West-central Alaska Research Articles

Kaltag fault, northern Yukon, Canada: Constraints on evolution of Arctic Alaska

The Kaltag fault has been linked to several strike-slip models of evolution of the western Arctic Ocean. Hundreds of kilometers of Cretaceous-Tertiary displacement have been hypothesized in models that emplace Arctic Alaska into its present position by either left- or right-lateral strike slip. However, regional-scale displacement is precluded by new potential-field data. Postulated transform emplacement of Arctic Alaska cannot be accommodated by motion on the Kaltag fault or adjacent structures. The Kaltag fault of the northern Yukon is an eastward extrapolation of its namesake in west-central Alaska: however, a connection cannot be demonstrated. Cretaceous-Tertiary displacement on the Alaskan Kaltag fault is probably accommodated elsewhere.

Isotopic Evidence for Early Proterozoic Age of the Idono Complex, West-Central Alaska

The Idono Complex of west-central Alaska is a fault-bounded, fragment of Early Proterozoic continental crust surrounded by overlap assemblages and younger terranes accreted in Mesozoic time. It is composed of granitic to dioritic orthogneiss, amphibolite, and metasedimentary rocks. Trace element compositions of the granitoids and amphibolite suggest rock formation in a subduction-related volcanic arc terrain. Nine zircon fractions from three samples of granitoid orthogneiss define a U-Pb discordia line intersecting concordia at and . The upper and lower intercepts are interpreted, respectively, as approximations of the time of granitoid crystallization and major episodic Pb-loss. The lower intercept age is similar to that indicated by some hornblende and biotite K-Ar ages. Other biotite, hornblende, and white mica K-Ar ages record later isotopic disturbance in Early Cretaceous time. Nd isotopic compositions at 2.06 Ga for tonalite orthogneiss () and amphibolite ( and +2.3) indicate that Early Proterozoic crust formation in the Idono Complex involved significant additions of mantle-derived magma. These rocks yield (depleted mantle) model ages of 2.0 to 2.1 Ga, similar to the U-Pb zircon upper intercept age. Involvement of Archean crust in the formation of at least some rocks of the Idono Complex is indicated by a granitic orthogneiss (), which yields a model age of 2.5 Ga. The ~2.06-Ga age of the Idono Complex is similar to that indicated for provenance(s) of widely distributed sedimentary rocks in western North America and may represent a fragment of the North American craton displaced by northward movement of the Pacific plate. At such, rocks of the Idono Complex may provide important insights into both Early Proterozoic evolution along the craton margin, and subsequent displacements.

Stratigraphy and sedimentology of the Mid‐Cretaceous deposits of the Yukon‐Koyukuk Basin, west central Alaska

The northeast trending Yukon‐Koyukuk basin of west central Alaska consists of two subbasins, the Kobuk‐Koyukuk subbasin to the north and east and the Lower Yukon subbasin to the southwest. The subbasins are separated by an arcuate Lower Cretaceous volcanic pile, the Hogatza trend, which is thought to be an accreted volcanic arc. The oldest part of the sedimentary fill of the subbasins consists of Valanginian to lower Albian(?) volcaniclastic rocks deposited on the flanks of the Hogatza trend. Following subsidence of the Hogatza trend, mid‐Cretaceous clastic sedimentary strata of mainly Albian and Cenomanian age, and possibly as thick as 8000 m, were shed into the basin; these deposits were derived from surrounding uplands or borderlands in the Seward Peninsula to the west, the Brooks Range to the north, and the Ruby geanticline to the southeast. These mid‐Cretaceous basin fill deposits can be divided into four main facies: (1) basin margin conglomerate facies, chiefly alluvial fan deposits that were transported basinward and rest in part unconformably on the surrounding uplands; (2) shelf facies, chiefly cross‐stratified and hummocky cross‐stratified sandstone deposited by wave‐generated currents on a shelf that rimmed the basin on its western and northern margins; (3) deltaic facies, chiefly sandstone and shale deposited in delta plain and delta front environments on a large constructional delta that prograded westward from the eastern basin margin across both subbasins and across the subsided southern part of the Hogatza trend; and (4) turbidite facies, chiefly interbedded sandstone and shale deposited as elongate deep‐sea fans and related deep‐sea clastic systems by flows that transported sediment to the axial parts of both subbasins, northeastward in the Lower Yukon subbasin and eastward to southward in the Kobuk‐Koyukuk subbasin. Sedimentation appears to have ended in the Santonian, followed by uplift, folding, and faulting of the basin fill. Less deformed, lower Tertiary nonmarine volcanic and volcaniclastic rocks unconformably overlie the more highly deformed Cretaceous strata.

Contrasting plutonic rock suites of the Yukon‐Koyukuk Basin and the Ruby geanticline, Alaska

The volcanogenic Yukon‐Koyukuk basin (YKB) and the Ruby geanticline (RG), a fault‐bounded miogeanticlinal assemblage of metamorphic‐plutonic rocks, are adjacent geologic provinces whose origins and mutual relationships have been enigmatic. The petrogenesis of the Cretaceous plutonic rock suites that occur in both provinces is a possible key to deciphering the complex regional geology and to placing constraints on the nature of the underlying crust and mantle. YKB plutons consist of two suites of distinctive compositions, age, and distribution forming opposite ends of a 500‐km‐long belt intruded into andesitic volcanic rocks of the Koyukuk terrane, a subduction‐related intraoceanic arc of Jurassic and Early Cretaceous age that collided with continental North America in Early Cretaceous time. Plutons in the western YKB yield mid‐Cretaceous K‐Ar ages of 99–113 Ma, have a high‐K2O character and consist of Si‐saturated to oversaturated monzonite, syenite, quartz syenite, and biotite granite with a considerable range in SiO2. Associated with the western YKB plutonic suite is an ultrapotassic rock province consisting of at least 12 intrusive complexes and dike swarms of feldspathoid‐bearing hypersolvus plutonic rocks. Their most striking chemical characteristic is consistently high K2O (to 16.6%), a K2O/Na2O ratio ≫ 1, and <60% SiO2. The western YKB plutonic suite extends into the southeastern Seward Peninsula, St. Lawrence Island, and eastern Siberia. Eastern YKB plutons in contrast form an epizonal, compositionally expanded calcalkaline suite ranging from tonalite to granite and characterized by high Na2O/K2O and Fe2O3/FeO ratios, abundant hornblende, mafic inclusions, and coeval volcanic rocks. They yield K‐Ar ages of 78–89 Ma, indicating a Late Cretaceous age of emplacement. The northeast trending Ruby geanticline contains the largest suite of plutonic rocks in west central Alaska. The plutons are composed chiefly of potassic, weakly to moderately peraluminous, hornblende‐free, ilmenite series biotite granite, two‐mica granite, and granodiorite. The plutons yield late Early Cretaceous K‐Ar and U‐Pb ages of 99–112 Ma and intrude (1) Precambrian‐Paleozoic greenschist facies metasedimentary rocks of the Ruby terrane, and (2) a thrust‐faulted sequence of mafic oceanic rocks of late Paleozoic to Jurassic age (the composite Angayucham‐Tozitna terrane). The compositional differences between the YKB plutonic suites suggests fundamental differences in source materials. The composition and geologic setting of the eastern YKB plutonic suite is typical of plutons formed by melting of mantle material or andesitic volcanic supracrustal material derived from the mantle with little involvement of continental crust or lithosphere. The western YKB plutonic suite extends into the continental rocks of the eastern Seward Peninsula, indicating similar source material beneath both provinces, possibly of a subcontinental character. This subcontinental mantle under the western YKB may wedge out to the east since K2O content of YKB plutonic rocks gradually decreases eastward across the basin. The RG plutonic suite differs from the adjacent eastern YKB plutonic suite in composition and age. The composition, geologic setting, and volume of the RG plutons indicates an origin by melting of continental crust material. The magmatic processes that caused melting may have resulted from crustal thickening, postcollisional relaxation following arc‐continent collision and suturing, or underthrusting of the Koyukuk terrane beneath the RG.

Two new Devonian spine-bearing pleurotomariacean gastropod genera from Alaska

Two new spine-bearing gastropods,Chlupacispira spinosan. gen. and sp. andSpinulrichospira cheeneetnukensisn. gen. and sp., are described from the late Early Devonian (Emsian) and early Middle Devonian (Eifelian), respectively, of west-central Alaska. These represent the earliest reported spiny pleurotomariacean gastropods. Otherwise, spinose pleurotomariaceans are known from strata no older than Carboniferous age.Spinulrichospira cheeneetnukensisn. gen. and sp. appears to represent a more highly ornamented derivative ofUlrichospiraDonald. Both new genera are part of the more highly ornamented fauna which occurred in warm equatorial waters of the Old World Realm during the Early and Middle Devonian, in contrast to more weakly ornamented shells of the Eastern Americas Realm and even more weakly ornamented (almost totally “plain”) shells of the Malvinokaffric Realm. The latter two realms are thought to represent subtropical to warm temperate and cool temperate to cool polar conditions, respectively.

First recognition of the Devonian alga Lancicula sergaensis Shuysky in North America (west-central Alaska)

First recognition of the Devonian alga <i>Lancicula sergaensis</i> Shuysky in North America (west-central Alaska)

Early Cretaceous Evolution of Yukon-Koyukuk Basin and Its Bearing on Development of Brookian Orogenic Belt, Alaska: ABSTRACT

The Brookian orogenic belt (Seward Peninsula, Brooks Range, Ruby geanticline) forms three sides of the Cretaceous Yukon-Koyukuk basin of west-central Alaska. Low Sri values from igneous rocks within the basin suggest that it is not underlain by older continental crust. The basin encloses a south-facing horseshoe-shaped trend of Lower Cretaceous andesitic volcanic rocks. Major-element chemistry of these rocks indicates that they are calc-alkaline and of island-arc affinity. Berriasian to Valanginian volcanic rocks in the basin are predominantly clastic and were deposited in shallow marine to subaerial environments. Marked subsidence began during Hauterivian time, accompanied by a change to highly potassic (shoshonitic) pyroclastic volcanism. During the Barremian (Aptian?), these tuffs were interbedded with Brookian-derived turbidites, deposited in a trough between the subsided volcanic platform and the uplifted Brookian metamorphic belt. Paleoflow was clockwise around the basin from west to east. By the Albian, significant volcanism had ceased, and the intervening trough filled with Brookian sediment. The Brookian orogeny apparently resulted from attempted subduction of the North American margin beneath the intraoceanic Koyukuk arc. The relatively long t mespan (approximately 30 Ma) between initial continental underthrusting (Tithonian?) in the Brooks Range and the shutoff of arc volcanism (Aptian?) suggests a very slow convergence rate (1-2 cm/year). End_of_Article - Last_Page 657------------

The Devonian goniatite genera Pinacites and Foordites from Alaska

The Devonian goniatite genera Pinacites and Foordites are described from west-central Alaska. This is the first time that Pinacites has been found in North America. The faunas are compared with those of the Eifelian Stage of Europe and in particular with the lower part of the Chotéč Limestone of Czechoslovakia.

The Yukon River Ingalik: Subsistence, the Fur Trade, and a Changing Resource Base

Throughout the period of historic contact, the Ingalik Indians, salmon fishermen of the lower-middle Yukon River in west-central Alaska, displayed a strong secondary dependence on large and small game animals. Patterns of fluctuating wildlife populations are examined and related to habitat changes, particularly those brought about by forest fires. Excessive trapping combined with ecological variations and a changing economic environment are shown to have been responsible for the decline in the fur trade apparent in the last two decades of the 19th century.

Potassium-Rich Alkaline Intrusive Rocks of Western Alaska

Alkaline intrusive rocks occur in a 300-km-long belt of small plutons and intrusive complexes that extends from west-central Alaska to the Bering Sea. The occurrence of a massif of similar rocks on the easternmost tip of the Chukotsk Peninsula suggests the belt may extend into Siberia. These are highly potassic and subsilicic rocks that were emplaced at about mid-Cretaceous time (Albian and Cenomanian). The belt of alkaline potassic rocks cuts across two different geologic provinces. The eastern half of the belt is in the Yukon-Koyukuk volcanogenic province of Mesozoic age; the western half is in the Seward Peninsula province, which is comprised mainly of metamorphic and sedimentary rocks ranging in age from Precambrian to Paleozoic. The Yukon-Koyukuk province is an unusual setting for alkaline rocks, commonly thought to be typical of stable continental platform or shield areas. The possibility that the volcanic rocks of the Yukon-Koyukuk province rest directly on oceanic crust together with the regional extent of the potassium-rich alkaline rocks suggests that the potassic magma may have originated in the mantle. Confinement of the alkaline rocks to the general boundary area between the two geologic provinces suggests this highly faulted area was a zone of structural weakness along which deep-seated alkaline magma was emplaced. This magma may have influenced the composition of other plutonic rocks that are closely associated in time and space with the alkaline potassic rocks.

Mesozoic Tectonics and Correlations in Yukon-Koyukuk Province, West-Central Alaska: ABSTRACT

The Yukon-Koyukuk province of west-central Alaska is a broad, wedge-shaped depression of Mesozoic and early Cenozoic rocks bordered on the west, north, and southeast by metamorphic rocks of Paleozoic and possibly older age. The province is not a simple sedimentary basin, as formerly believed, but a highly mobile tract that was subjected to repeated volcanism and plutonism during Cretaceous and early Tertiary times. Sedimentation was largely volcanogenic in character and confined, mostly, to a relatively short interval in mid-Cretaceous. Recent mapping on St. Lawrence Island suggests that the Mesozoic and early Cenozoic rocks of the Yukon-Koyukuk province swing in a broad arc across the Bering Sea and western St. Lawrence Island to join the Okhotsk volcanic belt of northea tern Siberia. Three different structural trends are recognizable among the intensely deformed pre-Tertiary rocks of the Yukon-Koyukuk province: East-west trends in the northern part of the province and adjoining Brooks Range, north-south trends along the western edge of the province from the western Brooks Range to the Yukon delta, and northeast-southwest trends along the Yukon and Koyukuk valleys. The province is transected between Ruby and Unalakleet by the Kaltag fault, a major strike-slip fault with 40-80 mi of probable right-lateral offset. Major structural discontinuities also occur along the Kobuk fault zone at the northern edge of the province, but the evidence for large-scale strike-slip displacement is inconclusive. End_of_Article - Last_Page 2500------------

PHYLOGENETIC RELATIONSHIPS OF AN UNKNOWN TETRAPLOID BROMUS, SECTION BROMOPSIS

A previously unreported member of the section Bromopsis, having taxonomic similarities to B. pumpellianus and B. pacificus, was recently found in westcentral Alaska. Its relationship to these species and to B. inermis and B. ciliatus has been investigated. Both this unreported member (Bromus X) and B. pacificus were shown for the first time to be tetraploids (2n = 28); forms of B. ciliatus originating in Russia and North America were diploids (2n = 14); and a number of Alaskan B. pumpellianus were octoploids (2n = 56). Chromosomes with large satellites were found in Bromus X, B. pumpellianus, and a tetraploid form of B. inermis but not in B. pacificus or B. ciliatus. Seed was produced and hybrids resulted from crosses of Bromus X with B. pumpellianus and tetraploid B. inermis but no seed developed from crosses with B. pacificus and B. ciliatus. A relatively high frequency of stainable pollen among the hybrids suggested that they were partially fertile. It was concluded that Bromus X was a very close relative of B. pumpellianus and B. inermis but was not closely related to B. pacificus or B. ciliatus.

Developments in Alaska in 1959

Exploratory activity in Alaska increased considerably during 1959. Geophysical surveying and drilling registered the largest increases. Twenty wells were active during 1959, compared with 12 in 1958. Eight exploratory wells spudded during the year, compared with 3 in 1958. Significant exploratory wells were drilled in 2 basins never before tested--the Nushagak Basin in southwestern Alaska and the Koyukuk Basin in west-central Alaska. Three development wells were completed in the Swanson River oil field. A major gas discovery was made on the Kenai Peninsula by the Union Oil Company of California and the Ohio Oil Company. The discovery well, Kenai Unit 14-6, was completed flowing dry gas at a 31,000 MCF/D rate from the Kenai formation. Two follow-up wells were also completed as gas producers. Twenty oil companies sent field parties into the various sedimentary basins. At the close of 1959 there were 32,611,079 acres under Federal oil and gas leases and 101,639 acres under State of Alaska oil and gas leases. The State of Alaska offered the first offshore oil and gas leases. Bidding for these was higher than had been anticipated. The State obtained $4,021,031.43 from the initial sale. A substantial decrease in surface mapping and a moderate decrease in geophysical activity are in prospect for 1960. Drilling will probably remain near the 1959 level.

Developments in Alaska in 1956

Exploration for oil and gas in Alaska during 1956 continued at about the same level as in 1955. Wildcat starts decreased in 1956 and there were no completions. Five drilling rigs were active in 1955 compared with three in 1956. Drilling was commenced or resumed on two Tertiary tests and one Mesozoic test in south-central Alaska and a Tertiary test, the Phillips-Kerr-McGee's Sullivan Unit No. 2, was spudded and drilling below 10,000 ft. at the end of the year in southeastern Alaska. Geophysical activities increased slightly from 7 party-months in 1955 to 9 party-months in 1956 whereas surface investigations remained at the 1955 level of 30 party-months. Applications for Federal oil and gas leases increased from approximately 800,000 acres in 1955 to about 2 3/4 million acr s in 1956. The majority of the newly acquired acreage is in the Yakutat area of southeastern Alaska and the Kateel River area of west-central Alaska. Significant developments, which indicate increased activity in 1957, include the awarding of a 10-year development contract to the Colorado Oil and Gas Corporation covering over one million acres in the Yakutat area, southeastern Alaska, and the approval of the Richfield Oil Company's 71,680-acre Swanson River Unit, on the Kenai Peninsula, Alaska. Richfield and Colorado Oil and Gas plan to commence drilling operations during the early part of 1957.