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USGS1997

Environmental geochemistry of shale-hosted Ag-Pb-Zn massive sulfide deposits in northwest Alaska: Natural background concentrations of metals in water from mineralized areas

Red Dog, Lik and Drenchwater are shale-hosted stratiform Ag-Pb-Zn massive sulfide deposits in the northwestern Brooks Range. Natural background concentrations of metals in waters from the undisturbed (unmined) Drenchwater prospect and Lik deposit were compared to pre-mining baseline studies conducted at Red Dog. The primary factors affecting water chemistry are the extent of exposure of the deposits, the grade of mineralization, the presence of carbonate reeks in the section, and the proportion of Fe-sulfide in the ore. Surface water samples from the Drenchwater prospect, which has pyrite-dominant mineralization exposed in outcrop, have pH values as low as 2.8 and high dissolved concentrations of metals including as much as 95 mg 1-1 Al, 270 mg 1-1 Fe, 8 ??1-1 Cd, 10 ??1-1 Pb, and 2600 ??1-1 Zn, with As up to 26 ??g1-1. Surface waters from the Red Dog deposit prior to mining were also acidic and metal-rich, however, dissolved metal concentrations in Red Dog waters were many times greater. The higher metal concentrations in Red Dog waters reflect the high Zn grades and the abundant sphalerite, pyrite, and galena that were present in outcrop prior to mining. In contrast, despite significant mineralization at the Lik deposit, carbonate rocks in the section buffer the system, resulting in less acidic, mostly near-neutral pH values with low concentrations of most metals except Zn.

Applied Geochemistry↗

USGS2020

Evolution of the submarine–subaerial edifice of Bogoslof volcano, Alaska, during its 2016–2017 eruption based on analysis of satellite imagery

The 2016–2017 eruption of Bogoslof volcano involved at least 70 detected eruptive events between mid-December 2016 and August 30, 2017. Acquisition of high-resolution satellite imagery throughout the duration of the eruptive period allowed us to document and map the various morphologic changes that occurred on the subaerial part of Bogoslof Island. The emplacement of pyroclastic-flow and surge deposits caused the island to increase in area by about 1.5 km 2 . The dominant volcanic landforms of the eruption were a series of tuff rings emplaced around various submarine vents. Many of the tuff rings were mantled with surface dunes and impressive amounts of ballistic ejecta, likely derived from erupting magma bodies or previously emplaced submarine lava domes. Debris-flow deposits and surface channels extending over tuff ring surfaces apparent in multiple satellite images are evidence for explosive ejection of seawater. In most cases, erupting vents were initially submarine or began at subaerial lava domes and were largely flooded by seawater suggesting that water-magma ratios were likely high. Under such conditions where water is abundant, eruptive products typically reflect a high degree of water involvement and are dominated by the formation of wet tephra jets and flows and associated deposits typically consist of fine ash and lapilli, contain accretionary lapilli and ash aggregates, and usually form tuff cones and mounds. We observed none of these features in our analysis of satellite data or during our examination of eruptive deposits on Bogoslof Island in 2018. On the contrary, the dominant landform associated with the Bogoslof eruption was tuff rings. The development of tuff rings and surface dunes are commonly associated with the formation of pyroclastic base surges that are by comparison emplaced relatively dry. Dry base surge deposits can be generated from phreatomagmatic explosions involving superheated steam. It is possible that shallow submarine, magma–wet sediment interactions were a characteristic and possibly a dominant eruptive process of the 2016–2017 Bogoslof eruption.

Alaska↗

USGS2026

Upper Triassic igneous rocks of the southern Kenai Peninsula, Alaska – Prelude to Early Jurassic subduction along the western Wrangellia composite terrane margin

New U–Pb zircon geochronology identifies a latest Triassic (ca 214–201 Ma) igneous suite of tuff, hypabyssal dikes, and a pluton on the southern Kenai Peninsula, Alaska. The igneous suite was emplaced within Upper Triassic sedimentary rocks along the southern margin of Western Wrangellia, the western-most fragment of the Wrangellia composite terrane. The igneous rocks range from mafic (50.6% SiO 2 ) to felsic (78.3% SiO 2 ), characteristically have less than 1.55% K 2 O, and generally have low trace element abundances. The tonalitic and trondhjemitic magmas were largely sourced in mafic-rich lower crust and incompletely assimilated quartz and other mineral xenocrysts are common. Fractionation involving plagioclase and amphibole is indicated for some magmas and composite intrusions and igneous xenoliths indicate magma mixing was possible. Paleozoic and Precambrian inherited zircons and initial 87 Sr/ 86 Sr (0.704103–0.705609) and 143 Nd/ 144 Nd (0.512396–0.512777) ratios indicate that the Western Wrangellia crustal sources are heterogeneous and contain sialic components. The latest Triassic magmatism reflects processes that preceded Early Jurassic subduction along the Wrangellia composite terrane and Pacific Ocean plate boundary. These processes involved heating and melting of mantle lithosphere and lower crust as mantle instabilities accompanied the breaking of the plate boundary linkages. The Late Triassic transition to subduction along the Wrangellia composite terrane margin coincided with the transition to subduction cessation in the Late Triassic arcs of the western Intermontane terranes of Canada. The shift to subduction along the outboard Wrangellia composite terrane margin marks the beginning of the Pacific Ocean–Cordillera plate interactions that came to dominate the tectonic evolution of the northern Cordillera from the Early Jurassic to today.

Alaska↗

USGS1975

Heavy-mineral trends in the Beaufort Sea

Sediments of the Beaufort Sea, off the North Slope of Alaska contain a great variety of heavy minerals. These include garnet, chrome spinel, augite, pigeonite, diopside, hornblende, enstatite, hypersthene, epidote, clinozoisite, zoisite, apatite, tourmaline, chloritoid, sphene, zircon, and opaque minerals. Much rarer constituents are glaucophane, lamprobolite, rutile, kyanite, staurolite, and riebeckite. The heavy-mineral fractions that were not treated with hydrochloric acid contain "iron-stained aggregates", grains of unidentifiable material encrusted with limonite. Samples containing relatively high percentages of iron-stained aggregates and altered opaque minerals occur in depths less than 10 m and within 16 km from shore. Garnet increases in abundance from east to west, which corresponds to a similar increase in garnet abundance in coastal outcrops of the Gubik Formation sands. Only garnet and iron-stained aggregates appear to have source-related distribution patterns. The other heavy minerals lack distinct distributive provinces, reflecting an environment dominated by intense mixing by ice-gouging and bioturbation and a homogenous source area. Waves and currents are not strong enough to sort sediments at depths greater than 10 m except during summer storms. The source of the Beaufort Sea heavy minerals is dominated by contributions from the Alaskan North Slope deposits of Tertiary and younger age. The Colville River, largest in the region, is probably the most influential in transporting sediments, but because of wave and current—mixing of sediments on the shelf, exact contributions from each river drainage cannot be ascertained. Coastal erosion of the Gubik Formation is probably at least as important as the Colville River in supplying heavy minerals to the Beaufort Sea.

Alaska↗

USGS2024

Event classification, seismicity, and eruption forecasting at Great Sitkin Volcano, Alaska: 1999–2023

The frequency content of volcanogenic seismicity is often used to classify events and their spatial and temporal progression is then used to map subsurface volcanic processes. The progression of volcano-seismic events and associated source processes also plays a critical role in eruption forecasting. Here we develop and evaluate a computerized methodology for characterizing volcano-seismic event types using Frequency Index and Average Peak Frequency. We apply and test this technique at Great Sitkin Volcano, Alaska, classifying over 9000 hypocenters between 1999 and 2023. This 24-year time span covers periods of seismic quiescence, earthquake activity on nearby tectonic (bookshelf) faults, precursory unrest from 2016 to 2021, and the explosive onset in May 2021 of the ongoing effusive eruption. We use the spatial and temporal evolution of classified event types to map the active volcanic and tectonic processes, develop a conceptual model of the subsurface magmatic system, and perform a retrospective analysis of eruption forecasts at Great Sitkin Volcano between 2016 and the present. The classification and progression of hypocenters suggests the subsurface Great Sitkin Volcano magmatic system consists of a mid- to lower- crustal source zone between 10 and 40 km depth and an upper crustal magma storage area between −1 and 10 km depth (hypocenter depth is referenced to sea level and negative depths reflect height above sea level). The earliest precursors occurred in July 2016 and consisted of deep long-period and volcano-tectonic earthquakes at mid-crustal depths suggesting the subsequent unrest and eruption were triggered by a deeper intrusion of magma. This mid-crustal seismic activity was immediately followed by the onset upper-crustal long-period events and volcano-tectonic earthquakes VTs suggesting a strong linkage between the shallow and deeper portions of the magmatic system. The upper crustal area was likely capped by the 1974 lava dome until the magmatic explosion on May 26, 2021.

Alaska↗

USGS2004

Paleozoic sedimentary rocks in the Red Dog Zn-Pb-Ag district and vicinity, western Brooks Range, Alaska: provenance, deposition, and metallogenic significance

Geochemical analyses of Paleozoic sedimentary rocks in the western Brooks Range reveal a complex evolutionary history for strata surrounding the large Zn-Pb-Ag deposits of the Red Dog district. Data for major elements, trace elements, and rare earth elements (REE) were obtained on 220 samples of unaltered and unmineralized siliciclastic rocks from the Upper Devonian-Lower Mississippian Endicott Group (Hunt Fork Shale, Noatak Sandstone, Kanayut Conglomerate, Kayak Shale), the overlying Carboniferous Lisburne Group (Kuna Formation, unnamed drowned shelf facies), and the Pennsylvanian-Permian Siksikpuk Formation. Major base metal sulfide deposits of the region are present only in the Kuna Formation, which in the Red Dog district comprises siliceous black shale and black chert, minor limestone (calcareous radiolarite), and sparse lithic turbidite and bedded siliceous rock. Gray and rare black shales of the Kayak Shale and common black shales of the Kuna Formation are anomalously low in iron (avg Fe/Ti = 6.25 and 6.34, respectively) relative to other Paleozoic shales in the region (9.58-10.6) and to average shales worldwide (10.1-10.5). In contrast, the bedded siliceous rocks contain appreciable hematite (avg Fe/Ti = 35.0) and high U/Ti and REE/Ti ratios that are interpreted to reflect low amounts of detrital material and a major Fe-rich eolian component. Geochemical data (e.g., MnO <0.01 wt %; avg Cr = 317 ppm), sizes of framboidal pyrite grains, and limited bioturbation suggest anoxic and denitrifying depositional conditions for most black shales of the Kuna Formation; low Mo/Ti ratios argue against euxinic (sulfate-reducing) conditions. Organic-rich black shales of the Kuna Formation with up to 8.4 wt percent C organic and gray to black shales of the Kayak Shale with up to 4.1 wt percent C organic typically have only sparse pyrite (<1 wt % S) and very low iron-limited S/C ratios (mostly <0.2). Immobile element plots (e.g., Th-Zr/10-Sc) suggest that source terranes for all of the formations were dominated by one or more felsic-rich continental arcs; a small proportion of recycled sediments is present locally. A minor mafic igneous component also occurs in several shales of the Kuna and Siksikpuk Formations. High average values for the chemical index of alteration [Al 2 O 3 /(Al 2 O 3 + CaO + Na 2 O + K 2 O)] ∞ 100 for shales of the Endicott Group (76.4-81.5) imply moderate to intense chemical weathering in source areas of these sediments. A lower average for black shales of the Kuna Formation (73.7) does not require such weathering. Textural and geochemical data record the effects of diagenetic and/or hydrothermal fluid flow in some of the Paleozoic rocks. Mobility of P, F, U, and light REE is documented in black shales of the Kuna Formation by phosphate replacements of carbonate clasts and of matrix material surrounding the clasts. A relatively low average Ce/Ce* value of 0.73 for P-poor black shales of the Kuna Formation ( < 0.05 wt % P 2 O 5 ) and a similar Ce/Ce* value of 0.78 for a siderite concretion in Kayak Shale suggest that these diagenetic fluids were oxidizing. Many shales of the Kuna Formation have high (K 2 O ∞ 100)/(K 2 O + Al 2 O 3 ) ratios of 21.0 to 23.0, which contrast with low ratios of generally <18.0 for shales of the underlying Endicott Group. The high ratios in shales of the Kuna Formation reflect preferential reaction of smectite to illite during the Jurassic-Cretaceous Brookian orogeny, owing to high silica activities in pore fluids that were generated by the dissolution of abundant biogenic silica. The distribution and composition of Paleozoic strata in the western Brooks Range may have played a fundamental role in Zn-Pb mineralization of the Red Dog district. In our model, deposition and early lithification of biogenic chert and bedded siliceous rocks in the upper part of the Kuna Formation served as a regional hydrologic seal, acting as a cap rock to heat and hydrothermal fluids during Late Mississippian base-metal mineralization. Equally important was the iron-poor composition of black shales of the Kuna Formation (i.e., low Fe/Ti ratios), which limited synsedimentary pyrite formation in precursor sediments, resulting in significant H 2 S production in pore waters through the interaction of aqueous sulfate with abundant organic matter. This H 2 S may have been critical to the subsurface deposition of the huge quantities of Zn and Pb in the district. On the basis of this model, we propose that low Fe/Ti and S/C ratios in black shale sequences are potential basin-scale exploration guides for giant sediment-hosted, stratiform Zn-Pb-Ag deposits.

Alaska↗

USGS2014

Source and progression of a submarine landslide and tsunami: The 1964 Great Alaska earthquake at Valdez

Like many subduction zone earthquakes, the deadliest aspects of the 1964 M = 9.2 Alaska earthquake were the tsunamis it caused. The worst of these were generated by local submarine landslides induced by the earthquake. These caused high runups, engulfing several coastal towns in Prince William Sound. In this paper, we study one of these cases in detail, the Port Valdez submarine landslide and tsunami. We combine eyewitness reports, preserved film, and careful posttsunami surveys with new geophysical data to inform numerical models for landslide tsunami generation. We review the series of events as recorded at Valdez old town and then determine the corresponding subsurface events that led to the tsunami. We build digital elevation models of part of the pretsunami and posttsunami fjord‐head delta. Comparing them reveals a ~1500 m long region that receded 150 m to the east, which we interpret as the primary delta landslide source. Multibeam imagery and high‐resolution seismic reflection data identify a ~400 m wide chute with hummocky deposits at its terminus, which may define the primary slide path. Using these elements we run hydrodynamic models of the landslide‐driven tsunamis that match observations of current direction, maximum inundation, and wave height at Valdez old town. We speculate that failure conditions at the delta front may have been influenced by manmade changes in drainage patterns as well as the fast retreat of Valdez and other glaciers during the past century.

Alaska↗

USGS2005

Physiological response of wild rainbow trout to angling: Impact of angling duration, fish size, body condition, and temperature

This study evaluated the immediate physiological response of wild rainbow trout to catch-and-release angling in the Alagnak River, southwest Alaska. Information was recorded on individual rainbow trout (n = 415) captured by angling including landing time and the time required to remove hooks (angling duration), the time to anesthetize fish in clove oil and withdraw blood, fish length and weight, and water temperature at capture locations. Plasma cortisol, glucose, ions (sodium, potassium, chloride), and lactate were analyzed to determine the effects of angling duration, fish size, body condition, and temperature. Levels of plasma ions did not change significantly during the observed physiological response and levels of plasma glucose were sometimes influenced by length (2000, 2001), body condition (2001), or temperature (2001). Levels of plasma cortisol and lactate in extended capture fish (angling duration greater than 2 min) were significantly higher than levels in rapid capture fish (angling duration less than 2 min). Rapid capture fish were significantly smaller than extended capture fish, reflecting that fish size influenced landing and handling times. Fish size was related to cortisol and lactate in 2002, which corresponded to the year when larger fish were captured and there were longer landing times. Body condition (i.e., weight/length regression residuals index), was significantly related to lactate in 2000 and 2001. Water temperatures were higher in 2001 (mean temperature ± S.E., 13 ± 2 o C) than in 2002 (10 ± 2 o C), and fish captured in 2001 had significantly higher cortisol and lactate concentrations than fish captured in 2002. The pattern of increase in plasma cortisol and lactate was due to the amount of time fish were angled, and the upper limit of the response was due to water temperature. The results of this study indicate the importance of minimizing the duration of angling in order to reduce the sublethal physiological disturbances in wild fish subjected to catch-and-release angling, particularly during warmer water temperatures. It is also important to note that factors such as fish size may influence both the duration of angling and subsequent physiological response.

Fisheries Research↗

USGS2024

Repeated coseismic uplift of coastal lagoons above the Patton Bay Splay Fault System, Montague Island, Alaska, USA

Coseismic slip on the Patton Bay splay fault system during the 1964 M w 9.2 Great Alaska Earthquake contributed to local tsunami generation and vertically uplifted shorelines as much as 11 m on Montague Island in Prince William Sound (PWS). Sudden uplift of 3.7–4.3 m caused coastal lagoons along the island's northwestern coast to gradually drain. The resulting change in depositional environment from marine lagoon to freshwater muskeg created a sharp, laterally continuous stratigraphic contact between silt and overlying peat. Here, we characterize the geomorphology, sedimentology, and diatom ecology across the 1964 earthquake contact and three similar prehistoric contacts within the stratigraphy of the Hidden Lagoons locality. We find that the contacts signal instances of abrupt coastal uplift that, within error, overlap the timing of independently constrained megathrust earthquakes in PWS—1964 Common Era, 760–870 yr BP, 2500–2700 yr BP, and 4120–4500 yr BP. Changes in fossil diatom assemblages across the inferred prehistoric earthquake contacts reflect ecological shifts consistent with repeated draining of a lagoon system caused by >3 m of coseismic uplift. Our observations provide evidence for four instances of combined megathrust-splay fault ruptures that have occurred in the past ∼4,200 years in PWS. The possibility that 1964-style combined megathrust-splay fault ruptures may have repeated in the past warrants their consideration in future seismic and tsunami hazards assessments.

Alaska↗

USGS2019

Petroleum systems framework of significant new oil discoveries in a giant Cretaceous (Aptian–Cenomanian) clinothem in Arctic Alaska

Recent oil discoveries in an Aptian–Cenomanian clinothem in Arctic Alaska demonstrate the potential for hundred-million- to billion-barrel oil accumulations in Nanushuk Formation topsets and Torok Formation foresets–bottomsets. Oil-prone source rocks and the clinothem are draped across the Barrow arch, a structural hinge between the Colville foreland basin and Beaufort Sea rifted margin. Stratigraphic traps lie in a favorable thermal maturity domain along multiple migration pathways across more than 30,000 km 2 (10,000 mi 2 ). Sediment from the Chukotkan orogen (Russia) filled the western Colville basin and spilled over the Beaufort rift shoulder, forming east- and north-facing shelf margins. Progradational shelf-margin trajectories change abruptly to “sawtooth” trajectories at mid-clinothem, the result of reduction in sediment influx. Two stratigraphic trap types are inferred in Nanushuk basal topsets in the eastern part of the clinothem: (1) lowstand systems tracts, inferred to reflect forced regression, include a narrow, thick progradational stacking pattern perched on a sequence boundary on the upper slope; and (2) highstand-progradational systems tracts include a broad, thin wedge of shingled parasequences above a toplap surface. Both include stratigraphically isolated sandstone sealed by mudstone. Trap geometries in Torok foreset and bottomset facies in the same area include basin-floor fan, slope-apron, and slope-channel deposits that pinch out upslope and are sealed by mudstone. Significant potential exists for the discovery of additional oil accumulations in these stratigraphic trap types in the eastern part of the clinothem. Less potential may exist in the western part because reservoir-seal pairs may not be well developed.

American Association of Petroleum Geologists Bulle↗

USGS2002

Marine predator surveys in Glacier Bay National Park and Preserve

Since 1999, vessel based surveys to estimate species composition, distribution and relative abundance of marine birds and mammals have been conducted along coastal and pelagic (offshore) transects in Glacier Bay, Alaska. Surveys have been conducted during winter (November-March) and summer (June). This annual report presents the results of those surveys conducted in March and June of 2001. Following completion of surveys in 2002 we will provide a final report of the results of all surveys conducted between 1999 and 2002. Glacier Bay supports diverse and abundant assemblages of marine birds and mammals. In 2001 we identified 58 species of bird, 7 species of marine mammal, and 6 species of terrestrial mammal on transects sampled during winter and summer. Of course all species are not equally abundant. Among all taxa, in both seasons, sea ducks were the numerically dominant group. In their roles as consumers and because of their generally large size, marine mammals are also likely important in the consumption of energy produced in the Glacier Bay ecosystem. Most common and abundant marine birds and mammals can be placed in either a fish based (e.g. alcids and pinnipeds), or a benthic invertebrate (e.g. sea ducks and sea otters) based food web. Distinct differences in the species composition and abundance of marine birds were observed between winter and summer surveys. Winter marine bird assemblages were dominated numerically (> 11,000; 65% of all birds) by a relatively few species of sea ducks (scoters, goldeneye, Bufflehead, Harlequin and Long-tailed ducks). The sea ducks were distributed almost exclusively along near shore habitats. The prevalence of sea ducks during the March surveys indicates the importance of Glacier Bay as a wintering area for this poorly understood group of animals that occupy a high trophic position in a principally benthic invertebrate (mussel and clam) food web. Marine mammal assemblages were generally consistent between seasons, although Humpback and Killer whales were not observed in winter 2001. Summer marine bird assemblages remained numerically dominated by sea ducks, but species composition shifted between the goldeneye whose density was 44/m 2 in winter to < 0.2/m 2 in summer, to scoters, whose density was 29/m 2 in winter to > 60/m 2 in summer. Large increases in Black-legged kittiwake, murrelet (Marbled and Kittlitz’s) and Common merganser densities were detected during summer surveys. Seasonal differences in abundance of species likely reflected differences in life history attributes (e.g. reproductive biology, foraging ecology) among species. Because of differences observed in species composition between the winter and summer, it is apparent that a single annual survey cannot accurately describe the populations of marine birds and mammals that occur in Glacier Bay. Preliminary analysis further suggests that interpretations of data resulting from this type of survey may depend to a large extent on the individual species. Because species exhibit differences in behavior, morphology, coloration, and distribution, accuracy and precision of abundance estimates likely vary among species. Confidence in survey results should be evaluated in consideration of life history and detection probabilities at the species level. However, survey results likely provide reasonable estimates of species composition and relative abundance, as well as accurate abundance estimates for those species whose detection closely approximates one.

Alaska↗

USGS2013

Pb-Sr-Nd isotopes in surficial materials at the Pebble Porphyry Cu-Au-Mo Deposit, Southwestern Alaska: can the mineralizing fingerprint be detected through cover?

The Cretaceous Pebble porphyry Cu-Au-Mo deposit is covered by tundra and glacigenic sediments. Pb-Sr-Nd measurements were done on sediments and soils to establish baseline conditions prior to the onset of mining operations and contribute to the development of exploration methods for concealed base metal deposits of this type. Pebble rocks have a moderate range for 206 Pb/ 204 Pb = 18.574 to 18.874, 207 Pb/ 204 Pb = 15.484 to 15.526, and 208, Pb/ 204 Pb = 38.053 to 38.266. Mineralized granodiorite shows a modest spread in 87 Sr/ 86 Sr (0.704354–0.707621) and 143 Nd/ 144 Nd (0.512639–0.512750). Age-corrected (89 Ma) values for the granodiorite yield relatively unradiogenic Pb (e.g., 207 Pb/ 204 Pb <15.52), low values of 87 Sr/ 86 Sr, and positive values of ɛNd (1.00–4.52) that attest to a major contribution of mantle-derived source rocks. Pond sediments and soils have similar Pb isotope signatures and 87 Sr/ 86 Sr and 143 Nd/ 144 Nd values that resemble the mineralized granodiorites. Glacial events have obscured the recognition of isotope signatures of mineralized rocks in the sediments and soils. Baseline radiogenic isotope compositions, prior to the onset of mining operations, reflect natural erosion, transport and deposition of heterogeneous till sheets that included debris from barren rocks, mineralized granodiorite and sulfides from the Pebble deposit, and other country rocks that pre- and postdate the mineralization events. Isotopic variations suggest that natural weathering of the deposit is generally reflected in these surficial materials. The isotope data provide geochemical constraints to glimpse through the extensive cover and together with other geochemical observations provide a vector to concealed mineralized rocks genetically linked with the Pebble deposit.

Alaska↗

USGS2013

Reconnaissance studies of potential petroleum source rocks in the Middle Jurassic Tuxedni Group near Red Glacier, eastern slope of Iliamna Volcano

Previous geological and organic geochemical studies have concluded that organic-rich marine shale in the Middle Jurassic Tuxedni Group is the principal source rock of oil and associated gas in Cook Inlet (Magoon and Anders, 1992; Magoon, 1994; Lillis and Stanley, 2011; LePain and others, 2012; LePain and others, submitted). During May 2009 helicopter-assisted field studies, 19 samples of dark-colored, fine-grained rocks were collected from exposures of the Red Glacier Formation of the Tuxedni Group near Red Glacier, about 70 km west of Ninilchik on the eastern flank of Iliamna Volcano (figs. 1 and 3). The rock samples were submitted to a commercial laboratory for analysis by Rock-Eval pyrolysis and to the U.S. Geological Survey organic geochemical laboratory in Denver, Colorado, for analysis of vitrinite reflectance. The results show that values of vitrinite reflectance (percent R o ) in our samples average about 2 percent, much higher than the oil window range of 0.6–1.3 percent (Johnsson and others, 1993). The high vitrinite reflectance values indicate that the rock samples experienced significant heating and furthermore suggest that these rocks may have generated oil and gas in the past but no longer have any hydrocarbon source potential. The high thermal maturity of the rock samples may have resulted from (1) the thermaleffects of igneous activity (including intrusion by igneous rocks), (2) deep burial beneath Jurassic, Cretaceous, and Tertiary strata that were subsequently removed by uplift and erosion, or (3) the combined effects of igneous activity and burial.

Alaska↗

USGS2015

Late Jurassic – early Cretaceous inversion of rift structures, and linkage of petroleum system elements across post-rift unconformity, U.S. Chukchi Shelf, arctic Alaska

Basin evolution of the U.S. Chukchi shelf involved multiple phases, including Late Devonian–Permian rifting, Permian–Early Jurassic sagging, Late Jurassic–Neocomian inversion, and Cretaceous–Cenozoic foreland-basin development. The focus of ongoing exploration is a petroleum system that includes sag-phase source rocks; inversion-phase reservoir rocks; structure spanning the rift, sag, and inversion phases; and hydrocarbon generation during the foreland-basin phase. Interpretation of 2-D seismic and sparse well data documents the presence, in the south-central part of the shelf, of a series of en-echelon, north-south trending monoclonal fold limbs that display up to 1+ km (3,300 ft) of structural relief. These folds, which are located above the tips of rift-phase normal faults, are interpreted as inversion structures formed by maximum compressive stress oriented obliquely to the strike of rift-phase normal faults. Erosional relief on a Jurassic unconformity, growth strata in the overlying Upper Jurassic to Neocomian strata, and east-dipping clinoforms in a high accommodation depocenter east of the inversion structures indicate profound structural influence on sedimentation. Oil-prone source rocks, reservoir-quality sandstone, migration pathways, and structural closure are linked intimately across the Jurassic unconformity, which reflects inversion. Thus, all these key petroleum systems elements were in place when Triassic source rocks entered the oil generation window during Cretaceous–Cenozoic stratigraphic burial.

Conference Paper↗

USGS2009

Timing of breeding and reproductive performance in murres and kittiwakes reflect mismatched seasonal prey dynamics

Seabirds are thought to time breeding to match the seasonal peak of food availability with peak chick energetic demands, but warming ocean temperatures have altered the timing of spring events, creating the potential for mismatches. The resilience of seabird populations to climate change depends on their ability to anticipate changes in the timing and magnitude of peak food availability and 'fine-tune' efforts to match ('Anticipation Hypothesis'). The degree that inter-annual variation in seabird timing of breeding and reproductive performance represents anticipated food availability versus energetic constraints ('Constraint Hypothesis') is poorly understood. We examined the relative merits of the Constraint and Anticipation Hypotheses by testing 2 predictions of the Constraint Hypothesis: (1) seabird timing of breeding is related to food availability prior to egg laying rather than the date of peak food availability, (2) initial reproductive output (e.g. laying success, clutch size) is related to pre-lay food availability rather than anticipated chick-rearing food availability. We analyzed breeding biology data of common murres Uria aalge and black-legged kittiwakes Rissa tridactyla and 2 proxies of the seasonal dynamics of their food availability (near-shore forage fish abundance and sea-surface temperature) at 2 colonies in Lower Cook Inlet, Alaska, USA, from 1996 to 1999. Our results support the Constraint Hypothesis: (1) for both species, egg laying was later in years with warmer sea-surface temperature and lower food availability prior to egg laying, but was not related to the date of peak food availability, (2) pre-egg laying food availability explained variation in kittiwake laying success and clutch size. Murre reproductive success was best explained by food availability during chick rearing. ?? 2009 Inter-Research.

Marine Ecology Progress Series↗

USGS2018

Strain partitioning in southeastern Alaska: Is the Chatham Strait Fault active?

A 1200 km-long transform plate boundary passes through southeastern Alaska and northwestern British Columbia and represents one of the most seismically active, but poorly understood continental margins of North America. Although most of the plate motion is accommodated by the right-lateral Queen Charlotte–Fairweather Fault (QCFF) System, which has produced at least six M > 7 earthquakes since 1920, seismic hazard ass essments also include the Chatham Strait Fault (CSF) as a potentially active, 400 km-long strike slip fault that cuts northward through southeastern Alaska, connecting with the Eastern Denali Fault. Nearly the entire length of the CSF is submerged beneath Chatham Strait and Lynn Canal and has never been systematically imaged using high-resolution marine geophysical approaches. In this study we present an integrated analysis of new marine seismic reflectiondata ac quired across Lynn Canal and tectonic block modeling constrained by data from continuous and campaign GPS sites. Seismic profiles cross the CSF at twelve locations spanning ∼50 km of fault length; they reveal thick (up to 300 m) packages of glaciomarine sedimentary facies emplaced on an unconformity su rface that formed during the Last Glacial Maximum (LGM). Localized warping of post-LGM stratigraphy (∼13.9 kyr B.P. to present) appears to correlate with sediment drape on basement topography and current-controlled deposition. There is no evidence for an active fault along the axis of Lynn Canal in the seismic reflection data. Crustal block models constrained by GPS data allow, but do not require, a maximum slip rate o f 2–3 mm/yr along the CSF; higher slip rates on the CSF result in significant misfit to GPS data in the surrounding region. Based on the combined marine geophysical and GPS observations, it is plausible that the CSF has not generated resolvable coseismic deformation in the last ∼13 ka and that the modern slip-rate is <1 mm/yr. We propose that models for strain transfer between the QCFF and the Denali Fault, and seismic hazard maps in general, may need to be reevaluated.

Alaska↗

USGS1983

Tin granites of Seward Peninsula, Alaska

Seven granite plutons, spatially and genetically related to tin metalization, are exposed in a 170-km-long belt across northwestern Seward Peninsula, Alaska. These plutons are cupolas and epizonal composite stocks that consist of several textural varieties of biotite granite, including medium- to coarse-grained seriate biotite granite, porphyritic biotite granite with an aplitic groundmass, and fine- to medium-grained equigranular biotite granite. The common accessory minerals are fluorite, allanite, apatite, and zircon. Other accessory minerals that are locally present include tourmaline, sphene, opaque oxide minerals, and late-forming (deuteric) muscovite and chlorite. The granites range in major-element contents as follows: SiO 2 , 72.5% to 76.6%; A1 2 O 3 , 12.7% to 14.3%; Na 2 O, 2.9% to 4.0%; K 2 O, 3.9% to 5.6%; and CaO, 0.6% to 1.2%. The sum of FeO + Fe 2 O 3 + MgO ranges from 0.3% to 2.4%; and the K 2 O to Na 2 O ratio from 1.1 to 1.8. The 0.1% to 0.9% F and 0.01% to 0.2% Cl reflect the over-all volatile-rich nature of the granites. The granites contain average or below-average concentrations of Co, Sc, Cr, and Zn, and generally above-average to distinctly high concentrations of Th, U, Hf, and Ta. The large cations emphasize the evolved nature of the granites; the Rb/Sr ratio is as high as 90 in some samples. Initial 87 Sr/ 86 Sr ratios range from 0.708 to as high as 0.720. The three Rb-Sr isochrons defined by the data agree with K-Ar age determinations and show that the stocks were emplaced during the Late Cretaceous, between about 70 and 80 m.y. ago. The field, petrologic, and geochemical data indicate that the plutons had a multistage origin that involved large-scale melting of sialic crust, emplacement of magmas derived from batholithic fractionation at depth, and subsequent evolution of these magmas to generate small volumes of more highly evolved residual magmas. Although evolution of the granite complexes was largely governed by crystal-melt fractionation, some minor-element variations in the highly evolved granites cannot be explained by this process. For example, the distribution of rubidium and the light rare-earths appears to have been influenced by volatile depletion at the final stages of crystallization. The field data, petrologic data, and variation trends, such as distinct shifts toward higher albite contents in the residual granites, suggest that the coexistence of a volatile phase was important in their evolution. These results require that models seeking to explain compositional gradients in high-level granite (rhyolite) systems fully consider the role of a coexisting volatile phase.

Alaska↗

USGS2009

Linking marine and freshwater growth in western Alaska Chinook salmon Oncorhynchus tshawytscha

The hypothesis that growth in Pacific salmon Oncorhynchus spp. is dependent on previous growth was tested using annual scale growth measurements of wild Chinook salmon Oncorhynchus tshawytscha returning to the Yukon and Kuskokwim Rivers, Alaska, from 1964 to 2004. First-year marine growth in individual O. tshawytscha was significantly correlated with growth in fresh water. Furthermore, growth during each of 3 or 4 years at sea was related to growth during the previous year. The magnitude of the growth response to the previous year's growth was greater when mean year-class growth during the previous year was relatively low. Length (eye to tail fork, LETF) of adult O. tshawytscha was correlated with cumulative scale growth after the first year at sea. Adult LETF was also weakly correlated with scale growth that occurred during freshwater residence 4 to 5 years earlier, indicating the importance of growth in fresh water. Positive growth response to previous growth in O. tshawytscha was probably related to piscivorous diet and foraging benefits of large body size. Faster growth among O. tshawytscha year classes that initially grew slowly may reflect high mortality in slow growing fish and subsequent compensatory growth in survivors. Oncorhynchus tshawytscha in this study exhibited complex growth patterns showing a positive relationship with previous growth and a possible compensatory response to environmental factors affecting growth of the age class.

Alaska↗