Now showing items 1-20 of 255

• #### Sediments of the Norris Glacier outwash area, upper Taku Inlet, southeastern Alaska

An 8-square mile outwash fan, composed of gravelly sediment, extends from the terminus of Norris Glacier to the waters of upper Taku Inlet, Southeastern Alaska. Thirty-seven surface sediment samples from the tidal portion of the fan form the bulk of this study. The tidal flat is largely composed of very poorly sorted muddy sediment and relatively well sorted sand which, for the most part, overlie outwash gravel. Mixing of various modal size classes has produced a complex sediment distribution pattern as well as a complicated size-sorting relationship. The sand-size fraction of the sediments consists of feldspar, quartz, rock fragments, amphiboles, pyroxenes, micas and opaques; the clay-size fraction consists of micas, chlorite, montmorillonite, feldspar and amphibole. The sediments are the product of glacial abrasion in the Juneau Ice Field area. The sand and mud are derived largely from Norris and Taku Glacier detritus; their nature indicates valley glacier detritus may be fairly rapidly sorted when subjected to hydraulic action. Absence of quartz and presence of feldspar in the clay-size fraction may indicate the physical properties of these minerals control the size to which they can be reduced by valley-glacier abrasion.
• #### The geology of three extrusive bodies in central Alaska Range

The Buzzard Creek basalt, Jumbo Dome, and Sugar Loaf Mountain occur in the Central Alaska Range. The purpose of this study is to determine the age, nature, geothermal potential, and possible genetic relationships between these igneous bodies. The areas were investigated by mapping, radiometric dating, and petrologic studies. The Buzzard Creek basalt appears to have formed by a maar eruption about 3,000 years ago. Seismic evidence suggests this basalt may be related to current subduction in the area. Jumbo Dome consists of calc-alkaline andesite and is probably Pleistocene in age. Sugar Loaf Mountain is composed of Mid-Tertiary rhyolite. Geochemistry suggests that the Sugar Loaf Mountain rhyolite and Jumbo Dome andesite may also be subduction-related. Differences in age and geochemistry indicate there is no genetic relationship between the rocks of the three areas. The ages, type of volcanic features', and snow melt patterns suggest that these three areas have low geothermal potential.
• #### An improved method of ice nucleus measurement

Ice nuclei, which initiate the ice nucleation process at a higher temperature than the homogeneous nucleation temperature, are essential for the initiation of the ice phase in clouds. Unfortunately, no standard method has been established for the measurement of ice nucleus concentration. The filter technique is a promising candidate if the tendency for ice nucleus concentrations to decreases as the volume sampled increases can be explained. For this study, an improved ventilation method for the development of exposed filters was developed and tested. The results were compared with results obtained in a static diffusion chamber. The volume effect was observed to be less with the new dynamic system. Further work needs to be done to find the optimum flow rate in order to reduce the vapor depletion problem to a minimum. The ratio of total counts of dynamic and static system appears to be a promising evaluation index.
• #### Increases And Fluctuations In Thermal Activity At Mount Wrangell, Alaska (Volcano, Glacier)

The objectives of this study were to document and interpret changes in thermal activity at two of three craters located on the rim of the ice-filled summit caldera of Mount Wrangell, an active glacier-clad shield volcano in south-central Alaska. The technique of "glacier calorimetry" was developed, through which changes in the volume of glacier ice in the craters and caldera were measured and related to changes in heat flow. Chemical analyses of gases and acid-thermal waters provided information on the underlying heat source. In 1965, thermal activity began increasing at both the North and West Craters. During the ensuing years, heat flow increased significantly at the North Crater, although in a highly fluctuating manner, while gradually declining at the West Crater. Pulses in heat flow at the North Crater occurred in 1966-68 and 1972-74, with both pulses followed by a four-year decline in activity. Increases in heat flow began again in 1978-79 and have continued unabated through the summer of 1983. Over 80 percent of the 4.4 x 10('7)m('3) ice volume within the crater in 1966 was melted by 1982, and the meltwaters have drained or evaporated from the crater. The subsequent rapid development of numerous fumaroles, the large dry-gas proportion of SO(,2) (27 percent), and the inferred presence of gaseous HCl indicate that a shallow degassing magma body is the source of heat driving the thermal system. Seismically induced fracturing above the magma body is hypothesized to explain the initial increases in thermal activity. The resulting massive influx of meltwaters into the subsurface is suggested as the cause of the fluctuations in heat flow. The continued increase in activity since 1979 suggests that the volume of meltwater being generated is no longer sufficient to quench the heat source beneath the crater.
• #### Upper crustal structure of southern Alaska: An interpretation of seismic refraction data from the Trans-Alaska Crustal Transect

Seismic refraction and wide-angle reflection data from the U.S. Geological Survey's Trans-Alaska Crustal Transect is used to investigate the upper crustal structure of southcentral Alaska. The data consist of two intersecting refraction lines: the 135-km Chugach profile which follows the E-W strike of the Chugach Mountains and the 126-km Cordova Peak profile which follows the N-S regional dip. The four shots of the Chugach profile and the five shots of the Cordova Peak profile were recorded on 120 portable seismic instruments spaced at 1-km intervals. Interpretation of data from the Chugach terrane indicates that near-surface unconsolidated sediment and glacial ice overlie rocks of unusually high average compressional velocities (5.4-6.9 km/s) in the upper 10 km of crust. A thick unit correlated with a metasedimentary and metavolcanic flysch sequence has velocities of 5.4-5.9 km/s. It is underlain by mafic to ultramafic metavolcanic rocks (6.0-6.4 km/s) correlated with the terrane basement. Mid-crustal layers beneath the Chugach terrane contain two velocity reversals (6.5 and 6.7 km/s) attributed to off-scraped oceanic sedimentary rocks which are underlain by mafic to ultramafic oceanic volcanic crust (7.0-7.2 km/s). Interpretation of data from the Prince William terrane indicates systematically lower velocities in Prince William terrane rocks as compared to Chugach terrane rocks at comparable depths. The upper 10 km of crust, having average compressional velocities of 3.0-6.2 km/s, is correlated with clastic sedimentary and volcanic rocks which are overlain by younger terrigenous sedimentary rocks. A 2-km thick layer at 10-12 km depth is correlated with mafic to ultramafic Prince William terrane basement rocks. The difference in velocity structure between the Chugach and Prince William terranes suggests that the Contact fault zone is a terrane boundary which extends to a depth of at least 10-12 km. Deep structure beneath the two terranes is not well constrained by the seismic refraction data. Potential field data support the interpretation that a thick low-velocity zone occurs at a 12-15 km depth and may contain subducted continental rocks of the Yakutat terrane, which is currently accreting to and being thrust beneath the North American continent along the Gulf of Alaska margin.
• #### A comparative study of contrasting structural styles in the range-front region of the northeastern Arctic National Wildlife Refuge, northeastern Brooks Range, Alaska

The range front of the northeastern Brooks Range in the Arctic National Wildlife Refuge (ANWR) is defined by anticlinoria cored by a 'basement' complex of weakly metamorphosed sedimentary, volcanic and intrusive rocks. These anticlinoria are interpreted to reflect horses in a northward-propagating regional duplex between a floor thrust at depth in the 'basement' complex and a roof thrust near the base of the cover sequence. Lateral variations in the geometry of these range-front anticlinoria reflect changes in lithology and deformational style of both the 'basement' and its cover. Two distinct structural geometries are displayed along the range front of northeastern ANWR. To the east, the large range-front anticlinorium is interpreted to reflect multiple horses of Cenozoic age within the stratified, slightly metamorphosed sedimentary and volcanic rocks of the pre-Mississippian 'basement'. During Cenozoic thrusting, these mechanically heterogeneous rocks deformed primarily via thrusting and related folding with minor penetrative strain. The Mississippian and younger cover sequence shortened via both thrust duplication and detachment folding above a detachment in the Mississippian Kayak Shale. In contrast, to the west the pre-Mississippian rocks consist primarily of the mechanically homogeneous Devonian Okpilak batholith. The batholith was transported northward during Cenozoic thrusting and now forms a major topographic and structural high near the range front. The batholith probably shortened during thrusting as a homogeneous mass via penetrative strain. Because the Kayak Shale is thin to absent in the vicinity of the batholith, Mississippian and younger rocks remained attached to the batholith and shortened via penetrative strain and minor imbrication. These two range-front areas form the central portion of two regional transects through northeastern ANWR. General area-balanced models for both transects suggest that the amount of total shortening is governed by the structural topography and the geometry of the basal detachment surface. While the structural topography of northeastern ANWR is reasonably well-constrained, the geometry of the basal detachment is not. Given a range in reasonable basal detachment geometries, shortening in both transects ranges from 16% to 61%. Detailed balanced cross sections based on subsurface and surface geologic data yield 46-48% shortening for both transects.
• #### Simulation of the recurrence probability of ice islands in the Arctic Ocean

Ice islands, the most extreme ice features in the Arctic Ocean, are hazards to offshore structures. To determine the probability of ice island trajectories in coastal areas of the Arctic Ocean, a random simulation model has been established. The model consists of the random ice island generation, the ice island dynamic model, and the Monte Carlo model for random geostrophic wind generation. Based on statistics of observation data, the generation location was assumed as uniformly distributed along the northern side of Ellesmere Islands. A 4 year interval of generation event is considered in the simulation. The number of new ice islands calved from ice shelves in one calving event is automatically produced by deducting each ice island area from the random area of ice shelf calved in one time. As a driving force source, geostrophic wind field was calculated from monthly-averaged pressure charts. As an significant force, the pack ice force was formulated by theoretical analysis combined with an existing empirical formula for each case. The results of probabilities of simulated ice island trajectories show that there are two zones of highest recurrence of ice islands, one near the Canadian Beaufort Sea, and another near the Chukchi Sea. There is a broad area of 1 to 10 year recurrence interval in the central ocean, and a high probability zone near the north end of Greenland. The simulation also yielded the frequencies of ice island ejection, the lifetime of ice islands, and the number of live ice islands in the Arctic Ocean. Two basic drift patterns of ice islands have been displayed by the simulation: short drift patterns in which ice islands move directly out of the ocean after generation, and the large-scale circulation pattern in which ice islands circulate around the Beaufort Sea from one to four times.
• #### Geochemical studies of fumarolic systems in the eastern Aleutian Volcanic Arc: Applications for understanding magmatic and volcanic processes

Geochemical studies of active and fossil fumaroles were conducted at Mount St. Augustine and the Valley of Ten Thousand Smokes (VTTS) to investigate fumarolic systems for providing information on volcanic and magmatic processes. Gases and condensates collected from high-temperature rooted fumaroles at Mount St. Augustine in 1979, 1982, and 1984 are characterized by systematic long-term trends in gas composition and stable isotopes that can be best explained by progressive magmatic outgassing coupled with increasing proportions of seawater in the fumarolic emissions. Seawater-magma interaction may initiate some of the early explosive phases of Mount St. Augustine eruptions. The distribution and morphology of rootless fumaroles formed on pyroclastic flows and a lava flow emplaced during the 1986 eruptive cycle of Mount St. Augustine were controlled by pre-eruption drainage and topography, as well as by the thickness, compaction, and settling of the flow deposits. The majority of chemical components present in encrustations collected from these active fumaroles were derived by acidic condensate leaching of the eruptive deposits. Trace-element distribution apparently followed a pattern of isomorphic substitution in the encrustation phases. A reconnaissance survey of surface Hg$\sp\circ$ contents in the VTTS supports the presence of a shallow intrusion beneath the dome-like feature known as the Turtle. Based on the Hg$\sp\circ$ data, the preferred model of the 1912 Novarupta vent is one generated by collapse of supporting vent walls into a cored-out explosive vent after the major eruptive phase. Vent morphology is funnel-like with subsidence concentrated in the narrow funnel center. The magnitude of the Novarupta Basin Hg$\sp\circ$ anomalies implies that a shallow ($\approx$1 km depth) incipient hydrothermal system has developed beneath the vent.
• #### Studies of the geophysics of sea ice

A non-linear growth model that solves the surface energy balance and heat conduction equations was developed to estimate thermal and physical properties of sea ice. The model incorporates several mechanisms that affect the salinity profile, including initial brine entrapment, brine expulsion, and gravity drainage, and is a non-linear extension of the model initially developed by Cox and Weeks (1988). Simulations were run to investigate the effects of the non-linear feedbacks which exist between the ice growth velocity and the thermal properties of the resulting ice. A comparison of the growth rate versus accumulated freeze-days was performed on the linear model, the non-linear model, and empirical formulas based on field observations. Allowing the model to run through the summer months with retarded ice growth and making an attempt at modelling summer desalinization processes produced second and third-year ice with proper temperature and salinity profiles. The ice growth model was then coupled to a Lambertian surface backscattering model for radar. By calculating the average dielectric constant of the penetration depth and using this value in the backscattering model, a comparison of the predicted signature variations in first-year sea ice was performed against observed backscattering values from ERS-1 SAR images of Dease Inlet, Alaska. The agreement between calculated and observed backscatter was surprisingly good considering that other factors may also influence radar returns. However, the more surprising result was the rescaling of the predicted first year ice signature by +6 dB produced a remarkable fit to observed backscattering values of multiyear ice. The predicted backscatter values and ice thicknesses were then used in conjunction with ERS-1 SAR imagery of the high Arctic to estimate areal coverage of the three major ice types in a 100 x 100 $km\sp2$ area. Heat and mass flux calculations were then performed to produce daily estimates of energy loss and salt infusion for the winter months of October 1991 through March 1992.
• #### Climate, seasonal snow cover and permafrost temperatures in Alaska north of the Brooks Range

Climatological data, active layer and permafrost measurements, and modeling were used to investigate the response of permafrost temperatures to changes in climate in Alaska north of the Brooks Range. Mean annual air temperature (MAAT) from 1987 to 1991 within about 110 km from the Arctic Coast was ${-12.4}\pm0.3\sp\circ C,$ while the mean annual permafrost surface temperature (MAPST) ranged from ${-9.0}\sp\circ C$ along the coast to ${-5.2}\sp\circ C$ inland. Air temperature changes alone can not explain the permafrost warming from the coast to inland. Measurements show that MAPST are about $3\sp\circ C$ to $6\sp\circ C$ warmer than MAAT in the region. The interaction of local microrelief and vegetation with snow appears to change the insulating effect of seasonal snow cover and may be the major factor which controls the permafrost temperature during the winter and thus the MAPST. Sensitivity analyses show that for the same MAAT conditions, changes in seasonal snow cover parameters can increase or decrease the MAPST about $7\sp\circ C.$ Snowfall was greater during the cold years and less during the warm years and was poorly correlated between stations. These results suggest that the effects of changes in air temperatures on permafrost temperatures historically may also have been modified by changes in snow cover. A numerical model was used to investigate the effect of changes in initial permafrost temperature conditions, MAAT, seasonal snow cover and thermal properties of soils on the permafrost temperatures. Permafrost may have started warming about the same time as the atmosphere did in the late 1800's, and the long term mean surface temperature of the permafrost may have been established prior to this time. Variations in the penetration depth of the warming signal may be related to differences in thermal properties of permafrost. Variations in the magnitude of the permafrost surface warming may be due to the effect of local factors such as soil type, vegetation, microrelief, soil moisture, and seasonal snow cover. The effect of the interaction of vegetation and snow cover may amplify the signal of temperature change in the permafrost.
• #### Stratigraphic variation across a Middle Devonian to Mississippian rift-basin margin and implications for subsequent fold and thrust geometry, northeastern Brooks Range, Alaska

A stratigraphic record from the eastern Brooks Range, Alaska, is interpreted to represent erosion and deposition of syn-rift and post-rift terrigenous clastic rocks across a Middle Devonian - Mississippian rift-basin margin. Middle Devonian - Mississippian terrigenous clastic rocks unconformably overlie complexly deformed Romanzof chert and constrain the age of latest mid-Paleozoic contractional deformation to pre-Middle Devonian time. The succession forms an abruptly southward-thickening basin-margin wedge characterized by abrupt facies changes, local evidence of active tectonism, multiple unconformities merging northward toward the basin margin, locally derived clastic deposits. The oldest deposits of this wedge are Middle - Upper(?) Devonian shallow-marine to alluvial-fan deposits (Ulungarat formation). Algal limestone with intercalated terrigenous clastic deposits and plant fossils (Mangaqtaaq formation) locally overlies the Ulungarat formation. The Ulungarat and Mangaqtaaq formations are interpreted to record syn-rift deposition. Coastal-plain to marine deposits of transgressive Kayak Shale overlie and intertongue with retrogradational Kekiktuk Conglomerate, recording coastal retreat and drowning of low-energy paleoshoreline. Deposits of the retrogradational Kekiktuk fluvial system thin and fine upward and to the north, reflecting depositional onlap of the basin-margin high. Kekiktuk Conglomerate and Kayak Shale are interpreted to overlie the post-rift unconformity and record the beginning of thermal subsidence. This stratigraphic succession provides a spatial and genetic link between structurally separated, stratigraphically distinct rocks of the Endicott Group. Thick, allochthonous rocks to the south record progradation and eventual retrogradation of a basin-filling wedge, whereas thin, autochthonous rocks to the north record transgressive overlap of the basin-margin sediment source area. The structural boundary between the north-central and northeastern Brooks Range coincides with the mid-Paleozoic rift-basin margin. North-vergent duplexes beneath the Kayak Shale consist of horses in the Middle Devonian-Mississippian clastic wedge to the south and thicker horses in pre-Middle Devonian rocks to the north. Above the Kayak Shale, north-vergent thrust-truncated folds are succeeded northward by detachment folds. These structural characteristics reflect the combined influence of abrupt lateral changes in stratigraphy across the rift-basin margin and stratigraphically controlled vertical variations in structural behavior.
• #### Detachment folds of the northeastern Brooks Range, Alaska: A basis for geometric and kinematic models of detachment folds

Detachment anticlines are defined by mechanically competent rock layers and form both by internal deformation of an adjacent weak layer and detachment above a lower competent unit. This study is important for: (1) Other fold-and-thrust belts. Detachment folds are probably very common in fold-and-thrust belts worldwide, but they are rarely recognized as such and are commonly mistaken for other fold-types. This is partly because a rigorous general model for detachment folds that allows for changes in detachment depth and for fixed-arc length kinematics is lacking in the geologic literature. A general detachment fold model is presented here that: (a) is based on observations of natural folds in the northeastern Brooks Range of Alaska; (b) does not assume constant detachment depth or hinge-migration kinematics and; (c) allows quantification of non-plane strain. The folds observed can be modeled kinematically as fixed-hinge buckle folds, whereas the fold geometry and distribution of strain indicators in each fold precludes the migrating-hinge kinematic interpretation that is common in published models. Layer-parallel shortening, initial fold asymmetry, initial stratigraphic thickness of the incompetent unit, and the nature of rheological gradations each predictably influence fold evolution. This study suggests a general scenario for the evolution of a typical detachment fold. The area defined by a detachment anticline increases rapidly during early stages of folding and this is accompanied by a decrease in depth to detachment beneath synclines and the formation of fixed-hinge parasitic and disharmonic folds. This trend continues until the interlimb angle of the primary fold reaches 90$\sp\circ$. Increased shortening requires volume-loss in the core and/or an increase in detachment depth beneath the fold. Finally, depending on the rheology of the system, the fold may lock and/or be truncated by a thrust fault. (2) Regional tectonics. The western part of the northeastern Brooks Range is mostly a passive-roof duplex, but this study shows that forward-propagating deformation occurred at various structural positions. (3) Economics. Detachment folds may form petroleum traps that require a treatment different than that for fault-bend or fault-propagation folds. Detachment fold traps may exist beneath the coastal plain of the Arctic National Wildlife Refuge.
• #### Merging remotely sensed data with geophysical models

Geophysical models are usually derived from the idealistic viewpoint that all required external parameters are, in principle, measurable. The models are then driven with the best available data for those parameters. In some cases, there are few measurements available, because of factors such as the location of the phenomena modeled. Satellite imagery provides a synoptic overview of a particular environment, supplying spatial and temporal variability as well as spectral data, making this an ideal source of data for some models. In other cases, although frequent satellite image observations are available, they are of little use to the modeler, because they do not provide values for the parameters demanded by the model. This thesis contains two examples of geophysical models that were derived expressly to utilize measurements and qualitative observations taken from satellite images as the major driving elements of the model. The methodology consists of designing a model such that it can be 'run' by numerical data extracted from image data sets, and using the image data for verification of the model or adjustment of parameters. The first example is a thermodynamic model of springtime removal of nearshore ice from an Arctic river delta area, using the Mackenzie River as a study site. In this example, a multi-date sequence of AVHRR images is used to provide the spatial and temporal patterns of melt, allowing the required physical observations in the model to be parameterized and tested. The second example is a dynamic model simulating the evolution of a volcanic ash cloud under the influence of atmospheric winds. In this case, AVHRR images are used to determine the position and size of the ash cloud as a function of time, allowing tuning of parameters and verification of the model.