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At least 1,513 records · Page 84Linked to original sources
USGS2019

Links between tectonics, magmatism, and mineralization in the formation of Late Cretaceous porphyry systems in the Yukon-Tanana upland, eastern Alaska, USA

Cretaceous-Paleocene porphyry Cu(±Mo±Au) occurrences are scattered throughout the Yukon-Tanana upland in eastern Alaska. Known occurrences in eastern Alaska are poorly characterized, despite a resurgence in exploration. Porphyry deposits in the upland are emplaced into structurally complex metamorphic rocks representing a variety of tectonic environments, resulting in diverse alteration and mineralization assemblages. New mapping, drill core logging, petrography, geochemistry, geochronology, and structural analysis allow improved characterization of the parameters of porphyry systems and identify key linkages to regional tectonic and magmatic events. New sericite 40 Ar/ 39 Ar and zircon U/Pb dates constrain porphyry systems to the Late Cretaceous-earliest Paleocene (ca. 71-63 Ma). Zircon Hf-isotope ratios and Ce and Eu concentrations indicate that Late Cretaceous-Paleocene intrusions emplaced into basement dominated by Triassic and Jurassic plutons are more isotopically juvenile, reflecting more oxidized conditions. In contrast, those emplaced into basement dominated by mid-Cretaceous plutons are more reduced crustal geochemical-affinity. Diversity in mineral assemblages in contrasting systems may reflect emplacement into crustal domains of varying compositions and oxidation states. Those formed within a domain containing more-oxidized Triassic and Jurassic plutons are molybdenite-rich and apparently lack gold. In contrast, systems formed within domains dominated by more reduced mid-Cretaceous plutons contain lower-sulfidation state mineral assemblages with reported gold.

Alaska, Yukon↗
USGS2020

Using hair cortisol to assess physiological stress in Alaska polar bears

The concentration of cortisol in hair (HCC) of polar bears (Ursus maritimus) may provide a retrospective view of physiological stress they experience and a link to their response to environmental change. To understand this relationship, we assayed HCC from polar bears captured in the Alaska Beaufort, Bering and Chukchi seas during 1983–1989 and 2004–2016. Cortisol accumulated in hair through summer and autumn and into the subsequent winter. HCC was similar between adult males and adult females. No difference in HCC across regions suggested all bears responded similarly to the environment. HCC in spring was elevated following years with a high winter Arctic Oscillation index and highly variable wind speed. HCC increased non-linearly with increasing duration of the continental shelf summer open water period up to 50 days and then decreased. HCC of spring samples declined with increasing body size, indicating that the stress response was more active in smaller bears or those in poor body condition. HCC of spring samples was greater and more variable in 2004–2006 than during either 1983–1989 or 2008–2016, and significantly so for females with 1st year cubs and subadult females. Elevated HCC in 2004–2006 coincided with years of reduced survival of southern Beaufort Sea polar bears and suggests that unidentified environmental perturbations impacted Alaska polar bears. Because HCC may be obtained by relatively non-invasive means, it has potential use for assessing polar bear populations that are difficult to study by capturing. Hence, information gained from HCC can inform polar bear conservation, especially on the vulnerability of subadult females and adult females with new cubs, and possible future environmental perturbations impacts on bear physiology.

Alaska↗
USGS2021

Permafrost characterization and feature identification using public domain airborne electromagnetic data, interior Alaska

The Alaska Division of Geological & Geophysical Surveys (DGGS) airborne electromagnetic (AEM) data are an excellent resource for permafrost characterization. AEM data can be used for pingo identification, estimating permafrost thickness, estimating surface talik thickness, evaluating permafrost health (temperature), talik identification and more. Data examples are shown from discontinuous permafrost areas just north of Fairbanks, Alaska, USA. Interpretations are made from 2D and 3D resistivity models created from 1D inversions of the Goldstream Valley AEM survey data (Emond, 2018a).

Alaska↗
USGS1975

Geologic setting and chemical characteristics of hot springs in west-central Alaska

Numerous hot springs occur in a variety of geologic provinces in west-central Alaska. Granitic plutons are common to all the provinces, and the hot springs are spatially associated with the contacts of these plutons. Of 23 hot springs whose bedrock geology is known, all are within 4.8 km (3 mi) of a granitic pluton. The occurrence of hot springs, however, appears to be independent of the age, composition, or magmatic history of the pluton. Most of the analyzed hot springs appear to have chemical and isotopic compositions indicating that they were derived from deeply circulating meteoric water. About 25 percent of the analyzed hot springs show a distinct saline character with high concentrations of chloride, sodium, potassium, and calcium indicating either much more complex water-rock reactions than in the other hot springs or the addition of another type of water. Chemical geothermometers suggest subsurface temperatures in the general range of 70° to 160°C. If the hot spring waters have derived their heat solely from deep circulation, they must have reached depths of 2 to 5 km (6,000-15,000 ft), assuming geothermal gradients of 30° to 50°C/km. If a shallow igneous heat source exists in the area or if dilution or mixing has occurred, these depths may be shallower. The geologic and chemical data, although preliminary, suggest that most of the hot springs of west-central Alaska have relatively low subsurface temperatures and limited reservoir capacities in comparison with geothermal areas presently being utilized for electrical power generation. :The springs may, however, have some potential for limited power generation locally, if and when heat-exchange technology becomes available, as well as for space heating and agricultural uses.

Alaska↗
USGS1974

Chemical variations across the Alaska-Aleutian Range batholith

A study of 79 chemical analyses of plutonic rocks in the northern part of the Alaska-Aleutian Range batholith shows that K 2 O and SiO 2 in Late Cretaceous and Tertiary rocks decrease toward the Pacific margin and that Al 2 O 3 and CaO increase. Plots for Fe 2 O 3 , FeO, MgO, and TiO 2 suggest a possible increase toward the Pacific margin; Na 2 O, H 2 O+, and MnO show no significant trends across the batholith. Oxide trends for groups of plutons in the western and eastern parts of the batholith in general are opposite that of the groups combined. For the Jurassic plutonic rocks, K 2 O may decrease toward the Pacific margin; other oxides show considerable scatter, and meaningful trends are not readily apparent. The oxide trends across the batholith are similar to trends across the central Sierra Nevada batholith of California, with the exception of SiO 2 and A1 2 O 3 , which show no significant changes. In the Coast Range batholith of British Columbia, potassium also increases away from the Pacific margin. The increase in K 2 O toward the continent in the Alaska-Aleutian Range batholith is similar to that found across Quaternary volcanic island arcs, where K 2 O content increases directly with increasing depth to inclined seismic zones. This increase suggests that generation of magma may have taken place along, or above, a paleoseismic zone. Geologic evidence supports such a model for the Jurassic plutonic rocks, which represent the roots of an early Mesozoic magmatic arc that probably formed above a descending oceanic plate. However, the hypothesis that magma for Late Cretaceous and Tertiary plutonic rocks was generated along seismic zones does not fit available geologic evidence, and an anatectic model cannot be ruled out.

Alaska↗
USGS1977

Intrusive rocks of the Yakutat-St. Elias area, south-central Alaska

Twenty-three plutons, exposed over a total area of nearly 1200 km 2 , have been studied in the Alaska part of the St. Elias Mountains between long 138° and 141°W. Results of potassium-argon age determinations combined with field relations, petrography, and major- and trace-element chemistry suggest six major intrusive events: (1) late Paleozoic gabbro to quartz diorite intruded Paleozoic metamorphic rocks that are probably equivalent to the Kaskawulsh Group in adjacent areas of Canada, (2) Triassic quartz diorite formed one small pluton in undated metamorphic rocks near Mt. St. Elias, (3) Jurassic tonalite and granite intruded upper Paleozoic(?) and lower Mesozoic(?) metamorphic rocks, (4) Late Cretaceous or Tertiary altered tonalite formed three widely separated plutons in metasedimentary rocks of Jurassic(?) and Cretaceous age in the Yakutat Group, (5) Eocene granodiorite and granite, and (6) late Cenozoic tonalite and granodiorite intruded both the Yakutat Group and upper Paleozoic(?) and lower Mesozoic(?) metamorphic rocks. The Paleozoic, Jurassic, and Cretaceous or Tertiary plutonic suites are restricted to particular geologic terranes, and the Jurassic and Eocene suites correlate with regional plutonic belts present elsewhere in southern Alaska. The distribution of the Tertiary plutons does not require large-scale horizontal displacements along the Fairweather and other major high-angle faults. The available data indicate that the mineral resource potential of the Yakutat-St. Elias area is low for those deposits that are generally related to magmatic processes.

Alaska↗
USGS1976

Microprobe study of olivine chromitites of the Goodnews Bay ultramafic complex, Alaska, and the occurrence of platinum

Electron-microprobe analyses of the dunite and olivine chromitite of the ultramaflc complex at Goodnews Bay, Alaska, show that the chromite contains more ferrous and ferric iron than is found in chromite from alpine or stratiform complexes. The iron-magnesium distribution coefficients between chromite and olivine demonstrate that more ferrous iron is partitioned to the chromite in the Goodnews Bay complex than to the chromite in stratiform chromitite but that the distribution is similar to that for alpine chromitite. The high iron content of the chromite and olivine at Goodnews Bay and the general geology of the complex indicate that it is a concentric-type ultramaflc complex similar to the platinum-bearing concentric complexes of southeastern Alaska. A platinum-alloy inclusion in the chromite found in the ultramaflc section at Goodnews Bay demonstrates that the olivine chromitite is the source of at least some of the platinum.

Alaska↗
USGS2023

Vulnerability and resilience of ice-rich permafrost to thermal erosion gullying in the Arctic Foothills infrastructure corridor, Alaska

The Arctic and its permafrost terrains are inherently dynamic, complex, and sensitive environments. Understanding the past and current changes occurring in these systems is key in predicting future variations, including the response of permafrost to climate change and to surface disturbances resulting from natural processes or anthropogenic activities. Here, we focus on advancing our understanding of the drivers controlling terrain vulnerability and resilience to thermal erosion gullying proximal to linear infrastructure in the lowlands of the Arctic Foothills (Alaska). This builds upon our previous work (Stephani et al. 2023) in the infrastructure corridor that includes the Dalton Highway and Trans Alaska Pipeline System (TAPS). To identify locations affected by thermal erosion gullying and recognize changes in the ~80-km long corridor section that travels through the Arctic Foothills, we compared time series of high-resolution satellite imagery from 2001 to 2023. When we captured the timing of gullying onset and/or growth, we examined prevailing climatic conditions (Sagwon Station) prior to these terrain changes. We integrated our findings with our field-based data (e.g., drilling) described in Stephani et al. (2023).

Alaska↗
USGS2025

Evidence for offset of Cretaceous plutons by the Tintina fault in eastern Alaska: Implications for regional metallogeny

Cretaceous magmatism in eastern interior Alaska is voluminous, but temporally and spatially diverse – suggestive of varying sources and drivers. More than 150 new U-Pb zircon and more than 500 geochemical analyses of Cretaceous plutonic units allow for the grouping of distinct plutonic suites. Magmatism was continuous from 120-66 Ma but can be grouped into temporally distinct pulses from ca. 115-100 Ma, 100-90 Ma, and 75-66 Ma. Geochemical diversity occurs during each pulse, further distinguishing multiple suites. Diverse metallogenic epochs are strongly correlated to pluton chemistry. Mineralization is largely absent prior to 108 Ma. From 108-100 Ma, plutonism is coeval with sparse, but notable Au-quartz veins with variable Bi, As, W, and Mo. From 100-90 Ma, intrusion-related mineralization zones from Au-Cu(-Bi) and U-Th in the northwest to central Au- Bi-As-Te(-W), and Mo-W to the southeast. Porphyry style Cu-Mo(-Au) occurrences occur with the latest Cretaceous plutons emplaced from 75-66 Ma . Restoration of ~450 km of dextral movement on the Tintina fault and comparison of metallogenic and geochemical characteristics of Alaska plutons suggest 100-90 Ma plutons may be the continuation of the metallogenically significant Tombstone, Mayo, and Tungsten suites from the Yukon.

Alaska↗
USGS2013

Challenges in making a seismic hazard map for Alaska and the Aleutians

We present a summary of the data and analyses leading to the revision of the time-independent probabilistic seismic hazard maps of Alaska and the Aleutians. These maps represent a revision of existing maps based on newly obtained data, and reflect best current judgments about methodology and approach. They have been prepared following the procedures and assumptions made in the preparation of the 2002 National Seismic Hazard Maps for the lower 48 States, and will be proposed for adoption in future revisions to the International Building Code. We present example maps for peak ground acceleration, 0.2 s spectral amplitude (SA), and 1.0 s SA at a probability level of 2% in 50 years (annual probability of 0.000404). In this summary, we emphasize issues encountered in preparation of the maps that motivate or require future investigation and research.

Alaska↗
USGS2011

A paleoseismic study along the central Denali Fault, Chistochina Glacier area, south-central Alaska

In the Chistochina Glacier area of south-central Alaska, the active trace of the Denali fault is well defined by prominent tectonic geomorphology, including scarps, grabens, and mole tracks associated with the 2002 Mw=7.9 Denali fault earthquake. Interpretation of a trench excavated across the 2002 rupture trace places a constraint on the timing of the penultimate earthquake to after 550 to 660 yr before 2002, consis- tent with other paleoseismic studies along the central Denali fault. Field measurement of offset moraine deposits and use of existing geochronologic data is the basis to estimate a minimum latest Pleistocene slip rate of 8–14 mm/yr.

Alaska↗
USGS1975

Radiometric dates from Alaska: A 1975 compilation

The following table of radiometric dates from Alaska includes published material through 1972 as well as some selected later data. The table includes 726 mineral and whole-rock dates determined by the K-Ar, Rb-Sr, fission-track U-Pb, and Pb-alpha techniques. The data are organized in alphabetical order of the 1:250,000 scale quadrangles in which the dated rocks are located. The latitude and longitude of each sample are given. In addition, each sample is located on a 1:250,000 quadrangle map by a grid system. The initial point of the grid is taken as the southwest corner of the quadrangle and the location of the sample is measured in inches east and inches north from that corner, e.g., "156E 126N" indicated 15.6 inches east and 12.6 inches north of the southwest corner of the quadrangle. Zeroes in the location columns for some dates indicate that accurate locations are not available. Rock type, dating method, mineral dated, radiometric age, sample identification number, and reference are also listed where possible. Short comments, mostly geographic locality names, are given for some dates. These comments have been taken from the original references. Sample identification numbers beginning with "AA" or "BB" have been assigned arbitrarily in cases where sample numbers were not assigned in the original references. Abbreviations are explained in the appendix at the end of table 1.

Alaska↗
USGS2015

The 2008 phreatomagmatic eruption of Okmok volcano, Aleutian Islands, Alaska: Chronology, deposits, and landform changes

Okmok volcano, Aleutian Islands, Alaska, explosively erupted over a five-week period between July 12 and August 23, 2008. The eruption was predominantly phreatomagmatic, producing fine-grained tephra that covered most of northeastern Umnak Island. The eruption had a maximum Volcanic Explosivity Index (VEI) of 4, with eruption column heights up to 16 km during the opening phase. Several craters and a master tuff cone formed in the caldera as a result of phreatomagmatic explosions and accumulated tephra-fall and surge deposits. Ascending magma continuously interacted with an extensive shallow groundwater table in the caldera, resulting in the phreatomagmatic character of the eruption. Syneruptive explosion and collapse processes enlarged a pre-existing lake, created a second, entirely new lake, and formed new, deep craters. A field of ephemeral collapse pits and collapse escarpments formed where rapid groundwater withdrawal removed material from beneath capping lava flows. This was the first significant phreatomagmatic event in the U.S. since the Ukinrek Maars eruption in 1977.

Alaska↗
USGS1946

Stratigraphy and structure of the area of the Killik, Chandler, Anaktuvuk, and Colville Rivers, Alaska

This report deals with results of field and laboratory studies carried out to May 1946 by the Geological Survey, largely as part of the Navy Department's program of petroleum investigations in northern Alaska. The immediate purpose of the work has been to collect and interpret stratigraphic and structural data pertinent to drilling in the Umiat area. Field studies were made during the summers of 1944 and 1945 in the drainage areas of the Colville, Killik, Chandler, and. Anaktuvuk Rivers (fig. 1). Paleontologic and microlithologic studies, together with stratigrahica1 and structural analysis of the areas, were carried out in the petroleum laboratory in Fairbanks. This work led. to preparation of a preliminary report submitted. in December, 1945. Additional detailed studies, in the Survey laboratories and offices in Washington, D. C., have resulted in some modification of earlier conclusions and changes in illustrations. The changes are minor and have no significant bearing on drilling in the Umiat area. The present report is accompanied. by revised conies of figures 1, 2, 3 (sheet 1) and. 5. These illustrations should be substituted for the comparable figures of the preliminary report. Figures 3 (sheet 2), 4, and. 6 of the earlier report stand unchanged and should, be added to the illustrations accompanying the present report. It is believed that work to date has revealed the general geologic picture of the area and. that the sequence of Upper Cretaceous rocks is established. The section of rocks in Umiat Test No. 1 has been approximately located within the sequence. This report, and the work on which it is based was done under the supervision of George O. Gates, who has contributed. much to the study.

Alaska↗
USGS1949

Heavy mineral zonation of Cretaceous and Tertiary rocks of the central area of northern Alaska

This report presents the general conclusions pertaining to the correlation of Cretaceous and Tertiary rocks in test wells and outcrops in the central area of northern Alaska (fig. 1) by means of heavy minerals. Approximately 1.000 drill and outcrop samples have been studied. In order to relate the materiel here presented to the regional geologic picture of the central area, the restored facies cross-sections A-A' and B-B' of T.G. Payne 1/ are used as a basis for the graphic presentation of the heavy mineral zonation. Section C-C' of Payne is not used because most of it lies outside the area treated in this report and because of new geological and geophysical interpretations in the Barrow area as the result of recent exploration. Instead, the heavy mineral zonation along a somewhat different section C-C' (see fig. 1) is discussed but not illustrated.

Alaska↗
USGS1952

Stratigraphy and paleontology of the Noatak and associated formations, Brooks Range, Alaska

A think complex sequence of olastic rocks, formerly named the Noatak formation, underlies the Lisburns formation (Mississippian) in the Brooks Range, northern Alaska. Five formations have been recognized as a result of of recent investigations by the author and other geologists of the Navy Oil Unit, U.S. Geological Survey. In the western Brooks Range, the three formations present below the Lisburns formation are: Utukok formation (Upper Touranisian to Lower Visean), Montak formation (restricted) (Upper Devonian to Lower Tournaisian (?)) and Hunt Fork (?) formation (Upper Devonian). In the central Brooks Range the Kayak formation (Lower to Upper Tournaisian), Kanayut formation (Upper Devonian to Lower Tournaisian (?)), and Hunt Fork formation (Upper Devonian) lie below the Lisburns formation. Lithologic and faunal characteristics of these sedimentary units reveal the regional pattern of sedimentation and suggest specific paleoecologic environments. The chart conglomerates in the Kanayut and Noatuk formations are thought to have formed in a belt characterized by fluctuating marine and nonmarine conditions, the chart source being a mid-Devonian ragolith. Analysis of formulas suggest certain correlations with the European, Siberian, and western Cordilleran sections. Described faunas of the Utukok and Kayak formations include 40 braciopod species and varieties, 9 echinodera species, 2 bryzoan species, 8 molluscan species, and 1 tribute species, of which 13 are new.

Alaska↗
USGS1953

Regional interpretation of the geology of the Kongakut - Firth Rivers area, Alaska

In 1952 the National Park Service became interested in setting aside a large wilderness area in northeastern Alaska. The area is approximately 7,000 square miles in size, with boundaries as follows: beginning at Camden Bay south along the Katakturuk River to lat. 68° N.; then east to long. 144° 33' W.; south to lat. 68° N.; then east to the Canadian Boundary. The two river areas which the Park Service is most interested in examining for ecology, botany, archaeology, and geology are the upper Firth and Kongakut Rivers.

Alaska↗
USGS1947

Preliminary report on the stratigraphy and structure of the Titaluk and upper Ikpikpuk Rivers, Alaska

Geological Survey Party No. 4 was assigned an investigation of the Ikpikpuk and Titaluk Rivers and East Fork of the Ikpikpuk River mainly for the purpose of obtaining stratigraphic information which would be of use in determining the parts of the Upper Cretaceous sequence that underlie the areas investigated by United Geophysical Company parties 43 and 46. A total of five days were spent in geological investigations of the Titaluk River with a Cub plane equipped with pontoons. The party was composed of the geologist and Don Hulshizer, pilot of Wien Alaska Airlines. The geologist and Ronald K. Sorem, field assistant, traversed the Ikpikpuk River by boat from the junction of Maybe Creek and the Kigalik River to near the junction of the East Fork. From the northernmost point reached by boat traverse, the river was examined by plane to north of latitude 70° N., but no outcrops or rubble indicating bedrock were seen. The East Fork was also examined from the air but no outcrops or rubble were observed.

Alaska↗