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161. [Article] Stratigraphy and sedimentation of the Neahkahnie Mountain - Angora Peak area, Tillamook and Clatsop Counties, Oregon
Four distinct lithologic units compose the Tertiary rocks of the Neahkahnie Mountain - Angora Peak area, located along the northwest Oregon coast near the town of Nehalem. The Tertiary units are the late ...Citation Citation
- Title:
- Stratigraphy and sedimentation of the Neahkahnie Mountain - Angora Peak area, Tillamook and Clatsop Counties, Oregon
- Author:
- Cressy, Frank Beecher
Four distinct lithologic units compose the Tertiary rocks of the Neahkahnie Mountain - Angora Peak area, located along the northwest Oregon coast near the town of Nehalem. The Tertiary units are the late Oligocene to early Miocene Oswald West mudstones, the middle Miocene Angora Peak sandstone member of the Astoria Formation, and middle Miocene intrusive and extrusive rocks of the Depoe Bay Basalt. These units are unconformably overlain by Pleistocene and Recent beach and dune sands, alluvium, and tidal flat muds. The Oswald West mudstones and the Angora Peak sandstone member are informal stratigraphic units proposed in this study. The Oswald West mudstones consist of over 1600 feet of well-bedded, highly burrowed, tuffaceous siltstones and silty mudstones interbedded with minor amounts of graded turbidite sandstones and submarine slump deposits. Foraminifera and trace fossils suggest deposition occurred in marine waters of upper bathyal depths. The Angora Peak sandstone consists of over 1800 feet of thin- to thick-bedded, locally cross-bedded, fine- to coarse-grained arkosic sandstones, pumiceous and basaltic conglomerates, carbonaceous and micaceous siltstones, and local coal beds. The inter-fingering shallow marine and fluvial sandstones are interpreted to have been deposited in a high-energy, wave dominated, deltaic environment which reworked the sediments into extensive delta-front sheet sands similar to those observed in the modern Niger and Rhone deltas. Mineralogy, heavy minerals, and conglomerate clast lithologies indicate that most of the sediments were derived from local uplifted areas of Eocene basalts and early Tertiary sediments and from the Oligocene Little Butte Volcanics in the western Cascades. Rare metamorphic and plutonic clasts, sedimentary quartzite, heavy minerals, and sandstone mineralogy suggest that metamorphic, igneous, and Paleozoic sedimentary terranes in eastern Oregon and Washington, British Columbia, Idaho, and Montana supplied some of the sediments, possibly via an ancestral Columbia River. Dikes, sills, and plugs of aphanitic to finely crystalline Depoe Bay Basalt intrude the older sedimentary rocks and locally are feeders for palagonitized pillow breccias which unconformably overlie the Angora Peak sandstone. The major intrusive body is a 1200-foot thick diabasic sill referred to as the Neahkahnie sill. The extrusive basalts formed in a subsiding marine basin in which over 1600 feet of pillow lavas, pillow breccias, and minor basalt flows were locally extruded. The area is cut by a series of west-northwest and north-trending faults. Two synclines and an anticline strike subparallel to the west-northwest trending faults.
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162. [Article] Hydro‐isostatic deflection and tectonic tilting in the central Andes: Initial results of a GPS survey of Lake Minchin shorelines
This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gl/.Citation Citation
- Title:
- Hydro‐isostatic deflection and tectonic tilting in the central Andes: Initial results of a GPS survey of Lake Minchin shorelines
- Author:
- Worden, Bruce, Donnellan, Andrea, Bills, Bruce G., Currey, Donald R., Emenger, Robert S., de Silva, Shanaka L., Lillquist, Karl D.
This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gl/.
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163. [Article] Petrology of the Bend pumice and Tumalo tuff, a Pleistocene Cascade eruption involving magma mixing
The Bend pumice and Tumalo tuff are products of a plinian eruption which occurred sometime between 0.89 and 2.6 m.y. The Bend pumice is a poorly consolidated, air-fall vitric lapilli tuff, which overlies ...Citation Citation
- Title:
- Petrology of the Bend pumice and Tumalo tuff, a Pleistocene Cascade eruption involving magma mixing
- Author:
- Hill, Brittain Eames
The Bend pumice and Tumalo tuff are products of a plinian eruption which occurred sometime between 0.89 and 2.6 m.y. The Bend pumice is a poorly consolidated, air-fall vitric lapilli tuff, which overlies a zone of reworked tephra. Perlitic obsidian in the reworked zone probably represents the remains of a dome which filled the eruptive vent and is chemically related to the Bend pumice magma. Detailed grain size analysis of the air-fall part of the Bend pumice shows that the eruptive vent was located approximately 10-20 km west of Bend, Oregon. Grain size variations in vertical section are probably related to fluctuations in the diameter of the vent rather than interruptions in deposition of the Bend pumice. The Tumalo tuff is nonwelded to moderately welded ash-flow tuff which directly overlies the Bend pumice. Lack of discernable normal grading in the upper 50 cm of the Bend pumice indicates that the Tumalo tuff was emplaced before the Bend pumice was completely deposited and leads to the conclusion that the Tumalo tuff is the product of collapse of the Bend pumice eruption column. The Tumalo tuff was formed by one episode of flow and has a well developed basal 2a layer. Variations in the distal character of layer 2a are thought to represent complex flow conditions in the head of the Tumalo tuff ash flow. Mixed pumices also are found in proximal Tumalo tuff deposits. The Bend pumice and Tumalo tuff are peraluminous and rhyodacitic. Within analytical uncertainties, they have identical major, minor, and trace element abundances. Both contain fresh hornblende in the mineral assemblage Plg + Opx + Mgt + Zr + Ap. The hornblende appears to have been a liquidus phase and indicates that the rhyodacite evolved under high pressure, hydrous conditions. A hight La to Ce ratio and a strong negative Eu anomaly in the B-T rhyodacite further indicates that the Bend pumice and Tumalo tuff evolved under physical conditions quite distinct from other rhyodacites in the central Oregon High Cascades analysed by Hughes (1982). Mixed pumices in the Tumalo tuff represent the incomplete mixing between Bend-Tumalo rhyodacite and a dacitic magma. Trace element modeling fails to provide an unequivocally common path of crystal fractionation between these two magmas. The magmas can not be directly related through thermogravitational diffusion or assimilation. While mixed pumice formation is usually attributed to mixing of genetically related magmas, Tumalo tuff mixed pumices were produced through the mixing of genetically unrelated magmas.
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Geologic mapping of the Longview-Kelso area and the measurement and description of a composite 650-meter thick stratigraphic section of the Cowlitz Formation (Tc) in Coal Creek using bio-, magneto-, litho-, ...
Citation Citation
- Title:
- Stratigraphy and sedimentology of the middle eocene Cowlitz Formation and adjacent sedimentary and volcanic units in the Longview-Kelso area, southwest Washington
- Author:
- McCutcheon, Mark S.
Geologic mapping of the Longview-Kelso area and the measurement and description of a composite 650-meter thick stratigraphic section of the Cowlitz Formation (Tc) in Coal Creek using bio-, magneto-, litho-, and sequence stratigraphy reveals a complex interplay of Cowlitz micaceous, lithic arkosic shelf to tidal/estuarine to delta plain facies associations, and Grays River basalt lava flows and interbedded basalt volcaniclastics from nearby Grays River eruptive centers (e.g., Mt. Solo and Rocky Point). The lower 100 meters of the Coal Creek section (informal unit 1, Chron 18r) consists of micaceous, lithic arkosic sandstone and siltstone and minor coals, was deposited as part of a highstand system tract (HST) at the base of 3rd order cycle number 3. This unit consists of four dominantly tidal shoaling-upward arkosic sandstone parasequences reflecting upper shoreface to delta plain depositional environments. The overlying unit 2 (Chron 18n) is defined by abundant Grays River basalt volcaniclastic interbeds that intertongue with Cowlitz lithic arkoses. This unit represents the latter part of 3rd order cycle 3, and consists of mostly fining- and thinning-upward parasequences of middle shoreface to delta plain successions of an aggradational to transgressive parasequence set. Near the top of unit 2 is a maximum marine flooding surface depositing lower shoreface lithic arkosic sandstone to shelf siltstones over upper shoreface micaceous lithic arkose. Unit 3 comprises 3rd order cycle 4 (Chron 17r), a lowstand system tract, and consists of 6 mostly fining- and thinning-upward parasequences of lower shoreface to delta plain facies associations. A parasequence or erosional boundary at the base of unit 5 (Chron 17r) consists of submarine channel-fill scoured into underlying micaceous siltstones, produced during a lowstand system tract (LST) of 3rd order cycle 5. This deep marine channel-fill sequence is overlain by thinlybedded to laminated overbank distal turbidites and hemipelagic siltstones that define the top of the Coal Creek section. These 5 informal units in Coal Creek lithologically and chronologically correlate to 5 similar informal units defined by Payne (1998) in the type section of Cowlitz Formation in Olequa Creek near Vader -30 km to the north. Middle Eocene Grays River Volcanics of the study area are mapped as two separate units: a lower unit over 150 meters thick in places, consisting of subaerial basaltic flows and invasive flows (Tgvl), intrusions (Tgvis and Tgvid), and volcaniclastics (Tgvsl); and an upper unit consisting of commonly mollusk-bearing, shallow marine basaltic sedimentary interbeds that intertongue with the Cowlitz Formation (Tgvs2), particularly Cowlitz unit 2 of the Coal Creek section. These volcaniclastic deposits are intrabasinal, derived from volcanic highlands to the west and northwest, and local phreatomagmatic tuff cones. The lower Grays River volcaniclastic unit typically overlies Grays River flows in the study area and is divided into 5 informal facies. Geochemically, Grays River flows in the study area fall within normal parameters (3 to 4% TiO2 and high iron tholeiitic basalts). However, basalt flows and bedded scoriaceous breccias near Rocky Point are anomalously low in TiO2 and are considered in this study to be a separate volcanic subunit (Rocky Point Basalts), time equivalent to and interfingering with Grays River lavas, but may represent mixing with shallower western Cascade calc-alkaline magma. Over 60 younger Grays River dikes intrude the Cowlitz Formation in Coal Creek. A dike low in the Coal Creek section is dated at 40 ± 0.36 Ma, and an invasive flow at Mt. Solo is dated at 36.98 ±.78 Ma. Volcanics capping the hills east of the Cowlitz River are chemically distinct as slightly younger western Cascade basaltic andesite flows, and two dikes east of the river are chemically distinct as western Cascade andesite. Overlying Grays River Volcanics and Cowlitz Formation in much of the study area, are clayey and commonly tuffaceous siltstones and silty sandstones, possibly of the late Eocene-early Oligocene Toutle Formation, a new unit to this area. The Toutle Formation is a mixture of wave and stream reworked micaceous and arkosic Cowlitz Formation and fresh silicic pyroclastic ash and pumice from the active western Cascade arc. An angular unconformity separates the Paleogene Grays River Volcanics, Cowlitz Formation, and Toutle Formation from the early to middle Miocene Columbia River Basalt Group. Based on lithology, geochemistry, stratigraphic relationships, and magnetic polarity, 6 individual Columbia River Basalt flows have been mapped in this study. The three lower Grande Ronde flows are of normal polarity and Ortley low MgO chemical composition. The lowermost flow (N2 Ortley #1) is absent in the Columbia Heights area, low MgO, about 10 meters thick and consists of pillow-palagonite sequences in the upper quarry on Mt. Solo. Aphyric N2 Ortley flow #2 is over 35 meters thick with well-developed upper and lower colonnade, and of intermediate MgO. N2 Ortley flow #3 is pillow-palagonite in the Storedahl Quarry and low MgO. A -4-meter thick tuffaceous overbank siltstone and basalt conglomeratic channel interbed separates the three low MgO Ortley flows from the overlying high MgO N2 Grande Ronde Sentinel Bluffs flow. A single exposure of well-developed large colonnade with sparse 1 cm labradorite laths, and reddish oxidized soil, defines the N Sand Hollow flow of the Frenchman Springs Member of the Wanapum Formation. The overlying Pomona Member is mapped based on previous work by other authors. Pliocene gravels and arkosic sand of the Troutdale Formation form upland terrace deposits up to 100 meters thick in southern parts of the study area, and represent the uplifted paleo-thalweg and overbank flood deposits of the downcutting, antecedent ancestral Columbia River. Well-rounded clasts are a mixture of extrabasinal granitic and metamorphic quartzite, and intrabasinal porphyritic basaltic andesite, dacite, and basalt from the western Cascades and Columbia River Basalts. Troutdale terrace gravels grade northward into contemporaneous volcanic pebble and cobble gravel terrace deposits produced along the ancestral Cowlitz River that are dominantly composed of porphyritic andesite gravel and volcanic sand from the western Cascades. Lower terraces along the Cowlitz River were deposited by the late Pleistocene Missoula Floods. All of these unconsolidated to semiconsolidated gravels and sands are prone to landslides, and the Aldercrest-Banyon landslide, the second worst landslide disaster in American history, occurred in the Troutdale Formation gravels. After eruption of the Grays River Volcanics and deposition of the Cowlitz Formation, the forearc underwent a period of transtension in the late-middle Eocene related to magmatic upwelling and reorganization of the subducting Farallon Plate. This event produced a northwest-trending set of oblique slip normal faults, along which Grays River dikes intruded. Starting in the early Miocene the region underwent a transpressional event, reactivating many of the northwest-trending faults, and producing the Columbia Heights Anticline, Hazel Dell Syncline, the Coal Creek Fault, and the Kelso Fault Zone. The paleotopography resulting from this event was stream eroded to a nearly flat plain before emplacement of the Columbia River Basalts, which are nearly horizontal today. Continued offset along the northwest-trending fault set has also offset the Columbia River Basalts. Continued oblique slip post-Miocene broad arching of the Coast Range and downcutting by the Columbia and Cowlitz Rivers has resulted in Pliocene and Pleistocene terraces, and produced an east-west fault set that offsets all earlier structural features.
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165. [Article] Sedimentary texture--a key to interpret deep-marine dynamics
The processes responsible for transporting and depositing thick sections of coarse-grained terrigenous clastics on the abyssal floor and for forming associated sedimentary structures are still conjectural. Many ...Citation Citation
- Title:
- Sedimentary texture--a key to interpret deep-marine dynamics
- Author:
- Allen, David William
The processes responsible for transporting and depositing thick sections of coarse-grained terrigenous clastics on the abyssal floor and for forming associated sedimentary structures are still conjectural. Many workers attribute coarse deep-sea sediments and their probable lithified equivalent, the graywackes of flysch deposits to some type of density movement. Deductions concerning the processes operating in a density flow generally are made from flume studies--in which an artificial situation may develop, or from lithified units--where the magnitude of post-depositional change is unknown. Both approaches contribute to our knowledge, but the unconsolidated elastics themselves should contain a unique key to understanding the dynamics of abyssal sedimentation. To test this theory, divisions of parallel lamination, found in deep-sea sand and silt, were selected for analysis. Since individual laminae closely approach discrete populations of particles assembled under contrasting conditions, their use carries environmental sampling to its practical limits. Northeast Pacific sediments of late Pleistocene and Holocene age, from deep-sea channel and abyssal plain environments, and representing two or three provenances were studied. A total of 115 light-colored and 84 dark-colored laminae were sampled from eight sequences at five locations. Samples averaged about 0.8 gram and were quantitatively processed using quarter-phi calibrated sieves and decantation techniques. Statistical evaluation of the procedure shows better than 95 percent sample recovery, and indicates that textural variance between laminae is significantly greater than within-sample variance. The classic concept of density transport--that coarsest material is carried by the nose of the current, and that clastic size grades tail-ward and upward in a uniformly decreasing manner--is not substantiated by moment measures, sand-silt-clay percentages or factor analysis of grain-size distributions, at least during deposition of the coarse division of parallel lamination. Coarse abyssal lamination develops within a narrow range of current velocity, the limits of which are defined texturally. Absolute velocity values for these limits can only be related, at the present time, to the few flume or in situ bottom current measurements available. Texture indicates that while the total amount of sand carried in suspension varies, lamination does not begin to form until a current is essentially depleted of all material coarser than fine sand--establishing an upper competency limit. At that time, coarse suspended material is distributed throughout the flow mostly in large eddies or vortices whose velocities are estimated on the order of about one meter/sec. Mean current velocity must be sufficient to maintain a dispersed traction carpet without deformation of bedform into ripples. This is postulated at about 50 cm/sec. A current model, based on textural evidence, is proposed to account for lamination. It is suggested that the critical stage in the formation of coarse abyssal lamination occurs while sediment is being dragged along the bottom as bedload. The flowing clastic traction carpet acquires kinetic energy as the current bypasses material lost from suspension. In turn, this energy results in grain shear. When the concentration of granular material in traction is large, it dissipates the energy of bottom shear mostly in collision contacts between gliding grains. The dispersive stresses developed tend to maintain grain separation and prevent settling. Eventually, turbulence in seawater entrapped between grains is suppressed and the net path of grans impelled by repeated collisions becomes quasi-laminar. Within this quasi-laminar traction system, dispersive pressure causes some migration of finer sizes toward the base of the carpet and a concentration of coarser grains in the upper bedload. As new material is introduced in large quantities from suspension, the zone of internal shear--the base of the moving carpet--is displaced progressively upward. As it passes, sediment compacts to a fraction of its dispersed thickness and a population of grains with a slightly finer size distribution than the carpet load comes to rest. This is buried by new deposition and a densely-packed, dark layer continues to accrete upward as long as a moving traction carpet is sustained and a dense rain of clastics is contributed from suspension. When a sand-laden eddy impinges on the bottom, it releases its coarsest load into traction and the dark layer then accreting increases significantly in grains larger than 44 microns. Any eddy, whether laden or not, on striking bottom adds to, or deducts its velocity from the velocity of the traction carpet and either increases or decreases bottom shear. Additional impulse given to tractive shear by eddies merely results in more effective size sorting. However, an eddy whose velocity of rotation is opposed to current movement may reduce shear below the critical necessary to maintain a thick carpet by dispersive pressure, The dispersed carpet collapses and instantaneously ceases moving. This less-densely packed layer has a slightly higher sand content than the accreted material below. When partially dried or weathered, alternate layers exhibit different moisture retention properties--the less-porous, accreted layers appearing dark and the more loosely packed layers appearing light.
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166. [Article] Genesis of the El Salvador porphyry copper deposit, Chile and distribution of epithermal alteration at Lassen Peak, California
The El Salvador porphyry copper deposit in the Indio Muerto district of northern Chile has been geologically investigated for more than 60 years and provides one of the best bases for understanding similar ...Citation Citation
- Title:
- Genesis of the El Salvador porphyry copper deposit, Chile and distribution of epithermal alteration at Lassen Peak, California
- Author:
- Lee, Robert G. (Robert George)
The El Salvador porphyry copper deposit in the Indio Muerto district of northern Chile has been geologically investigated for more than 60 years and provides one of the best bases for understanding similar environments of ore formation elsewhere in the world. Fourteen new zircon U/Pb isotopic ages obtained via in situ SHRIMP-RG analysis are here coupled with previous geological studies to allow refinement of the timing of Eocene porphyry magma emplacement responsible for copper and molybdenum mineralization that occurs in several ore bodies within the district. The earliest intrusions are rhyolites that crop out throughout the district, but are more abundant in the north. In contrast, the later granodiorite porphyries were emplaced only in the central and southern parts of the district. Two age periods of mineralization have been documented using zircon U/Pb geochronology. The low grade and small copper deposit at Old Camp in the northern district is associated with a quartz porphyry intrusion that yielded an age of 43.6 ± 0.5 Ma, whereas the main copper molybdenum deposit at Turquoise Gulch is associated with emplacement of the granodioritic L porphyry plug that yielded an age of 42.0 ± 0.5 Ma. The final intrusion is a series of latite porphyry dikes, which post-date ores and yielded a U/Pb zircon age of 41.6 ± 0.5 Ma. Inherited Eocene zircons with ages from ~45 Ma to ~47 Ma are found within younger porphyry intrusions and likely formed via magmatic recycling of older intrusions. Therefore, the zircon U/Pb ages suggest magmatism spanned approximately 5 million years from 47 to 42 Ma, with hydrothermal copper-molybdenum ores dominantly forming during the final stages of porphyry emplacement. Geochemical analyses by XRF, ICP-MS, electron microprobe and laser-ablation ICP-MS define a wide range of major, minor and trace element contents for the Eocene porphyry intrusions within the district. The early rhyolite and quartz porphyry intrusions have rare earth contents with strong negative europium anomalies and relatively low Sr/Y and Sm/Yb ratios consistent formation via fractional crystallization of plagioclase-rich mineral assemblages from more mafic parental melts. The granodiorite porphyries have no europium anomalies and a wide range of Sr/Y and Sm/Yb ratios that support an origin via fractional crystallization of garnet, hornblende ± titanite, and minor plagioclase from an andesitic parental melt. The granodiorite intrusions at M Gulch – Copper Hill are ~1 m.y. older than and have less evolved trace element ratios than the younger granodiorite intrusions associated with the main mineralization event. The evolving Eocene intrusions are the result of lower to middle crust melts ascending to mix with silica-rich differentiated melts derived from fractional crystallization of older andesitic magmas. Progressive decrease of Eu/Eu* ratios in the zircons with decreasing age gives direct evidence in support of the hypothesis that the main ore mineralization is directly related to the evolution of the upper crustal magma reservoir to progressively more oxidized conditions. A second goal of this study was to document the mineralogy and zonation of altered wall rock at Lassen National Volcanic Park in northern California, in order to understand the pressure, temperature, fluid composition, and epithermal processes along the southern flank of Lassen Peak. Extensive epithermal wall rock alteration occurs along the southern flank of the Cascadia volcano and includes both active and fossil geothermal systems. Geologic mapping coupled with mineral identification using a portable infrared spectrometer and X-ray diffraction outline several hydrothermal systems within the park. Currently active, steam-heated acid sulfate alteration is characterized by kaolinite, alunite, opal, and cristobalite with accessory iron sulfates. The active hydrothermal zones are proximal to thermal pools and fumaroles at Sulphur Works, Pilot Pinnacle, Little Hot Springs Valley, and Bumpass Hell. At least three fossil systems occur within andesite lavas and flow breccias of the eroded Pleistocene Brokeoff Volcano. Intermediate argillic alteration occurs at higher elevations on the flanks of the eroded volcano and is characterized by mixed layer illite-smectite, quartz, pyrite, and albite. Propylitic alteration occurs within the eroded lower elevations of Little Hot Springs Valley and is characterized by chlorite, calcite, quartz, pyrite, illite, albite, and rare epidote. Also present to a lesser extent is an advanced argillic alteration defined by pyrophyllite, dickite, alunite, kaolinite, and quartz formed at Pilot Pinnacle.
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167. [Article] An edaphic study of the Mt. Pisgah Arboretum water garden, Coast Fork of the Willamette River
Wetlands are widely identified as providing important and fundamental processes valuable for maintaining ecosystem health and diversity. Located in the southern Willamette Valley, the Mt. Pisgah Arboretum ...Citation Citation
- Title:
- An edaphic study of the Mt. Pisgah Arboretum water garden, Coast Fork of the Willamette River
- Author:
- Bergen, Cameron Francis
Wetlands are widely identified as providing important and fundamental processes valuable for maintaining ecosystem health and diversity. Located in the southern Willamette Valley, the Mt. Pisgah Arboretum contains some valuable remaining wetland habitat along the Coast Fork of the Willamette River. One goal of the Mt. Pisgah Arboretum is "to promote conservation, research, and awareness of ecology". To reach this goal, the Arboretum has identified the importance of maintenance and enhancement activities for onsite native habitats, including riparian and wetland habitats. Before restoration or enhancement activities can begin, it is essential to develop an understanding of current environmental conditions. The purpose of this research was to document both the characteristics and distribution of hydric soils and the hydrology, and to provide insight into the patterns and processes associated with a floodplain wetland. In this study, transect sampling of edaphic features was used to identify the distribution of hydric soils and the hydrologic nature of the Mt. Pisgah Arboretum Water Garden. Soil morphological data for particle size, matrix colors and redox features were evaluated and compared with observations of ground water hydrology, river hydrology and precipitation. Five stratigraphic units were identified underlying the Water Garden. A basalt Bedrock unit underlies the uplands associated with Mt. Pisgah and extends at least part way beneath the floodplain. The Clay unit was formed above the Bedrock unit, with some degree of encroachment onto the floodplain. Below 153 m are floodplain sediments, cobbles at depth, then a sand layer and silty clay loam at the surface. The Cobble unit overlaps the Bedrock unit at its base and is most likely Pleistocene age alluvium. The Sand unit is of Holocene age and is found only in the abandoned thalweg, tapering off laterally in both directions across the ancient channels. Draped above this all and slightly overlapping the upland Bedrock and Clay units, is the SiCL unit. The SiCL unit represents Holocene age alluvium, fine material deposited by slow moving water and overbank deposition. The Water Garden soils reflect this mosaic of parent materials on a complex slope. Water Garden soils sometimes met saturation requirements for hydric soils, but they did not always meet hydric soil indicator requirements. The hydrological data suggest that the soils in depressional areas of the Water Garden occupy a zone where water is exchanged between saturated sediments surrounding the channel of the Coast Fork and the channel itself. The hydrology of depressional areas with both ponded surface water and near surface saturation was principally the result of hyporheic upwelling. The soils in these depressional areas tended to form redox concentrations that met hydric soil indicator criteria. Hillslope soils in concave footslope positions exhibited hydrology indicating two separate zones of saturation, one near surface, the other at depth, related to infiltration and accumulation of precipitation. Few redoximorphic features were observed in hillslope soils, and the one hydric soil indicator that was used at these locations did not require redox. Accurate and detailed delineation of hydric soils on this landscape and clear determination of dominant sources of saturation provided an improved understanding of the complex nature of the Water Garden wetland. Results of this study show that hydric soils occupy both depressional and hillslope positions within the Water Garden. Delineation of a soil as hydric or non hydric was facilitated by the use of hydric soil indicator criteria, morphology and hydrology. This analysis provides the managers of the Mt. Pisgah Arboretum with an accurate representation of where hydric soils currently exist and the respective sources of saturation. With this information, managers are better equipped to develop restoration and enhancement options that better reflect the current environmental conditions in the Water Garden.
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This dissertation combines volcanological research of three convergent continental margins. Chapters 1 and 5 are general introductions and conclusions, respectively. Chapter 2 examines the spatiotemporal development ...
Citation Citation
- Title:
- Convergent margin magmatism in the central Andes and its near antipodes in western Indonesia : spatiotemporal and geochemical considerations
- Author:
- Salisbury, Morgan J.
This dissertation combines volcanological research of three convergent continental margins. Chapters 1 and 5 are general introductions and conclusions, respectively. Chapter 2 examines the spatiotemporal development of the Altiplano-Puna volcanic complex in the Lípez region of southwest Bolivia, a locus of a major Neogene ignimbrite flare- up, yet the least studied portion of the Altiplano-Puna volcanic complex of the Central Andes. New mapping and laser-fusion ⁴⁰Ar/³⁹Ar dating of sanidine and biotite from 56 locations, coupled with paleomagnetic data, refine the timing and volumes of ignimbrite emplacement in Bolivia and northern Chile to reveal that monotonous intermediate volcanism was prodigious and episodic throughout the complex. ⁴⁰Ar/³⁹Ar age determinations of 13 ignimbrites from northern Chile previously dated by the K-Ar method improve the overall temporal resolution of Altiplano-Puna volcanic complex development. Together with new and updated volume estimates, the new age determinations demonstrate a distinct onset of Altiplano-Puna volcanic complex ignimbrite volcanism with modest output rates beginning ~11 Ma, an episodic middle phase with the highest eruption rates between 8 and 3 Ma, followed by a general decline in volcanic output. The cyclic nature of individual caldera complexes and the spatiotemporal pattern of the volcanic field as a whole are consistent with both incremental construction of plutons as well as a composite Cordilleran batholith. Chapter 3 examines the spatiotemporal development of marine tephra deposits in deep sea sediment cores from the Sunda trench near Sumatra, which reveal evidence for seven large (minimum volume 0.6 – 6.3 km³), previously undocumented, explosive eruptions in this region over the last ~110,000 years, presumably sourced from mainland Sumatra. Sediment cores were collected within and adjacent to the Sunda trench from 3.3ºN to 4.6ºS at water depths between 1.8 and 5.5 km and distances of ~200 to 310 km from the active Sumatran volcanic arc. Glass shards within the tephra horizons were analyzed via the electron microprobe and laser ablation ICP-MS and define three compositional groups. Minimum volume estimates for the seven unique units are consistent with volcanic explosivity index (VEI; Newhall and Self, 1982) values of 4 - 5. The most frequent, widespread, and youngest deposits were found in the central region of the study area suggesting the central Sumatran arc as at the highest risk for large explosive eruptions. The first detailed chronological and geochemical data are presented for Tunupa volcano and nearby Huayrana lavas in chapter 4. New ⁴⁰Ar/³⁹Ar age determinations reveal edifice construction at ~1.5 Ma, a duration of ~90-240 k.y., and extrusion rates of 0.43 to 0.93 km³/k.y. Mineralogical compositional and textural data are consistent with shallow crustal storage (~7-18 km) and magma mixing. Volcano morphology, extrusion rates, mineralogy and textures are all similar to the Pleistocene to recent composite cones of the arc front, although new and available age data from the literature indicate that Western Cordilleran volcanism was concomitant with extrusion of both Huayrana (~11 Ma) and Tunupa (~1.5 Ma) lavas in the behind arc region. Arc-related volcanism was either widespread during these eruptive periods, or an additional melting mechanism was involved. Geochemical data, such as lower Ba/Nb ratios and enriched high field strength elemental concentrations, compared to volcanoes of the modern arc front suggest that Huayrana and Tunupa lavas were derived from a different source than the modern arc front. Geophysical and geochemical research in the central Andes indicate local variations in crustal and lithospheric thicknesses and compositions consistent with a dynamic continental lithosphere that has foundered in piecemeal fashion into the underlying asthenosphere throughout the mid to late Cenozoic. The data presented in this chapter for Tunupa and Huayrana indicate a complex petrogenetic origin and more research is necessary to determine the relative roles of arc and non-arc volcanism beneath the central Altiplano.
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Based on the principle that the history of a discipline is important to the discipline itself, this thesis devotes two chapters to ROBERT HOOKE AND THE FOUNDATION OF GEOLOGY and two chapters to modern ...
Citation Citation
- Title:
- Robert Hooke and the foundation of geology : a comparison of Steno and Hooke and the Hooke imprint on the Huttonian theory ; and, The tectonic evolution of the Oregon continental margin : rotation of segment boundaries and possible spacetime relationships in the Central High Cascades
- Author:
- Drake, Ellen T.
Based on the principle that the history of a discipline is important to the discipline itself, this thesis devotes two chapters to ROBERT HOOKE AND THE FOUNDATION OF GEOLOGY and two chapters to modern geology, viz. THE TECTONIC EVOLUTION OF THE OREGON CONTINENTAL MARGIN. The first part of this abstract covers the historical section of the thesis and the second part the scientific section. Robert Hooke was much more than the originator of Hooke's Law or an inventor who invented or perfected meteorological instruments and who pioneered equipment design for sounding the depths of the ocean and collecting ocean water samples at various depths. He supplied Isaac Newton with the concept of centripetal force which allowed Newton to formulate his Laws of Gravitation; he was the first to demonstrate the pressure and volume relationships of gases which were called Boyle's Law. His geological contributions had a profound influence on the development of geology, but they have been largely ignored by modern historians and geologists. The 17th-century Dane Nicolaus Steno has been honored by geologists as a founder of geology and the 18th-century Scotsman James Hutton is widely recognized as the father of modern geology. In a comparison of Steno's geological contributions with those of Hooke, the latter emerges as having made a more extensive and profound contribution. Furthermore, Hooke's system of the earth as presented in his Cutlerian Lectures, published posthumously as Discourse of Earthquakes in 1705, is almost identical to the theory James Hutton announced to the Royal Society of Edinburgh in 1785. This similarity is not a coincidence. That Hutton was thoroughly aware of Hooke's writings is shown not only by the extent to which the intelligentsia of the 18th century cited, quoted and adopted Hooke's ideas, but also by Hutton's own text in both his Abstract of 1785 and his Theory of the Earth in 1788. In the few places where Hutton disagreed with Hooke, Hutton's style became polemical. He seemed to argue against specific points originated by Hooke, which then act as a Hookian signature on the Huttonian Theory. Hooke's influence in the development of geological thought and especially on the foundation of the pre-continental-drift paradigm was a significant one. Robert Hooke, therefore, like Steno, deserves recognition by geologists as a founder of their science. The tectonic evolution of the Oregon continental margin centers around the process of subduction of the Juan de Fuca plate. Models have been advanced to explain the complex tectonic history of this area. The imbricate thrust model seems to test well at the Oregon margin. This model, however, is complicated by the additional processes of segmentation and rotation. This synthesized scenario of the Cenozoic tectonic evolution of the Oregon margin considers the subduction process, including volcanism in the Cascades, as a near-extinction system. This area presently could be experiencing a period of transition from compressional underthrusting to strike-slip motions resulting from extensional forces as the Juan de Fuca plate is coupled with the North America plate. A model involving the rotation of segment boundaries is proposed which may shed some light on time-space and petrologic relationships in the Cascades. The positions of these segment boundaries are supported by bathymetric data on the continental shelf and by the deformation of marine terraces along the Oregon and Washington coasts. Plane vector triangle solutions calculated by Riddihough (1977) for point interactions between the Juan de Fuca plate and the America plate show that the direction of convergence has not been constant over the last 8.5 m.y. The data suggest that the subducted slab had rotated at least 9° in a clockwise direction during the last 1.5 m.y. The model suggests that the arrival of a segment boundary in coincidence with conditions favorable for eruptions may have built the high Central Cascade peaks during the Pleistocene.
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Meyer's Canyon, a tributary of Bridge Creek in the John Day Basin, is a deeply incised valley fill in northcentral Oregon. The current channel is incised to the Cretaceous and Tertiary bedrock. To determine ...
Citation Citation
- Title:
- Valley fill and channel incision in Meyer's Canyon, northcentral Oregon
- Author:
- Peacock, Kathi A.
Meyer's Canyon, a tributary of Bridge Creek in the John Day Basin, is a deeply incised valley fill in northcentral Oregon. The current channel is incised to the Cretaceous and Tertiary bedrock. To determine the precedence of the current incision and the variation and timing of depositional sequences, the sediments exposed by incision were examined for clues. The incision evaluated in this study occurs along the length of the lower valley fill, approximately 2300 meters, with a maximum depth of about 22 meters near the medial section of the valley. The incision occurred near the beginning of the 20th century and widened from 1951 to 1979, after which tributary headward cutting only is occurring at one location. Colluvial aprons and aggradation within and at the margins of alluvial fans indicate depositional processes again dominate. Fill sediments date from the early Holocene. Volume of the fill prior to incision was estimated to be about 10.8 mcm (million cubic meters), of which 1.2 mcm (11%) was removed by the incision. Fill sediments are contributed by coalescing alluvial fans and alluvial plain sedimentation. The Upper Drainage and Permian Tributary could potentially donate 67% of the Lower Valley fill sediments though these portions of the drainage were not studied. Early sedimentation is dominated by coarse-grained fluvial transport, followed by numerous thick fine-grained sequences, topped by debris flow/mud drape couplets where proximal fan processes dominate. Sediment size decreases and sorting increases toward the fan margins. Valley plain deposition is currently and was, within the Holocene, enhanced and influenced by thick vegetation due to perennial groundwater saturation. Aggradation throughout the Lower Valley fill has dominated over the course of the Holocene, with only one previous episode of incision coincident with the Mt. Mazama eruption, about 6900 yrs BP. Rates of accumulation have changed over the course of the Holocene. Volume rate of accumulation was 140 m3/yr prior to the Mazama eruption and 210 m3/yr following the eruption at a proximal fan location. Within the alluvial fans and plains, sediment characteristics change with distance from source of sediment. At more distal fan and alluvial plain locations, an average volume accumulation rate of 260 m3/yr was estimated prior to the Mazama eruption, and 130 m3/yr following the eruption. These rates indicate that input at the proximal locations has been increasing in the late Holocene and that aggradation may again be dominating Meyer's Canyon sedimentation. Recurrence intervals of debris flows (proximal locations) or events capable of transporting matrix-supported gravels (distal and alluvial plain locations) show an average recurrence interval of 600 yrs pre-Mazama and 1500 yrs post-Z4azama. At proximal locations, the shortest interval is after about 1200 yrs before present (BP) when debris flows occurred about every 500 years. Shorter intervals also generally occurred in all pre-Mazama locations when coarse-sediment input was rapid, probably from the Pleistocene-Holocene climate shift from cool/wet to warmer/drier. Following the Z4azama eruption, the medial section of the Lower Valley fill had rapid input of coarse debris, while proximal fan locations had massive fine-grained input. This is interpreted as a complex response, i.e., rapid runoff reworked previously deposited sediments at proximal locations and sediments were deposited at more distal locations. Fine-grained sediment accumulation followed this period until about 1200 yrs BP. The strongest evidence for a causal mechanism for incision is a complex response at the previously saturated wet-meadow, medial portion of the Lower Valley fill due to loss of riparian vegetation which maintained an oversteepend alluvial slope. The previously saturated portion of the Lower Valley fill shows an increasing transportation slope over time. This slope was probably maintained by the hydrophytic vegetation, but loss of that vegetation due to Euroamerican influence could have led to a geomorphic threshold being crossed on the oversteepened slope and channel incision ensued. The incision is widest at this point and, if width is used as a surrogate for length of time of exposure, it is likely that incision began here.