Wrangell Mountains Research Articles

New data on the Upper Triassic bivalve Monotis in North America, and the new subgenus Pacimonotis

Re-examination of some North American collections of the late middle to early late Norian pterioid bivalve Monotis confirms the separation of M. (M.) alaskana Smith from the European type species M. salinaria (Schlotheim). The related M. (M.) haueri Kittl, as differentiated statistically, is more widely represented in southern Alaska, the Canadian Yukon, and Nevada than was formerly recognized. Species described originally from northeastern USSR and now confirmed as present in rocks from Alaska and western Canada include: M. (Entomonotis) pachypleura (Teller), M. (Eomonotis) daonellaeformis Kiparisova, and M. (E.) anjuensis Bytschkov and Efimova. Monotis subcircularis Gabb and related forms are transferred from the subgenus Entomonotis to the new subgenus Pacimonotis in order to give the former subgenus a more homogeneous morphologic range.Re-evaluation of the stratigraphic occurrences of these species shows that M. alaskana occurs above M. subcircularis and below M. haueri in the Wrangell Mountains, Alaska. Elsewhere in North America, however, Monotis haueri occurs at the same level as M. subcircularis and is thus a long-ranging form. These three taxa, as well as M. pachypleura, are representative of the upper Norian Cordilleranus Zone. Monotis daonellaeformis is among the oldest Monotis species of the middle Norian Columbianus Zone; M. anjuensis in North America is probably also from the Columbianus Zone but could overlap in occurrence with the upper Norian M. subcircularis.

Triassic corals and spongiomorphs from Hells Canyon, Wallowa terrane, Oregon

Twenty-one species of corals and three species of spongiomorphs occur in a series of richly fossiliferous, molluscan-dominated beds with silicified bioclasts in the Upper Triassic Martin Bridge Limestone of Hells Canyon, Oregon. Two of these,Maeandrostylis grandiseptusandRecticostastraea wallowaensisare new species.Recticostastraeais designated as a new genus.The fauna is early Norian and occurs in the island arc Wallowa terrane, one of many tectonostratigraphic terranes in western North America. Like other examples, it appears to have developed independently of the North American craton and to have links with Wrangellia. The fossil corals and spongiomorphs are para-autochthonous, occurring in a series of tempestite beds. They are interpreted to have inhabited a shallow-water carbonate platform that developed around a tropical island arc following cessation of volcanic activity. The corals and spongiomorphs are associated with abundant gastropods and a diverse epifaunal suspension-feeding bivalve fauna. Relative to the corals, branching spongiomorphs,Spongiomorpha ramosa, are more abundant and occur with relatively common branching, sheet to plate-like, colonial corals. Solitary corals are relatively rare. The associated bedded limestone includes a variety of shallow-water microfacies but throughout the Hells Canyon sequence, reef structure is absent.Together, the 24 coral and spongiomorph taxa show mixed paleogeographic affinities with Upper Triassic faunas known only from alpine regions of the western Tethys (five species), the Pamir Mountains, U.S.S.R. (two species), and the island of Timor (one species). Five additional species are pan-Tethyan and exceptionally cosmopolitan, but 11 species (45.8%) occur only in displaced terranes. Of these, a significant component (six species) is endemic to the Wallowa terrane. At least four Hells Canyon taxa, previously thought endemic to North American terranes, have recently been reported from the high-latitude Koryak terrane of northeastern U.S.S.R., a displaced tropical volcanic terrane of the northwestern Pacific. For Triassic corals, this is the first example of a clear link between western Pacific and eastern Pacific terranes. Less similarity exists with the Wrangell Mountains, Alaska, where identical age lower Norian silicified corals and spongiomorphs are known.

The upper Cenozoic White River "tillites" of southern Alaska: Subaerial slope and fan-delta deposits in a strike-slip setting

The White River tillites crop out over 8 km 2 either side of the Totschunda fault in the northern foothills of the Wrangell Mountains in southern Alaska and record glacially influenced slope and fan-delta deposition in a high-relief basin close to an active volcano between 10 and 2.7 Ma. Two measured sections, 1,200 and 450 m thick, show diamictite and conglomerates interbedded with augite andesite lavas. Facies range from massive diamictites, deposited subaerially as debris flows, to crudely stratified sandstones and conglomerates formed by sheet-flow processes on alluvial fans, and well-stratified and normally graded sandstones and conglomerates that accumulated subaqueously as turbidites; large-scale southward-dipping foresets as much as 17 m high identify a fan delta that prograded into a temporary lake. The White River succession is identified as a valley infill complex consisting of subaerial slope and lacustrine fan-delta deposits; there are no in situ primary deposits present in the succession. The component of sedimentation was limited to the supply of coarse-grained debris identified by the presence of glacially striated and faceted clasts. The White River deposits may be typical of glacial sequences preserved as isolated local basin fills in high-relief areas such as occur in convergent or strike-slip plate-margin settings. They provide only a limited and fragmentary record of paleoenvironmental changes compared to coeval glacially influenced marine rocks preserved in the Yakataga Formation (5 km thick) along the Gulf of Alaska coast.

Comparison of in situ and landsat derived reflectance of Alaskan Glaciers

Calculation of snow and ice albedo is necessary for developing physically realistic simulations of the Earth's climate using general circulation models, and useful for analysis of glacier mass balance change using remote sensing. Reflectances calculated from TM data and corrected for atmospheric effects correspond with in situ measured reflectances in the nadir-viewing mode, and are shown to be related to a glacier's mass balance if measured over a period of years. A reflectance of 0.895 for a test site in the Wrangell Mountains, Alaska, was calculated from TM Band 4 (0.76–0.90 μm) data and corrected for atmospheric effects. This value was comparable to the in situ reflectance of 0.90 measured in the same 0.76–0.90 μm wavelength region. For the same site, a reflectance value of 0.79 derived from integrating over most (0.40–3.0 μm) of the reflective portion of the electromagnetic spectrum was quite different from the integrated reflectance of 0.95 calculated for the spectral range 0.40–1.0 μm. This demonstrates the importance of using the full reflective energy spectrum for calculating the albedo of snow, and for obtaining a meaningful computation of a glacier's energy and mass balance change.

Comment and Reply on "Escape hypothesis for the Stikine block"

When stratigraphic, geochemical, and faunal data of Wrangellia and Wallowa-Seven Devils are viewed objectively, the data suggest—but do not require—that the two crustal blocks developed adjacent to one another. Sarewitz (1983) and Rusmore (1987) noted that the geochemical differences between the arclike Wallowa and Cadwallader volcanic rocks vs. the back-arclike Karmutsen volcanics on Wrangellia may be the result of their tectonic setting relative to the arc axis. We disagree with Mortimer's (1986) correlation of Wallowa volcanics with Stikinia volcanics because the subalkalic volcanic rocks of Wallowa are mainly Ladinian in age and compare poorly with the calc-alkalic augite porphyry of Carnian- Norian age that is typical of Stikine. Newton (1983) reported that Late Triassic molluscan fauna from the Wallowa terrane are most similar to those collected from the Wrangell Mountains. As addressed below, paleomagnetic data do not constrain Wallowa-Wrangellia-Stikine correlations. It appears, therefore, that faunal ties and timing of volcanism indicate that the Wallowa terrane is more similar to Wrangellia than to Stikine, and we incorporate this correlation into our model until direct evidence can support or deny it.

Pennsylvanian pluton stitching of Wrangellia and the Alexander terrane, Wrangell Mountains, Alaska

A quartz monzonite-syenite-alkali granite plutonic complex in eastern Alaska crosscuts the contact of the Alexander terrane and Wrangellia and intrudes the basement rocks of both terranes. Zircon U-Pb data indicate an intrusion age of 309 {plus minus} 5 Ma (Middle Pennsylvanian) for the pluton, and {sup 40}K-{sup 40}Ar age for hornblende separates indicate cooling to about 450 C during Middle Pennsylvanian-Early Permian time. The new field relations and age data demonstrate the Wrangellia and the Alexander terrane were contiguous during the Middle Pennsylvanian. This conclusion provides an important new constraint on paleogeographic reconstructions of the northwest Cordillera, and necessitates reassessment of stratigraphic and paleomagnetic data that were cited as evidence that the terranes evolved separately until the late Mesozoic.

A Mineral Collecting Adventure in the Skarn Zone,Wrangell Mountains, Alaska

A Mineral Collecting Adventure in the Skarn Zone,<i>Wrangell Mountains, Alaska</i>

Evidence for magma heterogeneity in the White River Ash (Yukon Territory)

Two Recent Plinian eruptions in the Wrangell Mountains (southeast Alaska) gave rise to two distinct ash-fall deposits that are collectively known as the White River Ash and cover much of the Yukon Territory, northwest Canada. Analysis of the pumiceous glass indicates that the magma chamber was compositionally inhomogeneous prior to each eruption. No compositional stratigraphy has been detected in the deposits, indicating either thorough mixing in the eruption cloud or thorough reworking after deposition. Thus each individual sample of ash represents a large part of the magma chamber, whereas larger pumice fragments are more homogeneous. Variations in temperature, 950–990 and 995–1030 °C, respectively, for the older and younger eruptions, and −log fo2 values, 9.3–8.3 and 8.3–7.7, derived from the Fe–Ti oxides, support the conclusion that the magma chamber was inhomogeneous.

Distribution, stratigraphy, petrochemistry, and palaeomagnetism of the late Pleistocene Old Crow tephra in Alaska and the Yukon

The late Quaternary Old Crow tephra is a two-pyroxene, calc-alkaline dacite whose known areal extent is broadly delimited by a triangle with apices at the Seward Peninsula, the Wrangell Mountains, and the Old Crow Basin in the northern Yukon. Everywhere the tephra is fine grained (Mdσ = 5.33 ± 0.32) and moderately to poorly sorted (6σ = 1.33 ± 0.29). Samples from the extremities of the fall-out zone have similarity coefficients of 0.95.A palaeomagnetic excursion is recorded in sediments just below Old Crow tephra in the Fairbanks area and at Imuruk Lake on the Seward Peninsula. A short, full reversal of the magnetic field has been preserved at the corresponding stratigraphic level near Old Crow in the northern Yukon. Chronological controls based on 14C and fission-track dates and sedimentation rates show that these palaeomagnetic features are almost certainly partial records of the complex geomagnetic Blake Event, which occurred 100–120 ka ago.New petrochemical data demonstrate that the source of Old Crow tephra is in the eastern Aleutian arc.

Comparative geochemistry and petrology of Triassic basaltic rocks from the Taku terrane on the Chilkat Peninsula and Wrangellia

Upper Triassic metabasalt from the Chilkat Peninsula in southeastern Alaska is lithologically similar to the Middle and (or) Upper Triassic Nikolai Greenstone from the Wrangell Mountains, east-central Alaska. Both basaltic sequences show comparable petrologic and geochemical features. The informally designated Chilkat metabasalt, which forms part of a Triassic sequence between the Denali fault and the Chilkoot lineament, is included in the Taku terrane, whereas the Nikolai Greenstone characterizes the allochthonous Wrangellia terrane. The Chilkat metabasalt consists mostly of massive to inconspicuously layered flows with well developed pillow structures at the top of the section; the sequence may reach a total thickness of 3000 m. The metabasalt is predominantly amygdaloidal and contains secondary minerals typical of greenschist-facies metamorphism. The Nikolai Greenstone is a widespread, mostly subaerial but locally pillowed, amygdaloidal basaltic sequence that reaches a total thickness of more than 3500 m; it is commonly less metamorphosed than the Chilkat metabasalt and contains secondary minerals typical of zeolite-facies to very low greenschist-facies metamorphism. Both the Chilkat and Nikolai basaltic rocks are predominantly tholeiitic. The major-element compositions of both sequences are very similar and cluster within a narrow range. The trace-element data for both sequences suggest a composition similar to the high-magnesian Columbia River flood basalts. Sediments associated with the Chilkat and Nikolai basaltic rocks share gross lithologic features and a similar depositional environment, even though the upper part of the Chilkat sequence appears to be a more offshore facies. The petrologic and geochemical data are compatible with the inference that the Taku terrane may have been coextensive with Wrangellia during the Late Triassic but has subsequently been laterally offset by dextral movement on the Denali fault.

Paleomagnetic evidence for a post-Cretaceous accretion of Wrangellia

In order to test the various estimates of the accretion age of some of the southern Alaska terranes, a paleomagnetic investigation of the MacColl Ridge Formation (latest Cretaceous age) in the Wrangell Mountains (Wrangellia) was undertaken. The data pass the fold test and reversal test, thus affirming the primary nature of the magnetic remanence. The mean paleomagnetic direction corresponds to a paleolatitude of 32°±9°, which is far south of the predicted paleolatitude, assuming a North American paleogeography. Therefore, these results apparently rule out a pre-Cenozoic accretion age of Wrangellia and its associated terranes composing the Southern Alaska superterrane. The data also imply about 130° counterclockwise rotation, which is the opposite sense of rotation usually observed in western North America.

Seven Devils terrane: Is it really a piece of Wrangellia?

The Seven Devils terrane of eastern Oregon and western Idaho has been correlated with the Wrangellia terrane, which is best exposed on Vancouver Island, British Columbia, and in the Wrangell Mountains, Alaska. However, geologic histories of these two terranes are different. The Triassic section of the Seven Devils terrane represents a volcanic arc of probable calc-alkalic affinity. It is characterized by abundant volcaniclastic lithologies, a complete suite of flow rocks, and fractionated rare-earth-element distribution patterns. The sequence of similar age in Wrangellia includes thick accumulations of tholeiitic pillow basalts, subaerial basalt flows, tuffs, and breccias. The tectonic environment in which these rocks were extruded is not known, but they are not an island-arc assemblage. Paleomagnetic and faunal evidence suggests that the Seven Devils terrane and Wrangellia could have formed close to one another. This evidence does not override the geologic arguments against correlation of these terranes. Lithologic, geochemical, and stratigraphic data all militate against consanguinity of the Seven Devils arc and Wrangellia.

Discovery of a Large-Magnitude, Late Pleistocene Volcanic Eruption in Alaska

An extensive blanket of dacitic tephra was deposited across Alaska and the western Yukon Territory by a plinian eruption about 80,000 years ago. The volume of tephra erupted probably exceeded 50 cubic kilometers and the source was probably in the Wrangell Mountains. This widespread layer of tephra serves as an excellent late Pleistocene stratigraphic marker.

Homing of Transplanted Alaskan Brown Bears

Homing of Transplanted Alaskan Brown Bears

Tectonic model and seismic potential of the eastern Gulf of Alaska and Yakataga Seismic Gap

Based on 13 new fault plane solutions and published seismological, geological, and geophysical data, we interpret the deformation along the Pacific‐North American plate margin in the eastern Gulf of Alaska. Three major tectonic units can be distinguished: (1) the North American plate, (2) the Pacific plate, and (3) a belt of mobile borderland terranes. The Pacific plate moves in a NNW direction at rates of about 6 cm/yr in relation to the North American plate. That motion results in mostly right‐lateral strike slip at the Queen Charlotte‐Fairweather fault system, a well‐known observation. A new finding,however, is that a small component (∼1 cm/yr) of convergence may also be present which results in minor subduction of the oceanic plate beneath portions of the continental margin. Heretofore the Queen Charlotte‐Fairweather fault zone and associated continental margin was interpreted as a classical, pure transform boundary. The Yakutat block, a borderland terrane about 400 km long and 100 to 200 km wide, is carried passively by the Pacific plate except that the block slowly overrides this plate at about 1 cm/yr. This motion is taken up by almost pure thrust faulting in a southwesterly direction along a 400‐km long SE striking shelf edge structure. At its NW edge the Yakutat block is in turn being thrust beneath the North American plate along the Pamplona zone‐Icy Bay lineament. The underthrusting of the Yakutat block results in a major orogeny, crustal shortening and uplift of the Chugach‐St. Elias range. The effects of this collision may extend as far as 500 km inland and cause some deformation at the Denali fault in the central Alaska Range. Subduction of the Pacific plate beneath the colliding margin appears responsible for development of an active volcanic arc up to 300 km inland which trends SE from the Wrangell Mountains to Yukon Territory, Canada, and perhaps to Mt. Edgecumbe volcano in southeast Alaska. The tectonic model proposed implies a high seismic hazard for the Queen Charlotte, Fairweather, and Chugach‐St. Elias fault systems. At these fault zones we estimate recurrence times for great events of about 100 years, but they may vary between 50 and 200 years. A temporarily very high potential for a great earthquake has been determined for the ‘Yakataga seismic gap’ located between Icy Bay and Kayak Island. Large or great thrust earthquakes on the detachment fault underlying the entire Yakutat wedge also appear possible but may only occur infrequently. Their recurrence times are estimated to be several

The Alaskan orocline, the paleomagnetism and the paleogeography of Alaska

Paleomagnetic data from southwestern Alaska when compared with similar data from North America do not support Carey's orocline hypothesis for Alaska's tectonic development. Further analysis of the Alaskan data and comparison with paleomagnetic data from the Wrangell Mountains point to a scenario for the development of southern Alaska involving the accretion of a number of island arcs and slivers of continental material against the northeast Pacific margin. This could be the result of the general northward motion of the Pacific plates from Mesozoic to the present. Perhaps the best analog for proto-Alaska would be the southwest Pacific today where there appears to be a number of small ocean basins being compressed together.

Holocene glacial and tree-line variations in the White River Valley and Skolai Pass, Alaska and Yukon Territory

Complex glacier and tree-line fluctuations in the White River valley on the northern flank of the St. Elias and Wrangell Mountains in southern Alaska and Yukon Territory are recognized by detailed moraine maps and drift stratigraphy, and are dated by dendrochronology, lichenometry, 14C ages, and stratigraphic relations of drift to the eastern (1230 14C yr BP) and northern (1980 14C yr BP) lobes of the White River Ash. The results show two major intervals of expansion, one concurrent with the well-known and widespread Little Ice Age and the other dated between 2900 and 2100 14C yr BP, with a culmination about 2600 and 2800 14C yr BP. Here, the ages of Little Ice Age moraines suggest fluctuating glacier expansion between ad 1500 and the early 20th century. Much of the 20th century has experienced glacier recession, but probably it would be premature to declare the Little Ice Age over. The complex moraine systems of the older expansion interval lie immediately downvalley from Little Ice Age moraines, suggesting that the two expansion intervals represent similar events in the Holocene, and hence that the Little Ice Age is not unique. Another very short-lived advance occurred about 1230 to 1050 14C yr BP. Spruce immigrated into the valley to a minimum altitude of 3500 ft (1067 m), about 600 ft (183 m) below the current spruce tree line of 4100 ft (1250 m), at least by 8020 14C yr BP. Subsequent intervals of high tree line were in accord with glacier recession; in fact, several spruce-wood deposits above current tree line occur bedded between Holocene tills. High deposits of fossil wood range up to 76 m above present tree line and are dated at about 5250, 3600 to 3000, and 2100 to 1230 14C yr BP. St. Elias glacial and tree-line fluctuations, which probably are controlled predominantly by summer temperature and by length of the growing and ablation seasons, correlate closely with a detailed Holocene tree-ring curve from California, suggesting a degree of synchronism of Holocene summer-temperature changes between the two areas. This synchronism is strengthened by comparison with the glacier record from British Columbia and Mt. Rainier, Likewise, broad synchronism of Holocene events exists across the Arctic between the St. Elias Mountains and Swedish Lappland. Finally, two sequences from the Southern Hemisphere show similar records, in so far as dating allows. Hence, we believe that a preliminary case can be made for broad synchronism of Holocene climatic fluctuations in several regions, although further data are needed and several areas, particularly Colorado and Baffin Island, show major differences in the regional pattern.

An Observation of Nocturnal Passerine Migration Through Panamá

This was a much wetter habitat than the plateau to the south of Rock Lake and contained a considerable amount of standing water masked by the vegetation. The discovery of Smith's Longspur breeding in the Wrangell Mountains area extends the known range in Alaska south of the Arctic Circle, and some 400 miles SSE of the nearest known breeding area in the Sheenjek valley on the south side of the Brooks Range. It is also interesting that the Wrangell Mountains sites are at much higher elevations than those reported from the Brooks Range. It now remains to be seen whether or not the species breeds in any of the intervening areas. Across the Canadian border there are records for southern Yukon Territory (Clarke 1945), and breeding evidence from along the Haines Highway in British Columbia (Weeden 1960).

Scientists eye steaming Mount Wrangell

Scientists of the U.S. Geological Survey and the University of Alaska are observing closely the increasingly high amounts of steam being emitted from the summit crater of 14,163‐ft (4.3169 km) Mount Wrangell, part of the Wrangell Mountains of Alaska. Although Mount Wrangell has been a special site of observations for the past 10 years, the monitoring of observations has been intensified because of the greatly increased heat flow, the amount of steam emitted, and the number of fumaroles this year. These are possible indications of an increasingly ‘restless’ volcanic mountain.The heat flow from and the number of fumaroles in Mount Wrangell have been increasing slowly for the past 10 years. Because of the ‘picked‐up tempo’ of volcanic activity the three scientists who have been making periodic observations of the mountain during the last decade have begun an intensified monitoring program. Carl Benson, a professor at the Geophysical Institute of the University of Alaska, has made at least one field trip each year to the top of the mountain to conduct experiments. Tom Miller, an igneous petrologist, and Larry Mayo, a glaciologist, both with the U.S. Geological Survey, have supported Benson's ground observations with aircraft observations. Their attention has been focused on the summit caldera.

Recent Observations in the Wrangell Mountains, Alaska

Little has been published on the avifauna of the Wrangell Mountains in southern Alaska. Therefore I wish to record some of my observations there during late June and July 1973. Of 56 species recorded in the study area, details are given below for 22 species which are of interest. In view of their known distribution in Canada, most of these species might be expected to occur in the Wrangell Mountains but have not previously been recorded there. The area studied was the immediate vicinity of Rock and Ptarmigan lakes in the eastern Wrangell Mountains, some 140 miles ESE of Glennallen on the Richardson Highway, and close to the Canadian border at 61o48' N 141015' W. The terrain was comprised of the two lakes and their connecting river valley, in which was a small unnamed lake, surrounding muskeg and spruce forest, and the adjacent mountains up to elevations a little in excess of 5000 ft above sea level. Willow and birch scrub extended from the lakes up the mountain slopes almost to the highest elevations, and above the spruce forest a belt of Populus balsamifera was usually present at 3900-4100 ft. Aythya marila. Greater Scaup. A pair on Rock Lake on 30 June, three pairs in the valley east of this lake on 4 July, and three females on Rock Lake on 15 July. No evidence of breeding was found. Bucephala islandica. Barrow's Goldeneye. There were more than six females on the unnamed lake on 1 July. On 3 July a female with six young was located on a pool on the south side of Rock Lake, and on the same date four females were seen on the river. Bucephala albeola. Bufflehead. A female appeared on Rock Lake on 16-17 July but was not seen again. Melanitta perspicillata. Surf Scoter. There were two pairs on Rock Lake on 3 July. Oxyura jamaicensis. Ruddy Duck. A female was present on the unnamed lake on 9 July. This duck has been recorded closest to the Wrangell Mountains in the Tetlin Lake area, about 100 miles to the north (Hansen, Condor 62:137, 1960). Aquila chrysaetos. Golden Eagle. Two adults, a subadult, and a juvenile were seen on various dates over the mountains immediately north of Rock Lake. A recently occupied aerie was located at about 5000 ft.