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• 1. [Article] A stratigraphic and geochemical investigation of ferruginous bauxite deposits in the Salem Hills, Marion County, Oregon

  Pacific Northwest ferruginous bauxite deposits have formed in four main areas of northwestern Oregon and southwestern Washington by laterization of flows of the Columbia River Basalt Group (CRBG). The ...

  Citation × Citation Citation Abstract Copy Title: A stratigraphic and geochemical investigation of ferruginous bauxite deposits in the Salem Hills, Marion County, Oregon Author: Hoffman, Charles William Year: 1981 Pacific Northwest ferruginous bauxite deposits have formed in four main areas of northwestern Oregon and southwestern Washington by laterization of flows of the Columbia River Basalt Group (CRBG). The deposits, averaging 36.3% Al2O3 , 31.8% Fe2O3 , and 5.9% Sio2, generally occur near the surface of hilltops in gently rolling areas. Two very different views have been advanced regarding the setting in which the deposits have formed. The first hypothesis calls for a blanket-type laterization by erosion of much of the original deposit upon uplift and dissection of the area. Title: A stratigraphic and geochemical investigation of ferruginous bauxite deposits in the Salem Hills, Marion County, Oregon Author: Hoffman, Charles William Year: 1981 Pacific Northwest ferruginous bauxite deposits have formed in four main areas of northwestern Oregon and southwestern Washington by laterization of flows of the Columbia River Basalt Group (CRBG). The deposits, averaging 36.3% Al2O3 , 31.8% Fe2O3 , and 5.9% Sio2, generally occur near the surface of hilltops in gently rolling areas. Two very different views have been advanced regarding the setting in which the deposits have formed. The first hypothesis calls for a blanket-type laterization by erosion of much of the original deposit upon uplift and dissection of the area. Close

• 2. [Article] Geophysical and geochemical analyses of selected Miocene coastal basalt features, Clatsop County, Oregon

  The proximity of Miocene Columbia River basalts to the "locally-erupted" coastal Miocene basalts in northwestern Oregon, and the compelling similarities between the two groups, suggest that the coastal ...

  Citation × Citation Citation Abstract Copy Title: Geophysical and geochemical analyses of selected Miocene coastal basalt features, Clatsop County, Oregon Author: Pfaff, Virginia Josette Year: 1980 The proximity of Miocene Columbia River basalts to the "locally-erupted" coastal Miocene basalts in northwestern Oregon, and the compelling similarities between the two groups, suggest that the coastal basalts, rather than being locally erupted, may be the westward extension of plateau basalts derived from eastern Oregon and Washington. The local-origin hypothesis is based largely on the interpretation of coastal dikes and sills as representing vent areas; however, a complex mechanism, as yet unsatisfactorily defined, would be required to cause the eruption of virtually identical magmas simultaneously from source areas 500 km apart. This study, therefore, has investigated the coastal basalt intrusions both laterally and vertically. Geochemical and paleomagnetic analysis was used to determine the occurrence and distribution of basalt units; gravity surveys enabled an examination of the subsurface extensions of basalt intrusions in sedimentary rocks. Title: Geophysical and geochemical analyses of selected Miocene coastal basalt features, Clatsop County, Oregon Author: Pfaff, Virginia Josette Year: 1980 The proximity of Miocene Columbia River basalts to the "locally-erupted" coastal Miocene basalts in northwestern Oregon, and the compelling similarities between the two groups, suggest that the coastal basalts, rather than being locally erupted, may be the westward extension of plateau basalts derived from eastern Oregon and Washington. The local-origin hypothesis is based largely on the interpretation of coastal dikes and sills as representing vent areas; however, a complex mechanism, as yet unsatisfactorily defined, would be required to cause the eruption of virtually identical magmas simultaneously from source areas 500 km apart. This study, therefore, has investigated the coastal basalt intrusions both laterally and vertically. Geochemical and paleomagnetic analysis was used to determine the occurrence and distribution of basalt units; gravity surveys enabled an examination of the subsurface extensions of basalt intrusions in sedimentary rocks. Close

• 3. [Article] Geochemical Evolution of Ferruginous Bauxite Deposits in Northwestern Oregon and Southwestern Washington

  Ferruginous bauxite deposits developed from flows of the Columbia River Basalt Group in northwestern Oregon and southwestern Washington. Samples of the iron pisolite and the gibbsite nodular zones from ...

  Citation × Citation Citation Abstract Copy Title: Geochemical Evolution of Ferruginous Bauxite Deposits in Northwestern Oregon and Southwestern Washington Author: Fassio, Joseph Michael Year: 1990 Ferruginous bauxite deposits developed from flows of the Columbia River Basalt Group in northwestern Oregon and southwestern Washington. Samples of the iron pisolite and the gibbsite nodular zones from the upper portion of the weathering profile of drill core from Columbia County, Oregon and Cowlitz County, Washington, were analyzed by instrumental neutron activation. The mineralogy was determined using Xray diffraction and Mossbauer spectroscopy. The gibbsite nodular zone is above the clay-rich saprolite or relic basalt zone. The nodules contain relic vesicles and well preserved relic plagioclase microlites. Gibbsite occurs with poorly crystalline goethite and hematite in the gibbsite nodular zone. Clays are absent in this part of the profile . The iron pisolite zone is at the top of the profile above the gibbsite nodular zone. Both maghemite and goethite pisolites occur in the lower part of the zone while maghemite pisolites are dominant in the upper part of the pisolite zone. The parent flow is the Frenchman Springs Member of the Wanapum Basalt for the Columbia County profile and the Pomona Member of the Saddle Mountains Basalt for the Cowlitz County profile. Distribution of the major, minor and trace elements through the profile shows three distinct sympathetic patterns consisting of lanthanide elements and Na; As, Sbi Th, Hf, and Ta; and transition metals Fe, Ti, V, and Cr. Ratios between the high-field strength elements Ta and Hf are nearly constant through the profile, and Hf appears to be the least mobile elements of the elements analyzed in the profile. Ratios of other elements were calculated against Hf, based on the assumption that it has remained largely immobile during weathering, to show element enrichment and depletion in the profile independent of mass-volume changes. Transition metals and Al show a progressive depletion through the upper gibbsite nodular and iron pisolite zone due to leaching in the profile. Lanthanide elements (except Ce), As, and Sb show an obvious enrichment in the iron pisolite zone relative to the gibbsite nodular zone. Volume reduction during weathering was calculated based on the immobility of Hf. In the gibbsite nodular zone, the volume reduction calculated for bulk samples is greater than for gibbsite nodule separates suggesting that a greater volume reduction occurred the matrix material surrounding the nodules. Ratios between gibbsite nodules and parent basalt of the immobile elements Hf, Ta, Fe, Ti, Th and Cr suggest that the nodules, where the relic textures are preserved, have undergone volume reduction. Based on the immobility of Hf, the gibbsite nodules lost approximately 40% of the original volume. The volume factors based on the immobility of Hf show that the pisolite zone experienced a greater volume reduction than the gibbsite nodular zone. Absolute gains and losses relative to the parent basalt show the following relative order of depletion: Na > La > Eu > Sm > Co > Mn > Ce > Sc > Ta > V > Cr > Lu > Th > Fe > Ga > Al. The lesser mobility of Ce and Lu relative to other lanthanide elements suggests fractionation of lanthanide elements in the bauxite profile. Aluminum is both enriched and depleted at different depths in the gibbsite nodular zone suggesting that Al is mobilized from the matrix and possibly the pisolite zone into Al enriched gibbsite nodules. Volume reduction and destruction of relic textures in the pisolite zone is accompanied by small-scale mobilization of Th, Cr, Fe, Hf, Ga, Sc, and Ta during the formation of iron pisolites. Formation of the iron pisolite zone above the gibbsite nodular zone may indicate a change in climate from a heavy year round to a seasonal rainfall pattern. Title: Geochemical Evolution of Ferruginous Bauxite Deposits in Northwestern Oregon and Southwestern Washington Author: Fassio, Joseph Michael Year: 1990 Ferruginous bauxite deposits developed from flows of the Columbia River Basalt Group in northwestern Oregon and southwestern Washington. Samples of the iron pisolite and the gibbsite nodular zones from the upper portion of the weathering profile of drill core from Columbia County, Oregon and Cowlitz County, Washington, were analyzed by instrumental neutron activation. The mineralogy was determined using Xray diffraction and Mossbauer spectroscopy. The gibbsite nodular zone is above the clay-rich saprolite or relic basalt zone. The nodules contain relic vesicles and well preserved relic plagioclase microlites. Gibbsite occurs with poorly crystalline goethite and hematite in the gibbsite nodular zone. Clays are absent in this part of the profile . The iron pisolite zone is at the top of the profile above the gibbsite nodular zone. Both maghemite and goethite pisolites occur in the lower part of the zone while maghemite pisolites are dominant in the upper part of the pisolite zone. The parent flow is the Frenchman Springs Member of the Wanapum Basalt for the Columbia County profile and the Pomona Member of the Saddle Mountains Basalt for the Cowlitz County profile. Distribution of the major, minor and trace elements through the profile shows three distinct sympathetic patterns consisting of lanthanide elements and Na; As, Sbi Th, Hf, and Ta; and transition metals Fe, Ti, V, and Cr. Ratios between the high-field strength elements Ta and Hf are nearly constant through the profile, and Hf appears to be the least mobile elements of the elements analyzed in the profile. Ratios of other elements were calculated against Hf, based on the assumption that it has remained largely immobile during weathering, to show element enrichment and depletion in the profile independent of mass-volume changes. Transition metals and Al show a progressive depletion through the upper gibbsite nodular and iron pisolite zone due to leaching in the profile. Lanthanide elements (except Ce), As, and Sb show an obvious enrichment in the iron pisolite zone relative to the gibbsite nodular zone. Volume reduction during weathering was calculated based on the immobility of Hf. In the gibbsite nodular zone, the volume reduction calculated for bulk samples is greater than for gibbsite nodule separates suggesting that a greater volume reduction occurred the matrix material surrounding the nodules. Ratios between gibbsite nodules and parent basalt of the immobile elements Hf, Ta, Fe, Ti, Th and Cr suggest that the nodules, where the relic textures are preserved, have undergone volume reduction. Based on the immobility of Hf, the gibbsite nodules lost approximately 40% of the original volume. The volume factors based on the immobility of Hf show that the pisolite zone experienced a greater volume reduction than the gibbsite nodular zone. Absolute gains and losses relative to the parent basalt show the following relative order of depletion: Na > La > Eu > Sm > Co > Mn > Ce > Sc > Ta > V > Cr > Lu > Th > Fe > Ga > Al. The lesser mobility of Ce and Lu relative to other lanthanide elements suggests fractionation of lanthanide elements in the bauxite profile. Aluminum is both enriched and depleted at different depths in the gibbsite nodular zone suggesting that Al is mobilized from the matrix and possibly the pisolite zone into Al enriched gibbsite nodules. Volume reduction and destruction of relic textures in the pisolite zone is accompanied by small-scale mobilization of Th, Cr, Fe, Hf, Ga, Sc, and Ta during the formation of iron pisolites. Formation of the iron pisolite zone above the gibbsite nodular zone may indicate a change in climate from a heavy year round to a seasonal rainfall pattern. Close

• 4. [Article] Extension and volcanism : tectonic development of the northwestern margin of the Basin and Range Province in southern Oregon

  The northwestern corner of the Basin and Range Province (NWBR) lies in southeast Oregon where extensive Late Miocene, mafic to bimodal basalt-rhyolite volcanism and extensional faults dominate a stark ...

  Citation × Citation Citation Abstract Copy Title: Extension and volcanism : tectonic development of the northwestern margin of the Basin and Range Province in southern Oregon Author: Scarberry, Kaleb C. The northwestern corner of the Basin and Range Province (NWBR) lies in southeast Oregon where extensive Late Miocene, mafic to bimodal basalt-rhyolite volcanism and extensional faults dominate a stark and arid landscape. Near Lake Abert, the Late Miocene volcanic section abuts Early Miocene, dominantly intermediate composition volcanoes at the Coleman Hills and Rabbit Hills. Such exposures are few in the NWBR and shed light on the volcanic and structural development of the region and significantly extend the known distribution of Early Miocene magmatism. New 40Ar-39Ar ages indicate that magmatism occurred at the Coleman Hills between 21.4 and 22.3 Ma and at the Rabbit Hills between 20.2 and 23.4 Ma. Magmatism at the Coleman Hills initiated with an episode of silicic volcanism that was rapidly followed by the intrusion and emplacement of dacite and andesite dikes and lavas. Early Miocene rhyolite of the Coleman Hills exhibits high 87Sr/86Sri relative to most Late Miocene rocks in the region (0.7051 vs. 0.7037) suggesting that crustal melting was important in their genesis. A notable lack of a Nb-trough, characteristic of subduction zone settings, and high 207Pb/204Pb, and 206Pb/204Pb suggests that these rocks were not generated within a volcanic arc setting and instead likely formed in a back-arc region that had been metasomatized by prior subduction. Crystal fractionation readily accounts for the generation of andesite and dacite compositions from basaltic andesite however a gap in the data between ~66 to 70% SiO2, isotopic variability and disequilibrium textures indicate that crustal anatexis, magma mixing and/or crustal assimilation may also be important processes in the evolutionary history of the Coleman Hills. Dike orientations within Early Miocene volcanoes often mimic the trend of faults that cut Late Miocene volcanic strata, suggesting that the faults may have utilized an existing structural fabric. Contact relations suggest as much as 500 m of volcanic paleo-topography existed near Lake Abert prior to deposition of the Middle Miocene Steens flood basalts at ~16 Ma. Late Miocene structural development of the Abert Rim fault is chronologically linked to the formation and northward propagation of the Walker Lane transform fault system in Nevada between 9 and 3 Ma. At Lake Abert, bimodal basalt-rhyolite volcansim from 8.8 to 7.7 Ma occurred within a zone of active NW-striking faulting. Volcanism during this interval was related to the initiation of faulting at Abert Rim fault. On the basis of morphology and geochemical correlation, Juniper Mountain and Venator Buttes are the source of silicic and mafic volcanism, respectively, between 8.8 to 7.7 Ma. Partial restorations demonstrate that the Abert Rim fault south of 42º40’ N latitude had >250 m of stratigraphic separation prior to deposition of the Rattlesnake Tuff at 7 Ma. The topography north of latitude 42º40’ N was gently sloping at 7 Ma, much like the High Lava Plains are today. Thus, faulting along the NNE-striking Abert Rim initiated between 7.7 and 7 Ma and after the formation of NW-striking fault systems. Title: Extension and volcanism : tectonic development of the northwestern margin of the Basin and Range Province in southern Oregon Author: Scarberry, Kaleb C. The northwestern corner of the Basin and Range Province (NWBR) lies in southeast Oregon where extensive Late Miocene, mafic to bimodal basalt-rhyolite volcanism and extensional faults dominate a stark and arid landscape. Near Lake Abert, the Late Miocene volcanic section abuts Early Miocene, dominantly intermediate composition volcanoes at the Coleman Hills and Rabbit Hills. Such exposures are few in the NWBR and shed light on the volcanic and structural development of the region and significantly extend the known distribution of Early Miocene magmatism. New 40Ar-39Ar ages indicate that magmatism occurred at the Coleman Hills between 21.4 and 22.3 Ma and at the Rabbit Hills between 20.2 and 23.4 Ma. Magmatism at the Coleman Hills initiated with an episode of silicic volcanism that was rapidly followed by the intrusion and emplacement of dacite and andesite dikes and lavas. Early Miocene rhyolite of the Coleman Hills exhibits high 87Sr/86Sri relative to most Late Miocene rocks in the region (0.7051 vs. 0.7037) suggesting that crustal melting was important in their genesis. A notable lack of a Nb-trough, characteristic of subduction zone settings, and high 207Pb/204Pb, and 206Pb/204Pb suggests that these rocks were not generated within a volcanic arc setting and instead likely formed in a back-arc region that had been metasomatized by prior subduction. Crystal fractionation readily accounts for the generation of andesite and dacite compositions from basaltic andesite however a gap in the data between ~66 to 70% SiO2, isotopic variability and disequilibrium textures indicate that crustal anatexis, magma mixing and/or crustal assimilation may also be important processes in the evolutionary history of the Coleman Hills. Dike orientations within Early Miocene volcanoes often mimic the trend of faults that cut Late Miocene volcanic strata, suggesting that the faults may have utilized an existing structural fabric. Contact relations suggest as much as 500 m of volcanic paleo-topography existed near Lake Abert prior to deposition of the Middle Miocene Steens flood basalts at ~16 Ma. Late Miocene structural development of the Abert Rim fault is chronologically linked to the formation and northward propagation of the Walker Lane transform fault system in Nevada between 9 and 3 Ma. At Lake Abert, bimodal basalt-rhyolite volcansim from 8.8 to 7.7 Ma occurred within a zone of active NW-striking faulting. Volcanism during this interval was related to the initiation of faulting at Abert Rim fault. On the basis of morphology and geochemical correlation, Juniper Mountain and Venator Buttes are the source of silicic and mafic volcanism, respectively, between 8.8 to 7.7 Ma. Partial restorations demonstrate that the Abert Rim fault south of 42º40’ N latitude had >250 m of stratigraphic separation prior to deposition of the Rattlesnake Tuff at 7 Ma. The topography north of latitude 42º40’ N was gently sloping at 7 Ma, much like the High Lava Plains are today. Thus, faulting along the NNE-striking Abert Rim initiated between 7.7 and 7 Ma and after the formation of NW-striking fault systems. Close

• 5. [Article] Rhyolitic magmatism of the High Lava Plains and adjacent Northwest Basin and Range, Oregon : implications for the evolution of continental crust

  Understanding continental crust formation and modification is a fundamental and longstanding geologic problem. Influx of mantle-derived basaltic magma and partial melting of the crust are two ways to drive ...

  Citation × Citation Citation Abstract Copy Title: Rhyolitic magmatism of the High Lava Plains and adjacent Northwest Basin and Range, Oregon : implications for the evolution of continental crust Author: Ford, Mark T., 1973- Understanding continental crust formation and modification is a fundamental and longstanding geologic problem. Influx of mantle-derived basaltic magma and partial melting of the crust are two ways to drive crustal differentiation. This process results in a low density upper crust and denser, more refractory lower crust, creating significant and vastly different geochemical reservoirs over time. The High Lava Plains (HLP) and Northwestern Basin and Range (NWBR) in central and eastern Oregon provide an excellent example of intraplate volcanism where we can examine the beginnings of segregation of a relatively young, recently accreted crust. The origins of continental magmatism and its relationship to plate tectonics, especially away from the continental margins, are only slowly becoming revealed. The western United States is the most volcanically active part of North America during Cenozoic time, and this activity includes the enigmatic volcanism of the HLP and NWBR. Rhyolitic volcanism in the HLP and NWBR is age-progressive but in a direction that is nearly perpendicular to North American Plate motion. Despite being erupted through a similar crust and with a similar composition of mafic input, the HLP province is strongly bimodal (basalt-rhyolite) while the NWBR province exhibits a continuum of compositions. High silica rhyolites are commonplace in the HLP, with approximately a 1:1 ratio of rhyolite to basalt, even though the crust is comprised of mafic accreted terranes. Asthenospheric flow, mantle melting and crustal extension coupled with southwesterly North American plate motion explain the age-progressive volcanism of the HLP and NWBR. Differential asthenopheric counterflow and mantle upwelling created by the down-going Cascadia slab, coupled with transtensional stresses related to the rotation of the North American plate and Basin and Range extension, decreasing to the north, can produce the observed variations in rhyolite compositions and volumes in the two adjacent provinces. These differences are caused by fundamentally different petrogenetic processes that take place in the crust. In the HLP, an increase in mantle-derived magma flux into the lower crust has created low silica rhyolite via partial melt that separated, coalesced and rose buoyantly. This low silica rhyolite may erupt, solidify in the upper crust, or differentiate by fractional crystallization to produce high-iron, high-silica rhyolite containing an anhydrous phase assemblage. In the NWBR, a smaller flux of basaltic magma, coupled with greater transtension resulted in small crustal processing zones where fractional crystallization coupled with magma mixing and recharge created a wide range of compositions. Partial melting to form rhyolites was limited. These rhyolites have lower iron, and hydrous phases (biotite, amphibole) are common. These processes modify the crust in different ways, leaving a stratified crust in the HLP but a less modified crust in the NWBR. Recent geophysical and isotopic studies bear out these differences and allow for a unified, internally consistent model for both provinces, one that relies only on partial melt generation driven by current plate movements and do not require a mantle plume contribution. The bimodal volcanism of the HLP is a direct consequence of the processes that cause the gravitational differentiation of the continental crust into upper and lower units. The model for the HLP is generally applicable to other localities that have predominantly mafic crust and a similar balance of crustal transtension and mantle-derived basaltic flux. One such place is Iceland, which has strongly bimodal (basalt – rhyolite) volcanism. In areas where silicic crust has become substantially more mafic due to a high flux of intraplated basalts, such as in the bimodal Snake River Plain, the model is also applicable. Title: Rhyolitic magmatism of the High Lava Plains and adjacent Northwest Basin and Range, Oregon : implications for the evolution of continental crust Author: Ford, Mark T., 1973- Understanding continental crust formation and modification is a fundamental and longstanding geologic problem. Influx of mantle-derived basaltic magma and partial melting of the crust are two ways to drive crustal differentiation. This process results in a low density upper crust and denser, more refractory lower crust, creating significant and vastly different geochemical reservoirs over time. The High Lava Plains (HLP) and Northwestern Basin and Range (NWBR) in central and eastern Oregon provide an excellent example of intraplate volcanism where we can examine the beginnings of segregation of a relatively young, recently accreted crust. The origins of continental magmatism and its relationship to plate tectonics, especially away from the continental margins, are only slowly becoming revealed. The western United States is the most volcanically active part of North America during Cenozoic time, and this activity includes the enigmatic volcanism of the HLP and NWBR. Rhyolitic volcanism in the HLP and NWBR is age-progressive but in a direction that is nearly perpendicular to North American Plate motion. Despite being erupted through a similar crust and with a similar composition of mafic input, the HLP province is strongly bimodal (basalt-rhyolite) while the NWBR province exhibits a continuum of compositions. High silica rhyolites are commonplace in the HLP, with approximately a 1:1 ratio of rhyolite to basalt, even though the crust is comprised of mafic accreted terranes. Asthenospheric flow, mantle melting and crustal extension coupled with southwesterly North American plate motion explain the age-progressive volcanism of the HLP and NWBR. Differential asthenopheric counterflow and mantle upwelling created by the down-going Cascadia slab, coupled with transtensional stresses related to the rotation of the North American plate and Basin and Range extension, decreasing to the north, can produce the observed variations in rhyolite compositions and volumes in the two adjacent provinces. These differences are caused by fundamentally different petrogenetic processes that take place in the crust. In the HLP, an increase in mantle-derived magma flux into the lower crust has created low silica rhyolite via partial melt that separated, coalesced and rose buoyantly. This low silica rhyolite may erupt, solidify in the upper crust, or differentiate by fractional crystallization to produce high-iron, high-silica rhyolite containing an anhydrous phase assemblage. In the NWBR, a smaller flux of basaltic magma, coupled with greater transtension resulted in small crustal processing zones where fractional crystallization coupled with magma mixing and recharge created a wide range of compositions. Partial melting to form rhyolites was limited. These rhyolites have lower iron, and hydrous phases (biotite, amphibole) are common. These processes modify the crust in different ways, leaving a stratified crust in the HLP but a less modified crust in the NWBR. Recent geophysical and isotopic studies bear out these differences and allow for a unified, internally consistent model for both provinces, one that relies only on partial melt generation driven by current plate movements and do not require a mantle plume contribution. The bimodal volcanism of the HLP is a direct consequence of the processes that cause the gravitational differentiation of the continental crust into upper and lower units. The model for the HLP is generally applicable to other localities that have predominantly mafic crust and a similar balance of crustal transtension and mantle-derived basaltic flux. One such place is Iceland, which has strongly bimodal (basalt – rhyolite) volcanism. In areas where silicic crust has become substantially more mafic due to a high flux of intraplated basalts, such as in the bimodal Snake River Plain, the model is also applicable. Close