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Determines the [sup40]Argon-[sup39]Argon geochronometry of pseudotachylytes from the North Cascade Mountains in Washington by vacuum encapsulation. [sup39]Argon loss; Textural and minerological features; Differences between total-gas age, argon-retention age and plateau age.

Investigates the basal Jurassic section at Puale Bay, Alaska using biochronologic and geochronometric data. Improvement in the calibration of early Jurassic time scale; Documentation of early Jurassic ammonoid succession; Reconsideration of the status of the bay as a Triassic-Jurassic boundary section.

Abstract: A set of 104 independently dated archaeomagnetic directions was used to extend the U.S. Southwest reference curve back to 375 cal BC and to calculate isolated mean VGPs centered on 960 cal BC and 2390 cal BC. Prior to this study, most U.S. Southwest reference curves extended to only ca. AD 585. This study employed Sternberg’s moving window technique with variably sized windows, rather than fixed windows, to smooth the dataset into a continuous curve. The size of each averaging window was determined by the density of data captured by the window, such that each window had a minimum data density of 5.0 and a minimum window size of 50 years. This approach differs from previous studies in the U.S. Southwest, which have applied a uniformly sized averaging window to a dataset regardless of the temporal distribution of the data. [ABSTRACT FROM AUTHOR]

Zircons from the Black Forest Bed, Petrifled Forest Member, Chinle Formation, in Petrified Forest National Park, yield ages that range from Late Triassic to Late Archean. Grains were analyzed by multigrain TIMS (thermal-ionization mass spectrometry), single-crystal TIMS, and SHRIMP (sensitive, high-resolution ion-microprobe). Multiple-grain analysis yielded a discordia trajectory with a lower intercept of 207 ± 2 Ma, which because of the nature of multiple-grain sampling of a detrital bed, is not considered conclusive. Analysis of 29 detrital-zircon grains by TIMS yielded UPb ages of 2706 ± 6 Ma to 206 ± 6 Ma. Eleven of these ages lie between 211 and 216 ± 6.8 Ma. Our statistical analysis of these grains indicates that the mean of the ages, 213 ± 1.7 Ma, reflects more analytical error than geologic variability in sources of the grains. Grains with ages of ca. 1400 Ma were derived from the widespread plutons of that age exposed throughout the southwestern Cordillera and central United States. Twelve grains analyzed by SHRIMP provide [sup 206]Pb*/[sup 238]U ages from 214 ± 2 Ma to 200 ± 4 Ma. We use these data to infer that cores of inherited material were present in many zircons and that single-crystal analysis provides an accurate estimation of the age of the bed. We further propose that, even if some degree of reworking has occurred, the very strong concentration of ages at ca. 213 Ma provides a maximum age for the Black Forest Bed of 213 ± 1.7 Ma. The actual age of the bed may be closer to 209 Ma. Dating continental successions is very difficult when distinct ash beds are not clearly identified, as is the case in the Chinle Formation. Detrital zircons in the Black Forest Bed, however, are dominated by an acicular morphology with preserved delicate terminations. The shape of these crystals and their inferred environment of deposition in slow-water settings suggest that the crystals were not far removed from their site of deposition in space and likely not far in time. Plinian ash clouds derived from explosive eruptions along the early Mesozoic Cordilleran margin provided the crystals to the Chinle basin, where local conditions insured their preservation. In the case of the Black Forest Bed, the products of one major eruption may dominate the volcanic contribution to the unit. Volcanic detritus in the Chinle Formation was derived from multiple, distinct sources. Coarse pebble- to cobble-size material may have originated in eastern California and/or western Arizona, where Triassic plutons are exposed. Fine-grained detritus, in contrast, was carried in ash clouds that derived from caldera eruptions in east-central California or western Nevada. [ABSTRACT FROM AUTHOR]

Examines crystal residence times using in situ uranium-lead (U-Pb) dating of zircons from the Bishop Tuff in east-central California. Maximum mean crystal residence time for zircon; Analysis of zircons from fallout and intercalated ash-flow deposits and unpolished rims; Storage of Bishop Tuff rhyolite prior to eruption.

Examines the use of optical dating of dune sand for the study of sea-level change in Massachusetts. Agreement on sea level between optical ages of barrier dune sand and evidence from salt-marsh peat sampled behind the barrier; Relevance of accurate records of relative sea-level change in modelling isostically forced crust and mantle behavior.

The age of the Ashburton Province, comprising an older divergent-margin megasequence and a younger convergent-margin megasequence, is poorly constrained. The Boolaloo Granodiorite, which intruded the divergent-margin megasequence on the western margin of the Ashburton Province, has given a SHRIMP zircon U–Pb age of 1786 ± 5 Ma, and therefore post-dates convergent-margin, backarc basin sequences, with established conventional zircon U–Pb ages of ca 1843–1828 Ma. However, it pre-dated deformation of convergent-margin, remnant-ocean sequences. Similarly aged (ca 1797–1791 Ma) granitoids are present in the adjacent Gascoyne Province, thereby identifying a magmatic fold-thrust belt that abutted a remnant ocean in the Ashburton Province. [ABSTRACT FROM AUTHOR]

Examines the reliability of 10Be produced in situ in dating moraines located in Wind River Mountains, Wyoming. Dryas; Cosmogenic-nuclide surface-exposure methods of dating boulder or bedrock surfaces.

Comments on the evidence presented by Anthony R. Prave (1999) regarding the age of the Proterozoic Kingston Peak Formation in Death Valley, California. Sordough Limestone Member as one of carbonate units within Kingston Peak Formation; Comparison of data from Death Valley rocks with chemostratigraphic record from other Neoproterozoic rocks; Reply of Prave.

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Acadian deformation front in the northern Appalachians of Maine and New Hampshire can now be closely located during the early Emsian (Early Devonian; 408-406 Ma). Tight correlations between paleontologically and isotopically dated rocks are possible only because of a new 408-Ma time scale tie point for the early Emsian. The deformation front lay between a belt of Lower Devonian fiysch and molasse that were deposited in an Acadian foreland basin and had not yet been folded and a belt of early Emsian plutons that intruded folded Lower Devonian rocks. This plutonic belt includes the newly dated Ore Mountain gabbro (U/Pb; 406 Ma), which hosts magmaticsulfide mineralization. Along the deformation front, a 407-Ma pluton that locally truncates Acadian folds (Katahdin) was the feeder to volcanic rocks (Traveler Rhyolite; 406-407 Ma) that are part of the foreland-basin succession involved in these same folds. The Emsian igneous rocks thus define a syncollisional magmatic province that straddled the deformation front. These findings bear on three alternative subduction geometries for the Acadian collision. [ABSTRACT FROM AUTHOR]

Analyzes the dating of single zircon grains from granite cuttings from southeastern Georgia. Age of the granite; Composition; Implications of the new data to the Suwannee terrane subcrop age province boundaries.

Presents a 40Ar/39Ar thermochronological analysis of Mesoproterozoic metamorphism in the Colorado Front Rage. Presence of staurolite, andalusite, cordierite, and garnet porphyroblasts overprinting earlier assemblages; Interpretation of muscovite and biotite ages; Observation of thermal effects for plutons, suggesting pervasive, regional crustal heating.

Isotopic determinations (K–Ar, Rb–Sr and Sm–Nd), and trace and rare-earth elemental analyses were made on a few biotite and clay fractions of Palaeozoic bentonite units from the eastern United States. The clay fractions were gently leached with dilute hydrochloric acid to study separately the acid-soluble minerals intimately associated with the extracted clay particles. The data highlight interesting potentials for this integrated approach to decipher complex tectonothermal evolutions of sedimentary basins. Biotite K–Ar ages are consistent with a Middle Ordovician stratigraphic age for the bentonite units with a mean age of 459±10 Ma. The clay residues give a Sm–Nd isochron age of 397±44 Ma, indicative of their crystallization during Acadian tectonothermal activity at about 200 °C. The clay leachates, which are considered to represent mineral phases different from clay material, yield a distinct Sm–Nd isochron age of 285±18 Ma which is indistinguishable from K–Ar ages obtained previously on the clays, suggesting a thermally induced diffusion of radiogenic 40Ar from clay particles during Alleghenian–Ouachita orogenic activity. The Rb–Sr system of the clay material seems to have been variably disturbed, except for the sample taken near the Allegheny Front for which an age of 179±4 Ma suggests a further localized activity of the thrust system at about 130–150 °C. Clearly the limited number of samples does not allow us to perfectly constrain an evolutionary model. However, analysis of the soluble minerals for their contents in metal and rare-earth elements suggests that metal-carrying fluids migrated during the Alleghenian–Ouachita orogenic activity in the eastern North American continent. Consequently, they could have contributed to the concentration of ore deposits in the region, but this possibility needs to be tested with a larger data base. [ABSTRACT FROM AUTHOR]

Documents the thermal history of rocks surrounding the 1.43 Ga Priest pluton in Manzano Mountain, central New Mexico, using metamorphic studies and 40Argon/39Argon dating. Proterozoic geology of the central and southern Manzano Mountains; Evidence for polymorphism preserved by rocks surrounding the Priest pluton; Regional metamorphic conditions.

Presents 40Ar/39Ar laser microprobe dates on detrital mineral grains extracted from sandstones of the Gravina belt, southeastern Alaska. Reconstruction of the geochronological characteristics of the Gravina belt source area; Age constraints for Gravina belt stratigraphy and regional tectonics; Multiple grain dating of primary volcaniclastic detritus.

Presents the results of a uranium-lead sphene dating of metamorphism in the Red Mountain pluton, Laramie Mountains, Wyoming. Boundaries of study area; Red Mountain pluton and its aureole; Sample descriptions; Strategies for interpreting uranium-lead data from low uranium materials.

Focuses on decisions of geologists at Washington DC to work out plans for a more accurate and complete geological timescale. Tasks that would be undertaken by an international network of laboratories; Comments of geochronologist Samuel Bowring on the project; Usefulness of uranium -- lead isotopes measurements in determining the accurate time during which the Cambrian explosion took place.

Reports that Earth scientist Wallace Broecker of Columbia University has settled out of court with Arizona State University (ASU) geographer Ronald Dorn who brought suit against him. Dorn's claim that Broecker defamed him in an article published in the periodical 'Science'; Rock dating technique espoused by Dorn and Broecker's appraisal of it which allegedly implied an accusation of data fabrication.

Explains that the rock-dating techniques of Arizona State University geographer Ronald I. Dorn have been called into question. Criticism of Dorn in `Science'; Dorn's rebuttal to the `Science' article; Questions about rock varnish samples taken from a Yucca Mountain site in Nevada; Dorn's method; Allegations that Dorn fabricated his results; Details of the investigation into the samples.

Reports that Arizona State University geographer Ronald I. Dorn is under investigation for possible scientific misconduct, as of March 19, 1998. Questions about the dating of rock varnish; Background of investigation; Interest in Dorn's methods; Dorn's research on petroglyphs in the petrified forest region of northeastern Arizona; Scholarly debate about rock varnish; Forthcoming paper in `Science.'

Presents news briefs related to science. United States legislation for permanent tax credit for research and development investments; Controversy over the Kennewick Man, found in the bank of Washington's Columbia River; Failure to date the remains; Claims laid by Native American tribes.

Oxygen isotope analyses of skarn minerals have long been used to fingerprint the variable fluid sources involved in skarn formation. The Empire Mountain skarn of the Sierra Nevada (California, USA) batholith is identified as a low‐δ18O skarn and is thought to form due to surface fluid involvement that was enhanced by fractures that formed during host rock brecciation. Although geochemically well characterized, the Empire Mountain skarn is less understood in terms of its hydrodynamic history. In this study, we develop a two‐dimensional model of oxygen isotope transport during high‐temperature fluid‐rock interactions to assess the mechanisms by which low‐δ18O garnets could form exclusive of brecciation. Highlighting regions nearest to the intrusion that could form garnet, we make three primary observations: (1) the oxygen isotope composition of the fluids, and not temperature, dominantly controls δ18Ogarnet values; (2) >6‰ increases in δ18Ogarnet, from negative to positive values, are observed over the maximum time frame of garnet thermodynamic stability; and (3) incremental emplacement of the intrusion can produce oscillations in δ18Ogarnet values. Without invoking brecciation, we find that low‐δ18O garnets can form without the presence of surface fluids; they instead source 18O‐depleted pore fluids from adjacent units. Further, surface fluids that do not equilibrate with the surrounding rock at depth become low‐δ18O fluid sources during later stages of pluton emplacement. This study underscores that pore fluids at depth, regardless of their equilibrium state, can act as dormant low‐δ18O fluid sources and may be responsible for low‐δ18O‐valued garnets. Plain Language Summary: An ancient hydrothermal system in the Sierra Nevada of California—Empire Mountain—is composed of garnets whose chemistry suggests that water from the surface of the Earth flowed several kilometers into the crust during the beginning stages of hydrothermal activity. The chemistry of the garnets also show that sources of fluid abruptly changed as the hydrothermal activity proceeded. Previous studies have attributed the presence of surface water and the abrupt changes in fluid source to the formation of fracture networks in the rocks that surrounded the causative magma body. However, we seek alternative explanations for these two observations using a numerical model. Our model simulates hydrothermal fluid flow and heat transport at Empire Mountain to assess changes in garnet chemistry during hydrothermal activity. Our results show that large fluctuations in garnet chemistry can occur without the formation of fractures. Additionally, we find that the chemistry of pore waters at depth and surface waters can be similar; instead of forming in the presence of surface water, garnets may form in the presence of chemically similar pore waters. Our findings highlight that the chemistry of garnets that form during hydrothermal activity may not be faithful recorders of distinct fluid sources during their growth. Key Points: Pore fluids at depth can have low oxygen isotope ratios, regardless of equilibrium stateSkarn garnets with low oxygen isotope ratios may form in the presence of these pore fluidsLarge variation in garnet oxygen isotope ratios may be due to influx of spatially heterogeneous pore fluids at depth [ABSTRACT FROM AUTHOR]

New U-Pb and Sm-Nd data from the Blue Mountains province, eastern Oregon and western Idaho, clarify terrane correlations and regional evolution of the western Laurentian plate margin during Mesozoic time. We report an Early Jurassic age for a red tuff unit at Pittsburg Landing, Idaho,which is 25 m.yr. older than previous Middle Jurassic estimates. In the Coon Hollow Formation at Pittsburg Landing and at the type location on the Snake River, chemical abrasion thermal ionization mass spectrometry U-Pb zircon ages on interbedded tuff and detrital zircon U-Pb maximum depositional ages indicate that deposition spanned ca. 160-150Ma, entirely during Late Jurassic time. Detrital zirconU-Pb ages represent local Wallowa arc basement and regional magmatic sources spanning ca. 290-140 Ma. Mudrock Nd isotope compositions of the Coon Hollow Formation record an increase in juvenilemagmatismconsistent with regional Late Jurassic trends in western North American magmatic systems. These data show that the Coon Hollow Formation is not part of a Middle Jurassic overlap assemblage, as has been historically interpreted. Instead, we propose that the Coon Hollow Formation is part of a belt of suprasubduction-zone extensional back-arc basins that formed in latest Jurassic time due to awell-documented period of trench retreat in the western United States. Our new data require that the underlyingWallowa terrane was accreted to and received detritus fromwesternNorth America by ca. 160Ma (early Late Jurassic). This minimum estimate for the age of terrane accretion in western Idaho and eastern Oregon is substantially earlier than previous estimates (~135-118 Ma). In the Blue Mountains region, westward expansion of Laurentia was accomplished by accretion of arc terranes to the North American craton prior to Late Jurassic time. [ABSTRACT FROM AUTHOR]

ABSTRACT Early Cenozoic terrestrial deposits in the western United States represent well-preserved archives of climatic and tectonic processes that together shaped the Earth's surface during the demise of a large continental plateau. This study examines a Cenozoic terrestrial sedimentary sequence in the central part of the Cordilleran orogen (Montana) using sedimentologic and geochemical techniques. At ∼49 Ma, we observe rapid major shifts in oxygen, carbon and strontium isotope records that are too large to directly reflect changes in meteoric water composition due to simple orographic rainout. The transition to low-δ18O values in pedogenic carbonate in concert with changes in the composition of clastic material at ∼49 Ma points to the input of evolved meteoric water to the hydrological cycle due to a change in the source of waters reaching Cordilleran intermontane regions in southwestern Montana. This drainage reorganization coincides with the initiation of magmatism and extension to the west in what is now Montana and Idaho. The sedimentological record shows evidence that depositional gradients increased in the study area ∼46 Ma, ∼3 Myr after the drainage reorganization occurred. This interval is most likely the time it took for extensional deformation to propagate to the study area itself. Evidence of freshening events in Laramide Basins to the southeast suggests that this drainage reorganization diverted waters to progressively fill these basins and highlights the impact of post-plateau extension-related landscape reorganization on river networks and lake dynamics. This study also emphasizes the importance of using multiple tools in deciphering topographic history through the study of terrestrial basin deposits, in that interpretation based on any single method employed would have compromised our ability to successfully identify the regional evolution of topography and drainage networks. [ABSTRACT FROM AUTHOR]

Abstract: In unglaciated areas of the Mississippi Valley region, the typical full loess–paleosol succession contains the Modern Soil developed in Peoria Silt, weakly developed Farmdale Geosol developed in Roxana Silt, Sangamon Geosol developed in Loveland Silt, and Yarmouth Geosol developed in Crowley''s Ridge Silt. Although a fifth loess called the Marianna Silt is reported at one area, the paleosol that separates the Crowley Ridge and Marianna Silts is not well defined. Previous thermoluminescence (TL) and optical stimulated luminescence (OSL) age chronology has suggested multiple phases of Sangamon Geosol developed in Loveland Silt, but clear morphological evidence of polygenetic Sangamon Geosol profiles have not been found. Recently, a thick loess–paleosol sequence has been studied in the middle Mississippi Valley in unglaciated southern Illinois, USA. Soil morphology and analytical results revealed five loesses and associated paleosol units. Two Sangamon Bt horizons were found separated by a thick ACtk horizon, interpreted to indicate two phases of Sangamon Geosol development. This well-preserved loess–paleosol succession provides one of the most complete mid–late Quaternary loess records in the middle Mississippi Valley to date, and is important for studying the stratigraphic framework and paleoclimate and environment changes. [Copyright &y& Elsevier]

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Mesoproterozoic metamorphic rocks exposed in the Llano Uplift, central Texas, were involved in a ∼1.15–1.12 Ga Grenville-age collisional orogenic event along the southern margin of Laurentia. This event is characterized by polyphase ductile deformation synchronous with transitional amphibolite-, granulite- to eclogite-facies metamorphism. Results of detailed structural mapping in the southeastern part of the Llano Uplift show that these rocks record a deformational history involving northeast-directed ductile thrusting and regional-scale polyphase folding, all related to northward collision of a southern continent and an arc with the southern margin of Laurentia. We use the structural, metamorphic, and kinematic history of the southeastern Llano uplift to critically evaluate collisional models for the Grenville orogen in Laurentia and discuss the implications for reconstructions of Rodinia. [ABSTRACT FROM AUTHOR]

Presents a geochemistry of Jurassic and Cretaceous plutons in California. Chemical and isotopic character of Jurassic and Cretaceous mafic rocks associated with the granitoid plutons; Major and trace element geochemistry as well as the isotopic composition of granitoid rocks separate from the mafic rocks.

Examines the oxygen isotope ratios of the Idaho batholith to determine its magmatic source. Preservation of magmatic oxygen isotope ratios in zircons; Evolution and alteration of the Idaho batholith; Evidence for contamination during the crystallization sequence in the mineral components of the batholith.

Looks at the geomorphic change in the Croatan National Forest in North Carolina's lower coastal plain from 1697 to 1997. Indepth look at the geomorphic changes in the landscape; Examination of the three critical points of geomorphic changes; Information on the similarity of human impact with those of Holocene seal level rise.