old crow tephra a new late pleistocene stratigraphic marker across north central alaska and western yukon territory old crow tephra is the first extensive pleistocene tephra unit to be documented in the northwestern part of north america it has a calc alkaline dacitic composition with abundant pyroxene plagioclase and fe ti oxides and minor hornblende biotite apatite and zircon thin clear bubble wall fragments are the dominant type of glass shard this tephra can be recognized by its glass and phenocryst compositions as determined by x ray fluorescence microprobe and instrumental neutron activation techniques it has an age between the limits of 60 000 and 120 000 yr set by 14c and fission track measurements respectively old crow tephra has been recognized in the koyukuk basin and fairbanks region of alaska and in the old crow lowlands of the northern yukon territory some 600 km to the east northeast the source vent is unknown but these occurrences considered in relation to the distant locations of potential quaternary volcanic sources demonstrate the widespread distribution of this tephra and underscore its importance as a regional stratigraphic marker

tephrochronology magnetostratigraphy and mammalian faunas of middle and early pleistocene sediments at two sites on the old crow river northern yukon territory canada abstract alluvial and lacustrine sediments exposed beneath late pleistocene glaciolacustrine silt and clay at two sites along the old crow river northern yukon territory are rich in fossils and contain tephra beds surprise creek tephra szt occurs in the lower part of the alluvial sequence at crh47 and little timber tephra ltt is present near the base of the exposure at crh94 surprise creek tephra has a glass fission track age of 0 17 0 07 ma and little timber tephra is 1 37 0 12 ma all sediments at crh47 have a normal remanent magnetic polarity and those near ltt at crh94 have a reversed polarity in agreement with the geomagnetic time scale small mammal remains from sediments near ltt support an early pleistocene age but the chronology is not so clear at crh47 because of the large error associated with the szt age determination tephrochronological and paleomagnetic considerations point to an mis 7 age for the interglacial beds just below szt at crh47 and at chester bluffs in east central alaska but mammalian fossils recovered from sediments close to szt suggest a late irvingtonian age therefore older than mis 7 further studies are needed to resolve this problem

a boundary element random walk model of mass transport in groundwater abstract a boundary element solution to the convective mass transport in groundwater is presented this solution produces a continuous velocity field and reduces the amount of data preparation time and bookkeeping by combining this solution and the random walk procedure a convective dispersive mass transport model is obtained this model may be easily used to simulate groundwater contamination problems the accuracy of the boundary element model has been verified by reproducing the analytical solution to a two dimensional convective mass transport problem the method was also used to simulate a convective dispersive problem

fission track ages of late cenozoic distal tephra beds in the yukon territory and alaska six distal tephra beds from the yukon territory and alaska have been dated by the fission track method zircon and glass ages were determined for the fort selkirk and lost chicken tephra beds but only glass ages for the others assuming that no track fading has occurred in the glass old crow and dawson tephra beds are younger than 120 000 and 52 000 years bp respectively mosquito gulch tephra is 1 22 ma old fort selkirk tephra is about 1 ma old the ester ash bed is 0 45 ma old and the best estimate of the age of lost chicken tephra is the range 1 7 2 6 ma it is evident from these results and from the known abundance of tephra beds within late cenozoic deposits of the yukon territory and alaska that application of the fission track method to distal tephra in conjunction with detailed characterization studies offers great potential for elucidation of the late cenozoic geologic history of alaska and the yukon territory

stratigraphy fossils and age of sediments at the upper pit of the lost chicken gold mine new information on the late pliocene environment of east central alaska abstract the upper pit at the lost chicken placer gold mine in east central alaska contains fossils that provide information on the flora and insect fauna of interior alaska just before the onset of global cooling at 2 5 myr fossils come from sediments interbedded with the lost chicken tephra dated at 2 9 0 4 myr early late pliocene and portray the floodplain and valley of a small creek within a region dominated by a coniferous forest richer in genera and species than the present one climate was wetter and less continental and there was probably little or no permafrost at least one other pliocene tephra the fortymile tephra occurs at the site and is also associated with plant and insect fossils among these fossils are extinct plants and insects like those found at other tertiary sites in northern canada and alaska the lost chicken sequence is the same age as the beaufort formation on meighen island more than 1000 km to the north like lost chicken meighen island sediments contain fossils representing a diverse boreal environment this shows that the latitudinal climate gradient during early late pliocene time was shallower than at present and the boreal forest had a far greater latitudinal span than now

distinction between the youngest toba tuff and oldest toba tuff from northern sumatra based on the area density of spontaneous fission tracks in their glass shards abstract determination of the area density of spontaneous fission tracks s in glass shards of toba tephra is a reliable way to distinguish between the youngest toba tuff ytt and the oldest toba tuff ott the s values for ytt uncorrected for partial track fading range from 70 to 181 tracks cm2 with a weighted mean of 108 5 tracks cm2 based on 15 samples corrected s values for ytt are in the range of 77 140 tracks cm2 with a weighted mean of 113 8 tracks cm2 within the range of uncorrected s values no significant difference in s exists between ytt samples collected from marine and continental depositional settings the uncorrected s for ott is 1567 114 tracks cm2 so that confusion with ytt is unlikely the s values of the toba tephra at bori morgaon and gandhigram in northwestern india indicate a ytt identity in agreement with geochemical data on their glass shards the presence of multiple glass populations and a glass fission track age determination therefore the view of others that ott is present at these sites and thereby indicates an early pleistocene age for the associated acheulean artifacts is incorrect

on the computation of darcian velocity and mass balance in the finite element modeling of groundwater flow darcian velocity has been conventionally calculated in the finite element modeling of groundwater flow by taking the derivatives of the computed pressure field this results in discontinuities in the velocity field at nodal points and element boundaries discontinuities become enormous when the computed pressure field is far from a linear distribution it is proposed in this paper that the finite element procedure that is used to simulate the pressure field or the moisture content field also be applied to darcy s law with the derivatives of the computed pressure field as the load function the problem of discontinuity is then eliminated and the error of mass balance over the region of interest is much reduced the reduction is from 23 8 to 2 2 by one numerical scheme and from 29 7 to 3 6 by another for a transient problem

magnetic reversal history of pleistocene sediments at old crow northwestern yukon territory the banks of the porcupine river near old crow in the unglaciated part of the northern yukon consist of about 55 m of interbedded fluvial and lacustrine gravels sands silts and clays this sequence was sampled closely for palaeomagnetic analysis and the sediments were found to possess a natural remanent magnetization nrm that is stable in direction with mean destructive fields of 200 300 oe 16 24 ka m therefore good palaeomagnetic data can be obtained from sediments that have been subjected to permafrost conditions although some samples show disturbance of nrm directions by cryoturbation magnetic polarity transitions are centred at 17 and 26 m above river level and the 15 samples between them define an interval of reversed polarity of considerable duration the behaviour of nrm during the transition periods is complex with declination switching abruptly by 180 but with the inclination displaying very erratic variation over a considerable time this may be caused by mixing in single samples of c

point counting technique for fission track dating of tephra glass shards and its relative standard error fission track dating of glass is potentially an efficient technique for tephrochronological studies however most glasses from quaternary tephra have very low spontaneous track densities and track counting is laborious the point counting technique is a simplified procedure for evaluating fission track areal densities in a glass shard population that allows the analyst to count larger numbers of tracks in significantly shorter times a field of view is coded as 1 if its center is on glass and is 0 otherwise and the total area of glass is estimated by counting the number of 1 s under simple assumptions the binomial distribution appears to provide a reasonable evaluation of the additional experimental error in a fission track density determination by this technique glass shard populations have been simulated assuming that glass areas are random samples from distributions analogous to real ones the effect of variation in glass area on the relative standard error of the estimated track density is found to be small the point counting technique significantly enhances the potential of fission track dating especially for very low track densities typical of most quaternary tephra as the additional error associated with its use is largely counterbalanced by the greater number of tracks that can be counted

pre glacial and interglacial pollen records over the last 3 ma from northwest canada why do holocene forests differ from those of previous interglaciations we synthesize pollen spectra from eleven dated stratigraphic sections from central and northern yukon palaeomagnetic and tephra dating indicates the earliest assemblages representing closed canopy pinus and picea forest are middle late pliocene age more open forest conditions indicated by increased poaceae and with evidence of permafrost are dated at ca 3 ma while pinus pollen is abundant at 3 ma it is reduced in records after 2 6 ma and subsequent pleistocene interglacial forest records are repeatedly dominated by picea along with alnus and small but significant amounts of abies surface sample comparisons indicate that abies was more widespread and abundant in past interglaciations than at present and that middle pleistocene piceaeabies forest grew in the northern yukon porcupine basin 500 km beyond modern abies limits in contrast pinus which occurs today in southern and central yukon was not a significant component of these pleistocene interglacial forests late holocene pollen assemblages with rare abies and high pinus are the most distinct in the past 2 6 ma possible factors driving holocene difference are paleoclimate paludification changes in megafaunal herbivory and an unusual fire regime anthropogenic burning is a factor unique to the holocene and if it is shown to be important in this case it would challenge our notion of what constitutes boreal wilderness

age of some pleistocene interglacial beds and associated fossils in eastern beringia defined by fission tracks in glass shards of chester bluff tephra

groundwater flow parameter estimation and quality modeling of the equus beds aquifer in kansas u s a abstract the salinity problems created in the burrton area as a result of poor oil field brine disposal practices of the past continue to be a major concern to the area depending on the equus beds aquifer for water including the city of wichita kansas in this paper an attempt is made to predict where and how fast the brine plume will move in this area and what the average chloride concentrations in different parts of the aquifer are in order to make such predictions it was necessary to get a calibrated model of the groundwater flow velocity field multiple regression analysis is used for parameter estimation of the steady state groundwater flow equation applied in the most critical area of the equus beds aquifer results of such an analysis produced a correlation coefficient of 0 992 between calculated and observed values of hydraulic head a chloride transport modeling effort is then carried out despite some serious data deficiencies the significance of which are evaluated through sensitivity analysis thus starting with the quasi steady state conditions of the early 1940 s it was possible to match the present chloride distribution satisfactorily chloride concentration predictions made for the year 2000 indicate that the quality of the wichita well field waters will not generally deteriorate from their present condition by that time

characterization identity distribution and source of late cenozoic tephra beds in the klondike district of the yukon canada a large number of distal silicic tephra beds have been preserved in the late cenozoic deposits of the klondike region yukon territory forty one tephra samples representing twelve distinctive beds are detailed in this study they range in composition from basaltic andesite to high silica rhyolite and were deposited during the late pliocene to late wisconsinan time interval seven tephra beds are derived from volcanoes in the wrangell volcanic field and four come from the more distant eastern aleutian arc alaska peninsula region but the source of the single andesitic tephra is unknown the widespread and well known old crow and sheep creek tephra beds have been identified in the klondike district but all the other tephra units are characterized in detail for the first time the ages of most tephra beds are poorly constrained but will undoubtedly become better known with the application of recently developed glass fission track methods hence prospects are favourable for the eventual development

paleomagnetic evidence for multiple late pliocene early pleistocene glaciations in the klondike area yukon territory the late pliocene early pleistocene terraces of the klondike area provide a conformal record of sedimentation which marks the transition from preglacial to glacial conditions and is one of the most complete records of glaciation and interglaciation in the northern cordillera preglacial sedimentation is recorded in the lower white channel gravel that contains a reverse normal polarity sequence a re aggradation of the nonglaciated valleys of the goldfields in response to late pliocene cooling is recorded by the upper white channel gravel which is characterized by a dominant normal polarity with a lower reversal likely associated with the first ice wedge casts klondike gravel interfingers with upper white channel and is normally magnetized indicating an equivalent magnetostratigraphic unit this glaciofluvial gravel provides evidence of extra basinal clasts marking a major reorganization of drainage associated with the first advance of the cordilleran ice sheet in western yukon territory correlated

fission track dating of a tephra layer related to poti malal and seguro drifts in the rio grande basin mendoza argentina two old drift units called poti malal and seguro have been differentiated in the rio grande basin based on relative age criteria stratigraphical relationships morphology and fission track dating a tephra dated at 0 226 0 025 ma was deposited on the poti malal drift and underlies the seguro outwash which is inferred to equate with marine oxygen isotope stage 6 the stratigraphical position and age suggest that the tephra post dates the poti malal glaciation and that it is older than the seguro drift the poti malal glaciation must be at least as old as early middle pleistocene and the seguro glaciation is assigned to the penultimate glaciation the tephra unit may have been deposited during marine oxygen isotope stage 7 copyright 2002 john wiley sons ltd

a boundary element and particle tracking model for advective transport in zoned aquifers abstract one of the most crucial factors that affects both the accuracy and efficiency of a groundwater transport model is the flow simulation method which produces the velocity field this paper presents a boundary element technique that incorporates the zoning approach to allow aquifer heterogeneity first the accuracy of the flow model is verified by comparing it with analytic solutions for well pumping and injection problems the boundary element flow model is then coupled with a particle tracking procedure for advection based transport simulation the combined model also calculates streamlines travel times and breakthrough curves and is therefore a powerful tool for simulation and management of advection dominated contamination problems

modelling of heat transfer with the random walk method part 1 application to thermal energy storage in porous aquifers abstract this study examines the application of the random walk method of resolution in the modelling of thermal energy storage in fractured aquifers the media under consideration have a high degree of fracturing but a low porosity the aperture of the fracture is small in relation to its overall extent the customary method of resolution developed to investigate the transport phenomenon in such fractured media is first presented briefly next the stochastic equations of the energy displacement in fractured aquifers are developed depending on the statistical location of the energy particles in the media a stochastic approach is used to determine the flow velocity field the method is applied to different configurations of fracture networks for a single well system of aquifer thermal energy storage the temperature fields corresponding to a fractured aquifer and its equivalent porous configuration are compared an equivalent porous medium is not an adequate substitute for a fractured aquifer in such modelling the effect of the porosity on the energy storage is obvious the temperature of the extracted water and the energy recovery factor increase with the fracture porosity except in a highly anisotropic medium the orientation distribution of the fractures has little effect on the heat transfer in an aquifer without regional flow in the presence of a natural hydraulic gradient the energy recovery factor tends towards zero and the production temperature towards the original ambient temperature of the aquifer in this case a charge and discharge well pair proves to be a suitable alternative to the single well system

discovery of a large magnitude late pleistocene volcanic eruption in alaska an extensive blanket of dacitic tephra was deposited across alaska and the western yukon territory by a plinian eruption about 80 000 years ago the volume of tephra erupted probably exceeded 50 cubic kilometers and the source was probably in the wrangell mountains this widespread layer of tephra serves as an excellent late pleistocene stratigraphic marker

on the characteristics of landslide tsunamis this review presents modelling techniques and processes that govern landslide tsunami generation with emphasis on tsunamis induced by fully submerged landslides the analysis focuses on a set of representative examples in simplified geometries demonstrating the main kinematic landslide parameters influencing initial tsunami amplitudes and wavelengths scaling relations from laboratory experiments for subaerial landslide tsunamis are also briefly reviewed it is found that the landslide acceleration determines the initial tsunami elevation for translational landslides while the landslide velocity is more important for impulsive events such as rapid slumps and subaerial landslides retrogressive effects stretch the tsunami and in certain cases produce enlarged amplitudes due to positive interference in an example involving a deformable landslide it is found that the landslide deformation has only a weak influence on tsunamigenesis however more research is needed to determine how landslide flow processes that involve strong deformation and long run out determine tsunami generation

isothermal plateau correction for partial fading of fission tracks in hydrated glass shards abstract the age of hydrated glass shards from the pleistocene ester ash bed at fairbanks alaska is determined using the population subtraction fission track method the age so determined is then corrected for partial annealing by use of the isothermal plateau fission track itp ft technique in this paper we describe the mathermatical basis of a procedure for obtaining the correct fission track age using the itp ft technique

isothermal plateau fission track age of the late pleistocene old crow tephra alaska hydrated glass shards of the widespread old crow tephra in alaska have been dated by the isothermal plateau fission track method which corrects for partial track fading if present the weighted mean age of 149 13 ka is older than previous estimates which range from 86 to 109 ka these new data support the view that old crow tephra was deposited during the cold interval that immediately preceded the last interglacial which is dated in the marine record at 125 ka b p it is likely therefore that the buried forest beds and numerous ice wedge pseudomorphs preserved in sediments just above old crow tephra in interior alaska relate to the last interglacial and not a younger warm event of early wisconsin age

on the eruption age and provenance of the old crow tephra abstract tephrochronology is used to correlate and reconstruct geographically disparate sedimentary records of changing environment climate and landscape throughout geologic time single tephra layers represent isochronous markers across broad regions thus accurate and precise radiometric constraints on the timing of eruption are critical to their utility the old crow tephra is found throughout eastern beringia and represents the largest preserved pleistocene ashfall event in the region despite its volume and significance as a stratigraphic marker the provenance of this tephra is debated and the interpreted eruption age of marine isotope stage mis 5 at 125 ka has vacillated to investigate provenance and eruption age we develop a geochemical fingerprint for the old crow tephra via titanomagnetite geochemistry and zircon crystallization cooling age via coupled u pb u th and u th he zircon geochronology our results indicate that old crow oxides are geochemically distinct from the commonly assumed source caldera system at the emmons lake volcanic center elvc zircon crystals from the old crow tephra range in age from proterozoic to pleistocene with concordant zircon u pb u th and u th he dates on the youngest population of grains suggesting crystallization in their parent magma and in turn eruption at 202 9 9 5 ka we discuss strengths and shortcomings of our radiogenic datasets in light of this result and review the far reaching implications of a change in old crow eruption age

gold run tephra a middle pleistocene stratigraphic and paleoenvironmental marker across west central yukon territory canada gold run tephra has been found at thistle creek sixtymile river and the klondike goldfields of west central yukon canada it is a hornblende bearing rhyolitic tephra with thicknesses of up to 10 cm at each site suggesting a widespread distribution across interior alaska and the yukon territory given the long distance to the nearest volcanic centre old crow flat creek and ta tephra beds are stratigraphically associated with gold run tephra at our study sites and have distinctive compositions gold run tephra is not accommodated by the current classification scheme for late cenozoic distal tephra beds in alaska and the yukon territory a scheme based on the physical and chemical attributes so that its provenance is unknown an early middle pleistocene age is supported by a glass fission track age of 0 74 0 06 ma a normal remanent magnetic polarity for the enclosing loess and the presence of a late irvingtonian faunal assemblage in the associated organic rich silts at thistle creek hence gol