Eastern Gulf Of Alaska Research Articles

Biology of the Pacific Pomfret (Brama japonica) in the North Pacific Ocean

Abundances of Pacific pomfret (Brama japonica), an epipelagic fish of the North Pacific Ocean, were estimated from gillnet catches during the summers of 1978–1989. Two size modes were common: small pomfret <1 yr old, and large fish ages 1–6. Large and small fish moved northward as temperatures increased, but large fish migrated farther north, often into the cool, low-salinity waters of the Central Subarctic Pacific. Lengths of small fish were positively correlated with latitude and negatively correlated with summer surface temperature. Interannual variations in the latitude of catches correlated with surface temperatures. Large catches were made in the eastern Gulf of Alaska (51–55°N) but modes of small pomfret were absent here, and large fish were rare at these latitudes farther to the west. Pomfret grow rapidly during their first two years of life. They are pectoral fin swimmers that swim continuously. They prey largely on gonatid squids in the region of the Subarctic Current in the Gulf of Alaska during summer. No evidence was found for aggregations on a scale ≤1 km. Differences in the incidence of tapeworm, spawning seasons, and size distributions suggest the possibility of discrete populations in the North Pacific Ocean.

Tectonic and geologic implications of the Zodiac fan, Aleutian Abyssal Plain, northeast Pacific

The Zodiac fan, a large body of upper Eocene through at least lower Oligocene turbidities, underlies the Aleutian Abyssal Plain and lies just south of the Aleutian Trench and the Alaskan Peninsula in the northeastern north Pacific. The fan deposits cover an area of more than 1 × 10 6 km 2 and contain at least 280,000 km 3 of terrigenous sediment. The most striking feature of the fan is its well-developed channel distributary system, which persists nearly to the plain limits. Levee overbank deposits associated with the channels are the dominant sedimentary style, a process believed to predominate on fans receiving primarily fine-grained detritus. Four major channels have been identified on the fan with the following relative age relation (> = older than): Sagittarius > Aquarius > Taurus, and Sagittarius >Seamap. Recently obtained seismic information implies that Seamap Channel may be older than Aquarius Channel. If Seamap is the youngest channel, a depositional interval of 8 m.y. (40−32 m.y. B.P.) is indicated for the fan as a whole. If Taurus is the youngest, as we believe, the interval of deposition is greater than 8 m.y. but is probably less than 16 m.y. (40−24 m.y. B.P.). Nanno-flora, pollen, and spores obtained from Deep Sea Drilling Site 183, near the northern margin of the fan, indicate that the fan formed in a nontropical (northern) environment, and that the source terrane had a climate similar to or slightly cooler than that recorded from coeval onshore deposits in the eastern Gulf of Alaska. On the basis of the interval of deposition, the volume of the fan, and sediment yield from climatically similar modern drainages, a minimum drainage of 500,000 km 2 is believed necessary to have supplied the sediment to form the fan, assuming no sediment was deposited before it reached the fan. Tertiary plate-motion models requiring large amounts of relative convergence along the Aleutian Trench are judged unworkable, as such reconstructions require the Zodiac fan to have formed 1,500 to 3,000 km from the nearest continental landmass and separated from it by topographic barriers, requiring that the drainage be inflated in size manyfold to overcome anticipated sediment losses during such a lengthy transport. As the minimum drainage (500,000 km 2 ) is already equal to one-half of the State of Alaska, any major expansion is judged unreasonable. This requires that relative convergence at the Aleutian Trench be limited to less than ∼500 km from 40 m.y. B.P. to present. A possible method by which this limitation can be satisfied is to allow a significant portion of southern Alaska to move in concert with the Pacific plate since the upper Eocene.

Paleogene metamorphism of an accretionary flysch terrane, eastern Gulf of Alaska

Reconnaissance field studies have identified a regional metamorphic complex, at least 200 km long and 25 to 50 km wide, in the eastern Chugach Mountains, southern Alaska. The metamorphic complex was developed within the Cretaceous Valdez Group, a flysch sequence that was accreted to the continental margin during the Late Cretaceous or early Tertiary. The metamorphic complex shows the progressive development from subgreenschist and green-schist facies slate, phyllite, and semischist that is regionally characteristic of the Valdez Group, to biotite-plagioclase-quartz schist (± muscovite, cordierite, andalusite, staurolite, garnet, and sillimanite) in an intermediate zone, and amphibolite facies muscovite-biotite-plagioclase-quartz schist and gneiss (± sillimanite and garnet) in a higher-grade central zone. K-feldspar is characteristically absent in the amphibolite facies schists, but it is present in migmatitic rocks that are common to the central zone. The mineral assemblages and petrographic relations indicate that metamorphism took place under low-pressure intermediate-facies series conditions; partial melting took place at the highest grade of metamorphism. Melting produced granodioritic magmas in migma-titic rocks of the central zone that were emplaced as discrete plutons in both the intermediate zone and peripheral rocks of the complex. The timing of metamorphism as indicated by field relations and K-Ar data is about 50 to 65 m.y. ago and overlaps or is coeval with the emplacement, throughout the Gulf of Alaska region, of early Tertiary plutons that are thought to be anatectic in origin. The distribution of these plutons suggests that a high-temperature low-pressure metamorphic belt, about 1,800 km long, developed along the leading edge of the continental margin in late Paleocene to early Eocene time.

Tectonic History and Progressive Development of Fold-Thrust Belt in Eastern Gulf of Alaska: ABSTRACT

The geology of the Gulf of Alaska, east of Kayak Island, records the temporal variation of three fundamentally different tectonic settings that developed owing to the interaction between plates along the western margin of North America. A late Meosozic to early Tertiary convergent margin setting is indicated by nearly contemporaneous plutonic belts, forearc-basin sequences, and accretionary terranes. In contrast, the middle Tertiary continental margin in the eastern Gulf of Alaska was relatively stable and is characterized by sedimentation in a subsiding basin with local extensional tectonism. The present tectonic setting was probably initiated during the End_Page 928------------------------------ Miocene and reflects a combinatyion of dextral strike-slip motion and oblique convergence. The proposed tectonic model suggests that most of the eastern Gulf of Alaska is underlain by early Tertiarty and older oceanic crust and that large-scale lateral displacement of the Yakutat block may not be necessary. Late Tertiary convergence in the central Gulf of Alaska, or Yakataga district, has controlled the structural evolution of a complexly deformed, fold-thrust belt consisting of numerous subparallel folds and north-dipping thrust faults. Moving from north to south, deformation in this belt becomes progressively younger and less intense; most of the structures in the offshore part of the Yakataga district are late Pliocene to Pleistocene in age. Industry leased a number of these relatively young structures in April 1976, and subsequently drilled ten dry holes. The drilling results suggest that geopressures, resulting in part from tectonic stresses, low geothermal gradients, and poor sand development appear to be characteristic of these structures and contributed to the lack of success.End_of_Article - Last_Page 929------------

Glacial Lithofacies of Neogene Yakataga Formation, Gulf of Alaska: ABSTRACT

Lithofacies, biofacies, and chronostratigraphic analyses indicate three relatively warm and three relatively cool paleoclimatic intervals within the 5,000-m thick strata of the lower Miocene through Holocene Yakataga Formation of the Robinson Mountains, eastern Gulf of Alaska. Glacial periods, recognized by the predominance of glacial lithofacies associated with populations of Neoglobigerina pachyderma s.l., are interpreted for the cool intervals. In the sections studied, the interglacial intervals have little or no glacial deposits. Yakataga Formation strata of glacial origin can be classified as having formed in either ice-contact or glacial-aqueous depositional environments. Ice-contact deposits consist of stratified or chaotic sedimentary materials, but erratics are rare in the Yakataga Formation. Glacial-aqueous deposits are of three principal types: glaciolacustrine, glaciofluvial, and glaciomarine. Glaciolacustrine deposits have not been identified in the Yakataga Formation. Glaciofluvial deposits, characterized by interbedded poorly sorted lenticular sand and gravel, occur in geographically limited and stratigraphically restricted intervals of the Yakataga Formation. Glaciomarine silts and clays, containing floating pebbles and cobbles and in-situ marine fossils, are the dominant glacial aqueous deposits i the Yakataga Formation. Detailed lithofacies associations and geometries define open-shelf and fjord glaciomarine subenvironments. Lithofacies sequences within these subenvironments suggest deposition by advancing and retreating ice masses. Mapping of the relative percentages of lithofacies types within the late Pliocene-Pleistocene glacial sequence defines the late Neogene paleogeography of the Yakataga district. This mapping suggests that the late Pliocene and Pleistocene distribution of alpine glaciers, coastal-outwash and alluvial flood plains, and fjords, and the associated rates of sedimentation, are analogous to the depositional pattern during the Holocene. End_of_Article - Last_Page 891------------

Factors Influencing Sediment Transport at Shelf Break: ABSTRACT

Recent investigations of sediment dynamics at the shelf break suggest that the factors which control the transport of sediment there can be grouped into two major categories: oceanographic and geologic. Oceanographic factors include tides, surface waves, internal waves, fronts, and meteorlogically driven currents that create a wide range of unsteady and quasi-steady water motions whose influences on sediment dispersal is poorly understood. These phenomena affect sediment along the shelf break in zones that have a large spatial and temporal variability. Geologic factors include shelf geometry and physiography, tectonism, and sediment type and supply. These factors influence shelf-break processes either by modifying oceanographic processes or by controlling the geologic set ing of the shelf break. An example of the former is that the physical structure of the bottom boundary layer depends strongly on the shape of the sea floor. An example of the latter is that sediment supply partly determines whether the shelf break is a net depositional or an erosional zone. Whereas both oceanographic and geologic factors interact to influence sediment transport processes at the shelf break, a particular continental margin may be dominated by one of these factors. For instance, such oceanographic agents as the Gulf Stream and winter storms control sediment dynamics over large segments of the east United States coast, whereas such geologic factors as large sediment supply and active tectonism overshadow oceanographic phenomena in the eastern Gulf of Alaska. Many intermediate conditions also exist. An example is at the complex shelf break off southern California, where active tectonism and oceanographic phenomena nearly equally influence shelf-edge processes. End_of_Article - Last_Page 943------------

Bathymetric map of oil and gas lease area available

A bathymetric map of oil and gas lease sale area 55 is available from the U.S. Geological Survey. The map, U.S.G.S. Open File Report 80‐649, ‘Bathymetric Map of the Outer Continental Shelf between Yakutat Bay and Dry Bay, Eastern Gulf of Alaska,’ is by Glenn P. Thrasher and Bruce W. Turner. The 1:250,000‐scale map was constructed from high‐resolution seismic reflection data from a U.S.G.S. contract survey conducted in 1979.

Development of a Synoptic Climatology for the Northeast Gulf of Alaska

Abstract The subjective, sea level pressure-based, synoptic climatologies of Sorkina and Putnins have been specialized for the coastal region of southern Alaska. The results of the subjective typing were compared to the automated correlation technique for surface chart typing proposed by Lund. The subjective and automated approaches point to four persistent patterns: the Aleutian low, the low over the southern Gulf of Alaska, high pressure centered over the eastern Gulf of Alaska, and high pressure centered over the western Gulf of Alaska. Beyond these four patterns, migratory low-pressure centers were divided into various groups. Matching individual synoptic charts to the map patterns, or sequencing, was done both subjectively and objectively by correlation with the National Meteorological Center grid point analyses. For the period 1968–77 75% of the twice daily sea level pressure charts could be sequenced objectively by matching one of the subjective subtypes with a correlation >0.7. Correlation-based t...

Benthic Foraminifers From Eastern Gulf of Alaska: ABSTRACT

Grab samples, dart cores, and gravity cores from the Gulf of Alaska between Montague Island and Yakutat Bay were analyzed for benthic foraminifers. Robust specimens of Cassidulina californica, C. limbata, and Cibicides lobatulus are abundant in coarse sediment on Tarr Bank, Middleton Island platform, and along much of the outer shelf east of Kayak Island. This fauna may be relict, as it occurs in areas known from seismic profiles to be nearly devoid of the Holocene clayey silt which blankets much of the shelf. In several samples, glauconite associated with this fauna provides evidence for a low rate of sedimentation. Another fauna, in which Epistominella pacifica occurs with Elphidium clavatum (typically a shallow-water, inner-shelf species), is found in Kayak Trough at depths ranging from 146 to 234 m and in Hinchinbrook Sea Valley at 205 m. Both of these depressions have topographic highs at their seaward terminations. In previous studies in other parts of the Gulf of Alaska, Epistominella pacifica is reported to be common above 300 m depth; however, farther south, off California, Oregon, and Washington, E. pacifica usually occurs at depths greater than 300 m. The association of E. pacifica and Elphidium clavatum might therefore be useful in determining paleoenvironments; their co-occurrence could indicate a depression, such as a trough or sea valley, somewhat restricted from open sea conditions End_Page 447------------------------------ Seismic records show that the Holocene clayey silt, which is common on much of the shelf, is thin or absent on Pamplona Ridge and in the outer part of Bering Trough. Samples from these areas, in water depths ranging from 163 to 315 m, have abundances of E. clavatum of 10% or more, and Buccella is present. This fauna may indicate deposition at shallower depths during a Pleistocene lower stand of sea level. End_of_Article - Last_Page 448------------

Geology of Continental Slope Adjacent to OCS Lease Sale 55, near Yakutat, Eastern Gulf of Alaska: ABSTRACT

Forty-two samples of probable outcrop dredged along the 250-km-long northwest-trending continental slope between long. 138°00^primeW and 142°30^primeW from water depths of 3,150 to 200 m provide control on the geology of the seaward margin of the Tertiary basin underlying the outer continental shelf (OCS) Lease Sale 55 on the Yakutat segment of the shelf. Six rock units apparently in approximately normal stratigraphic position are tentatively recognized. (1) Hard graywacke, argillite, and possible intrusive rocks of inferred Late Cretaceous age crop out from Cross Sound to 139°W and probably underlie much of the Fairweather Ground. (2) 800+ m of shallow-marine basaltic flow and pyroclastic rocks, at least in part of early Eocene age, makes up much of the rugged lower slope from 139°W to about 141°30^primeW, and rocks of this type are sporadically present west of Yakutat Sea Valley. (3) 500+ m of shallow-marine fine to coarse clastics including organic-rich shale of early and middle(?) Eocene age overlies and intertongues with the volcanic unit from 139°30^primeW to 142°30^primeW. (4) 1,500+ m of upper to middle bathyal organic-rich shale, tuffaceous shale, and sandstone of late Eocene and early(?) Oligocene age, apparently overlies the older units between about 140°W to 141°30^primeW. (5) 450+ m of siltstone with sandstone of probable late Oligocene age underlies the upper slope between 139°W and 140°W. (6) Neogene marine glacial deposits and mud occur on the OCS as slope-basin fills and slope-rise prisms, and as a mantle over the slope at the mouths of the Alsek Canyon and Yakutut Sea Valley. The lower Tertiary sequence sampled on the continental slope differs significantly from coeval rocks onshore or penetrated by wells on the Yakataga segment of the OCS to the west. The Eocene and Oligocene units sampled include abundant thermally mature source rocks. Most dredged sandstones are poor reservoirs, but some upper Eocene to lower Oligocene sandstones have fair to good porosity and permeability. Seismic reflection data indicate that the Eocene and Oligocene sequence dips beneath Lease Sale 55 where it could be a favorable exploratory target. End_of_Article - Last_Page 766------------

Late Neogene Depositional and Climatic Cycles in Yakataga Formation, Gulf of Alaska: ABSTRACT

Lithofacies and chronostratigraphic analyses indicate three relatively warm and three relatively cool paleoclimatic intervals within the 5,000-m-thick strata of the lower Miocene through Holocene Yakataga Formation of the Robinson Mountains, eastern Gulf of Alaska. The cycle identified correlates with the widely recognized paleoclimatic cycle of warm middle Miocene, fluctuating late Miocene, cool terminal Miocene, warm early Pliocene, and cool to cold late Pliocene and Pleistocene. Glacial periods, recognized by the predominance of glacial lithofacies associated with populations of Neoglobigerina pachyderma s.l., are interpreted for the cool intervals. In the sections studied, the interglacial intervals have little or no glacial deposits. End_of_Article - Last_Page 671------------

Earthquake and Wave Design Criteria for Offshore Platforms

A method that applies experience, projections of environmental conditions, response analyses, and reliability logic is used to develop earthquake and wave design criteria for one class of offshore platform system. The system consists of a steel, tubular-membered, template-type superstructure supported on piles and firm soils, in 300 ft of water. Three fictitious sites are considered: Eastern Gulf of Alaska, Southern California, and the Santa Barbara Channel. Ranges of design wave heights and ground motion velocities are given. Results are compared with those appropriate for a conventional building structure and with those from API RP 2A guidelines for fixed offshore platforms. Good agreement with API guidelines is indicated.

Glacial Origin of Megachannels of Upper Yakataga Formation (Pliocene-Pleistocene), Robinson Mountains, Gulf of Alaska: ABSTRACT

A tidewater glaciomarine depositional model is proposed for the upper Yakataga Formation in the Robinson Mountains, eastern Gulf of Alaska. The upper 2,000 to 3,000 m of the Yakataga Formation is characterized by major channels (depth > 90 m) cut into and filled with both fluvial and/or marine rocks. The channels are elongate and steep walled, and appear to be confined within or parallel with paleotopographically low areas. Detailed studies of five channels show maximum dimensions of 430 m in depth and about 3 km in width. Channel length was not determinable. Channel margins are in places grooved and striated. Channel-base conglomerates exhibit foreset bedding inclining down the channel axis, and are interpreted to be subglacial melt-water deposits. The channel fill is dominated by interbedded siltstones and pebbly siltstones (diamictites). The siltstones contain abundant in-situ marine fossils, commonly encrusting the tops of dropstones. These deposits are interpreted to be proglacial marine sediments and ice-rafted glacial erratics. Thin sandstones interbedded within the siltstones are graded and exhibit traction features. These are believed to originate as gravity-flow units from oversteepened clastic wedges deposited by subglacial melt-water discharge at the grounded terminus of a glacier. Diamictites are of two major types: (1) poorly sorted sandy siltstone units with patchy distribution of angular clasts (nonmarine tillite); and (2) moderately sorted, muddy siltstone units with in-situ fossils and evenly dispersed, slightly rounded clasts (marine tillite). Some of the diamictites are highly contorted, particularly those underlying younger channel bases. The contorted character is probably the result of loading sediments by an advancing ice lobe. Nonmarine sandstones and conglomerates are present as interbedded lenticular packages, and are interpreted as fluvial units deposited within a braided glacial-stream complex. These units occur both within channels and as clastic wedges within marine-shelf sequences. The channels are interpreted as fiords, and the modern fiords of the Yakataga area (Icy and Yakutat Bays) serve as modern depositional analogs. End_of_Article - Last_Page 411------------

Structure of Continental Shelf, Eastern Gulf of Alaska: ABSTRACT

Structure of Continental Shelf, Eastern Gulf of Alaska: ABSTRACT

Hydrocarbon Gases in Sediments of Eastern Gulf of Alaska: ABSTRACT

Hydrocarbon Gases in Sediments of Eastern Gulf of Alaska: ABSTRACT

Clay Mineralogy of Eastern Gulf of Alaska: ABSTRACT

Clay Mineralogy of Eastern Gulf of Alaska: ABSTRACT