedded lithic conglomerates and carbonaceous siltstones. The overlying Silver Point mudstone member consists of rhythmically interbedded mudstones and graded turbidite sandstones, overlain by bedded mudstones, thin siltstones, and local conglomerate lenses. Conglomerate clast lithologies and heavy mineral suites of the Silver Point and Angora. Peak members are similar to the sediment carried by the Columbia River today and may be an ancient deposit of that river system. The provenances for these strata were pre-Miocene andesitic and dacitic rocks of the western Cascades, the Coast Range Eocene Tillamook basalts, and plutonic, metamorphic, and sedimentary contributions from eastern Oregon and Washington, Idaho, Montana, and Canada, Channel fluvial sandstones, conglomerates, and well-bedded shallow marine sandstones of the Angora Peak member, probably interfingering with interbedded, deeper marine (600 feet-outer shelf) mudstones and turbidite sndstones of the Silver Point member, are interpreted to have been deposited near the mouth of a river, adjacent to the ocean as a delta. Dikes, sills, peperites, and irregular intrusive bodies of middle Miocene Depoe Bay Basalt intruded Oswald West mudstones and the Astoria Formation. These aphanitic to finely crystalline equigranular basaltic intrusives locally fed over 2000 feet of palagonitized pillow lavas and basaltic breccias which now form the highest peaks in the area. The intrusives locally penecontemporaneously deformed the Astoria strata into a series of large-scale soft sediment deformation folds and sedimentary breccias. Cape Foulweather Basalt intrudes all sedimentary units and cuts the Depoe Bay Basalts. The basalt is recognized by sparse, large plagioclase phenocrysts. The area is cut by several east-west and north-south trending high angle faults with up to 500 feet displacement A large north-south syncline in the central part of the thesis area delineates a structural and depositional basin. A smaller north-plunging anticline is present near the coast. Recent landslides in the Silver Point member have been particularly destructive along the coast and inland; they have been caused by wave, stream action, and by man over-steepening unstable slopes. Crushed basalt quarry rock and potential petroleum reservoirs in the Angora Peak sandstones in stratigraphic and structural tra traps, particularly in nearby offshore areas, are the main geological economic resources of the area. Title: Stratigraphy and sedimentation of the Onion Peak area, Clatsop County, Oregon Author: Smith, Thomas N. Six Tertiary units are exposed in the Onion Peak area near the town of Cannon Beach., Oregon. The units consist of: late Eocene to early Miocene Oswald West mudstones (informal), middle Miocene Astoria Formation (Angora Peak sandstone and Silver Point mudstone members.-informal), middle Miocene basalts of intrusive and extrusive Depoe Bay Basalt, and intrusive Cape Foulweather Basalt. Beach sands, Pleistocene marine terraces, and stream alluvium unconformably overlie these Tertiary units. The Oswald West mudstones consist of more than 1600 feet of well-bedded, burrowed, tuffaceou siltstones and mudstones with sub ordinate tuff and glauconitic sandstone beds. Foraminiferal and trace fossil evidence suggest that these mudstones were deposited in marine waters of upper bathyal depths. The overlying 1700-foot thick Astoria Formation is divided into two informal mappable members: the 1100-foot thick Angora Peak sandstone and the 600-foot thick Silver Point mudstone (proposed informally in this study). The Angora Peak sandstone member is composed of several hundred feet of thickly laminated feldspathic sandstones with local cross-bedded lithic conglomerates and carbonaceous siltstones. The overlying Silver Point mudstone member consists of rhythmically interbedded mudstones and graded turbidite sandstones, overlain by bedded mudstones, thin siltstones, and local conglomerate lenses. Conglomerate clast lithologies and heavy mineral suites of the Silver Point and Angora. Peak members are similar to the sediment carried by the Columbia River today and may be an ancient deposit of that river system. The provenances for these strata were pre-Miocene andesitic and dacitic rocks of the western Cascades, the Coast Range Eocene Tillamook basalts, and plutonic, metamorphic, and sedimentary contributions from eastern Oregon and Washington, Idaho, Montana, and Canada, Channel fluvial sandstones, conglomerates, and well-bedded shallow marine sandstones of the Angora Peak member, probably interfingering with interbedded, deeper marine (600 feet-outer shelf) mudstones and turbidite sndstones of the Silver Point member, are interpreted to have been deposited near the mouth of a river, adjacent to the ocean as a delta. Dikes, sills, peperites, and irregular intrusive bodies of middle Miocene Depoe Bay Basalt intruded Oswald West mudstones and the Astoria Formation. These aphanitic to finely crystalline equigranular basaltic intrusives locally fed over 2000 feet of palagonitized pillow lavas and basaltic breccias which now form the highest peaks in the area. The intrusives locally penecontemporaneously deformed the Astoria strata into a series of large-scale soft sediment deformation folds and sedimentary breccias. Cape Foulweather Basalt intrudes all sedimentary units and cuts the Depoe Bay Basalts. The basalt is recognized by sparse, large plagioclase phenocrysts. The area is cut by several east-west and north-south trending high angle faults with up to 500 feet displacement A large north-south syncline in the central part of the thesis area delineates a structural and depositional basin. A smaller north-plunging anticline is present near the coast. Recent landslides in the Silver Point member have been particularly destructive along the coast and inland; they have been caused by wave, stream action, and by man over-steepening unstable slopes. Crushed basalt quarry rock and potential petroleum reservoirs in the Angora Peak sandstones in stratigraphic and structural tra traps, particularly in nearby offshore areas, are the main geological economic resources of the area. Close

• 155. [Article] The geology of part of the Snake River Canyon and adjacent areas in Northeastern Oregon and Western Idaho

  The mapped area lies between the Wallowa Mountains of northeastern Oregon and the Seven Devils Mountains of western Idaho. Part of the Snake River canyon is included. A composite stratigraphic section ...

  Citation × Citation Citation Abstract Copy Title: The geology of part of the Snake River Canyon and adjacent areas in Northeastern Oregon and Western Idaho Author: Vallier, Tracy L. (Tracy Lowell), 1936- The mapped area lies between the Wallowa Mountains of northeastern Oregon and the Seven Devils Mountains of western Idaho. Part of the Snake River canyon is included. A composite stratigraphic section includes at least 30,000 feet of strata. Pre- Tertiary and Tertiary strata are separated by a profound unconformity. Pre -Tertiary layered rocks are mostly Permian and Triassic volcaniclastic and volcanic flow rocks. At least four pre -Tertiary intrusive suites occur. Tertiary rocks are Miocene and Pliocene plateau basalts. Quaternary glacial materials and stream deposits locally mantle the older rocks. Permian ( ?) rocks of the Windy Ridge Formation are the oldest rocks and consist of 2,000 to 3,000 feet of keratophyre, quartz keratophyre, and keratophric pyroclastic rocks. Unconformably ( ?) overlying the Windy Ridge Formation are 8,000 to 10,000 feet of volcaniclastic rocks and minor volcanic flow rocks of the Hunsaker Creek Formation of Middle Permian (Leonardian and Wordian) age. Spilitic flow rocks of the Kleinschmidt Volcanics are interlayered with and in part overlie the Hunsaker Creek Formation and comprise a sequence about 2,000 to 3,000 feet thick. The Paleozoic layered rocks were intruded by the Holbrook - Irondyke intrusives, composed of keratophyre porphyry, quartz keratophyre porphyry, diabase, and gabbro. The Paleozoic rocks were deformed by an orogeny between Middle Permian and Middle Triassic time. Plutonic rocks (Oxbow Complex) of gabbro, quartz diorite, diorite, and albite granite were intruded during Early Triassic (7) time. Movements along the Oxbow -Cuprum shear zone occurred during and after the intrusions. Middle Triassic (Ladinian) spilitic flow rocks and volcaniclastic rocks of the Grassy Ridge Formation overlie the older rocks with angular unconformity. Thicknesses are 3,000 to 4,000 feet in the northeast part of the map area; no rocks of the Grassy Ridge Formation are exposed in the southwest part. The Imnaha Formation of Late Triassic (Karnian) age overlies the Permian strata unconformably near Fish Lake in the western part of the area. The Doyle Creek Formation of Late Triassic (Karnian) age conformably overlies the Grassy Ridge Formation in the Snake River and Imnaha River canyons and may interfinger with the Imnaha Formation east of Fish Lake. The Doyle Creek Formation ranges in thickness from 3,000 to 5,000 feet and includes two members - the Ashby Creek Conglomerate and the Piedmont Point Member. The Martin Bridge Formation, represented by 1,750 feet of Late Triassic (Norian) limestone, conformably overlies the Doyle Creek Formation. At least two intrusive events apparently occurred during the Jurassic Period. The Jurassic ( ?) intrusives, were emplaced before regional metamorphism and consist of hypabyssal dikes and sills of diorite, quartz diorite, and dacite and andesite porphyries. Subsequently, the Upper ( ?) Jurassic intrusives were emplaced during a late stage of regional metamorphism and are represented by small stocks of gabbro, norite, quartz diorite, and gran - odiorite porphyry. A major orogeny during Middle and Late ( ?) Jurassic time deformed the rocks. Regional metamorphism produced mineral assemblages characteristic of the greenschist facies. Columbia River Basalt, 2,000 to 3,000 feet thick, erupted from fissures during late Miocene and early Pliocene time and covered an old erosion surface. Pliocene - Pleistocene uplift, alpine glaciation, and extensive stream erosion are responsible for the present topography. Title: The geology of part of the Snake River Canyon and adjacent areas in Northeastern Oregon and Western Idaho Author: Vallier, Tracy L. (Tracy Lowell), 1936- The mapped area lies between the Wallowa Mountains of northeastern Oregon and the Seven Devils Mountains of western Idaho. Part of the Snake River canyon is included. A composite stratigraphic section includes at least 30,000 feet of strata. Pre- Tertiary and Tertiary strata are separated by a profound unconformity. Pre -Tertiary layered rocks are mostly Permian and Triassic volcaniclastic and volcanic flow rocks. At least four pre -Tertiary intrusive suites occur. Tertiary rocks are Miocene and Pliocene plateau basalts. Quaternary glacial materials and stream deposits locally mantle the older rocks. Permian ( ?) rocks of the Windy Ridge Formation are the oldest rocks and consist of 2,000 to 3,000 feet of keratophyre, quartz keratophyre, and keratophric pyroclastic rocks. Unconformably ( ?) overlying the Windy Ridge Formation are 8,000 to 10,000 feet of volcaniclastic rocks and minor volcanic flow rocks of the Hunsaker Creek Formation of Middle Permian (Leonardian and Wordian) age. Spilitic flow rocks of the Kleinschmidt Volcanics are interlayered with and in part overlie the Hunsaker Creek Formation and comprise a sequence about 2,000 to 3,000 feet thick. The Paleozoic layered rocks were intruded by the Holbrook - Irondyke intrusives, composed of keratophyre porphyry, quartz keratophyre porphyry, diabase, and gabbro. The Paleozoic rocks were deformed by an orogeny between Middle Permian and Middle Triassic time. Plutonic rocks (Oxbow Complex) of gabbro, quartz diorite, diorite, and albite granite were intruded during Early Triassic (7) time. Movements along the Oxbow -Cuprum shear zone occurred during and after the intrusions. Middle Triassic (Ladinian) spilitic flow rocks and volcaniclastic rocks of the Grassy Ridge Formation overlie the older rocks with angular unconformity. Thicknesses are 3,000 to 4,000 feet in the northeast part of the map area; no rocks of the Grassy Ridge Formation are exposed in the southwest part. The Imnaha Formation of Late Triassic (Karnian) age overlies the Permian strata unconformably near Fish Lake in the western part of the area. The Doyle Creek Formation of Late Triassic (Karnian) age conformably overlies the Grassy Ridge Formation in the Snake River and Imnaha River canyons and may interfinger with the Imnaha Formation east of Fish Lake. The Doyle Creek Formation ranges in thickness from 3,000 to 5,000 feet and includes two members - the Ashby Creek Conglomerate and the Piedmont Point Member. The Martin Bridge Formation, represented by 1,750 feet of Late Triassic (Norian) limestone, conformably overlies the Doyle Creek Formation. At least two intrusive events apparently occurred during the Jurassic Period. The Jurassic ( ?) intrusives, were emplaced before regional metamorphism and consist of hypabyssal dikes and sills of diorite, quartz diorite, and dacite and andesite porphyries. Subsequently, the Upper ( ?) Jurassic intrusives were emplaced during a late stage of regional metamorphism and are represented by small stocks of gabbro, norite, quartz diorite, and gran - odiorite porphyry. A major orogeny during Middle and Late ( ?) Jurassic time deformed the rocks. Regional metamorphism produced mineral assemblages characteristic of the greenschist facies. Columbia River Basalt, 2,000 to 3,000 feet thick, erupted from fissures during late Miocene and early Pliocene time and covered an old erosion surface. Pliocene - Pleistocene uplift, alpine glaciation, and extensive stream erosion are responsible for the present topography. Close

• 156. [Article] Structure and stratigraphy of the Argenta area, Beaverhead County, Montana

  The thesis area comprises 53 square miles located in Beaverhead County, Montana, and includes the Argenta mining district. Sedimentary and igneous rocks of the area range in age from Precambrian to Recent. ...

  Citation × Citation Citation Abstract Copy Title: Structure and stratigraphy of the Argenta area, Beaverhead County, Montana Author: Hobbs, Billy Bob The thesis area comprises 53 square miles located in Beaverhead County, Montana, and includes the Argenta mining district. Sedimentary and igneous rocks of the area range in age from Precambrian to Recent. The aggregate thickness of the sedimentary rocks is approximately 13,500 feet. The only systems not represented are Ordovician and Silurian. The oldest sedimentary rocks consist of Late Precambrian shallow marine and nonmarine subarkose, argillite, and ortho-quartzite of the Belt Supergroup. These strata are divided into three informal map units. The Paleozoic sedimentary rocks that overlie the Belt strata were deposited in a marine and in part nonmarine environment. The Cambrian system is represented by the Late Cambrian Hasmark Formation, succeeded disconformably by the Late Devonian Jefferson Dolomite and Three Forks Formation. Overlying the Three Forks is a thick sequence of Early to Late Mississippian limestones which were separated into the Lodgepole Formation and Mission Canyon Limestone. The Amsden Formation of Late Mississippian to Early Pennsylvanian age overlies the Mission Canyon with probable disconformity. The Pennsylvanian Quadrant Formation conformably succeeds the Amsden. The Permian Phosphoria Formation is considered to disconformably(?) overlie the Quadrant Formation. Mesozoic strata were deposited in continental and marine environments. The marine Dinwoody Formation of Early Triassic age is disconformably succeeded by the Late Jurassic continental Morrison Formation. The dominantly continental Early Cretaceous Kootenai Formation overlies the Morrison Formation with a gradational and arbitrarily located boundary. Marine strata of the Colorado Group (?) of Early and Late Cretaceous age overlie the Kootenai Formation with probable disconformity. Rocks representing the Tertiary System are dominantly igneous. The Argenta Stock and related plutons consist of quartz monzonite, granodiorite, and latite porphyry of probable Middle or Late Eocene age. The Argenta Stock is a composite pluton comprising quartz monzonite and quartz monzonite porphyry. Lithic welded tuff and an olivine basalt flow of possible Eocene-Oligocene age are found only in the northwest part of the area. Rhyolitic vitric tuff discomformably overlain by a limestone-pebble conglomerate, both of probable Miocene age, are present only in the eastern part of the area. Quaternary strata in the area include both glacial and fluvial deposits. Pleistocene glaciers, representing at least three glacial stades, produced a glacial trough with valley-in-valley profile, and left moraines in the northwest part of the area. Resulting outwash deposits are found in terraces above the present Rattlesnake Creek drainage. Quaternary landslide and alluvial deposits are locally present. The Laramide Orogeny has produced a complex series of folds and faults which locally trend anomalously northeast-southwest. The main structural features are the Kelly Thrust and the Argenta Anticline. Horizontal compression from the west and northwest produced a series of dominantly northeast-plunging anticlines and synclines broken by steep faults in addition to the major structures. Title: Structure and stratigraphy of the Argenta area, Beaverhead County, Montana Author: Hobbs, Billy Bob The thesis area comprises 53 square miles located in Beaverhead County, Montana, and includes the Argenta mining district. Sedimentary and igneous rocks of the area range in age from Precambrian to Recent. The aggregate thickness of the sedimentary rocks is approximately 13,500 feet. The only systems not represented are Ordovician and Silurian. The oldest sedimentary rocks consist of Late Precambrian shallow marine and nonmarine subarkose, argillite, and ortho-quartzite of the Belt Supergroup. These strata are divided into three informal map units. The Paleozoic sedimentary rocks that overlie the Belt strata were deposited in a marine and in part nonmarine environment. The Cambrian system is represented by the Late Cambrian Hasmark Formation, succeeded disconformably by the Late Devonian Jefferson Dolomite and Three Forks Formation. Overlying the Three Forks is a thick sequence of Early to Late Mississippian limestones which were separated into the Lodgepole Formation and Mission Canyon Limestone. The Amsden Formation of Late Mississippian to Early Pennsylvanian age overlies the Mission Canyon with probable disconformity. The Pennsylvanian Quadrant Formation conformably succeeds the Amsden. The Permian Phosphoria Formation is considered to disconformably(?) overlie the Quadrant Formation. Mesozoic strata were deposited in continental and marine environments. The marine Dinwoody Formation of Early Triassic age is disconformably succeeded by the Late Jurassic continental Morrison Formation. The dominantly continental Early Cretaceous Kootenai Formation overlies the Morrison Formation with a gradational and arbitrarily located boundary. Marine strata of the Colorado Group (?) of Early and Late Cretaceous age overlie the Kootenai Formation with probable disconformity. Rocks representing the Tertiary System are dominantly igneous. The Argenta Stock and related plutons consist of quartz monzonite, granodiorite, and latite porphyry of probable Middle or Late Eocene age. The Argenta Stock is a composite pluton comprising quartz monzonite and quartz monzonite porphyry. Lithic welded tuff and an olivine basalt flow of possible Eocene-Oligocene age are found only in the northwest part of the area. Rhyolitic vitric tuff discomformably overlain by a limestone-pebble conglomerate, both of probable Miocene age, are present only in the eastern part of the area. Quaternary strata in the area include both glacial and fluvial deposits. Pleistocene glaciers, representing at least three glacial stades, produced a glacial trough with valley-in-valley profile, and left moraines in the northwest part of the area. Resulting outwash deposits are found in terraces above the present Rattlesnake Creek drainage. Quaternary landslide and alluvial deposits are locally present. The Laramide Orogeny has produced a complex series of folds and faults which locally trend anomalously northeast-southwest. The main structural features are the Kelly Thrust and the Argenta Anticline. Horizontal compression from the west and northwest produced a series of dominantly northeast-plunging anticlines and synclines broken by steep faults in addition to the major structures. Close

• 157. [Article] Subsurface geology of the southeastern Cuyama Valley, southern Coast Ranges, California

  The southeastern Cuyama Valley is located in the southern Coast Ranges, west of Bakersfield, California. Subsurface geologic mapping using data from 62 prospect wells was integrated with surface mapping to ...

  Citation × Citation Citation Abstract Copy Title: Subsurface geology of the southeastern Cuyama Valley, southern Coast Ranges, California Author: Spitz, Herbert M. The southeastern Cuyama Valley is located in the southern Coast Ranges, west of Bakersfield, California. Subsurface geologic mapping using data from 62 prospect wells was integrated with surface mapping to determine the depositional and deformational history of this part of the Neogene Cuyama basin. The northwest-trending Russell fault system was active from late Oligocene to Pliocene time. Deformation along this major wrench system began during deposition of the late Oligocene to early Miocene Soda Lake Shale Member of the Vaqueros Formation and ended prior to deposition of the Plio-Pleistocene Morales Formation. The southern extension of this fault system remains uncertain, but it probably joins the Ozena fault southeast of the South Cuyama oil field. Removal of 16 to 18 miles of right-lateral offset on the Russell fault system restores the Ozena fault to a position directly east of the La Panza fault. These two faults were probably a single fault, designated the La Panza-Ozena fault, which was active, north-side down, during the Oligocene. Nonmarine conglomerates of the Simmler and lower Caliente formations were deposited north of this fault during the Oligocene, prior to the initiation of notion on the Russell fault system. Three major faults of the Cox fault zone on the west and the Lundstrom-Becker and Becker-Heller faults on the east define the margins of the Cox trough, a complex north- to northeast-trending graben north of the Russell fault which was active from Saucesian through Relizian time (early Miocene). The Saltos Shale Member of the Monterey Formation increases in thickness by 3 to 7 times across the margins of the trough. Growth faulting associated with the Cox trough ended prior to deposition of the middle Johnston sand on all faults except the Cox 46-5 fault, which was active through deposition of the Branch Canyon Sandstone. Post-Pliocene tectonism in the Cuyama basin was limited to folding and reverse-faulting in response to regional north-south compression. The Central Cuyama dome and associated north-dipping reverse faults formed during an early episode of this deformation. Subsequent deformation occurred on the Morales fault, the South Cuyama fault, and the rejuvenated Ozena fault. This Quaternary thrusting obscured the structures of the older wrench-tectonic regime and resulted in the formation of the Caliente and Sierra Madre Ranges and the present-day Cuyama Valley. Title: Subsurface geology of the southeastern Cuyama Valley, southern Coast Ranges, California Author: Spitz, Herbert M. The southeastern Cuyama Valley is located in the southern Coast Ranges, west of Bakersfield, California. Subsurface geologic mapping using data from 62 prospect wells was integrated with surface mapping to determine the depositional and deformational history of this part of the Neogene Cuyama basin. The northwest-trending Russell fault system was active from late Oligocene to Pliocene time. Deformation along this major wrench system began during deposition of the late Oligocene to early Miocene Soda Lake Shale Member of the Vaqueros Formation and ended prior to deposition of the Plio-Pleistocene Morales Formation. The southern extension of this fault system remains uncertain, but it probably joins the Ozena fault southeast of the South Cuyama oil field. Removal of 16 to 18 miles of right-lateral offset on the Russell fault system restores the Ozena fault to a position directly east of the La Panza fault. These two faults were probably a single fault, designated the La Panza-Ozena fault, which was active, north-side down, during the Oligocene. Nonmarine conglomerates of the Simmler and lower Caliente formations were deposited north of this fault during the Oligocene, prior to the initiation of notion on the Russell fault system. Three major faults of the Cox fault zone on the west and the Lundstrom-Becker and Becker-Heller faults on the east define the margins of the Cox trough, a complex north- to northeast-trending graben north of the Russell fault which was active from Saucesian through Relizian time (early Miocene). The Saltos Shale Member of the Monterey Formation increases in thickness by 3 to 7 times across the margins of the trough. Growth faulting associated with the Cox trough ended prior to deposition of the middle Johnston sand on all faults except the Cox 46-5 fault, which was active through deposition of the Branch Canyon Sandstone. Post-Pliocene tectonism in the Cuyama basin was limited to folding and reverse-faulting in response to regional north-south compression. The Central Cuyama dome and associated north-dipping reverse faults formed during an early episode of this deformation. Subsequent deformation occurred on the Morales fault, the South Cuyama fault, and the rejuvenated Ozena fault. This Quaternary thrusting obscured the structures of the older wrench-tectonic regime and resulted in the formation of the Caliente and Sierra Madre Ranges and the present-day Cuyama Valley. Close

• 158. [Article] Geology of part of the northeast quarter of the Mitchell quadrangle, Oregon

  The thesis area covers 165 square miles including the greater part of the northeastern quarter of the Mitchell quadrangle in north-central Oregon. The three stratigraphic units exposed in the area consist ...

  Citation × Citation Citation Abstract Copy Title: Geology of part of the northeast quarter of the Mitchell quadrangle, Oregon Author: Patterson, Robert Lynn The thesis area covers 165 square miles including the greater part of the northeastern quarter of the Mitchell quadrangle in north-central Oregon. The three stratigraphic units exposed in the area consist of a variety of Tertiary volcanic rocks including the Clarno Formation, the John Day Formation, and the Columbia River Basalt. The Clarno Formation of Eocene to Early Oligocene age is the oldest rock unit in the thesis area. From oldest to youngest, the mapped units include volcanic boulder conglomerates, andesite and basalt flows and flow breccias, tuffaceous sandstones intercalated with lava flows, and twelve plugs and dikes of andesite and basalt. The composition, lack of sorting, stratigraphic and structural features, and the included flora indicate that the volcanic boulder conglomerates were deposited as mudflows on a terrain of high relief in a humid, subtropical climate. Andesite lavas were extruded intermittently during deposition of the mudflows resulting in complex intertonguing of the two rock types. Later, Clarno lavas poured from numerous isolated fissures and covered the volcanic boulder conglomerates. Near Middle Oligocene time the formation was folded into a broad NE-trending anticline which swings sharply eastward in the north-central half of the area. Following uplift, the Clarno rocks were eroded into a surface of moderate relief and thick fossil soils developed on scoriaceous portions of the lava flows. The John Day Formation of Late Oligocene to Early Miocene age consists of ash flows and ash falls of acid to intermediate pyroclastic material which were deposited unconformably on the deformed and eroded Clarno rocks. These pyroclastics were erupted from unknown local sources, and probably from distant volcanoes to the west. Much of the material deposited on the pre-Miocene anticlinal crest was reworked by streams and formed thick tuff beds on the flanks of the fold. In Middle Miocene time the consolidated John Day Formation was slightly folded parallel to pre-Miocene anticlinal trends and underwent erosion. The Columbia River Basalt was extruded from fissures onto the tuffs during Middle Miocene time. Following deposition the thick layers of basalt were gently folded as the pre-Miocene anticlinal trend again served as the pattern for mild uplift and deformation in latest Miocene time. Rocks younger than Middle Miocene were either never deposited on the basalt plateaus of the area, or have been completely removed by erosion. In Pleistocene and Recent time the plateaus were dissected and the flows were eroded from the anticlinal crest. Landslide, fluvial, and terrace deposits were laid down in the valleys. White volcanic ash was transported by wind from an unknown source and intercalated with Recent fluvial and slope wash deposits. Title: Geology of part of the northeast quarter of the Mitchell quadrangle, Oregon Author: Patterson, Robert Lynn The thesis area covers 165 square miles including the greater part of the northeastern quarter of the Mitchell quadrangle in north-central Oregon. The three stratigraphic units exposed in the area consist of a variety of Tertiary volcanic rocks including the Clarno Formation, the John Day Formation, and the Columbia River Basalt. The Clarno Formation of Eocene to Early Oligocene age is the oldest rock unit in the thesis area. From oldest to youngest, the mapped units include volcanic boulder conglomerates, andesite and basalt flows and flow breccias, tuffaceous sandstones intercalated with lava flows, and twelve plugs and dikes of andesite and basalt. The composition, lack of sorting, stratigraphic and structural features, and the included flora indicate that the volcanic boulder conglomerates were deposited as mudflows on a terrain of high relief in a humid, subtropical climate. Andesite lavas were extruded intermittently during deposition of the mudflows resulting in complex intertonguing of the two rock types. Later, Clarno lavas poured from numerous isolated fissures and covered the volcanic boulder conglomerates. Near Middle Oligocene time the formation was folded into a broad NE-trending anticline which swings sharply eastward in the north-central half of the area. Following uplift, the Clarno rocks were eroded into a surface of moderate relief and thick fossil soils developed on scoriaceous portions of the lava flows. The John Day Formation of Late Oligocene to Early Miocene age consists of ash flows and ash falls of acid to intermediate pyroclastic material which were deposited unconformably on the deformed and eroded Clarno rocks. These pyroclastics were erupted from unknown local sources, and probably from distant volcanoes to the west. Much of the material deposited on the pre-Miocene anticlinal crest was reworked by streams and formed thick tuff beds on the flanks of the fold. In Middle Miocene time the consolidated John Day Formation was slightly folded parallel to pre-Miocene anticlinal trends and underwent erosion. The Columbia River Basalt was extruded from fissures onto the tuffs during Middle Miocene time. Following deposition the thick layers of basalt were gently folded as the pre-Miocene anticlinal trend again served as the pattern for mild uplift and deformation in latest Miocene time. Rocks younger than Middle Miocene were either never deposited on the basalt plateaus of the area, or have been completely removed by erosion. In Pleistocene and Recent time the plateaus were dissected and the flows were eroded from the anticlinal crest. Landslide, fluvial, and terrace deposits were laid down in the valleys. White volcanic ash was transported by wind from an unknown source and intercalated with Recent fluvial and slope wash deposits. Close

• 159. [Article] Phylogeography, evolution, and conservation in forest-associated Pacific Northwest salamanders

  Phylogeographic studies of six Pacific Northwest forest-associated salamanders provide insight into historical and contemporary processes on population genetic structure. Among Larch Mountain Salamanders ...

  Citation × Citation Citation Abstract Copy Title: Phylogeography, evolution, and conservation in forest-associated Pacific Northwest salamanders Author: Wagner, R. Steven Phylogeographic studies of six Pacific Northwest forest-associated salamanders provide insight into historical and contemporary processes on population genetic structure. Among Larch Mountain Salamanders (Plethodon larselli), cytochrome b mitochondrial (mtDNA) sequences (381 bp) and random amplified polymorphic DNA (RAPDs; 34 loci) supported separate Management Units for northern and southern populations (12 populations, N 184 individuals) as delineated by the Columbia River. Southern populations exhibited significantly reduced expected heterozygosity at RAPD loci, which may be a consequence of a founder event or bottleneck. Similarly, significant population structure was found in Oregon Slender Salamanders (Batrachoseps wrighti). Cytochrome b sequences (744 bp) revealed two historical lineages among 22 populations (N = 339 individuals). RAPD markers further differentiated mid-range populations. Therefore, overlapping Management Units are warranted for northern-most, mid-range, and southern-most populations. Phylogenetic relationships, taxonomic identity, and population differentiation was examined among four morphologically conserved Torrent Salamanders species (Family Rhyacotritonidae). Analysis of three mitochondrial genes (cytochrome b, 16S, and 12S ribosomal RNA) indicated each species represented a well-supported monophyletic group. Results agreed with allozyme data (Good et. al. 1987, Good and Wake 1992) suggesting three groups of Torrent Salamanders (Rhyacotriton variegatus, R. cascadae, and the ancestor of R. olympicus and R. kezeri) diverged during the Miocene. A more recent divergence appears to have occurred between R. olympicus and R. kezeri during the late Pliocene/early Pleistocene. Populations within R. variegatus appear to be as diverged as R. olympicus and R. kezeri, supporting conservation unit designation within R. variegatus. MtDNA 16S ribosomal RNA sequences and allozymes (5 loci) identified Cascade and Southern Torrent Salamanders recently discovered in the Central Oregon Cascades. Results indicate a range extension for both species and suggest the Middle Fork of the Willamette River may provide a geographic barrier to dispersal. Phylogenetic analyses of Southern Torrent Salamanders (72 localities) based upon cytochrome b sequences revealed three divergent clades (north coast, Oregon, and California) that coincide with possible geographic barriers to dispersal. Merging mtDNA results with previous allozyme studies provides support for an Evolutionary Significant Unit for the California dade and separate Management Units for the north coast and Oregon clades. Title: Phylogeography, evolution, and conservation in forest-associated Pacific Northwest salamanders Author: Wagner, R. Steven Phylogeographic studies of six Pacific Northwest forest-associated salamanders provide insight into historical and contemporary processes on population genetic structure. Among Larch Mountain Salamanders (Plethodon larselli), cytochrome b mitochondrial (mtDNA) sequences (381 bp) and random amplified polymorphic DNA (RAPDs; 34 loci) supported separate Management Units for northern and southern populations (12 populations, N 184 individuals) as delineated by the Columbia River. Southern populations exhibited significantly reduced expected heterozygosity at RAPD loci, which may be a consequence of a founder event or bottleneck. Similarly, significant population structure was found in Oregon Slender Salamanders (Batrachoseps wrighti). Cytochrome b sequences (744 bp) revealed two historical lineages among 22 populations (N = 339 individuals). RAPD markers further differentiated mid-range populations. Therefore, overlapping Management Units are warranted for northern-most, mid-range, and southern-most populations. Phylogenetic relationships, taxonomic identity, and population differentiation was examined among four morphologically conserved Torrent Salamanders species (Family Rhyacotritonidae). Analysis of three mitochondrial genes (cytochrome b, 16S, and 12S ribosomal RNA) indicated each species represented a well-supported monophyletic group. Results agreed with allozyme data (Good et. al. 1987, Good and Wake 1992) suggesting three groups of Torrent Salamanders (Rhyacotriton variegatus, R. cascadae, and the ancestor of R. olympicus and R. kezeri) diverged during the Miocene. A more recent divergence appears to have occurred between R. olympicus and R. kezeri during the late Pliocene/early Pleistocene. Populations within R. variegatus appear to be as diverged as R. olympicus and R. kezeri, supporting conservation unit designation within R. variegatus. MtDNA 16S ribosomal RNA sequences and allozymes (5 loci) identified Cascade and Southern Torrent Salamanders recently discovered in the Central Oregon Cascades. Results indicate a range extension for both species and suggest the Middle Fork of the Willamette River may provide a geographic barrier to dispersal. Phylogenetic analyses of Southern Torrent Salamanders (72 localities) based upon cytochrome b sequences revealed three divergent clades (north coast, Oregon, and California) that coincide with possible geographic barriers to dispersal. Merging mtDNA results with previous allozyme studies provides support for an Evolutionary Significant Unit for the California dade and separate Management Units for the north coast and Oregon clades. Close

• 160. [Article] Subsurface structural evolution along the northern Whittier fault zone of the eastern Los Angeles basin, Southern California

  The Whittier fault forms the central part of a fault system extending from the East Montebello fault at Whittier Narrows to the Elsinore fault, which is traced as far as the Mexican border. The Whittier ...

  Citation × Citation Citation Abstract Copy Title: Subsurface structural evolution along the northern Whittier fault zone of the eastern Los Angeles basin, Southern California Author: Herzog, David W. The Whittier fault forms the central part of a fault system extending from the East Montebello fault at Whittier Narrows to the Elsinore fault, which is traced as far as the Mexican border. The Whittier fault forms a restraining bend in this fault system, resulting in uplift of the Puente Hills. The northwestern part of the Whittier fault in the Whittier oil field in the eastern Los Angeles basin strikes approximately N65°W and dips 70-75° northeast. The fault is near the range front of the Puente Hills northwest of Turnbull Canyon, and within the Puente Hills to the southeast. The central reach of the Whittier fault had normal separation in the Relizian and Luisian stages of the middle Miocene. From the Mohnian through Repettian stages of the late Miocene and early Pliocene, little, if any, offset occurred until the initiation of reverse offset in the Venturian stage of the late Pliocene. A component of right-lateral strike-slip may have been added near the end of the Pliocene, coinciding with the formation of the Elsinore fault. The Workman Hill and Whittier Heights faults may have formed in the late Pliocene to early Pleistocene, coinciding with the possible initiation of strike-slip on the Whittier fault. The present sense of slip on the Whittier fault southeast of the study area is nearly pure right-lateral strike-slip, with a slip rate of 2-3 mm/yr. The northwestern part of the Whittier fault has a component of reverse slip of approximately 1 mm/yr. The amount of strike-slip on this part of the fault was not determined by this study. The Rideout Heights, 304, and 184 low-amplitude anticlines formed in the Whittier oil field area in the late Miocene and early Pliocene. The Rideout Heights anticline is a southwest-verging fault-propagation fold trending northwesterly from the mouth of Turnbull Canyon through the Rideout Heights area. Strata are overturned in the southwest limb of the fold, and normally dipping in the northeast limb; the fold has been cut along its hinge by the Whittier fault. The 304 and 184 anticlines are north-verging and appear to be beddingplane shear folds in the northeast limb of the La Habra syncline. Recent strike-slip on the Whittier fault may have reactivated the 184 anticline, causing uplift of the footwall block south of Turnbull Canyon. North of Turnbull Canyon, the Whittier fault is at the range front with no evidence of Quaternary footwall uplift. The 304 anticline could be a fault-propagation fold from a previously-unknown southwest dipping blind reverse fault south of the Whittier fault; uplift on this fold could also be the cause of footwall uplift south of Turnbull Canyon. Active fault traces, possibly strike-slip, are on or near the Whittier fault south of Turnbull Canyon, but to the north, recent offsets appear to be northeast of the Whittier fault in the Puente Hills. These offsets may represent an attempt of the Whittier fault to straighten itself by bypassing the restraining bend at Turnbull Canyon. so, this movement is too recent to offset conglomerate beds more than a few tens of meters. Title: Subsurface structural evolution along the northern Whittier fault zone of the eastern Los Angeles basin, Southern California Author: Herzog, David W. The Whittier fault forms the central part of a fault system extending from the East Montebello fault at Whittier Narrows to the Elsinore fault, which is traced as far as the Mexican border. The Whittier fault forms a restraining bend in this fault system, resulting in uplift of the Puente Hills. The northwestern part of the Whittier fault in the Whittier oil field in the eastern Los Angeles basin strikes approximately N65°W and dips 70-75° northeast. The fault is near the range front of the Puente Hills northwest of Turnbull Canyon, and within the Puente Hills to the southeast. The central reach of the Whittier fault had normal separation in the Relizian and Luisian stages of the middle Miocene. From the Mohnian through Repettian stages of the late Miocene and early Pliocene, little, if any, offset occurred until the initiation of reverse offset in the Venturian stage of the late Pliocene. A component of right-lateral strike-slip may have been added near the end of the Pliocene, coinciding with the formation of the Elsinore fault. The Workman Hill and Whittier Heights faults may have formed in the late Pliocene to early Pleistocene, coinciding with the possible initiation of strike-slip on the Whittier fault. The present sense of slip on the Whittier fault southeast of the study area is nearly pure right-lateral strike-slip, with a slip rate of 2-3 mm/yr. The northwestern part of the Whittier fault has a component of reverse slip of approximately 1 mm/yr. The amount of strike-slip on this part of the fault was not determined by this study. The Rideout Heights, 304, and 184 low-amplitude anticlines formed in the Whittier oil field area in the late Miocene and early Pliocene. The Rideout Heights anticline is a southwest-verging fault-propagation fold trending northwesterly from the mouth of Turnbull Canyon through the Rideout Heights area. Strata are overturned in the southwest limb of the fold, and normally dipping in the northeast limb; the fold has been cut along its hinge by the Whittier fault. The 304 and 184 anticlines are north-verging and appear to be beddingplane shear folds in the northeast limb of the La Habra syncline. Recent strike-slip on the Whittier fault may have reactivated the 184 anticline, causing uplift of the footwall block south of Turnbull Canyon. North of Turnbull Canyon, the Whittier fault is at the range front with no evidence of Quaternary footwall uplift. The 304 anticline could be a fault-propagation fold from a previously-unknown southwest dipping blind reverse fault south of the Whittier fault; uplift on this fold could also be the cause of footwall uplift south of Turnbull Canyon. Active fault traces, possibly strike-slip, are on or near the Whittier fault south of Turnbull Canyon, but to the north, recent offsets appear to be northeast of the Whittier fault in the Puente Hills. These offsets may represent an attempt of the Whittier fault to straighten itself by bypassing the restraining bend at Turnbull Canyon. so, this movement is too recent to offset conglomerate beds more than a few tens of meters. Close