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Larsen J.F.,University of Alaska Fairbanks | Nye C.J.,354 College Road | Coombs M.L.,U.S. Geological Survey | Tilman M.,University of Alaska Fairbanks | And 2 more authors.
US Geological Survey Professional Paper | Year: 2010

Deposits from the 2006 eruption of Augustine Volcano, Alaska, record a complex history of magma mixing before and during the eruption. The eruption produced five major lithologies: low-silica andesite scoria (LSAS; 56.5 to 58.7 weight percent SiO2), mostly during the initial explosive phase; high-silica andesite pumice (HSA; 62.2 to 63.3 weight percent SiO2), prevalent during the continuous phase; dense low-silica andesite (DLSA; 56.4 to 59.3 weight percent SiO2), predominantly during the late effusive phase; and dense intermediate andesite (DIA) and banded clasts, present throughout the eruption but most abundant in the continuous phase. The DIA and banded clasts have compositions that fall between and partially overlap the ranges noted above. All rock types are phenocryst-rich (36 to 44 volume percent), containing plagioclase, orthopyroxene, augite, Fe-Ti oxides, olivine, and rare amphibole, apatite, and anhydrite. Glasses from tephra and flow-deposit clasts range from 66 to nearly 80 weight percent SiO2 and represent highly evolved melt relative to the bulk rock compositions. Fe-Ti oxides recorded fO2~2 log units above the Ni-NiO buffer and temperatures of 904±47°C and 838±14°C from LSAS and HSA samples, respectively, with the intermediate lithologies falling in the middle of these ranges. The dense low-silica andesite and scoria (collectively LSA) are compositionally nearly identical, and trace-element patterns show that the HSA is not the result of shallow crustal fractionation of the LSA. The petrological and geochemical data indicate that two-component magma mixing between the LSA and HSA caused the compositional spread in eruptive products. The phenocryst population in the LSA suggests that it represents a hybrid formed from the HSA and an unerupted, basaltic "replenishing" magma. On the basis of petrological and geophysical observations reported here and elsewhere in this volume, the HSA was stored as a crystal-rich mush with its top at ~5-km depth. An influx of basalt remobilized and partially mixed with a portion of the mush, forming the hybrid LSA. The lower viscosity LSA ascended towards the surface as a dike, erupting during the explosive phase in mid-January 2006. In late January, a large explosion produced the first significant volumes of HSA, followed by several days of rapid HSA effusion during the eruption's continuous phase. After a three-week hiatus marked by elevated gas output, signifying an open system, degassed LSA erupted during the final, effusive phase. Consistency in eruptive styles and compositions suggests that the HSA magma body may have been similarly rejuvenated during the past several eruptions. Source


Benowitz J.A.,University of Alaska Fairbanks | Haeussler P.J.,U.S. Geological Survey | Layer P.W.,University of Alaska Fairbanks | O'Sullivan P.B.,Apatite to Zircon Inc. | And 2 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2012

Topographic development inboard of the continental margin is a predicted response to ridge subduction. New thermochronology results from the western Alaska Range document ridge subduction related orogenesis. K-feldspar thermochronology (KFAT) of bedrock samples from the Tordrillo Mountains in the western Alaska Range complement existing U-Pb, 40Ar/ 39Ar and AFT (apatite fission track) data to provide constraints on Paleocene pluton emplacement, and cooling as well as Late Eocene to Miocene vertical movements and exhumation along fault-bounded blocks. Based on the KFAT analysis we infer rapid exhumation-related cooling during the Eocene in the Tordrillo Mountains. Our KFAT cooling ages are coeval with deposition of clastic sediments in the Cook Inlet, Matanuska Valley and Tanana basins, which reflect high-energy depositional environments. The Tordrillo Mountains KFAT cooling ages are also the same as cooling ages in the Iliamna Lake region, the Kichatna Mountains of the western Alaska Range, and Mt. Logan in the Wrangell-St. Elias Mountains, thus rapid cooling at this time encompasses a broad region inboard of, and parallel to, the continental margin extending for several hundred kilometers. We infer these cooling events and deposition of clastic rocks are related to thermal effects that track the eastward passage of a slab window in Paleocene-Eocene time related to the subduction of the proposed Resurrection-Kula spreading ridge. In addition, we conclude that the reconstructed KFAT max negative age-elevation relationship is likely related to a long period of decreasing relief in the Tordrillo Mountains. Copyright 2012 by the American Geophysical Union. Source


Shimer G.T.,Whitman College | Benowitz J.A.,University of Alaska Fairbanks | Layer P.W.,University of Alaska Fairbanks | McCarthy P.J.,University of Alaska Fairbanks | And 2 more authors.
Cretaceous Research | Year: 2016

Diagenetically altered volcanic ash deposits (bentonites) found in Cretaceous terrestrial and marine foreland basin sediments have the potential to be used for chronostratigraphy and subsurface correlation across Alaska's North Slope. Detailed age and geochemical studies of these volcanogenic deposits may also shed light on the tectonic evolution of the Arctic. Though these bentonites have been previously studied, there are few published results for regional bentonite ages and geochemistry due to challenges of dating weathered volcanic ash. We analyzed mineral separates from cored bentonites recovered from wells in the National Petroleum Reserve Alaska. The analyses confirm that an intense period of volcanic ash deposition on Alaska's North Slope began by the late Albian and persisted throughout the Cenomanian, an interval of rapid progradation and aggradation in the Colville basin. These results also add to a sparse record of radioisotopic ages from the Nanushuk Formation. A bentonite preserved in delta plain sediments in the upper Nanushuk Formation dates to 102.6 ± 1.5 Ma (late Albian), while a bentonite near the base of the overlying Seabee Formation was deposited at 98.2 ± 0.8 Ma, in the early Cenomanian. The two ages bracket a major flooding surface at the base of the Seabee Formation near Umiat, Alaska, placing it near the Albian-Cenomanian boundary (100.5 Ma). Several hundred feet up-section, the non-marine Tuluvak Formation contains bentonites with 40Ar/39Ar ages of 96.7 ± 0.7 to 94.2 ± 0.9 Ma (Cenomanian), several million years older than previously published K-Ar ages and biostratigraphic constraints suggest. Major and trace element geochemistry of a sub-sample of six bentonites from petroleum exploration wells at Umiat show a range in composition from andesite to rhyolite, with a continental arc source. The bentonites become more felsic from the late Albian (~102 Ma) to late Cenomanian (~94 Ma). A likely source for the bentonites is the Okhotsk-Chukotka Volcanic Belt (OCVB) of eastern Siberia, a continental arc which became active in the Albian and experienced episodes of effusivity throughout the Late Cretaceous. Chronostratigraphically anomalous 40Ar/39Ar ages coincide with peaks of magmatic activity in the OCVB, suggesting that these anomalously old ages may be due to magmatic contribution of xenocrysts or recycling of detrital minerals from older volcanic events. An alternative explanation for the chronostratigraphically anomalous ages is mixing of bentonites with detrital sediment derived from unroofing and erosion of metamorphic rocks in the Brooks Range, Herald Arch, and Chukotka throughout the mid to Late Cretaceous. © 2015 Elsevier Ltd. Source


Vallance J.W.,U.S. Geological Survey | Bull K.F.,354 College Road | Coombs M.L.,U.S. Geological Survey
US Geological Survey Professional Paper | Year: 2010

Each of the three phases of the 2006 eruption at Augustine Volcano had a distinctive eruptive style and flowage deposits. From January 11 to 28, the explosive phase comprised short vulcanian eruptions that punctuated dome growth and produced volcanowide pyroclastic flows and more energetic hot currents whose mobility was influenced by efficient mixing with and vaporization of snow. Initially, hot flows moved across winter snowpack, eroding it to generate snow, water, and pyroclastic slurries that formed mixed avalanches and lahars, first eastward, then northward, and finally southward, but subsequent flows produced no lahars or mixed avalanches. During a large explosive event on January 27, disruption of a lava dome terminated the explosive phase and emplaced the largest pyroclastic flow of the 2006 eruption northward toward Rocky Point. From January 28 to February 10, activity during the continuous phase comprised rapid dome growth and frequent dome-collapse pyroclastic flows and a lava flow restricted to the north sector of the volcano. Then, after three weeks of inactivity, during the effusive phase of March 3 to 16, the volcano continued to extrude the lava flow, whose steep sides collapsed infrequently to produce block-and-ash flows. The three eruptive phases were each unique not only in terms of eruptive style, but also in terms of the types and morphologies of deposits that were produced, and, in particular, of their lithologic components. Thus, during the explosive phase, low-silica andesite scoria predominated, and intermediate- and high-silica andesite were subordinate. During the continuous phase, the eruption shifted predominantly to high-silica andesite and, during the effusive phase, shifted again to dense low-silica andesite. Each rock type is present in the deposits of each eruptive phase and each flow type, and lithologic proportions are unique and consistent within the deposits that correspond to each eruptive phase. The chief factors that influenced pyroclastic currents and the characteristics of their deposits were genesis, grain size, and flow surface. Column collapse from short-lived vulcanian blasts, dome collapses, and collapses of viscous lavas on steep slopes caused the pyroclastic currents documented in this study. Column-collapse flows during the explosive phase spread widely and probably were affected by vaporization of ingested snow where they overran snowpack. Such pyroclastic currents can erode substrates formed of snow or ice through a combination of mechanical and thermal processes at the bed, thus enhancing the spread of these flows across snowpack and generating mixed avalanches and lahars. Grain-size characteristics of these initial pyroclastic currents and overburden pressures at their bases favored thermal scour of snow and coeval fluidization. These flows scoured substrate snow and generated secondary slurry flows, whereas subsequent flows did not. Some secondary flows were wetter and more laharic than others. Where secondary flows were quite watery, recognizable mixed-avalanche deposits were small or insignificant, and lahars were predominant. Where such flows contained substantial amounts of snow, mixed-avalanche deposits blanketed medial reaches of valleys and formed extensive marginal terraces and axial islands in distal reaches. Flows that contained significant amounts of snow formed cogenetic mixed avalanches that slid across surfaces protected by snowpack, whereas water-rich axial lahars scoured channels. Correlations of planimetric area (A) versus volume (V) for pyroclastic deposits with similar origins and characteristics exhibit linear trends, such that A=cV2/3, where c is a constant for similar groups of flows. This relationship was tested and calibrated for dome-collapse, column-collapse, and surgelike flows using area-volume data from this study and examples from Montserrat, Merapi, and Mount St. Helens. The ratio A/V2/3 = c gives a dimensionless measure of mobility calibrated for each of these three types of flow. Surgelike flows are highly mobile, with c ≈ 520; column-collapse flows have c ≈ 150; and dome-collapse flows have c ≈ 35, about that of simple rock avalanches. Such calibrated mobility factors have a potential use in volcano-hazard assessments. Source


Coombs M.L.,U.S. Geological Survey | Bull K.F.,354 College Road | Vallance J.W.,U.S. Geological Survey | Schneider D.J.,U.S. Geological Survey | And 3 more authors.
US Geological Survey Professional Paper | Year: 2010

During and after the 2006 eruption of Augustine Volcano, we compiled a geologic map and chronology of new lava and flowage deposits using observational flights, oblique and aerial photography, infrared imaging, satellite data, and field investigations. After approximately 6 months of precursory activity, the explosive phase of the eruption commenced with two explosions on January 11, 2006 (events 1 and 2) that produced snow-rich avalanches; little or no juvenile magma was erupted. Seismicity suggests that a small lava dome may have extruded on January 12, but, if so, it was subsequently destroyed. A series of six explosions on January 13-14 (events 3-8) produced widespread but thin (0-30 cm) pyroclastic-current deposits on the upper flanks above 300 m altitude and lobate, 0.5- to 2-m-thick pyroclastic flows that traveled down most flanks of the volcano. Between January 14 and 17, a smooth lava lobe formed in the east half of the roughly 400-m-wide summit crater and was only partially covered by later deposits. An explosion on January 17 (event 9) opened a crater in the new lava dome and produced a ballistic fall deposit and pyroclastic flow on the southwest flank. During the interval from January 17 to 27, a rubbly lava dome effused. On January 27, explosive event 10 generated a pyroclastic current that left a deposit, rich in dense clasts, on the north-northwest flank. Immediately following the pyroclastic current, a voluminous 4.7-km-long pyroclastic flow swept down the north flank. Three more explosive blasts on January 27 and 28 produced unknown but likely minor on-island deposits. The cumulative volume of erupted material from the explosive phase, including domes, flows, and fall deposits (Wallace and others, this volume), was 30×106 m 3 dense-rock equivalent (DRE). The continuous phase of the eruption (January 28 through February 10) began with a 4-day period of nearly continuous block-and-ash flows, which deposited small individual flow lobes that cumulatively formed fans to the north and northeast of the summit. A single larger pyroclastic flow on January 30 formed a braided deposit on the northwest flank. Roughly 9×106 m3 (DRE) of magma erupted during this period. Around February 2, the magma flux rate waned and a northward lava flow effused and reached a length of approximately 900 m by February 10. Approximately 11×106 m3 (DRE) of magma erupted during the second half of the continuous phase. After a 23-day hiatus, lava effusion recommenced in early March (the effusive phase) and was accompanied by frequent (but volumetrically minor) block-and-ash flows. From March 7 to 14, extrusion increased markedly; two blocky lava-flow lobes, each tens of meters thick, moved down the north and northeast flank of the volcano; and a new summit lava dome grew to be ~70 m taller than the pre-2006 summit. This phase produced 26×106 m3 (DRE) of lava. Active effusion had ceased about March 16, but, in April and May, three gravitational collapses from the west margin of the north lava flow produced additional block-and-ash flows. The basic sequence of the 2006 eruption closely matches that of eruptions in 1976 and 1986. Source

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